The main difference between food proteins and bug proteins in that this hypothesis is concerned, is that to achieve being replicated, bug proteins need a genetic code, while food proteins do not need it, as they have an idiot who “replicate” them (the idiot introduces voluntarily these proteins in its body by eating them again and again over a lifetime).



This hypothesis is intended to demonstrate that most human diseases are directly or indirectly due to the crosslinking (forever covalently binding) of certain food proteins with certain endogenous proteins by a human endogenous enzyme, transglutaminase.

The question is simple, is it really possible that one of our own enzymes bind a food protein to one of our own proteins forever in vivo? What would be the consequences if that happened?



Cell adhesion mediated by specific cell surface molecules plays a pivotal role in life sciences (Alsteens 2009).

Neuronal interactions, cellular communication, tissue development, inflammation, cancer and microbial infection are just a few examples of cellular processes regulated by cell adhesion molecules (Alsteens 2009).



Due to the lack of availability of studies investigating the effect of this mechanism on food proteins, to begin to address the questions this hypothesis propose we focus initially on analyzing bug studies related to similar issues.

Ok, lets look at those bugs!:

Microbial adherence is one of the most important determinants of pathogenesis (9).

Adhesion to host tissues generally is a prerequisite for colonization of the host by infecting microorganisms (Emody 2000).


Attachment of fungi to host cells is a critical part of fungal pathogenesis in animals (12).

During the past decade there has been an increased recognition of the importance of adhesion of fungi to host surfaces–both plant and animal–before penetration (Epstein 2006).

Even if we limit our discussion to fungal-substratum adhesion that occurs on the plant host surface before penetration, adhesion serves multiple functions: most obviously, adhesion keeps a fungus from being blown or rinsed from a potentially suitable environment. Adhesion prevents fungus displacement by water and/or wind (Epstein 2006).

Adhesion-reduced mutants of the plant pathogenic fungus Nectria haematococca were less virulent than the wild-type when deposited on the intact surface of cucurbit fruits, but were equally virulent when deposited into wounded fruits; the results indicated that adhesion is a virulence factor in the “natural” environment and suggested that adhesion prevents displacement by water and assists in efficient localization of secreted enzymes on the host surface (Jones and Epstein 1990).


Hwp1 Hyphal wall protein 1:

Hwp1 is a protein (glyco(manno)protein) located on the surface of a dimorphic fungus called Candida albicans.


Hwp1 is a transglutaminase substrate (N-terminal third of Hwp1 to be precise) (1).


Dimorphic: Candida Albicans fungus has two forms: round-to-oval yeast or blastospore form and filamentous hyphal form.


Hwp1 is expressed exclusively on hyphae surface of adhesive-invasive-competent hyphal filamentous form of Candida Albicans (3).


Candida Albicans (3 - 5 µm in diameter, round-to-oval, yeast or blastospore form)


Candida Albicans (up to 50 μm in lenght, filamentous, hyphal form)


Anatomy of Hwp1: Left: three-dimensional structure and Right: aminoacid sequence (9).



During both mucosal colonization and induction of disease, Candida Albicans interact with epithelial cells (14).

The interactions of Candida albicans with epithelial cells include adhesion and invasion (14).

Adhesion: The organism first adheres to epithelial cells. Adherence is mediated by multiple different adhesins that are present on the fungal cell surface. Some adhesins are expressed only by hyphae (like Hwp1 and Als3), whereas others are expressed by both hyphae and yeast-phase organisms (14).

Invasion: Next, the adherent organism can invade both into and between epithelial cells. Invasion into an epithelial cell can occur by induced endocytosis, whereby Als3 and other invasins on the fungal cell surface bind to E-cadherin and other target proteins on the epithelial cell surface. Binding to these epithelial cell proteins induces the epithelial cell to produce pseudopods that engulf the organism and pull it into the cell. C. albicans actively penetrates into epithelial cells by a mechanism that is currently poorly understood. It actively penetrates between epithelial cells by secreting aspartyl proteases that degrade E-cadherin and other interepithelial cell junctional proteins. Invasion into and between epithelial cells is a prerequisite for induction of epithelial cell damage (14):

Two major interactions of Candida Albicans with epithelial cells: Adhesion and Invasion (14).


The outcomes of these interactions are important in determining whether the organism can colonize a mucosal surface and subsequently cause disease (14).


Below: Both Candida Albicans forms; round-to-oval form  and filamentous hyphal form  have adhesion ability but the conversion from the round-to-oval form to the filamentous hyphal form enhances this ability (Höfken 2013):


Below: Possible mechanisms of epithelial or endothelial invasion: Left: Induced endocytosis: the interaction between yeast wall proteins such as als3 and Ssa1 with N-cadherin on the surface of endothelial cell triggers endocytosis. Center: Active penetration into a cell: cells are severely damaged in this process. Right: Active penetration between cells: active penetration between cells by degrading junction proteins and extracellular matrix (Höfken 2013):



In response to various environmental stimuli (in the presence of serum, high temperature, neutral pH, or nutrient-poor media), Candida albicans alters its morphology from a unicellular budding yeast to a multicellular hyphal form.


To stick and invade host tissues, Candida Albicans cells must first switch from the round-to-oval,  yeast or blastospore form to the filamentous, hyphal form, a phenotypic transition that involves dramatic changes both in cellular architecture and gene expression.


Many genes associated with this transition are essential for virulence, including both cell wall proteins and secreted enzymes (6).


The yeast-hypha transition is accompanied by the de novo synthesis of proteins that are targeted to hyphal surfaces (3).


One of these proteins expressed only on hyphae is Hwp1. Other remarkable protein expressed only on hyphae is called Als3.


Yeast-to-hyphal or filamentation (from white cells emerges a projection called hyphae)

Candida Albicans white cells undegoing filamentation by producing cellular projections called hyphaes.

Germ tubes emerge from yeast cells with no septum (wall) formed, then germ tubes elongate and only later a septum is formed in the hyphal tube


Hwp1 (and Als3) appear only on the hyphae during the yeast to hyphae morphological transition.


Several mechanisms have been proposed to contribute to the changes in surface composition of Candida albicans during morphogenesis. Ultrastructural studies support the occurrence of rearrangements or losses of wall components during morphogenesis , whereas unmasking of cryptic antigenic determinants has been suggested using monoclonal antibodies to localize specific antigens (3).




HWP1 encodes for a unique protein with combined mammalian and fungal functional domains (11).

Hwp1 combines features of mammalian transglutaminase substrate proteins with characteristics of fungal cell wall proteins to form an unconventional adhesin at the hyphal wall of Candida albicans (11).

Hwp1 has a half-mammalian- like, half-fungal hybrid primary structure (11).


Hwp1 can be subdivided in two domains: the serine/threonine rich domains I and II  and the antigenic, repetitive or proline/glutamine rich or transglutaminase substrate domain (4).




The central and C-terminal regions of Hwp1 contained a high percentage of serine and threonine residues, serine/threonine-rich regions are predicted to function in extending a ligand-binding domain into the extracellular space (4).

A C-terminal domain rich in hydroxy amino acids serves to extend a binding domain above the cell surface, may help explain the exposure of the antigenic domain of Hwp1 at the cell surface (4).

The features of Hwp1 are consistent with the paradigm that yeast cell surface proteins important for ligand binding have unique N-terminal domains that confer binding specificity but C-terminal domains with common features that permit cell wall anchoring and surface-exposure of N-terminal domains (4).



The N-terminal repetitive domain comprised approximately one-third of the overall amino acid sequence and is located in the N-terminal third of the protein (4).

The N-terminal antigenic domain was more hydrophilic and had an increased concentration of negatively charged residues compared with the remainder of Hwp1 (4).

Transglutaminase substrate Hwp1 domain is a proline- and glutamine-rich protein (3).

Transglutaminase substrate Hwp1 domain is a peptide that is largely composed of an acidic, repeated motif (degenerate amino acid repeat) (series of tandem repeats) 10 amino acids in length that is rich in proline and glutamine residues (3).

Below: Alignment of the repetitive amino acid sequences (repeats starting with amino acid 14) of Hwp1. Gaps have been introduced to maximize the similarities of the repeats (3):

The amino acid composition was notable in having 27% proline, 16% glutamine, and 12% aspartate residues (mole percents) (3).

Common features included proline residues at positions 2, 6, and 9 in all but two of the repeats. Cysteine residues predominated at position 3 and aspartate was found at position 4 (3).

The presence of a cysteine residue in each repeat probably leads to the formation of regularly spaced extracellular disulfide bonds that may be important for the surface conformation of hwp1 and possibly for intermolecular cross-linking of proteins on the cell surface. Given the specificity of hwp1 for hyphal surfaces, the presence of cysteines may be related to the enhanced sensitivity and increased protein release from hyphal forms following treatment with dithiothreitol (3).

The first six repeats had a tyrosine in the fifth position and glutamate in the tenth position, whereas the more carboxy proximal repeats had asparagine and aspartate, at the fifth and tenth positions, respectively (3).

The C terminus of hwp1 was threonine- and serine-rich providing abundant potential sites for O-glycosylation. The lack of agreement between the predicted molecular weight and the SDS-polycrylamide gel-determined molecular weight is typical for proteins with high proline content, although O-glycosylation of serine residues located near the C terminus might also contribute to the discrepancy (3).

The serine and threonines near the C-terminal end of hwp1 are likely to be sites for O-glycosylation and serve as wall spanning domains as has been proposed for other yeast surface proteins (3).


Hwp1 lacks Lys residues that are usually found adjacent to reactive Gln residues in TGase substrates (11).

This absence of Lys residues indicate that Hwp1 participates in cross-linking reactions solely as the Gln donor (11).


Computer modeling predicts that the N-terminal domain of Hwp1 is hydrophilic (11).

Hwp1 was found to be very acidic, having 63 negative charges at neutral pH contributing to the strong negative charge of surfaces of Candida albicans hyphae (4).

The presence of 31 acidic residues characterizes rHwp1N13 as a very soluble protein (11).

An additional feature of the repeats is the presence of acidic amino acids that would confer a negative charge to hyphal surfaces at physiological pH. The presence of anionic proteins on hyphal surfaces has been demonstrated by others (3).

The deduced amino acid composition (of antigenic Hwp1 domain) was hydrophilic throughout the entire sequence ( (3).


Computer modeling predicts that the N-terminal domain of Hwp1 is a predominantly coiled structure (11).

Chou-Fasman predictions of secondary structure showed the sequence to be primarily composed of turns with two helical sections in locations containing five and six consecutive glutamine residues (3).

The changes in ellipticity in the case of SPR3 were suggested to reflect β-turns that exist between the repeating amino acid units. Although there is evidence for similar organized structure in Hwp1, the amount is much lower than that found in SPR3 (11).

Biophysical analysis of the N-terminal domain of Hwp1 discloses a tight disulphide-bonded coil without α helices or β sheets (9).

The TGase substrate domain itself is a tight, disulfide-linked coil without discernible secondary structure (11).

Secondary Structure Features of the N Terminus of Hwp1: rHwp1N13 is a protein that is comprised predominantly of a coiled structure (11).

rHwp1N13 N-terminal domain exists completely as a coil. The N-terminal domain of Hwp1 is likely a rigid coiled structure (11).


Native Hwp1 is a complex mixture of forms that are cell-free, membrane-bound, as well as cell wall-linked (11).

Common occurrence of both membrane and cell wall anchored forms of cell wall localized proteins (11).

Hwp1 maturation into the cell wall-bound form: Presence of a 325-kDa GPI-anchored membrane species, which may be a precursor of a periplasmic 301-kDa intermediate destined to become covalently attached to glucan. Soluble 301-kDa Hwp1 may represent a precursor population of Hwp1 in transition between a membrane protein and the wall-anchored mature form. Small amounts of the 301-kDa species are present in the culture media of germ tubes, indicating that this form, if not bound to the cell wall, can diffuse out into the medium (11).

Furthermore, expressing a C-terminal truncated form of Hwp1 produced a protein of 301 kDa that was not bound to germ tube walls but instead found in the culture medium (11).

The latter results also showed that the C-terminal 26 amino acids of Hwp1 contain the necessary information to guide and link the protein to the β-glucan (11).

The amino acid sequence-based prediction that Hwp1 is a member of cell wall proteins covalently attached to the cell wall β-Glucan of Candida albicans (11).

The cell wall forms of Hwp1 comprise 75% of the total Hwp1 (11).

Although noncovalently bound forms of Hwp1 exist at the surface of germ tubes, it is likely that TGase associated with host buccal epithelial cell surfaces interacts only with the mature cell wall-attached Hwp1 during adhesion (11).

Below: Cell wall anchorage of Hwp1: The majority of Hwp1 is covalently bound to the cell wall; Hwp1 cross-linked to β(1,3)-glucan via β(1,6)-glucan in the cell wall (9):


Hwp1 appear to have N-terminal domain whose functions require exposure to the extracellular environment (4).

Data supports the existence of an N-terminal domain of Hwp1 that is set apart from the remainder of the protein (9).

Chymotrypsin sensitivity of native Hwp1 on germ tube surfaces further strengthening the argument for the separation of the N-terminal domain from the rest of the protein (11).

The surface probabilities were also highest for the sites with consecutive glutamine residues and for glutamines bounded by prolines (3).

Hwp1 is an outer protein with a cell surface-exposed NH2-terminal domain (1).

Hwp1 interacts with the cornified epithelium through the N-terminal domain (11).



The presence of amino acid repeats on hyphal surfaces is not surprising given that tandem amino acid repeats are widely distributed on surfaces of broad groups of microorganisms including fungi, parasites, viruses, and bacteria. The functions of repetitive surface proteins have frequently been found to involve the host and include attachment sites to host cells, evasion of phagocytosis, invasion of host cells, and neutralization epitopes (3).



Screens for germ tube–specific surface proteins that might function in adherence led to the identification of hyphal wall protein 1 (Hwp1), a protein with expression limited to surfaces of germ tubes and true hyphae (12).

Hwp1 is an important developmentally regulated adhesin (9).

Hwp1 is not found on surfaces of yeast forms or pseudohypahe forms of Candida Albicans (9).

Hwp1 is specifically localized in the walls (surfaces) of germ tubes and (true) hyphae of Candida albicans (3, 4).

Hwp1 is expressed on the surface of hyphae in the pathogen Candida albicans (3).

Hwp1 is exposed on surfaces of hyphae grown in mammalian hosts (3).

Hwp1 is present in hyphal but not yeast forms (3).


Using recombinant Hwp1 it was confirmed that Hwp1 is localized to hyphal surfaces of Candida Albicans (3).

Antiserum raised to a recombinant Hwp1 encoded by HWP1 cDNA should crossreact with hyphal surfaces. Serum from rabbits immunized with rhwp1 was tested for the ability to recognize antigens on C. albicans hyphal surfaces in immunofluorescence assays (3).

Immune serum stained hyphal surfaces but not the parent blastoconidia of C. albicans. Thus the antiserum was specific for rhwp1 and hyphal surfaces of C. albicans (3).

Hwp1 was found to be present on hyphal surfaces by immunofluorescence assays using monospecific antisera raised to the recombinant protein (3).

Antibodies from the screening antiserum that cross-reacted with hyphal surfaces of C. albicans in an immunofluorescence assay that employs germ tube forms as antigens. Yeast surfaces were negative or weakly fluorescent (3).

Below: Detection of hwp1 on hyphal surfaces of Candida Albicans grown in laboratory cultures. Candida Albicans yeasts bearing germ tubes were treated with various primary anisera followed by fluoresceinated goat anti rabbit IgG in indirect immunofluorescence assays; preimmune serum (left), mono-specific antiserum to rhwp1 (right) (3):

Hwp1 is exposed on hyphal surfaces and not on yeast surfaces of C. albicans (3).


Antibodies to Hwp1 were used to detect Hwp1 on Candida Albicans surfaces (11).

Below: Detection of hwp1 on hyphal surfaces of Candida Albicans grown in laboratory cultures. Immunofluorescence assay was performed to detect Hwp1 on the surface of germ tubes of a Candida Albicans without Hwp1 (left) and a Candida Albicans with Hwp1 (right) (11):


To determine if hwp1 was produced by C. albicans growing in mammalian hosts, immunofluorescence assays were performed on paraffinembedded tissues from beige mice that were heavily colonized with C. albicans (3).

Large numbers of organisms were seen in the lumen and keratinized superficial layers of the stomach following indirect immunofluorescence staining with polyvalent rabbit antiserum to C. albicans (3).

Monospecific antiserum to rhwp1 stained the filamentous but not yeast forms of C. albicans in the tissue sections, indicating that hwp1 is specific to hyphal forms during growth in the host as well as during growth in laboratory medium (3).

Below: Detection of hwp1 on hyphal surfaces of C. albicans grown in host tissues. Sections treated with various antisera from an 11-month-old females mouse that had been colonized for 9 months with C. albicans: preimmune serum (left), monospecific anti-serum to rhwp1 (right). The large arrowhead points to positive-staining hyphae, and the small arrowhead points to negative-staining yeast (3):



Anti-Hwp1 antibody staining reveals that Hwp1 is expressed in serum-induced hyphae of Candida Albicans (5).

Below: White cells and daughter buds of Candida Albicans did not stain with anti-Hwp1 antibody (5):

Below: White cells of Candida Albicans induced to form hyphae by suspending white cells in serum, the walls of the resulting germ tubes and hyphae stained selectively with Hwp1 antiserum (5):

Mother cells remained unstained, demonstrating that differentially expressed Hwp1 localized exclusively in the hyphal wall (5).

In an analysis of 500 white budding cells stained with anti-Hwp1 antibody, 0% of mother and 0% of daughter cells stained (5).

In a similar analysis of serum induced hypha-forming cells, 0% of the mother cells and 100% of the hyphae of each strain stained (5).

In an analysis of 500 untreated opaque phase cells 0% of the cells stained (5).

These results demonstrate that Hwp1 is differentially expressed in hyphae, and that budding opaque phase cells do not express Hwp1, even though they have been shown to express other hypha wall antigens (5).


Synthesis of this Hwp1 protein occurred exclusively during hyphal growth, showing that the bud-hypha transition controls the antigenic surface composition of hyphae by production of de novo proteins (3).

The hyphae-specific surface location was also seen on organisms colonizing the gastrointestinal mucosa of mice, indicating that Hwp1 is produced and developmentally regulated during growth in host tissues (3).


To determine if hwp1 was the sole antigen recognized by the screening hyphae-specific antiserum, blocking experiments were performed with this antiserum as well. Recombinant hwp1 did not block the fluorescence exhibited by the hypha-especific antiserum, indicating that antigens other than hwp1 are present on hyphal surfaces (3).




Hwp1 is a substrate for mammalian transglutaminase (1).


Potential transglutaminase susbtrates in the proved transglutaminase substrate N-terminal of Hwp1 (Hwp1: aminoacids 0 to 240, P46593 UNIPROT)


Potential transglutaminase susbtrates in the “only Q” zone of the proved transglutaminase substrate N-terminal of Hwp1 (Hwp1: aminoacids 40 to 164, P46593 UNIPROT)



Employing recombinant Hwp1 (rHwp1) it has been showed that the mature N-terminal third of Hwp1 serves as a substrate for mammalian TGases (11).

To determine if the N-terminal third region of Hwp1 is a substrate for TGase, we examined a recombinant protein, rHwp1∆C37,  a partial protein encoded by the N-terminal repetitive proline- and glutamine-rich domain (aminoacids 40 to 197 of Hwp1) for the ability to incorporate [14C]putrescine in the presence of TGase2 (1).

The recombinant protein was similar to other TGase2 substrates in the generation of multiple species of radiolabeled reaction products including monomers displaying increased migration , species of high molecular weight , and dimers bridged by putrescine. The production of all radiolabeled forms depended on the presence of active Tgase (1).

In addition to [14C]putrescine, TGase2 catalyzed the incorporation of another TGase cosubstrate, monodansylcadaverine, into rHwp1∆C37. The behavior of rHwp1∆C37 in interactions with TGase2 matched that of SPR proteins and other host TGase substrates and implicated the NH2-terminus of Hwp1 (aminoacids 40 to 197 of Hwp1) in cross-linking reactions with primary amines through Nε-(γ-glutamyl)lysine isodipeptide bonds (1).




Transglutaminase activity is produced by mammalian cells and not by Candida albicans (17).

With the Candida albicans adhesin Hwp1, the host transglutaminase covalently links the pathogen to the host epithelial cell (1).

Hwp1 is a substrate for transglutaminase activity derived from a host.

No intrinsic Candida albicans transglutaminase was detected. No endogenous TGase activity of Candida albicans was detected in whole organisms or in broken cell walls (1).

Candida Albicans adhesion represents a paradigm for microbial adhesion because it implicates essential host enzymes (1).


Hwp1 serves as a substrate for mammalian TGases (11).


Hwp1 participates in the formation of covalent bonds to primary amines and to a buccal epithelial cell (BEC) surface protein catalyzed by human transglutaminases (TGs) (13).


BEC TGases and TGase substrates participating in interactions with C. albicans were not identified (1).

The TGase activity on surfaces of buccal epithelial cells, attributed to TGase 1 by others utilizes rHwp1 as a substrate (11).


TG1 (keratinocyte transglutaminase) is active on the surface of buccal epithelial cells (BECs) where it functions in assembly of the cornified cell envelope, a scaffold of cross-linked proteins that gives mucosal cells their barrier properties. We found the participation of Hwp1 in the formation of a stabilized (heat and detergent resistant) TG-dependent adhesion to BECs, and speculated that TG1 on the surface of BECs catalyzed this reaction. In support of this conclusion, Candida Albicans without Hwp1 is unable to cause oropharyngeal candidiasis in mice. However, we were unable to verify the role of TG1 directly as mice without TG1 die of dehydration soon after birth (13).

A mechanism of adhesion based on transglutaminase activity on BECs could explain the requirement for Hwp1 in oral candidiasis (13).


In vitro, both native and recombinant Hwp1 are substrates for guinea pig liver tissue transglutaminase (TG2) (1, 13).

Hwp1 is a substrate for human tissue transglutaminase TG2 (13).

It is plausible that human tissue TG2 catalyzes transamidation reactions involving Hwp1 (13).



Hwp1 is a transglutaminase substrate located on hyphal surfaces of Candida Albicans (1).

The NH2-terminal domain of native Hwp1 appears to be a surface-exposed TGase substrate available for interactions with exogenous mammalian Tgases (1).


To asses presence of Hwp1 as a transglutaminase substrate on germ tube surfaces of Candida Albicans, strains with or without Hwp1 were used for comparison in TGase assays (1).

Below: Detection of TGase substrates of wild-type strain (with Hwp1, left) versus a mutant strain (without Hwp1, right) of Candida Albicans: Candida Albicans germinated cells were incubated with TGase2 (guinea pig liver tissue transglutaminase) and 5-(biotinamido) pentylamine (a transglutaminase substrate used for fluorescence staining of TGase substrates). TGase substrates were stained on germ tube surfaces of the strain with Hwp1 (left) but not on the strain without Hwp1 (right) thus indicating the requirement of Hwp1 for TGase substrate activity (1):


The results showed that Hwp1 functions as a substrate for transglutaminase and is the major substrate on hyphal surfaces.


Expression of Hwp1 on the surface of Candida albicans germ tubes was visualized by cross-linking 5-(biotinamido)pentylamine (5-BPA) to the germ tube surfaces by human recombinant tissue transglutaminase (hTG2) (13).

The primary amine, 5- (biotinamido)pentylamine and human recombinant tissue transglutaminase (hTG2) were used to visualize Hwp1 on the fungal surfaces (13).

Left column, light images; right column stained images (13).

Below: Candida Albicans without Hwp1 (SCH1211) (13):

Below: Candida Albicans with Hwp1 (SC5314) (13):



Searches of computer data bases showed that hwp1 was similar to a wide variety of proteins containing amino acid repeats having periodic proline residues and/or glutamine residues; however, no identical sequences were found (3).

Hwp1 (Hwp1 transglutaminase substrate domain) shares features with surface proteins of other lower eukaryotic microorganisms (3).

Hwp1 mimicks host cell transglutaminase (TGase)1 substrates (11).

In mimicking host ligands, Hwp1 adds an example from the fungal kingdom to known microbial adhesins that imitate mammalian proteins (11).



SPRs are proteins located in terminally differentiating mammalian stratified squamous epithelium and in cornified cell envelope (CE) (Tarcsa 1998).

Stratified squamous epithelium: The outermost layer of the skin and the inner lining of the mouth, esophagus, and vagina. It consists of squamous (flattened) epithelial cells arranged in layers upon a basal membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity. This type of epithelium is well suited to areas in the body subject to constant abrasion, as it is the thickest and layers can be sequentially sloughed off and replaced before the basement membrane is exposed (Wikipedia).

There are two types of stratified squamous epithelium: Nonkeratinized: Non-keratinized surfaces must be kept moist by bodily secretions to prevent them from drying out. Types of non-keratinized stratified squamous epithelium include cornea (see also corneal epithelium), oral cavity, esophagus, anal canal, vagina, and the internal portion of the lips. Keratinized: Keratinized surfaces are protected from abrasion by keratin and kept hydrated and protected from dehydration by glycolipids produced in the stratum granulosum. Examples of keratinized stratified squamous epithelium include epidermis of the palm of the hand and sole of the foot and the masticatory mucosa (Wikipedia).

Cornified cell envelope: It is a layer of protein assembled at the cell periphery in different epithelia that affords essential barrier function to stratified squamous epithelia. SPRs serve as structural protein precursors of the cornified cell envelope (Tarcsa 1998).

To date, three classes of SPRs have been identified: SPR1 (two members in all species examined so far), SPR2 (8–11 members), and a single member of the SPR3 class (Tarcsa 1998).

The expression of SPR proteins varies widely between different epithelia. For example, the epidermis expresses SPR1a (also known as cornifin α) and a limited number of SPR2 members. Other internal epithelia express more abundant amounts of other SPR2 members together with SPR1a. The SPR3 (also known as cornifin β) protein is most abundantly expressed in oral epithelia, the esophagus and rodent forestomach epithelium. Cultured keratinocytes from several epithelial sources tend to express both SPR1a and 1b, as well as a limited selection of other members. In addition, the amounts expressed vary during differentiation. Furthermore, a much wider selection of SPR members is expressed in epithelia in response to UV damage, chemical treatment and in hyperproliferative or malignant diseases (Tarcsa 1998).

All SPRs are built according to a common plan. They possess (head) amino- and (tail) carboxyl-terminal domains that are enriched in Gln, Lys, and Pro residues of sequences that are characteristic of and highly conserved between members within each class (Tarcsa 1998).

The SPR name in fact is based on a distinctive central domain consisting of a series of peptide repeats of either eight (SPR1 and SPR3) or nine (SPR2) residues that are enriched in Pro residues and of sequences that vary between the three classes, but members within each class are also highly conserved. The numbers of repeats range from as few as three in all human SPR2 proteins and the smallest mouse SPR2 protein to more than 23 in SPR3 proteins (Tarcsa 1998).


SPRs are transglutaminase substrates (1, 8, 11).

SPRs become cross-linked to other proteins by transglutaminases (Tarcsa 1998).

The Cornified cell envelope (CE) is made insoluble by extensive cross-linking of several defined structural proteins including the SPRs by disulfide bonds and Nε-(γ-glutamyl)lysine or N1,N8-bis(γ-glutamyl)spermidine isopeptide bonds formed by the action of transglutaminases (Tarcsa 1998).

TGases catalyze the usually irreversible formation of an intra- or more usually intermolecular isopeptide bond between acyl donor Gln and acceptor Lys residues. Unambiguous evidence that SPRs serve as CE precursors and are in fact good substrates for TGases in vivo comes from direct sequencing analyses. Many peptides recovered from limited proteolysis experiments of CEs isolated from human foreskin epidermis, cultured epidermal keratinocytes and mouse forestomach epithelium contained recognizable SPR sequences cross-linked through one or more isopeptide bonds to other CE proteins. Examination of these peptides revealed that Gln and Lys residues on only the head and tail sequences of the SPRs were involved in the cross-links. Furthermore, the data indicated that the SPRs functioned primarily as cross-bridging proteins, by mediating links between other proteins, often loricrin in the epidermis or loricrin, trichohyalin, and themselves in the forestomach CEs. They also form cross-links between themselves in the CEs of cultured keratinocytes that may serve as a model system for other internal stratified squamous epithelia (Tarcsa 1998).

Individual members of the SPR family can function as both amine donors and acceptors for the TGase 1 enzyme in vitro (Tarcsa 1998).

Although the three TGases commonly expressed in epithelia (TGases 1, 2 and 3) can cross-link SPR2 in vitro, TGase 3 is used almost exclusively for cross-linking SPRs to CE structures in epithelia in vivo (Tarcsa 1998).

Recombinant human SPR2 protein was used as a complete substrate by the TGase 1 and 3 enzymes present in the epidermis and other stratified squamous epithelia, but by the TGase 2 enzyme only very weakly, if at all (Tarcsa 1998).

Keratinocyte Transglutaminases create a host barrier defense by cross-linking SPR proteins and other proteins through covalent Nε-(γ-glutamyl)lysine iso-dipeptide bonds (1).

Cross-linking of epithelial cell proteins is essential. Mice lacking keratinocyte Transglutaminase die within a few hours of birth (1).

Although both SPR proteins and Hwp1 are TGase substrates, Hwp1 lacks the tripartite domain structure that serves to cross-link the head and tail domains of SPR proteins to themselves or to other proteins (11).

Perhaps Hwp1 contributes to proliferation of Candida albicans in keratinized epithelium by interacting with keratinocyte Tgases (1).


Hwp1 (Hwp1 transglutaminase substrate domain) has similarities to mammalian small proline-rich proteins (SPRs) (1).

Similarity in primary structure of rHwp1 to mammalian small proline-rich proteins (SPRs) (11).

Biophysical properties of the Hwp1 transglutaminase substrate domain were explored using a recombinant protein representative of the N-terminal domain of Hwp1 and were similar to other transglutaminase substrates, the small proline-rich proteins (SPRs) of cornified envelopes found in stratified squamous epithelia of mammals (11).

Similiarities of Hwp1 to small proline-rich (SPR) proteins are likely responsible for the TGase substrate properties of Hwp1 (11).

Role for Hwp1 in formation of stable complexes with human buccal epithelial cells (BECs) was suggested by the amino acid sequence of the NH2-terminal domain, which resembles mammalian TGase substrates , particularly the head and central domains of small proline-rich (SPR) proteins (1).


The predicted mature N terminus of Hwp1 resembles those of all three SPR in having Ser (S) as the first amino acid (11).

The presence of Tyr (Y) residues close to the N terminus as well as immediately prior to the first string of glutamine residues is similar to SPR2 and SPR3 (11).

The Hwp1 repeats (10 amino acids) are similar in size to SPR internal domain repeats (8 amino acids) and are also similar in the presence of Cys (C), Pro (P), and Glu (E) residues (11).

The Hwp1 repeats differ from those of the SPR family in the absence of Lys residues indicating that, unlike the SPR family, Hwp1 participates in cross-linking reactions solely as the Gln donor (11).

Below: Similarities between Hwp1 and SPR proteins: Comparison of the sequence of the N-terminal domain of Hwp1 and three members of the SPR family. Residues in bold type become cross-linked in the presence of epithelial cell Tgases in vitro. Glns in Hwp1 that are predicted to become cross-linked are shown underlined in bold type (11):

The similarity of Hwp1 to SPR proteins suggests that interactions of BEC TGases with SPR proteins are indicative of interactions with Hwp1 and that the primary role of Hwp1 is in the formation of stable attachments (1).

The similarity of Hwp1 to small proline-rich proteins that are expressed only in stratified squamous epithelium suggests that Hwp1 may be more important for mucosal than systemic candidiasis (9).


Long incubation periods (18 hours) are required for maximal cross-linking of SPR2 protein by epithelial cell TGase3 (1).


The overall coiled structure (of N-terminal domain of Hwp1) is similar to the human SPR protein family of cornified cell envelope proteins (11).



APRPs are proteins located in saliva in humans as well as other animals (Bennick 1982).

Saliva is a secretion produced by the salivary glands in the oral cavity. The macromolecules in saliva consist almost exclusively of a complex mixture of proteins called salivary proteins. The largest (~70% of the human salivary proteins) protein components of salivary proteins are the salivary proline-rich proteins. Salivary proline rich proteins are a family of proteins that can be divided into three groups acidic (30%), basic (23%) and glycosylated (17%) proteins. Acidic ones are the acidic proline-rich proteins or APRPs (Bennick 1982).

The primary structure of the acidic proline-rich proteins is unique and shows that the proteins do not belong to any known family of proteins (Bennick 1982).

Amino acid analysis of unfractionated human salivary proteins have demonstrated an unusually large amount of proline varying from 16 to 33% of all amino acids. This observation is all the more interesting because amylase which accounts for approximately 30% of the parotid protein only contains 2.3% proline which is similar to that of most proteins (Bennick 1982)

The human salivary proline-rich proteins (PRPs) are characterized by a predominance of the amino acids proline (25-42%), glycine (16-22%) and glutamic acid/glutamine (15-28%), which together make up 70-85% of the proteins (Kim 1993).

It is characteristic that proline accounts for 25 to 42% of the amino acids in salivary proline-rich proteins. In addition there are high contents of glutamine (glutamic acid) and glycine. Together these three residues account for 70 to 88% of all of the aminoacids in the proteins. (Bennick 1982)

In APRPs proline accounted for 24 to 27% of the residues and this amino acid together with glutamic acid (or glutamine) and glycine constituted 70 to 75% of all the amino acids (Bennick 1982).

In APRPs the sequence from residue 31 to the C-terminus consisted mostly of proline, glycine and glutamine. In this part of the protein there was a high degree of sequence repetition. The sequence pro-gln-gly-pro-pro (PQGPP) ocurred for example 5 times. This suggested the presence of recurrent folding of the peptide chain and gene multiplication. (Bennick 1982).

APRPs perform unique functions. The need for these functions arise because of the particular environment of the oral cavity, which is the only place in the organims where mineralized tissue is exposed to an external environment. The oral cavity also provides an environment where certain microorganisms may flourish, and ingested food is a potential source of chemical, mechanical and thermal irritation of the hard and soft tissues. (Bennick 1982)


APRPs are transglutaminase substrates (3).

APRPs are substrates for buccal epithelial cell transglutaminase (3).

A membrane-bound epithelial enzyme, transglutaminase, catalyzes the covalent cross-linking of acidic proline-rich proteins (APRPs) to surface proteins of buccal epithelial cells (BECs) (10).

Buccal epithelial cells transglutaminase (BECs TGase) catalyzes the covalent crosslinking of acidic proline-rich proteins (APRP) to BECs (10).

Some biological properties of human salivary proline-rich proteins have been demonstrated in vitro studies. The acidic salivary proline-rich proteins bind Ca2+ (Kim 1993).

APRPs will bind calcium with a strength which indicates that they may be important in maintaining the concentration of ionic calcium in saliva (Bennick 1982).

Transglutaminases require Ca2+ to act.


The significance of [125I]APRP cross-linking in Candida adhesion has not been determined as of yet (10).

APRPs may interfere with TGase catalyzed mechanisms of adhesion (10).

Because [125I]APRP are also crosslinked to BECs surface proteins, however, it is tempting to speculate that APRPs may compete with cross-linking between Candida and BEC cell surfaces and, thus, interfere with TGase-catalyzed components of Candida/BEC adhesion (10).

Presumably, the reactivity of APRP with TGase is dependent on their high glutamine content (10).

Because other members of the family share this characteristic, it would not be surprising if TGase reacts with all members of the proline-rich family of proteins (10).

This family comprises approximately 70% of the total protein in parotid saliva, suggesting that proline-rich proteins may have a significant effect on TGase-catalyzed components of Candida adhesion (10).


Hwp1 (Hwp1 transglutaminase substrate domain) shares features with host acidic salivary proline-rich proteins (3).

Like hwp1, aprp contain glutamine residues within proline-rich repeats, a conformational arrangement that may be favorable for formation of ε (γ glutamyl) lysine cross-links by transglutaminase, given the properties of other known substrates for epithelial cell transglutaminases (3).

The similarity of hwp1 to aprp could be important for interactions between C. albicans and the oral mucosa. The presence of a polyphenol-binding extracellular proline-rich wall protein of the plant fungal pathogen Colletotrichum graminicola lends support to the idea that the presence of proteins with aprp-like properties might be a common feature of fungal cell walls (3).

The high percentage of proline residues and the length of the repeat (10 amino acids) place hwp1 within a varied group of proline-rich proteins in which the proline residues are proposed to function in maintaining the polypeptide chains in extended conformations and to mediate noncovalent interactions between protein chains or, in the case of salivary proteins, to bind toxic plant polyphenols. Of particular interest is the presence of acidic salivary proline-rich proteins (aprp) in this group of proline-rich proteins (3).

Because Candida albicans expresses TGase reactive proteins and because APRP appears to be a good substrate for TGase, it is not surprising that TGase catalyzes cross-linking of [125I]APRP to Candida surface proteins (10).

An aprp-like, transglutaminase substrate might also be present on surfaces of C. albicans (3).


When C. albicans interacts with the oral epithelium in vivo, the organisms are coated with saliva, which has been reported to either inhibit or enhance the adherence of C. albicans to oral epithelial cells (20).

Umazume M, Ueta E, Osaki T (1995) Reduced inhibition of Candida albicans adhesion by saliva from patients receiving oral cancer therapy. J Clin Microbiol 33: 432–439.

Holmes AR, Bandara BM, Cannon RD (2002) Saliva promotes Candida albicans adherence to human epithelial cells. J Dent Res 81: 28–32

Also, it is possible that salivary proteins could potentially act as bridging molecules between the hyphae and oral epithelial cells and thereby facilitate endocytosis of C. Albicans (20).

To investigate these possibilities, we incubated C. albicans hyphae in the presence and absence of 20% saliva and then measured their interactions with both intact FaDu oral epithelial cells and oral epithelial cell surface proteins. Killed organisms were used in these experiments to obviate potential effects of saliva on the growth of the hyphae. We have previously shown that killed hyphae adhere to and are endocytosed by oral epithelial cells similarly to live hyphae. Incubating hyphae in saliva prior to adding them to the epithelial cells had no effect on the number of endocytosed or cell-associated organisms. As predicted by the endocytosis assay, saliva also had no detectable effect on the binding of epithelial cell surface proteins, including E-cadherin, to C. albicans hyphae. Therefore, salivary components do not act as bridging molecules between the organisms and epithelial cells (20).



[14C]putrescine in the presence of purified TGase, but not [14C]putrescine alone, was shown to be cross-linked into surface proteins of both morphogenetic forms (blastospore > hyphal forms) of Candida albicans (10).

TGase catalyzed cross-linking of [14C]putrescine into proteins was used to detect the expression of TGase reactive substrates on the cell-wall surface of both the blastospore and hyphal forms of Candida albicans what could cause Candida albicans adhesion by catalyzing Tgase cross-linking between Candida and oral epithelial cells (10).

Purified TGase was used to cross-link [14C]putrescine into TGase proteins on the surface of both the blastospore and hyphal forms of C. albicans. Following incubation, [14C]putrescine cross-linking to proteins on the cell walls of blastospores appeared to be constant when maintained in M199 over a 3-h period (BELOW; open bars). When blastospores were induced to germinate in the same medium, however, the amount of [14C]putrescine cross-linked to cell-wall proteins significantly increased over the same time period (BELOW; black bars) (10)

BELOW: Reaction of Candida cell-wall proteins with BEC TGase. lodinated proteins extracted from Candida albicans cell wall and proteins expressed on Candida albicans cell-wall surfaces react with BEC and purified TGase, respectively. Right: lodinated cell-wall extract alone (lane a) was cross-linked when incubated with BEC (lane b) and cross-linking was inhibited with iodoacetamide (lane c). Left: Over time, cross-linking of [14C]putrescine to Candida Albicans blatospores by purified TGase was constant (open bars), but cross-linking increased proportionally with time following germination (black bars). [14C]putrescine was covalently cross-linked into cell wall proteins of blatospore (inset; lane a) and hyphal forms (inset; lanes) (10):

Following incubation of Candida albicans with [14C]putrescine alone, only negligible amounts (<3% of that in experimental blastospores) were associated with the Candida cell-wall digest (10).

Fluorographs of the digested cell-wall material from both the blastospore and hyphal forms of Candida albicans showed a single high molecular weight component at the top of the resolving gel (10).

A representative fluorograph of blastospore and hyphal form digests (inset; lanes a and b, respectively) indicates that [14C]putrescine was indeed cross-linked to proteins in the cell-wall digests (10).

The expression of TGase reactive proteins appears to increase following transformation of Candida from the blastospore to the hyphal form. It is not clear whether this represents increased expression of a blastospore-phase protein in hyphal cell walls or a de novo expression of a hypha-specific TGase reactive protein (10).

Because [14C]putrescine-labeled complexes were too large to be separated by a 10% SDS-PAGE gel, it is not clear whether the complexes contained a high molecular weight protein(s) or TGase protein(s)/carbohydrate complex(es). Given the high content of manno-protein in the Candida cell wall, it seems likely that the latter is most probable (10).


TGase cross-links APRPs to candidal and epithelial surface proteins; APRPs were cross-linked to the Candida surface; When incubated with [125I]APRPs and purified TGase, both morphogenetic forms of Candida albicans bound dramatically more APRP than controls without TGase (10).

Incubation of Candida albicans with TGase and [125I]APRPs suggests that [125I]APRP can be cross-linked to TGase-reactive proteins on the surface of Candida albicans (10).

Both blastospore (open bars) and the hyphal form (closed bars) retained significant amounts of [1251]APRP (10).

Incubation with [125I]APRP alone resulted in the binding of <2% of that retained by the experimental blastospores (10).

Representative autoradiographs of the digested cell-wall material from both the blastospore and hyphal forms (lanes c and d, respectively) show high molecular weight complexes (10).

The molecular weights of these components are significantly higher than that of [125I]APRP incubated in the reaction buffer alone (lane a) or with [125I]APRP incubated with either morphogenetic form of Candida albicans (lane b, representative autoradiograph), suggesting that [125I]APRP is cross-linked with Candida cell-wall proteins into high-molecular-weight complexes (10).

The cross-linking of [125I]APRP to blastospores or hyphal forms was somewhat different than that of [14C]putrescine (10).

In general, less [125I]APRP remained cross-linked to the blastospore (open bar) than the hyphal form (closed bar); however, the difference between the amount of [14C]putrescine cross-linked to blastospores and hyphal forms is greater than [125I]APRP to the same morphogenetic forms under similar conditions (10).

Additionally, the pattern of [125I]APRP cross-linking to each morphogenetic form over time was not the same as for [14C]putrescine. For instance, the amount of [125I]APRP cross-linked to blastospore and hyphal forms decreased between 2 and 3 h, whereas in the same time period, [14C]putrescine cross-linking stayed the same or increased in blastospores and hyphal forms, respectively. It is possible that this is an experimental artifact; however, the same trends were repeated in two similar experiments and these results could not be explained by cell loss during extraction procedures (10).

Comparison between [14C]putrescine and [125I]APRP cross-linking to Candida albicans surface in the presence of TGase suggest that these two TGase substrates may be cross-linked to two different groups of proteins on the Candida cell surface (10).

BELOW: TGase-catalyzed cross-linking of APRPs to Candida Albicans. Purified TGase appeared to cross-link [125I]APRP to proteins on the surface of both blatospore and hyphal forms of Candida Albicans. Left: [125I]APRP could not be completely extracted from either blastospore (open bars) or hyphal forms (closed bars). Right: TGase/[125I]APRP-treated Candida Albicans indicate that cell-wall digests of the blastospore form (lane c) and the hyphal form (lane d) contain [125I]APRP cross-linked into a high-molecular-weight complex (arrow). [125I]APRP alone incubated with reaction buffer (lane a, representative sample) or with both forms of Candida (lane b, representative sample) were not cross-linked into higher-molecular-weight complexes (10):


Epithelial TGase may stabilize Candida adherence by cross-linking Candida and BEC surface proteins (10).

A component of blastospore form Candida/BEC adherence was shown to be resistant to detachment (10).

Iodoacetamide is the simplest irreversible inhibitor of transglutaminase. It inhibits transglutaminase-mediated protein cross-linking by binding to the active site of the enzyme and preventing substrate binding.

Inhibition of BEC TGase (using iodoacetamide) increased detachment of Candida Albicans blastospore forms from buccal epithelial cells under dissociation conditions (10).

Pretreatment of BECs with iodoacetamide was shown to inhibit the cross-linking of salivary components on the surface of BECs (10).

Pretreatment of BECs with iodoacetamide increased detachment of C. albicans blastospore forms from BECs under dissociation conditions (10).

Pretreatment of BECs with iodoacetamide decreased adherence of blastospore form of Candida albicans by approximately 75% (10).

Pretreatment of BECs with iodoacetamide (plot b) increased dissociation of blastospores significantly (~75%) over that of control samples not inhibited by iodoacetamide (plot a) (10).


BELOW: Implication of TGase in Candida Albicans blastospore forms / buccal epithelial cells (BECs) adhesion. Inhibition of BEC TGase increased detachment of Candida Albicans blastospore forms from BECs. Pretreatment of BECs with iodoacetamide increased dissociation of blastospore forms (plot a) over that of control samples not treated with iodoacetamide (plot b) (10):

In the present study, the increased dissociation of blastospores and hyphal forms from iodoacetamide-treated BECs suggests that inhibition of BEC TGase may destabilize Candida adhesion by inhibiting cross-linking between Candida and epithelial cell surfaces (10).

The inability of heating in SDS to increase the dissociation of either experimental or control blastospores suggests that TGase-catalyzed cross-linking may play a significant role in stabilization of C. albicans (ATCC 28366) adhesion to BECs (10).



Studying  attachment of Candida albicans to the oral mucosa it was shown that pretreatment of BECs with rHwp1N13 produced inhibition of germ tube stabilized adhesion by 78% (11).

The reasons that inhibition did not reach 100% are not known (11).

Why didn´t recombinant Hwp1 protein reach 100% of inhibition of Candida Albicans germ tube stabilized adhesion to buccal epithelial cells?

The presence of additional TGase substrates  is unlikely given that null hwp1/hwp1 mutants do not possess surface TGase substrate activity (11 making reference to 1).

One possibility is that the physical interaction of the germ tube surface to a BEC exposes cross-linking sites that are not accessible to masking by soluble rHwp1N13 molecules, thereby leading to incomplete inhibition of stabilized adhesion (11).




Adhesion of Candida albicans to host epithelial cells is a critical, essential first step in the infection process, for both colonization and subsequent induction of mucosal disease (14).

Candida albicans must be adherent enough to persist on mucosal surfaces (3).

The adhesion of different Candida species to host tissue correlates with their virulence, Candida Albicans adhering far better than other Candida species or the apathogenic yeast Saccharomyces cerevisiae to epithelial and endothelial cells as well as to extracellular matrix proteins (Emody 2000).

In addition, differences in relative virulence have also been observed in Candida Albicans strains with differing degrees of adherence, and spontaneous mutants defective in adherence to buccal epithelial cells are less virulent in animal models than the parent strain (Emody 2000).

As Candida Albicans adheres to many different types of host surfaces, it probably possesses a broad panel of adherence factors (Emody 2000).

Because adherence is essential for C. albicans to persist on mucosal surfaces, it is not surprising that this organism expresses multiple different surface structures that mediate adherence to epithelial cells (14).

A large number of specialized Candida albicans cell surface proteins called adhesins mediate binding of the fungus to host cells (13).

These various adhesins frequently exhibit differential expression on yeast versus hyphae, and mediate adherence by different mechanisms (14).

Although the numerous binding activities that have been attributed to hyphae involve a variety of host cells and host proteins, a frequent finding is that surface proteins of hyphae are involved in binding (3).

Hwp1 is one of these adhesins with an important role in adhesion.


Hwp1 is one of the Candida albicans surface proteins on hyphae.

Hwp1 on hyphae mediates fungal adhesion to epithelial cells (13).

The involvement of Hwp1 in a novel mechanism of adherence was predicted after the discovery that the amino acid sequence of the NH2-terminal domain resembled mammalian proteins that are substrates for transglutaminase (TGase) cross-linking enzymes (12).

Epithelial TGases in rodents and other mammals are essential for formation of a cross-linked network of squamous epithelial cell–specific proteins such as involucrin, loricrin, and the small, proline-rich proteins that make up the primary host defense barrier. The presence of Hwp1 might enable Candida albicans to form tight attachments to the oral mucosa through TGase-catalyzed cross-linking between Hwp1 and epithelial cell proteins. In in vitro experiments, both the amino-terminal domain of Hwp1 and germ tubes expressing Hwp1 became tightly attached to buccal epithelial cell surfaces through epithelial cell TGase activity (12).

Localization of the adhesive domain of Hwp1 is to the N-terminal third of the protein (repetitive proline/glutamine rich tgase substrate region) (9).

Hwp1 forms tight attachments to stratified squamous epithelial cells with cell surface-exposed TGase activity (11).


Hwp1 forms covalent cross-links to buccal epithelial cells in vitro by functioning as a substrate for mammalian transglutaminases (12).

Hwp1 becomes cross-linked to TGase-expressing surface squames of the oral mucosa (9).

Mammalian transglutaminase cross-links Candida albicans to human buccal epithelial cells (BECs) (1).

Hwp1 in hyphae of Candida Albicans is crosslinked to human buccal epithelium by mammalian transglutaminase (1).

TG activity on the surface of oral epithelial cells, produced by epithelial TG (TG1) would be correlated with tight binding of C. albicans via Hwp1 to the host cell surfaces (13).

TGase activity of the surfaces of BEC has been attributed to membrane-bound TGase 1 where it functions to cross-link proteins in the formation of the cornified cell envelope (11).

During stabilized adhesion of germ tubes to the surface of BEC, the insoluble TGase 1 probably contacts the most surface-exposed Hwp1 in the cell wall and cross-links the protein (and the organism) to yet unidentified host surface protein(s) (11).

Physical constraints would prevent BEC TGase 1 from penetrating the hyphal wall to interact with the non-wall species of Hwp1 (11).


Tgase covalently link Candida Albicans to the intestinal epithelium and endomysium by means of Hwp1 (2).

Hwp1 is used by Candida albicans to adhere to the intestinal epithelium. Adherence of the yeast to intestinal epithelial cells is mediated by transglutaminase. Mammalian transglutaminasa covalently link Candida Albicans to the intestinal epithelium and endomysium (2).

Tissue transglutaminase and endomysium components could become covalently linked to the yeast (2).

Adherence of the yeast to intestinal epithelial cells is mediated by transglutaminase, and a peptide that represents aminoacids 40–197 of Hwp1 (rHwp1ΔC37), which is a tissue transglutaminase substrate (2).

In immunofluorescence localization of TGase1 in epithelial tissues, diffuse signals were detected with anti-TGase1 antibodies in non-junctional plasma membranes such as microvilli (15).

Candida Albicans adhesion (filamentous, hyphal forms)


TGase stabilizes Candida albicans adhesion by covalently cross-linking Candida albicans Hwp1 and buccal epithelial cells (BECs) surface proteins

The primary role of Hwp1 is in the formation of stable attachments by means of cross-linking of germ tubes to buccal epithelial cells (BECs) (1).

Requirement for Hwp1 in stabilized attachment of germ tubes to buccal epithelial cells (BECs): unlike most pathogens that form various types of weak interactions with host cells, Candida albicans germ tubes form nondissociable complexes with human buccal epithelial cells (BECs) that are characteristic of transglutaminase (TGase)-mediated reactions in stability and in being prevented by TGase inhibitors (1).


HWP1 null mutants have been shown to be defective in forming stable attachments to buccal epithelium: Hwp1 is required for formation of stable complexes between C. albicans germ tubes and BECs: Candida albicans strains lacking Hwp1 were unable to form stable attachments to human buccal epithelial cells (BECs). The stable attachments greatly reduced in strains lacking Hwp1 as seen by comparing strains with or without Hwp1 in stabilized adhesion assays, which involved treatment of germ tube:BEC complexes with heat and the anionic detergent SDS to dissociate weak, noncovalent bonds.  The mutant strain without Hwp1 was greatly impaired in the ability to form stable attachments to BECs in that stabilized adhesion was only 23% of the other strains and was equivalent to values for other strains when TGase inhibitors were added (1).


rHwp1∆C37 forms stable attachments to BECs: rHwp1ΔC37, a recombinant protein that encompasses the NH2-terminal proline- and glutamine-rich domain of Hwp1 (aminoacids 40 to 197 of Hwp1) was radiolabeled and incubated with BECs in the presence or absence of iodoacetamide followed by treatment with heat and SDS. The results showed rHwp1∆C37 associated with BECs was sixfold greater when TGase was not inhibited thus providing further support for the role of Hwp1 in mediating stabilized adhesion (1).

Below: Radiolabeled rHwp1ΔC37 were incubated with BECs under adhesion assay conditions with or without iodoacetamide. Then associated rHwp1ΔC37 with BECs envelopes were determined (1):


Hyphae complexed to human BECs in vivo in specimens of pseudomembranous candidiasis of the buccal mucosa were not dissociated by heat and detergent treatments (up to 30 min) used in the stabilized adhesion assays, indicating predominance of stable attachments (1).

Germ tubes and hyphae of C. albicans exhibit highly polarized, apical growth and require mechanisms for anchoring. In mimicking mammalian TGase substrates, Hwp1 forms stable attachments between germ tubes and mammalian cells (1).

Stabilized adhesion was prevented by monodansylcadaverine, a competitive inhibitor of TGase-mediated protein cross-linking reactions, and by iodoacetamide, supporting the involvement of BEC Tgases (1).

Below: Candida Albicans strains with or without Hwp1 were used for comparison in TGase assays for stabilized adhesion. Transglutaminase inhibitors also were used (1):





The function of the N terminus of Hwp1 in stabilized adhesion to the surface of BECs was investigated by using rHwp1N13, comprising amino acids 1–148 of the mature protein (aminoacids 40 to 187 of Hwp1) as a competitor in adhesion assays (11).

The ability of rHwp1N13 to compete or inhibit stabilized adhesion of germ tubes to BECs was determined by preincubating rHwp1N13 or salivary amylase (negative control) with BECs prior to the addition of germ tubes. The adhesion of germ tubes to BECs in the presence of rHwp1N13 or salivary amylase was set relative to assays in the absence of added proteins (11).

Preincubation with rHwp1N13 prior to the addition of germ tubes reduced relative adhesion by 60% compared with controls without added protein or in the presence of the control protein, salivary amylase (11).

Inhibition was 78% of the maximum expected amount based on adhesion of the hwp1/hwp1 null mutant, which was 23% of control (11).

Below: Inhibition of germ tube adhesion by rHwp1N13. BECs were incubated with rHwp1N13 or salivary amilase prior to adding radiolabeled wild type germ tubes to measure the ability of rHwp1N13 to interfere with germ tube adhesion. Germ tube adhesion to BEC was determined relative to assays performed in the absence of added protein (No protein, set at 100% adhesion). The germ tube adhesion inhibition by rHwp1N13 was significant relative to no protein. Salivary amylase did not have an appreciable effect on germ tube adhesion (11):

The results show that the stabilized adhesion function of Hwp1 maps to the N-terminal portion of the protein (11).

The 78% inhibition of germ tube stabilized adhesion by pretreatment of BECs with rHwp1N13 is consistent with the importance of the TGase substrate domain of native Hwp1 in attachment of Candida albicans to the oral mucosa (11).

The reasons that inhibition did not reach 100% are not known (11).


Adhesive mechanism = covalently cross-linking of Candida albicans Hwp1 to BEC surface proteins by transglutaminase.

The host protein(s) participating in cross-links to Hwp1 remain unknown (9).

The presence of host innate and specific immune responses to adhesins and their potential role in preventing candidiasis have not been approached experimentally (9).


NOBILE 2006 (17):

Whether Hwp1 functions as an adhesin in the absence of host transglutaminase activity is less certain, though the possibility has never been ruled out. Indeed, a possible function for Hwp1 in C. albicans cell-cell adherence comes from the finding that it is induced by mating factor and is deposited on the surface of the bridge between mating partners. This localization might be expected for a cell-cell adhesin (17).


PHAN 2007 (20):

In our assays, the mutant without Als3 had significantly reduced adherence to endothelial cells, but normal adherence to oral epithelial cells (20).

It has been reported by others that a mutant without Als3 had reduced adherence to endothelial cells, buccal epithelial cells, and FaDu epithelial cells (20).

Also, we found that S. cerevisiae expressing Als3 bound to both endothelial cells and FaDu oral epithelial cells (20).

The discrepancy between the previous and current results is likely due to differences in the methodology of the assays. Specifically, the longer incubation time and the use of a 24-well tissue culture plate (rather than a 6-well plate) in the current endocytosis assay make it less sensitive to differences in adherence among strains (20).

In the affinity purification experiments, hyphaeof the  mutant without Als3 failed to bind to multiple host proteins, including N-cadherin and E-cadherin. Thus, these results are consistent with the model that Als3 binds to a broad range of host substrates. However, our data also indicate that there is some specificity in the binding of Als proteins to host constituents (20).


Coating the latex beads with rAls3-N resulted in a much greater increase in endocytosis than adherence. Also, coating beads with rAls1-N resulted in little to no increase in adherence, even though previous studies have clearly demonstrated that Als1 is an adhesin (20).

The likely explanation for the relative lack of adherence of the beads is that they were coated with fragments of Als1 or Als3 that lacked the tandem repeats present in the central portion of the full-length proteins (20).

Previously, we have found that the adhesive function of Als1 is dramatically reduced when some or all of the tandem repeats are eliminated (20).

Similarly, Oh et al. found that versions of Als3 with fewer tandem repeats mediated less adherence than did versions of Als3 with more tandem repeats. (20).

In addition, Rauceo et al. reported that the absence of tandem repeats reduced the binding affinity of Als5 (20).


S. cerevisiae expressing Als1 or Als3 binds to a variety of host constituents, including endothelial cells, epithelial cells, laminin, and fibronectin (20).



Transmission electron microscopic imaging of biopsy specimens from humans with oropharyngeal, vaginal and cutaneous candidiasis shows the presence of intraepithelial cell organisms, demonstrating that epithelial invasion occurs during these diseases (14).

Candida albicans is unique among oral pathogens in its ability to invade cornified layers of stratified squamous epithelium of the tongue, buccal surfaces, hard and soft palate, and esophagous (11).

In mice infected with C albicans, invasion and lysis of the villi in the intestine has been reported (2).

The switch from yeast to filamentous growth facilitates tissue invasion and is associated with the transition of Candida albicans from a harmless colonizer to a pathogen that causes symptomatic infections.

Hyphae appear to be the invasive form of the organisms, as the majority of intracellular organisms are hyphae, whereas yeast are typically located either between or on the surface of epithelial cells (14).

Epithelial cell invasion is important for the pathogenesis of mucosal candidiasis, because mutants of Candida albicans with reduced capacity to invade epithelial cells in vitro usually have reduced virulence in experimental animal models of mucosal candidiasis (14).

Candida Albicans (filamentous, hyphal forms invasion)


Candida Albicans (filamentous, hyphal forms invasion)


Adherent Candida albicans cells can invade epithelial surfaces both by penetrating into individual epithelial cells, and by degrading interepithelial cell junctions and passing between epithelial cells (14).

Invasion into epithelial cells is mediated by both induced endocytosis and active penetration, whereas degradation of epithelial cell junction proteins, such as E-cadherin, occurs mainly via proteolysis by secreted aspartyl proteinases (14).

The mechanisms by which Candida albicans invades epithelial cells have been investigated using in vitro models. The results of these studies suggest that Candida albicans can invade epithelial cells by two distinct mechanisms (14):


One mechanism is the induction of epithelial cell endocytosis by the organism. Endocytosis is induced by invasin-like proteins that are expressed on the surface of a C. albicans hypha. These proteins bind to epithelial cell surface proteins and induce the epithelial cell to produce pseudopods that engulf the organism and pull it inside the cell (14).

Candida albicans induces its own endocytosis by multiple epithelial cell lines, including HeLa cells, HET1-A esophageal cells, FaDu pharyngeal cells, OKF6/TERT-2 oral epithelial cells, and reconstituted human epithelia, which is a three-dimensional model of oral or vaginal epithelium (14).


Another mechanism of invasion is the active penetration of a hypha either into or between epithelial cells. This process requires fungal viability (14).

Candida albicans likely invades epithelial cells from different anatomic sites via different mechanisms. For example, the organism invades oral epithelial cell lines by both induced endocytosis and active penetration, whereas it invades a gastrointestinal epithelial cell line only by active penetration (14).


Below: High-power view of lingual longitudinal section of immunodeficient infected mouse being invaded by Candida Albicans showing that damage resulted from microabscesses formed in response to hyphal invasion of the keratinized layer (stained pink). Polymorphonuclear leukocytes (PMNL) form giant aggregates (pink arrow) that sever the keratin layer containing hyphae (red arrow) from the underlying epithelium. Although polymorphonuclear leukocytes (PMNL) occasionally appeared to be directed toward hyphae, PMNL more commonly formed intraepithelial microabscesses that fused and separated hyphae-laden keratin from the underlying stratum spinosum (12):

Below: Histopathologic sections of depapillated areas on the dorsal tongue surface of a rat, showing epithelial invasion by C. Albicans hyphal elements.  Candida albicans produce fungal hyphae that penetrated the lingual epithelium and stopped short of the prickle cell layer. C. albicans hyphae penetrating the lingual mucosa were 5.00 to 17.71 mm (Samaranayake 2001):


Hwp1 has not been related to invasion however some transglutaminase interaction could exist in the invasion process on the basis described in section more below  “E-CADHERIN: THE LINK BETWEEN ENDOCYTOSIS AND ACTIVE PENETRATION?” . If this transglutaminase interactions really exist, a protein substrate of transglutaminase different from Hwp1 (since Hwp1 has not been implicated in invasion) must exist on the surface of Candida Albicans. Will it be Als3 protein?

Hwp1 versus Als3:

Below: Comparisson of Hwp1 (Sundstrom 2002) and Als3 (Klotz 2010):


- Cell surface proteins (like all ALS family) (14)

- Adhesins; mediate binding to diverse host substrates (like all ALS family) (14)

- Mannoproteins (like all ALS)

- N-terminal domain contains the substrate binding region (like all ALS) (14)

- C-terminal domain is rich in serine and threonine (like all ALS) (14)

- Als1, Als3 and Als5 mediate adherence to multiple host constituents including oral epithelial cells (Hwp1 also mediates adherence to oral epithelial cells), whereas Als6 and Als9 bind to a much more limited range of host substrates and do not mediate adherence to epithelial cells. Als7 does not bind to any host substrates tested to date. The adherence function of Als2 and Als4 has not been investigated by this approach (heterologous expression of a C. albicans ALS gene in the normally non-adherent yeast, Saccharomyces cerevisiae) (14)

- Studies of C. albicans deletion mutants suggest that Als2, Als3 and Als4 mediate adherence to epithelial cells. Als1 has been found to mediate adherence to mouse tongues in an ex vivo assay, but not to exfoliated human buccal epithelial cells (14).

- Expressed only on the surface of Candida albicans hyphae. Some adhesins are preferentially expressed by specific morphological forms of C. albicans. For example, ALS3 and HWP1 are expressed by hyphae, but not yeast-phase organisms. In contrast, ALS1 is expressed by yeast cells under some conditions and for only a short time after hyphal formation is initiated (14).


- Repeats in central domain (variables number of tandem repeat sequences) (like all ALS) (14)

- N-termini of Als proteins predicts the presence of antiparallel β sheets, indicating that these proteins are members of the immunoglobulin superfamily (all ALS) Interestingly, the three-dimensional structures of the N-termini of most Als proteins are predicted to be similar to the three-dimensional structure of bacterial adhesins, including invasin of Yersina pseudotuberculosis, and collagen-binding protein and clumping factor A of Staphylococcus aureus (14)

- The N-terminal region of Hwp1 functions as a substrate for epithelial cell-associated transglutaminases that covalently link it to other proteins on the epithelial cell surface (14).


Als3 is another Candida Albicans surface protein.

Als3 is a Candida Albicans protein belonging to the The Candida albicans ALS (agglutinin-like sequence) gene family (14).

Als proteins have adherence properties (20).

The different Als proteins mediate adherence to broad variety of host substrates, and this adherence is likely critical for Candida albicans to infect host surfaces (20).

Each Als protein has three domains. The N-terminal domain contains the substrate-binding region. The central portion of the protein consists of a variable number of 36–amino acid tandem repeats. The C-terminal domain is rich in serine and threonine, and contains a GPI anchorage sequence that is predicted to be cleaved as the protein is exported to the cell surface (20).

Als3 not only mediates epithelial cell adherence, but also functions as an invasin that induces epithelial cell endocytosis (14).

Als3-mediated adhesion has not been related to transglutaminase but what about invasion?

Als3 has been made responsible of invasion.

Als3 can act as an invasin protein and induce endocytosis by normally nonphagocytic host cells (20).

Candida albicans hyphae invade endothelial cells and oral epithelial cells in vitro by inducing their own endocytosis (20).

Als3, like Hwp1 appears exclusively on hyphae and is the fungal surface protein that mediate this process (20).

Candida albicans mutant without Als3 has markedly impaired capacity to invade epithelial cells (14).

Each Als protein has three domains. The N-terminal domain contains the substrate-binding region. The central portion of the protein consists of a variable number of 36–amino acid tandem repeats. The C-terminal domain is rich in serine and threonine, and contains a GPI anchorage sequence that is predicted to be cleaved as the protein is exported to the cell surface (20).

Below: Als3 protein (Klotz 2010):



The interactions of Candida Albicans mutants without Als1 and without Als3 with human umbilical vein endothelial cells and the FaDu oral epithelial cell line were investigated. Using our standard differential fluorescence assay, we determined the capacity of each strain of C. albicans to adhere to these host cells and induce its own endocytosis (20).

Below: Hyphae of the mutant without Als1 interacted with both host cell types similarly to the wild-type strain, in contrast, there was a 90% reduction in the number of hyphae of the mutant without Als3 that were endocytosed by either endothelial or oral epithelial cells compared to the wild-type control strain (20):


Previously, we have reported that N-cadherin is one of the endothelial cell surface proteins that mediate endocytosis of C. Albicans (20).



Endocytosis of Candida albicans by endothelial cells is induced when the organism binds to N-cadherin on the endothelial cell surface (20).

Below: We used an affinity purification approach to determine whether Als1 or Als3 was required for Candida albicans hyphae to bind to N-cadherin in endothelial cell membrane extracts (20).

As predicted by the results of the endocytosis assay, mutant without Als1 bound to the same endothelial cell surface proteins, including N-cadherin, as did the wild-type strain. In contrast, the mutant without Als3 did not bind to N-cadherin, and it bound very weakly to the other endothelial cell surface proteins (20).

Als3 is required for C. albicans to bind to N-cadherin and other proteins on the endothelial cell surface (20).


The surface proteins on oral epithelial cells that are bound by Candida albicans hyphae have not been identified previously (20).

Below: We found that the wild-type strain of Candida albicans bound to at least four different proteins on the surface of FaDu oral epithelial cells. Many of these proteins appeared to be different from the endothelial cell proteins that were bound by wild-type C. albicans. FaDu oral epithelial cells express very low levels of N-cadherin (unpublished data), but high levels of E-cadherin. Therefore, we investigated whether E-cadherin was one of the epithelial cell proteins that was bound by C. albicans. Using an anti-E-cadherin monoclonal antibody, we detected a significant amount of this protein in immunoblots of epithelial cell membrane proteins that were bound by hyphae of wild-type C. Albicans. Furthermore, although the mutant without Als1 still bound to E-cadherin, the mutant without Als3 did not (20):

Candida albicans hyphae bind to epithelial cell E-cadherin in an Als3-dependent manner (20).

Invasion by endocytosis occurs as a result of the N-terminal region of Als3 binding to either N-cadherin or E- cadherin (20).


We verify that binding of Candida albicans hyphae to N-cadherin and E-cadherin on the surface of intact host cells required Als3 and was associated with induction of endocytosis (20).


Monolayers of endothelial cells or FaDu oral epithelial cells were infected with the various strains of C. albicans, after which N-cadherin and E-cadherin were detected by indirect immunofluorescence. To determine if the organisms were in the process of being endocytosed, the host cells were also stained for f-actin, which accumulates around such organisms. We observed that endothelial cell N-cadherin colocalized with hyphae of the wild-type strain and the mutant without Als1 that were being endocytosed (20).

In contrast, N-cadherin did not colocalize with hyphae of the mutant without Als3, and almost none of these hyphae were endocytosed. As expected, N-cadherin accumulated around hyphae of the als3D/als3D::ALS3 complemented strain similarly to the wild-type control strain (20).

Below: N-Cadherin from Endothelial Cells Colocalizes with Wild-type C. Albicans, but Not a mutant without Als3. Confocal micrographs of uninfected endothelial cells (A-C), or endothelial cells infected with the wild type strain (D-G), the mutant without Als3 (H-K) or the mutant without Als3 complemented with Als3 (L-O). The cells were stained for N-cadherin (A), (D), (H) and (L), actin microfilaments (B), (E), (I) and (M), and C. Albicans (F), (J) and (N). The merged images are shown in (C), (G), (K) and (O). Arrows indicate the accumulation of N-cadherin and microfilaments around the organisms (20):


A comparable pattern was observed with E-cadherin and oral epithelial cells (20).

E-cadherin colocalized with the endocytosed hyphae of all strains except for the mutant without Als3. Also, very few hyphae of the mutant without Als3 were endocytosed. The anti-N-cadherin and anti-E-cadherin monoclonal antibodies did not bind to C. albicans hyphae in the absence of endothelial or epithelial cells, indicating that these antibodies did not recognize any cross-reacting C. albicans antigens (20).

Below: E-Cadherin from Oral Epithelial Cells Colocalizes with Wild-type C. Albicans, but Not a mutant without Als3. Confocal micrographs of uninfected FaDu oral epithelial cells (A-C), or epithelial cells infected with the wild type strain (D-G), the mutant without Als3 (H-K) or the mutant without Als3 complemented with Als3 (L-O). The cells were stained for E-cadherin (A), (D), (H) and (L), actin microfilaments (B), (E), (I) and (M), and C. Albicans (F), (J) and (N). The merged images are shown in (C), (G), (K) and (O). Arrows indicate the accumulation of E-cadherin and microfilaments around the organisms (20):


Collectively, these findings support a model in which C. albicans Als3 binds to endothelial cell N-cadherin and oral epithelial cell E-cadherin, thereby stimulating actin-mediated endocytosis of the organism (20).


A striking finding was that the 3-D structures of the N-terminal regions of Als1 and Als3 are predicted to have recurrent β barrel domains and a global negative surface potential similar to the N-terminal regions of N-cadherin and E-cadherin (20).

The N-terminal regions of Als1 and Als3 bear a striking resemblance to the N-terminal ectodomains of N- and E-cadherin, however, the predicted N-Terminal Structures of Als1 and Als3 are different (20).


The predicted parameters of Als3 binding to N-cadherin were very similar to those of one molecule of N-cadherin binding to another (N-cadherin self-association), but different regions of Als1 and Als3 bind to N-cadherin. The N1 and N3 domains of Als1 bind to domains N1 and N2 of N-cadherin, whereas the N2 and N3 domains of Als3 bind to domains N1 and N2 (20).


Als1 and Als3 interact with N- and E-cadherin via different domains, and the geometry of Als3 binding to E-cadherin is similar to that of E-cadherin binding to itself (20).

The N-terminal regions of Als1 and Als3 are also predicted to interact differently with E-cadherin. The energetics of Als1 binding to E-cadherin are considerably weaker than either Als3 binding to E-cadherin or E-cadherin binding to itself (20).

The Als3-E-cadherin interaction is somewhat different from the Als3–N-cadherin interaction. The geometry of this interaction assumes a parallel orientation, such as that which occurs when E-cadherin binds to itself (20).


Als3 Is Predicted to Bind to Cadherins Differently than Als1 (20).

Als3 functions as a molecular mimic of mammalian cadherins, thereby facilitating C. albicans invasion of endothelial and oral epithelial cells (20).



Relatively few Candida albicans adhesins have been evaluated for invasin function (has Hwp1 be?); analysing additional adhesins will help clarify the relationship between adherence and invasion (14).

Als1 and Als3 act as invasins.

Als1 has lower invasive (endocytosis) capacity than Als3.

It is also virtually certain that C. albicans proteins other than Als1 and Als3 can induce epithelial cell endocytosis. For example, Candida albicans mutant without Als3 and without Als1 has only modestly reduced virulence in the mouse model of oropharyngeal candidiasis, indicating that other invasin-like proteins or other mechanisms of invasion can compensate for the absence of Als1 and Als3. Determining how C. albicans invades epithelial cells independently of Als1 and Als3 is an important future challenge (14).

Another Als Candida Albicans protein working as invasin is Als5.

Als5 induces weak endothelial cell endocytosis similar to that induced by Als1 (20).

Als3, Als1 and Als5 are Candida Albicans proteins belonging to the The C. albicans ALS (agglutinin-like sequence) gene family (14).

Computer-assisted modeling of the N-termini of Als1, Als3, and Als5 predicts the presence of antiparallel β sheets that define the immunoglobulin superfamily (20).


Saccharomyces cerevisiae expressing Als3 were avidly endocytosed by endothelial cells (20).

Als1 and Als3 have very similar binding specificities in vitro, and the N-terminal domain of either protein is sufficient to promote N- or E-cadherin-dependent endocytosis (18).

The fact that Als1 and Als3 have a shared function is consistent with their similarity in sequence (88% amino acid identity of their N-terminal 772 residues) and in their predicted N-terminal-domain structures (18).

Even though N-terminal regions of Als1 and Als3 share general features, they are predicted to differ both in their 3-D structure and their interactions with N- and E-cadherin. While Als1 appears to favor an extended conformation, Als3 tends toward a more folded structure that forms a more distinct molecular cleft (20).

Our molecular modeling results also suggest that Als1 binds more weakly to N- and E-cadherin than does Als3 (20).

Als3 binds with greater affinity to N- and E-cadherin than does Als1 (20).

Although the N-termini of Als1 and Als3 share considerable homology at the amino acid level, Als3 induced endocytosis much more efficiently than did Als1 (20).

Similar studies suggest that Als1 can also induce epithelial cell endocytosis, although with lower efficiency than Als3 (14).

The Als family of proteins contains six members in addition to Als1 and Als3. Most of these proteins are known to have adhesive function. When expressed in S. cerevisiae, Als5 induces weak endothelial cell endocytosis similar to that induced by Als1 (20).

Similar experiments suggested that Als1 and Als5 could also induce endocytosis, although with considerably less efficiency than Als3 (20).

Als6, Als7, and Als9 do not appear to mediate significant endothelial cell endocytosis, and Als2 and Als4 have not yet been tested in this assay (20).

Below: sequence comparisson between Als proteins related to its invasive ability and presence of potential transglutaminase substrates:


Als protein















Weak invasion







Weak invasion







No invasion







No invasion







No invasion







Als protein













Weak invasion






Weak invasion






No invasion






No invasion






No invasion






Als protein













Weak invasion






Weak invasion






No invasion






No invasion






No invasion







Als3 Is Required for C. albicans to Damage Endothelial Cells and Oral Epithelial Cells (20).

Infection of endothelial cells or oral epithelial cells with wild-type C. albicans hyphae in vitro causes significant damage to these cells, and endocytosis of the organisms is required to induce host cell damage (20).

Therefore, we investigated the extent of damage to endothelial cells and oral epithelial cells caused by the mutants without Als1 and Als3. Consistent with the results of the endocytosis assay, the mutant without Als1 caused the same amount of damage to both types of host cell as the wild-type strain. In contrast, the mutant without Als3 caused essentially no damage to either cell type. Complementing the als3D/als3D null mutant with a wild-type copy of ALS3 restored its capacity to damage these host cells. These data indicate that the inability of the mutant without Als3 to invade endothelial cells and oral epithelial cells is associated with a significantly reduced capacity to damage these cells (20).

A mutant without Asl3 caused virtually no damage to either endothelial cells or FaDu oral epithelial cells (20).

Below: A Candida Albicans mutant without Als3  has reduced capacity to damage Endothelial Cells and Oral Epithelial cells. The extent of host cell (endothelial and epithelial cells) damage by  Candida Albicans strains was determined by a 51Cr Release Assay (20):

A Candida Albicans mutant without Als3 was endocytosed poorly by endothelial cells and two different oral epithelial cell lines (20).


These current results are also consistent with our previous data that S. cerevisiae expressing Als3 are endocytosed by endothelial cells (20).

Similarly, Zhao et al. reported that a mutant without Als3 had markedly reduced capacity to damage reconstituted human epithelium. Our results suggest that the reduced capacity of the mutant without Als3 to damage endothelial cells and oral epithelial cells in vitro is due to its defect in invading these cells (20).

The invasion and damage defects of the mutant without Als3 suggest that Als3 is important for virulence during hematogenously disseminated and oropharyngeal candidiasis (20).


This process is passive on the part of the organism because killed hyphae are endocytosed similarly to live hyphae (14).

Studies with killed Candida Albicans indicate that induction of endocytosis is passive on the part of Candida Albicans because killed hyphae are endocytosed as avidly as are live hyphae (20).


Latex beads coated with the recombinant N-terminal portion of Als3 are avidly endocytosed by epithelial cells (14).


To determine if either Als1 or Als3 alone was able to induce endocytosis, we coated latex beads with the purified, recombinant N-terminal portion of Als1 (rAls1-N) or Als3 (rAls3-N), which consisted of amino acids 17 to 432 of either protein (20).

Latex beads coated with rAls3-N were efficiently and specifically endocytosed by both endothelial cells and oral epithelial cells (20).

Latex beads coated with just the N-terminus of Als3 were able to induce endocytosis. However, it was notable that coating the latex beads with rAls3-N resulted in a much greater increase in endocytosis than adherence (20).

The rAls3-N–coated beads were avidly endocytosed by CHO cells expressing either N-cadherin or E-cadherin, but not by CHO cells that did not express these cadherins (20).

The mutant without Als1 was endocytosed normally by oral epithelial cells, whereas latex beads coated with rAls1-N induced some endocytosis by these cells. The probable explanation for these apparently conflicting results is that mutant without Als1 still expressed Als3 on its surface. Thus, the presence of Als3 masked the effects of the absence of Als1 when the endocytosis assay was performed using whole organisms. Studies performed with latex beads coated with rAls1-N indicated that Als1 is capable of inducing epithelial cell endocytosis by itself (20).

Als1 and Als3 by Themselves Are Sufficient to Induce Endocytosis. However, beads coated with rAls3-N were endocytosed more avidly by CHO cells expressing N- or E-cadherin than were beads coated with rAls1-N (20).

Beads coated with rAls3-N were endocytosed much more efficiently than were beads coated with rAls1-N (20).


Als3, and to a lesser extent Als1, bind directly to N-cadherin and E-cadherin, and that this binding is sufficient to induce endocytosis. These data also indicate that the cadherin-binding domains of Als1 and Als3 are located in the N-termini of these proteins (20).


Tandem repeats influence the conformation and/or the substrate accessibility of the N-terminal region of Als3 and thereby enhance its binding affinity for host substrates. However, even the weak binding of the N-terminal portion of Als3 to N-cadherin or E-cadherin is sufficient to induce endocytosis (20).


Cadherins on the surface of human cells normally bind other cadherins for adhesion and signaling; however, we found that Als3 also binds to cadherins on endothelial cells and oral epithelial cells, and this binding induces these host cells to take up the fungus (20).

The results with the mutant without Als3 and the latex beads coated with rAls3-N demonstrate that endothelial cell N-cadherin and epithelial cell E-cadherin are two host cell ligands for Als3. Furthermore, binding of the N-terminus of Als3 to either of these cadherins is sufficient to induce endocytosis (20).

Als3 induces endocytosis by binding to E-cadherin and other proteins on the epithelial cell surface (14).

Als3 was required for Candida albicans to bind to multiple host cell surface proteins, including N-cadherin on endothelial cells and E-cadherin on oral epithelial cells (20).

The structure of Als3 is predicted to be quite similar to that of the two cadherins studied, and the parameters of the binding of Als3 to either cadherin are similar to those of cadherin–cadherin binding (20).

Molecular modeling of the interactions of the N-terminal region of Als3 with the ectodomains of N-cadherin and E-cadherin indicated that the binding parameters of Als3 to either cadherin are similar to those of cadherin–cadherin binding. Therefore, C. Albicans. Als3 is a functional and structural mimic of human cadherins, and provide new insights into how C. albicans invades host cells (20).

Computer-assisted molecular modeling of the binding of Als3 with N- and E-cadherin suggests that Als3 is a structural and functional mimic of these cadherins (20).

Als3 is a fungal invasin that mimics host cell cadherins and induces endocytosis by binding to N-cadherin on endothelial cells and E-cadherin on oral epithelial cells (20).

Binding of Als3 to E-cadherin is sufficient to induce endocytosis because latex beads coated with recombinant Als3 are internalized efficiently by Chinese hamster ovary cells expressing human E-cadherin (14).

Latex beads coated with the recombinant N-terminal portion of Als3 were endocytosed by Chinese hamster ovary cells expressing human N-cadherin or E-cadherin, whereas control beads coated with bovine serum albumin were not (20).


Endothelial cell endocytosis of Candida albicans is induced when the organism binds to N-cadherin and other endothelial cell surface proteins. This binding induces microfilament rearrangement, which results in the formation of pseudopods that engulf the organism and draw it into the cell (20).

Endothelial cell endocytosis of Candida albicans is dependent on extracellular calcium (transglutaminase is a calcium dependent enzyme) and is governed at least in part by the tyrosine phosphorylation of endothelial cell proteins (see more below transglutaminase type 1 (TGase1) was identified as a tyrosine-phosphorylated form colocalized with E-cadherin by Hiiragi 1999) (20).


The interaction of Als3 with E-cadherin activates the clathrin-dependent endocytosis pathway (14).

siRNA knockdown of components of this pathway, including clathrin, dynamin and cortactin, inhibits the endocytosis of Candida albicans by approximately 60% (14).

The fact that siRNA knockdown of the clathrin pathway results in incomplete inhibition of endocytosis suggests that additional signalling pathways also govern this process. These alternative signalling pathways are likely activated by receptors other than E-cadherin (14).

In support of this hypothesis, wild-type C. albicans hyphae bind to multiple epithelial cell surface proteins in addition to E-cadherin. Furthermore, Candida albicans mutant without Als3 fails to bind to several of these surface proteins, suggesting that Als3 has more than one epithelial cell target protein (14).

It is highly likely that additional host cell ligands, other than N- or E-cadherin, also contribute to the endocytosis of C. Albicans (20).

For many microbial pathogens, invasion of host cells is critical for the initiation and maintenance of infection, and many of these organisms have more than one mechanism for inducing their own uptake by host cells. Thus, it is logical to speculate that C. albicans has also evolved at least one Als3-independent mechanism to invade host cells (20).



Candida albicans can also invade epithelial cells by an active process that is independent of endocytosis called active penetration (14)

Active penetration seems to require hyphal formation and is not inhibited by when the epithelial cells are treated with a microfilament inhibitor, such as cytochalasin D (14).

During active penetration, the organism can either invade into an epithelial cell without inducing the formation of epithelial cell pseudopods or pass through the intercellular junction between epithelial cells (14).


The mechanism by which C. albicans actively penetrates into an epithelial cell is incompletely understood. It is possible that Saps contribute to this process (14).

Saps are Secreted Aspartyl proteases, one of the other Candida Albicans adhesins (14).


Pepstatin A, an inhibitor of aspartyl proteinases (Saps) has been reported to inhibit the invasion of C. albicans into corneocytes in mice with cutaneous candidiasis (14).

Some, but not all investigators have found that pepstatin A reduces C. albicans-induced damage to reconstituted human epithelia (14).

This reduced damage may be due in part to decreased epithelial cell invasion (14).

However, a limitation of the pepstatin A studies is that this protease inhibitor not only blocks C. albicans Sap activity, but it also inhibits the function of aspartyl proteases produced by the epithelial cell. Therefore, pepstatin A has effects on both the organism and the epithelial cell (14).

C. albicans mutants containing disruptions of various SAP genes have reduced capacity to damage vaginal and oral epithelial cells in some in vitro models, but not others (14).

It is possible that Sap activity is required for active penetration into epithelial cells because it alters the surface characteristics of C. albicans rather than degrading host cell proteins. Dalle et al. (2009) recently found that pepstatin A inhibits C. albicans invasion into epithelial cells only when the hyphae are pre-incubated with this inhibitor, prior to being added to the epithelial cells. They also found that triple mutants lacking either Sap1-3 or Sap4-6 have reduced capacity to invade epithelial cells. Importantly, these defects in epithelial cell invasion persist even when the mutants are killed (14).

 Taken together, these results suggest the possibility that Saps may activate by proteolysis some C. albicans surface proteins that are required for the organism to invade into an epithelial cell (14).


Saps may be more important for C. albicans to invade epithelial surfaces by passing between epithelial cells rather than penetrating into them. Several groups have observed that infection of epithelial cells by C. albicans in vitro results in proteolytic degradation of E-cadherin (14).

E-cadherin is concentrated at the intercellular junctions between epithelial cells, and its degradation is associated with loss of integrity of the epithelium (14).

In support of these in vitro findings, E-cadherin antigen is reduced in the oral epithelium of HIV-infected patients who have oropharyngeal candidiasis compared with those without this disease (14).

Degradation of E-cadherin is likely mediated at least in part by Saps because it can be blocked by pepstatin A (14).

In addition, a rim101Δ/Δ mutant, which has reduced expression of SAP4, SAP5 and SAP6, has reduced capacity to degrade E-cadherin and disrupt epithelium in a three-dimensional model. These defects are rescued by overexpression of SAP5 in this mutant (14).


Collectively, these results suggest that the major mechanism of C. albicans invasion between epithelial cells is by proteolysis of intercellular junctions, whereas invasion into individual epithelial cells occurs by both induction of epithelial cell endocytosis and active penetration via a mechanism that is yet to be defined (14).

A characteristic finding during oropharyngeal candidiasis is the destruction and loss of the superficial oral epithelium due to fungal invasion (14).

Similarly, when live C. albicans is incubated with epithelial cells in vitro, significant epithelial cell damage occurs (14).

Candida albicans must be at least partially endocytosed to cause epithelial cell damage because mutants with defects in inducing epithelial cell endocytosis cause less damage to these cells (14).

Similarly, inhibiting epithelial cell endocytosis of wild-type C. albicans with the microfilament inhibitor, cytochalasin D protects epithelial cells from damage (14).

However, epithelial cell damage is not a direct consequence of the endocytic process, because killed hyphae are avidly endocytosed, but cause no detectable damage (14).

Also, altering the capacity of a C. albicans mutant to induce endocytosis does not necessarily alter the amount of epithelial cell damage that it causes. For example, a rim101Δ/Δ mutant has impaired capacity to induce epithelial cell endocytosis and cause epithelial cell damage. Overexpression of PGA7 in the rim101Δ/Δ mutant results in increased epithelial cell endocytosis, but has no effect on the extent of epithelial damage that is induced. Thus, epithelial cell endocytosis can be dissociated from epithelial damage (14).

The mechanism by which C. albicans induces epithelial cell damage is incompletely understood. As mentioned above, Saps likely contribute to this process. However, C. albicans must cause epithelial cell damage by additional mechanisms because in some systems pepstatin A does not protect epithelial cells from damage, and sap mutant strains of C. albicans cause wild-type levels of epithelial cell damage (14).

It is possible that phospholipases secreted by C. albicans may also contribute to epithelial cell damage, although this hypothesis has not yet been rigorously tested (14).


Below the relationships found between transglutaminase and the two modes of invasion of Candida albicans (FOTGCREN):

E-cadherin & ENDOCYTOSIS:

Candida Albicans Als3 induces endocytosis by binding to E-cadherin and other proteins on the epithelial cell surface (14).

The interaction of Als3 with E-cadherin activates the clathrin-dependent endocytosis pathway (14).

It was found that clathrin is able to act as a Gln-donor in transglutaminase catalyzed reactions (Orru 2003).

The interaction between the Gln-donor clathrin and a Lys-donor cytoskeletal protein could be mediated by tissue transglutaminase (tTG) activity (Orru 2003).

It has been reported that tissue transglutaminase (tTG) is involved in the receptor-mediated endocytosis process in different cellular systems (Orru 2003).

Monodansylcadaverine, a synthetic amine substrate of tTG widely used to inhibit tTG activity in vivo, was shown to inhibit the internalization of ligands via the clathrin-mediated endocytotic pathway (Orru 2003).


Several groups have observed that infection of epithelial cells by Candida albicans in vitro results in proteolytic degradation of E-cadherin (14).

E-cadherin is concentrated at the intercellular junctions between epithelial cells, and its degradation is associated with loss of integrity of the epithelium (14).

In support of these in vitro findings, E-cadherin antigen is reduced in the oral epithelium of HIV-infected patients who have oropharyngeal candidiasis compared with those without this disease (14).

Degradation of E-cadherin is likely mediated at least in part by aspartyl proteinases (Saps) because it can be blocked by pepstatin A (14).

Degradation of E-cadherin can be blocked by pepstatin A (14).

Pepstatin A, an inhibitor of aspartyl proteinases (Saps), is also a transglutaminase inhibitor. Pepstatin A suppressed the activity of transglutaminase 1 (Egberts 2004).

Pepstatin A has been reported to inhibit the invasion of Candida albicans into corneocytes in mice with cutaneous candidiasis (14).

Some, but not all investigators have found that pepstatin A reduces Candida albicans-induced damage to reconstituted human epithelia. This reduced damage may be due in part to decreased epithelial cell invasion (14).


HIIRAGI 1999 (15):


Cell-cell adhesion is essential for the formation and maintenance of the integrity of animal body as a community of a wide variety of cells (15).

During development, the intercellular junctional complex, composed of tight junctions, adherens junctions (AJs), and desmosomes, is repeatedly destroyed and formed (15).

Below: Intercellular junctions in simple epithelia: Left: Schematic representation of intercellular junction complexes in polarized simple epithelia. Right: Electron micrograph showing intercellular ultrastructures. Mv: Microvilli; TJ:tight junction; AJ: adherens junction; DS: desmosome. Taken from (Michels 2010)

AJs are electron microscopically characterized by their electron-dense plasmalemmal undercoats through which actin filaments are densely associated with plasma membranes (15).

E-cadherin, a transmembrane protein responsible for Ca2+-dependent cell-cell adhesion, is concentrated and functions as a major adhesion molecule at AJs (15).

Below: Principal interactions of structural proteins at cadherin-based adherens junction. Actin filaments are linked to α-actinin and to membrane through vinculin. The head domain of vinculin associates to E-cadherin via α-, β-, and γ-catenins. The tail domain of vinculin binds to membrane lipids and to actin filaments (Wikipedia):

Furthermore, several constituents of AJ undercoats, which may connect E-cadherin to the actin-based cytoskeleton or regulate some AJ functions, have been identified, including α-, β-, and γ-catenins, vinculin, and p120 (15).

The molecular mechanism of the formation and destruction of AJs can thus be rephrased as the mechanism responsible for the assembly and disassembly of the multimolecular complexes consisting of E-cadherin and these undercoat-constitutive proteins (15).

Tyrosine phosphorylation of proteins has been shown to be directly involved in the assembly and disassembly of AJs (15).

Unexpectedly , when we attempted to identify heavily tyrosine-phosphorylated proteins in the isolated junctional fraction from the mouse liver, an enzyme with protein cross-linking activity, transglutaminase type 1 (TGase1), was identified as a tyrosine-phosphorylated form (15).

Below: Identification of heavily tyrosine-phosphorylated proteins in the junctional fraction from mouse liver: among the various isolated proteins,  three bands which were heavily tyrosine phosphorylated in the junctional fraction yielded several sequences that were identical to partial amino acid sequences of mouse N-cadherin, β-catenin, radixin, and human TGase1. Because the former three proteins are known to be directly involved in the functions of the junctional complex, some important function in junctions was also expected for TGase1 (15):


Transglutaminase type 1 was reported to be expressed only in keratinocytes (skin epidermis, involved in the covalent cross-linking of proteins in keratinocytes playing a central role by its cross-linking activity in the formation of the cornified cell envelope of terminally differentiated epidermis) but its expression in tissues other than the skin have not been examined (15).

However, transglutaminase type 1 was also expressed in large amounts in tissues containing simple epithelia, such as the lungs, liver, and kidneys (15).

Below: Analysis of TGase1 expression in mouse multiple tissues. TGase1 was detected in the liver, kidneys, lungs, and skin (15):

Fairly large amounts of TGase1 were concentrated in cell-to-cell adherens junctions in simple epithelial cells where they were covalently cross-linked to various proteins to form large multimolecular complexes (15).


Endogenous transglutaminase type 1 was immunofluorescently mostly colocalized with E-cadherin in cultured mouse simple epithelial cells (15).

Below: TGase1, E-cadherin and ZO-1 staining in cultured mouse epithelial cells. TGase1 and E-cadherin were diffusely co-distributed on lateral membranes (d´–f´ and arrowheads) with significant concentration at junctional regions (AJs) (arrows). In contrast, ZO-1 was concentrated more apically than TGase1 and E-cadherin. TGase1 was concentrated more basally than ZO-1 in junctional regions (a´–c´ and arrows). (15):


In the liver and kidney, immunoelectron microscopy revealed that endogenous transglutaminase type 1 was concentrated, although this localization was not exclusive, at cadherin-based adherens junctions in simple epithelial cells (15).

Below: Immunoelectron microscopy of ultrathin cryosections of liver and kidney epithelial cells revealed that TGase1 was concentrated at adherens junctions, although this was not exclusive, and not at tight junctions. When ultrathin cryosections of mouse liver (c– e) and kidney (f and g) were labeled with anti-TGase1 mAb, immunogold particles were detected in adherens junctions (AJ) but not in tight junctions (TJ) (15):

It should be emphasized here that even in simple epithelial cells TGase1 was not exclusively distributed at Ajs (15).


Transglutaminase cross-linking activity was also shown to be concentrated at intercellular junctions of simple epithelial cells (in vitro and in vivo labeling) (15).

Using a primary amine fot labeling TGase-specific substrates in liver fractions from mouse, it was shown that the endogenous TGase activity, i.e. the amount of amine-labeled proteins, was significantly concentrated in the junctional fraction. This reaction was actually dependent on the endogenous TGase activity, because it was completely suppressed by 10 mM EDTA or 10 mM cystamine (data not shown), potent inhibitors of Tgases (15).

Below: Mouse liver fractions transglutaminase labelled with an amine. The cross-linked proteins were enriched at the junctional fraction in the presence of CaCl2, but not in the presence of EDTA (a potent inhibitor of TGase activity), indicating that the endogenous TGase activity itself was concentrated in the junctional fraction. Analysis of the junctional fraction revealed that TGase1 was concentrated in the junctions not only as a full-length 97-kDa form but also as higher molecular mass forms. Because TGase itself was known to be cross-linked to various proteins through its own enzymatic activity, in the junctions this “auto-cross-linking” also would produce higher molecular mass forms of TGase1. The numerous biotin-labeled bands indicated that in the junctions TGase1 used various proteins as substrates, in amine cross-linking experiments with the isolated junctional fraction, various proteins appeared to be cross-linked by endogenous TGase1 activity (the identities of the major substrates for TGase1 in AJs remains to be elucidated). Thus, in the intercellular junctions, especially in AJs, we would expect the presence of highly complicated multimolecular complexes, in which constituents including TGase1 itself were covalently cross-linked through the TGase1 enzymatic activity. (15).


Below: Cultured mouse simple epithelial cells transglutaminase labelled with an amine to visualize endogenous transglutaminase (mTGase1) activity. The localization of the cross-linked proteins overlapped with endogenous TGase1 at cell-cell borders (a–c). In the absence of the amine (B-amine) (d´) or in the presence of cystamine (f´) in the culture medium there was no stained protein, indicating that this is a way to represent the cross-linking activity (e´) of endogenous TGase (d–f) (15):

The TGase activity was also enriched in the isolated junctional fraction from the liver and co-concentrated with E-cadherin at AJs in cultured simple epithelial cells (15).


The amine used in this study (5-(biotinamido)pentylamine) is not a specific substrate for TGase1 (15).

Among the four types of intracellular TGases (types 1–4), only type 2 (TGase2) was reported to be expressed in various types of cells. In agreement with previous reports, Western blotting with anti-TGase2 mAb revealed that TGase2 was abundant in the cytoplasm of the liver and was not concentrated in the junctional fraction in contrast to TGase1 (15).

Furthermore, in cultured simple epithelial cells, TGase2 was not detected by Western blotting or immunofluorescence staining with anti-Gase2 mAb (data not shown) (15)

Therefore, the TGase activity detected with 5-(biotinamido)pentylamine in this study was mostly attributed to TGase1, although the possibility cannot be ruled out that as yet unidentified type of TGase gives rise to part of observed activity, and the TGase1 concentrated at E-cadherin-based cell adhesion sites appeared to be active as a transglutaminase within cells. The “membrane-associated TGase activity,” which was previously reported in the liver, would be the same as the activity described in this study (15).


Because it is widely thought that the plasmalemmal undercoat structures in AJs are involved in the stabilization and/or up-regulation of cadherin-based cell adhesion, it is reasonable to speculate that TGase1-mediated cross-linking of proteins plays a role in further stabilization and up-regulation of cell-cell adhesion (15).

However, this cross-linking is not necessarily irreversible in vivo. The enzymatic activity that catalyzes the breakdown of g-glutamylamines to free amines and 5-oxo-proline, i.e. the breakdown of the TGase-mediated covalent bonds, was identified in rabbit tissues such as the kidneys, liver, and intestine. Recently, TGase2 and factor XIIIa themselves were reported to possess such hydrolytic activities (56). Therefore, it is possible that the TGase1-mediated covalent cross-linking is dynamically regulated in simple epithelial cells (15).

The formation of covalently cross-linked multimolecular complexes by transglutaminase type 1 is an important mechanism for maintenance of the structural integrity of simple epithelial cells, especially at cadherin-based adherens junctions (15).

TGase-dependent cross-linking of proteins plays some important role in the regulation of AJ assembly and disassembly (15).

The TGase1-mediated covalent cross-linking of proteins is directly involved in the formation and maintenance of intercellular junctions, especially adherens junctions Ajs (15).

Further studies are required to determine whether the TGase1-mediated cross-linking of proteins is one of the important and general mechanisms involved in formation, maintenance, and regulation of the structural integrity of simple epithelial cells, especially at Ajs (15).



Microbial translocation is defined as the migration of viable microorganisms or bacterial endotoxins (i.e., bacterial lipopolysaccharide (LPS), peptidoglycan, and lipopeptides) from the intestinal lumen to the mesenteric lymph nodes (MLN) and other extraintestinal sites.


The human gastrointestinal tract is colonized by a dense population of microorganisms, referred to as the bacterial flora. Although the gut provides a functional barrier between these organisms and the host, bacterial translocation is a common event in the healthy person. However, in critically ill patients, with various underlying diseases, this bacterial translocation may lead to infections and consequently to a further reduction in general health status (Wiest 2003).


From the historical observations of Berg and Garlington who defined bacterial translocation as “the passage of viable bacteria through the epithelial mucosa into the lamina propria and then to the mesenteric lymph nodes, and possibly other tissues,” this concept has been re defined several times. Currently, this definition has become broader and includes the passage of both viable and nonviable microbes and microbial products such as lipopolysaccharide (LPS) across ananatomically intact intestinal barrier (G D'Ettorre 2012).


Microbial translocation can be defined as the passage of both viable and non-viable microbes and microbial products across the intact intestinal barrier (Alexander 1998).


In mucosal candidiasis It exists the risk of that mucosally derived organisms translocate and cause systemic candidiasis. This would happen by translocation of Candida albicans to internal organs (12).


SUNDSTROM 2002 MICE MUCOSAL CANDIDIASIS ASSAY (12):                                          

Candida Albicans (with or without Hwp1 gene) were orally administrated to “abnormal” immunodeficient mice (12).

In murine systemic candidiasis, Candida albicans accumulates in the kidneys (12).

Quantitative kidney cultures were done to determine whether C. albicans (orally administrated) translocated across the GI tract (12).

Candida albicans was rarely found in the kidneys (12).

Kidneys of 1 tgε26 mouse that was given Candida Albicans with Hwp1 and 1 bg/bg-nu/nu mouse that received Candida Albicans without Hwp1 yielded positive cultures (12).

Only 2 of 25 ill orally colonized mice had Candida Albicans in its kidneys because of translocation of C. albicans from GI tract to internal organs (12).

These results are consistent with other reports, which showed that translocation across the GI tract occurs ≥3 weeks after monoassociation in bg/bg-nu/un and tge26 mice (12).


STAAB 2013 MICE MODEL OF TRANSLOCATION (13):                                          


A common route of infection in hospitalized patients is the translocation of endogenous Candida albicans from the intestinal tract into the blood stream associated with antibiotic use and hematological immunosuppression or chemotherapy that result in the loss of intestinal barrier function (13).

We examined the role of Hwp1 in gut translocation of Candida albicans to the bloodstream using a murine model (C57BL/6 mice) that mimics this route of infection (13).

C57BL/6 mice were first treated with antibiotics and fluconazole to reduce  the GI bacterial and fungal normal gut flora and to allow the colonization of Candida albicans in their GI tracts and later were fed Candida Albicans in their drinking water to establish the colonization (13).

We found that all three Candida albicans strains, Candida Albicans with Hwp1 (SC5315), Candida Albicans without Hwp1 (SCH1211) and Candida Albicans with Hwp1 re-introduced (reconstituted strain HR615), colonized the intestinal track of C57BL/6 mice to similar levels (13).

Candida Albicans mutant without Hwp1 was capable of colonizing the mouse gut to equal levels as Candida Albicans mutants with Hwp1 (13).

Hwp1 was not influential in gut colonization of mice. Gut colonization levels are independent upon HWP1 expression (13).


The gut-colonized mice were treated with intraperitoneal injections of cyclophosphamide to induce immunosuppression / intestinal mucosa damage, and allow translocation of C. albicans into the blood stream and dissemination to the liver (13).

All of the mice colonized with strains expressing at least one allele of HWP1 (n=14) died by day 8 post-immunosuppression while 2 of the 7 mice colonized with Candida Albicans without Hwp1 (SCH1211) strain survived to the end of the study (13).

The survival rate between Candida Albicans with Hwp1 (SC5314) and Candida Albicans without Hwp1 (SCH1211)-colonized mice was statistically different; however the survival rates of mice harboring Candida Albicans without Hwp1 (SCH1211) or Candida Albicans with Hwp1 re-introduced (reconstituted strain HR615) were not different from each other (13).

Candida albicans was recovered (at similar levels) from the livers of all the mice at the time of death demonstrating translocation from the GI tract to the organ independent of HWP1 expression (13).

However, Candida Albicans mutant without Hwp1 was less virulent relative to the wild type or the reconstituted strain in this model, thus defining a new phenotype for Hwp1 in murine candidiasis (13).

Below: Survival of mice post immunosuppression with cyclophosphamide. Mice colonized with the Candida Albicans without Hwp1 (SCH1211) strain were less virulent relative to Candida Albicans with Hwp1 (SC5314). Single expression of HWP1 -Candida Albicans with Hwp1 re-introduced (reconstituted strain HR615)- did not restore wild type survival kinetics in mice, although none of the mice colonized with HR615 survived to the end of the observation period. Hwp1 is required for full virulence in an animal model of gut translocation canididasis (13):

Expression of a single allele of HWP1 in HR615 did not fully correct the hwp1 null phenotype. The results observed with HR615 suggested that Hwp1 may participate in yet undefined functions that aid dissemination via the blood stream that require wild type Hwp1 expression levels (13).

Lack of Hwp1 affected the translocation of C. albicans from the mouse intestine into the bloodstream of mice (13).

These results suggested that wild type hyphal surface levels of Hwp1 were needed for rapid translocation of C. albicans from the mouse gut into the blood stream. However, once translocation had occurred, the lack of Hwp1 did not prevent dissemination to and establishment of C. albicans in the liver (13).

Wild type levels of Hwp1 were needed for the rapid translocation of C. albicans from the gut into the blood stream and infection of murine livers (13).

C. albicans cells unable to form filaments and express HWP1 (cph1Δ/Δ efg1Δ/Δ) do not translocate from the mouse gut to the liver as well as SC514; in contrast, cells locked in the filamentous morphology (tup1Δ/Δ) appear more virulent relative to wild type cells in this animal model of candidiasis (13).

Koh AY, Köhler JR, Coggshall KT, Van Rooijen N, Pier GB (2008) Mucosal damage and neutropenia are required for Candida albicans dissemination. PLOS Pathog 4: e35.

These results did not distinguish between the ability of C. albicans to undergo morphological transitions or the expression of adhesins (e.g. Hwp1, Als3p) associated with the hyphal morphology as factors affecting gut translocation (13).

Because hwp1 null strains are not deficient in filamentation, we were able to consider hyphae formation and HWP1 expression as separate variables in our studies (13).

The results here implied that expression of adhesins and perhaps other proteins associated with the hyphal form are the key variables essential for C. albicans translocation (13).

The mechanism by which Hwp1 aids translocation is not yet understood but we propose two models that are not mutually exclusive:


A critical amount of C. albicans self-aggregation is necessary to achieve a threshold fungal burden traversing the damaged intestinal mucosa from the lumen into the blood stream (13).

Dissemination to the liver of the hwp1 null strain was delayed as a consequence of decreased cell numbers trafficking from the GI tract to the blood stream (13).

In support of this hypothesis, the flocculent tup1Δ/Δ cells (cells locked in the filamentous morphology (tup1Δ/Δ)) are more virulent in this gut translocation candidiasis model relative to wild type even when gut colonization levels are two logs below wild type (13).


Alternatively, Hwp1 contributed to interactions with epithelial cells lining the intestinal mucosa and the lack of Hwp1 hampered the initial binding and subsequent translocation of the fungus into the blood stream (13).

Hwp1 also participates in adhesion of C. albicans to oral epithelial cells in a non-TG dependent manner, therefore it is plausible that the lack of such fungal-host interactions may affect the kinetics of GI translocation (13).


Hwp1 & NICHE:

STAAB 2013:


HWP1 gene expression differs from other niche-specific genes in that HWP1 is normally expressed in hyphal cells regardless of host niche (13).


Hwp1 engenders C. albicans with niche-specific capabilities to inhabit the oral cavity (13).

Hwp1 as a niche-specific virulence attribute required for colonization and local invasion of oral tissue (13).


Hwp1 & Casein:


STAAB 1999 (1):

Using a Candida Albicans Hwp1 recombinant protein, rHwp1∆C37, that en-compasses the NH2-terminal proline- and glutamine-rich domain, we examined its ability to incorporate [14C]putrescine in the presence of TGase2 (from guinea pig livers), levels of radioactivity associated with the recombinant protein were equivalent to those of casein, a known Tgase substrate, and were fourfold greater than for bovine serum albumin (BSA), a negative control (1).

Below: Incorporation of [14C]putrescine by rHwp1∆C37, casein, and BSA mediated by TGase (1):

Examination of proteins after TGase reactions showed no radioactivity associated with BSA; however, casein was similar to Candidas Albicans recombinant Hwp1 protein in the generation of multiple species of radiolabeled reaction products including monomers displaying increased migration , species of high molecular weight , and dimers bridged by putrescine. The production of all radiolabeled forms depended on the presence of active TGase. Casein was implicated in cross-linking reactions with primary amines through Nε-(γ-glutamyl)lysine isodipeptide bonds (1).


A number of small intestinal disorders may have a similar small intestinal biopsy appearance than those caused by gluten in celiac disease. Between them: specific lesions caused by Candida and nonspecific lesions caused by soy protein (and/or milk protein) (Freeman 2001).


Hwp1 & Gluten:

Candida colonization is hypothesized to be a trigger for celiac disease, which has inexplicably increased in prevalence over the past decades in tandem with increased antibiotic use and Candida infections.

HOMOLOGY Hwp1 & Gluten:


The virulence factor of C albicans—hyphal wall protein 1 (Hwp1)—contains aminoacid sequences that are identical or highly homologous to known coeliac disease-related α-gliadin and γ-gliadin T-cell epitopes (2).

Below: Comparison of the aminoacid sequences of wheat gliadins with those of the cell-wall component hyphal wall protein 1 (HWP1) of Candida albicans, in particular with HWP1 aminoacids 40–197, shows many identical and homologous sequences in the proteins (2):

Below: Hwp1 contains sequences of known coeliac disease-related T-cell epitopes from α gliadins and γ gliadins (2):

Hwp1 and γ gliadin have three and five identical PQQPQ repeats, respectively, that are also present in most T-cell stimulating epitopes (2).

The sequence YPQQPQ is present in Hwp1 and in the DQ2 γ-gliadin epitopes DQ2-γ-V, T-cell γ-III plus γ-IV, and an unspecified DQ2 T-cell epitope (2).

The homologous sequence FPQQPQ is present in epitope DQ2-γ-III (2).

Sequence PQQQ is present in HWP1 and in epitope DQ2-γ-IV (2).

The immunodominant sequence PQPQLPY from α gliadin is selectively deamidated by tissue transglutaminase to give PQPELPY. This sequence is highly homologous to sequences PQPDIPC and PQPDVPC that both arise twice in HWP1 (2).

CROSSLINKING Hwp1 versus Gluten:

Transglutaminases have a pivotal role in blood clotting and wound healing (ie, factor XIIIa), and usually form isopeptide bonds between glutamines and lysines in proteins, thus forming cross-linked protein networks (2).

The direct interaction between C albicans and transglutaminase was noted in an immunological study. When the yeast has been in contact with human tissue (including skin) or human cell lines it binds a component on its cell wall that is immunoreactive with rabbit polyclonal antibody against human plasma transglutaminase factor XIIIa (2).

Arrese JE, Pierard GE. Factor XIIIa-related antigen immunoreactivity of fungal cell wall: a biologically relevant feature? Dermatology 1995; 190: 119–23.

Factor XIIIa might, thus, be covalently linked to C albicans (2).

Factor XIIIa and tissue transglutaminase have large stretches of identical aminoacid sequences (2).

The gliadin T-cell epitopes and aminoacid sequences in HWP1 contain identical and highly homologous sequences, and transglutaminase binds to C albicans (2).

IMMUNOLOGY Hwp1 versus Gluten:


After an injury extracellular concentrations of tissue transglutaminase will raise and become available (2).

If a foreign protein (like Hwp1 or gliadin) were covalently linked (crosslinked) by transglutaminase to the intestinal epithelium, endomysium or any other human tissue or protein conjugates Hwp1-endomysium or tgase and Gliadin-endomysium or tgase would be exposed to the immune system and autoreactive antibodies to endomysium or tgase would be developed, since peptides from these proteins have become part of a foreign immunoreactive adjuvant (related to 2 but gliadin is not quoted).

Hypothetical gluten-tissue transglutaminase hybrids have, by contrast to the C albicans-transglutaminase species, never been found (2).


Chronic mucocutaneous candidiasis (CMC) is a rare disorder characterised by chronic and recurrent infections, predominantly caused by Candida albicans affecting the mucous membranes, nails and skin (Ee 2005).

Chronic mucocutaneous candidiasis is an immune deficiency disease (Kirkpatrick 1989).

In virtually all cases the etiologic agent of CMC is Candida Albicans (Kirkpatrick 1989).

BRINKERT 2009 (21):

Chronic mucocutaneous candidiasis may cause elevated gliadin antibodies (21).

We present a 4-year-old boy admitted to the hospital due to the typical symptoms of celiac disease with severe dystrophy, anaemia and elevated gliadin IgG antibodies. Upper endoscopy ruled out celiac disease but showed severe Candida esophagitis. Due to an impaired T-cell function especially following Candida antigen stimulation in vitro, plus recurrent Candida infections of the skin, the diagnosis of chronic mucocutaneous candidasis (CMC) was made. Under the treatment with fluconazol, trimethoprim/sulfmethoxazole and IVIG, the child improved impressively. Gliadin antibodies declined steadily (21).

The common symptoms growth retardation, anaemia and elevated gliadin antibodies are suggestive for celiac disease but very unspecific. The rare immunodeficiency CMC may cause elevated gliadin antibodies (21).

PALLER 2011:

Chronic mucocutaneous candidiasis (CMC) is characterized by recurrent infections of the skin, nails, and mucosae with Candida species, usually Candida Albicans.

This disorder appears to be a common phenotype for a variety of detfects in the immune response, most notably in the cellular branch of the immune system, an mainly the specific responses to antigens of Candida species.

Chronic mucocutaneous candidiasis (CMC) has been reported in association with elevated gliadin antibodies and a celiac disease-like presentation, with weaning of the antibodies during anti-candidal therapy.


Candida:  In rare cases,  an immunodeficiency state called chronic mucocutaneous candidasis (CMC) may cause elevated gliadin antibodies. A 4-year-old boy admitted to hospital with typical symptoms of celiac disease:  severe dystrophy, anaemia  and elevated gliadin IgG antibodies, was found on upper endoscopy to have Candida esophagitis, but no evidence of  celiac disease. Presence of impaired T-cell function along with recurrent Candida infections of the skin led to a diagnosis of  CMC. Treatment with systemic antifungals produced excellent  results and a steady decline in gliadin antibody levels.


A number of small intestinal disorders may have a similar small intestinal biopsy appearance than those caused by gluten in celiac disease. Between them: specific lesions caused by Candida (Freeman 2001).

Below: Upper line: Rat tongues dealing with Candida albicans: Left: tongue papillae almost intact. Right: tissue damaged, loss of filiform papillae. The normal peaked papillary tissue architecture is flattened. Flattening of tongue papillae (Campos 2009) Lower line: Intestinal mucosa of celiac disease patient intestine dealing with gluten: Left: Healthy small intestinal mucosa. Right: Mucosal surface damaged; “flat” mucosal surface with absence of villi, striking hyperplasia of the crypts, and extensive infiltration of the lamina propria by lymphocytes and plasma cells:

Below: Upper line: Rat tongues dealing with Candida albicans (Samaranayake 2001): Left: tongue papillae intact (uninfected epithelium). Normal filiform papillae. Right: tissue damaged; loss of filiform papillae and flat-surfaced lingual epithelium. Lower lines: Intestinal mucosa of celiac disease patient intestine dealing with gluten: Left: Healthy small intestinal mucosa. Right: Mucosal surface damaged; “flat” mucosal surface with absence of villi, striking hyperplasia of the crypts, and extensive infiltration of the lamina propria by lymphocytes and plasma cells:.



As obligate intracellular parasites, all viruses must have ways of entering target cells to initiate replication and infection (Sieczkarski 2002).

Cellular membranes present a barrier between the viral particle and intracellular site(s) of replication in the cytosol or nucleus (24).

In animal cells, viruses can enter target cells in two principal ways: by a direct mechanism at the cell surface (plasma membrane) or by following their internalization into cellular compartments (for example, endosomes) (Sieczkarski 2002).

Non-enveloped and enveloped viruses enter the cytosol directly at the plasma membrane or via host cell endocytic pathways (24).

While enveloped viruses are bound by a lipid bilayer, non-enveloped viruses are surrounded by a proteinaceous capsid. Both enveloped viruses and non-enveloped viruses have evolved complex mechanisms to enter cells (24).

The mechanisms employed by non-enveloped and enveloped viruses to cross membrane barriers differ significantly, most likely as a consequence of the biophysical constraints imposed by the viral envelope (24).

Non-enveloped viruses can enter the cytosol by directly penetrating the plasma membrane, as well as through a variety of endocytic mechanisms leading to penetration of internal membrane(s). Internal membranes crossed by non-enveloped viruses include the endosomal membrane (e.g. adenovirus), the Golg i ( e.g. papillomavirus;) and the endoplasmic reticulum (e.g. SV40). Strategies to disrupt or traverse host cell membranes must be included in the mechanisms of non-enveloped virus entry. However, the precise molecular and biophysical means by which non-enveloped viruses gain entry to the cytosol have not been clearly defined in all cases (24).


Infection with an enveloped virus requires the fusion of the viral envelope with a cellular membrane. In some cases, this can occur at the plasma membrane, as reported for HIV, where binding to plasma membrane-expressed forms of CD4 and chemokine receptors induce changes in the viral envelope glycoprotein that are thought to mediate membrane fusion under neutral pH conditions. Fusion of other enveloped viruses occurs within the low-pH environment of an acidic endosomal compartment. Enveloped viruses typically reach the endosomal compartment via trafficking in clathrin-coated vesicles, although a caveolar route of entry has been reported for human coronavirus 229E (24).

The entry of both enveloped and non-enveloped viral particles requires specific interactions between host cell molecules, or receptors, and viral encoded envelope or capsid proteins. One key result of this is to bring the virus into close contact with the plasma membrane (24).

In addition to primary receptors critical for virus attachment to the cell surface (e.g. CD4 for human immunodeficiency virus (HIV) ), important co-receptors have been identified (e.g. chemokine receptors CXCR4 or CCR5 for HIV) (24).

It is now becoming apparent that a wide variety of host cell molecules are important for virus internalisation in the absence of any direct association with the virus particle. This is leading to the notion of “entry factors ” , for example the tight junction proteins claudin-1 and occludin appear to have an indirect role in HCV entry (24).


Following attachment to the host cell surface, virus entry at the plasma membrane has been described for many viruses such as HIV and Poliovirus. Enveloped viruses can fuse directly with the plasma membrane, releasing the capsid directly into the cytosol, whilst non-enveloped viruses disrupt or form pore(s) in the plasma membrane to gain entry (24).

In contrast to fusion at the plasma membrane, many viruses (such as SV40 and Influenza A) utilise intracellular trafficking pathways to fuse with internal membranes in order to release their genomic material into the cytosol (24).

Receptor trafficking pathways often define particle internalisation routes and viruses typically enter the cell by a single defined pathway, although examples have been reported where viruses utilise multiple pathways in diverse cell types (24).

The majority of host cell membrane proteins internalise through clathrin-mediated endocytosis. The clathrin-mediated pathway is ubiquitous (24).

The majority of virus families utilize endocytosis as a means of entry into cells (Sieczkarski 2002).

This is not surprising, considering the many benefits that endocytosis oers. Many viruses have a low-pH-dependent conformational change that triggers fusion, penetration and/or uncoating and endocytosis is crucial to these viruses due to the acidification occurring within the endosomal pathway. It is also becoming appreciated that viruses without a strict low-pH step for entry also enter cells via endocytosis, as endosomes offer a convenient and often rapid transit system across the plasma membrane and through a crowded cytoplasm. For nuclear replicating viruses especially, the endosome can deliver its viral cargo to the vicinity of the nuclear pore, ready for translocation into the nucleoplasm (Sieczkarski 2002).

The first molecule shown to be required for endocytosis was clathrin, which mediates the primary route of endocytic internalization into cells. In response to an internalization signal (involving typically either a YXXΦ or di-leucine sequence in the cytoplasmic tail of a receptor), clathrin is assembled on the inside face of the plasma membrane to form a characteristic invagination or clathrin-coated pit (Sieczkarski 2002).

As befits its major role in endocytosis, clathrin has been shown to play a major role in the internalization of many viruses (Sieczkarski 2002).

Internalization from the plasma membrane is made by sorting and tracking events within intracellular vesicles (endocytic compartments).

Endocytic compartments are pleiomorphic structures that fuse with one another to promote ligand tracking. Subsequent to internalization, endosomes often undergo complex

tracking and sorting events. Two principal post-internalization endocytic tracking routes exist in the cell, which can be termed recycling or lysosome-targeted. Regulation of sorting and tracking is determined by inherent signals on the internalized receptor and by signalling events within the cell (Sieczkarski 2002).

The internalized vesicle acquires properties that are defined temporally and are thus termed `early' and `late' endosomes. The early endosome is an often pleiomorphic tubulo-vesicular structure and is a major sorting station where internalized cargo can be delivered back to the plasma membrane (the recycling pathway) or can progress to the late endosome. Late endosomes, comparatively, have a mostly juxtanuclear distribution, are more spherical and contain internal vesicles - leading to the term multi-vesicular bodies. They also dier from early endosomes in that they have a significantly lower pH (approximately pH 5.5 versus pH 6.2-6.5 in the early / recycling endosome). Late endosomes subsequently progress to lysosomes, which are characterized by the presence of degradative proteases and hydrolases, delivered by communication of endosomes with the transGolgi network (Sieczkarski 2002).

In the case of endocytic entry, internalization itself is generally not sucient for productive infection, as incoming viruses are still part of the extracellular space while in endosomes. Therefore, endocytosed viruses must penetrate or fuse with the endosomal membrane to be released into the cytoplasm. In addition, the endocytic pathway is often used by viruses requiring a specific localization within the cell for a successful infection (Sieczkarski 2002).

Viruses not only depend on the machinery of the cell for internalization but also for tracking within the cytoplasm and the ability to find the correct site for replication (Sieczkarski 2002).

gp120 & gp41 glycoproteins 120 & 41:

gp120 & gp41 are two proteins (glycoproteins) located on the surface of a virus called Human immunodeficiency virus (HIV).


gp120 & gp41 are both transglutaminase substrates (22,23)


HIV  (Russell Kightley)





Anatomy of gp160 and constituent parts: gp120 & gp41 Left: three-dimensional structure and Right: aminoacid sequence.


gp160; the source of gp120 and gp41:

Like all retroviruses, HIV displays a heterodimeric env protein (gp120-gp41 complex) which is synthesized as a polyprotein (gp160) and intracellularly cleaved (23).

Via a host-cell mediated process, gp160 is cleaved to form gp120 and the integral membrane protein gp41. As there is no covalent attachment between gp120 and gp41, free gp120 is released from the surface of virions and infected cells (Berman 1994).

Human immunodeficiency virus (HIV) env glycoproteins, like  those of other retroviruses, are synthesized as a precursor (gp160) that is cleaved to generate the surface (gp120) and transmembrane (gp41) env proteins, which are non-covalently  associated to each other (23).

The HIV envelope protein is a glycoprotein of about 160 kd (gp160) which is anchored in the membrane bilayer at its carboxyl terminal region. The N-terminal segment, gp120, protrudes into the aqueous environment surrounding the virion and the C-terminal segment, gp41, spans the membrane (Berman 1994).



The native and functional HIV-1 envelope glycoprotein (Env) complex is present on the virus surface as a trimer, each of the monomers made of noncovalently loosely associated gp120 surface and gp41 transmembrane glycoproteins (Visciano 2013).

gp120 contains the CD4-binding domains, while gp41 anchors the gp120-gp41 complex in the viral env or host-cell membrane (23).


gp120 is the portion of the HIV envelope protein which is on the surface of the virus (Berman 1994).

gp120 is known to possess the CD4 binding domain by which HIV attaches to its target cells (Berman 1994).

The amino acid sequence of gp120 contains five relatively conserved domains (C1, C2, C3, C4 and C5) interspersed with five hypervariable domains (V1, V2, V3, V4 and V5). The positions of the 18 cysteine residues in the gp120 primary sequence, and the positions of 13 of the approximately 24 N-linked glycosylation sites in the gp120 sequence are common to all gp120 sequences (Berman 1994).

The hypervariable domains (V domains) contain extensive amino acid substitutions, insertions and deletions. Sequence variations in these domains result in up to 30% overall sequence variability between gp120 molecules from the various viral isolates. Despite this variation, all gp120 sequences preserve the virus's ability to bind to the viral receptor CD4 and to interact with gp41 to induce fusion of the viral and host cell membranes (Berman 1994).

HIV-1 diversity facilitates evolution of resistance to antiretroviral therapy and escape from host immune responses (Abigail 2013).


HIV-1 gp120: Schematic of gp120 with the 5 conserved domains (C1–C5 and five variable domains (V1–V5). Sites for N-linked glycosylation are shown (Sanders 2008)


Diagram of sequence elements in gp120 and definition of its core. The branched symbols mark glycosylation sites (Chen 2005)



HIV-1 gp120: Schematic of gp120 with the 5 conserved domains (C1–C5 and five variable domains (V1–V5) (Briz 2006)


gp120 contains 9 conserved disulfide bridges. Also relevant is the so-called V3-loop. This is a surface-exposed highly immunogenic antibody-binding and hypervariable (immunological escape) region of gp120, which has been extensively sequenced.

gp120 domains (the location of the V3 loop in the gp120 molecule can be seen in this schematic visualization and the 9 conserved disulfide bridges in red)


gp120 from HIV BH10


One way in which antibodies would be expected to neutralize HIV-1 infection is by blocking the binding of the HIV-1 envelope glycoprotein, gp120, to its cellular receptor, CD4. However, it has been surprising that the CD4 blocking activity, readily demonstrated in sera from HIV-1 infected individuals and animals immunized with recombinant envelope glycoproteins, has not always correlated with neutralizing activity. Results obtained with monoclonal antibodies have shown that while some of the monoclonal antibodies that block the binding of gp120 to CD4 possess neutralizing activity, others do not (Berman 1994).


When the neutralizing activity of CD4 blocking monoclonal antibodies are compared to those directed to the principal neutralizing determinant (PND) located in the third variable domain (V3 domain) of gp120, the CD4 blocking antibodies appear to be significantly less potent. Thus, CD4 blocking monoclonal antibodies typically exhibit 50% inhibitory concentration values (IC50) in the 1-10 μg/ml range whereas PND directed monoclonal antibodies typically exhibit IC50 values in the 0.1 to 1.0 μg/ml range (Berman 1994).

There are also neutralizing epitopes in the V2 and C4 domains of gp120, in addition to the neutralizing epitopes in the V3 domain (Berman 1994).

There are neutralizing epitopes in the V2, V3, and C4 domains of HIV gp120 (Berman 1994).


In addition, an anti-Env glycan-specific neutralizing mAb, 2G12, was identified, which has an intriguing domain-swapped architecture. Initially N-linked glycans in conserved regions (C) 1, C2, C3, C4 and variable regions (V) 4 were identified with 2G12 binding. (McCoy 2013)


The transmembrane protein gp41 contains several glutamine residues, 75% of which are located on the outer surface of the virus (23).


Schematic Representation of gp41

Important functional regions include the fusion peptide (FP, purple box), the N- and C-terminal heptad repeat regions (NHR, green box, and CHR, red box, respectively), and the transmembrane region (TM, yellow box). The various domains are not drawn to scale

(Cardoso 2005)



The initial event in the HIV-infection process involves the binding of gp120, the coat glycoprotein of the virus, with a subset of peripheral T cells expressing the cell surface glycoprotein CD4 (23).

The CD4 molecule functions as a receptor for gp120, being essential for HIV entry into the host cell and for membrane fusion, which contributes to cell-to-cell transmission of  the virus and to its cytopathic effects characterized by syncytia formation and cell death (23).

CD4 may interact not only with gp120 but also with gp41, determining significant conformational changes in its own structure and in that of the transmembrane protein (23).




gp120 role:

First  surface gp120 contacts CD4. Binding of gp120 to its primary receptor on the cell surface, CD4.


HIV entry begins with the high affinity binding of gp120 to the host cell CD4, which induces a major conformational change in Env that exposes or creates a binding site on gp120 for the coreceptor, typically either CCR5 or CXCR4 (Anastassopoulou 2012).


Our current understanding of the role of these HIV receptors in viral fusion supports an interaction with the primary receptor CD4 that leads to conformational change(s) that primes the virus to interact with CCR5. It is this receptor complex formation which alters the conformation of gp160 and leads to pH-independent gp41 fusion (24).


gp41 role:

Sequential binding of gp120 to CD4 and the CCR5 or CXCR4 co-receptor lead to the release of gp41 with subsequent fusion of the viral and the host membrane.

The chemokine receptor, typically CCR5 or CXCR4, triggers molecular rearrangements in the gp41.



Despite being extensively studied, the entry process of HIV, the causative agent of AIDS remains a debated issue (24).

The events that follow the interaction of gp120 with the cell membrane and which precede the internalization of the viral contents in the cell have not yet been established (23).

In this respect, it has been postulated that, after gp120 binding, HIV enters the cell either by direct fusion of the virus envelope with the plasma membrane or by receptor-mediated endocytosis of a CD4-HIV complex (23).



Two main virus entry pathways: a | Clathrin-mediated endocytosis, for example, adenovirus. Endocytosis by caveolae can also occur, for example, SV40. b | Fusion at the cell membrane, for example, HIV.Fusion can also occur from inside an endosome, for example, influenza.

Virus entry: molecular mechanisms and biomedical applications




Below: Entry of enveloped viruses into cells. The virus particle bears viral attachment  proteins VAPs embedded in its plasma membrane, which interact with cell surface molecules (virus receptors) attaching the  virion to  the  cell surface. The membrane of the virion may then fuse directly with the plasma membrane releasing the genome into the cytoplasm. Alternatively the  virus particle is  internalized by  adsorptive or  receptor-mediated endocytosis and delivered to an endosome. The acidic pH triggers fusion of the viral membrane with the endosome membrane, liberating the genome (Lentz 1990):


It has long been thought that productive entry of HIV occurs via direct fusion of the viral envelope with the host cell plasma membrane (24).


Coreceptor engagement of CD4-bound gp120 induces additional reconfigurations, leading to the insertion of the gp41 fusion peptide (FP) into the host cell membrane and the formation of a pre-fusion complex. This pre-fusion intermediate is then refolded into an energetically favorable six-helix bundle that brings the two membranes in close proximity so that fusion can occur; the viral core is thereby released into the cytoplasm (Anastassopoulou 2012).



Fusion of the HIV cell to the host cell surface (NIAID)



HIV gp120 binds to CD4 on TT-cells and then to a coreceptor. Causes gp41 attachment to the cell membrane = virus-cell fusion and HIV infection (Immunotech Laboratories)



- One key piece of evidence for an endocytic-independent entry is the observation that infection is insensitive to neutralisation of endosomal pH. In fact, blocking endosome acidification was in some cases seen to augment viral infection, perhaps by sparing particles from lysosomal degredation (24).

- CD4 endocytosis was not required for HIV entry, suggesting that endocytosed particles were degraded by host cell lysosomes (24).



Whilst the clathrin-mediated entry pathway has been documented for almost as long as the classical plasma membrane entry mechanism, it has only recently been confirmed that this route offers a productive entry pathway for HIV (Daecke 2005) (24).

Recent data may even suggest that clathrin-mediated HIV entry is the only productive pathway for infection (Miyauchi et al. 2009) (24).


- EM studies have shown internalised virions in membrane-bound vesicles. Moreover, the rates of entry and uncoating of radio labelled virions in the human T-lymphoid cell line CEM were consistent with a receptor-mediated mechanism of entry (24).

- Live cell imaging has demonstrated that HIV fusion at the plasma membrane did not proceed further than mixing of lipids and could therefore not lead to productive infection (24).






The molecular events, underlying both virus-cell and cell-cell fusion, are poorly understood. In particular, little is known both about the molecular mechanisms which follow the interaction of gp120 with CD4, or other HIV receptors occurring on CD4- cells, and the molecular mechanisms of gp41 anchorage to the cell membrane. Since several proteins capable of interacting with cell surfaces (i.e. fibrinogen, fibrin, fibronectin, vitronectin and von-Willebrand factor) have been reported to be substrates for TGase, our studies have examined whether HIV env proteins possess this feature (23).



gp160 is completely ineffective as transglutaminase substrate (23).



Conversion by proteolysis, of gp160 into gp120 and gp41, exposes amino-acceptor glutaminyl residues not only in gp120 but also in gp41 (23).

gp120 & gp141 are both transglutaminase substrates (22,23)



MARINIELLO 1993 (gp120) (22):

Recombinant gp120, but not its precursor gp160, is an amino-acceptor substrate for TGase in vitro (23).

Human immunodeficiency virus envelope glycoprotein gp120, but not its precursor gp160, covalently incorporates both spermidine and glycine ethyl ester in the presence of Ca2+ and transglutaminase purified from guinea pig liver (22).

The examined ability to act as enzyme substrate of various glutamine-containing gp120 fragments, including the principal neutralizing determinant, the CD4 binding domain, and the sequence 254-274, suggested to be involved in post-binding events and in virus entry in the host cell, indicated the glutamine-265 as possible reactive acyl donor site of the protein (22).



Mariniello 1993 (gp120) (22) proved transglutaminase susbtrate in the HIV gp120 protein. Q255? (gp120: aminoacids 234 to 272, P04578 UNIPROT)


HIV strain













P04578 (ENV_HV1H2)













P04624 (ENV_HV1H3)













P19551 (ENV_HV1MF)













P18799 (ENV_HV1ND)













P04579 (ENV_HV1RH)













P03375 (ENV_HV1B1)













P03377 (ENV_HV1BR)













Q70626 (ENV_HV1LW)













P03378 (ENV_HV1A2)













P12488 (ENV_HV1BN)













P12490 (ENV_HV1N5)













P12491 (ENV_HV1Z3)













P20871 (ENV_HV1JR)













P31819 (ENV_HV1KB)













P35961 (ENV_HV1Y2)













Q73372 (ENV_HV1B9)













P05879 (ENV_HV1C4)














This Q2XX residue is located at the C terminus of the conserved C2 region of HIV gp120, near the beginning  of the variable V3 loop region.

The amimoacids preceding this Q255 (zone TIIVQLN and before) are contact residues for CD4 based on crystal structure analyses (Decker 2005):

Envelope gp120 alignments for HIV-2 (7312A and UC1), SIV (Mac239 and Ver-Tyo1), and HIV-1 (YU2 and HXB2).

Red dots indicate HIV-1 contact residues for CD4 based on crystal structure analyses

Asterisks below the sequence indicate conservation of amino acid identity across all five virus strains

(Decker 2005)

Residues of gp120 preceding the zone are in direct contact with CD4 (Kwong 1998):

gp120 structure-based sequence alignment.

The sequences are shown of HIV-1 B, C, O , HIV-2 , and SIV

The secondary-structure assignments are shown as arrows.

Solvent accessibility is indicated for each residue by an open circle if the fractional solvent accessibility is greater than 0.4, a half-filled circle if it is 0.1 to 0.4, and a filled circle if it is less than 0.1.

N-linked glycosylation is indicated by ‘m’ for the high-mannose additions

Residues of gp120 in direct contact with CD4 are indicated by an asterisk. Direct contact is a more restrictive criterion of interaction than the often-used loss of solvent accessible surface; residues of gp120 that have lost solvent-accessible surface but are not in direct contact include 278, 282.

(Kwong 1998)

Residues of gp120 preceding the zone are in direct contact with CD4 (LIAO 2013):

Sequence alignment of outer domain of the crystallized gp120, and diverse HIV-1 Env proteins recognized by CH103 (an antibody)

Secondary structure elements are labelled above the alignment.

Symbols in yellow or green denote gp120 outer domain contacts for CD4 and CH103, respectively, with open circles with rays representing side-chain contacts, and filled circles representing both main-chain and side-chain contacts.

(Liao 2013)


Interestingly, in the lethal HIV-1 group M , it seems to be better conserved lysine residues that glutamine residues:

gp120 alignment (Bunning 2009)


Location of transglutaminase substrate in gp120 from envelope protein of HIV


gp120 from HIV BH10 (in this case the Q position would be Q257)


VELJKOVIC 2003 (25):

It has been reported that the C-terminus of the second conserved region (C2) of the envelope glycoprotein gp120, encompassing peptide 273-295 RSANFTDNAKTIIVQLNESVEIN (NTM), is important for infectivity and neutralization of the human immunodeficiency virus type 1 (HIV-1) (25).

It was also demonstrated that human natural anti-vasoactive intestinal peptide (VIP) antibodies reactive with this gp120 region play an important role in control of HIV disease progression (25).

The bioinformatic analysis based on the time-frequency signal processing revealed non-obvious similarities between NTM and VIP (25).

When tested against a battery of sera from 46 AIDS patients, these peptides, in spite of a significant difference in their primary structures, showed a similar reactivity profiles (r = 0.83). Presented results point out that similarity in the periodical pattern of some physicochemical properties in primary structures of peptides plays a significant role in determination of their immunological crossreactivity (25).


P01282 (VIP_HUMAN) UniProtKB/Swiss-Prot Vasoactive intestinal peptide VIP Homo sapiens (Human) versus

P03375 (ENV_HV1B1) UniProtKB/Swiss-Prot Surface protein gp120 Human immunodeficiency virus type 1 group M subtype B (isolate BH10) (HIV-1)

RSANFTDNAKTIIVQLNESVEINCTRP (gp120 HIV 273-299 Q255 (Q287 in gp160))



Human immunodeficiency virus (HIV) disease progression varies greatly between individuals and it appears that host factors play an important role in determining the clinical outcome in HIV infection (25).

In order to define these host factors, Neurath and co-workers have investigated antibody profiles in two groups of HIV-infected patients: those who remained healthy for at least 10 years and those who developed AIDS within 5 years of the onset of infection (25).

They demonstrated that antibodies recognizing the peptide RSANFTDNAKTIIVQLNESVEINCTRP (amino acids 280–306 within the C2 region of the envelope glycoprotein gp120

from the BH-10 isolate of HIV-1) are significantly more prevalent in asymptomatic carriers than in patients who progressed to AIDS (6/9 in asymptomatic vs. 0/9 in AIDS patients) (25).

Based on these results, it appears that the absence or disappearance of these antibodies my represent a possible factor contributing to disease progression (25).

For this reason, it has been proposed that maintenance of a high level of these antibodies by immunotherapy, based on active immunization with antigens containing this peptide and/or administration of the corresponding antibodies, should be considered as a modality for therapy of HIV-1 infection. This assumption was strongly confirmed by recently reported results of therapy performed by passive immunization with human HIV-negative plasma enriched with antibodies reactive with the C2-derived peptide encompassing amino

acids 280–302 (25).

Despite the presence of the strongest T-cell epitope of gp120, which is active in vitro and an exposed B-cell epitope, the C-terminus of the C2 region encompassing amino acids 280–306 is not immunogenic in humans. Absence of the active B-cell epitope within this peptide indicates that antibodies in sera of HIV patients recognizing this region of HIV-1 gp120 represent autoreactive antibodies elicited by some human antigen. Vasoactive intestinal peptide (VIP) was identified as the human antigen likely inducing these natural antibodies, which are cross-reactive with peptide RSANFTDNAKTIIVQLNQSVEIN (denoted as peptide NTM) derived from the C2 region of HIV-1 gp120 (24).



MARINIELLO 1993 (gp41) (23):

We initially investigated whether one or more of gp41 glutamine residues could incorporate radioactive spermidine, in the presence of purified TGase and Ca2+ in vitro (23)

Since the transmembrane protein gp41 contains several glutamine residues, 75% of which are located on the outer surface of the virus, we initially investigated whether one or more of these residues could incorporate radioactive spermidine, in the presence of purified TGase and Ca2+ in vitro (23).

TGase-catalyzed incorporation of  spermidine into gp41 (23).

Recombinant gp41, the transmembrane glycoprotein of the human-immunodeficiency-virus (HIV) envelope, is an amino acceptor and donor substrate for transglutaminase in vitro (23).

gp41 is not only able to act as a TGase amino acceptor but also as an amino-donor substrate of transglutaminase (this presence of amine-donor site(s) in the gp41 was verified by the tgase-catalyzed incorporation of substance P (a peptide known to contain a reactive glutamine residue) into gp41, confirming the existence of one or more cross-linking lysine residues in the glycoprotein sequence (23).

Ability of TGase to produce gp41 homodimer and homopolymers. In vitro cross-linking of gp41 into both a homodimer and homopolymer(s) (23).

To explore and identify the specific gp41 site(s) involved in the reaction, we tested several overlapping synthetic peptides which covered all the glutamine residues present in the gp41 sequence located on the outer surface of the virus (23).

These synthetic peptides were tested as TGase substrates by TGase-catalyzed incorporation of  spermidine (23).

The gp41 peptides which were assayed contained all the glutamines present  in the external part of the transmembrane protein (23).

Below: Primary structure of HIV transmembrane gp41. The underlined fragments are the peptides assayed as TGase amino-acceptor substrates; the glutamine residues in white boxes represent  those found to be reactive sites for the enzyme. Outer (OUT) and inner (IN) viral environments (23):


gp41 fragments 27-42, 104-116, 128-144 and 143-154 are completely ineffective as TGase amino-acceptor substrates (23).






Gln51, Gln52, Gln66 (as amino-acceptor) and Lys77 (as amino-donor) residues were suggested as reactive sites in gp41 recognized by transglutaminase (23).



Mariniello 1993 (gp41) (23) proved transglutaminase susbtrates in the HIV gp41 protein (gp41: aminoacids 42 to 86, P04578 UNIPROT)




MARINIELLO 1993 (gp41) (23):

CD4 is unable to function as amino-acceptor substrate of transglutaminase when incubated alone with Tgase (23).

CD4 acquired amino-acceptor substrate property in the presence of similar amounts of gp41 (23).

There is an effect of CD4 on the ability of gp41 to act as TGase amino-acceptor substrate (23).

CD4 seems to acquire a TGase amino-acceptor substrate ability in the presence of  similar concentrations of gp41 (23).

Possible effect on the TGase-catalyzed  structural modification of gp41 by soluble CD4 (23).

The presence of CD4 in the reaction mixture negatively influenced spermidine incorporation into transmembrane protein monomer and dimer, apparently leading to an increased production of either homopolymers or heteropolymers. These phenomena were less evident when lower concentrations of CD4 were used (23).

These findings could be a consequence of non-covalent interaction between gp41 and CD4, affecting the molecular features of both proteins as amino acceptors for TGase. This hypothesis is supported by  the experiment showing an increase in CD4 immunoprecipitation, specifically observed when the protein was incubated with gp41 both in the presence and absence of either gp120 or bovine serum albumin (used as a control) (23).

Soluble CD4, even though unable to function as an amino-acceptor transglutaminase substrate, becomes active in the presence of gp41, negatively influencing the enzyme-catalyzed incorporation of the polyamine spermidine into the transmembrane protein (23).

A binding domain for a Gln66-containing and Lys77-containing region of transmembrane protein (amino acids 65-81) has been reported to be expressed on the surface of CD4+ cells. gp41 interaction with such a receptor is probably required for HIV fusion and internalization (23).

The existence of a putative gp41-binding site for a cell surface protein exposed after HIV interaction with CD4, might help to understand how CD4 activates the fusion potential of the

gp120-gp41 complex (23).



MARINIELLO 1993 (gp41) (23):

gp41 could cross-link to receptor(s) occurring on HIV-target cells and/or gp120 with both glutaminyl and lysyl residues (23).

Transglutaminase reactive sites in gp41 have been proposed for possible cross-linking reactions with gp120, CD4 or other receptor(s) occurring on the surface of HIV-target cells (23).

An extracellular or membrane-associated molecular form of transglutaminase could cross-link the virus env glycoproteins to each other and/or to CD4 or some different protein receptor occurring on the surface of the HIV-target cell (23).



MARINIELLO 1993 (gp41) (23):

Several functions have been ascribed to transglutaminases including receptor-mediated endocytosis (23).

The existence of membrane-associated TGase on the surface of human alveolar macrophages, human peripheral blood mononuclear cell and rabbit hepatocytes have been reported; the occurrence, in blood plasma, of the coagulation factor XIII, a zymogenic form of  the enzyme, is also well known (23).

It  has been demonstrated that rabbit hepatocyte surface-expressed TGase can serve as a crucial component of a binding site for exogenous fibrinogen or fibronectin, covalently incorporating these glycoproteins into high-molecular-mass complexes on the outside of the cell (23).

For both endothelial cells and  melanoma cells, the TGase-mediated covalent cross-linking of  the  cell-bound fibrinogen suggests the possibility that the surface-expressed enzyme may also serve as a binding site for glycoproteins in these cells (23).

These data emphasize the possibility that TGase catalyzes the formation of  a covalent bond between protein(s) of  the HIV envelope (env) and protein(s) present on the membrane of  HIV-target cells (23).

Possible role for human-immunodeficiency-virus (HIV) internalization, by some molecular forms of  transglutaminase either occurring extracellularly or associated to the membrane of  the HIV-target cells (23).

Possible role for transglutaminase in virus entry into host cells, via receptor-mediated endocytosis, and/or in HIV-induced CD4+ T-cell depletion via apoptosis (23).

In conclusion, the experimental evidence that soluble gp120 and gp41 can act as TGase substrates in vitro supports the notion that the enzyme could participate in the process of HIV entry into the host cells (23).

Primary amines and several peptides which share the property of being competitive inhibitors or substrates of TGase have been shown to inhibit internalization of ligands through coated pits and vesicles, we believe that  this potential new target for anti-HIV therapy deserves to be investigated first of all by testing  TGase inhibitors and alternative substrates as potential drugs blocking  virus entry (23).

Since it has been shown that the interaction of gp120 with CD4, as well as with other receptor(s) occurring in ganglion cells and hippocampal neurons, results in an increased intracellular Ca2+ concentration and that TGase, a strictly Ca2+-dependent enzyme, could be involved in receptor-mediated endocytosis, we hypothesized that transglutaminase may play a role in the process of virus internalization by cross-linking HIV env glycoproteins to specific receptor(s) occurring on the target-cell surface (23).



MARINIELLO 1993 (gp41) (23):

Programmed cell death (apoptosis) may be responsible for the deletion of reactive T cells that contributes to HIV-induced immunodeficiency. However, some of the stimuli inducing apoptosis were capable of both inducing TGase expression and increasing ε(γ-glutamyl)lysine-cross-link concentration. Therefore, the ability of HIV env glycoprotein to act as TGase substrates could be consistent also with a putative role played by the enzyme in virus-induced CD4+ T-cell depletion via apoptosis during the progression of HIV infection (23).


gp120 & gp41 & Casein:



TGase activity was assayed by a radiometric method, based on the Ca2+-dependent incorporation of [14C]spermidine into protein amino-acceptor substrates (23).

It was measured tgase-catalyzed incorporation of spermidine into casein, gp160, gp120 and gp41 (23).

Below: Transglutaminase amino-acceptor substrate activity of casein (N,N-dimethylated casein) compared to gp41, gp120 and gp160. Proteins were incubated with purified guinea pig liver transglutaminase in the presence of [14C]spermidine at 37ºC, pH 8.0 (23):

The ability of gp41 to incorporate radioactive spermidine was approximately 50% of that exhibited by N,N-dimethylated casein, a well known and very effective amino-acceptor substrate of the enzyme (23).

gp41 effectiveness to incorporate spermidine in the presence of active TGase was found to be more than twice as high as that exhibited by gp120 and only half of  that of N,N-dimethylated casein, a well known and very effective amino-acceptor substrate (23).





Human immunodeficiency virus infection / acquired immunodeficiency syndrome (HIV/AIDS) is a disease of the human immune system caused by infection with human immunodeficiency virus (HIV) (Wikipedia).


Two types of HIV have been characterized: HIV-1 and HIV-2. HIV-1 is the virus that was initially discovered. It is more virulent, more infective,and is the cause of the majority of HIV infections globally. The lower infectivity of HIV-2 compared to HIV-1 implies that fewer of those exposed to HIV-2 will be infected per exposure. Because of its relatively poor capacity for transmission, HIV-2 is largely confined to West Africa (Wikipedia)


Humans are not the natural hosts of either HIV-1 or HIV-2. Instead, these viruses have entered the human population as a result of zoonotic, or cross-species, transmission (Hahn 2000).


Phylogenetic studies1 of HIV-1 and HIV-2, alongside their counterpart simian immunodeficiency viruses (SIVs), show that the pandemic has resulted from as many as eight independent transmissions from African monkeys and apes (WOLFE 2004)


Numerous African primates are infected with simian immunodeficiency viruses (SIVs), and it is now well established that SIVs infecting chimpanzees (Pan troglodytes troglodytes) and western gorillas (Gorilla gorilla gorilla) in west central Africa are the progenitors of human immunodeficiency virus type 1 (HIV-1) (Van Heuverswyn 2007)


HIV´s most immediate ancestor is simian immunodeficiency virus of chimpanzees (SIVcpz), a retrovirus found in chimpanzees that inhabit the forests of west-central Africa. This observation is based on studies that have sampled retroviruses from humans and monkeys and sequenced key genes taken from them. Examination of genes using molecular methods can determine the genetic relatedness of retroviruses (as well as other organisms) by comparing the genetic sequence of a gene common to different viruses. The more the sequences resemble each other, the more related the viruses that harbor these genes are thought to be. Existing HIV strains have been sampled in order to make clear that their most likely common ancestor was not a human virus but SIVcpz (Lock 2010).


Evidence of simian immunodeficiency virus (SIV) infection has been reported for 26 different species of African nonhuman primates (Hahn 2000).


Two of these viruses, SIVcpz from chimpanzees and SIVsm from sooty mangabeys, are the cause of acquired immunodeficiency syndrome (AIDS) in humans (Hahn 2000).


Together, SIVcpz from chimpanzees and SIVsm from sooty mangabeys have been transmitted to humans on at least seven occasions. Current evidence indicates that the SIV

counterparts of HIV-1 and HIV-2 were intro-duced into the human population no fewer than seven times, and possibly more. Yet the HIV-1 group M viruses, which are responsible for the great majority of all HIV infections worldwide, appear to have arisen from just one such cross-species transmission event (Hahn 2000).




There are currently three scientific hypotheses to explain the origins of the HIV epidemic (Lock 2010).


Two hypotheses are iatrogenic (bad medical practices) and one is natural.




This theory argues that mass inoculation of over a million Africans with an experimental oral poliovirus vaccine (OPV) in the 1950s was responsible for contaminating a large number of humans with SIV. According to the OPV theory, this vaccine was contaminated with SIV because some batches were prepared using chimpanzee kidneys as a culture medium. Hooper found that the eraliest recorded cases of HIV were geographically clustered in areas where the experimental vaccine had been administered (Lock 2010).


Marx and his collaborators found that transmission of SIVsm was an extremely unusual occurrence amongst Africans who had SIV sm-infected monleys as pets despite their being often bitten; the “natural transfewr” rate appeared to be too low to explain an epidemic. Marx et al hypothesized that such rare cross-species infections could be amplified by the serial passage of SIV to humans from an original monkey-infected human through the reuse of (contaminated with SIV) needles. This is because, when a micro-organism passes from host to host, it is the most aggressive or virulent form that is most likely to be transmitted onwards. Not only would serial passage explain how larger numbers of people could be contaminated as reuse of needles was commonplace at the time, but it would account for SIV´s mutation into a more virulent HIV strain (Lock 2010).



What is known as the “natural transfer” theory hypothesizes that humans were contaminated by the chimpanzee virus through routine contact with their bodily fluids, most likely through the butchering and consumption of (infected with SIV) bushmeat. The natural transfer theory is supported by molecular studies that identify the period around 1931 as the most likely time the SIV began to diversify. Because diversification is a response to a changed environment, such as a new host, this suggests that around 1931 was when SIV “jumped species” into a human host (Hahn) (Lock 2010).


Molecular studies do not indicate whether diversification of SIV occurred in humans or chimps (Lock 2010).



There would be three route hypotheses to explain the origins of the HIV epidemic:


Contact with the blood and body fluids of animals during hunting or butchering can lead to transmission of many diseases and has been proposed as the primary mechanism for HIV origins. Non-human primate bushmeat from central Africa has a high frequency of SIV infection, confirming that individuals who hunt or butcher these animals are at risk for zoonosis.3 Although SIV and other primate retroviruses are known to infect laboratory and zoo workers, transmission of these viruses to human beings who are in regular contact with non-human primates in natural settings has not been documented (WOLFE 2004).

Our results show simian retroviral zoonosis in people who have direct contact with fresh non-human primate bushmeat (Wolfe 2004).










Among the great apes (the gorilla,the orangutan,the bonobo, and the chimpanzee) and ourselves, only humans and chimpanzees hunt and eat meat on a frequent basis. Since neither humans or chimpanzees are truly carnivorous - most traditional human societies eat a diet made up mostly of plant foods - we are considered omnivores (Craig B. Stanford).


When Jane Goodall first observed wild chimpanzees hunting and eating meat nearly 40 years ago, skeptics suggested that their behavior was aberrant and that the amount of meat eaten was trivial. Today, we know that chimpanzees everywhere eat mainly fruit, but are also predators in their forest ecosystems. In some sites the quantity of meat eaten by a chimpanzee community may approach one ton annually (Craig B. Stanford).


In the early 1960's, when Dr. Jane Goodall began her now famous study of the chimpanzees of Gombe National Park, Tanzania, it was thought that chimpanzees were strictly vegetarian. In fact, when Goodall first reported this behavior, many people were skeptical and claimed that meat was not a natural part of the chimpanzee diet. Today, hunting by chimpanzees at Gombe has been well documented (Teleki 1973; Goodall 1986), and hunting has also been observed at most other sites in Africa where chimpanzees have been studied, including Mahale Mountains National Park (Uehara et al. 1992) (also in Tanzania) and Tai National Park in Ivory Coast in West Africa (Boesch and Boesch 1989). At Gombe, we now know that chimpanzees may kill and eat more than 150 small and medium sized animals such as monkeys, wild pigs and small antelopes each year (Craig B. Stanford).


After three decades of research on the hunting behavior of chimpanzees at Gombe, we already know a great deal about their predatory patterns.  We know that although chimpanzees have been recorded to eat more than 35 types of vertebrate animals (Uehara 1997), the most important vertebrate prey species in their diet is the red colobus monkey. At Gombe, red colobus account for more than 80% of the prey items eaten. But Gombe chimpanzees do not select the colobus they will kill randomly; infant and juvenile colobus are caught in greater proportion than their availability (Stanford et al. 1994a, 1998a); 75% of all colobus killed are immature. Chimpanzees are largely fruit eaters, and meat composes only about 3% of the time they spent eating overall, less than in nearly all human societies (Craig B. Stanford).


Dr. Goodall received another startling answer to the question of what chimps eat when she observed groups of male chimpanzees conducting seemingly organized hunts of red colobus monkeys. After they had captured their prey, the chimps ate every part of the monkey, including the brain. They sometimes also shared some of the meat with female chimps who had not participated in the hunt.


Despite their hunting behavior, however, only a very tiny percentage–perhaps as small as two percent–of a wild chimp’s diet consists of meat or insects.


The origin of SIVcpz itself remains unclear. Phylogenetic analyses of the SIVcpz-US strain and seven other SIV lineages revealed that the SIVcpz genome has a recombinant origin. SIVcpz clustered closely with SIVrcm from red-capped mangabeys (Cercocebus torquatus) in the 5′ half of the genome, in Nef and in 3′LTR., and closely with SIV from several Cercopithecus species (C.nictitans, C. cephus, C. mona) in the vpu, tat, rev, and env genes (Souquière 2012)


Importantly, only simian species indigenous to the African continent are naturally infected. Thus far, natural SIV infections in Africa have been documented in the sooty mangabey (SM), (Cercocebus torquatus atys) (SIVsm strains) in Liberia, in Sierra Leone, and the Ivory Coast; in all four subspecies of African green monkeys (Cercopithecus aethiops) (SIVagm strains) in eastern, central, and western Africa; in the Sykes monkey (Cercopithecus mitis) (SIVsyk strains) in Kenya; in the mandrill (Mandrillus sphinx) (SIVmnd strains) in Gabon; and in chimpanzees (Pan troglodytes) (SIVcpz strains) in Gabon (Georges-Courbot 1998)


Chimpanzee, a saint?


HIV, like most other infectious diseases of humans, is a zoonotic epidemic and chimpanzees were the primate reservoir from which HIV arose (Lock 2010).


The monkey western red colobus is frequently hunted by the common chimpanzee (Wikipedia)


Karl Ammann

Karl Ammann

Research indicates chimpanzee predation may be a major selection factor upon some red colobus populations. Although chimpanzees primarily eat fruit, leaves, and insects, if the opportunity to eat meat rises, they will pursue it. In addition to sustenance, evidence suggests chimpanzees prey on red colobus to reinforce social status, as well as to attract sexual partners (Wikipedia).

The western red colobus is often hunted by the common chimpanzee. In 1994, western red colobus monkeys infected many chimpanzees with Ebola virus when they were hunted/eaten by the same chimpanzees (Wikipedia)




Eastern chimpanzees feeding on red colobus monkey (Ferrero-Labat)


After hunting, chimpanzees share the meat of a red colobus monkey amongst them (Roman M. Wittig)


Dominant Male Chimpanzee Sits Down with its Prey, a Red Colobus Monkey (Michael Nichols)


Chimpanzee feeding on red colobus monkey (Karl Ammann)


Chimpanzee Eating red colobus monkey Tropical forest, Western Uganda (Suzi Eszterhas)




Chimpanzees acquired siv from two smaller primates, the greater spotnosed monkey (Cercopithecus petaurista) and the red-capped mangabey (Cercocebus torquatus). “The chimpanzee virus is a recombinant of ancestors of these other viruses,” she added. “Chimpanzees acquired their infection like humans did, by hunting and consuming naturally infected primates.” (Hahn 2005)




Humans are not saints either.


Bushmeat is a term that refers to the use of wild animals, ranging from cane rats to gorillas, for food. The term typically refers to the practice in forests of Africa (Karesh 2009).


Bushmeat hunting, to provide animal proteins for the family and as a source of income, has been a longstanding common component of household economies in the Congo Basin and, more generally, throughout subSaharan Africa (Peers 2000).


Vegetarian or not, most people have a firm idea of what constitutes edible meat: cow, chicken, pig, turkey, etc. But in Africa, all wildlife species are used for meat including elephant, gorilla, chimpanzee and other primates, forest antelope (duikers), crocodile, porcupine, bush pig, cane rat, pangolin, monitor lizard, guinea fowl, etc. Hunters refer to the African forest as “the bush,” therefore wildlife and the meat derived from it is referred to as “bushmeat.”


Bushmeat is regularly consumed in African communities and may make up approximately 80% of all animal-based protein eaten in Central Africa (Diana Galant 2013).


80% of meat eaten in Cameroon is bushmeat including gorilla and chimp (ROB WAUGH)


Many primate species in addition to chimpanzees and sooty mangabeys are hunted and that 20% (or more) of these animals likely harbor SIV. Thus, if contact with infected blood or other secretions is indeed the primary route of transmission, hunters and food handlers may be at risk of infection with many more SIVs than just those from chimpanzees and sooty mangabeys (Peers 2000).


A substantial proportion of wild monkeys in Cameroon are SIV infected and that humans who hunt and handle bushmeat are exposed to a plethora of genetically highly divergent viruses (Peers 2000)


Cameroon is home to diverse wild-caught primate species that are extensively hunted for food and trade. Much of the primate meat sold for consumption derives from SIV infected monkeys (Peers 2000)


Scientists in Cameroon have warned that eating monkeys and apes could cause the next HIV. They are already tracking a HIV-like virus called simian foamy virus, and fear more viruses could spread and lead to a global health crisis. 80 per cent of the meat eaten in Cameroon is killed in the wild and is known as ‘bushmeat’, with gorilla, chimpanzee or monkey favourites (ROB WAUGH)


Serologic evidence of SIV infection has so far been documented in 26 primate species, and 20 of these viruses have been at least partially molecularly characterized (Peers 2000).


A study conducted in Cameroon from 1999 to 2001 tested wild monkeys, bushmeat animals, and pet monkeys for different strains of Simian Immunodeficiency Virus (SIV). SIV, the

zoonotic form of HIV, is the virus responsible for the AIDS pandemic. Study results showed SIV infection in 16.6% of all tested animals. The study’s limitations included not testing all primate species and under sampling of some species included in the study. For this reason, the percentage of primates infected with some strain of SIV could well be larger than 16.6%. The study also mentions that recombination between strains of HIV and SIV could pose a risk for a new zoonotic illness (Diana Galant 2013).


Carcasses of slaughthered spot-nosed guenons are transported from their forest homes for

Preparation and sale at a restaurant (Karl Ammann)



Adult gorilla victim of the illegal bushmeat trade.


Joseph Melloh uncovering the bushmeat trade in Central Africa


© E. Prescott 

                                                                                                                      Bush meat for sale at a market in Oshwe, Democratic Republic of Congo (Daniel Beltra)


Bushmeat in Pointe Noire, Democratic Republic of Congo (Hahn 2005)

It has been theorized that siv moved from chimpanzees and sooty mangabeys to humans—evolving into pathogenic hiv-1 and hiv-2 respectively— through exposure to primate blood, most likely as a result of the bushmeat trade. The hiv-1 group M epidemic likely began in the region of Kinshasa, Democratic Republic of Congo. Although wild chimpanzees are not found in the immediate vicinity of Kinshasa, the city is situated on the Congo River, which allowed for the easy transport of siv-infected bushmeat and of infected humans from rural to urban areas. At this bushmeat market in Pointe Noire, a butchered chimpanzee is shown in the middle of the photograph, along with other smoked and fresh meat.

bushmeat trade


In a recent publication, we described a new SIVcpz sequence (SIVcpzUS) and, on the basis of its analysis along with other data, con-cluded that the HIV-1 epidemic had arisen as a consequence of SIVcpz transmission from a particular chimpanzee subspecies, P. t. troglodytes, to humans. In that report, we demonstrated HIV-1 to be most closely related at a phylogenetic level to SIVcpz from P. t. troglodytes, presented indirect evidence for a higher prevalence of natural SIVcpz infection based on the discovery of viral recombination between SIVcpz viruses of different lineages, described geographic coincidence for all groups of HIV-1 (M, N, and O) and SIVcpz from P. t. troglodytes, and proposed hunting and field-dressing of chimpanzees (a common practice in west central Africa) as a plausible route of zoonotic transmission (Hahn 2000).


Investigators who screened 29 captive chimpanzees from Cameroon for evidence of SIVcpz infection and identified three animals (CAM3, CAM4, CAM5) as seropositive. Two of these animals had acquired their infection in the wild, whereas the remaining one represented a cage transmission. The two animals (CAM3 and CAM5) presumed to be naturally infected were found to be members of the P. t. Troglodytes subspecies, and their viruses fell within the SIVcpz(P.t.t.) radiation. Moreover, in the env region, the new Cameroonian chimpanzee viruses were significantly more closely related to SIVcpzUS and YBF30 (the single full-length representative of HIV-1 group N) than to any other virus within the HIV-1/SIVcpz(P.t.t.) radiation.  These data thus revealed a close geographical linkage of human and chimpanzee viruses and suggested that the cross-species transmission event that gave rise to HIV-1 group N occurred in Cameroon or its immediate vicinity Molecular viral epidemiological data showing that all known HIV-1 group N infections are restricted to Cameroon further support this hypothesis (Hahn 2000).

In summary, it seems clear that HIV-1 arose as a consequence of SIVcpz transmission from chimpanzees to humans and that the P. t. troglodytes subspecies represents a natural host and reservoir for this virus.



Humans and chimpanzees have shared the same habitat for eons, requiring that a historical change must have ocurred to explain why HIV emerged at some point during the 20th century (Lock 2010).


The seeds of the HIV-1 epidemic appear to have been planted in west equatorial Africa in the region encompassing Gabon, Equatorial Guinea, Cameroon, and the Republic of Congo (Congo-Brazzaville). It is only here that HIV-1 groups M, N, and O cocirculate in human populations and where chimpanzees (P. t. troglodytes) have been found to be infected with genetically closely related viruses. It is also within west equatorial Africa that the greatest diversity of HIV-1 group M viruses has been found (Hahn 2000).


In addition, chimpanzee and group N human viruses from Cameroon form a unique subcluster in phylogenetic trees of Env and Nef regions, implicating this particular geographic region as the site of origin for HIV-1 group N. The precise geographical origins of HIV-1 groups M and O are not known, but further screening and analysis of SIVcpz strains from chimpanzees (Hahn 2000).


Humans are exposed to a plethora of primate lentiviruses through hunting and handling of bushmeat in Cameroon, a country at the center of HIV-1 groups M, N, and O endemicity that is home to a diverse set of SIV-infected nonhuman primates (Peers 2000).


Mandrills and De Brazza’s guenons, and other monkeys and apes from rural villages in Cameroon, are infected with unique SIVs, some of which replicate in human cells in vitro (WOLFE 2004)



In the Belgian Congo between World Wars I and II, forced labor practices implemented under colonial rule led to widespread migration and famines and, it is thought, intensified recourse to bushmeat to survive (Lock 2010).


SIVcpz and SIVsm have been transmitted to humans as a result of cutaneous or mucous membrane exposure to infected animal blood. Among wild-living primates, biting and predation represent the most likely means of infection. In hu-mans, direct exposure to animal blood and secretions as a result of hunting, butchering, or other activities (such as consumption of uncooked contaminated meat) provides a plausible explanation for the transmission of lentiviruses from primates to humans (Hahn 2000).


Consumption of bushmeat in Central Africa and South America has greatly increased over the past 20 years. An article published in Emerging Infectious Diseases estimates that 1-3.4 million tons of bushmeat is consumed in Central Africa and 67-164 million kilograms of bushmeat is consumed in South America annually. Reasons cited for this increase include the expansion of commercial logging into isolated areas of Africa and disruption of control measures due to armed conflict in the area. Commercial logging creates an increased demand for bushmeat due to the increased worker population in the area and few dietary options available to workers and residents. Armed conflict disrupts local law enforcement and allows for unregulated hunting and consumption of bushmeat. The worldwide bushmeat market, estimated to be a multibillion-dollar business, not only allows for expanded consumption of infected bushmeat but for increased frequency of human exposure to zoonotic diseases through direct contact with fresh bushmeat. An article from Emerging Infectious Diseases claims “tracking, capturing, handling, butchering in the field, and transporting of carcasses involve risks of cross-species transmission.” Previous examples of cross-species transmission include simian foamy virus and human T-lymphotropic virus types 3 and 4. All have been found in people that hunt, butcher, or keep primates as pets (Galant 2013).


The bushmeat trade is an example of an anthropogenic factor that provides opportunities for the transmission of diseases from wildlife to humans. The expansion of the bushmeat trade over the past 2 decades has provided a venue for the emergence of zoonotic diseases by providing an increased opportunity for the transmission of organisms known to cause disease and organisms with an unknown impact on humans (Karesh 2009).


The sooty mangabey (Cercocebus atys) is an Old World monkey found in forests from Senegal east to Ghana (Wikipedia). Sooty mangabeys are naturally infected with a strain of Simian Immunodeficiency Virus (SIV), known as SIVsmm. Due to extensive human-mangabey contact in sub-Saharan Africa, SIVsmm has jumped from this species into humans on multiple occasions, resulting inHIV-2 virus Sooty mangabeys are frequently hunted for food, and orphans are kept as pets. Thus, there is the opportunity for frequent human contact with infected animals (Hahn 2000).



In west central Africa, for example, colobus, sun-tailed, and DeBrazza monkeys as well as mandrills, drills, and redcapped mangabeys are regularly hunted along with chimpanzees. Many of the SIVs known to infect these animals exhibit biological properties that render them at least candidates for natural transmission to humans, such as the ability to replicate efficiently in primary human lymphocytes. Thus, the potential for human exposure to a wide range of different SIVs has increased substantially in the past two decades, as have conditions that would be expected to facilitate their widespread dissemination (Hahn 2000).


It is notable that commercial logging of tropical forests represents an expanding industry in several African countries, in turn resulting in road construction into remote areas and the development of social and economic networks supporting this industry. As a consequence, the hunting and consumption of wild animals as a food source, traditionally a subsistence activity, has been transformed into a commercial enterprise termed the “bushmeat” trade (Hahn 2000).


The bushmeat trade has increased in the last decades. Commercial logging, which represents an important economic activity in Cameroon as well as many other west-central African countries, has led to road constructions into remote forest areas, human migration, and social and economic networks supporting this industry. Hunters are now penetrating previously inaccessible forest areas, making use of newly developed infrastructure to capture and transport bushmeat from remote areas to major city markets. Moreover, villages around logging concessions have grown from a few hundred to several thousand inhabitants in just a few years. These socioeconomic changes, combined with our estimates of SIV prevalence and genetic complexity in wild primates, suggest that the magnitude of human exposure to SIV has increased, as have the social and environmental conditions that would be expected to support the emergence of new zoonotic infections (Peers 2000).


The increased amount of hunting in central Africa that has resulted from a combination of urban demand for bushmeat and greater access to primate habitats provided by logging roads, has increased the frequency of human exposure to primate retroviruses and other disease-causing agents (Wolfe 2004)



The potential for recombination between currently circulating HIVs and newly introduced SIVs, possibly generating viruses with significantly altered biological properties, must be considered. Many examples of recombinant human and simian lentiviruses have been described. The majority of these have been reported for HIV-1 group M, primarily between members of different subtypes. Some of these recombinants have demonstrated their biological fitness by becoming major circulating forms of epidemiological significance. Examples include HIV-1 A/E and A/G recombinants that circulate epidemically throughout southeast Asia and Africa, respectively. Other examples include recombinant SIVsm, SIVagmSab, SIVrcm, and HIV-2. Together, these viruses highlight the fact that recombination is not a rare event in nature (Hahn 2000).


The public health implications of virus recombination derive from the potential for rapid acquisition of different biological properties, including drug resistance, altered tropism, and enhanced virulence. In particular, the discovery of viruses that are mosaics of different HIV-1 group M subtypes, or even of highly divergent lineages such as members of HIV-1 groups M and O, highlights the possibility that such viruses could evade serologic detection and may not be susceptible to vaccines that are based on a particular virus subtype or group (Hahn 2000).




Casein is a protein (phosphoprotein, phosphoric acid bound to the protein) that is grouped roughly spherical particles called casein micelles that are in colloidal dispersion in the animal milk. Naturally found only in the milk of mammals.


Casein is a substrate for transglutaminases (1).






Gluten is a protein (a complex mixture of storage proteins) found in various food grains. The main, toxic, wheat-gluten components are a family of closely related proline-rich and glutamine-rich alcohol-soluble proteins called gliadins (2).

The gliadins consist of protein subtypes A, α, β, γ, and ω (2).


Gluten is a substrate for transglutaminases ().




              1.      STAAB 1999: Staab JF, et al. (1999) Adhesive and mammalian transglutaminase substrate properties of Candida albicans Hwp1. Science 283:1535-1538

              2.      NIEUWENHUIZEN 2003: Nieuwenhuizen WF, et al. (2003) Is Candida albicans a trigger in the onset of coeliac disease?. Lancet 361: 2152–54

              3.      STAAB 1996: Staab JF, et al. (1996) Developmental Expression of a Tandemly Repeated, Proline- and Glutamine-rich Amino Acid Motif on Hyphal Surfaces of Candida albicans. The Journal of Biological Chemistry  271:6298-6305

              4.      STAAB 1998: Staab JF, et al. (1998) Genetic Organization and Sequence Analysis of the Hypha-specific Cell Wall Protein Gene HWP1 of Candida albicans. Yeast Vol. 14: 681–686

              5.      DANIELS 2003: Daniels KJ, et al. (2003) The Adhesin Hwp1 and the First Daughter Cell Localize to the a/a Portion of the Conjugation Bridge during Candida albicans Mating. Molecular Biology of the Cell Vol. 14, 4920–4930

              6.      ENE 2009: Ene IV et al. (2009) Hwp1 and Related Adhesins Contribute to both Mating and Biofilm Formation in Candida albicans. Eukaryotic Cell, Vol.8, Nº12, p. 1909–1913

              7.      LACHKE 2003: Lachke SA et al. (2003) Skin Facilitates Candida albicans Mating. Infection and Immunity Vol. 71, No. 9, p. 4970–4976

              8.      HITOMI 2005: Hitomi (2005) Transglutaminases in skin epidermis. Eur J Dermatol 2005; 15 (5): 313-9

              9.      SUNDSTROM 2002: Sundstrom P (2002) Adhesion in Candida spp. Cellular Microbiology 4(8), 461–469

        10.      BRADWAY 1993: Bradway SD et al. (1993) Do proline-rich proteins modulate a transglutaminase catalyzed mechanism of candidal adhesion?. Crit Rev Oral Biol Med 4: 293–99

        11.      STAAB 2004: Staab JF et al. (2004) Expression of Transglutaminase Substrate Activity on Candida albicans Germ Tubes through a Coiled, Disulfide-bonded N-terminal Domain of Hwp1 Requires C-terminal Glycosylphosphatidylinositol Modification. The Journal of  Biological Chemistry 279:40737-40747

        12.      SUNDSTROM 2002: Sundstrom P et al. (2002) Essential Role of the Candida albicans Transglutaminase Substrate, Hyphal Wall Protein 1, in Lethal Oroesophageal Candidiasis in Immunodeficient Mice. The Journal of Infectious Diseases 185:521–30

        13.      STAAB 2013: Staab JF et al. (2013) Niche-Specific Requirement for Hyphal Wall protein 1 in Virulence of Candida albicans. PLoS ONE 8(11): e80842

        14.      ZHU 2010: Zhu et al. (2010) Interactions of Candida albicans with epithelial cells. Cellular Microbiology 12(3), 273–282

        15.      HIIRAGI 1999: Hiiragi et al. (1999) Transglutaminase Type 1 and Its Cross-linking Activity Are Concentrated at Adherens Junctions in Simple Epithelial Cells. The Journal of Biological Chemistry 274, Nº 48, pp. 34148-34154

        16.      INCI 2013: Inci et al. (2013) Investigations of ALS1 and HWP1 genes in clinical isolates of Candida albicans. Turkish Journal of Medical Sciences 43: 125-130

        17.      NOBILE 2006: Nobile et al. (2006) Function of Candida albicans Adhesin Hwp1 in Biofilm Formation. Eukaryotic Cell 5(10):1604-1610

        18.      NOBILE 2008: Nobile et al. (2008) Complementary Adhesin Function in C. albicans Biofilm Formation. Current Biology 18, 1017–1024

        19.      BOURTOOM 2009: Bourtoom (2009) Edible protein films: properties enhancement. International Food Research Journal 16: 1-9

        20.      PHAN 2007: Phan et al. (2007) Als3 is a Candida albicans invasin that binds to cadherins and induces endocytosis by host cells. PLoS Biol 5(3): e64.

        21.      BRINKERT 2009: Brinkert et al. (2009) Chronic mucocutaneous candidiasis may cause elevated gliadin antibodies. Acta Paediatr. 98(10):1685-8.

        22.      MARINIELLO 1993 (gp120): Mariniello et al. (1993) Transglutaminase covalently incorporates amines into human immunodeficiency virus envelope glycoprotein gp120 in vitro. Int. J. Pept. Protein Res. 42, 204-206.

        23.      MARINIELLO 1993 (gp41): Mariniello et al. (1993) Human-immunodeficiency-virus transmembrane glycoprotein gp41 is an amino acceptor and donor substrate for transglutaminase in vitro. Eur. J. Biochem. 215, 99-104

        24.      THORLEY 2010: Thorley et al. (2010) Mechanisms of viral entry: sneaking in the front door. Protoplasma 244: 15-24.

        25.      VELJKOVIC 2003: Veljkovic eta al. (2003) Design of peptide mimetics of HIV-1 gp120 for prevention and therapy of HIV disease. J. Peptide Res. 62, 158–166.



Hippocrates of Cos

Greek physician (460 - 370 BC)


Let food be thy medicine and medicine be thy food

Maybe misquoted citation (Cardenas 2013) but see below:


The importance of food in medicine was recognized in the 5th Century BC by Hippocrates of Cos, who is considered the father of Western medicine. His work was compiled either directly or indirectly through his disciples, so that the existing knowledge on Hippocrates’ medicine consists of more than 60 texts known as The Hippocratic Corpus (Corpus Hippocraticum). This important text in the history of medicine expounds on the theory of diet. Up until Hippocrates, diseases had been seen as a consequence of divine intervention. With him, they became seen as a state caused by natural causes, including diet. There is no doubt about the relevance of food in The Hippocratic Corpus and its role in health and disease states (Cardenas 2013).


In order to fight diseases, Hippocratic doctors used two kinds of interventions. On the one hand, the previously existing therapeutic interventions such as medicines, incisions, and cauterization and on the other hand the new regimen or dietetic interventions. In a hierarchical order, the most important intervention was diet.  Secondly, medicines seemed to be considered as means of evacuation or purgation of impure fluids from the various cavities of the body. The dietetic intervention, which included a food regimen and exercises, was considered revolutionary at the time. The properties of foods were meticulously analyzed in the treatise On Regimen. Physicians were then able to prescribe a detailed food regimen to patients based on their individual nature, activity, age, season, etc. Thus it is considered that medicine in the Hippocratic era was in fact mainly a dietetic medicine, not a pharmacological or surgical medicine (Cardenas 2013).


“Persons in good health quickly lose their strength by taking purgative medicines, or using bad food


“It is a bad thing to give milk to persons having headache, and it is also bad to give it in fevers, and to persons whose hypochondria are swelled up, and troubled with borborygmi, and to thirsty persons; it is bad also, when given to those who have bilious discharges in acute fevers, and to those who have copious discharges of blood”


“Of course I know also that it makes a difference to a man's body whether bread be of bolted or of unbolted flour, whether it be of winnowed or of unwinnowed wheat, whether it be kneaded with much water or witli little, whether it be thoroughly kneaded or unkneaded, whether it be thoroughly baked or underbaked, and there are countless other differences. Barley-cake varies in just the same way. The properties too of each variety are powerful, and no one is like to any other. But how could he who has not considered these truths, or who considers them without learning, know anything about human ailments? For each of these differences produces in a human being an effect and a change of one sort or another, and upon these differences is based all the dieting of a man, whether he be in health, recovering from an illness, or suffering from one. Accordingly there could surely be notfiing more useful or more necessary to know than these things, and how the first discoverers, pursuing their inquiries excellently and with suitable application of reason to the nature of man”


Jean Seignalet

French physician (1936-2003)


"Food is an integral part of medicine and is more than less salt for hypertensives and less sugar for diabetics "


"I have sought to understand scientifically how a inadequate food could lead to a pathology."


“I have no doubt about this: food is both preventive and curative”


“In addition to the genetic predisposition of each one environmental factors are dominant in 90% of diseases


Two out of three cancers depend on food


“The acquired cancers (non-hereditary) (about 95%), even if found predisposed genes, are essentially caused by environmental factors: food, tobacco, asbestos or virus such as in the case of uterine cervical cancer”


“Radiation, chemicals, viruses and non-intestinal bacterias can only explain 40% of acquired cancers. Therefore, for the remaining 60% it seems logical to consider bacterial and food waste resulting intestinal origin of modern food”


"Modern food acts on a key body, the small intestine, providing molecules which can not degrade our enzymes. Large molecules, from food and bacterial origin cross the intestinal barrier and enter the blood. Are deposited in various tissues and clog the body. "


“Intracellular poisoning is the main reason for cell cancerization. Some foreign macromolecules progressively bother blocking the operation of various mechanisms and accumulation of waste breaks certain physiological balance. I am persuaded that this prolonged poisoning by lead cell ends alterations of nuclear DNA and cause genetic abnormalities that lead to cancer


“I have healed from a serious nervous depression by means of a dietary regime that excluded cereals and dairy products, which was rich in raw products”


Cow's milk is a very nutritious food ... for calves in the growing season. Humans digest milk only from our species, and at the the nursing period. The main milk protein, casein, is difficult to digest completely


You get interesting results yet their medical colleagues do not always believe in the benefits of feeding. JS: "Do not believe in this theory and it could be indifferent to them, I can understand. What surprises me most is that they do not want to experience. I have done my duty by exposing my theory. "



T. Colin Campbell

American biochemist



I consider nutrition to be THE premiere science in medicine – end of story.”


“I am most interested in, namely, the comprehensiveness of the nutrition effect on health and disease


“I focused on the role of nutrition on health maintenance and disease occurrence.”


“What we choose to eat also is one of the most emotionally intense topics of human discourse, ranking up there with sex, religion and politics. Yet, properly practiced nutrition, as a dietary lifestyle, can do more to create health and save health care costs than all the contemporary medical interventions put together.”

"We’ve distorted our diet seriously through the ages, and we have all the problems we have because of that distortion."


“The shorthand of the whole thing is we’re eating the wrong foods, basically, animal-­based foods, plus all this processed food, we’re eating the wrong food, and then turning around and relying on this silly notion that we can take a single chemical after we get a disease and hopefully make ourselves well. Ah, we get can get some benefits from that from time to time, but that’s not the long-­term solution to maintain health, it just doesn’t work that way.”


“I want people to talk about and to think about how should we be eating in a very empirical scientific sense, and not with an ideological bent to it.”


“All humans share virtually the same biochemistry and physiology, regardless of ethnicity, race and gender. They differ, both as individuals and as groups of people, in the DEGREE to which they respond to dietary insult. But the direction of the effect is essentially the same.”




"Well, it was very traditionally American, I suppose, rural America. I was raised on a dairy farm, and believed in the good old American diet, so to speak. I milked cows until I went away to school. .I went away to graduate school at Cornell University, and I thought the good old American diet is the best there is. The more dairy, meat and eggs we consumed, the better. When I did my doctoral research, my program was actually focused on the idea that we had to find more productive ways of producing more animal protein, in particular, that is, more meat, milk and eggs. The early part of my career was focused on protein, protein, protein. It was supposed to solve the world’s ills. And I was very much a part of that culture, and believed that that was the ultimate as far as good health is concerned. But as my career began to unfold, especially with all my students, and other colleagues, and a research career that involved experimental research, that is, actually doing the studies, designing studies, doing studies, and publishing the results, I eventually came to the view that I had to seriously question what this good old American diet was all about. When we started doing our research, we found that when we start consuming protein in excess of the amount we need, it elevates blood cholesterol and atherosclerosis and creates other problems. So I obviously made quite a change and quite a shift in my thinking over the last 50 years. And I find that my views are not just based on research that I did, but obviously the research that many, many others have done too. We did some research that I just found very provocative, and just caused me to really begin to question what we really believed as far as diet and health are concerned.”


“I’ve just finally come to the view that nutrition, if it’s properly understood and used, really has enormous potential to create health, maintain health, prevent disease, even cure disease, even cure advanced diseases. And so I just find the whole idea very, very exciting, I think it has a lot of potential, not only to help people be well, if they really understood what all this is about, but in the process, from a more societal point of view, I suppose, it could have a major impact on the runaway health care costs that we’re now experiencing in our country.”


“It's been estimated that the total number of people now living in the world who are going to die prematurely from smoking is of the order of 200 million people, that's a population approaching the size of the United States. When we compare the number of people dying from smoking with the number of people adversely affected by diet, for example, diet is often conceded as causing even great number of premature deaths than smoking. So, just using that very simple comparison, let me suggest that the number of people in the world today who are likely to die prematurely from poor diets, at least in the western sense, could easily be 200 or 300 hundred million people. These are big numbers, these are really big numbers. In fact, I would suggest that's a conservative number. Another way of looking at this is to say 60-70% of the people in the UK and the US and other western countries die prematurely from cancer, heart disease, diabetes and these other western kinds of disease. Since those diseases are preventable by dietary means, might we say that half, or 60-70% of these diseases really can be prevented at least until much older ages through dietary means. This is a large number of people, whatever that number is.”




“My research career started in 1956, so I’ve been around a long time. Most of my career was spent at Cornell University where I had a large research program. Initially, my research focused on diet and cancer. Through my research, I saw some very unusual things that did not appear in textbooks. It challenged my own thinking.”


“My experience and interest over the years has been concerned with the prevention of cancer primarily, particularly the prevention of cancer by dietary means. I happen to now believe very strongly that nutrition has a lot to do with whether or not we get cancer. It is an area that has been unfortunately underplayed and in some cases actually ignored by some of the central authorities who are involved in doing cancer research.”


“It's ironic that traditional cancer research organisations and health organisations will admit that at least a third of all cancers can be prevented by dietary means, but then in the next breath, they'll tell you that they really don't know how. Then you ask them how much money are you spending on this, and what you discover is that they're only spending about 1-2% of their budget at the most. There is some terrible discordance here.”




All that wealth for the few at the expense of health for the many


Money is made when fixing sick people, not in maintaining healthy people. Exceptionally well-endowed and powerful industries that survive on our money are not very serious about converting us to non-customers. Yes, the message is remarkably simple, but history shows us that considerable efforts, intentional or unintentional, have been made to ignore it, misunderstand it or make it complicated.”


“I think that the Food Pyramid is rather trivial and highly political. I have paid little attention to what they say or do, for I know how corrupt is that process.”


“Historically, we have been slaves to a nutrition-less health information system that, in effect, is designed to keep us in mental chains, thus to maintain the status quo.”


Do you feel passionate about this subject? : “Yes I do. I think in part because I was in the other camp in a way, not intentionally, but that's just where I was, from my childhood on through, into science - and I got into science because I thought science was a place where we were supposed to look at things honestly and make our decisions accordingly. I saw evidence that didn't agree with the way I was doing things or the way before us, and so I had to look at that. I thought it was a very simple matter and I got quite excited about this kind of thing, and in fact what I discoved was an enormous hostility and antagonism to the promotion of these ideas. I have to say I became somewhat cynical of the institutions of science because of that. I started thinking a lot about why it is that the institution of science itself behaves in such a way? And, what I'm now discovering is that science is not so ideal, in the way I once thought. I was very naive. The institution of science is closely related to who provides the funds for the science to be done, either directly or indirectly.”


“I think the indirect effect is even greater than the direct effects, and people in science advance their careers by how much research they do, and how much publicity they tend to get. And of course, they are going to advance their careers and get the publicity if they do the research that's generally accepted, in other words supporting the status quo. If a scientist comes along and says something different, they do it at their peril, because they just may not get the publications, they may not get the advancement in their care ers. That's a rather indirect effect, but nonetheless, it's a very serious effect, and they know it. And so, I think the institution of science, which has basically served a very reductionist way of thinking, that is producing little pills and magic bottles to do this that and everything else, that's what medical science has largely been, been fostering, been concerned with, and interested in.”


“And so one can sort of wonder why it is that we tend to focus on one thing at a time? Well, that’s the way things are sold, that’s the way they make money. And that’s the way things get patented in order to protect the intellectual property, in order for it to be marketed. And of course that’s the simplest way to think about things, too, just using one chemical at a time, or many just two or three, or so, working together in pill form. And it’s really quite ludicrous, when really one understands how things work in this very dynamic way in the tissues. On the one hand, one begins to recognize and understand that, and then to turn around and assume that we can take a single chemical, whether it’s a drug, or whether it’s a nutrient supplement, or whether it’s some other kind of thing, to just do one thing and try to correct a whole system by just sort of using one entity, and it just makes no sense. We can only expect to get unintended consequences, I think, by taking that approach. It’s just simply wrong.”

“And of course it serves the free market system and it serves our sense of how to control disease through cure, but, it doesn't serve the public. Prevention is really the way to go, and at the centre of the plate for prevention is nutrition, how we decide to eat and how we decide to behave otherwise, and that's a very comprehensive sort of lifestyle dietary change. That's where we get good health - that's what the public needs to know, and science is not delivering it. When I find I get hounded for my views by some of my colleagues, on these particular points, it makes me angry and in a sense pursue the question even more.”


“I’ve really become, I guess, in my older years, really pretty cynical about the whole medical system, the way we now do it. It’s not really creating health, we know that from the figures, it’s basically sustaining these extraordinary rates of disease that we now have. We’re not really getting anyplace, and we’re spending a heck of a lot of money getting no place. And it seems, the figures show, for example, in the United States, that we spent more per capita on medical care costs than any country in the world, probably somewhere in the neighborhood of about 50% more than the second highest country, I mean, we’re way up there. Yet when our medical system, when our health systems are judged by others in terms of quality of health care, we stand somewhere in the neighborhood of 30th to 40th in the list. So the question arises, why are we spending so much money and getting so little in return? Makes no sense.”


“We have lost respect for nature. We over-name things, we over-quantitate things. We live that way, partly because that’s the way our brains work, maybe we just can’t think in this kind of context – but we have to. That’s the way nature is and until we recognize that this is what life is all about, we’re not gonna make a lot of progress. What we’re going to do is continue to make a lot of money, and let the rich get richer and let the rest of the people serve as slaves. That’s one title I’m considering: The Master and Slave State. We’re focused on money, greed and competition, and the people who really want to go down that route become very hostile when you challenge them


“I say it’s “wealth for the few at the expense of health for the many”. It’s really what it’s all about. To come back to your question of HOW we do this – establishment does not understand nutrition, and whether they know it or not, they are consistently trying to keep this information from the public. So, I say, first of all, it’s about information control – let’s face it. And I show how industries have devised systems to keep things under control. Registered dietitians, for example, are the only ones allowed to practice nutrition professionally. There’s licensing, and the ones who are controlling the licensing is the American Dietetic Association (ADA). And the ADA is a front for the dairy industry, for crying out loud. Two years ago, I was invited to give a keynote there and we were given our registration bags and there on the outside it says “ADA partners” and you see The National Dairy Council, Coca Cola, Pepsi Cola. So I just took a picture and I just showed the others and I said “look at this criminal outfit”. They are the ones that not only control who is allowed to talk about food because of the licenses, but they also control the curriculum in universities as to what courses you have to take to get a Registered dietitian. So, all the Registered Dietitians in this country are working with a corrupt organization and getting trained in an area of nutrition that is controlled. When I tell them this, a lot of dietitians get upset, but all of a sudden they realize that they’ve been had. And the public has really been had.”


The meat and dairy industries have much power and influence in our society. How have these groups affected you personally and in your career: TCC: “They know well who I am, ever since about 1982, and they have tried, at times vigorously, to use less than professional means to silence me and to discredit my reputation. This has only spurred me onwards.”


“It is very frustrating to see, for example, medical professionals not being trained in nutrition, not understanding nutrition, and in many cases as they go out into practice almost denying that the dietary effect is all that important. I mean, that’s a pretty traditional view on the part of many physicians, of course. They have come to rely on drugs, as we all know, and surgery, and so forth, to sort of treat disease once it’s already present, and as I say, and treat it rather ineffectively in many cases.”




“I think the world is changing to some extent in the sense that, one of the biggest innovations in my view that has come along in the last few decades is the vastly improved facilities for communication, I mean virtually anyone in our society can, can get together some equipment and get some ideas and, and basically go out and promote some information and let the public know, and so now the dispersion of information is I think more democratic than it has been, at least I think. Whereas before, 20 or 30 years ago, 40 or 50 years ago, I mean, information was held in the hands of a few, and I think this is going to make a difference.”

“I'm rather optimistic and hopeful because I know it's related to the fact that 'the power and the money' has done tragic things, and they are still very powerful and there is a lot of 'power and money' there, but, thanks to the world of communication, books and the media, it can make a diffence. It really can make a diffence.”




“Every disease begins with genes but this is not a death sentence. For the most part, genes, good and bad, are controlled by nutrition [i.e., the food we consume]. With the right nutrition resources, our marvelous bodies, while always striving for health, manage which genes to express and which ones to keep quiescent. There is also considerable evidence that the initiation and expression of many autoimmune diseases may be influenced by nutrition (and therefore would be repressed by eating whole foods), sometimes initiated early in life, from what mothers eat while pregnant and nursing. We need more awareness of this phenomenon, and then we need to do more research on mother-child interactions.”


“We should not be relying on the idea that genes are determinants of our health. We should not be relying on the idea that nutrient supplementation is the way to get nutrition, because it’s not. I’m talking about whole, plant-based foods. The effect it produces is broad for treatment and prevention of a wide variety of ailments, from cancer to heart disease to diabetes.”


“There are hundreds of studies now showing that people who move from one country to another where the disease risk is very different, take on the risk of the disease of the country to which they move while they keep their genes the same. In other words, diseases don't occur because of a genetic predisposition, it may for individuals be somewhat different, but regardless of our genetic predisposition, we can control whether or not that we get the disease simply through dietary and lifestyle changes.”

“It is true that we have discovered a tremendous amount of information but this does not mean discovering what it all means. Indeed, our focus on details has created an enormous pile of contradictory observations--permitting too many people to construct ideas that please their palates and wallets more than educate their brains.”




“I don't care to pass personal blame or pose conspiracies, for we are all participants in this great war of words of what nutrition really means. Nonetheless, somewhere there is an origin and it is fostered by our professions, my nutrition and medical research community and my clinical colleagues' medical practice community. This is not surprising.The National Institutes of Health (NIH), which is the most influential research funding agency in the world, is comprised of 27 institutes, centers and programs and not one is named the Institute of Nutrition. Research funding is a mere pittance in a couple of the institutes and most of this is dedicated to the study of individual nutrients that I consider pharmacology, not nutrition. Further, there is not a single medical school in the country that teaches nutrition as a basic medical science. At best, a few may have an elective course that treats the subject in a most superficial manner. Public citizens, therefore, are left to fend for themselves against the hyped up claims of the food and drug industries.”


“And there’s nothing that can touch nutrition – if we understand it. We don’t understand nutrition. People in my community of research don’t understand nutrition. People in the medical practitioner community don’t understand it, we talked about that before, how there is no nutrition course requirement in medical school. And NIH is the premiere research funding agency in the world, it has the most money, has a great track record, is highly respected. It’s made up of 27 institutes, centers and programs, on cancer, diabetes, etc…Not one is called the Institute of Nutrition. The head of the NCI and the head of the Heart and Lung would say “oh, we got nutrition built into our fabric, in our system, in our research”, but when they asked them what percent would you actually say is focused on nutrition, they would give a figure of 2% or 3%. And the others don’t have any, so it’s just limited to a couple of the institutes, and only 2%-4%. And most of that is actually spent on clinical trials – where they spend a lot of the little money testing the ability of single nutrients, like “does vitamin C stop colon  polyps?” It’s done with an eye on the corporate sector – that’s what it’s about: What can we put into a pill and see if it works. And it’s just ridiculous. And to add insult to injury, since the director of an institute has to be a medical doctor, that means it has to be someone who is not trained in nutrition, by default. I have served on NIH review committees and it became very clear to me that they were consistently very opposed to nutrition – even though they use the word a lot, they don’t seriously study it.”


“I also have fervent views not to make claims for this dietary lifestyle that are not supported by reliable evidence. Predicting future events for this practice is not an exact science. Forecasting health and disease outcomes is a matter of odds, not a matter of certainty. We cannot say that all ailments will be controlled in all individuals by this nutritional strategy. But, on the basis of probability, it is abundantly clear that this dietary lifestyle has a breadth of effect that is greater than any combination of drugs and procedures ever used, without the accompanying side effects that are common to virtually all drugs.”


“If we are to understand the true value of nutrition. When done right, advanced heart disease can be cured, type 2 diabetes stopped and reversed, cancer can be prevented and, with some newer evidence, controlled after it appears. The range of diseases that can be prevented is more than impressive. The breadth and rapidity of the nutritional effect not only prevents disease but actually treats many of these diseases while restoring and maintaining health. The totality of these health effects are far more than almost anyone knows.”




Casein, a protein found in milk from mammals, is "the most significant carcinogen we consume””


“Let there be no doubt: cows milk protein is an exceptionally potent cancer promoter


“I was actually raised on a farm, and I milked cows until I was well into college, and obviously I ate that kind of diet. When I went away to college I was in pre-vetinary medicine as an undgraduate, but then in my graduate studies I did a PhD dissitation on figuring out ways to produce animal protein more efficiently, so we could eat more animals. That was my background. I was totally a farm boy, totally into that territory.”


You have made some strong statements about the negative effects of casein, a milk protein. Could you give our readers some of the major points you make about the detrimental nature of casein? What led you to those views?: TCC: “In experimental animals (rats and mice) we could turn on and off experimental cancer development by feeding and withdrawing casein at levels above minimum protein requirements. We also studied in great detail how this works and discovered some very profound and provocative phenomena that relate, more generally, to the broader issue of diet and health. This began with my work in the Philippines coordinating a nationwide program for feeding malnourished children and observing that those few families and their children consuming protein diets at levels similar to the U.S. got more cancer. A subsequent experimental animal study in India confirmed what I suspected that I was seeing. This then led to a long series of experimental animal studies that was largely confirmed in the studies of others, both in humans and in experimental animals.”




“Cooked food, yeah, there’s some information that when we’re overcooking foods, certain kinds of foods, we can, in fact, get some noxious chemicals on the basis of the information we now have, heterocyclic amines, as we call that class of compounds, and in the older literature there was another class of compounds referred to as the polycyclic aromatic hydrocarbons, that may result from essentially the burning of food, or having food exposed to fire. And so it’s the kind of information that would suggest that obviously cooking food a little bit is probably OK most of the time, but if we overdo it it’s not a good idea, and it’s one of the reasons I suggest we kind of stay toward the raw food side as much as possible.”


How have your colleagues responded to your efforts to reverse chronic diseases through diet: TCC: “As far as the research community is concerned, mostly with silence, although as I write this, I am suddenly seeing an increasing number of medical practitioners beginning to carry the banner forward. These people have, for the most part, seen first hand what they can do for their patients when they adopt this practice. I've lost some research colleagues but I've gained a lot more new colleagues.”


“Incidentally, that kind of eating was much better appreciated and accepted and promoted as long ago as the ancient Greek times. Some of the leading Greek philosophers and others who thought about medicine, diet and disease wrote surprisingly impressive observations on the relationship between eating that kind of food and good health. It all sort of disappeared around 300 or 400 AD, for some strange reason lots of things disappeared about that time, and I think that we're just now beginning to come out of the dark ages and going back to this; rediscovering what was well known to these medical people and philosophers.”

First version of the page 7 - 11 - 2013

Current version 21 - 4 - 2014


hits counter