Hemoglobin induces binding of several extracellular matrix proteins to Candida albicans. Identification of a common receptor for fibronectin, fibrinogen, and laminin

Specific Pathogen Recognition by Multiple Innate Immune Sensors in an Invertebrate

Detection of pathogens by all living organisms is the primary step needed to implement a coherent and efficient immune response. This implies a mediation by different soluble and/or membrane-anchored proteins related to innate immune receptors called PRRs (pattern-recognition receptors) to trigger immune signaling pathways. In most invertebrates, their roles have been inferred by analogy to those already characterized in vertebrate homologs. Despite the induction of their gene expression upon challenge and the presence of structural domains associated with the detection of pathogen-associated molecular patterns in their sequence, their exact role in the induction of immune response and their binding capacity still remain to be demonstrated. To this purpose, we developed a fast interactome approach, usable on any host–pathogen couple, to identify soluble proteins capable of directly or indirectly detecting the presence of pathogens. To investigate the molecular basis of immune recognition specificity, different pathogens (Gram-positive bacterium, Micrococcus luteus; Gram-negative, Escherichia coli; yeast, Saccharomyces cerevisiae; and metazoan parasites, Echinostoma caproni or Schistosoma mansoni) were exposed to hemocyte-free hemolymph from the gastropod Biomphalaria glabrata. Twenty-three different proteins bound to pathogens were identified and grouped into three different categories based on their primary function. Each pathogen was recognized by a specific but overlapping set of circulating proteins in mollusk’s hemolymph. While known PRRs such as C-type lectins were identified, other proteins not known to be primarily involved in pathogen recognition were found, including actin, tubulin, collagen, and hemoglobin. Confocal microscopy and specific fluorescent labeling revealed that extracellular actin present in snail hemolymph was able to bind to yeasts and induce their clotting, a preliminary step for their elimination by the snail immune system. Aerolysin-like proteins (named biomphalysins) were the only ones involved in the recognition of all the five pathogens tested, suggesting a sentinel role of these horizontally acquired toxins. These findings highlight the diversity and complexity of a highly specific innate immune sensing system. It paves the way for the use of such approach on a wide range of host–pathogen systems to provide new insights into the specificity and diversity of immune recognition by innate immune systems.

Targeting Fibronectin To Disrupt In Vivo Candida albicans Biofilms

New drug targets are of great interest for the treatment of fungal biofilms, which are routinely resistant to antifungal therapies. We theorized that the interaction of Candida albicans with matricellular host proteins would provide a novel target. Here, we show that an inhibitory protein (FUD) targeting Candida-fibronectin interactions disrupts biofilm formation in vitro and in vivo in a rat venous catheter model. The peptide appears to act by blocking the surface adhesion of Candida, halting biofilm formation.

Host contributions to construction of three device-associated Candida albicans biofilms

Among the most fascinating virulence attributes of Candida is the ability to transition to a biofilm lifestyle. As a biofilm, Candida cells adhere to a surface, such as a vascular catheter, and become encased in an extracellular matrix. During this mode of growth, Candida resists the normal immune response, often causing devastating disease. Based on scanning electron microscopy images, we hypothesized that host cells and proteins become incorporated into clinical biofilms. As a means to gain an understanding of these host-biofilm interactions, we explored biofilm-associated host components by using microscopy and liquid chromatography-mass spectrometry. Here we characterize the host proteins associated with several in vivo rat Candida albicans biofilms, including those from vascular catheter, denture, and urinary catheter models as well as uninfected devices. A conserved group of 14 host proteins were found to be more abundant during infection at each of the niches. The host proteins were leukocyte and erythrocyte associated and included proteins involved in inflammation, such as C-reactive protein, myeloperoxidase, and alarmin S100-A9. A group of 59 proteins were associated with both infected and uninfected devices, and these included matricellular and inflammatory proteins. In addition, site-specific proteins were identified, such as amylase in association with the denture device. Cellular analysis revealed neutrophils as the predominant leukocytes associating with biofilms. These experiments demonstrate that host cells and proteins are key components of in vivo Candida biofilms, likely with one subset associating with the device and another being recruited by the proliferating biofilm.

Contact-induced apical asymmetry drives the thigmotropic responses of Candida albicans hyphae

Filamentous hyphae of the human pathogen, Candida albicans, invade mucosal layers and medical silicones. In vitro, hyphal tips reorient thigmotropically on contact with small obstacles. It is not known how surface topography is sensed but hyphae lacking the cortical marker, Rsr1/Bud1, are unresponsive. We show that, on surfaces, the morphology of hyphal tips and the position of internal polarity protein complexes are asymmetrically skewed towards the substratum and biased towards the softer of two surfaces. In nano-fabricated chambers, the Spitzenkörper (Spk) responded to touch by translocating across the apex towards the point of contact, where its stable maintenance correlated with contour-following growth. In the rsr1Δ mutant, the position of the Spk meandered and these responses were attenuated. Perpendicular collision caused lateral Spk oscillation within the tip until after establishment of a new growth axis, suggesting Spk position does not predict the direction of growth in C. albicans. Acute tip reorientation occurred only in cells where forward growth was countered by hyphal friction sufficient to generate a tip force of ∼ 8.7 μN (1.2 MPa), more than that required to penetrate host cell membranes. These findings suggest mechanisms through which the organization of hyphal tip growth in C. albicans facilitates the probing, penetration and invasion of host tissue.

Hemolytic factor production by clinical isolates of Candida species

Most cases of fungal bloodstream infections (BIs) are attributed to Candida albicans; however, non-Candida albicans Candida species have recently been identified as common pathogens. Although hemolytic factor is known to be putative virulence factor contributing to pathogenicity in Candida species, its production is poorly evaluated. The present study was undertaken to analyze the production of hemolytic factor by C. albicans (10), C. tropicalis (13), and C. parapsilosis (8) isolates associated with BIs. Data of hemolysis zones on plate assay revealed that the majority of C. albicans isolates produced mild hemolytic activity whereas the majority of C. tropicalis produced strong activity. None of the tested C. parapsilosis isolates exhibited hemolysis on plate assay. We also evaluated the hemolytic activity in the cell-free broth. There were no significant differences (P > 0.05) in the secreted hemolytic activity among intra-species isolates. Different levels of secreted hemolytic factor were observed for Candida species, where C. tropicalis exhibited the highest production of hemolytic factor (P < 0.05) followed by C. albicans and C. parapsilosis. Inhibition of hemolysis (up to 89.12 %) from culture supernatant, following incubation with the lectin Concanavalin A (Con A), was observed for all three Candida species. This finding suggests that the secreted hemolytic factor of C. tropicalis and C. parapsilosis may be a mannoprotein, similar to that described for C. albicans.

Surface-expressed enolase contributes to the adhesion of Paracoccidioides brasiliensis to host cells

Paracoccidioidomycosis is a systemic mycosis caused by the dimorphic fungus Paracoccidioides brasiliensis. Understanding the interactions between P. brasiliensis and the host tissue depends on the study of the different steps of the process of colonization, especially adhesion, in which the pathogen recognizes ligands on the surface of host cells. This study aimed to verify the role of enolase in the host cell-fungus interaction and the ability of enolase to bind to extracellular matrix components, to determine its subcellular localization, and to study the P. brasiliensis enolase amino acid sequence. The data revealed that fibronectin is the major ligand of enolase. Evaluation of the location of enolase at an ultrastructural level revealed that it is distributed in various cellular compartments, but at a high level in the cell wall. The analysis of the amino acid sequence revealed an internal plasminogen-binding motif ((254)FYKADEKKY(262)), which is conserved in most organisms and described as an important interaction site of the enolase with the host cell surface. This suggests that enolase performs additional functions related to the glycolytic pathway and also plays a role of adhesion in P. brasiliensis. Therefore, this study increases the knowledge about the characteristics of enolase and its influence on the binding process of P. brasiliensis.

Entamoeba histolytica secretes two haem-binding proteins to scavenge haem

Entamoeba histolytica is a human pathogen which can grow using different sources of iron such as free iron, lactoferrin, transferrin, ferritin or haemoglobin. In the present study, we found that E. histolytica was also capable of supporting its growth in the presence of haem as the sole iron supply. In addition, when trophozoites were maintained in cultures supplemented with haemoglobin as the only iron source, the haem was released and thus it was introduced into cells. Interestingly, the Ehhmbp26 and Ehhmbp45 proteins could be related to the mechanism of iron acquisition in this protozoan, since they were secreted to the medium under iron-starvation conditions, and presented higher binding affinity for haem than for haemoglobin. In addition, both proteins were unable to bind free iron or transferrin in the presence of haem. Taken together, our results suggest that Ehhmbp26 and Ehhmbp45 could function as haemophores, secreted by this parasite to facilitate the scavenging of haem from the host environment during the infective process.

Retracted: Roles of Adherence and Matrix Metalloproteinases in Growth Patterns of Fungal Pathogens in Cornea

PURPOSE

To investigate the roles of adherence and matrix metalloproteinases (MMPs) in growth patterns of major fungal pathogens in cornea.

METHODS

Ninety-six eyes in 96 rabbits were equally divided into four groups receiving inoculation of fungal conidia of Aspergillus fumigatus, Candida albicans, Fusarium solani, and Penicillium citreo-viride, respectively, to induce fungal keratitis. Corneas in each group were obtained at 2, 8, 16 hr, and 1, 2, 3, 5, and 8 days after inoculation and were subjected to scanning electron microscopy, histopathological examination, and gelatin zymography. Eight saline-inoculated eyes in another eight rabbits served as controls.

RESULTS

All eyes in the fungus-inoculated groups developed fungal keratitis. The binding of conidia to corneal epithelial basement membrane was initiated earlier in the A. fumigatus and C. albicans groups than in the F. solani and P. citreo-viride groups. Destruction of basement membrane began at 1 to 3 days. Histopathologically, infiltration of inflammatory cells was more evident in the A. fumigatus and C. albicans groups than the F. solani and P. citreo-viride groups at 3 days. The hyphae of A. fumigatus and C. albicans traversed the cornea in a plane perpendicular to the stromal lamellae, whereas the hyphae of F. solani and P. citreo-viride lay parallel to the corneal lamellae. MMP-9 and MMP-2 were found in all infected corneas. At 3 days, proteolysis was most active; the level of MMP-9 was higher in the A. fumigatus and C. albicans groups than in the F. solani and P. citreo-viride groups. There were positive correlations among the number of binding conidia, degree of inflammation, and level of MMP-9 (p < 0.05).

CONCLUSIONS

The adherence ability, chemotaxis to neutrophils, and MMP-9 expression level differ in eyes with different fungal pathogens, which may contribute to the different growth patterns of fungi in cornea.

Candida albicans cell wall proteins

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

Induction of a high affinity fibronectin receptor in Candida albicans by caspofungin: requirements for beta (1,6) glucans and the developmental regulator Hbr1p

Candida albicans expresses at least two biochemically distinct fibronectin receptors. Hemoglobin induces expression of a low affinity receptor recognizing the fibronectin cell-binding domain, whereas growth in complex media induces a high affinity receptor recognizing the collagen-binding domain. We now show that sub-inhibitory concentrations of caspofungin and nikkomycin Z, but not fluconazole, induce the high affinity fibronectin receptor in a dose-dependent manner. Macromolecular complexes mechanically sheared from caspofungin-treated cells retained high affinity fibronectin binding that was sensitive to protease, disulfide reduction, and beta (1,3) glucanase digestion. The high affinity fibronectin receptor was not inducible in a Kre9 mutant strain of C. albicans deficient in beta (1,6) glucans. Conversely, a mutant strain lacking the fibronectin binding protein Als5p showed no defects in induction of high or low affinity fibronectin receptors. Heterozygous mutants of a regulator of white-opaque phenotypic switching, HBR1, lacked any detectable high affinity fibronectin receptor expression in response to caspofungin, and re-introduction of the gene restored activity. Therefore, sub-inhibitory dosages of caspofungin induce a high affinity fibronectin receptor that is distinct from the known receptor Als5p and is dependent on beta (1,6) glucans and HBR1.

Hemoglobin is an effective inducer of hyphal differentiation in Candida albicans

Hemoglobin is an abundant protein in the host vascular compartment and a source of iron, heme, and amino acids for many pathogens. The human fungal pathogen Candida albicans uses hemoglobin as an iron source as well as a signaling molecule to alter gene expression and induce adhesion to several extracellular matrix proteins. We now report that hemoglobin can promote true hyphal morphogenesis. Hemoglobin added to yeast cells at 37 degrees C rapidly induced expression of the hypha-specific genes HWP1 and ECE1 coincident with the pattern of hyphal development. A synthetic medium buffered with phosphate at pH 7.2 and containing physiological glucose (5 mM) and low ammonium ion (0.1 mM) was optimal for the response to hemoglobin. High glucose (110 mM), high ammonium ion (20 mM), and 0.1 mM glutamine were all inhibitory. Heme, free globin, or immobilized hemoglobin could not replicate the activity of hemoglobin to induce germ tubes or hypha-specific gene expression at 37 degrees C under optimized conditions. This implicates the previously described Hb-signaling receptor in hyphal formation. This response was also dependent upon the presence of the morphogenesis regulator Efg1p, but the MAP-kinase specific transcription factor Cph1p was not required. These data define a role for the host-factor hemoglobin in Efg1p-dependent hyphal development.

The effect of aminocandin (HMR 3270) on the in-vitro adherence of Candida albicans to polystyrene surfaces coated with extracellular matrix proteins or fibronectin

Aminocandin is a new representative of the echinocandins that could potentially affect the cellular morphology and metabolic status of Candida albicans cells within biofilms. This study investigated the influence of a sub-inhibitory concentration (MIC/2) of aminocandin on in-vitro growth of C. albicans and subsequent fungal adherence to plastic surfaces coated with fibronectin or extracellular matrix (ECM) proteins. Eleven strains of C. albicans were studied, of which six were susceptible and five were resistant to fluconazole. All 11 strains were susceptible to aminocandin in vitro, regardless of the culture medium used for the microdilution method. Aminocandin induced a significant (p <0.005) decrease in adherence when polystyrene was coated with ECM gel (ten strains) or fibronectin (seven strains). Growth in medium containing aminocandin (MIC/2) decreased the adherence of five (ECM gel) or three (fibronectin) of the six strains susceptible to fluconazole, and inhibition was observed for all five (ECM gel) or four (fibronectin) of the five fluconazole-resistant strains. Overall, the study demonstrated the anti-adherent properties of aminocandin with fluconazole-susceptible strains, and suggested that this activity was at least equivalent with fluconazole-resistant strains. Thus, the ability of aminocandin to inhibit the first step in the development of C. albicans biofilms appeared to be independent of the in-vitro resistance of C. albicans to fluconazole.

In vitro evaluation of virulence attributes of Candida spp. isolated from patients affected by diabetes mellitus

BACKGROUND

Diabetes mellitus is a common disease found worldwide and it has been previously suggested that oral candidal infections may be more frequent or severe in patients with this disease. Systemic and local factors may influence the balance between the host and yeasts, and favour the transformation of Candida isolates from commensal to pathogenic microorganisms. Candida species have developed specific virulence mechanisms that confer the ability to colonise host surfaces, to invade deeper host tissue, or to evade host defences. Few studies have investigated the expression of the virulence attributes of oral Candida isolates in patients with diabetes mellitus.

MATERIAL AND METHODS

The in vitro extracellular proteinase production and the in vitro ability to adhere to fibronectin of 229 Candida isolates of two geographic different groups of patients with diabetes mellitus and of healthy subjects were assessed.

RESULTS

Candida isolates of patients with diabetes mellitus expressed a higher ability to adhere than those of healthy subjects. Higher levels of adhesion were also recorded in patients with a lower oral Candida colonisation. No differences were observed in the in vitro expression of extracellular proteinase of Candida isolates of patients with diabetes mellitus and those of non-diabetic subjects. Isolates of patients with type 2 diabetes mellitus expressed greater levels of proteinase than isolates of type 1 diabetes mellitus.

CONCLUSIONS

Diabetes mellitus could be considered as an additional variable that may influence not only oral Candida carriage but also the ability of isolates to enhance the expression of virulence attributes.

Interaction of pathogenic fungi with host cells: Molecular and cellular approaches

This review provides an overview of several molecular and cellular approaches that are likely to supply insights into the host-fungus interaction. Fungi present intra- and/or extracellular host-parasite interfaces, the parasitism phenomenon being dependent on complementary surface molecules. The entry of the pathogen into the host cell is initiated by the fungus adhering to the cell surface, which generates an uptake signal that may induce its cytoplasmatic internalization. Furthermore, microbial pathogens use a variety of their surface molecules to bind to host extracellular matrix (ECM) components to establish an effective infection. On the other hand, integrins mediate the tight adhesion of cells to the ECM at sites referred to as focal adhesions and also play a role in cell signaling. The phosphorylation process is an important mechanism of cell signaling and regulation; it has been implicated recently in defense strategies against a variety of pathogens that alter host-signaling pathways in order to facilitate their invasion and survival within host cells. The study of signal transduction pathways in virulent fungi is especially important in view of their putative role in the regulation of pathogenicity. This review discusses fungal adherence, changes in cytoskeletal organization and signal transduction in relation to host-fungus interaction.

Hemoglobin regulates expression of an activator of mating-type locus alpha genes in Candida albicans

Phenotypic switching from the white to the opaque phase is a necessary step for mating in the pathogenic fungus Candida albicans. Suppressing switching during vascular dissemination of the organism may be advantageous, because opaque cells are more susceptible to host defenses. A repressor of white-opaque switching, HBR1 (hemoglobin response gene 1), was identified based on its specific induction following growth in the presence of exogenous hemoglobin. Deletion of a single HBR1 allele allowed opaque phase switching and mating competence, accompanied by a lack of detectable MTL alpha1 and alpha2 gene expression and enhanced MTLa1 gene expression. Conversely, overexpression of Hbr1p or exposure to hemoglobin increased MTLalpha gene expression. The a1/alpha2 repressed target gene CAG1 was derepressed in the same mutant in a hemoglobin-sensitive manner. Regulation of CAG1 by hemoglobin required an intact MTLa1 gene. Several additional Mtlp targets were perturbed in HBR1 mutants in a manner consistent with commitment to an a mating phenotype, including YEL007w, MFalpha, HST6, and RAM2. Therefore, Hbr1 is part of a host factor-regulated signaling pathway that controls white-opaque switching and mating in the absence of allelic deletion at the MTL locus.

Sensing the host environment: recognition of hemoglobin by the pathogenic yeast Candida albicans

Adhesion to host cells and tissues is important for several steps in the pathogenesis of disseminated Candida albicans infections. Although such adhesion is evident in vivo and for C. albicans grown in vitro in complex medium, some adhesive activities are absent when cultures are grown in defined media. However, addition of hemoglobin to defined media restores binding and adhesion to several host proteins. This activity of hemoglobin is independent of iron acquisition and is mediated by a cell surface hemoglobin receptor. In addition to regulating expression of adhesion receptors, hemoglobin rapidly induces expression of several genes. One of these, a heme oxygenase, allows the pathogen to utilize exogenous heme or hemoglobin to acquire iron and to produce the cytoprotective molecules alpha-biliverdin and carbon monoxide. The specific recognition of and responses to hemoglobin demonstrate a unique adaptation of C. albicans to be both a commensal and an opportunistic pathogen in humans.

Influence of sub-inhibitory concentrations of conventional antifungals on metabolism of Candida albicans and on its adherence to polystyrene and extracellular matrix proteins

Five antifungal agents with different mechanisms of action were compared for their ability to affect mitochondrial dehydrogenase activity and adherence capacity of Candida albicans to polystyrene and extracellular matrix proteins. Only amphotericin B inhibited mitochondrial dehydrogenase activity when the culture medium was supplemented with galactose. 5-Fluorocytosine and terbinafine did not affect this activity, whereas itraconazole and fluconazole improved it. Furthermore, in these experimental conditions, the effect of sub-inhibitory concentrations of antifungals on adherence was dependent on the tested antifungal and the adherence surface: amphotericin B inhibited adherence to polystyrene and fibrinogen, but improved adherence to extracellular matrix. For all surfaces tested, when culture medium was supplemented with galactose, fluorocytosine did not affect adherence, and itraconazole, fluconazole and terbinafine inhibited adherence. Our results also confirmed the influence of the carbohydrates: sub-minimum inhibitory concentrations (MIC) of itraconazole increased or did not modify the mitochondrial metabolism of yeasts when the culture medium was supplemented with galactose, but this antifungal always decreased mitochondrial metabolism when the culture medium was supplemented with glucose. These data indicate that antifungals used below their MIC values can have various effects. It is important to distinguish the effects of antifungals on the metabolism of C. albicans from effects on its adherence capacity. The former effects are linked to the viability of the yeast and the latter depends on the colonization of cellular as opposed to inert surfaces.

Different adhesins for type IV collagen on Candida albicans: identification of a lectin-like adhesin recognizing the 7S(IV) domain

Adherence of the opportunistic pathogen Candida albicans to basement membrane (BM) proteins is considered a crucial step in the development of candidiasis. In this study the interactions of C. albicans yeast cells with the three main domains of type IV collagen, a major BM glycoprotein, were analysed. C. albicans adhered to the three immobilized domains by different mechanisms. Adhesion to the N-terminal cross-linking domain (7S) required the presence of divalent cations, whereas interaction with the central collagenous domain (CC) was cation-independent. Recognition of the C-terminal non-collagenous domain (NC1) was partially cation-dependent. Binding inhibition assays with the corresponding domains in soluble form showed that these interactions were specific. Both Ca(2+) and Mg(2+) promoted adhesion to the 7S domain and the interaction was completely abolished by EDTA. Treatment of the 7S domain, or its subunits, with N-glycosidase F reduced yeast binding by approximately 70%. Moreover, several sugars known to be part of the N-linked oligosaccharide chains of collagen IV inhibited adhesion to immobilized 7S; N-acetylglucosamine, L-fucose and methylmannoside caused a similar inhibition whereas N-acetyllactosamine was a more effective inhibitor. In contrast, glucose, galactose, lactose or heparan sulfate did not affect yeast binding. Combinations of the inhibitory sugars at suboptimal inhibition concentrations did not reduce C. albicans adhesion more than the individual sugars, pointing to a single lectin as responsible for the interaction. These results taken together show that C. albicans utilizes several adhesins for interacting with type IV collagen, and that at least one of them is a lectin which recognizes the 7S(IV) oligosaccharide residues as its receptor.

Relative abundance of oligosaccharides in Candida species as determined by fluorophore-assisted carbohydrate electrophoresis

Fluorophore-assisted carbohydrate electrophoresis (FACE) is a straightforward, sensitive method for determining the presence and relative abundance of individual oligomannosyl residues in Candida mannoprotein, the major antigenic determinant located on the outer surface of the yeast cell wall. The single terminal aldehydes of oligomannosyl residues released by hydrolysis were tagged with the charged fluorophore 8-aminonaphthalene-1,3,6-trisulfonate (ANTS) and separated with high resolution on the basis of size by polyacrylamide gel electrophoresis. ANTS fluorescence labeling was not biased by oligomannoside length; therefore, band fluorescence intensity was directly related to the relative abundance of individual oligomannoside moieties in heterogeneous samples. FACE analysis revealed the major oligomannosides released by acid hydrolysis and beta-elimination of Fehling-precipitated mannan from Candida albicans, which were the same as those previously reported in studies based on mass and nuclear magnetic spectroscopic analysis. FACE was also amenable to the analysis of samples obtained by direct hydrolysis of whole yeast cells. Whole-cell acid hydrolysis and whole-cell beta-elimination of two isolates each of C. albicans, C. glabrata, C. krusei, C. lusitaniae, C. parapsilosis, C. rugosa, C. stellatoidea, and C. tropicalis resulted in oligomannoside gel banding patterns that were species and strain specific for the 16 isolates surveyed. Whereas some bands were specific for an individual isolate or species, other bands were shared by two or three species in various groupings. Differences in the mannoprotein composition of C. albicans A9 and four spontaneous cell surface mutants were also detected. Mannan "fingerprints," or banding pattern profiles, derived from the electrophoretic mobilities of individual bands relative to the migration of acid-hydrolyzed dextran (relative migration index) yielded profiles characteristic of individual isolates not revealed by standard assimilation and biochemical profiles. FACE represents an accessible, sensitive, and quantitative analytical tool enabling the characterization of yeast mannan complexity.

RGD-mediated adhesion in fungal pathogens of humans, plants and insects

Progress in the understanding of fungal adhesion has led to the identification of novel proteins recognizing the RGD tripeptide in matrix proteins and to the characterization of what appears to be an emerging subset of fungal adhesins that themselves contain an RGD sequence.

Evidence for the presence of pir-like proteins in Candida albicans

Pir proteins are unique proteins with internal repeat sequences that are reported to be present in the cell wall of Saccharomyces cerevisiae. They are covalently attached to the cell wall and can be released by mild alkali treatment. In this study the biotinylated cell wall preparations from Candida albicans and S. cerevisiae were extracted by alkali and beta-1,3 glucanase and analyzed in parallel. Among the four bands detected by streptavidin, two proteins were recognized by the antibody to the S. cerevisiae Pir protein Hsp150. The antibody also detected a high molecular mass protein secreted in the growth medium of C. albicans. Using S. cerevisiae HSP150/PIR2 gene as a probe, Southern and Northern hybridizations were performed with DNA and RNA of C. albicans. Hybridization with DNA digested with different restriction enzymes showed more than one hybridized fragment. An increased level of mRNA was found in heat shocked cells (37 degrees C for 45 min compared to 25 degrees C). Hybridization of ScHSP150 gene to mRNAs from cells grown in different media was also determined. Two transcripts of size approximately 3.5 kb and 2.0 kb were detected in mRNAs from cells grown in defined medium with glucose as carbon source or in the same medium supplemented with hemoglobin. The lower transcript of size 2.0 kb was absent in cells grown in medium with galactose as carbon source. A single band was also observed when cells were grown in rich medium. Together these results demonstrated the existence of beta1,3 glucan linked proteins in C. albicans, which are related to Pir family proteins of S. cerevisiae.

Oral colonization by Candida albicans

Candida albicans is a commensal yeast normally present in small numbers in the oral flora of a large proportion of humans. Colonization of the oral cavity by C. albicans involves the acquisition and maintenance of a stable yeast population. Micro-organisms are continually being removed from the oral cavity by host clearance mechanisms, and so, in order to survive and inhabit this eco-system, C. albicans cells have to adhere and replicate. The oral cavity presents many niches for C. albicans colonization, and the yeast is able to adhere to a plethora of ligands. These include epithelial and bacterial cell-surface molecules, extracellular matrix proteins, and dental acrylic. In addition, saliva molecules, including basic proline-rich proteins, adsorbed to many oral surfaces promote C. albicans adherence. Several adhesins present in the C. albicans cell wall have now been partially characterized. Adherence involves lectin, protein-protein, and hydrophobic interactions. As C. albicans cells evade host defenses and colonize new environments by penetrating tissues, they are exposed to new adherence receptors and respond by expressing alternative adhesins. The relatively small number of commensal Candida cells in the oral flora raises the possibility that strategies can be devised to prevent oral colonization and infection. However, the variety of oral niches and the complex adherence mechanisms of the yeast mean that such a goal will remain elusive until more is known about the contribution of each mechanism to colonization.

Cell surface proteins of Candida albicans: preparation of extracts and improved detection of proteins

We have reexamined the detection of the components in a beta-mercaptoethanol and ammonium carbonate buffer extract of surface proteins of Candida albicans and the effects of postextraction manipulation of the extract on recovery of extract components. Following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), preferential staining of some moieties was observed when bands detected by a commercial silver staining method or a Coomassie Brilliant Blue (CBB) staining method were compared. Additional protein bands that were either not detected or poorly detected by a single method alone were readily observed by a combined silver-CBB staining method. This method also detected alterations in the profile of extracted proteins from organisms grown in the presence of galactose or hemoglobin rather than glucose. Two-dimensional electrophoresis (2-DE) gel analysis by double stain showed better detection of several acidic and basic protein spots. Less than 10% of the extract as determined by a dye-binding assay was lost following either or both lyophilization and dialysis. These manipulations of the extract did not change the protein profile following SDS-PAGE as determined by the combined staining or Western blot analysis of a 70 kDa protein. These observations suggest that soluble cell wall proteins are not unusually sensitive to procedures routinely used in protein purification. In addition, these studies suggest that a modified staining method that combines both silver stain and CBB stain provides improved detection of cell wall proteins compared to either method alone.

A Candida albicans metallopeptidase degrades constitutive proteins of extracellular matrix

Among potential virulence factors of Candida albicans, enzymes seem to play an important role. Many studies concern the secreted aspartic proteinases (saps), and the degradation of some components of the subendothelial extracellular matrix by the isoenzyme sap2 has been proved. Nevertheless, other proteolytic enzymes could be involved in the pathogenicity of the yeast. We studied the degradation of four constitutive proteins of the extracellular matrix: type I and IV collagens, laminin and fibronectin, by a 95-kDa metallopeptidase, localised in the cell wall of C. albicans. Each of these constituents was incubated with the purified enzyme and its degradation products analysed by an electrophoretic method. We observed that type I collagen and fibronectin were totally degraded by the enzyme whereas type IV collagen and laminin were only partially degraded. The C. albicans metallopeptidase may play a role in the degradation of the subendothelial extracellular matrix components. This enzyme could facilitate the migration of the yeast in the tissues after crossing the endothelial layer, allowing the fungal invasion of target organs.

Interactions between Candida albicans and the human extracellular matrix component tenascin-C

Tenascins are large multimeric proteins that contain repeated structural motifs that include epidermal growth factor (EGF)-like repeats, fibronectin type III repeats and a globular fibrinogen-like domain, and are involved in tissue and organ morphogenesis, as well as in adhesion and migration of cells. C. albicans germ-tubes, but not blastospores, were able to bind to soluble human tenascin-C as revealed by an indirect immunofluorescence assay. However, materials present in cell wall extracts from both morphologies attached to tenascin-C immobilized in wells of a microtiter plate. The binding specificity was demonstrated by the inhibitory effect of antibodies against C. albicans cell wall components and an anti-tenascin antibody, but not anti-laminin antibody. Fibronectin, but not fibrinogen, inhibited binding, thus indicating a role of the fibronectin type III repeats in the interaction between the fungus and tenascin-C. Binding of C. albicans cell wall materials to tenascin was RGD- and divalent cation-independent.

Identification of a 21 kDa laminin-binding component of Candida albicans

Binding of Candida albicans strain JCM0239 to radiolabelled laminin was studied to identify possible receptors. When two forms of the organism were examined, laminin was found to bind to germinating blastoconidia. There was only weak binding to blastoconidia without germ tubes. Laminin-binding to germinating blastoconidia was inhibited by non-labelled laminin or fibrinogen but not by fibronectin, a fibronectin peptide (Gly-Arg-Gly-Asp-Ser), or a laminin peptide (Tyr-Ile-Gly-Ser-Arg). A cell extract from germinating blastoconidia was analyzed by using SDS-PAGE and Western blotting, and 125I-labelled laminin was shown to bind to a 21 kDa component.

Hemoglobin-induced binding of Candida albicans to the cell-binding domain of fibronectin is independent of the Arg-Gly-Asp sequence

Hemoglobin specifically induces fibronectin (FN) binding to the pathogenic yeast Candida albicans. When grown in the complex medium Sabouraud broth, C. albicans expresses receptors that bind to several domains of FN. Growth in defined medium supplemented with 0.1% hemoglobin, however, enhanced the binding of FN to a single class of receptors, with a Kd = 4.6 x 10(-8) M. Competitive binding assays using recombinant and proteolytic fragments of FN revealed that the cell-binding domain mediated this interaction. A recombinant 40-kDa fragment of FN consisting of type III repeats 9 to 13 had an inhibitory activity similar to that of the entire 120-kDa cell-binding domain, indicating that the C-terminal portion of the cell-binding domain contains the binding site. A recombinant 33-kDa fragment of the cell-binding domain and a 33-kDa fragment with the RGD sequence deleted had the same inhibitory activities, demonstrating that the RGD sequence recognized by some mammalian integrins is not required. The addition of hemoglobin to the culture medium also enhanced Candida cell adhesion to immobilized FN and to 120- and 40-kDa fragments of FN but not to the collagen-binding or fibrin I domains. Using ligand protection, we identified a surface protein from C. albicans with an apparent molecular mass of 55 kDa that was protected by both FN and the 40-kDa fragment derived from the cell-binding domain. Therefore, hemoglobin both induces FN binding and changes the relative affinities of C. albicans for the cell- and collagen-binding domains of FN.