Genetic organization and sequence analysis of the hypha-specific cell wall protein gene HWP1 of Candida albicans

Dose-Dependent Inhibitory Effect of Probiotic Lactobacillus plantarum on Streptococcus mutans-Candida albicans Cross-Kingdom Microorganisms

Dental caries is one of the most common chronic diseases worldwide. Streptococcus mutans and Candida albicans are two major pathogens associated with dental caries. Several recent studies revealed that Lactobacillus plantarum inhibits S. mutans and C. albicans in biofilms and in a rodent model of dental caries. The aim of this study was to investigate the dose-dependent effect of L. plantarum against S. mutans and C. albicans in a planktonic model that simulated a high-caries-risk clinical condition. Mono-, dual-, and multi-species models were utilized, with five doses of L. plantarum (ranging from 1.0 × 10⁴ to 1.0 × 10⁸ CFU/mL). Real-time PCR was used to assess the expression of the virulence genes of C. albicans and S. mutans and the genes of L. plantarum. Student's t-tests and one-way ANOVA, followed by post hoc tests, were employed to compare the cell viability and gene expression among groups. A dose-dependent inhibition on C. albicans and S. mutans was observed with increased dosages of L. plantarum. L. plantarum at 10⁸ CFU/mL demonstrated the highest antibacterial and antifungal inhibitory effect in the dual- and multi-species models. Specifically, at 20 h, the growth of C. albicans and S. mutans was suppressed by 1.5 and 5 logs, respectively (p < 0.05). The antifungal and antibacterial effects were attenuated in lower doses of L. plantarum (10⁴-10⁷ CFU/mL). The expression of C. albicans HWP1 and ECE 1 genes and S. mutans lacC and lacG genes were significantly downregulated with an added 10⁸ CFU/mL of L. plantarum (p < 0.05). The addition of 10⁸ CFU/mL L. plantarum further inhibited the hyphae or pseudohyphae formation of C. albicans. In summary, L. plantarum demonstrated dose-dependent antifungal and antibacterial effects against C. albicans and S. mutans. L. plantarum emerged as a promising candidate for the creation of novel antimicrobial probiotic products targeting dental caries prevention. Further research is warranted to identify the functional metabolites produced by L. plantarum at different dosages when interacting with C. albicans and S. mutans.

Inhibition of cell cycle-dependent hyphal and biofilm formation by a novel cytochalasin 19,20‑epoxycytochalasin Q in Candida albicans

Biofilm-mediated drug resistance is a key virulence factor of pathogenic microbes that cause a serious global health threat especially in immunocompromised individuals. Here, we investigated the antihyphal and antibiofilm activity of 19,20‑epoxycytochalasin Q (ECQ), a cytochalasin actin inhibitor isolated from medicinal mushroom Xylaria sp. BCC1067 against Candida albicans. Remarkably, 256 µg/ml of ECQ inhibited over 95% of C. albicans hyphal formation after 24 h-treatment. Combined ECQ and lipid-based biosurfactant effectively enhanced the antihyphal activity, lowering required ECQ concentrations. Hyphal fragmentation and reduction of biofilm biomass, shown by SEM and AFM visualization of ECQ-treated biofilms, were well corelated to the reduced metabolic activities of young and 24 h-preformed C. albicans biofilms. Induced intracellular accumulation of reactive oxygen species (ROS) also occurred in accompany with the leakage of shrunken cell membrane and defective cell wall at increasing ECQ concentrations. Transcriptomic analyses via RNA-sequencing revealed a massive change (> 1300 genes) in various biological pathways, following ECQ-treatment. Coordinated expression of genes, associated with cellular response to drugs, filamentous growth, cell adhesion, biofilm formation, cytoskeleton organization, cell division cycle, lipid and cell wall metabolisms was confirmed via qRT-PCR. Protein-protein association tool identified coupled expression between key regulators of cell division cyclin-dependent kinases (Cdc19/28) and a gamma-tubulin (Tub4). They coordinated ECQ-dependent hyphal specific gene targets of Ume6 and Tec1 during different phases of cell division. Thus, we first highlight the antihyphal and antibiofilm property of the novel antifungal agent ECQ against one of the most important life-threatening fungal pathogens by providing its key mechanistic detail in biofilm-related fungal infection.

Development and Use of a Monoclonal Antibody Specific for the Candida albicans Cell-Surface Protein Hwp1

The Candida albicans cell-surface protein Hwp1 functions in adhesion to the host and in biofilm formation. A peptide from the Gln-Pro-rich adhesive domain of Hwp1 was used to raise monoclonal antibody (MAb) 2-E8. MAb 2-E8 specificity for Hwp1 was demonstrated using a hwp1/hwp1 C. albicans isolate and strains that expressed at least one HWP1 allele. Immunofluorescence and atomic force microscopy experiments using MAb 2-E8 confirmed C. albicans germ-tube-specific detection of the Hwp1 protein. MAb 2-E8 also immunolabeled the tips of some Candida dubliniensis germ tubes grown under conditions that maximized HWP1 expression. The phylogeny of HWP1 and closely related genes suggested that the Gln-Pro-rich adhesive domain was unique to C. albicans and C. dubliniensis focusing the utility of MAb 2-E8 on these species. This new reagent can be used to address unanswered questions about Hwp1 and its interactions with other proteins in the context of C. albicans biology and pathogenesis.

Mycobiota-induced IgA antibodies regulate fungal commensalism in the gut and are dysregulated in Crohn's disease

Secretory immunoglobulin A (sIgA) plays an important role in gut barrier protection by shaping the resident microbiota community, restricting the growth of bacterial pathogens and enhancing host protective immunity via immunological exclusion. Here, we found that a portion of the microbiota-driven sIgA response is induced by and directed towards intestinal fungi. Analysis of the human gut mycobiota bound by sIgA revealed a preference for hyphae, a fungal morphotype associated with virulence. Candida albicans was a potent inducer of IgA class-switch recombination among plasma cells, via an interaction dependent on intestinal phagocytes and hyphal programming. Characterization of sIgA affinity and polyreactivity showed that hyphae-associated virulence factors were bound by these antibodies and that sIgA influenced C. albicans morphotypes in the murine gut. Furthermore, an increase in granular hyphal morphologies in patients with Crohn's disease compared with healthy controls correlated with a decrease in antifungal sIgA antibody titre with affinity to two hyphae-associated virulence factors. Thus, in addition to its importance in gut bacterial regulation, sIgA targets the uniquely fungal phenomenon of hyphal formation. Our findings indicate that antifungal sIgA produced in the gut can play a role in regulating intestinal fungal commensalism by coating fungal morphotypes linked to virulence, thereby providing a protective mechanism that might be dysregulated in patients with Crohn's disease.

Biomarkers for the diagnosis of invasive candidiasis in immunocompetent and immunocompromised patients

Blood culture methods show low sensitivity, so reliable non-culture diagnostic tests are needed to help clinicians with the introduction, de-escalation, and discontinuation of antifungal therapy in patients with suspected invasive candidiasis (IC). We evaluated different biomarkers for the diagnosis of IC in immunocompetent and immunocompromised patients at risk for developing invasive fungal diseases. The specificity of Candida albicans germ-tube antibodies (CAGTA) detection was high (89%-100%), but sensitivity did not exceed 61% even after raising the cut-off from 1/160 to 1/80. We developed enzyme-linked immunoassays detecting antibodies against C. albicans proteins (Als3-N, Hwp1-N, or Met6) that resulted more sensitive (66%-92%) but less specific than CAGTA assay. The combination of 1,3-beta-D-glucan (BDG) detection and CAGTA results provided the highest diagnostic usefulness in immunocompetent patients. However, in immunocompromised patients, anti-Met6 antibodies was the best biomarker, both, alone or in combination with BDG.

Identifying Candida albicans Gene Networks Involved in Pathogenicity

Candida albicans is a normal member of the human microbiome. It is also an opportunistic pathogen, which can cause life-threatening systemic infections in severely immunocompromized individuals. Despite the availability of antifungal drugs, mortality rates of systemic infections are high and new drugs are needed to overcome therapeutic challenges including the emergence of drug resistance. Targeting known disease pathways has been suggested as a promising avenue for the development of new antifungals. However, <30% of C. albicans genes are verified with experimental evidence of a gene product, and the full complement of genes involved in important disease processes is currently unknown. Tools to predict the function of partially or uncharacterized genes and generate testable hypotheses will, therefore, help to identify potential targets for new antifungal development. Here, we employ a network-extracted ontology to leverage publicly available transcriptomics data and identify potential candidate genes involved in disease processes. A subset of these genes has been phenotypically screened using available deletion strains and we present preliminary data that one candidate, PEP8, is involved in hyphal development and immune evasion. This work demonstrates the utility of network-extracted ontologies in predicting gene function to generate testable hypotheses that can be applied to pathogenic systems. This could represent a novel first step to identifying targets for new antifungal therapies.

Inhibition of Yeast-to-Hypha Transition and Virulence of Candida albicans by 2-Alkylaminoquinoline Derivatives

A rapid increase in Candida albicans infection and drug resistance has caused an emergent need for new clinical strategies against this fungal pathogen. In this study, we evaluated the inhibitory activity of a series of 2-alkylaminoquinoline derivatives against C. albicans isolates. A total of 28 compounds were assessed for their efficacy in inhibiting the yeast-to-hypha transition, which is considered one of the key virulence factors in C. albicans Several compounds showed strong activity to decrease the morphological transition and virulence of C. albicans cells. The two leading compounds, compound 1 (2-[piperidin-1-yl]quinolone) and compound 12 (6-methyl-2-[piperidin-1-yl]quinoline), remarkably attenuated C. albicans hyphal formation and cytotoxicity in a dose-dependent manner, but they showed no toxicity to either C. albicans cells or human cells. Intriguingly, compound 12 showed an excellent ability to inhibit C. albicans infection in the mouse oral mucosal infection model. This leading compound also interfered with the expression levels of hypha-specific genes in the cyclic AMP-protein kinase A and mitogen-activated protein kinase signaling pathways. Our findings suggest that 2-alkylaminoquinoline derivatives could potentially be developed as novel therapeutic agents against C. albicans infection due to their interference with the yeast-to-hypha transition.

Drugs currently under investigation for the treatment of invasive candidiasis

INTRODUCTION

The widespread implementation of immunosuppressants, immunomodulators, hematopoietic stem cell transplantation and solid organ transplantation in clinical practice has led to an expanding population of patients who are at risk for invasive candidiasis, which is the most common form of fungal disease among hospitalized patients in the developed world. The emergence of drug-resistant Candida spp. has added to the morbidity associated with invasive candidiasis and novel therapeutic strategies are urgently needed. Areas covered: In this paper, we explore investigational agents for the treatment of invasive candidiasis, with particular attention paid to compounds that have recently entered phase I or phase II clinical trials. Expert opinion: The antifungal drug development pipeline has been severely limited due to regulatory hurdles and a systemic lack of investment in novel compounds. However, several promising drug development strategies have recently emerged, including chemical screens involving Pathogen Box compounds, combination antifungal therapy, and repurposing of existing agents that were initially developed to treat other conditions, all of which have the potential to redefine the treatment of invasive candidiasis.

Niche-specific requirement for hyphal wall protein 1 in virulence of Candida albicans

Specialized Candida albicans cell surface proteins called adhesins mediate binding of the fungus to host cells. The mammalian transglutaminase (TG) substrate and adhesin, Hyphal wall protein 1 (Hwp1), is expressed on the hyphal form of C. albicans where it mediates fungal adhesion to epithelial cells. Hwp1 is also required for biofilm formation and mating thus the protein functions in both fungal-host and self-interactions. Hwp1 is required for full virulence of C. albicans in murine models of disseminated candidiasis and of esophageal candidiasis. Previous studies correlated TG activity on the surface of oral epithelial cells, produced by epithelial TG (TG1), with tight binding of C. albicans via Hwp1 to the host cell surfaces. However, the contribution of other Tgs, specifically tissue TG (TG2), to disseminated candidiasis mediated by Hwp1 was not known. A newly created hwp1 null strain in the wild type SC5314 background was as virulent as the parental strain in C57BL/6 mice, and virulence was retained in C57BL/6 mice deleted for Tgm2 (TG2). Further, the hwp1 null strains displayed modestly reduced virulence in BALB/c mice as did strain DD27-U1, an independently created hwp1Δ/Δ in CAI4 corrected for its ura3Δ defect at the URA3 locus. Hwp1 was still needed to produce wild type biofilms, and persist on murine tongues in an oral model of oropharyngeal candidiasis consistent with previous studies by us and others. Finally, lack of Hwp1 affected the translocation of C. albicans from the mouse intestine into the bloodstream of mice. Together, Hwp1 appears to have a minor role in disseminated candidiasis, independent of tissue TG, but a key function in host- and self-association to the surface of oral mucosa.

Distinct and redundant roles of the two MYST histone acetyltransferases Esa1 and Sas2 in cell growth and morphogenesis of Candida albicans

Candida albicans is associated with humans, as both a harmless commensal organism and a pathogen. Adaption to human body temperature is extremely important for its growth and morphogenesis. Saccharomyces cerevisiae Esa1, a member of the MYST family HATs (histone acetyltransferases) and the catalytic subunit of the NuA4 complex, and its homologues in other eukaryotes have been shown to be essential for cell growth. To investigate the functional roles of two MYST family HATs, Esa1 and Sas2 in C. albicans, we deleted ESA1 and SAS2 in the C. albicans genome and performed cell growth analyses. Our results demonstrated that C. albicans Esa1 is not essential for general growth but is essential for filamentous growth. The esa1/esa1 mutant cells exhibited sensitivity to thermal, genotoxic, and oxidative stresses but tolerance to cold, osmotic, and cell wall stresses. In contrast, the sas2/sas2 mutant adapted to growth at higher temperatures and promoted filament formation at lower temperatures, resembling the phenotype of a C. albicans strain overexpressing ESA1. Cells with deletions of both ESA1 and SAS2 were inviable, reflecting the functional redundancy in cell growth. C. albicans Esa1 and Sas2 have distinct and synergistic effects on histone acetylation at H4K5, H4K12, and H4K16. Esa1 contributes mainly to acetylation of H4K5 and H4K12, whereas Sas2 contributes to acetylation of H4K16. Our findings suggest that C. albicans Esa1 and Sas2 play opposite roles in cell growth and morphogenesis and contribute coordinately to histone acetylation and gene regulation.

Regulation of phenotypic transitions in the fungal pathogen Candida albicans

The human commensal fungus Candida albicans can cause not only superficial infections, but also life-threatening disease in immunocompromised individuals. C. albicans can grow in several morphological forms. The ability to switch between different phenotypic forms has been thought to contribute to its virulence. The yeast-filamentous growth transition and white-opaque switching represent two typical morphological switching systems, which have been intensively studied in C. albicans. The interplay between environmental factors and genes determines the morphology of C. albicans. This review focuses on the regulation of phenotypic changes in this pathogenic organism by external environmental cues and internal genes.

Insight into the antiadhesive effect of yeast wall protein 1 of Candida albicans

Ywp1 is a prominent glycosylphosphatidylinositol (GPI)-anchored glycoprotein of the cell wall of Candida albicans; it is present in the yeast form of this opportunistic fungal pathogen but absent from filamentous forms and chlamydospores. Yeast cells that lack Ywp1 are more adhesive and form thicker biofilms, implying an antiadhesive activity for Ywp1, with a possible role in yeast dispersal. The antiadhesive effect of Ywp1 is transplantable from yeast to hyphae, as hyphae that are forced to express YWP1 lose adhesion in an in vitro assay. Deletion of the GPI anchor results in loss of Ywp1 to the surrounding medium and reduction of the antiadhesive effect, implying an importance of time-dependent residency in the cell wall. Anchor-negative versions of Ywp1 possessing or lacking a C-terminal green fluorescent protein (GFP) tag were created in C. albicans and harvested from culture supernatants; in addition to serving as quantifiable markers for Ywp1 secretion, they revealed that the cleaved 11-kDa propeptide of Ywp1 remains strongly but noncovalently associated with the Ywp1 core. This association is resistant to highly acidic and basic solutions, 8 M urea, and 1% SDS (below 45°C). Above 50°C, SDS dissociates the isolated complex, but even higher temperatures are required to dissociate the propeptide from native Ywp1 that is anchored in a cell wall. This property has permitted detection, for the first time, of orthologs of Ywp1 in other members of the Candida clade. The cleaved propeptide, which carries the sole N-glycan of Ywp1, must participate in the antiadhesive effect of Ywp1.

Zap1 control of cell-cell signaling in Candida albicans biofilms

Biofilms of Candida albicans include both yeast cells and hyphae. Prior studies indicated that a zap1Δ/Δ mutant, defective in zinc regulator Zap1, has increased accumulation of yeast cells in biofilms. This altered yeast-hypha balance may arise from internal regulatory alterations or from an effect on the production of diffusible quorum-sensing (QS) molecules. Here, we develop biosensor reporter strains that express yeast-specific YWP1-RFP or hypha-specific HWP1-RFP, along with a constitutive TDH3-GFP normalization standard. Seeding these biosensor strains into biofilms allows a biological activity assay of the surrounding biofilm milieu. A zap1Δ/Δ biofilm induces the yeast-specific YWP1-RFP reporter in a wild-type biosensor strain, as determined by both quantitative reverse transcription-PCR (qRT-PCR) gene expression measurements and confocal microscopy. Remediation of the zap1Δ/Δ zinc uptake defect through zinc transporter gene ZRT2 overexpression reverses induction of the yeast-specific YWP1-RFP reporter. Gas chromatography-mass spectrometry (GC-MS) measurements of known organic QS molecules show that the zap1Δ/Δ mutant accumulates significantly less farnesol than wild-type or complemented strains and that ZRT2 overexpression does not affect farnesol accumulation. Farnesol is a well-characterized inhibitor of hypha formation; hence, a reduction in farnesol levels in zap1Δ/Δ biofilms is unexpected. Our findings argue that a Zap1- and zinc-dependent signal affects the yeast-hypha balance and that it is operative in the low-farnesol environment of the zap1Δ/Δ biofilm. In addition, our results indicate that Zap1 is a positive regulator of farnesol accumulation.

A novel allele of HWP1, isolated from a clinical strain of Candida albicans with defective hyphal growth and biofilm formation, has deletions of Gln/Pro and Ser/Thr repeats involved in cellular adhesion

Gene HWP1 encodes a major Candida albicans hyphae cell wall protein which is a substrate of mammalian transglutaminases, promoting the cross-link of the fungus to epithelial cells. Here, we describe a novel HWP1 allele, isolated from C. albicans blood isolates. Analysis of the translated sequence shows that three important regions are absent in the novel allele, HWP1-2, relative to the previously described allele, HWP1-1. Regions 1 and 2 consist of 10 amino acid repeats important for functional conformation of peptide chains and attachment of C. albicans cells to the mammalian epithelia. Region 3 consists of 34 amino acid residues rich in threonine and serine, with O-glycosylation sites that promote the cross-linking with other proteins on C. albicans surface. The HWP1-2 homozygous strain L757 and the heterozygous strain L296 (HWP1-1/HWP1-2) have significantly lower levels of HWP1 expression during hyphal growth and biofilm formation compared to strain SC5314 (HWP1-1/HWP1-1). However, strain L296 properly forms hyphae and biofilms in vitro while strain L757 has reduced hyphal growth (40.4%) and biofilm formation (90.8%). Our results indicate that the HWP1 locus has biofilm specific allelic differential expression and suggest that the HWP1-2 encoded protein is less efficient to maintain cell-to-cell and cell-to-surface adhesion during biofilm formation. This is the first report of a natural variant of HWP1.

Heterologous expression of Candida albicans cell wall-associated adhesins in Saccharomyces cerevisiae Reveals differential specificities in adherence and biofilm formation and in binding oral Streptococcus gordonii

Colonization and infection of the human host by opportunistic pathogen Candida albicans derive from an ability of this fungus to colonize mucosal tissues and prosthetic devices within the polymicrobial communities present. To determine the functions of C. albicans cell wall proteins in interactions with host or bacterial molecules, Saccharomyces cerevisiae was utilized as a surrogate host to express C. albicans cell wall proteins Als3p, Eap1p, Hwp1p, and Rbt1p. Salivary pellicle and fibrinogen were identified as novel substrata for Als3p and Hwp1p, while only Als3p mediated adherence of S. cerevisiae to basement membrane collagen type IV. Parental S. cerevisiae cells failed to form biofilms on salivary pellicle, polystyrene, or silicone, but cells expressing Als3p or Hwp1p exhibited significant attachment to each surface. Virulence factor Rbt1p also conferred lower-level binding to salivary pellicle and polystyrene. S. cerevisiae cells expressing Eap1p formed robust biofilms upon polystyrene surfaces but not salivary pellicle. Proteins Als3p and Eap1p, and to a lesser degree Hwp1p, conferred upon S. cerevisiae the ability to bind cells of the oral primary colonizing bacterium Streptococcus gordonii. These interactions, which occurred independently of amyloid aggregate formation, provide the first examples of specific C. albicans surface proteins serving as receptors for bacterial adhesins. Streptococcus gordonii did not bind parental S. cerevisiae or cells expressing Rbt1p. Taken collectively, these data suggest that a network of cell wall proteins comprising Als3p, Hwp1p, and Eap1p, with complementary adhesive functions, promotes interactions of C. albicans with host and bacterial molecules, thus leading to effective colonization within polymicrobial communities.

Candida infections of the genitourinary tract

All humans are colonized with Candida species, mostly Candida albicans, yet some develop diseases due to Candida, among which genitourinary manifestations are extremely common. The forms of genitourinary candidiasis are distinct from each other and affect different populations. While vulvovaginal candidiasis affects mostly healthy women, candiduria occurs typically in elderly, hospitalized, or immunocompromised patients and in neonates. Despite its high incidence and clinical relevance, genitourinary candidiasis is understudied, and therefore, important questions about pathogenesis and treatment guidelines remain to be resolved. In this review, we summarize the current knowledge about genitourinary candidiasis.

On mechanism of quorum sensing in Candida albicans by 3(R)-hydroxy-tetradecaenoic acid

Quorum sensing (QS) enables microorganisms to monitor their own density of population, and also their pathogenicity by intracellular signals, and synchronizing their specialized gene system in a particular cell density. QS system has been shown in Candida sp. as switching mechanism between successive phases in Candida cell morphology. The lag phase that occurs due to QS is commonly attributed to auto-stimulatory compounds, such as farnesol and farnesoic acid, which are released in the medium. The aim of this manuscript is to demonstrate the involvement of 3(R)-HTDE, a metabolite of linoleic acid, in the QS mechanism of Candida albicans. We show that 3(R)-HTDE, a β-oxidation metabolite of endogenously present linoleic acid, accelerates cell morphogenesis in C. albicans, with alteration of gene expressions necessary for hyphal formation at right density of population utilizing aerobic pathway of endogenous lipid metabolism. We also explore the mechanistic underpinnings of the process where we are able to show that alteration of gene expressions are necessary for hyphal formation at the right population density which is achieved by the proper utilization of an aerobic pathway of endogenous lipid metabolism. In addition, we showed how this mediates biofilm formation itself, and the understanding of these mechanisms can be crucial in designing successful interventional strategies to combat Candida related infections.

Aspergillus fumigatus MedA governs adherence, host cell interactions and virulence

In medically important fungi, regulatory elements that control development and asexual reproduction often govern the expression of virulence traits. We therefore cloned the Aspergillus fumigatus developmental modifier MedA and characterized its role in conidiation, host cell interactions and virulence. As in the model organism Aspergillus nidulans, disruption of medA in A. fumigatus dramatically reduced conidiation. However, the conidiophore morphology was markedly different between the two species. Further, gene expression analysis suggested that MedA governs conidiation through different pathways in A. fumigatus compared with A. nidulans. The A. fumigatusDeltamedA strain was impaired in biofilm production and adherence to plastic, as well as adherence to pulmonary epithelial cells, endothelial cells and fibronectin in vitro. The DeltamedA strain also had reduced capacity to damage pulmonary epithelial cells, and stimulate pro-inflammatory cytokine mRNA and protein expression. Consistent with these results, the A. fumigatusDeltamedA strain also exhibited reduced virulence in both an invertebrate and a mammalian model of invasive aspergillosis. Collectively, these results suggest that the downstream targets of A. fumigatus MedA mediate virulence, and may provide novel therapeutic targets for invasive aspergillosis.

Antigenic and phenotypic variations in fungi

Mechanisms to vary the phenotypic characteristics of fungi are diverse and can be important for their life cycle. This review summarizes phenotypic variability in fungi and divides this phenomenon into three topics: (i) morphological transitions, which are environmentally induced and involve the entire fungal population, (ii) reversible phenotypic switching between different colony morphologies, which is restricted to a small fraction of the population, and (iii) antigenic variation of surface antigens, which can be immuno-dominant epitopes happens in individual fungal cells.

Trifluoromethanesulfonic acid-based proteomic analysis of cell wall and secreted proteins of the ascomycetous fungi Neurospora crassa and Candida albicans

Cell wall proteins from purified Candida albicans and Neurospora crassa cell walls were released using trifluoromethanesulfonic acid (TFMS) which cleaves the cell wall glucan/chitin matrix and deglycosylates the proteins. The cell wall proteins were then characterized by SDS-PAGE and identified by proteomic analysis. The analyses for C. albicans identified 15 cell wall proteins and six secreted proteins. For N. crassa, the analyses identified 26 cell wall proteins and nine secreted proteins. Most of the C. albicans cell wall proteins are found in the cell walls of both yeast and hyphae cells, but some cell type-specific cell wall proteins were observed. The analyses showed that the pattern of cell wall proteins present in N. crassa vegetative hyphae and conidia (asexual spores) are quite different. Almost all of the cell wall proteins identified in N. crassa have close homologs in the sequenced fungal genomes, suggesting that these proteins have important conserved functions within the cell wall.

The GPI-modified proteins Pga59 and Pga62 of Candida albicans are required for cell wall integrity

The fungal cell wall is essential in maintaining cellular integrity and plays key roles in the interplay between fungal pathogens and their hosts. The PGA59 and PGA62 genes encode two short and related glycosylphosphatidylinositol-anchored cell wall proteins and their expression has been previously shown to be strongly upregulated when the human pathogen Candida albicans grows as biofilms. Using GFP fusion proteins, we have shown that Pga59 and Pga62 are cell-wall-located, N- and O-glycosylated proteins. The characterization of C. albicans pga59Δ/pga59Δ, pga62Δ/pga62Δ and pga59Δ/pga59Δ pga62Δ/pga62Δ mutants suggested a minor role of these two proteins in hyphal morphogenesis and that they are not critical to biofilm formation. Importantly, the sensitivity to different cell-wall-perturbing agents was altered in these mutants. In particular, simultaneous inactivation of PGA59 and PGA62 resulted in high sensitivity to Calcofluor white, Congo red and nikkomicin Z and in resistance to caspofungin. Furthermore, cell wall composition and observation by transmission electron microscopy indicated an altered cell wall structure in the mutant strains. Collectively, these data suggest that the cell wall proteins Pga59 and Pga62 contribute to cell wall stability and structure.

Recognition of Candida albicans Als3 by the germ tube-specific monoclonal antibody 3D9.3

Monoclonal antibody 3D9.3 (MAb 3D9.3) reacts with the surface of Candida albicans germ tubes and recognizes a protein epitope. We used a two-step chromatography procedure to purify and identify the antigen (3D9) from C. albicans strain 66396 germ tubes. MAb 3D9.3 recognized two intense protein bands at 140 and 180 kDa. A comparative analysis between theoretical and experimental mass spectrum peaks showed that both bands corresponded to Als3. This conclusion was supported by lack of reactivity between MAb 3D9.3 and an als3Delta/als3Delta mutant strain, and the fact that an immunoglobulin preparation enriched for Als3 specificity recognized the purified 3D9 antigen. PCR demonstrated that C. albicans strain 66396 has two different-sized ALS3 alleles that correspond to the two purified protein bands. Strain- and species-specificity of the 3D9 epitope were studied with various C. albicans strains and Candida species, such as closely related Candida dubliniensis. The 3D9 epitope was detected only in C. albicans, demonstrating the utility of MAb 3D9.3 for differentiation between C. albicans and C. dubliniensis. Adhesion assays demonstrated that MAb 3D9.3 blocks adhesion of C. albicans germ tubes to human buccal epithelial cells and vascular endothelial cells.

the hyphal-associated adhesin and invasin Als3 of Candida albicans mediates iron acquisition from host ferritin

Iron sequestration by host iron-binding proteins is an important mechanism of resistance to microbial infections. Inside oral epithelial cells, iron is stored within ferritin, and is therefore not usually accessible to pathogenic microbes. We observed that the ferritin concentration within oral epithelial cells was directly related to their susceptibility to damage by the human pathogenic fungus, Candida albicans. Thus, we hypothesized that host ferritin is used as an iron source by this organism. We found that C. albicans was able to grow on agar at physiological pH with ferritin as the sole source of iron, while the baker's yeast Saccharomyces cerevisiae could not. A screen of C. albicans mutants lacking components of each of the three known iron acquisition systems revealed that only the reductive pathway is involved in iron utilization from ferritin by this fungus. Additionally, C. albicans hyphae, but not yeast cells, bound ferritin, and this binding was crucial for iron acquisition from ferritin. Transcriptional profiling of wild-type and hyphal-defective C. albicans strains suggested that the C. albicans invasin-like protein Als3 is required for ferritin binding. Hyphae of an Δals3 null mutant had a strongly reduced ability to bind ferritin and these mutant cells grew poorly on agar plates with ferritin as the sole source of iron. Heterologous expression of Als3, but not Als1 or Als5, two closely related members of the Als protein family, allowed S. cerevisiae to bind ferritin. Immunocytochemical localization of ferritin in epithelial cells infected with C. albicans showed ferritin surrounding invading hyphae of the wild-type, but not the Δals3 mutant strain. This mutant was also unable to damage epithelial cells in vitro. Therefore, C. albicans can exploit iron from ferritin via morphology dependent binding through Als3, suggesting that this single protein has multiple virulence attributes.

Iron is an essential nutrient for all microbes. Many human pathogenic microbes have developed sophisticated strategies to acquire iron from the host as most compartments in the body contain little free iron. For example, in oral epithelial cells intracellular iron is bound to ferritin, a protein that is highly resistant to microbial attack. In fact, no microorganism has so far been shown to directly exploit ferritin as an iron source during interaction with host cells. This study demonstrates that the pathogenic fungus Candida albicans can use ferritin as the sole source of iron. Most intriguingly, C. albicans binds ferritin via a receptor that is only exposed on invasive hyphae. This receptor is Als3, which is a member of the Als-protein family. Als3 was previously demonstrated to be an adhesin with invasin-like properties. Mutants lacking Als3 failed to bind ferritin, grew poorly with ferritin as an iron source and were unable to damage epithelial cells. Strains of the baker's yeast expressing C. albicans Als3, but not two closely related proteins, Als1 or Als5, were able to bind ferritin. Therefore, C. albicans uses an additional morphology specific and unique iron uptake strategy based on ferritin while invading into host cells where ferritin is located.

Efg1-mediated recruitment of NuA4 to promoters is required for hypha-specific Swi/Snf binding and activation in Candida albicans

Efg1 is essential for hyphal development and virulence in the human pathogenic fungus Candida albicans. How Efg1 regulates gene expression is unknown. Here, we show that Efg1 interacts with components of the nucleosome acetyltransferase of H4 (NuA4) histone acetyltransferase (HAT) complex in both yeast and hyphal cells. Deleting YNG2, a subunit of the NuA4 HAT module, results in a significant decrease in the acetylation level of nucleosomal H4 and a profound defect in hyphal development, as well as a defect in the expression of hypha-specific genes. Using chromatin immunoprecipitation, Efg1 and the NuA4 complex are found at the UAS regions of hypha-specific genes in both yeast and hyphal cells, and Efg1 is required for the recruitment of NuA4. Nucleosomal H4 acetylation at the promoters peaks during initial hyphal induction in an Efg1-dependent manner. We also find that Efg1 bound to the promoters of hypha-specific genes is critical for recruitment of the Swi/Snf chromatin remodeling complex during hyphal induction. Our data show that the recruitment of the NuA4 complex by Efg1 to the promoters of hypha-specific genes is required for nucleosomal H4 acetylation at the promoters during hyphal induction and for subsequent binding of Swi/Snf and transcriptional activation.

Niche-specific gene expression during C. albicans infection

Infection by the opportunistic pathogen Candida albicans may occur in virtually any organ of the human host. Studies of C. albicans gene expression during experimental infection reveal that different stress responses are mounted during different types of infection, presumably because different environments present different challenges. In addition, at least two mechanisms allow expression of common genes or activities in multiple sites within the host: differential expression of isozymes in a multigene family and regulation of a common set of genes by multiple transcription factors. Thus, analysis of C. albicans gene expression illuminates details of host-pathogen interactions and the differences between sites within the host.

The Yak1 kinase is involved in the initiation and maintenance of hyphal growth in Candida albicans

Members of the dual-specificity tyrosine-phosphorylated and regulated kinase (DYRK) family perform a variety of functions in eukaryotes. We used gene disruption, targeted pharmacologic inhibition, and genome-wide transcriptional profiling to dissect the function of the Yak1 DYRK in the human fungal pathogen Candida albicans. C. albicans strains with mutant yak1 alleles showed defects in the yeast-to-hypha transition and in maintaining hyphal growth. They also could not form biofilms. Despite their in vitro filamentation defect, C. albicans yak1Delta/yak1Delta mutants remained virulent in animal models of systemic and oropharyngeal candidiasis. Transcriptional profiling showed that Yak1 was necessary for the up-regulation of only a subset of hypha-induced genes. Although downstream targets of the Tec1 and Bcr1 transcription factors were down-regulated in the yak1Delta/yak1Delta mutant, TEC1 and BCR1 were not. Furthermore, 63% of Yak1-dependent, hypha-specific genes have been reported to be negatively regulated by the transcriptional repressor Tup1 and inactivation of TUP1 in the yak1Delta/yak1Delta mutant restored filamentation, suggesting that Yak1 may function upstream of Tup1 in governing hyphal emergence and maintenance.

State of differentiation defines buccal epithelial cell affinity for cross-linking to Candida albicans Hwp1

Candida albicans utilizes mammalian cell-associated transglutaminase (TGase) activity to adhere covalently to human buccal epithelial cells (BECs) through Hyphal Wall Protein 1. Little is known about the factors leading to the identity and appearance of Hwp1 binding partners on cells lining the oral cavity. The observation that BECs vary in their ability to attach to C. albicans germ tubes and to bind recombinant Hwp1 (rHwp1) suggested that differentiation may play a role in affinity for germ tube attachment. Individual BECs were characterized for differentiation status and rHwp1 binding. rHwp1 bound to the more terminally differentiated cells displaying SPR3 and keratin 13 but not to less differentiated cells with abundant involucrin. Sequential expression of involucrin followed by SPR3 in oral keratinocytes was demonstrated using stratified organotypic cultures and a feeder layer system with the OKF6/TERT-2 cell line. Increased cross-linking of the lysine analogue 5-(biotinamido)pentylamine to cultured OKF6/TERT-2 cell proteins accompanied this increased expression of SPR3. Western blot analysis demonstrated the presence of rHwp1 cross-links to proteins from BECs or from OKF6/TERT-2 cells that had been mechanically dislodged from culture dishes. Therefore, the differentiation of SPR3 positive from involucrin positive cells is correlated with the acquisition of affinity for cross-linking to rHwp1 and covalent adhesion of germ tubes to BECs.

Inhibition on Candida albicans biofilm formation using divalent cation chelators (EDTA)

Candida albicans can readily form biofilms on both inanimate and biological surfaces. In this study we investigated a means of inhibiting biofilm formation using EDTA (Ethylenediaminetetra-acetic acid), a divalent cation chelating agent, which has been shown to affect C. albicans filamentation. Candida albicans biofilms were formed in 96-well microtitre plates. Cells were allowed to adhere for 1, 2, and 4 h at 37 degrees C, washed in PBS, and then treated with different concentrations of EDTA (0, 2.5, 25, and 250 mM). EDTA was also added to the standardized suspension prior to adding to the microtiter plate and to a preformed 24 h biofilm. All plates were then incubated at 37 degrees C for an additional 24 h to allow for biofilm formation. The extent and characteristics of biofilm formation were then microscopically assessed and with a semi-quantitative colorimetric technique based on the use of an XTT-reduction assay. Northern blot analysis of the hyphal wall protein (HWP1) expression was also monitored in planktonic and biofilm cells treated with EDTA. Microscopic analysis and colorimetric readings revealed that filamentation and biofilm formation were inhibited by EDTA in a concentration dependent manner. However, preformed biofilms were minimally affected by EDTA (maximum of 31% reduction at 250 mM). The HWP1 gene expression was reduced in EDTA-treated planktonic and biofilm samples. These results indicate that EDTA inhibits C. albicans biofilm formation are most likely through its inhibitory effect on filamentation and indicates the potential therapeutic effects of EDTA. This compound may serve a non-toxic means of preventing biofilm formation on infections with a C. albicans biofilm etiology.

Molecular phylogenetics of ascomycotal adhesins--a novel family of putative cell-surface adhesive proteins in fission yeasts

In this work, we identify a family of putative adhesins in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. The members of this family share a conserved tandem repeat related to those found in the Candida albicans Als family of adhesins. Unlike previously characterised adhesins that possess conserved ligand-binding domains at the N-terminus, this group of proteins carry ligand-binding domains at their C-termini. We demonstrate that one such domain--the uncharacterised GLEYA domain, is related to the lectin-like ligand-binding domain found in the Saccharomyces cerevisiae Flo proteins. Unlike the Flo and Als proteins, the fission yeast adhesins do not contain detectable glycosyl phosphatidyl inositol (GPI) membrane anchor signals to mediate their attachment to the cell wall, which may suggest a novel cell wall attachment mechanism. Further sequence analysis identified several putative adhesins in the sub-phylum of Pezizomycotina, where only a few adhesins have been described to date.

In vivo and ex vivo comparative transcriptional profiling of invasive and non-invasive Candida albicans isolates identifies genes associated with tissue invasion

The human pathogenic fungus Candida albicans can cause a wide range of infections and invade multiple organs. To identify C. albicans genes that are expressed during invasion of the liver, we used genome-wide transcriptional profiling in vivo and ex vivo. By analysing the different phases of intraperitoneal infection from attachment to tissue penetration in a time-course experiment and by comparing the profiles of an invasive with those of a non-invasive strain, we identified genes and transcriptional pattern which are associated with the invasion process. This includes genes involved in metabolism, stress, and nutrient uptake, as well as transcriptional programmes regulating morphology and environmental sensing. One of the genes identified as associated with liver invasion was DFG16, a gene crucial for pH-dependent hyphal formation, correct pH sensing, invasion at physiological pH and systemic infection.

Tcc1p, a novel protein containing the tetratricopeptide repeat motif, interacts with Tup1p to regulate morphological transition and virulence in Candida albicans

The transcriptional factor CaTup1p represses many genes involved in intracellular processes, including the yeast-hypha transition, in the human fungal pathogen Candida albicans. Using tandem affinity purification technology, we identified a novel protein that interacts with CaTup1p, named Tcc1p (Tup1p complex component). Tcc1p is a C. albicans-specific protein with a 736-amino-acid polypeptide with four tetratricopeptide repeat (TPR) motifs in the N-terminal portion. Tcc1p formed a protein complex with CaTup1p via the TPR domain of Tcc1p, independently of CaSsn6p-CaTup1p The tcc1Delta disruptant showed filamentous growth under conditions inducing the yeast form, as is true of the Catup1Delta mutant. Consistent with this result, the common set of hypha-specific genes was negatively regulated by both TCC1 and CaTUP1. These observations will provide new insights into CaTup1p-dependent transcriptional gene regulation in C. albicans.

Repression of CDC28 reduces the expression of the morphology-related transcription factors, Efg1p, Nrg1p, Rbf1p, Rim101p, Fkh2p and Tec1p and induces cell elongation in Candida albicans

The ability of the human fungal pathogen Candida albicans to transit its cell shape is important for its pathogenicity. To obtain additional evidence that the cell cycle of C. albicans is associated with its morphology, we generated and characterized a conditional mutant of C. albicans CDC28, a cyclin-dependent kinase. In the constructed strain, the expression of CDC28 was regulated by the MET3 promoter and could be repressed in the presence of methionine and cysteine. Cdc28p-depleted cells demonstrated highly polarized growth and wider filaments than serum-induced hyphae. Hyphae-specific genes, HWP1, RBT4 and ECE1, were activated in the elongated filaments caused by the Cdc28p depletion. Furthermore, the protein expression levels of the transcription factors involved in morphological transition, Efg1p, Nrg1p, Rbf1p, Rim101p, Fkh2p and Tec1p, decreased under conditions that repress CDC28 expression. Taken together, these data indicate that repression of CDC28 affected the protein levels of the morphology-related transcription factors, the regulation of hyphae-specific genes and cell shape in C. albicans.

Visualizing fungal infections in living mice using bioluminescent pathogenic Candida albicans strains transformed with the firefly luciferase gene

Animal studies with Candida albicans have provided models for understanding fungal virulence and antifungal drug development. To non-invasively monitor long-term Candida murine infections, clinical isolates were stably transformed with a codon-optimized luciferase gene to constitutively express luciferase. Chronic systemic infections were established in mice with engineered strains, and bioluminescent signals were apparent from kidneys by non-invasive imaging using charged-coupled device cameras. These infections were established in immune-competent mice, and bioluminescence was detectable in animals that showed no physiological consequence of infection, as well as those visually succumbing to the disease. Similarly, bioluminescence was measured from the vaginal tissue of mice infected vaginally. Fungal loads determined by plating vaginal lavages showed a similar pattern to the bioluminescent signals measured, and fungal infection could be detected in animals for over 30 days post infection by both modalities. The effect of the antifungal drug miconazole was tested in this model, and clearance in animals was apparent by both direct imaging and fungal load determination. The use of bioluminescence to monitor these and other models of Candida infections will greatly speed up the analysis of drug development studies, both in ease of visualizing infections and decreasing numbers of animals required to run such studies.

Expression of firefly luciferase in Candida albicans and its use in the selection of stable transformants

The infectious yeast Candida albicans is a model organism for understanding the mechanisms of fungal pathogenicity. We describe the functional expression of the firefly luciferase gene, a reporter commonly used to tag genes in many other cellular systems. Due to a non-standard codon usage by this yeast, the CUG codons were first mutated to UUG to allow functional expression. When integrated into the chromosome of C. albicans with a strong constitutive promoter, cells bioluminesce when provided with luciferin substrate in their media. When fused to the inducible promoter from the HWP1 gene, expression and bioluminescence was only detected in cultures conditioning hyphal growth. We further used the luciferase gene as a selection to isolate transformed cell lines from clinical isolates of C. albicans, using a high-density screening strategy that purifies transformed colonies by virtue of light emission. This strategy requires no drug or auxotrophic selectable marker, and we were thus able to generate stable transformants of clinical isolates that are identical to the parental strain in all aspects tested, other than their bioluminescence. The firefly luciferase gene can, therefore, be used as a sensitive reporter to analyze gene function both in laboratory and clinical isolates of this medically important yeast.

Cellular and molecular biology of Candida albicans estrogen response

Candida albicans is the most common etiological agent of vaginal candidiasis. Elevated host estrogen levels and the incidence of vaginal candidiasis are positively associated. Elevated estrogen levels may affect host and/or fungal cells. This study investigates the effect of 17-beta-estradiol, 17-alpha-estradiol, ethynyl estradiol, and estriol on several C. albicans strains at concentrations ranging from 10(-5) to 10(-10) M. The addition of 17-beta-estradiol or ethynyl estradiol to C. albicans cells caused an increase in the number of cells forming germ tubes and an increase in germ tube length in a dose- and strain-dependent manner. The addition of 17-alpha-estradiol or estriol did not have a significant effect on germ tube formation by the cultured cells. Exposure to exogenous estrogens did not significantly change the biomass of any C. albicans culture tested. The transcriptional profile of estrogen-treated C. albicans cells showed increased expression of CDR1 and CDR2 across several strain-estrogen concentration-time point combinations, suggesting that these genes are the most responsive to estrogen exposure. Analysis of strain DSY654, which lacks the CDR1 and CDR2 coding sequences, showed a significantly decreased number of germ tube-forming cells in the presence of 17-beta-estradiol. PDR16 was the most highly up-regulated gene in strain DSY654 under these growth conditions. The cell biology and gene expression data from this study led to a model that proposes how components of the phospholipid and sterol metabolic pathways may interact to affect C. albicans germ tube formation and length.

A putative dual-specific protein phosphatase encoded by YVH1 controls growth, filamentation and virulence in Candida albicans

In response to stimulants, such as serum, the yeast cells of the opportunistic fungal pathogen Candida albicans form germ tubes, which develop into hyphae. Yvh1p, one of the 29 protein phosphatases encoded in the C. albicans genome, has 45% identity with the dual-specific phosphatase Yvh1p of the model yeast Saccharomyces cerevisiae. In this study, Yvh1p expression was not observed during the initial step of germ tube formation, although Yvh1p was expressed constitutively in cell cycle progression of yeast or hyphal cells. In an attempt to analyse the function of Yvh1p phosphatase, the complete ORFs of both alleles were deleted by replacement with hph200-URA3-hph200 and ARG4. Although YVH1 has nine single-nucleotide polymorphisms in its coding sequence, both YVH1 alleles were able to complement the YVH1 gene disruptant. The vegetative growth of Deltayvh1 was significantly slower than the wild-type. The hyphal growth of Deltayvh1 on agar, or in a liquid medium, was also slower than the wild-type because of the delay in nuclear division and septum formation, although germ tube formation was similar between the wild-type and the disruptant. Despite the slow hyphal growth, the expression of several hypha-specific genes in Deltayvh1 was not delayed or repressed compared with that of the wild-type. Infection studies using mouse models revealed that the virulence of Deltayvh1 was less than that of the wild-type. Thus, YVH1 contributes to normal vegetative yeast or hyphal cell cycle progression and pathogenicity, but not to germ tube formation.

Regulation of the Cdc42/Cdc24 GTPase module during Candida albicans hyphal growth

The Rho G protein Cdc42 and its exchange factor Cdc24 are required for hyphal growth of the human fungal pathogen Candida albicans. Previously, we reported that strains ectopically expressing Cdc24 or Cdc42 are unable to form hyphae in response to serum. Here we investigated the role of these two proteins in hyphal growth, using quantitative real-time PCR to measure induction of hypha-specific genes together with time lapse microscopy. Expression of the hypha-specific genes examined depends on the cyclic AMP-dependent protein kinase A pathway culminating in the Efg1 and Tec1 transcription factors. We show that strains with reduced levels of CDC24 or CDC42 transcripts induce hypha-specific genes yet cannot maintain their expression in response to serum. Furthermore, in serum these mutants form elongated buds compared to the wild type and mutant budding cells, as observed by time lapse microscopy. Using Cdc24 fused to green fluorescent protein, we also show that Cdc24 is recruited to and persists at the germ tube tip during hyphal growth. Altogether these data demonstrate that the Cdc24/Cdc42 GTPase module is required for maintenance of hyphal growth. In addition, overexpression studies indicate that specific levels of Cdc24 and Cdc42 are important for invasive hyphal growth. In response to serum, CDC24 transcript levels increase transiently in a Tec1-dependent fashion, as do the G-protein RHO3 and the Rho1 GTPase activating protein BEM2 transcript levels. These results suggest that a positive feedback loop between Cdc24 and Tec1 contributes to an increase in active Cdc42 at the tip of the germ tube which is important for hypha formation.

Regulation of cell-surface genes and biofilm formation by the C. albicans transcription factor Bcr1p

The impact of many microorganisms on their environment depends upon their ability to form surface bound communities called biofilms [1]. Biofilm formation on implanted medical devices has severe consequences for human health by providing both a portal of entry and a sanctuary for invasive bacterial and fungal pathogens [1 and 2]. Biofilm regulators and adherence molecules are extensively defined for many bacterial pathogens [3, 4, and 5], but not for fungal pathogens such as Candida albicans. Elongated filaments called hyphae are a prominent feature of C. albicans biofilms, and known genes that promote biofilm formation are required for hyphal development [2, 6, 7 and 8]. From a new library of transcription-factor mutants, we identify Bcr1p, a zinc finger protein required for formation of biofilms but not hyphae. Expression analysis shows that Bcr1p activates cell-surface protein and adhesin genes, including several induced during hyphal development. BCR1 expression depends upon the hyphal regulator Tec1p. Thus, BCR1 is a downstream component of the hyphal regulatory network that couples expression of cell-surface genes to hyphal differentiation. Our results indicate that hyphal cells are specialized to present adherence molecules that support biofilm integrity.

Comprehensive Proteomic Analysis of Saccharomyces cerevisiae Cell Walls

The cell wall of yeast contains proteins that are covalently bound to the glycan network. These cell wall proteins (CWPs) mediate cell-cell interactions and may be involved in cell wall biosynthesis. Using tandem mass spectrometry, we have identified 19 covalently bound CWPs of Saccharomyces cerevisiae. Twelve of them are shown for the first time to be covalently incorporated into the cell wall. The identified proteins include 12 predicted glycosylphosphatidylinositol-modified CWPs, all four members of the Pir protein family, and three additional proteins (Scw4p, Scw10p, and Tos1p) that are, like Pir proteins, connected to the cell wall glycan network via an alkali-sensitive linkage. However, Scw4p, Scw10p, and Tos1p do not contain internal repeat sequences shown to be essential for Pir protein incorporation and may represent a separate class of CWPs. Strikingly, seven of the identified proteins (Gas1p, Gas3p, Gas5p, Crh1p, Utr2p, Scw4p, and Scw10p) are classified as glycoside hydrolases. Phenotypic analysis of deletion mutants lacking the corresponding CWP-encoding genes indicated that most of them have altered cell wall properties, which reinforces the importance of the identified proteins for proper cell wall formation. In particular, gas1Delta and ecm33Delta were highly sensitive to Calcofluor White and high temperature, whereas gas1Delta, scw4Delta, and tos1Delta were highly resistant to incubation with beta-1,3-glucanase. The CWP identification method developed here relies on directly generating tryptic peptides from isolated cell walls and is independent of the nature of the covalent linkages between CWPs and cell wall glycans. Therefore, it will probably be equally effective in many other fungi.

Transcriptional profiling in Candida albicans reveals new adaptive responses to extracellular pH and functions for Rim101p

The human pathogen Candida albicans grows and colonizes sites that can vary markedly in pH. The pH response in C. albicans is governed in part by the Rim101p pathway. In Saccharomyces cerevisiae, Rim101p promotes alkaline responses by repressing expression of NRG1, itself a transcriptional repressor. Our studies reveal that in C. albicans, Rim101p-mediated alkaline adaptation is not through repression of CaNRG1. Furthermore, our studies suggest that Rim101p and Nrg1p act in parallel pathways to regulate hyphal morphogenesis, an important contributor to virulence. To determine the wild-type C. albicans transcriptional response to acidic and alkaline pH, we utilized microarrays and identified 514 pH-responsive genes. Of these, several genes involved in iron acquisition were upregulated at pH 8, suggesting that alkaline pH induces iron starvation. Microarray analysis of rim101-/- cells indicated that Rim101p does not govern transcriptional responses at acidic pH, but does regulate a subset of transcriptional responses at alkaline pH, including the iron acquisition genes. We found that rim101-/- cells are sensitive to iron starvation, which suggests that one important aspect of the Rim101p-dependent alkaline pH response is to adapt to iron starvation conditions.

Transcriptional response of Candida albicans upon internalization by macrophages

The opportunistic fungal pathogen Candida albicans is both a benign gut commensal and a frequently fatal systemic pathogen. The interaction of C. albicans with the host's innate immune system is the primary factor in this balance; defects in innate immunity predispose the patient to disseminated candidiasis. Because of the central importance of phagocytic cells in defense against fungal infections, we have investigated the response of C. albicans to phagocytosis by mammalian macrophages using genomic transcript profiling. This analysis reveals a dramatic reprogramming of transcription in C. albicans that occurs in two successive steps. In the early phase cells shift to a starvation mode, including gluconeogenic growth, activation of fatty acid degradation, and downregulation of translation. In a later phase, as hyphal growth enables C. albicans to escape from the macrophage, cells quickly resume glycolytic growth. In addition, there is a substantial nonmetabolic response imbedded in the early phase, including machinery for DNA damage repair, oxidative stress responses, peptide uptake systems, and arginine biosynthesis. Further, a surprising percentage of the genes that respond specifically to macrophage contact have no known homologs, suggesting that the organism has undergone substantial evolutionary adaptations to the commensal or pathogen lifestyle. This transcriptional reprogramming is almost wholly absent in the related, but nonpathogenic, yeast Saccharomyces cerevisiae, suggesting that these large-scale and coordinated changes contribute significantly to the ability of this organism to survive and cause disease in vivo.

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

By serving as a microbial substrate for epithelial cell transglutaminase, Hwp1 (Hyphal wall protein 1) of Candida albicans participates in cross-links with proteins on the mammalian mucosa. Biophysical properties of the 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 of cornified envelopes found in stratified squamous epithelia. Recombinant Hwp1 lacks alpha and beta structures by circular dichroism and likely exists as a disulfide-cross-linked coiled-coil. The transglutaminase substrate property prompted a unique approach for investigating the features of surface Hwp1 on germ tubes. A lysine analog, 5-(biotinamido)pentylamine, was cross-linked to germ tubes catalyzed by transglutaminase 2 prior to cell fractionation, immunoprecipitation, and detection with streptavidin conjugates. The majority of the transglutaminase-modifiable Hwp1 was covalently attached to the beta-glucan of hyphae by the C terminus of Hwp1 via a glycosylphosphatidylinositol remnant anchor. A putative precursor of cell wall forms of Hwp1 was identified in the cell extract and in the culture medium. Hwp1 was modified by relatively short N-linked glycans, and the molecular size of the protein was reduced by hypomannosylation when expressed in O-glycosylation mutant strains. 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 C. albicans.

Identification of potential cell-surface proteins inCandida albicansand investigation of the role of a putative cell-surface glycosidase in adhesion and virulence

Cell-surface proteins are attractive targets for the development of novel antifungals as they are more accessible to drugs than are intracellular targets. By using a computational biology approach, we identified 180 potential cell-surface proteins in Candida albicans, including the known cell-surface adhesin Als1 and other cell-surface antigens, such as Pra1 and Csa1. Six proteins (named Csf1-6 for cell-surface factors) were selected for further biological characterization. First, we verified that the selected CSF genes are expressed in the yeast and/or hyphal form and then we investigated the effect of the loss of each CSF gene on cell-wall integrity, filamentation, adhesion to mammalian cells and virulence. As a result, we identified Csf4, a putative glycosidase with an apparent orthologue in Saccharomyces cerevisiae (Utr2), as an important factor for cell-wall integrity and maintenance. Interestingly, deletion of CSF4 also resulted in a defect in filamentation, a reduction in adherence to mammalian cells in an in vitro adhesion assay, and a prolongation of survival in an immunocompetent mouse model of disseminated candidiasis. A delay in colonization of key organs (e.g. kidney) was also observed, which is consistent with a reduction in virulence of the csf4-deletion strain. These data indicate a key role for extracellular glycosidases in fungal pathogenesis and represent a new site for therapeutic intervention to cure and prevent fungal disease.

Use of green fluorescent protein fusions to analyse the N- and C-terminal signal peptides of GPI-anchored cell wall proteins in Candida albicans

Glycophosphatidylinositol (GPI)-anchored proteins account for 26-35% of the Candida albicans cell wall. To understand the signals that regulate these proteins' cell surface localization, green fluorescent protein (GFP) was fused to the N- and C-termini of the C. albicans cell wall proteins (CWPs) Hwp1p, Als3p and Rbt5p. C. albicans expressing all three fusion proteins were fluorescent at the cell surface. GFP was released from membrane fractions by PI-PLC and from cell walls by beta-glucanase, which implied that GFP was GPI-anchored to the plasma membrane and then covalently attached to cell wall glucans. Twenty and 25 amino acids, respectively, from the N- and C-termini of Hwp1p were sufficient to target GFP to the cell surface. C-terminal substitutions that are permitted by the omega rules (G613D, G613N, G613S, G613A, G615S) did not interfere with GFP localization, whereas some non-permitted substitutions (G613E, G613Q, G613R, G613T and G615Q) caused GFP to accumulate in intracellular ER-like structures and others (G615C, G613N/G615C and G613D/G615C) did not. These results imply that (i) GFP fusions can be used to analyse the N- and C-terminal signal peptides of GPI-anchored CWPs, (ii) the omega amino acid in Hwp1p is G613, and (iii) C can function at the omega+2 position in C. albicans GPI-anchored proteins.