Control of the C. albicans cell wall damage response by transcriptional regulator Cas5

Impact of the Environment upon the Candida albicans Cell Wall and Resultant Effects upon Immune Surveillance

The fungal cell wall is an essential organelle that maintains cellular morphology and protects the fungus from environmental insults. For fungal pathogens such as Candida albicans, it provides a degree of protection against attack by host immune defences. However, the cell wall also presents key epitopes that trigger host immunity and attractive targets for antifungal drugs. Rather than being a rigid shield, it has become clear that the fungal cell wall is an elastic organelle that permits rapid changes in cell volume and the transit of large liposomal particles such as extracellular vesicles. The fungal cell wall is also flexible in that it adapts to local environmental inputs, thereby enhancing the fitness of the fungus in these microenvironments. Recent evidence indicates that this cell wall adaptation affects host-fungus interactions by altering the exposure of major cell wall epitopes that are recognised by innate immune cells. Therefore, we discuss the impact of environmental adaptation upon fungal cell wall structure, and how this affects immune recognition, focussing on C. albicans and drawing parallels with other fungal pathogens.

Host-Pathogen Interactions Mediated by MDR Transporters in Fungi: As Pleiotropic as it Gets!

Fungal infections caused by Candida, Aspergillus, and Cryptococcus species are an increasing problem worldwide, associated with very high mortality rates. The successful prevalence of these human pathogens is due to their ability to thrive in stressful host niche colonization sites, to tolerate host immune system-induced stress, and to resist antifungal drugs. This review focuses on the key role played by multidrug resistance (MDR) transporters, belonging to the ATP-binding cassette (ABC), and the major facilitator superfamilies (MFS), in mediating fungal resistance to pathogenesis-related stresses. These clearly include the extrusion of antifungal drugs, with C. albicans CDR1 and MDR1 genes, and corresponding homologs in other fungal pathogens, playing a key role in this phenomenon. More recently, however, clues on the transcriptional regulation and physiological roles of MDR transporters, including the transport of lipids, ions, and small metabolites, have emerged, linking these transporters to important pathogenesis features, such as resistance to host niche environments, biofilm formation, immune system evasion, and virulence. The wider view of the activity of MDR transporters provided in this review highlights their relevance beyond drug resistance and the need to develop therapeutic strategies that successfully face the challenges posed by the pleiotropic nature of these transporters.

The Role of Candida albicans Transcription Factor RLM1 in Response to Carbon Adaptation

Candida albicans is the main causative agent of candidiasis and one of the most frequent causes of nosocomial infections worldwide. In order to establish an infection, this pathogen supports effective stress responses to counter host defenses and adapts to changes in the availability of important nutrients, such as alternative carbon sources. These stress responses have clear implications on the composition and structure of Candida cell wall. Therefore, we studied the impact of lactate, a physiologically relevant carbon source, on the activity of C. albicans RLM1 transcriptional factor. RLM1 is involved in the cell wall integrity pathway and plays an important role in regulating the flow of carbohydrates into cell wall biosynthesis pathways. The role of C. albicans RLM1 in response to lactate adaptation was assessed in respect to several virulence factors, such as the ability to grow under cell wall damaging agents, filament, adhere or form biofilm, as well as to immune recognition. The data showed that growth of C. albicans cells in the presence of lactate induces the secretion of tartaric acid, which has the potential to modulate the TCA cycle on both the yeast and the host cells. In addition, we found that adaptation of C. albicans cells to lactate reduces their internalization by immune cells and consequent % of killing, which could be correlated with a lower exposure of the cell wall β-glucans. In addition, absence of RLM1 has a minor impact on internalization, compared with the wild-type and complemented strains, but it reduces the higher efficiency of lactate grown cells at damaging phagocytic cells and induces a high amount of IL-10, rendering these cells more tolerable to the immune system. The data suggests that RLM1 mediates cell wall remodeling during carbon adaptation, impacting their interaction with immune cells.

Roles of VPH2 and VMA6 in localization of V-ATPase subunits, cell wall functions and filamentous development in Candida albicans

The vacuolar-type H⁺-ATPase (V-ATPase) is known to be associated with various cellular processes. Several V-ATPase subunits have been identified in C. albicans. However, there are still a few V-ATPase subunits and assembly factors that remain uncharacterized. In this study, we identified one of putative V-ATPase assembly factors, Vph2, and V₀ subunit, Vma6, and explored their potential functions in C. albicans. Our results revealed that Vph2 and Vma6 were required for the correct distribution of V₀ subunit Vph1 and V₁ subunit Tfp1. Furthermore, Vph2 and Vma6 played an important role in endocytosis and vacuolar acidification. Disruption of VPH2 or VMA6 affected cell wall stress resistance and composition, accompanying induction of cell wall integrity (CWI) pathway. Besides, deletion of VPH2 or VMA6 led to weakened hyphal development in Spider medium that was not dependent on Hog1 activation. Moreover, the vph2Δ/Δ and vma6Δ/Δ mutants displayed attenuated virulence in a mouse model of systemic candidiasis. Taken together, our data indicated that Vph2 and Vma6 were essential for the proper localization of V-ATPase subunits, cell wall functions, filamentous growth and C. albicans pathogenesis, and provided the potential to better exploit V-ATPase-related proteins as antifungal targets.

Deletion of ADA2 Increases Antifungal Drug Susceptibility and Virulence in Candida glabrata

Candida glabrata, the second most frequent cause of candidiasis after Candida albicans, is an emerging human fungal pathogen that is intrinsically drug tolerant. Currently, studies of C. glabrata genes involved in drug tolerance are limited. Ada2, a component serving as a transcription adaptor of the Spt-Ada-Gcn5 acetyltransferase (SAGA) complex, is required for antifungal drug tolerance and virulence in C. albicans However, its roles in C. glabrata remain elusive. In this study, we found that ada2 mutants demonstrated severe growth defects at 40°C but only mild defects at 37°C or 25°C. In addition, C. glabrata ada2 mutants exhibited pleiotropic phenotypes, including susceptibility to three classes of antifungal drugs (i.e., azoles, echinocandins, and polyenes) and cell wall-perturbing agents but resistance to the endoplasmic reticulum stressor tunicamycin. According to RNA sequence analysis, the expression of 43 genes was downregulated and the expression of 442 genes was upregulated in the ada2 mutant compared to their expression in the wild type. C. glabrata ADA2, along with its downstream target ERG6, controls antifungal drug tolerance and cell wall integrity. Surprisingly, ada2 mutants were hypervirulent in a murine model of systemic infection, possibly due to the upregulation of multiple adhesin-like genes, increased agar invasion, and overstimulation of murine tumor necrosis factor alpha production.

Yeast casein kinase 2 governs morphology, biofilm formation, cell wall integrity, and host cell damage of Candida albicans

The regulatory networks governing morphogenesis of a pleomorphic fungus, Candida albicans are extremely complex and remain to be completely elucidated. This study investigated the function of C. albicans yeast casein kinase 2 (CaYck2p). The yck2Δ/yck2Δ strain displayed constitutive pseudohyphae in both yeast and hyphal growth conditions, and formed enhanced biofilm under non-biofilm inducing condition. This finding was further supported by gene expression analysis of the yck2Δ/yck2Δ strain which showed significant upregulation of UME6, a key transcriptional regulator of hyphal transition and biofilm formation, and cell wall protein genes ALS3, HWP1, and SUN41, all of which are associated with morphogenesis and biofilm architecture. The yck2Δ/yck2Δ strain was hypersensitive to cell wall damaging agents and had increased compensatory chitin deposition in the cell wall accompanied by an upregulation of the expression of the chitin synthase genes, CHS2, CHS3, and CHS8. Absence of CaYck2p also affected fungal-host interaction; the yck2Δ/yck2Δ strain had significantly reduced ability to damage host cells. However, the yck2Δ/yck2Δ strain had wild-type susceptibility to cyclosporine and FK506, suggesting that CaYck2p functions independently from the Ca+/calcineurin pathway. Thus, in C. albicans, Yck2p is a multifunctional kinase that governs morphogenesis, biofilm formation, cell wall integrity, and host cell interactions.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

The Candida albicans transcription factor Cas5 couples stress responses, drug resistance and cell cycle regulation

The capacity to coordinate environmental sensing with initiation of cellular responses underpins microbial survival and is crucial for virulence and stress responses in microbial pathogens. Here we define circuitry that enables the fungal pathogen Candida albicans to couple cell cycle dynamics with responses to cell wall stress induced by echinocandins, a front-line class of antifungal drugs. We discover that the C. albicans transcription factor Cas5 is crucial for proper cell cycle dynamics and responses to echinocandins, which inhibit β-1,3-glucan synthesis. Cas5 has distinct transcriptional targets under basal and stress conditions, is activated by the phosphatase Glc7, and can regulate the expression of target genes in concert with the transcriptional regulators Swi4 and Swi6. Thus, we illuminate a mechanism of transcriptional control that couples cell wall integrity with cell cycle regulation, and uncover circuitry governing antifungal drug resistance.Cas5 is a transcriptional regulator of responses to cell wall stress in the fungal pathogen Candida albicans. Here, Xie et al. show that Cas5 also modulates cell cycle dynamics and responses to antifungal drugs.

The Fungal Cell Wall: Structure, Biosynthesis, and Function

The molecular composition of the cell wall is critical for the biology and ecology of each fungal species. Fungal walls are composed of matrix components that are embedded and linked to scaffolds of fibrous load-bearing polysaccharides. Most of the major cell wall components of fungal pathogens are not represented in humans, other mammals, or plants, and therefore the immune systems of animals and plants have evolved to recognize many of the conserved elements of fungal walls. For similar reasons the enzymes that assemble fungal cell wall components are excellent targets for antifungal chemotherapies and fungicides. However, for fungal pathogens, the cell wall is often disguised since key signature molecules for immune recognition are sometimes masked by immunologically inert molecules. Cell wall damage leads to the activation of sophisticated fail-safe mechanisms that shore up and repair walls to avoid catastrophic breaching of the integrity of the surface. The frontiers of research on fungal cell walls are moving from a descriptive phase defining the underlying genes and component parts of fungal walls to more dynamic analyses of how the various components are assembled, cross-linked, and modified in response to environmental signals. This review therefore discusses recent advances in research investigating the composition, synthesis, and regulation of cell walls and how the cell wall is targeted by immune recognition systems and the design of antifungal diagnostics and therapeutics.

Schizosaccharomyces pombe rsv1 Transcription Factor and its Putative Homologues Preserved their Functional Homology and are Evolutionarily Conserved

Environmental glucose is an important regulator of biological processes, as it can launch different cell processes depending on its concentration. Thus, low glucose concentration can induce entry into quiescence, which ensures long-term viability for the cells or in other cases mycelial growth in the dimorphic species, which, in turn, provides the cells with fresh nutrients. Several genes, such as the genes of cAMP cascade, are involved in glucose sensing and response. Since this signal transduction pathway seemed to be an evolutionarily conserved process, we assumed that its genes were also conserved and preserved their functional homology. To obtain evidence, Schizosaccharomyces pombe rsv1 and its orthologous genes were investigated using in silico and experimental approaches. Our results supported that the Rsv1 zinc-finger transcription factors of Schizosaccharomyces japonicus and Schizosaccharomyces octosporus and the Candida albicans cas5p were really functional homologues of the S. pombe Rsv1. Namely, the homologous proteins were able to restore mutant phenotype of the S. pombe rsv1-deleted cells. Bioinformatic anaysis revealed that the most conserved parts of the proteins always contained the C2H2 domains and the complementation abilities of the counterpart genes were not uniform regarding the investigated features, which can be in connection with the conserved regions outside C2H2.

Chromosome 5 of Human Pathogen Candida albicans Carries Multiple Genes for Negative Control of Caspofungin and Anidulafungin Susceptibility

Candida albicans is an important fungal pathogen with a diploid genome that can adapt to caspofungin, a major drug from the echinocandin class, by a reversible loss of one copy of chromosome 5 (Ch5). Here, we explore a hypothesis that more than one gene for negative regulation of echinocandin tolerance is carried on Ch5. We constructed C. albicans strains that each lacked one of the following Ch5 genes: CHT2 for chitinase, PGA4 for glucanosyltransferase, and CSU51, a putative transcription factor. We demonstrate that independent deletion of each of these genes increased tolerance for caspofungin and anidulafungin, another echinocandin. Our data indicate that Ch5 carries multiple genes for negative control of echinocandin tolerance, although the final number has yet to be established.

Endoplasmic reticulum-derived reactive oxygen species (ROS) is involved in toxicity of cell wall stress to Candida albicans

The cell wall is an important cell structure in both fungi and bacteria, and hence becomes a common antimicrobial target. The cell wall-perturbing agents disrupt synthesis and function of cell wall components, leading to cell wall stress and consequent cell death. However, little is known about the detailed mechanisms by which cell wall stress renders fungal cell death. In this study, we found that ROS scavengers drastically attenuated the antifungal effect of cell wall-perturbing agents to the model fungal pathogen Candida albicans, and these agents caused remarkable ROS accumulation and activation of oxidative stress response (OSR) in this fungus. Interestingly, cell wall stress did not cause mitochondrial dysfunction and elevation of mitochondrial superoxide levels. Furthermore, the iron chelator 2,2'-bipyridyl (BIP) and the hydroxyl radical scavengers could not attenuate cell wall stress-caused growth inhibition and ROS accumulation. However, cell wall stress up-regulated expression of unfold protein response (UPR) genes, enhanced protein secretion and promoted protein folding-related oxidation of Ero1, an important source of ROS production. These results indicated that oxidation of Ero1 in the endoplasmic reticulum (ER), rather than mitochondrial electron transport and Fenton reaction, contributed to cell wall stress-related ROS accumulation and consequent growth inhibition. Our findings uncover a novel link between cell wall integrity (CWI), ER function and ROS production in fungal cells, and shed novel light on development of strategies promoting the antifungal efficacy of cell wall-perturbing agents against fungal infections.

Genomic and functional analyses unveil the response to hyphal wall stress in Candida albicans cells lacking β(1,3)-glucan remodeling

Background

The cell wall is essential for the yeast to hypha (Y-H) transition that enables Candida albicans to invade human tissues and evade the immune system. The main constituent, β(1,3)-glucan, is remodeled by glucanosyltransferases of the GH72 family. Phr1p is responsible of glucan remodeling at neutral-alkaline pH and is essential for morphogenesis and virulence. Due to the pH-regulated expression of PHR1, the phr1Δ phenotype is manifested at pH > 6 and its severity increases with the rise in pH. We exploited the pH-conditional nature of a PHR1 null mutant to analyze the impact of glucan remodeling on the hyphal transcriptional program and the role of chitin synthases in the hyphal wall stress (HWS) response.

Results

In hyphal growth inducing conditions, phr1Δ germ tubes are defective in elongation, accumulate chitin, and constitutively activate the signaling pathways mediated by the MAP kinases Mkc1p, Cek1p and Hog1p. The transcriptional profiles revealed an increase of transcript levels for genes involved in cell wall formation (CHS2 and CHS8, CRH11, PGA23, orf19.750, RBR1, RBT4, ECM331, PGA6, PGA13), protein N-glycosylation and sorting in the ER (CWH8 and CHS7), signaling (CPP1, SSK2), ion transport (FLC2, YVC1), stress response and metabolism and a reduced expression of adhesins. A transient up-regulation of DNA replication genes associated with entry into S-phase occurred whereas cell-cycle regulating genes (PCL1, PCL2, CCN1, GIN4, DUN1, CDC28) were persistently up-regulated. To test the physiological relevance of altered CHS gene expression, phr1Δ chsxΔ (x = 2,3,8) mutant phenotypes were analyzed during the Y-H transition. PHR1 deletion was synthetic lethal with CHS3 loss on solid M199 medium-pH 7.5 and with CHS8 deletion on solid M199-pH 8. On Spider medium, PHR1 was synthetic lethal with CHS3 or CHS8 at pH 8.

Conclusions

The absence of Phr1p triggers an adaptive response aimed to reinforce the hyphal cell wall and restore homeostasis. Chs3p is essential in preserving phr1Δ cell integrity during the Y-H transition. Our findings also unveiled an unanticipated essential role of Chs8p during filamentation on solid media. These results highlight the flexibility of fungal cells in maintaining cell wall integrity and contribute to assessments of glucan remodeling as a target for therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2853-5) contains supplementary material, which is available to authorized users.

Highly Dynamic and Specific Phosphatidylinositol 4,5-Bisphosphate, Septin, and Cell Wall Integrity Pathway Responses Correlate with Caspofungin Activity against Candida albicans

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] activates the yeast cell wall integrity pathway. Candida albicans exposure to caspofungin results in the rapid redistribution of PI(4,5)P2 and septins to plasma membrane foci and subsequent fungicidal effects. We studied C. albicans PI(4,5)P2 and septin dynamics and protein kinase C (PKC)-Mkc1 cell wall integrity pathway activation following exposure to caspofungin and other drugs. PI(4,5)P2 and septins were visualized by live imaging of C. albicans cells coexpressing green fluorescent protein (GFP)-pleckstrin homology (PH) domain and red fluorescent protein-Cdc10p, respectively. PI(4,5)P2 was also visualized in GFP-PH domain-expressing C. albicans mkc1 mutants. Mkc1p phosphorylation was measured as a marker of PKC-Mkc1 pathway activation. Fungicidal activity was assessed using 20-h time-kill assays. Caspofungin immediately induced PI(4,5)P2 and Cdc10p colocalization to aberrant foci, a process that was highly dynamic over 3 h. PI(4,5)P2 levels increased in a dose-response manner at caspofungin concentrations of ≤4× MIC and progressively decreased at concentrations of ≥8× MIC. Caspofungin exposure resulted in broad-based mother-daughter bud necks and arrested septum-like structures, in which PI(4,5)P2 and Cdc10 colocalized. PKC-Mkc1 pathway activation was maximal within 10 min, peaked in response to caspofungin at 4× MIC, and declined at higher concentrations. The caspofungin-induced PI(4,5)P2 redistribution remained apparent in mkc1 mutants. Caspofungin exerted dose-dependent killing and paradoxical effects at ≤4× and ≥8× MIC, respectively. Fluconazole, amphotericin B, calcofluor white, and H2O2 did not impact the PI(4,5)P2 or Cdc10p distribution like caspofungin did. Caspofungin exerts rapid PI(4,5)P2-septin and PKC-Mkc1 responses that correlate with the extent of C. albicans killing, and the responses are not induced by other antifungal agents. PI(4,5)P2-septin regulation is crucial in early caspofungin responses and PKC-Mkc1 activation.

Budding off: bringing functional genomics to Candida albicans

Candida species are the most prevalent human fungal pathogens, with Candida albicans being the most clinically relevant species. Candida albicans resides as a commensal of the human gastrointestinal tract but is a frequent cause of opportunistic mucosal and systemic infections. Investigation of C. albicans virulence has traditionally relied on candidate gene approaches, but recent advances in functional genomics have now facilitated global, unbiased studies of gene function. Such studies include comparative genomics (both between and within Candida species), analysis of total RNA expression, and regulation and delineation of protein-DNA interactions. Additionally, large collections of mutant strains have begun to aid systematic screening of clinically relevant phenotypes. Here, we will highlight the development of functional genomics in C. albicans and discuss the use of these approaches to addressing both commensalism and pathogenesis in this species.

Hypoxia and Temperature Regulated Morphogenesis in Candida albicans

Candida albicans is a common commensal in the human gut but in predisposed patients it can become an important human fungal pathogen. As a commensal, C. albicans adapts to low-oxygen conditions and represses its hyphal development by the transcription factor Efg1, which under normoxia activates filamentation. The repressive hypoxic but not the normoxic function of Efg1 required its unmodified N-terminus, was prevented by phosphomimetic residues at normoxic phosphorylation sites T179 and T206 and occurred only at temperatures ≤35°C. Genome-wide binding sites for native Efg1 identified 300 hypoxia-specific target genes, which overlapped partially with hypoxic binding sites for Ace2, a known positive regulator of hypoxic filamentation. Transcriptional analyses revealed that EFG1, ACE2 and their identified targets BCR1 and BRG1 encode an interconnected regulatory hub, in which Efg1/Bcr1 act as negative and Ace2/Brg1 act as positive regulators of gene expression under hypoxia. In this circuit, the hypoxic function of Ace2 was stimulated by elevated CO2 levels. The hyperfilamentous phenotype of efg1 and bcr1 mutants depended on Ace2/Brg1 regulators and required increased expression of genes encoding Cek1 MAP kinase and its downstream target Cph1. The intricate temperature-dependent regulatory mechanisms under hypoxia suggest that C. albicans restricts hyphal morphogenesis in oxygen-poor body niches, possibly to persist as a commensal in the human host.

Activation and alliance of regulatory pathways in C. albicans during mammalian infection

Gene expression dynamics have provided foundational insight into almost all biological processes. Here, we analyze expression of environmentally responsive genes and transcription factor genes to infer signals and pathways that drive pathogen gene regulation during invasive Candida albicans infection of a mammalian host. Environmentally responsive gene expression shows that there are early and late phases of infection. The early phase includes induction of zinc and iron limitation genes, genes that respond to transcription factor Rim101, and genes characteristic of invasive hyphal cells. The late phase includes responses related to phagocytosis by macrophages. Transcription factor gene expression also reflects early and late phases. Transcription factor genes that are required for virulence or proliferation in vivo are enriched among highly expressed transcription factor genes. Mutants defective in six transcription factor genes, three previously studied in detail (Rim101, Efg1, Zap1) and three less extensively studied (Rob1, Rpn4, Sut1), are profiled during infection. Most of these mutants have distinct gene expression profiles during infection as compared to in vitro growth. Infection profiles suggest that Sut1 acts in the same pathway as Zap1, and we verify that functional relationship with the finding that overexpression of either ZAP1 or the Zap1-dependent zinc transporter gene ZRT2 restores pathogenicity to a sut1 mutant. Perturbation with the cell wall inhibitor caspofungin also has distinct gene expression impact in vivo and in vitro. Unexpectedly, caspofungin induces many of the same genes that are repressed early during infection, a phenomenon that we suggest may contribute to drug efficacy. The pathogen response circuitry is tailored uniquely during infection, with many relevant regulatory relationships that are not evident during growth in vitro. Our findings support the principle that virulence is a property that is manifested only in the distinct environment in which host–pathogen interaction occurs.

A study of the invasive infection of a mammalian host by the pathogenic fungus Candida albicans reveals characteristic gene regulation patterns in response to the host environment, distinct from those seen when growing in vitro.

We have a limited understanding of how the expression of pathogens’ genes changes during infection of humans or other animal hosts, in contrast to in vitro models of infection. Here we profile the alteration in gene expression over time as a predictor of functional consequences during invasive growth of Candida in the kidney; a situation in which the limited number of pathogen cells makes gene expression challenging to assay. Our findings reveal that there are distinct early and late phases of infection, and identify new genes that govern the early zinc acquisition response necessary for proliferation in vivo—and thus required for infection. We also find that the response to drug treatment that manifests during infection can be distinct from that detected in vitro. We show that a well-known gene expression response to the antifungal drug caspofungin is naturally down-regulated in infecting cells, suggesting that the efficacy of the drug may be enhanced by a susceptible state of the pathogen during invasive proliferation.

Disruption of the transcriptional regulator Cas5 results in enhanced killing of Candida albicans by Fluconazole

Azole antifungal agents such as fluconazole exhibit fungistatic activity against Candida albicans. Strategies to enhance azole antifungal activity would be therapeutically appealing. In an effort to identify transcriptional pathways that influence the killing activity of fluconazole, we sought to identify transcription factors (TFs) involved in this process. From a collection of C. albicans strains disrupted for genes encoding TFs (O. R. Homann, J. Dea, S. M. Noble, and A. D. Johnson, PLoS Genet. 5:e1000783, 2009, http://dx.doi.org/10.1371/journal.pgen.1000783), four strains exhibited marked reductions in minimum fungicidal concentration (MFCs) in both RPMI and yeast extract-peptone-dextrose (YPD) media. One of these genes, UPC2, was previously characterized with regard to its role in azole susceptibility. Of mutants representing the three remaining TF genes of interest, one (CAS5) was unable to recover from fluconazole exposure at concentrations as low as 2 μg/ml after 72 h in YPD medium. This mutant also showed reduced susceptibility and a clear zone of inhibition by Etest, was unable to grow on solid medium containing 10 μg/ml fluconazole, and exhibited increased susceptibility by time-kill analysis. CAS5 disruption in highly azole-resistant clinical isolates exhibiting multiple resistance mechanisms did not alter susceptibility. However, CAS5 disruption in strains with specific resistance mutations resulted in moderate reductions in MICs and MFCs. Genome-wide transcriptional analysis was performed in the presence of fluconazole and was consistent with the suggested role of CAS5 in cell wall organization while also suggesting a role in iron transport and homeostasis. These findings suggest that Cas5 regulates a transcriptional network that influences the response of C. albicans to fluconazole. Further delineation of this transcriptional network may identify targets for potential cotherapeutic strategies to enhance the activity of the azole class of antifungals.

A novel role of the ferric reductase Cfl1 in cell wall integrity, mitochondrial function, and invasion to host cells in Candida albicans

Candida albicans is an important opportunistic pathogen, causing both superficial mucosal infections and life-threatening systemic diseases. Iron acquisition is an important factor for pathogen-host interaction and also a significant element for the pathogenicity of this organism. Ferric reductases, which convert ferric iron into ferrous iron, are important components of the high-affinity iron uptake system. Sequence analyses have identified at least 17 putative ferric reductase genes in C. albicans genome. CFL1 was the first ferric reductase identified in C. albicans. However, little is known about its roles in C. albicans physiology and pathogenicity. In this study, we found that disruption of CFL1 led to hypersensitivity to chemical and physical cell wall stresses, activation of the cell wall integrity (CWI) pathway, abnormal cell wall composition, and enhanced secretion, indicating a defect in CWI in this mutant. Moreover, this mutant showed abnormal mitochondrial activity and morphology, suggesting a link between ferric reductases and mitochondrial function. In addition, this mutant displayed decreased ability of adhesion to both the polystyrene microplates and buccal epithelial cells and invasion of host epithelial cells. These findings revealed a novel role of C. albicans Cfl1 in maintenance of CWI, mitochondrial function, and interaction between this pathogen and the host.

Participation of Candida albicans transcription factor RLM1 in cell wall biogenesis and virulence

Candida albicans cell wall is important for growth and interaction with the environment. RLM1 is one of the putative transcription factors involved in the cell wall integrity pathway, which plays an important role in the maintenance of the cell wall integrity. In this work we investigated the involvement of RLM1 in the cell wall biogenesis and in virulence. Newly constructed C. albicans Δ/Δrlm1 mutants showed typical cell wall weakening phenotypes, such as hypersensitivity to Congo Red, Calcofluor White, and caspofungin (phenotype reverted in the presence of sorbitol), confirming the involvement of RLM1 in the cell wall integrity. Additionally, the cell wall of C. albicans Δ/Δrlm1 showed a significant increase in chitin (213%) and reduction in mannans (60%), in comparison with the wild-type, results that are consistent with cell wall remodelling. Microarray analysis in the absence of any stress showed that deletion of RLM1 in C. albicans significantly down-regulated genes involved in carbohydrate catabolism such as DAK2, GLK4, NHT1 and TPS1, up-regulated genes involved in the utilization of alternative carbon sources, like AGP2, SOU1, SAP6, CIT1 or GAL4, and genes involved in cell adhesion like ECE1, ALS1, ALS3, HWP1 or RBT1. In agreement with the microarray results adhesion assays showed an increased amount of adhering cells and total biomass in the mutant strain, in comparison with the wild-type. C. albicans mutant Δ/Δrlm1 strain was also found to be less virulent than the wild-type and complemented strains in the murine model of disseminated candidiasis. Overall, we showed that in the absence of RLM1 the modifications in the cell wall composition alter yeast interaction with the environment, with consequences in adhesion ability and virulence. The gene expression findings suggest that this gene participates in the cell wall biogenesis, with the mutant rearranging its metabolic pathways to allow the use of alternative carbon sources.

Sap6, a secreted aspartyl proteinase, participates in maintenance the cell surface integrity of Candida albicans

Background

The polymorphic species Candida albicans is the major cause of candidiasis in humans. The secreted aspartyl proteinases (Saps) of C. albicans, encoded by a family of 10 SAP genes, have been investigated as the virulent factors during candidiasis. However, the biological functions of most Sap proteins are still uncertain. In this study, we applied co-culture system of C. albicans and THP-1 human monocytes to explore the pathogenic roles and biological functions of Sap proteinases.

Results

After 1 hr of co-culture of C. albicans strains and THP-1 human monocytes at 37°C, more than 60% of the THP-1-engulfed wild type and Δsap5 Candida cells were developing long hyphae. However, about 50% of THP-1-engulfed Δsap6 Candida cells were generating short hyphae, and more dead Candida cells were found in Δsap6 strain that was ingested by THP-1 cells (about 15% in Δsap6 strain vs. 2 ~ 2.5% in SC5314 and Δsap5 strains). The immunofluorescence staining demonstrated that the Sap6 is the major hyphal tip located Sap protein under THP-1 phagocytosis. The sap6-deleted strains (Δsap6, Δsap4/6, and Δsap5/6) appeared slower growth on Congo red containing solid medium at 25°C, and the growth defect was exacerbated when cultured at 37°C in Congo red or SDS containing medium. In addition, more proteins were secreted from Δsap6 strain and the β-mercaptoethanol (β-ME) extractable surface proteins from Δsap6 mutant were more abundant than that of extracted from wild type strain, which included the plasma membrane protein (Pma1p), the ER-chaperone protein (Kar2p), the protein transport-related protein (Arf1p), the cytoskeleton protein (Act1), and the mitochondrial outer membrane protein (porin 1). Moreover, the cell surface accessibility was increased in sap6-deleted strains.

Conclusion

From these results, we speculated that the cell surface constitution of C. albicans Δsap6 strain was defect. This may cause the more accessible of β-ME to disulfide-bridged cell surface components and may weaken the resistance of Δsap6 strain encountering phagocytosis of THP-1 cells. Sap6 protein displays a significant function involving in maintenance the cell surface integrity.

Candida albicans Czf1 and Efg1 coordinate the response to farnesol during quorum sensing, white-opaque thermal dimorphism, and cell death

Quorum sensing by farnesol in Candida albicans inhibits filamentation and may be directly related to its ability to cause both mucosal and systemic diseases. The Ras1-cyclic AMP signaling pathway is a target for farnesol inhibition. However, a clear understanding of the downstream effectors of the morphological farnesol response has yet to be unraveled. To address this issue, we screened a library for mutants that fail to respond to farnesol. Six mutants were identified, and the czf1Δ/czf1Δ mutant was selected for further characterization. Czf1 is a transcription factor that regulates filamentation in embedded agar and also white-to-opaque switching. We found that Czf1 is required for filament inhibition by farnesol under at least three distinct environmental conditions: on agar surfaces, in liquid medium, and when embedded in a semisolid agar matrix. Since Efg1 is a transcription factor of the Ras1-cyclic AMP signaling pathway that interacts with and regulates Czf1, an efg1Δ/efg1Δ czf1Δ/czf1Δ mutant was tested for filament inhibition by farnesol. It exhibited an opaque-cell-like temperature-dependent morphology, and it was killed by low farnesol levels that are sublethal to wild-type cells and both efg1Δ/efg1Δ and czf1Δ/czf1Δ single mutants. These results highlight a new role for Czf1 as a downstream effector of the morphological response to farnesol, and along with Efg1, Czf1 is involved in the control of farnesol-mediated cell death in C. albicans.

The P-type ATPase Spf1 is required for endoplasmic reticulum functions and cell wall integrity in Candida albicans

Endoplasmic reticulum (ER) is crucial for protein folding, glycosylation and secretion in eukaryotic organisms. These important functions are supported by high levels of Ca(2+) in the ER. We have recently identified a putative ER Ca(2+) pump in Candida albicans, called Spf1, which plays key roles in maintenance of cellular Ca(2+) homeostasis, morphogenesis and virulence. In this study, we purified Spf1 and confirmed that it is a P-type ATPase, suggesting its role in maintaining high levels of ER Ca(2+). Disruption of SPF1 caused severe defects in glycosylation of the ER-localized protein Cdc101 and secretory acid phosphatase, and a decrease in expression of SEC61 which encodes an important ER protein. Moreover, the spf1Δ/Δ mutant showed increased sensitivity to cell wall stresses, abnormal cell wall composition, delayed cell wall reconstruction and decreased flocculation and adherence, indicating its defect in cell wall integrity (CWI). We also revealed that disruption of SPF1 has an impact on gene expression related to CWI and morphogenesis. This study provides evidence that Spf1, as a P-type ATPase, is essential for ER functions and consequent CWI, implicating a role of ER Ca(2+) homeostasis in C. albicans physiology.

Chitin and glucan, the yin and yang of the fungal cell wall, implications for antifungal drug discovery and therapy

The structural carbohydrate polymers glucan and chitin compliment and reinforce each other in a dynamic process to maintain the integrity and physical strength of the fungal cell wall. The assembly of chitin and glucan in the cell wall of the budding yeast Saccharomyces cerevisiae and the polymorphic human pathogen Candida albicans are essential processes that involve a range of fungal-specific enzymes and regulatory networks. The fungal cell wall is, therefore, an attractive target for novel therapies as host cells lack many cell wall-related proteins. The most recent class of antifungal drug approved for clinical use, the echinocandins, targets the synthesis of cell wall β(1-3)glucan. The echinocandins are effective at treating invasive and bloodstream Candida infections and are now widely used in the clinic. However, there have been sporadic reports of breakthrough infections in patients undergoing echinocandin therapy. The acquisition of point mutations in the FKS genes that encode the catalytic β(1-3)glucan synthase subunits, the target of the echinocandins, has emerged as a dominant resistance mechanism. Cells with elevated chitin levels are also less susceptible to echinocandins and in addition, treatment with sub-MIC echinocandin activates cell wall salvage pathways that increase chitin synthesis to compensate for reduced glucan production. The development of drugs targeting the cell wall has already proven to be beneficial in providing an alternative class of drug for use in the clinic. Other cell wall targets such as chitin synthesis still hold great potential for drug development but careful consideration should be given to the capacity of fungi to manipulate their walls in a dynamic response to cell wall perturbations.

Hsp90 orchestrates transcriptional regulation by Hsf1 and cell wall remodelling by MAPK signalling during thermal adaptation in a pathogenic yeast

Thermal adaptation is essential in all organisms. In yeasts, the heat shock response is commanded by the heat shock transcription factor Hsf1. Here we have integrated unbiased genetic screens with directed molecular dissection to demonstrate that multiple signalling cascades contribute to thermal adaptation in the pathogenic yeast Candida albicans. We show that the molecular chaperone heat shock protein 90 (Hsp90) interacts with and down-regulates Hsf1 thereby modulating short term thermal adaptation. In the longer term, thermal adaptation depends on key MAP kinase signalling pathways that are associated with cell wall remodelling: the Hog1, Mkc1 and Cek1 pathways. We demonstrate that these pathways are differentially activated and display cross talk during heat shock. As a result ambient temperature significantly affects the resistance of C. albicans cells to cell wall stresses (Calcofluor White and Congo Red), but not osmotic stress (NaCl). We also show that the inactivation of MAP kinase signalling disrupts this cross talk between thermal and cell wall adaptation. Critically, Hsp90 coordinates this cross talk. Genetic and pharmacological inhibition of Hsp90 disrupts the Hsf1-Hsp90 regulatory circuit thereby disturbing HSP gene regulation and reducing the resistance of C. albicans to proteotoxic stresses. Hsp90 depletion also affects cell wall biogenesis by impairing the activation of its client proteins Mkc1 and Hog1, as well as Cek1, which we implicate as a new Hsp90 client in this study. Therefore Hsp90 modulates the short term Hsf1-mediated activation of the classic heat shock response, coordinating this response with long term thermal adaptation via Mkc1- Hog1- and Cek1-mediated cell wall remodelling.

Candida albicans is one of the most persistent yeast pathogens known to man, causing frequent mucosal infections (thrush) in otherwise healthy individuals, and potentially fatal bloodstream infections in immunocompromised patients. C. albicans colonises warm-blooded animals and occupies thermally buffered niches. Yet during its evolution this pathogen has retained the classic heat shock response whilst other stress responses have diverged significantly. We have established that the essential, evolutionarily conserved molecular chaperone, Hsp90, coordinates thermal adaptation. Hsp90 interacts with and modulates the activity of the heat shock transcription factor, Hsf1, thereby controlling the expression of heat shock proteins required for the clearance of proteins damaged by proteotoxic stresses. In addition, Hsp90 modulates the activities of key MAP kinase signalling pathways that mediate cell wall remodelling and long term adaptation to heat shock. Loss of any of these factors results in a significant reduction in thermotolerance.

Surface stress induces a conserved cell wall stress response in the pathogenic fungus Candida albicans

The human fungal pathogen Candida albicans can grow at temperatures of up to 45°C. Here, we show that at 42°C substantially less biomass was formed than at 37°C. The cells also became more sensitive to wall-perturbing compounds, and the wall chitin levels increased, changes that are indicative of wall stress. Quantitative mass spectrometry of the wall proteome using (15)N metabolically labeled wall proteins as internal standards revealed that at 42°C the levels of the β-glucan transglycosylases Phr1 and Phr2, the predicted chitin transglycosylases Crh11 and Utr2, and the wall maintenance protein Ecm33 increased. Consistent with our previous results for fluconazole stress, this suggests that a wall-remodeling response is mounted to relieve wall stress. Thermal stress as well as different wall and membrane stressors led to an increased phosphorylation of the mitogen-activated protein (MAP) kinase Mkc1, suggesting activation of the cell wall integrity (CWI) pathway. Furthermore, all wall and membrane stresses tested resulted in diminished cell separation. This was accompanied by decreased secretion of the major chitinase Cht3 and the endoglucanase Eng1 into the medium. Consistent with this, cht3 cells showed a similar phenotype. When treated with exogenous chitinase, cell clusters both from stressed cells and mutant strains were dispersed, underlining the importance of Cht3 for cell separation. We propose that surface stresses lead to a conserved cell wall remodeling response that is mainly governed by Mkc1 and is characterized by chitin reinforcement of the wall and the expression of remedial wall remodeling enzymes.

A novel role for the transcription factor Cwt1p as a negative regulator of nitrosative stress in Candida albicans

The ability of Candida albicans to survive in the presence of nitrosative stress during the initial contact with the host immune system is crucial for its ability to colonize mammalian hosts. Thus, this fungus must activate robust mechanisms to neutralize and repair nitrosative-induced damage. Until now, very little was known regarding the regulatory circuits associated with reactive nitrogen species detoxification in fungi. To gain insight into the transcriptional regulatory networks controlling nitrosative stress response (NRS) in C. albicans a compilation of transcriptional regulator-defective mutants were screened. This led to the identification of Cwt1p as a negative regulator of NSR. By combining genome-wide location and expression analyses, we have characterized the Cwt1p regulon and demonstrated that Cwt1p is directly required for proper repression of the flavohemoglobin Yhb1p, a key NO-detoxification enzyme. Furthermore, Cwt1p operates both by activating and repressing genes of specific functions solicited upon NSR. Additionally, we used Gene Set Enrichment Analysis to reinvestigate the C. albicans NSR-transcriptome and demonstrate a significant similarity with the transcriptional profiles of C. albicans interacting with phagocytic host-cells. In summary, we have characterized a novel negative regulator of NSR and bring new insights into the transcriptional regulatory network governing fungal NSR.

Functional control of the Candida albicans cell wall by catalytic protein kinase A subunit Tpk1

The cyclic AMP protein kinase A pathway governs numerous biological features of the fungal pathogen Candida albicans. The catalytic protein kinase A subunits, Tpk1 (orf19.4892) and Tpk2 (orf19.2277), have divergent roles, and most studies indicate a more pronounced role for Tpk2. Here we dissect two Tpk1-responsive properties: adherence and cell wall integrity. Homozygous tpk1/tpk1 mutants are hyperadherent, and a Tpk1 defect enables biofilm formation in the absence of Bcr1, a transcriptional regulator of biofilm adhesins. A quantitative gene expression-based assay reveals that tpk1/tpk1 and bcr1/bcr1 genotypes show mixed epistasis, as expected if Tpk1 and Bcr1 act mainly in distinct pathways. Overexpression of individual Tpk1-repressed genes indicates that cell surface proteins Als1, Als2, Als4, Csh1 and Csp37 contribute to Tpk1-regulated adherence. Tpk1 is also required for cell wall integrity, but has no role in the gene expression response to cell wall inhibition by caspofungin. Interestingly, increased expression of the adhesin gene ALS2 confers a cell wall defect, as manifested in hypersensitivity to the cell wall inhibitor caspofungin and a shallow cell wall structure. Our findings indicate that Tpk1 governs C. albicans cell wall properties through repression of select cell surface protein genes.

Fungal morphogenesis: in hot pursuit

Temperature affects diverse biological processes. In fungi such as the pathogen Candida albicans, temperature governs a morphogenetic switch between yeast and hyphal growth. A new report connects the thermosensor Hsp90 to a CDK-cyclin-transcription factor module that controls morphogenesis.

A Candida biofilm-induced pathway for matrix glucan delivery: implications for drug resistance

Extracellular polysaccharides are key constituents of the biofilm matrix of many microorganisms. One critical carbohydrate component of Candida albicans biofilms, β-1,3 glucan, has been linked to biofilm protection from antifungal agents. In this study, we identify three glucan modification enzymes that function to deliver glucan from the cell to the extracellular matrix. These enzymes include two predicted glucan transferases and an exo-glucanase, encoded by BGL2, PHR1, and XOG1, respectively. We show that the enzymes are crucial for both delivery of β-1,3 glucan to the biofilm matrix and for accumulation of mature matrix biomass. The enzymes do not appear to impact cell wall glucan content of biofilm cells, nor are they necessary for filamentation or biofilm formation. We demonstrate that mutants lacking these genes exhibit enhanced susceptibility to the commonly used antifungal, fluconazole, during biofilm growth only. Transcriptional analysis and biofilm phenotypes of strains with multiple mutations suggest that these enzymes act in a complementary fashion to distribute matrix downstream of the primary β-1,3 glucan synthase encoded by FKS1. Furthermore, our observations suggest that this matrix delivery pathway works independently from the C. albicans ZAP1 matrix formation regulatory pathway. These glucan modification enzymes appear to play a biofilm-specific role in mediating the delivery and organization of mature biofilm matrix. We propose that the discovery of inhibitors for these enzymes would provide promising anti-biofilm therapeutics.

Rapid redistribution of phosphatidylinositol-(4,5)-bisphosphate and septins during the Candida albicans response to caspofungin

We previously showed that phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P2] and septin regulation play major roles in maintaining Candida albicans cell wall integrity in response to caspofungin and other stressors. Here, we establish a link between PI(4,5)P2 signaling and septin localization and demonstrate that rapid redistribution of PI(4,5)P2 and septins is part of the natural response of C. albicans to caspofungin. First, we studied caspofungin-hypersusceptible C. albicans irs4 and inp51 mutants, which have elevated PI(4,5)P2 levels due to loss of PI(4,5)P2-specific 5'-phosphatase activity. PI(4,5)P2 accumulated in discrete patches, rather than uniformly, along surfaces of mutants in yeast and filamentous morphologies, as visualized with a green fluorescent protein (GFP)-pleckstrin homology domain. The patches also contained chitin (calcofluor white staining) and cell wall protein Rbt5 (Rbt5-GFP). By transmission electron microscopy, patches corresponded to plasma membrane invaginations that incorporated cell wall material. Fluorescently tagged septins Cdc10 and Sep7 colocalized to these sites, consistent with well-described PI(4,5)P2-septin physical interactions. Based on expression patterns of cell wall damage response genes, irs4 and inp51 mutants were firmly positioned within a group of caspofungin-hypersusceptible, septin-regulatory protein kinase mutants. irs4 and inp51 were linked most closely to the gin4 mutant by expression profiling, PI(4,5)P2-septin-chitin redistribution and other phenotypes. Finally, sublethal 5-min exposure of wild-type C. albicans to caspofungin resulted in redistribution of PI(4,5)P2 and septins in a manner similar to those of irs4, inp51, and gin4 mutants. Taken together, our data suggest that the C. albicans Irs4-Inp51 5'-phosphatase complex and Gin4 function upstream of PI(4,5)P2 and septins in a pathway that helps govern responses to caspofungin.

Identification and functional characterization of Rca1, a transcription factor involved in both antifungal susceptibility and host response in Candida albicans

The identification of novel transcription factors associated with antifungal response may allow the discovery of fungus-specific targets for new therapeutic strategies. A collection of 241 Candida albicans transcriptional regulator mutants was screened for altered susceptibility to fluconazole, caspofungin, amphotericin B, and 5-fluorocytosine. Thirteen of these mutants not yet identified in terms of their role in antifungal response were further investigated, and the function of one of them, a mutant of orf19.6102 (RCA1), was characterized by transcriptome analysis. Strand-specific RNA sequencing and phenotypic tests assigned Rca1 as the regulator of hyphal formation through the cyclic AMP/protein kinase A (cAMP/PKA) signaling pathway and the transcription factor Efg1, but also probably through its interaction with a transcriptional repressor, most likely Tup1. The mechanisms responsible for the high level of resistance to caspofungin and fluconazole observed resulting from RCA1 deletion were investigated. From our observations, we propose that caspofungin resistance was the consequence of the deregulation of cell wall gene expression and that fluconazole resistance was linked to the modulation of the cAMP/PKA signaling pathway activity. In conclusion, our large-scale screening of a C. albicans transcription factor mutant collection allowed the identification of new effectors of the response to antifungals. The functional characterization of Rca1 assigned this transcription factor and its downstream targets as promising candidates for the development of new therapeutic strategies, as Rca1 influences host sensing, hyphal development, and antifungal response.

Divergent targets of Candida albicans biofilm regulator Bcr1 in vitro and in vivo

Candida albicans is a causative agent of oropharyngeal candidiasis (OPC), a biofilm-like infection of the oral mucosa. Biofilm formation depends upon the C. albicans transcription factor Bcr1, and previous studies indicate that Bcr1 is required for OPC in a mouse model of infection. Here we have used a nanoString gene expression measurement platform to elucidate the role of Bcr1 in OPC-related gene expression. We chose for assays a panel of 134 genes that represent a range of morphogenetic and cell cycle functions as well as environmental and stress response pathways. We assayed gene expression in whole infected tongue samples. The results sketch a portrait of C. albicans gene expression in which numerous stress response pathways are activated during OPC. This one set of experiments identifies 64 new genes with significantly altered RNA levels during OPC, thus increasing substantially the number of known genes in this expression class. The bcr1Δ/Δ mutant had a much more limited gene expression defect during OPC infection than previously reported for in vitro growth conditions. Among major functional Bcr1 targets, we observed that ALS3 was Bcr1 dependent in vivo while HWP1 was not. We used null mutants and complemented strains to verify that Bcr1 and Hwp1 are required for OPC infection in this model. The role of Als3 is transient and mild, though significant. Our findings suggest that the versatility of C. albicans as a pathogen may reflect its ability to persist in the face of multiple stresses and underscore that transcriptional circuitry during infection may be distinct from that detailed during in vitro growth.

Investigating the function of Ddr48p in Candida albicans

Candidiasis now represents the fourth most frequent nosocomial infection both in the United States and worldwide. Candida albicans is an increasingly common threat to human health as a consequence of AIDS, steroid therapy, organ and tissue transplantation, cancer therapy, broad-spectrum antibiotics, and other immune defects. The pathogenic potential of C. albicans is intimately related to certain key processes, including biofilm formation and filamentation. Ddr48p is a damage response protein that is significantly upregulated during both biofilm formation and filamentation, but its actual function is unknown. Previous studies have indicated that this protein may be essential in C. albicans but not Saccharomyces cerevisiae. Here we examined the function of Ddr48p and investigated the role of this protein in biofilm formation and filamentation. We demonstrated that this protein is not essential in C. albicans and appears to be dispensable for filamentation. However, DDR48 is required for the flocculation response stimulated by 3-aminotriazole-induced amino acid starvation. Furthermore, we examined the response of this deletion strain to a wide variety of environmental stressors and antifungal compounds. We observed several mild sensitivity or resistance phenotypes and also found that Ddr48p contributes to the DNA damage response of C. albicans. The results of this study reveal that the role of this highly expressed protein goes beyond a general stress response and impinges on a key facet of pathogenesis, namely, the ability to sense and respond to changes in the host environment.

Portrait of Candida albicans adherence regulators

Cell-substrate adherence is a fundamental property of microorganisms that enables them to exist in biofilms. Our study focuses on adherence of the fungal pathogen Candida albicans to one substrate, silicone, that is relevant to device-associated infection. We conducted a mutant screen with a quantitative flow-cell assay to identify thirty transcription factors that are required for adherence. We then combined nanoString gene expression profiling with functional analysis to elucidate relationships among these transcription factors, with two major goals: to extend our understanding of transcription factors previously known to govern adherence or biofilm formation, and to gain insight into the many transcription factors we identified that were relatively uncharacterized, particularly in the context of adherence or cell surface biogenesis. With regard to the first goal, we have discovered a role for biofilm regulator Bcr1 in adherence, and found that biofilm regulator Ace2 is a major functional target of chromatin remodeling factor Snf5. In addition, Bcr1 and Ace2 share several target genes, pointing to a new connection between them. With regard to the second goal, our findings reveal existence of a large regulatory network that connects eleven adherence regulators, the zinc-response regulator Zap1, and approximately one quarter of the predicted cell surface protein genes in this organism. This limited yet sensitive glimpse of mutant gene expression changes had thus defined one of the broadest cell surface regulatory networks in C. albicans.

Antifungal resistance and new strategies to control fungal infections

Despite improvement of antifungal therapies over the last 30 years, the phenomenon of antifungal resistance is still of major concern in clinical practice. In the last 10 years the molecular mechanisms underlying this phenomenon were extensively unraveled. In this paper, after a brief overview of currently available antifungals, molecular mechanisms of antifungal resistance will be detailed. It appears that major mechanisms of resistance are essential due to the deregulation of antifungal resistance effector genes. This deregulation is a consequence of point mutations occurring in transcriptional regulators of these effector genes. Resistance can also follow the emergence of point mutations directly in the genes coding antifungal targets. In addition we further describe new strategies currently undertaken to discover alternative therapy targets and antifungals. Identification of new antifungals is essentially achieved by the screening of natural or synthetic chemical compound collections. Discovery of new putative antifungal targets is performed through genome-wide approaches for a better understanding of the human pathogenic fungi biology.

Interface of Candida albicans biofilm matrix-associated drug resistance and cell wall integrity regulation

Candida albicans frequently infects medical devices by growing as a biofilm, i.e., a community of adherent organisms entrenched in an extracellular matrix. During biofilm growth, Candida spp. acquire the ability to resist high concentrations of antifungal drugs. One recently recognized biofilm resistance mechanism involves drug sequestration by matrix β-1,3 glucan. Using a candidate gene approach, we investigated potential C. albicans β-1,3-glucan regulators, based on their homology to Saccharomyces cerevisiae, including SMI1 and protein kinase C (PKC) pathway components. We identified a role for the SMI1 in biofilm matrix glucan production and development of the associated drug resistance phenotype. This pathway appears to act through transcription factor Rlmp and glucan synthase Fks1p. The phenotypes of these mutant biofilms mimicked those of the smi1Δ/smi1Δ biofilm, and overexpression of FKS1 in the smi1Δ/smi1Δ mutant restored the biofilm resistant phenotype. However, control of this pathway is distinct from that of the upstream PKC pathway because the pkc1Δ/pkc1Δ, bck1Δ/bck1Δ, mkk2Δ/mkk2Δ, and mkc1Δ/mkc1Δ biofilms retained the resistant phenotype of the parent strain. In addition, resistance to cell-perturbing agents and gene expression data do not support a significant role for the cell wall integrity pathway during the biofilm formation. Here we show that Smi1p functions in conjunction with Rlm1p and Fks1p to produce drug-sequestering biofilm β-glucan. Our work provides new insight into how the C. albicans biofilm matrix production and drug resistance pathways intersect with the planktonic cell wall integrity pathway. This novel connection helps explain how pathogens in a multicellular biofilm community are protected from anti-infective therapy.

Application of the systematic "DAmP" approach to create a partially defective C. albicans mutant

An understanding of gene function often relies upon creating multiple kinds of alleles. Functional analysis in Candida albicans, a major fungal pathogen, has generally included characterization of mutant strains with insertion or deletion alleles and over-expression alleles. Here we use in C. albicans another type of allele that has been employed effectively in the model yeast Saccharomyces cerevisiae, a "Decreased Abundance by mRNA Perturbation" (DAmP) allele (Yan et al., 2008). DAmP alleles are created systematically through replacement of 30 noncoding regions with nonfunctional heterologous sequences, and thus are broadly applicable. We used a DAmP allele to probe the function of Sun41, a surface protein with roles in cell wall integrity, cell-cell adherence, hyphal formation, and biofilm formation that has been suggested as a possible therapeutic target (Firon et al., 2007; Hiller et al., 2007; Norice et al., 2007). A SUN41-DAmP allele results in approximately 10-fold reduced levels of SUN41 RNA, and yields intermediate phenotypes in most assays. We report that a sun41Δ/Δ mutant is defective in biofilm formation in vivo, and that the SUN41-DAmP allele complements that defect. This finding argues that Sun41 may not be an ideal therapeutic target for biofilm inhibition, since a 90% decrease in activity has little effect on biofilm formation in vivo. We anticipate that DAmP alleles of C. albicans genes will be informative for analysis of other prospective drug targets, including essential genes.

In vivo systematic analysis of Candida albicans Zn2-Cys6 transcription factors mutants for mice organ colonization

The incidence of fungal infections in immuno-compromised patients increased considerably over the last 30 years. New treatments are therefore needed against pathogenic fungi. With Candida albicans as a model, study of host-fungal pathogen interactions might reveal new sources of therapies. Transcription factors (TF) are of interest since they integrate signals from the host environment and participate in an adapted microbial response. TFs of the Zn2-Cys6 class are specific to fungi and are important regulators of fungal metabolism. This work analyzed the importance of the C. albicans Zn2-Cys6 TF for mice kidney colonization. For this purpose, 77 Zn2-Cys6 TF mutants were screened in a systemic mice model of infection by pools of 10 mutants. We developed a simple barcoding strategy to specifically detect each mutant DNA from mice kidney by quantitative PCR. Among the 77 TF mutant strains tested, eight showed a decreased colonization including mutants for orf19.3405, orf19.255, orf19.5133, RGT1, UGA3, orf19.6182, SEF1 and orf19.2646, and four an increased colonization including mutants for orf19.4166, ZFU2, orf19.1685 and UPC2 as compared to the isogenic wild type strain. Our approach was validated by comparable results obtained with the same animal model using a single mutant and the revertant for an ORF (orf19.2646) with still unknown functions. In an attempt to identify putative involvement of such TFs in already known C. albicans virulence mechanisms, we determined their in vitro susceptibility to pH, heat and oxidative stresses, as well as ability to produce hyphae and invade agar. A poor correlation was found between in vitro and in vivo assays, thus suggesting that TFs needed for mice kidney colonization may involve still unknown mechanisms. This large-scale analysis of mice organ colonization by C. albicans can now be extended to other mutant libraries since our in vivo screening strategy can be adapted to any preexisting mutants.

Efg1 Controls caspofungin-induced cell aggregation of Candida albicans through the adhesin Als1

Echinocandin drugs such as caspofungin (CASP), micafungin, and anidulafungin inhibit fungal cell wall biogenesis by blocking Fks1-mediated β-glucan deposition into the cell surface. Candins have become suitable drugs to treat life-threatening diseases caused by several fungal species, including Candida albicans, that are pathogenic for humans. Here, we present the discovery of a novel CASP-induced flocculation phenotype of C. albicans, which formed large cell aggregates in the presence of CASP. High concentrations of sugars such as mannose or glucose inhibit CASP-induced flocculation and improve survival of C. albicans cells exposed to CASP. Notably, exposure of C. albicans cells to CASP triggers Efg1-dependent expression of the adhesin ALS1 and induces invasive growth on agar plates. Indeed, cells lacking either Efg1 or Als1 show strongly diminished CASP-induced flocculation, and the absence of Efg1 leads to marked CASP hypersensitivity. On the other hand, CASP-induced invasive growth is enhanced in cells lacking Efg1. Hence, CASP stress drives an Efg1-dependent response, indicating that this multifunctional transcriptional regulator, which is otherwise involved in filamentation, white-to-opaque switching, and virulence, also modulates cell wall remodeling upon CASP challenge. Taken together, our data suggest that CASP-induced cell wall damage activates Efg1 in parallel with the known cell integrity stress signaling pathway to coordinate cell wall remodeling.

Effects of fluconazole on the secretome, the wall proteome, and wall integrity of the clinical fungus Candida albicans

Fluconazole is a commonly used antifungal drug that inhibits Erg11, a protein responsible for 14α-demethylation during ergosterol synthesis. Consequently, ergosterol is depleted from cellular membranes and replaced by toxic 14α-methylated sterols, which causes increased membrane fluidity and drug permeability. Surface-grown and planktonic cultures of Candida albicans responded similarly to fluconazole at 0.5 mg/liter, showing reduced biomass formation, severely reduced ergosterol levels, and almost complete inhibition of hyphal growth. There was no evidence of cell leakage. Mass spectrometric analysis of the secretome showed that its composition was strongly affected and included 17 fluconazole-specific secretory proteins. Relative quantification of (14)N-labeled query walls relative to a reference standard mixture of (15)N-labeled yeast and hyphal walls in combination with immunological analysis revealed considerable fluconazole-induced changes in the wall proteome as well. They were, however, similar for both surface-grown and planktonic cultures. Two major trends emerged: (i) decreased incorporation of hypha-associated wall proteins (Als3, Hwp1, and Plb5), consistent with inhibition of hyphal growth, and (ii) increased incorporation of putative wall repair-related proteins (Crh11, Pga4, Phr1, Phr2, Pir1, and Sap9). As exposure to the wall-perturbing drug Congo red led to a similar response, these observations suggested that fluconazole affects the wall. In keeping with this, the resistance of fluconazole-treated cells to wall-perturbing compounds decreased. We propose that fluconazole affects the integrity of both the cellular membranes and the fungal wall and discuss its potential consequences for antifungal therapy. We also present candidate proteins from the secretome for clinical marker development.

Signaling the glycoshield: maintenance of the Candida albicans cell wall

In fungi, the cell wall is a scaffold, an armor and an environmental gate. Sugar polymers including protein-O- or N-linked glycosyl chains or polysaccharides such as glucan or chitin are essential components to maintain cell wall functions. We describe mechanisms in the human fungal pathogen Candida albicans, by which the integrity of glycostructures are sensed and regulated. The results stress the importance of membrane sensors and MAP kinase pathways in the maintenance of cell wall structure and function.

Damage to the glycoshield activates PMT-directed O-mannosylation via the Msb2-Cek1 pathway in Candida albicans

Protein-O-mannosyltransferases (Pmt) transfer mannosyl residues to secretory proteins. Five isoforms of Pmt proteins in the human fungal pathogen Candida albicans have distinct functions in growth, morphogenesis and antifungal resistance. We found that PMT genes encoding the major isoforms Pmt1, Pmt2, Pmt4 are regulated differently in response to impaired glycostructures. While the PMT1 transcript level increased in cell wall mutants and under inhibition of N-glycosylation by tunicamycin, PMT2 and PMT4 transcripts were upregulated only by inhibition of Pmt1 activity. Reporter fusions revealed specific promoter sequences to be required for PMT1 repression in undamaged cells, which was de-repressed by tunicamycin. Constitutive PMT1 de-repression was observed in mutants lacking the Cek1 MAP kinase and its upstream sensor Msb2. In contrast, in msb2 and cek1 mutants, upregulation of PMT2/PMT4 by Pmt1 inhibition did not occur and basal expression of both transcripts were decreased. We identified Ace2 as a novel transcription factor, which upregulates PMT basal expression and induction in response to glycostructure damage. Mutants lacking Msb2, Cek1 and Ace2 were supersensitive to glycosylation and cell wall inhibitors. We propose that a Msb2, Cek1 and Ace2 signalling pathway addresses PMT genes as downstream targets and that different modes of regulation have evolved for PMT1 and PMT2/PMT4 genes.

Identification of sumoylation targets, combined with inactivation of SMT3, reveals the impact of sumoylation upon growth, morphology, and stress resistance in the pathogen Candida albicans

Posttranslational modifications of proteins play critical roles in the control of cellular differentiation, development, and environmental adaptation. In particular, the covalent attachment of the small ubiquitin-like modifier, SUMO, to target proteins (sumoylation) regulates cell cycle progression, transcription, nucleocytoplasmic transport, and stress responses. Here we combine proteomic, molecular, and cellular approaches to examine the roles of sumoylation in the major fungal pathogen of humans, Candida albicans. Using an N-terminally FLAG-tagged SUMO, 31 sumoylated proteins were identified in C. albicans with roles in stress responses (e.g., Hsp60, Hsp70 family members, Hsp104), the cytoskeleton and polarized growth (e.g., Tub1, Cct7, Mlc1), secretion, and endocytosis (e.g., Lsp1, Sec24, Sec7). The output from this proteomic screen was entirely consistent with the phenotypes of C. albicans mutants in which the single SUMO-encoding locus (SMT3) was inactivated or down-regulated. C. albicans smt3/smt3 cells displayed defects in growth, morphology, cell separation, nuclear segregation, and chitin deposition, suggesting important roles for sumoylation in cell cycle control. Smt3/smt3 cells also displayed sensitivity to thermal, oxidative, and cell wall stresses as well as to the antifungal drug caspofungin. Mutation of consensus sumoylation sites in Hsp60 and Hsp104 affected the resistance of C. albicans to thermal stress. Furthermore, signaling via the cell integrity pathway was defective in C. albicans smt3/smt3 cells. These observations provide mechanistic explanations for many of the observed phenotypic effects of Smt3 inactivation upon C. albicans growth and environmental adaptation. Clearly sumoylation plays key roles in fundamental cellular processes that underpin the pathogenicity of this medically important fungus.

Cell wall integrity is linked to mitochondria and phospholipid homeostasis in Candida albicans through the activity of the post-transcriptional regulator Ccr4-Pop2

The cell wall is essential for viability of fungi and is an effective drug target in pathogens such as Candida albicans. The contribution of post-transcriptional gene regulators to cell wall integrity in C. albicans is unknown. We show that the C. albicans Ccr4-Pop2 mRNA deadenylase, a regulator of mRNA stability and translation, is required for cell wall integrity. The ccr4/pop2 mutants display reduced wall β-glucans and sensitivity to the echinocandin caspofungin. Moreover, the deadenylase mutants are compromised for filamentation and virulence. We demonstrate that defective cell walls in the ccr4/pop2 mutants are linked to dysfunctional mitochondria and phospholipid imbalance. To further understand mitochondrial function in cell wall integrity, we screened a Saccharomyces cerevisiae collection of mitochondrial mutants. We identify several mitochondrial proteins required for caspofungin tolerance and find a connection between mitochondrial phospholipid homeostasis and caspofungin sensitivity. We focus on the mitochondrial outer membrane SAM complex subunit Sam37, demonstrating that it is required for both trafficking of phospholipids between the ER and mitochondria and cell wall integrity. Moreover, in C. albicans also Sam37 is essential for caspofungin tolerance. Our study provides the basis for an integrative view of mitochondrial function in fungal cell wall biogenesis and resistance to echinocandin antifungal drugs.

The Candida albicans Sur7 protein is needed for proper synthesis of the fibrillar component of the cell wall that confers strength

The Candida albicans plasma membrane plays important roles in interfacing with the environment, morphogenesis, and cell wall synthesis. The role of the Sur7 protein in cell wall structure and function was analyzed, since previous studies showed that this plasma membrane protein is needed to prevent abnormal intracellular growth of the cell wall. Sur7 localizes to stable patches in the plasma membrane, known as MCC (membrane compartment occupied by Can1), that are associated with eisosome proteins. The sur7Δ mutant cells displayed increased sensitivity to factors that exacerbate cell wall defects, such as detergent (SDS) and the chitin-binding agents calcofluor white and Congo red. The sur7Δ cells were also slightly more sensitive to inhibitors that block the synthesis of cell wall chitin (nikkomycin Z) and β-1,3-glucan (caspofungin). In contrast, Fmp45, a paralog of Sur7 that also localizes to punctate plasma membrane patches, did not have a detectable role in cell wall synthesis. Chemical analysis of cell wall composition demonstrated that sur7Δ cells contain decreased levels of β-glucan, a glucose polymer that confers rigidity on the cell wall. Consistent with this, sur7Δ cells were more sensitive to lysis, which could be partially rescued by increasing the osmolarity of the medium. Interestingly, Sur7 is present in static patches, whereas β-1,3-glucan synthase is mobile in the plasma membrane and is often associated with actin patches. Thus, Sur7 may influence β-glucan synthesis indirectly, perhaps by altering the functions of the cell signaling components that localize to the MCC and eisosome domains.

Secretion stress and antifungal resistance: an Achilles' heel of Aspergillus fumigatus?

The ability of Aspergillus fumigatus to establish and maintain an infection requires a continuous supply of nutrients to fuel energy production and growth. Like other filamentous fungi, A. fumigatus acquires nutrients by absorption, a mode of nutrition that depends upon the secretion of extracellular hydrolases to degrade the complex organic polymers in host tissues into reduced forms of carbon and nitrogen. If the folding capacity of the endoplasmic reticulum (ER) is exceeded during periods of high secretory activity, a signaling pathway known as the unfolded protein response (UPR) is activated to relieve the stress on the ER. Current evidence indicates that A. fumigatus relies upon this pathway to sustain the high rate of protease secretion needed to grow optimally in mammalian tissue. In addition, the UPR strengthens the ability of the secretory system to deliver cell wall and membrane components to the hyphal apex, which promotes the invasive growth of the expanding hyphae and protects the fungus from damage caused by antifungal drugs. The important contribution of UPR-dependent functions to the pathogenesis of invasive aspergillosis and antifungal susceptibility suggests that components of this pathway could be promising new targets for antifungal therapy.