Diverged binding specificity of Rim101p, the Candida albicans ortholog of PacC

Transcriptional expression of PHR2 is positively controlled by the calcium signaling transcription factor Crz1 through its binding motif in the promoter

IMPORTANCE

The fungal cell wall consists of glucans, mannoproteins, and chitin and is essential for cell viability, morphogenesis, and pathogenesis. The enzymes of the GH72 family are responsible for ß-(1,3)-glucan elongation and branching, which is crucial for the formation of the glucan-chitin polymer at the bud neck of yeast cells. In the human fungal pathogen Candida albicans, there are five GH72 enzyme-encoding genes: PHR1, PHR2, PHR3, PGA4, and PGA5. It is known that expression of PHR1 and PHR2 is controlled by the pH-responsive Rim101 pathway through the transcription factor Rim101. In this study, we have demonstrated that the transcription expression of PHR2 is also controlled by the transcription factor Crz1 through its binding motif in the promoter. Therefore, we have uncovered a dual-control mechanism by which PHR2 expression is negatively regulated via CaRim101 through the pH-responsive pathway and positively modulated by CaCrz1 through the calcium/calcineurin signaling pathway.

A glycosylated Phr1 protein is induced by calcium stress and its expression is positively controlled by the calcium/calcineurin signaling transcription factor Crz1 in Candida albicans

As one of the most important human fungal pathogens, Candida albicans senses and adapts to host niches with different pH values through the pH-responsive Rim101 pathway. Its transcription factor Rim101 activates the expression of alkaline pH-induced genes including PHR1 that encodes a glycosylphosphatidylinsitol-anchored β(1,3)-glucanosyltransferase critical for hyphal wall formation. The calcium/calcineurin signaling pathway is mediated by the transcription factor Crz1 in yeasts and other lower eukaryotes. Here we report that deletion of PHR1 leads to calcium sensitivity of C. albicans cells. In addition, expression of Phr1 is induced by calcium stress and under the control of Crz1 in C. albicans. EMSA assay demonstrates that Crz1 binds to one CDRE element in the PHR1 promoter. Alkaline treatment induces two species of glycosylated Phr1 proteins with different degrees of glycosylation, which is independent of Crz1. In contrast, only one species of Phr1 protein with a low degree of glycosylation is induced by calcium stress in a Crz1-dependent fashion. Therefore, we have provided an evidence that regulation of cell wall remodeling is integrated through differential degrees of Phr1 glycosylation by both the pH-regulated Rim101 pathway and the calcium/calcineurin signaling pathway in C. albicans. Video Abstract.

Titration of C-5 Sterol Desaturase Activity Reveals Its Relationship to Candida albicans Virulence and Antifungal Susceptibility Is Dependent upon Host Immune Status

The azole antifungals inhibit sterol 14α-demethylase (S14DM), which depletes cellular ergosterol and promotes synthesis of the dysfunctional lipid 14α-methylergosta-8,24(28)-dien-3β,6α-diol, ultimately arresting growth. Mutations that inactivate sterol Δ^5,6-desaturase (Erg3p), the enzyme that produces the sterol-diol upon S14DM inhibition, enhances Candida albicans growth in the presence of the azoles. However, erg3 null mutants are sensitive to some physiological stresses and can be less virulent than the wild type. These fitness defects may disfavor the selection of null mutants within patients. The objective of this study was to investigate the relationship between Erg3p activity, C. albicans pathogenicity, and the efficacy of azole therapy. An isogenic panel of strains was constructed that produce various levels of the ERG3 transcript. Analysis of the sterol composition confirmed a correspondingly wide range of Erg3p activity. Phenotypic analysis revealed that even moderate reductions in Erg3p activity are sufficient to greatly enhance C. albicans growth in the presence of fluconazole in vitro without impacting fitness. Moreover, even low levels of Erg3p activity are sufficient to support full virulence of C. albicans in the mouse model of disseminated infection. Finally, while the antifungal efficacy of fluconazole was similar for all strains in immunocompetent mice, there was an inverse correlation between Erg3p activity and the capacity of C. albicans to endure treatment in leukopenic mice. Collectively, these results establish that relative levels of Erg3p activity determine the antifungal efficacy of the azoles upon C. albicans and reveal the critical importance of host immunity in determining the clinical impact of this resistance mechanism. IMPORTANCE Mutations that completely inactivate Erg3p enable the prevalent human pathogen C. albicans to endure the azole antifungals in vitro. However, such null mutants are less frequently identified in azole-resistant clinical isolates than other resistance mechanisms, and previous studies have reported conflicting outcomes regarding antifungal resistance of these mutants in animal models of infection. The results of this study clearly establish a direct correlation between the level of Erg3p activity and the antifungal efficacy of fluconazole within a susceptible mammalian host. In addition, low levels of Erg3p activity are apparently more advantageous for C. albicans survival of azole therapy than complete loss of function. These findings suggest a more nuanced but more important role for Erg3p as a determinant of the clinical efficacy of the azole antifungals than previously appreciated. A revised model of the relationship between Erg3p activity, host immunity, and the antifungal susceptibility of C. albicans is proposed.

Rim101-upregulated Fets contribute to dark pigment formation in gray cells of Candida albicans

Candida albicans has long been known to switch between white and opaque phases; however, a third cell type, referred to as the 'gray' phenotype, was recently characterized. The three phenotypes have different colonial morphologies, with white cells forming white-colored colonies and opaque and gray cells forming dark-colored colonies. We previously showed that Wor1-upregulated ferroxidases (Fets) function as pigment multicopper oxidases that regulate the production of dark-pigmented melanin in opaque cells. In this study, we demonstrated that Fets also contributed to dark pigment formation in gray colonies but in a Wor1-independent manner. Deletion of both WOR1 and EFG1 locked cells in the gray phenotype in some rich media. However, the efg1/efg1 wor1/wor1 mutant could switch between white and gray in minimal media depending on the ambient pH. Specifically, mutant cells exhibited the white phenotype at pH 4.5 but switched to gray at pH 7.5. Consistent with phenotype switching, Fets expressions and melanin production were also regulated by ambient pH. Ectopic expression of the Rim101-405 allele in the mutant enabled the pH restriction to be bypassed and promoted gray cell formation in acidic media. Our data suggest that Rim101-upregulated Fets contribute to dark pigment formation in the gray cells.

CRISPR-Based Genetic Manipulation of Candida Species: Historical Perspectives and Current Approaches

The Candida genus encompasses a diverse group of ascomycete fungi that have captured the attention of the scientific community, due to both their role in pathogenesis and emerging applications in biotechnology; the development of gene editing tools such as CRISPR, to analyze fungal genetics and perform functional genomic studies in these organisms, is essential to fully understand and exploit this genus, to further advance antifungal drug discovery and industrial value. However, genetic manipulation of Candida species has been met with several distinctive barriers to progress, such as unconventional codon usage in some species, as well as the absence of a complete sexual cycle in its diploid members. Despite these challenges, the last few decades have witnessed an expansion of the Candida genetic toolbox, allowing for diverse genome editing applications that range from introducing a single point mutation to generating large-scale mutant libraries for functional genomic studies. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology is among the most recent of these advancements, bringing unparalleled versatility and precision to genetic manipulation of Candida species. Since its initial applications in Candida albicans, CRISPR-Cas9 platforms are rapidly evolving to permit efficient gene editing in other members of the genus. The technology has proven useful in elucidating the pathogenesis and host-pathogen interactions of medically relevant Candida species, and has led to novel insights on antifungal drug susceptibility and resistance, as well as innovative treatment strategies. CRISPR-Cas9 tools have also been exploited to uncover potential applications of Candida species in industrial contexts. This review is intended to provide a historical overview of genetic approaches used to study the Candida genus and to discuss the state of the art of CRISPR-based genetic manipulation of Candida species, highlighting its contributions to deciphering the biology of this genus, as well as providing perspectives for the future of Candida genetics.

Nutritional immunity: targeting fungal zinc homeostasis

Transition metals, such as Zn²⁺, are essential dietary constituents of all biological life, including mammalian hosts and the pathogens that infect them. Therefore, to thrive and cause infection, pathogens must successfully assimilate these elements from the host milieu. Consequently, mammalian immunity has evolved to actively restrict and/or pool metals to toxic concentrations in an effort to attenuate microbial pathogenicity - a process termed nutritional immunity. Despite host-induced Zn²⁺ nutritional immunity, pathogens such as Candida albicans, are still capable of causing disease and thus must be equipped with robust Zn²⁺ sensory, uptake and detoxification machinery. This review will discuss the strategies employed by mammalian hosts to limit Zn²⁺ during infection, and the subsequent fungal interventions that counteract Zn²⁺ nutritional immunity.

A variant ECE1 allele contributes to reduced pathogenicity of Candida albicans during vulvovaginal candidiasis

Vulvovaginal candidiasis (VVC), caused primarily by the human fungal pathogen Candida albicans, results in significant quality-of-life issues for women worldwide. Candidalysin, a toxin derived from a polypeptide (Ece1p) encoded by the ECE1 gene, plays a crucial role in driving immunopathology at the vaginal mucosa. This study aimed to determine if expression and/or processing of Ece1p differs across C. albicans isolates and whether this partly underlies differential pathogenicity observed clinically. Using a targeted sequencing approach, we determined that isolate 529L harbors a similarly expressed, yet distinct Ece1p isoform variant that encodes for a predicted functional candidalysin; this isoform was conserved amongst a collection of clinical isolates. Expression of the ECE1 open reading frame (ORF) from 529L in an SC5314-derived ece1Δ/Δ strain resulted in significantly reduced vaginopathogenicity as compared to an isogenic control expressing a wild-type (WT) ECE1 allele. However, in vitro challenge of vaginal epithelial cells with synthetic candidalysin demonstrated similar toxigenic activity amongst SC5314 and 529L isoforms. Creation of an isogenic panel of chimeric strains harboring swapped Ece1p peptides or HiBiT tags revealed reduced secretion with the ORF from 529L that was associated with reduced virulence. A genetic survey of 78 clinical isolates demonstrated a conserved pattern between Ece1p P2 and P3 sequences, suggesting that substrate specificity around Kex2p-mediated KR cleavage sites involved in protein processing may contribute to differential pathogenicity amongst clinical isolates. Therefore, we present a new mechanism for attenuation of C. albicans virulence at the ECE1 locus.

Regulation of zinc homeostatic genes by environmental pH in the filamentous fungus Aspergillus fumigatus

Aspergillus fumigatus can grow over a broad range of pH values even though zinc availability is greatly conditioned by ambient pH. It has been previously shown that regulation of zinc homeostatic genes in this fungus relies on the transcription factor ZafA. In addition, their expression is further modulated by the transcription factor PacC depending on ambient pH, which allows this fungus to grow in diverse types of niches, including soils and the lungs of immunosuppressed hosts. In this work the regulation by PacC of genes zrfB and zrfC that are expressed, respectively, under acidic and alkaline zinc-limiting conditions have been analysed in detail. Thus, data that extend the current model for PacC function, including the role of the full-length PacC⁷² protein and the PacC processed forms (PacC⁵³ and PacC²⁷ ) on gene expression has been provided, and a new mechanism for the repression of acid-expressed genes in alkaline media based on interference with the start of transcription has been described. Moreover, it was proposed that the transcription of both acid-expressed and alkaline-expressed genes under zinc-limiting conditions might also rely on a third factor (putatively Pontin/Reptin), which may be required to integrate the action of PacC and ZafA into gene specific transcriptional responses.

Vacuole and Mitochondria Patch (vCLAMP) Protein Vam6 Is Involved in Maintenance of Mitochondrial and Vacuolar Functions under Oxidative Stress in Candida albicans

Candida albicans is one of the most common opportunistic fungal pathogens in human beings. When infecting host cells, C. albicans is often exposed to oxidative stress from the host immune defense system. Maintenance of mitochondrial and vacuolar functions is crucial for its resistance to oxidative stress. However, the role of vacuole and mitochondria patchs (vCLAMPs) in cellular oxidative stress resistance and in the maintenance of organelle functions remains to be elucidated. Herein, the function of the vCLAMP protein Vam6 in response to oxidative stress was explored. The results showed that the vam6∆/∆ mutant exhibited obvious mitochondrial swelling, mtDNA damage, reduced activity of antioxidant enzymes, and abnormal vacuolar morphology under H₂O₂ treatment, indicating its important role in maintaining the structures and functions of both mitochondria and vacuoles under oxidative stress. Further studies showed that deletion of VAM6 attenuated hyphal development under oxidative stress. Moreover, loss of Vam6 obviously affected host tissue invasion and virulence of C. albicans. Taken together, this paper reveals the critical role of vCLAMPs in response to oxidative stress in C. albicans.

Roles for ALDH10 enzymes in γ-butyrobetaine synthesis, seed development, germination, and salt tolerance in Arabidopsis

Plant genomes generally contain two aldehyde dehydrogenase 10 (ALDH10) genes, which encode NAD+-dependent enzymes. These oxidize various aminoaldehydes that are produced by the catabolism of amino acids and polyamines. ALDH10s are closely related to the animal and fungal trimethylaminobutyraldehyde dehydrogenases (TMABADHs) that are involved in the synthesis of γ-butyrobetaine, the precursor of carnitine. Here, we explore the ability of the Arabidopsis thaliana proteins AtALDH10A8 and AtALDH10A9 to oxidize aminoaldehydes. We demonstrate that these enzymes display high TMABADH activities in vitro. Moreover, they can complement the Candida albicans tmabadhΔ/Δ null mutant. These findings illustrate the link between AtALDH10A8 and AtALDH10A9 and γ-butyrobetaine synthesis. An analysis of single and double knockout Arabidopsis mutant lines revealed that the double mutants had reduced γ-butyrobetaine levels. However, there were no changes in the carnitine contents of these mutants. The double mutants were more sensitive to salt stress. In addition, the siliques of the double mutants had a significant proportion of seeds that failed to mature. The mature seeds contained higher amounts of triacylglycerol, facilitating accelerated germination. Taken together, these results show that ALDH10 enzymes are involved in γ-butyrobetaine synthesis. Furthermore, γ-butyrobetaine fulfils a range of physiological roles in addition to those related to carnitine biosynthesis.

Zinc at the Host-Fungus Interface: How to Uptake the Metal?

Zinc is an essential nutrient for all living organisms. However, firm regulation must be maintained since micronutrients also can be toxic in high concentrations. This notion is reinforced when we look at mechanisms deployed by our immune system, such as the use of chelators or membrane transporters that capture zinc, when threatened with pathogens, like fungi. Pathogenic fungi, on the other hand, also make use of a variety of transporters and specialized zinc captors to survive these changes. In this review, we sought to explain the mechanisms, grounded in experimental analysis and described to date, utilized by pathogenic fungi to maintain optimal zinc levels.

The inositol polyphosphate kinase Ipk1 transcriptionally regulates mitochondrial functions in Candida albicans

Inositol polyphosphates (IPs) is an important family of signaling molecules that regulate multiple cellular processes, such as chromatin remodeling, transcription and mRNA export. Inositol polyphosphate kinases, as the critical enzymes for production and transformation of IPs, directly determine the intracellular levels of IPs and therefore are involved in many cellular processes. However, its roles in Candida albicans, the leading fungal pathogen in human beings, remain to be investigated. In this study, we identified the inositol polyphosphate kinase Ipk1 in C. albicans and found that it localizes in the nucleus. Moreover, in the ipk1Δ/Δ mutant, the activity of mitochondrial respiratory chain complexes and the mitochondrial function was severely impaired, which were associated with down-regulation of mitochondrial function-related genes revealed by transcription profiling analysis. The ipk1Δ/Δ mutant also displayed hypersensitivity to a series of environmental stresses, such as antifungal drugs, oxidants, cell wall perturbing agents and macrophage attacks, followed by attenuation of virulence in a mouse systematic infection model. These findings firstly reported the importance of inositol polyphosphate kinase Ipk1 in C. albicans, especially its role in mitochondrial function maintenance and pathogenicity.

The endophyte Serendipita indica reduces the sodium content of Arabidopsis plants exposed to salt stress: fungal ENA ATPases are expressed and regulated at high pH and during plant co-cultivation in salinity

Serendipita indica (formerly Piriformospora indica) is an endophytic fungus that colonizes plant roots producing a beneficial effect on plant growth and development under optimal and suboptimal conditions provoked by any biotic or abiotic stress, such as salt stress. Salinity induces osmotic and ionic imbalances in plants, mainly by altering the Na⁺ and K⁺ contents. However, the mechanism by which Serendipita improves plant growth has yet to be elucidated. Previous works suggest that this fungus improves the plant osmotic state but not much is known about whether it participates in readjustment of the ionic imbalance in plants. Here, we report that co-cultivation with Serendipita reduces the Na⁺ content of Arabidopsis plants under saline conditions. Additionally, we describe the functional characterization of the two Serendipita ENA ATPases, which are homologous to the main proteins involved in the salt tolerance of other fungi. Their heterologous expression in salt-sensitive yeast mutants shows that SiENA1 is involved in Na⁺ and K⁺ efflux, while SiENA5 seems to only be involved in Na⁺ detoxification. Both are induced and might have a relevant function at alkaline pH, condition in which there are few chlamydospores in the mycelium, as well as during co-cultivation with plants exposed to saline conditions.

Titrating Gene Function in the Human Fungal Pathogen Candida albicans through Poly-Adenosine Tract Insertion

Investigating a protein’s functional importance at the whole-organism level usually involves altering its expression level or its specific activity and observing the consequences with respect to physiology or phenotype. Several approaches designed to partially or completely abolish the function of a gene, including its deletion from the genome and the use of systems that facilitate conditional expression, have been widely applied. However, each has significant limitations that are especially problematic in pathogenic microbes when it is desirable to determine if a particular gene is required for infection in an animal model. In this study, we sought to determine if an alternative approach—the insertion of poly-A repeats within the coding sequence of the gene—is sufficient to modulate its function in the prevalent human fungal pathogen C. albicans. Our results confirm that this approach enables us to predictably and gradually titrate the expression level of a protein and thus to investigate the phenotypic consequences of various levels of gene/protein function.

A recent study demonstrated that the insertion of poly-adenosine (poly-A) tracts into an open reading frame can suppress expression of the encoded protein in both prokaryotic and eukaryotic species. Furthermore, the degree of suppression is proportional to the length of the poly-A insertion, which can therefore provide a reliable and predictable means to titrate a specific protein’s expression. The goal of this study was to determine if this methodology can be applied to modulate the expression of proteins in the prevalent human fungal pathogen, Candida albicans. Insertion of increasing numbers of AAA codons encoding lysine at the N terminus of the C. albicans lanosterol demethylase (Erg11p) progressively diminished expression without significantly reducing the levels of mRNA. This suggests that Erg11p expression was attenuated at the posttranscriptional level. A direct correlation between the number of AAA codons inserted and C. albicans susceptibility to the Erg11p inhibitor fluconazole was also noted, indicating a progressive loss of Erg11p activity. Finally, we constructed a series of C. albicans strains with 3 to 12 AAA codons inserted at the 5′ end of the ARO1 gene, which encodes a pentafunctional enzyme catalyzing five sequential steps of the aromatic amino acid biosynthetic pathway. Increasing numbers of AAA codons progressively reduced the growth rate of C. albicans in standard laboratory medium, indicating a progressive loss of ARO biosynthetic activity. These data unequivocally demonstrate the potential utility of the poly-A insertion method to examine the phenotypic consequences of titrating target protein function in C. albicans.

IMPORTANCE Investigating a protein’s functional importance at the whole-organism level usually involves altering its expression level or its specific activity and observing the consequences with respect to physiology or phenotype. Several approaches designed to partially or completely abolish the function of a gene, including its deletion from the genome and the use of systems that facilitate conditional expression, have been widely applied. However, each has significant limitations that are especially problematic in pathogenic microbes when it is desirable to determine if a particular gene is required for infection in an animal model. In this study, we sought to determine if an alternative approach—the insertion of poly-A repeats within the coding sequence of the gene—is sufficient to modulate its function in the prevalent human fungal pathogen C. albicans. Our results confirm that this approach enables us to predictably and gradually titrate the expression level of a protein and thus to investigate the phenotypic consequences of various levels of gene/protein function.

The Vacuolar Ca2+ ATPase Pump Pmc1p Is Required for Candida albicans Pathogenesis

Maintenance of Ca²⁺ homeostasis is important for fungal cells to respond to a multitude of stresses, as well as antifungal treatment, and for virulence in animal models. Here, we demonstrate that a P-type ATPase, Pmc1p, is required for Candida albicans to respond to a variety of stresses, affects azole susceptibility, and is required to sustain tissue invasive hyphal growth and to cause disease in a mouse model of disseminated infection. Defining the mechanisms responsible for maintaining proper Ca²⁺ homeostasis in this important human pathogen can ultimately provide opportunities to devise new chemotherapeutic interventions that dysregulate intracellular signaling and induce Ca²⁺ toxicity.

Calcium is a critically important secondary messenger of intracellular signal transduction in eukaryotes but must be maintained at low levels in the cytoplasm of resting cells to avoid toxicity. This is achieved by several pumps that actively transport excess cytoplasmic Ca²⁺ out of the cell across the plasma membrane and into other intracellular compartments. In fungi, the vacuole serves as the major storage site for excess Ca²⁺, with two systems actively transporting cytoplasmic calcium ions into the vacuole. The H⁺/Ca²⁺ exchanger, Vcx1p, harnesses the proton-motive force across the vacuolar membrane (generated by the V-ATPase) to drive Ca²⁺ transport, while the P-type ATPase Pmc1p uses ATP hydrolysis to translocate Ca²⁺ into the vacuole. Ca²⁺-dependent signaling is required for the prevalent human fungal pathogen Candida albicans to endure exposure to the azole antifungals and to cause disease within the mammalian host. The purpose of this study was to determine if the Pmc1p or Vcx1p Ca²⁺ pumps are required for C. albicans pathogenicity and if these pumps impact antifungal resistance. Our results indicate that Pmc1p is required by C. albicans to transition from yeast to hyphal growth, to form biofilms in vitro, and to cause disease in a mouse model of disseminated infection. Moreover, loss of Pmc1p function appears to enhance C. albicans azole tolerance in a temperature-dependent manner.

IMPORTANCE Maintenance of Ca²⁺ homeostasis is important for fungal cells to respond to a multitude of stresses, as well as antifungal treatment, and for virulence in animal models. Here, we demonstrate that a P-type ATPase, Pmc1p, is required for Candida albicans to respond to a variety of stresses, affects azole susceptibility, and is required to sustain tissue invasive hyphal growth and to cause disease in a mouse model of disseminated infection. Defining the mechanisms responsible for maintaining proper Ca²⁺ homeostasis in this important human pathogen can ultimately provide opportunities to devise new chemotherapeutic interventions that dysregulate intracellular signaling and induce Ca²⁺ toxicity.

Mgm1 is required for maintenance of mitochondrial function and virulence in Candida albicans

Mitochondria are dynamic organelles, and their shapes and sizes are regulated by mitochondrial fusion and fission. The proteins essential for mitochondrial fusion in Candida albicans have not been clearly characterized. In this study, Mgm1 was explored for its roles in mitochondrial function, cell cycle, hyphal growth and virulence in this pathogen. The deletion of MGM1 led to mitochondrial fragmentation and mtDNA loss and activated the checkpoint pathway to arrest the cell cycle in G1 phase. Moreover, loss of MGM1 led to defects in hyphal development and attenuation of virulence in a macrophage cell line and a mouse model of disseminated infection. These results reveal that Mgm1 plays an important role in mitochondrial dynamics and function, cell cycle progression, hyphal development and virulence in C. albicans.

The Beauveria bassiana Gas3 β-Glucanosyltransferase Contributes to Fungal Adaptation to Extreme Alkaline Conditions

Fungal β-1,3-glucanosyltransferases are cell wall-remodeling enzymes implicated in stress response, cell wall integrity, and virulence, with most fungal genomes containing multiple members. The insect-pathogenic fungus Beauveria bassiana displays robust growth over a wide pH range (pH 4 to 10). A random insertion mutant library screening for increased sensitivity to alkaline (pH 10) growth conditions resulted in the identification and mapping of a mutant to a β-1,3-glucanosyltransferase gene (Bbgas3). Bbgas3 expression was pH dependent and regulated by the PacC transcription factor, which activates genes in response to neutral/alkaline growth conditions. Targeted gene knockout of Bbgas3 resulted in reduced growth under alkaline conditions, with only minor effects of increased sensitivity to cell wall stress (Congo red and calcofluor white) and no significant effects on fungal sensitivity to oxidative or osmotic stress. The cell walls of ΔBbgas3 aerial conidia were thinner than those of the wild-type and complemented strains in response to alkaline conditions, and β-1,3-glucan antibody and lectin staining revealed alterations in cell surface carbohydrate epitopes. The ΔBbgas3 mutant displayed alterations in cell wall chitin and carbohydrate content in response to alkaline pH. Insect bioassays revealed impaired virulence for the ΔBbgas3 mutant depending upon the pH of the media on which the conidia were grown and harvested. Unexpectedly, a decreased median lethal time to kill (LT50, i.e., increased virulence) was seen for the mutant using intrahemocoel injection assays using conidia grown at acidic pH (5.6). These data show that BbGas3 acts as a pH-responsive cell wall-remodeling enzyme involved in resistance to extreme pH (>9).IMPORTANCE Little is known about adaptations required for growth at high (>9) pH. Here, we show that a specific fungal membrane-remodeling β-1,3-glucanosyltransferase gene (Bbgas3) regulated by the pH-responsive PacC transcription factor forms a critical aspect of the ability of the insect-pathogenic fungus Beauveria bassiana to grow at extreme pH. The loss of Bbgas3 resulted in a unique decreased ability to grow at high pH, with little to no effects seen with respect to other stress conditions, i.e., cell wall integrity and osmotic and oxidative stress. However, pH-dependent alternations in cell wall properties and virulence were noted for the ΔBbgas3 mutant. These data provide a mechanistic insight into the importance of the specific cell wall structure required to stabilize the cell at high pH and link it to the PacC/Pal/Rim pH-sensing and regulatory system.

Loss of Upc2p-Inducible ERG3 Transcription Is Sufficient To Confer Niche-Specific Azole Resistance without Compromising Candida albicans Pathogenicity

Inactivation of sterol Δ^5,6-desaturase (Erg3p) in the prevalent fungal pathogen Candida albicans is one of several mechanisms that can confer resistance to the azole antifungal drugs. However, loss of Erg3p activity is also associated with deficiencies in stress tolerance, invasive hyphal growth, and attenuated virulence in a mouse model of disseminated infection. This may explain why relatively few erg3-deficient strains have been reported among azole-resistant clinical isolates. In this study, we examined the consequences of Erg3p inactivation upon C. albicans pathogenicity and azole susceptibility in mouse models of mucosal and disseminated infection. While a C. albicanserg3Δ/Δ mutant was unable to cause lethality in the disseminated model, it induced pathology in a mouse model of vaginal infection. The erg3Δ/Δ mutant was also more resistant to fluconazole treatment than the wild type in both models of infection. Thus, complete loss of Erg3p activity confers azole resistance but also niche-specific virulence deficiencies. Serendipitously, we discovered that loss of azole-inducible ERG3 transcription (rather than complete inactivation) is sufficient to confer in vitro fluconazole resistance, without compromising C. albicans stress tolerance, hyphal growth, or pathogenicity in either mouse model. It is also sufficient to confer fluconazole resistance in the mouse vaginal model, but not in the disseminated model of infection, and thus confers niche-specific azole resistance without compromising C. albicans pathogenicity at either site. Collectively, these results establish that modulating Erg3p expression or activity can have niche-specific consequences on both C. albicans pathogenicity and azole resistance.

While conferring resistance to the azole antifungals in vitro, loss of sterol Δ^5,6-desaturase (Erg3p) activity has also been shown to reduce C. albicans pathogenicity. Accordingly, it has been presumed that this mechanism may not be significant in the clinical setting. The results presented here challenge this assumption, revealing a more complex relationship between Erg3p activity, azole resistance, C. albicans pathogenicity, and the specific site of infection. Most importantly, we have shown that even modest changes in ERG3 transcription are sufficient to confer azole resistance without compromising C. albicans fitness or pathogenicity. Given that previous efforts to assess the importance of ERG3 as a determinant of clinical azole resistance have focused almost exclusively on detecting null mutants, its role may have been grossly underestimated. On the basis of our results, a more thorough investigation of the contribution of the ERG3 gene to azole resistance in the clinical setting is warranted.

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.

Targeting Candida spp. to develop antifungal agents

Invasive fungal infections are a complex challenge throughout the world because of their high incidence, mainly in critically ill patients, and high mortality rates. The antifungal agents currently available are limited; thus, there is a need for the rapid development of new drugs. In silico methods are a modern strategy to explore interactions between new compounds and specific fungal targets, but they depend on precise genetic information. Here, we discuss the main Candida spp. target genes, including information about null mutants, virulence, cytolocalization, co-regulatory genes, and compounds that are related to protein expression. These data will provide a basis for the future in silico development of antifungal drugs.

From Genes to Networks: The Regulatory Circuitry Controlling Candida albicans Morphogenesis

Candida albicans is a commensal yeast of most healthy individuals, but also one of the most prevalent human fungal pathogens. During adaptation to the mammalian host, C. albicans encounters different niches where it is exposed to several types of stress, including oxidative, nitrosative (e.g., immune system), osmotic (e.g., kidney and oral cavity) stresses and pH variation (e.g., gastrointestinal (GI) tract and vagina). C. albicans has developed the capacity to respond to the environmental changes by modifying its morphology, which comprises the yeast-to-hypha transition, white-opaque switching, and chlamydospore formation. The yeast-to-hypha transition has been very well characterized and was shown to be modulated by several external stimuli that mimic the host environment. For instance, temperature above 37 ℃, serum, alkaline pH, and CO₂ concentration are all reported to enhance filamentation. The transition is characterized by the activation of an intricate regulatory network of signaling pathways, involving many transcription factors. The regulatory pathways that control either the stress response or morphogenesis are required for full virulence and promote survival of C. albicans in the host. Many of these transcriptional circuitries have been characterized, highlighting the complexity and the interconnections between the different pathways. Here, we present the major signaling pathways and the main transcription factors involved in the yeast-to-hypha transition. Furthermore, we describe the role of heat shock transcription factors in the morphogenetic transition, providing an edifying example of the complex cross talk between pathways involved in morphogenesis and stress response.

The PHR Family: The Role of Extracellular Transglycosylases in Shaping Candida albicans Cells

Candida albicans is an opportunistic microorganism that can become a pathogen causing mild superficial mycosis or more severe invasive infections that can be life-threatening for debilitated patients. In the etiology of invasive infections, key factors are the adaptability of C. albicans to the different niches of the human body and the transition from a yeast form to hypha. Hyphal morphology confers high adhesiveness to the host cells, as well as the ability to penetrate into organs. The cell wall plays a crucial role in the morphological changes C. albicans undergoes in response to specific environmental cues. Among the different categories of enzymes involved in the formation of the fungal cell wall, the GH72 family of transglycosylases plays an important assembly role. These enzymes cut and religate β-(1,3)-glucan, the major determinant of cell shape. In C. albicans, the PHR family encodes GH72 enzymes, some of which work in specific environmental conditions. In this review, we will summarize the work from the initial discovery of PHR genes to the study of the pH-dependent expression of PHR1 and PHR2, from the characterization of the gene products to the recent findings concerning the stress response generated by the lack of GH72 activity in C. albicans hyphae.

Target Abundance-Based Fitness Screening (TAFiS) Facilitates Rapid Identification of Target-Specific and Physiologically Active Chemical Probes

Conventional drug screening typically employs either target-based or cell-based approaches. The first group rely on biochemical assays to detect modulators of a purified target. However, hits frequently lack drug-like characteristics such as membrane permeability and target specificity. Cell-based screens identify compounds that induce a desired phenotype, but the target is unknown, which severely restricts further development and optimization. To address these issues, we have developed a second-generation target-based whole-cell screening approach that incorporates the principles of both chemical genetics and competitive fitness, which enables the identification of target-specific and physiologically active compounds from a single screen. We have chosen to validate this approach using the important human fungal pathogen Candida albicans with the intention of pursuing novel antifungal targets. However, this approach is broadly applicable and is expected to dramatically reduce the time and resources required to progress from screening hit to lead compound.

Traditional approaches to drug discovery are frustratingly inefficient and have several key limitations that severely constrain our capacity to rapidly identify and develop novel experimental therapeutics. To address this, we have devised a second-generation target-based whole-cell screening assay based on the principles of competitive fitness, which can rapidly identify target-specific and physiologically active compounds. Briefly, strains expressing high, intermediate, and low levels of a preselected target protein are constructed, tagged with spectrally distinct fluorescent proteins (FPs), and pooled. The pooled strains are then grown in the presence of various small molecules, and the relative growth of each strain within the mixed culture is compared by measuring the intensity of the corresponding FP tags. Chemical-induced population shifts indicate that the bioactivity of a small molecule is dependent upon the target protein’s abundance and thus establish a specific functional interaction. Here, we describe the molecular tools required to apply this technique in the prevalent human fungal pathogen Candida albicans and validate the approach using two well-characterized drug targets—lanosterol demethylase and dihydrofolate reductase. However, our approach, which we have termed target abundance-based fitness screening (TAFiS), should be applicable to a wide array of molecular targets and in essentially any genetically tractable microbe.

IMPORTANCE Conventional drug screening typically employs either target-based or cell-based approaches. The first group relies on biochemical assays to detect modulators of a purified target. However, hits frequently lack drug-like characteristics such as membrane permeability and target specificity. Cell-based screens identify compounds that induce a desired phenotype, but the target is unknown, which severely restricts further development and optimization. To address these issues, we have developed a second-generation target-based whole-cell screening approach that incorporates the principles of both chemical genetics and competitive fitness, which enables the identification of target-specific and physiologically active compounds from a single screen. We have chosen to validate this approach using the important human fungal pathogen Candida albicans with the intention of pursuing novel antifungal targets. However, this approach is broadly applicable and is expected to dramatically reduce the time and resources required to progress from screening hit to lead compound.

Overexpression of Candida albicans Secreted Aspartyl Proteinase 2 or 5 Is Not Sufficient for Exacerbation of Immunopathology in a Murine Model of Vaginitis

The secreted aspartyl proteinases of Candida albicans have long been implicated in virulence at the mucosal surface, including contributions to colonization and immunopathogenesis during vulvovaginal candidiasis. In an effort to disentangle hypha-associated virulence factor regulation from morphological transition, the purpose of this study was to determine if overexpression of SAP2 or SAP5 in an efg1Δ/Δ cph1Δ/Δ mutant could restore the capacity to cause immunopathology during murine vaginitis to this avirulent hypofilamentous strain. Two similar yet distinct genetic approaches were used to construct expression vectors to achieve SAP overexpression, and both genetic and functional assays confirmed elevated SAP activity in transformed strains. Similar to previous findings, intravaginal challenge of C57BL/6 mice with hypha-defective strains attained high levels of mucosal colonization but failed to induce robust vaginal immunopathology (neutrophil recruitment, interleukin-1β [IL-1β] secretion, and lactate dehydrogenase release) compared to that with the hypha-competent control. Moreover, constitutive expression of SAP2 or SAP5 in two distinct sets of such strains did not elicit immunopathological markers at levels above those observed during challenge with isogenic empty vector controls. Therefore, these results suggest that the physiological contributions of SAPs to vaginal immunopathology require hypha formation, other hypha-associated factors, or genetic interaction with EFG1 and/or CPH1 to cause symptomatic infection. Additionally, the outlined expression strategy and strain sets will be useful for decoupling other downstream morphogenetic factors from hyphal growth.

In Vivo Indicators of Cytoplasmic, Vacuolar, and Extracellular pH Using pHluorin2 in Candida albicans

Candida albicans is an opportunistic fungal pathogen that colonizes the reproductive and gastrointestinal tracts of its human host. It can also invade the bloodstream and deeper organs of immunosuppressed individuals, and thus it encounters enormous variations in external pH in vivo. Accordingly, survival within such diverse niches necessitates robust adaptive responses to regulate intracellular pH. However, the impact of antifungal drugs upon these adaptive responses, and on intracellular pH in general, is not well characterized. Furthermore, the tools and methods currently available to directly monitor intracellular pH in C. albicans, as well as other fungal pathogens, have significant limitations. To address these issues, we developed a new and improved set of pH sensors based on the pH-responsive fluorescent protein pHluorin. This includes a cytoplasmic sensor, a probe that localizes inside the fungal vacuole (an acidified compartment that plays a central role in intracellular pH homeostasis), and a cell surface probe that can detect changes in extracellular pH. These tools can be used to monitor pH within single C. albicans cells or in cell populations in real time through convenient and high-throughput assays.

Environmental or chemically induced stresses often trigger physiological responses that regulate intracellular pH. As such, the capacity to detect pH changes in real time and within live cells is of fundamental importance to essentially all aspects of biology. In this respect, pHluorin, a pH-sensitive variant of green fluorescent protein, has provided an invaluable tool to detect such responses. Here, we report the adaptation of pHluorin2 (PHL2), a substantially brighter variant of pHluorin, for use with the human fungal pathogen Candida albicans. As well as a cytoplasmic PHL2 indicator, we describe a version that specifically localizes within the fungal vacuole, an acidified subcellular compartment with important functions in nutrient storage and pH homeostasis. In addition, by means of a glycophosphatidylinositol-anchored PHL2-fusion protein, we generated a cell surface pH sensor. We demonstrated the utility of these tools in several applications, including accurate intracellular and extracellular pH measurements in individual cells via flow cytometry and in cell populations via a convenient plate reader-based protocol. The PHL2 tools can also be used for endpoint as well as time course experiments and to conduct chemical screens to identify drugs that alter normal pH homeostasis. These tools enable observation of the highly dynamic intracellular pH shifts that occur throughout the fungal growth cycle, as well as in response to various chemical treatments.

IMPORTANCECandida albicans is an opportunistic fungal pathogen that colonizes the reproductive and gastrointestinal tracts of its human host. It can also invade the bloodstream and deeper organs of immunosuppressed individuals, and thus it encounters enormous variations in external pH in vivo. Accordingly, survival within such diverse niches necessitates robust adaptive responses to regulate intracellular pH. However, the impact of antifungal drugs upon these adaptive responses, and on intracellular pH in general, is not well characterized. Furthermore, the tools and methods currently available to directly monitor intracellular pH in C. albicans, as well as other fungal pathogens, have significant limitations. To address these issues, we developed a new and improved set of pH sensors based on the pH-responsive fluorescent protein pHluorin. This includes a cytoplasmic sensor, a probe that localizes inside the fungal vacuole (an acidified compartment that plays a central role in intracellular pH homeostasis), and a cell surface probe that can detect changes in extracellular pH. These tools can be used to monitor pH within single C. albicans cells or in cell populations in real time through convenient and high-throughput assays.

Adaptation of Candida albicans to environmental pH induces cell wall remodelling and enhances innate immune recognition

Candida albicans is able to proliferate in environments that vary dramatically in ambient pH, a trait required for colonising niches such as the stomach, vaginal mucosal and the GI tract. Here we show that growth in acidic environments involves cell wall remodelling which results in enhanced chitin and β-glucan exposure at the cell wall periphery. Unmasking of the underlying immuno-stimulatory β-glucan in acidic environments enhanced innate immune recognition of C. albicans by macrophages and neutrophils, and induced a stronger proinflammatory cytokine response, driven through the C-type lectin-like receptor, Dectin-1. This enhanced inflammatory response resulted in significant recruitment of neutrophils in an intraperitoneal model of infection, a hallmark of symptomatic vaginal colonisation. Enhanced chitin exposure resulted from reduced expression of the cell wall chitinase Cht2, via a Bcr1-Rim101 dependent signalling cascade, while increased β-glucan exposure was regulated via a non-canonical signalling pathway. We propose that this "unmasking" of the cell wall may induce non-protective hyper activation of the immune system during growth in acidic niches, and may attribute to symptomatic vaginal infection.

Loss of Ssq1 leads to mitochondrial dysfunction, activation of autophagy and cell cycle arrest due to iron overload triggered by mitochondrial iron-sulfur cluster assembly defects in Candida albicans

Iron-sulfur clusters perform essential functions in enzymatic catalysis and homeostatic regulation. Here we for the first time identified Ssq1 as an essential component for iron-sulfur cluster assembly in Candida albicans. Ssq1 played an important role in cell growth. Shutting off SSQ1 led to accumulation of intracellular iron, especially in mitochondria, and disorder of intracellular iron regulation. In tetO-SSQ1, iron overloading triggered the oxidative damage of mitochondrial function. Surprisingly, disruption of SSQ1 activated autophagic pathway. The mitochondrial dysfunction was further aggravated when CCZ1 (which is essential for autophagy) and SSQ1 was simultaneously deleted, suggesting that autophagy played a critical role in maintenance of mitochondrial function in tetO-SSQ1. In addition, double deletion of SSQ1 and CCZ1 further elevated cellular iron levels in comparison with tetO-SSQ1, indicating that autophagy participated in maintenance of iron homeostasis. Furthermore, we found that loss of SSQ1 led to increasing protein expression of Rnr1 and redistribution of Rnr2 from the nucleus to cytoplasm, and further resulted in cell cycle arrest. The results implied that cell cycle arrest was caused by activating the checkpoint pathway because of impairing the iron-sulfur cluster assembly in tetO-SSQ1. Shutting off SSQ1 led to a significant defect in filamentous development. Interestingly, the tetO-SSQ1ccz1Δ/Δ growth was inhibited on hyphae-inducing solid media. Both tetO-SSQ1 and tetO-SSQ1ccz1Δ/Δ exhibited extremely attenuated virulence, indicating that Ssq1 might provide a promising target for antifungal drugs development. In summary, our findings provide new insights into the understanding of iron-sulfur cluster assembly-related gene in C. albicans.

Identification of small molecules that disrupt vacuolar function in the pathogen Candida albicans

The fungal vacuole is a large acidified organelle that performs a variety of cellular functions. At least a sub-set of these functions are crucial for pathogenic species of fungi, such as Candida albicans, to survive within and invade mammalian tissue as mutants with severe defects in vacuolar biogenesis are avirulent. We therefore sought to identify chemical probes that disrupt the normal function and/or integrity of the fungal vacuole to provide tools for the functional analysis of this organelle as well as potential experimental therapeutics. A convenient indicator of vacuolar integrity based upon the intracellular accumulation of an endogenously produced pigment was adapted to identify Vacuole Disrupting chemical Agents (VDAs). Several chemical libraries were screened and a set of 29 compounds demonstrated to reproducibly cause loss of pigmentation, including 9 azole antifungals, a statin and 3 NSAIDs. Quantitative analysis of vacuolar morphology revealed that (excluding the azoles) a sub-set of 14 VDAs significantly alter vacuolar number, size and/or shape. Many C. albicans mutants with impaired vacuolar function are deficient in the formation of hyphal elements, a process essential for its pathogenicity. Accordingly, all 14 VDAs negatively impact C. albicans hyphal morphogenesis. Fungal selectivity was observed for approximately half of the VDA compounds identified, since they did not alter the morphology of the equivalent mammalian organelle, the lysosome. Collectively, these compounds comprise of a new collection of chemical probes that directly or indirectly perturb normal vacuolar function in C. albicans.

Endosomal Trafficking Defects Can Induce Calcium-Dependent Azole Tolerance in Candida albicans

The azole antifungals arrest fungal growth through inhibition of ergosterol biosynthesis. We recently reported that a Candida albicans vps21Δ/Δ mutant, deficient in membrane trafficking through the late endosome/prevacuolar compartment (PVC), continues to grow in the presence of the azoles despite the depletion of cellular ergosterol. Here, we report that the vps21Δ/Δ mutant exhibits less plasma membrane damage upon azole treatment than the wild type, as measured by the release of a cytoplasmic luciferase reporter into the culture supernatant. Our results also reveal that the vps21Δ/Δ mutant has abnormal levels of intracellular Ca²⁺ and, in the presence of fluconazole, enhanced expression of a calcineurin-responsive RTA2-GFP reporter. Furthermore, the azole tolerance phenotype of the vps21Δ/Δ mutant is dependent upon both extracellular calcium levels and calcineurin activity. These findings underscore the importance of endosomal trafficking in determining the cellular consequences of azole treatment and indicate that this may occur through modulation of calcium- and calcineurin-dependent responses.

The Ccz1 mediates the autophagic clearance of damaged mitochondria in response to oxidative stress in Candida albicans

Autophagy plays a critical role in response to numerous cellular stresses, such as nutrient deprivation, hypoxia, starvation and organelle damage. The disruption of autophagy pathway affects multiple aspects of cellular stress response. Here we for the first time identified Ccz1 as an essential component for autophagy in Candida albicans. Our experiments demonstrated that loss of CCZ1 gene led to vacuolar fragmentation and disruption of the autophagy pathway. Our results also suggested that Ccz1 functioned in oxidative stress. In the ccz1Δ/Δ mutant, the levels of reactive oxidative species (ROS) sharply increased under H2O2 treatment. Further studies demonstrated that breakdown of the autophagic clearance pathway led to the accumulation of oxidative stress-damaged mitochondria, and consequently elevated cellular ROS levels in the ccz1Δ/Δ mutant. Furthermore, deletion of CCZ1 led to a significant defect in filamentous development at both 30°C and 37°C. The disruption of CCZ1 gene led to decreased capacity of macrophage killing and increased sensitivity to the macrophages. In addition, the ccz1Δ/Δ mutant exhibited attenuated virulence and decreased fungal burdens in the mouse systemic infection model, indicating that CCZ1 might provide a promising target for antifungal drugs development. In summary, our findings provide new insights into the understanding of autophagy-related gene in C. albicans.

ERG2 and ERG24 Are Required for Normal Vacuolar Physiology as Well as Candida albicans Pathogenicity in a Murine Model of Disseminated but Not Vaginal Candidiasis

Several important classes of antifungal agents, including the azoles, act by blocking ergosterol biosynthesis. It was recently reported that the azoles cause massive disruption of the fungal vacuole in the prevalent human pathogen Candida albicans. This is significant because normal vacuolar function is required to support C. albicans pathogenicity. This study examined the impact of the morpholine antifungals, which inhibit later steps of ergosterol biosynthesis, on C. albicans vacuolar integrity. It was found that overexpression of either the ERG2 or ERG24 gene, encoding C-8 sterol isomerase or C-14 sterol reductase, respectively, suppressed C. albicans sensitivity to the morpholines. In addition, both erg2Δ/Δ and erg24Δ/Δ mutants were hypersensitive to the morpholines. These data are consistent with the antifungal activity of the morpholines depending upon the simultaneous inhibition of both Erg2p and Erg24p. The vacuoles within both erg2Δ/Δ and erg24Δ/Δ C. albicans strains exhibited an aberrant morphology and accumulated large quantities of the weak base quinacrine, indicating enhanced vacuolar acidification compared with that of control strains. Both erg mutants exhibited significant defects in polarized hyphal growth and were avirulent in a mouse model of disseminated candidiasis. Surprisingly, in a mouse model of vaginal candidiasis, both mutants colonized mice at high levels and induced a pathogenic response similar to that with the controls. Thus, while targeting Erg2p or Erg24p alone could provide a potentially efficacious therapy for disseminated candidiasis, it may not be an effective strategy to treat vaginal infections. The potential value of drugs targeting these enzymes as adjunctive therapies is discussed.

Contribution of Fdh3 and Glr1 to Glutathione Redox State, Stress Adaptation and Virulence in Candida albicans

The major fungal pathogen of humans, Candida albicans, is exposed to reactive nitrogen and oxygen species following phagocytosis by host immune cells. In response to these toxins, this fungus activates potent anti-stress responses that include scavenging of reactive nitrosative and oxidative species via the glutathione system. Here we examine the differential roles of two glutathione recycling enzymes in redox homeostasis, stress adaptation and virulence in C. albicans: glutathione reductase (Glr1) and the S-nitrosoglutathione reductase (GSNOR), Fdh3. We show that the NADPH-dependent Glr1 recycles GSSG to GSH, is induced in response to oxidative stress and is required for resistance to macrophage killing. GLR1 deletion increases the sensitivity of C. albicans cells to H2O2, but not to formaldehyde or NO. In contrast, Fdh3 detoxifies GSNO to GSSG and NH3, and FDH3 inactivation delays NO adaptation and increases NO sensitivity. C. albicans fdh3⎔ cells are also sensitive to formaldehyde, suggesting that Fdh3 also contributes to formaldehyde detoxification. FDH3 is induced in response to nitrosative, oxidative and formaldehyde stress, and fdh3Δ cells are more sensitive to killing by macrophages. Both Glr1 and Fdh3 contribute to virulence in the Galleria mellonella and mouse models of systemic infection. We conclude that Glr1 and Fdh3 play differential roles during the adaptation of C. albicans cells to oxidative, nitrosative and formaldehyde stress, and hence during the colonisation of the host. Our findings emphasise the importance of the glutathione system and the maintenance of intracellular redox homeostasis in this major pathogen.

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.

Trafficking through the late endosome significantly impacts Candida albicans tolerance of the azole antifungals

The azole antifungals block ergosterol biosynthesis by inhibiting lanosterol demethylase (Erg11p). The resulting depletion of cellular ergosterol and the accumulation of "toxic" sterol intermediates are both thought to compromise plasma membrane function. However, the effects of ergosterol depletion upon the function of intracellular membranes and organelles are not well described. The purpose of this study was to characterize the effects of azole treatment upon the integrity of the Candida albicans vacuole and to determine whether, in turn, vacuolar trafficking influences azole susceptibility. Profound fragmentation of the C. albicans vacuole can be observed as an early consequence of azole treatment, and it precedes significant growth inhibition. In addition, a C. albicans vps21Δ/Δ mutant, blocked in membrane trafficking through the late endosomal prevacuolar compartment (PVC), is able to grow significantly more than the wild type in the presence of several azole antifungals under standard susceptibility testing conditions. Furthermore, the vps21Δ/Δ mutant is able to grow despite the depletion of cellular ergosterol. This phenotype resembles an exaggerated form of "trailing growth" that has been described for some clinical isolates. In contrast, the vps21Δ/Δ mutant is hypersensitive to drugs that block alternate steps in ergosterol biosynthesis. On the basis of these results, we propose that endosomal trafficking defects may lead to the cellular "redistribution" of the sterol intermediates that accumulate following inhibition of ergosterol biosynthesis. Furthermore, the destination of these intermediates, or the precise cellular compartments in which they accumulate, may be an important determinant of their toxicity and thus ultimately antifungal efficacy.

Role of TFP1 in vacuolar acidification, oxidative stress and filamentous development in Candida albicans

The vacuolar-type H(+)-ATPase (V-ATPase) is a multiprotein complex consisting of the V0 and V1 sectors, and is required for vacuolar acidification and virulence in the opportunistic fungal pathogen Candida albicans. In this study, we identified C. albicans Tfp1 as a putative subunit of V-ATPase, and explored its importance in multiple cellular processes. Our results revealed that Tfp1 played an essential role in vacuolar acidification and endocytic trafficking. In addition, the tfp1Δ/Δ mutant was sensitive to alkaline pH and elevated calcium concentrations, which is characteristic of loss of V-ATPase activity. The mutant also showed hypersensitivity to metal ions which might be attributed to a defect in sequestration of toxic ions to the vacuole through proton gradient produced by V-ATPase. Interestingly, deletion of TFP1 triggered endogenous oxidative stress even without exogenous oxidants. Compared with the wild-type strain, the tfp1Δ/Δ mutant showed significantly higher ROS levels and lower expression levels of redox-related genes with the addition of hydrogen peroxide (H2O2). Western blotting analysis showed that deletion of TFP1 significantly reduced the expression of Cap1 under H2O2 treatment, which contributes to the regulation of genes involved in the oxidative stress response. Furthermore, the tfp1Δ/Δ mutant showed significantly impaired filamentous development in hyphal induction media, and was avirulent in a mouse model of systemic candidiasis. Taken together, our results suggested that the putative V1 subunit Tfp1 is essential for vacuolar function and C. albicans pathogenesis, and provided a promising candidate for antifungal drugs.

Molecular genetic techniques for gene manipulation in Candida albicans

Candida albicans is one of the most common fungal pathogen in humans due to its high frequency as an opportunistic and pathogenic fungus causing superficial as well as invasive infections in immunocompromised patients. An understanding of gene function in C. albicans is necessary to study the molecular basis of its pathogenesis, virulence and drug resistance. Several manipulation techniques have been used for investigation of gene function in C. albicans, including gene disruption, controlled gene expression, protein tagging, gene reintegration, and overexpression. In this review, the main cassettes containing selectable markers used for gene manipulation in C. albicans are summarized; the advantages and limitations of these cassettes are discussed concerning the influences on the target gene expression and the virulence of the mutant strains.

Novel role of the Candida albicans ferric reductase gene CFL1 in iron acquisition, oxidative stress tolerance, morphogenesis and virulence

Ferric reductase catalyzes the reduction of ferric iron into ferrous iron and plays an essential role in high-affinity iron acquisition. In this study, we found that the cfl1Δ/Δ (orf19.1263) mutant was not defective in iron acquisition. However, deletion of CFL1 increased cellular iron accumulation by elevating surface ferric reductase activity in Candida albicans, revealing that there existed functional redundancy and/or a compensatory upregulation mechanism among ferric reductase genes. The absence of CFL1 resulted in increased expression levels of other alternative ferric reductase genes, including FRP1, CFL2 and FRE10. In addition, CFL1 played an important role in the response to different oxidative stresses. Further research revealed that the cfl1Δ/Δ mutant exhibited higher levels of both ROS production and SOD activity under oxidative conditions. Moreover, deletion of CFL1 led to a profound defect in filamentous development in an iron-independent manner at both 30 and 37 °C. The cfl1Δ/Δ mutant exhibited highly attenuated virulence and reduced fungal burdens in the mouse systemic infection model, indicating that CFL1 might be a potential target for antifungal drug development. In summary, our results provide new insights into the roles of ferric reductase gene in C. albicans.

The Cryptococcus neoformans Rim101 transcription factor directly regulates genes required for adaptation to the host

The Rim101 protein is a conserved pH-responsive transcription factor that mediates important interactions between several fungal pathogens and the infected host. In the human fungal pathogen Cryptococcus neoformans, the Rim101 protein retains conserved functions to allow the microorganism to respond to changes in pH and other host stresses. This coordinated cellular response enables this fungus to effectively evade the host immune response. Preliminary studies suggest that this conserved transcription factor is uniquely regulated in C. neoformans both by the canonical pH-sensing pathway and by the cyclic AMP (cAMP)/protein kinase A (PKA) pathway. Here we present comparative transcriptional data that demonstrate a strong concordance between the downstream effectors of PKA and Rim101. To define Rim101-dependent gene expression during a murine lung infection, we used nanoString profiling of lung tissue infected with a wild-type or rim101Δ mutant strain. In this setting, we demonstrated that Rim101 controls the expression of multiple cell wall-biosynthetic genes, likely explaining the enhanced immunogenicity of the rim101Δ mutant. Despite its divergent upstream regulation, the C. neoformans Rim101 protein recognizes a conserved DNA binding motif. Using these data, we identified direct targets of this transcription factor, including genes involved in cell wall regulation. Therefore, the Rim101 protein directly controls cell wall changes required for the adaptation of C. neoformans to its host environment. Moreover, we propose that integration of the cAMP/PKA and pH-sensing pathways allows C. neoformans to respond to a broad range of host-specific signals.

A novel role of the vacuolar calcium channel Yvc1 in stress response, morphogenesis and pathogenicity of Candida albicans

C. albicans is a common opportunistic pathogen, causing both superficial and life-threatening systemic infections. Calcium signaling is an intriguing aspect in its physiology, attributing to the roles in stress response and morphogenesis. Until recently, little was known about the mechanisms by which the calcium signaling-associated elements affect its pathogenicity. In this study, we found that Yvc1, a member of the transient receptor potential (TRP) family, localized on the vacuolar membrane. The yvc1Δ/Δ mutant displayed decreased ability of stress response, morphogenesis and attenuated virulence. The Spitzenkörper required for polarized growth were not detected in the hyphal tip of this mutant, suggesting a key role of Yvc1 in hyphal polarized growth and re-orientation to host signals. This study demonstrates, for the first time, that the putative vacuolar calcium channel Yvc1 plays an important role in C. albicans infection and survival in host tissues, which is associated with its pleiotropic effects in several fungal physiological processes, including stress response, morphogenesis, and polarized growth.

Engineering Candida albicans to secrete a host immunomodulatory factor

Gene knockout and transgenic mice are important tools that are widely used to dissect the mammalian hosts' responses to microbial invasion. A novel alternative is to engineer the pathogen itself to secrete host factors that stimulate or suppress specific immune defense mechanisms. Herein, we have described and validated an approach to facilitate the production and export of ectopic host proteins, from the most prevalent human fungal pathogen, Candida albicans. Our strategy utilized a prepropeptide from the C. albicans secreted aspartic proteinase, Sap2p. The prepeptide facilitates entry of Sap2p into the secretory pathway, while the propeptide maintains the protease as an inactive precursor, until proteolytic cleavage in the Golgi apparatus releases the mature protein. The Sap2p prepropeptide coding sequence was linked to that of two mammalian calcium-binding proteins, S100A8 and S100A9, which are associated with symptomatic vaginal candidiasis. The resulting expression constructs were then introduced into C. albicans. While the S100A8 protein is secreted into the growth medium intact, the S100A9 protein is apparently degraded during transit. Nonetheless, culture supernatants from both S100A8 and S100A9 expressing C. albicans strains acted as potent chemoattractants for a macrophage-like cell line and polymorphonuclear leukocytes. Thus, the pathogen-derived mammalian proteins possessed the expected biological activity.

Three prevacuolar compartment Rab GTPases impact Candida albicans hyphal growth

Disruption of vacuolar biogenesis in the pathogenic yeast Candida albicans causes profound defects in polarized hyphal growth. However, the precise vacuolar pathways involved in yeast-hypha differentiation have not been determined. Previously we focused on Vps21p, a Rab GTPase involved in directing vacuolar trafficking through the late endosomal prevacuolar compartment (PVC). Herein, we identify two additional Vps21p-related GTPases, Ypt52p and Ypt53p, that colocalize with Vps21p and can suppress the hyphal defects of the vps21Δ/Δ mutant. Phenotypic analysis of gene deletion strains revealed that loss of both VPS21 and YPT52 causes synthetic defects in endocytic trafficking to the vacuole, as well as delivery of the virulence-associated vacuolar membrane protein Mlt1p from the Golgi compartment. Transcription of all three GTPase-encoding genes is increased under hyphal growth conditions, and overexpression of the transcription factor Ume6p is sufficient to increase the transcription of these genes. While only the vps21Δ/Δ single mutant has hyphal growth defects, these were greatly exacerbated in a vps21Δ/Δ ypt52Δ/Δ double mutant. On the basis of relative expression levels and phenotypic analysis of gene deletion strains, Vps21p is the most important of the three GTPases, followed by Ypt52p, while Ypt53p has an only marginal impact on C. albicans physiology. Finally, disruption of a nonendosomal AP-3-dependent vacuolar trafficking pathway in the vps21Δ/Δ ypt52Δ/Δ mutant, further exacerbated the stress and hyphal growth defects. These findings underscore the importance of membrane trafficking through the PVC in sustaining the invasive hyphal growth form of C. albicans.

Novel insight into the expression and function of the multicopper oxidases in Candida albicans

Iron is an essential element required for most organisms. The high-affinity iron-uptake systems in the opportunistic pathogen Candida albicans are activated under iron-limited conditions and are also required for virulence. Here one component of high-affinity iron-uptake systems, the multicopper oxidase (MCO) genes, was characterized. We examined the expression of five MCO genes and demonstrated that CaFET3 and CaFET34 were the major MCO genes in response to iron deficiency. Complementation of the Saccharomyces cerevisiae fet3Δ mutant showed that CaFET34 could effectively rescue the growth phenotype in iron-limited medium. Deletion of CaFET33 and CaFET34 in C. albicans decreased cellular iron content and iron acquisition during iron starvation. However, the fet33Δ/Δ and fet34Δ/Δ mutants exhibited no obvious growth defect in solid iron-limited medium while the fet34Δ/Δ mutant showed a slight growth defect in liquid medium. Further analysis shows that other members of the five MCO genes, especially CaFET3, would compensate for the absence of CaFET33 and CaFET34. Furthermore, for the first time, we provide evidence that CaFET34 is implicated in hyphal development in an iron-independent manner and is required for C. albicans virulence in a mouse model of systemic infection. Together, our results not only expand our understanding about the expression of the MCO genes in C. albicans, but also provide a novel insight into the role of CaFET34 in iron metabolism, hyphal development and virulence.

Role of Ess1 in growth, morphogenetic switching, and RNA polymerase II transcription in Candida albicans

Candida albicans is a fungal pathogen that causes potentially fatal infections among immune-compromised individuals. The emergence of drug resistant C. albicans strains makes it important to identify new antifungal drug targets. Among potential targets are enzymes known as peptidyl-prolyl cis/trans isomerases (PPIases) that catalyze isomerization of peptide bonds preceding proline. We are investigating a PPIase called Ess1, which is conserved in all major human pathogenic fungi. Previously, we reported that C. albicans Ess1 is essential for growth and morphogenetic switching. In the present study, we re-evaluated these findings using more rigorous genetic analyses, including the use of additional CaESS1 mutant alleles, distinct marker genes, and the engineering of suitably-matched isogenic control strains. The results confirm that CaEss1 is essential for growth in C. albicans, but show that reduction of CaESS1 gene dosage by half (δ/+) does not interfere with morphogenetic switching. However, further reduction of CaEss1 levels using a conditional allele does reduce morphogenetic switching. We also examine the role of the linker α-helix that distinguishes C. albicans Ess1 from the human Pin1 enzyme, and present results of a genome-wide transcriptome analysis. The latter analysis indicates that CaEss1 has a conserved role in regulation of RNA polymerase II function, and is required for efficient termination of small nucleolar RNAs and repression of cryptic transcription in C. albicans.

Deletion and expression analysis of beta-(1,3)-glucanosyltransferase genes in Neurospora crassa

GPI(glycosylphosphatidylinositol)-anchored beta-(1,3)-glucanosyltransferases play an active role in cell wall biosynthesis in fungi. Neurospora crassa has 5 putative beta-(1,3)-glucanosyltransferase genes, namely, gel-1, gel-2, gel-3, gel-4, and gel-5, in its genome. Among them, the gel-3 gene is constitutively expressed at the highest level in growing hyphae, whereas gel-1 is expressed at the lowest level. The gel-3 deletion mutant displayed slow growth, while other gel gene disruptants exhibited normal growth. Although no gel gene disruption affected pH sensitivity and fertility, all Δgel mutants were resistant to cell wall degradation enzymes. Micafungin, a beta-(1,3)-glucan synthase inhibitor, induced gel-4 expression in the wild-type and 2 MAP kinase mutants mak-1 and mak-2 strains. In contrast, fludioxonil, an activator of OS-2 MAP kinase, strongly induced the gel-1 gene in the wild-type strain. Its induction was nearly abolished in the os-2 and in the atf-1/asl-1 mutant. These suggested that GEL-3 is a major factor in mycelial growth, while GEL-1 and GEL-4 may play important roles in cell wall remodeling in response to stress conditions or cell wall damage, respectively.

Ambient pH controls glycogen levels by regulating glycogen synthase gene expression in Neurospora crassa. New insights into the pH signaling pathway

Glycogen is a polysaccharide widely distributed in microorganisms and animal cells and its metabolism is under intricate regulation. Its accumulation in a specific situation results from the balance between glycogen synthase and glycogen phosphorylase activities that control synthesis and degradation, respectively. These enzymes are highly regulated at transcriptional and post-translational levels. The existence of a DNA motif for the Aspergillus nidulans pH responsive transcription factor PacC in the promoter of the gene encoding glycogen synthase (gsn) in Neurospora crassa prompted us to investigate whether this transcription factor regulates glycogen accumulation. Transcription factors such as PacC in A. nidulans and Rim101p in Saccharomyces cerevisiae play a role in the signaling pathway that mediates adaptation to ambient pH by inducing the expression of alkaline genes and repressing acidic genes. We showed here that at pH 7.8 pacC was over-expressed and gsn was down-regulated in wild-type N. crassa coinciding with low glycogen accumulation. In the pacC^KO strain the glycogen levels and gsn expression at alkaline pH were, respectively, similar to and higher than the wild-type strain at normal pH (5.8). These results characterize gsn as an acidic gene and suggest a regulatory role for PACC in gsn expression. The truncated recombinant protein, containing the DNA-binding domain specifically bound to a gsn DNA fragment containing the PacC motif. DNA-protein complexes were observed with extracts from cells grown at normal and alkaline pH and confirmed by ChIP-PCR analysis. The PACC present in these extracts showed equal molecular mass, indicating that the protein is already processed at normal pH, in contrast to A. nidulans. Together, these results show that the pH signaling pathway controls glycogen accumulation by regulating gsn expression and suggest the existence of a different mechanism for PACC activation in N. crassa.

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.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.