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

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Transcriptional regulation of the synthesis and secretion of farnesol in the fungus Candida albicans: examination of the Homann transcription regulator knockout collection

Candida albicans is an efficient colonizer of human gastrointestinal tracts and skin and is an opportunistic pathogen. C. albicans exhibits morphological plasticity, and the ability to switch between yeast and filamentous morphologies is associated with virulence. One regulator of this switch is the quorum sensing molecule farnesol that is produced by C. albicans throughout growth. However, the synthesis, secretion, regulation, and turnover of farnesol are not fully understood. To address this, we used our improved farnesol assay to screen a transcription regulator knockout library for differences in farnesol accumulation in whole cultures, pellets, and supernatants. All screened mutants produced farnesol and they averaged 9.2× more farnesol in the pellet than the supernatant. Nineteen mutants had significant differences with ten mutants producing more farnesol than their SN152+ wild-type control strain while nine produced less. Seven mutants exhibited greater secretion of farnesol while two exhibited less. We examined the time course for farnesol accumulation in six mutants with the greatest accumulation differences and found that those differences persisted throughout growth and they were not time dependent. Significantly, two high-accumulating mutants did not exhibit the decay in farnesol levels during stationary phase characteristic of wild-type C. albicans, suggesting that a farnesol modification/degradation mechanism is absent in these mutants. Identifying these transcriptional regulators provides new insight into farnesol's physiological functions regarding cell cycle progression, white-opaque switching, yeast-mycelial dimorphism, and response to cellular stress.

Integrative analysis of multi-omics data reveals inhibition of RB1 signaling promotes apatinib resistance of hepatocellular carcinoma

Although apatinib is a promising drug for the treatment of liver cancer, the underlying drug resistance mechanism is still unclear. Here, we constructed apatinib-resistant HepG2 cells. We then characterized the epigenomic, transcriptomic, and proteomic landscapes both in apatinib-resistant and non-resistant HepG2 cells. Differential expression, ATAC-seq, and proteomic data analyses were performed. We found that the cell cycle related protein RB1 may play an essential role in the process of apatinib resistant to hepatocarcinoma. Moreover, there were extensive variations at the transcriptome, epigenetic, and proteomic level. Finally, quantitative PCR (qPCR) and western blot analysis showed that expression level of RB1 in apatinib-resistant cell as well as the samples of patients in progressive disease were significantly lower than that in controls. Those results also showed that the RB1 pathway inhibitors CDK2-IN-73 and Palbociclib could relieve the resistance of apatinib resistant cells. Our results further enhance our understanding of the anti-tumorigenic and anti-angiogenic efficacy of apatinib in liver cancer and provide a novel perspective regarding apatinib resistance. Furthermore, we proved that CDKN2B inhibition of RB1 signaling promoted apatinib resistance in hepatocellular carcinoma. Those findings have greatly important biological significance for the resistance of apatinib and the treatment of liver cancer.

Genome-Wide DNA Changes Acquired by Candida albicans Caspofungin-Adapted Mutants

Drugs from the echinocandin (ECN) class are now recommended 'front-line' treatments of infections caused by a prevailing fungal pathogen, C. albicans. However, the increased use of ECNs is associated with a rising resistance to ECNs. As the acquisition of ECN resistance in C. albicans is viewed as a multistep evolution, determining factors that are associated with the decreased ECN susceptibility is of importance. We have recently identified two cohorts of genes that are either up- or downregulated in concert in order to control remodeling of cell wall, an organelle targeted by ECNs, in laboratory mutants with decreased ECN susceptibility. Here, we profiled the global DNA sequence of four of these adapted mutants in search of DNA changes that are associated with decreased ECN susceptibility. We find a limited number of 112 unique mutations representing two alternative mutational pathways. Approximately half of the mutations occurred as hotspots. Approximately half of mutations and hotspots were shared by ECN-adapted mutants despite the mutants arising as independent events and differing in some of their phenotypes, as well as in condition of chromosome 5. A total of 88 mutations are associated with 43 open reading frames (ORFs) and occurred inside of an ORF or within 1 kb of an ORF, predominantly as single-nucleotide substitution. Mutations occurred more often in the 5'-UTR than in the 3'-UTR by a 1.67:1 ratio. A total of 16 mutations mapped to eight genomic features that were not ORFs: Tca4-4 retrotransposon; Tca2-7 retrotransposon; lambda-4a long terminal repeat; mu-Ra long terminal repeat; MRS-7b Major Repeat Sequence; MRS-R Major Repeat Sequence; RB2-5a repeat sequence; and tL (CAA) leucine tRNA. Finally, eight mutations are not associated with any ORF or other genomic feature. Repeated occurrence of single-nucleotide substitutions in non-related drug-adapted mutants strongly indicates that these DNA changes are accompanying drug adaptation and could possibly influence ECN susceptibility, thus serving as factors facilitating evolution of ECN drug resistance due to classical mutations in FKS1.

Molecular Determinants Involved in Candida albicans Biofilm Formation and Regulation

Candida albicans is known for its pathogenicity, although it lives within the human body as a commensal member. The commensal nature of C. albicans is well controlled and regulated by the host's immune system as they live in the harmonized microenvironment. However, the development of certain unusual microhabitat conditions (change in pH, co-inhabiting microorganisms' population ratio, debilitated host-immune system) pokes this commensal fungus to transform into a pathogen in such a way that it starts to propagate very rapidly and tries to breach the epithelial barrier to enter the host's systemic circulations. In addition, Candida is infamous as a major nosocomial (hospital-acquired infection) agent because it enters the human body through venous catheters or medical prostheses. The hysterical mode of C. albicans growth builds its microcolony or biofilm, which is pathogenic for the host. Biofilms propose additional resistance mechanisms from host immunity or extracellular chemicals to aid their survival. Differential gene expressions and regulations within the biofilms cause altered morphology and metabolism. The genes associated with adhesiveness, hyphal/pseudo-hyphal growth, persister cell transformation, and biofilm formation by C. albicans are controlled by myriads of cell-signaling regulators. These genes' transcription is controlled by different molecular determinants like transcription factors and regulators. Therefore, this review has focused discussion on host-immune-sensing molecular determinants of Candida during biofilm formation, regulatory descriptors (secondary messengers, regulatory RNAs, transcription factors) of Candida involved in biofilm formation that could enable small-molecule drug discovery against these molecular determinants, and lead to disrupt the well-structured Candida biofilms effectively.

Hgc1 Independence of Biofilm Hyphae in Candida albicans

Biofilm and hypha formation are central to virulence of the fungal pathogen Candida albicans. The G1 cyclin gene HGC1 is required for hypha formation under diverse in vitro and in vivo growth conditions. Hgc1 is required for disseminated infection and is a linchpin in the argument that hyphal morphogenesis itself is required for pathogenicity. We report here that HGC1 is dispensable for hypha formation during biofilm formation both in vitro, under strong inducing conditions, and in vivo, in a mouse oropharyngeal candidiasis model. These findings are validated with two or more C. albicans isolates. Systematic screening of overexpressed cyclin genes indicates that CCN1 and CLN3 can compensate partially for Hgc1 function during biofilm growth. This conclusion is also supported by the severity of the hgc1Δ/Δ ccn1Δ/Δ double mutant biofilm defect. Our results suggest that hypha formation in biofilm is accomplished by combined action of multiple cyclins, not solely by Hgc1. IMPORTANCE The HGC1 gene encodes a cyclin that is required for virulence of the fungal pathogen Candida albicans. It is required to produce the elongated hyphal filaments of free-living planktonic cells that are associated with virulence. Here, we show that HGC1 is not required to produce hyphae in the alternative growth form of a biofilm community. We observe Hgc1-independent hyphae in two infection-relevant situations, biofilm growth in vitro and biofilm-like oropharyngeal infection. Our analysis suggests that hypha formation in the biofilm state reflects combined action of multiple cyclins.

Multiple Genes of Candida albicans Influencing Echinocandin Susceptibility in Caspofungin-Adapted Mutants

Candida albicans is an opportunistic human fungal pathogen that causes invasive infections in immunocompromised individuals. Despite the high anticandidal activity among the echinocandins (ECNs), a first-line therapy, resistance remains an issue. Furthermore, many clinical isolates display decreased ECN susceptibility, a physiological state which is thought to lead to resistance. Determining the factors that can decrease susceptibility is of high importance. We searched for such factors genome-wide by comparing the transcriptional profiles of five mutants that acquired decreased caspofungin susceptibility in vitro in the absence of canonical FKS1 resistance mutations. The mutants were derived from two genetic backgrounds and arose due to independent mutational events, some with monosomic chromosome 5 (Ch5). We found that the mutants exhibit common transcriptional changes. In particular, all mutants upregulate five genes from Ch2 in concert. Knockout experiments show that all five genes positively influence caspofungin and anidulafungin susceptibility and play a role in regulating the cell wall mannan and glucan contents. The functions of three of these genes, orf19.1766, orf19.6867, and orf19.5833, were previously unknown, and our work expands the known functions of LEU42 and PR26. Importantly, orf19.1766 and LEU42 have no human orthologues. Our results provide important clues as to basic mechanisms of survival in the presence of ECNs while identifying new genes controlling ECN susceptibility and revealing new targets for the development of novel antifungal drugs.

The Role of Sfp1 in Candida albicans Cell Wall Maintenance

The cell wall is the first interface for Candida albicans interaction with the surrounding environment and the host cells. Therefore, maintenance of cell wall integrity (CWI) is crucial for C. albicans survival and host-pathogen interaction. In response to environmental stresses, C. albicans undergoes cell wall remodeling controlled by multiple signaling pathways and transcription regulators. Here, we explored the role of the transcription factor Sfp1 in CWI. A deletion of the SFP1 gene not only caused changes in cell wall properties, cell wall composition and structure but also modulated expression of cell wall biosynthesis and remodeling genes. In addition, Cas5 is a known transcription regulator for C. albicans CWI and cell wall stress response. Interestingly, our results indicated that Sfp1 negatively controls the CAS5 gene expression by binding to its promoter element. Together, this study provides new insights into the regulation of C. albicans CWI and stress response.

Functional analysis of the Candida albicans kinome reveals Hrr25 as a regulator of antifungal susceptibility

Candida albicans is a leading cause of death due to systemic fungal infections. Poor patient outcomes are attributable to the limited number of antifungal classes and the increasing prevalence of drug resistance. Protein kinases have emerged as rewarding targets in the development of drugs for diverse diseases, yet kinases remain untapped in the quest for new antifungals. Here, we performed a comprehensive analysis of the C. albicans kinome to identify genes for which loss-of-function confers hypersensitivity to the two most widely deployed antifungals, echinocandins and azoles. Through this analysis, we found a role for the casein kinase 1 (CK1) homologue Hrr25 in regulating tolerance to both antifungals as well as target-mediated echinocandin resistance. Follow-up investigations established that Hrr25 regulates these responses through its interaction with the SBF transcription factor. Thus, we provide insights into the circuitry governing cellular responses to antifungals and implicate Hrr25 as a key mediator of drug resistance.

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Screening Candida albicans kinase mutants reveals 47 regulators of antifungal tolerance

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Hrr25 is important for growth and cell wall/membrane stress tolerance

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Hrr25 enables target-mediated echinocandin resistance

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Hrr25 interacts with the SBF transcription factor complex

Candida glycerinogenes Strains Overexpressing Transcription Factors have Improved Furfural Tolerance in Ethanol Production from Non-detoxified Cellulose Hydrolysate

Cellulose is one of the main raw materials for production of green ethanol, but the presence of the growth inhibitor furfural in non-detoxified lignocellulosic hydrolysates often seriously affects their utilization. In a previous study, we obtained strains of Candida glycerinogenes that were tolerant to furfural, but at concentrations above 2.5 g L^-1 there was a significant increase in the growth lag phase. In this work, transcription factor genes (SEF1, STB5, CAS5, and ETP1) associated with furfural tolerance were identified and employed to obtain modified strains permitting ethanol fermentation of concentrated and non-detoxified cellulose hydrolysates containing more than 2.5 g L^-1 furfural. Tolerance to furfural could be increased to 4.5 g L^-1 by overexpression of either STB5 or ETP1, which have different regulation patterns. Moreover, in non-detoxified and concentrated cellulose hydrolysate, overexpression of ETP1 significantly shortened the growth lag phase and ethanol fermentation time was reduced by 17-20%. In batch fermentations fed with concentrated non-detoxified lignocellulose hydrolysate, ethanol productivity and maximum ethanol concentration reached 2.4 g L^-1 h^-1 and 72.5 g L^-1, increases of 26.1% and 6.6%, respectively. The results provided a route for the economic use of lignocellulose for chemical production.

Caspofungin resistance in Candida albicans: genetic factors and synergistic compounds for combination therapies

Caspofungin and other echinocandins have been used for the treatment of human infections by the opportunistic yeast pathogen, Candida albicans. There has been an increase in infections by non-albicans Candida species such as Candida glabrata, Candida parapsilosis, Candida tropicalis, Candida krusei, and Candida auris in clinical or hospital settings. This is problematic to public health due to the increasing prevalence of echinocandin resistant species/strains. This review will present a summary on various studies that investigated the inhibitory action of caspofungin on 1,3-β-D-glucan synthesis, on cell wall structure, and biofilm formation of C. albicans. It will highlight some of the issues linked to caspofungin resistance or reduced caspofungin sensitivity in various Candida species and the potential benefits of antimicrobial peptides and other compounds in synergy with caspofungin.

The role of Candida albicans stress response pathways in antifungal tolerance and resistance

Human fungal pathogens are the causative agents of devastating diseases across the globe, and the increasing prevalence of drug resistance threatens to undermine the already limited treatment options. One prominent pathogen is the opportunistic fungus Candida albicans, which can cause both superficial and serious systemic infections in immunocompromised individuals. C. albicans antifungal drug resistance and antifungal tolerance are supported by diverse and expansive cellular stress response pathways. Some of the major players are the Ca²⁺-calmodulin-activated phosphatase calcineurin, the protein kinase C cell wall integrity pathway, and the molecular chaperone heat shock protein 90. Beyond these core signal transducers, several other enzymes and transcription factors have been implicated in both tolerance and resistance. Here, we highlight some of the major stress response pathways, key advances in identifying chemical matter to inhibit these pathways, and implications for C. albicans persistence in the host.

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Candida albicans can cause superficial and serious systemic infections in humans

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Stress response pathways regulate C. albicans antifungal resistance and tolerance

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Stress response regulators include calcineurin, Pkc1, Hsp90, and many others

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Stress response inhibitors could reduce the likelihood of fungi persisting in humans

A Markerless CRISPR-Mediated System for Genome Editing in Candida auris Reveals a Conserved Role for Cas5 in the Caspofungin Response

Candida auris is a multidrug-resistant human fungal pathogen that has recently emerged worldwide. It can cause life-threatening disseminated infections in humans, with mortality rates upwards of 50%. The molecular mechanisms underlying its multidrug resistance and pathogenic properties are largely unknown. Few methods exist for genome editing in C. auris, all of which rely on selectable markers that limit the number of modifications that can be made. Here, we present a markerless CRISPR/Cas9-mediated genome editing system in C. auris. Using this system, we successfully deleted genes of interest and subsequently reconstituted them at their native loci in isolates across all five C. auris clades. This system also enabled us to introduce precision genome edits to create translational fusions and single point mutations. Using Cas5 as a test case for this system, we discovered a conserved role for Cas5 in the caspofungin response between Candida albicans and C. auris. Overall, the development of a system for precise and facile genome editing in C. auris that can allow edits to be made in a high-throughput manner is a major step forward in improving our understanding of this important human fungal pathogen. IMPORTANCE Candida auris is a recently emerged multidrug-resistant fungal pathogen capable of causing life-threatening systemic infections in humans. Few tools are available for genome editing in C. auris. Here, we present a markerless genome editing system for C. auris that relies on CRISPR/Cas9 technology and works to modify the genomes of all known C. auris clades. Using this system, we discovered a conserved role for Cas5 in the caspofungin response between C. albicans and C. auris. Overall, the development of a system for facile genome editing in C. auris is a major step forward in improving our understanding of this important human fungal pathogen.

PsrA is a novel regulator contributes to antibiotic synthesis, bacterial virulence, cell motility and extracellular polysaccharides production in Serratia marcescens

Serratia marcescens is a Gram-negative bacterium of the Enterobacteriaceae family that can produce numbers of biologically active secondary metabolites. However, our understanding of the regulatory mechanisms behind secondary metabolites biosynthesis in S. marcescens remains limited. In this study, we identified an uncharacterized LysR family transcriptional regulator, encoding gene BVG90_12635, here we named psrA, that positively controlled prodigiosin synthesis in S. marcescens. This phenotype corresponded to PsrA positive control of transcriptional of the prodigiosin-associated pig operon by directly binding to a regulatory binding site (RBS) and an activating binding site (ABS) in the promoter region of the pig operon. We demonstrated that L-proline is an effector for the PsrA, which enhances the binding affinity of PsrA to its target promoters. Using transcriptomics and further experiments, we show that PsrA indirectly regulates pleiotropic phenotypes, including serrawettin W1 biosynthesis, extracellular polysaccharide production, biofilm formation, swarming motility and T6SS-mediated antibacterial activity in S. marcescens. Collectively, this study proposes that PsrA is a novel regulator that contributes to antibiotic synthesis, bacterial virulence, cell motility and extracellular polysaccharides production in S. marcescens and provides important clues for future studies exploring the function of the PsrA and PsrA-like proteins which are widely present in many other bacteria.

A Fungal Transcription Regulator of Vacuolar Function Modulates Candida albicans Interactions with Host Epithelial Cells

Microorganisms typically maintain cellular homeostasis despite facing large fluctuations in their surroundings. Microbes that reside on human mucosal surfaces may experience significant variations in nutrient and ion availability as well as pH. Whether the mechanisms employed by these microbial cells to sustain homeostasis directly impact on the interplay with the host’s mucosae remains unclear. Here, we report that the previously uncharacterized transcription regulator ZCF8 in the human-associated yeast Candida albicans maintains vacuole homeostasis when the fungus faces fluctuations in nitrogen. Genome-wide identification of genes directly regulated by Zcf8p followed by fluorescence microscopy to define their subcellular localization uncovered the fungal vacuole as a top target of Zcf8p regulation. Deletion and overexpression of ZCF8 resulted in alterations in vacuolar morphology and luminal pH and rendered the fungus resistant or susceptible to nigericin and brefeldin A, two drugs that impair vacuole and associated functions. Furthermore, we establish that the regulator modulates C. albicans attachment to epithelial cells in a manner that depends on the status of the fungal vacuole. Our findings, therefore, suggest that fungal vacuole physiology regulation is intrinsically linked to, and shapes to a significant extent, the physical interactions that Candida cells establish with mammalian mucosal surfaces.

Our pursuit for effective antifungal agents targeting fungal cell wall components, where are we?

Invasive mycotic infections account for an unacceptably high mortality rates in humans. These infections are initiated by the fungal cell wall which mediates host-fungi interactions. The cell wall is fused to the physiology of fungi, and it is involved in essential functions in the entire cell functionality. Components of the cell wall are synthesised and modified in the cell wall space by the activities of cell wall proteins through a range of signalling pathways that have only been described in many fungi, therefore making them suitable drug targets. The echinocandins class of cell wall-active drugs block cell wall β-1,3-glucan biosynthesis through inhibiting the catalytic subunit of the synthetic protein complex. Resistance to echinocandins can be through the acquisition of single nucleotide polymorphisms and/or through activation of cell wall signalling pathways resulting in altered cell wall proteome and elevated chitin content in the cell wall. Countering the cell wall remodelling process will enhance the effectiveness of β-1,3-glucan-active antifungal agents. Cell surface proteins are also important antifungal targets which can be used to develop rapid and robust diagnostics and more effective therapeutics. The cell wall remains a crucial target in fungi that needs to be harnessed to combat mycotic infections.

The transcription factor Cas5 suppresses hyphal morphogenesis during yeast-form growth in Candida albicans

Candida albicans is an opportunistic human pathogen that exists as yeast, hyphal or pseudohyphal forms depending on pH, nutrients, and temperature. The morphological transition from yeast to hyphae, which is required for the complete virulence of C. albicans, is controlled by many transcription factors that activate or repress hypha-specific genes. The C. albicans transcriptional factor Cas5, a key regulator of genes involved in cell wall integrity, affects the susceptibility of C. albicans to fluconazole, an inhibitor of ergosterol synthesis. In this study, we found that deletion of CAS5 in C. albicans decreased the expression levels of a set of ergosterol biosynthesis genes, such as ERG2, ERG3, ERG5, ERG6, ERG11, and ERG24, resulting in the accumulation of lanosterol and zymosterol, which are intermediate metabolites in the ergosterol biosynthesis pathway. Interestingly, it was observed that the cas5Δ/Δ mutant could not maintain the yeast form under non-hypha-inducing conditions, while the CAS5-overexpressing cells could not form hyphae under hypha-inducing conditions. Consistent with these observations, the cas5Δ/Δ mutant highly expressed hypha-specific genes, ALS3, ECE1, and HWP1, under non-hypha-inducing conditions. In addition, CAS5 transcription was significantly downregulated immediately after hyphal initiation in the wild-type strain. Furthermore, the cas5Δ/Δ mutant reduced the transcription of NRG1, which encodes a major repressor of hyphal morphogenesis, while Cas5 overexpression increased the transcription of NRG1 under hypha-inducing conditions. Collectively, this study suggests the potential role of Cas5 as a repressor of hypha-specific genes during yeast-form growth of C. albicans.

Fungal Cell Wall Proteins and Signaling Pathways Form a Cytoprotective Network to Combat Stresses

Candida species are part of the normal flora of humans, but once the immune system of the host is impaired and they escape from commensal niches, they shift from commensal to pathogen causing candidiasis. Candida albicans remains the primary cause of candidiasis, accounting for about 60% of the global candidiasis burden. The cell wall of C. albicans and related fungal pathogens forms the interface with the host, gives fungal cells their shape, and also provides protection against stresses. The cell wall is a dynamic organelle with great adaptive flexibility that allows remodeling, morphogenesis, and changes in its components in response to the environment. It is mainly composed of the inner polysaccharide rich layer (chitin, and β-glucan) and the outer protein coat (mannoproteins). The highly glycosylated protein coat mediates interactions between C. albicans cells and their environment, including reprograming of wall architecture in response to several conditions, such as carbon source, pH, high temperature, and morphogenesis. The mannoproteins are also associated with C. albicans adherence, drug resistance, and virulence. Vitally, the mannoproteins contribute to cell wall construction and especially cell wall remodeling when cells encounter physical and chemical stresses. This review describes the interconnected cell wall integrity (CWI) and stress-activated pathways (e.g., Hog1, Cek1, and Mkc1 mediated pathways) that regulates cell wall remodeling and the expression of some of the mannoproteins in C. albicans and other species. The mannoproteins of the surface coat is of great importance to pathogen survival, growth, and virulence, thus understanding their structure and function as well as regulatory mechanisms can pave the way for better management of candidiasis.

Environmentally contingent control of Candida albicans cell wall integrity by transcriptional regulator Cup9

The fungal pathogen Candida albicans is surrounded by a cell wall that is the target of caspofungin and other echinocandin antifungals. Candida albicans can grow in several morphological forms, notably budding yeast and hyphae. Yeast and hyphal forms differ in cell wall composition, leading us to hypothesize that there may be distinct genes required for yeast and hyphal responses to caspofungin. Mutants in 27 genes reported previously to be caspofungin hypersensitive under yeast growth conditions were all caspofungin hypersensitive under hyphal growth conditions as well. However, a screen of mutants defective in transcription factor genes revealed that Cup9 is required for normal caspofungin tolerance under hyphal and not yeast growth conditions. In a hyphal-defective efg1Δ/Δ background, Cup9 is still required for normal caspofungin tolerance. This result argues that Cup9 function is related to growth conditions rather than cell morphology. RNA-seq conducted under hyphal growth conditions indicated that 361 genes were up-regulated and 145 genes were down-regulated in response to caspofungin treatment. Both classes of caspofungin-responsive genes were enriched for cell wall-related proteins, as expected for a response to disruption of cell wall integrity and biosynthesis. The cup9Δ/Δ mutant, treated with caspofungin, had reduced RNA levels of 40 caspofungin up-regulated genes, and had increased RNA levels of 8 caspofungin down-regulated genes, an indication that Cup9 has a narrow rather than global role in the cell wall integrity response. Five Cup9-activated surface-protein genes have roles in cell wall integrity, based on mutant analysis published previously (PGA31 and IFF11) or shown here (ORF19.3499, ORF19.851, or PGA28), and therefore may explain the hypersensitivity of the cup9Δ/Δmutant to caspofungin. Our findings define Cup9 as a new determinant of caspofungin susceptibility.

Antifungal Drug Resistance: Molecular Mechanisms in Candida albicans and Beyond

Fungal infections are a major contributor to infectious disease-related deaths across the globe. Candida species are among the most common causes of invasive mycotic disease, with Candida albicans reigning as the leading cause of invasive candidiasis. Given that fungi are eukaryotes like their human host, the number of unique molecular targets that can be exploited for antifungal development remains limited. Currently, there are only three major classes of drugs approved for the treatment of invasive mycoses, and the efficacy of these agents is compromised by the development of drug resistance in pathogen populations. Notably, the emergence of additional drug-resistant species, such as Candida auris and Candida glabrata, further threatens the limited armamentarium of antifungals available to treat these serious infections. Here, we describe our current arsenal of antifungals and elaborate on the resistance mechanisms Candida species possess that render them recalcitrant to therapeutic intervention. Finally, we highlight some of the most promising therapeutic strategies that may help combat antifungal resistance, including combination therapy, targeting fungal-virulence traits, and modulating host immunity. Overall, a thorough understanding of the mechanistic principles governing antifungal drug resistance is fundamental for the development of novel therapeutics to combat current and emerging fungal threats.

Efg1 and Cas5 Orchestrate Cell Wall Damage Response to Caspofungin in Candida albicans

Echinocandins are recommended as the first-line drugs for the treatment of systemic candidiasis. Cas5 is a key transcription factor involved in the response to cell wall damage induced by echinocandins. In this study, through a genetic screen, we identified a second transcription factor, Efg1, that is also crucial for proper transcriptional responses to echinocandins. Like CAS5, deletion of EFG1 confers hypersensitivity to caspofungin. Efg1 is required for the induction of CAS5 in response to caspofungin. However, ectopically expressed CAS5 cannot rescue the growth defect of efg1 mutant in caspofungin-containing medium. Deleting EFG1 in the cas5 mutant exacerbates the cell wall stress upon caspofungin addition and renders caspofungin-resistant Candida albicans responsive to treatment. Genome-wide transcription profiling of efg1/efg1 and cas5/cas5 using transcriptome sequencing (RNA-Seq) indicates that Efg1 and Cas5 coregulate caspofungin-responsive gene expression, but they also independently control induction of some genes. We further show that Efg1 interacts with Cas5 by yeast two-hybrid and in vivo immunoprecipitation in the presence or absence of caspofungin. Importantly, Efg1 and Cas5 bind to some caspofungin-responsive gene promoters to coordinately activate their expression. Thus, we demonstrate that Efg1, together with Cas5, controls the transcriptional response to cell wall stress induced by caspofungin.

The Roles of Chromatin Accessibility in Regulating the Candida albicans White-Opaque Phenotypic Switch

Candida albicans, a diploid polymorphic fungus, has evolved a unique heritable epigenetic program that enables reversible phenotypic switching between two cell types, referred to as “white” and “opaque”. These cell types are established and maintained by distinct transcriptional programs that lead to differences in metabolic preferences, mating competencies, cellular morphologies, responses to environmental signals, interactions with the host innate immune system, and expression of approximately 20% of genes in the genome. Transcription factors (defined as sequence specific DNA-binding proteins) that regulate the establishment and heritable maintenance of the white and opaque cell types have been a primary focus of investigation in the field; however, other factors that impact chromatin accessibility, such as histone modifying enzymes, chromatin remodelers, and histone chaperone complexes, also modulate the dynamics of the white-opaque switch and have been much less studied to date. Overall, the white-opaque switch represents an attractive and relatively “simple” model system for understanding the logic and regulatory mechanisms by which heritable cell fate decisions are determined in higher eukaryotes. Here we review recent discoveries on the roles of chromatin accessibility in regulating the C. albicans white-opaque phenotypic switch.

The Transcription Factor SomA Synchronously Regulates Biofilm Formation and Cell Wall Homeostasis in Aspergillus fumigatus

The cell wall is essential for fungal viability and is absent from human hosts; thus, drugs disrupting cell wall biosynthesis have gained more attention. Caspofungin is a member of a new class of clinically approved echinocandin drugs to treat invasive aspergillosis by blocking β-1,3-glucan synthase, thus damaging the fungal cell wall. Here, we demonstrate that caspofungin and other cell wall stressors can induce galactosaminogalactan (GAG)-dependent biofilm formation in the human pathogen Aspergillus fumigatus. We further identified SomA as a master transcription factor playing a dual role in both biofilm formation and cell wall homeostasis. SomA plays this dual role by direct binding to a conserved motif upstream of GAG biosynthetic genes and genes involved in cell wall stress sensors, chitin synthases, and β-1,3-glucan synthase. Collectively, these findings reveal a transcriptional control pathway that integrates biofilm formation and cell wall homeostasis and suggest SomA as an attractive target for antifungal drug development.

Polysaccharides are key components of both the fungal cell wall and biofilm matrix. Despite having distinct assembly and regulation pathways, matrix exopolysaccharide and cell wall polysaccharides share common substrates and intermediates in their biosynthetic pathways. It is not clear, however, if the biosynthetic pathways governing the production of these polysaccharides are cooperatively regulated. Here, we demonstrate that cell wall stress promotes production of the exopolysaccharide galactosaminogalactan (GAG)-depend biofilm formation in the major fungal pathogen of humans Aspergillus fumigatus and that the transcription factor SomA plays a crucial role in mediating this process. A core set of SomA target genes were identified by transcriptome sequencing and chromatin immunoprecipitation coupled to sequencing (ChIP-Seq). We identified a novel SomA-binding site in the promoter regions of GAG biosynthetic genes agd3 and ega3, as well as its regulators medA and stuA. Strikingly, this SomA-binding site was also found in the upstream regions of genes encoding the cell wall stress sensors, chitin synthases, and β-1,3-glucan synthase. Thus, SomA plays a direct regulation of both GAG and cell wall polysaccharide biosynthesis. Consistent with these findings, SomA is required for the maintenance of normal cell wall architecture and compositions in addition to its function in biofilm development. Moreover, SomA was found to globally regulate glucose uptake and utilization, as well as amino sugar and nucleotide sugar metabolism, which provides precursors for polysaccharide synthesis. Collectively, our work provides insight into fungal adaptive mechanisms in response to cell wall stress where biofilm formation and cell wall homeostasis were synchronously regulated.

Signalling mechanisms involved in stress response to antifungal drugs

The emergence of antifungal resistance is a serious threat in the treatment of mycoses. The primary susceptible fungal cells may evolve a resistance after longer exposure to antifungal agents. The exposure itself causes stress condition, to which the fungus needs to adapt. This review provides detailed description of evolutionary conserved molecular mechanisms contributing to the adaptation response to stress caused by antifungal agents as well as their interconnection. The knowledge may help us to find new ways to delay the emergence of drug resistance as the same mechanisms are used regardless of what antifungal compound causes stress.

Transcriptional regulation of the caspofungin-induced cell wall damage response in Candida albicans

The human fungal pathogen Candida albicans maintains pathogenic and commensal states primarily through cell wall functions. The echinocandin antifungal drug caspofungin inhibits cell wall synthesis and is widely used in treating disseminated candidiasis. Signaling pathways are critical in coordinating the adaptive response to cell wall damage (CWD). C. albicans executes a robust transcriptional program following caspofungin-induced CWD. A comprehensive analysis of signaling pathways at the transcriptional level facilitates the identification of prospective genes for functional characterization and propels the development of novel antifungal interventions. This review article focuses on the molecular functions and signaling crosstalk of the C. albicans transcription factors Sko1, Rlm1, and Cas5 in caspofungin-induced CWD signaling.

Antifungal resistance-modifying multiplexing action of Momordica charantia protein and phosphorylated derivatives on the basis of growth-dependent gene coregulation in Candida albicans

Fungal growth-dependent gene coregulation is strongly implicated in alteration of gene-encoding target proteases ruling with an antifungal resistance niche and biology of resistant mutants. On the basis of multi-alterative processes in this platform, the resistance-modifying strategy is designed in ketoconazole resistant Candida albicans and evaluated with less selective Momordica charantia protein and allosterically phosphorylated derivatives at the Thr102, Thr24 and Thr255 sites, respectively. We demonstrate absolutely chemo-sensitizing efficacy regarding stepwise-modifying resistance in sensitivity, by a load of only 26.23-40.00 μg/l agents in Sabouraud's dextrose broth. Five successive modifying-steps realize the decreasing of ketoconazole E-test MIC50 from 11.10 to a lower level than 0.10 mg/l. With the ketoconazole resistance-modifying, colony undergoes a high-frequency morphological switch between high ploidy (opaque) and small budding haploid (white). A cellular event in the first modifying-step associates with relatively slow exponential growth (ie, a 4-h delay)-dependent action, mediated by agents adsorption. Moreover, multiple molecular roles are coupled with intracellularly and extracellularly binding to ATP-dependent RNA helicase dbp6; the 0.08-2.45 fold upregulation of TATA-box-binding protein, rRNA-processing protein and translation initiation factor 5A; and the 7.52-55.33% decrease of cytochrome P450 lanosterol 14α-demethylase, glucan 1, 3-β glucosidase, candidapepsin-1 and 1-acylglycerol-3-phosphate O-acyltransferase. Spatial and temporal gene coregulation, in the transcription and translation initiation stages with rRNA-processing, is a new coprocessing platform enabling target protease attenuations for resistance-impairing. An updated resistance-modifying measure of these agents in the low-dose antifungal strategic design may provide opportunities to a virtually safe therapy that is in high dose-dependency.

LAY SUMMARY

A new platform to modify resistance is fungal growth-dependent gene coregulation. MAP30 and phosphorylated derivatives are candidate resistance-modifying agents. Low-dose stepwise treatment absolutely modifies azole resistance in model fungus.

An expanded cell wall damage signaling network is comprised of the transcription factors Rlm1 and Sko1 in Candida albicans

The human fungal pathogen Candida albicans is constantly exposed to environmental challenges impacting the cell wall. Signaling pathways coordinate stress adaptation and are essential for commensalism and virulence. The transcription factors Sko1, Cas5, and Rlm1 control the response to cell wall stress caused by the antifungal drug caspofungin. Here, we expand the Sko1 and Rlm1 transcriptional circuit and demonstrate that Rlm1 activates Sko1 cell wall stress signaling. Caspofungin-induced transcription of SKO1 and several Sko1-dependent cell wall integrity genes are attenuated in an rlm1Δ/Δ mutant strain when compared to the treated wild-type strain but not in a cas5Δ/Δ mutant strain. Genome-wide chromatin immunoprecipitation (ChIP-seq) results revealed numerous Sko1 and Rlm1 directly bound target genes in the presence of caspofungin that were undetected in previous gene expression studies. Notable targets include genes involved in cell wall integrity, osmolarity, and cellular aggregation, as well as several uncharacterized genes. Interestingly, we found that Rlm1 does not bind to the upstream intergenic region of SKO1 in the presence of caspofungin, indicating that Rlm1 indirectly controls caspofungin-induced SKO1 transcription. In addition, we discovered that caspofungin-induced SKO1 transcription occurs through self-activation. Based on our ChIP-seq data, we also discovered an Rlm1 consensus motif unique to C. albicans. For Sko1, we found a consensus motif similar to the known Sko1 motif for Saccharomyces cerevisiae. Growth assays showed that SKO1 overexpression suppressed caspofungin hypersensitivity in an rlm1Δ/Δ mutant strain. In addition, overexpression of the glycerol phosphatase, RHR2, suppressed caspofungin hypersensitivity specifically in a sko1Δ/Δ mutant strain. Our findings link the Sko1 and Rlm1 signaling pathways, identify new biological roles for Sko1 and Rlm1, and highlight the complex dynamics underlying cell wall signaling.

Candida albicans is the most common human fungal pathogen isolated in clinical settings. The echinocandin drug caspofungin is used to treat invasive candidiasis; however, the emergence of increasing echinocandin resistance underscores the need for new antifungal strategies. Elucidating the signaling mechanisms that govern caspofungin-induced tolerance has the potential to identify candidate proteins that could serve as novel therapeutic targets. Here, we expand the Rlm1 and Sko1 cell wall transcriptional network and find that Rlm1 indirectly regulates Sko1 signaling. Furthermore, we identify Sko1- and Rlm1-specific biological roles in caspofungin adaptation, such as osmoregulation and secretion. Lastly, we discover a protective role for glycerol in caspofungin tolerance. Overall, these findings provide mechanistic insight into the genetic and cellular bases underlying cell wall signaling in C. albicans.

Genomic Insights Into Sugar Adaptation in an Extremophile Yeast Zygosaccharomyces rouxii

The osmotolerant Zygosaccharomyces rouxii is known for its trait to survive in extreme high sugar environments. This ability determines its role in the fermentation process and leads to yeast spoilage in the food industry. However, our knowledge of the gene expression in response to high sugar stress remains limited. Here, we conducted RNA-sequencing (RNA-seq) under different sugar concentrations of the spoilage yeast, Z. rouxii, which exhibit extremely high tolerance to sugar stress. The obtained differentially expressed genes (DEGs) are significantly different to that of the Saccharomyces cerevisiae, which is sensitive to extreme high sugar stress. Most of the DEGs participated in the “glucan synthesis,” “transmembrane transport,” “ribosome,” etc. In this work, we also demonstrated that the gene ZYRO0B03476g (ZrKAR2) encoding Kar2p can significantly affect the growth of Z. rouxii under high sugar stress. In addition, we combined with a previous study on the genome sequence of Z. rouxii, indicating that several gene families contain significantly more gene copies in the Z. rouxii lineage, which involved in tolerance to sugar stress. Our results provide a gene insight for understanding the high sugar tolerance trait, which may impact food and biotechnological industries and improve the osmotolerance in other organisms.

Inhibiting Fungal Echinocandin Resistance by Small-Molecule Disruption of Geranylgeranyltransferase Type I Activity

Echinocandin resistance in Candida is a great concern, as the echinocandin drugs are recommended as first-line therapy for patients with invasive candidiasis. However, therapeutic efforts to thwart echinocandin resistance have been hampered by a lack of fungal specific drug targets. Here, we show that deleting CDC43, the β subunit of geranylgeranyltransferase type I (GGTase I), confers hypersensitivity to echinocandins, which renders GGTase I a tractable target in combatting echinocandin resistance. The membrane localization of Rho1, which is critical for (1,3)-β-d-glucan synthase Fks1 activation, is disrupted in the cdc43 mutant, resulting in decreased amounts of glucans in the cell wall, thereby exacerbating the cell wall stress upon caspofungin addition. Guided by this insight, we found that selective chemical inhibition of GGTase I by L-269289 potentiates echinocandin activity and renders echinocandin-resistant Candida albicans responsive to treatment in vitro and in animal models for disseminated infection. Furthermore, L-269289 and echinocandins also act in a synergistic manner for the treatment of Candida tropicalis and Candida parapsilosis Importantly, deletion of CDC43 is lethal in Candida glabrata L-269289 is active on its own to kill C. glabrata, and its fungicidal activity is enhanced when combined with caspofungin. Thus, targeting GGTase I has therapeutic potential to address the clinical challenge of echinocandin-resistant candidiasis.

Candida albicans Antifungal Resistance and Tolerance in Bloodstream Infections: The Triad Yeast-Host-Antifungal

Candida albicans represents the most frequent isolated yeast from bloodstream infections. Despite the remarkable progress in diagnostic and therapeutic approaches, these infections continue to be a critical challenge in intensive care units worldwide. The economic cost of bloodstream fungal infections and its associated mortality, especially in debilitated patients, remains unacceptably high. Candida albicans is a highly adaptable microorganism, being able to develop resistance following prolonged exposure to antifungals. Formation of biofilms, which diminish the accessibility of the antifungal, selection of spontaneous mutations that increase expression or decreased susceptibility of the target, altered chromosome abnormalities, overexpression of multidrug efflux pumps and the ability to escape host immune defenses are some of the factors that can contribute to antifungal tolerance and resistance. The knowledge of the antifungal resistance mechanisms can allow the design of alternative therapeutically options in order to modulate or revert the resistance. We have focused this review on the main factors that are involved in antifungal resistance and tolerance in patients with C. albicans bloodstream infections.

Functional Genomic Screening Reveals Core Modulators of Echinocandin Stress Responses in Candida albicans

Candida albicans is a leading cause of death due to fungal infection. Treatment of systemic candidiasis often relies on echinocandins, which disrupt cell wall synthesis. Resistance is readily acquired via mutations in the drug target gene, FKS1. Both basal tolerance and resistance to echinocandins require cellular stress responses. We performed a systematic analysis of 3,030 C. albicans mutants to define circuitry governing cellular responses to echinocandins. We identified 16 genes for which deletion or transcriptional repression enhanced echinocandin susceptibility, including components of the Pkc1-MAPK signaling cascade. We discovered that the molecular chaperone Hsp90 is required for the stability of Pkc1 and Bck1, establishing key mechanisms through which Hsp90 mediates echinocandin resistance. We also discovered that perturbation of the CCT chaperonin complex causes enhanced echinocandin sensitivity, altered cell wall architecture, and aberrant septin localization. Thus, we provide insights into the mechanisms by which cellular chaperones enable crucial responses to echinocandin-induced stress.

Electron cryomicroscopy observation of acyl carrier protein translocation in type I fungal fatty acid synthase

During fatty acid biosynthesis, acyl carrier proteins (ACPs) from type I fungal fatty acid synthase (FAS) shuttle substrates and intermediates within a reaction chamber that hosts multiple spatially-fixed catalytic centers. A major challenge in understanding the mechanism of ACP-mediated substrate shuttling is experimental observation of its transient interaction landscape within the reaction chamber. Here, we have shown that ACP spatial distribution is sensitive to the presence of substrates in a catalytically inhibited state, which enables high-resolution investigation of the ACP-dependent conformational transitions within the enoyl reductase (ER) reaction site. In two fungal FASs with distinct ACP localization, the shuttling domain is targeted to the ketoacyl-synthase (KS) domain and away from other catalytic centers, such as acetyl-transferase (AT) and ER domains by steric blockage of the KS active site followed by addition of substrates. These studies strongly suggest that acylation of phosphopantetheine arm of ACP may be an integral part of the substrate shuttling mechanism in type I fungal FAS.

Ser/Thr protein phosphatases in fungi: structure, regulation and function

Reversible phospho-dephosphorylation of proteins is a major mechanism for the control of cellular functions. By large, Ser and Thr are the most frequently residues phosphorylated in eukar-yotes. Removal of phosphate from these amino acids is catalyzed by a large family of well-conserved enzymes, collectively called Ser/Thr protein phosphatases. The activity of these enzymes has an enormous impact on cellular functioning. In this work we pre-sent the members of this family in S. cerevisiae and other fungal species, and review the most recent findings concerning their regu-lation and the roles they play in the most diverse aspects of cell biology.

Linking Cellular Morphogenesis with Antifungal Treatment and Susceptibility in Candida Pathogens

Fungal infections are a growing public health concern, and an increasingly important cause of human mortality, with Candida species being amongst the most frequently encountered of these opportunistic fungal pathogens. Several Candida species are polymorphic, and able to transition between distinct morphological states, including yeast, hyphal, and pseudohyphal forms. While not all Candida pathogens are polymorphic, the ability to undergo morphogenesis is linked with the virulence of many of these pathogens. There are also many connections between Candida morphogenesis and antifungal drug treatment and susceptibility. Here, we review how Candida morphogenesis-a key virulence trait-is linked with antifungal drugs and antifungal drug resistance. We highlight how antifungal therapeutics are able to modulate morphogenesis in both sensitive and drug-resistant Candida strains, the shared signaling pathways that mediate both morphogenesis and the cellular response to antifungal drugs and drug resistance, and the connection between Candida morphology, drug resistance, and biofilm growth. We further review the development of anti-virulence drugs, and targeting Candida morphogenesis as a novel therapeutic strategy to target fungal pathogens. Together, this review highlights important connections between fungal morphogenesis, virulence, and susceptibility to antifungals.

Flow Cytometry Analysis of Fungal Ploidy

Ploidy, the number of sets of homologous chromosomes in a cell, can alter cellular physiology, gene regulation, and the spectrum of acquired mutations. Advances in single-cell flow cytometry have greatly improved the understanding of how genome size contributes to diverse biological processes including speciation, adaptation, pathogenesis, and tumorigenesis. For example, fungal pathogens can undergo whole genome duplications during infection of the human host and during acquisition of antifungal drug resistance. Quantification of ploidy is dramatically affected by the nucleic acid staining technique and the flow cytometry analysis of single cells. Ploidy in fungi is also impacted by samples that are heterogeneous for both ploidy and morphology, and control strains with known ploidy must be included in every flow cytometry experiment. To detect ploidy changes within fungal strains, the following protocol was developed to accurately and dependably interrogate single-cell ploidy. © 2018 by John Wiley & Sons, Inc.

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.

Gene Expression of Pneumocystis murina after Treatment with Anidulafungin Results in Strong Signals for Sexual Reproduction, Cell Wall Integrity, and Cell Cycle Arrest, Indicating a Requirement for Ascus Formation for Proliferation

The echinocandins are a class of antifungal agents that target β-1,3-d-glucan (BG) biosynthesis. In the ascigerous Pneumocystis species, treatment with these drugs depletes the ascus life cycle stage, which contains BG, but large numbers of forms which do not express BG remain in the infected lungs. In the present study, the gene expression profiles of Pneumocystis murina were compared between infected, untreated mice and mice treated with anidulafungin for 2 weeks to understand the metabolism of the persisting forms. Almost 80 genes were significantly up- or downregulated. Like other fungi exposed to echinocandins, genes associated with sexual replication, cell wall integrity, cell cycle arrest, and stress comprised the strongest upregulated signals in P. murina from the treated mice. The upregulation of the P. murina β-1,3-d-glucan endohydrolase and endo-1,3-glucanase was notable and may explain the disappearance of the existing asci in the lungs of treated mice since both enzymes can degrade BG. The biochemical measurement of BG in the lungs of treated mice and fluorescence microscopy with an anti-BG antibody supported the loss of BG. Downregulated signals included genes involved in cell replication, genome stability, and ribosomal biogenesis and function and the Pneumocystis-specific genes encoding the major surface glycoproteins (Msg). These studies suggest that P. murina attempted to undergo sexual replication in response to a stressed environment and was halted in any type of proliferative cycle, likely due to a lack of BG. Asci appear to be a required part of the life cycle stage of Pneumocystis, and BG may be needed to facilitate progression through the life cycle via sexual replication.

Tuning Hsf1 levels drives distinct fungal morphogenetic programs with depletion impairing Hsp90 function and overexpression expanding the target space

The capacity to respond to temperature fluctuations is critical for microorganisms to survive within mammalian hosts, and temperature modulates virulence traits of diverse pathogens. One key temperature-dependent virulence trait of the fungal pathogen Candida albicans is its ability to transition from yeast to filamentous growth, which is induced by environmental cues at host physiological temperature. A key regulator of temperature-dependent morphogenesis is the molecular chaperone Hsp90, which has complex functional relationships with the transcription factor Hsf1. Although Hsf1 controls global transcriptional remodeling in response to heat shock, its impact on morphogenesis remains unknown. Here, we establish an intriguing paradigm whereby overexpression or depletion of C. albicans HSF1 induces morphogenesis in the absence of external cues. HSF1 depletion compromises Hsp90 function, thereby driving filamentation. HSF1 overexpression does not impact Hsp90 function, but rather induces a dose-dependent expansion of Hsf1 direct targets that drives overexpression of positive regulators of filamentation, including Brg1 and Ume6, thereby bypassing the requirement for elevated temperature during morphogenesis. This work provides new insight into Hsf1-mediated environmentally contingent transcriptional control, implicates Hsf1 in regulation of a key virulence trait, and highlights fascinating biology whereby either overexpression or depletion of a single cellular regulator induces a profound developmental transition.

For human pathogens, the capacity to respond to elevated temperature is required for survival, with elevated temperature in the form of fever as a conserved host response to defend against infection. One of the leading fungal pathogens of humans in Candida albicans, which is capable of growing in both a yeast and filamentous state. The ability to transition between these forms is a key virulence trait, and one that is highly temperature-dependent. A pivotal regulator of filamentous growth is the temperature-responsive molecular chaperone Hsp90, which has complex relationships with the transcription factor Hsf1. Although Hsf1 regulates changes in gene expression in response to heat shock, its impact on morphogenesis remains unknown. Here, we uncover an intriguing phenomenon whereby overexpression or depletion of C. albicans HSF1 induces morphogenesis. We observe that HSF1 depletion compromises Hsp90 function, thereby driving filamentation. In contrast, HSF1 overexpression induces a dose-dependent expansion of its transcriptional targets that drives overexpression of positive regulators of filamentous growth. This work illuminates novel mechanisms through which tuning the levels of an environmentally contingent transcription factor drives a key developmental program.

FKS2 and FKS3 Genes of Opportunistic Human Pathogen Candida albicans Influence Echinocandin Susceptibility

Candida albicans, a prevailing opportunistic fungal pathogen of humans, has a diploid genome containing three homologous FKS genes that are evolutionarily conserved. One of these, the essential gene FKS1, encodes the catalytic subunit of glucan synthase, which is the target of echinocandin drugs and also serves as a site of drug resistance. The other two glucan synthase-encoding genes, FKS2 and FKS3, are also expressed, but their roles in resistance are considered unimportant. However, we report here that expression of FKS1 is upregulated in strains lacking either FKS2 or FKS3 Furthermore, in contrast to what is observed in heterozygous FKS1 deletion strains, cells lacking FKS2 or FKS3 contain increased amounts of cell wall glucan, are more resistant to echinocandin drugs, and consistently are tolerant to cell wall-damaging agents. Our data indicate that C. albicansFKS2 and FKS3 can act as negative regulators of FKS1, thereby influencing echinocandin susceptibility.

Antifungal drug resistance: evolution, mechanisms and impact

Microorganisms have a remarkable capacity to evolve resistance to antimicrobial agents, threatening the efficacy of the limited arsenal of antimicrobials and becoming a dire public health crisis. This is of particular concern for fungal pathogens, which cause devastating invasive infections with treatment options limited to only three major classes of antifungal drugs. The paucity of antifungals with clinical utility is in part due to close evolutionary relationships between these eukaryotic pathogens and their human hosts, which limits the unique targets to be exploited therapeutically. This review highlights the mechanisms by which fungal pathogens of humans evolve resistance to antifungal drugs, which provide crucial insights to enable development of novel therapeutic strategies to thwart drug resistance and combat fungal infectious disease.