Chlamydospore formation in Candida albicans requires the Efg1p morphogenetic regulator

The rod cell, a small form of Candida albicans, possesses superior fitness to the host gut and adaptation to commensalism

Candida albicans deploys various morphological forms through complex switching mechanisms, ensuring its survival and thriving as a commensal or pathogen in vastly different human niches. In this study, we demonstrate that a novel ''rod'' morphological form of C. albicans coexists and is interchangeable with previously reported white, gray, and opaque forms, constituting a tetra-stable phenotypic switching system. Rod cells arise from the efg1 mutant of SC5314 cells or from the clinical BJ1097 strain cultured under glucose-free conditions. They are characterized by a distinct gene expression profile and can be stably maintained through in vitro passaging or in vivo inhabitation of the gastrointestinal (GI) tract of mice. Remarkably, the majority of the efg1 mutant cells become rod cells in N-acetylglucosamine (GlcNAc)-containing medium, and the GlcNAc sensor Ngs1 is instrumental in converting the white or gray cells to the rod cells. Conversely, glucose inhibits rod cells through Cph1; consequently, the loss of Cph1 in the efg1 mutant cells permits their conversion to rod cells in glucose-replete media. Notably, rod cells of the efg1/ cph1 mutant display superior adaptation and longer persistence in the murine GI environment than wild-type white cells. Taken together, these findings establish rod cells as a previously unappreciated form that is not only morphologically and transcriptionally distinguishable but also defined by specific genetic and environmental determinants, shedding light on complex fungus-host interactions.

Prevalence and drug susceptibility of clinical Candida species in nasopharyngeal cancer patients in Vietnam

In the nature, Candida species are normal inhabitants and can be observed in a wide variety of vertebrates. In humans, especially for cancer patients who fall prey to opportunistic pathogens, this group of susceptible multi-drug resistant and biofilm-forming yeasts, are among the commonest ones. In this study, Candida species in 76 oral lesion samples from Vietnamese nasopharyngeal-cancer patients were isolated, morphologically identified using CHROMagar™, germ tube formation, and chlamydospore formation tests, and molecularly confirmed by PCR-RFLP. The drug susceptibility of these isolates was then tested, and the gene ERG11 was DNA sequenced to investigate the mechanism of resistance. The results showed that Candida albicans remained the most prevalent species (63.16% of the cases), followed by Candida glabrata, Candida tropicalis, and Candida krusei. The rates of resistance of non-albicans Candida for tested drugs were 85.71%, 53.57%, and 57.14% to fluconazole, clotrimazole, and miconazole, respectively. Although the drug-resistance rate of Candida albicans was lower than that of non-albicans Candida, it was higher than expected, suggesting an emerging drug-resistance phenomenon. Furthermore, ERG11 DNA sequencing revealed different mutations (especially K128T), implying the presence of multiple resistance mechanisms. Altogether, the results indicate an alarming drug-resistance situation in Candida species in Vietnamese cancer patients and emphasize the importance of species identification and their drug susceptibility prior to treatment.

Genetic modification of Candida maltosa, a non-pathogenic CTG species, reveals EFG1 function

Candida maltosa is closely related to important pathogenic Candida species, especially C. tropicalis and C. albicans, but it has been rarely isolated from humans. For this reason, through comparative studies, it could be a powerful model to understand the genetic underpinnings of the pathogenicity of Candida species. Here, we generated a cohesive assembly of the C. maltosa genome and developed genetic engineering tools that will facilitate studying this species at a molecular level. We used a combination of short and long-read sequencing to build a polished genomic draft composed of 14 Mbp, 45 contigs and close to 5700 genes. This assembly represents a substantial improvement from the currently available sequences that are composed of thousands of contigs. Genomic comparison with C. albicans and C. tropicalis revealed a substantial reduction in the total number of genes in C. maltosa. However, gene loss seems not to be associated to the avirulence of this species given that most genes that have been previously associated with pathogenicity were also present in C. maltosa. To be able to edit the genome of C. maltosa we generated a set of triple auxotrophic strains so that gene deletions can be performed similarly to what has been routinely done in pathogenic Candida species. As a proof of concept, we generated gene knockouts of EFG1, a gene that encodes a transcription factor that is essential for filamentation and biofilm formation in C. albicans and C. tropicalis. Characterization of these mutants showed that Efg1 also plays a role in biofilm formation and filamentous growth in C. maltosa, but it seems to be a repressor of filamentation in this species. The genome assembly and auxotrophic mutants developed here are a key step forward to start using C. maltosa for comparative and evolutionary studies at a molecular level.

Morphogenetic transitions in the adaptation of Candida albicans to the mammalian gut

Candida albicans is a pathobiont in humans that forms part of the mycobiota in healthy individuals and can cause different pathologies upon alterations of the host defenses. The mammalian gut is clinically relevant as this niche is the most common pool for bloodstream-derived infections. The ability of C. albicans to switch from yeast to hypha has been related to the commensal-to-pathogen transition and is, therefore, considered relevant in virulence. Recently, filaments have been implicated in the humoral response in the gut. C. albicans exhibits other morphologies that play different roles in pathogenicity and commensalism. This review focuses on the role of these morphological transitions in C. albicans proliferation and its establishment as a commensal in the mammalian gut, paying special attention to the transcription factors involved in their regulation.

Proteome dataset of Candida albicans (ATCC10231) opaque cell

OBJECTIVES

Candida albicans, a polymorphic yeast, is one of the most common, opportunistic fungal pathogens of humans. Among the different morphological forms, opaque form is one of the least-studied ones. This opaque phenotype is essential for mating and is also reported to be involved in colonizing the gastrointestinal tract. Considering the significance of the clinical and sexual reproduction of C. albicans, we have investigated the morphophysiological modulations in opaque form using a proteomic approach.

DATA DESCRIPTION

In the current investigation, we have used Micro-Liquid Chromatography-Mass Spectrometry (LC-MS/MS) analysis to create a protein profile for opaque-specific proteins. Whole-cell proteins from C. albicans (ATCC10231) cells that had been cultured for seven days on synthetic complete dextrose (SCD) medium in both as an opaque (test) and as a white (control) form cells were extracted, digested, and identified using LC-MS/MS. This information is meant to serve the scientific community and represents the proteome profile (SWATH Spectral Libraries) of C. albicans opaque form.

The participation of vacuoles and the regulation of various metabolic pathways under acid stress promote the differentiation of chlamydospore in Trichoderma harzianum T4

AIMS

Chlamydospores are a special, differentiated type with high environmental resistance. Consequently, the chlamydospores of Trichoderma harzianum T4 can used to industrialize the latter. This study aimed to investigate the key factors affecting the sporulation type of T. harzianum T4 and the mechanisms underlying this effect.

METHODS AND RESULTS

In the liquid fermentation of T. harzianum T4, ammonium sulfate (AS) inhibited conidia formation and chlamydospore production. Fermentation tests revealed that acid stress induced sporulation type alteration. Transcriptomic analysis was used to evaluate the adaptation strategy and mechanism underlying spore type alteration under acid stress. The fermentation experiments involving the addition of amino acids revealed that branched-chain amino acids benefited conidia production, whereas β-alanine benefited chlamydospore production. Confocal microscope fluorescence imaging and chloroquine intervention demonstrated that vacuole function was closely related to chlamydospore production.

CONCLUSION

The sporulation type of T. harzianum T4 can be controlled by adjusting the fermentation pH. T. harzianum T4 cells employ various self-protection measures against strong acid stress, including regulating their metabolism to produce a large number of chlamydospores for survival.

Farnesol and phosphorylation of the transcriptional regulator Efg1 affect Candida albicans white-opaque switching rates

Candida albicans is a normal member of the human microbiome and an opportunistic fungal pathogen. This species undergoes several morphological transitions, and here we consider white-opaque switching. In this switching program, C. albicans reversibly alternates between two cell types, named "white" and "opaque," each of which is normally stable across thousands of cell divisions. Although switching under most conditions is stochastic and rare, certain environmental signals or genetic manipulations can dramatically increase the rate of switching. Here, we report the identification of two new inputs which affect white-to-opaque switching rates. The first, exposure to sub-micromolar concentrations of (E,E)-farnesol, reduces white-to-opaque switching by ten-fold or more. The second input, an inferred PKA phosphorylation of residue T208 on the transcriptional regulator Efg1, increases white-to-opaque switching ten-fold. Combining these and other environmental inputs results in a variety of different switching rates, indicating that a given rate represents the integration of multiple inputs.

Anaerobic conditions are a major influence on Candida albicans chlamydospore formation

Candidiasis now represents the fourth most frequent nosocomial infection both in the USA and worldwide. Candida albicans is an increasingly common threat to human health as a consequence of AIDS, steroid therapy, organ and tissue transplantation, cancer therapy, broad-spectrum antibiotics, and other immune defects. Unfortunately, these infections carry unacceptably high morbidity, mortality rates and important economic repercussions (estimated total direct cost of approximately 2 billion dollars in 1998 in US hospitals alone). This pathogen can grow both in yeast and filamentous forms and the pathogenic potential of C. albicans is intimately related to certain key processes including filamentation. Chlamydospores are considered to be a dormant form of C. albicans that remain understudied. Chlamydospores have been widely used as a diagnostic tool to separate C. albicans and C. dubliniensis from other Candida species. More recently, media have been developed that use chlamydopsore formation to separate C. albicans and C. dubliniensis from each other. Chlamydospore formation can be stimulated by hypoxic conditions but only on limited specific media types. Here, we show that anaerobic conditions are enough to drive chlamydospore formation in C. albicans on the surface of nutrient-rich agar.

The Role of Glycoside Hydrolases in S. gordonii and C. albicans Interactions

Candida albicans can coaggregate with Streptococcus gordonii and cocolonize in the oral cavity. Saliva provides a vital microenvironment for close interactions of oral microorganisms. However, the level of fermentable carbohydrates in saliva is not sufficient to support the growth of multiple species. Glycoside hydrolases (GHs) that hydrolyze glycoproteins are critical for S. gordonii growth in low-fermentable-carbohydrate environments such as saliva. However, whether GHs are involved in the cross-kingdom interactions between C. albicans and S. gordonii under such conditions remains unknown. In this study, C. albicans and S. gordonii were cocultured in heart infusion broth with a low level of fermentable carbohydrate. Planktonic growth, biofilm formation, cell aggregation, and GH activities of monocultures and cocultures were examined. The results revealed that the planktonic growth of cocultured S. gordonii in a low-carbohydrate environment was elevated, while that of cocultured C. albicans was reduced. The biomass of S. gordonii in dual-species biofilms was higher than that of monocultures, while that of cocultured C. albicans was decreased. GH activity was observed in S. gordonii, and elevated activity of GHs was detected in S. gordonii-C. albicans cocultures, with elevated expression of GH-related genes of S. gordonii. By screening a mutant library of C. albicans, we identified a tec1Δ/Δ mutant strain that showed reduced ability to promote the growth and GH activities of S. gordonii compared with the wild-type strain. Altogether, the findings of this study demonstrate the involvement of GHs in the cross-kingdom metabolic interactions between C. albicans and S. gordonii in an environment with low level of fermentable carbohydrates. IMPORTANCE Cross-kingdom interactions between Candida albicans and oral streptococci such as Streptococcus gordonii have been reported. However, their interactions in a low-fermentable-carbohydrate environment like saliva is not clear. The current study revealed glycoside hydrolase-related cross-kingdom communications between S. gordonii and C. albicans under the low-fermentable-carbohydrate condition. We demonstrate that C. albicans can promote the growth and metabolic activities of S. gordonii by elevating the activities of cell-wall-anchored glycoside hydrolases of S. gordonii. C. albicans gene TEC1 is critical for this cross-kingdom metabolic communication.

EFG1, Everyone’s Favorite Gene in Candida albicans: A Comprehensive Literature Review

Candida sp. are among the most common fungal commensals found in the human microbiome. Although Candida can be found residing harmlessly on the surface of the skin and mucosal membranes, these opportunistic fungi have the potential to cause superficial skin, nail, and mucus membrane infections as well as life threatening systemic infections. Severity of infection is dependent on both fungal and host factors including the immune status of the host. Virulence factors associated with Candida sp. pathogenicity include adhesin proteins, degradative enzymes, phenotypic switching, and morphogenesis. A central transcriptional regulator of morphogenesis, the transcription factor Efg1 was first characterized in Candida albicans in 1997. Since then, EFG1 has been referenced in the Candida literature over three thousand times, with the number of citations growing daily. Arguably one of the most well studied genes in Candida albicans, EFG1 has been referenced in nearly all contexts of Candida biology from the development of novel therapeutics to white opaque switching, hyphae morphology to immunology. In the review that follows we will synthesize the research that has been performed on this extensively studied transcription factor and highlight several important unanswered questions.

Transcriptome Dynamics Underlying Chlamydospore Formation in Trichoderma virens GV29-8

Trichoderma spp. are widely used biocontrol agents which are antagonistic to a variety of plant pathogens. Chlamydospores are a type of propagules produced by many fungi that have thick walls and are highly resistant to adverse environmental conditions. Chlamydospore preparations of Trichoderma spp. can withstand various storage conditions, have a longer shelf life than conidial preparations and have better application potential. However, large-scale production of chlamydospores has proven difficult. To understand the molecular mechanisms governing chlamydospore formation (CF) in Trichoderma fungi, we performed a comprehensive analysis of transcriptome dynamics during CF across 8 different developmental time points, which were divided into 4 stages according to PCA analysis: the mycelium growth stage (S1), early and middle stage of CF (S2), flourishing stage of CF (S3), and late stage of CF and mycelia initial autolysis (S4). 2864, 3206, and 3630 DEGs were screened from S2 vs S1, S3 vs S2, and S4 vs S3, respectively. We then identified the pathways and genes that play important roles in each stage of CF by GO, KEGG, STC and WGCNA analysis. The results showed that DEGs in the S2 vs S1 were mainly enriched in organonitrogen compound metabolism, those in S3 vs S2 were mainly involved in secondary metabolite, cell cycle, and N-glycan biosynthesis, and DEGs in S4 vs S3 were mainly involved in lipid, glycogen, and chitin metabolic processes. We speculated that mycelial assimilation and absorption of exogenous nitrogen in the early growth stage (S1), resulted in subsequent nitrogen deficiency (S2). At the same time, secondary metabolites and active oxygen free radicals released during mycelial growth produced an adverse growth environment. The resulting nitrogen-deficient and toxin enriched medium may stimulate cell differentiation by initiating cell cycle regulation to induce morphological transformation of mycelia into chlamydospores. High expression of genes relating to glycogen, lipid, mannan, and chitin synthetic metabolic pathways during the flourishing (S3) and late stages (S4) of CF may be conducive to energy storage and cell wall construction in chlamydospores. For further verifying the functions of the amino sugar and nucleotide sugar metabolism (tre00520) pathway in the CF of T. virens GV29-8 strain, the chitin synthase gene (TRIVIDRAFT_90152), one key gene of the pathway, was deleted and resulted in the dysplasia of mycelia and an incapability to form normal chlamydospores, which illustrated the pathway affecting the CF of T. virens GV29-8 strain. Our results provide a new perspective for understanding the genetics of biochemical pathways involved in CF of Trichoderma spp.

Transcriptional control of hyphal morphogenesis in Candida albicans

Candida albicans is a multimorphic commensal organism and opportunistic fungal pathogen in humans. A morphological switch between unicellular budding yeast and multicellular filamentous hyphal growth forms plays a vital role in the virulence of C. albicans, and this transition is regulated in response to a range of environmental cues that are encountered in distinct host niches. Many unique transcription factors contribute to the transcriptional regulatory network that integrates these distinct environmental cues and determines which phenotypic state will be expressed. These hyphal morphogenesis regulators have been extensively investigated, and represent an increasingly important focus of study, due to their central role in controlling a key C. albicans virulence attribute. This review provides a succinct summary of the transcriptional regulatory factors and environmental signals that control hyphal morphogenesis in C. albicans.

Characterization of the mbsA Gene Encoding a Putative APSES Transcription Factor in Aspergillus fumigatus

The APSES family proteins are transcription factors (TFs) with a basic helix-loop-helix domain, known to regulate growth, development, secondary metabolism, and other biological processes in Aspergillus species. In the genome of the human opportunistic pathogenic fungus Aspergillus fumigatus, five genes predicted to encode APSES TFs are present. Here, we report the characterization of one of these genes, called mbsA (Afu7g05620). The deletion (Δ) of mbsA resulted in significantly decreased hyphal growth and asexual sporulation (conidiation), and lowered mRNA levels of the key conidiation genes abaA, brlA, and wetA. Moreover, ΔmbsA resulted in reduced spore germination rates, elevated sensitivity toward Nikkomycin Z, and significantly lowered transcripts levels of genes associated with chitin synthesis. The mbsA deletion also resulted in significantly reduced levels of proteins and transcripts of genes associated with the SakA MAP kinase pathway. Importantly, the cell wall hydrophobicity and architecture of the ΔmbsA asexual spores (conidia) were altered, notably lacking the rodlet layer on the surface of the ΔmbsA conidium. Comparative transcriptomic analyses revealed that the ΔmbsA mutant showed higher mRNA levels of gliotoxin (GT) biosynthetic genes, which was corroborated by elevated levels of GT production in the mutant. While the ΔmbsA mutant produced higher amount of GT, ΔmbsA strains showed reduced virulence in the murine model, likely due to the defective spore integrity. In summary, the putative APSES TF MbsA plays a multiple role in governing growth, development, spore wall architecture, GT production, and virulence, which may be associated with the attenuated SakA signaling pathway.

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.

A conserved regulator controls asexual sporulation in the fungal pathogen Candida albicans

Transcription factor Rme1 is conserved among ascomycetes and regulates meiosis and pseudohyphal growth in Saccharomyces cerevisiae. The genome of the meiosis-defective pathogen Candida albicans encodes an Rme1 homolog that is part of a transcriptional circuitry controlling hyphal growth. Here, we use chromatin immunoprecipitation and genome-wide expression analyses to study a possible role of Rme1 in C. albicans morphogenesis. We find that Rme1 binds upstream and activates the expression of genes that are upregulated during chlamydosporulation, an asexual process leading to formation of large, spherical, thick-walled cells during nutrient starvation. RME1 deletion abolishes chlamydosporulation in three Candida species, whereas its overexpression bypasses the requirement for chlamydosporulation cues and regulators. RME1 expression levels correlate with chlamydosporulation efficiency across clinical isolates. Interestingly, RME1 displays a biphasic pattern of expression, with a first phase independent of Rme1 function and dependent on chlamydospore-inducing cues, and a second phase dependent on Rme1 function and independent of chlamydospore-inducing cues. Our results indicate that Rme1 plays a central role in chlamydospore development in Candida species.

Characterization of a Candida albicans Mutant Defective in All MAPKs Highlights the Major Role of Hog1 in the MAPK Signaling Network

The success of Candida albicans as a pathogen relies on its ability to adapt and proliferate in different environmental niches. Pathways regulated by mitogen-activated protein kinases (MAPKs) are involved in sensing environmental conditions and developing an accurate adaptive response. Given the frequent cooperative roles of these routes in cellular functions, we have generated mutants defective in all combinations of the four described MAPKs in C. albicans and characterized its phenotype regarding sensitiveness to specific drugs, morphogenesis and interaction with host immune cells. We demonstrate that all MAPKs are dispensable in this yeast as a mutant defective in Cek1, Cek2, Mkc1 and Hog1 is viable although highly sensitive to oxidative and osmotic stress, displaying a specific pattern of sensitivity to antifungals. By comparing its phenotype with single, double and triple combinations of MAPK-deletion mutants we were able to unveil a Cek1-independent mechanism for Hog1 resistance to Congo red, and confirm the predominant effect of Hog1 on oxidative and osmotic adaptation. The quadruple mutant produces filaments under non-inducing conditions, but is unable to develop chlamydospores. Furthermore, cek1 cek2 mkc1 hog1 cells switch to the opaque state at high frequency, which is blocked by the ectopic expression of HOG1 suggesting a role of this kinase for phenotypic switching.

The stuA gene controls development, adaptation, stress tolerance, and virulence of the dermatophyte Trichophyton rubrum

The APSES family, comprising of the transcriptional regulators Asm1p, Phd1p, Sok2p, Efg1p, and StuA, is found exclusively in fungi and has been reported to control several cellular processes in these organisms. However, its function in dermatophytes has not yet been completely understood. Here, we generated two null mutant strains by deleting the stuA gene in the dermatophyte Trichophyton rubrum, the most common clinical isolate obtained from human skin and nail mycoses. The functional characterization of the knocked-out strains revealed the involvement of stuA in germination, morphogenesis of conidia and hyphae, pigmentation, stress responses, and virulence. Although the mutant strains could grow under several nutritional conditions, growth on the keratin medium, human nails, and skin was impaired. The co-culture of stuA mutants with human keratinocytes revealed enhanced development. Moreover, a stuA mutant grown on the keratin substrate showed a marked decrease in the transcript numbers of the hydrophobin encoding gene (hypA), suggesting the involvement of stuA in the molecular mechanisms underlying mechanosensing during the fungi-host interaction. In addition, bioinformatics analyses revealed the potential involvement of StuA in different biological processes such as oxidation-reduction, phosphorylation, proteolysis, transcription/translation regulation, and carbohydrate metabolism. Cumulatively, the present study suggested that StuA is a crosstalk mediator of many pathways and is an integral component of the infection process, implying that it could be a potential target for antifungal therapy.

Candida and Candidiasis-Opportunism Versus Pathogenicity: A Review of the Virulence Traits

One of the most important questions in microbiology nowadays, is how apparently harmless, commensal yeasts like Candida spp. can cause a rising number of infections. The occurrence of the disease requires firstly the attachment to the host cells, followed by the invasion of the tissue. The adaptability translates into a rapid ability to respond to stress factors, to take up nutrients or to multiply under different conditions. By forming complex intracellular networks such as biofilms, Candida spp. become not only more refractive to antifungal therapies but also more prone to cause disease. The inter-microbial interactions can enhance the virulence of a strain. In vivo, the fungal cells face a multitude of challenges and, as a result, they develop complex strategies serving one ultimate goal: survival. This review presents the virulence factors of the most important Candida spp., contributing to a better understanding of the onset of candidiasis and raising awareness of the highly complex interspecies interactions that can change the outcome of the disease.

Genetic interaction between Ptc2 and protein phosphatase 4 (PP4) in the regulation of DNA damage response and virulence in Candida albicans

In the pathogenic fungus Candida albicans, phosphoregulation of the checkpoint kinase Rad53 plays a crucial role in the filamentous growth response to genotoxic stresses. The protein phosphatase 4 (PP4) complex, containing Pph3 and either Psy2 or Psy4, is proved to play a critical role in Rad53 dephosphorylation. In previous studies, we characterized CaPtc2 (the ortholog of both Ptc2 and Ptc3 in Saccharomyces cerevisiae) as a potential DNA-damage-related protein phosphatase. In this study, we checked the genetic interaction of PTC2 with the PP4 complex in the DNA damage response pathway. The results suggest that Ptc2 shows a negative genetic interaction with Pph3, but positive genetic interaction with either Psy2 or Psy4 in response to genotoxic stress. Deletion of PTC2 alone resulted in no significant change in cell virulence, but double deletion of PTC2 PPH3 significantly decreased virulence, while double deletions of either PTC2 PSY2 or PTC2 PSY4 caused virulence levels similar to that shown by PSY2 or PSY4 single-gene deletion cells. Taken together, we propose that Ptc2 in C. albicans plays a compensatory role for Pph3 but is dependent on Psy2 and Psy4 in regulation of DNA damage and cell virulence.

Conserved and Divergent Functions of the cAMP/PKA Signaling Pathway in Candida albicans and Candida tropicalis

Fungal species undergo many morphological transitions to adapt to changing environments, an important quality especially in fungal pathogens. For decades, Candida albicans has been one of the most prevalent human fungal pathogens, and recently, the prevalence of Candida tropicalis as a causative agent of candidiasis has increased. In C. albicans, the ability to switch between yeast and hyphal forms is thought to be a key virulence factor and is regulated by multiple signaling cascades—including the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA), calcineurin, high-osmolarity glycerol (HOG), and mitogen-activated protein kinases (MAPK) signaling pathways—upon receiving environmental cues. The cAMP/PKA signaling pathway also triggers white-opaque switching in C. albicans. However, studies on C. tropicalis morphogenesis are limited. In this minireview, we discuss the regulation of the yeast-hypha transition, virulence, and white-opaque switching through the cAMP/PKA pathway in the closely related species C. albicans and C. tropicalis.

Identification of genes related to chlamydospore formation in Clonostachys rosea 67-1

Chlamydospores are specific structures that are of great significance to the commercialization of fungal biopesticides. To explore the genes associated with chlamydospore formation, a biocontrol fungus Clonostachys rosea 67‐1 that is capable of producing resistant spores under particular conditions was investigated by transcriptome sequencing and analysis. A total of 549,661,174 clean reads were obtained, and a series of differentially expressed genes potentially involved in fungal chlamydospore formation were identified. At 36 hr, 67 and 117 genes were up‐ and downregulated in C. rosea during chlamydospore production, compared with the control for conidiation, and 53 and 24 genes were up‐ and downregulated at 72 hr. GO classification suggested that the differentially expressed genes were related to cellular component, biological process, and molecular function categories. A total of 188 metabolism pathways were linked to chlamydospore production by KEGG analysis. Sixteen differentially expressed genes were verified by reverse transcription quantitative PCR, and the expression profiles were consistent with the transcriptome data. To the best of our knowledge, it is the first report on the genes associated with chlamydospore formation in C. rosea. The results provide insight into the molecular mechanisms underlying C. rosea sporulation, which will assist the development of fungal biocontrol agents.

Characterization of two homeodomain transcription factors with critical but distinct roles in virulence in the vascular pathogen Verticillium dahliae

Vascular wilt caused by Verticillium dahliae is a destructive disease that represents a chronic economic problem for crop production worldwide. In this work, we characterized two new regulators of pathogenicity in this species. Vph1 (VDAG_06555) was identified in a candidate gene approach as a putative homologue of the transcription factor Ste12. Vhb1 (VDAG_08786), identified in a forward genetics approach, is similar to the homeobox transcription factor Htf1, reported as a regulator of conidiogenesis in several fungi. Deletion of vph1 did not affect vegetative growth, whereas deletion of vhb1 greatly reduced sporulation rates in liquid medium. Both mutants failed to induce Verticillium wilt symptoms. However, unlike Δvph1, Δvhb1 could be re-isolated from the vascular system of some asymptomatic plants. Confocal microscopy further indicated that Δvph1 and Δvhb1 differed in their behaviour in planta; Δvph1 could not penetrate the root cortex, whereas Δvhb1 was impaired in its ability to colonize the xylem. In agreement with these observations, only Δvhb1 could penetrate cellophane paper. On cellophane, wild-type and Δvhb1 strains produced numerous short branches with swollen tips, resembling the hyphopodia formed on root surfaces, contrasting with Δvph1, which generated unbranched long filaments without swollen tips. A microarray analysis showed that these differences in growth were associated with differences in global transcription patterns, and allowed us to identify a large set of novel genes potentially involved in virulence in V. dahliae. Ste12 homologues are known regulators of invasive growth, but Vhb1 is the first putative Htf1 homologue identified with a critical role in virulence.

The APSES transcription factor Vst1 is a key regulator of development in microsclerotium- and resting mycelium-producing Verticillium species

Plant pathogens of the genus Verticillium pose a threat to many important crops worldwide. They are soil-borne fungi which invade the plant systemically, causing wilt symptoms. We functionally characterized the APSES family transcription factor Vst1 in two Verticillium species, V. dahliae and V. nonalfalfae, which produce microsclerotia and melanized hyphae as resistant structures, respectively. We found that, in V. dahliae Δvst1 strains, microsclerotium biogenesis stalled after an initial swelling of hyphal cells and cultures were never pigmented. In V. nonalfalfae Δvst1, melanized hyphae were also absent. These results suggest that Vst1 controls melanin biosynthesis independent of its role in morphogenesis. The absence of vst1 also had a great impact on sporulation in both species, affecting the generation of the characteristic verticillate conidiophore structure and sporulation rates in liquid medium. In contrast with these key roles in development, Vst1 activity was dispensable for virulence. We performed a microarray analysis comparing global transcription patterns of wild-type and Δvst1 in V. dahliae. G-protein/cyclic adenosine monophosphate (G-protein/cAMP) signalling and mitogen-activated protein kinase (MAPK) cascades are known to regulate fungal morphogenesis and virulence. The microarray analysis revealed a negative interaction of Vst1 with G-protein/cAMP signalling and a positive interaction with MAPK signalling. This analysis also identified Rho signalling as a potential regulator of morphogenesis in V. dahliae, positively interacting with Vst1. Furthermore, it exposed the association of secondary metabolism and development in this species, identifying Vst1 as a potential co-regulator of both processes. Characterization of the putative Vst1 targets identified in this study will aid in the dissection of specific aspects of development.

Candida albicans cell-type switching and functional plasticity in the mammalian host

Candida albicans is a ubiquitous commensal of the mammalian microbiome and the most prevalent fungal pathogen of humans. A cell-type transition between yeast and hyphal morphologies in C. albicans was thought to underlie much of the variation in virulence observed in different host tissues. However, novel yeast-like cell morphotypes, including opaque(a/α), grey and gastrointestinally induced transition (GUT) cell types, were recently reported that exhibit marked differences in vitro and in animal models of commensalism and disease. In this Review, we explore the characteristics of the classic cell types - yeast, hyphae, pseudohyphae and chlamydospores - as well as the newly identified yeast-like morphotypes. We highlight emerging knowledge about the associations of these different morphotypes with different host niches and virulence potential, as well as the environmental cues and signalling pathways that are involved in the morphological transitions.

Candida species Rewired Hyphae Developmental Programs for Chlamydospore Formation

Chlamydospore formation is a characteristic of many fungal species, among them the closely related human-pathogenic dimorphic yeasts Candida albicans and C. dubliniensis. Whereas function and regulation of filamentation are well-studied in these species, the basis of chlamydospore formation is mostly unknown. Here, we investigate the contribution of environmental and genetic factors and identified central proteins involved in species-specific regulation of chlamydosporulation. We show that specific nutrient levels strongly impact chlamydospore initiation, with starvation favoring sporulation and elevated levels of saccharides or peptone inhibiting it. Thresholds for these nutritional effects differ between C. albicans and C. dubliniensis, which explain species-specific chlamydospore formation on certain diagnostic media. A C. albicans nrg1Δ mutant phenocopied C. dubliniensis, putting Nrg1 regulation at the basis of species-specific chlamydospore formation under various conditions. By screening a series of potential chlamydospore regulators, we identified the TOR and cAMP pathways as crucial for sporulation. As rapamycin treatment blocked chlamydosporulation, a low basal Tor1 activity seems to be essential. In addition, TOR effector pathways play an important role, and loss of the NCR (nitrogen catabolite repression) gene regulators Gat1 and Gln3 reduced chlamydospore formation. A severe reduction was seen for a C. albicans gcn4Δ deletion strain, implicating a link between regulation of amino acid biosynthesis and chlamydospore development. On the other hand, deletion of the GTPase gene RAS1 and the adenylyl cyclase gene CYR1 caused a defect in chlamydospore formation that was mostly rescued by cAMP supplementation. Thus, cAMP-signaling is a second major pathway to control chlamydospore production. Finally, we confirmed light exposure to have a repressive effect on chlamydosporulation. However, permanent illumination only reduced, but not abolished chlamydospore production of C. albicans whereas C. dubliniensis sporulation was unaffected. In summary, we describe novel environmental factors which determine chlamydosporulation and propose a first model for the regulatory network of chlamydospore formation by different Candida species.

Candida albicans ISW2 Regulates Chlamydospore Suspensor Cell Formation and Virulence In Vivo in a Mouse Model of Disseminated Candidiasis

Formation of chlamydospores by Candida albicans was an established medical diagnostic test to confirm candidiasis before the molecular era. However, the functional role and pathological relevance of this in vitro morphological transition to pathogenesis in vivo remain unclear. We compared the physical properties of in vitro-induced chlamydospores with those of large C. albicans cells purified by density gradient centrifugation from Candida-infected mouse kidneys. The morphological and physical properties of these cells in kidneys of mice infected intravenously with wild type C. albicans confirmed that chlamydospores can form in infected kidneys. A previously reported chlamydospore-null Δisw2/Δisw2 mutant was used to investigate its role in virulence and chlamydospore induction. Virulence of the Δisw2/Δisw2 mutant strain was reduced 3.4-fold compared to wild type C. albicans or the ISW2 reconstituted strain. Altered host inflammatory reactions to the null mutant further indicate that ISW2 is a virulence factor in C. albicans. ISW2 deletion abolished chlamydospore formation within infected mouse kidneys, whereas the reconstituted strain restored chlamydospore formation in kidneys. Under chlamydospore inducing conditions in vitro, deletion of ISW2 significantly delayed chlamydospore formation, and those late induced chlamydospores lacked associated suspensor cells while attaching laterally to hyphae via novel spore-hypha septa. Our findings establish the induction of chlamydospores by C. albicans during mouse kidney colonization. Our results indicate that ISW2 is not strictly required for chlamydospores formation but is necessary for suspensor cell formation. The importance of ISW2 in chlamydospore morphogenesis and virulence may lead to additional insights into morphological differentiation and pathogenesis of C. albicans in the host microenvironment.

The transcriptional regulators SteA and StuA contribute to keratin degradation and sexual reproduction of the dermatophyte Arthroderma benhamiae

Most superficial fungal infections are caused by dermatophytes, a specialized group of filamentous fungi which exclusively infect keratinized host structures such as hair, skin and nails. Since little is known about the molecular basis of pathogenicity and sexual reproduction in dermatophytes, here we functionally addressed two central transcriptional regulators, SteA and StuA. In the zoophilic species Arthroderma benhamiae a strategy for targeted genetic manipulation was recently established, and moreover, the species is teleomorphic and thus allows performing assays based on mating. By comparative genome analysis homologs of the developmental regulators SteA and StuA were identified in A. benhamiae. Knock-out mutants of the corresponding genes as well as complemented strains were generated and phenotypically characterized. In contrast to A. benhamiae wild type and complemented strains, both mutants failed to produce sexual reproductive structures in mating experiments. Analysis of growth on keratin substrates indicated that loss of steA resulted in the inability of ΔsteA mutants to produce hair perforation organs, but did not affect mycelia formation during growth on hair and nails. By contrast, ΔstuA mutants displayed a severe growth defect on these substrates, but were still able to produce hair perforations. Hence, formation of hair perforation organs and fungal growth on hair per se are differentially regulated processes. Our findings on the major role of SteA and StuA during sexual development and keratin degradation in A. benhamiae provide insights into their role in dermatophytes and further enhance our knowledge of basic biology and pathogenicity of these fungi.

Characterization and control of Mucor circinelloides spoilage in yogurt

Consumer confidence in the food industry is severely affected by large-scale spoilage incidents. However, relatively little research exists on spoilage potential of members of the fungal subphylum Mucormycotina (e.g. Mucor), which includes dimorphic spoilage organisms that can switch between a yeast-like and hyphal phase depending on environmental conditions. The presence of Mucor circinelloides in yogurt may not cause spoilage, but growth and subsequent changes in quality (e.g. container bloating) can cause spoilage if not controlled. The purpose of this study was to evaluate the effects on M. circinelloides of pasteurization regimen, natamycin concentrations, and storage temperature in yogurt production, as measured by fungal proliferation and carbon dioxide production. A strain of M. circinelloides isolated from commercially spoiled yogurt showed greater yogurt-spoilage potential than clinical isolates and other industrial strains. D-values and z-values were determined for the spoilage isolate in milk as an evaluation of the fungus' ability to survive pasteurization. Natamycin was added to yogurt at 0, 5, 10, 15, and 20ppm (μg/ml) to determine its ability to inhibit M. circinelloides over the course of month-long challenge studies at 4°C, 15°C, and 25°C. Survivors were recovered on acidified PDA and carbon dioxide levels were recorded. The D-values at 54°C, 56°C, and 58°C for hyphae/sporangiospores were (in min) 38.31±0.02, 10.17±0.28, and 1.94±0.53, respectively, which yielded a z-value of 3.09°C. The D-values at 51°C, 53°C, and 55°C for yeast-like cells were (in min) 14.25±0.12, 6.87±1.19, and 2.44±0.35, respectively, which yielded a z-value of 0.34°C. These results indicated that M. circinelloides would not survive fluid milk pasteurization if contamination occurred prior to thermal treatment. CO2 production was only observed when M. circinelloides was incubated under low-oxygen conditions, and occurred only at temperatures above 4°C. Addition of 10ppm and greater of natamycin inhibited the growth and CO2 production of M. circinelloides under moderate temperature abuse when compared to the untreated control. These data suggest that yogurt spoilage (container bloating) caused by anaerobic growth of M. circinelloides is due to post-pasteurization contamination. Temperature abuse facilitated spoilage as CO2 production was observed in yogurt incubated at 15°C and 25°C, but not at 4°C. The addition of at least 10ppm of natamycin prevented M. circinelloides growth in both hyphal and yeast-like phases, as well as CO2 production in temperatures of up to 15°C for 30days.

AcstuA, which encodes an APSES transcription regulator, is involved in conidiation, cephalosporin biosynthesis and cell wall integrity of Acremonium chrysogenum

A transcriptional regulatory gene AcstuA was identified from Acremonium chrysogenum. AcstuA encodes a basic helix-loop-helix protein with similarity to StuA which regulates the core developmental processes of Aspergillus nidulans. Like disruption of stuA in A. nidulans, deficiency of AcstuA blocked the conidiation of A. chrysogenum through severely down-regulating the expression of AcbrlA and AcabaA which encode orthologs of the key fungal developmental regulators BrlA and AbaA. Disruption of AcstuA also drastically reduced cephalosporin production of A. chrysogenum. In agreement, the transcriptions of pcbAB, pbcC, cefD1, cefD2, cefEF and cefG were remarkably decreased in the AcstuA disruption mutant (ΔAcstuA). In addition to defects in conidiation and cephalosporin biosynthesis, ΔAcstuA produced abnormal swollen and fragmented hyphal cells during fermentation. The phenotypic alterations of hyphal cells caused by AcstuA deletion were restored by supplementation of NaCl in the medium, indicating that the deficiency of AcstuA has an influence on the cell wall integrity of A. chrysogenum. The transcriptions of two putative mannoprotein encoding genes Acmp2 and Acmp3 significantly reduced in ΔAcstuA, further indicating that cell wall integrity of the mutant is impaired. These results strongly suggested that AcstuA is involved in conidiation, cephalosporin production, hyphal fragmentation and cell wall integrity in A. chrysogenum.

Hypoxia and Temperature Regulated Morphogenesis in Candida albicans

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

Conserved and Distinct Functions of the “Stunted” (StuA)-Homolog Ust1 During Cell Differentiation in the Corn Smut Fungus Ustilago maydis

Ustilago maydis, causal agent of corn smut, can proliferate saprobically in a yeast form but its infectious filamentous form is an obligate parasite. Previously, we showed that Ust1, the first APSES (Asm1p, Phd1p, Sok2p, Efg1p, and StuAp) transcription factor functionally characterized in the phylum Basidiomycota, controlled morphogenesis and virulence in this species. Here, we further analyzed Ust1 function using multiple experimental approaches and determined that i) Ust1 activity was able to partially reverse stuA− conidiophore defects in Aspergillus nidulans; ii) in U. maydis, normal development and virulence were strongly dependent on precise induction or repression of Ust1 activity; iii) consistent with its role as a transcription factor regulating multiple processes, Ust1 accumulated in the nucleus at various stages of the life cycle; iv) however, it was undetectable at specific stages of pathogenic growth, indicating that Ust1 repression is part of normal development in planta; v) StuA response elements upstream of the ust1 open reading frame exhibited affinity for U. maydis DNA-binding proteins; vi) however, loss of regulated ust1 transcription had minor phenotypic effects; and vii) Ust1 was subject to post-translational phosphorylation but is not a target of cAMP signaling. Thus, the broad functional conservation between Ust1 and Ascomycota APSES proteins does not extend to the mechanisms regulating their activity.

The APSES family proteins in fungi: Characterizations, evolution and functions

The APSES protein family belongs to transcriptional factors of the basic helix-loop-helix (bHLH) class, the originally described members (APSES: Asm1p, Phd1p, Sok2p, Efg1p and StuAp) are used to designate this group of proteins, and they have been identified as key regulators of fungal development and other biological processes. APSES proteins share a highly conserved DNA-binding domain (APSES domain) of about 100 amino acids, whose central domain is predicted to form a typical bHLH structure. Besides APSES domain, several APSES proteins also contain additional domains, such as KilA-N and ankyrin repeats. In recent years, an increasing number of APSES proteins have been identified from diverse fungi, and they involve in numerous biological processes, such as sporulation, cellular differentiation, mycelial growth, secondary metabolism and virulence. Most fungi, including Aspergillus fumigatus, Aspergillus nidulans, Candida albicans, Fusarium graminearum, and Neurospora crassa, contain five APSES proteins. However, Cryptococcus neoformans only contains two APSES proteins, and Saccharomyces cerevisiae contains six APSES proteins. The phylogenetic analysis showed the APSES domains from different fungi were grouped into four clades (A, B, C and D), which is consistent with the result of homologous alignment of APSES domains using DNAman. The roles of APSES proteins in clade C have been studied in detail, while little is known about the roles of other APSES proteins in clades A, B and D. In this review, the biochemical properties and functional domains of APSES proteins are predicted and compared, and the phylogenetic relationship among APSES proteins from various fungi are analyzed based on the APSES domains. Moreover, the functions of APSES proteins in different fungi are summarized and discussed.

Modeling the transcriptional regulatory network that controls the early hypoxic response in Candida albicans

We determined the changes in transcriptional profiles that occur in the first hour following the transfer of Candida albicans to hypoxic growth conditions. The impressive speed of this response is not compatible with current models of fungal adaptation to hypoxia that depend on the depletion of sterol and heme. Functional analysis using Gene Set Enrichment Analysis (GSEA) identified the Sit4 phosphatase, Ccr4 mRNA deacetylase, and Sko1 transcription factor (TF) as potential regulators of the early hypoxic response. Cells mutated in these and other regulators exhibit a delay in their transcriptional responses to hypoxia. Promoter occupancy data for 29 TFs were combined with the transcriptional profiles of 3,111 in vivo target genes in a Network Component Analysis (NCA) to produce a model of the dynamic and highly interconnected TF network that controls this process. With data from the TF network obtained from a variety of sources, we generated an edge and node model that was capable of separating many of the hypoxia-upregulated and -downregulated genes. Upregulated genes are centered on Tye7, Upc2, and Mrr1, which are associated with many of the gene promoters that exhibit the strongest activations. The connectivity of the model illustrates the high redundancy of this response system and the challenges that lie in determining the individual contributions of specific TFs. Finally, treating cells with an inhibitor of the oxidative phosphorylation chain mimics most of the early hypoxic profile, which suggests that this response may be initiated by a drop in ATP production.

The APSES transcription factor Efg1 is a global regulator that controls morphogenesis and biofilm formation in Candida parapsilosis

Efg1 (a member of the APSES family) is an important regulator of hyphal growth and of the white-to-opaque transition in Candida albicans and very closely related species. We show that in Candida parapsilosis Efg1 is a major regulator of a different morphological switch at the colony level, from a concentric to smooth morphology. The rate of switching is at least 20-fold increased in an efg1 knockout relative to wild type. Efg1 deletion strains also have reduced biofilm formation, attenuated virulence in an insect model, and increased sensitivity to SDS and caspofungin. Biofilm reduction is more dramatic in in vitro than in in vivo models. An Efg1 paralogue (Efh1) is restricted to Candida species, and does not regulate concentric-smooth phenotype switching, biofilm formation or stress response. We used ChIP-seq to identify the Efg1 regulon. A total of 931 promoter regions bound by Efg1 are highly enriched for transcription factors and regulatory proteins. Efg1 also binds to its own promoter, and negatively regulates its expression. Efg1 targets are enriched in binding sites for 93 additional transcription factors, including Ndt80. Our analysis suggests that Efg1 has an ancient role as regulator of development in fungi, and is central to several regulatory networks.

Novel Regulatory Mechanisms of Pathogenicity and Virulence to Combat MDR in Candida albicans

Continuous deployment of antifungals in treating infections caused by dimorphic opportunistic pathogen Candida albicans has led to the emergence of drug resistance resulting in cross-resistance to many unrelated drugs, a phenomenon termed multidrug resistance (MDR). Despite the current understanding of major factors which contribute to MDR mechanisms, there are many lines of evidence suggesting that it is a complex interplay of multiple factors which may be contributed by still unknown mechanisms. Coincidentally with the increased usage of antifungal drugs, the number of reports for antifungal drug resistance has also increased which further highlights the need for understanding novel molecular mechanisms which can be explored to combat MDR, namely, ROS, iron, hypoxia, lipids, morphogenesis, and transcriptional and signaling networks. Considering the worrying evolution of MDR and significance of C. albicans being the most prevalent human fungal pathogen, this review summarizes these new regulatory mechanisms which could be exploited to prevent MDR development in C. albicans as established from recent studies.

Global transcriptome sequencing identifies chlamydospore specific markers in Candida albicans and Candida dubliniensis

Candida albicans and Candida dubliniensis are pathogenic fungi that are highly related but differ in virulence and in some phenotypic traits. During in vitro growth on certain nutrient-poor media, C. albicans and C. dubliniensis are the only yeast species which are able to produce chlamydospores, large thick-walled cells of unknown function. Interestingly, only C. dubliniensis forms pseudohyphae with abundant chlamydospores when grown on Staib medium, while C. albicans grows exclusively as a budding yeast. In order to further our understanding of chlamydospore development and assembly, we compared the global transcriptional profile of both species during growth in liquid Staib medium by RNA sequencing. We also included a C. albicans mutant in our study which lacks the morphogenetic transcriptional repressor Nrg1. This strain, which is characterized by its constitutive pseudohyphal growth, specifically produces masses of chlamydospores in Staib medium, similar to C. dubliniensis. This comparative approach identified a set of putatively chlamydospore-related genes. Two of the homologous C. albicans and C. dubliniensis genes (CSP1 and CSP2) which were most strongly upregulated during chlamydospore development were analysed in more detail. By use of the green fluorescent protein as a reporter, the encoded putative cell wall related proteins were found to exclusively localize to C. albicans and C. dubliniensis chlamydospores. Our findings uncover the first chlamydospore specific markers in Candida species and provide novel insights in the complex morphogenetic development of these important fungal pathogens.

Candida albicans: A Model Organism for Studying Fungal Pathogens

Candida albicans is an opportunistic human fungal pathogen that causes candidiasis. As healthcare has been improved worldwide, the number of immunocompromised patients has been increased to a greater extent and they are highly susceptible to various pathogenic microbes and C. albicans has been prominent among the fungal pathogens. The complete genome sequence of this pathogen is now available and has been extremely useful for the identification of repertoire of genes present in this pathogen. The major challenge is now to assign the functions to these genes of which 13% are specific to C. albicans. Due to its close relationship with yeast Saccharomyces cerevisiae, an edge over other fungal pathogens because most of the technologies can be directly transferred to C. albicans from S. cerevisiae and it is amenable to mutation, gene disruption, and transformation. The last two decades have witnessed enormous amount of research activities on this pathogen that leads to the understanding of host-parasite interaction, infections, and disease propagation. Clearly, C. albicans has emerged as a model organism for studying fungal pathogens along with other two fungi Aspergillus fumigatus and Cryptococcus neoformans. Understanding its complete life style of C. albicans will undoubtedly be useful for developing potential antifungal drugs and tackling Candida infections. This will also shed light on the functioning of other fungal pathogens.

Mannitol induces the conversion of conidia to chlamydospore-like structures that confer enhanced tolerance to heat, drought, and UV in Gibberella zeae

Fungi use mannitol to store carbon, balance redox, and mannitol serves as an antioxidant. Several fungi also increase stress tolerance by accumulating mannitol. The results of this study showed that conidia of the cereal head blight fungus Gibberella zeae were readily changed to chlamydospore-like structures (CLS) in cultures supplemented with high amounts of mannitol. CLS cellular features were atypical of chlamydospores, but accumulated high levels of glycogen, lipids, and chitin in the cytoplasm. In addition, CLS exhibited increased tolerance to environmental stresses, including UV, heat, and drought compared to normal conidia. Molecular approaches revealed that several genes associated with lipid metabolism, signal transduction, acetyl-CoA production, and chitin synthesis were involved in CLS formation. This is the first report to characterize conidia modifications similar to chlamydospores in G. zeae applying histological and molecular approaches. The results suggest CLS serve a role in G. zeae survival strategies under hot and dry field conditions.

Transcription factor Efg1 shows a haploinsufficiency phenotype in modulating the cell wall architecture and immunogenicity of Candida albicans

The Candida albicans transcription factor Efg1 is known to be involved in many different cellular processes, including morphogenesis, general metabolism, and virulence. Here we show that besides its manifold roles, Efg1 also has a prominent effect on cell wall structure and composition, strongly affecting the structural glucan part. Deletion of only one allele of EFG1 already results in severe phenotypes for cell wall biogenesis, comparable to those with deletion of both alleles, indicative of a severe haploinsufficiency for EFG1. The observed defects in structural setup of the cell wall, together with previously reported alterations in expression of cell surface proteins, result in altered immunogenic properties of strains with compromised Efg1 function. This is shown by interaction studies with macrophages and primary dendritic cells. The structural changes in the cell wall carbohydrate meshwork presented here, together with the manifold changes in cell wall protein composition and metabolism reported in other studies, contribute to the altered immune response mounted by innate immune cells and to the altered virulence phenotypes observed for strains lacking EFG1.

Target specificity of the Candida albicans Efg1 regulator

Efg1 is a central transcriptional regulator of morphogenesis and metabolism in Candida albicans. In vivo genome-wide ChIP chip and in vitro footprint analyses revealed the Efg1 recognition sequence (EGR-box) TATGCATA in the yeast growth form of this human fungal pathogen. Upstream regions of EFG1 and genes encoding transcriptional regulators of hyphal growth including TCC1, CZF1, TEC1, DEF1 and NRG1 contained EGR- and/or EGR-like boxes. Unexpectedly, after brief hyphal induction the genome-wide Efg1 binding pattern was completely altered and new binding sites of yet unknown specificity had appeared. Hyphal induction abolished Efg1 accumulation on EFG1 and TCC1 promoters and led to rapid decline of both transcripts, although the Efg1 protein persisted in cells. While EFG1 promoter activity in the yeast growth form did not depend on bound Efg1, its downregulation under hyphal induction depended on the presence of Efg1 and the protein kinase A isoform Tpk2. Deletion analyses of the EFG1 upstream region revealed that none of its resident EGR-boxes is uniquely responsible for EFG1 promoter downregulation. These results suggest different binding specificities of Efg1 in yeast growth and in hyphal induction and suggest a brief time window following hyphal induction, in which Efg1 exerts its repressive effect on target promoters.

Estradiol impairs the Th17 immune response against Candida albicans

Candida albicans is a commensal opportunistic pathogen that is also a member of gastrointestinal and reproductive tract microbiota. Exogenous factors, such as oral contraceptives, hormone replacement therapy, and estradiol, may affect susceptibility to Candida infection, although the mechanisms involved in this process have not been elucidated. We used a systemic candidiasis model to investigate how estradiol confers susceptibility to infection. We report that estradiol increases mouse susceptibility to systemic candidiasis, as in vivo and ex vivo estradiol-treated DCs were less efficient at up-regulating antigen-presenting machinery, pathogen killing, migration, IL-23 production, and triggering of the Th17 immune response. Based on these results, we propose that estradiol impairs DC function, thus explaining the increased susceptibility to infection during estrus.

Sch9 kinase integrates hypoxia and CO2 sensing to suppress hyphal morphogenesis in Candida albicans

The yeast-hypha transition is an important virulence trait of Candida albicans. We report that the AGC kinase Sch9 prevents hypha formation specifically under hypoxia at high CO(2) levels. sch9 mutants showed no major defects in growth and stress resistance but a striking hyperfilamentous phenotype under hypoxia (<10% O(2)), although only in the presence of elevated CO(2) levels (>1%) and at temperatures of <37°C during surface growth. The sch9 hyperfilamentous phenotype was independent of Rim15 kinase and was recreated by inhibition of Tor1 kinase by rapamycin or caffeine in a wild-type strain, suggesting that Sch9 suppression requires Tor1. Caffeine inhibition also revealed that both protein kinase A isoforms, as well as transcription factors Czf1 and Ace2, are required to generate the sch9 mutant phenotype. Transcriptomal analyses showed that Sch9 regulates most genes solely under hypoxia and in the presence of elevated CO(2). In this environment, Sch9 downregulates genes encoding cell wall proteins and nutrient transporters, while under normoxia Sch9 and Tor1 coregulate a minor fraction of Sch9-regulated genes, e.g., by inducing glycolytic genes. Other than in Saccharomyces cerevisiae, both sch9 and rim15 mutants showed decreased chronological aging under normoxia but not under hypoxia, indicating significant rewiring of the Tor1-Sch9-Rim15 pathway in C. albicans. The results stress the importance of environmental conditions on Sch9 function and establish a novel response circuitry to both hypoxia and CO(2) in C. albicans, which suppresses hypha formation but also allows efficient nutrient uptake, metabolism, and virulence.

Morphogenic regulator EFG1 affects the drug susceptibilities of pathogenic Candida albicans

This study shows that the morphogenic regulator EFG1 level affects the drug susceptibilities of Candida albicans when grown on solid growth media. The Deltaefg1 mutant showed sensitivity particularly to those drugs that target ergosterol or its metabolism. Efg1p disruption showed a gene-dosage effect on drug susceptibilities and resulted in enhanced susceptibility to drugs in the homozygous mutant as compared with the wild type, heterozygous and revertant strains. The enhanced sensitivity to drugs was independent of the status of ATP-binding cassette and MFS multidrug efflux pumps of C. albicans. The Deltaefg1 mutant displayed increased membrane fluidity that coincided with the downregulation of ERG11 and upregulation of OLE1 and ERG3, leading to enhanced passive diffusion of drugs. Interestingly, Deltaefg1 mutant cells displayed enhanced levels of endogenous ROS levels. Notably, the higher levels of ROS in the Deltaefg1 mutant could be reversed by the addition of antioxidants. However, the restoration of ROS levels did not reverse the drug sensitivities of the Deltaefg1 mutant. Taken together, we, for the first time, establish a new role to EFG1 in affecting the drug susceptibilities of C. albicans cells, independent of ROS and known drug efflux mechanisms.

Regulation of Ustilago maydis dimorphism, sporulation, and pathogenic development by a transcription factor with a highly conserved APSES domain

In Ustilago maydis, the causal agent of corn smut, the morphological transition from yeast to filamentous growth is inextricably linked to pathogenicity; budding haploid cells are saprobic and, upon mating of compatible strains, the fungus converts to dikaryotic filamentous growth and obligate parasitism. The filamentous dikaryon proliferates in the host plant, inducing tumor formation and undergoing additional morphological changes that eventually result in the production of melanized diploid teliospores. In an attempt to identify new trans-acting factors that regulate morphogenesis in U. maydis, we searched for the presence of common binding sequences in the promoter region of a set of 37 genes downregulated in the filamentous form. Putative cis-acting regulatory sequences fitting the consensus binding site for the Aspergillus nidulans transcription factor StuA were identified in 13 of these genes. StuA is a member of the APSES transcription factors which contain a highly conserved DNA-binding domain with a basic helix-loop-helix (bHLH)-like structure. This class of proteins comprises critical regulators of developmental processes in ascomycete fungi such as dimorphic growth, mating, and sporulation but has not been studied in any fungus of the phylum Basidiomycota. A search for StuA orthologs in the U. maydis genome identified a single closely related protein that we designated Ust1. Deletion of ust1 in budding haploid wild-type and solopathogenic strains led to filamentous growth and abolished mating, gall induction, and, consequently, in planta teliosporogenesis. Furthermore, cultures of ust1 null mutants produced abundant thick-walled, highly pigmented cells resembling teliospores which are normally produced only in planta. We showed that ssp1, a gene highly induced in teliospores produced in the host, is also abundantly expressed in cultures of ust1 null mutants containing these pigmented cells. Our results are consistent with a major role for ust1 in regulating dimorphism, virulence, and the sporulation program in U. maydis.

Sensing the environment: response of Candida albicans to the X factor

Virulence of the fungal human pathogen Candida albicans is also attributed to its ability to switch reversibly between yeast and hyphal growth forms. Morphogenesis in this yeast is influenced by the composition of the environment, activating sensors, which consequently play an important role in fungal pathogenicity. This review summarizes some of the main environmental sensors, their ligands and downstream signaling pathways in C. albicans. We will focus on proteins localized in the plasma membrane and on the interaction between cells and their environment. This will underline the convergence of several environmental signals onto the mitogen-activated protein kinase and protein kinase A pathways.