Bistable expression of WOR1, a master regulator of white-opaque switching in Candida albicans

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.

The transcription factor Ofi1 is critical for white-opaque switching in natural MTLa/α isolates of Candida albicans

Candida albicans, an opportunistic fungal pathogen, is able to switch between two distinct cell types: white and opaque. While white-to-opaque switching is typically repressed by the a1/α2 heterodimer in MTLa/α cells, it was recently reported that switching can also occur in some natural MTLa/α strains under certain environmental conditions. However, the regulatory program governing white-opaque switching in MTLa/α cells is not fully understood. Here, we collected 90 clinical isolates of C. albicans, 16 of which possess the ability to form opaque colonies. Among the known regulators implicated in white-opaque switching, only OFI1 exhibited significantly higher expression in these 16 strains compared to the reference strain SC5314. Importantly, ectopic expression of OFI1 in both clinical isolates and laboratory strains promoted switching frequency even in the absence of N-acetylglucosamine and high CO2 , the optimal condition for white-to-opaque switching in MTLa/α strains. Deleting OFI1 resulted in a reduction in opaque-formation frequency and the stability of the opaque cell in MTLa/α cells. Ofi1 binds to the promoters of WOR1 and WOR3 to induce their expression, which facilitates white-to-opaque switching. Ofi1 is conserved across the CTG species. Altogether, our study reported the identification of a transcription factor Ofi1 as the critical regulator that promotes white-to-opaque switching in natural MTLa/α isolates of C. albicans.

The Rfg1 and Bcr1 transcription factors regulate acidic pH-induced filamentous growth in Candida albicans

IMPORTANCE

Candida albicans is a human commensal and frequent pathogen that encounters a wide range of pH stresses. The ability of C. albicans to adapt to changes in extracellular pH is crucial for its success in colonization and pathogenesis. The Rim101 pH sensing pathway is well known to govern neutral-alkaline pH responses in this pathogen. Here, we report a novel Rfg1-Bcr1 regulatory pathway that governs acidic pH responses and regulates filamentous growth in C. albicans. In addition, the Rim101-Phr1 pathway, cAMP signaling pathway, transcription factors Efg1 and Flo8, and hyphal-specific G1 cyclin Hgc1 cooperate with this regulation. Our findings provide new insights into the regulatory mechanism of acidic pH response in C. albicans.

The defective gut colonization of Candida albicans hog1 MAPK mutants is restored by overexpressing the transcriptional regulator of the white opaque transition WOR1

The transcriptional master regulator of the white opaque transition of Candida albicans WOR1 is important for the adaptation to the commensal lifestyle in the mammalian gut, a major source of invasive candidiasis. We have generated cells that overproduce Wor1 in mutants defective in the Hog1 MAP kinase, defective in several stress responses and unable to colonize the mice gut. WOR1 overexpression allows hog1 to be established as a commensal in the murine gut in a commensalism model and even compete with wild-type C. albicans cells for establishment. This increased fitness correlates with an enhanced ability to adhere to biotic surfaces as well as increased proteinase and phospholipase production and a decrease in filamentation in vitro. We also show that hog1 WOR1^OE are avirulent in a systemic candidiasis model in mice.

Variation in transcription regulator expression underlies differences in white-opaque switching between the SC5314 reference strain and the majority of Candida albicans clinical isolates

Candida albicans, a normal member of the human microbiome and an opportunistic fungal pathogen, undergoes several morphological transitions. One of these transitions is white-opaque switching, where C. albicans alternates between 2 stable cell types with distinct cellular and colony morphologies, metabolic preferences, mating abilities, and interactions with the innate immune system. White-to-opaque switching is regulated by mating type; it is repressed by the a1/α2 heterodimer in a/α cells, but this repression is lifted in a/a and α/α mating type cells (each of which are missing half of the repressor). The widely used C. albicans reference strain, SC5314, is unusual in that white-opaque switching is completely blocked when the cells are a/α; in contrast, most other C. albicans a/α strains can undergo white-opaque switching at an observable level. In this paper, we uncover the reason for this difference. We show that, in addition to repression by the a1/α2 heterodimer, SC5314 contains a second block to white-opaque switching: 4 transcription regulators of filamentous growth are upregulated in this strain and collectively suppress white-opaque switching. This second block is missing in the majority of clinical strains, and, although they still contain the a1/α2 heterodimer repressor, they exhibit a/α white-opaque switching at an observable level. When both blocks are absent, white-opaque switching occurs at very high levels. This work shows that white-opaque switching remains intact across a broad group of clinical strains, but the precise way it is regulated and therefore the frequency at which it occurs varies from strain to strain.

Atmospheric humidity regulates same-sex mating in Candida albicans through the trehalose and osmotic signaling pathways

Sexual reproduction is prevalent in eukaryotic organisms and plays a critical role in the evolution of new traits and in the generation of genetic diversity. Environmental factors often have a direct impact on the occurrence and frequency of sexual reproduction in fungi. The regulatory effects of atmospheric relative humidity (RH) on sexual reproduction and pathogenesis in plant fungal pathogens and in soil fungi have been extensively investigated. However, the knowledge of how RH regulates the lifecycles of human fungal pathogens is limited. In this study, we report that low atmospheric RH promotes the development of mating projections and same-sex (homothallic) mating in the human fungal pathogen Candida albicans. Low RH causes water loss in C. albicans cells, which results in osmotic stress and the generation of intracellular reactive oxygen species (ROS) and trehalose. The water transporting aquaporin Aqy1, and the G-protein coupled receptor Gpr1 function as cell surface sensors of changes in atmospheric humidity. Perturbation of the trehalose metabolic pathway by inactivating trehalose synthase or trehalase promotes same-sex mating in C. albicans by increasing osmotic or ROS stresses, respectively. Intracellular trehalose and ROS signal the Hog1-osmotic and Hsf1-Hsp90 signaling pathways to regulate the mating response. We, therefore, propose that the cell surface sensors Aqy1 and Gpr1, intracellular trehalose and ROS, and the Hog1-osmotic and Hsf1-Hsp90 signaling pathways function coordinately to regulate sexual mating in response to low atmospheric RH conditions in C. albicans.

Glucose depletion enables Candida albicans mating independently of the epigenetic white-opaque switch

The human fungal pathogen Candida albicans can switch stochastically and heritably between a "white" phase and an "opaque" phase. Opaque cells are the mating-competent form of the species, whereas white cells are thought to be essentially "sterile". Here, we report that glucose depletion, a common nutrient stress, enables C. albicans white cells to undergo efficient sexual mating. The relative expression levels of pheromone-sensing and mating-associated genes (including STE2/3, MFA1, MFα1, FIG1, FUS1, and CEK1/2) are increased under glucose depletion conditions, while expression of mating repressors TEC1 and DIG1 is decreased. Cph1 and Tec1, factors that act downstream of the pheromone MAPK pathway, play opposite roles in regulating white cell mating as TEC1 deletion or CPH1 overexpression promotes white cell mating. Moreover, inactivation of the Cph1 repressor Dig1 increases white cell mating ~4000 fold in glucose-depleted medium relative to that in the presence of glucose. Our findings reveal that the white-to-opaque epigenetic switch may not be a prerequisite for sexual mating in C. albicans in nature.

Fun30 nucleosome remodeller regulates white-to-opaque switching in Candida albicans

Candida albicans ( C. albicans) is an opportunistic pathogen in humans and possesses a white-opaque heritable switching system. Wor1 is a master regulator of white-opaque switching and is essential for opaque cell formation in C. albicans. However, the regulatory network of Wor1 in white-opaque switching is still vague. In this study, we obtain a series of Wor1-interacting proteins using LexA-Wor1 as bait. Among these proteins, function unknown now 30 (Fun30) interacts with Wor1 in vitro and in vivo. Fun30 expression is upregulated in opaque cells at the transcriptional and protein levels. Loss of FUN30 attenuates white-to-opaque switching, while ectopic expression of FUN30 significantly increases white-to-opaque switching in an ATPase activity-dependent manner. Furthermore, FUN30 upregulation is dependent on CO ₂; loss of FLO8, a key CO ₂-sensing transcriptional regulator, abolishes FUN30 upregulation. Interestingly, deletion of FUN30 affects the WOR1 expression regulation feedback loop. Thus, our results indicate that the chromatin remodeller Fun30 interacts with Wor1 and is required for WOR1 expression and opaque cell formation.

Morphogenic plasticity: the pathogenic attribute of Candida albicans

Candida albicans is a commensal organism of the human gastrointestinal tract and a prevalent opportunistic pathogen. It exhibits different morphogenic forms to survive in different host niches with distinct environmental conditions (pH, temperature, oxidative stress, nutrients, serum, chemicals, radiation, etc.) and genetic factors (transcription factors and genes). The different morphogenic forms of C. albicans are yeast, hyphal, pseudohyphal, white, opaque, and transient gray cells, planktonic and biofilm forms of cells. These forms differ in the parameters like cellular phenotype, colony morphology, adhesion to solid surfaces, gene expression profile, and the virulent traits. Each form is functionally distinct and responds discretely to the host immune system and antifungal drugs. Hence, morphogenic plasticity is the key to virulence. In this review, we address the characteristics, the pathogenic potential of the different morphogenic forms and the conditions required for morphogenic transitions.

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.

Candida albicans-enteric viral interactions-The prostaglandin E2 connection and host immune responses

The human microbiome comprises trillions of microorganisms residing within different mucosal cavities and across the body surface. The gut microbiota modulates host susceptibility to viral infections in several ways, and microbial interkingdom interactions increase viral infectivity within the gut. Candida albicans, a frequently encountered fungal species in the gut, produces highly structured biofilms and eicosanoids such as prostaglandin E₂ (PGE₂), which aid in viral protection and replication. These biofilms encompass viruses and provide a shield from antiviral drugs or the immune system. PGE₂ is a key modulator of active inflammation with the potential to regulate interferon signaling upon microbial invasion or viral infections. In this review, we raise the perspective of gut interkingdom interactions involving C. albicans and enteric viruses, with a special focus on biofilms, PGE₂, and viral replication. Ultimately, we discuss the possible implications of C. albicans-enteric virus associations on host immune responses, particularly the interferon signaling pathway.

Effects of Hst3p inhibition in Candida albicans: a genome-wide H3K56 acetylation analysis

Candida spp. represent the third most frequent worldwide cause of infection in Intensive Care Units with a mortality rate of almost 40%. The classes of antifungals currently available include azoles, polyenes, echinocandins, pyrimidine derivatives, and allylamines. However, the therapeutical options for the treatment of candidiasis are drastically reduced by the increasing antifungal resistance. The growing need for a more targeted antifungal therapy is limited by the concern of finding molecules that specifically recognize the microbial cell without damaging the host. Epigenetic writers and erasers have emerged as promising targets in different contexts, including the treatment of fungal infections. In C. albicans, Hst3p, a sirtuin that deacetylates H3K56ac, represents an attractive antifungal target as it is essential for the fungus viability and virulence. Although the relevance of such epigenetic regulator is documented for the development of new antifungal therapies, the molecular mechanism behind Hst3p-mediated epigenetic regulation remains unrevealed. Here, we provide the first genome-wide profiling of H3K56ac in C. albicans resulting in H3K56ac enriched regions associated with Candida sp. pathogenicity. Upon Hst3p inhibition, 447 regions gain H3K56ac. Importantly, these genomic areas contain genes encoding for adhesin proteins, degradative enzymes, and white-opaque switching. Moreover, our RNA-seq analysis revealed 1330 upregulated and 1081 downregulated transcripts upon Hst3p inhibition, and among them, we identified 87 genes whose transcriptional increase well correlates with the enrichment of H3K56 acetylation on their promoters, including some well-known regulators of phenotypic switching and virulence. Based on our evidence, Hst3p is an appealing target for the development of new potential antifungal drugs.

Overexpression of the White Opaque Switching Master Regulator Wor1 Alters Lipid Metabolism and Mitochondrial Function in Candida albicans

Candida albicans is a commensal yeast that inhabits the gastrointestinal tract of humans; increased colonization of this yeast in this niche has implicated the master regulator of the white-opaque transition, Wor1, by mechanisms not completely understood. We have addressed the role that this transcription factor has on commensalism by the characterization of strains overexpressing this gene. We show that WOR1 overexpression causes an alteration of the total lipid content of the fungal cell and significantly alters the composition of structural and reserve molecular species lipids as determined by lipidomic analysis. These cells are hypersensitive to membrane-disturbing agents such as SDS, have increased tolerance to azoles, an augmented number of peroxisomes, and increased phospholipase activity. WOR1 overexpression also decreases mitochondrial activity and results in altered susceptibility to certain oxidants. All together, these changes reflect drastic alterations in the cellular physiology that facilitate adaptation to the gastrointestinal tract environment.

The intrinsically disordered region from PP2C phosphatases functions as a conserved CO2 sensor

Carbon dioxide not only plays a central role in the carbon cycle, but also acts as a crucial signal in living cells. Adaptation to changing CO₂ concentrations is critical for all organisms. Conversion of CO₂ to HCO₃^- by carbonic anhydrase and subsequent HCO₃^--triggered signalling are thought to be important for cellular responses to CO₂ (refs. ^1-3). However, carbonic anhydrases are suggested to transduce a change in CO₂ rather than be a direct CO₂ sensor^4,5, the mechanism(s) by which organisms sense CO₂ remain unknown. Here we demonstrate that a unique group of PP2C phosphatases from fungi and plants senses CO₂, but not HCO₃^-, to control diverse cellular programmes. Different from other phosphatases, these PP2Cs all have an intrinsically disordered region (IDR). They formed reversible liquid-like droplets through phase separation both in cells and in vitro, and were activated in response to elevated environmental CO₂ in an IDR-dependent manner. The IDRs in PP2Cs are characterized by a sequence of polar amino acids enriched in serine/threonine, which provides CO₂ responsiveness. CO₂-responsive activation of PP2Cs via the serine/threonine-rich IDR-mediated phase separation represents a direct CO₂ sensing mechanism and is widely exploited.

Multiple molecular events underlie stochastic switching between 2 heritable cell states in fungi

Eukaryotic transcriptional networks are often large and contain several levels of feedback regulation. Many of these networks have the ability to generate and maintain several distinct transcriptional states across multiple cell divisions and to switch between them. In certain instances, switching between cell states is stochastic, occurring in a small subset of cells of an isogenic population in a seemingly homogenous environment. Given the scarcity and unpredictability of switching in these cases, investigating the determining molecular events is challenging. White-opaque switching in the fungal species Candida albicans is an example of stably inherited cell states that are determined by a complex transcriptional network and can serve as an experimentally accessible model system to study characteristics important for stochastic cell fate switching in eukaryotes. In standard lab media, genetically identical cells maintain their cellular identity (either "white" or "opaque") through thousands of cell divisions, and switching between the states is rare and stochastic. By isolating populations of white or opaque cells, previous studies have elucidated the many differences between the 2 stable cell states and identified a set of transcriptional regulators needed for cell type switching and maintenance of the 2 cell types. Yet, little is known about the molecular events that determine the rare, stochastic switching events that occur in single cells. We use microfluidics combined with fluorescent reporters to directly observe rare switching events between the white and opaque states. We investigate the stochastic nature of switching by beginning with white cells and monitoring the activation of Wor1, a master regulator and marker for the opaque state, in single cells and throughout cell pedigrees. Our results indicate that switching requires 2 stochastic steps; first an event occurs that predisposes a lineage of cells to switch. In the second step, some, but not all, of those predisposed cells rapidly express high levels of Wor1 and commit to the opaque state. To further understand the rapid rise in Wor1, we used a synthetic inducible system in Saccharomyces cerevisiae into which a controllable C. albicans Wor1 and a reporter for its transcriptional control region have been introduced. We document that Wor1 positive autoregulation is highly cooperative (Hill coefficient > 3), leading to rapid activation and producing an "all or none" rather than a graded response. Taken together, our results suggest that reaching a threshold level of a master regulator is sufficient to drive cell type switching in single cells and that an earlier molecular event increases the probability of reaching that threshold in certain small lineages of cells. Quantitative molecular analysis of the white-opaque circuit can serve as a model for the general understanding of complex circuits.

Genome plasticity in Candida albicans: A cutting-edge strategy for evolution, adaptation, and survival

Candida albicans is the most implicated fungal species that grows as a commensal or opportunistic pathogen in the human host. It is associated with many life-threatening infections, especially in immunocompromised persons. The genome of Candida albicans is very flexible and can withstand a wide assortment of variations in a continuously changing environment. Thus, genome plasticity is central to its adaptation and has long been of considerable interest. C. albicans has a diploid heterozygous genome that is highly dynamic and can display variation from small to large scale chromosomal rearrangement and aneuploidy, which have implications in drug resistance, virulence, and pathogenicity. This review presents an up-to-date overview of recent genomic studies involving C. albicans. It discusses the accumulating evidence that shows how mitotic recombination events, ploidy dynamics, aneuploidy, and loss of heterozygosity (LOH) influence evolution, adaptation, and survival in C. albicans. Understanding the factors that affect the genome is crucial for a proper understanding of species and rapid development and adjustment of therapeutic strategies to mitigate their spread.

Heat Shock Transcription Factor CgHSF1 Is Required for Melanin Biosynthesis, Appressorium Formation, and Pathogenicity in Colletotrichum gloeosporioides

Heat shock transcription factors (HSFs) are a family of transcription regulators. Although HSFs’ functions in controlling the transcription of the molecular chaperone heat shock proteins and resistance to stresses are well established, their effects on the pathogenicity of plant pathogenic fungi remain unknown. In this study, we analyze the role of CgHSF1 in the pathogenicity of Colletotrichum gloeosporioides and investigate the underlying mechanism. Failure to generate the Cghsf1 knock-out mutant suggested that the gene is essential for the viability of the fungus. Then, genetic depletion of the Cghsf1 was achieved by inserting the repressive promoter of nitrite reductase gene (PniiA) before its coding sequence. The mutant showed significantly decrease in the pathogenicity repression of appressorium formation, and severe defects in melanin biosynthesis. Moreover, four melanin synthetic genes were identified as direct targets of CgHSF1. Taken together, this work highlights the role of CgHSF1 in fungal pathogenicity via the transcriptional activation of melanin biosynthesis. Our study extends the understanding of fungal HSF1 proteins, especially their involvement in pathogenicity.

Candida albicans MTLa2 regulates the mating response through both the a-factor and α-factor sensing pathways

The diploid fungal pathogen Candida albicans has three configurations at the mating type locus (MTL): heterozygous (a/α) and homozygous (a/a or α/α). C. albicans MTL locus encodes four transcriptional regulators (MTLa1, a2, α1, and α2). The conserved a1/α2 heterodimer controls not only mating competency but also white-opaque heritable phenotypic switching. However, the regulatory roles of MTLa2 and α1 are more complex and remain to be investigated. MTLa/a cells often express a cell type-specific genes and mate as the a-type partner, whereas MTLα/α cells express α-specific genes and mate as the α-type partner. In this study, we report that the MTLa2 regulator controls the formation of mating projections through both the a- and α-pheromone-sensing pathways and thus results in the bi-mater feature of "α cells" of C. albicans. Ectopic expression of MTLa2 in opaque α cells activates the expression of not only MFA1 and STE3 (a-pheromone receptor) but also MFα1 and STE2 (α-pheromone receptor). Inactivation of either the MFa-Ste3 or MFα-Ste2 pheromone-sensing pathway cannot block the MTLa2-induced development of mating projections. However, the case is different in MTLα1-ectopically expressed opaque a cells. Inactivation of the MFα-Ste2 but not the MFa-Ste3 pheromone-sensing pathway blocks MTLα1-induced development of mating projections. Therefore, MTLa2 and MTLα1 exhibit distinct regulatory features that control the mating response in C. albicans. These findings shed new light on the regulatory mechanism of bi-mating behaviors and sexual reproduction in C. albicans.

Parasexuality of Candida Species

While most fungi have the ability to reproduce sexually, multiple independent lineages have lost meiosis and developed parasexual cycles in its place. Emergence of parasexual cycles is particularly prominent in medically relevant fungi from the CUG paraphyletic group of Candida species. Since the discovery of parasex in C. albicans roughly two decades ago, it has served as the model for Candida species. Importantly, parasex in C. albicans retains hallmarks of meiosis including genetic recombination and chromosome segregation, making it a potential driver of genetic diversity. Furthermore, key meiotic genes play similar roles in C. albicans parasex and highlights parallels between these processes. Yet, the evolutionary role of parasex in Candida adaptation and the extent of resulting genotypic and phenotypic diversity remain as key knowledge gaps in this facultative reproductive program. Here, we present our current understanding of parasex, the mechanisms governing its regulation, and its relevance to Candida biology.

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

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

Role of the WOR1 Promoter of Candida albicans in Opaque Commitment

During induced differentiation, the process often involves a commitment event, after which induced cells, when returned to noninducing conditions, continue to differentiate. The commitment event is rarely identified. Candida albicans differentiates from the white to opaque phenotype, a prerequisite for mating and a process accompanying colonization of the lower gastrointestinal tract and skin. In analyses of white cell populations induced to synchronously differentiate from the white to opaque phenotype, opaque commitment occurs at approximately the same time as evagination and chitin ring formation in the process of daughter cell formation, several hours after the master switch gene WOR1 is upregulated. Mutational analyses of transcription factor binding regions P1, P2, P3, P4, and P6 of the WOR1 promoter reveal that individual deletion of any of the five transcription factor binding regions does not eliminate morphological differentiation to the opaque cell phenotype under opaque-inducing conditions, but individual deletion of P2, P3, or P4, blocks opaque commitment and maintenance of the opaque phenotype after transition to noninducing conditions. These results suggest that commitment occurs at the level of the WOR1 promoter and that morphological differentiation can be dissociated from phenotypic commitment.

Adenylyl Cyclase and Protein Kinase A Play Redundant and Distinct Roles in Growth, Differentiation, Antifungal Drug Resistance, and Pathogenicity of Candida auris

Candida auris is a globally emerging multidrug-resistant fungal pathogen. Its pathogenicity-related signaling networks are largely unknown. Here, we characterized the pathobiological functions of the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway in C. auris. We focused on adenylyl cyclase (CYR1), the PKA regulatory subunit (BCY1), and the PKA catalytic subunits (TPK1 and TPK2). We concluded that PKA acts both dependently and independently of Cyr1 in C. auris. Tpk1 and Tpk2 have major and minor roles, respectively, in PKA activity and functions. Both Cyr1 and PKA promote growth, thermotolerance, filamentous growth, and resistance to stress and antifungal drugs by regulating expression of multiple effector genes. In addition, Cyr1 and PKA subunits were involved in disinfectant resistance of C. auris. However, deletion of both TPK1 and TPK2 generally resulted in more severe defects than CYR1 deletion, indicating that Cyr1 and PKA play redundant and distinct roles. Notably, Tpk1 and Tpk2 have redundant but Cyr1-independent roles in haploid-to-diploid cell transition, which increases virulence of C. auris. However, Tpk1 and Tpk2 often play opposing roles in formation of biofilms and the cell wall components chitin and chitosan. Surprisingly, deletion of CYR1 or TPK1/TPK2, which resulted in severe in vitro growth defects at 37°C, did not attenuate virulence, and BCY1 deletion reduced virulence of C. auris in a systemic murine infection model. In conclusion, this study provides comprehensive insights into the role of the cAMP/PKA pathway in drug resistance and pathogenicity of C. auris and suggests a potential therapeutic option for treatment of C. auris-mediated candidemia.

Overexpression approaches to advance understanding of Candida albicans

Candida albicans is an opportunistic fungal pathogen that is responsible for infections linked to high mortality. Loss‐of‐function approaches, taking advantage of gene knockouts or inducible down‐regulation, have been successfully used in this species in order to understand gene function. However, overexpression of a gene provides an alternative, powerful tool to elucidate gene function and identify novel phenotypes. Notably, overexpression can identify pathway components that might remain undetected using loss‐of‐function approaches. Several repressible or inducible promoters have been developed which allow to shut off or turn on the expression of a gene in C. albicans upon growth in the presence of a repressor or inducer. In this review, we summarize recent overexpression approaches used to study different aspects of C. albicans biology, including morphogenesis, biofilm formation, drug tolerance, and commensalism.

BbWor1, a Regulator of Morphological Transition, Is Involved in Conidium-Hypha Switching, Blastospore Propagation, and Virulence in Beauveria bassiana

Morphological transition is an important adaptive mechanism in the host invasion process. Wor1 is a conserved fungal regulatory protein that controls the phenotypic switching and pathogenicity of Candida albicans. By modulating growth conditions, we simulated three models of Beauveria bassiana morphological transitions, including CTH (conidia to hyphae), HTC (hyphae to conidia), and BTB (blastospore to blastospore). Disruption of BbWor1 (an ortholog of Wor1) resulted in a distinct reduction in the time required for conidial germination (CTH), a significant increase in hyphal growth, and a decrease in the yield of conidia (HTC), indicating that BbWor1 positively controls conidium production and negatively regulates hyphal growth in conidium-hypha switching. Moreover, ΔBbWor1 prominently decreased blastospore yield, shortened the G0/G1 phase, and prolonged the G2/M phase under the BTB model. Importantly, BbWor1 contributed to conidium-hypha switching and blastospore propagation via different genetic pathways, and yeast one-hybrid testing demonstrated the necessity of BbWor1 to control the transcription of an allergen-like protein gene (BBA_02580) and a conidial wall protein gene (BBA_09998). Moreover, the dramatically weakened virulence of ΔBbWor1 was examined by immersion and injection methods. Our findings indicate that BbWor1 is a vital participant in morphological transition and pathogenicity in entomopathogenic fungi. IMPORTANCE As a well-known entomopathogenic fungus, Beauveria bassiana has a complex life cycle and involves transformations among single-cell conidia, blastospores, and filamentous hyphae. This study provides new insight into the regulation of the fungal cell morphological transitions by simulating three models. Our research identified BbWor1 as a core transcription factor of morphological differentiation that positively regulates the production of conidia and blastospores but negatively regulates hyphal growth. More importantly, BbWor1 affects fungal pathogenicity and the global transcription profiles within three models of growth stage transformation. The present study lays a foundation for the exploration of the transition mechanism of entomopathogenic fungi and provides material for the morphological study of fungi.

Signal-mediated localization of Candida albicans pheromone response pathway components

 

A MAPK cascade consists of three kinases, (MEKK, MEK and MAPK), that are sequentially activated in response to a stimulus and serve to transmit signals. In C. albicans and in yeast, an MAPK cascade is linked to the pheromone pathway through a scaffold protein (Cst5 and Ste5, respectively). Cst5 is much shorter and lacks key domains compared to Ste5, so in C. albicans, other elements, in particular the MEKK Ste11, play key roles in controlling the associations and localizations of network components.

Abstract

Candida albicans opaque cells release pheromones to stimulate cells of opposite mating type to activate their pheromone response pathway. Although this fungal pathogen shares orthologous proteins involved in the process with Saccharomyces cerevisiae, the pathway in each organism has unique characteristics. We have used GFP-tagged fusion proteins to investigate the localization of the scaffold protein Cst5, as well as the MAP kinases Cek1 and Cek2, during pheromone response in C. albicans. In wild-type cells, pheromone treatment directed Cst5-GFP to surface puncta concentrated at the tips of mating projections. These puncta failed to form in cells defective in either the Gα or β subunits. However, they still formed in response to pheromone in cells missing Ste11, but with the puncta distributed around the cell periphery in the absence of mating projections. These puncta were absent from hst7Δ/Δ cells, but could be detected in the ste11Δ/Δ hst7Δ/Δ double mutant. Cek2-GFP showed a strong nuclear localization late in the response, consistent with a role in adaptation, while Cek1-GFP showed a weaker, but early increase in nuclear localization after pheromone treatment. Activation loop phosphorylation of both Cek1 and Cek2 required the presence of Ste11. In contrast to Cek2-GFP, which showed no localization signal in ste11Δ/Δ cells, Cek1-GFP showed enhanced nuclear localization that was pheromone independent in the ste11Δ/Δ mutant. The results are consistent with CaSte11 facilitating Hst7-mediated MAP kinase phosphorylation and also playing a potentially critical role in both MAP kinase and Cst5 scaffold localization.

Transcription factor control of virulence in phytopathogenic fungi

Plant-pathogenic fungi are a significant threat to economic and food security worldwide. Novel protection strategies are required and therefore it is critical we understand the mechanisms by which these pathogens cause disease. Virulence factors and pathogenicity genes have been identified, but in many cases their roles remain elusive. It is becoming increasingly clear that gene regulation is vital to enable plant infection and transcription factors play an essential role. Efforts to determine their regulatory functions in plant-pathogenic fungi have expanded since the annotation of fungal genomes revealed the ubiquity of transcription factors from a broad range of families. This review establishes the significance of transcription factors as regulatory elements in plant-pathogenic fungi and provides a systematic overview of those that have been functionally characterized. Detailed analysis is provided on regulators from well-characterized families controlling various aspects of fungal metabolism, development, stress tolerance, and the production of virulence factors such as effectors and secondary metabolites. This covers conserved transcription factors with either specialized or nonspecialized roles, as well as recently identified regulators targeting key virulence pathways. Fundamental knowledge of transcription factor regulation in plant-pathogenic fungi provides avenues to identify novel virulence factors and improve our understanding of the regulatory networks linked to pathogen evolution, while transcription factors can themselves be specifically targeted for disease control. Areas requiring further insight regarding the molecular mechanisms and/or specific classes of transcription factors are identified, and direction for future investigation is presented.

The Glyoxylate Cycle Is Involved in White-Opaque Switching in Candida albicans

Candida albicans is a commensal yeast that inhabits the gastrointestinal tract of humans. The master regulator of the white-opaque transition WOR1 has been implicated in the adaptation to this commensal status. A proteomic analysis of cells overexpressing this transcription factor (WOR1^OE) suggested an altered metabolism of carbon sources and a phenotypic analysis confirmed this alteration. The WOR1^OE cells are deficient in using trehalose and xylose and are unable to use 2C sources, which is consistent with a reduction in the amount of Icl1, the isocitrate lyase enzyme. The icl1Δ/Δ mutants overexpressing WOR1 are deficient in the production of phloxine B positive cells, a main characteristic of opaque cells, a phenotype also observed in mating type hemizygous mtla1Δ icl1Δ/Δ cells, suggesting the involvement of Icl1 in the adaptation to the commensal state. In fact, icl1Δ/Δ cells have reduced fitness in mouse gastrointestinal tract as compared with essentially isogenic heterozygous ICL1/icl1Δ, but overproduction of WOR1 in an icl1Δ/Δ mutant does not restore fitness. These results implicate the glyoxylate shunt in the adaptation to commensalism of C. albicans by mechanisms that are partially independent of WOR1.

The microbial and host factors that govern Candida gastrointestinal colonization and dissemination

Candida species are among the most prevalent and abundant members of the gut mycobiota, with Candida albicans (CA) being the most prominent member. CA colonizes numerous mucosal surfaces, most notably the gastrointestinal (GI) and genitourinary tracts. In a healthy host, CA is a pathobiont that exists as a commensal but can become pathogenic if the host's immune system becomes suppressed. The microbial and/or host factors that dictate CA's ability to colonize mucosal surfaces and its ability to disseminate remain of great interest. Here, we review the recent advances and insights regarding Candida colonization and dissemination of the mammalian GI tract.

Candida pathogens induce protective mitochondria-associated type I interferon signalling and a damage-driven response in vaginal epithelial cells

Vaginal candidiasis is an extremely common disease predominantly caused by four phylogenetically diverse species: Candida albicans; Candida glabrata; Candida parapsilosis; and Candida tropicalis. Using a time course infection model of vaginal epithelial cells and dual RNA sequencing, we show that these species exhibit distinct pathogenicity patterns, which are defined by highly species-specific transcriptional profiles during infection of vaginal epithelial cells. In contrast, host cells exhibit a homogeneous response to all species at the early stages of infection, which is characterized by sublethal mitochondrial signalling inducing a protective type I interferon response. At the later stages, the transcriptional response of the host diverges in a species-dependent manner. This divergence is primarily driven by the extent of epithelial damage elicited by species-specific mechanisms, such as secretion of the toxin candidalysin by C. albicans. Our results uncover a dynamic, biphasic response of vaginal epithelial cells to Candida species, which is characterized by protective mitochondria-associated type I interferon signalling and a species-specific damage-driven response.

Recording of DNA-binding events reveals the importance of a repurposed Candida albicans regulatory network for gut commensalism

Candida albicans is a fungal component of the human gut microbiota and an opportunistic pathogen. C. albicans transcription factors (TFs), Wor1 and Efg1, are master regulators of an epigenetic switch required for fungal mating that also control colonization of the mammalian gut. We show that additional mating regulators, WOR2, WOR3, WOR4, AHR1, CZF1, and SSN6, also influence gut commensalism. Using Calling Card-seq to record Candida TF DNA-binding events in the host, we examine the role and relationships of these regulators during murine gut colonization. By comparing in-host transcriptomes of regulatory mutants with enhanced versus diminished commensal fitness, we also identify a set of candidate commensalism effectors. These include Cht2, a GPI-linked chitinase whose gene is bound by Wor1, Czf1, and Efg1 in vivo, that we show promotes commensalism. Thus, the network required for a C. albicans sexual switch is biochemically active in the host intestine and repurposed to direct commensalism.

Regulators of commensal and pathogenic life-styles of an opportunistic fungus-Candida albicans

The yeast Candida albicans is primarily a commensal of humans that colonizes the mucosal surfaces of the gastrointestinal and genital tracts. Yet, C. albicans can under certain circumstances undergo a shift from commensalism to pathogenicity. This transition is governed by fungal factors such as morphological transitions, environmental cues for instance relationships with gut microbiota and the host immune system. C. albicans utilizes distinct sets of regulatory programs to colonize or infect its host and to evade the host defense systems. Moreover, an orchestrated iron acquisition mechanism operates to adapt to specific niches with variable iron availability. Studies on regulatory networks and morphogenesis of these two distinct modes of C. albicans growth, suggest that both yeast and hyphal forms exist in both growth patterns and the regulatory circuits are inter-connected. Here, we summarize current knowledge about C. albicans commensal-to-pathogen shift, its regulatory elements and their contribution to human disease.

Wor1-regulated ferroxidases contribute to pigment formation in opaque cells of Candida albicans

Candida albicans is a harmless commensal resident in the human gut and a prevalent opportunistic pathogen. A key part of its commensalism and pathogenesis is its ability to switch between different morphological forms, including white‐to‐opaque switching. The Wor1 protein was previously identified as a master regulator of white‐to‐opaque switching in mating type locus (MTL) homozygous cells. The mechanisms by which the dark color of the opaque colonies is controlled and the pimpled surface of opaque cells is formed remain unknown. Candida albicans produces melanin pigment in vitro and during infection. However, the molecular mechanism underlying the regulation of melanin production is unclear. In this study, we demonstrated that ferroxidases (Fets) function as pigment multicopper oxidases and regulate the production of dark‐pigmented melanin in opaque cells. The FET genes presented distinct regulation patterns in response to different extracellular stimuli. In YPD (1% yeast extract, 2% peptone and 2% dextrose)‐rich medium, four of the five FET genes were up‐regulated by Wor1, especially at the human body temperature of 37 °C. In minimal medium with low ammonium concentrations, all five FET genes were up‐regulated by Wor1. However, at high ammonium concentrations, some FET genes were down‐regulated by Wor1. Wor1‐up‐regulated Fets contributed to dark pigment formation in opaque colonies, but not to the elongated shape of these opaque cells. Increased melanin externalization was associated with the pimpled surface of the opaque cells. Melanized C. albicans cells were more resistant to fungal clearance. Deletion of the five FET genes completely blocked melanin production in opaque cells and resulted in the generation of white elongated ‘opaque’ cells. In addition, the up‐regulated Fets are important for defense against oxidant attacks. The functional diversity of Fets may reflect the multiple strategies of C. albicans to rapidly adapt to diverse host niches.

Comparative genomics of white and opaque cell states supports an epigenetic mechanism of phenotypic switching in Candida albicans

Several Candida species can undergo a heritable and reversible transition from a 'white' state to a mating proficient 'opaque' state. This ability relies on highly interconnected transcriptional networks that control cell-type-specific gene expression programs over multiple generations. Candida albicans, the most prominent pathogenic Candida species, provides a well-studied paradigm for the white-opaque transition. In this species, a network of at least eight transcriptional regulators controls the balance between white and opaque states that have distinct morphologies, transcriptional profiles, and physiological properties. Given the reversible nature and the high frequency of white-opaque transitions, it is widely assumed that this switch is governed by epigenetic mechanisms that occur independently of any changes in DNA sequence. However, a direct genomic comparison between white and opaque cells has yet to be performed. Here, we present a whole-genome comparative analysis of C. albicans white and opaque cells. This analysis revealed rare genetic changes between cell states, none of which are linked to white-opaque switching. This result is consistent with epigenetic mechanisms controlling cell state differentiation in C. albicans and provides direct evidence against a role for genetic variation in mediating the switch.

A Sge1 homolog in Cytospora chrysosperma governs conidiation, virulence and the expression of putative effectors

SIX Gene Expression 1 (Sge1) is an important and well-recognized fungal-specific transcription regulator from the Gti1/Pac2 family that exhibits a conserved function in the vegetative growth, regulating the expression of effector genes and pathogenicity in plant pathogenic fungi. However, its functions in Cytospora chrysosperma, a notorious phytopathogenic fungus in forestry, remain poorly understood. Here, we characterized a Sge1 orthologue, CcSge1, in C. chrysosperma and deleted its Gti1/Pac2 domain for functional analysis. The CcSge1 deletion mutants showed obvious defects in hyphal growth, conidial production and response to hydrogen peroxide. Correspondingly, significantly lower expression of conidiation related genes were found in deletion mutants compared to that of the wild type. Importantly, the CcSge1 deletion mutants totally lost their pathogenicity to the host. Further analysis demonstrated that CcSge1 was responsible for the expression of putative effector genes and the transcription of CcSge1 was under tight control by pathogenicity-related MAP Kinase 1 (CcPmk1). What's more, one of the putative effector gene CCG_07874 was positively regulated by both CcSge1 and CcPmk1. Taken together, these data indicate that CcSge1is indispensable for hyphal radial growth, conidiation, the expression of effector genes and fungal virulence.

Transcriptional Circuits Regulating Developmental Processes in Candida albicans

Candida albicans is a commensal member of the human microbiota that colonizes multiple niches in the body including the skin, oral cavity, and gastrointestinal and genitourinary tracts of healthy individuals. It is also the most common human fungal pathogen isolated from patients in clinical settings. C. albicans can cause a number of superficial and invasive infections, especially in immunocompromised individuals. The ability of C. albicans to succeed as both a commensal and a pathogen, and to thrive in a wide range of environmental niches within the host, requires sophisticated transcriptional regulatory programs that can integrate and respond to host specific environmental signals. Identifying and characterizing the transcriptional regulatory networks that control important developmental processes in C. albicans will shed new light on the strategies used by C. albicans to colonize and infect its host. Here, we discuss the transcriptional regulatory circuits controlling three major developmental processes in C. albicans: biofilm formation, the white-opaque phenotypic switch, and the commensal-pathogen transition. Each of these three circuits are tightly knit and, through our analyses, we show that they are integrated together by extensive regulatory crosstalk between the core regulators that comprise each circuit.

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.

Candida albicans Double Mutants Lacking both EFG1 and WOR1 Can Still Switch to Opaque

Candida albicans, a pervasive opportunistic pathogen, undergoes a unique phenotypic transition from a "white" phenotype to an "opaque" phenotype. The switch to opaque impacts gene expression, cell morphology, wall structure, metabolism, biofilm formation, mating, virulence, and colonization of the skin and gastrointestinal (GI) tract. Although the regulation of switching is complex, a paradigm has evolved from a number of studies, in which, in its simplest form, the transcription factors Efg1 and Wor1 play central roles. When EFG1 is upregulated under physiological conditions, it represses WOR1, an activator of white-to-opaque switching, and the cell expresses the white phenotype; when EFG1 is downregulated, WOR1 is derepressed and activates expression of the opaque phenotype. Deletion of either EFG1 or WOR1 supports this yin-yang model of regulation. Here, we demonstrate that this simple model is insufficient, since strains in which WOR1 and EFG1 are simultaneously deleted can still be induced to switch en masse from white to opaque. Opaque cells of double mutants (efg1^-/- wor1^-/- ) are enlarged and elongate, form an enlarged vacuole, upregulate mCherry under the control of an opaque-specific promoter, form opaque cell wall pimples, express the opaque phenotype in lower GI colonization, and, if MTL homozygous, form conjugation tubes in response to pheromone and mate. These results can be explained if the basic and simplified model is expanded to include a WOR1-independent alternative opaque pathway repressed by EFG1 IMPORTANCE The switch from white to opaque in Candida albicans was discovered 33 years ago, but it is still unclear how it is regulated. A regulatory paradigm has emerged in which two transacting factors, Efg1 and Wor1, play central roles, Efg1 as a repressor of WOR1, which encodes an activator of the transition to the opaque phenotype. However, we show here that if both EFG1 and WOR1 are deleted simultaneously, bona fide opaque cells can still be induced en masse These results are not compatible with the simple paradigm, suggesting that an alternative opaque pathway (AOP) exists, which can activate expression of opaque and, like WOR1, is repressed by EFG1.

Coordinate genomic association of transcription factors controlled by an imported quorum sensing peptide in Cryptococcus neoformans

Qsp1 is a secreted quorum sensing peptide required for virulence of the fungal meningitis pathogen Cryptococcus neoformans. Qsp1 functions to control cell wall integrity in vegetatively growing cells and also functions in mating. Rather than acting on a cell surface receptor, Qsp1 is imported to act intracellularly via the predicted oligopeptide transporter Opt1. Here, we identify a transcription factor network as a target of Qsp1. Using whole-genome chromatin immunoprecipitation, we find Qsp1 controls the genomic associations of three transcription factors to genes whose outputs are regulated by Qsp1. One of these transcription factors, Cqs2, is also required for the action of Qsp1 during mating, indicating that it might be a shared proximal target of Qsp1. Consistent with this hypothesis, deletion of CQS2 impacts the binding of other network transcription factors specifically to Qsp1-regulated genes. These genetic and genomic studies illuminate mechanisms by which an imported peptide acts to modulate eukaryotic gene expression.

N-Acetylglucosamine (GlcNAc) Sensing, Utilization, and Functions in Candida albicans

The sensing and efficient utilization of environmental nutrients are critical for the survival of microorganisms in environments where nutrients are limited, such as within mammalian hosts. Candida albicans is a common member of the human microbiota as well as an opportunistic fungal pathogen. The amide derivative sugar N-acetlyglucosamine (GlcNAc) is an important signaling molecule for C. albicans that could be a major nutrient source for this fungus in host settings. In this article, we review progress made over the past two decades on GlcNAc utilization, sensing, and functions in C. albicans and its related fungal species. GlcNAc sensing and catabolic pathways have been intensively studied in C. albicans. The C. albicans protein Ngt1 represents the first identified GlcNAc-specific transporter in eukaryotic organisms. In C. albicans, GlcNAc not only induces morphological transitions including the yeast to hyphal transition and the white to opaque phenotypic switch, but it also promotes fungal cell death. The Ras-cAMP/PKA signaling pathway plays critical roles in regulating these processes. Given the importance of GlcNAc sensing and utilization in C. albicans, targeting GlcNAc associated pathways and key pathway components could be promising in the development of new antifungal strategies.

The PHO pathway regulates white-opaque switching and sexual mating in the human fungal pathogen Candida albicans

The opportunistic fungal pathogen Candida albicans is able to switch among several morphological phenotypes in response to environmental changes. White-opaque transition is a typical phenotypic switching system involved in the regulation of pathogenesis and sexual reproduction in C. albicans. Under regular laboratory culture conditions, to undergo white-to-opaque switching, cells must first undergo homozygosis at the mating-type locus (MTLa/a or α/α) since the a1/α2 heterodimer represses the expression of the Wor1 master regulator of switching in MTLa/α heterozygous strains. In this study, we report the roles of the PHO pathway of phosphate metabolism in the regulation of white-opaque switching and sexual mating in C. albicans. We find that deletion of the PHO pathway genes PHO81, PHO80, PHO2, and PHO4 induces the opaque phenotype in MTLa/α heterozygous cells. Low concentrations of external phosphate are conducive for the opaque phenotype in both MTL homozygous and heterozygous strains. Moreover, phosphate starvation can also increase the mating efficiency in C. albicans. Consistently, the pho80/pho80 mutant mimics an artificial phosphate starvation state and mates efficiently at both lower and higher phosphate concentrations. Our study establishes a link between the PHO pathway and white-opaque epigenetic switching in C. albicans.

A Set of Diverse Genes Influence the Frequency of White-Opaque Switching in Candida albicans

The fungal species Candida albicans is both a member of the human microbiome and a fungal pathogen. C. albicans undergoes several different morphological transitions, including one called white-opaque switching. Here, cells reversibly switch between two states, “white” and “opaque,” and each state is heritable through many cell generations. Each cell type has a distinct cellular and colony morphology and they differ in many other properties including mating, nutritional specialization, and interactions with the innate immune system. Previous genetic screens to gain insight into white-opaque switching have focused on certain classes of genes (for example transcriptional regulators or chromatin modifying enzymes). In this paper, we examined 172 deletion mutants covering a broad range of cell functions. We identified 28 deletion mutants with at least a fivefold effect on switching frequencies; these cover a wide variety of functions ranging from membrane sensors to kinases to proteins of unknown function. In agreement with previous reports, we found that components of the pheromone signaling cascade affect white-to-opaque switching; however, our results suggest that the major effect of Cek1 on white-opaque switching occurs through the cell wall damage response pathway. Most of the genes we identified have not been previously implicated in white-opaque switching and serve as entry points to understand new aspects of this morphological transition.

Epigenetic cell fate in Candida albicans is controlled by transcription factor condensates acting at super-enhancer-like elements

Cell identity in eukaryotes is controlled by transcriptional regulatory networks (TRNs) that define cell type-specific gene expression. In the opportunistic fungal pathogen Candida albicans, TRNs regulate epigenetic switching between two alternative cell states, ‘white’ and ‘opaque’, that exhibit distinct host interactions. Here, we reveal that the transcription factors (TFs) regulating cell identity contain prion-like domains (PrLDs) that enable liquid-liquid demixing and the formation of phase-separated condensates. Multiple white-opaque TFs can co-assemble into complex condensates as observed on single DNA molecules. Moreover, heterotypic interactions between PrLDs supports the assembly of multifactorial condensates at a synthetic locus within live eukaryotic cells. Mutation of the Wor1 PrLD revealed that substitution of acidic residues abolished its ability to phase separate and to co-recruit other TFs in live cells, as well as its function in C. albicans cell fate determination. Together, these studies reveal that PrLDs support the assembly of TF complexes that control fungal cell identity and highlight parallels with the ‘super-enhancers’ that regulate mammalian cell fate.

Thermodynamic Analysis of Bistability in Rayleigh-Bénard Convection

Bistability is often encountered in association with dissipative systems far from equilibrium, such as biological, physical, and chemical phenomena. There have been various attempts to theoretically analyze the bistabilities of dissipative systems. However, there is no universal theoretical approach to determine the development of a bistable system far from equilibrium. This study shows that thermodynamic analysis based on entropy production can be used to predict the transition point in the bistable region during Rayleigh–Bénard convection using the experimental relationship between the thermodynamic flux and driving force. The bistable region is characterized by two distinct features: the flux of the second state is higher than that of the first state, and the entropy production of the second state is lower than that of the first state. This thermodynamic interpretation provides new insights that can be used to predict bistable behaviors in various dissipative systems.

Label free proteomics and systematic analysis of secretome reveals effector candidates regulated by SGE1 and FTF1 in the plant pathogen Fusarium oxysporum f. sp. cubense tropical race 4

BACKGROUND

Phytopathogens secreted effectors during host colonization to suppress or trigger plant immunity. Identification of new effectors is one of the research focuses in recent years. There is only a limited knowledge about effectors of Fusarium oxysporum f. sp. Cubense tropical race 4 (Foc TR4), the causal agent of wilt disease in Cavendish banana.

RESULTS

Two transcription factors, SGE1 and FTF1, were constitutively over-expressed in Foc TR4 to partially mimic the in-planta state. Secreted proteins with high purity were prepared through a two-round extraction method. Then the secretome were analyzed via label free proteomics method. A total of 919 non-redundant proteins were detected, of which 74 proteins were predicted to be effector candidates. Among these candidates, 29 were up-regulated and 13 down-regulated in the strain over-expressing SGE1 and FTF1, 8 were up-regulated and 4 down-regulated in either SGE1 or FTF1 over expression strain.

CONCLUSIONS

Through label free proteomics analysis, a series of effector candidates were identified in secretome of Foc TR4. Our work put a foundation for functional research of these effectors.

Genetic regulation of the development of mating projections in Candida albicans

Candida albicans is a major human fungal pathogen, capable of switching among a range of morphological types, such as the yeast form, including white and opaque cell types and the GUT (gastrointestinally induced transition) cell type, the filamentous form, including hyphal and pseudohyphal cell types, and chlamydospores. This ability is associated with its commensal and pathogenic life styles. In response to pheromone, C. albicans cells are able to form long mating projections resembling filaments. This filamentous morphology is required for efficient sexual mating. In the current study, we report the genetic regulatory mechanisms controlling the development of mating projections in C. albicans. Ectopic expression of MTLα1 in “a” cells induces the secretion of α-pheromone and promotes the development of mating projections. Using this inducible system, we reveal that members of the pheromone-sensing pathway (including the pheromone receptor), the Ste11-Hst7-Cek1/2 mediated MAPK signalling cascade, and the RAM pathway are essential for the development of mating projections. However, the cAMP/PKA signalling pathway and a number of key regulators of filamentous growth such as Hgc1, Efg1, Flo8, Tec1, Ume6, and Rfg1 are not required for mating projection formation. Therefore, despite the phenotypic similarities between filaments and mating projections in C. albicans, distinct mechanisms are involved in the regulation of these two morphologies.

On Commensalism of Candida

Candida species are both opportunistic fungal pathogens and common members of the human mycobiome. Over the years, the main focus of the fungal field has been on understanding the pathogenic potential and disease manifestation of these organisms. Therefore, understanding of their commensal lifestyle, interactions with host epithelial barriers, and initial transition into pathogenesis is less developed. In this review, we will describe the current knowledge on the commensal lifestyle of these fungi, how they are able to adhere to and colonize host epithelial surfaces, compete with other members of the microbiota, and interact with the host immune response, as well as their transition into opportunistic pathogens by invading the gastrointestinal epithelium.

Unraveling How Candida albicans Forms Sexual Biofilms

Biofilms, structured and densely packed communities of microbial cells attached to surfaces, are considered to be the natural growth state for a vast majority of microorganisms. The ability to form biofilms is an important virulence factor for most pathogens, including the opportunistic human fungal pathogen Candida albicans. C. albicans is one of the most prevalent fungal species of the human microbiota that asymptomatically colonizes healthy individuals. However, C. albicans can also cause severe and life-threatening infections when host conditions permit (e.g., through alterations in the host immune system, pH, and resident microbiota). Like many other pathogens, this ability to cause infections depends, in part, on the ability to form biofilms. Once formed, C. albicans biofilms are often resistant to antifungal agents and the host immune response, and can act as reservoirs to maintain persistent infections as well as to seed new infections in a host. The majority of C. albicans clinical isolates are heterozygous (a/α) at the mating type-like (MTL) locus, which defines Candida mating types, and are capable of forming robust biofilms when cultured in vitro. These “conventional” biofilms, formed by MTL-heterozygous (a/α) cells, have been the primary focus of C. albicans biofilm research to date. Recent work in the field, however, has uncovered novel mechanisms through which biofilms are generated by C. albicans cells that are homozygous or hemizygous (a/a, a/Δ, α/α, or α/Δ) at the MTL locus. In these studies, the addition of pheromones of the opposite mating type can induce the formation of specialized “sexual” biofilms, either through the addition of synthetic peptide pheromones to the culture, or in response to co-culturing of cells of the opposite mating types. Although sexual biofilms are generally less robust than conventional biofilms, they could serve as a protective niche to support genetic exchange between mating-competent cells, and thus may represent an adaptive mechanism to increase population diversity in dynamic environments. Although conventional and sexual biofilms appear functionally distinct, both types of biofilms are structurally similar, containing yeast, pseudohyphal, and hyphal cells surrounded by an extracellular matrix. Despite their structural similarities, conventional and sexual biofilms appear to be governed by distinct transcriptional networks and signaling pathways, suggesting that they may be adapted for, and responsive to, distinct environmental conditions. Here we review sexual biofilms and compare and contrast them to conventional biofilms of C. albicans.

N-Acetylglucosamine Regulates Morphogenesis and Virulence Pathways in Fungi

N-acetylglucosamine (GlcNAc) is being increasingly recognized for its ability to stimulate cell signaling. This amino sugar is best known as a component of cell wall peptidoglycan in bacteria, cell wall chitin in fungi and parasites, exoskeletons of arthropods, and the extracellular matrix of animal cells. In addition to these structural roles, GlcNAc is now known to stimulate morphological and stress responses in a wide range of organisms. In fungi, the model organisms Saccharomyces cerevisiae and Schizosaccharomyces pombe lack the ability to respond to GlcNAc or catabolize it, so studies with the human pathogen Candida albicans have been providing new insights into the ability of GlcNAc to stimulate cellular responses. GlcNAc potently induces C. albicans to transition from budding to filamentous hyphal growth. It also promotes an epigenetic switch from White to Opaque cells, which differ in morphology, metabolism, and virulence properties. These studies have led to new discoveries, such as the identification of the first eukaryotic GlcNAc transporter. Other results have shown that GlcNAc can induce signaling in C. albicans in two ways. One is to act as a signaling molecule independent of its catabolism, and the other is that its catabolism can cause the alkalinization of the extracellular environment, which provides an additional stimulus to form hyphae. GlcNAc also induces the expression of virulence genes in the C. albicans, indicating it can influence pathogenesis. Therefore, this review will describe the recent advances in understanding the role of GlcNAc signaling pathways in regulating C. albicans morphogenesis and virulence.