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.

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.

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.

Streptococcus mutans suppresses filamentous growth of Candida albicans through secreting mutanocyclin, an unacylated tetramic acid.

Fungi and bacteria often co-exist and physically or chemically interact with each other in their natural niches. This inter-kingdom species interaction is exemplified by the gram-positive bacterial pathogen Streptococcus mutans and opportunistic fungal pathogen Candida albicans, which co-exist in the human mouth. It has been demonstrated that the dynamic interaction between these two species plays a critical role in their virulence and biofilm development. In this study, we discovered that S. mutans represses filamentous development and virulence in C. albicans through secreting a secondary metabolite, mutanocyclin (a tetramic acid). Mutanocyclin functions by regulating the PKA catabolic subunit Tpk2 and its preferential binding target Sfl1. Inactivation of Tpk2 in C. albicans results in an increased sensitivity to mutanocyclin, whereas overexpression of Tpk2 leads to an increased resistance. Dysfunction of SFL1 and its downstream target genes overrides the hyphal growth defect caused by mutanocyclin. Further investigation demonstrates that three glycosylphosphatidylinositol (GPI)-anchored proteins (Spr1, Hyr4, and Iff8), associated with cell wall biogenesis and remodeling, and a set of filamentous regulators also contribute to the mutanocyclin response. We propose that both transcriptional regulation and cell wall composition contribute to mutanocyclin-mediated filamentous inhibition. This repressive effect of mutanocyclin could function as a natural regulator of filamentous development in C. albicans.

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.

The Als3 Cell Wall Adhesin Plays a Critical Role in Human Serum Amyloid A1-Induced Cell Death and Aggregation in Candida albicans.

Antimicrobial peptides and proteins play critical roles in the host defense against invading pathogens. We recently discovered that recombinantly expressed human and mouse serum amyloid A1 (rhSAA1 and rmSAA1, respectively) proteins have potent antifungal activities against the major human fungal pathogen Candida albicans At high concentrations, rhSAA1 disrupts C. albicans membrane integrity and induces rapid fungal cell death. In the present study, we find that rhSAA1 promotes cell aggregation and targets the C. albicans cell wall adhesin Als3. Inactivation of ALS3 in C. albicans leads to a striking decrease in cell aggregation and cell death upon rhSAA1 treatment, suggesting that Als3 plays a critical role in SAA1 sensing. We further demonstrate that deletion of the transcriptional regulators controlling the expression of ALS3, such as AHR1, BCR1, and EFG1, in C. albicans results in similar effects to that of the als3/als3 mutant upon rhSAA1 treatment. Global gene expression profiling indicates that rhSAA1 has a discernible impact on the expression of cell wall- and metabolism-related genes, suggesting that rhSAA1 treatment could lead to a nutrient starvation effect on C. albicans cells.

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.

Environment-induced same-sex mating in the yeast Candida albicans through the Hsf1-Hsp90 pathway.

While sexual reproduction is pervasive in eukaryotic cells, the strategies employed by fungal species to achieve and complete sexual cycles is highly diverse and complex. Many fungi, including Saccharomyces cerevisiae and Schizosaccharomyces pombe, are homothallic (able to mate with their own mitotic descendants) because of homothallic switching (HO) endonuclease-mediated mating-type switching. Under laboratory conditions, the human fungal pathogen Candida albicans can undergo both heterothallic and homothallic (opposite- and same-sex) mating. However, both mating modes require the presence of cells with two opposite mating types (MTLa/a and α/α) in close proximity. Given the predominant clonal feature of this yeast in the human host, both opposite- and same-sex mating would be rare in nature. In this study, we report that glucose starvation and oxidative stress, common environmental stresses encountered by the pathogen, induce the development of mating projections and efficiently permit same-sex mating in C. albicans with an "a" mating type (MTLa/a). This induction bypasses the requirement for the presence of cells with an opposite mating type and allows efficient sexual mating between cells derived from a single progenitor. Glucose starvation causes an increase in intracellular oxidative species, overwhelming the Heat Shock transcription Factor 1 (Hsf1)- and Heat shock protein (Hsp)90-mediated stress-response pathway. We further demonstrate that Candida TransActivating protein 4 (Cta4) and Cell Wall Transcription factor 1 (Cwt1), downstream effectors of the Hsf1-Hsp90 pathway, regulate same-sex mating in C. albicans through the transcriptional control of the master regulator of a-type mating, MTLa2, and the pheromone precursor-encoding gene Mating α factor precursor (MFα). Our results suggest that mating could occur much more frequently in nature than was originally appreciated and that same-sex mating could be an important mode of sexual reproduction in C. albicans.

Integration of the tricarboxylic acid (TCA) cycle with cAMP signaling and Sfl2 pathways in the regulation of CO2 sensing and hyphal development in Candida albicans.

Morphological transitions and metabolic regulation are critical for the human fungal pathogen Candida albicans to adapt to the changing host environment. In this study, we generated a library of central metabolic pathway mutants in the tricarboxylic acid (TCA) cycle, and investigated the functional consequences of these gene deletions on C. albicans biology. Inactivation of the TCA cycle impairs the ability of C. albicans to utilize non-fermentable carbon sources and dramatically attenuates cell growth rates under several culture conditions. By integrating the Ras1-cAMP signaling pathway and the heat shock factor-type transcription regulator Sfl2, we found that the TCA cycle plays fundamental roles in the regulation of CO2 sensing and hyphal development. The TCA cycle and cAMP signaling pathways coordinately regulate hyphal growth through the molecular linkers ATP and CO2. Inactivation of the TCA cycle leads to lowered intracellular ATP and cAMP levels and thus affects the activation of the Ras1-regulated cAMP signaling pathway. In turn, the Ras1-cAMP signaling pathway controls the TCA cycle through both Efg1- and Sfl2-mediated transcriptional regulation in response to elevated CO2 levels. The protein kinase A (PKA) catalytic subunit Tpk1, but not Tpk2, may play a major role in this regulation. Sfl2 specifically binds to several TCA cycle and hypha-associated genes under high CO2 conditions. Global transcriptional profiling experiments indicate that Sfl2 is indeed required for the gene expression changes occurring in response to these elevated CO2 levels. Our study reveals the regulatory role of the TCA cycle in CO2 sensing and hyphal development and establishes a novel link between the TCA cycle and Ras1-cAMP signaling pathways.

Lactic acid bacteria differentially regulate filamentation in two heritable cell types of the human fungal pathogen Candida albicans.

Microorganisms rarely exist as single species in natural environments. The opportunistic fungal pathogen Candida albicans and lactic acid bacteria (LAB) are common members of the microbiota of several human niches such as the mouth, gut and vagina. Lactic acid bacteria are known to suppress filamentation, a key virulence feature of C. albicans, through the production of lactic acid and other metabolites. Here we report that C. albicans cells switch between two heritable cell types, white and opaque, to undergo filamentation to adapt to diversified environments. We show that acidic pH conditions caused by LAB and low temperatures support opaque cell filamentation, while neutral pH conditions and high temperatures promote white cell filamentation. The cAMP signalling pathway and the Rfg1 transcription factor play major roles in regulating the responses to these conditions. This cell type-specific response of C. albicans to different environmental conditions reflects its elaborate regulatory control of phenotypic plasticity.

The gray phenotype and tristable phenotypic transitions in the human fungal pathogen Candida tropicalis.

Phenotypic plasticity, the ability to switch between different morphological types, plays critical roles in environmental adaptation, leading to infections, and allowing for sexual reproduction in pathogenic Candida species. Candida tropicalis, which is both an emerging human fungal pathogen and an environmental fungus, can switch between two heritable cell types termed white and opaque. In this study, we report the discovery of a novel phenotype in C. tropicalis, named the gray phenotype. Similar to Candida albicans and Candida dubliniensis, white, gray, and opaque cell types of C. tropicalis also form a tristable switching system, where gray cells are relatively small and elongated. In C. tropicalis, gray cells exhibit intermediate levels of mating competency and virulence in a mouse systemic infection model compared to the white and opaque cell types, express a set of cell type-enriched genes, and exhibit both common and species-specific biological features. The key regulators of white-opaque transitions, Wor1 and Efg1, are not required for the gray phenotype. A comparative study of the gray phenotypes in C. tropicalis, C. albicans, and C. dubliniensis provides clues to explain the virulence properties and niche preferences of C. tropicalis.

Role of the N-acetylglucosamine kinase (Hxk1) in the regulation of white-gray-opaque tristable phenotypic transitions in C. albicans.

The amino sugar N-acetylglucosamine (GlcNAc) is a host-related environmental cue and a potent inducer of morphological transitions in the human fungal pathogen Candida albicans. It has been well established that GlcNAc promotes white-to-opaque switching and yeast-to-hyphal growth transition primarily through the Ras-cAMP signaling pathway. As a commensal yeast of humans, C. albicans can efficiently use GlcNAc as the carbon source. In this study, we sought to investigate whether the catabolic pathway of GlcNAc is involved in the regulation of white-gray-opaque tristable transitions in C. albicans. Phenotypic switching assays demonstrated that deletion of the GlcNAc kinase gene, HXK1, induced the gray and opaque phenotypes in a SC5314 background strain, which is heterozygous at the mating type locus (a/α) and is unable to switch to the gray or opaque phenotype under standard culture conditions. Cell type-enriched genes were exclusively expressed in the white, gray, and opaque cells of the hxk1/hxk1 mutant. Mating assays demonstrated that, similar to the counterparts of BJ1097 (a natural white-gray-opaque switchable strain), opaque cells of the hxk1/hxk1 mutant (Δ/α) mated more efficiently than white and gray cells. The transcription factors, Wor1 and Efg1, are required for the development of the opaque and white cell types in the hxk1/hxk1 mutant, respectively. However, deletion of the GlcNAc-specific transporter gene (NGT1), GlcNAc-6-phosphate deacetylase gene (DAC1), and glucosamine-6-phosphate deaminase gene (NAG1) in the same background strain had no obvious effect on white-gray-opaque transitions. Our findings suggest that the GlcNAc kinase, Hxk1, may function as a morphological regulator independent on its catabolic role in C. albicans.

Phenotypic diversity and correlation between white-opaque switching and the CAI microsatellite locus in Candida albicans.

Candida albicans is a commensal fungal pathogen that is often found as part of the human microbial flora. The aim of the present study was to establish a relationship between diverse genotypes and phenotypes of clinical isolates of C. albicans. Totally 231 clinical isolates were collected and used for genotyping and phenotypic switching analysis. Based on the microsatellite locus (CAI) genotyping assay, 65 different genotypes were identified, and some dominant types were found in certain human niches. For example, the genotypes of 30-44 and 30-45 were enriched in vaginal infection samples. C. albicans has a number of morphological forms including the single-celled yeasts, multicellular filaments, white, and opaque cell types. The relationship between the CAI genotype and the ability to undergo phenotypic switching was examined in the clinical isolates. We found that the strains with longer CAA/G repeats in both alleles of the CAI locus were more opaque competent. We also discovered that some MTL heterozygous (a/alpha) isolates could undergo white-opaque switching when grown on regular culture medium (containing glucose as the sole carbon source). Our study establishes a link between phenotypic switching and genotypes of the CAI microsatellite locus in clinical isolates of C. albicans.

Regulation of filamentation in the human fungal pathogen Candida tropicalis.

The yeast-filament transition is essential for the virulence of a variety of fungi that are pathogenic to humans. N-acetylglucosamine (GlcNAc) is a potent inducer of filamentation in Candida albicans and thermally dimorphic fungi such as Histoplasma capsulatum and Blastomyces dermatitidis. However, GlcNAc suppresses rather than promotes filamentation in Candida tropicalis, a fungal species that is closely related to C. albicans. Despite the intensive study in C. albicans, the regulatory mechanism of filamentation is poorly understood. In this study, we demonstrate that the cAMP signaling pathway plays a central role in the regulation of filamentation in C. tropicalis. By screening an overexpression library of 156 transcription factors, we have identified approximately 40 regulators of filamentous growth. Although most of the regulators (e.g., Tec1, Gat2, Nrg1, Sfl1, Sfl2 and Ash1) demonstrate a conserved role in the regulation of filamentation, similar to their homologues in C. albicans or Saccharomyces cerevisiae, a number of transcription factors (e.g., Wor1, Bcr1, Stp4, Efh1, Csr1 and Zcf17) play a specific role in C. tropicalis. Our findings indicate that multiple interconnected signaling pathways are involved in the regulation of filamentation in C. tropicalis. These mechanisms have conserved and divergent features among different Candida species.

Discovery of the gray phenotype and white-gray-opaque tristable phenotypic transitions in Candida dubliniensis.

Candida dubliniensis is closely related to Candida albicans, a major causative agent of candidiasis, and is primarily associated with oral colonization and infection in human immunodeficiency virus (HIV)-positive patients. Despite the high similarity of genomic and phenotypic features between the 2 species, C. dubliniensis is much less virulent and less prevalent than C. albicans. The ability to change morphological phenotypes is a striking feature of Candida species and is linked to virulence. In this study, we report a novel phenotype, the gray phenotype, in C. dubliniensis. Together with the previously reported white and opaque cell types, the gray phenotype forms a tristable phenotypic switching system in C. dubliniensis that is similar to the white-gray-opaque tristable switching system in C. albicans. Gray cells of C. dubliniensis are similar to their counterparts in C. albicans in terms of several biological aspects including cellular morphology, mating competence, and genetic regulatory mechanisms. However, the gray phenotypes of the 2 species have some distinguishing features. For example, the secreted aspartyl protease (Sap) activity is induced by bovine serum albumin (BSA) in gray cells of C. albicans, but not in gray cells of C. dubliniensis. Taken together, our results demonstrate that the biological features and regulatory mechanisms of white-gray-opaque tristable transitions are largely conserved in the 2 pathogenic Candida species.

N-Acetylglucosamine-Induced Cell Death in Candida albicans and Its Implications for Adaptive Mechanisms of Nutrient Sensing in Yeasts.

UNLABELLED: Single-celled organisms have different strategies to sense and utilize nutrients in their ever-changing environments. The opportunistic fungal pathogen Candida albicans is a common member of the human microbiota, especially that of the gastrointestinal (GI) tract. An important question concerns how C. albicans gained a competitive advantage over other microbes to become a successful commensal and opportunistic pathogen. Here, we report that C. albicans uses N-acetylglucosamine (GlcNAc), an abundant carbon source present in the GI tract, as a signal for nutrient availability. When placed in water, C. albicans cells normally enter the G0 phase and remain viable for weeks. However, they quickly lose viability when cultured in water containing only GlcNAc. We term this phenomenon GlcNAc-induced cell death (GICD). GlcNAc triggers the upregulation of ribosomal biogenesis genes, alterations of mitochondrial metabolism, and the accumulation of reactive oxygen species (ROS), followed by rapid cell death via both apoptotic and necrotic mechanisms. Multiple pathways, including the conserved cyclic AMP (cAMP) signaling and GlcNAc catabolic pathways, are involved in GICD. GlcNAc acts as a signaling molecule to regulate multiple cellular programs in a coordinated manner and therefore maximizes the efficiency of nutrient use. This adaptive behavior allows C. albicans' more efficient colonization of the gut. IMPORTANCE: The ability to rapidly and appropriately respond to nutrients in the environment is crucial to free-living microorganisms. To maximize the use of available nutrients, microorganisms often use a limiting nutritional component as a signal to coordinate multiple biological processes. The human fungal pathogen Candida albicans uses N-acetylglucosamine (GlcNAc) as a signal for the availability of external nutrient resources. GlcNAc induces rapid cell death in C. albicans due to the constitutive activation of oxidative metabolism and accumulation of reactive oxygen species (ROS), and multiple pathways are involved in its regulation. This study sheds light on the mechanisms of niche specialization of pathogenic fungi and raises the possibility that this cell death pathway could be an unexplored therapeutic target.

pH Regulates White-Opaque Switching and Sexual Mating in Candida albicans.

As a successful commensal and pathogen of humans, Candida albicans encounters a wide range of environmental conditions. Among them, ambient pH, which changes frequently and affects many biological processes in this species, is an important factor, and the ability to adapt to pH changes is tightly linked with pathogenesis and morphogenesis. In this study, we report that pH has a profound effect on white-opaque switching and sexual mating in C. albicans. Acidic pH promotes white-to-opaque switching under certain culture conditions but represses sexual mating. The Rim101-mediated pH-sensing pathway is involved in the control of pH-regulated white-opaque switching and the mating response. Phr2 and Rim101 could play a major role in acidic pH-induced opaque cell formation. Despite the fact that the cyclic AMP (cAMP) signaling pathway does not play a major role in pH-regulated white-opaque switching and mating, white and opaque cells of the cyr1/cyr1 mutant, which is defective in the production of cAMP, showed distinct growth defects under acidic and alkaline conditions. We further discovered that acidic pH conditions repressed sexual mating due to the failure of activation of the Ste2-mediated α-pheromone response pathway in opaque A: cells. The effects of pH changes on phenotypic switching and sexual mating could involve a balance of host adaptation and sexual reproduction in C. albicans.

The mitochondrial protein Mcu1 plays important roles in carbon source utilization, filamentation, and virulence in Candida albicans.

The fungus Candida albicans is both a pathogen and a commensal in humans. The ability to utilize different carbon sources available in diverse host niches is vital for both commensalism and pathogenicity. N-acetylglucosamine (GlcNAc) is an important signaling molecule as well as a carbon source in C. albicans. Here, we report the discovery of a novel gene MCU1 essential for GlcNAc utilization. Mcu1 is located in mitochondria and associated with multiple energy- and metabolism-related proteins including Por1, Atp1, Pet9, and Mdh1. Consistently, inactivating Por1 impaired GlcNAc utilization as well. Deletion of MCU1 also caused defects in utilizing non-fermentable carbon sources and amino acids. Furthermore, MCU1 is required for filamentation in several inducing conditions and virulence in a mouse systemic infection model. We also deleted TGL99 and GUP1, two genes adjacent to MCU1, and found that the gup1/gup1 mutant exhibited mild defects in the utilization of several carbon sources including GlcNAc, maltose, galactose, amino acids, and ethanol. Our results indicate that MCU1 exists in a cluster of genes involved in the metabolism of carbon sources. Given its importance in metabolism and lack of a homolog in humans, Mcu1 could be a potential target for developing antifungal agents.

White cells facilitate opposite- and same-sex mating of opaque cells in Candida albicans.

Modes of sexual reproduction in eukaryotic organisms are extremely diverse. The human fungal pathogen Candida albicans undergoes a phenotypic switch from the white to the opaque phase in order to become mating-competent. In this study, we report that functionally- and morphologically-differentiated white and opaque cells show a coordinated behavior during mating. Although white cells are mating-incompetent, they can produce sexual pheromones when treated with pheromones of the opposite mating type or by physically interacting with opaque cells of the opposite mating type. In a co-culture system, pheromones released by white cells induce opaque cells to form mating projections, and facilitate both opposite- and same-sex mating of opaque cells. Deletion of genes encoding the pheromone precursor proteins and inactivation of the pheromone response signaling pathway (Ste2-MAPK-Cph1) impair the promoting role of white cells (MTLa) in the sexual mating of opaque cells. White and opaque cells communicate via a paracrine pheromone signaling system, creating an environment conducive to sexual mating. This coordination between the two different cell types may be a trade-off strategy between sexual and asexual lifestyles in C. albicans.

Discovery of a "white-gray-opaque" tristable phenotypic switching system in candida albicans: roles of non-genetic diversity in host adaptation.

Non-genetic phenotypic variations play a critical role in the adaption to environmental changes in microbial organisms. Candida albicans, a major human fungal pathogen, can switch between several morphological phenotypes. This ability is critical for its commensal lifestyle and for its ability to cause infections. Here, we report the discovery of a novel morphological form in C. albicans, referred to as the "gray" phenotype, which forms a tristable phenotypic switching system with the previously reported white and opaque phenotypes. White, gray, and opaque cell types differ in a number of aspects including cellular and colony appearances, mating competency, secreted aspartyl proteinase (Sap) activities, and virulence. Of the three cell types, gray cells exhibit the highest Sap activity and the highest ability to cause cutaneous infections. The three phenotypes form a tristable phenotypic switching system, which is independent of the regulation of the mating type locus (MTL). Gray cells mate over 1,000 times more efficiently than do white cells, but less efficiently than do opaque cells. We further demonstrate that the master regulator of white-opaque switching, Wor1, is essential for opaque cell formation, but is not required for white-gray transitions. The Efg1 regulator is required for maintenance of the white phenotype, but is not required for gray-opaque transitions. Interestingly, the wor1/wor1 efg1/efg1 double mutant is locked in the gray phenotype, suggesting that Wor1 and Efg1 could function coordinately and play a central role in the regulation of gray cell formation. Global transcriptional analysis indicates that white, gray, and opaque cells exhibit distinct gene expression profiles, which partly explain their differences in causing infections, adaptation ability to diverse host niches, metabolic profiles, and stress responses. Therefore, the white-gray-opaque tristable phenotypic switching system in C. albicans may play a significant role in a wide range of biological aspects in this common commensal and pathogenic fungus.