The transcription factor Flo8 mediates CO2 sensing in the human fungal pathogen Candida albicans

Effects of CO2 in fungi

Carbon dioxide supplies carbon for photosynthetic species and is a major product of respiration for all life forms. Inside the human body where CO2 is a by-product of the tricarboxylic acid cycle, its level reaches 5% or higher. In the ambient atmosphere, ∼.04% of the air is CO2. Different organisms can tolerate different CO2 levels to various degrees, and experiencing higher CO2 is toxic and can lead to death. The fungal kingdom shows great variations in response to CO2 that has been documented by different researchers at different time periods. This literature review aims to connect these studies, highlight mechanisms underlying tolerance to high levels of CO2, and emphasize the effects of CO2 on fungal metabolism and morphogenesis.

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.

DYRK-family kinases regulate Candida albicans morphogenesis and virulence through the Ras1/PKA pathway

IMPORTANCE

Candida albicans is an opportunistic human fungal pathogen that frequently causes life-threatening infections in immunocompromised individuals. To cause disease, the fungus employs several virulence traits, including its ability to transition between yeast and filamentous states. Previous work identified a role for the kinase Yak1 in regulating C. albicans filamentation. Here, we demonstrate that Yak1 regulates morphogenesis through the canonical cAMP/PKA pathway and that this regulation is environmentally contingent, as host-relevant concentrations of CO2 bypass the requirement of Yak1 for C. albicans morphogenesis. We show a related kinase, Pom1, is important for filamentation in the absence of Yak1 under these host-relevant conditions, as deletion of both genes blocked filamentous growth under all conditions tested. Finally, we demonstrate that Yak1 is required for filamentation in a mouse model of C. albicans dermatitis using genetic and pharmacological approaches. Overall, our results expand our understanding of how Yak1 regulates an important virulence trait in 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.

Sub-lethal concentrations of chlorhexidine inhibit Candida albicans growth by disrupting ROS and metal ion homeostasis

Candida albicans is a normal resident of the human oral cavity. It is also the most common fungal pathogen, causing various oral diseases, particularly in immunocompromised individuals. Chlorhexidine digluconate (CHG) is a broad-spectrum antimicrobial agent widely used in dental practice and has been recommended to treat oral candidiasis. However, its action mechanism against the fungal pathogen C. albicans remains poorly understood. The aim of the present study was to investigate the effect of CHG at sub-lethal concentrations against C. albicans. CHG inhibited the growth of C. albicans in a dose- and time-dependent manner. Cells treated with CHG exhibited altered membrane permeability, reduced metabolic activity, and enhanced metal ion and reactive oxygen species (ROS) accumulation. Copper-sensing transcription factor Mac1, iron-sensing transcription factors Sfu1 and Sef2, and copper transporter Ctr1 regulated intracellular metal ion and ROS homeostasis in response to CHG. Deletion of MAC1, SFU1, or SEF2 increased intracellular ROS production and cell susceptibility to CHG. This study revealed a novel mechanism by which CHG induced apoptosis of C. albicans cells through the disruption of metal ion and ROS homeostasis, which may help to identify new targets for fungal infections.

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.

The Candida albicans Cdk8-dependent phosphoproteome reveals repression of hyphal growth through a Flo8-dependent pathway

Ssn3, also known as Cdk8, is a member of the four protein Cdk8 submodule within the multi-subunit Mediator complex involved in the co-regulation of transcription. In Candida albicans, the loss of Ssn3 kinase activity affects multiple phenotypes including cellular morphology, metabolism, nutrient acquisition, immune cell interactions, and drug resistance. In these studies, we generated a strain in which Ssn3 was replaced with a functional variant of Ssn3 that can be rapidly and selectively inhibited by the ATP analog 3-MB-PP1. Consistent with ssn3 null mutant and kinase dead phenotypes, inhibition of Ssn3 kinase activity promoted hypha formation. Furthermore, the increased expression of hypha-specific genes was the strongest transcriptional signal upon inhibition of Ssn3 in transcriptomics analyses. Rapid inactivation of Ssn3 was used for phosphoproteomic studies performed to identify Ssn3 kinase substrates associated with filamentation potential. Both previously validated and novel Ssn3 targets were identified. Protein phosphorylation sites that were reduced specifically upon Ssn3 inhibition included two sites in Flo8 which is a transcription factor known to positively regulate C. albicans morphology. Mutation of the two Flo8 phosphosites (threonine 589 and serine 620) was sufficient to increase Flo8-HA levels and Flo8 dependent transcriptional and morphological changes, suggesting that Ssn3 kinase activity negatively regulates Flo8.Under embedded conditions, when ssn3Δ/Δ and efg1Δ/Δ mutants were hyperfilamentous, FLO8 was essential for hypha formation. Previous work has also shown that loss of Ssn3 activity leads to increased alkalinization of medium with amino acids. Here, we show that the ssn3Δ/Δ medium alkalinization phenotype, which is dependent on STP2, a transcription factor involved in amino acid utilization, also requires FLO8 and EFG1. Together, these data show that Ssn3 activity can modulate Flo8 and its direct and indirect interactions in different ways, and underscores the potential importance of considering Ssn3 function in the control of transcription factor activities.

In Candida albicans, Ssn3 kinase activity co-regulates the transcription of numerous genes involved in hyphal growth, metabolism and nutrient acquisition, immune cell interactions, and drug resistance. Using a strain in which Ssn3 could be rapidly and selectively inhibited, we identified both known and novel Ssn3 targets. We identified two phosphosites in Flo8, a regulator of morphology and virulence, that were shown to negatively regulate Flo8 levels and activity. The data and tools presented here will enable a better understanding of how Ssn3 impacts transcriptional and post-transcriptional regulation in order to coordinate processes during physiological and morphological transitions as well as during steady state growth.

The association of air pollutants (CO2, MTBE) on Candida albicans and Candida glabrata drug resistance

Introduction

Therapeutic methods are very important in the prevalence of opportunistic fungal infections, which are an important cause of human diseases. In this study, air pollution agents that are in direct contact with microorganisms, and the effects of carbon sources using CO2 and MTBE on growth of fungi, and particularly the evaluation of changes in the expression of interfering genes in susceptibility and drug resistance in these fungi, were investigated.

Materials and Methods

Collecting samples and isolating Candida glabrata and Candida albicans with phenotypic methods were accomplished. We then evaluated the minimum inhibitory concentration (MIC) with the M27A4 protocol of CLSI. We adjusted 20 strains of C. albicans and 10 strains of C. glabrata whose sensitivity was evaluated in the MIC test with 5% CO2 and 5mg/ml methyl tert-butyl ether (MTBE) considered as air pollutants, and followed by re-evaluating MIC testing to separate azole-resistant strains. Interfering agents were also considered.

Results

Upregulation of some genes on the two mentioned yeasts had led to drug resistance in them; they were previously sensitive to both drugs. Correspondingly, 41% of C. glabrata samples in sputum showed sensitivity to these drugs. Upregulation of ERG11 (71%) and EPA1 (90%) were observed in resistant strains. Upregulation of genes associated with aspartate proteins and downregulation of SAP3 genes were recognized in C. glabrata in sputum and a 15% downregulation of bronchoalveolar lavage (BAL) isolate and 50% upregulation of SAP1 gene in C. albicans sensitive samples were observed and compared to fluconazole and itraconazole with the oral and joint sources. Remarkably, decreased SAP2 expression in oral sources and a 60% increase in resistant strains in C. albicans were observed. The downregulation of SAP3 expression showed in the joint samples. An increase in HWP1 expression (30%) was noted in isolated and drug-sensitive samples at the sputum and BAL source. CDR1 expression was increased in MTBE-affected species; however, it decreased in the vicinity of CT.

Conclusions

Air pollutants such as CO2 and MTBE eventually caused drug resistance in Candida, which can be one of the causes of drug resistance in candidiasis infections.

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.

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

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

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.

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.

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.

CO2 Signaling through the Ptc2-Ssn3 Axis Governs Sustained Hyphal Development of Candida albicans by Reducing Ume6 Phosphorylation and Degradation

Candida albicans is the most common cause of invasive fungal infections in humans. Its ability to sense and adapt to changing carbon dioxide levels is crucial for its pathogenesis. Carbon dioxide promotes hyphal development. The hypha-specific transcription factor Ume6 is rapidly degraded in air, but is stable under physiological CO₂ and hypoxia to sustain hyphal elongation. Here, we show that Ume6 stability is regulated by two parallel E3 ubiquitin ligases, SCF^Grr1 and Ubr1, in response to CO₂ and O₂, respectively. To uncover the CO₂ signaling pathway that regulates Ume6 stability, we performed genetic screens for mutants unable to respond to CO₂ for sustained filamentation. We find that the type 2C protein phosphatase Ptc2 is specifically required for CO₂-induced stabilization of Ume6 and hyphal elongation. In contrast, the cyclin-dependent kinase Ssn3 is found to be required for Ume6 phosphorylation and degradation in atmospheric CO₂ Furthermore, we find that Ssn3 is dephosphorylated in 5% CO₂ in a Ptc2-dependent manner, whereas deletion of PTC2 has no effect on Ssn3 phosphorylation in air. Our study uncovers the Ptc2-Ssn3 axis as a new CO₂ signaling pathway that controls hyphal elongation by regulating Ume6 stability in C. albicans IMPORTANCE The capacity to sense and adapt to changing carbon dioxide levels is crucial for all organisms. In fungi, CO₂ is a key determinant involved in fundamental biological processes, including growth, morphology, and virulence. In the pathogenic fungus Candida albicans, high CO₂ is directly sensed by adenylyl cyclase to promote hyphal growth. However, little is known about the mechanism by which hyphal development is maintained in response to physiological levels of CO₂ Here we report that a signal transduction system mediated by a phosphatase-kinase pair controls CO₂-responsive Ume6 phosphorylation and stability that in turn dictate hyphal elongation. Our results unravel a new regulatory mechanism of CO₂ signaling in fungi.

Functional divergence of a global regulatory complex governing fungal filamentation

Morphogenetic transitions are prevalent in the fungal kingdom. For a leading human fungal pathogen, Candida albicans, the capacity to transition between yeast and filaments is key for virulence. For the model yeast Saccharomyces cerevisiae, filamentation enables nutrient acquisition. A recent functional genomic screen in S. cerevisiae identified Mfg1 as a regulator of morphogenesis that acts in complex with Flo8 and Mss11 to mediate transcriptional responses crucial for filamentation. In C. albicans, Mfg1 also interacts physically with Flo8 and Mss11 and is critical for filamentation in response to diverse cues, but the mechanisms through which it regulates morphogenesis remained elusive. Here, we explored the consequences of perturbation of Mfg1, Flo8, and Mss11 on C. albicans morphogenesis, and identified functional divergence of complex members. We observed that C. albicans Mss11 was dispensable for filamentation, and that overexpression of FLO8 caused constitutive filamentation even in the absence of Mfg1. Harnessing transcriptional profiling and chromatin immunoprecipitation coupled to microarray analysis, we identified divergence between transcriptional targets of Flo8 and Mfg1 in C. albicans. We also established that Flo8 and Mfg1 cooperatively bind to promoters of key regulators of filamentation, including TEC1, for which overexpression was sufficient to restore filamentation in the absence of Flo8 or Mfg1. To further explore the circuitry through which Mfg1 regulates morphogenesis, we employed a novel strategy to select for mutations that restore filamentation in the absence of Mfg1. Whole genome sequencing of filamentation-competent mutants revealed chromosome 6 amplification as a conserved adaptive mechanism. A key determinant of the chromosome 6 amplification is FLO8, as deletion of one allele blocked morphogenesis, and chromosome 6 was not amplified in evolved lineages for which FLO8 was re-located to a different chromosome. Thus, this work highlights rewiring of key morphogenetic regulators over evolutionary time and aneuploidy as an adaptive mechanism driving fungal morphogenesis.

Fungal infections pose a severe burden to human health worldwide. Candida albicans is a leading cause of systemic fungal infections, with mortality rates approaching 40%. One of the key virulence traits of this fungus is its ability to transition between yeast and filamentous forms in response to diverse host-relevant cues. The model yeast Saccharomyces cerevisiae is also capable of filamentous growth in certain conditions, and previous work has identified a key transcriptional complex required for filamentation in both species. However, here we discover that the circuitry governed by this complex in C. albicans is largely distinct from that in the non-pathogenic S. cerevisiae. We also employ a novel selection strategy to perform experimental evolution, identifying chromosome triplication as a mechanism to restore filamentation in a non-filamentous mutant. This work reveals unique circuitry governing a key virulence trait in a leading fungal pathogen, identifying potential therapeutic targets to combat these life-threatening infections.

Multiple roles and diverse regulation of the Ras/cAMP/protein kinase A pathway in Candida albicans

Candida albicans is a major fungal pathogen of humans, causing both superficial and life-threatening systemic infections in immunocompromised people. The conserved Ras/cAMP/PKA pathway plays a key role in regulating multiple traits important for the virulence of C. albicans such as cell growth, yeast-hyphal transition, white-opaque switching, sexual reproduction and biofilm development. Diverse external signals influence cell physiology by activating this signaling pathway. The key components of the Ras/cAMP/PKA pathway include two Ras GTPases (Ras1 and Ras2), an adenylyl cyclase (Cyr1, also known as Cdc35), two cyclic nucleotide phosphodiesterases (Pde1 and Pde2) and the catalytic (Tpk1 and Tpk2) and regulatory (Bcy1) subunits of PKA kinase. Activation of this pathway dramatically alters the gene expression profile via several transcription factors, leading to the activation of specific biological processes. Here, we review the progress made in the past two decades to elucidate the molecular mechanisms by which the Ras/cAMP/PKA pathway senses diverse environmental cues and controls specific cellular responses and its connection with other signaling pathways in C. albicans.

Morphological changes and induction of antifungal resistance in Aspergillus fumigatus due to different CO2 levels

Background and Purpose

Aspergillosis is one of the most common opportunistic fungal infections in immunocompromised and neutropenic patients. Aspergillus fumigatus (A. fumigatus) is the most common causative agent of this infection. Due to variable CO₂ concentrations that pathogens are exposed to during the infection process and to understand the role of CO₂, we examined the effects of various CO₂ concentrations as one of the environmental factors on morphological changes and induction of antifungal resistance in A. fumigatus.

Materials and Methods

A. fumigatus strains were cultured and incubated under 1%, 3%, 5%, and 12% CO₂ atmospheres, each time for one, two, and four weeks. The control culture was maintained for one week without CO₂ atmosphere. Morphological changes were investigated and antifungal susceptibility test was performed according to the recommendations of the Clinical and Laboratory Standards Institute (CLSI) M38-A2 document. The results of different CO₂ atmospheres were compared with that of the control sample.

Results

We found that 1%, 3%, 5%, and 12% CO₂ atmospheres were associated with morphological colony changes. Macroscopically, the colonies were shallow dark green, smooth, crisp to powdery with reduced growth; microscopic examination revealed the absence of conidiation. The induction of antifungal resistance in the susceptible strains to itraconazole, voriconazole, and amphotericin B increased after exposure to 12% CO₂ atmosphere and four weeks of incubation. The MIC values for itraconazole, voriconazole, and amphotericin B were 16 g/ml, 1 g/ml, and 16 g/ml, respectively. These values for the control group were 0.125 g/ml, 0.125 g/ml, and 2 g/ml, respectively.

Conclusion

Exposure to different CO₂ atmospheres induced morphological changes in A. fumigatus, it seems to increase the MIC values, as well. In parallel, resistance to both itraconazole and voriconazole was also observed.

Genomic and Phenotypic Variation in Morphogenetic Networks of Two Candida albicans Isolates Subtends Their Different Pathogenic Potential

The transition from commensalism to pathogenicity of Candida albicans reflects both the host inability to mount specific immune responses and the microorganism’s dimorphic switch efficiency. In this study, we used whole genome sequencing and microarray analysis to investigate the genomic determinants of the phenotypic changes observed in two C. albicans clinical isolates (YL1 and YQ2). In vitro experiments employing epithelial, microglial, and peripheral blood mononuclear cells were thus used to evaluate C. albicans isolates interaction with first line host defenses, measuring adhesion, susceptibility to phagocytosis, and induction of secretory responses. Moreover, a murine model of peritoneal infection was used to compare the in vivo pathogenic potential of the two isolates. Genome sequence and gene expression analysis of C. albicans YL1 and YQ2 showed significant changes in cellular pathways involved in environmental stress response, adhesion, filamentous growth, invasiveness, and dimorphic transition. This was in accordance with the observed marked phenotypic differences in biofilm production, dimorphic switch efficiency, cell adhesion, invasion, and survival to phagocyte-mediated host defenses. The mutations in key regulators of the hyphal growth pathway in the more virulent strain corresponded to an overall greater number of budding yeast cells released. Compared to YQ2, YL1 consistently showed enhanced pathogenic potential, since in vitro, it was less susceptible to ingestion by phagocytic cells and more efficient in invading epithelial cells, while in vivo YL1 was more effective than YQ2 in recruiting inflammatory cells, eliciting IL-1β response and eluding phagocytic cells. Overall, these results indicate an unexpected isolate-specific variation in pathways important for host invasion and colonization, showing how the genetic background of C. albicans may greatly affect its behavior both in vitro and in vivo. Based on this approach, we propose that the co-occurrence of changes in sequence and expression in genes and pathways driving dimorphic transition and pathogenicity reflects a selective balance between traits favoring dissemination of the pathogen and traits involved in host defense evasion. This study highlights the importance of investigating strain-level, rather than species level, differences, when determining fungal–host interactions and defining commensal or pathogen behavior.

Host Sensing by Pathogenic Fungi

The ability to cause disease extends from the ability to grow within the host environment. The human host provides a dynamic environment to which fungal pathogens must adapt to in order to survive. The ability to grow under a particular condition (i.e., the ability to grow at mammalian body temperature) is considered a fitness attribute and is essential for growth within the human host. On the other hand, some environmental conditions activate signaling mechanisms resulting in the expression of virulence factors, which aid pathogenicity. Therefore, pathogenic fungi have evolved fitness and virulence attributes to enable them to colonize and infect humans. This review highlights how some of the major pathogenic fungi respond and adapt to key environmental signals within the human host.

Morphology Changes in Human Fungal Pathogens upon Interaction with the Host

Morphological changes are a very common and effective strategy for pathogens to survive in the mammalian host. During interactions with their host, human pathogenic fungi undergo an array of morphological changes that are tightly associated with virulence. Candida albicans switches between yeast cells and hyphae during infection. Thermally dimorphic pathogens, such as Histoplasma capsulatum and Blastomyces species transform from hyphal growth to yeast cells in response to host stimuli. Coccidioides and Pneumocystis species produce spherules and cysts, respectively, which allow for the production of offspring in a protected environment. Finally, Cryptococcus species suppress hyphal growth and instead produce an array of yeast cells—from large polyploid titan cells to micro cells. While the morphology changes produced by human fungal pathogens are diverse, they all allow for the pathogens to evade, manipulate, and overcome host immune defenses to cause disease. In this review, we summarize the morphology changes in human fungal pathogens—focusing on morphological features, stimuli, and mechanisms of formation in the host.

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.

Energy metabolism through the TCA cycle and mitochondrial electron transport are critical for the human fungal pathogen Candida albicans to survive and propagate in the host. This is, in part, due to the fact that C. albicans is a Crabtree-negative species, and thus exclusively uses respiration when oxygen is available. Here, we investigate the roles of the TCA cycle in hyphal development and CO₂ sensing in C. albicans. Through the use of ATP and the cellular signaling molecule CO₂, the TCA cycle integrates with the Ras1-cAMP signaling pathway, which is a central regulator of hyphal growth, to govern basic cellular biological processes. Together with Efg1, a downstream transcription factor of the cAMP signaling pathway, the heat shock factor-type transcription regulator Sfl2 controls CO₂-induced hyphal growth in C. albicans. Deletion of SFL2 results in the loss of global transcriptional responses under elevated CO₂ levels. Our study indicates that the TCA cycle not only occupies the central position of cellular metabolism but also regulates other biological processes such as CO₂ sensing and hyphal development through integration with the Ras1-cAMP signaling pathway in C. albicans.

Genome-wide functional analysis in Candida albicans

Candida albicans is an important etiological agent of superficial and life-threatening infections in individuals with compromised immune systems. To date, we know of several overlapping genetic networks that govern virulence attributes in this fungal pathogen. Classical use of deletion mutants has led to the discovery of numerous virulence factors over the years, and genome-wide functional analysis has propelled gene discovery at an even faster pace. Indeed, a number of recent studies using large-scale genetic screens followed by genome-wide functional analysis has allowed for the unbiased discovery of many new genes involved in C. albicans biology. Here we share our perspectives on the role of these studies in analyzing fundamental aspects of C. albicans virulence properties.

A Multistate Toggle Switch Defines Fungal Cell Fates and Is Regulated by Synergistic Genetic Cues

Heritable epigenetic changes underlie the ability of cells to differentiate into distinct cell types. Here, we demonstrate that the fungal pathogen Candida tropicalis exhibits multipotency, undergoing stochastic and reversible switching between three cellular states. The three cell states exhibit unique cellular morphologies, growth rates, and global gene expression profiles. Genetic analysis identified six transcription factors that play key roles in regulating cell differentiation. In particular, we show that forced expression of Wor1 or Efg1 transcription factors can be used to manipulate transitions between all three cell states. A model for tristability is proposed in which Wor1 and Efg1 are self-activating but mutually antagonistic transcription factors, thereby forming a symmetrical self-activating toggle switch. We explicitly test this model and show that ectopic expression of WOR1 can induce white-to-hybrid-to-opaque switching, whereas ectopic expression of EFG1 drives switching in the opposite direction, from opaque-to-hybrid-to-white cell states. We also address the stability of induced cell states and demonstrate that stable differentiation events require ectopic gene expression in combination with chromatin-based cues. These studies therefore experimentally test a model of multistate stability and demonstrate that transcriptional circuits act synergistically with chromatin-based changes to drive cell state transitions. We also establish close mechanistic parallels between phenotypic switching in unicellular fungi and cell fate decisions during stem cell reprogramming.

Systematic Genetic Screen for Transcriptional Regulators of the Candida albicans White-Opaque Switch

The human fungal pathogen Candida albicans can reversibly switch between two cell types named "white" and "opaque," each of which is stable through many cell divisions. These two cell types differ in their ability to mate, their metabolic preferences and their interactions with the mammalian innate immune system. A highly interconnected network of eight transcriptional regulators has been shown to control switching between these two cell types. To identify additional regulators of the switch, we systematically and quantitatively measured white-opaque switching rates of 196 strains, each deleted for a specific transcriptional regulator. We identified 19 new regulators with at least a 10-fold effect on switching rates and an additional 14 new regulators with more subtle effects. To investigate how these regulators affect switching rates, we examined several criteria, including the binding of the eight known regulators of switching to the control region of each new regulatory gene, differential expression of the newly found genes between cell types, and the growth rate of each mutant strain. This study highlights the complexity of the transcriptional network that regulates the white-opaque switch and the extent to which switching is linked to a variety of metabolic processes, including respiration and carbon utilization. In addition to revealing specific insights, the information reported here provides a foundation to understand the highly complex coupling of white-opaque switching to cellular physiology.

Environmental pH adaption and morphological transitions in Candida albicans

The human fungal pathogen Candida albicans encounters a wide range of pH stresses during its commensal and pathogenic lifestyles. It has been well studied that environmental pH regulates the yeast-filamentous growth transition in this fungus. White-opaque switching is another type of phenotypic transitions in C. albicans. White and opaque cells are two morphologically and functionally distinct cell types, which differ in many aspects including global gene expression profiles, virulence, mating competency, and susceptibility to antifungals. The switch between white and opaque cell types is heritable and epigenetically regulated. In a recently study, Sun et al. (Eukaryot Cell 14:1127-1134, 2015) reported that pH plays a critical role in the regulation of the white-opaque phenotypic switch and sexual mating in C. albicans via both the conserved Rim101-mediated pH sensing and cAMP signaling pathways. The effect of pH on the two biological processes may represent a balancing act between host environmental adaptation and sexual reproduction in this pathogenic fungus.

Switching of colony morphology and adhesion activity of Trichosporon asahii clinical isolates

Trichosporon asahii is a pathogenic yeast that causes trichosporonosis, a deep-seated infection, in immunocompromised hosts. Pathogenic factors involved in this infection have not been investigated in detail, but morphological phenotype switching is thought to be important for T. asahii pathogenesis. Therefore, we analyzed adhesion, which may be a key early step in T. asahii infection, after morphological phenotype switching. T. asahii clinical isolates show several colony morphologies. In this study, colonies showing white-farinose (W), off-white-smooth (O), off-white-rugose (OR), smooth (S), and yellowish-white (Y) morphologies were obtained from three isolates and compared in an adhesion assay performed in cell culture dishes. At least one type of colony morphology from each clinical isolate adhered strongly to the culture dish surface, although the colony type that displayed strong adherence varied among the strains. Thus, morphological phenotype switching altered the adhesion of T. asahii strains.

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.

Hypoxia and Temperature Regulated Morphogenesis in Candida albicans

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

SUMOylation of Wor1 by a novel SUMO E3 ligase controls cell fate in Candida albicans

Candida albicans is the most common human fungal pathogen, yet is a normal commensal resident of the human gut. CO(2) levels in the gut are much higher than in air, and it is known that elevated CO(2) concentration promotes C. albicans cells to undergo a phenotypic switch from white to opaque phase. Wor1, the master regulator of opaque cell formation, is required for both the white to opaque transition and opaque maintenance. To elucidate the regulatory mechanism of Wor1, we set out to identify Wor1-interacting proteins using a yeast two-hybrid screen. A SUMO E3 ligase named Wos1 (Wor1 SUMO-ligase 1) was identified to interact with Wor1 and regulate Wor1 SUMOylation. WOS1 expression is upregulated in response to high CO(2), and the induction by CO(2) is dependent on the transcription factor Flo8. Under high CO(2) conditions, Wos1 is required for the white to opaque switch and acts downstream of Flo8. At atmospheric CO(2) levels, overexpression of Wos1 enhances Wor1 SUMOylation and promotes the white to opaque switch. Wor1 is found to be SUMOylated at lysine 385, and loss of this mark by point mutation leads to a defect in CO(2) -mediated opaque cell induction. Together, our genetic and biological data show that Wos1-mediated Wor1 SUMOylation contributes to the regulation of CO(2) -induced white to opaque switching as well as heritable maintenance of the opaque cell type.

An expanded regulatory network temporally controls Candida albicans biofilm formation

Candida albicans biofilms are composed of highly adherent and densely arranged cells with properties distinct from those of free‐floating (planktonic) cells. These biofilms are a significant medical problem because they commonly form on implanted medical devices, are drug resistant and are difficult to remove. C. albicans biofilms are not static structures; rather they are dynamic and develop over time. Here we characterize gene expression in biofilms during their development, and by comparing them to multiple planktonic reference states, we identify patterns of gene expression relevant to biofilm formation. In particular, we document time‐dependent changes in genes involved in adhesion and metabolism, both of which are at the core of biofilm development. Additionally, we identify three new regulators of biofilm formation, Flo8, Gal4, and Rfx2, which play distinct roles during biofilm development over time. Flo8 is required for biofilm formation at all time points, and Gal4 and Rfx2 are needed for proper biofilm formation at intermediate time points.

Fungal morphogenesis

Morphogenesis in fungi is often induced by extracellular factors and executed by fungal genetic factors. Cell surface changes and alterations of the microenvironment often accompany morphogenetic changes in fungi. In this review, we will first discuss the general traits of yeast and hyphal morphotypes and how morphogenesis affects development and adaptation by fungi to their native niches, including host niches. Then we will focus on the molecular machinery responsible for the two most fundamental growth forms, yeast and hyphae. Last, we will describe how fungi incorporate exogenous environmental and host signals together with genetic factors to determine their morphotype and how morphogenesis, in turn, shapes the fungal microenvironment.

Synergistic regulation of hyphal elongation by hypoxia, CO(2), and nutrient conditions controls the virulence of Candida albicans

Candida albicans reversibly switches between yeast and hyphal morphologies, with hyphae being associated with virulence. Hyphal initiation and maintenance depend on host environment sensing. Hyphal maintenance in vitro requires chromatin remodeling of hypha-specific gene promoters, although disrupting chromatin-remodeling does not disrupt C. albicans hyphal elongation and virulence during invasive infection. We find that the combination of hypoxia and high CO2, but neither condition alone, maintains hyphal elongation, even in mutants lacking the nutrient-responsive chromatin-remodeling pathway. Ume6, the transcriptional activator of hypha-specific genes, is stabilized via regulation by Ofd1, a prolyl hydroxylase family member inhibited by hypoxia, and by an uncharacterized pathway that senses high CO2. Virulence and hyphal elongation in vivo are attenuated only when the parallelly acting Ume6 stabilization and chromatin-remodeling pathways are both blocked. The evolution of redundant signaling pathways allowing C. albicans to adapt to varied host environments may explain this commensal's success as a pathogen.

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

This study describes a novel “white-gray-opaque” tristable phenotypic switching system in the human fungal pathogen Candida albicans, revealing additional complexity in this organism's ability to adapt to changing environments.

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.

The capacity of the yeast Candida albicans to grow in several cellular forms—a phenomenon known as phenotypic plasticity—is critical for its survival and for its ability to thrive and cause infection in the human host. In this study, we report a novel form of C. albicans, the “gray” phenotype, which may enhance fitness and confer an adaptive advantage for this important pathogenic yeast in certain host environments. The gray cell type, together with the previously discovered “white” and “opaque” cell types, forms a tristable phenotypic switching system. The three phenotypes differ in their cellular and colony appearance, their global transcriptional profiles, their production of secreted aspartyl proteinases (enzymes that degrade host tissues and release nutrients), and their virulence in different infection models. Moreover, gray cells exhibit a level of mating competency that is intermediate between that of white and opaque cells. We further demonstrate that two key transcriptional regulators, Wor1 and Efg1, play central roles in the regulation of the “white-gray-opaque” tristable transitions. Our study reveals a multi-stable and heritable switching system, indicating that the adoption of distinct morphological forms in response to environmental change could be much more elaborate than previously thought.

Role of carbonic anhydrases in pathogenic micro-organisms: a focus on Aspergillus fumigatus

Aspergillus fumigatus is a ubiquitous saprophytic fungus responsible for organic material decomposition, and plays an important role in recycling environmental carbon and nitrogen. Besides its important role in the environment, this fungus has been reported as one of the most important fungal pathogens in immunocompromised patients. Due to changes in CO2 concentration that some pathogens face during the infection process, studies have been undertaken to understand the pathogenic roles of carbonic anhydrases (CAs), well-known CO2 hydration catalytic enzymes. As a basis for a discussion of the possible roles of CAs in A. fumigatus pathogenicity, this review describes the main characteristics of the A. fumigatus infection and the challenges for its treatment. In addition, it gathers findings from studies with CA inhibitor drugs as anti-infective agents in different pathogens.

N-acetylglucosamine-induced white-to-opaque switching in Candida albicans is independent of the Wor2 transcription factor

Candida albicans, a major opportunistic fungal pathogen of humans, can spontaneously undergo white-to-opaque switching, a prerequisite of mating. The phenotypes of white and opaque cells are heritable and bistable. The zinc-finger transcription factor Wor2 (White Opaque Regulator 2) has previously been identified as an important regulator of white-to-opaque switching. Deletion of WOR2 locks cells in the white phase when cultured on media containing glucose as the sole carbon source. In this study, we report that N-acetylglucosamine (GlcNAc) can induce white-to-opaque switching in the wor2/wor2 null mutant and stabilize the opaque phenotype of C. albicans. Moreover, overexpression of RAS1V13 (the activating form of RAS1) hypersensitizes white cells of the wor2/wor2 mutant to GlcNAc. These results suggest that Wor2 is not required for opaque cell formation at least under some culture conditions. Therefore C. albicans cells may adopt a different gene expression profile in response to GlcNAc to activate phenotypic switching.

Activation of the Cph1-dependent MAP kinase signaling pathway induces white-opaque switching in Candida albicans

Depending on the environmental conditions, the pathogenic yeast Candida albicans can undergo different developmental programs, which are controlled by dedicated transcription factors and upstream signaling pathways. C. albicans strains that are homozygous at the mating type locus can switch from the normal yeast form (white) to an elongated cell type (opaque), which is the mating-competent form of this fungus. Both white and opaque cells use the Ste11-Hst7-Cek1/Cek2 MAP kinase signaling pathway to react to the presence of mating pheromone. However, while opaque cells employ the transcription factor Cph1 to induce the mating response, white cells recruit a different downstream transcription factor, Tec1, to promote the formation of a biofilm that facilitates mating of opaque cells in the population. The switch from the white to the opaque cell form is itself induced by environmental signals that result in the upregulation of the transcription factor Wor1, the master regulator of white-opaque switching. To get insight into the upstream signaling pathways controlling the switch, we expressed all C. albicans protein kinases from a tetracycline-inducible promoter in a switching-competent strain. Screening of this library of strains showed that a hyperactive form of Ste11 lacking its N-terminal domain (Ste11^ΔN467) efficiently stimulated white cells to switch to the opaque phase, a behavior that did not occur in response to pheromone. Ste11^ΔN467-induced switching specifically required the downstream MAP kinase Cek1 and its target transcription factor Cph1, but not Cek2 and Tec1, and forced expression of Cph1 also promoted white-opaque switching in a Wor1-dependent manner. Therefore, depending on the activation mechanism, components of the pheromone-responsive MAP kinase pathway can be reconnected to stimulate an alternative developmental program, switching of white cells to the mating-competent opaque phase.

The pathogenic yeast Candida albicans can switch from the white yeast form to the mating-competent opaque form. Opaque cells are less virulent than white cells, but they can avoid recognition by phagocytes, indicating that white-opaque switching has evolved as an adaptation mechanism of C. albicans to specific host niches. Both white and opaque cells respond to mating pheromone by activating the Ste11-Hst7-Cek1/Cek2 MAP kinase pathway, but with different outcomes. Opaque cells utilize the transcription factor Cph1 to induce the mating response, whereas white cells recruit a different downstream transcription factor, Tec1, to promote biofilm formation. We used a comprehensive protein kinase expression library to gain insight into the signaling pathways that regulate switching from the white to the opaque phase and found that a hyperactive form of the upstream kinase Ste11 induced white opaque-switching, a behavior that did not occur in response to pheromone. Hyperactive Ste11 functions via the opaque-specific transcription factor Cph1 instead of the white-specific transcription factor Tec1 to promote this alternative developmental program. Therefore, depending on the activation mechanism, components of the pheromone-responsive MAP kinase pathway can be rewired to stimulate a transition from the more virulent white form to the less aggressive, but mating-competent opaque form.

Carbon dioxide-sensing in organisms and its implications for human disease

The capacity of organisms to sense changes in the levels of internal and external gases and to respond accordingly is central to a range of physiologic and pathophysiologic processes. Carbon dioxide, a primary product of oxidative metabolism is one such gas that can be sensed by both prokaryotic and eukaryotic cells and in response to altered levels, elicit the activation of multiple adaptive pathways. The outcomes of activating CO2-sensitive pathways in various species include increased virulence of fungal and bacterial pathogens, prey-seeking behavior in insects as well as taste perception, lung function, and the control of immunity in mammals. In this review, we discuss what is known about the mechanisms underpinning CO2 sensing across a range of species and consider the implications of this for physiology, disease progression, and the possibility of developing new therapeutics for inflammatory and infectious disease.

Bcr1 plays a central role in the regulation of opaque cell filamentation in Candida albicans

The human fungal pathogen Candida albicans has at least two types of morphological transitions: white to opaque cell transitions and yeast to hyphal transitions. Opaque cells have historically not been known to undergo filamentation under standard filament-inducing conditions. Here we find that Bcr1 and its downstream regulators Cup9, Nrg1 and Czf1 and the cAMP-signalling pathway control opaque cell filamentation in C. albicans. We have shown that deletion of BCR1, CUP9, NRG1 and CZF1 results in opaque cell filamentation under standard culture conditions. Disruption of BCR1 in white cells has no obvious effect on hyphal growth, suggesting that Bcr1 is an opaque-specific regulator of filamentation under the conditions tested. Moreover, inactivation of the cAMP-signalling pathway or disruption of its downstream transcriptional regulators, FLO8 and EFG1, strikingly attenuates filamentation in opaque cells of the bcr1/bcr1 mutant. Deletion of HGC1, a downstream gene of the cAMP-signalling pathway encoding G1 cyclin-related protein, completely blocks opaque cell filamentation induced by inactivation of BCR1. These results demonstrate that Bcr1 regulated opaque cell filamentation is dependent on the cAMP-signalling pathway. This study establishes a link between the white-opaque switch and the yeast-filamentous growth transition in C. albicans.

White-opaque switching in natural MTLa/α isolates of Candida albicans: evolutionary implications for roles in host adaptation, pathogenesis, and sex

Phenotypic transitions play critical roles in host adaptation, virulence, and sexual reproduction in pathogenic fungi. A minority of natural isolates of Candida albicans, which are homozygous at the mating type locus (MTL, a/a or α/α), are known to be able to switch between two distinct cell types: white and opaque. It is puzzling that white-opaque switching has never been observed in the majority of natural C. albicans strains that have heterozygous MTL genotypes (a/α), given that they contain all of the opaque-specific genes essential for switching. Here we report the discovery of white-opaque switching in a number of natural a/α strains of C. albicans under a condition mimicking aspects of the host environment. The optimal condition for white-to-opaque switching in a/α strains of C. albicans is to use N-acetylglucosamine (GlcNAc) as the sole carbon source and to incubate the cells in 5% CO2. Although the induction of white-to-opaque switching in a/α strains of C. albicans is not as robust as in MTL homozygotes in response to GlcNAc and CO2, opaque cells of a/α strains exhibit similar features of cellular and colony morphology to their MTL homozygous counterparts. Like MTL homozygotes, white and opaque cells of a/α strains differ in their behavior in different mouse infection models. We have further demonstrated that the transcriptional regulators Rfg1, Brg1, and Efg1 are involved in the regulation of white-to-opaque switching in a/α strains. We propose that the integration of multiple environmental cues and the activation and inactivation of a set of transcriptional regulators controls the expression of the master switching regulator WOR1, which determines the final fate of the cell type in C. albicans. Our discovery of white-opaque switching in the majority of natural a/α strains of C. albicans emphasizes its widespread nature and importance in host adaptation, pathogenesis, and parasexual reproduction.