MFalpha1, the gene encoding the alpha mating pheromone of Candida albicans

FvKex2 is required for development, virulence, and mycotoxin production in Fusarium verticillioides

Fusarium verticillioides is one of the most important fungal pathogens causing maize ear and stalk rots, thereby undermining global food security. Infected seeds are usually unhealthy for consumption due to contamination with fumonisin B1 (FB1) mycotoxin produced by the fungus as a virulence factor. Unveiling the molecular factors that determine fungal development and pathogenesis will help in the control and management of the diseases. Kex2 is a kexin-like Golgi-resident proprotein convertase that is involved in the activation of some important proproteins. Herein, we identified and functionally characterized FvKex2 in relation to F. verticillioides development and virulence by bioinformatics and functional genomics approaches. We found that FvKex2 is required for the fungal normal vegetative growth, because the growth of the ∆Fvkex2 mutant was significantly reduced on culture media compared to the wild-type and complemented strains. The mutant also produced very few conidia with morphologically abnormal shapes when compared with those from the wild type. However, the kexin-like protein was dispensable for the male role in sexual reproduction in F. verticillioides. In contrast, pathogenicity was nearly abolished on wounded maize stalks and sugarcane leaves in the absence of FvKEX2 gene, suggesting an essential role of Fvkex2 in the virulence of F. verticillioides. Furthermore, high-performance liquid chromatography analysis revealed that the ∆Fvkex2 mutant produced a significantly lower level of FB1 mycotoxin compared to the wild-type and complemented strains, consistent with the loss of virulence observed in the mutant. Taken together, our results indicate that FvKex2 is critical for vegetative growth, FB1 biosynthesis, and virulence, but dispensable for sexual reproduction in F. verticillioides. The study presents the kexin-like protein as a potential drug target for the management of the devastating maize ear and stalk rot diseases. Further studies should aim at uncovering the link between FvKex2 activity and FB1 biosynthesis genes. KEY POINTS: •The kexin-like protein FvKex2 contributes significantly to the vegetative growth of Fusarium verticillioides. •The conserved protein is required for fungal conidiation and conidial morphology, but dispensable for sexual reproduction. •Deletion of FvKEX2 greatly attenuates the virulence and mycotoxin production potential of F. verticillioides.

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

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

Parasexuality of Candida Species

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

Unraveling How Candida albicans Forms Sexual Biofilms

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

Propeptide genesis by Kex2-dependent cleavage of yeast wall protein 1 (Ywp1) of Candida albicans

Candida albicans is a prevalent fungal resident and opportunistic pathogen of humans, exhibiting a variety of ovoid and filamentous morphologies. Anchored within the cell wall of the ovoid yeast form of C. albicans is an abundant glycoprotein termed yeast wall protein 1 (Ywp1). Ywp1 has an antiadhesive effect that may facilitate yeast cell dispersal; it also contributes to the masking of the glucan matrix of the yeast cell wall, potentially providing shielding from recognition by the human immune system. Mature Ywp1 consists of an O-glycosylated core of 378 amino acids associated with an N-glycosylated propeptide that originates from an N-terminal segment of Ywp1. A tribasic (-RRR-) sequence in the immature Ywp1 polypeptide is separated by 8 amino acids from a dibasic (-KR-) sequence that is a canonical site for cleavage by the intracellular endopeptidase Kex2, and cleavage occurs at both of these sites to generate an 11 kilodalton (kDa) propeptide that remains strongly associated with the mature core of Ywp1. Previous studies demonstrated an absence of the 11 kDa propeptide in strains lacking Kex2, but the presence of lesser amounts of a 12 kDa propeptide ostensibly (and paradoxically) arising from cleavage at the dibasic site. Subsequent studies of wild type strains, however, suggested that post-secretion cleavages were carried out in vitro by acid proteases in unbuffered cultures to generate the 12 kDa propeptide. Here, intact and Gfp-tagged Ywp1 are utilized to show that neither of the two multibasic sites is normally cleaved in the absence of Kex2, but that uncleaved Ywp1 is still N-glycosylated and subsequently anchored to the cell wall. This furthers our understanding of the multistep cleavage of this highly conserved sequence, as well as the possible contributions of the cleaved propeptide to the maturation and functioning of Ywp1.

Repeat-containing effectors of filamentous pathogens and symbionts

Pathogenic and symbiotic filamentous microbes secrete effectors which suppress host immune responses and promote a successful colonization. Pathogen effectors are engaged in the arms race with their hosts and because of this they are subject to intense evolutionary pressure. Effectors particularly prone to rapid evolution display repeat-containing domains which can easily expand or contract and accumulate point mutations without altering their original function. In this review we address the diversity of function in such repeat-containing effectors, focus on new findings and point out avenues for future work.

The Long History of the Diverse Roles of Short ORFs: sPEPs in Fungi

Since the completion of the genome sequence of the model eukaryote Saccharomyces cerevisiae, there have been significant advancements in the field of genome annotation, in no small part due to the availability of datasets that make large-scale comparative analyses possible. As a result, since its completion there has been a significant change in annotated ORF size distribution in this first eukaryotic genome, especially in short ORFs (sORFs) predicted to encode polypeptides less than 150 amino acids in length. Due to their small size and the difficulties associated with their study, it is only relatively recently that these genomic features and the sORF-encoded peptides (sPEPs) they encode have become a focus of many researchers. Yet while this class of peptides may seem new and exciting, the study of this part of the proteome is nothing new in S. cerevisiae, a species where the biological importance of sPEPs has been elegantly illustrated over the past 30 years. Here the authors showcase a range of different sORFs found in S. cerevisiae and the diverse biological roles of their encoded sPEPs, and provide an insight into the sORFs found in other fungal species, particularly those pathogenic to humans.

Parasex Generates Phenotypic Diversity de Novo and Impacts Drug Resistance and Virulence in Candida albicans

Candida albicans is a diploid fungus that is a frequent cause of mucosal and systemic infections in humans. This species exhibits an unusual parasexual cycle in which mating produces tetraploid cells that undergo a nonmeiotic program of concerted chromosome loss to return to a diploid or aneuploid state. In this work, we used a multipronged approach to examine the capacity of parasex to generate diversity in C. albicans First, we compared the phenotypic properties of 32 genotyped progeny and observed wide-ranging differences in fitness, filamentation, biofilm formation, and virulence. Strikingly, one parasexual isolate displayed increased virulence relative to parental strains using a Galleria mellonella model of infection, establishing that parasex has the potential to enhance pathogenic traits. Next, we examined parasexual progeny derived from homothallic, same-sex mating events, and reveal that parasex can generate diversity de novo from identical parental strains. Finally, we generated pools of parasexual progeny and examined resistance of these pools to environmental stresses. Parasexual progeny were generally less fit than control strains across most test conditions, but showed an increased ability to grow in the presence of the antifungal drug fluconazole (FL). FL-resistant progeny were aneuploid isolates, often being diploid strains trisomic for both Chr3 and Chr6. Passaging of these aneuploid strains frequently led to loss of the supernumerary chromosomes and a concomitant decrease in drug resistance. These experiments establish that parasex generates extensive phenotypic diversity de novo, and that this process has important consequences for both virulence and drug resistance in C. albicans populations.

Structure-Activity Relationship of α Mating Pheromone from the Fungal Pathogen Fusarium oxysporum

During sexual development ascomycete fungi produce two types of peptide pheromones termed a and α. The α pheromone from the budding yeast Saccharomyces cerevisiae, a 13-residue peptide that elicits cell cycle arrest and chemotropic growth, has served as paradigm for the interaction of small peptides with their cognate G protein-coupled receptors. However, no structural information is currently available for α pheromones from filamentous ascomycetes, which are significantly shorter and share almost no sequence similarity with the S. cerevisiae homolog. High resolution structure of synthetic α-pheromone from the plant pathogenic ascomycete Fusarium oxysporum revealed the presence of a central β-turn resembling that of its yeast counterpart. Disruption of the-fold by d-alanine substitution of the conserved central Gly6-Gln7 residues or by random sequence scrambling demonstrated a crucial role for this structural determinant in chemoattractant activity. Unexpectedly, the growth inhibitory effect of F. oxysporum α-pheromone was independent of the cognate G protein-coupled receptors Ste2 and of the central β-turn but instead required two conserved Trp1-Cys2 residues at the N terminus. These results indicate that, despite their reduced size, fungal α-pheromones contain discrete functional regions with a defined secondary structure that regulate diverse biological processes such as polarity reorientation and cell division.

Specificity characterization of the α-mating factor hormone by Kex2 protease

Kex2 is a Ca²⁺-dependent serine protease from S. cerevisiae. Characterization of the substrate specificity of Kex2 is of particular interest because this protease serves as the prototype of a large family of eukaryotic subtilisin-related proprotein-processing proteases that cleave sites consisting of pairs or clusters of basic residues. Our goal was to study the prime region subsite S' of Kex2 because previous studies have only taken into account non-prime sites using AMC substrates but not the specificity of prime sites identified through structural modeling or predicted cleavage sites. Therefore, we used peptides derived from Abz-KR↓EADQ-EDDnp and Abz-YKR↓EADQ-EDDnp based on the pro-α-mating factor sequence. The specificity of Kex2 due to basic residues at P₁' is affected by the type of residue in the P₃ position. Some residues in P₁' with large or bulky side chains yielded poor substrate specificity. The k_cat/K_M values for peptides with P₂' substitutions containing Tyr in P₃ were higher than those obtained for the peptides without Tyr. In fact, P' and P modifications mainly promoted changes in k_cat and K_M, respectively. The pH profile of Kex2 was fit to a double-sigmoidal pH-titration curve. The specificity results suggest that Kex2 might be involved in the processing of the putative cleavage sites in a polypeptide involved in cell elongation, hyphal formation and the processing of a toxin, which result in host cell lysis. In summary, the specificity of Kex2 is dependent on the set of interactions with prime and non-prime subsites, resulting in synergism.

Evolutionary Selection on Barrier Activity: Bar1 Is an Aspartyl Protease with Novel Substrate Specificity

Peptide-based pheromones are used throughout the fungal kingdom for coordinating sexual responses between mating partners. Here, we address the properties and function of Bar1, an aspartyl protease that acts as a “barrier” and antagonist to pheromone signaling in multiple species. Candida albicans Bar1 was purified and shown to exhibit preferential cleavage of native α pheromone over pheromones from related fungal species. This result establishes that protease substrate specificity coevolved along with changes in its pheromone target. Pheromone cleavage by Bar1 occurred between residues Thr-5 and Asn-6 in the middle of the tridecapeptide sequence. Surprisingly, proteolytic activity was independent of the amino acid residues present at the scissile bond and instead relied on residues at the C terminus of α pheromone. Unlike most aspartyl proteases, Bar1 also exhibited a near-neutral pH optimum and was resistant to the class-wide inhibitor pepstatin A. In addition, genetic analysis was performed on C. albicansBAR1 and demonstrated that the protease not only regulates endogenous pheromone signaling but also can limit interspecies pheromone signaling. We discuss these findings and propose that the unusual substrate specificity of Bar1 is a consequence of its coevolution with the α pheromone receptor Ste2 for their shared peptide target.

Pheromones are important for intraspecies communication across the tree of life. In the fungal kingdom, extracellular proteases play a key role in antagonizing pheromone signaling in multiple species. This study examines the properties and function of Candida albicans Bar1, an aspartyl protease that cleaves and thereby inactivates α pheromone. We demonstrate that Bar1 plays important roles in regulating both intra- and interspecies pheromone signaling. The fungal protease shows preferential activity on the endogenous pheromone, but, surprisingly, cleavage activity is dependent on amino acid residues distal to the scissile bond. We propose that the unusual substrate specificity of Bar1 is a direct result of coevolution with Ste2, the receptor for α pheromone, for recognition of the same peptide target. The novel specificity of Bar1 reveals the complex forces shaping the evolution of mating pathways in fungi and uncovers a protease with potentially important applications in the biotechnology industry.

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 parasexual lifestyle of Candida albicans

Candida albicans is both a prevalent human commensal and the most commonly encountered human fungal pathogen. This lifestyle is dependent on the ability of the fungus to undergo rapid genetic and epigenetic changes, often in response to specific environmental cues. A parasexual cycle in C. albicans has been defined that includes several unique properties when compared to the related model yeast, Saccharomyces cerevisiae. Novel features include strict regulation of mating via a phenotypic switch, enhanced conjugation within a sexual biofilm, and a program of concerted chromosome loss in place of a conventional meiosis. It is expected that several of these adaptations co-evolved with the ability of C. albicans to colonize the mammalian host.

Candida survival strategies

Only few Candida species, e.g., Candida albicans, Candida glabrata, Candida dubliniensis, and Candida parapsilosis, are successful colonizers of a human host. Under certain circumstances these species can cause infections ranging from superficial to life-threatening disseminated candidiasis. The success of C. albicans, the most prevalent and best studied Candida species, as both commensal and human pathogen depends on its genetic, biochemical, and morphological flexibility which facilitates adaptation to a wide range of host niches. In addition, formation of biofilms provides additional protection from adverse environmental conditions. Furthermore, in many host niches Candida cells coexist with members of the human microbiome. The resulting fungal-bacterial interactions have a major influence on the success of C. albicans as commensal and also influence disease development and outcome. In this chapter, we review the current knowledge of important survival strategies of Candida spp., focusing on fundamental fitness and virulence traits of C. albicans.

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.

In eukaryotic organisms, cells often undergo differentiation into distinct cell types in order to fulfill specialized roles. To achieve a certain function, different cell types may behave coordinately to complete a task that they may otherwise be incapable of completing independently. The human fungal pathogen Candida albicans can exist as two functionally and morphologically distinct cell types: white and opaque. The white cell type is thought to be the default state and may be the majority cell population in nature. However, only the minority opaque cells are mating-competent. In this study, we report that white and opaque cells show a coordinated behavior in the process of mating. When in the presence of opaque cells with an opposite mating type, white cells release sexual pheromones, and thus create an environment conducive for both opposite- and same-sex mating of opaque cells. The two cell types communicate via a paracrine pheromone signaling system. We propose that this communal coordination between white and opaque cells may not only support the fungus to be a successful commensal and pathogen in the host, but may also increase the fitness of the fungus during evolution over time.

Sexual reproduction of human fungal pathogens

We review here recent advances in our understanding of sexual reproduction in fungal pathogens that commonly infect humans, including Candida albicans, Cryptococcus neoformans/gattii, and Aspergillus fumigatus. Where appropriate or relevant, we introduce findings on other species associated with human infections. In particular, we focus on rapid advances involving genetic, genomic, and population genetic approaches that have reshaped our view of how fungal pathogens evolve. Rather than being asexual, mitotic, and largely clonal, as was thought to be prevalent as recently as a decade ago, we now appreciate that the vast majority of pathogenic fungi have retained extant sexual, or parasexual, cycles. In some examples, sexual and parasexual unions of pathogenic fungi involve closely related individuals, generating diversity in the population but with more restricted recombination than expected from fertile, sexual, outcrossing and recombining populations. In other cases, species and isolates participate in global outcrossing populations with the capacity for considerable levels of gene flow. These findings illustrate general principles of eukaryotic pathogen emergence with relevance for other fungi, parasitic eukaryotic pathogens, and both unicellular and multicellular eukaryotic organisms.

The role of phenotypic switching in the basic biology and pathogenesis of Candida albicans

The "white-opaque" transition in Candida albicans was discovered in 1987. For the next fifteen years, a significant body of knowledge accumulated that included differences between the cell types in gene expression, cellular architecture and virulence in cutaneous and systemic mouse models. However, it was not until 2002 that we began to understand the role of switching in the life history of this pathogen, the role of the mating type locus and the molecular pathways that regulated it. Then in 2006, both the master switch locus WORI and the pheromone-induced white cell biofilm were discovered. Since that year, a number of new observations on the regulation and biology of switching have been made that have significantly increased the perceived complexity of this fascinating phenotypic transition.

Genetic control of conventional and pheromone-stimulated biofilm formation in Candida albicans

Candida albicans can stochastically switch between two phenotypes, white and opaque. Opaque cells are the sexually competent form of C. albicans and therefore undergo efficient polarized growth and mating in the presence of pheromone. In contrast, white cells cannot mate, but are induced – under a specialized set of conditions – to form biofilms in response to pheromone. In this work, we compare the genetic regulation of such “pheromone-stimulated” biofilms with that of “conventional” C. albicans biofilms. In particular, we examined a network of six transcriptional regulators (Bcr1, Brg1, Efg1, Tec1, Ndt80, and Rob1) that mediate conventional biofilm formation for their potential roles in pheromone-stimulated biofilm formation. We show that four of the six transcription factors (Bcr1, Brg1, Rob1, and Tec1) promote formation of both conventional and pheromone-stimulated biofilms, indicating they play general roles in cell cohesion and biofilm development. In addition, we identify the master transcriptional regulator of pheromone-stimulated biofilms as C. albicans Cph1, ortholog of Saccharomyces cerevisiae Ste12. Cph1 regulates mating in C. albicans opaque cells, and here we show that Cph1 is also essential for pheromone-stimulated biofilm formation in white cells. In contrast, Cph1 is dispensable for the formation of conventional biofilms. The regulation of pheromone- stimulated biofilm formation was further investigated by transcriptional profiling and genetic analyses. These studies identified 196 genes that are induced by pheromone signaling during biofilm formation. One of these genes, HGC1, is shown to be required for both conventional and pheromone-stimulated biofilm formation. Taken together, these observations compare and contrast the regulation of conventional and pheromone-stimulated biofilm formation in C. albicans, and demonstrate that Cph1 is required for the latter, but not the former.

Candida albicans is the predominant fungal pathogen afflicting humans, where many infections arise due to its proclivity to form biofilms. Biofilms are complex multicellular communities in which cells exhibit distinct properties to those grown in suspension. They are particularly relevant in the development of device-associated infections, and thus understanding biofilm regulation and biofilm architecture is a priority. C. albicans has the ability to form different types of biofilms under different environmental conditions. Here, we compare the regulation of biofilm formation in conventional biofilms, for which a core transcriptional network has recently been identified, with pheromone-stimulated biofilms, which occur when C. albicans white cells are exposed to pheromone. Our studies show that several regulatory components control biofilm formation under both conditions, including the network transcriptional regulators Bcr1, Brg1, Rob1, and Tec1. However, other transcriptional regulators are specific to each model of biofilm development. In particular, we demonstrate that Cph1, the master regulator of the pheromone response during mating, is essential for pheromone-stimulated biofilm formation but is dispensable for conventional biofilms. These studies provide an in-depth analysis of the regulation of pheromone-stimulated biofilms, and demonstrate that both shared and unique components operate in different models of biofilm formation in this human pathogen.

Regulation of phenotypic transitions in the fungal pathogen Candida albicans

The human commensal fungus Candida albicans can cause not only superficial infections, but also life-threatening disease in immunocompromised individuals. C. albicans can grow in several morphological forms. The ability to switch between different phenotypic forms has been thought to contribute to its virulence. The yeast-filamentous growth transition and white-opaque switching represent two typical morphological switching systems, which have been intensively studied in C. albicans. The interplay between environmental factors and genes determines the morphology of C. albicans. This review focuses on the regulation of phenotypic changes in this pathogenic organism by external environmental cues and internal genes.

Self-induction of a/a or alpha/alpha biofilms in Candida albicans is a pheromone-based paracrine system requiring switching

Like MTL-heterozygous (a/α) cells, white MTL-homozygous (a/a or α/α) cells of Candida albicans, to which a minority of opaque cells of opposite mating type have been added, form thick, robust biofilms. The latter biofilms are uniquely stimulated by the pheromone released by opaque cells and are regulated by the mitogen-activated protein kinase signal transduction pathway. However, white MTL-homozygous cells, to which opaque cells of opposite mating type have not been added, form thinner biofilms. Mutant analyses reveal that these latter biofilms are self-induced. Self-induction of a/a biofilms requires expression of the α-receptor gene STE2 and the α-pheromone gene MFα, and self-induction of α/α biofilms requires expression of the a-receptor gene STE3 and the a-pheromone gene MFa. In both cases, deletion of WOR1, the master switch gene, blocks cells in the white phenotype and biofilm formation, indicating that self-induction depends upon low frequency switching from the white to opaque phenotype. These results suggest a self-induction scenario in which minority opaque a/a cells formed by switching secrete, in a mating-type-nonspecific fashion, α-pheromone, which stimulates biofilm formation through activation of the α-pheromone receptor of majority white a/a cells. A similar scenario is suggested for a white α/α cell population, in which minority opaque α/α cells secrete a-pheromone. This represents a paracrine system in which one cell type (opaque) signals a second highly related cell type (white) to undergo a complex response, in this case the formation of a unisexual white cell biofilm.

Evolution of mating within the Candida parapsilosis species group

Candida orthopsilosis and Candida metapsilosis are closely related to Candida parapsilosis, a major cause of infection in premature neonates. Mating has not been observed in these species. We show that ∼190 isolates of C. parapsilosis contain only an MTLa idiomorph at the mating-type-like locus. Here, we describe the isolation and characterization of the MTL loci from C. orthopsilosis and C. metapsilosis. Among 16 C. orthopsilosis isolates, 9 were homozygous for MTLa, 5 were homozygous for MTLα, and 2 were MTLa/α heterozygotes. The C. orthopsilosis isolates belonged to two divergent groups, as characterized by restriction patterns at MTL, which probably represent subspecies. We sequenced both idiomorphs from each group and showed that they are 95% identical and that the regulatory genes are intact. In contrast, 18 isolates of C. metapsilosis contain only MTLα idiomorphs. Our results suggest that the role of MTL in determining cell type is being eroded in the C. parapsilosis species complex. The population structure of C. orthopsilosis indicates that mating may occur. However, expression of genes in the mating signal transduction pathway does not respond to exposure to alpha factor. C. parapsilosis is also nonresponsive, even when the GTPase-activating protein gene SST2 is deleted. In addition, splicing of introns in MTLa1 and MTLa2 is defective in C. orthopsilosis. Mating is not detected. The alpha factor peptide, which is the same sequence in C. parapsilosis, C. orthopsilosis, and C. metapsilosis, can induce a mating response in Candida albicans. It is therefore likely either that mating of C. orthopsilosis takes place under certain unidentified conditions or that the mating pathway has been adapted for other functions, such as cross-species communication.

Interspecies pheromone signaling promotes biofilm formation and same-sex mating in Candida albicans

The opportunistic pathogen Candida albicans undergoes a parasexual mating cycle in which cells must switch from the conventional "white" form to the alternative "opaque" form to become mating competent. Pheromones secreted by opaque cells induce the formation of polarized mating projections and result in cell-cell conjugation. In contrast, white cells are unable to undergo mating, but can still respond to pheromone by expression of adhesion genes that promote biofilm formation. In this study, we have analyzed the dual ability of pheromones to activate mating by opaque cells and biofilm formation by white cells. We first show that there is considerable plasticity in interactions between the α pheromone and its receptor, Ste2, by analysis of analogs of the α pheromone. Significantly, substituted forms of α pheromone can induce a response in opaque cells and this is sufficient to drive same-sex a-a cell fusion and homothallic mating. In addition, pheromone analogs were able to induce adhesion and biofilm formation in white cells of C. albicans. Because of the observed plasticity in pheromone signaling, we subsequently tested putative pheromones from multiple Candida species and identified nonnative ligands that can induce self-mating and biofilm responses in C. albicans. Our findings demonstrate that environmental signals can initiate C. albicans parasexual reproduction and biofilm formation, and highlight the role of the pheromone-signaling apparatus in mediating these functions.

Evolution of a new signal transduction pathway in Candida albicans

The evolution of new signal transductions pathways is poorly understood. Here I present a rare glimpse into the evolution of one such pathway, namely the white-cell pheromone response pathway in Candida albicans. In this pathway, the upper portion has been derived intact from the ancestral pathway for mating, the targeted transcription factor from an ancestral filamentation or biofilm pathway, and the upregulated genes from an ancestral biofilm pathway. Each component of this pathway, therefore, has been derived from a conserved pathway. I suggest that the evolution of this new pathway provides one possible paradigm for the evolution of other signal transduction pathways in new cell types.

Identification of a cell death pathway in Candida albicans during the response to pheromone

Mating in hemiascomycete yeasts involves the secretion of pheromones that induce sexual differentiation in cells of the opposite mating type. Studies in Saccharomyces cerevisiae have revealed that a subpopulation of cells experiences cell death during exposure to pheromone. In this work, we tested whether the phenomenon of pheromone-induced death (PID) also occurs in the opportunistic pathogen Candida albicans. Mating in C. albicans is uniquely regulated by white-opaque phenotypic switching; both cell types respond to pheromone, but only opaque cells undergo the morphological transition and cell conjugation. We show that approximately 20% of opaque cells, but not white cells, of laboratory strain SC5314 experience pheromone-induced death. Furthermore, analysis of mutant strains revealed that PID was significantly reduced in strains lacking Fig1 or Fus1 transmembrane proteins that are induced during the mating process and, we now show, are necessary for efficient mating in C. albicans. The level of PID was also Ca(2+) dependent, as chelation of Ca(2+) ions increased cell death to almost 50% of the population. However, in contrast to S. cerevisiae PID, pheromone-induced killing of C. albicans cells was largely independent of signaling via the Ca(2+)-dependent protein phosphatase calcineurin, even when combined with the loss of Cmk1 and Cmk2 proteins. Finally, we demonstrate that levels of PID vary widely between clinical isolates of C. albicans, with some strains experiencing close to 70% cell death. We discuss these findings in light of the role of prodeath and prosurvival pathways operating in yeast cells undergoing the morphological response to pheromone.

MAPK cell-cycle regulation in Saccharomyces cerevisiae and Candida albicans

The cell cycle is the sequential set of events that living cells undergo in order to duplicate. This process must be tightly regulated as alterations may lead to diseases such as cancer. The molecular events that control the cell cycle are directional and involve regulatory molecules such as cyclins and cyclin-dependent kinases (CDKs). The budding yeast Saccharomyces cerevisiae has become a model to study this complex system since it shares several mechanisms with higher eukaryotes. Signal transduction pathways are biochemical mechanisms that sense environmental changes and there is recent evidence that they control the progression through the cell cycle in response to several stimuli. In response to pheromone, the budding yeast arrests the cell cycle in the G1 phase at the START stage. Activation of the pheromone response pathway leads to the phosphorylation of Far1, which inhibits the function of complexes formed by G1 cyclins (Cln1 and Cln2) and the CDK (Cdc28), blocking the transition to the S phase. This response prepares the cells to fuse cytoplasms and nuclei to generate a diploid cell. Activation of the Hog1 MAP kinase in response to osmotic stress or arsenite leads to the transient arrest of the cell cycle in G1 phase, which is mediated by direct phosphorylation of the CDK inhibitor, Sic1, and by downregulation of cyclin expression. Osmotic stress also induces a delay in G2 phase by direct phosphorylation of Hsl7 via Hog1, which results in the accumulation of Swe1. As a consequence, cell cycle arrest allows cells to survive upon stress. Finally, cell wall damage can induce cell cycle arrest at G2 via the cell integrity MAPK Slt2. By linking MAPK signal transduction pathways to the cell cycle machinery, a tight and precise control of the cell division takes place in response to environmental changes. Research into similar MAPK-mediated cell cycle regulation in the opportunistic pathogen Candida albicans may result in the development of new antifungal therapies.

Sexual reproduction in the Candida clade: cryptic cycles, diverse mechanisms, and alternative functions

To have sex, or not to have sex, is a question posed by many microorganisms. In favor of a sexual lifestyle is the associated rearrangement of genetic material that confers potential fitness advantages, including resistance to antimicrobial agents. The asexual lifestyle also has benefits, as it preserves complex combinations of genes that may be optimal for pathogenesis. For this reason, it was thought that several pathogenic fungi favored strictly asexual modes of reproduction. Recent approaches using genome sequencing, population analysis, and experimental techniques have now revised this simplistic picture. It is now apparent that many pathogenic fungi have retained the ability to undergo sexual reproduction, although reproduction is primarily clonal in origin. In this review, we highlight the current understanding of sexual programs in the Candida clade of species. We also examine evidence that sexual-related processes can be used for functions in addition to mating and recombination in these organisms.

Tec1 mediates the pheromone response of the white phenotype of Candida albicans: insights into the evolution of new signal transduction pathways

The newly evolved pheromone response pathway of the white cell phenotype of the opportunistic human pathogen Candida albicans provides a unique view of how signal transduction pathways evolve.

The way in which signal transduction pathways evolve remains a mystery, primarily because we have few examples of ones that have newly evolved. There are numerous examples of how signal transduction pathways in the same organism selectively share components, most notably between the signal transduction pathways in Saccharomyces cerevisiae for the mating process, the filamentation process, cell wall integrity, ascospore formation, and osmoregulation. These examples, however, have not provided insights into how such pathways evolve. Here, through construction of an overexpression library for 107 transcription factors, and through mutational analyses, we have identified the transcription factor Tec1 as the last component of the newly evolved signal transduction pathway that regulates the pheromone response of the white cell phenotype in Candida albicans. The elucidation of this last component, Tec1, establishes a comprehensive description of the pheromone response pathway in the white cell phenotype of C. albicans, providing a unique perspective on how new signal transduction pathways may evolve. The three portions of this new regulatory pathway appear to have been derived from three different ancestral programs still functional in C. albicans. The upstream portion, including signals, receptors, the trimeric G protein complex, and the MAP kinase cascade, was derived intact from the upstream portion of the opaque pheromone response pathway of the mating process; Tec1, the transcription factor targeted by the MAP kinase pathway, was derived from a filamentation pathway; and the white-specific downstream target genes were derived from an ancestral biofilm process. The evolution of this pheromone response pathway provides a possible paradigm for how such signal transduction pathways evolve.

Signal transduction pathways regulate the response of cells to changes in the extracellular environment. Here, we report the identification of Tec1 as the single effector transcription factor of the pheromone response pathway of the human pathogen Candida albicans white cell type. This newly evolved pathway provides us with a unique opportunity to investigate signal transduction pathway evolution. In the C. albicans white-opaque transition, mating-competent opaque cells release mating pheromone that induces mating-incompetent white cells to form a biofilm which facilitates mating of the former. Each of the three major portions of the pathway that regulates white cell pheromone response appears to be derived from an ancestral pathway that is still intact and functional in C. albicans. The upstream portion—including the pheromone, its receptor, trimeric G protein complex, and a MAP kinase cascade—appears to be derived from the mating response pathway; transcription factor Tec1 from the filamentation pathway; and Tec1 target genes from the biofilm biosynthesis pathway. We posit that the sharing of upstream signaling components coordinates white and opaque cell pheromone responses, yet the divergence of downstream pathway components allows each cell type to elicit a unique phenotypic outcome. The white cell pheromone response pathway therefore provides a paradigm for how other such pathways may have evolved.

Fungal sex and pathogenesis

Human fungal pathogens are associated with diseases ranging from dandruff and skin colonization to invasive bloodstream infections. The major human pathogens belong to the Candida, Aspergillus, and Cryptococcus clades, and infections have high and increasing morbidity and mortality. Many human fungal pathogens were originally assumed to be asexual. However, recent advances in genome sequencing, which revealed that many species have retained the genes required for the sexual machinery, have dramatically influenced our understanding of the biology of these organisms. Predictions of a rare or cryptic sexual cycle have been supported experimentally for some species. Here, I examine the evidence that human pathogens reproduce sexually. The evolution of the mating-type locus in ascomycetes (including Candida and Aspergillus species) and basidiomycetes (Malassezia and Cryptococcus) is discussed. I provide an overview of how sex is suppressed in different species and discuss the potential associations with pathogenesis.

Genes selectively up-regulated by pheromone in white cells are involved in biofilm formation in Candida albicans

To mate, MTL-homozygous strains of the yeast pathogen Candida albicans must switch from the white to opaque phase. Mating-competent opaque cells then release pheromone that induces polarization, a G1 block and conjugation tube formation in opaque cells of opposite mating type. Pheromone also induces mating-incompetent white cells to become adhesive and cohesive, and form thicker biofilms that facilitate mating. The pheromone response pathway of white cells shares the upstream components of that of opaque cells, but targets a different transcription factor. Here we demonstrate that the genes up-regulated by the pheromone in white cells are activated through a common cis-acting sequence, WPRE, which is distinct from the cis-acting sequence, OPRE, responsible for up-regulation in opaque cells. Furthermore, we find that these white-specific genes play roles in white cell biofilm formation, and are essential for biofilm formation in the absence of an added source of pheromone, suggesting either an autocrine or pheromone-independent mechanism. These results suggest an intimate, complex and unique relationship between switching, mating and MTL-homozygous white cell biofilm formation, the latter a presumed virulence factor in C. albicans.

Candida albicans, like other microbial pathogens, form protective biofilms on host tissue, prosthetics and catheters. But C. albicans forms two types of biofilm, one by cells of majority strains that are heterozygous at the mating type locus, and another by white cells of minority strains that are homozygous at the mating type locus. These latter biofilms are enhanced by mating-competent minority opaque cells, a source of pheromone. The white cell biofilm response to pheromone is regulated by a pheromone response pathway that shares all of the upper components of the opaque cell mating response pathway, but targets a different transcription factor and activates different phase-specific downstream genes. Here we demonstrate that genes are up-regulated by pheromone in white cells through a common white cis-acting specific pheromone response element (WPRE), distinct from the element (OPRE) responsible for pheromone up-regulation of genes in opaque cells. In addition, the pheromone-induced white-specific genes play essential roles in biofilm formation. We further demonstrate that in the absence of minority opaque cells, the source of pheromone, majority white cells form biofilms through a process that is still dependent upon the same pheromone response pathway and targeted white-specific genes, suggesting either an autocrine system that involves the auto release of pheromone of the opposite mating type, or pheromone-independent activation. These observations indicate a unique interdependency of white cell biofilm formation, a presumed pathogenic trait, switching and mating in C. albicans.

Homothallic and heterothallic mating in the opportunistic pathogen Candida albicans

Candida albicans is the most common fungal pathogen in humans, causing both debilitating mucosal infections and potentially life-threatening systemic infections. Until recently, C. albicans was thought to be strictly asexual, existing only as an obligate diploid. A cryptic mating cycle has since been uncovered in which diploid a and alpha cells undergo efficient cell and nuclear fusion, resulting in tetraploid a/alpha mating products. Whereas mating between a and alpha cells has been established (heterothallism), we report here two pathways for same-sex mating (homothallism) in C. albicans. First, unisexual populations of a cells were found to undergo autocrine pheromone signalling and same-sex mating in the absence of the Bar1 protease. In both C. albicans and Saccharomyces cerevisiae, Bar1 is produced by a cells and inactivates mating pheromone alpha, typically secreted by alpha cells. C. albicans Deltabar1 a cells were shown to secrete both a and alpha mating pheromones; alpha-pheromone activated self-mating in these cells in a process dependent on Ste2, the receptor for alpha-pheromone. In addition, pheromone production by alpha cells was found to promote same-sex mating between wild-type a cells. These results establish that homothallic mating can occur in C. albicans, revealing the potential for genetic exchange even within unisexual populations of the organism. Furthermore, Bar1 protease has an unexpected but pivotal role in determining whether sexual reproduction can potentially be homothallic or is exclusively heterothallic. These findings also have implications for the mode of sexual reproduction in related species that propagate unisexually, and indicate a role for specialized sexual cycles in the survival and adaptation of pathogenic fungi.

A Candida albicans-specific region of the alpha-pheromone receptor plays a selective role in the white cell pheromone response

Candida albicans strains homozygous at the mating type locus can switch from white to opaque, and must do so to mate. Opaque cells then secrete mating pheromones that stimulate opaque cells of opposite mating type to undergo mating. These same pheromones stimulate mating-incompetent white cells to become cohesive and adhesive, and enhance white cell biofilm development, a pathogenic trait. Stimulation is mediated through the same receptor, G protein complex and mitogen-activated protein kinase pathway. Here we present evidence that a C. albicans-specific 55-amino-acid region of the first intracellular loop, IC1, of the alpha-pheromone receptor Ste2p, is required for the alpha-pheromone response of white cells, but not that of opaque cells. This represents a unique regulatory configuration in which activation of a common pathway by the same ligand, the same receptor and the same signal transduction pathway is dependent on a unique region of an intracellular loop of the common receptor in one of the two responding phenotypes.

Heterotrimeric G-protein subunit function in Candida albicans: both the alpha and beta subunits of the pheromone response G protein are required for mating

A pheromone-mediated signaling pathway that couples seven-transmembrane-domain (7-TMD) receptors to a mitogen-activated protein kinase module controls Candida albicans mating. 7-TMD receptors are typically connected to heterotrimeric G proteins whose activation regulates downstream effectors. Two Galpha subunits in C. albicans have been identified previously, both of which have been implicated in aspects of pheromone response. Cag1p was found to complement the mating pathway function of the pheromone receptor-coupled Galpha subunit in Saccharomyces cerevisiae, and Gpa2p was shown to have a role in the regulation of cyclic AMP signaling in C. albicans and to repress pheromone-mediated arrest. Here, we show that the disruption of CAG1 prevented mating, inactivated pheromone-mediated arrest and morphological changes, and blocked pheromone-mediated gene expression changes in opaque cells of C. albicans and that the overproduction of CAG1 suppressed the hyperactive cell cycle arrest exhibited by sst2 mutant cells. Because the disruption of the STE4 homolog constituting the only C. albicans gene for a heterotrimeric Gbeta subunit also blocked mating and pheromone response, it appears that in this fungal pathogen the Galpha and Gbeta subunits do not act antagonistically but, instead, are both required for the transmission of the mating signal.

Processing of predicted substrates of fungal Kex2 proteinases from Candida albicans, C. glabrata, Saccharomyces cerevisiae and Pichia pastoris

BACKGROUND

Kexin-like proteinases are a subfamily of the subtilisin-like serine proteinases with multiple regulatory functions in eukaryotes. In the yeast Saccharomyces cerevisiae the Kex2 protein is biochemically well investigated, however, with the exception of a few well known proteins such as the alpha-pheromone precursors, killer toxin precursors and aspartic proteinase propeptides, very few substrates are known. Fungal kex2 deletion mutants display pleiotropic phenotypes that are thought to result from the failure to proteolytically activate such substrates.

RESULTS

In this study we have aimed at providing an improved assembly of Kex2 target proteins to explain the phenotypes observed in fungal kex2 deletion mutants by in vitro digestion of recombinant substrates from Candida albicans and C. glabrata. We identified CaEce1, CA0365, one member of the Pry protein family and CaOps4-homolog proteins as novel Kex2 substrates.

CONCLUSION

Statistical analysis of the cleavage sites revealed extended subsite recognition of negatively charged residues in the P1', P2' and P4' positions, which is also reflected in construction of the respective binding pockets in the ScKex2 enzyme. Additionally, we provide evidence for the existence of structural constrains in potential substrates prohibiting proteolysis. Furthermore, by using purified Kex2 proteinases from S. cerevisiae, P. pastoris, C. albicans and C. glabrata, we show that while the substrate specificity is generally conserved between organisms, the proteinases are still distinct from each other and are likely to have additional unique substrate recognition.

Microtubule motor protein Kar3 is required for normal mitotic division and morphogenesis in Candida albicans

The kinesin-related protein Kar3 is a minus end-directed molecular motor that plays a multifunctional role in microtubule-directed nuclear movement. Previously, it was shown that Candida albicans Kar3p is critical for nuclear fusion during mating as kar3 mutants were defective in karyogamy. In this study, we confirm that Kar3p is required for nuclear congression in mating but that neither Kar3p nor the dynein motor protein Dyn1p is required for nuclear migration in the mating projection prior to cell fusion. In addition, we show that C. albicans Kar3p plays an important role in the cell and colony morphology of mitotically dividing cells, as evidenced by diminished filamentation of kar3 cells on Spider medium and an increased tendency of mutant cells to form pseudohyphal cells in liquid culture. Loss of Kar3p also led to defects in nuclear division, causing cells to grow slowly and exhibit reduced viability compared to wild-type cells. Slow growth was due, at least in part, to delayed cell cycle progression, as cells lacking Kar3p accumulated in anaphase of the cell cycle. Consistent with a role in mitotic division, Kar3 protein was shown to localize to the spindle pole bodies. Finally, kar3 cells exhibited unstable or aberrant mitotic spindles, a finding that accounts for the delay in cell cycle progression and decreased viability of these cells. We suggest that the altered morphology of kar3 cells is a direct consequence of the delay in anaphase, and this leads to increased polarized growth and pseudohypha formation.

The same receptor, G protein, and mitogen-activated protein kinase pathway activate different downstream regulators in the alternative white and opaque pheromone responses of Candida albicans

Candida albicans must undergo a switch from white to opaque to mate. Opaque cells then release mating type-specific pheromones that induce mating responses in opaque cells. Uniquely in C. albicans, the same pheromones induce mating-incompetent white cells to become cohesive, form an adhesive basal layer of cells on a surface, and then generate a thicker biofilm that, in vitro, facilitates mating between minority opaque cells. Through mutant analysis, it is demonstrated that the pathways regulating the white and opaque cell responses to the same pheromone share the same upstream components, including receptors, heterotrimeric G protein, and mitogen-activated protein kinase cascade, but they use different downstream transcription factors that regulate the expression of genes specific to the alternative responses. This configuration, although common in higher, multicellular systems, is not common in fungi, and it has not been reported in Saccharomyces cerevisiae. The implications in the evolution of multicellularity in higher eukaryotes are discussed.

Morphogenesis in Candida albicans

Candida albicans is termed a dimorphic fungus because it proliferates in either a yeast form or a hyphal form. The switch between these forms is the result of a complex interplay of external and internal factors and is coordinated in part by polarity-regulating proteins that are conserved among eukaryotic cells. However, yeast and hyphal cells are not the only morphological states of C. albicans. The opaque form required for mating, the pseudohyphal cell, and the chlamydospore represent distinct cell types that form in response to specific genetic or environmental conditions. In addition, hyperextended buds can form as a result of various cell cycle-related stresses. Recent studies are beginning to shed light on some of the molecular controls regulating the various morphogenetic forms of this fascinating human pathogen.

Barrier activity in Candida albicans mediates pheromone degradation and promotes mating

Mating in Candida albicans and Saccharomyces cerevisiae is regulated by the secretion of peptide pheromones that initiate the mating process. An important regulator of pheromone activity in S. cerevisiae is barrier activity, involving an extracellular aspartyl protease encoded by the BAR1 gene that degrades the alpha pheromone. We have characterized an equivalent barrier activity in C. albicans and demonstrate that the loss of C. albicans BAR1 activity results in opaque a cells exhibiting hypersensitivity to alpha pheromone. Hypersensitivity to pheromone is clearly seen in halo assays; in response to alpha pheromone, a lawn of C. albicans Deltabar1 mutant cells produces a marked zone in which cell growth is inhibited, whereas wild-type strains fail to show halo formation. C. albicans mutants lacking BAR1 also exhibit a striking mating defect in a cells, but not in alpha cells, due to overstimulation of the response to alpha pheromone. The block to mating occurs prior to cell fusion, as very few mating zygotes were observed in mixes of Deltabar1 a and alpha cells. Finally, in a barrier assay using a highly pheromone-sensitive strain, we were able to demonstrate that barrier activity in C. albicans is dependent on Bar1p. These studies reveal that a barrier activity to alpha pheromone exists in C. albicans and that the activity is analogous to that caused by Bar1p in S. cerevisiae.

The role of nutrient regulation and the Gpa2 protein in the mating pheromone response of C. albicans

Although traditionally classified as asexual, the human fungal pathogen Candida albicans can undergo highly efficient mating. A key component of this mating is the response to pheromone, which is mediated by a conserved kinase cascade that transduces the signal from the pheromone receptor to a transcriptional response in the nucleus. In this paper we show (i) that the detailed response of C. albicans to the alpha pheromone differs among clinical isolates, (ii) that the response depends critically on nutritional conditions, (iii) that the entire response is mediated by the Ste2 receptor, and (iv) that, in terms of genes induced, the response to alpha pheromone in C. albicans shows only marginal overlap with the response in Saccharomyces cerevisiae. We further investigated the nutritional control of pheromone induction and identify the GPA2 gene as a critical component. We found that Deltagpa2/Deltagpa2 mutants are hypersensitive to pheromone and, unlike wild-type strains, show efficient cell cycle arrest (including the formation of characteristic halos on solid medium) in response to mating pheromone. These results indicate that C. albicans, like several other fungal species but unlike S. cerevisiae, integrates signals from a nutrient-sensing pathway with those of the pheromone response MAP kinase pathway to generate the final transcriptional response.

Identification and characterization of MFA1, the gene encoding Candida albicans a-factor pheromone

In the opaque state, MTLa and MTLalpha strains of Candida albicans are able to mate, and this mating is directed by a pheromone-mediated signaling process. We have used comparisons of genome sequences to identify a C. albicans gene encoding a candidate a-specific mating factor. This gene is conserved in Candida dubliniensis and is similar to a three-gene family in the related fungus Candida parapsilosis but has extremely limited similarity to the Saccharomyces cerevisiae MFA1 (ScMFA1) and ScMFA2 genes. All these genes encode C-terminal CAAX box motifs characteristic of prenylated proteins. The C. albicans gene, designated CaMFA1, is found on chromosome 2 between ORF19.2165 and ORF19.2219. MFA1 encodes an open reading frame of 42 amino acids that is predicted to be processed to a 14-amino-acid prenylated mature pheromone. Microarray analysis shows that MFA1 is poorly expressed in opaque MTLa cells but is induced when the cells are treated with alpha-factor. Disruption of this C. albicans gene blocks the mating of MTLa cells but not MTLalpha cells, while the reintegration of the gene suppresses this cell-type-specific mating defect.

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

Candida albicans, a commensal organism and a pathogen of humans, can switch stochastically between a white phase and an opaque phase without an intermediate phase. The white and opaque phases have distinct cell shapes and gene expression programs. Once switched, each phase is stable for many cell divisions. White-opaque switching is under a1-alpha2 repression and therefore only happens in a or alpha cells. Mechanisms that control the switching are unknown. Here, we identify Wor1 (white-opaque regulator 1) as a master regulator of white-opaque switching. The deletion of WOR1 blocks opaque cell formation. The ectopic expression of WOR1 converts all cells to stable opaque cells in a or alpha cells. In addition, the ectopic expression of WOR1 in a/alpha cells is sufficient to induce opaque cell formation. Importantly, WOR1 expression displays an all-or-none pattern. It is undetectable in white cells, and it is highly expressed in opaque cells. The ectopic expression of Wor1 induces the transcription of WOR1 from the WOR1 locus, which correlates with the switch to opaque phase. We present genetic evidence for feedback regulation of WOR1 transcription. The feedback regulation explains the bistable and stochastic nature of white-opaque switching.

Recent advances in the genomic analysis of Candida albicans

The release of the diploid genomic sequence of Candida albicans and its recent community-based annotation have permitted a number of studies which have significantly advanced our understanding of the biology of this important human pathogen. These advances range from analysis of genomic changes to differential gene expression under a variety of conditions. A few general conclusions can be drawn from the data presently in hand; one can expect more and more new insights as the number and kind of experiments grows.

Opaque cells signal white cells to form biofilms in Candida albicans

Upon homozygosis from a/alpha to a/a or alpha/alpha, Candida albicans must still switch from the 'white' to 'opaque' phenotype to mate. It was, therefore, surprising to discover that pheromone selectively upregulated mating-associated genes in mating-incompetent white cells without causing G1 arrest or shmoo formation. White cells, like opaque cells, possess pheromone receptors, although their distribution and redistribution upon pheromone treatment differ between the two cell types. In speculating about the possible role of the white cell pheromone response, it is hypothesized that in overlapping white a/a and alpha/alpha populations in nature, rare opaque cells, through the release of pheromone, signal majority white cells of opposite mating type to form a biofilm that facilitates mating. In support of this hypothesis, it is demonstrated that pheromone induces cohesiveness between white cells, minority opaque cells increase two-fold the thickness of majority white cell biofilms, and majority white cell biofilms facilitate minority opaque cell chemotropism. These results reveal a novel form of communication between switch phenotypes, analogous to the inductive events during embryogenesis in higher eukaryotes.

Fungal heat-shock proteins in human disease

Heat-shock proteins (hsps) have been identified as molecular chaperones conserved between microbes and man and grouped by their molecular mass and high degree of amino acid homology. This article reviews the major hsps of Saccharomyces cerevisiae, their interactions with trehalose, the effect of fermentation and the role of the heat-shock factor. Information derived from this model, as well as from Neurospora crassa and Achlya ambisexualis, helps in understanding the importance of hsps in the pathogenic fungi, Candida albicans, Cryptococcus neoformans, Aspergillus spp., Histoplasma capsulatum, Paracoccidioides brasiliensis, Trichophyton rubrum, Phycomyces blakesleeanus, Fusarium oxysporum, Coccidioides immitis and Pneumocystis jiroveci. This has been matched with proteomic and genomic information examining hsp expression in response to noxious stimuli. Fungal hsp90 has been identified as a target for immunotherapy by a genetically recombinant antibody. The concept of combining this antibody fragment with an antifungal drug for treating life-threatening fungal infection and the potential interactions with human and microbial hsp90 and nitric oxide is discussed.

SST2, a regulator of G-protein signaling for the Candida albicans mating response pathway

Candida albicans contains a functional mating response pathway that is similar to the well-studied system of Saccharomyces cerevisiae. We have characterized a regulator of G protein signaling (RGS) homolog in C. albicans with sequence similarity to the SST2 gene of Saccharomyces cerevisiae. Disruption of this gene, which had been designated SST2, causes an opaque MTLa/MTLa derivative of strain SC5314 to show hypersensitivity to the C. albicans alpha-factor. This hypersensitivity generates an enhanced cell cycle arrest detected in halo assays but reduces the overall mating efficiency of the cells. Transcriptional profiling of the pheromone-regulated gene expression in the sst2 mutant shows a pattern of gene induction similar to that observed in wild-type cells, but the responsiveness is heightened. This involvement of an RGS in the sensitivity to pheromone is consistent with the prediction that the mating response pathway in C. albicans requires the activation of a heterotrimeric G protein.

Tetracycline-inducible gene expression and gene deletion in Candida albicans

The genetic analysis of Candida albicans, the major fungal pathogen of humans, is hampered by its diploid genome, the absence of a normal sexual cycle, and a nonstandard codon usage. Although effective methods to study gene function have been developed in the past years, systems to control gene expression in C. albicans are limited. We have established a system that allows induction of gene expression in C. albicans by the addition of tetracycline (Tet). By fusing genetically modified versions of the reverse Tet repressor from Escherichia coli and the transcription activation domain of the Gal4 protein from Saccharomyces cerevisiae, a C. albicans-adapted reverse Tet-dependent transactivator (rtTA) was created that was expressed from the constitutive ADH1 or the opaque-specific OP4 promoter. To monitor Tet-inducible gene expression, the caGFP reporter gene was placed under the control of a Tet-dependent promoter, obtained by fusing a minimal promoter from C. albicans to seven copies of the Tet operator sequence. Fluorescence of the cells demonstrated that gene expression could be efficiently induced by the addition of doxycycline in yeast, hyphal, and opaque cells of C. albicans. The Tet-inducible gene expression system was then used to manipulate the behavior of the various growth forms of C. albicans. Tet-induced expression of a dominant-negative CDC42 allele resulted in growth arrest as large, multinucleate cells. Filamentous growth was efficiently inhibited under all tested hyphal-growth-promoting conditions by Tet-inducible expression of the NRG1 repressor. Tet-induced expression of the MTLa1 gene in opaque cells of an MTLalpha strain forced the cells to switch to the white phase, whereas Tet-induced expression of the MTLa2 transcription factor induced shmooing. When the ecaFLP gene, encoding the site-specific recombinase FLP, was placed under the control of the Tet-dependent promoter, Tet-inducible deletion of genes which were flanked by the FLP target sequences was achieved with high efficiency to generate conditional null mutants. In combination with the dominant selection marker caSAT1, the Tet-inducible gene expression system was also applied in C. albicans wild-type strains, including drug-resistant clinical isolates that overexpressed the MDR1, CDR1, and CDR2 multidrug efflux pumps. This system, therefore, allows a growth medium-independent, Tet-inducible expression and deletion of genes in C. albicans and provides a convenient, versatile new tool to study gene function and manipulate cellular behavior in this model pathogenic fungus.

Candida albicans-conditioned medium protects yeast cells from oxidative stress: a possible link between quorum sensing and oxidative stress resistance

Candida albicans, the most frequent fungal pathogen of humans, encounters high levels of oxidants following ingestion by professional phagocytes and through contact with hydrogen peroxide-producing bacteria. In this study, we provide evidence that C. albicans is able to coordinately regulate the oxidative stress response at the global cell population level by releasing protective molecules into the surrounding medium. We demonstrate that conditioned medium, which is defined as a filter-sterilized supernatant from a C. albicans stationary-phase culture, is able to protect yeast cells from both hydrogen peroxide and superoxide anion-generating agents. Exponential-phase yeast cells preexposed to conditioned medium were able to survive levels of oxidative stress that would normally kill actively growing yeast cells. Heat treatment, digestion with proteinase K, pH adjustment, or the addition of the oxidant scavenger alpha-tocopherol did not alter the ability of conditioned medium to induce a protective response. Farnesol, a heat-stable quorum-sensing molecule (QSM) that is insensitive to proteolytic enzymes and is unaffected by pH extremes, is partly responsible for this protective response. In contrast, the QSM tyrosol did not alter the sensitivity of C. albicans cells to oxidants. Relative reverse transcription-PCR analysis indicates that Candida-conditioned growth medium induces the expression of CAT1, SOD1, SOD2, and SOD4, suggesting that protection may be mediated through the transcriptional regulation of antioxidant-encoding genes. Together, these data suggest a link between the quorum-sensing molecule farnesol and the oxidative stress response in C. albicans.

Effects of ploidy and mating type on virulence of Candida albicans

Candida albicans is the most common fungal pathogen of humans. The recent discovery of sexuality in this organism has led to the demonstration of a mating type locus which is usually heterozygous, although some isolates are homozygous. Tetraploids can be formed between homozygotes of the opposite mating type. However, the role of the mating process and tetraploid formation in virulence has not been investigated. We describe here experiments using a murine model of disseminated candidiasis which demonstrate that in three strains, including CAI-4, the most commonly used strain background, tetraploids are less virulent than diploids and can undergo changes in ploidy during infection. In contrast to reports with other strains, we find that MTL homozygotes are almost as virulent as the heterozygotes. These results show that the level of ploidy in Candida albicans can affect virulence, but the mating type configuration does not necessarily do so.