Canonical heterotrimeric G proteins regulating mating and virulence of Cryptococcus neoformans

Life Cycle of Cryptococcus neoformans

Cryptococcus neoformans is a ubiquitous environmental fungus and an opportunistic pathogen that causes fatal cryptococcal meningitis. Advances in genomics, genetics, and cellular and molecular biology of C. neoformans have dramatically improved our understanding of this important pathogen, rendering it a model organism to study eukaryotic biology and microbial pathogenesis. In light of recent progress, we describe in this review the life cycle of C. neoformans with a special emphasis on the regulation of the yeast-to-hypha transition and different modes of sexual reproduction, in addition to the impacts of the life cycle on cryptococcal populations and pathogenesis.

The cAMP/Protein Kinase a Pathway Regulates Virulence and Adaptation to Host Conditions in Cryptococcus neoformans

Nutrient sensing is critical for adaptation of fungi to environmental and host conditions. The conserved cAMP/PKA signaling pathway contributes to adaptation by sensing the availability of key nutrients such as glucose and directing changes in gene expression and metabolism. Interestingly, the cAMP/PKA pathway in fungal pathogens also influences the expression of virulence determinants in response to nutritional and host signals. For instance, protein kinase A (PKA) in the human pathogen Cryptococcus neoformans plays a central role in orchestrating phenotypic changes, such as capsule elaboration and melanin production, that directly impact disease development. In this review, we focus first on insights into the role of the cAMP/PKA pathway in nutrient sensing for the model yeast Saccharomyces cerevisiae to provide a foundation for understanding the pathway in C. neoformans. We then discuss key features of cAMP/PKA signaling in C. neoformans including new insights emerging from the analysis of transcriptional and proteomic changes in strains with altered PKA activity and expression. Finally, we highlight recent studies that connect the cAMP/PKA pathway to cell surface remodeling and the formation of titan cells.

G Protein α Subunit GpaB is Required for Asexual Development, Aflatoxin Biosynthesis and Pathogenicity by Regulating cAMP Signaling in Aspergillus flavus

The heterotrimeric G proteins are critical for signal transduction and function in numerous biological processes including vegetative growth, asexual development and fungal virulence in fungi. Here, we identified four G protein alpha subunits (GanA, GpaB, FadA and GaoC) in the notorious Aflatoxin-producing fungus Aspergillus flavus. GanA, GpaB and FadA have homologues in other fungal species, while GaoC is a novel one. Here, we showed that the loss function of gpaB displayed a defect in conidiophore formation and considerably reduced expression levels of conidia-specific genes brlA and abaA. A decreased viability of cell wall integrity stress and oxidative stress were also found in the ∆gpaB mutant. More importantly, aflatoxin (AF) biosynthesis and infection on crop seeds were severely impaired in the gpaB-deficient mutant. Further analyses demonstrated that the intracellular cAMP levels significantly reduced in the gpaB-deficient mutant compared to wildtype strains. Additionally, an alteration of PKA activities in the ∆gpaB mutant was also found. Overall, our results indicated that GpaB played diverse roles in asexual sporulation, AF biosynthesis and virulence by regulating cAMP signaling in Aspergillus flavus.

Overview of carbon and nitrogen catabolite metabolism in the virulence of human pathogenic fungi

It is estimated that fungal infections, caused most commonly by Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, result in more deaths annually than malaria or tuberculosis. It has long been hypothesized the fungal metabolism plays a critical role in virulence though specific nutrient sources utilized by human pathogenic fungi in vivo has remained enigmatic. However, the metabolic utilisation of preferred carbon and nitrogen sources, encountered in a host niche-dependent manner, is known as carbon catabolite and nitrogen catabolite repression (CCR, NCR), and has been shown to be important for virulence. Several sensory and uptake systems exist, including carbon and nitrogen source-specific sensors and transporters, that allow scavenging of preferred nutrient sources. Subsequent metabolic utilisation is governed by transcription factors, whose functions and essentiality differ between fungal species. Furthermore, additional factors exist that contribute to the implementation of CCR and NCR. The role of the CCR and NCR-related factors in virulence varies greatly between fungal species and a substantial gap in knowledge exists regarding specific pathways. Further elucidation of carbon and nitrogen metabolism mechanisms is therefore required in a fungal species- and animal model-specific manner in order to screen for targets that are potential candidates for anti-fungal drug development.

Comparative proteomic analysis of Gib2 validating its adaptor function in Cryptococcus neoformans

Cryptococcus neoformans causes often-fatal fungal meningoencephalitis in immunocompromised individuals. While the exact disease mechanisms remain elusive, signal transduction pathways mediated by key elements such as G-protein α subunit Gpa1, small GTPase Ras1, and atypical Gβ-like/RACK1 protein Gib2 are known to play important roles in C. neoformans virulence. Gib2 is important for normal growth, differentiation, and pathogenicity, and it also positively regulates cAMP levels in conjunction with Gpa1. Interestingly, Gib2 displays a scaffold protein property by interacting with a wide variety of cellular proteins. To explore Gib2 global regulatory functions, we performed two-dimensional differential gel electrophoresis (DIGE) analysis and found that GIB2 disruption results in an increased expression of 304 protein spots (43.4%) and a decreased expression of 396 protein spots (56.6%). Analysis of 96 proteins whose expression changes were deemed significant (≥ +/- 1.5- fold) revealed that 75 proteins belong to at least 12 functional protein groups. Among them, eight groups have the statistical stringency of p ≤ 0.05, and four groups, including Hsp70/71 heat shock protein homologs and ribosomal proteins, survived the Bonferroni correction. This finding is consistent with earlier established roles for the human Gβ-like/RACK1 and the budding yeast Saccharomyces cerevisiae Asc1. It suggests that Gib2 could also be part of the complex affecting ribosomal biogenesis and protein translation in C. neoformans. Since eukaryotic Hsp70/71 proteins are involved in the facilitation of nascent protein folding, processing, and protection of cells against stress, we also propose that Gib2-regulated stress responses are linked to fungal virulence. Collectively, our study supports a conserved role of Gβ-like/RACK/Gib2 proteins in the essential cellular process of ribosomal biogenesis and protein translation. Our study also highlights a multifaceted regulatory role of Gib2 in the growth and pathogenicity of C. neoformans.

Gα proteins Gvm2 and Gvm3 regulate vegetative growth, asexual development, and pathogenicityon apple in Valsa mali

In fungi, heterotrimeric guanine-nucleotide binding proteins (G-proteins) are key elements of signal transduction pathways, which control growth, asexual and sexual development, as well as virulence. In this study, we have identified two genes encoding heterotrimeric G protein alpha subunits, named Gvm2 and Gvm3, from Valsa mali, the causal agent of apple Valsa canker. Characterization of Gvm2 and Gvm3 mutants indicates that Gvm3 may be a crucial regulator of vegetative growth. Deletion of the corresponding gene results in a 20% reduction in growth rate. Besides, Gvm2 and Gvm3 seem to be involved in asexual reproduction, and mutants are hypersensitive to oxidative and cell membrane stresses. Interestingly, both G protein alpha subunits were most probably involved in V. mali virulence. In infection assays using Malus domestica cv. ‘Fuji’ leaves and twigs, the size of lesions caused by deletion mutants △Gvm2, or △Gvm3 are significantly reduced. Furthermore, many genes encoding hydrolytic enzymes—important virulence factors in V. mali—are expressed at a lower level in these deletion mutants. Our results suggest that Gvm2 and Gvm3 play an important role in virulence probably by regulation of expression of cell wall degrading enzymes. △Gvm2, and △Gvm3 mutants were further analyzed with respect to their impact on the transcript levels of genes in the cAMP/PKA pathway. The expression of the genes encoding adenylate cyclase VmAC, protein kinase A (PKA) regulatory subunit VmPKR, and PKA catalytic subunit VmPKA1 are down-regulated in both mutants. Further analyses indicated that intracellular cAMP level and PKA activity are down-regulated in the △Gvm3 mutant, but are basically unchanged in the △Gvm2 mutant. Overall, our findings indicate that both Gvm2 and Gvm3 play diverse roles in the modulation of vegetative growth, asexual development, and virulence in V. mali.

Hyphal chemotropism in fungal pathogenicity

The ability to grow as filamentous hyphae defines the lifestyle of fungi. Hyphae are exposed to a variety of chemical stimuli such as nutrients or signal molecules from mating partners and host organisms. How fungi sense and process this chemical information to steer hyphal growth is poorly understood. Saccharomyces cerevisiae and Neurospora crassa have served as genetic models for the identification of cellular components functioning in chemotropism. A recent study in the pathogen Fusarium oxysporum revealed distinct MAPK pathways governing hyphal growth towards nutrient sources and sex pheromones or plant signals, suggesting an unanticipated complexity of chemosensing during fungus-host interactions.

Impact of Protein Palmitoylation on the Virulence Potential of Cryptococcus neoformans

The localization and specialized function of Ras-like proteins are largely determined by posttranslational processing events. In a highly regulated process, palmitoyl groups may be added to C-terminal cysteine residues, targeting these proteins to specific membranes. In the human fungal pathogen Cryptococcus neoformans, Ras1 protein palmitoylation is essential for growth at high temperature but is dispensable for sexual differentiation. Ras1 palmitoylation is also required for localization of this protein on the plasma membrane. Together, these results support a model in which specific Ras functions are mediated from different subcellular locations. We therefore hypothesize that proteins that activate Ras1 or mediate Ras1 localization to the plasma membrane will be important for C. neoformans pathogenesis. To further characterize the Ras1 signaling cascade mediating high-temperature growth, we have identified a family of protein S-acyltransferases (PATs), enzymes that mediate palmitoylation, in the C. neoformans genome database. Deletion strains for each candidate gene were generated by homogenous recombination, and each mutant strain was assessed for Ras1-mediated phenotypes, including high-temperature growth, morphogenesis, and sexual development. We found that full Ras1 palmitoylation and function required one particular PAT, Pfa4, and deletion of the PFA4 gene in C. neoformans resulted in altered Ras1 localization to membranes, impaired growth at 37°C, and reduced virulence.

Crystal structure of Gib2, a signal-transducing protein scaffold associated with ribosomes in Cryptococcus neoformans

The atypical Gβ-like/RACK1 Gib2 protein promotes cAMP signalling that plays a central role in regulating the virulence of Cryptococcus neoformans. Gib2 contains a seven-bladed β transducin structure and is emerging as a scaffold protein interconnecting signalling pathways through interactions with various protein partners. Here, we present the crystal structure of Gib2 at a 2.2-Å resolution. The structure allows us to analyse the association between Gib2 and the ribosome, as well as to identify the Gib2 amino acid residues involved in ribosome binding. Our studies not only suggest that Gib2 has a role in protein translation but also present Gib2 as a physical link at the crossroads of various regulatory pathways important for the growth and virulence of C. neoformans.

Unisexual reproduction of Cryptococcus gattii

Cryptococcus gattii is a basidiomycetous human fungal pathogen that typically causes infection in tropical and subtropical regions and is responsible for an ongoing outbreak in immunocompetent individuals on Vancouver Island and in the Pacific Northwest of the US. Pathogenesis of this species may be linked to its sexual cycle that generates infectious propagules called basidiospores. A marked predominance of only one mating type (α) in clinical and environmental isolates suggests that a-α opposite-sex reproduction may be infrequent or geographically restricted, raising the possibility of an alternative unisexual cycle involving cells of only α mating type, as discovered previously in the related pathogenic species Cryptococcus neoformans. Here we report observation of hallmark features of unisexual reproduction in a clinical isolate of C. gattii (isolate 97/433) and describe genetic and environmental factors conducive to this sexual cycle. Our results are consistent with population genetic evidence of recombination in the largely unisexual populations of C. gattii and provide a useful genetic model for understanding how novel modes of sexual reproduction may contribute to evolution and virulence in this species.

A Ric8/synembryn homolog promotes Gpa1 and Gpa2 activation to respectively regulate cyclic AMP and pheromone signaling in Cryptococcus neoformans

The G protein α subunits Gpa1, Gpa2, and Gpa3 mediate signal transduction and are important in the growth and virulence of Cryptococcus neoformans. To understand how Gpa1 functions without a conventional Gβ subunit, we characterized a resistance to inhibitors of cholinesterase 8 (Ric8) homolog from C. neoformans, which shares amino acid sequence homology with other Ric8 proteins that exhibit guanine nucleotide exchange factor (GEF) activity toward Gα. We found that the ric8 mutant was reduced in capsule size and melanin formation, which could be suppressed by cyclic AMP (cAMP) supplementation or by introducing the activated GPA1(Q284L) allele. Consistent with the fact that Ric8 participates in cAMP signaling to regulate virulence, the ric8 mutant was attenuated in virulence toward mice. Interestingly, disruption of RIC8 also resulted in opposing effects on pheromone signaling, as the ric8 mutant showed reduced mating but an enhanced ability to induce the pheromone response in the mating partner. To identify Ric8 functional mechanisms, we examined the interactions between Ric8 and the three Gα proteins. Ric8 interacted with Gpa1 and Gpa2, but not Gpa3. The presence of Gpa1(Q284L) negatively affected its interaction with Ric8, whereas the activated Gpa2(Q203L) allele abolished the interaction. Collectively, these findings suggest that Ric8 functions as a GEF to facilitate the activation of Gpa1-cAMP signaling and to promote Gpa2, affecting mating efficiency. Our study highlights the distinct and conserved characteristics associated with G protein signaling and contributes to our overall understanding of how G protein α subunits function with or without a canonical Gβ partner in C. neoformans.

Noncanonical Gβ Gib2 is a scaffolding protein promoting cAMP signaling through functions of Ras1 and Cac1 proteins in Cryptococcus neoformans

Gβ-like/RACK1 functions as a key mediator of various pathways and contributes to numerous cellular functions in eukaryotic organisms. In the pathogenic fungus Cryptococcus neoformans, noncanonical Gβ Gib2 promotes cAMP signaling in cells lacking normal Gpa1 function while displaying versatility in interactions with Gα Gpa1, protein kinase Pkc1, and endocytic intersectin Cin1. To elucidate the Gib2 functional mechanism(s), we demonstrate that Gib2 is required for normal growth and virulence. We show that Gib2 directly binds to Gpa1 and Gγ Gpg1/Gpg2 and that it interacts with phosphodiesterase Pde2 and monomeric GTPase Ras1. Pde2 remains functionally dispensable, but Ras1 is found to associate with adenylyl cyclase Cac1 through the conserved Ras association domain. In addition, the ras1 mutant exhibits normal capsule formation, whereas the ras1 gpa1 mutant displays enhanced capsule formation, and the ras1 gpa1 cac1 mutant is acapsular. Collectively, these findings suggest that Gib2 promotes cAMP levels by relieving an inhibitory function of Ras1 on Cac1 in the absence of Gpa1. In addition, using GST affinity purification combined with mass spectrometry, we identified 47 additional proteins that interact with Gib2. These proteins have putative functions ranging from signal transduction, energy generation, metabolism, and stress response to ribosomal function. After establishing and validating a protein-protein interactive network, we believe Gib2 to be a key adaptor/scaffolding protein that drives the formation of various protein complexes required for growth and virulence. Our study reveals Gib2 as an essential component in deciphering the complexity of regulatory networks that control growth and virulence in C. neoformans.

Genomic insights into the atopic eczema-associated skin commensal yeast Malassezia sympodialis

Malassezia commensal yeasts are associated with a number of skin disorders, such as atopic eczema/dermatitis and dandruff, and they also can cause systemic infections. Here we describe the 7.67-Mbp genome of Malassezia sympodialis, a species associated with atopic eczema, and contrast its genome repertoire with that of Malassezia globosa, associated with dandruff, as well as those of other closely related fungi. Ninety percent of the predicted M. sympodialis protein coding genes were experimentally verified by mass spectrometry at the protein level. We identified a relatively limited number of genes related to lipid biosynthesis, and both species lack the fatty acid synthase gene, in line with the known requirement of these yeasts to assimilate lipids from the host. Malassezia species do not appear to have many cell wall-localized glycosylphosphatidylinositol (GPI) proteins and lack other cell wall proteins previously identified in other fungi. This is surprising given that in other fungi these proteins have been shown to mediate interactions (e.g., adhesion and biofilm formation) with the host. The genome revealed a complex evolutionary history for an allergen of unknown function, Mala s 7, shown to be encoded by a member of an amplified gene family of secreted proteins. Based on genetic and biochemical studies with the basidiomycete human fungal pathogen Cryptococcus neoformans, we characterized the allergen Mala s 6 as the cytoplasmic cyclophilin A. We further present evidence that M. sympodialis may have the capacity to undergo sexual reproduction and present a model for a pseudobipolar mating system that allows limited recombination between two linked MAT loci.

IMPORTANCE

Malassezia commensal yeasts are associated with a number of skin disorders. The previously published genome of M. globosa provided some of the first insights into Malassezia biology and its involvement in dandruff. Here, we present the genome of M. sympodialis, frequently isolated from patients with atopic eczema and healthy individuals. We combined comparative genomics with sequencing and functional characterization of specific genes in a population of clinical isolates and in closely related model systems. Our analyses provide insights into the evolution of allergens related to atopic eczema and the evolutionary trajectory of the machinery for sexual reproduction and meiosis. We hypothesize that M. sympodialis may undergo sexual reproduction, which has important implications for the understanding of the life cycle and virulence potential of this medically important yeast. Our findings provide a foundation for the development of genetic and genomic tools to elucidate host-microbe interactions that occur on the skin and to identify potential therapeutic targets.

The Cryptococcus neoformans capsule: a sword and a shield

The human fungal pathogen Cryptococcus neoformans is characterized by its ability to induce a distinct polysaccharide capsule in response to a number of host-specific environmental stimuli. The induction of capsule is a complex biological process encompassing regulation at multiple steps, including the biosynthesis, transport, and maintenance of the polysaccharide at the cell surface. By precisely regulating the composition of its cell surface and secreted polysaccharides, C. neoformans has developed intricate ways to establish chronic infection and dormancy in the human host. The plasticity of the capsule structure in response to various host conditions also underscores the complex relationship between host and parasite. Much of this precise regulation of capsule is achieved through the transcriptional responses of multiple conserved signaling pathways that have been coopted to regulate this C. neoformans-specific virulence-associated phenotype. This review focuses on specific host stimuli that trigger the activation of the signal transduction cascades and on the downstream transcriptional responses that are required for robust encapsulation around the cell.

Profiling a killer, the development of Cryptococcus neoformans

The ability of fungi to transition between unicellular and multicellular growth has a profound impact on our health and the economy. Many important fungal pathogens of humans, animals, and plants are dimorphic, and the ability to switch between morphological states has been associated with their virulence. Cryptococcus neoformans is a human fungal pathogen that causes life-threatening meningoencephalitis in immunocompromised and, in some cases, immunocompetent hosts. Cryptococcus neoformans grows vegetatively as a budding yeast and switches to hyphal growth during the sexual cycle, which is important in the study of cryptococcal pathogenicity because spores resulting from sexual development are infectious propagules and can colonize the lungs of a host. In addition, sexual reproduction contributes to the genotypic variability of Cryptococcus species, which may lead to increased fitness and virulence. Despite significant advances in our understanding of the mechanisms behind the development of C. neoformans, our knowledge is still incomplete. Recent studies have led to the emergence of many intriguing questions and hypotheses. In this review, we describe and discuss the most interesting aspects of C. neoformans development and address their impact on pathogenicity.

Cryptococcal titan cell formation is regulated by G-protein signaling in response to multiple stimuli

The titan cell is a recently described morphological form of the pathogenic fungus Cryptococcus neoformans. Occurring during the earliest stages of lung infection, titan cells are 5 to 10 times larger than the normal yeast-like cells, thereby resisting engulfment by lung phagocytes and favoring the persistence of infection. These enlarged cells exhibit an altered capsule structure, a thickened cell wall, increased ploidy, and resistance to nitrosative and oxidative stresses. We demonstrate that two G-protein-coupled receptors are important for induction of the titan cell phenotype: the Ste3a pheromone receptor (in mating type a cells) and the Gpr5 protein. Both receptors control titan cell formation through elements of the cyclic AMP (cAMP)/protein kinase A (PKA) pathway. This conserved signaling pathway, in turn, mediates its effect on titan cells through the PKA-regulated Rim101 transcription factor. Additional downstream effectors required for titan cell formation include the G(1) cyclin Pcl103, the Rho104 GTPase, and two GTPase-activating proteins, Gap1 and Cnc1560. These observations support developing models in which the PKA signaling pathway coordinately regulates many virulence-associated phenotypes in diverse human pathogens.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

The Gα subunit signals through the Ste50 protein during the mating pheromone response in the yeast Kluyveromyces lactis

Yeast mating signal transduction pathways require a heterotrimeric G protein composed of Gα, Gβ, and Gγ subunits connected to a mitogen-activated protein kinase (MAPK) module. While in Saccharomyces cerevisiae elimination of Gα induces constitutive activation of the mating pathway, in Kluyveromyces lactis it produces partial sterility, which indicates that K. lactis Gα (KlGα) is required to positively activate mating. We use physical interaction experiments to determine that KlGα interacts with the adaptor protein KlSte50p. The Ras association (RA) domain of KlSte50p favored interaction with the GDP-bound KlGα subunit, and when the KlGα protein is constitutively activated, the interaction drops significantly. Additionally, KlSte50p strongly associates with the MAPK kinase kinase (MAPKKK) KlSte11p through its sterile alpha motif (SAM) domain. Genetic experiments placed KlSte50p downstream of the G protein α subunit, indicating that KlGα may stimulate the mating pathway via KlSte50p. Fusion of KlSte50p to the KlGβ subunit partially eliminated the requirement of KlGα for mating, indicating that one contribution of KlGα to the mating pathway is to facilitate plasma membrane anchoring of KlSte50p.

Assessment of constitutive activity of a G protein-coupled receptor, CPR2, in Cryptococcus neoformans by heterologous and homologous methods

G protein-coupled receptors (GPCRs) comprise the largest superfamily of cell surface receptors and are primary targets for drug development. A variety of detection systems have been reported to study ligand-GPCR interactions. Using Saccharomyces cerevisiae to express foreign proteins has long been appreciated for its low cost, simplicity, and conserved cellular pathways. The yeast pheromone-responsive pathway has been utilized to assess a range of different GPCRs. We have identified a pheromone-like receptor, Cpr2, that is located outside of the MAT locus in the human fungal pathogen Cryptococcus neoformans. To characterize its function and potential ligands, we expressed CPR2 in a yeast heterologous expression system. To optimize for CPR2 expression in this system, pheromone receptor Ste3, regulator of G protein signaling (RGS) Sst2, and the cyclin-dependent kinase inhibitor Far1 were mutated. The lacZ gene was fused with the promoter of the FUS1 gene that is activated by the yeast pheromone signal and then introduced into yeast cells. Expression of CPR2 in this yeast heterologous expression system revealed that Cpr2 could activate the pheromone-responsive pathway without addition of potential ligands, suggesting it is a naturally occurring, constitutively active receptor. Mutation of a single amino acid, Leu(222), was sufficient to reverse the constitutive activity of Cpr2. In this chapter, we summarize methods used for assessing the constitutive activity of Cpr2 and its mutants, which could be beneficial for other GPCR studies.

The RGS protein Crg2 is required for establishment and progression of murine pulmonary cryptococcosis

Cryptococcal regulators of G protein signaling (CRG) are important for growth, differentiation, and virulence of Cryptococcus neoformans. Disruption of CRG2 resulted in dysregulated cAMP signaling and attenuated virulence, whereas disruption of CRG1 increased pheromone responses and enhanced virulence in the archetypal H99 strain. In tests with newly constructed near congenic mutants, a distinction between crg2Δ and crg1Δ gene expression was not apparent during macrophage interaction. Intranasal inoculation indicated that crg2Δ, crg1Δ, and wild-type strains reached the lungs within 0.5 hours of infection. However, CFUs were significantly decreased for crg2Δ at 2, 7, and 14 days post-infection. In contrast, crg1Δ proliferated to the same extent as the wild type (WT). Lung edema was not apparent in mice infected with crg2Δ 0.5 hours post-infection, which showed little cellular infiltrate in comparison to WT. Alveolar septal thickening was most evident in mice infected with crg1Δ, while mice infected with WT exhibited decreased septal thickening at later time points. Consistent with these observations, crg2Δ was less efficient in the elicitation of Th2 immune responses in a multiplex cytokine assay. Our results suggest that Crg2 is critical for establishment of early pulmonary infection and for persistence of infection, Crg1 regulates virulence in a strain-specific manner, and crg2Δ, crg1Δ and WT can all be distinguished on the basis of host tissue responses.

Pleiotropic function of intersectin homologue Cin1 in Cryptococcus neoformans

The manifestation of virulence traits in Cryptococcus neoformans is thought to rely on intracellular transport, a process not fully explored in this pathogenic fungus. Through interaction cloning, we identified a multi-modular protein, Cin1 (cryptococcal intersectin 1), whose domain structure is similar to that of the human endocytic protein ITSN1. Cin1 contains an N-terminal EH domain, a central coiled-coil region, a WH2 domain, two SH3 domains and a C-terminal RhoGEF (DH)-PH domain. Interestingly, alternative mRNA splicing resulted in two Cin1 isoforms, and Cin1 homologues are also restricted to basidiomycetous fungi. Disruption of the CIN1 gene had a pleiotropic effect on growth, normal cytokinesis, intracellular transports and the production of several virulence factors. Additionally, Cin1 interacts with cryptococcal Cdc42 and Wsp1 (a WASP homologue) proteins in vitro, suggesting a conserved role in the regulation of the actin cytoskeleton. However, deletion of RhoGEF or SH3 and RhoGEF domains did not result in any phenotypic changes, suggesting that functional redundancy exists in proteins containing similar domains or that the activities by other domains are necessary for Cin1 function. Our study presents the first evidence of a multi-modular protein whose function in intracellular transport underlies the growth, differentiation and virulence of a pathogenic microorganism.

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.

The beta subunit of the heterotrimeric G protein triggers the Kluyveromyces lactis pheromone response pathway in the absence of the gamma subunit

The Kluyveromyces lactis heterotrimeric G protein is a canonical Galphabetagamma complex; however, in contrast to Saccharomyces cerevisiae, where the Ggamma subunit is essential for mating, disruption of the KlGgamma gene yielded cells with almost intact mating capacity. Expression of a nonfarnesylated Ggamma, which behaves as a dominant-negative in S. cerevisiae, did not affect mating in wild-type and DeltaGgamma cells of K. lactis. In contrast to the moderate sterility shown by the single DeltaKlGalpha, the double DeltaKlGalpha DeltaKlGgamma mutant displayed full sterility. A partial sterile phenotype of the DeltaKlGgamma mutant was obtained in conditions where the KlGbeta subunit interacted defectively with the Galpha subunit. The addition of a CCAAX motif to the C-end of KlGbeta, partially suppressed the lack of both KlGalpha and KlGgamma subunits. In cells lacking KlGgamma, the KlGbeta subunit cofractionated with KlGalpha in the plasma membrane, but in the DeltaKlGalpha DeltaKlGgamma strain was located in the cytosol. When the KlGbeta-KlGalpha interaction was affected in the DeltaKlGgamma mutant, most KlGbeta fractionated to the cytosol. In contrast to the generic model of G-protein function, the Gbeta subunit of K. lactis has the capacity to attach to the membrane and to activate mating effectors in absence of the Ggamma subunit.

Remodeling of global transcription patterns of Cryptococcus neoformans genes mediated by the stress-activated HOG signaling pathways

The ability to sense and adapt to a hostile host environment is a crucial element for virulence of pathogenic fungi, including Cryptococcus neoformans. These cellular responses are evoked by diverse signaling cascades, including the stress-activated HOG pathway. Despite previous analysis of central components of the HOG pathway, its downstream signaling network is poorly characterized in C. neoformans. Here we performed comparative transcriptome analysis with HOG signaling mutants to explore stress-regulated genes and their correlation with the HOG pathway in C. neoformans. In this study, we not only provide important insights into remodeling patterns of global gene expression for counteracting external stresses but also elucidate novel characteristics of the HOG pathway in C. neoformans. First, inhibition of the HOG pathway increases expression of ergosterol biosynthesis genes and cellular ergosterol content, conferring a striking synergistic antifungal activity with amphotericin B and providing an excellent opportunity to develop a novel therapeutic method for treatment of cryptococcosis. Second, a number of cadmium-sensitive genes are differentially regulated by the HOG pathway, and their mutation causes resistance to cadmium. Finally, we have discovered novel stress defense and HOG-dependent genes, which encode a sodium/potassium efflux pump, protein kinase, multidrug transporter system, and elements of the ubiquitin-dependent system.

A constitutively active GPCR governs morphogenic transitions in Cryptococcus neoformans

Sex in fungi is driven by peptide pheromones sensed through seven-transmembrane pheromone receptors. In Cryptococcus neoformans, sexual reproduction occurs through an outcrossing/heterothallic a- sexual cycle or an inbreeding/homothallic - unisexual mating process. Pheromone receptors encoded by the mating-type locus (MAT) mediate reciprocal pheromone sensing during opposite-sex mating and contribute to but are not essential for unisexual mating. A pheromone receptor-like gene, CPR2, was discovered that is not encoded by MAT and whose expression is induced during a- mating. cpr2 mutants are fertile but have a fusion defect and produce abnormal hyphal structures, whereas CPR2 overexpression elicits unisexual reproduction. When heterologously expressed in Saccharomyces cerevisiae, Cpr2 activates pheromone responses in the absence of any ligand. This constitutive activity results from an unconventional residue, Leu(222), in place of a conserved proline in transmembrane domain six; a Cpr2(L222P) mutant is no longer constitutively active. Cpr2 engages the same G-protein activated signalling cascade as the Ste3a/alpha pheromone receptors, and thereby competes for pathway activation. This study established a new paradigm in which a naturally occurring constitutively active G protein-coupled receptor governs morphogenesis in fungi.

The LCB2 subunit of the sphingolip biosynthesis enzyme serine palmitoyltransferase can function as an attenuator of the hypersensitive response and Bax-induced cell death

Previous results showed that expression of the gene encoding the LONG-CHAIN BASE2 (LCB(2)) subunit of serine palmitoyltransferase (SPT), designated BcLCB(2), from nonheading Chinese cabbage (Brassica campestris ssp. chinensis) was up-regulated during hypersensitive cell death (HCD) induced by the Phytophthora boehmeriae elicitor PB90. Overexpression of BcLCB(2) in Nicotiana tabacum leaves suppressed the HCD normally initiated by elicitors and PB90-triggered H(2)O(2) accumulation. BcLCB(2) also functioned as a suppressor of mouse Bcl-2 associated X (Bax) protein-mediated HCD and cell death caused by Ralstonia solanacearum. BcLCB(2) overexpression suppressed Bax- and oxidant stress-triggered yeast cell death. Reactive oxygen species (ROS) accumulation induced by Bax was compromised in BcLCB(2)-overexpressing yeast cells. The findings that NbLCB(2) silencing in Nicotiana benthamiana enhanced elicitor-triggered HCD, combined with the fact that myriocin, a potent inhibitor of SPT, had no effect on Bax-induced programmed cell death, suggested that suppression of cell death was not involved in the dominant-negative effect that resulted from BcLCB(2) overexpression. A BcLCB(2) mutant assay showed that the suppression was not involved in SPT activity. The results suggest that plant HCD and stress-induced yeast cell death might share a common signal transduction pathway involving LCB(2), and that LCB(2) protects against cell death by inhibiting ROS accumulation, this inhibition being independent of SPT activity.

Magnificent seven: roles of G protein-coupled receptors in extracellular sensing in fungi

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors and are responsible for transducing extracellular signals into intracellular responses that involve complex intracellular-signaling networks. This review highlights recent research advances in fungal GPCRs, including classification, extracellular sensing, and G protein-signaling regulation. The involvement of GPCRs in pheromone and nutrient sensing has been studied extensively over the past decade. Following recent advances in fungal genome sequencing projects, a panoply of GPCR candidates has been revealed and some have been documented to play key roles sensing diverse extracellular signals, such as pheromones, sugars, amino acids, nitrogen sources, and even photons. Identification and deorphanization of additional putative GPCRs may require the development of new research tools. Here, we compare research on GPCRs in fungi with information derived from mammalian systems to provide a useful road map on how to better understand ligand-GPCR-G protein interactions in general. We also emphasize the utility of yeast as a discovery tool for systemic studies of GPCRs from other organisms.

The RGS protein Crg2 regulates both pheromone and cAMP signalling in Cryptococcus neoformans

G proteins orchestrate critical cellular functions by transducing extracellular signals into internal signals and controlling cellular responses to environmental cues. G proteins typically function as switches that are activated by G protein-coupled receptors (GPCRs) and negatively controlled by regulator of G protein signalling (RGS) proteins. In the human fungal pathogen Cryptococcus neoformans, three G protein alpha subunits (Gpa1, Gpa2 and Gpa3) have been identified. In a previous study, we identified the RGS protein Crg2 involved in regulating the pheromone response pathway through Gpa2 and Gpa3. In this study, a role for Crg2 was established in the Gpa1-cAMP signalling pathway that governs mating and virulence. We show that Crg2 physically interacts with Gpa1 and crg2 mutations increase cAMP production. crg2 mutations also enhance mating filament hyphae production, but reduce cell-cell fusion and sporulation efficiency during mating. Although crg2 mutations and the Gpa1 dominant active allele GPA1(Q284L) enhanced melanin production under normally repressive conditions, virulence was attenuated in a murine model. We conclude that Crg2 participates in controlling both Gpa1-cAMP-virulence and pheromone-mating signalling cascades and hypothesize it may serve as a molecular interface between these two central signalling conduits.

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.

The RGS protein Crg2 regulates pheromone and cyclic AMP signaling in Cryptococcus neoformans

Crg1 and Crg2 are regulators of G-protein signaling homologs found in the human fungal pathogen Cryptococcus neoformans. Crg1 negatively regulates pheromone responses and mating through direct inhibition of Galpha subunits Gpa2 and Gpa3. It has also been proposed that Crg2 has a role in mating, as genetic crosses involving Deltacrg2 mutants resulted in formation of hyperfilaments. We found that mutation of Gpa2 and Gpa3 partially suppressed the hyperfilamentation, mutation of Gpa3 alleviated Deltacrg2-specfic cell swelling, and mutation of the mitogen-activated protein kinase Cpk1 blocked both processes. These findings indicate that Gpa2 and Gpa3 function downstream of Crg2 and that Gpa3 is also epistatic to Crg2 in a Cpk1-dependent morphogenesis process linked to mating. Significantly, we found that Deltacrg2 mutants formed enlarged capsules that mimic cells expressing a constitutively active GPA1(Q284L) allele and that the levels of intracellular cyclic AMP (cAMP) were also elevated, suggesting that Crg2 also negatively regulates the Gpa1-cAMP signaling pathway. We further showed that Crg2 interacted with Gpa3 and Gpa1, but not Gpa2, in a pulldown assay and that Crg2 maintained a higher in vitro GTPase-activating protein activity toward Gpa3 and Gpa1 than to Gpa2. Finally, we found that dysregulation of cAMP due to the Crg2 mutation attenuated virulence in a murine model of cryptococcosis. Taken together, our study reveals Crg2 as an RGS (regulator of G-protein signaling) protein of multiregulatory function, including one that controls mating distinctly from Crg1 and one that serves as a novel inhibitor of Gpa1-cAMP signaling.