Molecular analysis of the Cryptococcus neoformans ADE2 gene, a selectable marker for transformation and gene disruption

3-bromopyruvate induces morphological alteration and may initiate programmed cell death in Cryptococcus neoformans cells

3-Bromopyruvate (3BP), known for its potent anticancer properties, also exhibits remarkable efficacy against the pathogenic fungus Cryptococcus neoformans. So far it has been proven that the main fungicidal activity of 3BP is based on ATP depletion and a reduction of intracellular level of glutathione. The presented study includes a broad range of methods to further investigate the mechanistic effects of 3BP on C. neoformans cells. The use of flow cytometry allowed a thorough examination of their survival during 3BP treatment, while observations using electron microscopy made it possible to note the changes in cellular morphology. Utilizing ruthenium red, the study suggests a mitochondrial pathway may initiate programmed cell death in response to 3BP. Analysis of free radical generation and gene expression changes supports this hypothesis. These findings enhance comprehension of 3BP's mechanisms in fungal cells, paving the way for its potential application as a therapeutic agent against cryptococcosis.

Genetic Transformation in Cryptococcus Species

Genetic transformation plays an imperative role in our understanding of the biology in unicellular yeasts and filamentous fungi, such as Saccharomyces cerevisiae, Aspergillus nidulans, Cryphonectria parasitica, and Magnaporthe oryzae. It also helps to understand the virulence and drug resistance mechanisms of the pathogenic fungus Cryptococcus that causes cryptococcosis in health and immunocompromised individuals. Since the first attempt at DNA transformation in this fungus by Edman in 1992, various methods and techniques have been developed to introduce DNA into this organism and improve the efficiency of homology-mediated gene disruption. There have been many excellent summaries or reviews covering the subject. Here we highlight some of the significant achievements and additional refinements in the genetic transformation of Cryptococcus species.

Development of an Agrobacterium-mediated transformation system for the cold-adapted fungi Pseudogymnoascus destructans and P. pannorum

The mechanisms of cold adaptation by fungi remain unknown. This topic is of high interest due to the emergence of white-nose syndrome (WNS), a skin infection of hibernating bats caused by Pseudogymnoascus destructans (Pd). Recent studies indicated that apart from Pd, there is an abundance of other Pseudogymnoascus species in the hibernacula soil. We developed an Agrobacterium tumefaciens-mediated transformation (ATMT) system for Pd and a related fungus Pseudogymnoascus pannorum (Pp) to advance experimental studies. URE1 gene encoding the enzyme urease was used as an easy to screen marker to facilitate molecular genetic analyses. A Uracil-Specific Excision Reagent (USER) Friendly pRF-HU2 vector containing Pd or Pp ure1::hygromycin (HYG) disruption cassette was introduced into A. tumefaciens AGL-1 cells by electroporation and the resulting strains were co-cultivated with conidia of Pd or Pp for various durations and temperatures to optimize the ATMT system. Overall, 680 Pd (0.006%) and 1800 Pp (0.018%) transformants were obtained from plating of 10(7) conidia; their recoveries were strongly correlated with the length of the incubation period (96h for Pd; 72h for Pp) and with temperature (15-18°C for Pd; 25°C for Pp). The homologous recombination in transformants was 3.1% for Pd and 16.7% for Pp. The availability of a standardized ATMT system would allow future molecular genetic analyses of Pd and related cold-adapted fungi.

A genomic safe haven for mutant complementation in Cryptococcus neoformans

Just as Koch’s postulates formed the foundation of early infectious disease study, Stanley Falkow’s molecular Koch’s postulates define best practice in determining whether a specific gene contributes to virulence of a pathogen. Fundamentally, these molecular postulates state that if a gene is involved in virulence, its removal will compromise virulence. Likewise, its reintroduction should restore virulence to the mutant. These approaches are widely employed in Cryptococcus neoformans, where gene deletion via biolistic transformation is a well-established technique. However, the complementation of these mutants is less straightforward. Currently, one of three approaches will be taken: the gene is reintroduced at the original locus, the gene is reintroduced into a random site in the genome, or the mutant is not complemented at all. Depending on which approach is utilized, the mutant may be complemented but other genes are potentially disrupted in the process. To counter the drawbacks of the current approaches to complementation we have created a new tool to assist in this key step in the study of a gene’s role in virulence. We have identified and characterized a small gene-free region in the C. neoformans genome dubbed the “safe haven”, and constructed a plasmid vector that targets DNA constructs to this preselected site. The plasmid vector integrates with high frequency, effectively complementing a mutant strain without disrupting adjacent genes. qRT-PCR of the flanking genes on either side of the safe haven site following integration of the targeting vector revealed no changes in their expression, and no secondary phenotypes were observed in a range of phenotypic assays including an intranasal murine infection model. Combined, these data confirm that we have successfully created a much-needed molecular resource for the Cryptococcus community, enabling the reliable fulfillment of the molecular Koch’s postulates.

Transgene induced co-suppression during vegetative growth in Cryptococcus neoformans

Introduction of DNA sequences into the genome often results in homology-dependent gene silencing in organisms as diverse as plants, fungi, flies, nematodes, and mammals. We previously showed in Cryptococcus neoformans that a repeat transgene array can induce gene silencing at a high frequency during mating (∼50%), but at a much lower frequency during vegetative growth (∼0.2%). Here we report a robust asexual co-suppression phenomenon triggered by the introduction of a cpa1::ADE2 transgene. Multiple copies of the cpa1::ADE2 transgene were ectopically integrated into the genome, leading to silencing of the endogenous CPA1 and CPA2 genes encoding the cyclosporine A target protein cyclophilin A. Given that CPA1-derived antisense siRNAs were detected in the silenced isolates, and that RNAi components (Rdp1, Ago1, and Dcr2) are required for silencing, we hypothesize that an RNAi pathway is involved, in which siRNAs function as trans factors to silence both the CPA1 and the CPA2 genes. The silencing efficiency of the CPA1 and CPA2 genes is correlated with the transgene copy number and reached ∼90% in the presence of >25 copies of the transgene. We term this transgene silencing phenomenon asexual co-suppression to distinguish it from the related sex-induced silencing (SIS) process. We further show that replication protein A (RPA), a single-stranded DNA binding complex, is required for transgene silencing, suggesting that RPA might play a similar role in aberrant RNA production as observed for quelling in Neurospora crassa. Interestingly, we also observed that silencing of the ADE2 gene occurred at a much lower frequency than the CPA1/2 genes even though it is present in the same transgene array, suggesting that factors in addition to copy number influence silencing. Taken together, our results illustrate that a transgene induced co-suppression process operates during C. neoformans vegetative growth that shares mechanistic features with quelling.

The development of gene transfer methods allows the production of transgenic lines in myriad eukaryotes. Frequently, transgenic DNA is integrated into the genome and transmitted as a heritable Mendelian trait. However, the introduced transgenes are in some cases not expressed (silenced). In addition, transgenes can also provoke silencing of endogenous genes with which they share sequence homology. This phenomenon was first observed in plants and named co-suppression. In fungi the best-documented co-suppression phenomenon occurs in vegetative tissue of the filamentous fungus Neurospora crassa and is termed quelling. Here we report a robust asexual co-suppression pathway that operates in the pathogenic fungus Cryptococcus neoformans and shares molecular components with quelling. Compared with the sex induced silencing (SIS) phenomenon previously discovered in C. neoformans, which efficiently silences genes during mating (∼50%) but not during vegetative growth (∼0.2%), asexual co-suppression operates efficiently during vegetative growth to suppress transgene expression and may also silence transposons and other repetitive sequences.

RNA interference in Cryptococcus neoformans

RNA interference (RNAi) is an experimental technique used to suppress individual gene expression in eukaryotic cells in a sequence-dependent manner. The process relies on double-stranded RNA (dsRNA) to target complementary messenger RNA for degradation. Here, we describe two plasmid-based strategies we have developed for RNAi in Cryptococcus neoformans. The pFrame vector utilizes the ACT1 promoter to enable the constitutive synthesis of hairpin RNA (hpRNA), the stem of which constitutes the dsRNA trigger. The pIBB103 vector relies on convergent, inducible GAL7 promoters to independently drive the synthesis of the sense and antisense strands of the interfering sequence; these strands anneal to form the initiating dsRNA molecule. Both vectors are designed to co-silence a "sentinel" gene with an easily scored phenotype to help identify clones in which RNAi is most effective. We provide guidelines for selecting a suitable interfering sequence to trigger RNAi in C. neoformans and describe the steps for subcloning into either vector, transforming C. neoformans by electroporation, screening clones for RNAi-related phenotypes, and evaluating the efficacy and specificity of gene silencing by RNAi.

Efficient implementation of RNA interference in the pathogenic yeast Cryptococcus neoformans

An improved method has been developed for RNA interference in Cryptococcus neoformans, using opposing promoters to facilitate cloning and RNA interference targeting URA5 to allow selection of cells in which silencing is most effective. These advances significantly reduce the variability of silencing and the effort required for interference plasmid construction.

A tetrad analysis of the basidiomycete fungus Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycete fungus that is found worldwide and causes disease in humans and animal species. The fungus grows asexually as a budding yeast. Under laboratory conditions it is capable of sexual reproduction between two mating types. After cell fusion a dikaryotic filament develops, at the tip of which a basidium gives rise to four chains of basidiospores. Because the chains each comprise 10-30 spores, rather than single spores, the analysis of individual meiotic events has not been attempted in C. neoformans in the style of tetrad analyses performed in other fungal species. Here, the basidiospores from >100 basidia were micromanipulated and the resultant >2500 progeny analyzed for three genetic markers to understand the sexual process in this fungus, leading to four observations: (i) Marker segregation provides genetic evidence for a single meiotic event within the basidium followed by multiple rounds of mitosis. (ii) Using each basidium as an unordered tetrad, the ADE2 and URA5 genes are linked to their centromeres, consistent with adjacent genomic regions rich in repetitive elements predicted to comprise Cryptococcus centromeres. (iii) Lack of germination of basidiospores is attributed to aneuploidy, rather than dormancy. (iv) Analysis of basidiospores derived from single chains demonstrates that each chain can contain different genotypes. This mechanism of sexual spore production would benefit the species with a high rate of dispersal and at the same time aid in simultaneous dissemination of both mating types to new locations in the environment.

Many globally isolated AD hybrid strains of Cryptococcus neoformans originated in Africa

Interspecific and intervarietal hybridization may contribute to the biological diversity of fungal populations. Cryptococcus neoformans is a pathogenic yeast and the most common fungal cause of meningitis in patients with AIDS. Most patients are infected with either of the two varieties of C. neoformans, designated as serotype A (C. neoformans var. grubii) or serotype D (C. neoformans var. neoformans). In addition, serotype AD strains, which are hybrids of these two varieties, are commonly isolated from clinical and environmental samples. While most isolates of serotype A and serotype D are haploid, AD strains are diploid or aneuploid, and contain two sets of chromosomes and two mating type alleles, MATa and MATα, one from each of the serotypes. The global population of serotype A is dominated by isolates with the MATα mating type (Aα); however, about half of the globally analyzed AD strains possess the extremely rare serotype A MATa allele (Aa). We previously described an unusual population of serotype A in Botswana, in which 25% of the strains contain the rare MATa allele. Here we utilized two methods, phylogenetic analysis of three genes and genotyping by scoring amplified fragment length polymorphisms, and discovered that AD hybrid strains possessing the rare serotype A MATa allele (genotype AaDα) cluster with isolates of serotype A from Botswana, whereas AD hybrids that possess the MATα serotype A allele (AαDa and AαDα) cluster with cosmopolitan isolates of serotype A. We also determined that AD hybrid strains are more resistant to UV irradiation than haploid serotype A strains from Botswana. These findings support two hypotheses: (i) AaDα strains originated in sub-Saharan Africa from a cross between strains of serotypes A and D; and (ii) this fusion produced hybrid strains with increased fitness, enabling the Botswanan serotype A MATa genome, which is otherwise geographically restricted, to survive, emigrate, and propagate throughout the world.

Hybridization between individuals of different species or varieties is common among fungi. However, the impact of hybridization on the evolution of pathogenic fungi is unresolved. Several hybrids of phytopathogenic fungi exhibit expanded host ranges. To our knowledge, this report is the first description of increased hybrid fitness (hybrid vigor) in a human pathogen, Cryptococcus neoformans, the most prevalent cause of fungal meningitis. We demonstrate that diploid hybrid strains are common among both environmental and clinical isolates of two varieties, represented by serotypes A and D. We determined that many globally isolated AD hybrid strains originated in sub-Saharan Africa and have increased resistance to ultraviolet radiation. We hypothesize that hybrid strains have increased fitness, which enabled them to emigrate from Africa and spread globally.

A new hexose transporter from Cryptococcus neoformans: molecular cloning and structural and functional characterization

We carried out a screen for Cryptococcus neoformans genes involved in resistance to copper ion toxicity and identified a new hexose transporter (Hxt) gene, HXT1. Hxt1 consists of 520 amino acids and functions to transport hexoses such as glucose. Although Hxt1 conferred copper resistance to Saccharomyces cerevisiae, disruption of the HXT1 gene showed that Hxt1 is not necessary for copper resistance. In virulence tests, an hxt1 mutant strain showed 12% less phenoloxidase activity than the wild-type strain, and no difference in the ability to form melanin was identified. In addition, the hxt1 mutant strain showed virulence similar to that of the wild-type strain in experiments with Caenorhabditis elegans. However, the hxt1 mutant strain generated larger capsules than were generated by the wild-type strain. Thus, Hxt1 appears to be involved in capsule formation.

Cas3p belongs to a seven-member family of capsule structure designer proteins

The polysaccharide capsule is the main virulence factor of the basidiomycetous yeast Cryptococcus neoformans. Four genes (CAP10, CAP59, CAP60, and CAP64) essential for capsule formation have been previously identified, although their roles in the biosynthetic pathway remain unclear. A genetic and bioinformatics approach allowed the identification of six CAP64-homologous genes, named CAS3, CAS31, CAS32, CAS33, CAS34, and CAS35, in the C. neoformans genome. This gene family is apparently specific in a subclass of the basidiomycete fungi. Single as well as double deletions of these genes in all possible combinations demonstrated that none of the CAP64-homologous genes were essential for capsule formation, although the cas35Delta strains displayed a hypocapsular phenotype. The chemical structure of the glucuronomannan (GXM) produced by the CAS family deletants revealed that these genes determined the position and the linkage of the xylose and/or O-acetyl residues on the mannose backbone. Hence, these genes are all involved in assembly of the GXM structure in C. neoformans.

A Sch9 protein kinase homologue controlling virulence independently of the cAMP pathway in Cryptococcus neoformans

The polysaccharide capsule is one of the established virulence factors in Cryptococcus neoformans that provides a barrier against the host-mediated immune response. Mutation of the gene encoding the Saccharomyces cerevisiae Sch9 protein kinase homologue resulted in cells with enlarged capsules in C. neoformans. Capsule production was abrogated in sch9 pka1 double mutants, indicating that protein kinase A (PKA) signaling is still necessary for capsule formation in sch9 mutants. The sch9 mutant also exhibited increased thermal tolerance, a phenotype similar to sch9 mutant strains of S. cerevisiae. In addition, the sch9 mutant was attenuated in mating and the highly encapsulated cells were attenuated in virulence, in contrast to the pkr1 mutant, lacking the regulatory subunit of protein kinase A, that produced similarly enlarged capsules yet was increased in virulence. Interestingly, the virulence for the sch9 mutant strain could be restored by introduction of a pkr1 mutation; and the sch9 pkr1 mutant strain was dramatically increased in size and capsule thickness, suggesting that Sch9 and PKA function via different targets involved in virulence. Our findings support a model in which Sch9 modulates capsule formation and contributes to the virulence of C. neoformans both independently of and in conjunction with the cAMP-PKA pathway.

Cyclic AMP-dependent protein kinase catalytic subunits have divergent roles in virulence factor production in two varieties of the fungal pathogen Cryptococcus neoformans

Our earlier findings established that cyclic AMP-dependent protein kinase functions in a signaling cascade that regulates mating and virulence of Cryptococcus neoformans var. grubii (serotype A). Mutants lacking the serotype A protein kinase A (PKA) catalytic subunit Pka1 are unable to mate, fail to produce melanin or capsule, and are avirulent in animal models, whereas mutants lacking the PKA regulatory subunit Pkr1 overproduce capsule and are hypervirulent. Because other mutations have been observed to confer different phenotypes in two diverged varieties of C. neoformans (grubii variety [serotype A] and neoformans variety [serotype D]), we analyzed the functions of the PKA genes in the serotype D neoformans variety. Surprisingly, the Pka1 catalytic subunit was not required for mating, haploid fruiting, or melanin or capsule production of serotype D strains. Here we identify a second PKA catalytic subunit gene, PKA2, that is present in both serotype A and D strains of C. neoformans. The divergent Pka2 catalytic subunit was found to regulate mating, haploid fruiting, and virulence factor production in serotype D strains. In contrast, Pka2 has no role in mating, melanin production, or capsule formation in serotype A strains. Our studies illustrate how different components of signaling pathways can be co-opted and functionally specialized during the evolution of related but distinct varieties or subspecies of a human fungal pathogen.

Genetics of Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus that primarily afflicts immunocompromised patients, infecting the central nervous system to cause meningoencephalitis that is uniformly fatal if untreated. C. neoformans is a basidiomycetous fungus with a defined sexual cycle that has been linked to differentiation and virulence. Recent advances in classical and molecular genetic approaches have allowed molecular descriptions of the pathways that control cell type and virulence. An ongoing genome sequencing project promises to reveal much about the evolution of this human fungal pathogen into three distinct varieties or species. C. neoformans shares features with both model ascomycetous yeasts (Saccharomyces cerevisiae, Schizosaccharomyces pombe) and basidiomycetous pathogens and mushrooms (Ustilago maydis, Coprinus cinereus, Schizophyllum commune), yet ongoing studies reveal unique features associated with virulence and the arrangement of the mating type locus. These advances have catapulted C. neoformans to center stage as a model of both fungal pathogenesis and the interesting approaches to life that the kingdom of fungi has adopted.

A MAP kinase cascade composed of cell type specific and non-specific elements controls mating and differentiation of the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle in which the alpha allele of the mating type locus is linked to virulence and haploid differentiation. Here we analysed a conserved MAP kinase cascade composed of mating-type specific (Ste11alpha, Ste12alpha) and non-specific (Ste7, Cpk1) elements. Gene disruption experiments demonstrate that this specialized MAP kinase pathway is required for both mating and cell type-specific differentiation but not for virulence. The Ste11alpha, Ste7 and Cpk1 kinases were found to act as a co-ordinate signalling module, whereas the Ste12alpha transcription factor functions with a redundant partner or in a branched or parallel signalling pathway. Our studies illustrate how MAP kinase cascades can be constructed from cell type-specific and non-specific components, yielding pathways that contribute to cell type-specific patterns of signalling and differentiation.

Role of mitochondrial carrier protein Mrs3/4 in iron acquisition and oxidative stress resistance of Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic basidiomycete that causes meningitis in immunocompromised patients. In this paper, we demonstrate that a previously described oxidant-sensitive mutant, oxy1, has constitutive ferric reductase and iron uptake, similar to a ferric reductase regulatory mutant, frr1. Through meiotic genetic analysis, we show that the two mutations are allelic. By complementation of frr1 with a genomic library, we isolated a gene, MRS3/4. The encoded protein is a putative solute transporter of the inner mitochondrial membrane. Disruption of this gene led to high ferric reductase, iron uptake and iron content, as well as increased sensitivity to hydrogen peroxide and slow growth in low iron medium. The disrupted gene is allelic with oxy1 and frr1. We sequenced the oxy1 and frr1 alleles of MRS3/4 and found that the frr1 mutation results in a premature stop codon, while the oxy1 mutation results in the substitution of a highly conserved glutamate residue with lysine. The Mrs3/4 protein appears to be involved in mitochondrial iron transport in eukaryotes. Resistance to strong oxidants requires stringent control of iron metabolism.

Killing of Caenorhabditis elegans by Cryptococcus neoformans as a model of yeast pathogenesis

We found that the well-studied nematode Caenorhabditis elegans can use various yeasts, including Cryptococcus laurentii and Cryptococcus kuetzingii, as a sole source of food, producing similar brood sizes compared with growth on its usual laboratory food source Escherichia coli OP50. C. elegans grown on these yeasts had a life span similar to (C. laurentii) or longer than (C. kuetzingii) those fed on E. coli. However, the human pathogenic yeast Cryptococcus neoformans killed C. elegans, and the C. neoformans polysaccharide capsule as well as several C. neoformans genes previously shown to be involved in mammalian virulence were also shown to play a role in C. elegans killing. These included genes associated with signal transduction pathways (GPA1, PKA1, PKR1, and RAS1), laccase production (LAC1), and the alpha mating type. C. neoformans adenine auxotrophs, which are less virulent in mammals, were also less virulent in C. elegans. These results support the model that mammalian pathogenesis of C. neoformans may be a consequence of adaptations that have evolved during the interaction of C. neoformans with environmental predators such as free-living nematodes and amoebae and suggest that C. elegans can be used as a simple model host in which C. neoformans pathogenesis can be readily studied.

A PCR-based strategy to generate integrative targeting alleles with large regions of homology

Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle for which genetic and molecular techniques are well developed. The entire genome sequence of one C. neoformans strain is nearing completion. The efficient use of this sequence is dependent upon the development of methods to perform more rapid genetic analysis including gene-disruption techniques. A modified PCR overlap technique to generate targeting constructs for gene disruption that contain large regions of gene homology is described. This technique was used to disrupt or delete more than a dozen genes with efficiencies comparable to those previously reported using cloning technology to generate targeting constructs. Moreover, it is shown that disruptions can be made using this technique in a variety of strain backgrounds, including the pathogenic serotype A isolate H99 and recently characterized stable diploid strains. In combination with the availability of the complete genomic sequence, this gene-disruption technique should pave the way for higher throughput genetic analysis of this important pathogenic fungus.

Mating-type-specific and nonspecific PAK kinases play shared and divergent roles in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle involving fusion of haploid MATalpha and MATa cells. Virulence has been linked to the mating type, and MATalpha cells are more virulent than congenic MATa cells. To study the link between the mating type and virulence, we functionally analyzed three genes encoding homologs of the p21-activated protein kinase family: STE20alpha, STE20a, and PAK1. In contrast to the STE20 genes that were previously shown to be in the mating-type locus, the PAK1 gene is unlinked to the mating type. The STE20alpha, STE20a, and PAK1 genes were disrupted in serotype A and D strains of C. neoformans, revealing central but distinct roles in mating, differentiation, cytokinesis, and virulence. ste20alpha pak1 and ste20a pak1 double mutants were synthetically lethal, indicating that these related kinases share an essential function. In summary, our studies identify an association between the STE20alpha gene, the MATalpha locus, and virulence in a serotype A clinical isolate and provide evidence that PAK kinases function in a MAP kinase signaling cascade controlling the mating, differentiation, and virulence of this fungal pathogen.

An auxotrophic pigmented Cryptococcus neoformans strain causing infection of the bone marrow

Cryptococcosis, caused by an encapsulated fungus, Cryptococcus neoformans, has emerged as a life-threatening infection in HIV-positive individuals and other immunocompromised hosts. This report describes an unusual strain of C. neoformans isolated from an AIDS patient that developed pigment on Sabouraud's medium. The yeast was auxotrophic for adenine due to a deletion in the coding region of ADE2, and was complemented by introduction of a functional copy of the ADE2 gene from C. neoformans. The yeast had an unusual myelotropism that was clinically evident as a pancytopenia with displacement of bone marrow precursors by yeast cells, and it had an unusual spectrum of infection in the human host. This is the first description of a nutritional auxotroph of C. neoformans isolated from a patient.

Adenylyl cyclase functions downstream of the Galpha protein Gpa1 and controls mating and pathogenicity of Cryptococcus neoformans

The signaling molecule cyclic AMP (cAMP) is a ubiquitous second messenger that enables cells to detect and respond to extracellular signals. cAMP is generated by the enzyme adenylyl cyclase, which is activated or inhibited by the Galpha subunits of heterotrimeric G proteins in response to ligand-activated G-protein-coupled receptors. Here we identified the unique gene (CAC1) encoding adenylyl cyclase in the opportunistic fungal pathogen Cryptococcus neoformans. The CAC1 gene was disrupted by transformation and homologous recombination. In stark contrast to the situation for Saccharomyces cerevisiae, in which adenylyl cyclase is essential, C. neoformans cac1 mutant strains were viable and had no vegetative growth defect. Furthermore, cac1 mutants maintained the yeast-like morphology of wild-type cells, in contrast to the constitutively filamentous phenotype found upon the loss of adenylyl cyclase in another basidiomycete pathogen, Ustilago maydis. Like C. neoformans mutants lacking the Galpha protein Gpal, cac1 mutants were mating defective and failed to produce two inducible virulence factors: capsule and melanin. As a consequence, cac1 mutant strains were avirulent in animal models of cryptococcal meningitis. Reintroduction of the wild-type CAC1 gene or the addition of exogenous cAMP suppressed cac1 mutant phenotypes. Moreover, the overexpression of adenylyl cyclase restored mating and virulence factor production in gpal mutant strains. Physiological studies revealed that the Galpha protein Gpa1 and adenylyl cyclase controlled cAMP production in response to glucose, and no cAMP was detectable in extracts from cac1 or gpa1 mutant strains. These findings provide direct evidence that Gpal and adenylyl cyclase function in a conserved signal transduction pathway controlling cAMP production, hyphal differentiation, and virulence of this human fungal pathogen.

Cas1p is a membrane protein necessary for the O-acetylation of the Cryptococcus neoformans capsular polysaccharide

The capsule is certainly the most obvious virulence factor for Cryptococcus neoformans. The main capsule constituents are glucuronoxylomannans (GXM). Several studies have focused on the structure and chemistry of the GXM component of the capsule, yet little is known about the genetic basis of the capsule construction. Using a monoclonal antibody specific to a sugar epitope, we isolated a capsule-structure mutant strain and cloned by complementation a gene named CAS1 that codes for a putative membrane protein. Although no sequence homology was found with any known protein in the different databases, protein analysis using the PROPSEARCH software classified Cas1p as a putative glycosyltransferase. Cas1p is a well-conserved evolutionary protein, as we identified one orthologue in the human genome, one in the drosophila genome and four in the plant Arabidopsis thaliana genome. Analysis of the capsule structure after CAS1 deletion showed that it is required for GXM O-acetylation.

Identification and characterization of the Cryptococcus neoformans phosphomannose isomerase-encoding gene, MAN1, and its impact on pathogenicity

The polysaccharide capsule surrounding Cryptococcus neoformans comprises manose, xylose and glucuronic acid, of which mannose is the major constituent. The GDP-mannose biosynthesis pathway is highly conserved in fungi and consists of three key enzymes: phosphomannose isomerase (PMI), phosphomannomutase (PMM) and GDP-mannose pyrophosphorylase (GMP). The MAN1 gene, encoding for the PMI enzyme, was isolated and sequenced from C. neoformans, and a disruption of the MAN1 gene was generated. One MAN1 disruption mutant, man1, which showed poor capsule formation, reduced polysaccharide secretion and morphological abnormalities, was chosen for virulence studies. In both the rabbit and the mouse models of invasive cryptococcosis, man1 was shown to be severely impaired in its virulence, with complete elimination of the yeast from the host. A reconstituted strain of man1 was constructed using gene replacement at the native locus. The wild-type and reconstituted strains were significantly more virulent than the knock-out mutant in both animal models. Our findings reveal that PMI activity is essential for the survival of C. neoformans in the host. The fact that the man1 mutant was not pathogenic suggests that blocking mannose synthesis could be fungicidal in the mammalian host and thus an excellent target for antifungal drug development.

Two cyclophilin A homologs with shared and distinct functions important for growth and virulence of Cryptococcus neoformans

Cyclophilin A is the target of the immunosuppressant cyclosporin A (CsA) and is encoded by a single unique gene conserved from yeast to humans. In the pathogenic fungus Cryptococcus neoformans, two homologous linked genes, CPA1 and CPA2, were found to encode two conserved cyclophilin A proteins. In contrast to Saccharomyces cerevisiae, in which cyclophilin A mutations confer CsA resistance but few other phenotypes, cyclophilin A mutations conferred dramatic phenotypes in C. neoformans. The Cpa1 and Cpa2 cyclophilin A proteins play a shared role in cell growth, mating, virulence and CsA toxicity. The Cpa1 and Cpa2 proteins also have divergent functions. cpa1 mutants are inviable at 39 degrees C and attenuated for virulence, whereas cpa2 mutants are viable at 39 degrees C and fully virulent. cpa1 cpa2 double mutants exhibited synthetic defects in growth and virulence. Cyclophilin A active site mutants restored growth of cpa1 cpa2 mutants at ambient but not at higher temperatures, suggesting that the prolyl isomerase activity of cyclophilin A has an in vivo function.

Molecular cloning, phylogenetic analysis and three-dimensional modeling of Cu,Zn superoxide dismutase ( CnSOD1 ) from three varieties of Cryptococcus neoformans

Cryptococcus neoformans (Cn), causal agent of fungal meningoencephalitis, has three varieties with variable host predilection. To explore mechanisms for these pathogenic differences, we have characterized Cu,Zn SOD gene (CnSOD1). A Saccharomyces cerevisiae sod1Delta mutant was complemented with Cn var. grubii yeast expression library. The complementing clone had an ORF of 462 bp and the deduced 154 aa sequence showed 61% identity with S. cerevisiae SOD1 and 53-65% with other eukaryotic SOD1s. Cn var. grubii CnSOD1 cDNA was used to clone corresponding cDNAs from var. neoformans and var. gattii. ORFs from three varieties revealed 20-29% differences in deduced aa (s) with a significant 6% non-synonymous aa substitution between Cn var. grubii and Cn var. gattii. Cosmid library screening and PCR cloning were used to obtain genomic SOD1, which was split by five introns with identical placements and a typical 5' splice junction sequence, GTNNGY. These introns also showed a large nt variation among the three Cn varieties. Phylogenetic analyses revealed CnSOD1 to be in a group distinct from other eukaryotic SOD1s and with a significant divergence of the var. grubii from var. gattii. The CnSOD1 -deduced protein was modeled based on the crystal structure of S. cerevisiae SOD1, which showed an excellent fit. Most of the non-synonymous aa substitutions occurred on the outside of the molecule and these may contribute to differences in antigenicity among the three varieties. Notably, Cn var. neoformans and var. gattii Cu,Zn SOD had three substitutions of glycine (Gly26, Gly92 and Gly123 for Asn26, Ser92 and Ser123) that may contribute to the observed lower thermostability of this enzyme vis-a-vis Cn var. grubii. This is the first nucleotide and structural comparison of a protein-encoding gene from the three Cn varieties, which may provide a framework for future studies on the role of Cu,Zn SOD in Cn pathogenesis.

Cyclic AMP-dependent protein kinase controls virulence of the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that infects the human central nervous system. This pathogen elaborates two specialized virulence factors: the antioxidant melanin and an antiphagocytic immunosuppressive polysaccharide capsule. A signaling cascade controlling mating and virulence was identified. The PKA1 gene encoding the major cyclic AMP (cAMP)-dependent protein kinase catalytic subunit was identified and disrupted. pka1 mutant strains were sterile, failed to produce melanin or capsule, and were avirulent. The PKR1 gene encoding the protein kinase A (PKA) regulatory subunit was also identified and disrupted. pkr1 mutant strains overproduced capsule and were hypervirulent in animal models of cryptococcosis. pkr1 pka1 double mutant strains exhibited phenotypes similar to that of pka1 mutants, providing epistasis evidence that the Pka1 catalytic subunit functions downstream of the Pkr1 regulatory subunit. The PKA pathway was also shown to function downstream of the Galpha protein Gpa1 and to regulate cAMP production by feedback inhibition. These findings define a Galpha protein-cAMP-PKA signaling pathway regulating differentiation and virulence of a human fungal pathogen.

A new dominant selectable marker for use in Cryptococcus neoformans

Cryptococcus neoformans is an excellent model system for studies on the molecular pathogenesis of fungal infections. There is only one dominant selectable market that can be used in the transformation of this organism, and we wanted to develop another. We found that various strains of C. neoformans are very sensitive to the aminoglycoside antibiotic nourseothricin, and that spontaneous resistance to this drug must be an extremely rare event. Resistance to nourseothricin is conferred by the product of the nourseothricin acetyltransferase gene (nat1) from Streptomyces noursei. In order to express this gene in C. neoformans, we created a fusion construct by driving expression of natl with the promoter sequence from a C. neoformans actin gene. Biolistic transformation of the serotype A C. neoformans strain H99 and the serotype D strain JEC21 with this construct resulted in transformation efficiencies of approximately 1,000 transformants microg(-1) of DNA and 20 transformants microg(-1) of DNA, respectively. Southern blots were performed using DNA from some of the H99 transformants, and this confirmed that all of the resistant isolates had the construct integrated in a random fashion within the genome. There was no cross-resistance of the nourseothricin-resistant transformants to hygromycin B, which is the other antibiotic used as a dominant selection marker in C. neoformans. The development of nourseothricin resistance as a second dominant selectable market will be helpful in future molecular studies on this important pathogenic fungus.

Identification and characterization of a highly conserved calcineurin binding protein, CBP1/calcipressin, in Cryptococcus neoformans

Calcineurin is the conserved target of the immunosuppressants cyclosporin A and FK506. Using the yeast two-hybrid system, we identified a novel calcineurin binding protein, CBP1, from the pathogenic fungus Cryptococcus neoformans. We show that CBP1 binds to calcineurin in vitro and in vivo, and FKBP12-FK506 inhibits CBP1 binding to calcineurin. Cryptococcus neoformans cbp1 mutant strains exhibit modest defects in growth under stress conditions and virulence, similar to but less severe than the phenotypes of calcineurin mutants. Saccharomyces cerevisiae mutants lacking the CBP1 homolog RCN1 are, like calcineurin mutants, sensitive to lithium cation stress. CBP1 shares a central peptide sequence motif, SPPxSPP, with related proteins in S.CEREVISIAE:, Schizosaccharomyces pombe, Drosophila melanogaster, Caenorhabditis elegans and humans, and peptides containing this motif altered calcineurin activity in vitro. Interestingly, the human CBP1 homolog DSCR1 is encoded by the Down's syndrome candidate region interval on chromosome 21, is highly expressed in the heart and central nervous system, and may play a role in calcineurin functions in heart development, neurite extension and memory.

RAS1 regulates filamentation, mating and growth at high temperature of Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycete yeast and opportunistic human pathogen of increasing clinical importance due to the increasing population of immunocompromised patients. To further investigate signal transduction cascades regulating fungal pathogenesis, we have identified the gene encoding a RAS homologue in this organism. The RAS1 gene was disrupted by transformation and homologous recombination. The resulting ras1 mutant strain was viable, but failed to grow at 37 degrees C, and exhibited significant defects in mating and agar adherence. The ras1 mutant strain was also avirulent in an animal model of cryptococcal meningitis. Reintroduction of the wild-type RAS1 gene complemented these ras1 mutant phenotypes and restored virulence in animals. A dominantly active RAS1 mutant allele, RAS1Q67L, induced a differentiation phenotype known as haploid fruiting, which involves filamentation, agar invasion and sporulation in response to nitrogen deprivation. The ras1 mutant mating defect was suppressed by overexpression of MAP kinase signalling elements and partially suppressed by exogenous cAMP. Additionally, cAMP also suppressed the agar adherence defect of the ras1 mutant. However, the ability of the ras1 mutant strain to grow at elevated temperature was not restored by cAMP or MAP kinase overexpression. Our findings support a model in which RAS1 signals in C. neoformans through cAMP-dependent, MAP kinase, and RAS-specific signalling cascades to regulate mating and filamentation, as well as growth at high temperature which is necessary for maintenance of infection.

Gene disruption by biolistic transformation in serotype D strains of Cryptococcus neoformans

Gene disruption by biolistic transformation in serotype D strains of Cryptococcus neoformans. Fungal Genetics and Biology 29, 38-48. Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle and well-developed genetic and molecular approaches. Two different transformation systems have been developed, and a number of genes have been disrupted by homologous recombination. However, the frequency of homologous recombination achieved by these approaches has differed dramatically between strains of the A and D serotypes. Transformation by electroporation in serotype D strains results in homologous recombination at frequencies of 1/1000 to 1/100,000, whereas transformation by the biolistic method has resulted in gene disruption at frequencies between 2 and 50% in serotype A strains. We find that gene disruption by homologous recombination can be achieved in the congenic serotype D strain series by biolistic transformation with frequencies of approximately 1 to 4%. By this approach, we have readily disrupted the genes encoding a MAPK homolog (CPK1), the calcineurin A catalytic subunit (CNA1), and a G protein alpha subunit (GPA1). By physical and genetic methods, we show that these mutations result from targeted recombination events without ectopic integrations. Because genetic approaches can be applied in the congenic serotype D strains, our observations represent a significant advance in molecular approaches to understand the physiology and virulence of this important human pathogen.

Comparison of the roles of calcineurin in physiology and virulence in serotype D and serotype A strains of Cryptococcus neoformans

The calcineurin gene was cloned and disrupted in serotype D strains of Cryptococcus neoformans. Serotype A and serotype D calcineurin mutants were inviable at 37 degrees C and avirulent in mice, whereas only serotype A mutants were cation stress sensitive. Thus, calcineurin plays conserved and divergent roles in serotype A and serotype D strains.

The G-protein beta subunit GPB1 is required for mating and haploid fruiting in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle. The gene encoding a heterotrimeric G-protein beta subunit, GPB1, was cloned and disrupted. gpb1 mutant strains are sterile, indicating a role for this gene in mating. GPB1 plays an active role in mediating responses to pheromones in early mating steps (conjugation tube formation and cell fusion) and signals via a mitogen-activated protein (MAP) kinase cascade in both MATalpha and MATa cells. The functions of GPB1 are distinct from those of the Galpha protein GPA1, which functions in a nutrient-sensing cyclic AMP (cAMP) pathway required for mating, virulence factor induction, and virulence. gpb1 mutant strains are also defective in monokaryotic fruiting in response to nitrogen starvation. We show that MATa cells stimulate monokaryotic fruiting of MATalpha cells, possibly in response to mating pheromone, which may serve to disperse cells and spores to locate mating partners. In summary, the Gbeta subunit GPB1 and the Galpha subunit GPA1 function in distinct signaling pathways: one (GPB1) senses pheromones and regulates mating and haploid fruiting via a MAP kinase cascade, and the other (GPA1) senses nutrients and regulates mating, virulence factors, and pathogenicity via a cAMP cascade.