Cyclic AMP signaling in Cryptococcus neoformans

As pathogenic microorganisms establish an infection, they must be able to sense host-specific signals and respond by elaborating determinants that allow for survival in these hostile conditions. Pathogen cell surface proteins detect these signals and activate signal transduction cascades that ultimately alter gene expression resulting in an adaptive cellular response. Here we review the mechanisms by which a pathogenic fungus uses the highly conserved cAMP signal transduction pathway to regulate cellular differentiation as well as its virulence potential.

Ras1 controls pheromone expression and response during mating in Cryptococcus neoformans

The Cryptococcus neoformans Ras1 signal transduction pathway controls mating, hyphal differentiation, and the ability of this opportunistic human fungal pathogen to grow at elevated temperatures. To further elucidate how Ras1 signals in this organism, the RAS1 gene was disrupted in the congenic serotype D strain background. Genetic epistasis experiments indicated that Ras1 regulates the mating response through the MAP kinase/pheromone response pathway. In fact, Ras1 is required for the transcriptional induction of elements of the pheromone response pathway. However, the ability of C. neoformans Ras1 to allow growth at 37 degrees C is mediated by a separate signaling pathway. Therefore a single Ras protein may differentially activate distinct downstream targets in response to different signals within the same organism. This conserved signaling motif has been coopted in C. neoformans to regulate mating and morphogenesis in addition to being required for its pathogenic potential.

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.

Ras1 and Ras2 contribute shared and unique roles in physiology and virulence of Cryptococcus neoformans

The Ras1 signal transduction pathway controls the ability of the pathogenic fungus Cryptococcus neoformans to grow at high temperatures and to mate. A second RAS gene was identified in this organism. RAS2 is expressed at a very low level compared to RAS1, and a ras2 mutation caused no alterations in vegetative growth rate, differentiation or virulence factor expression. The ras2 mutant strain was equally virulent to the wild-type strain in the murine inhalational model of cryptococcosis. Although a ras1 ras2 double mutant strain is viable, mutation of both RAS genes results in a decreased growth rate at all temperatures compared to strains with either single mutation. Overexpression of the RAS2 gene completely suppressed the ras1 mutant mating defect and partially suppressed its high temperature growth defect. After prolonged incubation at a restrictive temperature, the ras1 mutant demonstrated actin polarity defects that were also partially suppressed by RAS2 overexpression. These studies indicate that the C. neoformans Ras1 and Ras2 proteins share overlapping functions, but also play distinct signalling roles. Our findings also suggest a mechanism by which Ras1 controls growth of this pathogenic fungus at 37 degrees C, supporting a conserved role for Ras homologues in microbial cellular differentiation, morphogenesis and virulence.

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.

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.

The STE12alpha homolog is required for haploid filamentation but largely dispensable for mating and virulence in Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes meningitis in immunocompromised hosts. The organism has a known sexual cycle, and strains of the MATalpha mating type are more virulent than isogenic MATa strains in mice, and they are more common in the environment and infected hosts. A C. neoformans homolog of the STE12 transcription factor that regulates mating, filamentation, and virulence in Saccharomyces cerevisiae and Candida albicans was identified previously, found to be encoded by a novel region of the MATalpha mating type locus, and shown to enhance filamentous growth when overexpressed. We have disrupted the C. neoformans STE12 gene in a pathogenic serotype A isolate. ste12 mutant strains exhibit a severe defect in filamentation and sporulation (haploid fruiting) in response to nitrogen starvation. In contrast, ste12 mutant strains have only modest mating defects and are fully virulent in two animal models compared to the STE12 wild-type strain. In genetic epistasis experiments, STE12 functions in a MAP kinase cascade to regulate fruiting, but not mating. Thus, the C. neoformans STE12alpha transcription factor homolog plays a specialized function in haploid fruiting, but it is dispensable or redundant for mating and virulence. The association of the MATalpha locus with virulence may involve additional genes, and other transcription factors that regulate mating and virulence remain to be identified.

On the origins of congenic MATalpha and MATa strains of the pathogenic yeast Cryptococcus neoformans

The basidiomycetous yeast Cryptococcus neoformans infects humans and causes a meningoencephalitis that is uniformly fatal if untreated. The organism has a defined sexual cycle involving mating of haploid MATa and MATalpha strains, gene disruption by transformation and homologous recombination is now readily accomplished, and robust animal models for infection have been well established. In addition, a pair of congenic MATalpha and MATa haploid strains have been constructed that permit detailed studies on physiology and virulence by classical genetic approaches. These strains represent a valuable resource for further studies in this organism, and the genomic sequence of one of these strains, JEC21 (=B-4500), was recently chosen to be sequenced by an international consortium. Because of the importance of these strains for genetic studies in C. neoformans and the fact that the genomic sequence of one of these strains is in progress, we review here how these congenic strains were originally constructed.

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

Cryptococcus neoformans is an important fungal pathogen of man. The incidence of cryptococcal disease has increased dramatically in patients immunocompromised because of HIV infection, organ transplantation, or treatment with cytotoxic chemotherapy or corticosteroids. This organism is an excellent model for molecular dissection of fungal pathogenesis and virulence factors. Here we report the nucleotide sequence of the C. neoformans serotype D genomic ADE2 gene, which encodes a phosphoribosylaminoimidazole carboxylase required for purine biosynthesis. Importantly, this version of the ADE2 gene has been used as the selectable marker for virtually all gene disruptions by transformation and homologous recombination in C. neoformans. We compare the nucleotide and amino acid sequences of the ADE2 gene and product to other highly related adenine biosynthetic genes and enzymes from other yeasts and fungi. We also describe a series of convenient ADE2 cassettes for gene disruption construct preparation. Finally, we have identified the ade2 mutations in strains M001 and M049, adenine auxotrophic mutants derived from the serotype A strain H99. These mutant strains have served as recipients for targeted gene disruptions using the ADE2 gene. These studies should facilitate transformation and gene disruption approaches using the ADE2 selectable marker in this important human fungal pathogen.

Signal transduction pathways regulating differentiation and pathogenicity of Cryptococcus neoformans

The basidiomycetous yeast Cryptococcus neoformans is a human pathogen. Several phenotypes of this organism are defined as virulence traits including the polysaccharide capsule, melanin, and the ability to grow at 37 degreesC. The signaling pathways regulating the expression of these phenotypes and other important cellular processes are being defined on a molecular level. For example, the highly conserved signaling molecule calcineurin regulates high temperature growth in C. neoformans. A cryptococcal homolog of Saccharomyces cerevisiae STE12, the gene for a transcriptional regulator activated by the MAP kinase cascade, has also been identified. Additionally, the C. neoformans Galpha protein GPA1 and cAMP regulate mating, melanin production, encapsulation, and pathogenicity. This fungus is an excellent model to further dissect virulence-associated signaling pathways. The conserved role of Galpha proteins and cAMP-associated signaling pathways in fungal differentiation and pathogenicity is also reviewed.

Cryptococcus neoformans mating and virulence are regulated by the G-protein alpha subunit GPA1 and cAMP

This study explores signal transduction pathways that function during mating and infection in the opportunistic, human fungal pathogen Cryptococcus neoformans. The gene encoding a G-protein alpha subunit homolog, GPA1, was disrupted by homologous recombination. The gpa1 mutant strain was viable but exhibited a defect in mating in response to nitrogen starvation. Additionally, the gpa1 mutant strain failed to induce two well-established virulence factors-melanin synthesis, in response to glucose starvation; and capsule production, in response to iron limitation. As a consequence, virulence of the gpa1 mutant strain was significantly attenuated in an animal model of cryptococcal meningitis. Reintroduction of the wild-type GPA1 gene complemented the gpa1 mutant phenotypes and restored mating, melanin and capsule production, and virulence. Similarly, exogenous cAMP also suppressed the gpa1 mutant phenotypes, restoring mating and production of melanin and capsule. These observations support a model in which GPA1 has a role in sensing diverse environmental signals required for mating and virulence by regulating cAMP metabolism in C. neoformans.

Mycoplasma hominis pneumonia complicating bilateral lung transplantation: case report and review of the literature

Mycoplasma hominis is a commensal of humans. The organism has been predominantly associated with infections of the genitourinary tract. Extragenital infections have been described in neonates, in women during the postpartum period, and in immunocompromised patients. Pneumonia caused by M. hominis is very rare. This report describes the development of M. hominis pneumonia in a lung transplantation recipient and underscores the difficulty in establishing the correct diagnosis and the need for early and aggressive treatment with appropriate antimicrobial agents to insure a good outcome.

Inhibition of Cryptococcus neoformans replication by nitrogen oxides supports the role of these molecules as effectors of macrophage-mediated cytostasis

Activated macrophages are able to inhibit the replication of intracellular microbes and tumor cells. In the murine system, this cytostatic effect is associated with the oxidation of L-arginine to L-citrulline, nitrite, and nitrate and is thought to be mediated by an intermediate of this reaction, possibly nitric oxide (NO.). By exposing replicating Cryptococcus neoformans cells to conditions under which NO. is chemically generated, we have observed a cytostatic effect similar to that caused by activated murine macrophages. Nitric oxide is formed as a decomposition product of nitrite salts in acidic, aqueous solutions. Although C. neoformans replicates well in the presence of high nitrite concentrations at physiologic pH, its growth in acidic media can be inhibited by the addition of low concentrations of sodium nitrite. The degree of cytostasis is dependent on both the pH and the nitrite concentration of the NO. generating solution. The cytostatic effector molecule appears to be a gas since, in addition to inhibiting C. neoformans replication in solution, it is able to exert its inhibitory effect across a gas-permeable but ion-impermeable membrane. At high nitrite concentrations, a fungicidal effect occurs. We propose that the growth inhibition of C. neoformans upon exposure to chemically generated NO. or some related oxide of nitrogen represents a cell-free system simulating the cytostatic effect of activated murine macrophages.

Molecular characterization of TRP1, a gene coding for tryptophan synthetase in the basidiomycete Coprinus cinereus

We utilized a cloned gene (TRP5) encoding tryptophan synthetase (TSase) from Saccharomyces cerevisiae to identify and clone the corresponding gene (TRP1) from the basidiomycete Coprinus cinereus. The primary nucleotide (nt) sequence of this gene was determined and compared to sequences from other filamentous fungi, as well as to other genes coding for TSase. A transformation assay was used to demonstrate that 321 nt, which do not include CAAT or TATAAA elements and precede the translation initiation codon, are sufficient for expression in a variety of chromosomal locations. The coding region (2584 nt) is interrupted at nine positions, and putative splicing signals (5'-GTRNGT...YAG-3') are present in each case. The predicted translation product contains 702 amino acids (aa) and is very similar to other TSases, except in the region of aa 257-296 that connects the alpha and beta functional domains. Both the number and the identity of the aa differ in this region between C. cinereus. S. cerevisiae, and Neurospora crassa. Comparison of exon boundaries in the C. cinereus sequence to the three-dimensional structure of Salmonella typhimurium TSase indicates that there is no simple correlation between exons and major functional domains in this protein.