Cyclic AMP signaling in Cryptococcus neoformans

A Velvet Transcription Factor Specifically Activates Mating through a Novel Mating-Responsive Protein in the Human Fungal Pathogen Cryptococcus deneoformans

Sexual reproduction facilitates infection by the production of both a lineage advantage and infectious sexual spores in the ubiquitous human fungal pathogen Cryptococcus deneoformans. However, the regulatory determinants specific for initiating mating remain poorly understood. Here, we identified a velvet family regulator, Cva1, that strongly promotes sexual reproduction in C. deneoformans. This regulation was determined to be specific, based on a comprehensive phenotypic analysis of cva1Δ under 26 distinct in vitro and in vivo growth conditions. We further revealed that Cva1 plays a critical role in the initiation of early mating events, including sexual cell-cell fusion, but is not important for the late sexual development stages or meiosis. Thus, Cva1 specifically contributes to mating activation. Importantly, a novel mating-responsive protein, Cfs1, serves as the key target of Cva1 during mating, since its absence nearly blocks cell-cell fusion in C. deneoformans and its sister species C. neoformans. Together, our findings provide insight into how C. deneoformans ensures the regulatory specificity of mating.

IMPORTANCE The human fungal pathogen C. deneoformans is a model organism for studying fungal sexual reproduction, which is considered to be important to infection. However, the specific regulatory determinants for activation of sexual reproduction remain poorly understood. In this study, by combining transcriptomic and comprehensive phenotypic analysis, we identified a velvet family regulator Cva1 that specifically and critically elicits early mating events, including sexual cell-cell fusion. Significantly, Cva1 induces mating through the novel mating-responsive protein Cfs1, which is essential for cell-cell fusion in C. deneoformans and its sister species C. neoformans. Considering that Cva1 and Cfs1 are highly conserved in species belonging to Cryptococcaeceae, they may play conserved and specific roles in the initiation of sexual reproduction in this important fungal clade, which includes multiple human fungal pathogens.

Campafungins: Inhibitors of Candida albicans and Cryptococcus neoformans Hyphal Growth

Campafungin A is a polyketide that was recognized in the Candida albicans fitness test due to its antiproliferative and antihyphal activity. Its mode of action was hypothesized to involve inhibition of a cAMP-dependent PKA pathway. The originally proposed structure appeared to require a polyketide assembled in a somewhat unusual fashion. However, structural characterization data were never formally published. This background stimulated a reinvestigation in which campafungin A and three closely related minor constituents were purified from fermentations of a strain of the ascomycete fungus Plenodomus enteroleucus. Labeling studies, along with extensive NMR analysis, enabled assignment of a revised structure consistent with conventional polyketide synthetic machinery. The structure elucidation of campafungin A and new analogues encountered in this study, designated here as campafungins B, C, and D, is presented, along with a proposed biosynthetic route. The antimicrobial spectrum was expanded to methicillin-resistant Staphylococcus aureus, Candida tropicalis, Candida glabrata, Cryptococcus neoformans, Aspergillus fumigatus, and Schizosaccharomyces pombe, with MICs ranging as low as 4-8 μg mL^-1 in C. neoformans. Mode-of-action studies employing libraries of C. neoformans mutants indicated that multiple pathways were affected, but mutants in PKA/cAMP pathways were unaffected, indicating that the mode of action was distinct from that observed in C. albicans.

Environmental Factors That Contribute to the Maintenance of Cryptococcus neoformans Pathogenesis

The ability of microorganisms to colonise and display an intracellular lifestyle within a host body increases their fitness to survive and avoid extinction. This host–pathogen association drives microbial evolution, as such organisms are under selective pressure and can become more pathogenic. Some of these microorganisms can quickly spread through the environment via transmission. The non-transmittable fungal pathogens, such as Cryptococcus, probably return into the environment upon decomposition of the infected host. This review analyses whether re-entry of the pathogen into the environment causes restoration of its non-pathogenic state or whether environmental factors and parameters assist them in maintaining pathogenesis. Cryptococcus (C.) neoformans is therefore used as a model organism to evaluate the impact of environmental stress factors that aid the survival and pathogenesis of C. neoformans intracellularly and extracellularly.

The Aminoalkylindole BML-190 Negatively Regulates Chitosan Synthesis via the Cyclic AMP/Protein Kinase A1 Pathway in Cryptococcus neoformans

Cryptococcus neoformans can cause fatal meningoencephalitis in patients with AIDS or other immunocompromising conditions. Current antifungals are suboptimal to treat this disease; therefore, novel targets and new therapies are needed. Previously, we have shown that chitosan is a critical component of the cryptococcal cell wall and is required for survival in the mammalian host and that chitosan deficiency results in rapid clearance from the mammalian host. We had also identified several specific proteins that were required for chitosan biosynthesis, and we hypothesize that screening for compounds that inhibit chitosan biosynthesis would identify additional genes/proteins that influence chitosan biosynthesis. To identify these compounds, we developed a robust and novel cell-based flow cytometry screening method to identify small-molecule inhibitors of chitosan production. We screened the ICCB Known Bioactives library and identified 8 compounds that reduced chitosan in C. neoformans We used flow cytometry-based counterscreens and confirmatory screens, followed by a biochemical secondary screen to refine our primary screening hits to 2 confirmed hits. One of the confirmed hits that reduced chitosan content was the aminoalkylindole BML-190, a known inverse agonist of mammalian cannabinoid receptors. We demonstrated that BML-190 likely targets the C. neoformans G-protein-coupled receptor Gpr4 and, via the cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway, contributes to an intracellular accumulation of cAMP that results in decreased chitosan. Our discovery suggests that this approach could be used to identify additional compounds and pathways that reduce chitosan biosynthesis and could lead to potential novel therapeutics against C. neoformansIMPORTANCECryptococcus neoformans is a fungal pathogen that kills ∼200,000 people every year. The cell wall is an essential organelle that protects fungi from the environment. Chitosan, the deacetylated form of chitin, has been shown to be an essential component of the cryptococcal cell wall during infection of a mammalian host. In this study, we screened a set of 480 compounds, which are known to have defined biological activities, for activity that reduced chitosan production in C. neoformans Two of these compounds were confirmed using an alternative method of measuring chitosan, and one of these was demonstrated to impact the cAMP signal transduction pathway. This work demonstrates that the cAMP pathway regulates chitosan biosynthesis in C. neoformans and validates that this screening approach could be used to find potential antifungal agents.

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.

Investigation of Cryptococcus neoformans magnesium transporters reveals important role of vacuolar magnesium transporter in regulating fungal virulence factors

Cryptococcus neoformans is an important opportunistic fungal pathogen in humans. Recent studies have demonstrated that metals are critical factors for the regulation of fungal virulence in hosts. In this study, we systemically investigated the function of C. neoformans magnesium transporters in controlling the intracellular Mg balance and virulence-associated factors. We identified three Mg transporters in C. neoformans: Mgt1, Mgt2, and Mgt3. While we could not detect a Mg²⁺ -related growth phenotype in mgt1 and mgt3 knockout strains, a GAL7p-Mgt2 strain showed significant Mg-dependent growth defects in the presence of glucose. Further analysis demonstrated that MGT2 is a homolog of MNR2 in Saccharomyces cerevisiae, which is localized to the vacuolar membrane and participates in intracellular Mg transport. Interestingly, a transcriptome analysis showed that Mgt2 influenced the expression of 19 genes, which were independent of Mg²⁺ . We showed that melanin synthesis in C. neoformans required Mg²⁺ and Mgt2, and that capsule production was negatively regulated by Mg²⁺ and Mgt2. Repressing the expression of MGT2-induced capsule, which resulted in an increased fungal burden in the lungs. Cumulatively, this study sets the stage for further evaluation of the important role of Mg homeostasis in the regulation of melanin and capsule in C. neoformans.

Magnesium Ion Acts as a Signal for Capsule Induction in Cryptococcus neoformans

Cryptococcal meningitis caused by Cryptococcus neoformans, is a common opportunistic neural infection in immunocompromised individuals. Cryptococcus meningitis is associated with fungal burden with larger capsule size in cerebrospinal fluid (CSF). To understand the role of CSF constituents in capsule enlargement, we have evaluated the effect of artificial CSF on capsule induction in comparison with various other capsule inducing media. Two different strains of C. neoformans, an environmental and a clinical isolates were used in the present study. While comparing the various capsule inducing media for the two different strains of C. neoformans, it was observed that the capsule growth was significantly increased when grown in artificial CSF at pH 5.5, temperature 34°C for ATCC C. neoformans and 37°C for Clinical C. neoformans and with an incubation period of 72 h. In addition, artificial CSF supports biofilm formation in C. neoformans. While investigating the individual components of artificial CSF, we found that Mg²⁺ ions influence the capsule growth in both environmental and clinical strains of C. neoformans. To confirm our results we studied the expression of four major CAP genes namely, CAP10, CAP59, CAP60, and CAP64 in various capsule inducing media and in different concentrations of Mg²⁺ and Ca²⁺. Our results on gene expression suggest that, Mg²⁺ does have an effect on CAP gene expression, which are important for capsule biosynthesis and virulence. Our findings on the role of Mg²⁺ ion as a signal for capsule induction will promote a way to elucidate the control mechanisms for capsule biosynthesis in C. neoformans.

Stress signaling pathways for the pathogenicity of Cryptococcus

Sensing, responding, and adapting to the surrounding environment are crucial for all living organisms to survive, proliferate, and differentiate in their biological niches. This ability is also essential for Cryptococcus neoformans and its sibling species Cryptococcus gattii, as these pathogens have saprobic and parasitic life cycles in natural and animal host environments. The ability of Cryptococcus to cause fatal meningoencephalitis is highly related to its capability to remodel and optimize its metabolic and physiological status according to external cues. These cues act through multiple stress signaling pathways through a panoply of signaling components, including receptors/sensors, small GTPases, secondary messengers, kinases, transcription factors, and other miscellaneous adaptors or regulators. In this minireview, we summarize and highlight the importance of several stress signaling pathways that influence the pathogenicity of Cryptococcus and discuss future challenges in these areas.

The glucose sensor-like protein Hxs1 is a high-affinity glucose transporter and required for virulence in Cryptococcus neoformans

Cryptococcus is a major fungal pathogen that frequently causes systemic infection in patients with compromised immunity. Glucose, an important signal molecule and the preferred carbon source for Cryptococcus, plays a critical role in fungal development and virulence. Cryptococcus contains more than 50 genes sharing high sequence homology with hexose transporters in Saccharomyces cerevisiae. However, there is no report on their function in glucose sensing or transport. In this study, we investigated two hexose transporter-like proteins (Hxs1 and Hxs2) in Cryptococcus that share the highest sequence identity with the glucose sensors Snf3 and Rgt2 in S. cerevisiae. The expression of HXS1 is repressed by high glucose, while the HXS2 expression is not regulated by glucose. Functional studies showed that Hxs1 is required for fungal resistance to oxidative stress and fungal virulence. The hxs1Δ mutant exhibited a significant reduction in glucose uptake activity, indicating that Hxs1 is required for glucose uptake. Heterologous expression of Cryptococcus HXS1 rendered the S. cerevisiae mutant lacking all 20 hexose transporters a high glucose uptake activity, demonstrating that Hxs1 functions as a glucose transporter. Heterologous expression of HXS1 in the snf3Δ rgt2Δ double mutant did not complement its growth in YPD medium containing the respiration inhibitor antimycin A, suggesting that Hxs1 may not function as a glucose sensor. Taken together, our results demonstrate that Hxs1 is a high-affinity glucose transporter and required for fungal virulence.

Mechanisms of infection by the human fungal pathogen Cryptococcus neoformans

Brain infection by the fungus Cryptococcus neoformans results in inflammation of the meninges and brain parenchyma, a condition known as meningoencephalitis. One million people are estimated to suffer cryptococcal meningitis globally and >60% of these cases die within 3 months of diagnosis. Humans are believed to contract infection by inhalation of spores or dried yeast cells, which subsequently colonize the lung tissue. In the lungs, cryptococci may be cleared by the lung phagocytes, stay latent, cause pulmonary infection and/or disseminate to other body parts, preferentially the brain, culminating in cryptococcal meningoencephalitis. In this review, we discuss the pathogenesis of C. neoformans from the environment to the brain, the current understanding of the mechanisms of cryptococcal transmission into the brain and cryptococcal meningitis. We also give an insight into future cryptococcosis research and the development of novel therapies.

Pleiotropic roles of the Msi1-like protein Msl1 in Cryptococcus neoformans

Msi1-like (MSIL) proteins contain WD40 motifs and have a pleiotropic cellular function as negative regulators of the Ras/cyclic AMP (cAMP) pathway and components of chromatin assembly factor 1 (CAF-1), yet they have not been studied in fungal pathogens. Here we identified and characterized an MSIL protein, Msl1, in Cryptococcus neoformans, which causes life-threatening meningoencephalitis in humans. Notably, Msl1 plays pleiotropic roles in C. neoformans in both cAMP-dependent and -independent manners largely independent of Ras. Msl1 negatively controls antioxidant melanin production and sexual differentiation, and this was repressed by the inhibition of the cAMP-signaling pathway. In contrast, Msl1 controls thermotolerance, diverse stress responses, and antifungal drug resistance in a Ras/cAMP-independent manner. Cac2, which is the second CAF-1 component, appears to play both redundant and distinct functions compared to the functions of Msl1. Msl1 is required for the full virulence of C. neoformans. Transcriptome analysis identified a group of Msl1-regulated genes, which include stress-related genes such as HSP12 and HSP78. In conclusion, this study demonstrates pleiotropic roles of Msl1 in the human fungal pathogen C. neoformans, providing insight into a potential novel antifungal therapeutic target.

Toward an integrated model of capsule regulation in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that causes serious human disease in immunocompromised populations. Its polysaccharide capsule is a key virulence factor which is regulated in response to growth conditions, becoming enlarged in the context of infection. We used microarray analysis of cells stimulated to form capsule over a range of growth conditions to identify a transcriptional signature associated with capsule enlargement. The signature contains 880 genes, is enriched for genes encoding known capsule regulators, and includes many uncharacterized sequences. One uncharacterized sequence encodes a novel regulator of capsule and of fungal virulence. This factor is a homolog of the yeast protein Ada2, a member of the Spt-Ada-Gcn5 Acetyltransferase (SAGA) complex that regulates transcription of stress response genes via histone acetylation. Consistent with this homology, the C. neoformans null mutant exhibits reduced histone H3 lysine 9 acetylation. It is also defective in response to a variety of stress conditions, demonstrating phenotypes that overlap with, but are not identical to, those of other fungi with altered SAGA complexes. The mutant also exhibits significant defects in sexual development and virulence. To establish the role of Ada2 in the broader network of capsule regulation we performed RNA-Seq on strains lacking either Ada2 or one of two other capsule regulators: Cir1 and Nrg1. Analysis of the results suggested that Ada2 functions downstream of both Cir1 and Nrg1 via components of the high osmolarity glycerol (HOG) pathway. To identify direct targets of Ada2, we performed ChIP-Seq analysis of histone acetylation in the Ada2 null mutant. These studies supported the role of Ada2 in the direct regulation of capsule and mating responses and suggested that it may also play a direct role in regulating capsule-independent antiphagocytic virulence factors. These results validate our experimental approach to dissecting capsule regulation and provide multiple targets for future investigation.

Cryptococcus neoformans is a fungal pathogen that causes serious disease in immunocompromised individuals, killing over 600,000 people per year worldwide. A major factor in the ability of this microbe to cause disease is an extensive polysaccharide capsule that surrounds the cell and interferes with the host immune response to infection. This capsule expands dramatically in certain growth conditions, including those found in the mammalian host. We grew cells in multiple conditions and assessed gene expression and capsule size. This allowed us to identify a ‘transcriptional signature’ of genes whose expression correlates with capsule size; we speculated that a subset of these genes acts in capsule regulation. To test this hypothesis, we characterized one previously unstudied gene in this signature and found it to be a novel regulator of capsule expansion, fungal virulence, and mating. This gene encodes cryptococcal Ada2, a well-conserved protein that regulates genes involved in stress response and development. We used phenotypic analysis, RNA sequencing, and chromatin-immunoprecipitation sequencing (ChIP-Seq) to situate Ada2 in the complex network of genes that regulate capsule and other cryptococcal virulence factors. This approach, which yielded insights into the regulation of a critical fungal virulence factor, is applicable to similar questions in other pathogens.

The myriad roles of cyclic AMP in microbial pathogens: from signal to sword

All organisms must sense and respond to their external environments, and this signal transduction often involves second messengers such as cyclic nucleotides. One such nucleotide is cyclic AMP, a universal second messenger that is used by diverse forms of life, including mammals, fungi, protozoa and bacteria. In this review, we discuss the many roles of cAMP in bacterial, fungal and protozoan pathogens and its contributions to microbial pathogenesis. These roles include the coordination of intracellular processes, such as virulence gene expression, with extracellular signals from the environment, and the manipulation of host immunity by increasing cAMP levels in host cells during infection.

Dysregulation of serine biosynthesis contributes to the growth defect of a Mycobacterium tuberculosis crp mutant

Mycobacterium tuberculosis CRP(Mt), encoded by Rv3676 (crp), is a CRP-like transcription factor that binds with the serC-Rv0885 intergenic region. In the present study, we evaluated CRP(Mt) 's regulation of serC and Rv0885 in M. tuberculosis and M. bovis BCG, using site-specific mutagenesis, promoter fusions and reverse-transcriptase PCR (RT-PCR). The CRP(Mt) binding site was required for full expression of serC and Rv0885, and expression of both genes was reduced in M. tuberculosis and M. bovis BCG crp mutants. These data show that CRP(Mt) binding directly activates both serC and Rv0885 expression. M. tuberculosis serC restored the ability of an Escherichia coli serC mutant to grow in serine-dropout medium, demonstrating that M. tuberculosis serC encodes a phosphoserine aminotransferase. Serine supplementation, or overexpression of serC, accelerated the growth of M. tuberculosis and M. bovis BCG crp mutants in mycomedium, but not within macrophages. These results establish a role for CRP(Mt) in the regulation of amino acid biosynthesis, and show that reduced serine production contributes to the slow-growth phenotype of M. tuberculosis and M. bovis BCG crp mutants in vitro. Restoration of serine biosynthesis by serC expression will facilitate identification of additional CRP(Mt)-regulated factors required by M. tuberculosis during macrophage and host infection.

Unravelling secretion in Cryptococcus neoformans: more than one way to skin a cat

Secretion pathways in fungi are essential for the maintenance of cell wall architecture and for the export of a number of virulence factors. In the fungal pathogen, Cryptococcus neoformans, much evidence supports the existence of more than one route taken by secreted molecules to reach the cell periphery and extracellular space, and a significant degree of crosstalk between conventional and non-conventional secretion routes. The need for such complexity may be due to differences in the nature of the exported cargo, the spatial and temporal requirements for constitutive and non-constitutive protein secretion, and/or as a means of compensating for the extra burden on the secretion machinery imposed by the elaboration of the polysaccharide capsule. This review focuses on the role of specific components of the C. neoformans secretion machinery in protein and/or polysaccharide export, including Sec4, Sec6, Sec14, Golgi reassembly and stacking protein and extracellular exosome-like vesicles. We also address what is known about traffic of the lipid, glucosylceramide, a target of therapeutic antibodies and an important regulator of C. neoformans pathogenicity, and the role of signalling pathways in the regulation of secretion.

The cAMP/Protein Kinase A Pathway and Virulence in Cryptococcus neoformans

The basidiomycete fungus Cryptococcus neoformans is an important pathogen of immunocompromised people. The ability of the fungus to sense its environment is critical for proliferation and the generation of infectious propagules, as well as for adaptation to the mammalian host during infection. The conserved cAMP/protein kinase A pathway makes an important contribution to sensing, as demonstrated by the phenotypes of mutants with pathway defects. These phenotypes include loss of the ability to mate and to elaborate the key virulence factors capsule and melanin. This review summarizes recent work that reveals new targets of the pathway, new phenotypic consequences of signaling defects, and a more detailed understanding of connections with other aspects of cryptococcal biology including iron regulation, pH sensing, and stress.

Loss of cAMP-dependent protein kinase A affects multiple traits important for root pathogenesis by Fusarium oxysporum

The soilborne fungal pathogen Fusarium oxysporum causes vascular wilt and root rot diseases in many plant species. We investigated the role of cyclic AMP-dependent protein kinase A of F. oxysporum (FoCPKA) in growth, morphology, and root attachment, penetration, and pathogenesis in Arabidopsis thaliana. Affinity of spore attachment to root surfaces of A. thaliana, observed microscopically and measured by atomic force microscopy, was reduced by a loss-of-function mutation in the gene encoding the catalytic subunit of FoCPKA. The resulting mutants also failed to penetrate into the vascular system of A. thaliana roots and lost virulence. Even when the mutants managed to enter the vascular system via physically wounded roots, the degree of vascular colonization was significantly lower than that of the corresponding wild-type strain O-685 and no noticeable disease symptoms were observed. The mutants also had reduced vegetative growth and spore production, and their hyphal growth patterns were distinct from those of O-685. Coinoculation of O-685 with an focpkA mutant or a strain nonpathogenic to A. thaliana significantly reduced disease severity and the degree of root colonization by O-685. Several experimental tools useful for studying mechanisms of fungal root pathogenesis are also introduced.

Ste50 adaptor protein governs sexual differentiation of Cryptococcus neoformans via the pheromone-response MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) pathways control diverse cellular functions in pathogenic fungi, including sexual differentiation, stress response, and maintenance of cell wall integrity. Here we characterized a Cryptococcus neoformans gene, which is homologous to the yeast Ste50 that is known to play an important role in mating pheromone response and stress response as an adaptor protein to the Ste11 MAPK kinase kinase in Saccharomyces cerevisiae. The C. neoformans Ste50 was not involved in any of the stress responses or virulence factor production (capsule and melanin) that are controlled by the HOG and Ras/cAMP signaling pathways. However, Ste50 was required for mating in both serotype A and serotype D C. neoformans strains. The ste50Δ mutant was completely defective in cell-cell fusion and mating pheromone production. Double mutation of the STE50 gene blocked increased production of pheromone and the hyper-filamentation phenotype of cells deleted of the CRG1 gene, which encodes the RGS protein that negatively regulates pheromone responsive G-protein signaling via the MAPK pathway. Regardless of the presence of the basidiomycota-specific SH3 domains of Ste50 that are known to be required for full virulence of Ustilago maydis, Ste50 was dispensable for virulence of C. neoformans in a murine model of cryptococcosis. In conclusion, the Ste50 adaptor protein controls sexual differentiation of C. neoformans via the pheromone-responsive MAPK pathway but is not required for virulence.

PdeH, a high-affinity cAMP phosphodiesterase, is a key regulator of asexual and pathogenic differentiation in Magnaporthe oryzae

Cyclic AMP-dependent pathways mediate the communication between external stimuli and the intracellular signaling machinery, thereby influencing important aspects of cellular growth, morphogenesis and differentiation. Crucial to proper function and robustness of these signaling cascades is the strict regulation and maintenance of intracellular levels of cAMP through a fine balance between biosynthesis (by adenylate cyclases) and hydrolysis (by cAMP phosphodiesterases). We functionally characterized gene-deletion mutants of a high-affinity (PdeH) and a low-affinity (PdeL) cAMP phosphodiesterase in order to gain insights into the spatial and temporal regulation of cAMP signaling in the rice-blast fungus Magnaporthe oryzae. In contrast to the expendable PdeL function, the PdeH activity was found to be a key regulator of asexual and pathogenic development in M. oryzae. Loss of PdeH led to increased accumulation of intracellular cAMP during vegetative and infectious growth. Furthermore, the pdeHDelta showed enhanced conidiation (2-3 fold), precocious appressorial development, loss of surface dependency during pathogenesis, and highly reduced in planta growth and host colonization. A pdeHDelta pdeLDelta mutant showed reduced conidiation, exhibited dramatically increased (approximately 10 fold) cAMP levels relative to the wild type, and was completely defective in virulence. Exogenous addition of 8-Br-cAMP to the wild type simulated the pdeHDelta defects in conidiation as well as in planta growth and development. While a fully functional GFP-PdeH was cytosolic but associated dynamically with the plasma membrane and vesicular compartments, the GFP-PdeL localized predominantly to the nucleus. Based on data from cAMP measurements and Real-Time RTPCR, we uncover a PdeH-dependent biphasic regulation of cAMP levels during early and late stages of appressorial development in M. oryzae. We propose that PdeH-mediated sustenance and dynamic regulation of cAMP signaling during M. oryzae development is crucial for successful establishment and spread of the blast disease in rice.

Functional Characterization of cAMP-Regulated Gene, CAR1, in Cryptococcus neoformans

The cyclic AMP (cAMP) pathway plays a major role in growth, sexual differentiation, and virulence factor synthesis of pathogenic fungi. In Cryptococcus neoformans, perturbation of the cAMP pathway, such as a deletion in the gene encoding adenylyl cyclase (CAC1), causes defects in the production of virulence factors, including capsule and melanin production, as well as mating. Previously, we performed a comparative transcriptome analysis of the Ras- and cAMP-pathway mutants, which revealed 163 potential cAMP-regulated genes (38 genes at a 2-fold cutoff). The present study characterized the role of one of the cAMP pathway-dependent genes (serotype A identification number CNAG_ 06576.2). The expression patterns were confirmed by Northern blot analysis and the gene was designated cAMP-regulated gene 1 (CAR1). Interestingly, deletion of CAR1 did not affect biosynthesis of any virulence factors and the mating process, unlike the cAMP-signaling deficient cac1Δ mutant. Furthermore, the cac1Δ mutant exhibited wild-type levels of the stress-response phenotype against diverse environmental cues, indicating that Car1, albeit regulated by the cAMP-pathway, is not essential to confer a cAMP-dependent phenotype in C. neoformans.

Comparative transcriptome analysis reveals novel roles of the Ras and cyclic AMP signaling pathways in environmental stress response and antifungal drug sensitivity in Cryptococcus neoformans

The cyclic AMP (cAMP) pathway plays a central role in the growth, differentiation, and virulence of pathogenic fungi, including Cryptococcus neoformans. Three upstream signaling regulators of adenylyl cyclase (Cac1), Ras, Aca1, and Gpa1, have been demonstrated to control the cAMP pathway in C. neoformans, but their functional relationship remains elusive. We performed a genome-wide transcriptome analysis with a DNA microarray using the ras1Delta, gpa1Delta, cac1Delta, aca1Delta, and pka1Delta pka2Delta mutants. The aca1Delta, gpa1Delta, cac1Delta, and pka1Delta pka2Delta mutants displayed similar transcriptome patterns, whereas the ras1Delta mutant exhibited transcriptome patterns distinct from those of the wild type and the cAMP mutants. Interestingly, a number of environmental stress response genes are modulated differentially in the ras1Delta and cAMP mutants. In fact, the Ras signaling pathway was found to be involved in osmotic and genotoxic stress responses and the maintenance of cell wall integrity via the Cdc24-dependent signaling pathway. Notably, the Ras and cAMP mutants exhibited hypersensitivity to a polyene drug, amphotericin B, without showing effects on ergosterol biosynthesis, which suggested a novel method of antifungal combination therapy. Among the cAMP-dependent gene products that we characterized, two small heat shock proteins, Hsp12 and Hsp122, were found to be involved in the polyene antifungal drug susceptibility of C. neoformans.

Virulence in Cryptococcus species

Cryptococcus neoformans and Cryptococcus gattii are the cause of life-threatening meningoencephalitis in immunocompromised and immunocompetent individuals respectively. The increasing incidence of cryptococcal infection as a result of the AIDS epidemic, the recent emergence of a hypervirulent cryptococcal strain in Canada and the fact that mortality from cryptococcal disease remains high have stimulated intensive research into this organism. Here we outline recent advances in our understanding of C. neoformans and C. gattii, including intraspecific complexity, virulence factors, and key signaling pathways. We discuss the molecular basis of cryptococcal virulence and the interaction between these pathogens and the host immune system. Finally, we discuss future challenges in the study and treatment of cryptococcosis.

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.

The Mycobacterium bovis BCG cyclic AMP receptor-like protein is a functional DNA binding protein in vitro and in vivo, but its activity differs from that of its M. tuberculosis ortholog, Rv3676

Mycobacterium tuberculosis Rv3676 encodes a cyclic AMP (cAMP) receptor-like protein (CRP(Mt)) that has been implicated in global gene regulation and may play an important role during tuberculosis infection. The CRP(Mt) ortholog in Mycobacterium bovis BCG, CRP(BCG), is dysfunctional in an Escherichia coli CRP competition assay and has been proposed as a potential source of M. bovis BCG's attenuation. We compared CRP(BCG) and CRP(Mt) in vitro and in vivo, in M. bovis BCG and M. tuberculosis, to evaluate CRP(BCG)'s potential function in a mycobacterial system. Both proteins formed dimers in mycobacterial lysates, bound to the same target DNA sequences, and were similarly affected by the presence of cAMP in DNA binding assays. However, CRP(Mt) and CRP(BCG) differed in their relative affinities for specific DNA target sequences and in their susceptibilities to protease digestion. Surprisingly, CRP(BCG) DNA binding activity was stronger than that of CRP(Mt) both in vitro and in vivo, as measured by electrophoretic mobility shift and chromatin immunoprecipitation assays. Nutrient starvation-associated regulation of several CRP(Mt) regulon members also differed between M. bovis BCG and M. tuberculosis. We conclude that CRP(BCG) is a functional cAMP-responsive DNA binding protein with an in vivo DNA binding profile in M. bovis BCG similar to that of CRP(Mt) in M. tuberculosis. However, biologically significant functional differences may exist between CRP(BCG) and CRP(Mt) with respect to gene regulation, and this issue warrants further study.

Transcriptional regulation by protein kinase A in Cryptococcus neoformans

A defect in the PKA1 gene encoding the catalytic subunit of cyclic adenosine 5'-monophosphate (cAMP)-dependent protein kinase A (PKA) is known to reduce capsule size and attenuate virulence in the fungal pathogen Cryptococcus neoformans. Conversely, loss of the PKA regulatory subunit encoded by pkr1 results in overproduction of capsule and hypervirulence. We compared the transcriptomes between the pka1 and pkr1 mutants and a wild-type strain, and found that PKA influences transcript levels for genes involved in cell wall synthesis, transport functions such as iron uptake, the tricarboxylic acid cycle, and glycolysis. Among the myriad of transcriptional changes in the mutants, we also identified differential expression of ribosomal protein genes, genes encoding stress and chaperone functions, and genes for secretory pathway components and phospholipid synthesis. The transcriptional influence of PKA on these functions was reminiscent of the linkage between transcription, endoplasmic reticulum stress, and the unfolded protein response in Saccharomyces cerevisiae. Functional analyses confirmed that the PKA mutants have a differential response to temperature stress, caffeine, and lithium, and that secretion inhibitors block capsule production. Importantly, we also found that lithium treatment limits capsule size, thus reinforcing potential connections between this virulence trait and inositol and phospholipid metabolism. In addition, deletion of a PKA-regulated gene, OVA1, revealed an epistatic relationship with pka1 in the control of capsule size and melanin formation. OVA1 encodes a putative phosphatidylethanolamine-binding protein that appears to negatively influence capsule production and melanin accumulation. Overall, these findings support a role for PKA in regulating the delivery of virulence factors such as the capsular polysaccharide to the cell surface and serve to highlight the importance of secretion and phospholipid metabolism as potential targets for anti-cryptococcal therapy.

The biology of the Cryptococcus neoformans species complex

Cryptococcus neoformans is a major cause of fungal meningoencephalitis in immunocompromised patients. Despite recent advances in the genetics and molecular biology of C. neoformans, and improved techniques for molecular epidemiology, aspects of the ecology, population structure, and mode of reproduction of this environmental pathogen remain to be established. Application of recent insights into the life cycle of C. neoformans and its different ways of engaging in sexual reproduction under laboratory conditions has just begun to affect research on the ecology and epidemiology of this human pathogenic fungus. The melding of these disparate disciplines should yield rich dividends in our understanding of the evolution of microbial pathogens, providing insights relevant to diagnosis, treatment, and prevention.

Interaction of pathogenic fungi with host cells: Molecular and cellular approaches

This review provides an overview of several molecular and cellular approaches that are likely to supply insights into the host-fungus interaction. Fungi present intra- and/or extracellular host-parasite interfaces, the parasitism phenomenon being dependent on complementary surface molecules. The entry of the pathogen into the host cell is initiated by the fungus adhering to the cell surface, which generates an uptake signal that may induce its cytoplasmatic internalization. Furthermore, microbial pathogens use a variety of their surface molecules to bind to host extracellular matrix (ECM) components to establish an effective infection. On the other hand, integrins mediate the tight adhesion of cells to the ECM at sites referred to as focal adhesions and also play a role in cell signaling. The phosphorylation process is an important mechanism of cell signaling and regulation; it has been implicated recently in defense strategies against a variety of pathogens that alter host-signaling pathways in order to facilitate their invasion and survival within host cells. The study of signal transduction pathways in virulent fungi is especially important in view of their putative role in the regulation of pathogenicity. This review discusses fungal adherence, changes in cytoskeletal organization and signal transduction in relation to host-fungus interaction.

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.

Cryptococcus neoformans capsule biosynthesis and regulation

The capsule is certainly the most prominent virulence factor in Cryptococcus neoformans: acapsular strains are avirulent, and capsular polysaccharides have a deleterious effect on the immune system. Until very recently, very few genes involved in capsule biosynthesis had been identified - and this despite the existence of a detailed body of work concerning the capsule's composition, structure and their regulation by environmental factors. The tremendous development of experimental tools and techniques suited to the study of C. neoformans biology together with the sequencing of three complete genomes have, over the last three years, enabled the identification of a number of proteins which participate directly in biosynthesis of the capsule or which regulate its size. Even though this knowledge is still preliminary, it gives us a clearer picture of the various events needed for biosynthesis of this fascinating structure.