Multiple virulence factors of Cryptococcus neoformans are dependent on VPH1

The hybrid RAVE complex plays V-ATPase-dependent and -independent pathobiological roles in Cryptococcus neoformans

V-ATPase, which comprises 13-14 subunits, is essential for pH homeostasis in all eukaryotes, but its proper function requires a regulator to assemble its subunits. While RAVE (regulator of H+-ATPase of vacuolar and endosomal membranes) and Raboconnectin-3 complexes assemble V-ATPase subunits in Saccharomyces cerevisiae and humans, respectively, the function of the RAVE complex in fungal pathogens remains largely unknown. In this study, we identified two RAVE complex components, Rav1 and Wdr1, in the fungal meningitis pathogen Cryptococcus neoformans, and analyzed their roles. Rav1 and Wdr1 are orthologous to yeast RAVE and human Rabconnectin-3 counterparts, respectively, forming the hybrid RAVE (hRAVE) complex. Deletion of RAV1 caused severe defects in growth, cell cycle control, morphogenesis, sexual development, stress responses, and virulence factor production, while the deletion of WDR1 resulted in similar but modest changes, suggesting that Rav1 and Wdr1 play central and accessary roles, respectively. Proteomics analysis confirmed that Wdr1 was one of the Rav1-interacting proteins. Although the hRAVE complex generally has V-ATPase-dependent functions, it also has some V-ATPase-independent roles, suggesting a unique role beyond conventional intracellular pH regulation in C. neoformans. The hRAVE complex played a critical role in the pathogenicity of C. neoformans, and RAV1 deletion attenuated virulence and impaired blood-brain barrier crossing ability. This study provides comprehensive insights into the pathobiological roles of the fungal RAVE complex and suggests a novel therapeutic strategy for controlling cryptococcosis.

Bet-hedging antimicrobial strategies in macrophage phagosome acidification drive the dynamics of Cryptococcus neoformans intracellular escape mechanisms

The fungus Cryptococcus neoformans is a major human pathogen with a remarkable intracellular survival strategy that includes exiting macrophages through non-lytic exocytosis (Vomocytosis) and transferring between macrophages (Dragotcytosis) by a mechanism that involves sequential events of non-lytic exocytosis and phagocytosis. Vomocytosis and Dragotcytosis are fungal driven processes, but their triggers are not understood. We hypothesized that the dynamics of Dragotcytosis could inherit the stochasticity of phagolysosome acidification and that Dragotcytosis was triggered by fungal cell stress. Consistent with this view, fungal cells involved in Dragotcytosis reside in phagolysosomes characterized by low pH and/or high oxidative stress. Using fluorescent microscopy, qPCR, live cell video microscopy, and fungal growth assays we found that the that mitigating pH or oxidative stress reduced Dragotcytosis frequency, whereas ROS susceptible mutants of C. neoformans underwent Dragotcytosis more frequently. Dragotcytosis initiation was linked to phagolysosomal pH, oxidative stresses, and macrophage polarization state. Dragotcytosis manifested stochastic dynamics thus paralleling the dynamics of phagosomal acidification, which correlated with the inhospitality of phagolysosomes in differently polarized macrophages. Hence, randomness in phagosomal acidification randomly created a population of inhospitable phagosomes where fungal cell stress triggered stochastic C. neoformans non-lytic exocytosis dynamics to escape a non-permissive intracellular macrophage environment.

Vam6/Vps39/TRAP1-domain proteins influence vacuolar morphology, iron acquisition and virulence in Cryptococcus neoformans

The pathogenic fungus Cryptococcus neoformans must overcome iron limitation to cause disease in mammalian hosts. Previously, we reported a screen for insertion mutants with poor growth on haem as the sole iron source. In this study, we characterised one such mutant and found that the defective gene encoded a Vam6/Vps39/TRAP1 domain-containing protein required for robust growth on haem, an important iron source in host tissue. We designated this protein Vps3 based on reciprocal best matches with the corresponding protein in Saccharomyces cerevisiae. C. neoformans encodes a second Vam6/Vps39/TRAP1 domain-containing protein designated Vam6/Vlp1, and we found that this protein is also required for robust growth on haem as well as on inorganic iron sources. This protein is predicted to be a component of the homotypic fusion and vacuole protein sorting complex involved in endocytosis. Further characterisation of the vam6Δ and vps3Δ mutants revealed perturbed trafficking of iron acquisition functions (e.g., the high affinity iron permease Cft1) and impaired processing of the transcription factor Rim101, a regulator of haem and iron acquisition. The vps3Δ and vam6Δ mutants also had pleiotropic phenotypes including loss of virulence in a mouse model of cryptococcosis, reduced virulence factor elaboration and increased susceptibility to stress, indicating pleiotropic roles for Vps3 and Vam6 beyond haem use in C. neoformans. TAKE AWAYS: Two Vam6/Vps39/TRAP1-domain proteins, Vps3 and Vam6, support the growth of Cryptococcus neoformans on haem. Loss of Vps3 and Vam6 influences the trafficking and expression of iron uptake proteins. Loss of Vps3 or Vam6 eliminates the ability of C. neoformans to cause disease in a mouse model of cryptococcosis.

Vph1 is associated with the copper homeostasis of Cryptococcus neoformans serotype D

We clarified the roles of VPH1 in Cryptococcus neoformans serotype D by examining the detailed phenotypes of VPH1-deficient cells (Δvph1) in terms of their capability to grow in acidic and alkaline pH, at a high temperature, and under high osmotic conditions, in addition to the involvement of VPH1 in copper (Cu) homeostasis and the expression of some C. neoformans virulence factors. Δvph1 could grow well on minimal medium (YNB) but exhibited hypersensitivity to 20 μM Cu due to the failure to induce Cu-detoxifying metallothionein genes (CMT1 and CMT2). In contrast, Δvph1 exhibited defective growth on rich medium (YPD), and the induction of Cu transporter genes (CTR1 and CTR4) did not occur in this medium, implying that this strain was incapable of the uptake of Cu ions for growth. However, the addition of excess Cu promoted CTR gene expression and supported Δvph1 growth. These results suggested that the lack of the VPH1 gene disturbed Cu homeostasis in C. neoformans. Moreover, the loss of Vph1 function influenced the urease activity of C. neoformans.

A capsule-associated gene of Cryptococcus neoformans, CAP64, is involved in pH homeostasis

The CAP64 gene is known to be involved in capsule formation in the basidiomycete yeast Cryptococcus neoformans. A null mutant of CAP64, Δcap64, lacks a capsule around the cell wall and its acidic organelles are not stained with quinacrine. In order to clarify whether the Cap64 protein indeed maintains vacuole or vesicle acidification, so that the vesicle containing the capsule polysaccharide or DBB substrate are transported to the cell membrane side, the relationship between CAP64 and intracellular transport genes and between CAP64 and enzyme-secretion activity were analysed. Laccase activity was higher in the Δcap64 strain than in the wild-type strain, and the transcriptional levels of SAV1 and VPH1 were also higher in the Δcap64 strain than in the wild-type strain. The intracellular localization of the Cap64 protein was analysed by overexpressing an mCherry-tagged Cap64 and observing its fluorescence. The Cap64 protein was accumulated within cells in a patch-like manner. The quinacrine-stained cells were observed to analyse the acidified cell compartments; quinacrine was found to be accumulated in a patch-like manner, with the patches overlapping the fluorescence of CAP64-mCherry fusion protein. Quinacrine was thus accumulated in a patch-like fashion in the cells, and the mCherry-tagged Cap64 protein position was consistent with the position of quinacrine accumulation in cells. These results suggest that CAP64 might be involved in intracellular acidification and vesicle secretion via exocytosis.

The Environmental Effects on Virulence Factors and the Antifungal Susceptibility of Cryptococcus neoformans

Cryptococcus neoformans is a facultative intracellular pathogen responsible for fungal meningoencephalitis primarily in immunocompromised individuals. It has become evident the pathogenicity of C. neoformans is dependent on the fungal cell’s environment. The differential expression of virulence factors, based on the cell’s environmental conditions, is one mechanism allowing for the environmental control of the pathogenic ability of C. neoformans. Here, we discuss how these virulence factors (including melanin, the polysaccharide capsule, and Antiphagocytic protein 1) have been shown to be differentially expressed dependent on the cell’s environment. The genetics and signaling pathways leading to the environmental-dependent regulation of virulence factors will also be examined. Susceptibility to antifungal therapeutics is also regulated by the environment, and thus affects the pathogenic abilities of C. neoformans and disease outcomes. This review will also examine the role of the C. neoformans’s environment on antifungal susceptibilities, and the genetics and signaling pathways responsible for these susceptibility alterations. By examining the complex interplay between the environment and the pathogenicity of C. neoformans, we have a better understanding of the intricacies of the pathogen–environment interaction and how to exploit this interaction to develop the most effective treatment protocols.

Emerging insights on the role of V-ATPase in human diseases: Therapeutic challenges and opportunities

The control of the intracellular pH is vital for the survival of all organisms. Membrane transporters, both at the plasma and intracellular membranes, are key players in maintaining a finely tuned pH balance between intra- and extracellular spaces, and therefore in cellular homeostasis. V-ATPase is a housekeeping ATP-driven proton pump highly conserved among prokaryotes and eukaryotes. This proton pump, which exhibits a complex multisubunit structure based on cell type-specific isoforms, is essential for pH regulation and for a multitude of ubiquitous and specialized functions. Thus, it is not surprising that V-ATPase aberrant overexpression, mislocalization, and mutations in V-ATPase subunit-encoding genes have been associated with several human diseases. However, the ubiquitous expression of this transporter and the high toxicity driven by its off-target inhibition, renders V-ATPase-directed therapies very challenging and increases the need for selective strategies. Here we review emerging evidence linking V-ATPase and both inherited and acquired human diseases, explore the therapeutic challenges and opportunities envisaged from recent data, and advance future research avenues. We highlight the importance of V-ATPases with unique subunit isoform molecular signatures and disease-associated isoforms to design selective V-ATPase-directed therapies. We also discuss the rational design of drug development pipelines and cutting-edge methodological approaches toward V-ATPase-centered drug discovery. Diseases like cancer, osteoporosis, and even fungal infections can benefit from V-ATPase-directed therapies.

Laccase Properties, Physiological Functions, and Evolution

Discovered in 1883, laccase is one of the first enzymes ever described. Now, after almost 140 years of research, it seems that this copper-containing protein with a number of unique catalytic properties is widely distributed across all kingdoms of life. Laccase belongs to the superfamily of multicopper oxidases (MCOs)—a group of enzymes comprising many proteins with different substrate specificities and diverse biological functions. The presence of cupredoxin-like domains allows all MCOs to reduce oxygen to water without producing harmful byproducts. This review describes structural characteristics and plausible evolution of laccase in different taxonomic groups. The remarkable catalytic abilities and broad substrate specificity of laccases are described in relation to other copper-containing MCOs. Through an exhaustive analysis of laccase roles in different taxa, we find that this enzyme evolved to serve an important, common, and protective function in living systems.

Vacuolar proton-translocating ATPase is required for antifungal resistance and virulence of Candida glabrata

Vacuolar proton-translocating ATPase (V-ATPase) is located in fungal vacuolar membranes. It is involved in multiple cellular processes, including the maintenance of intracellular ion homeostasis by maintaining acidic pH within the cell. The importance of V-ATPase in virulence has been demonstrated in several pathogenic fungi, including Candida albicans. However, it remains to be determined in the clinically important fungal pathogen Candida glabrata. Increasing multidrug resistance of C. glabrata is becoming a critical issue in the clinical setting. In the current study, we demonstrated that the plecomacrolide V-ATPase inhibitor bafilomycin B₁ exerts a synergistic effect with azole antifungal agents, including fluconazole and voriconazole, against a C. glabrata wild-type strain. Furthermore, the deletion of the VPH2 gene encoding an assembly factor of V-ATPase was sufficient to interfere with V-ATPase function in C. glabrata, resulting in impaired pH homeostasis in the vacuole and increased sensitivity to a variety of environmental stresses, such as alkaline conditions (pH 7.4), ion stress (Na⁺, Ca²⁺, Mn²⁺, and Zn²⁺ stress), exposure to the calcineurin inhibitor FK506 and antifungal agents (azoles and amphotericin B), and iron limitation. In addition, virulence of C. glabrata Δvph2 mutant in a mouse model of disseminated candidiasis was reduced in comparison with that of the wild-type and VPH2-reconstituted strains. These findings support the notion that V-ATPase is a potential attractive target for the development of effective antifungal strategies.

Identification of a basidiomycete-specific Vilse-like GTPase activating proteins (GAPs) and its roles in the production of virulence factors in Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycetous pathogenic yeast that causes fatal infections in both immunocompetent and immunocompromised patients. Regulation on the production of its virulence factors is not fully understood. Here we reported the characterization of a gene, named CVH1(CNA06260), encoding a Drosophila Vilse-like RhoGAP homolog, which is hallmarked by three conserved functional domains: WW, MyTH4 and RhoGAP. Phylogenetic analysis suggests that CVH1 is highly conserved from protists to mammals and interestingly in basidiomycetes, but absent in plants or Ascomycota and other lower fungi. This phylogenetic distribution indicates an evolutionary link among these groups of organisms. Functional analyses demonstrated that CVH1 was involved in stress tolerance and virulence factor production. By disrupting CVH1, we created a second mutant cvh1Δ with the CRISPR-Cas9 editing tool. The mutant strain exhibited hypersensitivity to osmotic stress by 2 M sorbitol and NaCl, suggesting defects in the HOG signaling pathway and an interaction of Cvh1 with the HOG pathway. Hypersensitivity of cvh1Δ to 1% Congo red and 0.01% SDS suggests that the cell wall integrity was impaired in the mutant. And cvh1Δ hardly produced the pigment melanin and capsule. Our study for the first time demonstrates that the fungal Vilse-like RhoGAP CVH1 is an important regulator of multiple biological processes in C. neoformans, and provides novel insights into the regulatory circuit of stress resistance/cell wall integrity, and laccase and capsule synthesis in C. neoformans.

Transcriptional Profiling of Patient Isolates Identifies a Novel TOR/Starvation Regulatory Pathway in Cryptococcal Virulence

Human infection with Cryptococcus causes up to a quarter of a million AIDS-related deaths annually and is the most common cause of nonviral meningitis in the United States. As an opportunistic fungal pathogen, Cryptococcus neoformans is distinguished by its ability to adapt to diverse host environments, including plants, amoebae, and mammals. In the present study, comparative transcriptomics of the fungus within human cerebrospinal fluid identified expression profiles representative of low-nutrient adaptive responses. Transcriptomics of fungal isolates from a cohort of HIV/AIDS patients identified high expression levels of an alternative carbon nutrient transporter gene, STL1, to be associated with poor early fungicidal activity, an important clinical prognostic marker. Mouse modeling and pathway analysis demonstrated a role for STL1 in mammalian pathogenesis and revealed that STL1 expression is regulated by a novel multigene regulatory mechanism involving the CAC2 subunit of the chromatin assembly complex 1, CAF-1. In this pathway, the global regulator of virulence gene VAD1 was found to transcriptionally regulate a cryptococcal homolog of a cytosolic protein, Ecm15, in turn required for nuclear transport of the Cac2 protein. Derepression of STL1 by the CAC2-containing CAF-1 complex was mediated by Cac2 and modulated binding and suppression of the STL1 enhancer element. Derepression of STL1 resulted in enhanced survival and growth of the fungus in the presence of low-nutrient, alternative carbon sources, facilitating virulence in mice. This study underscores the utility of ex vivo expression profiling of fungal clinical isolates and provides fundamental genetic understanding of saprophyte adaption to the human host.IMPORTANCE Cryptococcus is a fungal pathogen that kills an estimated quarter of a million individuals yearly and is the most common cause of nonviral meningitis in the United States. The fungus is carried in about 10% of the adult population and, after reactivation, causes disease in a wide variety of immunosuppressed individuals, including the HIV infected and patients receiving transplant conditioning, cancer therapy, or corticosteroid therapy for autoimmune diseases. The fungus is widely carried in the soil but can also cause infections in plants and mammals. However, the mechanisms for this widespread ability to infect a variety of hosts are poorly understood. The present study identified adaptation to low nutrients as a key property that allows the fungus to inhabit these diverse environments. Further studies identified a nutrient transporter gene, STL1, to be upregulated under low nutrients and to be associated with early fungicidal activity, a marker of poor clinical outcome in a cohort of HIV/AIDS patients. Understanding molecular mechanisms involved in adaptation to the human host may help to design better methods of control and treatment of widely dispersed fungal pathogens such as Cryptococcus.

Comparative Ecology of Capsular Exophiala Species Causing Disseminated Infection in Humans

Exophiala spinifera and Exophiala dermatitidis (Fungi: Chaetothyriales) are black yeast agents potentially causing disseminated infection in apparently healthy humans. They are the only Exophiala species producing extracellular polysaccharides around yeast cells. In order to gain understanding of eventual differences in intrinsic virulence of the species, their clinical profiles were compared and found to be different, suggesting pathogenic strategies rather than coincidental opportunism. Ecologically relevant factors were compared in a model set of strains of both species, and significant differences were found in clinical and environmental preferences, but virulence, tested in Galleria mellonella larvae, yielded nearly identical results. Virulence factors, i.e., melanin, capsule and muriform cells responded in opposite direction under hydrogen peroxide and temperature stress and thus were inconsistent with their hypothesized role in survival of phagocytosis. On the basis of physiological profiles, possible natural habitats of both species were extrapolated, which proved to be environmental rather than animal-associated. Using comparative genomic analyses we found differences in gene content related to lipid metabolism, cell wall modification and polysaccharide capsule production. Despite the fact that both species cause disseminated infections in apparently healthy humans, it is concluded that they are opportunists rather than pathogens.

Candidiasis and the impact of flow cytometry on antifungal drug discovery

INTRODUCTION

Invasive candidiasis continues to be associated with significant morbidity and mortality as well as substantial health care costs nationally and globally. One of the contributing factors is the development of resistance to antifungal agents that are already in clinical use. Moreover, there are known treatment limitations with all of the available antifungal agents. Since traditional techniques in novel drug discovery are time consuming, high-throughput screening using flow cytometry presents as a potential tool to identify new antifungal agents that would be useful in the management of these patients. Areas covered: In this review, the authors discuss the use of automated high-throughput screening assays based upon flow cytometry to identify potential antifungals from a library comprised of a large number of bioactive compounds. They also review studies that employed the use of this research methodology that has identified compounds with antifungal activity. Expert opinion: High-throughput screening using flow cytometry has substantially decreased the processing time necessary for screening thousands of compounds, and has helped enhance our understanding of fungal pathogenesis. Indeed, the authors see this technology as a powerful tool to help scientists identify new antifungal agents that can be added to the clinician's arsenal in their fight against invasive candidiasis.

Identification of small molecules that disrupt vacuolar function in the pathogen Candida albicans

The fungal vacuole is a large acidified organelle that performs a variety of cellular functions. At least a sub-set of these functions are crucial for pathogenic species of fungi, such as Candida albicans, to survive within and invade mammalian tissue as mutants with severe defects in vacuolar biogenesis are avirulent. We therefore sought to identify chemical probes that disrupt the normal function and/or integrity of the fungal vacuole to provide tools for the functional analysis of this organelle as well as potential experimental therapeutics. A convenient indicator of vacuolar integrity based upon the intracellular accumulation of an endogenously produced pigment was adapted to identify Vacuole Disrupting chemical Agents (VDAs). Several chemical libraries were screened and a set of 29 compounds demonstrated to reproducibly cause loss of pigmentation, including 9 azole antifungals, a statin and 3 NSAIDs. Quantitative analysis of vacuolar morphology revealed that (excluding the azoles) a sub-set of 14 VDAs significantly alter vacuolar number, size and/or shape. Many C. albicans mutants with impaired vacuolar function are deficient in the formation of hyphal elements, a process essential for its pathogenicity. Accordingly, all 14 VDAs negatively impact C. albicans hyphal morphogenesis. Fungal selectivity was observed for approximately half of the VDA compounds identified, since they did not alter the morphology of the equivalent mammalian organelle, the lysosome. Collectively, these compounds comprise of a new collection of chemical probes that directly or indirectly perturb normal vacuolar function in C. albicans.

Identification of Multiple Cryptococcal Fungicidal Drug Targets by Combined Gene Dosing and Drug Affinity Responsive Target Stability Screening

Cryptococcus neoformans is a pathogenic fungus that is responsible for up to half a million cases of meningitis globally, especially in immunocompromised individuals. Common fungistatic drugs, such as fluconazole, are less toxic for patients but have low efficacy for initial therapy of the disease. Effective therapy against the disease is provided by the fungicidal drug amphotericin B; however, due to its high toxicity and the difficulty in administering its intravenous formulation, it is imperative to find new therapies targeting the fungus. The antiparasitic drug bithionol has been recently identified as having potent fungicidal activity. In this study, we used a combined gene dosing and drug affinity responsive target stability (GD-DARTS) screen as well as protein modeling to identify a common drug binding site of bithionol within multiple NAD-dependent dehydrogenase drug targets. This combination genetic and proteomic method thus provides a powerful method for identifying novel fungicidal drug targets for further development.

Cryptococcosis is a neglected fungal meningitis that causes approximately half a million deaths annually. The most effective antifungal agent, amphotericin B, was developed in the 1950s, and no effective medicine has been developed for this disease since that time. A key aspect of amphotericin B’s effectiveness is thought to be because of its ability to kill the fungus (fungicidal activity), rather than just stop or slow its growth. The present study utilized a recently identified fungicidal agent, bithionol, to identify potential fungicidal drug targets that can be used in developing modern fungicidal agents. A combined protein and genetic analysis approach was used to identify a class of enzymes, dehydrogenases, that the fungus uses to maintain homeostasis with regard to sugar nutrients. Similarities in the drug target site were found that resulted in simultaneous inhibition and killing of the fungus by bithionol. These studies thus identify a common, multitarget site for antifungal development.

Requirement of the isocitrate lyase gene ICL1 for VPS41-mediated starvation response in Cryptococcus neoformans

Cryptococcus neoformans is a major cause of fungal meningitis in individuals with impaired immunity. Our previous studies have shown that the VPS41 gene plays a critical role in the survival of Cryptococcus neoformans under nitrogen starvation; however, the molecular mechanisms underlying VPS41-mediated starvation response remain to be elucidated. In the present study, we show that, under nitrogen starvation, VPS41 strongly enhanced ICL1 expression in C. neoformans and that overexpression of ICL1 in the vps41 mutant dramatically suppressed its defects in starvation response due to the loss of VPS41 function. Moreover, targeted deletion of ICL1 resulted in a dramatic decline in viability of C. neoformans cells under nitrogen deprivation. Taken together, our data suggest a model in which VPS41 up-regulates ICL1 expression, directly or indirectly, to promote survival of C. neoformans under nitrogen starvation.

Virulence-Associated Enzymes of Cryptococcus neoformans

Enzymes play key roles in fungal pathogenesis. Manipulation of enzyme expression or activity can significantly alter the infection process, and enzyme expression profiles can be a hallmark of disease. Hence, enzymes are worthy targets for better understanding pathogenesis and identifying new options for combatting fungal infections. Advances in genomics, proteomics, transcriptomics, and mass spectrometry have enabled the identification and characterization of new fungal enzymes. This review focuses on recent developments in the virulence-associated enzymes from Cryptococcus neoformans. The enzymatic suite of C. neoformans has evolved for environmental survival, but several of these enzymes play a dual role in colonizing the mammalian host. We also discuss new therapeutic and diagnostic strategies that could be based on the underlying enzymology.

Role of Cln1 during melanization of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that has several well-described virulence determinants. A polysaccharide capsule and the ability to produce melanin are among the most important. Melanization occurs both in vitro, in the presence of catecholamine and indole compounds, and in vivo during the infection. Despite the importance of melanin production for cryptococcal virulence, the component and mechanisms involved in its synthesis have not been fully elucidated. In this work, we describe the role of a G1/S cyclin (Cln1) in the melanization process. Cln1 has evolved specifically with proteins present only in other basidiomycetes. We found that Cln1 is required for the cell wall stability and production of melanin in C. neoformans. Absence of melanization correlated with a defect in the expression of the LAC1 gene. The relation between cell cycle elements and melanization was confirmed by the effect of drugs that cause cell cycle arrest at a specific phase, such as rapamycin. The cln1 mutant was consistently more susceptible to oxidative damage in a medium that induces melanization. Our results strongly suggest a novel and hitherto unrecognized role for C. neoformans Cln1 in the expression of virulence traits.

Cryptococcal heat shock protein 70 homolog Ssa1 contributes to pulmonary expansion of Cryptococcus neoformans during the afferent phase of the immune response by promoting macrophage M2 polarization

Numerous virulence factors expressed by Cryptococcus neoformans modulate host defenses by promoting nonprotective Th2-biased adaptive immune responses. Prior studies demonstrate that the heat shock protein 70 homolog, Ssa1, significantly contributes to serotype D C. neoformans virulence through the induction of laccase, a Th2-skewing and CNS tropic factor. In the present study, we sought to determine whether Ssa1 modulates host defenses in mice infected with a highly virulent serotype A strain of C. neoformans (H99). To investigate this, we assessed pulmonary fungal growth, CNS dissemination, and survival in mice infected with either H99, an SSA1-deleted H99 strain (Δssa1), and a complement strain with restored SSA1 expression (Δssa1::SSA1). Mice infected with the Δssa1 strain displayed substantial reductions in lung fungal burden during the innate phase (days 3 and 7) of the host response, whereas less pronounced reductions were observed during the adaptive phase (day 14) and mouse survival increased only by 5 d. Surprisingly, laccase activity assays revealed that Δssa1 was not laccase deficient, demonstrating that H99 does not require Ssa1 for laccase expression, which explains the CNS tropism we still observed in the Ssa1-deficient strain. Lastly, our immunophenotyping studies showed that Ssa1 directly promotes early M2 skewing of lung mononuclear phagocytes during the innate phase, but not the adaptive phase, of the immune response. We conclude that Ssa1's virulence mechanism in H99 is distinct and laccase-independent. Ssa1 directly interferes with early macrophage polarization, limiting innate control of C. neoformans, but ultimately has no effect on cryptococcal control by adaptive immunity.

New technology and resources for cryptococcal research

Rapid advances in molecular biology and genome sequencing have enabled the generation of new technology and resources for cryptococcal research. RNAi-mediated specific gene knock down has become routine and more efficient by utilizing modified shRNA plasmids and convergent promoter RNAi constructs. This system was recently applied in a high-throughput screen to identify genes involved in host-pathogen interactions. Gene deletion efficiencies have also been improved by increasing rates of homologous recombination through a number of approaches, including a combination of double-joint PCR with split-marker transformation, the use of dominant selectable markers and the introduction of Cre-Loxp systems into Cryptococcus. Moreover, visualization of cryptococcal proteins has become more facile using fusions with codon-optimized fluorescent tags, such as green or red fluorescent proteins or, mCherry. Using recent genome-wide analytical tools, new transcriptional factors and regulatory proteins have been identified in novel virulence-related signaling pathways by employing microarray analysis, RNA-sequencing and proteomic analysis.

A role for LHC1 in higher order structure and complement binding of the Cryptococcus neoformans capsule

Polysaccharide capsules are important virulence factors for many microbial pathogens including the opportunistic fungus Cryptococcus neoformans. In the present study, we demonstrate an unusual role for a secreted lactonohydrolase of C. neoformans, LHC1 in capsular higher order structure. Analysis of extracted capsular polysaccharide from wild-type and lhc1Δ strains by dynamic and static light scattering suggested a role for the LHC1 locus in altering the capsular polysaccharide, both reducing dimensions and altering its branching, density and solvation. These changes in the capsular structure resulted in LHC1-dependent alterations of antibody binding patterns, reductions in human and mouse complement binding and phagocytosis by the macrophage-like cell line J774, as well as increased virulence in mice. These findings identify a unique molecular mechanism for tertiary structural changes in a microbial capsule, facilitating immune evasion and virulence of a fungal pathogen.

Polysaccharide capsules are important virulence factors in pathogenic microbes that provide a protective coat against host immunity. Cryptococcus neoformans is a pathogenic encapsulated yeast that is a major opportunistic infection, causing approximately 600,000 cases of meningitis per year in AIDS patients globally, and whose polysaccharide capsule is a major virulence factor. While extensive work has detailed the chemical components forming the cryptococcal capsule, the molecular events leading to the higher order assembly of the capsule, and its consequences on immune subterfuge remain unknown. In the present studies we used a proteomics method to identify a novel hydrolytic enzyme, lactonohydrolase (Lhc1) and used a variety of biophysical methods including dynamic and static light scattering as well as motility studies to show that extracted capsular polysaccharide undergoes remodeling in a LHC1-dependent fashion. This results in a more tightly compacted capsular structure that alters binding of anti-capsular antibodies and reduces binding by both human as well as mouse serum complement. Furthermore, LHC1-dependent capsular alterations serve to increase the virulence of the fungus in a mouse model, suggesting a novel role for this class of enzyme in capsular remodeling and immune evasion in microbial pathogenesis.

GPI (glycosylphosphatidylinositol)-linked aspartyl proteases regulate vacuole homoeostasis in Candida glabrata

A family of 11 GPI (glycosylphosphatidylinositol)-linked cell surface-associated aspartyl proteases (yapsins) in the human opportunistic fungal pathogen Candida glabrata is required for cell wall remodelling, pH homoeostasis, survival in macrophages and virulence in a murine model of disseminated candidiasis. In the present paper, we report new roles for yapsins in C. glabrata physiology and implicate them for the first time in the regulation of vacuole homoeostasis. In the present study we show that a C. glabrata mutant lacking all 11 yapsins, Cgyps1-11∆, possesses an enlarged vacuole and displays vma- (vacuolar membrane ATPase)-like phenotypes with elevated metal ion susceptibility in an alkaline pH medium and diminished Vma activity. The results of the present study also demonstrate a singular role for CgYps1 (C. glabrata yapsin 1) in the maintenance of ion homoeostasis under normal and calcineurin-inhibited conditions. Elevated polyphosphate levels and diminished cellular CPY (carboxypeptidase Y) activity in the Cgyps1-11∆ mutant highlight the yapsin requirement for a properly functioning vacuole. Lastly, a gross perturbation of cellular homoeostasis in the Cgyps1-11∆ mutant, even in the absence of external stressors, characterized by reduced levels of ATP and stress metabolites, elevated ROS (reactive oxygen species) levels, cell surface abnormalities, and a constitutively activated PKC (protein kinase C) signalling pathway underscore diverse physiological functions of yapsins in C. glabrata.

Microevolution during serial mouse passage demonstrates FRE3 as a virulence adaptation gene in Cryptococcus neoformans

Passage in mice of opportunistic pathogens such as Cryptococcus neoformans is known to increase virulence, but little is known about the molecular mechanisms involved in virulence adaptation. Serial mouse passage of nine environmental strains of serotype A C. neoformans identified two highly adapted virulent strains that showed a 4-fold reduction in time to death after four passages. Transcriptome sequencing expression studies demonstrated increased expression of a FRE3-encoded iron reductase in the two strains but not in a control strain that did not demonstrate increased virulence during mouse passage. FRE3 was shown to express an iron reductase activity and to play a role in iron-dependent growth of C. neoformans. Overexpression of FRE3 in the two original environmental strains increased growth in the macrophage cell line J774.16 and increased virulence. These data demonstrate a role for FRE3 in the virulence of C. neoformans and demonstrate how the increased expression of such a "virulence acquisition gene" during the environment-to-mammal transition, can optimize the virulence of environmental strains in mammalian hosts. IMPORTANCE Cryptococcus neoformans is a significant global fungal pathogen that also resides in the environment. Recent studies have suggested that the organism may undergo microevolution in the host. However, little is known about the permitted genetic changes facilitating the adaptation of environmental strains to mammalian hosts. The present studies subjected environmental strains isolated from several metropolitan areas of the United States to serial passages in mice. Transcriptome sequencing expression studies identified the increased expression of an iron reductase gene, FRE3, in two strains that adapted in mice to become highly virulent, and overexpression of FRE3 recapitulated the increased virulence after mouse passage. Iron reductase in yeast is important to iron uptake in a large number of microbial pathogens. These studies demonstrate the capacity of C. neoformans to show reproducible changes in the expression levels of small numbers of genes termed "virulence adaptation genes" to effectively increase pathogenicity during the environment-to-mammal transition.

Advances in targeting the vacuolar proton-translocating ATPase (V-ATPase) for anti-fungal therapy

Vacuolar proton-translocating ATPase (V-ATPase) is a membrane-bound, multi-subunit enzyme that uses the energy of ATP hydrolysis to pump protons across membranes. V-ATPase activity is critical for pH homeostasis and organelle acidification as well as for generation of the membrane potential that drives secondary transporters and cellular metabolism. V-ATPase is highly conserved across species and is best characterized in the model fungus Saccharomyces cerevisiae. However, recent studies in mammals have identified significant alterations from fungi, particularly in the isoform composition of the 14 subunits and in the regulation of complex disassembly. These differences could be exploited for selectivity between fungi and humans and highlight the potential for V-ATPase as an anti-fungal drug target. Candida albicans is a major human fungal pathogen and causes fatality in 35% of systemic infections, even with anti-fungal treatment. The pathogenicity of C. albicans correlates with environmental, vacuolar, and cytoplasmic pH regulation, and V-ATPase appears to play a fundamental role in each of these processes. Genetic loss of V-ATPase in pathogenic fungi leads to defective virulence, and a comprehensive picture of the mechanisms involved is emerging. Recent studies have explored the practical utility of V-ATPase as an anti-fungal drug target in C. albicans, including pharmacological inhibition, azole therapy, and targeting of downstream pathways. This overview will discuss these studies as well as hypothetical ways to target V-ATPase and novel high-throughput methods for use in future drug discovery screens.

Essential role for vacuolar acidification in Candida albicans virulence

Fungal infections are on the rise, with mortality above 30% in patients with septic Candida infections. Mutants lacking V-ATPase activity are avirulent and fail to acidify endomembrane compartments, exhibiting pleiotropic defects in secretory, endosomal, and vacuolar pathways. However, the individual contribution of organellar acidification to virulence and its associated traits is not known. To dissect their separate roles in Candida albicans pathogenicity we generated knock-out strains for the V0 subunit a genes VPH1 and STV1, which target the vacuole and secretory pathway, respectively. While the two subunits were redundant in many vma phenotypes, such as alkaline pH sensitivity, calcium homeostasis, respiratory defects, and cell wall integrity, we observed a unique contribution of VPH1. Specifically, vph1Δ was defective in acidification of the vacuole and its dependent functions, such as metal ion sequestration as evidenced by hypersensitivity to Zn(2+) toxicity, whereas stv1Δ resembled wild type. In growth conditions that elicit morphogenic switching, vph1Δ was defective in forming hyphae whereas stv1Δ was normal or only modestly impaired. Host cell interactions were evaluated in vitro using the Caco-2 model of intestinal epithelial cells, and murine macrophages. Like wild type, stv1Δ was able to inflict cellular damage in Caco-2 and macrophage cells, as assayed by LDH release, and escape by filamentation. In contrast, vph1Δ resembled a vma7Δ mutant, with significant attenuation in host cell damage. Finally, we show that VPH1 is required for fungal virulence in a murine model of systemic infection. Our results suggest that vacuolar acidification has an essential function in the ability of C. albicans to form hyphae and establish infection.

Three prevacuolar compartment Rab GTPases impact Candida albicans hyphal growth

Disruption of vacuolar biogenesis in the pathogenic yeast Candida albicans causes profound defects in polarized hyphal growth. However, the precise vacuolar pathways involved in yeast-hypha differentiation have not been determined. Previously we focused on Vps21p, a Rab GTPase involved in directing vacuolar trafficking through the late endosomal prevacuolar compartment (PVC). Herein, we identify two additional Vps21p-related GTPases, Ypt52p and Ypt53p, that colocalize with Vps21p and can suppress the hyphal defects of the vps21Δ/Δ mutant. Phenotypic analysis of gene deletion strains revealed that loss of both VPS21 and YPT52 causes synthetic defects in endocytic trafficking to the vacuole, as well as delivery of the virulence-associated vacuolar membrane protein Mlt1p from the Golgi compartment. Transcription of all three GTPase-encoding genes is increased under hyphal growth conditions, and overexpression of the transcription factor Ume6p is sufficient to increase the transcription of these genes. While only the vps21Δ/Δ single mutant has hyphal growth defects, these were greatly exacerbated in a vps21Δ/Δ ypt52Δ/Δ double mutant. On the basis of relative expression levels and phenotypic analysis of gene deletion strains, Vps21p is the most important of the three GTPases, followed by Ypt52p, while Ypt53p has an only marginal impact on C. albicans physiology. Finally, disruption of a nonendosomal AP-3-dependent vacuolar trafficking pathway in the vps21Δ/Δ ypt52Δ/Δ mutant, further exacerbated the stress and hyphal growth defects. These findings underscore the importance of membrane trafficking through the PVC in sustaining the invasive hyphal growth form of C. albicans.

Involvement of MoVMA11, a Putative Vacuolar ATPase c' Subunit, in Vacuolar Acidification and Infection-Related Morphogenesis of Magnaporthe oryzae

Many functions of vacuole depend on the activity of vacuolar ATPase which is essential to maintain an acidic lumen and create the driving forces for massive fluxes of ions and metabolites through vacuolar membrane. In filamentous fungus Magnaportheoryzae, subcellular colocalization and quinacrine staining suggested that the V1V0 domains of V-ATPase were fully assembled and the vacuoles were kept acidic during infection-related developments. Targeted gene disruption of MoVMA11 gene, encoding the putative c' subunit of V-ATPase, impaired vacuolar acidification and mimicked the phenotypes of yeast V-ATPase mutants in the poor colony morphology, abolished asexual and sexual reproductions, selective carbon source utilization, and increased calcium and heavy metals sensitivities, however, not in the typical pH conditional lethality. Strikingly, aerial hyphae of the MoVMA11 null mutant intertwined with each other to form extremely thick filamentous structures. The results also implicated that MoVMA11 was involved in cell wall integrity and appressorium formation. Abundant non-melanized swollen structures and rare, small appressoria without penetration ability were produced at the hyphal tips of the ΔMovma11 mutant on onion epidermal cells. Finally, the MoVMA11 null mutant lost pathogenicity on both intact and wounded host leaves. Overall, our data indicated that MoVMA11, like other fungal VMA genes, is associated with numerous cellular functions and highlighted that V-ATPase is essential for infection-related morphogenesis and pathogenesis in M. oryzae.

Susceptibility of intact germinating Arabidopsis thaliana to human fungal pathogens Cryptococcus neoformans and C. gattii

The fungus Cryptococcus contributes a large global burden of infectious death in both HIV-infected and healthy individuals. As Cryptococcus is an opportunistic pathogen, much of the evolutionary pressure shaping virulence occurs in environments in contact with plants and soil. The present studies investigated inoculation of intact seeds of the common weed Arabidopsis thaliana with fungal cells over a 21-day period. C. gattii was the more virulent plant pathogen, resulting in disrupted germination as well as increased stem lodging, fungal burden, and plant tissue colocalization. C. neoformans was a less virulent plant pathogen but exhibited prolonged tissue residence within the cuticle and vascular spaces. Arabidopsis mutants of the PRN1 gene, which is involved in abiotic and biotic signaling affecting phenylalanine-derived flavonoids, showed altered susceptibility to cryptoccocal infections, suggesting roles for this pathway in cryptococcal defense. The fungal virulence factor laccase was also implicated in plant pathogenesis, as a cryptococcal lac1Δ strain was less virulent than wild-type fungi and was unable to colonize seedlings. In conclusion, these studies expand knowledge concerning the ecological niche of Cryptococcus by demonstrating the pathogenic capacity of the anamorphic form of cryptococcal cells against healthy seedlings under physiologically relevant conditions. In addition, an important role of laccase in plant as well as human virulence may suggest mechanisms for laccase retention and optimization during evolution of this fungal pathogen.

Deletion of vacuolar proton-translocating ATPase V(o)a isoforms clarifies the role of vacuolar pH as a determinant of virulence-associated traits in Candida albicans

Vacuolar proton-translocating ATPase (V-ATPase) is a central regulator of cellular pH homeostasis, and inactivation of all V-ATPase function has been shown to prevent infectivity in Candida albicans. V-ATPase subunit a of the Vo domain (Voa) is present as two fungal isoforms: Stv1p (Golgi) and Vph1p (vacuole). To delineate the individual contribution of Stv1p and Vph1p to C. albicans physiology, we created stv1Δ/Δ and vph1Δ/Δ mutants and compared them to the corresponding reintegrant strains (stv1Δ/ΔR and vph1Δ/ΔR). V-ATPase activity, vacuolar physiology, and in vitro virulence-related phenotypes were unaffected in the stv1Δ/Δ mutant. The vph1Δ/Δ mutant exhibited defective V1Vo assembly and a 90% reduction in concanamycin A-sensitive ATPase activity and proton transport in purified vacuolar membranes, suggesting that the Vph1p isoform is essential for vacuolar V-ATPase activity in C. albicans. The vph1Δ/Δ cells also had abnormal endocytosis and vacuolar morphology and an alkalinized vacuolar lumen (pHvph1Δ/Δ = 6.8 versus pHvph1Δ/ΔR = 5.8) in both yeast cells and hyphae. Secreted protease and lipase activities were significantly reduced, and M199-induced filamentation was impaired in the vph1Δ/Δ mutant. However, the vph1Δ/Δ cells remained competent for filamentation induced by Spider media and YPD, 10% FCS, and biofilm formation and macrophage killing were unaffected in vitro. These studies suggest that different virulence mechanisms differentially rely on acidified vacuoles and that the loss of both vacuolar (Vph1p) and non-vacuolar (Stv1p) V-ATPase activity is necessary to affect in vitro virulence-related phenotypes. As a determinant of C. albicans pathogenesis, vacuolar pH alone may prove less critical than originally assumed.

The Cryptococcus neoformans capsule: a sword and a shield

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

Role of CTR4 in the Virulence of Cryptococcus neoformans

While research has identified an important contribution for metals, such as iron, in microbial pathogenesis, the roles of other transition metals, such as copper, remain mostly unknown. Recent evidence points to a requirement for copper homeostasis in the virulence of Cryptococcus neoformans based on a role for a CUF1 copper regulatory factor in mouse models and in a human patient cohort. C. neoformans is an important fungal pathogen that results in an estimated 600,000 AIDS-related deaths yearly. In the present studies, we found that a C. neoformans mutant lacking the CUF1-dependent copper transporter, CTR4, grows normally in rich medium at 37°C but has reduced survival in macrophages and attenuated virulence in a mouse model. This reduced survival and virulence were traced to a growth defect under nutrient-restricted conditions. Expression studies using a full-length CTR4-fluorescent fusion reporter construct demonstrated robust expression in macrophages, brain, and lung, the latter shown by ex vivo fluorescent imaging. Inductively coupled mass spectroscopy (ICP-MS) was used to probe the copper quota of fungal cells grown in defined medium and recovered from brain, which suggested a role for a copper-protective function of CTR4 in combination with cell metallothioneins under copper-replete conditions. In summary, these data suggest a role for CTR4 in copper-related homeostasis and subsequently in fungal virulence.

Crytococcus neoformans is a significant global fungal pathogen, and copper homeostasis is a relatively unexplored aspect of microbial pathogenesis that could lead to novel therapeutics. Previous studies correlated expression levels of a Ctr4 copper transporter to development of meningoencephalitis in a patient cohort of solid-organ transplants, but a direct role for Ctr4 in mammalian pathogenesis has not been demonstrated. The present studies utilize a Δctr4 mutant strain which revealed an important role for CTR4 in C. neoformans infections in mice and relate the gene product to homeostatic control of copper and growth under nutrient-restricted conditions. Robust expression levels of CTR4 during fungal infection were exploited to demonstrate expression and lung cryptococcal disease using ex vivo fluorescence imaging. In summary, these studies are the first to directly demonstrate a role for a copper transporter in fungal disease and provide an ex vivo imaging tool for further study of cryptococcal gene expression and pathogenesis.

Regulated expression of cyclic AMP-dependent protein kinase A reveals an influence on cell size and the secretion of virulence factors in Cryptococcus neoformans

Cyclic AMP-dependent protein kinase A (PKA) regulates elaboration of the virulence factors melanin and polysaccharide capsule in Cryptococcus neoformans. A mutation in PKA1 encoding the catalytic subunit is known to reduce virulence in mice while a defect in PKR1 encoding the regulatory subunit enhances disease. Here, we constructed strains with galactose-inducible and glucose-repressible versions of PKA1 and PKR1 by inserting the GAL7 promoter upstream of the genes. As expected, no capsule was found in dextrose-containing media for the P(GAL7):PKA1 strain, whereas a large capsule was formed on cells grown in galactose. Along with capsule thickness, high PKA activity also influenced cell size, ploidy and vacuole enlargement, as observed in previous reports of giant/titan cell formation. We employed the regulated strains to test the hypothesis that PKA influences secretion and found that elevated PKA expression positively regulates extracellular protease activity and negatively regulates urease secretion. Furthermore, proper PKA regulation and activity were required for wild-type levels of melanization and laccase activity, as well as correct localization of the enzyme. The latter phenotype is consistent with the discovery that PKA regulates the organization of intracellular membrane compartments. Overall, these results indicate that PKA influences secretion pathways directly related to virulence factor elaboration.

Two cation transporters Ena1 and Nha1 cooperatively modulate ion homeostasis, antifungal drug resistance, and virulence of Cryptococcus neoformans via the HOG pathway

Maintenance of cation homeostasis is essential for survival of all living organisms in their biological niches. It is also important for the survival of human pathogenic fungi in the host, where cation concentrations and pH will vary depending on different anatomical sites. However, the exact role of diverse cation transporters and ion channels in virulence of fungal pathogens remains elusive. In this study we functionally characterized ENA1 and NHA1, encoding a putative Na(+)/ATPase and Na(+)/H(+) antiporter, respectively, in Cryptococcus neoformans, a basidiomycete fungal pathogen which causes fatal meningoencephalitis. Expression of NHA1 and ENA1 is induced in response to salt and osmotic shock mainly in a Hog1-dependent manner. Phenotypic analysis of the ena1Δ, nha1Δ, and ena1Δnha1Δ mutants revealed that Ena1 controls cellular levels of toxic cations, such as Na(+) and Li(+) whereas both Ena1 and Nha1 are important for controlling less toxic K(+) ions. Under alkaline conditions, Ena1 was highly induced and required for growth in the presence of low levels of Na(+) or K(+) salt and Nha1 played a role in survival under K(+) stress. In contrast, Nha1, but not Ena1, was essential for survival at acidic conditions (pH 4.5) under high K(+) stress. In addition, Ena1 and Nha1 were required for maintenance of plasma membrane potential and stability, which appeared to modulate antifungal drug susceptibility. Perturbation of ENA1 and NHA1 enhanced capsule production and melanin synthesis. However, Nha1 was dispensable for virulence of C. neoformans although Ena1 was essential. In conclusion, Ena1 and Nha1 play redundant and discrete roles in cation homeostasis, pH regulation, membrane potential, and virulence in C. neoformans, suggesting that these transporters could be novel antifungal drug targets for treatment of cryptococcosis.

Inhibitors of V-ATPase proton transport reveal uncoupling functions of tether linking cytosolic and membrane domains of V0 subunit a (Vph1p)

Vacuolar ATPases (V-ATPases) are important for many cellular processes, as they regulate pH by pumping cytosolic protons into intracellular organelles. The cytoplasm is acidified when V-ATPase is inhibited; thus we conducted a high-throughput screen of a chemical library to search for compounds that acidify the yeast cytosol in vivo using pHluorin-based flow cytometry. Two inhibitors, alexidine dihydrochloride (EC(50) = 39 μM) and thonzonium bromide (EC(50) = 69 μM), prevented ATP-dependent proton transport in purified vacuolar membranes. They acidified the yeast cytosol and caused pH-sensitive growth defects typical of V-ATPase mutants (vma phenotype). At concentrations greater than 10 μM the inhibitors were cytotoxic, even at the permissive pH (pH 5.0). Membrane fractions treated with alexidine dihydrochloride and thonzonium bromide fully retained concanamycin A-sensitive ATPase activity despite the fact that proton translocation was inhibited by 80-90%, indicating that V-ATPases were uncoupled. Mutant V-ATPase membranes lacking residues 362-407 of the tether of Vph1p subunit a of V(0) were resistant to thonzonium bromide but not to alexidine dihydrochloride, suggesting that this conserved sequence confers uncoupling potential to V(1)V(0) complexes and that alexidine dihydrochloride uncouples the enzyme by a different mechanism. The inhibitors also uncoupled the Candida albicans enzyme and prevented cell growth, showing further specificity for V-ATPases. Thus, a new class of V-ATPase inhibitors (uncouplers), which are not simply ionophores, provided new insights into the enzyme mechanism and original evidence supporting the hypothesis that V-ATPases may not be optimally coupled in vivo. The consequences of uncoupling V-ATPases in vivo as potential drug targets are discussed.

Requirement of a Tsp2-type tetraspanin for laccase repression and stress resistance in the basidiomycete Cryptococcus neoformans

Fungal laccases have been widely used in industry. The expression of laccase often is repressible by the primary carbon source glucose in many fungi. The underlying basis is largely unclear. We demonstrate here that a gene, TSP2-1, was required for laccase repression by glucose in the basidiomycete Cryptococcus neoformans. TSP2-1 encodes a Tsp2-type tetraspanin. The disruption of TSP2-1 resulted in constant melanin formation and the expression of the laccase gene LAC1. This derepression phenotype was restorable by 10 mM exogenous cyclic AMP (cAMP). A capsule defect in the mutant tsp2-1Δ also was restored by cAMP. The results indicate an interaction of Tsp2-1 with the cAMP-dependent protein kinase A (PKA) pathway that has been shown to modulate laccase repression and capsule biosynthesis in this fungus. Other roles of TSP2-1, e.g., in maintaining cell membrane integrity and stress resistance, also were defined. This work reveals a Tsp2-1-dependent glucose repression in C. neoformans. The function of Tsp2-type tetraspanin Tsp2-1 is described for the first time.

Effects of microplusin, a copper-chelating antimicrobial peptide, against Cryptococcus neoformans

Microplusin is an antimicrobial peptide isolated from the cattle tick Rhipicephalus (Boophilus) microplus. Its copper-chelating ability is putatively responsible for its bacteriostatic activity against Micrococcus luteus as microplusin inhibits respiration in this species, which is a copper-dependent process. Microplusin is also active against Cryptococcus neoformans (MIC(50) = 0.09 μM), the etiologic agent of cryptococcosis. Here, we show that microplusin is fungistatic to C. neoformans and this inhibitory effect is abrogated by copper supplementation. Notably, microplusin drastically altered the respiratory profile of C. neoformans. In addition, microplusin affects important virulence factors of this fungus. We observed that microplusin completely inhibited fungal melanization, and this effect correlates with the inhibition of the related enzyme laccase. Also, microplusin significantly inhibited the capsule size of C. neoformans. Our studies reveal, for the first time, a copper-chelating antimicrobial peptide that inhibits respiration and growth of C. neoformans and modifies two major virulence factors: melanization and formation of a polysaccharide capsule. These features suggest that microplusin, or other copper-chelation approaches, may be a promising therapeutic for cryptococcosis.

Mating pheromone in Cryptococcus neoformans is regulated by a transcriptional/degradative "futile" cycle

Sexual reproduction in fungi requires induction of signaling pheromones within environments that are conducive to mating. The fungus Cryptococcus neoformans is currently the fourth greatest cause of infectious death in regions of Africa and undergoes mating in phytonutrient-rich environments to create spores with infectious potential. Here we show that under conditions where sexual development is inhibited, a ∼17-fold excess of MFα pheromone transcript is synthesized and then degraded by a DEAD box protein, Vad1, resulting in low steady state transcript levels. Transfer to mating medium or deletion of the VAD1 gene resulted in high level accumulation of MFα transcripts and enhanced mating, acting in concert with the mating-related HOG1 pathway. We then investigated whether the high metabolic cost of this apparently futile transcriptional cycle could be justified by a more rapid induction of mating. Maintenance of Vad1 activity on inductive mating medium by constitutive expression resulted in repressed levels of MFα that did not prevent but rather prolonged the time to successful mating from 5-6 h to 15 h (p < 0.0001). In sum, these data suggest that VAD1 negatively regulates the sexual cell cycle via degradation of constitutive high levels of MFα transcripts in a synthetic/degradative cycle, providing a mechanism of mRNA induction for time-critical cellular events, such as mating induction.

Survival defects of Cryptococcus neoformans mutants exposed to human cerebrospinal fluid result in attenuated virulence in an experimental model of meningitis

Cryptococcus neoformans is a fungal pathogen that encounters various microenvironments during growth in the mammalian host, including intracellular vacuoles, blood, and cerebrospinal fluid (CSF). Because the CSF is isolated by the blood-brain barrier, we hypothesize that CSF presents unique stresses that C. neoformans must overcome to establish an infection. We assayed 1,201 mutants for survival defects in growth media, saline, and human CSF. We assessed CSF-specific mutants for (i) mutant survival in both human bronchoalveolar lavage (BAL) fluid and fetal bovine serum (FBS), (ii) survival in macrophages, and (iii) virulence using both Caenorhabditis elegans and rabbit models of cryptococcosis. Thirteen mutants exhibited significant survival defects unique to CSF. The mutations of three of these mutants were recreated in the clinical serotype A strain H99: deletions of the genes for a cation ATPase transporter (ena1Δ), a putative NEDD8 ubiquitin-like protein (rub1Δ), and a phosphatidylinositol 4-kinase (pik1Δ). Mutant survival rates in yeast media, saline, and BAL fluid were similar to those of the wild type; however, survival in FBS was reduced but not to the levels in CSF. These mutant strains also exhibited decreased intracellular survival in macrophages, various degrees of virulence in nematodes, and severe attenuation of survival in a rabbit meningitis model. We analyzed the CSF by mass spectrometry for candidate compounds responsible for the survival defect. Our findings indicate that the genes required for C. neoformans survival in CSF ex vivo are necessary for survival and infection in this unique host environment.

Variability of phenotypic traits in Cryptococcus varieties and species and the resulting implications for pathogenesis

Variability of phenotypic characteristics in Cryptococcus neoformans var. grubii and var. neoformans as well as Cryptococcus gattii can have diverse effects on the virulence of these fungi and are thus important for pathogenesis. This article summarizes the diverse phenotypic changes that these fungi can manifest. We divide changes into those that affect the entire fungal population and are predominantly induced by environmental signals, and those that involve subpopulations of the fungal population and have to be selected. Last, the article summarizes the experimental evidence that epitopes on the polysaccharide capsule also vary, which may have implications for the pathogenesis as these findings would further diversify the fungal population.

Requirement for ergosterol in V-ATPase function underlies antifungal activity of azole drugs

Ergosterol is an important constituent of fungal membranes. Azoles inhibit ergosterol biosynthesis, although the cellular basis for their antifungal activity is not understood. We used multiple approaches to demonstrate a critical requirement for ergosterol in vacuolar H(+)-ATPase function, which is known to be essential for fungal virulence. Ergosterol biosynthesis mutants of S. cerevisiae failed to acidify the vacuole and exhibited multiple vma(-) phenotypes. Extraction of ergosterol from vacuolar membranes also inactivated V-ATPase without disrupting membrane association of its subdomains. In both S. cerevisiae and the fungal pathogen C. albicans, fluconazole impaired vacuolar acidification, whereas concomitant ergosterol feeding restored V-ATPase function and cell growth. Furthermore, fluconazole exacerbated cytosolic Ca(2+) and H(+) surges triggered by the antimicrobial agent amiodarone, and impaired Ca(2+) sequestration in purified vacuolar vesicles. These findings provide a mechanistic basis for the synergy between azoles and amiodarone observed in vitro. Moreover, we show the clinical potential of this synergy in treatment of systemic fungal infections using a murine model of Candidiasis. In summary, we demonstrate a new regulatory component in fungal V-ATPase function, a novel role for ergosterol in vacuolar ion homeostasis, a plausible cellular mechanism for azole toxicity in fungi, and preliminary in vivo evidence for synergism between two antifungal agents. New insights into the cellular basis of azole toxicity in fungi may broaden therapeutic regimens for patient populations afflicted with systemic fungal infections.

An unusual organelle in Cryptococcus neoformans links luminal pH and capsule biosynthesis

Cryptococcus neoformans is a basidiomycete that causes deadly infections in the immunocompromised. We previously generated a secretion mutant in this fungus by introducing a mutation in the SAV1 gene, which encodes a homolog of the Sec4/Rab8 subfamily GTPases. Under restrictive conditions there are two notable morphological changes in the sav1 mutant: accumulation of post-Golgi vesicles and the appearance of an unusual organelle, which we term the sav1 body (SB). The SB is an electron-transparent structure 0.2-1microm in diameter, with vesicles or other membranous structures associated with the perimeter. Surprisingly, the SB was heavily labeled with anti-glucuronoxylomannan (GXM) antibodies, suggesting that it contains a secreted capsule component, GXM. A structure similar to the SB, also labeled by anti-GXM antibodies, was induced in wild type cells treated with the vacuolar-ATPase inhibitor, bafilomycin A(1). Bafilomycin A(1) and other agents that increase intraluminal pH also inhibited capsule polysaccharide shedding and capsule growth. These studies highlight an unusual organelle observed in C. neoformans with a potential role in polysaccharide synthesis, and a link between luminal pH and GXM biosynthesis.

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.

Identification of ENA1 as a virulence gene of the human pathogenic fungus Cryptococcus neoformans through signature-tagged insertional mutagenesis

A library of more than 4,500 signature-tagged insertion mutants of the human pathogenic fungus Cryptococcus neoformans was generated, and a subset was screened in a murine inhalation model to identify genes required for virulence. New genes that regulate aspects of C. neoformans virulence were also identified by screening the entire library for in vitro phenotypes related to the ability to cause disease, including melanin production, growth at high temperature, and growth under conditions of nutrient limitation. A screen of 10% of the strain collection in mice identified an avirulent mutant strain with an insertion in the ENA1 gene, which is predicted to encode a fungus-specific sodium or potassium P-type ATPase. The results of the deletion of the gene and complementation experiments confirmed its key role in mammalian virulence. ena1 mutant strains exhibited no change in sensitivity to high salt concentrations but were sensitive to alkaline pH conditions, providing evidence that the fungus may have to survive at elevated pH during infection of the mammalian host. The mutation of the well-characterized virulence factor calcineurin (CNA1) also rendered C. neoformans strains sensitive to elevated pH. ENA1 transcripts in wild-type and cna1 mutant strains were upregulated in response to high pH, and cna1 ena1 double mutant strains exhibited increased sensitivity to elevated pH, indicating that at least two pathways in the fungus mediate survival under alkaline conditions. Signature-tagged mutagenesis is an effective strategy for the discovery of new virulence genes in fungal pathogens of animals.

PI3K signaling of autophagy is required for starvation tolerance and virulenceof Cryptococcus neoformans

Autophagy is a process by which cells recycle cytoplasm and defective organelles during stress situations such as nutrient starvation. It can also be used by host cells as an immune defense mechanism to eliminate infectious pathogens. Here we describe the use of autophagy as a survival mechanism and virulence-associated trait by the human fungal pathogen Cryptococcus neoformans. We report that a mutant form of C. neoformans lacking the Vps34 PI3K (vps34Delta), which is known to be involved in autophagy in ascomycete yeast, was defective in the formation of autophagy-related 8-labeled (Atg8-labeled) vesicles and showed a dramatic attenuation in virulence in mouse models of infection. In addition, autophagic vesicles were observed in WT but not vps34Delta cells after phagocytosis by a murine macrophage cell line, and Atg8 expression was exhibited in WT C. neoformans during human infection of brain. To dissect the contribution of defective autophagy in vps34Delta C. neoformans during pathogenesis, a strain of C. neoformans in which Atg8 expression was knocked down by RNA interference was constructed and these fungi also demonstrated markedly attenuated virulence in a mouse model of infection. These results demonstrated PI3K signaling and autophagy as a virulence-associated trait and survival mechanism during infection with a fungal pathogen. Moreover, the data show that molecular dissection of such pathogen stress-response pathways may identify new approaches for chemotherapeutic interventions.