A novel specificity protein 1 (SP1)-like gene regulating protein kinase C-1 (Pkc1)-dependent cell wall integrity and virulence factors in Cryptococcus neoformans

Viral Appropriation of Specificity Protein 1 (Sp1): The Role of Sp1 in Human Retro- and DNA Viruses in Promoter Activation and Beyond

Specificity protein 1 (Sp1) is a highly ubiquitous transcription factor and one employed by numerous viruses to complete their life cycles. In this review, we start by summarizing the relationships between Sp1 function, DNA binding, and structural motifs. We then describe the role Sp1 plays in transcriptional activation of seven viral families, composed of human retro- and DNA viruses, with a focus on key promoter regions. Additionally, we discuss pathways in common across multiple viruses, highlighting the importance of the cell regulatory role of Sp1. We also describe Sp1-related epigenetic and protein post-translational modifications during viral infection and how they relate to Sp1 binding. Finally, with these insights in mind, we comment on the potential for Sp1-targeting therapies, such as repurposing drugs currently in use in the anti-cancer realm, and what limitations such agents would have as antivirals.

Involvement of the Cell Wall-Integrity Pathway in Signal Recognition, Cell-Wall Biosynthesis, and Virulence in Magnaporthe oryzae

The fungal cell wall is the first layer exposed to the external environment. The cell wall has key roles in regulating cell functions, such as cellular stability, permeability, and protection against stress. Understanding the structure of the cell wall and the mechanism of its biogenesis is important for the study of fungi. Highly conserved in fungi, including Magnaporthe oryzae, the cell wall-integrity (CWI) pathway is the primary signaling cascade regulating cell-wall structure and function. The CWI pathway has been demonstrated to correlate with pathogenicity in many phytopathogenic fungi. In the synthesis of the cell wall, the CWI pathway cooperates with multiple signaling pathways to regulate cell morphogenesis and secondary metabolism. Many questions have arisen regarding the cooperation of different signaling pathways with the CWI pathway in regulating cell-wall synthesis and pathogenicity. In this review, we summarized the latest advances in the M. oryzae CWI pathway and cell-wall structure. We discussed the CWI pathway components and their involvement in different aspects, such as virulence factors, the possibility of the pathway as a target for antifungal therapies, and crosstalk with other signaling pathways. This information will aid in better understanding the universal functions of the CWI pathway in regulating cell-wall synthesis and pathogenicity in M. oryzae. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Molecular Dissection of Crz1 and Its Dynamic Subcellular Localization in Cryptococcus neoformans

Across lower eukaryotes, the transcription factor Crz1 is dephosphorylated by calcineurin, which facilitates Crz1 translocation to the nucleus to regulate gene expression. In the fungal pathogen Cryptococcus neoformans, calcineurin-Crz1 signaling maintains calcium homeostasis, thermotolerance, cell wall integrity, and morphogenesis. How Crz1 distinguishes different stressors and differentially regulates cellular responses is poorly understood. Through monitoring Crz1 subcellular localization over time, we found that Crz1 transiently localizes to granules after exposure to high temperature or calcium. These granules also host the phosphatase calcineurin and Pub1, a ribonucleoprotein stress granule marker, suggesting a role of stress granules in modulating calcineurin-Crz1 signaling. Additionally, we constructed and analyzed an array of Crz1 truncation mutants. We identified the intrinsically disordered regions in Crz1 contribute to proper stress granule localization, nuclear localization, and function. Our results provide the groundwork for further determination of the mechanisms behind the complex regulation of Crz1.

Fungal resilience and host-pathogen interactions: Future perspectives and opportunities

We are constantly exposed to the threat of fungal infection. The outcome-clearance, commensalism or infection-depends largely on the ability of our innate immune defences to clear infecting fungal cells versus the success of the fungus in mounting compensatory adaptive responses. As each seeks to gain advantage during these skirmishes, the interactions between host and fungal pathogen are complex and dynamic. Nevertheless, simply compromising the physiological robustness of fungal pathogens reduces their ability to evade antifungal immunity, their virulence, and their tolerance against antifungal therapy. In this article I argue that this physiological robustness is based on a 'Resilience Network' which mechanistically links and controls fungal growth, metabolism, stress resistance and drug tolerance. The elasticity of this network probably underlies the phenotypic variability of fungal isolates and the heterogeneity of individual cells within clonal populations. Consequently, I suggest that the definition of the fungal Resilience Network represents an important goal for the future which offers the clear potential to reveal drug targets that compromise drug tolerance and synergise with current antifungal therapies.

Updating Insights into the Regulatory Mechanisms of Calcineurin-Activated Transcription Factor Crz1 in Pathogenic Fungi

Ca2+, as a second messenger in cells, enables organisms to adapt to different environmental stresses by rapidly sensing and responding to external stimuli. In recent years, the Ca2+ mediated calcium signaling pathway has been studied systematically in various mammals and fungi, indicating that the pathway is conserved among organisms. The pathway consists mainly of complex Ca2+ channel proteins, calcium pumps, Ca2+ transporters and many related proteins. Crz1, a transcription factor downstream of the calcium signaling pathway, participates in regulating cell survival, ion homeostasis, infection structure development, cell wall integrity and virulence. This review briefly summarizes the Ca2+ mediated calcium signaling pathway and regulatory roles in plant pathogenic fungi. Based on discussing the structure and localization of transcription factor Crz1, we focus on the regulatory role of Crz1 on growth and development, stress response, pathogenicity of pathogenic fungi and its regulatory mechanisms. Furthermore, we explore the cross-talk between Crz1 and other signaling pathways. Combined with the important role and pathogenic mechanism of Crz1 in fungi, the new strategies in which Crz1 may be used as a target to explore disease control in practice are also discussed.

Transcription Factor Crz1 from Cryptococcus humicola Conferred Aluminum Resistance and Interacted with Calcineurin

Calcineurin was activated by aluminum stress and increased aluminum resistance. To investigate how the calcineurin pathway regulates aluminum stress in Cryptococcus humicola, the expressions of Crz1 under stresses were analyzed by quantitative real-time PCR. Calcium, cadmium, and aluminum induced the expression of Crz1. Cna1, calcineurin catalytic subunit A (CNA1) encoding gene, was constructed into pGBKT7 and Crz1 gene was constructed into pGADT7. The resultant plasmids, pGADT7-Crz1 and pGBKT7-Cna1, were transformed into Y2HGold and Y187 yeast strain, respectively. Yeast two-hybridization results showed an interaction between CNA1 and Crz1. The role of Crz1 gene in stresses resistance including hydrogen peroxide, calcium, cadmium, and aluminum was assayed by constructing transgenic yeast. The growth of Crz1 transgenic yeasts was much better than that of the control yeast under these stress conditions. These results suggested that Crz1 participated in resistance to stresses and Crz1 showed an interaction with CNA1.

The monothiol glutaredoxin Grx4 influences thermotolerance, cell wall integrity, and Mpk1 signaling in Cryptococcus neoformans

Monothiol glutaredoxins are important regulators of iron homeostasis that play conserved roles in the sensing and trafficking of iron-sulfur clusters. We previously characterized the role of the monothiol glutaredoxin Grx4 in iron homeostasis, the interaction with the iron regulator Cir1, and virulence in Cryptococcus neoformans. This important fungal pathogen causes cryptococcal meningoencephalitis in immunocompromised individuals worldwide. Here, we demonstrate that Grx4 is required for proliferation at elevated temperatures (both 37°C and 39°C) and under stress conditions. In particular, the grx4Δ mutant was hypersensitive to SDS, calcofluor white (CFW), and caffeine, suggesting that Grx4 is required for membrane and cell wall integrity (CWI). In this context, we found that Grx4 regulated the phosphorylation of the Mpk1 mitogen-activated protein kinase (MAPK) of the CWI pathway in cells grown at elevated temperature or upon treatment with CFW, caffeine, or SDS. The grx4Δ mutant also displayed increased sensitivity to FK506 and cyclosporin A, two inhibitors of the calcineurin pathway, indicating that Grx4 may influence growth at higher temperatures in parallel with calcineurin signaling. Upon thermal stress or calcium treatment, loss of Grx4 also caused partial mis-localization of Crz1, the transcription factor that is a calcineurin substrate. The phenotypes of the grx4Δ, crz1Δ, and cna1Δ (calcineurin) mutants suggest shared contributions to the regulation of temperature, cell wall, and other stresses. In summary, we show that Grx4 is also a key regulator of the responses to a variety of stress conditions in addition to its roles in iron homeostasis in C. neoformans.

Cell Wall Integrity Pathway Involved in Morphogenesis, Virulence and Antifungal Susceptibility in Cryptococcus neoformans

Due to its location, the fungal cell wall is the compartment that allows the interaction with the environment and/or the host, playing an important role during infection as well as in different biological functions such as cell morphology, cell permeability and protection against stress. All these processes involve the activation of signaling pathways within the cell. The cell wall integrity (CWI) pathway is the main route responsible for maintaining the functionality and proper structure of the cell wall. This pathway is highly conserved in the fungal kingdom and has been extensively characterized in Saccharomyces cerevisiae. However, there are still many unknown aspects of this pathway in the pathogenic fungi, such as Cryptococcus neoformans. This yeast is of particular interest because it is found in the environment, but can also behave as pathogen in multiple organisms, including vertebrates and invertebrates, so it has to adapt to multiple factors to survive in multiple niches. In this review, we summarize the components of the CWI pathway in C. neoformans as well as its involvement in different aspects such as virulence factors, morphological changes, and its role as target for antifungal therapies among others.

A Signature-Tagged Mutagenesis (STM)-based murine-infectivity assay for Cryptococcus neoformans

Signature-tagged mutagenesis (STM) is a high-throughput genetic technique that can be used to investigate the function of genes by constructing a large number of mutant strains with unique DNA identification tags, pooling them, and screening them for a particular phenotypic trait. STM was first designed for the identification of genes that contribute to the virulence or infectivity of a pathogen in its host. Recently, this method has also been applied for the identification of mutants with specific phenotypes, such as antifungal drug resistance and proliferation. In the present study, we describe an STM method for the identification of genes contributing to the infectivity of Cryptococcus neoformans using a mutant library, in which each strain was tagged with a unique DNA sequence.

Structure-guided approaches to targeting stress responses in human fungal pathogens

Fungi inhabit extraordinarily diverse ecological niches, including the human body. Invasive fungal infections have a devastating impact on human health worldwide, killing ∼1.5 million individuals annually. The majority of these deaths are attributable to species of Candida, Cryptococcus, and Aspergillus Treating fungal infections is challenging, in part due to the emergence of resistance to our limited arsenal of antifungal agents, necessitating the development of novel therapeutic options. Whereas conventional antifungal strategies target proteins or cellular components essential for fungal growth, an attractive alternative strategy involves targeting proteins that regulate fungal virulence or antifungal drug resistance, such as regulators of fungal stress responses. Stress response networks enable fungi to adapt, grow, and cause disease in humans and include regulators that are highly conserved across eukaryotes as well as those that are fungal-specific. This review highlights recent developments in elucidating crystal structures of fungal stress response regulators and emphasizes how this knowledge can guide the design of fungal-selective inhibitors. We focus on the progress that has been made with highly conserved regulators, including the molecular chaperone Hsp90, the protein phosphatase calcineurin, and the small GTPase Ras1, as well as with divergent stress response regulators, including the cell wall kinase Yck2 and trehalose synthases. Exploring structures of these important fungal stress regulators will accelerate the design of selective antifungals that can be deployed to combat life-threatening fungal diseases.

Zinc Finger Proteins in the Human Fungal Pathogen Cryptococcus neoformans

Zinc is one of the essential trace elements in eukaryotes and it is a critical structural component of a large number of proteins. Zinc finger proteins (ZNFs) are zinc-finger domain-containing proteins stabilized by bound zinc ions and they form the most abundant proteins, serving extraordinarily diverse biological functions. In recent years, many ZNFs have been identified and characterized in the human fungal pathogen Cryptococcus neoformans, a fungal pathogen causing fatal meningitis mainly in immunocompromised individuals. It has been shown that ZNFs play important roles in the morphological development, differentiation, and virulence of C. neoformans. In this review, we, first, briefly introduce the ZNFs and their classification. Then, we explain the identification and classification of the ZNFs in C. neoformans. Next, we focus on the biological role of the ZNFs functionally characterized so far in the sexual reproduction, virulence factor production, ion homeostasis, pathogenesis, and stress resistance in C. neoformans. We also discuss the perspectives on future function studies of ZNFs in C. neoformans.

Paxillin-Mediated Recruitment of Calcineurin to the Contractile Ring Is Required for the Correct Progression of Cytokinesis in Fission Yeast

Paxillin is a scaffold protein that participates in focal adhesion signaling in mammalian cells. Fission yeast paxillin ortholog, Pxl1, is required for contractile actomyosin ring (CAR) integrity and collaborates with the β-glucan synthase Bgs1 in septum formation. We show here that Pxl1's main function is to recruit calcineurin (CN) phosphatase to the actomyosin ring; and thus the absence of either Pxl1 or calcineurin causes similar cytokinesis defects. In turn, CN participates in the dephosphorylation of the Cdc15 F-BAR protein, which recruits and concentrates Pxl1 at the CAR. Our findings suggest the existence of a positive feedback loop between Pxl1 and CN and establish that Pxl1 is a crucial component of the CN signaling pathway during cytokinesis.

The Cys2His2 zinc finger protein Zfp1 regulates sexual reproduction and virulence in Cryptococcus neoformans

Cryptococcus neoformans is a ubiquitous yeast pathogen that often infects the human central nervous system (CNS) to cause meningitis in immunocompromised individuals. Although numerous signaling pathways and factors important for fungal sexual reproduction and virulence have been investigated, their precise mechanism of action remains to be further elucidated. In this study, we identified and characterized a novel zinc finger protein Zfp1 that regulates fungal sexual reproduction and virulence in C. neoformans. qRT-PCR and ZFP1 promoter regulatory activity assays revealed a ubiquitous expression pattern of ZFP1 in all stages during mating. Subcellular localization analysis indicates that Zfp1 is targeted to the cytoplasm of C. neoformans. In vitro assays of stress responses showed that zfp1Δ mutants and the ZFP1 overexpressed strains ZFP1^OE are hypersensitive to SDS, but not Congo red, indicating that Zfp1 may regulate cell membrane integrity. Zfp1 is also essential for fungal sexual reproduction because basidiospore production was blocked in bilateral mating between zfp1Δ mutants or ZFP1 overexpressed strains. Fungal nuclei development assay showed that nuclei in the bilateral mating of zfp1Δ mutants or ZFP1 overexpressed strains failed to undergo meiosis after fusion, indicating Zfp1 is important for regulating meiosis during mating. Although zfp1Δ mutants showed normal growth and produced normal major virulence factors, virulence was attenuated in a murine model. Interestingly, we found that the ZFP1 overexpressed strains were avirulent in a murine systemic-infection model. Overall, our study showed that the zinc finger protein Zfp1 is essential for fungal sporulation and virulence 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.

Glucosamine stimulates pheromone-independent dimorphic transition in Cryptococcus neoformans by promoting Crz1 nuclear translocation

Morphotype switch is a cellular response to external and internal cues. The Cryptococcus neoformans species complex can undergo morphological transitions between the yeast and the hypha form, and such morphological changes profoundly affect cryptococcal interaction with various hosts. Filamentation in Cryptococcus was historically considered a mating response towards pheromone. Recent studies indicate the existence of pheromone-independent signaling pathways but their identity or the effectors remain unknown. Here, we demonstrated that glucosamine stimulated the C. neoformans species complex to undergo self-filamentation. Glucosamine-stimulated filamentation was independent of the key components of the pheromone pathway, which is distinct from pheromone-elicited filamentation. Glucosamine stimulated self-filamentation in H99, a highly virulent serotype A clinical isolate and a widely used reference strain. Through a genetic screen of the deletion sets made in the H99 background, we found that Crz1, a transcription factor downstream of calcineurin, was essential for glucosamine-stimulated filamentation despite its dispensability for pheromone-mediated filamentation. Glucosamine promoted Crz1 translocation from the cytoplasm to the nucleus. Interestingly, multiple components of the high osmolality glycerol response (HOG) pathway, consisting of the phosphorelay system and some of the Hog1 MAPK module, acted as repressors of glucosamine-elicited filamentation through their calcineurin-opposing effect on Crz1’s nuclear translocation. Surprisingly, glucosamine-stimulated filamentation did not require Hog1 itself and was distinct from the conventional general stress response. The results demonstrate that Cryptococcus can resort to multiple genetic pathways for morphological transition in response to different stimuli. Given that the filamentous form attenuates cryptococcal virulence and is immune-stimulatory in mammalian models, the findings suggest that morphogenesis is a fertile ground for future investigation into novel means to compromise cryptococcal pathogenesis.

Cryptococcal meningitis claims half a million lives each year. There is no clinically available vaccine and the current antifungal therapies have serious limitations. Thus identifying cryptococcal specific programs that can be targeted for antifungal or vaccine development is of great value. We have shown previously that switching from the yeast to the hypha form drastically attenuates/abolishes cryptococcal virulence. Cryptococcal cells in the filamentous form also trigger host immune responses that can protect the host from a subsequent lethal challenge. However, self-filamentation is rarely observed in serotype A isolates that are responsible for the vast majority of cryptococcosis cases. In this study, we found that glucosamine stimulated self-filamentation in genetically distinct strains of the Cryptococcus species complex, including the most commonly used serotype A reference strain H99. We demonstrated that filamentation elicited by glucosamine did not depend on the pheromone pathway, but it requires the calcineurin transcription factor Crz1. Glucosamine promotes nuclear translocation of Crz1, which is positively controlled by the phosphatase calcineurin and is suppressed by the HOG pathway. These findings raise the possibility of manipulating genetic pathways controlling fungal morphogenesis against diseases caused by the Cryptococcus species complex.

Creation, characterization and utilization of Cryptococcus neoformans mutants sensitive to micafungin

We constructed deletion mutants of Cryptococcus neoformans var neoformans (serotype D) genes encoding late ergosterol biosynthetic pathway enzymes and found that the mutations enhanced susceptibility to various drugs including micafungin, one of the echinocandins, to which wild-type Cryptococcus strains show no susceptibility. Furthermore, through isolation of a mutant resistant to micafungin from a micafungin-sensitive erg mutant and genetic analysis of it, we found that the responsible mutation occurred in the hotspot 2 of FKS1 encoding β-1, 3-glucan synthase, indicating that micafungin inhibited the growth of the erg mutant via inhibiting Fks1 activity. Addition of ergosterol to the culture of the erg mutants recovered the resistance to micafungin, suggesting that the presence of ergosterol in membrane inhibits the accession of micafungin to its target. We found that a loss of one of genes encoding subunits of v-ATPase, VPH1, made Cryptococcus cells sensitive to micafungin. Our observation that the erg2 vph1 double mutant was more sensitive to micafungin than either single mutant suggests that these two genes act differently in becoming resistant to micafungin. The erg mutants allowed us to study the physiological significance of β-1, 3-glucan synthesis in C. neoformans; the inhibition of β-1, 3-glucan synthesis induced cell death and changes in cellular morphology. By observing the erg mutant cells recovering from the growth inhibition imposed by micafungin, we recognized β-1, 3-glucan synthesis would suppress filamentous growth in C. neoformans.

Elucidation of the calcineurin-Crz1 stress response transcriptional network in the human fungal pathogen Cryptococcus neoformans

Calcineurin is a highly conserved Ca²⁺/calmodulin-dependent serine/threonine-specific protein phosphatase that orchestrates cellular Ca²⁺ signaling responses. In Cryptococcus neoformans, calcineurin is activated by multiple stresses including high temperature, and is essential for stress adaptation and virulence. The transcription factor Crz1 is a major calcineurin effector in Saccharomyces cerevisiae and other fungi. Calcineurin dephosphorylates Crz1, thereby enabling Crz1 nuclear translocation and transcription of target genes. Here we show that loss of Crz1 confers phenotypes intermediate between wild-type and calcineurin mutants, and demonstrate that deletion of the calcineurin docking domain results in the inability of Crz1 to translocate into the nucleus under thermal stress. RNA-sequencing revealed 102 genes that are regulated in a calcineurin-Crz1-dependent manner at 37°C. The majority of genes were down-regulated in cna1Δ and crz1Δ mutants, indicating these genes are normally activated by the calcineurin-Crz1 pathway at high temperature. About 58% of calcineurin-Crz1 target genes have unknown functions, while genes with known or predicted functions are involved in cell wall remodeling, calcium transport, and pheromone production. We identified three calcineurin-dependent response element motifs within the promoter regions of calcineurin-Crz1 target genes, and show that Crz1 binding to target gene promoters is increased upon thermal stress in a calcineurin-dependent fashion. Additionally, we found a large set of genes independently regulated by calcineurin, and Crz1 regulates 59 genes independently of calcineurin. Given the intermediate crz1Δ mutant phenotype, and our recent evidence for a calcineurin regulatory network impacting mRNA in P-bodies and stress granules independently of Crz1, calcineurin likely acts on factors beyond Crz1 that govern mRNA expression/stability to operate a branched transcriptional/post-transcriptional stress response network necessary for fungal virulence. Taken together, our findings reveal the core calcineurin-Crz1 stress response cascade is maintained from ascomycetes to a pathogenic basidiomycete fungus, but its output in C. neoformans appears to be adapted to promote fungal virulence.

The ubquitiously conserved serine/threonine-specific protein phosphatase calcineurin is crucial for virulence of several opportunistic human fungal pathogens including Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. We show that Crz1 acts downstream of calcineurin, to 1) govern expression of genes involved in cell wall integrity, and calcium and small molecule transport, and 2) contribute to stress survival and virulence of C. neoformans. Our studies reveal that calcineurin also controls mRNA expression levels of other genes independently of Crz1. We propose that calcineurin operates in a branched signal transduction cascade controlling targets at both the transcriptional and post-transcriptional levels.

Characterization of calcineurin from Cryptococcus humicola and the application of calcineurin in aluminum tolerance

BACKGROUND

Calcineurin (CaN) is a Ca2+- and calmodulin (CaM)-dependent serine/threonine phosphatase. Previous studies have found that CaN is involved in the regulation of the stress responses.

RESULTS

In this study, the growth of Cryptococcus humicola was inhibited by the CaN inhibitor tacrolimus (FK506) under aluminum (Al) stress. The expression of CNA encoding a catalytic subunit A (CNA) and its interaction with CaM were upregulated when the concentration of Al was increased. A CaM-binding domain and key amino acids responsible for interaction with CaM were identified. ∆CNAb with a deletion from S454 to A639 was detected to bind to CaM, while ∆CNAa with a deletion from R436 to A639 showed no binding to CaM. The binding affinities of CNA1 and CNA2, in which I439 or I443 were replaced by Ala, were decreased relative to wild-type CNA. The phosphatase activities of ∆CNAa, CNA1 and CNA2 were lower than the wild-type protein. These results suggest that the region between R436 and S454 is essential for the interaction with CaM and I439, I443 are key amino acids in this region. The ability of the CNA transgenic yeast to develop resistance to Al was significantly higher than that of control yeast. Residual Al in the CNA transgenic yeast culture media was significantly lower than the amount of Al originally added to the media or the residual Al remaining in the control yeast culture media. These findings suggest that CNA confers Al tolerance, and the mechanism of Al tolerance may involve absorption of active Al.

CONCLUSIONS

Al stress up-regulated the expression of CNA. CaM-binding domain and key amino acids responsible for interaction with CaM were identified and both are required for phosphatase activities. CNA conferred yeast Al resistance indicating that the gene has a potential to improve Al-tolerance through gene engineering.

Calcineurin in fungal virulence and drug resistance: Prospects for harnessing targeted inhibition of calcineurin for an antifungal therapeutic approach

Increases in the incidence and mortality due to the major invasive fungal infections such as aspergillosis, candidiasis and cryptococcosis caused by the species of Aspergillus, Candida and Cryptococcus, are a growing threat to the immunosuppressed patient population. In addition to the limited armamentarium of the current classes of antifungal agents available (pyrimidine analogs, polyenes, azoles, and echinocandins), their toxicity, efficacy and the emergence of resistance are major bottlenecks limiting successful patient outcomes. Although these drugs target distinct fungal pathways, there is an urgent need to develop new antifungals that are more efficacious, fungal-specific, with reduced or no toxicity and simultaneously do not induce resistance. Here we review several lines of evidence which indicate that the calcineurin signaling pathway, a target of the immunosuppressive drugs FK506 and cyclosporine A, orchestrates growth, virulence and drug resistance in a variety of fungal pathogens and can be exploited for novel antifungal drug development.

Modulation of Replicative Lifespan in Cryptococcus neoformans: Implications for Virulence

The fungal pathogen, Cryptococcus neoformans, has been shown to undergo replicative aging. Old cells are characterized by advanced generational age and phenotypic changes that appear to mediate enhanced resistance to host and antifungal-based killing. As a consequence of this age-associated resilience, old cells accumulate during chronic infection. Based on these findings, we hypothesized that shifting the generational age of a pathogenic yeast population would alter its vulnerability to the host and affect its virulence. SIR2 is a well-conserved histone deacetylase, and a pivotal target for the development of anti-aging drugs. We tested its effect on C. neoformans’ replicative lifespan (RLS). First, a mutant C. neoformans strain (sir2Δ) was generated, and confirmed a predicted shortened RLS in sir2Δ cells consistent with its known role in aging. Next, RLS analysis showed that treatment of C. neoformans with Sir2p-agonists resulted in a significantly prolonged RLS, whereas treatment with a Sir2p-antagonist shortened RLS. RLS modulating effects were dependent on SIR2 and not observed in sir2Δ cells. Because SIR2 loss resulted in a slightly impaired fitness, effects of genetic RLS modulation on virulence could not be compared with wild type cells. Instead we chose to chemically modulate RLS, and investigated the effect of Sir2p modulating drugs on C. neoformans cells in a Galleria mellonella infection model. Consistent with our hypothesis that shifts in the generational age of the infecting yeast population alters its vulnerability to host cells, we observed decreased virulence of C. neoformans in the Galleria host when RLS was prolonged by treatment with Sir2p agonists. In contrast, treatment with a Sir2p antagonist, which shortens RLS enhanced virulence in Galleria. In addition, combination of Sir2p agonists with antifungal therapy enhanced the antifungal’s effect. Importantly, no difference in virulence was observed with drug treatment when sir2Δ cells were used for infection, which confirmed target specificity and ruled out non-specific effects of the drugs on the Galleria host. Thus, this study suggests that RLS modulating drugs, such as Sir2p agonists, shift lifespan and vulnerability of the fungal population, and should be further investigated as a potential class of novel antifungal drug targets that can enhance antifungal efficacy.

Rewiring of Signaling Networks Modulating Thermotolerance in the Human Pathogen Cryptococcus neoformans

Thermotolerance is a crucial virulence attribute for human pathogens, including the fungus Cryptococcus neoformans that causes fatal meningitis in humans. Loss of the protein kinase Sch9 increases C. neoformans thermotolerance, but its regulatory mechanism has remained unknown. Here, we studied the Sch9-dependent and Sch9-independent signaling networks modulating C. neoformans thermotolerance by using genome-wide transcriptome analysis and reverse genetic approaches. During temperature upshift, genes encoding for molecular chaperones and heat shock proteins were upregulated, whereas those for translation, transcription, and sterol biosynthesis were highly suppressed. In this process, Sch9 regulated basal expression levels or induced/repressed expression levels of some temperature-responsive genes, including heat shock transcription factor (HSF1) and heat shock proteins (HSP104 and SSA1). Notably, we found that the HSF1 transcript abundance decreased but the Hsf1 protein became transiently phosphorylated during temperature upshift. Nevertheless, Hsf1 is essential for growth and its overexpression promoted C. neoformans thermotolerance. Transcriptome analysis using an HSF1 overexpressing strain revealed a dual role of Hsf1 in the oxidative stress response and thermotolerance. Chromatin immunoprecipitation demonstrated that Hsf1 binds to the step-type like heat shock element (HSE) of its target genes more efficiently than to the perfect- or gap-type HSE. This study provides insight into the thermotolerance of C. neoformans by elucidating the regulatory mechanisms of Sch9 and Hsf1 through the genome-scale identification of temperature-dependent genes.

The putative autophagy regulator Atg7 affects the physiology and pathogenic mechanisms of Cryptococcus neoformans

AIM

We investigated the involvement of the autophagy protein 7 (Atg7) in physiology and pathogenic potential of Cryptococcus neoformans.

MATERIALS & METHODS

The C. neoformans gene encoding Atg7 was deleted by biolistic transformation for characterization of autophagy mechanisms, pigment formation, cell dimensions, interaction with phagocytes and pathogenic potential in vivo.

RESULTS & CONCLUSION

ATG7 deletion resulted in defective autophagy mechanisms, enhanced pigmentation and increased cellular size both in vitro and in vivo. The atg7Δ mutant had decreased survival in the lung of infected mice, higher susceptibility to the killing machinery of different host phagocytes and reduced ability to kill an invertebrate host. These results connect Atg7 with mechanisms of pathogenicity in the C. neoformans model.

Crz1p Regulates pH Homeostasis in Candida glabrata by Altering Membrane Lipid Composition

The asexual facultative aerobic haploid yeast Candida glabrata is widely used in the industrial production of various organic acids. To elucidate the physiological function of the C. glabrata transcription factor Crz1p (CgCrz1p) and its role in tolerance to acid stress, we deleted or overexpressed the corresponding gene, CgCRZ1 Deletion of CgCRZ1 resulted in a 60% decrease in the dry weight of cells (DCW) and a 50% drop in cell viability compared with those of the wild type at pH 2.0. Expression of lipid metabolism-associated genes was also significantly downregulated. Consequently, the proportion of C_18:1 fatty acids, the ratio of unsaturated to saturated fatty acids, and the ergosterol content decreased by 30%, 46%, and 30%, respectively. Additionally, membrane integrity, fluidity, and H⁺-ATPase activity were reduced by 45%, 9%, and 50%, respectively. In contrast, overexpression of CgCrz1p increased C_18:1 and ergosterol contents by 16% and 40%, respectively. Overexpression also enhanced membrane integrity, fluidity, and H⁺-ATPase activity by 31%, 6%, and 20%, respectively. Moreover, in the absence of pH buffering, the DCW and pyruvate titers increased by 48% and 60%, respectively, compared to that of the wild type. Together, these results suggest that CgCrz1p regulates tolerance to acidic conditions by altering membrane lipid composition in C. glabrataIMPORTANCE This study provides insight into the metabolism of Candida glabrata under acidic conditions, such as those encountered during the industrial production of organic acids. We found that overexpression of the transcription factor CgCrz1p improved viability, biomass, and pyruvate yields at a low pH. Analysis of plasma membrane lipid composition indicated that CgCrz1p might play an important role in its integrity and fluidity and that it enhanced the pumping of protons in acidic environments. We propose that altering the structure of the cell membrane may provide a successful strategy for increasing C. glabrata productivity at a low pH.

Calcineurin Targets Involved in Stress Survival and Fungal Virulence

Calcineurin governs stress survival, sexual differentiation, and virulence of the human fungal pathogen Cryptococcus neoformans. Calcineurin is activated by increased Ca²⁺ levels caused by stress, and transduces signals by dephosphorylating protein substrates. Herein, we identified and characterized calcineurin substrates in C. neoformans by employing phosphoproteomic TiO₂ enrichment and quantitative mass spectrometry. The identified targets include the transactivator Crz1 as well as novel substrates whose functions are linked to P-bodies/stress granules (PBs/SGs) and mRNA translation and decay, such as Pbp1 and Puf4. We show that Crz1 is a bona fide calcineurin substrate, and Crz1 localization and transcriptional activity are controlled by calcineurin. We previously demonstrated that thermal and other stresses trigger calcineurin localization to PBs/SGs. Several calcineurin targets localized to PBs/SGs, including Puf4 and Pbp1, contribute to stress resistance and virulence individually or in conjunction with Crz1. Moreover, Pbp1 is also required for sexual development. Genetic epistasis analysis revealed that Crz1 and the novel targets Lhp1, Puf4, and Pbp1 function in a branched calcineurin pathway that orchestrates stress survival and virulence. These findings support a model whereby calcineurin controls stress and virulence, at the transcriptional level via Crz1, and post-transcriptionally by localizing to PBs/SGs and acting on targets involved in mRNA metabolism. The calcineurin targets identified in this study share little overlap with known calcineurin substrates, with the exception of Crz1. In particular, the mRNA binding proteins and PBs/SGs residents comprise a cohort of novel calcineurin targets that have not been previously linked to calcineurin in mammals or in Saccharomyces cerevisiae. This study suggests either extensive evolutionary rewiring of the calcineurin pathway, or alternatively that these novel calcineurin targets have yet to be characterized as calcineurin targets in other organisms. These findings further highlight C. neoformans as an outstanding model to define calcineurin-responsive virulence networks as targets for antifungal therapy.

Calcineurin is a Ca²⁺/calmodulin-dependent protein phosphatase essential for stress survival, sexual development, and virulence of the human fungal pathogen Cryptococcus neoformans and other major pathogenic fungi of global human health relevance. However, no calcineurin substrates are known in pathogenic fungi. Employing state-of-the-art phosphoproteomic approaches we identified calcineurin substrates, including calcineurin itself and the conserved Crz1 transcriptional activator known to function in calcium signaling and stress survival. Remarkably, our study also identified novel calcineurin targets involved in RNA processing, stability, and translation, which colocalize together with calcineurin in stress granules/P-bodies upon thermal stress. These findings support a model whereby calcineurin functions in a branched pathway, via Crz1 and several of the identified novel targets, that governs transcriptional and posttranscriptional circuits to drive stress survival, sexual development, and fungal virulence. Our study underscores C. neoformans as an experimental model to define basic paradigms of calcineurin signaling in global thermostress responsive virulence networks that can be targeted for fungal therapy.

Computational Analysis Reveals a Key Regulator of Cryptococcal Virulence and Determinant of Host Response

Cryptococcus neoformans is a ubiquitous, opportunistic fungal pathogen that kills over 600,000 people annually. Here, we report integrated computational and experimental investigations of the role and mechanisms of transcriptional regulation in cryptococcal infection. Major cryptococcal virulence traits include melanin production and the development of a large polysaccharide capsule upon host entry; shed capsule polysaccharides also impair host defenses. We found that both transcription and translation are required for capsule growth and that Usv101 is a master regulator of pathogenesis, regulating melanin production, capsule growth, and capsule shedding. It does this by directly regulating genes encoding glycoactive enzymes and genes encoding three other transcription factors that are essential for capsule growth: GAT201, RIM101, and SP1. Murine infection with cryptococci lacking Usv101 significantly alters the kinetics and pathogenesis of disease, with extended survival and, unexpectedly, death by pneumonia rather than meningitis. Our approaches and findings will inform studies of other pathogenic microbes.

Cryptococcus neoformans causes fatal meningitis in immunocompromised individuals, mainly HIV positive, killing over 600,000 each year. A unique feature of this yeast, which makes it particularly virulent, is its polysaccharide capsule; this structure impedes host efforts to combat infection. Capsule size and structure respond to environmental conditions, such as those encountered in an infected host. We have combined computational and experimental tools to elucidate capsule regulation, which we show primarily occurs at the transcriptional level. We also demonstrate that loss of a novel transcription factor alters virulence factor expression and host cell interactions, changing the lethal condition from meningitis to pneumonia with an exacerbated host response. We further demonstrate the relevant targets of regulation and kinetically map key regulatory and host interactions. Our work elucidates mechanisms of capsule regulation, provides methods and resources to the research community, and demonstrates an altered pathogenic outcome that resembles some human conditions.

ALL2, a Homologue of ALL1, Has a Distinct Role in Regulating pH Homeostasis in the Pathogen Cryptococcus neoformans

Cryptococcus neoformans is a facultative intracellular fungal pathogen that has a polysaccharide capsule and causes life-threatening meningoencephalitis. Its capsule, as well as its ability to survive in the acidic environment of the phagolysosome, contributes to the pathogen's resilience in the host environment. Previously, we reported that downregulation of allergen 1 (ALL1) results in the secretion of a shorter, more viscous exopolysaccharide with less branching and structural complexity, as well as altered iron homeostasis. Now, we report on a homologous coregulated gene, allergen 2 (ALL2). ALL2's function was characterized by generating null mutants in C. neoformans. In contrast to ALL1, loss of ALL2 attenuated virulence in the pulmonary infection model. The all2Δ mutant shed a less viscous exopolysaccharide and exhibited higher sensitivity to hydrogen peroxide than the wild type, and as a result, the all2Δ mutant was more resistant to macrophage-mediated killing. Transcriptome analysis further supported the distinct function of these two genes. Unlike ALL1's involvement in iron homeostasis, we now present data on ALL2's unique function in maintaining intracellular pH in low-pH conditions. Thus, our data highlight that C. neoformans, a human-pathogenic basidiomycete, has evolved a unique set of virulence-associated genes that contributes to its resilience in the human niche.

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.

Systematic functional profiling of transcription factor networks in Cryptococcus neoformans

Cryptococcus neoformans causes life-threatening meningoencephalitis in humans, but its overall biological and pathogenic regulatory circuits remain elusive, particularly due to the presence of an evolutionarily divergent set of transcription factors (TFs). Here, we report the construction of a high-quality library of 322 signature-tagged gene-deletion strains for 155 putative TF genes previously predicted using the DNA-binding domain TF database, and examine their in vitro and in vivo phenotypic traits under 32 distinct growth conditions. At least one phenotypic trait is exhibited by 145 out of 155 TF mutants (93%) and ∼85% of them (132/155) are functionally characterized for the first time in this study. The genotypic and phenotypic data for each TF are available in the C. neoformans TF phenome database (http://tf.cryptococcus.org). In conclusion, our phenome-based functional analysis of the C. neoformans TF mutant library provides key insights into transcriptional networks of basidiomycetous fungi and human fungal pathogens.

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.

Calcineurin-Crz1 signaling in lower eukaryotes

Calcium ions are ubiquitous intracellular messengers. An increase in the cytosolic Ca(2+) concentration activates many proteins, including calmodulin and the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin. The phosphatase is conserved from yeast to humans (except in plants), and many target proteins of calcineurin have been identified. The most prominent and best-investigated targets, however, are the transcription factors NFAT (nuclear factor of activated T cells) in mammals and Crz1 (calcineurin-responsive zinc finger 1) in yeast. In recent years, many orthologues of Crz1 have been identified and characterized in various species of fungi, amoebae, and other lower eukaryotes. It has been shown that the functions of calcineurin-Crz1 signaling, ranging from ion homeostasis through cell wall biogenesis to the building of filamentous structures, are conserved in the different organisms. Furthermore, frequency-modulated gene expression through Crz1 has been discovered as a striking new mechanism by which cells can coordinate their response to a signal. In this review, I focus on the latest findings concerning calcineurin-Crz1 signaling in fungi, amoebae and other lower eukaryotes. I discuss the potential of Crz1 and its orthologues as putative drug targets, and I also discuss possible parallels with calcineurin-NFAT signaling in mammals.

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.

Phosphoproteome of Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated pathogenic yeast, which causes life threatening meningitis in immunocompromised individuals. C. neoformans var. grubii is the most prevalent and virulent form among the two varieties of C. neoformans - C. neoformans var. grubii and C. neoformans var. neoformans. The virulence of C. neoformans is mainly conferred by its capsule and melanin. cAMP dependent PKA-induced phosphorylation events are reported to be associated with the expression of these virulence traits, which highlights the importance of phosphoproteins in virulence and infection. Therefore, we performed global profiling of phosphoproteome of C. neoformans to enable a better understanding of molecular regulation of its virulence and pathogenesis. High resolution mass spectrometry of TiO2 enriched phosphopeptides from C. neoformans var. grubii grown in culture led to the identification of 1089 phosphopeptides derived from 648 proteins including about 45 kinases. Motif enrichment analysis revealed that most CDK family substrates were found to be phosphorylated. This indicates that cyclin-dependent kinases were among the active kinases in the pathogen in culture. These studies provide a framework for understanding virulence mechanisms in the context of signalling pathways in pathogenic yeast. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.

BIOLOGICAL SIGNIFICANCE

C. neoformans is a pathogenic yeast responsible for cryptococcal meningitis. Melanin and polysaccharide capsule have been established as some of the key virulence factors that play a major role in the pathogenesis of C. neoformans. Recent studies have shown the role of kinase mediated signalling pathways in governing biosynthesis of these virulence factors. This study revealed 1540 phosphorylation sites in 648 proteins providing a comprehensive view of phosphoproteins in C. neoformans. This should serve as a useful resource to explore activated signalling pathways in C. neoformans and their association with its virulence and pathogenesis.

The CRZ1/SP1-like gene links survival under limited aeration, cell integrity and biofilm formation in the pathogenic yeast Cryptococcus neoformans

AIMS

Limited aeration has been demonstrated to cause slowdown in proliferation and delayed budding, resulting eventually in a unique unbudded G2-arrest in the obligate aerobic pathogenic yeast Cryptococcus neoformans. Also, the ability to adapt to decreased oxygen levels during pathogenesis has been identified as a virulence factor in C. neoformans. The aim of this study was to identify and characterize genes that are necessary for the proliferation slowdown and G2-arrest caused by limited aeration.

METHODS

Random mutants were prepared and screened for lack of typical slowdown of proliferation under limited aeration. The CNAG_00156.2 gene coding for a zinc-finger transcription factor was identified in mutants showing most distinctive phenotype. Targeted deletion strain and reconstituted strain were prepared to characterize and confirm the gene functions. This gene was also identified in a parallel studies as homologous both to calcineurin responsive (Crz1) and PKC1-dependent (SP1-like) transcription factors.

RESULTS

We have confirmed the role of the cryptococcal homologue of CRZ1/SP1-like transcription factor in cell integrity, and newly demonstrated its role in slowdown of proliferation and survival under reduced aeration, in biofilm formation and in susceptibility to fluconazole.

CONCLUSIONS

Our data demonstrate a tight molecular link between slowdown of proliferation during hypoxic adaptation and maintenance of cell integrity in C. neoformans and present a new role for the CRZ1 family of transcription factors in fungi. The exact positioning of this protein in cryptococcal signalling cascades remains to be clarified.

Allergen1 regulates polysaccharide structure in Cryptococcus neoformans

Cryptococcus neoformans is an important human, fungal pathogen that sheds polysaccharide (exo-PS) into host tissues. While shed exo-PS mediates numerous untoward effects (including promoting increased intracranial pressure), little is known about the regulation of this phenomenon. Since downregulation of the Allergen 1 (ALL1) gene is associated with high ICP, we investigated the relationship between ALL1 expression and exo-PS structure using a variety of biophysical techniques. The Δall1 mutants of two serotypes produced a shorter exo-PS with less branching and structural complexity than the parental strains. Consistent with lower branching, these exo-PSs manifested higher intrinsic viscosity than the parental strains. The Δall1 mutant strains manifested differences in epitope expression and significant resistance to phagocytosis. Exo-PS of Δall1 mutant exhibited anti-phagocytic properties. Comparative transcriptome analysis of mutant and parental strain under iron-deprived conditions indicated a role of ALL1 in iron homeostasis, characterized by differential regulation of genes that mediate iron reduction and transport. Together, our results demonstrate a role of ALL1 in regulating conformational aspects of PS structure and iron homeostasis. These findings provide a mechanism to explain how changes in ALL1 expression influence virulence of switch variants and suggest that structural changes and polymer length are epigenetically regulated.

Phospholipase C of Cryptococcus neoformans regulates homeostasis and virulence by providing inositol trisphosphate as a substrate for Arg1 kinase

Phospholipase C (PLC) of Cryptococcus neoformans (CnPlc1) is crucial for virulence of this fungal pathogen. To investigate the mechanism of CnPlc1-mediated signaling, we established that phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a major CnPlc1 substrate, which is hydrolyzed to produce inositol trisphosphate (IP(3)). In Saccharomyces cerevisiae, Plc1-derived IP(3) is a substrate for the inositol polyphosphate kinase Arg82, which converts IP(3) to more complex inositol polyphosphates. In this study, we show that in C. neoformans, the enzyme encoded by ARG1 is the major IP(3) kinase, and we further demonstrate that catalytic activity of Arg1 is essential for cellular homeostasis and virulence in the Galleria mellonella infection model. IP(3) content was reduced in the CnΔplc1 mutant and markedly increased in the CnΔarg1 mutant, while PIP(2) was increased in both mutants. The CnΔplc1 and CnΔarg1 mutants shared significant phenotypic similarity, including impaired thermotolerance, compromised cell walls, reduced capsule production and melanization, defective cell separation, and the inability to form mating filaments. In contrast to the S. cerevisiae ARG82 deletion mutant (ScΔarg82) strain, the CnΔarg1 mutant exhibited dramatically enlarged vacuoles indicative of excessive vacuolar fusion. In mammalian cells, PLC-derived IP(3) causes Ca(2+) release and calcineurin activation. Our data show that, unlike mammalian PLCs, CnPlc1 does not contribute significantly to calcineurin activation. Collectively, our findings provide the first evidence that the inositol polyphosphate anabolic pathway is essential for virulence of C. neoformans and further show that production of IP(3) as a precursor for synthesis of more complex inositol polyphosphates is the key biochemical function of CnPlc1.

The Crz1/Sp1 transcription factor of Cryptococcus neoformans is activated by calcineurin and regulates cell wall integrity

Cryptococcus neoformans survives host temperature and regulates cell wall integrity via a calcium-dependent phosphatase, calcineurin. However, downstream effectors of C. neoformans calcineurin are largely unknown. In S. cerevisiae and other fungal species, a calcineurin-dependent transcription factor Crz1, translocates to nuclei upon activation and triggers expression of target genes. We now show that the C. neoformans Crz1 ortholog (Crz1/Sp1), previously identified as a protein kinase C target during starvation, is a bona fide target of calcineurin under non-starvation conditions, during cell wall stress and growth at high temperature. Both the calcineurin-defective mutant, Δcna1, and a CRZ1/SP1 mutant (Δcrz1) were susceptible to cell wall perturbing agents. Furthermore, expression of the chitin synthase encoding gene, CHS6, was reduced in both mutants. We tracked the subcellular localization of Crz1-GFP in WT C. neoformans and Δcna1 in response to different stimuli, in the presence and absence of the calcineurin inhibitor, FK506. Exposure to elevated temperature (30–37°C vs 25°C) and extracellular calcium caused calcineurin-dependent nuclear accumulation of Crz1-GFP. Unexpectedly, 1M salt and heat shock triggered calcineurin-independent Crz1-GFP sequestration within cytosolic and nuclear puncta. To our knowledge, punctate cytosolic distribution, as opposed to nuclear targeting, is a unique feature of C. neoformans Crz1. We conclude that Crz1 is selectively activated by calcium/calcineurin-dependent and independent signals depending on the environmental conditions.

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.

Adaptation of Cryptococcus neoformans to mammalian hosts: integrated regulation of metabolism and virulence

The basidiomycete fungus Cryptococcus neoformans infects humans via inhalation of desiccated yeast cells or spores from the environment. In the absence of effective immune containment, the initial pulmonary infection often spreads to the central nervous system to result in meningoencephalitis. The fungus must therefore make the transition from the environment to different mammalian niches that include the intracellular locale of phagocytic cells and extracellular sites in the lung, bloodstream, and central nervous system. Recent studies provide insights into mechanisms of adaptation during this transition that include the expression of antiphagocytic functions, the remodeling of central carbon metabolism, the expression of specific nutrient acquisition systems, and the response to hypoxia. Specific transcription factors regulate these functions as well as the expression of one or more of the major known virulence factors of C. neoformans. Therefore, virulence factor expression is to a large extent embedded in the regulation of a variety of functions needed for growth in mammalian hosts. In this regard, the complex integration of these processes is reminiscent of the master regulators of virulence in bacterial pathogens.