Two glutathione peroxidases in the fungal pathogen Cryptococcus neoformans are expressed in the presence of specific substrates

With age comes resilience: how mitochondrial modulation drives age-associated fluconazole tolerance in Cryptococcus neoformans

Cryptococcus neoformans (Cn) is an opportunistic fungal microorganism that causes life-threatening meningoencephalitis. During the infection, the microbial population is heterogeneously composed of cells with varying generational ages, with older cells accumulating during chronic infections. This is attributed to their enhanced resistance to phagocytic killing and tolerance of antifungals like fluconazole (FLC). In this study, we investigated the role of ergosterol synthesis, ATP-binding cassette (ABC) transporters, and mitochondrial metabolism in the regulation of age-dependent FLC tolerance. We find that old Cn cells increase the production of ergosterol and exhibit upregulation of ABC transporters. Old cells also show transcriptional and phenotypic characteristics consistent with increased metabolic activity, leading to increased ATP production. This is accompanied by increased production of reactive oxygen species, which results in mitochondrial fragmentation. This study demonstrates that the metabolic changes occurring in the mitochondria of old cells drive the increase in ergosterol synthesis and the upregulation of ABC transporters, leading to FLC tolerance.

IMPORTANCE

Infections caused by Cryptococcus neoformans cause more than 180,000 deaths annually. Estimated 1-year mortality for patients receiving care ranges from 20% in developed countries to 70% in developing countries, suggesting that current treatments are inadequate. Some fungal cells can persist and replicate despite the usage of current antifungal regimens, leading to death or treatment failure. Aging in fungi is associated with enhanced tolerance against antifungals and resistance to killing by host cells. This study shows that age-dependent increase in mitochondrial reactive oxygen species drive changes in the regulation of membrane transporters and ergosterol synthesis, ultimately leading to the heightened tolerance against fluconazole in old C. neoformans cells. Understanding the underlying molecular mechanisms of this age-associated antifungal tolerance will enable more targeted antifungal therapies for cryptococcal infections.

Glutathione-mediated redox regulation in Cryptococcus neoformans impacts virulence

The fungal pathogen Cryptococcus neoformans is well adapted to its host environment. It has several defence mechanisms to evade oxidative and nitrosative agents released by phagocytic host cells during infection. Among them, melanin production is linked to both fungal virulence and defence against harmful free radicals that facilitate host innate immunity. How C. neoformans manipulates its redox environment to facilitate melanin formation and virulence is unclear. Here we show that the antioxidant glutathione is inextricably linked to redox-active processes that facilitate melanin and titan cell production, as well as survival in macrophages and virulence in a murine model of cryptococcosis. Comparative metabolomics revealed that disruption of glutathione biosynthesis leads to accumulation of reducing and acidic compounds in the extracellular environment of mutant cells. Overall, these findings highlight the importance of redox homeostasis and metabolic compensation in pathogen adaptation to the host environment and suggest new avenues for antifungal drug development.

Achieving Resilience in Aging: How Mitochondrial Modulation Drives Age-associated Fluconazole Tolerance in Cryptococcus neoformans

Cryptococcus neoformans ( Cn ) is an opportunistic fungal microorganism that causes life-threatening meningoencephalitis. During the infection, the microbial population is heterogeneously composed of cells with varying generational ages, with older cells accumulating during chronic infections. This is attributed to their enhanced resistance to phagocytic killing and tolerance of antifungals like fluconazole (FLC). In this study, we investigated the role of ergosterol synthesis, ATP-binding cassette (ABC) transporters, and mitochondrial metabolism in the regulation of age-dependent FLC tolerance. We find that old Cn cells increase the production of ergosterol and exhibit upregulation of ABC transporters. Old cells also show transcriptional and phenotypic characteristics consistent with increased metabolic activity, leading to increased ATP production. This is accompanied by increased production of reactive oxygen species (ROS), which results in mitochondrial fragmentation. This study demonstrates that the metabolic changes occurring in the mitochondria of old cells drive the increase in ergosterol synthesis and the upregulation of ABC transporters, leading to FLC tolerance.

IMPORTANCE

Infections caused by Cryptococcus neoformans cause more than 180,000 deaths annually. Estimated one-year mortality for patients receiving care ranges from 20% in developed countries to 70% in developing countries, suggesting that current treatments are inadequate. Some fungal cells can persist and replicate despite the usage of current antifungal regimens, leading to death or treatment failure. In replicative aging, older cells display a resilient phenotype, characterized by their enhanced tolerance against antifungals and resistance to killing by host cells. This study shows that age-dependent increase in mitochondrial reactive oxygen species drive changes in ABC transporters and ergosterol synthesis, ultimately leading to the heightened tolerance against fluconazole in old C. neoformans cells. Understanding the underlying molecular mechanisms of this age-associated antifungal tolerance will enable more targeted antifungal therapies for cryptococcal infections.

Measuring Stress Phenotypes in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic human fungal pathogen capable of surviving in a wide range of environments and hosts. It has been developed as a model organism to study fungal pathogenesis due to its fully sequenced haploid genome and optimized gene deletion and mutagenesis protocols. These methods have greatly aided in determining the relationship between Cryptococcus genotype and phenotype. Furthermore, the presence of congenic mata and matα strains associated with a defined sexual cycle has helped further understand cryptococcal biology. Several in vitro stress conditions have been optimized to closely mimic the stress that yeast encounter in the environment or within the infected host. These conditions have proven to be extremely useful in elucidating the role of several genes in allowing yeast to adapt and survive in hostile external environments. This chapter describes various in vitro stress conditions that could be used to test the sensitivity of different mutant strains, as well as the protocol for preparing them. We have also included a list of mutants that could be used as a positive control strain when testing the sensitivity of the desired strain to a specific stress.

Dual RNA-Seq Profiling Unveils Mycoparasitic Activities of Trichoderma atroviride against Haploid Armillaria ostoyae in Antagonistic Interaction Assays

Armillaria ostoyae, a species among the destructive forest pathogens from the genus Armillaria, causes root rot disease on woody plants worldwide. Efficient control measures to limit the growth and impact of this severe underground pathogen are under investigation. In a previous study, a new soilborne fungal isolate, Trichoderma atroviride SZMC 24276 (TA), exhibited high antagonistic efficacy, which suggested that it could be utilized as a biocontrol agent. The dual culture assay results indicated that the haploid A. ostoyae-derivative SZMC 23085 (AO) (C18/9) is highly susceptible to the mycelial invasion of TA. In the present study, we analyzed the transcriptome of AO and that of TA in in vitro dual culture assays to test the molecular arsenal of Trichoderma antagonism and the defense mechanisms of Armillaria. We conducted time-course analysis and functional annotation and analyzed enriched pathways and differentially expressed genes including biocontrol-related candidate genes from TA and defense-related candidate genes from AO. The results indicated that TA deployed several biocontrol mechanisms when confronted with AO. In response, AO initiated multiple defense mechanisms to protect against the fungal attack. To our knowledge, the present study offers the first transcriptome analysis of a biocontrol fungus attacking AO. Overall, this study provides insights that aid the further exploration of plant pathogen-biocontrol agent interaction mechanisms. IMPORTANCE Armillaria species can survive for decades in the soil on dead woody debris, develop rapidly under favorable conditions, and harmfully infect newly planted forests. Our previous study found Trichoderma atroviride to be highly effective in controlling Armillaria growth; therefore, our current work explored the molecular mechanisms that might play a key role in Trichoderma-Armillaria interactions. Direct confrontation assays combined with time course-based dual transcriptome analysis provided a reliable system for uncovering the interactive molecular dynamics between the fungal plant pathogen and its mycoparasitic partner. Furthermore, using a haploid Armillaria isolate allowed us to survey the deadly prey-invading activities of the mycoparasite and the ultimate defensive strategies of its prey. Our current study provides detailed insights into the essential genes and mechanisms involved in Armillaria defense against Trichoderma and the genes potentially involved in the efficiency of Trichoderma to control Armillaria. In addition, using a sensitive haploid Armillaria strain (C18/9), with its complete genome data already available, also offers the opportunity to test possible variable molecular responses of Armillaria ostoyae toward diverse Trichoderma isolates with various biocontrol abilities. Initial molecular tests of the dual interactions may soon help to develop a targeted biocontrol intervention with mycoparasites against plant pathogens.

The small heat shock protein Hsp12.1 has a major role in the stress response and virulence of Cryptococcus gattii

Cryptococcus gattii is one of the etiological agents of cryptococcosis. To achieve a successful infection, C. gattii cells must overcome the inhospitable host environment and deal with the highly specialized immune system and poor nutrients availability. Inside the host, C. gattii uses a diversified set of tools to maintain homeostasis and establish infection, such as the expression of remarkable and diverse heat shock proteins (Hsps). Grouped by molecular weight, little is known about the Hsp12 subset in pathogenic fungi. In this study, the function of the C. gattii HSP12.1 and HSP12.2 genes was characterized. Both genes were upregulated during murine infection and heat shock. The hsp12.1 Δ null mutant cells were sensitive to plasma membrane and oxidative stressors. Moreover, HSP12 deletion induced C. gattii reactive oxygen species (ROS) accumulation associated with a differential expression pattern of oxidative stress-responsive genes compared to the wild type strain. Apart from these findings, the deletion of the paralog gene HSP12.2 did not lead to any detectable phenotype. Additionally, the double-deletion mutant strain hsp12.1 Δ /hsp12.2 Δ presented a similar phenotype to the single-deletion mutant hsp12.1 Δ, suggesting a minor participation of Hsp12.2 in these processes. Furthermore, HSP12.1 disruption remarkably affected C. gattii virulence and phagocytosis by macrophages in an invertebrate model of infection, demonstrating its importance for C. gattii pathogenicity.

The Role of the Glutathione System in Stress Adaptation, Morphogenesis and Virulence of Pathogenic Fungi

Morphogenesis and stress adaptation are key attributes that allow fungal pathogens to thrive and infect human hosts. During infection, many fungal pathogens undergo morphological changes, and this ability is highly linked to virulence. Furthermore, pathogenic fungi have developed multiple antioxidant defenses to cope with the host-derived oxidative stress produced by phagocytes. Glutathione is a major antioxidant that can prevent cellular damage caused by various oxidative stressors. While the role of glutathione in stress detoxification is known, studies of the glutathione system in fungal morphological switching and virulence are lacking. This review explores the role of glutathione metabolism in fungal adaptation to stress, morphogenesis, and virulence. Our comprehensive analysis of the fungal glutathione metabolism reveals that the role of glutathione extends beyond stressful conditions. Collectively, glutathione and glutathione-related proteins are necessary for vitality, cellular development and pathogenesis.

Basidiomycota Fungi and ROS: Genomic Perspective on Key Enzymes Involved in Generation and Mitigation of Reactive Oxygen Species

Our review includes a genomic survey of a multitude of reactive oxygen species (ROS) related intra- and extracellular enzymes and proteins among fungi of Basidiomycota, following their taxonomic classification within the systematic classes and orders, and focusing on different fungal lifestyles (saprobic, symbiotic, pathogenic). Intra- and extracellular ROS metabolism-involved enzymes (49 different protein families, summing 4170 protein models) were searched as protein encoding genes among 63 genomes selected according to current taxonomy. Extracellular and intracellular ROS metabolism and mechanisms in Basidiomycota are illustrated in detail. In brief, it may be concluded that differences between the set of extracellular enzymes activated by ROS, especially by H2O2, and involved in generation of H2O2, follow the differences in fungal lifestyles. The wood and plant biomass degrading white-rot fungi and the litter-decomposing species of Agaricomycetes contain the highest counts for genes encoding various extracellular peroxidases, mono- and peroxygenases, and oxidases. These findings further confirm the necessity of the multigene families of various extracellular oxidoreductases for efficient and complete degradation of wood lignocelluloses by fungi. High variations in the sizes of the extracellular ROS-involved gene families were found, however, among species with mycorrhizal symbiotic lifestyle. In addition, there are some differences among the sets of intracellular thiol-mediation involving proteins, and existence of enzyme mechanisms for quenching of intracellular H2O2 and ROS. In animal- and plant-pathogenic species, extracellular ROS enzymes are absent or rare. In these fungi, intracellular peroxidases are seemingly in minor role than in the independent saprobic, filamentous species of Basidiomycota. Noteworthy is that our genomic survey and review of the literature point to that there are differences both in generation of extracellular ROS as well as in mechanisms of response to oxidative stress and mitigation of ROS between fungi of Basidiomycota and Ascomycota.

A Conserved Gcn2-Gcn4 Axis Links Methionine Utilization and the Oxidative Stress Response in Cryptococcus neoformans

The fungal pathogen Cryptococcus neoformans relies on post-transcriptional mechanisms of gene regulation to adapt to stressors it encounters in the human host, such as oxidative stress and nutrient limitation. The kinase Gcn2 regulates translation in response to stress by phosphorylating the initiation factor eIF2, and it is a crucial factor in withstanding oxidative stress in C. neoformans, and amino acid limitation in many fungal species. However, little is known about the role of Gcn2 in nitrogen limitation in C. neoformans. In this study, we demonstrate that Gcn2 is required for C. neoformans to utilize methionine as a source of nitrogen, and that the presence of methionine as a sole nitrogen source induces eIF2 phosphorylation. The stress imposed by methionine leads to an oxidative stress response at both the levels of transcription and translation, as seen through polysome profiling as well as increased abundance of select oxidative stress response transcripts. The transcription factor Gcn4 is also required for methionine utilization and oxidative stress resistance, and RT-qPCR data suggests that it regulates expression of certain transcripts in response to oxidative stress. The results of this study suggest a connection between nitrogen metabolism and oxidative stress in C. neoformans that is mediated by Gcn4, possibly indicating the presence of a compound stress response in this clinically important fungal pathogen.

Basic principles of the virulence of Cryptococcus

Among fungal pathogens, Cryptococcus neoformans has gained great importance among the scientific community of several reasons. This fungus is the causative agent of cryptococcosis, a disease mainly associated to HIV immunosuppression and characterized by the appearance of meningoencephalitis. Cryptococcal meningitis is responsible for hundreds of thousands of deaths every year. Research of the pathogenesis and virulence mechanisms of this pathogen has focused on three main different areas: Adaptation to the host environment (nutrients, pH, and free radicals), mechanism of immune evasion (which include phenotypic variations and the ability to behave as a facultative intracellular pathogen), and production of virulence factors. Cryptococcus neoformans has two phenotypic characteristics, the capsule and synthesis of melanin that have a profound effect in the virulence of the yeast because they both have protective effects and induce host damage as virulence factors. Finally, the mechanisms that result in dissemination and brain invasion are also of key importance to understand cryptococcal disease. In this review, I will provide a brief overview of the main mechanisms that makes C. neoformans a pathogen in susceptible patients.

Abbreviations: RNS: reactive nitrogen species; BBB: brain blood barrier; GXM: glucuronoxylomannan; GXMGal: glucuronoxylomannogalactan

Regulatory Mechanism of the Atypical AP-1-Like Transcription Factor Yap1 in Cryptococcus neoformans

The human meningitis fungal pathogen, Cryptococcus neoformans, contains the atypical yeast AP-1-like protein Yap1. Yap1 lacks an N-terminal cysteine-rich domain (n-CRD), which is present in other fungal Yap1 orthologs, but has a C-terminal cysteine-rich domain (c-CRD). However, the role of c-CRD and its regulatory mechanism remain unknown. Here, we report that Yap1 is transcriptionally regulated in response to oxidative, osmotic, and membrane-destabilizing stresses partly in an Mpk1-dependent manner, supporting its role in stress resistance. The c-CRD domain contributed to the role of Yap1 only in resistance to certain oxidative stresses and azole drugs but not in other cellular functions. Yap1 has a minor role in the survival of C. neoformans in a murine model of systemic cryptococcosis.

AP-1-like transcription factors play evolutionarily conserved roles as redox sensors in eukaryotic oxidative stress responses. In this study, we aimed to elucidate the regulatory mechanism of an atypical yeast AP-1-like protein, Yap1, in the stress response and virulence of Cryptococcus neoformans. YAP1 expression was induced and involved not only by oxidative stresses, such as H₂O₂ and diamide, but also by other environmental stresses, such as osmotic and membrane-destabilizing stresses. Yap1 was distributed throughout both the cytoplasm and the nucleus under basal conditions and more enriched within the nucleus in response to diamide but not to other stresses. Deletion of the C-terminal cysteine-rich domain (c-CRD), where the nuclear export signal resides, increased nuclear enrichment of Yap1 under basal conditions and altered resistance to oxidative stresses but did not affect the role of Yap1 in other stress responses and cellular functions. As a potential upstream regulator of Yap1, we discovered that Mpk1 is positively involved, but Hog1 is mostly dispensable. Pleiotropic roles for Yap1 in diverse biological processes were supported by transcriptome data showing that 162 genes are differentially regulated by Yap1, with further analysis revealing that Yap1 promotes cellular resistance to toxic cellular metabolites produced during glycolysis, such as methylglyoxal. Finally, we demonstrated that Yap1 plays a minor role in the survival of C. neoformans within hosts.

IMPORTANCE The human meningitis fungal pathogen, Cryptococcus neoformans, contains the atypical yeast AP-1-like protein Yap1. Yap1 lacks an N-terminal cysteine-rich domain (n-CRD), which is present in other fungal Yap1 orthologs, but has a C-terminal cysteine-rich domain (c-CRD). However, the role of c-CRD and its regulatory mechanism remain unknown. Here, we report that Yap1 is transcriptionally regulated in response to oxidative, osmotic, and membrane-destabilizing stresses partly in an Mpk1-dependent manner, supporting its role in stress resistance. The c-CRD domain contributed to the role of Yap1 only in resistance to certain oxidative stresses and azole drugs but not in other cellular functions. Yap1 has a minor role in the survival of C. neoformans in a murine model of systemic cryptococcosis.

Secreted peroxidases VmPODs play critical roles in the conidiation, H2O2 sensitivity and pathogenicity of Valsa mali

Apple Valsa canker, caused by the necrotrophic pathogen Valsa mali, is a devastating disease of apples and causes great financial loss in East Asia. Improving the understanding of apple - V. mali interactions will contribute to disease management. In this study, three predicted secreted peroxidases (VmPOD1, VmPOD2 and VmPOD3) were uncovered based on the secretome and genome information of V. mali. Phylogenetic analysis showed that VmPOD1 is a catalase peroxidase, VmPOD2 is a chloroperoxidase, and VmPOD3 is a plant peroxidase-like peroxidase. The secretion function of the corresponding genes was confirmed using the yeast invertase secretion system. The deletion of VmPODs did not affect the vegetative growth when the mutants (ΔVmPOD1, ΔVmPOD2 and ΔVmPOD3) and the wild-type strain 03-8 were grown on PDA medium at 25 °C in the dark. However, the respective mutants showed impaired conidiation ability with fewer pycnidia, and all gene deletion mutants grew more slowly than 03-8 on PDA supplemented with H₂O₂ (Final concentration: 0.06 mol/L H₂O₂)_. In addition, VmPOD1 and VmPOD2 were found to be significantly up-regulated at an early infection stage, and VmPOD3 showed sustained high expression during the whole infection progress of V. mali. In addition, the virulence of ΔVmPOD3 was significantly reduced, implying that VmPOD3 plays a critical role during the interaction between V. mali and apple. All of the defective phenotypes could be nearly restored by re-introducing the wild-type VmPOD1, VmPOD2 or VmPOD3 allele. The results enhanced our understanding of the secreted peroxidase, which could also act as a type of virulence factor from the necrotrophic pathogen V. mali and provided new insight into the role of the pathogen-secreted peroxidase.

Thioredoxin and Glutaredoxin Systems Required for Oxidative Stress Resistance, Fungicide Sensitivity, and Virulence of Alternaria alternata

This study determined the function of thioredoxin and glutaredoxin systems in the phytopathogenic fungus Alternaria alternata via analyzing mutants obtained from the targeted deletion of genes encoding thioredoxin peroxidase (Tsa1), thioredoxin reductase (Trr1), and glutathione reductase (Glr1). Trr1 and Glr1, but not Tsa1, are required for growth and conidiation. The reduced growth and conidiation seen in the Trr1 or Glr1 deletion mutant can be restored by glutathione. Deletion mutants showing growth inhibition by oxidants are defective for H₂O₂ detoxification and induce smaller lesions on citrus leaves. Trr1 and Glr1, but not Tsa1, also contribute to NaCl resistance. Glr1 is required for sorbitol resistance and is responsible for resistance to mancozeb and boscalid but not chlorothalonil fungicides, a novel phenotype that has not been reported in fungi. Trr1 is required for resistance to boscalid and chlorothalonil fungicides but confers susceptibility to mancozeb. The Tsa1 deletion mutant displays wild-type sensitivity to the tested fungicides. The expression of Tsa1 and Trr1 is regulated by the oxidative stress responsive regulators Yap1, Hog1, and Skn7. The expression of Tsa1, but not Trr1, is also regulated indirectly by the NADPH oxidase. The results indicate that the capability to resist oxidative stress is required for virulence of A. alternataIMPORTANCE The thioredoxin and glutaredoxin systems are important thiol antioxidant systems in cells, and knowledge of these two systems in the plant-pathogenic fungus A. alternata is useful for finding new strategies to reduce the virulence of this pathogen. In this study, we demonstrated that thiol antioxidant system-related genes (Tsa1, Trr1, and Glr1) are required for H₂O₂ detoxification and virulence in A. alternata Moreover, deletion of Trr1 results in hypersensitivity to the fungicides chlorothalonil and boscalid, and Glr1 deletion mutants are highly sensitive to mancozeb, which is the fungicide mostly used in citrus fields. Therefore, our findings demonstrate that the ability to detoxify reactive oxygen species (ROS) plays a critical role in pathogenesis on citrus and provide novel insights into the physiological functions of thiol-containing systems in fungicide sensitivity for A. alternata.

Cryptococcal pathogenic mechanisms: a dangerous trip from the environment to the brain

Cryptococcus neoformans is an opportunistic pathogenic yeast that causes serious infections, most commonly of the central nervous system (CNS). C. neoformans is mainly found in the environment and acquired by inhalation. It could be metaphorically imagined that cryptococcal disease is a "journey" for the microorganism that starts in the environment, where this yeast loads its suitcase with virulence traits. C. neoformans first encounters the infected mammalian host in the lungs, a site in which it must choose the right elements from its "virulence suitcase" to survive the pulmonary immune response. However, the lung is often only the first stop in this journey, and in some individuals the fungal trip continues to the brain. To enter the brain, C. neoformans must "open" the main barrier that protects this organ, the blood brain barrier (BBB). Once in the brain, C. neoformans expresses a distinct set of protective attributes that confers a strong neurotropism and the ability to cause brain colonisation. In summary, C. neoformans is a unique fungal pathogen as shown in its ability to survive in the face of multiple stress factors and to express virulence factors that contribute to the development of disease.

Molecular dynamics simulation studies suggests unconventional roles of non-secretary laccases from enteropathogenic gut bacteria and Cryptococcus neoformans serotype D

Laccase in Cryptococcus neoformans is covalently linked to the carbohydrate moiety of the cell wall, which allows it to get access to the different substrates for catalyzing their oxidation and therefore plays a vital role in the virulence. The laccase gene (3.0 kb) from C. neoformans serotype D was amplified, cloned and sequenced for protein modeling, docking and simulation studies. The three dimensional homology models of laccase protein from C. neoformans and other pathogenic gut bacteria were docked with selected biomolecules like prostaglandins (PG), membrane phospholipids, neurotransmitters (serotonin) using GOLD software. The GOLDscore values of laccase from C. neoformans docked with prostaglandinH₂ (59.76), prostaglandinG₂ (59.45), prostaglandinE₂ (60.99), phosphatidylinositol (54.95), phosphatidylcholine (46.26), phosphatidylserine (55.26), arachidonic acid (53.08) and serotonin (46.22) were similar to the laccase from enteropathogenic bacteria but showed a better binding affinity as compared to that of the non-pathogenic bacteria (e.g. Bacillus safensis, Bacillus pumilus and Bacillus subtilis). The RMSD of MD simulation study done for 25 ns using laccase protein from C. neoformans complexed with phosphatidylcholine was found to be highly stable, followed by the laccase-PGE₂ and laccase-serotonin complexes. Furthermore, the binding free energy results were found to support the docking and MD simulation results. The present study implies that few candidate ligands can be intermediate substrate in the catalysis of microbial laccases, which can further play some crucial role in the cell signaling and pathogenesis of enteropathogenic gut micro flora and C. neoformans.

Fungal Strategies to Evade the Host Immune Recognition

The recognition of fungal cells by the host immune system is key during the establishment of a protective anti-fungal response. Even though the immune system has evolved a vast number of processes to control these organisms, they have developed strategies to fight back, avoiding the proper recognition by immune components and thus interfering with the host protective mechanisms. Therefore, the strategies to evade the immune system are as important as the virulence factors and attributes that damage the host tissues and cells. Here, we performed a thorough revision of the main fungal tactics to escape from the host immunosurveillance processes. These include the composition and organization of the cell wall, the fungal capsule, the formation of titan cells, biofilms, and asteroid bodies; the ability to undergo dimorphism; and the escape from nutritional immunity, extracellular traps, phagocytosis, and the action of humoral immune effectors.

Anti-Immune Strategies of Pathogenic Fungi

Pathogenic fungi have developed many strategies to evade the host immune system. Multiple escape mechanisms appear to function together to inhibit attack by the various stages of both the adaptive and the innate immune response. Thus, after entering the host, such pathogens fight to overcome the immune system to allow their survival, colonization and spread to different sites of infection. Consequently, the establishment of a successful infectious process is closely related to the ability of the pathogen to modulate attack by the immune system. Most strategies employed to subvert or exploit the immune system are shared among different species of fungi. In this review, we summarize the main strategies employed for immune evasion by some of the major pathogenic fungi.

The glutathione peroxidase-mediated reactive oxygen species resistance, fungicide sensitivity and cell wall construction in the citrus fungal pathogen Alternaria alternata

The ability to detoxify reactive oxygen species (ROS) is critical for pathogenicity in the necrotrophic fungus Alternaria alternata. We report a glutathione peroxidase 3 (AaGPx3) involved in the complex signalling network that is essential for the detoxification of cellular stresses induced by ROS and for A. alternata pathogenesis in citrus. AaGPx3 deletion mutants displayed increased sensitivity to H2 O2 and many ROS-generating compounds. AaGPx3 is required for correct fungal development as the AaGPx3 mutant strains showed a severe reduction in conidiation. AaGPx3 mutants accumulated higher chitin content than the wild-type and were less sensitive to the cell wall-targeting compounds calcofluor white and Congo red, as well as the fungicides fludioxonil and vinclozolin, suggesting a role of the glutathione systems in fungal cell wall construction. Virulence assays revealed that AaGPx3 is required for full virulence. The expression of AaGPx3 was downregulated in fungal strains carrying defective NADPH oxidase (Nox) or the oxidative stress responsive regulators YAP1 and HOG1, all implicated in ROS resistance. These results further support the important role of ROS detoxification during A. alternata pathogenesis in citrus. Overall, our study provides genetic evidence to define the central role of AaGPx3 in the biological and pathological functions of A. alternata.

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.

Functional analysis of the role of glutathione peroxidase (GPx) in the ROS signaling pathway, hyphal branching and the regulation of ganoderic acid biosynthesis in Ganoderma lucidum

Ganoderma lucidum, a hallmark of traditional Chinese medicine, has been widely used as a pharmacologically active compound. Although numerous research studies have focused on the pharmacological mechanism, fewer studies have explored the basic biological features of this species, restricting the further development and application of this important mushroom. Because of the ability of this mushroom to reduce and detoxify the compounds produced by various metabolic pathways, glutathione peroxidase (GPx) is one of the most important antioxidant enzymes with respect to ROS. Although studies in both animals and plants have suggested many important physiological functions of GPx, there are few systematic research studies concerning the role of this enzyme in fungi, particularly in large basidiomycetes. In the present study, we cloned the GPx gene and created GPx-silenced strains by the down-regulation of GPx gene expression using RNA interference. The results indicated an essential role for GPx in controlling the intracellular H2O2 content, hyphal branching, antioxidant stress tolerance, cytosolic Ca(2+) content and ganoderic acid biosynthesis. Further mechanistic investigation revealed that GPx is regulated by intracellular H2O2 levels and suggested that crosstalk occurs between GPx and intracellular H2O2. Moreover, evidence was obtained indicating that GPx regulation of hyphal branching via ROS might occur independently of the cytosolic Ca(2+) content. Further mechanistic investigation also revealed that the effects of GPx on ganoderic acid synthesis via ROS are regulated by the cytosolic Ca(2+) content. Taken together, these findings indicate that ROS have a complex influence on growth, development and secondary metabolism in fungi and that GPx serves an important function. The present study provides an excellent framework to identify GPx functions and highlights a role for this enzyme in ROS regulation.

Sulphiredoxin plays peroxiredoxin-dependent and -independent roles via the HOG signalling pathway in Cryptococcus neoformans and contributes to fungal virulence

Mechanisms of oxidative stress resistance are crucial virulence factors for survival and proliferation of fungal pathogens within the human host. In this study we have identified and functionally characterized the role of sulphiredoxin, Srx1, in oxidative stress resistance of Cryptococcus neoformans causing fungal meningoencephalitis and regulation of peroxiredoxins, Tsa1 and Tsa3, and thioredoxins, Trx1 and Trx2. The C. neoformans HOG (High Osmolarity Glycerol response) pathway was essential for the transcriptional regulation of SRX1 under peroxide stress conditions. A gene deletion study revealed that Srx1 was required for cells to counteract peroxide stress, but not other oxidative damaging agents. HOG1 was found to be essential for the induction of adaptive response to peroxide stress with concurrent repression of ergosterol biosynthesis in an SRX1-independent manner. Consistent with this, phosphorylation of C. neoformans Hog1 was modulated by both low and high doses of exogenous hydrogen peroxide treatment. Immunoblot analysis using the C. neoformans Tsa1 specific antibody revealed that both Srx1 and Trx1 were essential for recycling of oxidized Tsa1. In addition to its role in peroxide sensing and response C. neoformans Srx1 was also found to be required for a peroxiredoxin-independent function in promoting fungicide-dependent cell swelling and growth arrest. Finally we showed the importance of C. neoformans Srx1 in fungal pathogenesis by demonstrating its requirement for full virulence using a mouse infection model.

Two Rac paralogs regulate polarized growth in the human fungal pathogen Cryptococcus neoformans

A genome wide analysis of the human fungal pathogen Cryptococcus neoformans var. grubii has revealed a number of duplications of highly conserved genes involved in morphogenesis. Previously, we reported that duplicate Cdc42 paralogs provide C. neoformans with niche-specific responses to environmental stresses: Cdc42 is required for thermotolerance, while Cdc420 supports the formation of titan cells. The related Rho-GTPase Rac1 has been shown in C. neoformans var. neoformans to play a major role in filamentation and to share Cdc42/Cdc420 binding partners. Here we report the characterization of a second Rac paralog in C. neoformans, Rac2, and describe its overlapping function with the previously described CnRac, Rac1. Further, we demonstrate that the Rac paralogs play a primary role in polarized growth via the organization of reactive oxygen species and play only a minor role in the organization of actin. Finally, we provide preliminary evidence that pharmacological inhibitors of Rac activity and actin stability have synergistic activity.

Global transcriptome profile of Cryptococcus neoformans during exposure to hydrogen peroxide induced oxidative stress

The ability of the opportunistic fungal pathogen Cryptococcus neoformans to resist oxidative stress is one of its most important virulence related traits. To cope with the deleterious effect of cellular damage caused by the oxidative burst inside the macrophages, C. neoformans has developed multilayered redundant molecular responses to neutralize the stress, to repair the damage and to eventually grow inside the hostile environment of the phagosome. We used microarray analysis of cells treated with hydrogen peroxide (H(2)O(2)) at multiple time points in a nutrient defined medium to identify a transcriptional signature associated with oxidative stress. We discovered that the composition of the medium in which fungal cells were grown and treated had a profound effect on their capacity to degrade exogenous H(2)O(2). We determined the kinetics of H(2)O(2) breakdown by growing yeast cells under different conditions and accordingly selected an appropriate media composition and range of time points for isolating RNA for hybridization. Microarray analysis revealed a robust transient transcriptional response and the intensity of the global response was consistent with the kinetics of H(2)O(2) breakdown by treated cells. Gene ontology analysis of differentially expressed genes related to oxidation-reduction, metabolic process and protein catabolic processes identified potential roles of mitochondrial function and protein ubiquitination in oxidative stress resistance. Interestingly, the metabolic pathway adaptation of C. neoformans to H(2)O(2) treatment was remarkably distinct from the response of other fungal organisms to oxidative stress. We also identified the induction of an antifungal drug resistance response upon the treatment of C. neoformans with H(2)O(2). These results highlight the complexity of the oxidative stress response and offer possible new avenues for improving our understanding of mechanisms of oxidative stress resistance in C. neoformans.

Zap1 regulates zinc homeostasis and modulates virulence in Cryptococcus gattii

Zinc homeostasis is essential for fungal growth, as this metal is a critical structural component of several proteins, including transcription factors. The fungal pathogen Cryptococcus gattii obtains zinc from the stringent zinc-limiting milieu of the host during the infection process. To characterize the zinc metabolism in C. gattii and its relationship to fungal virulence, the zinc finger protein Zap1 was functionally characterized. The C. gattii ZAP1 gene is an ortholog of the master regulatory genes zafA and ZAP1 that are found in Aspergillus fumigatus and Saccharomyces cerevisiae, respectively. There is some evidence to support an association between Zap1 and zinc metabolism in C. gattii: (i) ZAP1 expression is highly induced during zinc deprivation, (ii) ZAP1 knockouts demonstrate impaired growth in zinc-limiting conditions, (iii) Zap1 regulates the expression of ZIP zinc transporters and distinct zinc-binding proteins and (iv) Zap1 regulates the labile pool of intracellular zinc. In addition, the deletion of ZAP1 reduces C. gattii virulence in a murine model of cryptococcosis infection. Based on these observations, we postulate that proper zinc metabolism plays a crucial role in cryptococcal virulence.

Mechanisms of cryptococcal virulence and persistence

Cryptococcus neoformans is an environmental yeast that is a leading cause of fatal mycosis in AIDS patients and a major cause of meningoencephalitis and CNS-related mortality around the globe. Although C. neoformans infection is mostly a manifestation of immune deficiency, up to 25% of cases reported in the USA occur in patients without recognizable immune defects, indicating that C. neoformans can develop mechanisms that allow it to evade immune defenses and persist in noncompromised hosts. This article discusses mechanisms and routes of infection and the most important elements of host response as well as the mechanisms that promote cryptococcal survival within the host. Metabolic adaptation to physiological host conditions and the mechanisms limiting immune recognition, interfering with phagocytosis and extending intracellular survival of C. neoformans are highlighted. We describe the mechanisms by which C. neoformans can alter adaptive host responses, especially cell-mediated immunity, which is required for clearance of this microbe. We also review cryptococcal strategies of survival in the CNS and briefly discuss adaptations developing in response to medical treatment.

Ferrochelatase is a conserved downstream target of the blue light-sensing White collar complex in fungi

Light is a universal signal perceived by organisms, including fungi, in which light regulates common and unique biological processes depending on the species. Previous research has established that conserved proteins, originally called White collar 1 and 2 from the ascomycete Neurospora crassa, regulate UV/blue light sensing. Homologous proteins function in distant relatives of N. crassa, including the basidiomycetes and zygomycetes, which diverged as long as a billion years ago. Here we conducted microarray experiments on the basidiomycete fungus Cryptococcus neoformans to identify light-regulated genes. Surprisingly, only a single gene was induced by light above the commonly used twofold threshold. This gene, HEM15, is predicted to encode a ferrochelatase that catalyses the final step in haem biosynthesis from highly photoreactive porphyrins. The C. neoformans gene complements a Saccharomyces cerevisiae hem15Delta strain and is essential for viability, and the Hem15 protein localizes to mitochondria, three lines of evidence that the gene encodes ferrochelatase. Regulation of HEM15 by light suggests a mechanism by which bwc1/bwc2 mutants are photosensitive and exhibit reduced virulence. We show that ferrochelatase is also light-regulated in a white collar-dependent fashion in N. crassa and the zygomycete Phycomyces blakesleeanus, indicating that ferrochelatase is an ancient target of photoregulation in the fungal kingdom.

Interaction of pathogenic yeasts with phagocytes: survival, persistence and escape

Pathogenic yeasts, either from the environment or the normal flora, have to face phagocytic cells that constitute the first line of defence during infection. In order to evade or counteract attack by phagocytes, pathogenic yeasts have acquired a repertoire of strategies to survive, colonize and infect the host. In this review we focus on the interaction of yeasts, such as Candida, Histoplasma or Cryptococcus species, with macrophages or neutrophils. We discuss strategies used by these fungi to prevent phagocytosis or to counteract phagocytic activities. We go on to describe the strategies that permit intracellular survival within phagocytes and that may eventually lead to damage of and escape from the phagocyte.

Isocitrate dehydrogenase is important for nitrosative stress resistance in Cryptococcus neoformans, but oxidative stress resistance is not dependent on glucose-6-phosphate dehydrogenase

The opportunistic intracellular fungal pathogen Cryptococcus neoformans depends on many antioxidant and denitrosylating proteins and pathways for virulence in the immunocompromised host. These include the glutathione and thioredoxin pathways, thiol peroxidase, cytochrome c peroxidase, and flavohemoglobin denitrosylase. All of these ultimately depend on NADPH for either catalytic activity or maintenance of a reduced, functional form. The need for NADPH during oxidative stress is well established in many systems, but a role in resistance to nitrosative stress has not been as well characterized. In this study we investigated the roles of two sources of NADPH, glucose-6-phosphate dehydrogenase (Zwf1) and NADP(+)-dependent isocitrate dehydrogenase (Idp1), in production of NADPH and resistance to oxidative and nitrosative stress. Deletion of ZWF1 in C. neoformans did not result in an oxidative stress sensitivity phenotype or changes in the amount of NADPH produced during oxidative stress compared to those for the wild type. Deletion of IDP1 resulted in greater sensitivity to nitrosative stress than to oxidative stress. The amount of NADPH increased 2-fold over that in the wild type during nitrosative stress, and yet the idp1Delta strain accumulated more mitochondrial damage than the wild type during nitrosative stress. This is the first report of the importance of Idp1 and NADPH for nitrosative stress resistance.

PeroxiBase: a database with new tools for peroxidase family classification

Peroxidases (EC 1.11.1.x), which are encoded by small or large multigenic families, are involved in several important physiological and developmental processes. They use various peroxides as electron acceptors to catalyse a number of oxidative reactions and are present in almost all living organisms. We have created a peroxidase database (http://peroxibase.isb-sib.ch) that contains all identified peroxidase-encoding sequences (about 6000 sequences in 940 organisms). They are distributed between 11 superfamilies and about 60 subfamilies. All the sequences have been individually annotated and checked. PeroxiBase can be consulted using six major interlink sections ‘Classes’, ‘Organisms’, ‘Cellular localisations’, ‘Inducers’, ‘Repressors’ and ‘Tissue types’. General documentation on peroxidases and PeroxiBase is accessible in the ‘Documents’ section containing ‘Introduction’, ‘Class description’, ‘Publications’ and ‘Links’. In addition to the database, we have developed a tool to classify peroxidases based on the PROSITE profile methodology. To improve their specificity and to prevent overlaps between closely related subfamilies the profiles were built using a new strategy based on the silencing of residues. This new profile construction method and its discriminatory capacity have been tested and validated using the different peroxidase families and subfamilies present in the database. The peroxidase classification tool called PeroxiScan is accessible at the following address: http://peroxibase.isb-sib.ch/peroxiscan.php.

Metabolic adaptation in Cryptococcus neoformans during early murine pulmonary infection

SUMMARY

The pathogenic fungus Cryptococcus neoformans generally initiates infection in mammalian lung tissue and subsequently disseminates to the brain. We performed serial analysis of gene expression (SAGE) on C. neoformans cells recovered from the lungs of mice and found elevated expression of genes for central carbon metabolism including functions for acetyl-CoA production and utilization. Deletion of the highly expressed ACS1 gene encoding acetyl-CoA synthetase revealed a requirement for growth on acetate and for full virulence. Transcripts for transporters (e.g. for monosaccharides, iron, copper and acetate) and for stress-response proteins were also elevated thus indicating a nutrient-limited and hostile host environment. The pattern of regulation was reminiscent of the control of alternative carbon source utilization and stress response by the Snf1 protein kinase in Saccharomyces cerevisiae. A snf1 mutant of C. neoformans showed defects in alternative carbon source utilization, the response to nitrosative stress, melanin production and virulence. However, loss of Snf1 did not influence the expression of a set of genes for carbon metabolism that were elevated upon lung infection. Taken together, the results reveal specific metabolic adaptations of C. neoformans during pulmonary infection and indicate a role for ACS1 and SNF1 in virulence.

Comparison of the thiol-dependent antioxidant systems in the ectomycorrhizal Laccaria bicolor and the saprotrophic Phanerochaete chrysosporium

Sequencing of the Laccaria bicolor and Phanerochaete chrysosporium genomes, together with the availability of many fungal genomes, allow careful comparison to be made of these two basidiomycetes, which possess a different way of life (either symbiotic or saprophytic), with other fungi. Central to the antioxidant systems are superoxide dismutases, catalases and thiol-dependent peroxidases (Tpx). The two reducing systems (thioredoxin (Trx) and glutathione/glutaredoxin (Grx)) are of particular importance against oxidative insults, both for detoxification, through the regeneration of thiol-peroxidases, and for developmental, physiological and signalling processes. Among those thiol-dependent antioxidant systems, special emphasis is given to the redoxin and methionine sulfoxide reductase (Msr) multigenic families. The genes coding for these enzymes were identified in the L. bicolor and P. chrysosporium genomes, were correctly annotated, and the gene content, organization and distribution were compared with other fungi. Expression of the Laccaria genes was also compiled from microarray data. A complete classification, based essentially on gene structure, on phylogenetic and sequence analysis, and on existing experimental data, was proposed. Comparison of the gene content of fungi from all phyla did not show huge differences for multigenic families in the reactive oxygen species (ROS) detoxification network, although some protein subgroups were absent in some fungi.

Cryptococcus neoformans, a fungus under stress

Cryptococcus neoformans is a human fungal pathogen that survives exposure to stresses during growth in the human host, including oxidative and nitrosative stress, high temperature, hypoxia, and nutrient deprivation. There have been many genes implicated in resistance to individual stresses. Notably, the catalases do not have the expected role in resistance to external oxidative stress, but specific peroxidases appear to be critical for resistance to both oxidative and nitrosative stresses. Signal transduction through the HOG1 and calcineurin/calmodulin pathways has been implicated in the stress response. Microarray and proteomic analyses have indicated that the common responses to stress are induction of metabolic and oxidative stress genes, and repression of genes encoding translational machinery.

Exploration of whole-genome responses of the human AIDS-associated yeast pathogen Cryptococcus neoformans var grubii: nitric oxide stress and body temperature

Cryptococcus neoformans var grubii is an opportunistic basidiomycete yeast pathogen that is a significant cause of HIV/AIDS-related deaths worldwide. We describe a whole-genome oligonucleotide microarray for this pathogen. These arrays have been used to elucidate the transcriptional responses of the genome to heat shock as well as to two conditions relevant to human infections: body temperature and nitric oxide (NO) stress produced by the NO donor DPTA-NONOate. This analysis revealed an NO-inducible C. neoformans-specific four-gene family that showed a highly similar transcriptional profile to that of FHB1, a previously described NO dioxygenase/flavohemoglobin required for virulence. NO treatment also induced genes involved in the synthesis of the antioxidant mannitol, a polyol that accumulates in the cerebrospinal fluid of infected patients. Exposure to NO also caused increased expression of the sole C. neoformans var grubii protein with HHE/hemerythrin cation binding motifs. Notably, a similar gene in E. coli, ytfE, has been shown to be NO-inducible and protects bacterial cells from killing by NO. Genes induced by NO were highly enriched for those repressed at 37 degrees C, indicating an unexpected interplay between temperature and NO regulation in this basidiomycete. Resources described here should facilitate future investigations of this lethal human yeast pathogen.

Loss of cell wall alpha(1-3) glucan affects Cryptococcus neoformans from ultrastructure to virulence

Yeast cell walls are critical for maintaining cell integrity, particularly in the face of challenges such as growth in mammalian hosts. The pathogenic fungus Cryptococcus neoformans additionally anchors its polysaccharide capsule to the cell surface via alpha(1-3) glucan in the wall. Cryptococcal cells disrupted in their alpha glucan synthase gene were sensitive to stresses, including temperature, and showed difficulty dividing. These cells lacked surface capsule, although they continued to shed capsule material into the environment. Electron microscopy showed that the alpha glucan that is usually localized to the outer portion of the cell wall was absent, the outer region of the wall was highly disorganized, and the inner region was hypertrophic. Analysis of cell wall composition demonstrated complete loss of alpha glucan accompanied by a compensatory increase in chitin/chitosan and a redistribution of beta glucan between cell wall fractions. The mutants were unable to grow ina mouse model of infection, but caused death in nematodes. These studies integrate morphological and biochemical investigations of the role of alpha glucan in the cryptococcal cell wall.

The Cryptococcus neoformans catalase gene family and its role in antioxidant defense

In the present study, we sought to elucidate the contribution of the Cryptococcus neoformans catalase gene family to antioxidant defense. We employed bioinformatics techniques to identify four members of the C. neoformans catalase gene family and created mutants lacking single or multiple catalase genes. Based on a phylogenetic analysis, CAT1 and CAT3 encode putative spore-specific catalases, CAT2 encodes a putative peroxisomal catalase, and CAT4 encodes a putative cytosolic catalase. Only Cat1 exhibited detectable biochemical activity in vitro, and Cat1 activity was constitutive in the yeast form of this organism. Although they were predicted to be important in spores, neither CAT1 nor CAT3 was essential for mating or spore viability. Consistent with previous studies of Saccharomyces cerevisiae, the single (cat1, cat2, cat3, and cat4) and quadruple (cat1 cat2 cat3 cat4) catalase mutant strains exhibited no oxidative-stress phenotypes under conditions in which either exogenous or endogenous levels of reactive oxygen species were elevated. In addition, there were no significant differences in the mean times to mortality between groups of mice infected with C. neoformans catalase mutant strains (the cat1 and cat1 cat2 cat3 cat4 mutants) and those infected with wild-type strain H99. We conclude from the results of this study that C. neoformans possesses a robust antioxidant system, composed of functionally overlapping and compensatory components that provide protection against endogenous and exogenous oxidative stresses.

Differential gene expression profiling of vocal fold polyps and Reinke's edema by complementary DNA microarray

OBJECTIVES

Our purpose was to determine whether complementary DNA (cDNA) microarray analysis (MA) can establish distinct gene expression profiles for 2 phenotypically similar vocal fold lesions: Reinke's edema (RE) and polyps. Established transcript profiles can provide insight into the molecular and cellular processes involved in these diseases.

METHODS

Eleven RE specimens and 17 polyps were analyzed with MA for 8,745 genes. Further MA profiling was attempted within each lesion group to identify molecular markers for reflux exposure and smoking. Prediction analysis was used to predict lesion classification for 2 unclassified samples. A real-time polymerase chain reaction was performed to corroborate MA transcript levels for selected significant genes.

RESULTS

Sixty-five genes were found to differentiate RE and polyps (p = .0088). For RE, 19 genes were differentiated for reflux exposure (p = .016). No genes were found to differentiate smokers from nonsmokers. For polyps, no genes were found to differentiate for reflux (p = .16) and smoking (p = .565). Categorization of unclassified lesions was possible with a minimum of 13 genes.

CONCLUSIONS

We demonstrate the feasibility of benign lesion classification based on MA. Microarray analysis is useful not only for improving diagnosis and classification of such lesions, but also for potentially generating prognostic indicators and targets for therapy.

Posttranslational, translational, and transcriptional responses to nitric oxide stress in Cryptococcus neoformans: implications for virulence

The ability of the fungal pathogen Cryptococcus neoformans to evade the mammalian innate immune response and cause disease is partially due to its ability to respond to and survive nitrosative stress. In this study, we use proteomic and genomic approaches to elucidate the response of C. neoformans to nitric oxide stress. This nitrosative stress response involves both transcriptional, translational, and posttranslational regulation. Proteomic and genomic analyses reveal changes in expression of stress response genes. In addition, genes involved in cell wall organization, respiration, signal transduction, transport, transcriptional control, and metabolism show altered expression under nitrosative conditions. Posttranslational modifications of transaldolase (Tal1), aconitase (Aco1), and the thiol peroxidase, Tsa1, are regulated during nitrosative stress. One stress-related protein up-regulated in the presence of nitric oxide stress is glutathione reductase (Glr1). To further investigate its functional role during nitrosative stress, a deletion mutant was generated. We show that this glr1Delta mutant is sensitive to nitrosative stress and macrophage killing in addition to being avirulent in mice. These studies define the response to nitrosative stress in this important fungal pathogen.