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

A Toxoplasma gondii Gluconeogenic Enzyme Contributes to Robust Central Carbon Metabolism and Is Essential for Replication and Virulence

The expression of gluconeogenic enzymes is typically repressed when glucose is available. The protozoan parasite Toxoplasma gondii utilizes host glucose to sustain high rates of intracellular replication. However, despite their preferential utilization of glucose, intracellular parasites constitutively express two isoforms of the gluconeogenic enzyme fructose 1,6-bisphosphatase (TgFBP1 and TgFBP2). The rationale for constitutive expression of FBPases in T. gondii remains unclear. We find that conditional knockdown of TgFBP2 results in complete loss of intracellular growth in vitro under glucose-replete conditions and loss of acute virulence in mice. TgFBP2 deficiency was rescued by expression of catalytically active FBPase and was associated with altered glycolytic and mitochondrial TCA cycle fluxes, as well as dysregulation of glycolipid, amylopectin, and fatty acid biosynthesis. Futile cycling between gluconeogenic and glycolytic enzymes may constitute a regulatory mechanism that allows T. gondii to rapidly adapt to changes in nutrient availability in different host cells.

Transcriptional control of sexual development in Cryptococcus neoformans

Developmental processes are essential for the normal life cycles of many pathogenic fungi, and they can facilitate survival in challenging environments, including the human host. Sexual development of the human fungal pathogen Cryptococcus neoformans not only produces infectious particles (spores) but has also enabled the evolution of new disease-related traits such as drug resistance. Transcription factor networks are essential to the development and pathogenesis of C. neoformans, and a variety of sequence-specific DNA-binding proteins control both key developmental transitions and virulence by regulating the expression of their target genes. In this review we discuss the roles of known transcription factors that harbor important connections to both development and virulence. Recent studies of these transcription factors have identified a common theme in which metabolic, stress, and other responses that are required for sexual development appear to have been co-opted for survival in the human host, thus facilitating pathogenesis. Future work elucidating the connection between development and pathogenesis will provide vital insights into the evolution of complex traits in eukaryotes as well as mechanisms that may be used to combat fungal pathogens.

In Vitro Analysis of Metabolites Secreted during Infection of Lung Epithelial Cells by Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated basidiomycetous yeast commonly associated with pigeon droppings and soil. The opportunistic pathogen infects humans through the respiratory system and the metabolic implications of C. neoformans infection have yet to be explored. Studying the metabolic profile associated with the infection could lead to the identification of important metabolites associated with pulmonary infection. Therefore, the aim of the study was to simulate cryptococcal infection at the primary site of infection, the lungs, and to identify the metabolic profile and important metabolites associated with the infection at low and high multiplicity of infections (MOI). The culture supernatant of lung epithelial cells infected with C. neoformans at MOI of 10 and 100 over a period of 18 hours were analysed using gas chromatography mass spectrometry. The metabolic profiles obtained were further analysed using multivariate analysis and the pathway analysis tool, MetaboAnalyst 2.0. Based on the results from the multivariate analyses, ten metabolites were selected as the discriminatory metabolites that were important in both the infection conditions. The pathways affected during early C. neoformans infection of lung epithelial cells were mainly the central carbon metabolism and biosynthesis of amino acids. Infection at a higher MOI led to a perturbance in the β-alanine metabolism and an increase in the secretion of pantothenic acid into the growth media. Pantothenic acid production during yeast infection has not been documented and the β-alanine metabolism as well as the pantothenate and CoA biosynthesis pathways may represent underlying metabolic pathways associated with disease progression. Our study suggested that β-alanine metabolism and the pantothenate and CoA biosynthesis pathways might be the important pathways associated with cryptococcal infection.

Deubiquitinase Ubp5 Is Required for the Growth and Pathogenicity of Cryptococcus gattii

Cryptococcus gattii is a resurgent fungal pathogen that primarily infects immunocompetent hosts. Thus, it poses an increasingly significant impact on global public health; however, the mechanisms underlying its pathogenesis remain largely unknown. We conducted a detailed characterization of the deubiquitinase Ubp5 in the biology and virulence of C. gattii using the hypervirulent strain R265, and defined its properties as either distinctive or shared with C. neoformans. Deletion of the C. gattii Ubp5 protein by site-directed disruption resulted in a severe growth defect under both normal and stressful conditions (such as high temperature, high salt, cell wall damaging agents, and antifungal agents), similar to the effects observed in C. neoformans. However, unlike C. neoformans, the C. gattii ubp5Δ mutant displayed a slight enhancement of capsule and melanin production, indicating the evolutionary convergence and divergence of Ubp5 between these two sibling species. Attenuated virulence of the Cg-ubp5Δ mutant was not solely due to its reduced thermotolerance at 37°C, as shown in both worm and mouse survival assays. In addition, the assessment of fungal burden in mammalian organs further indicated that Ubp5 was required for C. gattii pulmonary survival and, consequently, extrapulmonary dissemination. Taken together, our work highlights the importance of deubiquitinase Ubp5 in the virulence composite of both pathogenic cryptococcal species, and it facilitates a better understanding of C. gattii virulence mechanisms.

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.

The Role of Amino Acid Permeases and Tryptophan Biosynthesis in Cryptococcus neoformans Survival

Metabolic diversity is an important factor during microbial adaptation to different environments. Among metabolic processes, amino acid biosynthesis has been demonstrated to be relevant for survival for many microbial pathogens, whereas the association between pathogenesis and amino acid uptake and recycling are less well-established. Cryptococcus neoformans is an opportunistic fungal pathogen with many habitats. As a result, it faces frequent metabolic shifts and challenges during its life cycle. Here we studied the C. neoformans tryptophan biosynthetic pathway and found that the pathway is essential. RNAi indicated that interruptions in the biosynthetic pathway render strains inviable. However, auxotroph complementation can be partially achieved by tryptophan uptake when a non preferred nitrogen source and lower growth temperature are applied, suggesting that amino acid permeases may be the target of nitrogen catabolism repression (NCR). We used bioinformatics to search for amino acid permeases in the C. neoformans and found eight potential global permeases (AAP1 to AAP8). The transcriptional profile of them revealed that they are subjected to regulatory mechanisms which are known to respond to nutritional status in other fungi, such as (i) quality of nitrogen (Nitrogen Catabolism Repression, NCR) and carbon sources (Carbon Catabolism Repression, CCR), (ii) amino acid availability in the extracellular environment (SPS-sensing) and (iii) nutritional deprivation (Global Amino Acid Control, GAAC). This study shows that C. neoformans has fewer amino acid permeases than other model yeasts, and that these proteins may be subjected to complex regulatory mechanisms. Our data suggest that the C. neoformans tryptophan biosynthetic pathway is an excellent pharmacological target. Furthermore, inhibitors of this pathway cause Cryptococcus growth arrest in vitro.

The endosomal sorting complex required for transport machinery influences haem uptake and capsule elaboration in Cryptococcus neoformans

Iron availability is a key determinant of virulence in the pathogenic fungus Cryptococcus neoformans. Previous work revealed that the ESCRT (endosomal sorting complex required for transport) protein Vps23 functions in iron acquisition, capsule formation and virulence. Here, we further characterized the ESCRT machinery to demonstrate that defects in the ESCRT-II and III complexes caused reduced capsule attachment, impaired growth on haem and resistance to non-iron metalloprotoporphyrins. The ESCRT mutants shared several phenotypes with a mutant lacking the pH-response regulator Rim101, and in other fungi, the ESCRT machinery is known to activate Rim101 via proteolytic cleavage. We therefore expressed a truncated and activated version of Rim101 in the ESCRT mutants and found that this allele restored capsule formation but not growth on haem, thus suggesting a Rim101-independent contribution to haem uptake. We also demonstrated that the ESCRT machinery acts downstream of the cAMP/protein kinase A pathway to influence capsule elaboration. Defects in the ESCRT components also attenuated virulence in macrophage survival assays and a mouse model of cryptococcosis to a greater extent than reported for loss of Rim101. Overall, these results indicate that the ESCRT complexes function in capsule elaboration, haem uptake and virulence via Rim101-dependent and independent mechanisms.

Targets of the Sex Inducer homeodomain proteins are required for fungal development and virulence in Cryptococcus neoformans

In the yeast Saccharomyces cerevisiae, the regulation of cell types by homeodomain transcription factors is a key paradigm; however, many questions remain regarding this class of developmental regulators in other fungi. In the human fungal pathogen Cryptococcus neoformans, the homeodomain transcription factors Sxi1α and Sxi2a are required for sexual development that produces infectious spores, but the molecular mechanisms by which they drive this process are unknown. To better understand homeodomain control of fungal development, we determined the targets of the Sxi2a-Sxi1α heterodimer using whole genome expression analyses paired with in silico and in vitro binding site identification methods. We identified Sxi-regulated genes that contained a site bound directly by the Sxi proteins that is required for full regulation in vivo. Among the targets of the Sxi2a-Sxi1α complex were many genes known to be involved in sexual reproduction, as well as several well-studied virulence genes. Our findings suggest that genes involved in sexual development are also important in mammalian disease. Our work advances the understanding of how homeodomain transcription factors control complex developmental events and suggests an intimate link between fungal development and virulence.

Cryptococcus neoformans meningitis with negative cryptococcal antigen: Evaluation of a new immunochromatographic detection assay

Detection of cryptococcal antigen in serum or cerebrospinal fluid allows cryptococcal meningitis diagnosis within few hours with >90% sensitivity. In an HIV-positive patient with Cryptococcus neoformans meningitis, initial antigen detection by immunoagglutination was negative. We thus evaluated a new immunochromatographic detection assay that exhibited a higher sensitivity.

Mitochondrial Protein Nfu1 Influences Homeostasis of Essential Metals in the Human Fungal Pathogen Cryptococcus neoformans

Mitochondrial protein Nfu1 plays an important role in the assembly of mitochondrial Fe-S clusters and intracellular iron homeostasis in the model yeast Saccharomyces cerevisiae. In this study, we identified the Nfu1 ortholog in the human fungal pathogen Cryptococcus neoformans. Our data showed that C. neoformans Nfu1 localized in the mitochondria and influenced homeostasis of essential metals such as iron, copper and manganese. Marked growth defects were observed in the mutant lacking NFU1, which suggests a critical role of Nfu1 in Fe-S cluster biosynthesis and intracellular metal homeostasis in C. neoformans.

Metabolism in fungal pathogenesis

Fungal pathogens must assimilate local nutrients to establish an infection in their mammalian host. We focus on carbon, nitrogen, and micronutrient assimilation mechanisms, discussing how these influence host-fungus interactions during infection. We highlight several emerging trends based on the available data. First, the perturbation of carbon, nitrogen, or micronutrient assimilation attenuates fungal pathogenicity. Second, the contrasting evolutionary pressures exerted on facultative versus obligatory pathogens have led to contemporary pathogenic fungal species that display differing degrees of metabolic flexibility. The evolutionarily ancient metabolic pathways are conserved in most fungal pathogen, but interesting gaps exist in some species (e.g., Candida glabrata). Third, metabolic flexibility is generally essential for fungal pathogenicity, and in particular, for the adaptation to contrasting host microenvironments such as the gastrointestinal tract, mucosal surfaces, bloodstream, and internal organs. Fourth, this metabolic flexibility relies on complex regulatory networks, some of which are conserved across lineages, whereas others have undergone significant evolutionary rewiring. Fifth, metabolic adaptation affects fungal susceptibility to antifungal drugs and also presents exciting opportunities for the development of novel therapies.

Developmental cell fate and virulence are linked to trehalose homeostasis in Cryptococcus neoformans

Among pathogenic environmental fungi, spores are thought to be infectious particles that germinate in the host to cause disease. The meningoencephalitis-causing yeast Cryptococcus neoformans is found ubiquitously in the environment and sporulates in response to nutrient limitation. While the yeast form has been studied extensively, relatively little is known about spore biogenesis, and spore germination has never been evaluated at the molecular level. Using genome transcript analysis of spores and molecular genetic approaches, we discovered that trehalose homeostasis plays a key role in regulating sporulation of C. neoformans, is required for full spore viability, and influences virulence. Specifically, we found that genes involved in trehalose metabolism, including a previously uncharacterized secreted trehalase (NTH2), are highly overrepresented in dormant spores. Deletion of the two predicted trehalases in the C. neoformans genome, NTH1 and NTH2, resulted in severe defects in spore production, a decrease in spore germination, and an increase in the production of alternative developmental structures. This shift in cell types suggests that trehalose levels modulate cell fate decisions during sexual development. We also discovered that deletion of the NTH2 trehalase results in hypervirulence in a murine model of infection. Taken together, these data show that the metabolic adaptations that allow this fungus to proliferate ubiquitously in the environment play unexpected roles in virulence in the mammalian host and highlight the complex interplay among the processes of metabolism, development, and pathogenesis.

Iron and copper as virulence modulators in human fungal pathogens

Fungal pathogens have evolved sophisticated machinery to precisely balance the fine line between acquiring essential metals and defending against metal toxicity. Iron and copper are essential metals for many processes in both fungal pathogens and their mammalian hosts, but reduce viability when present in excess. However, during infection, the host uses these two metals differently. Fe has a long-standing history of influencing virulence in pathogenic fungi, mostly in regards to Fe acquisition. Numerous studies demonstrate the requirement of the Fe acquisition pathway of Candida, Cryptococcus and Aspergillus for successful systemic infection. Fe is not free in the host, but is associated with Fe-binding proteins, leading fungi to develop mechanisms to interact with and to acquire Fe from these Fe-bound proteins. Cu is also essential for cell growth and development. Essential Cu-binding proteins include Fe transporters, superoxide dismutase (SOD) and cytochrome c oxidase. Although Cu acquisition plays critical roles in fungal survival in the host, recent work has revealed that Cu detoxification is extremely important. Here, we review fungal responses to altered metal conditions presented by the host, contrast the roles of Fe and Cu during infection, and outline the critical roles of fungal metal homeostasis machinery at the host-pathogen axis.

Defects in phosphate acquisition and storage influence virulence of Cryptococcus neoformans

Nutrient acquisition and sensing are critical aspects of microbial pathogenesis. Previous transcriptional profiling indicated that the fungal pathogen Cryptococcus neoformans, which causes meningoencephalitis in immunocompromised individuals, encounters phosphate limitation during proliferation in phagocytic cells. We therefore tested the hypothesis that phosphate acquisition and polyphosphate metabolism are important for cryptococcal virulence. Deletion of the high-affinity uptake system interfered with growth on low-phosphate medium, perturbed the formation of virulence factors (capsule and melanin), reduced survival in macrophages, and attenuated virulence in a mouse model of cryptococcosis. Additionally, analysis of nutrient sensing functions for C. neoformans revealed regulatory connections between phosphate acquisition and storage and the iron regulator Cir1, cyclic AMP (cAMP)-dependent protein kinase A (PKA), and the calcium-calmodulin-activated protein phosphatase calcineurin. Deletion of the VTC4 gene encoding a polyphosphate polymerase blocked the ability of C. neoformans to produce polyphosphate. The vtc4 mutant behaved like the wild-type strain in interactions with macrophages and in the mouse infection model. However, the fungal load in the lungs was significantly increased in mice infected with vtc4 deletion mutants. In addition, the mutant was impaired in the ability to trigger blood coagulation in vitro, a trait associated with polyphosphate. Overall, this study reveals that phosphate uptake in C. neoformans is critical for virulence and that its regulation is integrated with key signaling pathways for nutrient sensing.

The Cryptococcus neoformans transcriptome at the site of human meningitis

Cryptococcus neoformans is the leading cause of fungal meningitis worldwide. Previous studies have characterized the cryptococcal transcriptome under various stress conditions, but a comprehensive profile of the C. neoformans transcriptome in the human host has not been attempted. Here, we extracted RNA from yeast cells taken directly from the cerebrospinal fluid (CSF) of two AIDS patients with cryptococcal meningitis prior to antifungal therapy. The patients were infected with strains of C. neoformans var. grubii of molecular type VNI and VNII. Using RNA-seq, we compared the transcriptional profiles of these strains under three environmental conditions (in vivo CSF, ex vivo CSF, and yeast extract-peptone-dextrose [YPD]). Although we identified a number of differentially expressed genes, single nucleotide variants, and novel genes that were unique to each strain, the overall expression patterns of the two strains were similar under the same environmental conditions. Specifically, yeast cells obtained directly from each patient’s CSF were more metabolically active than cells that were incubated ex vivo in CSF. Compared with growth in YPD, some genes were identified as significantly upregulated in both in vivo and ex vivo CSF, and they were associated with genes previously recognized for contributing to pathogenicity. For example, genes with known stress response functions, such as RIM101, ENA1, and CFO1, were regulated similarly in the two clinical strains. Conversely, many genes that were differentially regulated between the two strains appeared to be transporters. These findings establish a platform for further studies of how this yeast survives and produces disease.

Cryptococcus neoformans, an environmental, opportunistic yeast, is annually responsible for an estimated million cases of meningitis and over 600,000 deaths, mostly among HIV-infected patients in sub-Saharan Africa and Asia. Using RNA-seq, we analyzed the gene expression of two strains of C. neoformans obtained from the cerebrospinal fluid (CSF) of infected patients, thus creating a comprehensive snapshot of the yeasts’ genetic responses within the human body. By comparing the gene expression of each clinical strain under three conditions (in vivo CSF, ex vivo CSF, and laboratory culture), we identified genes and pathways that were uniquely regulated by exposure to CSF and likely crucial for the survival of C. neoformans in the central nervous system. Further analyses revealed genetic diversity between the strains, providing evidence for cryptococcal evolution and strain specificity. This ability to characterize transcription in vivo enables the elucidation of specific genetic responses that promote disease production and progression.

Histoplasma capsulatum depends on de novo vitamin biosynthesis for intraphagosomal proliferation

During infection of the mammalian host, Histoplasma capsulatum yeasts survive and reside within macrophages of the immune system. Whereas some intracellular pathogens escape into the host cytosol, Histoplasma yeasts remain within the macrophage phagosome. This intracellular Histoplasma-containing compartment imposes nutritional challenges for yeast growth and replication. We identified and annotated vitamin synthesis pathways encoded in the Histoplasma genome and confirmed by growth in minimal medium that Histoplasma yeasts can synthesize all essential vitamins with the exception of thiamine. Riboflavin, pantothenate, and biotin auxotrophs of Histoplasma were generated to probe whether these vitamins are available to intracellular yeasts. Disruption of the RIB2 gene (riboflavin biosynthesis) prevented growth and proliferation of yeasts in macrophages and severely attenuated Histoplasma virulence in a murine model of respiratory histoplasmosis. Rib2-deficient yeasts were not cleared from lung tissue but persisted, consistent with functional survival mechanisms but inability to replicate in vivo. In addition, depletion of Pan6 (pantothenate biosynthesis) but not Bio2 function (biotin synthesis) also impaired Histoplasma virulence. These results indicate that the Histoplasma-containing phagosome is limiting for riboflavin and pantothenate and that Histoplasma virulence requires de novo synthesis of these cofactor precursors. Since mammalian hosts do not rely on vitamin synthesis but instead acquire essential vitamins through diet, vitamin synthesis pathways represent druggable targets for therapeutics.

Old Cryptococcus neoformans cells contribute to virulence in chronic cryptococcosis

Does cell age matter in virulence? The emergence of persister cells during chronic infections is critical for persistence of infection, but little is known how this occurs. Here, we demonstrate for the first time that the replicative age of the fungal pathogen Cryptococcus neoformans contributes to persistence during chronic meningoencephalitis. Generationally older C. neoformans cells are more resistant to hydrogen peroxide stress, macrophage intracellular killing, and antifungal agents. Older cells accumulate in both experimental rat infection and in human cryptococcosis. Mathematical modeling supports the concept that the presence of older C. neoformans cells emerges from in vivo selection pressures. We propose that advanced replicative aging is a new unanticipated virulence trait that emerges during chronic fungal infection and facilitates persistence. Therapeutic interventions that target old cells could help in the clearance of chronic infections.

Our findings that the generational age of Cryptococcus neoformans cells matters in pathogenesis introduces a novel concept to eukaryotic pathogenesis research. We propose that emerging properties of aging C. neoformans cells and possibly also other fungal pathogens contribute to persistence and virulence. Whereas the replicative life span of strains may not matter for virulence per se, age-related resilience and thus the generational age of individual C. neoformans cells within a pathogen population could greatly affect persistence of the pathogen population and therefore impact outcome.

An encapsulation of iron homeostasis and virulence in Cryptococcus neoformans

Vertebrate hosts actively sequester iron, and fungal and other pathogens must therefore adapt to a severe limitation in iron availability to cause disease. Recent studies reveal that the pathogenic fungus Cryptococcus neoformans overcomes iron limitation by multiple mechanisms that target transferrin and heme. The regulation of iron uptake is mediated by an interconnected set of transcription factors that include the master iron regulator Cir1 and the pH-responsive factor Rim101. These factors integrate iron homeostasis with a myriad of other functions including pH sensing, nutrient and stress signaling pathways, virulence factor elaboration, and cell wall biogenesis.

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

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

Fungal cell wall dynamics and infection site microenvironments: signal integration and infection outcome

Upon entrance into the host, fungi encounter a myriad of host effector products and microenvironments that they sense and adapt to for survival. Alterations of the structure and composition of the cell wall is a major fungal adaptation mechanism to evade these environments. Here we discuss recent findings of host-microenvironmental induced fungal cell wall changes, including structure, composition, and protein content, and their effects on host immune responses. A take home message from these recent studies is an emerging understanding of how integration of multiple signals, of both fungal and host responses to dynamic infection site microenvironments, determines outcomes of infection. A challenge moving forward is to further understand these mechanisms and harness them for therapeutic benefit.

3-Bromopyruvate: a novel antifungal agent against the human pathogen Cryptococcus neoformans

We have investigated the antifungal activity of the pyruvic acid analogue: 3-bromopyruvate (3-BP). Growth inhibition by 3-BP of 110 strains of yeast-like and filamentous fungi was tested by standard spot tests or microdilution method. The human pathogen Cryptococcus neoformans exhibited a low Minimal Inhibitory Concentration (MIC) of 0.12-0.15 mM 3-BP. The high toxicity of 3-BP toward C. neoformans correlated with high intracellular accumulation of 3-BP and also with low levels of intracellular ATP and glutathione. Weak cytotoxicity towards mammalian cells and lack of resistance conferred by the PDR (Pleiotropic Drug Resistance) network in the yeast Saccharomyces cerevisiae, are other properties of 3-BP that makes it a novel promising anticryptococcal drug.

Cryptococcus neoformans copper detoxification machinery is critical for fungal virulence

Copper (Cu) is an essential metal that is toxic at high concentrations. Thus, pathogens often rely on host Cu for growth, but host cells can hyperaccumulate Cu to exert antimicrobial effects. The human fungal pathogen Cryptococcus neoformans encodes many Cu-responsive genes, but their role in infection is unclear. We determined that pulmonary C. neoformans infection results in Cu-specific induction of genes encoding the Cu-detoxifying metallothionein (Cmt) proteins. Mutant strains lacking CMTs or expressing Cmt variants defective in Cu-coordination exhibit severely attenuated virulence and reduced pulmonary colonization. Consistent with the upregulation of Cmt proteins, C. neoformans pulmonary infection results in increased serum Cu concentrations and increases and decreases alveolar macrophage expression of the Cu importer (Ctr1) and ATP7A, a transporter implicated in phagosomal Cu compartmentalization, respectively. These studies indicate that the host mobilizes Cu as an innate antifungal defense but C. neoformans senses and neutralizes toxic Cu to promote infection.

Cryptococcus neoformans requires the ESCRT protein Vps23 for iron acquisition from heme, for capsule formation, and for virulence

Iron availability is a key regulator of virulence factor elaboration in Cryptococcus neoformans, the causative agent of fungal meningoencephalitis in HIV/AIDS patients. In addition, iron is an essential nutrient for pathogen proliferation in mammalian hosts but little is known about the mechanisms of iron sensing and uptake in fungal pathogens that attack humans. In this study, we mutagenized C. neoformans by Agrobacterium-mediated T-DNA insertion and screened for mutants with reduced growth on heme as the sole iron source. Among 34 mutants, we identified a subset with insertions in the gene for the ESCRT-I (endosomal sorting complex required for transport) protein Vps23 that resulted in a growth defect on heme, presumably due to a defect in uptake via endocytosis or misregulation of iron acquisition from heme. Remarkably, vps23 mutants were also defective in the elaboration of the cell-associated capsular polysaccharide that is a major virulence factor, while overexpression of Vps23 resulted in cells with a slightly enlarged capsule. These phenotypes were mirrored by a virulence defect in the vps23 mutant in a mouse model of cryptococcosis and by hypervirulence of the overexpression strain. Overall, these results reveal an important role for trafficking via ESCRT functions in both heme uptake and capsule formation, and they further reinforce the connection between iron and virulence factor deployment in C. neoformans.

Coordination of hypoxia adaptation and iron homeostasis in human pathogenic fungi

In mammals, hypoxia causes facilitated erythropoiesis that requires increased iron availability with established links between oxygen and iron in regulation of the transcription factor hypoxia-inducible factor. Therefore, cellular responses to hypoxia and iron starvation are linked in mammals and are host conditions that pathogens encounter during infection. In human pathogenic fungi, molecular mechanisms underlying hypoxia adaptation and iron homeostasis have been investigated. However, the interconnected regulation of hypoxia adaptation and iron homeostasis remains to be fully elucidated. This review discusses the potential transcriptional regulatory links between hypoxia adaptation and iron homeostasis in human pathogenic fungi. Transcriptome analyses demonstrate that core regulators of hypoxia adaptation and iron homeostasis are involved in regulation of several common genes responsible for iron acquisition and ergosterol biosynthesis. Importantly, iron starvation increases susceptibility of fungal cells to antifungal drugs and decreased levels of ergosterol, while key hypoxia regulators are also involved in responses to antifungal drugs and mediating ergosterol levels. We suggest that pathogenic fungi have developed a coordinated regulatory system in response to hypoxia and iron starvation through (i) regulation of expression of hypoxia-responsive and iron-responsive genes via cross-linked key regulators, and/or (ii) regulation of factors involved in ergosterol biosynthesis. Thus, both oxygen and iron availability are intimately tied with fungal virulence and responses to existing therapeutics and further elucidation of their interrelationship should have significant clinical implications.

A defect in ATP-citrate lyase links acetyl-CoA production, virulence factor elaboration and virulence in Cryptococcus neoformans

The interaction of Cryptococcus neoformans with phagocytic cells of the innate immune system is a key step in disseminated disease leading to meningoencephalitis in immunocompromised individuals. Transcriptional profiling of cryptococcal cells harvested from cell culture medium or from macrophages found differential expression of metabolic and other functions during fungal adaptation to the intracellular environment. We focused on the ACL1 gene for ATP-citrate lyase, which converts citrate to acetyl-CoA, because this gene showed elevated transcript levels in macrophages and because of the importance of acetyl-CoA as a central metabolite. Mutants lacking ACL1 showed delayed growth on medium containing glucose, reduced cellular levels of acetyl-CoA, defective production of virulence factors, increased susceptibility to the antifungal drug fluconazole and decreased survival within macrophages. Importantly, acl1 mutants were unable to cause disease in a murine inhalation model, a phenotype that was more extreme than other mutants with defects in acetyl-CoA production (e.g. an acetyl-CoA synthetase mutant). Loss of virulence is likely due to perturbation of critical physiological interconnections between virulence factor expression and metabolism in C. neoformans. Phylogenetic analysis and structural modelling of cryptococcal Acl1 identified three indels unique to fungal protein sequences; these differences may provide opportunities for the development of pathogen-specific inhibitors.

Systems biology of host-fungus interactions: turning complexity into simplicity

Modeling interactions between fungi and their hosts at the systems level requires a molecular understanding both of how the host orchestrates immune surveillance and tolerance, and how this activation, in turn, affects fungal adaptation and survival. The transition from the commensal to pathogenic state, and the co-evolution of fungal strains within their hosts, necessitates the molecular dissection of fungal traits responsible for these interactions. There has been a dramatic increase in publically available genome-wide resources addressing fungal pathophysiology and host–fungal immunology. The integration of these existing data and emerging large-scale technologies addressing host–pathogen interactions requires novel tools to connect genome-wide data sets and theoretical approaches with experimental validation so as to identify inherent and emerging properties of host–pathogen relationships and to obtain a holistic view of infectious processes. If successful, a better understanding of the immune response in health and microbial diseases will eventually emerge and pave the way for improved therapies.

Peroxisomal and mitochondrial β-oxidation pathways influence the virulence of the pathogenic fungus Cryptococcus neoformans

An understanding of the connections between metabolism and elaboration of virulence factors during host colonization by the human-pathogenic fungus Cryptococcus neoformans is important for developing antifungal therapies. Lipids are abundant in host tissues, and fungal pathogens in the phylum basidiomycota possess both peroxisomal and mitochondrial β-oxidation pathways to utilize this potential carbon source. In addition, lipids are important signaling molecules in both fungi and mammals. In this report, we demonstrate that defects in the peroxisomal and mitochondrial β-oxidation pathways influence the growth of C. neoformans on fatty acids as well as the virulence of the fungus in a mouse inhalation model of cryptococcosis. Disease attenuation may be due to the cumulative influence of altered carbon source acquisition or processing, interference with secretion, changes in cell wall integrity, and an observed defect in capsule production for the peroxisomal mutant. Altered capsule elaboration in the context of a β-oxidation defect was unexpected but is particularly important because this trait is a major virulence factor for C. neoformans. Additionally, analysis of mutants in the peroxisomal pathway revealed a growth-promoting activity for C. neoformans, and subsequent work identified oleic acid and biotin as candidates for such factors. Overall, this study reveals that β-oxidation influences virulence in C. neoformans by multiple mechanisms that likely include contributions to carbon source acquisition and virulence factor elaboration.

Aimless mutants of Cryptococcus neoformans: failure to disseminate

The pathogenic fungus Cryptococcus neoformans exhibits a striking propensity to cause central nervous system (CNS) disease in people with HIV/AIDS. Given that cryptococcal infections are generally initiated by pulmonary colonization, dissemination requires that the fungus withstand phagocytic killing, cross the alveolar-capillary interface in the lung, survive in the circulatory system and breach the blood-brain barrier. We know little about the molecular mechanisms underlying dissemination, but there is a rapidly growing list of mutants that fail to cause CNS disease. These mutants reveal a remarkable diversity of functions and therefore illustrate the complexity of the cryptococcal-host interaction. The challenge now is to extend the analysis of these mutants to acquire a detailed understanding of each step in dissemination.