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

Transcriptional and Post-Translational Roles of Calcineurin in Cationic Stress and Glycerol Biosynthesis in Cryptococcus neoformans

Stress management is an adaptive advantage for survival in adverse environments. Pathogens face this challenge during host colonization, requiring an appropriate stress response to establish infection. The fungal pathogen Cryptococcus neoformans undergoes thermal, oxidative, and osmotic stresses in the environment and animal host. Signaling systems controlled by Ras1, Hog1, and calcineurin respond to high temperatures and osmotic stress. Cationic stress caused by Na+, K+, and Li+ can be overcome with glycerol, the preferred osmolyte. Deleting the glycerol phosphate phosphatase gene (GPP2) prevents cells from accumulating glycerol due to a block in the last step of its biosynthetic pathway. Gpp2 accumulates in a phosphorylated form in a cna1Δ strain, and a physical interaction between Gpp2 and Cna1 was found; moreover, the gpp2Δ strain undergoes slow growth and has attenuated virulence in animal models of infection. We provide biochemical evidence that growth in 1 M NaCl increases glycerol content in the wild type, whereas gpp2Δ, cna1Δ, and cnb1Δ mutants fail to accumulate it. The deletion of cnb1Δ or cna1Δ renders yeast cells sensitive to cationic stress, and the Gfp-Gpp2 protein assumes an abnormal localization. We suggest a mechanism in which calcineurin controls Gpp2 at the post-translational level, affecting its localization and activity, leading to glycerol biosynthesis. Also, we showed the transcriptional profile of glycerol-deficient mutants and established the cationic stress response mediated by calcineurin; among the biological processes differentially expressed are carbon utilization, translation, transmembrane transport, glutathione metabolism, oxidative stress response, and transcription regulation. To our knowledge, this is the first time that this transcriptional profile has been described. These results have implications for pathogen stress adaptability.

Gene-specific transcriptional activation by the Aspergillus fumigatus AtrR factor requires a conserved C-terminal domain

Treatment of fungal infections associated with the filamentous fungus Aspergillus fumigatus is becoming more problematic as this organism is developing resistance to the main chemotherapeutic drug at an increasing rate. Azole drugs represent the current standard-of-care in the treatment of aspergillosis with this drug class acting by inhibiting a key step in the biosynthesis of the fungal sterol ergosterol. Azole compounds block the activity of the lanosterol α-14 demethylase, encoded by the cyp51A gene. A common route of azole resistance involves an increase in transcription of cyp51A. This transcriptional increase requires the function of a Zn2Cys6 DNA-binding domain-containing transcription activator protein called AtrR. AtrR was identified through its action as a positive regulator of expression of an ATP-binding cassette transporter (abcC/cdr1B here called abcG1). Using both deletion and alanine scanning mutagenesis, we demonstrate that a conserved C-terminal domain in A. fumigatus is required for the expression of abcG1 but dispensable for cyp51A transcription. This domain is also found in several other fungal pathogen AtrR homologs consistent with a conserved gene-selective function of this protein segment being conserved. Using RNA sequencing (RNA-seq), we find that this gene-specific transcriptional defect extends to several other membrane transporter-encoding genes including a second ABC transporter locus. Our data reveal that AtrR uses at least two distinct mechanisms to induce gene expression and that normal susceptibility to azole drugs cannot be provided by maintenance of wild-type expression of the ergosterol biosynthetic pathway when ABC transporter expression is reduced.

IMPORTANCE

Aspergillus fumigatus is the primary human filamentous fungal pathogen. The principal chemotherapeutic drug used to control infections associated with A. fumigatus is the azole compound. These drugs are well-tolerated and effective, but resistance is emerging at an alarming rate. Most resistance is associated with mutations that lead to overexpression of the azole target enzyme, lanosterol α-14 demethylase, encoded by the cyp51A gene. A key regulator of cyp51A gene expression is the transcription factor AtrR. Very little is known of the molecular mechanisms underlying the effect of AtrR on gene expression. Here, we use deletion and clustered amino acid substitution mutagenesis to map a region of AtrR that confers gene-specific activation on target genes of this transcription factor. This region is highly conserved across AtrR homologs from other pathogenic species arguing that its importance in transcriptional regulation is maintained across evolution.

Loss of a conserved C-terminal region of the Aspergillus fumigatus AtrR transcriptional regulator leads to a gene-specific defect in target gene expression

Treatment of fungal infections associated with the filamentous fungus Aspergillus fumigatus is becoming more problematic as this organism is developing resistance to the main chemotherapeutic drug at an increasing rate. Azole drugs represent the current standard-of-care in treatment of aspergillosis with this drug class acting by inhibiting a key step in biosynthesis of the fungal sterol ergosterol. Azole compounds block the activity of the lanosterol α-14 demethylase, encoded by the cyp51A gene. A common route of azole resistance involves an increase in transcription of cyp51A. This transcriptional increase requires the function of a Zn2Cys6 DNA-binding domain-containing transcription activator protein called AtrR. AtrR was identified through its action as a positive regulator of expression of an ATP-binding cassette transporter (abcC/cdr1B here called abcG1). Using both deletion and alanine scanning mutagenesis, we demonstrate that a conserved C-terminal domain in A. fumigatus is required for expression of abcG1 but dispensable for cyp51A transcription. This domain is also found in several other fungal pathogen AtrR homologues consistent with a conserved gene-selective function of this protein segment being conserved. Using RNA-seq, we find that this gene-specific transcriptional defect extends to several other membrane transporter-encoding genes including a second ABC transporter locus. Our data reveal that AtrR uses at least two distinct mechanisms to induce gene expression and that normal susceptibility to azole drugs cannot be provided by maintenance of wild-type expression of the ergosterol biosynthetic pathway when ABC transporter expression is reduced.

Pleiotropic roles of LAMMER kinase, Lkh1 in stress responses and virulence of Cryptococcus neoformans

Dual-specificity LAMMER kinases are highly evolutionarily conserved in eukaryotes and play pivotal roles in diverse physiological processes, such as growth, differentiation, and stress responses. Although the functions of LAMMER kinase in fungal pathogens in pathogenicity and stress responses have been characterized, its role in Cryptococcus neoformans, a human fungal pathogen and a model yeast of basidiomycetes, remains elusive. In this study, we identified a LKH1 homologous gene and constructed a strain with a deleted LKH1 and a complemented strain. Similar to other fungi, the lkh1Δ mutant showed intrinsic growth defects. We observed that C. neoformans Lkh1 was involved in diverse stress responses, including oxidative stress and cell wall stress. Particularly, Lkh1 regulates DNA damage responses in Rad53-dependent and -independent manners. Furthermore, the absence of LKH1 reduced basidiospore formation. Our observations indicate that Lkh1 becomes hyperphosphorylated upon treatment with rapamycin, a TOR protein inhibitor. Notably, LKH1 deletion led to defects in melanin synthesis and capsule formation. Furthermore, we found that the deletion of LKH1 led to the avirulence of C. neoformans in a systemic cryptococcosis murine model. Taken together, Lkh1 is required for the stress response, sexual differentiation, and virulence of C. neoformans.

Casein kinase 2 complex: a central regulator of multiple pathobiological signaling pathways in Cryptococcus neoformans

The casein kinase 2 (CK2) complex has garnered extensive attention over the past decades as a potential therapeutic target for diverse human diseases, including cancer, diabetes, and obesity, due to its pivotal roles in eukaryotic growth, differentiation, and metabolic homeostasis. While CK2 is also considered a promising antifungal target, its role in fungal pathogens remains unexplored. In this study, we investigated the functions and regulatory mechanisms of the CK2 complex in Cryptococcus neoformans, a major cause of fungal meningitis. The cryptococcal CK2 complex consists of a single catalytic subunit, Cka1, and two regulatory subunits, Ckb1 and Ckb2. Our findings show that Cka1 plays a primary role as a protein kinase, while Ckb1 and Ckb2 have major and minor regulatory functions, respectively, in growth, cell cycle control, morphogenesis, stress response, antifungal drug resistance, and virulence factor production. Interestingly, triple mutants lacking all three subunits (cka1Δ ckb1Δ ckb2Δ) exhibited more severe phenotypic defects than the cka1Δ mutant alone, suggesting that Ckb1/2 may have Cka1-independent functions. In a murine model of systemic cryptococcosis, cka1Δ and cka1Δ ckb1Δ ckb2Δ mutants showed severely reduced virulence. Transcriptomic, proteomic, and phosphoproteomic analyses further revealed that the CK2 complex controls a wide array of effector proteins involved in transcriptional regulation, cell cycle control, nutrient metabolisms, and stress responses. Most notably, CK2 disruption led to dysregulation of key signaling cascades central to C. neoformans pathogenicity, including the Hog1, Mpk1 MAPKs, cAMP/PKA, and calcium/calcineurin signaling pathways. In summary, our study provides novel insights into the multifaceted roles of the fungal CK2 complex and presents a compelling case for targeting it in the development of new antifungal drugs.IMPORTANCEThe casein kinase 2 (CK2) complex, crucial for eukaryotic growth, differentiation, and metabolic regulation, presents a promising therapeutic target for various human diseases, including cancer, diabetes, and obesity. Its potential as an antifungal target is further highlighted in this study, which explores CK2's functions in C. neoformans, a key fungal meningitis pathogen. The CK2 complex in C. neoformans, comprising the Cka1 catalytic subunit and Ckb1/2 regulatory subunits, is integral to processes like growth, cell cycle, morphogenesis, stress response, drug resistance, and virulence. Our findings of CK2's role in regulating critical signaling pathways, including Hog1, Mpk1 MAPKs, cAMP/PKA, and calcium/calcineurin, underscore its importance in C. neoformans pathogenicity. This study provides valuable insights into the fungal CK2 complex, reinforcing its potential as a target for novel antifungal drug development and pointing out a promising direction for creating new antifungal agents.

Hog1-mediated stress tolerance in the pathogenic fungus Trichosporon asahii

Trichosporon asahii is an opportunistic pathogenic fungus that causes severe and sometimes fatal infections in immunocompromised patients. Hog1, a mitogen-activated protein kinase, regulates the stress resistance of some pathogenic fungi, however its role in T. asahii has not been investigated. Here, we demonstrated that the hog1 gene-deficient T. asahii mutant is sensitive to high temperature, cell membrane stress, oxidative stress, and antifungal drugs. Growth of the hog1 gene-deficient T. asahii mutant was delayed at 40 °C. The hog1 gene-deficient T. asahii mutant also exhibited sensitivity to sodium dodecyl sulfate, hydrogen peroxide, menadione, methyl methanesulfonate, UV exposure, and antifungal drugs such as amphotericin B under a glucose-rich condition. Under a glucose-restricted condition, the hog1 gene-deficient mutant exhibited sensitivity to NaCl and KCl. The virulence of the hog1 gene-deficient mutant against silkworms was attenuated. Moreover, the viability of the hog1 gene-deficient mutant decreased in the silkworm hemolymph. These phenotypes were restored by re-introducing the hog1 gene into the gene-deficient mutant. Our findings suggest that Hog1 plays a critical role in regulating cellular stress responses in T. asahii.

The high osmolarity glycerol (HOG) pathway in fungi†

While fungi are widely occupying nature, many species are responsible for devastating mycosis in humans. Such niche diversity explains how quick fungal adaptation is necessary to endow the capacity of withstanding fluctuating environments and to cope with host-imposed conditions. Among all the molecular mechanisms evolved by fungi, the most studied one is the activation of the phosphorelay signalling pathways, of which the high osmolarity glycerol (HOG) pathway constitutes one of the key molecular apparatus underpinning fungal adaptation and virulence. In this review, we summarize the seminal knowledge of the HOG pathway with its more recent developments. We specifically described the HOG-mediated stress adaptation, with a particular focus on osmotic and oxidative stress, and point out some lags in our understanding of its involvement in the virulence of pathogenic species including, the medically important fungi Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus, compared to the model yeast Saccharomyces cerevisiae. Finally, we also highlighted some possible applications of the HOG pathway modifications to improve the fungal-based production of natural products in the industry.

HOG1 has an essential role in the stress response, virulence and pathogenicity of Cryptococcus gattii

Cryptococcus gattii (C. gattii) is a lethal pathogen that causes the majority of cryptococcosis cases in previously healthy individuals. This pathogen poses an increasing threat to global public health, but the mechanisms underlying the pathogenesis have remained to be fully elucidated. In the present study, the role of high-osmolarity glycerol (HOG)1 in the stress reaction and virulence control of C. gattii was characterized by deleting the HOG1 gene using the clinical isolate strain CZ2012, and finally, the virulence and pathogenic traits of the deletion strain were defined. Deletion of the HOG1 gene resulted in notable growth defects under stress conditions (high salt and antifungal drugs), but different traits were observed under oxidative stress conditions (hydrogen peroxide). Similarly, the C. gattii hog1Δ strains (deletion of HOG1) also displayed decreased capsule production and melanin synthesis. Furthermore, mice infected with the hog1Δ strain had longer survival times than those infected with the wild-type strain and the reconstituted strain. The hog1Δ strain recovered from infected organs exhibited significant growth defects in terms of decreased colony count and size. The present results suggested that HOG1 has a significant role in the virulence of C. gattii and these results may help to elucidate the pathogenesis of C. gattii.

Landscape of gene expression variation of natural isolates of Cryptococcus neoformans in response to biologically relevant stresses

Cryptococcus neoformans is an opportunistic fungal pathogen that at its peak epidemic levels caused an estimated million cases of cryptococcal meningitis per year worldwide. This species can grow in diverse environmental (trees, soil and bird excreta) and host niches (intracellular microenvironments of phagocytes and free-living in host tissues). The genetic basic for adaptation to these different conditions is not well characterized, as most experimental work has relied on a single reference strain of C. neoformans. To identify genes important for yeast infection and disease progression, we profiled the gene expression of seven C. neoformans isolates grown in five representative in vitro environmental and in vivo conditions. We characterized gene expression differences using RNA-Seq (RNA sequencing), comparing clinical and environmental isolates from two of the major lineages of this species, VNI and VNBI. These comparisons highlighted genes showing lineage-specific expression that are enriched in subtelomeric regions and in lineage-specific gene clusters. By contrast, we find few expression differences between clinical and environmental isolates from the same lineage. Gene expression specific to in vivo stages reflects available nutrients and stresses, with an increase in fungal metabolism within macrophages, and an induction of ribosomal and heat-shock gene expression within the subarachnoid space. This study provides the widest view to date of the transcriptome variation of C. neoformans across natural isolates, and provides insights into genes important for in vitro and in vivo growth stages.

Enhanced resistance of Trichoderma harzianum LZDX-32-08 to hygromycin B induced by sea salt

OBJECTIVES

To determine the effect of sea salt on the resistance of Trichoderma harzianum LZDX-32-08 to hygromycin B and speculate the possible mechanisms involved via transcriptome analysis.

RESULTS

Sea salt addition in media to simulate marine environment significantly increased the tolerance of marine-derived fungus Trichoderma harzianum LZDX-32-08 to hygromycin B from 40 to 500 μg/ml. Meanwhile, sea salt addition also elicited the hygromycin B resistance of 5 other marine or terrestrial fungi. Transcriptomic analyses of T. harzianum cultivated on PDA, PDA supplemented with sea salt and PDA with both sea salt and hygromycin B revealed that genes coding for P-type ATPases, multidrug resistance related transporters and acetyltransferases were up-regulated, while genes coding for Ca²⁺/H⁺ antiporter and 1,3-glucosidase were down-regulated, indicating probable increased efflux and inactivation of hygromycin B as well as enhanced biofilm formation, which could jointly contribute to the drug resistance.

CONCLUSIONS

Marine environment or high ion concentration in the environment could be an importance inducer for antifungal resistance. Possible mechanisms and related key genes were proposed for understanding the molecular basis and overcoming this resistance.

Broadening the spectrum of fluorescent protein tools for use in the encapsulated human fungal pathogen Cryptococcus neoformans

Green fluorescent protein (GFP) and its counterparts are modern molecular biology research tools indispensable in many experimental systems. Within fungi, researchers studying Saccharomyces cerevisiae and other model ascomycetes have access to a wide variety of fluorescent proteins. Unfortunately, many of these tools have not crossed the phylum divide into the Basidiomycota, where only GFP S65T, Venus, Ds-Red, and mCherry are currently available. To address this, we searched the literature for potential candidates to be expressed in the human fungal pathogen Cryptococcus neoformans and identified a suite of eight more modern fluorescent proteins that span the visible spectrum. A single copy of each fluorophore was heterologously expressed in Safe Haven 1 and their fluorescence intensities compared in this encapsulated yeast. mTurquoise2, mTFP1, Clover, mNeonGreen, mRuby3, and Citrine were highly visible under the microscope, whereas Superfolder GFP and mMaroon1 were not. Expressed fluorophores did not impact growth or virulence as demonstrated by an in vitro spotting assay and murine inhalation model, respectively.

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.

Unraveling Melanin Biosynthesis and Signaling Networks in Cryptococcus neoformans

Melanins are dark green, brown, or black pigments that serve as antioxidant, reactive oxygen species (ROS) scavengers that protect fungal pathogens from radiation and host immune responses. Cryptococcus neoformans, the major etiological agent of fungal meningoencephalitis, also utilizes melanin as a key virulence factor. In this basidiomycete pathogen, melanin production is regulated by the cAMP and high-osmolarity glycerol response (HOG) pathways, and yet its complex signaling networks remain poorly described. In this study, we uncovered novel melanin synthesis regulatory networks consisting of core transcription factors (TFs), including Bzp4, Usv101, Hob1, and Mbs1, and core kinases Gsk3 and Kic1. These networks were identified through coupling systematic analyses of the expression and epistatic relationships of TF and kinase mutant libraries in the presence of diverse melanin substrates with transcriptome profiling of the core TF mutants. Thus, this report provides comprehensive insight into the melanin-regulating pathways in C. neoformans and other fungal pathogens.

Melanin is an antioxidant polyphenol pigment required for the pathogenicity of many fungal pathogens, but comprehensive regulatory mechanisms remain unidentified. In this study, we systematically analyzed melanin-regulating signaling pathways in Cryptococcus neoformans and identified four melanin-regulating core transcription factors (TFs), Bzp4, Usv101, Mbs1, and Hob1, required for induction of the laccase gene (LAC1). Bzp4, Usv101, and Mbs1 independently regulate LAC1 induction, whereas Hob1 controls Bzp4 and Usv101 expression. Both Bzp4 and Usv101 are localized in the cytoplasm under nutrient-rich conditions (i.e., in the presence of yeast extract-peptone-dextrose [YPD] medium) but translocate into the nucleus upon nutrient starvation (i.e., in the presence of yeast nitrogen base [YNB] medium without glucose), and Mbs1 is constitutively localized in the nucleus. Notably, the cAMP pathway is not involved in regulation of the four TFs, but the high-osmolarity glycerol response (HOG) pathway negatively regulates induction of BZP4 and LAC1. Next, we searched for potential kinases upstream of the core TFs and identified nine core kinases; their deletion led to defective melanin production and LAC1 induction. Deletion of GSK3 or KIC1 abolished induction of LAC1 and BZP4 and perturbed nuclear translocation of Bzp4. Notably, Gsk3 also regulated expression of HOB1, USV101, and MBS1, indicating that it is a critical melanin-regulating kinase. Finally, an RNA sequencing-based transcriptome analysis of the wild-type strain and of bzp4Δ, usv101Δ, hob1Δ, and mbs1Δ strains under nutrient-rich and nutrient-starved conditions revealed that the melanin-regulating core TFs govern redundant and distinct classes of genes involved in a variety of biological processes.

The endophyte Serendipita indica reduces the sodium content of Arabidopsis plants exposed to salt stress: fungal ENA ATPases are expressed and regulated at high pH and during plant co-cultivation in salinity

Serendipita indica (formerly Piriformospora indica) is an endophytic fungus that colonizes plant roots producing a beneficial effect on plant growth and development under optimal and suboptimal conditions provoked by any biotic or abiotic stress, such as salt stress. Salinity induces osmotic and ionic imbalances in plants, mainly by altering the Na⁺ and K⁺ contents. However, the mechanism by which Serendipita improves plant growth has yet to be elucidated. Previous works suggest that this fungus improves the plant osmotic state but not much is known about whether it participates in readjustment of the ionic imbalance in plants. Here, we report that co-cultivation with Serendipita reduces the Na⁺ content of Arabidopsis plants under saline conditions. Additionally, we describe the functional characterization of the two Serendipita ENA ATPases, which are homologous to the main proteins involved in the salt tolerance of other fungi. Their heterologous expression in salt-sensitive yeast mutants shows that SiENA1 is involved in Na⁺ and K⁺ efflux, while SiENA5 seems to only be involved in Na⁺ detoxification. Both are induced and might have a relevant function at alkaline pH, condition in which there are few chlamydospores in the mycelium, as well as during co-cultivation with plants exposed to saline conditions.

Stress-Activated Protein Kinases in Human Fungal Pathogens

The ability of fungal pathogens to survive hostile environments within the host depends on rapid and robust stress responses. Stress-activated protein kinase (SAPK) pathways are conserved MAPK signaling modules that promote stress adaptation in all eukaryotic cells, including pathogenic fungi. Activation of the SAPK occurs via the dual phosphorylation of conserved threonine and tyrosine residues within a TGY motif located in the catalytic domain. This induces the activation and nuclear accumulation of the kinase and the phosphorylation of diverse substrates, thus eliciting appropriate cellular responses. The Hog1 SAPK has been extensively characterized in the model yeast Saccharomyces cerevisiae. Here, we use this a platform from which to compare SAPK signaling mechanisms in three major fungal pathogens of humans, Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. Despite the conservation of SAPK pathways within these pathogenic fungi, evidence is emerging that their role and regulation has significantly diverged. However, consistent with stress adaptation being a common virulence trait, SAPK pathways are important pathogenicity determinants in all these major human pathogens. Thus, the development of drugs which target fungal SAPKs has the exciting potential to generate broad-acting antifungal treatments.

Nutrient and Stress Sensing in Pathogenic Yeasts

More than 1.5 million fungal species are estimated to live in vastly different environmental niches. Despite each unique host environment, fungal cells sense certain fundamentally conserved elements, such as nutrients, pheromones and stress, for adaptation to their niches. Sensing these extracellular signals is critical for pathogens to adapt to the hostile host environment and cause disease. Hence, dissecting the complex extracellular signal-sensing mechanisms that aid in this is pivotal and may facilitate the development of new therapeutic approaches to control fungal infections. In this review, we summarize the current knowledge on how two important pathogenic yeasts, Candida albicans and Cryptococcus neoformans, sense nutrient availability, such as carbon sources, amino acids, and ammonium, and different stress signals to regulate their morphogenesis and pathogenicity in comparison with the non-pathogenic model yeast Saccharomyces cerevisiae. The molecular interactions between extracellular signals and their respective sensory systems are described in detail. The potential implication of analyzing nutrient and stress-sensing systems in antifungal drug development is also discussed.

Monovalent cation transporters at the plasma membrane in yeasts

Maintenance of proper intracellular concentrations of monovalent cations, mainly sodium and potassium, is a requirement for survival of any cell. In the budding yeast Saccharomyces cerevisiae, monovalent cation homeostasis is determined by the active extrusion of protons through the Pma1 H⁺ -ATPase (reviewed in another chapter of this issue), the influx and efflux of these cations through the plasma membrane transporters (reviewed in this chapter), and the sequestration of toxic cations into the vacuoles. Here, we will describe the structure, function, and regulation of the plasma membrane transporters Trk1, Trk2, Tok1, Nha1, and Ena1, which play a key role in maintaining physiological intracellular concentrations of Na⁺ , K⁺ , and H⁺ , both under normal growth conditions and in response to stress.

Structure and function of yeast and fungal Na+ /H+ antiporters

Sodium proton antiporters (or sodium proton exchangers [NHEs]) are a critical family of membrane proteins that exchange sodium for protons across cell membranes. In yeast and plants, their primary function is to keep the sodium concentration low inside the cytoplasm. One class of NHE constitutively expressed in yeast is the plasma membrane Na⁺ /H⁺ antiporter, and another class is expressed on the endosomal/vacuolar membrane. At present, four bacterial plasma membrane antiporter structures are known and nuclear magnetic resonance structures are available for the membrane spanning transmembrane helices of mammalian and yeast NHEs. Additionally, a vast amount of mutational data are available on the role of individual amino acids and critical motifs involved in transport. We combine this information to obtain a more detailed picture of the yeast NHE plasma membrane protein and review mechanisms of transport, conserved motifs, unique residues important in function, and regulation of these proteins. The Na⁺ /H⁺ antiporter of Schizosaccharomyces pombe, SpNHE1, is an interesting model protein in an easy to study system and is representative of fungal Na⁺ /H⁺ antiporters. © IUBMB Life, 70(1):23-31, 2018.

The water channel protein aquaporin 1 regulates cellular metabolism and competitive fitness in a global fungal pathogen Cryptococcus neoformans

In this study, an aquaporin protein, Aqp1, in Cryptococcus neoformans, which can lead either saprobic or parasitic lifestyles and causes life-threatening fungal meningitis was identified and characterized. AQP1 expression was rapidly induced (via the HOG pathway) by osmotic or oxidative stress. In spite of such transcriptional regulation, Aqp1 was found to be largely unnecessary for adaptation to diverse environmental stressors, regardless of the presence of the polysaccharide capsule. The latter is shown here to be a key environmental-stress protectant for C. neoformans. Furthermore, Aqp1 was not required for the development and virulence of C. neoformans. Deletion of AQP1 increased hydrophobicity of the cell surface. The comparative metabolic profiling analysis of the aqp1Δ mutant and AQP1-overexpressing strains revealed that deletion of AQP1 significantly increased cellular accumulation of primary and secondary metabolites, whereas overexpression of AQP1 depleted such metabolites, suggesting that this water channel protein performs a critical function in metabolic homeostasis. In line with this result, it was found that the aqp1Δ mutant (which is enriched with diverse metabolites) survived better than the wild type and a complemented strain, indicating that Aqp1 is likely to be involved in competitive fitness of this fungal pathogen.

Physiological Differences in Cryptococcus neoformans Strains In Vitro versus In Vivo and Their Effects on Antifungal Susceptibility

Cryptococcus neoformans is an environmentally ubiquitous fungal pathogen that primarily causes disease in people with compromised immune systems, particularly those with advanced AIDS. There are estimated to be almost 1 million cases per year of cryptococcal meningitis in patients infected with human immunodeficiency virus, leading to over 600,000 annual deaths, with a particular burden in sub-Saharan Africa. Amphotericin B (AMB) and fluconazole (FLC) are key components of cryptococcal meningitis treatment: AMB is used for induction, and FLC is for consolidation, maintenance and, for occasional individuals, prophylaxis. However, the results of standard antifungal susceptibility testing (AFST) for AMB and FLC do not correlate well with therapeutic outcomes and, consequently, no clinical breakpoints have been established. While a number of explanations for this absence of correlation have been proffered, one potential reason that has not been adequately explored is the possibility that the physiological differences between the in vivo infection environment and the in vitro AFST environment lead to disparate drug susceptibilities. These susceptibility-influencing factors include melanization, which does not occur during AFST, the size of the polysaccharide capsule, which is larger in infecting cells than in those grown under normal laboratory conditions, and the presence of large polyploid "titan cells," which rarely occur under laboratory conditions. Understanding whether and how C. neoformans differentially expresses mechanisms of resistance to AMB and FLC in the AFST environment compared to the in vivo environment could enhance our ability to interpret AFST results and possibly lead to the development of more applicable testing methods.

PiHOG1, a stress regulator MAP kinase from the root endophyte fungus Piriformospora indica, confers salinity stress tolerance in rice plants

In this study, yeast HOG1 homologue from the root endophyte Piriformospora indica (PiHOG1) was isolated and functionally characterized. Functional expression of PiHOG1 in S. cerevisiae ∆hog1 mutant restored osmotolerance under high osmotic stress. Knockdown (KD) transformants of PiHOG1 generated by RNA interference in P. indica showed that genes for the HOG pathway, osmoresponse and salinity tolerance were less stimulated in KD-PiHOG1 compared to the wild-type under salinity stress. Furthermore, KD lines are impaired in the colonization of rice roots under salinity stress of 200 mM NaCl, and the biomass of the host plants, their shoot and root lengths, root number, photosynthetic pigment and proline contents were reduced as compared to rice plants colonized by WT P. indica. Therefore, PiHOG1 is critical for root colonisation, salinity tolerance and the performance of the host plant under salinity stress. Moreover, downregulation of PiHOG1 resulted not only in reduced and delayed phosphorylation of the remaining PiHOG1 protein in colonized salinity-stressed rice roots, but also in the downregulation of the upstream MAP kinase genes PiPBS2 and PiSSK2 involved in salinity tolerance signalling in the fungus. Our data demonstrate that PiHOG1 is not only involved in the salinity response of P. indica, but also helping host plant to overcome salinity stress.

Systemic Approach to Virulence Gene Network Analysis for Gaining New Insight into Cryptococcal Virulence

Cryptococcus neoformans is pathogenic yeast, responsible for highly lethal infections in compromised patients around the globe. C. neoformans typically initiates infections in mammalian lung tissue and subsequently disseminates to the central nervous system where it causes significant pathologies. Virulence genes of C. neoformans are being characterized at an increasing rate, however, we are far from a comprehensive understanding of their roles and genetic interactions. Some of these reported virulence genes are scattered throughout different databases, while others are not yet included. This study gathered and analyzed 150 reported virulence associated factors (VAFs) of C. neoformans. Using the web resource STRING database, our study identified different interactions between the total VAFs and those involved specifically in lung and brain infections and identified a new strain specific virulence gene, SHO1, involved in the mitogen-activated protein kinase signaling pathway. As predicted by our analysis, SHO1 expression enhanced C. neoformans virulence in a mouse model of pulmonary infection, contributing to enhanced non-protective immune Th2 bias and progressively enhancing fungal growth in the infected lungs. Sequence analysis indicated 77.4% (116) of total studied VAFs are soluble proteins, and 22.7% (34) are transmembrane proteins. Motifs involved in regulation and signaling such as protein kinases and transcription factors are highly enriched in Cryptococcus VAFs. Altogether, this study represents a pioneering effort in analysis of the virulence composite network of C. neoformans using a systems biology approach.

Unique roles of the unfolded protein response pathway in fungal development and differentiation

Cryptococcus neoformans, a global fungal meningitis pathogen, employs the unfolded protein response pathway. This pathway, which consists of an evolutionarily conserved Ire1 kinase/endoribonuclease and a unique transcription factor (Hxl1), modulates the endoplasmic reticulum stress response and pathogenicity. Here, we report that the unfolded protein response pathway governs sexual and unisexual differentiation of C. neoformans in an Ire1-dependent but Hxl1-independent manner. The ire1∆ mutants showed defects in sexual mating, with reduced cell fusion and pheromone-mediated formation of the conjugation tube. Unexpectedly, these mating defects did not result from defective pheromone production because expression of the mating pheromone gene (MFα1) was strongly induced in the ire1∆ mutant. Ire1 controls sexual differentiation by modulating the function of the molecular chaperone Kar2 and by regulating mating-induced localisation of mating pheromone transporter (Ste6) and receptor (Ste3/Cprα). Deletion of IRE1, but not HXL1, also caused significant defects in unisexual differentiation in a Kar2-independent manner. Moreover, we showed that Rim101 is a novel downstream factor of Ire1 for production of the capsule, which is a unique structural determinant of C. neoformans virulence. Therefore, Ire1 uniquely regulates fungal development and differentiation in an Hxl1-independent manner.

Pho4 mediates phosphate acquisition in Candida albicans and is vital for stress resistance and metal homeostasis

This study provides the first evidence that the phosphate-responsive transcription factor Pho4 is vital for survival of Candida albicans to diverse and physiologically relevant stresses. Pho4 is important for C. albicans pathogenesis, and thus these findings illustrate how metabolic adaptation promotes C. albicans survival in the host.

During interactions with its mammalian host, the pathogenic yeast Candida albicans is exposed to a range of stresses such as superoxide radicals and cationic fluxes. Unexpectedly, a nonbiased screen of transcription factor deletion mutants revealed that the phosphate-responsive transcription factor Pho4 is vital for the resistance of C. albicans to these diverse stresses. RNA-Seq analysis indicated that Pho4 does not induce stress-protective genes directly. Instead, we show that loss of Pho4 affects metal cation toxicity, accumulation, and bioavailability. We demonstrate that pho4Δ cells are sensitive to metal and nonmetal cations and that Pho4-mediated polyphosphate synthesis mediates manganese resistance. Significantly, we show that Pho4 is important for mediating copper bioavailability to support the activity of the copper/zinc superoxide dismutase Sod1 and that loss of Sod1 activity contributes to the superoxide sensitivity of pho4Δ cells. Consistent with the key role of fungal stress responses in countering host phagocytic defenses, we also report that C. albicans pho4Δ cells are acutely sensitive to macrophage-mediated killing and display attenuated virulence in animal infection models. The novel connections between phosphate metabolism, metal homeostasis, and superoxide stress resistance presented in this study highlight the importance of metabolic adaptation in promoting C. albicans survival in the host.

Calcineurin and Calcium Channel CchA Coordinate the Salt Stress Response by Regulating Cytoplasmic Ca2+ Homeostasis in Aspergillus nidulans

The eukaryotic calcium/calmodulin-dependent protein phosphatase calcineurin is crucial for the environmental adaption of fungi. However, the mechanism of coordinate regulation of the response to salt stress by calcineurin and the high-affinity calcium channel CchA in fungi is not well understood. Here we show that the deletion of cchA suppresses the hyphal growth defects caused by the loss of calcineurin under salt stress in Aspergillus nidulans Additionally, the hypersensitivity of the ΔcnaA strain to extracellular calcium and cell-wall-damaging agents can be suppressed by cchA deletion. Using the calcium-sensitive photoprotein aequorin to monitor the cytoplasmic Ca(2+) concentration ([Ca(2+)]c) in living cells, we found that calcineurin negatively regulates CchA on calcium uptake in response to external calcium in normally cultured cells. However, in salt-stress-pretreated cells, loss of either cnaA or cchA significantly decreased the [Ca(2+)]c, but a deficiency in both cnaA and cchA switches the [Ca(2+)]c to the reference strain level, indicating that calcineurin and CchA synergistically coordinate calcium influx under salt stress. Moreover, real-time PCR results showed that the dysfunction of cchA in the ΔcnaA strain dramatically restored the expression of enaA (a major determinant for sodium detoxification), which was abolished in the ΔcnaA strain under salt stress. These results suggest that double deficiencies of cnaA and cchA could bypass the requirement of calcineurin to induce enaA expression under salt stress. Finally, YvcA, a member of the transient receptor potential channel (TRPC) protein family of vacuolar Ca(2+) channels, was proven to compensate for calcineurin-CchA in fungal salt stress adaption.IMPORTANCE The feedback inhibition relationship between calcineurin and the calcium channel Cch1/Mid1 has been well recognized from yeast. Interestingly, our previous study (S. Wang et al., PLoS One 7:e46564, 2012, http://dx.doi.org/10.1371/journal.pone.0046564) showed that the deletion of cchA could suppress the hyphal growth defects caused by the loss of calcineurin under salt stress in Aspergillus nidulans In this study, our findings suggest that fungi are able to develop a unique mechanism for adapting to environmental salt stress. Compared to cells cultured normally, the NaCl-pretreated cells had a remarkable increase in transient [Ca(2+)]c Furthermore, we show that calcineurin and CchA are required to modulate cellular calcium levels and synergistically coordinate calcium influx under salt stress. Finally, YvcA, a member of of the TRPC family of vacuolar Ca(2+) channels, was proven to compensate for calcineurin-CchA in fungal salt stress adaption. The findings in this study provide insights into the complex regulatory links between calcineurin and CchA to maintain cytoplasmic Ca(2+) homeostasis in response to different environments.

The cAMP/protein kinase A signaling pathway in pathogenic basidiomycete fungi: Connections with iron homeostasis

A number of pathogenic species of basidiomycete fungi are either life-threatening pathogens of humans or major economic pests for crop production. Sensing the host is a key aspect of pathogen proliferation during disease, and signal transduction pathways are critically important for detecting environmental conditions and facilitating adaptation. This review focuses on the contributions of the cAMP/protein kinase A (PKA) signaling pathway in Cryptococcus neoformans, a species that causes meningitis in humans, and Ustilago maydis, a model phytopathogen that causes a smut disease on maize. Environmental sensing by the cAMP/PKA pathway regulates the production of key virulence traits in C. neoformans including the polysaccharide capsule and melanin. For U. maydis, the pathway controls the dimorphic transition from budding growth to the filamentous cell type required for proliferation in plant tissue. We discuss recent advances in identifying new components of the cAMP/PKA pathway in these pathogens and highlight an emerging theme that pathway signaling influences iron acquisition.

Coordinate responses to alkaline pH stress in budding yeast

Alkalinization of the medium represents a stress condition for the budding yeast Saccharomyces cerevisiae to which this organism responds with profound remodeling of gene expression. This is the result of the modulation of a substantial number of signaling pathways whose participation in the alkaline response has been elucidated within the last ten years. These regulatory inputs involve not only the conserved Rim101/PacC pathway, but also the calcium-activated phosphatase calcineurin, the Wsc1-Pkc1-Slt2 MAP kinase, the Snf1 and PKA kinases and oxidative stress-response pathways. The uptake of many nutrients is perturbed by alkalinization of the environment and, consequently, an impact on phosphate, iron/copper and glucose homeostatic mechanisms can also be observed. The analysis of available data highlights cases in which diverse signaling pathways are integrated in the gene promoter to shape the appropriate response pattern. Thus, the expression of different genes sharing the same signaling network can be coordinated, allowing functional coupling of their gene products.

The role of the de novo pyrimidine biosynthetic pathway in Cryptococcus neoformans high temperature growth and virulence

Fungal infections are often difficult to treat due to the inherent similarities between fungal and animal cells and the resulting host toxicity from many antifungal compounds. Cryptococcus neoformans is an opportunistic fungal pathogen of humans that causes life-threatening disease, primarily in immunocompromised patients. Since antifungal therapy for this microorganism is limited, many investigators have explored novel drug targets aim at virulence factors, such as the ability to grow at mammalian physiological temperature (37°C). To address this issue, we used the Agrobacterium tumefaciens gene delivery system to create a random insertion mutagenesis library that was screened for altered growth at elevated temperatures. Among several mutants unable to grow at 37°C, we explored one bearing an interruption in the URA4 gene. This gene encodes dihydroorotase (DHOase) that is involved in the de novo synthesis of pyrimidine ribonucleotides. Loss of the C. neoformans Ura4 protein, by targeted gene interruption, resulted in an expected uracil/uridine auxotrophy and an unexpected high temperature growth defect. In addition, the ura4 mutant displayed phenotypic defects in other prominent virulence factors (melanin, capsule and phospholipase) and reduced stress response compared to wild type and reconstituted strains. Accordingly, this mutant had a decreased survival rate in macrophages and attenuated virulence in a murine model of cryptococcal infection. Quantitative PCR analysis suggests that this biosynthetic pathway is induced during the transition from 30°C to 37°C, and that transcriptional regulation of de novo and salvage pyrimidine pathway are under the control of the Ura4 protein.

Cryptococcus neoformans and Cryptococcus gattii, the etiologic agents of cryptococcosis

Cryptococcus neoformans and Cryptococcus gattii are the two etiologic agents of cryptococcosis. They belong to the phylum Basidiomycota and can be readily distinguished from other pathogenic yeasts such as Candida by the presence of a polysaccharide capsule, formation of melanin, and urease activity, which all function as virulence determinants. Infection proceeds via inhalation and subsequent dissemination to the central nervous system to cause meningoencephalitis. The most common risk for cryptococcosis caused by C. neoformans is AIDS, whereas infections caused by C. gattii are more often reported in immunocompetent patients with undefined risk than in the immunocompromised. There have been many chapters, reviews, and books written on C. neoformans. The topics we focus on in this article include species description, pathogenesis, life cycle, capsule, and stress response, which serve to highlight the specializations in virulence that have occurred in this unique encapsulated melanin-forming yeast that causes global deaths estimated at more than 600,000 annually.

Distinct and redundant roles of protein tyrosine phosphatases Ptp1 and Ptp2 in governing the differentiation and pathogenicity of Cryptococcus neoformans

Protein tyrosine phosphatases (PTPs) serve as key negative-feedback regulators of mitogen-activated protein kinase (MAPK) signaling cascades. However, their roles and regulatory mechanisms in human fungal pathogens remain elusive. In this study, we characterized the functions of two PTPs, Ptp1 and Ptp2, in Cryptococcus neoformans, which causes fatal meningoencephalitis. PTP1 and PTP2 were found to be stress-inducible genes, which were controlled by the MAPK Hog1 and the transcription factor Atf1. Ptp2 suppressed the hyperphosphorylation of Hog1 and was involved in mediating vegetative growth, sexual differentiation, stress responses, antifungal drug resistance, and virulence factor regulation through the negative-feedback loop of the HOG pathway. In contrast, Ptp1 was not essential for Hog1 regulation, despite its Hog1-dependent induction. However, in the absence of Ptp2, Ptp1 served as a complementary PTP to control some stress responses. In differentiation, Ptp1 acted as a negative regulator, but in a Hog1- and Cpk1-independent manner. Additionally, Ptp1 and Ptp2 localized to the cytosol but were enriched in the nucleus during the stress response, affecting the transient nuclear localization of Hog1. Finally, Ptp1 and Ptp2 played minor and major roles, respectively, in the virulence of C. neoformans. Taken together, our data suggested that PTPs could be exploited as novel antifungal targets.

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.

pH signaling in human fungal pathogens: a new target for antifungal strategies

Fungi are exposed to broadly fluctuating environmental conditions, to which adaptation is crucial for their survival. An ability to respond to a wide pH range, in particular, allows them to cope with rapid changes in their extracellular settings. PacC/Rim signaling elicits the primary pH response in both model and pathogenic fungi and has been studied in multiple fungal species. In the predominant human pathogenic fungi, namely, Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans, this pathway is required for many functions associated with pathogenesis and virulence. Aspects of this pathway are fungus specific and do not exist in mammalian cells. In this review, we highlight recent advances in our understanding of PacC/Rim-mediated functions and discuss the growing interest in this cascade and its factors as potential drug targets for antifungal strategies. We focus on both conserved and distinctive features in model and pathogenic fungi, highlighting the specificities of PacC/Rim signaling in C. albicans, A. fumigatus, and C. neoformans. We consider the role of this pathway in fungal virulence, including modulation of the host immune response. Finally, as now recognized for other signaling cascades, we highlight the role of pH in adaptation to antifungal drug pressure. By acting on the PacC/Rim pathway, it may therefore be possible (i) to ensure fungal specificity and to limit the side effects of drugs, (ii) to ensure broad-spectrum efficacy, (iii) to attenuate fungal virulence, (iv) to obtain additive or synergistic effects with existing antifungal drugs through tolerance inhibition, and (v) to slow the emergence of resistant mutants.

The Cryptococcus neoformans Rim101 transcription factor directly regulates genes required for adaptation to the host

The Rim101 protein is a conserved pH-responsive transcription factor that mediates important interactions between several fungal pathogens and the infected host. In the human fungal pathogen Cryptococcus neoformans, the Rim101 protein retains conserved functions to allow the microorganism to respond to changes in pH and other host stresses. This coordinated cellular response enables this fungus to effectively evade the host immune response. Preliminary studies suggest that this conserved transcription factor is uniquely regulated in C. neoformans both by the canonical pH-sensing pathway and by the cyclic AMP (cAMP)/protein kinase A (PKA) pathway. Here we present comparative transcriptional data that demonstrate a strong concordance between the downstream effectors of PKA and Rim101. To define Rim101-dependent gene expression during a murine lung infection, we used nanoString profiling of lung tissue infected with a wild-type or rim101Δ mutant strain. In this setting, we demonstrated that Rim101 controls the expression of multiple cell wall-biosynthetic genes, likely explaining the enhanced immunogenicity of the rim101Δ mutant. Despite its divergent upstream regulation, the C. neoformans Rim101 protein recognizes a conserved DNA binding motif. Using these data, we identified direct targets of this transcription factor, including genes involved in cell wall regulation. Therefore, the Rim101 protein directly controls cell wall changes required for the adaptation of C. neoformans to its host environment. Moreover, we propose that integration of the cAMP/PKA and pH-sensing pathways allows C. neoformans to respond to a broad range of host-specific signals.

Stress signaling pathways for the pathogenicity of Cryptococcus

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

Current understanding of HOG-MAPK pathway in Aspergillus fumigatus

Aspergillus fumigatus is an important opportunistic fungal pathogen that causes lethal systemic invasive aspergillosis. It must be able to adapt to stress in the microenvironment during host invasion and systemic spread. The high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) signaling pathway is a key element that controls adaptation to environmental stress. It plays a critical role in the virulence of several fungal pathogens. In this review, we summarize the current knowledge about the functions of different components of the HOG-MAPK pathway in A. fumigatus through mutant analysis or inferences from the genome annotation, focusing on their roles in adaptation to stress, regulation of infection-related morphogenesis, and effect on virulence. We also briefly compare the functions of the HOG pathway in A. fumigatus with those in the model fungi Saccharomyces cerevisiae and Aspergillus nidulans as well as several other human and plant pathogens including Candida albicans, Cryptococcus neoformans, and Magnaporthe oryzae. The genes described in this review mainly include tcsB, fos1, skn7, sho1, pbs2, and sakA whose deletion mutants have already been established in A. fumigatus. Among them, fos1 has been considered a virulence factor in A. fumigatus, indicating that components of the HOG pathway may be suitable as targets for developing new fungicides. However, quite a few of the genes of this pathway, such as sskA (ssk1), sskB, steC, and downstream regulator genes, are not well characterized. System biology approaches may contribute to a more comprehensive understanding of HOG pathway functions with dynamic details.