Hsf1 and Hsp90 orchestrate temperature-dependent global transcriptional remodelling and chromatin architecture in Candida albicans

Comparative analyses of the biological characteristics, fluconazole resistance, and heat adaptation mechanisms of Candida auris and members of the Candida haemulonii complex

Candida auris and the emerging fungal pathogens comprising the Candida haemulonii complex (C. haemulonii and C. duobushaemulonii) are phylogenetically closely related. Notably, each can cause serious nosocomial infection and acquire multidrug resistance. We isolated various strains of C. auris and C. haemulonii complex from patient specimens. The results of growth curve analysis and the spot assay showed that C. auris was the most tolerant to high temperatures, while differences were found among the five strains of the C. haemulonii complex. We selected a representative strain from each of the three types of fungi that exhibit differences in heat resistance for further research. In addition, three strains were resistant to fluconazole, whereas sensitivity to common antifungal agents differed, as determined by the micro liquid-based dilution method. Moreover, C. auris was more virulent in Galleria mellonella than members of the C. haemulonii complex. In addition, there were notable differences in biofilm formation, hydrophobicity, adhesion, and enzyme activity among the three Candida strains. The results of transcriptomics, real-time quantitative PCR, and biochemical analyses showed that C. auris was most tolerant to heat stress due to the expression of genes in regulating pyruvate consumption and the accumulation of intracellular pyruvate. These results provide valuable references for further studies of the biological characteristics, pathogenesis, and treatment of Candida infection.

IMPORTANCE

Candida auris and the Candida haemulonii complex are multidrug-resistant fungi that have emerged in recent years, posing a significant threat to human health. The biological characteristics of two strains of the Candida haemulonii complex and one strain of C. auris isolated from clinical patient samples were analyzed. Our primary focus was to compare the heat resistance between C. auris and the C. haemulonii complex, with a particular emphasis on understanding the differences in the heat resistance mechanisms. The main distinction between environmental and pathogenic fungi is that the latter can survive at human body temperature. Despite their close phylogenetic relationship, the C. haemulonii complex and C. auris exhibit significant differences in heat resistance. Studying these heat resistance mechanisms may aid in our understanding of the evolutionary process of environmental fungi transforming into pathogenic fungi.

When HSFs bring the heat-mapping the transcriptional circuitries of HSF-type regulators in Candida albicans

Heat shock factor (HSF)-type regulators are stress-responsive transcription factors widely distributed among eukaryotes, including fungi. They carry a four-stranded winged helix-turn-helix DNA-binding domain considered as the signature domain for HSFs. The genome of the opportunistic yeast Candida albicans encodes four HSF members, namely, Sfl1, Sfl2, Skn7, and the essential regulator, Hsf1. C. albicans HSFs do not only respond to heat shock and/or temperature variation but also to CO2 levels, oxidative stress, and quorum sensing, acting this way as central decision makers. In this minireview, I follow on the heels of my mSphere of Influence commentary (2020) to provide an overview of the repertoire of HSF regulators in Saccharomyces cerevisiae and C. albicans and describe how their genetic perturbation in C. albicans, coupled with genome-wide expression and location analyses, allow to map their transcriptional circuitry. I highlight how they can regulate, in common, a crucial developmental program: filamentous growth.

Extracellular Hsp90 of Candida albicans contributes to the virulence of the pathogen by activating the NF-κB signaling pathway and inducing macrophage pyroptosis

Strategies aimed at targeting fungal extracellular heat shock protein 90 (eHsp90) using vaccines and antibodies have demonstrated encouraging potential in the prevention and management of invasive fungal diseases (IFDs). However, the precise underlying mechanism by which eHsp90 contributes to the heightened virulence of Candida albicans (C. albicans) remains an enigma, awaiting further elucidation. In our current research, we have found that the 47-kDa fragment of C. albicans Hsp90 (CaHsp90), which serves as the primary antigenic determinant, is not degraded within C. albicans cells. Moreover, we have discovered that extracellular CaHsp90 (eCaHsp90) is derived from the components of lysed C. albicans cells. We also generated recombinant CaHsp90 in Escherichia coli, and found that eCaHsp90 spreads beyond the initial C. albicans colonization site, thereby enhancing the overall virulence of the organism. Our results further clarify that eCaHsp90 activates the nuclear factor kappa-B (NF-κB) signaling pathway and upregulates the expression of NACHT, LRR, and PYD domains-containing protein 3 (NLRP3). This upregulation results in the activation of Gasdermin D (GSDMD) and subsequent macrophage pyroptosis, ultimately increasing the virulence of C. albicans. This study provides valuable insights into the mechanism by which eCaHsp90 contributes to the virulence of C. albicans, offering a pharmacological basis for antifungal strategies targeting fungal eHsp90.

HDAC6 modulates the cognitive behavioral function and hippocampal tissue pathological changes of APP/PS1 transgenic mice through HSP90-HSF1 pathway

The aim of this study was to investigate histone deacetylase 6 (HDAC6) modifies the heat shock protein 90 (HSP90) and heat shock transcription factor 1 (HSF1) affect the levels of pathological markers such as Aβ oligomers (Aβo) and Tau phosphorylation (p-Tau) in APP/PS1 double transgenic mice hippocampal tissues or HT22 neurons as well as the changes in cognitive behavioral functions of mice. (1) APP/PS1 transgenic mice (6 months old, 25 ~ 30 g) were randomly assigned to 5 experimental groups, C57BL/6J mice (6 months old, 25 ~ 30 g) were used as 4 control groups, with 8 mice in each group. All mice underwent intracerebroventricular (i.c.v.) cannulation, and the experimental groups were administered with normal saline (APP + NS group), HDAC6 agonist tubastatin A hydrochloride (TSA) (APP + TSA group) or HDAC6 agonist theophylline (Theo) (APP + Theo group), HSP90 inhibitor Ganetespib (Gane) (APP + Gane group), or a combination of pre-injected Gane by TSA (APP + Gane + TSA group); the control group received i.c.v. injections of Gane (Gane group), TSA (TSA group), Theo (Theo group) or NS (NS group), respectively. (2) Mouse hippocampal neurons HT22 were randomly divided into a control group (Control) and an Aβ1-42 intervention group (Aβ). Within the Aβ group, further divisions were made for knockdown HSP90 (Aβ + siHSP90 group), overexpression HSP90 (Aβ + OE-HSP90 group), knockdown HSF1(Aβ + siHSF1 group) and knockdown HSF1 followed by overexpression HSP90 (Aβ + siHSF1 + OE-HSP90 group), resulting in a total of 6 groups. Morris water maze test was used to evaluate the cognitive behavior of the mice. Western blot and immunohistochemistry or immunofluorescence were performed to detect the levels of HDAC6, HSP90, HSF1, Aβ1-42, Tau protein, and p-Tau in the hippocampal tissue or HT22 cells. qRT-PCR was used to measure the levels of hdac6, hsp90, and hsf1 mRNA in the hippocampus or nerve cells. (1) The levels of HDAC6, Aβ1-42 and p-Tau were elevated, while HSP90 and HSF1 were decreased in the hippocampal tissue of APP/PS1 transgenic mice (all P < 0.01). Inhibiting HDAC6 upregulated the expressions of HSP90 and HSF1 in the hippocampal tissue of APP/PS1 mice, while decreasing the levels of Aβ1-42 and p-Tau as well as improving the spatial cognitive behavior in mice (P < 0.05 or P < 0.01). The opposite effects were observed upon HDAC6 activation. However, inhibiting HSP90 reduced the expression of HSF1 (P < 0.01) and increased the levels of Aβ1-42 and p-Tau (P < 0.05 or P < 0.01) but did not significantly affect the expression of HDAC6 (P > 0.05). No significant changes were observed in the aforementioned indicators in the 4 control groups (P > 0.05). (2) In the Aβ1-42 intervention group, HDAC6 and Aβ1-42, p-Tau expression levels were elevated, while HSP90 and HSF1 expressions were all decreased, and cell viability was reduced (P < 0.05 or P < 0.01). Overexpression of HSP90 upregulated HSF1 expression, decreased the levels of Aβ1-42 and p-Tau, and increased cell viability (P < 0.05 or P < 0.01). Knocking down HSP90 had the opposite effect; and knocking down HSF1 increased the levels of Aβ1-42 and p-Tau and decreased cells viability (all P < 0.01), but did not result in significant changes in the expression levels of HSP90 (P > 0.05). Inhibiting HDAC6 can upregulate the expressions of HSP90 and HSF1 but reduce the levels of Aβ1-42 and p-Tau in the hippocampus of APP/PS1 mice and improvement of cognitive behavioral function in mice; Overexpression of HSP90 can increase HSF1 but decrease Aβ1-42 and p-Tau levels in the hippocampal neurons and increase cell activity. It is suggested that HDAC6 may affect the formation of Aβ oligomers and the changes in Tau protein phosphorylation levels in the hippocampus of AD transgenic mouse as well as the alterations in cognitive behavioral functions by regulating the HSP90-HSF1 pathway.

To each its own: Mechanisms of cross-talk between GPI biosynthesis and cAMP-PKA signaling in Candida albicans versus Saccharomyces cerevisiae

Candida albicans is an opportunistic fungal pathogen that can switch between yeast and hyphal morphologies depending on the environmental cues it receives. The switch to hyphal form is crucial for the establishment of invasive infections. The hyphal form is also characterized by the cell surface expression of hyphae-specific proteins, many of which are GPI-anchored and important determinants of its virulence. The coordination between hyphal morphogenesis and the expression of GPI-anchored proteins is made possible by an interesting cross-talk between GPI biosynthesis and the cAMP-PKA signaling cascade in the fungus; a parallel interaction is not found in its human host. On the other hand, in the nonpathogenic yeast, Saccharomyces cerevisiae, GPI biosynthesis is shut down when filamentation is activated and vice versa. This too is achieved by a cross-talk between GPI biosynthesis and cAMP-PKA signaling. How are diametrically opposite effects obtained from the cross-talk between two reasonably well-conserved pathways present ubiquitously across eukarya? This Review attempts to provide a model to explain these differences. In order to do so, it first provides an overview of the two pathways for the interested reader, highlighting the similarities and differences that are observed in C. albicans versus the well-studied S. cerevisiae model, before going on to explain how the different mechanisms of regulation are effected. While commonalities enable the development of generalized theories, it is hoped that a more nuanced approach, that takes into consideration species-specific differences, will enable organism-specific understanding of these processes and contribute to the development of targeted therapies.

Ultrasound and neuroinflammation: immune modulation via the heat shock response

Current pharmacological therapeutic approaches targeting chronic inflammation exhibit transient efficacy, often with adverse effects, limiting their widespread use - especially in the context of neuroinflammation. Effective interventions require the consideration of homeostatic function, pathway dysregulation, and pleiotropic effects when evaluating therapeutic targets. Signalling molecules have multiple functions dependent on the immune context, and this complexity results in therapeutics targeting a single signalling molecule often failing in clinical translation. Additionally, the administration of non-physiologic levels of neurotrophic or anti-inflammatory factors can alter endogenous signalling, resulting in unanticipated effects. Exacerbating these challenges, the central nervous system (CNS) is isolated by the blood brain barrier (BBB), restricting the infiltration of many pharmaceutical compounds into the brain tissue. Consequently, there has been marked interest in therapeutic techniques capable of modulating the immune response in a pleiotropic manner; ultrasound remains on this frontier. While ultrasound has been used therapeutically in peripheral tissues - accelerating healing in wounds, bone fractures, and reducing inflammation - it is only recently that it has been applied to the CNS. The transcranial application of low intensity pulsed ultrasound (LIPUS) has successfully mitigated neuroinflammation in vivo, in models of neurodegenerative disease across a broad spectrum of ultrasound parameters. To date, the underlying biological effects and signalling pathways modulated by ultrasound are poorly understood, with a diverse array of reported molecules implicated. The distributed nature of the beneficial response to LIPUS implies the involvement of an, as yet, undetermined upstream signalling pathway, homologous to the protective effect of febrile range hyperthermia in chronic inflammation. As such, we review the heat shock response (HSR), a protective signalling pathway activated by thermal and mechanical stress, as the possible upstream regulator of the anti-inflammatory effects of ultrasound.

Candida auris undergoes adhesin-dependent and -independent cellular aggregation

Candida auris is a fungal pathogen of humans responsible for nosocomial infections with high mortality rates. High levels of resistance to antifungal drugs and environmental persistence mean these infections are difficult to treat and eradicate from a healthcare setting. Understanding the life cycle and the genetics of this fungus underpinning clinically relevant traits, such as antifungal resistance and virulence, is of the utmost importance to develop novel treatments and therapies. Epidemiological and genomic studies have identified five geographical clades (I-V), which display phenotypic and genomic differences. Aggregation of cells, a phenotype primarily of clade III strains, has been linked to reduced virulence in some infection models. The aggregation phenotype has thus been associated with conferring an advantage for (skin) colonisation rather than for systemic infection. However, strains with different clade affiliations were compared to infer the effects of different morphologies on virulence. This makes it difficult to distinguish morphology-dependent causes from clade-specific or even strain-specific genetic factors. Here, we identify two different types of aggregation: one induced by antifungal treatment which is a result of a cell separation defect; and a second which is controlled by growth conditions and only occurs in strains with the ability to aggregate. The latter aggregation type depends on an ALS-family adhesin which is differentially expressed during aggregation in an aggregative C. auris strain. Finally, we demonstrate that macrophages cannot clear aggregates, suggesting that aggregation might after all provide a benefit during systemic infection and could facilitate long-term persistence in the host.

The Snf5-Hsf1 transcription module synergistically regulates stress responses and pathogenicity by maintaining ROS homeostasis in Sclerotinia sclerotiorum

The SWI/SNF complex is guided to the promoters of designated genes by its co-operator to activate transcription in a timely and appropriate manner to govern development, pathogenesis, and stress responses in fungi. Nevertheless, knowledge of the complexes and their co-operator in phytopathogenic fungi is still fragmented. We demonstrate that the heat shock transcription factor SsHsf1 guides the SWI/SNF complex to promoters of heat shock protein (hsp) genes and antioxidant enzyme genes using biochemistry and pharmacology. This is accomplished through direct interaction with the complex subunit SsSnf5 under heat shock and oxidative stress. This results in the activation of their transcription and mediates histone displacement to maintain reactive oxygen species (ROS) homeostasis. Genetic results demonstrate that the transcription module formed by SsSnf5 and SsHsf1 is responsible for regulating morphogenesis, stress tolerance, and pathogenicity in Sclerotinia sclerotiorum, especially by directly activating the transcription of hsp genes and antioxidant enzyme genes counteracting plant-derived ROS. Furthermore, we show that stress-induced phosphorylation of SsSnf5 is necessary for the formation of the transcription module. This study establishes that the SWI/SNF complex and its co-operator cooperatively regulate the transcription of hsp genes and antioxidant enzyme genes to respond to host and environmental stress in the devastating phytopathogenic fungi.

Heat shock factor 1 drives regulatory T-cell induction to limit murine intestinal inflammation

The heat shock response is a critical component of the inflammatory cascade that prevents misfolding of new proteins and regulates immune responses. Activation of clusters of differentiation (CD)4+ T cells causes an upregulation of heat shock transcription factor, heat shock factor 1 (HSF1). We hypothesized that HSF1 promotes a pro-regulatory phenotype during inflammation. To validate this hypothesis, we interrogated cell-specific HSF1 knockout mice and HSF1 transgenic mice using in vitro and in vivo techniques. We determined that while HSF1 expression was induced by anti-CD3 stimulation alone, the combination of anti-CD3 and transforming growth factor β, a vital cytokine for regulatory T cell (Treg) development, resulted in increased activating phosphorylation of HSF1, leading to increased nuclear translocation and binding to heat shock response elements. Using chromatin immunoprecipitation (ChIP), we demonstrate the direct binding of HSF1 to foxp3 in isolated murine CD4+ T cells, which in turn coincided with induction of FoxP3 expression. We defined that conditional knockout of HSF1 decreased development and function of Tregs and overexpression of HSF1 led to increased expression of FoxP3 along with enhanced Treg suppressive function. Adoptive transfer of CD45RBHigh CD4 colitogenic T cells along with HSF1 transgenic CD25+ Tregs prevented intestinal inflammation when wild-type Tregs did not. Finally, overexpression of HSF1 provided enhanced barrier function and protection from murine ileitis. This study demonstrates that HSF1 promotes Treg development and function and may represent both a crucial step in the development of induced regulatory T cells and an exciting target for the treatment of inflammatory diseases with a regulatory T-cell component. SIGNIFICANCE STATEMENT: The heat shock response (HSR) is a canonical stress response triggered by a multitude of stressors, including inflammation. Evidence supports the role of the HSR in regulating inflammation, yet there is a paucity of data on its influence in T cells specifically. Gut homeostasis reflects a balance between regulatory clusters of differentiation (CD)4+ T cells and pro-inflammatory T-helper (Th)17 cells. We show that upon activation within T cells, heat shock factor 1 (HSF1) translocates to the nucleus, and stimulates Treg-specific gene expression. HSF1 deficiency hinders Treg development and function and conversely, HSF1 overexpression enhances Treg development and function. While this work, focuses on HSF1 as a novel therapeutic target for intestinal inflammation, the findings have significance for a broad range of inflammatory conditions.

The Rfg1 and Bcr1 transcription factors regulate acidic pH-induced filamentous growth in Candida albicans

IMPORTANCE

Candida albicans is a human commensal and frequent pathogen that encounters a wide range of pH stresses. The ability of C. albicans to adapt to changes in extracellular pH is crucial for its success in colonization and pathogenesis. The Rim101 pH sensing pathway is well known to govern neutral-alkaline pH responses in this pathogen. Here, we report a novel Rfg1-Bcr1 regulatory pathway that governs acidic pH responses and regulates filamentous growth in C. albicans. In addition, the Rim101-Phr1 pathway, cAMP signaling pathway, transcription factors Efg1 and Flo8, and hyphal-specific G1 cyclin Hgc1 cooperate with this regulation. Our findings provide new insights into the regulatory mechanism of acidic pH response in C. albicans.

Candida albicans and Antifungal Peptides

Candida albicans, a ubiquitous opportunistic fungal pathogen, plays a pivotal role in human health and disease. As a commensal organism, it normally resides harmlessly within the human microbiota. However, under certain conditions, C. albicans can transition into a pathogenic state, leading to various infections collectively known as candidiasis. With the increasing prevalence of immunocompromised individuals and the widespread use of invasive medical procedures, candidiasis has become a significant public health concern. The emergence of drug-resistant strains further complicates treatment options, highlighting the urgent need for alternative therapeutic strategies. Antifungal peptides (AFPs) have gained considerable attention as potential candidates for combating Candida spp. infections. These naturally occurring peptides possess broad-spectrum antimicrobial activity, including specific efficacy against C. albicans. AFPs exhibit several advantageous properties, such as rapid killing kinetics, low propensity for resistance development, and diverse mechanisms of action, making them promising alternatives to conventional antifungal agents. In recent years, extensive research has focused on discovering and developing novel AFPs with improved efficacy and selectivity against Candida species. Advances in biotechnology and synthetic peptide design have enabled the modification and optimization of natural peptides, enhancing their stability, bioavailability, and therapeutic potential. Nevertheless, several challenges must be addressed before AFPs can be widely implemented in clinical practice. These include optimizing peptide stability, enhancing delivery methods, overcoming potential toxicity concerns, and conducting comprehensive preclinical and clinical studies. This commentary presents a short overview of candidemia and AFP; articles and reviews published in the last 10 years were searched on The National Library of Medicine (National Center for Biotechnology Information-NIH-PubMed). The terms used were C. albicans infections, antimicrobial peptides, antifungal peptides, antifungal peptides mechanisms of action, candidemia treatments and guidelines, synthetic peptides and their challenges, and antimicrobial peptides in clinical trials as the main ones. Older publications were cited if they brought some relevant concept or helped to bring a perspective into our narrative. Articles older than 20 years and those that appeared in PubMed but did not match our goal to bring updated information about using antifungal peptides as an alternative to C. albicans infections were not considered.

Candida albicans exhibits heterogeneous and adaptive cytoprotective responses to antifungal compounds

Candida albicans, an opportunistic human pathogen, poses a significant threat to human health and is associated with significant socio-economic burden. Current antifungal treatments fail, at least in part, because C. albicans can initiate a strong drug tolerance response that allows some cells to grow at drug concentrations above their minimal inhibitory concentration. To better characterize this cytoprotective tolerance program at the molecular single-cell level, we used a nanoliter droplet-based transcriptomics platform to profile thousands of individual fungal cells and establish their subpopulation characteristics in the absence and presence of antifungal drugs. Profiles of untreated cells exhibit heterogeneous expression that correlates with cell cycle stage with distinct metabolic and stress responses. At 2 days post-fluconazole exposure (a time when tolerance is measurable), surviving cells bifurcate into two major subpopulations: one characterized by the upregulation of genes encoding ribosomal proteins, rRNA processing machinery, and mitochondrial cellular respiration capacity, termed the Ribo-dominant (Rd) state; and the other enriched for genes encoding stress responses and related processes, termed the Stress-dominant (Sd) state. This bifurcation persists at 3 and 6 days post-treatment. We provide evidence that the ribosome assembly stress response (RASTR) is activated in these subpopulations and may facilitate cell survival.

Characterizing the Response of Puccinia striiformis f. sp. tritici to Periods of Heat Stress that Are Common in Kansas and the Great Plains Region of North America

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici, is considered a disease of cool environments, and it has been observed that high temperatures can suppress disease development. However, recent field observations in Kansas suggest that the pathogen may be recovering from heat stress more quickly than expected. Previous research indicates that some strains of this pathogen were adapted to warm temperature regimes but did not consider how the pathogen responds to periods of heat stress that are common in the Great Plains region of North America. Therefore, the objectives of this study were to characterize the response of contemporary isolates of P. striiformis f. sp. tritici to periods of heat stress and to look for evidence of temperature adaptations within the pathogen population. These experiments evaluated nine isolates of the pathogen: eight isolates collected in Kansas between 2010 and 2021 and a historical reference isolate. Treatments compared the latent period and colonization rate of isolates given a cool temperature regime (12 to 20°C) and as they recovered from 7 days of heat stress (22 to 35°C). Results documented that contemporary isolates of the pathogen had similar latent periods and colonization rates as the historical reference under the cool temperature regime. Following exposure to 7 days of heat stress, the contemporary isolates had shorter latent periods and higher colonization rates than the historical isolate. There was also variability in how the contemporary isolates recovered from heat stress, with some isolates collected during 2019 to 2021 recovering sooner than those collected just 5 to 10 years ago.

Glucose depletion enables Candida albicans mating independently of the epigenetic white-opaque switch

The human fungal pathogen Candida albicans can switch stochastically and heritably between a "white" phase and an "opaque" phase. Opaque cells are the mating-competent form of the species, whereas white cells are thought to be essentially "sterile". Here, we report that glucose depletion, a common nutrient stress, enables C. albicans white cells to undergo efficient sexual mating. The relative expression levels of pheromone-sensing and mating-associated genes (including STE2/3, MFA1, MFα1, FIG1, FUS1, and CEK1/2) are increased under glucose depletion conditions, while expression of mating repressors TEC1 and DIG1 is decreased. Cph1 and Tec1, factors that act downstream of the pheromone MAPK pathway, play opposite roles in regulating white cell mating as TEC1 deletion or CPH1 overexpression promotes white cell mating. Moreover, inactivation of the Cph1 repressor Dig1 increases white cell mating ~4000 fold in glucose-depleted medium relative to that in the presence of glucose. Our findings reveal that the white-to-opaque epigenetic switch may not be a prerequisite for sexual mating in C. albicans in nature.

An Overlooked and Underrated Endemic Mycosis-Talaromycosis and the Pathogenic Fungus Talaromyces marneffei

Talaromycosis is an invasive mycosis endemic in tropical and subtropical Asia and is caused by the pathogenic fungus Talaromyces marneffei. Approximately 17,300 cases of T. marneffei infection are diagnosed annually, and the reported mortality rate is extremely high (~1/3). Despite the devastating impact of talaromycosis on immunocompromised individuals, particularly HIV-positive persons, and the increase in reported occurrences in HIV-uninfected persons, diagnostic and therapeutic approaches for talaromycosis have received far too little attention worldwide. In 2021, scientists living in countries where talaromycosis is endemic raised a global demand for it to be recognized as a neglected tropical disease. Therefore, T. marneffei and the infectious disease induced by this fungus must be treated with concern. T. marneffei is a thermally dimorphic saprophytic fungus with a complicated mycological growth process that may produce various cell types in its life cycle, including conidia, hyphae, and yeast, all of which are associated with its pathogenicity. However, understanding of the pathogenic mechanism of T. marneffei has been limited until recently. To achieve a holistic view of T. marneffei and talaromycosis, the current knowledge about talaromycosis and research breakthroughs regarding T. marneffei growth biology are discussed in this review, along with the interaction of the fungus with environmental stimuli and the host immune response to fungal infection. Importantly, the future research directions required for understanding this serious infection and its causative pathogenic fungus are also emphasized to identify solutions that will alleviate the suffering of susceptible individuals worldwide.

Bioinformatics Analysis, Expression Profiling, and Functional Characterization of Heat Shock Proteins in Wolfi-poria cocos

Heat shock proteins (HSPs) play critical roles in regulating different mechanisms under high-temperature conditions. HSPs have been identified and well-studied in different plants. However, there is a lack of information about their genomic organization and roles in medicinal plants and fungi, especially in Wolfi-poria cocos (W. cocos). We identified sixteen heat shock proteins (HSPs) in W. cocos and analyzed in terms of phylogenetic analysis, gene structure, motif distribution patterns, physiochemical properties, and expression comparison in different strains. Based on phylogenetic analysis, HSPs were divided into five subgroups (WcHSP100, WcHSP90, WcHSP70, WcHSP60, and WcsHSP). Subgroups WcHSP100s, WcHSP90s, WcHSP70s, WcHSP60, and WcsHSPs were further divided into 3, 2, 3, 1, and 6 subfamilies, respectively. Moreover, the expression profiling of all HSP genes in five strains of W. cocos under different temperature extremes revealed that expression of most HSPs were induced by high temperature. However, every subfamily showed different expression suggesting distinctive role in heat stress tolerance. WcHSP70-4, WcHSP90-1, and WcHSP100-1 showed the highest response to high temperature stress. Heterologous expression of WcHSP70-4, WcHSP90-1, and WcHSP100-1 genes in Escherichia coli enhanced survival rate of E. coli during heat stress. These findings suggest the role of W. cocos heat shock genes in the high temperature stress tolerance.

FungiExpresZ: an intuitive package for fungal gene expression data analysis, visualization and discovery

Bioinformatics analysis and visualization of high-throughput gene expression data require extensive computer programming skills, posing a bottleneck for many wet-lab scientists. In this work, we present an intuitive user-friendly platform for gene expression data analysis and visualization called FungiExpresZ. FungiExpresZ aims to help wet-lab scientists with little to no knowledge of computer programming to become self-reliant in bioinformatics analysis and generating publication-ready figures. The platform contains many commonly used data analysis tools and an extensive collection of pre-processed public ribonucleic acid sequencing (RNA-seq) datasets of many fungal species, including important human, plant and insect pathogens. Users may analyse their data alone or in combination with public RNA-seq data for an integrated analysis. The FungiExpresZ platform helps wet-lab scientists to overcome their limitations in genomics data analysis and can be applied to analyse data of any organism. FungiExpresZ is available as an online web-based tool (https://cparsania.shinyapps.io/FungiExpresZ/) and an offline R-Shiny package (https://github.com/cparsania/FungiExpresZ).

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

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

HSF1 and Its Role in Huntington's Disease Pathology

PURPOSE OF REVIEW

Heat shock factor 1 (HSF1) is the master transcriptional regulator of the heat shock response (HSR) in mammalian cells and is a critical element in maintaining protein homeostasis. HSF1 functions at the center of many physiological processes like embryogenesis, metabolism, immune response, aging, cancer, and neurodegeneration. However, the mechanisms that allow HSF1 to control these different biological and pathophysiological processes are not fully understood. This review focuses on Huntington's disease (HD), a neurodegenerative disease characterized by severe protein aggregation of the huntingtin (HTT) protein. The aggregation of HTT, in turn, leads to a halt in the function of HSF1. Understanding the pathways that regulate HSF1 in different contexts like HD may hold the key to understanding the pathomechanisms underlying other proteinopathies. We provide the most current information on HSF1 structure, function, and regulation, emphasizing HD, and discussing its potential as a biological target for therapy.

DATA SOURCES

We performed PubMed search to find established and recent reports in HSF1, heat shock proteins (Hsp), HD, Hsp inhibitors, HSF1 activators, and HSF1 in aging, inflammation, cancer, brain development, mitochondria, synaptic plasticity, polyglutamine (polyQ) diseases, and HD.

STUDY SELECTIONS

Research and review articles that described the mechanisms of action of HSF1 were selected based on terms used in PubMed search.

RESULTS

HSF1 plays a crucial role in the progression of HD and other protein-misfolding related neurodegenerative diseases. Different animal models of HD, as well as postmortem brains of patients with HD, reveal a connection between the levels of HSF1 and HSF1 dysfunction to mutant HTT (mHTT)-induced toxicity and protein aggregation, dysregulation of the ubiquitin-proteasome system (UPS), oxidative stress, mitochondrial dysfunction, and disruption of the structural and functional integrity of synaptic connections, which eventually leads to neuronal loss. These features are shared with other neurodegenerative diseases (NDs). Currently, several inhibitors against negative regulators of HSF1, as well as HSF1 activators, are developed and hold promise to prevent neurodegeneration in HD and other NDs.

CONCLUSION

Understanding the role of HSF1 during protein aggregation and neurodegeneration in HD may help to develop therapeutic strategies that could be effective across different NDs.

Curcumin affects function of Hsp90 and drug efflux pump of Candida albicans

Candida albicans is a pathogenic yeast that causes candidiasis in immunocompromised patients. The overuse of antifungal drugs has led to the development of resistance to such drugs by this fungus, which is a major challenge in antifungal chemotherapy. One approach to this problem involves the utilization of new natural products as an alternative source of antifungals. Curcumin, one such natural product, has been widely studied as a drug candidate and is reported to exhibit antifungal activity against C. albicans. Although studies of the mechanism of curcumin against human cancer cells have shown that it inhibits heat shock protein 90 (Hsp90), little is known about its function against C. albicans. In this paper, using a doxycycline-mediated HSP90 strain and an HSP90-overexpressing strain of C. albicans, we demonstrated that the curcumin triggered a decrease in Hsp90 by affecting it at the post-transcriptional level. This also led to the downregulation of HOG1 and CDR1, resulting in a reduction of the stress response and efflux pump activity of C. albicans. However, the inhibition of HSP90 by curcumin was not due to the inhibition of transcription factors HSF1 or AHR1. We also found that curcumin can not only decrease the transcriptional expression of CDR1, but also inhibit the efflux pump activity of Cdr1. Hence, we conclude that disruption of HSP90 by curcumin could impair cell growth, stress responses and efflux pump activity of C. albicans.

Response and regulatory mechanisms of heat resistance in pathogenic fungi

Abstract

Both the increasing environmental temperature in nature and the defensive body temperature response to pathogenic fungi during mammalian infection cause heat stress during the fungal existence, reproduction, and pathogenic infection. To adapt and respond to the changing environment, fungi initiate a series of actions through a perfect thermal response system, conservative signaling pathways, corresponding transcriptional regulatory system, corresponding physiological and biochemical processes, and phenotypic changes. However, until now, accurate response and regulatory mechanisms have remained a challenge. Additionally, at present, the latest research progress on the heat resistance mechanism of pathogenic fungi has not been summarized. In this review, recent research investigating temperature sensing, transcriptional regulation, and physiological, biochemical, and morphological responses of fungi in response to heat stress is discussed. Moreover, the specificity thermal adaptation mechanism of pathogenic fungi in vivo is highlighted. These data will provide valuable knowledge to further understand the fungal heat adaptation and response mechanism, especially in pathogenic heat-resistant fungi.

Key points

• Mechanisms of fungal perception of heat pressure are reviewed.

• The regulatory mechanism of fungal resistance to heat stress is discussed.

• The thermal adaptation mechanism of pathogenic fungi in the human body is highlighted.

Heat Shock Transcription Factor CgHSF1 Is Required for Melanin Biosynthesis, Appressorium Formation, and Pathogenicity in Colletotrichum gloeosporioides

Heat shock transcription factors (HSFs) are a family of transcription regulators. Although HSFs’ functions in controlling the transcription of the molecular chaperone heat shock proteins and resistance to stresses are well established, their effects on the pathogenicity of plant pathogenic fungi remain unknown. In this study, we analyze the role of CgHSF1 in the pathogenicity of Colletotrichum gloeosporioides and investigate the underlying mechanism. Failure to generate the Cghsf1 knock-out mutant suggested that the gene is essential for the viability of the fungus. Then, genetic depletion of the Cghsf1 was achieved by inserting the repressive promoter of nitrite reductase gene (PniiA) before its coding sequence. The mutant showed significantly decrease in the pathogenicity repression of appressorium formation, and severe defects in melanin biosynthesis. Moreover, four melanin synthetic genes were identified as direct targets of CgHSF1. Taken together, this work highlights the role of CgHSF1 in fungal pathogenicity via the transcriptional activation of melanin biosynthesis. Our study extends the understanding of fungal HSF1 proteins, especially their involvement in pathogenicity.

Transcriptional Control of Hypoxic Hyphal Growth in the Fungal Pathogen Candida albicans

The ability of Candida albicans, an important human fungal pathogen, to develop filamentous forms is a crucial determinant for host invasion and virulence. While hypoxia is one of the predominant host cues that promote C. albicans filamentous growth, the regulatory circuits that link oxygen availability to filamentation remain poorly characterized. We have undertaken a genetic screen and identified the two transcription factors Ahr1 and Tye7 as central regulators of the hypoxic filamentation. Both ahr1 and tye7 mutants exhibited a hyperfilamentous phenotype specifically under an oxygen-depleted environment suggesting that these transcription factors act as negative regulators of hypoxic filamentation. By combining microarray and ChIP-chip analyses, we have characterized the set of genes that are directly modulated by Ahr1 and Tye7. We found that both Ahr1 and Tye7 modulate a distinct set of genes and biological processes. Our genetic epistasis analysis supports our genomic finding and suggests that Ahr1 and Tye7 act independently to modulate hyphal growth in response to hypoxia. Furthermore, our genetic interaction experiments uncovered that Ahr1 and Tye7 repress the hypoxic filamentation via the Efg1 and Ras1/Cyr1 pathways, respectively. This study yielded a new and an unprecedented insight into the oxygen-sensitive regulatory circuit that control morphogenesis in a fungal pathogen.

CSU52, a novel regulator functions as a repressor of L-sorbose utilization in Candida albicans

Background and Objectives

Monosomy of chromosome 5 associated with utilization of non-canonical sugar L-sorbose is one of the well-studied aneuploidies in Candida albicans. Stress-induced ploidy changes are crucial determinants for pathogenicity and genetic diversity in C. albicans. The five scattered regulatory regions (A, B, C, 135, and 139) comprising of two functionally redundant pathways (SUR1 and SUR2) were found to be responsible for the growth on L-sorbose. So far, three genes such as CSU51, CSU53 and CSU57 have been identified in region A, region 135 and region C, respectively. In this study we have verified the role of region B in this regulatory pathway.

Materials and Methods

We employed a combinatorial gene deletion approach to verify the role of region B followed by co-over expression studies and qRT-PCR to identify the regulatory role of this region.

Results

We confirmed the role of region B in the regulation of SOU1 gene expression. The qRT-PCR results showed that regulation occurs at transcriptional level along with other two regions in SUR1 pathway. A previously uncharacterized open reading frame in region B has been implicated in this regulation and designated as CSU52. Integrating multiple copies of CSU52 in the genome at tandem, suppresses the growth of recipient strain on L-sorbose, establishing it as a repressor of SOU1 gene.

Conclusion

This finding completes the identification of regulators in SUR1 pathway. This result paves the way to study the underlying molecular mechanisms of SOU1 gene regulation that in-turn helps to understand stress induced aneuploidy.

The ATP-dependent SWI/SNF and RSC chromatin remodelers cooperatively induce unfolded protein response genes during endoplasmic reticulum stress

The SWI/SNF subfamily remodelers (SWI/SNF and RSC) generally promote gene expression by displacing or evicting nucleosomes at the promoter regions. Their action creates a nucleosome-depleted region where transcription machinery accesses the DNA. Their function has been shown critical for inducing stress-responsive transcription programs. Although the role of SWI/SNF and RSC complexes in transcription regulation of heat shock responsive genes is well studied, their involvement in other pathways such as unfolded protein response (UPR) and protein quality control (PQC) is less known. This study shows that SWI/SNF occupies the promoters of UPR, HSP and PQC genes in response to unfolded protein stress, and its recruitment at UPR promoters depends on Hac1 transcription factor and other epigenetic factors like Ada2 and Ume6. Disruption of SWI/SNF's activity does not affect the remodeling of these promoters or gene expression. However, inactivation of RSC and SWI/SNF together diminishes induction of most of the UPR, HSP and PQC genes tested. Furthermore, RSC and SWI/SNF colocalize at these promoters, suggesting that these two remodelers functionally cooperate to induce stress-responsive genes under proteotoxic conditions.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Suppression of hyphal formation and virulence of Candida albicans by natural and synthetic compounds

Candida albicans undergoes a morphological yeast-to-hyphal transition during infection, which plays a significant role in its pathogenesis. The filamentous morphology of the hyphal form has been identified as a virulence factor as it facilitates surface adherence, intertwining with biofilm, invasion, and damage to host tissues and organs. Hence, inhibition of filamentation in addition to biofilm formation is considered a viable strategy against C. albicans infections. Furthermore, a good understanding of the signaling pathways involved in response to environmental cues driving hyphal growth is also critical to an understanding of C. albicans pathogenicity and to develop novel therapies. In this review, first the clinical significance and transcriptional control of C. albicans hyphal morphogenesis are addressed. Then, various strategies employed to suppress filamentation, prevent biofilm formation, and reduce virulence are discussed. These strategies include the inhibition of C. albicans filament formation using natural or synthetic compounds, and their combination with other agents or nanoformulations.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

Anticipatory Stress Responses and Immune Evasion in Fungal Pathogens

In certain niches, microbes encounter environmental challenges that are temporally linked. In such cases, microbial fitness is enhanced by the evolution of anticipatory responses where the initial challenge simultaneously activates pre-emptive protection against the second impending challenge. The accumulation of anticipatory responses in domesticated yeasts, which have been termed 'adaptive prediction', has led to the emergence of 'core stress responses' that provide stress cross-protection. Protective anticipatory responses also seem to be common in fungal pathogens of humans. These responses reflect the selective pressures that these fungi have faced relatively recently in their evolutionary history. Consequently, some pathogens have evolved 'core environmental responses' which exploit host signals to trigger immune evasion strategies that protect them against imminent immune attack.

Transcriptional control of hyphal morphogenesis in Candida albicans

Candida albicans is a multimorphic commensal organism and opportunistic fungal pathogen in humans. A morphological switch between unicellular budding yeast and multicellular filamentous hyphal growth forms plays a vital role in the virulence of C. albicans, and this transition is regulated in response to a range of environmental cues that are encountered in distinct host niches. Many unique transcription factors contribute to the transcriptional regulatory network that integrates these distinct environmental cues and determines which phenotypic state will be expressed. These hyphal morphogenesis regulators have been extensively investigated, and represent an increasingly important focus of study, due to their central role in controlling a key C. albicans virulence attribute. This review provides a succinct summary of the transcriptional regulatory factors and environmental signals that control hyphal morphogenesis in C. albicans.

The Heat Shock Transcription Factor HsfA Is Essential for Thermotolerance and Regulates Cell Wall Integrity in Aspergillus fumigatus

The deleterious effects of human-induced climate change have long been predicted. However, the imminent emergence and spread of new diseases, including fungal infections through the rise of thermotolerant strains, is still neglected, despite being a potential consequence of global warming. Thermotolerance is a remarkable virulence attribute of the mold Aspergillus fumigatus. Under high-temperature stress, opportunistic fungal pathogens deploy an adaptive mechanism known as heat shock (HS) response controlled by heat shock transcription factors (HSFs). In eukaryotes, HSFs regulate the expression of several heat shock proteins (HSPs), such as the chaperone Hsp90, which is part of the cellular program for heat adaptation and a direct target of HSFs. We recently observed that the perturbation in cell wall integrity (CWI) causes concomitant susceptibility to elevated temperatures in A. fumigatus, although the mechanisms underpinning the HS response and CWI cross talking are not elucidated. Here, we aim at further deciphering the interplay between HS and CWI. Our results show that cell wall ultrastructure is severely modified when A. fumigatus is exposed to HS. We identify the transcription factor HsfA as essential for A. fumigatus viability, thermotolerance, and CWI. Indeed, HS and cell wall stress trigger the coordinated expression of both hsfA and hsp90. Furthermore, the CWI signaling pathway components PkcA and MpkA were shown to be important for HsfA and Hsp90 expression in the A. fumigatus biofilms. Lastly, RNA-sequencing confirmed that hsfA regulates the expression of genes related to the HS response, cell wall biosynthesis and remodeling, and lipid homeostasis. Our studies collectively demonstrate the connection between the HS and the CWI pathway, with HsfA playing a crucial role in this cross-pathway regulation, reinforcing the importance of the cell wall in A. fumigatus thermophily.

Functional connections between cell cycle and proteostasis in the regulation of Candida albicans morphogenesis

Morphological plasticity is a key virulence trait for many fungal pathogens. For the opportunistic fungal pathogen Candida albicans, transitions among yeast, pseudohyphal, and hyphal forms are critical for virulence, because the morphotypes play distinct roles in the infection process. C. albicans morphogenesis is induced in response to many host-relevant conditions and is regulated by complex signaling pathways and cellular processes. Perturbation of either cell-cycle progression or protein homeostasis induces C. albicans filamentation, demonstrating that these processes play a key role in morphogenetic control. Regulators such as cyclin-dependent kinases, checkpoint proteins, the proteasome, the heat shock protein Hsp90, and the heat shock transcription factor Hsf1 all influence morphogenesis, often through interconnected effects on the cell cycle and proteostasis. This review highlights the major cell-cycle and proteostasis regulators that modulate morphogenesis and discusses how these two processes intersect to regulate this key virulence trait.

Systematic Analysis of Functionally Related Gene Clusters in the Opportunistic Pathogen, Candida albicans

The proper balance of gene expression is essential for cellular health, organismal development, and maintaining homeostasis. In response to complex internal and external signals, the cell needs to modulate gene expression to maintain proteostasis and establish cellular identity within its niche. On a genome level, single-celled prokaryotic microbes display clustering of co-expressed genes that are regulated as a polycistronic RNA. This phenomenon is largely absent from eukaryotic microbes, although there is extensive clustering of co-expressed genes as functional pairs spread throughout the genome in Saccharomyces cerevisiae. While initial analysis demonstrated conservation of clustering in divergent fungal lineages, a comprehensive analysis has yet to be performed. Here we report on the prevalence, conservation, and significance of the functional clustering of co-regulated genes within the opportunistic human pathogen, Candida albicans. Our analysis reveals that there is extensive clustering within this organism-although the identity of the gene pairs is unique compared with those found in S. cerevisiae-indicating that this genomic arrangement evolved after these microbes diverged evolutionarily, rather than being the result of an ancestral arrangement. We report a clustered arrangement in gene families that participate in diverse molecular functions and are not the result of a divergent orientation with a shared promoter. This arrangement coordinates the transcription of the clustered genes to their neighboring genes, with the clusters congregating to genomic loci that are conducive to transcriptional regulation at a distance.

High-Resolution Genome-Wide Occupancy in Candida spp. Using ChEC-seq

To persist in their dynamic human host environments, fungal pathogens must sense and adapt by modulating their gene expression to fulfill their cellular needs. Understanding transcriptional regulation on a global scale would uncover cellular processes linked to persistence and virulence mechanisms that could be targeted for antifungal therapeutics. Infections associated with the yeast Candida albicans, a highly prevalent fungal pathogen, and the multiresistant related species Candida auris are becoming a serious public health threat. To define the set of a gene regulated by a transcriptional regulator in C. albicans, chromatin immunoprecipitation (ChIP)-based techniques, including ChIP with microarray technology (ChIP-chip) or ChIP-DNA sequencing (ChIP-seq), have been widely used. Here, we describe a new set of PCR-based micrococcal nuclease (MNase)-tagging plasmids for C. albicans and other Candida spp. to determine the genome-wide location of any transcriptional regulator of interest using chromatin endogenous cleavage (ChEC) coupled to high-throughput sequencing (ChEC-seq). The ChEC procedure does not require protein-DNA cross-linking or sonication, thus avoiding artifacts related to epitope masking or the hyper-ChIPable euchromatic phenomenon. In a proof-of-concept application of ChEC-seq, we provided a high-resolution binding map of the SWI/SNF chromatin remodeling complex, a master regulator of fungal fitness in C. albicans, in addition to the transcription factor Nsi1 that is an ortholog of the DNA-binding protein Reb1 for which genome-wide occupancy was previously established in Saccharomyces cerevisiae The ChEC-seq procedure described here will allow a high-resolution genomic location definition which will enable a better understanding of transcriptional regulatory circuits that govern fungal fitness and drug resistance in these medically important fungi.IMPORTANCE Systemic fungal infections caused by Candida albicans and the "superbug" Candida auris are becoming a serious public health threat. The ability of these yeasts to cause disease is linked to their faculty to modulate the expression of genes that mediate their escape from the immune surveillance and their persistence in the different unfavorable niches within the host. Comprehensive knowledge on gene expression control of fungal fitness is consequently an interesting framework for the identification of essential infection processes that could be hindered by chemicals as potential therapeutics. Here, we expanded the use of ChEC-seq, a technique that was initially developed in the yeast model Saccharomyces cerevisiae to identify genes that are modulated by a transcriptional regulator, in pathogenic yeasts from the genus Candida This robust technique will allow a better characterization of key gene expression regulators and their contribution to virulence and antifungal resistance in these pathogenic yeasts.

A Transcriptional Regulatory Map of Iron Homeostasis Reveals a New Control Circuit for Capsule Formation in Cryptococcus neoformans

Iron is essential for the growth of the human fungal pathogen Cryptococcus neoformans within the vertebrate host, and iron sensing contributes to the elaboration of key virulence factors, including the formation of the polysaccharide capsule. C. neoformans employs sophisticated iron acquisition and utilization systems governed by the transcription factors Cir1 and HapX. However, the details of the transcriptional regulatory networks that are governed by these transcription factors and connections to virulence remain to be defined. Here, we used chromatin immunoprecipitation followed by next-generation sequencing (ChIP-seq) and transcriptome analysis (RNA-seq) to identify genes directly regulated by Cir1 and/or HapX in response to iron availability. Overall, 40 and 100 genes were directly regulated by Cir1, and 171 and 12 genes were directly regulated by HapX, under iron-limited and replete conditions, respectively. More specifically, we found that Cir1 directly controls the expression of genes required for iron acquisition and metabolism, and indirectly governs capsule formation by regulating specific protein kinases, a regulatory connection not previously revealed. HapX regulates the genes responsible for iron-dependent pathways, particularly under iron-depleted conditions. By analyzing target genes directly bound by Cir1 and HapX, we predicted the binding motifs for the transcription factors and verified that the purified proteins bind these motifs in vitro Furthermore, several direct target genes were coordinately and reciprocally regulated by Cir1 and HapX, suggesting that these transcription factors play conserved roles in the response to iron availability. In addition, biochemical analyses revealed that Cir1 and HapX are iron-containing proteins, implying that the regulatory networks of Cir1 and HapX may be influenced by the incorporation of iron into these proteins. Taken together, our identification of the genome-wide transcriptional networks provides a detailed understanding of the iron-related regulatory landscape, establishes a new connection between Cir1 and kinases that regulate capsule, and underpins genetic and biochemical analyses that reveal iron-sensing mechanisms for Cir1 and HapX in C. neoformans.

Adapting to survive: How Candida overcomes host-imposed constraints during human colonization

Successful human colonizers such as Candida pathogens have evolved distinct strategies to survive and proliferate within the human host. These include sophisticated mechanisms to evade immune surveillance and adapt to constantly changing host microenvironments where nutrient limitation, pH fluctuations, oxygen deprivation, changes in temperature, or exposure to oxidative, nitrosative, and cationic stresses may occur. Here, we review the current knowledge and recent findings highlighting the remarkable ability of medically important Candida species to overcome a broad range of host-imposed constraints and how this directly affects their physiology and pathogenicity. We also consider the impact of these adaptation mechanisms on immune recognition, biofilm formation, and antifungal drug resistance, as these pathogens often exploit specific host constraints to establish a successful infection. Recent studies of adaptive responses to physiological niches have improved our understanding of the mechanisms established by fungal pathogens to evade the immune system and colonize the host, which may facilitate the design of innovative diagnostic tests and therapeutic approaches for Candida infections.

The Proteasome Governs Fungal Morphogenesis via Functional Connections with Hsp90 and cAMP-Protein Kinase A Signaling

Protein homeostasis is critical for proliferation and viability of all organisms. For Candida albicans, protein homeostasis also modulates the transition between yeast and filamentous forms, which is critical for virulence. A key regulator of morphogenesis is the molecular chaperone Hsp90, which mediates proteostasis under physiological and stress conditions. Hsp90 regulates morphogenesis by repressing cyclic AMP-protein kinase A (cAMP-PKA) signaling, such that inhibition of Hsp90 causes filamentation in the absence of an inducing cue. We explored the effect of perturbation of another facet of protein homeostasis and discovered that morphogenesis is also regulated by the proteasome, a large 33-subunit protein complex consisting of a 20S catalytic core and two 19S regulatory particles, which controls degradation of intracellular proteins. We identified a conserved role of the proteasome in morphogenesis as pharmacological inhibition of the proteasome induced filamentation of C. albicans and the related species Candida dubliniensis, Candida tropicalis, Candida krusei, and Candida parapsilosis For C. albicans, genetic depletion of any of 29 subunits of the 19S or 20S particle induced filamentation. Filaments induced by inhibition of either the proteasome or Hsp90 have shared structural characteristics, such as aberrant nuclear content, and shared genetic dependencies, such as intact cAMP-PKA signaling. Consistent with a functional connection between these facets of protein homeostasis that modulate morphogenesis, we observed that proteasome inhibition results in an accumulation of ubiquitinated proteins that overwhelm Hsp90 function, relieving Hsp90-mediated repression of morphogenesis. Together, our findings provide a mechanism whereby interconnected facets of proteostasis regulate C. albicans morphogenesis.IMPORTANCE Fungi cause life-threatening infections and pose a serious threat to human health as there are very few effective antifungal drugs. Candida albicans is a major human fungal pathogen and cause of morbidity and mortality in immunocompromised individuals. A key trait that enables C. albicans virulence is its ability to transition between yeast and filamentous forms. Understanding the mechanisms regulating this virulence trait can facilitate the development of much-needed, novel therapeutic strategies. A key regulator of morphogenesis is the molecular chaperone Hsp90, which is crucial for proteostasis. Here, we expanded our understanding of how proteostasis regulates fungal morphogenesis and identified the proteasome as a repressor of filamentation in C. albicans and related species. Our work suggests that proteasome inhibition overwhelms Hsp90 function, thereby inducing morphogenesis. This work provides a foundation for understanding the role of the proteasome in fungal virulence and offers potential for targeting the proteasome to disarm fungal pathogens.

Nine Things Genomics Can Tell Us About Candida auris

Candida auris is a recently emerged multidrug-resistant fungal pathogen causing severe illness in hospitalized patients. C. auris is most closely related to a few environmental or rarely observed but cosmopolitan Candida species. However, C. auris is unique in the concern it is generating among public health agencies for its rapid emergence, difficulty to treat, and the likelihood for further and more extensive outbreaks and spread. To date, five geographically distributed and genetically divergent lineages have been identified, none of which includes isolates that were collected prior to 1996. Indeed, C. auris' ecological niche(s) and emergence remain enigmatic, although a number of hypotheses have been proposed. Recent genomic and transcriptomic work has also identified a variety of gene and chromosomal features that may have conferred C. auris with several important clinical phenotypes including its drug-resistance and growth at high temperatures. In this review we discuss nine major lines of enquiry into C. auris that big-data technologies and analytical approaches are beginning to answer.

Rapid deacetylation of yeast Hsp70 mediates the cellular response to heat stress

Hsp70 is a highly conserved molecular chaperone critical for the folding of new and denatured proteins. While traditional models state that cells respond to stress by upregulating inducible HSPs, this response is relatively slow and is limited by transcriptional and translational machinery. Recent studies have identified a number of post-translational modifications (PTMs) on Hsp70 that act to fine-tune its function. We utilized mass spectrometry to determine whether yeast Hsp70 (Ssa1) is differentially modified upon heat shock. We uncovered four lysine residues on Ssa1, K86, K185, K354 and K562 that are deacetylated in response to heat shock. Mutation of these sites cause a substantial remodeling of the Hsp70 interaction network of co-chaperone partners and client proteins while preserving essential chaperone function. Acetylation/deacetylation at these residues alter expression of other heat-shock induced chaperones as well as directly influencing Hsf1 activity. Taken together our data suggest that cells may have the ability to respond to heat stress quickly though Hsp70 deacetylation, followed by a slower, more traditional transcriptional response.

Regulation of the Hsf1-dependent transcriptome via conserved bipartite contacts with Hsp70 promotes survival in yeast

Protein homeostasis and cellular fitness in the presence of proteotoxic stress is promoted by heat shock factor 1 (Hsf1), which controls basal and stress-induced expression of molecular chaperones and other targets. The major heat shock proteins and molecular chaperones Hsp70 and Hsp90, in turn, participate in a negative feedback loop that ensures appropriate coordination of the heat shock response with environmental conditions. Features of this regulatory circuit in the budding yeast Saccharomyces cerevisiae have been recently defined, most notably regarding direct interaction between Hsf1 and the constitutively expressed Hsp70 protein Ssa1. Here, we sought to further examine the Ssa1/Hsf1 regulation. We found that Ssa1 interacts independently with both the previously defined CE2 site in the Hsf1 C-terminal transcriptional activation domain and with an additional site that we identified within the N-terminal activation domain. Consistent with both sites bearing a recognition signature for Hsp70, we demonstrate that Ssa1 contacts Hsf1 via its substrate-binding domain and that abolishing either regulatory site results in loss of Ssa1 interaction. Removing Hsp70 regulation of Hsf1 globally dysregulated Hsf1 transcriptional activity, with synergistic effects on both gene expression and cellular fitness when both sites are disrupted together. Finally, we report that Hsp70 interacts with both transcriptional activation domains of Hsf1 in the related yeast Lachancea kluyveri Our findings indicate that Hsf1 transcriptional activity is tightly regulated to ensure cellular fitness and that a general and conserved Hsp70-HSF1 feedback loop regulates cellular proteostasis in yeast.

Regulatory mechanisms controlling morphology and pathogenesis in Candida albicans

Candida albicans, a major human fungal pathogen, can cause a wide variety of both mucosal and systemic infections, particularly in immunocompromised individuals. Multiple lines of evidence suggest a strong association between virulence and the ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous cells in response to host environmental cues. Most previous studies on mechanisms important for controlling the C. albicans morphological transition have focused on signaling pathways and sequence-specific transcription factors. However, in recent years a variety of novel mechanisms have been reported, including those involving global transcriptional regulation and translational control. A large-scale functional genomics screen has also revealed new roles in filamentation for certain key biosynthesis pathways. This review article will highlight several of these exciting recent discoveries and discuss how they are relevant to the development of novel antifungal strategies. Ultimately, components of mechanisms that control C. albicans morphogenesis and pathogenicity could potentially serve as viable antifungal targets.

Regulation of the heat shock transcription factor Hsf1 in fungi: implications for temperature-dependent virulence traits

The impact of fungal pathogens on human health is devastating. For fungi and other pathogens, a key determinant of virulence is the capacity to thrive at host temperatures, with elevated temperature in the form of fever as a ubiquitous host response to defend against infection. A prominent feature of cells experiencing heat stress is the increased expression of heat shock proteins (Hsps) that play pivotal roles in the refolding of misfolded proteins in order to restore cellular homeostasis. Transcriptional activation of this heat shock response is orchestrated by the essential heat shock transcription factor, Hsf1. Although the influence of Hsf1 on cellular stress responses has been studied for decades, many aspects of its regulation and function remain largely enigmatic. In this review, we highlight our current understanding of how Hsf1 is regulated and activated in the model yeast Saccharomyces cerevisiae, and highlight exciting recent discoveries related to its diverse functions under both basal and stress conditions. Given that thermal adaption is a fundamental requirement for growth and virulence in fungal pathogens, we also compare and contrast Hsf1 activation and function in other fungal species with an emphasis on its role as a critical regulator of virulence traits.

Role of heat shock proteins 70/90 in exercise physiology and exercise immunology and their diagnostic potential in sports

Heat shock proteins (HSPs) are molecular chaperones facilitating the unfolding or folding of secondary structures of proteins, their client proteins, in cellular stress situations. Various internal and external physiological and mechanical stress factors induce a homeostatic imbalance, followed by an increased expression of HSP70 and HSP90. Exercise is a stress factor, too, and its cumulative physiological perturbation manifests at a cellular level by threatening the protein homeostasis of various cell types. Consequently, an increase of HSP70/90 was described in plasma and mononuclear cells and various organs and tissues, such as muscle, liver, cardiac tissue, and brain, after an acute bout of exercise. The specific response of HSP70/90 seems to be strongly related to the modality of exercise, with several dependent factors such as duration, intensity, exercise type, subjects’ training status, and environmental factors, e.g., temperature. It is suggested that HSP70/90 play a major role in immune regulation and cell protection during exercise and in the efficiency of regeneration and reparation processes. During long-term training, HSP70/90 are involved in preconditioning and adaptation processes that might also be important for disease prevention and therapy. With regard to their highly sensitive and individual response to specific exercise and training modalities, this review discusses whether and how HSP70 and HSP90 can be applied as biomarkers for monitoring exercise and training.

Environment-induced same-sex mating in the yeast Candida albicans through the Hsf1-Hsp90 pathway

While sexual reproduction is pervasive in eukaryotic cells, the strategies employed by fungal species to achieve and complete sexual cycles is highly diverse and complex. Many fungi, including Saccharomyces cerevisiae and Schizosaccharomyces pombe, are homothallic (able to mate with their own mitotic descendants) because of homothallic switching (HO) endonuclease-mediated mating-type switching. Under laboratory conditions, the human fungal pathogen Candida albicans can undergo both heterothallic and homothallic (opposite- and same-sex) mating. However, both mating modes require the presence of cells with two opposite mating types (MTLa/a and α/α) in close proximity. Given the predominant clonal feature of this yeast in the human host, both opposite- and same-sex mating would be rare in nature. In this study, we report that glucose starvation and oxidative stress, common environmental stresses encountered by the pathogen, induce the development of mating projections and efficiently permit same-sex mating in C. albicans with an "a" mating type (MTLa/a). This induction bypasses the requirement for the presence of cells with an opposite mating type and allows efficient sexual mating between cells derived from a single progenitor. Glucose starvation causes an increase in intracellular oxidative species, overwhelming the Heat Shock transcription Factor 1 (Hsf1)- and Heat shock protein (Hsp)90-mediated stress-response pathway. We further demonstrate that Candida TransActivating protein 4 (Cta4) and Cell Wall Transcription factor 1 (Cwt1), downstream effectors of the Hsf1-Hsp90 pathway, regulate same-sex mating in C. albicans through the transcriptional control of the master regulator of a-type mating, MTLa2, and the pheromone precursor-encoding gene Mating α factor precursor (MFα). Our results suggest that mating could occur much more frequently in nature than was originally appreciated and that same-sex mating could be an important mode of sexual reproduction in C. albicans.

Genetic Analysis of Candida auris Implicates Hsp90 in Morphogenesis and Azole Tolerance and Cdr1 in Azole Resistance

Candida auris is an emerging fungal pathogen and a serious global health threat as the majority of clinical isolates display elevated resistance to currently available antifungal drugs. Despite the increased prevalence of C. auris infections, the mechanisms governing drug resistance remain largely elusive. In diverse fungi, the evolution of drug resistance is enabled by the essential molecular chaperone Hsp90, which stabilizes key regulators of cellular responses to drug-induced stress. Hsp90 also orchestrates temperature-dependent morphogenesis in Candida albicans, a key virulence trait. However, the role of Hsp90 in the pathobiology of C. auris remains unknown. In order to study regulatory functions of Hsp90 in C. auris, we placed HSP90 under the control of a doxycycline-repressible promoter to enable transcriptional repression. We found that Hsp90 is essential for growth in C. auris and that it enables tolerance of clinical isolates with respect to the azoles, which inhibit biosynthesis of the membrane sterol ergosterol. High-level azole resistance was independent of Hsp90 but dependent on the ABC transporter CDR1, deletion of which resulted in abrogated resistance. Strikingly, we discovered that C. auris undergoes a morphogenetic transition from yeast to filamentous growth in response to HSP90 depletion or cell cycle arrest but not in response to other cues that induce C. albicans filamentation. Finally, we observed that this developmental transition is associated with global transcriptional changes, including the induction of cell wall-related genes. Overall, this report provides a novel insight into mechanisms of drug tolerance and resistance in C. auris and describes a developmental transition in response to perturbation of a core regulator of protein homeostasis.IMPORTANCE Fungal pathogens pose a serious threat to public health. Candida auris is an emerging fungal pathogen that is often resistant to commonly used antifungal drugs. However, the mechanisms governing drug resistance and virulence in this organism remain largely unexplored. In this study, we adapted a conditional expression system to modulate the transcription of an essential gene, HSP90, which regulates antifungal resistance and virulence in diverse fungal pathogens. We showed that Hsp90 is essential for growth in C. auris and is important for tolerance of the clinically important azole antifungals, which block ergosterol biosynthesis. Further, we established that the Cdr1 efflux transporter regulates azole resistance. Finally, we discovered that C. auris transitions from yeast to filamentous growth in response to Hsp90 inhibition, accompanied by global transcriptional remodeling. Overall, this work provides a novel insight into mechanisms regulating azole resistance in C. auris and uncovers a distinct developmental program regulated by Hsp90.

Genetic and epigenetic determinants establish a continuum of Hsf1 occupancy and activity across the yeast genome

Heat shock factor 1 is the master transcriptional regulator of molecular chaperones and binds to the same cis-acting heat shock element (HSE) across the eukaryotic lineage. In budding yeast, Hsf1 drives the transcription of ∼20 genes essential to maintain proteostasis under basal conditions, yet its specific targets and extent of inducible binding during heat shock remain unclear. Here we combine Hsf1 chromatin immunoprecipitation sequencing (seq), nascent RNA-seq, and Hsf1 nuclear depletion to quantify Hsf1 binding and transcription across the yeast genome. We find that Hsf1 binds 74 loci during acute heat shock, and these are linked to 46 genes with strong Hsf1-dependent expression. Notably, Hsf1's induced DNA binding leads to a disproportionate (∼7.5-fold) increase in nascent transcription. Promoters with high basal Hsf1 occupancy have nucleosome-depleted regions due to the presence of "pioneer factors." These accessible sites are likely critical for Hsf1 occupancy as the activator is incapable of binding HSEs within a stably positioned, reconstituted nucleosome. In response to heat shock, however, Hsf1 accesses nucleosomal sites and promotes chromatin disassembly in concert with the Remodels Structure of Chromatin (RSC) complex. Our data suggest that the interplay between nucleosome positioning, HSE strength, and active Hsf1 levels allows cells to precisely tune expression of the proteostasis network.