Tec1 and Ste12 transcription factors play a role in adaptation to low pH stress and biofilm formation in the human opportunistic fungal pathogen Candida glabrata

In vivo RNA sequencing reveals a crucial role of Fus3-Kss1 MAPK pathway in Candida glabrata pathogenicity

Candida glabrata is an important and increasingly common pathogen of humans, particularly in immunocompromised hosts. Despite this, little is known about how this fungus causes disease. Here, we applied RNA sequencing and an in vivo invasive infection model to identify the attributes that allow this organism to infect hosts. Fungal transcriptomes show a dramatic increase in the expression of Fus3 and Kss1, two mitogen-activated protein kinases (MAPKs), during invasive infection. We further demonstrate that they are both highly induced under a combination of serum and high CO2 conditions. Deletion of both FUS3 and KSS1, but neither gene alone, results in a reduced fungal burden in organs, as well as in the gastrointestinal tract in the DSS (Dextran Sulfate Sodium)-induced colitis model. Similarly, the defect in persistence in macrophages and attenuated adhesion to epithelial cells are observed when FUS3 and KSS1 are both disrupted. The fus3 kss1 double mutant also displays defects in the induction of virulence attributes such as genes required for iron acquisition and adhesion and in the anti-fungal drug tolerance. The putative downstream transcription factors Ste12 (1), Ste12 (2), Tec1, and Tec2 are found to be involved in the regulation of these virulence attributes. Collectively, our study indicates that an evolutionary conserved MAPK pathway, which regulates mating and filamentous growth in Saccharomyces cerevisiae, is critical for C. glabrata pathogenicity.

IMPORTANCE

The MAPK signaling pathway, mediated by closely related kinases Fus3 and Kss1, is crucial for controlling mating and filamentous growth in Saccharomyces cerevisiae, but this pathway does not significantly impact hyphal development and pathogenicity in Candida albicans, a commensal-pathogenic fungus of humans. Furthermore, deletion of Cpk1, the ortholog of Fus3 in pathogenic fungus Cryptococcus neoformans, has no effect on virulence. Here, we demonstrate that the MAPK pathway is crucial for the pathogenicity of Candida glabrata, a fungus that causes approximately one-third of cases of hematogenously disseminated candidiasis in the United States. This pathway regulates multiple virulence attributes including the induction of iron acquisition genes and adhesins, as well as persistence in macrophages and organs. Our work provides insights into C. glabrata pathogenesis and highlights an example in which regulatory rewiring of a conserved pathway confers a virulent phenotype in a pathogen.

Candida albicans and Candida glabrata: global priority pathogens

SUMMARYA significant increase in the incidence of Candida-mediated infections has been observed in the last decade, mainly due to rising numbers of susceptible individuals. Recently, the World Health Organization published its first fungal pathogen priority list, with Candida species listed in medium, high, and critical priority categories. This review is a synthesis of information and recent advances in our understanding of two of these species-Candida albicans and Candida glabrata. Of these, C. albicans is the most common cause of candidemia around the world and is categorized as a critical priority pathogen. C. glabrata is considered a high-priority pathogen and has become an increasingly important cause of candidemia in recent years. It is now the second most common causative agent of candidemia in many geographical regions. Despite their differences and phylogenetic divergence, they are successful as pathogens and commensals of humans. Both species can cause a broad variety of infections, ranging from superficial to potentially lethal systemic infections. While they share similarities in certain infection strategies, including tissue adhesion and invasion, they differ significantly in key aspects of their biology, interaction with immune cells, host damage strategies, and metabolic adaptations. Here we provide insights on key aspects of their biology, epidemiology, commensal and pathogenic lifestyles, interactions with the immune system, and antifungal resistance.

Modulators of MAPK pathway activity during filamentous growth in Saccharomyces cerevisiae

Mitogen-activated protein kinase (MAPK) pathways control the response to intrinsic and extrinsic stimuli. In the budding yeast Saccharomyces cerevisiae, cells undergo filamentous growth, which is regulated by the fMAPK pathway. To better understand the regulation of the fMAPK pathway, a genetic screen was performed to identify spontaneous mutants with elevated activity of an fMAPK pathway-dependent growth reporter (ste4  FUS1-HIS3). In total, 159 mutants were isolated and analyzed by secondary screens for invasive growth by the plate-washing assay and filament formation by microscopy. Thirty-two mutants were selected for whole-genome sequencing, which identified new alleles in genes encoding known regulators of the fMAPK pathway. These included gain-of-function alleles in STE11, which encodes the MAPKKK, as well as loss-of-function alleles in KSS1, which encodes the MAP kinase, and loss-of-function alleles in RGA1, which encodes a GTPase-activating protein (GAP) for CDC42. New alleles in previously identified pathway modulators were also uncovered in ALY1, AIM44, RCK2, IRA2, REG1, and in genes that regulate protein folding (KAR2), glycosylation (MNN4), and turnover (BLM10). Mutations leading to C-terminal truncations in the transcription factor Ste12p were also uncovered that resulted in elevated reporter activity, identifying an inhibitory domain of the protein from residues 491 to 688. We also find that a diversity of filamentous growth phenotypes can result from combinatorial effects of multiple mutations and by loss of different regulators of the response. The alleles identified here expand the connections surrounding MAPK pathway regulation and reveal new features of proteins that function in the signaling cascade.

Modulators of MAPK pathway activity during filamentous growth in Saccharomyces cerevisiae

Mitogen-activated protein kinase (MAPK) pathways control the response to intrinsic and extrinsic stimuli. In the budding yeast Saccharomyces cerevisiae, cells undergo filamentous growth, which is regulated by the fMAPK pathway. To better understand the regulation of the fMAPK pathway, a genetic screen was performed to identify spontaneous mutants with elevated activity of an fMAPK-pathway dependent growth reporter (ste4 FUS1-HIS3). In total, 159 mutants were isolated and analyzed by secondary screens for invasive growth by the plate-washing assay, and filament formation by microscopy. Thirty-two mutants were selected for whole-genome sequencing, which identified new alleles in genes encoding known regulators of the fMAPK pathway. These included gain-of-function alleles in STE11, which encodes the MAPKKK, as well as loss-of-function alleles in KSS1, which encodes the MAP kinase, and RGA1, which encodes a GTPase activating protein (GAP) for CDC42. New alleles in previously identified pathway modulators were also uncovered in ALY1, AIM44, RCK2, IRA2, REG1 and in genes that regulate protein folding (KAR2), glycosylation (MNN4), and turnover (BLM10). C-terminal truncations in the transcription factor Ste12p were also uncovered that resulted in elevated reporter activity, presumably identifying an inhibitory domain in the C-terminus of the protein. We also show that a wide variety of filamentous growth phenotypes result from mutations in different regulators of the response. The alleles identified here expand the connections surrounding MAPK pathway regulation and reveal new features of proteins that function in the signaling cascade.

Transcription Factors Tec1 and Tec2 Play Key Roles in the Hyphal Growth and Virulence of Mucor lusitanicus Through Increased Mitochondrial Oxidative Metabolism

Mucormycosis is a lethal and difficult-to-treat fungal infection caused by fungi of the order Mucorales. Mucor lusitanicus, a member of Mucorales, is commonly used as a model to understand disease pathogenesis. However, transcriptional control of hyphal growth and virulence in Mucorales is poorly understood. This study aimed to investigate the role of Tec proteins, which belong to the TEA/ATTS transcription factor family, in the hyphal development and virulence of M. lusitanicus. Unlike in the genome of Ascomycetes and Basidiomycetes, which have a single Tec homologue, in the genome of Mucorales, two Tec homologues, Tec1 and Tec2, were found, except in that of Phycomyces blakesleeanus, with only one Tec homologue. tec1 and tec2 overexpression in M. lusitanicus increased mycelial growth, mitochondrial content and activity, expression of the rhizoferrin synthetase-encoding gene rfs, and virulence in nematodes and wax moth larvae but decreased cAMP levels and protein kinase A (PKA) activity. Furthermore, tec1- and tec2-overexpressing strains required adequate mitochondrial metabolism to promote the virulent phenotype. The heterotrimeric G beta subunit 1-encoding gene deletant strain (Δgpb1) increased cAMP-PKA activity, downregulation of both tec genes, decreased both virulence and hyphal development, but tec1 and tec2 overexpression restored these defects. Overexpression of allele-mutated variants of Tec1(S332A) and Tec2(S168A) in the putative phosphorylation sites for PKA increased both virulence and hyphal growth of Δgpb1. These findings suggest that Tec homologues promote mycelial development and virulence by enhancing mitochondrial metabolism and rhizoferrin accumulation, providing new information for the rational control of the virulent phenotype of M. lusitanicus.

Pre-colonization with the fungus Candida glabrata exacerbates infection by the bacterial pathogen Clostridioides difficile in a murine model

The contributions of commensal fungi to human health and disease are not well understood. Candida species such as C. albicans and C. glabrata are opportunistic pathogenic fungi and common colonizers of the human intestinal tract. They have been shown to affect the host immune system and interact with the gut microbiome and pathogenic microorganisms. Therefore, Candida species could be expected to play important ecological roles in the host gastrointestinal tract. Previously, our group demonstrated that pre-colonization of mice with C. albicans protected them against lethal C. difficile infection (CDI). Here, we show that mice pre-colonized with C. glabrata succumbed to CDI more rapidly than mice that were not pre-colonized suggesting an enhancement in C. difficile pathogenesis. Further, when C. difficile was added to pre-formed C. glabrata biofilms, an increase in matrix and overall biomass was observed. These effects were also shown with C. glabrata clinical isolates. Interestingly, the presence of C. difficile increased C. glabrata biofilm susceptibility to caspofungin, indicating potential effects on the fungal cell wall. Defining this intricate and intimate relationship will lead to an understanding of the role of Candida species in the context of CDI and novel aspects of Candida biology. IMPORTANCE Most microbiome studies have only considered the bacterial populations while ignoring other members of the microbiome such as fungi, other eukaryotic microorganisms, and viruses. Therefore, the role of fungi in human health and disease has been significantly understudied compared to their bacterial counterparts. This has generated a significant gap in knowledge that has negatively impacted disease diagnosis, understanding, and the development of therapeutics. With the development of novel technologies, we now have an understanding of mycobiome composition, but we do not understand the roles of fungi in the host. Here, we present findings showing that Candida glabrata, an opportunistic pathogenic yeast that colonizes the mammalian gastrointestinal tract, can impact the severity and outcome of a Clostridioides difficile infection (CDI) in a murine model. These findings bring attention to fungal colonizers during CDI, a bacterial infection of the gastrointestinal tract.

The transcription factor Ste12-like increases the mycelial abiotic stress tolerance and regulates the fruiting body development of Flammulina filiformis

Introduction

Flammulina filiformis is one of the most commercially important edible fungi worldwide, with its nutritional value and medicinal properties. It becomes a good model species to study the tolerance of abiotic stress during mycelia growth in edible mushroom cultivation. Transcription factor Ste12 has been reported to be involved in the regulation of stress tolerance and sexual reproduction in fungi.

Methods

In this study, identification and phylogenetic analysis of ste12-like was performed by bioinformatics methods. Four ste12-like overexpression transformants of F. filiformis were constructed by Agrobacterium tumefaciens-mediated transformation.

Results and Discussion

Phylogenetic analysis showed that Ste12-like contained conserved amino acid sequences. All the overexpression transformants were more tolerant to salt stress, cold stress and oxidative stress than wild-type strains. In the fruiting experiment, the number of fruiting bodies of overexpression transformants increased compared with wild-type strains, but the growth rate of stipes slowed down. It suggested that gene ste12-like was involved in the regulation of abiotic stress tolerance and fruiting body development in F. filiformis.

Mixed Fungal Biofilms: From Mycobiota to Devices, a New Challenge on Clinical Practice

Most current protocols for the diagnosis of fungal infections are based on culture-dependent methods that allow the evaluation of fungal morphology and the identification of the etiologic agent of mycosis. Most current protocols for the diagnosis of fungal infections are based on culture-dependent methods that enable the examination of the fungi for further identification of the etiological agent of the mycosis. The isolation of fungi from pure cultures is typically recommended, as when more than one species is identified, the second agent is considered a contaminant. Fungi mostly survive in highly organized communities that provoke changes in phenotypic profile, increase resistance to antifungals and environmental stresses, and facilitate evasion from the immune system. Mixed fungal biofilms (MFB) harbor more than one fungal species, wherein exchange can occur that potentialize the effects of these virulence factors. However, little is known about MFB and their role in infectious processes, particularly in terms of how each species may synergistically contribute to the pathogenesis. Here, we review fungi present in MFB that are commensals of the human body, forming the mycobiota, and how their participation in MFB affects the maintenance of homeostasis. In addition, we discuss how MFB are formed on both biotic and abiotic surfaces, thus being a significant reservoir of microorganisms that have already been associated in infectious processes of high morbidity and mortality.