StuA-Regulated Processes in the Dermatophyte Trichophyton rubrum: Transcription Profile, Cell-Cell Adhesion, and Immunomodulation

The StuA Transcription Factor and Alternative Splicing Mechanisms Drive the Levels of MAPK Hog1 Transcripts in the Dermatophyte Trichophyton rubrum

Trichophyton rubrum is a human fungal pathogen that causes dermatophytosis, an infection that affects keratinized tissues. Integrated molecular signals coordinate mechanisms that control pathogenicity. Transcriptional regulation is a core regulation of relevant fungal processes. Previous RNA sequencing data revealed that the absence of the transcription factor StuA resulted in the differential expression of the MAPK-related high glycerol osmolarity gene (hog1) in T. rubrum. Here we validated the role of StuA in regulating the transcript levels of hog1. We showed through RT-qPCR that transcriptional regulation controls hog1 levels in response to glucose, keratin, and co-culture with human keratinocytes. In addition, we also detected hog1 pre-mRNA transcripts that underwent alternative splicing, presenting intron retention in a StuA-dependent mechanism. Our findings suggest that StuA and alternative splicing simultaneously, but not dependently, coordinate hog1 transcript levels in T. rubrum. As a means of preventing and treating dermatophytosis, our results contribute to the search for new potential drug therapies based on the molecular aspects of signaling pathways in T. rubrum.

The Transcription Factor StuA Regulates the Glyoxylate Cycle in the Dermatophyte Trichophyton rubrum under Carbon Starvation

Trichophyton rubrum is the primary causative agent of dermatophytosis worldwide. This fungus colonizes keratinized tissues and uses keratin as a nutritional source during infection. In T. rubrum-host interactions, sensing a hostile environment triggers the adaptation of its metabolic machinery to ensure its survival. The glyoxylate cycle has emerged as an alternative metabolic pathway when glucose availability is limited; this enables the conversion of simple carbon compounds into glucose via gluconeogenesis. In this study, we investigated the impact of stuA deletion on the response of glyoxylate cycle enzymes during fungal growth under varying culture conditions in conjunction with post-transcriptional regulation through alternative splicing of the genes encoding these enzymes. We revealed that the ΔstuA mutant downregulated the malate synthase and isocitrate lyase genes in a keratin-containing medium or when co-cultured with human keratinocytes. Alternative splicing of an isocitrate lyase gene yielded a new isoform. Enzymatic activity assays showed specific instances where isocitrate lyase and malate synthase activities were affected in the mutant strain compared to the wild type strain. Taken together, our results indicate a relevant balance in transcriptional regulation that has distinct effects on the enzymatic activities of malate synthase and isocitrate lyase.

PdStuA Is a Key Transcription Factor Controlling Sporulation, Hydrophobicity, and Stress Tolerance in Penicillium digitatum

Penicillium digitatum has become one of the main pathogens in citrus due to its high spore production and easy spread. In this study, the function of the APSES transcription factor StuA in P. digitatum was characterized, and the results indicated that it was involved in conidium and conidiophore development. No conidiophores were observed in the mycelium of the ∆PdStuA mutant that had grown for two days, while an abnormal conidiophore was found after another two days of incubation, and only small thin phialides as well as a very small number of spores were formed at the top of the hyphae. Moreover, it was observed that the ∆PdStuA mutant showed various defects, such as reduced hydrophobicity and decreased tolerance to cell wall inhibitors and H2O2. Compared to the original P. digitatum, the colony diameter of the ∆PdStuA mutant was not significantly affected, but the growth of aerial hyphae was obviously induced. In in vivo experiments, the spore production of the ∆PdStuA mutant grown on citrus fruit was remarkably decreased; however, there was no significant difference in the lesion diameter between the mutant and original strain. It could be inferred that less spore production might result in reduced spread in citrus, thereby reducing the green mold infection in citrus fruit during storage. This study provided a gene, PdStuA, which played key role in the sporulation of P. digitatum, and the results might provide a reference for the molecular mechanisms of sporulation in P. digitatum.

An update on the myriad antifungal resistance mechanisms in dermatophytes and the place of experimental and existential therapeutic agents for Trichophyton complex implicated in tinea corporis and cruris

INTRODUCTION

There is an epidemic emergence of increased resistance in dermatophytes with to antifungal drugs with ergosterol1 (Erg1) and Erg11 mutations to terbinafine and azoles. Apart from mutations, mechanisms that predict clinical failure include efflux pumps, cellular kinases, heat shock proteins (Hsp), and biofilms. Apart from itraconazole and SUBATM (Super-Bioavailable) itraconazole, measures that can be used in terbinafine failure include efflux-pump inhibitors, Hsp inhibitors and judicious use of antifungal drugs (topical + systemic) combinations.

AREAS COVERED

A PubMed search was done for the relevant studies and reviews published in the last 22 years using keywords dermatophytes OR Trichophyton, anti-fungal, resistance, mechanism and fungal AND resistance mechanisms. Our aim was to look for literature on prevalent species and we specifically researched studies on Trichophyton genus. We have analyzed varied antifungal drug mechanisms and detailed varied experimental and approved drugs to treat recalcitrant dermatophytosis.

EXPERT OPINION

Apart from administering drugs with low minimum inhibitory concentration, combinations of oral and topical antifungals (based on synergy data) and new formulations of existing drugs are useful in recalcitrant cases. There is a need for research into resistance mechanism of the existent Trichophyton strains in therapeutic failures in tinea corporis & cruris instead of data derived from laboratory strains which may not mirror clinical failures.

Rapid and sustained T cell-based immunotherapy against invasive fungal disease via a combined two step procedure

Introduction

Aspergillus fumigatus (Asp) infections constitute a major cause of morbidity and mortality in patients following allogeneic hematopoietic stem cell transplantation (HSCT). In the context of insufficient host immunity, antifungal drugs show only limited efficacy. Faster and increased T-cell reconstitution correlated with a favorable outcome and a cell-based therapy approach strongly indicated successful clearance of fungal infections. Nevertheless, complex and cost- or time-intensive protocols hampered their implementation into clinical application.

Methods

To facilitate the clinical-scale manufacturing process of Aspergillus fumigatus-specific T cells (ATCs) and to enable immediate (within 24 hours) and sustained (12 days later) treatment of patients with invasive aspergillosis (IA), we adapted and combined two complementary good manufacturing practice (GMP)-compliant approaches, i) the direct magnetic enrichment of Interferon-gamma (IFN-γ) secreting ATCs using the small-scale Cytokine Secretion Assay (CSA) and ii) a short-term in vitro T-cell culture expansion (STE), respectively. We further compared stimulation with two standardized and commercially available products: Asp-lysate and a pool of overlapping peptides derived from different Asp-proteins (PepMix).

Results

For the fast CSA-based approach we detected IFN-γ⁺ ATCs after Asp-lysate- as well as PepMix-stimulation but with a significantly higher enrichment efficiency for stimulation with the Asp-lysate when compared to the PepMix. In contrast, the STE approach resulted in comparably high ATC expansion rates by using Asp-lysate or PepMix. Independent of the stimulus, predominantly CD4⁺ helper T cells with a central-memory phenotype were expanded while CD8⁺ T cells mainly showed an effector-memory phenotype. ATCs were highly functional and cytotoxic as determined by secretion of granzyme-B and IFN-γ.

Discussion

For patients with IA, the immediate adoptive transfer of IFN-γ⁺ ATCs followed by the administration of short-term in vitro expanded ATCs from the same donor, might be a promising therapeutic option to improve the clinical outcome.

Alternative Splicing in Trichophyton rubrum Occurs in Efflux Pump Transcripts in Response to Antifungal Drugs

Dermatophytes are challenging to treat because they have developed many strategies to neutralize the stress triggered by antifungals. Drug tolerance is achieved by mechanisms such as drug efflux and biofilm formation, and cellular efflux is a consequence of the synergistic and compensatory regulation of efflux pumps. Alternative splicing (AS) has also been considered as a mechanism that enhances fungal adaptive responses. We used RNA-seq data from the dermatophyte Trichophyton rubrum exposed to undecanoic acid (UDA) to search for and validate AS in genes encoding efflux pumps. The magnitude of this phenomenon was evaluated using UDA and other antifungals (caspofungin, itraconazole, and terbinafine) in planktonic and biofilm cultures. In addition to the conventional isoforms, the efflux pump encoded by TERG_04309 presented two intron-retained isoforms. Biofilms trigger the simultaneous production of at least two isoforms. The intron-retained isoforms showed short lengths and topologically different organization. Furthermore, we identified the putative interaction of efflux pumps (TERG_04309 and TERG_04224). Co-expression of these genes suggests a synergistic action in antifungal resistance. Our data provide new insights into drug tolerance related to differential isoform usage and the co-expression of stress-responsive genes, which may lead to higher antifungal resistance, mainly in biofilms.

Peptidase Regulation in Trichophyton rubrum Is Mediated by the Synergism Between Alternative Splicing and StuA-Dependent Transcriptional Mechanisms

Trichophyton rubrum is the most common causative agent of dermatophytosis worldwide and uses keratinized substrates such as skin and nails as its main source of nutrition during infection. Its pathogenic character relies on colonization and viability maintenance at the target host sites. Since fungal physiology must adapt and respond to host conditions for the successful establishment of infection, biological mechanisms are constantly being triggered by T. rubrum to guarantee its survival in the host environment. The ability of this fungus to sense and modulate the secretion of specific proteases according to environmental pH signaling is considered as a pivotal virulence factor for effective invasion and persistence of infection in the host. Transcriptional regulation of genes encoding specific proteases, such as peptidases, is a key biological process that drives physiological modulation to meet fungal requirements. It accomplishes a robust balance among transcript isoforms that can be directed to perform distinct cellular functions. Thus, alternative splicing mechanisms are suitable for fungal cells to establish a balance toward reprogramming protein translation to impair or boost physiological conditions. In this study, we investigated the role of alternative splicing, especially intron retention events, in generating isoforms of virulence factors in T. rubrum mediated by transcriptional coordination of the protein StuA, a recently described transcription factor in this fungus. By analyzing the previous gene expression data provided by RNA-sequencing and after validation by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR), we observed that two peptidase-coding genes (TERG_00734 and TERG_04614) could be direct targets of alternative splicing in the presence of keratin. Furthermore, protease isoforms generated by alternative splicing in T. rubrum were also detected in a co-culture with human keratinocytes, highlighting the role of these proteins in keratin deconstruction. Our results strongly suggest the influence of StuA on the regulation of virulence factors in T. rubrum and dermatophyte infections by triggering the transcription of the peptidase genes mentioned above in an alternative splicing-independent balance. The results elucidate how fungal cells drive alternate splicing to promote physiological adaptations and show that transcriptional regulation and virulence traits are robust elements required for dermatophyte infection.

State-of-the-Art Dermatophyte Infections: Epidemiology Aspects, Pathophysiology, and Resistance Mechanisms

The burden of fungal infections is not widely appreciated. Although these infections are responsible for over one million deaths annually, it is estimated that one billion people are affected by severe fungal diseases. Mycoses of nails and skin, primarily caused by fungi known as dermatophytes, are the most common fungal infections. Trichophyton rubrum appears to be the most common causative agent of dermatophytosis, followed by Trichophyton interdigitale. An estimated 25% of the world’s population suffers from dermatomycosis. Although these infections are not lethal, they compromise the quality of life of infected patients. The outcome of antidermatophytic treatments is impaired by various conditions, such as resistance and tolerance of certain dermatophyte strains. The adage “know your enemy” must be the focus of fungal research. There is an urgent need to increase awareness about the significance of these infections with precise epidemiological data and to improve knowledge regarding fungal biology and pathogenesis, with an emphasis on adaptive mechanisms to tackle adverse conditions from host counteractions. This review outlines the current knowledge about dermatophyte infections, with a focus on signaling pathways required for fungal infection establishment and a broad perspective on cellular and molecular factors involved in antifungal resistance and tolerance.