Plasma Membrane Fusion Is Specifically Impacted by the Molecular Structure of Membrane Sterols During Vegetative Development of Neurospora crassa

Targeted regulation of sterol biosynthesis genes according to perturbations in ergosterol biosynthesis in fungi

INTRODUCTION

The synthesis and regulation of ergosterol are vital for fungal growth and stress adaptation. While ergosterol-mediated feedback regulation is a recognized mechanism controlling sterol biosynthesis in fungi, prior research suggests the presence of additional regulatory mechanisms. However, the specifics of the alternative regulatory mechanisms have not been systematically investigated.

OBJECTIVES

We proposed that a regulatory network is likely to discern disturbances in sterol biosynthesis and trigger responses accordingly. This study aimed to validate the hypothesis and investigate the regulatory mechanisms.

METHODS

Quantitative Real-time PCR and HPLC-MS/MS were used to explore and compare the regulation of sterol biosynthesis in different fungi. Key transcription factors involved in the alternative regulatory mechanism in Neurospora crassa were identified by phenotypic profiling of a transcription factor mutant library. ChIP-qPCR, fluorescence confocal imaging, RNA sequencing, and gene set enrichment analysis (GSEA) were used to reveal the mechanism of each transcription factor.

RESULTS

Unlike the canonical ergosterol-mediated feedback regulation in fungi like C. neoformans, our study demonstrated that the inhibitions of ergosterol biosynthesis at specific steps triggered distinct transcriptional responses of erg genes in fungi, including N. crassa and Aspergillus fumigatus. In N. crassa, the responses were orchestrated by different transcription factors. Specifically, the inhibition of ERG24 and ERG2 activated transcription factors SAH-2 and AtrR, resulting in the upregulation of erg24, erg2, erg25, and erg3. Furthermore, the inhibition of ERG11/CYP51 activated transcription factor NcSR, leading to the upregulation of erg11 and erg6. Phenotypic profiles of mutants of various N. crassa erg genes and the aforementioned transcription factors implied that the targeted regulation of ergosterol biosynthesis could fortify fungal viability within complex habitats.

CONCLUSION

Our study reveals a novel regulatory mechanism in fungi: targeted upregulation of specific sterol biosynthesis genes in response to given perturbations in ergosterol biosynthesis, exhibiting a higher degree of precision and sophistication in sterol biosynthesis regulation.

Antifungal azoles trigger a xenobiotic detoxification pathway and chitin synthesis in Neurospora crassa

The presence of antifungal drugs is prompting the fungal microorganisms to react by mechanisms broader than the resistance. The fungi evolved mechanisms, by which they respond to various stress conditions, including the presence of antifungal compounds. In this work, we studied the response of model filamentous fungus Neurospora crassa to azole antifungals in the broader context of the adaptation mechanisms. We demonstrated the increase in expression of filamentous fungi-specific genes encoding cytochrome enzymes of CYP65 clan and plasma membrane-localized ABCC transporters. Azoles appear not to conjugate with glutathione. Surprisingly, the azoles caused changes in the hyphae organization and the amount of chitin in cell wall by the same manner that was thought to be echinocandin-specific. The response to individual azoles appeared to be influenced by the structure of azole compound (prochloraz - main outlier). Taken together, these findings demonstrate the importance of study of stress response mechanisms, specifically in filamentous fungi. Many aspects of the reaction within azoles seem to be similar, though specificities are occurring.

A focus on yeast mating: From pheromone signaling to cell-cell fusion

Cells live in a chemical environment and are able to orient towards chemical cues. Unicellular haploid fungal cells communicate by secreting pheromones to reproduce sexually. In the yeast models Saccharomyces cerevisiae and Schizosaccharomyces pombe, pheromonal communication activates similar pathways composed of cognate G-protein-coupled receptors and downstream small GTPase Cdc42 and MAP kinase cascades. Local pheromone release and sensing, at a mobile surface polarity patch, underlie spatial gradient interpretation to form pairs between two cells of distinct mating types. Concentration of secretion at the point of cell-cell contact then leads to local cell wall digestion for cell fusion, forming a diploid zygote that prevents further fusion attempts. A number of asymmetries between mating types may promote efficiency of the system. In this review, we present our current knowledge of pheromone signaling in the two model yeasts, with an emphasis on how cells decode the pheromone signal spatially and ultimately fuse together. Though overall pathway architectures are similar in the two species, their large evolutionary distance allows to explore how conceptually similar solutions to a general biological problem can arise from divergent molecular components.

Analysis of the Putative Nucleoporin POM33 in the Filamentous Fungus Sordaria macrospora

In the filamentous fungus Sordaria macrospora (Sm), the STRIPAK complex is required for vegetative growth, fruiting-body development and hyphal fusion. The SmSTRIPAK core consists of the striatin homolog PRO11, the scaffolding subunit of phosphatase PP2A, SmPP2AA, and its catalytic subunit SmPP2Ac1. Among other STRIPAK proteins, the recently identified coiled-coil protein SCI1 was demonstrated to co-localize around the nucleus. Pulldown experiments with SCI identified the transmembrane nucleoporin (TM Nup) SmPOM33 as a potential nuclear-anchor of SmSTRIPAK. Localization studies revealed that SmPOM33 partially localizes to the nuclear envelope (NE), but mainly to the endoplasmic reticulum (ER). We succeeded to generate a Δpom33 deletion mutant by homologous recombination in a new S. macrospora Δku80 recipient strain, which is defective in non-homologous end joining. Deletion of Smpom33 did neither impair vegetative growth nor sexual development. In pulldown experiments of SmPOM33 followed by LC/MS analysis, ER-membrane proteins involved in ER morphology, protein translocation, glycosylation, sterol biosynthesis and Ca²⁺-transport were significantly enriched. Data are available via ProteomeXchange with identifier PXD026253. Although no SmSTRIPAK components were identified as putative interaction partners, it cannot be excluded that SmPOM33 is involved in temporarily anchoring the SmSTRIPAK to the NE or other sites in the cell.