The C ₂ H ₂ transcription factor SltA is required for germination and hyphal development in Aspergillus fumigatus

Abstract

Germination of inhaled Aspergillus fumigatus conidia is a necessary sequitur for infection. Germination of conidia starts with the breaking of dormancy, which is initiated by an increase of the cellular perimeter in a process termed isotropic growth. This swelling phase is followed by polarized growth, resulting in the formation of a germ tube. The multinucleate tubular cells exhibit tip growth from the hyphae, after which lateral branches emerge to form the mycelial network. The regulatory mechanisms governing conidial germination are not well defined. In this study, we identified a novel role for the transcription factor SltA in the orchestration of germination and hyphal development. Conidia lacking sltA fail to appropriately regulate isotropic growth and begin to swell earlier and subsequently switch to polarized growth faster. Additionally, hyphal development is distorted in a ∆ sltA isolate as hyphae are hyper-branching and wider, and show branching at the apical tip. ∆ sltA conidia are more tolerant to cell wall stressors on minimal medium compared to the wild-type (WT) strain. A transcriptome analysis of different stages of early growth was carried out to assess the regulatory role of SltA. Null mutants generated for three of the most dysregulated genes showed rapid germ tube emergence. Distinct from the phenotype observed for ∆ sltA , conidia from these strains lacked defects in isotropic growth, but switched to polarized growth faster. Here, we characterize and describe several genes in the regulon of SltA, highlighting the complex nature of germination. IMPORTANCE Aspergillus fumigatus is the main human fungal pathogen causing aspergillosis. For this fungus, azoles are the most commonly used antifungal drugs for treatment of aspergillosis. However, the prevalence of azole resistance is alarmingly increasing and linked with elevated mortality. Germination of conidia is crucial within its asexual life cycle and plays a critical role during the infection in the human host. Precluding germination could be a promising strategy considering the role of germination in Aspergillus spp. pathogenicity. Here, we identify a novel role for SltA in appropriate maintenance of dormancy, germination, and hyphal development. Three genes in the regulon of SltA were also essential for appropriate germination of conidia. With an expanding knowledge of germination and its different morphotypes, more advances can be made toward potential anti-germination targets for therapy.

The EH domain-containing protein, EdeA, is involved in endocytosis, cell wall integrity, and pathogenicity in Aspergillus fumigatus

“…Fresh conidia of related strains (1 × 10 5 conidia/mL) were inoculated onto glass coverslips in liquid MM and YG media and grown statically at 37°C for 7 h, 8 h, and 9 h. An Olympus Th4-200 fluorescence microscope with a 20× objective (Olympus Corporation, Tokyo, Japan) was used to capture images of each strain. Images were used to measure the germination rate of A. fumigatus , and conidia with an emerging germ tube in the form of a drop were scored as germinated ( 51 ). The number of germlings in each image was divided by the total number of conidia and germlings in the entire image.…”

A phylogenetic approach to explore the Aspergillus fumigatus conidial surface-associated proteome and its role in pathogenesis

“…One protein (encoded by Afu3g13755) did not have a homologue in the A. fumigatus A1163 background strain and was not examined further. We examined the expression of these 62 genes in a recently published RNAseq dataset that describes gene modulation during conidial germination 19 (Fig. 2d).…”