S10.5c Insight into the role of secondary metabolism in the pathogenesis of Scedosporium apiospermum

Abstract

S10.5 Fungal respiratory infections in Cystic Fibrosis, September 24, 2022, 10:30 AM - 12:00 PMSecondary metabolism is a general term defining biosynthetic pathways that occur in plants, bacteria, and fungi and lead to the production of highly diversified molecular structures. Among the diverse functions that were attributed to these molecules, it is now obvious that they are predominantly involved in chemical warfare with competitors in their environments. In fungi, the two main classes of fungal secondary metabolites are polyketides (PKs) and nonribosomal peptides (NRPs). The biosynthesis of such bioactive molecules is performed by large multifunctional enzymes (NRP synthases, NRPS, and polyketide synthases, PKS) encoded by genes usually located within clusters. Conversely to Aspergillus fumigatus, only a few data are currently available for other pathogenic fungi. In this context, our research group is interested in delineating the role of secondary metabolism in Scedosporium apiospermum, a multi-resistant mold known to colonize chronically the airways of patients with cystic fibrosis (CF). Taking advantage of the availability of the S. apiospermum genome sequence, we first conducted an in silico analysis aiming at exploring the PKs and NRPs battery of the fungus. A total of 9 genes encoding PKs, 9 encoding NRPs, and 5 encoding hybrid NRPs/PKs enzymes were identified. All 3 of the PKs gene clusters presented homologies with those involved in the biosynthesis of pseurotin A. transbergamotene, and ovalicin, or the tremorgenic toxin b-aflatrem while a fourth one is involved in the biosynthesis of melanin. Among the NRPs encoding genes, 6 exhibited sufficient similarity scores with other fungal NRPs to predict the class of the generated peptide: siderophores (2), epidithiodioxopiperazines (2), and cyclopeptides (2). Nevertheless, substrate prediction methods for NRPs domains failed, thus questioning about the nature of the produced peptides. We thus focused our attention on the characterization of some NRP and PK biosynthetic pathways. Since iron acquisition is known to be crucial for the survival of microorganisms as for the virulence of numerous pathogens, we first investigated clusters predicted to be responsible for the biosynthesis of siderophores in S. apiospermum. For instance, we disrupted the SAPIO_CDS2806 gene, an ortholog of sidD which drives the production of the extracellular hydroxamate-type siderophore fusarinin C in Aspergillus fumigatus. A comparison of culture supernatants from sidD mutants and their parent strain revealed that S. apiospermum secretes a unique extracellular siderophore, namely Nα‐methylcoprogen B and that sidD gene was essential for the biosynthesis of this siderophore. sidD mutation resulted in the lack of growth under iron limiting conditions. Interestingly, pyoverdine supported the growth of the parent strain only, suggesting that Nα‐methylcoprogen B is required for iron acquisition from this Pseudomonas aeruginosa siderophore. Finally, the deletion of sidD resulted in the loss of virulence in a murine model of scedosporiosis. Altogether, our results demonstrate that S. apiospermum sidD gene drives the synthesis of a unique extracellular siderophore, namely Nα‐methylcoprogen B, which is essential for fungal growth and virulence. Above all, we also provide unprecedented data suggesting that this fungal siderophore scavenges iron from pyoverdine, which might explain the antagonism between S. apiospermum and P. aeruginosa in CF.