TRXR2, a thioredoxin reductase-encoding gene, contributes to protection against the oxidative stress and virulence in Scedosporium apiospermum

Scedosporium apiospermum has received an increased attention over the past decades, especially because of its low susceptibility to current antifungals and its clinical relevance in cystic fibrosis. Nevertheless, little is still known about its pathogenic mechanisms. During the inflammatory reaction, macrophages and neutrophils release antimicrobial compounds, especially reactive oxygen species (ROS). To cope with ROS, pathogens have developed various strategies, including synthesis of some immunoprotective secondary metabolites, and enzymatic mechanisms relying on antioxidant enzymes. A recent transcriptomic study showed that the TRXR2 gene, encoding one of its two thioredoxin reductases (TrxRs), is overexpressed upon exposure to oxidative stress. To investigate the role of TrxR2, the encoding gene was disrupted using the CRISPR-Cas9 technology. An ultrastructural study revealed that TRXR2 gene knock-out induced the loss of the outer cell wall layer of hyphae, thereby increasing their surface hydrophobicity. The effects on the tolerance of the fungus to chemically-induced oxidative stresses were also investigated, revealing a higher susceptibility to cumene hydroperoxide. Additionally, an overexpression of BoyT gene (encoding the other TrxR) was seen as a compensatory mechanism, but bioinformatic analysis also suggested distinct roles for the two TrxRs in relation with distinct subcellular localizations. Finally, conidia exhibited a reduced capacity to adhere to epithelial cells and a diminished virulence in the Galleria mellonella model. In conclusion, these results suggest that TrxR2 plays a contributory role in the pathogenicity of S. apiospermum, but is not a major determinant of its virulence, which justifies future studies on the other components of the thioredoxin system.