Despite current treatment options, invasive infections caused by Aspergillus fumigatus continue having a high associated mortality rate. Nutrient supply and metabolic versatility are known to be essential virulence traits, a better understanding of which might derive in the identification of novel suitable molecular targets for the development of more efficient chemotherapy. We had previously demonstrated that regulation of sulphur assimilation is essential for A. fumigatus pathogenicity. Therefore, in this study we aimed to perform an in-depth characterization of the trans-sulfuration pathway to define the relevance of the sulphur containing amino acids for fungal virulence.
We have constructed several single, double and conditional mutants eliminating key genes of the trans-sulfuration pathway to create cysteine and methionine auxotrophic strains. Subsequently, we have investigated the nutrient requirements, oxidative resistance and virulence capacities of those strains to determine the relevance of the sulphurated amino acids biosynthesis for A. fumigatus growth in vivo.
A cysteine auxotrophic strain has reduced virulence capacities, proving that the amount of cysteine in the lungs is limited. This mutant further evidences the robustness of the subordinate glutathione redox system in A. fumigatus. Interestingly, we show that growth of a methionine synthase conditional mutant can only be rescued in the presence of all amino acids, but is avirulent in a murine model of invasive pulmonary aspergillosis. This demonstrates that the amount of readily available amino acids in the lung tissue is rather scarce and, furthermore, that this enzyme is essential for intrapulmonary growth. Taking into account the difference between fungal and mammalian methionine synthases, we propose that it constitutes an ideal metabolic target and will pursue the development of a specific drug to fight A. fumigatus infection.
Cysteine biosynthesis is important for intrapulmonary growth, whereas the action of the methionine synthase gene product is essential for A. fumigatus virulence.