Fungi remarkably produce a variety of secondary metabolites as a consequence of different environmental stimuli. These compounds can ultimately provide fitness attributes to the producing organism. Recently, we characterized two components of the A. fumigatus cell wall integrity pathway (CWI), pkcA and rlmA and observed that in addition to the cell wall related-phenotypes, the perturbation of the signaling circuit coordinated by the PkcA-MpkA-RlmA module impacts on the production of fumiquinazolines (Fq). FqC is the major Fq produced by A. fumigatus which accumulation was associated with conidia formation. Here we show that pkcAG579R and ΔrlmA mutant strains produce lower FqC (24.7% and 27.9%, respectively) and that FqC concentrations were 10.5- fold lower in the ΔmpkA strain. This decrease is accompanied by global down-regulation in mRNA expression of the Fq cluster genes during the asexual development. Aiming to understand if other cell stresses could influence the production of FqC, we performed a screening using different null mutants and found that the deletion of the transcription factor SebA, (primarily involved in heat shock and oxidative stress) overproduced FqC (about 4.5-fold increase) indicating that sebA is a negative regulator of FqC production. A. fumigatus is sensitive to FqC and this tolerance is decreased in the CWI pathway mutants and increased in the ΔsebA strain. In addition, FqC can induce pore formation on the membrane of macrophages and highly stimulates the secretion the pro-inflammatory cytokine TNF-α by this cell type. We also used the soil amoeba Dictyostelium discoideum to study the phagocytic interaction of this organism with conidia from the ΔsebA strain. Interestingly, conidia of the ΔsebA were significantly less phagocytized by D. discoideum and the opposite occurred when conidia from the CWI pathway mutants were tested. Our results suggest that Fq production is regulated at different levels in A. fumigatus and that FqC can serve as a defense compound against other microorganisms or soil predators. Although we could not detect FqC in the lung of infected mice, this molecule is potentially cytotoxic to fungi and mammalian cells.
Full conference title:
- Asperfest 14 (2017)