Deletion of core septin gene aspB in Aspergillus fumigatus results in fungicidal activity of caspofungin

Septins are a family of GTP-binding proteins found in many eukaryotic lineages. Although highly conserved throughout many eukaryotes, their functions vary across species. In Aspergillus fumigatus , the etiological agent of invasive aspergillosis, septins participate in a variety of processes, including conidiation, septation, and responses to cell wall stress. Previous studies determined that the Δ aspB septin deletion strain had a greater sensitivity to anti-cell wall drugs, especially the echinocandins, yet mechanisms behind this augmented sensitivity are unknown. We performed cell viability staining of the septin deletion strains after caspofungin exposure and found that the Δ aspA , Δ aspB , and Δ aspC strains had significantly lower cell viability. Concomitant with the reduced viability, these septin deletion strains are more susceptible to caspofungin on solid media. These results indicate that the septin cytoskeleton is important for A. fumigatus survival in the presence of caspofungin. Due to the potential of improved therapeutic outcome, we followed up using a neutropenic murine model of invasive aspergillosis. Animals infected with the Δ aspB strain and treated with caspofungin showed improved survival compared to the animals infected with the Δ akuB KU80 wild-type or complemented strains. Additionally, histological analysis showed reduced fungal burden in the Δ aspB infected, caspofungin-treated group. To gain an insight into how AspB coordinate fungal response to caspofungin, we used proteomics and identified proteins involved in the septin-dependent response to caspofungin, including four candidate interactors involved in cell wall stress response. Deletion of these candidate genes resulted in increased susceptibility to caspofungin and moderately reduced viability post-drug exposure. Taken together, these data suggest that septin AspB contributes to the fungistatic response to caspofungin.