The Aspergillus fumigatus septation components Bud4 and Rho4 are essential for virulence

Aspergillus fumigatus is the primary etiologic agent of pulmonary mold infections, the most severe of which is invasive pulmonary aspergillosis (IPA). As the currently approved antifungal drugs are threatened by growing resistance rates and host toxicity, novel therapeutic options are urgently needed. Our recent work suggests that septation inhibition may be one such strategy, as aseptate strains of A. fumigatus are hypersusceptible to the echinocandin antifungal drugs and display significantly reduced virulence in animal models of IPA. In the current study, we expand upon our understanding of the anillin-like protein, Bud4, and Rho GTPase, Rho4, which regulate septation in Aspergillus nidulans and A. fumigatus, respectively. Whereas AnBud4 is not essential for septation in A. nidulans, deletion of the A. fumigatus ortholog (∆bud4) resulted in complete loss of septation. A similar phenotype was also observed in ∆rho4, consistent with previous studies. Supporting a critical role for hyphal septation in the establishment of invasive aspergillosis, we find that ∆bud4 and ∆rho4 display avirulence in a mouse model of IPA characterized by a loss of tissue-invasive growth. Furthermore, both deletion mutations were able to convert anti-Aspergillus echinocandin activity from static to cidal. Through titratable bud4 expression using a doxycycline-repressible promoter system, we also find that even partial repression of septation significantly increases echinocandin susceptibility. Taken together, these studies support a therapeutic strategy which could incorporate a septation inhibitor in combination with echinocandins to reduce tissue-invasive growth and improve antifungal susceptibility.