The Small GTPase RacA Mediates Intracellular Reactive Oxygen Species Production, Polarized Growth, and Virulence in the Human Fungal Pathogen Aspergillus fumigatus

Abstract

ABSTRACT Aspergillus fumigatus is the predominant mold pathogen in immunocompromised patients. In this study, we present the first characterization of the small GTPase RacA in A. fumigatus . To gain insight into the function of racA in the growth and pathogenesis of A. fumigatus , we constructed a strain that lacks a functional racA gene. The Δ racA strain showed significant morphological defects, including a reduced growth rate and abnormal conidiogenesis on glucose minimal medium. In the Δ racA strain, apical dominance in the leading hyphae is lost and, instead, multiple axes of polarity emerge. Intriguingly, superoxide production at the hyphal tips was reduced by 25% in the Δ racA strain. Treatment of wild-type hyphae with diphenylene iodonium, an inhibitor of NADPH oxidase, resulted in phenotypes similar to that of the Δ racA strain. These data suggest that Δ racA strain phenotypes may be due to a reduction or alteration in the production of reactive oxygen species. Most surprisingly, despite these developmental and growth abnormalities, the Δ racA strain retained at least wild-type virulence in both an insect model and two immunologically distinct murine models of invasive pulmonary aspergillosis. These results demonstrate that in vitro growth phenotypes do not always correlate with in vivo virulence and raise intriguing questions about the role of RacA in Aspergillus virulence.