Upstream and Downstream Regulation of Asexual Development in Aspergillus fumigatus

Abstract

ABSTRACT The opportunistic human pathogen Aspergillus fumigatus produces a large quantity of asexual spores (conidia), which are the primary agent causing invasive aspergillosis in immunocompromised patients. We investigated the mechanisms controlling asexual sporulation (conidiation) in A. fumigatus via examining functions of four key regulators, GpaA (Gα), AfFlbA (RGS), AfFluG, and AfBrlA, previously studied in Aspergillus nidulans . Expression analyses of gpaA , Af flbA , Af fluG , Af brlA , and Af wetA throughout the life cycle of A. fumigatus revealed that, while transcripts of Af flbA and Af fluG accumulate constantly, the latter two downstream developmental regulators are specifically expressed during conidiation. Both loss-of-function Af flbA and dominant activating GpaA Q204L mutations resulted in reduced conidiation with increased hyphal proliferation, indicating that GpaA signaling activates vegetative growth while inhibiting conidiation. As GpaA is the primary target of AfFlbA, the dominant interfering GpaA G203R mutation suppressed reduced conidiation caused by loss of Af flbA function. These results corroborate the hypothesis that functions of G proteins and RGSs are conserved in aspergilli. We then examined functions of the two major developmental activators AfFluG and AfBrlA. While deletion of Af brlA eliminated conidiation completely, null mutation of Af fluG did not cause severe alterations in A. fumigatus sporulation in air-exposed culture, implying that, whereas the two aspergilli may have a common key downstream developmental activator, upstream mechanisms activating brlA may be distinct. Finally, both Af fluG and Af flbA mutants showed reduced conidiation and delayed expression of Af brlA in synchronized developmental induction, indicating that these upstream regulators contribute to the proper progression of conidiation.