Basal UPR activity in Aspergillus fumigatus regulates adaptation to nutrient stress and is critical for the establishment of corneal infection

Abstract

The Aspergillus fumigatus unfolded protein response (UPR) is a two-component relay consisting of the ER-bound IreA protein, which splices and activates the mRNA of the transcription factor HacA. Spliced hacA accumulates under conditions of acute ER stress in vitro, and UPR null mutants are hypovirulent in a murine model of invasive pulmonary infection. In this report, we demonstrate that a hacA deletion mutant is completely unable to establish infection in a model of fungal keratitis, a corneal infection and an important cause of ocular morbidity and unilateral blindness worldwide. Contrary to our initial prediction, however, we demonstrate that hacA splicing is not increased above baseline conditions in the cornea, nor is the expression of genes classically associated with UPR activation, such as protein chaperones. We employed transcriptomics on wild-type and{Delta} hacA strains in gelatin media, as a proxy for the corneal environment, and found that hacA supports the expression of numerous primary and secondary metabolic processes that likely promote adaptation to nutrient limitation. Taken together, our results support a model in which the cornea, similar to growth on protein in vitro, is a source of sub-acute ER stress for A. fumigatus, but one nevertheless that requires the UPR pathway for proper adaptation. The data also suggest that this pathway could be a target for novel antifungals that improve visual outcomes for fungal keratitis patients. AUTHOR SUMMARYFungal keratitis has emerged as a leading cause of ocular morbidity and unilateral blindness worldwide. Relative to other infectious contexts, however, little is known about the fungal genes or pathways that regulate invasive growth and virulence in the corneal environment. In this report, we demonstrate that genetic disruption of the Aspergillus fumigatus unfolded protein response (UPR) abolishes the ability of the mold to establish infection in a mouse model of FK. Despite this critical role for virulence, however, we did not detect a concerted activation of the pathway beyond levels observed on standard medium, suggesting that the host environment is not an acute source of endoplasmic reticulum stress. Transcriptomic profiling of the wild-type and UPR-deficient strains under host-relevant nutrient conditions revealed a critical role for the pathway in regulating primary and secondary metabolism, cell wall biology, and mitochondrial function, all of which likely modulate fungal growth within and interactions with the host. These results expand our understanding of UPR regulation and function in this important mold pathogen and suggest the pathway could serve as a target for novel antifungals that improve visual outcomes in the setting of fungal keratitis.