Deletion of the putative stretch-activated ion channel Mid1 is hypervirulent in Aspergillus fumigatus

Calcium ion is a universal and physiologically important molecule affecting almost every cellular function, while Mid1 (mating induced death) is a stretch-activated ion channel localized in the plasma membrane, which can replenish the fungus with extracellular calcium. Aspergillus fumigatus is one of the most important fungal pathogens but little is known about its calcium channels in the plasma membrane. In this study, a homolog of Saccharomyces cerevisiae Mid1 was identified in A. fumigatus and its encoding gene afmid1 was deleted by replacing it with pyr4 selectable marker, designated as Δafmid1. Analysis of the phenotypes demonstrated that Δafmid1 had growth defects under solid and liquid cultural conditions. The mutant Δafmid1 had delayed germination in minimal medium and abnormal morphogenesis in the medium containing ethylene glycol tetraacetic acid (EGTA), an agent with major affinity for Ca2+ and minor affinity for Mn2+. The sensitivity against cell wall disturbing agents, osmotic stress, alkaline environments, high temperatures or starvation in Δafmid1 resembled wild type. However, the mutant Δafmid1 demonstrated more sensitivity to oxidative agents (H2O2 and Menadione) than wild type. Most surprisingly, deletion of afmid1 from A. fumigatus led to hypervirulence in the immunosuppressed mice model.