Mutual independence of alkaline- and calcium-mediated signalling in Aspergillus fumigatus refutes the existence of a conserved druggable signalling nexus.

Author(s): Loss O,  Bertuzzi M,  Yan Y,  Fedorova N,  McCann BL,  Armstrong-James D,  Espeso EA,  Read ND,  Nierman WC,  Bignell EM

Journal: Mol Microbiol

Date: 2017 Dec

Major Program(s) or Research Group(s):

PubMed ID: 28922497

PMC ID: PMC5725717

Abstract: Functional coupling of calcium- and alkaline responsive signalling occurs in multiple fungi to afford efficient cation homeostasis. Host microenvironments exert alkaline stress and potentially toxic concentrations of Ca2+ , such that highly conserved regulators of both calcium- (Crz) and pH- (PacC/Rim101) responsive signalling are crucial for fungal pathogenicity. Drugs targeting calcineurin are potent antifungal agents but also perturb human immunity thereby negating their use as anti-infectives, abrogation of alkaline signalling has, therefore, been postulated as an adjunctive antifungal strategy. We examined the interdependency of pH- and calcium-mediated signalling in Aspergillus fumigatus and found that calcium chelation severely impedes hyphal growth indicating a critical requirement for this ion independently of ambient pH. Transcriptomic responses to alkaline pH or calcium excess exhibited minimal similarity. Mutants lacking calcineurin, or its client CrzA, displayed normal alkaline tolerance and nuclear translocation of CrzA was unaffected by ambient pH. Expression of a highly conserved, alkaline-regulated, sodium ATPase was tolerant of genetic or chemical perturbations of calcium-mediated signalling, but abolished in null mutants of the pH-responsive transcription factor PacC, and PacC proteolytic processing occurred normally during calcium excess. Taken together our data demonstrate that in A. fumigatus the regulatory hierarchy governing alkaline tolerance circumvents calcineurin signalling.