The amino acid permease Byc1 is involved in calcineurin-dependent thermos-tolerance in Cryptococcus neoformans

Cryptococcosis is one of the most serious infectious fungal diseases. It is known to infect the human respiratory tract and central nervous system and cause meningitis and encephalitis, especially in patients with AIDS. In the sub-Saharan area, cryptococcosis causes more mortality than tuberculosis. Cryptococcus neoformans is the etiological agent. There has been a growing number of reports of antifungal resistance in C. neoformans, and this fungus is also primarily resistant to the newest antifungal drug class, the echinocandins. Despite advances in modern medicine, the morbidity and mortality of C. neoformans infections remain unacceptably high. In C. neoformans, calcineurin, a serine/threonine phosphatase, is required for growth at the human host temperature, and calcineurin mutants are not virulent in murine models of cryptococcal meningitis. Therefore, calcineurin has long been considered an antifungal target. However, calcineurin is highly conserved in fungi and humans, and thus direct pharmacological inhibition of calcineurin can risk lowering host immunity as the human calcineurin is required for T-cell responses. A previous study demonstrated that there is a link between calcineurin, an amino acid permease, and protein kinase As in Mucorales. In this study, we further reveal the existence of a novel link between calcineurin and an amino acid permease, Byc1, for a thermo-tolerance phenotype in this fungus. We found that calcineurin negatively regulates the expression of the BYC1 gene and that byc1{Delta} mutants are partially resistant to FK506. Moreover, we revealed that overexpression of the BYC1 gene recapitulates the calcineurin mutants thermos-sensitive phenotypes, including FK506 sensitivity and lowered virulence in a murine infection model. We also found that protein kinase A (Pka1) mutants are also partially resistant to FK506. These results support the link between calcineurin, Byc1, and Pka1 in C. neoformans and will serve as a platform for developing novel antifungal drugs against the calcineurin pathway without directly targeting the phosphatase.