Loss of the high-affinity vacuolar Ca2+ pump Pmc1p confers echinocandin tolerance in Candida albicans through enhancing calcineurin-based responses.

Signaling through the calcium-activated calcineurin phosphatase promotes fungal survival of stressful conditions including those imposed by antifungal medications. Calcium is an essential secondary messenger that regulates diverse cellular processes in eukaryotes; however, it is also profoundly toxic and cytoplasmic concentrations must be tightly controlled. In fungi, the vacuole serves as a major calcium reservoir with the H+-exchanger Vcx1p and P-type ATPase Pmc1p sequestering intracellular calcium. Upon stimulation, these stores can be released through the Yvc1p ion channel to create transient cytoplasmic pulses that activate calcium-dependent responses. Despite the importance of calcineurin signaling in sustaining fungal viability upon antifungal insult, the contribution of many other proteins responsible for intracellular calcium homeostasis during antifungal exposure remains poorly understood. Here, we investigated whether Yvc1p, Vcx1p, and Pmc1p influence Candida albicans capacity to endure exposure to the first-line echinocandin antifungals. Our results demonstrate that loss of Pmc1p function confers high-levels of tolerance, with pmc1?/? mutant cells sustaining less damage, surviving and capable of proliferation in the presence of supra-MIC concentrations of the echinocandins. Moreover, this phenotype is dependent upon elevated signaling through the calcineurin pathway. Finally, we demonstrate that at least a subset of drugs previously identified as echinocandin antagonists activate calcineurin signaling in a Pmc1-dependent manner to drive echinocandin tolerance. These findings reveal how genetic or pharmacological modulation of calcium homeostasis can have profound consequences on the outcome of C. albicans - echinocandin interaction.