An osmotically induced cytosolic Ca2+ transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance of Saccharomyces cerevisiae

Hyperosmotic stress caused by NaCl, LiCl, or sorbitol induces an immediate and short duration (similar to 1 min) transient cytosolic Ca2+ ([Ca2+](cyt)) increase (Ca2+-dependent aequorin luminescence) in Saccharomyces cerevisiae cells. The amplitude of the osmotically induced [Ca2+] cyt transient was attenuated by the addition of chelating agents EGTA or BAPTA, cation channel pore blockers, competitive inhibitors of Ca2+ transport, or mutations (cch1Delta or mid1Delta) that reduce Ca2+ influx, indicating that Ca2+ is a source for the transient. An osmotic pre-treatment (30 min) administered by inoculating cells into media supplemented with either NaCl (0.4 or 0.5 m) or sorbitol (0.8 or 1.0 m) enhanced the subsequent growth of these cells in media containing 1 m NaCl or 2 m sorbitol. Inclusion of EGTA in the osmotic pretreatment media or the cch1Delta mutation reduced cellular capacity for NaCl but not hyperosmotic adaptation. The stress-adaptive effect of hyperosmotic pretreatment was mimicked by exposing cells briefly to 20 mm CaCl2. Thus, NaCl- or sorbitol-induced hyperosmotic shock causes a [Ca2+](cyt) transient that is facilitated by Ca2+ influx, which enhances ionic but not osmotic stress adaptation. NaCl-induced ENA1 expression was inhibited by EGTA, cch1Delta mutation, and FK506, indicating that the [Ca2+](cyt) transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance.