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 (∼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 (cch1Δ or mid1Δ) that reduce Ca²⁺ influx, indicating that Ca²⁺ Ca_ext2+ is a source for the transient. An osmotic pretreatment (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 cch1Δ 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, cch1Δ mutation, and FK506, indicating that the [Ca²⁺]_cyt transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance.