Supralinear Dependence of the IP3 Receptor-to-Mitochondria Local Ca2+ Transfer on the Endoplasmic Reticulum Ca2+ Loading.

Calcium signal propagation from endoplasmic reticulum (ER) to mitochondria regulates a multitude of mitochondrial and cell functions, including oxidative ATP production and cell fate decisions. Ca²⁺ transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP₃) receptors (IP3R) can locally expose the mitochondrial Ca²⁺ uniporter (mtCU) to high [Ca²⁺] nanodomains. The Ca²⁺ loading state of the ER (Ca^2 +_ER) can vary broadly in physiological and pathological scenarios, however, the correlation between Ca^2 +_ER and the local Ca²⁺ transfer is unclear. Here, we studied IP₃-induced Ca²⁺ transfer to mitochondria at different Ca^2 +_ER in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca²⁺] ([Ca²⁺]_c) and mitochondrial matrix [Ca²⁺] ([Ca²⁺]_m). Preincubation of intact cells in high versus low extracellular [Ca²⁺] caused disproportionally greater increase in [Ca²⁺]_m than [Ca²⁺]_c responses to IP₃-mobilizing agonist. Increasing Ca^2 +_ER by small Ca²⁺ boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca²⁺ uptake from IP₃-induced Ca²⁺ release. The IP₃-induced local [Ca²⁺] spikes exposing the mitochondrial surface measured using a genetically targeted sensor appeared to linearly correlate with Ca^2 +_ER, indicating that amplification happened in the mitochondria. Indeed, overexpression of an EF-hand deficient mutant of the mtCU gatekeeper MICU1 reduced the cooperativity of mitochondrial Ca²⁺ uptake. Interestingly, the IP₃-induced [Ca²⁺]_m signal plateaued at high Ca^2 +_ER, indicating activation of a matrix Ca²⁺ binding/chelating species. Mitochondria thus seem to maintain a "working [Ca²⁺]_m range" via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca²⁺]_m signals in this working range.