Supralinear Dependence of the IP Receptor-to-Mitochondria Local Ca Transfer on the Endoplasmic Reticulum Ca 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. Ca 2+ transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP 3 ) receptors (IP3R) can locally expose the mitochondrial Ca 2+ uniporter (mtCU) to high [Ca 2+ ] nanodomains. The Ca 2+ 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 Ca 2+ transfer is unclear. Here, we studied IP 3 -induced Ca 2+ transfer to mitochondria at different Ca 2 + ER in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca 2+ ] ([Ca 2+ ] c ) and mitochondrial matrix [Ca 2+ ] ([Ca 2+ ] m ). Preincubation of intact cells in high versus low extracellular [Ca 2+ ] caused disproportionally greater increase in [Ca 2+ ] m than [Ca 2+ ] c responses to IP 3 -mobilizing agonist. Increasing Ca 2 + ER by small Ca 2+ boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca 2+ uptake from IP 3 -induced Ca 2+ release. The IP 3 -induced local [Ca 2+ ] 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 Ca 2+ uptake. Interestingly, the IP 3 -induced [Ca 2+ ] m signal plateaued at high Ca 2 + ER , indicating activation of a matrix Ca 2+ binding/chelating species. Mitochondria thus seem to maintain a “working [Ca 2+ ] m range” via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca 2+ ] m signals in this working range.