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 transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP) receptors (IP3R) can locally expose the mitochondrial Ca uniporter (mtCU) to high [Ca] nanodomains. The Ca loading state of the ER (Ca) can vary broadly in physiological and pathological scenarios, however, the correlation between Ca and the local Ca transfer is unclear. Here, we studied IP-induced Ca transfer to mitochondria at different Ca in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca] ([Ca]) and mitochondrial matrix [Ca] ([Ca]). Preincubation of intact cells in high versus low extracellular [Ca] caused disproportionally greater increase in [Ca] than [Ca] responses to IP-mobilizing agonist. Increasing Ca by small Ca boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca uptake from IP-induced Ca release. The IP-induced local [Ca] spikes exposing the mitochondrial surface measured using a genetically targeted sensor appeared to linearly correlate with Ca, 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 uptake. Interestingly, the IP-induced [Ca] signal plateaued at high Ca, indicating activation of a matrix Ca binding/chelating species. Mitochondria thus seem to maintain a "working [Ca] range" via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca] signals in this working range.