Supralinear Dependence of the IP 3 Receptor-to-Mitochondria Local Ca 2+ Transfer on the Endoplasmic Reticulum Ca 2+ 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.
Competing Interests: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
(© The Author(s) 2024.)