ER-mitochondria distance is a critical parameter for efficient mitochondrial Ca2+ uptake and oxidative metabolism

IP₃ receptor (IP₃R)-mediated Ca²⁺ transfer at the mitochondria-endoplasmic reticulum (ER) contact sites (MERCS) drives mitochondrial Ca²⁺ uptake and oxidative metabolism and is linked to different pathologies, including Parkinson's disease (PD). The dependence of Ca²⁺ transfer efficiency on the ER-mitochondria distance remains unexplored. Employing molecular rulers that stabilize ER-mitochondrial distances at 5 nm resolution, and using genetically encoded Ca²⁺ indicators targeting the ER lumen and the sub-mitochondrial compartments, we now show that a distance of ~20 nm is optimal for Ca²⁺ transfer and mitochondrial oxidative metabolism due to enrichment of IP₃R at MERCS. In human iPSC-derived astrocytes from PD patients, 20 nm MERCS were specifically reduced, which correlated with a reduction of mitochondrial Ca²⁺ uptake. Stabilization of the ER-mitochondrial interaction at 20 nm, but not at 10 nm, fully rescued mitochondrial Ca²⁺ uptake in PD astrocytes. Our work determines with precision the optimal distance for Ca²⁺ flux between ER and mitochondria and suggests a new paradigm for fine control over mitochondrial function.