ER-mitochondria distance is a critical parameter for efficient mitochondrial Ca ²⁺ uptake and oxidative metabolism

IP ₃ receptor (IP ₃ R)-mediated Ca ²⁺ transfer at the mitochondria-endoplasmic reticulum (ER) contact sites (MERCS) drives mitochondrial Ca ²⁺ uptake and oxidative metabolism and is linked to different pathologies, including Parkinson’s disease (PD). The dependence of Ca ²⁺ 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 Ca ²⁺ indicators targeting the ER lumen and the sub-mitochondrial compartments, we now show that a distance of ∼20 nm is optimal for Ca ²⁺ transfer and mitochondrial oxidative metabolism due to enrichment of IP ₃ R at MERCS. In human iPSC-derived astrocytes from PD patients, 20 nm MERCS were specifically reduced which correlated with a reduction of mitochondrial Ca ²⁺ uptake. Our work determines with precision the optimal distance for Ca ²⁺ flux between ER and mitochondria and suggests a new paradigm for fine control over mitochondrial function.