Polo Kinase Phosphorylates Miro to Control ER-Mitochondria Contact Sites and Mitochondrial Ca2+ Homeostasis in Neural Stem Cell Development

Mitochondria play central roles in buffering intracellular Ca2+ transients. While basal mitochondria! Ca2+ (Ca-mito(2+)) is needed to maintain organellar physiology, Ca-mito(2+) overload can lead to cell death. How Ca-mito(2+) homeostasis is regulated is not well understood. Here we show that Miro, a known component of the mitochondrial transport machinery, regulates Drosophila neural stem cell (NSC) development through Ca-mito(2+) homeostasis control, independent of its role in mitochondrial transport. Miro interacts with Ca2+ transporters at the ER-mitochondria contact site (ERMCS). Its inactivation causes Ca-mito(2+) depletion and metabolic impairment, whereas its overexpression results in Ca-mito(2+) overload, mitochondrial morphology change, and apoptotic response. Both conditions impaired NSC lineage progression. Ca-mito(2+) homeostasis is influenced by Polo-mediated phosphorylation of a conserved residue in Miro, which positively regulates Miro localization to, and the integrity of, ERMCS. Our results elucidate a regulatory mechanism underlying Ca-mito(2+) homeostasis and how its dysregulation may affect NSC metabolism/development and contribute to disease.