Na⁺ controls hypoxic signalling by the mitochondrial respiratory chain

All metazoans depend on the consumption of O₂ by the mitochondrial oxidative phosphorylation system (OXPHOS) to produce energy. In addition, the OXPHOS uses O₂ to produce reactive oxygen species that can drive cell adaptations^1-4, a phenomenon that occurs in hypoxia^4-8 and whose precise mechanism remains unknown. Ca²⁺ is the best known ion that acts as a second messenger⁹, yet the role ascribed to Na⁺ is to serve as a mere mediator of membrane potential¹⁰. Here we show that Na⁺ acts as a second messenger that regulates OXPHOS function and the production of reactive oxygen species by modulating the fluidity of the inner mitochondrial membrane. A conformational shift in mitochondrial complex I during acute hypoxia¹¹ drives acidification of the matrix and the release of free Ca²⁺ from calcium phosphate (CaP) precipitates. The concomitant activation of the mitochondrial Na⁺/Ca²⁺ exchanger promotes the import of Na⁺ into the matrix. Na⁺ interacts with phospholipids, reducing inner mitochondrial membrane fluidity and the mobility of free ubiquinone between complex II and complex III, but not inside supercomplexes. As a consequence, superoxide is produced at complex III. The inhibition of Na⁺ import through the Na⁺/Ca²⁺ exchanger is sufficient to block this pathway, preventing adaptation to hypoxia. These results reveal that Na⁺ controls OXPHOS function and redox signalling through an unexpected interaction with phospholipids, with profound consequences for cellular metabolism.