Mitochondrial phosphate carrier-dependence of mitochondrial calcium chelation and respiration in skeletal muscle

The past decade has seen remarkable progress in the molecular resolution of the influx and efflux components of mitochondrial Ca ²⁺ handling, but little progress in matrix Ca ²⁺ chelation that is central to mitochondrial calcium signaling. Among possible chelators, inorganic phosphate (Pi) is dynamic, and its uptake can lessen during Ca ²⁺ uptake the rundown of the membrane potential, the primary driving force for Ca ²⁺ uptake. Thus, PiC, the major mitochondrial Pi transporter, is well-positioned to regulate mitochondrial Ca ²⁺ handling. To test this, we depleted (KD) PiC in murine skeletal muscle. We show that PiC depletion enhances the matrix free Ca ²⁺ rise across a range of Ca ²⁺ uptake activities, and surprisingly, causes elevated mitochondrial Ca ²⁺ uptake, which seems to arise from an increased abundance of the mitochondrial Ca ²⁺ uniporter. With protein levels of non-mitochondrial Ca ²⁺ handling proteins unaltered, the greater mitochondrial Ca ²⁺ uptake may drive a suppressed cytoplasmic Ca ²⁺ response to tetanic stimulation, via lesser Ca ²⁺ -mediated positive feedback on Ca ²⁺ release channels, contributing to an exercise deficit in KD mice. Alternatively, less buffering of matrix Ca ²⁺ might be beneficial. We test these possibilities by lowering MCU, the uniporter’s pore forming subunit, in skeletal muscle of adult PiC KD mice, and find a worsened exercise deficit. This study establishes the requirement for PiC to maintain a bound fraction of Ca ²⁺ in the matrix, and reveals a fitness benefit for elevated [Ca ²⁺ ]m in striated muscle depleted of PiC.