Alternate Pathways for Inorganic Phosphate Uptake into Mitochondria

Abstract

The mitochondrial phosphate carrier (PiC), encoded by the nuclear gene SLC25A3, was purified more than 30 years ago. It is widely believed that PiC serves as the primary means of inorganic phosphate (Pi) uptake into mitochondria for oxidative phosphorylation (oxphos) and for buffering the vast amount of calcium that mitochondria can take up. Yet, the recent discovery of mutations in human PiC, and the development of a PiC floxed mouse only now allow PiC function to be directly studied in vivo. Human SLC25A3 mutations produce a severe clinical phenotype at birth and with striated muscle as a key affected tissue, and mice with cardiac-specific PiC loss eventually develop abnormal cardiac function. Yet, evidence of ample ATP and near normal function despite PiC depletion in humans and mice with PiC depletion suggest that PiC expression far exceeds Pi needs and/or that functional compensation can be substantial. Here we test the hypothesis that Pi enters mitochondria by alternate route(s), allowing substantial mitochondrial ATP production despite near absence of PiC. We generated mice with Tamoxifen(Tam)-inducible skeletal muscle (Skm) PiC knockdown (Tam+Cre+); 3 weeks after Tam, PiC protein was 95% PiC loss; maximal oxphos was ∼35% of control with pyruvate+malate as substrate and rose to ∼50% of control with fatty acid+malate. In contrast, maximal oxphos in Tam+Cre+ mitochondria was abolished with succinate as substrate. Together these observations suggest that, in the context of PiC loss, compensatory Pi uptake may be occur via the dicarboxylate carrier, which exchanges succinate or malate for Pi. Whether this occurs in vivo requires investigation.