Requirement for the mitochondrial pyruvate carrier in mammalian development revealed by a hypomorphic allelic series

Glucose and oxygen are two of the most important molecules transferred from mother to fetus during eutherian pregnancy, and the metabolic fates of these nutrients converge at the transport and metabolism of pyruvate in mitochondria. Pyruvate enters the mitochondrial matrix through the mitochondrial pyruvate carrier (MPC), a complex in the inner mitochondrial membrane that consists of two essential components, MPC1 and MPC2. Here, we define the requirement for mitochondrial pyruvate metabolism during development with a progressive allelic series of Mpc1 deficiency in mouse. Mpc1 deletion was homozygous lethal in midgestation, but Mpc1 hypomorphs and tissue-specific deletion of Mpc1 presented as early perinatal lethality. The allelic series demonstrated that graded suppression of MPC resulted in dose-dependent metabolic and transcriptional changes. Steady-state metabolomics analysis of brain and liver from Mpc1 hypomorphic embryos identified compensatory changes in amino acid and lipid metabolism. Flux assays in Mpc1-deficient embryonic fibroblasts also reflected these changes, including a dramatic increase in mitochondrial alanine utilization. The mitochondrial alanine transaminase GPT2 was found to be necessary and sufficient for increased alanine flux upon MPC inhibition. These data show that impaired mitochondrial pyruvate transport results in biosynthetic deficiencies that can be mitigated in part by alternative anaplerotic substrates in utero.