The transport of pyruvate into mitochondria requires a specific carrier, the mitochondrial pyruvate carrier (MPC). The MPC represents a central node of carbon metabolism, and its activity is likely to play a key role in bioenergetics. Until now, investigation of the MPC activity has been limited. However, the recent molecular identification of the components of the carrier has allowed us to engineer a genetically encoded biosensor and to monitor the activity of the MPC in real time in a cell population or in a single cell. We report that the MPC activity is low in cancer cells, which mainly rely on glycolysis to generate ATP, a characteristic known as the Warburg effect. We show that this low activity can be reversed by increasing the concentration of cytosolic pyruvate, thus increasing oxidative phosphorylation. This biosensor represents a unique tool to investigate carbon metabolism and bioenergetics in various cell types.

Mitochondrial import of pyruvate by the mitochondrial pyruvate carrier (MPC) is a central step which links cytosolic and mitochondrial intermediary metabolism. To investigate the role of the MPC in mammalian physiology and development, we generated a mouse strain with complete loss of MPC1 expression. This resulted in embryonic lethality at around E13.5. Mouse embryonic fibroblasts (MEFs) derived from mutant mice displayed defective pyruvate-driven respiration as well as perturbed metabolic profiles, and both defects could be restored by reexpression of MPC1. Labeling experiments using 13C-labeled glucose and glutamine demonstrated that MPC deficiency causes increased glutaminolysis and reduced contribution of glucose-derived pyruvate to the TCA cycle. Morphological defects were observed in mutant embryonic brains, together with major alterations of their metabolome including lactic acidosis, diminished TCA cycle intermediates, energy deficit and a perturbed balance of neurotransmitters. Strikingly, these changes were reversed when the pregnant dams were fed a ketogenic diet, which provides acetyl-CoA directly to the TCA cycle and bypasses the need for a functional MPC. This allowed the normal gestation and development of MPC deficient pups, even though they all died within a few minutes post-delivery. This study establishes the MPC as a key player in regulating the metabolic state necessary for embryonic development, neurotransmitter balance and post-natal survival.

Peroxisome proliferator-activated receptors (PPARs) have been suggested as the master regulators of adipose tissue formation. However, their role in regulating brown fat functionality has not been resolved. To address this question, we generated mice with inducible brown fat-specific deletions of PPARα, β/δ, and γ, respectively. We found that both PPARα and β/δδ are dispensable for brown fat function. In contrast, we could show that ablation of PPARγ in vitro and in vivo led to a reduced thermogenic capacity accompanied by a loss of inducibility by β-adrenergic signaling, as well as a shift from oxidative fatty acid metabolism to glucose utilization. We identified glycerol kinase (Gyk) as a partial mediator of PPARγ function and could show that Gyk expression correlates with brown fat thermogenic capacity in human brown fat biopsies. Thus, Gyk might constitute the link between PPARγ-mediated regulation of brown fat function and activation by β-adrenergic signaling.

Identification of pre-diabetic individuals with decreased functional ß-cell mass is essential for the prevention of diabetes. However, in vivo detection of early asymptomatic ß-cell defect remains unsuccessful. Metabolomics emerged as a powerful tool in providing read-outs of early disease states before clinical manifestation. We aimed at identifying novel plasma biomarkers for loss of functional ß-cell mass in the asymptomatic pre-diabetic stage. Non-targeted and targeted metabolomics were applied on both lean ß-Phb2 -/mice (ß-cell-specific prohibitin-2 knockout) and obese db/db mice (leptin receptor mutant), two distinct mouse models requiring neither chemical nor diet treatments to induce spontaneous decline of functional ß-cell mass promoting progressive diabetes development.Non-targeted metabolomics on ß-Phb2 -/mice identified 48 and 82 significantly affected metabolites in liver and plasma, respectively. Machine learning analysis pointed to deoxyhexose sugars consistently reduced at the asymptomatic pre-diabetic stage, including in db/db mice, showing strong correlation with the gradual loss of ß-cells. Further targeted metabolomics by GC-MS uncovered the identity of the deoxyhexose with 1,5-anhydroglucitol displaying the most significant changes. In conclusion, this study identified 1,5-anhydroglucitol associated with the loss of functional ß-cell mass and uncovered metabolic similarities between liver and plasma, providing insights into the systemic effects caused by early decline in ß-cells.