Effects of UCP3 overexpression on mouse muscle mitochondrial function in-vivo

Uncoupling protein 3 (UCP3) is a mitochondrial trans-membrane protein that is highly expressed in muscle. It shares high homology with UCP1, which mediates thermogenesis in brown adipose tissue, and it has been proposed that UCP3 may also dissipate the proton electrochemical gradient across the inner mitochondrial membrane and ‘uncouple’ mitochondrial oxidation and ATP synthesis. Modulation of UCP3 expression alters oxygen consumption and proton leak in isolated mitochondria in vitro, but its precise function in-vivo has yet to be fully elucidated. The aim of this study was to determine whether muscle-specific overexpression of UCP3 in transgenic (TG) mice alters the coupling of mitochondrial oxidative phosphorylation in vivo. Energy production was assessed in the hindlimb of anesthetized control (WT) and UCP3-TG mice by determining the rate of Pi → ATP flux (VATP) using 31P saturation-transfer MRS. In a separate series of experiments, substrate oxidation via the tricarboxylic acid cycle (VTCA) was determined using a novel biopsy-based technique. Anesthetized mice were infused with 2-13C acetate for varying durations up to 90 minutes; plasma samples and the soleus-gastrocnemius muscle complex of each hindlimb were obtained at different intervals during the infusion from each mouse. Concentrations and 13C enrichments of the muscle metabolite pools were measured by 1H[13C]-MRS of the extracted tissue at 500MHz. VTCA was calculated by metabolic modeling of the time-courses of enrichment of the muscle 13C4-glutamate, 13C3-glutamate and 13C4-glutamine pools. VATP was decreased by 19% in the muscle of UCP3-TG mice compared to WT mice (4.55 ± 0.29 vs 5.65 ± 0.27 μmol/(g-min), P=0.02). In contrast, Monte-Carlo analysis of the metabolic modeling data indicated that VTCA was increased by 22% in the muscle of UCP3-TG mice with respect to WT mice (120.3 ± 9.9 vs 94.3 ± 5.8 nmol/(g-min), P=0.014). CONCLUSION: Overexpression of UCP3 in the muscle of TG mice increased rates of mitochondrial substrate oxidation and decreased energy production in vivo, signifying reduced mitochondrial efficiency. These data are consistent with UCP3 mediating the uncoupling of oxidative phosphorylation in these mice.