Loss of mitochondrial calcium uniporter rewires skeletal muscle metabolism and substrate preference.

Skeletal muscle mitochondria readily accumulate Ca2+ in response to SR store-releasing stimuli thanks to the activity of the mitochondrial calcium uniporter (MCU), the highly selective channel responsible for mitochondrial Ca2+ uptake. MCU positively regulates myofiber size in physiological conditions and counteracts pathological loss of muscle mass. Here we show that skeletal muscle-specific MCU deletion inhibits myofiber mitochondrial Ca2+ uptake, impairs muscle force and exercise performance, and determines a slow to fast switch in MHC expression. Mitochondrial Ca2+ uptake is required for effective glucose oxidation, as demonstrated by the fact that in muscle-specific MCU-/- myofibers oxidative metabolism is impaired and glycolysis rate is increased. Although defective, mitochondrial activity is partially sustained by increased fatty acid (FA) oxidation. In MCU-/- myofibers, PDP2 overexpression drastically reduces FA dependency, demonstrating that decreased PDH activity is the main trigger of the metabolic rewiring of MCU-/- muscles. Accordingly, PDK4 overexpression in MCUfl/fl myofibers is sufficient to increase FA-dependent respiration. Finally, as a result of the muscle-specific MCU deletion, a systemic catabolic response impinging on both liver and adipose tissue metabolism occurs.