Signaling pathways regulating skeletal muscle metabolism and growth

Skeletal muscle can perceive cellular energy status and substrate availability and demonstrates remarkable plasticity in response to environmental changes. Nonetheless, how skeletal muscle and its resident stem cells (satellite cells; SCs) sense and respond to nutrient flux remains largely undefined. The dynamic post-translational modification O-GlcNAcylation has been shown to serve as a cellular nutrient sensor in a wide range of cells and tissues, yet its role in skeletal muscle and SCs remains unexplored. Here, we ablated skeletal muscle O-GlcNAc transferase (OGT), and thus O-GlcNAcylation, and found the knockout mice exhibited enhanced glucose uptake, insulin sensitivity, and resistance to high-fat diet induced obesity. Additionally, mKO mice had a 3-fold increase in circulating levels of interleukin-15 (IL-15), a potent anti-obesity cytokine, potentially through epigenetic regulation of Il15 by OGT. To further investigate if there was a causal relationship between OGT ablation and the lean phenotype, we generated muscle specific OGT and interleukin-15 receptor alpha (IL-15ra) double knockout mice (mDKO). As a result, mDKO mice had blunted IL-15 secretion and minimal protection against HFD-induced obesity. Together, these data indicate the skeletal muscle OGT-IL15 axis plays an essential role in the maintenance of skeletal muscle and whole-body metabolic homeostasis.

As satellite cells (SCs) play an indispensable role in postnatal muscle growth and adult regenerative myogenesis, we investigated the role of O-GlcNAcylation in SC function. To this end, we conditionally ablated OGT in SCs (cKO) and found cKO mice had impaired SC proliferation, in vivo cycling properties, population stability, metabolic regulation, and adult regenerative myogenesis. Together these findings show that SCs require O-GlcNAcylation, presumably to gauge nutritional signals, for proper function and metabolic homeostasis.

Another critical yet often neglected player in myogenesis are mitochondria. Traditionally depicted as a power plant in cells, mitochondria are critical for numerous nonconventional, energy-independent cellular process. To investigate the role of both mitochondrial energy production and alternative mitochondrial functions in myogenic regulation, we ablated ATP synthase subunit beta (ATP5b) and ubiquinol-cytochrome c reductase (UQCRFS1) in C2C12 myoblasts to disrupt mitochondrial ATP production and mitochondrial membrane potential, respectively. Ablation of UQCRFS1, but not ATP5b, impaired myoblast proliferation, although lack of either gene compromised myoblast fusion. Interestingly, addition of the potent myogenic stimulator IGF-1 rescued ATP5b fusion but could not override UQCRFS1 knockout effects on proliferation or differentiation. These data demonstrate mitochondrial ATP production is not the "metabolic switch" that governs myogenic progression but rather an alternative mitochondrial function.

In summary, skeletal muscle and their resident stem cell population (SCs) both use O-GlcNAcylation, feasibly to sense and respond to nutritional cues, for the maintenance of metabolic homeostasis and normal physiology. A deeper understand of both muscle and SC metabolic regulation may provide therapeutic targets to improve global metabolism and muscle growth.