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. / Doctor of Philosophy / Skeletal muscle is responsible for approximately 20% of basal energy expenditure and 70-90% of insulin-mediated glucose disposal, and as such changes in skeletal muscle metabolism and insulin sensitivity have profound impacts on whole body metabolism. Skeletal muscle is a plastic tissue that can perceive nutrient availability, which permits metabolic adaptations to environmental changes. Deletion of the nutrient sensing pathway O-GlcNAcylation in skeletal muscle (mKO) protected mice from high-fat diet induced obesity and ameliorates whole-body insulin sensitivity. Skeletal muscle can secrete myokines to elicit endocrine effects on other tissues in the body, and as such, we proposed perturbation of this nutrient sensing pathway in skeletal muscle alters myokine secretion to elicit responses in other metabolically active tissues to support its energy requirements. Indeed, circulating levels of interleukin-15, a potent anti-obesity myokine, increased 3-fold in mKO mice. To determine the contribution of IL-15 to the mKO phenotype, we used a genetic approach to blunt IL-15 secretion from skeletal muscle (mDKO), which partially negated the lean mKO phenotype. Our findings show the ability of skeletal muscle to "sense" changes in nutrients through O-GlcNAcylation is necessary for proper muscle and whole-body metabolism. Moreover, this nutrient sensing mechanism is also important for proper muscle stem cell function, also known as satellite cells (SCs). Loss of O-GlcNAcylation in SCs impairs their ability to regenerate muscle after injury, which can be attributed to a reduced capacity to proliferate and an inability to maintain a healthy SC population. Interestingly, SCs lacking O-GlcNAcylation have a greater mitochondrial content. Using a myoblast cell line, we investigated the contribution of mitochondria to myogenesis, the formation of muscle, and found mitochondrial energy production is dispensable in the myogenic process. Our studies show skeletal muscle and SCs rely on highly integrated signaling cascades that sense and respond to intrinsic metabolic changes and extrinsic nutritional cues to function properly.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101750 |
| Date | 05 January 2021 |
| Creators | Zumbaugh, Morgan Daughtry |
| Contributors | Animal and Poultry Sciences, Gerrard, David E., Shi, Tim Hao, Rhoads, Robert P., Johnson, Sally E. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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