Regulation and function of feeding-induced mTORC1 in skeletal muscle

Abstract

Skeletal muscle is crucial to metabolic health. Environmental cues, such as feeding and physical activity, are essential for the maintenance of muscle mass. These cues converge on the signaling node, mammalian target of rapamycin (mTOR) complex 1 (mTORC1). This signaling node is the primary site of regulation for anabolism in cells and most tissues. Furthermore, mTORC1 is involved in suppressing cellular catabolism, complicating the role of mTORC1 in balancing cell size. This complexity has limited understanding of the physiological role of mTORC1 in regulating skeletal muscle mass. Here, I utilize genetic models to dissect the role of feeding-induced mTORC1 activation to discover how this regulates skeletal muscle mass and function. Exogenous growth factors and hormones activate mTORC1 in response to various signals (e.g. insulin in response to feeding). This occurs through phosphorylation of 5 residues on TSC2, which abrogates TSC2-mediated inhibition of mTORC1. I utilized a phosphorylation-site mutant mouse model (TSC2-5A) to investigate how AKT regulates mTORC1 during development, across the whole body, and in a skeletal muscle-specific fashion. In collaborative studies, I show that it is AKT’s ability to phosphorylate TSC2 that determines if mTORC1 is activated upon refeeding in skeletal muscle. Loss of this ability throughout all cell types results in adult mice with a decrease in lean mass, skeletal muscle mass, and muscle fiber size. Loss of postprandial mTORC1 activation corresponds to loss of the nearly 50% increase in protein synthesis induced by feeding in wild-type mice. Primary myotubes derived from TSC2-5A mice show a decrease in both basal and insulin-stimulated rates of protein synthesis. These data suggest that loss of AKT-mediated phosphorylation of TSC2 reduces mTORC1 activation and postprandial protein synthesis resulting in smaller muscle fibers. To investigate the tissue-specific role of AKT-mediated phosphorylation of TSC2 to regulate mTORC1, I generated skeletal muscle specific phospho-mutant mice (SkM-TSC2-5A). These mice showed the same lack of postprandial mTORC1 activation, despite an elevated activation of AKT, and loss of postprandial protein synthesis as documented in the whole-body phospho-mutant mice. Despite this, SkM-TSC2-5A mice show no difference in body weight, lean mass, muscle weight, or myofiber size relative to their wild-type counterparts. I show that the AKT-mediated phosphorylation sites on TSC2 are dispensable for the mechanical activation of mTORC1, leading to the conclusion that postprandial mTORC1 activation and protein synthesis are dispensable in skeletal muscle that maintains sensitivity to mechanical stimuli. Lastly, I show that the skeletal muscle-specific loss of these sites results in enhanced endurance capacity without changes in muscle strength. This is not due to changes in glycogen content or muscle fiber type but is accompanied by a modest increase in skeletal muscle mitochondrial content, which could underlie this phenotype. In sum, this study defines AKT-mediated phosphorylation of TSC2, not nutrients, as the dominant signal underlying feeding-induced mTORC1 activation in skeletal muscle. Furthermore, feeding-induced mTORC1 activation is dispensable for skeletal muscle mass and strength; instead, it may have a negative effect on endurance capacity in mice.