Autophagy and Protein Turnover Responses to Exercise-Nutrient Interactions in Human Skeletal Muscle

Skeletal muscle is a dynamic tissue comprising the largest protein reservoir of the human body with a rate of turnover of ~1-2% per day. Protein turnover is regulated by the coordination of intracellular systems regulating protein synthesis and breakdown that converge in a spatiotemporal manner on lysosomal organelles responsible for integrating a variety of contractile and nutritional stimuli. One such system, autophagy, which literally means to ‘self-eat,’ involves capturing of cellular material for deliver to, and disintegration by, the lysosome. The autophagic ‘cargo’ is subsequently recycled for use in synthetic reactions and thus maintenance of protein balance. As a dynamic system, autophagy responds to intracellular perturbations to homeostasis elicited by exercise and changes in nutrient availability in an attempt to restore energy balance. However, little is known regarding how autophagy is modulated following exercise in response to changes in nutrient availability. This thesis is comprised of three independent studies in which the effects of divergent forms of exercise-nutrient interactions are investigated in relation to autophagy-mediated protein turnover processes in skeletal muscle.

Study 1 assessed whether resistance-based exercise undertaken following a short-term period of dietary energy restriction activates autophagic cell signalling, and whether high-protein availability during recovery from exercise attenuates the autophagic response. This latter supposition was based on the anabolic properties of amino acids that may temporarily repress autophagy in vitro. In contrast to one of the original hypotheses, protein availability promoted the largest accumulation of proteins implicated in the induction of skeletal muscle autophagy and thus, turnover-remodelling, which is required to support the elevated synthetic demands imposed by resistance exercise contraction in a state of energy deficit.

Study 2 investigated the effects of alcohol intoxication during recovery from vigorous exercise on autophagy and whether concomitant protein availability could ‘rescue’ alcohol-exposed muscle tissue from the toxic effects of alcohol metabolism. It was hypothesised that the largest autophagic response (e.g., the accumulation of specific autophagy-related proteins in different subcellular compartments) would be seen when alcohol with carbohydrate, but not protein, was co-ingested, thereby promoting greater rates of protein degradation. However, the results from this study showed that alcohol availability consistently attenuated the abundance of numerous autophagy-related proteins that culminated in cell death responses. Protein availability, in part, through a compensatory induction of mitochondrial biogenesis, facilitated ‘sparing’ of the alcohol-exposed tissue from these deleterious effects of alcohol metabolism, thus revealing the intrinsic anti-apoptotic effects of exogenous protein. While excess alcohol consumption should be avoided following sport and/or exercise training, protein co-ingestion may relieve some of the intracellular damage and facilitate recovery-adaption.

Study 3 investigated the impact of acutely elevating systemic fatty acid availability and its impact on skeletal muscle protein turnover. Participants received either a lipid infusion with or without the addition of exercise, or a saline control, and following these infusions ingested a bolus amount of protein. It was hypothesised that lipid availability would attenuate markers of cellular anabolism (i.e., translation initiation) that would be ameliorated by exercise. Whereas the lipid infusion alone induced an elevated autophagic flux, combining the lipid infusion with exercise inhibited this activation of autophagy and, in response to protein ingestion, promoted the largest intracellular anabolic protein translational response. In addition, exercise performed during the lipid infusion resulted in a novel mitochondria-specific autophagic response independent of canonical routes of autophagic degradation. Therefore, the anabolic sensitivity of skeletal muscle to protein ingestion, despite high-circulating free fatty acids, was ‘rescued’ by strenuous exercise performed during this infusion and was associated with the disposal of mitochondrial organelles presumably damaged by lipid availability. Combining strenuous exercise with high-protein availability in the context of excess circulating lipids is a powerful stimulus for promoting muscle protein turnover-remodelling.

Taken collectively, the results from this thesis demonstrate that nutrient availability alters the responsiveness of skeletal muscle protein turnover, in particular autophagy, to exercise stimuli. The optimal nutrient ‘pairing’ with exercise in regards to optimising muscle quality and quantity for athletes and non-athletes alike, in the context of dietary energy restriction or excess alcohol and fat availability, is the consumption of high-quality sources of protein.