The adoption of a regular exercise program has immense benefits for whole body health, and for improving the quality of skeletal muscle. This is important as muscle is involved in metabolism, locomotion, and force production, making it a large contributor to the quality of life. The coordinated behavior of several intracellular organelles is responsible for the maintenance of skeletal muscle health, and these organelles are adaptable in response to both acute and chronic exercise. While the adaptations of mitochondria to exercise are well-established, potential alterations in muscle lysosomes are less appreciated. Lysosomes degrade and recycle debris during the terminal step of various forms of autophagy, such as mitophagy, the pathway involved in the removal of dysfunctional mitochondria. This lysosomal activity is important for the maintenance of cellular protein and organelle homeostasis. Recent work has shown that lysosome biogenesis begins with every acute bout of exercise, driven by the nuclear translocation of regulatory transcription factors such as TFEB and TFE3, which mediate the transcription of autophagy and lysosomal genes. These transcription factors also play a role in other pathways such as chaperone-mediated autophagy (CMA) and the regeneration of existing lysosomes through the autophagic-lysosome reformation (ALR) pathway. When performed repeatedly, acute bouts of exercise elicit a longer-term adaptive response, leading to the formation of active lysosomes, which increase lysosomal degradative capacity in skeletal muscle. This review addresses the current knowledge surrounding the effects of acute and chronic exercise on lysosomal adaptations in skeletal muscle, highlighting a novel pathway of muscle plasticity.

Efficient signal transduction that mediates mitochondrial turnover is a strong determinant of metabolic health in skeletal muscle. Of these pathways, our focus was aimed towards the enhancement of antioxidant capacity, mitophagy, and mitochondrial biogenesis. While physical activity is an excellent inducer of mitochondrial turnover, its ability to ubiquitously activate and enhance mitochondrial turnover prevents definitive differentiation of the contribution made by each pathway. Therefore, we employed three agents, Sulforaphane (SFN), Urolithin A (UroA), and ZLN005 (ZLN), which are activators of important biological markers involved in antioxidant signaling, mitophagy, and biogenesis, respectively. We investigated the time-dependent changes in proteins related to each mechanism in C2C12 myotubes. SFN treatment resulted in increased nuclear localization of the transcription factor Nuclear factor (erythroid-derived 2)-like 2 (Nrf-2) after 4 ​hour (h), with subsequent 2-fold increases in the antioxidant enzymes Nicotinamide Quinone Oxidoreductase 1 (NQO1) and Heme-Oxygenase-1 (HO-1) by 24 ​h and 48 ​h. Mitochondrial respiration and ATP production were significantly increased by both 24 h and 48 ​h. UroA showed a 2-fold increase in AMP-activated Protein Kinase (AMPK) after 4 ​h, which led to a modest 30% increase in whole cell mitophagy markers p62 and LC3, after 48 ​h. This was accompanied by a reduction in cellular Reactive Oxygen Species (ROS), detected with the CellROX Green reagent. Mitophagy flux measurements showed mitophagy activation as both LC3-Ⅱ and p62 flux increased with UroA at 24-h and 48-h time points, respectively. Finally, AMPK activation was observed by 4 ​h, in addition to a 2-fold increase in Mitochondrial Transcription Factor A (TFAM) promoter activity by 24 ​h of ZLN treatment following transient transfection of a TFAM promoter-luciferase construct. Mitochondrial respiration and ATP production were enhanced by 24 ​h. Our results suggest that early time points of treatment increase upstream pathway activity, whereas later time points represent the increased phenotypic expression of related downstream markers. Our findings suggest that the spatiotemporal progression of these mechanisms following drug treatment is another important factor to consider when examining subcellular changes towards mitochondrial turnover in muscle.