The Impact of Aging, Exercise and Muscle Disuse on Skeletal Muscle Autophagy, Mitophagy and Lysosome Biogenesis

Inactivity and age-related sarcopenia similarly lead to a loss of muscle mass and function. This has largely been attributed to deficits in the synthesis of proteins and organelles, yet the processes that govern the clearance of such constituents remain unexplored. The maintenance of the latter is integral, as damaged proteins/organelles perpetuate functional impairments within the tissue. The autophagy-lysosome system (ALS), which entails the selective tagging of damaged proteins and organelles, followed by their digestion in the lysosomes, is one such method. When confined to the mitochondria, the energy producing organelles, this process is termed mitophagy. Our work is focused on the regulation of intracellular degradation through the ALS, in both disuse and aging models, with implications for mitochondrial and tissue health. To examine the dynamic effects of muscle disuse, we unilaterally denervated the hindlimb muscle of rats for 1, 3 or 7 days. Our results indicate that autophagy and mitophagy flux are biphasic, being upregulated in the early time points (i.e., 1 and 3 days) and downregulated at the latter time point. Increases in lysosomal protein levels were promoted by the upregulation and nuclear activation of the transcriptional regulator of lysosomal protein synthesis (Tfeb). Utilizing electron microscopy, we measured an increase in vacuolar inclusions, indicative of lysosomal dysfunction with prolonged denervation. In aged muscle we similarly report elevations in ALS components and higher nuclear TFEB versus young mice. Uniquely, female mice had a greater abundance of ALS-related proteins and indices of autophagosomal turnover. This indicates that biological sex influences the capacity for autophagy. Paradoxically, catabolic events are also transiently upregulated in response to exercise, serving to "prune" the tissue's faulty parts, including the mitochondria. Thus, we subjected young and aged mice to exhaustive exercise. Young male mice were able to activate autophagy and lysosome biogenesis to a greater extent than female counterparts. This effect was blunted in aged mouse muscle, independent of sex. We have uncovered how autophagy and mitophagy are differentially regulated in denervated and aged muscle. Further, we were able to show that biological sex influences the regulation of the autophagy-lysosome system in young, aged, and exercised muscle.