Lysosomal Alterations in Skeletal Muscle Plasticity – An Investigation of Age, Exercise and Disuse
Authors: Triolo Matthew , Slavin Mikhaela , Kim Yuho , Carter Heather N. , Hood David A. 
Published: April 2020

Skeletal muscle displays a high degree of plasticity in response to a variety of contractile activity stimuli. This is exemplified in response to exercise training, in which muscle mitochondrial content is enhanced to drive an increase in metabolic capacity. The opposite is true with chronic muscle disuse and advancing age, in which the muscle loses its oxidative capacity. This plasticity is regulated by alterations in the synthesis and degradation of mitochondria. A major intracellular degradation system is the autophagy‐lysosome pathway. This includes the targeting of damaged intracellular components by autophagosomes, followed by delivery of these to the lysosomes for degradation. However, there is little understanding of the regulation of the lysosomes in response to physiological stimuli. Thus, our objective is to understand how muscle lysosomes are regulated in response to physiological stimuli, including exercise, disuse and age. Thus, we utilized unilateral chronic contractile activity (CCA;7 days) to elicit endurance exercise adaptations, and denervation of the peroneal nerve (7 days) to elicit disuse adaptations. These opposing activity paradigms resulted in parallel increases (50%) and decreases (50%) in mitochondrial content within muscle, respectively. To assess the effects of age, 36‐month‐old rats and 24‐month‐old mice were used. CCA elicited increases in lysosomal proteins Lamp1, Lamp2 and Cathepsin D by 2.9‐,1.8‐ and 1.3‐fold respectively, an effect that was blunted in aged rats. Both denervated and aged muscle exhibited elevated levels of these proteins, such as 3.0‐ and 1.6‐fold increases in Lamp1 and Lamp2 in denervated muscle, and 5‐fold increases in these proteins with age. To understand how these opposing processes (i.e. CCA vs age/disuse) elicit directionally similar changes in these proteins, we measured TFEB, a transcription factor that drives the formation of lysosomes. CCA elicited modest, 50% elevations in TFEB protein, whereas denervation and aging increased TFEB by 70% and 180%, respectively. However, TFEB localization was elevated differentially, by 10%, 25% and 100% in CCA, denervation and aging, respectively. Importantly, electron micrographs of denervated and aged muscle exhibited evidence of lipofuscin accumulation, suggesting impairments in lysosomal function with these wasting conditions, and this could account for an accumulation of dysfunctional lysosomal proteins. To evaluate the transcriptional control of lysosomal biogenesis, young and aged mice were injected with the TFEB‐promoter luciferase reporter. Acute exercise elevated TFEB promoter activity by 1.6‐fold in young muscle, and 2.4‐fold in aged muscle, whereas nuclear TFEB was enhanced to a greater extent in young muscle. This could explain the blunted CCA‐induced lysosomal adaptations in aged muscle. Our data suggest increases in lysosomal proteins are evident with both enhanced (i.e. CCA) and reduced (i.e denervation and age) contractile stimuli. Future work will evaluate whether the changes in lysosomal proteins represent functional lysosomes or dysfunctional accumulation of organelles.

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NSERC and CIHR