Neuromuscular electrical stimulation enhances the ability of serum extracellular vesicles to regenerate aged skeletal muscle after injury.

Exercise promotes healthy aging of skeletal muscle. This benefit may be mediated by youthful factors in the circulation released in response to an exercise protocol. While numerous studies to date have explored soluble proteins as systemic mediators of rejuvenating effect of exercise on tissue function, here we showed that the beneficial effect of skeletal muscle contractile activity on aged muscle function is mediated, at least in part, by regenerative properties of circulating extracellular vesicles (EVs). Muscle contractile activity elicited by neuromuscular electrical stimulation (NMES) decreased intensity of expression of the tetraspanin surface marker, CD63, on circulating EVs. Moreover, NMES shifted the biochemical Raman fingerprint of circulating EVs in aged animals with significant changes in lipid and sugar content in response to NMES when compared to controls. As a demonstration of the physiological relevance of these EV changes, we showed that intramuscular administration of EVs derived from aged animals subjected to NMES enhanced aged skeletal muscle healing after injury. These studies suggest that repetitive muscle contractile activity enhances the regenerative properties of circulating EVs in aged animals.