Eccentric contractions increase the phosphorylation of tuberous sclerosis complex‐2 (TSC2) and alter the targeting of TSC2 and the mechanistic target of rapamycin to the lysosome

Abstract

Key points: Mechanical stimuli play a major role in the regulation of skeletal muscle mass. Signalling through a protein kinase called the mechanistic target of rapamycin (mTOR) is essential for mechanically induced changes in muscle mass; however, the mechanism(s) via which mechanical stimuli regulate mTOR signalling have not been defined. In this study, mouse skeletal muscles were stimulated with eccentric contractions (ECs) to determine if the mechanical activation of mTOR signalling is associated with changes in the phosphorylation of the tuberous sclerosis complex‐2 (TSC2) and the targeting of both mTOR and TSC2 to the lysosome. Our results demonstrate that ECs induce hyper‐phosphorylation of TSC2, enhanced lysosomal targeting of mTOR and nearly abolish the lysosomal targeting of TSC2. These novel observations suggest that alterations in the lysosomal targeting of mTOR/TSC2 could play a fundamental role in the mechanism via which mechanical stimuli regulate mTOR signalling and ultimately skeletal muscle mass. Abstract The goal of this study was to determine whether the mechanical activation of mechanistic target of rapamycin (mTOR) signalling is associated with changes in phosphorylation of tuberous sclerosis complex‐2 (TSC2) and targeting of mTOR and TSC2 to the lysosome. As a source of mechanical stimulation, mouse skeletal muscles were subjected to eccentric contractions (ECs). The results demonstrated that ECs induced hyper‐phosphorylation of TSC2 and at least part of this increase occurred on residue(s) that fall within RxRxxS/T consensus motif(s). Furthermore, in control muscles, we found that both mTOR and TSC2 are highly enriched at the lysosome. Intriguingly, ECs enhanced the lysosomal association of mTOR and almost completely abolished the lysosomal association of TSC2. Based on these results, we developed a new model that could potentially explain how mechanical stimuli activate mTOR signalling. Furthermore, this is the first study to reveal that the activation of mTOR is associated with the translocation of TSC2 away from the lysosome. Since a large number of signalling pathways rely on TSC2 to control mTOR signalling, our results have potentially revealed a fundamental mechanism via which not only mechanical, but also various other types of stimuli, control mTOR signalling.