May the Force move you: TSC‐ing the mechanical activation of mTOR (2013)

That fact that lifting heavy loads causes muscles to grow bigger and stronger has been recognized for over 2000 years. However, it is only in the last 15 years that the necessary molecular tools have been developed to begin to understand how the mechanical signal of load across the muscle is converted into a chemical signal that results in a change in muscle phenotype. In this issue of The Journal of Physiology, that understanding takes another significant step forward courtesy of insight revealed by Jacobs and colleagues (2013). Up until now, it was clear that the mechanistic target of rapamycin (mTOR) was activated in a load-dependent manner (Baar & Esser, 1999), and that the increase in mTOR activity was required for the load-induced increase in muscle protein synthesis and hypertrophy (Goodman et al. 2011). This makes sense in that mTOR (1) can increase protein synthesis and limit protein breakdown, (2) can be activated by growth signals (activity, feeding, and hormones), and (3) is inhibited by stress (fasting, unfolded proteins, alcohol consumption, aging, etc.). But the issue of how mTOR was activated by activity in muscle remained unclear. In most cells, mTOR is activated when a growth factor binds to a receptor tyrosine kinase and activates phosphoinositide 3-kinase and protein kinase B/Akt (PKB). Once activated, PKB phosphorylates and inactivates tuberin (TSC2), a GTPase activating protein that turns off the Ras-homologue enriched in brain (Rheb). Since Rheb is the direct activator of mTOR, inactivating TSC2 (turning the inhibitor off) is the switch that activates mTOR, promoting protein synthesis and cell growth. The canonical growth factor pathway can activate mTOR. However, in muscle, the activation of mTOR by loading occurs in a growth factor-independent manner (Philp et al. 2011). That is, muscle loading enhances mTOR activity in the absence of classical growth factor-activated signals. This suggests the existence of a mechanoreceptor, a protein that can directly ‘feel the force’ and transduce it into a chemical signal. Jacobs and her colleagues show in this issue of The Journal of Physiology that, like growth factors, mechanotransduction targets TSC2. Using immunoprecipitation and λ phosphatase, the authors showed that TSC2 was phosphorylated in response to resistance exercise. Unlike growth factors, the phosphorylation of TSC2 did not occur at the previously described Thr1462 site. Instead, Jacobs showed that TSC2 was phosphorylated at other Ser/Thr residues within an RxRxx motif. The next interesting observation was that the phosphorylation of this motif was associated with a move of TSC2 from the membrane fraction to the cytoplasm. The highlight of the work was their use of frequency scatterplots to co-localize TSC2 and LAMP2, a marker of the lysosome. Using ultra-thin sections they showed that resistance exercise causes a 90% decrease in the association of TSC2 and LAMP2, suggesting that TSC2 had been moved away from the lysosome. This is important because the lysosome is where the mTOR activator Rheb is located. Together, these data suggest that a mechanoreceptor senses loading, induced by eccentric contractions in this case, and turns on an RxRxx-targeted kinase that phosphorylates TSC2 and moves it off the lysosome and away from its target Rheb, allowing mTOR activation (Fig. 1). A schematic of the activation of mTOR following resistance exercise Loading activates an RxRxx targeted kinase resulting in the phosphorylation of TSC2. Phosphorylated TSC2 moves from the lysosome (LAMP2 positive membranes) into the cytoplasm and can no longer keep Rheb in the GDP bound state. Activation of Rheb together with amino acid stimulated recruitment of mTOR to the lysosome results in the activation of mTOR, increase in muscle protein synthesis, and, if repeated at a sufficient frequency, muscle hypertrophy. Not only does TSC2 move away from the lysosome, but Jacobs also showed that mTOR moves to the lysosome 1 h after resistance exercise. However, whether the movement of mTOR to the lysosome is dependent on the mechanical load or amino acids is not entirely clear. We do know that within the first 90 min following resistance exercise there is a load-dependent increase in the amino acid leucine in the active muscle (MacKenzie et al. 2009). Since leucine causes the translocation of mTOR to the lysosome through the leucyl tRNA synthase–Rag–Ragulator pathway (Sancak et al. 2010), this suggests that mTOR is moving to the lysosome as a result of the increase in amino acids and not a direct mechanical signal. This observation also explains why taking extra amino acids immediately following resistance exercise can increase protein synthesis and muscle growth, probably by amplifying the movement of mTOR to the lysosome. As with any good research paper, the findings by Jacobs and colleagues raise further questions. What load-induced kinase targets RxRxx motifs in TSC2? How is such a kinase activated by load? Conversely, in the absence of load, what are the mechanisms that tether TSC2 to the lysosome? Can TSC2 be ‘encouraged’ to dissociate from the lysosome, perhaps to a therapeutic end? Answers to these questions, would finally give us a complete understanding of how muscle feels the force. Jacobs and colleagues have given us a good start.