Abstract
BackgroundThe mammalian target of rapamycin complex 2 (mTORC2), containing the essential protein rictor, regulates cellular metabolism and cytoskeletal organization by phosphorylating protein kinases, such as PKB/Akt, PKC, and SGK. Inactivation of mTORC2 signaling in adult skeletal muscle affects its metabolism, but not muscle morphology and function. However, the role of mTORC2 in adult muscle stem cells (MuSCs) has not been investigated.MethodUsing histological, biochemical, and molecular biological methods, we characterized the muscle phenotype of mice depleted for rictor in the Myf5-lineage (RImyfKO) and of mice depleted for rictor in skeletal muscle fibers (RImKO). The proliferative and myogenic potential of MuSCs was analyzed upon cardiotoxin-induced injury in vivo and in isolated myofibers in vitro.ResultsSkeletal muscle of young and 14-month-old RImyfKO mice appeared normal in composition and function. MuSCs from young RImyfKO mice exhibited a similar capacity to proliferate, differentiate, and fuse as controls. In contrast, the number of MuSCs was lower in young RImyfKO mice than in controls after two consecutive rounds of cardiotoxin-induced muscle regeneration. Similarly, the number of MuSCs in RImyfKO mice decreased with age, which correlated with a decline in the regenerative capacity of mutant muscle. Interestingly, reduction in the number of MuSCs was also observed in 14-month-old RImKO muscle.ConclusionsOur study shows that mTORC2 signaling is dispensable for myofiber formation, but contributes to the homeostasis of MuSCs. Loss of mTORC2 does not affect their myogenic function, but impairs the replenishment of MuSCs after repeated injuries and their maintenance during aging. These results point to an important role of mTORC2 signaling in MuSC for muscle homeostasis.
Highlights
The mammalian target of rapamycin complex 2, containing the essential protein rictor, regulates cellular metabolism and cytoskeletal organization by phosphorylating protein kinases, such as PKB/Akt, PKC, and SGK
Our study shows that mammalian target of rapamycin complex 2 (mTORC2) signaling is dispensable for myofiber formation, but contributes to the homeostasis of Muscle stem cell (MuSC)
These experiments show that rictor-depleted MuSCs have the same proliferation and differentiation potential as control cells, consistent with results obtained in cultured primary myoblasts [16]
Summary
The mammalian target of rapamycin complex 2 (mTORC2), containing the essential protein rictor, regulates cellular metabolism and cytoskeletal organization by phosphorylating protein kinases, such as PKB/Akt, PKC, and SGK. Genetic inactivation of mTORC2 in skeletal muscle fibers via deletion of Rictor, using human skeletal actin (HSA)-Cre or muscle creatine kinase (MCK)-Cre, does not result in an overt muscle phenotype [11] but impairs insulin-stimulated glucose transport and increases glycogen synthase activity [12]. Deletion of Rictor using Myf5-Cre (i.e., depleting rictor in progenitor cells that give rise to myoblasts and brown adipocytes) revealed an important function of mTORC2 for brown adipocyte differentiation and growth [15]. Embryonic myogenesis was largely unaffected, which was in stark contrast to Myf5-Cre-driven depletion of raptor (inactivation of mTORC1), which caused perinatal death of the mice [16]
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