mTOR regulates skeletal muscle regeneration in vivo through kinase-dependent and kinase-independent mechanisms.
Rapamycin-sensitive signaling is required for skeletal muscle differentiation and remodeling. In cultured myoblasts, the mammalian target of rapamycin (mTOR) has been reported to regulate differentiation at different stages through distinct mechanisms, including one that is independent of mTOR kinase activity. However, the kinase-independent function of mTOR remains controversial, and no in vivo studies have examined those mTOR myogenic mechanisms previously identified in vitro. In this study, we find that rapamycin impairs injury-induced muscle regeneration. To validate the role of mTOR with genetic evidence and to probe the mechanism of mTOR function, we have generated and characterized transgenic mice expressing two mutants of mTOR under the control of human skeletal actin (HSA) promoter: rapamycin-resistant (RR) and RR/kinase-inactive (RR/KI). Our results show that muscle regeneration in rapamycin-administered mice is restored by RR-mTOR expression. In the RR/KI-mTOR mice, nascent myofiber formation during the early phase of regeneration proceeds in the presence of rapamycin, but growth of the regenerating myofibers is blocked by rapamycin. Igf2 mRNA levels increase drastically during early regeneration, which is sensitive to rapamycin in wild-type muscles but partially resistant to rapamycin in both RR- and RR/KI-mTOR muscles, consistent with mTOR regulation of Igf2 expression in a kinase-independent manner. Furthermore, systemic ablation of S6K1, a target of mTOR kinase, results in impaired muscle growth but normal nascent myofiber formation during regeneration. Therefore, mTOR regulates muscle regeneration through kinase-independent and kinase-dependent mechanisms at the stages of nascent myofiber formation and myofiber growth, respectively.
Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, USA.
Yejing
Y
Ge
Ai-Luen
AL
Wu
Christine
C
Warnes
Jianming
J
Liu
Chongben
C
Zhang
Hideki
H
Kawasome
Naohiro
N
Terada
Marni D
MD
Boppart
Christopher J
CJ
Schoenherr
Jie
J
Chen
D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D006128:Growth / Q000187:drug effects; D006801:Humans; D007335:Insulin-Like Growth Factor II / Q000235:genetics / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000254:growth & development / Q000502:physiology*; D009154:Mutation; D010770:Phosphotransferases / Q000378:metabolism*; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D012038:Regeneration / Q000187:drug effects / Q000502:physiology*; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000172:deficiency / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases
D002352:Carrier Proteins; D012333:RNA, Messenger; D007335:Insulin-Like Growth Factor II; D010770:Phosphotransferases; D017853:Phosphotransferases (Alcohol Group Acceptor); C546842:MTOR protein, human; C546843:mTOR protein, mouse; D038744:Ribosomal Protein S6 Kinases, 90-kDa; C512964:Rps6ka1 protein, mouse; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus