The Roles Of MTOR Signaling In Muscle Regeneration And Tendon Differentiation

Part I:The Role and mechanism of Rab5 in skeletal muscle regenerationRab5 is a master regulator for endosome biogenesis and transport while its in vivo physiological function remains elusive.Here,we found that Rab5a is upregulated in several in vivo and in vitro myogenesis models.In line with this,myogenic ablation of Rab5a in mice impaired injury-induced skeletal muscle regeneration.Gain-and loss-of function studies revealed that Rab5a is essential for myoblast differentiation.We further identified that Rab5a directly interacts with Insulin Receptor Substrate 1(IRSI),an essential scaffold protein for IGF signaling.Rab5a regulates the activation of IRS1 via coordinating the association between IRS1 and IGF receptor(IGFR).This Overexpression of AKT/PKB,but not p38MAPK,could rescue the defective differentiation phenotype of Rab5a-depleted myoblasts.Consistent with this,Rab5a is critical for activating both myogenesis-induced and IGF-evoked AKT-mTOR signaling.Myogenic deletion of Rab5a also reduced the activation of AKT-mTOR axis in regenerating skeletal muscles.Importantly,Rab5a coordinates the association of IGFR and IRS1 which is essential for mediating IGF-evoked AKT-mTOR activation during myoblast differentiation.Moreover,the association of Rab5a to IRS1 is required for regulating IRS-AKT-mTOR signaling.Our study thus uncovers the physiological roles of Rab5a in regulating muscle regeneration and delineates a mechanism modulating IRS-AKT-mTOR signaling.Part II:The Role of AKT-mTOR Signaling on the tenogenesis of mesenchymal stem cellsTendon repair is a clinical challenge because of the limited understanding on tenogenesis.The synthesis of type I collagen(Collagen I)and other extracellular matrix are essential for tendon differentiation and homeostasis.Current studies on tenogenesis focused mostly on the tenogenic transcriptional factors while the signaling controlling tenogenesis on translational level remains largely unknown.Here,we showed that mechanistic target of rapamycin(mTOR)signaling was activated by protenogenic grov*wth factor,transforming growth factors betal,and insulin-like growth factor-I.The expression of mTOR was upregulated during tenogenesis of mesenchymal stem cells(MSCs).Moreover,mTOR was downregulated in human tendinopathy tissues and was inactivated upon statin treatment.Both inhibition and depletion of AKT or mTOR significantly reduced type I collagen production and impaired tenogenesis of MSCs.Tendon specific-ablation of mTOR resulted in tendon defect and reduction of Collagen I.However,there is no evident downregulation of tendon associated collagens at the transcription level.Our study demonstrated that AKT-mTOR axis is a key mediator of tendon differentiation and provided a novel therapeutic target for tendinopathy and tendon injuries.