Aims: Rotator cuff muscle atrophy and fatty infiltration affect the clinical outcomes of rotator cuff tear patients. However, there is no effective treatment for fatty infiltration at this time. High-intensity interval training (HIIT) helps to activate beige adipose tissue. The goal of this study was to test the role of HIIT in improving muscle quality in a rotator cuff tear model via the β3 adrenergic receptor (β3AR).Methods: Three-month-old C57BL/6 J mice underwent a unilateral rotator cuff injury procedure. Mice were forced to run on a treadmill with the HIIT programme during the first to sixth weeks or seventh to 12th weeks after tendon tear surgery. To study the role of β3AR, SR59230A, a selective β3AR antagonist, was administered to mice ten minutes before each exercise through intraperitoneal injection. Supraspinatus muscle, interscapular brown fat, and inguinal subcutaneous white fat were harvested at the end of the 12th week after tendon tear and analyzed biomechanically, histologically, and biochemically.Results: Histological analysis of supraspinatus muscle showed that HIIT improved muscle atrophy, fatty infiltration, and contractile force compared to the no exercise group. In the HIIT groups, supraspinatus muscle, interscapular brown fat, and inguinal subcutaneous white fat showed increased expression of tyrosine hydroxylase and uncoupling protein 1, and upregulated the β3AR thermogenesis pathway. However, the effect of HIIT was not present in mice injected with SR59230A, suggesting that HIIT affected muscles via β3AR.Conclusion: HIIT improved supraspinatus muscle quality and function after rotator cuff tears by activating systemic sympathetic nerve fibre near adipocytes and β3AR.Cite this article: Bone Joint Res 2023;12(8):455–466.

Introduction: Promoting muscle regeneration through stem cell therapy has potential risks. We investigated the effect of umbilical cord mesenchymal stem cells (UMSCs) Exosomes (Exo) Follistatin on muscle regeneration.Material and methods: The Exo was derived from UMSCs cells and was utilized to affect the mice muscle injury model and C2C12 cells myotubes atrophy model. The western blot, qRT-PCR and IF were utilized to determine the effects of Exo on the levels of Follistatin, MyHC, MyoD, Myostatin, MuRF1, MAFbx, α-SMA, Collagen I, Smad2, and AKT. In addition, HE and Masson staining were used to assess muscle tissue damage in mice.Results: The level of Follistatin in Exo was significantly higher than that in UMSCs. UMSCs-Exo increased the levels of Follistatin, MyHC, MyoD, and p-Smad2 and decreased the levels of Myostatin, MuRF1, MAFbx, α-SMA, Collagen I, p-AKT, and p-mTOR in mice or C2C12 cells. In addition, UMSCs-Exo decreased levels of inflammation and fibrosis in mice. However, UMSCs-Exo-si-Follistatin reversed the effect of UMSCs-Exo. Transfection of oe-Smad2 up-regulated the protein levels of Collagen I, α-SMA, and changed the ratio of p-Smad2/Smad2 expression to 0.33, and 0.34, 0.73. LY294002 decreased the levels of MyHC, MyoD, and the ratio of p-AKT/ AKT and p-mTOR/mTOR expression to 0.12, 0.17, 0.33, and 0.41, increased the levels of MuRF1 and MAFbx to 0.36 and 0.34.Conclusions: This study demonstrated that Follistatin in UMSCs-Exo inhibits fibrosis and promotes muscle regeneration in mice by regulating Smad and AKT signaling.