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Despite the regenerative capacity of muscle, tissue volume is not restored after volumetric muscle loss (VML), perhaps due to a loss-of-structural extracellular matrix. We recently demonstrated the structural and functional restoration of muscle tissue in a mouse model of VML using an engineered "bioconstruct," comprising an extracellular matrix scaffold (decellularized muscle), muscle stem cells (MuSCs), and muscle-resident cells (MRCs). To test the ability of the cell-based bioconstruct to restore whole-muscle biomechanics, we measured biomechanical parameters in uninjured muscles, muscles injured to produce VML lesions, and in muscles that were injured and then treated by implanting either the scaffolds alone or with bioconstructs containing the scaffolds, MuSCs, and MRCs. We measured the active and passive forces over a range of lengths, viscoelastic force relaxation, optimal length, and twitch dynamics. Injured muscles showed a narrowed length-tension curve or lower force over a narrower range of muscle lengths, and increased passive force. When treated with bioconstructs, but not with scaffolds alone, injured muscles showed active and passive length-tension relationships that were not different from uninjured muscles. Moreover, injured muscles treated with bioconstructs exhibited reduced fibrosis compared to injured muscles either untreated or treated with scaffolds alone. The cell-based bioconstruct is a promising treatment approach for future translational efforts to restore whole-muscle biomechanics in muscles with VML lesions., Competing Interests: The authors declare no competing interests.

The development of cell therapy for repairing damaged or diseased skeletal muscle has been hindered by the inability to significantly expand immature, transplantable myogenic stem cells (MuSCs) in culture. To overcome this limitation, a deeper understanding of the mechanisms regulating the transition between activated, proliferating MuSCs and differentiation-primed, poorly engrafting progenitors is needed. Here, we show that methyltransferase Setd7 facilitates such transition by regulating the nuclear accumulation of β-catenin in proliferating MuSCs. Genetic or pharmacological inhibition of Setd7 promotes in vitro expansion of MuSCs and increases the yield of primary myogenic cell cultures. Upon transplantation, both mouse and human MuSCs expanded with a Setd7 small-molecule inhibitor are better able to repopulate the satellite cell niche, and treated mouse MuSCs show enhanced therapeutic potential in preclinical models of muscular dystrophy. Thus, Setd7 inhibition may help bypass a key obstacle in the translation of cell therapy for muscle disease., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Volumetric muscle loss (VML) is associated with loss of skeletal muscle function, and current treatments show limited efficacy. Here we show that bioconstructs suffused with genetically-labelled muscle stem cells (MuSCs) and other muscle resident cells (MRCs) are effective to treat VML injuries in mice. Imaging of bioconstructs implanted in damaged muscles indicates MuSCs survival and growth, and ex vivo analyses show force restoration of treated muscles. Histological analysis highlights myofibre formation, neovascularisation, but insufficient innervation. Both innervation and in vivo force production are enhanced when implantation of bioconstructs is followed by an exercise regimen. Significant improvements are also observed when bioconstructs are used to treat chronic VML injury models. Finally, we demonstrate that bioconstructs made with human MuSCs and MRCs can generate functional muscle tissue in our VML model. These data suggest that stem cell-based therapies aimed to engineer tissue in vivo may be effective to treat acute and chronic VML.