Stem Cell Differentiation to Muscle Induced by a Suitably Compliant Matrix

Stem cells can in principle develop into a wide range of soft tissue cell types, but the importance of the 'softness' of the tissue microenvironment in differentiation is only now becoming clear. In fact, tissue compliance varies considerably between tissues, and muscle is one soft tissue that at rest appears to have a well-defined matricellular elasticity (10-15 kPa) which is readily achieved with polymeric hydrogels. We use a molecular layer of collagen-I on crosslinked polyacrylamide hydrogels with muscle-like elasticity to induce adult human mesenchymal stem cells (MSCs) cultured in 10% serum to become spindle-shaped myoblasts that express key early markers (Pax7, MyoD). When replated at high density, these cells further fuse into multi-nucleated myotubes expressing late skeletal muscle markers. On hydrogels that are too stiff, the MSCs do not differentiate – unless we add a potent glucocorticoid used in the clinic to treat muscular dystrophy. This single glucocorticoid also tends to downregulate adhesive/cytoskeletal structures involved in substrate attachment and sensing, and it appears to suppress osteogenic markers, consistent with bone loss in treated patients. This replaces typical soluble induction cocktails of multiple glucocorticoids plus other factors, and in such a case, we find muscle induction occurs regardless of matrix elasticity. Nonetheless, the best results at low dose clearly occur in combination with an optimally compliant matrix.