Osteogenic potential of human skeletal muscle stem cells

Objectives: Skeletal muscle is a reservoir of myogenic precursors or stem cells with the potential to participate in tissue regeneration therapies. We used expression of the α7 integrin gene that encodes the surface adhesion receptor as a means for the rapid isolation of pure populations of myoblasts from human muscle. The capacity of receptor-positive (α7⁺) myoblasts to differentiate into osteogenic and other lineages were examined. Methods: α7⁺ myoblasts were harvested from human adult and fetal muscle by FACS. Isolated cells were analyzed for myogenic marker expression including CD56/NCAM, M-cadherin and c-Met. To assess the potential of the α7⁺ myoblasts to differentiate into osteogenic lineage, cells were treated with BMP-2. Osteogenic differentiation was assessed by expression of alkaline phosphatase (ALP) activity, osteocalcin (OC) and Runx2 gene expression. We also examined modulation of integrin expression, adhesive and motility functions following differentiation into osteogenic lineage. Results: The α7⁺ myoblasts were found to strongly express myogenic markers. Importantly, these cells could be induced to undergo myogenic differentiation and fuse to form myotubes. However, following BMP-2 treatment, cells failed to fuse and transdifferentiated along an osteogenic pathway expressing ALP activity, OC and Runx2. Similarly, α7⁺ cells differentiated along an adipocytic pathway with specific inducers. BMP-2 treatment downregulated α7 integrin expression in myoblasts at the level of the promoter, and attenuated adhesion and migration on laminin substrates. In contrast, BMP-2 induced α2 integrin with enhanced adhesion and motility on collagen. Conclusion: These observations show that α7⁺ myoblasts are pluripotent stem cells capable of differentiation along osteogenic and adipocytic lineages. Following differentiation to osteoblasts, there is a coordinated switch in integrin expression profile that potentiates adhesion and motility on interstitial matrix molecules. Our results suggest that α7⁺ multipotent muscle stem cells may provide a means to enhance tissue regeneration in orofacial tissues. This work supported by NIH-DE15404.