The extracellular matrix protein betaIG-H3 is expressed at myotendinous junctions and supports muscle cell adhesion

Molecules of the extracellular matrix (ECM) play important roles in the development and maintenance of myotendinous junctions (MTJs), specialized regions of muscle to bone union. In this report we provide evidence that skeletal muscle cells synthesize the collagen- and fibronectin-binding ECM protein betaIG-H3 and that betaIG-H3 is localized to MTJs. In situ hybridization experiments revealed that during E16.5-E18.5 of murine development, betaIG-H3 RNA transcripts were expressed where developing skeletal muscle fibers contact primordial cartilage and bone. Immunohistochemical analysis verified that the betaIG-H3 protein itself localized distinctively at MTJs, and ultrastructural analysis suggested that betaIG-H3 associates with extracellular fibers and the surface of cells. In vitro, recombinant betaIG-H3 functioned as an adhesion substratum for skeletal muscle cells. Adhesion was significantly reduced by anti-integrin alpha7 and beta1 antibodies, suggesting that betaIG-H3 binds to skeletal muscle cells via alpha7beta1 integrin. Localization of betaIG-H3 to the termini of skeletal muscle fibers and the binding of betaIG-H3 to cells and to molecules of the ECM suggests that betaIG-H3 may play an organizational and structural role in developing MTJs, linking skeletal muscle to components of the ECM.