The α₅β₁ integrin associates with a dystrophin-containing lattice during muscle development

The organization of the α₅β₁ integrin on skeletal muscle was studied in culture and in sections from adult and embryonic tissue using monoclonal antibodies specific for the α₅ subunit. The α₅β₁ integrin showed changes in organization and in the molecules with which it colocalizes. On early myoblasts, possessing a fibroblast-like morphology, the α₅ integrin organization was indistinguishable from that on fibroblasts; it was expressed prominently and localized in numerous focal contacts around the cell periphery. In bipolar myoblasts and early myotubes, the α₅ integrin was expressed only weakly and localized in a small number of focal contact-like structures. As myogenesis proceeded there was an apparent increase in integrin expression and a change in organization. In addition to the focal contact-like structures that persist throughout myogenesis in vitro, a dense lattice-like structure of integrin appeared. Fibrillar fibronectin, talin, and non-muscle α-actinin did not colocalize with the α₅β₁ integrin in the lattice structure as they did in the focal contact-like structures. However, dystrophin, which displayed a diffuse distribution earlier, now colocalized with the α₅β₁ integrin in the punctate lattice. Coincident with the registration of myofibrils into visible sarcomeres, the prominent dense, lattice structure disappeared leaving the focal contact-like structures as the only regions of organized α₅β₁ integrin. Despite the presence of the β₁ integrin in neuromuscular or myotendinous junctions in vivo and on myotubes in vitro, the α₅β₁ integrin was not present in either junction. These observations suggest that the α₅β₁ integrin is involved in the adhesion of muscle to the extracellular matrix, the organization of the dystrophin-containing lattice, and the organization of nascent myofibrils which emanate from the focal contact- and stress fiber-like structures in muscle. Other integrins appear to anchor myofibrils at the myotendinous and neuromuscular junctions.