THE ALPHA-5-BETA-1 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. © 1992.