SPATIAL AND TEMPORAL EXPRESSION OF THE BETA-1D INTEGRIN DURING MOUSE DEVELOPMENT

The beta 1D protein is a recently characterized isoform of the integri n beta 1 subunit that is present in cardiac and skeletal muscles. In t his study, we have examined the expression of beta 1D in different typ es of skeletal muscle and in cardiac muscle and studied its distributi on during mouse development, using new monoclonal antibodies specific for beta 1D Immunoprecipitation studies revealed that, while beta 1A i s strongly expressed in proliferating C2C12 myoblasts, beta 1D is only expressed after their differentiation to myotubes. In these myotubes, beta 1D is associated with different alpha subunits, namely alpha 3A, alpha 5, alpha 7A, or alpha 7B. Initially, during embryogenesis, the beta 1A subunit is the only beta 1 variant expressed in skeletal and c ardiac muscle. The beta 1D subunit is first detected in skeletal muscl e at E17.5, whereas in cardiac muscle its expression begins around the time of birth. Later the expression of beta 1A in skeletal and cardia c muscle becomes restricted to capillary cells, whereas beta 1D eventu ally becomes the only variant expressed in adult cardiac and skeletal muscle cells. The switch from the beta 1A to the beta 1D subunit in ca rdiac muscle cells coincides with the expression of alpha 7. In adults there is a distinct concentration of beta 1D at the myotendinous junc tions of muscle fibers and at costameres in both cardiac and skeletal muscle. In addition, beta 1D is present at intercalated discs in cardi ac muscle and at neuromuscular junctions in skeletal muscle cells. The amount of beta 1D in different types of skeletal muscle (fast, slow, and mixed-type) was similar, but cardiac muscle expressed almost five times as much of this protein. We suggest that beta 1D plays a role in the maintenance of the cytoarchitecture of mature muscle and in the f unctional integrity of the muscle cells. (C) 1997 Wiley-Liss, Inc.