Integrin on developing and adult skeletal muscle

Avian integrin is a complex of integral membrane glycoproteins that appears to function as a dual receptors for both intracellular cytoskeletal and extracellular matrix components. Antibodies were raised against this complex and used to (1) immunolocalize integrin on cryosections of developing and adult muscle tissue and on developing myotube cultures in vitro and (2) immunoaffinity purify integrin from various fiber-type specific muscles. Integrin localization was compared with that of its putative cytoskeletal-associated and extracellular matrix ligands, talin and vinculin and fibronectin and laminin, respectively. The goal was to identify putative sites of interaction between the muscle sarcolemma and the cytoskeleton and the extracellular matrix and to reveal any differences in the molecular composition at these sites. Integrin's distribution on the sarcolemma of early (Day 12) embryonic limb muscle was random and punctate. On late embryonic (Days 17-19) limb muscle tissue its distribution was generally uniform but with occasional increased densities at specific sites along the sarcolemma. Posthatch (>3 weeks) fast twitch muscle showed a highly regionalized distribution. These regions of integrin concentration coincided with densities of acetylcholine receptors, revealed by TRITC α-bungarotoxin labeling, and regions of muscle-tendon interaction, identified by morphological criteria. Tissue culture studies also demonstrated integrin densities at analogous sites in vitro, e.g., acetylcholine receptor clusters and sites at which myofibrils terminate at the sarcolemma. These integrin-rich sites were also shown to be Triton X-100 insoluble and therefore presumably are linked to the cytoskeleton or extracellular matrix. The localization of integrin on developing and adult muscle tissue was compared with that of fibronectin, laminin, vinculin, and talin using double, immunofluorescently labeled cryosections. In general, integrin did not colocalize exclusively with any one of its putative ligands. In the embryo, discrete densities of both talin and vinculin were observed at the myotendinous junction, whereas integrin immunoreactivity was widely distributed on muscle, vasculature, nerve, and connective tissue with no discernible sites of increased density. Laminin was primarily associated with muscle and nerve whereas fibronectin was prominent on connective tissue. On posthatch tissue, the distributions of talin, vinculin, laminin, and fibronectin were similar to those in the embryo, whereas the distribution of integrin was restricted to specific sites. The distribution of integrin was also examined for fiber-type specific differences on adult muscle tissue. In contrast to the discrete localization of integrin on fast twitch muscle, its distribution on slow, tonic muscle, e.g., anterior latissimus dorsi, was generally uniform surrounding the myofibers and extending along their entire length. The above observations lead to the following conclusions. (1) Integrin participates in several different putative associations between the sarcolemma and the extracellular matrix and/or the cytoskeleton. These include the neuromuscular and myotendinous junctions but may also include some other uncharacterized junctions. Its uniform distribution on ALD myofibers implies sarcolemmal/ECM or cytoskeletal interaction along the entire fiber length. (2). The wide distribution of integrin on embryonic muscle tissue contrasts the restricted distributions of both vinculin and talin. This suggests that integrin either interacts with other as yet unidentified cytoskeletal associated molecules at other sites or that it is not coupled to the cytoskeleton at these regions. (3). Integrin-rich regions can be associated with different extraceiiular matrix and cytoskeletal-associated moleceules. This suggests its participation in different kinds of molecular organizations. Explanations for these different organizations include differences in isoforms of junctional molecules including integrin or the participation of novel components.