Integrin in Acetylcholine Receptor Clustering

The clustering of acetylcholine receptors (AChR) on skeletal muscle fibers is an early event in the formation of neuromuscular junctions. Recent studies show that laminin as well as agrin can induce AChR clustering. Since the a 7 b 1 integrin is a major laminin receptor in skeletal muscle, we determined if this integrin participates in laminin and/or agrin-induced AChR clustering. The alternative cytoplasmic domain variants, a 7A and a 7B, and the extracellular spliced forms, a 7X1 and a 7X2, were studied for their ability to engage in AChR clustering. Immunofluorescence microscopy of C2C12 myofibers shows that the a 7 b 1 integrin colocalizes with laminin-induced AChR clusters and to a much lesser extent with agrin-induced AChR clusters. However, together laminin and agrin promote a synergistic response and all AChR colocalize with the integrin. Laminin also induces the physical association of the integrin and AChR. High concentrations of antia 7 antibodies inhibit colocalization of the integrin with AChR clusters as well as the enhanced response promoted by both laminin and agrin. Engaging the integrin with low concentrations of antia 7 antibody initiates cluster formation in the absence of agrin or laminin. Whereas both the a 7A and a 7B cytoplasmic domain variants cluster with AChR, only those isoforms containing the a 7X2 extracellular domain were active. These results demonstrate that the a 7 b 1 integrin has a physiologic role in laminin-induced AChR clustering, that alternative splicing is integral to this function of the a 7 chain, and that laminin, agrin, and the a 7 b 1 integrin interact in a common or convergent pathway in the formation of neuromuscular junctions.