Synaptic Integrins in Developing, Adult, and Mutant Muscle: Selective Association of α1, α7A, and α7B Integrins with the Neuromuscular Junction

Abstract

Differentiation of both pre- and postsynaptic structures at the skeletal neuromuscular junction is organized by the basal lamina that occupies the synaptic cleft. As β1 integrins are a major class of receptors for basal lamina components, we stained muscles with antibodies to the 10 integrin α subunits known to form dimers with β1, to determine if any of these molecules were concentrated at synaptic sites on muscle fibers. In both developing and adult muscle, the integrin α1 chain was selectively associated with presynaptic cells (Schwann cells and/or nerve terminals), while α7 was present on both synaptic and extrasynaptic portions of the muscle fiber surface. Thus α1 and α7 integrins are present in synaptic membranes. Expression of the α7 chain was analyzed further by staining with antibodies specific for three alternatively spliced products of the α7 gene (A, B, and C), all of which were expressed in muscle. The α7A and α7B isoforms were confined to synaptic sites in adult muscle, while α7C was present both synaptically and extrasynaptically. In developing muscle, α7A appeared postnatally and specifically at the synapse; α7B was present throughout the muscle fiber perinatally, becoming confined to the synapse in the second postnatal week; and α7C was present extrasynaptically both perinatally and in adulthood. Thus, two of the α7 integrins are synapse-specific, and all three show distinct spatiotemporal patterns of expression within a single cell type. Finally, we asked whether perturbation of laminin expression affected the distribution of the α7 integrins. In normal mice, laminin β2 is concentrated in synaptic basal lamina. In β2-null mutant mice, α7A was still present at synaptic sites, but α7B was absent. This result provides genetic evidence that basal lamina composition is a determinant of integrin distribution.