Distinct α(7A)β1 and α(7B)β1 integrin expression patterns during mouse development: α(7A) is restricted to skeletal muscle but α(7B) is expressed in striated muscle, vasculature, and nervous system

Abstract

The laminin binding α(7A)β1 integrin has been described as a major integrin in skeletal muscle. The RNA coding for the cytoplasmic domain of α7 integrin undergoes alternative splicing to generate two major forms, denoted α(7A) and α(7B). In the current paper, we have examined the developmental expression patterns of the eTA and α(7B) splice variants in the mouse. The α7 integrin expression is compared to that of the nonintegrin laminin receptor dystroglycan and to that of laminin-α1 and laminin-α2 chains. α(7A) integrin was found by in situ hybridization to be specific to skeletal muscle. Antibodies specific for α(7B) integrin and in situ hybridization revealed the presence of α7 mRNA and α(7B) protein in the E10 myotome and later in primary and secondary myotubes. In the heart, α(7B) integrin was not detectable in the endocardium or myocardium during embryonic and fetal heart development. Northern blot analysis and immunohistochemistry revealed a postnatal induction of α(7B) in the myocardium. In addition to striated muscle, α(7B) integrin was localized to previously unreported nonmuscle locations such as a subset of vascular endothelia and restricted sites in the nervous system. Comparison of the α7 integrin expression pattern with that of different laminin isoforms and dystroglycan revealed a coordinated temporal expression of dystroglycan, α7 integrin, and laminin-α2, but not laminin-α1, in the forming skeletal muscle. We conclude that the α(7A) and α(7B) integrin variants are expressed in a developmentally regulated, tissue-specific pattern suggesting different functions for the two splice forms.