Laminin-induced change in conformation of preexisting alpha7beta1 integrin signals secondary myofiber formation.

Two distinct populations of myoblasts, distinguishable by alpha7 integrin expression have been hypothesized to give rise to two phases of myofiber formation in embryonic limb development. We show here that alpha7 integrin is detectable far earlier than previously reported on both "primary" and "secondary" lineage myoblasts and myofibers. An antibody (1211) that recognizes an intracellular epitope allowed detection of alpha7 integrin previously missed using an antibody (H36) that recognizes an extracellular epitope. We found that when myoblasts were isolated and cultured from different developmental stages, H36 only detected alpha7 integrin that was in direct contact with its ligand, laminin. Moreover, alpha7 integrin detection by H36 was reversible and highly localized to subcellular points of contact between myoblasts and laminin-coated 2.8-microm microspheres. Prior to secondary myofiber formation in limb embryogenesis, laminin was present but not in close proximity to clusters of primary myofibers that expressed alpha7 integrin detected by antibody 1211 using deconvolution microscopy. These results suggest that the timing of the interaction of preexisting alpha7 integrin with its ligand, laminin, is a major determinant of allosteric changes that result in an activated form of alpha7 integrin capable of transducing signals from the extracellular matrix commensurate with secondary myofiber formation. Copyright 2001 Academic Press.