The laminin-binding activity of the alpha 7 integrin receptor is defined by developmentally regulated splicing in the extracellular domain

The expression pattern of the laminin-binding alpha 7 beta 1 Integrin is developmentally regulated in skeletal, cardiac, and smooth muscle. The X1/X2 alternative splicing in the extracellular domain of alpha 7 is found in the variable region between conserved alpha-chain homology repeat domains III and IV, a site implicated in ligand binding. To assess differences in X1/X2 isoform activity, we generated MCF-7 cell lines transfected with alpha 7-X1/X2 cDNAs. Transfectants expressing the alpha 7-X2 variant adhered rapidly to laminin 1, whereas those expressing alpha 7-X1 failed to attach. That alpha 7-X1 exists in an inactive state was established in assays using an activating beta 1 antibody that induced X1-dependent cell adhesion and spreading. Furthermore, the activation of alpha 7-X1 was cell type specific, and when expressed in HT1080 cells, the Integrin was converted into a fully functional receptor capable of promoting adhesion. Thus, the expression of the alpha 7-X1/X2 Integrin is a novel mechanism that regulates receptor affinity states in a cell-specific context and may modulate integrin-dependent events during muscle development and repair.