Muscle β1D integrin reinforces the cytoskeleton-matrix link:: Modulation of integrin adhesive function by alternative splicing

Expression of muscle-specific beta 1D integrin with an alternatively spliced cytoplasmic domain in CHO and GD25, beta 1 integrin-minus cells leads to their phenotypic conversion, beta 1D-transfected nonmuscle cells display rounded morphology, lack of pseudopodial activity, retarded spreading, reduced migration, and significantly enhanced contractility compared with their beta 1A-expressing counterparts. The transfected beta 1D is targeted to focal adhesions and efficiently displaces the endogenous beta 1A and alpha v beta 3 integrins from the sites of cell-matrix contact, This displacement is observed on several types of extracellular matrix substrata and leads to elevated stability of focal adhesions in beta 1D transfectants. Whereas a significant part of cellular beta 1A integrin is extractable in digitonin, the majority of the transfected beta 1D is digitonin-insoluble and is strongly associated with the detergent-insoluble cytoskeleton. Increased interaction of beta 1D integrin with the actin cytoskeleton is consistent with and might be mediated by its enhanced binding to talin, In contrast, beta 1A interacts more strongly with alpha-actinin, than beta 1D. Inside-out driven activation of the beta 1D ectodomain increases ligand binding and fibronectin matrix assembly by beta 1D transfectants, Phenotypic effects of beta 1D integrin expression in nonmuscle cells are due to its enhanced interactions with both cytoskeletal and extracellular ligands. They parallel the transitions that muscle cells undergo during differentiation. Modulation of pi integrin adhesive function by alternative splicing serves as a physiological mechanism reinforcing the cytoskeleton-matrix link in muscle cells. This reflects the major role for beta 1D integrin in muscle, where extremely stable association is required for contraction.