β1D integrin splice variant stabilizes integrin dynamics and reduces integrin signaling by limiting paxillin recruitment

Heterodimeric integrin receptors control cell adhesion, migration and extracellular matrix assembly. While the alpha integrin subunit determines extracellular ligand specificity, the beta integrin chain binds to an acidic residue of the ligand, and cytoplasmic adapter protein families such as talins, kindlins and paxillin, to form mechanosensing cell matrix adhesions. Alternative splicing of the beta 1 integrin cytoplasmic tail creates ubiquitously expressed beta 1A, and the heart and skeletal muscle-specific beta 1D form. To study the physiological difference between these forms, we developed fluorescent beta 1 integrins and analyzed their dynamics, localization, and cytoplasmic adapter recruitment and effects on cell proliferation. On fibronectin, GFP-tagged beta 1A integrin showed dynamic exchange in peripheral focal adhesions, and long, central fibrillar adhesions. In contrast, GFP-beta 1D integrins exchanged slowly, forming immobile and short central adhesions. While adhesion recruitment of GFP-beta 1A integrin was sensitive to C-terminal tail mutagenesis, GFP-beta 1D integrin was recruited independently of the distal NPXY motif. In addition, a P786A mutation in the proximal, talin-binding NPXY(783 )motif switched beta 1D to a highly dynamic integrin. In contrast, the inverse A786P mutation in beta 1A integrin interfered with paxillin recruitment and proliferation. Thus, differential beta 1 integrin splicing controls integrin-dependent adhesion signaling, to adapt to the specific physiological needs of differentiated muscle cells.