Laminin stimulates spreading of platelets through integrin α6β1-dependent activation of GPVI

The extracellular matrix protein, laminin, supports platelet adhesion through binding to integrin alpha(6)beta(1) In the present study, we demonstrate that human laminin, purified from placenta, also stimulates formation of filopodia and lamellipodia in human and mouse platelets through a pathway that is dependent on alpha(6)beta(1) and the collagen receptor GPVI. The integrin alpha(6)beta(1) is essential for adhesion to laminin, as demonstrated using an alpha(6)-blocking antibody, whereas GPVI is dispensable for this response, as shown using knock-out mouse platelets. On the other hand, lamellipodia formation on laminin is completely inhibited in the absence of GPVI, although filopodia formation remains and is presumably mediated via alpha(6)beta(1) Lamellipodia and filopodia formation are inhibited in Syk-deficient platelets, demonstrating a key role for the kinase in signaling downstream of GPVI and integrin alpha(6)beta(1) GPVI was confirmed as a receptor for laminin using surface plasmon resonance spectroscopy and by demonstration of lamellipodia formation on laminin in the presence of collagenase. These results identify GPVI as a novel receptor for laminin and support a model in which integrin alpha(6)beta(1) brings laminin to GPVI, which in turn mediates lamellipodia formation. We speculate that laminin contributes to platelet spreading in vivo through a direct interaction with GPVI.