Integrin α₂β₁ mediates outside-in regulation of platelet spreading on collagen through activation of Src kinases and PLCγ2

Collagen plays a critical role in hemostasis by promoting adhesion and activation of platelets at sites of vessel injury. In the present model of platelet–collagen interaction, adhesion is mediated via the inside-out regulation of integrin α₂β₁ and activation through the glycoprotein VI (GPVI)–Fc receptor (FcR) γ-chain complex. The present study extends this model by demonstrating that engagement of α₂β₁ by an integrin-specific sequence from within collagen or by collagen itself generates tyrosine kinase–based intracellular signals that lead to formation of filopodia and lamellipodia in the absence of the GPVI–FcR γ-chain complex. The same events do not occur in platelet suspensions. α₂β₁ activation of adherent platelets stimulates tyrosine phosphorylation of many of the proteins in the GPVI–FcR γ-chain cascade, including Src, Syk, SLP-76, and PLCγ2 as well as plasma membrane calcium ATPase and focal adhesion kinase. α₂β₁-mediated spreading is dramatically inhibited in the presence of the Src kinase inhibitor PP2 and in PLCγ2-deficient platelets. Spreading is abolished by chelation of intracellular Ca²⁺. Demonstration that adhesion of platelets to collagen via α₂β₁ generates intracellular signals provides a new insight into the mechanisms that control thrombus formation and may explain the unstable nature of β₁-deficient thrombi and why loss of the GPVI–FcR γ-chain complex has a relatively minor effect on bleeding.