PECAM-1 Is a Negative Regulator of Platelet Activation by Laminin.

Interaction of resting platelets with exposed components of the subendothelial matrix is an important activating event that takes place early on at sites of vascular injury. Platelet responses to collagen are mediated both by the integrin α2β1 and the glycoprotein (GP)VI/Fc receptor (FcR) γ chain complex. Upon collagen binding, Src family kinases phosphorylate tyrosines within Immunoreceptor Tyrosine-based Inhibitory Motifs (ITAMs) that are present within the cytoplasmic domain of the FcRγ chain. These phosphorylated ITAMs serve as docking sites for the protein-tyrosine kinase Syk, which, when recruited and activated, initiates the rapid assembly of a series of adaptor proteins and protein-tyrosine kinases that are crucial for platelet activation responses. Recently, platelet activation by laminin, which binds the related integrin α6β1, was shown to also require signaling through the GPVI/FcRγ chain complex. Because the adhesion and signaling receptor, PECAM-1, serves as a negative regulator of collagen-induced platelet activation, we sought to determine whether PECAM-1 might similarly regulate platelet activation by laminin. We found that PECAM-1 became rapidly tyrosine phosphorylated on its cytoplasmic ITIMs following adhesion of either human or murine platelets to immobilized laminin, suggesting that it may be involved in modulating platelet responses to laminin. PECAM-1-deficient murine platelets exhibited both an increased rate and extent of cell spreading on immobilized laminin compared to their PECAM-1-positive, wild-type (wt) counterparts. Granule secretion, as determined by release of serotonin, was also augmented. Taken together, these data support the notion that PECAM-1 is a negative regulator of platelet responses to laminin.