HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY A novel Syk-dependent mechanism of platelet activation by the C-type lectin receptor CLEC-2

The snake venom rhodocytin has been reported to bind to integrin 21 and glycoprotein (GP) Ib on platelets, but it is also able to induce activation indepen-dent of the 2 receptors and of GPVI. Using rhodocytin affinity chromatography, we have identified a novel C-type lectin recep-tor, CLEC-2, in platelets that confers sig-naling responses to rhodocytin when ex-pressed in a cell line. CLEC-2 has a single tyrosine residue in a YXXL motif in its cytosolic tail, which undergoes tyrosine phosphorylation upon platelet activation by rhodocytin or an antibody to CLEC-2, but not to collagen, thrombin receptor agonist peptide (TRAP), or convulxin. Ty-rosine phosphorylation of CLEC-2 and other signaling proteins by rhodocytin is inhibited by the Src family kinase inhibi-tor PP2. Further, activation of murine platelets by rhodocytin is abolished in the absence of Syk and PLC2, and partially reduced in the absence of LAT, SLP-76, and Vav1/Vav3. These findings define a novel signaling pathway in platelets whereby activation of CLEC-2 by rhodocy-tin leads to tyrosine phosphorylation of its cytosolic tail, binding of Syk and initia-tion of downstream tyrosine phosphoryla-tion events, and activation of PLC2. CLEC-2 is the first C-type lectin receptor to be found on platelets which signals through this novel pathway. (Blood. 2006