[Identification of the novel platelet activation receptor CLEC-2 and Its pathological and physiological roles]

We have identified a novel class of platelet activation receptor, CLEC-2, as a receptor for rhodocytin, a platelet-activating snake venom. CLEC-2 activation leads to tyrosine phosphorylation of the YITL motif in its cytoplasmic tail, the binding of Syk, initiation of downstream tyrosine phosphorylation events, and activation of phodpholipas Cgamma2, which result in platelet aggregation. We also identified podoplanin as an internal ligand for CLEC-2. Podoplanin is expressed on the surface of tumor cells and facilitates tumor metastasis by inducing platelet aggregation. We showed that an antibody that blocked the binding between CLEC-2 and podoplanin inhibited tumor metastasis using an experimental lung metastasis model in mice. Podoplanin is also expressed in lymphatic endothelial cells, but the physiological role of the interaction between CLEC-2 and podoplanin in lymphatic endothelial cells has not been elucidated. We generated CLEC-2-deficient mice, and found that these mice die in the embryonic/neonatal stages, associated with disorganized and blood-filled lymphatic vessels and severe edema. Moreover, through the transplantation of fetal liver cells from CLEC-2 +/+ or CLEC-2-/- embryos, we were able to demonstrate that CLEC-2 is involved in thrombus stabilization in vitro and in vivo, possibly through homophilic interactions without any apparent increase in the bleeding tendency. These findings revealed that CLEC-2 plays a crucial role in not only tumor metastasis, but also in lymphangiogenesis and thrombus stabilization. We propose that CLEC-2 could be an ideal novel target protein for an anti-platelet drug, which inhibits pathological thrombus formation but not physiological hemostasis, as well as a novel target protein for an anti-metastatic drug.