Abstract 16134: Semaphorin 4D Supports Communication Between Platelets in the Early Stages of Thrombus Formation
Platelets normally circulate as single cells, but form stable contacts with each other in response to vascular injury or disease. This process is dependent on cell adhesion molecules and enables platelet surface ligands to bind receptors on adjacent platelets. Our recent works suggests that one such ligand is Semaphorin 4D (Sema4D). We have previously reported that Sema4D is expressed on the surface of mouse and human platelets and have shown that mice lacking Sema4D have a selective defect in collagen-induced platelet activation and impaired responses in models of vascular injury. Here, we have examined the molecular mechanism involved in Sema4D action in platelets. The results show that Sema4D(-/-) platelets exhibit significantly less activation of the tyrosine kinase, Syk, downstream of the collagen receptor, GPVI/FcRγ complex, as well as an additional ITAM (immunoreceptor tyrosine-based activation motif) receptor, Clec-2. Consequently, PLCγ2 is hypophosphorylated and the release of intracellular Ca2+ is diminished leading to decreased activation of integrin αIIbβ3 and subsequent decreased platelet aggregation in the absence of Sema4D. Conversely, activation of Syk and integrin αIIbβ3 is indistinguishable from wild type platelets when platelets are prevented from coming in contact with each other. Thus, we propose a model in which Sema4D on the surface of one platelet binds its receptor on another platelet during the early stages of thrombus formation. The defect in Syk activation and platelet aggregation can be overcome by the addition of soluble recombinant Sema4D extracellular domain (rSema4D) providing additional evidence that Sema4D on the platelet surface acts as a ligand to promote Syk activation. The best characterized receptors for Sema4D are Plexin B family members (B1 and B2) and CD72. We have found that human platelets express Plexin B2 and CD72 while mouse platelets appear to express only Plexin B2. Sema4D(-/-) platelets also showed decreased activation of Rac1, which is known to be regulated downstream of Plexins. In conclusion, our data identify Sema4D as a novel regulator of contact-dependent signaling downstream of ITAM receptors in platelets and suggest that a Plexin family member mediates responses to Sema4D in platelets.
- © 2011 by American Heart Association, Inc.