Loss of Sema4D Signaling in Platelets Impairs the Formation and Stability of Arterial Thrombi In Vivo and Reduces Myocardial Infarct Size.

Contact-dependent signaling between platelets helps to promote thrombus growth and stability. One mechanism for contact-dependent signaling involves the binding of cell surface ligands to corresponding receptors on the surface of adjacent cells. In our efforts to identify novel participants in this process, we have recently reported that platelets express on their surface the semaphorin family member, sema4D, and its two known receptors, CD72 and plexin-B1 (Zhu, et al, PNAS, 2007). We have also shown that although their initial tail bleeding time is normal, platelets from sema4D(−/−) mice have a defect in collagen-induced signaling and platelet aggregation in vitro. In the present studies, we used matched sema4D(−/−) and wild type (WT) mice to examine the consequences of impaired sema4D signaling in models of platelet function in vivo. In the first model, irradiated Rose Bengal dye was used to produce an arteriolar injury in an exteriorized cremaster muscle. Platelets were identified with a fluorescent CD41 antibody and detected in real time using digital microscopy. The results showed that thrombus formation occurred in all of the mice that were tested, but while stable occlusion was observed in approximately half of the control mice, none of the sema4D(−/−) mice developed stable occlusions during the period of observation (p<0.02). Similarly, when a laser was used to produce a focal injury in cremaster muscle arterioles, both the initial rate of platelet accumulation and the peak extent of accumulation were approximately 50% lower in the sema4D(−/−) mice than in the matched controls. To test the contribution of sema4D to platelet responses in a larger artery, the right common carotid was injured by transient exposure to FeCl3 and changes in flow were measured using a Doppler probe. The results showed that the time to occlusion was 35% greater in the sema4D(−/−) mice than in controls (p<0.02). Furthermore, stable occlusion occurred in only 9 of 16 (56%) sema4D(−/−) mice Vs. 7 of 9 (78%) WT mice. Finally, myocardial infarct size was measured in an ischemia/reperfusion injury model 48 hrs after transient ligation of the left anterior descending coronary artery. Although infarction occurred in all cases, infarct volume was 56% smaller in the sema4D(−/−) mice than the matched controls (p<0.01). In summary, these results show that there is a substantial impairment of platelet function in vivo in mice that lack sema4D. This impairment was observed in both arterioles and arteries using several different methods to evoke platelet activation. When combined with our earlier observations, the results show that signaling by sema4D and its receptors provides a novel mechanism to promote thrombus growth and stability.