Diminished contact-dependent reinforcement of Syk activation underlies impaired thrombus growth in mice lacking Semaphorin 4D

Abstract

We recently reported that Semaphorin 4D (Sema4D) and its receptors are expressed on the platelet surface and showed that Sema4D (

Hierarchical organization in the hemostatic response and its relationship to the platelet-signaling network

“…As suggested by earlier studies, 5 inhibition of thrombin activity significantly impairs full platelet activation and formation of a definable core region. In the present study we also show that contact-dependent signaling by semaphorin 4D 12,13 regulates the size of the core, while shell size is most heavily influenced by ADP and regulators of G i2 -mediated signaling. Thus, the ability of multiple agonists to activate platelets in vitro can be seen not as representing redundancy but as a means to sculpt and limit the hemostatic response in vivo.…”

“…by guest www.bloodjournal.org From conditions that permit platelet-platelet interactions. 12,13 Because greater packing density would be expected to facilitate contactdependent signaling, we hypothesized that loss of Sema4D would preferentially impact the core region, with only a secondary effect on formation of the outer shell. Consistent with our previous report, 23 overall platelet accumulation measured with anti-CD41 was substantially reduced in Sema4D 2/2 mice ( Figure 5A).…”

“…In contrast to the Sema4D knockout, platelets from G i2 a(1/G184S) mice display a gain of function phenotype because the substitution renders G i2 a insensitive to feedback inactivation by RGS proteins. 15,22 This increases platelet function in vitro, in part, by increasing the duration of signaling downstream of the platelet ADP receptor, P2Y 12 , and enhances overall thrombus size in vivo. 22 New measurements performed here show that this increase is due solely to expansion of the P-selectin-negative shell region ( Figure 5C-D).…”

“…The impact of a thrombin inhibitor and a P2Y 12 antagonist on hemostatic plug architecture Two well-characterized inhibitors of thrombin and P2Y 12 receptors were used to map the effects of thrombin and ADP on the hierarchical organization of the hemostatic response. G i2 is the primary mediator of platelet responses downstream of the ADP receptor, P2Y 12 .…”

“…G i2 is the primary mediator of platelet responses downstream of the ADP receptor, P2Y 12 . 24,25 Consistent with the results in the G i2 a(1/G184S) mice, addition of the direct-acting P2Y 12 antagonist, cangrelor, significantly reduced the size of the hemostatic plug shell without substantially affecting the core region ( Figure 6A-B and supplemental Video 5), indicating that ADP is critical for recruitment and retention of platelets in the shell. Thrombin, on the other hand, appears to be the primary factor in platelet activation within the core.…”