| Platelet accumulation and activation is the direct cause of many vasculardisease such as acute coronary syndrome and stroke. It is so important to study themechanism of platelet accumulation and activation to reduce thrombotic disease.Platelet activation is by receptor mediated signaling in response to stimuli of varyingpotency such as collagen, thrombin, ADP and thromboxane A2(TxA2). Themulti-signaling made co-contribution for platelet accumulation and thrombin formation,finally formed a structure contained activated platelet and fibrin.A recent study demonstrated that platelet accumulation following vascular injury invivo is hierarchically organized resulting in a structure comprised of a core offully-activated platelets that is overlaid with an unstable shell of less activated platelets.This structure results from different elements of the platelet signaling network givingrise to regions that differ in platelet activation state, packing density, and stability. It wasthus proposed that regional differences in platelet activation reflect regional differencesin the distribution of platelet agonists. This provides new insights into heterogeneousplatelet activation during platelet accumulation in vivo. Thromboxane A2(TxA2), adominant prostanoid product of cyclooxygenase1(COX-1) generated in platelets, is aplatelet agonist and plays an important role in the maintenance of vascular hemostasis.It is a major therapeutic target of anti-platelet therapy. But its contribution to theregional architecture of a platelet mass is unknown.To determine the TxA2signaling in hierarchical organization of a thrombus,multicolor intravital microscopy was used to observe platelet accumulation andactivation in thromboxane A2receptor knockout (TP-/-) mice, low dose aspirin treatedWT mice and COX-1knockdown mice following laser-induced injury in mousecremaster arterioles.TP-/-mice showed reduced total platelet (CD41) accumulation following vascular injury, consistent with a previous report. Interestingly, the core area of the thrombus inwhich the platelets are fully activated (P-selectin+), was similar between TP-/-and WTduring thrombus formation. This suggests that TxA2signaling via the TP receptorprimarily influences platelet recruitment and retention in the outer shell region of aplatelet mass, but not full platelet activation in the core region.Aspirin inhibits TxA2production through acetylation of COX-1, and is widelyused as both primary and secondary prevention of cardiovascular diseases. We foundthat aspirin treatment reduced total platelet accumulation following laser-induced injuryin vivo. In contrast to our findings in the TP-/-mice, low dose aspirin also resulted inreduced platelet activation and core region formation at later time points. We found thesimilar result in COX-1knockdown mice that both the “shell” and “core” area werereduced compare to the WT mice. This suggests that additional COX-1metabolites maycontribute to full platelet activation independent of TP receptor signaling.As a conclusion, our studies show for the first time the role of TxA2signaling inproducing the hierarchical structure of a platelet mass formed in response to vascularinjury in vivo. The results show:1) directly blocking TxA2-TP signaling by targetingTP influences the outer shell but not the core.2) In ASA-treated or COX-1knockdown(KD) mice, both the shell and core area are reduced.Our data indicate that TxA2signaling is critical for recruitment and/or retention ofplatelets in the outer shell region of a developing hemostatic plug. The data also suggestthat additional COX-1metabolites may contribute to full platelet activation in the coreregion.These findings further highlight the importance of discrete spatial localization ofplatelet agonists within an evolving platelet plug in order to achieve the optimalhemostatic response. The results open a new theory for thrombus formation and providean experimental basis for the regulation of thrombotic-related disorders. |
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