Regulation of Platelet Reactivity by S1P Signaling

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The preeminent cause of morbidity and mortality in humans is cardiovascular disease (CVD). Race is considered to be an important determinant of the outcome of CVD since compared to whites, blacks have a twofold higher incidence of CVD even when other confounding factors are accounted for. Myocardial infarction and stroke result from formation of occlusive platelet thrombus at the site of atherosclerotic plaque rupture. Recently, Dr. Edelstein and colleagues have shown that differences in platelet reactivity to protease activated receptor 4-activating peptide (PAR4-AP) is heritable and is associated with racial disparity. They have identified two probable candidates, a variant of PAR4 protein (120A-T) and increased expression levels of phosphatidyl choline transfer protein (PCTP) to be contributing for the observed increase in platelet reactivity to PAR4-AP and ethnic differences seen. However, how these two independent proteins may regulate platelet reactivity to PAR4-AP is not well understood. We hypothesize that the hyperresponsive PAR4 variant associates with neutral sphingomyelinase (n-SMase), a key enzyme involved in synthesis of S1P, a potent bioactive lipid that binds to its receptor on the platelet surface to potentiate platelet reactivity. Enhanced levels of PCTP provide an increased amount of sphingomyelin (SM) required for S1P synthesis. Together, they enhanced recruitment of platelets in the growing thrombus thus influencing the outcome of CVD. To test this hypothesis, we propose the following three specific aims. Specific Aim 1: To evaluate the role of S1P synthetic pathway in influencing PAR4-induced signaling in platelets. We will evaluate if thrombin/PAR4-AP will differentially induce n-SMase activity as well as S1P levels in a PAR4 genotype specific manner. Next, we will determine if association of PAR4 with n-SMase is genotype specific and whether this association will influence association of PAR4 with G?q. Furthermore, we will test if blockade of enzymes of S1P synthetic pathway and S1PR1/2 receptors for S1P on platelets will attenuate PAR4-AP-induced platelet function in PAR4 genotype dependent manner. We will also determine if genetic ablation of key enzymes of S1P synthesis using CRISPR/Cas9 technology in MEG-01 cells expressing PAR4 hyperreactive variant will rescue platelet hyperreactivity. In Specific Aim 2, we will evaluate the regulation of S1P synthesis in platelets by CIB1. It has been shown that CIB1 bind Sphk1/2 and regulate their activity in cells. We will first determine if Cib1 regulate thrombin-induced S1P synthesis using Cib1-/- mouse platelets. In Specific Aim 3, we will assess the role of platelet?derived S1P in regulation of thrombosis and transient ischemic stroke (tMCAO). We will evaluate if platelet specific deletion of Cib1 or inhibition of S1P signaling in mice expressing human PAR4 variants will differentially alter in vivo thrombotic and ischemic stroke outcomes. Since racial disparity in thrombosis and ischemic stroke has been well documented, our study may identify the factor responsible for this disparity and provide avenues for therapeutic intervention.
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