Adenosine Diphosphate to Stop Loss of Blood - Essay Example

How does adenosine diphosphate (ADP) activate platelets? Loss of blood is stopped on damage of a blood vessel and healing occurs in a series of overlapping processes, in which platelets play a vital role (Waugh 2006). Exposure of sub endothelial collagen, activation of platelets, and the initiation of the coagulation cascade follow a vascular injury. (Kahner et al. 2006). The platelets are activated by the released ADP in an autocrine and paracrine fashion, from damaged cells at places of vascular injury (Offerman et al. 2006). Activation of platelets at sites of vascular injury Foster et al. (2001) mentioned that ADP acts through multiple receptors and is an important mediator of platelet aggregation. ADP induces platelet shape change, calcium flux, and inhibition of adenylyl cyclase, leading to aggregation. Kahner et al. (2006) also stated that rapid shape change, release of granular contents, generation of lipid mediators, and aggregation occur during platelet activation. Various surface receptors involved in platelet activation include G protein coupled receptors (GPCRs), integrins and glycoprotein receptors. Adenosine diphosphate (ADP) (released from platelet granules) and thromboxane A2 (generated within platelets) are the secondary mediators that activate other resting platelets, resulting in the amplification of initial physiological haemostatic response. Adenosine nucleotides are released following platelet activation signal through the P2 purinergic receptors on the platelet membrane. The two types of P2 receptors are ligand-gated cation channels (P2X), or GPCRs (P2Y). Till date, seven different P2X receptors (P2X1)) and eight distinct P2Y receptors (P2Y1,2,4,6,11-14) have been identified which were cloned from mammalian tissues. P2Y1, P2Y12, andP2X1 subtypes are the P2 purinergic receptors expressed on platelets and their physiologic agonists are ADP, ADP, and adenosine triphosphate (ATP), respectively. These receptors, when activated by the agonists, initiate a complex signaling cascade that ultimately results in platelet activation and thrombus formation. The physiological agonist for the P2Y1 receptor is ADP. This receptor can also be stimulated in vitro by 2MeSADP, ADP, APDaS and ADPbS in the decreasing order of potency and is involved platelet shape change and aggregation brought about by ADP. P2Y12 is coupled to Gi2 protein. Upon stimulation, the Ga and Gbc subunits of the heterotrimeric G protein dissociate and activate various signaling pathways. The Gai2 is found to be responsible for ADP-mediated inhibition of adenylyl cyclase and subsequent reduction in cytosolic cAMP concentrations. This is one of the important events for ADP-induced fibrinogen receptor activation and platelet aggregation. A variety of cellular factors such as phosphoinositide-3kinases (PI3K), Akt/protein kinase B, Rap1b, Src family tyrosine kinases, and G protein-gated potassium channels (GIRKs) are activated by the released Gbc dimmers. Several biological processes such as platelet aggregation, shear-induced platelet aggregation, cytoplasmic calcium mobilization, secretion from alpha, and dense granules, platelet procoagulant activity, thromboxane A2 generation, thrombus growth and stability, and various protein phosphorylation events, are regulated by the Gi-coupled P2Y12 receptor signaling pathway. The Gi-coupled P2Y12 signaling pathway also causes fibrinogen receptor activation and platelet aggregation by coupling with G12/13 or the Gz pathways. Recent studies also report that several other Src kinases, in addition to c-Src kinase can be activated downstream of selective P2Y1 or P2Y12 receptor stimulation. It has also been found that Src family kinase selective inhibitor, PP2, abolished P2Y12- receptordependent Ser473 phosphorylation of Akt. ADP-induced cPLA2 phosphorylation and thromboxane A2 generation were abolished by Src inhibition, indicating that they are upstream regulators of both these events. Hence, Src kinases appear to play an important role in P2Y12 signal transduction ((Kahner et al. 2006). Dorsam et al. (2004) in a review stated that P2Y12 receptor is the target of P2Y12 receptor antagonists that have proven therapeutic value. Hollopeter et al. (2002) demonstrated that the P2Y12 receptor is selectively distributed in tissues, which becomes a very potent target for the new antithrombotic drugs which are being developed. Andre et al. (2003) reported that platelet adhesion/activation, thrombus growth, and stability are the key steps in thrombosis in which P2Y12 is involved. The receptor P2Y1 has a major role in the platelet shape change induced by ADP. Platelets undergo shape change from smooth discs to speculated spheres, activate the fibrinogen receptor, leading to the aggregation of platelets, releasing granule contents, and producing thromboxane A2 on exposure to ADP (Jin et al. 1998). Upon cloning a P2Y1 receptor cDNA clone platelet cDNA library of human, the receptor P2Y1mediated mobilization of calcium from intracellular stores and shape change in platelets. which is induced by ADP. Certain P2Y1 selective antagonists such as adenosine 3*-phosphate 5*- phosphate (A3P5P, adenosine 2*phosphate 5*- phosphate (A2P5P) and adenosine 3*-phosphate 5*phosphosulfate (A3P5PS), were also investigated for ADP -induced intracellular calcium increases. Their finding supported the idea that several P2 receptors are involved in the regulation of different aspects of platelet stimulus- response coupling. In platelet shape change induced by ADP, the P2TAC -adenylyl cyclase, and the P2X1-, coupled to rapid calcium influx were not found to play an important role. Savi et al. (1998) determined if the P2Y1 purinoceptor is the receptor responsible for ADP induced platelet aggregation and the target of clopidogrel. The PCR product analysis confirmed the presence of mRNA of the P2Y1 receptor in platelets. The scientific meeting held at LA, Thuile, Italian Alps, in March, 2000 discussed about the role of these three purinergic receptors. All the investigators working in this field had a similar opinion. (Cattaneo &Gachet 2001). Kunapuli et al. (2003) confirmed the importance of both P2Y1 and P2Y12 receptors in platelet recruitment to a thrombus forming under flow conditions. Blood from a patient with a defective P2Y12 receptor, when perfused over collagen or immobilized fibrinogen, formed significantly smaller thrombi on both surfaces compared with control blood P2Y1 receptor antagonism also led to the formation of smaller thrombi. Nancy et al. (2001) described the relative importance of the different ADP receptor subtypes, P2Y12 and P2Y1, in 2 types of platelet aggregation under arterial flow conditions: direct shear-induced platelet aggregation in a cone-and plate viscometer and platelet adhesion, followed by aggregation, on an insolubilized vWf-collagen 1 surface using a whole blood perfusion system. Their results indicated that ADP receptor antagonists P2Y12 and P2Y1 in combination effectively inhibited the direct shear-induced platelet aggregation and also platelet aggregation subsequent to initial adhesion under flow conditions. Their conclusion from the study was that the inhibitors of these pathways can be used in a potential manner as antiarterial thrombotic agents. Remijn et al. (2002) investigated the role of P2Y12 in platelet adhesion and thrombus formation under physiological flow by using blood from a patient with a defect in the gene encoding P2Y12. Using human blood deficient in the ADP receptor P2Y12, this was the first study reported on platelet adhesion and thrombus formation in flow. Their findings for the first time indicate that P2Y12-dependent activation of platelets by ADP is involved in platelet adhesion to fibrinogen as well as in collagen-induced formation of densely packed large thrombi under physiological flow conditions Nitric oxide (NO) is an established regulator of platelet function. Oberprieler et al. (2007) studied the importance of Ca2+ and phosphoinositol-3-kinase (PI3kinase) as targets for NO signaling, in the physiological context of platelet adhesion using adenosine diphosphate (ADP) stimulated adhesion to immobilized fibrinogen. As indicated by biochemical studies in suspended platelets, Ca2+ mobilization and PI3kinase represent targets for NO signaling. These targets have never been studied in combination or in the physiological context of ADP-induced adhesion. There was an in inhibition of adhesion by wortmannin by NO, as reported by the authors. This suggested an action independent of PI3kinase most likely on Ca2+ mobilization as observed in suspended platelets. The contribution of platelet ADP receptors to the initiation of intravascular coagulation was studied by Leon et al (2004). The P2Y1 and P2Y12 receptors on association with ADP was involved in the fast activation of the blood-based tissue factor (TF). The intraplatelet ADP secreted into the extracellular environment binds to its cell membrane receptors, thereby enhancing the-granule release when platelet interacts with collagen. Therefore, prestored TF and P-selectin are likely to be exposed which consequently activates GPIIb/IIIa (_IIb_2 integrin). This further enhances secretion via outside-in signaling. Adhesive interactions between the activated platelets and the neutrophils results in the functional activation of the rapidly exposed TF. The ADP receptors play an important role in sustaining the functional competence of the intravascular TF. An important observation following this event is initiation of coagulation and thrombus development. There appears to be a close temporal and spatial coupling between the platelet recruitment. Leon et al (2004) characterized the role of P2Y12 and P2Y1 receptors in thrombin generation. They concluded that the these receptors participate in a different manner in the generation of thrombin. Both receptors participate in leukocyte TF exposure in whole blood, whereas mostly the P2Y12 receptor has a major role in the TF-induced thrombin formation in PRP, probably by mediating negative PS exposure on platelets. Their results, together with the increased thromboresistance observed in mice when both ADP receptors are blocked, provide further evidence for the relevance of molecules targeting these receptors as antithrombotic agents. In conclusion, platelets express at least 3 distinct purinergic receptors that bind ADP-the ligand-gated ion channel P2X1 receptor and 2 distinct Gprotein- linked P2 receptors, P2Y12 and P2Y1. ADP binding to the P2X1 receptor causes the movement of Ca from the exterior of the platelet to the cytoplasm. Binding of ADP to P2Y1 activates platelet phospholipase C and mobilizes Ca from the intracellular stores, leading to the protein kinase C activation and washed platelets aggregation . ADP binding to P2Y12 inhibits platelet adenylyl cyclase, lowers platelet cyclic adenosine monophosphate levels, and potentiates ADP-induced aggregation. References Read More