Platelet activation and coagulation normally do not occur in an intact blood vessel. After blood vessel wall injury, platelet plug formation is initiated by the adherence of the platelets to subendothelial collagen [1,2]. In high shear arterial blood, platelets are first slowed down from their blood flow velocity by interacting with the collagen-bound von Willebrand factor and are subsequently stopped by binding directly to the collagen by their glycoprotein (GP) receptor complex [2,3]. The activation of these collagen receptors on platelets after their binding to the collagen activates phospholipase C-mediated cascades (Fig. 1) [1–3]. This results in the mobilization of calcium from the dense tubular system [4,5]. An increase in intracellular calcium is associated with the activation of several kinases necessary for morphologic change, the presentation of the procoagulant surface, the secretion of platelet granular content, the activation of GPs, and the activation of phospholipase A2 (see Fig. 1) [2,5–7]. The presentation of the procoagulant surface results in the colocalization of different coagulation factors on the surface of the activated platelet, which triggers a series of zymogen conversions, resulting in the release of active thrombin from prothrombin [8]. Adenosine diphosphate (ADP), adenosine triphosphate, and serotonin are released from the dense platelet granule. Activated phospholipase A2 enzymes release arachidonic acid (AA) by the cleaving of fatty acids, especially phosphatidylcholine and phosphatidylethanolamine, at their sn-2 position [9–11]. AA is a precursor for thromboxane A2 (TBXA2) synthesis. In the first step in platelets, prostaglandin (PG)-endoperoxide synthase 1 (PTGS1; also known as cyclooxygenase 1) catalyzes the transformation of AA into cyclic endoperoxide PG G2 and H2 [9]. In platelets, PGG2 and PGH2 are then mainly converted by TBXA synthase into TBXA2 [9].
Fig. 1
Effects of antiplatelet drugs in platelet aggregation pathway. (PA154444041; http://www.pharmgkb.org/do/serve?objId=PA154444041o cAMP, cyclic AMP; GNAS, guanine nucleotide binding protein a s; IP3, inositol ...
The mechanism of action of aspirin is the inhibition of PTGS1, thereby preventing the production of PGs and, particularly in platelets, inhibiting TBXA2 production [10–12]. In ex vivo platelet aggregation testing, aspirin affects predominantly AA-stimulated platelet aggregation through a direct pathway, and also collagen-stimulated platelet aggregation through indirect pathways. A review by Lopez Farre et al. [12] discusses further mechanisms associated with platelet response to aspirin.
The processes described above result in the local accumulation of molecules such as thrombin, TBXA2, and ADP, which are important for the further recruitment of platelets and the amplification of activation signals as described above. The secreted agonists activate their respective G protein-coupled receptors: coagulation factor II (thrombin) receptors (F2R also known as protease-activated receptor 1; F2RL3 also known as protease-activated receptor 4), TBXA2 receptor (TBXA2R), and ADP receptors (P2RY1 and P2RY12) [10,11,13–15]. The P2RY12 receptor couples to Gi, and when activated by ADP, inhibits adenylate cyclase [16]. This interaction counteracts the stimulation of cyclic AMP formation by endothelial-derived PGs, which alleviates the inhibitory effect of cyclic AMP on inositol 1,4,5-trisphosphate-mediated calcium release [14,16–20]. P2RY12 has a major role in arterial thrombosis and pharmacologic targeting of this receptor, which is an important strategy in the treatment of cardiovascular diseases [21]. Thienopyridines (ticlopidine, clopidogrel, prasugrel), a class of oral anti-platelet agents, permanently inhibit P2RY12 signaling by irreversibly binding the receptor and blocking ADP-induced platelet activation and aggregation [22].
F2R, TBXA2R, and P2RY1 couple to Gq-phospholipase C–inositol 1,4,5-trisphosphate–Ca2+ pathway, inducing shape change and platelet aggregation [14,23,24]. In addition, receptor signaling by G12/13 (F2R; TBXA2R) contributes to morphologic changes through the activation of kinases [23,24]. Platelet adhesion, cytoskeletal reorganization, secretion, and amplification loops are all different steps toward the formation of a platelet plug. These cascades finally result in the activation of the fibrinogen receptor (GPIIb/GPIIIa) expressed on platelet cells [14,25,26]. This activation results in the exposure of the binding sites for fibrinogen, which are not available in inactive platelets. The binding of fibrinogen results in the linkage of the activated platelets through fibrinogen bridges, thereby mediating aggregation [3]. The inhibition of this receptor by GPIIb/GPIIIa inhibitors blocks platelet aggregation induced by any agonist [27,28].
The individual platelet response is variable because of polymorphisms in genes involved in the activation and aggregation of platelets, in conjunction with environmental factors, and contributes to diseases such as arterial thrombosis ([29–33], and http://www.bloodomics.org/web/). In addition to the variation in platelet physiology, platelet sensitivity to drugs targeting platelet activation and aggregation is also influenced by gene polymorphisms and clinical and environmental variables [29,31,34].
