Platelet activation

In order for the recruitment of platelets to continue during the aggregation phase it is necessary that the activated state of the adhered and aggregated platelets can be propagated to newly arriving platelets. This propagation of activation is executed via potent autocrine and paracrine signaling pathways. Activated platelets will secrete abundantly of ADP and ATP from dense granule that in turn may activate neighboring platelets via ADP and ATP sensitive receptors. ADP is a ligand to the receptors P2Y1 and P2Y12, located on the platelet surface [25, 26]. The P2Y receptors are seven transmembrane spanning proteins that transmit outside-in signaling by coupling to G-proteins [27]. The P2X1 receptor is a ligand gated cation channel and was not until the year 2000 disclosed as a specific ATP receptor. Its main function has been suggested as potentiating mechanism for other activation pathways [28]. Following initial platelet activation, the signal molecule thromboxane A2 (TxA2) is rapidly synthesized and released by platelets. The target for TxA2 is the TPa receptor, a seven transmembrane spanning receptor that is coupled to G-proteins on the cytosolic side. TxA2 is a lipid signal molecule synthesized by arachidonic acid metabolism, and the synthesis can be effectively inhibited by aspirin treatment, resulting in the attenuation of platelet activation [29, 30]. During thrombus formation, the simultaneous activation of the coagulation cascade results in generation of the key coagulation enzyme thrombin. Thrombin has a dual role in haemostasis, as it in addition to facilitating the fibrin network formation also has a function as an activator of two distinct platelet receptors, namely the protease activated receptors (PARs) 1 and 4. Thrombin acts on the PARs by cleaving off a small peptide from its exposed extracellular part of the N-terminus. The cleavage results in a new tethered N-terminus that functions as a ligand and thereby mediates the activation of the receptor. The outside-in signaling event is further propagated on the intracellular side by coupled G-proteins [31, 32]. Although PAR1 and PAR4 couple to the same G-protein subtypes, PAR1 and PAR4 apparently differ in terms of affinity for thrombin and the duration of intracellular signalling. It has been proposed that PAR1 has a higher affinity for thrombin than PAR4 [33-35]; however, there is evidence that PAR4 is also activated by thrombin at low concentrations [35]. Furthermore, GPIba binds thrombin with high affinity and has been proposed as a cofactor for the PAR activation mechanism by co-localization of thrombin with the PAR receptors, thus facilitating their cleavage [36]. A large number of the platelet signaling receptors are coupled with G-proteins on the cytosolic side and such receptors are therefore termed G-protein coupled receptors (GPCRs). There are several types (and subtypes) of G-proteins in platelets and the most prominent a re Gq, Gi, Gz and G13. Their respective activation will propagate through different intracellular signaling pathways and lead to differential activation responses (reviewed in [37]). The platelet receptors are an exceptionally important part of the functionality of the haemostatic system and deficiency of functional platelet receptors often result in bleeding disorders (reviewed in [38]).

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