Biphasic Kinetics of Activation and Signaling for PAR1 and PAR4 Thrombin Receptors in Platelets

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Abstract
Thrombin activates platelets in an ordered sequence of events that includes shape change, increase in cytoplasmic Ca2+, activation of the αIIbβ3 integrin, granule secretion, aggregation, and formation of a stable hemostatic plug. Activation of this process has also been implicated in the pathogenesis of atherosclerosis, stroke, and thrombosis. There are two identified thrombin-activated receptors on the surface of human platelets. PAR1 is a high-affinity thrombin receptor, and PAR4 is a low apparent affinity thrombin receptor of uncertain function. The goal of these studies is to determine the kinetics of thrombin activation of PAR1 and PAR4 and to relate the individual inputs from each receptor to platelet Ca2+ signaling, secondary autocrine stimulation, and aggregation. Using a combination of PAR-specific peptide ligands and anti-PAR1 reagents, we separated the biphasic thrombin Ca2+ response of platelets into two discrete componentsa rapid spike response caused by PAR1, followed by a slower prolonged response from PAR4. Despite having a 20−70-fold slower rate of activation, PAR4 produces the majority of the integrated Ca2+ signal that is sustained by the continuous presence of catalytically active thrombin. Surprisingly, PAR4 activation is much more effective than PAR1 activation in mounting secondary autocrine Ca2+ signals from secreted ADP. The strong ADP response due to activated PAR4, however, requires prior activation of PAR1 as would normally occur during treatment of platelets with thrombin. Thus, the late signal generated by activated PAR4 is not redundant with the early signal from PAR1 and instead serves to greatly extend the high intracellular Ca2+ levels that support the late phase of the platelet aggregation process.