Introduction
Platelet activation is critical for normal hemostasis, and platelet-dependent arterial thrombosis underlies most myocardial infarctions. Thrombin is the most potent activator of platelets.1,2 For this reason, understanding the process by which thrombin activates platelets is necessary for understanding hemostasis and thrombosis and may yield novel anti-platelet therapies. This chapter focuses on our recent studies of the receptors that mediate activation of human platelets by thrombin.3,4
Thrombin signaling is mediated, at least in part, by a family of G protein-coupled protease-activated receptors (PARs), for which PAR1 is the prototype.5,6 PAR1 is activated when thrombin binds to and cleaves its amino terminal exodomain to unmask a new receptor amino terminus.5 This new amino terminus then serves as a tethered peptide ligand, binding intramolecularly to the body of the receptor to effect transmembrane signaling.5,7,8 The synthetic peptide SFLLRN, which mimics the first six amino acids of the new amino terminus unmasked by receptor cleavage, functions as a PAR1 agonist and activates the receptor independent of thrombin and proteolysis.5,9,10 Such peptides have been used as pharmacological probes of PAR function in various cell types.
Our understanding of the role of PARs in platelet activation is evolving rapidly. PAR1 mRNA and protein were detected in human platelets,5,11-13 SFLLRN-activated human platelets,5,9,10 and PAR1-blocking antibodies inhibited human platelet activation by low, but not high, concentrations of thrombin.11,12 These data suggested a role for PAR1 in activation of human platelets by thrombin but left open the possibility that other receptors contribute.
Curiously, PAR1 appeared to play no role in mouse platelets.14-16 PAR1-activating peptides did not activate rodent platelets, and platelets from PAR1-deficient mice responded like wild-type platelets to thrombin.16 The latter observation prompted a search for additional thrombin receptors and led to the identification of PAR3.17 PAR3 is activated by thrombin and is expressed in mouse platelets. PAR3 blocking antibodies inhibited mouse platelet activation by low, but not high, concentrations of thrombin,18 and knockout of PAR3 abolished mouse platelet responses to low, but not high, concentrations of thrombin.3 These results established that PAR3 is necessary for normal thrombin signaling in mouse platelets but also pointed to the existence of another mouse platelet thrombin receptor. Such a receptor, PAR4, was recently identified.3,19 PAR4 appears to function in both mouse and human platelets.3 The role of PAR3 in human platelets, if any, remains to be determined, and whether still unidentified receptors contribute to thrombin activation of platelets is unknown. Nonetheless, available data suggest a testable, working model in which PAR3 and PAR4 mediate thrombin activation of mouse platelets and PAR1 and PAR4 mediate activation of human platelets.
To determine the roles of PAR1, PAR3, and PAR4 in activation of human platelets by thrombin, we examined PAR mRNA and protein expression in platelets and probed PAR function using specific peptide agonists. We also examined the effect of receptor desensitization, receptor blocking antibodies, and a PAR1 antagonist, used alone and in combination, on platelet activation.4
