Lack of canonical PAR4 activation is associated with reduced arterial and venous thrombosis in mice.

BackgroundProtease-activated receptor 4 (PAR4) is the only functional thrombin receptor on mouse platelets. Expression and activation of platelet PAR4 was shown to influence hemostatic plug stability and contributes to thrombosis in different murine arterial and venous thrombosis models. PAR4 activation by thrombin and other serine proteases occurs at the canonical activation site at Arginine (Arg) 59 in mice and Arg47 in humans. If murine PAR4 has functional non-canonical activation sites as shown for other PARs is unknown. ObjectiveTo investigate canonical and potentially non-canonical PAR4 signaling in mice, we generated a mouse model expressing a functional, thrombin-cleavage resistant PAR4 by changing Arg59 to Alanine (Ala) 59 in murine PAR4 (PAR4R59A). PAR4R59A mice were used to assess the impact of impaired canonical (thrombin)-dependent PAR4 signaling on hemostasis and thrombosis in mice. MethodsWe analyzed platelet aggregation, platelet integrin activation and -granule release ex vivo. Hemostasis and thrombosis in PAR4R59A and their control mice was compared using the jugular vein needle puncture injury-induced hemostasis model, and the ferric chloride-induced carotid artery and electrolytic injury-induced femoral vein thrombosis models. ResultsPlatelets of PAR4R59A mice did not response to thrombin but responded normally to PAR4 agonist peptide (PAR4AP) stimulation in aggregation assays. Platelets of PAR4R59A and their control mice exhibited comparable responses to ADP, convulxin or PAR4AP regarding integrin activation and -granular release. PAR4R59A mice exhibited impaired hemostasis in the jugular vein needle puncture model, and were protected from ferric chloride-induced arterial thrombosis and from electrolytic injury induced thrombosis in the femoral vein. ConclusionThe novel PAR4R59A mouse expresses a thrombin-insensitive but still functional PAR4. We propose that the new mouse line will increase the in vivo investigation of canonical PAR4 signaling pathways and may reveal unknown non-canonical PAR4 signaling in different pathologies.