Human PAR1 expressed on mouse platelets contributes to hemostasis and arterial occlusion

Thrombin (FIIa) signaling through protease-activated receptors (PARs) is a relevant platelet activation mechanism. Human (h) PAR1 antagonists are approved for antithrombotic therapy, but bleeding is a concerning complication. In addition to PARs, platelet glycoprotein (GP) Ibα also binds FIIa enhancing human platelet response to lower agonist concentrations ex vivo . Signaling through GPCRs is well understood, but how GPIbα association with distinct PARs modulates FIIa-dependent platelet activation in vivo remains unclear. One obstacle in addressing this question is the distinct human platelet PAR1/PAR4 expression as opposed to mouse (m) PAR3/PAR4. Previous attempts to express functioning hPAR1 in mouse platelets using platelet-specific promoters or targeting into the mPAR3 locus have not been successful. Here we report our studies using a hPAR1 transgene with a floxed STOP sequence and strong synthetic (CAG) promoter targeted into the mouse Rosa26 locus. Generated Rosa26-hPAR1Tg ^fl mice were then sequentially crossbred with PF4–Cre and PAR3 ^−/− mice, yielding mP3 ^−/− hP1Tg mice whose platelets expressed hPAR1 with endogenous mPAR4. These mice, unlike PAR3 ^−/− , had no excessive bleeding after tail clipping, but bled profusely after administration of the hPAR1 antagonist, vorapaxar. Accordingly, mP3 ^−/− hP1Tg mice had more frequent and stable post injury occlusion of the carotid artery than PAR3 ^−/− mice, but this difference was abolished by vorapaxar treatment. We anticipate that studies in this mouse strain will help unravel the regulation of PAR-mediated thrombin–induced platelet activation in vivo with findings more directly relevant to human pathophysiology.