A Mouse Model of the PAR4 Sequence Variant P310L Shows Decreased Platelet Reactivity and Thrombosis

Protease activated receptors 1 and 4 (PAR1, PAR4) on platelets mediate thrombin signaling and are vital for thrombus development. Hypercoagulation and thrombin generation are major risk factors for thrombosis. Sustained thrombin signaling through PAR4 promotes platelet activation, phosphatidylserine exposure, and subsequent thrombin generation. A single-nucleotide polymorphism (SNP, rs2227376) leads to an amino acid change in extracellular loop 3 (ECL3) of PAR4, which decreases PAR4 activation (PAR4-P310L). This SNP is also associated with a significantly lower risk for venous thromboembolism (VTE) in a GWAS meta-analysis. We aim to determine the mechanism by which PAR4-P310L (rs2227376) impacts platelet function and reduces the risk of VTE using mice with a homologous mutation (PAR4-P322L). Ex vivo platelet function was determined using flow cytometry and aggregometry. Arterial and venous thrombosis was examined using the ferric-chloride induced carotid artery injury and inferior vena cava (IVC) stasis models, respectively. Platelets from heterozygous (PAR4-322P/L) and homozygous (PAR4-322L/L) mice had decreased thrombin-mediated aggregation and decreased PAR4 reactivity compared to wild types. These mice also had a prolonged time to occlusion in arterial thrombosis, with 55% of PAR4-322P/L mice unable to develop stable thrombi within 30 minutes. In venous thrombosis, wild-type mice developed clotted IVCs that averaged 31 mg after 48 hours, while global PAR4-knockout mice averaged 21 mg (p<0.01), PAR4-322P/L 23 mg (p<0.01), and PAR4-322L/L 28 mg. Since platelet procoagulant activity is known to drive venous thrombosis, we used lactadherin to show that PAR4-P322L platelets express less phosphatidylserine in response to thrombin (5 nM) and convulxin (1 nM). Platelet-rich plasma from PAR4-P322L mice also generated less thrombin compared to wild-types, and whole blood from mice lacking PAR4 showed delayed clot formation in rotational thromboelastometry. Altogether, this suggests a direct impact of PAR4 signaling on platelet-mediated thrombin generation. Our new mouse model shows that the polymorphism in ECL3, PAR4-322L, decreases platelet reactivity and thrombosis. We also show PAR4-322L leads to less procoagulant platelets and decreases the endogenous thrombin potential. This decreased ability to generate thrombin offers a mechanism for PAR4's role in VTE. Our findings highlight a key role of PAR4 signaling in platelet activation and procoagulant activity in thrombosis.