Protection of β2GPI Deficient Mice from Thrombosis Reflects a Defect in PAR3-facilitated Platelet Activation

BackgroundAntibodies to {beta}2-glycoprotein I ({beta}2GPI) cause thrombosis in antiphospholipid syndrome, however the role of {beta}2GPI itself in regulation of coagulation pathways in vivo is not well understood. MethodsWe developed {beta}2GPI-deficient mice (Apoh-/-) by deleting exon 2 and 3 of Apoh using CRISPR/Cas9 and compared the propensity of wild-type (WT) and Apoh-/-mice to develop thrombosis using rose bengal and FeCl3-induced carotid thrombosis, laser-induced cremaster arteriolar injury, and inferior vena cava (IVC) stasis models. We also compared tail bleeding times and assessed platelet activation in WT and Apoh-/- mice in the absence and presence of exogenous {beta}2GPI. ResultsCompared to WT littermates, Apoh-/- mice demonstrated a prolonged time to occlusion of the carotid artery after exposure to rose bengal or FeCl3, and reduced platelet and fibrin accumulation in cremasteric arterioles after laser injury. Similarly, significantly smaller thrombi were retrieved from the IVC of Apoh-/-mice 48 hours after IVC occlusion. The activated partial thromboplastin time (aPTT) and prothrombin time, as well as aPTT reagent- and tissue factor-induced thrombin generation times using plasma from Apoh-/- and WT mice revealed no differences. However, we observed significant prolongation of tail bleeding in Apoh-/- mice, and reduced P-selectin expression and binding of fibrinogen to the activated 2b{beta}3 integrin on platelets from these mice after stimulation with low thrombin concentrations; these changes were reversed by exogenous {beta}2GPI. An antibody to PAR3 blocked thrombin-induced activation of WT, but not Apoh-/- platelets, as well as the ability of {beta}2GPI to restore the activation response of Apoh-/- platelets to thrombin. {beta}2GPI deficiency did not affect platelet activation by a PAR4-activator peptide, or ADP. ConclusionsIn mice, {beta}2GPI may mediate procoagulant activity by enhancing the ability of PAR3 to present thrombin to PAR4, promoting platelet activation at low thrombin concentrations. Key PointsO_LI{beta}2GPI deficient mice are protected from experimental arterial, venous, and microvascular thrombosis. C_LIO_LI{beta}2GPI deficient mice display prolonged tail bleeding times and reduced PAR3-facilitated platelet activation by low concentrations of thrombin. C_LI