Lactadherin blocks thrombosis and hemostasis in vivo: correlation with platelet phosphatidylserine exposure

Summary.  Background: Platelet membrane phosphatidylserine (PS) is considered to be essential for hemostasis and thrombosis, but the in vivo topography of platelet PS has not been characterized. We hypothesized that platelet PS exposure would be identified on adherent platelets at the site of vascular injury and that blockade of PS would impede hemostasis and thrombosis. Objective: To localize and estimate the extent of platelet PS exposure and evaluate the impact of PS blockade in vivo. Methods: Lactadherin, a PS‐binding milk protein, was utilized together with annexin V to detect both partial and complete membrane PS exposure on platelets in a mouse model of thrombosis and to evaluate the functional need for PS. Preliminary experiments were performed with synthetic membranes and with purified platelets. Results: The number of lactadherin‐binding sites on synthetic membranes was proportional to PS content, whereas annexin V required a threshold of 2.5–8% PS. Approximately 95% of thrombin‐stimulated platelets exposed PS, but the quantity was below the threshold for annexin V binding at physiologic Ca2+ concentrations. In mice, most adherent and aggregated platelets on the walls of ferric chloride‐treated mesenteric veins exposed low levels of PS, rather than having complete exposure. In mice, blockade of PS with lactadherin inhibited platelet prothrombinase and factor Xase activity, and prolonged tail bleeding time and the time to carotid artery thrombosis. Conclusions: In vivo PS exposure contributes to both hemostasis and thrombosis. In this model of vascular injury, most platelets exhibit partial rather than complete PS exposure.