Clot Contraction Drives the Translocation of Procoagulant Platelets to Thrombus Surface

Objective- After activation at the site of vascular injury, platelets differentiate into 2 subpopulations, exhibiting either proaggregatory or procoagulant phenotype. Although the functional role of proaggregatory platelets is well established, the physiological significance of procoagulant platelets, the dynamics of their formation, and spatial distribution in thrombus remain elusive. Approach and Results- Using transmission electron microscopy and fluorescence microscopy of arterial thrombi formed in vivo after ferric chloride-induced injury of carotid artery or mechanical injury of abdominal aorta in mice, we demonstrate that procoagulant platelets are located at the periphery of the formed thrombi. Real-time cell tracking during thrombus formation ex vivo revealed that procoagulant platelets originate from different locations within the thrombus and subsequently translocate towards its periphery. Such redistribution of procoagulant platelets was followed by generation of fibrin at thrombus surface. Using in silico model, we show that the outward translocation of procoagulant platelets can be driven by the contraction of the forming thrombi, which mechanically expels these nonaggregating cells to thrombus periphery. In line with the suggested mechanism, procoagulant platelets failed to translocate and remained inside the thrombi formed ex vivo in blood derived from nonmuscle myosin ( MYH9)-deficient mice. Ring-like distribution of procoagulant platelets and fibrin around the thrombus observed with blood of humans and wild-type mice was not present in thrombi of MYH9-knockout mice, confirming a major role of thrombus contraction in this phenomenon. Conclusions- Contraction of arterial thrombus is responsible for the mechanical extrusion of procoagulant platelets to its periphery, leading to heterogeneous structure of thrombus exterior.