Background: Phosphatidylserine-expressing platelets do not have active integrin ␣ IIb ␤ 3 but somehow retain fibrinogen. Results:The adhesive ␣-granule proteins fibrinogen and thrombospondin are concentrated in a fibrin polymerization-dependent "cap" on phosphatidylserine-expressing platelets that promotes their incorporation into thrombi. Conclusion:This suggests a revised model for the adhesive properties of phosphatidylserine-expressing platelets. Significance: The role of phosphatidylserine-expressing platelets in thrombus formation and its mechanism are re-evaluated.

Objective— Phosphatidylserine (PS) externalization by platelets upon activation is a key event in hemostasis and thrombosis. It is currently believed that strong stimulation of platelets forms 2 subpopulations, only 1 of which expresses PS. Methods and Results— Here, we demonstrate that physiological stimulation leads to the formation of not 1 but 2 types of PS-expressing activated platelets, with dramatically different properties. One subpopulation sustained increased calcium level after activation, whereas another returned to the basal low-calcium state. High-calcium PS-positive platelets had smaller size, high surface density of fibrin(ogen), no active integrin α IIb β 3 , depolarized mitochondrial membranes, gradually lost cytoplasmic membrane integrity, and were poorly aggregated. In contrast, the low-calcium PS-positive platelets had normal size, retained mitochondrial membrane potential and cytoplasmic membrane integrity, and combined retention of fibrin(ogen) with active α IIb β 3 and high proaggregatory function. Formation of low-calcium PS-positive platelets was promoted by platelet concentration increase or shaking and was decreased by integrin α IIb β 3 antagonists, platelet dilution, or in platelets from kindlin-3–deficient and Glanzmann thrombasthenia patients. Conclusion— Identification of a novel PS-expressing platelet subpopulation with low calcium regulated by integrin α IIb β 3 can be important for understanding the mechanisms of PS exposure and thrombus formation.

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In this study we have investigated the process of spatial fibrin clot formation in non-steered platelet-free plasma at the temperatures from 20°C to 43°C using thrombodynamics – the novel in vitro hemostasis assay, which imitates the process of hemostatic clot growth in vivo. During data processing the following parameters were calculated: initial (V i ) and stationary (V st ) rates of clot growth which characterize initiation and propagation phases of clotting process, and clot size on the 30 th minute. The temperature dependence of extrinsic and intrinsic tenase activities, which determine values of the initial and stationary clot growth rates, respectively, have been also measured. It was established that the temperature lowering from 37°C to 24°C extends mainly on the initiation phase of clot growth, while the stationary rate of clot growth changes insignificantly. Meanwhile none of the thrombodynamics parameters shows the dramatic change of plasma coagulation system condition at the temperature of 24°C (acute hypothermia). Using the thrombodynamics assay an assumption, that the temperature lowering does not change the state of plasma hemostasis system significantly has been confirmed.