Two types of procoagulant platelets are formed upon physiological activation and are controlled by integrin α(IIb)β(3)

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.

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.

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. Conclusions: 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.