Key Points• The major subpopulation of platelets involved in thrombus development form via regulated necrosis involving cyclophilin D.• Necrotic platelets may be targeted independent of platelet activation.A subpopulation of platelets fulfills a procoagulant role in hemostasis and thrombosis by enabling the thrombin burst required for fibrin formation and clot stability at the site of vascular injury. Excess procoagulant activity is linked with pathological thrombosis. The identity of the procoagulant platelet has been elusive. The cell death marker 4-[N-(S-glutathionylacetyl)amino]phenylarsonous acid (GSAO) rapidly enters a subpopulation of agonist-stimulated platelets via an organic anion-transporting polypeptide and is retained in the cytosol through covalent reaction with protein dithiols. Labeling with GSAO, together with exposure of P-selectin, distinguishes necrotic from apoptotic platelets and correlates with procoagulant potential. GSAO 1 platelets form in occluding murine thrombi after ferric chloride injury and are attenuated with megakaryocyte-directed deletion of the cyclophilin D gene. These platelets form a procoagulant surface, supporting fibrin formation, and reduction in GSAO 1 platelets is associated with reduction in platelet thrombus size and fibrin formation. Analysis of platelets from human subjects receiving aspirin therapy indicates that these procoagulant platelets form despite aspirin therapy, but are attenuated by inhibition of the necrosis pathway. These findings indicate that the major subpopulation of platelets involved in fibrin formation are formed via regulated necrosis involving cyclophilin D, and that they may be targeted independent of platelet activation. (Blood. 2015;126(26):2852-2862

Platelets are critical mediators of thrombosis and hemostasis. In response to agonist, platelets aggregate to form a thrombus via ligand binding of the glycoprotein IIb/IIIa receptor. However, activated platelets are heterogeneous in nature and a subset of platelets stimulated by strong agonists support the assembly of the coagulation complexes. It is proposed that these "procoagulant" platelets have a unique role in hemostasis and thrombosis as the link between primary and secondary hemostasis, localizing the thrombin burst required for fibrin formation to micro-domains within the platelet thrombus. Loss of procoagulant potential leads to bleeding while an increase is linked with propensity to thrombosis. While many features of the procoagulant platelet are known, the exact nature of the procoagulant platelet remains controversial. It is noted that many of the morphological and biochemical features of procoagulant platelets are also features of the cyclophilin D necrosis pathway. This review will focus on the distinct roles of platelet subpopulations, the identity of the procoagulant platelet, and the potential role of the cell death pathways in regulating platelet procoagulant response.

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