Key Points• Factor XIII-A is exposed in protruding caps on the activated platelet surface.• Platelet FXIII-A exerts antifibrinolytic function by cross-linking a 2 AP to fibrin.Factor XIII (FXIII) stabilizes thrombi against fibrinolysis by cross-linking a 2-antiplasmin (a 2 AP) to fibrin. Cellular FXIII (FXIII-A) is abundant in platelets, but the extracellular functions of this pool are unclear because it is not released by classical secretion mechanisms. We examined the function of platelet FXIII-A using Chandler model thrombi formed from FXIIIdepleted plasma. Platelets stabilized FXIII-depleted thrombi in a transglutaminase-dependent manner. FXIII-A activity on activated platelets was unstable and was rapidly lost over 1 hour. Inhibiting platelet activation abrogated the ability of platelets to stabilize thrombi. Incorporating a neutralizing antibody to a 2 AP into FXIII-depleted thrombi revealed that the stabilizing effect of platelet FXIII-A on lysis was a 2 AP dependent. Platelet FXIII-A activity and antigen were associated with the cytoplasm and membrane fraction of unstimulated platelets, and these fractions were functional in stabilizing FXIII-depleted thrombi against lysis. Fluorescence confocal microscopy and flow cytometry revealed exposure of FXIII-A on activated membranes, with maximal signal detected with thrombin and collagen stimulation. FXIII-A was evident in protruding caps on the surface of phosphatidylserine-positive platelets. Our data show a functional role for platelet FXIII-A through exposure on the activated platelet membrane where it exerts antifibrinolytic function by cross-linking a 2 AP to fibrin. (Blood. 2014;124(26):3982-3990) IntroductionFactor XIII (FXIII) plays an essential role in normal hemostasis where it contributes to the regulation of fibrinolysis, 1-3 the maintenance of pregnancy, 4 wound healing, and angiogenesis. 5 The function of FXIII in hemostasis is further emphasized in its deficiency, which results in hemorrhage 6,7 and slow wound healing. 4,7 FXIII circulates in plasma as a heterotetramer (FXIII-A 2 B 2 ) where the catalytic A (FXIII-A 2 ) subunits are almost exclusively in complex with the inhibitory carrier B (FXIII-B 2 ) subunits. 8 FXIII is activated by the combined action of thrombin and Ca 21 to form the active transglutaminase (TG) FXIIIa. 9,10 FXIIIa confers stability to the fibrin matrix by cross-linking fibrin, substantially altering its rheologic properties.11,12 TG catalyzes formation of covalent e-(g-glutamyl) lysyl bonds 13 in which the lysine e-amino group is cross-linked to the glutamine g-carboxymide group. 14The cross-linking of g-chains confers a degree of rigidity to the fibrin network, which is further stabilized by the cross-linking of a-chains 15 to generate high-molecular-weight polymers. 10,16 FXIIIa also cross-links inhibitors of fibrinolysis to fibrin, further increasing its insolubility and resistance to plasmin. These inhibitors include a 2 -antiplasmin (a 2 AP), 17 thrombin activatable fibrinolysis inhibitor, 18 and plasminogen ac...

Key Points• Under physiological flow rates, plasminogen primarily accumulates on fibrin(ogen), emanating from platelets and initiates fibrinolysis.• Plasminogen is localized to defined "caps" on the surface of PS-exposing platelets in a fibrin(ogen)-dependent manner.The interaction of plasminogen with platelets and their localization during thrombus formation and fibrinolysis under flow are not defined. Using a novel model of whole blood thrombi, formed under flow, we examine dose-dependent fibrinolysis using fluorescence microscopy. Fibrinolysis was dependent upon flow and the balance between fibrin formation and plasminogen activation, with tissue plasminogen activator-mediated lysis being more efficient than urokinase plasminogen activator-mediated lysis. Fluorescently labeled plasminogen radiates from platelet aggregates at the base of thrombi, primarily in association with fibrin. Hirudin attenuates, but does not abolish plasminogen binding, denoting the importance of fibrin. Flow cytometry revealed that stimulation of platelets with thrombin/convulxin significantly increased the plasminogen signal associated with phosphatidylserine (PS)-exposing platelets. Binding was attenuated by tirofiban and GlyPro-Arg-Pro amide, confirming a role for fibrin in amplifying plasminogen binding to PSexposing platelets. Confocal microscopy revealed direct binding of plasminogen and fibrinogen to different platelet subpopulations. Binding of plasminogen and fibrinogen co-localized with PAC-1 in the center of spread platelets. In contrast, PS-exposing platelets were PAC-1 negative, and bound plasminogen and fibrinogen in a protruding "cap." These data show that different subpopulations of platelets harbor plasminogen by diverse mechanisms and provide an essential scaffold for the accumulation of fibrinolytic proteins that mediate fibrinolysis under flow. (Blood. 2015;125(16):2568-2578 IntroductionPlatelet accumulation is central to the hemostatic response. Platelets are activated in vivo by numerous agonists of varying potency, including thrombin, collagen, adenosine 59diphosphate, and thromboxane A2. Platelets exhibit a nonuniform response to activation, with distinct populations forming with different surface characteristics.1 Aggregating platelets are characterized by a spherical shape, binding of fibrinogen, and expression of the active integrin a IIb b 3, and predominantly function in clot retraction. Highly activated platelets are observed on collagen fibers 1 and in the core region of a thrombus nearest the vascular injury.2 These platelets are characterized by membrane exposure of phosphatidylserine (PS), a rounded balloon-like structure, sustained increase in cytosolic Ca 21, and binding of coagulation factors. 3,4 PS-exposing platelets, also termed procoagulant platelets, substantially enhance the activity of the prothrombinase complex, 5,6 and subsequent thrombin and fibrin formation. 7 An additional subpopulation of platelets, termed "coated" platelets, are generated in response to strong dual agonist stimulatio...

Autoactivation of FXII by polyP, of the size found in platelets, proceeds via an active single-chain intermediate. scFXII-polyP70 shows activity towards physiological substrates, and may represent the primary event in initiating contact activation in vivo.

Key Points• PolyP significantly augments the plasminogen activator capacity of FXIIa.• Platelet-bound fibrin acts as a reservoir for plasminogen, FXII(a), and polyP.Activated factor XII (FXIIa) has plasminogen activator capacity but its relative contribution to fibrinolysis is considered marginal compared with urokinase and tissue plasminogen activator. Polyphosphate (polyP) is released from activated platelets and mediates FXII activation. Here, we investigate the contribution of polyP to the plasminogen activator function of aFXIIa. We show that both polyP 70 , of the chain length found in platelets (60-100 mer), and platelet-derived polyP significantly augment the plasminogen activation capacity of aFXIIa. PolyP 70 stimulated the autoactivation of FXII and subsequent plasminogen activation, indicating that once activated, aFXIIa remains bound to polyP 70 . Indeed, complex formation between polyP 70 and aFXIIa provides protection against autodegradation. Plasminogen activation by bFXIIa was minimal and not enhanced by polyP 70 , highlighting the importance of the anion binding site. PolyP 70 did not modulate plasmin activity but stimulated activation of Glu and Lys forms of plasminogen by aFXIIa. Accordingly, polyP 70 was found to bind to FXII, aFXIIa, and plasminogen, but not bFXIIa. Fibrin and polyP 70 acted synergistically to enhance aFXIIa-mediated plasminogen activation. The plasminogen activator activity of the aFXIIa-polyP 70 complex was modulated by C1 inhibitor and histidine-rich glycoprotein, but not plasminogen activator inhibitors 1 and 2. Platelet polyP and FXII were found to colocalize on the activated platelet membrane in a fibrin-dependent manner and decorated fibrin strands extending from platelet aggregates. We show that in the presence of platelet polyP and the downstream substrate fibrin, aFXIIa is a highly efficient and favorable plasminogen activator. Our data are the first to document a profibrinolytic function of platelet polyP. (Blood. 2016;128(24):2834-2845