Polyphosphate colocalizes with factor XII on platelet-bound fibrin and augments its plasminogen activator activity

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 alpha FXIIa. We show that both polyP(70), of the chain length found in platelets (60-100mer), and platelet-derived polyP significantly augment the plasminogen activation capacity of alpha FXIIa. PolyP(70) stimulated the autoactivation of FXII and subsequent plasminogen activation, indicating that once activated, alpha FXIIa remains bound to polyP(70). Indeed, complex formation between polyP(70) and alpha FXIIa provides protection against autodegradation. Plasminogen activation by beta FXIIa 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 alpha FXIIa. Accordingly, polyP(70) was found to bind to FXII, alpha FXIIa, and plasminogen, but not beta FXIIa. Fibrin and polyP(70) acted synergistically to enhance alpha FXIIa-mediated plasminogen activation. The plasminogen activator activity of the alpha FXIIa-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, alpha FXIIa is a highly efficient and favorable plasminogen activator. Our data are the first to document a profibrinolytic function of platelet polyP.