The fibrinogen αC domain binds plasminogen and enhances plasmin generation on thrombin-activated platelets

Fibrinolysis, the breakdown of fibrin-rich clots, is a tightly regulated process that ensures timely clot resolution and maintenance of vascular integrity. Platelets play a critical role in this process, but mechanisms remain incompletely understood.

Define the contribution of the fibrinogen αC domain to plasminogen binding and plasmin generation on platelets.

Plasminogen binding to resting or thrombin-stimulated platelets isolated from wild-type (Fga^WT/WT), fibrinogen-knockout (Fga^-/-), or fibrinogen αC-truncated (Fga^270/270) mice was quantified using flow cytometry. Platelet-mediated plasmin generation was determined using the plasmin-specific chromogenic substrate S-2251 and quantified using time-squared analysis.

Thrombin stimulation of Fga^WT/WT or Fga^270/270 platelets enhanced plasminogen binding by 4.3-fold and 2.8-fold, respectively, while thrombin stimulation did not enhance plasminogen binding to Fga^-/- platelets. Activated thrombin-activatable fibrinolysis inhibitor (TAFIa) decreased plasminogen binding to activated Fga^WT/WT platelets by 39%; TAFIa treatment had no impact on plasminogen binding to Fga^270/270 or Fga^-/- platelets. Thrombin stimulation enhanced plasmin generation on Fga^WT/WT platelets by 22.0-fold, and this was reduced by 32% by TAFIa. Thrombin stimulation enhanced plasmin generation on Fga^270/270 and Fga^-/- platelets by only 4.7-fold and 6.7-fold, respectively, but TAFIa did not modify this response.

C-terminal lysine residue(s) generated during thrombin-mediated platelet stimulation within the αC domain of fibrinogen confer maximal plasminogen binding and subsequent plasmin generation that is sensitive to suppression by TAFIa.