Role of platelets in regulating activated coagulation factor XI activity

Area of Science:

• Hemostasis and Thrombosis
• Platelet Biology
• Coagulation Cascade

Background:

• Factor XI (FXI) binding to platelets is known, but its functional role is unclear.
• Platelets are crucial for hemostasis and thrombosis, while FXI promotes thrombosis.
• Platelets generate thrombin but also release FXIa inhibitors, creating a functional paradox.

Purpose of the Study:

• Investigate how platelets support or inhibit activated Factor XI (FXIa) activity.
• Elucidate the secretory and binding mechanisms involved in platelet-FXIa interactions.

Main Methods:

• Assessed FXIa activity in purified systems and human plasma with and without platelets.
• Analyzed FXI and Factor XII activation.
• Utilized Western blot and immunoprecipitation to study FXIa binding to platelets.
• Examined FXIa variants lacking anion-binding sites.

Main Results:

• Platelets enhanced FXIa activity, Factor IX activation, and fibrin generation.
• Platelets inhibited FXI and Factor XII activation.
• The platelet secretome inhibited FXIa, but this inhibition was blocked by activated platelets.
• FXIa binding to platelets is zinc-dependent and not mediated by glycoprotein Ib.
• Platelet-bound FXIa showed increased capacity to activate Factor IX, likely due to protection from inhibitors.

Conclusions:

• Platelet surface interaction may allosterically modulate FXIa activity.
• Platelets enhance FXIa's procoagulant function by protecting it from inhibition.
• This interaction highlights a novel mechanism in platelet-mediated thrombosis.