Fibrin stimulates platelets to increase factor VIIIa binding site expression

Area of Science:

• Hematology
• Biochemistry
• Thrombosis Research

Background:

• Factor VIII (FVIII) is a crucial cofactor for Factor X activation (FXase) on platelet membranes.
• Platelet activation by thrombin increases FVIII binding sites but to a limited extent.
• The role of developing thrombus components in modulating FVIII binding is not fully understood.

Purpose of the Study:

• To investigate whether molecules present in developing thrombi, specifically fibrin and fibrinogen, can upregulate FVIII binding site expression on activated platelets.
• To determine the functional consequence of any observed changes in FVIII binding on platelet-mediated FXase activity.

Main Methods:

• Flow cytometry was used to quantify the binding of fluorescein-labeled FVIIIa to activated platelets.
• A FXase assay measured platelet-dependent prothrombinase activity.
• Platelets were stimulated with various agonists, including thrombin, and co-incubated with normal and mutant fibrinogen and fibrin.

Main Results:

• Thrombin-activated platelets expressed a baseline of 214 +/- 67 FVIIIa binding sites.
• Co-incubation with 5 µg/mL polymerized fibrin increased FVIIIa binding sites 3- to 8-fold (1470 +/- 130), with a parallel increase in FXase activity.
• Fibrinogen did not enhance binding site expression, and fibrin's effect was blocked by GPIIbIIIa inhibitors and GPRP peptide, indicating dependence on fibrin polymerization and specific gamma-chain interactions.

Conclusions:

• Polymerized fibrin acts as a potent platelet co-stimulus, significantly upregulating FVIIIa binding site expression.
• This fibrin-mediated upregulation enhances the procoagulant function of platelets by increasing FVIII cofactor availability.
• The findings highlight a novel mechanism by which fibrin contributes to efficient thrombus formation.