Human factor XII binding to the glycoprotein Ib-IX-V complex inhibits thrombin-induced platelet aggregation

Area of Science:

• Hematology
• Biochemistry
• Molecular Biology

Background:

• Factor XII deficiency is linked to thrombosis, implying an antithrombotic role for factor XII.
• High molecular weight kininogen (HK) inhibits thrombin-induced platelet aggregation via the glycoprotein (GP) Ib-IX-V complex.
• The precise mechanism of factor XII's antithrombotic activity remains unclear.

Purpose of the Study:

• To elucidate the biochemical mechanism by which factor XII influences thrombin-induced platelet aggregation.
• To investigate the role of factor XII activation and its interaction with platelet receptors.

Main Methods:

• Assessing the effect of factor XII zymogen and activated factor XIIa on thrombin-induced platelet aggregation.
• Investigating factor XII binding to platelets and its interaction with the GP Ib-IX-V complex.
• Utilizing biosensor technology to monitor protein-protein interactions between factor XII, HK, and GP Ibalpha.

Main Results:

• Activated factor XIIa, but not factor XII zymogen, inhibited thrombin-induced platelet aggregation.
• Factor XIIa's inhibitory effect was independent of its catalytic activity but required conformational changes.
• Factor XII displaced HK binding to platelets, indicating interaction with the GP Ib complex, and both bind to GP Ibalpha.

Conclusions:

• Activated factor XIIa inhibits thrombin-induced platelet aggregation through interaction with the platelet GP Ib receptor.
• Factor XII may regulate thrombin's interaction with GP Ib, potentially by co-localizing with HK.
• This interaction suggests a novel mechanism for factor XII in controlling platelet aggregation and thrombosis in vivo.