Glycoprotein Ibalpha-mediated platelet adhesion and aggregation to immobilized thrombin under conditions of flow.

Objective: Thrombin interacts with platelets via the protease-activated receptors (PARs) 1 and 4, and via glycoprotein Ibalpha (GPIbalpha). Recently, it was shown that platelets are able to adhere to immobilized thrombin under static conditions via GPIbalpha.

Methods And Results: Here, we show that platelets are also able to adhere to and form stable aggregates on immobilized thrombin under conditions of flow. Adhesion and aggregation to thrombin was dependent on the interaction with GPIbalpha, as addition of glycocalicin or an antibody blocking the interaction between thrombin and GPIbalpha inhibited platelet adhesion. Additionally, platelet adhesion to recombinant thrombin mutants, which are unable to bind GPIbalpha, was severely suppressed. Furthermore, platelet adhesion to thrombin was dependent on activation of PARs, and partly on granule secretion and thromboxane-A2 synthesis. Immobilization of thrombin on a fibrin network resulted in substantially increased adhesion compared with fibrin alone. The adhesion to fibrin alone was completely abolished by addition of dRGDW, whereas fibrin-bound thrombin still showed substantial platelet adhesion in the presence of dRGDW, indicating that fibrin-bound thrombin is able to directly capture platelets under flow.

Conclusions: These results indicate that platelets are able to adhere to thrombin under flow conditions, which is dependent on the interaction with GPIbalpha.