Background: Thrombin is the key enzyme for platelet activation and coagulation. Thrombin interacts with platelets through protease-activated receptors (PARs) 1 and 4 and von Willebrand factor binding glycoprotein 1b. Many studies reported high affinity, and low-affinity thrombin binding sites with an estimated number of sites per platelet, however, all these studies considered PAR receptors as a single site, and the exact number of each receptor was not known. Receptor number for PAR4 was reported only recently by Li et al. in 2020.

Aims: To analyze the interaction of thrombin with platelets using Microscale thermophoresis (MST).

Methods: Microscale thermophoresis (MST) is a technology that can analyze the interactions between biomolecules and is used to measure the affinity between two biomolecules. In this study, we are pioneers in using MST to analyze the interaction of thrombin to platelets. The exact number of each receptor per platelet was used for calculations. We used fluorescently labeled thrombin and washed platelets in the presence of various inhibitors of thrombin exosites, GP1b, PAR1, or PAR4 to study the affinity of thrombin towards its receptors.

Results: GP1b was found to be the low-affinity binding site as blocking of GP1b by its antibody did not affect the thrombin affinity significantly. Blockage of exosite 1 affected thrombin affinity the most as the PAR1 receptor is the high-affinity site and also the PAR1 receptor number is larger than that of PAR4. PAR-specific inhibitors vorapaxar or BMS-986120 did not affect thrombin binding to the platelets.

Conclusions: The affinity of thrombin towards its receptors on platelets is in the order of PAR1>PAR4>GP1b. MST is a useful and non-harmful technique that can be used to study the interaction of biomolecules with platelets.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/study-of-the-affinity-of-thrombin-towards-its-receptors-on-platelets/