The role of platelet thrombin receptors PAR1 and PAR4 in health and disease

Abstract: Blood cells are continuously flowing in our systems maintaining haemostasis in the arteries and veins. If a vessel is damaged, the smallest cell fragments in the blood (platelets) are directed to cover the wound and plug the leakage to prevent blood loss. Most of the time platelets stop the blood leak without any difficulties. During other, pathological, circumstances, platelets continue to form a thrombus, preventing the blood flow and may cause myocardial infarction or stroke.Thrombin is the most potent platelet agonist and is a product created in the coagulation cascade. This thesis is focused on the interactions between the two platelet thrombin receptors; protease activated receptors 1 (PAR1) and PAR4 in vitro. We have investigated potential differences between these receptors in several situations associated with cardiovascular disease.First we studied interactions between PAR1 and PAR4 and the oral pathogen Porphyromonas gingivalis (which secretes enzymes, gingipains, with properties similar to thrombin). Here we showed that P. gingivalis is signaling mainly, but not exclusively, via PAR4. Our second study showed that the cross-talk between the stress hormone epinephrine and thrombin occur exclusively through PAR4 if the key-substance ATP is present and cyclooxygenase-1 inhibited by aspirin. The third study investigated platelet secretion, with focus on the protein plasminogen activator inhibitor 1(PAI-1), an inhibitor of the fibrinolytic process responsible for dissolving a formed clot. Here we showed that PAI-1 secretion and synthesis was more sensitive to stimulation through PAR1 than PAR4. Finally this thesis describes differences between PAR1 and PAR4 in cell-signaling pathways regulating the stability of a platelet aggregate, where PAR4 seems to be of importance to create stable platelet aggregates and that this stability is dependent on ADP activation via P2Y12 and cell signaling via PI3-kinase.Until now, PAR1 has been considered to be the most important thrombin receptor, due to its high affinity for thrombin. However, there must be a reason why platelets express two different thrombin receptors. This thesis highlights several situations where PAR4 plays a complementary and important role in platelet signaling and haemostasis.In conclusion, this thesis suggests that PAR4 plays a major role in calcium signaling and the induction of sustained aggregation, while PAR1 shows a more prominent role in platelet secretion and synthesis. This thesis also reveals new interactions between platelet thrombin receptors and the ADP-, ATP- and epinephrine receptors. The results described in this thesis contribute to an increased knowledge of the platelet thrombin receptors and their interplay in situations such as infection, stress, fibrinolysis, and platelet aggregation.