The functional roles of GPVI, GPIba, and PARs in the generation of platelet-derived reactive oxygen species.
Platelet receptor ligation initiates multiple signalling pathways leading to platelet-specific events such as shape change, secretion, integrin αІІbβ3 activation and platelet aggregation. In addition to these events, albeit less understood, platelets generate reactive oxygen species (ROS). In phagocytic cells, ROS act as a defence mechanism, while in nonphagocytic cells, such as platelets, the function of ROS has not been well described. Previous studies have shown that platelet-generated ROS serve to activate platelets and are involved in cellular signalling. This thesis focuses on the specific roles of two key platelet receptors pivotal in haemostasis and thrombosis, the glycoprotein (GP)lb-IX-V complex and GPVI, and the signalling pathways induced through these receptors that lead to ROS production. An important role for focal adhesion kinase (FAK) in the GPVI pathway is described and it is shown that GPIbα, like GPVI, plays a role in platelet-derived ROS generation. Signalling proteins involved in protease activated receptor (PAR)-induced ROS formation are identified, and a functional role for PAR4 is described that is independent of PARI. Furthermore, a novel association between PAR4 and GPIbα in platelet-derived ROS generation is proposed. NOX1 is determined as the main source of ROS in platelets, regardless of the agonist employed, and is found to mediate thromboxane A2 (TxA2) generation, independent of platelet aggregation. In conclusion, ROS generation is an attractive potential target in cardiovascular disease. It remains to be established whether targeting ROS in cardiovascular disease provides an alternative, safer and more specific therapy than other anti-platelet drugs such as aspirin.