Platelets play a fundamental role in thrombosis, but they are increasingly regarded as discrete immune cells due to their pro-inflammatory actions on leukocytes and the endothelium. The P2Y12 receptor is responsible for amplifying and sustaining platelet activation. As a result, P2Y12 inhibitors, such as ticagrelor and clopidogrel, form an essential component of acute coronary syndrome treatment in preventing thrombotic events. Furthermore, P2Y12 receptors have also been identified on vascular smooth muscle cells where they mediate cell contraction and mitogenic responses. This thesis investigates the role of P2Y12 in mediating the vessel wall response to injury and to inflammation, specifically in atherogenesis, and considers the use of P2Y12 inhibitors in modulating these effects.
I have shown, using a murine ferric chloride injury model, that platelet P2Y12 is crucial in mediating thrombus and neointima formation following vessel injury, and that P2Y12 inhibition at the time of injury was sufficient to prevent their formation.
To investigate the role of P2Y12 in atherogenesis, a novel ApoE/P2Y12 double knockout mouse strain was generated and fed a high-fat western diet for 12 weeks, before assessing atherosclerotic burden. Compared to ApoE-/- controls, P2Y12-deficency led to a moderate reduction in lesion area, particularly in areas of the vasculature prone to turbulent blood flow, such as the aortic arch. These vessels exhibited characteristics of established disease, with advanced lesions, and so atheroma was also assessed after 4 weeks on a western diet. I demonstrated, using bone marrow transplantations, that platelet P2Y12-deficiency profoundly reduced platelet reactivity compared to ApoE-/- controls, but had no effect on atheroma; whereas vessel wall P2Y12-deficiency significantly attenuated lesion area and led to a less mature lesion phenotype. Treating mice with ticagrelor or clopidogrel for 4 weeks achieved effective platelet P2Y12 inhibition, compared to mice treated with mannitol as a control, but did not block the vessel wall P2Y12 effect on atherogenesis.
These data describe a previously unknown role for vessel wall P2Y12 in potentiating early atherogenesis. Despite its major contribution to platelet reactivity, platelet P2Y12 had no effect at this early stage, and yet is vital in later disease in atherothrombosis and modulating neointima formation following vessel injury. This work emphasises the complexity of cellular roles within atherogenesis, and how the impact of these cell populations may change as disease evolves. Further work is required to clarify the extent to which vessel wall and platelet P2Y12 influence atherogenesis throughout disease progression, in order to provide a clearer understanding of potential pharmacological targets in the prevention and management of atherosclerosis.
