Discrete spatiotemporal control of cAMP-regulated platelet functions by A-kinase anchoring proteins

Platelets are small cells that play a key role in blood clotting that prevents the loss of excessive blood from the circulation when we cut ourselves. Upon activation, platelets become sticky and adhere to the walls of the blood vessels to form a clot. People who suffer from heart disease have platelets that are stickier than normal although these reasons for this are unclear. The inappropriate activation of blood platelets inside our arteries plays a key role in both heart attacks and strokes, and is a principal cause of mortality and morbidity in the UK. To protect ourselves from the increased stickiness of platelets, endothelial cells that line blood vessels release a chemical messenger called prostacyclin (PGI2), which inhibits platelet activity, making them less 'sticky' and reducing thrombosis. Subjects with heart disease, diabetes and obesity have platelets that do not respond very well to PGI2, making them more vulnerable to thrombosis. In this project we aim to investigate the molecular mechanisms that regulate platelet sensitivity to PGI2, which are currently undefined. We will use blood from human volunteers to isolate platelets and study the proteins that are activated by PGI2. By understanding how PGI2 controls platelets in normal individuals we can begin to compare this with people with disease to understand how the pathway breaks down. The study will increase our understanding of how PGI2 protects against heart disease and may lead to the development of new antithrombotic strategies.

The inappropriate activation of blood platelets is associated with arterial thrombosis that underlines clinical events such as myocardial infarction. Elevation in cAMP leading to activation of protein kinase A (PKA) represents a major inhibitory pathway for blood platelet function and arterial thrombosis. Subjects with cardiovascular disease have reduced sensitivity to inhibition by cAMP and PKA, which is thought to contribute to their elevated risk of arterial thrombosis. However, the molecular basis of their resistance to the inhibitory effects of cAMP/PKA is unknown. Platelets possess two isoforms of PKA (PKA-I and II), although the specific functions of these isoforms in regulating platelet activity are unknown. In many cells PKA signalling is isoform specific and directed by A-kinase anchoring proteins (AKAPs). In preliminary experiments, inhibition of PKA-I/AKAP interactions reduced platelet inhibition by prostacyclin and PKA-mediated signalling events, suggesting that a type I AKAP may regulate PKA signalling in platelets. In the present application we will (a) examine the role of PKA isoforms in platelet signalling and function, (b) determine how this is affected by coupling with AKAPs, and (c) determine which AKAPs are present in platelets. We will use pharmacological inhibitors of PKA in platelets and gene silencing of PKA isoforms in megakaryocytes to dissect the potential non-redundant roles of PKA isozymes in these cells. This will be complemented by the use of highly selective peptides that uncouple AKAP-PKA interactions in an isoform specific manner, to determine the role of AKAPs in PKA platelet signalling and function. Furthermore, chemical proteomics methodology will be used to identify platelet AKAPs. These studies will produce highly novel data on the PKA isoform specific events and their regulation by AKAPs, and provide a platform for future studies examining how specific PKA isozyme-AKAP coupling modulate distinct aspects of platelet function.

The knowledge gained from the proposed studies will provide novel information of the regulation of blood platelet activity. Since these cells participate in normal haemostasis, preventing blood loss upon injury, and in pathophysiological thrombosis, knowledge of their function provides important information on human biology. We believe our work will be of particular interest to clinicians, biomedical and clinical scientists and to the pharmaceutical industry to aid in the development of new therapeutic regimes to combat thrombotic disease. The inhibition platelet function with drugs such as aspirin and ADP receptor antagonists is supported by extensive experimental data. However, clinically, current approaches lack efficacy in many patients, and are associated with common and serious side effects, which include bleeding. The increasing rates of incidence of arterial thrombosis and underlying obesity-related metabolic disorders, will only add to this burden and therefore more refined, efficacious and safer approaches are required. cAMP activated intracellular signalling pathways are the most potent endogenous mechanisms for platelet inhibition. In contrast to clinically used platelet inhibitors (e.g. Aspirin), which only block certain functions of platelets while leaving others untouched, cAMP causes broad inhibition of all aspects of platelet function and is therefore critical to 'global' modulation of platelet activity. Therefore a detailed understanding of cAMP regulatory mechanisms in platelets is essential to (i) aid clinical understanding of why the cAMP pathway is compromised in disease states and therefore how it contributes to the pathogenesis of arterial thrombosis, (ii) understand the molecular mechanisms that lead to a comprehensive inhibition of platelet activation providing a broader frame work for clinical and biomedical scientists to design new strategies to combat excessive platelet function, and (iii) to provide new therapeutic targets for drug development by the pharmaceutical industry.