Evaluation of the role of ARAP1 in platelet function and thrombosis

Platelets are small cells in the blood that are important to stop a bleeding but can also cause a stroke or heart attack when they clump together in a blood vessel in the brain or heart. We recently found that platelets that like to clump together more than other platelets have increased activity of a protein called PI3 kinase. The PI3kinase protein can communicate with other proteins and one of these is a protein called ARAP1. The aim of this study is to investigate whether the ARAP1 protein is responsible for making platelets hyperactive, which may thereby contribute to strokes and heart attacks.

Platelets are small cells in the blood that are essential for hemostasis, but can also contribute to thrombosis and cardiovascular disease when activated inappropriately. We have previously shown that platelet hyperactivity can contribute to resistance to anti-platelet therapy, potentially increasing the risk of stroke and myocardial infarction in patients with cardiovascular disease receiving anti platelet therapy. One of the main intracellular signalling kinases that contributes to platelet hyperactivity is the lipid kinase PI3kinase, which phosphorylates lipids such as PI(4,5)P2 in the membrane resulting in the generation of PI(3,4,5)P3. The mechanism by which PI3 kinase increases platelet function is still largely unknown but it likely to be mediated though binding of proteins to PI(3,4,5)P3. We recently showed that one of the proteins that binds to PI(3,4,5)P3, and therefore may contribute to platelet hyperactivity, is the Arf and Rho GTPase activating protein ARAP1. Human platelets express relatively high levels of ARAP1 compared to other tissues and ARAP1 is the only ARAP expressed in platelets. Indeed, we confirmed ARAP1 expression in human and mouse platelets, and show that ARAP1 is tyrosine phosphorylated upon platelet activation in a manner that is highly dependent on secondary signalling. At present the regulation and function of ARAP1 in platelets is completely unknown and this application therefore aims to obtain preliminary data using platelets that lack ARAP1 to demonstrate a role for ARAP1 in platelet function and thrombosis. This study will provide more insight in the underlying cause of platelet hyperactivity and potentially identify a novel target to prevent heart attacks and strokes.