GRK5 regulation of GPCR signaling in platelets.

About This Grant

Project Summary Cardiovascular and cerebrovascular diseases have historically been among the most common causes of death in the United States. Venous thromboembolism (VTE), heart attack, and stroke are all diseases in which platelets play a role through inappropriate platelet activation and subsequent platelet plug formation. Most platelet activators stimulate platelets via G protein- coupled receptors (GPCRs). Our goals are to elucidate the mechanisms responsible for the feedback regulation of GPCRs during platelet activation and to investigate how the thrombin receptor can be negatively regulated by GPCR kinase 5 (GRK5). Changes in GRK expression have been linked to many cardiovascular diseases. The roles of GRK subtypes are highly cell-, receptor-, and disease-specific. We first reported that GRK5 normally limits platelet activation downstream of GPCRs and that removing the GRK5-dependent limits on GPCR signaling produces a prothrombotic state. More recently, we reported genome-wide association studies (GWAS) of pathway-specific functional responses to thrombin receptor-agonist peptides in subsets of 1,562 individuals. We identified a genetic variant in GRK5 and the sensitivity of platelets to activate through the thrombin receptor PAR1. The minor allele (G) is associated with decreased GRK5 expression in platelets, greater sensitivity of platelets to activate through PAR1, and increased risks of cardiovascular diseases. These studies in humans, combined with findings from platelet studies in GRK5-/- mice, and the preliminary data provided herein provide strong evidence that GRK5 is a critical negative regulator of platelet activation via PAR1 and P2Y receptors. We hypothesize that GRK5 is an important regulator of GPCR-mediated platelet function and can be manipulated to manage platelet dysfunction in disease. The hypothesis will be tested with three specific aims. In Aim 1, we will delineate the mechanisms by which GRK5 regulates platelet activation via a subset of GPCRs. In Aim 2, we will determine how the activity of GRK5 is regulated in platelets. In Aim 3, we will investigate the role of GRK5 in pathological thrombus formation, including VTE and stroke. The proposed studies will 1) provide the first insight into how GRK5 regulates GPCR signaling and how GRK5 itself can be regulated in platelets, and 2) lay the groundwork for optimizing antiplatelet therapy for the nearly 2% of the US population who are homozygous for the high-risk GRK5 G-allele.