Abstract 8556: Inhibition of Grk2 Augments Cardiac Beta2-Adrenergic Receptor-Dependent Contractility via Prevention of Receptor Interaction with Phosphodiesterase Type 4d

Abstract

β 1 - and β 2 -adrenergic receptors (βARs) are G-protein coupled receptors (GPCRs) that play clearly distinct roles in cardiac physiology/pathology. For instance, cardiomyocyte contraction is readily stimulated by β 1 AR but not β 2 AR signaling. This might be explained by differences in assembly of macromolecular signaling complexes: β 1 AR forms a signaling complex with phosphodiesterase (PDE) type 4D8 directly, and agonist binding dissociates this complex. Conversely, GPCR kinase (GRK)2-induced β 2 AR phosphorylation leads to recruitment of a complex consisting of β-arrestin (βarr), a universal GPCR adapter/scaffolding molecule, and another PDE4D variant, PDE4D5. This PDE4D recruitment is postulated to constrain β 2 AR pro-contractile signaling by limiting compartmentalization of 3′-5′-adenosine monophosphate signaling. Thus, herein, we sought to investigate the effect of inhibition of this complex recruitment on β 2 AR pro-contractile signaling in vivo. In order to block β 2 AR-PDE4D interaction in cardiac myocytes in vivo, we crossed β 1 AR knockout (B1KO) mice, which do not express β 1 AR, with M27 mice, which overexpress, specifically in cardiac myocytes, the GRK2 inhibitor GRK2ct (or βARKct). By blocking GRK2-mediated phosphorylation, βarr-dependent PDE4D recruitment to β 2 AR is prevented. We studied the offspring both under normal conditions and after surgically induced myocardial infarction (MI). Contractility was significantly augmented in M27/B1KO mice compared to B1KO's, both in healthy mice (ejection fraction (EF): 69+1.8% vs. 57+1.7% respectively, p<0.05, n=8) and at 4 weeks after MI (EF: 42.6+0.1% vs. 25+3.9% respectively, p<0.05, n=8). In addition, GRK2ct expression attenuated cardiac dilatation of B1KO's at 4 weeks post-MI. At the molecular level, M27/B1KO hearts displayed significantly less membrane recruitment of both PDE4D5 and PDE4D3 compared to B1KO hearts, as evidenced by western blotting in cardiac membrane preparations, indicating less β 2 AR-PDE4D interaction. Thus, cardiac GRK2 inhibition by GRK2ct increases β 2 AR-dependent contractility both normally and in post-MI heart failure. The underlying mechanism appears to involve inhibition of the GRK2/βarr-mediated PDE4D recruitment to the receptor.