The PDZ Binding Motif of the β1 Adrenergic Receptor Modulates Receptor Trafficking and Signaling in Cardiac Myocytes

Abstract

β₁ and β₂adrenergic receptors (AR) regulate the intrinsic contraction rate in neonatal mouse cardiac myocytes through distinct signaling pathways. It has been shown that stimulation of β₁ARs leads to a protein kinase A-dependent increase in contraction rate. In contrast, stimulation of β₂ARs has a biphasic effect on contraction rate, with an initial protein kinase A-independent increase followed by a sustained decrease that is blocked by pertussis toxin. The β₂AR undergoes agonist-induced endocytosis in cardiac myocytes while the β₁AR remains on the cell surface. It has been shown that a PDZ domain binding motif at the carboxyl terminus of β₁AR interacts with the postsynaptic density protein PSD-95 when both are expressed in HEK293 cells. We found that mutation of this PDZ binding motif in the β₁AR (β₁AR-PDZ) enabled agonist-induced internalization in cardiac myocytes. Moreover, stimulation of β₁AR-PDZ had a biphasic effect on the myocyte contraction rate similar to that observed following stimulation of the β₂AR. The secondary decrease in the contraction rate was mediated by G_i and could be blocked by pertussis toxin. Furthermore, a non-selective endocytosis inhibitor, concanavalin A, inhibited the internalization of wild type β₂AR and the mutated β₁AR-PDZ, and blocked the coupling of both receptors to G_i. Finally, treating myocytes with a membrane-permeable peptide representing β₁AR PDZ motif caused the endogenous β₁AR to behave like β₁AR-PDZ. These studies suggest that association of the β₁AR with PSD-95 or a related protein dictates signaling specificity by retaining the receptor at the cell surface and preventing interaction with G_i.