The PDZ-binding motif of the β 2 -adrenoceptor is essential for physiologic signaling and trafficking in cardiac myocytes

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Abstract
β1- and β2-adrenergic receptors (AR) regulate cardiac myocyte function through distinct signaling pathways. In addition to regulating cardiac rate and contractility, β1AR and β2AR may play different roles in the pathogenesis of heart failure. Studies on neonatal cardiac myocytes from β1AR and β2AR knockout mice suggest that subtype-specific signaling is determined by subtype-specific membrane targeting and trafficking. Stimulation of β2ARs has a biphasic effect on contraction rate, with an initial increase followed by a sustained Gi-dependent decrease. Recent studies show that a PDZ domain-binding motif at the carboxyl terminus of human β2AR interacts with ezrin-binding protein 50/sodium–hydrogen exchanger regulatory factor, a PDZ-domain-containing protein. The human β2AR carboxyl terminus also binds to N-ethylmaleimide-sensitive factor, which does not contain a PDZ domain. We found that mutation of the three carboxyl-terminal amino acids in the mouse β2AR (β2AR-AAA) disrupts recycling of the receptor after agonist-induced internalization in cardiac myocytes. Nevertheless, stimulation of the β2AR-AAA produced a greater contraction rate increase than that of the wild-type β2AR. This enhanced stimulation of contraction rate can be attributed in part to the failure of the β2AR-AAA to couple to Gi. We also observed that coupling of endogenous, wild-type β2AR to Gi in β1AR knockout myocytes is inhibited by treatment with a membrane-permeable peptide representing the β2AR carboxyl terminus. These studies demonstrate that association of the carboxyl terminus of the β2AR with ezrin-binding protein 50/sodium–hydrogen exchanger regulatory factor, N-ethylmaleimide-sensitive factor, or some related proteins dictates physiologic signaling specificity and trafficking in cardiac myocytes.