In the heart, beta-adrenergic receptor (?-AR) stimulation increases contractile performance and heart rate as part of the 'fight-or-flight'stress response. Over stimulation of the sympathetic nervous system is a common theme in hypertension and is a primary contributor to heart failure. Increased sympathetic tone impinges on G protein-coupled receptors (GPCR) such as the ?1- AR to promote their desensitization and internalization. The human ?1-AR is internalized in response to ?-agonists such as isoproterenol, followed by recycling (i.e. re-insertion back into the membrane) and resensitization of its signaling pathway. We determined that the recycling of the ?1-AR is dependent on two domains one is serine312 in the third intracellular loop of the ?1-AR, and the other is an "ESKV" sequence that corresponds to a type-1 PDZ ligand in the carboxy tail of the ?1-AR. The ESKV sequence binds to a scaffold composed of the MAGUK protein SAP97, A-kinase anchoring protein 79 (AKAP79) and the cAMP-dependent protein kinase (PKA), that we have termed the ?1-adrenergic receptosome. This scaffold is tethered to the ?1-AR-PDZ via SAP97, which in turn binds to the AKAP79/PKA complex to assemble a trimeric scaffold (or receptosome) at the PDZ domain of the ?1-AR. We have also shown that the components of this complex act in concert to facilitate the recycling of the agonist internalized ?1-AR, through a common mechanism involving PKA-mediated phosphorylation of Ser312. Consequently, our primary hypothesis is, "that recycling and resensitization of the ?1-AR is dependent upon the targeting of PKA to the ?1-AR via the ?1-adrenergic receptosome". We propose to determine the domains within each member of the ?1-adrenergic receptosome that are functionally involved in recycling and resensitization of the ?1-AR in HEK-293 cells and rat/mouse neonatal ventricular myocytes.
In aims #1 and 2, we will identify the specific domains in AKAP79 and in SAP97, respectively that facilitate the recycling and resensitization of the ?1- AR. Then we will determine if AKAP79 and/or SAP97 are essential for recycling of the ?1-AR in cardiac ventricular myocytes prepared from neonatal rats or knockout mice. Finally in aim #3: we will characterize the molecular determinants of the PDZ motif of the ?1-AR that regulate its binding to SAP97 and facilitate the recycling and resensitization of the ?1-AR in HEK-293 and in heart cells. Results of these studies will provide new information concerning the mechanisms of ?1-AR resensitization that may ultimately provide new therapeutic paradigms to improve ?-blocker therapy, which is a main tool for the treatment of hypertension and heart failure.
In the heart, beta-1-adrenergic receptor stimulation increases contractile performance and heart rate as part of the 'fight-or-flight'stress response. New information indicates that the beta-1-adrenergic receptor is connected to other cellular networks that affect its fate and markedly broaden the range of its actions within the cell. By investigating the function of these networks, we believe that we can improve on the performance of beta-blockers, which are widely-used in treating two major health problems in America, namely hypertension and heart failure.
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