RGS Proteins in Sympathetic Nervous System Regulation: Focus on Adrenal RGS4

The sympathetic nervous system (SNS) consists largely of two different types of components: neurons that release the neurotransmitter norepinephrine (NE, noradrenaline) to modulate homeostasis of the innevrvated effector organ or tissue and adrenal chromaffin cells, which synthesize and secrete the hormone epinephrine (Epi, adrenaline) and some NE into the blood circulation to act at distant organs and tissues that are not directly innervated by the SNS. Like almost every physiological process in the human body, G protein-coupled receptors (GPCRs) tightly modulate both NE release from sympathetic neuronal terminals and catecholamine (CA) secretion from the adrenal medulla. Regulator of G protein Signaling (RGS) proteins, acting as guanosine triphosphatase (GTPase)-activating proteins (GAPs) for the Gα subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins), play a central role in silencing G protein signaling from a plethora of GPCRs. Certain RGS proteins and, in particular, RGS4, have been implicated in regulation of SNS activity and of adrenal chromaffin cell CA secretion. More specifically, recent studies have implicated RGS4 in regulation of NE release from cardiac sympathetic neurons by means of terminating free fatty acid receptor (FFAR)-3 calcium signaling and in regulation of NE and Epi secretion from the adrenal medulla by means of terminating cholinergic calcium signaling in adrenal chromaffin cells. Thus, in this review, we provide an overview of the current literature on the involvement of RGS proteins, with a particular focus on RGS4, in these two processes, i.e., NE release from sympathetic nerve terminals & CA secretion from adrenal chromaffin cells. We also highlight the therapeutic potential of RGS4 pharmacological manipulation for diseases characterized by sympathetic dysfunction or SNS hyperactivity, such as heart failure and hypertension.