RGS6/Gβ5 Complex Accelerates I _KACh Gating Kinetics in Atrial Myocytes and Modulates Parasympathetic Regulation of Heart Rate

Abstract

Rationale:The parasympathetic reduction in heart rate involves the sequential activation of m2 muscarinic cholinergic receptors (m 2 Rs), pertussis toxin-sensitive (Gi/o) heterotrimeric G proteins, and the atrial potassium channel I KACh . Molecular mechanisms regulating this critical signal transduction pathway are not fully understood.Objective: To determine whether the G protein signaling regulator Rgs6/G␤5 modulates m 2 R-I KACh signaling and cardiac physiology. Methods and Results: Cardiac expression of Rgs6, and its interaction with G␤5, was demonstrated by immunoblotting and immunoprecipitation. Rgs6؊/؊ mice were generated by gene targeting, and the cardiac effects of Rgs6 ablation were analyzed by whole-cell recordings in isolated cardiomyocytes and ECG telemetry. Loss of Rgs6 yielded profound delays in m 2 R-I KACh deactivation kinetics in both neonatal atrial myocytes and adult sinoatrial nodal cells. Rgs6 ؊/؊ mice exhibited mild resting bradycardia and altered heart rate responses to pharmacological manipulations that were consistent with enhanced m 2 R-I KACh signaling. Conclusions:The cardiac Rgs6/G␤5 complex modulates the timing of parasympathetic influence on atrial myocytes and heart rate in mice. (Circ Res. 2010;107:1350-1354.) Key Words: G protein Ⅲ muscarinic Ⅲ knockout Ⅲ cardiac Ⅲ GIRK Ⅲ regulators of G protein signaling C ardiac output is shaped to a great extent by sympathetic and parasympathetic influences. Parasympathetic input tempers heart rate (HR), counteracts the proarrhythmic effects of sympathetic activation, and is mediated by acetylcholine (ACh). 1 ACh is released from postganglionic parasympathetic neurons and binds to m 2 muscarinic receptors (m 2 Rs) on pacemaker cells and atrial myocytes, triggering activation of pertussis toxin-sensitive (Gi/o) heterotrimeric G proteins. 2 Once activated, G proteins dissociate into G␣-GTP and G␤␥ subunits, leading to modulation of adenylyl cyclase and multiple ion channels. Central among these reactions is the binding of G␤␥ to the atrial potassium channel I KACh , a heterotetramer composed of G protein-gated inwardly rectifying K ϩ (Girk)1 and Girk4 channel subunits. 3 Binding of G␤␥ to I KACh enhances its gating, which leads to cell hyperpolarization and, ultimately, decreased HR. 4 The duration of G protein signaling is controlled by members of the RGS (regulator of G protein signaling) family. 5 RGS proteins stimulate inactivation of G␣-GTP, facilitating its reassembly with G␤␥. RGS proteins play a critical role in shaping bradycardic effects of m 2 R receptor activation. 6 -8 Indeed, eliminating RGS influence by expressing G␣ subunits insensitive to RGS action results in a substantial enhancement of I KACh regulation by m 2 R signaling, via both G␣ o and G␣ i2 pathways. 7,8 Although more than 30 RGS proteins have been identified, the involvement of specific RGS proteins in the regulation of parasympathetic input is not fully understood. Here, we report an unexpected role of the Rgs6/G␤5 complex, previously thought to be neuron-specif...

Regulator of G Protein Signaling 6 (RGS6) Protein Ensures Coordination of Motor Movement by Modulating GABAB Receptor Signaling

“…The net result is a loss of mossy fiber/CGN/parallel fibermediated Purkinje cell excitation and reduced GABAergic inhibition of the deep cerebellar nuclei, an imbalance known to manifest phenotypically as an impaired coordination of motor movement (39). Indeed, the 2-fold delay in GIRK channel deactivation seen here is consistent with that observed in cardiac myocytes and hippocampal neurons lacking RGS protein-mediated regulation of M2 receptor and GABA B R-induced GIRK currents, respectively (22,23,45). In each case, loss of RGS protein expression leads to increased agonist sensitivity and exaggeration of agonist-induced phenotypic changes.…”

Absence of the Regulator of G-protein Signaling, RGS4, Predisposes to Atrial Fibrillation and Is Associated with Abnormal Calcium Handling

“…Parasympathetic control of heart rate in the SA node has been attributed to both RGS4 and RGS6 (9,11,12). Our studies were consistent with published studies and with an important role of RGS4 in the SA node in damping down the normal vagal input.…”

GIRK channel modulation by assembly with allosterically regulated RGS proteins

“…GIRK activity is controlled in vivo by the regulator of G protein signaling (RGS) family (10)(11)(12)(13). Regulator of G-protein signaling (RGS) proteins accelerate rates of G-protein deactivation by acting as GTPase-activating proteins (GAPs) for Gprotein α-subunits (14)(15)(16).…”