Endogenous RGS proteins modulate SA and AV nodal functions in isolated heart: implications for sick sinus syndrome and AV block

Abstract

G protein-coupled receptors play a pivotal role in regulating cardiac automaticity. Their function is controlled by regulator of G protein signaling (RGS) proteins acting as GTPase-activating proteins for Galpha subunits to suppress Galpha(i) and Galpha(q) signaling. Using knock-in mice in which Galpha(i2)-RGS binding and negative regulation are disrupted by a genomic Galpha(i2)G184S (GS) point mutation, we recently (Fu Y, Huang X, Zhong H, Mortensen RM, D'Alecy LG, Neubig RR. Circ Res 98: 659-666, 2006) showed that endogenous RGS proteins suppress muscarinic receptor-mediated bradycardia. To determine whether this was due to direct regulation of cardiac pacemakers or to alterations in the central nervous system or vascular responses, we examined isolated, perfused hearts. Isoproterenol-stimulated beating rates of heterozygote (+/GS) and homozygote (GS/GS) hearts were significantly more sensitive to inhibition by carbachol than were those of wild type (+/+). Even greater effects were seen in the absence of isoproterenol; the potency of muscarinic-mediated bradycardia was enhanced fivefold in GS/GS and twofold in +/GS hearts compared with +/+. A(1)-adenosine receptor-mediated bradycardia was unaffected. In addition to effects on the sinoatrial node, +/GS and GS/GS hearts show significantly increased carbachol-induced third-degree atrioventricular (AV) block. Atrial pacing studies demonstrated an increased PR interval and AV effective refractory period in GS/GS hearts compared with +/+. Thus loss of the inhibitory action of endogenous RGS proteins on Galpha(i2) potentiates muscarinic inhibition of cardiac automaticity and conduction. The severe carbachol-induced sinus bradycardia in Galpha(i2)G184S mice suggests a possible role for alterations of Galpha(i2) or RGS proteins in sick sinus syndrome and pathological AV block.

RGS4 Regulates Parasympathetic Signaling and Heart Rate Control in the Sinoatrial Node

“…10 Similarly, Iso-treated hearts from mice lacking RGS4 show M 2 R-dependent atrioventricular node block and cardiac arrest, implicating its potential role as a regulator of the crosstalk mechanisms between ␤-adrenergic receptors and M 2 R. 27 However, it remains to be determined whether atrioventricular node conduction is altered in RGS4-deficient mice and whether loss of RGS4 in regions of the heart outside of the SAN significantly increases the susceptibility of hearts to conduction defects and arrhythmogenesis in vivo.…”

“…These proteins have recently emerged as inhibitory candidates of parasympathetic signaling in autorhythmic cells of the SAN because expression of RGS-resistant G␣ i2 or G␣ o in mice reduced pacemaker cell automaticity. 9,10 However, the pan-specific RGS protein inhibition in these models precluded identification of the specific RGS proteins involved. Although a large number of mammalian RGS proteins are expressed in the heart, [11][12][13] their specific roles as regulators of parasympathetic pathway effectors are not well understood.…”

Thinking Outside of the “RGS Box”: New Approaches to Therapeutic Targeting of Regulators of G Protein Signaling: Fig. 1.

“…In contrast, RGS5 knockout mice were shown in one study to be hypotensive (Cho et al, 2008), possibly suggesting a tightly regulated balance between different RGS proteins in the control of physiological responses. RGS proteins have also been demonstrated, by us and others, to regulate cardiac rhythmicity through actions in the sinoatrial and atrioventricular nodes in the heart (Fu et al, 2006(Fu et al, , 2007Bender et al, 2008;Cifelli et al, 2008).…”

Regulators of G-Protein Signaling and Their Gα Substrates: Promises and Challenges in Their Use as Drug Discovery Targets

“…This "RGS-insensitivity" point mutation (glycine to serine) was originally identified by DiBello et al (1998) in GPA1, the G␣ subunit of the yeast S. cerevisiae, functions equivalently in mammalian G␣ subunits such as G␣ i1 , G␣ o , and G␣ q (Lan et al, 1998;Clark and Traynor, 2004) and has also been shown to leave all other functions of G␣ intact, including intrinsic nucleotide binding and hydrolysis activities, as well as coupling to G␤␥, receptor, and effectors (Lan et al, 1998;Chen et al, 2004;Day et al, 2004;Fu et al, 2004;Ikeda and Jeong, 2004). Separate mouse strains bearing this RGS-insensitivity point mutation (G184S) within their G␣ o or G␣ i2 gene loci, respectively, have been generated (Fu et al, 2004;Huang et al, 2006); these mice possess select changes in various organ system functions controlled by GPCR signaling, including central nervous system, cardiovascular, and endocrine functions (Fu et al, , 2007Huang et al, 2006Huang et al, , 2008Goldenstein et al, 2009;Icaza et al, 2009;Signarvic et al, 2010;Talbot et al, 2010). The most recent findings of Ruiz de Azua et al (2010), that RGS4 is a negative regulator of M3 mAChR signaling to insulin secretion in pancreatic ␤-cells, articulate well with the findings of Huang et al (2008) that RGS-insensitive G␣ i2 (G184S) knock-in mice exhibit increased glucose tolerance when on a high-fat diet.…”