Targeted inactivation of alphai2 or alphai3 disrupts activation of the cardiac muscarinic K+ channel, IK+Ach, in intact cells

Biased signaling of Ca2+-sensing receptors in cardiac myocytes regulates GIRK channel activity

Ca²⁺-sensing receptors (CaSRs) belong to the class C of G protein-coupled receptors and are activated by extracellular Ca²⁺. CaSRs display biased G protein signaling by coupling to different classes of heterotrimeric G proteins depending on agonist and cell type. In this study we used fluorescent biosensors to directly analyze G protein coupling to CaSRs and downstream signaling in living cells. In HEK 293 cells, CaSRs displayed biased signaling: elevation of extracellular Ca²⁺ or application of the alternative agonist spermine caused activation of G_i- and G_q-proteins. Adult cardiac myocytes express endogenous CaSRs, which have been implicated in regulating Ca²⁺ signaling and contractility. Biased signaling of CaSRs has not been investigated in these cells. To evaluate efficiencies of G_i- and G_q-signaling via CaSRs in rat atrial myocytes, we measured G protein-activated K⁺ (GIRK) channels. Activation of GIRK requires binding of Gβγ subunits released from G_i proteins, whereas G_q-signaling results in inhibition of GIRK channel activity. Stimulation of CaSRs by Ca²⁺ or spermine failed to directly activate G_i and GIRK channels. When GIRK channels were pre-activated via endogenous M₂ receptors, stimulation of CaSRs caused pronounced inhibition of GIRK currents. This effect was specific to CaSR activation: GIRK current inhibition was sensitive to NPS-2143, a negative allosteric modulator of CaSRs, and abrogated by FR900359, a direct inhibitor of G_q. GIRK current inhibition was also sensitive to the PKC inhibitor chelerythrine, suggesting that following activation of CaSR and G_q, GIRK currents are modulated by PKC phosphorylation. We conclude from this data that cardiac CaSRs do not activate G_i and affect GIRK currents preferentially via the G_q/PKC pathway.

Differential effects of inhibitory G protein isoforms on G protein-gated inwardly rectifying K+ currents in adult murine atria

G protein-gated inwardly rectifying K⁺ (GIRK) channels are the major inwardly rectifying K⁺ currents in cardiac atrial myocytes and an important determinant of atrial electrophysiology. Inhibitory G protein α-subunits can both mediate activation via acetylcholine but can also suppress basal currents in the absence of agonist. We studied this phenomenon using whole cell patch clamping in murine atria from mice with global genetic deletion of Gα_i2, combined deletion of Gα_i1/Gα_i3, and littermate controls. We found that mice with deletion of Gα_i2 had increased basal and agonist-activated currents, particularly in the right atria while in contrast those with Gα_i1/Gα_i3 deletion had reduced currents. Mice with global genetic deletion of Gα_i2 had decreased action potential duration. Tissue preparations of the left atria studied with a multielectrode array from Gα_i2 knockout mice showed a shorter effective refractory period, with no change in conduction velocity, than littermate controls. Transcriptional studies revealed increased expression of GIRK channel subunit genes in Gα_i2 knockout mice. Thus different G protein isoforms have differential effects on GIRK channel behavior and paradoxically Gα_i2 act to increase basal and agonist-activated GIRK currents. Deletion of Gα_i2 is potentially proarrhythmic in the atria.

Knockdown of cardiac Kir3.1 gene with siRNA can improve bradycardia in an experimental sinus bradycardia rat model

The objective of this study was to explore whether the inhibition of potassium inwardly rectifying channel (Kir3.1) with short interfering RNA (siRNA) can improve bradycardia in an experimental sinus bradycardia rat model. 54 Sprague Dawley (SD) rats were randomly divided into three groups: experimental group, control group, and sham group. Sinus bradycardia model was established in SD rats through chemical ablation of sinoatrial (SA) node with 20% formaldehyde. Variations of Kir3.1 expression at mRNA and protein level were examined with qPCR and Western blotting. Electrocardiograms (ECG) of rats at 3 days and 1, 2, 3, and 4 weeks after chemical ablation and lentivirus injection were recorded and differences were compared among the three groups. The differences among multiple groups were analyzed by one-way analysis of variance (ANOVA). It was found through RT-PCR and Western blot that the mRNA and protein levels of Kir3.1 at sinoatrial node areas were decreased by 42 ± 7% and 31 ± 7% in comparison with control group, respectively (P < 0.05 in both comparisons) after 4 weeks of chemical ablation/injection. Whole-cell patch clamp data showed that the lentiviral construct could significantly inhibit the potassium current of a muscarinic acetylcholine-sensitive K⁺ channel, I_KACh. ECG data showed that the heart rate of experimental group increased after 3 days of chemical ablation/injection and lasted for at least 4 weeks after the chemical ablation/injection (heart rate increased 15.4 ± 3.8% in comparison with control group, P < 0.05). Inhibition of Kir3.1 could rescue sinus bradycardia induced by chemical ablation of SA node with 20% formaldehyde at least partly through inhibiting I_KACh channel.

RGS-Insensitive G Proteins as In Vivo Probes of RGS Function

Guanine nucleotide-binding proteins of the inhibitory (Gi/o) class play critical physiological roles and the receptors that activate them are important therapeutic targets (e.g., mu opioid, serotonin 5HT1a, etc.). Gi/o proteins are negatively regulated by regulator of G protein signaling (RGS) proteins. The redundant actions of the 20 different RGS family members have made it difficult to establish their overall physiological role. A unique G protein mutation (G184S in Gαi/o) prevents RGS binding to the Gα subunit and blocks all RGS action at that particular Gα subunit. The robust phenotypes of mice expressing these RGS-insensitive (RGSi) mutant G proteins illustrate the profound action of RGS proteins in cardiovascular, metabolic, and central nervous system functions. Specifically, the enhanced Gαi2 signaling through the RGSi Gαi2(G184S) mutant knock-in mice shows protection against cardiac ischemia/reperfusion injury and potentiation of serotonin-mediated antidepressant actions. In contrast, the RGSi Gαo mutant knock-in produces enhanced mu-opioid receptor-mediated analgesia but also a seizure phenotype. These genetic models provide novel insights into potential therapeutic strategies related to RGS protein inhibitors and/or G protein subtype-biased agonists at particular GPCRs.

μ-Opioid receptor coupling to Gα(o) plays an important role in opioid antinociception

Opioid analgesics elicit their effects via activation of the mu-opioid receptor (MOR), a G protein-coupled receptor known to interact with Gα(i/o)-type G proteins. Work in vitro has suggested that MOR couples preferentially to the abundant brain Gα(i/o) isoform, Gα(o). However, studies in vivo evaluating morphine-mediated antinociception have not supported these findings. The aim of the present work was to evaluate the contribution of Gα(o) to MOR-dependent signaling by measuring both antinociceptive and biochemical endpoints in a Gα(o) null transgenic mouse strain. Male wild-type and Gα(o) heterozygous null (Gα(o) ⁺/⁻) mice were tested for opioid antinociception in the hot plate test or the warm-water tail withdrawal test as measures of supraspinal or spinal antinociception, respectively. Reduction in Gα(o) levels attenuated the supraspinal antinociception produced by morphine, methadone, and nalbuphine, with the magnitude of suppression dependent on agonist efficacy. This was explained by a reduction in both high-affinity MOR expression and MOR agonist-stimulated G protein activation in whole brain homogenates from Gα(o) ⁺/⁻ and Gα(o) homozygous null (Gα(o)⁻/⁻) mice, compared with wild-type littermates. On the other hand, morphine spinal antinociception was not different between Gα(o) ⁺/⁻ and wild-type mice and high-affinity MOR expression was unchanged in spinal cord tissue. However, the action of the partial agonist nalbuphine was compromised, showing that reduction in Gα(o) protein does decrease spinal antinociception, but suggesting a higher Gα(o) protein reserve. These results provide the first in vivo evidence that Gα(o) contributes to maximally efficient MOR signaling and antinociception.

Sex dimorphic actions of rosiglitazone in generalised peroxisome proliferator-activated receptor-gamma (PPAR-gamma)-deficient mice

AIMS/HYPOTHESIS

The aim of this study was to determine the dependency on peroxisome proliferator-activated receptor-gamma (PPAR-gamma) of insulin sensitisation and glucose homeostasis by thiazolidinediones using a global Ppar-gamma (also known as Pparg)-knockout mouse model.

METHODS

Global Mox2-Cre-Ppar-gamma-knockout (MORE-PGKO) mice were treated with rosiglitazone and analysed for insulin sensitivity and glucose metabolism. Metabolic and hormonal variables were determined. Adipose and other tissues were measured and analysed for gene expression.

RESULTS

Rosiglitazone induced regrowth of fat in female but not male MORE-PGKO mice, and only in specific depots. Insulin sensitivity increased but, surprisingly, was not associated with the typical changes in adipokines, plasma NEFA or tissue triacylglycerol. However, increases in alternatively activated macrophage markers, which have been previously associated with metabolic improvement, were observed in the regrown fat. Rosiglitazone improved glucose homeostasis but not insulin sensitivity in male MORE-PGKO mice, with further increase of insulin associated with an apparent expansion of pancreatic islets.

CONCLUSIONS/INTERPRETATION

Stimulating fat growth by rosiglitazone is sufficient to improve insulin sensitivity in female mice with 95% PPAR-gamma deficiency. This increase in insulin sensitivity is not likely to be due to changes typically seen in adipokines or lipids but may involve changes in macrophage polarisation that occur independent of PPAR-gamma. In contrast, rosiglitazone improves glucose homeostasis in male mice with similar PPAR-gamma deficiency by increasing insulin production independent of changes in adiposity. Further, the insulin-sensitising effect of rosiglitazone is dependent on PPAR-gamma in this male lipodystrophic model.

Genetic disruption of G proteins, G(i2)alpha or G(o)alpha, does not abolish inotropic and chronotropic effects of stimulating muscarinic cholinoceptors in atrium

BACKGROUND AND PURPOSE

Classically, stimulation of muscarinic cholinoceptors exerts negative inotropic and chronotropic effects in the atrium of mammalian hearts. These effects are crucial to the vagal regulation of the heart beat. This effect is assumed to be mediated via GTP binding (G) proteins, because they can be abolished by Pertussis toxin. However, it is unknown which G proteins are involved.

EXPERIMENTAL APPROACH

We studied contractility in isolated left or right atrium from genetically manipulated mice with deletion of one of two G proteins, either of the alpha subunit of G(i2) protein (G(i2)alpha) or of the alpha subunit of G(o) protein (G(o)alpha). Preparations were stimulated with carbachol alone or after pretreatment with the beta-adrenoceptor agonist isoprenaline. For comparison, the effects of carbachol on L-type Ca(2+)-channels in isolated ventricular cardiomyocytes were studied.

KEY RESULTS

The negative inotropic and chronotropic effects of carbachol alone or in the presence of isoprenaline were identical in atria from knockout or wild-type mice. However, the effect of carbachol on isoprenaline-activated L-type Ca(2+)-channel in isolated ventricular cardiomyocytes was greatly attenuated in both types of knockout mice studied.

CONCLUSIONS AND IMPLICATIONS

These data imply that there is either redundancy of G proteins for signal transduction or that Pertussis toxin-sensitive proteins other than G(i2)alpha and G(o)alpha mediate the vagal stimulation in the atrium. Moreover, different G proteins mediate the effect of carbachol in ventricle compared with atrium.

Targeted G-protein inhibition as a novel approach to decrease vagal atrial fibrillation by selective parasympathetic attenuation

AIMS

The parasympathetic nervous system is thought to play a key role in atrial fibrillation (AF). Since parasympathetic signalling is primarily mediated by the heterotrimeric G-protein, Galpha(i)betagamma, we hypothesized that targeted inhibition of Galpha(i) interactions in the posterior left atrium (PLA) would modify the substrate for vagal AF.

METHODS AND RESULTS

Cell-penetrating(cp)-Galpha(i)1/2 and cp-Galpha(i)3 C-terminal peptides were assessed for their ability to attenuate cholinergic-parasympathetic signalling in isolated feline atrial myocytes and in canine left atrium (LA). Confocal fluorescence microscopy indicated that cp-Galpha(i)1/2 and/or cp-Galpha(i)3 peptides moderated carbachol attenuation of cellular Ca(2+) transients in isolated atrial myocytes. High-density epicardial mapping of dog PLA, left atrial pulmonary veins (PVs), and left atrial appendage (LAA) indicated that the delivery of cp-Galpha(i)1/2 peptide or cp-Galpha(i)3 peptide into the PLA prolonged effective refractory periods at baseline and during vagal stimulation in the PLA and to varying extents also in the LAA and PV regions. After delivery of cp-Galpha(i) peptides into the PLA, AF inducibility during vagal stimulation was significantly diminished.

CONCLUSION

These results demonstrate the feasibility of using specific G(i)-protein inhibition to achieve selective parasympathetic denervation in the PLA, with a resulting change in vagal responsiveness across the entire LA.

The role of inhibitory heterotrimeric G proteins in the control of in vivo heart rate dynamics

Multiple isoforms of inhibitory Galpha-subunits (Galphai1,2,3, as well as Galphao) are present within the heart, and their role in modulating pacemaker function remains unresolved. Do inhibitory Galpha-subunits selectively modulate parasympathetic heart rate responses? Published findings using a variety of experimental approaches have implicated roles for Galphai2, Galphai3, and Galphao in parasympathetic signal transduction. We have compared in vivo different groups of mice with global genetic deletion of Gialpha1/Galphai3, Galphai2, and Galphao against littermate controls using implanted ECG telemetry. Significant resting tachycardia was observed in Galphai2(-/-) and Galphao(-/-) mice compared with control and Galphai1(-/-)/Galphai3(-/-) mice (P < 0.05). Loss of diurnal heart rate variation was seen exclusively in Galphao(-/-) mice. Using heart rate variability (HRV) analysis, compared with littermate controls (4.02 ms2 +/- 1.17; n = 6, Galphai2(-/-)) mice have a selective attenuation of high-frequency (HF) power (0.73 ms2 +/- 0.31; n = 5, P < 0.05). Galphai1(-/-)/Galphai3(-/-) and Galphao(-/-) cohorts have nonsignificant changes in HF power. Galphao(-/-) mice have a different basal HRV signature. The observed HRV phenotype in Galphai2(-/-) mice was qualitatively similar to atropine (1 mg/kg)-treated controls [and mice treated with the GIRK channel blocker tertiapinQ (0.05 mg/kg)]. Maximal cardioinhibitory response to the M(2)-receptor agonist carbachol (0.5 mg/kg) compared with basal heart rate was attenuated in Galphai2(-/-) mice (0.08 +/- 0.04; n = 6) compared to control (0.27 +/- 0.04; n = 7 P < 0.05). Our data suggest a selective defect of parasympathetic heart rate modulation in mice with Galphai2 deletion. Mice with Galphao deletion also have a defect in short-term heart rate dynamics, but this is qualitatively different to the effects of atropine, tertiapinQ, and Galphai2 deletion. In contrast, Galphai1 and Galphai3 do not appear to be essential for parasympathetic responses in vivo.

G(o) controls the hyperpolarization-activated current in embryonic stem cell-derived cardiocytes

Hyperpolarization current (I(f)) is an important player in controlling heart rate and is stimulated by cAMP and inhibited by members of the pertussis toxin-sensitive G-protein G(i)/G(o) family. We have successfully derived cardiocytes from embryonic stem cells lacking G(o) or G(i2) and G(i3). We have established that both basal and isoproterenol-stimulated activities of I(f) in these cardiocytes have typical nodal-atrial characteristics and are unaffected by targeted gene inactivation of the G proteins G(o) or G(i2) and G(i3). Under basal conditions, both G(o) and G(i) are required for muscarinic inhibition of I(f) activity via a mechanism that involves the generation of nitric oxide, whereas, with prior stimulation by beta-agonists, only G(o) is required and G(i) and nitric oxide production are not. Our findings establish an essential role for G(o) in the antiadrenergic effect of muscarinic agent on I(f).

Gene-trapped mouse embryonic stem cell-derived cardiac myocytes and human genetics implicate AKAP10 in heart rhythm regulation

Sudden cardiac death due to abnormal heart rhythm kills 400,000-460,000 Americans each year. To identify genes that regulate heart rhythm, we are developing a screen that uses mouse embryonic stem cells (mESCs) with gene disruptions that can be differentiated into cardiac cells for phenotyping. Here, we show that the heterozygous disruption of the Akap10 (D-AKAP2) gene that disrupts the final 51 aa increases the contractile response of cultured cardiac cells to cholinergic signals. In both heterozygous and homozygous mutant mice derived from these mESCs, the same Akap10 disruption increases the cardiac response to cholinergic signals, suggesting a dominant interfering effect of the Akap10 mutant allele. The mutant mice have cardiac arrhythmias and die prematurely. We also found that a common variant of AKAP10 in humans (646V, 40% of alleles) was associated with increased basal heart rate and decreased heart rate variability (markers of low cholinergic/vagus nerve sensitivity). These markers predict an increased risk of sudden cardiac death. Although the molecular mechanism remains unknown, our findings in mutant mESCs, mice, and a common human AKAP10 SNP all suggest a role for AKAP10 in heart rhythm control. Our stem cell-based screen may provide a means of identifying other genes that control heart rhythm.

Missing links: mechanisms of protean agonism

The concept of pharmacological efficacy has been much discussed recently with significant interest both in inverse agonists and in protean agonists (i.e., compounds with functional selectivity for different effector responses). Although first proposed in the mid-1990s, the pharmacological and therapeutic importance of these concepts is now receiving wider support. Two articles in recent issues of Molecular Pharmacology, Lane et al. (p. 1349, current issue) and Galandrin and Bouvier (Mol Pharmacol 70:1575-1584, 2006), provide new mechanistic information on functionally selective ligands at the pharmacologically important D2 dopamine receptor and the beta(1) and beta(2) adrenergic receptors. Each article bridges a gap between recent biophysical studies showing distinct receptor conformations produced by different ligands and the increasing number of reports of discordant outputs by a single ligand to two effector readouts. The Lane et al. study clearly demonstrates G protein-specific actions of D(2) dopamine receptor ligands. These range from equivalent responses for Galpha(o) and Galpha(i) activation by norapomorphine and 7-hydroxy-2-dipropylaminotetralin to S-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine, which is an agonist for Galpha(o) activation and an inverse agonist at Galpha(i1) and Galpha(i2). Likewise, Galandrin and Bouvier describe a two-dimensional Cartesian efficacy approach in which propranolol is an agonist for extracellular signal-regulated kinase activation, probably through beta-arrestin, while functioning as an inverse agonist for adenylyl cyclase activation. Thus, these two important articles further solidify the concepts of functional selectivity and protean agonism and begin to define the first postreceptor step in actions of protean agonist ligands.

Go but not Gi2 or Gi3 is required for muscarinic regulation of heart rate and heart rate variability in mice

Muscarinic receptor-mediated cardiac parasympathetic activity is essential for regulating heart rate and heart rate variability (HRV). It has not been clear which G(i)/G(o) protein is responsible for these effects. We addressed this question using knockout mice that lack G protein alpha(i2), alpha(i3), or alpha(o) specifically. Unlike previously reported, our alpha(o)-null mice had significantly more survivors with normal life span. Isolated hearts from alpha(o)-null mice demonstrated much less sensitivity to the negative chronotropic effects of the muscarinic agonist carbachol to lower heart rate at baseline and a more profound effect under the stimulation of the beta-adrenergic agonist isoproterenol. In the presence of parasympathetic activation indirectly produced by methoxamine, an alpha(1)-adrenergic agonist, alpha(o)-null mice showed markedly decreased HRV compared with wild-type control mice. These differences in heart rate and HRV were not observed in alpha(i2)-null or alpha(i3)-null mice. Our findings establish an essential role for alpha(o) G protein in the anti-adrenergic effect of carbachol on heart rate regulation.

Hypotension, lipodystrophy, and insulin resistance in generalized PPARgamma-deficient mice rescued from embryonic lethality

We rescued the embryonic lethality of global PPARgamma knockout by breeding Mox2-Cre (MORE) mice with floxed PPARgamma mice to inactivate PPARgamma in the embryo but not in trophoblasts and created a generalized PPARgamma knockout mouse model, MORE-PPARgamma knockout (MORE-PGKO) mice. PPARgamma inactivation caused severe lipodystrophy and insulin resistance; surprisingly, it also caused hypotension. Paradoxically, PPARgamma agonists had the same effect. We showed that another mouse model of lipodystrophy was hypertensive, ruling out the lipodystrophy as a cause. Further, high salt loading did not correct the hypotension in MORE-PGKO mice. In vitro studies showed that the vasculature from MORE-PGKO mice was more sensitive to endothelial-dependent relaxation caused by muscarinic stimulation, but was not associated with changes in eNOS expression or phosphorylation. In addition, vascular smooth muscle had impaired contraction in response to alpha-adrenergic agents. The renin-angiotensin-aldosterone system was mildly activated, consistent with increased vascular capacitance or decreased volume. These effects are likely mechanisms contributing to the hypotension. Our results demonstrated that PPARgamma is required to maintain normal adiposity and insulin sensitivity in adult mice. Surprisingly, genetic loss of PPARgamma function, like activation by agonists, lowered blood pressure, likely through a mechanism involving increased vascular relaxation.

Mechanism of action and structural requirements of constrained peptide inhibitors of RGS proteins

Regulators of G-protein signaling (RGS) accelerate guanine triphosphate hydrolysis by Galpha-subunits and profoundly inhibit signaling by G protein-coupled receptors. The distinct expression patterns and pathophysiologic regulation of RGS proteins suggest that inhibitors may have therapeutic potential. We previously reported the design of a constrained peptide inhibitor of RGS4 (1: Ac-Val-Lys-[Cys-Thr-Gly-Ile-Cys]-Glu-NH2, S-S) based on the structure of the Galphai switch 1 region but its mechanism of action was not established. In the present study, we show that 1 inhibits RGS4 by mimicking and competing for binding with the switch 1 region of Galphai and that peptide 1 shows selectivity for RGS4 and RGS8 versus RGS7. Structure-activity relationships of analogs related to 1 are described that illustrate key features for RGS inhibition. Finally, we demonstrate activity of the methylene dithioether-bridged peptide inhibitor, 2, to modulate muscarinic receptor-regulated potassium currents in atrial myocytes. These data support the proposed mechanism of action of peptide RGS inhibitors, demonstrate their action in native cells, and provide a starting point for the design of RGS inhibitor drugs.

Endogenous RGS Proteins and Gα Subtypes Differentially Control Muscarinic and Adenosine-Mediated Chronotropic Effects

Cardiac automaticity is controlled by G protein-coupled receptors, such as adrenergic, muscarinic, and adenosine receptors. The strength and duration of G protein signaling is attenuated by regulator of G protein signaling (RGS) proteins acting as GTPase-activating proteins for Galpha subunits; however, little is known about the role of endogenous RGS proteins in cardiac function. We created point mutations in Galpha subunits that disrupt Galpha-RGS binding and introduced them into embryonic stem (ES) cells by homologous recombination. Spontaneously contacting cardiocytes derived from the ES cells were used to evaluate the role of endogenous RGS proteins in chronotropic regulation. The RGS-insensitive GalphaoG184S homozygous knock-in (GalphaoGS/GS) cells demonstrated enhanced adenosine A1 and muscarinic M2 receptor-mediated bradycardic responses. In contrast, Galphai2GS/GS cells showed enhanced responses to M2 but not A1 receptors. Similarly M2 but not A1 bradycardic responses were dramatically enhanced in Galphai2GS/GS mice. Blocking G protein-coupled inward rectifying K+ (GIRK) channels largely abolished the mutation-induced enhancement of the M2 receptor-mediated response but had a minimal effect on A1 responses. The Galphas-dependent stimulation of beating rate by the beta2 adrenergic receptor agonist procaterol was significantly attenuated in GalphaoGS/GS and nearly abolished in Galphai2GS/GS cells because of enhanced signaling via a pertussis toxin sensitive mechanism. Thus, endogenous RGS proteins potently reduce the actions of Galpha(i/o)-linked receptors on cardiac automaticity. Furthermore, M2 and A1 receptors differentially use Galphai2 and Galphao and associated downstream effectors. Thus, alterations in RGS function may play a role in pathophysiological processes and RGS proteins could represent novel cardiovascular therapeutic targets.

Effects on heart rate of an anti-M2 acetylcholine receptor immune response in mice

Autoantibodies in vitro modulating the M2 acetylcholine receptor (M2ACh-R) were observed in patients with idiopathic dilated cardiomyopathy (IDC) or Chagas' cardiomyopathy (ChC). We investigated the in vivo consequences on heart rate of such antibodies in mice immunized with a peptide derived from the second extracellular loop of the M2ACh-R compared with mice immunized with an irrelevant peptide. Sera of mice immunized with the M2ACh-R-derived peptide recognized the M2ACh-R on immunoblots and enhanced agonist activity of carbachol toward the M2AChR transfected in CHO cells. In vivo, no difference could be shown in heart rate or heart rate variability between the two groups of mice. The decrease in heart rate induced by carbachol was more pronounced, however, in the M2ACh-R immunized mice. The increase in heart rate induced by atropine, gallamine, and isoproterenol was significantly attenuated in the M2ACh-R immunized mice. Analysis of heart rate variability further argued for an increased parasympathetic response to different drugs in the M2ACh-R immunized mice. Antibodies raised against the M2AChR can behave as positive M2AChR allosteric modulators in vivo. They might be protective in boosting the activity of the parasympathetic drive to the heart, especially in patients with a high sympathetic tone.

Kinetic modeling of Na(+)-induced, Gbetagamma-dependent activation of G protein-gated K(+) channels

G protein-activated K(+)(GIRK) channels are activated by numerous neurotransmitters that act on Gi/o proteins, via a direct interaction with the Gbetagamma subunit of G proteins. In addition, GIRK channels are positively regulated by intracellular Na(+) via a direct interaction (fast pathway) and via a GGbetagamma-dependent mechanism (slow pathway). The slow modulation has been proposed to arise from the recently described phenomenon of Na(+)-induced reduction of affinity of interaction between GalphaGDP and Gbetagamma subunits of G proteins. In this scenario, elevated Na(+) enhances basal dissociation of G protein heterotrimers, elevating free cellular Gbetagamma and activating GIRK. However, it is not clear whether this hypothesis can account for the quantitative and kinetic aspects of the observed regulation. Here, we report the development of a quantitative model of slow, Na(+)-dependent, G protein-mediated activation of GIRK. Activity of GIRK1F137S channels, which are devoid of direct interaction with Na(+), was measured in excised membrane patches and used as an indicator of free GGbetagamma levels. The change in channel activity was used to calculate the Na(+)-dependent change in the affinity of G protein subunit interaction. Under a wide range of initial conditions, the model predicted that a relatively small decrease in the affinity of interaction of GalphaGDP and GGbetagamma (about twofold under most conditions) accounts for the twofold activation of GIRK induced by Na(+), in agreement with biochemical data published previously. The model also correctly described the slow time course of Na(+) effect and explained the previously observed enhancement of Na(+)-induced activation of GIRK by coexpressed Galphai3. This is the first quantitative model that describes the basal equilibrium between free and bound G protein subunits and its consequences on regulation of a GGbetagamma effector.

Cardiomyocyte-specific knockout and agonist of peroxisome proliferator-activated receptor-gamma both induce cardiac hypertrophy in mice

Peroxisome proliferator-activated receptor (PPAR)-gamma is required for adipogenesis but is also found in the cardiovascular system, where it has been proposed to oppose inflammatory pathways and act as a growth suppressor. PPAR-gamma agonists, thiazolidinediones (TZDs), inhibit cardiomyocyte growth in vitro and in pressure overload models. Paradoxically, TZDs also induce cardiac hypertrophy in animal models. To directly determine the role of cardiomyocyte PPAR-gamma, we have developed a cardiomyocyte-specific PPAR-gamma-knockout (CM-PGKO) mouse model. CM-PGKO mice developed cardiac hypertrophy with preserved systolic cardiac function. Treatment with a TZD, rosiglitazone, induced cardiac hypertrophy in both littermate control mice and CM-PGKO mice and activated distinctly different hypertrophic pathways from CM-PGKO. CM-PGKO mice were found to have increased expression of cardiac embryonic genes (atrial natriuretic peptide and beta-myosin heavy chain) and elevated nuclear factor kappaB activity in the heart, effects not found by rosiglitazone treatment. Rosiglitazone increased cardiac phosphorylation of p38 mitogen-activated protein kinase independent of PPAR-gamma, whereas rosiglitazone induced phosphorylation of extracellular signal-related kinase 1/2 in the heart dependent of PPAR-gamma. Phosphorylation of c-Jun N-terminal kinases was not affected by rosiglitazone or CM-PGKO. Surprisingly, despite hypertrophy, Akt phosphorylation was suppressed in CM-PGKO mouse heart. These data show that cardiomyocyte PPAR-gamma suppresses cardiac growth and embryonic gene expression and inhibits nuclear factor kappaB activity in vivo. Further, rosiglitazone causes cardiac hypertrophy at least partially independent of PPAR-gamma in cardiomyocytes and through different mechanisms from CM-PGKO.

Modulation of Endothelin-A Receptor, Gα Subunit, and RGS2 Expression during H9c2 Cardiomyoblast Differentiation

In cardiac myocytes, growth responses depend on activation of G protein-coupled receptors interacting with Gq/11 protein subfamily members. Endothelin receptors of the ETA subtype belong to this receptor group inducing hypertrophic responses. To understand the role of ETA receptors and signal transduction proteins in modulating cell growth, we analyzed the pharmacological profile of this receptor, its level of expression together with those of Galpha subunits and the RGS2 protein in cardiomyoblasts differentiating into the cardiac phenotype. H9c2 rat cardiomyoblasts were grown in the presence of 10% fetal bovine serum (FBS) or 1% FBS plus all-trans-retinoic acid to induce the cardiac phenotype. The pharmacological properties of ETA receptors were investigated by competition-binding experiments, whereas the protein expression profile was analyzed by immunoblot and immunocytochemistry. The pharmacological profile of ETA receptors changed during differentiation of cardiomyoblasts into cardiomyocytes, and the amount of expressed receptor appeared to increase. Immunocytochemistry also showed a marked increase of receptor expression on cell membranes of differentiated cardiomyocytes. Among the other signaling proteins examined, both Galphaq/11 and RGS2 expression decreased in cells with the cardiac phenotype. Our results demonstrate that the expression of key proteins (ETA receptor, Galphaq/11, and RGS2) involved in signal transduction of hypertrophic stimuli is modulated during cell differentiation and correlates with the cardiac phenotype.

Differential subunit composition of the G protein-activated inward-rectifier potassium channel during cardiac development

Parasympathetic slowing of the heart rate is predominantly mediated by acetylcholine-dependent activation of the G protein-gated potassium (K+) channel (IK,ACh). This channel is composed of 2 inward-rectifier K+ (Kir) channel subunits, Kir3.1 and Kir3.4, that display distinct functional properties. Here we show that subunit composition of IK,ACh changes during embryonic development. At early stages, IK,ACh is primarily formed by Kir3.1, while in late embryonic and adult cells, Kir3.4 is the predominant subunit. This change in subunit composition results in reduced rectification of IK,ACh, allowing for marked K+ currents over the whole physiological voltage range. As a consequence, IK,ACh is able to generate the membrane hyperpolarization that underlies the strong negative chronotropy occurring in late- but not early-stage atrial cardiomyocytes upon application of muscarinic agonists. Both strong negative chronotropy and membrane hyperpolarization can be induced in early-stage cardiomyocytes by viral overexpression of the mildly rectifying Kir3.4 subunit. Thus, a switch in subunit composition is used to adopt IK,ACh to its functional role in adult cardiomyocytes.

Gαi1 and Gαi3 Differentially Interact with, and Regulate, the G Protein-activated K+ Channel

G protein-activated K(+) channels (GIRKs; Kir3) are activated by direct binding of Gbetagamma subunits released from heterotrimeric G proteins. In native tissues, only pertussis toxin-sensitive G proteins of the G(i/o) family, preferably Galpha(i3) and Galpha(i2), are donors of Gbetagamma for GIRK. How this specificity is achieved is not known. Here, using a pull-down method, we confirmed the presence of Galpha(i3-GDP) binding site in the N terminus of GIRK1 and identified novel binding sites in the N terminus of GIRK2 and in the C termini of GIRK1 and GIRK2. The non-hydrolyzable GTP analog, guanosine 5'-3-O-(thio)triphosphate, reduced the binding of Galpha(i3) by a factor of 2-4. Galpha(i1-GDP) bound to GIRK1 and GIRK2 much weaker than Galpha(i3-GDP). Titrated expression of components of signaling pathway in Xenopus oocytes and their activation by m2 muscarinic receptors revealed that G(i3) activates GIRK more efficiently than G(i1), as indicated by larger and faster agonist-evoked currents. Activation of GIRK by purified Gbetagamma in excised membrane patches was strongly augmented by coexpression of Galpha(i3) and less by Galpha(i1). Differences in physical interactions of GIRK with GDP-bound Galpha subunits, or Galphabetagamma heterotrimers, may dictate different extents of Galphabetagamma anchoring, influence the efficiency of GIRK activation by Gbetagamma, and play a role in determining signaling specificity.

G-proteins as transducers in transmembrane signalling

The G-protein-mediated signalling system has evolved as one of the most widely used transmembrane signalling mechanisms in mammalian organisms. All mammalian cells express G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. G-protein-mediated signalling is involved in many physiological and pathological processes. This review summarizes some general aspects of G-protein-mediated signalling and focusses on recent data especially from studies in mutant mice which have elucidated some of the cellular and biological functions of heterotrimeric G-prtoteins.

Gating properties of GIRK channels activated by Galpha(o)- and Galpha(i)-coupled muscarinic m2 receptors in Xenopus oocytes: the role of receptor precoupling in RGS modulation

'Regulators of G protein Signalling' (RGSs) accelerate the activation and deactivation kinetics of G protein-gated inwardly rectifying K(+) (GIRK) channels. In an apparent paradox, RGSs do not reduce steady-state GIRK current amplitudes as expected from the accelerated rate of deactivation when reconstituted in Xenopus oocytes. We present evidence here that this kinetic anomaly is dependent on the degree of G protein-coupled receptor (GPCR) precoupling, which varies with different Galpha(i/o)-RGS complexes. The gating properties of GIRK channels (Kir3.1/Kir3.2a) activated by muscarinic m2 receptors at varying levels of G protein expression were examined with or without the co-expression of either RGS4 or RGS7 in Xenopus oocytes. Different levels of specific m2 receptor-Galpha coupling were established by uncoupling endogenous pertussis toxin (PTX)-sensitive Galpha(i/o) subunits with PTX, while expressing varying amounts of a single PTX-insensitive subunit (Galpha(i1(C351G)), Galpha(i2(C352G)), Galpha(i3(C351G)), Galpha(oA(C351G)), or Galpha(oB(C351G))). Co-expression of each of the PTX-insensitive Galpha(i/o) subunits rescued acetylcholine (ACh)-elicited GIRK currents (I(K,ACh)) in a concentration-dependent manner, with Galpha(o) isoforms being more effective than Galpha(i) isoforms. Receptor-independent 'basal' GIRK currents (I(K,basal)) were reduced with increasing expression of PTX-insensitive Galpha subunits and were accompanied by a parallel rise in I(K,ACh). These effects together are indicative of increased Gbetagamma scavenging by the expressed Galpha subunit and the subsequent formation of functionally coupled m2 receptor-G protein heterotrimers (Galpha((GDP))betagamma). Co-expression of RGS4 accelerated all the PTX-insensitive Galpha(i/o)-coupled GIRK currents to a similar extent, yet reduced I(K,ACh) amplitudes 60-90 % under conditions of low Galpha(i/o) coupling. Kinetic analysis indicated the RGS4-dependent reduction in steady-state GIRK current was fully explained by the accelerated deactivation rate. Thus kinetic inconsistencies associated with RGS4-accelerated GIRK currents occur at a critical threshold of G protein coupling. In contrast to RGS4, RGS7 selectively accelerated Galpha(o)-coupled GIRK currents. Co-expression of Gbeta5, in addition to enhancing the kinetic effects of RGS7, caused a significant reduction (70-85 %) in steady-state GIRK currents indicating RGS7-Gbeta5 complexes disrupt Galpha(o) coupling. Altogether these results provide further evidence for a GPCR-Galphabetagamma-GIRK signalling complex that is revealed by the modulatory affects of RGS proteins on GIRK channel gating. Our functional experiments demonstrate that the formation of this signalling complex is markedly dependent on the concentration and composition of G protein-RGS complexes.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

G alpha COOH-terminal minigene vectors dissect heterotrimeric G protein signaling

The COOH-termini of heterotrimeric guanine nucleotide-binding protein (G protein) alpha subunits (Galpha) are critical for both binding to their cognate G protein-coupled receptors (GPCRs) and determining specificity. Additionally, synthetic peptides corresponding to the COOH-terminus can serve as competitive inhibitors of receptor-G protein interactions, presumably by blocking the site on the GPCR that normally binds the G protein. To selectively antagonize G protein signal transduction events, we have generated minigene vectors that encode 14 unique COOH-terminal sequence for the 16 Galpha subunits. Minigene vectors expressing Galpha COOH-terminal peptides, or the control minigene vector, which expresses the inhibitory Galpha subunit (G(i)) peptide in random order, can be systematically introduced into cells by transfection and used to determine which G protein underlies a given GPCR-mediated response. Because Galpha COOH-terminal minigene vectors selectively block signal transduction through a given G protein, they are a powerful tool for dissecting out which G protein mediates a given biochemical or physiological function. This also provides a novel strategy for exploring the coupling mechanisms of receptors that interact with multiple G proteins, as well as for teasing out the downstream responses mediated by a specific G protein.

The role of muscarinic K(+) channels in the negative chronotropic effect of a muscarinic agonist

Acetylcholine causes bradycardia through M2 muscarinic receptors in sinoatrial node cells. I examined with electrocardiogram how the muscarinic K(+) (K(ACh)) channel participates in the sinus bradycardia induced by a muscarinic agonist in the Langendorff preparation of rabbit hearts. In the presence of 100 nM propranolol, 1 nM to 10 microM carbachol (CCh) induced sinus bradycardia in a concentration-dependent manner. Tertiapin (100 or 300 nM), which selectively blocks K(ACh) channels in cardiac myocytes, significantly inhibited the effect of > or =300 nM but not < or =100 nM CCh. The effect of CCh was divided into tertiapin-sensitive and -insensitive components. The former component was induced by >100 nM CCh in a concentration-dependent manner and accounted for approximately 75% of the maximum effect of CCh. The K(ACh) channel in atrial myocytes was also activated by this range of concentrations of CCh as measured with the patch-clamp method. The tertiapin-insensitive component was induced by 1 to 300 nM CCh in a concentration-dependent manner and accounted for approximately 25% of the maximum effect of CCh. The sinus rate in the presence of 1 microM CCh and 300 nM tertiapin was similar to that in the presence of 2 mM CsCl, a blocker of the hyperpolarization-activated I(f) current. Furthermore, no tertiapin-insensitive component existed in the presence of 2 mM CsCl. Therefore, the negative chronotropic effect of > or =300 nM CCh is mainly mediated by K(ACh) channels, whereas that of < or =100 nM CCh may result from suppression of the I(f) current.

G(alpha)(i) controls the gating of the G protein-activated K(+) channel, GIRK

GIRK (Kir3) channels are activated by neurotransmitters coupled to G proteins, via a direct binding of G(beta)(gamma). The role of G(alpha) subunits in GIRK gating is elusive. Here we demonstrate that G(alpha)(i) is not only a donor of G(beta)(gamma) but also regulates GIRK gating. When overexpressed in Xenopus oocytes, GIRK channels show excessive basal activity and poor activation by agonist or G(beta)(gamma). Coexpression of G(alpha)(i3) or G(alpha)(i1) restores the correct gating parameters. G(alpha)(i) acts neither as a pure G(beta)(gamma) scavenger nor as an allosteric cofactor for G(beta)(gamma). It inhibits only the basal activity without interfering with G(beta)(gamma)-induced response. Thus, GIRK is regulated, in distinct ways, by both arms of the G protein. G(alpha)(i) probably acts in its GDP bound form, alone or as a part of G(alpha)(beta)(gamma) heterotrimer.

Disruption of cytoskeletal integrity impairs Gi-mediated signaling due to displacement of Gi proteins

beta1 integrins play a crucial role as cytoskeletal anchorage proteins. In this study, the coupling of the cytoskeleton and intracellular signaling pathways was investigated in beta1 integrin deficient (-/-) embryonic stem cells. Muscarinic inhibition of the L-type Ca2+ current (ICa) and activation of the acetylcholine-activated K+ current (IK,ACh) was found to be absent in beta1 integrin-/- cardiomyocytes. Conversely, beta adrenoceptor-mediated modulation of ICa was unaffected by the absence of beta1 integrins. This defect in muscarinic signaling was due to defective G protein coupling. This was supported by deconvolution microscopy, which demonstrated that Gi exhibited an atypical subcellular distribution in the beta1 integrin-/- cardiomyocytes. A critical role of the cytoskeleton was further demonstrated using cytochalasin D, which displaced Gi and impaired muscarinic signaling. We conclude that cytoskeletal integrity is required for correct localization and function of Gi-associated signaling microdomains.

Targeted inactivation of Galpha(i) does not alter cardiac function or beta-adrenergic sensitivity

Inhibitory Galpha(i) protein increases in the myocardium during hypertrophy and has been associated with beta-adrenergic receptor (beta-AR) desensitization, contractile dysfunction, and progression of cardiac disease. The role of Galpha(i) proteins in mediating basal cardiac function and beta-AR response in nonpathological myocardium, however, is uncertain. Transgenic mice with targeted inactivation of Galpha(i2) or Galpha(i3) were examined for in vivo cardiac function with the use of conscious echocardiography and for ex vivo cardiac response to inotropic stimulation with the use of Langendorff blood-perfused isolated hearts and adult ventricular cardiomyocytes. Echocardiography revealed that percent fractional shortening and heart rate were similar among wild-type, Galpha(i2)-null, and Galpha(i3)-null mice. Comparable baseline diastolic and contractile performance was also observed in isolated hearts and isolated ventricular myocytes from wild-type mice and mice lacking Galpha(i) proteins. Isoproterenol infusion enhanced diastolic and contractile performance to a similar degree in wild-type, Galpha(i2)-null, and Galpha(i3)-null mice. These data demonstrate no observable role for inhibitory G proteins in mediating basal cardiac function or sensitivity to beta-AR stimulation in nonpathological myocardium.

G-protein mediated gating of inward-rectifier K+ channels

G-protein regulated inward-rectifier potassium channels (GIRK) are part of a superfamily of inward-rectifier K+ channels which includes seven family members. To date four GIRK subunits, designated GIRK1-4 (also designated Kir3.1-4), have been identified in mammals, and GIRK5 has been found in Xenopus oocytes. GIRK channels exist in vivo both as homotetramers and heterotetramers. In contrast to the other mammalian GIRK family members, GIRK1 can not form functional channels by itself and has to assemble with GIRK2, 3 or 4. As the name implies, GIRK channels are modulated by G-proteins; they are also modulated by phosphatidylinositol 4,5-bisphosphate, intracellular sodium, ethanol and mechanical stretch. Recently a family of GTPase activating proteins known as regulators of G-protein signaling were shown to be the missing link for the fast deactivation kinetics of GIRK channels in native cells, which contrast with the slow kinetics observed in heterologously expressed channels. GIRK1, 2 and 3 are highly abundant in brain, while GIRK4 has limited distribution. Here, GIRK1/2 seems to be the predominant heterotetramer. In general, neuronal GIRK channels are involved in the regulation of the excitability of neurons and may contribute to the resting potential. Interestingly, only the GIRK1 and 4 subunits are distributed in the atrial and sinoatrial node cells of the heart and are involved in the regulation of cardiac rate. Our main objective of this review is to assess the current understanding of the G-protein modulation of GIRK channels and their physiological importance in mammals.

Enhanced negative chronotropy by inhibitory receptors in transgenic heart overexpressing beta(2)-adrenoceptors

Transgenic (TG) mice overexpressing beta(2)-adrenoceptors (AR) in the heart have enhanced beta-adrenergic activity. Since the degree of beta-adrenergic activation influences the negative chronotropic control of heart rate (HR), we studied the inhibitory effect of cholinergic and purinergic stimulation on HR in TG and wild-type (WT) control mice. Bradycardia in response to vagal nerve stimulation and administration of acetylcholine or adenosine was studied in anesthetised animals and perfused hearts. Basal HR was significantly higher in TG than WT mice (P<0.01). Electrical stimulation of vagal nerves (1-32 Hz) induced a Hz-dependent reduction in HR and the response was more pronounced in TG than WT groups (P<0.01). In perfused hearts, HR reduction by acetylcholine (ACh) was more pronounced with EC(50) 110-fold lower in TG than WT hearts. Adenosine-induced bradycardia, which was abolished by a P(1) antagonist, was more pronounced in TG hearts. After pre-treatment with pertussis toxin (PT, 100 microg/kg), bradycardia by vagal nerve stimulation or ACh remained unchanged in WT, but markedly inhibited in TG hearts (both P<0.01). Conversely, inhibiting guanylyl cyclase with LY83583 (30 microM) or nitric oxide synthase with L-NMMA (100 microM) attenuated HR reduction by vagal nerve stimulation in WT but not in TG hearts. Immunobloting assay showed similar G(ialpha2) abundance in TG and WT hearts. Thus, cardiac overexpression of beta(2)AR with high beta-adrenergic activity leads to hypersensitivity of inhibitory receptors controlling HR due to increase in activity of PT-sensitive G-proteins.

MEKK1 suppresses oxidative stress-induced apoptosis of embryonic stem cell-derived cardiac myocytes

A combination of in vitro embryonic stem (ES) cell differentiation and targeted gene disruption has defined complex regulatory events underlying oxidative stress-induced cardiac apoptosis, a model of postischemic reperfusion injury of myocardium. ES cell-derived cardiac myocytes (ESCM) having targeted disruption of the MEKK1 gene were extremely sensitive, relative to wild-type ESCM, to hydrogen peroxide-induced apoptosis. In response to oxidative stress, MEKK1-/- ESCM failed to activate c-Jun kinase (JNK) but did activate p38 kinase similar to that observed in wild-type ESCM. The increased apoptosis was mediated through enhanced tumor necrosis factor alpha production, a response that was positively and negatively regulated by p38 and the MEKK1-JNK pathway, respectively. Thus, MEKK1 functions in the survival of cardiac myocytes by inhibiting the production of a proapoptotic cytokine. MEKK1 regulation of the JNK pathway is a critical response for the protection against oxidative stress-induced apoptosis in cardiac myocytes.

Galpha(i2), Galpha(i3)and Galpha(o) are all required for normal muscarinic inhibition of the cardiac calcium channels in nodal/atrial-like cultured cardiocytes

The cardiac L-type calcium current (I(Ca,L)) is an important regulator of myocardial contractility. It is activated by sympathetic stimulation and inhibited by parasympathetic activity via muscarinic acetylcholine receptors. Muscarinic inhibition of I(Ca,L) occurs via activation of pertussis toxin (PTX)-sensitive heterotrimeric G-proteins. Although recent studies have shown that expression of G(oalpha) is important for this effect in adult mouse ventricular cells, two other PTX-sensitive G-proteins (G(i2) and G(i3)) are also expressed in cardiocytes and are activated. Their role in the regulation of I(Ca,L) has not been examined. In addition, it is not known whether nodal/atrial cardiac cells use the same G-proteins. We show that gene inactivation of each of the three PTX-sensitive Galpha-proteins (alpha(i2), alpha(i3), and alpha(o)) affects muscarinic inhibition of cardiac I(Ca,L) in embryonic stem (ES) cell-derived cardiocytes. Inactivation of either alpha(i2) or alpha(i3) markedly slows the time course of muscarinic inhibition of I(Ca,L), and in cells where both alpha(i2) and alpha(i3) are inactivated the effects are not additive. We also establish an essential role for alpha(o)in this atrial/nodal-like cardiocyte system and show that alpha(o)acts proximal to NO generation. NO generation plays a critical role in I(Ca,L) regulation since the nitric oxide synthase (NOS) antagonist, l -NMMA, blocked the inhibition of I(Ca,L) in WT and in alpha(i2)/alpha(i3)-null cells. In WT cells, the NO generating agent SIN-1 inhibited I(Ca,L) and the addition of carbachol resulted in faster inhibition, suggesting that pathways in addition to NO are also activated. This study shows that alpha(i2) and alpha(i3) play a critical role in the normal inhibition of cardiocyte I(Ca,L). Thus, all muscarinic receptor activated G-proteins (G(i2), G(i3) and G(o)) are necessary for normal inhibition and act through both NO and non-NO signaling pathways.

Simultaneous Cre catalyzed recombination of two alleles to restore neomycin sensitivity and facilitate homozygous mutations

Cells homozygous for neo-expressing mutations can be derived by culturing heterozygotes with elevated G418. We demonstrate that this strategy is significantly less efficient if hyg is substituted for neo. Therefore, to introduce additional mutations Cre recombinase was used to remove floxed neo from both alleles of homozygotes at two different loci. The rate-determining step in Cre excision appeared independent of substrate copy number. Incorporating cytosine deaminase and Herpes simplex virus thymidine kinase allowed negative selection for both targeting and Cre excision. The resulting G418-sensitive homozygous mutants should allow mutagenesis at additional loci and avoid untoward effects of retained selection markers.

A dominant-negative strategy for studying roles of G proteins in vivo

G proteins play a critical role in transducing a large variety of signals into intracellular responses. Increasingly, there is evidence that G proteins may play other roles as well. Dominant-negative constructs of the alpha subunit of G proteins would be useful in studying the roles of G proteins in a variety of processes, but the currently available dominant-negative constructs, which target Mg2+-binding sites, are rather leaky. A variety of studies have implicated the carboxyl terminus of G protein alpha subunits in both mediating receptor-G protein interaction and in receptor selectivity. Thus we have made minigene plasmid constructs that encode oligonucleotide sequences corresponding to the carboxyl-terminal undecapeptide of Galphai, Galphaq, or Galphas. To determine whether overexpression of the carboxyl-terminal peptide would block cellular responses, we used as a test system the activation of the M2 muscarinic receptor activated K+ channels in HEK 293 cells. The minigenes were transiently transfected along with G protein-regulated inwardly rectifying K+ channels (GIRK) into HEK 293 cells that stably express the M2 muscarinic receptor. The presence of the Galphai carboxyl-terminal peptide results in specific inhibition of GIRK activity in response to agonist stimulation of the M2 muscarinic receptor. The Galphai minigene construct completely blocks agonist-mediated M2 mAChR K+ channel response whereas the control minigene constructs (empty vector, pcDNA3.1, and the Galpha carboxyl peptide in random order, pcDNA-GalphaiR) had no effect on agonist-mediated M2 muscarinic receptor GIRK response. The inhibitory effects of the Galphai minigene construct were specific because overexpression of peptides corresponding to the carboxyl terminus of Galphaq or Galphas had no effect on M2 muscarinic receptor stimulation of the K+ channel.

Establishment of beta-adrenergic modulation of L-type Ca2+ current in the early stages of cardiomyocyte development

beta-Adrenergic modulation of the L-type Ca2+ current (ICaL) was characterized for different developmental stages in murine embryonic stem cell-derived cardiomyocytes using the whole-cell patch-clamp technique at 37 degreesC. Cardiomyocytes first appeared in embryonic stem cell-derived embryoid bodies grown for 7 days (7d). ICaL was insensitive to isoproterenol, forskolin, and 8-bromo-cAMP in very early developmental stage (VEDS) cardiomyocytes (from 7+1d to 7+2d) but highly stimulated by these substances in late developmental stage (LDS) cardiomyocytes (from 7+9d to 7+12d), indicating that all signaling cascade components became functionally coupled during development. In early developmental stage (EDS) cells (from 7+3d to 7+5d), the stimulatory response to forskolin and 8-bromo-cAMP was relatively weak. The forskolin effect was strongly augmented by ATP-gamma-S. At this stage, basal ICaL was stimulated by the nonselective phosphodiesterase (PDE) inhibitor isobutylmethylxanthine, by PDE inhibitors selective for the PDE II, III, and IV isoforms, as well as by the phosphatase inhibitor okadaic acid. Stimulation of ICaL by the catalytic subunit of the cAMP-dependent protein kinase A (PKA) was found to be similar (about 3 times) throughout development and in adult mouse ventricular cardiomyocytes, indicating that no structural changes of the Ca2+ channel related to phosphorylation occurred during development. ICaL was stimulated by isoproterenol in the presence of a PKA inhibitor and GTP-gamma-S in LDS but not VEDS cardiomyocytes, suggesting the development of a membrane-delimited stimulatory pathway mediated through the stimulatory GTP binding protein, Gs. We conclude that uncoupling and/or low expression of Gs protein accounted for the ICaL insensitivity to beta-adrenergic stimulation in VEDS cardiomyocytes. Furthermore, in EDS cells at the 7+4d stage, the reduced beta-adrenergic response is due, at least in part, to high intrinsic PDE and phosphatase activities.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

G protein alpha subunit G alpha z couples neurotransmitter receptors to ion channels in sympathetic neurons

The functional roles subserved by G(alpha)z, a G protein alpha subunit found predominantly in neuronal tissues, have remained largely undefined. Here, we report that G(alpha)z coupled neurotransmitter receptors to N-type Ca2+ channels when transiently overexpressed in rat sympathetic neurons. The G(alpha)z-mediated inhibition was voltage dependent and PTX insensitive. Recovery from G(alpha)z-mediated inhibition was extremely slow but accelerated by coexpression with RGS proteins. G(alpha)z selectively interacted with a subset of receptors that ordinarily couple to N-type Ca2+ channels via PTX-sensitive Go/i proteins. In addition, G(alpha)z rescued the activation of heterologously expressed GIRK channels in PTX-treated neurons. These results suggest that G(alpha)z is capable of coupling receptors to ion channels and might underlie PTX-insensitive ion channel modulation observed in neurons under physiological and pathological conditions.

Abnormal heart rate regulation in GIRK4 knockout mice

Acetylcholine (ACh) released from the stimulated vagus nerve decreases heart rate via modulation of several types of ion channels expressed in cardiac pacemaker cells. Although the muscarinic-gated potassium channel I(KACh) has been implicated in vagally mediated heart rate regulation, questions concerning the extent of its contribution have remained unanswered. To assess the role of I(KACh) in heart rate regulation in vivo, we generated a mouse line deficient in I(KACh) by targeted disruption of the gene coding for GIRK4, one of the channel subunits. We analyzed heart rate and heart rate variability at rest and after pharmacological manipulation in unrestrained conscious mice using electrocardiogram (ECG) telemetry. We found that I(KACh) mediated approximately half of the negative chronotropic effects of vagal stimulation and adenosine on heart rate. In addition, this study indicates that I(KACh) is necessary for the fast fluctuations in heart rate responsible for beat-to-beat control of heart activity, both at rest and after vagal stimulation. Interestingly, noncholinergic systems also appear to modulate heart activity through I(KACh). Thus, I(KACh) is critical for effective heart rate regulation in mice.