Coupling of muscarinic m1, m2 and m3 acetylcholine receptors, expressed in Chinese hamster ovary cells, to pertussis toxin-sensitive/insensitive guanine nucleotide-binding proteins

Importance of receptor expression in the classification of novel ligands at the M2 muscarinic acetylcholine receptor

BACKGROUND AND PURPOSE

Affinity-based, selective orthosteric ligands for the muscarinic acetylcholine receptors (mAChRs) are difficult to develop due to high sequence homology across the five subtypes. Selectivity can also be achieved via the selective activation of a particular subtype or signalling pathway. Promisingly, a prior study identified compounds 6A and 7A as functionally selective and Gi biased compounds at the M2 mAChR. Here, we have investigated the activation of individual G protein subfamilies and the downstream signalling profiles of 6A and 7A at the M2 mAChR.

EXPERIMENTAL APPROACH

G protein activation was measured with the TRUPATH assay in M2 mAChR FlpIn CHO cells. Activity in downstream signalling pathways was determined using the cAMP CAMYEL BRET sensor and assay of ERK 1/2 phosphorylation.

KEY RESULTS

M2 mAChRs coupled to Gɑi1 , GɑoA and Gɑs , but not Gɑq , in response to canonical orthosteric agonists. Compounds 6A and 7A did not elicit any G protein activation, cAMP inhibition or stimulation, or ERK 1/2 phosphorylation. Instead, a Schild analysis indicates a competitive, antagonistic interaction of compounds 6A and 7A with ACh in the Gɑi1 activation assay. Overexpression of the M2 mAChR may suggest an expression-dependent activation profile of compounds 6A and 7A.

CONCLUSIONS AND IMPLICATIONS

These data confirm that the M2 mAChR preferentially couples to Gɑi/o and to a lesser extent to Gɑs in response to canonical orthosteric ligands. However, this study was not able to detect Gɑi bias of compounds 6A and 7A, highlighting the importance of cellular background when classifying new ligands.

Helix 8 and the i3 loop of the muscarinic M3 receptor are crucial sites for its regulation by the Gβ5-RGS7 complex

The muscarinic M3 receptor (M3R) is a Gq-coupled receptor and is known to interact with many intracellular regulatory proteins. One of these molecules is Gβ5-RGS7, the permanently associated heterodimer of G protein β-subunit Gβ5 and RGS7, a regulator of G protein signaling. Gβ5-RGS7 can attenuate M3R-stimulated release of Ca²⁺ from intracellular stores or enhance the influx of Ca²⁺ across the plasma membrane. Here we show that deletion of amino acids 304–345 from the central portion of the i3 loop renders M3R insensitive to regulation by Gβ5-RGS7. In addition to the i3 loop, interaction of M3R with Gβ5-RGS7 requires helix 8. According to circular dichroism spectroscopy, the peptide corresponding to amino acids 548–567 in the C-terminus of M3R assumes an α-helical conformation. Substitution of Thr553 and Leu558 with Pro residues disrupts this α-helix and abolished binding to Gβ5-RGS7. Introduction of the double Pro substitution into full-length M3R (M3R^TP/LP) prevents trafficking of the receptor to the cell surface. Using atropine or other antagonists as pharmacologic chaperones, we were able to increase the level of surface expression of the TP/LP mutant to levels comparable to that of wild-type M3R. However, M3R-stimulated calcium signaling is still severely compromised. These results show that the interaction of M3R with Gβ5-RGS7 requires helix 8 and the central portion of the i3 loop.

Regulator of G protein signaling 2 deficiency causes endothelial dysfunction and impaired endothelium-derived hyperpolarizing factor-mediated relaxation by dysregulating Gi/o signaling

Regulator of G protein signaling 2 (RGS2) is a GTPase-activating protein for G(q/11)α and G(i/o)α subunits. RGS2 deficiency is linked to hypertension in mice and humans, although causative mechanisms are not understood. Because endothelial dysfunction and increased peripheral resistance are hallmarks of hypertension, determining whether RGS2 regulates microvascular reactivity may reveal mechanisms relevant to cardiovascular disease. Here we have determined the effects of systemic versus endothelium- or vascular smooth muscle-specific deletion of RGS2 on microvascular contraction and relaxation. Contraction and relaxation of mesenteric resistance arteries were analyzed in response to phenylephrine, sodium nitroprusside, or acetylcholine with or without inhibitors of nitric oxide (NO) synthase or K(+) channels that mediate endothelium-derived hyperpolarizing factor (EDHF)-dependent relaxation. The results showed that deleting RGS2 in vascular smooth muscle had minor effects. Systemic or endothelium-specific deletion of RGS2 strikingly inhibited acetylcholine-evoked relaxation. Endothelium-specific deletion of RGS2 had little effect on NO-dependent relaxation but markedly impaired EDHF-dependent relaxation. Acute, inducible deletion of RGS2 in endothelium did not affect blood pressure significantly. Impaired EDHF-mediated vasodilatation was rescued by blocking G(i/o)α activation with pertussis toxin. These findings indicated that systemic or endothelium-specific RGS2 deficiency causes endothelial dysfunction resulting in impaired EDHF-dependent vasodilatation. RGS2 deficiency enables endothelial G(i/o) activity to inhibit EDHF-dependent relaxation, whereas RGS2 sufficiency facilitates EDHF-evoked relaxation by squelching endothelial G(i/o) activity. Mutation or down-regulation of RGS2 in hypertension patients therefore may contribute to endothelial dysfunction and defective EDHF-dependent relaxation. Blunting G(i/o) signaling might improve endothelial function in such patients.

[³⁵S]GTPγS binding as an index of total G-protein and Gα-subtype-specific activation by GPCRs

On activation, G-protein-coupled receptors (GPCRs) exert many of their cellular actions through -promoting guanine nucleotide exchange on Gα subunits of heterotrimeric G proteins to release Gα-GTP and free βγ-subunits. In membrane preparations, GTP can be substituted by ³⁵S-labeled guanosine- 5'-O-(3-thio)triphosphate ([³⁵S]GTPγS) and on agonist stimulation a quasi-stable [³⁵S]GTPγS-Gα -complex forms and accumulates. Separation of [³⁵S]GTPγS-Gα complexes from free [³⁵S]GTPγS allows differences between basal and agonist-stimulated rates of [³⁵S]GTPγS-Gα complex formation- to be used to obtain pharmacological information on receptor-G-protein information transfer. Further, by releasing Gα-subunits into solution following the [³⁵S]GTPγS binding step, Gα-subunit-specific antibodies can be used to investigate the Gα-protein subpopulations activated by receptors by immunoprecipitation of [³⁵S]GTPγS-Gα complexes and quantification by scintillation counting. Here, we describe a total [³⁵S]GTPγS binding assay and a modification of this method that incorporates a Gα-specific immunoprecipitation step.

Pharmacology of polymorphic variants of the human 5-HT1A receptor

The 5-HT1A receptor is a critical mediator of serotonergic (5-HT) function. We have identified 13 potential single nucleotide polymorphisms resulting in amino acid changes throughout the human 5-HT1A receptor. The pharmacological profiles of these 13 polymorphic variants were then characterized using a high-throughput assay based on ligand-dependent transformation of NIH/3T3 cells. The majority of the polymorphic variants displayed wild-type pharmacological profiles in response to a panel of well-established agonists at the 5-HT1A receptor. However, the A50V polymorphic variant, which had an alanine to valine substitution in transmembrane 1, exhibited a loss of detectable response to 5-HT. Interestingly, all other agonists tested, including buspirone, lisuride, and (+)8-OH-DPAT, exhibited efficacies similar to that of the wild-type receptor. The competitive antagonist, methiothepin, also displayed a 19-fold decrease in potency at the A50V variant receptor. However, both 5-HT and methiothepin were able to compete for [3H]WAY-100635 binding to the A50V variant with affinities similar to the wild-type receptor. Moreover, the Bmax of [3H]WAY-100635 binding was 14-fold lower for the A50V variant than for the wild-type receptor. Thus, the A50V receptor variant exhibited ligand-specific functional alterations in addition to lower expression levels. These data suggest a previously unappreciated role for transmembrane 1 in mediating 5-HT response at the 5-HT1A receptor. Furthermore, individuals that potentially harbor the A50V polymorphism might display aberrant affective behaviors and altered responses to drugs targeting the 5-HT1A receptor.

Allosteric modulation of muscarinic receptor signaling: alcuronium-induced conversion of pilocarpine from an agonist into an antagonist

Previous studies on allosteric interactions at muscarinic receptors have often focused on ligand-receptor binding interactions, because ligand binding seemed to reflect functional consequences. The prototypal allosteric agent alcuronium is known to bind with similar affinity to the M(2) subtype of muscarinic acetylcholine receptors whether or not the receptors are occupied by the agonist pilocarpine. To determine allosteric modulation of receptor signaling by alcuronium, the effects of pilocarpine were measured in contracting guinea pig left atria and on G-protein coupling in M(2)-transfected Chinese hamster ovary (CHO) cell membranes. Alcuronium dose-dependently suppressed pilocarpine-induced reduction of isometric contraction force in atria (pIC(50, Alc) = 5.63) without any effect on the EC(50) of pilocarpine, consistent with an allosteric mechanism. In contrast, alcuronium shifted the concentration-effect curve of the agonist oxotremorine M to the right without affecting the maximal effect, in a formally competitive manner (pK(A, Alc) = 5.54). If pilocarpine remained receptor bound in the presence of alcuronium, this indicates that pilocarpine can no longer act as an agonist. In support of this hypothesis, pilocarpine acted as a competitive antagonist against oxotremorine M in the presence of 10 microM alcuronium. Measuring guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding in CHO-M(2) membranes yielded similar results. Alcuronium suppressed pilocarpine-induced stimulation of [(35)S]GTPgammaS binding (pIC(50, Alc) = 5.47) without shift in EC(50), whereas it competitively shifted the response to oxotremorine M (pK(A, Alc) = 5.97). [(3)H]Oxotremorine M binding data corresponded with the functional findings. In conclusion, alcuronium converted the agonist pilocarpine into an antagonist-a novel type of functional allosteric interaction.

Homologous and heterologous uncoupling of muscarinic M(3) and alpha(1B) adrenoceptors to Galpha(q/11) in SH-SY5Y human neuroblastoma cells

1. The present study employed a [(35)S]-GTPgammaS binding protocol in conjunction with immunoprecipitation (IP) of the Galpha subunits to investigate the desensitization of G(q/11)-coupled receptors at the level of the G-protein activation. Membranes from SH-SY5Y cells expressing the recombinant human alpha(1B)-adrenoceptor (alpha(1B)-AR) (and endogenously expressing the M(3) muscarinic acetylcholine receptor (M(3)-AChR)) exhibited G(q/11) activation in a concentration-dependent manner in response to noradrenaline or methacholine. 2. Pre-treatment of intact cells with agonist prior to membrane preparation and use in the [(35)S]-GTPgammaS IP assay demonstrated that both receptors were homologously desensitized by pre-treatment with agonist since the G(q/11) activation in response to a secondary challenge with agonist was markedly reduced. Stimulation of alpha(1B)-AR was effective at heterologously desensitizing the M(3)-AChR. The PKC inhibitor, Ro-31-8220 (10 microM) was ineffective at preventing the agonist-mediated receptor desensitization. 3. [(32)P]P(i)-labelled cells allowed the detection of increases in receptor phosphorylation. Phorbol 12,13 dibutyrate (PDBu) (1 microM) was effective at producing a Ro-31-8220 (10 microM)-sensitive, detectable increase in alpha(1B)-AR but not M(3)-AChR phosphorylation. Noradrenaline (30 microM) stimulated alpha(1B)-AR phosphorylation, which could be partially inhibited by Ro-31-8220 (10 microM). The phosphorylation of M(3)-AChR was increased by methacholine (100 microM) incubation and this effect appeared to be insensitive to Ro-31-8220 (10 microM). 4. These findings demonstrate that [(35)S]-GTPgammaS-Galpha-subunit IP can be used to estimate receptor desensitization as a decline in receptor-G-protein coupling. Both the alpha(1B)-AR and M(3)-AChR undergo rapid homologous desensitization that is associated with an increase in receptor phosphorylation. The heterologous desensitization of M(3)-AChR produced by alpha(1B)-AR stimulation is not associated with a detectable increase in M(3)-AChR phosphorylation, suggesting that receptor phosphorylation is not necessarily a prerequisite for desensitization.

G(q/11) and G(i/o) activation profiles in CHO cells expressing human muscarinic acetylcholine receptors: dependence on agonist as well as receptor-subtype

1. Profiles of G protein activation have been assessed using a [35S]-GTPgammaS binding/immunoprecipitation strategy in Chinese hamster ovary cells expressing either M1, M2, M3 or M4 muscarinic acetylcholine (mACh) receptor subtypes, where expression levels of M1 and M3, or M2 and M4 receptors were approximately equal. 2. Maximal [35S]-GTPgammaS binding to G(q/11)alpha stimulated by M1/M3 receptors, or G(i1-3)alpha stimulated by M2/M4 receptors occurred within approximately 2 min of agonist addition. The increases in G(q/11)alpha-[35S]-GTPgammaS binding after M1 and M3 receptor stimulation differed substantially, with M1 receptors causing a 2-3 fold greater increase in [35S]-GTPgammaS binding and requiring 5 fold lower concentrations of methacholine to stimulate a half-maximal response. 3. Comparison of M2 and M4 receptor-mediated G(i1-3)alpha-[35S]-GTPgammaS binding also revealed differences, with M2 receptors causing a greater increase in G(i1-3)alpha activation and requiring 10 fold lower concentrations of methacholine to stimulate a half-maximal response. 4. Comparison of methacholine- and pilocarpine-mediated effects revealed that the latter partial agonist is more effective in activating G(i3)alpha compared to G(i1/2)alpha for both M2 and M4 receptors. More marked agonist/partial agonist differences were observed with respect to M1/M3-mediated stimulations of G(q/11)alpha- and G(i1-3)alpha-[35S]-GTPgammaS binding. Whereas coupling to these Galpha subclasses decreased proportionately for M1 receptor stimulation by these agonists, pilocarpine possesses a greater intrinsic activity at M3 receptors for G(i)alpha versus G(q/11)alpha activation. 5. These data demonstrate that mACh receptor subtype and the nature of the agonist used govern the repertoire of G proteins activated. They also provide insights into how the diversity of coupling can be pharmacologically exploited, and provide a basis for a better understanding of how multiple receptor subtypes can be differentially regulated.

Basal signaling activity of human dopamine D2L receptor demonstrated with an ecdysone-inducible mammalian expression system

Basal signaling activity of the dopamine D2L (long isoform) receptor was investigated using an ecdysone-inducible mammalian expression system. Whereas basal signaling activity had been demonstrated for the D1, D2S (short isoform), D3, and D5 receptor subtypes, this issue has yet to be clearly resolved for the D2L receptors. An ecdysone-inducible mammalian expression system was used to express the dopamine D2L receptor in human embryonic kidney cells (HEK293). The two ecdysone analogs, muristerone A and ponasterone A, induced D2L receptor expression dose-dependently from 120 (non-induced) to 2000 fmol/mg protein which is similar to physiological D2 receptor density in dopaminergic brain regions. With this approach, we demonstrate significant basal D2L receptor activity. Basal and agonist-stimulated signaling activity, determined with assays of cAMP levels and [35S]GTPgammaS binding to assess G protein coupling, correlated with the level of receptor expression. Furthermore, among several antagonists tested, only fluphenazine and trifluoperazine exhibited clearly detectable inverse agonist activity. This differs from the D2S receptors, where most antagonists were reported to display full inverse agonism. Differences between inverse agonist effects of D2L and D2S antagonists may be relevant to the treatment of diseases involving dopamine D2 receptors.

Pharmacological characterization of type 1alpha metabotropic glutamate receptor-stimulated [35S]-GTPgammaS binding

1. The activation of G proteins by type 1alpha metabotropic glutamate receptors (mGluRs) in membranes from recombinant baby hamster kidney cells expressing the cloned rat mGluR1alpha receptor has been studied by use of a [35S]-guanosine 5'-[gamma-thio]triphosphate ([35S]-GTPgammaS) binding assay. 2. L-Glutamate increased the rate of [35S]-GTPgammaS binding in a concentration-dependent manner (-logEC50 (M) 5.25 +/- 0.07), with an optimal (62.4 +/- 1.6%) increase over basal binding being observed following 60 min incubation at 30 degrees C with 70 pM [35S]-GTPgammaS, 1 microM GDP, 10 mM MgCl2, 100 mM NaCl and 100 microg membrane protein ml(-1). The L-glutamate (100 microM)-stimulated increase in [35S]-GTPgammaS binding was totally prevented in the presence of the group I mGluR antagonist (S)-4-carboxy-3-hydroxyphenylglycine (300 microM). 3. Quantitative analysis of the affinity and number of G proteins activated by a maximally effective concentration of L-glutamate revealed an equilibrium dissociation constant (K(D)) for [35S]-GTPgammaS binding of 0.76 +/- 0.20 nM and a maximal number of GTPgammaS-liganded G proteins (Bmax) of 361 +/- 30 fmol mg(-1) protein. 4. Metabotropic glutamate receptor agonists, quisqualate (-logEC50 (M) 6.74 +/- 0.06), 1S,3R-ACPD (4.64 +/- 0.08) and (S)-3,5-dihydroxyphenylglycine (5.16 +/- 0.23) also increased [35S]-GTPgammaS binding in a concentration-dependent manner, with the latter two agents behaving as partial agonists. 5. (+)-alpha-Methylcarboxyphenylglycine (300 microM) caused a parallel rightward shift of the L-glutamate concentration-effect curve for [35S]-GTPgammaS binding, allowing an antagonist equilibrium dissociation constant (K(D)) of 34.0 +/- 7.8 microM to be calculated for this mGluR antagonist. 6. Pretreatment of BHK-mGluR1alpha cells with a concentration of pertussis toxin (PTX) shown to be maximally effective (100 ng ml(-1), 24 h) before membrane preparation resulted in a marked decrease in agonist-stimulated [35S]-GTPgammaS binding (by 66.0 +/- 0.9%), and an altered concentration-effect relationship for agonist-stimulated [35S]-GTPgammaS binding by the residual PTX-insensitive G-protein population. 7. The modulation of [35S]-GTPgammaS binding by agonists and antagonists in membranes from recombinant cells provides an excellent system in which to study mGluR interactions with PTX-sensitive and -insensitive G proteins.

Somatostatin5 receptor-mediated [35S]guanosine-5'-O-(3-thio)triphosphate binding: agonist potencies and the influence of sodium chloride on intrinsic activity

We studied the activation of the human somatostatin5 receptor recombinantly expressed in CHO-K1 cells by using some newly available agonists and antagonists. Somatostatin-28 bound to this receptor with a higher affinity than somatostatin-14 and was more potent in increasing [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding. Somatostatin-14-induced [35S]GTPgammaS binding to membranes from this cell line was decreased in a concentration-related manner by increasing concentrations of GDP and sodium chloride. At 50 mM (low) sodium, agonist EC50 values for stimulating [35S]GTPgammaS binding were lower than those at 150 mM (high) sodium and were closer to their respective affinity estimates (dissociation equilibrium constants) for binding to the receptor in the absence of sodium. Both agonist binding to the high affinity state of the receptor and agonist-induced [35S]GTPgammaS binding were abolished by pertussis toxin pretreatment. The putative somatostatin5 receptor-selective ligand L-362,855, unlike somatostatin-14 and somatostatin-28, showed differential intrinsic activity for stimulation of [35S]GTPgammaS binding, behaving as a partial agonist in high sodium and a full agonist in low sodium. In contrast, BIM-23056 did not behave as an agonist under any conditions studied but was able to antagonize somatostatin-14-induced [35S]GTPgammaS binding. We conclude that measurement of [35S]GTPgammaS binding mediated by somatostatin receptor activation in the presence of different concentrations of sodium chloride provides a useful functional assay for assessing the relative agonist efficacies of novel ligands identified from radioligand binding studies.

Muscarinic M3 receptor coupling and regulation

Current concepts regarding the regulation and coupling of muscarinic m3 receptors to G-proteins and various effectors are discussed. The last few years have provided much evidence that although muscarinic m1, m3 and m5 subtypes couple predominantly via pertussis toxin-insensitive G-proteins (Gq/11) to activate phosphoinositidase C (PIC), interactions with other G-proteins (Gi, Go, Gs) can be readily observed in cells expressing recombinant muscarinic receptors even at relatively low levels. The significance of this diversity and the potential for agonist "trafficking" could open up opportunities for novel approaches to selective agonist action. Finally, mechanisms underlying the regulation of muscarinic m3 coupling through Gq/11 to PIC are discussed. In particular, our recent studies on precursor lipid depletion, whether regulation is receptor or cell specific and the identification and role of receptor kinases are briefly reviewed.

[35S]GTP gamma S binding: a tool to evaluate functional activity of a cloned opioid receptor transiently expressed in COS cells

In this paper we propose a powerful procedure to measure functional activation of the mouse delta-opioid receptor transiently expressed in mammalian cells. Receptor stimulation was assessed using a population of electroporated COS cells, transfected at a 50% efficiency. Under those conditions, agonist-promoted activation of the receptor was measured by [35S]GTP gamma S binding. Both BW373U86, an alkaloid compound, and DADLE, a peptide agonist, elicited increase of specific [35S]GTP gamma S binding representing 300% of basal level. Maximal activation was compared to that obtained for the cloned receptor stably expressed in CHO cells. Agonist efficacy was similar in both expressions systems, demonstrating the high sensitivity of the proposed method applied to transient expression. Finally dose-response curves were found highly reproducible across transfection experiments, opening the possibility for a direct comparison of distinct recombinant receptor preparations. This method represents a powerful tool for the study of opioid signal transduction at the receptor level. It may also be extended to investigate signalling properties of other Gi/Go coupled receptors.

Muscarinic m1 receptor-stimulated adenylate cyclase activity in Chinese hamster ovary cells is mediated by Gs alpha and is not a consequence of phosphoinositidase C activation

The mechanism underlying muscarinic m1 receptor-mediated increases in adenosine 3',5'-cyclic monophosphate (cAMP) was investigated in Chinese hamster ovary (CHO) cells expressing human recombinant m1 muscarinic receptors (CHO-ml cells). Stimulation of CHO-ml cells with carbachol resulted in marked accumulation of Ins(1,4,5)P3 and cAMP, in an atropine-sensitive manner, with EC50 values (log M) of -5.16 +/- 0.06 and -3.93 +/- 0.07 respectively. Basal and agonist-stimulated cAMP accumulation were unaffected by a 5 min pretreatment with l microM phorbol 12,13-dibutyrate and were not attenuated by pertussis toxin (100 ng/ml, 20h). Agonist-stimulated cAMP accumulation was also observed in CHO-ml cell membranes incubated in a buffer containing 100 nM free Ca2+. Guanosine 5'- [gamma-thio]triphosphate (10 microM) potentiated agonist-stimulated cAMP accumulation in CHO-ml cell membranes, implicating a G-protein involvement in this response. Co-incubation of carbachol with forskolin (10 microM) produced a greater than additive accumulation of cAMP in CHO-ml cells. Furthermore, a C-terminal-directed anti-Gs alpha serum attenuated both carbachol-stimulated (in CHO-ml cell membranes) and isoprenaline-stimulated (in CHO-beta 2 cell membranes) cAMP accumulation with a similar dose-dependency. These results suggest that muscarinic agonist-stimulated cAMP accumulation in CHO-ml cells occurs via activation of Gs alpha and not as a consequence of phosphoinositidase C activation.