An investigation of whether agonist-selective receptor conformations occur with respect to M2 and M4 muscarinic acetylcholine receptor signalling via Gi/o and Gs proteins

Ligand bias at the muscarinic acetylcholine receptor family: Opportunities and challenges

Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors (GPCRs) that are activated by the endogenous neurotransmitter, acetylcholine (ACh). Disruption of mAChR signalling has been associated with a variety of neurological disorders and non-neurological diseases. Consequently, the development of agonists and antagonists of the mAChRs has been a major avenue in drug discovery. Unfortunately, mAChR ligands are often associated with on-target side effects for two reasons. The first reason is due to the high sequence conservation at the orthosteric ACh binding site among all five receptor subtypes (M1-M5), making on-target subtype selectivity a major challenge. The second reason is due to on-target side effects of mAChR drugs that are associated with the pleiotropic nature of mAChR signalling at the level of a single mAChR subtype. Indeed, there is growing evidence that within the myriad of signalling events produced by mAChR ligands, some will have therapeutic benefits, whilst others may promote cholinergic side effects. This paradigm of drug action, known as ligand bias or biased agonism, is an attractive feature for next-generation mAChR drugs, as it holds the promise of developing drugs devoid of on-target adverse effects. Although relatively simple to detect and even quantify in vitro, ligand bias, as observed in recombinant systems, does not always translate to in vivo systems, which remains a major hurdle in GPCR drug discovery, including the mAChR family. Here we report recent studies that have attempted to detect and quantify ligand bias at the mAChR family, and briefly discuss the challenges associated with biased agonist drug development. This article is part of the Special Issue on "Ligand Bias".

Fine Tuning Muscarinic Acetylcholine Receptor Signaling Through Allostery and Bias

The M1 and M4 muscarinic acetylcholine receptors (mAChRs) are highly pursued drug targets for neurological diseases, in particular for Alzheimer's disease and schizophrenia. Due to high sequence homology, selective targeting of any of the M1-M5 mAChRs through the endogenous ligand binding site has been notoriously difficult to achieve. With the discovery of highly subtype selective mAChR positive allosteric modulators in the new millennium, selectivity through targeting an allosteric binding site has opened new avenues for drug discovery programs. However, some hurdles remain to be overcome for these promising new drug candidates to progress into the clinic. One challenge is the potential for on-target side effects, such as for the M1 mAChR where over-activation of the receptor by orthosteric or allosteric ligands can be detrimental. Therefore, in addition to receptor subtype selectivity, a drug candidate may need to exhibit a biased signaling profile to avoid such on-target adverse effects. Indeed, recent studies in mice suggest that allosteric modulators for the M1 mAChR that bias signaling toward specific pathways may be therapeutically important. This review brings together details on the signaling pathways activated by the M1 and M4 mAChRs, evidence of biased agonism at these receptors, and highlights pathways that may be important for developing new subtype selective allosteric ligands to achieve therapeutic benefit.

β1-adrenoceptor-stimulated lactate production in cultured astrocytes is predominantly glycogen-independent

Noradrenaline (NA) promotes breakdown of the glucose-polymer, glycogen, and hence enhances glycolytic production of lactate in astrocytes. Here, in cultured rat cerebrocortical astrocytes, we examined the contributions of different adrenoceptor subtypes to NA-modulated glucose metabolism, and the relationship of NA-induced glycogenolysis to lactate production. Stimulation of astrocytic glucose metabolism by NA was mediated predominantly via β1-adrenoceptors and cAMP. Constitutive β 1-adrenoceptor activity - in the absence of exogenous NA - contributed to the basal rate of glycogen turnover. Although mRNAs encoding both β 1- and β 2-adrenoceptors were detected in these astrocytes, β 2-adrenoceptors contributed little to NA-induced modulation of glucose metabolism. Activation of α2- and α 1-adrenoceptors in these cells decreased cAMP and increased cytosolic Ca²⁺, respectively, but did not modulate NA-induced glycogenolysis: α 2-adrenoceptors because glycogenolysis was induced maximally by NA concentrations that only began to inhibit cAMP production; and α 1-adrenoceptors possibly because of desensitisation and depletion of Ca²⁺ stores. Under basal conditions, astrocytes converted glucose to extracellular lactate in near stoichiometric manner. When glucose-starved astrocytes were given fresh glucose-containing medium, lactate accumulation displayed a brief lag period before attaining a steady-state rate. During this lag period NA, acting at β 1-adrenoceptors, increased the rate of lactate accumulation both in the absence and presence of an inhibitor of glycogen turnover. At the steady-state, the rate of glucose incorporation into accumulated glycogen was ~ 5% of that into lactate, but NA enhanced lactate output by 20-50%: this further indicates that NA, via β 1-adrenoceptors and cAMP, can enhance astrocytic lactate production independently of its effect on glycogen turnover.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

M(4) muscarinic receptors and locomotor activity regulation

M(4) muscarinic receptors (M(4) MR) represent a subfamily of G-protein coupled receptors serving a substantial role in spontaneous locomotor activity regulation, cognition and modulation of cholinergic system. With increasing body of literature discussing the role of M(4) MR some controversies arose. Thus, we try here to summarize the current evidence regarding the M(4) MR, with the special focus on their role in Locomotor activity control. We review the molecular function of M(4) MR in specific brain areas implicated in locomotor regulation, and shortly in other CNS processes that could be connected to locomotor activity. We also focus on brain areas implicated in locomotor activity biorhythm changes like suprachiasmatic nucleus, subparaventricular zone posterior hypothalamic area, striatum and thalamus. Gender-related aspects and differences in locomotor activity in males and females are discussed further.

Superagonism at G protein-coupled receptors and beyond

Ligands targeting GPCRs can be categorized according to their intrinsic efficacy to trigger a specific, receptor-mediated response. A ligand endowed with the same level of efficacy as the endogenous agonist can be classified as a full agonist, whereas a compound that displays greater efficacy, that is, higher receptor signalling output than the endogenous agonist, can be called a superagonist. Subsequent to GPCR activation, an intracellular signalling cascade is set in motion, which may generate substantial amplification of the signal. This may obscure superagonism in pharmacological assays and, therefore, the definition of superagonism necessitates a combination of operational approaches, reduction of spare receptors or estimation of receptor activation close to the receptor level to quantify relative agonist efficacies in a particular system. The first part of this review will compare GPCR superagonism with superagonism in the field of immunology, where this term is well established. In the second part, known GPCR superagonists will be reviewed. Then, the experimental and analytical challenges in the deconvolution of GPCR superagonism will be addressed. Finally, the potential benefit of superagonism is discussed. The molecular mechanisms behind GPCR superagonism are not completely understood. However, crystallography shows that agonist binding alone is not sufficient for a fully active receptor state and that binding of the G protein is at least equally important. Accordingly, the emerging number of reported superagonists implies that ligand-induced receptor conformations more active than the ones stabilized by the endogenous agonist are indeed feasible. Superagonists may have therapeutic potential when receptor function is impaired or to induce negative feedback mechanisms. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Muscarine-like compounds derived from a pyrolysis product of cellulose

Starting from a hydroxylactone anhydrosugar available from catalytic cellulose pyrolysis, five new muscarine-like compounds have been synthesized and studied for their binding affinity to human subtype muscarine receptors, obtaining results supported by docking calculation.

Agonists with supraphysiological efficacy at the muscarinic M2 ACh receptor

BACKGROUND AND PURPOSE

Artificial agonists may have higher efficacy for receptor activation than the physiological agonist. Until now, such 'superagonism' has rarely been reported for GPCRs. Iperoxo is an extremely potent muscarinic receptor agonist. We hypothesized that iperoxo is a 'superagonist'.

EXPERIMENTAL APPROACH

Signalling of iperoxo and newly synthesized structural analogues was compared with that of ACh at label-free M2 muscarinic receptors applying whole cell dynamic mass redistribution, measurement of G-protein activation, evaluation of cell surface agonist binding and computation of operational efficacies.

KEY RESULTS

In CHO-hM2 cells, iperoxo significantly exceeds ACh in Gi /Gs signalling competence. In the orthosteric loss-of-function mutant M2 -Y104(3.33) A, the maximum effect of iperoxo is hardly compromised in contrast to ACh. 'Superagonism' is preserved in the physiological cellular context of MRC-5 human lung fibroblasts. Structure-signalling relationships including iperoxo derivatives with either modified positively charged head group or altered tail suggest that 'superagonism' of iperoxo is mechanistically based on parallel activation of the receptor protein via two orthosteric interaction points.

CONCLUSION AND IMPLICATIONS

Supraphysiological agonist efficacy at muscarinic M2 ACh receptors is demonstrated for the first time. In addition, a possible underlying molecular mechanism of GPCR 'superagonism' is provided. We suggest that iperoxo-like orthosteric GPCR activation is a new avenue towards a novel class of receptor activators.

Multiple active receptor conformation, agonist efficacy and maximum effect of the system: the conformation-based operational model of agonism

The operational model of agonism assumes that the maximum effect a particular receptor system can achieve (the Em parameter) is fixed. Em estimates are above but close to the asymptotic maximum effects of endogenous agonists. The concept of Em is contradicted by superagonists and those positive allosteric modulators that significantly increase the maximum effect of endogenous agonists. An extension of the operational model is proposed that assumes that the Em parameter does not necessarily have a single value for a receptor system but has multiple values associated to multiple active receptor conformations. The model provides a mechanistic link between active receptor conformation and agonist efficacy, which can be useful for the analysis of agonist response under different receptor scenarios.

Quantitative analysis reveals multiple mechanisms of allosteric modulation of the mGlu5 receptor in rat astroglia

Positive and negative allosteric modulators (PAMs and NAMs, respectively) of the type 5 metabotropic glutamate (mGlu5) receptor have demonstrable therapeutic potential in an array of neurological and psychiatric disorders. Here, we have used rat cortical astrocytes to investigate how PAMs and NAMs mediate their activity and reveal marked differences between PAMs with respect to their modulation of orthosteric agonist affinity and efficacy. Affinity cooperativity factors (α) were assessed using [(3)H]2-methyl-6-(phenylethynyl)-pyridine (MPEP)-PAM competition binding in the absence and presence of orthosteric agonist, whereas efficacy cooperativity factors (β) were calculated from net affinity/efficacy cooperativity parameters (αβ) obtained from analyses of the abilities of PAMs to potentiate [(3)H]inositol phosphate accumulation in astrocytes stimulated with a submaximal (EC(20)) concentration of orthosteric agonist. We report that whereas 3,3'-difluorobenzaldazine (DFB) and 3-cyano-N-(1,3-diphenyl-1H-prazol-5-yl)benzamide (CDPPB) primarily exert their allosteric modulatory effects through modifying the apparent orthosteric agonist affinity at the astrocyte mGlu5 receptor, the effects of S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidinl-1-yl}-methanone (ADX47273) are mediated primarily via efficacy-driven modulation. In [(3)H]MPEP-NAM competition binding assays, both MPEP and 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP) defined similar specific binding components, with affinities that were unaltered in the presence of orthosteric agonist, indicating wholly negative efficacy-driven modulations. It is noteworthy that whereas M-5MPEP only partially inhibited orthosteric agonist-stimulated [(3)H]inositol phosphate accumulation in astrocytes, it could completely suppress Ca(2+) oscillations stimulated by quisqualate or (S)-3,5-dihydroxyphenylglycine. In contrast, MPEP was fully inhibitory with respect to both functional responses. The finding that M-5MPEP has different functional effects depending on the endpoint measured is discussed as a possible example of permissive allosteric antagonism.

Lack of association of a spontaneous mutation of the Chrm2 gene with behavioral and physiologic phenotypic differences in inbred mice

The nucleotide substitution C797T in the Chrm2 gene causes substitution of leucine for proline at position 266 (P266L) of the CHRM2 protein. Because Chrm2 codes for the type 2 muscarinic receptor, this mutation could influence physiologic and behavioral phenotypes of mice. Chrm2 mRNA was not differentially expressed in 2 brain regions with high cholinergic innervation in a mouse strain that does (BALB/cByJ) or does not (C57BL/6J) have the mutation. In addition, strains of mice with and without the C797T point mutation in Chrm2 did not differ significantly in muscarinic binding properties. Variation across strains was detected in terms of acoustic startle, prepulse inhibition, and the physiologic effects of the muscarinic agonist oxotremorine. However, interstrain differences in these measures did not correlate with the presence of the mutation. Although we were unable to associate a measurable phenotype with the Chrm2 mutation, assessment of the mutation on other genetic backgrounds or in the context of other traits might reveal differential effects. Therefore, despite our negative findings, evaluation of characteristics that involve muscarinic function should be undertaken with caution when comparing mice with different alleles of the Chrm2 gene.

An optical dynamic mass redistribution assay reveals biased signaling of dualsteric GPCR activators

Increasing attention is paid in basic science and in drug discovery to pathway selective intracellular signaling as a novel approach to achieve precise control of cell function via G protein-coupled receptors (GPCRs). With respect to signaling, GPCRs are often promiscuous in that more than one intracellular biochemical pathway is activated upon receptor stimulation by the endogenous transmitter or by exogenous drugs. We studied signaling by a novel class of GPCR activators that were designed to bind simultaneously to the orthosteric transmitter-binding site and the allosteric site of muscarinic acetylcholine receptors. An optical biosensor technique was applied to measure activation-induced dynamic mass redistribution (DMR) in CHO cells stably expressing the muscarinic receptor subtype of interest. The use of tools to modulate signaling and measuring G protein activation directly proved that DMR is a valid and comfortable approach to gain real-time insight into intracellular signaling pathway activation and to identify signaling pathway-selective drugs.

Contrasting effects of allosteric and orthosteric agonists on m1 muscarinic acetylcholine receptor internalization and down-regulation

A new class of subtype-selective muscarinic acetylcholine (mACh) receptor agonist that activates the receptor through interaction at a site distinct from the orthosteric acetylcholine binding site has been reported recently. Here, we have compared the effects of orthosteric (oxotremorine-M, arecoline, pilocarpine) and allosteric [4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine (AC-42); 1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone (77-LH-28-1)] agonists on M(1) mACh receptor internalization and down-regulation, as well as functional coupling in a Chinese hamster ovary (CHO) cell line. In contrast to full and partial orthosteric agonists, which cause significant receptor internalization and down-regulation, prolonged exposure to AC-42 did not significantly alter either cell-surface or total cellular M(1) mACh receptor expression. 77-LH-28-1, an AC-42 homolog, did cause some receptor internalization, but not down-regulation. The presence of atropine completely prevented the orthosteric agonist-induced adaptive changes in receptor populations; however, in contrast, the copresence of atropine and AC-42 significantly increased both cell-surface receptor and total M(1) mACh receptor expression. Maximal phosphoinositide hydrolysis responses to the partial agonist arecoline were similar in CHO-M(1) cells pretreated for 24 h with either AC-42 or vehicle; in contrast, these responses were markedly reduced when cells were pretreated with oxotremorine-M or pilocarpine. These data indicate that, whereas AC-42 binding to the M(1) mACh receptor can initiate signal transduction, the AC-42-liganded receptor is resistant to the usual mechanisms regulating receptor internalization and down-regulation. In addition, our data suggest unusual interactions between allosteric agonists and orthosteric antagonists to regulate cell-surface and total cellular receptor expression.

Muscarinic inhibition of hippocampal and striatal adenylyl cyclase is mainly due to the M(4) receptor

The five muscarinic acetylcholine receptors (M(1)-M(5)) are differentially expressed in the brain. M(2) and M(4) are coupled to inhibition of stimulated adenylyl cyclase, while M(1), M(3) and M(5) are mainly coupled to the phosphoinositide pathway. We studied the muscarinic receptor regulation of adenylyl cyclase activity in the rat hippocampus, compared to the striatum and amygdala. Basal and forskolin-stimulated adenylyl cyclase activity was higher in the striatum but the muscarinic inhibition was much lower. Highly selective muscarinic toxins MT1 and MT2-affinity order M(1) > or = M(4) >> others-and MT3-highly selective M(4) antagonist-did not show significant effects on basal or forskolin-stimulated cyclic AMP production but, like scopolamine, counteracted oxotremorine inhibition. Since MTs have negligible affinity for M(2), M(4) would be the main subtype responsible for muscarinic inhibition of forskolin-stimulated enzyme. Dopamine stimulated a small fraction of the enzyme (3.1% in striatum, 1.3% in the hippocampus). Since MT3 fully blocked muscarinic inhibition of dopamine-stimulated enzyme, M(4) receptor would be responsible for this regulation.

Membrane cholesterol content influences binding properties of muscarinic M2 receptors and differentially impacts activation of second messenger pathways

We investigated the influence of membrane cholesterol content on preferential and non-preferential signaling through the M(2) muscarinic acetylcholine receptor expressed in CHO cells. Cholesterol depletion by 39% significantly decreased the affinity of M(2) receptors for [(3)H]-N-methylscopolamine ([(3)H]-NMS) binding and increased B(max) in intact cells and membranes. Membranes displayed two-affinity agonist binding sites for carbachol and cholesterol depletion doubled the fraction of high-affinity binding sites. In intact cells it also reduced the rate of agonist-induced receptor internalization and changed the profile of agonist binding from a single site to two affinity states. Cholesterol enrichment by 137% had no effects on carbachol E(max) of cAMP synthesis inhibition and on cAMP synthesis stimulation and inositolphosphates (IP) accumulation at higher agonist concentrations (non-preferred pathways). On the other hand, cholesterol depletion significantly increased E(max) of cAMP synthesis inhibition or stimulation without change in potency, and decreased E(max) of IP accumulation. Noteworthy, modifications of membrane cholesterol had no effect on membrane permeability, oxidative activity, protein content, or relative expression of G(s), G(i/o), and G(q/11) alpha subunits. These results demonstrate distinct changes of M(2) receptor signaling through both preferential and non-preferential G-proteins consequent to membrane cholesterol depletion that occur at the level of receptor/G-protein/effector protein interactions in the cell membrane. The significant decrease of IP accumulation by cholesterol depletion was also observed in cells expressing M(3) receptors and by both cholesterol depletion and enrichment in cells expressing M(1) receptors indicating relevance of reduced G(q/11) signaling for the pathogenesis of Alzheimer's disease.

Dualsteric GPCR targeting: a novel route to binding and signaling pathway selectivity

Selective modulation of cell function by G protein-coupled receptor (GPCR) activation is highly desirable for basic research and therapy but difficult to achieve. We present a novel strategy toward this goal using muscarinic acetylcholine receptors as a model. The five subtypes bind their physiological transmitter in the highly conserved orthosteric site within the transmembrane domains of the receptors. Orthosteric muscarinic activators have no binding selectivity and poor signaling specificity. There is a less well conserved allosteric site at the extracellular entrance of the binding pocket. To gain subtype-selective receptor activation, we synthesized two hybrids fusing a highly potent oxotremorine-like orthosteric activator with M(2)-selective bis(ammonio)alkane-type allosteric fragments. Radioligand binding in wild-type and mutant receptors supplemented by receptor docking simulations proved M(2) selective and true allosteric/orthosteric binding. G protein activation measurements using orthosteric and allosteric blockers identified the orthosteric part of the hybrid to engender receptor activation. Hybrid-induced dynamic mass redistribution in CHO-hM(2) cells disclosed pathway-specific signaling. Selective receptor activation (M(2)>M(1)>M(3)) was verified in living tissue preparations. As allosteric sites are increasingly recognized on GPCRs, the dualsteric concept of GPCR targeting represents a new avenue toward potent agonists for selective receptor and signaling pathway activation.

Selectivity of agonists for the active state of M1 to M4 muscarinic receptor subtypes

We measured the intrinsic relative activity (RA(i)) of muscarinic agonists to detect possible selectivity for receptor subtypes and signaling pathways. RA(i) is a relative measure of the microscopic affinity constant of an agonist for the active state of a GPCR expressed relative to that of a standard agonist. First, we estimated RA(i) values for a panel of agonists acting at the M(4) muscarinic receptor coupled to three distinct G-protein pathways: G(i) inhibition of cAMP accumulation, G(s) stimulation of cAMP accumulation, and G alpha(15) stimulation of phosphoinositide hydrolysis. Our results show similar RA(i) values for each agonist, suggesting that the same active state of the M(4) receptor triggers the activation of the three G proteins. We also estimated RA(i) values for agonists across M(1) to M(4) muscarinic subtypes stably transfected in Chinese hamster ovary cells. Our results show selectivity of McN-A-343 [4-I-[3-chlorophenyl]carbamoyloxy)-2-butynyltrimethylammnonium chloride] for the M(1) and M(4) subtypes and selectivity of pilocarpine for the M(1) and M(3) subtypes. The other agonists tested lacked marked selectivity among M(1) to M(4) receptors. Finally, we estimated RA(i) values from published literature on M(1), M(2), and M(3) muscarinic responses and obtained results consistent with our own studies. Our results show that the RA(i) estimate is a useful receptor-dependent measure of agonist activity.

G protein coupling and signaling pathway activation by m1 muscarinic acetylcholine receptor orthosteric and allosteric agonists

The M(1) muscarinic acetylcholine (mACh) receptor is among a growing number of G protein-coupled receptors that are able to activate multiple signaling cascades. AC-42 (4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine) is an allosteric agonist that can selectively activate the M(1) mACh receptor in the absence of an orthosteric ligand. Allosteric agonists have the potential to stabilize unique receptor conformations, which may in turn cause differential activation of signal transduction pathways. In the present study, we have investigated the signaling pathways activated by AC-42, its analog 77-LH-28-1 (1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone), and a range of orthosteric muscarinic agonists [oxotremorine-M (oxo-M), arecoline, and pilocarpine] in Chinese hamster ovary cells recombinantly expressing the human M(1) mACh receptor. Each agonist was able to activate Galpha(q/11)-dependent signaling, as demonstrated by an increase in guanosine 5'-O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding to Galpha(q/11) proteins and total [(3)H]inositol phosphate accumulation assays in intact cells. All three orthosteric agonists caused significant enhancements in [(35)S]GTPgammaS binding to Galpha(i1/2) subunits over basal; however, neither allosteric ligand produced a significant response. In contrast, both orthosteric and allosteric agonists are able to couple to the Galpha(s)/cAMP pathway, enhancing forskolin-stimulated cAMP accumulation. These data provide support for the concept that allosteric and orthosteric mACh receptor agonists both stabilize receptor conformations associated with Galpha(q/11)- and Galpha(s)-dependent signaling; however, AC-42 and 77-LH-28-1, unlike oxo-M, arecoline, and pilocarpine, do not seem to promote M(1) mACh receptor-Galpha(i1/2) coupling, suggesting that allosteric agonists have the potential to activate distinct subsets of downstream effectors.

Rit contributes to neurite outgrowth triggered by the alpha subunit of Go

Heterotrimeric GTP-binding protein transduce signals initiated by a variety of hormones and neurotransmitters. Go, a member of the Go/Gi family, is the most abundant heterotrimeric GTP-binding protein in nervous tissues and has been implicated in neuronal differentiation. The mechanism by which Go modulates neuronal differentiation has not been, however, fully elucidated. Here, we identified small GTPase Rit as an interacting partner of the alpha-subunit of Go (Goalpha). The biochemical characterizations of Goalpha::Rit interaction revealed that Rit is a candidate downstream effector for Goalpha. Furthermore, dominant negative Rit inhibited Goalpha-induced neurite outgrowth and Erk phosphorylation in Neuro2a cells. These results suggest that Rit may be involved in the signaling pathway for Goalpha-mediated neuronal differentiation.

Relaxant action mechanism of berberine identified as the active principle of Argemone ochroleuca Sweet in guinea-pig tracheal smooth muscle

In this study we investigated the relaxant effect of the aerial parts of Argemone ochroleuca (Papaveraceae), which is used in Mexican traditional medicine for the treatment of various respiratory diseases such as cough, bronchitis and asthma. The alkaloid berberine was identified as one of the active relaxant principles (EC50 = 118.50 +/-3.91 microM) in the dichloromethane extract of A. ochroleuca (EC50 = 78.03 +/- 2.15 microg mL(-1) with 95.12 +/- 3.56% of relaxation). Berberine concentration-dependently relaxed the carbachol-induced precontractions but not histamine- or KCl-induced precontraction. The relaxant effect of berberine was unaffected by the presence of propranolol (3 microM), glibenclamide (10 microM) or ODQ (10microM). However, 2', 5'-dideoxyadenosine (10 microM) blocked the log concentration-response curves of berberine. On the other hand, berberine produced a leftward shift of the log concentration-response curves of isoproterenol, forskolin and nitroprusside. Additionally, berberine produced a parallel rightward shift of the concentration-response curve of carbachol in a competitive manner with a pA2 of 3.87 +/- 0.045. The above results suggest that the relaxant effect of berberine on tracheal muscle is due to its antagonistic effect on muscarinic acetylcholine receptors.

The alpha subunit of Go interacts with promyelocytic leukemia zinc finger protein and modulates its functions

Heterotrimeric GTP-binding proteins (G proteins) mediate signal transduction generated by neurotransmitters and hormones. Go, a member of the Go/Gi family, is the most abundant heterotrimeric G protein in the brain. Most mechanistic analyses on Go activation demonstrate that its action is mediated by the Gbetagamma dimer; downstream effectors for its alpha subunit (Goalpha) have not been clearly defined. Here, we employ the yeast two-hybrid system to screen for Goalpha-interacting partners in a cDNA library from human fetal brain. The transcription factor promyelocytic leukemia zinc finger protein (PLZF) specifically bound to Goalpha. Interactions between PLZF and Goalpha were confirmed using in vitro and in vivo affinity binding assays. Activated Goalpha interacted directly with PLZF, and enhanced its function as a transcriptional and cell growth suppressor. Notably, PLZF activity was additionally promoted by the Go/ialpha-coupled cannabinoid receptor (CB) in HL60 cells endogenously expressing CB and PLZF. These results collectively suggest that Goalpha modulates the function of PLZF via direct interactions. Our novel findings provide insights into the diverse cellular roles of Goalpha and its coupled receptor.

G alpha(q)-coupled muscarinic acetylcholine receptors enhance nicotinic acetylcholine receptor signaling in Caenorhabditis elegans mating behavior

In this study, we address why metabotropic and ionotropic cholinergic signaling pathways are used to facilitate motor behaviors. We demonstrate that a G alpha(q)-coupled muscarinic acetylcholine receptor (mAChR) signaling pathway enhances nicotinic acetylcholine receptor (nAChR) signaling to facilitate the insertion of the Caenorhabditis elegans male copulatory spicules into the hermaphrodite during mating. Previous studies showed that ACh (acetylcholine) activates nAChRs on the spicule protractor muscles to induce the attached spicules to extend from the tail. Using the mAChR agonist Oxo M (oxotremorine M), we identified a GAR-3(mAChR)-G alpha(q) pathway that promotes protractor muscle contraction by upregulating nAChR signaling before mating. GAR-3(mAChR) is expressed in the protractor muscles and in the spicule-associated SPC and PCB cholinergic neurons. However, ablation of these neurons or impairing cholinergic transmission reduces drug-induced spicule protraction, suggesting that drug-stimulated neurons directly activate muscle contraction. Behavioral analysis of gar-3 mutants indicates that, in wild-type males, GAR-3(mAChR) expression in the SPC and PCB neurons is required for the male to sustain rhythmic spicule muscle contractions during attempts to breach the vulva. We propose that the GAR-3(mAChR)/G alpha(q) pathway sensitizes the spicule neurons and muscles before and during mating so that the male can respond to hermaphrodite vulva efficiently.

Interaction between prostaglandin E2 and l-cis-diltiazem, a specific blocker of cyclic nucleotide gated channels in bovine aortic endothelial cells

Prostaglandins are known to transduce their signals via 7 transmembrane prostanoid receptors, which typically signal through coupling to G proteins and downstream second messenger molecules and protein kinase activation. Recently we have shown that cyclic nucleotides affect prostaglandins binding to bovine aortic endothelial cells independent of protein kinases. Here we show that incubation of bovine aortic endothelial cells with permeable analogs of cAMP or cGMP leads to a rapid and reversible reduction in PGE(2) binding to the cells. Since cyclic nucleotides are known modulators of cyclic nucleotide gated channels, we examined the effect of a specific cyclic nucleotide gated channel blocker l-cis-diltiazem on prostaglandin E(2) (PGE(2)) binding to bovine aortic endothelial cells. L-cis-diltiazem is shown to displace PGE(2) binding to bovine aortic endothelial cells in a dose dependent manner. In addition the effect of PGE(2) and l-cis-diltiazem on thapsigargin induced calcium elevation in the cells was compared. Both agents reduced in bovine aortic endothelial cells the thapsigargin induced calcium elevation by about half. PGE(2) also retarded the time course of the response to thapsigargin. Simultaneous treatment of the cells with both PGE(2) and l-cis-diltiazem did not yield an inhibitory effect beyond that observed with l-cis-diltiazem alone. Together our data point at the cyclic nucleotide gated channels as a feasible candidate for association with the PGE(2) binding site in bovine aortic endothelial cells.

Constitutive activity and inverse agonism at the M2 muscarinic acetylcholine receptor

Introduction of a single-point mutation (Asn to Tyr) at position 410 at the junction between transmembrane domain 6 and the third extracellular loop of the human M(2) muscarinic acetylcholine (mACh) receptor generated a mutant receptor (N410Y) that possesses many of the hallmark features of a constitutively active mutant receptor. These included enhanced agonist binding affinity and potency, in addition to agonist-independent accumulation of [(3)H]inositol phosphates in cells coexpressing the chimeric Galpha(qi5) protein and the N410Y mutant M(2) mACh receptor. Constitutive activity was sensitive to inhibition by a range of muscarinic ligands, including those used clinically in the management of overactive bladder (oxybutynin, tolterodine, and darifenacin), indicating that these ligands behave as inverse agonists at the M(2) mACh receptor. Long-term (24-h) treatment of Chinese hamster ovary cells expressing the N410Y mutant M(2) mACh receptor with certain mACh receptor inverse agonists (atropine, darifenacin, and pirenzepine) elicited a concentration-dependent up-regulation of cell surface receptor expression. However, not all ligands possessing negative efficacy in the [(3)H]inositol phosphate accumulation assays were capable of significantly up-regulating receptor expression, perhaps indicating a spectrum of negative efficacies among ligands traditionally defined as mACh receptor antagonists. Finally, structurally distinct agonists exhibited differences in their relative potencies for the activation of Galpha(i/o) versus Galpha(s), consistent with agonist-directed trafficking of signaling at the N410Y mutant, but not at the wild-type M(2) mACh receptor. This indicates that the N410Y mutation of the M(2) mACh receptor alters receptor-G-protein coupling in an agonist-dependent manner, in addition to generating a constitutively active receptor phenotype.