Activation of muscarinic acetylcholine receptors via their allosteric binding sites

Allostery: The Good, the Bad, and the Ugly

With the advent of functional screening, more allosteric molecules are being discovered and developed as possible therapeutic entities. Allosteric proteins are unique because of two specific properties: 1) separate binding sites for allosteric modulators and guests and 2) mandatory alteration of receptor conformation upon binding of allosteric modulators. For G protein-coupled receptors, these properties produce many beneficial effects on pharmacologic systems that are described here. Allosteric discovery campaigns also bring with them added considerations that must be addressed for the endeavor to be successful, and these are described herein as well. SIGNIFICANCE STATEMENT: Recent years have seen the increasing presence of allosteric molecules as possible therapeutic drug candidates. The scientific procedures to characterize these are unique and require special techniques, so it is imperative that scientists understand the new concepts involved in allosteric function. This review examines the reasons why allosteric molecules should be considered as new drug entities and the techniques required to optimize the discovery process for allosteric molecules.

Fusion with Promiscuous Gα16 Subunit Reveals Signaling Bias at Muscarinic Receptors

A complex evaluation of agonist bias at G-protein coupled receptors at the level of G-protein classes and isoforms including non-preferential ones is essential for advanced agonist screening and drug development. Molecular crosstalk in downstream signaling and a lack of sufficiently sensitive and selective methods to study direct coupling with G-protein of interest complicates this analysis. We performed binding and functional analysis of 11 structurally different agonists on prepared fusion proteins of individual subtypes of muscarinic receptors and non-canonical promiscuous α-subunit of G₁₆ protein to study agonist bias. We have demonstrated that fusion of muscarinic receptors with Gα₁₆ limits access of other competitive Gα subunits to the receptor, and thus enables us to study activation of Gα₁₆ mediated pathway more specifically. Our data demonstrated agonist-specific activation of G₁₆ pathway among individual subtypes of muscarinic receptors and revealed signaling bias of oxotremorine towards Gα₁₆ pathway at the M₂ receptor and at the same time impaired Gα₁₆ signaling of iperoxo at M₅ receptors. Our data have shown that fusion proteins of muscarinic receptors with α-subunit of G-proteins can serve as a suitable tool for studying agonist bias, especially at non-preferential pathways.

Functionally selective and biased agonists of muscarinic receptors

Disruption of cholinergic signalling via muscarinic receptors is associated with various pathologies, like Alzheimer's disease or schizophrenia. Selective muscarinic agonists possess therapeutic potential in the treatment of diabetes, pain or Sjögren's syndrome. The orthosteric binding site of all subtypes of the muscarinic receptor is structurally identical, making the development of affinity-based selective agonists virtually impossible. Some agonists, however, are functionally selective; they activate only a subset of receptors or signalling pathways. Others may stabilise specific conformations of the receptor leading to non-uniform modulation of individual signalling pathways (biased agonists). Functionally selective and biased agonists represent a promising approach for selective activation of individual subtypes of muscarinic receptors. In this work we review chemical structures, receptor binding and agonist-specific conformations of currently known functionally selective and biased muscarinic agonists in the context of their intricate intracellular signalling. Further, we take a perspective on the possible use of biased agonists for tissue and organ-specific activation of muscarinic receptors.

Pharmaceutical Assessment Suggests Locomotion Hyperactivity in Zebrafish Triggered by Arecoline Might Be Associated with Multiple Muscarinic Acetylcholine Receptors Activation

Arecoline is one of the nicotinic acid-based alkaloids, which is found in the betel nut. In addition to its function as a muscarinic agonist, arecoline exhibits several adverse effects, such as inducing growth retardation and causing developmental defects in animal embryos, including zebrafish, chicken, and mice. In this study, we aimed to study the potential adverse effects of waterborne arecoline exposure on zebrafish larvae locomotor activity and investigate the possible mechanism of the arecoline effects in zebrafish behavior. The zebrafish behavior analysis, together with molecular docking and the antagonist co-exposure experiment using muscarinic acetylcholine receptor antagonists were conducted. Zebrafish larvae aged 96 h post-fertilization (hpf) were exposed to different concentrations (0.001, 0.01, 0.1, and 1 ppm) of arecoline for 30 min and 24 h, respectively, to find out the effect of arecoline in different time exposures. Locomotor activities were measured and quantified at 120 hpf. The results showed that arecoline caused zebrafish larvae locomotor hyperactivities, even at a very low concentration. For the mechanistic study, we conducted a structure-based molecular docking simulation and antagonist co-exposure experiment to explore the potential interactions between arecoline and eight subtypes, namely, M1a, M2a, M2b, M3a, M3b, M4a, M5a, and M5b, of zebrafish endogenous muscarinic acetylcholine receptors (mAChRs). Arecoline was predicted to show a strong binding affinity to most of the subtypes. We also discovered that the locomotion hyperactivity phenotypes triggered by arecoline could be rescued by co-incubating it with M1 to M4 mAChR antagonists. Taken together, by a pharmacological approach, we demonstrated that arecoline functions as a highly potent hyperactivity-stimulating compound in zebrafish that is mediated by multiple muscarinic acetylcholine receptors.

The operational model of allosteric modulation of pharmacological agonism

Proper determination of agonist efficacy is indispensable in the evaluation of agonist selectivity and bias to activation of specific signalling pathways. The operational model (OM) of pharmacological agonism is a useful means for achieving this goal. Allosteric ligands bind to receptors at sites that are distinct from those of endogenous agonists that interact with the orthosteric domain on the receptor. An allosteric modulator and an orthosteric agonist bind simultaneously to the receptor to form a ternary complex, where the allosteric modulator affects the binding affinity and operational efficacy of the agonist. Allosteric modulators are an intensively studied group of receptor ligands because of their selectivity and preservation of physiological space-time pattern of the signals they modulate. We analysed the operational model of allosterically-modulated agonism (OMAM) including modulation by allosteric agonists. Similar to OM, several parameters of OMAM are inter-dependent. We derived equations describing mutual relationships among parameters of the functional response and OMAM. We present a workflow for the robust fitting of OMAM to experimental data using derived equations.

Carbachol dimers with primary carbamate groups as homobivalent modulators of muscarinic receptors

Although agonists and antagonists of muscarinic receptors have been known for long time, there is renewed interest in compounds (such as allosteric or bitopic ligands, or biased agonists) able to differently and selectively modulate these receptors. As a continuation of our previous research, we designed a new series of dimers of the well-known cholinergic agonist carbachol. The new compounds were tested on the five cloned human muscarinic receptors (hM_1-5) expressed in CHO cells by means of equilibrium binding experiments, showing a dependence of the binding affinity on the length and position of the linker connecting the two monomers. Kinetic binding studies revealed that some of the tested compounds were able to slow the rate of NMS dissociation, suggesting allosteric behavior, also supported by docking simulations. Assessment of ERK1/2 phosphorylation on hM₁, hM₂ and hM₃ activation showed that the new compounds are endowed with muscarinic antagonist properties. At hM₂ receptors, some compounds were able to stimulate GTPγS binding but not cAMP accumulation, suggesting a biased behavior. Classification, Molecular and cellular pharmacology.

Agonist-Specific Conformations of the M2 Muscarinic Acetylcholine Receptor Assessed by Molecular Dynamics

Binding of muscarinic ligands, both antagonists and agonists, and their effects on the conformation of the M₂ acetylcholine receptor were modeled in silico and compared to experimental data. After docking of antagonists to the M₂ receptor in an inactive conformation (3UON, 5ZK3, 5ZKB, or 5ZKB) and agonists in an active conformation (4MQS), 100 ns of conventional molecular dynamics (MD) followed by 500 ns of accelerated MD was run. Conventional MD revealed ligand-specific interactions with the receptor. Antagonists stabilized the receptor in an inactive conformation during accelerated MD. The receptor in complex with various agonists attained different conformations specific to individual agonists. The magnitude of the TM6 movement correlated with agonist efficacy at the non-preferential G_s pathway. The shape of the intracellular opening where the receptor interacts with a G-protein was different for the classical agonist carbachol, super-agonist iperoxo, and G_i/o-biased partial agonists JR-6 and JR-7, being compatible with experimentally observed agonist bias at the G-protein level. Moreover, a wash-resistant binding of the unique agonist xanomeline associated with interactions with membrane lipids was formed during accelerated MD. Thus, accelerated MD is suitable for modeling of ligand-specific receptor binding and receptor conformations that is essential for the design of experiments aimed at identification of the secondary binding sites and understanding molecular mechanisms underlying receptor activation.

Current Advances in Allosteric Modulation of Muscarinic Receptors

Allosteric modulators are ligands that bind to a site on the receptor that is spatially separated from the orthosteric binding site for the endogenous neurotransmitter. Allosteric modulators modulate the binding affinity, potency, and efficacy of orthosteric ligands. Muscarinic acetylcholine receptors are prototypical allosterically-modulated G-protein-coupled receptors. They are a potential therapeutic target for the treatment of psychiatric, neurologic, and internal diseases like schizophrenia, Alzheimer’s disease, Huntington disease, type 2 diabetes, or chronic pulmonary obstruction. Here, we reviewed the progress made during the last decade in our understanding of their mechanisms of binding, allosteric modulation, and in vivo actions in order to understand the translational impact of studying this important class of pharmacological agents. We overviewed newly developed allosteric modulators of muscarinic receptors as well as new spin-off ideas like bitopic ligands combining allosteric and orthosteric moieties and photo-switchable ligands based on bitopic agents.

Novel M2 -selective, Gi -biased agonists of muscarinic acetylcholine receptors

BACKGROUND AND PURPOSE

More than 30% of currently marketed medications act via GPCRs. Thus, GPCRs represent one of the most important pharmacotherapeutic targets. In contrast to traditional agonists activating multiple signalling pathways, agonists activating a single signalling pathway represent a new generation of drugs with increased specificity and fewer adverse effects.

EXPERIMENTAL APPROACH

We have synthesized novel agonists of muscarinic ACh receptors and tested their binding and function (on levels of cAMP and inositol phosphates) in CHO cells expressing individual subtypes of muscarinic receptors, primary cultures of rat aortic smooth muscle cells and suspensions of digested native tissues from rats. Binding of the novel compounds to M₂ receptors was modelled in silico.

KEY RESULTS

Two of the tested new compounds (1-(thiophen-2-ylmethyl)-3,6-dihydro-2H-pyridinium and 1-methyl-1-(thiophen-2-ylmethyl)-3,6-dihydro-2H-pyridinium) only inhibited cAMP synthesis in CHO cells, primary cultures, and native tissues, with selectivity for M₂ muscarinic receptors and displaying bias towards the G_i signalling pathway at all subtypes of muscarinic receptors. Molecular modelling revealed interactions with the orthosteric binding site in a way specific for a given agonist followed by agonist-specific changes in the conformation of the receptor.

CONCLUSIONS AND IMPLICATIONS

The identified compounds may serve as lead structures in the search for novel non-steroidal and non-opioid analgesics acting via M₂ and M₄ muscarinic receptors with reduced side effects associated with activation of the phospholipase C signalling pathway.

The muscarinic antagonist gallamine induces proliferation of airway smooth muscle cells regardless of the cell phenotype

BACKGROUND

Muscarinic receptor antagonists are a usual treatment for chronic airway diseases, with increased bronchoconstriction, like asthma and chronic obstructive pulmonary disease. These diseases are usually accompanied by airway remodeling, involving airway smooth muscle cell (ASMC) proliferation. The purpose of this study was to examine the effect of the muscarinic receptor modulator gallamine on rabbit tracheal ASMC proliferation.

METHODS

ASMCs were incubated with gallamine (1 nM-10 mM), atropine (1 fM-10 mM), and/or acetylcholine (1 nM-1 mM), in the presence or absence of FBS (1% or 10%). Cell proliferation was estimated by incorporation of radioactive thymidine, the Cell Titer AQueous One Solution method and cell number counting after Trypan blue exclusion. The mechanisms mediating cell proliferation were studied using the PI3K and MAPK inhibitors LY294002 (20 μM) and PD98059 (100 μM), respectively. Cell phenotype was studied by indirect immunofluorescence for α-actin, Myosin Heavy Chain and desmin.

RESULTS

ASMC incubation with the muscarinic receptor allosteric modulator gallamine or the muscarinic receptor antagonist atropine increased methyl-[³H]thymidine incorporation and cell number in a dose-dependent manner. ASMC proliferation was mediated via PI3K and MAPK activation and was transient. Gallamine antagonized the mitogenic effect of 1% FBS. Furthermore, gallamine had a similar effect on contractile ASMCs, without synergizing with or affecting acetylcholine induced proliferation, or altering the percentage of ASMCs expressing contractile phenotype marker proteins.

CONCLUSIONS

Gallamine, in the absence of any agonist, has a transient mitogenic effect on ASMCs, regardless of the cell phenotype, mediated by the PI3K and the MAPK signaling pathways.

Development of a human vasopressin V1a-receptor antagonist from an evolutionary-related insect neuropeptide

Characterisation of G protein-coupled receptors (GPCR) relies on the availability of a toolbox of ligands that selectively modulate different functional states of the receptors. To uncover such molecules, we explored a unique strategy for ligand discovery that takes advantage of the evolutionary conservation of the 600-million-year-old oxytocin/vasopressin signalling system. We isolated the insect oxytocin/vasopressin orthologue inotocin from the black garden ant (Lasius niger), identified and cloned its cognate receptor and determined its pharmacological properties on the insect and human oxytocin/vasopressin receptors. Subsequently, we identified a functional dichotomy: inotocin activated the insect inotocin and the human vasopressin V1b receptors, but inhibited the human V1aR. Replacement of Arg8 of inotocin by D-Arg8 led to a potent, stable and competitive V1aR-antagonist ([D-Arg8]-inotocin) with a 3,000-fold binding selectivity for the human V1aR over the other three subtypes, OTR, V1bR and V2R. The Arg8/D-Arg8 ligand-pair was further investigated to gain novel insights into the oxytocin/vasopressin peptide-receptor interaction, which led to the identification of key residues of the receptors that are important for ligand functionality and selectivity. These observations could play an important role for development of oxytocin/vasopressin receptor modulators that would enable clear distinction of the physiological and pathological responses of the individual receptor subtypes.

Changes in Membrane Cholesterol Differentially Influence Preferential and Non-preferential Signaling of the M1 and M3 Muscarinic Acetylcholine Receptors

We have found earlier that changes in membrane cholesterol content have distinct impact on signaling via the M1, M2, or M3 receptors expressed in CHO cells (CHO-M1 through CHO-M3). Now we investigated whether gradual changes in membrane cholesterol exerts differential effects on coupling of the M1 and M3 muscarinic receptors to preferential signaling pathways through Gq/11 and non-preferential Gs G-proteins signaling. Changes in membrane cholesterol resulted in only marginal alterations of antagonist and agonist affinity of the M1 and M3 receptors, and did not influence precoupling of either subtype. Changes in membrane cholesterol did not influence parameters of carbachol-stimulated GTP-γ(35)S binding in CHO-M1 membranes while reduction as well as augmentation of membrane cholesterol lowered the efficacy but increased the potency of carbachol in CHO-M3 membranes. Gradual increase or decrease in membrane cholesterol concentration dependently attenuated agonist-induced inositolphosphates release while only cholesterol depletion increased basal values in both cell lines. Similarly, membrane cholesterol manipulation modified basal and agonist-stimulated cAMP synthesis via Gs in the same way in both cell lines. These results demonstrate that changes in membrane cholesterol concentration differentially impact preferential and non-preferential M1 and M3 receptor signaling. They point to the activated G-protein/effector protein interaction as the main site of action in alterations of M1 receptor-mediated stimulation of second messenger pathways. On the other hand, modifications in agonist-stimulated GTP-γ(35)S binding in CHO-M3 membranes indicate that in this case changes in ligand-activated receptor/G-protein interaction may also play a role.

Outline of Therapeutic Interventions With Muscarinic Receptor-Mediated Transmission

Muscarinc receptor-mediated signaling takes part in many physiological functions ranging from complex higher nervous activity to vegetative responses. Specificity of action of the natural muscarinic agonist acetylcholine is effected by action on five muscarinic receptor subtypes with particular tissue and cellular localization, and coupling preference with different G-proteins and their signaling pathways. In addition to physiological roles it is also implicated in pathologic events like promotion of carcinoma cells growth, early pathogenesis of neurodegenerative diseases in the central nervous system like Alzheimer´s disease and Parkinson´s disease, schizophrenia, intoxications resulting in drug addiction, or overactive bladder in the periphery. All of these disturbances demonstrate involvement of specific muscarinic receptor subtypes and point to the importance to develop selective pharmacotherapeutic interventions. Because of the high homology of the orthosteric binding site of muscarinic receptor subtypes there is virtually no subtype selective agonist that binds to this site. Activation of specific receptor subtypes may be achieved by developing allosteric modulators of acetylcholine binding, since ectopic binding domains on the receptor are less conserved compared to the orthosteric site. Potentiation of the effects of acetylcholine by allosteric modulators would be beneficial in cases where acetylcholine release is reduced due to pathological conditions. When presynaptic function is severly compromised, the utilization of ectopic agonists can be a thinkable solution.

Dopamine receptor signaling and current and future antipsychotic drugs

All currently efficacious antipsychotic drugs have as part of their mechanism the ability to attenuate some or all of the signaling through the dopamine D(2) receptor. More recently, the dopamine D(1) receptor has been hypothesized to be a promising target for the treatment of negative and/or cognitive aspects of schizophrenia that are not improved by current antipsychotics. Although cAMP has been presumed to be the primary messenger for signaling through the dopamine receptors, the last decade has unveiled a complexity that has provided exciting avenues for the future discovery of antipsychotic drugs (APDs). We review the signaling mechanisms of currently approved APDs at dopamine D(2) receptors, and note that aripiprazole is a compound that is clearly differentiated from other approved drugs. Although aripiprazole has been postulated to cause dopamine stabilization due to its partial D(2) agonist properties, a body of literature suggests that an alternative mechanism, functional selectivity, is of primary importance. Finally, we review the signaling at dopamine D(1) receptors, and the idea that drugs that activate D(1) receptors may have use as APDs for improving negative and cognitive symptoms. We address the current state of drug discovery in the D(1) area and its relationship to novel signaling mechanisms. Our conclusion is that although the first APD targeting dopamine receptors was discovered more than a half-century ago, recent research advances offer the possibility that novel and/or improved drugs will emerge in the next decade.

Subtype differences in pre-coupling of muscarinic acetylcholine receptors

Based on the kinetics of interaction between a receptor and G-protein, a myriad of possibilities may result. Two extreme cases are represented by: 1/Collision coupling, where an agonist binds to the free receptor and then the agonist-receptor complex “collides” with the free G-protein. 2/Pre-coupling, where stable receptor/G-protein complexes exist in the absence of agonist. Pre-coupling plays an important role in the kinetics of signal transduction. Odd-numbered muscarinic acetylcholine receptors preferentially couple to G_q/11, while even-numbered receptors prefer coupling to G_i/o. We analyzed the coupling status of the various subtypes of muscarinic receptors with preferential and non-preferential G-proteins. The magnitude of receptor-G-protein coupling was determined by the proportion of receptors existing in the agonist high-affinity binding conformation. Antibodies directed against the C-terminus of the α-subunits of the individual G-proteins were used to interfere with receptor-G-protein coupling. Effects of mutations and expression level on receptor-G-protein coupling were also investigated. Tested agonists displayed biphasic competition curves with the antagonist [³H]-N-methylscopolamine. Antibodies directed against the C-terminus of the α-subunits of the preferential G-protein decreased the proportion of high-affinity sites, and mutations at the receptor-G-protein interface abolished agonist high-affinity binding. In contrast, mutations that prevent receptor activation had no effect. Expression level of preferential G-proteins had no effect on pre-coupling to non-preferential G-proteins. Our data show that all subtypes of muscarinic receptors pre-couple with their preferential classes of G-proteins, but only M₁ and M₃ receptors also pre-couple with non-preferential G_i/o G-proteins. Pre-coupling is not dependent on agonist efficacy nor on receptor activation. The ultimate mode of coupling is therefore dictated by a combination of the receptor subtype and the class of G-protein.

Negative cooperativity in binding of muscarinic receptor agonists and GDP as a measure of agonist efficacy

BACKGROUND AND PURPOSE

Conventional determination of agonist efficacy at G-protein coupled receptors is measured by stimulation of guanosine-5'-γ-thiotriphosphate (GTPγS) binding. We analysed the role of guanosine diphosphate (GDP) in the process of activation of the M₂ muscarinic acetylcholine receptor and provide evidence that negative cooperativity between agonist and GDP binding is an alternative measure of agonist efficacy.

EXPERIMENTAL APPROACH

Filtration and scintillation proximity assays measured equilibrium binding as well as binding kinetics of [³⁵S]GTPγS and [³H]GDP to a mixture of G-proteins as well as individual classes of G-proteins upon binding of structurally different agonists to the M₂ muscarinic acetylcholine receptor.

KEY RESULTS

Agonists displayed biphasic competition curves with the antagonist [³H]-N-methylscopolamine. GTPγS (1 µM) changed the competition curves to monophasic with low affinity and 50 µM GDP produced a similar effect. Depletion of membrane-bound GDP increased the proportion of agonist high-affinity sites. Carbachol accelerated the dissociation of [³H]GDP from membranes. The inverse agonist N-methylscopolamine slowed GDP dissociation and GTPγS binding without changing affinity for GDP. Carbachol affected both GDP association with and dissociation from G(i/o) G-proteins but only its dissociation from G(s/olf) G-proteins.

CONCLUSIONS AND IMPLICATIONS

These findings suggest the existence of a low-affinity agonist-receptor conformation complexed with GDP-liganded G-protein. Also the negative cooperativity between GDP and agonist binding at the receptor/G-protein complex determines agonist efficacy. GDP binding reveals differences in action of agonists versus inverse agonists as well as differences in activation of G(i/o) versus G(s/olf) G-proteins that are not identified by conventional GTPγS binding.

A novel fast mechanism for GPCR-mediated signal transduction--control of neurotransmitter release

In addition to calcium influx, charge movement in the G protein–coupled M₂-muscarinic receptor is required for the control of acetylcholine release.

Reliable neuronal communication depends on accurate temporal correlation between the action potential and neurotransmitter release. Although a requirement for Ca²⁺ in neurotransmitter release is amply documented, recent studies have shown that voltage-sensitive G protein–coupled receptors (GPCRs) are also involved in this process. However, how slow-acting GPCRs control fast neurotransmitter release is an unsolved question. Here we examine whether the recently discovered fast depolarization-induced charge movement in the M₂-muscarinic receptor (M₂R) is responsible for M₂R-mediated control of acetylcholine release. We show that inhibition of the M₂R charge movement in Xenopus oocytes correlated well with inhibition of acetylcholine release at the mouse neuromuscular junction. Our results suggest that, in addition to Ca²⁺ influx, charge movement in GPCRs is also necessary for release control.

Allosteric Modulation of Muscarinic Acetylcholine Receptors

An allosteric modulator is a ligand that binds to an allosteric site on the receptor and changes receptor conformation to produce increase (positive cooperativity) or decrease (negative cooperativity) in the binding or action of an orthosteric agonist (e.g., acetylcholine). Since the identification of gallamine as the first allosteric modulator of muscarinic receptors in 1976, this unique mode of receptor modulation has been intensively studied by many groups. This review summarizes over 30 years of research on the molecular mechanisms of allosteric interactions of drugs with the receptor and for new allosteric modulators of muscarinic receptors with potential therapeutic use. Identification of positive modulators of acetylcholine binding and function that enhance neurotransmission and the discovery of highly selective allosteric modulators are mile-stones on the way to novel therapeutic agents for the treatment of schizophrenia, Alzheimer’s disease and other disorders involving impaired cognitive function.

Prediction of functionally selective allosteric interactions at an M3 muscarinic acetylcholine receptor mutant using Saccharomyces cerevisiae

Saccharomyces cerevisiae is a tractable yeast species for expression and coupling of heterologous G protein-coupled receptors with the endogenous pheromone response pathway. Although this platform has been used for ligand screening, no studies have probed its ability to predict novel pharmacology and functional selectivity of allosteric ligands. As a proof of concept, we expressed a rat M(3) muscarinic acetylcholine receptor (mAChR) bearing a mutation (K(7.32)E) recently identified to confer positive cooperativity between acetylcholine and the allosteric modulator brucine in various strains of S. cerevisiae, each expressing a different human Galpha/yeast Gpa1 protein chimera, and probed for G protein-biased allosteric modulation. Subsequent assays performed in this system revealed that brucine was a partial allosteric agonist and positive modulator of carbachol when coupled to Gpa1/G(q) proteins, a positive modulator (no agonism) when coupled to Gpa1/G(12) proteins, and a neutral modulator when coupled to Gpa1/G(i) proteins. It is noteworthy that these results were validated at the human M(3)K(7.32)E mAChR expressed in a mammalian (Chinese hamster ovary) cell background by determination of calcium mobilization and membrane ruffling as surrogate measures of G(q) and G(12) protein activation, respectively. Furthermore, the combination of this functionally selective allosteric modulator with G protein-biased yeast screens allowed us to ascribe a potential G protein candidate (G(12)) as a key mediator for allosteric modulation of M(3)K(7.32)E mAChR-mediated ERK1/2 phosphorylation, which was confirmed by small interfering RNA knockdown experiments. These results highlight how the yeast platform can be used to identify functional selectivity of allosteric ligands and to facilitate dissection of convergent signaling pathways.

Seven transmembrane receptors as shapeshifting proteins: the impact of allosteric modulation and functional selectivity on new drug discovery

It is useful to consider seven transmembrane receptors (7TMRs) as disordered proteins able to allosterically respond to a number of binding partners. Considering 7TMRs as allosteric systems, affinity and efficacy can be thought of in terms of energy flow between a modulator, conduit (the receptor protein), and a number of guests. These guests can be other molecules, receptors, membrane-bound proteins, or signaling proteins in the cytosol. These vectorial flows of energy can yield standard canonical guest allostery (allosteric modification of drug effect), effects along the plane of the cell membrane (receptor oligomerization), or effects directed into the cytosol (differential signaling as functional selectivity). This review discusses these apparently diverse pharmacological effects in terms of molecular dynamics and protein ensemble theory, which tends to unify 7TMR behavior toward cells. Special consideration will be given to functional selectivity (biased agonism and biased antagonism) in terms of mechanism of action and potential therapeutic application. The explosion of technology that has enabled observation of diverse 7TMR behavior has also shown how drugs can have multiple (pluridimensional) efficacies and how this can cause paradoxical drug classification and nomenclatures.

Ligand detection in the allosteric world

Historically, traditional screening for ligands has been optimized to detect standard orthosteric agonists and antagonists. However, with increasing emphasis on cellular functional screens, more allosteric ligands are being discovered as potential drugs. In addition, there are theoretical reasons (increased selectivity, better control of physiological systems, separate control of affinity and efficacy) allosteric ligands may be preferred therapeutic chemical targets. These factors may make it desirable to design high-throughput screens to specifically detect functionally allosteric ligands. This article discusses the unique features of allosteric ligands as drugs as well as the special conditions that should be considered to optimize a high-throughput screen toward the detection of allosteric drugs. Finally, the likelihood of detecting allosteric ligands that have direct effects on cells (either conventional agonism or functionally selective effects) is discussed as well as the optimization of detection of such ligands in screening assays.

Divergence of allosteric effects of rapacuronium on binding and function of muscarinic receptors

Background

Many neuromuscular blockers act as negative allosteric modulators of muscarinic acetylcholine receptors by decreasing affinity and potency of acetylcholine. The neuromuscular blocker rapacuronium has been shown to have facilitatory effects at muscarinic receptors leading to bronchospasm. We examined the influence of rapacuronium on acetylcholine (ACh) binding to and activation of individual subtypes of muscarinic receptors expressed in Chinese hamster ovary cells to determine its receptor selectivity.

Results

At equilibrium rapacuronium bound to all subtypes of muscarinic receptors with micromolar affinity (2.7-17 μM) and displayed negative cooperativity with both high- and low-affinity ACh binding states. Rapacuronium accelerated [³H]ACh association with and dissociation from odd-numbered receptor subtypes. With respect to [³⁵S]GTPγS binding rapacuronium alone behaved as an inverse agonist at all subtypes. Rapacuronium concentration-dependently decreased the potency of ACh-induced [³⁵S]GTPγS binding at M₂ and M₄ receptors. In contrast, 0.1 μM rapacuronium significantly increased ACh potency at M₁, M₃, and M₅ receptors. Kinetic measurements at M₃ receptors showed acceleration of the rate of ACh-induced [³⁵S]GTPγS binding by rapacuronium.

Conclusions

Our data demonstrate a novel dichotomy in rapacuronium effects at odd-numbered muscarinic receptors. Rapacuronium accelerates the rate of ACh binding but decreases its affinity under equilibrium conditions. This results in potentiation of receptor activation at low concentrations of rapacuronium (1 μM) but not at high concentrations (10 μM). These observations highlight the relevance and necessity of performing physiological tests under non-equilibrium conditions in evaluating the functional effects of allosteric modulators at muscarinic receptors. They also provide molecular basis for potentiating M₃ receptor-mediated bronchoconstriction.

Weak toxin WTX from Naja kaouthia cobra venom interacts with both nicotinic and muscarinic acetylcholine receptors

Iodinated [125I] weak toxin from Naja kaouthia (WTX) cobra venom was injected into mice, and organ-specific binding was monitored. Relatively high levels of [125I]WTX were detected in the adrenal glands. Rat adrenal membranes were therefore used for analysis of [125I]WTX-binding sites. Specific [125I]WTX binding was partially inhibited by both alpha-cobratoxin, a blocker of the alpha7 and muscle-type nicotinic acetylcholine receptors (nAChRs), and by atropine, an antagonist of the muscarinic acetylcholine receptor (mAChR). Binding to rat adrenal nAChR had a Kd of 2.0+/-0.8 microM and was inhibited by alpha-cobratoxin but not by a short-chain alpha-neurotoxin antagonist of the muscle-type nAChR, suggesting a specific interaction with the alpha7-type nAChR. WTX binding was reduced not only by atropine but also by other muscarinic agents (oxotremorine and muscarinic toxins from Dendroaspis angusticeps), indicating an interaction with mAChR. This interaction was further characterized using individual subtypes of human mAChRs expressed in Chinese hamster ovary cells. WTX concentrations up to 30 microM did not inhibit binding of [3H]acetylcholine to any subtype of mAChR by more than 50%. Depending on receptor subtype, WTX either increased or had no effect on the binding of the muscarinic antagonist [3H]N-methylscopolamine, which binds to the orthosteric site, a finding indicative of an allosteric interaction. Furthermore, WTX alone activated G-protein coupling with all mAChR subtypes and reduced the efficacy of acetylcholine in activating G-proteins with the M1, M4, and M5 subtypes. Our data demonstrate an orthosteric WTX interaction with nAChR and an allosteric interaction with mAChRs.

Flexible modulation of agonist efficacy at the human A3 adenosine receptor by the imidazoquinoline allosteric enhancer LUF6000

Background

A series of 1H-imidazo- [4,5-c]quinolin-4-amine derivatives, represented by LUF6000 (N-(3,4-dichloro-phenyl)-2-cyclohexyl-1H-imidazo [4,5-c]quinolin-4-amine), are allosteric modulators of the human A₃ adenosine receptor (AR). Here we studied the modulation by LUF6000 of the maximum effect (E_max) of structurally diverse agonists at the A₃ AR stably expressed in CHO cells.

Results

In an assay of [³⁵S]GTPγS binding, the E_max of the A₃ AR agonist Cl-IB-MECA at the A₃ AR was lower than that of the non-selective AR agonist NECA. LUF6000 exerted an E_max-enhancing effect at a concentration of 0.1 μM or higher, and was shown to increase the E_max of Cl-IB-MECA and other low-efficacy agonists to a larger extent than that of the high-efficacy agonist NECA. Interestingly, LUF6000 converted a nucleoside A₃ AR antagonist MRS542, but not a non-nucleoside antagonist MRS1220, into an agonist. LUF6000 alone did not show any effect. Mathematical modeling was performed to explain the differential effects of LUF6000 on agonists with various E_max. A simple explanation for the observation that LUF6000 has a much stronger effect on Cl-IB-MECA than on NECA derived from the mathematical modeling is that NECA has relatively strong intrinsic efficacy, such that the response is already close to the maximum response. Therefore, LUF6000 cannot enhance E_max much further.

Conclusion

LUF6000 was found to be an allosteric enhancer of E_max of structurally diverse agonists at the A₃ AR, being more effective for low-E_max agonists than for high-E_max agonists. LUF6000 was demonstrated to convert an antagonist into an agonist, which represents the first example in G protein-coupled receptors. The observations from the present study are consistent with that predicted by mathematical modeling.

Antifibrillatory properties of mivacurium in a canine model of atrial fibrillation

The electrophysiologic actions of the competitive neuromuscular blocker mivacurium (0.05-0.8 mg/kg IV; N = 10) and atropine sulfate [0.01-0.16 mg/Kg intravenously (IV), N = 6] were determined under control conditions, during right vagus nerve stimulation, and during anterior right ganglionated plexus stimulation. Both drugs suppressed shortening of right atrial monophasic action potential (MAP) duration, right atrial refractoriness, and right superior pulmonary vein sleeve refractoriness produced by vagus nerve or ganglionated plexus stimulation and suppressed the induction of atrial fibrillation. Suppression of atrial fibrillation by atropine was accompanied by improved sinus and atrioventricular (AV) nodal function, increasing the ventricular heart rate observed during sinus rhythm and atrial fibrillation and eliminating the depressant actions of vagus nerve stimulation on sinoatrial (SA) and AV nodal function. Unlike atropine, mivacurium selectively antagonized the effects of vagus nerve and ganglionated plexus stimulation on atrial and pulmonary vein sleeve myocardium (shortening of action potential duration/refractoriness and increased atrial vulnerability) without altering sinus or AV nodal function under control conditions or during vagus nerve stimulation. The selective inhibition of parasympathetic nervous system effects in atrium versus sinus and AV nodes by mivacurium may represent a selective mechanism for the suppression of atrial fibrillation without altering SA and AV nodal function.

Antifibrillatory actions of cisatracurium: an atrial specific M2 receptor antagonist

INTRODUCTION

Muscarinic receptor antagonists are proposed to prevent atrial fibrillation (AF), but also facilitate AV conduction, limiting clinical usefulness.

METHODS

Cisatracurium, a neuromuscular blocker, was administered to anesthetized dogs (0.05-0.8 mg/kg IV) and was administered to superfused pulmonary vein (PV) tissues in vitro.

RESULTS

Dose-dependent suppression of AF induced by premature atrial stimuli was observed under control conditions (n = 3), right vagus nerve stimulation (n = 7), and anterior right ganglionated plexus stimulation (n = 3). AF was prevented (P < 0.0001) concurrent with suppression of the decreased atrial MAP duration/ERP accompanying vagus nerve stimulation without altering AH intervals or sinus cycle length. Although atropine (0.001-0.016 mg/kg, n = 4) suppressed AF (P < 0.04) in association with suppression of atrial MAP shortening induced by vagus nerve stimulation, atropine also prevented sinus cycle length and AH interval prolongation with vagus nerve stimulation, and decreased AV effective and functional refractory periods. In vitro, both cisatracurium and atropine prevented (1) action potential shortening produced by acetylcholine administration and (2) action potential shortening and arrhythmia triggering within PV sleeves produced by local autonomic nerve stimulation, atropine producing competitive inhibition, and cisatracurium producing noncompetitive M(2) muscarinic receptor blockade.

CONCLUSIONS

Cisatracurium demonstrates a dose-dependent (1) suppression of AF and atrial action potential shortening accompanying vagus nerve stimulation without facilitating sinus or atrioventricular nodal function and (2) noncompetitive blockade of action potential shortening and triggered firing induced in isolated PVs by local autonomic nerve stimulation. The data are consistent with allosteric binding of cisatracurium to the M(2) muscarinic receptor in canine atrium.

Two-state models and the analysis of the allosteric effect of gallamine at the M2 muscarinic receptor

We measured the influence of gallamine on the functional responses and binding properties of selected agonists at the M(2) muscarinic receptor and analyzed the data within the context of the allosteric ternary complex model. Our analysis showed that gallamine modified agonist affinity without influencing efficacy. To explain this behavior, we investigated the allosteric ternary complex model at a deeper level of analysis to assess allosterism in terms of the differential affinity of gallamine for ground and active states of the receptor. Our simulations showed that two-state models based on a single orthosteric site for the agonist linked to an allosteric site for gallamine could not account for affinity-only modulation, even if multiple conformations of ground and active states were considered. We also expanded the tandem two-site model (J Biol Chem 275:18836-18844, 2000) within the context of the allosteric ternary complex model and analyzed the resulting hybrid model at the level of receptor states. This model posits that the agonist first binds to a relay site and then shuttles to the activation site to turn on the receptor. If it is assumed that allosterism occurs at the relay site and not the activation site, then this model can account for affinity-only modulation in a manner consistent with the allosteric ternary complex model.

Influence of positive allosteric modulators on GABA(B) receptor coupling in rat brain: a scintillation proximity assay characterisation of G protein subtypes

Little is known concerning coupling of cerebral GABA(B) receptors to G protein subtypes, and the influence of positive allosteric modulators (PAMs) has not been evaluated. These questions were addressed by an antibody-capture/scintillation proximity assay strategy. GABA concentration-dependently enhanced the magnitude of [(35)S]GTPgammaS binding to Galphao and, less markedly, Galphai(1/3) in cortex, whereas Gq and Gs/olf were unaffected. (R)-baclofen and SKF97581 likewise activated Galphao and Galphai(1/3), expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABA(B) antagonist, CGP55845A, abolished agonist-induced activation of Galphao and Galphai(1/3) in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist-induced [(35)S]GTPgammaS binding to Galphao in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at Galphai(1/3). Similarly, in human embryonic kidney cells expressing GABA(B(1a+2)) or GABA(B(1b+2)) receptors, allosteric modulators did not detectably enhance efficacy of GABA at Galphai(1/3), though they increased its potency. To summarise, GABA(B) receptors coupled both to Galphao and to Galphai, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi(1/3). It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.

Allosteric agonist modulators

This article discusses a model to describe the effects of molecules that bind to a site on the receptor separate from that of the endogenous agonist to actively produce receptor signals (direct agonism). In addition, these molecules also modify the biological responses of the endogenous agonist (either potentiation or antagonism). The effects of such compounds in high-throughput screening assays are described as well as their effects on the dose-response curves to conventional agonists.

Allosteric modulation of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors (mAChRs) are prototypical Family A G protein coupled-receptors. The five mAChR subtypes are widespread throughout the periphery and the central nervous system and, accordingly, are widely involved in a variety of both physiological and pathophysiological processes. There currently remains an unmet need for better therapeutic agents that can selectively target a given mAChR subtype to the relative exclusion of others. The main reason for the lack of such selective mAChR ligands is the high sequence homology within the acetylcholine-binding site (orthosteric site) across all mAChRs. However, the mAChRs possess at least one, and likely two, extracellular allosteric binding sites that can recognize small molecule allosteric modulators to regulate the binding and function of orthosteric ligands. Extensive studies of prototypical mAChR modulators, such as gallamine and alcuronium, have provided strong pharmacological evidence, and associated structure-activity relationships (SAR), for a "common" allosteric site on all five mAChRs. These studies are also supported by mutagenesis experiments implicating the second extracellular loop and the interface between the third extracellular loop and the top of transmembrane domain 7 as contributing to the common allosteric site. Other studies are also delineating the pharmacology of a second allosteric site, recognized by compounds such as staurosporine. In addition, allosteric agonists, such as McN-A-343, AC-42 and N-desmethylclozapine, have also been identified. Current challenges to the field include the ability to effectively detect and validate allosteric mechanisms, and to quantify allosteric effects on binding affinity and signaling efficacy to inform allosteric modulator SAR.

Allosteric enhancers, allosteric agonists and ago-allosteric modulators: where do they bind and how do they act?

Many small-molecule agonists also display allosteric properties. Such ago-allosteric modulators act as co-agonists, providing additive efficacy--instead of partial antagonism--and they can affect--and often improve--the potency of the endogenous agonist. Surprisingly, the apparent binding sites of several ordinary allosteric enhancers and ago-allosteric modulators seem to overlap with those of the endogenous agonists. Different molecular scenarios are proposed to explain this discrepancy from classical allosteric models. In one scenario, the ago-allosteric modulator can interchange between different binding modes. In another, dimeric, receptor scenario, the endogenous agonist binds to one protomer while the ago-allosteric modulator binds to the other, 'allosteric' protomer. It is suggested that testing for ago-allosteric properties should be an integral part of the agonist drug discovery process because a compound that acts with--rather than against--the endogenous agonist could be an optimal agonist drug.

Wash-resistantly bound xanomeline inhibits acetylcholine release by persistent activation of presynaptic M(2) and M(4) muscarinic receptors in rat brain

We studied the effects of 3-[3-hexyloxy-1,2,5-thiadiazo-4-yl]-1,2,5,6-tetrahydro-1-methylpyridine (xanomeline) wash-resistant binding on presynaptic muscarinic regulation of electrically evoked [(3)H]acetylcholine (ACh) release from rat brain slices. In both cortical and striatal tissues that possess M(2) and M(4) autoreceptors, respectively, immediate application of 10 microM xanomeline had no effect on evoked [(3)H]ACh release or its inhibition by 10 microM carbachol. In contrast, preincubation with 1, 10, or 100 microM xanomeline for 15 min decreased evoked release of ACh measured after 53 min of washing in xanomeline-free medium in a concentration-dependent manner. The maximal inhibitory effect equaled the immediate effect of the muscarinic full agonist carbachol, and it was completely (at 1 and 10 microM xanomeline) or partially (at 100 microM xanomeline) blocked by 1 microM N-methylscopolamine. Neither presence of N-methylscopolamine during 100 microM xanomeline treatment nor previous irreversible inactivation of the classical receptor binding site using propylbenzylcholine mustard in cortical slices prevented the inhibitory effect of wash-resistantly bound xanomeline. Treatment of cortical slices with xanomeline slightly decreased the number of muscarinic binding sites, and it markedly decreased affinity for N-methylscopolamine. When applied as in acetylcholine release experiments, xanomeline did not impair presynaptic alpha(2)-adrenoceptor-mediated regulation of noradrenaline release. The functional studies in brain tissue reported in this work demonstrate that xanomeline can function as a wash-resistant agonist of native presynaptic muscarinic M(2) and M(4) receptors with both competitive and allosteric components of action.

Small-molecule agonists for the glucagon-like peptide 1 receptor

The peptide hormone glucagon-like peptide (GLP)-1 has important actions resulting in glucose lowering along with weight loss in patients with type 2 diabetes. As a peptide hormone, GLP-1 has to be administered by injection. Only a few small-molecule agonists to peptide hormone receptors have been described and none in the B family of the G protein coupled receptors to which the GLP-1 receptor belongs. We have discovered a series of small molecules known as ago-allosteric modulators selective for the human GLP-1 receptor. These compounds act as both allosteric activators of the receptor and independent agonists. Potency of GLP-1 was not changed by the allosteric agonists, but affinity of GLP-1 for the receptor was increased. The most potent compound identified stimulates glucose-dependent insulin release from normal mouse islets but, importantly, not from GLP-1 receptor knockout mice. Also, the compound stimulates insulin release from perfused rat pancreas in a manner additive with GLP-1 itself. These compounds may lead to the identification or design of orally active GLP-1 agonists.

Muscarinic M2 receptors directly activate Gq/11 and Gs G-proteins

Muscarinic M(2) receptors preferentially couple with the G(i/o) class of G-proteins to inhibit cAMP synthesis. However, they can also stimulate net synthesis of cAMP and inositol phosphate (IP) accumulation. We investigated in intact Chinese hamster ovary (CHO) cells expressing human M(2) receptors (CHO-M(2) cells) whether direct interaction of M(2) receptors with G(s) and G(q/11) G-proteins is responsible for the latter effects. Suppression of the G(s)alpha subunit using RNA interference abolished stimulation of cAMP synthesis induced by 1 mM carbachol in both control and pertussis toxin-treated CHO-M(2) cells but had no effect on the inhibition of forskolin-stimulated cAMP synthesis. Carbachol stimulated accumulation of IP with an EC(50) of 79 microM. Removal of the G(q),G(11), or both alpha subunits reduced this response by 78, 54, and 92%, respectively, whereas suppression of the G(s)alpha subunit had no effect. Similar results obtained in CHO cells expressing M(1) receptors that preferentially couple with G(s) and G(q/11) G-proteins confirmed the efficiency of siRNA treatments. Stimulation of M(2) receptors in control and pertussis toxin-treated cells by a series of full agonists with respect to inhibition of adenylyl cyclase displayed different efficacies in stimulating IP accumulation. Carbachol, acetylcholine, and oxotremorine-M [N,N,N-trimethyl-4-(2-oxo-1-pyrolidinyl)-2-butyn-1-ammonium] behaved as full agonists, furmethide (N,N,N-trimethyl-2-furanmethammonium) and methylfurmethide [(5-methyl-2-furyl)methyltrimethylammonium] were partial agonists, and oxotremorine (1-[4-(1-pyrrolidinyl)-2-butynyl]-2-pyrrolidinone) had no effect. Our results provide direct evidence of M(2) receptor coupling with the alpha subunits of G(s) and G(q/11) G-proteins and demonstrate induction of multiple receptor conformational states dependent on both the concentration and the nature of the agonist used.

Allosteric agonists of 7TM receptors: expanding the pharmacological toolbox

Approximately 1% of the genome of higher organisms encodes seven-transmembrane (7TM) G-protein-coupled receptors, which control an extensive range of physiological processes and represent drug targets for nearly half of all drugs that are prescribed currently. To date, most drugs that target 7TM receptors interact via the same domain as the endogenous agonist, called the orthosteric site. However, the advent of functional screening assays has greatly increased the number of allosteric ligands identified. Such ligands bind to topographically distinct sites on 7TM receptors. In addition to modulating the affinity of orthosteric ligands, allosteric ligands can also alter the efficacy of orthosteric ligands and activate 7TM receptors in their own right. In this article, we briefly review the current status of putative allosteric agonists of 7TM receptors, and discuss the promises and challenges that this class of ligand might pose for pharmacologists and the drug-discovery industry.

Structure-activity relationships of new 1H-imidazo[4,5-c]quinolin-4-amine derivatives as allosteric enhancers of the A3 adenosine receptor

1H-Imidazo[4,5-c]quinolin-4-amine derivatives have been synthesized as allosteric modulators of the human A3 adenosine receptor (AR). Structural modifications were made at the 4-amino and 2 positions. The compounds were tested in both binding and functional assays, and many were found to be allosteric enhancers of the action of A3AR agonists by several different criteria. First, a potentiation of the maximum efficacy of the agonist Cl-IB-MECA was observed for numerous derivatives. Also, a number of these compounds decreased the rate of dissociation of the agonist [125I]I-AB-MECA from the A3AR. Most prominently, compound 43 (LUF6000) was found to enhance agonist efficacy in a functional assay by 45% and decrease dissociation rate similarly without influencing agonist potency. The structural requirements for allosteric enhancement at the A3AR were distinct from the requirements to inhibit equilibrium binding. Thus, we have prepared allosteric enhancers of the human A3AR that have an improved allosteric effect in comparison to the inhibition of equilibrium binding at the orthosteric site.

Receptors as microprocessors: pharmacological nuance on metabotropic glutamate receptors 1alpha

G protein-coupled receptors have revealed themselves to be complex information-processing units that may be exploited for subtle therapeutic signaling effects. Thus, ligands may not only turn receptors on and off, but may also select from their repertoire of signaling effects to further refine drug response.

Ago-allosteric modulation and other types of allostery in dimeric 7TM receptors

Conventionally, an allosteric modulator is neutral in respect of efficacy and binds to a receptor site distant from the orthosteric site of the endogenous agonist. However, recently compounds being ago-allosteric modulators have been described i.e., compounds acting both as agonists on their own and as enhancers for the endogenous agonists in both increasing agonist potency and providing additive efficacy-superagonism. The additive efficacy can also be observed with agonists, which are neutral or even negative modulators of the potency of the endogenous ligand. Based on the prevailing dimeric concept for 7TM receptors, it is proposed that the ago-allosteric modulators bind in the orthosteric binding site, but-importantly-in the "other" or allosteric protomer of the dimer. Hereby, they can act both as additive co-agonists, and through intermolecular cooperative effects between the protomers, they may influence the potency of the endogenous agonist. It is of interest that at least some endogenous agonists can only occupy one protomer of a dimeric 7TM receptor complex at a time and thereby they leave the orthosteric binding site in the allosteric protomer free, potentially for binding of exogenous, allosteric modulators. If the allosteric modulator is an agonist, it is an ago-allosteric modulator; if it is neutral, it is a classical enhancer. Molecular mapping in hetero-dimeric class-C receptors, where the endogenous agonist clearly binds only in one protomer, supports the notion that allosteric modulators can act through binding in the "other" protomer. It is suggested that for the in vivo, clinical setting a positive ago-allosteric modulator should be the preferred agonist drug.

Differences in kinetics of xanomeline binding and selectivity of activation of G proteins at M(1) and M(2) muscarinic acetylcholine receptors

Xanomeline is a functionally selective M(1)/M(4) muscarinic acetylcholine receptor agonist that nevertheless binds with high affinity to all five subtypes of muscarinic receptors. A novel mode of interaction of this ligand with the muscarinic M(1) receptors characterized by persistent binding and receptor activation after extensive washout has been shown previously. In the present study, using human M(1) and M(2) receptors expressed in Chinese hamster ovary cells and [(3)H]N-methylscopolamine as a tracer, we show that persistent binding of xanomeline also occurs at the M(2) receptor with similar affinity as at the M(1) receptor (K(I) = 294 and 296 nM, respectively). However, kinetics of formation of xanomeline wash-resistant binding to M(2) receptors was markedly slower than to M(1) receptors. Xanomeline was a potent fast-acting full agonist in stimulating guanosine 5'-O-(3-[(35)S]thio)triphosphate binding at M(1) receptors, whereas at M(2) receptors it behaved as a potent partial agonist (40% of carbachol maximal response) only upon preincubation for 1 h. Development of xanomeline agonistic effects at the M(2) receptor was slower than its ability to attenuate carbachol responses. We also demonstrate that xanomeline discriminates better between G protein subtypes at M(1) than at M(2) receptors. Our data support the notion that xanomeline interacts with multiple sites on the muscarinic receptor, resulting in divergent conformations that exhibit differential effects on ligand binding and receptor activation. These conformations are both time- and concentration-dependent and vary between the M(1) and the M(2) receptor.

Function-specific blockage of M(1) and M(3) muscarinic acetylcholine receptors by VX and echothiophate

Certain organophosphate (OP) cholinesterase inhibitors (ChEIs) are also known to bind to the muscarinic acetylcholine receptor (mAChR). The functional consequences of such binding were investigated here using the following OP compounds: VX, echothiophate, sarin, and soman. VX (charged at physiological pH) and echothiophate (formally charged) inhibited a specific signal transduction pathway in CHO cells expressing either the M(1) or M(3) mAChR. Hence, they blocked carbamylcholine (CCh)-induced cyclic adenosine monophosphate (cAMP) synthesis (muM) and had almost no effect on CCh-induced phosphoinositide (PI) hydrolysis. These substances were inactive on forskolin-induced cAMP inhibition signaling in CHO cells expressing M(2) mAChR. In binding studies, using [(3)H]-N-methyl scopolamine ([(3)H]NMS) as the competitor ligand, the ChEIs, VX and echothiophate exhibited binding to rat cortical mAChR with K(i) values in the muM range. The non-charged compounds, sarin and soman, were inert in modulating both cAMP metabolism and PI hydrolysis in CHO cells expressing M(1), M(2), and M(3) mAChRs, and no binding was observed in presence of [(3)H]NMS. These data suggest that VX and echothiophate act as function-specific blockers via a non-classical path of antagonistic activity, implying the involvement of allosteric/ectopic-binding site in M(1) and M(3) mAChRs. The functionally selective antagonistic behavior of echothiophate and VX makes them potential tools for dissecting the interactions of the mAChR with different G proteins.

Probing the Molecular Mechanism of Interaction between 4-n-Butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl]-piperidine (AC-42) and the Muscarinic M1 Receptor: Direct Pharmacological Evidence That AC-42 Is an Allosteric Agonist

4-n-Butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl]-piperidine hydrogen chloride (AC-42) is a selective agonist of the muscarinic M(1) receptor previously suggested to interact with an "ectopic" site on this receptor. However, the pharmacological properties of this site (i.e., whether it overlaps to any extent with the classic orthosteric site or represents a novel allosteric site) remain undetermined. In the present study, atropine or pirenzepine significantly inhibited the ability of either carbachol or AC-42 to stimulate inositol phosphate accumulation or intracellular calcium mobilization in Chinese hamster ovary (CHO) cells stably expressing the human M(1) receptor. However, the interaction between either of these antagonists and AC-42 was characterized by Schild slopes significantly less than unity. Increasing the concentrations of atropine revealed that the Schild regression was curvilinear, consistent with a negative allosteric interaction. More direct evidence for an allosteric mode of action of AC-42 was obtained in [(3)H]N-methylscopolamine ([(3)H]NMS) binding studies, in that both AC-42 and the prototypical modulator gallamine failed to fully inhibit specific [(3)H]NMS binding in a manner that was quantitatively described by an allosteric model applied to both modulator data sets. Furthermore, AC-42 and gallamine significantly retarded the rate of [(3)H]NMS dissociation from CHO-hM(1) cell membranes, conclusively demonstrating their ability to bind to a topographically distinct site to change M(1) receptor conformation. These data provide the first direct evidence that AC-42 is an allosteric agonist that activates M(1) receptors in the absence of the orthosteric agonist.

The effects of the neurosteroids: pregnenolone, progesterone and dehydroepiandrosterone on muscarinic receptor-induced responses in Xenopus oocytes expressing M1 and M3 receptors

The neurosteroids pregnenolone, progesterone, and dehydroepiandrosterone (DHEA) occur naturally in the nervous system. They act on neural tissues, participate in neuronal signaling, and are reported to alter neuronal excitability via nongenomic mechanisms. Muscarinic receptors have important roles in neuronal functions in the brain and autonomic nervous system. In this study, we investigated the effects of pregnenolone, progesterone, and DHEA on M(1) and M(3) muscarinic receptors using the Xenopus oocyte expression system. Pregnenolone and progesterone inhibited the acetylcholine (ACh)-mediated responses of M(1) and M(3) receptors expressed in Xenopus oocytes, whereas DHEA did not. The half-maximal inhibitory concentrations (IC(50)) for pregnenolone inhibition of M(1) receptor- and M(3) receptor-mediated currents were 11.4 and 6.0 microM respectively; the IC(50) values for progesterone inhibition of M(1) receptor- and M(3) receptor-mediated currents were 2.5 and 3.0 microM respectively. The selective protein kinase C (PKC) inhibitor GF109203X had little effect on the pregnenolone or progesterone inhibition of the ACh-induced currents in Xenopus oocytes expressing M(1) or M(3) receptors. The inhibitory effects of pregnenolone and progesterone were overcome at higher concentrations of ACh. Pregnenolone and progesterone inhibited the [(3)H]quinuclidinyl benzilate (QNB) binding to M(1) and M(3) receptor expressed in Xenopus oocytes, and Scatchard plot analysis of [(3)H]QNB binding revealed that pregnenolone and progesterone altered the K(d) value and the B(max), indicating noncompetitive inhibition. In conclusion, pregnenolone and progesterone inhibited M(1) and M(3) receptor functions noncompetitively by the mechanism independent of PKC and by interfering with ACh binding to the receptors.

Sera from patients with idiopathic dilated cardiomyopathy decrease ICa in cardiomyocytes isolated from rabbits

Autoantibodies against muscarinic and adrenergic receptors have been found in the sera of patients with idiopathic dilated cardiomyopathy (IDC) and Chagas disease, but it is still unclear whether they can functionally interact with their respective receptors to modulate cardiac functions. In this study, our goal was to detect the presence of those antibodies in the sera of patients with IDC and characterize their electrophysiological effects on cardiomyocytes from rabbits. By using ELISA immunoassays, we detected high titers of antibodies against muscarinic M2 receptors in the sera of all IDC patients, whereas the detection of antibodies against the beta1-receptor occurred in 50% of them. Electrophysiological experiments using the whole cell configuration of the patch-clamp technique showed that sera from 43% of IDC patients induced a significant decrease (approximately 26%) in isoproterenol-stimulated L-type Ca2+ currents in rabbit ventricular myocytes, whereas the sera from healthy blood donors failed to do so. As expected, IDC sera also decreased the action potential duration (by 10.5%) due to a shortening of the plateau phase. Sera that reduced isoproterenol-stimulated L-type Ca2+ currents did not cause any effect on K+ currents. We conclude that sera from IDC patients have autoantibodies, which interact with muscarinic M2 receptors of rabbit cardiomyocytes, acting in an agonist-like fashion. This action results in changes in electrogenesis, which, as often observed in patients with IDC, could initiate ventricular arrhythmias that lead to sudden death.