Allosteric interactions at muscarinic cholinoceptors

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".

SBMLKinetics: a tool for annotation-independent classification of reaction kinetics for SBML models

BACKGROUND

Reaction networks are widely used as mechanistic models in systems biology to reveal principles of biological systems. Reactions are governed by kinetic laws that describe reaction rates. Selecting the appropriate kinetic laws is difficult for many modelers. There exist tools that attempt to find the correct kinetic laws based on annotations. Here, I developed annotation-independent technologies that assist modelers by focusing on finding kinetic laws commonly used for similar reactions.

RESULTS

Recommending kinetic laws and other analyses of reaction networks can be viewed as a classification problem. Existing approaches to determining similar reactions rely heavily on having good annotations, a condition that is often unsatisfied in model repositories such as BioModels. I developed an annotation-independent approach to find similar reactions via reaction classifications. I proposed a two-dimensional kinetics classification scheme (2DK) that analyzed reactions along the dimensions of kinetics type (K type) and reaction type (R type). I identified approximately ten mutually exclusive K types, including zeroth order, mass action, Michaelis-Menten, Hill kinetics, and others. R types were organized by the number of distinct reactants and the number of distinct products in reactions. I constructed a tool, SBMLKinetics, that inputted a collection of SBML models and then calculated reaction classifications as the probability of each 2DK class. The effectiveness of 2DK was evaluated on BioModels, and the scheme classified over 95% of the reactions.

CONCLUSIONS

2DK had many applications. It provided a data-driven annotation-independent approach to recommending kinetic laws by using type common for the kind of models in combination with the R type of the reactions. Alternatively, 2DK could also be used to alert users that a kinetic law was unusual for the K type and R type. Last, 2DK provided a way to analyze groups of models to compare their kinetic laws. I applied 2DK to BioModels to compare the kinetics of signaling networks with the kinetics of metabolic networks and found significant differences in K type distributions.

Identification of a Novel Allosteric Site at the M5 Muscarinic Acetylcholine Receptor

The M5 muscarinic acetylcholine receptor (mAChR) has emerged as an exciting therapeutic target for the treatment of addiction and behavioral disorders. This has been in part due to promising preclinical studies with the M5 mAChR selective negative allosteric modulator (NAM), ML375. The binding site of ML375 remains unknown, however, making it difficult to develop improved M5 mAChR selective modulators. To determine the possible location of the ML375 binding site, we used radioligand binding and functional assays to show that ML375 does not interact with the well-characterized "common" mAChR allosteric site located in the receptor's extracellular vestibule, nor a previously proposed second allosteric site recognized by the modulator, amiodarone. Molecular docking was used to predict potential allosteric sites within the transmembrane (TM) domain of the M5 mAChR. These predicted sites were assessed using M5-M2 mAChR receptor chimeras and further targeted with site-directed mutagenesis, which enabled the identification of a putative binding site for ML375 at the interface of TMs 2-4. Collectively, these results identify a third allosteric site at the M5 mAChR and highlight the ability of allosteric modulators to selectively target highly conserved proteins.

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.

100 years of modelling ligand-receptor binding and response: A focus on GPCRs

Experimental pharmacologists rely on the application of models to describe biological observations in order to learn about a drug's effective concentration, the strength with which it binds its target and drives a response (at either molecular or system level), and the nature of more complex drug actions (allosterism/functional selectivity). Models in current use build upon decades of basic principles, going back to the beginning of the last century. Yet often, researchers are only partially familiar with these underlying principles, creating the potential for confusion due to failure to recognise the underpinning assumptions of the models that are used. Here, we describe the history of receptor theory as it underpins receptor stimulus-response models in use today, emphasising particularly attributes and models relevant to GPCRs-and point to some current aims of model development.

Utility of an "Allosteric Site-Impaired" M2 Muscarinic Acetylcholine Receptor as a Novel Construct for Validating Mechanisms of Action of Synthetic and Putative Endogenous Allosteric Modulators

Muscarinic acetylcholine receptors (mAChRs) are exemplar models for understanding G protein-coupled receptor (GPCR) allostery, possessing a "common" allosteric site in an extracellular vestibule (ECV) for synthetic modulators including gallamine, strychnine, and brucine. In addition, there is intriguing evidence of endogenous peptides/proteins that may target this region at the M₂ mAChR. A common feature of synthetic and endogenous M₂ mAChR negative allosteric modulators (NAMs) is their cationic nature. Using a structure-based approach, we previously designed a mutant M₂ mAChR (N410K+T423K) to specifically abrogate binding of ECV cationic modulators (Dror et al., 2013). Herein, we used this "allosteric site-impaired" receptor to investigate allosteric interactions of synthetic modulators as well as basic peptides (poly-l-arginine, endogenously produced protamine, and major basic protein). Using [³H]N-methylscopolamine equilibrium and kinetic binding and functional assays of guanosine 5'-O-[γ-thio]triphosphate [³⁵S] binding and extracellular signal-regulated kinases 1 and 2 phosphorylation, we found modest effects of the mutations on potencies of orthosteric antagonists and an increase in the affinity of the cognate agonist, acetylcholine, likely reflecting the effect of the mutations on the access/egress of these ligands into the orthosteric pocket. More importantly, we noted a significant abrogation in affinity for all synthetic or peptidic modulators at the mutant mAChR, validating their allosteric nature. Collectively, these findings provide evidence for a hitherto-unappreciated role of endogenous cationic peptides interacting allosterically at the M₂ mAChR and identify the allosteric site-impaired GPCR as a tool for validating NAM activity as well as a potential candidate for future chemogenetic strategies to understand the physiology of endogenous allosteric substances.

Discovery of new GPCR ligands to illuminate new biology

Although a plurality of drugs target G-protein-coupled receptors (GPCRs), most have emerged from classical medicinal chemistry and pharmacology programs and resemble one another structurally and functionally. Though effective, these drugs are often promiscuous. With the realization that GPCRs signal via multiple pathways, and with the emergence of crystal structures for this family of proteins, there is an opportunity to target GPCRs with new chemotypes and confer new signaling modalities. We consider structure-based and physical screening methods that have led to the discovery of new reagents, focusing particularly on the former. We illustrate their use against previously untargeted or orphan GPCRs, against allosteric sites, and against classical orthosteric sites that selectively activate one downstream pathway over others. The ligands that emerge are often chemically novel, which can lead to new biological effects.

Novel Allosteric Modulators of G Protein-coupled Receptors

G protein-coupled receptors (GPCRs) are allosteric proteins, because their signal transduction relies on interactions between topographically distinct, yet conformationally linked, domains. Much of the focus on GPCR allostery in the new millennium, however, has been on modes of targeting GPCR allosteric sites with chemical probes due to the potential for novel therapeutics. It is now apparent that some GPCRs possess more than one targetable allosteric site, in addition to a growing list of putative endogenous modulators. Advances in structural biology are also shedding new insights into mechanisms of allostery, although the complexities of candidate allosteric drugs necessitate rigorous biological characterization.

Muscarinic acetylcholine receptors: novel opportunities for drug development

The muscarinic acetylcholine receptors are a subfamily of G protein-coupled receptors that regulate numerous fundamental functions of the central and peripheral nervous system. The past few years have witnessed unprecedented new insights into muscarinic receptor physiology, pharmacology and structure. These advances include the first structural views of muscarinic receptors in both inactive and active conformations, as well as a better understanding of the molecular underpinnings of muscarinic receptor regulation by allosteric modulators. These recent findings should facilitate the development of new muscarinic receptor subtype-selective ligands that could prove to be useful for the treatment of many severe pathophysiological conditions.

What ligand-gated ion channels can tell us about the allosteric regulation of G protein-coupled receptors

The GABA(A) receptor is the target for a number of important allosteric drugs used in medicine, including benzodiazepines and anesthetics. These modulators have variable effects on the potency and maximal response of macroscopic currents elicited by different GABA(A) receptor agonists, yet this modulation is consistent with a two-state model in which the allosteric ligand has invariant affinity constants for the active and inactive states. Analysis of the effects of an allosteric agonist, like etomidate, on the population current provides a means of estimating the gating constant of the unliganded GABA(A) receptor (∼10(-4)). In contrast, allosteric interactions at the M(2) muscarinic receptor are often inconsistent with a two-state model. Analyzing allosterism within the constraints of a two-state model, nonetheless, provides an unbiased measure of probe dependence as well as clues to the mechanism of allosteric modulation. The rather simple allosteric effect of affinity-only modulation is difficult to explain and suggests modulation of a peripheral orthosteric ligand-docking site on the M(2) muscarinic receptor.

Development of a radioligand, [(3)H]LY2119620, to probe the human M(2) and M(4) muscarinic receptor allosteric binding sites

In this study, we characterized a muscarinic acetylcholine receptor (mAChR) potentiator, LY2119620 (3-amino-5-chloro-N-cyclopropyl-4-methyl-6-[2-(4-methylpiperazin-1-yl)-2-oxoethoxy]thieno[2,3-b]pyridine-2-carboxamide) as a novel probe of the human M2 and M4 allosteric binding sites. Since the discovery of allosteric binding sites on G protein-coupled receptors, compounds targeting these novel sites have been starting to emerge. For example, LY2033298 (3-amino-5-chloro-6-methoxy-4-methyl-thieno(2,3-b)pyridine-2-carboxylic acid cyclopropylamid) and a derivative of this chemical scaffold, VU152100 (3-amino-N-(4-methoxybenzyl)-4,6-dim​ethylthieno[2,3-b]pyridine carboxamide), bind to the human M4 mAChR allosteric pocket. In the current study, we characterized LY2119620, a compound similar in structure to LY2033298 and binds to the same allosteric site on the human M4 mAChRs. However, LY2119620 also binds to an allosteric site on the human M2 subtype. [(3)H]NMS ([(3)H]N-methylscopolamine) binding experiments confirm that LY2119620 does not compete for the orthosteric binding pocket at any of the five muscarinic receptor subtypes. Dissociation kinetic studies using [(3)H]NMS further support that LY2119620 binds allosterically to the M2 and M4 mAChRs and was positively cooperative with muscarinic orthosteric agonists. To probe directly the allosteric sites on M2 and M4, we radiolabeled LY2119620. Cooperativity binding of [(3)H]LY2119620 with mAChR orthosteric agonists detects significant changes in Bmax values with little change in Kd, suggesting a G protein-dependent process. Furthermore, [(3)H]LY2119620 was displaced by compounds of similar chemical structure but not by previously described mAChR allosteric compounds such as gallamine or WIN 62,577 (17-β-hydroxy-17-α-ethynyl-δ-4-androstano[3,2-b]pyrimido[1,2-a]benzimidazole). Our results therefore demonstrate the development of a radioligand, [(3)H]LY2119620 to probe specifically the human M2 and M4 muscarinic receptor allosteric binding sites.

The summary on non-reactivation cholinergic properties of oxime reactivators: the interaction with muscarinic and nicotinic receptors

Organophosphorus inhibitors (OP) of acetylcholinesterase (AChE) represent a group of highly toxic compounds. The treatment of OP intoxication is, however, insufficiently ensured. Currently, two main categories of drugs-anticholinergics and oxime reactivators- are employed as antidotes. Oximes have been reported to act at several levels of the cholinergic transmission, and among the non-reactivation effects, the interaction with cholinergic receptors stands out. This review addresses issues correlated with non-reactivating effects of oxime reactivators with a special focus on the muscarinic and nicotinic receptors, but involvement of other cholinergic structures such as AChE and choline uptake carriers are discussed too. It can be concluded that the oxime reactivators show a variation in their antagonistic effect on the muscarinic and nicotinic receptors, which is likely to be of significance in the treatment of OP poisoning. In vitro data reported oximes to exert higher efficacy on the muscarinic M2 subtype than on the AChE. However, this effect seemed to be subtype specific since the antagonistic M3 effect was lower. Also, and importantly, the antimuscarinic effect was larger than that on nicotinic receptors. Even though atropine showed a much higher muscarinic antagonism, it is supposed that non-reactivation properties of oxime reactivators play a significant role in the treatment of OP poisoning.

Quantification of allosteric interactions at G protein-coupled receptors using radioligand binding assays

Allosteric interactions involve the simultaneous binding of two ligands to the same receptor. An allosteric modulator causes a conformational change in the receptor protein that yields a change in the binding or signaling of an orthosteric agent, i.e., an agonist or competitive antagonist that binds to the endogenous agonist binding site. Because of the complex nature of allosteric phenomena, the detection and quantification of their effects on orthosteric ligand binding relies on the use of both equilibrium and non-equilibrium assays to ensure proper interpretation of the findings. Outlined in this unit are the most common experimental approaches for measuring allosteric effects on orthosteric ligand affinity at G protein-coupled receptors. There is also a discussion of the analysis of experimental data derived from such assays.

The best of both worlds? Bitopic orthosteric/allosteric ligands of g protein-coupled receptors

It is now acknowledged that G protein-coupled receptors, the largest class of drug targets, adopt multiple active states that can be preferentially stabilized by orthosteric ligands or allosteric modulators, thus giving rise to the phenomenon of pathway-biased signaling. In the past few years, researchers have begun to explore the potential of linking orthosteric and allosteric pharmacophores to yield bitopic hybrid ligands. This approach is an extension of the more traditional bivalent ligand concept and shares some of the same challenges, including the choice and role of the linker between the two pharmacophores and the validation of mechanism of action. Nonetheless, the promise of bitopic ligands is the generation of novel chemical tools that have improved affinity and/or selectivity profiles. Previously identified functionally selective compounds (and medicines) also may act via a bitopic mechanism, suggesting that the phenomenon is more widespread than currently appreciated.

Allosteric Modulation of G Protein Coupled Receptors by Cytoplasmic, Transmembrane and Extracellular Ligands

G protein coupled receptors (GPCRs) bind diverse classes of ligands, and depending on the receptor, these may bind in their transmembrane or the extracellular domains, demonstrating the principal ability of GPCRs to bind ligand in either domains. Most recently, it was also observed that small molecule ligands can bind in the cytoplasmic domain, and modulate binding and response to extracellular or transmembrane ligands. Thus, all three domains in GPCRs are potential sites for allosteric ligands, and whether a ligand is allosteric or orthosteric depends on the receptor. Here, we will review the evidence supporting the presence of putative binding pockets in all three domains of GPCRs and discuss possible pathways of communication between these pockets.

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.

Growth hormone secretagogues and growth hormone releasing peptides act as orthosteric super-agonists but not allosteric regulators for activation of the G protein Galpha(o1) by the Ghrelin receptor

Some growth hormone secretagogues act as agonists at the ghrelin receptor and have been described as "ago-allosteric" ligands because of an ability to also modulate the maximum efficacy and potency of ghrelin (Holst et al., 2005). In membranes prepared from cells coexpressing the human ghrelin receptor and the G protein Galpha(o1), N-[1(R)-1, 2-dihydro-1-ethanesulfonylspiro-3H-indole-3,4'-piperidin)-1'-yl]carbonyl-2-(phenylmethoxy)-ethyl-2-amino-2-methylpropanamide (MK-677), growth hormone-releasing peptide 6 (GHRP-6), and the 2(R)-hydroxypropyl derivative of 3-amino-3-methyl-N-(2,3,4,5-tetrahydro-2-oxo-1-([2'-(1H-tetrazol-5-yl) (1,1'-biphenyl)-4-yl]methyl)-1H-1-benzazepin-3(R)-yl)-butanamide (L-692,585) each functioned as direct agonists, and each displayed higher efficacy than ghrelin. The effect of multiple, fixed concentrations of each of these ligands on the function and concentration-dependence of ghrelin and the effect of multiple, fixed concentrations of ghrelin on the action of MK-677, GHRP-6, and L-692,585 was analyzed globally according to a modified version of an operational model of allosterism that accounts for allosteric modulation of affinity, efficacy, and allosteric agonism. Each of the data sets was best fit by a model of simple competition between a partial and a full agonist. Both positive and negative allosteric modulators are anticipated to alter the kinetics of binding of an orthosteric agonist. However, none of the proposed ago-allosteric regulators tested had any effect on the dissociation kinetics of (125)I-[His]-ghrelin, and GHRP-6 and MK-677 were able to fully displace (125)I-[His]-ghrelin from the receptor. At least in the system tested, each of the ligands acted in a simple competitive fashion with ghrelin as demonstrated by analysis according to a model whereby ghrelin is a partial agonist with respect to each of the synthetic agonists tested.

Muscarinic toxins: tools for the study of the pharmacological and functional properties of muscarinic receptors

Muscarinic receptors mediate metabotropic actions of acetylcholine in the CNS and PNS and autocrine functions of acetylcholine in non-neuronal systems. Because of the lack of highly selective muscarinic ligands, the precise location, functional role, and roles in various diseases of the five muscarinic receptor subtypes remain unclear. Muscarinic toxins isolated from the venom of Dendroaspis snakes have a natural high affinity and selectivity, associated with roles as competitive antagonists, allosteric modulators, and potential agonists. These toxins may therefore be invaluable tools for studying muscarinic receptors. We review data on the structural and pharmacological characterization of the muscarinic toxins, focusing on recent structure-function studies on toxin-receptor interactions. We discuss the potential benefits of using these toxins for investigating muscarinic function in vivo.

Allosteric modulation of the muscarinic M4 receptor as an approach to treating schizophrenia

Current antipsychotics provide symptomatic relief for patients suffering from schizophrenia and related psychoses; however, their effectiveness is variable and many patients discontinue treatment due to side effects. Although the etiology of schizophrenia is still unclear, a leading hypothesis implicates an imbalanced dopaminergic system. Muscarinic acetylcholine (ACh) receptors regulate dopamine levels in key areas of the brain involved in psychosis, with the M(4) subtype emerging as a key regulator of dopaminergic hyperactivity. Unfortunately, no selective small molecule tools exist to provide pharmacological validation of this hypothesis. Here, we describe the discovery of a small molecule modulator, LY2033298, that is highly selective for human M(4) receptors by virtue of targeting an allosteric site on this receptor. Pharmacological assays confirmed the selectivity of LY2033298 for the M(4) receptor and revealed the highest degree of positive allosteric enhancement of ACh potency thus far identified. Radioligand binding assays also show this compound to directly potentiate agonist binding while having minimal effects on antagonist binding. Mutational analysis identified a key amino acid (D(432)) in the third extracellular loop of the human M(4) receptor to be critical for selectivity and agonist potentiation by LY2033298. Importantly, LY2033298 was active in animal models predictive of clinical antipsychotic drug efficacy indicating its potential use as a first-in-class, selective, allosteric muscarinic antipsychotic agent.

Autoantibodies enhance agonist action and binding to cardiac muscarinic receptors in chronic Chagas' disease

Chronic Chagasic patient immunoglobulins (CChP-IgGs) recognize an acidic amino acid cluster at the second extracellular loop (el2) of cardiac M(2)-muscarinic acetylcholine receptors (M(2)AChRs). These residues correspond to a common binding site for various allosteric agents. We characterized the nature of the M(2)AChR/CChP-IgG interaction in functional and radioligand binding experiments applying the same mainstream strategies previously used for the characterization of other allosteric agents. Dose-response curves of acetylcholine effect on heart rate were constructed with data from isolated heart experiments in the presence of CChP or normal blood donor (NBD) sera. In these experiments, CChP sera but not NBD sera increased the efficacy of agonist action by augmenting the onset of bradyarrhythmias and inducing a Hill slope of 2.5. This effect was blocked by gallamine, an M(2)AChR allosteric antagonist. Correspondingly, CChP-IgGs increased acetylcholine affinity twofold and showed negative cooperativity for [(3)H]-N-methyl scopolamine ([(3)H]-NMS) in allosterism binding assays. A peptide corresponding to the M(2)AChR-el2 blocked this effect. Furthermore, dissociation assays showed that the effect of gallamine on the [(3)H]-NMS off-rate was reverted by CChP-IgGs. Finally, concentration-effect curves for the allosteric delay of W84 on [(3)H]-NMS dissociation right shifted from an IC(50) of 33 nmol/L to 78 nmol/L, 992 nmol/L, and 1670 nmol/L in the presence of 6.7 x 10(- 8), 1.33 x 10(- 7), and 2.0 x 10(- 7) mol/L of anti-el2 affinity-purified CChP-IgGs. Taken together, these findings confirmed a competitive interplay of these ligands at the common allosteric site and revealed the novel allosteric nature of the interaction of CChP-IgGs at the M(2)AChRs as a positive cooperativity effect on acetylcholine action.

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.

Critical role for the second extracellular loop in the binding of both orthosteric and allosteric G protein-coupled receptor ligands

The second extracellular (E2) loop of G protein-coupled receptors (GPCRs) plays an essential but poorly understood role in the binding of non-peptidic small molecules. We have utilized both orthosteric ligands and allosteric modulators of the M2 muscarinic acetylcholine receptor, a prototypical Family A GPCR, to probe possible E2 loop binding dynamics. We developed a homology model based on the crystal structure of bovine rhodopsin and predicted novel cysteine substitutions that should dramatically reduce E2 loop flexibility via disulfide bond formation and significantly inhibit the binding of both types of ligands. This prediction was validated experimentally using radioligand binding, dissociation kinetics, and cell-based functional assays. The results argue for a flexible "gatekeeper" role of the E2 loop in the binding of both allosteric and orthosteric GPCR ligands.

Structure-Function Studies of Allosteric Agonism at M2Muscarinic Acetylcholine Receptors

The M2 muscarinic acetylcholine receptor (mAChR) possesses at least one binding site for allosteric modulators that is dependent on the residues (172)EDGE(175), Tyr(177), and Thr(423). However, the contribution of these residues to actions of allosteric agonists, as opposed to modulators, is unknown. We created mutant M2 mAChRs in which the charge of the (172)EDGE(175) sequence had been neutralized and each Tyr(177) and Thr(423) was substituted with alanine. Radioligand binding experiments revealed that these mutations had a profound inhibitory effect on the prototypical modulators gallamine, alcuronium, and heptane-1,7-bis-[dimethyl-3'-phthalimidopropyl]-ammonium bromide (C7/3-phth) but minimal effects on the orthosteric antagonist [3H]N-methyl scopolamine. In contrast, the allosteric agonists 4-I-[3-chlorophenyl]carbamoyloxy)-2-butynyltrimethylammnonium chloride (McN-A-343), 4-n-butyl-1-[4-(2-methylphenyl)-4-oxo-1-butyl] piperidine hydrogen chloride (AC-42), and the novel AC-42 derivative 1-[3-(4-butyl-1-piperidinyl)propyl]-3,4-dihydro-2(1H)-quinolinone (77-LH-28-1) demonstrated an increased affinity or proportion of high-affinity sites at the combined EDGE-YT mutation, indicating a different mode of binding to the prototypical modulators. Subsequent functional assays of extracellular signal-regulated kinase (ERK)1/2 phosphorylation and guanosine 5'-(gamma-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding revealed minimal effects of the mutations on the orthosteric agonists acetylcholine (ACh) and pilocarpine but a significant increase in the efficacy of McN-A-343 and potency of 77-LH-28-1. Additional mutagenesis experiments found that these effects were predominantly mediated by Tyr(177) and Thr(423), rather than the (172)EDGE(175) sequence. The functional interaction between each of the allosteric agonists and ACh was characterized by high negative cooperativity but was consistent with an increased allosteric agonist affinity at the combined EDGE-YT mutant M2 mAChR. This study has thus revealed a differential role of critical allosteric site residues on the binding and function of allosteric agonists versus allosteric modulators of M2 mAChRs.

Allosteric interaction of the neuromuscular blockers vecuronium and pancuronium with recombinant human muscarinic M2 receptors

Neuromuscular blocking drugs produce muscle weakness by interaction with nicotinic-acetylcholine receptors. Cardiovascular side effects have been reported. In this study the neuromuscular blocking drug vecuronium and the controls gallamine and pancuronium slowed the rate of atropine induced [(3)H]N-methylscopolamine dissociation from Chinese hamster ovary cells expressing recombinant human muscarinic M2 receptors K(off) values min(-1); vecuronium (125 nM), atropine 0.45+/-0.07+blocker 0.04+/-0.02; gallamine (21 nM), atropine 0.42+/-0.05+blocker 0.15+/-0.04; pancuronium(21 nM), atropine 0.36+/-0.03+blocker 0.03+/-0.01). These data indicate that vecuronium, gallamine and pancuronium interact with an allosteric site on the muscarinic M2 receptor (located on the heart) and this may explain some of their cardiac side effects.

Allosteric Modulation of G Protein–Coupled Receptors

The past decade has witnessed a significant growth in the identification of allosteric modulators of G protein-coupled receptors (GPCRs), i.e., ligands that interact with binding sites that are topographically distinct from the orthosteric site recognized by the receptor's endogenous agonist. Because of their ability to modulate receptor conformations in the presence of orthosteric ligand, allosteric modulators can "fine-tune" classical pharmacological responses. This is advantageous in terms of a potential for engendering greater GPCR subtype-selectivity, but represents a significant challenge for detecting and validating allosteric behaviors. Although allosteric sites need not have evolved to accommodate endogenous ligands, there are a number of examples of where such modulators have been shown to contribute to physiological or pathophysiological processes. Studies are also beginning to unravel the structural basis of allosteric modulation of GPCRs. It remains to be determined whether such modulation represents interactions within monomers versus across dimers.

Muscarinic receptor subtypes in neuronal and non-neuronal cholinergic function

1 Muscarinic M1-M5 receptors mediate the metabotropic actions of acetylcholine in the nervous system. A growing body of data indicate they also mediate autocrine functions of the molecule. The availability of novel and selective muscarinic agonists and antagonists, as well as in vivo gene disruption techniques, has clarified the roles of muscarinic receptors in mediating both functions of acetylcholine. 2 Selective M1 agonists or mixed M1 agonists/M2 antagonists may provide an approach to the treatment of cognitive disorders, while M3 antagonism, or mixed M2/M3 antagonists, are approved for the treatment of contractility disorders including overactive bladder and chronic obstructive pulmonary disease. Preclinical data suggest that selective agonism of the M4 receptor will provide novel anti-nociceptive agents, while therapeutics-based upon agonism or antagonism of the muscarinic M5 receptor have yet to be reported. 3 The autocrine functions of muscarinic receptors broadly fall into two areas - control of cell growth or proliferation and mediation of the release of chemical mediators from epithelial cells, ultimately causing muscle relaxation. The former particularly are involved in embryological development, oncogenesis, keratinocyte function and immune responsiveness. The latter regulate contractility of smooth muscle in the vasculature, airways and urinary bladder. 4 Most attention has focused on muscarinic M1 or M3 receptors which mediate lymphocyte immunoresponsiveness, cell migration and release of smooth muscle relaxant factors. Muscarinic M4 receptors are implicated in the regulation of keratinocyte adhesion and M2 receptors in stem cell proliferation and development. Little data are available concerning the M5 receptor, partly due to the difficulties in defining the subtype pharmacologically. 5 The autocrine functions of acetylcholine, like those in the nervous system, involve activation of several muscarinic receptor subtypes. Consequently, the role of these subtypes in autocrine, as well neuronal cholinergic systems, significantly expands their importance in physiology and pathophysiology.

Identification of Various Allosteric Interaction Sites on M1Muscarinic Receptor Using125I-Met35-Oxidized Muscarinic Toxin 7

Monoiodinated, Met35-oxidized muscarinic toxin 7 (MT7ox) was synthesized, and its affinity constants for free or N-methyl scopolamine (NMS)-occupied hM1 receptor were measured directly by equilibrium and kinetic binding experiments. Identical values were obtained with the two types of assay methods, 14 pM and 0.9 nM in free or NMS-liganded receptor states, respectively, highlighting a strong negative cooperativity between this allosteric toxin and NMS. Identical results were obtained with indirect binding experiments with [3H]NMS using the ternary complex model, clearly demonstrating the reciprocal nature of this cooperativity. Furthermore, the effects of various orthosteric and allosteric agents on the dissociation kinetic of 125I-MT7ox were measured and show that, except for the MT1 toxin, all of the ligands studied [NMS, atropine, gallamine, brucine, tacrine, staurosporine, and (9S,10S,12R)-2,3,9,10,11-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester (KT5720)] interact allosterically with muscarinic toxin 7. Equilibrium binding experiments with 125I-MT7ox and [3H]NMS were conducted to reveal the effects of these ligands on the free receptor, and affinity constants (pKx values) were calculated using the allosteric ternary complex model. Our results suggest that MT7 toxin interacts with hM1 receptor at a specific allosteric site, which may partially overlap those identified previously for "classic" or "atypical" allosteric agents and highlight the potential of this new allosteric tracer in studying allosterism at muscarinic receptors.

Comparative study on pharmacological effects of DM-phencynonate hydrochloride and its optical isomers

AIM

The 3-azabicyclo(3,3,1)nonanyl-9-alpha-yl-alpha-cyclopentyl-alpha-phenyl-alpha-glycolate (DM-phencynonate hydrochloride, DMCPG) is a demethylated metabolite of 3-methyl-3-azabicyclo(3,3,1)nonanyl-9-alpha-yl-alpha-cyclopentyl-alpha-phenyl-alpha-glycolate (phencynonate hydrochloride, CPG). (+/-)DMCPG had one chiral center and two enantiomers [R(-) and S(+)DMCPG]. Here we carried out a comparative study of the pharmacological profiles of these optical isomers.

METHODS

Affinity and relative efficacy were tested using a radioligand-binding assay with muscarinic acetylcholine receptors from the rat cerebral cortex. Pharmacological activity was assessed in three individual experiments: (1) potentiating the effect of a sub-threshold hypnotic dose of sodium pentobarbital; (2) inhibiting oxotremorine-induced salivation; and (3) inhibiting the contractile response to carbachol.

RESULTS

In the competitive binding assay, R(-)DMCPG (K(i)=763.75 nmol/L) was 4- and 2-fold more potent than (+/-)DMCPG (K(i)=3186 nmol/L) and S(+)DMCPG (K(i)=1699 nmol/L) in inhibiting the binding of [(3)H]QNB. The R(-) and S (+) configurations showed positive cooperation (n(H)>1) with the muscarinic receptor, whereas (+/-)DMCPG had a negative cooperation (n(H)<1) relationship with the muscarinic receptor in a radio-binding assay. Both the R(-) and S(+) configurations could potentiate the effect of sub-threshold hypnotic dose of sodium pentobarbital in a dose-dependent manner (the ED(50) values were 2.53 and 18.65 mg/kg, respectively), but (+/-)DMCPG did not display significant central depressant effects at doses from 10 to 29.15 mg/kg (P>0.05). (+/-)DMCPG and its optical isomers suppressed the guinea pig ileum contractile response to carbachol. The IC(50) values were 7.78 x 10(-9), 1.88 x 10(-7), and 1.038 x 10(-7) nmol/L, respectively. In the anti-salivation study, (+/-)DMCPG and its enantiomers depressed oxotremorine- induced salivation in a dose-dependent manner, and the order of potency was R(-)DMCPG (ED(50)=0.44 mg/kg) > (+/-)DMCPG (ED(50)=2.88 mg/kg) >S(+)DMCPG (ED(50)=5.05 mg/kg).

CONCLUSION

(+/-)DMCPG and its optical isomers have differences in their pharmacological potencies as anticholinergic agents, and the R(-) configuration is more active than the S(+) configuration.

Fluorophore-tagged GPCR ligands

Fluorescently tagged drug molecules can be successfully employed to visualize the location of their receptor target at the single-cell level. Furthermore, if their binding to the receptor is reversible, one can now obtain detailed pharmacological information such as affinity using single-molecule detection techniques. When coupled to the growing exploitation of fluorescence-based read-outs in high throughput and high content screening, it is clear that fluorescent molecules offer a safer, more powerful and more versatile alternative to radioligands in molecular pharmacology and drug discovery. GPCR pharmacology has benefited enormously from the application of fluorescence-based technologies and we now possess a much greater understanding of this receptor family's basic molecular mechanisms of action through the careful design and judicious use of fluorescent peptide and small-molecule-based ligands.

New heterocyclic ligands for the adenosine receptors P1 and for the ATP receptors P2

Extracellular adenosine and adenine nucleotides induce various cellular responses through activation of P1 and P2 receptors. P1 receptors preferentially recognize adenosine and four different G protein-coupled receptors (A(1), A(2A), A(2B), and A(3) subtypes) have been identified. On the other hand, P2 receptors are activated by adenine and/or uridine nucleotides and classified into two families: ionotropic P2X and G protein-coupled P2Y receptors. In this article, we summarize our studies which led to development of new potent and selective heterocyclic ligands for the adenosine receptors P1 and for the ATP receptors P2X(7).

G-protein-coupled receptor allosterism: the promise and the problem(s)

Allosteric modulators of G-protein-coupled receptors interact with binding sites that are topographically distinct from the orthosteric site recognized by the receptor's endogenous agonist. Allosteric ligands offer a number of advantages over orthosteric drugs, including the potential for greater receptor subtype selectivity and a more ‘physiological’ regulation of receptor activity. However, the manifestations of allosterism at G-protein-coupled receptors are quite varied, and significant challenges remain for the optimization of screening methods to ensure the routine detection and validation of allosteric ligands.

Fluorescent pirenzepine derivatives as potential bitopic ligands of the human M1 muscarinic receptor

Following a recent description of fluorescence resonance energy transfer between enhanced green fluorescent protein (EGFP)-fused human muscarinic M1 receptors and Bodipy-labeled pirenzepine, we synthesized seven fluorescent derivatives of this antagonist in order to further characterize ligand-receptor interactions. These compounds carry Bodipy [558/568], Rhodamine Red-X [560/580], or Fluorolink Cy3 [550/570] fluorophores connected to pirenzepine through various linkers. All molecules reversibly bind with high affinity to M1 receptors (radioligand and energy transfer binding experiments) provided that the linker contains more than six atoms. The energy transfer efficiency exhibits modest variations among ligands, indicating that the distance separating EGFP from the fluorophores remains almost constant. This also supports the notion that the fluorophores may bind to the receptor protein. Kinetic analyses reveal that the dissociation of two Bodipy derivatives (10 or 12 atom long linkers) is sensitive to the presence of the allosteric modulator brucine, while that of all other molecules (15-24 atom long linkers) is not. The data favor the idea that these analogues might interact with both the acetylcholine and the brucine binding domains.

Probing the Pharmacophore for Allosteric Ligands of Muscarinic M2 Receptors: SAR and QSAR Studies in a Series of Bisquaternary Salts of Caracurine V and Related Ring Systems

Allosteric effects on muscarinic acetylcholine M(2) receptors were examined in a series of bisquaternary salts of the Strychnos alkaloid caracurine V (6) and related iso-caracurine V, tetrahydrocaracurine V, and bisnortoxiferine ring systems. The compounds inhibited dissociation of the orthosteric antagonist [(3)H]N-methylscopolamine (NMS) from porcine cardiac M(2) receptors with EC(0.5,diss) values from 4 to 3270 nM. The majority of compounds hardly changed [(3)H]NMS equilibrium binding, indicating similar binding affinities in free and NMS-occupied M(2) receptors. The most potent agents were found in the caracurine V, iso-caracurine V, and tetrahydrocaracurine V series and carried nonpolar alkyl groups with a maximal chain length of three carbon atoms. 3D QSAR (CoMSIA) analysis explained the wide range of binding affinities by steric and electrostatic properties of the side chains. Furthermore, the findings suggest that the spatial orientation of the "caracurine" aromatic rings compared with the bisnortoxiferine ring skeleton is favorable to optimal allostere-receptor interactions.

Designing human m1 muscarinic receptor-targeted hydrophobic eigenmode matched peptides as functional modulators

A new proprietary de novo peptide design technique generated ten 15-residue peptides targeting and containing the leading nontransmembrane hydrophobic autocorrelation wavelengths, "modes", of the human m(1) muscarinic cholinergic receptor, m(1)AChR. These modes were also shared by the m(4)AChR subtype (but not the m(2), m(3), or m(5) subtypes) and the three-finger snake toxins that pseudoirreversibly bind m(1)AChR. The linear decomposition of the hydrophobically transformed m(1)AChR amino acid sequence yielded ordered eigenvectors of orthogonal hydrophobic variational patterns. The weighted sum of two eigenvectors formed the peptide design template. Amino acids were iteratively assigned to template positions randomly, within hydrophobic groups. One peptide demonstrated significant functional indirect agonist activity, and five produced significant positive allosteric modulation of atropine-reversible, direct-agonist-induced cellular activation in stably m(1)AChR-transfected Chinese hamster ovary cells, reflected in integrated extracellular acidification responses. The peptide positive allosteric ligands produced left-shifts and peptide concentration-response augmentation in integrated extracellular acidification response asymptotic sigmoidal functions and concentration-response behavior in Hill number indices of positive cooperativity. Peptide mode specificity was suggested by negative crossover experiments with human m(2)ACh and D(2) dopamine receptors. Morlet wavelet transformation of the leading eigenvector-derived, m(1)AChR eigenfunctions locates seven hydrophobic transmembrane segments and suggests possible extracellular loop locations for the peptide-receptor mode-matched, modulatory hydrophobic aggregation sites.

Investigation of the interaction of a putative allosteric modulator, N-(2,3-diphenyl-1,2,4-thiadiazole-5-(2H)-ylidene) methanamine hydrobromide (SCH-202676), with M1 muscarinic acetylcholine receptors

The interaction between a novel G protein-coupled receptor modulator, N-(2,3-diphenyl-1,2,4-thiadiazole-5-(2H)-ylidene) methanamine hydrobromide (SCH-202676), and the M(1) muscarinic acetylcholine receptor (mAChR) was investigated. In contrast to the prototypical mAChR allosteric modulator, heptane 1,7-bis-(dimethyl-3'-phthalimidopropyl)-ammonium bromide (C(7)/3-phth), SCH-202676 had no effect on the dissociation kinetics of [(3)H]N-methylscopolamine ([(3)H]NMS) at M(1) mAChRs stably expressed in Chinese hamster ovary (CHO) cell membranes. However, SCH-202676 completely inhibited the binding of [(3)H]NMS in membrane preparations, with a Hill slope significantly greater than unity, indicative of positive cooperativity in the binding of the inhibitor. Moreover, SCH-202676 caused dextral shifts of the [(3)H]NMS saturation binding curve that were greater than expected for a competitive interaction. The addition of C(7)/3-phth (100 microM) had no significant effect on the inhibitory potency of SCH-202676. In contrast to the findings in cell membranes, the interaction between SCH-202676 and [(3)H]NMS in intact M(1) CHO cells yielded saturation and inhibition isotherms that were compatible with the predictions for a competitive interaction. Intact cell assays of acetylcholine-mediated phosphoinositide hydrolysis in the absence or presence of SCH-202676 revealed a mixed competitive/noncompetitive mode of interaction that was dependent on the concentration of SCH-202676. These data reveal that the nature of the interaction between SCH-202676 and the M(1) mAChR is dependent on whether it is studied using intact versus broken cell preparations. It is proposed that SCH-202676 uses a dual mode of ligand-receptor interaction involving both extra- and intracellular attachment points on the M(1) mAChR that are distinct from the allosteric binding site recognized by prototypical mAChR modulators such as C(7)/3-phth.

Interactions of edrophonium with neostigmine in the rat trachea

The muscarinic M(3) receptor of airway smooth muscle has both an orthosteric binding site and an allosteric binding site. Edrophonium may bind to the allosteric site, resulting in the inhibition of the action of the orthosteric site. Therefore, we examined the effects of edrophonium on neostigmine-induced contractile and phosphatidylinositol responses of rat trachea. Neostigmine (100 micro M in final concentration) was added, and ring tension was examined by the addition of edrophonium. After the completion of the experiment, Krebs-Henseleit (K-H) solution containing both edrophonium and neostigmine was changed three times with fresh K-H solution, and the tension was recorded. Tracheal slices were incubated with [(3)H]myo-inositol and 100 micro M neostigmine in the presence or absence of edrophonium. The [(3)H]inositol monophosphate (IP(1)) was measured. Data were expressed as mean +/- SE. Statistical significance (P < 0.05) was determined with analysis of variance. Neostigmine-induced tension and IP(1) accumulation were attenuated by edrophonium at concentrations of 100 micro M or more. This attenuation was reversed to more than 80% of control levels by washing with fresh K-H solution. The results suggest that edrophonium would bind to the allosteric site, resulting in the inhibition of the action of the orthosteric site of muscarinic M(3) receptors of rat trachea.

IMPLICATIONS

We examined the effects of edrophonium on neostigmine-induced contractile and phosphatidylinositol responses of rat trachea. Neostigmine-induced tension and inositol monophosphate accumulation were attenuated by edrophonium. This attenuation was reversed by washing. The results suggest that edrophonium would bind to the allosteric site.

Identification of essential residues involved in the allosteric modulation of the human A(3) adenosine receptor

We examined the effects on allosteric modulation and ligand binding of the mutation of amino acid residues of the human A(3) adenosine receptor (A(3)AR) that are hypothesized to be near one of three loci: the putative sodium binding site, the putative ligand binding site, and the DRY motif in transmembrane helical domain 3. The effects of three heterocyclic allosteric modulators [the imidazoquinoline 2-cyclopentyl-4-phenylamino-1H-imidazo[4,5-c]quinoline (DU124183), the pyridinylisoquinoline 4-methoxy-N-[7-methyl-3-(2-pyridinyl)-1-isoquinolinyl]benzamide (VUF5455), and the amiloride analog 5-(N,N-hexamethylene)-amiloride] on the dissociation of the agonist radioligand, N(6)- (4-amino-3-[(125)I]iodobenzyl)-5'-N-methylcarboxamidoadenosine, were compared at wild-type (WT) and mutant A(3)ARs. The F182A(5.43) and N274A(7.45) mutations eliminated the allosteric effects of all three modulators but had little effect on agonist binding. The N30A(1.50) and D58N(2.50) mutations abolished the allosteric effects of DU124183 and VUF5455, but not HMA, whereas the D107N(3.49) mutation abolished the effects of DU124183, but not HMA or VUF5455. The T94A(3.36), H95A(3.37), K152A(EL2), W243A(6.48), L244A(6.49), and S247A(6.52) mutations did not influence allosteric effects of the modulators. Sodium ions (100 mM), which modulate agonist binding at a variety of receptors, caused an approximately 80% inhibition of agonist binding in WT A(3)ARs but did not show any effect on D58N(2.50), D107N(3.49), and F182A(5.43) mutant receptors. In contrast, NaCl induced a modest increase of agonist binding in N30A(1.50) and N274A(7.45) mutant receptors. NaCl decreased the dissociation rate of the antagonist radioligand [(3)H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i]purin-5-one (PSB-11) at the WT A(3)ARs, but not the D58N(2.50) mutant receptor. The results were interpreted using a rhodopsin-based molecular model of the A(3)AR to suggest multiple binding modes of the allosteric modulators.

Systematic development of high affinity bis(ammonio)alkane-type allosteric enhancers of muscarinic ligand binding

Bis(ammonio)alkane compounds carrying lateral phthalimidopropyl substituents on the nitrogen atoms belong to the archetypal muscarinic allosteric agents. Herein, a series of symmetrical and nonsymmetrical compounds was synthesized in which the phthalimide residues were replaced by differently substituted imide moieties. The allosteric action was measured in porcine heart muscarinic M(2) receptors using [(3)H]N-methylscopolamine (NMS) as a ligand for the orthosteric receptor site in equilibrium binding and dissociation experiments. 1,8-Naphthalimido residues conferred an up to 100-fold gain in affinity leading into the low nanomolar range, while the inhibition of NMS binding was maintained. Additional propyl chain methylation was accompanied by an allosteric elevation of orthosteric ligand binding. In general, the gain in allosteric activity achieved by ring variation plus propyl chain methylation on one side of the molecule could not be augmented by symmetrical variations. The elevation of the ligand binding can be explained by different quantitative structure-activity relationships for the affinities to the free and the orthoster-liganded receptor.

Alkyl diphenylacetyl, 9H-xanthene- and 9H-thioxanthene-carbonyl carbamates as positive allosteric modulators of mGlu1 receptors

Starting from the random-screening hit 1a, a series of alkyl diphenylacetyl, 9H-xanthene- and 9H-thioxanthene-carbonyl carbamates 1 has been prepared. These derivatives turned out to be selective positive allosteric modulators of mGlu1 receptors. These compounds do not directly activate mGlu1 receptors but markedly potentiate agonist stimulated responses, increasing potency and maximum efficacy.

Analogs of WIN 62,577 define a second allosteric site on muscarinic receptors

WIN 51,708 (17-beta-hydroxy-17-alpha-ethynyl-5-alpha-androstano[3,2-b]pyrimido[1,2-a]benzimidazole) and WIN 62,577 (17-beta-hydroxy- 17-alpha-ethynyl-delta(4)-androstano[3,2-b]pyrimido[1,2-a]benzimidazole) are potent and centrally active antagonists at rat, but not human, NK(1) receptors. The interactions of these compounds and some analogs with [(3)H]N-methyl scopolamine ([(3)H]NMS) and unlabeled acetylcholine (ACh) at M(1)-M(4) muscarinic receptors have been studied using equilibrium and nonequilibrium radioligand binding methods. The results are consistent with the predictions of the allosteric ternary complex model. The WIN compounds have log affinities for the unliganded receptor in the range 5 to 6.7, and exhibit positive, negative, or neutral cooperativity with [(3)H]NMS and ACh, depending on the receptor subtype and nature of the interacting ligands. WIN 62,577 is an allosteric enhancer of ACh affinity at M(3) receptors. Although interacting allosterically, WIN 62,577 and WIN 51,708 do not affect [(3)H]NMS dissociation from M(3) receptors. Certain analogs have higher affinities than WIN 62,577, and truncated forms of WIN 62,577, including steroids, also act allosterically. One analog, 17-beta-hydroxy-17-alpha-Delta(4)-androstano[3,2-b]pyrido[2,3-b]indole (PG987), has the unique effect of speeding [(3)H]NMS dissociation; its largest effect, 2.5-fold, is at M(3) receptors. The interaction between PG987 and other allosteric agents on [(3)H]NMS dissociation from M(3) receptors indicate that PG987 binds reversibly to a site distinct from that to which gallamine and strychnine bind: in contrast, PG987 seems to bind to the same site on M(3) receptors as KT5720, staurosporine, and WIN 51,708. Therefore, in addition to the allosteric site that binds strychnine (and probably chloromethyl brucine, another allosteric enhancer) there is a second, nonoverlapping, pharmacologically distinct allosteric site on M(3) receptors that also supports positive cooperativity with ACh.

New insights in insurmountable antagonism

Antagonists that produce parallel rightward shifts of agonist dose-response curves with no alteration of the maximal response are traditionally classified as surmountable, while insurmountable antagonists also depress the maximal response. Although the longevity of the antagonist-receptor complex is quoted in many studies to explain insurmountable antagonism, slowly interconverting receptor conformations, allosteric binding sites, and receptor internalization have been evoked as alternative explanations. To complicate matters even further, insurmountable antagonism is not only drug-related; it may also depend on the tissue, species and experimental design. For the sake of drug development, it is important to elucidate the molecular mechanisms of insurmountable antagonism. New experimental approaches, such as intact cell studies and the use of computer-assisted simulations based on dynamic receptor models, herald the advent of better insight in the future.

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

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

Substituted pentacyclic carbazolones as novel muscarinic allosteric agents: synthesis and structure-affinity and cooperativity relationships

Two series of pentacyclic carbazolones, 22 and 23, have been synthesized utilizing a facile intramolecular Dielsminus signAlder reaction and are allosteric modulators at muscarinic acetylcholine receptors. Their affinities and cooperativities with acetylcholine and the antagonist N-methylscopolamine (NMS) at M(1)minus signM(4) receptors have been analyzed and compared. All of the synthesized compounds are negatively cooperative with acetylcholine. In contrast, the majority of the compounds exhibit positive cooperativity with NMS, particularly at M(2) and M(4) receptors. The subtype selectivity, in terms of affinity, was in general M(2) > M(1) > M(4) > M(3). The largest increases in affinity produced by a single substitution of the core structure were given by the 1-OMe (22b) and 1-Cl (22d) derivatives. The position of the N in the ring did not appear to be important for binding affinity or cooperativity. Two compounds 22y and 23i, both trisubstituted analogues, were the most potent compounds synthesized, with dissociation constants of 30minus sign100 nM for the M(2) NMS-liganded and unliganded receptor, respectively. The results indicate that the allosteric site, like the primary binding site, is capable of high-affinity interactions with molecules of relatively low molecular weight.