Allosteric interactions of staurosporine and other indolocarbazoles with N-[methyl-(3)H]scopolamine and acetylcholine at muscarinic receptor subtypes: identification of a second allosteric site

Mutational disruption of a conserved disulfide bond in muscarinic acetylcholine receptors attenuates positive homotropic cooperativity between multiple allosteric sites and has subtype-dependent effects on the affinities of muscarinic allosteric ligands

The 2nd outer loop (o2) of muscarinic acetylcholine receptors (mAChRs) contains a highly conserved cysteine residue that is believed to participate in a disulfide bond and is flanked on either side by epitopes that are critical to the binding of many muscarinic allosteric modulators. We determined the allosteric binding parameters of the modulators gallamine, W84, and tetrahydroaminoacridine (THA) at M2 and M3 mAChRs in which these cysteine residues had been mutated to alanines. THA is known to bind to mAChRs with a strong positive homotropic cooperativity (a Hill slope of approximately 2) that implies that it must interact with multiple allosteric sites. The disulfide cysteine mutations in M2 receptors reduced the allosteric potencies of the tested modulators as if the critical adjacent residue (Tyr177) itself had been mutated. However, in M3 receptors, the disulfide cysteine mutations had no effect on the potencies of gallamine or W84 and even increased the potency of THA. It was most interesting that the strong, positive, homotropic interactions of THA at both M2 and M3 receptors were markedly reduced by the cysteine mutations. In addition, gallamine also displayed positive homotropic cooperativity in its interactions with M3 receptors (but not M2 receptors), and this cooperativity was not evident in the cysteine mutants. Thus, it seems that these cysteine residues play a role in linking cooperating allosteric sites, although it is not currently possible to say whether these multiple sites lie within one receptor or on two linked receptors of a dimer or higher order oligomer.

Interaction Studies of Multiple Binding Sites on M4 Muscarinic Acetylcholine Receptors

This study investigated the reciprocal cross-interactions between two distinct allosteric sites on the M(4) muscarinic acetylcholine receptor (mAChR) in the absence or presence of different orthosteric ligands. Initial studies revealed that two novel benzimidazole allosteric modulators, 17-beta-hydroxy-17-alpha-ethy nyl-delta(4)-androstano[3,2-b]pyrimido[1,2-a]benzimidazole (WIN 62,577) and 17-beta-hydroxy-17-alpha-ethynyl-5-alpha-androstano[3,2-b]pyrimido[1,2-a]benzimidazole (WIN 51,708), exhibited different degrees of positive, negative, or close-to-neutral cooperativity with the orthosteric site on M(1) or M(4) mAChRs, depending on the chemical nature of the orthosteric radioligand that was used [[(3)H]N-methylscopolamine ([(3)H]NMS) versus [(3)H]quinuclidinylbenzilate ([(3)H]QNB)]. The largest window for observing an effect (negative cooperativity) was noted for the combination of WIN 62,577 and [(3)H]QNB at the M(4) mAChR. Experiments involving the combination of these two ligands with unlabeled agonists [acetylcholine, 4-(m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium (McN-A-343), or xanomeline] revealed low degrees of negative cooperativity between WIN 62,577 and each agonist, whereas stronger negative cooperativity was observed against atropine. It is interesting that when these experiments were repeated using the prototypical modulators heptane-1,7-bis-(dimethyl-3'-phthalimidopropyl)-ammonium bromide (C(7)/3-phth), alcuronium, or brucine (which act at a separate allosteric site), WIN 62,577 exhibited negative cooperativity with each modulator when the orthosteric site was unoccupied, but this switched to neutral cooperativity when the receptor was occupied by [(3)H]QNB. Dissociation kinetic experiments using [(3)H]NMS and combination of C(7)/3-phth with WIN 62,577 also provided evidence for neutral cooperativity between the two allosteric sites when the orthosteric site is occupied. Together, these results provide insight into the nature of the interaction between two distinct allosteric sites on the M(4) mAChR and how this interaction is perturbed upon occupancy of the orthosteric site.

Keynote review: Allosterism in membrane receptors

Allosteric modulation of membrane receptors has been intensively studied in the past three decades and is now considered to be an important indirect mechanism for the control of receptor function. The allosteric site on the GABA(A) receptor is the target for the most widely prescribed sleep medicines, the benzodiazepines. Cinacalcet, an allosteric enhancer of the calcium-sensing receptor, is used to treat secondary hyperparathyroidism. Allosteric ligands might be especially valuable to control receptors for which the design of selective orthosteric agonists or antagonists has been elusive, such as muscarinic acetylcholine receptors.

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.

The existence of a second allosteric site on the M1 muscarinic acetylcholine receptor and its implications for drug design

Fully flexible docking of KT5720, an allosteric modulator of the muscarinic receptors, was performed on a dynamic model of the M(1) muscarinic acetylcholine receptor. The results confirmed the existence of a second allosteric site, located on the intracellular face of the receptor. These results would be beneficial for the design of modulators of this receptor to be used as an effective alternative against the Alzheimer's disease.

Identification of multiple allosteric sites on the M1muscarinic acetylcholine receptor

Staurosporine and four staurosporine derivatives were docked on the rhodopsin-based homology model of the M1 muscarinic acetylcholine receptor in order to localize the possible allosteric sites of this receptor. It was found that there were three major allosteric sites, two of which are located at the extracellular face of the receptor, and one in the intracellular domain of the receptor. In the present study, the localization of these binding sites is described for the first time. The present study confirms the existence of multiple allosteric sites on the M1 muscarinic receptor, and lays the ground for further experimental and computational analysis to better understand how muscarinic receptors are modulated via their allosteric sites. These findings will also help to design and develop novel drugs acting as allosteric modulators of the M1 receptor, which can be used in the treatment of the Alzheimer's disease.

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.

Allosteric binding sites on muscarinic acetylcholine receptors

In this issue of Molecular Pharmacology, Tränkle et al. (p. 1597) present new findings regarding the existence of a second allosteric site on the M2 muscarinic acetylcholine receptor (M2 mAChR). The M2 mAChR is a prototypic class A G protein-coupled receptor (GPCR) that has proven to be a very useful model system to study the molecular mechanisms involved in the binding of allosteric GPCR ligands. Previous studies have identified several allosteric muscarinic ligands, including the acetylcholinesterase inhibitor tacrine and the bis-pyridinium derivative 4,4'-bis-[(2,6-dichloro-benzyloxy-imino)-methyl]-1,1'-propane-1,3-diyl-bis-pyridinium dibromide (Duo3), which, in contrast to conventional allosteric muscarinic ligands, display concentration-effect curves with slope factors >1. By analyzing the interactions of tacrine and Duo3 with other allosteric muscarinic agents predicted to bind to the previously identified ;common' allosteric binding site, Tränkle et al. provide evidence suggesting that two allosteric agents and one orthosteric ligand may be able to bind to the M2 mAChR simultaneously. Moreover, studies with mutant mAChRs indicated that the M2 receptor epitopes involved in the binding of tacrine and Duo3 may not be identical. Molecular modeling and ligand docking studies suggested that the additional allosteric site probably represents a subdomain of the receptor's allosteric binding cleft. Because allosteric binding sites have been found on many other GPCRs and drugs interacting with these sites are thought to have great therapeutic potential, the study by Tränkle et al. should be of considerable general interest.

Atypical Muscarinic Allosteric Modulation: Cooperativity between Modulators and Their Atypical Binding Topology in Muscarinic M2and M2/M5Chimeric Receptors

The binding and function of muscarinic acetylcholine receptors can be modulated allosterically. Some allosteric muscarinic ligands are "atypical", having steep concentration-effect curves and not interacting competitively with "typical" allosteric modulators. For atypical agents, a second allosteric site has been proposed. Different approaches have been used to gain further insight into the interaction with M2 receptors of two atypical agents, tacrine and the bispyridinium compound 4,4'-bis-[(2,6-dichloro-benzyloxy-imino)-methyl]-1,1'-propane-1,3-diyl-bispyridinium dibromide (Duo3). Interaction studies, using radioligand binding assays and the allosteric ligands obidoxime, Mg2+, and the new tool hexamethonium to antagonize the allosteric actions of the atypical ligands, showed different modes of interaction for tacrine and Duo3 at M2 receptors. A negatively cooperative interaction was observed between hexamethonium and tacrine (but not Duo3). A tacrine dimer that exhibited increased allosteric potency relative to tacrine but behaved like a typical allosteric modulator was competitively inhibited by hexamethonium. M2/M5-receptor mutants revealed a dependence of tacrine and Duo3 affinity on different receptor epitopes. This was confirmed by docking simulations using a three-dimensional model of the M2 receptor. These showed that the allosteric site could accommodate two molecules of tacrine simultaneously but only one molecule of Duo3, which binds in different mode from typical allosteric agents. Therefore, the atypical actions of tacrine and Duo3 involve different modes of receptor interaction, but their sites of attachment seem to be the "common" allosteric binding domain at the entrance to the orthosteric ligand binding pocket of the M2-receptor. Additional complex behavior may be rationalized by allosteric interactions transmitted within a receptor dimer.

Nonpeptide and Peptide Growth Hormone Secretagogues Act Both as Ghrelin Receptor Agonist and as Positive or Negative Allosteric Modulators of Ghrelin Signaling

Two nonpeptide (L692,429 and MK-677) and two peptide [GH-releasing peptide (GHRP)-6 and ghrelin] agonists were compared in binding and in signal transduction assays: calcium mobilization, inositol phosphate turnover, cAMP-responsive element (CRE), and serum-responsive element (SRE) controlled transcription, as well as arrestin mobilization. MK-677 acted as a simple agonist having an affinity of 6.5 nm and activated all signal transduction systems with similar high potency (0.2–1.4 nm). L-692,429 also displayed a very similar potency in all signaling assays (25–60 nm) but competed with a 1000-fold lower apparent affinity for ghrelin binding and surprisingly acted as a positive allosteric receptor modulator by increasing ghrelin’s potency 4- to 10-fold. In contrast, the potency of GHRP-6 varied 600-fold (0.1–61 nm) depending on the signal transduction assay, and it acted as a negative allosteric modulator of ghrelin signaling. Unexpectedly, the maximal signaling efficacy for ghrelin was increased above what was observed with the hormone itself during coadministration with the nonendogenous agonists. It is concluded that agonists for the ghrelin receptor vary both in respect of their intrinsic agonist properties and in their ability to modulate ghrelin signaling. A receptor model is presented wherein ghrelin normally only activates one receptor subunit in a dimer and where the smaller nonendogenous agonists bind in the other subunit to act both as coagonists and as either neutral (MK-677), positive (L-692,429), or negative (GHRP-6) modulators of ghrelin function. It is suggested that an optimal drug candidate could be an agonist that also is a positive modulator of ghrelin signaling.

Asparagine, Valine, and Threonine in the Third Extracellular Loop of Muscarinic Receptor Have Essential Roles in the Positive Cooperativity of Strychnine-Like Allosteric Modulators

We have investigated allosteric interactions of four closely related strychnine-like substances: Wieland-Gumlich aldehyde (WGA), propargyl Wieland-Gumlich aldehyde, strychnine, and brucine with N-methylscopolamine (NMS) on M(3) subtype of muscarinic receptor genetically modified in the second or the third extracellular loop to corresponding loops of M(2) subtype (M(3)o2 and M(3)o3 chimera). The M(3)o2 chimeric receptor The exhibited no change in either affinity of strychnine, brucine, and WGA or in cooperativity of brucine or WGA, whereas both parameters for propargyl-WGA changed. In contrast, there was a change in affinity of all tested modulators (except for brucine) and in their cooperativity in the M(3)o3 chimera. Directions of affinity changes in both chimeras were always toward values of the donor M(2) subtype, but changes in cooperativity were variable. Compared with the native M(3) receptor, strychnine displayed a slight increase in positive cooperativity and propargyl-WGA a robust decrease in negative cooperativity at M(3)o2 chimera. Similar changes were found in the M(3)o3 chimera. Interestingly, cooperativity of brucine and WGA at the M(3)o3 chimera changed from negative to positive. This is the first evidence of constitution of positive cooperativity of WGA by switching sequences of two parental receptors, both exhibiting negative cooperativity. Gradual replacement of individual amino acids revealed that only three residues (NVT of the o3 loop of the M(2) receptor) are involved in this effect. Data suggest that these amino acids are essential for propagation of a conformation change resulting in positive cooperativity induced by these modulators.

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.

Allosteric modulation of G-protein coupled receptors

The superfamily of G-protein coupled receptors (GPCRs) has more than 1000 members and is the largest family of proteins in the body. GPCRs mediate signalling of stimuli as diverse as light, ions, small molecules, peptides and proteins and are the targets for many pharmaceuticals. Most GPCR ligands are believed to activate (agonists) or inhibit (competitive antagonists) receptor signalling by binding the receptor at the same site as the endogenous agonist, the orthosteric site. In contrast, allosteric ligands modulate receptor function by binding to different regions in the receptor, allosteric sites. In recent years, combinatorial chemistry and high throughput screening have helped identify several allosteric GPCR modulators with novel structures, several of which already have become valuable pharmacological tools and may be candidates for clinical testing in the near future. This mini review outlines the current status and perspectives of allosteric modulation of GPCR function with emphasis on the pharmacology of endogenous and synthesised modulators, their receptor interactions and the therapeutic prospects of allosteric ligands compared to orthosteric ligands.

Vinburnine decelerates [3H]N-methylscopolamine binding to recombinant human muscarinic M1–M4 acetylcholine receptors

The kinetics of [3H]N-methylscopolamine binding to membranes of Chinese hamster ovary (CHO) cells expressing muscarinic M(1)-M(4) acetylcholine receptors was studied. [3H]N-methylscopolamine dissociation was used for the "single-point" analysis of allosteric modulation by vinburnine (L-eburnamonine). [3H]N-methylscopolamine dissociation was decelerated by vinburnine with EC(50) values of 29.5, 4.1, 9.5 and 15.0 microM for muscarinic M(1)-M(4) receptors, respectively. Acetylcholine doubled the EC(50) of vinburnine for muscarinic M(3) receptors. These kinetic EC(50) values correlated with equilibrium binding constants, supporting the ternary allosteric model. Vinburnine also decelerated the association of [3H]N-methylscopolamine binding, resulting in opposite cooperativity for muscarinic M(1) and M(2) receptors.

Allosteric site on muscarinic acetylcholine receptors: identification of two amino acids in the muscarinic M2 receptor that account entirely for the M2/M5 subtype selectivities of some structurally diverse allosteric ligands in N-methylscopolamine-occupied receptors

Two epitopes have been identified recently to be responsible for the high-affinity binding of alkane-bisammonium and caracurine V type allosteric ligands to N-methylscopolamine (NMS)-occupied M2 muscarinic acetylcholine receptors, relative to M5 receptors: the amino acid M2-Thr423 at the top of transmembrane region (TM) 7 and an epitope comprising the second extracellular loop (o2) of the M2 receptor including the flanking regions of TM4 and TM5. We aimed to find out whether a single amino acid could account for the contribution of this epitope to binding affinity. Allosteric interactions were investigated in wild-type and mutant receptors in which the orthosteric binding site was occupied by [3H]NMS (5 mM Na,K,Pi buffer, pH 7.4, 23 degrees C). Using M2/M5 chimeric and point-mutated receptors, the relevant epitope was narrowed down to M2-Tyr177. A double point-mutated M2 receptor in which both M2-Tyr177 and M2-Thr423 were replaced by the corresponding amino acids of M5 revealed that these two amino acids account entirely for the (approximately 100-fold) M2/M5 selectivity of the alkane-bisammonium and the caracurine V type allosteric ligands. At NMS-free M2 receptors, the caracurine V derivative also displayed approximately 100-fold M2/M5 selectivity, but the double point mutation reduced the M2 affinity by only approximately 10-fold; thus, additional epitopes may influence selectivity for the free receptors. A three-dimensional model of the M2 receptor was used to simulate allosteric agent docking to NMS-occupied receptors. M2-Tyr177 and M2-Thr423 seem to be located near the junction of the allosteric and the orthosteric areas of the M2 receptor ligand binding cavity.

Interactions of orthosteric and allosteric ligands with [3H]dimethyl-W84 at the common allosteric site of muscarinic M2 receptors

An optimized assay for the binding of [3H]dimethyl-W84 to its allosteric site on M2 muscarinic receptors has been used to directly measure the affinities of allosteric ligands. Their potencies agree with those deduced indirectly by their modulation of the equilibrium binding and kinetics of [3H]N-methylscopolamine ([3H]NMS) binding to the orthosteric site. The affinities and cooperativities of orthosteric antagonists with [3H]dimethyl-W84 have also been quantitated. These affinities agree with those measured directly in a competition assay using [3H]NMS. All these data are compatible with the predictions of the allosteric ternary complex model. The association and dissociation kinetics of [3H]dimethyl-W84 are rapid but the estimate of its association rate constant is nevertheless comparable with that found for the orthosteric radioligand, [3H]NMS. This is unexpected, given that the allosteric site to which [3H]dimethyl-W84 binds is thought to be located on the external face of the receptor and above the [3H]NMS binding site that is buried within the transmembrane helices. The atypical allosteric ligands tacrine and 4,4'-bis-[(2,6-dichloro-benzyloxy-imino)-methyl]-1,1'-propane-1,3-diyl-bis-pyridinium dibromide (Duo3) inhibit [3H]dimethyl-W84 binding with the same potencies and comparably steep slope factors as found for inhibition of [3H]NMS binding. Tacrine and Duo3 decrease [3H]dimethyl-W84 affinity, not the number of binding sites. It is suggested that these atypical ligands either bind to the two known spatially separated allosteric sites on muscarinic receptors with positive cooperativity or their binding to the common allosteric site modulates receptor-receptor interactions such that homotropic positive cooperativity within a dimer or higher oligomer is generated.

Agonist actions of "beta-blockers" provide evidence for two agonist activation sites or conformations of the human beta1-adrenoceptor

Previous work with 4-[3-[(1,1-dimethylethyl)amino]2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one (CGP 12177) has led to the suggestion that there are two different agonist conformations of the human beta1-adrenoceptor: 1) where classic agonists (catecholamines) and beta-antagonists act, and 2) where CGP 12177 is an agonist and relatively resistant to inhibition by beta-adrenoceptor antagonists. In the present study, we have used studies of cAMP response element-regulated gene transcription to confirm the presence of these two beta1-adrenoceptor sites/conformations and to provide strong evidence that a range of clinically used beta-adrenoceptor blockers (beta-blockers) exhibit differential agonists and/or antagonist actions at the two sites. [2-(3-Carbamoyl-4-hydroxyphenoxy)-ethylamino]-3-[4-(1-methyl-4-trifluormethyl-2-imidazolyl)-phenoxy]-2-propanolmethanesulphonate (CGP 20712A) and atenolol act as classic antagonists at the catecholamine binding site but have much lower affinity for the secondary CGP 12177 site. CGP 12177 and carvedilol are potent antagonists at the catecholamine site but mediate substantial agonist actions on gene transcription via the secondary antagonist-resistant site at higher concentrations. Agonist effects of beta-blockers are not, however, confined to this secondary site, and we show that some (particularly acebutolol and labetolol) act primarily via the catecholamine site, whereas others (pindolol and alprenolol) can stimulate both. The different responses to beta-blockers seen in the clinic may therefore be caused in part by these beta-blocker agonist responses and the differential activation of the two sites or conformations.

Regulation of muscarinic acetylcholine receptor signaling

Multiple mechanisms regulate the signaling of the five members of the family of the guanine nucleotide binding protein (G protein)-coupled muscarinic acetylcholine (ACh) receptors (mAChRs). Following activation by classical or allosteric agonists, mAChRs can be phosphorylated by a variety of receptor kinases and second messenger-regulated kinases. The phosphorylated mAChR subtypes can interact with beta-arrestin and presumably other adaptor proteins as well. As a result, the various mAChR signaling pathways may be differentially altered, leading to short-term or long-term desensitization of a particular signaling pathway, receptor-mediated activation of the mitogen-activated protein kinase pathway downstream of mAChR phosphorylation, as well as long-term potentiation of mAChR-mediated phospholipase C stimulation. Agonist activation of mAChRs may also induce receptor internalization and down-regulation, which proceed in a highly regulated manner, depending on receptor subtype and cell type. In this review, our current understanding of the complex regulatory processes that underlie signaling of mAChR is summarized.

Kinase inhibitors: not just for kinases anymore

Kinase inhibitors are widely employed as biological reagents and as leads for drug design. Their use is often complicated by their lack of specificity. Although binding conserved ATP sites accounts for some of their nonspecificity, some compounds inhibit proteins not known to bind ATP. It has been found that promiscuous hits from high-throughput screening may act as aggregates. To explore whether this mechanism might explain the action of widely used nonspecific kinase inhibitors, 15 such compounds were studied. Eight of these, rottlerin, quercetin, K-252c, bisindolylmaleimide I, bisindolylmaleimide IX, U0126, indirubin, and indigo, inhibited three diverse non-kinase enzymes. Inhibition was time-dependent and sensitive to enzyme concentration; by light scattering, the compounds formed particles of 100-1000 nm diameter. These observations suggest that these eight kinase inhibitors, at least at micromolar concentrations, are promiscuous and act as aggregates. Results obtained from the use of these compounds at micromolar or higher concentrations against individual enzymes should be interpreted cautiously.

Allosteric drugs acting at muscarinic acetylcholine receptors

The binding properties of muscarinic acetylcholine receptors are affected by various drugs acting at a second (allosteric) binding site, usually (but not always) at supratherapeutic concentrations. Allosteric drugs acting at GABA receptors present advantages over competitive drugs; this explains the interest raised by allosteric effects on muscarinic receptors. A theoretical and practicable definition of allosteric drugs acting at muscarinic receptors will be given in this work, together with a summary of recent data concerning the number, position, and structural requirements of their binding sites.

Cooperative interactions at M2 muscarinic acetylcholine receptors: structure/activity relationships in stepwise shortened bispyridinium- and bis(ammonio)alkane-type allosteric modulators

Muscarinic M2-receptors allow for divergent modes of allosteric action, depending on the structure of the allosteric modulator. Phthalimido-substituted bis(ammonio)alkane-type modulators belong to the common mode allosteric agents, whereas a physicochemically closely related bispyridinium-oxime with dichlorobenzyl-substituents at both ends is an atypical agent. Here, we compared the actions of stepwise shortened compounds composed of the phthalimido moiety and middle chains of either the bispyridinium- or the bis(ammonio)alkane-type. Allosteric interactions were measured in pig M2 receptors with the orthosteric probe [3H]N-methylscopolamine ([3H]NMS) to label the acetylcholine binding site of the receptors. Dissociation and equilibrium binding experiments revealed parallel structure/activity-relationships in both series of compounds with regard to the cooperativity of interaction with [3H]NMS and to the underlying binding affinities in radioligand-occupied and free receptors. In conclusion, the findings are in line with the hypothesis that the phthalimido-moiety, but not the middle chain, is pivotal for the topology of interaction with the M2-receptor protein.

Chemical synthesis of MT1 and MT7 muscarinic toxins: critical role of Arg-34 in their interaction with M1 muscarinic receptor

Two muscarinic toxins, MT1 and MT7, were obtained by one-step solid-phase synthesis using the 9-fluorenylmethoxycarbonyl-based method. The synthetic and natural toxins, isolated from the snake venom or recombinantly expressed, display identical physicochemical properties and pharmacological profiles. High protein recovery allowed us to specify the selectivity of these toxins for various muscarinic receptor subtypes. Thus, sMT7 has a selectivity for the M1 receptor that is at least 20,000 times that for the other subtypes. The stability of the toxin-receptor complexes indicates that sMT1 interacts reversibly with the M1 receptor, unlike sMT7, which binds it quasi-irreversibly. The effect of the synthetic toxins on the atropine-induced [3H]N-methylscopolamine (NMS) dissociation confirms that sMT7 targets the allosteric site on the M1 receptor, whereas sMT1 seems interact on the orthosteric one. The great decreases in the binding potencies observed after the R34A modification in sMT1 and sMT7 toxins highlight the functional role of this conserved residue in their interactions with the M1 receptor. Interestingly, after the R34A modification, the sMT7 toxin binds reversibly on the M1 receptor. Furthermore, the potency of sMT7-R34A for the NMS-occupied receptor is lower compared with unmodified toxin, supporting the role of this residue in the allosteric interaction of sMT7. All these results and the different charge distributions observed at the two toxin surfaces of their structure models support the hypothesis that the two toxins recognize the M1 receptor differently.

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.

Metal ion-mediated agonism and agonist enhancement in melanocortin MC1 and MC4 receptors

An endogenous metal-ion site in the melanocortin MC1 and MC4 receptors was characterized mainly in transiently transfected COS-7 cells. ZnCl(2) alone stimulated signaling through the Gs pathway with a potency of 11 and 13 microm and an efficacy of 50 and 20% of that of alpha-melanocortin stimulating hormone (alpha-MSH) in the MC1 and MC4 receptors, respectively. In the presence of peptide agonist, Zn(II) acted as an enhancer on both receptors, because it shifted the dose-response curves to the left: most pronounced was a 6-fold increase in alpha-MSH potency on the MC1 receptor. The effect of the metal ion appeared to be additive, because the maximal cAMP response for alpha-MSH in the presence of Zn(II) was 60% above the maximal response for the peptide alone. The affinity of Zn(II) could be increased through binding of the metal ion in complex with small hydrophobic chelators. The binding affinities and profiles were similar for a number of the 2,2'-bipyridine and 1,10-phenanthroline analogs in complex with Zn(II) in the MC1 and MC4 receptors. However, the potencies and efficacies of the metal-ion complexes were very different in the two receptors, and close to full agonism was obtained in the MC1 receptor. Metal ion-chelator complexes having antagonistic properties were also found. An initial attempt to map the metal-ion binding site in the MC1 receptor indicated that Cys(271) in extracellular loop 3 and possibly Asp(119) at the extracellular end of TM-III, which are both conserved among all MC receptors, are parts of the site. It is concluded that the function of the MC1 and MC4 receptors can be positively modulated by metal ions acting both as partial agonists and as potentiators for other agonists, including the endogenous peptide ligand alpha-MSH at Zn(II) concentrations that could be physiological. Furthermore, the metal ion-chelator complexes may serve as leads in the development of novel melanocortin receptor modulators.

Towards a high-affinity allosteric enhancer at muscarinic M1 receptors

Loss of forebrain acetylcholine (ACh) is an early neurochemical lesion in Alzheimer's Disease (AD), and muscarinic receptors for ACh are involved in memory and cognition, so a muscarinic agonist could provide 'replacement therapy' in this disease. Muscarinic receptors, which couple to G-proteins, occur throughout the CNS, and in the periphery they mediate the responses of the parasympathetic nervous system, so selectivity is crucial. The five subtypes of muscarinic receptor, M1-M5, have a distinct regional distribution, with M2 and M3 mediating most of the peripheral effects, M2 predominating in hindbrain areas, and M1 predominating in the cortex and hippocampus--the brain regions most associated with memory and cognition, which has lead to a search for a truly M1-selective muscarinic agonist. That search has so far been unsuccessful, but acetylcholinesterase inhibitors such as donepezil (Aricept), which potentiate cholinergic neurotransmission, have a therapeutic role in the management of AD; so the M1 receptor remains a therapeutic target. Our approach is to develop allosteric enhancers--compounds which bind to the receptor at an 'allosteric' site which is distinct from the 'primary' site to which the endogenous ligand binds, and which enhance the affinity (or efficacy) of the endogenous ligand. We have developed radioligand binding assays and analyses for the detection and quantitatitation of allosteric interactions of a test agent with labelled and unlabelled 'primary' ligands, and we report here some results of the initial phase of a chemical synthesis project to develop potent and selective allosteric enhancers at muscarinic M1 receptors.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

Discovery of an ectopic activation site on the M(1) muscarinic receptor

Receptors have well-conserved regions that are recognized and activated by hormones and neurotransmitters. Most drugs bind to these sites and mimic or block the action of the native ligands. Using a high-throughput functional screen, we identified a potent and selective M(1) muscarinic receptor agonist from a novel structural class. Using a series of chimeric receptors, we demonstrated that this ligand activates the receptor through a region that is not conserved among receptor subtypes, explaining its unprecedented selectivity. This region of the receptor is distinct from the conserved region that is recognized by traditional ligands. The finding that receptors for small-molecule transmitters can have multiple, structurally distinct activation sites has broad implications for the study of receptor structure/function and the potential for the discovery of novel ligands with high selectivity.

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.

Seven-transmembrane receptors: crystals clarify

The X-ray structure of the photoreceptor rhodopsin has provided the first atomic-resolution structure of a seven-transmembrane (7-TM) G-protein-coupled receptor. This has provided an improved template for interpreting the huge body of structure--activity, mutagenesis and affinity labelling data available for related 7-TM receptors, such as muscarinic acetylcholine receptors. Ligand contacts, and the intramolecular interactions that stabilize the ground state structure, can be identified with some degree of confidence. We now have a firm basis for attempts to predict the structure of the receptor--G-protein complex, and understand the mechanism by which the agonist--receptor complex activates the G protein.

Allosteric site on muscarinic acetylcholine receptors: a single amino acid in transmembrane region 7 is critical to the subtype selectivities of caracurine V derivatives and alkane-bisammonium ligands

Diverse muscarinic allosteric ligands exhibit greatest affinity toward the M2 receptor subtype and lowest affinity toward M5. In this study, we evaluated the potencies with which two groups of highly M2/M5 selective allosteric agents modulate the dissociation of [3H]N-methylscopolamine from M2/M5 chimeric and point-mutated receptors. These allosteric ligands included two alkane-bisammonium compounds and a series of caracurine V derivatives, which are structurally closely related to (but stereochemically different from) the prototype allosteric ligand alcuronium. Like alcuronium, the caracurine V and alkane-bisammonium compounds displayed significantly increased affinities compared with M5 toward the chimera that included the M2 second outer loop (o2) plus surrounding regions. Unlike alcuronium, the compounds had enhanced affinities for a chimera with M2 sequence in transmembrane region (TM) 7; site-directed mutagenesis in wild-type and chimeric receptors indicated that the threonine residue at M2(423) was entirely responsible for the sensitivity toward TM7. Subsequent studies demonstrated that this TM7 epitope is likewise present in the M4 receptor, as M4(436)serine. The M2(423)threonine residue is near the M2(419)asparagine identified previously to influence gallamine binding. These studies demonstrate that a stereochemical difference can be sufficient to translate into divergent epitope sensitivities. Nonetheless, these allosteric ligands seem to derive affinity from two main regions of the receptor: o2 plus flanking regions and o3/TM7. These two epitopes are sufficient to explain the M2/M5 selectivity of the presently investigated compounds; this is the first time that the subtype selectivity of muscarinic allosteric agents has been completely accounted for by distinct receptor epitopes.

Evidence that the anti-spasmogenic effect of the beta-adrenoceptor agonist, isoprenaline, on guinea-pig trachealis is not mediated by cyclic AMP-dependent protein kinase

1. The spasmolytic and anti-spasmogenic activity of beta-adrenoceptor agonists on airways smooth muscle is thought to involve activation of the cyclic AMP/cyclic AMP-dependent protein kinase (PKA) cascade. Here we have tested the hypothesis that PKA mediates the anti-spasmogenic activity of isoprenaline and other cyclic AMP-elevating agents in guinea-pig isolated trachea by utilizing a number of cell permeant cyclic AMP analogues that act as competitive 'antagonists' of PKA. 2. Anion-exchange chromatography of guinea-pig tracheae resolved two peaks of PKA activity that corresponded to the type I ( approximately 5%) and type II ( approximately 93%) isoenzymes. 3. Pre-treatment of tracheae with zardaverine (30 microM), vasoactive intestinal peptide (VIP) (1 microM) and the non-selective activator of PKA, Sp-8-CPT-cAMPS (10 microM), produced a non-parallel rightwards shift in the concentration-response curves that described acetylcholine (ACh)-induced tension generation. The type II-selective PKA inhibitor, Rp-8-CPT-cAMPS (300 microM), abolished this effect. 4. Pre-treatment of tracheae with Sp-8-Br-PET-cGMPS (30 microM) produced a non-parallel rightwards shift of the concentration-response curves that described ACh-induced tension generation. The selective cyclic GMP-dependent protein kinase (PKG) inhibitor, Rp-8-pCPT-cGMPS (300 microM), abolished this effect. 5. Pre-treatment of tracheae with isoprenaline (1 microM) produced a 10 fold shift to the right of the ACh concentration-response curve by a mechanism that was unaffected by Rp-8-Br-cAMPS (300 microM, selective inhibitor of type I PKA), Rp-8-CPT-cAMPS (300 microM) and Rp-8-pCPT-cGMPS (300 microM). 6. We conclude that the anti-spasmogenic activity of Sp-8-CPT-cAMPS, zardaverine and VIP in guinea-pig trachea is attributable to activation of the cyclic AMP/PKA cascade whereas isoprenaline suppresses ACh-induced contractions by a mechanism(s) that is independent of PKA and PKG.

The endocannabinoid nervous system: unique opportunities for therapeutic intervention

The active principle in marijuana, Delta(9)-tetrahydrocannabinol (THC), has been shown to have wide therapeutic application for a number of important medical conditions, including pain, anxiety, glaucoma, nausea, emesis, muscle spasms, and wasting diseases. Delta(9)-THC binds to and activates two known cannabinoid receptors found in mammalian tissue, CB1 and CB2. The development of cannabinoid-based therapeutics has focused predominantly on the CB1 receptor, based on its predominant and abundant localization in the CNS. Like most of the known cannabinoid agonists, Delta(9)-THC is lipophilic and relatively nonselective for both receptor subtypes. Clinical studies show that nonselective cannabinoid agonists are relatively safe and provide therapeutic efficacy, but that they also induce psychotropic side effects. Recent studies of the biosynthesis, release, transport, and disposition of anandamide are beginning to provide an understanding of the role of lipid transmitters in the CNS. This review attempts to link current understanding of the basic biology of the endocannabinoid nervous system to novel opportunities for therapeutic intervention. This new knowledge may facilitate the development of cannabinoid receptor-targeted therapeutics with improved safety and efficacy profiles.

Interactions between allosteric modulators and 4-DAMP and other antagonists at muscarinic receptors: potential significance of the distance between the N and carboxyl C atoms in the molecules of antagonists

Allosteric enhancement of the affinity of muscarinic receptors for their ligands offers a new way to influence cholinergic neurotransmission. The structure of the allosteric binding domain(s) and the features of agonists, antagonists and modulators which determine the occurrence of either positive or negative cooperativity require clarification. We tested interactions between allosteric modulators alcuronium, strychnine and brucine and eight antagonists at muscarinic receptors expressed in CHO cells. In experiments with unlabeled antagonists, all three modulators enhanced the affinity for 4-diphenylacetoxy-N-dimethylpiperidinium (4-DAMP) at the M2 receptors, and strychnine did so also at the M4 receptors. Positive interactions were also observed between alcuronium and L-hyoscyamine (M2) and scopolamine (M2), between strychnine and butylscopolamine (M4), L-hyoscyamine (M2 and M4) and scopolamine (M4), and between brucine and scopolamine (M2). Positive effects of alcuronium, strychnine and brucine on the affinity of the M2 receptors for 4-DAMP have been confirmed by direct measurements of the binding of [3H]-4-DAMP. A comparison of molecular models of several antagonists which are esters revealed that antagonists in which the distance between the N and the carboxyl C atoms corresponds to five chemical bonds are more likely to display positive cooperativity with alcuronium at the M2 receptors than the antagonists in which the N-carboxyl C distance corresponds to four chemical bonds.

The allosteric interaction of otenzepad (AF-DX 116) at muscarinic M2 receptors in guinea pig atria

The effects of the muscarinic receptor antagonist, otenzepad, in combination with the competitive antagonists N-methylscopolamine, dexetimide and atropine, or the allosteric modulators, C(7)/3'-phth, gallamine and alcuronium, were measured in the guinea pig electrically driven left atrium using the agonists, carbachol or acetylcholine. Otenzepad, in combination with C(7)/3'-phth or gallamine, gave concentration-ratios close to additive and in agreement with theoretical model predictions for combination of two allosteric modulators acting at a common site. However, when otenzepad was combined with alcuronium, dexetimide or N-methylscopolamine, supra-additive effects were observed. For either competitive antagonist in combination with otenzepad, the degree of supra-additivity was more evident after 2-h equilibration than after 40 min. When otenzepad was combined with atropine, no supra-additivity was observed with carbachol as the agonist, but was evident with acetylcholine. Otenzepad was also unable to fully inhibit [3H]N-methylscopolamine binding when the radioligand was employed at a concentration of approximately 100 x K(D). It is concluded that the action of otenzepad involves an allosteric site and a number of possibilities are discussed for its location.