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

Hybridization into a Bitopic Ligand Increased Muscarinic Receptor Activation for Isopilocarpine but Not for Pilocarpine Derivatives

Pilocarpine (1), a secondary metabolite of several Pilocarpus species, is a therapeutically used partial agonist of muscarinic acetylcholine receptors (mAChRs). The available pharmacological data and structure-activity relationships do not provide comparable data for all five receptor subtypes. In this study, pilocarpine (1), its epimer isopilocarpine (2), racemic analogues pilosinine (3) and desmethyl pilosinine (4), and the respective hybrid ligands with a naphmethonium fragment (5-C6 to 8-C6) were synthesized and analyzed in mini-G nano-BRET assays at the five mAChRs. In line with earlier studies, pilocarpine was the most active compound among the orthosteric ligands 1-4. Computational docking of pilocarpine and isopilocarpine to the active M2 receptor suggests that the trans-configuration of isopilocarpine leads to a loss of the hydrogen bond from the lactone carbonyl to N6.52, explaining the lower activity of isopilocarpine. Hybrid formation of pilocarpine (1) and isopilocarpine (2) led to an inverted activity rank, with the trans-configured isopilocarpine hybrid (6-C6) being more active. The hydrogen bond of interest is formed by the isopilocarpine hybrid (6-C6) but not by the pilocarpine hybrid (5-C6). Hybridization thus leads to a modified binding mode of the orthosteric moiety, as the binding mode of the hybrid is dominated by the high-affinity allosteric moiety.

Structural basis of lysophosphatidylserine receptor GPR174 ligand recognition and activation

Lysophosphatidylserine (LysoPS) is a lipid mediator that induces multiple cellular responses through binding to GPR174. Here, we present the cryo-electron microscopy (cryo-EM) structure of LysoPS-bound human GPR174 in complex with G_s protein. The structure reveals a ligand recognition mode, including the negatively charged head group of LysoPS forms extensive polar interactions with surrounding key residues of the ligand binding pocket, and the L-serine moiety buries deeply into a positive charged cavity in the pocket. In addition, the structure unveils a partially open pocket on transmembrane domain helix (TM) 4 and 5 for a lateral entry of ligand. Finally, the structure reveals a G_s engaging mode featured by a deep insertion of a helix 5 (αH5) and extensive polar interactions between receptor and αH5. Taken together, the information revealed by our structural study provides a framework for understanding LysoPS signaling and a rational basis for designing LysoPS receptor-targeting drugs.

Structure of the human galanin receptor 2 bound to galanin and Gq reveals the basis of ligand specificity and how binding affects the G-protein interface

Galanin is a neuropeptide expressed in the central and peripheral nervous systems, where it regulates various processes including neuroendocrine release, cognition, and nerve regeneration. Three G-protein coupled receptors (GPCRs) for galanin have been discovered, which is the focus of efforts to treat diseases including Alzheimer’s disease, anxiety, and addiction. To understand the basis of the ligand preferences of the receptors and to assist structure-based drug design, we used cryo-electron microscopy (cryo-EM) to solve the molecular structure of GALR2 bound to galanin and a cognate heterotrimeric G-protein, providing a molecular view of the neuropeptide binding site. Mutant proteins were assayed to help reveal the basis of ligand specificity, and structural comparison between the activated GALR2 and inactive hβ2AR was used to relate galanin binding to the movements of transmembrane (TM) helices and the G-protein interface.

Biased Profile of Xanomeline at the Recombinant Human M4 Muscarinic Acetylcholine Receptor

Many Food and Drug Administration (FDA)-approved drugs are structural analogues of the endogenous (natural) ligands of G protein-coupled receptors (GPCRs). However, it is becoming appreciated that chemically distinct ligands can bind to GPCRs in conformations that lead to different cellular signaling events, a phenomenon termed biased agonism. Despite this, the rigorous experimentation and analysis required to identify biased agonism are often not undertaken in most clinical candidates and go unrealized. Recently, xanomeline, a muscarinic acetylcholine receptor (mAChR) agonist, has entered phase III clinical trials for the treatment of schizophrenia. If successful, xanomeline will be the first novel FDA-approved antipsychotic drug in almost 50 years. Intriguingly, xanomeline's potential for biased agonism at the mAChRs and, in particular, the M₄ mAChR, the most promising receptor target for schizophrenia, has not been assessed. Here, we quantify the biased agonism profile of xanomeline and three other mAChR agonists in Chinese hamster ovary cells recombinantly expressing the M₄ mAChR. Agonist activity was examined across nine distinct signaling readouts, including the activation of five different G protein subtypes, ERK1/2 phosphorylation, β-arrestin recruitment, calcium mobilization, and cAMP regulation. Relative to acetylcholine (ACh), xanomeline was biased away from ERK1/2 phosphorylation and calcium mobilization compared to Gα_i2 protein activation. These findings likely have important implications for our understanding of the therapeutic action of xanomeline and call for further investigation into the in vivo consequences of biased agonism in drugs targeting the M₄ mAChR for the treatment of schizophrenia.

Insights into the mechanism of atropine's anti-myopia effects: evidence against cholinergic hyperactivity and modulation of dopamine release

Background and Purpose

The ability of the muscarinic cholinergic antagonist atropine to inhibit myopia development in humans and animal models would suggest that cholinergic hyperactivity may underlie myopic growth. To test this, we investigated whether cholinergic agonists accelerate ocular growth rates in chickens. Furthermore, we investigated whether atropine alters ocular growth by downstream modulation of dopamine levels, a mechanism postulated to underlie its antimyopic effects.

Experimental Approach

Muscarinic (muscarine and pilocarpine), nicotinic (nicotine) and non‐specific (oxotremorine and carbachol) cholinergic agonists were administered to chicks developing form‐deprivation myopia (FDM) or chicks that were otherwise untreated. Vitreal levels of dopamine and its primary metabolite 3,4‐dihydroxyphenylacetic acid (DOPAC) were examined using mass spectrometry MS in form‐deprived chicks treated with atropine (360, 15 or 0.15 nmol). Further, we investigated whether dopamine antagonists block atropine's antimyopic effects.

Key Results

Unexpectedly, administration of each cholinergic agonist inhibited FDM but did not affect normal ocular development. Atropine only affected dopamine and DOPAC levels at its highest dose. Dopamine antagonists did not alter the antimyopia effects of atropine.

Conclusion and Implications

Muscarinic, nicotinic and non‐specific cholinergic agonists inhibited FDM development. This indicates that cholinergic hyperactivity does not underlie myopic growth and questions whether atropine inhibits myopia via cholinergic antagonism. This study also demonstrates that changes in retinal dopamine release are not required for atropine's antimyopic effects. Finally, nicotinic agonists may represent a novel and more targeted approach for the cholinergic control of myopia as they are unlikely to cause the anterior segment side effects associated with muscarinic treatment.

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.

Angiotensin and biased analogs induce structurally distinct active conformations within a GPCR

Biased agonists of G protein-coupled receptors (GPCRs) preferentially activate a subset of downstream signaling pathways. In this work, we present crystal structures of angiotensin II type 1 receptor (AT1R) (2.7 to 2.9 angstroms) bound to three ligands with divergent bias profiles: the balanced endogenous agonist angiotensin II (AngII) and two strongly β-arrestin-biased analogs. Compared with other ligands, AngII promotes more-substantial rearrangements not only at the bottom of the ligand-binding pocket but also in a key polar network in the receptor core, which forms a sodium-binding site in most GPCRs. Divergences from the family consensus in this region, which appears to act as a biased signaling switch, may predispose the AT1R and certain other GPCRs (such as chemokine receptors) to adopt conformations that are capable of activating β-arrestin but not heterotrimeric G_q protein signaling.

Exploring Biased Agonism at FPR1 as a Means to Encode Danger Sensing

Ligand-based selectivity in signal transduction (biased signaling) is an emerging field of G protein-coupled receptor (GPCR) research and might allow the development of drugs with targeted activation profiles. Human formyl peptide receptor 1 (FPR1) is a GPCR that detects potentially hazardous states characterized by the appearance of N-formylated peptides that originate from either bacteria or mitochondria during tissue destruction; however, the receptor also responds to several non-formylated agonists from various sources. We hypothesized that an additional layer of FPR signaling is encoded by biased agonism, thus allowing the discrimination of the source of threat. We resorted to the comparative analysis of FPR1 agonist-evoked responses across three prototypical GPCR signaling pathways, i.e., the inhibition of cAMP formation, receptor internalization, and ERK activation, and analyzed cellular responses elicited by several bacteria- and mitochondria-derived ligands. We also included the anti-inflammatory annexinA1 peptide Ac2-26 and two synthetic ligands, the W-peptide and the small molecule FPRA14. Compared to the endogenous agonists, the bacterial agonists displayed significantly higher potencies and efficacies. Selective pathway activation was not observed, as both groups were similarly biased towards the inhibition of cAMP formation. The general agonist bias in FPR1 signaling suggests a source-independent pathway selectivity for transmission of pro-inflammatory danger signaling.

Luciferase complementation based-detection of G-protein-coupled receptor activity

Protein complementation assays (PCA) are used as pharmacological tools, enabling a wide array of applications, ranging from studies of protein-protein interactions to second messenger effects. Methods to detect activities of G protein-coupled receptors (GPCRs) have particular relevance for drug screening. Recent development of an engineered luciferase NanoLuc created the possibility of generating a novel PCA, which in turn could open a new avenue for developing drug screening assays. Here we identified a novel split position for NanoLuc and demonstrated its use in a series of fusion constructs to detect the activity of GPCRs. The split construct can be applied to a variety of pharmacological screening systems.

Cortical network dynamics are coupled with cognitive aging in rats

Changes in neuronal network activity and increased interindividual variability in memory are among the most consistent features of growing older. Here, we examined the relationship between these hallmarks of aging. Young and aged rats were trained on a water maze task where aged individuals reliably display an increased range of spatial memory capacities relative to young. Two weeks later, neuronal activity was induced pharmacologically with a low dose of pilocarpine and control animals received vehicle. Activity levels were proxied by quantifying the immediate early gene products Arc and c-Fos. While no relationship was observed between basal, resting activity, and individual differences in spatial memory in any brain region, pilocarpine-induced marker expression was tightly coupled with memory in all areas of the prefrontal cortex (PFC) and hippocampus examined. The nature of this association, however, differed across regions and in relation to age-related cognitive outcome. Specifically, in the medial PFC, induced activity was greatest in aged rats with cognitive impairment and correlated with water maze performance across all subjects. In the hippocampus, the range of induced marker expression was comparable between groups and similarly coupled with memory in both impaired and unimpaired aged rats. Together the findings highlight that the dynamic range of neural network activity across multiple brain regions is a critical component of neurocognitive aging.

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

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

The muscarinic agonist pilocarpine modifies cocaine-reinforced and food-reinforced responding in rats: comparison with the cholinesterase inhibitor tacrine

Activation of muscarinic receptors in the brain antagonizes the actions of cocaine, blocking both its discriminative stimulus and reinforcing properties. Pilocarpine is a nonselective muscarinic agonist that is used clinically, but has not been well characterized for its actions during cocaine-reinforced behavior. This study evaluated its effects on cocaine-reinforced and food-reinforced behaviors in rats, using the cholinesterase inhibitor tacrine as a comparator. Intraperitoneal pilocarpine or tacrine at doses of 1.0 mg/kg or more attenuated self-administration of low-dose cocaine (0.1 mg/kg injection) but also increased oral movements. Pilocarpine was less potent than tacrine in decreasing responding supported by low or intermediate amounts of liquid food. Combined treatment with pilocarpine and tacrine was more effective than either compound alone in attenuating self-administration of intermediate-dose cocaine. At a low (0.66 mg/kg) dose which did not modify reinforced responding, pilocarpine increased nonspecific behavior (sniffing, rearing, and activity) in cocaine-reinforced but not in food-reinforced animals; with greater doses increasing cholinergic or gastrointestinal signs. These effects were most consistently correlated with changes in reinforcement in rats responding for cocaine relative to food-reinforced animals. Overall, pilocarpine exhibited modest selectivity for attenuating self-administration of low-dose cocaine without affecting a nondrug reinforcer.

Repurposing Pilocarpine Hydrochloride for Treatment of Candida albicans Infections

Candida albicans is the most common human fungal pathogen with an estimated crude mortality rate of 40%. The ability of the organism to switch from the yeast to hyphal form and produce biofilms are important virulence factors. C. albicans infections are combatted by the host immune system. However, Candida triggers a strong inflammatory response that, if not appropriately regulated, can damage host tissues. Therefore, it is important that the host immune response eliminates the fungus but limits tissue damage. This study provides evidence that targeting cholinergic receptors cannot only curb the virulence of C. albicans by inhibiting filamentous growth and biofilm formation but can also appropriately regulate the host immune response to induce rapid clearance with limited damage to vital tissues. This article provides evidence that repurposing licensed drugs that target cholinergic receptors may offer novel therapeutic solutions for the prevention or treatment of fungal infections.

Acetylcholine modulates the virulence of Candida albicans and regulates an appropriate immune response to infection in a Galleria mellonella infection model. Indeed, the evidence suggests that C. albicans possesses a functional cholinergic receptor that can regulate filamentous growth and biofilm formation. Furthermore, G. mellonella immune cell subsets possess repertories of cholinergic receptors which regulate an effective and appropriate cellular immune response to C. albicans infection. This study aimed to investigate the cholinergic receptor subtype involved in regulation of filamentous growth and biofilm formation by C. albicans and determine the roles of cholinergic receptors in modulation of G. mellonella immune cell subsets. The general muscarinic receptor agonist, pilocarpine hydrochloride, inhibited C. albicans biofilm formation and pathogenicity, a phenomenon that could be reversed using the general muscarinic receptor antagonist, scopolamine. Pilocarpine hydrochloride protected G. mellonella larvae from C. albicans infection via inhibition of C. albicans filamentation and appropriate regulation of cellular immunity. However, scopolamine abrogated the capacity of pilocarpine hydrochloride to protect G. mellonella larvae from C. albicans infection. Furthermore, acetylcholine and pilocarpine hydrochloride exhibited differential modulatory capabilities on Galleria mellonella hemocyte responses to C. albicans. The data in this article demonstrate that a muscarinic receptor modulates C. albicans filamentation and biofilm formation. Furthermore, the results suggest that G. mellonella hemocyte subsets possess unique repertoires of cholinergic receptors that regulate their differentiation, activation, and function in contrasting manners. Therefore, targeting cholinergic receptors by repurposing currently licensed cholinergic drugs may offer novel therapeutic solutions for the prevention or treatment of fungal infections.

IMPORTANCECandida albicans is the most common human fungal pathogen with an estimated crude mortality rate of 40%. The ability of the organism to switch from the yeast to hyphal form and produce biofilms are important virulence factors. C. albicans infections are combatted by the host immune system. However, Candida triggers a strong inflammatory response that, if not appropriately regulated, can damage host tissues. Therefore, it is important that the host immune response eliminates the fungus but limits tissue damage. This study provides evidence that targeting cholinergic receptors cannot only curb the virulence of C. albicans by inhibiting filamentous growth and biofilm formation but can also appropriately regulate the host immune response to induce rapid clearance with limited damage to vital tissues. This article provides evidence that repurposing licensed drugs that target cholinergic receptors may offer novel therapeutic solutions for the prevention or treatment of fungal infections.

Analysis of Biased Agonism

Agonists and most natural ligands bind to receptors in their inactive state and quickly induce an active receptor conformation that initiates cell signaling. The active receptor state initiates signaling because of its structural complementariness with coupling proteins that activate signaling pathways, such as G proteins and G protein-coupled receptor kinases. Agonist bias refers to the propensity of an agonist to direct receptor signaling through one pathway relative to another. Thus, if the agonist exhibits much higher affinity for active state 1 compared to active state 2, it will cause a robust activation of receptor coupling protein 1 but not 2, and ultimately, a preferential stimulation of signaling pathway 1. Biased agonists are potentially more selective therapeutic agents because there are numerous cases where the therapeutic and adverse effects of an agonist are mediated by distinct pathways involving G proteins and β-arrestin. Given the mechanism for agonist bias, the most straightforward approach for quantifying bias involves the estimation of agonist affinity for the inactive receptor state and the active receptor states involved in signaling through different pathways. The approach provides quantitative estimates of the sensitivities of different signaling pathways, enabling one to determine to what extent the observed selectivity is caused by agonist or system bias. In addition, the approach is a powerful adjunct to in silico docking studies and can be applied to in vivo assays, structure-activity relationships, and the analysis of published agonist concentration-response curves.

Acetylcholine signaling system in progression of lung cancers

The neurotransmitter acetylcholine (ACh) acts as an autocrine growth factor for human lung cancer. Several lines of evidence show that lung cancer cells express all of the proteins required for the uptake of choline (choline transporter 1, choline transporter-like proteins) synthesis of ACh (choline acetyltransferase, carnitine acetyltransferase), transport of ACh (vesicular acetylcholine transport, OCTs, OCTNs) and degradation of ACh (acetylcholinesterase, butyrylcholinesterase). The released ACh binds back to nicotinic (nAChRs) and muscarinic receptors on lung cancer cells to accelerate their proliferation, migration and invasion. Out of all components of the cholinergic pathway, the nAChR-signaling has been studied the most intensely. The reason for this trend is due to genome-wide data studies showing that nicotinic receptor subtypes are involved in lung cancer risk, the relationship between cigarette smoke and lung cancer risk as well as the rising popularity of electronic cigarettes considered by many as a "safe" alternative to smoking. There are a small number of articles which review the contribution of the other cholinergic proteins in the pathophysiology of lung cancer. The primary objective of this review article is to discuss the function of the acetylcholine-signaling proteins in the progression of lung cancer. The investigation of the role of cholinergic network in lung cancer will pave the way to novel molecular targets and drugs in this lethal malignancy.

A method for the quantification of biased signalling at constitutively active receptors

BACKGROUND AND PURPOSE

Biased agonism, the ability of an agonist to differentially activate one of several signal transduction pathways when acting at a given receptor, is an increasingly recognized phenomenon at many receptors. The Black and Leff operational model lacks a way to describe constitutive receptor activity and hence inverse agonism. Thus, it is impossible to analyse the biased signalling of inverse agonists using this model. In this theoretical work, we develop and illustrate methods for the analysis of biased inverse agonism.

EXPERIMENTAL APPROACH

Methods were derived for quantifying biased signalling in systems that demonstrate constitutive activity using the modified operational model proposed by Slack and Hall. The methods were illustrated using Monte Carlo simulations.

KEY RESULTS

The Monte Carlo simulations demonstrated that, with an appropriate experimental design, the model parameters are 'identifiable'. The method is consistent with methods based on the measurement of intrinsic relative activity (RA_i ) (ΔΔlogR or ΔΔlog(τ/K_a )) proposed by Ehlert and Kenakin and their co-workers but has some advantages. In particular, it allows the quantification of ligand bias independently of 'system bias' removing the requirement to normalize to a standard ligand.

CONCLUSIONS AND IMPLICATIONS

In systems with constitutive activity, the Slack and Hall model provides methods for quantifying the absolute bias of agonists and inverse agonists. This provides an alternative to methods based on RA_i and is complementary to the ΔΔlog(τ/K_a ) method of Kenakin et al. in systems where use of that method is inappropriate due to the presence of constitutive activity.

Establishment of a radiotelemetric recording technique in mice to investigate gastric slow waves: Modulatory role of putative neurotransmitter systems

NEW FINDINGS

What is the central question of this study? Gastric slow waves originating from the interstitial cells of Cajal-smooth muscle syncytium are usually studied in culture or in tissue segments, but nobody has described recordings of slow waves from awake, freely moving mice. Can radiotelemetry be used to record slow waves, and do they respond predictably to drug treatment? What is the main finding and its importance? Radiotelemetry can be used to record slow waves from awake, freely moving mice, permitting an examination of drug actions in vivo, which is crucial to drug discovery projects for characterizing the effects of drugs and metabolites on gastrointestinal function.

ABSTRACT

The mouse is the most commonly used species in preclinical research, and isolated tissues are used to study slow waves from the interstitial cells of Cajal-smooth muscle syncytium of the gastrointestinal tract. The aim of this study was to establish a radiotelemetric technique in awake mice to record gastric myoelectric activity from the antrum to gain insight into the effects of endogenous modulatory systems on slow waves. Under general anaesthesia, two biopotential wires from a telemetry transmitter were sutured into the antrum of male ICR (imprinting control region) mice. The animals were allowed 1 week to recover from surgery before the i.p. administration of drugs to stimulate or inhibit slow waves. The basal dominant frequency of slow waves was 6.96 ± 0.43 c.p.m., and the percentages of power in the bradygastric, normogastric and tachygastric ranges were 6.89 ± 0.98, 37.32 ± 1.72 and 34.38 ± 0.77%, respectively (n = 74). Nicotine at 1 mg kg^-1 increased normogastric power, but at 3 mg kg^-1 it increased bradygastric power (P < 0.05). Metoclopramide at 10 mg kg^-1 increased normogastric power; sodium nitroprusside at 10 mg kg^-1 had latent effects on tachygastric power (P < 0.05); and l-NAME at 10 mg kg^-1 had no effect (P > 0.05). Nicotine and bethanechol also caused varying degrees of hypothermia (>1°C reductions; P < 0.05). In conclusion, radiotelemetry can be used to record slow waves from awake, freely moving mice. In light of our findings, we recommend that studies assessing slow waves should also assess body temperature simultaneously.

A Complementary Scale of Biased Agonism for Agonists with Differing Maximal Responses

Compelling data in the literature from the recent years leave no doubt about the pluridimensional nature of G protein-coupled receptor function and the fact that some ligands can couple with different efficacies to the multiple pathways that a receptor can signal through, a phenomenon most commonly known as functional selectivity or biased agonism. Nowadays, transduction coefficients (log(τ/K_A)), based on the Black and Leff operational model of agonism, are widely used to calculate bias. Nevertheless, combining both affinity and efficacy in a single parameter can result in compounds showing a defined calculated bias of one pathway over other though displaying varying experimental bias preferences. In this paper, we present a novel scale (log(τ)), that attempts to give extra substance to different compound profiles in order to better classify compounds and quantify their bias. The efficacy-driven log(τ) scale is not proposed as an alternative to the affinity&efficacy-driven log(τ/K_A) scale but as a complement in those situations where partial agonism is present. Both theoretical and practical approaches using μ-opioid receptor agonists are presented.

Systematic errors in detecting biased agonism: Analysis of current methods and development of a new model-free approach

Discovering biased agonists requires a method that can reliably distinguish the bias in signalling due to unbalanced activation of diverse transduction proteins from that of differential amplification inherent to the system being studied, which invariably results from the non-linear nature of biological signalling networks and their measurement. We have systematically compared the performance of seven methods of bias diagnostics, all of which are based on the analysis of concentration-response curves of ligands according to classical receptor theory. We computed bias factors for a number of β-adrenergic agonists by comparing BRET assays of receptor-transducer interactions with Gs, Gi and arrestin. Using the same ligands, we also compared responses at signalling steps originated from the same receptor-transducer interaction, among which no biased efficacy is theoretically possible. In either case, we found a high level of false positive results and a general lack of correlation among methods. Altogether this analysis shows that all tested methods, including some of the most widely used in the literature, fail to distinguish true ligand bias from "system bias" with confidence. We also propose two novel semi quantitative methods of bias diagnostics that appear to be more robust and reliable than currently available strategies.

Estimation of the receptor-state affinity constants of ligands in functional studies using wild type and constitutively active mutant receptors: Implications for estimation of agonist bias

We describe a method for estimating the affinities of ligands for active and inactive states of a G protein-coupled receptor (GPCR). Our protocol involves measuring agonist-induced signaling responses of a wild type GPCR and a constitutively active mutant of it under control conditions and after partial receptor inactivation or reduced receptor expression. Our subsequent analysis is based on the assumption that the activating mutation increases receptor isomerization into the active state without affecting the affinities of ligands for receptor states. A means of confirming this assumption is provided. Global nonlinear regression analysis yields estimates of 1) the active (K_act) and inactive (K_inact) receptor-state affinity constants, 2) the isomerization constant of the unoccupied receptor (K_q-obs), and 3) the sensitivity constant of the signaling pathway (K_E-obs). The latter two parameters define the output response of the receptor, and hence, their ratio (K_q-obs/K_E) is a useful measure of system bias. If the cellular system is reasonably stable and the K_q-obs and K_E-obs values of the signaling pathway are known, the K_act and K_inact values of additional agonists can be estimated in subsequent experiments on cells expressing the wild type receptor. We validated our method through computer simulation, an analytical proof, and analysis of previously published data. Our approach provides 1) a more meaningful analysis of structure-activity relationships, 2) a means of validating in silico docking experiments on active and inactive receptor structures and 3) an absolute, in contrast to relative, measure of agonist bias.

Receptor Activity-modifying Protein-directed G Protein Signaling Specificity for the Calcitonin Gene-related Peptide Family of Receptors

The calcitonin gene-related peptide (CGRP) family of G protein-coupled receptors (GPCRs) is formed through the association of the calcitonin receptor-like receptor (CLR) and one of three receptor activity-modifying proteins (RAMPs). Binding of one of the three peptide ligands, CGRP, adrenomedullin (AM), and intermedin/adrenomedullin 2 (AM2), is well known to result in a Gα_s-mediated increase in cAMP. Here we used modified yeast strains that couple receptor activation to cell growth, via chimeric yeast/Gα subunits, and HEK-293 cells to characterize the effect of different RAMP and ligand combinations on this pathway. We not only demonstrate functional couplings to both Gα_s and Gα_q but also identify a Gα_i component to CLR signaling in both yeast and HEK-293 cells, which is absent in HEK-293S cells. We show that the CGRP family of receptors displays both ligand- and RAMP-dependent signaling bias among the Gα_s, Gα_i, and Gα_q/11 pathways. The results are discussed in the context of RAMP interactions probed through molecular modeling and molecular dynamics simulations of the RAMP-GPCR-G protein complexes. This study further highlights the importance of RAMPs to CLR pharmacology and to bias in general, as well as identifying the importance of choosing an appropriate model system for the study of GPCR pharmacology.

Activation of M1/4 receptors phase advances the hamster circadian clock during the day

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) can be reset by the cholinergic agonist carbachol. In hamsters, intraSCN carbachol produces phase advances during the day. This phenomenon has previously been attributed to the muscarinic receptors, as carbachol-induced phase shifts are blocked by pretreatment with the muscarinic antagonist atropine. The SCN contains all five muscarinic receptors, leaving open the question as to which muscarinic receptors mediate these shifts. Here we test two selective muscarinic agonists, the M1/4 agonist McN-A-343 and the M2/3 agonist bethanechol, in addition to the non-selective cholinergic agonist carbachol. Consistent with previous reports, carbachol produced significant phase advances when injected to the SCN during the mid-subjective day. At the doses used here, McN-A-343, but not bethanechol, also produced significant phase shifts when injected to the SCN during the mid-subjective day. Phase shifts to McN-A-343 were as large as those produced by carbachol, suggesting that activation of the M1/4 receptors alone can fully account for the daytime phase advances produced by cholinergic agonists. Given acetylcholine's role in arousal, and the similarity between phase advances to carbachol/McN-A-343 and to exercise and arousal manipulations, it is possible that acetylcholine may contribute to non-photic resetting of the circadian clock.

Biased ligand quantification in drug discovery: from theory to high throughput screening to identify new biased μ opioid receptor agonists

BACKGROUND AND PURPOSE

Biased GPCR ligands are able to engage with their target receptor in a manner that preferentially activates distinct downstream signalling and offers potential for next generation therapeutics. However, accurate quantification of ligand bias in vitro is complex, and current best practice is not amenable for testing large numbers of compound. We have therefore sought to apply ligand bias theory to an industrial scale screening campaign for the identification of new biased μ receptor agonists.

EXPERIMENTAL APPROACH

μ receptor assays with appropriate dynamic range were developed for both Gαi -dependent signalling and β-arrestin2 recruitment. Δlog(Emax /EC50 ) analysis was validated as an alternative for the operational model of agonism in calculating pathway bias towards Gαi -dependent signalling. The analysis was applied to a high throughput screen to characterize the prevalence and nature of pathway bias among a diverse set of compounds with μ receptor agonist activity.

KEY RESULTS

A high throughput screening campaign yielded 440 hits with greater than 10-fold bias relative to DAMGO. To validate these results, we quantified pathway bias of a subset of hits using the operational model of agonism. The high degree of correlation across these biased hits confirmed that Δlog(Emax /EC50 ) was a suitable method for identifying genuine biased ligands within a large collection of diverse compounds.

CONCLUSIONS AND IMPLICATIONS

This work demonstrates that using Δlog(Emax /EC50 ), drug discovery can apply the concept of biased ligand quantification on a large scale and accelerate the deliberate discovery of novel therapeutics acting via this complex pharmacology.

Human Neuropeptide S Receptor Is Activated via a Gαq Protein-biased Signaling Cascade by a Human Neuropeptide S Analog Lacking the C-terminal 10 Residues

Human neuropeptide S (NPS) and its cognate receptor regulate important biological functions in the brain and have emerged as a future therapeutic target for treatment of a variety of neurological and psychiatric diseases. The human NPS (hNPS) receptor has been shown to dually couple to Gαs- and Gαq-dependent signaling pathways. The human NPS analog hNPS-(1-10), lacking 10 residues from the C terminus, has been shown to stimulate Ca(2+)mobilization in a manner comparable with full-length hNPSin vitrobut seems to fail to induce biological activityin vivo Here, results derived from a number of cell-based functional assays, including intracellular cAMP-response element (CRE)-driven luciferase activity, Ca(2+)mobilization, and ERK1/2 phosphorylation, show that hNPS-(1-10) preferentially activates Gαq-dependent Ca(2+)mobilization while exhibiting less activity in triggering Gαs-dependent CRE-driven luciferase activity. We further demonstrate that both Gαq- and Gαs-coupled signaling pathways contribute to full-length hNPS-mediated activation of ERK1/2, whereas hNPS-(1-10)-promoted ERK1/2 activation is completely inhibited by the Gαqinhibitor UBO-QIC but not by the PKA inhibitor H89. Moreover, the results of Ala-scanning mutagenesis of hNPS-(1-13) indicated that residues Lys(11)and Lys(12)are structurally crucial for the hNPS receptor to couple to Gαs-dependent signaling. In conclusion, our findings demonstrate that hNPS-(1-10) is a biased agonist favoring Gαq-dependent signaling. It may represent a valuable chemical probe for further investigation of the therapeutic potential of human NPS receptor-directed signalingin vivo.

Gaddum Memorial Lecture 2014: receptors as an evolving concept: from switches to biased microprocessors

This review is based on the JR Vane Medal Lecture presented at the BPS Winter Meeting in December 2014 by T. Kenakin. A recording of the lecture is included as supporting information and can also be viewed online here: https://www.youtube.com/watch?v=xrP81AQ8l-8. Pharmacological models used to describe drug agonism and antagonism have evolved over the past 20 years from a parsimonious model describing single active and inactive receptor states to models of multiconformational receptor systems modified by ligand conformational selection. These latter models describe the observed, presently underexploited, pharmacological mechanism of ligand-directed biased signalling. Biased signals can be quantified with transduction coefficients (ΔΔLog(τ/KA) values), a scale grounded in the Black/Leff operational model; this enables the optimization of biased profiles through medicinal chemistry. The past decades have also brought the availability of new technologies to measure multiple functional effects mediated by seven transmembrane receptors. These have confirmed that drugs can have many efficacies, which may be collaterally linked, that is there is no linear sequence of activities required. In addition, new functional screening assays have introduced increasing numbers of allosteric ligands into drug discovery. These molecules are permissive (they do not necessarily preclude endogenous signalling in vivo); therefore, they may allow better fine tuning of pathological physiology. The permissive quality of allosteric ligands can also change the quality of endogenous signalling efficacy ('induced bias') as well as the quantity of signal; in this regard, indices related to ΔΔLog(τ/KA) values (namely ΔLog(αβ) values) can be used to quantify these effects for optimization in the drug discovery process. All of these added scales of drug activity will, hopefully, allow better targeting of candidate molecules towards therapies.

The Effective Application of Biased Signaling to New Drug Discovery

The ability of agonists to selectively activate some but not all signaling pathways linked to pleiotropically signaling receptors has opened the possibility of obtaining molecules that emphasize beneficial signals, de-emphasize harmful signals, and concomitantly deemphasize harmful signals while blocking the harmful signals produced by endogenous agonists. The detection and quantification of biased effects is straightforward, but two important factors should be considered in the evaluation of biased effects in drug discovery. The first is that efficacy, and not bias, determines whether a given agonist signal will be observed; bias only dictates the relative concentrations at which agonist signals will appear when they do appear. Therefore, a Cartesian coordinate system plotting relative efficacy (on a scale of Log relative Intrinsic Activities) as the ordinates and Log(bias) as the abscissae is proposed as a useful tool in evaluating possible biased molecules for progression in discovery programs. Second, it should be considered that the current scales quantifying bias limit this property to the allosteric vector (ligand/receptor/coupling protein complex) and that whole-cell processing of this signal can completely change measured bias from in vitro predictions.

The Measurement of Receptor Signaling Bias

This chapter describes a method to quantify biased signaling effects of agonists; this approach can furnish a scale for medicinal chemists to optimize biased profiles. Biased ligands have different pharmacological properties on a molecular level (stabilization of different receptor active states) and thus can have different pharmacological profiles therapeutically. The calculation of transduction ratios (ΔΔlog(τ/K A)) values (where τ is efficacy and K A a measure of affinity) allows the identification of agonists that demonstrate unique signaling either for different signaling pathways linked to the receptor, differences in cellular host, effects of receptor mutation, or receptor selectivity profiles. The procedure includes statistical methods to assess the significance of these effects.

Efficacy and ligand bias at the μ-opioid receptor

In order to describe drug action at a GPCR, a full understanding of the pharmacological terms affinity, efficacy and potency is necessary. This is true whether comparing the ability of different agonists to produce a measurable response in a cell or tissue, or determining the relative ability of an agonist to activate a single receptor subtype and produce multiple responses. There is a great deal of interest in the μ-opioid receptor (MOP receptor) and the ligands that act at this GPCR not only because of the clinically important analgesic effects produced by MOP agonists but also because of their liability to induce adverse effects such as respiratory depression and dependence. Our understanding of the mechanisms underlying these effects, as well as the ability to develop new, more effective MOP receptor drugs, depends upon the accurate determination of the efficacy with which these ligands induce coupling of MOP receptors to downstream signalling events. In this review, which is written with the minimum of mathematical content, the basic meaning of terms including efficacy, intrinsic activity and intrinsic efficacy is discussed, along with their relevance to the field of MOP receptor pharmacology, and in particular in relation to biased agonism at this important GPCR.

Investigating G protein signalling bias at the glucagon-like peptide-1 receptor in yeast

BACKGROUND AND PURPOSE

The glucagon-like peptide 1 (GLP-1) receptor performs an important role in glycaemic control, stimulating the release of insulin. It is an attractive target for treating type 2 diabetes. Recently, several reports of adverse side effects following prolonged use of GLP-1 receptor therapies have emerged: most likely due to an incomplete understanding of signalling complexities.

EXPERIMENTAL APPROACH

We describe the expression of the GLP-1 receptor in a panel of modified yeast strains that couple receptor activation to cell growth via single Gα/yeast chimeras. This assay enables the study of individual ligand-receptor G protein coupling preferences and the quantification of the effect of GLP-1 receptor ligands on G protein selectivity.

KEY RESULTS

The GLP-1 receptor functionally coupled to the chimeras representing the human Gαs, Gαi and Gαq subunits. Calculation of the dissociation constant for a receptor antagonist, exendin-3 revealed no significant difference between the two systems. We obtained previously unobserved differences in G protein signalling bias for clinically relevant therapeutic agents, liraglutide and exenatide; the latter displaying significant bias for the Gαi pathway. We extended the use of the system to investigate small-molecule allosteric compounds and the closely related glucagon receptor.

CONCLUSIONS AND IMPLICATIONS

These results provide a better understanding of the molecular events involved in GLP-1 receptor pleiotropic signalling and establish the yeast platform as a robust tool to screen for more selective, efficacious compounds acting at this important class of receptors in the future.

Analysis of functional selectivity through G protein-dependent and -independent signaling pathways at the adrenergic α(2C) receptor

Although G protein-coupled receptors (GPCRs) are traditionally categorized as Gs-, Gq-, or Gi/o-coupled, their signaling is regulated by multiple mechanisms. GPCRs can couple to several effector pathways, having the capacity to interact not only with more than one G protein subtype but also with alternative signaling or effector proteins such as arrestins. Moreover, GPCR ligands can have different efficacies for activating these signaling pathways, a characteristic referred to as biased agonism or functional selectivity. In this work our aim was to detect differences in the ability of various agonists acting at the α2C type of adrenergic receptors (α2C-ARs) to modulate cAMP accumulation, cytoplasmic Ca(2+) release, β-arrestin recruitment and receptor internalization. A detailed comparative pharmacological characterization of G protein-dependent and -independent signaling pathways was carried out using adrenergic agonists (norepinephrine, phenylephrine, brimonidine, BHT-920, oxymetazoline, clonidine, moxonidine, guanabenz) and antagonists (MK912, yohimbine). As initial analysis of agonist Emax and EC50 values suggested possible functional selectivity, ligand bias was quantified by applying the relative activity scale and was compared to that of the endogenous agonist norepinephrine. Values significantly different from 0 between pathways indicated an agonist that promoted different level of activation of diverse effector pathways most likely due to the stabilization of a subtly different receptor conformation from that induced by norepinephrine. Our results showed that a series of agonists acting at the α2C-AR displayed different degree of functional selectivity (bias factors ranging from 1.6 to 36.7) through four signaling pathways. As signaling via these pathways seems to have distinct functional and physiological outcomes, studying all these stages of receptor activation could have further implications for the development of more selective therapeutics with improved efficacy and/or fewer side effects.

Advances in G protein-coupled receptor allostery: from function to structure

It is now widely accepted that G protein-coupled receptors (GPCRs) are highly dynamic proteins that adopt multiple active states linked to distinct functional outcomes. Furthermore, these states can be differentially stabilized not only by orthosteric ligands but also by allosteric ligands acting at spatially distinct binding sites. The key pharmacologic characteristics of GPCR allostery include improved selectivity due to either greater sequence divergence between receptor subtypes and/or subtype-selective cooperativity, a ceiling level to the effect, probe dependence (whereby the magnitude and direction of the allosteric effect change with the nature of the interacting ligands), and the potential for biased signaling. Recent chemical biology developments are beginning to demonstrate how the incorporation of analytical pharmacology and operational modeling into the experimental workflow can enrich structure-activity studies of allostery and bias, and have also led to the discovery of a new class of hybrid orthosteric/allosteric (bitopic) molecules. The potential for endogenous allosteric modulators to play a role in physiology and disease remains to be fully appreciated but will likely represent an important area for future studies. Finally, breakthroughs in structural and computational biology are beginning to unravel the mechanistic basis of GPCR allosteric modulation at the molecular level.

What is pharmacological 'affinity'? Relevance to biased agonism and antagonism

The differences between affinity measurements made in binding studies and those relevant to receptor function are described. There are theoretical and practical reasons for not utilizing binding data and, in terms of the quantification of signaling bias, it is unnecessary to do so. Finally, the allosteric control of ligand affinity through receptor-signaling protein interaction is discussed within the context of biased antagonism. In this regard, it is shown that both the bias and relative efficacy of a ligand are essential data for fully predicting biased effects in vivo.

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.

Divergent transducer-specific molecular efficacies generate biased agonism at a G protein-coupled receptor (GPCR)

The concept of "biased agonism" arises from the recognition that the ability of an agonist to induce a receptor-mediated response (i.e. "efficacy") can differ across the multiple signal transduction pathways (e.g. G protein and β-arrestin (βarr)) emanating from a single GPCR. Despite the therapeutic promise of biased agonism, the molecular mechanism(s) whereby biased agonists selectively engage signaling pathways remain elusive. This is due in large part to the challenges associated with quantifying ligand efficacy in cells. To address this, we developed a cell-free approach to directly quantify the transducer-specific molecular efficacies of balanced and biased ligands for the angiotensin II type 1 receptor (AT1R), a prototypic GPCR. Specifically, we defined efficacy in allosteric terms, equating shifts in ligand affinity (i.e. KLo/KHi) at AT1R-Gq and AT1R-βarr2 fusion proteins with their respective molecular efficacies for activating Gq and βarr2. Consistent with ternary complex model predictions, transducer-specific molecular efficacies were strongly correlated with cellular efficacies for activating Gq and βarr2. Subsequent comparisons across transducers revealed that biased AT1R agonists possess biased molecular efficacies that were in strong agreement with the signaling bias observed in cellular assays. These findings not only represent the first measurements of the thermodynamic driving forces underlying differences in ligand efficacy between transducers but also support a molecular mechanism whereby divergent transducer-specific molecular efficacies generate biased agonism at a GPCR.

Estimation of ligand affinity constants for receptor states in functional studies involving the allosteric modulation of G protein-coupled receptors: implications for ligand bias

INTRODUCTION

The affinity constants of a ligand for active and inactive states of a receptor ultimately determine its capacity to activate downstream signaling events. In this report, we describe a reverse-engineering strategy for estimating these microscopic constants.

METHODS

Our approach involves analyzing responses measured downstream in the signaling pathway of a G protein-coupled receptor under conditions of allosteric modulation and reduced receptor expression or partial receptor inactivation. The analysis also yields estimates of the isomerization constant of the unoccupied receptor, the sensitivity constant of the signaling pathway, and the more empirical parameters of the receptor population including the observed affinities and efficacies of allosteric and orthosteric ligands - including inverse agonists - and the efficacy of the unoccupied receptor (i.e., constitutive activity).

RESULTS AND DISCUSSION

We validate our approach with an analytical proof and by analysis of simulated data. We also use our method to analyze data from the literature. We show that the values of the microscopic constants of orthosteric and allosteric ligands are constant regardless of the allosteric interaction and the nature of the receptor-signaling pathway as long as the same active state mediates the response. Our analysis is useful for quantifying probe-dependent allosteric interactions and the selectivity of agonists for different signaling pathways. Knowing the isomerization constant and sensitivity constant of a signaling pathway in a given cell line or tissue preparation enables future investigators to estimate the affinity constants of agonists for receptor states simply through analysis of their concentration-response curves. Our approach also provides a means of validating in silico estimates of ligand affinity for crystal structures of active and inactive states of the receptor.

Quantifying biased β-arrestin signaling

It is now established that agonists do not uniformly activate pleiotropic signaling mechanisms initiated by receptors but rather can bias signals according to the unique receptor conformations they stabilize. One of the important emerging signaling systems where this can occur is through β-arrestin. This chapter discusses biased signaling where emphasis or de-emphasis of β-arrestin signaling is postulated (or been shown) to be beneficial. The chapter specifically focuses on methods to quantify biased effects; these methods furnish scales that can be used in the process of optimizing biased agonism (and antagonism) for therapeutic benefit. Specifically, methods to derive ΔΔLog(τ/K A) or ΔΔLog(Relative Activity) values are described to do this.

Quantification of ligand bias for clinically relevant β2-adrenergic receptor ligands: implications for drug taxonomy

The concepts of functional selectivity and ligand bias are becoming increasingly appreciated in modern drug discovery programs, necessitating more informed approaches to compound classification and, ultimately, therapeutic candidate selection. Using the β2-adrenergic receptor as a model, we present a proof of concept study that assessed the bias of 19 β-adrenergic ligands, including many clinically used compounds, across four pathways [cAMP production, extracellular signal-regulated kinase 1/2 (ERK1/2) activation, calcium mobilization, and receptor endocytosis] in the same cell background (human embryonic kidney 293S cells). Efficacy-based clustering placed the ligands into five distinct groups with respect to signaling signatures. In some cases, apparent functional selectivity originated from off-target effects on other endogenously expressed adrenergic receptors, highlighting the importance of thoroughly assessing selectivity of the responses before concluding receptor-specific ligand-biased signaling. Eliminating the nonselective compounds did not change the clustering of the 10 remaining compounds. Some ligands exhibited large differences in potency for the different pathways, suggesting that the nature of the receptor-effector complexes influences the relative affinity of the compounds for specific receptor conformations. Calculation of relative effectiveness (within pathway) and bias factors (between pathways) for each of the compounds, using an operational model of agonism, revealed a global signaling signature for all of the compounds relative to isoproterenol. Most compounds were biased toward ERK1/2 activation over the other pathways, consistent with the notion that many proximal effectors converge on this pathway. Overall, we demonstrate a higher level of ligand texture than previously anticipated, opening perspectives for the establishment of pluridimensional correlations between signaling profiles, drug classification, therapeutic efficacy, and safety.

Selective κ opioid antagonists nor-BNI, GNTI and JDTic have low affinities for non-opioid receptors and transporters

Background

Nor-BNI, GNTI and JDTic induce selective κ opioid antagonism that is delayed and extremely prolonged, but some other effects are of rapid onset and brief duration. The transient effects of these compounds differ, suggesting that some of them may be mediated by other targets.

Results

In binding assays, the three antagonists showed no detectable affinity (K_i≥10 µM) for most non-opioid receptors and transporters (26 of 43 tested). There was no non-opioid target for which all three compounds shared detectable affinity, or for which any two shared sub-micromolar affinity. All three compounds showed low nanomolar affinity for κ opioid receptors, with moderate selectivity over μ and δ (3 to 44-fold). Nor-BNI bound weakly to the α_2C-adrenoceptor (K_i = 630 nM). GNTI enhanced calcium mobilization by noradrenaline at the α_1A-adrenoceptor (EC₅₀ = 41 nM), but did not activate the receptor, displace radioligands, or enhance PI hydrolysis. This suggests that it is a functionally-selective allosteric enhancer. GNTI was also a weak M₁ receptor antagonist (K_B = 3.7 µM). JDTic bound to the noradrenaline transporter (K_i = 54 nM), but only weakly inhibited transport (IC₅₀ = 1.1 µM). JDTic also bound to the opioid-like receptor NOP (K_i = 12 nM), but gave little antagonism even at 30 µM. All three compounds exhibited rapid permeation and active efflux across Caco-2 cell monolayers.

Conclusions

Across 43 non-opioid CNS targets, only GNTI exhibited a potent functional effect (allosteric enhancement of α_1A-adrenoceptors). This may contribute to GNTI's severe transient effects. Plasma concentrations of nor-BNI and GNTI may be high enough to affect some peripheral non-opioid targets. Nonetheless, κ opioid antagonism persists for weeks or months after these transient effects dissipate. With an adequate pre-administration interval, our results therefore strengthen the evidence that nor-BNI, GNTI and JDTic are highly selective κ opioid antagonists.

Effects of asparagine mutagenesis of conserved aspartic acids in helix 2 (D2.50) and 3 (D3.32) of M1-M4 muscarinic receptors on the irreversible binding of nitrogen mustard analogs of acetylcholine and McN-A-343

We investigated how asparagine mutagenesis of conserved aspartic acids in helix 2 (D2.50) and 3 (D3.32) of M1-M4 muscarinic receptors alters the irreversible binding of acetylcholine mustard and BR384 (4-[(2-bromoethyl)methyl-amino]-2-butynyl N-(3-chlorophenyl)carbamate), a nitrogen mustard derivative of McN-A-343 ([4-[[N-(3-chlorophenyl)carbamoyl]oxy]-2-butynyl] trimethylammonium chloride). The D2.50N mutation moderately increased the affinity of the aziridinium ions of acetylcholine mustard and BR384 for M2-M4 receptors and had little effect on the rate constant for receptor alkylation. The D3.32N mutation greatly reduced the rate constant for receptor alkylation by acetylcholine mustard but not by BR384, although the affinity of BR384 was reduced. The combination of both mutations (D2.50N/D3.32N) substantially reduced the rate constant for receptor alkylation by BR384 relative to that of wild type and mutant D2.50N and D3.32N receptors. The change in binding affinity caused by the mutations suggests that the D2.50N mutation alters the interaction of acetylcholine mustard with D3.32 of the M1 and M3 receptors but not that of the M4 receptor. BR384 exhibited the converse relationship. The simplest explanation is that acetylcholine mustard and BR384 alkylate at least two residues on M1-M4 receptors and that the D2.50N mutation alters the rate of alkylation of D3.32 relative to another residue, perhaps D2.50 itself.

Effects of modulating M3 muscarinic receptor activity on azoxymethane-induced liver injury in mice

Previously, we reported that azoxymethane (AOM)-induced liver injury is robustly exacerbated in M3 muscarinic receptor (M3R)-deficient mice. We used the same mouse model to test the hypothesis that selective pharmacological modulation of M3R activity regulates the liver injury response. Initial experiments confirmed that giving a selective M3R antagonist, darifenacin, to AOM-treated mice mimicked M3R gene ablation. Compared to vehicle controls, mice treated with the M3R antagonist had reduced survival and increased liver nodularity and fibrosis. We next assessed AOM-induced liver injury in mice treated with a selective M3R agonist, pilocarpine. After pilocarpine treatment, stimulation of post-M3R signaling in the liver was evidenced by ERK and AKT activation. In contrast to the damaging effects of the M3R antagonist, administering pilocarpine to AOM-treated mice significantly attenuated hepatic stellate cell activation, collagen deposition, bile ductule proliferation, and liver fibrosis and nodularity. As anticipated from these findings, livers from pilocarpine-treated mice exhibited reduced expression of key players in fibrosis (α1 collagen, α-smooth muscle actin, TGF-β1, PGDF, TGF-β1R, PGDFR) and decreased mRNA levels for molecules that regulate extracellular matrix formation (TIMP-1, TIMP-2, MMP-2, MMP-13). Cleaved caspase-3, nitrotyrosine and BrdU immunostaining provided evidence that pilocarpine treatment reduced hepatocyte apoptosis and oxidative stress, while increasing hepatocyte proliferation. Collectively, these findings identify several downstream mechanisms whereby M3R activation ameliorates toxic liver injury. These novel observations provide a proof-of-principle that selectively stimulating M3R activation to prevent or diminish liver injury is a therapeutic strategy worthy of further investigation.

New concepts in pharmacological efficacy at 7TM receptors: IUPHAR review 2

The present-day concept of drug efficacy has changed completely from its original description as the property of agonists that causes tissue activation. The ability to visualize the multiple behaviours of seven transmembrane receptors has shown that drugs can have many efficacies and also that the transduction of drug stimulus to various cellular stimulus-response cascades can be biased towards some but not all pathways. This latter effect leads to agonist 'functional selectivity', which can be favourable for the improvement of agonist therapeutics. However, in addition, biased agonist potency becomes cell type dependent with the loss of the monotonic behaviour of stimulus-response mechanisms, leading to potential problems in agonist quantification. This has an extremely important effect on the discovery process for new agonists since it now cannot be assumed that a given screening or lead optimization assay will correctly predict therapeutic behaviour. This review discusses these ideas and how new approaches to quantifying agonist effect may be used to circumvent the cell type dependence of agonism. This article, written by a corresponding member of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR), reviews our current understanding of the interaction of ligands with seven transmembrane receptors. Further information on these pharmacological concepts is being incorporated into the IUPHAR/BPS database GuideToPharmacology.org.

Biasing the parathyroid hormone receptor: relating in vitro ligand efficacy to in vivo biological activity

Recent advances in our understanding of the pluridimensional nature of GPCR signaling have provided new insights into how orthosteric ligands regulate receptors, and how the phenomenon of functional selectivity or ligand "bias" might be exploited in pharmaceutical design. In contrast to the predictions of simple two-state models of GPCR function, where ligands affect all aspects of GPCR signaling proportionally, current models assume that receptors exist in multiple "active" conformations that differ in their ability to couple to different downstream effectors, and that structurally distinct ligands can bias signaling by preferentially stabilizing different active states. The type 1 parathyroid hormone receptor (PTH(1)R) offers unique insight into both the opportunities and challenges of exploiting ligand bias in pharmaceutical design, not only because numerous "biased" PTH analogs have been described but also because many of them have been characterized for biological activity in vivo. The PTH(1)R has pleiotropic signaling capacity, coupling to G(s), G(q/11), and G(i/o) family heterotrimeric G proteins, and binding arrestins, which mediate receptor desensitization and arrestin-dependent signaling. Here, we compare the activity of six different PTH(1)R ligands in a common HEK293 cell background using three readouts of receptor activation, cAMP production, intracellular calcium influx, and ERK1/2 activation, demonstrating the range of signal bias that can be experimentally observed in a "typical" screening program. When the in vitro activity profiles of these ligands are compared to their reported effects on bone mass in murine models, it is apparent that ligands activating cAMP production produce an anabolic response that does not correlate with the ability to also elicit calcium signaling. Paradoxically, one ligand that exhibits inverse agonism for cAMP production and arrestin-dependent ERK1/2 activation in vitro, (D-Trp(12), Tyr(34))-bPTH(7-34), reportedly produces an anabolic bone response in vivo despite an activity profile that is dramatically different from that of other active ligands. This underscores a major challenge facing efforts to rationally design "biased" GPCR ligands for therapeutic application. While it is clearly plausible to identify functionally selective ligands, the ability to predict how bias will affect drug response in vivo, is often lacking, especially in complex disorders.

TNF-α-induced CXCL8 production by A549 cells: involvement of the non-neuronal cholinergic system

It was recently suggested that the non-neuronal cholinergic system has a regulatory role in pulmonary inflammation. We investigated this system's involvement in the control of cytokine production by the A549 human alveolar epithelial cell line. CXCL8 and acetylcholine (ACh) concentrations were measured using ELISA and LC-MS/MS, respectively. The mRNA expression of muscarinic receptor (MR) subtypes was determined using RT-PCR. In A549 cells, TNF-α increased the release of CXCL8 and ACh and the expression of the subtype 3 MR (M3R). Furthermore, TNF-α-induced CXCL8 secretion was (i) inhibited by the MR antagonist tiotropium and the M3R antagonist 4-DAMP and (ii) enhanced by the M1/M3R agonist pilocarpine and the cholinesterase inhibitor physostigmine. Taken as a whole, these results suggest that ACh release by A549 cells enhances TNF-α-induced CXCL8 secretion through activation of the M3R. Western blot analysis revealed that pilocarpine and physostigmine enhanced the TNF-α-induced phosphorylation of ERK1/2 and p38 MAPK and the degradation of IκBα. Inhibition of these pathways with specific inhibitors abrogated the pilocarpine-induced CXCL8 release. Our results suggest that the TNF-α-induced secretion of CXCL8 in A549 cells is regulated by the release of ACh, the latter's binding to the M3R and the downstream activation of NF-κB and the ERK1/2 and p38 MAPK signaling pathways. Our findings suggest that MR antagonists may have anti-inflammatory effects by preventing pro-inflammatory events driven by endogenous, non-neuronal ACh.

The potential for selective pharmacological therapies through biased receptor signaling

The discovery that not all agonists uniformly activate cellular signaling pathways (biased signaling) has greatly changed the drug discovery process for agonists and the strategy for treatment of disease with agonists. Technological advances have enabled complex receptor behaviors to be viewed independently and through these assays, the bias for an agonist can be quantified. It is predicted that therapeutic phenotypes will be linked, through translational studies, to quantified scales of bias to guide medicinal chemists in the drug discovery process.

Overview of muscarinic receptor subtypes

The physiological role of muscarinic receptors is highly complex and, although not completely understood, has become clearer over the last decade. Recent pharmacological evidence with novel compounds, together with data from transgenic mice, suggests that all five subtypes have defined functions in the nervous system as well as mediating the non neuronal, hormonal actions of acetylcholine. Numerous novel agonists, allosteric regulators, and antagonists have now been identified with authentic subtype specificity in vitro and in vivo. These compounds provide additional pharmacological opportunities for selective subtype modulation as well as a new generation of muscarinic receptor-based therapeutics.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

A simple method for quantifying functional selectivity and agonist bias

Activation of seven-transmembrane (7TM) receptors by agonists does not always lead to uniform activation of all signaling pathways mediated by a given receptor. Relative to other ligands, many agonists are "biased" toward producing subsets of receptor behaviors. A hallmark of such "functional selectivity" is cell type dependence; this poses a particular problem for the profiling of agonists in whole cell test systems removed from the therapeutic one(s). Such response-specific cell-based variability makes it difficult to guide medicinal chemistry efforts aimed at identifying and optimizing therapeutically meaningful agonist bias. For this reason, we present a scale, based on the Black and Leff operational model, that contains the key elements required to describe 7TM agonism, namely, affinity (K(A) (-1)) for the receptor and efficacy (τ) in activating a particular signaling pathway. Utilizing a "transduction coefficient" term, log(τ/K(A)), this scale can statistically evaluate selective agonist effects in a manner that can theoretically inform structure-activity studies and/or drug candidate selection matrices. The bias of four chemokines for CCR5-mediated inositol phosphate production versus internalization is quantified to illustrate the practical application of this method. The independence of this method with respect to receptor density and the calculation of statistical estimates of confidence of differences are specifically discussed.

Signalling bias in new drug discovery: detection, quantification and therapeutic impact

Agonists of seven-transmembrane receptors, also known as G protein-coupled receptors (GPCRs), do not uniformly activate all cellular signalling pathways linked to a given seven-transmembrane receptor (a phenomenon termed ligand or agonist bias); this discovery has changed how high-throughput screens are designed and how lead compounds are optimized for therapeutic activity. The ability to experimentally detect ligand bias has necessitated the development of methods for quantifying agonist bias in a way that can be used to guide structure-activity studies and the selection of drug candidates. Here, we provide a viewpoint on which methods are appropriate for quantifying bias, based on knowledge of how cellular and intracellular signalling proteins control the conformation of seven-transmembrane receptors. We also discuss possible predictions of how biased molecules may perform in vivo, and what potential therapeutic advantages they may provide.