Applications and limitations of fitting of the operational model to determine relative efficacies of agonists

Importance of receptor expression in the classification of novel ligands at the M2 muscarinic acetylcholine receptor

BACKGROUND AND PURPOSE

Affinity-based, selective orthosteric ligands for the muscarinic acetylcholine receptors (mAChRs) are difficult to develop due to high sequence homology across the five subtypes. Selectivity can also be achieved via the selective activation of a particular subtype or signalling pathway. Promisingly, a prior study identified compounds 6A and 7A as functionally selective and Gi biased compounds at the M2 mAChR. Here, we have investigated the activation of individual G protein subfamilies and the downstream signalling profiles of 6A and 7A at the M2 mAChR.

EXPERIMENTAL APPROACH

G protein activation was measured with the TRUPATH assay in M2 mAChR FlpIn CHO cells. Activity in downstream signalling pathways was determined using the cAMP CAMYEL BRET sensor and assay of ERK 1/2 phosphorylation.

KEY RESULTS

M2 mAChRs coupled to Gɑi1, GɑoA and Gɑs, but not Gɑq, in response to canonical orthosteric agonists. Compounds 6A and 7A did not elicit any G protein activation, cAMP inhibition or stimulation, or ERK 1/2 phosphorylation. Instead, a Schild analysis indicates a competitive, antagonistic interaction of compounds 6A and 7A with ACh in the Gɑi1 activation assay. Overexpression of the M2 mAChR may suggest an expression-dependent activation profile of compounds 6A and 7A.

CONCLUSIONS AND IMPLICATIONS

These data confirm that the M2 mAChR preferentially couples to Gɑi/o and to a lesser extent to Gɑs in response to canonical orthosteric ligands. However, this study was not able to detect Gɑi bias of compounds 6A and 7A, highlighting the importance of cellular background when classifying new ligands.

LINKED ARTICLES

This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Structural Insights into Partial Activation of the Prototypic G Protein-Coupled Adenosine A2A Receptor

The adenosine A2A receptor (A2AAR) belongs to the rhodopsin-like G protein-coupled receptor (GPCR) family, which constitutes the largest class of GPCRs. Partial agonists show reduced efficacy as compared to physiological agonists and can even act as antagonists in the presence of a full agonist. Here, we determined an X-ray crystal structure of the partial A2AAR agonist 2-amino-6-[(1H-imidazol-2-ylmethyl)sulfanyl]-4-p-hydroxyphenyl-3,5-pyridinedicarbonitrile (LUF5834) in complex with the A2AAR construct A2A-PSB2-bRIL, stabilized in its inactive conformation and being devoid of any mutations in the ligand binding pocket. The determined high-resolution structure (2.43 Å) resolved water networks and crucial binding pocket interactions. A direct hydrogen bond of the p-hydroxy group of LUF5834 with T883.36 was observed, an amino acid that was mutated to alanine in the most frequently used A2AAR crystallization constructs thus preventing the discovery of its interactions in most of the previous A2AAR co-crystal structures. G protein dissociation studies confirmed partial agonistic activity of LUF5834 as compared to that of the full agonist N-ethylcarboxamidoadenosine (NECA). In contrast to NECA, the partial agonist was still able to bind to the receptor construct locked in its inactive conformation by an S913.39K mutation, although with an affinity lower than that at the native receptor. This could explain the compound's partial agonistic activity: while full A2AAR agonists bind exclusively to the active conformation, likely following conformational selection, partial agonists bind to active as well as inactive conformations, showing higher affinity for the active conformation. This might be a general mechanism of partial agonism also applicable to other GPCRs.

Agonist-selective activation of individual G-proteins by muscarinic receptors

Selective activation of individual subtypes of muscarinic receptors is a promising way to safely alleviate a wide range of pathological conditions in the central nervous system and the periphery as well. The flexible G-protein interface of muscarinic receptors allows them to interact with several G-proteins with various efficacy, potency, and kinetics. Agonists biased to the particular G-protein mediated pathway may result in selectivity among muscarinic subtypes and, due to the non-uniform expression of individual G-protein alpha subunits, possibly achieve tissue specificity. Here, we demonstrate that novel tetrahydropyridine-based agonists exert specific signalling profiles in coupling with individual G-protein α subunits. These signalling profiles profoundly differ from the reference agonist carbachol. Moreover, coupling with individual Gα induced by these novel agonists varies among subtypes of muscarinic receptors which may lead to subtype selectivity. Thus, the novel tetrahydropyridine-based agonist can contribute to the elucidation of the mechanism of pathway-specific activation of muscarinic receptors and serve as a starting point for the development of desired selective muscarinic agonists.

Distinct roles of the extracellular surface residues of glucagon-like peptide-1 receptor in β-arrestin 1/2 signaling

Glucagon-like peptide-1 receptor (GLP-1R) is a prime drug target for type 2 diabetes and obesity. The ligand initiated GLP-1R interaction with G protein has been well studied, but not with β-arrestin 1/2. Therefore, bioluminescence resonance energy transfer (BRET), mutagenesis and an operational model were used to evaluate the roles of 85 extracellular surface residues on GLP-1R in β-arrestin 1/2 recruitment triggered by three representative GLP-1R agonists (GLP-1, exendin-4 and oxyntomodulin). Residues selectively regulated β-arrestin 1/2 recruitment for diverse ligands, and β-arrestin isoforms were identified. Mutation of residues K130-S136, L142 and Y145 on the transmembrane helix 1 (TM1)-extracellular domain (ECD) linker decreased β-arrestin 1 recruitment but increased β-arrestin 2 recruitment. Other extracellular loop (ECL) mutations, including P137A, Q211A, D222A and M303A selectively affected β-arrestin 1 recruitment while D215A, L217A, Q221A, S223A, Y289A, S301A, F381A and I382A involved more in β-arrestin 2 recruitment for the ligands. Oxyntomodulin engaged more broadly with GLP-1R extracellular surface to drive β-arrestin 1/2 recruitment than GLP-1 and exendin-4; I147, W214 and L218 involved in β-arrestin 1 recruitment, while L141, D215, L218, D293 and F381 in β-arrestin 2 recruitment for oxyntomodulin particularly. Additionally, the non-conserved residues on β-arrestin 1/2 C-domains contributed to interaction with GLP-1R. Further proteomic profiling of GLP-1R stably expressed cell line upon ligand stimulation with or without β-arrestin 1/2 overexpression demonstrated both commonly and biasedly regulated proteins and pathways associated with cognate ligands and β-arrestins. Our study offers valuable information about ligand induced β-arrestin recruitment mediated by GLP-1R and consequent intracellular signaling events.

A critical re-evaluation of the slope factor of the operational model of agonism: When to exponentiate operational efficacy

Agonist efficacy denoting the "strength" of agonist action is a cornerstone in the proper assessment of agonist selectivity and signalling bias. The simulation models are very accurate but complex and hard to fit experimental data. The parsimonious operational model of agonism (OMA) has become successful in the determination of agonist efficacies and ranking them. In 1983, Black and Leff introduced the slope factor to the OMA to make it more flexible and allow for fitting steep as well as flat concentration-response curves. First, we performed a functional analysis to indicate the potential pitfalls of the OMA. Namely, exponentiation of operational efficacy may break relationships among the OMA parameters. The fitting of the Black & Leff equation to the theoretical curves of several models of functional responses and the experimental data confirmed the fickleness of the exponentiation of operational efficacy affecting estimates of operational efficacy as well as other OMA parameters. In contrast, fitting The OMA based on the Hill equation to the same data led to better estimates of model parameters. In conclusion, Hill equation-based OMA should be preferred over the Black & Leff equation when functional-response curves differ in the slope factor. Otherwise, the Black & Leff equation should be used with extreme caution acknowledging potential pitfalls.

Quantification of receptor binding from response data obtained at different receptor levels: a simple individual sigmoid fitting and a unified SABRE approach

Methods that allow quantification of receptor binding (occupancy) by measuring response (effect) data only are of interest as they can be used to allow characterization of binding properties (e.g., dissociation constant, K_d) without having to perform explicit ligand binding experiments that require different setups (e.g., use of labeled ligands). However, since response depends not just on the binding affinity-determined receptor occupancy, but also on receptor activation, which is affected by ligand efficacy (plus constitutive activity, if present), and downstream pathway amplification, this requires the acquisition and fitting of multiple concentration-response data. Here, two alternative methods, which both are straightforward to implement using nonlinear regression software, are described to fit such multiple responses measured at different receptor levels that can be obtained, for example, by partial irreversible receptor inactivation (i.e., Furchgott method) or different expression levels. One is a simple method via straightforward fitting of each response with sigmoid functions and estimation of K_d from the obtained E_max and EC₅₀ values as K_d = (E_max·EC'₅₀ - E'_max·EC₅₀)/(E_max - E'_max). This is less error-prone than the original Furchgott method of double-reciprocal fit and simpler than alternatives that require concentration interpolations, thus, should allow more widespread use of this so-far underutilized approach to estimate binding properties. Relative efficacies can then be compared using E_max·K_d/EC₅₀ values. The other is a complex method that uses the SABRE receptor model to obtain a unified fit of the multiple concentration-response curves with a single set of parameters that include binding affinity K_d, efficacy ε, amplification γ, and Hill coefficient n. Illustrations with simulated and experimental data are presented including with activity data of three muscarinic agonists measured in rabbit myocardium.

Insights into the operational model of agonism of receptor dimers

INTRODUCTION

Accurate ranking of efficacies and potencies of agonists is essential in the discovery of new selective agonists. For the purpose of system-independent ranking of agonists, the operational model of agonism (OMA) has become a standard. Many receptors function as oligomers which makes functional responses more complex, requiring an extension of the original OMA.

AREAS COVERED

Explicit equations of the operational model of agonism of receptor dimers (OMARD) were derived. The OMARD can be applied to any receptor possessing two orthosteric sites. The behavior of OMARD was analyzed to demonstrate its complexity and relation to experimental data. Properties of OMARD and OMA equations were compared to demonstrate their pros and cons.

EXPERT OPINION

Extension of OMA by slope factors gives simple equations of functional response that are easy to fit experimental data but results may be inaccurate because of exponentiation of operational efficacy. Also, such equations cannot accommodate bell-shaped curves. Explicit equations of OMARD give accurate results but are complex and tedious to fit experimental data. All operational models use inter-dependent parameters that are a hurdle in the fitting. A good understanding of OMARD behavior helps to overcome such obstacles.

Cancer-Associated Mutations of the Adenosine A2A Receptor Have Diverse Influences on Ligand Binding and Receptor Functions

The adenosine A_2A receptor (A_2AAR) is a class A G-protein-coupled receptor (GPCR). It is an immune checkpoint in the tumor micro-environment and has become an emerging target for cancer treatment. In this study, we aimed to explore the effects of cancer-patient-derived A_2AAR mutations on ligand binding and receptor functions. The wild-type A_2AAR and 15 mutants identified by Genomic Data Commons (GDC) in human cancers were expressed in HEK293T cells. Firstly, we found that the binding affinity for agonist NECA was decreased in six mutants but increased for the V275A mutant. Mutations A165V and A265V decreased the binding affinity for antagonist ZM241385. Secondly, we found that the potency of NECA (EC₅₀) in an impedance-based cell-morphology assay was mostly correlated with the binding affinity for the different mutants. Moreover, S132L and H278N were found to shift the A_2AAR towards the inactive state. Importantly, we found that ZM241385 could not inhibit the activation of V275A and P285L stimulated by NECA. Taken together, the cancer-associated mutations of A_2AAR modulated ligand binding and receptor functions. This study provides fundamental insights into the structure–activity relationship of the A_2AAR and provides insights for A_2AAR-related personalized treatment in cancer.

Neurosteroids and steroid hormones are allosteric modulators of muscarinic receptors

The membrane cholesterol was found to bind and modulate the function of several G-protein coupled receptors including muscarinic acetylcholine receptors. We investigated the binding of 20 steroidal compounds including neurosteroids and steroid hormones to muscarinic receptors. Corticosterone, progesterone and some neurosteroids bound to muscarinic receptors with the affinity of 100 nM or greater. We established a structure-activity relationship for steroid-based allosteric modulators of muscarinic receptors. Further, we show that corticosterone and progesterone allosterically modulate the functional response of muscarinic receptors to acetylcholine at physiologically relevant concentrations. It can play a role in stress control or in pregnancy, conditions where levels of these hormones dramatically oscillate. Allosteric modulation of muscarinic receptors via the cholesterol-binding site represents a new pharmacological approach at diseases associated with altered cholinergic signalling.

Employing the Operational Model to Measure System-Independent OTR Efficacy

Ideally, pharmacological analysis is based on quantitative measurements applied toward unveiling biological mechanisms and guiding medicinal chemistry efforts in drug discovery and basic research. The most robust analysis frameworks generate findings which can be dissociated from the specific experimental setting to provide insights of a broader scope. With insufficient efficacy being the leading cause of drug failures in late-stage clinical trials, more rigorous preclinical definitions might assist in better translation. This chapter details a new method for accessing the Black and Leff operational model of agonism using a stable Flp-In™ T-REx™ HEK293 cell line under tetracycline-dependent control of OTR expression. We cover steps for performing the G_q-coupled HTRF^® IP-One assay, curve-fitting data, calculating and statistically representing system-independent drug activity, predicting responses in different sensitivities, and plotting of multivariate analyses.

Fusion with Promiscuous Gα16 Subunit Reveals Signaling Bias at Muscarinic Receptors

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

Functionally selective and biased agonists of muscarinic receptors

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

The operational model of allosteric modulation of pharmacological agonism

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

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

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

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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