Allosteric modulation of M1 muscarinic acetylcholine receptor internalization and subcellular trafficking

Biased receptor signalling and intracellular trafficking profiles of structurally distinct formylpeptide receptor 2 agonists

BACKGROUND

There is increasing interest in developing FPR2 agonists (compound 43, ACT-389949 and BMS-986235) as potential pro-resolving therapeutics, with ACT-389949 and BMS-986235 having entered phase I clinical development. FPR2 activation leads to diverse downstream outputs. ACT-389949 was observed to cause rapid tachyphylaxis, while BMS-986235 and compound 43 induced cardioprotective effects in preclinical models. We aim to characterise the differences in ligand-receptor engagement and downstream signalling and trafficking bias profile.

EXPERIMENTAL APPROACH

Concentration-response curves to G protein dissociation, β-arrestin recruitment, receptor trafficking and second messenger signalling were generated using FPR2 ligands (BMS-986235, ACT-389949, compound 43 and WKYMVm), in HEK293A cells. Log(τ/KA) was obtained from the operational model for bias analysis using WKYMVm as a reference ligand. Docking of FPR2 ligands into the active FPR2 cryoEM structure (PDBID: 7T6S) was performed using ICM pro software.

KEY RESULTS

Bias analysis revealed that WKYMVm and ACT-389949 shared a very similar bias profile. In comparison, BMS-986235 and compound 43 displayed approximately 5- to 50-fold bias away from β-arrestin recruitment and trafficking pathways, while being 35- to 60-fold biased towards cAMP inhibition and pERK1/2. Molecular docking predicted key amino acid interactions at the FPR2 shared between WKYMVm and ACT-389949, but not with BMS-986235 and compound 43.

CONCLUSION AND IMPLICATIONS

In vitro characterisation demonstrated that WKYMVm and ACT-389949 differ from BMS-986235 and compound 43 in their signalling and protein coupling profile. This observation may be explained by differences in the ligand-receptor interactions. In vitro characterisation provided significant insights into identifying the desired bias profile for FPR2-based pharmacotherapy.

Opportunities and challenges for the development of M1 muscarinic receptor positive allosteric modulators in the treatment for neurocognitive deficits

Targeting allosteric sites of M1 muscarinic acetylcholine receptors (M1 receptors) is a promising strategy to treat neurocognitive disorders, such as Alzheimer's disease and schizophrenia. Indeed, the last two decades have seen an impressive body of work focussing on the design and development of positive allosteric modulators (PAMs) for the M1 receptor. This has led to the identification of a structurally diverse range of highly selective M1 PAMs. In preclinical models, M1 PAMs have shown rescue of cognitive deficits and improvement of endpoints predictive of symptom domains of schizophrenia. Yet, to date only a few M1 PAMs have reached early-stage clinical trials, with many of them failing to progress further due to on-target mediated cholinergic adverse effects that have plagued the development of this class of ligand. This review covers the recent preclinical and clinical studies in the field of M1 receptor drug discovery for the treatment of Alzheimer's disease and schizophrenia, with a specific focus on M1 PAM, highlighting both the undoubted potential but also key challenges for the successful translation of M1 PAMs from bench-side to bedside. 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.

Cholinergic Receptor Muscarinic 1 Co-Localized with Mitochondria in Cultured Dorsal Root Ganglion Neurons, and Its Deletion Disrupted Mitochondrial Ultrastructure in Peripheral Neurons: Implications in Alzheimer's Disease

Background

Loss of Cholinergic Receptor Muscarinic 1 (CHRM1) has been linked to the pathogenesis of Alzheimer's disease (AD). Our recent study found significantly lower CHRM1 protein levels in AD patient cortices, linked to reduced survival. Furthermore, using knockout mice (Chrm1-/-) we demonstrated that deletion of Chrm1 alters cortical mitochondrial structure and function, directly establishing a connection between its loss and mitochondrial dysfunction in the context of AD. While CHRM1's role in the brain has been extensively investigated, its impact on peripheral neurons in AD remains a crucial area of research, especially considering reported declines in peripheral nerve conduction among AD patients.

Objective

The objective was to characterize Chrm1 localization and mitochondrial deficits in Chrm1-/- dorsal root ganglion (DRG) neurons.

Methods

Recombinant proteins tagged with Green or Red Fluorescent Protein (GFP/RFP) were transiently expressed to investigate the localization of Chrm1 and mitochondria, as well as mitochondrial movement in the neurites of cultured primary mouse DRG neurons, using confocal time-lapse live cell imaging. Transmission electron microscopy was performed to examine the ultrastructure of mitochondria in both wild-type and Chrm1-/- DRGs.

Results

Fluorescence imaging revealed colocalization and comigration of N-terminal GFP-tagged Chrm1 and mitochondrial localization signal peptide-tagged RFP-labelled mitochondria in the DRGs neurons. A spectrum of mitochondrial structural abnormalities, including disruption and loss of cristae was observed in 87% neurons in Chrm1-/- DRGs.

Conclusions

This study suggests that Chrm1 may be localized in the neuronal mitochondria and loss of Chrm1 in peripheral neurons causes sever mitochondrial structural aberrations resembling AD pathology.

Hippocampal versus cortical deletion of cholinergic receptor muscarinic 1 in mice differentially affects post-translational modifications and supramolecular assembly of respiratory chain-associated proteins, mitochondrial ultrastructure, and respiration: implications in Alzheimer's disease

Introduction: In a previous retrospective study using postmortem human brain tissues, we demonstrated that loss of Cholinergic Receptor Muscarinic 1 (CHRM1) in the temporal cortex of a subset of Alzheimer's patients was associated with poor survival, whereas similar loss in the hippocampus showed no such association. Mitochondrial dysfunction underlies Alzheimer's pathogenesis. Therefore, to investigate the mechanistic basis of our findings, we evaluated cortical mitochondrial phenotypes in Chrm1 knockout (Chrm1^-/-) mice. Cortical Chrm1 loss resulted in reduced respiration, reduced supramolecular assembly of respiratory protein complexes, and caused mitochondrial ultrastructural abnormalities. These mouse-based findings mechanistically linked cortical CHRM1 loss with poor survival of Alzheimer's patients. However, evaluation of the effect of Chrm1 loss on mouse hippocampal mitochondrial characteristics is necessary to fully understand our retrospective human tissue-based observations. This is the objective of this study. Methods: Enriched hippocampal and cortical mitochondrial fractions (EHMFs/ECMFs, respectively) derived from wild-type and Chrm1^-/- mice were used to measure respiration by quantifying real-time oxygen consumption, supramolecular assembly of oxidative phosphorylation (OXPHOS)-associated proteins by blue native polyacrylamide gel electrophoresis, post-translational modifications (PTMs) by isoelectric focusing (IEF), and mitochondrial ultrastructure by electron microscopy. Results: In contrast to our previous observations in Chrm1^-/- ECMFs, EHMFs of Chrm1^-/- mice significantly increased respiration with a concomitant increase in the supramolecular assembly of OXPHOS-associated proteins, specifically Atp5a and Uqcrc2, with no mitochondrial ultrastructural alterations. IEF of ECMFs and EHMFs from Chrm1^-/- mice showed a decrease and an increase, respectively in a negatively charged (pH∼3) fraction of Atp5a relative to the wild-type mice, with a corresponding decrease or increase in the supramolecular assembly of Atp5a and respiration indicating a tissue-specific signaling effect. Discussion: Our findings indicate that loss of Chrm1 in the cortex causes structural, and physiological alterations to mitochondria that compromise neuronal function, whereas Chrm1 loss in the hippocampus may benefit neuronal function by enhancing mitochondrial function. This brain region-specific differential effect of Chrm1 deletion on mitochondrial function supports our human brain region-based findings and Chrm1^-/- mouse behavioral phenotypes. Furthermore, our study indicates that Chrm1-mediated brain region-specific differential PTMs of Atp5a may alter complex-V supramolecular assembly which in turn regulates mitochondrial structure-function.

Loss of cholinergic receptor muscarinic 1 impairs cortical mitochondrial structure and function: implications in Alzheimer's disease

Introduction: Cholinergic Receptor Muscarinic 1 (CHRM1) is a G protein-coupled acetylcholine (ACh) receptor predominantly expressed in the cerebral cortex. In a retrospective postmortem brain tissues-based study, we demonstrated that severely (≥50% decrease) reduced CHRM1 proteins in the temporal cortex of Alzheimer's patients significantly correlated with poor patient outcomes. The G protein-mediated CHRM1 signal transduction cannot sufficiently explain the mechanistic link between cortical CHRM1 loss and the appearance of hallmark Alzheimer's pathophysiologies, particularly mitochondrial structural and functional abnormalities. Therefore, the objective of this study was to analyze the molecular, ultrastructural, and functional properties of cortical mitochondria using CHRM1 knockout (Chrm1-/-) and wild-type mice to identify mitochondrial abnormalities. Methods: Isolated and enriched cortical mitochondrial fractions derived from wild-type and Chrm1-/- mice were assessed for respiratory deficits (oxygen consumption) following the addition of different substrates. The supramolecular assembly of mitochondrial oxidative phosphorylation (OXPHOS)-associated protein complexes (complex I-V) and cortical mitochondrial ultrastructure were investigated by blue native polyacrylamide gel electrophoresis and transmission electron microscopy (TEM), respectively. A cocktail of antibodies, specific to Ndufb8, Sdhb, Uqcrc2, Mtco1, and Atp5a proteins representing different subunits of complexes I-V, respectively was used to characterize different OXPHOS-associated protein complexes. Results: Loss of Chrm1 led to a significant reduction in cortical mitochondrial respiration (oxygen consumption) concomitantly associated with reduced oligomerization of ATP synthase (complex V) and supramolecular assembly of complexes I-IV (Respirasome). Overexpression of Chrm1 in transformed cells (lacking native Chrm1) significantly increased complex V oligomerization and respirasome assembly leading to enhanced respiration. TEM analysis revealed that Chrm1 loss led to mitochondrial ultrastructural defects and alteration in the tinctorial properties of cortical neurons causing a significant increase in the abundance of dark cortical neurons (Chrm1-/- 85% versus wild-type 2%). Discussion: Our findings indicate a hitherto unknown effect of Chrm1 deletion in cortical neurons affecting mitochondrial function by altering multiple interdependent factors including ATP synthase oligomerization, respirasome assembly, and mitochondrial ultrastructure. The appearance of dark neurons in Chrm1-/- cortices implies potentially enhanced glutamatergic signaling in pyramidal neurons under Chrm1 loss condition. The findings provide novel mechanistic insights into Chrm1 loss with the appearance of mitochondrial pathophysiological deficits in Alzheimer's disease.

Tacrine-Based Hybrids: Past, Present, and Future

Alzheimer's disease (AD) is a neurodegenerative disorder which is characterized by β-amyloid (Aβ) aggregation, τ-hyperphosphorylation, and loss of cholinergic neurons. The other important hallmarks of AD are oxidative stress, metal dyshomeostasis, inflammation, and cell cycle dysregulation. Multiple therapeutic targets may be proposed for the development of anti-AD drugs, and the "one drug-multiple targets" strategy is of current interest. Tacrine (THA) was the first clinically approved cholinesterase (ChE) inhibitor, which was withdrawn due to high hepatotoxicity. However, its high potency in ChE inhibition, low molecular weight, and simple structure make THA a promising scaffold for developing multi-target agents. In this review, we summarized THA-based hybrids published from 2006 to 2022, thus providing an overview of strategies that have been used in drug design and approaches that have resulted in significant cognitive improvements and reduced hepatotoxicity.

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.

Targeting the M1 muscarinic acetylcholine receptor in Alzheimer's disease

Alzheimer's disease (AD) remains a major cause of morbidity and mortality worldwide, and despite extensive research, only a few drugs are available for management of the disease. One strategy has been to up-regulate cholinergic neurotransmission to improve cognitive function, but this approach has dose-limiting adverse effects. To avoid these adverse effects, new drugs that target specific receptor subtypes of the cholinergic system are needed, and the M1 subtype of muscarinic acetylcholine receptor (M1-mAChR) has been shown to be a good target for this approach. By using several strategies, M1-mAChR ligands have been developed and trialled in preclinical animal models and in human studies, with varying degrees of success. This article reviews the different approaches to targeting the M1-mAChR in AD and discusses the advantages and limitations of these strategies. The factors to consider in targeting the M1-mAChR in AD are also discussed.

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.

GRKs as Modulators of Neurotransmitter Receptors

Many receptors for neurotransmitters, such as dopamine, norepinephrine, acetylcholine, and neuropeptides, belong to the superfamily of G protein-coupled receptors (GPCRs). A general model posits that GPCRs undergo two-step homologous desensitization: the active receptor is phosphorylated by kinases of the G protein-coupled receptor kinase (GRK) family, whereupon arrestin proteins specifically bind active phosphorylated receptors, shutting down G protein-mediated signaling, facilitating receptor internalization, and initiating distinct signaling pathways via arrestin-based scaffolding. Here, we review the mechanisms of GRK-dependent regulation of neurotransmitter receptors, focusing on the diverse modes of GRK-mediated phosphorylation of receptor subtypes. The immediate signaling consequences of GRK-mediated receptor phosphorylation, such as arrestin recruitment, desensitization, and internalization/resensitization, are equally diverse, depending not only on the receptor subtype but also on phosphorylation by GRKs of select receptor residues. We discuss the signaling outcome as well as the biological and behavioral consequences of the GRK-dependent phosphorylation of neurotransmitter receptors where known.

The luminescent HiBiT peptide enables selective quantitation of G protein-coupled receptor ligand engagement and internalization in living cells

G protein-coupled receptors (GPCRs) are prominent targets to new therapeutics for a range of diseases. Comprehensive assessments of their cellular interactions with bioactive compounds, particularly in a kinetic format, are imperative to the development of drugs with improved efficacy. Hence, we developed complementary cellular assays that enable equilibrium and real-time analyses of GPCR ligand engagement and consequent activation, measured as receptor internalization. These assays utilize GPCRs genetically fused to an N-terminal HiBiT peptide (1.3 kDa), which produces bright luminescence upon high-affinity complementation with LgBiT, an 18-kDa subunit derived from NanoLuc. The cell impermeability of LgBiT limits signal detection to the cell surface and enables measurements of ligand-induced internalization through changes in cell-surface receptor density. In addition, bioluminescent resonance energy transfer is used to quantify dynamic interactions between ligands and their cognate HiBiT-tagged GPCRs through competitive binding with fluorescent tracers. The sensitivity and dynamic range of these assays benefit from the specificity of bioluminescent resonance energy transfer and the high signal intensity of HiBiT/LgBiT without background luminescence from receptors present in intracellular compartments. These features allow analyses of challenging interactions having low selectivity or affinity and enable studies using endogenously tagged receptors. Using the β-adrenergic receptor family as a model, we demonstrate the versatility of these assays by utilizing the same HiBiT construct in analyses of multiple aspects of GPCR pharmacology. We anticipate that this combination of target engagement and proximal functional readout will prove useful to the study of other GPCR families and the development of new therapeutics.

Split luciferase-based assay for simultaneous analyses of the ligand concentration- and time-dependent recruitment of β-arrestin2

Functional selectivity of agonists has gained increasing interest in G protein-coupled receptor (GPCR) research, e.g. due to expectations of drugs with reduced adverse effects. Different agonist-dependent GPCR conformations are conceived to selectively activate a balanced or imbalanced intracellular signalling response, involving e.g. different Gα subtypes, Gβγ-subunits and β-arrestins. To discriminate between the different signalling pathways (bias), sensitive techniques are needed that do not interfere with signalling. We applied split luciferase complementation to the GPCR/β-arrestin2 interaction and thoroughly analysed the influence of its implementation on intracellular signalling. This led to an assay enabling the functional characterization of ligands at the hH₁R, the hM_1,5R and the hNTS₁R in live HEK293T cells. As demonstrated at the hM_1,5R, the assay was sensitive enough to identify iperoxo as a superagonist. Time-dependent analyses of the recruitment of β-arrestin2 became possible, allowing the identification of class A and class B GPCRs, due to the differential duration of their interaction with β-arrestin2 and their recycling to the cell membrane. The developed β-arrestin2 recruitment assay, which provides concentration- and time-dependent information on the interaction between GPCRs and β-arrestin2 upon stimulation of the receptor, should be broadly applicable and of high value for the analysis of agonist bias.

Direct SUMOylation of M1 muscarinic acetylcholine receptor increases its ligand-binding affinity and signal transduction

SUMOylation is a significant post-translational modification (PTM) by the small ubiquitin-related modifier (SUMO). Increasing evidence shows SUMOylation regulates GPCR signaling; however, very few GPCRs have been shown to be SUMOylation targets to date. In this study, we identified M1 muscarinic acetylcholine receptor (M1 mAChR), a member of the GPCRs, as a new SUMO substrate. When the mAChR was activated by the agonist carbachol, the colocalization of the M1 mAChR and SUMO-1 protein markedly decreased in immunoprecipitation and immunofluorescence assays. SUMOylation of the M1 mAChR played an important role in increasing the ligand-binding affinity to M1 mAChR, signaling efficiencies, and receptor endocytosis. Through the site-directed mutagenesis approach, K327 was identified as the SUMOylation site of the M1 mAChR. Mutation of the consensus SUMOylation site of the M1 mAChR reduces not only the colocalization of SUMO-1, but also the ligand-binding affinity and signal transduction. The function of M1 mAChR was regulated by SUMOylation through the stabilization of active-state conformation revealed by molecular dynamics simulations. Our results provide evidence that M1 SUMOylation is an important PTM involved in regulation of the affinity for agonists and for activation of signaling pathways.-Xu, J., Tan, P., Li, H., Cui, Y., Qiu, Y., Wang, H., Zhang, X., Li, J., Zhu, L., Zhou, W., Chen, H. Direct SUMOylation of M1 muscarinic acetylcholine receptor increases its ligand-binding affinity and signal transduction.

Glucagon-like peptide-1 receptor internalisation controls spatiotemporal signalling mediated by biased agonists

The glucagon-like peptide-1 receptor (GLP-1R) is a major therapeutic target in the treatment of type 2 diabetes due to its roles in regulating blood glucose and in promoting weight loss. Like many GPCRs, it is pleiotropically coupled, can be activated by multiple ligands and is subject to biased agonism. The GLP-1R undergoes agonist mediated receptor internalisation that may be associated with spatiotemporal control of signalling and biased agonism, although to date, this has not been extensively explored. Here, we investigate GLP-1R trafficking and its importance with regard to signalling, including the localisation of key signalling molecules, mediated by biased peptide agonists that are either endogenous GLP-1R ligands or are used clinically. Each of the agonists promoted receptor internalisation through a dynamin and caveolae dependent mechanism and traffic the receptor to both degradative and recycling pathways. This internalisation is important for signalling, with cAMP and ERK1/2 phoshorylation (pERK1/2) generated by both plasma membrane localised and internalised receptors. Further assessment of pERK1/2 revealed that all peptides induced nuclear ERK activity, but ligands, liraglutide and oxyntomodulin that are biased towards pERK1/2 relative to cAMP (when compared to GLP-1 and exendin-4), also stimulated pERK1/2 activity in the cytosol. This compartmentalisation of ERK1/2 signalling was reliant on receptor internalisation, with restriction of receptor localisation to the plasma membrane limiting ERK1/2 signalling to the cytosol. Thus, this study implicates a role of receptor internalisation in spatiotemporal control of ERK1/2 signalling that may contribute to GLP-1R biased agonism.

Assessment of the Molecular Mechanisms of Action of Novel 4-Phenylpyridine-2-One and 6-Phenylpyrimidin-4-One Allosteric Modulators at the M1 Muscarinic Acetylcholine Receptors

Positive allosteric modulators (PAMs) that target the M1 muscarinic acetylcholine (ACh) receptor (M1 mAChR) are potential treatments for cognitive deficits in conditions such as Alzheimer disease and schizophrenia. We recently reported novel 4-phenylpyridine-2-one and 6-phenylpyrimidin-4-one M1 mAChR PAMs with the potential to display different modes of positive allosteric modulation and/or agonism but whose molecular mechanisms of action remain undetermined. The current study compared the pharmacology of three such novel PAMs with the prototypical first-generation PAM, benzyl quinolone carboxylic acid (BQCA), in a recombinant Chinese hamster ovary (CHO) cell line stably expressing the human M1 mAChR. Interactions between the orthosteric agonists and the novel PAMs or BQCA suggested their allosteric effects were solely governed by modulation of agonist affinity. The greatest degree of positive co-operativity was observed with higher efficacy agonists, whereas minimal potentiation was observed when the modulators were tested against the lower efficacy agonist, xanomeline. Each PAM was investigated for its effects on the endogenous agonist ACh on three different signaling pathways [extracellular signal-regulated kinases 1/2 phosphorylation, inositol monophosphate (IP1) accumulation, and β-arrestin-2 recruitment], revealing that the allosteric potentiation generally tracked with the efficiency of stimulus-response coupling, and that there was little pathway bias in the allosteric effects. Thus, despite the identification of novel allosteric scaffolds targeting the M1 mAChR, the molecular mechanism of action of these compounds is largely consistent with a model of allostery previously described for BQCA, suggesting that this may be a more generalized mechanism for M1 mAChR PAM effects than previously appreciated.

Regulation and trafficking of muscarinic acetylcholine receptors

Fidelity of signal transduction relies on cells expressing the appropriate number of functional receptors. Fluctuation in the total number of muscarinic acetylcholine receptors has been implicated in a range of physiological and pathophysiological processes, and the mechanisms responsible for this regulation represent potential molecular targets for therapeutic intervention. This article will review the current literature on the endocytic trafficking of muscarinic receptors and how knowledge of the trafficking of related receptors might influence future studies. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Potent Prearranged Positive Allosteric Modulators of the Glucagon-like Peptide-1 Receptor

Drugs that allosterically modulate G protein-coupled receptor (GPCR) activity display higher specificity and may improve disease treatment. However, the rational design of compounds that target the allosteric site is difficult, as conformations required for receptor activation are poorly understood. Guided by photopharmacology, a set of prearranged positive allosteric modulators (PAMs) with restricted degrees of freedom was designed and tested against the glucagon-like peptide-1 receptor (GLP-1R), a GPCR involved in glucose homeostasis. Compounds incorporating a trans-stilbene comprehensively outperformed those with a cis-stilbene, as well as the benchmark BETP, as GLP-1R PAMs. We also identified major effects of ligand conformation on GLP-1R binding kinetics and signal bias. Thus, we describe a photopharmacology-directed approach for rational drug design, and introduce a new class of stilbene-containing PAM for the specific regulation of GPCR activity.

Muscarinic receptor regulates extracellular signal regulated kinase by two modes of arrestin binding

Binding of agonists to G-protein-coupled receptors (GPCRs) activates heterotrimeric G proteins and downstream signaling. Agonist-bound GPCRs are then phosphorylated by protein kinases and bound by arrestin to trigger desensitization and endocytosis. Arrestin plays another important signaling function. It recruits and regulates activity of an extracellular signal-regulated kinase (ERK) cascade. However, molecular details and timing of ERK activation remain fundamental unanswered questions that limit understanding of how arrestin-dependent GPCR signaling controls cell functions. Here we validate and model a system that tracks the dynamics of interactions of arrestin with receptors and of ERK activation using optical reporters. Our intermolecular FRET measurements in living cells are consistent with β-arrestin binding to M₁ muscarinic acetylcholine receptors (M₁Rs) in two different binding modes, transient and stable. The stable mode persists for minutes after agonist removal. The choice of mode is governed by phosphorylation on key residues in the third intracellular loop of the receptor. We detect a similar intramolecular conformational change in arrestin in either binding mode. It develops within seconds of arrestin binding to the M₁ receptor, and it reverses within seconds of arrestin unbinding from the transient binding mode. Furthermore, we observed that, when stably bound to phosphorylated M₁R, β-arrestin scaffolds and activates MEK-dependent ERK. In contrast, when transiently bound, β-arrestin reduces ERK activity via recruitment of a protein phosphatase. All this ERK signaling develops at the plasma membrane. In this scaffolding hypothesis, a shifting balance between the two arrestin binding modes determines the degree of ERK activation at the membrane.

Quantitative image cytometry for analyzing intracellular trafficking of G protein-coupled receptors on a chemical-trapping single cell array

G protein-coupled receptors (GPCRs) are important targets in medical and pharmaceutical research fields, because they play key roles in a variety of biological processes. Recently, intracellular trafficking of GPCRs involving endosomal internalization and recycling to the plasma membrane has been studied as a regulation mechanism for GPCR activities. However, the absence of a quantitative single-cell analysis method has hampered conditional GPCR trafficking studies and the possibility of gaining significant insights into the mechanism of regulation of GPCR signaling. Here, we report a facile image cytometry method to analyze the trafficking of GPCRs. In this method, GPCR-expressing cells were arrayed with a photo-responsive cell-immobilizing reagent in a single-cell manner, and the tagged GPCR was visualized by pulse-labeling with a fluorescent dye through sortase-mediated peptide-tag ligation. We quantified the intracellular distribution changes of a pH-dependent GPCR, G2A, by time-course observation under mildly acidic and slightly basic pH conditions. The difference in pH-dependent G2A trafficking between individual cells was automatically detected by an image analysis custom software program, and simultaneously, the average distribution ratios were also determined for understanding the properties of G2A. The present method should be applicable for investigating the dynamic intracellular trafficking of a wide variety of GPCRs under various conditions in a high-throughput manner.

Receptor Species-dependent Desensitization Controls KCNQ1/KCNE1 K+ Channels as Downstream Effectors of Gq Protein-coupled Receptors

Activation of G_q protein-coupled receptors (G_qPCRs) might induce divergent cellular responses, related to receptor-specific activation of different branches of the G_q signaling pathway. Receptor-specific desensitization provides a mechanism of effector modulation by restricting the spatiotemporal activation of signaling components downstream of G_q We quantified signaling events downstream of G_qPCR activation with FRET-based biosensors in CHO and HEK 293 cells. KCNQ1/KCNE1 channels (I_Ks) were measured as a functional readout of receptor-specific activation. Activation of muscarinic M₁ receptors (M₁-Rs) caused robust and reversible inhibition of I_Ks. In contrast, activation of α_1B-adrenergic receptors (α_1B-ARs) induced transient inhibition of I_Ks, which turned into delayed facilitation after agonist withdrawal. As a novel finding, we demonstrate that G_qPCR-specific kinetics of I_Ks modulation are determined by receptor-specific desensitization, evident at the level of Gα_q activation, phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion, and diacylglycerol production. Sustained I_Ks inhibition during M₁-R stimulation is attributed to robust membrane PIP₂ depletion, whereas the rapid desensitization of α_1B-AR delimits PIP₂ reduction and augments current activation by protein kinase C (PKC). Overexpression of Ca²⁺-independent PKCδ did not affect the time course of α_1B-AR-induced diacylglycerol formation, excluding a contribution of PKCδ to α_1B-AR desensitization. Pharmacological inhibition of Ca²⁺-dependent PKC isoforms abolished fast α_1B receptor desensitization and augmented I_Ks reduction, but did not affect I_Ks facilitation. These data indicate a contribution of Ca²⁺-dependent PKCs to α_1B-AR desensitization, whereas I_Ks facilitation is induced by Ca²⁺-independent PKC isoforms. In contrast, neither inhibition of Ca²⁺-dependent/Ca²⁺-independent isoforms nor overexpression of PKCδ induced M₁ receptor desensitization, excluding a contribution of PKC to M₁-R-induced I_Ks modulation.

Specific cell surface labeling of GPCRs using split GFP

Specific cell surface labeling is essential for visualizing the internalization processes of G-protein coupled receptors (GPCRs) and for gaining mechanistic insight of GPCR functions. Here we present a rapid, specific, and versatile labeling scheme for GPCRs at living-cell membrane with the use of a split green fluorescent protein (GFP). Demonstrated with two GPCRs, GPR17 and CysLT₂R, we show that two β-stands (β-stands 10 and 11) derived from a superfolder GFP (sfGFP) can be engineered to one of the three extracellular loop of a GPCR. The complementary fragment of sfGFP has nine β-strands (β-stands 1-9) that carries the mature fluorophore, and can be proteolytically derived from the full-length sfGFP. Separately the GFP fragments are non-fluorescent, but become fluorescent upon assembly, thus allowing specific labeling of the target proteins. The two GFP fragments rapidly assemble and the resulting complex is extremely tight under non-denaturing conditions, which allows real-time and quantitative assessment of the internalized GPCRs. We envision that this labeling scheme will be of great use for labeling other membrane proteins in various biological and pharmacological applications.

An Antibody Biosensor Establishes the Activation of the M1 Muscarinic Acetylcholine Receptor during Learning and Memory

Establishing the in vivo activation status of G protein-coupled receptors would not only indicate physiological roles of G protein-coupled receptors but would also aid drug discovery by establishing drug/receptor engagement. Here, we develop a phospho-specific antibody-based biosensor to detect activation of the M₁ muscarinic acetylcholine receptor (M₁ mAChR) in vitro and in vivo. Mass spectrometry phosphoproteomics identified 14 sites of phosphorylation on the M₁ mAChR. Phospho-specific antibodies to four of these sites established that serine at position 228 (Ser²²⁸) on the M₁ mAChR showed extremely low levels of basal phosphorylation that were significantly up-regulated by orthosteric agonist stimulation. In addition, the M₁ mAChR-positive allosteric modulator, 1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid, enhanced acetylcholine-mediated phosphorylation at Ser²²⁸. These data supported the hypothesis that phosphorylation at Ser²²⁸ was an indicator of M₁ mAChR activation. This was further supported in vivo by the identification of phosphorylated Ser²²⁸ on the M₁ mAChR in the hippocampus of mice following administration of the muscarinic ligands xanomeline and 1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid. Finally, Ser²²⁸ phosphorylation was seen to increase in the CA1 region of the hippocampus following memory acquisition, a response that correlated closely with up-regulation of CA1 neuronal activity. Thus, determining the phosphorylation status of the M₁ mAChR at Ser²²⁸ not only provides a means of establishing receptor activation following drug treatment both in vitro and in vivo but also allows for the mapping of the activation status of the M₁ mAChR in the hippocampus following memory acquisition thereby establishing a link between M₁ mAChR activation and hippocampus-based memory and learning.

Discovery of a Novel Class of Negative Allosteric Modulator of the Dopamine D2 Receptor Through Fragmentation of a Bitopic Ligand

Recently, we have demonstrated that N-((trans)-4-(2-(7-cyano-3,4-dihydroisoquinolin-2(1H)-yl)ethyl)cyclohexyl)-1H-indole-2-carboxamide (SB269652) (1) adopts a bitopic pose at one protomer of a dopamine D2 receptor (D2R) dimer to negatively modulate the binding of dopamine at the other protomer. The 1H-indole-2-carboxamide moiety of 1 extends into a secondary pocket between the extracellular ends of TM2 and TM7 within the D2R protomer. To target this putative allosteric site, we generated and characterized fragments that include and extend from the 1H-indole-2-carboxamide moiety of 1. N-Isopropyl-1H-indole-2-carboxamide (3) displayed allosteric pharmacology and sensitivity to mutations of the same residues at the top of TM2 as was observed for 1. Using 3 as an "allosteric lead", we designed and synthesized an extensive fragment library to generate novel SAR and identify N-butyl-1H-indole-2-carboxamide (11d), which displayed both increased negative cooperativity and affinity for the D2R. These data illustrate that fragmentation of extended compounds can expose fragments with purely allosteric pharmacology.

β-Arrestin Recruitment and Biased Agonism at Free Fatty Acid Receptor 1

FFAR1/GPR40 is a seven-transmembrane domain receptor (7TMR) expressed in pancreatic β cells and activated by FFAs. Pharmacological activation of GPR40 is a strategy under consideration to increase insulin secretion in type 2 diabetes. GPR40 is known to signal predominantly via the heterotrimeric G proteins Gq/11. However, 7TMRs can also activate functionally distinct G protein-independent signaling via β-arrestins. Further, G protein- and β-arrestin-based signaling can be differentially modulated by different ligands, thus eliciting ligand-specific responses ("biased agonism"). Whether GPR40 engages β-arrestin-dependent mechanisms and is subject to biased agonism is unknown. Using bioluminescence resonance energy transfer-based biosensors for real-time monitoring of cell signaling in living cells, we detected a ligand-induced GPR40-β-arrestin interaction, with the synthetic GPR40 agonist TAK-875 being more effective than palmitate or oleate in recruiting β-arrestins 1 and 2. Conversely, TAK-875 acted as a partial agonist of Gq/11-dependent GPR40 signaling relative to both FFAs. Pharmacological blockade of Gq activity decreased FFA-induced insulin secretion. In contrast, knockdown or genetic ablation of β-arrestin 2 in an insulin-secreting cell line and mouse pancreatic islets, respectively, uniquely attenuated the insulinotropic activity of TAK-875, thus providing functional validation of the biosensor data. Collectively, these data reveal that in addition to coupling to Gq/11, GPR40 is functionally linked to a β-arrestin 2-mediated insulinotropic signaling axis. These observations expose previously unrecognized complexity for GPR40 signal transduction and may guide the development of biased agonists showing improved clinical profile in type 2 diabetes.

Unraveling the molecular architecture of a G protein-coupled receptor/β-arrestin/Erk module complex

β-arrestins serve as signaling scaffolds downstream of G protein-coupled receptors, and thus play a crucial role in a plethora of cellular processes. Although it is largely accepted that the ability of β-arrestins to interact simultaneously with many protein partners is key in G protein-independent signaling of GPCRs, only the precise knowledge of these multimeric arrangements will allow a full understanding of the dynamics of these interactions and their functional consequences. However, current experimental procedures for the determination of the three-dimensional structures of protein-protein complexes are not well adapted to analyze these short-lived, multi-component assemblies. We propose a model of the receptor/β-arrestin/Erk1 signaling module, which is consistent with most of the available experimental data. Moreover, for the β-arrestin/Raf1 and the β-arrestin/ERK interactions, we have used the model to design interfering peptides and shown that they compete with both partners, hereby demonstrating the validity of the predicted interaction regions.

The Viral G Protein-Coupled Receptor ORF74 Hijacks β-Arrestins for Endocytic Trafficking in Response to Human Chemokines

Kaposi’s sarcoma-associated herpesvirus-infected cells express the virally encoded G protein-coupled receptor ORF74. Although ORF74 is constitutively active, it binds human CXC chemokines that modulate this basal activity. ORF74-induced signaling has been demonstrated to underlie the development of the angioproliferative tumor Kaposi’s sarcoma. Whereas G protein-dependent signaling of ORF74 has been the subject of several studies, the interaction of this viral GPCR with β-arrestins has hitherto not been investigated. Bioluminescence resonance energy transfer experiments demonstrate that ORF74 recruits β-arrestins and subsequently internalizes in response to human CXCL1 and CXCL8, but not CXCL10. Internalized ORF74 traffics via early endosomes to recycling and late endosomes. Site-directed mutagenesis and homology modeling identified four serine and threonine residues at the distal end of the intracellular carboxyl-terminal of ORF74 that are required for β-arrestin recruitment and subsequent endocytic trafficking. Hijacking of the human endocytic trafficking machinery is a previously unrecognized action of ORF74.

A new mechanism of allostery in a G protein-coupled receptor dimer

SB269652 (1) is the first drug-like allosteric modulator of the dopamine D₂ receptor (D₂R), but contains structural features associated with orthosteric D₂R antagonists. Using a functional complementation system to control the identity of individual protomers within a dimeric D₂R complex, we converted the pharmacology of the interaction between SB269652 and dopamine from allosteric to competitive by impairing ligand binding to one of the protomers, indicating that the allostery requires D₂R dimers. Additional experiments identified a “bitopic” pose for SB269652 extending from the orthosteric site into a secondary pocket at the extracellular end of the transmembrane (TM) domain, involving TM2 and TM7. Engagement of this secondary pocket was a requirement for the allosteric pharmacology of SB269652. This suggests a novel mechanism whereby a bitopic ligand binds in an extended pose on one G protein-coupled receptor protomer to allosterically modulate the binding of a ligand to the orthosteric site of a second protomer.