Ligand bias at the muscarinic acetylcholine receptor family: Opportunities and challenges

Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors (GPCRs) that are activated by the endogenous neurotransmitter, acetylcholine (ACh). Disruption of mAChR signalling has been associated with a variety of neurological disorders and non-neurological diseases. Consequently, the development of agonists and antagonists of the mAChRs has been a major avenue in drug discovery. Unfortunately, mAChR ligands are often associated with on-target side effects for two reasons. The first reason is due to the high sequence conservation at the orthosteric ACh binding site among all five receptor subtypes (M1-M5), making on-target subtype selectivity a major challenge. The second reason is due to on-target side effects of mAChR drugs that are associated with the pleiotropic nature of mAChR signalling at the level of a single mAChR subtype. Indeed, there is growing evidence that within the myriad of signalling events produced by mAChR ligands, some will have therapeutic benefits, whilst others may promote cholinergic side effects. This paradigm of drug action, known as ligand bias or biased agonism, is an attractive feature for next-generation mAChR drugs, as it holds the promise of developing drugs devoid of on-target adverse effects. Although relatively simple to detect and even quantify in vitro, ligand bias, as observed in recombinant systems, does not always translate to in vivo systems, which remains a major hurdle in GPCR drug discovery, including the mAChR family. Here we report recent studies that have attempted to detect and quantify ligand bias at the mAChR family, and briefly discuss the challenges associated with biased agonist drug development. This article is part of the Special Issue on "Ligand Bias".

Understanding the impact of nuclear-localized GPCRs on cellular signalling

G protein-coupled receptors (GPCRs) have historically been associated with signalling events driven from the plasma membrane. More recently, signalling from endosomes has been recognized as a feature of internalizing receptors. However, there was little consideration given to the notion that GPCRs can be targeted to distinct subcellular locations that did not involve an initial trafficking to the cell surface. Here, we focus on the evidence for and the potential impact of GPCR signalling specifically initiated from the nuclear membrane. We also discuss the possibilities for selectively targeting this and other internal pools of receptors as novel venues for drug discovery.

Probing phosphorylation events in biological membranes: The transducer function

The extracellular environment is sensed by receptors in the plasma membrane. Some of these receptors initiate cytoplasmic signaling cascades involving phosphorylation: the addition of a phosphate group to a specific amino acid, such as tyrosine, in a protein. Receptor Tyrosine Kinases (RTKs) are one large class of membrane receptors that can directly initiate signaling cascades through their intracellular kinase domains, which both catalyze tyrosine phosphorylation and get phosphorylated. In the first step of signaling, the ligands stabilize phosphorylation-competent RTK dimers and oligomers, which leads to the phosphorylation of specific tyrosine residues in the activation loop of the kinases. Here we discuss quantitative measurements of tyrosine phosphorylation efficiencies for RTKs, described by the "transducer function". The transducer function links the phosphorylation (the response) and the binding of the activating ligand to the receptor (the stimulus). We overview a methodology that allows such measurements in direct response to ligand binding. We discuss experiments which demonstrate that EGF is a partial agonist, and that two tyrosines in the intracellular domain of EGFR, Y1068 and Y1173, are differentially phosphorylated in the EGF-bound EGFR dimers.

SHU9119 and MBP10 are biased ligands at the human melanocortin-4 receptor

The melanocortin-4 receptor (MC4R), a G protein-coupled receptor, is critically involved in regulating energy homeostasis as well as modulation of reproduction and sexual function. Two peptide antagonists (SHU9119 and MBP10) were derived from the endogenous agonist α-melanocyte stimulating hormone. But their pharmacology at human MC4R is not fully understood. Herein, we performed detailed pharmacological studies of SHU9119 and MBP10 on wild-type (WT) and six naturally occurring constitutively active MC4Rs. Both ligands had no or negligible agonist activity in Gαs-cAMP signaling on WT MC4R, but stimulated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation on WT and mutant MC4Rs. Mechanistic studies revealed that SHU9119 and MBP10 stimulated ERK1/2 signaling of MC4R by different mechanisms, with SHU9119-stimulated ERK1/2 signaling mediated by phosphatidylinositol 3-kinase (PI3K) and MBP10-initiated ERK1/2 activation through PI3K and β-arrestin. In summary, our studies demonstrated that SHU9119 and MBP10 were biased ligands for MC4R, preferentially activating ERK1/2 signaling through different mechanisms. SHU9119 acted as a biased ligand and MBP10 behaved as a biased allosteric modulator.

Design, Synthesis, and In Vitro Characterization of Proteolytically-Stable Opioid-Neurotensin Hybrid Peptidomimetics

Linking an opioid to a nonopioid pharmacophore represents a promising approach for reducing opioid-induced side effects during pain management. Herein, we describe the optimization of the previously reported opioid-neurotensin hybrids (OPNT-hybrids), SBL-OPNT-05 & -10, containing the μ-/δ-opioid agonist H-Dmt-d-Arg-Aba-β-Ala-NH2 and NT(8-13) analogs optimized for NTS2 affinity. In the present work, the constrained dipeptide Aba-β-Ala was modified to investigate the optimal linker length between the two pharmacophores, as well as the effect of expanding the aromatic moiety within constrained dipeptide analogs, via the inclusion of a naphthyl moiety. Additionally, the N-terminal Arg residue of the NT(8-13) pharmacophore was substituted with β3 hArg. For all analogs, affinity was determined at the MOP, DOP, NTS1, and NTS2 receptors. Several of the hybrid ligands showed a subnanomolar affinity for MOP, improved binding for DOP compared to SBL-OPNT-05 & -10, as well as an excellent NTS2-affinity with high selectivity over NTS1. Subsequently, the Gαi1 and β-arrestin-2 pathways were evaluated for all hybrids, along with their stability in rat plasma. Upon MOP activation, SBL-OPNT-13 and -18 were the least effective at recruiting β-arrestin-2 (E max = 17 and 12%, respectively), while both compounds were also found to be partial agonists at the Gαi1 pathway, despite improved potency compared to DAMGO. Importantly, these analogs also showed a half-life in rat plasma in excess of 48 h, making them valuable tools for future in vivo investigations.

A molecular mechanism to diversify Ca2+ signaling downstream of Gs protein-coupled receptors

A long-held tenet in inositol-lipid signaling is that cleavage of membrane phosphoinositides by phospholipase Cβ (PLCβ) isozymes to increase cytosolic Ca2+ in living cells is exclusive to Gq- and Gi-sensitive G protein-coupled receptors (GPCRs). Here we extend this central tenet and show that Gs-GPCRs also partake in inositol-lipid signaling and thereby increase cytosolic Ca2+. By combining CRISPR/Cas9 genome editing to delete Gαs, the adenylyl cyclase isoforms 3 and 6, or the PLCβ1-4 isozymes, with pharmacological and genetic inhibition of Gq and G11, we pin down Gs-derived Gβγ as driver of a PLCβ2/3-mediated cytosolic Ca2+ release module. This module does not require but crosstalks with Gαs-dependent cAMP, demands Gαq to release PLCβ3 autoinhibition, but becomes Gq-independent with mutational disruption of the PLCβ3 autoinhibited state. Our findings uncover the key steps of a previously unappreciated mechanism utilized by mammalian cells to finetune their calcium signaling regulation through Gs-GPCRs.

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.

Get Ready to Sharpen Your Tools: A Short Guide to Heterotrimeric G Protein Activity Biosensors

G protein-coupled receptors (GPCRs) are the largest class of transmembrane receptors encoded in the human genome, and they initiate cellular responses triggered by a plethora of extracellular stimuli ranging from neurotransmitters and hormones to photons. Upon stimulation, GPCRs activate heterotrimeric G proteins (Gαβγ) in the cytoplasm, which then convey signals to their effectors to elicit cellular responses. Given the broad biological and biomedical relevance of GPCRs and G proteins in physiology and disease, there is great interest in developing and optimizing approaches to measure their signaling activity with high accuracy and across experimental systems pertinent to their functions in cellular communication. This review provides a historical perspective on approaches to measure GPCR-G protein signaling, from quantification of second messengers and other indirect readouts of activity to biosensors that directly detect the activity of G proteins. The latter is the focus of a more detailed overview of the evolution of design principles for various optical biosensors of G protein activity with different experimental capabilities. We will highlight advantages and limitations of biosensors that detect different G protein activation hallmarks, like dissociation of Gα and Gβγ or nucleotide exchange on Gα, as well as their suitability to detect signaling mediated by endogenous versus exogenous signaling components or in physiologically relevant systems like primary cells. Overall, this review intends to provide an assessment of the state-of-the-art for biosensors that directly measure G protein activity to allow readers to make informed decisions on the selection and implementation of currently available tools. SIGNIFICANCE STATEMENT: G protein activity biosensors have become essential and widespread tools to assess GPCR signaling and pharmacology. Yet, investigators face the challenge of choosing from a growing list of G protein activity biosensors. This review provides an overview of the features and capabilities of different optical biosensor designs for the direct detection of G protein activity in cells, with the aim of facilitating the rational selection of systems that align with the specific scientific questions and needs of investigators.

Linkers in Bitopic Agonists Shape Bias Profile among Transducers for the Dopamine D2 and D3 Receptors

Bitopic ligands bind both orthosteric and allosteric or secondary binding sites within the same receptor, often resulting in an improvement of receptor selectivity, potency, and efficacy. In particular, for both agonists and antagonists of the dopamine D2 and D3 receptors (D2R and D3R), the primary therapeutic targets for several neurological and neuropsychiatric disorders, bitopic ligand design has proved advantageous in achieving better pharmacological profiles in vitro. Although the two pharmacophores within a bitopic ligand are typically considered the main drivers of conformational change for a receptor, the role of the linker that connects the two has not yet been systematically studied for its relevance in receptor activity profiles. Here, we present a comprehensive analysis of sumanirole and PF592,379-based indole-containing bitopic compounds in agonist activity at D2R and D3R, with a focus on linker chemical space and stereochemistry through testing six distinct chirally resolved linkers and a simple aliphatic linker. The structure activity relationships (SARs) of these linkers are examined extensively, beyond the conventional level, by characterizing the activation of all putative transducers over a 44 min time course. Our multiparametric analysis reveals previously unappreciated specific linker-dependent effects on primary pharmacophores, receptors, transducer activation kinetics, and bias, highlighting the utility of this comprehensive approach and the significance of the linker type in shaping transducer bias profiles.

Natural Product-Inspired Dopamine Receptor Ligands

Due to their evolutionary bias as ligands for biologically relevant drug targets, natural products offer a unique opportunity as lead compounds in drug discovery. Given the involvement of dopamine receptors in various physiological and behavioral functions, they are linked to numerous diseases and disorders such as Parkinson's disease, schizophrenia, and substance use disorders. Consequently, ligands targeting dopamine receptors hold considerable therapeutic and investigative promise. As this perspective will highlight, dopamine receptor targeting natural products play a pivotal role as scaffolds with unique and beneficial pharmacological properties, allowing for natural product-inspired drug design and lead optimization. As such, dopamine receptor targeting natural products still have untapped potential to aid in the treatment of disorders and diseases related to central nervous system (CNS) and peripheral nervous system (PNS) dysfunction.

Know your molecule: pharmacological characterization of drug candidates to enhance efficacy and reduce late-stage attrition

Despite advances in chemical, computational and biological sciences, the rate of attrition of drug candidates in clinical development is still high. A key point in the small-molecule discovery process that could provide opportunities to help address this challenge is the pharmacological characterization of hit and lead compounds, culminating in the selection of a drug candidate. Deeper characterization is increasingly important, because the 'quality' of drug efficacy, at least for G protein-coupled receptors (GPCRs), is now understood to be much more than activation of commonly evaluated pathways such as cAMP signalling, with many more 'efficacies' of ligands that could be harnessed therapeutically. Such characterization is being enabled by novel assays to characterize the complex behaviour of GPCRs, such as biased signalling and allosteric modulation, as well as advances in structural biology, such as cryo-electron microscopy. This article discusses key factors in the assessments of the pharmacology of hit and lead compounds in the context of GPCRs as a target class, highlighting opportunities to identify drug candidates with the potential to address limitations of current therapies and to improve the probability of them succeeding in clinical development.

Quantitative and qualitative differences in the activation of a fibroblast growth factor receptor by different FGF ligands

The FGF system is the most complex of all receptor tyrosine kinase signaling networks with 18 FGF ligands and four FGFRs that deliver morphogenic signals to pattern most embryonic structures. Even when a single FGFR is expressed in the tissue, different FGFs can trigger dramatically different biological responses via this receptor. Here we show both quantitative and qualitative differences in the signaling of one of the FGF receptors, FGFR1c, in response to different FGFs. We provide an overview of the recent discovery that FGFs engage in biased signaling via FGFR1c. We discuss the concept of ligand bias, which represents qualitative differences in signaling as it is a measure of differential ligand preferences for different downstream responses. We show how FGF ligand bias manifests in functional data in cultured chondrocyte cells. We argue that FGF-ligand bias contributes substantially to FGF-driven developmental processes, along with known differences in FGF expression levels, FGF-FGFR binding coefficients and differences in FGF stability in vivo.

Inverse Regulation of C-C Chemokine Receptor 3 Oligomerization by Downstream Proteins Indicates Biased Signal Transduction Pathways

Oligomerization is one of the important mechanisms for G protein-coupled receptors (GPCRs) to modulate their activity in signal transduction. However, details of how and why the oligomerization of GPCRs regulates their functions under physiological conditions remain largely unknown. Here, using single-molecule photobleaching technology, we show that chemokine ligand 5 (CCL5) and chemokine ligand 8 (CCL8) are similar to the previously reported chemokine ligand 11 (CCL11) and chemokine ligand 24 (CCL24), which can regulate the oligomerization of chemokine receptor 3 (CCR3). Our results further demonstrate that downstream proteins, β-arrestin 2 and Gi protein complex, on the CCR3 signal transduction pathway, can inversely regulate the oligomeric states of CCR3 induced by its binding ligands. This unexpected discovery suggests complex relationships between the oligomeric behaviors of CCR3 and the components of ligands-CCR3-downstream proteins, reflecting the potentially functional impact of the oligomerization on the multiple activation pathways of GPCR, such as biased activation.

Smooth operator(s): dialing up and down neurotransmitter responses by G-protein regulators

G-protein-coupled receptors (GPCRs) are essential mediators of neuromodulation and prominent pharmacological targets. While activation of heterotrimeric G-proteins (Gαβɣ) by GPCRs is essential in this process, much less is known about the postreceptor mechanisms that influence G-protein activity. Neurons express G-protein regulators that shape the amplitude and kinetics of GPCR-mediated synaptic responses. Although many of these operate by directly altering how G-proteins handle guanine-nucleotides enzymatically, recent discoveries have revealed alternative mechanisms by which GPCR-stimulated G-protein responses are modulated at the synapse. In this review, we cover the molecular basis for, and consequences of, the action of two G-protein regulators that do not affect the enzymatic activity of G-proteins directly: Gα inhibitory interacting protein (GINIP), which binds active Gα subunits, and potassium channel tetramerization domain-containing 12 (KCTD12), which binds active Gβγ subunits.

Functional consequences of spatial, temporal and ligand bias of G protein-coupled receptors

G protein-coupled receptors (GPCRs) regulate every aspect of kidney function by mediating the effects of various endogenous and exogenous substances. A key concept in GPCR function is biased signalling, whereby certain ligands may selectively activate specific pathways within the receptor's signalling repertoire. For example, different agonists may induce biased signalling by stabilizing distinct active receptor conformations - a concept that is supported by advances in structural biology. However, the processes underlying functional selectivity in receptor signalling are extremely complex, involving differences in subcellular compartmentalization and signalling dynamics. Importantly, the molecular mechanisms of spatiotemporal bias, particularly its connection to ligand binding kinetics, have been detailed for GPCRs critical to kidney function, such as the AT1 angiotensin receptor (AT1R), V2 vasopressin receptor (V2R) and the parathyroid hormone 1 receptor (PTH1R). This expanding insight into the multifaceted nature of biased signalling paves the way for innovative strategies for targeting GPCR functions; the development of novel biased agonists may represent advanced pharmacotherapeutic approaches to the treatment of kidney diseases and related systemic conditions, such as hypertension, diabetes and heart failure.

The application of artificial intelligence to accelerate G protein-coupled receptor drug discovery

The application of artificial intelligence (AI) approaches to drug discovery for G protein-coupled receptors (GPCRs) is a rapidly expanding area. Artificial intelligence can be used at multiple stages during the drug discovery process, from aiding our understanding of the fundamental actions of GPCRs to the discovery of new ligand-GPCR interactions or the prediction of clinical responses. Here, we provide an overview of the concepts behind artificial intelligence, including the subfields of machine learning and deep learning. We summarise the published applications of artificial intelligence to different stages of the GPCR drug discovery process. Finally, we reflect on the benefits and limitations of artificial intelligence and share our vision for the exciting potential for further development of applications to aid GPCR drug discovery. In addition to making the drug discovery process "faster, smarter and cheaper," we anticipate that the application of artificial intelligence will create exciting new opportunities for GPCR drug discovery. 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.

Exploiting Cell-Based Assays to Accelerate Drug Development for G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are relevant targets for health and disease as they regulate various aspects of metabolism, proliferation, differentiation, and immune pathways. They are implicated in several disease areas, including cancer, diabetes, cardiovascular diseases, and mental disorders. It is worth noting that about a third of all marketed drugs target GPCRs, making them prime pharmacological targets for drug discovery. Numerous functional assays have been developed to assess GPCR activity and GPCR signaling in living cells. Here, we review the current literature of genetically encoded cell-based assays to measure GPCR activation and downstream signaling at different hierarchical levels of signaling, from the receptor to transcription, via transducers, effectors, and second messengers. Singleplex assay formats provide one data point per experimental condition. Typical examples are bioluminescence resonance energy transfer (BRET) assays and protease cleavage assays (e.g., Tango or split TEV). By contrast, multiplex assay formats allow for the parallel measurement of multiple receptors and pathways and typically use molecular barcodes as transcriptional reporters in barcoded assays. This enables the efficient identification of desired on-target and on-pathway effects as well as detrimental off-target and off-pathway effects. Multiplex assays are anticipated to accelerate drug discovery for GPCRs as they provide a comprehensive and broad identification of compound effects.

Bias translation: The final frontier?

Biased signalling is a natural result of GPCR allosteric function and should be expected from any and all synthetic and natural agonists. Therefore, it may be encountered in all agonist discovery projects and must be considered as a beneficial (or possible detrimental) feature of new candidate molecules. While bias is detected easily, the synoptic nature of GPCR signalling makes translation of simple in vitro bias to complex in vivo systems problematic. The practical outcome of this is a difficulty in predicting the therapeutic value of biased signalling due to the failure of translation of identified biased signalling to in vivo agonism. This is discussed in this review as well as some new ways forward to improve this translation process and better exploit this powerful pharmacologic mechanism.

Anti-inflammatory action of new hybrid N-acyl-[1,2]dithiolo-[3,4-c]quinoline-1-thione

Most of pharmaceutical agents display a number of biological activities. It is obvious that testing even one compound for thousands of biological activities is not practically possible. A computer-aided prediction is therefore the method of choice in this case to select the most promising bioassays for particular compounds. Using the PASS Online software, we determined the probable anti-inflammatory action of the 12 new hybrid dithioloquinolinethiones derivatives. Chemical similarity search in the World-Wide Approved Drugs (WWAD) and DrugBank databases did not reveal close structural analogues with the anti-inflammatory action. Experimental testing of anti-inflammatory activity of the synthesized compounds in the carrageenan-induced inflammation mouse model confirmed the computational predictions. The anti-inflammatory activity of the studied compounds (2a, 3a-3k except for 3j) varied between 52.97% and 68.74%, being higher than the reference drug indomethacin (47%). The most active compounds appeared to be 3h (68.74%), 3k (66.91%) and 3b (63.74%) followed by 3e (61.50%). Thus, based on the in silico predictions a novel class of anti-inflammatory agents was discovered.

Ligand bias underlies differential signaling of multiple FGFs via FGFR1

The differential signaling of multiple FGF ligands through a single fibroblast growth factor (FGF) receptor (FGFR) plays an important role in embryonic development. Here, we use quantitative biophysical tools to uncover the mechanism behind differences in FGFR1c signaling in response to FGF4, FGF8, and FGF9, a process which is relevant for limb bud outgrowth. We find that FGF8 preferentially induces FRS2 phosphorylation and extracellular matrix loss, while FGF4 and FGF9 preferentially induce FGFR1c phosphorylation and cell growth arrest. Thus, we demonstrate that FGF8 is a biased FGFR1c ligand, as compared to FGF4 and FGF9. Förster resonance energy transfer experiments reveal a correlation between biased signaling and the conformation of the FGFR1c transmembrane domain dimer. Our findings expand the mechanistic understanding of FGF signaling during development and bring the poorly understood concept of receptor tyrosine kinase ligand bias into the spotlight.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins encoded in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Specific pharmacological and Gi/o protein responses of some native GPCRs in neurons

G protein-coupled receptors (GPCRs) constitute the largest family of membrane proteins and are important drug targets. The discovery of drugs targeting these receptors and their G protein signaling properties are based on assays mainly performed with modified receptors expressed in heterologous cells. However, GPCR responses may differ in their native environment. Here, by using highly sensitive Gi/o sensors, we reveal specific properties of Gi/o protein-mediated responses triggered by GABAB, α2 adrenergic and cannabinoid CB1 receptors in primary neurons, different from those in heterologous cells. These include different profiles in the Gi/o protein subtypes-mediated responses, and differences in the potencies of some ligands even at similar receptor expression levels. Altogether, our results show the importance of using biosensors compatible with primary cells for evaluating the activities of endogenous GPCRs in their native environment.

EGFR signaling and pharmacology in oncology revealed with innovative BRET-based biosensors

Mutations of receptor tyrosine kinases (RTKs) are associated with the development of many cancers by modifying receptor signaling and contributing to drug resistance in clinical settings. We present enhanced bystander bioluminescence resonance energy transfer-based biosensors providing new insights into RTK biology and pharmacology critical for the development of more effective RTK-targeting drugs. Distinct SH2-specific effector biosensors allow for real-time and spatiotemporal monitoring of signal transduction pathways engaged upon RTK activation. Using EGFR as a model, we demonstrate the capacity of these biosensors to differentiate unique signaling signatures, with EGF and Epiregulin ligands displaying differences in efficacy, potency, and responses within different cellular compartments. We further demonstrate that EGFR single point mutations found in Glioblastoma or non-small cell lung cancer, impact the constitutive activity of EGFR and response to tyrosine kinase inhibitor. The BRET-based biosensors are compatible with microscopy, and more importantly characterize the next generation of therapeutics directed against RTKs.

The β-Blocker Carvedilol and Related Aryloxypropanolamines Promote ERK1/2 Phosphorylation in HEK293 Cells with K A Values Distinct From Their Equilibrium Dissociation Constants as β 2-Adrenoceptor Antagonists: Evidence for Functional Affinity

The determination of affinity by using functional assays is important in drug discovery because it provides a more relevant estimate of the strength of interaction of a ligand to its cognate receptor than radioligand binding. However, empirical evidence for so-called, "functional affinity" is limited. Herein, we determined whether the affinity of carvedilol, a β-adrenoceptor antagonist used to treat heart failure that also promotes extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation, differed between these two pharmacological activities. Four structurally related β-adrenoceptor antagonists (alprenolol, carazolol, pindolol, propranolol) that also activated ERK1/2 were included as comparators to enhance our understanding of how these drugs work in the clinical setting. In HEK293 cells stably expressing the human β 2-adrenoceptor carvedilol and related aryloxypropanolamines were partial agonists of ERK1/2 phosphorylation with potencies ([A]50s) that were lower than their equilibrium dissociation constants (K Bs) as β 2-adrenoceptor antagonists. As the [A]50 of a partial agonist is a good approximation of its K B, then these data indicated that the affinities of carvedilol and related ligands for these two activities were distinct. Moreover, there was a significant negative rank order correlation between the [A]50 of each ligand to activate ERK1/2 and their intrinsic activities (i.e., as intrinsic activity for ERK1/2 phosphorylation increased, so did affinity). Genome editing revealed that the transducer that coupled the β 2-adrenoceptor to ERK1/2 phosphorylation in response to carvedilol and other β 2-adrenoceptor antagonists was Gαs. Collectively, these data support the concept of "functional affinity" and indicate that the ability of the β 2-adrenoceptor to recruit Gαs may influence the affinity of the activating ligand. SIGNIFICANCE STATEMENT: In HEK293 cells overexpressing the human β2-adrenoceptor carvedilol and four related aryloxypropanolamines behaved as β2-adrenoceptor antagonists and partial agonists of ERK1/2 phosphorylation with rank orders of affinity that were distinct. These data imply that carvedilol and other β-blockers can stabilize the β2-adrenoceptor in different affinity conformations that are revealed when functionally distinct responses are measured. This is the basis for the pharmacological concept of "functional affinity."

GproteinDb in 2024: new G protein-GPCR couplings, AlphaFold2-multimer models and interface interactions

G proteins are the major signal proteins of ∼800 receptors for medicines, hormones, neurotransmitters, tastants and odorants. GproteinDb offers integrated genomic, structural, and pharmacological data and tools for analysis, visualization and experiment design. Here, we present the first major update of GproteinDb greatly expanding its coupling data and structural templates, adding AlphaFold2 structure models of GPCR-G protein complexes and advancing the interactive analysis tools for their interfaces underlying coupling selectivity. We present insights on coupling agreement across datasets and parameters, including constitutive activity, agonist-induced activity and kinetics. GproteinDb is accessible at https://gproteindb.org.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically-relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed new insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally-occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Allostery: The Good, the Bad, and the Ugly

With the advent of functional screening, more allosteric molecules are being discovered and developed as possible therapeutic entities. Allosteric proteins are unique because of two specific properties: 1) separate binding sites for allosteric modulators and guests and 2) mandatory alteration of receptor conformation upon binding of allosteric modulators. For G protein-coupled receptors, these properties produce many beneficial effects on pharmacologic systems that are described here. Allosteric discovery campaigns also bring with them added considerations that must be addressed for the endeavor to be successful, and these are described herein as well. SIGNIFICANCE STATEMENT: Recent years have seen the increasing presence of allosteric molecules as possible therapeutic drug candidates. The scientific procedures to characterize these are unique and require special techniques, so it is imperative that scientists understand the new concepts involved in allosteric function. This review examines the reasons why allosteric molecules should be considered as new drug entities and the techniques required to optimize the discovery process for allosteric molecules.

Key aspects of modern GPCR drug discovery

G-protein-coupled receptors (GPCRs) are the largest and most versatile cell surface receptor family with a broad repertoire of ligands and functions. We've learned an enormous amount about discovering drugs of this receptor class since the first GPCR was cloned and expressed in 1986, such that it's now well-recognized that GPCRs are the most successful target class for approved drugs. Here we take the reader through a GPCR drug discovery journey from target to the clinic, highlighting the key learnings, best practices, challenges, trends and insights on discovering drugs that ultimately modulate GPCR function therapeutically in patients. The future of GPCR drug discovery is inspiring, with more desirable drug mechanisms and new technologies enabling the delivery of better and more successful drugs.

Quantitative receptor model for responses that are left- or right-shifted versus occupancy (are more or less concentration sensitive): the SABRE approach

Simple one-to three-parameter models routinely used to fit typical dose-response curves and calculate EC50 values using the Hill or Clark equation cannot provide the full picture connecting measured response to receptor occupancy, which can be quite complex due to the interplay between partial agonism and (pathway-dependent) signal amplification. The recently introduced SABRE quantitative receptor model is the first one that explicitly includes a parameter for signal amplification (γ) in addition to those for binding affinity (K d), receptor-activation efficacy (ε), constitutive activity (ε R0), and steepness of response (Hill slope, n). It can provide a unified framework to fit complex cases, where fractional response and occupancy do not match, as well as simple ones, where parameters constrained to specific values can be used (e.g., ε R0 = 0, γ = 1, or n = 1). Here, it is shown for the first time that SABRE can fit not only typical cases where response curves are left-shifted compared to occupancy (κ = K d/EC50 > 1) due to signal amplification (γ > 1), but also less common ones where they are right-shifted (i.e., less concentration-sensitive; κ = K d/EC50 < 1) by modeling them as apparent signal attenuation/loss (γ < 1). Illustrations are provided with μ-opioid receptor (MOPr) data from three different experiments with one left- and one right-shifted response (G protein activation and β-arrestin2 recruitment, respectively; EC50,Gprt < K d < EC50,βArr). For such cases of diverging pathways with differently shifted responses, partial agonists can cause very weak responses in the less concentration-sensitive pathway without having to be biased ligands due to the combination of low ligand efficacy and signal attenuation/loss-an illustration with SABRE-fitted oliceridine data is included.

Identification of 5-HT2A receptor signaling pathways associated with psychedelic potential

Serotonergic psychedelics possess considerable therapeutic potential. Although 5-HT2A receptor activation mediates psychedelic effects, prototypical psychedelics activate both 5-HT2A-Gq/11 and β-arrestin2 transducers, making their respective roles unclear. To elucidate this, we develop a series of 5-HT2A-selective ligands with varying Gq efficacies, including β-arrestin-biased ligands. We show that 5-HT2A-Gq but not 5-HT2A-β-arrestin2 recruitment efficacy predicts psychedelic potential, assessed using head-twitch response (HTR) magnitude in male mice. We further show that disrupting Gq-PLC signaling attenuates the HTR and a threshold level of Gq activation is required to induce psychedelic-like effects, consistent with the fact that certain 5-HT2A partial agonists (e.g., lisuride) are non-psychedelic. Understanding the role of 5-HT2A Gq-efficacy in psychedelic-like psychopharmacology permits rational development of non-psychedelic 5-HT2A agonists. We also demonstrate that β-arrestin-biased 5-HT2A receptor agonists block psychedelic effects and induce receptor downregulation and tachyphylaxis. Overall, 5-HT2A receptor Gq-signaling can be fine-tuned to generate ligands distinct from classical psychedelics.

Ligand selectivity hotspots in serotonin GPCRs

Serotonin is a neurotransmitter regulating numerous physiological processes also modulated by drugs, for example, schizophrenia, depression, migraine, and obesity. However, these drugs typically have adverse effects caused by promiscuous binding across 12 serotonin and more than 20 homologous receptors. Recently, structures of the entire serotonin receptor family uncovered molecular ligand recognition. Here, we present a map of 19 'selectivity hotspots', that is, nonconserved binding site residues governing selectivity via favorable target interactions or repulsive 'off-target' contacts. Furthermore, we review functional rationale from observed ligand-binding affinities and mutagenesis effects. Unifying knowledge underlying specific probes and drugs is critical toward the functional characterization of different receptors and alleviation of adverse effects.

Quantification of ligand and mutation-induced bias in EGFR phosphorylation in direct response to ligand binding

Signaling bias is the ability of a receptor to differentially activate downstream signaling pathways in response to different ligands. Bias investigations have been hindered by inconsistent results in different cellular contexts. Here we introduce a methodology to identify and quantify bias in signal transduction across the plasma membrane without contributions from feedback loops and system bias. We apply the methodology to quantify phosphorylation efficiencies and determine absolute bias coefficients. We show that the signaling of epidermal growth factor receptor (EGFR) to EGF and TGFα is biased towards Y1068 and against Y1173 phosphorylation, but has no bias for epiregulin. We further show that the L834R mutation found in non-small-cell lung cancer induces signaling bias as it switches the preferences to Y1173 phosphorylation. The knowledge gained here challenges the current understanding of EGFR signaling in health and disease and opens avenues for the exploration of biased inhibitors as anti-cancer therapies.

Gα Protein Signaling Bias at Serotonin 1A Receptor

Serotonin 1A receptor (5-HT1AR) is a clinically relevant target because of its involvement in several central and peripheral functions, including sleep, temperature homeostasis, processing of emotions, and response to stress. As a G protein coupled receptor (GPCR) activating numerous Gα i/o/z family members, 5-HT1AR can potentially modulate multiple intracellular signaling pathways in response to different therapeutics. Here, we applied a cell-based bioluminescence resonance energy transfer assay to quantify how ten structurally diverse 5-HT1AR agonists exert biased signaling by differentially stimulating Gα i/o/z family members. Our concentration-response analysis of the activation of each Gα i/o/z protein revealed unique potency and efficacy profiles of selected agonists when compared with the reference 5-hydroxytryptamine, serotonin. Overall, our analysis of signaling bias identified groups of ligands sharing comparable G protein activation selectivity and also drugs with unique selectivity profiles. We observed, for example, a strong bias of F-15599 toward the activation of Gα i3 that was unique among the agonists tested: we found a biased factor of +2.19 when comparing the activation of Gα i3 versus Gα i2 by F-15599, while it was -0.29 for 8-hydroxy-2-(di-n-propylamino) tetralin. Similarly, vortioxetine showed a biased factor of +1.06 for Gα z versus Gα oA, while it was -1.38 for vilazodone. Considering that alternative signaling pathways are regulated downstream of each Gα protein, our data suggest that the unique pharmacological properties of the tested agonists could result in multiple unrelated cellular outcomes. Further investigation is needed to reveal how this type of ligand bias could affect cellular responses and to illuminate the molecular mechanisms underlying therapeutic profile and side effects of each drug. SIGNIFICANCE STATEMENT: Serotonin 1a receptor (5-HT1AR) activates several members of the Gi/o/z protein family. Here, we examined ten structurally diverse and clinically relevant agonists acting on 5-HT1AR and identified distinctive bias patterns among G proteins. Considering the diversity of their intracellular effectors and signaling properties, this data reveal novel mechanisms underlying both therapeutic and undesirable effects.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Quantitative assessment of ligand bias from bias plots: The bias coefficient "kappa"

The current methods for quantifying ligand bias involve the construction of bias plots and the calculations of bias coefficients that can be compared using statistical methods. However, widely used bias coefficients can diverge in their abilities to identify ligand bias and can give false positives. As the empirical bias plots are considered the most reliable tools in bias identification, here we develop an analytical description of bias plot trajectories and introduce a bias coefficient, kappa, which is calculated from these trajectories. The new bias coefficient complements the tool-set in ligand bias identification in cell signaling research.

A stepwise activation model for the insulin receptor

The binding of insulin to the insulin receptor (IR) triggers a cascade of receptor conformational changes and autophosphorylation, leading to the activation of metabolic and mitogenic pathways. Recent advances in the structural and functional analyses of IR have revealed the conformations of the extracellular domains of the IR in inactive and fully activated states. However, the early activation mechanisms of this receptor remain poorly understood. The structures of partially activated IR in complex with aptamers provide clues for understanding the initial activation mechanism. In this review, we discuss the structural and functional features of IR complexed with various ligands and propose a model to explain the sequential activation mechanism. Moreover, we discuss the structures of IR complexed with biased agonists that selectively activate metabolic pathways and provide insights into the design of selective agonists and their clinical implications.

G Protein-Biased Agonists for Intracellular Angiotensin Receptors Promote Collagen Secretion in Myofibroblasts

Photoactivatable ligands remain valuable tools to study the spatiotemporal aspects of cellular signaling. However, the synthesis, handling, and biological validation of such compounds remain challenging, especially when dealing with peptides. We report an optimized synthetic strategy, where laborious preparation of dimethoxy-nitrobenzyl-tyrosine building blocks was replaced by direct functionalization of amino acid side chains while peptides remained coupled to resin, reducing both preparation time and cost. Our caged peptides were designed to investigate cellular responses mediated by intracellular angiotensin II receptors (iATR) upon interaction with known biased and unbiased ligands. The pathophysiological roles of iATRs remain poorly understood, and we sought to develop ligands to explore this. Initial validation showed that our caged ligands undergo rapid photolysis and produced functionally active peptides upon UV exposure. We also show, for the first time, that different biased ligands (β-arrestin- vs G protein-biased analogues) evoked distinct responses when uncaged in adult rat myofibroblasts. Intracellularly targeted versions of Ang II (unbiased) or G protein-biased analogues (TRV055, TRV056) were more effective than β-arrestin-biased Ang II analogues (SI, TRV026, and TRV27) in inducing collagen secretion, suggesting a divergent role in regulating the fibrotic response.

Divergent Pharmacology and Biased Signaling of the Four Melanocortin-4 Receptor Isoforms in Rainbow Trout (Oncorhynchus mykiss)

The melanocortin-4 receptor (MC4R) is essential for the modulation of energy balance and reproduction in both fish and mammals. Rainbow trout (Oncorhynchus mykiss) has been extensively studied in various fields and provides a unique opportunity to investigate divergent physiological roles of paralogues. Herein we identified four trout mc4r (mc4ra1, mc4ra2, mc4rb1, and mc4rb2) genes. Four trout Mc4rs (omMc4rs) were homologous to those of teleost and mammalian MC4Rs. Multiple sequence alignments, a phylogenetic tree, chromosomal synteny analyses, and pharmacological studies showed that trout mc4r genes may have undergone different evolutionary processes. All four trout Mc4rs bound to two peptide agonists and elevated intracellular cAMP levels dose-dependently. High basal cAMP levels were observed at two omMc4rs, which were decreased by Agouti-related peptide. Only omMc4rb2 was constitutively active in the ERK1/2 signaling pathway. Ipsen 5i, ML00253764, and MCL0020 were biased allosteric modulators of omMc4rb1 with selective activation upon ERK1/2 signaling. ML00253764 behaved as an allosteric agonist in Gs-cAMP signaling of omMc4rb2. This study will lay the foundation for future physiological studies of various mc4r paralogs and reveal the evolution of MC4R in vertebrates.

Structure-Activity Assessment and In-Depth Analysis of Biased Agonism in a Set of Phenylalkylamine 5-HT2A Receptor Agonists

Serotonergic psychedelics are described to have activation of the serotonin 2A receptor (5-HT2A) as their main pharmacological action. Despite their relevance, the molecular mechanisms underlying the psychedelic effects induced by certain 5-HT2A agonists remain elusive. One of the proposed hypotheses is the occurrence of biased agonism, defined as the preferential activation of certain signaling pathways over others. This study comparatively monitored the efficiency of a diverse panel of 4-position-substituted (and N-benzyl-derived) phenylalkylamines to induce recruitment of β-arrestin2 (βarr2) or miniGαq to the 5-HT2A, allowing us to assess structure-activity relationships and biased agonism. All test compounds exhibited agonist properties with a relatively large range of both EC50 and Emax values. Interestingly, the lipophilicity of the 2C-X phenethylamines was correlated with their efficacy in both assays but yielded a stronger correlation in the miniGαq- than in the βarr2-assay. Molecular docking suggested that accommodation of the 4-substituent of the 2C-X analogues in a hydrophobic pocket between transmembrane helices 4 and 5 of 5-HT2A may contribute to this differential effect. Aside from previously used standard conditions (lysergic acid diethylamide (LSD) as a reference agonist and a 2 h activation profile to assess a compound's activity), serotonin was included as a second reference agonist, and the compounds' activities were also assessed using the first 30 min of the activation profile. Under all assessed circumstances, the qualitative structure-activity relationships remained unchanged. Furthermore, the use of two reference agonists allowed for the estimation of both "benchmark bias" (relative to LSD) and "physiology bias" (relative to serotonin).

Identification of 5-HT2A Receptor Signaling Pathways Responsible for Psychedelic Potential

Serotonergic psychedelics possess considerable therapeutic potential. Although 5-HT2A receptor activation mediates psychedelic effects, prototypical psychedelics activate both 5-HT2A-Gq/11 and β-arrestin2 signaling, making their respective roles unclear. To elucidate this, we developed a series of 5-HT2A-selective ligands with varying Gq efficacies, including β-arrestin-biased ligands. We show that 5-HT2A-Gq but not 5-HT2A-β-arrestin2 efficacy predicts psychedelic potential, assessed using head-twitch response (HTR) magnitude in male mice. We further show that disrupting Gq-PLC signaling attenuates the HTR and a threshold level of Gq activation is required to induce psychedelic-like effects, consistent with the fact that certain 5-HT2A partial agonists (e.g., lisuride) are non-psychedelic. Understanding the role of 5-HT2A-Gq efficacy in psychedelic-like psychopharmacology permits rational development of non-psychedelic 5-HT2A agonists. We also demonstrate that β-arrestin-biased 5-HT2A receptor agonists induce receptor downregulation and tachyphylaxis, and have an anti-psychotic-like behavioral profile. Overall, 5-HT2A receptor signaling can be fine-tuned to generate ligands with properties distinct from classical psychedelics.

Fine-tuning GPCR-mediated neuromodulation by biasing signaling through different G protein subunits

G-protein-coupled receptors (GPCRs) mediate neuromodulation through the activation of heterotrimeric G proteins (Gαβγ). Classical models depict that G protein activation leads to a one-to-one formation of Gα-GTP and Gβγ species. Each of these species propagates signaling by independently acting on effectors, but the mechanisms by which response fidelity is ensured by coordinating Gα and Gβγ responses remain unknown. Here, we reveal a paradigm of G protein regulation whereby the neuronal protein GINIP (Gα inhibitory interacting protein) biases inhibitory GPCR responses to favor Gβγ over Gα signaling. Tight binding of GINIP to Gαi-GTP precludes its association with effectors (adenylyl cyclase) and, simultaneously, with regulator-of-G-protein-signaling (RGS) proteins that accelerate deactivation. As a consequence, Gαi-GTP signaling is dampened, whereas Gβγ signaling is enhanced. We show that this mechanism is essential to prevent the imbalances of neurotransmission that underlie increased seizure susceptibility in mice. Our findings reveal an additional layer of regulation within a quintessential mechanism of signal transduction that sets the tone of neurotransmission.

A MOR Antagonist with High Potency and Antagonist Efficacy among Diastereomeric C9-Alkyl-Substituted N-Phenethyl-5-(3-hydroxy)phenylmorphans

The 5-(3-hydroxy)phenylmorphan structural class of compounds are unlike the classical morphinans, 4,5-epoxymorphinans, and 6,7-benzomorphans, in that they have an equatorially oriented aromatic ring rather than the axial orientation of that ring found in the classical opioids. This modified and simplified opioid-like structure has been shown to retain antinociceptive activity, depending on its stereochemistry and substituents, and some of them have been found to be much more potent than morphine. A simple C9-hydroxy-5-(3-hydroxy)phenylmorphan enantiomer was found to be about 500 times more potent than morphine in vivo. We have previously examined C9-alkenyl and hydroxyalkyl substituents in the N-phenethyl-5-(3-hydroxy)phenylmorphan class of compounds. Comparable C9-alkyl (methyl through butyl) substituents, with their sets of diastereomers, have not been explored. All these compounds have now been synthesized to determine the effect chain-length and stereochemistry at the C9 position in the molecule might have on their interaction with opioid receptors. We now report the synthesis and in vitro activity of 16 compounds, the C9-methyl, ethyl, propyl, and butyl diastereomers, using the inhibition of forskolin-induced cAMP accumulation assay. Several potent (sub-nanomolar and nanomolar) MOR compounds were found to be selective agonists with varying efficacy. Of greatest interest, a selective MOR antagonist was discovered; it did not display any DOR or KOR agonist activity in vitro, was three times more potent than naltrexone, and was found to antagonize the EC90 of fentanyl at MOR to a greater extent than naltrexone.

Investigation of adenosine A1 receptor-mediated β-arrestin 2 recruitment using a split-luciferase assay

Background: Adenosine A1 receptor (A1AR) plays a prominent role in neurological and cardiac diseases and inflammatory processes. Its endogenous ligand adenosine is known to be one of the key players in the sleep-wake cycle. Like other G protein-coupled receptors (GPCRs), stimulation of A1AR leads to the recruitment of arrestins in addition to the activation of G proteins. So far, little is known about the role of these proteins in signal transduction and regulation of A1AR compared to the activation of G proteins. In this work, we characterized a live cell assay for A1AR-mediated β-arrestin 2 recruitment. We have applied this assay to a set of different compounds that interact with this receptor. Methods: Based on NanoBit® technology, a protein complementation assay was developed in which the A1AR is coupled to the large part of the nanoluciferase (LgBiT), whereas its small part (SmBiT) is fused to the N-terminus of β-arrestin 2. Stimulation of A1AR results in the recruitment of β-arrestin 2 and subsequent complementation of a functional nanoluciferase. For comparison, corresponding data on the effect of receptor stimulation on intracellular cAMP levels were collected for some data sets using the GloSensor™ assay. Results: The assay gives highly reproducible results with a very good signal-to-noise ratio. Capadenoson, in contrast to adenosine, CPA, or NECA, shows only partial agonism in this assay with respect to the recruitment of β-arrestin 2, whereas it shows full agonism in the case of the inhibitory effect of A1AR on cAMP production. By using a GRK2 inhibitor, it becomes clear that the recruitment is at least partially dependent on the phosphorylation of the receptor by this kinase. Interestingly, this was also the first time that we demonstrate the A1AR-mediated recruitment of β-arrestin 2 by stimulation with a valerian extract. Conclusion: The presented assay is a useful tool for the quantitative study of A1AR-mediated β-arrestin 2 recruitment. It allows data collection for stimulatory, inhibitory, and modulatory substances and is also suitable for more complex substance mixtures such as valerian extract.

A novel G protein-biased agonist at the μ opioid receptor induces substantial receptor desensitisation through G protein-coupled receptor kinase

BACKGROUND AND PURPOSE

G protein-biased μ opioid receptor agonists have the potential to induce less receptor desensitisation and tolerance than balanced opioids. Here, we investigated if the cyclic endomorphin analogue Tyr-c[D-Lys-Phe-Tyr-Gly] (Compound 1) is a G protein-biased μ agonist and characterised its ability to induce rapid receptor desensitisation in mammalian neurones.

EXPERIMENTAL APPROACH

The signalling and trafficking properties of opioids were characterised using bioluminescence resonance energy transfer assays, enzyme-linked immunosorbent assay and phosphosite-specific immunoblotting in human embryonic kidney 293 cells. Desensitisation of opioid-induced currents were studied in rat locus coeruleus neurones using whole-cell patch-clamp electrophysiology. The mechanism of Compound 1-induced μ receptor desensitisation was probed using kinase inhibitors.

KEY RESULTS

Compound 1 has similar intrinsic activity for G protein signalling as morphine. As predicted for a G protein-biased μ agonist, Compound 1 induced minimal agonist-induced internalisation and phosphorylation at intracellular μ receptor serine/threonine residues known to be involved in G protein-coupled receptor kinase (GRK)-mediated desensitisation. However, Compound 1 induced robust rapid μ receptor desensitisation in locus coeruleus neurons, to a greater degree than morphine. The extent of Compound 1-induced desensitisation was unaffected by activation or inhibition of protein kinase C (PKC) but was significantly reduced by inhibition of GRK.

CONCLUSION AND IMPLICATIONS

Compound 1 is a novel G protein-biased μ agonist that induces substantial rapid receptor desensitisation in mammalian neurons. Surprisingly, Compound 1-induced desensitisation was demonstrated to be GRK dependent despite its G protein bias. Our findings refute the assumption that G protein-biased agonists will evade receptor desensitisation and tolerance.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

The complex molecular pharmacology of the dopamine D2 receptor: Implications for pramipexole, ropinirole, and rotigotine

With L-DOPA, dopamine agonists such as pramipexole, ropinirole and rotigotine constitute key therapeutic options for the management of motor symptoms of Parkinson's disease. These compounds exert their beneficial effect on motor behaviours by activating dopamine D₂-class receptors and thereby compensating for the declining dopaminergic transmission in the dorsal striatum. Despite a strong similarity in their mechanism of action, these three dopamine agonists present distinct clinical profiles, putatively underpinned by differences in their pharmacological properties. In this context, this review aims at contributing to close the gap between clinical observations and data from molecular neuropharmacology by exploring the properties of pramipexole, ropinirole and rotigotine from both the clinical and molecular perspectives. Indeed, this review first summarizes and compares the clinical features of these three dopamine agonists, and then explores their binding profiles at the different dopamine receptor subtypes. Moreover, the signalling profiles of pramipexole, ropinirole and rotigotine at the D₂ receptor are recapitulated, with a focus on biased signalling and the potential therapeutic implications. Overall, this review aims at providing a unifying framework of interpretation for both clinicians and fundamental pharmacologists interested in a deep understanding of the pharmacological properties of pramipexole, ropinirole and rotigotine.

Biased Agonism: Lessons from Studies of Opioid Receptor Agonists

In ligand bias different agonist drugs are thought to produce distinct signaling outputs when activating the same receptor. If these signaling outputs mediate therapeutic versus adverse drug effects, then agonists that selectively activate the therapeutic signaling pathway would be extremely beneficial. It has long been thought that μ-opioid receptor agonists that selectively activate G protein- over β-arrestin-dependent signaling pathways would produce effective analgesia without the adverse effects such as respiratory depression. However, more recent data indicate that most of the therapeutic and adverse effects of agonist-induced activation of the μ-opioid receptor are actually mediated by the G protein-dependent signaling pathway, and that a number of drugs described as G protein biased in fact may not be biased, but instead may be low-intrinsic-efficacy agonists. In this review we discuss the current state of the field of bias at the μ-opioid receptor and other opioid receptor subtypes.

Development of N-(1-Adamantyl)benzamides as Novel Anti-Inflammatory Multitarget Agents Acting as Dual Modulators of the Cannabinoid CB2 Receptor and Fatty Acid Amide Hydrolase

Cannabinoid type 2 receptor (CB2R), belonging to the endocannabinoid system, is overexpressed in pathologies characterized by inflammation, and its activation counteracts inflammatory states. Fatty acid amide hydrolase (FAAH) is an enzyme responsible for the degradation of the main endocannabinoid anandamide; thus, the simultaneous CB2R activation and FAAH inhibition may be a synergistic anti-inflammatory strategy. Encouraged by principal component analysis (PCA) data identifying a wide chemical space shared by CB2R and FAAH ligands, we designed a small library of adamantyl-benzamides, as potential dual agents, CB2R agonists, and FAAH inhibitors. The new compounds were tested for their CB2R affinity/selectivity and CB2R and FAAH activity. Derivatives 13, 26, and 27, displaying the best pharmacodynamic profile as CB2R full agonists and FAAH inhibitors, decreased pro-inflammatory and increased anti-inflammatory cytokines production. Molecular docking simulations complemented the experimental findings by providing a molecular rationale behind the observed activities. These multitarget ligands constitute promising anti-inflammatory agents.