Biased ligands at G-protein-coupled receptors: promise and progress. Trends Pharmacol Sci. 2014;35:308-316. [PMID: 24878326 DOI: 10.1016/j.tips.2014.04.007]

Sequence-Specific Regulation of Endocytic Lifetimes Modulates Arrestin-Mediated Signaling at the µ Opioid Receptor

Functional selectivity at the µ opioid receptor (µR), a prototypical G-protein-coupled receptor that is a physiologically relevant target for endogenous opioid neurotransmitters and analgesics, has been a major focus for drug discovery in the recent past. Functional selectivity is a cumulative effect of the magnitudes of individual signaling pathways, e.g., the Gα_i-mediated and the arrestin-mediated pathways for µR. The present work tested the hypothesis that lifetimes of agonist-induced receptor-arrestin clusters at the cell surface control the magnitude of arrestin signaling, and therefore functional selectivity, at µR. We show that endomorphin-2 (EM2), an arrestin-biased ligand for µR, lengthens surface lifetimes of receptor-arrestin clusters significantly compared with morphine. The lengthening of lifetimes required two specific leucines on the C-terminal tail of µR. Mutation of these leucines to alanines decreased the magnitude of arrestin-mediated signaling by EM2 without affecting G-protein signaling, suggesting that lengthened endocytic lifetimes were required for arrestin-biased signaling by EM2. Lengthening surface lifetimes by pharmacologically slowing endocytosis was sufficient to increase arrestin-mediated signaling by both EM2 and the clinically relevant agonist morphine. Our findings show that distinct ligands can leverage specific sequence elements on µR to regulate receptor endocytic lifetimes and the magnitude of arrestin-mediated signaling, and implicate these sequences as important determinants of functional selectivity in the opioid system.

G Protein-coupled Receptor Biased Agonism

G protein-coupled receptors are the largest family of targets for current therapeutics. The classic model of their activation was binary, where agonist binding induced an active conformation and subsequent downstream signaling. Subsequently, the revised concept of biased agonism emerged, where different ligands at the same G protein-coupled receptor selectively activate one downstream pathway versus another. Advances in understanding the mechanism of biased agonism have led to the development of novel ligands, which have the potential for improved therapeutic and safety profiles. In this review, we summarize the theory and most recent breakthroughs in understanding biased signaling, examine recent laboratory investigations concerning biased ligands across different organ systems, and discuss the promising clinical applications of biased agonism.

Cannabinoid receptor ligand bias: implications in the central nervous system

The G protein-coupled cannabinoid receptors CB₁, CB₂, GPR18, and GPR55 regulate neurotransmission, pain, and inflammation and have been intensively investigated as potential drug targets. Each of these GPCRs is coupled to multiple effector proteins mediating divergent cellular signals. The ligand bias of cannabinoid-targeted compounds is only beginning to be quantified. Research into cannabinoid bias is now revealing correlations between bias in cell culture and functional outcomes in vivo. We present an example study of cannabinoid bias in the context of Huntington disease. In future, an understanding of cannabinoid receptor structure and quantification of ligand bias will optimize drug selection matched to patient population and disease.

NF1 Is a Direct G Protein Effector Essential for Opioid Signaling to Ras in the Striatum

It is well recognized that G-protein-coupled receptors (GPCRs) can activate Ras-regulated kinase pathways to produce lasting changes in neuronal function. Mechanisms by which GPCRs transduce these signals and their relevance to brain disorders are not well understood. Here, we identify a major Ras regulator, neurofibromin 1 (NF1), as a direct effector of GPCR signaling via Gβγ subunits in the striatum. We find that binding of Gβγ to NF1 inhibits its ability to inactivate Ras. Deletion of NF1 in striatal neurons prevents the opioid-receptor-induced activation of Ras and eliminates its coupling to Akt-mTOR-signaling pathway. By acting in the striatal medium spiny neurons of the direct pathway, NF1 regulates opioid-induced changes in Ras activity, thereby sensitizing mice to psychomotor and rewarding effects of morphine. These results delineate a novel mechanism of GPCR signaling to Ras pathways and establish a critical role of NF1 in opioid addiction.

Cardioprotective Angiotensin-(1-7) Peptide Acts as a Natural-Biased Ligand at the Angiotensin II Type 1 Receptor

Hyperactivity of the renin-angiotensin-aldosterone system through the angiotensin II (Ang II)/Ang II type 1 receptor (AT1-R) axis constitutes a hallmark of hypertension. Recent findings indicate that only a subset of AT1-R signaling pathways is cardiodeleterious, and their selective inhibition by biased ligands promotes therapeutic benefit. To date, only synthetic biased ligands have been described, and whether natural renin-angiotensin-aldosterone system peptides exhibit functional selectivity at AT1-R remains unknown. In this study, we systematically determined efficacy and potency of Ang II, Ang III, Ang IV, and Ang-(1-7) in AT1-R-expressing HEK293T cells on the activation of cardiodeleterious G-proteins and cardioprotective β-arrestin2. Ang III and Ang IV fully activate similar G-proteins than Ang II, the prototypical AT1-R agonist, despite weaker potency of Ang IV. Interestingly, Ang-(1-7) that binds AT1-R fails to promote G-protein activation but behaves as a competitive antagonist for Ang II/Gi and Ang II/Gq pathways. Conversely, all renin-angiotensin-aldosterone system peptides act as agonists on the AT1-R/β-arrestin2 axis but display biased activities relative to Ang II as indicated by their differences in potency and AT1-R/β-arrestin2 intracellular routing. Importantly, we reveal Ang-(1-7) a known Mas receptor-specific ligand, as an AT1-R-biased agonist, selectively promoting β-arrestin activation while blocking the detrimental Ang II/AT1-R/Gq axis. This original pharmacological profile of Ang-(1-7) at AT1-R, similar to that of synthetic AT1-R-biased agonists, could, in part, contribute to its cardiovascular benefits. Accordingly, in vivo, Ang-(1-7) counteracts the phenylephrine-induced aorta contraction, which was blunted in AT1-R knockout mice. Collectively, these data suggest that Ang-(1-7) natural-biased agonism at AT1-R could fine-tune the physiology of the renin-angiotensin-aldosterone system.

Human DP and EP2 prostanoid receptors take on distinct forms depending on the diverse binding of different ligands

Human D-type prostanoid (DP) and E-type prostanoid 2 (EP2) receptors are G protein-coupled receptors and are regarded as the most closely related receptors among prostanoid receptors because they are generated by tandem duplication. The DP receptor-cognate ligand, prostaglandin D₂ (PGD₂ ) has the ability to activate not only DP receptors but also EP2 receptors. Likewise, the EP2 receptor-cognate ligand, prostaglandin E₂ (PGE₂ ) has the ability to activate DP receptors in addition to EP receptors in order to stimulate cAMP formation. However, since PGD₂ and/or PGE₂ activate DP and EP2 receptors to similar maximal levels, that is, their similar efficacies, differences between the ligands in each receptor have not yet been determined in detail except for their different affinities. Herein we demonstrated, using an in silico simulation to predict binding patterns among DP or EP2 receptors and PGD₂ , PGE₂ , or prostaglandin F_2α as the reference prostanoid, that DP and EP2 receptors plausibly take on distinct forms depending on the diverse binding of different ligands. Since these ligands have the potential to make these receptors form distinct conformations with discrete signaling pathways, they are consequently regarded as endogenous biased ligands. Moreover, by using functional assays, the susceptibilities of the DP receptors to the noncognate ligands were approximately 10 times lower than those of EP2 receptors. Thus, EP2 receptors seem to be able to distinguish endogenous ligands better than DP receptors, thereby both receptors are plausibly gaining role-sharing functions with respect to one another as the copies of duplicated gene.

Overview of Critical Parameters for the Design and Execution of a High-Throughput Screen for Allosteric Ligands

Allosteric ligands modulate the activity of receptor targets by binding to sites that are distinct from the orthosteric (native agonist) binding site. Allosteric modulators have potential therapeutic advantages over orthosteric agonists and antagonists, including improved selectivity, and maintenance of the spatial and temporal fidelity of native signaling patterns. The identification of allosteric ligands presents unique challenges because of the requirement for screening in the presence of an orthosteric agonist, the small signal window that is produced by many allosteric modulators, the proclivity of allosteric modulators to exhibit activity switching within a chemotype (e.g., one compound may be a positive allosteric modulator while a close analog is a negative allosteric modulator), and probe dependence (differential interactions with different orthosteric agonists). Described in this unit are emerging strategies for the identification of allosteric ligands by high-throughput screening (HTS), including the use of multiple-add/multiple-read HTS assays and tool molecule-based screening formats. © 2016 by John Wiley & Sons, Inc.

Current Knowledge and Perspectives on Histamine H1 and H2 Receptor Pharmacology: Functional Selectivity, Receptor Crosstalk, and Repositioning of Classic Histaminergic Ligands

H₁ and H₂ histamine receptor antagonists, although developed many decades ago, are still effective for the treatment of allergic and gastric acid-related conditions. This article focuses on novel aspects of the pharmacology and molecular mechanisms of histamine receptors that should be contemplated for optimizing current therapies, repositioning histaminergic ligands for new therapeutic uses, or even including agonists of the histaminergic system in the treatment of different pathologies such as leukemia or neurodegenerative disorders. In recent years, new signaling phenomena related to H₁ and H₂ receptors have been described that make them suitable for novel therapeutic approaches. Crosstalk between histamine receptors and other membrane or nuclear receptors can be envisaged as a way to modulate other signaling pathways and to potentiate the efficacy of drugs acting on different receptors. Likewise, biased signaling at histamine receptors seems to be a pharmacological feature that can be exploited to investigate nontraditional therapeutic uses for H₁ and H₂ biased agonists in malignancies such as acute myeloid leukemia and to avoid undesired side effects when used in standard treatments. It is hoped that the molecular mechanisms discussed in this review contribute to a better understanding of the different aspects involved in histamine receptor pharmacology, which in turn will contribute to increased drug efficacy, avoidance of adverse effects, or repositioning of histaminergic ligands.

Conformationally selective RNA aptamers allosterically modulate the β2-adrenoceptor

G-protein-coupled receptor (GPCR) ligands function by stabilizing multiple, functionally distinct receptor conformations. This property underlies the ability of 'biased agonists' to activate specific subsets of a given receptor's signaling profile. However, stabilizing distinct active GPCR conformations to enable structural characterization of mechanisms underlying GPCR activation remains difficult. These challenges have accentuated the need for receptor tools that allosterically stabilize and regulate receptor function through unique, previously unappreciated mechanisms. Here, using a highly diverse RNA library combined with advanced selection strategies involving state-of-the-art next-generation sequencing and bioinformatics analyses, we identify RNA aptamers that bind a prototypical GPCR, the β2-adrenoceptor (β2AR). Using biochemical, pharmacological, and biophysical approaches, we demonstrate that these aptamers bind with nanomolar affinity at defined surfaces of the receptor, allosterically stabilizing active, inactive, and ligand-specific receptor conformations. The discovery of RNA aptamers as allosteric GPCR modulators significantly expands the diversity of ligands available to study the structural and functional regulation of GPCRs.

Allosteric communication pipelines in G-protein-coupled receptors

The binding of ligands to G-protein-coupled receptors (GPCRs) in the extracellular region transmits the signal to the intracellular region to initiate coupling to effector proteins. The mechanism of this allosteric communication remains largely unexplored. Knowledge of the residues involved in the pipeline of the allosteric communication from the extracellular to the intracellular region will provide means to (a) design ligands with bias in potency towards one signaling pathway over others, and (b) design allosteric modulators that show subtype selectivity in GPCRs. In this review we describe the current state of the computational methods that provide insights into the allosteric communication in GPCRs and elucidate how this information can be used to design allosteric modulators.

Analyzing the roles of multi-functional proteins in cells: The case of arrestins and GRKs

Most proteins have multiple functions. Obviously, conventional methods of manipulating the level of the protein of interest in the cell, such as over-expression, knockout or knockdown, affect all of its functions simultaneously. The key advantage of these methods is that over-expression, knockout or knockdown does not require any knowledge of the molecular mechanisms of the function(s) of the protein of interest. The disadvantage is that these approaches are inadequate to elucidate the role of an individual function of the protein in a particular cellular process. An alternative is the use of re-engineered proteins, in which a single function is eliminated or enhanced. The use of mono-functional elements of a multi-functional protein can also yield cleaner answers. This approach requires detailed knowledge of the structural basis of each function of the protein in question. Thus, a lot of preliminary structure-function work is necessary to make it possible. However, when this information is available, replacing the protein of interest with a mutant in which individual functions are modified can shed light on the biological role of those particular functions. Here, we illustrate this point using the example of protein kinases, most of which have additional non-enzymatic functions, as well as arrestins, known multi-functional signaling regulators in the cell.

A Novel Allosteric Activator of Free Fatty Acid 2 Receptor Displays Unique Gi-functional Bias

The short chain fatty acid receptor FFA2 is able to stimulate signaling via both G_i- and G_q/G₁₁-promoted pathways. These pathways are believed to control distinct physiological end points but FFA2 receptor ligands appropriate to test this hypothesis have been lacking. Herein, we characterize AZ1729, a novel FFA2 regulator that acts as a direct allosteric agonist and as a positive allosteric modulator, increasing the activity of the endogenously produced short chain fatty acid propionate in G_i-mediated pathways, but not at those transduced by G_q/G₁₁. Using AZ1729 in combination with direct inhibitors of G_i and G_q/G₁₁ family G proteins demonstrated that although both arms contribute to propionate-mediated regulation of phospho-ERK1/2 MAP kinase signaling in FFA2-expressing 293 cells, the G_q/G₁₁-mediated pathway is predominant. We extend these studies by employing AZ1729 to dissect physiological FFA2 signaling pathways. The capacity of AZ1729 to act at FFA2 receptors to inhibit β-adrenoreceptor agonist-promoted lipolysis in primary mouse adipocytes and to promote chemotaxis of isolated human neutrophils confirmed these as FFA2 processes mediated by G_i signaling, whereas, in concert with blockade by the G_q/G₁₁ inhibitor FR900359, the inability of AZ1729 to mimic or regulate propionate-mediated release of GLP-1 from mouse colonic preparations defined this physiological response as an end point transduced via activation of G_q/G₁₁.

Mapping Cannabinoid 1 Receptor Allosteric Site(s): Critical Molecular Determinant and Signaling Profile of GAT100, a Novel, Potent, and Irreversibly Binding Probe

One of the most abundant G-protein coupled receptors (GPCRs) in brain, the cannabinoid 1 receptor (CB1R), is a tractable therapeutic target for treating diverse psychobehavioral and somatic disorders. Adverse on-target effects associated with small-molecule CB1R orthosteric agonists and inverse agonists/antagonists have plagued their translational potential. Allosteric CB1R modulators offer a potentially safer modality through which CB1R signaling may be directed for therapeutic benefit. Rational design of candidate, druglike CB1R allosteric modulators requires greater understanding of the architecture of the CB1R allosteric endodomain(s) and the capacity of CB1R allosteric ligands to tune the receptor's information output. We have recently reported the synthesis of a focused library of rationally designed, covalent analogues of Org27569 and PSNCBAM-1, two prototypic CB1R negative allosteric modulators (NAMs). Among the novel, pharmacologically active CB1R NAMs reported, the isothiocyanate GAT100 emerged as the lead by virtue of its exceptional potency in the [(35)S]GTPγS and β-arrestin signaling assays and its ability to label CB1R as a covalent allosteric probe with significantly reduced inverse agonism in the [(35)S]GTPγS assay as compared to Org27569. We report here a comprehensive functional profiling of GAT100 across an array of important downstream cell-signaling pathways and analysis of its potential orthosteric probe-dependence and signaling bias. The results demonstrate that GAT100 is a NAM of the orthosteric CB1R agonist CP55,940 and the endocannabinoids 2-arachidonoylglycerol and anandamide for β-arrestin1 recruitment, PLCβ3 and ERK1/2 phosphorylation, cAMP accumulation, and CB1R internalization in HEK293A cells overexpressing CB1R and in Neuro2a and STHdh(Q7/Q7) cells endogenously expressing CB1R. Distinctively, GAT100 was a more potent and efficacious CB1R NAM than Org27569 and PSNCBAM-1 in all signaling assays and did not exhibit the inverse agonism associated with Org27569 and PSNCBAM-1. Computational docking studies implicate C7.38(382) as a key feature of GAT100 ligand-binding motif. These data help inform the engineering of newer-generation, druggable CB1R allosteric modulators and demonstrate the utility of GAT100 as a covalent probe for mapping structure-function correlates characteristic of the druggable CB1R allosteric space.

Development of therapeutic antibodies to G protein-coupled receptors and ion channels: Opportunities, challenges and their therapeutic potential in respiratory diseases

The development of recombinant antibody therapeutics continues to be a significant area of growth in the pharmaceutical industry with almost 50 approved monoclonal antibodies on the market in the US and Europe. Therapeutic drug targets such as soluble cytokines, growth factors and single transmembrane spanning receptors have been successfully targeted by recombinant monoclonal antibodies and the development of new product candidates continues. Despite this growth, however, certain classes of important disease targets have remained intractable to therapeutic antibodies due to the complexity of the target molecules. These complex target molecules include G protein-coupled receptors and ion channels which represent a large target class for therapeutic intervention with monoclonal antibodies. Although these targets have typically been addressed by small molecule approaches, the exquisite specificity of antibodies provides a significant opportunity to provide selective modulation of these important regulators of cell function. Given this opportunity, a significant effort has been applied to address the challenges of targeting these complex molecules and a number of targets are linked to the pathophysiology of respiratory diseases. In this review, we provide a summary of the importance of GPCRs and ion channels involved in respiratory disease and discuss advantages offered by antibodies as therapeutics at these targets. We highlight some recent GPCRs and ion channels linked to respiratory disease mechanisms and describe in detail recent progress made in the strategies for discovery of functional antibodies against challenging membrane protein targets such as GPCRs and ion channels.

Delineating biased ligand efficacy at 7TM receptors from an experimental perspective

During the last 10 years, the concept of "biased agonism" also called "functional selectivity" swamped the pharmacology of 7 transmembrane receptors and paved the way for developing signaling pathway-selective drugs with increased efficacy and less adverse effects. Initially thought to select the activation of only a subset of the signaling pathways by the reference agonist, bias ligands revealed higher complexity as they have been shown to stabilize variable receptor conformations that associate with distinct signaling events from the reference. Today, one major challenge relies on the in vitro determination of the bias and classification of these ligands, as a prerequisite for future in vivo and clinical translation. In this review, current experimental considerations for the bias evaluation related to choice of the cellular model, of the signaling pathway as well as of the assays are presented and discussed.

G Protein-Coupled Receptors (GPCRs) in Alzheimer's Disease: A Focus on BACE1 Related GPCRs

The G protein coupled receptors (GPCRs) have been considered as one of the largest families of validated drug targets, which involve in almost overall physiological functions and pathological processes. Meanwhile, Alzheimer’s disease (AD), the most common type of dementia, affects thinking, learning, memory and behavior of elderly people, that has become the hotspot nowadays for its increasing risks and incurability. The above fields have been intensively studied, and the link between the two has been demonstrated, whereas the way how GPCRs perturb AD progress are yet to be further explored given their complexities. In this review, we summarized recent progress regarding the GPCRs interacted with β-site APP cleaving enzyme 1 (BACE1), a key secretase in AD pathogenesis. Then we discussed the current findings on the regulatory roles of GPCRs on BACE1, and the possibility for pharmaceutical treatment of AD patients by the allosteric modulators and biased ligands of GPCRs. We hope this review can provide new insights into the understanding of mechanistic link between GPCRs and BACE1, and highlight the potential of GPCRs as therapeutic target for AD.

Characterisation of Signalling by the Endogenous GPER1 (GPR30) Receptor in an Embryonic Mouse Hippocampal Cell Line (mHippoE-18)

Estrogen can modulate neuronal development and signalling by both genomic and non-genomic pathways. Many of its rapid, non-genomic effects on nervous tissue have been suggested to be mediated via the activation of the estrogen sensitive G-protein coupled receptor (GPER1 or GPR30). There has been much controversy over the cellular location, signalling properties and endogenous activators of GPER1. Here we describe the pharmacology and signalling properties of GPER1 in an immortalized embryonic hippocampal cell line, mHippoE-18. This cell line does not suffer from the inherent problems associated with the study of this receptor in native tissue or the problems associated with heterologously expression in clonal cell lines. In mHippoE-18 cells, 17β-Estradiol can mediate a dose-dependent rapid potentiation of forskolin-stimulated cyclic AMP levels but does not appear to activate the ERK1/2 pathway. The effect of 17β-Estradiol can be mimicked by the GPER1 agonist, G1, and also by tamoxifen and ICI 182,780 which activate GPER1 in a variety of other preparations. The response is not mimicked by the application of the classical estrogen receptor agonists, PPT, (an ERα agonist) or DPN, (an ERβ agonist), further suggesting that this effect of 17β-Estradiol is mediated through the activation of GPER1. However, after exposure of the cells to the GPER1 specific antagonists, G15 and G36, the stimulatory effects of the above agonists are replaced by dose-dependent inhibitions of forskolin-stimulated cyclic AMP levels. This inhibitory effect is mimicked by aldosterone in a dose-dependent way even in the absence of the GPER1 antagonists. The results are discussed in terms of possible "Biased Antagonism" whereby the antagonists change the conformation of the receptor resulting in changes in the agonist induced coupling of the receptor to different second messenger pathways.

Bitter taste receptors: Novel insights into the biochemistry and pharmacology

Bitter taste receptors (T2Rs) belong to the super family of G protein-coupled receptors (GPCRs). There are 25 T2Rs expressed in humans, and these interact with a large and diverse group of bitter ligands. T2Rs are expressed in many extra-oral tissues and can perform diverse physiological roles. Structure-function studies led to the identification of similarities and dissimilarities between T2Rs and Class A GPCRs including amino acid conservation and novel motifs. However, the efficacy of most of the T2R ligands is not yet elucidated and the biochemical pharmacology of T2Rs is poorly understood. Recent studies on T2Rs characterized novel ligands including blockers for these receptors that include inverse agonist and antagonists. In this review we discuss the techniques used for elucidating bitter blockers, concept of ligand bias, generic amino acid numbering, the role of cholesterol, and conserved water molecules in the biochemistry and pharmacology of T2Rs.

GPR40 receptor activation leads to CREB phosphorylation and improves cognitive performance in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is a very complex neurodegenerative disorder as neuronal loss is a prominent and initial feature of AD. This loss correlates with cognitive deficits more closely than amyloid load. GPR40 receptor belongs to the class of G-protein coupled receptors, is expressed in wide parts of the brain including the hippocampus which is involved in spatial learning and memory. Till now, there are few studies investigating the functional role of GPR40 in brain. In this study, we evaluated the functional role of GPR40 receptor in the A-beta AD mice model. Administration of Aβ1-42 (410pmol) intracerebroventricularly (i.c.v.) once at the beginning of experiment significantly impaired cognitive performance (in step-through passive test), the ability of spatial learning and memory in (Morris water maze test), working memory, attention, anxiety in (Novel object recognition test), and spatial working and reference-memory in (Hole board discrimination test) compared with the control group. The results revealed that GPR40 receptor treatment groups significantly ameliorated model mice cognitive performance. All GPR40 receptor agonist GW9508, treatment groups enhanced the learning and memory ability in Step-through passive test, Morris water maze test, Hole board discrimination test, Novel object recognition test. Furthermore, we have observed that activation of GPR40 receptor provoked the phosphorylation of the cAMP response element binding protein (CREB) and significant increase in neurotropic factors including brain derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NT-3), neurotrohin-4 (NT-4) in mouse hippocampal neurons and contribute to neurogenesis. These results suggest that GPR40 is a suitable therapeutic candidate for neurogenesis and neuroprotection in the treatment and prevention of AD.

The role of kinetic context in apparent biased agonism at GPCRs

Biased agonism describes the ability of ligands to stabilize different conformations of a GPCR linked to distinct functional outcomes and offers the prospect of designing pathway-specific drugs that avoid on-target side effects. This mechanism is usually inferred from pharmacological data with the assumption that the confounding influences of observational (that is, assay dependent) and system (that is, cell background dependent) bias are excluded by experimental design and analysis. Here we reveal that ‘kinetic context', as determined by ligand-binding kinetics and the temporal pattern of receptor-signalling processes, can have a profound influence on the apparent bias of a series of agonists for the dopamine D₂ receptor and can even lead to reversals in the direction of bias. We propose that kinetic context must be acknowledged in the design and interpretation of studies of biased agonism.

Biased agonists act at a receptor to preferentially induce distinct intracellular signalling responses over others. Here the authors show how kinetics of ligand binding and signaling responses greatly influence observed bias profiles, and hence must be considered when studying biased agonists.

Recent updates on GPCR biased agonism

G protein-coupled receptors (GPCRs) are the most important targets for drug discovery and not surprisingly ∼40% of all drugs currently in the market act on these receptors. Currently, one of the most active areas in GPCRs signaling is biased agonism, a phenomenon that occurs when a given ligand is able to preferentially activate one (or some) of the possible signaling pathways. In this review, we highlight the most recent findings about biased agonism, including an extension of this concept to intracellular signaling, allosterism, strategies for assessment and interpretation, and perspectives of therapeutic applications for biased agonists.

Feedback regulation of G protein-coupled receptor signaling by GRKs and arrestins

GPCRs are ubiquitous in mammalian cells and present intricate mechanisms for cellular signaling and communication. Mechanistically, GPCR signaling was identified to occur vectorially through heterotrimeric G proteins that are negatively regulated by GRK and arrestin effectors. Emerging evidence highlights additional roles for GRK and Arrestin partners, and establishes the existence of interconnected feedback pathways that collectively define GPCR signaling. GPCRs influence cellular dynamics and can mediate pathologic development, such as cancer and cardiovascular remolding. Hence, a better understanding of their overall signal regulation is of great translational interest and research continues to exploit the pharmacologic potential for modulating their activity.

The emerging role of GPR50 receptor in brain

GPR50 receptor one of the member of G protein-coupled receptors (GPCRs) is extensively expressed in the pituitary, hypothalamus,cortex, midbrain, pons, amygdala, and in several brainstem nuclei. The exact function of this receptor in brain is remains unclear. This review presents current knowledge regarding the function of GPR50 receptor in brain, with a focus on role of this receptor in the hypothalamus-pituitary-adrenal (HPA) axis and the glucocorticoid receptor (GR) signaling, leptin signaling, adaptive thermogenesis, torpor, neurite outgrowth, and self-renewal and neuronal differentiation of neural progenitor cells NPCs. Although the results are encouraging, further research is needed to clarify GPR50 role in neurobiology of mood disorders, adaptive thermogenesis, torpor, and in the pathophysiology of neurological disorders.

Bombesin-like receptor 3 regulates blood pressure and heart rate via a central sympathetic mechanism

Bombesin-like receptor 3 (BRS-3) is an orphan G protein-coupled receptor that regulates energy expenditure, food intake, and body weight. We examined the effects of BRS-3 deletion and activation on blood pressure and heart rate. In free-living, telemetered Brs3 null mice the resting heart rate was 10% lower than wild-type controls, while the resting mean arterial pressure was unchanged. During physical activity, the heart rate and blood pressure increased more in Brs3 null mice, reaching a similar heart rate and higher mean arterial pressure than control mice. When sympathetic input was blocked with propranolol, the heart rate of Brs3 null mice was unchanged, while the heart rate in control mice was reduced to the level of the null mice. The intrinsic heart rate, measured after both sympathetic and parasympathetic blockade, was similar in Brs3 null and control mice. Intravenous infusion of the BRS-3 agonist MK-5046 increased mean arterial pressure and heart rate in wild-type but not in Brs3 null mice, and this increase was blocked by pretreatment with clonidine, a sympatholytic, centrally acting α2-adrenergic agonist. In anesthetized mice, hypothalamic infusion of MK-5046 also increased both mean arterial pressure and heart rate. Taken together, these data demonstrate that BRS-3 contributes to resting cardiac sympathetic tone, but is not required for activity-induced increases in heart rate and blood pressure. The data suggest that BRS-3 activation increases heart rate and blood pressure via a central sympathetic mechanism.

Biased agonism: An emerging paradigm in GPCR drug discovery

G protein coupled receptors have historically been one of the most druggable classes of cellular proteins. The members of this large receptor gene family couple to primary effectors, G proteins, that have built in mechanisms for regeneration and amplification of signaling with each engagement of receptor and ligand, a kinetic event in itself. In recent years GPCRs, have been found to interact with arrestin proteins to initiate signal propagation in the absence of G protein interactions. This pinnacle observation has changed a previously held notion of the linear spectrum of GPCR efficacy and uncovered a new paradigm in GPCR research and drug discovery that relies on multidimensionality of GPCR signaling. Ligands were found that selectively confer activity in one pathway over another, and this phenomenon has been referred to as 'biased agonism' or 'functional selectivity'. While great strides in the understanding of this phenomenon have been made in recent years, two critical questions still dominate the field: How can we rationally design biased GPCR ligands, and ultimately, which physiological responses are due to G protein versus arrestin interactions? This review will discuss the current understanding of some of the key aspects of biased signaling that are related to these questions, including mechanistic insights in the nature of biased signaling and methods for measuring ligand bias, as well as relevant examples of drug discovery applications and medicinal chemistry strategies that highlight the challenges and opportunities in this rapidly evolving field.

Biased Agonism of the Angiotensin II Type I Receptor

Angiotensin II (AngII) type I receptor (AT1R) recognizes AngII, a cardiovascular peptide hormone that acts as a terminal effector of the renin-angiotensin system (RAS). AT1R belongs to the rhodopsin-like peptidergic family of G protein-coupled receptors (GPCRs) and serves as a therapeutic target for the treatment of cardiovascular diseases, such as hypertension, cardiac hypertrophy and heart failure. Classically, AT1R was considered to signal only through G proteins. However, recent studies have revealed that AT1R is capable of activating G protein-independent signaling that is mediated by β-arrestins. β-arrestin is a cytosolic scaffold that is recruited to the activated GPCRs. In vitro and ex vivo studies have demonstrated that the activation of the AT1R-β-arrestin pathway stimulates contractility and exerts prosurvival effects in cardiomyocytes. TRV027, a potent synthetic β-arrestin-biased ligand for AT1R, specifically activates AT1R-β-arrestin signaling without stimulating G proteins. In preclinical studies, TRV027 not only produced vasodilation by antagonizing the AT1R-Gαq pathway but also enhanced cardiac performance by activating AT1R-β-arrestin signaling. Because of this unique pharmacological profile, TRV027 is now being evaluated in a phase II clinical trial as a novel therapeutic for acute heart failure (AHF).

Delta Opioid Receptor (DOR) Ligands and Pharmacology: Development of Indolo- and Quinolinomorphinan Derivatives Based on the Message-Address Concept

The pharmacology of the delta opioid receptor (DOR) has lagged, mainly due to the lack of an agonist with high potency and selectivity in vivo. The DOR is now receiving increasing attention, and there has been progress in the synthesis of better novel ligands. The discovery of a selective receptor DOR antagonist, naltrindole (NTI), stimulated the design and synthesis of (±)TAN-67, which was designed based on the message-address concept and the accessory site theory. Intensive studies using (±)TAN-67 determined the DOR-mediated various pharmacological effects, such as antinociceptive effects for painful diabetic neuropathy and cardiovascular protective effects. We improved the agonist activity of TAN-67 to afford SN-28, which was modified to KNT-127, a novel compound that improved the blood-brain barrier permeability. In addition, KNT-127 showed higher selectivity for the DOR and had potent agonist activity following systemic administration. Interestingly, KNT-127 produced no convulsive effects, unlike prototype DOR agonists. The KNT-127 type derivatives with a quinolinomorphinan structure are expected to be promising candidates for the development of therapeutic DOR agonists.

New paradigms in GPCR drug discovery

G protein-coupled receptors (GPCRs) remain a major domain of pharmaceutical discovery. The identification of GPCR lead compounds and their optimization are now structure-based, thanks to advances in X-ray crystallography, molecular modeling, protein engineering and biophysical techniques. In silico screening provides useful hit molecules. New pharmacological approaches to tuning the pleotropic action of GPCRs include: allosteric modulators, biased ligands, GPCR heterodimer-targeted compounds, manipulation of polypharmacology, receptor antibodies and tailoring of drug molecules to fit GPCR pharmacogenomics. Measurements of kinetics and drug efficacy are factors influencing clinical success. With the exception of inhibitors of GPCR kinases, targeting of intracellular GPCR signaling or receptor cycling for therapeutic purposes remains a futuristic concept. New assay approaches are more efficient and multidimensional: cell-based, label-free, fluorescence-based assays, and biosensors. Tailoring GPCR drugs to a patient's genetic background is now being considered. Chemoinformatic tools can predict ADME-tox properties. New imaging technology visualizes drug action in vivo. Thus, there is reason to be optimistic that new technology for GPCR ligand discovery will help reverse the current narrowing of the pharmaceutical pipeline.

CMKLR1 and GPR1 mediate chemerin signaling through the RhoA/ROCK pathway

Chemerin is an adipose-derived hormone that regulates immunity and energy homesotasis. To date, all known chemerin functions have been attributed to activation of the G protein-coupled receptor chemokine-like receptor-1 (CMKLR1). Chemerin is also the only known ligand for a second receptor, G protein-coupled receptor-1 (GPR1), whose signaling and function remains unknown. This study investigated the in vitro signal transduction mechanisms of CMKLR1 and GPR1 using a panel of luciferase-reporters and pathway-specific inhibitors. Herein we report the novel finding that chemerin signals through a RhoA and rho-associated protein kinase (ROCK)-dependent pathway for activation of the transcriptional regulator serum-response factor (SRF). Despite similarities in RhoA/ROCK, Gαi/o, and MAPK signaling, we also demonstrate species-specific and receptor-dependent variations in GPR1 and CMKLR1 signaling and expression of the SRF target genes EGR1, FOS and VCL. Moreover, we demonstrate that signaling through p38, Gαi/o, RhoA, and ROCK is required for chemerin-mediated chemotaxis of L1.2 lymphocytes and AGS gastric adenocarcinoma cells. These results provide, to our knowledge, the first empirical evidence that GPR1 is a functional chemerin receptor and identify RhoA/SRF as a novel chemerin-signaling axis via both CMKLR1 and GPR1.

PTH receptor-1 signalling-mechanistic insights and therapeutic prospects

Parathyroid hormone/parathyroid hormone-related protein receptor (PTH/PTHrP type 1 receptor; commonly known as PTHR1) is a family B G-protein-coupled receptor (GPCR) that regulates skeletal development, bone turnover and mineral ion homeostasis. PTHR1 transduces stimuli from PTH and PTHrP into the interior of target cells to promote diverse biochemical responses. Evaluation of the signalling properties of structurally modified PTHR1 ligands has helped to elucidate determinants of receptor function and mechanisms of downstream cellular and physiological responses. Analysis of PTHR1 responses induced by structurally modified ligands suggests that PTHR1 can continue to signal through a G-protein-mediated pathway within endosomes. Such findings challenge the longstanding paradigm in GPCR biology that the receptor is transiently activated at the cell membrane, followed by rapid deactivation and receptor internalization. Evaluation of structurally modified PTHR1 ligands has further led to the identification of ligand analogues that differ from PTH or PTHrP in the type, strength and duration of responses induced at the receptor, cellular and organism levels. These modified ligands, and the biochemical principles revealed through their use, might facilitate an improved understanding of PTHR1 function in vivo and enable the treatment of disorders resulting from defects in PTHR1 signalling. This Review discusses current understanding of PTHR1 modes of action and how these findings might be applied in future therapeutic agents.

GPCR profiling: from hits to leads and from genotype to phenotype

GPCRs remain one of the most important classes of drug targets and, therefore, GPCR assay development and high-throughput GPCR ligand profiling continue to be major efforts in drug discovery. This article focuses on GPCR platform strategies from hits to leads with miniaturized complex pharmacology approaches. Three main areas of GPCR profiling are discussed including pharmacologically relevant hit identification, the pharmacology dossier applied to parallel structure activity and structure liability relationships and high-throughput mechanism studies from genotype to phenotype.

Des-aspartate-angiotensin I causes specific release of PGE2 and PGI2 in HUVEC via the angiotensin AT1 receptor and biased agonism

DAA-I (des-aspartate-angiotensin I), an endogenous angiotensin, had been shown earlier to ameliorate animal models of cardiovascular diseases via the angiotensin AT1 receptor and prostaglandins. The present study investigated further the action of DAA-I on the release of PGE2, PGI2, PGF2α and TXA2 in HUVEC. 10(-11)-10(-8)M DAA-I and 15min incubation specifically released PGE2 and PGI2. The release was inhibited by losartan and indomethacin but not by PD123319 and NS398 indicating that the angiotensin AT1 receptor and COX-1 mediate the release. At concentrations higher than 10(-7)M, DAA-I mimics the action of angiotensin II by releasing TXA2 but had no effect on the production of PGF2α. At similar concentrations and 4h incubation, DAA-I increased the release of the 4 prostaglandins via the angiotensin AT1 receptor and COX-2, again mimicking the action of angiotensin II. HUVEC that were preincubated with DAA-I or angiotensin II, released similar profiles of prostaglandins when incubated with arachidonic acid after the angiotensin had been washed off. We postulate that the internalized DAA-I/receptor complex remains active and mediates the conversion of arachidonic acid to the respective prostaglandins. The release of PGE2 and PGI2 via the angiotensin AT1 receptor and COX-1 is a novel specific action of DAA-I and is likely responsible for its beneficial effects seen in earlier studies. This specific action is definable as a biased agonism of the angiotensin AT1 receptor, which identifies DAA-I as a novel biased agonist and potential therapeutic that is able to produce specific prostaglandins at nanomolar concentrations.

Dysregulated Inflammatory Signaling upon Charcot-Marie-Tooth Type 1C Mutation of SIMPLE Protein

Endosomal trafficking is a key mechanism to modulate signal propagation and cross talk. Ubiquitin adaptors, along with endosomal sorting complex required for transport (ESCRT) complexes, are also integrated to terminate ligand-receptor activation in late endosomes and multivesicular bodies (MVBs). Within these pathways, we recently demonstrated that the protein SIMPLE is a novel player in MVB regulation. SIMPLE is also clinically important and its mutation accounts for the Charcot-Marie-Tooth type 1C (CMT1C) disease. MVB defects of mutation and deletion of SIMPLE, however, are distinct. Here, we show that MVB defects found in mutation but not deletion of SIMPLE lead to impaired turnover and accumulation of ESCRT-0 protein Hrs punctain late endosomes. We further uncover increased colocalization of ubiquitin ligase TRAF6 and Hrs in late endosomes. Upon stimulation with interkeukin-1 or transforming growth factor , prolonged activation of p38 kinase/JNK is detected, while nuclear accumulation of NF-κB and phosphorylation of SMAD2 is reduced with CMT1C mutation. The aberrant kinetics we observed in inflammatory signaling may contribute to increased tumor susceptibility and changes in the levels of chemokines/cytokines that result from CMT1C mutation. We propose that altered endosomal trafficking due to malformations of MVBs and subsequent atypical signaling kinetic may account for a toxic gain of function in CMT1C pathogenesis.

Using Bioluminescence Resonance Energy Transfer (BRET) to Characterize Agonist-Induced Arrestin Recruitment to Modified and Unmodified G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) represent ∼25% of current drug targets. Ligand binding to these receptors activates G proteins and arrestins, which are involved in differential signaling pathways. Because functionally selective or biased ligands activate one of these two pathways, they may be superior medications for certain diseases states. The identification of such ligands requires robust drug screening assays for both G protein and arrestin activity. This unit describes protocols for two bioluminescence resonance energy transfer (BRET)-based assays used to monitor arrestin recruitment to GPCRs. One assay requires modification of GPCRs by fusion to a BRET donor or acceptor moiety, whereas the other can detect arrestin recruitment to unmodified GPCRs.

In vivo veritas, the next frontier for functionally selective GPCR ligands

The realization that G-protein coupled receptors (GPCR) engage several cell signaling mechanisms simultaneously has led to a multiplication of research aimed at developing biased ligands exerting a selective action on subsets of responses downstream of a given receptor. Several tools have been developed to identify such ligands using recombinant cell systems. However the validation of biased ligand activity in animal models remains a serious challenge. Here we present a general strategy that can be used to validate biased ligand activity in vivo and supports it as a strategy for further drug development. In doing so, we placed special attention on strategies allowing to discriminate between G-protein and beta-arrestin mediated mechanisms. We also underscore differences between in vitro and in vivo systems and suggest avenues for tool development to streamline the translation of biased ligands development to pre-clinical animal models.

Noribogaine is a G-protein biased κ-opioid receptor agonist

Noribogaine is the long-lived human metabolite of the anti-addictive substance ibogaine. Noribogaine efficaciously reaches the brain with concentrations up to 20 μM after acute therapeutic dose of 40 mg/kg ibogaine in animals. Noribogaine displays atypical opioid-like components in vivo, anti-addictive effects and potent modulatory properties of the tolerance to opiates for which the mode of action remained uncharacterized thus far. Our binding experiments and computational simulations indicate that noribogaine may bind to the orthosteric morphinan binding site of the opioid receptors. Functional activities of noribogaine at G-protein and non G-protein pathways of the mu and kappa opioid receptors were characterized. Noribogaine was a weak mu antagonist with a functional inhibition constants (Ke) of 20 μM at the G-protein and β-arrestin signaling pathways. Conversely, noribogaine was a G-protein biased kappa agonist 75% as efficacious as dynorphin A at stimulating GDP-GTP exchange (EC50=9 μM) but only 12% as efficacious at recruiting β-arrestin, which could contribute to the lack of dysphoric effects of noribogaine. In turn, noribogaine functionally inhibited dynorphin-induced kappa β-arrestin recruitment and was more potent than its G-protein agonistic activity with an IC50 of 1 μM. This biased agonist/antagonist pharmacology is unique to noribogaine in comparison to various other ligands including ibogaine, 18-MC, nalmefene, and 6'-GNTI. We predict noribogaine to promote certain analgesic effects as well as anti-addictive effects at effective concentrations>1 μM in the brain. Because elevated levels of dynorphins are commonly observed and correlated with anxiety, dysphoric effects, and decreased dopaminergic tone, a therapeutically relevant functional inhibition bias to endogenously released dynorphins by noribogaine might be worthy of consideration for treating anxiety and substance related disorders.

Pharmacological Profile of Nociceptin/Orphanin FQ Receptors Interacting with G-Proteins and β-Arrestins 2

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions by selectively activating an opioid like receptor named N/OFQ peptide receptor (NOP). Biased agonism is emerging as an important and therapeutically relevant pharmacological concept in the field of G protein coupled receptors including opioids. To evaluate the relevance of this phenomenon in the NOP receptor, we used a bioluminescence resonance energy transfer technology to measure the interactions of the NOP receptor with either G proteins or β-arrestin 2 in the absence and in presence of increasing concentration of ligands. A large panel of receptor ligands was investigated by comparing their ability to promote or block NOP/G protein and NOP/arrestin interactions. In this study we report a systematic analysis of the functional selectivity of NOP receptor ligands. NOP/G protein interactions (investigated in cell membranes) allowed a precise estimation of both ligand potency and efficacy yielding data highly consistent with the known pharmacological profile of this receptor. The same panel of ligands displayed marked differences in the ability to promote NOP/β-arrestin 2 interactions (evaluated in whole cells). In particular, full agonists displayed a general lower potency and for some ligands an inverted rank order of potency was noted. Most partial agonists behaved as pure competitive antagonists of receptor/arrestin interaction. Antagonists displayed similar values of potency for NOP/Gβ₁ or NOP/β-arrestin 2 interaction. Using N/OFQ as reference ligand we computed the bias factors of NOP ligands and a number of agonists with greater efficacy at G protein coupling were identified.

Agonistic aptamer to the insulin receptor leads to biased signaling and functional selectivity through allosteric modulation

Due to their high affinity and specificity, aptamers have been widely used as effective inhibitors in clinical applications. However, the ability to activate protein function through aptamer-protein interaction has not been well-elucidated. To investigate their potential as target-specific agonists, we used SELEX to generate aptamers to the insulin receptor (IR) and identified an agonistic aptamer named IR-A48 that specifically binds to IR, but not to IGF-1 receptor. Despite its capacity to stimulate IR autophosphorylation, similar to insulin, we found that IR-A48 not only binds to an allosteric site distinct from the insulin binding site, but also preferentially induces Y1150 phosphorylation in the IR kinase domain. Moreover, Y1150-biased phosphorylation induced by IR-A48 selectively activates specific signaling pathways downstream of IR. In contrast to insulin-mediated activation of IR, IR-A48 binding has little effect on the MAPK pathway and proliferation of cancer cells. Instead, AKT S473 phosphorylation is highly stimulated by IR-A48, resulting in increased glucose uptake both in vitro and in vivo. Here, we present IR-A48 as a biased agonist able to selectively induce the metabolic activity of IR through allosteric binding. Furthermore, our study also suggests that aptamers can be a promising tool for developing artificial biased agonists to targeted receptors.

Studying GPCR/cAMP pharmacology from the perspective of cellular structure

Signal transduction via G-protein coupled receptors (GPCRs) relies upon the production of cAMP and other signaling cascades. A given receptor and agonist pair, produce multiple effects upon cellular physiology which can be opposite in different cell types. The production of variable cellular effects via the signaling of the same GPCR in different cell types is a result of signal organization in space and time (compartmentation). This organization is usually based upon the physical and chemical properties of the membranes in which the GPCRs reside and the repertoire of downstream effectors and co-factors that are available at that location. In this review we explore mechanisms of GPCR signal compartmentation and broadly review the state-of-the-art methodologies which can be utilized to study them. We provide a clear rationale for a “localized” approach to the study of the pharmacology and physiology of GPCRs and particularly the secondary messenger cAMP.