When simple agonism is not enough: emerging modalities of GPCR ligands

GHRH and the prostate

In the late 1960s and early 1970s, hypothalamic regulatory hormones were isolated, characterized and sequenced. Later, it was demonstrated hypothalamic and ectopic production of growth hormone-releasing hormone (GHRH) in normal and tumor tissues, of both humans and animals. Pituitary-type GHRH receptors (pGHRH-R) had been demonstrated to be expressed predominantly in the anterior pituitary gland but also found in other somatic cells, and significantly present in various human cancers; in addition, the expression of splice variants (SVs) of GHRH receptor (GHRH-R) has been found not only in the pituitary but in extrapituitary tissues, including human neoplasms. In relation to the prostate, besides the pGHRH-R, it has been detected the presence of truncated splice variants of GHRH-R (SV1-SV4) in normal human prostate and human prostate cancer (PCa) specimens; lastly, a novel SV of GHRH-R has been detected in human PCa. Signaling pathways activated by GHRH include AC/cAMP/PKA, Ras/Raf/ERK, PI3K/Akt/mTOR and JAK2/STAT3, which are involved in processes such as cell survival, proliferation and cytokine secretion. The neuropeptide GHRH can also transactivate the epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor (HER)-2. Thus, GHRH-Rs have become drug targets for several types of clinical conditions, including prostate-related conditions such as prostatitis, benign hyperplasia and cancer. Over the last fifty years, the development of GHRH-R receptor antagonists has been unstoppable, improving their potency, stability and affinity for the receptor. The last series of GHRH-R antagonists, AVR, exhibits superior anticancer and anti-inflammatory activities in both in vivo and in vitro assays.

Designing drugs and chemical probes with the dualsteric approach

Traditionally, drugs are monovalent, targeting only one site on the protein surface. This includes orthosteric and allosteric drugs, which bind the protein at orthosteric and allosteric sites, respectively. Orthosteric drugs are good in potency, whereas allosteric drugs have better selectivity and are solutions to classically undruggable targets. However, it would be difficult to simultaneously reach high potency and selectivity when targeting only one site. Also, both kinds of monovalent drugs suffer from mutation-caused drug resistance. To overcome these obstacles, dualsteric modulators have been proposed in the past twenty years. Compared to orthosteric or allosteric drugs, dualsteric modulators are bivalent (or bitopic) with two pharmacophores. Each of the two pharmacophores bind the protein at the orthosteric and an allosteric site, which could bring the modulator with special properties beyond monovalent drugs. In this study, we comprehensively review the current development of dualsteric modulators. Our main effort reason and illustrate the aims to apply the dualsteric approach, including a "double win" of potency and selectivity, overcoming mutation-caused drug resistance, developments of function-biased modulators, and design of partial agonists. Moreover, the strengths of the dualsteric technique also led to its application outside pharmacy, including the design of highly sensitive fluorescent tracers and usage as molecular rulers. Besides, we also introduced drug targets, designing strategies, and validation methods of dualsteric modulators. Finally, we detail the conclusions and perspectives.

Subtle Structural Modification of a Synthetic Cannabinoid Receptor Agonist Drastically Increases its Efficacy at the CB1 Receptor

The emergence of synthetic cannabinoid receptor agonists (SCRAs) as illicit psychoactive substances has posed considerable public health risks, including fatalities. Many SCRAs exhibit much higher efficacy and potency compared with the phytocannabinoid Δ9-tetrahydrocannabinol (THC) at the cannabinoid receptor 1 (CB1R), leading to dramatic differences in signaling levels that can be toxic. In this study, we investigated the structure-activity relationships of aminoalkylindole SCRAs at CB1Rs, focusing on 5F-pentylindoles containing an amide linker attached to different head moieties. Using in vitro bioluminescence resonance energy transfer assays, we identified a few SCRAs exhibiting significantly higher efficacy in engaging the Gi protein and recruiting β-arrestin than the reference CB1R full agonist CP55940. Importantly, the extra methyl group on the head moiety of 5F-MDMB-PICA, as compared to that of 5F-MMB-PICA, led to a large increase in efficacy and potency at the CB1R. This pharmacological observation was supported by the functional effects of these SCRAs on glutamate field potentials recorded in hippocampal slices. Molecular modeling and simulations of the CB1R models bound with both of the SCRAs revealed critical structural determinants contributing to the higher efficacy of 5F-MDMB-PICA and how these subtle differences propagated to the receptor-G protein interface. Thus, we find that apparently minor structural changes in the head moiety of SCRAs can cause major changes in efficacy. Our results highlight the need for close monitoring of the structural modifications of newly emerging SCRAs and their potential for toxic drug responses in humans.

A subtle structural modification of a synthetic cannabinoid receptor agonist drastically increases its efficacy at the CB1 receptor

The emergence of synthetic cannabinoid receptor agonists (SCRAs) as illicit psychoactive substances has posed considerable public health risks that include fatalities. Many SCRAs exhibit much higher efficacy and potency, compared with the phytocannabinoid Δ9-tetrahydrocannabinol (THC), at the cannabinoid receptor 1 (CB1R), a G protein-coupled receptor involved in modulating neurotransmitter release. In this study, we investigated structure activity relationships (SAR) of aminoalkylindole SCRAs at CB1Rs, focusing on 5F-pentylindoles containing an amide linker attached to different head moieties. Using in vitro bioluminescence resonance energy transfer (BRET) assays, we identified a few of SCRAs exhibiting significantly higher efficacy in engaging the Gi protein and recruiting β-arrestin than the reference CB1R full agonist CP55940. Importantly, adding a methyl group at the head moiety of 5F-MMB-PICA yielded 5F-MDMB-PICA, an agonist exhibiting a large increase in efficacy and potency at the CB1R. This pharmacological observation was supported by a functional assay of the effects of these SCRAs on glutamate field potentials recorded in hippocampal slices. Molecular modeling and simulations of the CB1R bound with either of the SCRAs revealed critical structural determinants contributing to the higher efficacy of 5F-MDMB-PICA, and how these subtle differences propagated to the receptor-G protein interface. Thus, we find that apparently minor structural changes in the head moiety of SCRAs can cause major changes in efficacy. Our results highlight the need for close monitoring of structural modifications of newly emerging SCRAs and their potential for toxic drug responses in humans.

Allosteric Modulators of Dopamine D2 Receptors for Fine-Tuning of Dopaminergic Neurotransmission in CNS Diseases: Overview, Pharmacology, Structural Aspects and Synthesis

Allosteric modulation of G protein-coupled receptors (GPCRs) is nowadays a hot topic in medicinal chemistry. Allosteric modulators, i.e., compounds which bind in a receptor site topologically distinct from orthosteric sites, exhibit a number of advantages. They are more selective, safer and display a ceiling effect which prevents overdosing. Allosteric modulators of dopamine D₂ receptor are potential drugs against a number of psychiatric and neurological diseases, such as schizophrenia and Parkinson's disease. In this review, an insightful summary of current research on D₂ receptor modulators is presented, ranging from their pharmacology and structural aspects of ligand-receptor interactions to their synthesis.

The Binding Mode to Orthosteric Sites and/or Exosites Underlies the Therapeutic Potential of Drugs Targeting Cannabinoid CB2 Receptors

The classical terms agonists and antagonists for G protein coupled receptors (GPCRs) have often become misleading. Even the biased agonism concept does not describe all the possibilities already demonstrated for GPCRs. The cannabinoid CB₂ receptor (CB₂R) emerged as a promising target for a variety of diseases. Reasons for such huge potential are centered around the way drugs sit in the orthosteric and/or exosites of the receptor. On the one hand, a given drug in a specific CB₂R conformation leads to a signaling cascade that differs qualitatively and/or quantitatively from that triggered by another drug. On the other hand, a given drug may lead to different signaling outputs in two different tissues (or cell contexts) in which the conformation of the receptor is affected by allosteric effects derived from interactions with other proteins or with membrane lipids. This highlights the pharmacological complexity of this receptor and the need to further unravel the binding mode of CB₂R ligands in order to fine-tune signaling effects and therapeutic propositions.

Probing Allosteric Regulation Mechanism of W7.35 on Agonist-Induced Activity for μOR by Mutation Simulation

The residue located at 15 positions before the most conserved residue in TM7 (7.35 of Ballesteros-Weinstein number) plays important roles in ligand binding and the receptor activity for class A GPCRs. Nevertheless, its regulation mechanism has not been clearly clarified in experiments, and some controversies also exist for its impact on μ-opioid receptors (μOR) bound by agonists. Thus, we chose the μ-opioid receptor (μOR) of class A GPCRs as a representative and utilized a microsecond accelerated molecular dynamics simulation (aMD) coupled with a protein structure network (PSN) to explore the effect of W318^7.35 on its functional activity induced by the agonist endomorphin2 mainly by a comparison of the wild system and its W7.35A mutant. When endomorphin2 binds to the wild-type μOR, TM6 in μOR moves outward to form an open intracellular conformation that is beneficial to accommodating the β-arrestin transducer, rather than the G-protein transducer due to the clash with the α5 helix of G-protein, thus acting as a β-arrestin biased agonist. However, the W318A mutation induces the intracellular part of μOR to form a closed state, which disfavors coupling with either G-protein or β-arrestin. The allosteric pathway analysis further reveals that the binding of endomorphin2 to the wild-type μOR transmits more activation signals to the β-arrestin binding site while the W318A mutation induces more structural signals to transmit to common binding residues of the G protein and β-arrestin. More interestingly, the residue at the 7.35 position regulates the shortest allosteric pathway in indirect ways by influencing the interactions between other ligand-binding residues and endomorphin2. W293^6.48 and F289^6.44 are important for regulating the different activities of μOR induced either by the agonist or by the mutation. Y336^7.53, F343^8.50, and D340^8.47 play crucial roles in activating the β-arrestin biased signal induced by the agonist endomorphin2, while L158^3.43 and V286^6.41 devote important contributions to the change in the activity of endomorphin2 from the β-arrestin biased agonist to the antagonist upon the W318A mutation.

Allosteric Molecular Switches in Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGlu) are class C G protein-coupled receptors of eight subtypes that are omnipresently expressed in the central nervous system. mGlus have relevance in several psychiatric and neurological disorders, therefore they raise considerable interest as drug targets. Allosteric modulators of mGlus offer advantages over orthosteric ligands owing to their increased potential to achieve subtype selectivity, and this has prompted discovery programs that have produced a large number of reported allosteric mGlu ligands. However, the optimization of allosteric ligands into drug candidates has proved to be challenging owing to induced-fit effects, flat or steep structure-activity relationships and unexpected changes in theirpharmacology. Subtle structural changes identified as molecular switches might modulate the functional activity of allosteric ligands. Here we review these switches discovered in the metabotropic glutamate receptor family..

Tuning Down the Pain - An Overview of Allosteric Modulation of Opioid Receptors: Mechanisms of Modulation, Allosteric Sites, Modulator Syntheses

Opioid signaling plays a central role in pain perception. As such, it remains the main target in the development of antinociceptive agents, despite serious side effects involved. In recent years, hopes for improved opioid painkillers are rising, together with our understanding of allosterism and biased signaling mechanisms. In this review, we focus on recently discovered allosteric modulators of opioid receptors, insights into phenomena underlying their action, as well as on how they extend our understanding of mechanisms of previously known compounds. A brief overlook of their synthesis is also presented.

Voltage-induced structural modifications on M2 muscarinic receptor and their functional implications when interacting with the superagonist iperoxo

It has been reported that muscarinic type-2 receptors (M2R) are voltage sensitive in an agonist-specific manner. In this work, we studied the effects of membrane potential on the interaction of M2R with the superagonist iperoxo (IXO), both functionally (using the activation of the ACh-gated K⁺ current (I_KACh) in cardiomyocytes) and by molecular dynamics (MD) simulations. We found that IXO activated I_KACh with remarkable high potency and clear voltage dependence, displaying a larger effect at the hyperpolarized potential. This result is consistent with a greater affinity, as validated by a slower (τ = 14.8 ± 2.3 s) deactivation kinetics of the IXO-evoked I_KACh than that at the positive voltage (τ = 6.7 ± 1.2 s). The voltage-dependent M2R-IXO interaction induced I_KACh to exhibit voltage-dependent features of this current, such as the 'relaxation gating' and the modulation of rectification. MD simulations revealed that membrane potential evoked specific conformational changes both at the external access and orthosteric site of M2R that underlie the agonist affinity change provoked by voltage on M2R. Moreover, our experimental data suggest that the 'tyrosine lid' (Y104, Y403, and Y426) is not the previously proposed voltage sensor of M2R. These findings provide an insight into the structural and functional framework of the biased signaling induced by voltage on GPCRs.

A structural basis for how ligand binding site changes can allosterically regulate GPCR signaling and engender functional selectivity

Signaling bias is the propensity for some agonists to preferentially stimulate G protein-coupled receptor (GPCR) signaling through one intracellular pathway versus another. We previously identified a G protein-biased agonist of the D₂ dopamine receptor (D2R) that results in impaired β-arrestin recruitment. This signaling bias was predicted to arise from unique interactions of the ligand with a hydrophobic pocket at the interface of the second extracellular loop and fifth transmembrane segment of the D2R. Here, we showed that residue Phe189 within this pocket (position 5.38 using Ballesteros-Weinstein numbering) functions as a microswitch for regulating receptor interactions with β-arrestin. This residue is relatively conserved among class A GPCRs, and analogous mutations within other GPCRs similarly impaired β-arrestin recruitment while maintaining G protein signaling. To investigate the mechanism of this signaling bias, we used an active-state structure of the β₂-adrenergic receptor (β2R) to build β2R-WT and β2R-Y199^5.38A models in complex with the full β2R agonist BI-167107 for molecular dynamics simulations. These analyses identified conformational rearrangements in β2R-Y199^5.38A that propagated from the extracellular ligand binding site to the intracellular surface, resulting in a modified orientation of the second intracellular loop in β2R-Y199^5.38A, which is predicted to affect its interactions with β-arrestin. Our findings provide a structural basis for how ligand binding site alterations can allosterically affect GPCR-transducer interactions and result in biased signaling.

Novel mechanism of modulation at a ligand-gated ion channel; action of 5-Cl-indole at the 5-HT3 A receptor

BACKGROUND AND PURPOSE

The 5-HT₃ receptor is a prototypical member of the Cys-loop ligand-gated ion channel (LGIC) superfamily and an established therapeutic target. In addition to activation via the orthosteric site, receptor function can be modulated by allosteric ligands. We have investigated the pharmacological action of Cl-indole upon the 5-HT₃ A receptor and identified that this positive allosteric modulator possesses a novel mechanism of action for LGICs.

EXPERIMENTAL APPROACH

The impact of Cl-indole upon the 5-HT₃ receptor was assessed using single cell electrophysiological recordings and [³ H]-granisetron binding in HEK293 cells stably expressing the 5-HT₃ receptor.

KEY RESULTS

Cl-indole failed to evoke 5-HT₃ A receptor-mediated responses (up to 30 μM) or display affinity for the [³ H]-granisetron binding site. However, in the presence of Cl-indole, termination of 5-HT application revealed tail currents mediated via the 5-HT₃ A receptor that were independent of the preceding 5-HT concentration but were antagonized by the 5-HT₃ receptor antagonist, ondansetron. These tail currents were absent in the 5-HT₃ AB receptor. Furthermore, the presence of 5-HT revealed a concentration-dependent increase in the affinity of Cl-indole for the orthosteric binding site of the human 5-HT₃ A receptor.

CONCLUSIONS AND IMPLICATIONS

Cl-indole acts as both an orthosteric agonist and an allosteric modulator, but the presence of an orthosteric agonist (e.g. 5-HT) is a prerequisite to reveal both actions. Precedent for ago-allosteric action is available, yet the essential additional presence of an orthosteric agonist is now reported for the first time. This widening of the pharmacological mechanisms to modulate LGICs may offer further therapeutic opportunities.

Superagonism at G protein-coupled receptors and beyond

Ligands targeting GPCRs can be categorized according to their intrinsic efficacy to trigger a specific, receptor-mediated response. A ligand endowed with the same level of efficacy as the endogenous agonist can be classified as a full agonist, whereas a compound that displays greater efficacy, that is, higher receptor signalling output than the endogenous agonist, can be called a superagonist. Subsequent to GPCR activation, an intracellular signalling cascade is set in motion, which may generate substantial amplification of the signal. This may obscure superagonism in pharmacological assays and, therefore, the definition of superagonism necessitates a combination of operational approaches, reduction of spare receptors or estimation of receptor activation close to the receptor level to quantify relative agonist efficacies in a particular system. The first part of this review will compare GPCR superagonism with superagonism in the field of immunology, where this term is well established. In the second part, known GPCR superagonists will be reviewed. Then, the experimental and analytical challenges in the deconvolution of GPCR superagonism will be addressed. Finally, the potential benefit of superagonism is discussed. The molecular mechanisms behind GPCR superagonism are not completely understood. However, crystallography shows that agonist binding alone is not sufficient for a fully active receptor state and that binding of the G protein is at least equally important. Accordingly, the emerging number of reported superagonists implies that ligand-induced receptor conformations more active than the ones stabilized by the endogenous agonist are indeed feasible. Superagonists may have therapeutic potential when receptor function is impaired or to induce negative feedback mechanisms. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

An analysis of glucocorticoid receptor-mediated gene expression in BEAS-2B human airway epithelial cells identifies distinct, ligand-directed, transcription profiles with implications for asthma therapeutics

BACKGROUND AND PURPOSE

International asthma guidelines recommend that inhaled glucocorticoids be used as a monotherapy in all patients with mild to moderate disease because of their ability to suppress airways inflammation. Current evidence suggests that the therapeutic benefit of glucocorticoids is due to the transactivation and transrepression of anti-inflammatory and pro-inflammatory genes respectively. However, the extent to which clinically relevant glucocorticoids are equivalent in their ability to modulate gene expression is unclear.

EXPERIMENTAL APPROACH

A pharmacodynamics investigation of glucocorticoid receptor (GR)-mediated gene transactivation in BEAS-2B human airway epithelial cells was performed using a glucocorticoid response element luciferase reporter coupled with an analysis of glucocorticoid-inducible genes encoding proteins with anti-inflammatory and adverse-effect potential.

KEY RESULTS

Using transactivation as a functionally relevant output, a given glucocorticoid displayed a unique, gene expression 'fingerprint' where intrinsic efficacy and GR density were essential determinants. We showed that depending on the gene selected for analysis, a given glucocorticoid can behave as an antagonist, partial agonist, full agonist or even 'super agonist'. In the likely event that different, tissue-dependent gene expression profiles are reproduced in vivo, then the anti-inflammatory and adverse-effect potential of many glucocorticoids currently available as asthma therapeutics may not be equivalent.

CONCLUSIONS AND IMPLICATIONS

The generation of gene expression 'fingerprints' in target and off-target human tissues could assist the rational design of GR agonists with improved therapeutic ratios. This approach could identify compounds that are useful in the management of severe asthma and other inflammatory disorders where systemic exposure is desirable.

G protein-coupled receptors: an overview of signaling mechanisms and screening assays

The existence of cellular receptors, a group of specialized biomolecules to which endogenous and exogenous compounds bind and exert an effect, is one of the most exciting aspects of cell biology. Among the different receptor types recognized today, G-protein-coupled receptors (GPCRs) constitute, undoubtedly, one of the most important classes, in part due to their versatility, but particularly, due to their central role in a multitude of physiological states. The unveiling of GPCR function and mode of action is a challenging task that prevails until our days, as the full potential of these receptors is far from being established. Such an undertaking calls for a joint effort of multidisciplinary teams that must combine state-of-the-art technologies with in-depth knowledge of cell biology to probe such specialized molecules. This review provides a concise coverage of the scientific progress that has been made in GPCR research to provide researchers with an updated overview of the field. A brief outline of the historical breakthroughs is followed by a discussion of GPCR signaling mechanisms and by a description of the role played by assay technologies.

Insights into a defined secondary binding region on β-adrenoceptors and putative roles in ligand binding and drug design

Putative roles of a secondary binding region shared among beta-adrenoceptors.

Using constitutive activity to define appropriate high-throughput screening assays for orphan g protein-coupled receptors

Orphan G protein-coupled receptors represent an underexploited resource for drug discovery but pose a considerable challenge for assay development because their cognate G protein signaling pathways are often unknown. In this methodological chapter, we describe the use of constitutive activity, that is, the inherent ability of receptors to couple to their cognate G proteins in the absence of ligand, to inform the development of high-throughput screening assays for a particular orphan receptor. We specifically focus on a two-step process, whereby constitutive G protein coupling is first determined using yeast Gpa1/human G protein chimeras linked to growth and β-galactosidase generation. Coupling selectivity is then confirmed in mammalian cells expressing endogenous G proteins and driving accumulation of transcription factor-fused luciferase reporters specific to each of the classes of G protein. Based on these findings, high-throughput screening campaigns can be performed on the already miniaturized mammalian reporter system.

Auto-inhibition at a ligand-gated ion channel: a cross-talk between orthosteric and allosteric sites

BACKGROUND AND PURPOSE

A ligand is believed to produce either positive or negative responses, or to block both of them. However, an indole compound was found to promote both positive and negative effects at the 5-HT3 AB receptor, which displays a low level of spontaneous activity. The present study attempted to delineate the mechanisms underlying this phenomenon.

EXPERIMENTAL APPROACH

The spontaneously active V291S 5-HT3 A receptor was used to explore the properties of 5-hydroxyindole (5-HoI) and 5-methoxyindole (5-MoI), structural analogues of 5-HT, either alone or in combination with orthosteric probes.

KEY RESULTS

Two types of efficacy switching were initiated by altering ligand structure and concentration. At lower concentrations, a subtle structural change at position 5 of the indole molecule resulted in opposite effects. 5-HoI apparently elicited partial allosteric inverse agonism, whereas 5-MoI induced allosteric agonism. Interestingly, at a higher concentration, these indoles produced distinct auto-inhibition, manifested as a switch from positive to negative effects. 5-HoI induced a transition from orthosteric agonism to allosteric inverse agonism, whereas 5-MoI produced a shift from allosteric agonism to orthosteric inverse agonism. The auto-inhibition appears to involve communication between orthosteric and allosteric sites of the active receptor conformation and/or between inactive and active conformations. An additive effect of orthosteric and allosteric inverse agonism and insensitivity of allosteric agonism to orthosteric antagonism were also demonstrated.

CONCLUSIONS AND IMPLICATIONS

Together, the results suggest that the moiety at position 5 of the indole structure is a critical determinant of a ligand's properties at the 5-HT3 A receptor, providing new insights into understanding ligand-receptor interactions.

Agonists with supraphysiological efficacy at the muscarinic M2 ACh receptor

BACKGROUND AND PURPOSE

Artificial agonists may have higher efficacy for receptor activation than the physiological agonist. Until now, such 'superagonism' has rarely been reported for GPCRs. Iperoxo is an extremely potent muscarinic receptor agonist. We hypothesized that iperoxo is a 'superagonist'.

EXPERIMENTAL APPROACH

Signalling of iperoxo and newly synthesized structural analogues was compared with that of ACh at label-free M2 muscarinic receptors applying whole cell dynamic mass redistribution, measurement of G-protein activation, evaluation of cell surface agonist binding and computation of operational efficacies.

KEY RESULTS

In CHO-hM2 cells, iperoxo significantly exceeds ACh in Gi /Gs signalling competence. In the orthosteric loss-of-function mutant M2 -Y104(3.33) A, the maximum effect of iperoxo is hardly compromised in contrast to ACh. 'Superagonism' is preserved in the physiological cellular context of MRC-5 human lung fibroblasts. Structure-signalling relationships including iperoxo derivatives with either modified positively charged head group or altered tail suggest that 'superagonism' of iperoxo is mechanistically based on parallel activation of the receptor protein via two orthosteric interaction points.

CONCLUSION AND IMPLICATIONS

Supraphysiological agonist efficacy at muscarinic M2 ACh receptors is demonstrated for the first time. In addition, a possible underlying molecular mechanism of GPCR 'superagonism' is provided. We suggest that iperoxo-like orthosteric GPCR activation is a new avenue towards a novel class of receptor activators.

Role of the N-terminal region in G protein-coupled receptor functions: negative modulation revealed by 5-HT2B receptor polymorphisms

The putative role of the N-terminal region of rhodopsin-like 7 transmembrane biogenic amine receptors in agonist-induced signaling has not yet been clarified despite recent advances in 7 transmembrane receptor structural biology. Given the existence of N-terminal nonsynonymous polymorphisms (R6G;E42G) within the HTR2B gene in a drug-abusing population, we assessed whether these polymorphisms affect 5-hydroxytryptamine 2B (5-HT2B) receptor in vitro pharmacologic and coupling properties in transfected COS-7 cells. Modification of the 5-HT2B receptor N terminus by the R6G;E42G polymorphisms increases such agonist signaling pathways as inositol phosphate accumulation as assessed by either classic or operational models. The N-terminal R6G;E42G mutations of the 5-HT2B receptor also increase cell proliferation and slow its desensitization kinetics compared with the wild-type receptor, further supporting a role for the N terminus in transduction efficacy. Furthermore, by coexpressing a tethered wild-type 5-HT2B receptor N terminus with a 5-HT2B receptor bearing a N-terminal deletion, we were able to restore original coupling. This reversion to normal activity of a truncated 5-HT2B receptor by coexpression of the membrane-tethered wild-type 5-HT2B receptor N terminus was not observed using a membrane-tethered 5-HT2B receptor R6G;E42G N terminus. These data suggest that the N terminus exerts a negative control over basal as well as agonist-stimulated receptor activity that is lost in the R6G;E42G mutant. Our findings reveal a new and unanticipated role of the 5-HT2B receptor N terminus as a negative modulator, affecting both constitutive and agonist-stimulated activity. Moreover, our data caution against excluding the N terminus and extracellular loops in structural studies of this 7 transmembrane receptor family.

Recent structural advances of β1 and β2 adrenoceptors yield keys for ligand recognition and drug design

Because they represent attractive drug targets, adrenoceptors have been widely studied. Recent progress in structural data of β-adrenoceptors allows us to understand and predict key interactions in ligand recognition and receptor activation. Nevertheless, an important aspect of this process has only begun to be explored: the stabilization of a conformational state of these receptors upon contact with a ligand and the capacity of a ligand to influence receptor conformation through allosteric modulation, biased signaling, and selectivity. The aim of the present Perspective is to identify the well-defined orthosteric binding site and possible allosteric sites and to analyze the importance of the ligand-receptor interaction in the stabilization of certain receptor conformations. For this purpose, we have reviewed recent advances made through the use of X-ray data from ligand-β-adrenoceptor (including ADRB1 and ADRB2) crystal structures. Most importantly, implications in the medicinal chemistry field are explored in relation to drug design.

Insights into the structural biology of G-protein coupled receptors impacts drug design for central nervous system neurodegenerative processes

In the last few years, there have been important new insights into the structural biology of G-protein coupled receptors. It is now known that allosteric binding sites are involved in the affinity and selectivity of ligands for G-protein coupled receptors, and that signaling by these receptors involves both G-protein dependent and independent pathways. The present review outlines the physiological and pharmacological implications of this perspective for the design of new drugs to treat disorders of the central nervous system. Specifically, new possibilities are explored in relation to allosteric and orthosteric binding sites on dopamine receptors for the treatment of Parkinson's disease, and on muscarinic receptors for Alzheimer's disease. Future research can seek to identify ligands that can bind to more than one site on the same receptor, or simultaneously bind to two receptors and form a dimer. For example, the design of bivalent drugs that can reach homo/hetero-dimers of D2 dopamine receptor holds promise as a relevant therapeutic strategy for Parkinson's disease. Regarding the treatment of Alzheimer's disease, the design of dualsteric ligands for mono-oligomeric rinic receptors could increase therapeutic effectiveness by generating potent compounds that could activate more than one signaling pathway.

The allosteric vestibule of a seven transmembrane helical receptor controls G-protein coupling

Seven transmembrane helical receptors (7TMRs) modulate cell function via different types of G proteins, often in a ligand-specific manner. Class A 7TMRs harbour allosteric vestibules in the entrance of their ligand-binding cavities, which are in the focus of current drug discovery. However, their biological function remains enigmatic. Here we present a new strategy for probing and manipulating conformational transitions in the allosteric vestibule of label-free 7TMRs using the M₂ acetylcholine receptor as a paradigm. We designed dualsteric agonists as 'tailor-made' chemical probes to trigger graded receptor activation from the acetylcholine-binding site while simultaneously restricting spatial flexibility of the receptor's allosteric vestibule. Our findings reveal for the first time that a 7TMR's allosteric vestibule controls the extent of receptor movement to govern a hierarchical order of G-protein coupling. This is a new concept assigning a biological role to the allosteric vestibule for controlling fidelity of 7TMR signalling.

Class A seven transmembrane helical receptors harbour vestibules at the entrance to the ligand-binding domain. Here, Bock et al. use probes to monitor the conformation of the M2 muscarinic receptor and show that the vestibule alters the extent of receptor movement.

Metabotropic glutamate receptor 5-positive allosteric modulators for the treatment of schizophrenia (2004–2012)

The mGlu5, a class C G-protein-coupled receptor and member of the group I mGlu receptor family, has been demonstrated to play a role in a number of therapeutic areas within the CNS, including schizophrenia, dementia, epilepsy, cognition, drug abuse, and fragile X syndrome. Small-molecule modulation of mGlu5 via positive allosteric modulators (PAMs) is being pursued as a promising approach for the treatment of schizophrenia and has been validated preclinically in a number of animal models. This article provides a brief historical overview of mGlu5 PAMs in the primary literature followed by a comprehensive overview of the patent literature since 2004. Schizophrenia is a complex disorder and although no mGlu5 PAMs have progressed into clinical trials in patients, the target continues to show promise as an attractive non-dopaminergic therapy. The successful development of mGlu5 PAMs for clinical testing must address several issues, including challenges associated with ‘molecular switches’, allosteric-agonist activity and stimulus bias.

G protein-coupled receptor transactivation: from molecules to mice

G protein-coupled receptors (GPCRs) mediate a diverse range of physiological functions via activation of complex signaling systems. Organization of GPCRs in to dimers and oligomers provides a mechanism for both signal diversity and specificity in cellular responses, yet our understanding of the physiological significance of dimerization, particularly homodimerization, has not been forthcoming. This chapter will describe how we have investigated the physiological importance of GPCR homodimerization, using the luteinizing hormone/chorionic gonadotropin receptor as a model GPCR. Using transactivation as a mode of assessing receptor dimerization, we describe our cellular system and functional assays for assessment of transactivation in vitro and detail our strategy for generating a mouse model to assess GPCR transactivation in vivo.

Multiple active receptor conformation, agonist efficacy and maximum effect of the system: the conformation-based operational model of agonism

The operational model of agonism assumes that the maximum effect a particular receptor system can achieve (the Em parameter) is fixed. Em estimates are above but close to the asymptotic maximum effects of endogenous agonists. The concept of Em is contradicted by superagonists and those positive allosteric modulators that significantly increase the maximum effect of endogenous agonists. An extension of the operational model is proposed that assumes that the Em parameter does not necessarily have a single value for a receptor system but has multiple values associated to multiple active receptor conformations. The model provides a mechanistic link between active receptor conformation and agonist efficacy, which can be useful for the analysis of agonist response under different receptor scenarios.

Modulation of G protein-coupled adenosine receptors by strategically functionalized agonists and antagonists immobilized on gold nanoparticles

Gold nanoparticles (AuNPs) allow the tuning of pharmacokinetic and pharmacodynamic properties by active or passive targeting of drugs for cancer and other diseases. We have functionalized gold nanoparticles by tethering specific ligands, agonists and antagonists, of adenosine receptors (ARs) to the gold surface as models for cell surface interactions with G protein-coupled receptors (GPCRs). The AuNP conjugates with chain-extended AR ligands alone (PEGylated nucleosides and nonnucleosides, anchored to the Au via thioctic acid) were found to be insoluble in water due to hydrophobic entities in the ligand. Therefore, we added a second, biologically inactive pendant moiety to increase the water solubility, consisting of a PEGylated chain terminating in a carboxylic or phosphate group. The purity and stability of the immobilized biologically active ligand were examined by ultrafiltration and HPLC. Pharmacological receptor binding studies on these GPCR ligand-derivatized AuNPs (2-5 nm in diameter), performed using membranes of mammalian cells stably expressing human A1, A2A, and A3ARs, showed that the desired selectivity was retained with K(i) values (nanomolar) of A3AR agonist 21b and A2AAR antagonists 24 and 26a of 14 (A3), 34 (A2A), and 69 (A2A), respectively. The corresponding monomers displayed K i values of 37, 61, and 1,420 nM, respectively. In conclusion, we have synthesized stable, water-soluble AuNP derivatives of tethered A3 and A2AAR ligands that retain the biological properties of their monomeric ligands and are intended for therapeutic and imaging applications. This is the first prototypical application to gold carriers of small molecule (nonpeptide) GPCR ligands, which are under investigation for treatment of cancer and inflammatory diseases.

A potent class of GPR40 full agonists engages the enteroinsular axis to promote glucose control in rodents

Type 2 diabetes is characterized by impaired glucose homeostasis due to defects in insulin secretion, insulin resistance and the incretin response. GPR40 (FFAR1 or FFA1) is a G-protein-coupled receptor (GPCR), primarily expressed in insulin-producing pancreatic β-cells and incretin-producing enteroendocrine cells of the small intestine. Several GPR40 agonists, including AMG 837 and TAK-875, have been disclosed, but no GPR40 synthetic agonists have been reported that engage both the insulinogenic and incretinogenic axes. In this report we provide a molecular explanation and describe the discovery of a unique and potent class of GPR40 full agonists that engages the enteroinsular axis to promote dramatic improvement in glucose control in rodents. GPR40 full agonists AM-1638 and AM-6226 stimulate GLP-1 and GIP secretion from intestinal enteroendocrine cells and increase GSIS from pancreatic islets, leading to enhanced glucose control in the high fat fed, streptozotocin treated and NONcNZO10/LtJ mouse models of type 2 diabetes. The improvement in hyperglycemia by AM-1638 was reduced in the presence of the GLP-1 receptor antagonist Ex(9–39)NH₂.

Action of molecular switches in GPCRs--theoretical and experimental studies

G protein coupled receptors (GPCRs), also called 7TM receptors, form a huge superfamily of membrane proteins that, upon activation by extracellular agonists, pass the signal to the cell interior. Ligands can bind either to extracellular N-terminus and loops (e.g. glutamate receptors) or to the binding site within transmembrane helices (Rhodopsin-like family). They are all activated by agonists although a spontaneous auto-activation of an empty receptor can also be observed. Biochemical and crystallographic methods together with molecular dynamics simulations and other theoretical techniques provided models of the receptor activation based on the action of so-called "molecular switches" buried in the receptor structure. They are changed by agonists but also by inverse agonists evoking an ensemble of activation states leading toward different activation pathways. Switches discovered so far include the ionic lock switch, the 3-7 lock switch, the tyrosine toggle switch linked with the nPxxy motif in TM7, and the transmission switch. The latter one was proposed instead of the tryptophan rotamer toggle switch because no change of the rotamer was observed in structures of activated receptors. The global toggle switch suggested earlier consisting of a vertical rigid motion of TM6, seems also to be implausible based on the recent crystal structures of GPCRs with agonists. Theoretical and experimental methods (crystallography, NMR, specific spectroscopic methods like FRET/BRET but also single-molecule-force-spectroscopy) are currently used to study the effect of ligands on the receptor structure, location of stable structural segments/domains of GPCRs, and to answer the still open question on how ligands are binding: either via ensemble of conformational receptor states or rather via induced fit mechanisms. On the other hand the structural investigations of homoand heterodimers and higher oligomers revealed the mechanism of allosteric signal transmission and receptor activation that could lead to design highly effective and selective allosteric or ago-allosteric drugs.

Allosteric modulators of rhodopsin-like G protein-coupled receptors: opportunities in drug development

Rhodopsin-like (class A) G protein-coupled receptors (GPCRs) are one of the most important classes of drug targets. The discovery that these GPCRs can be allosterically modulated by small drug molecules has opened up new opportunities in drug development. It will allow the drugability of "difficult targets", such as GPCRs activated by large (glyco)proteins, or by very polar or highly lipophilic physiological agonists. Receptor subtype selectivity should be more easily achievable with allosteric than with orthosteric ligands. Allosteric modulation will allow a broad spectrum of pharmacological effects largely expanding that of orthosteric ligands. Furthermore, allosteric modulators may show an improved safety profile as compared to orthosteric ligands. Only recently, the explicit search for allosteric modulators has been started for only a few rhodopsin-like GPCRs. The first negative allosteric modulators (allosteric antagonists) of chemokine receptors, maraviroc (CCR5 receptor), used in HIV therapy, and plerixafor (CXCR4 receptor) for stem cell mobilization, have been approved as drugs. The development of allosteric modulators for rhodopsin-like GPCRs as novel drugs is still at an early stage; it appears highly promising.

A biased ligand for OXE-R uncouples Gα and Gβγ signaling within a heterotrimer

Differential targeting of heterotrimeric G protein versus β-arrestin signaling are emerging concepts in G protein-coupled receptor (GPCR) research and drug discovery, and biased engagement by GPCR ligands of either β-arrestin or G protein pathways has been disclosed. Herein we report on a new mechanism of ligand bias to titrate the signaling specificity of a cell-surface GPCR. Using a combination of biomolecular and virtual screening, we identified the small-molecule modulator Gue1654, which inhibits Gβγ but not Gα signaling triggered upon activation of Gα(i)-βγ by the chemoattractant receptor OXE-R in both recombinant and human primary cells. Gue1654 does not interfere nonspecifically with signaling directly at or downstream of Gβγ. This hitherto unappreciated mechanism of ligand bias at a GPCR highlights both a new paradigm for functional selectivity and a potentially new strategy to develop pathway-specific therapeutics.

Protective Role of the ACE2/Ang-(1-9) Axis in Cardiovascular Remodeling

Despite reduction in cardiovascular (CV) events and end-organ damage with the current pharmacologic strategies, CV disease remains the primary cause of death in the world. Pharmacological therapies based on the renin angiotensin system (RAS) blockade are used extensively for the treatment of hypertension, heart failure, and CV remodeling but in spite of their success the prevalence of end-organ damage and residual risk remain still high. Novel approaches must be discovered for a more effective treatment of residual CV remodeling and risk. The ACE2/Ang-(1–9) axis is a new and important target to counterbalance the vasoconstrictive/proliferative RAS axis. Ang-(1–9) is hydrolyzed slower than Ang-(1–7) and is able to bind the Ang II type 2 receptor. We review here the current experimental evidence suggesting that activation of the ACE2/Ang-(1–9) axis protects the heart and vessels (and possibly the kidney) from adverse cardiovascular remodeling in hypertension as well as in heart failure.

Low affinity GPCRs for metabolic intermediates: challenges for pharmacologists

The discovery that a number of metabolites and metabolic intermediates can act through G protein-coupled receptors has attracted great interest in the field and has led to new therapeutic targets for diseases such as hypertension, type 2 diabetes, inflammation, and metabolic syndrome. However, the low apparent affinity of these ligands for their cognate receptors poses a number of challenges for pharmacologists interested in investigating receptor structure, function or physiology. Furthermore, the endogenous ligands matched to their receptors have other, well established metabolic roles and thus selectivity is difficult to achieve. This review discusses some of the issues researchers face when working with these receptors and highlights the ways in which a number of these obstacles have been overcome.

Mutation of Phe318 within the NPxxY(x)(5,6)F motif in melanin-concentrating hormone receptor 1 results in an efficient signaling activity

Melanin-concentrating hormone receptor 1 (MCHR1) is a G-protein-coupled receptor (GPCR) that plays an important role in feeding by coupling to Gα(q)- and Gα(i)-mediated signal transduction pathways. To interrogate the molecular basis for MCHR1 activation, we analyzed the effect of a series of site-directed mutations on rat MCHR1 function. In the highly conserved NPxxY(x)(5,6)F domain of GPCRs, the phenylalanine residue is involved in structural constraints; replacement with alanine generally leads to impaired/lost GPCR function. However, Phe-to-Ala (F318A) mutation in MCHR1 had no significant effect on the level of cell surface expression and receptor signaling. By analyzing a further series of mutants, we found that Phe-to-Lys substitution (F318K) caused the most significant reduction in the EC(50) value of MCH for calcium mobilization without affecting receptor expression at the cell surface. Interestingly, GTPγS-binding, which monitors Gα(i) activation, was not modulated by F318K. Our results, combined with computer modeling, provide new insight into the role of Phe in the NPxxY(x)(5,6)F motif as a structurally critical site for receptor dynamics and a determinant of Gα protein interaction.

Immunomodulation using agonists and antagonists: potential clinical applications

INTRODUCTION

Extensive studies have gone into understanding the differential role of the innate and adaptive arms of the immune system in the context of various diseases. Receptor-ligand interactions are responsible for mediating cross-talk between the innate and adaptive arms of the immune system, so as to effectively counter the pathogenic challenge. While TLRs remain the best studied innate immune receptor, many other receptor families are now coming to the fore for their role in various pathologies. Research has focused on the discovery of novel agonists and antagonists for these receptors as potential therapeutics.

AREAS COVERED

In this review, we present an overview of the recent advances in the discovery of drugs targeting important receptors such as G-protein coupled receptors, TRAIL-R, IL-1β receptor, PPARs, etc. All these receptors play a critical role in the modulation of the immune response. We focus on the recent paradigms applied for the generation of specific and effective therapeutics for these receptors and their status in clinical trials.

EXPERT OPINION

Non-specific activation by antagonist/agonist is a difficult problem to dodge. This demands innovation in ligand designing with the use of strategies such as allosterism and dual-specific ligands. Rigorous preclinical and clinical studies are required in transforming a compound to a therapeutic.

Developing chemical genetic approaches to explore G protein-coupled receptor function: validation of the use of a receptor activated solely by synthetic ligand (RASSL)

Molecular evolution and chemical genetics have been applied to generate functional pairings of mutated G protein-coupled receptors (GPCRs) and nonendogenous ligands. These mutant receptors, referred to as receptors activated solely by synthetic ligands (RASSLs) or designer receptors exclusively activated by designer drugs (DREADDs), have huge potential to define physiological roles of GPCRs and to validate receptors in animal models as therapeutic targets to treat human disease. However, appreciation of ligand bias and functional selectivity of different ligands at the same receptor suggests that RASSLs may signal differently than wild-type receptors activated by endogenous agonists. We assessed this by generating forms of wild-type human M(3) muscarinic receptor and a RASSL variant that responds selectively to clozapine N-oxide. Although the RASSL receptor had reduced affinity for muscarinic antagonists, including atropine, stimulation with clozapine N-oxide produced effects very similar to those generated by acetylcholine at the wild-type M(3)-receptor. Such effects included the relative movement of the third intracellular loop and C-terminal tail of intramolecular fluorescence resonance energy transfer sensors and the ability of the wild type and evolved mutant to regulate extracellular signal-regulated kinase 1/2 phosphorylation. Each form interacted similarly with β-arrestin 2 and was internalized from the cell surface in response to the appropriate ligand. Furthermore, the pattern of phosphorylation of specific serine residues within the evolved receptor in response to clozapine N-oxide was very similar to that produced by acetylcholine at the wild type. Such results provide confidence that, at least for the M(3) muscarinic receptor, results obtained after transgenic expression of this RASSL are likely to mirror the actions of acetylcholine at the wild type receptor.

GPCR agonist binding revealed by modeling and crystallography

Despite recent progress in structural coverage of the G-protein-coupled receptor (GPCR) family, high plasticity of these membrane proteins poses additional challenges for crystallographic studies of their complexes with different classes of ligands, especially agonists. The ability to predict computationally the binding of natural and clinically relevant agonists and corresponding changes in the receptor pocket, starting from inactive GPCR structures, is therefore of great interest for understanding GPCR biology and drug action. Comparison of computational models published in 2009 and 2010 with recently determined agonist-bound structures of β-adrenergic and adenosine A(2A) receptors reveals high accuracy of the predicted agonist binding poses (0.8 Å and 1.7 Å respectively) and receptor interactions. In the case of the β(2)AR, energy-based models with limited backbone flexibility have also allowed characterization of side-chain rotations and a finite backbone shift in the pocket region as determinants of full, partial or inverse agonism. Development of accurate models of agonist binding for other GPCRs will be instrumental for functional and pharmacological studies, complementing biochemical and crystallographic techniques.

Diversity and modularity of G protein-coupled receptor structures

G protein-coupled receptors (GPCRs) comprise the most 'prolific' family of cell membrane proteins, which share a general mechanism of signal transduction, but greatly vary in ligand recognition and function. Crystal structures are now available for rhodopsin, adrenergic, and adenosine receptors in both inactive and activated forms, as well as for chemokine, dopamine, and histamine receptors in inactive conformations. Here we review common structural features, outline the scope of structural diversity of GPCRs at different levels of homology, and briefly discuss the impact of the structures on drug discovery. Given the current set of GPCR crystal structures, a distinct modularity is now being observed between the extracellular (ligand-binding) and intracellular (signaling) regions. The rapidly expanding repertoire of GPCR structures provides a solid framework for experimental and molecular modeling studies, and helps to chart a roadmap for comprehensive structural coverage of the whole superfamily and an understanding of GPCR biological and therapeutic mechanisms.

Medicinal chemistry for 2020

Rapid advances in our collective understanding of biomolecular structure and, in concert, of biochemical systems, coupled with developments in computational methods, have massively impacted the field of medicinal chemistry over the past two decades, with even greater changes appearing on the horizon. In this perspective, we endeavor to profile some of the most prominent determinants of change and speculate as to further evolution that may consequently occur during the next decade. The five main angles to be addressed are: protein-protein interactions; peptides and peptidomimetics; molecular diversity and pharmacological space; molecular pharmacodynamics (significance, potential and challenges); and early-stage clinical efficacy and safety. We then consider, in light of these, the future of medicinal chemistry and the educational preparation that will be required for future medicinal chemists.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

Extracellular loop 2 of the free fatty acid receptor 2 mediates allosterism of a phenylacetamide ago-allosteric modulator

Allosteric agonists are powerful tools for exploring the pharmacology of closely related G protein-coupled receptors that have nonselective endogenous ligands, such as the short chain fatty acids at free fatty acid receptors 2 and 3 (FFA2/GPR43 and FFA3/GPR41, respectively). We explored the molecular mechanisms mediating the activity of 4-chloro-α-(1-methylethyl)-N-2-thiazolylbenzeneacetamide (4-CMTB), a recently described phenylacetamide allosteric agonist and allosteric modulator of endogenous ligand function at human FFA2, by combining our previous knowledge of the orthosteric binding site with targeted examination of 4-CMTB structure-activity relationships and mutagenesis and chimeric receptor generation. Here we show that 4-CMTB is a selective agonist for FFA2 that binds to a site distinct from the orthosteric site of the receptor. Ligand structure-activity relationship studies indicated that the N-thiazolyl amide is likely to provide hydrogen bond donor/acceptor interactions with the receptor. Substitution at Leu(173) or the exchange of the entire extracellular loop 2 of FFA2 with that of FFA3 was sufficient to reduce or ablate, respectively, allosteric communication between the endogenous and allosteric agonists. Thus, we conclude that extracellular loop 2 of human FFA2 is required for transduction of cooperative signaling between the orthosteric and an as-yet-undefined allosteric binding site of the FFA2 receptor that is occupied by 4-CMTB.