Fluorescent agonists and antagonists for vasopressin/oxytocin G protein-coupled receptors: usefulness in ligand screening assays and receptor studies

Targeting the Oxytocin Receptor for Breast Cancer Management: A Niche for Peptide Tracers

Breast cancer is a leading cause of death in women, and its management highly depends on early disease diagnosis and monitoring. This remains challenging due to breast cancer's heterogeneity and a scarcity of specific biomarkers that could predict responses to therapy and enable personalized treatment. This Perspective describes the diagnostic landscape for breast cancer management, molecular strategies targeting receptors overexpressed in tumors, the theranostic potential of the oxytocin receptor (OTR) as an emerging breast cancer target, and the development of OTR-specific optical and nuclear tracers to study, visualize, and treat tumors. A special focus is on the chemistry and pharmacology underpinning OTR tracer development, preclinical in vitro and in vivo studies, challenges, and future directions. The use of peptide-based tracers targeting upregulated receptors in cancer is a highly promising strategy complementing current diagnostics and therapies and providing new opportunities to improve cancer management and patient survival.

Bringing GPCR Structural Biology to Medical Applications: Insights from Both V2 Vasopressin and Mu-Opioid Receptors

G-protein coupled receptors (GPCRs) are versatile signaling proteins that regulate key physiological processes in response to a wide variety of extracellular stimuli. The last decade has seen a revolution in the structural biology of clinically important GPCRs. Indeed, the improvement in molecular and biochemical methods to study GPCRs and their transducer complexes, together with advances in cryo-electron microscopy, NMR development, and progress in molecular dynamic simulations, have led to a better understanding of their regulation by ligands of different efficacy and bias. This has also renewed a great interest in GPCR drug discovery, such as finding biased ligands that can either promote or not promote specific regulations. In this review, we focus on two therapeutically relevant GPCR targets, the V2 vasopressin receptor (V2R) and the mu-opioid receptor (µOR), to shed light on the recent structural biology studies and show the impact of this integrative approach on the determination of new potential clinical effective compounds.

Design and synthesis of first environment-sensitive coumarin fluorescent agonists for MrgX2

Mas related G-protein-coupled receptor member X2 (MrgX2) has been identified as the crucial receptor in drug induced pseudo-allergic reactions and allergic diseases. In this research, the first type of fluorescent agonists (ZX1, ZX2 and ZX3) for MrgX2 were developed by conjugating environment-sensitive fluorophore coumarin to MrgX2 selective agonists (R)-ZINC-3573. Their environment-sensitive property was confirmed by the dramatically increase of fluorescent intensity after binding to the hydrophobic ligand binding domain MrgX2, which help to overcome the high background signal. Based on these characteristics, they can be used for selective visualization of MrgX2 in living cells even with their own background interference. Among these fluorescent agonists, compound ZX2 possessed splendid spectroscopic properties, outstanding pharmacological activities (EC₅₀ = 0.93 μM, K_D = 1.97 μM). And a competitive binding assay was established with ZX2 to analysis the binding affinity of MrgX2 agonists, which shown high coherence with the results of cell membrane chromatography. To our knowledge, these probes are the first fluorescent ligands of MrgX2 with agonistic activity and environment-sensitive property, which is expected to use for the development of MrgX2 molecular pharmacology and serve as a convenient high-throughput screening tool for the drug candidates targeting MrgX2.

Measuring Affinity of Ligands to the Oxytocin Receptor Using Radioligand Binding

Two aims of oxytocin receptor (OTR)-targeted drug discovery are development of selective OTR-binding PET tracers and development of brain-permeable selective OTR agonists. By allowing measurement of central OTR binding site occupancy after administration of intranasal oxytocin, OTR PET tracers inform an understanding of the conflicting effects on pro-social behaviors seen with administration of intranasal oxytocin in human studies. By mitigating pharmacokinetic and pharmacodynamic limitations of intranasal oxytocin, development of brain-permeable selective OTR agonists may produce therapies for mental disorders that involve asocial symptoms. A key step in development of new OTR-targeting PET radioligands and small molecule agonists is measurement of OTR affinity. One technique that can quantitate the affinity of candidate ligands for the OTR is competition radioligand binding. This chapter describes the materials, methods, and considerations of experimental design required to conduct the steps of competition radioligand binding for OTR drug discovery.

Directed evolution for high functional production and stability of a challenging G protein-coupled receptor

Membrane proteins such as G protein-coupled receptors (GPCRs) carry out many fundamental biological functions, are involved in a large number of physiological responses, and are thus important drug targets. To allow detailed biophysical and structural studies, most of these important receptors have to be engineered to overcome their poor intrinsic stability and low expression levels. However, those GPCRs with especially poor properties cannot be successfully optimised even with the current technologies. Here, we present an engineering strategy, based on the combination of three previously developed directed evolution methods, to improve the properties of particularly challenging GPCRs. Application of this novel combination approach enabled the successful selection for improved and crystallisable variants of the human oxytocin receptor, a GPCR with particularly low intrinsic production levels. To analyse the selection results and, in particular, compare the mutations enriched in different hosts, we developed a Next-Generation Sequencing (NGS) strategy that combines long reads, covering the whole receptor, with exceptionally low error rates. This study thus gave insight into the evolution pressure on the same membrane protein in prokaryotes and eukaryotes. Our long-read NGS strategy provides a general methodology for the highly accurate analysis of libraries of point mutants during directed evolution.

New small molecule fluorescent probes for G protein-coupled receptors: valuable tools for drug discovery

G protein-coupled receptors (GPCRs) are essential signaling proteins and tractable therapeutic targets. To develop new drug candidates, GPCR drug discovery programs require versatile, sensitive pharmacological tools for ligand binding and compound screening. With the availability of new imaging modalities and proximity-based ligand binding technologies, fluorescent ligands offer many advantages and are increasingly being used, yet labeling small molecules remains considerably more challenging relative to peptides. Focusing on recent fluorescent small molecule studies for family A GPCRs, this review addresses some of the key challenges, synthesis approaches and structure-activity relationship considerations, and discusses advantages of using high-resolution GPCR structures to inform conjugation strategies. While no single approach guarantees successful labeling without loss of affinity or selectivity, the choice of fluorophore, linker type and site of attachment have proved to be critical factors that can significantly affect their utility in drug discovery programs, and as discussed, can sometimes lead to very unexpected results.

Crystal structure of the human oxytocin receptor

The peptide hormone oxytocin modulates socioemotional behavior and sexual reproduction via the centrally expressed oxytocin receptor (OTR) across several species. Here, we report the crystal structure of human OTR in complex with retosiban, a nonpeptidic antagonist developed as an oral drug for the prevention of preterm labor. Our structure reveals insights into the detailed interactions between the G protein-coupled receptor (GPCR) and an OTR-selective antagonist. The observation of an extrahelical cholesterol molecule, binding in an unexpected location between helices IV and V, provides a structural rationale for its allosteric effect and critical influence on OTR function. Furthermore, our structure in combination with experimental data allows the identification of a conserved neurohypophyseal receptor-specific coordination site for Mg²⁺ that acts as potent, positive allosteric modulator for agonist binding. Together, these results further our molecular understanding of the oxytocin/vasopressin receptor family and will facilitate structure-guided development of new therapeutics.

Oxytocin and Rodent Models of Addiction

Interest for the use of oxytocin as a treatment for addiction began over 40years ago. Better known for its roles in parturition, lactation and pair bonding, oxytocin also has anxiolytic properties, reduces immune and inflammatory responses, and has a role in learning and memory. In this chapter, oxytocin effects on addiction processes are described by highlighting research findings that have used oxytocin within current preclinical animal models of addiction, relapse, or craving. First, we provide a brief background of the endogenous oxytocin system followed by descriptions of the behavioral models used to study addiction, including models of drug taking and seeking. Then we review recent preclinical studies that have used oxytocin as a therapeutic intervention throughout multiple stages of the addiction cycle from a behavioral and neurobiological perspective. These models encompass the entire range of the addiction cycle including acquisition and maintenance of drug taking, withdrawal and craving during periods of drug abstinence, and ultimately relapse. We then posit several theories about how oxytocin interacts with both drug and social reward, as well as presenting a mechanistic account of how specific oxytocin receptor localization may contribute to oxytocin's efficacy as an addiction therapeutic.

Oxytocin receptors are expressed on dopamine and glutamate neurons in the mouse ventral tegmental area that project to nucleus accumbens and other mesolimbic targets

The mesolimbic dopamine (DA) circuitry determines which behaviors are positively reinforcing and therefore should be encoded in the memory to become a part of the behavioral repertoire. Natural reinforcers, like food and sex, activate this pathway, thereby increasing the likelihood of further consummatory, social, and sexual behaviors. Oxytocin (OT) has been implicated in mediating natural reward and OT-synthesizing neurons project to the ventral tegmental area (VTA) and nucleus accumbens (NAc); however, direct neuroanatomical evidence of OT regulation of DA neurons within the VTA is sparse. To phenotype OT-receptor (OTR) expressing neurons originating within the VTA, we delivered Cre-inducible adeno-associated virus that drives the expression of fluorescent marker into the VTA of male mice that had Cre-recombinase driven by OTR gene expression. OTR-expressing VTA neurons project to NAc, prefrontal cortex, the extended amygdala, and other forebrain regions but less than 10% of these OTR-expressing neurons were identified as DA neurons (defined by tyrosine hydroxylase colocalization). Instead, almost 50% of OTR-expressing cells in the VTA were glutamate (GLU) neurons, as indicated by expression of mRNA for the vesicular GLU transporter (vGluT). About one-third of OTR-expressing VTA neurons did not colocalize with either DA or GLU phenotypic markers. Thus, OTR expression by VTA neurons implicates that OT regulation of reward circuitry is more complex than a direct action on DA neurotransmission. J. Comp. Neurol. 525:1094-1108, 2017. © 2016 Wiley Periodicals, Inc.

Fluorescent approaches for understanding interactions of ligands with G protein coupled receptors

G protein coupled receptors are responsible for a wide variety of signaling responses in diverse cell types. Despite major advances in the determination of structures of this class of receptors, the underlying mechanisms by which binding of different types of ligands specifically elicits particular signaling responses remain unclear. The use of fluorescence spectroscopy can provide important information about the process of ligand binding and ligand dependent conformational changes in receptors, especially kinetic aspects of these processes that can be difficult to extract from X-ray structures. We present an overview of the extensive array of fluorescent ligands that have been used in studies of G protein coupled receptors and describe spectroscopic approaches for assaying binding and probing the environment of receptor-bound ligands with particular attention to examples involving yeast pheromone receptors. In addition, we discuss the use of fluorescence spectroscopy for detecting and characterizing conformational changes in receptors induced by the binding of ligands. Such studies have provided strong evidence for diversity of receptor conformations elicited by different ligands, consistent with the idea that GPCRs are not simple on and off switches. This diversity of states constitutes an underlying mechanistic basis for biased agonism, the observation that different stimuli can produce different responses from a single receptor. It is likely that continued technical advances will allow fluorescence spectroscopy to play an important role in continued probing of structural transitions in G protein coupled receptors. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Fluorescent ligands to investigate GPCR binding properties and oligomerization

Fluorescent ligands for GPCRs (G-protein-coupled receptors) have been synthesized for a long time but their use was usually restricted to receptor localization in the cell by fluorescent imaging microscopy. During the last two decades, the emergence of new fluorescence-based strategies and the concomitant development of fluorescent measurement apparatus have dramatically widened the use of fluorescent ligands. Among the various strategies, TR (time-resolved)-FRET (fluorescence resonance energy transfer) approaches exhibit an interesting potential to study GPCR interactions with various partners. We have derived various sets of ligands that target different GPCRs with fluorophores, which are compatible with TR-FRET strategies. Fluorescent ligands labelled either with a fluorescent donor (such as europium or terbium cryptate) or with a fluorescent acceptor (such as fluorescein, dy647 or Alexa Fluor® 647), for example, kept high affinities for their cognate receptors. These ligands turn out to be interesting tools to develop FRET-based binding assays. We also used these fluorescent ligands to analyse GPCR oligomerization by measuring FRET between ligands bound to receptor dimers. In contrast with FRET strategies, on the basis of receptor labelling, the ligand-based approach we developed is fully compatible with the study of wild-type receptors and therefore with receptors expressed in native tissues. Therefore, by using fluorescent analogues of oxytocin, we demonstrated the existence of oxytocin receptor dimers in the mammary gland of lactating rats.

Oxytocin and vasopressin agonists and antagonists as research tools and potential therapeutics

We recently reviewed the status of peptide and nonpeptide agonists and antagonists for the V(1a), V(1b) and V(2) receptors for arginine vasopressin (AVP) and the oxytocin receptor for oxytocin (OT). In the present review, we update the status of peptides and nonpeptides as: (i) research tools and (ii) therapeutic agents. We also present our recent findings on the design of fluorescent ligands for V(1b) receptor localisation and for OT receptor dimerisation. We note the exciting discoveries regarding two novel naturally occurring analogues of OT. Recent reports of a selective VP V(1a) agonist and a selective OT agonist point to the continued therapeutic potential of peptides in this field. To date, only two nonpeptides, the V(2) /V(1a) antagonist, conivaptan and the V(2) antagonist tolvaptan have received Food and Drug Administration approval for clinical use. The development of nonpeptide AVP V(1a), V(1b) and V(2) antagonists and OT agonists and antagonists has recently been abandoned by Merck, Sanofi and Pfizer. A promising OT antagonist, Retosiban, developed at Glaxo SmithKline is currently in a Phase II clinical trial for the prevention of premature labour. A number of the nonpeptide ligands that were not successful in clinical trials are proving to be valuable as research tools. Peptide agonists and antagonists continue to be very widely used as research tools in this field. In this regard, we present receptor data on some of the most widely used peptide and nonpeptide ligands, as a guide for their use, especially with regard to receptor selectivity and species differences.

Visualizing microtubule-dependent vasopressin type 2 receptor trafficking using a new high-affinity fluorescent vasopressin ligand

The vasopressin receptor type 2 (V2R) is the major target of vasopressin (VP) in renal epithelial cells. Although it is known that VP induces V2R internalization, accumulation in the perinuclear area, and degradation, the V2R intracellular trafficking pathways remain elusive. We visualized this process by developing a new fluorescent VP analog tagged by tetramethylrhodamine (TMR)-[Lys-(PEG)(2)-Suc-TMR(8)]VP or (VP(TMR)). This ligand is fully functional as revealed by its high binding affinity toward V2R [(K(d)) =157 ± 52 nM] and ability to increase intracellular cAMP 32-fold. VP(TMR) induced V2R internalization in LLC-PK1 cells expressing either a FLAG-tagged receptor (FLAG-V2R) or V2R C-terminally tagged with green fluorescent protein (GFP) (V2R-GFP). After internalization, VP(TMR) and V2R-GFP colocalized in the perinuclear area, suggesting that the hormone and receptor traffic along the same pathway. VP(TMR) and V2R colocalized initially with the early endosome markers EEA1 and Rab5, and later with the recycling and late endosome markers Rab11 and Rab25. Epifluorescence microscopy of LLC-PK1 cells expressing GFP-tagged microtubules (MT) showed that VP(TMR)-containing vesicles travel along the MT network, and even remain attached to MT during the metaphase and anaphase of mitosis. Colchicine, a MT-depolymerizing agent, abolished perinuclear accumulation of VP(TMR), and Western blot analysis showed that VP-induced V2R-GFP degradation is markedly retarded, but not abolished, by colchicine (10 μM). We conclude that the new VP(TMR) ligand is suitable for dissecting V2R and VP internalization and trafficking in cells, and that V2R trafficking and down-regulation is an MT-dependent mechanism.

GPCR oligomers in pharmacology and signaling

G protein-coupled receptors (GPCRs) represent one of the largest families of cell surface receptors, and are the target of more than half of the current therapeutic drugs on the market. When activated by an agonist, the GPCR undergoes conformational changes that facilitate its interaction with heterotrimeric G proteins, which then relay signals to downstream intracellular effectors. Although GPCRs were thought to function as monomers, many studies support the hypothesis that G protein coupling involves the formation of GPCR homo- and/or hetero-complexes. These complex systems have been suggested to exhibit specific signaling cascades, pharmacological, internalization, and recycling properties. In this review, we summarize recent advances in our understanding of the structure, function and dynamics of GPCR complexes, as well as the findings obtained in animal models.

Design, synthesis, and pharmacological characterization of fluorescent peptides for imaging human V1b vasopressin or oxytocin receptors

Among the four known vasopressin and oxytocin receptors, the specific localization of the V1b isoform is poorly described because of the lack of selective pharmacological tools. In an attempt to address this need, we decided to design, synthesize, and characterize fluorescent selective V1b analogues. Starting with the selective V1b agonist [deamino-Cys(1),Leu(4),Lys(8)]vasopressin (d[Leu(4),Lys(8)]VP) synthesized earlier, we added blue, green, or red fluorophores to the lysine residue at position 8 either directly or by the use of linkers of different lengths. Among the nine analogues synthesized, two exhibited very promising properties. These are d[Leu(4),Lys(Alexa 647)(8)]VP (3) and d[Leu(4),Lys(11-aminoundecanoyl-Alexa 647)(8)]VP (9). They remained full V1b agonists with nanomolar affinity and specifically decorated the plasma membrane of CHO cells stably transfected with the human V1b receptor. These new selective fluorescent peptides will allow the cellular localization of V1b or OT receptor isoforms in native tissues.

Time-resolved FRET between GPCR ligands reveals oligomers in native tissues

G protein-coupled receptor (GPCR) oligomers have been proposed to play critical roles in cell signaling, but confirmation of their existence in a native context remains elusive, as no direct interactions between receptors have been reported. To demonstrate their presence in native tissues, we developed a time-resolved FRET strategy that is based on receptor labeling with selective fluorescent ligands. Specific FRET signals were observed with four different receptors expressed in cell lines, consistent with their dimeric or oligomeric nature in these transfected cells. More notably, the comparison between FRET signals measured with sets of fluorescent agonists and antagonists was consistent with an asymmetric relationship of the two protomers in an activated GPCR dimer. Finally, we applied the strategy to native tissues and succeeded in demonstrating the presence of oxytocin receptor dimers and/or oligomers in mammary gland.