Homogeneous time-resolved fluorescence-based assay to screen for ligands targeting the growth hormone secretagogue receptor type 1a

Structural insights into ligand recognition, selectivity, and activation of bombesin receptor subtype-3

Bombesin receptor subtype-3 (BRS3) is an important orphan G protein-coupled receptor that regulates energy homeostasis and insulin secretion. As a member of the bombesin receptor (BnR) family, the lack of known endogenous ligands and high-resolution structure has hindered the understanding of BRS3 signaling and function. We present two cryogenic electron microscopy (cryo-EM) structures of BRS3 in complex with the heterotrimeric Gq protein in its active states: one bound to the pan-BnR agonist BA1 and the other bound to the synthetic BRS3-specific agonist MK-5046. These structures reveal the architecture of the orthosteric ligand pocket underpinning molecular recognition and provide insights into the structural basis for BRS3's selectivity and low affinity for bombesin peptides. Examination of conserved micro-switches suggests a shared activation mechanism among BnRs. Our findings shed light on BRS3's ligand selectivity and signaling mechanisms, paving the way for exploring its therapeutic potential for diabetes, obesity, and related metabolic disorders.

GHSR in a Subset of GABA Neurons Controls Food Deprivation-Induced Hyperphagia in Male Mice

The growth hormone secretagogue receptor (GHSR), primarily known as the receptor for the hunger hormone ghrelin, potently controls food intake, yet the specific Ghsr-expressing cells mediating the orexigenic effects of this receptor remain incompletely characterized. Since Ghsr is expressed in gamma-aminobutyric acid (GABA)-producing neurons, we sought to investigate whether the selective expression of Ghsr in a subset of GABA neurons is sufficient to mediate GHSR's effects on feeding. First, we crossed mice that express a tamoxifen-dependent Cre recombinase in the subset of GABA neurons that express glutamic acid decarboxylase 2 (Gad2) enzyme (Gad2-CreER mice) with reporter mice, and found that ghrelin mainly targets a subset of Gad2-expressing neurons located in the hypothalamic arcuate nucleus (ARH) and that is predominantly segregated from Agouti-related protein (AgRP)-expressing neurons. Analysis of various single-cell RNA-sequencing datasets further corroborated that the primary subset of cells coexpressing Gad2 and Ghsr in the mouse brain are non-AgRP ARH neurons. Next, we crossed Gad2-CreER mice with reactivable GHSR-deficient mice to generate mice expressing Ghsr only in Gad2-expressing neurons (Gad2-GHSR mice). We found that ghrelin treatment induced the expression of the marker of transcriptional activation c-Fos in the ARH of Gad2-GHSR mice, yet failed to induce food intake. In contrast, food deprivation-induced refeeding was higher in Gad2-GHSR mice than in GHSR-deficient mice and similar to wild-type mice, suggesting that ghrelin-independent roles of GHSR in a subset of GABA neurons is sufficient for eliciting full compensatory hyperphagia in mice.

Illuminating Neuropeptide Y Y4 Receptor Binding: Fluorescent Cyclic Peptides with Subnanomolar Binding Affinity as Novel Molecular Tools

The neuropeptide Y (NPY) Y4 receptor (Y4R), a member of the family of NPY receptors, is physiologically activated by the linear 36-amino acid peptide pancreatic polypeptide (PP). The Y4R is involved in the regulation of various biological processes, most importantly pancreatic secretion, gastrointestinal motility, and regulation of food intake. So far, Y4R binding affinities have been mostly studied in radiochemical binding assays. Except for a few fluorescently labeled PP derivatives, fluorescence-tagged Y4R ligands with high affinity have not been reported. Here, we introduce differently fluorescence-labeled (Sulfo-Cy5, Cy3B, Py-1, Py-5) Y4R ligands derived from recently reported cyclic hexapeptides showing picomolar Y4R binding affinity. With pKi values of 9.22-9.71 (radioligand competition binding assay), all fluorescent ligands (16-19) showed excellent Y4R affinity. Y4R saturation binding, binding kinetics, and competition binding with reference ligands were studied using different fluorescence-based methods: flow cytometry (Sulfo-Cy5, Cy3B, and Py-1 label), fluorescence anisotropy (Cy3B label), and NanoBRET (Cy3B label) binding assays. These experiments confirmed the high binding affinity to Y4R (equilibrium pKd: 9.02-9.9) and proved the applicability of the probes for fluorescence-based Y4R competition binding studies and imaging techniques such as single-receptor molecule tracking.

Sodium is a negative allosteric regulator of the ghrelin receptor

The functional properties of G protein-coupled receptors (GPCRs) are intimately associated with the different components in their cellular environment. Among them, sodium ions have been proposed to play a substantial role as endogenous allosteric modulators of GPCR-mediated signaling. However, this sodium effect and the underlying mechanisms are still unclear for most GPCRs. Here, we identified sodium as a negative allosteric modulator of the ghrelin receptor GHSR (growth hormone secretagogue receptor). Combining 23Na-nuclear magnetic resonance (NMR), molecular dynamics, and mutagenesis, we provide evidence that, in GHSR, sodium binds to the allosteric site conserved in class A GPCRs. We further leveraged spectroscopic and functional assays to show that sodium binding shifts the conformational equilibrium toward the GHSR-inactive ensemble, thereby decreasing basal and agonist-induced receptor-catalyzed G protein activation. All together, these data point to sodium as an allosteric modulator of GHSR, making this ion an integral component of the ghrelin signaling machinery.

Median eminence blood flow influences food intake by regulating ghrelin access to the metabolic brain

Central integration of peripheral appetite-regulating signals ensures maintenance of energy homeostasis. Thus, plasticity of circulating molecule access to neuronal circuits involved in feeding behavior plays a key role in the adaptive response to metabolic changes. However, the mechanisms involved remain poorly understood despite their relevance for therapeutic development. Here, we investigated the role of median eminence mural cells, including smooth muscle cells and pericytes, in modulating gut hormone effects on orexigenic/anorexigenic circuits. We found that conditional activation of median eminence vascular cells impinged on local blood flow velocity and altered ghrelin-stimulated food intake by delaying ghrelin access to target neurons. Thus, activation of median eminence vascular cells modulates food intake in response to peripheral ghrelin by reducing local blood flow velocity and access to the metabolic brain.

Molecular basis for allosteric agonism and G protein subtype selectivity of galanin receptors

Peptide hormones and neuropeptides are complex signaling molecules that predominately function through G protein-coupled receptors (GPCRs). Two unanswered questions remaining in the field of peptide-GPCR signaling systems pertain to the basis for the diverse binding modes of peptide ligands and the specificity of G protein coupling. Here, we report the structures of a neuropeptide, galanin, bound to its receptors, GAL1R and GAL2R, in complex with their primary G protein subtypes G_i and G_q, respectively. The structures reveal a unique binding pose of galanin, which almost ‘lays flat’ on the top of the receptor transmembrane domain pocket in an α-helical conformation, and acts as an ‘allosteric-like’ agonist via a distinct signal transduction cascade. The structures also uncover the important features of intracellular loop 2 (ICL2) that mediate specific interactions with G_q, thus determining the selective coupling of G_q to GAL2R. ICL2 replacement in G_i-coupled GAL1R, μOR, 5-HT_1AR, and G_s-coupled β₂AR and D1R with that of GAL2R promotes G_q coupling of these receptors, highlighting the dominant roles of ICL2 in G_q selectivity. Together our results provide insights into peptide ligand recognition and allosteric activation of galanin receptors and uncover a general structural element for G_q coupling selectivity.

The basis for the diverse peptide-binding modes and the G protein selectivity of peptide GPCRs remains elusive. Here, the authors offer a structural basis for allosteric-like agonism and G protein selectivity of a neuropeptide GPCR, galanin receptor.

Concerted conformational dynamics and water movements in the ghrelin G protein-coupled receptor

There is increasing support for water molecules playing a role in signal propagation through G protein-coupled receptors (GPCRs). However, exploration of the hydration features of GPCRs is still in its infancy. Here, we combined site-specific labeling with unnatural amino acids to molecular dynamics to delineate how local hydration of the ghrelin receptor growth hormone secretagogue receptor (GHSR) is rearranged upon activation. We found that GHSR is characterized by a specific hydration pattern that is selectively remodeled by pharmacologically distinct ligands and by the lipid environment. This process is directly related to the concerted movements of the transmembrane domains of the receptor. These results demonstrate that the conformational dynamics of GHSR are tightly coupled to the movements of internal water molecules, further enhancing our understanding of the molecular bases of GPCR-mediated signaling.

Molecular recognition of an acyl-peptide hormone and activation of ghrelin receptor

Ghrelin, also called “the hunger hormone”, is a gastric peptide hormone that regulates food intake, body weight, as well as taste sensation, reward, cognition, learning and memory. One unique feature of ghrelin is its acylation, primarily with an octanoic acid, which is essential for its binding and activation of the ghrelin receptor, a G protein-coupled receptor. The multifaceted roles of ghrelin make ghrelin receptor a highly attractive drug target for growth retardation, obesity, and metabolic disorders. Here we present two cryo-electron microscopy structures of G_q-coupled ghrelin receptor bound to ghrelin and a synthetic agonist, GHRP-6. Analysis of these two structures reveals a unique binding pocket for the octanoyl group, which guides the correct positioning of the peptide to initiate the receptor activation. Together with mutational and functional data, our structures define the rules for recognition of the acylated peptide hormone and activation of ghrelin receptor, and provide structural templates to facilitate drug design targeting ghrelin receptor.

Ghrelin is a gastric peptide hormone and its acylation is required for binding to and activation of the ghrelin receptor in the brain, which initiates appetite. Here, the authors present cryo-EM structures of the G_q-coupled ghrelin receptor bound to ghrelin and the synthetic agonist GHRP-6 and they describe how the acylated peptide hormone is recognised by the receptor, which is of interest for drug design.

SNAP-tag-enabled super-resolution imaging reveals constitutive and agonist-dependent trafficking of GPR56 in pancreatic β-cells

Objective

Members of the adhesion G protein-coupled receptor (aGPCR) subfamily are important actors in metabolic processes, with GPR56 (ADGRG1) emerging as a possible target for type 2 diabetes therapy. GPR56 can be activated by collagen III, its endogenous ligand, and by a synthetic seven amino-acid peptide (TYFAVLM; P7) contained within the GPR56 Stachel sequence. However, the mechanisms regulating GPR56 trafficking dynamics and agonist activities are not yet clear.

Methods

Here, we introduced SNAPf-tag into the N-terminal segment of GPR56 to monitor GPR56 cellular activity in situ. Confocal and super-resolution microscopy were used to investigate the trafficking pattern of GPR56 in native MIN6 β-cells and in MIN6 β-cells where GPR56 had been deleted by CRISPR-Cas9 gene editing. Insulin secretion, changes in intracellular calcium, and β-cell apoptosis were determined by radioimmunoassay, single-cell calcium microfluorimetry, and measuring caspase 3/7 activities, respectively, in MIN6 β-cells and human islets.

Results

SNAP-tag labelling indicated that GPR56 predominantly underwent constitutive internalisation in the absence of an exogenous agonist, unlike GLP-1R. Collagen III further stimulated GPR56 internalisation, whereas P7 was without significant effect. The overexpression of GPR56 in MIN6 β-cells did not affect insulin secretion. However, it was associated with reduced β-cell apoptosis, while the deletion of GPR56 made MIN6 β-cells more susceptible to cytokine-induced apoptosis. P7 induced a rapid increase in the intracellular calcium in MIN6 β-cells (in a GPR56-dependent manner) and human islets, and it also caused a sustained and reversible increase in insulin secretion from human islets. Collagen III protected human islets from cytokine-induced apoptosis, while P7 was without significant effect.

Conclusions

These data indicate that GPR56 exhibits both agonist-dependent and -independent trafficking in β-cells and suggest that while GPR56 undergoes constitutive signalling, it can also respond to its ligands when required. We have also identified that constitutive and agonist-dependent GPR56 activation is coupled to protect β-cells against apoptosis, offering a potential therapeutic target to maintain β-cell mass in type 2 diabetes.

•

GPR56 predominantly underwent constitutive internalisation in β-cells in the absence of exogenous agonist.

•

The GPR56 agonists, collagen III and P7, showed differential effects on GPR56 trafficking and islet functions.

•

Constitutive and agonist-dependent GPR56 activation is coupled to protection of β-cells against apoptosis.

Allosteric modulation of ghrelin receptor signaling by lipids

The membrane is an integral component of the G protein-coupled receptor signaling machinery. Here we demonstrate that lipids regulate the signaling efficacy and selectivity of the ghrelin receptor GHSR through specific interactions and bulk effects. We find that PIP2 shifts the conformational equilibrium of GHSR away from its inactive state, favoring basal and agonist-induced G protein activation. This occurs because of a preferential binding of PIP2 to specific intracellular sites in the receptor active state. Another lipid, GM3, also binds GHSR and favors G protein activation, but mostly in a ghrelin-dependent manner. Finally, we find that not only selective interactions but also the thickness of the bilayer reshapes the conformational repertoire of GHSR, with direct consequences on G protein selectivity. Taken together, this data illuminates the multifaceted role of the membrane components as allosteric modulators of how ghrelin signal could be propagated.

A Decade's Progress in the Development of Molecular Imaging Agents Targeting the Growth Hormone Secretagogue Receptor

The growth hormone secretagogue receptor 1a (GHSR), also called the ghrelin receptor, is a G protein-coupled receptor known to play an important metabolic role in the regulation of various physiological processes, including energy expenditure, growth hormone secretion, and cell proliferation. This receptor has been implicated in numerous health issues including obesity, gastrointestinal disorders, type II diabetes, and regulation of body weight in patients with Prader-Willi syndrome, and there has been growing interest in studying its mechanism of behavior to unlock further applications of GHSR-targeted therapeutics. In addition, the GHSR is expressed in various types of cancer including prostate, breast, and testicular cancers, while aberrant expression has been reported in cardiac disease. Targeted molecular imaging of the GHSR could provide insights into its role in biological processes related to these disease states. Over the past decade, imaging probes targeting this receptor have been discovered for the imaging modalities PET, SPECT, and optical imaging. High-affinity analogues of ghrelin, the endogenous ligand for the GHSR, as well as small molecule inhibitors have been developed and evaluated both in vitro and in pre-clinical models. This review provides a comprehensive overview of the molecular imaging agents targeting the GHSR reported to the end of 2019.

Development of Nonpeptidic Inverse Agonists of the Ghrelin Receptor (GHSR) Based on the 1,2,4-Triazole Scaffold

GHSR controls, among others, growth hormone and insulin secretion, adiposity, feeding, and glucose metabolism. Therefore, an inverse agonist ligand capable of selectively targeting GHSR and reducing its high constitutive activity appears to be a good candidate for the treatment of obesity-related metabolic diseases. In this context, we present a study that led to the development of several highly potent and selective inverse agonists of GHSR based on the 1,2,4-triazole scaffold. We demonstrate that, depending on the nature of the substituents on positions 3, 4, and 5, this scaffold leads to ligands that exert an intrinsic inverse agonist activity on GHSR-catalyzed G protein activation through the stabilization of a specific inactive receptor conformation. Thanks to an in vivo evaluation, we also show that one of the most promising ligands not only exerts an effect on insulin secretion in rat pancreatic islets but also affects the orexigenic effects of ghrelin in mice.

Development of High-Specificity Fluorescent Probes to Enable Cannabinoid Type 2 Receptor Studies in Living Cells

Pharmacological modulation of cannabinoid type 2 receptor (CB₂R) holds promise for the treatment of numerous conditions, including inflammatory diseases, autoimmune disorders, pain, and cancer. Despite the significance of this receptor, researchers lack reliable tools to address questions concerning the expression and complex mechanism of CB₂R signaling, especially in cell-type and tissue-dependent contexts. Herein, we report for the first time a versatile ligand platform for the modular design of a collection of highly specific CB₂R fluorescent probes, used successfully across applications, species, and cell types. These include flow cytometry of endogenously expressing cells, real-time confocal microscopy of mouse splenocytes and human macrophages, as well as FRET-based kinetic and equilibrium binding assays. High CB₂R specificity was demonstrated by competition experiments in living cells expressing CB₂R at native levels. The probes were effectively applied to FACS analysis of microglial cells derived from a mouse model relevant to Alzheimer's disease.

Biotinylated non-ionic amphipols for GPCR ligands screening

We present herein the synthesis of biotin-functionalized polymers (BNAPols) that have been developed for the fixation of membrane proteins (MPs) onto surfaces. BNAPols were synthesized by free-radical polymerization of a tris(hydroxymethyl)acrylamidomethane (THAM)-derived amphiphilic monomer in the presence of a thiol-based transfer agent with an azido group. Then a Huisgen-cycloaddition reaction was performed with Biotin-(PEG)₈-alkyne that resulted in formation of the biotinylated polymers. The designed structure of BNAPols was confirmed by NMR spectroscopy, and a HABA/avidin assay was used for estimating the percentage of biotin grafted on the polymer end chain. The colloidal characterization of these biotin-functionalized polymers was done using both dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) techniques. BNAPols were used to stabilize a model G protein-coupled receptor (GPCR), the human Growth Hormone Secretagogue Receptor (GHSR), out of its membrane environment. Subsequent immobilization of the BNAPols:GHSR complex onto a streptavidin-coated surface allowed screening of ligands based on their ability to bind the immobilized receptor. This opens the way to the use of biotinylated NAPols to immobilize functional, unmodified, membrane proteins, providing original sensor devices for multiple applications including innovative ligand screening assays.

Fluorescent ligands: Bringing light to emerging GPCR paradigms

In recent years, several novel aspects of GPCR pharmacology have been described, which are thought to play a role in determining the in vivo efficacy of a compound. Fluorescent ligands have been used to study many of these, which have also required the development of new experimental approaches. Fluorescent ligands offer the potential to use the same fluorescent probe to perform a broad range of experiments, from single-molecule microscopy to in vivo BRET. This review provides an overview of the in vitro use of fluorescent ligands in further understanding emerging pharmacological paradigms within the GPCR field, including ligand-binding kinetics, allosterism and intracellular signalling, along with the use of fluorescent ligands to study physiologically relevant therapeutic agents.

Development of a novel fluorescent ligand of growth hormone secretagogue receptor based on the N-Terminal Leap2 region

Liver-expressed antimicrobial peptide 2 (LEAP2) was recently recognized as an endogenous ligand for the growth hormone secretagogue receptor (GHSR), which also is a receptor for the hormone ghrelin. LEAP2 blocks ghrelin-induced activation of GHSR and inhibits GHSR constitutive activity. Since fluorescence-based imaging and pharmacological analyses to investigate the biology of GHSR require reliable probes, we developed a novel fluorescent GHSR ligand based on the N-terminal LEAP2 sequence, hereafter named F-LEAP2. In vitro, F-LEAP2 displayed binding affinity and inverse agonism to GHSR similar to LEAP2. In a heterologous expression system, F-LEAP2 labeling was specifically observed in the surface of GHSR-expressing cells, in contrast to fluorescent ghrelin labeling that was mainly observed inside the GHSR-expressing cells. In mice, centrally-injected F-LEAP2 reduced ghrelin-induced food intake, in a similar fashion to LEAP2, and specifically labeled cells in GHSR-expressing brain areas. Thus, F-LEAP2 represents a valuable tool to study the biology of GHSR in vitro and in vivo.

Structure and dynamics of G protein-coupled receptor-bound ghrelin reveal the critical role of the octanoyl chain

Ghrelin plays a central role in controlling major biological processes. As for other G protein-coupled receptor (GPCR) peptide agonists, the structure and dynamics of ghrelin bound to its receptor remain obscure. Using a combination of solution-state NMR and molecular modeling, we demonstrate that binding to the growth hormone secretagogue receptor is accompanied by a conformational change in ghrelin that structures its central region, involving the formation of a well-defined hydrophobic core. By comparing its acylated and nonacylated forms, we conclude that the ghrelin octanoyl chain is essential to form the hydrophobic core and promote access of ghrelin to the receptor ligand-binding pocket. The combination of coarse-grained molecular dynamics studies and NMR should prove useful in improving our mechanistic understanding of the complex conformational space explored by a natural peptide agonist when binding to its GPCR. Such information should also facilitate the design of new ghrelin receptor-selective drugs.

Brain accessibility delineates the central effects of circulating ghrelin

Ghrelin is a hormone produced in the gastrointestinal tract that acts via the growth hormone secretagogue receptor. In the central nervous system, ghrelin signalling is able to recruit different neuronal targets that regulate the behavioural, neuroendocrine, metabolic and autonomic effects of the hormone. Notably, several studies using radioactive or fluorescent variants of ghrelin have found that the accessibility of circulating ghrelin into the mouse brain is both strikingly low and restricted to some specific brain areas. A variety of studies addressing central effects of systemically injected ghrelin in mice have also provided indirect evidence that the accessibility of plasma ghrelin into the brain is limited. Here, we review these previous observations and discuss the putative pathways that would allow plasma ghrelin to gain access into the brain together with their physiological implications. Additionally, we discuss some potential features regarding the accessibility of plasma ghrelin into the human brain based on the observations reported by studies that investigate the consequences of ghrelin administration to humans.

Biomedical applications of radioiodinated peptides

The overexpression of peptide receptors in certain tumors as compared to endogeneous expression levels represents the molecular basis for the design of peptide-based tools for targeted nuclear imaging and therapy. Receptor targeting with radiolabelled peptides became a very important imaging and/or therapeutic approach in nuclear medicine and oncology. A great variety of peptides has been radiolabelled with clinical relevant radionuclides, such as radiometals and radiohalogens. However, to the best of our knowledge concise and updated reviews providing information about the biomedical application of radioiodinated peptides are still missing. This review outlines the synthetic efforts in the preparation of radioiodinated peptides highlighting the importance of radioiodine in nuclear medicine, giving an overview of the most relevant radioiodination strategies that have been employed and describes relevant examples of their use in the biomedical field.

Receptor-Ligand Interaction Measured by Inductively Coupled Plasma Mass Spectrometry and Selenium Labeling

In the search for an alternative strategy to the radioactivity measurement conventionally performed to probe receptor-ligand interactions in pharmacological assays, we demonstrated that selenium labeling of the studied ligand combined with elemental mass spectrometry was as efficient and robust as the reference method but devoid of its environmental and health hazards. The proof-of-concept was illustrated on two GPCR receptors, vasopressin (V_1A) and cholecystokinin B (CCK-B), involving peptides as endogenous ligands. We proposed several methodologies to produce selenium-labeled ligands according to peptide sequences along with binding affinity constraints. A selection of selenopeptides that kept high affinities toward the targeted receptor were engaged in saturation and competitive binding experiments with subsequent sensitive RP-LC-ICP-MS measurements. Experimental values of affinity constant ( K_i) were perfectly correlated to literature data, illustrating the general great potency of replacing radioactive iodine by selenium for ligand labeling to further undergo unaffected pharmacology experiments efficiently monitored by elemental mass spectrometry.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

Conditional and Reversible Activation of Class A and B G Protein-Coupled Receptors Using Tethered Pharmacology

Understanding the activation and internalization of G protein-coupled receptors (GPCRs) using conditional approaches is paramount to developing new therapeutic strategies. Here, we describe the design, synthesis, and testing of ExONatide, a benzylguanine-linked peptide agonist of the glucagon-like peptide-1 receptor (GLP-1R), a class B GPCR required for maintenance of glucose levels in humans. ExONatide covalently binds to SNAP-tagged GLP-1R-expressing cells, leading to prolonged cAMP generation, Ca²⁺ rises, and intracellular retention of the receptor. These effects were readily switched OFF following cleavage of the introduced disulfide bridge using the cell-permeable reducing agent beta-mercaptoethanol (BME). A similar approach could be extended to a class A GPCR using GhrelON, a benzylguanine-linked peptide agonist of the growth hormone secretagogue receptor 1a (GHS-R1a), which is involved in food intake and growth. Thus, ExONatide and GhrelON allow SNAP-tag-directed activation of class A and B GPCRs involved in gut hormone signaling in a reversible manner. This tactic, termed reductively cleavable agONist (RECON), may be useful for understanding GLP-1R and GHS-R1a function both in vitro and in vivo, with applicability across GPCRs.

From the Promiscuous Asenapine to Potent Fluorescent Ligands Acting at a Series of Aminergic G-Protein-Coupled Receptors

Monoamine neurotransmitters such as serotonin, dopamine, histamine, and noradrenaline have important and varied physiological functions and similar chemical structures. Representing important pharmaceutical drug targets, the corresponding G-protein-coupled receptors (termed aminergic GPCRs) belong to the class of cell membrane receptors and share many levels of similarity as well. Given their pharmacological and structural closeness, one could hypothesize the possibility to derivatize a ubiquitous ligand to afford rapidly fluorescent probes for a large set of GPCRs to be used for instance in FRET-based binding assays. Here we report fluorescent derivatives of the nonselective agent asenapine which were designed, synthesized, and evaluated as ligands of 34 serotonin, dopamine, histamine, melatonin, acetylcholine, and adrenergic receptors. It appears that this strategy led rapidly to the discovery and development of nanomolar affinity fluorescent probes for 14 aminergic GPCRs. Selected probes were tested in competition binding assays with unlabeled competitors in order to demonstrate their suitability for drug discovery purposes.

Ghrelin receptor inverse agonists as a novel therapeutic approach against obesity-related metabolic disease

AIMS

Ghrelin is implicated in the control of energy balance and glucose homeostasis. The ghrelin receptor exhibits ligand-independent constitutive activity, which can be pharmacologically exploited to induce inverse ghrelin actions. Because ghrelin receptor inverse agonists (GHSR-IA) might be effective for the treatment of obesity-related metabolic disease, we tested 2 novel synthetic compounds GHSR-IA1 and GHSR-IA2.

MATERIALS AND METHODS

In functional cell assays, electrophysiogical and immunohistochemical experiments, we demonstrated inverse agonist activity for GHSR-IA1 and GHSR-IA2. We used healthy mice, Zucker diabetic fatty (ZDF) rats and diet-induced obese (DIO) mice to explore effects on food intake (FI), body weight (BW), conditioned taste aversion (CTA), oral glucose tolerance (OGT), pancreatic islet morphology, hepatic steatosis (HS), and blood lipids.

RESULTS

Both compounds acutely reduced FI in mice without inducing CTA. Chronic GHSR-IA1 increased metabolic rate in chow-fed mice, suppressed FI, and improved OGT in ZDF rats. Moreover, the progression of islet hyperplasia to fibrosis in ZDF rats slowed down. GHSR-IA2 reduced FI and BW in DIO mice, and reduced fasting and stimulated glucose levels compared with pair-fed and vehicle-treated mice. GHSR-IA2-treated DIO mice showed decreased blood lipids. GHSR-IA1 treatment markedly decreased HS in DIO mice.

CONCLUSIONS

Our study demonstrates therapeutic actions of novel ghrelin receptor inverse agonists, suggesting a potential to treat obesity-related metabolic disorders including diabetes mellitus.

Quantitative MALDI-MS Binding Assays: An Alternative to Radiolabeling

Radiolabeling of ligands is still the gold standard in the study of high-affinity receptor-ligand interactions. In an effort toward safer and simpler alternatives to the use of radioisotopes, we developed a quantitative and highly sensitive matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) method that relies on the use of chemically tagged ligands designed to be specifically detectable when present as traces in complex biological mixtures such as cellular lysates. This innovative technology allows easy, sensitive detection and accurate quantification of analytes at the sub-nanomolar level. After statistical validation, we were able to perform pharmacological evaluations of G protein-coupled receptor (V1A-R)-ligand interactions. Both saturation and competitive binding assays were successfully processed.

Fluorescence- and bioluminescence-based approaches to study GPCR ligand binding

Ligand binding is a vital component of any pharmacologist's toolbox and allows the detailed investigation of how a molecule binds to its receptor. These studies enable the experimental determination of binding affinity of labelled and unlabelled compounds through kinetic, saturation (Kd ) and competition (Ki ) binding assays. Traditionally, these studies have used molecules labelled with radioisotopes; however, more recently, fluorescent ligands have been developed for this purpose. This review will briefly cover receptor ligand binding theory and then discuss the use of fluorescent ligands with some of the different technologies currently employed to examine ligand binding. Fluorescent ligands can be used for direct measurement of receptor-associated fluorescence using confocal microscopy and flow cytometry as well as in assays such as fluorescence polarization, where ligand binding is monitored by changes in the free rotation when a fluorescent ligand is bound to a receptor. Additionally, fluorescent ligands can act as donors or acceptors for fluorescence resonance energy transfer (FRET) with the development of assays based on FRET and time-resolved FRET (TR-FRET). Finally, we have recently developed a novel bioluminescence resonance energy transfer (BRET) ligand binding assay utilizing a small (19 kDa), super-bright luciferase subunit (NanoLuc) from a deep sea shrimp. In combination with fluorescent ligands, measurement of RET now provides an array of methodologies to study ligand binding. While each method has its own advantages and drawbacks, binding studies using fluorescent ligands are now a viable alternative to the use of radioligands. 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.

Identification of the epidermal growth factor receptor as the receptor for Salmonella Rck-dependent invasion

The Salmonella Rck outer membrane protein binds to the cell surface, which leads to bacterial internalization via a Zipper mechanism. This invasion process requires induction of cellular signals, including phosphorylation of tyrosine proteins, and activation of c-Src and PI3K, which arises as a result of an interaction with a host cell surface receptor. In this study, epidermal growth factor receptor (EGFR) was identified as the cell signaling receptor required for Rck-mediated adhesion and internalization. First, Rck-mediated adhesion and internalization were shown to be altered when EGFR expression and activity were modulated. Then, immunoprecipitations were performed to demonstrate the Rck-EGFR interaction. Furthermore, surface plasmon resonance biosensor and homogeneous time-resolved fluorescence technologies were used to demonstrate the direct interaction of Rck with the extracellular domain of human EGFR. Finally, our study strongly suggests a noncompetitive binding of Rck and EGF to EGFR. Overall, these results demonstrate that Rck is able to bind to EGFR and thereby establish a tight adherence to provide a signaling cascade, which leads to internalization of Rck-expressing bacteria.-Wiedemann, A., Mijouin, L., Ayoub, M. A., Barilleau, E., Canepa, S., Teixeira-Gomes, A. P., Le Vern, Y., Rosselin, M., Reiter, E., Velge, P. Identification of the epidermal growth factor receptor as the receptor for Salmonella Rck-dependent invasion.

New ligands of the ghrelin receptor based on the 1,2,4-triazole scaffold by introduction of a second chiral center

Introducing a second chiral center on our previously described 1,2,4-triazole, allowed us to increase diversity and elongate the 'C-terminal part' of the molecule. Therefore, we were able to explore mimics of the substance P analogs described as inverse agonists. Some compounds presented affinities in the nanomolar range and potent biological activities, while one exhibited a partial inverse agonist behavior similar to a Substance P analog.

The novel nonapeptide acein targets angiotensin converting enzyme in the brain and induces dopamine release

BACKGROUND AND PURPOSE

Using an in-house bioinformatics programme, we identified and synthesized a novel nonapeptide, H-Pro-Pro-Thr-Thr-Thr-Lys-Phe-Ala-Ala-OH. Here, we have studied its biological activity, in vitro and in vivo, and have identified its target in the brain.

EXPERIMENTAL APPROACH

The affinity of the peptide was characterized using purified whole brain and striatal membranes from guinea pigs and rats . Its effect on behaviour in rats following intra-striatal injection of the peptide was investigated. A photoaffinity UV cross-linking approach combined with subsequent affinity purification of the ligand covalently bound to its receptor allowed identification of its target.

KEY RESULTS

The peptide bound with high affinity to a single class of binding sites, specifically localized in the striatum and substantia nigra of brains from guinea pigs and rats. When injected within the striatum of rats, the peptide stimulated in vitro and in vivo dopamine release and induced dopamine-like motor effects. We purified the target of the peptide, a ~151 kDa protein that was identified by MS/MS as angiotensin converting enzyme (ACE I). Therefore, we decided to name the peptide acein.

CONCLUSION AND IMPLICATIONS

The synthetic nonapeptide acein interacted with high affinity with brain membrane-bound ACE. This interaction occurs at a different site from the active site involved in the well-known peptidase activity, without modifying the peptidase activity. Acein, in vitro and in vivo, significantly increased stimulated release of dopamine from the brain. These results suggest a more important role for brain ACE than initially suspected.

Structure-activity relationship for peptídic growth hormone secretagogues

Growth hormone releasing peptides (GHRPs) could be widely used by cheating athletes because they produce growth hormone (GH) secretion, so may generate an ergogenic effect in the body. Knowledge of the essential amino acids needed in GHRP structure for interaction with the target biological receptor GHSR1a, the absorption through different administration routes, and the maintenance of pharmacological activity of potential biotransformation products may help in the fight against their abuse in sport. Several GHRPs and truncated analogues with the common core Ala-Trp-(D-Phe)-Lys have been studied with a radio-competitive assay for the GHSR1a receptor against the radioactive natural ligand ghrelin. Relevant chemical modifications influencing the activity for positions 1, 2, 3, and 7 based on the structure aa-aa-aa-Ala-Trp-(D-Phe)-Lys have been obtained. To test in vivo the applicability of the activities observed, the receptor assay activity in samples from excretion studies performed after nasal administration of GHRP-1, GHRP-2, GHRP-6, Hexarelin, and Ipamorelin was confirmed. Overall results obtained allow to infer structure-activity information for those GHRPs and to detect GHSR1a binding (intact GHRPs plus active metabolites) in excreted urines. Copyright © 2016 John Wiley & Sons, Ltd.

Selective homodimerization of unprotected peptides using hybrid hydroxydimethylsilane derivatives

A straightforward way to dimerize unprotected peptide sequences is presented; it relies on a chemoselective condensation of hybrid peptides bearing a hydroxydimethylsilyl group at a chosen position to generate siloxane bonds upon freeze-drying.

Palatability Can Drive Feeding Independent of AgRP Neurons

Feeding behavior is exquisitely regulated by homeostatic and hedonic neural substrates that integrate energy demand as well as the reinforcing and rewarding aspects of food. Understanding the net contribution of homeostatic and reward-driven feeding has become critical because of the ubiquitous source of energy-dense foods and the consequent obesity epidemic. Hypothalamic agouti-related peptide-secreting neurons (AgRP neurons) provide the primary orexigenic drive of homeostatic feeding. Using models of neuronal inhibition or ablation, we demonstrate that the feeding response to a fast ghrelin or serotonin receptor agonist relies on AgRP neurons. However, when palatable food is provided, AgRP neurons are dispensable for an appropriate feeding response. In addition, AgRP-ablated mice present exacerbated stress-induced anorexia and palatable food intake--a hallmark of comfort feeding. These results suggest that, when AgRP neuron activity is impaired, neural circuits sensitive to emotion and stress are engaged and modulated by food palatability and dopamine signaling.

Agonism, Antagonism, and Inverse Agonism Bias at the Ghrelin Receptor Signaling

The G protein-coupled receptor GHS-R1a mediates ghrelin-induced growth hormone secretion, food intake, and reward-seeking behaviors. GHS-R1a signals through Gq, Gi/o, G13, and arrestin. Biasing GHS-R1a signaling with specific ligands may lead to the development of more selective drugs to treat obesity or addiction with minimal side effects. To delineate ligand selectivity at GHS-R1a signaling, we analyzed in detail the efficacy of a panel of synthetic ligands activating the different pathways associated with GHS-R1a in HEK293T cells. Besides β-arrestin2 recruitment and ERK1/2 phosphorylation, we monitored activation of a large panel of G protein subtypes using a bioluminescence resonance energy transfer-based assay with G protein-activation biosensors. We first found that unlike full agonists, Gq partial agonists were unable to trigger β-arrestin2 recruitment and ERK1/2 phosphorylation. Using G protein-activation biosensors, we then demonstrated that ghrelin promoted activation of Gq, Gi1, Gi2, Gi3, Goa, Gob, and G13 but not Gs and G12. Besides, we identified some GHS-R1a ligands that preferentially activated Gq and antagonized ghrelin-mediated Gi/Go activation. Finally, we unambiguously demonstrated that in addition to Gq, GHS-R1a also promoted constitutive activation of G13. Importantly, we identified some ligands that were selective inverse agonists toward Gq but not of G13. This demonstrates that bias at GHS-R1a signaling can occur not only with regard to agonism but also to inverse agonism. Our data, combined with other in vivo studies, may facilitate the design of drugs selectively targeting individual signaling pathways to treat only the therapeutically relevant function.

Fit-for-Purpose Radio Receptor Assay for the Determination of Growth Hormone Secretagogues in Urine

The everlasting pharmacological development is continuously producing new substances with potential doping abuse. Among these, secretagogues are very prone to misuse by athletes for their properties to release growth hormone (GH) and some limitations in the actual analytical methods to detect them. In this paper, an in-depth study on the key variables of the radio receptor method previously developed by our group is performed and a fit-for-purpose protocol is established. Thus, this sensitive and robust screening method is proposed as an intelligent and preventive antidoping method to detect new growth hormone secretagogues (GHSs) in exceptional suspicious urine samples obtained from athletes and will support the current detection methods based on liquid chromatography–mass spectrometry (LC-MS).

Probing the pharmacology of G protein-coupled receptors with fluorescent ligands

G protein-coupled receptors control a wide range of physiological processes and are the target for many clinically used drugs. Understanding the way in which receptors bind agonists and antagonists, their organisation in the membrane and their regulation after agonist binding are important properties which are key to developing new drugs. One way to achieve this knowledge is through the use of fluorescent ligands, which have been used to study the expression and function of receptors in endogenously expressing systems. Fluorescent ligands with appropriate imaging properties can be used in conjunction with confocal microscopy to investigate the regulation of receptors after activation. Alternatively, through the use of single molecule microscopy, they can probe the spatial organisation of receptors within the membrane. This review focuses on the techniques in which fluorescent ligands have been used and the novel aspects of G protein-coupled receptor pharmacology which have been uncovered. This article is part of the Special Issue entitled 'Fluorescent Tools in Neuropharmacology'.

Time-resolved FRET binding assay to investigate hetero-oligomer binding properties: proof of concept with dopamine D1/D3 heterodimer

G protein-coupled receptors (GPCRs) have been described to form hetero-oligomers. The importance of these complexes in physiology and pathology is considered crucial, and heterodimers represent promising new targets to discover innovative therapeutics. However, there is a lack of binding assays to allow the evaluation of ligand affinity for GPCR hetero-oligomers. Using dopamine receptors and more specifically the D1 and D3 receptors as GPCR models, we developed a new time-resolved FRET (TR-FRET) based assay to determine ligand affinity for the D1/D3 heteromer. Based on the high-resolution structure of the dopamine D3 receptor (D3R), six fluorescent probes derived from a known D3R partial agonist (BP 897) were designed, synthesized and evaluated as high affinity and selective ligands for the D3/D2 receptors, and for other dopamine receptor subtypes. The highest affinity ligand 21 was then employed in the development of the D1/D3 heteromer assay. The TR-FRET was monitored between a fluorescent tag donor carried by the D1 receptor (D1R) and a fluorescent acceptor D3R ligand 21. The newly reported assay, easy to implement on other G protein-coupled receptors, constitutes an attractive strategy to screen for heteromer ligands.

Folding and stability of integral membrane proteins in amphipols

Amphipols (APols) are a family of amphipathic polymers designed to keep transmembrane proteins (TMPs) soluble in aqueous solutions in the absence of detergent. APols have proven remarkably efficient at (i) stabilizing TMPs, as compared to detergent solutions, and (ii) folding them from a denatured state to a native, functional one. The underlying physical-chemical mechanisms are discussed.

Direct Ras Inhibitors Identified from a Structurally Rigidified Bicyclic Peptide Library

A one-bead-two-compound (OBTC) library of structurally rigidified bicyclic peptides was chemically synthesized on TentaGel microbeads (90 μm), with each bead displaying a unique bicyclic peptide on its surface and a linear encoding peptide of the same sequence in its interior. Screening of the library against oncogenic K-Ras G12V mutant identified two classes of Ras ligands. The class I ligands apparently bind to the effector-binding site and inhibit the Ras-Raf interaction, whereas the class II ligand appears to bind to a yet unidentified site different from the effector-binding site. These Ras ligands provide useful research tools and may be further developed into therapeutic agents.

Luminescent lanthanide cryptates: from the bench to the bedside

The design and application of luminescent lanthanide cryptates for sensing biological interactions is highlighted through the review of the work performed in our laboratory and with academic collaborations. The path from the initial applications probing biochemical interaction in vitro to "state-of-the-art" cellular assays toward clinical applications using homogeneous time-resolved fluorescence technology is described. An overview of the luminescent lanthanide macrocyclic compounds developed at Cisbio in the recent past is given with an emphasis on specific constraints required by specific applications. Recent assays for drug-discovery and diagnostic purposes using both antibody-based and suicide-enzyme-based technology are illustrated. New perspectives in the field of molecular medicine and time-resolved microscopy are discussed.

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.

A novel homogeneous time-resolved fluoroimmunoassay for carcinoembryonic antigen based on water-soluble quantum dots

Quantum dots are not widely used in clinical diagnosis. However, the homogeneous time-resolved fluorescence assay possesses many advantages over current methods for the detection of carcinoembryonic antigen (CEA), a primary marker for many cancers and diseases. Therefore, a novel luminescent terbium chelates- (LTCs) and quantum dots-based homogeneous time-resolved fluorescence assay was developed to detect CEA. Glutathione-capped quantum dots (QDs) were prepared from oil-soluble QDs with a 565 nm emission peak. Conjugates (QDs-6 F11) were prepared with QDs and anti-CEA monoclonal antibody. LTCs were prepared and conjugates (LTCs-S001) were prepared with another anti-CEA monoclonal antibody. The fluorescence lifetime of QDs was optimized for sequential analysis. The Förster distance (R0) was calculated as 61.9 Å based on the overlap of the spectra of QDs-6 F11 and LTCs-S001. Using a double-antibody sandwich approach, the above antibody conjugates were used as energy acceptor and donor, respectively. The signals from QDs were collected in time-resolved mode and analyzed for the detection of CEA. The results show that the QDs were suitable for time-resolved fluoroassays. The spatial distance of the donor-acceptor pair was calculated to be 61.9 Å. The signals from QDs were proportional to CEA concentration. The standard curve was LogY = 2.75566 + 0.94457 LogX (R = 0.998) using the fluorescence counts (Y) of QDs and the concentrations of CEA (X). The calculated sensitivity was 0.4 ng/mL. The results indicate that water-soluble QDs are suitable for the homogenous immunoassay. This work has expanded future applications of QDs in homogeneous clinical bioassays. Furthermore, a QDs-based homogeneous multiplex immunoassay will be investigated as a biomarker for infectious diseases in future research.

Structural and biophysical characterisation of G protein-coupled receptor ligand binding using resonance energy transfer and fluorescent labelling techniques

The interaction between ligands and the G protein-coupled receptors (GPCRs) to which they bind has long been the focus of intensive investigation. The signalling cascades triggered by receptor activation, due in most cases to ligand binding, are of great physiological and medical importance; indeed, GPCRs are targeted by in excess of 30% of small molecule therapeutic medicines. Attempts to identify further pharmacologically useful GPCR ligands, for receptors with known and unknown endogenous ligands, continue apace. In earlier days direct assessment of such interactions was restricted largely to the use of ligands incorporating radioactive isotope labels as this allowed detection of the ligand and monitoring its interaction with the GPCR. This use of such markers has continued with the development of ligands labelled with fluorophores and their application to the study of receptor-ligand interactions using both light microscopy and resonance energy transfer techniques, including homogenous time-resolved fluorescence resonance energy transfer. Details of ligand-receptor interactions via X-ray crystallography are advancing rapidly as methods suitable for routine production of substantial amounts and stabilised forms of GPCRs have been developed and there is hope that this may become as routine as the co-crystallisation of serine/threonine kinases with ligands, an approach that has facilitated widespread use of rapid structure-based ligand design. Conformational changes involved in the activation of GPCRs, widely predicted by biochemical and biophysical means, have inspired the development of intramolecular FRET-based sensor forms of GPCRs designed to investigate the events following ligand binding and resulting in a signal propagation across the cell membrane. Finally, a number of techniques are emerging in which ligand-GPCR binding can be studied in ways that, whilst indirect, are able to monitor its results in an unbiased and integrated manner. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.