Development of a scintiplate assay for recombinant human alpha(2B)-adrenergic receptor

A Simple Theoretical Model for Fluorescence Polarization Binding Assay Development

Fluorescence polarization is a screening technology that is radioactivity free, homogeneous, and ratiometric. The signal measured with this technology is a weighted value of free and bound ligand. As a consequence, saturation curves are accessible only after calculation of the corresponding concentrations of free and bound ligand. To make this technology more accessible to assay development, the authors propose a simple mathematical model that predicts fluorescence polarization values from ligand and receptor total concentrations, depending on the corresponding dissociation constant. This model was validated using data of Bodipy-NDP-αMSH binding to MC5, obtained after either ligand saturation of a receptor preparation or, conversely, receptor saturation of a ligand solution. These experimental data were also used to calculate the actual concentration of free and bound ligand and receptor and to obtain pharmacological constants by Scatchard analysis. A general method is proposed, which facilitates the design of fluorescence polarization binding assays by relying on the representation of theoretical polarization values. This approach is illustrated by the application to 2 systems of very different affinities.

A Cellular Assay for Inhibitors of the Fatty Acid Biosynthetic Pathway Using Scintillating Microplates

A simplified method for monitoring the incorporation of radiolabeled acetate into lipids in a cellular system is described. The assay eliminates the commonly employed labor-intensive organic extraction step by plating the cells in 96-well tissue culture-treated ScintiPlates(®) that enable direct measurement of radiolabeled cell membrane-embedded lipids. Since the scintillant is entrenched in the plates, radioactivity in close proximity to the scintillant is measured without the need for liquid scintillation cocktail. The utility of this method for evaluating inhibitors of the de novo fatty acid synthetic pathway is demonstrated here with fatty acid synthase (FASN). Due to the upregulation of FASN activity in many tumor types, development of inhibitors to block the FASN activity in cells shows promise as an attractive and tractable approach for therapeutic intervention.

From Recombinant Expression to Crystals: A Step-by-Step Guide to GPCR Crystallography

G protein-coupled receptors (GPCRs) are the primary targets of drugs prescribed for many human pathophysiological conditions such as hypertension, allergies, schizophrenia, asthma, and various types of cancer. High-resolution structure determination of GPCRs has been a key focus area in GPCR biology to understand the basic mechanism of their activation and signaling and to materialize the long-standing dream of structure-based drug design on these versatile receptors. There has been tremendous effort at this front in the past two decades and it has culminated into crystal structures of 27 different receptors so far. The recent progress in crystallization and structure determination of GPCRs has been driven by innovation and cutting-edge developments at every step involved in the process of crystallization. Here, we present a step-by-step description of various steps involved in GPCR crystallization starting from recombinant expression to obtaining diffracting crystals. We also discuss the next frontiers in GPCR biology that are likely to be a primary focus for crystallography efforts in the next decade or so.

Scintillation proximity assay

Scintillation proximity assay technologies provide a rapid non-separation method to measure common biological interactions using radioactively tagged molecules. This unit identifies potential uses of the technology for the measurement of receptor-ligand binding, cAMP accumulation, GTP binding to heterotrimeric G proteins, protease activity and cellular uptake.

Development of homogeneous high-affinity agonist binding assays for 5-HT2 receptor subtypes

The serotonin (5-hydroxytryptamine) 5-HT2 receptor subfamily consists of three members, 5-HT2A, 5-HT2B, and 5-HT2C. These receptors share high homology in their amino acid sequence, have similar signaling pathways, and have been indicated to play important roles in feeding, anxiety, aggression, sexual behavior, mood, and pain. Subtype-selective agonists and antagonists have been explored as drugs for hypertension, Parkinson's disease, sleep disorders, anxiety, depression, schizophrenia, and obesity. In this study, we report the development of homogeneous agonist binding assays in a scintillation proximity assay (SPA) format to determine the high-affinity binding state of agonist compounds for the human 5-HT2C, 5-HT2A, and 5-HT2B receptors. The 5-HT2 agonist 1-(4- [125I]iodo-2,5-dimethoxyphenyl)-2-aminopropane ([125I]DOI) was used to label the high-affinity sites for the 5-HT2A and 5-HT2C receptors. The high-affinity sites for the 5-HT2B receptor were labeled with [3H]lysergic acid diethylamide. Total receptor expression was determined with the 5-HT2 antagonist [3H]mesulergine for the 5-HT2B and 5-HT2C receptors, and [3H]ketanserin for the 5-HT2A receptor. The agonist high-affinity binding sites accounted for 2.3% (5-HT(2C) receptor), 4.0% (5-HT2A receptor), and 22% (5-HT2B receptor) of the total receptor population. Competition binding studies using known agonists indicated high Z' values of the agonist binding assays in SPA format (Z' > 0.70). The Ki values of 5-HT, (R)(-)DOI, and VER-3323 for the 5-HT2A, 5-HT2B, and 5-HT2C receptors by SPA format were equivalent to published data determined by filtration binding assays. These results indicate that agonist binding assays in SPA format can be easily adapted to a high throughput assay to screen for selective 5-HT2C receptor agonists, as well as for selectivity profiling of the compounds.

Receptor–ligand binding assays: Technologies and Applications

Receptor-ligand interactions play a crucial role in biological systems and their measurement forms an important part of modern pharmaceutical development. Numerous assay formats are available that can be used to screen and quantify receptor ligands. In this review, we give an overview over both radioactive and non-radioactive assay technologies with emphasis on the latter. While radioreceptor assays are fast, easy to use and reproducible, their major disadvantage is that they are hazardous to human health, produce radioactive waste, require special laboratory conditions and are thus rather expensive on a large scale. This has led to the development of non-radioactive assays based on optical methods like fluorescence polarization, fluorescence resonance energy transfer or surface plasmon resonance. In light of their application in high-throughput screening environments, there has been an emphasis on so called "mix-and-measure" assays that do not require separation of bound from free ligand. The advent of recombinant production of receptors has contributed to the increased availability of specific assays and some aspects of the expression of recombinant receptors will be reviewed. Applications of receptor-ligand binding assays described in this review will relate to screening and the quantification of pharmaceuticals in biological matrices.

Functional studies with membrane-bound and detergent-solubilized alpha2-adrenergic receptors expressed in Sf9 cells

A chip-based biosensor technology using surface plasmon resonance (SPR) was developed for studying the interaction of ligands and G protein-coupled receptors (GPCRs). GPCRs, the fourth largest superfamily in the human genome, are the largest class of targets for drug discovery. We have expressed the three subtypes of alpha(2)-adrenergic receptor (alpha(2)-AR), a prototypical GPCR as functional fusion proteins in baculovirus-infected insect cells. The localization of the expressed receptor was observed in intracellular organelles, as detected by eGFP fluorescence. In addition, the deletion mutants of alpha(2B)-AR, with a deletion in the 3rd intracellular loop, exhibited unaltered K(d) values and enhanced stability, thus making them more promising candidates for crystallization. SPR demonstrated that small molecule ligands can bind the detergent-solubilized receptor, thus proving that alpha(2)-AR is active even in a lipid-free environment. The K(d) values obtained from the biosensor analysis and traditional ligand binding studies correlate well with each other. This is the first demonstration of the binding of a small molecule to the detergent-solubilized state of alpha(2)-ARs and interaction of low-molecular mass-ligands in real time in a label-free environment. This technology will also allow the development of high throughput platform for screening a large number of compounds for generation of leads.

Development of a homogeneous binding assay for histamine receptors

Histamine is critically involved in a wide range of physiological and pathological processes through its actions at different receptors. Thus, histamine receptors have been actively pursued as therapeutic targets in the pharmaceutical industry for the treatment of a variety of diseases. There are currently four histamine receptors that have been cloned, all of which are G protein-coupled receptors. Studies from both academia and pharmaceutical companies have identified compounds that modulate the function of specific histamine receptors. These efforts led to the successful introduction of histamine H(1) and H(2) receptor antagonists for the treatment of allergy and excess gastric acid secretion, respectively. Histamine H(3) receptor ligands are currently under investigation for the treatment of obesity and neurological disorders. The recently identified histamine H(4) receptor is preferentially expressed in the immune tissues, suggesting a potential role in normal immune functions and possibly in the pathogenesis of inflammatory diseases. Even with the long history of histamine research and the important applications of histamine receptor ligands, assays to measure the affinity of compounds binding to histamine receptors are still routinely analyzed using a filtration assay, a very low-throughput assay involving washing and filtration steps. This article describes a simple, robust, and homogeneous binding assay based on the scintillation proximity assay (SPA) technology that provides results equivalent to those obtained using the more complex filtration assay. The SPA format is easily adapted to high-throughput screening because it is amenable to automation. In summary, this technique allows high-throughput screening of compounds against multiple histamine receptors and, thus, facilitates drug discovery efforts.

Functional expression and direct visualization of the human α2B-adrenergic receptor and α2B-AR-green fluorescent fusion protein in mammalian cell using Semliki Forest virus vectors

The alpha 2B -adrenergic receptor ( alpha 2B -AR), a member of the G protein-coupled receptor (GPCR) superfamily, was expressed at high levels from Semliki Forest virus (SFV) vectors in mammalian cells. Constructs were engineered by fusing enhanced green fluorescent protein (eGFP) and the SFV capsid to opposite ends of the alpha 2B -AR. The receptor fusions alpha 2B -AR-eGFP and CAP- alpha 2B -AR expressed in CHO-K1 cells generated alpha 2B values of 176 and 122pmol/mg of membrane protein, respectively, and showed similar ligand binding characteristics, alpha 2B -AR subtype-selectivity, and G protein activation as reported for stable expression in CHO-K1 cells. Cryo-electron microscopy and eGFP-based fluorescence indicated the same subcellular receptor distribution. SFV expression is well suited for studies on the pharmacology, biochemistry, and cell biology of GPCRs, and for large-scale recombinant protein production in mammalian suspension culture to generate sufficient receptor quantities for structural biology.