Considerations for using fluorescence polarization in the screening of G protein-coupled receptors

Application of a Fluorescence Anisotropy-Based Assay to Quantify Transglutaminase 2 Activity in Cell Lysates

Transglutaminase 2 (TGase 2) is a multifunctional protein which is involved in various physiological and pathophysiological processes. The latter also include its participation in the development and progression of malignant neoplasms, which are often accompanied by increased protein synthesis. In addition to the elucidation of the molecular functions of TGase 2 in tumor cells, knowledge of its concentration that is available for targeting by theranostic agents is a valuable information. Herein, we describe the application of a recently developed fluorescence anisotropy (FA)-based assay for the quantitative expression profiling of TGase 2 by means of transamidase-active enzyme in cell lysates. This assay is based on the incorporation of rhodamine B-isonipecotyl-cadaverine (R-I-Cad) into N,N-dimethylated casein (DMC), which results in an increase in the FA signal over time. It was shown that this reaction is not only catalyzed by TGase 2 but also by TGases 1, 3, and 6 and factor XIIIa using recombinant proteins. Therefore, control measurements in the presence of a selective irreversible TGase 2 inhibitor were mandatory to ascertain the specific contribution of TGase 2 to the overall FA rate. To validate the assay regarding the quality of quantification, spike/recovery and linearity of dilution experiments were performed. A total of 25 cancer and 5 noncancer cell lines were characterized with this assay method in terms of their activatable TGase 2 concentration (fmol/µg protein lysate) and the results were compared to protein synthesis data obtained by Western blotting. Moreover, complementary protein quantification methods using a biotinylated irreversible TGase 2 inhibitor as an activity-based probe and a commercially available ELISA were applied to selected cell lines to further validate the results obtained by the FA-based assay. Overall, the present study demonstrates that the FA-based assay using the substrate pair R-I-Cad and DMC represents a facile, homogenous and continuous method for quantifying TGase 2 activity in cell lysates.

Binding Characterization of Cyclic Peptide Ligands to Target Proteins and Chemical Epitopes Using ELISA and Fluorescence Polarization Assays

Enzyme-linked immunosorbent assay (ELISA) is a plate-based immunological assay designed to detect and quantify peptides, proteins, antibodies, and hormones. Fluorescence polarization (FP) is a solution-phase technique that can be used to determine equilibrium dissociation constant of ligand for the protein of interest. Here we describe the protocols for different ELISAs and for Fluorescence Polarization, and how they can be used to determine relative or absolute binding of macrocyclic peptides to the target proteins. In ELISA, the target protein is used as the antigen, and the binding of antigen is quantified using cyclic peptides and enzyme-linked antibodies. In Fluorescence Polarization assays, a cyclic ligand is fluorescent dye-labeled and titrated with serial concentrations of the non-labeled target protein to determine the equilibrium dissociation constant (K_D) of ligand for protein. Detailed descriptions of sample preparation and the ELISA and FP experiments are provided in this chapter.

A2AR Binding Kinetics in the Ligand Depletion Regime

Ligand binding plays a fundamental role in stimulating the downstream signaling of membrane receptors. Here, ligand-binding kinetics of the full-length human adenosine A_2A receptor (A_2AR) reconstituted in detergent micelles were measured using a fluorescently labeled ligand via fluorescence anisotropy. Importantly, to optimize the signal-to-noise ratio, these experiments were conducted in the ligand depletion regime. In the ligand depletion regime, the assumptions used to determine analytical solutions for one-site binding models for either one or two ligands in competition are no longer valid. We therefore implemented a numerical solution approach to analyze kinetic binding data as experimental conditions approach the ligand depletion regime. By comparing the results from the numerical and the analytical solutions, we highlight the ligand-receptor ratios at which the analytical solution begins to lose predictive accuracy. Using the numerical solution approach, we determined the kinetic rate constants of the fluorescent ligand, FITC-APEC, and those for three unlabeled ligands using competitive association experiments. The association and dissociation rate constants of the unlabeled ligands determined from the competitive association experiments were then independently validated using competitive dissociation data. Based on this study, a numerical solution is recommended to determine kinetic ligand-binding parameters for experiments conducted in the ligand-depletion regime.

Highly sensitive naphthalimide-based fluorescence polarization probe for detecting cancer cells

Fluorescence polarization (FP)-based signal is a self-referencing fluorescence signal, and it is less dependent on dye concentration and environmental interferences, which makes FP measurement an attractive alternative sensing technology to fluorescence intensity-based detection. However, most of the fluorescence polarization probes were constructed by introducing fluorescein, rhodamine, and cyanine dyes, which have relatively shorter excited-state lifetimes compared with BODIPY and naphthalimide dyes. Herein, a first naphthalimide based fluorescence polarization probe (BIO) was designed and synthesized for selective and direct detection of cancer cells. The relatively longer excited-state lifetimes and high photostability of naphthalimide makes BIO more sensitive and accuracy in quantitative determination of HeLa cells in homogeneous solution without cell lysis and further separation steps. The detection limit of BIO for HeLa cells was about 85 cells mL(-1), the linear range was from 2.5 × 10(2) cells mL(-1) to 1 × 10(6) cells mL(-1) and the response time is no more than 25 min. Moreover, due to the relatively high photostability of naphthalimide, BIO was particularly suitable for live cell imaging under continuous irradiation with confocal microscopy, and the specific interaction of BIO with CD44-overexpressing cell lines was clearly visualized. Importantly, this BIO based sensing platform offers a direct and real-time tool for cancer cell diagnosis when complemented with the use of naphthalimide-based fluorescence polarization probe.

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.

Overview of high-throughput screening

High-throughput screening (HTS) is a key process used in drug discovery to identify hits from compound libraries that may become leads for medicinal chemistry optimization. This updated overview discusses the utilization of compound libraries, compounds derived from combinatorial and parallel synthesis campaigns and natural product sources; creation of mother and daughter plates; and compound storage, handling, and bar coding in HTS. The unit also presents an overview of established and emerging assay technologies (i.e., time-resolved fluorescence, fluorescence polarization, fluorescence-correlation spectroscopy, functional whole cell assays, and high-content assays) and their integration in automation hardware and IT systems. This revised unit provides updated descriptions of state-of-the-art instrumentation and technologies in this rapidly changing environment. The section on assay methodologies now also covers enzyme complementation assays and methods for high-throughput screening of ion channel activities. Finally, a section on criteria for assay robustness is included discussing the Z'-factor, which is now a widely accepted criterion for evaluation and validation of high throughput screening assays.

Strategies to discover unexpected targets for drugs active at G protein-coupled receptors

G protein-coupled receptors (GPCRs) are an evolutionarily conserved family of signaling molecules comprising approximately 2% of the human genome; this receptor family remains a central focus in basic pharmacology studies and drug discovery efforts. Detailed studies of drug action at GPCRs over the past decade have revealed existing and novel ligands that exhibit polypharmacology-that is, drugs with activity at more than one receptor target for which they were designed. These "off-target" drug actions can be a liability that causes adverse side effects; however, in several cases, drugs with less selectivity demonstrate better clinical efficacy. Here we review physical screening and cheminformatic approaches that define drug activity at the GPCR receptorome. In many cases, such profiling has revealed unexpected targets that explain therapeutic actions as well as off-targets underlying drug side effects. Such drug-receptor profiling has also provided new insights into mechanisms of action of existing drugs and has suggested directions for future drug development.

Development of a high-throughput fluorescence polarization assay for the discovery of phosphopantetheinyl transferase inhibitors

An alarming number of clinically relevant bacterial pathogens are becoming resistant to many antibiotics, thereby fueling intense research into the discovery of novel therapeutic targets. Phosphopantetheinyl transferases (PPTases) represent a promising target for antibacterial development because these enzymes are crucial for the biosynthesis of a multitude of a pathogen's collection of essential metabolites and virulence factors biosynthesized via polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) pathways. Here we describe the development of a fluorescence polarization (FP) assay that is amenable for high-throughput screening to identify PPTase inhibitors. The FP assay was validated against a panel of competitive ligands and displayed an excellent Z' score.

G-protein-coupled receptor-focused drug discovery using a target class platform approach

In recent years, several large pharmaceutical companies have taken a novel approach to drug discovery biology and chemistry in that they channel their efforts with respect to particular target classes, such as G-protein-coupled receptors (GPCRs), toward dedicated, specialized teams. Benefits of such an organizational structure are the prospects of establishing several target-family-specific experimental techniques and skill sets, thereby enabling a comprehensive functional profiling of drug candidates in different pharmacological respects. In this context, the recently increased number of reports on GPCR ligand-biased signaling has further spurred the efforts in the pharmaceutical industry toward broader biological characterization of the test compounds, for example employing high-content screening to analyze different GPCR ligand-induced signaling pathways. The knowledge of the disease-relevant functional properties of the small molecule GPCR ligands enables target-specific chemical optimization and GPCR-subclass-directed library design. In the case of GPCRs, where little--although at present slowly expanding--structural information on the targets is available, the modeling of GPCR structures crucially depends on biological validation (typically supported by site-directed mutagenesis of the GPCR ligand binding site). In this review, we aim to recapitulate efforts in the pharmaceutical industry to address GPCR-directed drug discovery in a target-class-directed platform approach: establishing GPCR-specific biological assay panels and creating computational chemistry methods for finding and optimizing small molecules modulating the activity of GPCRs.

A fluorescence polarization assay for identifying ligands that bind to vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a homodimeric proangiogenic protein that induces endothelial cell migration and proliferation primarily through interactions with its major receptors, VEGFR-1 and VEGFR-2. Inhibitors of one or both of these VEGF-receptor interactions could be beneficial as therapeutics for diseases caused by dysfunctional angiogenesis (e.g., cancer). Others have reported small peptides that bind to the VEGF dimer at surface regions that are recognized by the receptors. Here we report the development of a fluorescence polarization assay based on the binding to VEGF of a derivative of one of these peptides that has been labeled with BODIPY-tetramethylrhodamine (BODIPY(TMR)). This 384-well format assay is tolerant to dimethyl sulfoxide (DMSO, up to 4% [v/v]) and has a Z' factor of 0.76, making it useful for identifying molecules that associate with the receptor-binding surface of the VEGF dimer.

Development, validation and implementation of immobilized metal affinity for phosphochemicals (IMAP)-based high-throughput screening assays for low-molecular-weight compound libraries

This protocol describes assay development, validation and implementation of automated immobilized metal affinity for phosphochemicals (IMAP)-based fluorescence polarization (FP) and time-resolved fluorescence resonance energy transfer (TR-FRET) high-throughput screening (HTS) assays for identification of low-molecular-weight kinase inhibitors. Both procedures are performed in miniaturized kinase reaction volumes and involve the stepwise addition of test or control compounds, enzyme and substrate/ATP. Kinase reactions are stopped by subsequent addition of IMAP-binding buffer. Assay attributes of the IMAP FP and TR-FRET methodologies are described. HTS assays developed using these procedures should result in Z-factors and low assay variability necessary for robust HTS assays. Providing that the required reagents and equipment are available, one scientist should be able to develop a 384-well, miniaturized HTS assay in approximately 6-8 weeks. Specific automated HTS assay conditions will determine the number of assay plates processed in a screening session, but two scientists should expect to process between 100 and 150 assay plates in one 8-h screening day.

Pharmacological Profiling of Chemokine Receptor–Directed Compounds Using High-Content Screening

High-content screening, typically defined as automated fluorescence microscopy combined with image analysis, is now well established as a means to study test compound effects in cellular disease-modeling systems. In this work, the authors establish several high-content screening assays in the 384-well format to measure the activation of the CC-type chemokine receptors 2B and 3 (CCR2B, CCR3). As a cellular model system, the authors use Chinese hamster ovary cells, stably transfected with 1 of the respective receptors. They characterize receptor stimulation by human monocyte chemoattractant protein-1 for CCR2B and by human eotaxin-1 for CCR3: Receptor internalization and receptor-induced phosphorylation of ERK1/2 (pERK) were quantified using fluorescence imaging and image analysis. The 4 assay formats were robust, displayed little day-to-day variability, and delivered good Z′ statistics for both CCRs. For each of the 2 receptors, the authors evaluated the potency of inhibitory compounds in the internalization format and the pERK assay and compared the results with those from other assays (ligand displacement binding, Ca2+ mobilization, guanosine triphosphate exchange, chemotaxis). Both physiological agonists and test compounds differed significantly with respect to potencies and efficacies in the various profiling assays. The diverse assay formats delivered partially overlapping and partially complementary information, enabling the authors to reduce the probability of test compound—related technology artifacts and to specify the mode of action for individual test compounds. Transfer of the high-content screening format to a fully automated medium-throughput screening platform for CCR3 enabled the profiling of large compound numbers with respect to G protein signaling and possible tolerance-inducing liabilities. ( Journal of Biomolecular Screening 2008:40-53)

High Content Screening to Monitor G Protein-Coupled Receptor Internalisation

G protein-coupled receptors (GPCRs) fulfil a broad diversity of physiological functions in areas such as neurotransmission, respiration, cardiovascular action, pain and more. Consequently, they are considered as the most successful group of therapeutic targets on the pharmaceutical market, and the search for compounds that interfere with GPCR function in a specific and selective way is a major focus of the pharmaceutical industry. High Content Screening (HCS), a combination of fluorescence microscopic imaging and automated image analysis, has become a frequently employed tool to study test compound effects in cellular disease modelling systems. One way to functionally analyse the effect of test compounds on GPCRs by HCS relies on the broadly observed phenomenon of desensitisation. Agonist stimulation of most GPCRs leads to their intracellular phosphorylation and subsequent internalisation, resulting in the termination of receptor signalling and the seclusion of the GPCR from further extracellular stimulation. Complementary to other functional GPCR drug discovery assays, GPCR internalisation assays enable a desensitisation-focussed pharmacological analysis of test compounds.

Early evaluation of compound QT prolongation effects: A predictive 384-well fluorescence polarization binding assay for measuring hERG blockade

INTRODUCTION

A large number of drugs from a variety of pharmacological classes have been demonstrated to cause adverse effects on cardiac rhythm, including the life-threatening arrhythmia Torsades de Pointes. These side effects are often associated with prolongation of the QT interval and are mediated via blockade of the human ether-a-go-go related gene (hERG) encoded potassium channel. In order to manage this risk in the pharmaceutical industry it is desirable to evaluate QT prolongation as early as possible in the drug discovery process.

METHODS

Here we describe the development of a 384-well fluorescence polarization (FP) binding assay compatible with high-throughput assessment of compound blockade of the hERG channel during the lead optimisation process. To characterise the fluorescent ligand that was developed, competition binding studies, kinetic studies and electrophysiology studies were performed. Furthermore, to validate the assay as a key screening method a series of competition binding studies were performed and correlated with functional data obtained via patch-clamp.

RESULTS

Evaluation of the assay indicates that high quality data is obtained (Z'>0.6), that the K(i) values determined are equivalent to more traditional radiometric methods and that it is predictive for functional hERG blockade as assessed by patch clamp.

DISCUSSION

Whilst FP assays, utilizing a variety of fluors, have become well established for the evaluation of G-protein-coupled receptor (GPCRs) and kinase ligand interactions, this technique has not been applied widely to the study of ion channels. Therefore, this represents a novel assay format that is amenable to the evaluation of thousands of compounds per day. Whilst other assay formats have proven predictive or high throughput, this assay represents one of few that combines both attributes, moreover it represents the most cost effective assay, making it truly amenable to early assessment of hERG blockade.

The expression of recombinant human LOX-1 and identifying its mimic ligands by fluorescence polarization-based high throughput screening

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) was identified as a major receptor for oxidized low-density lipoprotein (oxLDL) in endothelial cells. LOX-1 critically mediates the endothelial dysfunction and the progression of atherosclerosis by oxLDL stimulation. It might be an important target for vascular endothelium. In order to obtain human LOX-1 and identify its mimic ligand for facilitating the study of LOX-1 function, a recombinant plasmid pPIC9K-His-hLOX-1 was structured and expressed human LOX-1 in Pichia pastoris GS115. Western blot analysis ensured the expressed recombinant human LOX-1 protein and a receptor-ligand binding assay showed that it had high binding affinity with oxLDL. With this receptor protein, a competitive fluorescence polarization-based high throughput screening method was established in a 384-well microplate to isolate the mimic ligands of human LOX-1. The evaluating parameter Z' value of 0.72 for this method showed that fluorescence polarization-based high throughput screening assay was robust and the results had a high reliability. By the fluorescence polarization-based high throughput screening assay, a total of 20,316 chemicals were screened, and 2 chemicals were identified that they have a high affinity with human LOX-1. Competitive uptake DiI-oxLDL assay by human LOX-1 transfected CHO-K1 cells further confirmed that two chemicals block the uptake of DiI-oxLDL. And the preliminary results indicated that isolated mimic ligands may act as a function of antagonist. The discovery of human LOX-1 mimic ligand would benefit to further study the function of LOX-1 and identify a novel avenue for prevention and treatment atherosclerosis.

High-throughput fluorescence polarization method for identifying ligands of LOX-1

AIM

To develop and optimize a competitive fluorescence polarization (FP) method, and use it as a high-throughput screening (HTS) assay for drug discovery.

METHODS

Human lectin-like oxidized low-density lipoprotein receptor-1 (hLOX-1) and oxidized low-density lipoprotein (oxLDL) were used to establish a high-throughput fluorescence polarization assay to screen ligands of human LOX-1. A 96-well plate assay was performed with a fast plate reader. Three fluorescein isothiocyanate-labeled hLOX-1 concentrations (100, 200, and 400 nmol/L) were selected to be titrated by oxLDL (from 0.05 nmol/L to 100 micromol/L) in order to obtain optimal reactive concentrations. The concentration of Me2SO used (0%, 1%, 3%, 5%) and incubation time (15 min, 30 min, 1 h, 2 h) were optimized. The Z' factor was calculated to estimate the quality of FP-based HTS.

RESULTS

Concentrations of 200 nmol/L for human LOX-1 and 50 micromol/L for oxLDL were used in the actual assay. Concentrations of 0% to 5% Me2SO and different reaction times did not affect the FP-based HTS. The Z' value was 0.66. By using this detection and screening system, 12 700 compounds were screened and 3 ligands with an IC50 of less than 4.5 micromol/L were found.

CONCLUSION

The established competitive FP-based assay is sensitive, stable, highly reproducible and robust, and suitable for HTS for ligands of the hLOX-1 receptor.

G Protein‐Coupled Receptor Internalization Assays in the High‐Content Screening Format

High-content screening (HCS), a combination of fluorescence microscopic imaging and automated image analysis, has become a frequently applied tool to study test compound effects in cellular disease-modeling systems. This chapter describes the measurement of G protein-coupled receptor (GPCR) internalization in the HCS format using a high-throughput, confocal cellular imaging device. GPCRs are the most successful group of therapeutic targets on the pharmaceutical market. Accordingly, the search for compounds that interfere with GPCR function in a specific and selective way is a major focus of the pharmaceutical industry today. This chapter describes methods for the ligand-induced internalization of GPCRs labeled previously with either a fluorophore-conjugated ligand or an antibody directed against an N-terminal tag of the GPCR. Both labeling techniques produce robust assay formats. Complementary to other functional GPCR drug discovery assays, internalization assays enable a pharmacological analysis of test compounds. We conclude that GPCR internalization assays represent a valuable medium/high-throughput screening format to determine the cellular activity of GPCR ligands.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Discovery of novel inhibitors of Bcl-xL using multiple high-throughput screening platforms

Bcl-xL is a member of the Bcl-2 family of proteins that are implicated to play a vital role in several diseases including cancer. Bcl-xL suppresses apoptosis; thus the inhibition of Bcl-xL function could restore the apoptotic process. To identify antagonists of Bcl-xL function, two ultra-high-throughput screens were implemented. An activity assay utilized fluorescence polarization, based on the binding of fluorescein-labeled peptide [the BH3 domain of BAD protein (F-Bad 6)] to Bcl-xL. A 384-well plate assay with mixtures of 10 drug compounds per well, combined with a fast plate reader, resulted in a throughput of 46,080 data points/day. Utilizing this screening format, 370,400 compounds were screened in duplicate and 425 inhibitors with an IC(50) below 100 microM were identified. The second assay format, affinity selection/mass spectrometry (ASMS), used ultrafiltration to separate Bcl-xL binders from nonbinders in mixtures of 2400 compounds. The bound species were subsequently separated from the protein and analyzed by flow injection electrospray mass spectrometry. Utilizing the ASMS format, 263,382 compounds were screened in duplicate and 29 binders with affinities below 100 microM were identified. Two novel classes of Bcl-xL inhibitors were identified by both methods and confirmed to bind (13)C-labeled Bcl-xL using heteronuclear magnetic resonance spectroscopy.

High-throughput screening of interactions between G protein-coupled receptors and ligands using confocal optics microscopy

Interactions of extracellular ligands with proteins in the cellular plasma membrane are the starting point for various intracellular signaling cascades. In the pharmaceutical industry, particular attention has been paid to G protein- coupled receptors (GPCRs), which are involved in various disease processes. In so-called high-throughput screening (HTS) campaigns, large medicinal chemistry compound libraries were searched for bioactive molecules that would either induce or inhibit the activity of a specific disease-relevant GPCR. In the respective drug discovery assays, the test compound typically competes with the physiological ligand for a binding site on the receptor. The transmembrane receptor is prepared in the form of membrane fragments or, as described here, in so-called virus-like particles (VLiPs). As hundreds of thousands of test compounds must be analyzed, there is a strict need for low volume binding assays to save the expensive bioreagents, and to reduce the consumption of the test compounds. In this chapter, we describe the application of confocal optics microscopy to measure GPCR ligand interactions in low microliter assay volumes.

Fluorescence polarization assays for high-throughput screening of neuropeptide FF receptors

We have developed the first fluorescence polarization assays of human neuropeptide FF2 receptors in 384-well microtiter plates. Assays are completed in a single well with no transfer, separation, or wash steps. The performance is suitable for high-throughput drug screening applications with regard to speed of analysis, magnitude of displaceable signal, precision, and sensitivity of various reagents. The rank order of potency of agonists and antagonists agrees well relative to the published radiometric filtration assays: DMe NPFF > NPFF > frog PP (Rana temporaria pancreatic polypeptide) > PQRFamide > BIBP 3226. The effect of highly colored compounds is very small on the polarization signal up to micromolar concentrations. The method serves as a simple and fast alternative to radioligand binding assays of antiobesity drug candidates related to NPFF receptors.

A high-throughput screen for identification of molecular mimics of Smac/DIABLO utilizing a fluorescence polarization assay

Resistance to apoptosis is afforded by inhibitor of apoptosis proteins (IAPs) which bind to and inhibit the caspases responsible for cleavage of substrates leading to apoptotic cell death. Smac (or DIABLO), a proapoptotic protein released from the mitochondrial intermembrane space into the cytosol, promotes apoptosis by binding to IAPs, thus reversing their inhibitory effects on caspases. We have developed a high-throughput fluorescence polarization assay utilizing a fluorescein-labeled peptide similar to the "IAP binding" domain of Smac N terminus complexed with the BIR3 domain of X-linked IAP (XIAP) to identify small-molecule mimics of the action of Smac. The IC(50)s of peptides and a tetrapeptidomimetic homologous to the N terminus of Smac demonstrated the specificity and utility of this assay. We have screened the National Cancer Institute "Training Set" of 230 compounds, with well-defined biological actions, and the "Diversity Set" of 2000 chemically diverse structures for compounds which significantly reduced fluorescence polarization. Highly fluorescing or fluorescence-quenching compounds (false positives) were distinguished from those which interfered with Smac peptide binding to the XIAP-BIR3 in a dose-dependent manner (true positives). This robust assay offers potential for high-throughput screening discovery of novel compounds simulating the action of Smac/DIABLO.