Ligand binding to transmembrane receptors on intact cells or membrane vesicles measured in a homogeneous 1-microliter assay format

High-Throughput Screening with HyperCyt® Flow Cytometry to Detect Small Molecule Formylpeptide Receptor Ligands

High-throughput flow cytometry (HTFC), enabled by faster automated sample processing, represents a promising high- content approach for compound library screening. HyperCyt® is a recently developed automated HTFC analysis system by which cell samples are rapidly aspirated from microplate wells and delivered to the flow cytometer. The formylpeptide receptor (FPR) family of G protein–coupled receptors contributes to the localization and activation of tissue-damaging leukocytes at sites of chronic inflammation. Here, the authors describe development and application of an HTFC screening approach to detect potential anti-inflammatory compounds that block ligand binding to FPR. Using a homogeneous no-wash assay, samples were routinely processed at 1.5 s/well (~2500 cells analyzed/sample), allowing a 96-well plate to be processed in less than 2.5 min. Assay sensitivity and accuracy were validated by detection of a previously documented active compound with relatively low FPR affinity (sulfinpyrazone, inhibition constant [Ki]=14 μM) from among a collection of 880 compounds in the Prestwick Chemical Library. The HyperCyt® system was therefore demonstrated to be a robust, sensitive, and highly quantitative method with which to screen lead compound libraries in a 96-well format.

A Modular, Fully Integrated Ultra-High-Throughput Screening System Based on Confocal Fluorescence Analysis Techniques

The rapid increase in size of compound libraries, as well as new targets emerging from the Human Genome Project, require progress in ultra-high-throughput screening (uHTS) systems. In a joint effort with scientists and engineers from the biotech and the pharmaceutical industry, a modular, fully integrated system for miniaturized uHTS was developed. The goal was to achieve high data quality in small assay volumes (1-4 μL) combined with reliable and unattended operation. Two new confocal fluorescence readers have been designed. One of the instruments is a 4-channel confocal fluorescence reader, measuring with 4 objectives in parallel. The fluorescence readout is based on single-molecule detection methods, allowing high sensitivity at low tracer concentrationsand delivering an information-rich output. The other instrument isa confocal fluorescence im aging reader, where the imagesare analyzed in terms of generic patternsand quantified in units of intensity per pixel. Both readers are spanning the application range from assays with isolated targets in homogenous solution or membrane vesiclebased assays (4-channel reader) to cell-based assays (imaging reader). Results from a comprehensive test on these assay types demonstrate the high quality and robustness of this screening system.

High-throughput flow cytometry for drug discovery

High-throughput flow cytometry exploits a novel many-samples/one-file approach to dramatically speed data acquisition, limit aspirated sample volume to as little as 2 μl/well and produce multisample data sets that facilitate automated analysis of samples in groups as well as individually. It has been successfully applied to both cell- and microsphere-based bioassays in 96- and 384-well formats, to screen tens-of-thousands of compounds and identify novel bioactive structures. High-content multiparametric analysis capabilities have been exploited for assay multiplexing, allowing the assessment of biologic selectivity and specificity to be an integral component of primary screens. These and other advances in the last decade have contributed to the application of flow cytometry as a uniquely powerful tool for probing biologic and chemical diversity and complex systems biology.

High-Throughput Lead Finding and Optimisation for GPCR Targets

Driven by past successes and the detailed knowledge of signalling cascades and physiological processes, G-protein-coupled receptors are taking a prominent place in the portfolios of many pharmaceutical companies. To successfully address this target class, scientists need not only a good understanding of the specific receptor under investigation, but also the right tools from assay technology, reagent production to a hit-to-lead process that acknowledges the importance of parameters beyond potency and embraces the gain in knowledge of the last decade. This manuscripts attempts to summarise some of the changes and progress made across the pharmaceutical industry to design an efficient and effective strategy for finding and optimising small molecules modulating the activity of GPCRs.

Calculation of photon-count number distributions via master equations

Fitting of photon-count number histograms is a way of analysis of fluorescence intensity fluctuations, a successor to fluorescence correlation spectroscopy. First versions of the theory for calculating photon-count number distributions have assumed constant emission intensity by a molecule during a counting time interval. For a long time a question has remained unanswered: to what extent is this assumption violated in experiments? Here we present a theory of photon-count number distributions that takes account of intensity fluctuations during a counting time interval. Theoretical count-number distributions are calculated via a numerical solution of Master equations (ME), which is a set of differential equations describing diffusion, singlet-triplet transitions, and photon emission. Detector afterpulsing and dead-time corrections are also included. The ME-theory is tested by fitting a series of photon-count number histograms corresponding to different lengths of the counting time interval. Compared to the first version of fluorescence intensity multiple distribution analysis theory introduced in 2000, the fit quality is significantly improved. It is discussed how a theory of photon-count number distributions, which assumes constant emission intensity during a counting time interval, may also yield a good fit quality. We argue that the spatial brightness distribution used in calculations of the fit curve is not the true spatial brightness distribution. Instead, a number of dynamic processes, which cause fluorescence intensity fluctuations, are indirectly taken into account via the profile adjustment parameters.

Rapid analysis of Forster resonance energy transfer by two-color global fluorescence correlation spectroscopy: trypsin proteinase reaction

In this study we introduce the combination of two-color global fluorescence correlation spectroscopy (2CG-FCS) and Förster resonance energy transfer (FRET) as a very powerful combination for monitoring biochemical reactions on the basis of single molecule events. 2CG-FCS, which is a new variation emerging from the family of fluorescence correlation spectroscopy, globally analyzes the simultaneously recorded auto- and cross-correlation data from two photon detectors monitoring the fluorescence emission of different colors. Overcoming the limitations inherent in mere auto- and cross-correlation analysis, 2CG-FCS is sensitive in resolving and quantifying fluorescent species that differ in their diffusion characteristics and/or their molecular brightness either in one or both detection channels. It is able to account for effects that have often been considered as sources of severe artifacts in two-color and FRET measurements, the most prominent artifacts comprising photobleaching, cross talk, or concentration variations in sample preparation. Because of its very high statistical accuracy, the combination of FRET and 2CG-FCS is suited for high-throughput applications such as drug screening. Employing beam scanning during data acquisition even further enhances this capability and allows measurement times of <2 s. The improved performance in monitoring a FRET sample was verified by following the protease cleavage reaction of a FRET-active peptide. The FRET-inactive subpopulation of uncleaved substrate could be correctly assigned, revealing a substantial portion of inactive or missing acceptor label. The results were compared to those obtained by two-dimensional fluorescence intensity distribution analysis.

Investigating cellular signaling reactions in single attoliter vesicles

Understanding cellular signaling mediated by cell surface receptors is key to modern biomedical research and drug development. The discovery of a growing number of potential molecular targets and therapeutic compounds requires downscaling and accelerated functional screening. Receptor-mediated cellular responses are typically investigated on single cells or cell populations. Here, we show how to monitor cellular signaling reactions at a yet unreached miniaturization level. On the basis of our observations, cytochalasin induces mammalian cells to extrude from their plasma membrane submicrometer-sized native vesicles. They comprise functional cell surface receptors correctly exposing their extracellular ligand binding sites on the outer vesicle surface and retaining cytosolic proteins in the vesicle interior. As a prototypical example, ligand binding to the ionotropic 5-HT(3) receptor and subsequent transmembrane Ca(2+) signaling were monitored in single attoliter vesicles. Thus, native vesicles are the smallest autonomous containers capable of performing cellular signaling reactions under physiological conditions. Because a single cell delivers about 50 native vesicles, which can be isolated and addressed as individuals, our concept allows multiple functional analyses of individual cells having a limited availability and opens new vistas for miniaturized bioanalytics.

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.

Endothelin receptor in virus-like particles: ligand binding observed by fluorescence fluctuation spectroscopy

The functional analysis of transmembrane receptor proteins is frequently hampered by the difficulty to produce sufficiently homogeneous receptor preparations that preserve the physiological biomembrane integration of the receptor protein. To improve the receptor protein density in the lipid bilayer and to maintain the physiological lipid-protein environment, a novel method has been established that enables the selective integration of transmembrane receptors into a virus-like particle (VLiP). Here we have studied the binding of tetramethylrhodamine-labeled endothelin-1 (TMR-ET-1) to VLiP-integrated endothelin A receptor (ET(A)R) by fluorescence fluctuation spectroscopy. The concentration of TMR-ET-1 was determined by fluorescence correlation spectroscopy (FCS). These measurements also confirmed that the free ligand is monomeric in solution in our experiments. Fluorescence intensity distribution analysis (FIDA) was used to quantify the fraction of ligands bound to ET(A)Rs in the VLiPs. For the interaction between ET-1 and VLiP-integrated ET(A)Rs, K(D) values of 0.5 nM and 0.3 nM were determined from ligand and receptor titration experiments, respectively. For comparison, a FIDA analysis was also carried out with ET(A)Rs in membrane fragments derived from an ET(A)R-overexpressing mammalian cell line, which yielded a similar K(D) of 0.2 nM. In addition, we examined the binding competition of a set of reference compounds to VLiP-ET(A)Rs in the presence of ET-1 and obtained K(i) values similar to those reported in the literature. Our results demonstrate that integration into VLiPs does not change the binding properties of the ET(A)Rs. FIDA analysis of VLiP-integrated receptors shows great promise for highly miniaturized and fast compound testing in the pharmaceutical industry.

Novel fluorescence based receptor binding assay method for receptors lacking ligand conjugates with preserved affinity: study on estrogen receptor alpha

In this study a novel general approach is presented that allows for a straightforward design of receptor binding assays. This principle of a receptor binding assay is applied to the estrogen receptor, which is important in the management of breast cancer and for the estimation of the estrogenic potency of chemicals in the environment. The inhibitory concentrations to reduce cell proliferation in 50% of controls for 17-beta-estradiol, 4-hydroxy tamoxifen, and tamoxifen are determined to be 61 nM, 33 nM, and 17 microM, respectively. The measurement time of the nanoparticle based immunoassay format is 3 s. The Z' factor, which is calculated to be 0.89, reflects the excellent assay performance.

Confocal optics microscopy for biochemical and cellular high-throughput screening

In recent years, both academia and pharmaceutical industry have produced significant advances in confocal detection and spectroscopy by laser-induced fluorescence. Confocal fluorescence studies provide information on identity, size, diffusion coefficient and concentration of the fluorescently labeled entity. This enables the establishment of sophisticated biochemical drug screening assays using the multitude of fluorescence parameters that can be observed (e.g. molecular brightness, fluorescence lifetime, anisotropy, resonance energy transfer). In cellular screening assays, confocality introduces spatial resolution in the vertical direction and reduces background fluorescence from outside the focal plane. Confocal HTS systems focusing on femtoliter-sized observation volumes allow for assay volumes far beyond current limits.

Highly sensitive fluorescence detection technology currently available for HTS

Homogeneous fluorescence methods are providing an important tool for HTS technologies. A wide range of different techniques have been established on the market, with read-outs ranging from total fluorescence intensity to statistical analysis of fluorescence fluctuations for biochemical assays or fluorescence imaging techniques for cellular systems. Each method has its own advantages and limitations, which have to be accounted for when designing a specific assay. Here, recently developed fluorescence techniques and some of their applications, with a particular focus on sensitivity, are summarized and their principles are presented.

Single-molecule detection technologies in miniaturized high-throughput screening: fluorescence intensity distribution analysis

Single-molecule detection technologies are becoming a powerful readout format to support ultra-high-throughput screening. These methods are based on the analysis of fluorescence intensity fluctuations detected from a small confocal volume element. The fluctuating signal contains information about the mass and brightness of the different species in a mixture. The authors demonstrate a number of applications of fluorescence intensity distribution analysis (FIDA), which discriminates molecules by their specific brightness. Examples for assays based on brightness changes induced by quenching/dequenching of fluorescence, fluorescence energy transfer, and multiple-binding stoichiometry are given for important drug targets such as kinases and proteases. FIDA also provides a powerful method to extract correct biological data in the presence of compound fluorescence.