A 1536-well cAMP assay for Gs- and Gi-coupled receptors using enzyme fragmentation complementation

Tannic acid acts as an agonist of the dopamine D2L receptor, regulates immune responses, and ameliorates experimentally induced colitis in mice

Tannic acid (TA) is an herbal polyphenol containing a galloyl group that has been prescribed to treat gastroenteritis, diarrhea, and irritable bowel syndrome. TA has anti-inflammatory, anti-cancer, and anti-viral properties; however, the molecular mechanisms of these potential therapeutic effects are still largely unknown. Here, we examined the ability of TA to induce anti-inflammatory responses. TA was found to be an agonist of the dopamine D2L receptor. TA reduced interferon (IFN)-γ and interleukin (IL)-1β secretion but upregulated tumor necrosis factor α and IL-10 secretion from lipopolysaccharide (LPS)-stimulated mouse splenocytes. TA also reduced IFN-γ secretion but enhanced IL-10 secretion from anti-cluster of differentiation (CD) 3/CD28 antibody-stimulated splenocytes. An immune subset study confirmed that TA regulated cytokine secretion by various types of immune cells in the context of stimulation with LPS or anti-CD3/CD28 antibodies. Administration of TA to mice with experimentally induced colitis strikingly suppressed weight loss, colon shrinkage, and IL-17 secretion from mesenteric lymph node lymphocytes in response to CD3/CD28 stimulation. These data suggest that TA suppresses inflammatory responses in colitis by regulating cytokine secretion by immune cells in the colon.

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Tannic acid is an agonist of the dopamine D2L receptor.

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Tannic acid suppresses IFN-γ secretion by LPS-stimulated splenocytes.

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Tannic acid modulates anti-CD3/CD28 antibody-stimulated cytokine levels in CD4⁺ T cells.

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Tannic acid ameliorates dextran sodium salt (DSS)-induced colitis in C57BL/6 mice.

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Tannic acid reduces production of IL-17 in DSS-induced colitis.

Present and future approaches to screening of G-protein-coupled receptors

As G-protein-coupled receptors (GPCRs) mediate a multitude of cellular signal transduction events, affecting more or less all human disease areas, it is, therefore, no surprise that they comprise the largest family of current drug targets. Screening of compounds interacting with GPCRs has developed during the past decade from receptor binding assays, to various functional determination of coupling to G-proteins, and, more recently, G-protein-independent signal transduction events. Additional opportunities have been presented in drug discovery through novel pharmacological properties obtained for receptor dimers and by identification of ligands for orphan GPCRs. Furthermore, high-throughput formats and automation has substantially facilitated and accelerated the screening process providing powerful tools in improving modern drug discovery.

Characterization of G-protein coupled receptor modulators using homogeneous cAMP assays

More than two-thirds of all known G-protein coupled receptors are known to modulate the function of adenylate cyclase resulting in altered levels of cAMP. In turn, cAMP fluctuations transform agonist binding events into physiological changes in cell behavior. The advent of nonradioactive, homogeneous methods of measuring intracellular cAMP has enabled the rapid growth of drug discovery and research applications for these GPCR targets. In this chapter, we describe a nonradioactive, chemiluminescent cAMP detection method using enzyme fragment complementation technology to detect a wide range of GPCR modulators which is also suitable for high-throughput screening.

Tools for GPCR drug discovery

G-protein-coupled receptors (GPCRs) mediate many important physiological functions and are considered as one of the most successful therapeutic targets for a broad spectrum of diseases. The design and implementation of high-throughput GPCR assays that allow the cost-effective screening of large compound libraries to identify novel drug candidates are critical in early drug discovery. Early functional GPCR assays depend primarily on the measurement of G-protein-mediated 2nd messenger generation. Taking advantage of the continuously deepening understanding of GPCR signal transduction, many G-protein-independent pathways are utilized to detect the activity of GPCRs, and may provide additional information on functional selectivity of candidate compounds. With the combination of automated imaging systems and label-free detection systems, such assays are now suitable for high-throughput screening (HTS). In this review, we summarize the most widely used GPCR assays and recent advances in HTS technologies for GPCR drug discovery.

A new homogeneous high-throughput screening assay for profiling compound activity on the human ether-a-go-go-related gene channel

Long QT syndrome, either inherited or acquired from drug treatments, can result in ventricular arrhythmia (torsade de pointes) and sudden death. Human ether-a-go-go-related gene (hERG) channel inhibition by drugs is now recognized as a common reason for the acquired form of long QT syndrome. It has been reported that more than 100 known drugs inhibit the activity of the hERG channel. Since 1997, several drugs have been withdrawn from the market due to the long QT syndrome caused by hERG inhibition. Food and Drug Administration regulations now require safety data on hERG channels for investigative new drug (IND) applications. The assessment of compound activity on the hERG channel has now become an important part of the safety evaluation in the process of drug discovery. During the past decade, several in vitro assay methods have been developed and significant resources have been used to characterize hERG channel activities. However, evaluation of compound activities on hERG have not been performed for large compound collections due to technical difficulty, lack of throughput, and/or lack of biological relevance to function. Here we report a modified form of the FluxOR thallium flux assay, capable of measuring hERG activity in a homogeneous 1536-well plate format. To validate the assay, we screened a 7-point dilution series of the LOPAC 1280 library collection and reported rank order potencies of ten common hERG inhibitors. A correlation was also observed for the hERG channel activities of 10 known hERG inhibitors determined in this thallium flux assay and in the patch clamp experiment. Our findings indicate that this thallium flux assay can be used as an alternative method to profile large-volume compound libraries for compound activity on the hERG channel.

Development of a high-throughput assay for monitoring cAMP levels in cardiac ventricular myocytes

G-protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors involved in cell signal transduction. Many of these GPCRs convey their pharmacological actions by regulating intracellular levels of 3',5'-cyclic adenosine monophosphate (cAMP). Although the heart expresses more than 100 GPCRs, drug agonists for approximately one third of these GPCRs have not been identified. The goal of this project was to initiate the development of a high-throughput screening assay for monitoring cAMP in the heart. Neonatal rat cardiac ventricular myocytes were isolated and cultured on coverslips (whole-cell patch clamp recording) or in 96-well plates (fluorescent imaging plate reader measurements). Cells were infected with adenovirus expressing either beta-galactosidase (AdLacZ) or a mutant cyclic nucleotide-gated (CNG) channel containing the double mutation C460W/E583M (AdCNG). Addition of 2 microM forskolin along with 100 microM 3-isobutyl-1-methylxanthine, to increase intracellular cAMP, activated a cation current in myocytes infected with the AdCNG. In myocytes loaded with the fluorescent Ca indicator Fluo-4, stimulation with forskolin, epinephrine, norepinephrine, or the beta-adrenergic receptor agonist isoproterenol increased the fluorescent signal indicative of Ca influx through the CNG channel. In conclusion, CNG channels are readily expressed in cultured cardiac myocytes and may be utilized in high-throughput screening assays of intracellular cAMP.

Biochemical Detection of cGMP From Past to Present: An Overview

Cyclic guanosine monophosphate (cGMP), generated via the guanylate cyclase (GC)-catalyzed conversion from GTP, is unequivocally recognized as crucial second messenger, intimately involved in the regulation of a broad range of physiological processes such as long term potentiation, blood pressure regulation, or platelet aggregation (for review: Hobbs 2000). Since its first identification in rat urine by Ashman and co-workers (1963), various approaches have been conceived and established to quantify cGMP in biological samples, or to detect cGMP as the reaction product of enzymatic assays, allowing the determination of kinetic parameters. These approaches have evolved from laborious handling of small numbers of samples with average sensitivity to highly developed biochemical detection assays allowing the processing of very large numbers of samples. The present article focuses upon the history of biochemical cGMP detection from the pioneering work of the early years to the actual state-of-the-art approaches for the detection of this important biological messenger.

A 1,536-well [(35)S]GTPgammaS scintillation proximity binding assay for ultra-high-throughput screening of an orphan galphai-coupled GPCR

Members of the superfamily of seven transmembrane receptors, known as G protein-coupled receptors (GPCRs), are important targets for many therapeutic areas in drug discovery. A homogeneous guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) scintillation proximity assay (SPA) binding assay targeting a Galphai-coupled GPCR recombinantly expressed in membranes of Chinese hamster ovary (CHO) cells was developed and miniaturized into 1,536-well plate format. The primary ultra-high-throughput screen of the entire compound collection was accomplished on the Kalypsys (San Diego, CA) robotic platform at a concentration of 8 muM using the 1,536-well [(35)S]GTPgammaS SPA binding functional assay. The signal-to-noise ratio of the primary screen was approximately 2.1-fold, and the plate coefficient of variation for the compound field was approximately 11%. The hit rate from the primary screen for receptor agonists at >35% activity was approximately 0.3%. Primary hits were cherry-picked, confirmed in triplicate, counterscreened against untransfected CHO cell membranes, and further analyzed in a cyclic AMP functional assay, resulting in 34 leads for optimization.

Comparison on functional assays for Gq-coupled GPCRs by measuring inositol monophospate-1 and intracellular calcium in 1536-well plate format

Cell-based functional assays used for compound screening and lead optimization play an important role in drug discovery for G-protein coupled receptors (GPCRs). Cell-based assays can define the role of a compound as an agonist, antagonist or inverse agonist and can provide detailed information about the potency and efficacy of a compound. In addition, cell-based screens can be used to identify allosteric modulators that interact with sites other than the binding site of the endogenous ligand. Intracellular calcium assays which use a fluorescent calcium binding dye (such as Fluo-3, Fluo-4 or Fura-2) have been used in compound screening campaigns to measure the activity of Gq-coupled GPCRs. However, such screening methodologies require a special instrumentation to record the rapid change in intracellular free calcium concentration over time. The radioactive inositol 1,4,5- triphosphate (IP(3)) assay measures (3)H-inositol incorporation and is another traditional assay for the assessment of Gq-coupled GPCR activity, but it is not suitable for screening of large size compound collections because it requires a cell wash step and generates radioactive waste. To avoid these limitations, we have optimized and miniaturized a TR-FRET based IP-One assay that measures inositol monophosphate in a 1536-well plate format. This assay is homogenous, non-radioactive and does not require a kinetic readout. It has been tested with the cell lines expressing M(1) acetylcholine, FFAR1, vasopressin V1b, or Neuropeptide S receptors. The activities of antagonists determined in the IP-One assay correlated well with these measured in the intracellular calcium assay while the correlation of agonist activities might vary from cell line to cell line. This IP-One assay offers an alternative method for high throughput screening of Gq-coupled GPCRs without using costly kinetic plate readers.

Quantitative high-throughput screening using a live-cell cAMP assay identifies small-molecule agonists of the TSH receptor

The thyroid-stimulating hormone (TSH; thyrotropin) receptor belongs to the glycoprotein hormone receptor subfamily of 7-transmembrane spanning receptors. TSH receptor (TSHR) is expressed mainly in thyroid follicular cells and is activated by TSH, which regulates the growth and function of thyroid follicular cells. Recombinant TSH is used in diagnostic screens for thyroid cancer, especially in patients after thyroid cancer surgery. Currently, no selective small-molecule agonists of the TSHR are available. To screen for novel TSHR agonists, the authors miniaturized a commercially available cell-based cyclic adenosine 3',5' monophosphate (cAMP) assay into a 1536-well plate format. This assay uses an HEK293 cell line stably transfected with the TSHR coupled to a cyclic nucleotide gated ion channel as a biosensor. From a quantitative high-throughput screen of 73,180 compounds in parallel with a parental cell line (without the TSHR), 276 primary active compounds were identified. The activities of the selected active compounds were further confirmed in an orthogonal homogeneous time-resolved fluorescence cAMP-based assay. Forty-nine compounds in several structural classes have been confirmed as the small-molecule TSHR agonists that will serve as a starting point for chemical optimization and studies of thyroid physiology in health and disease.

Miniaturization and automation of an ubiquitin ligase cascade enzyme-linked immunosorbent assay in 1,536-well format

Enzyme-linked immunosorbent assays (ELISAs) are a long established and widely used assay format for drug discovery and diagnostics. They offer many advantages over homogeneous assay formats, including high sensitivity and separation (wash) steps that remove detection-interfering compounds. Many high-throughput screening assays are now performed in miniaturized formats (1,536- and 3,456-well plates) for higher throughput and lower reagent consumption. With miniaturization, separation steps in assays such as ELISA can become difficult to implement. Here we report on the implementation of the Kalypsys, Inc. (San Diego, CA) 1,536-well plate washer to enable the successful miniaturization and full automation of an ELISA that monitors ubiquitin ligase activity. The 1,536-well plate ELISA was robust and used for the high-throughput screening of a large screening collection (>1 million compounds).

Beta galactosidase complementation: a cell-based luminescent assay platform for drug discovery

Many cell-based assays interrogating cell pathway activation employ protocols that require microscopic imaging techniques. However, such assays are not in general widely adopted for primary screening. Protein complementation, particularly of enzymes, provides an alternative approach for cell pathway analysis, with a principal advantage that is amenable to high throughput screening using microtiter plate protocols. Notably, alpha complementation of the enzyme beta-galactosidase has been exploited as a technology in this regard, using substrates that generates luminescent signals. This review describes the various uses of this flexible technology to cell-based assay development.

Ultra-High-Throughput Screening for Antagonists of A Gi-Coupled Receptor in A 2.2-μl 3,456-Well Plate Format CyclicAMP Assay

3',5'-Cyclic adenosine monophosphate (cAMP) is a common intracellular second messenger that enables cells to respond to external stimuli. Measurement of intracellular cAMP concentrations is thus widely used for studying guanosine triphosphate binding protein-coupled receptors (GPCRs), which make up a large class of pharmaceutical drug targets. Although several assay technologies exist to measure cAMP, most are not suitable for ultra-high-throughput screening (uHTS), as is often required for screening large (greater than 1 million) chemical libraries for the identification of suitable leads for drug development. Here we report that the enzyme fragment complementation assay, a homogeneous gain of signal assay based on complementation of two fragments of a beta-galactosidase enzyme, is compatible with uHTS requirements of a 2.2-microl total assay volume in 3,456-well plate format. We describe the miniaturization of this assay into 3,456-well plate format exhibiting comparable sensitivity and plate statistics to those of a 384-well assay and the application of this assay in uHTS for the identification of antagonists of a Gi-coupled receptor.

A miniaturized cell-based fluorescence resonance energy transfer assay for insulin-receptor activation

This report describes the development, optimization, and implementation of a miniaturized cell-based assay for the identification of small-molecule insulin mimetics and potentiators. Cell-based assays are attractive formats for compound screening because they present the molecular targets in their cellular environment. A fluorescence resonance energy transfer (FRET) cell-based assay that measures the insulin-dependent colocalization of Akt2 fused with either cyan fluorescent protein or yellow fluorescent protein to the cellular membrane was developed. This ratiometric FRET assay was miniaturized into a robust, yet sensitive 3456-well nanoplate assay with Z' factors of approximately 0.6 despite a very small assay window (less than twofold full activation with insulin). The FRET assay was used for primary screening of a large compound collection for insulin-receptor agonists and potentiators. To prioritize compounds for further development, primary hits were tested in two additional assays, a biochemical time-resolved fluorescence resonance energy transfer assay to measure insulin-receptor phosphorylation and a translocation-based imaging assay. Results from the three assays were combined to yield 11 compounds as potential leads for the development of insulin mimetics or potentiators.

Screening the receptorome

The term 'receptorome' is now being used to describe receptors, ion channels and transporters in the human genome that are potential drug targets. These proteins comprise a considerable fraction of the human genome, and include the G protein-coupled receptors, which are the targets for many medications. In this review, we summarize recent advances in the field, including the concept that the ultimate goal of drug discovery may not be the development of highly selective single-target drugs, the idea that potential side-effects can also be the goal of multi-target drug screening, and a discussion of the application of computational screening and public domain databases available to interested investigators.

A Homogeneous Cell-Based Assay to Measure Nuclear Translocation Using β-Galactosidase Enzyme Fragment Complementation

Positional complementation describes the use of homogeneous assays using beta- galactosidase (beta gal) enzyme fragment complementation to detect cellular protein translocation. This phenomenon occurs when the protein of interest, recombinantly expressed as a fusion protein with a modified alpha fragment of beta gal, translocates to a cellular compartment expressing an enzyme acceptor fragment of the enzyme. When these fragments interact, high-affinity complementation occurs, and a signal is generated that is then detected upon cell lysis. In the present paper the use of positional complementation is exemplified by measuring nuclear translocation of the glucocorticoid receptor in Chinese hamster ovary-K1 cells. The approach thus provides for homogeneous protocols, in an endpoint microtiter plate assay format, without the use of either imaging or reporter gene techniques. Consequently, these characteristics suggest that the technique is suitable for automated instrumentation protocols used in high throughput screening campaigns designed to identify activators or inhibitors of nuclear translocation.

Application of division arrest technology to cell-based HTS: comparison with frozen and fresh cells

Cell-based functional assays are becoming popular in many HTS laboratories because of recent advances in detection and automation technologies. However, the supply of large amounts of live cells with consistent cellular response for day-to-day screening operations over several days/weeks is a tremendous challenge. The high cost of cell culture, labor-intensive nature of the work, and inherent variability in cellular responses from time to time tend to be prohibitive for extensive applications of cell-based assays in HTS. We therefore tested division-arrested cells that were prepared in a single batch and frozen at -80 degrees C before use in several cell-based assays and in a robotic screening campaign. Chinese hamster ovary cells expressing a Gq-coupled receptor were analyzed for the agonist-induced intracellular Ca2+ response measured on a fluorescent imaging plate reader. In this case, the division-arrested cells showed consistent agonist-induced intracellular Ca2+ concentration response as reflected by signal-to-basal ratio and EC50 even 48 h after cell plating. In comparison, the responses from untreated frozen cells and fresh cells declined significantly approximately 30 h after cell plating. In other cell-based assays tested (cyclic AMP assay, reporter gene beta-lactamase assay, and ion-channel assay), the division-arrested cells performed as well as frozen, or fresh cells. We thus conclude that the use of alternate strategies such as frozen cells or division-arrested cells may alleviate the need for several batches of cell plating each day during HTS while maintaining the desired robotic throughput and assay quality.

Miniaturization of absorbance assays using the fluorescent properties of white microplates

Miniaturization of high-throughput screening (HTS) assays has several obvious advantages, including increased throughput and lower cost by reduction in reagent consumption. Although absorbance assays are widely used in research laboratories, their application for HTS in a low-volume format has been met with mixed success because they are difficult to miniaturize. Challenges for the miniaturization of absorbance assays include low signal due to short path lengths and meniscus distortions in small well sizes. Here we describe a method to miniaturize absorbance assays to standard, white, low-volume 384-well and 1536-well microplates using a fluorometric plate reader for detection. The premise of this absorbance assay is based on the fluorescent properties of white microplates and the ability of a colored product to quench the fluorescence signal from the plate by absorbing either the excitation light or the emission light. This method was applied to the detection of inorganic phosphate using Quinaldine red and Malachite green dyes and to the monitoring of alkaline phosphatase hydrolysis of p-nitrophenyl phosphate. These assays can be carried out in low volumes, give robust screening statistics, and can be accomplished with a simple, inexpensive fluorometric plate reader.