High Throughput Quantitation of cAMP Production Mediated by Activation of Seven Transmembrane Domain Receptors

Luciferase-based GloSensor™ cAMP assay: Temperature optimization and application to cell-based kinetic studies

G protein-coupled receptors (GPCRs) are an important receptor superfamily and common therapeutic targets. The second messenger cyclic adenosine monophosphate (cAMP) is a key mediator in many GPCR signaling pathways. Monitoring intracellular cAMP levels can help identify orthosteric agonists and antagonists, as well as allosteric modulators. In this regard, luminescence-based biosensors have revolutionized our ability to monitor GPCR signaling kinetics. The GloSensor™ cAMP assay enables real-time monitoring of signaling downstream of many GPCRs. However, it is crucial to optimize assay conditions such as temperature. As well, it has not been reported whether the effects of temperature on biosensor activity are reversible. Here, we describe the temperature sensitivity and reversibility of the GloSensor™ cAMP assay, and which GloSensor™ version is optimal for measuring cytosolic cAMP. We also present a detailed protocol for monitoring cAMP levels in live cells expressing endogenous or exogenous GPCRs. Temperature optimization studies were carried out using HEK293H cells transiently transfected with the adenosine receptor A2a and the GloSensor™ plasmid (pGloSensor-20F or -22F). We found that preincubation and luminescence reading at room temperature were optimal as compared to higher temperatures. As well, the GloSensor-22F biosensor had a superior signal-to-background ratio and the effect of temperature on biosensor activity was reversible. However, thermal instability of the biosensor may pose a problem for in vivo studies. Nevertheless, the GloSensor™ cAMP assay can be applied to analyze signaling by a wide range of GPCRs for drug discovery purposes.

Modeling Progressive Fibrosis with Pluripotent Stem Cells Identifies an Anti-fibrotic Small Molecule

Progressive organ fibrosis accounts for one-third of all deaths worldwide, yet preclinical models that mimic the complex, progressive nature of the disease are lacking, and hence, there are no curative therapies. Progressive fibrosis across organs shares common cellular and molecular pathways involving chronic injury, inflammation, and aberrant repair resulting in deposition of extracellular matrix, organ remodeling, and ultimately organ failure. We describe the generation and characterization of an in vitro progressive fibrosis model that uses cell types derived from induced pluripotent stem cells. Our model produces endogenous activated transforming growth factor β (TGF-β) and contains activated fibroblastic aggregates that progressively increase in size and stiffness with activation of known fibrotic molecular and cellular changes. We used this model as a phenotypic drug discovery platform for modulators of fibrosis. We validated this platform by identifying a compound that promotes resolution of fibrosis in in vivo and ex vivo models of ocular and lung fibrosis.

Comparison of the Full-Length and 152~528 Truncate of Human Cyclic Nucleotide Phosphodiesterase 4B2 for the Characterization of Inhibitors

AIM AND OBJECTIVE

Human full-length cyclic nucleotide phosphodiesterase isozyme 4B2 (hPDE4B2) as the target for screening and characterizing inhibitors suffers from low activity yield and the coexistence of two conformational states bearing different affinities for (R)-rolipram. Hence, the 152~528 truncate of hPDE4B2 existing only in the low-affinity conformation state for (R)-rolipram was compared against the full-length hPDE4B2 to characterize inhibitors.

MATERIALS AND METHODS

With 6His-SUMO tag at the N-terminus, both the full-length hPDE 4B2 (SF-hPDE4B2) and the 152~528 truncate (ST-hPDE4B2) were expressed in Escherichia coli cells, purified through Ni-NTA column and compared for the characterization of inhibitors. The inhibition constants (Ki) of some synthesized rolipram analogues against both targets were determined with 96-well microplate through the coupled action of monophosphatase on AMP and spectrophotometric assay of phosphate with malachite green.

RESULTS

After affinity purification with Ni2+-NTA column, ST-hPDE4B2 showed about 30-fold higher specific activity and 100-fold higher activity yield than SF-hPDE4B2; Ki of (R)-rolipram on ST-hPDE4B2 was consistent with that on the low-affinity state of the untagged full-length hPDE4B2 expressed in insect cells. Of some representative rolipram analogues as inhibitors, a dual-logarithm model quantitatively described their monotonic association, and Ki from 0.010 mM to 8.5 mM against SF-hPDE4B2 was predicted from Ki against ST-hPDE4B2, supporting the discovery of consistent hits by the use of both targets with a pair of properly-set cutoffs.

CONCLUSION

ST-hPDE4B2 with much higher activity yield may be a favorable alternative target to characterize/screen rolipram analogues as hPDE4B inhibitors in high-throughput mode.

Insights into GPCR pharmacology from the measurement of changes in intracellular cyclic AMP; advantages and pitfalls of differing methodologies

It is clear that the G protein-coupled receptor family play a key role in the pharmaceutical industry, with a significant proportion of approved drugs targeting this protein class. While our growing understanding of the complexity of G protein-coupled receptor pharmacology is playing a key role in the future success of these endeavours, with allosteric mechanisms now well integrated into the industrial community and G protein-independent signalling mechanisms establishing themselves as novel phenomenon to be exploited, it is still possible to underestimate the complexity of G protein signal transduction mechanisms and the impact that inappropriate study of these mechanisms can have on data interpretation. In this manuscript we review different approaches to measuring the cAMP signal transduction pathway, with particular emphasis on key parameters influencing the data quality and biological relevance.

Receptor-drug interaction: europium employment for studying the biochemical pathway of g-protein-coupled receptor activation

In medicinal chemistry field, the biochemical pathways, involved in 7-transmembrane domains G-protein coupled receptors (GPCRs) activation, are commonly studied to establish the activity of ligands towards GPCRs. The most studied steps are the measurement of activated GTP-alpha subunit and stimulated intracellular cAMP. At the present, many researchers defined agonist or antagonist activity of potential GPCRs drugs employing [(35)S]GTPgammaS or [(3)H]cAMP as probes. Recently, the corresponding lanthanide labels Eu-GTP and Eu-cAMP as alternative to radiochemicals have been developed because they are highly sensitive, easy to automate, easily synthesized, they display a much longer shelf-life and they can be used in multilabel experiments. In the present review, the receptor-drug interaction by europium employment for studying the biochemical pathway of GPCR activation has been focused. Moreover, comparative studies between lanthanide label probes and the corresponding radiolabeled compounds have been carried out.

Measuring spatiotemporal dynamics of cyclic AMP signaling in real-time using FRET-based biosensors

Cyclic AMP governs many fundamental signaling events in eukaryotic cells. Although cAMP signaling has been a major research focus for a long time, recent technological developments are revealing novel aspects of this paradigmatic pathway. In this chapter, we give an overview over current fluorescence resonance energy transfer (FRET)-based sensors for detection of cAMP dynamics, and their application in monitoring local, compartmentalized cAMP signals within living cells. A basic step-by-step protocol is given for conducting a FRET experiment in primary cells with a unimolecular cAMP sensor, which can easily be adapted to a user's specific requirements.

Cell-based assays for high-throughput screening

Cell-based assays represent approximately half of all high-throughput screens currently performed. Here, we review in brief the history and status of high-throughput screening (HTS), and summarize some of the challenges and benefits associated with the use of cell-based assays in HTS. Approaches for successful experimental design and execution of cell-based screens are introduced, including strategies for assay development, implementation of primary and secondary screens, and target identification. In doing so, we hope to provide a comprehensive review of the cell-based HTS process and an introduction to the methodologies and techniques used.

Screening technologies for G protein-coupled receptors: from HTS to uHTS

The discovery of drugs for G protein-coupled receptors (GPCRs) has traditionally been very successful, even before the structural nature of these molecular targets was elucidated. Over the years, this family of proteins has become more important in the understanding and treatment of different human pathologies, representing today close to 30% of the molecular targets of all marketed drugs. The sequencing of the human genome unveiled the existence of many new GPCRs and this has increased even more the interest of this family of proteins as potential drug targets. Today the search for compounds that interfere or modulate the function of GPCRs is one of the major focuses of pharmaceutical companies. The understanding of the molecular events that take place upon receptor activation, together with the need of testing large chemical libraries, has resulted in the development of a variety of methods and technologies to measure the activity of these receptors. In this chapter we will review most of the assay technologies currently in use for "in vitro" pharmacological screening, their evolution, their capabilities, and their limitations.

Introduction: cell-based assays for high-throughput screening

Cell-based assays represent approximately half of all high-throughput screens (HTS) currently performed. Here we review the history and status of HTS, and summarize some of the challenges and benefits associated with the use of cell-based assays in HTS, drawing upon themes that will reemerge in subsequent chapters in this book. Approaches for successful experimental design and execution of cell-based HTS are introduced, including strategies for assay development, implementation of primary and secondary screens, and target identification. In doing so, we hope to provide a comprehensive review of the cell-based HTS process and an introduction to the methodologies and techniques described in this book.

Cell-based assays in GPCR drug discovery

G protein-coupled receptors (GPCRs) transmit extracellular signals into the intracellular space, and play key roles in the physiological regulation of virtually every cell and tissue. Characteristic for the GPCR superfamily of cell surface receptors are their seven transmembrane-spanning alpha-helices, an extracellular N terminus and intracellular C-terminal tail. Besides transmission of extracellular signals, their activity is modulated by cellular signals in an auto- or transregulatory fashion. The molecular complexity of GPCRs and their regulated signaling networks triggered the interest in academic research groups to explore them further, and their drugability and role in pathophysiology triggers pharmaceutical research towards small molecular weight ligands and therapeutic antibodies. About 30% of marketed drugs target GPCRs, which underlines the importance of this target class. This review describes current and emerging cellular assays for the ligand discovery of GPCRs.

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.

A flow-through fluorescence polarization detection system for measuring GPCR-mediated modulation of cAMP production

A flow-through fluorescence polarization (FP) detection system that makes use of a novel high-performance liquid chromatography (HPLC) fluorescence detector modified with polarization filters was developed. This flow-through FP detection system was evaluated by using a novel and very cost-effective bioassay for cyclic adenosine monophosphate (cAMP). The bioassay was first evaluated and optimized in an FP plate reader format and subsequently in a flow-through bioassay setup. The principle of the bioassay is based on the competition of cAMP and a fluorescent cAMP derivative for the cAMP binding domain of protein kinase A. cAMP could accurately be determined over a range of 0.8 to 30 pmol/well in the plate reader FP assay and over a range of 0.3 to 50 pmol/well in the flow-through FP assay setup. High Z' factors (i.e., 0.89 for the plate reader and 0.93 for the flow-through FP cAMP assay, respectively) indicated robust assays. Finally, functional cAMP signaling of the human histamine H(3) G-protein-coupled receptor (GPCR) in cell cultures was measured with both assay formats with good sensitivities and assay windows. The pEC(50) values obtained in both assay formats were in accordance with those obtained with standard methods. The flow-through FP detection system could thus be used as a cost-effective alternative to FP plate reader assays. Moreover, the novel flow-through FP detection system for cAMP constitutes a good analytical tool to be used in the GPCR research field as an alternative to the use of FP plate readers or radioactive laboratories nowadays used for cAMP measurements.

Heterotrimeric G proteins and the single-transmembrane domain IGF-II/M6P receptor: functional interaction and relevance to cell signaling

The G protein-coupled receptor (GPCR) family represents the largest and most versatile group of cell surface receptors. Classical GPCR signaling constitutes ligand binding to a seven-transmembrane domain receptor, receptor interaction with a heterotrimeric G protein, and the subsequent activation or inhibition of downstream intracellular effectors to mediate a cellular response. However, recent reports on direct, receptor-independent G protein activation, G protein-independent signaling by GPCRs, and signaling of nonheptahelical receptors via trimeric G proteins have highlighted the intrinsic complexities of G protein signaling mechanisms. The insulin-like growth factor-II/mannose-6 phosphate (IGF-II/M6P) receptor is a single-transmembrane glycoprotein whose principal function is the intracellular transport of lysosomal enzymes. In addition, the receptor also mediates some biological effects in response to IGF-II binding in both neuronal and nonneuronal systems. Multidisciplinary efforts to elucidate the intracellular signaling pathways that underlie these effects have generated data to suggest that the IGF-II/M6P receptor might mediate transmembrane signaling via a G protein-coupled mechanism. The purpose of this review is to outline the characteristics of traditional and nontraditional GPCRs, to relate the IGF-II/M6P receptor's structure with its role in G protein-coupled signaling and to summarize evidence gathered over the years regarding the putative signaling of the IGF-II/M6P receptor mediated by a G protein.

A new chemical tool for exploring the role of the PDE4D isozyme in leukocyte function

Nicotinamide (2) is a potent and selective inhibitor of the PDE4D isozyme and as a chemical tool selectively blocks eosinophil mediator release and chemotaxis thus linking the role of PDE4D to eosinophil function.

Identification of human dopamine D1-like receptor agonist using a cell-based functional assay

AIM

To establish a cell-based assay to screen human dopamine D1 and D5 receptor agonists against compounds from a natural product compound library.

METHODS

Synthetic responsive elements 6 cAMP response elements (CRE) and a mini promoter containing a TATA box were inserted into the pGL3 basic vector to generate the reporter gene construct pCRE/TA/Luci. CHO cells were co-transfected with the reporter gene construct and human D1 or D5 receptor cDNA in mammalian expression vectors. Stable cell lines were established for agonist screening. A natural product compound library from over 300 herbs has been established. The extracts from these herbs were used for human D1 and D5 receptor agonist screenings.

RESULTS

A number of extracts were identified that activated both D1 and D5 receptors. One of the herb extracts, SBG492, demonstrated distinct pharmacological characteristics with human D1 and D5 receptors. The EC(50) values of SBG492 were 342.7 microg/mL for the D1 receptor and 31.7 microg/mL for the D5 receptor.

CONCLUSION

We have established a cell-based assay for high-throughput drug screening to identify D1-like receptor agonists from natural products. Several extracts that can active D1-like receptors were discovered. These compounds could be useful tools for studies on the functions of these receptors in the brain and could potentially be developed into therapeutic drugs for the treatment of central nervous system diseases.

High throughput screening technologies for direct cyclic AMP measurement

A study comparing five different cAMP detection technologies in terms of sensitivity, robustness, and feasibility for HTS is presented. In this report, the following methods are described: a nonhomogeneous DELFIA, and the homogeneous methods based on time-resolved fluorescence (HTRF), luminescent singlet oxygen channeling or ALPHAScreen, FP, and high-affinity enzyme complementation. DELFIA had the highest sensitivity, whereas ALPHAScreen and HTRF shared several advantages, including high sensitivity, broad dynamic range, and minimal reagent addition steps. For G(s)-coupled antagonist screens, we found HTRF and ALPHAScreen the more sensitive and HTS-compatible techniques.

cAMP detection methods in HTS: selecting the best from the rest

The number of technologies that enable high-throughput functional screening of G-protein-coupled receptors has expanded markedly over the past 5 years. Consequently, choosing the most appropriate technology can be a daunting task, particularly for Gi- or Gs-coupled receptors. The most common systems for cyclic AMP detection are reviewed, highlighting the practical and theoretical aspects that are important in their application to high-throughput screening. Current technologies can do the job, but it is likely that the future may require development of technologies that provide even greater biological information.

A homogeneous enzyme fragment complementation cyclic AMP screen for GPCR agonists

In the new high-throughput screening (HTS) campaign, receptor functional assays, 3',5'-cyclic adenosine monophosphate (cAMP), intracellular [Ca(2)+](i), phosphatidylinositol turnover, and reporter-based assays are being used as primary screens as they are now developed as homogeneous and automation-friendly assays. FlashPlate assay and scintillation proximity assay using radiolabeled cAMP have been used for measuring cAMP. A nonradioactive homogeneous HTS assay using HitHunter trade mark enzyme fragment complementation (EFC) technology was evaluated for measuring cAMP in adherent and suspension cells overexpressing a Galpha(s)-coupled receptor. In the EFC-cAMP assay, the beta-galactosidase (beta-gal) donor fragment-cAMP (ED-cAMP) conjugate complements with the beta-gal enzyme acceptor (EA) fragment to form an active beta-gal enzyme. Binding of ED-cAMP conjugate to the anti-cAMP antibody prevents its complementation with the EA fragment to form an active enzyme. Cyclic AMP in the samples compete with ED-cAMP to bind to the anti-cAMP antibody, thus increasing the free ED-cAMP that can complement with the EA fragment to form an active enzyme that is assayed with a luminescent substrate. Thus, this assay results in a positive signal unlike other technologies, wherein the signal is completed by cAMP in the sample. Glucagon-like peptide (GLP)-1 binds to GLP-1 receptor (with a Kd of 0.2 nM) signals through Galpha(s) to activate adenylate cyclase, which results in an increase of intracellular cAMP (EC(50) of 0.3 nM). GLP-1 stimulation of cAMP levels measured by the EFC method was similar in both adherent and suspension cell formats (EC(50)~0.3 nM) at different cell numbers. The assay was further validated with forskolin, exendin, and several active GLP-1 peptide analogues. The stimulation of cAMP by GLP-1 and forskolin was effectively inhibited by the adenylate cyclase inhibitors MDL-12330A and SQ-22536, confirming that the increased cAMP is through the AC pathway. The assay tolerates dimethyl sulfoxide (DMSO) up to 10%, and tartrazine does not interfere with the assay with the adherent cells up to 1 mM and affects minimally up to 10 microM in suspension cells. The assay is very robust, with a Z' value of 0.7 to 0.8. The assay was validated with several plates of low molecular weight nonpeptide compounds and peptide agonists with different potencies. The suspension cell protocol is a robust homogeneous assay that involves fewer steps than the adherent cell protocol and is suitable for HTS. The cAMP assay using EFC technology is advantageous in that it has a greater dynamic range of detection; is nonradioactive, very sensitive, robust; has minimal interference from DMSO and colored compounds; and is amenable for automation. An added advantage of this assay is that the cAMP is measured as a positive signal, thereby reducing the incidence of false positives.

G Protein-Coupled Receptors in Drug Discovery

G protein-coupled receptors (GPCRs) represent one of the most important drug discovery targets such that compounds targeted against GPCRs represent the single largest drug class currently on the market. With the revolutionary advances in human genome sciences and the identification of numerous orphan GPCRs, it is even more important to identify ligands for these orphan GPCRs so that their physiological and pathological roles can be delineated. To this end, major pharmaceutical industries are investing enormous amounts of time and money to achieve this object. This review is a bird's eye view on the various aspects of GPCRs in drug discovery.

Functional cell-based uHTS in chemical genomic drug discovery

The availability of genomic information significantly increases the number of potential targets available for drug discovery, although the function of many targets and their relationship to disease is unknown. In a chemical genomic research approach, ultra-high throughput screening (uHTS) of genomic targets takes place early in the drug discovery process, before target validation. Target-selective modulators then provide drug leads and pharmacological research tools to validate target function. Effective implementation of a chemical genomic strategy requires assays that can perform uHTS for large numbers of genomic targets. Cell-based functional assays are capable of the uHTS throughput required for chemical genomic research, and their functional nature provides distinct advantages over ligand-binding assays in the identification of target-selective modulators.

[Assay in high throughput screening]

HTS (High Throughput Screening) has been put into practice in recent years. HTS is aiming to discover lead compounds for medicinal drugs. High efficiency must be achieved in all the processes including sample preparation, assay procedure, automation and data management. This review will focus on the aspects concerned with the assay technology and the efficiency in HTS. One of the major trends in HTS is assay miniaturization using high-density microplates with 384 and 1536 wells. This allows us to increase the throughput and to decrease the cost. The so-called "mix and measure" or "homogeneous" assay system, which has no separation steps such as washing or filtration, is effective for this purpose. The homogeneous assays, such as scintillation proximity assay (SPA), fluorescence energy transfer (HTRF, LANCE) and fluorescence polarization (FP), are frequently used. The reporter gene assay or the cell proliferation assay can be adapted for the homogeneous assay using high-density plates. In addition, HTS measuring the intracellular Ca2+ influx is also possible using a CCD Imager. The assay quality as well as the efficiency is also important especially in HTS. The Z'-factor provides a good tool for evaluating the quality of assays.

Homogeneous cell-based fluorescence polarization assay for the direct detection of cAMP

A fluorescence polarization-based functional assay for cyclic AMP (cAMP) production in cells has been proven effective for the detection of agonist-stimulated cAMP production in a HEK 293 recombinant cell line expressing the corticotropin-releasing factor subtype 2alpha (CRF2alpha) receptor. Assays were completed in a single well of a 384-well microplate with no transfer, separation, or wash steps incurred. The assay performance is excellent for adaptation to the high throughput screening environment in terms of speed of analysis, magnitude of displaced signal, precision, and detection limits for cAMP quantitation. Relative potencies of agonists and antagonists are maintained with respect to radiometric assays. The assay withstands up to 5% DMSO and up to 10 microM concentrations of highly colored compound. These attributes suggest that accurate assessment of drug binding can be measured using this assay.

Discovery of cytokine mimics using cell-based systems

The successful cloning and subsequent clinical application of recombinant cytokines and/or growth factors has generated a number of important therapeutics. In contrast to the G-protein-coupled receptors, identification of small-molecule agonists of the cytokine and/or growth factor receptor family has proved difficult. The first small peptides and non-peptidic small-molecule agonists for several receptors have recently been reported. The initial identification and/or crucial characterization of these molecules as true mimics was dependent on the use of cell-based functional assays. This article will review recent cell-based assay technologies that are suitable for HTS and that are being applied to the discovery of novel cytokine and growth factor mimics.

High-throughput screening: new technology for the 21st century

New technologies in high-throughput screening have significantly increased throughput and reduced assay volumes. Key advances over the past few years include new fluorescence methods, detection platforms and liquid-handling technologies. Screening 100,000 samples per day in miniaturized assay volumes will soon become routine. Furthermore, new technologies are now being applied to information-rich cell-based assays, and this is beginning to remove one of the key bottlenecks downstream from primary screening.

A novel high throughput chemiluminescent assay for the measurement of cellular cyclic adenosine monophosphate levels

The second messenger 3', 5'-cyclic AMP (cAMP) is a highly regulated molecule that is governed by G protein-coupled receptor activation and other cellular processes. Measurement of cAMP levels in cells is widely used as an indicator of receptor function in drug discovery applications. We have developed a nonradioactive ELISA for the accurate quantitation of cAMP levels produced in cell-based assays. This novel competitive assay utilizes chemiluminescent detection that affords both a sensitivity and a dynamic assay range that have not been previously reported with any other assay methodologies. The assay has been automated in 96- and 384-well formats, providing assay data that are equivalent to, if not better than, data generated by hand. This report demonstrates the application of this novel assay technology to the functional analysis of a specific G protein-coupled receptor, neuropeptide receptor Y1, on SK-N-MC cells. Our data indicate the feasibility of utilizing this assay methodology for monitoring cAMP levels in a wide range of functional cell-based assays for high throughput screening.

High-throughput and ultra-high-throughput screening: solution- and cell-based approaches

The trend towards assay miniaturization for high-throughput and ultra-high-throughput screening continues to spur development of homogeneous, fluorescence-based assays in higher density, smaller volume microplate formats. Recently, first-generation microfluidic devices have been designed for performing continuous-flow biochemical and cell-based assays. These devices provide orders-of-magnitude reduction in reagent consumption, and offer the potential for implementing high-throughput screening in formats that integrate up-front compound handling with unique assay functionality.