Miniaturization of intracellular calcium functional assays to 1536-well plate format using a fluorometric imaging plate reader

Optical Image Sensors for Smart Analytical Chemiluminescence Biosensors

Optical biosensors have emerged as a powerful tool in analytical biochemistry, offering high sensitivity and specificity in the detection of various biomolecules. This article explores the advancements in the integration of optical biosensors with microfluidic technologies, creating lab-on-a-chip (LOC) platforms that enable rapid, efficient, and miniaturized analysis at the point of need. These LOC platforms leverage optical phenomena such as chemiluminescence and electrochemiluminescence to achieve real-time detection and quantification of analytes, making them ideal for applications in medical diagnostics, environmental monitoring, and food safety. Various optical detectors used for detecting chemiluminescence are reviewed, including single-point detectors such as photomultiplier tubes (PMT) and avalanche photodiodes (APD), and pixelated detectors such as charge-coupled devices (CCD) and complementary metal-oxide-semiconductor (CMOS) sensors. A significant advancement discussed in this review is the integration of optical biosensors with pixelated image sensors, particularly CMOS image sensors. These sensors provide numerous advantages over traditional single-point detectors, including high-resolution imaging, spatially resolved measurements, and the ability to simultaneously detect multiple analytes. Their compact size, low power consumption, and cost-effectiveness further enhance their suitability for portable and point-of-care diagnostic devices. In the future, the integration of machine learning algorithms with these technologies promises to enhance data analysis and interpretation, driving the development of more sophisticated, efficient, and accessible diagnostic tools for diverse applications.

High-throughput screening strategies for space-based radiation countermeasure discovery

As humanity begins to venture further into space, approaches to better protect astronauts from the hazards found in space need to be developed. One particular hazard of concern is the complex radiation that is ever present in deep space. Currently, it is unlikely enough spacecraft shielding could be launched that would provide adequate protection to astronauts during long-duration missions such as a journey to Mars and back. In an effort to identify other means of protection, prophylactic radioprotective drugs have been proposed as a potential means to reduce the biological damage caused by this radiation. Unfortunately, few radioprotectors have been approved by the FDA for usage and for those that have been developed, they protect normal cells/tissues from acute, high levels of radiation exposure such as that from oncology radiation treatments. To date, essentially no radioprotectors have been developed that specifically counteract the effects of chronic low-dose rate space radiation. This review highlights how high-throughput screening (HTS) methodologies could be implemented to identify such a radioprotective agent. Several potential target, pathway, and phenotypic assays are discussed along with potential challenges towards screening for radioprotectors. Utilizing HTS strategies such as the ones proposed here have the potential to identify new chemical scaffolds that can be developed into efficacious radioprotectors that are specifically designed to protect astronauts during deep space journeys. The overarching goal of this review is to elicit broader interest in applying drug discovery techniques, specifically HTS towards the identification of radiation countermeasures designed to be efficacious towards the biological insults likely to be encountered by astronauts on long duration voyages.

Coupling Droplet Microfluidics with Mass Spectrometry for Ultrahigh-Throughput Analysis of Complex Mixtures up to and above 30 Hz

High- and ultrahigh-throughput label-free sample analysis is required by many applications, extending from environmental monitoring to drug discovery and industrial biotechnology. HTS methods predominantly are based on a targeted workflow, which can limit their scope. Mass spectrometry readily provides chemical identity and abundance for complex mixtures, and here, we use microdroplet generation microfluidics to supply picoliter aliquots for analysis at rates up to and including 33 Hz. This is demonstrated for small molecules, peptides, and proteins up to 66 kDa on three commercially available mass spectrometers from salty solutions to mimic cellular environments. Designs for chip-based interfaces that permit this coupling are presented, and the merits and challenges of these interfaces are discussed. On an Orbitrap platform droplet infusion rates of 6 Hz are used for analysis of cytochrome c, on a DTIMS Q-TOF similar rates were obtained, and on a TWIMS Q-TOF utilizing IM-MS software rates up to 33 Hz are demonstrated. The potential of this approach is demonstrated with proof of concept experiments on crude mixtures including egg white, unpurified recombinant protein, and a biotransformation supernatant.

High throughput screening of complex biological samples with mass spectrometry – from bulk measurements to single cell analysis

We review the state of the art in HTS using mass spectrometry with minimal sample preparation from complex biological matrices. We focus on industrial and biotechnological applications.

When Robots Are Good: Fully Automated Thermo LAS Robotic Assay System with Dual FLIPRTETRA and TAP SelecT Robotic Cell Culture System

Cell-based assays for identification of biologically active small molecules from chemical libraries are becoming increasingly popular for HTS aimed at a variety of drug targets. Functional assays require good coordination between several independent processes during the run. This article describes our custom designed, fully automated Thermo LAS robotic system with integrated FLIPRTETRA and several additional peripheral elements such as a BioTek ELX washer unit, a PE Evolution pipettor, and PE FlexDrop dispenser. The Thermo LAS robotic control software, Polara, ensures that each and every plate of sensitive cells in a large batch experiences the same procedure as an individual plate assayed in the hands of a scientist. Such robotic systems can process hundreds of plates a day and require large-scale automated support for cell preparation. The TAP SelecT is an automated robotic system that can plate 100– 300 plates of cells per day with defined accuracy and precision. In addition to plating cells, SelecT can also pass and expand cell lines. Here, we present a case study of a GPCR-mediated Ca-flux assay, where this robotic team enables high-throughput logistics even for an extremely sensitive cell-based assay.

A Model for Efficient Assay Development and Screening at a Small Research Site

Creating an automated assay team with multiple skills that can support diverse screening responsibilities is a key challenge for drug discovery research sites. Development of ultra high throughput screening (uHTS) screens, development of lead identification and lead optimization assays within medicinal chemistry supported projects, and automation and miniaturization of assays are best performed by a dedicated team of varying backgrounds, experience, and skill sets. This article discusses how integration with therapeutic franchises, regular communication processes, and formal and informal cross-training facilitate the establishment of a streamlined and efficient model at our site for supporting multiple projects at relatively modest financial outlay and recruitment levels.

Cell microarrays

In the postgenomic era, DNA and protein arrays are increasing the speed at which knowledge is gathered on gene expression in cells and tissues. At the same time, researchers realize that a miniaturized and parallelized analysis of whole cells may equally expedite the acquisition of data describing cellular properties and function. Researchers are starting to explore means of generating and using cell microarrays to investigate cells at higher throughput. In this initial phase of exploration, cell microarrays are being developed for various cellular analyses including the effects of gene expression, cellular reactions to the biomolecular environment, and profiling of cell surface molecules. This article will provide an overview of different types of eukaryotic cell microarrays described to date, how they are generated, and their fields of application.

Novel patient cell-based HTS assay for identification of small molecules for a lysosomal storage disease

Small molecules have been identified as potential therapeutic agents for lysosomal storage diseases (LSDs), inherited metabolic disorders caused by defects in proteins that result in lysosome dysfunctional. Some small molecules function assisting the folding of mutant misfolded lysosomal enzymes that are otherwise degraded in ER-associated degradation. The ultimate result is the enhancement of the residual enzymatic activity of the deficient enzyme. Most of the high throughput screening (HTS) assays developed to identify these molecules are single-target biochemical assays. Here we describe a cell-based assay using patient cell lines to identify small molecules that enhance the residual arylsulfatase A (ASA) activity found in patients with metachromatic leukodystrophy (MLD), a progressive neurodegenerative LSD. In order to generate sufficient cell lines for a large scale HTS, primary cultured fibroblasts from MLD patients were transformed using SV40 large T antigen. These SV40 transformed (SV40t) cells showed to conserve biochemical characteristics of the primary cells. Using a specific colorimetric substrate para-nitrocatechol sulfate (pNCS), detectable ASA residual activity were observed in primary and SV40t fibroblasts from a MLD patient (ASA-I179S) cultured in multi-well plates. A robust fluorescence ASA assay was developed in high-density 1,536-well plates using the traditional colorimetric pNCS substrate, whose product (pNC) acts as “plate fluorescence quencher” in white solid-bottom plates. The quantitative cell-based HTS assay for ASA generated strong statistical parameters when tested against a diverse small molecule collection. This cell-based assay approach can be used for several other LSDs and genetic disorders, especially those that rely on colorimetric substrates which traditionally present low sensitivity for assay-miniaturization. In addition, the quantitative cell-based HTS assay here developed using patient cells creates an opportunity to identify therapeutic small molecules in a disease-cellular environment where potentially disrupted pathways are exposed and available as targets.

Parsimonious discovery of synergistic drug combinations

Combination therapies that enhance efficacy or permit reduced dosages to be administered have seen great success in a variety of therapeutic applications. More fundamentally, the discovery of epistatic pathway interactions not only informs pharmacologic intervention but can be used to better understand the underlying biological system. There is, however, no systematic and efficient method to identify interacting activities as candidates for combination therapy and, in particular, to identify those with synergistic activities. We devised a pooled, self-deconvoluting screening paradigm for the efficient comprehensive interrogation of all pairs of compounds in 1000-compound libraries. We demonstrate the power of the method to recover established synergistic interactions between compounds. We then applied this approach to a cell-based screen for anti-inflammatory activities using an assay for lipopolysaccharide/interferon-induced acute phase response of a monocytic cell line. The described method, which is >20 times as efficient as a naïve approach, was used to test all pairs of 1027 bioactive compounds for interleukin-6 suppression, yielding 11 pairs of compounds that show synergy. These 11 pairs all represent the same two activities: β-adrenergic receptor agonists and phosphodiesterase-4 inhibitors. These activities both act through cyclic AMP elevation and are known to be anti-inflammatory alone and to synergize in combination. Thus we show proof of concept for a robust, efficient technique for the identification of synergistic combinations. Such a tool can enable qualitatively new scales of pharmacological research and chemical genetics.

Receptor screening technologies in the evaluation of Amazonian ethnomedicines with potential applications to cognitive deficits

ETHNOPHARMACOLOGICAL RELEVANCE

Amazonian peoples utilize a variety of psychoactive plants that may contain novel biologically active compounds. Efforts to investigate such remedies in terms of neuropharmacology have been limited.

AIM OF THIS STUDY

This study identified Amazonian ethnomedicines with potential for the treatment of cognitive deficits in schizophrenia and dementias, and characterized their interactions with CNS neurotransmitter receptors in vitro.

MATERIALS AND METHODS

Approximately 300 Amazonian species with folk uses or constituents indicative of central nervous system activity were incorporated into a database constructed from literature searches, herbarium surveys, and interviews with traditional practitioners. Approximately 130 of these targeted species were collected in Loreto province, Peru, and 228 fractions derived from them were screened in 31 radioreceptor assays via the resources of the NIMH Psychoactive Drug Screening Program. A subset was also screened in functional assays at selected serotonin, muscarinic, and adrenergic receptors.

RESULTS

Ninety-one samples displayed ≥60% inhibition of radioligand binding activity in receptor assays; 135 samples displayed agonist or antagonist activity (or both) in functional assays.

CONCLUSIONS

Potential CNS activity was detected in about 40% of the samples screened, with some correlations to both folk uses and phytochemical constituents. These results may point to novel and potentially therapeutic CNS active compounds.

Elusive equilibrium: the challenge of interpreting receptor pharmacology using calcium assays

Calcium is a key intracellular signal that controls manifold cellular processes over a wide temporal range. The development of calcium-sensitive fluorescent dyes and proteins revolutionized our ability to visualize this important second messenger and its complex signalling characteristics. The subsequent advent of high throughput plate-based fluorescence readers has resulted in the calcium assay becoming the most widely utilized assay system for the characterization of novel receptor ligands. In this review we discuss common approaches to calcium assays, paying particular attention to the potential issues associated with interpretation of receptor pharmacology using this system. Topics covered include dye saturation and forced-coupling of receptors to the calcium pathway, but special consideration is given to the influence of non-equilibrium conditions in this rapid signalling system. Modelling the calcium transient in a kinetic mode allows the influence of ligand kinetics, receptor reserve and read time to be explored. This demonstrates that observed ligand pharmacology at very early time points can be quite different to that determined after longer incubations, even resulting in reversal of agonist potency orders that may be misinterpreted as agonist biased signalling. It also shows that estimates of antagonist affinity, whether by Schild analysis or inhibition curves, are similarly affected by hemi-equilibrium conditions. Finally we end with a discussion on practical approaches to accurately estimate the affinity of insurmountable antagonists using calcium assays.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6.

A multiplex calcium assay for identification of GPCR agonists and antagonists

Activation of G(q) protein-coupled receptors can be monitored by measuring the increase in intracellular calcium with fluorescent dyes. Recent advances in fluorescent kinetic plate readers and liquid-handling technology have made it possible to follow these transient changes in intracellular calcium in a 1,536-well plate format for high-throughput screening (HTS). Here, we have applied the latest generation of fluorescence kinetic plate readers to multiplex the agonist and antagonist screens of a G protein-coupled receptor (GPCR). This multiplexed assay format provides an efficient and cost-effective method for HTS of G(q)-coupled GPCR targets.

Rapid and label-free screening of enzyme inhibitors using segmented flow electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) is an attractive analytical tool for high-throughput screening because of its rapid scan time and ability to detect compounds without need for labels. Impediments to the use of ESI-MS for screening have been the relatively large sample consumed and slow sample introduction rates associated with commonly used flow injection analysis. We have previously shown that by segmenting nanoliter plugs of sample with air, an array of discrete samples can be delivered to a platinum-coated emitter tip for ESI-MS analysis with throughput as high as 0.8 Hz and carry-over between samples less than 0.1%. This method was applied to screening for inhibitors of acetylcholinesterase as a demonstration of the potential of segmented flow ESI-MS for such applications. Each enzyme assay consumed 10 nL of sample. At 1 microL/min infusion rate, 102 samples were analyzed, corresponding to a 0.65 Hz sample analysis rate. Linear quantification of choline was achieved from 200 microM to 10 mM using this method and Z' values were over 0.8 for the assay. Detailed pharmacologic dose-response curves of selected inhibitors were also measured in high-throughput fashion to validate the method.

Identification of Metabotropic Glutamate Receptor Subtype 5 Potentiators Using Virtual High-Throughput Screening

Selective potentiators of glutamate response at metabotropic glutamate receptor subtype 5 (mGluR5) have exciting potential for the development of novel treatment strategies for schizophrenia. A total of 1,382 compounds with positive allosteric modulation (PAM) of the mGluR5 glutamate response were identified through high-throughput screening (HTS) of a diverse library of 144,475 substances utilizing a functional assay measuring receptor-induced intracellular release of calcium. Primary hits were tested for concentration-dependent activity, and potency data (EC₅₀ values) were used for training artificial neural network (ANN) quantitative structure−activity relationship (QSAR) models that predict biological potency from the chemical structure. While all models were trained to predict EC₅₀, the quality of the models was assessed by using both continuous measures and binary classification. Numerical descriptors of chemical structure were used as input for the machine learning procedure and optimized in an iterative protocol. The ANN models achieved theoretical enrichment ratios of up to 38 for an independent data set not used in training the model. A database of ∼450,000 commercially available drug-like compounds was targeted in a virtual screen. A set of 824 compounds was obtained for testing based on the highest predicted potency values. Biological testing found 28.2% (232/824) of these compounds with various activities at mGluR5 including 177 pure potentiators and 55 partial agonists. These results represent an enrichment factor of 23 for pure potentiation of the mGluR5 glutamate response and 30 for overall mGluR5 modulation activity when compared with those of the original mGluR5 experimental screening data (0.94% hit rate). The active compounds identified contained 72% close derivatives of previously identified PAMs as well as 28% nontrivial derivatives of known active compounds.

Multiplexed random peptide library and phospho-specific antibodies facilitate human polo-like kinase 1 inhibitor screen

One of the challenges to develop time-resolved fluorescence resonance energy transfer (TR-FRET) assay for serine/threonine (Ser/Thr) protein kinase is to select an optimal peptide substrate and a specific phosphor Ser/Thr antibody. This report describes a multiplexed random screen-based development of TR-FRET assay for ultra-high-throughput screening (uHTS) of small molecule inhibitors for a potent cancer drug target polo-like kinase 1 (Plk1). A screen of a diverse peptide library in a 384-well plate format identified several highly potent substrates that share the consensus motif for phosphorylation by Plk1. Their potencies were comparable to FKD peptide, a designed peptide substrate derived from well-described Plk1 substrate Cdc25C. A specific anti-phosphor Ser/Thr antibody p(S/T)F antibody that detects the phosphorylation of FKD peptide was screened out of 87 antibodies with time-resolved fluorometry technology in a 96-well plate format. Using FKD peptide and p(S/T)F antibody, we successfully developed a robust TR-FRET assay in 384-well plate format, and further miniaturized this assay to 1,536-well plate format to perform uHTS. We screened about 1.2 million compounds for Plk1 inhibitors using a Plk1 deletion mutant that only has the kinase domain and subsequently screened the same compound library using a full-length active-mutant Plk1. These uHTSs identified a number of hit compounds, and some of them had selectivity to either the deletion mutant or the full-length protein. Our results prove that a combination of random screen for substrate peptide and phospho-specific antibodies is very powerful strategy to develop TR-FRET assays for protein kinases.

Aequorin functional assay for characterization of G-protein-coupled receptors: implementation with cryopreserved transiently transfected cells

Assay technologies that measure intracellular Ca(2+) release are among the predominant methods for evaluation of GPCR function. These measurements have historically been performed using cell-permeable fluorescent dyes, although the use of the recombinant photoprotein aequorin (AEQ) as a Ca(2+) sensor has gained popularity with recent advances in instrumentation. The requirement of the AEQ system for cells expressing both the photoprotein and the GPCR target of interest has necessitated the labor-intensive development of cell lines stably expressing both proteins. With the goal of streamlining this process, transient transfections were used to either (1) introduce AEQ into cells stably expressing the GPCR of interest or (2) introduce the GPCR into cells stably expressing the AEQ protein, employing the human muscarinic M(1) receptor as a model system. Robust results were obtained from cryopreserved cells prepared by both strategies, yielding agonist and antagonist pharmacology in good agreement with literature values. Good reproducibility was observed between multiple transient transfection events. These results indicate that transient transfection is a viable and efficient method for production of cellular reagents for use in AEQ assays.

Photoproteins: important new tools in drug discovery

The G protein-coupled receptor (GPCR) family is a major target for drug discovery, and most, if not all, GPCRs can couple to Ca2+ signaling. Consequently, there are a number of cellbased, primary, high-throughput screening (HTS) assays used for drug discovery that assess changes in intracellular Ca2+ as a functional readout of GPCR activation. Historically, changes in intracellular Ca2+ levels have been readily detected using fluorescent dyes that emit light in proportion to changes in intracellular Ca2+ concentration. An alternative approach to indirectly measure changes in Ca2+ concentrations involves the use of recombinantly expressed biosensor photoproteins, of which aequorin is a prototypic example. These biosensors have the advantage that they provide an intense luminescent signal in response to elevations in intracellular Ca2+. This exquisite sensitivity, the high signal-to-noise ratios, and the ability to target expression to discrete subcellular sites (in order to detect Ca2+ microdomains) have made photoproteins a principal choice in a wide range of GPCR drug discovery programs. Photoproteins are also finding increasing use in detecting activation of other molecular target classes such as ligand-gated ion channels and transporters. This review focuses upon the use of calcium photoproteins principally for use in GPCR drug discovery and HTS.

A fluorescence-based thiol quantification assay for ultra-high-throughput screening for inhibitors of coenzyme A production

Here we report the development and miniaturization of a cell-free enzyme assay for ultra-high-throughput screening (uHTS) for inhibitors of two potential drug targets for obesity and cancer: fatty acid synthase (FAS) and acetyl-coenzyme A (CoA) carboxylase (ACC) 2. This assay detects CoA, a product of the FAS-catalyzed condensation of malonyl-CoA and acetyl-CoA. The free thiol of CoA can react with 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin (CPM), a profluorescent coumarin maleimide derivative that becomes fluorescent upon reaction with thiols. FAS produces long-chain fatty acid and CoA from the condensation of malonyl-CoA and acetyl-CoA. In our FAS assay, CoA released in the FAS reaction forms a fluorescence adduct with CPM that emits at 530 nm when excited at 405 nm. Using this detection method for CoA, we measured the activity of sequential enzymes in the fatty acid synthesis pathway to develop an ACC2/FAS-coupled assay where ACC2 produces malonyl-CoA from acetyl-CoA. We miniaturized the FAS and ACC2/FAS assays to 3,456- and 1,536-well plate format, respectively, and completed uHTSs for small molecule inhibitors of this enzyme system. This report shows the results of assay development, miniaturization, and inhibitor screening for these potential drug targets.

Characterization of the 5-HT2b Receptor in Evaluation of Aequorin Detection of Calcium Mobilization for Miniaturized GPCR High-Throughput Screening

Fluorescent detection of calcium mobilization has been used successfully to identify modulators of G-protein—coupled receptors (GPCRs); however, inherent issues with fluorescence may limit its potential for high-throughput screening miniaturization. The data presented here demonstrate that the calcium-sensitive photoprotein aequorin (AequoScreen™), when compared with FLUO-4 in the same cellular background, allows for miniaturization of functional kinetic calcium flux assays, in which the rank order of potency and efficacy was maintained for a series of diverse small-molecule modulators. Small-volume (<10 µL) 384- and 1536-well aequorin assays were implemented by integration of acoustic dispensing (Echo 550™) and kinetic flash luminometry (CyBi Lumax™). The enhanced high signal-to-background ratios observed relative to fluorescence were readily manipulated by altering per-well cell densities and yielded acceptable screening statistics in miniaturized format for both agonist and antagonist screening scenarios. In addition, the authors demonstrate the feasibility of using agonist concentrations less than EC50 in a miniaturized antagonist assay. These features, coupled with improved sample handling, should enhance sensitivity and provide the benefits of miniaturization including cost reduction and throughput gains. ( Journal of Biomolecular Screening 2008:486-493)

Enzyme microarrays assembled by acoustic dispensing technology

Miniaturizing bioassays to the nanoliter scale for high-throughput screening reduces the consumption of reagents that are expensive or difficult to handle. Through the use of acoustic dispensing technology, nanodroplets containing 10 microM ATP (3 microCi/microL (32)P) and reaction buffer in 10% glycerol were positionally dispensed to the surface of glass slides to form 40-nL compartments (100 droplets/slide) for Pim1 (proviral integration site 1) kinase reactions. The reactions were activated by dispensing 4 nL of various levels of a pyridocarbazolo-cyclopentadienyl ruthenium complex Pim1 inhibitor, followed by dispensing 4 nL of a Pim1 kinase and peptide substrate solution to achieve final concentrations of 150 nM enzyme and 10 microM substrate. The microarray was incubated at 30 degrees C (97% R(h)) for 1.5 h. The spots were then blotted to phosphocellulose membranes to capture phosphorylated substrate. With phosphor imaging to quantify the washed membranes, the assay showed that, for doses of inhibitor from 0.75 to 3 microM, Pim1 was increasingly inhibited. Signal-to-background ratios were as high as 165, and average coefficients of variation for the assay were approximately 20%. Coefficients of variation for dispensing typical working buffers were under 5%. Thus, microarrays assembled by acoustic dispensing are promising as cost-effective tools that can be used in protein assay development.

Use of Cryopreserved Cells for Enabling Greater Flexibility in Compound Profiling

Measurement of intracellular calcium release following agonist challenge within cells expressing the relevant membrane protein is a commonly used format to derive structure-activity relationship (SAR) data within a compound profiling assay. The Fluorometric Imaging Plate Reader (FLIPR) has become the gold standard for this purpose. FLIPR traditionally uses cells that are maintained in continuous culture for compound profiling of iterative chemistry campaigns. This supply dictates that assays can only be run on 4 of 5 weekdays, or alternative cell culture machinery is required such that plating can occur remotely at the weekend. The data reported here demonstrate that high-quality compound profiling data can be generated from the use of cryopreserved cells and that these cells can also be plated at various densities to generate equivalent data between 24 and 72 h post-plating. Hence, the authors report a method that allows data generation throughout the week and without the requirement of highly automated cell culture or continuous culture. ( Journal of Biomolecular Screening 2008:354-362)

Development of quantitative cell-based enzyme assays in microdroplets

We describe the development of an enzyme assay inside picoliter microdroplets. The enzyme alkaline phosphatase is expressed in Escherichia coli cells and presented in the periplasm. Droplets act as discrete reactors which retain and localize any reaction product. The catalytic turnover of the substrate is measured in individual droplets by monitoring the fluorescence at several time points within the device and exhibits kinetic behavior similar to that observed in bulk solution. Studies on wild type and a mutant enzyme successfully demonstrated the feasibility of using microfluidic droplets to provide time-resolved kinetic measurements.

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).

High-throughput screening assays for the identification of chemical probes

High-throughput screening (HTS) assays enable the testing of large numbers of chemical substances for activity in diverse areas of biology. The biological responses measured in HTS assays span isolated biochemical systems containing purified receptors or enzymes to signal transduction pathways and complex networks functioning in cellular environments. This Review addresses factors that need to be considered when implementing assays for HTS and is aimed particularly at investigators new to this field. We discuss assay design strategies, the major detection technologies and examples of HTS assays for common target classes, cellular pathways and simple cellular phenotypes. We conclude with special considerations for configuring sensitive, robust, informative and economically feasible HTS assays.

Microfluidic device for electric field-driven single-cell capture and activation

A microchip that performs directed capture and chemical activation of surface-modified single cells has been developed. The cell capture system is comprised of interdigitated gold electrodes microfabricated on a glass substrate within PDMS channels. The cell surface is labeled with thiol functional groups using endogenous RGD receptors, and adhesion to exposed gold pads on the electrodes is directed by applying a driving electric potential. Multiple cell types can thus be sequentially and selectively captured on desired electrodes. Single-cell capture efficiency is optimized by varying the duration of field application. Maximum single-cell capture is attained for the 10-min trial, with 63 +/- 9% (n = 30) of the electrode pad rows having a single cell. In activation studies, single M1WT3 CHO cells loaded with the calcium-sensitive dye fluo-4 AM were captured; exposure to the muscarinic agonist carbachol increased the fluorescence to 220 +/- 74% (n = 79) of the original intensity. These results demonstrate the ability to direct the adhesion of selected living single cells on electrodes in a microfluidic device and to analyze their response to chemical stimuli.

Real-time and high throughput monitoring of cAMP in live cells using a fluorescent membrane potential-sensitive dye

Adenosine-3',5'-cyclic monophosphate (cAMP) conveys the signals from G-protein coupled receptors (GPCRs) and regulates a variety of downstream cellular events. However, there are few robust assays available for measuring cAMP in live cells. Most of the existing cAMP assays require cell lysis and/or have relatively low throughput. We report a live cell-based cAMP assay that has been developed to record the real-time changes in intracellular cAMP. By employing a mutated cyclic-nucleotide-gated ion channel (CNGC) as the cAMP biosensor, the change of cAMP level is coupled to the change of transmembrane potential that is measured through a new fluorescent membrane potential (MP)-sensitive dye, HLB 021-152. We have successfully used HLB 021-152 for homogeneously monitoring cAMP stimulations in live cells under both serum-containing and serum-free environments. Upon stimulating the endogenous or heterogenous GPCRs on CNGC-cloned human embryonic kidney 293 cells with agonists, the fluorescence signal of HLB 021-152 increases rapidly. It has much greater assay dynamic range than DiSBAC2(3), the existing "gold standard" dye for measuring cellular MP. This new MP dye can be readily formulated for high throughput screening of GPCR modulators either with serum or without serum.

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.

Identification of small molecule antagonists of the human mas-related gene-X1 receptor

The recently identified mas-related-gene (MRG) family of receptors, located primarily in sensory neurons of the dorsal root ganglion, has been implicated in the perception of pain. Thus, antagonists of this class of receptors have been postulated to be useful analgesics. Toward this end, we developed a cell-based beta-lactamase (BLA) reporter gene assay to identify small molecule antagonists of the human MRG-X1 receptor from a library of compounds. Single-cell clones expressing functional receptors were selected using the BLA reporter gene technology. The EC50 for the MRG agonist peptide, BAM15, appeared to be comparable between the BLA assay and the intracellular Ca2+ transient assays in these cells. Ultra high-throughput screening of approximately 1 million compounds in a 1.8-microl cell-based BLA reporter gene assay was conducted in a 3456-well plate format. Compounds exhibiting potential antagonist profile in the BLA assay were confirmed in the second messenger Ca2+ transient assay. A cell-based receptor trafficking assay was used to further validate the mechanism of action of these compounds. Several classes of compounds, particularly the 2,3-disubstituted azabicyclo-octanes, appear to be relatively potent antagonists at the human MRG-X1 receptors, as confirmed by the receptor trafficking assay and radioligand binding studies. Furthermore, the structure-activity relationship reveals that within this class of compounds, the diphenylmethyl moiety is constant at the 2-substituent, whereas the 3-substituent is directly correlated with the antagonist activity of the compound.

Cherry-Picking in an Orchard: Unattended LC/MS Analysis from an Autosampler with >32,000 Samples Online

The 1536-well microplate format has widely supplanted the 384-well microplate format for high-throughput screening and for IC50 assays. Previously, liquid chromatography/mass spectrometry (LC/MS) analyses of such samples required manual transfers of the wells of interest from a 1536-well plate into a 384-well plate. Because this manual transfer introduced a source of potential error, it became clear that amore appropriate solutionwould be to sample directly from the 1536-well plates. Currently, commercially available 1536-well plate autosamplers are not compatible with Waters LC/MS systems. The authors have modified their CTC PAL autosampler to support injection from up to twenty-four 1536-well plates. This allows them to cherry-pick any sample from up to 36,864 wells on the autosampler. Because of its success at this Institute, sampling from 1536-well plates has not only become the preferredmethod for LC/MS analysis from IC50 plates but also become the standard format used for the handling of and the sampling from large combinatorial libraries.

Multiplexed G-protein-coupled receptor Ca2+ flux assays for high-throughput screening

An early drug discovery approach focusing on gene families can benefit from strategies that exploit common signaling mechanisms to more effectively identify and characterize novel chemical lead structures. Multiplexing, defined as the screening of multiple targets within the same experiment, is an example of this strategy. Here, the authors describe a technique that allows multiplexing of a common assay type used to study G-protein-coupled receptors: changes in intracellular Ca2+ levels as measured by Molecular Device's fluorometric imaging plate reader (FLIPR). The multiplexed FLIPR assays showed the expected pharmacological properties of single assays, with good reproducibility and Z* factors. The authors used them to screen large compound libraries in 2 multiplexed assay designs. The 1st used a single-cell line expressing 2 different receptors and the 2nd a mixture of 2 cell lines of the same type each expressing distinct receptors. Screening using these multiplexed assays produced significant savings in reagents, time, and human resources and allowed the authors to quickly identify specific and selective hits.

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.

The highly efficacious actions of N-desmethylclozapine at muscarinic receptors are unique and not a common property of either typical or atypical antipsychotic drugs: is M1 agonism a pre-requisite for mimicking clozapine's actions?

RATIONALE

Recent studies have suggested that the salutary actions of clozapine in schizophrenia may be due to selective activation of M(1) muscarinic receptors by clozapine and/or its major active metabolite N-desmethylclozapine.

OBJECTIVE

We systematically tested this hypothesis by screening a large number of psychoactive compounds, including many atypical antipsychotic drugs, for agonist activity at cloned, human M(1), M(3) and M(5) muscarinic receptors.

RESULTS

Only three of the 14 atypical antipsychotic drugs we tested were found to possess partial agonist actions at M(1) muscarinic receptors (fluperlapine, JL13, clozapine). A few additional miscellaneous compounds had a modest degree of M(1) agonist actions. Only carbachol and N-desmethylclozapine had appreciable M(3) muscarinic agonism at M(3) muscarinic receptors, although several were M(5) partial agonists including MK-212, N-desmethylclozapine and xanomeline.

CONCLUSION

Although M(1) muscarinic receptor-selective partial agonists have shown promise in some preclinical antipsychotic drug models, these studies indicate that it is unlikely that the salutary actions of clozapine and similar atypical antipsychotic drugs are mediated solely by M(1) muscarinic receptor activation. It is possible, however, that the M(1) agonism of N-desmethylclozapine contributes to the uniquely beneficial actions of clozapine. Thus, these results are consistent with the notion that a balanced degree of activity at multiple biogenic amine receptors, including M(1) muscarinic agonism, is responsible for the uniquely beneficial actions of clozapine.

Image-based screening: a technology in transition

Image-based screening (IBS) has proven itself with whole-well assays in which throughput and assay miniaturization are priorities. Recent interest, however, has centered upon the use of automated imaging technology to conduct assays at subcellular resolution. These in vitro assays have the potential to increase lead quality at early stages in drug discovery. Subcellular IBS is not yet mature and, although some assays provide reliable data at reasonable throughput, many others have yet to demonstrate robust application. Developments in image acquisition, analysis and informatics technologies are ongoing and are expected to broaden the usefulness of subcellular IBS.

G-protein-coupled receptor screening technologies

The choice of assay for high throughput screening (HTS) is of strategic importance to the successful identification of chemical entities that can be developed into drugs. During the past decade several technologies have emerged permitting large compound collections to be screened against biologically relevant models in a high throughput fashion. In this review, we summarise the technologies that are available for screening G-protein-coupled receptors (GPCRs) and discuss the issues that impact upon the choice of screening methodology.: