Nuclear Export Inhibitors and Kinase Inhibitors Identified Using a MAPK-Activated Protein Kinase 2 Redistribution®Screen

High throughput microscopy and single cell phenotypic image-based analysis in toxicology and drug discovery

Measuring single cell responses to the universe of chemicals (drugs, natural products, environmental toxicants etc.) is of paramount importance to human health as phenotypic variability in sensing stimuli is a hallmark of biology that is considered during high throughput screening. One of the ways to approach this problem is via high throughput, microscopy-based assays coupled with multi-dimensional single cell analysis methods. Here, we will summarize some of the efforts in this vast and growing field, focusing on phenotypic screens (e.g., Cell Painting), single cell analytics and quality control, with particular attention to environmental toxicology and drug screening. We will discuss advantages and limitations of high throughput assays with various end points and levels of complexity.

Identification of Akt Pathway Inhibitors Using Redistribution Screening on the FLIPR and the IN Cell 3000 Analyzer

The PI3-kinase/Akt pathway is an important cell survival pathway that is deregulated in the majority of human cancers. Despite the apparent druggability of several kinases in the pathway, no specific catalytic inhibitors have been reported in the literature. The authors describe the development of a fluorometric imaging plate reader (FLIPR)-based Akt1 translocation assay to discover inhibitors of Akt1 activation. Screening of a diverse chemical library of 45,000 compounds resulted in identification of several classes of Akt1 translocation inhibitors. Using a combination of classical in vitro assays and translocation assays directed at different steps of the Akt pathway, the mechanisms of action of 2 selected chemical classes were further defined. Protein translocation assays emerge as powerful tools for hit identification and characterization. ( Journal of Biomolecular Screening 2005:20-29)

The development of high-content screening (HCS) technology and its importance to drug discovery

INTRODUCTION

High-content screening (HCS) was introduced about twenty years ago as a promising analytical approach to facilitate some critical aspects of drug discovery. Its application has spread progressively within the pharmaceutical industry and academia to the point that it today represents a fundamental tool in supporting drug discovery and development.

AREAS COVERED

Here, the authors review some of significant progress in the HCS field in terms of biological models and assay readouts. They highlight the importance of high-content screening in drug discovery, as testified by its numerous applications in a variety of therapeutic areas: oncology, infective diseases, cardiovascular and neurodegenerative diseases. They also dissect the role of HCS technology in different phases of the drug discovery pipeline: target identification, primary compound screening, secondary assays, mechanism of action studies and in vitro toxicology.

EXPERT OPINION

Recent advances in cellular assay technologies, such as the introduction of three-dimensional (3D) cultures, induced pluripotent stem cells (iPSCs) and genome editing technologies (e.g., CRISPR/Cas9), have tremendously expanded the potential of high-content assays to contribute to the drug discovery process. Increasingly predictive cellular models and readouts, together with the development of more sophisticated and affordable HCS readers, will further consolidate the role of HCS technology in drug discovery.

Structural Basis of Targeting the Exportin CRM1 in Cancer

Recent studies have demonstrated the interference of nucleocytoplasmic trafficking with the establishment and maintenance of various cancers. Nucleocytoplasmic transport is highly regulated and coordinated, involving different nuclear transport factors or receptors, importins and exportins, that mediate cargo transport from the cytoplasm into the nucleus or the other way round, respectively. The exportin CRM1 (Chromosome region maintenance 1) exports a plethora of different protein cargoes and ribonucleoprotein complexes. Structural and biochemical analyses have enabled the deduction of individual steps of the CRM1 transport cycle. In addition, CRM1 turned out to be a valid target for anticancer drugs as it exports numerous proto-oncoproteins and tumor suppressors. Clearly, detailed understanding of the flexibility, regulatory features and cooperative binding properties of CRM1 for Ran and cargo is a prerequisite for the design of highly effective drugs. The first compound found to inhibit CRM1-dependent nuclear export was the natural drug Leptomycin B (LMB), which blocks export by competitively interacting with a highly conserved cleft on CRM1 required for nuclear export signal recognition. Clinical studies revealed serious side effects of LMB, leading to a search for alternative natural and synthetic drugs and hence a multitude of novel therapeutics. The present review examines recent progress in understanding the binding mode of natural and synthetic compounds and their inhibitory effects.

Pharmacological manipulation of transcription factor protein-protein interactions: opportunities and obstacles

Much research on transcription factor biology and their genetic pathways has been undertaken over the last 30 years, especially in the field of developmental biology and cancer. Yet, very little is known about the molecular modalities of highly dynamic interactions between transcription factors, genomic DNA, and protein partners. Methodological breakthroughs such as RNA-seq (RNA-sequencing), ChIP-seq (chromatin immunoprecipitation sequencing), RIME (rapid immunoprecipitation mass spectrometry of endogenous proteins), and single-molecule imaging will dramatically accelerate the discovery rate of their molecular mode of action in the next few years. From a pharmacological viewpoint, conventional methods used to target transcription factor activity with molecules mimicking endogenous ligands fail to achieve high specificity and are limited by a lack of identification of new molecular targets. Protein-protein interactions are likely to represent one of the next major classes of therapeutic targets. Transcription factors, known to act mostly via protein-protein interaction, may well be at the forefront of this type of drug development. One hurdle in this field remains the difficulty to collate structural data into meaningful information for rational drug design. Another hurdle is the lack of chemical libraries meeting the structural requirements of protein-protein interaction disruption. As more attempts at modulating transcription factor activity are undertaken, valuable knowledge will be accumulated on the modality of action required to modulate transcription and how these findings can be applied to developing transcription factor drugs. Key discoveries will spawn into new therapeutic approaches not only as anticancer targets but also for other indications, such as those with an inflammatory component including neurodegenerative disorders, diabetes, and chronic liver and kidney diseases.

Low-molecular-weight MK2 inhibitors: a tough nut to crack!

The p38 pathway has been at the center of interest for anti-inflammatory drug discovery for many years as it is crucial for the biosynthesis of TNF-α, IL-1β and other mediators. Most of the anti-inflammatory effects of p38 inhibition are mediated through MAPK-activated protein kinase-2 (MK2), a direct downstream target of p38, which makes MK2 a very interesting drug target. Within the last 5 years, several classes of low-molecular-weight MK2 inhibitors were disclosed in the patent and primary literature. Advanced compounds could be optimized to nanomolar potencies and inhibit TNF-α release, as well as the phosphorylation of the MK2 substrate heat-shock protein 27 in cellular assays. This article will review the recent progress in this field and will highlight and discuss the most promising compound series disclosed so far.

Visualization of human immunodeficiency virus protease inhibition using a novel Förster resonance energy transfer molecular probe

The in vivo high-throughput screening (HTS) of human immunodeficiency virus (HIV) protease inhibitors is a significant challenge because of the lack of reliable assays that allow the visualization of HIV targets within living cells. In this study, we developed a new molecular probe that utilizes the principles of Förster resonance energy transfer (FRET) to visualize HIV-1 protease inhibition within living cells. The probe is constructed by linking two fluorescent proteins: AcGFP1 (a mutant green fluorescent protein) and mCherry (a red fluorescent protein) with an HIV-1 protease cleavable p2/p7 peptide. The cleavage of the linker peptide by HIV-1 protease leads to separation of AcGFP1 from mCherry, quenching FRET between AcGFP1 and mCherry. Conversely, the addition of a protease inhibitor prevents the cleavage of the linker peptide by the protease, allowing FRET from AcGFP1 to mCherry. Thus, HIV-1 protease inhibition can be determined by measuring the FRET signal's change generated from the probe. Both in vitro and in vivo studies demonstrated the feasibility of applying the probe for quantitative analyses of HIV-1 protease inhibition. By cotransfecting HIV-1 protease and the probe expression plasmids into 293T cells, we showed that the inhibition of HIV-1 protease by inhibitors can be visualized or quantitatively determined within living cells through ratiometric FRET microscopy imaging measurement. It is expected that this new probe will allow high-content screening (HCS) of new anti-HIV drugs.

Utilizing a novel tandem oral dosing strategy to enhance exposure of low-solubility drug candidates in a preclinical setting

Time and resource constraints necessitate increasingly early decision making to accelerate or stop preclinical drug discovery programs. Early discovery drug candidates may be potent inhibitors of new targets, but all too often exhibit poor pharmaceutical and pharmacokinetic properties that limit the in vivo exposure. Low solubility of a drug candidate often leads to poor oral bioavailability and poor dose linearity that creates an issue for efficacy and target safety studies, where high drug exposures are desired. When solubility issues are encountered, enabling formulations are often used to improve the exposure. However, this approach often requires a substantial and lengthy investment to develop the formulation. In our study, two drug candidates with poor aqueous solubility were dosed in rats as simple suspension formulations using a novel tandem dosing strategy, which employs dosing orally in 2.5 h increments up to three times to simulate an oral infusion by avoiding saturation of absorption associated with bolus dosing. These compounds were also dosed using the same suspension formulations and a standard dosing strategy. The resulting in vivo exposures were compared. It was found that this novel tandem dosing strategy significantly improved the in vivo exposures.

Analysis of cellular phosphatidylinositol (3,4,5)-trisphosphate levels and distribution using confocal fluorescent microscopy

We have developed an immunocytochemistry method for the semiquantitative detection of phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P3) at the cell plasma membrane. This protocol combines the use of a glutathione S-transferase-tagged pleckstrin homology (PH) domain of the general phosphoinositides-1 receptor (GST-GRP1PH) with fluorescence confocal microscopy and image segmentation using cell mask software analysis. This methodology allows the analysis of PI(3,4,5)P3 subcellular distribution in resting and epidermal growth factor (EGF)-stimulated HEK293T cells and in LIM1215 (wild-type phosphoinositide 3-kinase (PI3K)) and LIM2550 (H1047R mutation in PI3K catalytic domain) colonic carcinoma cells. Formation of PI(3,4,5)P3 was observed 5min following EGF stimulation and resulted in an increase of the membrane/cytoplasm fluorescence ratio from 1.03 to 1.53 for HEK293T cells and from 2.2 to 3.3 for LIM1215 cells. Resting LIM2550 cells stained with GST-GRP1PH had an elevated membrane/cytoplasm fluorescence ratio of 9.8, suggesting constitutive PI3K activation. The increase in the membrane/cytoplasm fluorescent ratio was inhibited in a concentration-dependent manner by the PI3K inhibitor LY294002. This cellular confocal imaging assay can be used to directly assess the effects of PI3K mutations in cancer cell lines and to determine the potential specificity and effectiveness of PI3K inhibitors in cancer cells.

High content screening: seeing is believing

High content screening (HCS) combines the efficiency of high-throughput techniques with the ability of cellular imaging to collect quantitative data from complex biological systems. HCS technology is integrated into all aspects of contemporary drug discovery, including primary compound screening, post-primary screening capable of supporting structure-activity relationships, and early evaluation of ADME (absorption, distribution, metabolism and excretion)/toxicity properties and complex multivariate drug profiling. Recently, high content approaches have been used extensively to interrogate stem cell biology. Despite these dramatic advances, a number of significant challenges remain related to the use of more biology- and disease-relevant cell systems, the development of informative reagents to measure and manipulate cellular events, and the integration of data management and informatics.

Human kinome drug discovery and the emerging importance of atypical allosteric inhibitors

IMPORTANCE OF THE FIELD

Protein kinases are important targets for drug discovery because they possess critical roles in many human diseases. Several protein kinase inhibitors have entered clinical development with others having already been approved for treating a host of diseases. However, many kinase inhibitors suffer from non-selectivity because they interact with the ATP binding region which has similar structures amongst the protein kinases and this non-selectivity sometimes can cause side effects. As a consequence, there is much interest in developing drugs that inhibit kinases through non-classical mechanisms with the hope of avoiding the side effects of previous kinase drugs.

AREAS COVERED IN THIS REVIEW

This review covers emerging information on kinase biology and discusses new approaches to design selective inhibitors that do not compete with ATP.

WHAT THE READER WILL GAIN

The reader will gain a better understanding of the importance of the field of allosteric inhibitor drug discovery and how this has required the adoption of a new generation of high-throughput screening techniques.

TAKE HOME MESSAGE

Discovery and development of allosteric modulators will result in a family of novel kinase therapies with greater selectivity and more varied ways to control activity of disease causing kinase targets.

High-throughput automated confocal microscopy imaging screen of a kinase-focused library to identify p38 mitogen-activated protein kinase inhibitors using the GE InCell 3000 analyzer

The integration of fluorescent microscopy imaging technologies and image analysis into high-content screening (HCS) has been applied throughout the drug discovery pipeline to identify, evaluate, and advance compounds from early lead generation through preclinical candidate selection. In this chapter we describe the development, validation, and implementation of an HCS assay to screen compounds from a kinase-focused small-molecule library to identify inhibitors of the p38 pathway using the GE InCell 3000 automated imaging platform. The assay utilized a genetically modified HeLa cell line stably expressing mitogen-activated, protein-activating protein kinase-2 fused to enhanced green fluorescent protein (MK2-EGFP) and measured the subcellular distribution of the MK2-EGFP as a direct readout of p38 activation. The MK2-EGFP translocation assay performed in 384-well glass bottom microtiter plates exhibited a robust Z-factor of 0.46 and reproducible EC50 and IC50 determinations for activators and inhibitors, respectively. A total of 32,891 compounds were screened in singlicate at 50 microM and 156 were confirmed as inhibitors of p38-mediated MK2-EGFP translocation in follow-up IC50 concentration response curves. Thirty-one compounds exhibited IC50s less than 1 microM, and at least one novel structural class of p38 inhibitor was identified using this HCA/HCS chemical biology screening approach.

A potent and selective p38 inhibitor protects against bone damage in murine collagen-induced arthritis: a comparison with neutralization of mouse TNFalpha

BACKGROUND AND PURPOSE

The p38 kinase regulates the release of proinflammatory cytokines including tumour-necrosis factor-alpha (TNFalpha) and is regarded as a potential therapeutic target in rheumatoid arthritis (RA). Using the novel p38 inhibitor Org 48762-0, we investigated the therapeutic potential of p38 inhibition and compared this to anti-mouse (m)TNFalpha antibody treatment in murine collagen-induced arthritis (CIA).

EXPERIMENTAL APPROACH

Pharmacological profiles of Org 48762-0 were characterized in kinase assays, cellular assays and in lipopolysaccharide (LPS)-induced inflammation in mice. The effects of Org 48762-0 and of mTNFalpha-neutralization on established arthritis were examined in murine CIA.

KEY RESULTS

Org 48762-0 potently inhibited p38alpha kinase with a high degree of kinase selectivity. In cellular assays, Org 48762-0 reduced LPS-induced TNFalpha release. Oral administration of Org 48762-0 in mice showed drug-like pharmacokinetic properties and inhibited LPS-induced cytokine production. These pharmacological characteristics of Org 48762-0 prompted a comparison of therapeutic efficacy with mTNFalpha-neutralization in CIA. Org 48762-0 and anti-mTNFalpha antibody treatment equally inhibited development of arthritis when evaluated macroscopically. Radiological analyses revealed protection against bone damage for both treatments, although statistical difference was reached with Org 48762-0 treatment only. Further, micro-computed tomographical and histopathological analyses confirmed the protective effects of Org 48762-0 on joint damage.

CONCLUSIONS AND IMPLICATIONS

Pharmacological targeting of p38 kinase provided good protection against joint tissue damage in CIA. In our experiments, neutralization of mTNFalpha produced less prominent suppression of bone damage. Our data suggest a therapeutic potential for selective and potent p38 inhibitors in RA.

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

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

High-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.

An HTS Approach to Screen for Antagonists of the Nuclear Export Machinery Using High Content Cell-Based Assays

Intracellular localization is essential for the regulated activity of many signaling molecules associated with disease-relevant pathways. High content screening is a powerful technology to monitor the impact of small molecules or interfering RNAs on translocation of proteins within intact cells. Several assays have been developed to measure the nucleocytoplasmic shuttling of proteins like nuclear factor kappaB, FoxO, or nuclear factor of activated T-cells involved in distinct signaling networks. However, since all these proteins bear a leucine-rich nuclear export signal (NES), modulators of the NES-dependent export machinery can lead to misinterpretation of the assay readout. Here we report the generation of U2nesRELOC, a cell-based system for the identification of nuclear export inhibitors and specific silencers of the nuclear export machinery, and its adaptation to high throughput screening. The assay is based on mammalian cells stably expressing green fluorescent protein (GFP)-labeled Rev protein, which contains a strong heterologous NES. The fluorescent signal of untreated U2nesRELOC cells localizes exclusively to the cytoplasm. Upon treatment with the nuclear export inhibitor leptomycin B the GFP-labeled reporter protein accumulates rapidly in the cell nucleus. The assay has been adapted to 96-multiwell format and fully automated. Pilot experiments with a panel of 50 test compounds using three different concentrations per compound resulted in very consistent data sets with excellent reproducibility and an average Z' value of 0.76. In summary, U2nesRELOC is a cell-based nuclear export assay suitable for high throughput screening, providing counterscreens for pathway deconvolution.

High Content Screening to Monitor G Protein-Coupled Receptor Internalisation

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

High content kinetic assays of neuronal signaling implemented on BD pathway HT

A great deal of information can be gained from kinetic fluorescence-based measurement of cellular responses; however, until recently the use of such approaches has been limited by the manual nature of the instrumentation available. Higher-throughput kinetic studies of signaling pathways are greatly facilitated by new confocal, liquid handling-enabled, high content screening (HCS) platforms. In the present work, we have implemented one such instrument, the BD(TM) Pathway HT bioimager (BD Biosciences, Rockville, MD), for studying regulation of neuronal signaling pathways. We have established a neuronal calcium oscillation model, whereby rate of oscillation, amplitude of oscillation, and level of synchronicity across the culture can be measured. We have implemented membrane potential measurement using fluorescence resonance energy transfer-based dyes, for single cell characterization on this platform, showing the benefits of a truly flexible excitation and recording system; this dye combination cannot be readily implemented on all HCS platforms because of constraints of excitation wavelengths. We have validated long-term intracellular calcium imaging experiments, using innovative dyes and BD Pathway HT's spinning disk-based confocal excitation. To maximize both throughput and reproducibility, walk-away automation integration of this bioimaging technology has been implemented, producing an affordable, compact platform for fully automated kinetic HCS.

Generation and characterization of a stable MK2-EGFP cell line and subsequent development of a high-content imaging assay on the Cellomics ArrayScan platform to screen for p38 mitogen-activated protein kinase inhibitors

This chapter describes the generation and characterization of a stable MK2-EGFP expressing HeLa cell line and the subsequent development of a high-content imaging assay on the Cellomics ArrayScan platform to screen for p38 MAPK inhibitors. Mitogen-activated protein kinase activating protein kinase-2 (MK2) is a substrate of p38 MAPK kinase, and p38-induced phosphorylation of MK-2 induces a nucleus to cytoplasm translocation (Engel et al., 1998; Neininger et al., 2001; Zu et al., 1995). Through a process of heterologous expression of a MK2-EGFP fusion protein in HeLa cells using retroviral infection, antibiotic selection, and flow sorting, we were able to isolate a cell line in which the MK2-EGFP translocation response could be robustly quantified on the Cellomics ArrayScan platform using the nuclear translocation algorithm. A series of assay development experiments using the A4-MK2-EGFP-HeLa cell line are described to optimize the assay with respect to cell seeding density, length of anisomycin stimulation, dimethyl sulfoxide tolerance, assay signal window, and reproducibility. The resulting MK2-EGFP translocation assay is compatible with high-throughput screening and was shown to be capable of identifying p38 inhibitors. The MK2-EGF translocation response is susceptible to other classes of inhibitors, including nonselective kinase inhibitors, kinase inhibitors that inhibit upstream kinases in the p38 MAPK signaling pathway, and kinases involved in cross talk between different modules (ERKs, JNKs, and p38s) of the MAPK signaling pathways. An example of mining "high-content" image-based multiparameter data to extract additional information on the effects of compound treatment of cells is presented.

Novel fluorescent proteins for high-content screening

The development of fast microscopic imaging devices has enabled the application of automated fluorescence microscopy to pharmaceutical high-throughput drug-discovery assays, referred to as high-content screening (HCS). Initially, green fluorescent protein and its derivatives from Aequorea Victoria, and later anthozoan fluorescent proteins (FPs) have become potent tools as live-cell markers in HCS assays. We illustrate the broad applicability of classic and novel FPs to drug-discovery assays, giving example applications of the use of FPs in multiplexed imaging as fluorescent timers, photosensitizers and pulse-chase labels, and for robotically integrated compound testing.

Identification of RAS-Mitogen-Activated Protein Kinase Signaling Pathway Modulators in an ERF1 Redistribution® Screen

The RAS-mitogen-activated protein kinase (MAPK) signaling pathway has a central role in regulating the proliferation and survival of both normal and tumor cells. This pathway has been 1 focus area for the development of anticancer drugs, resulting in several compounds, primarily kinase inhibitors, in clinical testing. The authors have undertaken a cell-based, high-throughput screen using a novel ERF1 Redistribution® assay to identify compounds that modulate the signaling pathway. The hit compounds were subsequently tested for activity in a functional cell proliferation assay designed to selectively detect compounds inhibiting the proliferation of MAPK pathway-dependent cancer cells. The authors report the identification of 2 cell membrane-permeable compounds that exhibit activity in the ERF1 Redistribution® assay and selectively inhibit proliferation of MAPK pathway-dependent malignant melanoma cells at similar potencies (IC50=< 5 μM). These compounds have drug-like structures and are negative in RAF, MEK, and ERK in vitro kinase assays. Drugs belonging to these compound classes may prove useful for treating cancers caused by excessive MAPK pathway signaling. The results also show that cell-based, high-content Redistribution® screens can detect compounds with different modes of action and reveal novel targets in a pathway known to be disease relevant.

Cellular imaging in drug discovery

Traditional screening paradigms often focus on single targets. To facilitate drug discovery in the more complex physiological environment of a cell or organism, powerful cellular imaging systems have been developed. The emergence of these detection technologies allows the quantitative analysis of cellular events and visualization of relevant cellular phenotypes. Cellular imaging facilitates the integration of complex biology into the screening process, and addresses both high-content and high-throughput needs. This review describes how cellular imaging technologies contribute to the drug discovery process.

MAPKAP kinases - MKs - two's company, three's a crowd

Downstream of mitogen-activated protein kinases (MAPKs), three structurally related MAPK-activated protein kinases (MAPKAPKs or MKs) - MK2, MK3 and MK5 - signal to diverse cellular targets. Although there is no known common function for all three MKs, these kinases are involved in important processes: MKs regulate gene expression at the transcriptional and post-transcriptional level, control cytoskeletal architecture and cell-cycle progression, and are implicated in inflammation and cancer.

Development and implementation of three mitogen-activated protein kinase (MAPK) signaling pathway imaging assays to provide MAPK module selectivity profiling for kinase inhibitors: MK2-EGFP translocation, c-Jun, and ERK activation

This chapter describes the development and implementation of three independent imaging assays for the major mitogen-activated protein kinase (MAPK) signaling modules: p38, JNK, and ERK. There are more than 500 protein kinases encoded in the human genome that share an ATP-binding site and catalytic domain conserved in both sequence and structure. The majority of kinase inhibitors have been found to be competitive with ATP, raising concerns regarding kinase selectivity and potency in an environment of millimolar intracellular concentrations of ATP, as well as the potential for off-target effects via the many other cellular proteins that bind and/or utilize ATP. The apparent redundancy of the kinase isoforms and functions in the MAPK signaling modules present additional challenges for kinase inhibitor selectivity and potency. Imaging assays provide a method to address many of these concerns. Cellular imaging approaches facilitate analysis of the targets expressed in the context of their endogenous substrates and scaffolding proteins and in a complex environment for which subcellular localization, cross talk between pathways, phosphatase regulatory control, and intracellular ATP concentrations are relevant to the functions of the kinase. The assays described herein provide a strategy to profile kinase inhibitors for MAPK pathway selectivity while simultaneously providing information on cell morphology or toxicity. Results suggest that the MAPK pathways are indeed susceptible to nonselective kinase inhibitors such as staurosporin and inhibitors that inhibit upstream MAPK Kinase Kinases (MKKKs) and MAPK Kinases (MKKs) in the MAPK signaling pathway, especially those involved in cross talk between the pathways. However, selective MAPK inhibitors were identified that exhibited pathway selectivity as evidenced by significantly lower IC(50) values for their respective p38, JNK, or ERK signaling pathway assays.

Photoconvertible Fluorescent Protein EosFP: Biophysical Properties and Cell Biology Applications

EosFP is a fluorescent protein from the coral Lobophyllia hemprichii that changes its fluorescence emission from green to red upon irradiation with near-UV light. Here we present the spectroscopic properties of wild-type EosFP and a variety of monomeric and dimeric mutants and provide a structural interpretation of its oligomerization and photoconversion, which is based on X-ray structure analysis of the green and red species that we reported recently. Because functional expression of the monomeric EosFP variant is limited to temperatures of 30 degrees C, we have developed a tandem dimer. This construct, in which two EosFP subunits are connected by a flexible 12 amino acid linker, expresses well after fusion with the androgen and endothelin A receptors at 37 degrees C. A variety of applications in cellular imaging, developmental biology and automated high-content screening applications are presented, which demonstrate that EosFP is a powerful tool for in vivo monitoring of cellular processes.

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

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

Assay Development and Case History of a 32K‐Biased Library High‐Content MK2‐EGFP Translocation Screen to Identify p38 Mitogen‐Activated Protein Kinase Inhibitors on the ArrayScan 3.1 Imaging Platform

This chapter describes the conversion and assay development of a 96-well MK2-EGFP translocation assay into a higher density 384-well format high-content assay to be screened on the ArrayScan 3.1 imaging platform. The assay takes advantage of the well-substantiated hypothesis that mitogen-activated protein kinase-activating protein kinase-2 (MK2) is a substrate of p38 MAPK kinase and that p38-induced phosphorylation of MK-2 induces a nucleus-to-cytoplasm translocation. This chapter also presents a case history of the performance of the MK2-EGFP translocation assay, run as a "high-content" screen of a 32K kinase-biased library to identify p38 inhibitors. The assay performed very well and a number of putative p38 inhibitor hits were identified. Through the use of multiparameter data provided by the nuclear translocation algorithm and by checking images, a number of compounds were identified that were potential artifacts due to interference with the imaging format. These included fluorescent compounds, or compounds that dramatically reduced cell numbers due to cytotoxicity or by disrupting cell adherence. A total of 145 compounds produced IC(50) values <50.0 muM in the MK2-EGFP translocation assay, and a cross target query of the Lilly-RTP HTS database confirmed their inhibitory activity against in vitro kinase targets, including p38a. Compounds were confirmed structurally by LCMS analysis and profiled in cell-based imaging assays for MAPK signaling pathway selectivity. Three of the hit scaffolds identified in the MK2-EGFP translocation HCS run on the ArrayScan were selected for a p38a inhibitor hit-to-lead structure activity relationship (SAR) chemistry effort.

Probing the primary screening efficiency by multiple replicate testing: a quantitative analysis of hit confirmation and false screening results of a biochemical assay

Despite a large body of references on assay development, assay optimization, strategies, and methodologies for high-throughput screening (HTS), there have been few reports on investigations of the efficiency of primary screening in a systematic and quantitative manner for a typical HTS process. Recently, the authors investigated the primary hit comparison and the effect of measurement variability by screening a library of approximately 25,000 random compounds in multiple replicate tests in a nuclear receptor recruitment assay with 2 different assay detection technologies. In this report, we utilized these sets of multiple replicate screening data from a different perspective and conducted a systematic data analysis in order to gain some insights into the hit-finding efficiency of a typical primary screening process. Specifically, hit confirmation, false-positive (declaration) rates, and false-negative rates at different hit cutoff limits were explored and calculated from the 2 different assay formats. Results and analyses provided some quantitative estimation regarding the reliability and efficiency of the primary screening process. For the 2 assay formats tested in this report, the confirmation rate (activity repeated at or above a certain hit limit) was found to be 65% or above. It was also suggested that, at least in this case, applying some hit-selection strategies, it is possible to decrease the number of false-negative or false-positive hits without significantly increasing the efforts in primary screening.

Translocation Biosensors to Study Signal-Specific Nucleo-Cytoplasmic Transport, Protease Activity and Protein-Protein Interactions

Regulated nucleo-cytoplasmic transport is crucial for cellular homeostasis and relies on protein interaction networks. In addition, the spatial division into the nucleus and the cytoplasm marks two intracellular compartments that can easily be distinguished by microscopy. Consequently, combining the rules for regulated nucleo-cytoplasmic transport with autofluorescent proteins, we developed novel cellular biosensors composed of glutathione S-transferase, mutants of green fluorescent protein and rational combinations of nuclear import and export signals. Addition of regulatory sequences resulted in three classes of biosensors applicable for the identification of signal-specific nuclear export and import inhibitors, small molecules that interfere with protease activity and compounds that prevent specific protein-protein interactions in living cells. As a unique feature, our system exploits nuclear accumulation of the cytoplasmic biosensors as the reliable readout for all assays. Efficacy of the biosensors was systematically investigated and also demonstrated by using a fully automated platform for high throughput screening (HTS) microscopy and assay analysis. The introduced modular biosensors not only have the potential to further dissect nucleo-cytoplasmic transport pathways but also to be employed in numerous screening applications for the early stage evaluation of potential drug candidates.

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.

Emerging classes of protein–protein interaction inhibitors and new tools for their development

Protein-protein interactions play a key role in the signal transduction pathways that regulate cellular function. Three years ago, few descriptions of small molecule protein-protein interaction inhibitors (SMPPIIs) existed in the literature. Today, the number of examples of both the biology and chemistry of such interaction inhibitors is growing rapidly. This growth occurs at the convergence of medicinal chemistry, signaling biology and novel assay technology for profiling emerging compound classes and modes of action. Protein translocation assays provide a unique new tool for identifying, profiling, and optimizing SMPPIIs. This review summarizes recent work in the field, and outlines future developments we can anticipate.