UBC13 is a non-canonical Ubiquitin Conjugating Enzyme (E2) that has been implicated in a variety of cellular signaling processes due to its ability to catalyze formation of Lysine 63-linked polyubiquitin chains on various substrates. In particular, UBC13 is required for signaling by a variety of receptors important in immune regulation, making it a candidate target for inflammatory diseases. UBC13 is also critical for double-strand DNA repair, and thus a potential radiosensitizer and chemosensitizer target for oncology. We developed a high-throughput screening (HTS) assay for UBC13 based on the method of time-resolved fluorescence resonance energy transfer (TR-FRET). The TR-FRET assay combines fluorochrome (Fl)-conjugated ubiquitin (fluorescence acceptor) with terbium (Tb)-conjugated ubiquitin (fluorescence donor), such that the assembly of mixed chains of Fl- and Tb-ubiquitin creates a robust TR-FRET signal. We defined conditions for optimized performance of the TR-FRET assay in both 384 and 1536-well formats. Chemical library screens (total 456,865 compounds) were conducted in high-throughput mode using various compound collections, affording superb Z' scores (typically > 0.7) and thus validating the performance of the assays. Altogether, the HTS assays described here are suitable for large-scale, automated screening of chemical libraries in search of compounds with inhibitory activity against UBC13.
PML is a tumor suppressor that promotes apoptosis through both p53-dependent and - independent mechanisms, participates in Rb-mediated cell cycle arrest, inhibits neoangiogenesis, and contributes to maintenance of genomic stability. PML also plays a role in host defense against viruses, conferring antiviral activity. When active, PML localizes to subnuclear structures named PML oncogenic domains (PODs) or PML nuclear bodies (PML-NBs), whereas inactive PML is located diffusely throughout the nucleus of cells, thus providing a morphological indicator. Known activators of PML include arsenicals and interferons, however, these agents induce a plethora of toxic effects, limiting their effectiveness. The objective of the current study was to develop a high content screening (HCS) assay for the identification of chemical activators of PML. We describe methods for automated analysis of POD formation using high throughput microscopy (HTM) to localize PML immunofluorescence in conjunction with image analysis software for POD quantification. Using this HCS assay in 384 well format, we performed pilot screens of a small synthetic chemical library and mixture-based combinatorial libraries, demonstrating the robust performance of the assay. HCS counter-screening assays were also developed for hit characterization, based on immunofluorescence analyses of the subcellular location of phosphorylated H2AX or phosphorylated CHK1, which increase in a punctate nuclear pattern in response to DNA damage. Thus, the HCS assay devised here represents a high throughput screen that can be utilized to discover POD-inducing compounds that may restore the tumor suppressor activity of PML in cancers or possibly promote anti-viral states.
PML; POD; nuclear bodies; apoptosis; high content screening
The standard (STD) 5 × 5 hybrid median filter (HMF) was previously described as a nonparametric local backestimator of spatially arrayed microtiter plate (MTP) data. As such, the HMF is a useful tool for mitigating global and sporadic systematic error in MTP data arrays. Presented here is the first known HMF correction of a primary screen suffering from systematic error best described as gradient vectors. Application of the STD 5 × 5 HMF to the primary screen raw data reduced background signal deviation, thereby improving the assay dynamic range and hit confirmation rate. While this HMF can correct gradient vectors, it does not properly correct periodic patterns that may present in other screening campaigns. To address this issue, 1 × 7 median and a row/column 5 × 5 hybrid median filter kernels (1 × 7 MF and RC 5 × 5 HMF) were designed ad hoc, to better fit periodic error patterns. The correction data show periodic error in simulated MTP data arrays is reduced by these alternative filter designs and that multiple corrective filters can be combined in serial operations for progressive reduction of complex error patterns in a MTP data array.
Kinases are attractive drug targets because of the central roles they play in signal transduction pathways and human diseases. Their well-formed ATP-binding pockets make ideal targets for small molecule inhibitors. For drug discovery purposes, many peptide-based kinase assays have been developed that measure substrate phosphorylation using fluorescence-based readouts. However, for some kinases these assays may not be appropriate. In the case of the LIM kinases (LIMK), an inability to phosphorylate peptide substrates resulted in previous high-throughput screens (HTS) using radioactive labeling of recombinant cofilin protein as the readout. We describe the development of a HTS-compatible assay that measures relative ATP levels using luciferase-generated luminescence as a function of LIMK activity. The assay was inexpensive to perform and proof-of-principle screening of kinase inhibitors demonstrated that compound potency against LIMK could be determined; ultimately the assay was used for successful prosecution of automated HTS. Following HTS, the secondary assay format was changed to obtain more accurate measures of potency and mechanism of action using more complex (and expensive) assays. The luciferase assay nonetheless provides an inexpensive and reliable primary assay for HTS that allowed for the identification of LIMK inhibitors to initiate discovery programs for the eventual treatment of human diseases.
LIM kinase; assay; luminescence; high-throughput screen
The authors conducted a high-throughput screening campaign for inhibitors of SV40 large T antigen ATPase activity to identify candidate antivirals that target the replication of polyomaviruses. The primary assay was adapted to 1536-well microplates and used to screen the National Institutes of Health Molecular Libraries Probe Centers Network library of 306 015 compounds. The primary screen had an Z value of ~0.68, signal/background = 3, and a high (5%) DMSO tolerance. Two counterscreens and two secondary assays were used to prioritize hits by EC50, cytotoxicity, target specificity, and off-target effects. Hits that inhibited ATPase activity by >44% in the primary screen were tested in dose–response efficacy and eukaryotic cytotoxicity assays. After evaluation of hit cytotoxicity, drug likeness, promiscuity, and target specificity, three compounds were chosen for chemical optimization. Chemical optimization identified a class of bisphenols as the most effective biochemical inhibitors. Bisphenol A inhibited SV40 large T antigen ATPase activity with an IC50 of 41 μM in the primary assay and 6.2 μM in a cytoprotection assay. This compound class is suitable as probes for biochemical investigation of large T antigen ATPase activity, but because of their cytotoxicity, further optimization is necessary for their use in studying polyomavirus replication in vivo.
SV40; chemistry; large T antigen; HTS; antivirals
Methylation is a ubiquitous covalent modification used to control the function of diverse biomolecules including hormones, neurotransmitters, xenobiotics, proteins, nucleic acids and lipids. Histone methyltransferases (HMTs) are currently of high interest as drug targets because of their role in epigenetic regulation, however most HMT assay methods are either not amenable to an HTS environment or are applicable to a limited number of enzymes. We developed a generic methyltransferase assay method using fluorescent immunodetection of AMP, which is formed from the MT reaction product S-adenosylhomocysteine in a dual enzyme coupling step. The detection range of the assay, its suitability for HTS, including stability of reagents following dispensing and after addition to reactions as well as the potential for interference from drug like molecules was investigated. In addition, the use of the assay for measuring inhibitor potencies with peptide or intact protein substrates was examined through pilot screening with selected reference enzymes including HMT G9a. By combining a novel enzymatic coupling step with the well characterized Transcreener® AMP/GMP assay, we have developed a robust HTS assay for HMTs which should be broadly applicable to other types of methyltransferases as well.
epigenetics; histone methyltransferase; DNA methyltransferase; methyltransferase assay; high throughput screening
Misregulation of the Wnt pathway has been shown to be responsible for a variety of human diseases, most notably cancers. Screens for inhibitors of this pathway have been performed almost exclusively using cultured mammalian cells or with purified proteins. We have previously developed a biochemical assay using Xenopus egg extracts to recapitulate key cytoplasmic events in the Wnt pathway. Using this biochemical system, we show that a recombinant form of the Wnt coreceptor, LRP6, regulates the stability of two key components of the Wnt pathway (β-catenin and Axin) in opposing fashion. We have now fused β-catenin and Axin to firefly and Renilla luciferase, respectively, and demonstrate that the fusion proteins behave similarly as their wild-type counterparts. Using this dual luciferase readout, we adapted the Xenopus extracts system for high-throughput screening. Results from these screens demonstrate signal distribution curves that reflect the complexity of the library screened. Of several compounds identified as cytoplasmic modulators of the Wnt pathway, one was further validated as a bona fide inhibitor of the Wnt pathway in cultured mammalian cells and Xenopus embryos. We show that other embryonic pathways may be amendable to screening for inhibitors/modulators in Xenopus egg extracts.
Wnt signaling; Xenopus extracts; β-catenin; Axin; LRP6; flavonoids
Activation of the antioxidant response element (ARE) up-regulates enzymes involved in detoxification of electrophiles and reactive oxygen species. The induction of ARE genes is regulated by the interaction between redox sensor protein, Keap1, and the transcription factor, Nrf2. Fluorescently labeled Nrf2 peptides containing the ETGE motif were synthesized and optimized as tracers in the development of a fluorescence polarization (FP) assay to identify small molecule inhibitors of Keap1-Nrf2 interaction. The tracers were optimized to increase the dynamic range of the assay and their binding affinities to the Keap1 Kelch domain. The binding affinities of Nrf2 peptide inhibitors obtained in our FP assay using FITC-9mer Nrf2 peptide amide as the probe were in good agreement with those obtained previously by a surface plasmon resonance (SPR) assay. The FP assay exhibits considerable tolerance towards DMSO and produced a Z'-factor greater than 0.6 in a 384-well format. Further optimization of the probe led to cyanine-labeled 9mer Nrf2 peptide amide, which can be used along with the FITC-9mer Nrf2 peptide amide in a high throughput screening (HTS) assay to discover small molecule inhibitors of Keap1-Nrf2 interaction.
Nrf2; Keap1; ARE; fluorescence polarization; high throughput screening; oxidative response
Automated microscopes have enabled the unprecedented collection of images at a rate that precludes visual inspection. Automated image analysis is required to identify interesting samples and extract quantitative information for high content screening (HCS). However, researchers are impeded by the lack of metrics and software tools to identify image-based aberrations that pollute data, limiting an experiment's quality. We have developed and validated approaches to identify those image acquisition artifacts that prevent optimal extraction of knowledge from high-throughput microscopy experiments. We have implemented these as a versatile, open-source toolbox of algorithms and metrics readily usable by biologists to improve data quality in a wide variety of biological experiments.
Aberrant protein-protein interactions are attractive drug targets in a variety of neurodegenerative diseases due to the common pathology of accumulation of protein aggregates. In amyotrophic lateral sclerosis, mutations in SOD1 cause the formation of aggregates and inclusions that may sequester other proteins and disrupt cellular processes. It has been demonstrated that mutant SOD1, but not wild-type SOD1, interacts with the axonal transport motor dynein and that this interaction contributes to motor neuron cell death, suggesting that disrupting this interaction may be a potential therapeutic target. However, it can be challenging to configure a high-throughput screening (HTS)–compatible assay to detect inhibitors of a protein-protein interaction. Here we describe the development and challenges of an HTS for small-molecule inhibitors of the mutant SOD1-dynein interaction. We demonstrate that the interaction can be formed by coexpressing the A4V mutant SOD1 and dynein intermediate complex in cells and that this interaction can be disrupted by compounds added to the cell lysates. Finally, we show that some of the compounds identified from a pilot screen to inhibit the protein-protein interaction with this method specifically disrupt the interaction between the dynein complex and mtSOD1 but not the dynein complex itself when applied to live cells.
CNS and PNS diseases; protein-protein interactions; fluorescence methods; cell-based assays
Class O forkhead box (FOXO) transcription factors are downstream targets of the PI3K/AKT signaling pathway, which is upregulated in many tumors. AKT phosphorylates and inactivates FOXO1 by relocating it to the cytoplasm. Because FOXO1 functions as a tumor suppressor by negatively regulating cell-cycle progression and cell survival, compounds that promote FOXO1 localization to the nucleus might have therapeutic value in oncology. Here we describe the identification of such compounds by using an image-based, high content screen. Compounds that were active in retaining FOXO1 in the nucleus were tested to determine their pathway-specificity and isoform-specificity by using high content assays for Rev and FOXO3, respectively.
high content screening; FOXO1; AKT; FOXO3
There are no effective antivirals currently available for the treatment of flavivirus infection in humans. As such, the identification and characterization of novel drug target sites are critical to developing new classes of antiviral drugs. The flavivirus NS5 N-terminal capping enzyme (CE) is vital for the formation of the viral RNA cap structure, which directs viral polyprotein translation and stabilizes the 5′ end of the viral genome. The structure of the flavivirus CE has been solved, and a detailed understanding of the CE–guanosine triphosphate (GTP) and CE–RNA cap interactions is available. Because of the essential nature of the interaction for viral replication, disrupting CE–GTP binding is an attractive approach for drug development. The authors have previously developed a robust assay for monitoring CE–GTP binding in real time. They adapted this assay for high-throughput screening and performed a pilot screen of 46 323 commercially available compounds. A number of small-molecule inhibitors capable of displacing a fluorescently labeled GTP in vitro were identified, and a second functional assay was developed to identify false positives. The results presented indicate that the flavivirus CE cap-binding site is a valuable new target site for antiviral drug discovery and should be further exploited for broad-spectrum anti-flaviviral drug development.
flavivirus; NS5 N-terminal capping enzyme (CE); high-throughput screening; drug development; anti-infective drugs; fluorescence polarization (FP)
The molecular pathology of many protein misfolding, toxic gain-of-function diseases, such as amyotrophic lateral sclerosis (ALS), is not well understood. Although protein misfolding and aggregation are common themes in these diseases, efforts to identify cellular factors that regulate this process in an unbiased fashion and on a global scale have been lacking. Using an adapted version of an extant β-gal-based protein solubility assay, an expression screen for cellular modulators of solubility of an ALS-causing mutant SOD1 was carried out in mammalian cells. Following fluorescence-activated cell sorting enrichment of a mouse spinal cord cDNA library for gene products that increased SOD1 solubility, high-throughput screening of the cDNA pools from this enriched fraction was employed to identify pools containing relevant modulators. Positive pools, containing approximately 10 cDNA clones each, were diluted and rescreened iteratively until individual clones that improved SOD1 folding/solubility were identified. Genes with profound effects in the solubility assay were selected for validation by independent biochemical assays. Six of 10 validated genes had a significant effect on SOD1 solubility and folding in a SOD1 promoter-driven β-gal assay, indicating that global screening of cellular targets using such protein solubility/folding assay is viable and can be adapted for other misfolding diseases.
cDNA expression cloning screen; amyotrophic lateral sclerosis; superoxide dismutase 1; protein solubility assay
Inhibitors of human dimethylarginine dimethylaminohydrolase-1 (DDAH-1) are of therapeutic interest for controlling pathological nitric oxide production. Only a limited number of biologically useful inhibitors have been identified, so structurally diverse lead compounds are desired. In contrast with previous assays that do not possesses adequate sensitivity for optimal screening, herein is reported a high-throughput assay that uses an alternative thiol-releasing substrate, S-methyl-L-thiocitrulline, and a thiol-reactive fluorophore, 7-diethylamino-3-(4′-maleimidylphenyl)-4-methylcoumarin, to enable continuous detection of product formation by DDAH-1. The assay is applied to query two commercial libraries totaling 4,446 compounds and two representative hits are described, including a known DDAH-1 inhibitor. This is the most sensitive DDAH-1 assay reported to date, and enables screening of compound libraries using [S] =KM conditions, while displaying Z′ factors from 0.6 – 0.8. Therefore, this strategy now makes possible high-throughput screening for human DDAH-1 inhibitors in pursuit of molecular probes and drugs to control excessive nitric oxide production.
Dimethylarginine dimethylaminohydrolase; high-throughput screening; nitric oxide; CPM
The thyroid hormone receptors (TR) are members of the nuclear hormone receptor (NHR) superfamily that regulate development, growth, and metabolism. Upon ligand binding, TR releases bound corepressors and recruits coactivators to modulate target gene expression. Steroid Receptor Coactivator 2 (SRC2) is an important coregulator that interacts with TRβ to activate gene transcription. To identify novel inhibitors of the TRβ and SRC2 interaction, we performed a quantitative high throughput screen (qHTS) of a TRβ-SRC2 fluorescence polarization assay against more than 290,000 small molecules. The qHTS assayed compounds at six concentrations up to 92 uM to generate titration-response curves and determine the potency and efficacy of all compounds. The qHTS dataset enabled the characterization of actives for structure-activity relationships as well as for potential artifacts such as fluorescence interference. Selected qHTS actives were tested in the screening assay using fluoroprobes labeled with Texas Red or fluorescein. The retest identified 19 series and 4 singletons as active in both assays with 40% or greater efficacy, free of compound interference and not toxic to mammalian cells. Selected compounds were tested as independent samples and a methylsulfonylnitrobenzoate series inhibited the TRβ-SRC2 interaction with 5 uM IC50. This series represents a new class of thyroid hormone receptor-coactivator modulators.
thyroid receptor; small molecule; HTS; coactivator; protein-protein interaction
The type II secretion (T2S) system in Gram-negative bacteria is comprised of the Sec and Tat pathways for translocating proteins into the periplasm and an outer membrane secretin for transporting proteins into the extracellular space. To discover Sec/Tat/T2S pathway inhibitors as potential new therapeutics, we used a Pseudomonas aeruginosa bioluminescent reporter strain responsive to SecA depletion and inhibition to screen compound libraries and characterize the hits. The reporter strain placed a luxCDABE operon under regulation of a SecA depletion-responsive up-regulated promoter in a secA deletion background complemented with an ectopic lac-regulated secA copy. Bioluminescence was indirectly proportional to the IPTG concentration and stimulated by azide, a known SecA ATPase inhibitor. A total of 96 compounds (0.1% of 73,000) were detected as primary hits due to stimulation of luminescence with a z-score ≥5. Direct secretion assays of the 9 most potent hits, representing 5 chemical scaffolds, revealed that they do not inhibit SecA-mediated secretion of β-lactamase into the periplasm, but do inhibit T2S-mediated extracellular secretion of elastase with IC50 values from 5 – 25 μM. In addition, 7 of the 9 compounds also inhibited the T2S-mediated extracellular secretion of phospholipases C by P. aeruginosa and of protease activity by Burkholderia pseudomallei.
P. aeruginosa; type II secretion; high throughput screening; inhibitors
Despite advances toward understanding the prevention and treatment of many cancers, patients who suffer from oral squamous cell carcinoma (OSCC) confront a survival rate that has remained unimproved for more than two decades indicating our ability to treat them pharmacologically has reached a plateau. In an ongoing effort to improve the clinical outlook for this disease, we previously reported that an essential component of the mechanism by which the proteasome inhibitor bortezomib (PS-341, Velcade) induced apoptosis in OSCC required the activation of a terminal unfolded protein response (UPR). Predicated on these studies, we hypothesized that high throughput screening (HTS) of large diverse chemical libraries might identify more potent or selective small molecule activators of the apoptotic arm of the UPR to control or kill OSCC. We have developed complementary cell-based assays using stably transfected CHO-K1 cell lines that individually assess the PERK/eIF2α/CHOP (apoptotic) or the IRE1/XBP1 (adaptive) UPR sub-pathways. A ~66K compound collection was screened at the University of Michigan Center for Chemical Genomics that included a unique library of pre-fractionated natural product extracts. The mycotoxin methoxycitrinin was isolated from a natural extract and found to selectively activate the CHOP-luciferase reporter at 80μM. A series of citrinin derivatives were isolated from these extracts, including a unique congener that has not been previously described. In an effort to identify more potent compounds we examined the ability of citrinin and the structurally related mycotoxins ochratoxin A and patulin to activate the UPR. Strikingly, we found that patulin at 2.5 – 10μM induced a terminal UPR in a panel of OSCC cells that was characterized by an increase in CHOP, GADD34 and ATF3 gene expression and XBP1 splicing. A luminescent caspase assay and the induction of several BH3-only genes indicated that patulin could induce apoptosis in OSCC cells. These data support the use of this complementary HTS strategy to identify novel modulators of UPR signaling and tumor cell death.
unfolded protein response; endoplasmic reticulum stress; cell-based assay; luciferase reporter; natural products
High-throughput screening data repositories, such as PubChem, represent valuable resources for the development of small molecule chemical probes and can serve as entry points for drug discovery programs. While the loose data format offered by PubChem allows for great flexibility, important annotations, such as the assay format and technologies employed, are not explicitly indexed. We have previously developed a BioAssay Ontology (BAO) and curated over 350 assays with standardized BAO terms. Here we describe the use of BAO annotations to analyze a large set of assays that employ luciferase- and β-lactamase-based technologies. We identified promiscuous chemotypes pertaining to different sub-categories of assays and specific mechanisms by which these chemotypes interfere in reporter gene assays. Our results show that the data in PubChem can be used to identify promiscuous compounds that interfere non-specifically with particular technologies. Furthermore, we show that BAO is a valuable toolset for the identification of related assays and for the systematic generation of insights that are beyond the scope of individual assays or screening campaigns.
compound promiscuity; assay ontology; reporter gene assays; high-throughput screening data analysis; cheminformatics
JAK3 has become an ideal target for the therapeutic treatment of immune-related diseases, as well as for the prevention of organ allograft rejection. A number of JAK3 inhibitors have been identified by in vitro biochemical enzymatic assays, but the majority display significant off-target effects on JAK2. Therefore, there is an urgent need to develop new experimental approaches to identify compounds that specifically inhibit JAK3. Here, we showed that in 32D/IL-2Rβ cells, STAT5 becomes phosphorylated by IL-3/JAK2- or IL-2/JAK3-dependent pathway. Importantly, the selective JAK3 inhibitor CP-690,550 blocked the phosphorylation as well as the nuclear translocation of STAT5 following treatment of cells with IL-2, but not with IL-3. In an attempt to use the cells for large-scale chemical screens to identify JAK3 inhibitors, we established a cell line 32D/IL-2Rβ/6×STAT5 stably expressing a well-characterized STAT5 reporter gene. Treatment of this cell line with IL-2 or IL-3 dramatically increased the reporter activity in a high-throughput format. As expected, JAK3 inhibitors, CP-690,550 and JAK3 inhibitor VI, selectively inhibited the activity of the 6×STAT5 reporter following treatment with IL-2. By contrast, the pan-JAK inhibitor Curcumin non-selectively inhibited the activity of this reporter following treatment with either IL-2 or IL-3. Thus, this study indicates that our STAT5 reporter cell line can be used as an efficacious cellular model for chemical screens to identify low-molecular-weight inhibitors specific for JAK3.
Assay development; Cytokine; JAK3 inhibitors; STAT5 reporter; high-throughput chemical screening
DNA gyrase, a type II topoisomerase that introduces negative supercoils into DNA, is a validated antibacterial drug target. The holoenzyme is composed of 2 subunits, gyrase A (GyrA) and gyrase B (GyrB), which form a functional A2B2 heterotetramer required for bacterial viability. A novel fluorescence polarization (FP) assay has been developed and optimized to detect inhibitors that bind to the adenosine triphosphate (ATP) binding domain of GyrB. Guided by the crystal structure of the natural product novobiocin bound to GyrB, a novel novobiocin–Texas Red probe (Novo-TRX) was designed and synthesized for use in a high-throughput FP assay. The binding kinetics of the interaction of Novo-TRX with GyrB from Francisella tularensis has been characterized, as well as the effect of common buffer additives on the interaction. The assay was developed into a 21-μL, 384-well assay format and has been validated for use in high-throughput screening against a collection of Food and Drug Administration–approved compounds. The assay performed with an average Z′ factor of 0.80 and was able to identify GyrB inhibitors from a screening library.
fluorescence polarization; gyrase; assay development; high-throughput screen; anthracycline
Methylation of arginine residues, catalyzed by protein arginine methyltransferases (PRMTs), is one important protein post-translational modification involved in epigenetic regulation of gene expression. A fast and effective assay for PRMT can provide valuable information for dissecting the biological functions of PRMTs, as well as for screening small-molecule inhibitors of arginine methylation. Currently, among the methods used for PRMT activity measurement, many contain laborious separation procedures, which restrict the applications of these assays for high-throughput screening (HTS) in drug discovery. The authors report here a mix-and-measure method to measure PRMT activity based on the principle of scintillation proximity assay (SPA). In this assay, 3H-AdoMet was used as methyl donor, and biotin-modified histone H4 peptide served as a methylation substrate. Following the methylation reaction catalyzed by PRMTs, streptavidin-coated SPA beads were added to the reaction solution, and SPA signals were detected by a MicroBeta scintillation counter. No separation step is needed, which simplifies the assay procedure and greatly enhances the assay speed. Particularly, the miniaturization and robustness suggest that this method is suited for HTS of PRMT inhibitors.
protein arginine methyltransferases; PRMT; scintillation proximity assay; SPA; high-throughput screening; HTS
Hepatitis C virus (HCV) is a considerable global health problem for which new classes of therapeutics are needed. We developed a high-throughput assay to identify compounds that selectively block translation initiation from the HCV internal ribosome entry site (HCV IRES). Rabbit reticulocyte lysate conditions were optimized to faithfully report on authentic HCV IRES-dependent translation relative to a 5′ capped mRNA control. We screened a library of ~430,000 small molecules for IRES inhibition, leading to ~1,700 initial hits. After secondary counter screening the vast majority of hits proved to be luciferase and general translation inhibitors. Despite well-optimized in vitro translation conditions, in the end we found no selective HCV IRES inhibitors but did discover a new scaffold of general translation inhibitor. The analysis of these molecules, and the finding that a large fraction of false positives resulted from off-target effects, highlights the challenges inherent in screens for RNA-specific inhibitors.
Hepatitis C virus (HCV); IRES; luciferase; high-throughput screen; rabbit reticulocyte lysate
The ribosome-inhibiting toxin ricin binds exposed β1→4 linked galactosyls on multiple glycolipids and glycoproteins on the cell surface of most eukaryotic cells. After endocytosis, internal cell trafficking is promiscuous, with only a small proportion of ricin proceeding down a productive (cytotoxic) trafficking route to the endoplasmic reticulum (ER). Here, the catalytic ricin A chain traverses the membrane to inactivate the cytosolic ribosomes, which can be monitored by measuring reduction in protein biosynthetic capacity or cell viability. Although some markers have been discovered for the productive pathway, many molecular details are lacking. To identify a more comprehensive set of requirements for ricin intoxication, the authors have developed an RNAi screen in Drosophila S2 cells, screening in parallel the effects of individual RNAi treatments alone and when combined with a ricin challenge. Initial screening of 806 gene knockdowns has revealed a number of candidates for both productive and nonproductive ricin trafficking, including proteins required for transport to the Golgi, plus potential toxin interactors within the ER and cytosol.
ricin; RNAi; S2 cells; screen; PDI; ERAD