TRIM25 Enhances the Antiviral Action of Zinc-Finger Antiviral Protein (ZAP).

The host factor and interferon (IFN)-stimulated gene (ISG) product, zinc-finger antiviral protein (ZAP), inhibits a number of diverse viruses by usurping and intersecting with multiple cellular pathways. To elucidate its antiviral mechanism, we perform a loss-of-function genome-wide RNAi screen to identify cellular cofactors required for ZAP antiviral activity against the prototype alphavirus, Sindbis virus (SINV). In order to exclude off-target effects, we carry out stringent confirmatory assays to verify the top hits. Important ZAP-liaising partners identified include proteins involved in membrane ion permeability, type I IFN signaling, and post-translational protein modification. The factor contributing most to the antiviral function of ZAP is TRIM25, an E3 ubiquitin and ISG15 ligase. We demonstrate here that TRIM25 interacts with ZAP through the SPRY domain, and TRIM25 mutants lacking the RING or coiled coil domain fail to stimulate ZAP's antiviral activity, suggesting that both TRIM25 ligase activity and its ability to form oligomers are critical for its cofactor function. TRIM25 increases the modification of both the short and long ZAP isoforms by K48- and K63-linked polyubiquitin, although ubiquitination of ZAP does not directly affect its antiviral activity. However, TRIM25 is critical for ZAP's ability to inhibit translation of the incoming SINV genome. Taken together, these data uncover TRIM25 as a bona fide ZAP cofactor that leads to increased ZAP modification enhancing its translational inhibition activity.

Suramin inhibits cullin-RING E3 ubiquitin ligases.

Cullin-RING E3 ubiquitin ligases (CRL) control a myriad of biological processes by directing numerous protein substrates for proteasomal degradation. Key to CRL activity is the recruitment of the E2 ubiquitin-conjugating enzyme Cdc34 through electrostatic interactions between E3's cullin conserved basic canyon and the acidic C terminus of the E2 enzyme. This report demonstrates that a small-molecule compound, suramin, can inhibit CRL activity by disrupting its ability to recruit Cdc34. Suramin, an antitrypansomal drug that also possesses antitumor activity, was identified here through a fluorescence-based high-throughput screen as an inhibitor of ubiquitination. Suramin was shown to target cullin 1's conserved basic canyon and to block its binding to Cdc34. Suramin inhibits the activity of a variety of CRL complexes containing cullin 2, 3, and 4A. When introduced into cells, suramin induced accumulation of CRL substrates. These observations help develop a strategy of regulating ubiquitination by targeting an E2-E3 interface through small-molecule modulators.

The "TSH Receptor Glo Assay" - A High-Throughput Detection System for Thyroid Stimulation.

BACKGROUND: To identify novel small molecules against the TSH receptor, we developed a sensitive transcription-based luciferase high-throughput screening (HTS) system named the TSHR-Glo Assay (TSHR-Glo). METHODS: This assay uses double-transfected Chinese hamster ovary cells stably expressing the human TSHR and a cAMP-response element (CRE) construct fused to an improved luciferase reporter gene. RESULTS: The assay was highly responsive toward TSH in a dose-dependent manner with a TSH sensitivity of 10(-10)M (10 ± 1.12 µU/ml) and thyroid-stimulating antibodies, a hallmark of Graves' disease, could also be detected. The assay was validated against the standard indicator of HTS performance - the Z-factor (Z') - producing a score of 0.895. Using the TSHR-Glo assay, we screened 48,224 compounds from a diverse chemical library in duplicate plates at a fixed dose of 17 µM. Twenty molecules with the greatest activity out of 62 molecules that were identified by this technique were subsequently screened against the parent luciferase stable cell line in order to eliminate false positive stimulators. CONCLUSION: Using this approach, we were able to identify specific agonists against the TSH receptor leading to the characterization of several TSH agonist molecules. Hence, the TSHR-Glo assay was a one-step cell-based HTS assay, which was successful in the discovery of novel small molecular agonists and for the detection of stimulating antibodies to the TSH receptor.

A high throughput Cre-lox activated viral membrane fusion assay identifies pharmacological inhibitors of HIV entry.

Enveloped virus entry occurs when viral and cellular membranes fuse releasing particle contents into the target cell. Human immunodeficiency virus (HIV) entry occurs by cell-free virus or virus transferred between infected and uninfected cells through structures called virological synapses. We developed a high-throughput cell-based assay to identify small molecule inhibitors of cell-free or virological synapse-mediated entry. An HIV clone carrying Cre recombinase as a Gag-internal gene fusion releases active Cre into cells upon viral entry activating a recombinatorial gene switch changing dsRed to GFP-expression. A screen of a 1998 known-biological profile small molecule library identified pharmacological HIV entry inhibitors that block both cell-free and cell-to-cell infection. Many top hits were noted as HIV inhibitors in prior studies, but not previously recognized as entry antagonists. Modest therapeutic indices for simvastatin and nigericin were observed in confirmatory HIV infection assays. This robust assay is adaptable to study HIV and heterologous viral pseudotypes.

Distribution Analyzer, a methodology for identifying and clustering outlier conditions from single-cell distributions, and its application to a Nanog reporter RNAi screen.

BACKGROUND: Chemical or small interfering (si) RNA screens measure the effects of many independent experimental conditions, each applied to a population of cells (e.g., all of the cells in a well). High-content screens permit a readout (e.g., fluorescence, luminescence, cell morphology) from each cell in the population. Most analysis approaches compare the average effect on each population, precluding identification of outliers that affect the distribution of the reporter in the population but not its average. Other approaches only measure changes to the distribution with a single parameter, precluding accurate distinction and clustering of interesting outlier distributions. RESULTS: We describe a methodology to identify outlier conditions by considering the cell-level measurements from each condition as a sample of an underlying distribution. With appropriate selection of a distance metric, all effects can be embedded in a fixed-dimensionality Euclidean basis, facilitating identification and clustering of biologically interesting outliers. We demonstrate that measurement of distances with the Hellinger distance metric offers substantial computational efficiencies over alternative metrics. We validate this methodology using an RNA interference (RNAi) screen in mouse embryonic stem cells (ESC) with a Nanog reporter. The methodology clusters effects of multiple control siRNAs into their true identities better than conventional approaches describing the median cell fluorescence or the commonly used Kolmogorov-Smirnov distance between the observed fluorescence distribution and the null distribution. It identifies outlier genes with effects on the reporter distribution that would have been missed by other methods. Among them, siRNA targeting Chek1 leads to a wider Nanog reporter fluorescence distribution. Similarly, siRNA targeting Med14 or Med27 leads to a narrower Nanog reporter fluorescence distribution. We confirm the roles of these three genes in regulating pluripotency by mRNA expression and alkaline phosphatase staining using independent short hairpin (sh) RNAs. CONCLUSIONS: Using our methodology, we describe each experimental condition by a probability distribution. Measuring distances between probability distributions permits a multivariate rather than univariate readout. Clustering points derived from these distances allows us to obtain greater biological insight than methods based solely on single parameters. We find several outliers from a mouse ESC RNAi screen that we confirm to be pluripotency regulators. Many of these outliers would have been missed by other analysis methods.

A high-throughput chemical screen reveals that harmine-mediated inhibition of DYRK1A increases human pancreatic beta cell replication.

Types 1 and 2 diabetes affect some 380 million people worldwide. Both ultimately result from a deficiency of functional pancreatic insulin-producing beta cells. Beta cells proliferate in humans during a brief temporal window beginning around the time of birth, with a peak percentage (∼2%) engaged in the cell cycle in the first year of life. In embryonic life and after early childhood, beta cell replication is barely detectable. Whereas beta cell expansion seems an obvious therapeutic approach to beta cell deficiency, adult human beta cells have proven recalcitrant to such efforts. Hence, there remains an urgent need for antidiabetic therapeutic agents that can induce regeneration and expansion of adult human beta cells in vivo or ex vivo. Here, using a high-throughput small-molecule screen (HTS), we find that analogs of the small molecule harmine function as a new class of human beta cell mitogenic compounds. We also define dual-specificity tyrosine-regulated kinase-1a (DYRK1A) as the likely target of harmine and the nuclear factors of activated T cells (NFAT) family of transcription factors as likely mediators of human beta cell proliferation and differentiation. Using three different mouse and human islet in vivo-based models, we show that harmine is able to induce beta cell proliferation, increase islet mass and improve glycemic control. These observations suggest that harmine analogs may have unique therapeutic promise for human diabetes therapy. Enhancing the potency and beta cell specificity of these compounds are important future challenges.

New small molecule agonists to the thyrotropin receptor.

BACKGROUND: Novel small molecular ligands (SMLs) to the thyrotropin receptor (TSHR) have potential as improved molecular probes and as therapeutic agents for the treatment of thyroid dysfunction and thyroid cancer. METHODS: To identify novel SMLs to the TSHR, we developed a transcription-based luciferase-cAMP high-throughput screening system and we screened 48,224 compounds from a 100K library in duplicate. RESULTS: We obtained 62 hits using the cut-off criteria of the mean±three standard deviations above the baseline. Twenty molecules with the greatest activity were rescreened against the parent CHO-luciferase cell for nonspecific activation, and we selected two molecules (MS437 and MS438) with the highest potency for further study. These lead molecules demonstrated no detectible cross-reactivity with homologous receptors when tested against luteinizing hormone (LH)/human chorionic gonadotropin receptor and follicle stimulating hormone receptor-expressing cells. Molecule MS437 had a TSHR-stimulating potency with an EC50 of 13×10(-8) M, and molecule MS438 had an EC50 of 5.3×10(-8) M. The ability of these small molecule agonists to bind to the transmembrane domain of the receptor and initiate signal transduction was suggested by their activation of a chimeric receptor consisting of an LHR ectodomain and a TSHR transmembrane. Molecular modeling demonstrated that these molecules bound to residues S505 and E506 for MS438 and T501 for MS437 in the intrahelical region of transmembrane helix 3. We also examined the G protein activating ability of these molecules using CHO cells co-expressing TSHRs transfected with luciferase reporter vectors in order to measure Gsα, Gßγ, Gαq, and Gα12 activation quantitatively. The MS437 and MS438 molecules showed potent activation of Gsα, Gαq, and Gα12 similar to TSH, but neither the small molecule agonists nor TSH showed activation of the Gßγ pathway. The small molecules MS437 and MS438 also showed upregulation of thyroglobulin (Tg), sodium iodine symporter (NIS), and TSHR gene expression. CONCLUSIONS: Pharmacokinetic analysis of MS437 and MS438 indicated their pharmacotherapeutic potential, and their intraperitoneal administration to normal female mice resulted in significantly increased serum thyroxine levels, which could be maintained by repeated treatments. These molecules can therefore serve as lead molecules for further development of powerful TSH agonists.

A genome-wide RNAi screen identifies opposing functions of Snai1 and Snai2 on the Nanog dependency in reprogramming.

Nanog facilitates embryonic stem cell self-renewal and induced pluripotent stem cell generation during the final stage of reprogramming. From a genome-wide small interfering RNA screen using a Nanog-GFP reporter line, we discovered opposing effects of Snai1 and Snai2 depletion on Nanog promoter activity. We further discovered mutually repressive expression profiles and opposing functions of Snai1 and Snai2 during Nanog-driven reprogramming. We found that Snai1, but not Snai2, is both a transcriptional target and protein partner of Nanog in reprogramming. Ectopic expression of Snai1 or depletion of Snai2 greatly facilitates Nanog-driven reprogramming. Snai1 (but not Snai2) and Nanog cobind to and transcriptionally activate pluripotency-associated genes including Lin28 and miR-290-295. Ectopic expression of miR-290-295 cluster genes partially rescues reprogramming inefficiency caused by Snai1 depletion. Our study thus uncovers the interplay between Nanog and mesenchymal factors Snai1 and Snai2 in the transcriptional regulation of pluripotency-associated genes and miRNAs during the Nanog-driven reprogramming process.

Interactions between the L1 cell adhesion molecule and ezrin support traction-force generation and can be regulated by tyrosine phosphorylation.

An Ig superfamily cell-adhesion molecule, L1, forms an adhesion complex at the cell membrane containing both signaling molecules and cytoskeletal proteins. This complex mediates the transduction of extracellular signals and generates actin-mediated traction forces, both of which support axon outgrowth. The L1 cytoplasmic region binds ezrin, an adapter protein that interacts with the actin cytoskeleton. In this study, we analyzed L1-ezrin interactions in detail, assessed their role in generating traction forces by L1, and identified potential regulatory mechanisms controlling ezrin-L1 interactions. The FERM domain of ezrin binds to the juxtamembrane region of L1, demonstrated by yeast two-hybrid interaction traps and protein binding analyses in vitro. A lysine-to-leucine substitution in this domain of L1 (K1147L) shows reduced binding to the ezrin FERM domain. Additionally, in ND7 cells, the K1147L mutation inhibits retrograde movement of L1 on the cell surface that has been linked to the generation of the traction forces necessary for axon growth. A membrane-permeable peptide consisting of the juxtamembrane region of L1 that can disrupt endogenous L1-ezrin interactions inhibits neurite extension of cerebellar cells on L1 substrates. Moreover, the L1-ezrin interactions can be modulated by tyrosine phosphorylation of the L1 cytoplasmic region, namely, Y1151, possibly through Src-family kinases. Replacement of this tyrosine together with Y1176 with either aspartate or phenylalanine changes ezrin binding and alters colocalization with ezrin in ND7 cells. Collectively, these data suggest that L1-ezrin interactions mediated by the L1 juxtamembrane region are involved in traction-force generation and can be regulated by the phosphorylation of L1.

Mechanism of PLC-mediated Kir3 current inhibition.

A large number of ion channels maintain their activity through direct interactions with phosphatidylinositol bisphosphate (PIP2). For such channels, hydrolysis of PIP2 causes current inhibition. It has become controversial whether the inhibitory effects on channel activity represent direct effects of PIP2 hydrolysis or of downstream PKC action. We studied Phospholipase C (PLC)-dependent inhibition of G protein-activated inwardly rectifying K+ (Kir3) channels. By monitoring simultaneously channel activity and PIP2 hydrolysis, we determined that both direct PIP2 depletion and PKC actions contribute to Kir3 current inhibition. We show that the PKC-induced effects strongly depend on PIP2 levels in the membrane. At the same time, we show that PKC destabilizes Kir3/PIP2 interactions and enhances the effects of PIP2 depletion on channel activity. These results demonstrate that PIP2 depletion and PKC-mediated effects reinforce each other and suggest that both of these interdependent mechanisms contribute to Kir3 current inhibition. This mechanistic insight may explain how even minor changes in PIP2 levels can have profound effects on Kir3 activity. We also show that stabilization of Kir3/PIP2 interactions by Gbetagamma attenuates both PKC and Gq-mediated current inhibition, suggesting that diverse pathways regulate Kir3 activity through modulation of channel interactions with PIP2.

MAP kinase pathway-dependent phosphorylation of the L1-CAM ankyrin binding site regulates neuronal growth.

The growth of neuronal processes depends critically on the function of adhesion proteins that link extracellular ligands to the cytoskeleton. The neuronal adhesion protein L1-CAM serves as a receptor for nerve growth-promoting proteins, a process that is inhibited by the interaction between L1-CAM and the cytoskeleton adaptor ankyrin. Using a novel reporter based on intramolecular bioluminescence resonance energy transfer, we have determined that the MAP kinase pathway regulates the phosphorylation of the FIGQY motif in the adhesion protein L1-CAM and its interaction with ankyrin B. MAP kinase pathway inhibitors block L1-CAM-mediated neuronal growth. However, this blockade is partially rescued by inhibitors of L1-CAM-ankyrin binding. These results demonstrate that the MAP kinase pathway regulates L1-CAM-mediated nerve growth by modulating ankyrin binding, suggesting that nerve growth can be regulated at the level of individual receptors.

G protein-induced trafficking of voltage-dependent calcium channels.

Calcium channels are well known targets for inhibition by G protein-coupled receptors, and multiple forms of inhibition have been described. Here we report a novel mechanism for G protein-mediated modulation of neuronal voltage-dependent calcium channels that involves the destabilization and subsequent removal of calcium channels from the plasma membrane. Imaging experiments in living sensory neurons show that, within seconds of receptor activation, calcium channels are cleared from the membrane and sequestered in clathrin-coated vesicles. Disruption of the L1-CAM-ankyrin B complex with the calcium channel mimics transmitter-induced trafficking of the channels, reduces calcium influx, and decreases exocytosis. Our results suggest that G protein-induced removal of plasma membrane calcium channels is a consequence of disrupting channel-cytoskeleton interactions and might represent a novel mechanism of presynaptic inhibition.

Teaching resources. Regulation of complexes by cytoskeletal elements: integrins serve as force transducers linking mechanical stimuli and biochemical signals.

This Teaching Resource provides a summary and slides derived from a lecture on the integrin family members and the downstream signaling pathways they activate; it is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture focuses on the capacity of integrins to serve as mechanotransducers, with particular emphasis on signaling mechanisms downstream of integrins.

Ankyrin binding mediates L1CAM interactions with static components of the cytoskeleton and inhibits retrograde movement of L1CAM on the cell surface.

The function of adhesion receptors in both cell adhesion and migration depends critically on interactions with the cytoskeleton. During cell adhesion, cytoskeletal interactions stabilize receptors to strengthen adhesive contacts. In contrast, during cell migration, adhesion proteins are believed to interact with dynamic components of the cytoskeleton, permitting the transmission of traction forces through the receptor to the extracellular environment. The L1 cell adhesion molecule (L1CAM), a member of the Ig superfamily, plays a crucial role in both the migration of neuronal growth cones and the static adhesion between neighboring axons. To understand the basis of L1CAM function in adhesion and migration, we quantified directly the diffusion characteristics of L1CAM on the upper surface of ND-7 neuroblastoma hybrid cells as an indication of receptor-cytoskeleton interactions. We find that cell surface L1CAM engages in diffusion, retrograde movement, and stationary behavior, consistent with interactions between L1CAM and two populations of cytoskeleton proteins. We provide evidence that the cytoskeletal adaptor protein ankyrin mediates stationary behavior while inhibiting the actin-dependent retrograde movement of L1CAM. Moreover, inhibitors of L1CAM-ankyrin interactions promote L1CAM-mediated axon growth. Together, these results suggest that ankyrin binding plays a crucial role in the anti-coordinate regulation of L1CAM-mediated adhesion and migration.