A homogenous luminescent proximity assay for 14-3-3 interactions with both phosphorylated and nonphosphorylated client peptides

A Personalized 14-3-3 Disease-Targeting Workflow Yields Repositioning Drug Candidates

Rare diseases typically evade the application of the standard drug discovery and development pipelines due to their understudied molecular etiology and the small market size. Herein, we report a rare disease-directed workflow that rapidly studies the molecular features of the disorder, establishes a high-throughput screening (HTS) platform, and conducts an HTS of thousands of approved drugs to identify and validate repositioning drug candidates. This study examines the pediatric neurological disorder caused by de novo mutations in YWHAG, the gene encoding the scaffolding protein 14-3-3γ, and the workflow discovers nuclear relocalization and a severe drop in 14-3-3γ binding to its phosphorylated protein partners as the key molecular features of the pathogenic hotspot YWHAG mutations. We further established a robust in vitro HTS platform and screened ca. 3000 approved drugs to identify the repositioning drug candidates that restore the deficient 14-3-3γ-phosphotarget interactions. Our workflow can be applied to other 14-3-3-related disorders and upscaled for many other rare diseases.

A drug library screen identifies Carbenoxolone as novel FOXO inhibitor that overcomes FOXO3-mediated chemoprotection in high-stage neuroblastoma

The transcription factor FOXO3 has been associated in different tumor entities with hallmarks of cancer, including metastasis, tumor angiogenesis, maintenance of tumor-initiating stem cells, and drug resistance. In neuroblastoma (NB), we recently demonstrated that nuclear FOXO3 promotes tumor angiogenesis in vivo and chemoresistance in vitro. Hence, inhibiting the transcriptional activity of FOXO3 is a promising therapeutic strategy. However, as no FOXO3 inhibitor is clinically available to date, we used a medium-throughput fluorescence polarization assay (FPA) screening in a drug-repositioning approach to identify compounds that bind to the FOXO3-DNA-binding-domain (DBD). Carbenoxolone (CBX), a glycyrrhetinic acid derivative, was identified as a potential FOXO3-inhibitory compound that binds to the FOXO3-DBD with a binding affinity of 19 µM. Specific interaction of CBX with the FOXO3-DBD was validated by fluorescence-based electrophoretic mobility shift assay (FAM-EMSA). CBX inhibits the transcriptional activity of FOXO3 target genes, as determined by chromatin immunoprecipitation (ChIP), DEPP-, and BIM promoter reporter assays, and real-time RT-PCR analyses. In high-stage NB cells with functional TP53, FOXO3 triggers the expression of SESN3, which increases chemoprotection and cell survival. Importantly, FOXO3 inhibition by CBX treatment at pharmacologically relevant concentrations efficiently repressed FOXO3-mediated SESN3 expression and clonogenic survival and sensitized high-stage NB cells to chemotherapy in a 2D and 3D culture model. Thus, CBX might be a promising novel candidate for the treatment of therapy-resistant high-stage NB and other "FOXO-resistant" cancers.

Epilepsy-associated gene Nedd4-2 mediates neuronal activity and seizure susceptibility through AMPA receptors

The neural precursor cell expressed developmentally down-regulated gene 4–2, Nedd4-2, is an epilepsy-associated gene with at least three missense mutations identified in epileptic patients. Nedd4-2 encodes a ubiquitin E3 ligase that has high affinity toward binding and ubiquitinating membrane proteins. It is currently unknown how Nedd4-2 mediates neuronal circuit activity and how its dysfunction leads to seizures or epilepsies. In this study, we provide evidence to show that Nedd4-2 mediates neuronal activity and seizure susceptibility through ubiquitination of GluA1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, (AMPAR). Using a mouse model, termed Nedd4-2^andi, in which one of the major forms of Nedd4-2 in the brain is selectively deficient, we found that the spontaneous neuronal activity in Nedd4-2^andi cortical neuron cultures, measured by a multiunit extracellular electrophysiology system, was basally elevated, less responsive to AMPAR activation, and much more sensitive to AMPAR blockade when compared with wild-type cultures. When performing kainic acid-induced seizures in vivo, we showed that elevated seizure susceptibility in Nedd4-2^andi mice was normalized when GluA1 is genetically reduced. Furthermore, when studying epilepsy-associated missense mutations of Nedd4-2, we found that all three mutations disrupt the ubiquitination of GluA1 and fail to reduce surface GluA1 and spontaneous neuronal activity when compared with wild-type Nedd4-2. Collectively, our data suggest that impaired GluA1 ubiquitination contributes to Nedd4-2-dependent neuronal hyperactivity and seizures. Our findings provide critical information to the future development of therapeutic strategies for patients who carry mutations of Nedd4-2.

Many patients with neurological disorders suffer from an imbalance in neuronal and circuit excitability and present with seizure or epilepsy as the common comorbidity. Human genetic studies have identified many epilepsy-associated genes, but the pathways by which those genes are connected to brain circuit excitability are largely unknown. Our study focused on one of the epilepsy-associated genes, Nedd4-2, and aimed to dissect the molecular mechanism underlying Nedd4-2-associated epilepsy. Nedd4-2 encodes a ubiquitin E3 ligase. Several neuronal ion channels have been identified as its substrates, including the GluA1 subunit of AMPAR. Our results first demonstrate up-regulation of spontaneous neuronal activity and seizure susceptibility when Nedd4-2 is reduced in a mouse model. These deficits can be corrected when GluA1/AMPAR is pharmacologically or genetically inhibited. In addition, we found that three epilepsy-associated missense mutations of Nedd4-2 inhibit the ubiquitination of GluA1 and fail to reduce GluA1 surface expression or spontaneous neuronal activity when compared to wild-type Nedd4-2. These findings suggest the reduction of GluA1 ubiquitination as a crucial deficit underlying insufficient function of Nedd4-2 and provide critical information to the development of therapies for patients who carry mutations of Nedd4-2.

Discovery of Sanggenon G as a natural cell-permeable small-molecular weight inhibitor of X-linked inhibitor of apoptosis protein (XIAP)

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Discovery of a novel XIAP-inhibitory natural compound from Morus root bark (Sanggenon G).

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Sanggenon G binds specific to the BIR3 domain of XIAP in a low μM range.

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Sanggenon G interferes with XIAP-BIR3-substrate binding in living cells.

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Sanggenon G acts as chemosensitizer in tumor cell lines with high XIAP expression.

Defects in the regulation of apoptosis are one main cause of cancer development and may result from overexpression of anti-apoptotic proteins such as the X-linked inhibitor of apoptosis protein (XIAP). XIAP is frequently overexpressed in human leukemia and prostate and breast tumors. Inhibition of apoptosis by XIAP is mainly coordinated through direct binding to the initiator caspase-9 via its baculovirus-IAP-repeat-3 (BIR3) domain. XIAP inhibits caspases directly making it to an attractive target for anti-cancer therapy. In the search for novel, non-peptidic XIAP inhibitors in this study we focused on the chemical constituents of sāng bái pí (mulberry root bark). Most promising candidates of this plant were tested biochemically in vitro by a fluorescence polarization (FP) assay and in vivo via protein fragment complementation analysis (PCA). We identified the Diels Alder adduct Sanggenon G (SG1) as a novel, small-molecular weight inhibitor of XIAP. As shown by FP and PCA analyses, SG1 binds specifically to the BIR3 domain of XIAP with a binding affinity of 34.26 μM. Treatment of the transgenic leukemia cell line Molt3/XIAP with SG1 enhances caspase-8, -3 and -9 cleavage, displaces caspase-9 from XIAP as determined by immunoprecipitation experiments and sensitizes these cells to etoposide-induced apoptosis. SG1 not only sensitizes the XIAP-overexpressing leukemia cell line Molt3/XIAP to etoposide treatment but also different neuroblastoma cell lines endogenously expressing high XIAP levels. Taken together, Sanggenon G (SG1) is a novel, natural, non-peptidic, small-molecular inhibitor of XIAP that can serve as a starting point to develop a new class of improved XIAP inhibitors.

A time-resolved fluorescence resonance energy transfer assay for high-throughput screening of 14-3-3 protein-protein interaction inhibitors

Protein-protein interaction networks mediate diverse biological processes by regulating various signaling hubs and clusters. 14-3-3 proteins, a family of phosphoserine/threonine-binding molecules, serve as major interaction hubs in eukaryotic cells and have emerged as promising therapeutic targets for various human diseases. In order to identify chemical probes for mechanistic studies and for potential therapeutic development, we have developed highly sensitive bioassays to monitor the interaction of 14-3-3 with a client protein. In this study, we describe a homogenous time-resolved fluorescence resonance energy transfer (TR-FRET) assay to detect the interaction of 14-3-3 with Bad, a proapoptotic member of the Bcl-2 family. Through a series of titration studies in which europium-labeled 14-3-3 serves as an FRET donor and a Dy647-labeled phosphorylated Bad, the peptide acts as an FRET acceptor, we have achieved a robust TR-FRET assay that is suitable for high-throughput screening (HTS) with an excellent signal-to-background ratio of >20 and Z' values >0.7. This assay was further miniaturized to a 1,536-well format for ultra-HTS (uHTS), and exhibited a similar robust performance. The utility and performance of the assay for uHTS were validated by (i) known inhibitors, including peptide R18 and small molecule FOBISIN101, and (ii) screening of a 51,200 compound library. This simple and robust assay is generally applicable to detect the interaction of 14-3-3 with other client proteins. It provides a sensitive and easy-to-use tool to facilitate the discovery of 14-3-3 protein inhibitors as well as to study 14-3-3-mediated protein-protein interactions.

14-3-3ζ interacts with hepatocyte nuclear factor 1α and enhances its DNA binding and transcriptional activation

14-3-3 proteins regulate numerous cellular processes through interaction with a variety of proteins, and have been identified as HNF1α binding partner by mass spectrometry analysis in our previous study. In the present study, the interaction between 14-3-3ζ and HNF1α has been further validated by in vivo and in vitro assays. Moreover, we have found that overexpression of 14-3-3ζ potentiated the transcriptional activity of HNF1α in cultured cells, and silencing of 14-3-3ζ by RNA interference in HepG2 cells specifically affected the HNF1α-dependent gene expression. Furthermore, we have demonstrated that 14-3-3ζ is recruited to endogenous HNF1α responsive promoters and enhances HNF1α binding to its cognate DNA sequences. In addition, we have also provided evidence that the association between HNF1α and 14-3-3ζ is phosphorylation-dependent. Taken together, these results suggest that 14-3-3ζ may be an endogenous physiologic regulator of HNF1α.

14-3-3 proteins as potential therapeutic targets

The 14-3-3 family of phosphoserine/phosphothreonine-binding proteins dynamically regulates the activity of client proteins in various signaling pathways that control diverse physiological and pathological processes. In response to environmental cues, 14-3-3 proteins orchestrate the highly regulated flow of signals through complex networks of molecular interactions to achieve well-controlled physiological outputs, such as cell proliferation or differentiation. Accumulating evidence now supports the concept that either an abnormal state of 14-3-3 protein expression, or dysregulation of 14-3-3/client protein interactions, contributes to the development of a large number of human diseases. In particular, clinical investigations in the field of oncology have demonstrated a correlation between upregulated 14-3-3 levels and poor survival of cancer patients. These studies highlight the rapid emergence of 14-3-3 proteins as a novel class of molecular target for potential therapeutic intervention. The current status of 14-3-3 modulator discovery is discussed.

Two High Throughput Screen Assays for Measurement of TNF-α in THP-1 Cells

Tumor Necrosis Factor-α (TNF-α), a secreted cytokine, plays an important role in inflammatory diseases and immune disorders, and is a potential target for drug development. The traditional assays for detecting TNF-α, enzyme linked immunosorbent assay (ELISA) and radioimmunoassay, are not suitable for the large size compound screens. Both assays suffer from a complicated protocol, multiple plate wash steps and/or excessive radioactive waste. A simple and quick measurement of TNF-α production in a cell based assay is needed for high throughput screening to identify the lead compounds from the compound library. We have developed and optimized two homogeneous TNF-α assays using the HTRF (homogeneous time resolved fluorescence) and AlphaLISA assay formats. We have validated the HTRF based TNF-α assay in a 1536-well plate format by screening a library of 1280 pharmacologically active compounds. The active compounds identified from the screen were confirmed in the AlphaLISA TNF-α assay using a bead-based technology. These compounds were also confirmed in a traditional ELISA assay. From this study, several beta adrenergic agonists have been identified as TNF-α inhibitors. We also identified several novel inhibitors of TNF-α, such as BTO-1, CCG-2046, ellipticine, and PD 169316. The results demonstrated that both homogeneous TNF-α assays are robust and suitable for high throughput screening.