CPSF3-dependent pre-mRNA processing as a druggable node in AML and Ewing's sarcoma.

The post-genomic era has seen many advances in our understanding of cancer pathways, yet resistance and tumor heterogeneity necessitate multiple approaches to target even monogenic tumors. Here, we combine phenotypic screening with chemical genetics to identify pre-messenger RNA endonuclease cleavage and polyadenylation specificity factor 3 (CPSF3) as the target of JTE-607, a small molecule with previously unknown target. We show that CPSF3 represents a synthetic lethal node in a subset of acute myeloid leukemia (AML) and Ewing's sarcoma cancer cell lines. Inhibition of CPSF3 by JTE-607 alters expression of known downstream effectors in AML and Ewing's sarcoma lines, upregulates apoptosis and causes tumor-selective stasis in mouse xenografts. Mechanistically, it prevents the release of newly synthesized pre-mRNAs, resulting in read-through transcription and the formation of DNA-RNA hybrid R-loop structures. This study implicates pre-mRNA processing, and specifically CPSF3, as a druggable target providing an avenue to therapeutic intervention in cancer.

Author Correction: CPSF3-dependent pre-mRNA processing as a druggable node in AML and Ewing's sarcoma.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Discovery of a ZIP7 inhibitor from a Notch pathway screen.

The identification of activating mutations in NOTCH1 in 50% of T cell acute lymphoblastic leukemia has generated interest in elucidating how these mutations contribute to oncogenic transformation and in targeting the pathway. A phenotypic screen identified compounds that interfere with trafficking of Notch and induce apoptosis via an endoplasmic reticulum (ER) stress mechanism. Target identification approaches revealed a role for SLC39A7 (ZIP7), a zinc transport family member, in governing Notch trafficking and signaling. Generation and sequencing of a compound-resistant cell line identified a V430E mutation in ZIP7 that confers transferable resistance to the compound NVS-ZP7-4. NVS-ZP7-4 altered zinc in the ER, and an analog of the compound photoaffinity labeled ZIP7 in cells, suggesting a direct interaction between the compound and ZIP7. NVS-ZP7-4 is the first reported chemical tool to probe the impact of modulating ER zinc levels and investigate ZIP7 as a novel druggable node in the Notch pathway.

Conserved methionine dictates substrate preference in Nramp-family divalent metal transporters.

Natural resistance-associated macrophage protein (Nramp) family transporters catalyze uptake of essential divalent transition metals like iron and manganese. To discriminate against abundant competitors, the Nramp metal-binding site should favor softer transition metals, which interact either covalently or ionically with coordinating molecules, over hard calcium and magnesium, which interact mainly ionically. The metal-binding site contains an unusual, but conserved, methionine, and its sulfur coordinates transition metal substrates, suggesting a vital role in their transport. Using a bacterial Nramp model system, we show that, surprisingly, this conserved methionine is dispensable for transport of the physiological manganese substrate and similar divalents iron and cobalt, with several small amino acid replacements still enabling robust uptake. Moreover, the methionine sulfur's presence makes the toxic metal cadmium a preferred substrate. However, a methionine-to-alanine substitution enables transport of calcium and magnesium. Thus, the putative evolutionary pressure to maintain the Nramp metal-binding methionine likely exists because it-more effectively than any other amino acid-increases selectivity for low-abundance transition metal transport in the presence of high-abundance divalents like calcium and magnesium.

Structure of a force-conveying cadherin bond essential for inner-ear mechanotransduction.

Hearing and balance use hair cells in the inner ear to transform mechanical stimuli into electrical signals. Mechanical force from sound waves or head movements is conveyed to hair-cell transduction channels by tip links, fine filaments formed by two atypical cadherins known as protocadherin 15 and cadherin 23 (refs 4, 5). These two proteins are involved in inherited deafness and feature long extracellular domains that interact tip-to-tip in a Ca(2+)-dependent manner. However, the molecular architecture of this complex is unknown. Here we combine crystallography, molecular dynamics simulations and binding experiments to characterize the protocadherin 15-cadherin 23 bond. We find a unique cadherin interaction mechanism, in which the two most amino-terminal cadherin repeats (extracellular cadherin repeats 1 and 2) of each protein interact to form an overlapped, antiparallel heterodimer. Simulations predict that this tip-link bond is mechanically strong enough to resist forces in hair cells. In addition, the complex is shown to become unstable in response to Ca(2+) removal owing to increased flexure of Ca(2+)-free cadherin repeats. Finally, we use structures and biochemical measurements to study the molecular mechanisms by which deafness mutations disrupt tip-link function. Overall, our results shed light on the molecular mechanics of hair-cell sensory transduction and on new interaction mechanisms for cadherins, a large protein family implicated in tissue and organ morphogenesis, neural connectivity and cancer.

Enzymatic blockade of the ubiquitin-proteasome pathway.

Ubiquitin-dependent processes control much of cellular physiology. We show that expression of a highly active, Epstein-Barr virus-derived deubiquitylating enzyme (EBV-DUB) blocks proteasomal degradation of cytosolic and ER-derived proteins by preemptive removal of ubiquitin from proteasome substrates, a treatment less toxic than the use of proteasome inhibitors. Recognition of misfolded proteins in the ER lumen, their dislocation to the cytosol, and degradation are usually tightly coupled but can be uncoupled by the EBV-DUB: a misfolded glycoprotein that originates in the ER accumulates in association with cytosolic chaperones as a deglycosylated intermediate. Our data underscore the necessity of a DUB activity for completion of the dislocation reaction and provide a new means of inhibition of proteasomal proteolysis with reduced cytotoxicity.

Characterization and structural studies of the Plasmodium falciparum ubiquitin and Nedd8 hydrolase UCHL3.

Like their human hosts, Plasmodium falciparum parasites rely on the ubiquitin-proteasome system for survival. We previously identified PfUCHL3, a deubiquitinating enzyme, and here we characterize its activity and changes in active site architecture upon binding to ubiquitin. We find strong evidence that PfUCHL3 is essential to parasite survival. The crystal structures of both PfUCHL3 alone and in complex with the ubiquitin-based suicide substrate UbVME suggest a rather rigid active site crossover loop that likely plays a role in restricting the size of ubiquitin adduct substrates. Molecular dynamics simulations of the structures and a model of the PfUCHL3-PfNedd8 complex allowed the identification of shared key interactions of ubiquitin and PfNedd8 with PfUCHL3, explaining the dual specificity of this enzyme. Distinct differences observed in ubiquitin binding between PfUCHL3 and its human counterpart make it likely that the parasitic DUB can be selectively targeted while leaving the human enzyme unaffected.

Streptococcus pyogenes pSM19035 requires dynamic assembly of ATP-bound ParA and ParB on parS DNA during plasmid segregation.

The accurate partitioning of Firmicute plasmid pSM19035 at cell division depends on ATP binding and hydrolysis by homodimeric ATPase delta(2) (ParA) and binding of omega(2) (ParB) to its cognate parS DNA. The 1.83 A resolution crystal structure of delta(2) in a complex with non-hydrolyzable ATPgammaS reveals a unique ParA dimer assembly that permits nucleotide exchange without requiring dissociation into monomers. In vitro, delta(2) had minimal ATPase activity in the absence of omega(2) and parS DNA. However, stoichiometric amounts of omega(2) and parS DNA stimulated the delta(2) ATPase activity and mediated plasmid pairing, whereas at high (4:1) omega(2) : delta(2) ratios, stimulation of the ATPase activity was reduced and delta(2) polymerized onto DNA. Stimulation of the delta(2) ATPase activity and its polymerization on DNA required ability of omega(2) to bind parS DNA and its N-terminus. In vivo experiments showed that delta(2) alone associated with the nucleoid, and in the presence of omega(2) and parS DNA, delta(2) oscillated between the nucleoid and the cell poles and formed spiral-like structures. Our studies indicate that the molar omega(2) : delta(2) ratio regulates the polymerization properties of (delta*ATP*Mg(2+))(2) on and depolymerization from parS DNA, thereby controlling the temporal and spatial segregation of pSM19035 before cell division.

Structure-Based Design and Preclinical Characterization of Selective and Orally Bioavailable Factor XIa Inhibitors: Demonstrating the Power of an Integrated S1 Protease Family Approach.

The serine protease factor XI (FXI) is a prominent drug target as it holds promise to deliver efficacious anticoagulation without an enhanced risk of major bleeds. Several efforts have been described targeting the active form of the enzyme, FXIa. Herein, we disclose our efforts to identify potent, selective, and orally bioavailable inhibitors of FXIa. Compound 1, identified from a diverse library of internal serine protease inhibitors, was originally designed as a complement factor D inhibitor and exhibited submicromolar FXIa activity and an encouraging absorption, distribution, metabolism, and excretion (ADME) profile while being devoid of a peptidomimetic architecture. Optimization of interactions in the S1, S1ß, and S1' pockets of FXIa through a combination of structure-based drug design and traditional medicinal chemistry led to the discovery of compound 23 with subnanomolar potency on FXIa, enhanced selectivity over other coagulation proteases, and a preclinical pharmacokinetics (PK) profile consistent with bid dosing in patients.

Scaffold Morphing Identifies 3-Pyridyl Azetidine Ureas as Inhibitors of Nicotinamide Phosphoribosyltransferase (NAMPT).

Small molecules that inhibit the metabolic enzyme NAMPT have emerged as potential therapeutics in oncology. As part of our effort in this area, we took a scaffold morphing approach and identified 3-pyridyl azetidine ureas as a potent NAMPT inhibiting motif. We explored the SAR of this series, including 5 and 6 amino pyridines, using a convergent synthetic strategy. This lead optimization campaign yielded multiple compounds with excellent in vitro potency and good ADME properties that culminated in compound 27.

Structures of human N-Acetylglucosamine kinase in two complexes with N-Acetylglucosamine and with ADP/glucose: insights into substrate specificity and regulation.

N-Acetylglucosamine (GlcNAc), a major component of complex carbohydrates, is synthesized de novo or salvaged from lysosomally degraded glycoconjugates and from nutritional sources. The salvage pathway requires that GlcNAc kinase converts GlcNAc to GlcNAc-6-phosphate, a component utilized in UDP-GlcNAc biosynthesis or energy metabolism. GlcNAc kinase belongs to the sugar kinase/Hsp70/actin superfamily that catalyze phosphoryl transfer from ATP to their respective substrates, and in most cases catalysis is associated with a large conformational change in which the N-terminal small and C-terminal large domains enclose the substrates. Here we report two crystal structures of homodimeric human GlcNAc kinase, one in complex with GlcNAc and the other in complex with ADP and glucose. The active site of GlcNAc kinase is located in a deep cleft between the two domains of the V-shaped monomer. The enzyme adopts a "closed" configuration in the GlcNAc-bound complex and GlcNAc interacts with residues of both domains. In addition, the N-acetyl methyl group contacts residues of the other monomer in the homodimer, a unique feature compared to other members of the sugar kinase/Hsp70/actin superfamily. This contrasts an "open" configuration in the ADP/glucose-bound structure, where glucose cannot form these interactions, explaining its low binding affinity for GlcNAc kinase. Our results support functional implications derived from apo crystal structures of GlcNAc kinases from Chromobacter violaceum and Porphyromonas gingivalis and show that Tyr205, which is phosphorylated in thrombin-activated platelets, lines the GlcNAc binding pocket. This suggests that phosphorylation of Tyr205 may modulate GlcNAc kinase activity and/or specificity.

A NMDA-receptor calcium influx assay sensitive to stimulation by glutamate and glycine/D-serine.

N-methyl-D-aspartate-receptors (NMDARs) are ionotropic glutamate receptors that function in synaptic transmission, plasticity and cognition. Malfunction of NMDARs has been implicated in a variety of nervous system disorders, making them attractive therapeutic targets. Overexpression of functional NMDAR in non-neuronal cells results in cell death by excitotoxicity, hindering the development of cell-based assays for NMDAR drug discovery. Here we report a plate-based, high-throughput approach to study NMDAR function. Our assay enables the functional study of NMDARs with different subunit composition after activation by glycine/D-serine or glutamate and hence presents the first plate-based, high throughput assay that allows for the measurement of NMDAR function in glycine/D-serine and/or glutamate sensitive modes. This allows to investigate the effect of small molecule modulators on the activation of NMDARs at different concentrations or combinations of the co-ligands. The reported assay system faithfully replicates the pharmacology of the receptor in response to known agonists, antagonists, positive and negative allosteric modulators, as well as the receptor's sensitivity to magnesium and zinc. We believe that the ability to study the biology of NMDARs rapidly and in large scale screens will enable the identification of novel therapeutics whose discovery has otherwise been hindered by the limitations of existing cell based approaches.

Crystal Structure and Conformational Change Mechanism of a Bacterial Nramp-Family Divalent Metal Transporter.

The widely conserved natural resistance-associated macrophage protein (Nramp) family of divalent metal transporters enables manganese import in bacteria and dietary iron uptake in mammals. We determined the crystal structure of the Deinococcus radiodurans Nramp homolog (DraNramp) in an inward-facing apo state, including the complete transmembrane (TM) segment 1a (absent from a previous Nramp structure). Mapping our cysteine accessibility scanning results onto this structure, we identified the metal-permeation pathway in the alternate outward-open conformation. We investigated the functional impact of two natural anemia-causing glycine-to-arginine mutations that impaired transition metal transport in both human Nramp2 and DraNramp. The TM4 G153R mutation perturbs the closing of the outward metal-permeation pathway and alters the selectivity of the conserved metal-binding site. In contrast, the TM1a G45R mutation prevents conformational change by sterically blocking the essential movement of that helix, thus locking the transporter in an inward-facing state.

Crystal structure of a squalene cyclase in complex with the potential anticholesteremic drug Ro48-8071.

Squalene-hopene cyclase (SHC) catalyzes the conversion of squalene into pentacyclic compounds. It is the prokaryotic counterpart of the eukaryotic oxidosqualene cyclase (OSC) that catalyzes the steroid scaffold formation. Because of clear sequence homology, SHC can serve as a model for OSC, which is an attractive target for anticholesteremic drugs. We have established the crystal structure of SHC complexed with Ro48-8071, a potent inhibitor of OSC and therefore of cholesterol biosynthesis. Ro48-8071 is bound in the active-center cavity of SHC and extends into the channel that connects the cavity with the membrane. The binding site of Ro48-8071 is largely identical with the expected site of squalene; it differs from a previous model based on photoaffinity labeling. The knowledge of the inhibitor binding mode in SHC is likely to help develop more potent inhibitors for OSC.

Structure and Sequence Analyses of Clustered Protocadherins Reveal Antiparallel Interactions that Mediate Homophilic Specificity.

Clustered protocadherin (Pcdh) proteins mediate dendritic self-avoidance in neurons via specific homophilic interactions in their extracellular cadherin (EC) domains. We determined crystal structures of EC1-EC3, containing the homophilic specificity-determining region, of two mouse clustered Pcdh isoforms (PcdhγA1 and PcdhγC3) to investigate the nature of the homophilic interaction. Within the crystal lattices, we observe antiparallel interfaces consistent with a role in trans cell-cell contact. Antiparallel dimerization is supported by evolutionary correlations. Two interfaces, located primarily on EC2-EC3, involve distinctive clustered Pcdh structure and sequence motifs, lack predicted glycosylation sites, and contain residues highly conserved in orthologs but not paralogs, pointing toward their biological significance as homophilic interaction interfaces. These two interfaces are similar yet distinct, reflecting a possible difference in interaction architecture between clustered Pcdh subfamilies. These structures initiate a molecular understanding of clustered Pcdh assemblies that are required to produce functional neuronal networks.

Structural determinants of cadherin-23 function in hearing and deafness.

The hair-cell tip link, a fine filament directly conveying force to mechanosensitive transduction channels, is composed of two proteins, protocadherin-15 and cadherin-23, whose mutation causes deafness. However, their molecular structure, elasticity, and deafness-related structural defects are unknown. We present crystal structures of the first and second extracellular cadherin repeats of cadherin-23. Overall, structures show typical cadherin folds, but reveal an elongated N terminus that precludes classical cadherin interactions and contributes to an N-terminal Ca(2+)-binding site. The deafness mutation D101G, in the linker region between the repeats, causes a slight bend between repeats and decreases Ca(2+) affinity. Molecular dynamics simulations suggest that cadherin-23 repeats are stiff and that either removing Ca(2+) or mutating Ca(2+)-binding residues reduces rigidity and unfolding strength. The structures define an uncharacterized cadherin family and, with simulations, suggest mechanisms underlying inherited deafness and how cadherin-23 may bind with itself and with protocadherin-15 to form the tip link.

Structure of a herpesvirus-encoded cysteine protease reveals a unique class of deubiquitinating enzymes.

All members of the herpesviridae contain within their large tegument protein a cysteine protease module that displays deubiquitinating activity. We report the crystal structure of the cysteine protease domain of murine cytomegalovirus M48 (M48(USP)) in a complex with a ubiquitin (Ub)-based suicide substrate. M48(USP) adopts a papain-like fold, with the active-site cysteine forming a thioether linkage to the suicide substrate. The Ub core participates in an extensive hydrophobic interaction with an exposed beta hairpin loop of M48(USP). This Ub binding mode contributes to Ub specificity and is distinct from that observed in other deubiquitinating enzymes. Both the arrangement of active-site residues and the architecture of the interface with Ub lead us to classify this domain as the founding member of a previously unknown class of deubiquitinating enzymes.

Crystal structure of CD26/dipeptidyl-peptidase IV in complex with adenosine deaminase reveals a highly amphiphilic interface.

Dipeptidyl-peptidase IV (DPPIV or CD26) is a homodimeric type II membrane glycoprotein in which the two monomers are subdivided into a beta-propeller domain and an alpha/beta-hydrolase domain. As dipeptidase, DPPIV modulates the activity of various biologically important peptides and, in addition, DPPIV acts as a receptor for adenosine deaminase (ADA), thereby mediating co-stimulatory signals in T-lymphocytes. The 3.0-A resolution crystal structure of the complex formed between human DPPIV and bovine ADA presented here shows that each beta-propeller domain of the DPPIV dimer binds one ADA. At the binding interface, two hydrophobic loops protruding from the beta-propeller domain of DPPIV interact with two hydrophilic and heavily charged alpha-helices of ADA, giving rise to the highest percentage of charged residues involved in a protein-protein contact reported thus far. Additionally, four glycosides linked to Asn229 of DPPIV bind to ADA. In the crystal structure of porcine DPPIV, the observed tetramer formation was suggested to mediate epithelial and lymphocyte cell-cell adhesion. ADA binding to DPPIV could regulate this adhesion, as it would abolish tetramerization.

Thiol-modifying inhibitors for understanding squalene cyclase function.

The function of squalene-hopene cyclase from Alicyclobacillus acidocaldarius was studied by labelling critical cysteine residues of the enzyme, either native or inserted by site-directed mutagenesis, with different thiol-reacting molecules. The access of the substrate to the active centre cavity through a nonpolar channel that contains a narrow constriction harbouring a cysteine residue (C435) was probed by labelling experiments on both a C435S mutant, lacking C435 of the channel constriction, and a C25S/C50S/C455S/C537S mutant, bearing C435 as the only cysteine residue. Labelling experiments with tritiated 3-carboxy-4-nitrophenyl-dithio-1,1',2-trisnorsqualene (CNDT-squalene) showed that the cysteine residue at the channel constriction was covalently modified by the squalene-like inhibitor. Time-dependent inactivation of the C25S/C50S/C455S/C537S mutant by a number of squalene analogues and other agents with thiol-modifying activity suggested that modifying C435 caused the obstruction of the channel constriction thus blocking access of the substrate to the active site. The tryptic fragment comprising C435 of the quadruple mutant labelled with the most effective inhibitor had the expected altered molecular mass, as determined by LC-ESI-MS measurements. The arrangement of the substrate in the active site cavity was studied by using thiol reagents as probes in labelling experiments with the double mutant D376C/C435S in which D376, supposedly the substrate-protonating residue, was substituted by cysteine. The inhibitory effect was evaluated in terms of the reduced ability to cyclize oxidosqualene, as the mutant is unable to catalyse the reaction of squalene to hopene. Among the inhibitors tested, the substrate analogue squalene-maleimide proved to be a very effective time-dependent inhibitor.