Priyadarshini Jaishankar

Pharmacological dimerization and activation of the exchange factor eIF2B antagonizes the integrated stress response

The general translation initiation factor eIF2 is a major translational control point. Multiple signaling pathways in the integrated stress response phosphorylate eIF2 serine-51, inhibiting nucleotide exchange by eIF2B. ISRIB, a potent drug-like small molecule, renders cells insensitive to eIF2α phosphorylation and enhances cognitive function in rodents by blocking long-term depression. ISRIB was identified in a phenotypic cell-based screen, and its mechanism of action remained unknown. We now report that ISRIB is an activator of eIF2B. Our reporter-based shRNA screen revealed an eIF2B requirement for ISRIB activity. Our results define ISRIB as a symmetric molecule, show ISRIB-mediated stabilization of activated eIF2B dimers, and suggest that eIF2B4 (δ-subunit) contributes to the ISRIB binding site. We also developed new ISRIB analogs, improving its EC50 to 600 pM in cell culture. By modulating eIF2B function, ISRIB promises to be an invaluable tool in proof-of-principle studies aiming to ameliorate cognitive defects resulting from neurodegenerative diseases.

Novel non-peptidic vinylsulfones targeting the S2 and S3 subsites of parasite cysteine proteases.

We describe here the identification of non-peptidic vinylsulfones that inhibit parasite cysteine proteases in vitro and inhibit the growth of Trypanosoma brucei brucei parasites in culture. A high resolution (1.75 A) co-crystal structure of 8a bound to cruzain reveals how the non-peptidic P2/P3 moiety in such analogs bind the S2 and S3 subsites of the protease, effectively recapitulating important binding interactions present in more traditional peptide-based protease inhibitors and natural substrates.

Ceapins are a new class of unfolded protein response inhibitors, selectively targeting the ATF6α branch

The membrane-bound transcription factor ATF6α plays a cytoprotective role in the unfolded protein response (UPR), required for cells to survive ER stress. Activation of ATF6α promotes cell survival in cancer models. We used cell-based screens to discover and develop Ceapins, a class of pyrazole amides, that block ATF6α signaling in response to ER stress. Ceapins sensitize cells to ER stress without impacting viability of unstressed cells. Ceapins are highly specific inhibitors of ATF6α signaling, not affecting signaling through the other branches of the UPR, or proteolytic processing of its close homolog ATF6β or SREBP (a cholesterol-regulated transcription factor), both activated by the same proteases. Ceapins are first-in-class inhibitors that can be used to explore both the mechanism of activation of ATF6α and its role in pathological settings. The discovery of Ceapins now enables pharmacological modulation all three UPR branches either singly or in combination. DOI: http://dx.doi.org/10.7554/eLife.11878.001

USP7 small-molecule inhibitors interfere with ubiquitin binding

The ubiquitin system regulates essential cellular processes in eukaryotes. Ubiquitin is ligated to substrate proteins as monomers or chains and the topology of ubiquitin modifications regulates substrate interactions with specific proteins. Thus ubiquitination directs a variety of substrate fates including proteasomal degradation. Deubiquitinase enzymes cleave ubiquitin from substrates and are implicated in disease; for example, ubiquitin-specific protease-7 (USP7) regulates stability of the p53 tumour suppressor and other proteins critical for tumour cell survival. However, developing selective deubiquitinase inhibitors has been challenging and no co-crystal structures have been solved with small-molecule inhibitors. Here, using nuclear magnetic resonance-based screening and structure-based design, we describe the development of selective USP7 inhibitors GNE-6640 and GNE-6776. These compounds induce tumour cell death and enhance cytotoxicity with chemotherapeutic agents and targeted compounds, including PIM kinase inhibitors. Structural studies reveal that GNE-6640 and GNE-6776 non-covalently target USP7 12 Å distant from the catalytic cysteine. The compounds attenuate ubiquitin binding and thus inhibit USP7 deubiquitinase activity. GNE-6640 and GNE-6776 interact with acidic residues that mediate hydrogen-bond interactions with the ubiquitin Lys48 side chain, suggesting that USP7 preferentially interacts with and cleaves ubiquitin moieties that have free Lys48 side chains. We investigated this idea by engineering di-ubiquitin chains containing differential proximal and distal isotopic labels and measuring USP7 binding by nuclear magnetic resonance. This preferential binding protracted the depolymerization kinetics of Lys48-linked ubiquitin chains relative to Lys63-linked chains. In summary, engineering compounds that inhibit USP7 activity by attenuating ubiquitin binding suggests opportunities for developing other deubiquitinase inhibitors and may be a strategy more broadly applicable to inhibiting proteins that require ubiquitin binding for full functional activity.

Ligand-Induced Proton Transfer and Low-Barrier Hydrogen Bond Revealed by X-ray Crystallography.

Ligand binding can change the pKa of protein residues and influence enzyme catalysis. Herein, we report three ultrahigh resolution X-ray crystal structures of CTX-M β-lactamase, directly visualizing protonation state changes along the enzymatic pathway: apo protein at 0.79 Å, precovalent complex with nonelectrophilic ligand at 0.89 Å, and acylation transition state (TS) analogue at 0.84 Å. Binding of the noncovalent ligand induces a proton transfer from the catalytic Ser70 to the negatively charged Glu166, and the formation of a low-barrier hydrogen bond (LBHB) between Ser70 and Lys73, with a length of 2.53 Å and the shared hydrogen equidistant from the heteroatoms. QM/MM reaction path calculations determined the proton transfer barrier to be 1.53 kcal/mol. The LBHB is absent in the other two structures although Glu166 remains neutral in the covalent complex. Our data represents the first X-ray crystallographic example of a hydrogen engaged in an enzymatic LBHB, and demonstrates that desolvation of the active site by ligand binding can provide a protein microenvironment conducive to LBHB formation. It also suggests that LBHBs may contribute to stabilization of the TS in general acid/base catalysis together with other preorganized features of enzyme active sites. These structures reconcile previous experimental results suggesting alternatively Glu166 or Lys73 as the general base for acylation, and underline the importance of considering residue protonation state change when modeling protein-ligand interactions. Additionally, the observation of another LBHB (2.47 Å) between two conserved residues, Asp233 and Asp246, suggests that LBHBs may potentially play a special structural role in proteins.

Mechanistic and Structural Understanding of Uncompetitive Inhibitors of Caspase-6

Inhibition of caspase-6 is a potential therapeutic strategy for some neurodegenerative diseases, but it has been difficult to develop selective inhibitors against caspases. We report the discovery and characterization of a potent inhibitor of caspase-6 that acts by an uncompetitive binding mode that is an unprecedented mechanism of inhibition against this target class. Biochemical assays demonstrate that, while exquisitely selective for caspase-6 over caspase-3 and -7, the compound’s inhibitory activity is also dependent on the amino acid sequence and P1’ character of the peptide substrate. The crystal structure of the ternary complex of caspase-6, substrate-mimetic and an 11 nM inhibitor reveals the molecular basis of inhibition. The general strategy to develop uncompetitive inhibitors together with the unique mechanism described herein provides a rationale for engineering caspase selectivity.

Structure-Based Design of Potent and Ligand-Efficient Inhibitors of CTX-M Class A β-Lactamase

The emergence of CTX-M class A extended-spectrum β-lactamases poses a serious health threat to the public. We have applied structure-based design to improve the potency of a novel noncovalent tetrazole-containing CTX-M inhibitor (K(i) = 21 μM) more than 200-fold via structural modifications targeting two binding hot spots, a hydrophobic shelf formed by Pro167 and a polar site anchored by Asp240. Functional groups contacting each binding hot spot independently in initial designs were later combined to produce analogues with submicromolar potencies, including 6-trifluoromethyl-3H-benzoimidazole-4-carboxylic acid [3-(1H-tetrazol-5-yl)-phenyl]-amide, which had a K(i) value of 89 nM and reduced the MIC of cefotaxime by 64-fold in CTX-M-9 expressing Escherichia coli . The in vitro potency gains were accompanied by improvements in ligand efficiency (from 0.30 to 0.39) and LipE (from 1.37 to 3.86). These new analogues represent the first nM-affinity noncovalent inhibitors of a class A β-lactamase. Their complex crystal structures provide valuable information about ligand binding for future inhibitor design.

Broad-spectrum allosteric inhibition of herpesvirus proteases.

Herpesviruses rely on a homodimeric protease for viral capsid maturation. A small molecule, DD2, previously shown to disrupt dimerization of Kaposi’s sarcoma-associated herpesvirus protease (KSHV Pr) by trapping an inactive monomeric conformation and two analogues generated through carboxylate bioisosteric replacement (compounds 2 and 3) were shown to inhibit the associated proteases of all three human herpesvirus (HHV) subfamilies (α, β, and γ). Inhibition data reveal that compound 2 has potency comparable to or better than that of DD2 against the tested proteases. Nuclear magnetic resonance spectroscopy and a new application of the kinetic analysis developed by Zhang and Poorman [Zhang, Z. Y., Poorman, R. A., et al. (1991) J. Biol. Chem. 266, 15591–15594] show DD2, compound 2, and compound 3 inhibit HHV proteases by dimer disruption. All three compounds bind the dimer interface of other HHV proteases in a manner analogous to binding of DD2 to KSHV protease. The determination and analysis of cocrystal structures of both analogues with the KSHV Pr monomer verify and elaborate on the mode of binding for this chemical scaffold, explaining a newly observed critical structure–activity relationship. These results reveal a prototypical chemical scaffold for broad-spectrum allosteric inhibition of human herpesvirus proteases and an approach for the identification of small molecules that allosterically regulate protein activity by targeting protein–protein interactions.

Tailoring small molecules for an allosteric site on procaspase-6.

Although they represent attractive therapeutic targets, caspases have so far proven recalcitrant to the development of drugs targeting the active site. Allosteric modulation of caspase activity is an alternate strategy that potentially avoids the need for anionic and electrophilic functionality present in most active‐site inhibitors. Caspase‐6 has been implicated in neurodegenerative disease, including Huntington’s and Alzheimer’s diseases. Herein we describe a fragment‐based lead discovery effort focused on caspase‐6 in its active and zymogen forms. Fragments were identified for procaspase‐6 using surface plasmon resonance methods and subsequently shown by X‐ray crystallography to bind a putative allosteric site at the dimer interface. A fragment‐merging strategy was employed to produce nanomolar‐affinity ligands that contact residues in the L2 loop at the dimer interface, significantly stabilizing procaspase‐6. Because rearrangement of the L2 loop is required for caspase‐6 activation, our results suggest a strategy for the allosteric control of caspase activation with drug‐like small molecules.

Structure of the nucleotide exchange factor eIF2B reveals mechanism of memory-enhancing molecule.

ISRIB mechanism of action In rodents, a druglike small molecule named ISRIB enhances cognition and reverses cognitive deficits after traumatic brain injury. ISRIB activates a protein complex called eIF2B that is required for the synthesis of new proteins. Tsai et al. report the visualization of eIF2B bound to ISRIB at near-atomic resolution by cryo–electron microscopy. Biochemical studies revealed that ISRIB is a “molecular staple” that promotes assembly of the fully active form of eIF2B. Zyryanova et al. report similar structures together with information on the binding of ISRIB analogs and their effects on protein translation. Science, this issue p. eaaq0939, p. 1533 A drug-like inhibitor of the integrated stress response bridges tetrameric subcomplexes of eIF2B, assembling active holoenzyme. INTRODUCTION Regulation by the integrated stress response (ISR) converges on the phosphorylation of translation initiation factor eIF2 in response to a variety of stresses. Phosphorylation converts eIF2 from a substrate to a competitive inhibitor of its dedicated guanine nucleotide exchange factor, eIF2B, inhibiting translation. ISRIB is a drug-like eIF2B activator that reverses the effects of eIF2 phosphorylation, enhances cognition, and corrects cognitive deficits after brain injury in rodents. Because ISRIB shows promise for treating neurological disorders a deeper understanding of its mechanism of action is crucial. Previous work identified eIF2B as a target of ISRIB and suggested that the molecule stabilizes and activates the enzyme. However, the molecule’s mode of binding and means of activation remain unknown. RATIONALE To identify the binding site and mechanism of action of ISRIB, we used cryo–electron microscopy (cryo-EM) to determine an atomic-resolution structure of decameric human eIF2B bound to ISRIB. We validated the structural model using mutational analysis and the synthesis of ISRIB analogs. Combined with pre–steady-state kinetic analysis of eIF2B complex assembly, these findings enabled us to derive a functional model of ISRIB action. RESULTS A robust recombinant expression and purification protocol for all subunits of human eIF2B produced a stable eIF2B holoenzyme that sedimented as a decamer. Under conditions of elevated ionic strength, an eIF2Bα dimer [eIF2B(α2)] dissociated from the remainder of the decamer, whereas ISRIB prevented disassembly. Sedimentation velocity experiments determined that in the absence of eIF2Bα, the remaining subunits form tetrameric complexes [eIF2B(βγδε)]. Loss of eIF2B(α2) largely abolished eIF2B’s nucleotide exchange activity. To explain these findings, we determined a structure of human eIF2B bound to ISRIB at 2.8 Å average resolution. The structure revealed that ISRIB binds within a deep cleft at a two-fold symmetry interface between the eIF2Bβ and eIF2Bδ subunits in the decamer. Greater resolution within the binding pocket enabled precise positioning of ISRIB, which we validated by probing with designed ISRIB analogs and mutational analysis. For example, stereospecific addition of a methyl group to ISRIB abrogated activity, whereas an eIF2B(δL179A) mutation accommodated this analog and restored activity. Further, a predicted C-H-π interaction between eIF2B(βH188) and ISRIB was confirmed by mutation of βH188 to other aromatic residues, which resulted in enhanced stability of the complex. To determine how ISRIB enhances incorporation of eIF2B(α2) into the complex, we analyzed the eIF2B(βγδε) tetramer structurally and functionally. Cryo-EM imaging and analytical ultracentrifugation revealed that ISRIB staples two eIF2B(βγδε) tetramers together to form an octamer across its two-fold symmetry axis. The resulting octamer displays a composite surface for avid eIF2B(α2) binding, explaining ISRIB’s mechanism of action. Consistent with this model, saturating half-binding sites in the tetramer with ISRIB prevented dimerization and failed to activate the enzyme. Additional loss-of-function and gain-of-function dimerization mutants produced complexes that were insensitive to ISRIB. CONCLUSION From this work, the regulation of eIF2B assembly from stable subcomplexes emerges as a rheostat for eIF2B activity that tunes translation during the ISR and can be further modulated by ISRIB acting as a “molecular staple.” As a two-fold symmetric small molecule, ISRIB bridges a central symmetry axis of the decameric eIF2B complex, stabilizing it in an activated state. ISRIB’s action as an assembly-promoting enzyme activator provides a plausible model for its ability to ameliorate the inhibitory effects of eIF2α phosphorylation. Understanding the different modes of regulation of this vital translational control point will be of particular importance in the nervous system where ISRIB has been shown to have a range of effects, and will further enable ISRIB’s development as a promising therapeutic agent in combating cognitive disorders. ISRIB bound to human eIF2B. View of cryo-EM density for eIF2B(αβγδε)2, colored in distinct shades for each subunit copy: (red, α; blue, β; green, γ; gold, δ; gray, ε). Density assigned to ISRIB is depicted in CPK coloring (red, O; blue, N; green, Cl) and artistically contrasted from its target protein. Regulation by the integrated stress response (ISR) converges on the phosphorylation of translation initiation factor eIF2 in response to a variety of stresses. Phosphorylation converts eIF2 from a substrate to a competitive inhibitor of its dedicated guanine nucleotide exchange factor, eIF2B, thereby inhibiting translation. ISRIB, a drug-like eIF2B activator, reverses the effects of eIF2 phosphorylation, and in rodents it enhances cognition and corrects cognitive deficits after brain injury. To determine its mechanism of action, we solved an atomic-resolution structure of ISRIB bound in a deep cleft within decameric human eIF2B by cryo–electron microscopy. Formation of fully active, decameric eIF2B holoenzyme depended on the assembly of two identical tetrameric subcomplexes, and ISRIB promoted this step by cross-bridging a central symmetry interface. Thus, regulation of eIF2B assembly emerges as a rheostat for eIF2B activity that tunes translation during the ISR and that can be further modulated by ISRIB.