Taiji Oashi

Turning Defense into Offense: Defensin Mimetics as Novel Antibiotics Targeting Lipid II

We have previously reported on the functional interaction of Lipid II with human alpha-defensins, a class of antimicrobial peptides. Lipid II is an essential precursor for bacterial cell wall biosynthesis and an ideal and validated target for natural antibiotic compounds. Using a combination of structural, functional and in silico analyses, we present here the molecular basis for defensin-Lipid II binding. Based on the complex of Lipid II with Human Neutrophil peptide-1, we could identify and characterize chemically diverse low-molecular weight compounds that mimic the interactions between HNP-1 and Lipid II. Lead compound BAS00127538 was further characterized structurally and functionally; it specifically interacts with the N-acetyl muramic acid moiety and isoprenyl tail of Lipid II, targets cell wall synthesis and was protective in an in vivo model for sepsis. For the first time, we have identified and characterized low molecular weight synthetic compounds that target Lipid II with high specificity and affinity. Optimization of these compounds may allow for their development as novel, next generation therapeutic agents for the treatment of Gram-positive pathogenic infections.

Electrostatic interactions mediate binding of obscurin to small ankyrin 1: biochemical and molecular modeling studies.

Small ankyrin 1 (sAnk1; also known as Ank1.5) is an integral protein of the sarcoplasmic reticulum (SR) in skeletal and cardiac muscle cells, where it is thought to bind to the C-terminal region of obscurin, a large modular protein that surrounds the contractile apparatus. Using fusion proteins in vitro, in combination with site-directed mutagenesis and surface plasmon resonance measurements, we previously showed that the binding site on sAnk1 for obscurin consists, in part, of six lysine and arginine residues. Here we show that four charged residues in the high-affinity binding site on obscurin for sAnk1 (between residues 6316 and 6345), consisting of three glutamates and a lysine, are necessary, but not sufficient, for this site on obscurin to bind to sAnk1 with high affinity. We also identify specific complementary mutations in sAnk1 that can partially or completely compensate for the changes in binding caused by charge-switching mutations in obscurin. We used molecular modeling to develop structural models of residues 6322-6339 of obscurin bound to sAnk1. The models, based on a combination of Brownian and molecular dynamics simulations, predict that the binding site on sAnk1 for obscurin is organized as two ankyrin-like repeats, with the last α-helical segment oriented at an angle to nearby helices, allowing lysine 6338 of obscurin to form an ionic interaction with aspartate 111 of sAnk1. This prediction was validated by double-mutant cycle experiments. Our results are consistent with a model in which electrostatic interactions between specific pairs of side chains on obscurin and sAnk1 promote binding and complex formation.

The small molecule IMR-1 inhibits the Notch transcriptional activation complex to suppress tumorigenesis

In many cancers, aberrant Notch activity has been demonstrated to play a role in the initiation and maintenance of the neoplastic phenotype and in cancer stem cells, which may allude to its additional involvement in metastasis and resistance to therapy. Therefore, Notch is an exceedingly attractive therapeutic target in cancer, but the full range of potential targets within the pathway has been underexplored. To date, there are no small-molecule inhibitors that directly target the intracellular Notch pathway or the assembly of the transcriptional activation complex. Here, we describe an in vitro assay that quantitatively measures the assembly of the Notch transcriptional complex on DNA. Integrating this approach with computer-aided drug design, we explored potential ligand-binding sites and screened for compounds that could disrupt the assembly of the Notch transcriptional activation complex. We identified a small-molecule inhibitor, termed Inhibitor of Mastermind Recruitment-1 (IMR-1), that disrupted the recruitment of Mastermind-like 1 to the Notch transcriptional activation complex on chromatin, thereby attenuating Notch target gene transcription. Furthermore, IMR-1 inhibited the growth of Notch-dependent cell lines and significantly abrogated the growth of patient-derived tumor xenografts. Taken together, our findings suggest that a novel class of Notch inhibitors targeting the transcriptional activation complex may represent a new paradigm for Notch-based anticancer therapeutics, warranting further preclinical characterization. Cancer Res; 76(12); 3593-603. ©2016 AACR.

Importance of domain closure for the autoactivation of ERK2.

Extracellular signal-regulated kinases 1 and 2 (ERK1 and -2, respectively) play a critical role in regulating cell division and have been implicated in cancer. In addition to activation by MAPK/ERK kinases 1 and 2 (MEK1 and -2, respectively), certain mutants of ERK2 can be activated by autophosphorylation. To identify the mechanism of autoactivation, we have performed a series of molecular dynamics simulations of ERK1 and -2 in various stages of activation as well as the constitutively active Q103A, I84A, L73P, and R65S ERK2 mutants. Our simulations indicate the importance of domain closure for autoactivation and activity regulation, with that event occurring prior to folding of the activation lip and of loop L16. Results indicate that the second phosphorylation event, that of T183, disrupts hydrogen bonding involving D334, thereby allowing the kinase to lock into the active conformation. On the basis of the simulations, three predictions were made. G83A was suggested to impede activation; K162M was suggested to perturb the interface between the N- and C-domains leading to activation, and Q64C was hypothesized to stop folding of loop L16, thereby perturbing the homodimerization interface. Functional analysis of the mutants validated the predictions concerning the G83A and Q64C mutants. The K162M mutant did not autoactivate as predicted, however, which may be due to the location of the residue on the protein surface near the ED substrate docking domain.

Automated selection of compounds with physicochemical properties to maximize bioavailability and druglikeness.

Adequate bioavailability is one of the essential properties for an orally administered drug. Lipinski and others have formulated simplified rules in which compounds that satisfy selected physiochemical properties, for example, molecular weight (MW) ≤ 500 or the logarithm of the octanol-water partition coefficient, log P(o/w) < 5, are anticipated to likely have pharmacokinetic properties appropriate for oral administration. However, these schemes do not simultaneously consider the combination of the physiochemical properties, complicating their application in a more automated fashion. To overcome this, we present a novel method to select compounds with a combination of physicochemical properties that maximize bioavailability and druglikeness based on compounds in the World Drug Index database. In the study four properties, MW, log P(o/w), number of hydrogen bond donors, and number of hydrogen acceptors, were combined into a 4-dimensional (4D) histogram, from which a scoring function was defined on the basis of a 4D dependent multivariate Gaussian model. The resulting equation allows for assigning compounds a bioavailability score, termed 4D-BA, such that chemicals with higher 4D-BA scores are more likely to have oral druglike characteristics. The descriptor is validated by applying the function to drugs previously categorized in the Biopharmaceutics Classification System, and examples of application of the descriptor are given in the context of previously published studies targeting heme oxygenase and SHP2 phosphatase. The approach is anticipated to be useful in early lead identification studies in combination with clustering methods to maximize chemical and structural diversity when selecting compounds for biological assays from large database screens. It may also be applied to prioritize synthetically feasible chemical modifications during lead compound optimization.

Small-molecule inhibitors of ERK-mediated immediate early gene expression and proliferation of melanoma cells expressing mutated BRaf.

Constitutive activation of the extracellular-signal-regulated kinases 1 and 2 (ERK1/2) are central to regulating the proliferation and survival of many cancer cells. The current inhibitors of ERK1/2 target ATP binding or the catalytic site and are therefore limited in their utility for elucidating the complex biological roles of ERK1/2 through its phosphorylation and regulation of over 100 substrate proteins. To overcome this limitation, a combination of computational and experimental methods was used to identify low-molecular-mass inhibitors that are intended to target ERK1/2 substrate-docking domains and selectively interfere with ERK1/2 regulation of substrate proteins. In the present study, we report the identification and characterization of compounds with a thienyl benzenesulfonate scaffold that were designed to inhibit ERK1/2 substrates containing an F-site or DEF (docking site for ERK, FXF) motif. Experimental evidence shows the compounds inhibit the expression of F-site containing immediate early genes (IEGs) of the Fos family, including c-Fos and Fra1, and transcriptional regulation of the activator protein-1 (AP-1) complex. Moreover, this class of compounds selectively induces apoptosis in melanoma cells containing mutated BRaf and constitutively active ERK1/2 signalling, including melanoma cells that are inherently resistant to clinically relevant kinase inhibitors. These findings represent the identification and initial characterization of a novel class of compounds that inhibit ERK1/2 signalling functions and their potential utility for elucidating ERK1/2 and other signalling events that control the growth and survival of cancer cells containing elevated ERK1/2 activity.

Hydrophobic residues in small ankyrin 1 participate in binding to obscurin

Abstract Small ankyrin-1 is a splice variant of the ANK1 gene that binds to obscurin A. Previous studies have identified electrostatic interactions that contribute to this interaction. In addition, molecular dynamics (MD) simulations predict four hydrophobic residues in a ‘hot spot’ on the surface of the ankyrin-like repeats of sAnk1, near the charged residues involved in binding. We used site-directed mutagenesis, blot overlays and surface plasmon resonance assays to study the contribution of the hydrophobic residues, V70, F71, I102 and I103, to two different 30-mers of obscurin that bind sAnk1, Obsc6316–6345 and Obsc6231–6260. Alanine mutations of each of the hydrophobic residues disrupted binding to the high affinity binding site, Obsc6316–6345. In contrast, V70A and I102A mutations had no effect on binding to the lower affinity site, Obsc6231–6260. Alanine mutagenesis of the five hydrophobic residues present in Obsc6316–6345 showed that V6328, I6332, and V6334 were critical to sAnk1 binding. Individual alanine mutants of the six hydrophobic residues of Obsc6231–6260 had no effect on binding to sAnk1, although a triple alanine mutant of residues V6233/I6234/I6235 decreased binding. We also examined a model of the Obsc6316–6345-sAnk1 complex in MD simulations and found I102 of sAnk1 to be within 2.2Å of V6334 of Obsc6316–6345. In contrast to the I102A mutation, mutating I102 of sAnk1 to other hydrophobic amino acids such as phenylalanine or leucine did not disrupt binding to obscurin. Our results suggest that hydrophobic interactions contribute to the higher affinity of Obsc6316–6345 for sAnk1 and to the dominant role exhibited by this sequence in binding.

Abstract 4554: A docking domain directed inhibitor targeting ERK regulated F-site substrates.

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) are ubiquitously expressed in almost all tissues and cell types. Accordingly, ERK1/2 regulate a diversity of cellular processes by phosphorylating more than 150 substrate proteins. With its constitutive activation being frequently implicated in human malignancies, ERK has been an appealing target in anti-cancer drug development. However, currently available ERK1/2 inhibitors are ATP-competitive. Since these ERK inhibitors block all ERK functions, the potential for side effects in normal tissues and cells may limit their potential of being developed into therapeutic agents. To overcome this limitation, we used a combination of computational and experimental methods to develop small molecular weight inhibitors that target ERK-substrate docking domains. Here we report the identification, characterization and optimization of a compound with a thienyl benzenesulfonate scaffold that specifically inhibits substrates containing an F-site or DEF (docking site for ERK, FXF) motif. Our X-ray crystallography data showed that this compound bound to a pocket in the vicinity of the F-site docking domain on ERK. Biological analysis further demonstrated that this compound and its analogs could preferentially inhibit phosphorylation and function of F-site-containing ERK substrates, and that these compounds preferentially inhibited growth of melanoma cells harboring a V600E B-Raf mutant. Additionally, this compound was shown to inhibit the phosphorylation of specific PKC isoforms via an ERK dependent mechanism. These findings represent the first identification and validation of chemicals that selectively inhibit ERK substrates and signaling events through ATP-independent mechanisms. These compounds have potential utility for elucidating the complex biological roles of ERK1/2 and development into novel anti-cancer agents. Citation Format: Jun Zhang, Taiji Oashi, Ramin Samadani, Kerrick Nevels, Kimberly Burkhard, Deva Priyakumar, Prabhu Raman, Steven Fletcher, Edvin Pozharsky, Paul Shapiro, Alexander MacKerell. A docking domain directed inhibitor targeting ERK regulated F-site substrates. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4554. doi:10.1158/1538-7445.AM2013-4554

Identification of ERK2-Substrate Protein Inhibitors via Virtual Screening, Biological Assays and X-Ray Crystallography

Extracellular signal regulated kinases (ERK1/2) are involved in signaling events that regulate cell division and proliferation. Hyperactivation of ERK has been implicated in the pathogenesis of many human cancers. The F-site recruitment site (FRS) (L198, H230, Y231, L232, L235, and Y261) as well as common docking (CD) (D316 and D319) and ED (T157 and T158) domain in ERK2 is used to facilitate interactions with substrate proteins. Thus, small molecules targeting FRS and/or CD/ED domain have the potential to modulate ERK2 specific functions, potentially leading to the development of novel therapeutic agents. MD simulations of ERK2 from which structurally diverse conformations were selected were used to identify putative binding sites for low molecular weight compounds in the vicinity of the FRS and CD/ED sites. Identified sites were then targeted in individual database screens of over 1.5 million compounds. Following two levels of database screening, fingerprint based similarity clustering and analysis of physicochemical properties that maximize bioavailability, final compounds for biological assay were selected for each site. Inhibition of ERK2-specific phosphorylation was confirmed and dose-dependency was measured in several cancer cell lines using colony survival assays. Direct binding of active compounds to ERK2 was validated by fluorescence quenching experiments. ERK2 was crystallized in complex with several active compounds, showing binding in the critical site in FRS. These identified compounds provide novel tools to study the biological functions of ERK2 as well as act as lead compounds for the development of novel therapeutic candidates for cancer.

Surface-Exposed Hydrophobic Residues on Small Ankyrin-1 Mediate Binding to Obscurin

Small ankyrin-1 (sAnk1, Ank1.5) is a splice variant of the ANK1 gene that binds to the large modular protein, obscurin A, with nanomolar affinity, a reaction that may help to organize the sarcoplasmic reticulum in striated muscle. A subset of lysine and arginine residues in the 2 ankyrin repeats of sAnk1 interact specifically with 4 glutamate residues in a stretch of 30 amino acids of obscurin to mediate binding. Homology modeling and molecular dynamics simulations have revealed a “hot spot” of 4 hydrophobic residues exposed on the surface of the ankyrin repeat domain of sAnk1. We used site-directed mutagenesis of bacterially expressed fusion proteins, followed by blot overlays and surface plasmon resonance assays, to study the contribution of these 4 residues, V70, F71, I102 and I103, to binding to the 30-mer of obscurin. Alanine mutations of each of these four residues inhibited binding to residues 6316-6345 of obscurin (Obsc6316-6345). In contrast, V70A and I102A mutations had no effect on binding to a second sAnk1 binding site on obscurin, located within residues 6231-6260 (Obsc6231-6260 ). Using the same methods, we mutated the 5 hydrophobic residues present in Obsc6316-6345 to alanine and identified V6328, I6332, and V6334 as critical for proper binding. Our results suggest that hydrophobic interactions as well as electrostatic interactions are important for the binding of sAnk1 to Obsc6316-6345, consistent with studies of the complexes formed by other ankyrin repeat proteins with their ligands. Hydrophobic interactions are likely to contribute to the difference in affinity of sAnk1 for Obsc6316-6345 and Obsc6231-6260, and for the dominant role played by the more C-terminal sequence in binding.Supported by grant R01- AR056330 from the NIH to RJB and training grants T32 GM08181 (to RJB) and T32 AR07592 (to M. Schneider).