The 2020 conference season at the EMBL Advanced Training Centre kicked off with the EMBL Conference: Expanding the Druggable Proteome with Chemical Biology (5 – 7 February 2020). Meet the three poster prize winners from the conference – Patrick Zanon, Enric Ros and Rens de Vries.
Identification of novel antibiotic targets using covalent inhibitors and residue-specific proteomics
Bacterial resistance towards all marketed antibiotics poses an imminent threat to global health. In order to overcome this antibiotic crisis, drugs with novel mechanisms-of-action are desperately needed. Covalent inhibitors are especially promising in this regard as they are already prevalent as antibiotics (e.g. β-lactams and fosfomycin), allow targeting protein pockets that are hard to address with non-covalent interactions alone and hold the promise to overcome some mechanisms of resistance development. Furthermore, covalent inhibitors are uniquely suited to identify new binding pockets on proteins using residue-specific proteomics and in this way to broaden the scope of targetable protein targets.
The vast majority of covalent inhibitors so far either hijack the enzymatic activity of the protein by modification of catalytic serines and tyrosines or address cysteines through their inherent outstanding nucleophilicity. Nevertheless, the number of potentially addressable proteins in the bacterial proteome is significantly limited by the requirement for these amino acids to be present in target proteins. By developing electrophilic groups that are selective for other amino acids (e.g. lysine), we strive to expand the number of exploitable interaction sites for covalent inhibitors in the bacterial proteome. Furthermore, to assess the reactivity and selectivity of covalent inhibitors and to streamline the discovery of novel antibiotic targets, we develop new methods for residue-specific activity-based protein profiling.[2,3] In this way, we are convinced, that we will be able to make important contributions to overcome the antibiotic crisis.
 R. A. Bauer, Drug Discov. Today 2015, 20, 1061–1073.
 K. M. Backus et al., Nature 2016, 534, 570.
 P. R. A. Zanon, L. Lewald, S. M. Hacker Angew. Chem. Int. Ed., doi: 10.1002/anie.201912075.
(1) Technical University of Munich, Germany
Incorporating 1,2,4,5-tetrazines into proteins: A method for targeted drug release
Bioorthogonal reactions, namely reactions that can take place under biocompatible conditions, are having a major impact in the development of new research tools and novel therapeutic strategies. In the latter case, the discovery of the reaction commonly referred to as “click-to-release” (CtR), which triggers the liberation of a given cargo (normally a drug or a fluorophore), has led to several applications in drug delivery. This reaction happens between a 1,2,4,5-Tetrazine (Tz) fragment and certain alkenes or alkynes and, in order to achieve drug delivery specifically at the site of action, one of the two reactant counterparts should be conjugated to a biomolecule acting as a carrier, ideally a protein.
We have synthetized the previously unreported 3-bromo-1,2,4,5-tetrazine and used its excellent reactivity to attain chemoselective protein labelling onto lysines. Due to the chemical features of the formed amino-Tz. The resulting labelled lysines can undergo fast CtR reactions with trans-cyclooctenes, thereby releasing a desired cargo under physiological conditions. To showcase the applicability of this approach, we have labelled the monoclonal antibody Trastuzumab (anti-Her2) and demonstrated the specific release of the cytotoxic drug doxorubicin upon reaction in a mammalian cell culture context, resulting in a decrease in cell viability.
Additionally, we have also used 3-bromo-1,2,4,5-tetrazine to synthetize an amino-Tz containing non-natural amino acid and used it to achieve protein labelling through its genetic incorporation by amber codon suppression in Escherichia coli. The resulting site-selectively labelled proteins can also trigger fast, high yielding CtR reactions.
To summarize, we have successfully applied a new compound, 3-bromo-1,2,4,5-tetrazine, as a reagent to achieve either chemoselective or site selective protein labelling. We have applied the bioconjugated proteins to demonstrate their potential use for targeted drug delivery in a relevant cellular model, opening new therapeutically useful methodologies.
(1) IRB Barcelona, Spain
Modulation of nuclear receptors through ligand architecture
Nuclear receptors (NRs) have been one of the primary drug targets over the last decades for their ability to regulate gene expression. The traditional approach of modulating NRs is to design small synthetic molecules that interact with the ligand-binding domain (LBD) of the NR. Ligands can thereby either enhance or inhibit gene transcription. Apart from the effects on transcription, recent research shows that minor changes in the ligand scaffold can have a significant impact on the behavior of the NR. In this research, we show how small-molecules can change both the dimerization behavior of NRs and the recruitment of allosteric modulators.
The Retinoic X Receptor α (RXRα) is known as a master regulator among NRs through its ability to heterodimerize with, and thereby modulate, other NRs. We show, using a novel NanoBIT complexation assay, that small directed changes in the RXR ligand scaffold can lead to selective formation of specific hetero- and homodimers. Using our structural data and focused compound library, a model was developed to help to understand this effect of the ligand. This information can serve as a blueprint to design small-molecules that selectively target specific NRs via RXR. This makes RXR as an exciting and versatile target for NR modulation, especially when classical modulation of the partner NR is not possible.
Recently, small-molecules have been found to bind to allosteric sites of NRs. Allosteric ligands are of interest since they do not compete with the endogenous ligand of the NR and often shown an increased selectivity towards their target. We show, using X-ray crystallography and biochemical assays, that there is communication between orthosteric and allosteric ligands in the RAR-related orphan receptor γ t (RORγt). We successfully solved eleven new ternary crystal structures of RORγt in the presence of both orthosteric and allosteric ligands. These structures mechanistically show how binding of the orthosteric ligand leads to positive cooperative binding of the allosteric ligand.
(1) Eindhoven University of Technology, The Netherlands
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