Correction to "Rational Chemical Design of Molecular Glue Degraders".

[This corrects the article DOI: 10.1021/acscentsci.2c01317.].

Rational Chemical Design of Molecular Glue Degraders.

Targeted protein degradation with molecular glue degraders has arisen as a powerful therapeutic modality for eliminating classically undruggable disease-causing proteins through proteasome-mediated degradation. However, we currently lack rational chemical design principles for converting protein-targeting ligands into molecular glue degraders. To overcome this challenge, we sought to identify a transposable chemical handle that would convert protein-targeting ligands into molecular degraders of their corresponding targets. Using the CDK4/6 inhibitor ribociclib as a prototype, we identified a covalent handle that, when appended to the exit vector of ribociclib, induced the proteasome-mediated degradation of CDK4 in cancer cells. Further modification of our initial covalent scaffold led to an improved CDK4 degrader with the development of a but-2-ene-1,4-dione ("fumarate") handle that showed improved interactions with RNF126. Subsequent chemoproteomic profiling revealed interactions of the CDK4 degrader and the optimized fumarate handle with RNF126 as well as additional RING-family E3 ligases. We then transplanted this covalent handle onto a diverse set of protein-targeting ligands to induce the degradation of BRD4, BCR-ABL and c-ABL, PDE5, AR and AR-V7, BTK, LRRK2, HDAC1/3, and SMARCA2/4. Our study undercovers a design strategy for converting protein-targeting ligands into covalent molecular glue degraders.

An Activity-Based Oxaziridine Platform for Identifying and Developing Covalent Ligands for Functional Allosteric Methionine Sites: Redox-Dependent Inhibition of Cyclin-Dependent Kinase 4.

Activity-based protein profiling (ABPP) is a versatile strategy for identifying and characterizing functional protein sites and compounds for therapeutic development. However, the vast majority of ABPP methods for covalent drug discovery target highly nucleophilic amino acids such as cysteine or lysine. Here, we report a methionine-directed ABPP platform using Redox-Activated Chemical Tagging (ReACT), which leverages a biomimetic oxidative ligation strategy for selective methionine modification. Application of ReACT to oncoprotein cyclin-dependent kinase 4 (CDK4) as a representative high-value drug target identified three new ligandable methionine sites. We then synthesized a methionine-targeting covalent ligand library bearing a diverse array of heterocyclic, heteroatom, and stereochemically rich substituents. ABPP screening of this focused library identified 1oxF11 as a covalent modifier of CDK4 at an allosteric M169 site. This compound inhibited kinase activity in a dose-dependent manner on purified protein and in breast cancer cells. Further investigation of 1oxF11 found prominent cation-π and H-bonding interactions stabilizing the binding of this fragment at the M169 site. Quantitative mass-spectrometry studies validated 1oxF11 ligation of CDK4 in breast cancer cell lysates. Further biochemical analyses revealed cross-talk between M169 oxidation and T172 phosphorylation, where M169 oxidation prevented phosphorylation of the activating T172 site on CDK4 and blocked cell cycle progression. By identifying a new mechanism for allosteric methionine redox regulation on CDK4 and developing a unique modality for its therapeutic intervention, this work showcases a generalizable platform that provides a starting point for engaging in broader chemoproteomics and protein ligand discovery efforts to find and target previously undruggable methionine sites.

Deubiquitinase-targeting chimeras for targeted protein stabilization.

Many diseases are driven by proteins that are aberrantly ubiquitinated and degraded. These diseases would be therapeutically benefited by targeted protein stabilization (TPS). Here we present deubiquitinase-targeting chimeras (DUBTACs), heterobifunctional small molecules consisting of a deubiquitinase recruiter linked to a protein-targeting ligand, to stabilize the levels of specific proteins degraded in a ubiquitin-dependent manner. Using chemoproteomic approaches, we discovered the covalent ligand EN523 that targets a non-catalytic allosteric cysteine C23 in the K48-ubiquitin-specific deubiquitinase OTUB1. We showed that a DUBTAC consisting of our EN523 OTUB1 recruiter linked to lumacaftor, a drug used to treat cystic fibrosis that binds ΔF508-cystic fibrosis transmembrane conductance regulator (CFTR), robustly stabilized ΔF508-CFTR protein levels, leading to improved chloride channel conductance in human cystic fibrosis bronchial epithelial cells. We also demonstrated stabilization of the tumor suppressor kinase WEE1 in hepatoma cells. Our study showcases covalent chemoproteomic approaches to develop new induced proximity-based therapeutic modalities and introduces the DUBTAC platform for TPS.

A strategy to assess the cellular activity of E3 ligase components against neo-substrates using electrophilic probes.

While there are hundreds of predicted E3 ligases, characterizing their applications for targeted protein degradation has proved challenging. Here, we report a chemical biology approach to evaluate the ability of modified recombinant E3 ligase components to support neo-substrate degradation. Bypassing the need for specific E3 ligase binders, we use maleimide-thiol chemistry for covalent functionalization followed by E3 electroporation (COFFEE) in live cells. We demonstrate that electroporated recombinant von Hippel-Lindau (VHL) protein, covalently functionalized at its ligandable cysteine with JQ1 or dasatinib, induces degradation of BRD4 or tyrosine kinases, respectively. Furthermore, by applying COFFEE to SPSB2, a Cullin-RING ligase 5 receptor, as well as to SKP1, the adaptor protein for Cullin-RING ligase 1 F box (SCF) complexes, we validate this method as a powerful approach to define the activity of previously uncharacterized ubiquitin ligase components, and provide further evidence that not only E3 ligase receptors but also adaptors can be directly hijacked for neo-substrate degradation.

Manumycin polyketides act as molecular glues between UBR7 and P53.

Molecular glues are an intriguing therapeutic modality that harness small molecules to induce interactions between proteins that typically do not interact. However, such molecules are rare and have been discovered fortuitously, thus limiting their potential as a general strategy for therapeutic intervention. We postulated that natural products bearing one or more electrophilic sites may be an unexplored source of new molecular glues, potentially acting through multicovalent attachment. Using chemoproteomic platforms, we show that members of the manumycin family of polyketides, which bear multiple potentially reactive sites, target C374 of the putative E3 ligase UBR7 in breast cancer cells, and engage in molecular glue interactions with the neosubstrate tumor-suppressor TP53, leading to p53 transcriptional activation and cell death. Our results reveal an anticancer mechanism of this natural product family, and highlight the potential for combining chemoproteomics and multicovalent natural products for the discovery of new molecular glues.

A Nimbolide-Based Kinase Degrader Preferentially Degrades Oncogenic BCR-ABL.

Targeted protein degradation (TPD) and proteolysis-targeting chimeras (PROTACs) have arisen as powerful therapeutic modalities for degrading specific proteins in a proteasome-dependent manner. However, a major limitation of TPD is the lack of E3 ligase recruiters. Recently, we discovered the natural product nimbolide as a covalent recruiter for the E3 ligase RNF114. Here, we show the broader utility of nimbolide as an E3 ligase recruiter for TPD applications. We demonstrate that a PROTAC linking nimbolide to the kinase and BCR-ABL fusion oncogene inhibitor dasatinib, BT1, selectively degrades BCR-ABL over c-ABL in leukemia cancer cells, compared to previously reported cereblon or VHL-recruiting BCR-ABL degraders that show opposite selectivity or, in some cases, inactivity. Thus, we further establish nimbolide as an additional general E3 ligase recruiter for PROTACs, and we demonstrate the importance of expanding upon the arsenal of E3 ligase recruiters, as such molecules confer differing selectivity for the degradation of neo-substrate proteins.

Discovery of a Covalent Kinase Inhibitor from a DNA-Encoded Small-Molecule Library × Protein Library Selection.

We previously reported interaction determination using unpurified proteins (IDUP), a method to selectively amplify DNA sequences encoding ligand:target pairs from a mixture of DNA-linked small molecules and unpurified protein targets in cell lysates. In this study, we applied IDUP to libraries of DNA-encoded bioactive compounds and DNA-tagged human kinases to identify ligand:protein binding partners out of 32 096 possible combinations in a single solution-phase library × library experiment. The results recapitulated known small molecule:protein interactions and also revealed that ethacrynic acid is a novel ligand and inhibitor of MAP2K6 kinase. Ethacrynic acid inhibits MAP2K6 in part through alkylation of a nonconserved cysteine residue. This work validates the ability of IDUP to discover ligands for proteins of biomedical relevance.

Expanding the Scope of Electrophiles Capable of Targeting K-Ras Oncogenes.

There is growing interest in reversible and irreversible covalent inhibitors that target noncatalytic amino acids in target proteins. With a goal of targeting oncogenic K-Ras variants (e.g., G12D) by expanding the types of amino acids that can be targeted by covalent inhibitors, we survey a set of electrophiles for their ability to label carboxylates. We functionalized an optimized ligand for the K-Ras switch II pocket with a set of electrophiles previously reported to react with carboxylates and characterized the ability of these compounds to react with model nucleophiles and oncogenic K-Ras proteins. Here, we report that aziridines and stabilized diazo groups preferentially react with free carboxylates over thiols. Although we did not identify a warhead that potently labels K-Ras G12D, we were able to study the interactions of many electrophiles with K-Ras, as most of the electrophiles rapidly label K-Ras G12C. We characterized the resulting complexes by crystallography, hydrogen/deuterium exchange, and differential scanning fluorimetry. Our results both demonstrate the ability of a noncatalytic cysteine to react with a diverse set of electrophiles and emphasize the importance of proper spatial arrangements between a covalent inhibitor and its intended nucleophile. We hope that these results can expand the range of electrophiles and nucleophiles of use in covalent protein modulation.

An aptamer-functionalized chemomechanically modulated biomolecule catch-and-release system.

The efficient extraction of (bio)molecules from fluid mixtures is vital for applications ranging from target characterization in (bio)chemistry to environmental analysis and biomedical diagnostics. Inspired by biological processes that seamlessly synchronize the capture, transport and release of biomolecules, we designed a robust chemomechanical sorting system capable of the concerted catch and release of target biomolecules from a solution mixture. The hybrid system is composed of target-specific, reversible binding sites attached to microscopic fins embedded in a responsive hydrogel that moves the cargo between two chemically distinct environments. To demonstrate the utility of the system, we focus on the effective separation of thrombin by synchronizing the pH-dependent binding strength of a thrombin-specific aptamer with volume changes of the pH-responsive hydrogel in a biphasic microfluidic regime, and show a non-destructive separation that has a quantitative sorting efficiency, as well as the system's stability and amenability to multiple solution recycling.

Novel selection methods for DNA-encoded chemical libraries.

Driven by the need for new compounds to serve as biological probes and leads for therapeutic development and the growing accessibility of DNA technologies including high-throughput sequencing, many academic and industrial groups have begun to use DNA-encoded chemical libraries as a source of bioactive small molecules. In this review, we describe the technologies that have enabled the selection of compounds with desired activities from these libraries. These methods exploit the sensitivity of in vitro selection coupled with DNA amplification to overcome some of the limitations and costs associated with conventional screening methods. In addition, we highlight newer techniques with the potential to be applied to the high-throughput evaluation of DNA-encoded chemical libraries.

A DNA-based molecular probe for optically reporting cellular traction forces.

We developed molecular tension probes (TPs) that report traction forces of adherent cells with high spatial resolution, can in principle be linked to virtually any surface, and obviate monitoring deformations of elastic substrates. TPs consist of DNA hairpins conjugated to fluorophore-quencher pairs that unfold and fluoresce when subjected to specific forces. We applied TPs to reveal that cellular traction forces are heterogeneous within focal adhesions and localized at their distal edges.

Identification of ligand-target pairs from combined libraries of small molecules and unpurified protein targets in cell lysates.

We describe the development and validation of interaction determination using unpurified proteins (IDUP), a method that selectively amplifies DNA sequences identifying ligand+target pairs from a mixture of DNA-linked small molecules and unpurified protein targets in cell lysates. By operating in cell lysates, IDUP preserves native post-translational modifications and interactions with endogenous binding partners, thereby enabling the study of difficult-to-purify targets and increasing the potential biological relevance of detected interactions compared with methods that require purified proteins. In IDUP, target proteins are associated with DNA oligonucleotide tags either non-covalently using a DNA-linked antibody or covalently using a SNAP-tag. Ligand-target binding promotes hybridization of a self-priming hairpin that is extended by a DNA polymerase to create a DNA strand that contains sequences identifying both the target and its ligand. These sequences encoding ligand+target pairs are selectively amplified by PCR and revealed by high-throughput DNA sequencing. IDUP can respond to the effect of affinity-modulating adaptor proteins in cell lysates that would be absent in ligand screening or selection methods using a purified protein target. This capability was exemplified by the 100-fold amplification of DNA sequences encoding FRB+rapamycin or FKBP+rapamycin in samples overexpressing both FRB and FKBP (FRB·rapamycin+FKBP, Kd ≈ 100 fM; FKBP·rapamycin+FRB, Kd = 12 nM). In contrast, these sequences were amplified 10-fold less efficiently in samples overexpressing either FRB or FKBP alone (rapamycin+FKBP, Kd ≈ 0.2 nM; rapamcyin+FRB, Kd = 26 µM). Finally, IDUP was used to process a model library of DNA-linked small molecules and a model library of cell lysates expressing SNAP-target fusions combined in a single sample. In this library×library experiment, IDUP resulted in enrichment of sequences corresponding to five known ligand+target pairs ranging in binding affinity from Kd = 0.2 nM to 3.2 µM out of 67,858 possible combinations, with no false positive signals enriched to the same extent as that of any of the bona fide ligand+target pairs.

Interaction-dependent PCR: identification of ligand-target pairs from libraries of ligands and libraries of targets in a single solution-phase experiment.

Interaction-dependent PCR (IDPCR) is a solution-phase method to identify binding partners from combined libraries of small-molecule ligands and targets in a single experiment. Binding between DNA-linked targets and DNA-linked ligands induces formation of an extendable duplex. Extension links codes that identify the ligand and target into one selectively amplifiable DNA molecule. In a model selection, IDPCR resulted in the enrichment of DNA encoding all five known protein-ligand pairs out of 67 599 possible sequences.

Photopolymerized poly(ethylene glycol)/poly(L-lysine) hydrogels for the delivery of neural progenitor cells.

Neural progenitor cells (NPCs) have shown promise in a number of models of disease and injury, but for these cells to be safe and effective, they must be directed to differentiate appropriately following transplantation. We have developed a photopolymerized hydrogel composed of macromers of poly(ethylene glycol) (PEG) bound to poly(L-lysine) (PLL) that supports NPC survival and directs differentiation. Green fluorescent protein (GFP) positive NPCs were encapsulated in these gels and demonstrated survival up to 17 days. When encapsulated in the gels at a photoinitiator concentration of 5.0 mg/ml, few NPCs (0.5 +/- 0.25%) demonstrated apoptosis. Furthermore, 55 +/- 6% of the NPCs cultured within the gels in epidermal growth factor (EGF) containing media differentiated into a mature neuronal cell type (neurofilament 200 positive) while the remainder 44 +/- 8% were undifferentiated (nestin positive). A small percentage, 1 +/- 0.4%, expressed the astrocytic marker glial acidic fibrilary protein. Photopolymerized PEG/PLL gels promote the survival and direct the differentiation of NPCs, making this system a promising delivery vehicle for NPCs in the treatment of injuries and diseases of the central nervous system.