Discovery and Characterization of a Chemical Probe Targeting the Zinc-Finger Ubiquitin-Binding Domain of HDAC6.

Histone deacetylase 6 (HDAC6) inhibition is an attractive strategy for treating numerous cancers, and HDAC6 catalytic inhibitors are currently in clinical trials. The HDAC6 zinc-finger ubiquitin-binding domain (UBD) binds free C-terminal diglycine motifs of unanchored ubiquitin polymer chains and protein aggregates, playing an important role in autophagy and aggresome assembly. However, targeting this domain with small molecule antagonists remains an underdeveloped avenue of HDAC6-focused drug discovery. We report SGC-UBD253 (25), a chemical probe potently targeting HDAC6-UBD in vitro with selectivity over nine other UBDs, except for weak USP16 binding. In cells, 25 is an effective antagonist of HDAC6-UBD at 1 µM, with marked proteome-wide selectivity. We identified SGC-UBD253N (32), a methylated derivative of 25 that is 300-fold less active, serving as a negative control. Together, 25 and 32 could enable further exploration of the biological function of the HDAC6-UBD and investigation of the therapeutic potential of targeting this domain.

Huntingtin structure is orchestrated by HAP40 and shows a polyglutamine expansion-specific interaction with exon 1.

Huntington's disease results from expansion of a glutamine-coding CAG tract in the huntingtin (HTT) gene, producing an aberrantly functioning form of HTT. Both wildtype and disease-state HTT form a hetero-dimer with HAP40 of unknown functional relevance. We demonstrate in vivo and in cell models that HTT and HAP40 cellular abundance are coupled. Integrating data from a 2.6 Å cryo-electron microscopy structure, cross-linking mass spectrometry, small-angle X-ray scattering, and modeling, we provide a near-atomic-level view of HTT, its molecular interaction surfaces and compacted domain architecture, orchestrated by HAP40. Native mass spectrometry reveals a remarkably stable hetero-dimer, potentially explaining the cellular inter-dependence of HTT and HAP40. The exon 1 region of HTT is dynamic but shows greater conformational variety in the polyglutamine expanded mutant than wildtype exon 1. Our data provide a foundation for future functional and drug discovery studies targeting Huntington's disease and illuminate the structural consequences of HTT polyglutamine expansion.

Structure-Activity Relationship of USP5 Inhibitors.

USP5 is a deubiquitinase that has been implicated in a range of diseases, including cancer, but no USP5-targeting chemical probe has been reported to date. Here, we present the progression of a chemical series that occupies the C-terminal ubiquitin-binding site of a poorly characterized zinc-finger ubiquitin binding domain (ZnF-UBD) of USP5 and competitively inhibits the catalytic activity of the enzyme. Exploration of the structure-activity relationship, complemented with crystallographic characterization of the ZnF-UBD bound to multiple ligands, led to the identification of 64, which binds to the USP5 ZnF-UBD with a KD of 2.8 µM and is selective over nine proteins containing structurally similar ZnF-UBD domains. 64 inhibits the USP5 catalytic cleavage of a di-ubiquitin substrate in an in vitro assay. This study provides a chemical and structural framework for the discovery of a chemical probe to delineate USP5 function in cells.

Identification and Structure-Activity Relationship of HDAC6 Zinc-Finger Ubiquitin Binding Domain Inhibitors.

HDAC6 plays a central role in the recruitment of protein aggregates for lysosomal degradation and is a promising target for combination therapy with proteasome inhibitors in multiple myeloma. Pharmacologically displacing ubiquitin from the zinc-finger ubiquitin-binding domain (ZnF-UBD) of HDAC6 is an underexplored alternative to catalytic inhibition. Here, we present the discovery of an HDAC6 ZnF-UBD-focused chemical series and its progression from virtual screening hits to low micromolar inhibitors. A carboxylate mimicking the C-terminal extremity of ubiquitin, and an extended aromatic system stacking with W1182 and R1155, are necessary for activity. One of the compounds induced a conformational remodeling of the binding site where the primary binding pocket opens up onto a ligand-able secondary pocket that may be exploited to increase potency. The preliminary structure-activity relationship accompanied by nine crystal structures should enable further optimization into a chemical probe to investigate the merit of targeting the ZnF-UBD of HDAC6 in multiple myeloma and other diseases.

Small Molecule Antagonists of the Interaction between the Histone Deacetylase 6 Zinc-Finger Domain and Ubiquitin.

Inhibitors of HDAC6 have attractive potential in numerous cancers. HDAC6 inhibitors to date target the catalytic domains, but targeting the unique zinc-finger ubiquitin-binding domain (Zf-UBD) of HDAC6 may be an attractive alternative strategy. We developed X-ray crystallography and biophysical assays to identify and characterize small molecules capable of binding to the Zf-UBD and competing with ubiquitin binding. Our results revealed two adjacent ligand-able pockets of HDAC6 Zf-UBD and the first functional ligands for this domain.