Nucleophile-Fragment Screening for Site-Specific Covalent Ligand Discovery in Opioid Receptor Signaling

PROJECT SUMMARY
Substance abuse disorders including opioid dependence are characterized by brain oxidative stress and neu-
roinflammation. In addition to dopamine autooxidation, chronic drug consumption increases the expression of
pro-oxidant enzymes such as NADPH-oxidase (NOX), leading to excessive production of reactive oxygen spe-
cies (ROS) primarily in the form hydrogen peroxide (H2O2). Endogenous H2O2 can transiently and/or irreversibly
oxidize cysteinyl residues depending upon its concentration and duration of exposure. In proteins, H2O2 reacts
with redox-sensitive cysteine thiols to form sulfenic acid (Cys-SOH). Cysteine S-sulfenation has emerged as a
major post-translational modification that exerts significant effects on protein function, analogous to phosphory-
lation. A small but compelling literature suggests that changes in cysteine redox state affect mu opioid receptor
(MOR) function, as thiol alkylation, site-directed mutagenesis, and redox-modulating agents can alter ligand
binding and downstream signaling events. In addition to structural and regulatory roles, oxidation of the cysteine
thiol blocks the reaction of this residue with a,b-unsaturated carbonyls and alkyl halides, significantly limiting the
utility of conventional electrophile-fragment screening as a tool for covalent ligand discovery. To address this
issue, we have recently developed a strategy that employs nucleophilic covalent fragments to target S-sulfenated
(oxidized) cysteines. These sulfenic acid-reactive activity-based protein profiling (ABPP) probes have been cou-
pled with state-of-the-art quantitative proteomics to identify S-sulfenated cysteines in human cells, which presage
the development of covalent fragments therapeutically targeting redox-active cysteines. Here, the following Spe-
cific Aims are proposed: 1) Increase the size and structural diversity of our nucleophile-fragment libraries; 2) Map
cysteine redox reactivity changes and S-sulfenated (oxidized) cysteine ligandability in differentiated mature SH-
SY5Y neurons that are unstimulated (control) or treated with morphine agonist (stimulated). These studies are
rationalized based on proof-of-concept experiments which demonstrate that unique ligandable sites are identified
when fragments are functionalized with nucleophilic reactive groups that react with S-sulfenated (oxidized) cys-
teine residues. Deliverables from these studies are a novel chemoproteomic method, chemical matter that can
be mined as a source of small-molecule probes and as starting points for drug discovery.

Grant Number: 5R21DA059363-03
NIH Institute/Center: NIH
Principal Investigator: Kate Carroll