Uncovering the roles of oxysterols in neuropathic pain

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Chronic neuropathic pain afflicts 15-20 million people in the U.S.1 Defining the molecular mechanisms involved is key to developing novel non-opioid analgesics.2 We recently discovered that the Gαi protein-coupled receptor (GPCR) Gpr183 transcript increases in glial cells of the dorsal horn of the spinal cord (DH-SC) following nerve injury and contributes to ensuing neuropathic pain.3 The primary endogenous ligand for GPR183 is the potent signaling oxysterol 7α,25-dihydroxycholesterol (7α,25-OHC).4-6 However, little is known about the roles of 7α,25- OHC/GPR183 in pain beyond our recent publication.3 Moreover, commercial GPR183 antagonists are very limited and have chemical properties that raise questions about their in vivo utility.3,7 To address this, we initiated an in silico drug discovery approach that screened 5.5 million commercial compounds with similarities to a GPR183 pharmacophore model, then docked and ranked the highest similar docking scores.3 The top 16 compounds were tested in calcium mobilization (FLIPR) assays to identify several small-molecule selective GPR183 antagonists with IC50 values in the nM range, exemplified by SAE-14.3 SAE-14 was counter-screened against the GPCRome at NIMH PDSP8 with no significant activity for over 300 receptors. Systemic SAE-14 administration reversed mechano-and cold allodynia (behavioral hypersensitivities) induced by nerve injury in male and female rodents with no observable side effects or engaging the endogenous opioid system.3 These effects were mimicked by intrathecal (i.th.) injection of SAE-14, identifying the spinal cord as a potential site of GPR183 antagonist action.3 Moreover, pain behavioral phenotypes were recapitulated by i.th. injection of 7α,25- OHC or a fluorinated analog that activated Gαi/o-linked (pertussis toxin), GPR183-dependent mitogen-activated protein kinase (MAPK) signaling (preliminary data). MAPK signaling in the DH-SC is crucial in regulating neuroinflammatory/pro-nociceptive genes and maintaining persistent pain sensitization.9,10 GPR183-dependent MAPK signaling was supported by unbiased transcriptomic analysis of the DH-SC from SAE-14-treated rats with neuropathic pain (preliminary data). SAE-14 lacks sufficient metabolic stability and solubility/absorption (preliminary data) and structure-activity relationship studies on SAE-14 are necessary for optimization and discovery of druggable GPR183 antagonists. Based on our published work3 and preliminary data, we hypothesize that 7α,25-OHC/GPR183 signaling in the DH-SC contributes to the development and maintenance of neuropathic pain states. Proposed studies in Aim 1 will investigate the role of 7α,25-OHC/GPR183 on MAPK activation and neuroinflammation in the DH-SC. Then in Aim 2, we will develop GPR183 antagonists based on SAE-14 and use a testing funnel to screen to find those with optimal stability and bioavailability for in vivo pharmacological profiling in rodent neuropathic pain models in Aim 3. Our findings will further our understanding of GPR183 signaling in neuropathic pain and identify the first small molecule GPR183 antagonists with properties suitable for non-addictive analgesics development addressing a key mission of the HEAL initiative.