Validation of σ2R/TMEM97 as a neuropathic pain target: Specificity, human expression and mechanism of action.
The Sigma 2 receptor ({sigma}2R) was described pharmacologically more than three decades ago, but its molecular identity remained obscure until recently when it was identified as transmembrane protein 97 (TMEM97). We and others have shown that {sigma}2R/TMEM97 ligands alleviate mechanical hypersensitivity in mouse neuropathic pain models with a time course wherein maximal anti-nociceptive effect is approximately 24 hours following dosing. We sought to understand this unique anti-neuropathic pain effect by addressing two key questions: do these {sigma}2R/TMEM97 compounds act selectively via the receptor, and what is their downstream mechanism on nociceptive neurons? Using male and female conventional knockout (KO) mice for Tmem97, we find that a new {sigma}2R/TMEM97 binding compound, FEM-1689, requires the presence of the gene to produce anti-nociception in the spared nerve injury model in mice. Using primary mouse dorsal root ganglion (DRG) neurons, we demonstrate that FEM-1689 inhibits the integrated stress response (ISR) and promotes neurite outgrowth via a {sigma}2R/TMEM97-specific action. We extend the clinical translational value of these findings by showing that FEM-1689 reduces ISR and p-eIF2 levels in human sensory neurons and that it alleviates the pathogenic engagement of ISR by methylglyoxal. We also demonstrate that {sigma}2R/TMEM97 is expressed in human nociceptors and satellite glial cells. These results validate {sigma}2R/TMEM97 as a promising target for further development for the treatment of neuropathic pain. Significance StatementNeuropathic pain is a major medical problem that is poorly treated with existing therapeutics. Our findings demonstrate that targeting {sigma}2R/TMEM97 with a newly described modulator reduces pain hypersensitivity in a mouse model with exquisite selectivity. We also identify integrated stress response (ISR) inhibition as a potential mechanism of action that links the receptor to cellular signaling events that have preclinical and clinical validation for pain relief. Our work suggests that {sigma}2R/TMEM97 can be selectively engaged by specific small molecules to produce ISR inhibition in a subset of cells that are critical for neuropathic pain. {sigma}2R/TMEM97-targeted therapeutics thus have the potential to offer effective pain relief without engagement of opioid receptors.