New approach to treating neuropathic pain offers the possibility of non-opioid pain relief for millions

By Pooja Toshniwal PahariaReviewed by Danielle Ellis, B.Sc.Feb 5 2024

In a recent study published in PNAS, researchers investigated the selective activity of sigmoid 2 receptor (σ2R) or transmembrane protein 97 (TMEM97) ligands on murine neuropathic pain models, their effects on nociceptive neurons and the mechanism of action after 24 hours.

Background

Neuropathic pain, a chronic condition, presents a significant therapeutic challenge due to the limited effectiveness and considerable side effects of current medications. A novel medication, TMEM97, has been found as a possible remedy, with roles in calcium signaling, cholesterol transport, and homeostasis. TMEM97 is associated with neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

About the study

In the present study, researchers investigated whether the anti-neuropathic pain benefits of σ2R/TMEM97 ligands were attributable to their binding to σ2R/TMEM97, which exhibits antinociception activity in animal models. They investigated whether targeting TMEM97 could alleviate pain by interfering with the integrated stress response (ISR) associated with neuropathic pain produced by traumatic nerve damage, metabolic diseases, and autoimmune conditions.

The team used a knockout (KO) murine model of the TMEM97 gene and the TMEM97-binding ligand, FEM-1689, to explore potential causal associations of σ2R/TMEM97 with antinociception in murine neuropathic pain models. They used RNAscope in situ hybridization on human dorsal root ganglia (DRG) from organ donors to investigate σ2R/TMEM97 expression in nociceptors.

The researchers developed FEM-1689 based on favorable outcomes in neuropathic pain models treated with UKH-1114. They explored whether σ2R/TMEM97 ligands diminish pain hypersensitivity by acting on σ2R/TMEM97 using a global TMEM97-KO mouse. They treated male and female wild-type and TMEM97KO rats with intravenous injections of FEM-1689 at 10 nM, 100 nM, and 1,000.0 nM, consistent with target binding for 16 hours.

The team investigated whether FEM-1689 would block the ISR in DRG neurons. They developed mouse dorsal root ganglia neurons from wild-type and TMEM97KO animals, administered FEM-1689 for 16 hours, and evaluated changes in phosphorylated eukaryotic initiation factor 2 (p-eIF2α) expression using immunocytochemistry (ICC). They explored the temporal dynamics of the FEM-1689 molecule in lowering p-eIF2α expression. They administered 100 nM FEM-1689 to wild-type murine DRG neurons over 0.5 hours, one hour, three hours, six hours, 12 hours, and 16 hours and measured p-eIF2α immunoreactivity.

The team investigated whether additional drugs binding to TMEM97 reduce the integrated stress response and whether ISR inhibition was specific to TMEM97 regulators that enhance antinociception. Computational docking analysis showed distinct interactions of FEM-1689 and SAS-0132 modulators with the expanded binding sites of TMEM97. They investigated whether the effects of the FEM-1689 compound on lowering p-eIF2α levels could reduce ISR-dependent and methylglyoxal (MGO)-induced mechanical hypersensitivity. To apply their findings from murine DRG studies to individuals, they treated human dorsal root ganglia neuronal cells from organ donor individuals with the TMEM97-binding ligand over 16 hours.

Results

FEM-1689 requires the presence of the Tmem97 gene to generate antinociception in the sparing nerve damage model in mice. FEM-1689 suppressed the ISR and promoted neurite outgrowth through a σ2R/TMEM97-specific action, decreasing ISR and p-eIF2α expression in human sensory neurons and relieving pathogenic ISR engagement by MGO. Human SCN10A-positive putative nociceptors and satellite glial cells express σ2R/TMEM97.

Murine DRG neurons and satellite glial cells produced Tmem97 messenger ribonucleic acid (mRNA) in a manner comparable to the human DRG. The antinociceptive effect of FEM-1689 in the SNI model was missing in TMEM97KO mice. The findings support targeting σ2R/TMEM97 to treat neuropathic pain. FEM-1689 was more specific for σ2R/TMEM97 than 40 CNS proteins, excluding σ1R and the norepinephrine transporter (NET).

Following nerve damage, male and female TMEM97KO and wild-type animals acquired significant and sustained mechanical hypersensitivity. The increased dosage of the TMEM97-binding ligand, FEM-1689, significantly corrected mechanical hypersensitivity in male and female wild-type mice for four days after a single treatment. However, FEM-1689 failed to diminish mechanical hypersensitivity in TMEM97KO mice, indicating that its effects were σ2R/TMEM97-dependent.

FEM-1689, a drug that suppressed the integrated stress response in murine DRG neurons, caused no change in p-ACC levels or AMPK-p-ACC pathway activity in cultured mouse DRG neurons after 16 hours, indicating a distinct antinociceptive route. TMEM97-KO neurons exhibited lower baseline p-eIF2α levels than wild-type neurons. ISRIB, an ISR inhibitor, lowers p-eIF2α levels in both kinds. FEM-1689 effectively corrected MGO-induced and ISR-dependent pain hypersensitivity in human sensory neurons, highlighting the potential for developing TMEM97 modulators for diabetic neuropathic pain.

The study findings showed that FEM-1689, a σ2R/TMEM97 ligand, can reduce neuropathic-type pain and block the ISR in mice. This impact needs direct regulation of σ2R/TMEM97, not σ1R or other proteins or receptors. FEM-1689 inhibited eIF2α phosphorylation within human dorsal root ganglia, reducing MGO-induced integrated stress responses. Targeting TMEM97 may diminish mechanical hypersensitivity in pain sufferers by decreasing the ISR.