Trigeminal nerve damage-induced neuropathic pain is a severely debilitating chronic orofacial pain syndrome. Spinal chemokine CXCL13 and its receptor CXCR5 were recently demonstrated to play a pivotal role in the pathogenesis of spinal nerve ligation-induced neuropathic pain. Whether and how CXCL13/CXCR5 in the trigeminal ganglion (TG) mediates orofacial pain are unknown. The partial infraorbital nerve ligation (pIONL) was used to induce trigeminal neuropathic pain in mice. The expression of ATF3, CXCL13, CXCR5, and phosphorylated extracellular signal-regulated kinase (pERK) in the TG was detected by immunofluorescence staining and western blot. The effect of shRNA targeting on CXCL13 or CXCR5 on pain hypersensitivity was checked by behavioral testing. pIONL induced persistent mechanical allodynia and increased the expression of ATF3, CXCL13, and CXCR5 in the TG. Inhibition of CXCL13 or CXCR5 by shRNA lentivirus attenuated pIONL-induced mechanical allodynia. Additionally, pIONL-induced neuropathic pain and the activation of ERK in the TG were reduced in Cxcr5 −/− mice. Furthermore, MEK inhibitor (PD98059) attenuated mechanical allodynia and reduced TNF-α and IL-1β upregulation induced by pIONL. TNF-α inhibitor (Etanercept) and IL-1β inhibitor (Diacerein) attenuated pIONL-induced orofacial pain. Finally, intra-TG injection of CXCL13 induced mechanical allodynia, increased the activation of ERK and the production of TNF-α and IL-1β in the TG of WT mice, but not in Cxcr5 −/− mice. Pretreatment with PD98059, Etanercept, or Diacerein partially blocked CXCL13-induced mechanical allodynia, and PD98059 also reduced CXCL13-induced TNF-α and IL-1β upregulation. CXCL13 and CXCR5 contribute to orofacial pain via ERK-mediated proinflammatory cytokines production. Targeting CXCL13/CXCR5/ERK/TNF-α and IL-1β pathway in the trigeminal ganglion may offer effective treatment for orofacial neuropathic pain.

Background: Neuropathic pain that caused by lesion or dysfunction of the nervous system is associated with gene expression changes in the sensory pathway. Long noncoding RNAs (lncRNAs) have been reported to be able to regulate gene expression. Identifying lncRNA expression patterns in the spinal cord under normal and neuropathic pain conditions is essential for understanding the genetic mechanisms behind the pathogenesis of neuropathic pain. Results: Spinal nerve ligation (SNL) induced rapid and persistent pain hypersensitivity, characterized by mechanical allodynia and heat hyperalgesia. Meanwhile, astrocytes and microglia were dramatically activated in the ipsilateral spinal cord dorsal horn at 10 days after SNL. Further lncRNA microarray and mRNA microarray analysis showed that the expression profiles of lncRNA and mRNA between SNL and sham-operated mice were greatly changed at 10 days. The 511 differentially expressed (>2 fold) lncRNAs (366 up-regulated, 145 down-regulated) and 493 mRNAs (363 upregulated, 122 down-regulated) were finally identified. The expression patterns of several lncRNAs and mRNAs were further confirmed by qPCR. Functional analysis of differentially expressed (DE) mRNAs showed that the most significant enriched biological processes of up-regulated genes in SNL include immune response, defense response, and inflammation response, which are important pathogenic mechanisms underlying neuropathic pain. 35 DE lncRNAs have neighboring or overlapping DE mRNAs in genome, which is related to Toll-like receptor signaling, cytokine–cytokine receptor interaction, and peroxisome proliferator-activated receptor signaling pathway. Conclusion: Our findings uncovered the expression pattern of lncRNAs and mRNAs in the mice spinal cord under neuropathic pain condition. These lncRNAs and mRNAs may represent new therapeutic targets for the treatment of neuropathic pain.

G-protein-coupled receptors are considered to be cell-surface sensors of extracellular signals, thereby having a crucial role in signal transduction and being the most fruitful targets for drug discovery. G-protein-coupled receptor 151 (GPR151) was reported to be expressed specifically in the habenular area. Here we report the expression and the epigenetic regulation of GRP151 in the spinal cord after spinal nerve ligation (SNL) and the contribution of GPR151 to neuropathic pain in male mice. SNL dramatically increased GPR151 expression in spinal neurons.GPR151mutation or spinal inhibition by shRNA alleviated SNL-induced mechanical allodynia and heat hyperalgesia. Interestingly, the CpG island in theGPR151gene promoter region was demethylated, the expression of DNA methyltransferase 3b (DNMT3b) was decreased, and the binding of DNMT3b withGPR151promoter was reduced after SNL. Overexpression of DNMT3b in the spinal cord decreasedGPR151expression and attenuated SNL-induced neuropathic pain. Furthermore, Krüppel-like factor 5 (KLF5), a transcriptional factor of the KLF family, was upregulated in spinal neurons, and the binding affinity of KLF5 withGPR151promoter was increased after SNL. Inhibition of KLF5 reducedGPR151expression and attenuated SNL-induced pain hypersensitivity. Further mRNA microarray analysis revealed that mutation ofGPR151reduced the expression of a variety of pain-related genes in response to SNL, especially mitogen-activated protein kinase (MAPK) signaling pathway-associated genes. This study reveals that GPR151, increased by DNA demethylation and the enhanced interaction with KLF5, contributes to the maintenance of neuropathic pain via increasing MAPK pathway-related gene expression.SIGNIFICANCE STATEMENTG-protein-coupled receptors (GPCRs) are targets of various clinically approved drugs. Here we report that SNL increased GPR151 expression in the spinal cord, and mutation or inhibition of GPR151 alleviated SNL-induced neuropathic pain. In addition, SNL downregulated the expression of DNMT3b, which caused demethylation ofGPR151gene promoter, facilitated the binding of transcriptional factor KLF5 with theGPR151promoter, and further increased GPR151 expression in spinal neurons. The increased GPR151 may contribute to the pathogenesis of neuropathic pain via activating MAPK signaling and increasing pain-related gene expression. Our study reveals an epigenetic mechanism underlying GPR151 expression and suggests that targeting GPR151 may offer a new strategy for the treatment of neuropathic pain.

Nerve injury–induced changes of gene expression in dorsal root ganglion (DRG) are critical for neuropathic pain genesis. However, how these changes occur remains elusive. Here we report the down-regulation of zinc finger protein 382 (ZNF382) in injured DRG neurons after nerve injury. Rescuing this down-regulation attenuates nociceptive hypersensitivity. Conversely, mimicking this down-regulation produces neuropathic pain symptoms, which are alleviated by C-X-C motif chemokine 13 (CXCL13) knockdown or its receptor CXCR5 knockout. Mechanistically, an identified cis-acting silencer at distal upstream of the Cxcl13 promoter suppresses Cxcl13 transcription via binding to ZNF382. Blocking this binding or genetically deleting this silencer abolishes the ZNF382 suppression on Cxcl13 transcription and impairs ZNF382-induced antinociception. Moreover, ZNF382 down-regulation disrupts the repressive epigenetic complex containing histone deacetylase 1 and SET domain bifurcated 1 at the silencer-promoter loop, resulting in Cxcl13 transcriptional activation. Thus, ZNF382 down-regulation is required for neuropathic pain likely through silencer-based epigenetic disinhibition of CXCL13, a key neuropathic pain player, in DRG neurons.

Peripheral nerve injury‐induced spinal microglial proliferation plays a pivotal role in neuropathic pain. So far, key intracellular druggable molecules involved in this process are not identified. The nuclear factor of activated T‐cells (NFAT1) is a master regulator of immune cell proliferation. Whether and how NFAT1 modulates spinal microglial proliferation during neuropathic pain remain unknown. Here it is reported that NFAT1 is persistently upregulated in microglia after spinal nerve ligation (SNL), which is regulated by TET2‐mediated DNA demethylation. Global or microglia‐specific deletion of Nfat1 attenuates SNL‐induced pain and decreases excitatory synaptic transmission of lamina II neurons. Furthermore, deletion of Nfat1 decreases microglial proliferation and the expression of multiple microglia‐related genes, such as cytokines, transmembrane signaling receptors, and transcription factors. Particularly, SNL increases the binding of NFAT1 with the promoter of Itgam, Tnf, Il‐1b, and c‐Myc in the spinal cord. Microglia‐specific overexpression of c‐MYC induces pain hypersensitivity and microglial proliferation. Finally, inhibiting NFAT1 and c‐MYC by intrathecal injection of inhibitor or siRNA alleviates SNL‐induced neuropathic pain. Collectively, NFAT1 is a hub transcription factor that regulates microglial proliferation via c‐MYC and guides the expression of the activated microglia genome. Thus, NFAT1 may be an effective target for treating neuropathic pain.