Epigenomic landscape of the human dorsal root ganglion: sex differences and transcriptional regulation of nociceptive genes

Gene expression is influenced by chromatin architecture via controlled access of regulatory factors to DNA. To better understand regulation of gene expression in the human dorsal root ganglion (hDRG) we used bulk and spatial transposase-accessible chromatin technology followed by sequencing (ATAC-seq). We detected a total of 3005 differentially accessible chromatin regions (DARs) between sexes using bulk ATAC-seq. DARs in female hDRG mapped mainly to the X chromosome. In males, DARs were found in autosomal genes. We also found differential transcription factor binding motifs within DARs. EGR1/3 and SP1/4 were abundant in females, and JUN, FOS and other AP-1 family members in males. With the aim of dissecting the open chromatin profile in hDRG neurons, we used spatial ATAC-seq. Consistent with our bulk ATAC-seq data, most of the DARs in female hDRG were located in X chromosome genes. Neuron cluster showed higher chromatin accessibility in GABAergic, glutamatergic, and interferon-related genes in females, and in Ca2+-signaling-related genes in males. Sex differences in open chromatin transcription factor binding sites in neuron-proximal barcodes were consistent with the bulk data, having EGR1 transcription factor activity in females and AP-1 family members in males. Accordingly, we showed higher expression of EGR1 in female hDRG compared to male with in-situ hybridization. Our findings point to epigenomic sex differences in the hDRG that likely underlie divergent transcriptional responses that determine mechanistic sex differences in pain.

Vinorelbine causes a neuropathic pain-like state in mice via STING and MNK1 signaling associated with type I interferon induction

Type I interferons (IFNs) increase the excitability of dorsal root ganglia (DRGs) neurons via MNK-eIF4E signaling to promote pain sensitization in mice. Activation of stimulator of interferon response cGAMP interactor 1 (STING) signaling is pivotal for type I IFN induction. We hypothesized that vinorelbine, a chemotherapeutic and activator of STING, would cause a neuropathic pain-like state in mice via STING signaling in DRG neurons associated with IFN production. Vinorelbine caused tactile allodynia and grimacing in wild-type (WT) mice and increased p-IRF3, type I IFNs, and p-eIF4E in peripheral nerves. Supporting our hypothesis, vinorelbine failed to induce IRF3-IFNs-MNK-eIF4E in StingGt/Gt mice and, subsequently, failed to cause pain. The vinorelbine-elicited increase of p-eIF4E was not observed in Mknk1-/- (MNK1 knockout) mice in peripheral nerves consistent with the attenuated pro-nociceptive effect of vinorelbine in these mice. Our findings show that activation of STING signaling in the periphery causes a neuropathic pain-like state through type I IFN signaling to DRG nociceptors.

Vinorelbine causes a neuropathic pain-like state in mice via STING and MNK1 signaling associated with type I interferon induction

Type I interferons (IFNs) increase the excitability of dorsal root ganglion (DRG) neurons via activation of MNK-eIF4E translation signaling to promote pain sensitization in mice. Activation of STING signaling is a key component of type I IFN induction. Manipulation of STING signaling is an active area of investigation in cancer and other therapeutic areas. Vinorelbine is a chemotherapeutic that activates STING and has been shown to cause pain and neuropathy in oncology clinical trials in patients. There are conflicting reports on whether STING signaling promotes or inhibits pain in mice. We hypothesized that vinorelbine would cause a neuropathic pain-like state in mice via STING and signaling pathways in DRG neurons associated with type I IFN induction. Vinorelbine (10 mg/kg, i.v.) induced tactile allodynia and grimacing in WT male and female mice and increased p-IRF3 and type I IFN protein in peripheral nerves. In support of our hypothesis, vinorelbine-mediated pain was absent in male and female StingGt/Gt mice. Vinorelbine also failed to induce IRF3 and type I IFN signaling in these mice. Since type I IFNs engage translational control via MNK1-eIF4E in DRG nociceptors, we assessed vinorelbine-mediated p-eIF4E changes. Vinorelbine increased p-eIF4E in DRG in WT animals but not in StingGt/Gt or Mknk1-/- (MNK1 KO) mice. Consistent with these biochemical findings, vinorelbine had an attenuated pro-nociceptive effect in male and female MNK1 KO mice. Our findings support the conclusion that activation of STING signaling in the peripheral nervous system causes a neuropathic pain-like state that is mediated by type I IFN signaling to DRG nociceptors.

Spinal dopaminergic D1 and D5 receptors contribute to reserpine-induced fibromyalgia-like pain in rats

Fibromyalgia is a complex pain syndrome without a precise etiology. Reduced monoamines levels in serum and cerebrospinal fluid in fibromyalgia patients has been reported and could lead to a dysfunction of descending pain modulatory system producing the painful syndrome. This study evaluated the role of D₁-like dopamine receptors in the reserpine-induced fibromyalgia-like pain model in female Wistar rats. Reserpine-treated animals were intrathecally injected with different dopamine receptors agonists and antagonists, and small interfering RNAs (siRNAs) against D₁ and D₅ receptor subtypes. Withdrawal and muscle pressure thresholds were assessed with von Frey filaments and the Randall-Selitto test, respectively. Expression of D₁-like receptors in lumbar spinal cord and dorsal root ganglion was determined using real time polymerase chain reaction (qPCR). Reserpine induced tactile allodynia and muscle hyperalgesia. Intrathecal dopamine and D₁-like receptor agonist SKF-38393 induced nociceptive hypersensitivity in naïve rats, whilst this effect was prevented by the D₁-like receptor antagonist SCH-23390. Moreover, SCH-23390 induced a sex-dependent antiallodynic effect in reserpine-treated rats. Furthermore, transient silencing of D₁ and D₅ receptors significantly reduced reserpine-induced hypersensitivity in female rats. Reserpine slightly increased mRNA D₅ receptor expression in dorsal spinal cord, but not in DRG. This work provides new insights about the involvement of the spinal dopaminergic D₁/D₅ receptors in reserpine-induced hypersensitivity in rats.

Sex-dependent pronociceptive role of spinal α5 -GABAA receptor and its epigenetic regulation in neuropathic rodents

Extrasynaptic α₅ -subunit containing GABA_A (α₅ -GABA_A ) receptors participate in chronic pain. Previously, we reported a sex difference in the action of α₅ -GABA_A receptors in dysfunctional pain. However, the underlying mechanisms remain unknown. The aim of this study was to examine this sexual dimorphism in neuropathic rodents and the mechanisms involved. Female and male Wistar rats or ICR mice were subjected to nerve injury followed by α₅ -GABA_A receptor inverse agonist intrathecal administration, L-655,708. The drug produced an antiallodynic effect in nerve-injured female rats and mice, and a lower effect in males. We hypothesized that changes in α₅ -GABA_A receptor, probably influenced by hormonal and epigenetic status, might underlie this sex difference. Thus, we performed qPCR and western blot. Nerve injury increased α₅ -GABA_A mRNA and protein in female dorsal root ganglia (DRG) and decreased them in DRG and spinal cord of males. To investigate the hormonal influence over α₅ -GABA_A receptor actions, we performed nerve injury to ovariectomized rats and reconstituted them with 17β-estradiol (E2). Ovariectomy abrogated L-655,708 antiallodynic effect and E2 restored it. Ovariectomy decreased α₅ -GABA_A receptor and estrogen receptor α protein in DRG of neuropathic female rats, while E2 enhanced them. Since DNA methylation might contribute to α₅ -GABA_A receptor down-regulation in males, we examined CpG island DNA methylation of α₅ -GABA_A receptor coding gene through pyrosequencing. Nerve injury increased methylation in male, but not female rats. Pharmacological inhibition of DNA methyltransferases increased α₅ -GABA_A receptor and enabled L-655,708 antinociceptive effect in male rats. These results suggest that α₅ -GABA_A receptor is a suitable target to treat chronic pain in females.

Blockade of spinal α5-GABAA receptors differentially reduces reserpine-induced fibromyalgia-type pain in female rats

The role of spinal α5 subunit-containing GABAA (α5-GABAA) receptors in chronic pain is controversial. The purpose of this study was to investigate the participation of spinal α5-GABAA receptors in the reserpine-induced pain model. Reserpine administration induced tactile allodynia and muscle hyperalgesia in female and male rats. Intrathecal injection of L-655,708 and TB 21007 (7 days after the last reserpine injection) decreased tactile allodynia and, at a lesser extent, muscle hyperalgesia in female rats. The effects of these drugs produced a lower antiallodynic and antihyperalgesic effect in male than in female rats. Contrariwise, these drugs produced tactile allodynia and muscle hyperalgesia in naïve rats and these effects were lower in naïve male than female rats. Intrathecal L-838,417 prevented or reversed L-655,708-induced antiallodynia in reserpine-treated female rats. Repeated treatment with α5-GABAA receptor small interfering RNA (siRNA), but not scramble siRNA, reduced reserpine-induced allodynia in female rats. Accordingly, α5-GABAA receptor siRNA induced nociceptive hypersensitivity in naïve female rats. Reserpine enhanced α5-GABAA receptors expression in spinal cord and dorsal root ganglia (DRG), while it increased CD11b (OX-42) and glial fibrillary acidic protein (GFAP) fluorescence intensity in the lumbar spinal cord. In contrast, reserpine diminished K+-Cl- co-transporter 2 (KCC2) protein in the lumbar spinal cord. Data suggest that spinal α5-GABAA receptors play a sex-dependent proallodynic effect in reserpine-treated rats. In contrast, these receptors have a sex-dependent antiallodynic role in naïve rats.

ATF2, but not ATF3, participates in the maintenance of nerve injury-induced tactile allodynia and thermal hyperalgesia

Transcription factors are proteins that modulate the transcriptional rate of target genes in the nucleus in response to extracellular or cytoplasmic signals. Activating transcription factors 2 (ATF2) and 3 (ATF3) respond to environmental signals and maintain cellular homeostasis. There is evidence that inflammation and nerve injury modulate ATF2 and ATF3 expression. However, the function of these transcription factors in pain is unknown. The purpose of this study was to investigate the contribution of ATF2 and ATF3 to nerve injury-induced neuropathic pain. L5/6 spinal nerve ligation induced tactile allodynia and thermal hyperalgesia. Moreover, nerve damage enhanced ATF2 and ATF3 protein expression in injured L5/6 dorsal root ganglia and spinal cord but not in uninjured L4 dorsal root ganglia. Nerve damage also enhanced ATF2 immunoreactivity in dorsal root ganglia and spinal cord 7 to 21 days post-injury. Repeated intrathecal post-treatment with a small-interfering RNA targeted against ATF2 (ATF2 siRNA) or anti-ATF2 antibody partially reversed tactile allodynia and thermal hyperalgesia. In contrast, ATF3 siRNA or anti-ATF3 antibody did not modify nociceptive behaviors. ATF2 immunoreactivity was found in dorsal root ganglia and spinal cord co-labeling with NeuN mainly in non-peptidergic (IB4⁺) but also in peptidergic (CGRP⁺) neurons. ATF2 was found mainly in small- and medium-sized neurons. These results suggest that ATF2, but not ATF3, is found in strategic sites related to spinal nociceptive processing and participates in the maintenance of neuropathic pain in rats.