Eukaryotic initiation factor 4 gamma 2 contributes to neuropathic pain through down-regulation of Kv1.2 and the mu opioid receptor in mouse primary sensory neurones

Repressing iron overload ameliorates central post-stroke pain via the Hdac2-Kv1.2 axis in a rat model of hemorrhagic stroke

JOURNAL/nrgr/04.03/01300535-202412000-00027/figure1/v/2024-04-08T165401Z/r/image-tiff Thalamic hemorrhage can lead to the development of central post-stroke pain. Changes in histone acetylation levels, which are regulated by histone deacetylases, affect the excitability of neurons surrounding the hemorrhagic area. However, the regulatory mechanism of histone deacetylases in central post-stroke pain remains unclear. Here, we show that iron overload leads to an increase in histone deacetylase 2 expression in damaged ventral posterolateral nucleus neurons. Inhibiting this increase restored histone H3 acetylation in the Kcna2 promoter region of the voltage-dependent potassium (Kv) channel subunit gene in a rat model of central post-stroke pain, thereby increasing Kcna2 expression and relieving central pain. However, in the absence of nerve injury, increasing histone deacetylase 2 expression decreased Kcna2 expression, decreased Kv current, increased the excitability of neurons in the ventral posterolateral nucleus area, and led to neuropathic pain symptoms. Moreover, treatment with the iron chelator deferiprone effectively reduced iron overload in the ventral posterolateral nucleus after intracerebral hemorrhage, reversed histone deacetylase 2 upregulation and Kv1.2 downregulation, and alleviated mechanical hypersensitivity in central post-stroke pain rats. These results suggest that histone deacetylase 2 upregulation and Kv1.2 downregulation, mediated by iron overload, are important factors in central post-stroke pain pathogenesis and could serve as new targets for central post-stroke pain treatment.

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

RALY participates in nerve trauma-induced nociceptive hypersensitivity through triggering Eif4g2 gene expression in primary sensory neurons

BACKGROUND AND PURPOSE

Peripheral nerve trauma-induced dysregulation of pain-associated genes in the primary sensory neurons of dorsal root ganglion (DRG) contributes to neuropathic pain genesis. RNA-binding proteins participate in gene transcription. We hypothesized that RALY, an RNA-binding protein, participated in nerve trauma-induced dysregulation of DRG pain-associated genes and nociceptive hypersensitivity.

METHODS AND RESULTS

Immunohistochemistry staining showed that RALY was expressed exclusively in the nuclei of DRG neurons. Peripheral nerve trauma caused by chronic constriction injury (CCI) of unilateral sciatic nerve produced time-dependent increases in the levels of Raly mRNA and RALY protein in injured DRG. Blocking this increase through DRG microinjection of adeno-associated virus 5 (AAV5)-expressing Raly shRNA reduced the CCI-induced elevation in the amount of eukaryotic initiation factor 4 gamma 2 (Eif4g2) mRNA and Eif4g2 protein in injured DRG and mitigated the development and maintenance of CCI-induced nociceptive hypersensitivity, without altering basal (acute) response to noxious stimuli and locomotor activity. Mimicking DRG increased RALY through DRG microinjection of AAV5 expressing Raly mRNA up-regulated the expression of Eif4g2 mRNA and Eif4g2 protein in the DRG and led to hypersensitive responses to noxious stimuli in the absence of nerve trauma. Mechanistically, CCI promoted the binding of RALY to the promoter of Eif4g2 gene and triggered its transcriptional activity.

CONCLUSION AND IMPLICATIONS

Our findings indicate that RALY participates in nerve trauma-induced nociceptive hypersensitivity likely through transcriptionally triggering Eif4g2 expression in the DRG. RALY may be a potential target in neuropathic pain management.

Transcription factor EBF1 mitigates neuropathic pain by rescuing Kv1.2 expression in primary sensory neurons

Nerve injury-induced alternations of gene expression in primary sensory neurons of the dorsal root ganglion (DRG) are molecular basis of neuropathic pain genesis. Transcription factors regulate gene expression. In this study, we examined whether early B cell factor 1 (EBF1), a transcription factor, in the DRG, participated in neuropathic pain caused by chronic constriction injury (CCI) of the sciatic nerve. EBF1 was distributed exclusively in the neuronal nucleus and coexpressed with cytoplasmic/membrane Kv1.2 in individual DRG neurons. The expression of Ebf1 mRNA and protein was time-dependently downregulated in the ipsilateral lumbar (L) 3/4 DRGs after unilateral CCI. Rescuing this downregulation through microinjection of the adeno-associated virus 5 expressing full-length Ebf1 mRNA into the ipsilateral L3/4 DRGs reversed the CCI-induced decrease of DRG Kv1.2 expression and alleviated the development and maintenance of mechanical, heat and cold hypersensitivities. Conversely, mimicking the downregulation of DRG EBF1 through microinjection of AAV5-expressing Ebf1 shRNA into unilateral L3/4 DRGs produced a reduction of Kv1.2 expression in the ipsilateral L3/4 DRGs, spontaneous pain, and the enhanced responses to mechanical, heat and cold stimuli in naive mice. Mechanistically, EBF1 not only bound to the Kcna2 gene (encoding Kv1.2) promoter but also directly activated its activity. CCI decreased the EBF1 binding to the Kcna2 promoter in the ipsilateral L3/4 DRGs. Our findings suggest that DRG EBF1 downregulation contributes to neuropathic pain likely by losing its binding to Kcna2 promoter and subsequently silencing Kv1.2 expression in primary sensory neurons. Exogenous EBF1 administration may mitigate neuropathic pain by rescuing DRG Kv1.2 expression.

Systemic administration of NIS-lncRNA antisense oligonucleotide alleviates neuropathic pain

OBJECTIVE

The antisense oligonucleotide (ASO) is an FDA-approved strategy in the treatment of neurological diseases. We have shown the viability of using intrathecal ASO to suppress nerve injury-specific long noncoding RNA (NIS-lncRNA) in dorsal root ganglion (DRG), resulting in a stable and long-lasting antinociceptive effect on NP. This study examined whether systemic administration of NIS-lncRNA ASO relieved the chronic constriction injury (CCI)-induced nociceptive hypersensitivity.

METHODS

A single subcutaneous injection of NIS-lncRNA ASO at a dose of 1,000 µg was carried out 7 days after CCI or sham surgery in male mice. Behavioral tests were performed one day before surgery and at different days after surgery. DRG and spinal cord were finally collected for quantitative real-time RT-PCR and Western blot assays.

RESULTS

NIS-lncRNA ASO significantly alleviated CCI-induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia starting on day 14 or 21 post-ASO injection and lasting for at least 7 days on the ipsilateral side. Additionally, CCI-induced spontaneous pain and ipsilateral dorsal horn neuronal and astrocyte hyperactivation were blocked on day 28 after NIS-lncRNA ASO injection. As predicted, the CCI-induced increases in the levels of NIS-lncRNA and its downstream target C-C motif chemokine ligand 2 in the ipsilateral lumbar 3 and 4 DRGs were attenuated on day 28 following NIS-lncRNA ASO injection.

CONCLUSION

Our findings indicate that systemic administration of NIS-lncRNA ASO also produces a stable and long-lasting antinociceptive effect on neuropathic pain. NIS-lncRNA ASO may have potential clinical application in the treatment of this disorder.

Sensory neuron-specific long noncoding RNA in small non-peptidergic dorsal root ganglion neurons selectively impairs nerve injury-induced mechanical hypersensitivity

AIMS

Nerve injury-induced mechanical hypersensitivity is one of major clinical symptoms in neuropathic pain patients. Understanding molecular mechanisms underlying this symptom is crucial for developing effective therapies. The present study was to investigate whether sensory neuron-specific long noncoding RNA (SS-lncRNA) predominantly expressed in small non-peptidergic dorsal root ganglion (DRG) neurons repaired nerve injury-induced mechanical hypersensitivity.

MATERIALS AND METHODS

SS-lncRNA downregulation in the mas-related G protein-coupled receptor member D (Mrgprd)-expressed DRG neurons was rescued and mimicked by crossbreeding MrgprdCreERT2/+ lines with Rosa26SS-lncRNA knock-in mice and SS-lncRNAfl/fl mice, respectively, followed by tamoxifen injection.

KEY FINDINGS

Rescuing SS-lncRNA downregulation in the Mrgprd-expressed DRG neurons significantly reversed the spinal nerve ligation (SNL)-induced reduction of the calcium-activated potassium channel subfamily N member 1 (KCNN1) in these DRG neurons and alleviated the SNL-induced mechanical hypersensitivity, without affecting the SNL-induced heat and cold nociceptive hypersensitivities, on the ipsilateral side. Conversely, mimicking SS-lncRNA downregulation in the Mrgprd-expressed DRG neurons reduced basal KCNN1 expression in these DRG neurons and produced the enhanced response to mechanical stimulation, but not thermal and cold stimuli, on bilateral sides. Mechanistically, SS-lncRNA downregulation caused a reduction in its binding to lysine-specific demethylase 6B (KDM6B) and consequent recruitment of less KDM6B to Kcnn1 promoter and an increase of H3K27me3 enrichment in this promoter in injured DRG.

SIGNIFICANCE

Our findings suggest that SS-lncRNA downregulation in small non-peptidergic sensory neurons is required specifically for nerve injury-induced mechanical hypersensitivity likely through silencing KCNN1 expression caused by KDM6B-gated increase of H3K27me3 enrichment in Kcnn1 promoter in these neurons.

An Overview of the Mechanisms Involved in Neuralgia

Neuralgia is a frequently occurring condition that causes chronic pain and burdens both patients and their families. Earlier research indicated that anti-inflammatory treatment, which was primarily utilized to address conditions like neuralgia, resulted in positive outcomes. However, recent years have witnessed the emergence of various novel mechanisms associated with pain-related disorders. This review provides a concise overview of the inflammatory mechanisms involved in neuralgia. It also examines recent advancements in research, exploring the influence of ion channels and synaptic proteins on neuralgia and its complications. Additionally, the interactions between these mechanisms are discussed with the aim of suggesting innovative therapeutic approaches and research directions for the management of neuralgia.

N6-Methyladenosine methylase METTL3 contributes to neuropathic pain by epigenetic silencing of mu opioid receptor

We aimed at exploring the role and mechanism of METTL3-mediated m6A modification in neuropathic pain. Male Sprague-Dawley rats were randomly divided into four groups: Sham operation group (Sham group), chronic constriction injury (CCI) of the sciatic nerve model group (NPP group), intrathecal injection of virus down-regulated METTL3 + CCI model group (M3 + NPP group) and intrathecal injection of negative control virus + CCI model group (Scr + NPP group). The M3 + NPP group and the Scr + NPP group were intrathecally injected with virus nineteen days before operation. The paw withdrawal mechanical thresholds and paw withdrawal latency were respectively recorded one day before operation, three days, five days and seven days after operation. The rats were sacrificed on the seventh day after operation, and their spinal cord tissues were taken. The frozen sections of rats were performed to observe the expression of green fluorescent protein of the virus. The methylation level of RNA, the protein expression of m6A-related enzyme (METTL3) and mu opioid receptor (MOR) in spinal cord tissues of the four groups were measured. Downregulation of METTL3 had no effect on the overall methylation level of the spinal cord, but it could regulate the methylation level of the OPRM1 gene RNA encoding MOR, partially restore the expression of MOR, and relieve pain in rats. In the process of NPP, METTL3 may inhibit the expression of MOR by regulating the methylation level of OPRM1 gene RNA encoding MOR, and ultimately promote the occurrence and development of NPP.

Phosphorylated Upstream Frameshift 1-dependent Nonsense-mediated μ-Opioid Receptor mRNA Decay in the Spinal Cord Contributes to the Development of Neuropathic Allodynia-like Behavior in Rats

BACKGROUND

Nonsense-mediated messenger RNA (mRNA) decay increases targeted mRNA degradation and has been implicated in the regulation of gene expression in neurons. The authors hypothesized that nonsense-mediated μ-opioid receptor mRNA decay in the spinal cord is involved in the development of neuropathic allodynia-like behavior in rats.

METHODS

Adult Sprague-Dawley rats of both sexes received spinal nerve ligation to induce neuropathic allodynia-like behavior. The mRNA and protein expression contents in the dorsal horn of animals were measured by biochemical analyses. Nociceptive behaviors were evaluated by the von Frey test and the burrow test.

RESULTS

On Day 7, spinal nerve ligation significantly increased phosphorylated upstream frameshift 1 (UPF1) expression in the dorsal horn (mean ± SD; 0.34 ± 0.19 in the sham ipsilateral group vs. 0.88 ± 0.15 in the nerve ligation ipsilateral group; P < 0.001; data in arbitrary units) and drove allodynia-like behaviors in rats (10.58 ± 1.72 g in the sham ipsilateral group vs. 1.19 ± 0.31 g in the nerve ligation ipsilateral group, P < 0.001). No sex-based differences were found in either Western blotting or behavior tests in rats. Eukaryotic translation initiation factor 4A3 (eIF4A3) triggered SMG1 kinase (0.06 ± 0.02 in the sham group vs. 0.20 ± 0.08 in the nerve ligation group, P = 0.005, data in arbitrary units)-mediated UPF1 phosphorylation, leading to increased nonsense-mediated mRNA decay factor SMG7 binding and µ-opioid receptor mRNA degradation (0.87 ± 0.11-fold in the sham group vs. 0.50 ± 0.11-fold in the nerve ligation group, P = 0.002) in the dorsal horn of the spinal cord after spinal nerve ligation. Pharmacologic or genetic inhibition of this signaling pathway in vivo ameliorated allodynia-like behaviors after spinal nerve ligation.

CONCLUSIONS

This study suggests that phosphorylated UPF1-dependent nonsense-mediated μ-opioid receptor mRNA decay is involved in the pathogenesis of neuropathic pain.

EDITOR’S PERSPECTIVE

The Downregulation of Opioid Receptors and Neuropathic Pain

Neuropathic pain (NP) refers to pain caused by primary or secondary damage or dysfunction of the peripheral or central nervous system, which seriously affects the physical and mental health of 7-10% of the general population. The etiology and pathogenesis of NP are complex; as such, NP has been a hot topic in clinical medicine and basic research for a long time, with researchers aiming to find a cure by studying it. Opioids are the most commonly used painkillers in clinical practice but are regarded as third-line drugs for NP in various guidelines due to the low efficacy caused by the imbalance of opioid receptor internalization and their possible side effects. Therefore, this literature review aims to evaluate the role of the downregulation of opioid receptors in the development of NP from the perspective of dorsal root ganglion, spinal cord, and supraspinal regions. We also discuss the reasons for the poor efficacy of opioids, given the commonness of opioid tolerance caused by NP and/or repeated opioid treatments, an angle that has received little attention to date; in-depth understanding might provide a new method for the treatment of NP.

microRNAs profiling of small extracellular vesicles from midbrain tissue of Parkinson's disease

Small extracellular vesicles (sEVs) are generated by all types of cells during physiological or pathological conditions. There is growing interest in tissue-derived small extracellular vesicles (tdsEVs) because they can be isolated from a single tissue source. Knowing the representation profile of microRNA (miRNA) in midbrain tissue-derived sEVs (bdsEVs) and their roles is imperative for understanding the pathological mechanism and improving the diagnosis and treatment of Parkinson's disease (PD). bdsEVs from a rat model of PD and a sham group were separated and purified using ultracentrifugation, size-exclusion chromatography (SEC), and ultrafiltration. Then, miRNA profiling of bdsEVs in both groups was performed using next-generation sequencing (NGS). The expression levels of 180 miRNAs exhibited significant differences between the two groups, including 114 upregulated and 66 downregulated genes in bdsEVs of PD rats compared with the sham group (p < 0.05). Targets of the differentially expressed miRNAs were predicted by miRanda and RNAhybrid, and their involvement in the signaling pathways and cellular function has been analyzed through the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Gene Ontology (GO). Furthermore, we explored the expression levels of miR-103-3p, miR-107-3p, miR-219a-2-3p, and miR-379-5p in bdsEVs, sEVs derived from plasma, and plasma of both groups of rats. Interestingly, the expression levels of miR-103-3p, miR-107-3p, miR-219a-2-3p, and miR-379-5p were elevated in bdsEVs and sEVs from plasma; in contrast, their expression levels were decreased in plasma of the rat model of PD. In summary, miRNAs may play a significant role in the onset and development of PD, and miRNAs need to be selected carefully as a research subject for exploring the pathological mechanism and the potential therapeutic targets and diagnostic markers of PD.

Effect of intrathecal NIS-lncRNA antisense oligonucleotides on neuropathic pain caused by nerve trauma, chemotherapy, or diabetes mellitus

BACKGROUND

Blocking increased expression of nerve injury-specific long non-coding RNA (NIS-lncRNA) in injured dorsal root ganglia (DRG) through DRG microinjection of NIS-lncRNA small hairpin interfering RNA or generation of NIS-lncRNA knockdown mice mitigates neuropathic pain. However, these strategies are impractical in the clinic. This study employed a Food and Drug Administration (FDA)-approved antisense oligonucleotides strategy to examine the effect of NIS-lncRNA ASOs on neuropathic pain.

METHODS

Effects of intrathecal injection of NIS-lncRNA antisense oligonucleotides on day 7 or 14 after chronic constriction injury (CCI) of the sciatic nerve, fourth lumbar (L4) spinal nerve ligation, or intraperitoneal injection of paclitaxel or streptozotocin on the expression of DRG NIS-lncRNA and C-C chemokine ligand 2 (CCL2, an NIS-lncRNA downstream target) and nociceptive hypersensitivity were examined. We also assessed whether NIS-lncRNA antisense oligonucleotides produced cellular toxicity.

RESULTS

Intrathecal NIS-lncRNA antisense oligonucleotides attenuated CCI-induced mechanical allodynia, heat hyperalgesia, cold hyperalgesia, and ongoing nociceptive responses, without changing basal or acute nociceptive responses and locomotor function. Intrathecal NIS-lncRNA antisense oligonucleotides also blocked CCI-induced increases in NIS-lncRNA and CCL2 in the ipsilateral L3 and L4 DRG and hyperactivities of neurones and astrocytes in the ipsilateral L3 and L4 spinal cord dorsal horn. Similar results were found in antisense oligonucleotides-treated mice after spinal nerve ligation or intraperitoneal injection of paclitaxel or streptozotocin. Normal morphologic structure and no cell loss were observed in the DRG and spinal cord of antisense oligonucleotides-treated mice.

CONCLUSION

These findings further validate the role of NIS-lncRNA in trauma-, chemotherapy-, or diabetes-induced neuropathic pain and demonstrate potential clinical application of NIS-lncRNA antisense oligonucleotides for neuropathic pain management.

Transcription factor ETS proto-oncogene 1 contributes to neuropathic pain by regulating histone deacetylase 1 in primary afferent neurons

Nerve injury can induce aberrant changes in ion channels, enzymes, and cytokines/chemokines in the dorsal root ganglia (DRGs); these changes are due to or at least partly governed by transcription factors that contribute to the genesis of neuropathic pain. However, the involvement of transcription factors in neuropathic pain is poorly understood. In this study, we report that transcription factor (TF) ETS proto-oncogene 1 (ETS1) is required for the initiation and development of neuropathic pain. Sciatic nerve chronic constrictive injury (CCI, a clinical neuropathic pain model) increases ETS1 expression in the injured male mouse DRG. Blocking this upregulation alleviated CCI-induced mechanical allodynia and thermal hyperalgesia, with no apparent effect on locomotor function. Mimicking this upregulation results in the genesis of nociception hypersensitivity; mechanistically, nerve injury-induced ETS1 upregulation promotes the expression of histone deacetylase 1 (HDAC1, a key initiator of pain) via enhancing its binding activity to the HDAC1 promotor, leading to the elevation of spinal central sensitization, as evidenced by increased expression of p-ERK1/2 and GFAP in the dorsal spinal horn. It appears that the ETS1/HDAC1 axis in DRG may have a critical role in the development and maintenance of neuropathic pain, and ETS1 is a potential therapeutic target in neuropathic pain.

Upregulation of LncRNA71132 in the spinal cord regulates hypersensitivity in a rat model of bone cancer pain

ABSTRACT

Bone cancer pain (BCP) is a pervasive clinical symptom which impairs the quality life. Long noncoding RNAs (lncRNAs) are enriched in the central nervous system and play indispensable roles in numerous biological processes, while its regulatory function in nociceptive information processing remains elusive. Here, we reported that functional modulatory role of ENSRNOT00000071132 (lncRNA71132) in the BCP process and sponging with miR-143 and its downstream GPR85-dependent signaling cascade. Spinal lncRNA71132 was remarkably increased in the rat model of bone cancer pain. The knockdown of spinal lncRNA71132 reverted BCP behaviors and spinal c-Fos neuronal sensitization. Overexpression of spinal lncRNA71132 in naive rat generated pain behaviors, which were accompanied by increased spinal c-Fos neuronal sensitization. Furthermore, it was found that lncRNA71132 participates in the modulation of BCP by inversely regulating the processing of miR-143-5p. In addition, an increase in expression of spinal lncRNA71132 resulted in the decrease in expression of miR-143 under the BCP state. Finally, it was found that miR-143-5p regulates pain behaviors by targeting GPR85. Overexpression of miR-143-5p in the spinal cord reverted the nociceptive behaviors triggered by BCP, accompanied by a decrease in expression of spinal GPR85 protein, but no influence on expression of gpr85 mRNA. The findings of this study indicate that lncRNA71132 works as a miRNA sponge in miR-143-5p-mediated posttranscriptional modulation of GPR85 expression in BCP. Therefore, epigenetic interventions against lncRNA71132 may potentially work as novel treatment avenues in treating nociceptive hypersensitivity triggered by bone cancer.

E74-like factor 1 contributes to nerve trauma-induced nociceptive hypersensitivity through transcriptionally activating matrix metalloprotein-9 in dorsal root ganglion neurons

ABSTRACT

Nerve trauma-induced alternations of gene expression in the neurons of dorsal root ganglion (DRG) participate in nerve trauma-caused nociceptive hypersensitivity. Transcription factors regulate gene expression. Whether the transcription factor E74-like factor 1 (ELF1) in the DRG contributes to neuropathic pain is unknown. We report here that peripheral nerve trauma caused by chronic constriction injury (CCI) of unilateral sciatic nerve or unilateral fourth lumbar spinal nerve ligation led to the time-dependent increases in the levels of Elf1 mRNA and ELF1 protein in injured DRG, but not in the spinal cord. Preventing this increase through DRG microinjection of adeno-associated virus 5 expressing Elf1 shRNA attenuated the CCI-induced upregulation of matrix metallopeptidase 9 (MMP9) in injured DRG and induction and maintenance of nociceptive hypersensitivities, without changing locomotor functions and basal responses to acute mechanical, heat, and cold stimuli. Mimicking this increase through DRG microinjection of AAV5 expressing full-length Elf1 upregulated DRG MMP9 and produced enhanced responses to mechanical, heat, and cold stimuli in naive mice. Mechanistically, more ELF1 directly bond to and activated Mmp9 promoter in injured DRG neurons after CCI. Our data indicate that ELF1 participates in nerve trauma-caused nociceptive hypersensitivity likely through upregulating MMP9 in injured DRG. E74-like factor 1 may be a new target for management of neuropathic pain.

Targeting the somatosensory system with AAV9 and AAV2retro viral vectors

Adeno-associated viral (AAV) vectors allow for site-specific and time-dependent genetic manipulation of neurons. However, for successful implementation of AAV vectors, major consideration must be given to the selection of viral serotype and route of delivery for efficient gene transfer into the cell type being investigated. Here we compare the transduction pattern of neurons in the somatosensory system following injection of AAV9 or AAV2retro in the parabrachial complex of the midbrain, the spinal cord dorsal horn, the intrathecal space, and the colon. Transduction was evaluated based on Cre-dependent expression of tdTomato in transgenic reporter mice, following delivery of AAV9 or AAV2retro carrying identical constructs that drive the expression of Cre/GFP. The pattern of distribution of tdTomato expression indicated notable differences in the access of the two AAV serotypes to primary afferent neurons via peripheral delivery in the colon and to spinal projections neurons via intracranial delivery within the parabrachial complex. Additionally, our results highlight the superior sensitivity of detection of neuronal transduction based on reporter expression relative to expression of viral products.

C/EBPβ Participates in Nerve Trauma-Induced TLR7 Upregulation in Primary Sensory Neurons

Nerve trauma-induced toll-like receptor 7 (TLR7) expression level increases in primary sensory neurons in injured dorsal root ganglion (DRG) avails to neuropathic pain, but the reason is still unknown. In the current study, we showed that unilateral lumbar 4 (L4) spinal nerve ligation (SNL) upregulated CCAAT/enhancer-binding protein-β (C/EBPβ) expression in ipsilateral L4 DRG. Preventing this elevation attenuated the SNL-induced upregulation of TLR7 in the ipsilateral L4 DRG and inhibited cold/thermal hyperalgesia and mechanical allodynia. In injected DRG, mimicking nerve trauma-induced C/EBPβ upregulation increased TLR7 levels, augmented responses to cold/thermal/mechanical stimuli, and caused ipsilateral spontaneous pain with no SNL. Mechanistically, SNL upregulated binding of increased C/EBPβ to Tlr7 promoter in ipsilateral L4 DRG. Accorded that C/EBPβ could trigger the activation of Tlr7 promoter and co-expressed with Tlr7 mRNA in individual DRG neurons, our findings strongly suggest the role of C/EBPβ in nerve trauma-mediated TLR7 upregulation in injured primary sensory neurons.

Intrathecal administration of the fat-mass and obesity-associated protein inhibitor mitigates neuropathic pain in female rats

Several intracellular signals are involved in the sexual dimorphism of chronic pain. Our previous studies demonstrated that the fat-mass and obesity-associated protein (FTO), a demethylase of RNA N6-methyladenosine, in the injured dorsal root ganglion (DRG) contributed to the development and maintenance of nerve injury-induced nociceptive hypersensitivity in male rats and male mice. However, whether these effects of DRG FTO are in a sex-dependent manner is still unknown. The present study sought to investigate the effect of intrathecal administration of a specific FTO inhibitor, meclofenamic acid (MA), on chronic constriction injury (CCI)-induced nociceptive hypersensitivity in female rats. Intrathecal injection of MA attenuated the CCI-induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia in both the induction and maintenance periods, without changing acute/basal pain and locomotor function, in female rats. Intrathecal MA also blocked the CCI-induced hyperactivations of neurons and astrocytes in the ipsilateral L4 and L5 dorsal horns of female rats. Mechanistically, intrathecal MA prevented the CCI-induced increase in the histone methyltransferase G9a expression and reversed the G9a-controlled downregulation of mu-opioid receptor and Kv1.2 proteins in the ipsilateral L4 and L5 DRGs of female rats. These findings indicate that the effects of the FTO inhibitor on nerve injury-induced nociceptive hypersensitivity in female rats are similar to those in male rats reported previously. Our data also further confirm the role of DRG FTO in neuropathic pain and suggest potential clinical application of the FTO inhibitors for the prevention and treatment of this disorder in both men and women.

Emerging roles of circular RNAs in neuropathic pain

Neuropathic pain is a major type of chronic pain caused by the disease or injury of the somatosensory nervous system. It afflicts about 10% of the general population with a significant proportion of patients’ refractory to conventional medical treatment. This highlights the importance of a better understanding of the molecular pathogenesis of neuropathic pain so as to drive the development of novel mechanism‐driven therapy. Circular RNAs (circRNAs) are a type of non‐coding, regulatory RNAs that exhibit tissue‐ and disease‐specific expression. An increasing number of studies reported that circRNAs may play pivotal roles in the development of neuropathic pain. In this review, we first summarize circRNA expression profiling studies on neuropathic pain. We also highlight the molecular mechanisms of specific circRNAs (circHIPK3, circAnks1a, ciRS‐7, cZRANB1, circZNF609 and circ_0005075) that play key functional roles in the pathogenesis of neuropathic pain and discuss their potential diagnostic, prognostic, and therapeutic utilization in the clinical management of neuropathic pain.

Effect of Pharmacological Inhibition of Fat-Mass and Obesity-Associated Protein on Nerve Trauma-Induced Pain Hypersensitivities

Genetic knockout or knockdown of fat-mass and obesity-associated protein (FTO), a demethylase that participates in RNA N6-methyladenosine modification in injured dorsal root ganglion (DRG), has been demonstrated to alleviate nerve trauma-induced nociceptive hypersensitivities. However, these genetic strategies are still impractical in clinical neuropathic pain management. The present study sought to examine the effect of intrathecal administration of two specific FTO inhibitors, meclofenamic acid (MA) and N-CDPCB, on the development and maintenance of nociceptive hypersensitivities caused by unilateral L5 spinal nerve ligation (SNL) in rats. Intrathecal injection of either MA or N-CDPCB diminished dose-dependently the SNL-induced mechanical allodynia, heat hyperalgesia, cold hyperalgesia, and spontaneous ongoing nociceptive responses in both development and maintenance periods, without altering acute/basal pain and locomotor function. Intrathecal MA also reduced the SNL-induced neuronal and astrocyte hyperactivities in the ipsilateral L5 dorsal horn. Mechanistically, intrathecal injection of these two inhibitors blocked the SNL-induced increase in the histone methyltransferase G9a expression and rescued the G9a-controlled downregulation of mu opioid receptor and Kv1.2 proteins in the ipsilateral L5 DRG. These findings further indicate the role of DRG FTO in neuropathic pain and suggest potential clinical application of the FTO inhibitors for management of this disorder.

Nerve trauma-caused downregulation of opioid receptors in primary afferent neurons: Molecular mechanisms and potential managements

Neuropathic pain is the most common clinical disorder destroying the quality of patient life and leading to a marked economic and social burden. Opioids are still last option for pharmacological treatment of this disorder, but their antinociceptive effects are limited in part due to the downregulation of opioid receptors in the primary afferent neurons after peripheral nerve trauma. How this downregulation occurs is not completely understood, but recent studies have demonstrated that peripheral nerve trauma drives the alterations in epigenetic modifications (including DNA methylation, histone methylation and mciroRNAs), expression of transcription factors, post-transcriptional modifications (e.g., RNA methylation) and protein translation initiation in the neurons of nerve trauma-related dorsal root ganglion (DRG) and that these alternations may be associated with nerve trauma-caused downregulation of DRG opioid receptors. This review presents how opioid receptors are downregulated in the DRG after peripheral nerve trauma, specifically focusing on distinct molecular mechanisms underlying transcriptional and translational processes. This review also discusses how this downregulation contributes to the induction and maintenance of neuropathic pain. A deeper understanding of these molecular mechanisms likely provides a novel avenue for prevention and/or treatment of neuropathic pain.