DNA methyltransferase DNMT3a contributes to neuropathic pain by repressing Kcna2 in primary afferent neurons

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Micronutrient supplementation affects DNA methylation in male gonads with potential intergenerational epigenetic inheritance involving the embryonic development through glutamate receptor-associated genes

BACKGROUND

DNA methylation has an important role in intergenerational inheritance. An increasing number of studies have reported evidence of germline inheritance of DNA methylation induced by nutritional signals in mammals. Vitamins and minerals as micronutrients contribute to growth performance in vertebrates, including Atlantic salmon (Salmo salar), and also have a role in epigenetics as environmental factors that alter DNA methylation status. It is important to understand whether micronutrients in the paternal diet can influence the offspring through alterations of DNA methylation signatures in male germ cells.

RESULTS

Here, we show the effect of micronutrient supplementation on DNA methylation profiles in the male gonad through a whole life cycle feeding trial of Atlantic salmon fed three graded levels of micronutrient components. Our results strongly indicate that micronutrient supplementation affects the DNA methylation status of genes associated with cell signalling, synaptic signalling, and embryonic development. In particular, it substantially affects DNA methylation status in the promoter region of a glutamate receptor gene, glutamate receptor ionotropic, NMDA 3A-like (grin3a-like), when the fish are fed both medium and high doses of micronutrients. Furthermore, two transcription factors, histone deacetylase 2 (hdac2) and a zinc finger protein, bind to the hyper-methylated site in the grin3a-like promoter. An estimated function of hdac2 together with a zinc finger indicates that grin3a-like has a potential role in intergenerational epigenetic inheritance and the regulation of embryonic development affected by paternal diet.

CONCLUSIONS

The present study demonstrates alterations of gene expression patterns and DNA methylation signatures in the male gonad when Atlantic salmon are fed different levels of micronutrients. Alterations of gene expression patterns are of great interest because the gonads are supposed to have limited metabolic activities compared to other organs, whereas alterations of DNA methylation signatures are of great importance in the field of nutritional epigenetics because the signatures affected by nutrition could be transferred to the next generation. We provide extensive data resources for future work in the context of potential intergenerational inheritance through the male germline.

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.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

DNA Methylation and Non-Coding RNAs during Tissue-Injury Associated Pain

While about half of the population experience persistent pain associated with tissue damages during their lifetime, current symptom-based approaches often fail to reduce such pain to a satisfactory level. To provide better patient care, mechanism-based analgesic approaches must be developed, which necessitates a comprehensive understanding of the nociceptive mechanism leading to tissue injury-associated persistent pain. Epigenetic events leading the altered transcription in the nervous system are pivotal in the maintenance of pain in tissue injury. However, the mechanisms through which those events contribute to the persistence of pain are not fully understood. This review provides a summary and critical evaluation of two epigenetic mechanisms, DNA methylation and non-coding RNA expression, on transcriptional modulation in nociceptive pathways during the development of tissue injury-associated pain. We assess the pre-clinical data and their translational implication and evaluate the potential of controlling DNA methylation and non-coding RNA expression as novel analgesic approaches and/or biomarkers of persistent pain.

Long non-coding RNA rhabdomyosarcoma 2-associated transcript contributes to neuropathic pain by recruiting HuR to stabilize DNA methyltransferase 3 alpha mRNA expression in dorsal root ganglion neuron

Introduction

Long non-coding RNAs (lncRNAs) act as key regulators in multiple human diseases. In particular, the dysfunction of lncRNAs in dorsal root ganglion (DRG) contributes to the pathogenesis of neuropathic pain (NP). Nevertheless, the role and mechanism of most lncRNAs in NP remain unclear.

Methods

Two classic chronic NP models, including L4 spinal nerve ligation (SNL) model and chronic constriction injury (CCI) of the sciatic nerve, were performed. Mechanical allodynia and heat hyperalgesia were used to evaluate neuropathic pain. DRG microinjection was used to deliver agents into DRG. qRT-PCR, immunofluorescence, immunoprecipitation, western blotting, siRNA transfection, AAV transduction were performed to investigate the phenotypes and molecular basis.

Results

Here, we discovered that Rmst as a lncRNA was specifically expressed in Atf3 ⁺ injured DRG neurons and significantly upregulated following peripheral nerve damage. Rmst overexpression by direct DRG injection of AAV5-Rmst causes neuropathic symptoms in the absence of nerve damage. Conversely, blocking Rmst expression in injured DRGs alleviated nerve injury-induced pain hypersensitivities and downregulated Dnmt3a expression. Furthermore, we found peripheral nerve damage induced Rmst increase could interact with RNA-binding protein HuR to stabilize the Dnmt3a mRNA.

Conclusion

Our findings reveal a crucial role of Rmst in damaged DRG neurons under NP condition and provide a novel target for drug development against NP.

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.

ZNF382 controls mouse neuropathic pain via silencer-based epigenetic inhibition of Cxcl13 in DRG neurons

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.

Epigenetic control of ion channel expression and cell-specific splicing in nociceptors: Chronic pain mechanisms and potential therapeutic targets

Ion channels underlie all forms for electrical signaling including the transmission of information about harmful events. Voltage-gated calcium ion channels have dual function, they support electrical signaling as well as intracellular calcium signaling through excitation-dependent calcium entry across the plasma membrane. Mechanisms that regulate ion channel forms and actions are essential for myriad cell functions and these are targeted by drugs and therapeutics. When disrupted, the cellular mechanisms that control ion channel activity can contribute to disease pathophysiology. For example, alternative pre-mRNA splicing is a major step in defining the precise composition of the transcriptome across different cell types from early cellular differentiation to programmed apoptosis. An estimated 30% of disease-causing mutations are associated with altered alternative splicing, and mis-splicing is a feature of numerous highly prevalent diseases including neurodegenerative, cancer, and chronic pain. Here we discuss the important role of epigenetic regulation of gene expression and cell-specific alternative splicing of calcium ion channels in nociceptors, with emphasis on how these processes are disrupted in chronic pain, the potential therapeutic benefit of correcting or compensating for aberrant ion channel splicing in chronic pain.

Epigenetic modifications in neuropathic pain

Neuropathic pain (NP) is a common symptom in many diseases of the somatosensory nervous system, which severely affects the patient’s quality of life. Epigenetics are heritable alterations in gene expression that do not cause permanent changes in the DNA sequence. Epigenetic modifications can affect gene expression and function and can also mediate crosstalk between genes and the environment. Increasing evidence shows that epigenetic modifications, including DNA methylation, histone modification, non-coding RNA, and RNA modification, are involved in the development and maintenance of NP. In this review, we focus on the current knowledge of epigenetic modifications in the development and maintenance of NP. Then, we illustrate different facets of epigenetic modifications that regulate gene expression and their crosstalk. Finally, we discuss the burgeoning evidence supporting the potential of emerging epigenetic therapies, which has been valuable in understanding mechanisms and offers novel and potent targets for NP therapy.

Nucleo-cytoplasmic RNA distribution responsible for maintaining neuroinflammatory microenvironment

Subcellular localization of transcripts is highly associated with regulation of gene expression, synthesis of protein, and also the development of the human brain cortex. Although many mechanisms are prevalent in the occurrence of neuroinflammation, the mechanisms based on differences in subcellular localization of transcripts have not been explored. To characterize the dynamic profile of nuclear and cytoplasmic transcripts during the progress of haemorrhage-induced neuroinflammation, we isolated nucleo-cytoplasmic RNA fractions of oxyhaemoglobin (oxy-Hb) treated microglia cells and sequenced both fractions. We discovered that cytoplasmic retained genes were the major forces to maintain the neuroinflammatory microenvironment with 10 hub genes and 40 conserved genes were identified. Moreover, antisense RNA Gm44096 and lincRNA Gm47270, which co-expressed with a crowd of inflammatory genes in the cytoplasm, were discovered as regulatory strategies for sustaining the neuroinflammatory microenvironment. Thus, our study provides a new perspective on understanding haemorrhage-induced neuroinflammation and also reveals a mechanism of lncRNA responsible for maintaining the neuroinflammatory microenvironment.

A Compound Mitigates Cancer Pain and Chemotherapy-Induced Neuropathic Pain by Dually Targeting nNOS-PSD-95 Interaction and GABAA Receptor

Metastatic bone pain and chemotherapy-induced peripheral neuropathic pain are the most common clinical symptoms in cancer patients. The current clinical management of these two disorders is ineffective and/or produces severe side effects. The present study employed a dual-target compound named as ZL006-05 and examined the effect of systemic administration of ZL006-05 on RM-1-induced bone cancer pain and paclitaxel-induced neuropathic pain. Intravenous injection of ZL006-05 dose-dependently alleviated RM-1-induced mechanical allodynia, heat hyperalgesia, cold hyperalgesia, and spontaneously ongoing nociceptive responses during both induction and maintenance periods, without analgesic tolerance, affecting basal/acute pain and locomotor function. Similar behavioral results were observed in paclitaxel-induced neuropathic pain. This injection also decreased neuronal and astrocyte hyperactivities in the lumbar dorsal horn after RM-1 tibial inoculation or paclitaxel intraperitoneal injection. Mechanistically, intravenous injection of ZL006-05 potentiated the GABAA receptor agonist-evoked currents in the neurons of the dorsal horn and anterior cingulate cortex and also blocked the paclitaxel-induced increase in postsynaptic density-95-neuronal nitric oxide synthase interaction in dorsal horn. Our findings strongly suggest that ZL006-05 may be a new candidate for the management of cancer pain and chemotherapy-induced peripheral neuropathic pain.

TrkA specific signalling pathways are critical for mechanical allodynia development and bone alterations in a mouse model of rheumatoid arthritis

ABSTRACT

Rheumatoid arthritis is frequently associated with chronic pain that still remains difficult to treat. Targeting nerve growth factor (NGF) seems very effective to reduce pain in at least, osteoarthritis and chronic low back pain but leads to some potential adverse events. Our aim was to better understand the involvement of the intracellular signalling pathways activated by NGF through its specific tyrosine kinase type A (TrkA) receptor in the pathophysiology of rheumatoid arthritis using the complete Freund adjuvant model in our knock-in TrkA/C mice. Our multimodal study demonstrated that knock-in TrkA/C mice exhibited a specific decrease of mechanical allodynia, weight bearing deficit, peptidergic (CGRP+) and sympathetic (TH+) peripheral nerve sprouting in the joints, a reduction in osteoclast activity and bone resorption markers, and a decrease of CD68 positive cells in the joint with no apparent changes in joint inflammation compared to WT mice following arthritis. Finally, transcriptomic analysis show several differences in DRGs mRNA expression of putative mechanotransducers such as ASIC3 and TRAAK as well as intracellular pathways such as c-Jun in the joint/DRGs. These results suggest that TrkA specific intracellular signalling pathways are specifically involved in mechanical hypersensitivity and bone alterations following arthritis using TrkA/C mice.

The role of dorsal root ganglia alpha-7 nicotinic acetylcholine receptor in complete Freund's adjuvant-induced chronic inflammatory pain

BACKGROUND

Alpha-7 nicotinic acetylcholine receptor (α7 nAChR) was reported to have a critical role in the regulation of pain sensitivity and neuroinflammation. However, the expression level of α7 nAChR in dorsal root ganglion (DRG) and the underlying neuroinflammatory mechanisms associated with hyperalgesia are still unknown.

METHODS

In the present study, the expression and mechanism of α7 nAChR in chronic inflammatory pain was investigated using a complete Freund's adjuvant (CFA)-induced chronic inflammatory pain model. Subsequently, a series of assays including immunohistochemistry, western blotting, and quantitative real-time polymerase chain reaction (qRT-PCR) were performed.

RESULTS

α7 nAChR was mostly colocalized with NeuN in DRG and upregulated after CFA injection. Microinjection of α7 nAChR siRNA into ipsilateral L4/5 DRGs aggravated the CFA-induced pain hypersensitivity. Intrathecal α7 nAChR agonist GTS-21 attenuated the development of CFA-induced mechanical and temperature-related pain hypersensitivities. In neuronal the SH-SY5Y cell line, the knockdown of α7 nAChRs triggered the upregulation of TRAF6 and NF-κB under CFA-induced inflammatory conditions, while agitation of α7 nAChR suppressed the TRAF6/NF-κB activation. α7 nAChR siRNA also exacerbated the secretion of pro-inflammatory mediators from LPS-induced SH-SY5Y cells. Conversely, α7 nAChR-specific agonist GTS-21 diminished the release of interleukin-1beta (IL-1β), IL-6, IL-8, and tumor necrosis factor-α (TNFα) in SH-SY5Y cells under inflammatory conditions. Mechanistically, the modulation of pain sensitivity and neuroinflammatory action of α7 nAChR may be mediated by the TRAF6/NF-κB signaling pathway.

CONCLUSIONS

The findings of this study suggest that α7 nAChR may be potentially utilized as a therapeutic target for therapeutics of chronic inflammatory pain.

Orthodontic force induces nerve injury-like transcriptomic changes driven by TRPV1-expressing afferents in mouse trigeminal ganglia

Orthodontic force produces mechanical irritation and localized inflammation in the periodontium, which causes pain in most patients. Nocifensive behaviors resulting from orthodontic force in mice can be substantially attenuated by intraganglionic injection of resiniferatoxin (RTX), a neurotoxin that specifically ablates a subset of neurons expressing transient receptor potential vanilloid 1 (TRPV1). In the current study, we determined changes in the transcriptomic profiles in the trigeminal ganglia (TG) following the application of orthodontic force, and assessed the roles of TRPV1-expressing afferents in these transcriptomic changes. RTX or vehicle was injected into the TG of mice a week before the placement of an orthodontic spring exerting 10 g of force. After 2 days, the TG were collected for RNA sequencing. The application of orthodontic force resulted in 1279 differentially expressed genes (DEGs) in the TG. Gene ontology analysis showed downregulation of gliogenesis and ion channel activities, especially of voltage-gated potassium channels. DEGs produced by orthodontic force correlated more strongly with DEGs resulting from nerve injury than from inflammation. Orthodontic force resulted in the differential expression of multiple genes involved in pain regulation, including upregulation of Atf3, Adcyap1, Bdnf, and Csf1, and downregulation of Scn10a, Kcna2, Kcnj10, and P2ry1. Orthodontic force-induced DEGs correlated with DEGs specific to multiple neuronal and non-neuronal subtypes following nerve injury. These transcriptomic changes were abolished in the mice that received the RTX injection. These results suggest that orthodontic force produces transcriptomic changes resembling nerve injury in the TG and that nociceptive inputs through TRPV1-expressing afferents leads to subsequent changes in gene expression not only in TRPV1-positive neurons, but also in TRPV1-negative neurons and non-neuronal cells throughout the ganglia. Orthodontic force-induced transcriptomic changes might be an active regenerative program of trigeminal ganglia in response to axonal injury following orthodontic force.

Downregulation of a Dorsal Root Ganglion-Specifically Enriched Long Noncoding RNA is Required for Neuropathic Pain by Negatively Regulating RALY-Triggered Ehmt2 Expression

Nerve injury-induced maladaptive changes of gene expression in dorsal root ganglion (DRG) neurons contribute to neuropathic pain. Long non-coding RNAs (lncRNAs) are emerging as key regulators of gene expression. Here, a conserved lncRNA is reported, named DRG-specifically enriched lncRNA (DS-lncRNA) for its high expression in DRG neurons. Peripheral nerve injury downregulates DS-lncRNA in injured DRG due, in part, to silencing of POU domain, class 4, transcription factor 3, a transcription factor that interacts with the DS-lncRNA gene promoter. Rescuing DS-lncRNA downregulation blocks nerve injury-induced increases in the transcriptional cofactor RALY-triggered DRG Ehmt2 mRNA and its encoding G9a protein, reverses the G9a-controlled downregulation of opioid receptors and Kcna2 in injured DRG, and attenuates nerve injury-induced pain hypersensitivities in male mice. Conversely, DS-lncRNA downregulation increases RALY-triggered Ehmt2/G9a expression and correspondingly decreases opioid receptor and Kcna2 expression in DRG, leading to neuropathic pain symptoms in male mice in the absence of nerve injury. Mechanistically, downregulated DS-lncRNA promotes more binding of increased RALY to RNA polymerase II and the Ehmt2 gene promoter and enhances Ehmt2 transcription in injured DRG. Thus, downregulation of DS-lncRNA likely contributes to neuropathic pain by negatively regulating the expression of RALY-triggered Ehmt2/G9a, a key neuropathic pain player, in DRG neurons.

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.

Improving NKCC1 Function Increases the Excitability of DRG Neurons Exacerbating Pain Induced After TRPV1 Activation of Primary Sensory Neurons

Background Our aim was to investigate the effects of the protein expression and the function of sodium, potassium, and chloride co-transporter (NKCC1) in the dorsal root ganglion (DRG) after activation of transient receptor potential vanilloid 1 receptor (TRPV1) in capsaicin-induced acute inflammatory pain and the possible mechanism of action.

Methods Male Sprague–Dawley rats were randomly divided into control, capsaicin, and inhibitor groups. The expression and distribution of TRPV1 and NKCC1 in rat DRG were observed by immunofluorescence. Thermal radiation and acetone test were used to detect the pain threshold of heat and cold noxious stimulation in each group. The expressions of NKCC1 mRNA, NKCC1 protein, and p-NKCC1 in the DRG were detected by PCR and western blotting (WB). Patch clamp and chloride fluorescent probe were used to observe the changes of GABA activation current and intracellular chloride concentration. After intrathecal injection of protein kinase C (PKC) inhibitor (GF109203X) or MEK/extracellular signal-regulated kinase (ERK) inhibitor (U0126), the behavioral changes and the expression of NKCC1 and p-ERK protein in L_4–6 DRG were observed.

Result: TRPV1 and NKCC1 were co-expressed in the DRG. Compared with the control group, the immunofluorescence intensity of NKCC1 and p-NKCC1 in the capsaicin group was significantly higher, and the expression of NKCC1 in the nuclear membrane was significantly higher than that in the control group. The expression of NKCC1 mRNA and protein of NKCC1 and p-NKCC1 in the capsaicin group were higher than those in the control group. After capsaicin injection, GF109203X inhibited the protein expression of NKCC1 and p-ERK, while U0126 inhibited the protein expression of NKCC1. In the capsaicin group, paw withdrawal thermal latency (WTL) was decreased, while cold withdrawal latency (CWL) was prolonged. Bumetanide, GF109203X, or U0126 could reverse the effect. GABA activation current significantly increased in the DRG cells of the capsaicin group, which could be reversed by bumetanide. The concentration of chloride in the DRG cells of the capsaicin group increased, but decreased after bumetanide, GF109203X, and U0126 were administered.

Conclusion Activation of TRPV1 by exogenous agonists can increase the expression and function of NKCC1 protein in DRG, which is mediated by activation of PKC/p-ERK signaling pathway. These results suggest that DRG NKCC1 may participate in the inflammatory pain induced by TRPV1.

MMP24 Contributes to Neuropathic Pain in an FTO-Dependent Manner in the Spinal Cord Neurons

Nerve injury-induced gene expression change in the spinal cord is critical for neuropathic pain genesis. RNA N⁶-methyladenosine (m⁶A) modification represents an additional layer of gene regulation. We showed that spinal nerve ligation (SNL) upregulated the expression of matrix metallopeptidase 24 (MMP24) protein, but not Mmp24 mRNA, in the spinal cord neurons. Blocking the SNL-induced upregulation of spinal MMP24 attenuated local neuron sensitization, neuropathic pain development and maintenance. Conversely, mimicking MMP24 increase promoted the spinal ERK activation and produced evoked nociceptive hypersensitivity. Methylated RNA Immunoprecipitation Sequencing (MeRIP-seq) and RNA Immunoprecipitation (RIP) assay indicated the decreased m⁶A enrichment in the Mmp24 mRNA under neuropathic pain condition. Moreover, fat-mass and obesity-associated protein (FTO) was colocalized with MMP24 in spinal neurons and shown increased binding to the Mmp24 mRNA in the spinal cord after SNL. Overexpression or suppression of FTO correlates with promotion or inhibition of MMP24 expression in cultured spinal cord neurons. In conclusion, SNL promoted the m⁶A eraser FTO binding to the Mmp24 mRNA, which subsequently facilitated the translation of MMP24 in the spinal cord, and ultimately contributed to neuropathic pain genesis.

Epigenetic upregulation of acid-sensing ion channel 1 contributes to gastric hypersensitivity in adult offspring rats with prenatal maternal stress

Functional dyspepsia is a common functional gastrointestinal disorder. Gastric hypersensitivity (GHS) is a hallmark of this disorder, but the cellular mechanisms remain largely unknown. Stressors during gestational period could have effects on the offspring's tissue structure and function, which may predispose to gastrointestinal diseases. The aim of this study was to test whether prenatal maternal stress (PMS) induces GHS and to investigate role of acid-sensing ion channel (ASIC)/nuclear factor-κB (NF-κB) signaling by examining Asic1 methylation status in adult offspring rats. Gastric hypersensitivity in response to gastric distension was examined by electromyography recordings. Changes in neuronal excitability were determined by whole-cell patch-clamp recording techniques. Demethylation of CpG islands of Asic1 was determined by methylation-specific PCR and bisulfite sequencing assay. Prenatal maternal stress produced GHS in adult offspring rats. Treatment with amiloride, an inhibitor of ASICs, significantly attenuated GHS and reversed hyperexcitability of gastric-specific dorsal root ganglion (DRG) neurons labeled by the dye DiI. Expression of ASIC1 and NF-κBp65 was markedly enhanced in T7 to T10 DRGs. Furthermore, PMS led to a significant demethylation of CpG islands in the Asic1 promoter. A chromatin immunoprecipitation assay showed that PMS also enhanced the ability of NF-κBp65 to bind the promoter of Asic1 gene. Blockade of NF-κB using lentiviral-p65shRNA reversed upregulation of ASIC1 expression, GHS, and the hyperexcitability of DRG neurons. These data suggest that upregulation of ASIC1 expression is attributed to Asic1 promoter DNA demethylation and NF-κB activation, and that the enhanced interaction of the Asic1 and NF-κBp65 contributes to GHS induced by PMS.

A Role for Transmembrane Protein 16C/Slack Impairment in Excitatory Nociceptive Synaptic Plasticity in the Pathogenesis of Remifentanil-induced Hyperalgesia in Rats

Remifentanil is widely used to control intraoperative pain. However, its analgesic effect is limited by the generation of postoperative hyperalgesia. In this study, we investigated whether the impairment of transmembrane protein 16C (TMEM16C)/Slack is required for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) activation in remifentanil-induced postoperative hyperalgesia. Remifentanil anesthesia reduced the paw withdrawal threshold from 2 h to 48 h postoperatively, with a decrease in the expression of TMEM16C and Slack in the dorsal root ganglia (DRG) and spinal cord. Knockdown of TMEM16C in the DRG reduced the expression of Slack and elevated the basal peripheral sensitivity and AMPAR expression and function. Overexpression of TMEM16C in the DRG impaired remifentanil-induced ERK1/2 phosphorylation and behavioral hyperalgesia. AMPAR-mediated current and neuronal excitability were downregulated by TMEM16C overexpression in the spinal cord. Taken together, these findings suggest that TMEM16C/Slack regulation of excitatory synaptic plasticity via GluA1-containing AMPARs is critical in the pathogenesis of remifentanil-induced postoperative hyperalgesia in rats.

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

BACKGROUND

Nerve injury-induced changes in gene expression in the dorsal root ganglion (DRG) contribute to neuropathic pain genesis. Eukaryotic initiation factor 4 gamma 2 (eIF4G2) is a general repressor of cap-dependent mRNA translation. Whether DRG eIF4G2 participates in nerve injury-induced alternations in gene expression and nociceptive hypersensitivity is unknown.

METHODS

The expression and distribution of eIF4G2 mRNA and protein in mouse DRG after spinal nerve ligation (SNL) were assessed. Effects of eIF4G2 siRNA microinjected through a glass micropipette into the injured DRG on the SNL-induced DRG mu opioid receptor (MOR) and Kv1.2 downregulation and nociceptive hypersensitivity were examined. In addition, effects of DRG microinjection of adeno-associated virus 5-expressing eIF4G2 (AAV5-eIF4G2) on basal DRG MOR and Kv1.2 expression and nociceptive thresholds were analysed.

RESULTS

eIF4G2 protein co-expressed with Kv1.2 and MOR in DRG neurones. Levels of eIF4G2 mRNA (1.7 [0.24] to 2.3 [0.14]-fold of sham, P<0.01) and protein (1.6 [0.14] to 2.5 [0.22]-fold of sham, P<0.01) in injured DRG were time-dependently increased on days 3-14 after SNL. Blocking increased eIF4G2 through microinjection of eIF4G2 siRNA into the injured DRG attenuated SNL-induced downregulation of DRG MOR and Kv1.2 and development and maintenance of nociceptive hypersensitivities. DRG microinjection of AAV5-eIF4G2 reduced DRG MOR and Kv1.2 expression and elicited hypersensitivities to mechanical, heat and cold stimuli in naïve mice.

CONCLUSIONS

eIF4G2 contributes to neuropathic pain through participation in downregulation of Kv1.2 and MOR in injured DRG and is a potential target for treatment of this disorder.

AXL signaling in primary sensory neurons contributes to chronic compression of dorsal root ganglion-induced neuropathic pain in rats

Low back pain is a chronic, highly prevalent, and hard-to-treat condition in the elderly. Clinical studies indicate that AXL, which belongs to the tyrosine kinase receptor subfamily, mediates pathological pain. However, it is not clear exactly how AXL regulates pain behaviors. In this study, we used a model of chronic compression of dorsal root ganglion-induced neuropathic pain to recreate clinical intervertebral foramen stenosis and related lumbocrural pain to explore whether AXL in primary sensory neurons contributes to this neuropathic pain in rats. Using double-labeling immunofluorescence, we observed that both phosphorylated AXL and AXL were localized primarily on isolectin B4-positive and calcitonin gene-related peptide-positive neurons, while AXL was also localized in neurofilament-200-positive neurons. Chronic compression of dorsal root ganglion-induced pain was associated with the upregulation of AXL mRNA and protein in injured dorsal root ganglia. Repeated intrathecal administration of the AXL inhibitor, TP0903, or the AXL small interfering RNA effectively alleviated chronic compression of dorsal root ganglion-induced pain hypersensitivities. Moreover, repeated intrathecal administration of either TP0903 or AXL small interfering RNA reduced the expression of mammalian target of rapamycin in injured dorsal root ganglia, suggesting that mammalian target of rapamycin may mediate AXL’s actions. These results indicate that the upregulation of dorsal root ganglion AXL may be part of a peripheral mechanism of neuropathic pain via an intracellular mammalian target of rapamycin-signaling pathway. Thus, while AXL inhibitors have so far primarily shown clinical efficacy in tumor treatment, AXL intervention could also serve as a potential target for the treatment of neuropathic pain.

CREB Participates in Paclitaxel-Induced Neuropathic Pain Genesis Through Transcriptional Activation of Dnmt3a in Primary Sensory Neurons

Chemotherapy-induced peripheral neuropathic pain (CIPNP) often occurs in cancer patients treated with antineoplastic drugs. Therapeutic management of CIPNP is very limited, at least in part due to the largely unknown mechanisms that underlie CIPNP genesis. Here, we showed that systemic administration of the chemotherapeutic drug paclitaxel significantly and time-dependently increased the levels of cyclic AMP response element-binding protein (CREB) in dorsal root ganglion (DRG) neurons. Blocking this increase through DRG microinjection of Creb siRNA attenuated paclitaxel-induced mechanical, heat, and cold nociceptive hypersensitivities. Mimicking this increase through DRG microinjection of the adeno-associated virus 5 expressing full-length Creb mRNA led to enhanced responses to basal mechanical, heat, and cold stimuli in mice in absence of paclitaxel treatment. Mechanically, paclitaxel-induced increase of DRG CREB protein augmented Dnmt3a promoter activity and participated in the paclitaxel-induced upregulation of DNMT3a protein in the DRG. CREB overexpression also elevated the expression of DNMT3a in in vivo and in vitro DRG neurons of naïve mice. Given that DNMT3a is an endogenous instigator of CIPNP and that CREB co-expresses with DNMT3a in DRG neurons, CREB may be a key player in CIPNP through transcriptional activation of the Dnmt3a gene in primary sensory neurons. CREB is thus a likely potential target for the therapeutic management of this disorder.

Guanosine-5'-triphosphate cyclohydrolase 1 regulated long noncoding RNAs are potential targets for microglial activation in neuropathic pain

Several studies have confirmed that microglia are involved in neuropathic pain. Inhibition of guanosine-5'-triphosphate cyclohydrolase 1 (GTPCH1) can reduce the inflammation of microglia. However, the precise mechanism by which GTPCH1 regulates neuropathic pain remains unclear. In this study, BV2 microglia were transfected with adenovirus to knockdown GTPCH1 expression. High throughput sequencing analysis revealed that the mitogen-activated protein kinase (MAPK) related pathways and proteins were the most significantly down-regulated molecular function. Co-expression network analysis of Mapk14 mRNA and five long noncoding RNAs (lncRNAs) revealed their correlation. Quantitative reverse transcription-polymerase chain reaction revealed that among five lncRNAs, ENSMUST00000205634, ENSMUST00000218450 and ENSMUST00000156079 were related to the downregulation of Mapk14 mRNA expression. These provide some new potential targets for the involvement of GTPCH1 in neuropathic pain. This study is the first to note the differential expression of lncRNAs and mRNA in GTPCH1 knockdown BV2 microglia. Findings from this study reveal the mechanism by which GTPCH1 activates microglia and provide new potential targets for microglial activation in neuropathic pain.

Peripheral Neuropathic Pain: From Experimental Models to Potential Therapeutic Targets in Dorsal Root Ganglion Neurons

Neuropathic pain exerts a global burden caused by the lesions in the somatosensory nerve system, including the central and peripheral nervous systems. The mechanisms of nerve injury-induced neuropathic pain involve multiple mechanisms, various signaling pathways, and molecules. Currently, poor efficacy is the major limitation of medications for treating neuropathic pain. Thus, understanding the detailed molecular mechanisms should shed light on the development of new therapeutic strategies for neuropathic pain. Several well-established in vivo pain models were used to investigate the detail mechanisms of peripheral neuropathic pain. Molecular mediators of pain are regulated differentially in various forms of neuropathic pain models; these regulators include purinergic receptors, transient receptor potential receptor channels, and voltage-gated sodium and calcium channels. Meanwhile, post-translational modification and transcriptional regulation are also altered in these pain models and have been reported to mediate several pain related molecules. In this review, we focus on molecular mechanisms and mediators of neuropathic pain with their corresponding transcriptional regulation and post-translational modification underlying peripheral sensitization in the dorsal root ganglia. Taken together, these molecular mediators and their modification and regulations provide excellent targets for neuropathic pain treatment.

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.

Pharmacology and Mechanism of Action of HSK16149, a Selective Ligand of α2δ Subunit of Voltage-Gated Calcium Channel with Analgesic Activity in Animal Models of Chronic Pain

Chronic pain is a public health problem because current treatments are unsatisfactory with small therapeutic index. Although pregabalin is effective for treating chronic pain, the clinical use is limited because of its side effects. Therefore, improving its therapeutic index is essential. In this study, HSK16149 was found to be a novel ligand of voltage-gated calcium channel (VGCC) α ₂ δ subunit. HSK16149 inhibited [³H]gabapentin binding to the α ₂ δ subunit and was 23 times more potent than pregabalin. In two rat models of neuropathic pain, the minimum effective dose (MED) of HSK16149 was 10 mg/kg, and the efficacy was similar to that of 30 mg/kg pregabalin. Moreover, the efficacy of HSK16149 could persist up to 24 hours postadministration at 30 mg/kg, whereas the efficacy of pregabalin lasted only for 12 hours at 30 mg/kg in streptozotocin-induced diabetic neuropathy model, indicating that HSK16149 might be a longer-acting drug candidate. HSK16149 could also inhibit mechanical allodynia in intermittent cold stress model and decrease phase II pain behaviors in formalin-induced nociception model. In addition, the locomotor activity test showed that the MED of HSK16149 was similar to that of pregabalin, whereas in the Rotarod test, the MEDs of HSK16149 and pregabalin were 100 and 30 mg/kg, respectively. These findings indicated that HSK16149 might have a better safety profile on the central nervous system. In summary, HSK16149 is a potent ligand of VGCC α ₂ δ subunit with a better therapeutic index than pregabalin. Hence, it could be an effective and safe drug candidate for treating chronic pain. SIGNIFICANCE STATEMENT: As a novel potent ligand of voltage-gated calcium channel α ₂ δ subunit, HSK16149 has the potential to be an effective and safe drug candidate for the treatment of chronic pain.

Molecular Transport through a Biomimetic DNA Channel on Live Cell Membranes

Biological membrane channels, considered as molecular gatekeepers, control the transportation of molecules and ions across live cell membranes. Developing synthetic passable channels with predictable structures, high transport efficiency, and low cytotoxicity on live cells is of great interest for replicating the functions of endogenous protein channels, but remains challenging. The development of DNA nanotechnology provides possible solutions for making synthetic channels with precise structures and controllable functionalization. Therefore, in this work, we constructed a phosphorothioate-modified DNA nanopore able to structurally mimic biological channels for molecular transport across live cell membranes. With its stable structure with small hollow size (<2 nm) and the ability to interact with the lipid molecules, this DNA nanopore could show stable insertion into the plasma membrane. We further proved that this membrane-spanning channel could transport ions and antitumor drugs to neurons and cancer cells, respectively, and do so within a certain time window. We expect that this live cell membrane-spanning synthetic DNA nanopore will provide a tool for studying cellular communication, building synthetic cells, and achieving controlled transmembrane transport to cells.

Fgr contributes to hemorrhage-induced thalamic pain by activating NF-κB/ERK1/2 pathways

Thalamic pain, a type of central poststroke pain, frequently occurs following ischemia/hemorrhage in the thalamus. Current treatment of this disorder is often ineffective, at least in part due to largely unknown mechanisms that underlie thalamic pain genesis. Here, we report that hemorrhage caused by microinjection of type IV collagenase or autologous whole blood into unilateral ventral posterior lateral nucleus and ventral posterior medial nucleus of the thalamus increased the expression of Fgr, a member of the Src family nonreceptor tyrosine kinases, at both mRNA and protein levels in thalamic microglia. Pharmacological inhibition or genetic knockdown of thalamic Fgr attenuated the hemorrhage-induced thalamic injury on the ipsilateral side and the development and maintenance of mechanical, heat, and cold pain hypersensitivities on the contralateral side. Mechanistically, the increased Fgr participated in hemorrhage-induced microglial activation and subsequent production of TNF-α likely through activation of both NF-κB and ERK1/2 pathways in thalamic microglia. Our findings suggest that Fgr is a key player in thalamic pain and a potential target for the therapeutic management of this disorder.

Halogenated derivatives of cytidine: Structural analysis and binding affinity

Cytidine is a well-known inhibitor of DNA methyltransferase (MTN) enzyme for preventing cancer cells growth. Based on therapeutic benefits, it could be considered as a “lead compound” to be optimized through structural modification for arising better binding affinity in this case. Halogenated derivatives of cytidine were investigated in this work to examine structural and biological features employing in silico approach. To this aim, geometries of the original cytidine and four of its halogenated derivatives were minimized to prepare ligands for interacting with MTN enzyme target in molecular docking simulations. The results for singular ligand structures introduced I-cytidine as an optimized lead compound for contributing to proper interactions with MTN enzyme; the trend was confirmed by molecular docking simulations. As a final remark, I-cytidine could be considered as better ligand for complexation with the MTN enzyme in comparison with the original cytidine.

K+ Channels in Primary Afferents and Their Role in Nerve Injury-Induced Pain

Sensory abnormalities generated by nerve injury, peripheral neuropathy or disease are often expressed as neuropathic pain. This type of pain is frequently resistant to therapeutic intervention and may be intractable. Numerous studies have revealed the importance of enduring increases in primary afferent excitability and persistent spontaneous activity in the onset and maintenance of peripherally induced neuropathic pain. Some of this activity results from modulation, increased activity and /or expression of voltage-gated Na+ channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. K+ channels expressed in dorsal root ganglia (DRG) include delayed rectifiers (Kv1.1, 1.2), A-channels (Kv1.4, 3.3, 3.4, 4.1, 4.2, and 4.3), KCNQ or M-channels (Kv7.2, 7.3, 7.4, and 7.5), ATP-sensitive channels (KIR6.2), Ca2+-activated K+ channels (KCa1.1, 2.1, 2.2, 2.3, and 3.1), Na+-activated K+ channels (KCa4.1 and 4.2) and two pore domain leak channels (K2p; TWIK related channels). Function of all K+ channel types is reduced via a multiplicity of processes leading to altered expression and/or post-translational modification. This also increases excitability of DRG cell bodies and nociceptive free nerve endings, alters axonal conduction and increases neurotransmitter release from primary afferent terminals in the spinal dorsal horn. Correlation of these cellular changes with behavioral studies provides almost indisputable evidence for K+ channel dysfunction in the onset and maintenance of neuropathic pain. This idea is underlined by the observation that selective impairment of just one subtype of DRG K+ channel can produce signs of pain in vivo. Whilst it is established that various mediators, including cytokines and growth factors bring about injury-induced changes in DRG function and excitability, evidence presently available points to a seminal role for interleukin 1β (IL-1β) in control of K+ channel function. Despite the current state of knowledge, attempts to target K+ channels for therapeutic pain management have met with limited success. This situation may change with the advent of personalized medicine. Identification of specific sensory abnormalities and genetic profiling of individual patients may predict therapeutic benefit of K+ channel activators.

The Role of DNA Methylation in Transcriptional Regulation of Pro-Nociceptive Genes in Rat Trigeminal Ganglia

Epigenetic modulation by DNA methylation is associated with aberrant gene expression in sensory neurons, which consequently leads to pathological pain responses. In this study, we sought to investigate whether peripheral inflammation alters global DNA methylation in trigeminal ganglia (TG) and results in abnormal expression of pro-nociceptive genes. Our results show that peripheral inflammation remotely reduced the level of global DNA methylation in rat TG with a concurrent reduction in DNMT1 and DNMT3a expression. Using unbiased steps, we selected the following pro-nociceptive candidate genes that are potentially regulated by DNA methylation: TRPV1, TRPA1, P2X3, and PIEZO2. Inhibition of DNMT with 5-Aza-dC in dissociated TG cells produced dose-dependent upregulation of TRPV1, TRPA1, and P2X3. Systemic treatment of animals with 5-Aza-dC significantly increased the expression of TRPV1, TRPA1, and PIEZO2 in TG. Furthermore, the overexpression of DNMT3a, as delivered by a lentiviral vector, significantly downregulated TRPV1 and PIEZO2 expression and also reliably decreased TRPA1 and P2X3 transcripts. MeDIP revealed that this overexpression also significantly enhanced methylation of CGIs associated with TRPV1 and TRPA1. In addition, bisulfite sequencing data indicated that the CGI associated with TRPA1 was methylated in a pattern catalyzed by DNMT3a. Taken together, our results show that all 4 pro-nociceptive genes are subject to epigenetic modulation via DNA methylation, likely via DNMT3a under inflammatory conditions. These findings provide the first evidence for the functional importance of DNA methylation as an epigenetic factor in the transcription of pro-nociceptive genes in TG that are implicated in pathological orofacial pain responses.

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.

Induction of TGF-β receptor I expression in a DNA methylation-independent manner mediated by DNMT3A downregulation is involved in early-onset severe preeclampsia

Preeclampsia, especially early-onset severe preeclampsia is one of the leading causes of maternal and fetal morbidity and mortality. Although it has been well known that the pathophysiology of early-onset severe preeclampsia begins with abnormal placentation and aberrant activation of TGF-β signaling inhibits trophoblast cell invasion, the mechanisms underlying dysregulation of TGF-β signaling in early-onset severe preeclampsia remain elusive to date. Here, we revealed that induction of TGFBR1/TGF-β signaling mediated by DNMT3A downregulation plays a critical role in early-onset severe preeclampsia. Our results show that DNMT3A downregulation elevates TGFBR1 expression in trophoblast cells. Moreover, inhibition of TGFBR1 and TGF-β/Smad signaling can rescue the deficiencies of trophoblast cell migration and invasion caused by DNMT3A knockdown. Mechanistically, DNMT3A suppresses the transcription of TGFBR1 through recruiting EZH2 to its promoter but not changing DNA methylation of TGFBR1 promoter. In human samples, we detected lowly expressed DNMT3A, highly expressed TGFBR1 and hyperactivation of TGF-β/Smad signaling in decidua-embedded extravillous trophoblasts in early-onset severe preeclampsia, which provides the clinical evidence for the correlation between DNMT3A and TGFBR1. Collectively, our findings demonstrate that DNA methylation-independent induction of TGFBR1 mediated by DNMT3A downregulation is relevant to the development of early-onset severe preeclampsia.

Epigenetic restoration of voltage-gated potassium channel Kv1.2 alleviates nerve injury-induced neuropathic pain

Voltage-gated potassium channels (Kv) are important regulators of neuronal excitability for its role of regulating resting membrane potential and repolarization. Recent studies show that Kv channels participate in neuropathic pain, but the detailed underlying mechanisms are far from being clear. In this study, we used siRNA, miR-137 agomir, and antagomir to regulate the expression of Kv1.2 in spinal cord and dorsal root ganglia (DRG) of naïve and chronic constriction injury (CCI) rats. Kv currents and neuron excitability in DRG neurons were examined by patch-clamp whole-cell recording to verify the change in Kv1.2 function. The results showed that Kv1.2 was down-regulated in DRG and spinal dorsal horn (SDH) by CCI. Knockdown of Kv1.2 by intrathecally injecting Kcna2 siRNA induced significant mechanical and thermal hypersensitivity in naïve rats. Concomitant with the down-regulation of Kv1.2 was an increase in the expression of the miR-137. The targeting and regulating of miR-137 on Kcna2 was verified by dual-luciferase reporter system and intrathecal injecting miR-137 agomir. Furthermore, rescuing the expression of Kv1.2 in CCI rats, achieved through inhibiting miR-137, restored the abnormal Kv currents and excitability in DRG neurons, and alleviated mechanical allodynia and thermal hyperalgesia. These results indicate that the miR-137-mediated Kv1.2 impairment is a crucial etiopathogenesis for the nerve injury-induced neuropathic pain and can be a novel potential therapeutic target for neuropathic pain management.

Relationship between Pain Behavior and Changes in KCNA2 Expression in the Dorsal Root Ganglia of Rats with Osteoarthritis

Objective. To investigate the relationship between pain behavior and potassium voltage-gated channel subfamily A member 2 (KCNA2) expression in dorsal root ganglia (DRGs) of rats with osteoarthritis (OA). Methods. Male Sprague-Dawley rats were randomly divided into three groups: blank control group (group C), normal saline group (group S), and group OA. Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were measured one day before injection and one, two, four, and six weeks after injection. At one, two, four, and six weeks after injection, pathological knee joint changes and activated transcription factor-3 (ATF-3) and KCNA2 expressions in DRGs were analyzed. Results. Compared with preinjection, PWMT and PWTL at two, four, and six weeks after injection were significantly decreased in the group OA (P<0.05 or 0.01). Compared with group C, PWMT and PWTL at two, four, and six weeks after injection were significantly decreased in the group OA (P<0.05 or 0.01). In the group OA, slight local articular cartilage surface destruction was found at week one. The cartilage surface destruction gradually developed, and the exacerbation of cartilage matrix reduction and bone hyperplasia were increasingly aggravated and eventually evolved into advanced OA in the second to sixth weeks. Compared with group C, ATF-3 expression was significantly increased, and KCNA2 expression was significantly decreased in the group OA at two, four, and six weeks after injection (P<0.05 or 0.01). Compared to baseline, ATF-3 expression was significantly increased, and KCNA2 expression was significantly decreased in the group OA (P<0.05 or 0.01). Conclusion. Pain behavior in OA rats was associated with decreased KCNA2 expression in DRGs.

N6-Methyladenosine Demethylase FTO Contributes to Neuropathic Pain by Stabilizing G9a Expression in Primary Sensory Neurons

Nerve injury-induced change in gene expression in primary sensory neurons of dorsal root ganglion (DRG) is critical for neuropathic pain genesis. N6-methyladenosine (m6A) modification of RNA represents an additional layer of gene regulation. Here, it is reported that peripheral nerve injury increases the expression of the m6A demethylase fat-mass and obesity-associated proteins (FTO) in the injured DRG via the activation of Runx1, a transcription factor that binds to the Fto gene promoter. Mimicking this increase erases m6A in euchromatic histone lysine methyltransferase 2 (Ehmt2) mRNA (encoding the histone methyltransferase G9a) and elevates the level of G9a in DRG and leads to neuropathic pain symptoms. Conversely, blocking this increase reverses a loss of m6A sites in Ehmt2 mRNA and destabilizes the nerve injury-induced G9a upregulation in the injured DRG and alleviates nerve injury-associated pain hypersensitivities. FTO contributes to neuropathic pain likely through stabilizing nerve injury-induced upregulation of G9a, a neuropathic pain initiator, in primary sensory neurons.

Toll-like receptor 7 contributes to neuropathic pain by activating NF-κB in primary sensory neurons

Toll like receptor 7 (TLR7) is expressed in neurons of the dorsal root ganglion (DRG), but whether it contributes to neuropathic pain is elusive. We found that peripheral nerve injury caused by ligation of the fourth lumbar (L₄) spinal nerve (SNL) or chronic constriction injury of sciatic nerve led to a significant increase in the expression of TLR7 at mRNA and protein levels in mouse injured DRG. Blocking this increase through microinjection of the adeno-associated virus (AAV) 5 expressing TLR7 shRNA into the ipsilateral L₄ DRG alleviated the SNL-induced mechanical, thermal and cold pain hypersensitivities in both male and female mice. This microinjection also attenuated the SNL-induced increases in the levels of phosphorylated extracellular signal-regulated kinase ½ (p-ERK1/2) and glial fibrillary acidic protein (GFAP) in L₄ dorsal horn on the ipsilateral side during both development and maintenance periods. Conversely, mimicking this increase through microinjection of AAV5 expressing full-length TLR7 into unilateral L_3/4 DRGs led to elevations in the amounts of p-ERK1/2 and GFAP in the dorsal horn, augmented responses to mechanical, thermal and cold stimuli, and induced the spontaneous pain on the ipsilateral side in the absence of SNL. Mechanistically, the increased TLR7 activated the NF-κB signaling pathway through promoting the translocation of p65 into the nucleus and phosphorylation of p65 in the nucleus from the injured DRG neurons. Our findings suggest that DRG TLR7 contributes to neuropathic pain by activating NF-κB in primary sensory neurons. TLR7 may be a potential target for therapeutic treatment of this disorder.

Intra-axonal mechanisms driving axon regeneration

Traumatic injury to the peripheral and central nervous systems very often causes axotomy, where an axon loses connections with its target resulting in loss of function. The axon segments distal to the injury site lose connection with the cell body and degenerate. Axotomized neurons in the periphery can spontaneously mount a regenerative response and reconnect to their denervated target tissues, though this is rarely complete in humans. In contrast, spontaneous regeneration rarely occurs after axotomy in the spinal cord and brain. Here, we concentrate on the mechanisms underlying this spontaneous regeneration in the peripheral nervous system, focusing on events initiated from the axon that support regenerative growth. We contrast this with what is known for axonal injury responses in the central nervous system. Considering the neuropathy focus of this special issue, we further draw parallels and distinctions between the injury-response mechanisms that initiate regenerative gene expression programs and those that are known to trigger axon degeneration.

Cell-specific exon methylation and CTCF binding in neurons regulate calcium ion channel splicing and function

Cell-specific alternative splicing modulates myriad cell functions and is disrupted in disease. The mechanisms governing alternative splicing are known for relatively few genes and typically focus on RNA splicing factors. In sensory neurons, cell-specific alternative splicing of the presynaptic Ca_V channel Cacna1b gene modulates opioid sensitivity. How this splicing is regulated is unknown. We find that cell and exon-specific DNA hypomethylation permits CTCF binding, the master regulator of mammalian chromatin structure, which, in turn, controls splicing in a DRG-derived cell line. In vivo, hypomethylation of an alternative exon specifically in nociceptors, likely permits CTCF binding and expression of Ca_V2.2 channel isoforms with increased opioid sensitivity in mice. Following nerve injury, exon methylation is increased, and splicing is disrupted. Our studies define the molecular mechanisms of cell-specific alternative splicing of a functionally validated exon in normal and disease states – and reveal a potential target for the treatment of chronic pain.

C-X-C Motif Chemokine 10 Contributes to the Development of Neuropathic Pain by Increasing the Permeability of the Blood-Spinal Cord Barrier

Neuropathic pain is among the most debilitating forms of chronic pain. Studies have suggested that chronic pain pathogenesis involves neuroimmune interactions and blood-spinal cord barrier (BSCB) disruption. However, the underlying mechanisms are poorly understood. We modeled neuropathic pain in rats by inducing chronic constriction injury (CCI) of the sciatic nerve and analyzed the effects on C-X-C motif chemokine 10 (CXCL10)/CXCR3 activation, BSCB permeability, and immune cell migration from the circulation into the spinal cord. We detected CXCR3 expression in spinal neurons and observed that CCI induced CXCL10/CXCR3 activation, BSCB disruption, and mechanical hyperalgesia. CCI-induced BSCB disruption enabled circulating T cells to migrate into the spinal parenchyma. Intrathecal administration of an anti-CXCL10 antibody not only attenuated CCI-induced hyperalgesia, but also reduced BSCB permeability, suggesting that CXCL10 acts as a key regulator of BSCB integrity. Moreover, T cell migration may play a critical role in the neuroimmune interactions involved in the pathogenesis of CCI-induced neuropathic pain. Our results highlight CXCL10 as a new potential drug target for the treatment of nerve injury-induced neuropathic pain.

Role of Melatonin in the Regulation of Pain

Melatonin is a pleiotropic hormone synthesized and secreted mainly by the pineal gland in vertebrates. Melatonin is an endogenous regulator of circadian and seasonal rhythms. Melatonin is involved in many physiological and pathophysiological processes demonstrating antioxidant, antineoplastic, anti-inflammatory, and immunomodulatory properties. Accumulating evidence has revealed that melatonin plays an important role in pain modulation through multiple mechanisms. In this review, we examine recent evidence for melatonin on pain regulation in various animal models and patients with pain syndromes, and the potential cellular mechanisms.

Long noncoding RNA H19 in the injured dorsal root ganglion contributes to peripheral nerve injury-induced pain hypersensitivity

Peripheral nerve injury-induced changes in gene transcription and translation in the dorsal root ganglion (DRG) play a critical role in the development and maintenance of neuropathic pain. Long noncoding RNAs (lncRNAs) regulate gene expression. Here, we report that peripheral nerve injury caused by ligation of the fourth spinal nerve (SNL) led to a time-dependent increase in the expression in H19, an lncRNA, in the injured DRG. Microinjection of a specific H19 siRNA, but not negative control scrambled siRNA, into the injured DRG 4 days before SNL alleviated mechanical allodynia and thermal hyperalgesia on days 3 and 5 post-SNL. Additionally, DRG microinjection of the H19 siRNA on day 7 after SNL reduced mechanical allodynia and thermal hyperalgesia on days 10 and 12 post-SNL. DRG microinjection of neither siRNA affected locomotor activity and acute basal responses to mechanical and thermal stimuli. Our findings suggest that H19 participates in the peripheral mechanism underlying the development and maintenance of neuropathic pain. H19 may be a potential target for treatment of this disorder.

Alternative Splicing of Nrcam Gene in Dorsal Root Ganglion Contributes to Neuropathic Pain

NrCAM, a neuronal cell adhesion molecule in the L1 family of the immunoglobulin superfamily, is subjected to extensively alternative splicing and involved in neural development and some disorders. The aim of this study was to explore the role of Nrcam mRNA alternative splicing in neuropathic pain. A next generation RNA sequencing analysis of dorsal root ganglions (DRGs) showed the differential expression of two splicing variants of Nrcam, Nrcam⁺¹⁰ and Nrcam^-10, in the injured DRG after the fourth lumbar spinal nerve ligation (SNL) in mice. SNL increased the exon 10 insertion, resulting in an increase in the amount of Nrcam⁺¹⁰ and a corresponding decrease in the level of Nrcam^-10 in the injured DRG. An antisense oligonucleotide (ASO) that specifically targeted exon 10 of Nrcam gene (Nrcam ASO) repressed RNA expression of Nrcam⁺¹⁰ and increased RNA expression of Nrcam^-10 in in vitro DRG cell culture. Either DRG microinjection or intrathecal injection of Nrcam ASO attenuated SNL-induced the development of mechanical allodynia, thermal hyperalgesia, or cold allodynia. Nrcam ASO also relieved SNL- or chronic compression of DRG (CCD)-induced the maintenance of pain hypersensitivities in male and female mice. PERSPECTIVE: We conclude that the relative levels of alternatively spliced Nrcam variants are critical for neuropathic pain genesis. Targeting Nrcam alternative splicing via the antisense oligonucleotides may be a new potential avenue in neuropathic pain management.

Epigenetic reduction of miR-214-3p upregulates astrocytic colony-stimulating factor-1 and contributes to neuropathic pain induced by nerve injury

Emerging evidence has indicated that colony-stimulating factor-1 (CSF1) modulates neuroinflammation in the central nervous system and the development of neuropathic pain, while the underlying mechanism remains unknown. Here, we identified the increased expression of CSF1 derived from activated astrocytes in the ipsilateral dorsal horn in rats with spinal nerve ligation (SNL). Suppression of CSF1 expression alleviated neuroinflammation, neuronal hyperexcitability, and glutamatergic receptor subunit upregulation in the dorsal horn and improved SNL-induced pain behavior. We also found reduced miR-214-3p expression in the ipsilateral dorsal horn following an SNL procedure; miR-214-3p directly bound to the 3'-UTR of CSF1 mRNA and negatively regulated CSF1 expression. Intrathecal delivery of miR-214-3p mimic reversed the enhanced expression of CSF1 and astrocyte overactivity and alleviated the IL-6 upregulation and pain behavior induced by SNL. Moreover, suppression of spinal miR-214-3p increased astrocyte reactivity, promoted CSF1 and IL-6 production, and induced pain hypersensitivity in naive animals. Furthermore, SNL induced the expression of DNA methyltransferase 3a (DNMT3a) that was associated with the hypermethylation of the miR-214-3p promoter, leading to reduced miR-214-3p expression in the model rodents. Treatment with the DNMT inhibitor zebularine significantly reduced cytosine methylation in the miR-214-3p promoter; this reduced methylation consequently increased the expression of miR-214-3p and decreased the content of CSF1 in the ipsilateral dorsal horn and, further, attenuated IL-6 production and pain behavior in rats with SNL. Together, our data indicate that the DNMT3a-mediated epigenetic suppression of miR-214-3p enhanced CSF1 production in astrocytes, which subsequently induced neuroinflammation and pain behavior in SNL model rats.

DNA methylation and behavioral changes induced by neonatal spinal transection

Although the importance of epigenetic mechanisms in behavioral development has been gaining attention in recent years, research has largely focused on the brain. To our knowledge, no studies to date have investigated epigenetic changes in the developing spinal cord to determine the dynamic manner in which the spinal epigenome may respond to environmental input during behavioral development. Animal studies demonstrate that spinal cord plasticity is heightened during early development, is somewhat preserved following neonatal transection, and that spinal injured animals are responsive to sensory feedback. Because epigenetic alterations have been implicated in brain plasticity and are highly responsive to experience, these alterations are promising candidates for molecular substrates of spinal plasticity as well. Thus, the current study investigated behavioral changes in the development of weight-bearing locomotion and epigenetic modifications in the spinal cord of infant rats following a neonatal low-thoracic spinal transection or sham surgery on postnatal day (P)1. Specifically, global levels of methylation and methylation status of the brain-derived neurotrophic factor (Bdnf) gene, a neurotrophin heavily involved in both CNS and behavioral plasticity, particularly in development, were examined in lumbar tissue harvested on P10 from sham and spinal-transected subjects. Behavioral results demonstrate that compared to shams, spinal-transected subjects exhibit significantly reduced partial-weight bearing hindlimb activity. Molecular data demonstrate group differences in global lumbar methylation levels as well as exon-specific group differences in Bdnf methylation. This study represents an initial step toward understanding the relationship between epigenetic mechanisms and plasticity associated with spinal cord and locomotor development.

MZF1 in the Dorsal Root Ganglia Contributes to the Development and Maintenance of Neuropathic Pain via Regulation of TRPV1

Previous studies have demonstrated that myeloid zinc finger 1 (MZF1) in the dorsal root ganglion (DRG) participates in neuropathic pain induced by chronic-constriction injury (CCI) via regulation of voltage-gated K⁺ channels (Kv). Emerging evidence indicates that transient receptor potential vanilloid 1 (TRPV1) is involved in the development and maintenance of neuropathic pain. Although it is known that the transcription of TRPV1 is regulated by Kruppel-like zinc-finger transcription factor 7 (Klf7)—and that the structure of TRPV1 is similar to that of Kv—few studies have systematically investigated the relationship between MZF1 and TRPV1 in neuropathic pain. In the present study, we demonstrated that CCI induced an increase in MZF1 and TRPV1 in lumbar-level 4/5 (L4/5) DRGs at 3 days post-CCI and that this increase was persistent until at least 14 days post-CCI. DRG microinjection of rAAV5-MZF1 into the DRGs of naïve rats resulted in a decrease in paw-withdrawal threshold (PWT) and paw-withdrawal latency (PWL) compared with that of the rAAV5-EGFP group, which started at four weeks and lasted until at least eight weeks after microinjection. Additionally, prior microinjection of MZF1 siRNA clearly ameliorated CCI-induced reduction in PWT and PWL at 3 days post-CCI and lasted until at least 7 days post-CCI. Correspondingly, microinjection of MZF1 siRNA subsequent to CCI alleviated the established mechanical allodynia and thermal hyperalgesia induced by CCI, which occurred at 3 days postinjection and lasted until at least 10 days postinjection. Microinjection of rAAV5-MZF1 increased the expression of TRPV1 in DRGs. Microinjection of MZF1 siRNA diminished the CCI-induced increase of TRPV1, but not P2X7R, in DRGs. These findings suggest that MZF1 may contribute to neuropathic pain via regulation of TRPV1 expression in DRGs.