Promoter demethylation of cystathionine-β-synthetase gene contributes to inflammatory pain in rats

CBP Expression Contributes to Neuropathic Pain via CREB and MeCP2 Regulation in the Spared Nerve Injury Rat Model

Background and Objectives: This study aimed to investigate the relationship between neuropathic pain and CREB-binding protein (CBP) and methyl-CpG-binding protein 2 (MeCP2) expression levels in a rat model with spared nerve injury (SNI). Materials and Methods: Rat (male Sprague-Dawley white rats) models with surgical SNI (n = 6) were prepared, and naive rats (n = 5) were used as controls. The expression levels of CBP and MeCP2 in the spinal cord and dorsal root ganglion (DRG) were compared through immunohistochemistry at 7 and 14 days after surgery. The relationship between neuropathic pain and CBP/MeCP2 was also analyzed through intrathecal siRNA administration. Results: SNI induced a significant increase in the number of CBPs in L4 compared with contralateral DRG as well as with naive rats. The number of MeCP2 cells in the dorsal horn on the ipsilateral side decreased significantly compared with the contralateral dorsal horn and the control group. SNI induced a significant decrease in the number of MeCP2 neurons in the L4 ipsilateral DRG compared with the contralateral DRG and naive rats. The intrathecal injection of CBP siRNA significantly inhibited mechanical allodynia induced by SNI compared with non-targeting siRNA treatment. MeCP2 siRNA injection showed no significant effect on mechanical allodynia. Conclusions: The results suggest that CBP and MeCP2 may play an important role in the generation of neuropathic pain following peripheral nerve injury.

N4-acetylcytidine acetylation of neurexin 2 in the spinal dorsal horn regulates hypersensitivity in a rat model of cancer-induced bone pain

Cancer-induced bone pain (CIBP) significantly impacts the quality of life and survival of patients with advanced cancer. Despite the established role of neurexins in synaptic structure and function, their involvement in sensory processing during injury has not been extensively studied. In this study using a rat model of CIBP, we observed increased neurexin 2 expression in spinal cord neurons. Knockdown of neurexin 2 in the spinal cord reversed CIBP-related behaviors, sensitization of spinal c-Fos neurons, and pain-related negative emotional behaviors. Additionally, increased acetylation of neurexin 2 mRNA was identified in the spinal dorsal horn of CIBP rats. Decreasing the expression of N-acetyltransferase 10 (NAT10) reduced neurexin 2 mRNA acetylation and neurexin 2 expression. In PC12 cells, we confirmed that neurexin 2 mRNA acetylation enhanced its stability, and neurexin 2 expression was regulated by NAT10. Finally, we discovered that the NAT10/ac4C-neurexin 2 axis modulated neuronal synaptogenesis. This study demonstrated that the NAT10/ac4C-mediated posttranscriptional modulation of neurexin 2 expression led to the remodeling of spinal synapses and the development of conscious hypersensitivity in CIBP rats. Therefore, targeting the epigenetic modification of neurexin 2 mRNA ac4C may offer a new therapeutic approach for the treatment of nociceptive hypersensitivity in CIBP.

Trim14-IκBα Signaling Regulates Chronic Inflammatory Pain in Rats and Osteoarthritis Patients

Chronic inflammatory pain is the highest priority for people with osteoarthritis when seeking medical attention. Despite the availability of NSAIDs and glucocorticoids, central sensitization and peripheral sensitization make pain increasingly difficult to control. Previous studies have identified the ubiquitination system as an important role in the chronic inflammatory pain. Our study displayed that the E3 ubiquitin ligase tripartite motif-containing 14 (Trim14) was abnormally elevated in the serum of patients with osteoarthritis and pain, and the degree of pain was positively correlated with the degree of Trim14 elevation. Furthermore, CFA-induced inflammatory pain rat model showed that Trim14 was significantly increased in the L3-5 spinal dorsal horn (SDH) and dorsal root ganglion (DRG), and in turn the inhibitor of nuclear factor Kappa-B isoform α (IκBα) was decreased after Trim14 elevation. After intrathecal injection of Trim14 siRNA to inhibit Trim14 expression, IκBα expression was reversed and increased, and the pain behaviors and anxiety behaviors of rats were significantly relieved. Overall, these findings suggested that Trim14 may contribute to chronic inflammatory pain by degrading IκBα, and that Trim14 may become a novel therapeutic target for chronic inflammatory pain.

Whole-genome methylation profiling reveals regions associated with painful temporomandibular disorders and active recovery processes

ABSTRACT

Temporomandibular disorders (TMDs), collectively representing one of the most common chronic pain conditions, have a substantial genetic component, but genetic variation alone has not fully explained the heritability of TMD risk. Reasoning that the unexplained heritability may be because of DNA methylation, an epigenetic phenomenon, we measured genome-wide DNA methylation using the Illumina MethylationEPIC platform with blood samples from participants in the Orofacial Pain: Prospective Evaluation and Risk Assessment (OPPERA) study. Associations with chronic TMD used methylation data from 496 chronic painful TMD cases and 452 TMD-free controls. Changes in methylation between enrollment and a 6-month follow-up visit were determined for a separate sample of 62 people with recent-onset painful TMD. More than 750,000 individual CpG sites were examined for association with chronic painful TMD. Six differentially methylated regions were significantly ( P < 5 × 10 -8 ) associated with chronic painful TMD, including loci near genes involved in the regulation of inflammatory and neuronal response. A majority of loci were similarly differentially methylated in acute TMD consistent with observed transience or persistence of symptoms at follow-up. Functional characterization of the identified regions found relationships between methylation at these loci and nearby genetic variation contributing to chronic painful TMD and with gene expression of proximal genes. These findings reveal epigenetic contributions to chronic painful TMD through methylation of the genes FMOD , PM20D1 , ZNF718 , ZFP57 , and RNF39 , following the development of acute painful TMD. Epigenetic regulation of these genes likely contributes to the trajectory of transcriptional events in affected tissues leading to resolution or chronicity of pain.

Upregulation of lysine-specific demethylase 6B aggravates inflammatory pain through H3K27me3 demethylation-dependent production of TNF-α in the dorsal root ganglia and spinal dorsal horn in rats

AIMS

Lysine-specific demethylase 6B (KDM6B) serves as a key mediator of gene transcription. It regulates expression of proinflammatory cytokines and chemokines in variety of diseases. Herein, the role and the underlying mechanisms of KDM6B in inflammatory pain were studied.

METHODS

The inflammatory pain was conducted by intraplantar injection of complete Freund's adjuvant (CFA) in rats. Immunofluorescence, Western blotting, qRT-PCR, and chromatin immunoprecipitation (ChIP)-PCR were performed to investigate the underlying mechanisms.

RESULTS

CFA injection led to upregulation of KDM6B and decrease in the level of H3K27me3 in the dorsal root ganglia (DRG) and spinal dorsal horn. The mechanical allodynia and thermal hyperalgesia following CFA were alleviated by the treatment of intrathecal injection of GSK-J4, and by microinjection of AAV-EGFP-KDM6B shRNA in the sciatic nerve or in lumbar 5 dorsal horn. The increased production of tumor necrosis factor-α (TNF-α) following CFA in the DRGs and dorsal horn was inhibited by these treatments. ChIP-PCR showed that CFA-induced increased binding of nuclear factor κB with TNF-α promoter was repressed by the treatment of microinjection of AAV-EGFP-KDM6B shRNA.

CONCLUSIONS

These results suggest that upregulated KDM6B via facilitating TNF-α expression in the DRG and spinal dorsal horn aggravates inflammatory pain.

Narrative Review of the Complex Interaction between Pain and Trauma in Children: A Focus on Biological Memory, Preclinical Data, and Epigenetic Processes

The incidence and collective impact of early adverse experiences, trauma, and pain continue to increase. This underscores the urgent need for translational efforts between clinical and preclinical research to better understand the underlying mechanisms and develop effective therapeutic approaches. As our understanding of these issues improves from studies in children and adolescents, we can create more precise preclinical models and ultimately translate our findings back to clinical practice. A multidisciplinary approach is essential for addressing the complex and wide-ranging effects of these experiences on individuals and society. This narrative review aims to (1) define pain and trauma experiences in childhood and adolescents, (2) discuss the relationship between pain and trauma, (3) consider the role of biological memory, (4) decipher the relationship between pain and trauma using preclinical data, and (5) examine the role of the environment by introducing the importance of epigenetic processes. The ultimate scope is to better understand the wide-ranging effects of trauma, abuse, and chronic pain on children and adolescents, how they occur, and how to prevent or mitigate their effects and develop effective treatment strategies that address both the underlying causes and the associated physiological and psychological effects.

miR-1306-3p directly activates P2X3 receptors in primary sensory neurons to induce visceral pain in rats

ABSTRACT

Mounting evidence indicates that microRNAs (miRNAs) play critical roles in various pathophysiological conditions and diseases, but the physiological roles of extracellular miRNAs on the disease-related ion channels remain largely unknown. Here, we showed that miR-1306-3p evoked action potentials and induced inward currents of the acutely isolated rat dorsal root ganglion (DRG) neurons. The miR-1306-3p-induced effects were significantly inhibited by A317491, a potent inhibitor of the P2X3 receptor (P2X3R), or disappeared after the knockdown of P2X3Rs in DRG neurons. We further identified R180, K315, and R52 as the miR-1306-3p interaction sites on the extracellular domain of P2X3Rs, which were distinct from the orthosteric ATP-binding sites. Intrathecal injection of miR-1306-3p produced visceral pain but not somatic pain in normal control rats. Conversely, intrathecal application of a miR-1306-3p antagomir and A317491 significantly alleviated visceral pain in a rat model of chronic visceral pain. Together, our findings suggest that miR-1306-3p might function as an endogenous ligand to activate P2X3Rs, eventually leading to chronic visceral pain.

Intraganglionic reactive oxygen species mediate inflammatory pain and hyperalgesia through TRPA1 in the rat

Reactive oxygen species (ROS) are generated in nociceptive pathways in response to inflammation and injury. ROS are accumulated within the sensory ganglia following peripheral inflammation, but the functional role of intraganlionic ROS in inflammatory pain is not clearly understood. The aims of this study were to investigate whether peripheral inflammation leads to prolonged ROS accumulation within the trigeminal ganglia (TG), whether intraganglionic ROS mediate pain hypersensitivity via activation of TRPA1, and whether TRPA1 expression is upregulated in TG during inflammatory conditions by ROS. We demonstrated that peripheral inflammation causes excess ROS production within TG during the period when inflammatory mechanical hyperalgesia is most prominent. Additionally, scavenging intraganglionic ROS attenuated inflammatory mechanical hyperalgesia and a pharmacological blockade of TRPA1 localized within TG also mitigated inflammatory mechanical hyperalgesia. Interestingly, exogenous administration of ROS into TG elicited mechanical hyperalgesia and spontaneous pain-like responses via TRPA1, and intraganglionic ROS induced TRPA1 upregulation in TG. These results collectively suggest that ROS accumulation in TG during peripheral inflammation contributes to pain and hyperalgesia in a TRPA1 dependent manner, and that ROS further exacerbate pathological pain responses by upregulating TRPA1 expression. Therefore, any conditions that exacerbate ROS accumulation within somatic sensory ganglia can aggravate pain responses and treatments reducing ganglionic ROS may help alleviate inflammatory pain.

Transcriptomic, Epigenomic, and Neuroanatomic Signatures Differ in Chronic Prurigo, Atopic Dermatitis, and Brachioradial Pruritus

Scratching and scratch-induced injuries, including neuroanatomical alterations, are key characteristics of chronic pruritus entities of different origins. The aim of this study was to link gene expression (array hybridization, qPCR) with DNA methylation (array hybridization) and neuroanatomy (PGP9.5 staining) in chronic nodular prurigo (CNPG), atopic dermatitis (AD), brachioradial pruritus (BRP), and matched healthy controls. Specific signatures of gene expression and DNA methylation clearly discriminated pruritic lesional skin from nonpruritic skin in CNPG and from healthy skin of volunteers, respectively. Although intraepidermal nerve fiber density was indiscriminately reduced, the level of epidermal branching, assessed by a semiquantitative pattern analysis, differentiated the entities (CNPG > BRP > AD). Correspondingly, repellent SEMA3A showed the highest expression in AD, whereas axonal growth-promoting nerve GF was most prominent in CNPG and BRP. Overexpression of genes for nerve fiber regeneration (NELL2/NFKB/ARTN) was found in AD and CNPG but not in BRP. Our findings suggest that differential branching patterns rather than mere innervation density separate chronic itch conditions and reflect disease-specific local expression profiles. In pruritic dermatoses (AD and CNPG), nerve injury and subsequent sprouting may primarily result from chronic scratching, whereas genuine neuropathy is expected to underlie BRP.

Hydrogen Sulfide Biomedical Research in China-20 Years of Hindsight

Hydrogen sulfide (H2S) is an important gasotransmitter that is produced by mammalian cells and performs profound physiological and pathophysiological functions. Biomedical research on H2S metabolism and function in China began 20 years ago, which pioneered the examination of the correlation of abnormal H2S metabolism and cardiovascular diseases. Over the last two decades, research teams in China have made numerous breakthrough discoveries on the effects of H2S metabolism on hypertension, atherosclerosis, pulmonary hypertension, shock, angiogenesis, chronic obstructive pulmonary disease, pain, iron homeostasis, and testicle function, to name a few. These research developments, carried by numerous research teams all over China, build nationwide research network and advance both laboratory study and clinical applications. An integrated and collaborative research strategy would further promote and sustain H2S biomedical research in China and in the world.

Role of Epigenetic Mechanisms in Chronic Pain

Pain is an unpleasant but essential-to-life sensation, usually resulting from tissue damage. When pain persists long after the injury has resolved, it becomes pathological. The precise molecular and cellular mechanisms causing the transition from acute to chronic pain are not fully understood. A key aspect of pain chronicity is that several plasticity events happen along the neural pathways involved in pain. These long-lasting adaptive changes are enabled by alteration in the expression of relevant genes. Among the different modulators of gene transcription in adaptive processes in the nervous system, epigenetic mechanisms play a pivotal role. In this review, I will first outline the main classes of epigenetic mediators and then discuss their implications in chronic pain.

Genetic and epigenetic mechanisms influencing acute to chronic postsurgical pain transitions in pediatrics: Preclinical to clinical evidence

Background

Chronic postsurgical pain (CPSP) in children remains an important problem with no effective preventive or therapeutic strategies. Recently, genomic underpinnings explaining additional interindividual risk beyond psychological factors have been proposed.

Aims

We present a comprehensive review of current preclinical and clinical evidence for genetic and epigenetic mechanisms relevant to pediatric CPSP.

Methods

Narrative review.

Results

Animal models are relevant to translational research for unraveling genomic mechanisms. For example, Cacng2, p2rx7, and bdnf mutant mice show altered mechanical hypersensitivity to injury, and variants of the same genes have been associated with CPSP susceptibility in humans; similarly, differential DNA methylation (H1SP) and miRNAs (miR-96/7a) have shown translational implications. Animal studies also suggest that crosstalk between neurons and immune cells may be involved in nociceptive priming observed in neonates. In children, differential DNA methylation in regulatory genomic regions enriching GABAergic, dopaminergic, and immune pathways, as well as polygenic risk scores for enhanced prediction of CPSP, have been described. Genome-wide studies in pediatric CPSP are scarce, but pathways identified by adult gene association studies point to potential common mechanisms.

Conclusions

Bench-to-bedside genomics research in pediatric CPSP is currently limited. Reverse translational approaches, use of other -omics, and inclusion of pediatric/CPSP endophenotypes in large-scale biobanks may be potential solutions. Time of developmental vulnerability and longitudinal genomic changes after surgery warrant further investigation. Emergence of promising precision pain management strategies based on gene editing and epigenetic programing emphasize need for further research in pediatric CPSP-related genomics.

Emerging roles of cystathionine β-synthase in various forms of cancer

The expression of the reverse transsulfuration enzyme cystathionine-β-synthase (CBS) is markedly increased in many forms of cancer, including colorectal, ovarian, lung, breast and kidney, while in other cancers (liver cancer and glioma) it becomes downregulated. According to the clinical database data in high-CBS-expressor cancers (e.g. colon or ovarian cancer), high CBS expression typically predicts lower survival, while in the low-CBS-expressor cancers (e.g. liver cancer), low CBS expression is associated with lower survival. In the high-CBS expressing tumor cells, CBS, and its product hydrogen sulfide (H₂S) serves as a bioenergetic, proliferative, cytoprotective and stemness factor; it also supports angiogenesis and epithelial-to-mesenchymal transition in the cancer microenvironment. The current article reviews the various tumor-cell-supporting roles of the CBS/H₂S axis in high-CBS expressor cancers and overviews the anticancer effects of CBS silencing and pharmacological CBS inhibition in various cancer models in vitro and in vivo; it also outlines potential approaches for biomarker identification, to support future targeted cancer therapies based on pharmacological CBS inhibition.

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.

The Role of H2S in the Gastrointestinal Tract and Microbiota

The pathways and mechanisms of the production of H₂S in the gastrointestinal tract are briefly described, including endogenous H₂S produced by the organism and H₂S from microorganisms in the gastrointestinal tract. In addition, the physiological regulatory functions of H₂S on gastrointestinal motility, sensation, secretion and absorption, endocrine system, proliferation and differentiation of stem cells, and the possible mechanisms involved are introduced. In view of the complexity of biosynthesis, physiological roles, and the mechanism of H₂S, this chapter focuses on the interactions and dynamic balance among H₂S, gastrointestinal microorganisms, and the host. Finally, we focus on some clinical gastrointestinal diseases, such as inflammatory bowel disease, colorectal cancer, functional gastrointestinal disease, which might occur or develop when the above balance is broken. Pharmacological regulation of H₂S or the intestinal microorganisms related to H₂S might provide new therapeutic approaches for some gastrointestinal diseases.

Methyltransferase-like 3 contributes to inflammatory pain by targeting TET1 in YTHDF2-dependent manner

ABSTRACT

The methyltransferase-like 3 (Mettl3) is a key component of the large N6-adenosine-methyltransferase complex in mammalian responsible for RNA N6-methyladenosine (m6A) modification, which plays an important role in gene post-transcription modulation. Although RNA m6A is enriched in mammalian neurons, its regulatory function in nociceptive information processing remains elusive. Here, we reported that Complete Freund's Adjuvant (CFA)-induced inflammatory pain significantly decreased global m6A level and m6A writer Mettl3 in the spinal cord. Mimicking this decease by knocking down or conditionally deleting spinal Mettl3 elevated the levels of m6A in ten-eleven translocation methylcytosine dioxygenases 1 (Tet1) mRNA and TET1 protein in the spinal cord, leading to production of pain hypersensitivity. By contrast, overexpressing Mettl3 reversed a loss of m6A in Tet1 mRNA and blocked the CFA-induced increase of TET1 in the spinal cord, resulting in the attenuation of pain behavior. Furthermore, the decreased level of spinal YT521-B homology domain family protein 2 (YTHDF2), an RNA m6A reader, stabilized upregulation of spinal TET1 because of the reduction of Tet1 mRNA decay by the binding to m6A in Tet1 mRNA in the spinal cord after CFA. This study reveals a novel mechanism for downregulated spinal cord METTL3 coordinating with YTHDF2 contributes to the modulation of inflammatory pain through stabilizing upregulation of TET1 in spinal neurons.

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.

Oxidative, Reductive, and Nitrosative Stress Effects on Epigenetics and on Posttranslational Modification of Enzymes in Cardiometabolic Diseases

Oxidative (OS), reductive (RS), and nitrosative (NSS) stresses produce carbonylation, glycation, glutathionylation, sulfhydration, nitration, and nitrosylation reactions. OS, RS, and NSS are interrelated since RS results from an overactivation of antioxidant systems and NSS is the result of the overactivation of the oxidation of nitric oxide (NO). Here, we discuss the general characteristics of the three types of stress and the way by which the reactions they induce (a) damage the DNA structure causing strand breaks or inducing the formation of 8-oxo-d guanosine; (b) modify histones; (c) modify the activities of the enzymes that determine the establishment of epigenetic cues such as DNA methyl transferases, histone methyl transferases, acetyltransferases, and deacetylases; (d) alter DNA reparation enzymes by posttranslational mechanisms; and (e) regulate the activities of intracellular enzymes participating in metabolic reactions and in signaling pathways through posttranslational modifications. Furthermore, the three types of stress may establish new epigenetic marks through these reactions. The development of cardiometabolic disorders in adult life may be programed since early stages of development by epigenetic cues which may be established or modified by OS, RS, and NSS. Therefore, the three types of stress participate importantly in mediating the impact of the early life environment on later health and heritability. Here, we discuss their impact on cardiometabolic diseases. The epigenetic modifications induced by these stresses depend on union and release of chemical residues on a DNA sequence and/or on amino acid residues in proteins, and therefore, they are reversible and potentially treatable.

Time course of DNA methylation in pain conditions: From experimental models to humans

BACKGROUND AND OBJECTIVE

Throughout the last decade, research has uncovered associations between pain and epigenetic alterations caused by environmental factors. Specifically, studies have demonstrated correlations between pain conditions and altered DNA methylation patterns. Thus, DNA methylation has been revealed as a possible modulator or contributor to pain conditions, providing a potential therapeutic target for treatment by DNA methylation modification. To develop such treatments, it is necessary to clarify a wide number of aspects on how DNA methylation affects pain perception; first and foremost, the temporal dynamics. The objective of the present review is to provide an overview of current knowledge on temporal dynamics of DNA methylation in response to pain, and to investigate if a timeframe can be established based on the data of currently published studies.

DATABASES AND DATA TREATMENT

PubMed, MEDLINE, Google Scholar and Embase were searched comprehensively for studies of DNA methylation in neuropathic, inflammatory and alternative animal pain models, and in chronic pain patients including Complex Regional Pain Syndrome, chronic postsurgical pain, chronic widespread pain, fibromyalgia and Crohn's disease.

RESULTS

We identified 34 articles highlighting variations in temporal dynamics of DNA methylation across species and between different types of pain. These studies represent a starting point to uncover new insights in the DNA methylation time course in pain.

CONCLUSIONS

No timeframe can currently be made for the DNA methylation response to pain in any of the reviewed conditions, highlighting an important focus area for future research.

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.

Adult Stress Promotes Purinergic Signaling to Induce Visceral Pain in Rats with Neonatal Maternal Deprivation

Chronic visceral pain is one of the primary symptoms of patients with irritable bowel syndrome (IBS), which affects up to 15% of the population world-wide. The detailed mechanisms of visceral pain remain largely unclear. Our previous studies have shown that neonatal maternal deprivation (NMD) followed by adult multiple stress (AMS) advances the occurrence of visceral pain, likely due to enhanced norepinephrine (NE)-β₂ adrenergic signaling. This study was designed to explore the roles of P2X3 receptors (P2X3Rs) in the chronic visceral pain induced by combined stress. Here, we showed that P2X3Rs were co-expressed in β₂ adrenergic receptor (β₂-AR)-positive dorsal root ganglion neurons and that NE significantly enhanced ATP-induced Ca²⁺ signals. NMD and AMS not only significantly increased the protein expression of P2X3Rs, but also greatly enhanced the ATP-evoked current density, number of action potentials, and intracellular Ca²⁺ concentration of colon-related DRG neurons. Intrathecal injection of the P2X3R inhibitor A317491 greatly attenuated the visceral pain and the ATP-induced Ca²⁺ signals in NMD and AMS rats. Furthermore, the β₂-AR antagonist butoxamine significantly reversed the expression of P2X3Rs, the ATP-induced current density, and the number of action potentials of DRG neurons. Overall, our data demonstrate that NMD followed by AMS leads to P2X3R activation, which is most likely mediated by upregulation of β₂ adrenergic signaling in primary sensory neurons, thus contributing to visceral hypersensitivity.

CBS-Induced H2S Generation in Hippocampus Inhibits EA-Induced Analgesia

Hydrogen sulfide (H₂S) is an important mediator participating in both physiological and pathological systems and related to the inflammatory process. Acupuncture has a therapeutic effect on inflammatory pain. However, whether H₂S generated in the central nervous system (CNS) is a mediator of electroacupuncture (EA) treatment for inflammatory pain is unknown. We injected complete Freund's adjuvant (CFA) to induce inflammatory pain and applied EA treatment as an interventional strategy for pain relief. The results presented here show that S-adenosyl-l-methionine (SAM), an allosteric activator of cystathionine-β-synthetase (CBS), may reverse the therapeutic effect of EA. CBS-induced H₂S generation might get involved in the mechanism of EA-induced analgesia in the hippocampus on chronic inflammatory pain.

Cystathionine-β-Synthase: Molecular Regulation and Pharmacological Inhibition

Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used "CBS inhibitors" (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

Hypomethylation of nerve growth factor (NGF) promotes binding of C/EBPα and contributes to inflammatory hyperalgesia in rats

Background

Chronic pain usually accompanied by tissue damage and inflammation. However, the pathogenesis of chronic pain remains unclear.

Methods

We investigated the role of nerve growth factor (NGF) in chronic inflammatory pain induced by complete Freund’s adjuvant (CFA), explored the methylation status of CpG islands in the promoter region of the NGF gene, and clarified the function and mechanism of C/EBPα-NGF signaling pathway from epigenetic perspective in the chronic inflammatory pain model.

Results

CFA induced significant hyperalgesia and continuous upregulation of NGF mRNA and protein levels in the L4–6 dorsal root ganglions (DRGs) in rats. Hypomethylation of CpG islands occurred in the NGF gene promoter region after CFA treatment. At the same time, the miR-29b expression level was significantly increased, while the DNA methyltransferase 3b (DNMT3b) level reduced significantly. Moreover, CFA treatment promoted binding of C/EBPα to the NGF gene promoter region and C/EBPα siRNA treatment obviously decreased expression of NGF levels and also alleviate inflammatory hyperalgesia significantly in rats.

Conclusion

Collectively, the results indicated that CFA leads to the upregulation of miR-29b level, which represses the expression of DNMT3b, enhances the demethylation of the NGF gene promoter region, and promotes the binding of C/EBPα with the NGF gene promoter, thus results in the upregulation of NGF gene expression and maintenance of chronic inflammatory pain.

Hypomethylation of nerve growth factor (NGF) promotes binding of C/EBPα and contributes to inflammatory hyperalgesia in rats

BACKGROUND

Chronic pain usually accompanied by tissue damage and inflammation. However, the pathogenesis of chronic pain remains unclear.

METHODS

We investigated the role of nerve growth factor (NGF) in chronic inflammatory pain induced by complete Freund's adjuvant (CFA), explored the methylation status of CpG islands in the promoter region of the NGF gene, and clarified the function and mechanism of C/EBPα-NGF signaling pathway from epigenetic perspective in the chronic inflammatory pain model.

RESULTS

CFA induced significant hyperalgesia and continuous upregulation of NGF mRNA and protein levels in the L4-6 dorsal root ganglions (DRGs) in rats. Hypomethylation of CpG islands occurred in the NGF gene promoter region after CFA treatment. At the same time, the miR-29b expression level was significantly increased, while the DNA methyltransferase 3b (DNMT3b) level reduced significantly. Moreover, CFA treatment promoted binding of C/EBPα to the NGF gene promoter region and C/EBPα siRNA treatment obviously decreased expression of NGF levels and also alleviate inflammatory hyperalgesia significantly in rats.

CONCLUSION

Collectively, the results indicated that CFA leads to the upregulation of miR-29b level, which represses the expression of DNMT3b, enhances the demethylation of the NGF gene promoter region, and promotes the binding of C/EBPα with the NGF gene promoter, thus results in the upregulation of NGF gene expression and maintenance of chronic inflammatory pain.

Proteomic Biomarkers of Retinal Inflammation in Diabetic Retinopathy

Diabetic retinopathy (DR), a sight-threatening neurovasculopathy, is the leading cause of irreversible blindness in the developed world. DR arises as the result of prolonged hyperglycemia and is characterized by leaky retinal vasculature, retinal ischemia, retinal inflammation, angiogenesis, and neovascularization. The number of DR patients is growing with an increase in the elderly population, and therapeutic approaches are limited, therefore, new therapies to prevent retinal injury and enhance repair are a critical unmet need. Besides vascular endothelial growth factor (VEGF)-induced vascular proliferation, several other mechanisms are important in the pathogenesis of diabetic retinopathy, including vascular inflammation. Thus, combining anti-VEGF therapy with other new therapies targeting these pathophysiological pathways of DR may further optimize treatment outcomes. Technological advancements have allowed for high-throughput proteomic studies examining biofluids such as aqueous humor, vitreous humor, tear, and serum. Many DR biomarkers have been identified, especially proteins involved in retinal inflammatory processes. This review attempts to summarize the proteomic biomarkers of DR-associated retinal inflammation identified over the last several years.

Pharmacogenomics in chronic pain therapy: from disease to treatment and challenges for clinical practice

Pharmacogenomics (PGx) has emerged as an encouraging tool in chronic pain therapy. Genetic variations associated with drug effectiveness or adverse reactions (amitriptyline/nortriptyline/codeine/oxycodone/tramadol-CYP2D6, amitriptyline-CYP2C19, carbamazepine-HLA-A, carbamazepine/oxcarbazepine-HLA-B) can be used to guide chronic pain management. Despite this evidence, many obstacles still need to be overcome for the effective clinical implementation of PGx. To translate the pharmacogenetic testing into actionable clinical decisions, the Clinical Pharmacogenetics Implementation Consortium has been developing guidelines for several drug–gene pairs. This review will show the applicability of PGx in chronic pain from disease to treatment; report the drug–gene pairs with strongest evidences in the clinic; and the challenges for the clinical implementation of PGx.

Social Environment and Epigenetics

Our social environment, from the microscopic to the macro-social, affects us for the entirety of our lives. One integral line of research to examine how interpersonal and societal environments can get "under the skin" is through the lens of epigenetics. Epigenetic mechanisms are adaptations made to our genome in response to our environment which include tags placed on and removed from the DNA itself to how our DNA is packaged, affecting how our genes are read, transcribed, and interact. These tags are affected by social environments and can persist over time; this may aid us in responding to experiences and exposures, both the enriched and the disadvantageous. From memory formation to immune function, the experience-dependent plasticity of epigenetic modifications to micro- and macro-social environments may contribute to the process of learning from comfort, pain, and stress to better survive in whatever circumstances life has in store.

Hydrogen Sulfide Donor NaHS Improves Metabolism and Reduces Muscle Atrophy in Type 2 Diabetes: Implication for Understanding Sarcopenic Pathophysiology

Sarcopenia, a loss of muscle mass and functionality, constitutes a major contributor to disability in diabetes. Hydrogen sulfide (H₂S) dynamics and muscle mass regulatory signaling were studied in GK rats, a model for type 2 diabetes (T2D). GK rats exhibited a number of features that are consistent with sarcopenia and T2D including loss of muscle mass and strength, in addition to glucose intolerance, insulin resistance, and impaired β-cell responsiveness to glucose. Mechanistically, activation levels of Akt, a key modulator of protein balance, were decreased in T2D. Consequently, we confirmed reduced activity of mTOR signaling components and higher expression of atrophy-related markers typified by FoxO1/atrogin-1/MuRF1 and myostatin-Smad2/3 signaling during the course of diabetes. We observed in GK rat reduced antioxidant capacity (↓GSH/GSSG) and increased expression and activity of NADPH oxidase in connection with augmented rate of oxidation of lipids, proteins, and DNA. H₂S bioavailability and the expression of key enzymes involved in its synthesis were suppressed as a function of diabetes. Interestingly, GK rats receiving NaHS displayed increased muscle Akt/mTOR signaling and decreased expression of myostatin and the FoxO1/MuRF1/atrogin-dependent pathway. Moreover, diabetes-induced heightened state of oxidative stress was also ameliorated in response to NaHS therapy. Overall, the current data support the notion that a relationship exists between sarcopenia, heightened state of oxidative stress, and H₂S deficiency at least in the context of diabetes. Moreover, treatment with a potent H₂S donor at an early stage of diabetes is likely to mitigate the development of sarcopenia/frailty and predictably reduces its devastating sequelae of amputation.

Upregulation of CXCR4 through promoter demethylation contributes to inflammatory hyperalgesia in rats

AIM AND METHODS

Chronic pain associated with inflammation is a common clinical problem, and the underlying mechanisms yet are incompletely defined. DNA methylation has been implicated in the pathogenesis of chronic pain. However, the specific genes regulated by DNA methylation under inflammatory pain condition remain largely unknown. Here, we investigated how chemokine receptor CXCR4 expression is regulated by DNA methylation and how it contributes to inflammatory pain induced by complete Freund's adjuvant (CFA) in rats.

RESULTS

Intraplantar injection of CFA could not only induce significant hyperalgesia in rats, but also significantly increase the expression of CXCR4 mRNA and protein in the dorsal root ganglion (DRG). Intrathecal injection of CXCR4 antagonist AMD3100 significantly relieved hyperalgesia in inflammatory rats in a time- and dose-dependent manner. Bisulfite sequencing and methylation-specific PCR demonstrate that CFA injection led to a significant demethylation of CpG island at CXCR4 gene promoter. Consistently, the expression of DNMT3b was significantly downregulated after CFA injection. Online software prediction reveals three binding sites of p65 in the CpG island of CXCR4 gene promoter, which has confirmed by the chromatin immunoprecipitation assay, CFA treatment significantly increases the recruitment of p65 to CXCR4 gene promoter. Inhibition of NF-kB signaling using p65 inhibitor pyrrolidine dithiocarbamate significantly prevented the increases of the CXCR4 expression.

CONCLUSION

Upregulation of CXCR4 expression due to promoter demethylation followed by increased recruitment of p65 to promoter of CXCR4 gene contributes to inflammatory hyperalgesia. These findings provide a theoretical basis for the treatment of chronic pain from an epigenetic perspective.

MBD1 Contributes to the Genesis of Acute Pain and Neuropathic Pain by Epigenetic Silencing of Oprm1 and Kcna2 Genes in Primary Sensory Neurons

The transmission of normal sensory and/or acute noxious information requires intact expression of pain-associated genes within the pain pathways of nervous system. Expressional changes of these genes after peripheral nerve injury are also critical for neuropathic pain induction and maintenance. Methyl-CpG-binding domain protein 1 (MBD1), an epigenetic repressor, regulates gene transcriptional activity. We report here that MBD1 in the primary sensory neurons of DRG is critical for the genesis of acute pain and neuropathic pain as DRG MBD1-deficient mice exhibit the reduced responses to acute mechanical, heat, cold, and capsaicin stimuli and the blunted nerve injury-induced pain hypersensitivities. Furthermore, DRG overexpression of MBD1 leads to spontaneous pain and evoked pain hypersensitivities in the WT mice and restores acute pain sensitivities in the MBD1-deficient mice. Mechanistically, MDB1 represses Oprm1 and Kcna2 gene expression by recruiting DNA methyltransferase DNMT3a into these two gene promoters in the DRG neurons. DRG MBD1 is likely a key player under the conditions of acute pain and neuropathic pain.SIGNIFICANCE STATEMENT In the present study, we revealed that the mice with deficiency of methyl-CpG-binding domain protein 1 (MBD1), an epigenetic repressor, in the DRG displayed the reduced responses to acute noxious stimuli and the blunted neuropathic pain. We also showed that DRG overexpression of MBD1 produced the hypersensitivities to noxious stimuli in the WT mice and rescued acute pain sensitivities in the MBD1-deficient mice. We have also provided the evidence that MDB1 represses Oprm1 and Kcna2 gene expression by recruiting DNA methyltransferase DNMT3a into these two gene promoters in the DRG neurons. DRG MBD1 may participate in the genesis of acute pain and neuropathic pain likely through regulating DNMT3a-controlled Oprm1 and Kcna2 gene expression in the DRG neurons.

Cystathionine β-Synthase in Physiology and Cancer

Cystathionine β-synthase (CBS) regulates homocysteine metabolism and contributes to hydrogen sulfide (H₂S) biosynthesis through which it plays multifunctional roles in the regulation of cellular energetics, redox status, DNA methylation, and protein modification. Inactivating mutations in CBS contribute to the pathogenesis of the autosomal recessive disease CBS-deficient homocystinuria. Recent studies demonstrating that CBS promotes colon and ovarian cancer growth in preclinical models highlight a newly identified oncogenic role for CBS. On the contrary, tumor-suppressive effects of CBS have been reported in other cancer types, suggesting context-dependent roles of CBS in tumor growth and progression. Here, we review the physiological functions of CBS, summarize the complexities regarding CBS research in oncology, and discuss the potential of CBS and its key metabolites, including homocysteine and H₂S, as potential biomarkers for cancer diagnosis or therapeutic targets for cancer treatment.

Prenatal maternal stress induces visceral hypersensitivity of adult rat offspring through activation of cystathionine-β-synthase signaling in primary sensory neurons

Irritable bowel syndrome is a disorder of unknown etiology characterized by widespread, chronic abdominal pain associated with altered bowel movements. Increasing amounts of evidence indicate that stressors presented during gestational periods could have long-term effects on the offspring's tissue structure and function, which may predispose to gastrointestinal diseases. The aim of the present study is to determine whether prenatal maternal stressis a adverse factor affecting gastrointestinal sensitivity and to investigate possible mechanisms underlying prenatal maternal stress-induced visceral hypersensitivity in adult offspring. Prenatal maternal stress was induced in pregnant Sprague-Dawley rats by exposure to heterotypic intermitent stress from gestational day 7 to delivery. Prenatal maternal stress significantly increased visceromotor response to colorectal distention in adult offspring from the age of 6 weeks to 10 weeks. Prenatal maternal stress also enhanced neuronal excitability including depolarization of resting membrane potentials, reduction in rheobase, and an increase in the number of action potentials evoked by 2× and 3× rheobase current stimultion of colon-specific dorsal root ganglion neurons. Prenatal maternal stress remarkably enhanced expression of cystathionine-β-synthase and Nav1.7 in T13-L2 thoracolumbar dorsal root ganglions both at protein and mRNA levels. Intraperitoneal injection of aminooxyacetic acid, an inhibitor of cystathionine-β-synthase, attenuated prenatal maternal stress-induced visceral hypersensitivity in a dose-dependent manner. A consecutive seven-day administration of aminooxyacetic acid reversed the hyperexcitability of colon-specific dorsal root ganglion neurons and markedly reduced Nav1.7 expression. These results indicate that the presence of multiple psychophysical stressors during pregnancy is associated with visceral hypersensitivity in offspring, which is likely mediated by an upregualtion of cystathionine-β-synthase and Nav1.7 expression. Prenatal maternal stress might be a significant contributor to irritable bowel syndrome, and cystathionine-β-synthase might be a potential target for treatment for chronic visceral hypersensitivity in patients with irritable bowel syndrome.

Melatonin impedes Tet1-dependent mGluR5 promoter demethylation to relieve pain

Melatonin (N-acetyl-5-methoxytryptamine)/MT2 receptor-dependent epigenetic modification represents a novel pathway in the treatment of neuropathic pain. Because spinal ten-eleven translocation methylcytosine dioxygenase 1 (Tet1)-dependent epigenetic demethylation has recently been linked to pain hypersensitivity, we hypothesized that melatonin/MT2-dependent analgesia involves spinal Tet1-dependent demethylation. Here, we showed that spinal Tet1 gene transfer by intrathecal delivery of Tet1-encoding vectors to naïve rats produced profound and long-lasting nociceptive hypersensitivity. In addition, enhanced Tet1 expression, Tet1-metabotropic glutamate receptor subtype 5 (mGluR5) promoter coupling, demethylation at the mGluR5 promoter, and mGluR5 expression in dorsal horn neurons were observed. Rats subjected to spinal nerve ligation and intraplantar complete Freund's adjuvant injection displayed tactile allodynia and behavioral hyperalgesia associated with similar changes in the dorsal horn. Notably, intrathecal melatonin injection reversed the protein expression, protein-promoter coupling, promoter demethylation, and pain hypersensitivity induced by Tet1 gene transfer, spinal nerve ligation, and intraplantar complete Freund's adjuvant injection. All the effects caused by melatonin were blocked by pretreatment with a MT2 receptor-selective antagonist. In conclusion, melatonin relieves pain by impeding Tet1-dependent demethylation of mGluR5 in dorsal horn neurons through the MT2 receptor. Our findings link melatonin/MT2 signaling to Tet1-dependent epigenetic demethylation of nociceptive genes for the first time and suggest melatonin as a promising therapy for the treatment of pain.

DNMT3a methylation in neuropathic pain

Background

Mu opioid receptor (MOR) plays a crucial role in mediating analgesic effects of opioids and is closely associated with the pathologies of neuropathic pain. Previous studies have reported that peripheral nerve injury downregulates MOR expression, but the epigenetic mechanisms remain unknown.

Objective

Therefore, we investigated DNA methyltransferase3a (DNMT3a) expression or methylation changes within MOR promoter in the spinal cord in a neuropathic pain induced by a chronic constriction injury (CCI) mouse model and further determined whether these injury-associated changes are reversible by pharmacological interventions.

Methods

A CCI mouse model was established and tissue specimens of lumbar spinal cords were collected. The nociception threshold was evaluated by a Model Heated 400 Base. DNMT3a and MOR mRNA and protein level were detected by real-time-polymerase chain reaction and Western blot, respectively. Methylation of DNMT3a gene was measured by methylation-specific PCR.

Results

Our data showed that chronic nerve injury led to a significant upregulation of DNMT3a expression that was associated with increased methylation of MOR gene promoter and decreased MOR protein expression in the spinal cord. Inhibition of DNMT3a catalytic activity with DNMT inhibitor RG108 significantly blocked the increase in methylation of the MOR promoter, and then upregulated MOR expression and attenuated thermal hyperalgesia in neuropathic pain mice.

Conclusion

This study demonstrates that an increase of DNMT3a expression and MOR methylation epigenetically play an important role in neuropathic pain. Targeting DNMT3a to the promoter of MOR gene by DNMT inhibitor may be a promising approach to the development of new neuropathic pain therapy.

Promoted Interaction of C/EBPα with Demethylated Cxcr3 Gene Promoter Contributes to Neuropathic Pain in Mice

DNA methylation has been implicated in the pathogenesis of chronic pain. However, the specific genes regulated by DNA methylation under neuropathic pain condition remain largely unknown. Here we investigated how chemokine receptor CXCR3 is regulated by DNA methylation and how it contributes to neuropathic pain induced by spinal nerve ligation (SNL) in mice. SNL increased Cxcr3 mRNA and protein expression in the neurons of the spinal cord. Meanwhile, the CpG (5'-cytosine-phosphate-guanine-3') island in the Cxcr3 gene promoter region was demethylated, and the expression of DNA methyltransferase 3b (DNMT3b) was decreased. SNL also increased the binding of CCAAT (cytidine-cytidine-adenosine-adenosine-thymidine)/enhancer binding protein α (C/EBPα) with Cxcr3 promoter and decreased the binding of DNMT3b with Cxcr3 promoter in the spinal cord. C/EBPα expression was increased in spinal neurons after SNL, and inhibition of C/EBPα by intrathecal small interfering RNA attenuated SNL-induced pain hypersensitivity and reduced Cxcr3 expression. Furthermore, SNL-induced mechanical allodynia and heat hyperalgesia were markedly reduced in Cxcr3^-/- mice. Spinal inhibition of Cxcr3 by shRNA or CXCR3 antagonist also attenuated established neuropathic pain. Moreover, CXCL10, the ligand of CXCR3, was increased in spinal neurons and astrocytes after SNL. Superfusing spinal cord slices with CXCL10 enhanced spontaneous EPSCs and potentiated NMDA-induced and AMPA-induced currents of lamina II neurons. Finally, intrathecal injection of CXCL10 induced CXCR3-dependent pain hypersensitivity in naive mice. Collectively, our results demonstrated that CXCR3, increased by DNA demethylation and the enhanced interaction with C/EBPα, can be activated by CXCL10 to facilitate excitatory synaptic transmission and contribute to the maintenance of neuropathic pain.

SIGNIFICANCE STATEMENT

Peripheral nerve injury induces changes of gene expression in the spinal cord that may contribute to the pathogenesis of neuropathic pain. CXCR3 is a chemokine receptor. Whether it is involved in neuropathic pain and how it is regulated after nerve injury remain largely unknown. Our study demonstrates that spinal nerve ligation downregulates the expression of DNMT3b, which may cause demethylation of Cxcr3 gene promoter and facilitate the binding of CCAAT/enhancer binding protein α with Cxcr3 promoter and further increase CXCR3 expression in spinal neurons. The upregulated CXCR3 may contribute to neuropathic pain by facilitating central sensitization. Our study reveals an epigenetic mechanism underlying CXCR3 expression and also suggests that targeting the expression or activation of CXCR3 signaling may offer new therapeutics for neuropathic pain.

Protein Kinase C Mediates the Corticosterone-induced Sensitization of Dorsal Root Ganglion Neurons Innervating the Rat Stomach

BACKGROUND/AIMS

Gastric hypersensitivity contributes to abdominal pain in patients with functional dyspepsia. Recent studies showed that hormones induced by stress are correlated with visceral hypersensitivity. However, the precise mechanisms underlying gastric hypersensitivity remain largely unknown. The aim of the present study was designed to investigate the roles of corticosterone (CORT) on excitability of dorsal root ganglion (DRG) neurons innervating the stomach.

METHODS

DRG neurons innervating the stomach were labeled by DiI injection into the stomach wall. Patch clamp recordings were employed to examine neural excitability and voltage-gated sodium channel currents. Electromyograph technique was used to determine the responses of neck muscles to gastric distension.

RESULTS

Incubation of acutely isolated DRG neurons with CORT significantly depolarized action potential threshold and enhanced the number of action potentials induced by current stimulation of the neuron. Under voltage-clamp mode, incubation of CORT enhanced voltage-gated sodium current density of the recorded neurons. Pre-incubation of GF109203X, an inhibitor of protein kinase C, blocked the CORT-induced hyperexcitability and potentiation of sodium currents. However, pre-incubation of H-89, an inhibitor of protein kinase A, did not alter the sodium current density. More importantly, intraperitoneal injection of CORT produced gastric hypersensitivity of healthy rats, which was blocked by pre-administration of GF109203X but not H-89.

CONCLUSIONS

Our data strongly suggest that CORT rapidly enhanced neuronal excitability and sodium channel functions, which is most likely mediated by protein kinase C but not protein kinase A signaling pathway in DRG neurons innervating the stomach, thus underlying the gastric hypersensitivity induced by CORT injection.

Receptor Mechanisms Mediating the Pro-Nociceptive Action of Hydrogen Sulfide in Rat Trigeminal Neurons and Meningeal Afferents

Hydrogen sulfide (H₂S), a well-established member of the gasotransmitter family, is involved in a variety of physiological functions, including pro-nociceptive action in the sensory system. Although several reports have shown that H₂S activates sensory neurons, the molecular targets of H₂S action in trigeminal (TG) nociception, implicated in migraine, remains controversial. In this study, using suction electrode recordings, we investigate the effect of the H₂S donor, sodium hydrosulfide (NaHS), on nociceptive firing in rat meningeal TG nerve fibers. The effect of NaHS was also explored with patch-clamp and calcium imaging techniques on isolated TG neurons. NaHS dramatically increased the nociceptive firing in TG nerve fibers. This effect was abolished by the TRPV1 inhibitor capsazepine but was partially prevented by the TRPA1 blocker HC 030031. In a fraction of isolated TG neurons, NaHS transiently increased amplitude of capsaicin-induced currents. Moreover, NaHS by itself induced inward currents in sensory neurons, which were abolished by the TRPV1 inhibitor capsazepine suggesting involvement of TRPV1 receptors. In contrast, the inhibitor of TRPA1 receptors HC 030031 did not prevent the NaHS-induced currents. Imaging of a large population of TG neurons revealed that NaHS induced calcium transients in 41% of tested neurons. Interestingly, this effect of NaHS in some neurons was inhibited by the TRPV1 antagonist capsazepine whereas in others it was sensitive to the TRPA1 blocker HC 030031. Our data suggest that both TRPV1 and TRPA1 receptors play a role in the pro-nociceptive action of NaHS in peripheral TG nerve endings in meninges and in somas of TG neurons. We propose that activation of TRPV1 and TRPA1 receptors by H₂S during neuro-inflammation conditions contributes to the nociceptive firing in primary afferents underlying migraine pain.

DNA Hydroxymethylation by Ten-eleven Translocation Methylcytosine Dioxygenase 1 and 3 Regulates Nociceptive Sensitization in a Chronic Inflammatory Pain Model

Background

Ten-eleven translocation methylcytosine dioxygenase converts 5-methylcytosine in DNA to 5-hydroxymethylcytosine, which plays an important role in gene transcription. Although 5-hydroxymethylcytosine is enriched in mammalian neurons, its regulatory function in nociceptive information processing is unknown.

Methods

The global levels of 5-hydroxymethylcytosine and ten-eleven translocation methylcytosine dioxygenase were measured in spinal cords in mice treated with complete Freund’s adjuvant. Immunoblotting, immunohistochemistry, and behavioral tests were used to explore the downstream ten-eleven translocation methylcytosine dioxygenase-dependent signaling pathway.

Results

Complete Freund’s adjuvant-induced nociception increased the mean levels (± SD) of spinal 5-hydroxymethylcytosine (178 ± 34 vs. 100 ± 21; P = 0.0019), ten-eleven translocation methylcytosine dioxygenase-1 (0.52 ± 0.11 vs. 0.36 ± 0.064; P = 0.0088), and ten-eleven translocation methylcytosine dioxygenase-3 (0.61 ± 0.13 vs. 0.39 ± 0.08; P = 0.0083) compared with levels in control mice (n = 6/group). The knockdown of ten-eleven translocation methylcytosine dioxygenase-1 or ten-eleven translocation methylcytosine dioxygenase-3 alleviated thermal hyperalgesia and mechanical allodynia, whereas overexpression cytosinethem in naïve mice (n = 6/group). Down-regulation of spinal ten-eleven translocation methylcytosine dioxygenase-1 and ten-eleven translocation methylcytosine dioxygenase-3 also reversed the increases in Fos expression (123 ± 26 vs. 294 ± 6; P = 0.0031; and 140 ± 21 vs. 294 ± 60; P = 0.0043, respectively; n = 6/group), 5-hydroxymethylcytosine levels in the Stat3 promoter (75 ± 16.1 vs. 156 ± 28.9; P = 0.0043; and 91 ± 19.1 vs. 156 ± 28.9; P = 0.0066, respectively; n = 5/group), and consequent Stat3 expression (93 ± 19.6 vs. 137 ± 27.5; P = 0.035; and 72 ± 15.2 vs. 137 ± 27.5; P = 0.0028, respectively; n = 5/group) in complete Freund’s adjuvant-treated mice.

Conclusions

This study reveals a novel epigenetic mechanism for ten-eleven translocation methylcytosine dioxygenase-1 and ten-eleven translocation methylcytosine dioxygenase-3 in the modulation of spinal nociceptive information via targeting of Stat3.

Drugging the pain epigenome

More than 20% of adults worldwide experience different types of chronic pain, which are frequently associated with several comorbidities and a decrease in quality of life. Several approved painkillers are available, but current analgesics are often hampered by insufficient efficacy and/or severe adverse effects. Consequently, novel strategies for safe, highly efficacious treatments are highly desirable, particularly for chronic pain. Epigenetic mechanisms such as DNA methylation, histone modifications and microRNAs (miRNAs) strongly affect the regulation of gene expression, potentially for long periods over years or even generations, and have been associated with pathophysiological pain. Several studies, mostly in animals, revealed that inhibitors of DNA methylation, activators and inhibitors of histone modification and modulators of miRNAs reverse a number of pathological changes in the pain epigenome, which are associated with altered expression of pain-relevant genes. This epigenetic modulation might then reduce the nociceptive response and provide novel therapeutic options for analgesic therapy of chronic pain states. However, a number of challenges, such as nonspecific effects and poor delivery to target cells and tissues, hinder the rapid development of such analgesics. In this Review, we critically summarize data on epigenetics and pain, focusing on challenges in clinical development as well as possible new approaches to the drug modulation of the pain epigenome.

Inhibition of cystathionine β-synthetase suppresses sodium channel activities of dorsal root ganglion neurons of rats with lumbar disc herniation

The pathogenesis of pain in lumbar disc herniation (LDH) remains poorly understood. We have recently demonstrated that voltage-gated sodium channels (VGSCs) in dorsal root ganglion (DRG) neurons were sensitized in a rat model of LDH. However, the detailed molecular mechanism for sensitization of VGSCs remains largely unknown. This study was designed to examine roles of the endogenous hydrogen sulfide synthesizing enzyme cystathionine β-synthetase (CBS) in sensitization of VGSCs in a previously validated rat model of LDH. Here we showed that inhibition of CBS activity by O-(Carboxymethyl) hydroxylamine hemihydrochloride (AOAA) significantly attenuated pain hypersensitivity in LDH rats. Administration of AOAA also reduced neuronal hyperexcitability, suppressed the sodium current density, and right-shifted the V_1/2 of the inactivation curve, of hindpaw innervating DRG neurons, which is retrogradely labeled by DiI. In vitro incubation of AOAA did not alter the excitability of acutely isolated DRG neurons. Furthermore, CBS was colocalized with Na_V1.7 and Na_V1.8 in hindpaw-innervating DRG neurons. Treatment of AOAA markedly suppressed expression of Na_V1.7 and Na_V1.8 in DRGs of LDH rats. These data suggest that targeting the CBS-H₂S signaling at the DRG level might represent a novel therapeutic strategy for chronic pain relief in patients with LDH.

Tet1-dependent epigenetic modification of BDNF expression in dorsal horn neurons mediates neuropathic pain in rats

Ten-eleven translocation methylcytosine dioxygenase 1 (Tet1) mediates the conversion of 5-methylcytosine (5 mC) to 5-hydroxymethylcytosine (5 hmC), hence promoting DNA demethylation. Although recent studies have linked the DNA demethylation of specific genes to pain hypersensitivity, the role of spinal Tet1-dependent DNA demethylation in nociception hypersensitivity development remains elusive. Here, we report correlated with behavioral allodynia, spinal nerve ligation (SNL) upregulated Tet1 expression in dorsal horn neurons that hydroxylate 5 mC to 5 hmC at CpG dinucleotides in the bdnf promoter to promote spinal BDNF expression at day 7 after operation. Focal knockdown of spinal Tet1 expression decreased Tet1 binding and 5 hmC enrichment, further increased 5 mC enrichment at CpG sites in the bdnf promoter and decreased spinal BDNF expression accompanied by the alleviation of the developed allodynia. Moreover, at day 7 after operation, SNL-enhanced Tet1 expression also inhibited the binding of DNA methyltransferases (DNMTs, i.e., DNMT1, DNMT3a, and DNMT3b) to the bdnf promoter, a requirement for transcriptional silencing by catalysing 5-cytosine (5C) to 5 mC. Together, these data suggest at CpG sites of the bdnf promoter, SNL-enhanced Tet1 expression promotes DNA demethylation both by converting 5 mC to 5 hmC and inhibiting DNMT binding to regulate spinal BDNF expression, hence contributing to behavioral allodynia development.

Hydroxymethylation of microRNA-365-3p Regulates Nociceptive Behaviors via Kcnh2

DNA 5-hydroxylmethylcytosine (5hmC) catalyzed by ten-eleven translocation methylcytosine dioxygenase (TET) occurs abundantly in neurons of mammals. However, the in vivo causal link between TET dysregulation and nociceptive modulation has not been established. Here, we found that spinal TET1 and TET3 were significantly increased in the model of formalin-induced acute inflammatory pain, which was accompanied with the augment of genome-wide 5hmC content in spinal cord. Knockdown of spinal TET1 or TET3 alleviated the formalin-induced nociceptive behavior and overexpression of spinal TET1 or TET3 in naive mice produced pain-like behavior as evidenced by decreased thermal pain threshold. Furthermore, we found that TET1 or TET3 regulated the nociceptive behavior by targeting microRNA-365-3p (miR-365-3p). Formalin increased 5hmC in the miR-365-3p promoter, which was inhibited by knockdown of TET1 or TET3 and mimicked by overexpression of TET1 or TET3 in naive mice. Nociceptive behavior induced by formalin or overexpression of spinal TET1 or TET3 could be prevented by downregulation of miR-365-3p, and mimicked by overexpression of spinal miR-365-3p. Finally, we demonstrated that a potassium channel, voltage-gated eag-related subfamily H member 2 (Kcnh2), validated as a target of miR-365-3p, played a critical role in nociceptive modulation by spinal TET or miR-365-3p. Together, we concluded that TET-mediated hydroxymethylation of miR-365-3p regulates nociceptive behavior via Kcnh2.

SIGNIFICANCE STATEMENT

Mounting evidence indicates that epigenetic modifications in the nociceptive pathway contribute to pain processes and analgesia response. Here, we found that the increase of 5hmC content mediated by TET1 or TET3 in miR-365-3p promoter in the spinal cord is involved in nociceptive modulation through targeting a potassium channel, Kcnh2. Our study reveals a new epigenetic mechanism underlying nociceptive information processing, which may be a novel target for development of antinociceptive drugs.

H2S-induced thiol-based redox switches: Biochemistry and functional relevance for inflammatory diseases

During the last decades, small inorganic molecules like reactive oxygen species (ROS), nitric oxide (NO), carbon monoxide (CO) and even the highly toxic hydrogen sulfide (H2S) have been evolved as important signaling molecules that trigger crucial cellular processes by regulating the activity of kinases, phosphatases and transcription factors. These redox molecules use similar target structures and therefore, the composition of the complex "redox environment" determines the final outcome of signaling processes and may subsequently also affect the behavior of a cell in an inflammatory environment. Here, we discuss the role of H2S in this complex interplay with a focus on the transcription factors Nrf2 and NFκB.

Enhanced binding capability of nuclear factor-κB with demethylated P2X3 receptor gene contributes to cancer pain in rats

Supplemental Digital Content is Available in the Text.

Epigenetic regulations of P2X3 receptors play a crucial role in cancer pain. Targeting p65 binding to demethylated P2X3 receptor gene suppresses cancer pain.

Nuclear factor-kappa B (NF-κB) signaling is implicated in both cancer development and inflammation processes. However, the roles and mechanisms of NF-κB signaling in the development of cancer-induced pain (CIP) remain unknown. This study was designed to investigate the roles of the p65 subunit of NF-κB in regulation of the purinergic receptor (P2X3R) plasticity in dorsal root ganglion (DRG) of CIP rats. We showed here that tumor cell injection produced mechanical and thermal hyperalgesia, and an enhanced body weight–bearing difference, which was correlated with an upregulation of p65 and P2X3R expression in lumber DRGs and a potentiation of ATP-evoked responses of tibia-innervating DRG neurons. Inhibition of NF-κB signaling using p65 inhibitor pyrrolidine dithiocarbamate, BAY-11-7082, or lentiviral-p65 short-hairpin RNA significantly attenuated CIP and reversed the activities of P2X3R. Interestingly, tumor cell injection led to a significant demethylation of CpG island in p2x3r gene promoter and enhanced ability of p65 to bind the promoter of p2x3r gene. Our findings suggest that upregulation of P2X3R expression was mediated by the enhanced binding capability of p65 with demethylated promoter of p2x3r gene, thus contributing to CIP. NF-κBp65 might be a potential target for treating CIP, a neuropathic pain generated by tumor cell–induced injury to nerves that innervate the skin.

Could targeting epigenetic processes relieve chronic pain states?

PURPOSE OF REVIEW

Aberrations in the epigenetic landscape have previously been associated with human diseases such as cancer and schizophrenia, and drugs that target epigenetic processes are currently used as therapeutic agents. This article will review the evidence obtained from animal studies indicating that epigenetic processes might regulate long-term pain states and then discuss the possibility that targeting epigenetic mechanisms might be useful for the management of chronic pain.

RECENT FINDINGS

Recent animal studies have reported injury-induced changes in epigenetic processes in the central nervous system. The picture that has emerged is that of very complex epigenetic programs that depend on the injury. However, some studies have reported the successful use of nonspecific epigenetic tools to improve the hypersensitivity that develops in animal models of long-term pain states.

SUMMARY

The field of epigenetics and pain is rapidly emerging but further investigation is needed to fully comprehend the contribution of epigenetic processes to chronic pain states. Although therapeutic approaches targeting these mechanisms might seem worthwhile, we cannot assert that currently available global tools such as histone deacetylase inhibitors can be used successfully for the long-term treatment of chronic pain states.

Promoted Interaction of Nuclear Factor-κB With Demethylated Purinergic P2X3 Receptor Gene Contributes to Neuropathic Pain in Rats With Diabetes

Painful diabetic neuropathy is a common complication of diabetes produced by mechanisms that as yet are incompletely defined. The aim of this study was to investigate the roles of nuclear factor-κB (NF-κB) in the regulation of purinergic receptor P2X ligand-gated ion channel 3 (P2X3R) plasticity in dorsal root ganglion (DRG) neurons of rats with painful diabetes. Here, we showed that hindpaw pain hypersensitivity in streptozocin-induced diabetic rats was attenuated by treatment with purinergic receptor antagonist suramin or A-317491. The expression and function of P2X3Rs was markedly enhanced in hindpaw-innervated DRG neurons in diabetic rats. The CpG (cytosine guanine dinucleotide) island in the p2x3r gene promoter region was significantly demethylated, and the expression of DNA methyltransferase 3b was remarkably downregulated in DRGs in diabetic rats. The binding ability of p65 (an active form of NF-κB) with the p2x3r gene promoter region and p65 expression were enhanced significantly in diabetes. The inhibition of p65 signaling using the NF-κB inhibitor pyrrolidine dithiocarbamate or recombinant lentiviral vectors designated as lentiviral vector-p65 small interfering RNA remarkably suppressed P2X3R activities and attenuated diabetic pain hypersensitivity. Insulin treatment significantly attenuated pain hypersensitivity and suppressed the expression of p65 and P2X3Rs. Our findings suggest that the p2x3r gene promoter DNA demethylation and enhanced interaction with p65 contributes to P2X3R sensitization and diabetic pain hypersensitivity.

Hydrogen sulfide-induced itch requires activation of Cav3.2 T-type calcium channel in mice

The contributions of gasotransmitters to itch sensation are largely unknown. In this study, we aimed to investigate the roles of hydrogen sulfide (H₂S), a ubiquitous gasotransmitter, in itch signaling. We found that intradermal injection of H₂S donors NaHS or Na₂S, but not GYY4137 (a slow-releasing H₂S donor), dose-dependently induced scratching behavior in a μ-opioid receptor-dependent and histamine-independent manner in mice. Interestingly, NaHS induced itch via unique mechanisms that involved capsaicin-insensitive A-fibers, but not TRPV1-expressing C-fibers that are traditionally considered for mediating itch, revealed by depletion of TRPV1-expressing C-fibers by systemic resiniferatoxin treatment. Moreover, local application of capsaizapine (TRPV1 blocker) or HC-030031 (TRPA1 blocker) had no effects on NaHS-evoked scratching. Strikingly, pharmacological blockade and silencing of Ca_v3.2 T-type calcium channel by mibefradil, ascorbic acid, zinc chloride or Ca_v3.2 siRNA dramatically decreased NaHS-evoked scratching. NaHS induced robust alloknesis (touch-evoked itch), which was inhibited by T-type calcium channels blocker mibefradil. Compound 48/80-induced itch was enhanced by an endogenous precursor of H₂S (L-cysteine) but attenuated by inhibitors of H₂S-producing enzymes cystathionine γ-lyase and cystathionine β-synthase. These results indicated that H₂S, as a novel nonhistaminergic itch mediator, may activates Ca_v3.2 T-type calcium channel, probably located at A-fibers, to induce scratching and alloknesis in mice.