Nerve Injury-Induced Chronic Pain Is Associated with Persistent DNA Methylation Reprogramming in Dorsal Root Ganglion

Identification of Potential Intervention Targets Involved in Prior Exercise that Attenuates Peripheral Neuropathic Pain by Integrating Transcriptome and Whole-genome Bisulfite Sequencing Analyses

Changes in DNA methylation and subsequent alterations in gene expression have opened a new direction in research related to the pathogenesis of peripheral neuropathic pain (PNP). This study aimed to reveal epigenetic perturbations underlying DNA methylation in the dorsal root ganglion (DRG) of rats with peripheral nerve injury in response to prior exercise and identify potential target genes involved. Male Sprague-Dawley rats were divided into three groups, namely, chronic constriction injury (CCI) of the sciatic nerve, CCI with prior 6-week swimming training (CCI_Ex), and sham operated (Sham). Mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were used as the main observation indicators to evaluate behavioral changes associated with pain. In this study, 6-week swimming training before CCI prevented later chronic pain. In particular, CCI rats with prior exercise showed a significant increase in the MWT and TWL of the injured lateral hind paw compared with CCI rats without exercise on days 14, 21, and 28 after CCI. Whole-genome bisulfite sequencing from the injured lumbar (L4-L6) DRGs on the 28th day after surgery was detected. We also generated DNA methylation maps of the two comparisons (sham group vs. CCI and CCI groups vs. CCI_Ex group), and 396 overlapping differentially methylated region-related genes were found between the two comparisons. Moreover, we integrated RNA sequencing to understand the mechanism by which differential DNA methylation after CCI may influence gene expression. Finally, Ryr1 and Xirp2 were identified through association analysis of two omics and quantitative reverse-transcription polymerase chain reaction, respectively. The methylation levels of Ryr1 and Xirp2 were upregulated with a corresponding increase in their mRNA expression in the DRGs of CCI rats, whereas prior exercise downregulated Ryr1 methylation and restore its expression level. Functional enrichment analysis of both omics found that the calcium signaling pathway was significantly enriched. Therefore, the potential intervention targets (Ryr1 and Xirp2) in L4-L6 DRGs may be involved in prior exercise that attenuates PNP induced by CCI. This study provides crucial insights into the epigenetic regulation of PNP responses to prior exercise.

α2δ-1-Linked NMDA and AMPA Receptors in Neuropathic Pain and Gabapentinoid Action

Chronic neuropathic pain is a debilitating condition that presents a significant therapeutic challenge. Unlike nociceptive pain, neuropathic pain is predominantly driven by glutamate NMDA receptors (NMDARs) and/or Ca2+-permeable AMPA receptors (CP-AMPARs) at synapses between primary afferent nerves and excitatory neurons in the spinal dorsal horn. The α2δ-1 protein, encoded by Cacna2d1 and historically recognized as a subunit of voltage-activated Ca2+ channels, is the primary target of gabapentinoids, such as gabapentin and pregabalin, which are widely prescribed for neuropathic pain and epilepsy. However, gabapentinoids have minimal effects on Ca2+ channel activity. Recent studies reveal that α2δ-1 plays a pivotal role in amplifying nociceptive input to the spinal cord in neuropathic pain. This action is mediated through its dynamic physical interactions with phosphorylated NMDARs and GluA1/GluA2 subunits via its intrinsically disordered C-terminal region. α2δ-1 not only promotes synaptic trafficking of NMDARs but also disrupts heteromeric assembly of GluA1/GluA2 subunits in the spinal dorsal horn. The central function of α2δ-1 is to elevate intracellular Ca2+ concentrations at both presynaptic and postsynaptic sites, augmenting nociceptive transmission. Consequently, α2δ-1 serves as a dual regulator coordinating synaptic expression of NMDARs and GluA1 homomeric CP-AMPARs, a function that underlies the therapeutic actions of gabapentinoids. By inhibiting α2δ-1, gabapentinoids reduce the hyperactivity of synaptic α2δ-1-bound NMDARs and CP-AMPARs, thereby dampening the excessive excitatory synaptic transmission characteristic of neuropathic pain. These newly identified roles of α2δ-1 in orchestrating glutamatergic synaptic plasticity suggest that gabapentinoids could be repurposed for treating other neurological disorders involving dysregulated synaptic NMDARs and CP-AMPARs.

Unveiling the Mechanisms of Pain in Endometriosis: Comprehensive Analysis of Inflammatory Sensitization and Therapeutic Potential

Endometriosis is a complicated, estrogen-dependent gynecological condition with a high morbidity rate. Pain, as the most common clinical symptom of endometriosis, severely affects women's physical and mental health and exacerbates socioeconomic burden. However, the specific mechanisms behind the occurrence of endometriosis-related pain remain unclear. It is currently believed that the occurrence of endometriosis pain is related to various factors, such as immune abnormalities, endocrine disorders, the brain-gut axis, angiogenesis, and mechanical stimulation. These factors induce systemic chronic inflammation, which stimulates the nerves and subsequently alters neural plasticity, leading to nociceptive sensitization and thereby causing chronic pain. In this paper, we compile and review the articles published on the study of nociceptive sensitization and endometriosis pain mechanisms. Starting from the factors influencing the chronic pain associated with endometriosis, we explain the relationship between these factors and chronic inflammation and further elaborate on the potential mechanisms by which chronic inflammation induces nociceptive sensitization. We aim to reveal the possible mechanisms of endometriosis pain, as well as nociceptive sensitization, and offer potential new targets for the treatment of endometriosis pain.

Histone Methyltransferase G9a in Primary Sensory Neurons Promotes Inflammatory Pain and Transcription of Trpa1 and Trpv1 via Bivalent Histone Modifications

Transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1) channels are crucial for detecting and transmitting nociceptive stimuli. Inflammatory pain is associated with sustained increases in TRPA1 and TRPV1 expression in primary sensory neurons. However, the epigenetic mechanisms driving this upregulation remain unknown. G9a (encoded by Ehmt2) catalyzes H3K9me2 and generally represses gene transcription. In this study, we found that intrathecal administration of UNC0638, a specific G9a inhibitor, or G9a-specific siRNA, substantially reduced complete Freund's adjuvant (CFA)-induced pain hypersensitivity. Remarkably, CFA treatment did not induce persistent pain hypersensitivity in male and female mice with conditional Ehmt2 knock-out in dorsal root ganglion (DRG) neurons. RNA sequencing and quantitative PCR analyses showed that CFA treatment caused a sustained increase in mRNA levels of Trpa1 and Trpv1 in the DRG. Ehmt2 knock-out in DRG neurons elevated baseline Trpa1 and Trpv1 mRNA levels but notably reversed CFA-induced increases in their expression. Chromatin immunoprecipitation revealed that CFA treatment reduced G9a and H3K9me2 levels while increasing H3K9ac and H3K4me3-activating histone marks-at Trpa1 and Trpv1 promoters in the DRG. Strikingly, conditional Ehmt2 knock-out in DRG neurons not only diminished H3K9me2 but also reversed CFA-induced increases in H3K9ac and H3K4me3 at Trpa1 and Trpv1 promoters. Our findings suggest that G9a in primary sensory neurons constitutively represses Trpa1 and Trpv1 transcription under normal conditions but paradoxically enhances their transcription during tissue inflammation. This latter action accounts for inflammation-induced TRPA1 and TRPV1 upregulation in the DRG. Thus, G9a could be targeted for alleviating persistent inflammatory pain.

The Voltage-Gated Calcium Channel α2δ Subunit in Neuropathic Pain

Neuropathic pain (NP) is a chronic pain caused by injury or disease of the somatosensory nervous system, or it can be directly caused by disease. It often presents with clinical features like spontaneous pain, hyperalgesia, and dysesthesia. At present, voltage-gated calcium ion channels (VGCCs) are known to be closely related to the development of NP, especially the α2δ subunit. The α2δ subunit is a regulatory subunit of VGCCs. It exists mainly in the brain and peripheral nervous system, especially in nerve cells, and it plays a crucial part in regulating presynaptic and postsynaptic functions. Furthermore, the α2δ subunit influences neuronal excitation and pain signaling by promoting its expression and localization through binding to VGCC-related subunits. The α2δ subunit is widely used in the management of NP as a target of antiepileptic drugs gabapentin and pregabalin. Although drug therapy is one of the treatments for NP, its clinical application is limited due to the adverse reactions caused by drug therapy. Therefore, further research on the therapeutic target α2δ subunit is needed, and attempts are made to obtain an effective treatment for relieving NP without side effects. This review describes the current associated knowledge on the function of the α2δ subunit in perceiving and modulating NP.

Impaired Molecular Mechanisms Contributing to Chronic Pain in Patients with Hidradenitis Suppurativa: Exploring Potential Biomarkers and Therapeutic Targets

Hidradenitis suppurativa (HS) is a chronic skin condition that primarily affects areas with dense hair follicles and apocrine sweat glands, such as the underarms, groin, buttocks, and lower breasts. Intense pain and discomfort in HS have been commonly noted, primarily due to the lesions' effects on nearby tissues. Pain is a factor that can influence DNA methylation patterns, though its exact role in HS is not fully understood. We aim to identify molecular markers of chronic pain in HS patients. We performed DNA methylome of peripheral blood DNA derived from a group of 24 patients with HS and 24 healthy controls, using Illumina methylation array chips. We identified 253 significantly differentially methylated CpG sites across 253 distinct genes regulating pain sensitization in HS, including 224 hypomethylated and 29 hypermethylated sites. Several genes with pleiotropic roles include transporters (ABCC2, SLC39A8, SLC39A9), wound healing (MIR132, FGF2, PDGFC), ion channel regulators (CACNA1C, SCN1A), oxidative stress mediators (SCN8A, DRD2, DNMT1), cytochromes (CYP19A, CYP1A2), cytokines (TGFB1, IL4), telomere regulators (CSNK1D, SMAD3, MTA1), circadian rhythm (IL1R2, ABCG1, RORA), ultradian rhythms (PHACTR1, TSC2, ULK1), hormonal regulation (PPARA, NR3C1, ESR2), and the serotonin system (HTR1D, HTR1E, HTR3C, HTR4, TPH2). They also play roles in glucose metabolism (POMC, IRS1, GNAS) and obesity (DRD2, FAAH, MMP2). Gene ontology and pathway enrichment analysis identified 43 pathways, including calcium signaling, cocaine addiction, and nicotine addiction. This study identified multiple differentially methylated genes involved in chronic pain in HS, which may serve as biomarkers and therapeutic targets. Understanding their epigenetic regulation is crucial for personalized pain management and could enhance the identification of high-risk patients, leading to better preventative therapies and improved maternal and neonatal outcomes.

Nerve injury inhibits Oprd1 and Cnr1 transcription through REST in primary sensory neurons

The transcription repressor REST in the dorsal root ganglion (DRG) is upregulated by peripheral nerve injury and promotes the development of chronic pain. However, the genes targeted by REST in neuropathic pain development remain unclear. The expression levels of four opioid receptor genes (Oprm1, Oprd1, Oprl1 and Oprk1) and the cannabinoid CB1 receptor (Cnr1) gene in the DRG regulate nociception. In this study, we determined the role of REST in controlling their expression in the DRG induced by spared nerve injury (SNI). SNI induced chronic pain hypersensitivity in wild-type mice and was accompanied by increased levels of Rest transcript and protein. Transcriptomic analyses of wild-type mouse DRGs suggested that SNI upregulates the expression of Rest transcripts and downregulates the transcripts of all four opioid receptor genes and the Cnr1 gene. Quantitative reverse transcription polymerase chain reaction analyses of these tissues validated these results. Analysis of publicly available bioinformatic data suggested that REST binds to the promoter regions of Oprm1 and Cnr1. Chromatin immunoprecipitation analyses indicated the presence of REST at these promoters. Full-length Rest conditional knockout in primary sensory neurons reduced SNI-induced pain hypersensitivity and rescued the SNI-induced reduction in the expression of Oprd1 and Cnr1 in mouse DRG. Our results suggest that nerve injury represses the transcription of at least the Oprd1 and Cnr1 genes via REST in primary sensory neurons and that REST is a potential therapeutic target for neuropathic pain. Thus, inhibiting REST activity could potentially reduce chronic neuropathic pain and augment opioid/cannabinoid analgesic actions by increasing the transcription of Oprd1 and Cnr1 genes in DRG neurons.

Nerve injury augments Cacna2d1 transcription via CK2-mediated phosphorylation of the histone deacetylase HDAC2 in dorsal root ganglia

The development of chronic neuropathic pain involves complex synaptic and epigenetic mechanisms. Nerve injury causes sustained upregulation of α2δ-1 (encoded by the Cacna2d1 gene) in the dorsal root ganglion (DRG), contributing to pain hypersensitivity by directly interacting with and augmenting presynaptic NMDA receptor activity in the spinal dorsal horn. Under normal conditions, histone deacetylase 2 (HDAC2) is highly enriched at the Cacna2d1 gene promoter in the DRG, which constitutively suppresses Cacna2d1 transcription. However, nerve injury leads to HDAC2 dissociation from the Cacna2d1 promoter, promoting the enrichment of active histone marks and Cacna2d1 transcription in primary sensory neurons. In this study, we determined the mechanism by which nerve injury diminishes HDAC2 occupancy at the Cacna2d1 promoter in the DRG. Spinal nerve injury in rats increased serine-394 phosphorylation of HDAC2 in the DRG. Coimmunoprecipitation showed that nerve injury enhanced the physical interaction between HDAC2 and casein kinase II (CK2) in the DRG. Furthermore, repeated intrathecal treatment with CX-4945, a potent and specific CK2 inhibitor, markedly reversed nerve injury-induced pain hypersensitivity, HDAC2 phosphorylation, and α2δ-1 expression levels in the DRG. In addition, treatment with CX-4945 largely restored HDAC2 enrichment at the Cacna2d1 promoter and reduced the elevated levels of acetylated H3 and H4 histones, particularly H3K9ac and H4K5ac, at the Cacna2d1 promoter in the injured DRG. These findings suggest that nerve injury increases CK2 activity and CK2-HDAC2 interactions, which enhance HDAC2 phosphorylation in the DRG. This, in turn, diminishes HDAC2 enrichment at the Cacna2d1 promoter, thereby promoting Cacna2d1 transcription.

Critical Windows: Exploring the Association Between Perinatal Trauma, Epigenetics, and Chronic Pain

Chronic pain is highly prevalent and burdensome, affecting millions of people worldwide. Although it emerges at any point in life, it often manifests in adolescence. Given that adolescence is a unique developmental period, additional strains associated with persistent and often idiopathic pain lead to significant long-term consequences. While there is no singular cause for the chronification of pain, epigenetic modifications that lead to neural reorganization may underpin central sensitization and subsequent manifestation of pain hypersensitivity. Epigenetic processes are particularly active during the prenatal and early postnatal years. We demonstrate how exposure to various traumas, such as intimate partner violence while in utero or adverse childhood experiences, can significantly influence epigenetic regulation within the brain and in turn modify pain-related processes. We provide compelling evidence that the burden of chronic pain is likely initiated early in life, often being transmitted from mother to offspring. We also highlight two promising prophylactic strategies, oxytocin administration and probiotic use, that have the potential to attenuate the epigenetic consequences of early adversity. Overall, we advance understanding of the causal relationship between trauma and adolescent chronic pain by highlighting epigenetic mechanisms that underlie this transmission of risk, ultimately informing how to prevent this rising epidemic.

Chronic pain in the elderly: Exploring cellular and molecular mechanisms and therapeutic perspectives

Chronic pain is a debilitating condition frequently observed in the elderly, involving numerous pathological mechanisms within the nervous system. Diminished local blood flow, nerve degeneration, variations in fiber composition, alterations in ion channels and receptors, accompanied by the sustained activation of immune cells and release of pro-inflammatory cytokines, lead to overactivation of the peripheral nervous system. In the central nervous system, chronic pain is strongly associated with the activation of glial cells, which results in central sensitization and increased pain perception. Moreover, age-related alterations in neural plasticity and disruptions in pain inhibitory pathways can exacerbate chronic pain in older adults. Finally, the environmental influences on the development of chronic pain in the elderly must be considered. An understanding of these mechanisms is essential for developing novel treatments for chronic pain, which can significantly improve the quality of life for this vulnerable population.

Opioid and Cannabinoid Systems in Pain: Emerging Molecular Mechanisms and Use in Clinical Practice, Health, and Fitness

Pain is an unpleasant sensory and emotional experience. Adequate pain control is often challenging, particularly in patients with chronic pain. Despite advances in pain management, drug addiction, overtreatment, or substance use disorders are not rare. Hence the need for further studies in the field. The substantial progress made over the last decade has revealed genes, signalling pathways, molecules, and neuronal networks in pain control thus opening new clinical perspectives in pain management. In this respect, data on the epigenetic modulation of opioid and cannabinoid receptors, key actors in the modulation of pain, offered new perspectives to preserve the activity of opioid and endocannabinoid systems to increase the analgesic efficacy of opioid- and cannabinoid-based drugs. Similarly, upcoming data on cannabidiol (CBD), a non-psychoactive cannabinoid in the marijuana plant Cannabis sativa, suggests analgesic, anti-inflammatory, antioxidant, anticonvulsivant and ansiolitic effects and supports its potential application in clinical contexts such as cancer, neurodegeneration, and autoimmune diseases but also in health and fitness with potential use in athletes. Hence, in this review article, we summarize the emerging epigenetic modifications of opioid and cannabinoid receptors and focus on CBD as an emerging non-psychoactive cannabinoid in pain management in clinical practice, health, and fitness.

HDAC inhibitors as a potential therapy for chemotherapy-induced neuropathic pain

Cancer, a chronic disease characterized by uncontrolled cell development, kills millions of people globally. The WHO reported over 10 million cancer deaths in 2020. Anticancer medications destroy healthy and malignant cells. Cancer treatment induces neuropathy. Anticancer drugs cause harm to spinal cord, brain, and peripheral nerve somatosensory neurons, causing chemotherapy-induced neuropathic pain. The chemotherapy-induced mechanisms underlying neuropathic pain are not fully understood. However, neuroinflammation has been identified as one of the various pathways associated with the onset of chemotherapy-induced neuropathic pain. The neuroinflammatory processes may exhibit varying characteristics based on the specific type of anticancer treatment delivered. Neuroinflammatory characteristics have been observed in the spinal cord, where microglia and astrocytes have a significant impact on the development of chemotherapy-induced peripheral neuropathy. The patient's quality of life might be affected by sensory deprivation, loss of consciousness, paralysis, and severe disability. High cancer rates and ineffective treatments are associated with this disease. Recently, histone deacetylases have become a novel treatment target for chemotherapy-induced neuropathic pain. Chemotherapy-induced neuropathic pain may be treated with histone deacetylase inhibitors. Histone deacetylase inhibitors may be a promising therapeutic treatment for chemotherapy-induced neuropathic pain. Common chemotherapeutic drugs, mechanisms, therapeutic treatments for neuropathic pain, and histone deacetylase and its inhibitors in chemotherapy-induced neuropathic pain are covered in this paper. We propose that histone deacetylase inhibitors may treat several aspects of chemotherapy-induced neuropathic pain, and identifying these inhibitors as potentially unique treatments is crucial to the development of various chemotherapeutic combination treatments.

Epigenetic Landscapes of Pain: DNA Methylation Dynamics in Chronic Pain

Chronic pain is a prevalent condition with a multifaceted pathogenesis, where epigenetic modifications, particularly DNA methylation, might play an important role. This review delves into the intricate mechanisms by which DNA methylation and demethylation regulate genes associated with nociception and pain perception in nociceptive pathways. We explore the dynamic nature of these epigenetic processes, mediated by DNA methyltransferases (DNMTs) and ten-eleven translocation (TET) enzymes, which modulate the expression of pro- and anti-nociceptive genes. Aberrant DNA methylation profiles have been observed in patients with various chronic pain syndromes, correlating with hypersensitivity to painful stimuli, neuronal hyperexcitability, and inflammatory responses. Genome-wide analyses shed light on differentially methylated regions and genes that could serve as potential biomarkers for chronic pain in the epigenetic landscape. The transition from acute to chronic pain is marked by rapid DNA methylation reprogramming, suggesting its potential role in pain chronicity. This review highlights the importance of understanding the temporal dynamics of DNA methylation during this transition to develop targeted therapeutic interventions. Reversing pathological DNA methylation patterns through epigenetic therapies emerges as a promising strategy for pain management.

Dysfunction of Small-Conductance Ca2+-Activated Potassium (SK) Channels Drives Amygdala Hyperexcitability and Neuropathic Pain Behaviors: Involvement of Epigenetic Mechanisms

Neuroplasticity in the amygdala and its central nucleus (CeA) is linked to pain modulation and pain behaviors, but cellular mechanisms are not well understood. Here, we addressed the role of small-conductance Ca2+-activated potassium (SK) channels in pain-related amygdala plasticity. The facilitatory effects of the intra-CeA application of an SK channel blocker (apamin) on the pain behaviors of control rats were lost in a neuropathic pain model, whereas an SK channel activator (NS309) inhibited pain behaviors in neuropathic rats but not in sham controls, suggesting the loss of the inhibitory behavioral effects of amygdala SK channels. Brain slice electrophysiology found hyperexcitability of CeA neurons in the neuropathic pain condition due to the loss of SK channel-mediated medium afterhyperpolarization (mAHP), which was accompanied by decreased SK2 channel protein and mRNA expression, consistent with a pretranscriptional mechanisms. The underlying mechanisms involved the epigenetic silencing of the SK2 gene due to the increased DNA methylation of the CpG island of the SK2 promoter region and the change in methylated CpG sites in the CeA in neuropathic pain. This study identified the epigenetic dysregulation of SK channels in the amygdala (CeA) as a novel mechanism of neuropathic pain-related plasticity and behavior that could be targeted to control abnormally enhanced amygdala activity and chronic neuropathic pain.

Pathology of pain and its implications for therapeutic interventions

Pain is estimated to affect more than 20% of the global population, imposing incalculable health and economic burdens. Effective pain management is crucial for individuals suffering from pain. However, the current methods for pain assessment and treatment fall short of clinical needs. Benefiting from advances in neuroscience and biotechnology, the neuronal circuits and molecular mechanisms critically involved in pain modulation have been elucidated. These research achievements have incited progress in identifying new diagnostic and therapeutic targets. In this review, we first introduce fundamental knowledge about pain, setting the stage for the subsequent contents. The review next delves into the molecular mechanisms underlying pain disorders, including gene mutation, epigenetic modification, posttranslational modification, inflammasome, signaling pathways and microbiota. To better present a comprehensive view of pain research, two prominent issues, sexual dimorphism and pain comorbidities, are discussed in detail based on current findings. The status quo of pain evaluation and manipulation is summarized. A series of improved and innovative pain management strategies, such as gene therapy, monoclonal antibody, brain-computer interface and microbial intervention, are making strides towards clinical application. We highlight existing limitations and future directions for enhancing the quality of preclinical and clinical research. Efforts to decipher the complexities of pain pathology will be instrumental in translating scientific discoveries into clinical practice, thereby improving pain management from bench to bedside.

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.

Nerve injury inhibits Oprd1 and Cnr1 transcription through REST in primary sensory neurons

The transcription repressor REST in the dorsal root ganglion (DRG) is upregulated by peripheral nerve injury and promotes the development of chronic pain. However, the genes targeted by REST in neuropathic pain development remain unclear. The expression levels of 4 opioid receptor (Oprm1, Oprd1, Oprl1, Oprk1) and the cannabinoid CB1 receptor (Cnr1) genes in the DRG regulate nociception. In this study, we determined the role of REST in the control of their expression in the DRG induced by spared nerve injury (SNI) in both male and female mice. Transcriptomic analyses of male mouse DRGs followed by quantitative reverse transcription polymerase chain reaction analyses of both male and female mouse DRGs showed that SNI upregulated expression of Rest and downregulated mRNA levels of all 4 opioid receptor and Cnr1 genes, but Oprm1 was upregulated in female mice. Analysis of publicly available bioinformatic data suggested that REST binds to the promoter regions of Oprm1 and Cnr1. Chromatin immunoprecipitation analyses indicated differing levels of REST at these promoters in male and female mice. Full-length Rest conditional knockout in primary sensory neurons reduced SNI-induced pain hypersensitivity and rescued the SNI-induced reduction in the expression of Oprd1 and Cnr1 in the DRG in both male and female mice. Our results suggest that nerve injury represses the transcription of Oprd1 and Cnr1 via REST in primary sensory neurons and that REST is a potential therapeutic target for neuropathic pain.

DNA demethylation in the hypothalamus promotes transcription of Agtr1a and Slc12a2 and hypertension development

Increased expression of angiotensin II AT1A receptor (encoded by Agtr1a) and Na+-K+-Cl- cotransporter-1 (NKCC1, encoded by Slc12a2) in the hypothalamic paraventricular nucleus (PVN) contributes to hypertension development. However, little is known about their transcriptional control in the PVN in hypertension. DNA methylation is a critical epigenetic mechanism that regulates gene expression. Here, we determined whether transcriptional activation of Agtr1a and Slc12a2 results from altered DNA methylation in spontaneously hypertensive rats (SHR). Methylated DNA immunoprecipitation and bisulfite sequencing-PCR showed that CpG methylation at Agtr1a and Slc12a2 promoters in the PVN was progressively diminished in SHR compared with normotensive Wistar-Kyoto rats (WKY). Chromatin immunoprecipitation-quantitative PCR revealed that enrichment of DNA methyltransferases (DNMT1 and DNMT3A) and methyl-CpG binding protein 2, a DNA methylation reader protein, at Agtr1a and Slc12a2 promoters in the PVN was profoundly reduced in SHR compared with WKY. By contrast, the abundance of ten-eleven translocation enzymes (TET1-3) at Agtr1a and Slc12a2 promoters in the PVN was much greater in SHR than in WKY. Furthermore, microinjecting of RG108, a selective DNMT inhibitor, into the PVN of WKY increased arterial blood pressure and correspondingly potentiated Agtr1a and Slc12a2 mRNA levels in the PVN. Conversely, microinjection of C35, a specific TET inhibitor, into the PVN of SHR markedly reduced arterial blood pressure, accompanied by a decrease in Agtr1a and Slc12a2 mRNA levels in the PVN. Collectively, our findings suggest that DNA hypomethylation resulting from the DNMT/TET switch at gene promoters in the PVN promotes transcription of Agtr1a and Slc12a2 and hypertension development.

Curcumin can improve spinal cord injury by inhibiting DNA methylation

Spinal cord injury (SCI) is a serious central nervous system disease. Traumatic SCI often causes persistent neurological deficits below the injury level. Epigenetic changes occur after SCI. Studies have shown DNA methylation to be a key player in nerve regeneration and remodeling, and in regulating some pathophysiological characteristics of SCI. Curcumin is a natural polyphenol from turmeric. It has anti-inflammatory, antioxidant, and neuroprotective effects, and can mitigate the cell and tissue damage caused by SCI. This report analyzed the specific functions of DNA methylation in central nervous system diseases, especially traumatic brain injury and SCI. DNA methylation can regulate the level of gene expressions in the central nervous system. Therefore, pharmacological interventions regulating DNA methylation may be promising for SCI.

The Epigenetics of Neuropathic Pain: A Systematic Update

Epigenetics deals with alterations to the gene expression that occur without change in the nucleotide sequence in the DNA. Various covalent modifications of the DNA and/or the surrounding histone proteins have been revealed, including DNA methylation, histone acetylation, and methylation, which can either stimulate or inhibit protein expression at the transcriptional level. In the past decade, an exponentially increasing amount of data has been published on the association between epigenetic changes and the pathomechanism of pain, including its most challenging form, neuropathic pain. Epigenetic regulation of the chromatin by writer, reader, and eraser proteins has been revealed for diverse protein targets involved in the pathomechanism of neuropathic pain. They include receptors, ion channels, transporters, enzymes, cytokines, chemokines, growth factors, inflammasome proteins, etc. Most work has been invested in clarifying the epigenetic downregulation of mu opioid receptors and various K+ channels, two types of structures mediating neuronal inhibition. Conversely, epigenetic upregulation has been revealed for glutamate receptors, growth factors, and lymphokines involved in neuronal excitation. All these data cannot only help better understand the development of neuropathic pain but outline epigenetic writers, readers, and erasers whose pharmacological inhibition may represent a novel option in the treatment of pain.

POU2F1/DNMT3a Pathway Participates in Neuropathic Pain by Hypermethylation-Mediated LRFN4 Downregulation Following Oxaliplatin Treatment

Evidence demonstrates that DNA methylation is associated with the occurrence and development of various neurological diseases. However, the potential target genes undergoing DNA methylation, as well as their involvement in the chemotherapy drug oxaliplatin-induced neuropathic pain, are still unclear. Here, Lrfn4, which showed hypermethylation in the promoter regions, was screened from the SRA methylation database (PRJNA587622) following oxaliplatin treatment. MeDIP and qPCR assays identified that oxaliplatin treatment increased the methylation in Lrfn4 promoter region and decreased the expression of LRFN4 in the spinal dorsal horn. The assays with gain and loss of LRFN4 function demonstrated that LRFN4 downregulation in spinal dorsal horn contributed to the oxaliplatin-induced mechanical allodynia and cold hyperalgesia. Moreover, oxaliplatin treatment increased the DNA methyltransferases DNMT3a expression and the interaction between DNMT3a and Lrfn4 promoter, while inhibition of DNMT3a prevented the downregulation of LRFN4a induced by oxaliplatin. We also observed that the transcriptional factor POU2F1 can bind to the predicted sites in DNMT3a promoter region, oxaliplatin treatment upregulated the expression of transcriptional factor POU2F1 in dorsal horn neurons. Intrathecal injection of POU2F1 siRNA prevented the DNMT3a upregulation and the LRFN4 downregulation induced by oxaliplatin. Additionally, intrathecal injection of DNMT3a siRNA or POU2F1 siRNA alleviated the mechanical allodynia induced by oxaliplatin. These findings suggested that transcription factor POU2F1 upregulated the expression of DNMT3a, which subsequently decreased LRFN4 expression through hypermethylation modification in spinal dorsal horn, thereby mediating neuropathic pain following oxaliplatin treatment.

CpG Methylation Levels in HPA Axis Genes Predict Chronic Pain Outcomes Following Trauma Exposure

Chronic post-traumatic musculoskeletal pain (CPTP) is a common outcome of traumatic stress exposure. Biological factors that influence the development of CPTP are poorly understood, though current evidence indicates that the hypothalamic-pituitary-adrenal (HPA) axis plays a critical role in its development. Little is known about molecular mechanisms underlying this association, including epigenetic mechanisms. Here, we assessed whether peritraumatic DNA methylation levels at 248 5'-C-phosphate-G-3' (CpG) sites in HPA axis genes (FKBP5, NR3C1, CRH, CRHR1, CRHR2, CRHBP, POMC) predict CPTP and whether identified CPTP-associated methylation levels influence expression of those genes. Using participant samples and data collected from trauma survivors enrolled into longitudinal cohort studies (n = 290), we used linear mixed modeling to assess the relationship between peritraumatic blood-based CpG methylation levels and CPTP. A total of 66 (27%) of the 248 CpG sites assessed in these models statistically significantly predicted CPTP, with the three most significantly associated CpG sites originating from the POMC gene region (ie, cg22900229 [β = .124, P < .001], cg16302441 [β = .443, P < .001], cg01926269 [β = .130, P < .001]). Among the genes analyzed, both POMC (z = 2.36, P = .018) and CRHBP (z = 4.89, P < .001) were enriched in CpG sites significantly associated with CPTP. Further, POMC expression was inversely correlated with methylation levels in a CPTP-dependent manner (6-months NRS<4: r = -.59, P < .001; 6-months NRS ≥ 4: r = -.18, P = .2312). Our results suggest that methylation of HPA axis genes including POMC and CRHBP predict risk for and may contribute to vulnerability to CPTP. PERSPECTIVE: Peritraumatic blood levels of CpG methylation sites in HPA axis genes, particularly CpG sites in the POMC gene, predict CPTP development. This data substantially advances our understanding of epigenetic predictors and potential mediators of CPTP, a highly common, morbid, and hard-to-treat form of chronic pain.

NMDA Receptors at Primary Afferent-Excitatory Neuron Synapses Differentially Sustain Chemotherapy- and Nerve Trauma-Induced Chronic Pain

The spinal dorsal horn contains vesicular glutamate transporter-2 (VGluT2)-expressing excitatory neurons and vesicular GABA transporter (VGAT)-expressing inhibitory neurons, which normally have different roles in nociceptive transmission. Spinal glutamate NMDAR hyperactivity is a crucial mechanism of chronic neuropathic pain. However, it is unclear how NMDARs regulate primary afferent input to spinal excitatory and inhibitory neurons in neuropathic pain. Also, the functional significance of presynaptic NMDARs in neuropathic pain has not been defined explicitly. Here we showed that paclitaxel treatment or spared nerve injury (SNI) similarly increased the NMDAR-mediated mEPSC frequency and dorsal root-evoked EPSCs in VGluT2 dorsal horn neurons in male and female mice. By contrast, neither paclitaxel nor SNI had any effect on mEPSCs or evoked EPSCs in VGAT neurons. In mice with conditional Grin1 (gene encoding GluN1) KO in primary sensory neurons (Grin1-cKO), paclitaxel treatment failed to induce pain hypersensitivity. Unexpectedly, SNI still caused long-lasting pain hypersensitivity in Grin1-cKO mice. SNI increased the amplitude of puff NMDA currents in VGluT2 neurons and caused similar depolarizing shifts in GABA reversal potentials in WT and Grin1-cKO mice. Concordantly, spinal Grin1 knockdown diminished SNI-induced pain hypersensitivity. Thus, presynaptic NMDARs preferentially amplify primary afferent input to spinal excitatory neurons in neuropathic pain. Although presynaptic NMDARs are required for chemotherapy-induced pain hypersensitivity, postsynaptic NMDARs in spinal excitatory neurons play a dominant role in traumatic nerve injury-induced chronic pain. Our findings reveal the divergent synaptic connectivity and functional significance of spinal presynaptic and postsynaptic NMDARs in regulating cell type-specific nociceptive input in neuropathic pain with different etiologies.SIGNIFICANCE STATEMENT Spinal excitatory neurons relay input from nociceptors, whereas inhibitory neurons repress spinal nociceptive transmission. Chronic nerve pain is associated with aberrant NMDAR activity in the spinal dorsal horn. This study demonstrates, for the first time, that chemotherapy and traumatic nerve injury preferentially enhance the NMDAR activity at primary afferent-excitatory neuron synapses but have no effect on primary afferent input to spinal inhibitory neurons. NMDARs in primary sensory neurons are essential for chemotherapy-induced chronic pain, whereas nerve trauma causes pain hypersensitivity predominantly via postsynaptic NMDARs in spinal excitatory neurons. Thus, presynaptic and postsynaptic NMDARs at primary afferent-excitatory neuron synapses are differentially engaged in chemotherapy- and nerve injury-induced chronic pain and could be targeted respectively for treating these painful conditions.

Intraperitoneal 5-Azacytidine Alleviates Nerve Injury-Induced Pain in Rats by Modulating DNA Methylation

To investigate the role of DNA methylation in modulating chronic neuropathic pain (NPP), identify possible target genes of DNA methylation involved in this process, and preliminarily confirm the medicinal value of the DNA methyltransferases (DNMTs) inhibitor 5-azacytidine (5-AZA) in NPP by targeting gene methylation. Two rat NPP models, chronic constriction injury (CCI) and spinal nerve ligation (SNL), were used. The DNA methylation profiles in the lumbar spinal cord were assayed using an Arraystar Rat RefSeq Promoter Array. The underlying genes with differential methylation were then identified and submitted to Gene Ontology and pathway analysis. Methyl-DNA immunoprecipitation quantitative PCR (MeDIP-qPCR) and quantitative reverse transcription-PCR (RT-qPCR) were used to confirm gene methylation and expression. The protective function of 5-AZA in NPP and gene expression were evaluated via behavioral assays and RT-qPCR, respectively. Analysis of the DNA methylation patterns in the lumbar spinal cord indicated that 1205 differentially methylated fragments in CCI rats were located within DNA promoter regions, including 638 hypermethylated fragments and 567 hypomethylated fragments. The methylation levels of Grm4, Htr4, Adrb2, Kcnf1, Gad2, and Pparg, which are associated with long-term potentiation (LTP) and glutamatergic synapse pathways, were increased with a corresponding decrease in their mRNA expression, in the spinal cords of CCI rats. Moreover, we found that the intraperitoneal injection of 5-AZA (4 mg/kg) attenuated CCI- or SNL-induced mechanical allodynia and thermal hyperalgesia. Finally, the mRNA expression of hypermethylated genes such as Grm4, Htr4, Adrb2, Kcnf1, and Gad2 was reversed after 5-AZA treatment. CCI induced widespread methylation changes in the DNA promoter regions in the lumbar spinal cord. Intraperitoneal 5-AZA alleviated hyperalgesia in CCI and SNL rats, an effect accompanied by the reversed expression of hypermethylated genes. Thus, DNA methylation inhibition represents a promising epigenetic strategy for protection against chronic NPP following nerve injury. Our study lays a theoretical foundation for 5-AZA to become a clinical targeted drug.

Epigenetic and epitranscriptomic regulation of axon regeneration

Effective axonal regeneration in the adult mammalian nervous system requires coordination of elevated intrinsic growth capacity and decreased responses to the inhibitory environment. Intrinsic regenerative capacity largely depends on the gene regulatory network and protein translation machinery. A failure to activate these pathways upon injury is underlying a lack of robust axon regeneration in the mature mammalian central nervous system. Epigenetics and epitranscriptomics are key regulatory mechanisms that shape gene expression and protein translation. Here, we provide an overview of different types of modifications on DNA, histones, and RNA, underpinning the regenerative competence of axons in the mature mammalian peripheral and central nervous systems. We highlight other non-neuronal cells and their epigenetic changes in determining the microenvironment for tissue repair and axon regeneration. We also address advancements of single-cell technology in charting transcriptomic and epigenetic landscapes that may further facilitate the mechanistic understanding of differential regenerative capacity in neuronal subtypes. Finally, as epigenetic and epitranscriptomic processes are commonly affected by brain injuries and psychiatric disorders, understanding their alterations upon brain injury would provide unprecedented mechanistic insights into etiology of injury-associated-psychiatric disorders and facilitate the development of therapeutic interventions to restore brain function.

Photophysical Mechanisms of Photobiomodulation Therapy as Precision Medicine

Despite a significant focus on the photochemical and photoelectrical mechanisms underlying photobiomodulation (PBM), its complex functions are yet to be fully elucidated. To date, there has been limited attention to the photophysical aspects of PBM. One effect of photobiomodulation relates to the non-visual phototransduction pathway, which involves mechanotransduction and modulation to cytoskeletal structures, biophotonic signaling, and micro-oscillatory cellular interactions. Herein, we propose a number of mechanisms of PBM that do not depend on cytochrome c oxidase. These include the photophysical aspects of PBM and the interactions with biophotons and mechanotransductive processes. These hypotheses are contingent on the effect of light on ion channels and the cytoskeleton, the production of biophotons, and the properties of light and biological molecules. Specifically, the processes we review are supported by the resonant recognition model (RRM). This previous research demonstrated that protein micro-oscillations act as a signature of their function that can be activated by resonant wavelengths of light. We extend this work by exploring the local oscillatory interactions of proteins and light because they may affect global body circuits and could explain the observed effect of PBM on neuro-cortical electroencephalogram (EEG) oscillations. In particular, since dysrhythmic gamma oscillations are associated with neurodegenerative diseases and pain syndromes, including migraine with aura and fibromyalgia, we suggest that transcranial PBM should target diseases where patients are affected by impaired neural oscillations and aberrant brain wave patterns. This review also highlights examples of disorders potentially treatable with precise wavelengths of light by mimicking protein activity in other tissues, such as the liver, with, for example, Crigler-Najjar syndrome and conditions involving the dysregulation of the cytoskeleton. PBM as a novel therapeutic modality may thus behave as "precision medicine" for the treatment of various neurological diseases and other morbidities. The perspectives presented herein offer a new understanding of the photophysical effects of PBM, which is important when considering the relevance of PBM therapy (PBMt) in clinical applications, including the treatment of diseases and the optimization of health outcomes and performance.

HDAC2 in Primary Sensory Neurons Constitutively Restrains Chronic Pain by Repressing α2δ-1 Expression and Associated NMDA Receptor Activity

α2δ-1 (encoded by the Cacna2d1 gene) is a newly discovered NMDA receptor-interacting protein and is the therapeutic target of gabapentinoids (e.g., gabapentin and pregabalin) frequently used for treating patients with neuropathic pain. Nerve injury causes sustained α2δ-1 upregulation in the dorsal root ganglion (DRG), which promotes NMDA receptor synaptic trafficking and activation in the spinal dorsal horn, a hallmark of chronic neuropathic pain. However, little is known about how nerve injury initiates and maintains the high expression level of α2δ-1 to sustain chronic pain. Here, we show that nerve injury caused histone hyperacetylation and diminished enrichment of histone deacetylase-2 (HDAC2), but not HDAC3, at the Cacna2d1 promoter in the DRG. Strikingly, Hdac2 knockdown or conditional knockout in DRG neurons in male and female mice consistently induced long-lasting mechanical pain hypersensitivity, which was readily reversed by blocking NMDA receptors, inhibiting α2δ-1 with gabapentin or disrupting the α2δ-1-NMDA receptor interaction at the spinal cord level. Hdac2 deletion in DRG neurons increased histone acetylation levels at the Cacna2d1 promoter, upregulated α2δ-1 in the DRG, and potentiated α2δ-1-dependent NMDA receptor activity at primary afferent central terminals in the spinal dorsal horn. Correspondingly, Hdac2 knockdown-induced pain hypersensitivity was blunted in Cacna2d1 knockout mice. Thus, our findings reveal that HDAC2 functions as a pivotal transcriptional repressor of neuropathic pain via constitutively suppressing α2δ-1 expression and ensuing presynaptic NMDA receptor activity in the spinal cord. HDAC2 enrichment levels at the Cacna2d1 promoter in DRG neurons constitute a unique epigenetic mechanism that governs acute-to-chronic pain transition.SIGNIFICANCE STATEMENT Excess α2δ-1 proteins produced after nerve injury directly interact with glutamate NMDA receptors to potentiate synaptic NMDA receptor activity in the spinal cord, a prominent mechanism of nerve pain. Because α2δ-1 upregulation after nerve injury is long lasting, gabapentinoids relieve pain symptoms only temporarily. Our study demonstrates for the first time the unexpected role of intrinsic HDAC2 activity at the α2δ-1 gene promoter in limiting α2δ-1 gene transcription, NMDA receptor-dependent synaptic plasticity, and chronic pain development after nerve injury. These findings challenge the prevailing view about the role of general HDAC activity in promoting chronic pain. Restoring the repressive HDAC2 function and/or reducing histone acetylation at the α2δ-1 gene promoter in primary sensory neurons could lead to long-lasting relief of nerve pain.

Direct cell-to-cell transfer in stressed tumor microenvironment aggravates tumorigenic or metastatic potential in pancreatic cancer

Pancreatic cancer exhibits a characteristic tumor microenvironment (TME) due to enhanced fibrosis and hypoxia and is particularly resistant to conventional chemotherapy. However, the molecular mechanisms underlying TME-associated treatment resistance in pancreatic cancer are not fully understood. Here, we developed an in vitro TME mimic system comprising pancreatic cancer cells, fibroblasts and immune cells, and a stress condition, including hypoxia and gemcitabine. Cells with high viability under stress showed evidence of increased direct cell-to-cell transfer of biomolecules. The resulting derivative cells (CD44^high/SLC16A1^high) were similar to cancer stem cell-like-cells (CSCs) with enhanced anchorage-independent growth or invasiveness and acquired metabolic reprogramming. Furthermore, CD24 was a determinant for transition between the tumorsphere formation or invasive properties. Pancreatic cancer patients with CD44^low/SLC16A1^low expression exhibited better prognoses compared to other groups. Our results suggest that crosstalk via direct cell-to-cell transfer of cellular components foster chemotherapy-induced tumor evolution and that targeting of CD44 and MCT1(encoded by SLC16A1) may be useful strategy to prevent recurrence of gemcitabine-exposed pancreatic cancers.

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.

Sensory neuron dysfunction in orthotopic mouse models of colon cancer

Reports of neurological sequelae related to colon cancer are largely restricted to rare instances of paraneoplastic syndromes, due to autoimmune reactions. Systemic inflammation associated with tumor development influences sensory neuron function in other disease models, though the extent to which this occurs in colorectal cancer is unknown. We induced orthotopic colorectal cancer via orthotopic injection of two colorectal cancer cell lines (MC38 and CT26) in two different mouse strains (C57BL/6 and Balb/c, respectively). Behavioral tests of pain sensitivity and activity did not detect significant alterations in sensory sensitivity or diminished well-being throughout tumor development. However, immunohistochemistry revealed widespread reductions in intraepidermal nerve fiber density in the skin of tumor-bearing mice. Though loss of nerve fiber density was not associated with increased expression of cell injury markers in dorsal root ganglia, lumbar dorsal root ganglia neurons of tumor-bearing animals showed deficits in mitochondrial function. These neurons also had reduced cytosolic calcium levels in live-cell imaging and reduced spontaneous activity in multi-electrode array analysis. Bulk RNA sequencing of DRGs from tumor-bearing mice detected activation of gene expression pathways associated with elevated cytokine and chemokine signaling, including CXCL10. This is consistent with the detection of CXCL10 (and numerous other cytokines, chemokines and growth factors) in MC38 and CT26 cell-conditioned media, and the serum of tumor-bearing mice. Our study demonstrates in a pre-clinical setting that colon cancer is associated with latent sensory neuron dysfunction and implicates cytokine/chemokine signaling in this process. These findings may have implications for determining risk factors and treatment responsiveness related to neuropathy in colorectal cancer.

Cellular and Molecular Machinery of Neuropathic Pain: an Emerging Insight

Purpose of Review

Neuropathic pain (NP) has been ubiquitously characterized by lesion and its linked somatosensory system either the central nervous system (CNS) or peripheral nervous system (PNS) This PNS episode is the most prevalent site of NP origin and is found to be associated with afferent nerve fibers carrying pain signals from injured/trauma site to the CNS including the brain. Several kinds of pharmacotherapeutic drugs shuch as analgesics, anti-convulsants, and anti-depressants are being employed for the its possible interventions. The NP has been a great interest to follow different pathophysiological mechanisms which are often considered to correlate with the metabolic pathways and its mediated disease. There is paucity of knowledge to make such mechanism via NP.

Recent Finding

Most notably, recent pandemic outbreak of COVID-19 has also been reported in chronic pain mediated diabetes, inflammatory disorders, and cancers. There is an increasing incidence of NP and its complex mechanism has now led to identify the possible investigations of responsible genes and proteins via bioinformatics tools. The analysis might be more instrumental as collecting the genes from pain genetic database, analyzing the variants through differential gene expression (DEG) and constructing the protein–protein interaction (PPI) networks and thereby determining their upregulating and downregulating pathways.

Summary

This review sheds a bright light towards several mechanisms at both cellular and molecular level, correlation of NP-mediated disease mechanism and possible cell surface biomarkers (receptors), and identified genes could be more promising for their pharmacological targets.

Cannabinoid CB2 receptors are upregulated via bivalent histone modifications and control primary afferent input to the spinal cord in neuropathic pain

Type-2 cannabinoid receptors (CB2, encoded by the Cnr2 gene) are mainly expressed in immune cells, and CB2 agonists normally have no analgesic effect. However, nerve injury upregulates CB2 in the dorsal root ganglion (DRG), following which CB2 stimulation reduces neuropathic pain. It is unclear how nerve injury increases CB2 expression or how CB2 activity is transformed in neuropathic pain. In this study, immunoblotting showed that spinal nerve ligation (SNL) induced a delayed and sustained increase in CB2 expression in the DRG and dorsal spinal cord synaptosomes. RNAscope in situ hybridization also showed that SNL substantially increased CB2 mRNA levels, mostly in medium and large DRG neurons. Furthermore, we found that the specific CB2 agonist JWH-133 significantly inhibits the amplitude of dorsal root-evoked glutamatergic excitatory postsynaptic currents in spinal dorsal horn neurons in SNL rats, but not in sham control rats; intrathecal injection of JWH-133 reversed pain hypersensitivity in SNL rats, but had no effect in sham control rats. In addition, chromatin immunoprecipitation-qPCR analysis showed that SNL increased enrichment of two activating histone marks (H3K4me3 and H3K9ac) and diminished occupancy of two repressive histone marks (H3K9me2 and H3K27me3) at the Cnr2 promoter in the DRG. In contrast, SNL had no effect on DNA methylation levels around the Cnr2 promoter. Our findings suggest that peripheral nerve injury promotes CB2 expression in primary sensory neurons via epigenetic bivalent histone modifications and that CB2 activation reduces neuropathic pain by attenuating nociceptive transmission from primary afferent nerves to the spinal cord.

Association between CpG island DNA methylation in the promoter region of RELN and positive and negative types of schizophrenia

Objective

To explore the association between CpG island methylation in the promoter region of RELN and positive (type I) and negative (type II) types of schizophrenia, and investigate serum interleukin (IL)-1β, IL-6, and myelin basic protein (MBP) in schizophrenia.

Methods

Levels of CpG island methylation in the promoter region of RELN were detected in peripheral blood of patients with schizophrenia (experimental group) and healthy individuals (control group), and serum IL-1β, IL-6, and MBP were measured.

Results

The positive rate of CpG island methylation in the promoter region of RELN was higher in the experimental group than in the control group; however, there were no significant differences between type I and II patients. There were differences in Positive and Negative Syndrome Scale (PANSS) scores and serum IL-1β, IL-6, and MBP between type I and II patients. Furthermore, there were positive correlations between serum IL-1β, IL-6, and MBP and PANSS scores (negative symptoms) in type II patients.

Conclusion

CpG island methylation in the promoter region of RELN was associated with schizophrenia, but not with its clinical type. There may be different pathological mechanisms in type I and II schizophrenia, and type II schizophrenia may be associated with serum IL-1β, IL-6, and MBP.

The Epigenetic Mechanisms Involved in Chronic Pain in Rodents: A Mini- Review

Chronic pain is a common distressing neurological disorder and about 30% of the global population suffers from it. In addition to being highly prevalent, chronic pain causes a heavy economic and social burden. Although substantial progress has been achieved to dissect the underlying mechanism of chronic pain in the past few decades, the incidence and treatment of this neurological illness is yet not properly managed in clinical practice. While nerve injury-, chemotherapy- or inflammation-induced functional regulation of gene expression in the dorsal root ganglion and spinal cord are extensively reported to be involved in the pathogenic process of chronic pain, the specific mechanism of these altered transcriptional profile still remains unclear. Recent studies have shown that epigenetic mechanisms, including DNA/RNA methylation, histone modification and circular RNAs regulation, are involved in the occurrence and development of chronic pain. In this review, we provide a description of research on the role of epigenetic mechanism in chronic pain, summarize the latest clinical and preclinical advance in this field, and propose the potential directions for further research to elucidate the molecular mechanism underlying the pathogenesis of 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.

Epigenetic Mechanisms of Neural Plasticity in Chronic Neuropathic Pain

Neuropathic pain is a challenging clinical problem and remains difficult to treat. Altered gene expression in peripheral sensory nerves and neurons due to nerve injury is well documented and contributes critically to the synaptic plasticity in the spinal cord and the initiation and maintenance of chronic pain. However, our understanding of the epigenetic mechanisms regulating the transcription of pro-nociceptive (e.g., NMDA receptors and α2δ-1) and antinociceptive (e.g., potassium channels and opioid and cannabinoid receptors) genes are still limited. In this review, we summarize recent studies determining the roles of histone modifications (including methylation, acetylation, and ubiquitination), DNA methylation, and noncoding RNAs in neuropathic pain development. We review the epigenetic writer, reader, and eraser proteins that participate in the transcriptional control of the expression of key ion channels and neurotransmitter receptors in the dorsal root ganglion after traumatic nerve injury, which is commonly used as a preclinical model of neuropathic pain. A better understanding of epigenetic reprogramming involved in the transition from acute to chronic pain could lead to the development of new treatments for neuropathic pain.

Epigenetics and Postsurgical Pain: A Scoping Review

OBJECTIVE

Multiple factors are involved in the physiology and variability of postsurgical pain, a great part of which can be explained by genetic and environmental factors and their interaction. Epigenetics refers to the mechanism by which the environment alters the stability and expression of genes. We conducted a scoping review to examine the available evidence in both animal models and clinical studies on epigenetic mechanisms involved in the regulation of postsurgical and chronic postsurgical pain.

METHODS

The Arksey and O'Malley framework and the PRISMA-ScR (Preferred Reporting Items for Systematic Review and Meta-Analysis, scoping reviews extension) guidelines were used. The PubMed, Web of Science, and Google Scholar databases were searched, and the original articles cited in reviews located through the search were also reviewed. English-language articles without time limits were retrieved. Articles were selected if the abstract addressed information on the epigenetic or epigenomic mechanisms, histone, or DNA methylation and microribonucleic acids involved in postsurgical and chronic postsurgical pain in animal models and clinical studies.

RESULTS

The initial search provided 174 articles, and 95 were used. The available studies to date, mostly in animal models, have shown that epigenetics contributes to the regulation of gene expression in the pathways involved in postsurgical pain and in maintaining long-term pain.

CONCLUSION

Research on possible epigenetic mechanisms involved in postsurgical pain and chronic postsurgical pain in humans is scarce. In view of the evidence available in animal models, there is a need to evaluate epigenetic pain mechanisms in the context of human and clinical studies.

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.

Molecular Changes in the Dorsal Root Ganglion during the Late Phase of Peripheral Nerve Injury-induced Pain in Rodents: A Systematic Review

The dorsal root ganglion is widely recognized as a potential target to treat chronic pain. A fundamental understanding of quantitative molecular and genomic changes during the late phase of pain is therefore indispensable. The authors performed a systematic literature review on injury-induced pain in rodent dorsal root ganglions at minimally 3 weeks after injury. So far, slightly more than 300 molecules were quantified on the protein or messenger RNA level, of which about 60 were in more than one study. Only nine individual sequencing studies were performed in which the most up- or downregulated genes varied due to heterogeneity in study design. Neuropeptide Y and galanin were found to be consistently upregulated on both the gene and protein levels. The current knowledge regarding molecular changes in the dorsal root ganglion during the late phase of pain is limited. General conclusions are difficult to draw, making it hard to select specific molecules as a focus for treatment.

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.

MeCP2 Epigenetic Silencing of Oprm1 Gene in Primary Sensory Neurons Under Neuropathic Pain Conditions

Opioids are the last option for the pharmacological treatment of neuropathic pain, but their antinociceptive effects are limited. Decreased mu opioid receptor (MOR) expression in the peripheral nervous system may contribute to this. Here, we showed that nerve injury induced hypermethylation of the Oprm1 gene promoter and an increased expression of methyl-CpG binding protein 2 (MeCP2) in injured dorsal root ganglion (DRG). The downregulation of MOR in the DRG is closely related to the augmentation of MeCP2, an epigenetic repressor, which could recruit HDAC1 and bind to the methylated regions of the Oprm1 gene promoter. MeCP2 knockdown restored the expression of MOR in injured DRG and enhanced the analgesic effect of morphine, while the mimicking of this increase via the intrathecal infusion of viral vector-mediated MeCP2 was sufficient to reduce MOR in the DRG. Moreover, HDAC1 inhibition with suberoylanilide hydroxamic acid, an HDAC inhibitor, also prevented MOR reduction in the DRG of neuropathic pain mice, contributing to the augmentation of morphine analgesia effects. Mechanistically, upregulated MeCP2 promotes the binding of a high level of HDCA1 to hypermethylated regions of the Oprm1 gene promoter, reduces the acetylation of histone H3 (acH3) levels of the Oprm1 gene promoter, and attenuates Oprm1 transcription in injured DRG. Thus, upregulated MeCP2 and HDAC1 in Oprm1 gene promoter sites, negatively regulates MOR expression in injured DRG, mitigating the analgesic effect of the opioids. Targeting MeCP2/HDAC1 may thus provide a new solution for improving the therapeutic effect of opioids in a clinical setting.

The methyl donor S-adenosyl methionine reverses the DNA methylation signature of chronic neuropathic pain in mouse frontal cortex

Supplemental Digital Content is Available in the Text.

Chronic administration of S-adenosylmethionine reverses neuropathic pain–induced changes in DNA methylation in the mouse frontal cortex.

Chronic pain is associated with persistent but reversible structural and functional changes in the prefrontal cortex (PFC). This stable yet malleable plasticity implicates epigenetic mechanisms, including DNA methylation, as a potential mediator of chronic pain–induced cortical pathology. We previously demonstrated that chronic oral administration of the methyl donor S-adenosyl methionine (SAM) attenuates long-term peripheral neuropathic pain and alters global frontal cortical DNA methylation. However, the specific genes and pathways associated with the resolution of chronic pain by SAM remain unexplored.

Whole Genomic DNA Methylation Profiling of CpG Sites in Promoter Regions of Dorsal Root Ganglion in Diabetic Neuropathic Pain Mice

DNA methylation and demethylation play an important role in neuropathic pain. In general, DNA methylation of CpG sites in the promoter region impedes gene expression, whereas DNA demethylation contributes to gene expression. Here, we evaluated the methylation status of CpG sites in genomic DNA promoter regions in dorsal root ganglions (DRGs) of diabetic neuropathic pain (DNP) mice. In our research, streptozotocin (STZ) was intraperitoneally injected into mice to construct DNP models. The DNP mice showed higher fasting blood glucose (above 11.1 mmol/L), lower body weight, and mechanical allodynia than control mice. Whole-genome bisulfite sequencing (WGBS) revealed an altered methylation pattern in CpG sites in the DNA promoter regions in DRGs of DNP mice. The results showed 376 promoter regions with hypermethylated CpG sites and 336 promoter regions with hypomethylated CpG sites. In addition, our data indicated that altered DNA methylation occurs primarily on CpG sites in DNA promoter regions. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that differentially methylated CpG sites annotated genes were involved in activities of the nervous and sensory systems. Enrichment analysis indicated that genes in these pathways contributed to diabetes or pain. In conclusion, our study enriched the role of DNA methylation in DNP.

DNMT3b SUMOylation Mediated MMP-2 Upregulation Contribute to Paclitaxel Induced Neuropathic Pain

Paclitaxel is a common chemotherapeutic agent in cancer treatment, while it often causes chemotherapy-induced peripheral neuropathy (CIPN), which manifested as hyperalgesia and allodynia, and its mechanism remains largely unknown. The previous study has shown that matrix metalloproteinase-2 (MMP-2) plays a pivotal role in spinal nerve ligation (SNL) induced neuropathic pain, but its function in CIPN and exact molecular mechanisms underlying upregulation is not explored. Our present study revealed that MMP-2 is also upregulated in paclitaxel induced neuropathic pain (NP), and knockdown it by siRNA can ameliorate mechanical allodynia. Since DNA methylation is closely related to gene transcription, we explored the methylation status of the MMP-2 gene and demonstrated that MMP-2 upregulation is related to the reduced methylation level of its promoter. DNA methylation is mediated by DNA methyltransferases (DNMTs), and previous studies suggested that three main types of DNMTs can undergo SUMOylation. Our next study revealed that SUMO1 modification of DNMT3b is significantly enhanced. Intrathecal administration of SUMOylation inhibitor, ginkgolic acid (GA), could reverse enhanced SUMO1 modification of DNMT3b and upregulation of MMP-2 in the model rats. Further investigation suggested that DNMT3b binding activity to the promoter region of the MMP-2 gene is significantly decreased in paclitaxel treated rats, and the administration of GA can reverse these effects, which is also accompanied by changes in the promoter methylation status of the MMP-2 gene. Our study demonstrates that MMP-2 up-regulation mediated by DNMT3b SUMOylation is essential for paclitaxel induced NP development, which brings us new therapeutic options for CIPN.

Interleukin-10 resolves pain hypersensitivity induced by cisplatin by reversing sensory neuron hyperexcitability

Understanding the mechanisms that drive transition from acute to chronic pain is essential to identify new therapeutic targets. The importance of endogenous resolution pathways acting as a "brake" to prevent development of chronic pain has been largely ignored. We examined the role of interleukin-10 (IL-10) in resolution of neuropathic pain induced by cisplatin. In search of an underlying mechanism, we studied the effect of cisplatin and IL-10 on spontaneous activity (SA) in dorsal root ganglia neurons. Cisplatin (2 mg/kg daily for 3 days) induced mechanical hypersensitivity that resolved within 3 weeks. In both sexes, resolution of mechanical hypersensitivity was delayed in Il10 mice, in WT mice treated intrathecally with neutralizing anti-IL-10 antibody, and in mice with cell-targeted deletion of IL-10R1 on advillin-positive sensory neurons. Electrophysiologically, small- to medium-sized dorsal root ganglia neurons from cisplatin-treated mice displayed an increase in the incidence of SA. Cisplatin treatment also depolarized the resting membrane potential, and decreased action potential voltage threshold and rheobase, while increasing ongoing activity at -45 mV and the amplitude of depolarizing spontaneous fluctuations. In vitro addition of IL-10 (10 ng/mL) reversed the effect of cisplatin on SA and on the depolarizing spontaneous fluctuation amplitudes, but unexpectedly had little effect on the other electrophysiological parameters affected by cisplatin. Collectively, our findings challenge the prevailing concept that IL-10 resolves pain solely by dampening neuroinflammation and demonstrate in a model of chemotherapy-induced neuropathic pain that endogenous IL-10 prevents transition to chronic pain by binding to IL-10 receptors on sensory neurons to regulate their activity.

The transition from acute to chronic pain: dynamic epigenetic reprogramming of the mouse prefrontal cortex up to 1 year after nerve injury

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DNA methylation undergoes rapid and large changes in mouse prefrontal cortex at multiple time points postinjury, implicating hundreds of genes in a time-dependent manner.

Chronic pain is associated with persistent structural and functional changes throughout the neuroaxis, including in the prefrontal cortex (PFC). The PFC is important in the integration of sensory, cognitive, and emotional information and in conditioned pain modulation. We previously reported widespread epigenetic reprogramming in the PFC many months after nerve injury in rodents. Epigenetic modifications, including DNA methylation, can drive changes in gene expression without modifying DNA sequences. To date, little is known about epigenetic dysregulation at the onset of acute pain or how it progresses as pain transitions from acute to chronic. We hypothesize that acute pain after injury results in rapid and persistent epigenetic remodelling in the PFC that evolves as pain becomes chronic. We further propose that understanding epigenetic remodelling will provide insights into the mechanisms driving pain-related changes in the brain. Epigenome-wide analysis was performed in the mouse PFC 1 day, 2 weeks, 6 months, and 1 year after peripheral injury using the spared nerve injury in mice. Spared nerve injury resulted in rapid and persistent changes in DNA methylation, with robust differential methylation observed between spared nerve injury and sham-operated control mice at all time points. Hundreds of differentially methylated genes were identified, including many with known function in pain. Pathway analysis revealed enrichment in genes related to stimulus response at early time points, immune function at later time points, and actin and cytoskeletal regulation throughout the time course. These results emphasize the importance of considering pain chronicity in both pain research and in treatment optimization.

Adult mouse dorsal root ganglia neurons form aberrant glutamatergic connections in dissociated cultures

Cultured sensory neurons can exhibit complex activity patterns following stimulation in terms of increased excitability and interconnected responses of multiple neurons. Although these complex activity patterns suggest a network-like configuration, research so far had little interest in synaptic network formation ability of the sensory neurons. To identify interaction profiles of Dorsal Root Ganglia (DRG) neurons and explore their putative connectivity, we developed an in vitro experimental approach. A double transgenic mouse model, expressing genetically encoded calcium indicator (GECI) in their glutamatergic neurons, was produced. Dissociated DRG cultures from adult mice were prepared with a serum-free protocol and no additional growth factors or cytokines were utilized for neuronal sensitization. DRG neurons were grown on microelectrode arrays (MEA) to induce stimulus-evoked activity with a modality-free stimulation strategy. With an almost single-cell level electrical stimulation, spontaneous and evoked activity of GCaMP6s expressing neurons were detected under confocal microscope. Typical responses were analyzed, and correlated calcium events were detected across individual DRG neurons. Next, correlated responses were successfully blocked by glutamatergic receptor antagonists, which indicated functional synaptic coupling. Immunostaining confirmed the presence of synapses mainly in the axonal terminals, axon-soma junctions and axon-axon intersection sites. Concisely, the results presented here illustrate a new type of neuron-to-neuron interaction in cultured DRG neurons conducted through synapses. The developed assay can be a valuable tool to analyze individual and collective responses of the cultured sensory neurons.

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.