Epigenetic regulation of persistent pain

Histone deacetylase as emerging pharmacological therapeutic target for neuropathic pain: From epigenetic to selective drugs

BACKGROUND

Neuropathic pain remains a formidable challenge for modern medicine. The first-line pharmacological therapies exhibit limited efficacy and unfavorable side effect profiles, highlighting an unmet need for effective therapeutic medications. The past decades have witnessed an explosion in efforts to translate epigenetic concepts into pain therapy and shed light on epigenetics as a promising avenue for pain research. Recently, the aberrant activity of histone deacetylase (HDAC) has emerged as a key mechanism contributing to the development and maintenance of neuropathic pain.

AIMS

In this review, we highlight the distinctive role of specific HDAC subtypes in a cell-specific manner in pain nociception, and outline the recent experimental evidence supporting the therapeutic potential of HDACi in neuropathic pain.

METHODS

We have summarized studies of HDAC in neuropathic pain in Pubmed.

RESULTS

HDACs, widely distributed in the neuronal and non-neuronal cells of the dorsal root ganglion and spinal cord, regulate gene expression by deacetylation of histone or non-histone proteins and involving in increased neuronal excitability and neuroinflammation, thus promoting peripheral and central sensitization. Importantly, pharmacological manipulation of aberrant acetylation using HDAC-targeted inhibitors (HDACi) has shown promising pain-relieving properties in various preclinical models of neuropathic pain. Yet, many of which exhibit low-specificity that may induce off-target toxicities, underscoring the necessity for the development of isoform-selective HDACi in pain management.

CONCLUSIONS

Abnormally elevated HDACs promote neuronal excitability and neuroinflammation by epigenetically modulating pivotal gene expression in neuronal and immune cells, contributing to peripheral and central sensitization in the progression of neuropathic pain, and HDACi showed significant efficacy and great potential for alleviating neuropathic pain.

Hippocampal synaptic plasticity injury mediated by SIRT1 downregulation is involved in chronic pain-related cognitive dysfunction

AIMS

Cognitive dysfunction associated with chronic pain may be caused by impaired synaptic plasticity. Considering the impact of silent information regulator 1 (SIRT1) on synaptic plasticity, we explored the exact role of SIRT1 in cognitive impairment caused by chronic pain.

METHODS

We evaluated the memory ability of mice with the fear conditioning test (FCT) after spared nerve injury (SNI) model. Western blotting and immunofluorescence were used to analyze the expression levels of SIRT1. Hippocampal synaptic plasticity was detected with Golgi staining, transmission electron microscopy, and long-term potentiation (LTP). In the intervention study, AAV9-CaMKIIα-Cre-EGFP was injected to SIRT1flox/flox mice to knockdown the expression levels of SIRT1. Besides, SNI mice were injected with AAV2/9-CaMKIIα-SIRT1-3*Flag-GFP or SRT1720 to increase the expression levels or enzymatic activity of SIRT1.

RESULTS

Our current results indicated that cognitive function in SNI mice was impaired, SIRT1 expression in glutaminergic neurons in the hippocampal CA1 area was downregulated, and synaptic plasticity was altered. Selective knockdown of SIRT1 in hippocampus damaged synaptic plasticity and cognitive function of healthy mice. In addition, the impaired synaptic plasticity and cognitive dysfunction of SNI mice could be improved by the upregulation of SIRT1 expression or enzyme activity.

CONCLUSIONS

Reduced SIRT1 expression in hippocampus of SNI mice may induce cognitive impairment associated with chronic pain by mediating the impaired synaptic plasticity.

Pain-sensorimotor interactions: New perspectives and a new model

How pain and sensorimotor behavior interact has been the subject of research and debate for many decades. This article reviews theories bearing on pain-sensorimotor interactions and considers their strengths and limitations in the light of findings from experimental and clinical studies of pain-sensorimotor interactions in the spinal and craniofacial sensorimotor systems. A strength of recent theories is that they have incorporated concepts and features missing from earlier theories to account for the role of the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions of pain in pain-sensorimotor interactions. Findings acquired since the formulation of these recent theories indicate that additional features need to be considered to provide a more comprehensive conceptualization of pain-sensorimotor interactions. These features include biopsychosocial influences that range from biological factors such as genetics and epigenetics to psychological factors and social factors encompassing environmental and cultural influences. Also needing consideration is a mechanistic framework that includes other biological factors reflecting nociceptive processes and glioplastic and neuroplastic changes in sensorimotor and related brain and spinal cord circuits in acute or chronic pain conditions. The literature reviewed and the limitations of previous theories bearing on pain-sensorimotor interactions have led us to provide new perspectives on these interactions, and this has prompted our development of a new concept, the Theory of Pain-Sensorimotor Interactions (TOPSMI) that we suggest gives a more comprehensive framework to consider the interactions and their complexity. This theory states that pain is associated with plastic changes in the central nervous system (CNS) that lead to an activation pattern of motor units that contributes to the individual's adaptive sensorimotor behavior. This activation pattern takes account of the biological, psychological, and social influences on the musculoskeletal tissues involved in sensorimotor behavior and on the plastic changes and the experience of pain in that individual. The pattern is normally optimized in terms of biomechanical advantage and metabolic cost related to the features of the individual's musculoskeletal tissues and aims to minimize pain and any associated sensorimotor changes, and thereby maintain homeostasis. However, adverse biopsychosocial factors and their interactions may result in plastic CNS changes leading to less optimal, even maladaptive, sensorimotor changes producing motor unit activation patterns associated with the development of further pain. This more comprehensive theory points towards customized treatment strategies, in line with the management approaches to pain proposed in the biopsychosocial model of pain.

Translational research updates in female health anesthesiology: a narrative review

Background and Objective

Females represent 49.6% of the global population and constitute a significant proportion of surgical patients and hospital admissions. Little is known about the bi-directional effects of sex and anesthetics or the impact of anesthetic interventions on long-term female health outcomes. Sex differences in pain pathways can influence pain experience and treatment effectiveness. The impact of anesthetic management on the recurrence of breast cancer is poorly understood, as are the long-term consequences of cardiovascular disease and safe and effective treatments in pregnancy. This review aims to outline recent advances in translational science in female health anesthesiology research and highlight critical research opportunities in pain, cancer outcomes, and cardiovascular disorders.

Methods

We searched PubMed and summarized relevant articles published in English between December 2021 and June 2022.

Key Content and Findings

Studies reveal sex differences in pain pathways and highlight the importance of sex as a biological variable in experimental designs and translational medicine. Sex differences have also been observed in side effects attributed to opioid analgesics. We summarize some of the neural circuits that might underlie these differences. In the perioperative setting, specific anesthetics are implicated in metastatic seeding potential and acute and chronic pain outcomes, suggesting the importance of anesthetic selection in comprehensive care during oncologic surgery. In the peridelivery setting, preeclampsia, a cardiovascular disorder of pregnancy, affects maternal outcomes; however, biomarkers can risk-stratify females at risk for preeclampsia and hold promise for identifying the risk of adverse neurological and other health outcomes.

Conclusions

Research that builds diagnostic and predictive tools in pain and cardiovascular disease will help anesthesiologists minimize sex-related risks and side effects associated with anesthetics and peri-hospital treatments. Sex-specific anesthesia care will improve outcomes, as will the provision of practical information to patients and clinicians about the effectiveness of therapies and behavioral interventions. However, more research studies and specific analytic plans are needed to continue addressing sex-based outcomes in anesthesiology.

Molecular and Epigenetic Aspects of Opioid Receptors in Drug Addiction and Pain Management in Sport

Opioids are substances derived from opium (natural opioids). In its raw state, opium is a gummy latex extracted from Papaver somniferum. The use of opioids and their negative health consequences among people who use drugs have been studied. Today, opioids are still the most commonly used and effective analgesic treatments for severe pain, but their use and abuse causes detrimental side effects for health, including addiction, thus impacting the user's quality of life and causing overdose. The mesocorticolimbic dopaminergic circuitry represents the brain circuit mediating both natural rewards and the rewarding aspects of nearly all drugs of abuse, including opioids. Hence, understanding how opioids affect the function of dopaminergic circuitry may be useful for better knowledge of the process and to develop effective therapeutic strategies in addiction. The aim of this review was to summarize the main features of opioids and opioid receptors and focus on the molecular and upcoming epigenetic mechanisms leading to opioid addiction. Since synthetic opioids can be effective for pain management, their ability to induce addiction in athletes, with the risk of incurring doping, is also discussed.

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.

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.

Roles of the Neuron-Restrictive Silencer Factor in the Pathophysiological Process of the Central Nervous System

The neuron-restrictive silencer factor (NRSF), also known as repressor element 1 (RE-1) silencing transcription factor (REST) or X2 box repressor (XBR), is a zinc finger transcription factor that is widely expressed in neuronal and non-neuronal cells. It is a master regulator of the nervous system, and the function of NRSF is the basis of neuronal differentiation, diversity, plasticity, and survival. NRSF can bind to the neuron-restrictive silencer element (NRSE), recruit some co-repressors, and then inhibit transcription of NRSE downstream genes through epigenetic mechanisms. In neurogenesis, NRSF functions not only as a transcriptional silencer that can mediate the transcriptional inhibition of neuron-specific genes in non-neuronal cells and thus give neuron cells specificity, but also as a transcriptional activator to induce neuronal differentiation. Many studies have confirmed the association between NRSF and brain disorders, such as brain injury and neurodegenerative diseases. Overexpression, underexpression, or mutation may lead to neurological disorders. In tumorigenesis, NRSF functions as an oncogene in neuronal tumors, such as neuroblastomas, medulloblastomas, and pheochromocytomas, stimulating their proliferation, which results in poor prognosis. Additionally, NRSF-mediated selective targets gene repression plays an important role in the development and maintenance of neuropathic pain caused by nerve injury, cancer, and diabetes. At present, several compounds that target NRSF or its co-repressors, such as REST-VP16 and X5050, have been shown to be clinically effective against many brain diseases, such as seizures, implying that NRSF and its co-repressors may be potential and promising therapeutic targets for neural disorders. In the present review, we introduced the biological characteristics of NRSF; reviewed the progress to date in understanding the roles of NRSF in the pathophysiological processes of the nervous system, such as neurogenesis, brain disorders, neural tumorigenesis, and neuropathic pain; and suggested new therapeutic approaches to such brain diseases.

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.

Exercise attenuates low back pain and alters epigenetic regulation in intervertebral discs in a mouse model

BACKGROUND CONTEXT

Chronic low back pain (LBP) is a multifactorial disorder with complex underlying mechanisms, including associations with intervertebral disc (IVD) degeneration in some individuals. It has been demonstrated that epigenetic processes are involved in the pathology of IVD degeneration. Epigenetics refers to several mechanisms, including DNA methylation, that have the ability to change gene expression without inducing any change in the underlying DNA sequence. DNA methylation can alter the entire state of a tissue for an extended period of time and thus could potentially be harnessed for long-term pain relief. Lifestyle factors, such as physical activity, have a strong influence on epigenetic regulation. Exercise is a commonly prescribed treatment for chronic LBP, and sex-specific epigenetic adaptations in response to endurance exercise have been reported. However, whether exercise interventions that attenuate LBP are associated with epigenetic alterations in degenerating IVDs has not been evaluated.

PURPOSE

We hypothesize that the therapeutic efficacy of physical activity is mediated, at least in part, at the epigenetic level. The purpose of this study was to use the SPARC-null mouse model of LBP associated with IVD degeneration to clarify (1) if IVD degeneration is associated with altered expression of epigenetic regulatory genes in the IVDs, (2) if epigenetic regulatory machinery is sensitive to therapeutic environmental intervention, and (3) if there are sex-specific differences in (1) and/or (2).

STUDY DESIGN

Eight-month-old male and female SPARC-null and age-matched control (WT) mice (n=108) were assigned to exercise (n=56) or sedentary (n=52) groups. Deletion of SPARC is associated with progressive IVD degeneration and behavioral signs of LBP. The exercise group received a circular plastic home cage running wheel on which they could run freely. The sedentary group received an identical wheel secured in place to prevent rotation. After 6 months, the results obtained in each group were compared.

METHODS

After 6 months of exercise, LBP-related behavioral indices were determined, and global DNA methylation (5-methylcytosine) and epigenetic regulatory gene mRNA expression in IVDs were assessed. This project was supported by the Canadian Institutes for Health Research. The authors have no conflicts of interest.

RESULTS

Lumbar IVDs from WT sedentary and SPARC-null sedentary mice had similar levels of global DNA methylation (%5-mC) and comparable mRNA expression of epigenetic regulatory genes (Dnmt1,3a,b, Mecp2, Mbd2a,b, Tet1-3) in both sexes. Exercise attenuated LBP-related behaviors, decreased global DNA methylation in both WT (p<.05) and SPARC-null mice (p<.01) and reduced mRNA expression of Mecp2 in SPARC-null mice (p<.05). Sex-specific effects of exercise on expression of mRNA were also observed.

CONCLUSIONS

Exercise alleviates LBP in a mouse model. This may be mediated, in part, by changes in the epigenetic regulatory machinery in degenerating IVDs. Epigenetic alterations due to a lifestyle change could have a long-lasting therapeutic impact by changing tissue homeostasis in IVDs.

CLINICAL SIGNIFICANCE

This study confirmed the therapeutic benefits of exercise on LBP and suggests that exercise results in sex-specific alterations in epigenetic regulation in IVDs. Elucidating the effects of exercise on epigenetic regulation may enable the discovery of novel gene targets or new strategies to improve the treatment of chronic LBP.

Repurposing cancer drugs identifies kenpaullone which ameliorates pathologic pain in preclinical models via normalization of inhibitory neurotransmission

Inhibitory GABA-ergic neurotransmission is fundamental for the adult vertebrate central nervous system and requires low chloride concentration in neurons, maintained by KCC2, a neuroprotective ion transporter that extrudes intracellular neuronal chloride. To identify Kcc2 gene expression‑enhancing compounds, we screened 1057 cell growth-regulating compounds in cultured primary cortical neurons. We identified kenpaullone (KP), which enhanced Kcc2/KCC2 expression and function in cultured rodent and human neurons by inhibiting GSK3ß. KP effectively reduced pathologic pain-like behavior in mouse models of nerve injury and bone cancer. In a nerve-injury pain model, KP restored Kcc2 expression and GABA-evoked chloride reversal potential in the spinal cord dorsal horn. Delta-catenin, a phosphorylation-target of GSK3ß in neurons, activated the Kcc2 promoter via KAISO transcription factor. Transient spinal over-expression of delta-catenin mimicked KP analgesia. Our findings of a newly repurposed compound and a novel, genetically-encoded mechanism that each enhance Kcc2 gene expression enable us to re-normalize disrupted inhibitory neurotransmission through genetic re-programming.

Astrocytic c-Jun N-terminal kinase-histone deacetylase-2 cascade contributes to glutamate transporter-1 decrease and mechanical allodynia following peripheral nerve injury in rats

Decrease of glutamate transporter-1 (GLT-1) in the spinal dorsal horn after nerve injury induces enhanced excitatory transmission and causes persistent pain. Histone deacetylases (HDACs)-catalyzed deacetylation might contribute to the decrease of GLT-1, while the detailed mechanisms have yet to be fully elaborated. Spinal nerve ligation (SNL) induced significant increases of HDAC2 and decreases of GLT-1 in spinal astrocytes. Intrathecal infusion of the HDAC2 inhibitors attenuated the decrease of GLT-1 and enhanced phosphorylation of glutamate receptors. GLT-1 and phosphorylated c-Jun N-terminal kinase (JNK) were highly colocalized in the spinal cord, and a large number of pJNK positive cells were HDAC2 positive. Intrathecally infusion of the JNK inhibitor SP600125 significantly inhibited SNL-induced upregulation of HDAC2. SNL-induced HDAC2 up-regulation could be inhibited by the neutralizing anti-tumor necrosis factor-α (TNF-α) binding protein etanercept or the microglial inhibitor minocycline. In cultured astrocytes, TNF-α induced enhanced phosphorylation of JNK and a significant increase of HDAC2, as well as a remarkable decrease of GLT-1, which could be prevented by SP600125 or the HDAC2 specific inhibitor CAY10683. Our data suggest that astrocytic JNK-HDAC2 cascade contributes to GLT-1 decrease and mechanical allodynia following peripheral nerve injury. Neuroimmune activation after peripheral nerve injury could induce epigenetic modification changes in astrocytes and contribute to chronic pain maintenance.

Chronic Orofacial Pain: Models, Mechanisms, and Genetic and Related Environmental Influences

Chronic orofacial pain conditions can be particularly difficult to diagnose and treat because of their complexity and limited understanding of the mechanisms underlying their aetiology and pathogenesis. Furthermore, there is considerable variability between individuals in their susceptibility to risk factors predisposing them to the development and maintenance of chronic pain as well as in their expression of chronic pain features such as allodynia, hyperalgesia and extraterritorial sensory spread. The variability suggests that genetic as well as environmental factors may contribute to the development and maintenance of chronic orofacial pain. This article reviews these features of chronic orofacial pain, and outlines findings from studies in animal models of the behavioural characteristics and underlying mechanisms related to the development and maintenance of chronic orofacial pain and trigeminal neuropathic pain in particular. The review also considers the role of environmental and especially genetic factors in these models, focussing on findings of differences between animal strains in the features and underlying mechanisms of chronic pain. These findings are not only relevant to understanding underlying mechanisms and the variability between patients in the development, expression and maintenance of chronic orofacial pain, but also underscore the importance for considering the strain of the animal to model and explore chronic orofacial pain processes.

MicroRNA-547-5p-mediated interleukin-33/suppressor of tumorigenicity 2 signaling underlies the genesis and maintenance of neuropathic pain and is targeted by the therapy with bone marrow stromal cells

Interleukin-33 (IL-33)/suppressor of tumorigenicity 2 (ST2) signaling is known to promote inflammation and the genesis and maintenance of neuropathic pain. However, it remained mostly unknown how IL-33/ST2 signaling can be enhanced by neuropathic stimulations. Here, we report that the chronic constriction nerve injury (CCI)-induced increases in the expression of IL-33 and ST2 and a decrease in microRNA (miRNA)-547-5p not only in the dorsal root ganglia (DRG) but also in spinal dorsal horn (SDH) ipsilateral to the CCI. We found that increasing endogenous miRNA-547-5p by the intrathecal (i.t.) infusion of agomir-miR-547-5p did not produce any effect in naive rats but blocked the CCI-induced increases in the IL-33 and ST2, and pain sensitivity. The reducing endogenous miRNA-547-5p by the i.t. delivering antagomir-miR-547-5p into naive rats caused significant changes in IL-33 and ST2 expressions in both the DRG and SDH, and pain sensitivity, which were similar to those induced by the CCI. Since increasing IL-33 by the i.t. infusion of recombinant IL-33 produced no change in the expression of miR-547-5p, and the CCI still reduced miR-547-5p expression in rats with the IL-33 knockdown, we conclude that the reduction of miR-547-5p can be an upstream event leading to the enhancement of IL-33/ST2 signaling induced by the CCI. The intravenous application of bone marrow stromal cells (BMSCs) reduced the depression of miR-547-5p in both the DRG and SDH, and pain hypersensitivity produced by the CCI or antagomir-miR547-5p application. However, the BMSC effect was significantly occluded by the pretreatment with miR-547-5p agomir or the IL-33 knockdown, demonstrating a novel mechanism underlying the BMSC therapy.

Methylation quantitative trait locus analysis of chronic postsurgical pain uncovers epigenetic mediators of genetic risk

Background: Overlap of pathways enriched by single nucleotide polymorphisms and DNA-methylation underlying chronic postsurgical pain (CPSP), prompted pilot study of CPSP-associated methylation quantitative trait loci (meQTL). Materials & methods: Children undergoing spine-fusion were recruited prospectively. Logistic-regression for genome- and epigenome-wide CPSP association and DNA-methylation-single nucleotide polymorphism association/mediation analyses to identify meQTLs were followed by functional genomics analyses. Results: CPSP (n = 20/58) and non-CPSP groups differed in pain-measures. Of 2753 meQTLs, DNA-methylation at 127 cytosine-guanine dinucleotides mediated association of 470 meQTLs with CPSP (p < 0.05). At PARK16 locus, CPSP risk meQTLs were associated with decreased DNA-methylation at RAB7L1 and increased DNA-methylation at PM20D1. Corresponding RAB7L1/PM20D1 blood eQTLs (GTEx) and cytosine-guanine dinucleotide-loci enrichment for histone marks, transcription factor binding sites and ATAC-seq peaks suggest altered transcription factor-binding. Conclusion: CPSP-associated meQTLs indicate epigenetic mechanisms mediate genetic risk. Clinical trial registration: NCT01839461, NCT01731873 (ClinicalTrials.gov).

DNA methylation changes in genes involved in inflammation and depression in fibromyalgia: a pilot study

OBJECTIVES

The present pilot study aims to investigate DNA methylation changes of genes related to fibromyalgia (FM) development and its main comorbid symptoms, including sleep impairment, inflammation, depression and other psychiatric disorders. Epigenetic modifications might trigger or perpetuate complex interplay between pain transduction/transmission, central pain processing and experienced stressors in vulnerable individuals.

METHODS

We conducted DNA methylation analysis by targeted bisulfite NGS sequencing testing differential methylation in 112 genomic regions from leukocytes of eight women with FM and their eight healthy sisters as controls.

RESULTS

Tests for differentially methylated regions and cytosines brought focus on the GRM2 gene, encoding the metabotropic glutamate receptor2. The slightly increased DNA methylation observed in the GRM2 region of FM patients may confirm the involvement of the glutamate pathway in this pathological condition. Logistic regression highlighted the simultaneous association of methylation levels of depression and inflammation-related genes with FM.

CONCLUSIONS

Altogether, the results evidence the glutamate pathway involvement in FM and support the idea that a combination of methylated and unmethylated genes could represent a risk factor to FM or its consequence, more than single genes. Further studies on the identified biomarkers could contribute to unravel the causative underlying FM mechanisms, giving reliable directions to research, improving the diagnosis and effective therapies.

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.

MeCP2 mediates transgenerational transmission of chronic pain

Pain symptoms can be transmitted across generations, but the mechanisms underlying these outcomes remain poorly understood. Here, we identified an essential role for primary somatosensory cortical (S1) glutamate neuronal DNA methyl-CpG binding protein 2 (MeCP2) in the transgenerational transmission of pain. In a female mouse chronic pain model, the offspring displayed significant pain sensitization. In these mice, MeCP2 expression was increased in S1 glutamate (Glu^S1) neurons, correlating with increased neuronal activity. Downregulation of Glu^S1 neuronal MeCP2 in maternal mice with pain abolished offspring pain sensitization, whereas overexpression of MeCP2 in naïve maternal mice induced pain sensitization in offspring. Notably, single-cell sequencing and chromatin immunoprecipitation analysis showed that the expression of a wide range of genes was changed in offspring and maternal Glu^S1 neurons, some of which were regulated by MeCP2. These results collectively demonstrate the putative importance of MeCP2 as a key regulator in pain transgenerational transmission through actions on Glu^S1 neuronal maladaptation.

The role and pharmacological properties of the P2X7 receptor in neuropathic pain

Neuropathic Pain (NPP) is caused by direct or indirect damage to the nervous system and is a common symptom of many diseases. Clinically, drugs are usually used to suppress pain, such as (lidocaine, morphine, etc.), but the effect is short-lived, poor analgesia, and there are certain dependence and side effects. Therefore, the investigation of the treatment of NPP has become an urgent problem in medical, attracting a lot of research attention. P2X7 is dependent on Adenosine triphosphate (ATP) ion channel receptors and has dual functions for the development of nerve damage and pain. In this review, we explored the link between the P2X7 receptor (P2X7R) and NPP, providing insight into the P2X7R and NPP, discussing the pathological mechanism of P2 X7R in NPP and the biological characteristics of P2X7R antagonist inhibiting its over-expression for the targeted therapy of NPP.

Repressor element 1-silencing transcription factor drives the development of chronic pain states

Chronic pain is an unmet clinical problem with vast individual, societal, and economic impact. Pathologic activity of the peripheral somatosensory afferents is one of the major drivers of chronic pain. This overexcitable state of somatosensory neurons is, in part, produced by the dysregulation of genes controlling neuronal excitability. Despite intense research, a unifying theory behind neuropathic remodelling is lacking. Here, we show that transcriptional suppressor, repressor element 1-silencing transcription factor (REST; neuron-restrictive silencing factor, NRSF), is necessary and sufficient for the development of hyperalgesic state after chronic nerve injury or inflammation. Viral overexpression of REST in mouse dorsal root ganglion (DRG) induced prominent mechanical and thermal hyperalgesia in vivo. Sensory neuron-specific, inducible Rest knockout prevented the development of such hyperalgesic state in 3 different chronic pain models. Genetic deletion of Rest reverted injury-induced hyperalgesia. Moreover, viral overexpression of REST in the same neurons in which its gene has been genetically deleted restored neuropathic hyperalgesia. Finally, sensory neuron specific Rest knockout prevented injury-induced downregulation of REST target genes in DRG neurons. This work identified REST as a major regulator of peripheral somatosensory neuron remodelling leading to chronic pain. The findings might help to develop a novel therapeutic approache to combat chronic pain.

Epigenetic suppression of liver X receptor β in anterior cingulate cortex by HDAC5 drives CFA-induced chronic inflammatory pain

Background

Liver X receptors (LXRs), including LXRα and LXRβ, are key regulators of transcriptional programs for both cholesterol homeostasis and inflammation in the brain. Here, the modes of action of LXRs and the epigenetic mechanisms regulating LXRβ expression in anterior cingulate cortex (ACC) of chronic inflammatory pain (CIP) are investigated.

Methods

The deficit of LXR isoform and analgesic effect of LXR activation by GW3965 were evaluated using the mouse model of CIP induced by hindpaw injection of complete Freund’s adjuvant (CFA). The mechanisms involved in GW-mediated analgesic effects were analyzed with immunohistochemical methods, ELISA, co-immunoprecipitation (Co-IP), Western blot, and electrophysiological recording. The epigenetic regulation of LXRβ expression was investigated by chromatin immunoprecipitation, quantitative real-time PCR, and sequencing.

Results

We revealed that CFA insult led to LXRβ reduction in ACC, which was associated with upregulated expression of histone deacetylase 5 (HDAC5), and knockdown of LXRβ by shRNA led to thermal hyperalgesia. Co-IP showed that LXRβ interacted with NF-κB p65 physically. LXRβ activation by GW3965 exerted analgesic effects by inhibiting the nuclear translocation of NF-κB, reducing the phosphorylation of mitogen-activated protein kinases (MAPKs) in ACC, and decreasing the promoted input-output and enhanced mEPSC frequency in ACC neurons after CFA exposure. In vitro experiments confirmed that HDAC5 triggered histone deacetylation on the promoter region of Lxrβ, resulting in downregulation of Lxrβ transcription.

Conclusion

These findings highlight an epigenetic mechanism underlying LXRβ deficits linked to CIP, and LXRβ activation may represent a potential novel target for the treatment of CIP with an alteration in inflammation responses and synaptic transmission in ACC.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1507-3) contains supplementary material, which is available to authorized users.

Src activation in the hypothalamic arcuate nucleus may play an important role in pain hypersensitivity

Src family of kinases (SFKs) has been found to play an important role in the regulation of nociception. However, how each member of this family acts in the central nervous system (CNS) structures involved in the relay and/or modulation of nociceptive signals, and thereby contributes to the formation and maintenance of pain hypersensitivity, is still a challenge. In this work, a combined study using biochemical, genetic and behavioral approaches was conducted. We found that the expression of activated SFKs in the hypothalamic arcuate nucleus (ARC) area was significantly increased following the development of inflammation induced by injection of complete freund's adjuvant (CFA) into the hind paw of rats. Furthermore, we identified that Src, but not Fyn or Lyn in the Src family, was activated, and that Src knockdown in the ARC area blocked the inflammation-induced increases in the expression of activated SFKs, the N-Methyl-D-aspartate receptor (NMDAR) GluN2B subunit and phosphorylated GluN2B at Y1472 in this region. Moreover, the CFA injection-induced allodynia and hyperalgesia, and the analgesic effect produced by systemic application of the SFK inhibitor, SU6656, were significantly diminished. However, the Src knockdown did not induce any change in the expression of activated SFKs  and the NMDAR GluN2B subunit in normal rats which were not injected with CFA. Neither the Src knockdown nor the systemic application of SU6656 affected the mechanical and thermal sensitivity of the normal rats. Thus, Src activation in the ARC may be a key event for formation and maintenance of pain hypersensitivity associated with peripheral inflammation.

Designing and conducting proof-of-concept chronic pain analgesic clinical trials

Introduction:

The evolution of pain treatment is dependent on successful development and testing of interventions. Proof-of-concept (POC) studies bridge the gap between identification of a novel target and evaluation of the candidate intervention's efficacy within a pain model or the intended clinical pain population.

Methods:

This narrative review describes and evaluates clinical trial phases, specific POC pain trials, and approaches to patient profiling.

Results:

We describe common POC trial designs and their value and challenges, a mechanism-based approach, and statistical issues for consideration.

Conclusion:

Proof-of-concept trials provide initial evidence for target use in a specific population, the most appropriate dosing strategy, and duration of treatment. A significant goal in designing an informative and efficient POC study is to ensure that the study is safe and sufficiently sensitive to detect a preliminary efficacy signal (ie, a potentially valuable therapy). Proof-of-concept studies help avoid resources wasted on targets/molecules that are not likely to succeed. As such, the design of a successful POC trial requires careful consideration of the research objective, patient population, the particular intervention, and outcome(s) of interest. These trials provide the basis for future, larger-scale studies confirming efficacy, tolerability, side effects, and other associated risks.

Histone deacetylase inhibitors prevent persistent hypersensitivity in an orofacial neuropathic pain model

Chronic orofacial pain is a significant health problem requiring identification of regulating processes. Involvement of epigenetic modifications that is reported for hindlimb neuropathic pain experimental models, however, is less well studied in cranial nerve pain models. Three independent observations reported here are the (1) epigenetic profile in mouse trigeminal ganglia (TG) after trigeminal inflammatory compression (TIC) nerve injury mouse model determined by gene expression microarray, (2) H3K9 acetylation pattern in TG by immunohistochemistry, and (3) efficacy of histone deacetylase (HDAC) inhibitors to attenuate development of hypersensitivity. After TIC injury, ipsilateral whisker pad mechanical sensitization develops by day 3 and persists well beyond day 21 in contrast to sham surgery. Global acetylation of H3K9 decreases at day 21 in ipsilateral TG . Thirty-four genes are significantly (p < 0.05) overexpressed in the ipsilateral TG by at least two-fold at either 3 or 21 days post-trigeminal inflammatory compression injury. The three genes most overexpressed three days post-trigeminal inflammatory compression nerve injury are nerve regeneration-associated gene ATF3, up 6.8-fold, and two of its regeneration-associated gene effector genes, Sprr1a and Gal, up 174- and 25-fold, respectively. Although transcription levels of 25 of 32 genes significantly overexpressed three days post-trigeminal inflammatory compression return to constitutive levels by day 21, these three regeneration-associated genes remain significantly overexpressed at the later time point. On day 21, when tissues are healed, other differentially expressed genes include 39 of the top 50 upregulated and downregulated genes. Remarkably, preemptive manipulation of gene expression with two HDAC inhibitors (HDACi's), suberanilohydroxamic acid (SAHA) and MS-275, reduces the magnitude and duration of whisker pad mechanical hypersensitivity and prevents the development of a persistent pain state. These findings suggest that trigeminal nerve injury leads to epigenetic modifications favoring overexpression of genes involved in nerve regeneration and that maintaining transcriptional homeostasis with epigenetic modifying drugs could help prevent the development of persistent pain.

Altered glial glutamate transporter expression in descending circuitry and the emergence of pain chronicity

BACKGROUND

The glutamate type 1 transporter (GLT1) plays a major role in glutamate homeostasis in the brain. Although alterations of GLT1 activity have been linked to persistent pain, the significance of these changes is poorly understood. Focusing on the rostral ventromedial medulla, a key site in pain modulation, we examined the expression and function of GLT1 and related transcription factor kappa B-motif binding phosphoprotein (KBBP) in rats after adjuvant-induced hind paw inflammation.

RESULTS

After inflammation, GLT1 and KBBP showed an early upregulation and gradual transition to downregulation that lasted throughout the eight-week observation period. Nitration of GLT1 was reduced at 30 min and increased at eight weeks after inflammation, suggesting an initial increase and later decrease in transporter activity. Mechanical hyperalgesia and paw edema exhibited an initial developing phase with peak hyperalgesia at 4 to 24 h, a subsequent attenuating phase, followed by a late persistent phase that lasted for months. The downregulation of GLT1 occurred at a time when hyperalgesia transitioned into the persistent phase. In the rostral ventromedial medulla, pharmacological block with dihydrokainic acid and RNAi of GLT1 and KBBP increased nociception and overexpression of GLT1 reversed persistent hyperalgesia. Further, the initial upregulation of GLT1 and KBBP was blocked by local anesthetic block, and pretreatment with dihydrokainic acid facilitated the development of hyperalgesia.

CONCLUSIONS

These results suggest that the initial increased GLT1 activity depends on injury input and serves to dampen the development of hyperalgesia. However, later downregulation of GLT1 fosters the net descending facilitation as injury persists, leading to the emergence of persistent pain.

When pain gets stuck: the evolution of pain chronification and treatment resistance

It is well-recognized that, despite similar pain characteristics, some people with chronic pain recover, whereas others do not. In this review, we discuss possible contributions and interactions of biological, social, and psychological perturbations that underlie the evolution of treatment-resistant chronic pain. Behavior and brain are intimately implicated in the production and maintenance of perception. Our understandings of potential mechanisms that produce or exacerbate persistent pain remain relatively unclear. We provide an overview of these interactions and how differences in relative contribution of dimensions such as stress, age, genetics, environment, and immune responsivity may produce different risk profiles for disease development, pain severity, and chronicity. We propose the concept of "stickiness" as a soubriquet for capturing the multiple influences on the persistence of pain and pain behavior, and their stubborn resistance to therapeutic intervention. We then focus on the neurobiology of reward and aversion to address how alterations in synaptic complexity, neural networks, and systems (eg, opioidergic and dopaminergic) may contribute to pain stickiness. Finally, we propose an integration of the neurobiological with what is known about environmental and social demands on pain behavior and explore treatment approaches based on the nature of the individual's vulnerability to or protection from allostatic load.

Continuing war on pain: a personalized approach to the therapy with nonsteroidal anti-inflammatory drugs and opioids

Successful pain management requires the delivery of analgesia with minimal risk of adverse drug reactions. Nonsteroidal anti-inflammatory drugs and opioids remain the mainstay of treatment for the majority of patients. Unfortunately, almost 50% of all patients experience inadequate pain relief and serious side effects. Allelic variants in genes coding for target proteins, transporters and enzymes, which govern analgesic drugs action and their fate in the organism, might explain inter-individual variability in pain severity and in drug-induced pain relief and toxicities. Additionally, it seems that epigenetic changes contribute to the highly variable response to pain treatment. Therefore, pharmacogenomic testing might be a valuable tool for personalization of pain treatment, with a multidisciplinary team approach involved.

Threshold Effect of G9a/Glp on Peripheral Nerve Injury Induced Hypersensitivity

Background

Previous studies disclosed the pivotal role of methyltransferase complex G9a/Glp in the pathogenesis of neuropathic hypersensitivity induced by peripheral nerve injury. We observed that higher dose of G9a inhibitor improved nociceptive behavior, but the lower dose worsened pain. The aim of this study is to extensively observe the differential effect of various dosages of G9a/Glp inhibitors on nerve injury-induced allodynia.

Materials and methods

After approval by the institutional ethical committee on pain research in conscious animals, C57BL/6 mice were used for measuring nociceptive behavior evoked with von Frey filaments after spared nerve injury. G9a/Glp inhibitor BIX01294 or UNC0638 was injected through the pre-buried intrathecal catheter. The dose–response curves of behavioral changes were depicted when inhibitors were administered once in bolus at the 14th day post spared nerve injury. Withdrawal behaviors were compared during the 49 days’ observation window after spared nerve injury with various dosages of inhibitors injected intrathecally for 14 days.

Results

Dose–behavior curves of a single bolus of both BIX01294 and UNC0638 displayed a “V”-shaped responses of allodynia withdrawal from lower through higher dose when measured at the 14th day post spared nerve injury. A threshold dose of 10.0 µg for BIX01294 and 80.0 µg for UNC0638 significantly worsened allodynia. However, daily bolus intrathecal injection for 14 days of both inhibitors lower or higher than these threshold doses prominently improved nociceptive behavior, producing contrasting results. On the same animal, threshold dose followed by a lower or higher dose with a 14 days’ interval also showed contrast effect on nociceptive behavior, and a lower or higher dose to threshold dose sequence of inhibitor administration was vice versa.

Conclusions

Methyltransferase complex G9a/Glp has a threshold role in mediating peripheral nerve injury-induced hypersensitivity at its low level versus high level through inhibiting and facilitating the nociceptive behavior, respectively.

Genome-wide methylation analysis of a large population sample shows neurological pathways involvement in chronic widespread musculoskeletal pain

Chronic widespread musculoskeletal pain (CWP), has a considerable heritable component, which remains to be explained. Epigenetic factors may contribute to and account for some of the heritability estimate. We analysed epigenome-wide methylation using MeDIPseq in whole blood DNA from 1708 monozygotic and dizygotic Caucasian twins having CWP prevalence of 19.9%. Longitudinally stable methylation bins (lsBINs), were established by testing repeated measurements conducted ≥3 years apart, n = 292. DNA methylation variation at lsBINs was tested for association with CWP in a discovery set of 50 monozygotic twin pairs discordant for CWP, and in an independent dataset (n = 1608 twins), and the results from the 2 samples were combined using Fisher method. Functional interpretation of the most associated signals was based on functional genomic annotations, gene ontology, and pathway analyses. Of 723,029 signals identified as lsBINs, 26,399 lsBINs demonstrated the same direction of association in both discovery and replication datasets at nominal significance (P ≤ 0.05). In the combined analysis across 1708 individuals, whereas no lsBINs showed genome-wide significance (P < 10-8), 24 signals reached p≤9E-5, and these included association signals mapping in or near to IL17A, ADIPOR2, and TNFRSF13B. Bioinformatics analyses of the associated methylation bins showed enrichment for neurological pathways in CWP. We estimate that the variance explained by epigenetic factors in CWP is 6%. This, the largest study to date of DNA methylation in CWP, points towards epigenetic modification of neurological pathways in CWP and provides proof of principle of this method in teasing apart the complex risk factors for CWP.

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.

Can Chronic Pain Be Prevented?

All chronic pain begins at some discrete point in time. Significant strides in the understanding of mechanisms and risk factors associated with the transition from a new, or acute, pain experience to a chronic pain condition have been made over the past 20 years. These insights provide the hope of one day being able to modify or even halt this pathophysiologic progression. This article reviews some of the current knowledge of this transition as well as the evidence currently available to best prevent and treat it using persistent surgical pain as a model.

EZH2 regulates spinal neuroinflammation in rats with neuropathic pain

Alteration in gene expression along the pain signaling pathway is a key mechanism contributing to the genesis of neuropathic pain. Accumulating studies have shown that epigenetic regulation plays a crucial role in nociceptive process in the spinal dorsal horn. In this present study, we investigated the role of enhancer of zeste homolog-2 (EZH2), a subunit of the polycomb repressive complex 2, in the spinal dorsal horn in the genesis of neuropathic pain in rats induced by partial sciatic nerve ligation. EZH2 is a histone methyltransferase, which catalyzes the methylation of histone H3 on K27 (H3K27), resulting in gene silencing. We found that levels of EZH2 and tri-methylated H3K27 (H3K27TM) in the spinal dorsal horn were increased in rats with neuropathic pain on day 3 and day 10 post nerve injuries. EZH2 was predominantly expressed in neurons in the spinal dorsal horn under normal conditions. The number of neurons with EZH2 expression was increased after nerve injury. More strikingly, nerve injury drastically increased the number of microglia with EZH2 expression by more than sevenfold. Intrathecal injection of the EZH2 inhibitor attenuated the development and maintenance of mechanical and thermal hyperalgesia in rats with nerve injury. Such analgesic effects were concurrently associated with the reduced levels of EZH2, H3K27TM, Iba1, GFAP, TNF-α, IL-1β, and MCP-1 in the spinal dorsal horn in rats with nerve injury. Our results highly suggest that targeting the EZH2 signaling pathway could be an effective approach for the management of neuropathic pain.

Suberoylanilide hydroxamic acid prevents downregulation of spinal glutamate transporter-1 and attenuates spinal nerve ligation-induced neuropathic pain behavior

Glutamate transporter-1 (GLT-1) reduction causes dysregulation of excitatory-inhibitory balance, contributing toward neuropathic pain development. However, the mechanisms underlying GLT-1 downregulation are still unclear. Histone acetylation plays a pivotal role in the regulation of gene expression. We sought to examine the contribution of histone acetylation on pain hypersensitivity and GLT-1 downregulation in neuropathic pain development. Histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) was intrathecally infused to rats through osmotic pumps from -5 days to 7 days after spinal nerve ligation (SNL). Behavioral tests indicated that SAHA could significantly prevent SNL-induced mechanical allodynia and thermal hyperalgesia. The effect was dose related and lasted to 10 days after SNL when the SAHA infusion was stopped on day 7. Immunohistochemistry, western blot, and real-time reverse transcription PCR analysis showed that SAHA significantly prevented SNL-induced downregulation of GLT-1 in the spinal dorsal horn. In addition, SNL-induced weakened acetylation of histone H3 (AcH3) was significantly inhibited by SAHA. Immunofluorescent histochemistry showed that both GLT-1 and AcH3 had high expressions in the dorsal horn. Double staining indicated that several GLT-1-positive cells were colocalized with AcH3. Our data provide evidence that histone deacetylation may contribute toward the loss of GLT-1 and this could be a new consideration for the development of more effective strategies for treating neuropathic pain.

Recent advances in understanding neuropathic pain: glia, sex differences, and epigenetics

Neuropathic pain results from diseases or trauma affecting the nervous system. This pain can be devastating and is poorly controlled. The pathophysiology is complex, and it is essential to understand the underlying mechanisms in order to identify the relevant targets for therapeutic intervention. In this article, we focus on the recent research investigating neuro-immune communication and epigenetic processes, which gain particular attention in the context of neuropathic pain. Specifically, we analyze the role of glial cells, including microglia, astrocytes, and oligodendrocytes, in the modulation of the central nervous system inflammation triggered by neuropathy. Considering epigenetics, we address DNA methylation, histone modifications, and the non-coding RNAs in the regulation of ion channels, G-protein-coupled receptors, and transmitters following neuronal damage. The goal was not only to highlight the emerging concepts but also to discuss controversies, methodological complications, and intriguing opinions.

Dysfunctional stress responses in chronic pain

Many dysfunctional and chronic pain conditions overlap. This review describes the different modes of chronic deregulation of the adaptive response to stress which may be a common factor for these conditions. Several types of dysfunction can be identified within the hypothalamo-pituitary-adrenal axis: basal hypercortisolism, hyper-reactivity, basal hypocortisolism and hypo-reactivity. Neuroactive steroid synthesis is another component of the adaptive response to stress. Dehydroepiandrosterone (DHEA) and its sulfated form DHEA-S, and progesterone and its derivatives are synthetized in cutaneous, nervous, and adipose cells. They are neuroactive factors that act locally. They may have a role in the localization of the symptoms and their levels can vary both in the central nervous system and in the periphery. Persistent changes in neuroactive steroid levels or precursors can induce localized neurodegeneration. The autonomic nervous system is another component of the stress response. Its dysfunction in chronic stress responses can be expressed by decreased basal parasympathethic activity, increased basal sympathetic activity or sympathetic hyporeactivity to a stressful stimulus. The immune and genetic systems also participate. The helper-T cells Th1 secrete pro-inflammatory cytokines such as IL-1-β, IL-2, IL-6, IL-8, IL-12, IFN-γ, and TNF-α, whereas Th2 secrete anti-inflammatory cytokines: IL-4, IL-10, IGF-10, IL-13. Chronic deregulation of the Th1/Th2 balance can occur in favor of anti- or pro-inflammatory direction, locally or systemically. Individual vulnerability to stress can be due to environmental factors but can also be genetically influenced. Genetic polymorphisms and epigenetics are the main keys to understanding the influence of genetics on the response of individuals to constraints.

Is there any therapeutic value for the use of histone deacetylase inhibitors for chronic pain?

Chronic pain is a complex clinical condition that reduces the quality of life for billions of people. In recent years, the role of epigenetic modulation in the control of long-term neuronal plasticity has attracted the attention of pain researchers. The epigenetic mechanisms include covalent modifications of DNA and/or histone proteins. Mounting evidence suggests that the activity of histone deacetylases (HDACs) and levels of histone acetylation are dynamic and that these enzymes modulate pain-related synaptic plasticity. Therefore, HDACs play essential roles in chronic pain development and maintenance. In this mini review, we will discuss the role of HDACs in the pathogenesis of chronic pain and will consider the therapeutic value of HDAC inhibitors in treating chronic pain.

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.

Selective repression of gene expression in neuropathic pain by the neuron-restrictive silencing factor/repressor element-1 silencing transcription (NRSF/REST)

Neuropathic pain often develops following nerve injury as a result of maladaptive changes that occur in the injured nerve and along the nociceptive pathways of the peripheral and central nervous systems. Multiple cellular and molecular mechanisms likely account for these changes; however, the exact nature of these mechanisms remain largely unknown. A growing number of studies suggest that alteration in gene expression is an important step in the progression from acute to chronic pain states and epigenetic regulation has been proposed to drive this change in gene expression. In this review, we discuss recent evidence that the DNA-binding protein neuron-restrictive silencing factor/repressor element-1 silencing transcription factor (NRSF/REST) is an important component in the development and maintenance of neuropathic pain through its role as a transcriptional regulator for a select subset of genes that it normally represses during development.

Early-life stress-induced visceral hypersensitivity and anxiety behavior is reversed by histone deacetylase inhibition

Stressful life events, especially in childhood, can have detrimental effects on health and are associated with a host of psychiatric and gastrointestinal disorders including irritable bowel syndrome (IBS). Early-life stress can be recapitulated in animals using the maternal separation (MS) model, exhibiting many key phenotypic outcomes including visceral hypersensitivity and anxiety-like behaviors. The molecular mechanisms of MS are unclear, but recent studies point to a role for epigenetics. Histone acetylation is a key epigenetic mark that is altered in numerous stress-related disease states. Here, we investigated the role of histone acetylation in early-life stress-induced visceral hypersensitivity. Interestingly, increased number of pain behaviors and reduced threshold of visceral sensation were associated with alterations in histone acetylation in the lumbosacral spinal cord, a key region in visceral pain processing. Moreover, we also investigated whether the histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), could reverse early-life stress-induced visceral hypersensitivity and stress-induced fecal pellet output in the MS model. Significantly, SAHA reversed both of these parameters. Taken together, these data describe, for the first time, a key role of histone acetylation in the pathophysiology of early-life stress-induced visceral hypersensitivity in a well-established model of IBS. These findings will inform new research aimed at the development of novel pharmaceutical approaches targeting the epigenetic machinery for novel anti-IBS drugs.

Neuroplasticity underlying the comorbidity of pain and depression

Acute pain induces depressed mood, and chronic pain is known to cause depression. Depression, meanwhile, can also adversely affect pain behaviors ranging from symptomology to treatment response. Pain and depression independently induce long-term plasticity in the central nervous system (CNS). Comorbid conditions, however, have distinct patterns of neural activation. We performed a review of the changes in neural circuitry and molecular signaling pathways that may underlie this complex relationship between pain and depression. We also discussed some of the current and future therapies that are based on this understanding of the CNS plasticity that occurs with pain and depression.

Role of α5-containing nicotinic receptors in neuropathic pain and response to nicotine

Nicotinic receptors in the central nervous system (nAChRs) are known to play important roles in pain processing and modulate behavioral responses to analgesic drugs, including nicotine. The presence of the α5-neuronal nicotinic accessory subunit in the nicotinic receptor complex is increasingly understood to modulate reward and aversive states, addiction, and possibly pathological pain. In the current study, using α5-knockout (KO) mice and subunit-specific antibodies, we assess the role of α5-containing neuronal nicotinic receptors in neuropathic pain and in the analgesic response to nicotine. After chronic constriction injury (CCI) or partial sciatic nerve ligation (PSNL), no differences in mechanical, heat, or cold hyperalgesia were found in wild-type (WT) versus α5-KO littermate mice. The number of α5-containing nAChRs was decreased (rather than increased) after CCI in the spinal cord and in the thalamus. Nevertheless, thermal analgesic response to nicotine was marginally reduced in CCI α5-KO mice at 4 days after CCI, but not at later timepoints or after PSNL. Interestingly, upon daily intermittent nicotine injections in unoperated mice, WT animals developed tolerance to nicotine-induced analgesia to a larger extent than α5-KO mice. Our results suggest that α5-containing nAChRs mediate analgesic tolerance to nicotine but do not play a major role in neuropathic pain.