HDAC inhibitors attenuate the development of hypersensitivity in models of neuropathic pain

Established and emerging roles of protein kinases in regulating primary sensory neurons in injury-and inflammation-associated pain

INTRODUCTION

Recent seminal neuroscience research has significantly increased our knowledge on cellular and molecular responses of various cells in the pain pathway to peripheral nerve injuries and inflammatory processes. Transcriptomic and epigenetic analysis of primary sensory neurons (PSNs) in animal models of peripheral injuries revealed new insights into altered gene expression profiles and epigenetic modifications, which, via increasing spinal nociceptive input, lead to the development of pain. Among the various classes of molecules involved in driving differential gene expression, protein kinases, the enzymes that catalyze the phosphorylation of molecules, are emerging to control histone modification and chromatin remodeling needed for the alteration in transcriptional activity.

AREAS COVERED

Here, we focused on how protein kinases contribute to transcriptomic changes and pathways of induced reprogramming within PSNs upon peripheral nerve injury and inflammation. We conducted systematic literature search across multiple databases, including PubMed, NIH ClinicalTrials.gov portal and GEOData from 1980 to 2024 and compared protein kinase expression frequencies between publicly available RNA sequencing datasets of PSNs and investigated differences in protein kinase expression levels after peripheral nerve injury.

EXPERT OPINION

Novel findings support a new concept that protein kinases constitute regulatory hubs of reprogramming of PSNs, which offers novel analgesic approaches.

Understanding the comorbidities among psychiatric disorders, chronic low-back pain, and spinal degenerative disease using observational and genetically informed analyses

Psychiatric disorders and symptoms are associated with differences in pain perception and sensitivity. These differences can have important implications in treating spinal degenerative disease (SDD) and chronic low-back pain (CLBP). Leveraging data from the UK Biobank (UKB) and the All of Us Research Program (AoU), we investigated the effects linking psychiatric disorders (alcohol use disorder, anxiety, attention deficit hyperactivity disorder, bipolar disorder, cannabis use disorder, depression, opioid use disorder, posttraumatic stress disorder, and schizophrenia) to SDD and CLBP. We applied multi-nominal regression models, polygenic risk scoring (PRS), and one-sample Mendelian randomization (MR) to triangulate the effects underlying the associations observed. We also performed gene ontology and drug-repurposing analyses to dissect the biology shared among mental illnesses, SDD, and CLBP. Comparing individuals affected only by SDD (UKB N=37,745, AoU N=3,477), those affected only by CLBP (UKB N=15,496, AoU N=23,325), and those affected by both conditions (UKB N=11,463, AoU N= 13,451) to controls (UKB N=337,362, AoU N= 117,162), observational and genetically informed analyses highlighted that the strongest effects across the three case groups were observed for alcohol use disorder, anxiety, depression, and posttraumatic stress disorder. Additionally, schizophrenia and its PRS appeared to have an inverse relationship with CLBP, SDD, and their comorbidity. One-sample MR highlighted a potential direct effect of internalizing disorders on the outcomes investigated that was particularly strong on SDD. Our drug-repurposing analyses identified histone deacetylase inhibitors as targeting molecular pathways shared among psychiatric disorders, SDD, and CLBP. In conclusion, these findings support that the comorbidity among psychiatric disorders, SDD, and CLBP is due to the contribution of direct effects and shared biology linking these health outcomes. These pleiotropic mechanisms together with sociocultural factors play a key role in shaping the SDD-CLBP comorbidity patterns observed across the psychopathology spectrum.

Exploring the early drivers of pain in Parkinson's disease

Pain is a common and complex non-motor symptom in people with Parkinson's disease (PWP). Little is known about the genetic drivers of pain in PWP, and progress in its study has been challenging. Here, we conducted two genome-wide association studies (GWAS) to identify genetic variants associated with pain experienced during the earliest stages of Parkinson's disease. The study population consisted of 4,159 PWP of European ancestry who were mapped to five previously-described, longitudinal pain trajectories. In the first GWAS, the extreme pain trajectories (highest burden versus no significant pain over time) were compared, and in the second GWAS, a multinomial approach was undertaken. While no variant reached genome-wide significance, we identified promising associations, such as rs117108018 (ORGWAS-Extreme=8.96, pGWAS-Extreme=2.5 × 10- 7), a brain/nerve eQTL for L3MBTL3 and EPB41L2, and rs61881484 (pGWAS-Multinomial=2 × 10- 7), which intersects a transcription factor peak targeting CREB1, critical in sensory neuron synaptic plasticity and neuropathic pain regulation. Gene-based tests implicated CTNNB1 (pGWAS-Extreme=3.2 × 10- 5), KLK7 (pGWAS-Extreme=7 × 10- 5), and SLITRK3 (pGWAS-Multinomial=3.2 × 10- 5), which have been associated with neurodevelopment. At the pathway-level, there was an enrichment for genes involved in neurotransmitter regulation and opioid dependence. This study implicates neuropathic pain mechanisms as prominent drivers of elevated pain in PWP, suggests potential therapeutic genetic targets for further research.

The effect of ponicidin on CFA-induced chronic inflammatory pain and its mechanism based on network pharmacology and molecular docking

Purpose

Inflammation is a frequent precursor to the development of chronic pain. Ponicidin, a compound derived from traditional Chinese medicine, possesses immunomodulatory and anti-inflammatory properties. However, whether ponicidin mitigates inflammatory pain through its anti-inflammatory effects and potential target molecules remains to be further explored. In this study, we investigated the analgesic effects of ponicidin in a mouse model of Complete Freund's Adjuvant (CFA)-induced inflammatory pain and employed network pharmacology to predict the potential therapeutic targets of ponicidin for pain treatment.

Methods

Initially, we established a mouse model of inflammatory pain induced by Complete Freund's Adjuvant (CFA). Following the establishment of the model, the analgesic effects of ponicidin were assessed using behavioral tests, and further validation was conducted through hematoxylin and eosin (H&E) staining, enzyme-linked immunosorbent assay (ELISA), and immunofluorescence methods. Subsequently, we analyzed the potential analgesic targets of ponicidin using network pharmacology approaches and molecular docking.

Results

In this study, we observed that ponicidin has a significant alleviating effect on CFA-induced inflammatory pain. Our results suggest that ponicidin may alleviate inflammatory pain by reducing inflammatory responses in the spinal cord and hind paw of CFA model mice. Furthermore, we found that ponicidin can mitigate the activation of macrophages in the subcutaneous tissue of the hind paw and microglia in the dorsal horn of the spinal cord. Network pharmacology analysis suggests that ponicidin may exert its analgesic effects through a multi-target, multi-pathway mechanism. Key transcription factors such as nuclear factor NF-κB p105 subunit (NFKB1), RELA, SP1, signal transducer and activator of transcription 3 (STAT3), and repressor element 1 silencing transcription factor (REST) may be involved in the underlying mechanisms of ponicidin's analgesic action. Through molecular docking and experimental validation, we have identified toll-like receptor 4 (TLR4) and hypoxia-inducible factor 1-alpha (HIF1A) as key targets of ponicidin's analgesic effects.

Conclusions

Ponicidin alleviates inflammatory pain by reducing inflammatory responses in the spinal cord and hind paw of the CFA model mice. TLR4 and HIF1A may as key targets for the analgesic effects of ponicidin.

A novel SIRT1 activator attenuates neuropathic pain by inhibiting spinal neuronal activation via the SIRT1-mGluR1/5 pathway

Neuropathic pain is a type of pain caused by an injury or disease of the somatosensory nervous system. Currently, there is still absence of effective therapeutic drugs for neuropathic pain, so developing new therapeutic drugs is urgently needed. In the present study, we observed the effect of Comp 6d, a novel silent information regulator 1 (SIRT1) activator synthesized in our laboratory, on neuropathic pain and investigated the mechanisms involved. We found that both intrathecal and intraperitoneal injections of Comp 6d effectively alleviated neuropathic pain induced by chronic constriction injury (CCI) or spared nerve injury (SNI). However, the effect of Comp 6d on neuropathic pain was abolished in SIRT1 knockout mice. These results demonstrated that Comp 6d alleviated neuropathic pain by specifically activating SIRT1 in the spinal cord. Moreover, long-term intraperitoneal injection of Comp 6d had no significant side effects on heart, liver and kidney in SNI mice. Further study showed that the improvement of neuropathic pain by Comp 6d was mediated by the downregulation of mGluR1/5 levels and the subsequent inhibition of spinal neuronal activation. Taken together, the present findings suggest that the novel SIRT1 activator Comp 6d inhibits the activation of spinal cord neurons via the SIRT1-mGluR1/5 pathway, thereby attenuating neuropathic pain. Comp 6d is expected to be an effective therapeutic agent for neuropathic pain.

Sodium butyrate restored TRESK current controlling neuronal hyperexcitability in a mouse model of oxaliplatin-induced peripheral neuropathic pain

Chemotherapy-induced peripheral neuropathy (CIPN) and its related pain are common challenges for patients receiving oxaliplatin chemotherapy. Oxaliplatin accumulation in dorsal root ganglion (DRGs) is known to impair gene transcription by epigenetic dysregulation. We hypothesized that sodium butyrate, a pro-resolution short-chain fatty acid, inhibited histone acetylation in DRGs and abolished K+ channel dysregulation-induced neuronal hyperexcitability after oxaliplatin treatment. Mechanical allodynia and cold hyperalgesia of mice receiving an accumulation of 15 ​mg/kg oxaliplatin, with or without intraperitoneal sodium butyrate supplementation, were assessed using von Frey test and acetone evaporation test. Differential expressions of histone deacetylases (HDACs) and pain-related K+ channels were quantified with rt-qPCR and protein assays. Immunofluorescence assays of histone acetylation at H3K9/14 were performed in primary DRG cultures treated with sodium butyrate. Current clamp recording of action potentials and persistent outward current of Twik-related-spinal cord K+ (TRESK) channel were recorded in DRG neurons with small diameters extract. Accompanied by mechanical allodynia and cold hyperalgesia, HDAC1 was upregulated in mice receiving oxaliplatin treatment. Sodium butyrate enhanced global histone acetylation at H3K9/14 in DRG neurons. In vivo sodium butyrate supplementation restored oxaliplatin-induced Kcnj9 and Kcnk18 expression and pain-related behaviors in mice for at least 14 days. Oxaliplatin-induced increase in action potentials frequencies and decrease in magnitudes of KCNK18-related current were reversed in mice receiving sodium butyrate supplementation. This study suggests that sodium butyrate was a useful agent to relieve oxaliplatin-mediated neuropathic pain.

SGK1-HDAC4-HMGB1 signaling pathway in the spinal cord dorsal horn participates in diabetic neuropathic pain

PURPOSE

This study aimed to determine whether serum-and glucocorticoid-inducible kinase1 (SGK1) activation-dependent histone deacetylase 4 (HDAC4) phosphorylation, nucleocytoplasmic trafficking, and subsequent regulation of high-mobility group protein box 1 (HMGB1) expression are involved in type 2 diabetic neuropathic pain (DNP).

METHODS

The type 2 diabetic neuropathic pain model was established in rats by feeding them with a high-fat and high-sugar diet for 8 weeks and then fasting them for 12 h, followed by a single intraperitoneal injection of streptozotocin (STZ, 35 mg/kg). SGK1 was inhibited in the spinal cord by intrathecal administration of the SGK1 inhibitor GSK-650394.

RESULTS

The present study revealed that pSGK1/tSGK1 was persistently upregulated in the spinal cord of rats with type-2 DNP. The downregulation of pSGK1/tSGK1 through the intrathecal injection of the SGK1 inhibitor GSK-650394 significantly ameliorated the pain hypersensitivity, relieved the abnormal expression of pHDAC4/tHDAC4 and HMGB1, and affected HDAC4 nucleocytoplasmic trafficking in DNP rats.

CONCLUSION

Our data suggest that SGK1 in the spinal cord modulates type-2 DNP by regulating the HDAC4/HMGB1 pathway.

Upregulated spinal histone deacetylases induce nociceptive sensitization by inhibiting the GABA system in chronic constriction injury-induced neuropathy in rats

Introduction

Neuropathic pain (NP) affects countless people worldwide; however, few effective treatments are currently available. Histone deacetylases (HDACs) participate in epigenetic modifications in neuropathy-induced nociceptive sensitization. Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter that can inhibit NP. The present study aimed to examine the role of spinal HDAC and its isoforms in neuropathy.

Methods

Male Wistar Rat with chronic constriction injury (CCI)-induced peripheral neuropathy and HDAC inhibitor, panobinostat, was administrated intrathecally. We performed quantitative real-time polymerase chain reaction (RT-qPCR), western blot, and immunohistochemical analysis of lumbar spinal cord dorsal horn and nociceptive behaviors (thermal hyperalgesia and mechanical allodynia) measurements.

Results

Herein, RT-qPCR analysis revealed that spinal hdac3, hdac4, and hdac6 were upregulated in CCI rats. Western blotting and immunofluorescence staining further confirmed that HDAC3, HDAC4, and HDAC6 were significantly upregulated, whereas GABA and its synthesis key enzyme glutamic acid decarboxylase (GAD) 65 were dramatically downregulated. Intrathecal panobinostat attenuated nociceptive behavior and restored the downregulated spinal GAD65 and GABA expression in CCI rats.

Conclusions

HDAC upregulation might induce nociception through GAD65 and GABA inhibition in CCI-induced neuropathy. These findings strongly suggest that HDACs negatively regulate inhibitory neurotransmitters, constituting a potential therapeutic strategy for an epigenetic approach to manage NP.

Investigating neuroepigenetic alterations in chronic low back pain with positron emission tomography

ABSTRACT

Epigenetics has gained considerable interest as potential mediators of molecular alterations that could underlie the prolonged sensitization of nociceptors, neurons, and glia in response to various environmental stimuli. Histone acetylation and deacetylation, key processes in modulating chromatin, influence gene expression; elevated histone acetylation enhances transcriptional activity, whereas decreased acetylation leads to DNA condensation and gene repression. Altered levels of histone deacetylase (HDAC) have been detected in various animal pain models, and HDAC inhibitors have demonstrated analgesic effects in these models, indicating HDACs' involvement in chronic pain pathways. However, animal studies have predominantly examined epigenetic modulation within the spinal cord after pain induction, which may not fully reflect the complexity of chronic pain in humans. Moreover, methodological limitations have previously impeded an in-depth study of epigenetic changes in the human brain. In this study, we employed [ 11 C]Martinostat, an HDAC-selective radiotracer, positron emission tomography to assess HDAC availability in the brains of 23 patients with chronic low back pain (cLBP) and 11 age-matched and sex-matched controls. Our data revealed a significant reduction of [ 11 C]Martinostat binding in several brain regions associated with pain processing in patients with cLBP relative to controls, highlighting the promising potential of targeting HDAC modulation as a therapeutic strategy for cLBP.

The ketogenic diet mitigates opioid-induced hyperalgesia by restoring short-chain fatty acids-producing bacteria in the gut

ABSTRACT

Opioids are commonly prescribed to patients with chronic pain. Chronic opioid usage comes with a slew of serious side effects, including opioid-induced hyperalgesia (OIH). The patients with long-term opioid treatment experience paradoxical increases in nociceptive hypersensitivity, namely, OIH. Currently, treatment options for OIH are extremely lacking. In this study, we show that the ketogenic diet recovers the abnormal pain behavior caused by chronic morphine treatment in male mice, and we further show that the therapeutic effect of the ketogenic diet is mediated through gut microbiome. Our 16S rRNA sequencing demonstrates that chronic morphine treatment causes changes in mouse gut microbiota, specifically a decrease in short-chain fatty acids-producing bacteria, and the sequencing data also show that the ketogenic diet rescues those bacteria in the mouse gut. More importantly, we show that supplementation with short-chain fatty acids (butyrate, propionate, and acetate) can delay the onset of OIH, indicating that short-chain fatty acids play a direct role in the development of OIH. Our findings suggest that gut microbiome could be targeted to treat OIH, and the ketogenic diet can be used as a complementary approach for pain relief in patients with chronic opioid treatment. We only used male mice in this study, and thus, our findings cannot be generalized to both sexes.

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.

Role of HDAC5 Epigenetics in Chronic Craniofacial Neuropathic Pain

The information provided from the papers reviewed here about the role of epigenetics in chronic craniofacial neuropathic pain is critically important because epigenetic dysregulation during the development and maintenance of chronic neuropathic pain is not yet well characterized, particularly for craniofacial pain. We have noted that gene expression changes reported vary depending on the nerve injury model and the reported sample collection time point. At a truly chronic timepoint of 10 weeks in our model of chronic neuropathic pain, functional groupings of genes examined include those potentially contributing to anti-inflammation, nerve repair/regeneration, and nociception. Genes altered after treatment with the epigenetic modulator LMK235 are discussed. All of these differentials are key in working toward the development of diagnosis-targeted therapeutics and likely for the timing of when the treatment is provided. The emphasis on the relevance of time post-injury is reiterated here.

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

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

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.

5HT2A modulation attenuates pancreatic cancer induced pain mouse model by inhibiting HDAC

PURPOSE

Patients have been severely suffered from cancer associated pain, and pancreatic cancer is the most severe form of cancer associated with pain. There are very few options available to manage it. The present report evaluated the effect of 5HT2A on pancreatic cancer associated pain.

METHODS

Pancreatic cancer was induced by injecting SW 1,990 cells (~3×106 in a 20 μL suspension) into the pancreas and formed a 2-3-mm vesicle using an inoculator fitted with a 26-gauge needle in BALB/c-nu mice. Survival rate and body weight of the mice were observed. Pain behaviour testing was performed at the end of each week (third and fourth week) after surgery. Inflammatory mediators and HDAC 2 proteins were determined in the spinal tissue using quantitative real-time polymerase chain reaction.

RESULTS

There was improvement in the survival rate and body weight in 5HT2A antagonist treated group than pancreatic cancer group of mice. Moreover, 5HT2A antagonist ameliorated the alteration in pain behaviour of pancreatic cancer mice. mRNA expression of HDAC2 and level of inflammatory cytokines were reduced in the spinal tissue of 5HT 2A antagonist treated group than pancreatic cancer group of mice.

CONCLUSIONS

Data revealed that 5HT2A antagonist ameliorates pain associated with pancreatic cancer mice by HDAC inhibition and inflammatory cytokines. The result of investigation supports that modulation of 5HT2A receptor could be used clinically to protects neuropathic pain in pancreatic cancer.

A phenotypic screening platform for chronic pain therapeutics using all-optical electrophysiology

ABSTRACT

Chronic pain associated with osteoarthritis (OA) remains an intractable problem with few effective treatment options. New approaches are needed to model the disease biology and to drive discovery of therapeutics. We present an in vitro model of OA pain, where dorsal root ganglion (DRG) sensory neurons were sensitized by a defined mixture of disease-relevant inflammatory mediators, here called Sensitizing PAin Reagent Composition or SPARC. Osteoarthritis-SPARC components showed synergistic or additive effects when applied in combination and induced pain phenotypes in vivo. To measure the effect of OA-SPARC on neural firing in a scalable format, we used a custom system for high throughput all-optical electrophysiology. This system enabled light-based membrane voltage recordings from hundreds of neurons in parallel with single cell and single action potential resolution and a throughput of up to 500,000 neurons per day. A computational framework was developed to construct a multiparameter OA-SPARC neuronal phenotype and to quantitatively assess phenotype reversal by candidate pharmacology. We screened ∼3000 approved drugs and mechanistically focused compounds, yielding data from over 1.2 million individual neurons with detailed assessment of functional OA-SPARC phenotype rescue and orthogonal "off-target" effects. Analysis of confirmed hits revealed diverse potential analgesic mechanisms including ion channel modulators and other mechanisms including MEK inhibitors and tyrosine kinase modulators. Our results suggest that the Raf-MEK-ERK axis in DRG neurons may integrate the inputs from multiple upstream inflammatory mediators found in osteoarthritis patient joints, and MAPK pathway activation in DRG neurons may contribute to chronic pain in patients with osteoarthritis.

Role of Histone Deacetylase Inhibitor in Diabetic Painful Neuropathy

Diabetic painful neuropathy (DPN) is one of the most detrimental complications of diabetes. Alterations in neuroinflammatory mediators play significant roles in the development of DPN. Infiltration of the neutrophils and monocyte/macrophages contributes substantial role in the degenerative process of the distal sciatic nerve by forming neutrophil extracellular traps (NETs) under diabetic condition. Citrullination of histones due to increase in protein arginine deiminase (PAD) enzyme activity under hyperglycemia may promote NET formation, which can further increase the cytokine production by activating macrophages and proliferation of neutrophils. This study reveals that the increase in histone deacetylases (HDAC) is crucial in DPN and inhibition of HDAC using HDAC inhibitor (HDACi) FK228 would suppress NETosis and alleviate diabetic nerve degeneration and pain. FK228, also known as romidepsin, is FDA approved for the treatment of cutaneous T-cell lymphoma yet the molecular mechanisms of this drug are not completely understood in DPN. In this study, type 2 diabetic (T2D) mice with pain were treated with HDACi, FK228 1 mg/kg; I.P. 2 × /week for 3 weeks. The results demonstrate that FK228 treatment can alleviate thermal hyperalgesia and mechanical allodynia significantly along with changes in the expression of HDACs in the dorsal root ganglia (DRG) and spinal cord dorsal horn neurons of diabetic animals. The results also indicate that FK228 treatment can alter the expression of neutrophil elastase (NE), extracellular or cell free DNA (cfDNA), citrullinated histone-3 (CitH3), PADI4, growth-associated protein (GAP)-43, and glucose transporter (GLUT)-4. Overall, this study suggests that FK228 could amend the expression of nerve regeneration markers and inflammatory mediators in diabetic animals and may offer an alternative treatment approach for DPN.

The Protein Acetylation after Traumatic Spinal Cord Injury: Mechanisms and Therapeutic Opportunities

Spinal cord injury (SCI) leads to deficits of various normal functions and is difficult to return to a normal state. Histone and non-histone protein acetylation after SCI is well documented and regulates spinal cord plasticity, axonal growth, and sensory axon regeneration. However, our understanding of protein acetylation after SCI is still limited. In this review, we summarize current research on the role of acetylation of histone and non-histone proteins in regulating neuron growth and axonal regeneration in SCI. Furthermore, we discuss inhibitors and activators targeting acetylation-related enzymes, such as α-tubulin acetyltransferase 1 (αTAT1), histone deacetylase 6 (HDAC6), and sirtuin 2 (SIRT2), to provide promising opportunities for recovery from SCI. In conclusion, a comprehensive understanding of protein acetylation and deacetylation in SCI may contribute to the development of SCI treatment.

Epigenetic HDAC5 Inhibitor Reverses Craniofacial Neuropathic Pain in Mice

Identifying and resolving molecular complexities underlying chronic neuropathic pain is a significant challenge. Among the numerous classes of histone deacetylases, Class I (HDAC 1-3) and Class III (sirtuins) have been best studied in experimental pain models where inhibitor pre-treatments but not post-treatments abrogate the development of pain-related behaviors. Post-treatment here in week 3 with less well-studied Class IIa HDAC4/5 selective inhibitor LMK235 diminishes the trigeminal ganglia increases of HDAC5 RNA and protein in two chronic orofacial neuropathic pain models to levels measured in naïve mice at week 10 post-model induction. HDAC4 RNA reported in lower limb inflammatory pain models is not evident in the trigeminal models. Many other gene alterations persisting at week 10 in the trigeminal ganglia (TG) are restored to naïve levels in mice treated with LMK235. Important pain-related upregulated genes Hoxc8,b9,d8; P2rx4, Cckbr, growth hormone (Gh), and schlafen (Slfn4) are greatly reduced in LMK235-treated mice. Fold increase in axon regeneration/repair genes Sostdc1, TTr, and Folr1 after injury are doubled by LMK235 treatment. LMK235 reduces the excitability of trigeminal ganglia neurons in culture isolated from nerve injured mice compared to vehicle-treated controls, with no effect on neurons from naïve mice. Electrophysiological characterization profile includes a shift where ∼20% of the small neurons recorded under LMK235-treated conditions are high threshold, whereas none of the neurons under control conditions have high thresholds. LMK235 reverses long-standing mechanical and cold hypersensitivity in chronic trigeminal neuropathic pain models in males and females (5,10 mg/kg), preventing development of anxiety- and depression-like behaviors. PERSPECTIVE: Data here support HDAC5 as key epigenetic factor in chronic trigeminal neuropathic pain persistence, validated with the study of RNA alterations, TG neuronal excitability, and pain-related behaviors. HDAC5 inhibitor given in week 3 restores RNA balance at 10 weeks, while upregulation remains for response to wound healing and chronic inflammation RNAs.

Evaluation of epigenetic (HDAC, DNMT) and pain (Gad65, TGF) factors following photobiomodulation therapy in a neuropathic pain model

Photobiomodulation therapy (PBMT) is converted to the most common analgesic treatment before the whole mechanism is yet to be discovered. This study for the first time was designed to investigate alternations of epigenetic factors after pain and PBMT. The CCI model was chosen to induce pain. Pain evaluation tests including plantar, acetone, von Frey, and pinch were done weekly. Then spinal cord tissue was isolated for evaluating mRNA expression of DNMT3a, HDAC1, and NRSF using RT-qPCR method, and protein expression factors of HDAC2 and DNMT3a using western blotting. GAD65 and TGF-β proteins were assessed by the IHC method. PBMT increased the pain threshold up to the point where it roughly met the pain threshold of the control group. After three weeks of treatment, both PBMT protocols demonstrated a reduction in allodynia and hyperalgesia. While some molecules, such as TGF-β and Gad65, increased following PBMT, we observed no inhibition of NRSF, HDAC1, and DNMT3a expression despite implementing two different protocols.

Development and Pharmacochemical Characterization Discover a Novel Brain-Permeable HDAC11-Selective Inhibitor with Therapeutic Potential by Regulating Neuroinflammation in Mice

Recent studies have shown that the epigenetic protein histone deacetylase 11 (HDAC11) is highly expressed in the brain and critically modulates neuroimmune functions, making it a potential therapeutic target for neurological disorders. Herein, we report the development of PB94, which is a novel HDAC11 inhibitor. PB94 exhibited potency and selectivity against HDAC11 with IC50 = 108 nM and >40-fold selectivity over other HDAC isoforms. Pharmacokinetic/pharmacodynamic evaluation indicated that PB94 possesses promising drug-like properties. Additionally, PB94 was radiolabeled with carbon-11 as [11C]PB94 for positron emission tomography (PET), which revealed significant brain uptake and metabolic properties suitable for drug development in live animals. Furthermore, we demonstrated that neuropathic pain was associated with brain upregulation of HDAC11 and that pharmacological inhibition of HDAC11 by PB94 ameliorated neuropathic pain in a mouse model. Collectively, our findings support further development of PB94 as a selective HDAC11 inhibitor for neurological indications, including pain.

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.

Epigenetic regulation of beta-endorphin synthesis in hypothalamic arcuate nucleus neurons modulates neuropathic pain in a rodent pain model

Although beta-endorphinergic neurons in the hypothalamic arcuate nucleus (ARC) synthesize beta-endorphin (β-EP) to alleviate nociceptive behaviors, the underlying regulatory mechanisms remain unknown. Here, we elucidated an epigenetic pathway driven by microRNA regulation of β-EP synthesis in ARC neurons to control neuropathic pain. In pain-injured rats miR-203a-3p was the most highly upregulated miRNA in the ARC. A similar increase was identified in the cerebrospinal fluid of trigeminal neuralgia patients. Mechanistically, we found histone deacetylase 9 was downregulated following nerve injury, which decreased deacetylation of histone H3 lysine-18, facilitating the binding of NR4A2 transcription factor to the miR-203a-3p gene promoter, thereby upregulating miR-203a-3p expression. Further, increased miR-203a-3p was found to maintain neuropathic pain by targeting proprotein convertase 1, an endopeptidase necessary for the cleavage of proopiomelanocortin, the precursor of β-EP. The identified mechanism may provide an avenue for the development of new therapeutic targets for neuropathic pain treatment.

Glioma Stem Cells Are Sensitized to BCL-2 Family Inhibition by Compromising Histone Deacetylases

Glioblastoma (GBM) remains an incurable disease with an extremely high five-year recurrence rate. We studied apoptosis in glioma stem cells (GSCs) in response to HDAC inhibition (HDACi) combined with MEK1/2 inhibition (MEKi) or BCL-2 family inhibitors. MEKi effectively combined with HDACi to suppress growth, induce cell cycle defects, and apoptosis, as well as to rescue the expression of the pro-apoptotic BH3-only proteins BIM and BMF. A RNAseq analysis of GSCs revealed that HDACi repressed the pro-survival BCL-2 family genes MCL1 and BCL-XL. We therefore replaced MEKi with BCL-2 family inhibitors and observed enhanced apoptosis. Conversely, a ligand for the cancer stem cell receptor CD44 led to reductions in BMF, BIM, and apoptosis. Our data strongly support further testing of HDACi in combination with MEKi or BCL-2 family inhibitors in glioma.

The Selective CB2 Agonist COR167 Reduced Symptoms in a Mice Model of Trauma-Induced Peripheral Neuropathy through HDAC-1 Inhibition

Neuropathic pain is a chronic disabling condition with a 7-10% of prevalence in the general population that is largely undertreated. Available analgesic therapies are poorly effective and are often accompanied by numerous side effects. Growing evidence indicates cannabinoids are a valuable treatment opportunity for neuropathic pain. The endocannabinoid system is an important regulator of pain perception through the CB1 receptors, but CB1 agonists, while largely effective, are not always satisfactory pain-relieving agents in clinics because of their serious adverse effects. Recently, several CB2 agonists have shown analgesic, anti-hyperalgesic, and anti-allodynic activity in the absence of CB1-induced psychostimulant effects, offering promise in neuropathic pain management. The aim of this study was to evaluate the anti-neuropathic activity of a novel selective CB2 agonist, COR167, in a preclinical model of peripheral neuropathy, the spared nerve injury (SNI). Oral COR167, in a dose-dependent manner, attenuated mechanical allodynia and thermal hyperalgesia after acute and repeated administration, showing the absence of tolerance induction. At anti-neuropathic doses, COR167 did not show any alteration in the locomotor behavior. SNI mice showed increased microglial levels of HDAC1 protein in the ipsilateral side of the spinal cord, along with NF-kB activation. COR167 treatment prevented the HDAC1 overexpression and the NF-kB activation and increased the levels of the anti-inflammatory cytokine IL-10 through a CB2-mediated mechanism. Oral administration of COR167 shows promising therapeutic potential in the management of neuropathic pain conditions.

Neuropathic pain as a trigger for histone modifications in limbic circuitry

Chronic pain involves both central and peripheral neuronal plasticity that encompasses changes in the brain, spinal cord, and peripheral nociceptors. Within the forebrain, mesocorticolimbic regions associated with emotional regulation have recently been shown to exhibit lasting gene expression changes in models of chronic pain. To better understand how such enduring transcriptional changes might be regulated within brain structures associated with processing of pain or affect, we examined epigenetic modifications involved with active or permissive transcriptional states (histone H3 lysine 4 mono and trimethylation, and histone H3 lysine 27 acetylation) in periaqueductal gray (PAG), lateral hypothalamus (LH), nucleus accumbens (NAc), and ventral tegmental area (VTA) 5 weeks after sciatic nerve injury in mice to model chronic pain. For both male and female mice in chronic pain, we observed an overall trend for a reduction of these epigenetic markers in periaqueductal gray, LH, and NAc, but not VTA. Moreover, we discovered that some epigenetic modifications exhibited changes associated with pain history, while others were associated with individual differences in pain sensitivity. When taken together, these results suggest that nerve injury leads to chronic chromatin-mediated suppression of transcription in key limbic brain structures and circuits, which may underlie enduring changes in pain processing and sensitivity within these systems.

Spinal HDAC6 mediates nociceptive behaviors induced by chronic constriction injury via neuronal activation and neuroinflammation

Neuropathic pain (NP) is often accompanied by psychiatric comorbidities and currently lacks effective treatment. Prior research has shown that HDAC6 plays a crucial role in pain sensitization, but the specific mechanisms remain unclear. HDAC6 inhibitors have been found to alleviate mechanical allodynia caused by inflammation and peripheral nerve damage. In this study, we investigated the cellular mechanisms of HDAC6 in the development and maintenance of neuropathic pain. Our findings indicate that HDAC6 expression in the spinal cord (SC) is upregulated in a time-dependent manner following chronic constriction injury (CCI). HDAC6 is primarily expressed in neurons and microglia in the spinal cord. CCI-induced HDAC6 production was abolished by intrathecal injection of a microglia inhibitor. ACY-1215, a specific HDAC6 inhibitor, significantly reduced CCI-induced mechanical allodynia, but not thermal hyperalgesia. ACY-1215 also inhibited neuron activation and suppressed CCI-induced pyroptosis and neuroinflammatory responses. In summary, our results suggest that HDAC6 contributes to the development and maintenance of NP through neuronal activation and neuroinflammation. HDAC6 may be a promising target for treating NP.

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

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

Sp1 Inhibits PGC-1α via HDAC2-Catalyzed Histone Deacetylation in Chronic Constriction Injury-Induced Neuropathic Pain

BACKGROUND

Our previous study has illuminated that PGC-1α downregulation promoted chronification of pain after burn injury. RNA-seq analysis predicted association between Sp1 and chronic constriction injury (CCI)-provoked neuropathic pain. Further ChIP-Atlas data investigation suggested the binding to Sp1 to PGC-1α. Thereby, we performed this study to illustrate the functional relevance of the Sp1/PGC-1α axis in neuropathic pain.

METHODS

Neuropathic pain was induced by CCI in vivo in rats, followed by assessment of neuropathic pain-like behaviors. The expression of Sp1 and correlated genes was determined in CCI rat spinal cord tissues. Furthermore, microglia were exposed to lipopolysaccharide (LPS) to mimic inflammation and then cocultured with neurons. Knockdown and ectopic expression methods were used in vivo and in vitro to define the role the Sp1/HDAC2/PGC-1α axis.

RESULTS

Sp1 expression was upregulated in spinal cord tissues of CCI rats. Silencing Sp1 ameliorated CCI-induced neuropathic pain, as reflected by elevated paw withdrawal threshold and paw withdrawal latency, as well as alleviated microglia activation, neuronal dysfunction, inflammatory responses, mitochondrial dysfunction, and oxidative stress in spinal cord tissues. Sp1 knockdown also reversed LPS-induced microglial inflammation and neuronal dysfunction. Sp1 promoted histone deacetylation in the PGC-1α promoter and inhibited PGC-1α expression via recruiting HDAC2. PGC-1α overexpression diminished CCI-induced neuropathic pain and LPS-induced inflammation and mitochondrial dysfunction, based on which Sp1 aggravated microglial inflammation and neuronal dysfunction in neuropathic pain.

CONCLUSION

This study elucidated the promoting effects of Sp1 on CCI-induced neuropathic pain via the HDAC2/PGC-1α axis.

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.

Paeonol inhibits chronic constriction injury-induced astrocytic activation and neuroinflammation in rats via the HDAC/miR-15a pathway

Neuropathic pain affects millions of people in the worldwide, but the major therapeutics perform limited effectiveness. Paeonol (PAE) is widely distributed in Paeonis albiflora, and has manifested anti-inflammatory and antioxidative effects in multiple diseases. The present study aims to elucidate the effect of Paeonol (PAE) on neuropathic pain (NP) and the potential targets. Chronic constriction injury model was established to mimic NP in vivo in rats. The expression of GFAP, HDAC2, AHDAC3, Ac-H3K9, Histone-H3, Ac-H4K12, Histone-H4, TNF-α, IL-1β, and IL-6 was assessed by real-time polymerase chain reaction, western blot, and/or enzyme-linked immunosorbent assay kits. Ultimately, results indicated that intervention of PAE significantly blocked neuroinflammation and astrocytic activation via blocking HDAC/miR-15a signaling in CCI rats. These data revealed PAE is a novel therapeutic target for the treatment of neuropathic pain.

Dual HDAC/BRD4 Inhibitors Relieves Neuropathic Pain by Attenuating Inflammatory Response in Microglia After Spared Nerve Injury

Despite the effort on developing new treatments, therapy for neuropathic pain is still a clinical challenge and combination therapy regimes of two or more drugs are often needed to improve efficacy. Accumulating evidence shows an altered expression and activity of histone acetylation enzymes in chronic pain conditions and restoration of these aberrant epigenetic modifications promotes pain-relieving activity. Recent studies showed a synergistic activity in neuropathic pain models by combination of histone deacetylases (HDACs) and bromodomain and extra-terminal domain (BET) inhibitors. On these premises, the present study investigated the pharmacological profile of new dual HDAC/BRD4 inhibitors, named SUM52 and SUM35, in the spared nerve injury (SNI) model in mice as innovative strategy to simultaneously inhibit HDACs and BETs. Intranasal administration of SUM52 and SUM35 attenuated thermal and mechanical hypersensitivity in the absence of locomotor side effects. Both dual inhibitors showed a preferential interaction with BRD4-BD2 domain, and SUM52 resulted the most active compound. SUM52 reduced microglia-mediated spinal neuroinflammation in spinal cord sections of SNI mice as showed by reduction of IBA1 immunostaining, inducible nitric oxide synthase (iNOS) expression, p65 nuclear factor-κB (NF-κB) and p38 MAPK over-phosphorylation. A robust decrease of the spinal proinflammatory cytokines content (IL-6, IL-1ß) was also observed after SUM52 treatment. Present results, showing the pain-relieving activity of HDAC/BRD4 dual inhibitors, indicate that the simultaneous modulation of BET and HDAC activity by a single molecule acting as multi-target agent might represent a promise for neuropathic pain relief.

Oxidative Stress and Mitochondrial Dysfunction Associated with Peripheral Neuropathy in Type 1 Diabetes

Significance: This review highlights the many intracellular processes generating reactive oxygen species (ROS) in the peripheral nervous system in the context of type 1 diabetes. The major sources of superoxide and hydrogen peroxide (H₂O₂) are described, and scavenging systems are explained. Important roles of ROS in regulating normal redox signaling and in a disease setting, such as diabetes, contributing to oxidative stress and cellular damage are outlined. The primary focus is the role of hyperglycemia in driving elevated ROS production and oxidative stress contributing to neurodegeneration in diabetic neuropathy (within the dorsal root ganglia [DRG] and peripheral nerve). Recent Advances: Contributors to ROS production under high intracellular glucose concentration such as mitochondria and the polyol pathway are discussed. The primarily damaging impact of ROS on multiple pathways including mitochondrial function, endoplasmic reticulum (ER) stress, autophagy, and epigenetic signaling is covered. Critical Issues: There is a strong focus on mechanisms of diabetes-induced mitochondrial dysfunction and how this may drive ROS production (in particular superoxide). The mitochondrial sites of superoxide/H₂O₂ production via mitochondrial metabolism and aerobic respiration are reviewed. Future Directions: Areas for future development are highlighted, including the need to clarify diabetes-induced changes in autophagy and ER function in neurons and Schwann cells. In addition, more clarity is needed regarding the sources of ROS production at mitochondrial sites under high glucose concentration (and lack of insulin signaling). New areas of study should be introduced to investigate the role of ROS, nuclear lamina function, and epigenetic signaling under diabetic conditions in peripheral nerve.

Theophylline Induces Remyelination and Functional Recovery in a Mouse Model of Peripheral Neuropathy

Charcot-Marie-Tooth disease (CMT) is a large group of inherited peripheral neuropathies that are primarily due to demyelination and/or axonal degeneration. CMT type 1A (CMT1A), which is caused by the duplication of the peripheral myelin protein 22 (PMP22) gene, is a demyelinating and the most frequent CMT subtype. Hypermyelination, demyelination, and secondary loss of large-caliber axons are hallmarks of CMT1A, and there is currently no cure and no efficient treatment to alleviate the symptoms of the disease. We previously showed that histone deacetylases 1 and 2 (HDAC1/2) are critical for Schwann cell developmental myelination and remyelination after a sciatic nerve crush lesion. We also demonstrated that a short-term treatment with Theophylline, which is a potent activator of HDAC2, enhances remyelination and functional recovery after a sciatic nerve crush lesion in mice. In the present study, we tested whether Theophylline treatment could also lead to (re)myelination in a PMP22-overexpressing mouse line (C22) modeling CMT1A. Indeed, we show here that a short-term treatment with Theophylline in C22 mice increases the percentage of myelinated large-caliber axons and the expression of the major peripheral myelin protein P0 and induces functional recovery. This pilot study suggests that Theophylline treatment could be beneficial to promote myelination and thereby prevent axonal degeneration and enhance functional recovery in CMT1A patients.

Histone Deacetylase Inhibitors Counteract CGRP Signaling and Pronociceptive Sensitization in a Rat Model of Medication Overuse Headache

Chronic triptan exposurein rodents recapitulates medication overuse headache (MOH), causing cephalic pain sensitization and trigeminal ganglion overexpression of pronociceptive proteins including CGRP. Because of these transcriptional derangements, as well as the emerging role of epigenetics in chronic pain, in the present study, we evaluated the effects of the histone deacetylase inhibitors (HDACis) panobinostat and givinostat, in rats chronically exposed to eletriptan for one month. Both panobinostat and givinostat counteracted overexpression of genes coding for CGRP and its receptor subunit RAMP1, having no effects on CLR and RCP receptor subunits in the trigeminal ganglion (TG) of eletriptan-exposed rats. Within the trigeminal nucleus caudalis (TNc), transcripts for these genes were neither upregulated by eletriptan nor altered by concomitant treatment with panobinostat or givinostat. HDACis counteracted hypersensitivity to capsaicin-induced vasodilatation in the trigeminal territory, as well as photophobic behavior and cephalic allodyniain eletriptan-exposed rats. Eletriptan did not affect CGRP, CLR, and RAMP1 expression in cultured trigeminal ganglia, whereas both inhibitors reduced transcripts for CLR and RAMP-1. The drugs, however, increased luciferase expression driven by CGRP promoter in cultured cells. Our findings provide evidence for a key role of HDACs and epigenetics in MOH pathogenesis, highlighting the therapeutic potential of HDAC inhibition in the prevention of migraine chronification.

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.

SIRT1: A promising therapeutic target for chronic pain

Chronic pain remains an unresolved problem. Current treatments have limited efficacy. Thus, novel therapeutic targets are urgently required for the development of more effective analgesics. An increasing number of studies have proved that sirtuin 1 (SIRT1) agonists can relieve chronic pain. In this review, we summarize recent progress in understanding the roles and mechanisms of SIRT1 in mediating chronic pain associated with peripheral nerve injury, chemotherapy-induced peripheral neuropathy, spinal cord injury, bone cancer, and complete Freund's adjuvant injection. Emerging studies have indicated that SIRT1 activation may exert positive effects on chronic pain relief by regulating inflammation, oxidative stress, and mitochondrial dysfunction. Therefore, SIRT1 agonists may serve as potential therapeutic drugs for chronic pain.

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.

Characterization of Acetylation of Histone H3 at Lysine 9 in the Trigeminal Ganglion of a Rat Trigeminal Neuralgia Model

Trigeminal neuralgia (TN) is a chronic neuropathic pain disorder characterized by spontaneous and elicited paroxysms of electric-shock-like or stabbing pain in a region of the face. The epigenetic regulation of TN is still obscure. In current study, a rat TN model subject to carbamazepine (CBZ) treatment was established, and transcriptome- and genome-scale profiling of H3K9ac and HDAC3 was performed by RNA-seq and ChIP-seq. We observed that H3K9ac levels in the trigeminal ganglion were lower in the TN rats compared with those in the control, and CBZ treatment led to recovery of H3K9ac levels. Further, we found that HDAC3 was overactivated, which interfered with H3K9 acetylation due to higher phosphorylation in TN compared with that in the control. Finally, the phosphokinase leucine-rich repeat kinase 2 (LRRK2) was demonstrated to contribute to HDAC3 activity via the MAPK signaling pathway. Taken together, we identified a regulatory mechanism in which the phosphate groups transferred from activated ERK and LRRK2 to HDAC3 caused genome-scale deacetylation at H3K9 and resulted in the silencing of a large number of genes in TN. The kinases or important enzymes within this regulatory axis may represent important targets for TN therapy and prevention.

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.

Genetic basis of thermal nociceptive sensitivity and brain weight in a BALB/c reduced complexity cross

Thermal nociception involves the transmission of temperature-related noxious information from the periphery to the CNS and is a heritable trait that could predict transition to persistent pain. Rodent forward genetics complement human studies by controlling genetic complexity and environmental factors, analysis of end point tissue, and validation of variants on appropriate genetic backgrounds. Reduced complexity crosses between nearly identical inbred substrains with robust trait differences can greatly facilitate unbiased discovery of novel genes and variants. We found BALB/cByJ mice showed enhanced sensitivity on the 53.5°C hot plate and mechanical stimulation in the von Frey test compared to BALB/cJ mice and replicated decreased gross brain weight in BALB/cByJ versus BALB/cJ. We then identified a quantitative trait locus (QTL) on chromosome 13 for hot plate sensitivity (LOD = 10.7; p < 0.001; peak = 56 Mb) and a QTL for brain weight on chromosome 5 (LOD = 8.7; p < 0.001). Expression QTL mapping of brain tissues identified H2afy (56.07 Mb) as the top transcript with the strongest association at the hot plate locus (FDR = 0.0002) and spliceome analysis identified differential exon usage within H2afy associated with the same locus. Whole brain proteomics further supported decreased H2AFY expression could underlie enhanced hot plate sensitivity, and identified ACADS as a candidate for reduced brain weight. To summarize, a BALB/c reduced complexity cross combined with multiple-omics approaches facilitated identification of candidate genes underlying thermal nociception and brain weight. These substrains provide a powerful, reciprocal platform for future validation of candidate variants.

The Class I HDAC Inhibitor, MS-275, Prevents Oxaliplatin-Induced Chronic Neuropathy and Potentiates Its Antiproliferative Activity in Mice

Oxaliplatin, the first-line chemotherapeutic agent against colorectal cancer (CRC), induces peripheral neuropathies, which can lead to dose limitation and treatment discontinuation. Downregulation of potassium channels, which involves histone deacetylase (HDAC) activity, has been identified as an important tuner of acute oxaliplatin-induced hypersensitivity. MS-275, a class I histone deacetylase inhibitor (HDACi), prevents acute oxaliplatin-induced peripheral neuropathy (OIPN). Moreover, MS-275 exerts anti-tumor activity in several types of cancers, including CRC. We thus hypothesized that MS-275 could exert both a preventive effect against OIPN and potentially a synergistic effect combined with oxaliplatin against CRC development. We first used RNAseq to assess transcriptional changes occurring in DRG neurons from mice treated by repeated injection of oxaliplatin. Moreover, we assessed the effects of MS-275 on chronic oxaliplatin-induced peripheral neuropathy development in vivo on APC^Min/+ mice and on cancer progression when combined with oxaliplatin, both in vivo on APC^Min/+ mice and in a mouse model of an orthotopic allograft of the CT26 cell line as well as in vitro in T84 and HT29 human CRC cell lines. We found 741 differentially expressed genes (DEGs) between oxaliplatin- and vehicle-treated animals. While acute OIPN is known as a channelopathy involving HDAC activity, chronic OIPN exerts weak ion channel transcriptional changes and no HDAC expression changes in peripheral neurons from OIPN mice. However, MS-275 prevents the development of sensory neuropathic symptoms induced by repeated oxaliplatin administration in APC^Min/+ mice. Moreover, combined with oxaliplatin, MS-275 also exerts synergistic antiproliferative and increased survival effects in CT26-bearing mice. Consistently, combined drug associations exert synergic apoptotic and cell death effects in both T84 and HT29 human CRC cell lines. Our results strongly suggest combining oxaliplatin and MS-275 administration in CRC patients in order to potentiate the antiproliferative action of chemotherapy, while preventing its neurotoxic effect.

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.

Identification of miRNAs That Mediate Protective Functions of Anti-Cancer Drugs During White Matter Ischemic Injury

We have previously shown that two anti-cancer drugs, CX-4945 and MS-275, protect and preserve white matter (WM) architecture and improve functional recovery in a model of WM ischemic injury. While both compounds promote recovery, CX-4945 is a selective Casein kinase 2 (CK2) inhibitor and MS-275 is a selective Class I histone deacetylase (HDAC) inhibitor. Alterations in microRNAs (miRNAs) mediate some of the protective actions of these drugs. In this study, we aimed to (1) identify miRNAs expressed in mouse optic nerves (MONs); (2) determine which miRNAs are regulated by oxygen glucose deprivation (OGD); and (3) determine the effects of CX-4945 and MS-275 treatment on miRNA expression. RNA isolated from MONs from control and OGD-treated animals with and without CX-4945 or MS-275 treatment were quantified using NanoString nCounter^® miRNA expression profiling. Comparative analysis of experimental groups revealed that 12 miRNAs were expressed at high levels in MONs. OGD upregulated five miRNAs (miR-1959, miR-501-3p, miR-146b, miR-201, and miR-335-3p) and downregulated two miRNAs (miR-1937a and miR-1937b) compared to controls. OGD with CX-4945 upregulated miR-1937a and miR-1937b, and downregulated miR-501-3p, miR-200a, miR-1959, and miR-654-3p compared to OGD alone. OGD with MS-275 upregulated miR-2134, miR-2141, miR-2133, miR-34b-5p, miR-153, miR-487b, miR-376b, and downregulated miR-717, miR-190, miR-27a, miR-1959, miR-200a, miR-501-3p, and miR-200c compared to OGD alone. Interestingly, miR-501-3p and miR-1959 were the only miRNAs upregulated by OGD, and downregulated by OGD plus CX-4945 and MS-275. Therefore, we suggest that protective functions of CX-4945 or MS-275 against WM injury maybe mediated, in part, through miRNA expression.

Histone post-translational modifications as potential therapeutic targets for pain management

Effective pharmacological management of pain associated with tissue pathology is an unmet medical need. Transcriptional modifications in nociceptive pathways are pivotal for the development and the maintenance of pain associated with tissue damage. Accumulating evidence has shown the importance of the epigenetic control of transcription in nociceptive pathways via histone post-translational modifications (PTMs). Hence, histone PTMs could be targets for novel effective analgesics. Here, we discuss the current understanding of histone PTMs in the modulation of gene expression affecting nociception and pain phenotypes following tissue injury. We also provide a critical view of the translational implications of preclinical models and discuss opportunities and challenges of targeting histone PTMs to relieve pain in clinically relevant tissue injuries.

Dual BET/HDAC inhibition to relieve neuropathic pain: Recent advances, perspectives, and future opportunities

Despite the intense research on developing new therapies for neuropathic pain states, available treatments have limited efficacy and unfavorable safety profiles. Epigenetic alterations have a great influence on the development of cancer and neurological diseases, as well as neuropathic pain. Histone acetylation has prevailed as one of the well investigated epigenetic modifications in these diseases. Altered spinal activity of histone deacetylase (HDAC) and Bromo and Extra terminal domain (BET) have been described in neuropathic pain models and restoration of these aberrant epigenetic modifications showed pain-relieving activity. Over the last decades HDACs and BETs have been the focus of drug discovery studies, leading to the development of numerous small-molecule inhibitors. Clinical trials to evaluate their anticancer activity showed good efficacy but raised toxicity concerns that limited translation to the clinic. To maximize activity and minimize toxicity, these compounds can be applied in combination of sub-maximal doses to produce additive or synergistic interactions (combination therapy). Recently, of particular interest, dual BET/HDAC inhibitors (multi-target drugs) have been developed to assure simultaneous modulation of BET and HDAC activity by a single molecule. This review will summarize the most recent advances with these strategies, describing advantages and limitations of single drug treatment vs combination regimens. This review will also provide a focus on dual BET/HDAC drug discovery investigations as future therapeutic opportunity for human therapy of neuropathic pain.

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.

Epigenetics Involvement in Oxaliplatin-Induced Potassium Channel Transcriptional Downregulation and Hypersensitivity

Peripheral neuropathy is the most frequent dose-limiting adverse effect of oxaliplatin. Acute pain symptoms that are induced or exacerbated by cold occur in almost all patients immediately following the first infusions. Evidence has shown that oxaliplatin causes ion channel expression modulations in dorsal root ganglia neurons, which are thought to contribute to peripheral hypersensitivity. Most dysregulated genes encode ion channels involved in cold and mechanical perception, noteworthy members of a sub-group of potassium channels of the K2P family, TREK and TRAAK. Downregulation of these K2P channels has been identified as an important tuner of acute oxaliplatin-induced hypersensitivity. We investigated the molecular mechanisms underlying this peripheral dysregulation in a murine model of neuropathic pain triggered by a single oxaliplatin administration. We found that oxaliplatin-mediated TREK-TRAAK downregulation, as well as downregulation of other K+ channels of the K2P and Kv families, involves a transcription factor known as the neuron-restrictive silencer factor (NRSF) and its epigenetic co-repressors histone deacetylases (HDACs). NRSF knockdown was able to prevent most of these K+ channel mRNA downregulation in mice dorsal root ganglion neurons as well as oxaliplatin-induced acute cold and mechanical hypersensitivity. Interestingly, pharmacological inhibition of class I HDAC reproduces the antinociceptive effects of NRSF knockdown and leads to an increased K+ channel expression in oxaliplatin-treated mice.

Histone deacetylase 3 in hippocampus contributes to memory impairment after chronic constriction injury of sciatic nerve in mice

ABSTRACT

Chronic neuropathic pain is frequently accompanied by memory impairment, yet the underlying mechanisms remain unclear. Here, we showed that mice displayed memory impairment starting at 14 days and lasting for at least 21 days after chronic constriction injury (CCI) of unilateral sciatic nerve in mice. Systemic administration of the pan histone deacetylase (HDAC) inhibitor sodium butyrate attenuated this memory impairment. More specifically, we found that hippocampus HDAC3 was involved in this process because the levels of its mRNA and protein increased significantly in the hippocampus at 14 and 21 days after CCI, but not sham surgery. Systemic administration of the selective HDAC3 antagonist RGFP966 attenuated CCI-induced memory impairment, improved hippocampal long-term potentiation impairment, and rescued reductions of dendritic spine density and synaptic plasticity-associated protein in the hippocampus. In addition, HDAC3 overexpression in the hippocampus led to memory impairment without affecting basal nociceptive responses in naive mice. Our findings suggest that HDAC3 contributes to memory impairment after CCI by impairing synaptic plasticity in hippocampus. Histone deacetylase 3 might serve as a potential molecular target for therapeutic treatment of memory impairment under neuropathic pain conditions.