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

The Role of Methylation Modification in Neural Injury and Repair

The diverse methylation modifications of DNA, histones and RNA have emerged as pivotal regulatory mechanisms of gene expression in multiple biological processes at the epigenetic level. They function by coordinating gene expression through impacting gene transcription, mRNA processing and maturation, protein translation and metabolism. Changes in methylation profiles of nucleic acids and histones have been observed in many different types neural injuries in both the central nervous system and the peripheral nervous system, such as 5-methylcytosine in DNA, N6-methyladenosine in RNA and methylation of lysine residues in various histones. Importantly, altering these modifications plays key roles in regulation of neural injury and repair. In this review, we highlight recent research advances of the methylation-related epigenetic modifications in multiple aspects of neural injury and regeneration, including neural protection, axon regeneration, microenvironment modulation and neural functional recovery. We also discuss the current unsolved problems in the field and propose potential future research directions.

Repetitive neonatal pain increases spinal cord DNA methylation of the µ-opioid receptor

BACKGROUND

Repetitive neonatal painful procedures experienced in the neonatal intensive care unit (NICU) are known to alter the development of the nociceptive system and have long-lasting consequences. Recent evidence indicates that NICU stay affects the methylation of the opioid receptor mu 1 encoding gene (Mor-1). Additionally, a preclinical model of neonatal procedural pain established lower adult post-operative MOR-1 levels in the spinal cord. Thus, we hypothesized that neonatal procedural pain increases the DNA methylation status of Mor-1 in the spinal cord and dorsal root ganglia (DRGs).

METHODS

To this end, repetitive neonatal procedural pain was induced in animals, during the first postnatal week, a period equivalent to preterm human brain development. On postnatal day 10 methylation of Mor-1 promotor was assessed in the spinal cord and the DRG using bisulfite pyrosequencing.

RESULTS

Our findings demonstrated that neonatal procedural pain increased spinal cord Mor-1 promotor DNA methylation in the ipsilateral side as compared to the contralateral side, an effect that was not observed in the control animals, nor in the DRG.

CONCLUSION

This study is the first to highlight a localized and noxious-stimuli-dependent effect of repetitive neonatal procedural pain on Mor-1 promotor methylation and emphasizes the need to explore the effects of repetitive neonatal procedural pain on the epigenome.

IMPACT

This study reveals that repetitive neonatal procedural pain is associated with increased DNA methylation of the Mor-1 promoter in the spinal cord of neonatal rats. This is the first study to identify an effect of neonatal procedural pain on DNA methylation, emphasizing the critical need for further investigation into the epigenetic consequences of neonatal procedural pain. These insights could lead to better management and treatment strategies to mitigate the long-term impacts of early pain exposure on neurodevelopment and behavior.

Synaptic Structure and Transcriptomic Profiling of Reward and Sensory Brain Areas in Male Mice of Fentanyl Addiction

Background

Opioid-based medications are powerful analgesics commonly prescribed for pain management, but they are also highly addictive. The over-prescription of opioids analgesics has triggered current opioid crisis, which now has expanded to heroin and illicit synthetic opioids like fentanyl and its analogues. The side effects of fentanyl abuse have been well recognized, yet the underlying molecular adaptations across brain regions upon fentanyl exposure remain elusive.

Methods

The transmission electron microscopy (TEM) and next-generation RNA-sequencing (RNA-seq) were used to investigate the ultrastructure synaptic alterations and transcriptional profiling changes of reward and sensory brain regions in mice after fentanyl exposure.

Results

The naloxone-precipitated acute withdrawal symptoms were observed in mice exposed to fentanyl. Results of TEM showed an increase in the number of synapses, widening of synaptic gaps, and thickening of postsynaptic density in the NAc of the fentanyl addiction mice, accompanied by obvious mitochondrial swelling. RNA-seq identified differentially expressed genes (DEGs) in prefrontal cortex of mice brains after fentanyl exposure, and the expression of some addiction-related genes such as Calm4, Cdh1, Drd1/2/3/4, F2rl2, Gabra6, Ht2cr, Oprk1 and Rxfp3 showed the most striking changes among experimental groups. KEGG enrichment analysis indicated that these DEGs were related to the development of addiction behavior, dopaminergic/GABAergic/serotonergic synapse, synapse assembly/synaptic plasticity/synaptic vesicle cycle, cAMP/MAPK signaling pathway, neuroactive ligand-receptor interactions. These transcriptomic changes may be correlated with the structural and behavioral changes observed in fentanyl-exposed mice.

Discussion

The findings of this study contribute to a better understanding of the molecular mechanism of addiction behavior, which is essential for the development of optimized therapy strategies for addicts.

Lung-innervating nociceptor sensory neurons promote pneumonic sepsis during carbapenem-resistant Klebsiella pneumoniae lung infection

Carbapenem-resistant Klebsiella pneumoniae (CRKP) causes Gram-negative lung infections and fatal pneumonic sepsis for which limited therapeutic options are available. The lungs are densely innervated by nociceptor sensory neurons that mediate breathing, cough, and bronchoconstriction. The role of nociceptors in defense against Gram-negative lung pathogens is unknown. Here, we found that lung-innervating nociceptors promote CRKP pneumonia and pneumonic sepsis. Ablation of nociceptors in mice increased lung CRKP clearance, suppressed trans-alveolar dissemination of CRKP, and protected mice from hypothermia and death. Furthermore, ablation of nociceptors enhanced the recruitment of neutrophils and Ly6Chi monocytes and cytokine induction. Depletion of Ly6Chi monocytes, but not of neutrophils, abrogated lung and extrapulmonary CRKP clearance in ablated mice, suggesting that Ly6Chi monocytes are a critical cellular population to regulate pneumonic sepsis. Further, neuropeptide calcitonin gene-related peptide suppressed the induction of reactive oxygen species in Ly6Chi monocytes and their CRKP-killing abilities. Targeting nociceptor signaling could be a therapeutic approach for treating multidrug-resistant Gram-negative infection and pneumonic sepsis.

Involvement of HDAC2-mediated kcnq2/kcnq3 genes transcription repression activated by EREG/EGFR-ERK-Runx1 signaling in bone cancer pain

Bone cancer pain (BCP) represents a prevalent symptom among cancer patients with bone metastases, yet its underlying mechanisms remain elusive. This study investigated the transcriptional regulation mechanism of Kv7(KCNQ)/M potassium channels in DRG neurons and its involvement in the development of BCP in rats. We show that HDAC2-mediated transcriptional repression of kcnq2/kcnq3 genes, which encode Kv7(KCNQ)/M potassium channels in dorsal root ganglion (DRG), contributes to the sensitization of DRG neurons and the pathogenesis of BCP in rats. Also, HDAC2 requires the formation of a corepressor complex with MeCP2 and Sin3A to execute transcriptional regulation of kcnq2/kcnq3 genes. Moreover, EREG is identified as an upstream signal molecule for HDAC2-mediated kcnq2/kcnq3 genes transcription repression. Activation of EREG/EGFR-ERK-Runx1 signaling, followed by the induction of HDAC2-mediated transcriptional repression of kcnq2/kcnq3 genes in DRG neurons, leads to neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats. Consequently, the activation of EREG/EGFR-ERK-Runx1 signaling, along with the subsequent transcriptional repression of kcnq2/kcnq3 genes by HDAC2 in DRG neurons, underlies the sensitization of DRG neurons and the pathogenesis of BCP in rats. These findings uncover a potentially targetable mechanism contributing to bone metastasis-associated pain in cancer patients.

Pathology of pain and its implications for therapeutic interventions

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

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.

Mu opioid receptor expressing neurons in the rostral ventromedial medulla are the source of mechanical hypersensitivity induced by repeated restraint stress

Repeated exposure to psychophysical stress often causes an increase in sensitivity and response to pain. This phenomenon is commonly called stress-induced hyperalgesia (SIH). Although psychophysical stress is a well-known risk factor for numerous chronic pain syndromes, the neural mechanism underlying SIH has not yet been elucidated. The rostral ventromedial medulla (RVM) is a key output element of the descending pain modulation system. Descending signals from the RVM have a major impact on spinal nociceptive neurotransmission. In the present study, to clarify changes in the descending pain modulatory system in rats with SIH, we examined the expression of Mu opioid receptor (MOR) mRNA, MeCP2 and global DNA methylation in the RVM after repeated restraint stress for 3 weeks. Additionally, we microinjected neurotoxin dermorphin-SAP into the RVM. The repeated restraint stress for 3 weeks induced mechanical hypersensitivity in the hind paw, a significant increase in the expression of MOR mRNA and MeCP2, and a significant decrease in global DNA methylation in the RVM. The MeCP2 binding to MOR gene promoter in the RVM was significantly decreased in rats with repeated restraint stress. Furthermore, microinjection of dermorphin-SAP into the RVM prevented the mechanical hypersensitivity induced by repeated restraint stress. Although, because of the lack of specific antibody to MOR, we could not show a quantitative analysis in the number of MOR-expressing neurons after the microinjection, these results suggest that MOR-expressing neurons in the RVM induce SIH after repeated restraint stress.

REAC Antalgic Neuro Modulation in Chronic Post Herpetic Neuralgia

BACKGROUND

Chronic post-herpetic neuralgia (CPHN) is a symptomatic condition that afflicts adults and elderly individuals. The chronicity of this symptomatology can be conditioned by the epigenetic modifications induced by the virus on the processes of neurotransmission and sensitivity to pain. The aim of this study is to investigate whether manipulating endogenous bioelectrical activity (EBA), responsible for neurotransmission processes and contributing to the induction of epigenetic modifications, can alleviate pain symptoms.

METHODS

This manipulation was carried out with the antalgic neuromodulation (ANM) treatment of radioelectric asymmetric conveyer (REAC) technology. Pain assessment before and after treatment was performed using a numerical analog scale (NAS) and a simple descriptive scale (SDS).

RESULTS

The results of the analysis showed an over four-point decrease in NAS scale score and over one point decrease in SDS scale score, with a statistical significance for both tests of p < 0.005.

CONCLUSIONS

The results obtained in this study demonstrate how REAC ANM manipulation of EBA can lead to improvement in epigenetically conditioned symptoms such as CPHN. These results should prompt further research to expand knowledge and ensure optimized therapeutic outcomes.

Endogenous opiates and behavior: 2021

This paper is the forty-fourth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2021 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonizts and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Global research trends on epigenetics and neuropathic pain: A bibliometric analysis

Objective

Neuropathic pain (NP) is a common disease that manifests with pathological changes in the somatosensory system. In recent years, the interactions of NP with the epigenetic mechanism have been increasingly elucidated. However, only a few studies have used bibliometric tools to systematically analyze knowledge in this field. The objective of this study is to visually analyze the trends, hotspots, and frontiers in epigenetics and NP research by using a bibliometric method.

Methods

Studies related to epigenetics and NP were searched from the Science Citation Index-Expanded of the Web of Science Core Collection database. Search time is from inception to November 30, 2022. No restrictions were placed on language. Only articles and reviews were included as document types. Data on institutions, countries, authors, journal distribution, and keywords were imported into CiteSpace software for visual analysis.

Results

A total of 867 publications met the inclusion criteria, which spanned the period from 2000 to 2022. Over the years, the number of publications and the frequency of citations exhibited a clear upward trend in general, reaching a peak in 2021. The major contributing countries in terms of the number of publications were China, the United States, and Japan. The top three institutions were Rutgers State University, Xuzhou Medical University, and Nanjing Medical University. Molecular Pain, Pain, and Journal of Neuroinflammation contributed significantly to the volume of issues. Among the top 10 authors in terms of the number of publications, Tao Yuan-Xiang contributed 30 entries, followed by Zhang Yi with 24 and Wu Shao-Gen with 20. On the basis of the burst and clusters of keywords, "DNA methylation," "Circular RNA," "acetylation," "long non-coding RNA," and "microglia" are global hotspots in the field.

Conclusion

The bibliometric analysis indicates that the number of publications related to epigenetics and NP is exhibiting a rapid increase. Keyword analysis shows that "DNA methylation," "Circular RNA," "acetylation," "long non-coding RNA" and "microglia" are the most interesting terms for researchers in the field. More rigorous clinical trials and additional studies that explore relevant mechanisms are required in the future.

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.

HDACi: The Columbus' Egg in Improving Cancer Treatment and Reducing Neurotoxicity?

Histone deacetylases (HDACs) are a group of enzymes that modify gene expression through the lysine acetylation of both histone and non-histone proteins, leading to a broad range of effects on various biological pathways. New insights on this topic broadened the knowledge on their biological activity and even more questions arose from those discoveries. The action of HDACs is versatile in biological pathways and, for this reason, inhibitors of HDACs (HDACis) have been proposed as a way to interfere with HDACs' involvement in tumorigenesis. In 2006, the first HDACi was approved by FDA for the treatment of cutaneous T-cell lymphoma; however, more selective HDACis were recently approved. In this review, we will consider new information on HDACs' expression and their regulation for the treatment of central and peripheral nervous system diseases.

DNA Methylation: A Target in Neuropathic Pain

Neuropathic pain (NP), caused by an injury or a disease affecting the somatosensory nervous system of the central and peripheral nervous systems, has become a global health concern. Recent studies have demonstrated that epigenetic mechanisms are among those that underlie NP; thus, elucidating the molecular mechanism of DNA methylation is crucial to discovering new therapeutic methods for NP. In this review, we first briefly discuss DNA methylation, demethylation, and the associated key enzymes, such as methylases and demethylases. We then discuss the relationship between NP and DNA methylation, focusing on DNA methyltransferases including methyl-CpG-binding domain (MBD) family proteins and ten-eleven translocation (TET) enzymes. Based on experimental results of neuralgia in animal models, the mechanism of DNA methylation-related neuralgia is summarized, and useful targets for early drug intervention in NP are discussed.