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

Gut microbes-spinal connection is required for itch sensation

The gut microbiota has been linked to a number of neurological disorders. However, it is unclear whether the gut microbiota is involved in the genesis of chronic itch, a refractory condition that afflicts patients both physically and mentally. Here, we report that depletion of gut microbiota enhances tolerance to itch in mice orally administered with antibiotics (ABX) and mice free of germ. Of note, oral gavage with Bacteroides fragilis (B. fragilis), a prominent species of the genus Bacteroides with most differential change, corrected the ABX-induced itch dysfunction through its driven metabolite acetyl-l-carnitine (ALC). Mechanistically, gut microbiota or B. fragilis depletion caused a decrease in RNA N6-methyladenosine (m6A) demethylase FTO expression in the dorsal horn and a consequent increase in RNA m6A sites in Mas-related G protein-coupled receptor F (MrgprF) mRNA, leading to decreased MRGPRF protein. The downregulation of FTO was triggered by inactivation of ETS proto-oncogene 1 (ETS1), a transcription factor that binds to the Fto promoter. These findings support a gut microbe - spinal connection in modulation of itch sensation in RNA m6A epigenetic-dependent manner and highlight a critical role of ALC in linking the altered B. fragilis and itch dysfunction.

TET1 participates in oxaliplatin-induced neuropathic pain by regulating microRNA-30b/Nav1.6

Chemotherapy-induced neuropathic pain poses significant clinical challenges and severely impacts patient quality of life. Sodium ion channels are crucial in regulating neuronal excitability and pain. Our research indicates that the microRNA-30b (miR-30b) in rat dorsal root ganglia (DRG) contributes to chemotherapy-induced neuropathic pain by regulating the Nav1.6 protein. Additionally, ten-eleven translocation methylcytosine dioxygenase 1 (TET1) plays a crucial role in pain generation by altering gene expression. We established a chemotherapy-induced neuropathy model using intraperitoneal oxaliplatin (OXA) injections and measured TET1 and Nav1.6 protein in the DRG. Using lentivirus and Tet1flox/flox mice, we modulated TET1 expression and assessed pain behaviors, DRG neuronal excitability, Nav1.6 currents, miR-30b-5p, and demethylation of the Mir30b promoter region. We employed chromatin immunoprecipitation to pinpoint TET1-binding sites on the Mir30b promoter. The impacts of miR-30b agomir or antagomir on Nav1.6 expression and pain responses were assessed postintrathecal injections. The results showed that OXA reduced TET1, increasing neuronal excitability, Nav1.6 currents, and miR-30b-5p in the DRG. TET1 knockdown exacerbated these effects and induced pain behaviors. Conversely, TET1 overexpression reversed these effects. TET1 also targeted and enhanced demethylation at the Mir30b promoter (-1103 bp to -1079 bp). miR-30b agomir reduces Nav1.6, whereas miR-30b antagomir reverses TET1's effects on Nav1.6 and pain. In OXA-induced neuropathy, decreased TET1 reduces miR-30b, elevating Nav1.6 expression and currents and contributing to pain. We hypothesize that TET1 mediates this process by regulating the demethylation of the Mir30b promoter.

Kv3.1 Interaction with UBR5 Is Required for Chronic Inflammatory Pain

Chronic inflammatory pain caused by neuronal hyperactivity is a common and refractory disease. Kv3.1, a member of the Kv3 family of voltage-dependent K+ channels, is a major determinant of the ability of neurons to generate high-frequency action potentials. However, little is known about its role in chronic inflammatory pain. Here, we show that although Kv3.1 mRNA expression was unchanged, Kv3.1 protein expression was decreased in the dorsal spinal horn of mice after plantar injection of complete Freund's adjuvant (CFA), a mouse model of inflammatory pain. Upregulating Kv3.1 expression alleviated CFA-induced mechanical allodynia and heat hyperalgesia, whereas downregulating Kv3.1 induced nociception-like behaviors. Additionally, we found that ubiquitin protein ligase E3 component n-recognin 5 (UBR5), a key factor in the initiation of chronic pain, binds directly to Kv3.1 to drive its ubiquitin degradation. Intrathecal injection of the peptide TP-CH-401, a Kv3.1 ubiquitination motif sequence, rescued the decrease in Kv3.1 expression and Kv currents through competitive binding to UBR5, and consequently attenuated mechanical and thermal hypersensitivity. These findings demonstrate a previously unrecognized pathway of Kv3.1 abrogation by UBR5 and indicate that Kv3.1 is critically involved in the regulation of nociceptive behavior. Kv3.1 is thus a promising new target for treating inflammatory pain.

Epigenetic mechanisms of bone cancer pain

The management and treatment of bone cancer pain (BCP) remain significant clinical challenges, imposing substantial economic burdens on patients and society. Extensive research has demonstrated that BCP induces changes in the gene expression of peripheral sensory nerves and neurons, which play crucial roles in the onset and maintenance of BCP. However, our understanding of the epigenetic mechanisms of BCP underlying the transcriptional regulation of pro-nociceptive (such as inflammatory factors and the transient receptor potential family) and anti-nociceptive (such as potassium channels and opioid receptors) genes remains limited. This article reviews the epigenetic regulatory mechanisms in BCP, analyzing the roles of histone modifications, DNA methylation, and noncoding RNAs (ncRNAs) in the expression of pro-nociceptive and anti-nociceptive genes. Finally, we provide a comprehensive view of the functional mechanisms of epigenetic regulation in BCP and explore the potential of these epigenetic molecules as therapeutic targets for BCP.

TET1 Participates in Complete Freund's Adjuvant-induced Trigeminal Inflammatory Pain by Regulating Kv7.2 in a Mouse Model

Trigeminal inflammatory pain is one of the most severe pain-related disorders in humans; however, the underlying mechanisms remain largely unknown. In this study, we investigated the possible contribution of interaction between ten-eleven translocation methylcytosine dioxygenase 1 (TET1) and the voltage-gated K+ channel Kv7.2 (encoded by Kcnq2) to orofacial inflammatory pain in mice. We found that complete Freund's adjuvant (CFA) injection reduced the expression of Kcnq2/Kv7.2 in the trigeminal ganglion (TG) and induced orofacial inflammatory pain. The involvement of Kv7.2 in CFA-induced orofacial pain was further confirmed by Kv7.2 knockdown or overexpression. Moreover, TET1 knockdown in Tet1flox/flox mice significantly reduced the expression of Kv7.2 and M currents in the TG and led to pain-like behaviors. Conversely, TET1 overexpression by lentivirus rescued the CFA-induced decreases of Kcnq2 and M currents and alleviated mechanical allodynia. Our data suggest that TET1 is implicated in CFA-induced trigeminal inflammatory pain by positively regulating Kv7.2 in TG neurons.

RNA-binding proteins in pain

RNAs are meticulously controlled by proteins. Through direct and indirect associations, every facet in the brief life of an mRNA is subject to regulation. RNA-binding proteins (RBPs) permeate biology. Here, we focus on their roles in pain. Chronic pain is among the largest challenges facing medicine and requires new strategies. Mounting pharmacologic and genetic evidence obtained in pre-clinical models suggests fundamental roles for a broad array of RBPs. We describe their diverse roles that span RNA modification, splicing, stability, translation, and decay. Finally, we highlight opportunities to expand our understanding of regulatory interactions that contribute to pain signaling. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Regulation RNA in Disease and Development > RNA in Disease.

DExH-box helicase 9 modulates hippocampal synapses and regulates neuropathic pain

Experimental studies have shown that neuropathic pain impairs hippocampal synaptic plasticity. Here, we sought to determine the underlying mechanisms responsible for synaptic changes in neuropathic painful mouse hippocampal neurons. Beyond demonstrating proof-of-concept for the location of DExH-box helicase 9 (DHX9) in the nucleus, we found that it did exist in the cytoplasm and DHX9 depletion resulted in structural and functional changes at synapses in the hippocampus. A decrease of DHX9 was observed in the hippocampus after peripheral nerve injury; overexpression of DHX9 in the hippocampus significantly alleviated the nociceptive responses and improved anxiety behaviors. Mimicking DHX9 decrease evoked spontaneous pain behavioral symptoms and anxiety emotion in naïve mice. Mechanistically, we found that DHX9 bound to dendrin (Ddn) mRNA, which may have altered the level of synaptic- and dendritic-associated proteins. The data suggest that DHX9 contributes to synapses in hippocampal neurons and may modulate neuropathic pain and its comorbidity aversive emotion.

Specific inhibition of TET1 in the spinal dorsal horn alleviates inflammatory pain in mice by regulating synaptic plasticity

DNA demethylation mediated by ten-eleven translocation 1 (TET1) is a critical epigenetic mechanism in which gene expression is regulated via catalysis of 5-methylcytosine to 5-hydroxymethylcytosine. Previously, we demonstrated that TET1 is associated with the genesis of chronic inflammatory pain. However, how TET1 participates in enhanced nociceptive responses in chronic pain remains poorly understood. Here, we report that conditional knockout of Tet1 in dorsal horn neurons via intrathecal injection of rAAV-hSyn-Cre in Tet1fl/fl mice not only reversed the inflammation-induced upregulation of synapse-associated proteins (post-synaptic density protein 95 (PSD95) and synaptophysin (SYP)) in the dorsal horn but also ameliorated abnormalities in dendritic spine morphology and alleviated pain hypersensitivities. Pharmacological blockade of TET1 by intrathecal injection of a TET1-specific inhibitor-Bobcat 339-produced similar results, as did knockdown of Tet1 by intrathecal injection of siRNA. Thus, our data strongly suggest that increased TET1 expression during inflammatory pain upregulates the expression of multiple synapse-associated proteins and dysregulates synaptic morphology in dorsal horn neurons, suggesting that Tet1 may be a potential target for analgesic strategies.

FOXD3-mediated transactivation of ALKBH5 promotes neuropathic pain via m6A-dependent stabilization of 5-HT3A mRNA in sensory neurons

The N6-methyladenosine (m6A) modification of RNA is an emerging epigenetic regulatory mechanism that has been shown to participate in various pathophysiological processes. However, its involvement in modulating neuropathic pain is still poorly understood. In this study, we elucidate a functional role of the m6A demethylase alkylation repair homolog 5 (ALKBH5) in modulating trigeminal-mediated neuropathic pain. Peripheral nerve injury selectively upregulated the expression level of ALKBH5 in the injured trigeminal ganglion (TG) of rats. Blocking this upregulation in injured TGs alleviated trigeminal neuropathic pain, while mimicking the upregulation of ALKBH5 in intact TG neurons sufficiently induced pain-related behaviors. Mechanistically, histone deacetylase 11 downregulation induced by nerve injury increases histone H3 lysine 27 acetylation (H3K27ac), facilitating the binding of the transcription factor forkhead box protein D3 (FOXD3) to the Alkbh5 promoter and promoting Alkbh5 transcription. The increased ALKBH5 erases m6A sites in Htr3a messenger RNA (mRNA), resulting in an inability of YT521-B homology domain 2 (YTHDF2) to bind to Htr3a mRNA, thus causing an increase in 5-HT3A protein expression and 5-HT3 channel currents. Conversely, blocking the increased expression of ALKBH5 in the injured TG destabilizes nerve injury-induced 5-HT3A upregulation and reverses mechanical allodynia, and the effect can be blocked by 5-HT3A knockdown. Together, FOXD3-mediated transactivation of ALKBH5 promotes neuropathic pain through m6A-dependent stabilization of Htr3a mRNA in TG neurons. This mechanistic understanding may advance the discovery of new therapeutic targets for neuropathic pain management.

METTL14-mediated m6A epitranscriptomic modification contributes to chemotherapy-induced neuropathic pain by stabilizing GluN2A expression via IGF2BP2

Epigenetics is a biological process that modifies and regulates gene expression, affects neuronal function, and contributes to pain. However, the mechanism by which epigenetics facilitates and maintains chronic pain is poorly understood. We aimed to determine whether N6-methyladenosine (m6A) specifically modified by methyltransferase-like 14 (METTL14) alters neuronal activity and governs pain by sensitizing the GluN2A subunit of the N-methyl-d-aspartate receptor (NMDAR) in the dorsal root ganglion (DRG) neurons in a model of chemotherapy-induced neuropathic pain (CINP). Using dot blotting, immunofluorescence, gain/loss-of-function, and behavioral assays, we found that m6A levels were upregulated in L4-L6 DRG neurons in CINP in a DBP/METTL14-dependent manner, which was also confirmed in human DRGs. Blocking METTL14 reduced m6A methylation and attenuated pain hypersensitivity. Mechanistically, METTL14-mediated m6A modification facilitated the synaptic plasticity of DRG neurons by enhancing the GluN2A subunit of NMDAR, and inhibiting METTL14 blocked this effect. In contrast, overexpression of METTL14 upregulated m6A modifications, enhanced presynaptic NMDAR activity in DRG neurons, and facilitated pain sensation. Our findings reveal a previously unrecognized mechanism of METTL14-mediated m6A modification in DRG neurons to maintain neuropathic pain. Targeting these molecules may provide a new strategy for pain treatment.

TET1-mediated epigenetic regulation of tumor necrosis factor-α in trigeminal ganglia contributes to chronic temporomandibular joint pain

Chronic temporomandibular joint (TMJ) pain profoundly affects patients' quality of life. Trigeminal tumor necrosis factor-α (TNFα) plays a pivotal role in mediating TMJ pain in mice, yet the underlying epigenetic mechanisms remain enigmatic. To unravel these epigenetic intricacies, we employed a multifaceted approach. Hydroxymethylated DNA immunoprecipitation (hMeDIP) and chromatin immunoprecipitation (ChIP) followed by qPCR were employed to investigate the demethylation of TNFα gene (Tnfa) and its regulation by ten-eleven translocation methylcytosine dioxygenase 1 (TET1) in a chronic TMJ pain mouse model. The global levels of 5-hydroxymethylcytosine (5hmc) and percentage of 5hmc at the Tnfa promoter region were measured in the trigeminal ganglia (TG) and spinal trigeminal nucleus caudalis (Sp5C) following complete Freund's adjuvant (CFA) or saline treatment. TET1 knockdown and pain behavioral testing were conducted to ascertain the role of TET1-mediated epigenetic regulation of TNFα in the pathogenesis of chronic TMJ pain. Our finding revealed an increase in 5hmc at the Tnfa promoter region in both TG and Sp5C of CFA-treated mice. TET1 was upregulated in the mouse TG, and the ChIP result showed TET1 direct binding to the Tnfa promoter, with higher efficiency in the CFA-treated group. Immunofluorescence revealed the predominant expression of TET1 in trigeminal neurons. TET1 knockdown in the TG significantly reversed CFA-induced TNFα upregulation and alleviated chronic TMJ pain. In conclusion, our study implicates TET1 as a vital epigenetic regulator contributing to chronic inflammatory TMJ pain via trigeminal TNFα signaling. Targeting TET1 holds promise for epigenetic interventions in TMJ pain management.

DNA N6-methyladenine methylase N6AMT1 controls neuropathic pain through epigenetically modifying Kcnj16 in dorsal horn neurons

ABSTRACT

Nerve injury-induced aberrant changes in gene expression in spinal dorsal horn neurons are critical for the genesis of neuropathic pain. N6-methyladenine (m 6 A) modification of DNA represents an additional layer of gene regulation. Here, we report that peripheral nerve injury significantly decreased the level of m 6 A-specific DNA methyltransferase 1 ( N6amt1 ) in dorsal horn neurons. This decrease was attributed, at least partly, to a reduction in transcription factor Nr2f6 . Rescuing the decrease in N6amt1 reversed the loss of m 6 A at the promoter for inwardly rectifying potassium channel subfamily J member 16 ( Kcnj16 ), mitigating the nerve injury-induced upregulation of Kcnj16 expression in the dorsal horn and alleviating neuropathic pain hypersensitivities. Conversely, mimicking the downregulation of N6amt1 in naive mice erased DNA m 6 A at the Kcnj16 promoter, elevated Kcnj16 expression, and led to neuropathic pain-like behaviors. Therefore, decreased N6amt1 caused by NR2F6 is required for neuropathic pain, likely through its regulation of m 6 A-controlled KCNJ16 in dorsal horn neurons, suggesting that DNA m 6 A modification may be a potential new target for analgesic and treatment strategies.

Spinal-Specific Super Enhancer in Neuropathic Pain

Dysfunctional gene expression in nociceptive pathways plays a critical role in the development and maintenance of neuropathic pain. Super enhancers (SEs), composed of a large cluster of transcriptional enhancers, are emerging as new players in the regulation of gene expression. However, whether SEs participate in nociceptive responses remains unknown. Here, we report a spinal-specific SE (SS-SE) that regulates chronic constriction injury (CCI)-induced neuropathic pain by driving Ntmt1 and Prrx2 transcription in dorsal horn neurons. Peripheral nerve injury significantly enhanced the activity of SS-SE and increased the expression of NTMT1 and PRRX2 in the dorsal horn of male mice in a bromodomain-containing protein 4 (BRD4)-dependent manner. Both intrathecal administration of a pharmacological BRD4 inhibitor JQ1 and CRISPR-Cas9-mediated SE deletion abolished the increased NTMT1 and PRRX2 in CCI mice and attenuated their nociceptive hypersensitivities. Furthermore, knocking down Ntmt1 or Prrx2 with siRNA suppressed the injury-induced elevation of phosphorylated extracellular-signal-regulated kinase (p-ERK) and glial fibrillary acidic protein (GFAP) expression in the dorsal horn and alleviated neuropathic pain behaviors. Mimicking the increase in spinal Ntmt1 or Prrx2 in naive mice increased p-ERK and GFAP expression and led to the genesis of neuropathic pain-like behavior. These results redefine our understanding of the regulation of pain-related genes and demonstrate that BRD4-driven increases in SS-SE activity is responsible for the genesis of neuropathic pain through the governance of NTMT1 and PRRX2 expression in dorsal horn neurons. Our findings highlight the therapeutic potential of BRD4 inhibitors for the treatment of neuropathic pain.SIGNIFICANCE STATEMENT SEs drive gene expression by recruiting master transcription factors, cofactors, and RNA polymerase, but their role in the development of neuropathic pain remains unknown. Here, we report that the activity of an SS-SE, located upstream of the genes Ntmt1 and Prrx2, was elevated in the dorsal horn of mice with neuropathic pain. SS-SE contributes to the genesis of neuropathic pain by driving expression of Ntmt1 and Prrx2 Both inhibition of SS-SE with a pharmacological BRD4 inhibitor and genetic deletion of SS-SE attenuated pain hypersensitivities. This study suggests an effective and novel therapeutic strategy for neuropathic pain.

EGR1-Driven METTL3 Activation Curtails VIM-Mediated Neuron Injury in Epilepsy

Uncovering mechanisms underlying epileptogenesis aids in preventing further epilepsy progression and to lessen seizure severity and frequency. The purpose of this study is to explore the antiepileptogenic and neuroprotective mechanisms of EGR1 in neuron injuries encountered in epilepsy. Bioinformatics analysis was conducted to identify the key genes related to epilepsy. The mice were rendered epileptic using the kainic acid protocol, followed by measurement of seizure severity, high amplitude and frequency, pathological changes of hippocampal tissues and neuron apoptosis. Furthermore, an in vitro epilepsy model was constructed in the neurons isolated from newborn mice, which was then subjected to loss- and gain-of-function investigations, followed by neuron injury and apoptosis assessment. Interactions among EGR1, METTL3, and VIM were analyzed by a series of mechanistic experiments. In the mouse and cell models of epilepsy, VIM was robustly induced. However, its knockdown reduced hippocampal neuron injury and apoptosis. Meanwhile, VIM knockdown decreased inflammatory response and neuron apoptosis in vivo. Mechanistic investigations indicated that EGR1 transcriptionally activated METTL3, which in turn downregulated VIM expression through m6A modification. EGR1 activated METTL3 and reduced VIM expression, thereby impairing hippocampal neuron injury and apoptosis, preventing epilepsy progression. Taken together, this study demonstrates that EGR1 alleviates neuron injuries in epilepsy by inducing METTL3-mediated inhibition of VIM, which provides clues for the development of novel antiepileptic treatments.

The m6A reader YTHDF2 alleviates the inflammatory response by inhibiting IL-6R/JAK2/STAT1 pathway-mediated high-mobility group box-1 release

Background

Sepsis is a common severe complication in major burn victims and is characterized by a dysregulated systemic response to inflammation. YTH domain family 2 (YTHDF2), a well-studied N6-methyladenosine (m6A) reader that specifically recognizes and binds to m6A-modified transcripts to mediate their degradation, is connected to pathogenic and physiological processes in eukaryotes, but its effect on sepsis is still unknown. We aimed to discover the effects and mechanisms of YTHDF2 in sepsis.

Methods

Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and western blot analyses were used to measure the expression of YTHDF2, the interleukin 6 receptor (IL-6R), high-mobility group box-1 (HMGB1), Janus kinase 2 (JAK2) and signal transducer and activator of transcription 1 (STAT1) under different in vitro conditions. Enzyme-linked immunosorbent assays were utilized to evaluate the expression of HMGB1, IL-6, IL-1β and tumor necrosis factor-α. To confirm that YTHDF2 specifically targets IL-6R mRNA, RNA immunoprecipitation and dual-luciferase reporter assays were performed. Finally, we utilized a mouse model of lipopolysaccharide (LPS)-induced sepsis to verify the effects of YTHDF2 in vivo.

Results

According to our findings, YTHDF2 was expressed at a low level in peripheral blood mononuclear cells from septic mice and patients as well as in LPS-induced RAW264.7 cells. Overexpression of YTHDF2 alleviated the inflammatory response by inhibiting HMGB1 release and JAK2/STAT1 signalling in LPS-stimulated cells. Mechanistically, YTHDF2 suppressed JAK2/STAT1 signalling by directly recognizing the m6A-modified site in IL-6R and decreasing the stability of IL-6R mRNA, thereby inhibiting HMGB1 release. In vivo experiments showed that YTHDF2 played a protective role in septic mice by suppressing the IL-6R/JAK2/STAT1/HMGB1 axis.

Conclusions

In summary, these findings demonstrate that YTHDF2 plays an essential role as an inhibitor of inflammation to reduce the release of HMGB1 by inhibiting the IL-6R/JAK2/STAT1 pathway, indicating that YTHDF2 is a novel target for therapeutic interventions in sepsis.

The role of N6-methyladenosine modification in rodent models of neuropathic pain: from the mechanism to therapeutic potential

Neuropathic pain (NP) is a common chronic pain condition resulted from lesions or diseases of somatosensory nervous system, but the pathogenesis remains unclear. A growing body of evidence supports the relationship between pathogenesis and N6-methyladenosine (m6A) modifications of RNA. However, studies on the role of m6A modifications in NP are still at an early stage. Elucidating different etiologies is important for understanding the specific pathogenesis of NP. This article provides a comprehensive review on the role of m6A methylation modifications including methyltransferases ("writers"), demethylases ("erasers"), and m6A binding proteins ("readers") in NP models. Further analysis of the pathogenic mechanism relationship between m6A and NP provided novel theoretical and practical significance for clinical treatment of NP.

Downregulation of ciRNA-Kat6b in dorsal spinal horn is required for neuropathic pain by regulating Kcnk1 in miRNA-26a-dependent manner

AIMS

Nerve injury-induced maladaptive changes in gene expression in the spinal neurons are essential for neuropathic pain genesis. Circular RNAs (ciRNA) are emerging as key regulators of gene expression. Here, we identified a nervous-system-tissues-specific ciRNA-Kat6 with conservation in humans and mice. We aimed to investigate whether and how spinal dorsal horn ciRNA-Kat6b participates in neuropathic pain.

METHODS

Unilateral sciatic nerve chronic constrictive injury (CCI) surgery was used to prepare the neuropathic pain model. The differentially expressed ciRNAs were obtained by RNA-Sequencing. The identification of nervous-system-tissues specificity of ciRNA-Kat6b and the measurement of ciRNA-Kat6b and microRNA-26a (miRNA-26a) expression level were carried out by quantitative RT-PCR. The ciRNA-Kat6b that targets miRNA-26a and miRNA-26a that targets Kcnk1 were predicted by bioinformatics analysis and verified by in vitro luciferase reports test and in vivo experiments including Western-blot, immunofluorescence, and RNA-RNA immunoprecipitation. The correlation between neuropathic pain and ciRNA-Kat6b, miRNA-26a, or Kcnk1 was examined by the hypersensitivity response to heat and mechanical stimulus.

RESULTS

Peripheral nerve injury downregulated ciRNA-Kat6b in the dorsal spinal horn of male mice. Rescuing this downregulation blocked nerve injury-induced increase of miRNA-26a, reversed the miRNA-26a-triggered decrease of potassium channel Kcnk1, a key neuropathic pain player, in the dorsal horn, and alleviates CCI-induced pain hypersensitivities. On the contrary, mimicking this downregulation increased the miRNA-26a level and decreased Kcnk1 in the spinal cord, resulting in neuropathic pain-like syndrome in naïve mice. Mechanistically, the downregulation of ciRNA-Kat6b reduced the accounts of miRNA-26a binding to ciRNA-Kat6b, and elevated the binding accounts of miRNA-26a to the 3' untranslated region of Kcnk1 mRNA and degeneration of Kcnk1 mRNA, triggering in the reduction of KCNK1 protein in the dorsal horn of neuropathic pain mice.

CONCLUSION

The ciRNA-Kat6b/miRNA-26a/Kcnk1 pathway in dorsal horn neurons regulates the development and maintenance of neuropathic pain, ciRNA-Kat6b may be a potential new target for analgesic and treatment strategies.

DHX9/DNA-tandem repeat-dependent downregulation of ciRNA-Fmn1 in the dorsal horn is required for neuropathic pain

Circular RNAs (ciRNAs) are emerging as new players in the regulation of gene expression. However, how ciRNAs are involved in neuropathic pain is poorly understood. Here, we identify the nervous-tissue-specific ciRNA-Fmn1 and report that changes in ciRNA-Fmn1 expression in spinal cord dorsal horn neurons play a key role in neuropathic pain after nerve injury. ciRNA-Fmn1 was significantly downregulated in ipsilateral dorsal horn neurons after peripheral nerve injury, at least in part because of a decrease in DNA helicase 9 (DHX9), which regulates production of ciRNA-Fmn1 by binding to DNA-tandem repeats. Blocking ciRNA-Fmn1 downregulation reversed nerve-injury-induced reductions in both the binding of ciRNA-Fmn1 to the ubiquitin ligase UBR5 and the level of ubiquitination of albumin (ALB), thereby abrogating the nerve-injury-induced increase of ALB expression in the dorsal horn and attenuating the associated pain hypersensitivities. Conversely, mimicking downregulation of ciRNA-Fmn1 in naïve mice reduced the UBR5-controlled ubiquitination of ALB, leading to increased expression of ALB in the dorsal horn and induction of neuropathic-pain-like behaviors in naïve mice. Thus, ciRNA-Fmn1 downregulation caused by changes in binding of DHX9 to DNA-tandem repeats contributes to the genesis of neuropathic pain by negatively modulating UBR5-controlled ALB expression in the dorsal horn.

Spinal CBX2 contributes to neuropathic pain by activating ERK signaling pathway in male mice

The deregulated spinal cord proteins induced by nerve injury are the key to neuropathic pain. Integrated transcriptome and translatome analyses can screen out deregulated proteins controlled by only post-transcriptional regulation. By comparing RNA sequencing (RNA-seq) and ribosome profiling sequencing (Ribo-seq) data, we identified an upregulated protein, chromobox 2 (CBX2), with its mRNA level unchanged in the spinal cord after peripheral nerve injury. CBX2 was mainly distributed in the spinal cord neurons. Blocking the SNL-induced increase of spinal CBX2 attenuated the neuronal and astrocytes hyperactivities and pain hypersensitivities in both the development and maintenance phases. Conversely, mimicking the upregulation of CBX2 in the spinal cord facilitated the activities of neurons and astrocytes and produced evoked nociceptive hypersensitivity and spontaneous pain. Our results also revealed that activating the ERK pathway, upregulating CXCL13 in neurons, and CXCL13 further inducing astrocyte activation were possible downstream signaling mechanisms of CBX2 in pain processing. In conclusion, upregulation of CBX2 after nerve injury leads to nociceptive hyperalgesia by promoting neuronal and astrocyte hyperactivities through the ERK pathway. Inhibiting CBX2 upregulation may be therapeutically beneficial.

Wnt3a/YTHDF1 Regulated Oxaliplatin-Induced Neuropathic Pain Via TNF-α/IL-18 Expression in the Spinal Cord

Oxaliplatin is widely used in cancer treatment, however, many patients will suffer from neuropathic pain (NP) induced by it at the same time. Therefore exploring the mechanism and founding novel target for this problem are needed. In this study, YTHDF1 showed upregulation in oxaliplatin treated mice. As m6A is known as conserved and it widely functions in numerous physiological and pathological processes. Therefore, we focused on exploring the molecular mechanism of whether and how YTHDF1 functions in NP induced by oxaliplatin. IHC and western blotting were conducted to measure proteins. Intrathecal injection for corresponding siRNAs in C57/BL6 mice or spinal microinjection for virus in YTHDF1flox/flox mice were applied to specially knockdown the expression of molecular. Von Frey, acetone test and ethyl chloride (EC) test were applied to evaluate NP behavior. YTHDF1, Wnt3a, TNF-α and IL-18 were increased in oxaliplatin treated mice, restricted the molecular mentioned above respectively can significantly attenuate oxaliplatin-induced NP, including the mechanical allodynia and cold allodynia. Silencing YTHDF1 and inhibiting Wnt3a and Wnt signaling pathways can reduce the enhancement of TNF-α and IL-18, and the decreasing of the upregulation of YTHDF1 can be found when inhibiting Wnt3a and Wnts signaling pathways in oxaliplatin treated mice. Our study indicated a novel pathway that can contribute to oxaliplatin-induced NP, the Wnt3a/YTHDF1 to cytokine pathway, which upregulating YTHDF1 functioned as the downstream of Wnt3a signal and promoted the translation of TNF-α and IL-18 in oxaliplatin treated mice.

The Cytidine N-Acetyltransferase NAT10 Participates in Peripheral Nerve Injury-Induced Neuropathic Pain by Stabilizing SYT9 Expression in Primary Sensory Neurons

RNA N4-acetylcytidine (ac4C) modification is increasingly recognized as an important layer of gene regulation; however, the involvement of ac4C in pain regulation has not been studied. Here, we report that N-acetyltransferase 10 protein (NAT10; the only known ac4C "writer") contributes to the induction and development of neuropathic pain in an ac4C-dependent manner. Peripheral nerve injury increases the levels of NAT10 expression and overall ac4C in injured dorsal root ganglia (DRGs). This upregulation is triggered by the activation of upstream transcription factor 1 (USF1), a transcription factor that binds to the Nat10 promoter. Knock-down or genetic deletion of NAT10 in the DRG abolishes the gain of ac4C sites in Syt9 mRNA and the augmentation of SYT9 protein, resulting in a marked antinociceptive effect in nerve-injured male mice. Conversely, mimicking NAT10 upregulation in the absence of injury evokes the elevation of Syt9 ac4C and SYT9 protein and induces the genesis of neuropathic-pain-like behaviors. These findings demonstrate that USF1-governed NAT10 regulates neuropathic pain by targeting Syt9 ac4C in peripheral nociceptive sensory neurons. Our findings establish NAT10 as a critical endogenous initiator of nociceptive behavior and a promising new target for treating neuropathic pain.SIGNIFICANCE STATEMENT The cytidine N4-acetylcytidine (ac4C), a new epigenetic RNA modification, is crucial for the translation and stability of mRNA, but its role for chronic pain remains unclear. Here, we demonstrate that N-acetyltransferase 10 (NAT10) acts as ac4C N-acetyltransferase and plays an important role in the development and maintenance of neuropathic pain. NAT10 was upregulated via the activation of the transcription factor upstream transcription factor 1 (USF1) in the injured dorsal root ganglion (DRG) after peripheral nerve injury. Since pharmacological or genetic deleting NAT10 in the DRG attenuated the nerve injury-induced nociceptive hypersensitivities partially through suppressing Syt9 mRNA ac4C and stabilizing SYT9 protein level, NAT10 may serve as an effective and novel therapeutic target for neuropathic pain.

TET1-TRPV4 Signaling Contributes to Bone Cancer Pain in Rats

Bone cancer pain (BCP) is excruciating for cancer patients, with limited clinical treatment options and significant side effects, due to the complex and unclear pathogenesis of bone cancer pain. Peripheral sensitization in dorsal root ganglion (DRG) neurons is a recognized cellular mechanism for bone cancer pain. The pathological mechanism of chronic pain is increasingly being affected by epigenetic mechanisms. In this study, we unbiasedly showed that the DNA hydroxymethylase ten-eleven translocation 1 (TET1) expression was significantly increased in the L4-6 DRG of BCP rats and ten-eleven translocation 2 (TET2) expression did not change significantly. Notably, TET1 inhibition by intrathecal injection of Bobcat339 (a TET1 inhibitor) effectively relieved mechanical hyperalgesia in BCP rats. Peripheral sensitization in chronic pain relies on the activation and overexpression of ion channels on neurons. Here, we demonstrated that TRPV4, one of the transient receptor potential ion channel family members, was significantly elevated in the L4-6 DRG of BCP rats. In addition, TRPV4 inhibition by intrathecal injection of HC067047 (a TRPV4 inhibitor) also significantly attenuated mechanical hyperalgesia in BCP rats. Interestingly, we found that TET1 inhibition downregulated TRPV4 expression in the L4-6 DRG of BCP rats. As a result, these findings suggested that TET1 may contribute to bone cancer pain by upregulating TRPV4 expression in the L4-6 DRG of BCP rats and that TET1 or TRPV4 may become therapeutic targets for bone cancer pain.

CircRalgapa1 facilitates morphine tolerance via miR-873a-5p/A20 axis in mice

Morphine tolerance (MT) caused by long-term use of morphine is a major medical problem. The underlying molecular mechanisms of morphine tolerance remain unclear. Here, we establish the morphine tolerance model in mice and verify whether a novel circRNA, circRalgapa1 is involved in morphine tolerance and its specific molecular mechanism. We show that the expression of circRalgapa1 in the spinal cord is significantly down-expressed in the spinal cord of morphine-tolerant mice. CircRalgapa1 is mainly located in the neuronal cytoplasm and co-localizes with miR-873a-5p. Mechanically, circRalgapa1 acts as competing endogenous RNAs (ceRNAs) to regulate the inhibitory of miR-873a-5p on A20 (also known as tumor necrosis factor α-induced protein 3, TNFAIP3). Functionally, overexpression of circRalgapa1 by intrathecal injection of adeno-associated virus (AAV- circRalgapa1) attenuated the formation of morphine tolerance and partially reversed the development of morphine tolerance. Moreover, overexpression of miR-873a-5p blocked the effect of AAV-circRalgapa1 on alleviating morphine tolerance in mice. In conclusion, chronic morphine administration-mediated down-regulation of circRalgapa1 in the spinal cord contributes to morphine tolerance via miR-873a-5p/A20 axis in mice. Overexpression of circRalgapa1 may be a promising RNA-based therapy for morphine tolerance.

Methyltransferase-like 3 modulates visceral hypersensitivity through regulating the nuclear export of circKcnk9 in YTHDC1-dependent manner

Background: Accumulating evidence shows that N6-methyladenosine (m⁶A) modulators contribute to the process of chronic pain. However, the exact mechanisms of m⁶A writers involved in visceral hypersensitivity of Irritable bowel syndrome (IBS) remain unclear. This article aimed to reveal a new mechanism for the progression of IBS. Methods: The IBS-like model was established by neonatal colorectal distention (CRD). The relationship between m⁶A and circKcnk9 was analyzed by bioinformatics, immunofluorescence and RNA fluorescence in situ hybridization (FISH) assays. Visceral hypersensitivity was assessed based on the electromyography (EMG) response of the abdominal external oblique muscle to CRD. In vivo and in vitro studies (including EMG stereotactic infusion, Western blot and qRT-PCR) were utilized to explore the biological functions of related indicators. The bioinformatics, RIP experiments and RNA pull-down assays were used to explore the potential molecular mechanisms. Results: We identified that neonatal CRD increased the level of the m⁶A via methyltransferase-like 3 (METTL3) in the hippocampal neurons. Subsequently, knockdown of METTL3 could alleviate visceral hypersensitivity in IBS-like rats. By contrast, overexpression of METTL3 could induce visceral hypersensitivity and activate hippocampal neurons in control rats. Moreover, YTHDC1, the only m⁶A-associated protein predicted by bioinformatics to bind to circKcnk9, modulated visceral hypersensitivity through regulating the nuclear export of circKcnk9 in an m⁶A-dependent manner. Notably, FISH data suggested that the increased nuclear staining of circKcnk9 caused by siYTHDC1 could be recovered by overexpression of YTHDC1 wild type (WT) but not YTHDC1 negative control (NC) in PC12 cells. Conclusions: Our findings reveal a new regulatory mechanism in progress of IBS, that is, METTL3 modulates visceral hypersensitivity through regulating the nuclear export of circKcnk9 in YTHDC1-dependent manner.

Increased ten-eleven translocation methylcytosine dioxygenase one in dorsal root ganglion contributes to inflammatory pain in CFA rats

DNA hydroxylation catalyzed by Tet dioxygenases occurs abundantly in neurons in mammals. However, effects of ten-eleven translocation methylcytosine dioxygenase 1 (TET1) expression and hydroxymethylation status on neuron injury remain unclear. This study was designed to explore the effects of TET1 and TET2 expression in the inflammatory pain of rats induced by complete Freund's adjuvant (CFA). Mechanical paw withdrawal threshold (PWT) and thermal withdrawal latency (TWL) were detected to assess pain behavior. The expression of TET1 and TET2 were measured in the dorsal root ganglion (DRG) with western blotting analysis. Immunofluorescence staining is employed to detect the expression and co-location of TRPV1 with TET1. Intrathecal administration of Bobcat339 was used to inhibit TET1 function in dorsal root ganglion. The paw withdrawal threshold and thermal withdrawal latency of rats were significantly reduced after CFA Injection. Western blot results showed that the expression of TET1 was significantly increased at 3 days after CFA injection, but TET2 had no statistical difference. Immunofluorescence results showed that TET1 was co-localized with TRPV1. Intrathecal administration of Bobcat339 improved mechanical and thermal pain threshold in CFA rats. Our findings highlight the role of TET1 in chronic inflammatory pain model. The expression of TET1 was increased in CFA rats, and suppression of TET1 will ameliorate inflammatory pain.

Non-coding RNA and n6-methyladenosine modification play crucial roles in neuropathic pain

After peripheral nerve injury, pain signals are transmitted from primary sensory neurons in the dorsal root ganglion (DRG) to the central nervous system. Epigenetic modification affects neuropathic pain through alterations in the gene expression in pain-related areas and glial cell activation. Recent studies have shown that non-coding RNA and n6-methyladenosine (m6A) methylation modification play pivotal regulatory roles in the occurrence and maintenance of neuropathic pain. Dysregulation of the RNA m6A level via dynamic changes in methyltransferase and demethylase after central or peripheral nerve injury commonly regulates pain-associated genes, contributing to the induction and maintenance of neuropathic pain. The dynamic process has significant implications for the development and maintenance of neuropathic pain. However, the underlying mechanisms by which non-coding RNA and m6A RNA modification regulate neuropathic pain are not well-characterized. This article elucidates the multiple mechanisms of non-coding RNA and m6A methylation in the context of neuropathic pain, and summarizes its potential functions as well as recent advances.

Comprehensive analysis of m6A methylation modification in chronic spinal cord injury in mice

Chronic spinal cord injury (CSCI) is a catastrophic disease of the central nervous system (CNS), resulting in partial or complete loss of neurological function. N6-methyladenosine (m6A) is the most common form of reversible posttranslational modification at the RNA level. However, the role of m6A modification in CSCI remains unknown. In this study, we established a CSCI model using a water-absorbable polyurethane polymer, with behavioral assessment, electrophysiological analysis, and histochemical staining for validation. Methylated RNA immunoprecipitation sequencing (meRIP-seq) and messenger RNA sequencing (mRNA-seq) were jointly explored to compare the differences between CSCI spinal tissue and normal spinal tissue. Furthermore, real-time quantitative reverse transcription pcr (qRT-PCR), western blot analysis, and immunofluorescence staining were used to analyze m6A modification-related proteins. We found that water-absorbable polyurethane polymer simulated well chronic spinal cord compression. Basso mouse scale scores and electrophysiological analysis showed continuous neurological function decline after chronic compression of the spinal cord. meRIP-seq identified 642 differentially modified m6A genes, among which 263 genes were downregulated and 379 genes were upregulated. mRNA-seq showed that 1544 genes were upregulated and 290 genes were downregulated after CSCI. Gene Ontology terms and enriched Kyoto Encyclopedia of Genes and Genomes pathways were also identified. qRT-PCR, western blotting, and immunofluorescence staining showed that Mettl14, Ythdf1, and Ythdf3 were significantly upregulated after CSCI. Our study revealed a comprehensive profile of m6A modifications in CSCI which may act as a valuable key for future research on CSCI.

Identification and characterization of N6-methyladenosine circular RNAs in the spinal cord of morphine-tolerant rats

Morphine tolerance (MT) is a tricky problem, the mechanism of it is currently unknown. Circular RNAs (circRNAs) serve significant functions in the biological processes (BPs) of the central nervous system. N6-methyladenosine (m⁶A), as a key post-transcriptional modification of RNA, can regulate the metabolism and functions of circRNAs. Here we explore the patterns of m⁶A-methylation of circRNAs in the spinal cord of morphine-tolerant rats. In brief, we constructed a morphine-tolerant rat model, performed m⁶A epitranscriptomic microarray using RNA samples collected from the spinal cords of morphine-tolerant rats and normal saline rats, and implemented the bioinformatics analysis. In the spinal cord of morphine-tolerant rats, 120 circRNAs with different m⁶A modifications were identified, 54 of which were hypermethylated and 66 of which were hypomethylated. Functional analysis of these m⁶A circRNAs found some important pathways involved in the pathogenesis of MT, such as the calcium signaling pathway. In the m⁶A circRNA-miRNA networks, several critical miRNAs that participated in the occurrence and development of MT were discovered to bind to these m⁶A circRNAs, such as miR-873a-5p, miR-103-1-5p, miR-107-5p. M⁶A modification of circRNAs may be involved in the pathogenesis of MT. These findings may lead to new insights into the epigenetic etiology and pathology of MT.

The Involvement of Caspases in Neuroinflammation and Neuronal Apoptosis in Chronic Pain and Potential Therapeutic Targets

Chronic pain is a common, complex and unpleasant sensation following nerve injury, tissue trauma, inflammatory diseases, infection and cancer. It affects up to 25% of adults and is increasingly recognized as the leading cause of distress, disability and disease burden globally. Chronic pain is often refractory to most current analgesics, thus emphasizing the requirement for improved therapeutic medications. It is of great importance to elucidate the specific pathogenesis of chronic pain with different etiologies. Recent progress has advanced our understanding in the contribution of neuroinflammation and glial cells (microglia and astrocyte) activation in the plasticity of excitatory nociceptive synapses and the development of chronic pain phenotypes. Oxidative stress-associated neuronal apoptosis is also identified to be a pivotal step for central pain sensitization. The family of cysteine aspartate specific proteases (Caspases) has been well known to be key signaling molecules for inflammation and apoptosis in several neurological conditions. Recent studies have highlighted the unconventional and emerging role of caspases in microgliosis, astrocytes morphogenesis, chemokines release, cytokines secretion and neuronal apoptosis in initiating and maintaining synaptogenesis, synaptic strength and signal transduction in persistent pain hypersensitivity, suggesting the possibility of targeting caspases pathway for prevention and treatment of chronic pain. In this review, we will discuss and summarize the advances in the distinctive properties of caspases family in the pathophysiology of chronic pain, especially in neuropathic pain, inflammatory pain, cancer pain and musculoskeletal pain, with the aim to find the promising therapeutic candidates for the resolution of chronic pain to better manage patients undergoing chronic pain in clinics.

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.

miRNA-22 Upregulates Mtf1 in Dorsal Horn Neurons and Is Essential for Inflammatory Pain

Chronic inflammatory pain seriously affects patients' quality of life because of a paucity of effective clinical treatments caused, at least in part, by lack of full understanding of the underlying mechanisms. miRNAs are known to be involved in inflammatory pain via silencing or degrading of target mRNA in the cytoplasm. The present study provides a novel mechanism by which miRNA-22 positively regulates metal-regulatory transcription factor 1 (Mtf1) in the nuclei of neurons in the dorsal horn of the spinal cord. We found that miRNA-22 was significantly increased in the dorsal horn of mice with either inflammatory pain induced by plantar injection of complete Freund's adjuvant (CFA) or neuropathic pain induced by unilateral sciatic nerve chronic constrictive injury (CCI). Knocking down or blocking miRNA-22 alleviated CFA-induced mechanical allodynia and heat hyperalgesia, whereas overexpressing miRNA-22 produced pain-like behaviors. Mechanistically, the increased miRNA-22 binds directly to the Mtf1 promoter to recruit RNA polymerase II and elevate Mtf1 expression. The increased Mtf1 subsequently enhances spinal central sensitization, as evidenced by increased expression of p-ERK1/2, GFAP, and c-Fos in the dorsal horn. Our findings suggest that the miRNA-22-Mtf1 signaling axis in the dorsal horn plays a critical role in the induction and maintenance of inflammatory pain. This signaling pathway may be a promising therapeutic target in inflammatory pain.

N(6)-adenosine-methyltransferase-14 promotes glioma tumorigenesis by repressing argininosuccinate synthase 1 expression in an m6A-dependent manner

Glioma is one of the leading causes of tumor-related deaths worldwide, but its potential mechanism remains unclear. This study aimed to explore the biological role and potential mechanism of argininosuccinate synthase 1 (ASS1) in glioma. The relative expression levels of ASS1 in glioma specimens and cell lines were calculated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and Western blotting. The biological functions of ASS1 were demonstrated using the 5-ethynyl-2'-deoxyuridine (EdU) assay, transwell assay, and in vivo experiments. In addition, methylated RNA immunoprecipitation (MeRIP), RNA immunoprecipitation (RIP), and luciferase reporter assays were performed to explore the molecular mechanism of ASS1 in glioma. ASS1 expression levels were found to be downregulated in glioma specimens and cell lines. Functionally, we confirmed that ASS1 inhibited glioma cell proliferation, migration, invasion, and growth both. Furthermore, we found that ASS1 was a target of N(6)-adenosine-methyltransferase-14 (METTL14)-mediated N6-methyladenosine (m6A) modification. Overexpression of METTL14 markedly elevated ASS1 mRNA m6A modification and suppressed ASS1 mRNA expression. We also revealed that METTL14-mediated ASS1 mRNA degradation relied on the YTH m6A RNA-binding protein 2 (YTHDF2)-dependent pathway. We confirmed that decreased ASS1 expression promoted the cell proliferation, migration, and invasion in glioma, and that the METTL14/ASS1/YTHDF2 regulatory axis may be an effective therapeutic target for glioma.

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.