Promoted Interaction of C/EBPα with Demethylated Cxcr3 Gene Promoter Contributes to Neuropathic Pain in Mice

CXCL10 Enhances Acid-Sensing Ion Channel Currents in Rat Dorsal Root

Both CXCL10/CXCR3 and acid-sensing ion channels (ASICs) are expressed in nociceptive sensory neurons and participate in various pain processes, but it is still unclear whether there is a link between them. Herein, we report that CXCL10 enhances the electrophysiological activity of ASICs in rat dorsal root ganglia (DRG) neurons. A brief (10 min) application of CXCL10 increased acid-evoked ASIC currents in a concentration-dependent manner. CXCL10 increased the maximum response of ASICs to acidic stimuli without changing their sensitivity. CXCL10 enhanced ASIC currents in DRG cells through CXCR3, as this enhancement was completely blocked by AMG487, a selective CXCR3 antagonist. CXCL10 also increased ASIC3 currents in CHO cells coexpressing ASIC3 and CXCR3 but not in cells expressing ASIC3 alone. The CXCL10-mediated increase in ASIC currents was prevented by the application of either the G protein inhibitor GDP-β-S or the p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190 but not by the ERK inhibitor U0126 or the JNK inhibitor SP600125. Moreover, CXCL10 increased the number of action potentials triggered by acidic stimuli via CXCR3. CXCL10 dose-dependently exacerbated acid-induced nociceptive behavior in rats through peripheral CXCR3. These results indicated that CXCL10/CXCR3 signaling enhanced ASIC-mediated electrophysiological activity in DRG neurons and nociception in rats via a p38 MAPK-dependent pathway, revealing a novel mechanism underlying pain. CXCL10/CXCR3 signaling may be an effective target in the treatment of pain associated with tissue acidification.

Identification and validation of aging-related genes in neuropathic pain using bioinformatics

Background

Neuropathic pain (NP) is a debilitating and refractory chronic pain with a higher prevalence especially in elderly patients. Cell senescence considered a key pathogenic factor in NP. The objective of this research is to discover genes associated with aging in peripheral blood of individuals with NP using bioinformatics techniques.

Methods

Two cohorts (GSE124272 and GSE150408) containing peripheral blood samples of NP were downloaded from the GEO database. By merging the two cohorts, differentially expressed aging-related genes (DE-ARGs) were obtained by intersection with aging-related genes. The potential biological mechanisms of DE-ARGs were further analyzed through GO and KEGG. Three machine learning methods, namely, LASSO, SVM-RFE, and Random Forest, were utilized to identify diagnostic biomarkers. A Nomogram model was developed to assess their diagnostic accuracy. The validation of biomarker expression and diagnostic effectiveness was conducted in three distinct pain cohorts. The CIBERSORT algorithm was employed to evaluate the immune cell composition in the peripheral blood of patients with NP and investigate its association with the expression of diagnostic biomarkers.

Results

This study identified a total of 24 DE-ARGs, mainly enriched in "Chemokine signaling pathway," "Inflammatory mediator regulation of TRP channels," "HIF-1 signaling pathway" and "FOXO signaling pathway". Three machine learning algorithms identified a total of four diagnostic biomarkers (CEBPA, CEACAM1, BTG3 and IL-1R1) with good diagnostic performance and the similar expression difference trend in different types of pain cohorts. The expression levels of CEACAM1 and IL-1R1 exhibit a positive correlation with the percentage of neutrophils.

Conclusion

Using machine learning techniques, our research identified four diagnostic biomarkers related to aging in peripheral blood, providing innovative approaches for the diagnosis and treatment of NP.

Resveratrol suppresses liver cancer progression by downregulating AKR1C3: targeting HCC with HSA nanomaterial as a carrier to enhance therapeutic efficacy

The enzyme AKR1C3 plays a crucial role in hormone and drug metabolism and is associated with abnormal expression in liver cancer, leading to tumor progression and poor prognosis. Nanoparticles modified with HSA can modulate the tumor microenvironment by enhancing photodynamic therapy to induce apoptosis in tumor cells and alleviate hypoxia. Therefore, exploring the potential regulatory mechanisms of resveratrol on AKR1C3 through the construction of HSA-RSV NPs carriers holds significant theoretical and clinical implications for the treatment of liver cancer. The aim of this study is to investigate the targeted regulation of AKR1C3 expression through the loading of resveratrol (RSV) on nanomaterials HSA-RSV NPs (Nanoparticles) in order to alleviate tumor hypoxia and inhibit the progression of hepatocellular carcinoma (HCC), and to explore its molecular mechanism. PubChem database and PharmMapper server were used to screen the target genes of RSV. HCC-related differentially expressed genes (DEGs) were analyzed through the GEO dataset, and relevant genes were retrieved from the GeneCards database, resulting in the intersection of the three to obtain candidate DEGs. GO and KEGG enrichment analyses were performed on the candidate DEGs to analyze the potential cellular functions and molecular signaling pathways affected by the main target genes. The cytohubba plugin was used to screen the top 10 target genes ranked by Degree and further intersected the results of LASSO and Random Forest (RF) to obtain hub genes. The expression analysis of hub genes and the prediction of malignant tumor prognosis were conducted. Furthermore, a pharmacophore model was constructed using PharmMapper. Molecular docking simulations were performed using AutoDockTools 1.5.6 software, and ROC curve analysis was performed to determine the core target. In vitro cell experiments were carried out by selecting appropriate HCC cell lines, treating HCC cells with different concentrations of RSV, or silencing or overexpressing AKR1C3 using lentivirus. CCK-8, clone formation, flow cytometry, scratch experiment, and Transwell were used to measure cancer cell viability, proliferation, migration, invasion, and apoptosis, respectively. Cellular oxygen consumption rate was analyzed using the Seahorse XF24 analyzer. HSA-RSV NPs were prepared, and their characterization and cytotoxicity were evaluated. The biological functional changes of HCC cells after treatment were detected. An HCC subcutaneous xenograft model was established in mice using HepG2 cell lines. HSA-RSV NPs were injected via the tail vein, with a control group set, to observe changes in tumor growth, tumor targeting of NPs, and biological safety. TUNEL, Ki67, and APC-hypoxia probe staining were performed on excised tumor tissue to detect tumor cell proliferation, apoptosis, and hypoxia. Lentivirus was used to silence or overexpress AKR1C3 simultaneously with the injection of HSA-RSV NPs via the tail vein to assess the impact of AKR1C3 on the regulation of HSA-RSV NPs in HCC progression. Bioinformatics analysis revealed that AKR1C3 is an important target gene involved in the regulation of HCC by RSV, which is associated with the prognosis of HCC patients and upregulated in expression. In vitro cell experiments showed that RSV significantly inhibits the respiratory metabolism of HCC cells, suppressing their proliferation, migration, and invasion and promoting apoptosis. Silencing AKR1C3 further enhances the toxicity of RSV towards HCC cells. The characterization and cytotoxicity experiments of nanomaterials demonstrated the successful construction of HSA-RSV NPs, which exhibited stronger inhibitory effects on HCC cells. In vivo, animal experiments further confirmed that targeted downregulation of AKR1C3 by HSA-RSV NPs suppresses the progression of HCC and tumor hypoxia while exhibiting tumor targeting and biological safety. Targeted downregulation of AKR1C3 by HSA-RSV NPs can alleviate HCC tumor hypoxia and inhibit the progression of HCC.

NOX4-mediated astrocyte ferroptosis in Alzheimer's disease

This study investigates NADPH oxidase 4 (NOX4) involvement in iron-mediated astrocyte cell death in Alzheimer's Disease (AD) using single-cell sequencing data and transcriptomes. We analyzed AD single-cell RNA sequencing data, identified astrocyte marker genes, and explored biological processes in astrocytes. We integrated AD-related chip data with ferroptosis-related genes, highlighting NOX4. We validated NOX4's role in ferroptosis and AD in vitro and in vivo. Astrocyte marker genes were enriched in AD, emphasizing their role. NOX4 emerged as a crucial player in astrocytic ferroptosis in AD. Silencing NOX4 mitigated ferroptosis, improved cognition, reduced Aβ and p-Tau levels, and alleviated mitochondrial abnormalities. NOX4 promotes astrocytic ferroptosis, underscoring its significance in AD progression.

Deciphering the SOX4/MAPK1 regulatory axis: a phosphoproteomic insight into IQGAP1 phosphorylation and pancreatic Cancer progression

OBJECTIVE

This study aims to elucidate the functional role of IQGAP1 phosphorylation modification mediated by the SOX4/MAPK1 regulatory axis in developing pancreatic cancer through phosphoproteomics analysis.

METHODS

Proteomics and phosphoproteomics data of pancreatic cancer were obtained from the Clinical Proteomic Tumor Analysis Consortium (CPTAC) database. Differential analysis, kinase-substrate enrichment analysis (KSEA), and independent prognosis analysis were performed on these datasets. Subtype analysis of pancreatic cancer patients was conducted based on the expression of prognostic-related proteins, and the prognosis of different subtypes was evaluated through prognosis analysis. Differential analysis of proteins in different subtypes was performed to identify differential proteins in the high-risk subtype. Clinical correlation analysis was conducted based on the expression of prognostic-related proteins, pancreatic cancer typing results, and clinical characteristics in the pancreatic cancer proteomics dataset. Functional pathway enrichment analysis was performed using GSEA/GO/KEGG, and most module proteins correlated with pancreatic cancer were selected using WGCNA analysis. In cell experiments, pancreatic cancer cells were grouped, and the expression levels of SOX4, MAPK1, and the phosphorylation level of IQGAP1 were detected by RT-qPCR and Western blot experiments. The effect of SOX4 on MAPK1 promoter transcriptional activity was assessed using a dual-luciferase assay, and the enrichment of SOX4 on the MAPK1 promoter was examined using a ChIP assay. The proliferation, migration, and invasion functions of grouped pancreatic cancer cells were assessed using CCK-8, colony formation, and Transwell assays. In animal experiments, the impact of SOX4 on tumor growth and metastasis through the regulation of MAPK1-IQGAP1 phosphorylation modification was studied by constructing subcutaneous and orthotopic pancreatic cancer xenograft models, as well as a liver metastasis model in nude mice.

RESULTS

Phosphoproteomics and proteomics data analysis revealed that the kinase MAPK1 may play an important role in pancreatic cancer progression by promoting IQGAP1 phosphorylation modification. Proteomics analysis classified pancreatic cancer patients into two subtypes, C1 and C2, where the high-risk C2 subtype was associated with poor prognosis, malignant tumor typing, and enriched tumor-related pathways. SOX4 may promote the occurrence of the high-risk C2 subtype of pancreatic cancer by regulating MAPK1-IQGAP1 phosphorylation modification. In vitro cell experiments confirmed that SOX4 promoted IQGAP1 phosphorylation modification by activating MAPK1 transcription while silencing SOX4 inhibited the proliferation, migration, and invasion of pancreatic cancer cells by reducing the phosphorylation level of MAPK1-IQGAP1. In vivo, animal experiments further confirmed that silencing SOX4 suppressed the growth and metastasis of pancreatic cancer by reducing the phosphorylation level of MAPK1-IQGAP1.

CONCLUSION

The findings of this study suggest that SOX4 promotes the phosphorylation modification of IQGAP1 by activating MAPK1 transcription, thereby facilitating the growth and metastasis of pancreatic cancer.

Patch-seq: Advances and Biological Applications

Multimodal analysis of gene-expression patterns, electrophysiological properties, and morphological phenotypes at the single-cell/single-nucleus level has been arduous because of the diversity and complexity of neurons. The emergence of Patch-sequencing (Patch-seq) directly links transcriptomics, morphology, and electrophysiology, taking neuroscience research to a multimodal era. In this review, we summarized the development of Patch-seq and recent applications in the cortex, hippocampus, and other nervous systems. Through generating multimodal cell type atlases, targeting specific cell populations, and correlating transcriptomic data with phenotypic information, Patch-seq has provided new insight into outstanding questions in neuroscience. We highlight the challenges and opportunities of Patch-seq in neuroscience and hope to shed new light on future neuroscience research.

Photobiomodulation reduces neuropathic pain after spinal cord injury by downregulating CXCL10 expression

BACKGROUND

Many studies have recently highlighted the role of photobiomodulation (PBM) in neuropathic pain (NP) relief after spinal cord injury (SCI), suggesting that it may be an effective way to relieve NP after SCI. However, the underlying mechanisms remain unclear. This study aimed to determine the potential mechanisms of PBM in NP relief after SCI.

METHODS

We performed systematic observations and investigated the mechanism of PBM intervention in NP in rats after SCI. Using transcriptome sequencing, we screened CXCL10 as a possible target molecule for PBM intervention and validated the results in rat tissues using reverse transcription-polymerase chain reaction and western blotting. Using immunofluorescence co-labeling, astrocytes and microglia were identified as the cells responsible for CXCL10 expression. The involvement of the NF-κB pathway in CXCL10 expression was verified using inhibitor pyrrolidine dithiocarbamate (PDTC) and agonist phorbol-12-myristate-13-acetate (PMA), which were further validated by an in vivo injection experiment.

RESULTS

Here, we demonstrated that PBM therapy led to an improvement in NP relative behaviors post-SCI, inhibited the activation of microglia and astrocytes, and decreased the expression level of CXCL10 in glial cells, which was accompanied by mediation of the NF-κB signaling pathway. Photobiomodulation inhibit the activation of the NF-κB pathway and reduce downstream CXCL10 expression. The NF-κB pathway inhibitor PDTC had the same effect as PBM on improving pain in animals with SCI, and the NF-κB pathway promoter PMA could reverse the beneficial effect of PBM.

CONCLUSIONS

Our results provide new insights into the mechanisms by which PBM alleviates NP after SCI. We demonstrated that PBM significantly inhibited the activation of microglia and astrocytes and decreased the expression level of CXCL10. These effects appear to be related to the NF-κB signaling pathway. Taken together, our study provides evidence that PBM could be a potentially effective therapy for NP after SCI, CXCL10 and NF-kB signaling pathways might be critical factors in pain relief mediated by PBM after SCI.

Expression of NF-κB-associated lncRNAs in different types of migraine

PURPOSE

NF-κB partakes in the pathophysiology of neurologic conditions. We quantified levels of NF-κB-associated genes in 119 patients with migraine versus healthy controls.

METHODS

We measured levels of NF-κB-associated genes in 42 patients with migraine compared with age- and sex-matched controls.

RESULTS

Comparison between patients without aura and controls revealed down-regulation of PACER [expression ratio (95% CI) 0.15 (0.06-0.36), P value < 0.0001]. Similar results were detected when comparing expression of PACER in patients with aura and controls [expression ratio (95% CI) 0.05 (0.02-0.12), P value < 0.0001]. Both DILC and CEBPA were over-expressed in patients with aura [expression ratio (95% CI) 4.9 (2.96-7.83), P value < 0.0001 and expression ratio (95% CI) 3.65 (2.39-5.24), P value < 0.0001, respectively] and in patients without aura compared with controls [expression ratio (95% CI) 3.6 (2.21-5.69), P value < 0.0001 and expression ratio (95% CI) 4.5 (2.53-7.11), P value < 0.0001, respectively]. ADINR was over-expressed in patients with aura [expression ratio (95% CI) 4.98 (3.09-8.33), P value < 0.0001] as well as patients without aura compared with controls [expression ratio (95% CI) 13.15 (7.41-23.58), P value < 0.0001]. Notably, ADINR levels were lower in patients with aura compared with patients without aura. When comparing ATG5 levels in patients with aura and controls, significant up-regulation was detected [expression ratio (95% CI) 4.4 (3.01-6.32), P value < 0.0001]. This pattern was also detected in patients without aura compared with controls [expression ratio (95% CI) 3.5 (2.28-5.35), P < 0.0001]. Finally, expression of DICER1-AS1 was elevated in patients with aura compared with patients without aura [expression ratio (95% CI) 2.47 (1.14-5.85), P = 0.03]. This lncRNA was under-expressed in patients without aura compared with controls [expression ratio (95% CI) 0.4 (0.21-1.31), P = 0.03]. CEBPA, ATG5 and ADINR had the best AUC values for distinguishing patients with aura from controls (AUC values = 0.91, 0.85 and 0.83, respectively). The AUC values for separation between patients without aura and controls were 0.90, 0.86 and 0.75 for CEBPA, ATG5 and ADINR, respectively.

CONCLUSION

Taken together, several genes in the NF-κB pathway has been revealed to be dysregulated in migraineurs and expression of these genes can be used as markers for this neurological condition.

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

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

Transcriptomic, Epigenomic, and Neuroanatomic Signatures Differ in Chronic Prurigo, Atopic Dermatitis, and Brachioradial Pruritus

Scratching and scratch-induced injuries, including neuroanatomical alterations, are key characteristics of chronic pruritus entities of different origins. The aim of this study was to link gene expression (array hybridization, qPCR) with DNA methylation (array hybridization) and neuroanatomy (PGP9.5 staining) in chronic nodular prurigo (CNPG), atopic dermatitis (AD), brachioradial pruritus (BRP), and matched healthy controls. Specific signatures of gene expression and DNA methylation clearly discriminated pruritic lesional skin from nonpruritic skin in CNPG and from healthy skin of volunteers, respectively. Although intraepidermal nerve fiber density was indiscriminately reduced, the level of epidermal branching, assessed by a semiquantitative pattern analysis, differentiated the entities (CNPG > BRP > AD). Correspondingly, repellent SEMA3A showed the highest expression in AD, whereas axonal growth-promoting nerve GF was most prominent in CNPG and BRP. Overexpression of genes for nerve fiber regeneration (NELL2/NFKB/ARTN) was found in AD and CNPG but not in BRP. Our findings suggest that differential branching patterns rather than mere innervation density separate chronic itch conditions and reflect disease-specific local expression profiles. In pruritic dermatoses (AD and CNPG), nerve injury and subsequent sprouting may primarily result from chronic scratching, whereas genuine neuropathy is expected to underlie BRP.

Astrocytes in Chronic Pain: Cellular and Molecular Mechanisms

Chronic pain is challenging to treat due to the limited therapeutic options and adverse side-effects of therapies. Astrocytes are the most abundant glial cells in the central nervous system and play important roles in different pathological conditions, including chronic pain. Astrocytes regulate nociceptive synaptic transmission and network function via neuron–glia and glia–glia interactions to exaggerate pain signals under chronic pain conditions. It is also becoming clear that astrocytes play active roles in brain regions important for the emotional and memory-related aspects of chronic pain. Therefore, this review presents our current understanding of the roles of astrocytes in chronic pain, how they regulate nociceptive responses, and their cellular and molecular mechanisms of action.

Identification of Epigenetic Interactions between MicroRNA-30c-5p and DNA Methyltransferases in Neuropathic Pain

Neuropathic pain is a prevalent and severe chronic syndrome, often refractory to treatment, whose development and maintenance may involve epigenetic mechanisms. We previously demonstrated a causal relationship between miR-30c-5p upregulation in nociception-related neural structures and neuropathic pain in rats subjected to sciatic nerve injury. Furthermore, a short course of an miR-30c-5p inhibitor administered into the cisterna magna exerts long-lasting antiallodynic effects via a TGF-β1-mediated mechanism. Herein, we show that miR-30c-5p inhibition leads to global DNA hyper-methylation of neurons in the lumbar dorsal root ganglia and spinal dorsal horn in rats subjected to sciatic nerve injury. Specifically, the inhibition of miR-30-5p significantly increased the expression of the novo DNA methyltransferases DNMT3a and DNMT3b in those structures. Furthermore, we identified the mechanism and found that miR-30c-5p targets the mRNAs of DNMT3a and DNMT3b. Quantitative methylation analysis revealed that the promoter region of the antiallodynic cytokine TGF-β1 was hypomethylated in the spinal dorsal horn of nerve-injured rats treated with the miR-30c-5p inhibitor, while the promoter of Nfyc, the host gene of miR-30c-5p, was hypermethylated. These results are consistent with long-term protection against neuropathic pain development after nerve injury. Altogether, our results highlight the key role of miR-30c-5p in the epigenetic mechanisms' underlying neuropathic pain and provide the basis for miR-30c-5p as a therapeutic target.

NFAT1 Orchestrates Spinal Microglial Transcription and Promotes Microglial Proliferation via c-MYC Contributing to Nerve Injury-Induced Neuropathic Pain

Peripheral nerve injury-induced spinal microglial proliferation plays a pivotal role in neuropathic pain. So far, key intracellular druggable molecules involved in this process are not identified. The nuclear factor of activated T-cells (NFAT1) is a master regulator of immune cell proliferation. Whether and how NFAT1 modulates spinal microglial proliferation during neuropathic pain remain unknown. Here it is reported that NFAT1 is persistently upregulated in microglia after spinal nerve ligation (SNL), which is regulated by TET2-mediated DNA demethylation. Global or microglia-specific deletion of Nfat1 attenuates SNL-induced pain and decreases excitatory synaptic transmission of lamina II neurons. Furthermore, deletion of Nfat1 decreases microglial proliferation and the expression of multiple microglia-related genes, such as cytokines, transmembrane signaling receptors, and transcription factors. Particularly, SNL increases the binding of NFAT1 with the promoter of Itgam, Tnf, Il-1b, and c-Myc in the spinal cord. Microglia-specific overexpression of c-MYC induces pain hypersensitivity and microglial proliferation. Finally, inhibiting NFAT1 and c-MYC by intrathecal injection of inhibitor or siRNA alleviates SNL-induced neuropathic pain. Collectively, NFAT1 is a hub transcription factor that regulates microglial proliferation via c-MYC and guides the expression of the activated microglia genome. Thus, NFAT1 may be an effective target for treating neuropathic pain.

Role of Epigenetic Mechanisms in Chronic Pain

Pain is an unpleasant but essential-to-life sensation, usually resulting from tissue damage. When pain persists long after the injury has resolved, it becomes pathological. The precise molecular and cellular mechanisms causing the transition from acute to chronic pain are not fully understood. A key aspect of pain chronicity is that several plasticity events happen along the neural pathways involved in pain. These long-lasting adaptive changes are enabled by alteration in the expression of relevant genes. Among the different modulators of gene transcription in adaptive processes in the nervous system, epigenetic mechanisms play a pivotal role. In this review, I will first outline the main classes of epigenetic mediators and then discuss their implications in chronic pain.

Targeting G protein coupled receptors for alleviating neuropathic pain

Pain sensation is a normal physiological response to alert and prevent further tissue damage. It involves the perception of external stimuli by somatosensory neurons, then transmission of the message to various other types of neurons present in the spinal cord and brain to generate an appropriate response. Currently available analgesics exhibit very modest efficacy, and that too in only a subset of patients with chronic pain conditions, particularly neuropathic pain. The G protein-coupled receptors (GPCRs) are expressed on presynaptic, postsynaptic terminals, and soma of somatosensory neurons, which binds to various types of ligands to modulate neuronal activity and thus pain sensation in both directions. Fundamentally, neuropathic pain arises due to aberrant neuronal plasticity, which includes the sensitization of peripheral primary afferents (dorsal root ganglia and trigeminal ganglia) and the sensitization of central nociceptive neurons in the spinal cord or trigeminal nucleus or brain stem and cortex. Owing to the expression profiles of GPCRs in somatosensory neurons and other neuroanatomical regions involved in pain processing and transmission, this article shall focus only on four families of GPCRs: 1- Opioid receptors, 2-Cannabinoid receptors, 3-Adenosine receptors, and 4-Chemokine receptors.

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.

Decoding gene expression signatures in mice trigeminal ganglion across trigeminal neuropathic pain stages via high-throughput sequencing

Trigeminal neuropathic pain (TNP) arises due to peripheral nerve injury, the mechanisms underlying which are little known. The altered gene expression profile in sensory ganglia is critical for neuropathic pain generation and maintenance. We, therefore, assessed the transcriptome of the trigeminal ganglion (TG) from mice at different periods of pain progression. Trigeminal neuropathic pain was established by partial infraorbital nerve transection (pIONT). High-throughput RNA sequencing was applied to detect the mRNA profiles of TG collected at 3 and 10 days after modeling. Injured TG displayed dramatically altered mRNA expression profiles compared to Sham. Different gene expression profiles were obtained at 3 and 10 days after pIONT. Moreover, 314 genes were significantly upregulated, and 81 were significantly downregulated at both 3 and 10 days post-pIONT. Meanwhile, enrichment analysis of these persistent differentially expressed genes (DEGs) showed that the MAPK pathway was the most significantly enriched pathway for upregulated DEGs, validated by immunostaining. In addition, TG cell populations defined by single-nuclei RNA sequencing displayed cellular localization of DEGs at a single-cell resolution. Protein-protein interaction (PPI) and sub-PPI network analyses constructed networks and identified the top 10 hub genes for DEGs at different time points. The present data provide novel information on the gene expression signatures of TG during the development and maintenance phases of TNP, and the identified hub genes and pathways may serve as potential targets for treatment.

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.

Paeonol Ameliorates Chronic Itch and Spinal Astrocytic Activation via CXCR3 in an Experimental Dry Skin Model in Mice

Paeonol is a bioactive phenol presents mainly in Paeonia suffruticosa Andr. (Paeoniaceae), Paeonia lactiflora Pall., and Dioscorea japonica Thunb. (Dioscoreaceae), harboring various pharmacological activities including anti-inflammatory, antioxidant, immune regulatory activity and reverse chemoresistance. Recent reports revealed paeonol exhibited good effects on chronic dermatitis, such as atopic dermatitis (AD) and psoriasis. However, whether paeonol is effective for dry skin disease and its mechanism of action still remain unclear. In this study, we analysed the effects of paeonol on a mouse model of dry skin treated with acetone-ether-water (AEW), which showed impressive activities in reducing scratching behavior and skin inflammation. To elucidate the underlying molecular targets for the anti-pruritic ability of paeonol, we screened the expression of possible chemokine pathways in the spinal cord. The expression of CXCR3 was significantly alleviated by paeonol, which increased greatly in the spinal neurons of AEW mice. In addition, treatment of paeonol significantly inhibited AEW-induced expression of astrocyte activity-dependent genes including Tlr4, Lcn2 and Hspb1 et al. The inhibitory effects of paeonol on scratching behavior and astrocytic activation in the spinal cord induced by AEW were abolished when CXCR3 was antagonized or genetically ablated. Taken together, our results indicated that paeonol can ameliorate AEW-induced inflammatory response and itching behavior, and reduce the expression of spinal astrocyte activity-dependent genes induced by AEW, which are driven by CXCR3.

DNA Methylation and Non-Coding RNAs during Tissue-Injury Associated Pain

While about half of the population experience persistent pain associated with tissue damages during their lifetime, current symptom-based approaches often fail to reduce such pain to a satisfactory level. To provide better patient care, mechanism-based analgesic approaches must be developed, which necessitates a comprehensive understanding of the nociceptive mechanism leading to tissue injury-associated persistent pain. Epigenetic events leading the altered transcription in the nervous system are pivotal in the maintenance of pain in tissue injury. However, the mechanisms through which those events contribute to the persistence of pain are not fully understood. This review provides a summary and critical evaluation of two epigenetic mechanisms, DNA methylation and non-coding RNA expression, on transcriptional modulation in nociceptive pathways during the development of tissue injury-associated pain. We assess the pre-clinical data and their translational implication and evaluate the potential of controlling DNA methylation and non-coding RNA expression as novel analgesic approaches and/or biomarkers of persistent pain.

Role of CXCL10 in Spinal Cord Injury

Spinal cord injury (SCI) results in acute inflammatory responses and secondary damages, including neuronal and glial cell death, axonal damage and demyelination, and blood-brain barrier (BBB) damage, eventually leading to neuronal dysfunction and other complications. C-X-C motif Chemokine Ligand 10 (CXCL10) is expressed after the injury, playing multiple roles in the development and progression of SCI. Moreover, the CXCL10 antagonist can restrict inflammatory immune responses and promote neuronal regeneration and functional recovery. In this review, we summarize the structure and biological functions of CXCL10, and the roles of the CXCL10 / CXCR3 axis in acute inflammatory responses, secondary damages, and complications during SCI, thus providing a potential theoretical basis by highlighting CXCL10 as a new potential drug target for the treatment of SCI.

ZNF382 controls mouse neuropathic pain via silencer-based epigenetic inhibition of Cxcl13 in DRG neurons

Nerve injury-induced changes of gene expression in dorsal root ganglion (DRG) are critical for neuropathic pain genesis. However, how these changes occur remains elusive. Here we report the down-regulation of zinc finger protein 382 (ZNF382) in injured DRG neurons after nerve injury. Rescuing this down-regulation attenuates nociceptive hypersensitivity. Conversely, mimicking this down-regulation produces neuropathic pain symptoms, which are alleviated by C-X-C motif chemokine 13 (CXCL13) knockdown or its receptor CXCR5 knockout. Mechanistically, an identified cis-acting silencer at distal upstream of the Cxcl13 promoter suppresses Cxcl13 transcription via binding to ZNF382. Blocking this binding or genetically deleting this silencer abolishes the ZNF382 suppression on Cxcl13 transcription and impairs ZNF382-induced antinociception. Moreover, ZNF382 down-regulation disrupts the repressive epigenetic complex containing histone deacetylase 1 and SET domain bifurcated 1 at the silencer-promoter loop, resulting in Cxcl13 transcriptional activation. Thus, ZNF382 down-regulation is required for neuropathic pain likely through silencer-based epigenetic disinhibition of CXCL13, a key neuropathic pain player, in DRG neurons.

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.

Neuromedin B receptor stimulation of Cav3.2 T-type Ca2+ channels in primary sensory neurons mediates peripheral pain hypersensitivity

Background: Neuromedin B (Nmb) is implicated in the regulation of nociception of sensory neurons. However, the underlying cellular and molecular mechanisms remain unknown.

Methods: Using patch clamp recording, western blot analysis, immunofluorescent labelling, enzyme-linked immunosorbent assays, adenovirus-mediated shRNA knockdown and animal behaviour tests, we studied the effects of Nmb on the sensory neuronal excitability and peripheral pain sensitivity mediated by Cav3.2 T-type channels.

Results: Nmb reversibly and concentration-dependently increased T-type channel currents (I_T) in small-sized trigeminal ganglion (TG) neurons through the activation of neuromedin B receptor (NmbR). This NmbR-mediated I_T response was G_q protein-coupled, but independent of protein kinase C activity. Either intracellular application of the QEHA peptide or shRNA-mediated knockdown of G_β abolished the NmbR-induced I_T response. Inhibition of protein kinase A (PKA) or AMP-activated protein kinase (AMPK) completely abolished the Nmb-induced I_T response. Analysis of phospho-AMPK (p-AMPK) revealed that Nmb significantly activated AMPK, while AMPK inhibition prevented the Nmb-induced increase in PKA activity. In a heterologous expression system, activation of NmbR significantly enhanced the Cav3.2 channel currents, while the Cav3.1 and Cav3.3 channel currents remained unaffected. Nmb induced TG neuronal hyperexcitability and concomitantly induced mechanical and thermal hypersensitivity, both of which were attenuated by T-type channel blockade. Moreover, blockade of NmbR signalling prevented mechanical hypersensitivity in a mouse model of complete Freund's adjuvant-induced inflammatory pain, and this effect was attenuated by siRNA knockdown of Cav3.2.

Conclusions: Our study reveals a novel mechanism by which NmbR stimulates Cav3.2 channels through a G_βγ-dependent AMPK/PKA pathway. In mouse models, this mechanism appears to drive the hyperexcitability of TG neurons and induce pain hypersensitivity.

Comparison of the Effects of Chemokine Receptors CXCR2 and CXCR3 Pharmacological Modulation in Neuropathic Pain Model-In Vivo and In Vitro Study

Recent findings have highlighted the roles of CXC chemokine family in the mechanisms of neuropathic pain. Our studies provide evidence that single/repeated intrathecal administration of CXCR2 (NVP-CXCR2-20) and CXCR3 ((±)-NBI-74330) antagonists explicitly attenuated mechanical/thermal hypersensitivity in rats after chronic constriction injury of the sciatic nerve. After repeated administration, both antagonists showed strong analgesic activity toward thermal hypersensitivity; however, (±)-NBI-74330 was more effective at reducing mechanical hypersensitivity. Interestingly, repeated intrathecal administration of both antagonists decreased the mRNA and/or protein levels of pronociceptive interleukins (i.e., IL-1beta, IL-6, IL-18) in the spinal cord, but only (±)-NBI-74330 decreased their levels in the dorsal root ganglia after nerve injury. Furthermore, only the CXCR3 antagonist influenced the spinal mRNA levels of antinociceptive factors (i.e., IL-1RA, IL-10). Additionally, antagonists effectively reduced the mRNA levels of pronociceptive chemokines; NVP-CXCR2-20 decreased the levels of CCL2, CCL6, CCL7, and CXCL4, while (±)-NBI-74330 reduced the levels of CCL3, CCL6, CXCL4, and CXCL9. Importantly, the results obtained from the primary microglial and astroglial cell cultures clearly suggest that both antagonists can directly affect the release of these ligands, mainly in microglia. Interestingly, NVP-CXCR2-20 induced analgesic effects after intraperitoneal administration. Our research revealed important roles for CXCR2 and CXCR3 in nociceptive transmission, especially in neuropathic pain.

The Potential Roles of Glial Cells in the Neuropathogenesis of Cerebral Malaria

Cerebral malaria (CM) is a severe neurological complication of malaria caused by the Plasmodium falciparum parasite. It is one of the leading causes of death in children under 5 years of age in Sub-Saharan Africa. CM is associated with blood-brain barrier disruption and long-term neurological sequelae in survivors of CM. Despite the vast amount of research on cerebral malaria, the cause of neurological sequelae observed in CM patients is poorly understood. In this article, the potential roles of glial cells, astrocytes, and microglia, in cerebral malaria pathogenesis are reviewed. The possible mechanisms by which glial cells contribute to neurological damage in CM patients are also examined.

CXCR3 signalling partially contributes to the pathogenesis of neuropathic pain in male rodents

BACKGROUND

Currently, there is a lack of effective therapy for chronic pain. Increasing evidence has shown that chemokines and their correlative receptors involved in the neuron-glial cell cross-talk could contribute to the pathogenesis of neuropathic pain. Our previous studies suggested that CXCR3 expression was elevated in the spinal dorsal horn after nerve injury.

OBJECTIVES

In this study, we aimed to explore the role of CXCR3 signalling in chronic pain modulation.

METHODS

Reverse transcription quantitative PCR and Western blotting were used to measure the expression of CXCR3 and its ligands in the spinal cord following chronic constriction injury (CCI) of the sciatic nerve. Cxcr3 -knockout mice were used to observe the effect of the receptor on pain-related behaviour and microglial activation. Immunohistochemistry was used to investigate the expression of two activation markers for spinal microglia, Iba-1 and phosphorylated-p38 (p-p38) in these mice.

RESULTS

The expression of CXCR3 and its ligand CXCL11 was upregulated in the lumbar dorsal horn of the spinal cord in CCI models. In Cxcr3 -knockout mice, CCI-induced tactile allodynia and thermal hyperalgesia were observed to be alleviated during the early stage of pain processing. Meanwhile, the expression of the glial activation markers, namely, Iba-1 and p-p38, was decreased.

CONCLUSION

Our results demonstrate that CXCR3 could be a key modulator involved in pain modulation of the spinal cord; therefore, CXCR3-related signalling pathways could be potential targets for the treatment of intractable pathological pain.

Global gene expression and chromatin accessibility of the peripheral nervous system in animal models of persistent pain

BACKGROUND

Efforts to understand genetic variability involved in an individual's susceptibility to chronic pain support a role for upstream regulation by epigenetic mechanisms.

METHODS

To examine the transcriptomic and epigenetic basis of chronic pain that resides in the peripheral nervous system, we used RNA-seq and ATAC-seq of the rat dorsal root ganglion (DRG) to identify novel molecular pathways associated with pain hypersensitivity in two well-studied persistent pain models induced by chronic constriction injury (CCI) of the sciatic nerve and intra-plantar injection of complete Freund's adjuvant (CFA) in rats.

RESULTS

Our RNA-seq studies identify a variety of biological process related to synapse organization, membrane potential, transmembrane transport, and ion binding. Interestingly, genes that encode transcriptional regulators were disproportionately downregulated in both models. Our ATAC-seq data provide a comprehensive map of chromatin accessibility changes in the DRG. A total of 1123 regions showed changes in chromatin accessibility in one or both models when compared to the naïve and 31 shared differentially accessible regions (DAR)s. Functional annotation of the DARs identified disparate molecular functions enriched for each pain model which suggests that chromatin structure may be altered differently following sciatic nerve injury and hind paw inflammation. Motif analysis identified 17 DNA sequences known to bind transcription factors in the CCI DARs and 33 in the CFA DARs. Two motifs were significantly enriched in both models.

CONCLUSIONS

Our improved understanding of the changes in chromatin accessibility that occur in chronic pain states may identify regulatory genomic elements that play essential roles in modulating gene expression in the DRG.

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

ABSTRACT

The methyltransferase-like 3 (Mettl3) is a key component of the large N6-adenosine-methyltransferase complex in mammalian responsible for RNA N6-methyladenosine (m6A) modification, which plays an important role in gene post-transcription modulation. Although RNA m6A is enriched in mammalian neurons, its regulatory function in nociceptive information processing remains elusive. Here, we reported that Complete Freund's Adjuvant (CFA)-induced inflammatory pain significantly decreased global m6A level and m6A writer Mettl3 in the spinal cord. Mimicking this decease by knocking down or conditionally deleting spinal Mettl3 elevated the levels of m6A in ten-eleven translocation methylcytosine dioxygenases 1 (Tet1) mRNA and TET1 protein in the spinal cord, leading to production of pain hypersensitivity. By contrast, overexpressing Mettl3 reversed a loss of m6A in Tet1 mRNA and blocked the CFA-induced increase of TET1 in the spinal cord, resulting in the attenuation of pain behavior. Furthermore, the decreased level of spinal YT521-B homology domain family protein 2 (YTHDF2), an RNA m6A reader, stabilized upregulation of spinal TET1 because of the reduction of Tet1 mRNA decay by the binding to m6A in Tet1 mRNA in the spinal cord after CFA. This study reveals a novel mechanism for downregulated spinal cord METTL3 coordinating with YTHDF2 contributes to the modulation of inflammatory pain through stabilizing upregulation of TET1 in spinal neurons.

Nerve Growth Factor Enhances Tooth Mechanical Hyperalgesia Through C-C Chemokine Ligand 19 in Rats

The nerve growth factor (NGF) plays an important role in the regulation of neuropathic pain. It has been demonstrated that calcitonin gene-related peptide (CGRP), a well-known contributor to neurogenic inflammation, increases neuroinflammatory pain induced by NGF. The inflammatory mediator that NGF most strongly induces is C-C chemokine ligand 19 (CCL19), which can recruit inflammatory cells by binding to the receptor CCR7 followed by promoting the response of neuroinflammation. However, the regulatory mechanism of NGF and CCL19 in tooth movement orofacial pain and the interaction between both are still unclear. In this study, male Sprague–Dawley rats were used to study the modulation of NGF on orofacial pain through CCL19 and the role of each in tooth movement pain in rats. The expression levels of CCL19 mRNA and protein were determined by real-time PCR and immunofluorescence, respectively. Pain levels were assessed by measuring the rats' bite force, which drops as pain rises. Meanwhile, by verifying the relationship between CGRP and CCL19, it was laterally confirmed that NGF could modulate tooth movement-induced mechanical hyperalgesia through CCL19. The results showed that the expression level of CCL19 rose with the increased NGF, and neurons expressing CGRP can express stronger CCL19. Compared with the baseline level, the bite force for all rats dropped sharply on day 1, reached its lowest level on day 3, and recovered gradually on day 5. All results indicated that NGF played an important role in tooth movement orofacial pain via positively regulating CCL19 expression in the trigeminal ganglia of rats. Additionally, CCL19 increased the sensitivity to experimental tooth movement orofacial pain. NGF can regulate CCL19 expression, although it may regulate other inflammatory pathways as well. This is the first report on the interactions and modulations of tooth movement orofacial pain by NGF through CCL19 in rats.

G protein-coupled receptor GPR151 is involved in trigeminal neuropathic pain through the induction of Gβγ/extracellular signal-regulated kinase-mediated neuroinflammation in the trigeminal ganglion

ABSTRACT

Trigeminal nerve injury-induced neuropathic pain is a debilitating chronic orofacial pain syndrome but lacks effective treatment. G protein-coupled receptors (GPCRs), especially orphan GPCRs (oGPCRs) are important therapeutic targets in pain medicine. Here, we screened upregulated oGPCRs in the trigeminal ganglion (TG) after partial infraorbital nerve transection (pIONT) and found that Gpr151 was the most significantly upregulated oGPCRs. Gpr151 mRNA was increased from pIONT day 3 and maintained for more than 21 days. Furthermore, GPR151 was expressed in the neurons of the TG after pIONT. Global mutation or knockdown of Gpr151 in the TG attenuated pIONT-induced mechanical allodynia. In addition, the excitability of TG neurons was increased after pIONT in wild-type (WT) mice, but not in Gpr151-/- mice. Notably, GPR151 bound to Gαi protein, but not Gαq, Gα12, or Gα13, and activated the extracellular signal-regulated kinase (ERK) through Gβγ. Extracellular signal-regulated kinase was also activated by pIONT in the TG of WT mice, but not in Gpr151-/- mice. Gene microarray showed that Gpr151 mutation reduced the expression of a large number of neuroinflammation-related genes that were upregulated in WT mice after pIONT, including chemokines CCL5, CCL7, CXCL9, and CXCL10. The mitogen-activated protein kinase inhibitor (PD98059) attenuated mechanical allodynia and reduced the upregulation of these chemokines after pIONT. Collectively, this study not only revealed the involvement of GPR151 in the maintenance of trigeminal neuropathic pain but also identified GPR151 as a Gαi-coupled receptor to induce ERK-dependent neuroinflammation. Thus, GPR151 may be a potential drug target for the treatment of trigeminal neuropathic pain.

CXCL10 and CXCR3 in the Trigeminal Ganglion Contribute to Trigeminal Neuropathic Pain in Mice

Purpose

Trigeminal neuropathic pain is very common clinically, but effective treatments are lacking. Chemokines and their receptors have been implicated in the pathogenesis of chronic pain. This study explored the role of the chemokine CXCL10 and its receptor, CXCR3, in trigeminal neuropathic pain in mice.

Materials and Methods

Trigeminal neuropathic pain was established by partial infraorbital nerve ligation (pIONL) in wild-type and Cxcr3^−/− mice. Facial mechanical allodynia was evaluated by behavioral testing. A lentivirus containing Cxcr3 shRNA (LV-Cxcr3 shRNA) was microinjected into the trigeminal ganglion (TG) to knock down Cxcr3 expression. Quantitative polymerase chain reaction assays and immunofluorescence staining were used to examine Cxcl10/Cxcr3 mRNA expression and protein distribution. Western blotting was performed to examine activation of extracellular signal-regulated kinase (ERK) and AKT in the TG. Intra-TG injection of an AKT inhibitor was performed to examine the role of AKT in trigeminal neuropathic pain.

Results

pIONL induced persistent trigeminal neuropathic pain, which was alleviated in Cxcr3^−/− mice. Intra-TG injection of LV-Cxcr3 shRNA attenuated pIONL-induced mechanical allodynia. Furthermore, pIONL increased the expression of CXCR3 and its major ligand, CXCL10, in TG neurons. Intra-TG injection of CXCL10 induced pain hypersensitivity in wild-type mice but not in Cxcr3^−/− mice. CXCL10 also induced activation of ERK and AKT in the TG of wild-type mice. Finally, pIONL-induced activation of ERK and AKT was reduced in Cxcr3^−/− mice. Intra-TG injection of the AKT inhibitor alleviated pIONL-induced mechanical allodynia in WT mice but not in Cxcr3^−/− mice.

Conclusion

CXCL10 acts on CXCR3 to induce ERK and AKT activation in TG neurons and contributes to the maintenance of trigeminal neuropathic pain.

Macrophagic and microglial complexity after neuronal injury

Central nervous system (CNS) injuries do not heal properly in contrast to normal tissue repair, in which functional recovery typically occurs. The reason for this dichotomy in wound repair is explained in part by macrophage and microglial malfunction, affecting both the extrinsic and intrinsic barriers to appropriate axonal regeneration. In normal healing tissue, macrophages promote the repair of injured tissue by regulating transitions through different phases of the healing response. In contrast, inflammation dominates the outcome of CNS injury, often leading to secondary damage. Therefore, an understanding of the molecular mechanisms underlying this dichotomy is critical to advance in neuronal repair therapies. Recent studies highlight the plasticity and complexity of macrophages and microglia beyond the classical view of the M1/M2 polarization paradigm. This plasticity represents an in vivo continuous spectrum of phenotypes with overlapping functions and markers. Moreover, macrophage and microglial plasticity affect many events essential for neuronal regeneration after injury, such as myelin and cell debris clearance, inflammation, release of cytokines, and trophic factors, affecting both intrinsic neuronal properties and extracellular matrix deposition. Until recently, this complexity was overlooked in the translation of therapies modulating these responses for the treatment of neuronal injuries. However, recent studies have shed important light on the underlying molecular mechanisms of this complexity and its transitions and effects on regenerative events. Here we review the complexity of macrophages and microglia after neuronal injury and their roles in regeneration, as well as the underlying molecular mechanisms, and we discuss current challenges and future opportunities for treatment.

CXCL10/CXCR3 Signaling in the DRG Exacerbates Neuropathic Pain in Mice

Chemokines and receptors have been implicated in the pathogenesis of chronic pain. Here, we report that spinal nerve ligation (SNL) increased CXCR3 expression in dorsal root ganglion (DRG) neurons, and intra-DRG injection of Cxcr3 shRNA attenuated the SNL-induced mechanical allodynia and heat hyperalgesia. SNL also increased the mRNA levels of CXCL9, CXCL10, and CXCL11, whereas only CXCL10 increased the number of action potentials (APs) in DRG neurons. Furthermore, in Cxcr3^-/- mice, CXCL10 did not increase the number of APs, and the SNL-induced increase of the numbers of APs in DRG neurons was reduced. Finally, CXCL10 induced the activation of p38 and ERK in ND7-23 neuronal cells and DRG neurons. Pretreatment of DRG neurons with the P38 inhibitor SB203580 decreased the number of APs induced by CXCL10. Our data indicate that CXCR3, activated by CXCL10, mediates p38 and ERK activation in DRG neurons and enhances neuronal excitability, which contributes to the maintenance of neuropathic pain.

Time course of DNA methylation in pain conditions: From experimental models to humans

BACKGROUND AND OBJECTIVE

Throughout the last decade, research has uncovered associations between pain and epigenetic alterations caused by environmental factors. Specifically, studies have demonstrated correlations between pain conditions and altered DNA methylation patterns. Thus, DNA methylation has been revealed as a possible modulator or contributor to pain conditions, providing a potential therapeutic target for treatment by DNA methylation modification. To develop such treatments, it is necessary to clarify a wide number of aspects on how DNA methylation affects pain perception; first and foremost, the temporal dynamics. The objective of the present review is to provide an overview of current knowledge on temporal dynamics of DNA methylation in response to pain, and to investigate if a timeframe can be established based on the data of currently published studies.

DATABASES AND DATA TREATMENT

PubMed, MEDLINE, Google Scholar and Embase were searched comprehensively for studies of DNA methylation in neuropathic, inflammatory and alternative animal pain models, and in chronic pain patients including Complex Regional Pain Syndrome, chronic postsurgical pain, chronic widespread pain, fibromyalgia and Crohn's disease.

RESULTS

We identified 34 articles highlighting variations in temporal dynamics of DNA methylation across species and between different types of pain. These studies represent a starting point to uncover new insights in the DNA methylation time course in pain.

CONCLUSIONS

No timeframe can currently be made for the DNA methylation response to pain in any of the reviewed conditions, highlighting an important focus area for future research.

C/EBPα-mediated transcriptional activation of miR-134-5p entails KPNA3 inhibition and modulates focal hypoxic-ischemic brain damage in neonatal rats

Hypoxic-ischemic brain damage (HIBD) is a frequent cause of mortality and neurological handicaps in infants and children worldwide. To understand better the pathogenesis and management of HIBD, we established a HIBD model by common carotid artery ligation followed by systemic hypoxia in neonatal rats, and in other studies induced neuronal death in rat pheochromocytoma (PC12) cells by 12 h of oxygen-glucose deprivation (OGD). The level of KPNA3 declined in rats following experimental HIBD and in PC12 cells following OGD. KPNA3 overexpression protected neonatal rats against HIBD and PC12 cells against OGD-induced cell death. KPNA3 demonstrated to be the target of miR-134-5p could be activated by the transcriptional factor CCAAT/enhancer binding protein alpha (C/EBPα). The expression of miR-134-5p and C/EBPα was elevated in rats following experimental HIBD and in PC12 cells following OGD. In the parallel experiments, C/EBPα knockdown and miR-134 inhibition protected against HIBD pathology in neonatal rats and against OGD-induced neuronal death in PC12 cells. These findings reveal that the C/EBPα/miR-134-5p/KPNA3 axis mediates hypoxic-ischemic brain damage and neuronal death, thus presenting a potential therapeutic target for the treatment of human newborns at risk for HIBD.

CXCR3 contributes to neuropathic pain via ERK activation in the anterior cingulate cortex

The anterior cingulate cortex (ACC) is activated by noxious stimuli and is involved in the affective component of pain processing; but its role in the sensory component of pain remains largely unknown. Studies have verified that Chemokine (C-X-C motif) receptor 3 (CXCR3) is involved in nociceptive sensitization in the spinal cord after peripheral nerve injury; however, the expression of CXCR3 in the ACC and its role in neuropathic pain has not been reported. Here, we showed that CXCR3 co-localized with neurons in the ACC and the upregulation of CXCR3 corresponded with hypersensitive behaviors after a chronic constriction injury of the sciatic nerve. Pharmacological blockade of CXCR3 using local injection of its inhibitor, AMG487, into the ACC significantly attenuated hyperalgesia induced by chronic constriction injury and suppressed the phosphorylation of extracellular signal-regulated kinase (ERK). Collectively, these results suggest that CXCR3 in the ACC is involved in hyperalgesia induced by peripheral nerve injury and ERK may be a downstream target.

Evaluating the Role of CXCR3 in Pain Modulation: A Literature Review

CXCR3 is a well-known receptor involved in immune cell recruitment and inflammation. Pathological inflammation leads to pain stimulation and hence nociception. Therefore, we decided to review the recent research on CXCR3 to identify its precise role in the modulation of pain in a variety of clinical conditions targeting various regions of the body. Studies were selected from PubMed Medline, which relate CXCR3 to the progression of diseases with either bone cancer pain, neuropathic pain, cystitis pain, osteoarthritis and rheumatoid arthritis pain, dental pain, in particular, periodontitis and pulpitis. In all the diseases studied, a high prevalence of CXCR3 and/or its ligand were identified where CXCR3 is a key player in the pathophysiological process of many inflammatory conditions. CXCR3 and its ligands, particularly CXCL10, modulate nociception via actions in the dorsal root ganglia and dorsal horn of the spinal cord, in cases of bone cancer pain, neuropathic, and joint pain. However, with the other studied disease, no direct link to pain has been made, although it contributes to the pathological progression of the diseases and hence would be a causal factor for the pain. Furthermore, CXCR3 appears to play a role in desensitizing the opioid receptor in the descending modulatory pathway within the brain stem as well as modulating opioid-induced hyperalgesia in the dorsal horn of the spinal cord. Further research is required for understanding the exact mechanisms of CXCR3 in pain modulation centrally and peripherally. A greater understanding of the immunological activities and pharmacological consequence of CXCR3 and its ligands could help in the discovery of newer drugs for modulating pain arising from pathogenic or inflammatory sources. Given the significance of the CXCR3 for nociception, its utilization may prove to be beneficial as a target for analgesia.

Liquiritin Alleviates Pain Through Inhibiting CXCL1/CXCR2 Signaling Pathway in Bone Cancer Pain Rat

Bone cancer pain (BCP) is an intractable clinical problem, and lacked effective drugs for treating it. Recent research showed that several chemokines in the spinal cord are involved in the pathogenesis of BCP. In this study, the antinociceptive effects of liquiritin, which is an active component extracted from Glycyrrhizae Radix, were tested and the underlying mechanisms targeting spinal dorsal horn (SDH) were investigated. The BCP group displayed a significant decrease in the mechanical withdrawal threshold on days 6, 12, and 18 when compared with sham groups. Intrathecal administration of different doses of liquiritin alleviated mechanical allodynia in BCP rats. The results of immunofluorescent staining and western blotting showed that liquiritin inhibited BCP-induced activation of astrocytes in the spinal cord. Moreover, intrathecal administration of liquiritin effectively inhibited the activation of CXCL1/CXCR2 signaling pathway and production of IL-1β and IL-17 in BCP rats. In astroglial-enriched cultures, Lipopolysaccharides (LPS) elicited the release of chemokine CXCL1, and the release was decreased in a dose-dependent manner by liquiritin. In primary neurons, liquiritin indirectly reduced the increase of CXCR2 by astroglial-enriched-conditioned medium but not directly on the CXCR2 target site. These results suggested that liquiritin effectively attenuated BCP in rats by inhibiting the activation of spinal astrocytic CXCL1 and neuronal CXCR2 pathway. These findings provided evidence regarding the the antinociceptive effect of liquiritin on BCP.

C-X-C Motif Chemokine 10 Contributes to the Development of Neuropathic Pain by Increasing the Permeability of the Blood-Spinal Cord Barrier

Neuropathic pain is among the most debilitating forms of chronic pain. Studies have suggested that chronic pain pathogenesis involves neuroimmune interactions and blood-spinal cord barrier (BSCB) disruption. However, the underlying mechanisms are poorly understood. We modeled neuropathic pain in rats by inducing chronic constriction injury (CCI) of the sciatic nerve and analyzed the effects on C-X-C motif chemokine 10 (CXCL10)/CXCR3 activation, BSCB permeability, and immune cell migration from the circulation into the spinal cord. We detected CXCR3 expression in spinal neurons and observed that CCI induced CXCL10/CXCR3 activation, BSCB disruption, and mechanical hyperalgesia. CCI-induced BSCB disruption enabled circulating T cells to migrate into the spinal parenchyma. Intrathecal administration of an anti-CXCL10 antibody not only attenuated CCI-induced hyperalgesia, but also reduced BSCB permeability, suggesting that CXCL10 acts as a key regulator of BSCB integrity. Moreover, T cell migration may play a critical role in the neuroimmune interactions involved in the pathogenesis of CCI-induced neuropathic pain. Our results highlight CXCL10 as a new potential drug target for the treatment of nerve injury-induced neuropathic pain.

Identification of neuropathic pain-associated genes and pathways via random walk with restart algorithm

BACKGROUND

Neuropathic pain (NP) develops from neuropathic lesions or diseases affecting the nervous system, and has become a serious public health issue due to its complex symptoms, high incidence and long duration. At present, the exact pathogenesis of NP is still unclear. In this study, we sought to identify the genes as well as the related molecular mechanisms associated with NP occurrence and development.

METHODS

We firstly identified the differentially expressed genes between NP spinal nerve ligation (SNL) rats and control sham rats and then projected them onto a STRING network for functional association analysis. Then, Random Walk with Restart (RWR) was conducted to find some new NP-related genes, with their potential functions sequentially analyzed by GO annotation and KEGG pathway analysis.

RESULTS

Some new NP-related genes, like Gng13, C3 and Cxcl2, were identified by RWR analysis. Meanwhile, some biological functions like inflammatory responses, chemotaxis and immune responses, as well as some signaling pathways, such as those involved in neuroactive ligand-receptor interactions, complement and blood coagulation cascade reactions, and cytokine-receptor interactions that the new NP- related genes were most activated were found to be associated with NP occurrence and development.

CONCLUSIONS

This study extends our knowledge of NP occurrence and development and provides new therapeutic targets for future NP treatment.

DNA methylation and behavioral changes induced by neonatal spinal transection

Although the importance of epigenetic mechanisms in behavioral development has been gaining attention in recent years, research has largely focused on the brain. To our knowledge, no studies to date have investigated epigenetic changes in the developing spinal cord to determine the dynamic manner in which the spinal epigenome may respond to environmental input during behavioral development. Animal studies demonstrate that spinal cord plasticity is heightened during early development, is somewhat preserved following neonatal transection, and that spinal injured animals are responsive to sensory feedback. Because epigenetic alterations have been implicated in brain plasticity and are highly responsive to experience, these alterations are promising candidates for molecular substrates of spinal plasticity as well. Thus, the current study investigated behavioral changes in the development of weight-bearing locomotion and epigenetic modifications in the spinal cord of infant rats following a neonatal low-thoracic spinal transection or sham surgery on postnatal day (P)1. Specifically, global levels of methylation and methylation status of the brain-derived neurotrophic factor (Bdnf) gene, a neurotrophin heavily involved in both CNS and behavioral plasticity, particularly in development, were examined in lumbar tissue harvested on P10 from sham and spinal-transected subjects. Behavioral results demonstrate that compared to shams, spinal-transected subjects exhibit significantly reduced partial-weight bearing hindlimb activity. Molecular data demonstrate group differences in global lumbar methylation levels as well as exon-specific group differences in Bdnf methylation. This study represents an initial step toward understanding the relationship between epigenetic mechanisms and plasticity associated with spinal cord and locomotor development.

Vascular Endothelial Growth Factor A Signaling Promotes Spinal Central Sensitization and Pain-related Behaviors in Female Rats with Bone Cancer

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Cancer pain is a pervasive clinical symptom impairing life quality. Vascular endothelial growth factor A has been well studied in tumor angiogenesis and is recognized as a therapeutic target for anti-cancer treatment. This study tested the hypothesis that vascular endothelial growth factor A and vascular endothelial growth factor receptor 2 contribute to bone cancer pain regulation associated with spinal central sensitization.

Methods

This study was performed on female rats using a metastatic breast cancer bone pain model. Nociceptive behaviors were evaluated by mechanical allodynia, thermal hyperalgesia, spontaneous pain, and CatWalk gait analysis. Expression levels were measured by real-time quantitative polymerase chain reaction, western blot, and immunofluorescence analysis. Excitatory synaptic transmission was detected by whole-cell patch-clamp recordings. The primary outcome was the effect of pharmacologic intervention of spinal vascular endothelial growth factor A/vascular endothelial growth factor receptor 2–signaling on bone cancer pain behaviors.

Results

The mRNA and protein expression of vascular endothelial growth factor A and vascular endothelial growth factor receptor 2 were upregulated in tumor-bearing rats. Spinal blocking vascular endothelial growth factor A or vascular endothelial growth factor receptor 2 significantly attenuated tumor-induced mechanical allodynia (mean ± SD: vascular endothelial growth factor A, 7.6 ± 2.6 g vs. 5.3 ± 3.3 g; vascular endothelial growth factor receptor 2, 7.8 ± 3.0 g vs. 5.2 ± 3.4 g; n = 6; P &lt; 0.0001) and thermal hyperalgesia (mean ± SD: vascular endothelial growth factor A, 9.0 ± 2.4 s vs. 7.4 ± 2.7 s; vascular endothelial growth factor receptor 2, 9.3 ± 2.5 s vs. 7.5 ± 3.1 s; n = 6; P &lt; 0.0001), as well as spontaneous pain and abnormal gaits. Exogenous vascular endothelial growth factor A enhanced excitatory synaptic transmission in a vascular endothelial growth factor receptor 2–dependent manner, and spinal injection of exogenous vascular endothelial growth factor A was sufficient to cause pain hypersensitivity via vascular endothelial growth factor receptor 2–mediated activation of protein kinase C and Src family kinase in naïve rats. Moreover, spinal blocking vascular endothelial growth factor A/vascular endothelial growth factor receptor 2 pathways suppressed protein kinase C-mediated N-methyl-d-aspartate receptor activation and Src family kinase-mediated proinflammatory cytokine production.

Conclusions

Vascular endothelial growth factor A/vascular endothelial growth factor receptor 2 contributes to central sensitization and bone cancer pain via activation of neuronal protein kinase C and microglial Src family kinase pathways in the spinal cord.

Brain-derived neurotrophic factor stimulation of T-type Ca2+ channels in sensory neurons contributes to increased peripheral pain sensitivity

Although brain-derived neurotrophic factor (BDNF) is implicated in the nociceptive signaling of peripheral sensory neurons, the underlying mechanisms remain largely unknown. Here, we elucidated the effects of BDNF on the neuronal excitability of trigeminal ganglion (TG) neurons and the pain sensitivity of rats mediated by T-type Ca2+ channels. BDNF reversibly and dose-dependently enhanced T-type channel currents through the activation of tropomyosin receptor kinase B (TrkB). Antagonism of phosphatidylinositol 3-kinase (PI3K) but not of its downstream target, the kinase AKT, abolished the BDNF-induced T-type channel response. BDNF application activated p38 mitogen-activated protein kinase (MAPK), and this effect was prevented by inhibition of PI3K but not of protein kinase A (PKA). Antagonism of either PI3K or p38 MAPK prevented the BDNF-induced stimulation of PKA activity, whereas PKA inhibition blocked the BDNF-mediated increase in T-type currents. BDNF increased the rate of action potential firing in TG neurons and enhanced the pain sensitivity of rats to mechanical stimuli. Moreover, inhibition of TrkB signaling abolished the increased mechanical sensitivity in a rat model of chronic inflammatory pain, and this effect was attenuated by either T-type channel blockade or knockdown of the channel Cav3.2. Together, our findings indicate that BDNF enhances T-type currents through the stimulation of TrkB coupled to PI3K-p38-PKA signaling, thereby inducing neuronal hyperexcitability of TG neurons and pain hypersensitivity in rats.

The Role of CXCR3 in Neurological Diseases

BACKGROUND

Neurological diseases have become an obvious challenge due to insufficient therapeutic intervention. Therefore, novel drugs for various neurological disorders are in desperate need. Recently, compelling evidence has demonstrated that chemokine receptor CXCR3, which is a G protein-coupled receptor in the CXC chemokine receptor family, may play a pivotal role in the development of neurological diseases. The aim of this review is to provide evidence for the potential of CXCR3 as a therapeutic target for neurological diseases.

METHODS

English journal articles that focused on the invovlement of CXCR3 in neurological diseases were searched via PubMed up to May 2017. Moreover, reference lists from identified articles were included for overviews.

RESULTS

The expression level of CXCR3 in T cells was significantly elevated in several neurological diseases, including multiple sclerosis (MS), glioma, Alzheimer's disease (AD), chronic pain, human T-lymphotropic virus type 1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and bipolar disorder. CXCR3 antagonists showed therapeutic effects in these neurological diseases.

CONCLUSION

These studies provided hard evidence that CXCR3 plays a vital role in the pathogenesis of MS, glioma, AD, chronic pain, HAM/TSP and bipolar disorder. CXCR3 is a crucial molecule in neuroinflammatory and neurodegenerative diseases. It regulates the activation of infiltrating cells and resident immune cells. However, the exact functions of CXCR3 in neurological diseases are inconclusive. Thus, it is important to understand the topic of chemokines and the scope of their activity in neurological diseases.

Targeting the β2-integrin LFA-1, reduces adverse neuroimmune actions in neuropathic susceptibility caused by prenatal alcohol exposure

Recently, moderate prenatal alcohol exposure (PAE) was shown to be a risk factor for peripheral neuropathy following minor nerve injury. This effect coincides with elevated spinal cord astrocyte activation and ex vivo immune cell reactivity assessed by proinflammatory cytokine interleukin (IL) -1β protein expression. Additionally, the β2-integrin adhesion molecule, lymphocyte function-associated antigen-1 (LFA-1), a factor that influences the expression of the proinflammatory/anti-inflammatory cytokine network is upregulated. Here, we examine whether PAE increases the proinflammatory immune environment at specific anatomical sites critical in the pain pathway of chronic sciatic neuropathy; the damaged sciatic nerve (SCN), the dorsal root ganglia (DRG), and the spinal cord. Additionally, we examine whether inhibiting LFA-1 or IL-1β actions in the spinal cord (intrathecal; i.t., route) could alleviate chronic neuropathic pain and reduce spinal and DRG glial activation markers, proinflammatory cytokines, and elevate anti-inflammatory cytokines. Results show that blocking the actions of spinal LFA-1 using BIRT-377 abolishes allodynia in PAE rats with sciatic neuropathy (CCI) of a 10 or 28-day duration. This effect is observed (utilizing immunohistochemistry; IHC, with microscopy analysis and protein quantification) in parallel with reduced spinal glial activation, IL-1β and TNFα expression. DRG from PAE rats with neuropathy reveal significant increases in satellite glial activation and IL-1β, while IL-10 immunoreactivity is reduced by half in PAE rats under basal and neuropathic conditions. Further, blocking spinal IL-1β with i.t. IL-1RA transiently abolishes allodynia in PAE rats, suggesting that IL-1β is in part, necessary for the susceptibility of adult-onset peripheral neuropathy caused by PAE. Chemokine mRNA analyses from SCN, DRG and spinal cord reveal that increased CCL2 occurs following CCI injury regardless of PAE and BIRT-377 treatment. These data demonstrate that PAE creates dysregulated proinflammatory IL-1β and TNFα /IL-10 responses to minor injury in the sciatic-DRG-spinal pain pathway. PAE creates a risk for developing peripheral neuropathies, and LFA-1 may be a novel therapeutic target for controlling dysregulated neuroimmune actions as a consequence of PAE.