The increased prevalence of GluA2-lacking, Ca2+-permeable AMPA receptors (CP-AMPARs) at spinal cord sensory synapses amplifies nociceptive transmission and maintains chronic neuropathic pain. Nerve injury–induced upregulation of α2δ-1 disrupts the assembly of GluA1/GluA2 heteromers, favoring the synaptic incorporation of GluA1 homotetramers in the spinal dorsal horn. Although GluA1-GluA3 subunits are broadly expressed, whether α2δ-1 regulates GluA3-containing AMPARs remains unknown. Here, we unexpectedly found that coexpression with α2δ-1 — but not α2δ-2 or α2δ-3 — diminished GluA3 AMPAR currents and protein levels, an effect blocked by pregabalin, an α2δ-1 C-terminus peptide, or proteasome inhibition. Both nerve injury and α2δ-1 overexpression reduced protein levels of GluA3 and GluA2/GluA3 heteromers in the spinal cord. Furthermore, α2δ-1 coexpression or nerve injury increased GluA3 ubiquitination, with K861 at the C-terminus of GluA3 identified as a key ubiquitination site mediating α2δ-1–induced GluA3 degradation. Additionally, intrathecal delivery of the Gria3 gene reversed nerve injury–induced nociceptive hypersensitivity and synaptic CP-AMPARs by restoring protein levels of GluA3 and GluA2/GluA3 heteromers in the spinal cord. These findings reveal that α2δ-1 promotes GluA1 homotetramer assembly and synaptic CP-AMPAR expression by driving ubiquitin-proteasome–mediated degradation of GluA3, providing insights into the molecular mechanisms of neuropathic pain and the therapeutic actions of gabapentinoids.

Gaining a new footing on molecular mechanisms of neuropathic pain in patients.

Methyltransferase METTL3 regulates neuropathic pain through m6A methylation modification of SOCS1

Neuropathic pain, a prevalent chronic condition in clinical settings, has attracted widespread societal attention. This condition is characterized by a persistent pain state accompanied by affective and cognitive disruptions, significantly impacting patients' quality of life. However, current clinical therapies fall short of addressing its complexity. Thus, exploring the underlying molecular mechanism of neuropathic pain and identifying potential targets for intervention is highly warranted. The transient receptor potential (TRP) receptors, a class of widely distributed channel proteins, in the nervous system, play a crucial role in sensory signaling, cellular calcium regulation, and developmental influences. TRP ion channels are also responsible for various sensory responses including heat, cold, pain, and stress. This review highlights recent advances in understanding TRPs in various rodent models of neuropathic pain, aiming to uncover potential therapeutic targets for clinical management.

Establishing experimental models to study neuropathic pain has been challenging due to the complex mechanism underlying the condition. Although in vivo models have been useful in the observation of behavioural pain responses, it should be acknowledged that species-to-species variance can lead to differences in terms of molecular mechanism and genetic expression. The study of molecular and signal transduction of neuropathic pain using in vivo models faces limitations due to ethical considerations involving pain induction in animals and the intricacy of molecular interactions in the pathophysiology of the condition. Hence, developing relevant in vitro models to study neuropathic pain is important, as it considers the physiological microenvironment and reduces the use of experimental animals. Several considerations should be taken into account in developing an in vitro model of neuropathic pain, including the use of either primary culture of cell lines with considerations to their origins; human or animal, the method of neuropathic pain-like induction and the relevant assays to assess pain. This review recapitulates previous research employing in vitro models in investigating the molecular mechanism of neuropathic pain, intending to provide an alternative to the growing concerns on in vivo neuropathic pain models.

Investigating Post-transcriptional Mechanism of Neuropathic Pain

Investigating post-transcriptional mechanisms of neuropathic pain

TNF signal, ROS, and caspase-11 play important role in HIV gp120 with morphine-Induced Neuropathic Pain.

Neuropathic pain is directly developed from lesions or somatosensory nervous system diseases that are associated with emotion regulation. In general population, the incidence of neuropathic pain ranges from 7% to 10%, but the underlying mechanism remains largely unknown. Neuropathic pain is often associated with structural and functional abnormalities in multiple brain regions, and its regulation has been shown to correspond with the forebrain, including nucleus accumbens (NAc), medial prefrontal cortex (mPFC) and periaqueductal gray (PAG). To investigate the molecular mechanism of neuropathic pain across different brain regions, we identified the differentially expressed genes (DEGs) between the spared nerve injury model (SNI) mice suffering neuropathic pain and the control Sham mice in NAc, mPFC and PAG three brain regions, and mapped these genes onto a comprehensively functional association network. Thereafter, novel neuropathic pain genes in these three regions were identified using With Random Walk with Restart (RWR) analysis, such as Asic3, Cd200r1 and MT2, besides well-known Capn11 and CYP2E1. Interactions or cross talks among DEGs in NAc, mPFC and PAG three brain regions were discovered. Our results provide novel insights into neuropathic pain and help to explore therapeutic targets in the treatment.

Neuropathic pain is a common, debilitating clinical issue. Here, the weighted gene co-expression network analysis (WGCNA) was used to identify the specific modules and hub genes that are related to neuropathic pain. The microarray dataset of a neuropathic rat model induced by tibial nerve transection (TNT), including dorsal root ganglion (DRG) tissues from TNT model (n=7) and sham (n=8) rats, was downloaded from the ArrayExpress database (E-MTAB-2260). The co-expression network modules were identified by the WGCNA package. The protein–protein interaction (PPI) network was constructed, and the node with highest level of connectivity in the network were identified as the hub gene. A total of 1739 genes and seven modules were identified. The most significant module was the brown module, which contained 215 genes that were primarily associated with the biological process (BP) of the defense response and molecular function of calcium ion binding. Furthermore, C–C motif chemokine ligand 2 (Ccl2), Fos and tissue inhibitor of metalloproteinase 1 (Timp1) which were identified as the hub genes in the PPI network and two subnetworks separately. The in vivo studies validated that mRNA and protein levels of Ccl2, Fos and Timp1 were up-regulated in DRG and spinal cord tissues after TNT. The present study offers novel insights into the molecular mechanisms of neuropathic pain in the context of peripheral nerve injury.

For my Ph.D. research topic, I isolated endogenous morphine-like analgesic dipeptide, kyotorphin, which mediates Met-enkephalin release, and discovered kyotorphin synthetase, a putative receptor and antagonist. Furthermore, I succeeded in purifying μ-opioid receptor and functional reconstitution with purified G proteins. After receiving my full professor position at Nagasaki University in 1996, I worked on two topics of research, molecular mechanisms of chronic pain through lysophosphatidic acid (LPA) and identification and characterization of neuroprotective protein, prothymosin α. In a series of studies, we have shown that LPA signaling defines the molecular mechanisms of neuropathic pain and fibromyalgia in terms of development and maintenance. Above all, the discovery of feed-forward system in LPA production and pain memory may contribute to better understanding of chronic pain and future analgesic drug discovery. Regarding prothymosin α, we first discovered it as neuronal necrosis-inhibitory molecule through two independent mechanisms, such as toll-like receptor and F0/F1 ATPase, both which protect neurons through indirect mechanisms. Prothymosin α is released by non-classical and non-vesicular mechanisms on various stresses, such as ischemia, starvation, and heat-shock. Thus it may be called a new type of neuroprotective damage-associated molecular patterns (DAMPs)/Alarmins. Heterozygotic mice showed a defect in memory-learning and neurogenesis as well as anxiogenic behaviors. Small peptide, P6Q derived from prothymosin α retains neuroprotective actions, which include blockade of cerebral hemorrhage caused by late treatment with tissue plasminogen activator in the stroke model in mice.

Objective To explore the roles of zinc-fingers and homeoboxes (ZHX) in the dorsal root ganglia (DRG) in peripheral nerve injury-induced pain hypersensitivity, and provide a new idea for molecular mechanism of neuropathic pain (NP). Methods (1) Twenty-four 8-week-old male C57BL6 mice were randomly divided into sham-operated group and chronic constriction injury (CCI) of the sciatic nerve model group (n=12). One d before modeling and 7 d after modeling, the changes of paw withdrawal frequency (PWF) to mechanical stimuli and paw withdrawal latency (PWL) to thermal stimulation were detected. Seven d after modeling, reverse transcription real-time quantitative PCR (RT-qPCR) was used to detect the ZHX1, ZHX2, and ZHX3 mRNA expressions, and Western blotting was employed to detect the ZHX2 protein expression. (2) Thirty-six 8-week-old male C57BL6 mice were randomly divided into equivalent dose solvent CCI group, nonsense negative control sequence [siNC] CCI group and ZHX2 siRNA CCI group, and equivalent dose solvent sham-operated group, siNC sham-operated group and ZHX2 siRNA sham-operated group (n=6); each treatment was given to the DRG. One d before drug injection and 7 d after drug injection, the changes of PWF to mechanical stimuli and PWL to thermal stimulation were detected; RT-qPCR was used to detect the ZHX2 mRNA expression. Results (1) Seven d after modeling, PWF was significantly increased and PWL was statistically shorten in the hind-paw of the ipsilateral side in the CCI group as compared with those in the sham-operated group (P<0.05); DRG ZHX2 mRNA expression in the CCI group increased for 1.71 times as compared with that in the sham-operated group, with significant difference (P<0.05); DRG ZHX2 protein expression in the CCI group increased for 2.15 times as compared with that in the sham-operated group, with significant difference (P<0.05). (2) As compared with the equivalent dose solvent sham-operated group, siNC sham-operated group and ZHX2 siRNA sham-operated group, equivalent dose solvent CCI group and siNC CCI group had significantly increased PWF and DRG ZHX2 mRNA expression, and statistically shorten PWL (P<0.05); and ZHX2 siRNA CCI group had significantly decreased PWF of the ipsilateral side and DRG ZHX2 mRNA expression, and statistically longer PWL of the ipsilateral side as compared with equivalent dose solvent CCI group and siNC CCI group (P<0.05). Conclusion Knockdown periphery nerve injury-induced DRG ZHX2 up-regulation attenuates pain hypersensitivity following periphery nerve injury, and ZHX2 may be a new potential target to treat NP. Key words: Dorsal root ganglia; Zinc-fingers and homeoboxes 2; Nerve injury; Neuropathic pain

Human lysozyme is one of the most extensively studied proteins yet its exact physiological functions still remain a mystery. Here, we highlight a hitherto unrecognized role of lysozyme in neuropathic pain and neuronal excitability. Injection of lysozyme alone in healthy rats leads to heightened sensitivity in them towards both mechanical and thermal pain stimuli. Also, in rat models of neuropathic pain, lysozyme is over – expressed and its down – regulation by siRNA or inhibition of its activity by its disaccharide inhibitor ‐ chitobiose relieves pain confirming a causal role of lysozyme in neuropathic pain. Moreover, as the catalytically incompetent lysozyme or its chemically inhibited counterparts fail to provoke pain indicate that it deploys its catalytic site for eliciting pain. Bath application of lysozyme remarkably increased both the spontaneous excitatory potentials and currents (sEPSPs &amp; sEPSCs) and the amplitudes of afferent mediated evoked excitatory synaptic inputs (eFPSPs &amp; eEPSCs) in Aδ fibres in a fibre specific manner in the dorsal horn neurons. It also increased paired pulsed ratio only in the C‐fibres. When complexed with chitobiose it does not lead to any change in neuronal excitability. Thus, lysozyme excites neurons by deploying its catalytic region in a fibre specific manner. Further, we show that it deploys its catalytic site to recruit Annexin A2 to transduce TLR4→NFkβ signaling pathway which may culminate in the fusion of neurotransmitter vesicles at the synapse to manifest neuronal excitability and eventually pain. Thus, glycan inhibitors of lysozyme and antagonists of the pathway recruited by it to inflict pain pave not only the way for novel approaches for dissecting the molecular mechanism of neuropathic pain but also for treating this unmet medical need efficaciously.Support or Funding InformationCouncil of Scientific and Industrial Research, India to ASThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Neuropathic pain is a kind of pain caused by primary or secondary impairment or dysfunction of peripheral or central nervous system. Patients with neuropathic pain were often with poor clinical outcome. We screened the differentially expressed genes between sciatic nerve injury and dorsal root ganglion gene in the sham operation model. Microarray and the spared nerve injury module were used to explore the molecular mechanism of neuropathic pain by injuries and the differentially expressed genes (DEGs) were identified out. Besides, the bioinformatics methods were used to figure out the signaling pathways and expression regulation pattern these DEGs were enriched in, which may provide a basis for the molecular research and medicine target of therapy. Besides, protein-protein interaction network analysis was performed on these selected intersection genes. A total of 40 DEGs were screened out and only pctp gene was down-regulated, the left 39 genes were all up-regulated. Then, GO and KEGG enrichment analysis were performed on these intersection genes by DAVID software. Furthermore, protein-protein interaction network analysis was used to analyze the critical genes of neuropathic pain. Finally, four genes, that is, Jun, Gal, Cd74, and C1qb were identified to have strong interactions with other genes, which may function as the prognostic and predictive genes of neuropathic pain caused by peripheral injuries. Our results suggested that four differentially expressed genes, Jun, Gal, Cd74, and C1qb, had the potential to serve as prognostic or predictive markers for neuropathic pain, suggesting a potential application in the improvement of prognostic tools and treatments.

BackgroundMechanisms of neuropathic pain are still largely unknown. Molecular changes in spinal dorsal horn may contribute to the initiation and development of neuropathic pain. Circular RNAs (circRNAs) have been identified as microRNA sponges and involved in various biological processes, but whether their expression profile changes in neuropathic pain condition is not reported.MethodsTo test whether neuropathic pain influences circRNA expression, we developed a sciatic chronic constriction injury (CCI) model in rats. The CCI ipsilateral spinal dorsal horns of lumbar enlargement segments (L3–L5) were collected, and the total RNA was extracted and subjected to Arraystar Rat circRNA Microarray. Quantitative real-time polymerase chain reaction (qPCR) was used to confirm the circRNA expression profile. To estimate functions of differential circRNAs, bioinformatics analyses including gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes Pathway analyses were performed for the top 100 circRNAs and circRNA–microRNA networks were constructed for the top 10 circRNAs.ResultscircRNA microarrays showed that 469 circRNAs were differentially expressed between CCI and sham-operated rats (fold change ≥2). In all, 363 of them were significantly upregulated, and the other 106 were downregulated in the CCI group. Three of them (circRNA_013779, circRNA_008008, and circRNA_003724) overexpressed >10 times after CCI insult. Expression levels of eight circRNAs were verified using qPCR. GO analysis revealed that thousands of predicted target genes were involved in the biological processes, cellular component, and molecular function; in addition, dozens of these genes were enriched in the Hippo signaling pathway, MAPK signaling pathway, and so on. Competing endogenous RNAs analysis showed that circRNA_008008 and circRNA_013779 are the two largest nodes in the circRNA–microRNA interaction network of the top 10 circRNAs.ConclusionCCI resulted in a comprehensive expression profile of circRNAs in the spinal dorsal horn in rats. CircRNAs in the dorsal horn could be helpful to reveal molecular mechanisms of neuropathic pain.

Flow cytometry analysis of inflammatory cells isolated from the sciatic nerve and DRG after chronic constriction injury in mice

An Improved Rodent Model of Trigeminal Neuropathic Pain by Unilateral Chronic Constriction Injury of Distal Infraorbital Nerve.

BackgroundAnaphase-promoting complex/cyclosome (APC/C) and its co-activator Cdh1 are important ubiquitin-ligases in proliferating cells and terminally differentiated neurons. In recent years, APC/C-Cdh1 has been reported as an important complex contributing to synaptic development and transmission. Interestingly, cortical APC/C-Cdh1 is found to play a critical role in the maintenance of neuropathic pain, but it is not clear whether APC/C-Cdh1 in spinal dorsal cord is involved in molecular mechanisms of neuropathic pain conditions.ResultsImmunostaining showed that Cdh1 was mainly distributed in dorsal horn neurons of the spinal cord in rats. Its expression was downregulated in the ipsilateral dorsal horn at 14 days after spared nerve injury. Rescued expression of Cdh1 in spinal cord by intrathecal administration of recombinant lentivirus encoding Cdh1 (Lenti-Cdh1-GFP) significantly attenuated spared nerve injury-induced mechanical allodynia. Furthermore, rescued expression of spinal Cdh1 significantly reduced surface membrane expression of GluR1, but increased the expression of GluR1-related erythropoietin-producing human hepatocellular receptor A4 and its ligand EphrinA1 in dorsal horn of spared nerve injury-treated animals.ConclusionsThis study indicates that a downregulation of Cdh1 expression in spinal dorsal horn is involved in molecular mechanisms underlying the maintenance of neuropathic pain. Upregulation of spinal Cdh1 may be a promising approach to treat neuropathic pain.

Nerve injury or dysfunction in the peripheral and central nervous systems are the leading causes for the development of neuropathies, which are frequently associated with allodynia and hyperalgesia. Treatment of these disorders is often unsatisfactory due to side effects or insufficient analgesia of the currently available drugs. Therefore, elucidating the molecular mechanisms of neuropathic pain is an important prerequisite for the rational development of novel analgesic drugs for the therapy of neuropathic pain. Several proteomic approaches have been performed to explore protein modifications in the nervous system associated with neuropathies in different animal models, which might contribute to the detection of new drug targets. Furthermore, there are proteomic studies investigating human cerebrospinal fluid from patients suffering from neuropathies. The results of these studies and the potential clinical value of the proteomic data are summarized and discussed in this review.

Neuropathic pain is often caused by nerve injury or dysfunction in the peripheral and central nervous system and is frequently associated with allodynia and hyperalgesia. The underlying molecular mechanisms of neuropathic pain are largely unknown, and therefore, pharmacologic treatment is insufficient in many cases. To elucidate translational and posttranslational modifications in the nervous system that arise after nerve injury, a number of proteomic studies have been performed using different animal neuropathy models. The results of these proteomic approaches are summarized in this review to provide a better overview of proteins that are involved into the pathogenesis of nerve injury and neuropathic pain. This might allow a better understanding of the pathophysiologic signaling pathways in this impairment, facilitate the discovery of specific biomarkers, and thus promote the development of novel pain therapies.