Gene Expression Profiling of Cutaneous Injured and Non-Injured Nociceptors in SNI Animal Model of Neuropathic Pain

TFAP2A is involved in neuropathic pain by regulating Grin1 expression in glial cells of the dorsal root ganglion

Neuropathic pain is a highly prevalent and refractory condition, yet its mechanism remains poorly understood. While NR1, the essential subunit of NMDA receptors, has long been recognized for its pivotal role in nociceptive transmission, its involvement in presynaptic stimulation is incompletely elucidated. Transcription factors can regulate the expression of both pro-nociceptive and analgesic factors. Our study shows that transcription factor TFAP2A was up-regulated in the dorsal root ganglion (DRG) neurons, satellite glial cells (SGCs), and Schwann cells following spinal nerve ligation (SNL). Intrathecal injection of siRNA targeting Tfap2a immediately or 7 days after SNL effectively alleviated SNL-induced pain hypersensitivity and reduced Tfap2a expression levels. Bioinformatics analysis revealed that TFAP2A may regulate the expression of the Grin1 gene, which encodes NR1. Dual-luciferase reporter assays confirmed TFAP2A's positive regulation of Grin1 expression. Notably, both Tfap2a and Grin1 were expressed in the primary SGCs and upregulated by lipopolysaccharides. The expression of Grin1 was also down-regulated in the DRG following Tfap2a knockdown. Furthermore, intrathecal injection of siRNA targeting Grin1 immediately or 7 days post-SNL effectively alleviated SNL-induced mechanical allodynia and thermal hyperalgesia. Finally, intrathecal Tfap2a siRNA alleviated SNL-induced neuronal hypersensitivity, and incubation of primary SGCs with Tfap2a siRNA decreased NMDA-induced upregulation of proinflammatory cytokines. Collectively, our study reveals the role of TFAP2A-Grin1 in regulating neuropathic pain in peripheral glia, offering a new strategy for the development of novel analgesics.

Microglial STING activation alleviates nerve injury-induced neuropathic pain in male but not female mice

Microglia, resident immune cells in the central nervous system, play a role in neuroinflammation and the development of neuropathic pain. We found that the stimulator of interferon genes (STING) is predominantly expressed in spinal microglia and upregulated after peripheral nerve injury. However, mechanical allodynia, as a marker of neuropathic pain following peripheral nerve injury, did not require microglial STING expression. In contrast, STING activation by specific agonists (ADU-S100, 35 nmol) significantly alleviated neuropathic pain in male mice, but not female mice. STING activation in female mice leads to increase in proinflammatory cytokines that may counteract the analgesic effect of ADU-S100. Microglial STING expression and type I interferon-ß (IFN-ß) signaling were required for the analgesic effects of STING agonists in male mice. Mechanistically, downstream activation of TANK-binding kinase 1 (TBK1) and the production of IFN-ß, may partly account for the analgesic effect observed. These findings suggest that STING activation in spinal microglia could be a potential therapeutic intervention for neuropathic pain, particularly in males.

Spinal Caspase-6 Contributes to Intrathecal Morphine-induced Acute Itch and Contact Dermatitis-induced Chronic Itch Through Regulating the Phosphorylation of Protein Kinase Mζ in Mice

Patients receiving neuraxial treatment with morphine for pain relief often experience a distressing pruritus. Neuroinflammation-mediated plasticity of sensory synapses in the spinal cord is critical for the development of pain and itch. Caspase-6, as an intracellular cysteine protease, is capable of inducing central nociceptive sensitization through regulating synaptic transmission and plasticity. Given the tight interaction between protein kinase Mζ (PKMζ) and excitatory synaptic plasticity, this pre-clinical study investigates whether caspase-6 contributes to morphine-induced itch and chronic itch via PKMζ. Intrathecal morphine and contact dermatitis were used to cause pruritus in mice. Morphine antinociception, itch-induced scratching behaviors, spinal activity of caspase-6, and phosphorylation of PKMζ and ERK were examined. Caspase-6 inhibitor Z-VEID-FMK, exogenous caspase-6 and PKMζ inhibitor ZIP were utilized to reveal the mechanisms and prevention of itch. Herein, we report that morphine induces significant scratching behaviors, which is accompanied by an increase in spinal caspase-6 cleavage and PKMζ phosphorylation (but not expression). Intrathecal injection of Z-VEID-FMK drastically reduces morphine-induced scratch bouts and spinal phosphorylation of PKMζ, without abolishing morphine analgesia. Moreover, intrathecal strategies of ZIP dose-dependently reduce morphine-induced itch-like behaviors. Spinal phosphorylation of ERK following neuraxial morphine is down-regulated by ZIP therapy. Recombinant caspase-6 directly exhibits scratching behaviors and spinal phosphorylation of ERK, which is compensated by PKMζ inhibition. Also, spinal inhibition of caspase-6 and PKMζ reduces the generation and maintenance of dermatitis-induced chronic itch. Together, these findings demonstrate that spinal caspase-6 modulation of PKMζ phosphorylation is important in the development of morphine-induced itch and dermatitis-induced itch in mice.

Immunohistochemical phenotype of sensory neurons associated with sympathetic plexuses in the trigeminal ganglia of adult nerve growth factor transgenic mice

Following peripheral nerve injury, postganglionic sympathetic axons sprout into the affected sensory ganglia and form perineuronal sympathetic plexuses with somata of sensory neurons. This sympathosensory coupling contributes to the onset and persistence of injury-induced chronic pain. We have documented the presence of similar sympathetic plexuses in the trigeminal ganglia of adult mice that ectopically overexpress nerve growth factor (NGF), in the absence of nerve injury. In this study, we sought to further define the phenotype(s) of these trigeminal sensory neurons having sympathetic plexuses in our transgenic mice. Using quantitative immunofluorescence staining analyses, we show that the invading sympathetic axons specifically target sensory somata immunopositive for several biomarkers: NGF high-affinity receptor tyrosine kinase A (trkA), calcitonin gene-related peptide (CGRP), neurofilament heavy chain (NFH), and P2X purinoceptor 3 (P2X3). Based on these phenotypic characteristics, the majority of the sensory somata surrounded by sympathetic plexuses are likely to be NGF-responsive nociceptors (i.e., trkA expressing) that are peptidergic (i.e., CGRP expressing), myelinated (i.e., NFH expressing), and ATP sensitive (i.e., P2X3 expressing). Our data also show that very few sympathetic plexuses surround sensory somata expressing other nociceptive (pain) biomarkers, including substance P and acid-sensing ion channel 3. No sympathetic plexuses are associated with sensory somata that display isolectin B4 binding. Though the cellular mechanisms that trigger the formation of sympathetic plexus (with and without nerve injury) remain unknown, our new observations yield an unexpected specificity with which invading sympathetic axons appear to target a precise subtype of nociceptors. This selectivity likely contributes to pain development and maintenance associated with sympathosensory coupling.

Peripheral Branch Injury Induces Oxytocin Receptor Expression at the Central Axon Terminals of Primary Sensory Neurons

Considerable evidence suggests that oxytocin, as a regulatory nonapeptide, participates in modulatory mechanisms of nociception. Nonetheless, the role of this hypothalamic hormone and its receptor in the sensory pathway has yet to be fully explored. The present study performed immunohistochemistry, enzyme-linked immunosorbent assay, and RT-qPCR analysis to assess changes in the expression of the neuronal oxytocin receptor in female rats following tight ligation of the sciatic nerve after 1, 3, and 7 days of survival. Oxytocin receptor immunoreactivity was present in both dorsal root ganglia and lumbar spinal cord segments, but not accumulated at the site of the ligation of the peripheral nerve branch. We found a time-dependent change in the expression of oxytocin receptor mRNA in L5 dorsal root ganglion neurons, as well as an increase in the level of the receptor protein in the lumbar segment of the spinal cord. A peak in the expression was observed on day 3, which downturned slightly by day 7 after the nerve ligation. These results show that OTR expression is up-regulated in response to peripheral nerve lesions. We assume that the importance of OTR is to modify spinal presynaptic inputs of the sensory neurons upon injury-induced activation, thus to be targets of the descending oxytocinergic neurons from supraspinal levels. The findings of this study support the concept that oxytocin plays a role in somatosensory transmission.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Somatosensory neuron types and their neural networks as revealed via single-cell transcriptomics

Single-cell RNA sequencing (scRNA-seq) has allowed profiling cell types of the dorsal root ganglia (DRG) and their transcriptional states in physiology and chronic pain. However, the evaluation criteria used in previous studies to classify DRG neurons varied, which presents difficulties in determining the various types of DRG neurons. In this review, we aim to integrate findings from previous transcriptomic studies of the DRG. We first briefly introduce the history of DRG-neuron cell-type profiling, and discuss the advantages and disadvantages of different scRNA-seq methods. We then examine the classification of DRG neurons based on single-cell profiling under physiological and pathological conditions. Finally, we propose further studies on the somatosensory system at the molecular, cellular, and neural network levels.

Iron Metabolism and Ferroptosis in Peripheral Nerve Injury

Peripheral nerve injury (PNI) is a major clinical problem that may lead to different levels of sensory and motor dysfunction including paralysis. Due to the high disability rate and unsatisfactory prognosis, the exploration and revealment of the mechanisms involved in the PNI are urgently required. Ferroptosis, a recently identified novel form of cell death, is an iron-dependent process. It is a unique modality of cell death, closely associated with iron concentrations, generation of reactive oxygen species, and accumulation of the lipid reactive oxygen species. These processes are regulated by multiple cellular metabolic pathways, including iron overloading, lipid peroxidation, and the glutathione/glutathione peroxidase 4 pathway. Furthermore, ferroptosis is accompanied by morphological changes in the mitochondria, such as increased membrane density and shrunken mitochondria; this association between ferroptosis and mitochondrial damage has been detected in various diseases, including spinal cord injury and PNI. The inhibition of ferroptosis can promote the repair of damaged peripheral nerves, reduce mitochondrial damage, and promote the recovery of neurological function. In this review, we intend to discuss the detailed mechanisms of ferroptosis and summarize the current researches on ferroptosis with respect to nerve injury. This review also aims at providing new insights on targeting ferroptosis for PNI treatment.

Integrated analyses reveal evolutionarily conserved and specific injury response genes in dorsal root ganglion

Rodent dorsal root ganglion (DRG) is widely used for studying axonal injury. Extensive studies have explored genome-wide profiles on rodent DRGs under peripheral nerve insults. However, systematic integration and exploration of these data still be limited. Herein, we re-analyzed 21 RNA-seq datasets and presented a web-based resource (DRGProfile). We identified 53 evolutionarily conserved injury response genes, including well-known injury genes (Atf3, Npy and Gal) and less-studied transcriptional factors (Arid5a, Csrnp1, Zfp367). Notably, we identified species-preference injury response candidates (e.g. Gpr151, Lipn, Anxa10 in mice; Crisp3, Csrp3, Vip, Hamp in rats). Temporal profile analysis reveals expression patterns of genes related to pre-regenerative and regenerating states. Finally, we found a large sex difference in response to sciatic nerve injury, and identified four male-specific markers (Uty, Eif2s3y, Kdm5d, Ddx3y) expressed in DRG. Our study provides a comprehensive integrated landscape for expression change in DRG upon injury which will greatly contribute to the neuroscience community.

Article Type: Original Article Title: Linalyl Acetate Ameliorates Mechanical Hyperalgesia Through Suppressing Inflammation by TSLP/IL-33 Signaling

Neuropathic pain is a debilitating chronic disorder, significantly causing personal and social burdens, in which activated neuroinflammation is one major contributor. Thymic stromal lymphopoietin (TSLP) and interleukin (IL)-33 is important for chronic inflammation. Linalyl acetate (LA) is main component of lavender oil with an anti-inflammatory property through TSLP signaling. The aim of the study is to investigate how LA regulates mechanical hyperalgesia after sciatic nerve injury (SNI). Adult Sprague-Dawley male rats were separated into 3 groups: control group, SNI group and SNI with LA group. LA was administrated intraperitoneally one day before SNI. Pain behavior test was evaluated through calibration forceps testing. Ipsilateral sciatic nerves (SNs), dorsal root ganglions (DRGs) and spinal cord were collected for immunofluorescence staining and Western blotting analyses. SNI rats were more sensitive to hyperalgesia response to mechanical stimulus since operation, which was accompanied by spinal cord glial cells reactions and DRG neuro-glial interaction. LA could relieve the pain sensation, proinflammatory cytokines and decrease the expression of TSLP/TSLPR complex. Also, LA could reduce inflammation through reducing IL-33 signaling. This study is the first to indicate that LA can modulate pain through TSLP/TSLPR and IL-33 signaling after nerve injury.

Chronic Pain after Bone Fracture: Current Insights into Molecular Mechanisms and Therapeutic Strategies

Bone fracture following traumatic injury or due to osteoporosis is characterized by severe pain and motor impairment and is a major cause of global mortality and disability. Fracture pain often originates from mechanical distortion of somatosensory nerve terminals innervating bones and muscles and is maintained by central sensitization. Chronic fracture pain (CFP) after orthopedic repairs is considered one of the most critical contributors to interference with the physical rehabilitation and musculoskeletal functional recovery. Analgesics available for CFP in clinics not only have poor curative potency but also have considerable side effects; therefore, it is important to further explore the pathogenesis of CFP and identify safe and effective therapies. The typical physiopathological characteristics of CFP are a neuroinflammatory response and excitatory synaptic plasticity, but the specific molecular mechanisms involved remain poorly elucidated. Recent progress has deepened our understanding of the emerging properties of chemokine production, proinflammatory mediator secretion, caspase activation, neurotransmitter release, and neuron-glia interaction in initiating and sustaining synaptogenesis, synaptic strength, and signal transduction in central pain sensitization, indicating the possibility of targeting neuroinflammation to prevent and treat CFP. This review summarizes current literature on the excitatory synaptic plasticity, microgliosis, and microglial activation-associated signaling molecules and discusses the unconventional modulation of caspases and stimulator of interferon genes (STING) in the pathophysiology of CFP. We also review the mechanisms of action of analgesics in the clinic and their side effects as well as promising therapeutic candidates (e.g., specialized pro-resolving mediators, a caspase-6 inhibitor, and a STING agonist) for pain relief by the attenuation of neuroinflammation with the aim of better managing patients undergoing CFP in the clinical setting.

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

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

Sex-Specific Transcriptomic Signatures in Brain Regions Critical for Neuropathic Pain-Induced Depression

Neuropathic pain is a chronic debilitating condition with a high comorbidity with depression. Clinical reports and animal studies have suggested that both the medial prefrontal cortex (mPFC) and the anterior cingulate cortex (ACC) are critically implicated in regulating the affective symptoms of neuropathic pain. Neuropathic pain induces differential long-term structural, functional, and biochemical changes in both regions, which are thought to be regulated by multiple waves of gene transcription. However, the differences in the transcriptomic profiles changed by neuropathic pain between these regions are largely unknown. Furthermore, women are more susceptible to pain and depression than men. The molecular mechanisms underlying this sexual dimorphism remain to be explored. Here, we performed RNA sequencing and analyzed the transcriptomic profiles of the mPFC and ACC of female and male mice at 2 weeks after spared nerve injury (SNI), an early time point when the mice began to show mild depressive symptoms. Our results showed that the SNI-induced transcriptomic changes in female and male mice were largely distinct. Interestingly, the female mice exhibited more robust transcriptomic changes in the ACC than male, whereas the opposite pattern occurred in the mPFC. Cell type enrichment analyses revealed that the differentially expressed genes involved genes enriched in neurons, various types of glia and endothelial cells. We further performed gene set enrichment analysis (GSEA), which revealed significant de-enrichment of myelin sheath development in both female and male mPFC after SNI. In the female ACC, gene sets for synaptic organization were enriched, and gene sets for extracellular matrix were de-enriched after SNI, while such signatures were absent in male ACC. Collectively, these findings revealed region-specific and sexual dimorphism at the transcriptional levels induced by neuropathic pain, and provided novel therapeutic targets for chronic pain and its associated affective disorders.

Systematic analysis of inflammation and pain pathways in a mouse model of gout

Gout is a prevalent and painful inflammatory arthritis, and its global burden continues to rise. Intense pain induced by gout attacks is a major complication of gout. However, systematic studies of gout inflammation and pain are lacking. Using a monosodium urate (MSU) crystal-induced gout model, we performed genome-wide transcriptome analysis of the inflamed ankle joint, dorsal root ganglion (DRG), and spinal cord of gouty mice. Our results revealed important transcriptional changes, including highly elevated inflammation and broad activation of immune pathways in both the joint and the nervous system, in gouty mice. Integrated analysis showed that there was a remarkable overlap between our RNAseq and human genome-wide association study (GWAS) of gout; for example, the risk gene, stanniocalcin-1 (STC1) showed significant upregulation in all three tissues. Interestingly, when compared to the transcriptomes of human osteoarthritis (OA) and rheumatoid arthritis (RA) joint tissues, we identified significant upregulation of cAMP/cyclic nucleotide-mediated signaling shared between gouty mice and human OA with high knee pain, which may provide excellent drug targets to relieve gout pain. Furthermore, we investigated the common and distinct transcriptomic features of gouty, inflammatory pain, and neuropathic pain mouse models in their DRG and spinal cord tissues. Moreover, we discovered distinct sets of genes with significant differential alternative splicing or differential transcript usage in each tissue, which were largely not detected by conventional differential gene expression analysis approaches. Based on these results, our study provided a more accurate and comprehensive depiction of transcriptomic alterations related to gout inflammation and pain.

Genetic and functional evidence for gp130/IL6ST-induced transient receptor potential ankyrin 1 upregulation in uninjured but not injured neurons in a mouse model of neuropathic pain

ABSTRACT

Peripheral nerve injuries result in pronounced alterations in dorsal root ganglia, which can lead to the development of neuropathic pain. Although the polymodal mechanosensitive transient receptor potential ankyrin 1 (TRPA1) ion channel is emerging as a relevant target for potential analgesic therapies, preclinical studies do not provide unequivocal mechanistic insight into its relevance for neuropathic pain pathogenesis. By using a transgenic mouse model with a conditional depletion of the interleukin-6 (IL-6) signal transducer gp130 in Nav1.8 expressing neurons (SNS-gp130-/-), we provide a mechanistic regulatory link between IL-6/gp130 and TRPA1 in the spared nerve injury (SNI) model. Spared nerve injury mice developed profound mechanical hypersensitivity as indicated by decreased withdrawal thresholds in the von Frey behavioral test in vivo, as well as a significant increase in mechanosensitivity of unmyelinated nociceptive primary afferents in ex vivo skin-nerve recordings. In contrast to wild type and control gp130fl/fl animals, SNS-gp130-/- mice did not develop mechanical hypersensitivity after SNI and exhibited low levels of Trpa1 mRNA in sensory neurons, which were partially restored by adenoviral gp130 re-expression in vitro. Importantly, uninjured but not injured neurons developed increased responsiveness to the TRPA1 agonist cinnamaldehyde, and neurons derived from SNS-gp130-/- mice after SNI were significantly less responsive to cinnamaldehyde. Our study shows for the first time that TRPA1 upregulation is attributed specifically to uninjured neurons in the SNI model, and this depended on the IL-6 signal transducer gp130. We provide a solution to the enigma of TRPA1 regulation after nerve injury and stress its significance as an important target for neuropathic pain disorders.

Proteome and Network Analysis Provides Novel Insights Into Developing and Established Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating side-effect of cancer therapies. So far, the development of CIPN cannot be prevented, neither can established CIPN be reverted, often leading to the cessation of necessary chemotherapy. Thus, there is an urgent need to explore the mechanistic basis of CIPN to facilitate its treatment. Here we used an integrated approach of quantitative proteome profiling and network analysis in a clinically relevant rat model of paclitaxel-induced peripheral neuropathy. We analysed lumbar rat DRG at two critical time points: (1) day 7, right after cessation of paclitaxel treatment, but prior to neuropathy development (pre-CIPN); (2) 4 weeks after paclitaxel initiation, when neuropathy has developed (peak-CIPN). In this way we identified a differential protein signature, which shows how changes in the proteome correlate with the development and maintenance of CIPN, respectively. Extensive biological pathway and network analysis reveals that, at pre-CIPN, regulated proteins are prominently implicated in mitochondrial (dys)function, immune signalling, neuronal damage/regeneration, and neuronal transcription. Orthogonal validation in an independent rat cohort confirmed the increase of β-catenin (CTNNB1) at pre-CIPN. More importantly, detailed analysis of protein networks associated with β-catenin highlights translationally relevant and potentially druggable targets. Overall, this study demonstrates the enormous value of combining animal behaviour with proteome and network analysis to provide unprecedented insights into the molecular basis of CIPN. In line with emerging approaches of network medicine our results highlight new avenues for developing improved therapeutic options aimed at preventing and treating CIPN.

Evaluation of Associated Genes with Traumatic Pain: A Systematic Review

OBJECTIVES

The knowledge about the molecular pathway of traumatic pain relief is less documented. This systematic review study aimed to identify the genes and molecular pathways associated with various traumatic pains.

METHODS

The online databases such as EMBASE, MEDLINE, PubMed, Cochrane Library, International Clinical Trials Registry Platform, Clinical Trials, Google Scholar, Wiley, ISI Web of Knowledge, and Scopus were searched. Two review authors searched and screened all records' titles and abstracts, and the third expert reviewer author resolved their disagreement. The study's design, various trauma injuries, types of genes, and molecular pathways were recorded. The genes and molecular pathways data were obtained via GeneCards®: The Human Gene Database (https://www.genecards.org).

RESULTS

Studies on a variety of trauma injuries regarding nerve and Spinal Cord Injuries (SCIs) (12 records), Hypertrophic scar with Severe Pain (one record), severe post-traumatic musculoskeletal pain (MSP) (one record), and orthopedic trauma (one record) were included. The main molecular pathways such as the immune system, apoptosis, and death receptor signaling, T-cell antigen receptor (TCR) signaling pathway, oxidative stress, interleukin(s) mediated signaling pathway, biological oxidations, metabolic pathways (especially amino acid metabolism and amino group), focal adhesion, the proliferation of vascular, epithelial, and connective tissue cells, angiogenesis and neural development were identified.

CONCLUSION

The immune system, apoptosis, and metabolic pathways are crucial for understanding the roles of genes in traumatic pain. It is recommended that these identified pathways and related genes be considered therapeutical targets for pain management in patients with trauma injuries. In addition, different forms of trauma injuries require different pathways and related genes to be considered.

The Tunicate Metabolite 2-(3,5-Diiodo-4-methoxyphenyl)ethan-1-amine Targets Ion Channels of Vertebrate Sensory Neurons

Marine tunicates produce defensive amino-acid-derived metabolites, including 2-(3,5-diiodo-4-methoxyphenyl)ethan-1-amine (DIMTA), but their mechanisms of action are rarely known. Using an assay-guided approach, we found that out of the many different sensory cells in the mouse dorsal root ganglion (DRG), DIMTA selectively affected low-threshold cold thermosensors. Whole-cell electrophysiology experiments using DRG cells, channels expressed in Xenopus oocytes, and human cell lines revealed that DIMTA blocks several potassium channels, reducing the magnitude of the afterhyperpolarization and increasing the baseline intracellular calcium concentration [Ca²⁺]_i of low-threshold cold thermosensors. When injected into mice, DIMTA increased the threshold of cold sensation by >3 °C. DIMTA may thus serve as a lead in the further design of compounds that inhibit problems in the cold-sensory system, such as cold allodynia and other neuropathic pain conditions.

MicroRNA-124-3p attenuates the development of nerve injury-induced neuropathic pain by targeting early growth response 1 in the dorsal root ganglia and spinal dorsal horn

Emerging evidence indicates the early growth response 1 (Egr1) plays an important role in the pathogenesis of chronic pain. However, the regulation of Egr1 expression in the DRG and spinal cord in neuropathic pain remains unclear. In the current study, the neuropathic pain was conducted by lumber 5 spinal nerve ligation (SNL) in rats. The role of miR-124-3p in Egr1 expression was examined. Our results showed that the SNL led to a significant increase in the expression of Egr1 mRNA and protein in the DRG and dorsal horn. This increased expression of Egr1 correlated with a reduction of miR-124-3p in the same region. Prior i.t. injection of Egr1 decoy AYX1 inhibited the expression of Egr1 and attenuated the neuropathic pain-like hypersensitivity following SNL. The dual-luciferase reporter assay revealed the luciferase activity of the Egr1 3'-UTR plasmid was inhibited by the miR-124-3p agomir. But this inhibition was completely reversed in the mutant 3'-UTR Egr1 group. In vivo, the SNL-induced behavioral signs of neuropathic pain and the increases in Egr1 mRNA and protein in the DRG and dorsal horn were prevented by prior to i.t. injection of miR-124-3p agomir. While, i.t. injection of miR-124-3p antagomir in naïve rats resulted in mechanical allodynia and thermal hyperalgesia and an overexpression of Egr1 in the DRG and dorsal horn. Together, our results suggest that the miR-124-3p-regulated Egr1 expression in the DRG and dorsal horn contributes to the development of neuropathic pain. Targeting miR-124-3p might be a promising therapeutic strategy in the treatment of chronic pain.

Nox4-dependent upregulation of S100A4 after peripheral nerve injury modulates neuropathic pain processing

Previous studies suggested that reactive oxygen species (ROS) produced by NADPH oxidase 4 (Nox4) affect the processing of neuropathic pain. However, mechanisms underlying Nox4-dependent pain signaling are incompletely understood. In this study, we aimed to identify novel Nox4 downstream interactors in the nociceptive system. Mice lacking Nox4 specifically in sensory neurons were generated by crossing Advillin-Cre mice with Nox4^fl/fl mice. Tissue-specific deletion of Nox4 in sensory neurons considerably reduced mechanical hypersensitivity and neuronal action potential firing after peripheral nerve injury. Using a proteomic approach, we detected various proteins that are regulated in a Nox4-dependent manner after injury, including the small calcium-binding protein S100A4. Immunofluorescence staining and Western blot experiments confirmed that S100A4 expression is massively up-regulated in peripheral nerves and dorsal root ganglia after injury. Furthermore, mice lacking S100A4 showed increased mechanical hypersensitivity after peripheral nerve injury and after delivery of a ROS donor. Our findings suggest that S100A4 expression is up-regulated after peripheral nerve injury in a Nox4-dependent manner and that deletion of S100A4 leads to an increased neuropathic pain hypersensitivity.

Spinal caspase-6 regulates AMPA receptor trafficking and dendritic spine plasticity through netrin-1 in postoperative pain after orthopedic surgery for tibial fracture in mice

Chronic postoperative pain hinders functional recovery after bone fracture and orthopedic surgery. Recently reported evidence indicates that caspase-6 is important in excitatory synaptic plasticity and pathological pain. Meanwhile, netrin-1 controls postsynaptic recruitment of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and synaptogenesis. The present work aimed to examine whether caspase-6 and netrin-1 contribute to fracture-induced postoperative allodynia. A mouse model of tibial fracture by intramedullary pinning was generated for inducing postoperative pain. Then, paw withdrawal threshold, spinal caspase-6 activity, netrin-1 secretion, AMPAR trafficking, and spine morphology were examined. Caspase-6 inhibition and netrin-1 knockdown by shRNA were performed to elucidate the pathogenetic mechanism of allodynia and its prevention. Whole-cell patch-clamp recording was performed to assess caspase-6's function in spinal AMPAR-induced current. Tibial fractures after orthopedic operation initiated persistent postsurgical mechanical and cold allodynia, accompanied by increased spinal active caspase-6, netrin-1 release, GluA1-containing AMPAR trafficking, spine density, and AMPAR-induced current in dorsal horn neurons. Caspase-6 inhibition reduced fracture-associated allodynia, netrin-1 secretion, and GluA1 trafficking. Netrin-1 deficiency impaired fracture-caused allodynia, postsynaptic GluA1 recruitment, and spine plasticity. The specific GluA2-lacking AMPAR antagonist NASPM also dose dependently prevented postoperative pain. The reduction of fracture-mediated postoperative excitatory synaptic AMPAR current in the dorsal horn by caspase-6 inhibition was compromised by recombinant netrin-1. Exogenous caspase-6 induced pain hypersensitivity, reversing by netrin-1 knockdown or coapplication of NASPM. Thus, spinal caspase-6 modulation of GluA1-containing AMPAR activation and spine morphology through netrin-1 secretion is important in the development of fracture-related postsurgical pain in the mouse.

Activation of the regeneration-associated gene STAT3 and functional changes in intact nociceptors after peripheral nerve damage in mice

BACKGROUND

In the context of neuropathic pain, the contribution of regeneration to the development of positive symptoms is not completely understood. Several efforts have been done to described changes in axotomized neurons, however, there is scarce data on changes occurring in intact neurons, despite experimental evidence of functional changes. To address this issue, we analysed by immunohistochemistry the presence of phosphorylated signal transducer and activator of transcription 3 (pSTAT3), an accepted marker of regeneration, within DRGs where axotomized neurons were retrogradely labelled following peripheral nerve injury. Likewise, we have characterized abnormal electrophysiological properties in intact fibres after partial nerve injury.

METHODS/RESULTS

We showed that induction of pSTAT3 in sensory neurons was similar after partial or total transection of the sciatic nerve and to the same extent within axotomized and non-axotomized neurons. We also examined pSTAT3 presence on non-peptidergic and peptidergic nociceptors. Whereas the percentage of neurons marked by IB4 decrease after injury, the proportion of CGRP neurons did not change, but its expression switched from small- to large-diameter neurons. Besides, the percentage of CGRP+ neurons expressing pSTAT3 increased significantly 2.5-folds after axotomy, preferentially in neurons with large diameters. Electrophysiological recordings showed that after nerve damage, most of the neurons with ectopic spontaneous activity (39/46) were non-axotomized C-fibres with functional receptive fields in the skin far beyond the site of damage.

CONCLUSIONS

Neuronal regeneration after nerve injury, likely triggered from the site of injury, may explain the abnormal functional properties gained by intact neurons, reinforcing their role in neuropathic pain.

SIGNIFICANCE

Positive symptoms in patients with peripheral neuropathies correlate to abnormal functioning of different subpopulations of primary afferents. Peripheral nerve damage triggers regenerating programs in the cell bodies of axotomized but also in non-axotomized nociceptors which is in turn, develop abnormal spontaneous and evoked discharges. Therefore, intact nociceptors have a significant role in the development of neuropathic pain due to their hyperexcitable peripheral terminals. Therapeutical targets should focus on inhibiting peripheral hyperexcitability in an attempt to limit peripheral and central sensitization.

Revisiting PNS Plasticity: How Uninjured Sensory Afferents Promote Neuropathic Pain

Despite the widespread study of how injured nerves contribute to chronic pain, there are still major gaps in our understanding of pain mechanisms. This is particularly true of pain resulting from nerve injury, or neuropathic pain, wherein tactile or thermal stimuli cause painful responses that are particularly difficult to treat with existing therapies. Curiously, this stimulus-driven pain relies upon intact, uninjured sensory neurons that transmit the signals that are ultimately sensed as painful. Studies that interrogate uninjured neurons in search of cell-specific mechanisms have shown that nerve injury alters intact, uninjured neurons resulting in an activity that drives stimulus-evoked pain. This review of neuropathic pain mechanisms summarizes cell-type-specific pathology of uninjured sensory neurons and the sensory ganglia that house their cell bodies. Uninjured neurons have demonstrated a wide range of molecular and neurophysiologic changes, many of which are distinct from those detected in injured neurons. These intriguing findings include expression of pain-associated molecules, neurophysiological changes that underlie increased excitability, and evidence that intercellular signaling within sensory ganglia alters uninjured neurons. In addition to well-supported findings, this review also discusses potential mechanisms that remain poorly understood in the context of nerve injury. This review highlights key questions that will advance our understanding of the plasticity of sensory neuron subpopulations and clarify the role of uninjured neurons in developing anti-pain therapies.

Transcriptional Reprogramming of Distinct Peripheral Sensory Neuron Subtypes after Axonal Injury

Primary somatosensory neurons are specialized to transmit specific types of sensory information through differences in cell size, myelination, and the expression of distinct receptors and ion channels, which together define their transcriptional and functional identity. By profiling sensory ganglia at single-cell resolution, we find that all somatosensory neuronal subtypes undergo a similar transcriptional response to peripheral nerve injury that both promotes axonal regeneration and suppresses cell identity. This transcriptional reprogramming, which is not observed in non-neuronal cells, resolves over a similar time course as target reinnervation and is associated with the restoration of original cell identity. Injury-induced transcriptional reprogramming requires ATF3, a transcription factor that is induced rapidly after injury and necessary for axonal regeneration and functional recovery. Our findings suggest that transcription factors induced early after peripheral nerve injury confer the cellular plasticity required for sensory neurons to transform into a regenerative state.

Pharmacological target-focused transcriptomic analysis of native vs cultured human and mouse dorsal root ganglia

Dorsal root ganglion (DRG) neurons detect sensory inputs and are crucial for pain processing. They are often studied in vitro as dissociated cell cultures with the assumption that this reasonably represents in vivo conditions. However, to the best of our knowledge, no study has directly compared genome-wide transcriptomes of DRG tissue in vivo versus in vitro or between laboratories and culturing protocols. Comparing RNA sequencing-based transcriptomes of native to cultured (4 days in vitro) human or mouse DRG, we found that the overall expression levels of many ion channels and G-protein-coupled receptors specifically expressed in neurons are markedly lower although still expressed in culture. This suggests that most pharmacological targets expressed in vivo are present under the condition of dissociated cell culture, but with changes in expression levels. The reduced relative expression for neuronal genes in human DRG cultures is likely accounted for by increased expression of genes in fibroblast-like and other proliferating cells, consistent with their mitotic status in these cultures. We found that the expression of a subset of genes typically expressed in neurons increased in human and mouse DRG cultures relative to the intact ganglion, including genes associated with nerve injury or inflammation in preclinical models such as BDNF, MMP9, GAL, and ATF3. We also found a striking upregulation of a number of inflammation-associated genes in DRG cultures, although many were different between mouse and human. Our findings suggest an injury-like phenotype in DRG cultures that has important implications for the use of this model system for pain drug discovery.

A preliminary study on DRGs and spinal cord of a galanin receptor 2-EGFP transgenic mouse

The neuropeptide galanin functions via three G-protein coupled receptors, Gal_1-3-R. Both Gal₁-R and ₂-R are involved in pain signaling at the spinal level. Here a Gal₂-R-EGFP transgenic (TG) mouse was generated and studied in pain tests and by characterizing Gal₂-R expression in both sensory ganglia and spinal cord. After peripheral spared nerve injury, mechanical allodynia developed and was ipsilaterally similar between wild type (WT) and TG mice. A Gal₂-R-EGFP-positive signal was primarily observed in small and medium-sized dorsal root ganglion (DRG) neurons and in spinal interneurons and processes. No significant difference in size distribution of DRG neuronal profiles was found between TG and WT mice. Both percentage and fluorescence intensity of Gal₂-R-EGFP-positive neuronal profiles were overall significantly upregulated in ipsilateral DRGs as compared to contralateral DRGs. There was an ipsilateral reduction in substance P-positive and calcitonin gene-related peptide (CGRP)-positive neuronal profiles, and this reduction was more pronounced in TG as compared to WT mice. Moreover, Gal₂-R-EGFP partly co-localized with three pain-related neuropeptides, CGRP, neuropeptide Y and galanin, both in intact and injured DRGs, and with galanin also in local neurons in the superficial dorsal horn. Taken together, the present results provide novel information on the localization and phenotype of DRG and spinal neurons expressing the second galanin receptor, Gal₂-R, and on phenotypic changes following peripheral nerve injury. Gal2-R may also be involved in autoreceptor signaling.

Mechanisms for Reducing Neuropathic Pain

Injury typically results in the development of neuropathic pain, but the pain normally decreases and disappears in paralleled with wound healing. The pain results from cells resident at, and recruited to, the injury site releasing pro-inflammatory cytokines and other mediators leading to the development of pro-inflammatory environment and causing nociceptive neurons to develop chronic ectopic electrical activity, which underlies neuropathic pain. The pain decreases as some of the cells that induce pro-inflammation, changing their phenotype leading to the blocking the release of pro-inflammatory mediators while releasing anti-inflammatory mediators, and blocking nociceptive neuron chronic spontaneous electrical activity. Often, despite apparent wound healing, the neuropathic pain becomes chronic. This raises the question of how chronic pain can be eliminated. While many of the cells and mediators contributing to the development and maintenance of neuropathic pain are known, a better understanding is required of how the injury site environment can be controlled to permanently eliminate the pro-inflammatory environment and silence the chronically electrically active nociceptive neurons. This paper examines how methods that can promote the transition of the pro-inflammatory injury site to an anti-inflammatory state, by changing the composition of local cell types, modifying the activity of pro- and anti-inflammatory receptors, inducing the release of anti-inflammatory mediators, and silencing the chronically electrically active nociceptive neurons. It also examines the hypothesis that factors released from platelet-rich plasma applied to chronic pain sites can permanently eliminate chronic inflammation and its associated chronic pain.

Transcriptional Profiling of Non-injured Nociceptors After Spinal Cord Injury Reveals Diverse Molecular Changes

Traumatic spinal cord injury (SCI) has devastating implications for patients, including a high predisposition for developing chronic pain distal to the site of injury. Chronic pain develops weeks to months after injury, consequently, patients are treated after irreparable changes have occurred. Nociceptors are central to chronic pain; however, the diversity of this cellular population presents challenges to understanding mechanisms and attributing pain modalities to specific cell types. To begin to address how peripheral sensory neurons below the injury level may contribute to the below-level pain reported by SCI patients, we examined SCI-induced changes in gene expression in lumbar dorsal root ganglia (DRG) below the site of injury. SCI was performed at the T10 vertebral level, with injury produced by a vessel clip with a closing pressure of 15 g for 1 min. Alterations in gene expression produce long-term sensory changes, therefore, we were interested in studying SCI-induced transcripts before the onset of chronic pain, which may trigger changes in downstream signaling pathways and ultimately facilitate the transmission of pain. To examine changes in the nociceptor subpopulation in DRG distal to the site of injury, we retrograde labeled sensory neurons projecting to the hairy hindpaw skin with fluorescent dye and collected the corresponding lumbar (L2–L6) DRG 4 days post-injury. Following dissociation, labeled neurons were purified by fluorescence-activated cell sorting (FACS). RNA was extracted from sorted sensory neurons of naïve, sham, or SCI mice and sequenced. Transcript abundances validated that the desired population of nociceptors were isolated. Cross-comparisons to data sets from similar studies confirmed, we were able to isolate our cells of interest and identify a unique pattern of gene expression within a subpopulation of neurons projecting to the hairy hindpaw skin. Differential gene expression analysis showed high expression levels and significant transcript changes 4 days post-injury in SCI cell populations relevant to the onset of chronic pain. Regulatory interrelationships predicted by pathway analysis implicated changes within the synaptogenesis signaling pathway as well as networks related to inflammatory signaling mechanisms, suggesting a role for synaptic plasticity and a correlation with pro-inflammatory signaling in the transition from acute to chronic pain.

Injury-Induced Effectors of Neuropathic Pain

Injuries typically result in the development of neuropathic pain, which decreases in parallel with wound healing. However, the pain may remain after the injury appears to have healed, which is generally associated with an ongoing underlying pro-inflammatory state. Injury induces many cells to release factors that contribute to the development of a pro-inflammatory state, which is considered an essential first step towards wound healing. However, pain elimination requires a transition of the injury site from pro- to anti-inflammatory. Therefore, developing techniques that eliminate chronic pain require an understanding of the cells resident at and recruited to injury sites, the factors they release, that promote a pro-inflammatory state, and promote the subsequent transition of that site to be anti-inflammatory. Although a relatively large number of cells, factors, and gene expression changes are involved in these processes, it may be possible to control a relatively small number of them leading to the reduction and elimination of chronic neuropathic pain. This first of two papers examines the roles of the most salient cells and mediators associated with the development and maintenance of chronic neuropathic pain. The following paper examines the cells and mediators involved in reducing and eliminating chronic neuropathic pain.

Optogenetic Inhibition of CGRPα Sensory Neurons Reveals Their Distinct Roles in Neuropathic and Incisional Pain

Cutaneous somatosensory neurons convey innocuous and noxious mechanical, thermal, and chemical stimuli from peripheral tissues to the CNS. Among these are nociceptive neurons that express calcitonin gene-related peptide-α (CGRPα). The role of peripheral CGRPα neurons (CANs) in acute and injury-induced pain has been studied using diphtheria toxin ablation, but their functional roles remain controversial. Because ablation permanently deletes a neuronal population, compensatory changes may ensue that mask the physiological or pathophysiological roles of CANs, particularly for injuries that occur after ablation. Therefore, we sought to define the role of intact CANs in vivo under baseline and injury conditions by using noninvasive transient optogenetic inhibition. We assessed pain behavior longitudinally from acute to chronic time points. We generated adult male and female mice that selectively express the outward rectifying proton pump archaerhodopsin-3 (Arch) in CANs, and inhibited their peripheral cutaneous terminals in models of neuropathic (spared nerve injury) and inflammatory (skin-muscle incision) pain using transdermal light activation of Arch. After nerve injury, brief activation of Arch reversed the chronic mechanical, cold, and heat hypersensitivity, alleviated the spontaneous pain, and reversed the sensitized mechanical currents in primary afferent somata. In contrast, Arch inhibition of CANs did not alter incision-induced hypersensitivity. Instead, incision-induced mechanical and heat hypersensitivity was alleviated by peripheral blockade of CGRPα peptide-receptor signaling. These results reveal that CANs have distinct roles in the time course of pain during neuropathic and incisional injuries and suggest that targeting peripheral CANs or CGRPα peptide-receptor signaling could selectively treat neuropathic or postoperative pain, respectively.SIGNIFICANCE STATEMENT The contribution of sensory afferent CGRPα neurons (CANs) to neuropathic and inflammatory pain is controversial. Here, we left CANs intact during neuropathic and perioperative incision injury by using transient transdermal optogenetic inhibition of CANs. We found that peripheral CANs are required for neuropathic mechanical, cold, and heat hypersensitivity, spontaneous pain, and sensitization of mechanical currents in afferent somata. However, they are dispensable for incisional pain transmission. In contrast, peripheral pharmacological inhibition of CGRPα peptide-receptor signaling alleviated the incisional mechanical and heat hypersensitivity, but had no effect on neuropathic pain. These results show that CANs have distinct roles in neuropathic and incisional pain and suggest that their targeting via novel peripheral treatments may selectively alleviate neuropathic versus incisional pain.

Microglia in Pain: Detrimental and Protective Roles in Pathogenesis and Resolution of Pain

The previous decade has seen a rapid increase in microglial studies on pain, with a unique focus on microgliosis in the spinal cord after nerve injury and neuropathic pain. Numerous signaling molecules are altered in microglia and contribute to the pathogenesis of pain. Here, we discuss how microglial signaling regulates spinal cord synaptic plasticity in acute and chronic pain conditions with different degrees and variations of microgliosis. We highlight that microglial mediators such as pro- and anti-inflammatory cytokines are powerful neuromodulators that regulate synaptic transmission and pain via neuron-glial interactions. We also reveal an emerging role of microglia in the resolution of pain, in part via specialized pro-resolving mediators including resolvins, protectins, and maresins. We also discuss a possible role of microglia in chronic itch.

Is Optogenetic Activation of Vglut1-Positive Aβ Low-Threshold Mechanoreceptors Sufficient to Induce Tactile Allodynia in Mice after Nerve Injury?

Mechanical allodynia is a cardinal feature of pathological pain. Recent work has demonstrated the necessity of Aβ-low-threshold mechanoreceptors (Aβ-LTMRs) for mechanical allodynia-like behaviors in mice, but it remains unclear whether these neurons are sufficient to produce pain under pathological conditions. We generated a transgenic mouse in which channelrhodopsin-2 (ChR2) is conditionally expressed in vesicular glutamate transporter 1 (Vglut1) sensory neurons (Vglut1-ChR2), which is a heterogeneous population of large-sized sensory neurons with features consistent with Aβ-LTMRs. In naive male Vglut1-ChR2 mice, transdermal hindpaw photostimulation evoked withdrawal behaviors in an intensity- and frequency-dependent manner, which were abolished by local anesthetic and selective A-fiber blockade. Surprisingly, male Vglut1-ChR2 mice did not show significant differences in light-evoked behaviors or real-time aversion after nerve injury despite marked hypersensitivity to punctate mechanical stimuli. We conclude that optogenetic activation of cutaneous Vglut1-ChR2 neurons alone is not sufficient to produce pain-like behaviors in neuropathic mice.SIGNIFICANCE STATEMENT Mechanical allodynia, in which innocuous touch is perceived as pain, is a common feature of pathological pain. To test the contribution of low-threshold mechanoreceptors (LTMRs) to nerve-injury-induced mechanical allodynia, we generated and characterized a new transgenic mouse (Vglut1-ChR2) to optogenetically activate cutaneous vesicular glutamate transporter 1 (Vglut1)-positive LTMRs. Using this mouse, we found that light-evoked behaviors were unchanged by nerve injury, which suggests that activation of Vglut1-positive LTMRs alone is not sufficient to produce pain. The Vglut1-ChR2 mouse will be broadly useful for the study of touch, pain, and itch.

Monoclonal Antibody Targeting the Matrix Metalloproteinase 9 Prevents and Reverses Paclitaxel-Induced Peripheral Neuropathy in Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a disabling condition accompanying several cancer drugs, including the front-line chemotherapeutic agent paclitaxel. Although CIPN can force dose reduction or even discontinuation of chemotherapy, affecting survival in cancer patients, there is no US Food and Drug Administration-approved treatment for CIPN. CIPN in mice is characterized by neuropathic pain (eg, mechanical allodynia) in association with oxidative stress and neuroinflammation in dorsal root ganglia (DRGs), as well as retraction of intraepidermal nerve fibers. Here, we report that paclitaxel-induced mechanical allodynia is associated with transcriptional increase in matrix metalloproteinase (MMP) 2 and 9 and decrease in metallopeptidase inhibitor 1 (TIMP1), a strong endogenous MMP9 inhibitor. Consistently, MMP9 protein levels are increased in DRG neurons in vivo and in vitro after paclitaxel treatment, and it is demonstrated, for the first time, that intrathecal injections of exogenous TIMP1 or a monoclonal antibody targeting MMP9 (MMP9 mAb) significantly prevented and reversed paclitaxel-induced mechanical allodynia in male and female mice. Analyses of DRG tissues showed that MMP9 mAb significantly decreased oxidative stress and neuroinflammatory mediators interleukin-6 and tumor necrosis factor α, as well as prevented paclitaxel-induced loss of intraepidermal nerve fibers. These findings suggest that MMP signaling plays a key role in paclitaxel-induced peripheral neuropathy, and MMP9 mAb may offer new therapeutic approaches for the treatment of CIPN. PERSPECTIVE: Chemotherapy-induced peripheral neuropathy (CIPN) remains ineffectively managed in cancer patients, potentially leading to the discontinuation of an otherwise life-saving treatment. Here, we demonstrate that a monoclonal antibody targeting MMP9 alleviates neuropathic pain and several mechanisms linked to CIPN. This study is particularly relevant, because a humanized MMP9 antibody is already in advanced clinical trials for the treatment of colitis and cancer, and it may be straightforwardly repurposed for the relief of CIPN.

Nociceptor Translational Profiling Reveals the Ragulator-Rag GTPase Complex as a Critical Generator of Neuropathic Pain

Nociceptors, sensory neurons in the DRG that detect damaging or potentially damaging stimuli, are key drivers of neuropathic pain. Injury to these neurons causes activation of translation regulation signaling, including the mechanistic target of rapamycin complex 1 (mTORC1) and mitogen-activated protein kinase interacting kinase (MNK) eukaryotic initiation factor (eIF) 4E pathways. This is a mechanism driving changes in excitability of nociceptors that is critical for the generation of chronic pain states; however, the mRNAs that are translated to lead to this plasticity have not been elucidated. To address this gap in knowledge, we used translating ribosome affinity purification in male and female mice to comprehensively characterize mRNA translation in Scn10a-positive nociceptors in chemotherapy-induced neuropathic pain (CIPN) caused by paclitaxel treatment. This unbiased method creates a new resource for the field, confirms many findings in the CIPN literature and also find extensive evidence for new target mechanisms that may cause CIPN. We provide evidence that an underlying mechanism of CIPN is sustained mTORC1 activation driven by MNK1-eIF4E signaling. RagA, a GTPase controlling mTORC1 activity, is identified as a novel target of MNK1-eIF4E signaling. This demonstrates a novel translation regulation signaling circuit wherein MNK1-eIF4E activity drives mTORC1 via control of RagA translation. CIPN and RagA translation are strongly attenuated by genetic ablation of eIF4E phosphorylation, MNK1 elimination or treatment with the MNK inhibitor eFT508. We identify a novel translational circuit for the genesis of neuropathic pain caused by chemotherapy with important implications for therapeutics.SIGNIFICANCE STATEMENT Neuropathic pain affects up to 10% of the population, but its underlying mechanisms are incompletely understood, leading to poor treatment outcomes. We used translating ribosome affinity purification technology to create a comprehensive translational profile of DRG nociceptors in naive mice and at the peak of neuropathic pain induced by paclitaxel treatment. We reveal new insight into how mechanistic target of rapamycin complex 1 is activated in neuropathic pain pointing to a key role of MNK1-eIF4E-mediated translation of a complex of mRNAs that control mechanistic target of rapamycin complex 1 signaling at the surface of the lysosome. We validate this finding using genetic and pharmacological techniques. Our work strongly suggests that MNK1-eIF4E signaling drives CIPN and that a drug in human clinical trials, eFT508, may be a new therapeutic for neuropathic pain.

Region-Resolved Quantitative Proteome Profiling Reveals Molecular Dynamics Associated With Chronic Pain in the PNS and Spinal Cord

To obtain a thorough understanding of chronic pain, large-scale molecular mapping of the pain axis at the protein level is necessary, but has not yet been achieved. We applied quantitative proteome profiling to build a comprehensive protein compendium of three regions of the pain neuraxis in mice: the sciatic nerve (SN), the dorsal root ganglia (DRG), and the spinal cord (SC). Furthermore, extensive bioinformatics analysis enabled us to reveal unique protein subsets which are specifically enriched in the peripheral nervous system (PNS) and SC. The immense value of these datasets for the scientific community is highlighted by validation experiments, where we monitored protein network dynamics during neuropathic pain. Here, we resolved profound region-specific differences and distinct changes of PNS-enriched proteins under pathological conditions. Overall, we provide a unique and validated systems biology proteome resource (summarized in our online database painproteome.em.mpg.de), which facilitates mechanistic insights into somatosensory biology and chronic pain—a prerequisite for the identification of novel therapeutic targets.

TRPV1 Agonist, Capsaicin, Induces Axon Outgrowth after Injury via Ca2+/PKA Signaling

Preconditioning nerve injuries activate a pro-regenerative program that enhances axon regeneration for most classes of sensory neurons. However, nociceptive sensory neurons and central nervous system neurons regenerate poorly. In hopes of identifying novel mechanisms that promote regeneration, we screened for drugs that mimicked the preconditioning response and identified a nociceptive ligand that activates a preconditioning-like response to promote axon outgrowth. We show that activating the ion channel TRPV1 with capsaicin induces axon outgrowth of cultured dorsal root ganglion (DRG) sensory neurons, and that this effect is blocked in TRPV1 knockout neurons. Regeneration occurs only in NF200-negative nociceptive neurons, consistent with a cell-autonomous mechanism. Moreover, we identify a signaling pathway in which TRPV1 activation leads to calcium influx and protein kinase A (PKA) activation to induce a preconditioning-like response. Finally, capsaicin administration to the mouse sciatic nerve activates a similar preconditioning-like response and induces enhanced axonal outgrowth, indicating that this pathway can be induced in vivo. These findings highlight the use of local ligands to induce regeneration and suggest that it may be possible to target selective neuronal populations for repair, including cell types that often fail to regenerate.