ATF2, but not ATF3, participates in the maintenance of nerve injury-induced tactile allodynia and thermal hyperalgesia

Neuron-specific RNA-sequencing reveals different responses in peripheral neurons after nerve injury

Peripheral neurons are heterogeneous and functionally diverse, but all share the capability to switch to a pro-regenerative state after nerve injury. Despite the assumption that the injury response is similar among neuronal subtypes, functional recovery may differ. Understanding the distinct intrinsic regenerative properties between neurons may help to improve the quality of regeneration, prioritizing the growth of axon subpopulations to their targets. Here, we present a comparative analysis of regeneration across four key peripheral neuron populations: motoneurons, proprioceptors, cutaneous mechanoreceptors, and nociceptors. Using Cre/Ai9 mice that allow fluorescent labeling of neuronal subtypes, we found that nociceptors showed the greater regeneration after a sciatic crush, followed by motoneurons, mechanoreceptors, and, finally, proprioceptors. By breeding these Cre mice with Ribotag mice, we isolated specific translatomes and defined the regenerative response of these neuronal subtypes after axotomy. Only 20% of the regulated genes were common, revealing a diverse response to injury among neurons, which was also supported by the differential influence of neurotrophins among neuron subtypes. Among differentially regulated genes, we proposed MED12 as a specific regulator of the regeneration of proprioceptors. Altogether, we demonstrate that the intrinsic regenerative capacity differs between peripheral neuron subtypes, opening the door to selectively modulate these responses.

Spinal nerve ligation: An experimental model to study neuropathic pain in rats and mice

Neuropathic pain, defined as the most terrible of all tortures, which a nerve wound may inflict, is a common chronic painful condition caused by gradual damage or dysfunction of the somatosensory nervous system. As with many chronic diseases, neuropathic pain has a profound economic and emotional impact worldwide and represents a major public health issue from a treatment standpoint. This condition involves multiple sensory symptoms including impaired transmission and perception of noxious stimuli, burning, shooting, spontaneous pain, mechanical or thermal allodynia and hyperalgesia. Current pharmacological options for the treatment of neuropathic pain are limited, ineffective and have unacceptable side effects. In this framework, a deeper understanding of the pathophysiology and molecular mechanisms associated with neuropathic pain is key to the development of promising new therapeutical approaches. For this purpose, a plethora of experimental models that mimic common clinical features of human neuropathic pain have been characterized in rodents, with the spinal nerve ligation (SNL) model being one of the most widely used. In this chapter, we provide a detailed surgical procedure of the SNL model used to induce neuropathic pain in rats and mice. We further describe the behavioral approaches used for stimulus-evoked and spontaneous pain assessment in rodents. Finally, we demonstrate that our SNL model induces multiple pain behaviors in rats and mice.

Activation of α 6 -containing GABA A receptors induces antinociception under physiological and pathological conditions

ABSTRACT

The loss of GABAergic inhibition is a mechanism that underlies neuropathic pain. Therefore, rescuing the GABAergic inhibitory tone through the activation of GABA A receptors is a strategy to reduce neuropathic pain. This study was designed to elucidate the function of the spinal α 6 -containing GABA A receptor in physiological conditions and neuropathic pain in female and male rats. Results show that α 6 -containing GABA A receptor blockade or transient α 6 -containing GABA A receptor knockdown induces evoked hypersensitivity and spontaneous pain in naive female rats. The α 6 subunit is expressed in IB4 + and CGRP + primary afferent neurons in the rat spinal dorsal horn and dorsal root ganglia but not astrocytes. Nerve injury reduces α 6 subunit protein expression in the central terminals of the primary afferent neurons and dorsal root ganglia, whereas intrathecal administration of positive allosteric modulators of the α 6 -containing GABA A receptor reduces tactile allodynia and spontaneous nociceptive behaviors in female, but not male, neuropathic rats and mice. Overexpression of the spinal α 6 subunit reduces tactile allodynia and restores α 6 subunit expression in neuropathic rats. Positive allosteric modulators of the α 6 -containing GABA A receptor induces a greater antiallodynic effect in female rats and mice compared with male rats and mice. Finally, α 6 subunit is expressed in humans. This receptor is found in CGRP + and P2X3 + primary afferent fibers but not astrocytes in the human spinal dorsal horn. Our results suggest that the spinal α 6 -containing GABA A receptor has a sex-specific antinociceptive role in neuropathic pain, suggesting that this receptor may represent an interesting target to develop a novel treatment for neuropathic pain.

Proteomic Analysis of the Spinal Dorsal Horn in Mice with Neuropathic Pain After Exercise

Purpose

Neuropathic pain (NP) is a chronic pain state with a complex etiology that currently lacks effective treatment in clinical practice. Studies have found that exercise training can alleviate NP hyperalgesia, but the specific mechanism remains unclear. Here, we sought to identify proteins and signaling pathways critical for mediating the effects of treadmill training on NP in a mouse model of spared nerve injury (SNI).

Methods

We used Tandem Mass Tag (TMT) technology for proteins and signaling pathways identification. Functional enrichment analyses were conducted using DAVID and Metascape software. Ingenuity pathway analysis was used to conduct functional annotation and analyze alterations in canonical pathways and molecular networks. Reverse transcription quantitative PCR (RT-qPCR) was used to confirm the results of proteomics analysis.

Results

A total of 270 differentially expressed proteins were screened in the detrained and trained groups (P ≤0.05). Enrichment and ingenuity pathway analysis revealed the effects of treadmill training on autophagy, cAMP-mediated signaling, calcium signaling and NP signaling in dorsal horn nerves. Treadmill training reduced the expression of Akt3, Atf2, Gsk3b, Pik3c3, Ppp2ca, and Sqstm1, and increased the expression of Pik3cb in the autophagic pathway.

Conclusion

Our results suggest that treadmill training may alleviate nociceptive hyperalgesia in NP mice by modulating the autophagic pathway, providing unique mechanistic insights into the analgesic effects of exercise.

Bestrophin-1 Participates in Neuropathic Pain Induced by Spinal Nerve Transection but not Spinal Nerve Ligation

Previous studies have reported that L5/L6 spinal nerve ligation (SNL), but not L5 spinal nerve transection (SNT), enhances anoctamin-1 in injured and uninjured dorsal root ganglia (DRG) of rats suggesting some differences in function of the type of nerve injury. The role of bestrophin-1 in these conditions is unknown. The aim of this study was to investigate the role of bestrophin-1 in rats subjected to L5 SNT and L5/L6 SNL. SNT up-regulated bestrophin-1 protein expression in injured L5 and uninjured L4 DRG at day 7, whereas it enhanced GAP43 mainly in injured, but also in uninjured DRG. In contrast, SNL enhanced GAP43 at day 1 and 7, while bestrophin-1 expression increased only at day 1 after nerve injury. Accordingly, intrathecal injection of the bestrophin-1 blocker CaCC_inh-A01 (1-10 µg) reverted SNT- or SNL-induced tactile allodynia in a concentration-dependent manner. Intrathecal injection of CaCC_inh-A01 (10 µg) prevented SNT-induced upregulation of bestrophin-1 and GAP43 at day 7. In contrast, CaCC_inh-A01 did not affect SNL-induced up-regulation of GAP43 nor bestrophin-1. Bestrophin-1 was mainly expressed in small- and medium-size neurons in naïve rats, while SNT increased bestrophin-1 immunoreactivity in CGRP+, but not in IB4+ neuronal cells in DRG. Intrathecal injection of bestrophin-1 plasmid (pCMVBest) induced tactile allodynia and increased bestrophin-1 expression in DRG and spinal cord in naïve rats. CaCC_inh-A01 reversed bestrophin-1 overexpression-induced tactile allodynia and restored bestrophin-1 expression. Our data suggest that bestrophin-1 plays a relevant role in neuropathic pain induced by SNT, but not by SNL. PERSPECTIVE: SNT, but not SNL, up-regulates bestrophin-1 and GAP43 protein expression in injured L5 and uninjured L4 DRG. SNT increases bestrophin-1 immunoreactivity in CGRP+ neurons in DRG. Bestrophin-1 overexpression induces allodynia. CaCCinh-A01 reduces allodynia and restores bestrophin-1 expression. Our data suggest bestrophin-1 is differentially regulated depending on the neuropathic pain model.

The impact of tumor immunogenicity on cancer pain phenotype using syngeneic oral cancer mouse models

Head and neck squamous cell carcinoma (HNSCC) patients report severe function-induced pain at the site of the primary tumor. The current hypothesis is that oral cancer pain is initiated and maintained in the cancer microenvironment due to secretion of algogenic mediators from tumor cells and surrounding immune cells that sensitize the primary sensory neurons innervating the tumor. Immunogenicity, which is the ability to induce an adaptive immune response, has been widely studied using cancer cell transplantation experiments. However, oral cancer pain studies have primarily used xenograft transplant models in which human-derived tumor cells are inoculated in an athymic mouse lacking an adaptive immune response; the role of inflammation in oral cancer-induced nociception is still unknown. Using syngeneic oral cancer mouse models, we investigated the impact of tumor cell immunogenicity and growth on orofacial nociceptive behavior and oral cancer-induced sensory neuron plasticity. We found that an aggressive, weakly immunogenic mouse oral cancer cell line, MOC2, induced rapid orofacial nociceptive behavior in both male and female C57Bl/6 mice. Additionally, MOC2 tumor growth invoked a substantial injury response in the trigeminal ganglia as defined by a significant upregulation of injury response marker ATF3 in tongue-innervating trigeminal neurons. In contrast, using a highly immunogenic mouse oral cancer cell line, MOC1, we found a much slower onset of orofacial nociceptive behavior in female C57Bl/6 mice only as well as sex-specific differences in the tumor-associated immune landscape and gene regulation in tongue innervating sensory neurons. Together, these data suggest that cancer-induced nociceptive behavior and sensory neuron plasticity can greatly depend on the immunogenic phenotype of the cancer cell line and the associated immune response.

The glial cell's role in antinociceptive differential effects of oxytocin upon female and male rats

BACKGROUND

Sex plays a crucial role in pain processing and response to analgesic drugs. Indeed, spinal glia seems to be significant in the sexual dimorphism observed in the above effects. Recently, studies have associated oxytocin with antinociceptive effects, but these have been mainly performed in male animals; consequently, the influence of sex has been poorly explored.

METHODS

Using a model of spinal nociception that produces pain through activation of the spinal glia, that is, intrathecal (i.t.) lipopolysaccharide (LPS) injection, we analysed the changes in the analgesic response to i.t. oxytocin in female and male rats by behavioural (punctate mechanical hypersensitivity), electrophysiological (unitary extracellular recordings of wide dynamic range [WDR] cells) and molecular biology (real-time PCR of proinflammatory genes) experiments.

RESULTS

We found that LPS-induced hypersensitivity was longer in female (>96 h) than in male (≈4 h) rats. Besides, spinal oxytocin preferentially prevents the LPS-induced hypersensitivity in male rather than female rats. Indeed, LPS increases the spinal neuronal-evoked activity associated with the activation of peripheral Aδ- and C-fibres and post-discharge in males, whereas only C-fibre discharge was enhanced in females. The electrophysiological data correlate with the fact that spinal oxytocin only prevented TNF-α and IL-1β synthesis in male rats.

CONCLUSIONS

Therefore, these data suggest that oxytocin-mediated analgesia depends on a sexual dimorphism involving activation of the spinal glia. These results reinforced the idea that different strategies are required to treat pain in men and women, and that oxytocin could be used preferentially to treat pain with a significant inflammatory component in men.

SIGNIFICANCE STATEMENT

Oxytocin is a molecule that emerges as a potent analgesic in preclinical and clinical studies. We investigated the contribution of glia to the response of oxytocin-induced analgesia and how sex influences in this response show that different strategies are required to treat pain in men and women, and that oxytocin could be used preferentially to treat pain with a significant inflammatory component in men.

Spared Nerve Injury Causes Sexually Dimorphic Mechanical Allodynia and Differential Gene Expression in Spinal Cords and Dorsal Root Ganglia in Rats

Neuropathic pain is more prevalent in women. However, females are under-represented in animal experiments, and the mechanisms of sex differences remain inadequately understood. We used the spared nerve injury (SNI) model in rats to characterize sex differences in pain behaviour, unbiased RNA-Seq and proteomics to study the mechanisms. Male and female rats were subjected to SNI- and sham-surgery. Mechanical and cold allodynia were assessed. Ipsilateral lumbar dorsal root ganglia (DRG) and spinal cord (SC) segments were collected for RNA-seq analysis with DESeq2 on Day 7. Cerebrospinal fluid (CSF) samples for proteomic analysis and DRGs and SCs for analysis of IB-4 and CGRP, and IBA1 and GFAP, respectively, were collected on Day 21. Females developed stronger mechanical allodynia. There were no differences between the sexes in CGRP and IB-4 in the DRG or glial cell markers in the SC. No CSF protein showed change following SNI. DRG and SC showed abundant changes in gene expression. Sexually dimorphic responses were found in genes related to T-cells (cd28, ctla4, cd274, cd4, prf1), other immunological responses (dpp4, c5a, cxcr2 and il1b), neuronal transmission (hrh3, thbs4, chrna4 and pdyn), plasticity (atf3, c1qc and reg3b), and others (bhlhe22, mcpt1l, trpv6). We observed significantly stronger mechanical allodynia in females and numerous sexually dimorphic changes in gene expression following SNI in rats. Several genes have previously been linked to NP, while some are novel. Our results suggest gene targets for further studies in the development of new, possibly sex-specific, therapies for NP.

microRNA-125b and its downstream Smurf1/KLF2/ATF2 axis as important promoters on neurological function recovery in rats with spinal cord injury

The purpose of this study is to investigate the role of microRNA-125b (miR-125b) and its mechanism in spinal cord injury (SCI) by targeting Smurf1. After loss- and gain-function approaches were conducted in SCI rat models and neural stem cells (NSCs) isolated from foetal rats, the Basso-Beattie-Bresnahan (BBB) score was calculated, and related protein expression was determined by Western blot analysis and cell apoptosis by TUNEL staining. NSC viability was detected by CCK-8, migration abilities by Transwell assay and apoptosis by flow cytometry. The relationship between miR-125b, Smurf1 and KLF2 was evaluated by dual-luciferase reporter gene experiments, Co-IP and in vivo ubiquitin modification assays. Inhibition of miR-125b and KLF2 and the up-regulation of Smurf1 and ATF2 were observed in SCI rats. BBB scores were elevated, the expression of Nestin, NeuN, GFAP, NF-200 and Bcl-2 protein was enhanced but that of Bax protein was reduced, and cell apoptosis was inhibited in SCI rats after up-regulating miR-125b or silencing ATF2. Smurf1 was a target gene of miR-125b, which promoted KLF2 degradation through its E3 ubiquitin ligase function, and KLF2 repressed the expression of ATF2 in NSCs. The results in vivo were replicated in vitro. miR-125b overexpression promotes neurological function recovery after SCI.

Transient receptor potential vanilloid subtype 1 depletion mediates mechanical allodynia through cellular signal alterations in small-fiber neuropathy

Transient receptor potential vanilloid subtype 1 (TRPV1) is a polymodal nociceptor that monitors noxious thermal sensations. Few studies have addressed the role of TRPV1 in mechanical allodynia in small-fiber neuropathy (SFN) caused by sensory nerve damage. Accordingly, this article reviews the putative mechanisms of TRPV1 depletion that mediates mechanical allodynia in SFN. The intraepidermal nerve fibers (IENFs) degeneration and sensory neuronal injury are the primary characteristics of SFN. Intraepidermal nerve fibers are mainly C-polymodal nociceptors and Aδ-fibers, which mediated allodynic pain after neuronal sensitization. TRPV1 depletion by highly potent neurotoxins induces the upregulation of activating transcription factor 3 and IENFs degeneration which mimics SFN. TRPV1 is predominately expressed by the peptidergic than nonpeptidergic nociceptors, and these neurochemical discrepancies provided the basis of the distinct pathways of thermal analgesia and mechanical allodynia. The depletion of peptidergic nociceptors and their IENFs cause thermal analgesia and sensitized nonpeptidergic nociceptors respond to mechanical allodynia. These distinct pathways of noxious stimuli suggested determined by the neurochemical-dependent neurotrophin cognate receptors such as TrkA and Ret receptors. The neurogenic inflammation after TRPV1 depletion also sensitized Ret receptors which results in mechanical allodynia. The activation of spinal TRPV1(+) neurons may contribute to mechanical allodynia. Also, an imbalance in adenosinergic analgesic signaling in sensory neurons such as the downregulation of prostatic acid phosphatase and adenosine A1 receptors, which colocalized with TRPV1 as a membrane microdomain also correlated with the development of mechanical allodynia. Collectively, TRPV1 depletion-induced mechanical allodynia involves a complicated cascade of cellular signaling alterations.

PKC- and PKA-dependent phosphorylation modulates TREK-1 function in naïve and neuropathic rats

PKC and PKA phosphorylation inhibit TREK-1 channels downstream of G_s protein-coupled receptor activation in vitro. However, the role of phosphorylation of TREK-1 in neuropathic pain is unknown. The purpose of this study was to investigate whether altered TREK-1 channel function by PKA and PKC modulators contributes to antiallodynia in neuropathic rats. Furthermore, we investigated if the in vitro described sites for PKC and PKA phosphorylation (S300 and S333, respectively) participate in the modulation of TREK-1 in naïve and neuropathic rats. L5/L6 spinal nerve ligation (SNL) induced tactile allodynia. Intrathecal injection of BL-1249 (TREK-1 activator) reversed nerve injury-induced tactile allodynia, whereas spadin (TREK-1 blocker) produced tactile allodynia in naïve rats and reversed the antiallodynic effect induced by BL-1249 in neuropathic rats. Intrathecal administration of rottlerin or Rp-cAMPs (PKC and PKA inhibitors, respectively) enhanced the antiallodynia observed with BL-1249 in neuropathic rats. In contrast, pretreatment with PdBu or forskolin (PKC and PKA activators, respectively) reduced the BL-1249-induced antiallodynia. Intrathecal injection of two high-activity TREK-1 recombinant channels, using a in vivo transfection method with lipofectamine, with mutations at PKC/PKA phosphosites (S300A and S333A) reversed tactile allodynia in neuropathic rats, with no effect in naïve rats. In contrast, transfection of two low-activity TREK-1 recombinant channels with phosphomimetic mutations at those sites (S300D and S333D) produced tactile allodynia in naïve rats and interfered with antiallodynic effects of rottlerin/BL-1249 or Rp-cAMPs/BL-1249. Data suggest that TREK-1 channel activity can be dynamically tuned in vivo by PKC/PKA to provoke allodynia and modulate its antiallodynic role in neuropathic pain.

Activating transcription factor 3 modulates protein kinase C epsilon activation in diabetic peripheral neuropathy

Background

Skin denervation that develops in patients with diabetes mellitus as a neuropathic manifestation is known as diabetic peripheral neuropathy (DPN). Skin denervation is parallel to neuronal injuries that alter intracellular signaling. To date, the correlation between nerve injury and the activation of intracellular responses to neuropathic manifestations has not been elucidated; specifically, whether activating transcription factor 3 (ATF3) is responsible for neuronal injury and a critical molecule that modulates the activation of intracellular protein kinase C epsilon (p-PKCε) and pain development in DPN is a crucial question.

Methods

To address, ATF3 knockout (atf3^−/− group, C57/B6 genetic background) and wild-type mice (atf3^+/+ group) received a single dose of streptozotocin (200 mg/kg) to generate a mouse model of DPN.

Results

Both atf3^+/+ and atf3^−/− mice exhibited hyperglycemia and the same pathology of skin denervation at posttreatment month 2, but only atf3^+/+ mice developed thermal hyperalgesia (P<0.001) and mechanical allodynia (P=0.002). The atf3^+/+ group, but not the atf3^−/− group, had preferential ATF3 upregulation on p-PKCε(+) neurons with a ratio of 37.7%±6.1% in p-PKCε(+):ATF3(+) neurons (P<0.001). In addition, B-cell lymphoma-extra large (Bcl-_XL), an antiapoptotic Bcl2 family protein, exhibited parallel patterns to p-PKCε (ie, Bcl-_XL upregulation was reversed in atf3^−/− mice). These two molecules were colocalized and increased by approximately two-fold in the atf3^+/+ group compared with the atf3^−/− group (30.0%±3.4% vs 13.7% ± 6.2%, P=0.003). Furthermore, linear analysis results showed that the densities of p-PKCε and Bcl-_XL had a reverse linear relationship with the degrees of thermal hyperalgesia and mechanical allodynia.

Conclusion

Collectively, this report suggested that ATF3 is a critical upstream molecule that modulates p-PKCε and Bcl-_XL expression, which consequently mediated the development of neuropathic manifestation in DPN.