Schwann Cell Plasticity is Regulated by a Weakened Intrinsic Antioxidant Defense System in Acute Peripheral Nerve Injury

Role of Oxygen and Its Radicals in Peripheral Nerve Regeneration: From Hypoxia to Physoxia to Hyperoxia

Oxygen is compulsory for mitochondrial function and energy supply, but it has numerous more nuanced roles. The different roles of oxygen in peripheral nerve regeneration range from energy supply, inflammation, phagocytosis, and oxidative cell destruction in the context of reperfusion injury to crucial redox signaling cascades that are necessary for effective axonal outgrowth. A fine balance between reactive oxygen species production and antioxidant activity draws the line between physiological and pathological nerve regeneration. There is compelling evidence that redox signaling mediated by the Nox family of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases plays an important role in peripheral nerve regeneration. Further research is needed to better characterize the role of Nox in physiological and pathological circumstances, but the available data suggest that the modulation of Nox activity fosters great therapeutic potential. One of the promising approaches to enhance nerve regeneration by modulating the redox environment is hyperbaric oxygen therapy. In this review, we highlight the influence of various oxygenation states, i.e., hypoxia, physoxia, and hyperoxia, on peripheral nerve repair and regeneration. We summarize the currently available data and knowledge on the effectiveness of using hyperbaric oxygen therapy to treat nerve injuries and discuss future directions.

METTL3-Mediated N6-Methyladenosine Modification of lncRNA D26496 Suppresses the Proliferation and Migration of Schwann Cells after Sciatic Nerve Injury

Previous reports showed that LncRNA D26496 was downregulated and N6-methyladenosine (m6A) methyltransferase METTL3 was upregulated in sciatic nerve injury (SNI). YTH-Domain Family Member 2 (YTHDF2) regulated RNA degradation through recognizing m6A sites. However, whether METTL3-mediated m6A of D26496 plays a role in development of SNI is unknown. Therefore, in this study, we established a rat SNI model and a H₂O₂-induced Schwann cell injury model to investigate the role of D26496 in modulating SNI and how the expression of D26496 was regulated during this process. D26496 expression was downregulated in both models. Rats with SNI displayed severe oxidative stress, manifested as increased MDA production and decreased SOD and GSH activity. Moreover, overexpression of D26496 alleviated H₂O₂-induced Schwann cell injury likely by promoting cell proliferation and migration and suppressing cell apoptosis and oxidative stress. Mechanism studies found that METTL3 expression was upregulated after SNI, and silencing METTL3 reduced the D26496 m6A level, but upregulated D26496 expression. Subsequent studies found that YTHDF2 was upregulated after SNI, and abundant m6A modified D26496 in the precipitated protein-RNA complexes by anti-YTHDF2 antibody, whereas silencing YTHDF2 promoted D26496 expression but had no effect on m6A levels of D29496. Silencing D26496 reversed the protective effect of knocking down METTL3 or knocking down YTHDF2 on H₂O₂-induced cell damage. In vivo, D26496 overexpression alleviated SNI-induced neuropathic pain and oxidative stress. In conclusion, our results suggested that D26496 m6A modification mediated by METTL3 and recognition of D26496 m6A sites by YTHDF2 induced D26496 degradation, thereby participating in the progression of SNI.

Beneficial Effect of Bee Venom and Its Major Components on Facial Nerve Injury Induced in Mice

Peripheral nerve injury (PNI) is a health problem that affects many people worldwide. This study is the first to evaluate the potential effect of bee venom (BV) and its major components in a model of PNI in the mouse. For that, the BV used in this study was analyzed using UHPLC. All animals underwent a distal section-suture of facial nerve branches, and they were randomly divided into five groups. Group 1: injured facial nerve branches without any treatment. Group 2: the facial nerve branches were injured, and the normal saline was injected similarly as in the BV-treated group. Group 3: injured facial nerve branches with local injections of BV solution. Group 4: injured facial nerve branches with local injections of a mixture of PLA2 and melittin. Group 5: injured facial nerve branches with local injection of betamethasone. The treatment was performed three times a week for 4 weeks. The animals were submitted to functional analysis (observation of whisker movement and quantification of nasal deviation). The vibrissae muscle re-innervation was evaluated by retrograde labeling of facial motoneurons in all experimental groups. UHPLC data showed 76.90 ± 0.13%, 11.73 ± 0.13%, and 2.01 ± 0.01%, respectively, for melittin, phospholipase A2, and apamin in the studied BV sample. The obtained results showed that BV treatment was more potent than the mixture of PLA2 and melittin or betamethasone in behavioral recovery. The whisker movement occurred faster in BV-treated mice than in the other groups, with a complete disappearance of nasal deviation two weeks after surgery. Morphologically, a normal fluorogold labeling of the facial motoneurons was restored 4 weeks after surgery in the BV-treated group, but no such restoration was ever observed in other groups. Our findings indicate the potential of the use of BV injections to enhance appropriate functional and neuronal outcomes after PNI.

Translocator protein alleviates allodynia and improves Schwann cell function against diabetic peripheral neuropathy via activation of the Nrf2-dependent antioxidant system and promoting autophagy

AIMS

In diabetes, autophagy and the nuclear factor erythroid-derived-2-like 2 (Nrf2)-dependent antioxidant system are impaired. Translocator protein (TSPO) agonist Ro5-4864 alleviates neuropathic pain, including diabetic peripheral neuropathy (DPN). However, the precise mechanisms remain unclear. Thus, we investigated the effects of Ro5-4864 on autophagy and the Nrf2-dependent antioxidant system in the sciatic nerves of DPN rats.

METHODS

All rats were randomly assigned to Sham or DPN group. After type 2 diabetes modelling (established by high-fat diet and streptozotocin injection) followed by behavioural tests, established DPN rats were randomly assigned to the DPN group, the Ro (TSPO agonist Ro5-4864) group, the Ro + 3-MA (autophagy inhibitor) group and the Ro + ML385 (Nrf2 inhibitor) group. Behavioural assessments were performed at baseline, on days 3, 7, 14, 21 and 28. Sciatic nerves were collected on day 28 for immunofluorescence, morphological and western blot analyses.

RESULTS

Ro5-4864 alleviated allodynia and increased myelin sheath thickness and myelin protein expression after DPN. Beclin-1 (p < 0.01) and LC3-II/LC3-I ratio (p < 0.01) decreased and p62 (p < 0.01) accumulated in the DPN rats. Ro5-4864 administration increased the Beclin-1 and LC3-II/LC3-I ratio and decreased p62 accumulation. Furthermore, nuclear Nrf2 contents (p < 0.01) and cytoplasmic HO-1 (p < 0.01) and NQO1 (p < 0.01) expressions were significantly inhibited in the DPN rat, which was also improved by Ro5-4864. All the beneficial effects were abrogated by 3-MA or ML385.

CONCLUSION

TSPO exhibited a potent analgesic effect and improved Schwann cell function and regeneration against DPN by activating the Nrf2-dependent antioxidant system and promoting autophagy.

Biological nerve conduit model with de-epithelialized human amniotic membrane and adipose-derived mesenchymal stem cell sheet for repair of peripheral nerve defects

In this study, a biological conduit, consisting of an adipocyte-derived mesenchymal stem cell (AdMSCs) sheet and amniotic membrane (AM), was designed for the reconstruction of peripheral nerve defects. To evaluate the effect of the produced conduit on neural regeneration, a 10-mm sciatic nerve defect was created in rats, and experiments were carried out on six groups, i.e., sham control group (SC), negative control group (NC), nerve autograft group (NG), the biological conduit (AdMSCs + AM) group, the commercial PGA tube conduit (PGA) group, and the conduit only consisting of AM (AM) group. The effects of different nerve repair methods on the peripheral nerve and gastrocnemius muscle were evaluated by functional, histological, and immunohistochemical tests. When the number of myelinated axons was compared between the groups of AdMSCs + AM and PGA, it was higher in the AdMSCs + AM group (p < 0.05). The percentage of gastrocnemius collagen bundle area of AdMSCs + AM group was found to be statistically lower than the PGA group (p < 0.05). The muscle fiber diameter of AdMSCs + AM group was lower than that of the NG group, but significantly higher than that of the PGA group and the AM group (p < 0.001). Muscle weight index was significantly higher in the AdMSCs + AM group compared to the PGA group (p < 0.05). It was observed that nerve regeneration was faster in the AdMSCs + AM group, and there was an earlier improvement in pin-prick score and sciatic functional index compared to the PGA group and the AM group. In conclusion, the biological conduit prepared from the AdMSCs sheet and AM is regarded as a new biological conduit that can be used as an alternative treatment method to nerve autograft in clinical applications.

Nox, Nox, Are You There? The Role of NADPH Oxidases in the Peripheral Nervous System

Significance: Reactive oxygen species (ROS) contribute to multiple aspects of peripheral nervous system (PNS) biology ranging from physiological processes (e.g., axonal outgrowth and regeneration) to pathophysiology (e.g., nerve degeneration). Although ROS are derived from multiple sources, NADPH oxidase (Nox) family members are dedicated to ROS generation. Noxs are expressed in the PNS, and their overexpression is associated with detrimental effects on nerve function and contributes, at least in part, to peripheral neuropathies. Recent Advances: Of the seven members, studies mostly focused on Nox1, Nox2, and Nox4, which are expressed in the PNS in a cell-specific manner. We have also recently identified human Nox5 in sural nerve biopsies. When maintained at homeostatic levels, Noxs regulate several aspects of peripheral nerve health, most notably neurite outgrowth and axonal regeneration following nerve lesion. While Nox2 and Nox4 dysregulation is a major source of oxidative stress in PNS disorders, including neuropathic pain and diabetic peripheral neuropathy, recent evidence also implicates Nox1 and Nox5. Critical Issues: Although there is compelling evidence for a direct role of Noxs on nerve function, little is known about their subcellular localization, intercellular regulation, and interaction. These, together with redox signaling, are considered crucial components of nerve redox status. In addition, the lack of isoform-specific inhibitors limits conclusions about the physiological role of Noxs in the PNS and their therapeutic potential in peripheral neuropathies. Future Directions: Future research using isoform-specific genetic and pharmacological approaches are therefore needed to better understand the significance of Nox enzymes in PNS (patho) physiology. Antioxid. Redox Signal. 37, 613-630.

Investigation of the Hydrogen Sulfide Signaling Pathway in Schwann Cells during Peripheral Nerve Degeneration: Multi-Omics Approaches

N-ethylmaleimide (NEM) inhibits peripheral nerve degeneration (PND) by targeting Schwann cells in a hydrogen sulfide (H₂S)-pathway-dependent manner, but the underlying molecular and pharmacological mechanisms are unclear. We investigated the effect of NEM, an α,β-unsaturated carboxyl compound, on H₂S signaling in in vitro- and ex vivo-dedifferentiated Schwann cells using global proteomics (LC-MS) and transcriptomics (whole-genome and small RNA-sequencing (RNA-seq)) methods. The multi-omics analyses identified several genes and proteins related to oxidative stress, such as Sod1, Gnao1, Stx4, Hmox2, Srxn1, and Edn1. The responses to oxidative stress were transcriptionally regulated by several transcription factors, such as Atf3, Fos, Rela, and Smad2. In a functional enrichment analysis, cell cycle, oxidative stress, and lipid/cholesterol metabolism were enriched, implicating H₂S signaling in Schwann cell dedifferentiation, proliferation, and myelination. NEM-induced changes in the H₂S signaling pathway affect oxidative stress, lipid metabolism, and the cell cycle in Schwann cells. Therefore, regulation of the H₂S signaling pathway by NEM during PND could prevent Schwann cell demyelination, dedifferentiation, and proliferation.

Transcription factor SS18L1 regulates the proliferation, migration and differentiation of Schwann cells in peripheral nerve injury

Transcription factors bind to specific DNA sequences, modulate the transcription of target genes, and regulate various biological processes, including peripheral nerve regeneration. Our previous analysis showed that SS18L1, a gene encoding the transcription factor SS18-like protein 1, was differentially expressed in the distal sciatic nerve stumps after rat sciatic nerve transection injury, but its effect on peripheral nerve injury has not been reported. In the current study, we isolated and cultured primary Schwann cells, and examined the role of SS18L1 for the biological functions of the cells. Depletion of SS18L1 by siRNA in Schwann cells enhanced cell proliferation and inhibited cell migration, as determined by EdU assay and transwell migration assay, respectively. In addition, silencing of SS18L1 inhibited Schwann cell differentiation induced by HRG and cAMP. Bioinformatics analyses revealed an interaction network of SS18L1, including DF2, SMARCD1, SMARCA4, and SMARCE1, which may be implicated in the regulatory functions of SS18L1 on the proliferation, migration and differentiation of Schwann cells. In conclusion, our results revealed a temporal expression profile of SS18L1 in peripheral nerve injury and its potential roles during the process of nerve recovery.

Exosomes derived from differentiated human ADMSC with the Schwann cell phenotype modulate peripheral nerve-related cellular functions

Peripheral nerve regeneration remains a significant clinical challenge due to the unsatisfactory functional recovery and public health burden. Exosomes, especially those derived from mesenchymal stem cells (MSCs), are promising as potential cell-free therapeutics and gene therapy vehicles for promoting neural regeneration. In this study, we reported the differentiation of human adipose derived MSCs (hADMSCs) towards the Schwann cell (SC) phenotype (hADMSC-SCs) and then isolated exosomes from hADMSCs with and without differentiation (i.e., dExo vs uExo). We assessed and compared the effects of uExo and dExo on antioxidative, angiogenic, anti-inflammatory, and axon growth promoting properties by using various peripheral nerve-related cells. Our results demonstrated that hADMSC-SCs secreted more neurotrophic factors and other growth factors, compared to hADMSCs without differentiation. The dExo isolated from hADMSC-SCs protected rat SCs from oxidative stress and enhanced HUVEC migration and angiogenesis. Compared to uExo, dExo also had improved performances in downregulating pro-inflammatory gene expressions and cytokine secretions and promoting axonal growth of sensory neurons differentiated from human induced pluripotent stem cells. Furthermore, microRNA (miRNA) sequencing analysis revealed that exosomes and their parent cells shared some similarities in their miRNA profiles and exosomes displayed a distinct miRNA signature. Many more miRNAs were identified in dExo than in uExo. Several upregulated miRNAs, like miRNA-132-3p and miRNA-199b-5p, were highly related to neuroprotection, anti-inflammation, and angiogenesis. The dExo can effectively modulate various peripheral nerve-related cellular functions and is promising for cell-free biological therapeutics to enhance neural regeneration.

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Exosomes were isolated from hADMSCs with and without differentiation towards SC phenotype (i.e., dExo vs uExo).

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hADMSC-SCs secreted more growth factors compared to hADMSCs without differentiation.

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The dExo protected rat SCs from oxidative stress and enhanced endothelial cell migration and angiogenesis.

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dExo promoted axonal growth of sensory neurons differentiated from hiPSCs.

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miRNA sequencing analysis unveiled and compared the exosomal and cellular miRNA profiles.

The Effects of Nuclear Factor Erythroid 2 (NFE2)-Related Factor 2 (Nrf2) Activation in Preclinical Models of Peripheral Neuropathic Pain

Oxidative stress, resulting from an imbalance between the formation of damaging free radicals and availability of protective antioxidants, can contribute to peripheral neuropathic pain conditions. Reactive oxygen and nitrogen species, as well as products of the mitochondrial metabolism such as superoxide anions, hydrogen peroxide, and hydroxyl radicals, are common free radicals. Nuclear factor erythroid 2 (NFE2)-related factor 2 (Nrf2) is a transcription factor encoded by the NFE2L2 gene and is a member of the cap 'n' collar subfamily of basic region leucine zipper transcription factors. Under normal physiological conditions, Nrf2 remains bound to Kelch-like ECH-associated protein 1 in the cytoplasm that ultimately leads to proteasomal degradation. During peripheral neuropathy, Nrf2 can translocate to the nucleus, where it heterodimerizes with muscle aponeurosis fibromatosis proteins and binds to antioxidant response elements (AREs). It is becoming increasingly clear that the Nrf2 interaction with ARE leads to the transcription of several antioxidative enzymes that can ameliorate neuropathy and neuropathic pain in rodent models. Current evidence indicates that the antinociceptive effects of Nrf2 occur via reducing oxidative stress, neuroinflammation, and mitochondrial dysfunction. Here, we will summarize the preclinical evidence supporting the role of Nrf2 signaling pathways and Nrf2 inducers in alleviating peripheral neuropathic pain.

Studies on the Regulatory Roles and Related Mechanisms of lncRNAs in the Nervous System

Long noncoding RNAs (lncRNAs) have attracted extensive attention due to their regulatory role in various cellular processes. Emerging studies have indicated that lncRNAs are expressed to varying degrees after the growth and development of the nervous system as well as injury and degeneration, thus affecting various physiological processes of the nervous system. In this review, we have compiled various reported lncRNAs related to the growth and development of central and peripheral nerves and pathophysiology (including advanced nerve centers, spinal cord, and peripheral nervous system) and explained how these lncRNAs play regulatory roles through their interactions with target-coding genes. We believe that a full understanding of the regulatory function of lncRNAs in the nervous system will contribute to understand the molecular mechanism of changes after nerve injury and will contribute to discover new diagnostic markers and therapeutic targets for nerve injury diseases.

Mesenchymal stem cell treatment for peripheral nerve injury: a narrative review

Peripheral nerve injuries occur as the result of sudden trauma and lead to reduced quality of life. The peripheral nervous system has an inherent capability to regenerate axons. However, peripheral nerve regeneration following injury is generally slow and incomplete that results in poor functional outcomes such as muscle atrophy. Although conventional surgical procedures for peripheral nerve injuries present many benefits, there are still several limitations including scarring, difficult accessibility to donor nerve, neuroma formation and a need to sacrifice the autologous nerve. For many years, other therapeutic approaches for peripheral nerve injuries have been explored, the most notable being the replacement of Schwann cells, the glial cells responsible for clearing out debris from the site of injury. Introducing cultured Schwann cells to the injured sites showed great benefits in promoting axonal regeneration and functional recovery. However, there are limited sources of Schwann cells for extraction and difficulties in culturing Schwann cells in vitro. Therefore, novel therapeutic avenues that offer maximum benefits for the treatment of peripheral nerve injuries should be investigated. This review focused on strategies using mesenchymal stem cells to promote peripheral nerve regeneration including exosomes of mesenchymal stem cells, nerve engineering using the nerve guidance conduits containing mesenchymal stem cells, and genetically engineered mesenchymal stem cells. We present the current progress of mesenchymal stem cell treatment of peripheral nerve injuries.

Nrf2 knockout altered brain iron deposition and mitigated age-related motor dysfunction in aging mice

The transcription factor NF-E2-related factor 2 (Nrf2) is a central regulator of cellular antioxidant and detoxification response. The association between Nrf2 activity and iron-related oxidative stress in neurodegenerative diseases has been studied, and Nrf2 was found to transcriptionally regulate the expression of iron transporters and ferroptosis-related factors. However, the role of Nrf2 in age-related motor dysfunction and its link to iron metabolism dysregulation in brain have not been fully elucidated. In this study, with different ages of Nrf2 knockout (KO) and wild type (WT) mice, we investigated the effects of Nrf2 deficiency on brain oxidative stress, iron metabolism and the motor coordination ability of mice. In contrast to the predicted neuroprotective role of Nrf2 in oxidative stress-related diseases, we found that Nrf2 KO remarkably improved the motor coordination of aged mice, which was associated with the reduced ROS level and decreased apoptosis of dopaminergic neurons in substantia nigra (SN) of 18-month-old Nrf2 KO mice. With high-iron and Parkinson's disease (PD) mouse models, we revealed that Nrf2 KO prevented the deposition of brain iron, particularly in SN and striatum, which may subsequently delay motor dysfunction in aged mice. The regulation of Nrf2 KO on brain iron metabolism was likely mediated by decreasing the ferroportin 1 (FPN1) level on brain microvascular endothelial cells, thus hindering the process of iron entry into the brain. Nrf2 may be a potential therapeutic target in age-related motor dysfunction diseases for its role in regulating brain iron homeostasis.

Heme Oxygenase 1 in Schwann Cells Regulates Peripheral Nerve Degeneration Against Oxidative Stress

During Wallerian degeneration, Schwann cells lose their characteristic of myelinating axons and shift into the state of developmental promyelinating cells. This recharacterized Schwann cell guides newly regrowing axons to their destination and remyelinates reinnervated axons. This Schwann cell dynamics during Wallerian degeneration is associated with oxidative events. Heme oxygenases (HOs) are involved in the oxidative degradation of heme into biliverdin/bilirubin, ferrous iron, and carbon monoxide. Overproduction of ferrous iron by HOs increases reactive oxygen species, which have deleterious effects on living cells. Thus, the key molecule for understanding the exact mechanism of Wallerian degeneration in the peripheral nervous system is likely related to oxidative stress-mediated HOs in Schwann cells. In this study, we demonstrate that demyelinating Schwann cells during Wallerian degeneration highly express HO1, not HO2, and remyelinating Schwann cells during nerve regeneration decrease HO1 activation to levels similar to those in normal myelinating Schwann cells. In addition, HO1 activation during Wallerian degeneration regulates several critical phenotypes of recharacterized repair Schwann cells, such as demyelination, transdedifferentiation, and proliferation. Thus, these results suggest that oxidative stress in Schwann cells after peripheral nerve injury may be regulated by HO1 activation during Wallerian degeneration and oxidative-stress-related HO1 activation in Schwann cells may be helpful to study deeply molecular mechanism of Wallerian degeneration.