Microarray and qPCR Analyses of Wallerian Degeneration in Rat Sciatic Nerves

[Construction of a predictive model for postoperative pain relief after microscopic spermatic cord surgery for spermatic cord pain]

OBJECTIVE

To explore the relevant factors affecting the efficacy of microscopic spermatic cord surgery and build a predictive model for postoperative pain relief.

METHODS

A retrospective analysis was conducted on the clinical data of 324 patients with spermatic cord pain who visited the Department of Urology at Peking University People's Hospital between October 2015 and April 2023. This cohort included 212 patients with varicocele-related spermatic cord pain and 112 patients with idiopathic spermatic cord pain. All the patients underwent microsurgical procedures: varicocele-related pain was treated with microsurgical varicocelectomy, and idiopathic pain was treated with microsurgical denervation of the spermatic cord. The patients were categorized into effective and ineffective groups based on whether their pain had decreased by more than 50% six months post-surgery compared with pre-surgery levels. Baseline data were preliminarily screened for clinical indicators using t tests and univariate analysis. Clinical predictor variables [age, duration of pain, diameter of varicocele, patient health questionnaire-9 (PHQ-9) score, generalized anxiety disorder-7 (GAD-7) score] were selected using Lasso regression. A clinical prediction model for effective pain relief following microscopic spermatic cord surgery was constructed using Logistic regression and presented as a nomogram. The model's internal validation was performed using the bootstrap method. Its predictive power and clinical utility were evaluated through the concor-dance index, the area under the receiver operating characteristic curve, and calibration plots.

RESULTS

Post-microscopic varicocele ligation, 156 patients (73.58%) experienced significant pain relief, as did 94 patients (83.93%) following microscopic denervation. Independent predictors for postoperative outcomes included age, PHQ-9 score, GAD-7 score, chronic pain duration, and varicocele diameter, differing slightly between varicocele-related and idiopathic pain groups. The models demonstrated excellent predictive ability, with areas under the curve of 0.909 and 0.913 for varicocele and idiopathic groups, respectively, and high concordance indices.

CONCLUSION

The postoperative efficacy prediction model based on age, pain duration, PHQ-9 score, GAD-7 score, and varicocele diameter has good predictive ability and clinical applicability, and can be used in clinical practice.

Exosomes repairment for sciatic nerve injury: a cell-free therapy

Sciatic nerve injury (SNI) is a common type of peripheral nerve injury typically resulting from trauma, such as contusion, sharp force injuries, drug injections, pelvic fractures, or hip dislocations. It leads to both sensory and motor dysfunctions, characterized by pain, numbness, loss of sensation, muscle atrophy, reduced muscle tone, and limb paralysis. These symptoms can significantly diminish a patient's quality of life. Following SNI, Wallerian degeneration occurs, which activates various signaling pathways, inflammatory factors, and epigenetic regulators. Despite the availability of several surgical and nonsurgical treatments, their effectiveness remains suboptimal. Exosomes are extracellular vesicles with diameters ranging from 30 to 150 nm, originating from the endoplasmic reticulum. They play a crucial role in facilitating intercellular communication and have emerged as highly promising vehicles for drug delivery. Increasing evidence supports the significant potential of exosomes in repairing SNI. This review delves into the pathological progression of SNI, techniques for generating exosomes, the molecular mechanisms behind SNI recovery with exosomes, the effectiveness of combining exosomes with other approaches for SNI repair, and the changes and future outlook for utilizing exosomes in SNI recovery.

FOSL1 modulates Schwann cell responses in the wound microenvironment and regulates peripheral nerve regeneration

Peripheral glial Schwann cells switch to a repair state after nerve injury, proliferate to supply lost cell population, migrate to form regeneration tracks, and contribute to the generation of a permissive microenvironment for nerve regeneration. Exploring essential regulators of the repair responses of Schwann cells may benefit the clinical treatment for peripheral nerve injury. In the present study, we find that FOSL1, a AP-1 member that encodes transcription factor FOS Like 1, is highly expressed at the injured sites following peripheral nerve crush. Interfering FOSL1 decreases the proliferation rate and migration ability of Schwann cells, leading to impaired nerve regeneration. Mechanism investigations demonstrate that FOSL1 regulates Schwann cell proliferation and migration by directly binding to the promoter of EPH Receptor B2 (EPHB2) and promoting EPHB2 transcription. Collectively, our findings reveal the essential roles of FOSL1 in regulating the activation of Schwann cells and indicate that FOSL1 can be targeted as a novel therapeutic approach to orchestrate the regeneration and functional recovery of injured peripheral nerves.

Transcription Factor BCL11A Regulates Schwann Cell Behavior During Peripheral Nerve Regeneration

Nerve injury-induced Schwann cell dedifferentiation helps to construct a favorable microenvironment for axon growth. Transcription factors regulate cell reprogramming and thus may be critical for Schwann cell phenotype switch during peripheral nerve regeneration. Here, we show that transcription factor B-cell lymphoma/leukemia 11A (BCL11A) is up-regulated in Schwann cells of injured peripheral nerves. Bcl11a silencing suppresses Schwann cell viability, decreases Schwann cell proliferation and migration rates, and impairs the debris clearance ability of Schwann cells. Reduced Bcl11a in injured peripheral nerves results in restricted axon elongation and myelin wrapping, leading to recovery failure. Mechanistically, we demonstrate that BCL11A may mediate Schwann cell activity through binding to the promoter of nuclear receptor subfamily 2 group F member 2 (Nr2f2) and regulating Nr2f2 expression. Collectively, we conclude that BCL11A is essential for Schwann cell activation and peripheral nerve regeneration, providing a potential therapeutic target for the treatment of peripheral nerve injury.

Transcriptional Control of Peripheral Nerve Regeneration

Transcription factors are master regulators of various cellular processes under diverse physiological and pathological conditions. Many transcription factors that are differentially expressed after injury to peripheral nerves play important roles in nerve regeneration. Considering that rapid and timely regrowth of injured axons is a prerequisite for successful target reinnervation, here, we compile transcription factors that mediates axon elongation, including axon growth suppressor Klf4 and axon growth promoters c-Myc, Sox11, STAT3, Atf3, c-Jun, Smad1, C/EBPδ, and p53. Besides neuronal changes, Schwann cell phenotype modulation is also critical for nerve regeneration. The activation of Schwann cells at early time points post injury provides a permissive microenvironment whereas the re-differentiation of Schwann cells at later time points supports myelin sheath formation. Hence, c-Jun and Sox2, two critical drivers for Schwann cell reprogramming, as well as Krox-20 and Sox10, two essential regulators of Schwann cell myelination, are reviewed. These transcription factors may serve as promising targets for promoting the functional recovery of injured peripheral nerves.

Basic mechanisms of peripheral nerve injury and treatment via electrical stimulation

Previous studies on the mechanisms of peripheral nerve injury (PNI) have mainly focused on the pathophysiological changes within a single injury site. However, recent studies have indicated that within the central nervous system, PNI can lead to changes in both injury sites and target organs at the cellular and molecular levels. Therefore, the basic mechanisms of PNI have not been comprehensively understood. Although electrical stimulation was found to promote axonal regeneration and functional rehabilitation after PNI, as well as to alleviate neuropathic pain, the specific mechanisms of successful PNI treatment are unclear. We summarize and discuss the basic mechanisms of PNI and of treatment via electrical stimulation. After PNI, activity in the central nervous system (spinal cord) is altered, which can limit regeneration of the damaged nerve. For example, cell apoptosis and synaptic stripping in the anterior horn of the spinal cord can reduce the speed of nerve regeneration. The pathological changes in the posterior horn of the spinal cord can modulate sensory abnormalities after PNI. This can be observed in cases of ectopic discharge of the dorsal root ganglion leading to increased pain signal transmission. The injured site of the peripheral nerve is also an important factor affecting post-PNI repair. After PNI, the proximal end of the injured site sends out axial buds to innervate both the skin and muscle at the injury site. A slow speed of axon regeneration leads to low nerve regeneration. Therefore, it can take a long time for the proximal nerve to reinnervate the skin and muscle at the injured site. From the perspective of target organs, long-term denervation can cause atrophy of the corresponding skeletal muscle, which leads to abnormal sensory perception and hyperalgesia, and finally, the loss of target organ function. The mechanisms underlying the use of electrical stimulation to treat PNI include the inhibition of synaptic stripping, addressing the excessive excitability of the dorsal root ganglion, alleviating neuropathic pain, improving neurological function, and accelerating nerve regeneration. Electrical stimulation of target organs can reduce the atrophy of denervated skeletal muscle and promote the recovery of sensory function. Findings from the included studies confirm that after PNI, a series of physiological and pathological changes occur in the spinal cord, injury site, and target organs, leading to dysfunction. Electrical stimulation may address the pathophysiological changes mentioned above, thus promoting nerve regeneration and ameliorating dysfunction.

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.

Robust temporal changes of cellular senescence and proliferation after sciatic nerve injury

Cellular senescence and proliferation are essential for wound healing and tissue remodeling. However, senescence-proliferation cell fate after peripheral nerve injury has not been clearly revealed. Here, post-injury gene expression patterns in rat sciatic nerve stumps (SRP113121) and L4-5 dorsal root ganglia (SRP200823) obtained from the National Center for Biotechnology Information were analyzed to decipher cellular senescence and proliferation-associated genetic changes. We first constructed a rat sciatic nerve crush model. Then, β-galactosidase activities were determined to indicate the existence of cellular senescence in the injured sciatic nerve. Ki67 and EdU immunostaining was performed to indicate cellular proliferation in the injured sciatic nerve. Both cellular senescence and proliferation were less vigorous in the dorsal root ganglia than in sciatic nerve stumps. These results reveal the dynamic changes of injury-induced cellular senescence and proliferation from both genetic and morphological aspects, and thus extend our understanding of the biological processes following peripheral nerve injury. The study was approved by the Animal Ethics Committee of Nantong University, China (approval No. 20190226-001) on February 26, 2019.

Expression of Protein Acetylation Regulators During Peripheral Nerve Development, Injury, and Regeneration

Protein acetylation, regulated by acetyltransferases and deacetylases, is an important post-translational modification that is involved in numerous physiological and pathological changes in peripheral nerves. There is still no systematical analysis on the expression changes of protein acetylation regulators during sciatic nerve development, injury, and regeneration. Here, we sequenced and analyzed the transcriptome of mouse sciatic nerves during development and after injury. We found that the changes in the expression of most regulators followed the rule that “development is consistent with regeneration and opposite to injury.” Immunoblotting with pan-acetylated antibodies also revealed that development and regeneration are a process of increased acetylation, while injury is a process of decreased acetylation. Moreover, we used bioinformatics methods to analyze the possible downstream molecules of two key regulators, histone deacetylase 1 (Hdac1) and lysine acetyltransferase 2b (Kat2b), and found that they were associated with many genes that regulate the cell cycle. Our findings provide an insight into the association of sciatic nerve development, injury, and regeneration from the perspective of protein acetylation.

Endocytosis-associated patterns in nerve regeneration after peripheral nerve injury

Background

Clearance of myelin debris and remyelination of myelin are necessary steps for peripheral nerve remodeling and regeneration. It has yet to be clarified which genes or proteins are involved in endocytosis or exocytosis in the removal of myelin debris during peripheral nerve repair.

Methods

For this project, a rat model of subacute stage of sciatic nerve injury was established first. Subsequently, normal Schwann cells (NSCs) and activated Schwann cells (ASCs) were harvest before and after peripheral nerve injury (PNI). Following methylated DNA immunoprecipitation sequencing (MeDIP-seq) and tandem mass tags (TMT) labeling analysis of NSCs and ASCs, what common biomarkers changes in peripheral nervous systems remain to be elucidated.

Results

A total of 14,770 different expression genes (DEGs) and 3249 different expression proteins (DEPs) were screened between ASCs and NSCs. For the exosomes, the diameter and particles concentration of exosomes were 141.7 ​nm and 2.97 ​× ​10⁷ particles/mL, respectively. The size distribution of exosomes was 50–200 ​nm. ASCs showed higher cellular uptake ability than the NSCs by cellular uptake test. Moreover, RAB7A, ARF6, ARF1, VPS45, RAB11A, DNM3, and NEDD4 were the core markers and may control the molecular mechanism of the Endocytosis pathway.

Conclusion

These biomarkers may play significant roles in the initiation phase of demyelination and axon regeneration.

The translational potential of this article

This study explores that the endocytosis-associated patterns of Schwann cells may be new therapeutic strategy for nerve tissue engineering and nerve regeneration.

Erectile dysfunction resulting from pelvic surgery is associated with changes in cavernosal gene expression indicative of cavernous nerve injury

Pelvic surgery, even without direct cavernous nerve injury, carries a high risk of post-operative erectile dysfunction. The present studies were aimed at identifying molecular mechanisms by which pelvic surgery results in erectile dysfunction. As a model of pelvic surgery, male Sprague-Dawley rats underwent pelvic laparotomy, avoiding direct cavernous nerve injury. A second group of animals, serving as a model of direct cavernous nerve injury, underwent bilateral transection of the cavernous nerve. Cavernosometry demonstrated, that even in the absence of direct nerve injury, the pelvic surgery model exhibited significant erectile dysfunction 3 days post-operatively. Gene expression profiling also demonstrated that even in this animal model of nerve-sparing pelvic surgery, the profile of differentially expressed genes in cavernosal tissue was indicative of cavernous nerve injury. In addition, although 6 hr after surgery there were significant changes in circulating cytokine/chemokine levels, an inflammatory response in the major pelvic ganglion, cavernous nerve and cavernosal tissue was only observed 3 days post-surgery. Our results validate a rat model of pelvic surgery exhibiting erectile dysfunction and suggest systemic release of cytokines/chemokines following surgical trauma might mediate a pathological inflammatory response in tissues distal to the site of surgical trauma, indirectly resulting in cavernous nerve injury and erectile dysfunction.

Identification of key genes involved in axon regeneration and Wallerian degeneration by weighted gene co-expression network analysis

Peripheral nerve injury repair requires a certain degree of cooperation between axon regeneration and Wallerian degeneration. Therefore, investigating how axon regeneration and degeneration work together to repair peripheral nerve injury may uncover the molecular mechanisms and signal cascades underlying peripheral nerve repair and provide potential strategies for improving the low axon regeneration capacity of the central nervous system. In this study, we applied weighted gene co-expression network analysis to identify differentially expressed genes in proximal and distal sciatic nerve segments from rats with sciatic nerve injury. We identified 31 and 15 co-expression modules from the proximal and distal sciatic nerve segments, respectively. Functional enrichment analysis revealed that the differentially expressed genes in proximal modules promoted regeneration, while the differentially expressed genes in distal modules promoted neurodegeneration. Next, we constructed hub gene networks for selected modules and identified a key hub gene, Kif22, which was up-regulated in both nerve segments. In vitro experiments confirmed that Kif22 knockdown inhibited proliferation and migration of Schwann cells by modulating the activity of the extracellular signal-regulated kinase signaling pathway. Collectively, our findings provide a comparative framework of gene modules that are co-expressed in injured proximal and distal sciatic nerve segments, and identify Kif22 as a potential therapeutic target for promoting peripheral nerve injury repair via Schwann cell proliferation and migration. All animal experiments were approved by the Institutional Animal Ethics Committee of Nantong University, China (approval No. S20210322-008) on March 22, 2021.

A Novel NAD Signaling Mechanism in Axon Degeneration and its Relationship to Innate Immunity

Axon degeneration represents a pathological feature of many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease where axons die before the neuronal soma, and axonopathies, such as Charcot-Marie-Tooth disease and hereditary spastic paraplegia. Over the last two decades, it has slowly emerged that a central signaling pathway forms the basis of this process in many circumstances. This is an axonal NAD-related signaling mechanism mainly regulated by the two key proteins with opposing roles: the NAD-synthesizing enzyme NMNAT2, and SARM1, a protein with NADase and related activities. The crosstalk between the axon survival factor NMNAT2 and pro-degenerative factor SARM1 has been extensively characterized and plays an essential role in maintaining the axon integrity. This pathway can be activated in necroptosis and in genetic, toxic or metabolic disorders, physical injury and neuroinflammation, all leading to axon pathology. SARM1 is also known to be involved in regulating innate immunity, potentially linking axon degeneration to the response to pathogens and intercellular signaling. Understanding this NAD-related signaling mechanism enhances our understanding of the process of axon degeneration and enables a path to the development of drugs for a wide range of neurodegenerative diseases.

Transcriptomic analysis reveals essential microRNAs after peripheral nerve injury

Studies have shown that microRNAs (miRNAs) mediate posttranscriptional regulation of target genes and participate in various physiological and pathological processes, including peripheral nerve injury. However, it is hard to select key miRNAs with essential biological functions among a large number of differentially expressed miRNAs. Previously, we collected injured sciatic nerve stumps at multiple time points after nerve crush injury, examined gene changes at different stages (acute, sub-acute, and post-acute), and obtained mRNA expression profiles. Here, we jointly analyzed mRNAs and miRNAs, and investigated upstream miRNAs of differentially expressed mRNAs using Ingenuity Pathway Analysis bioinformatic software. A total of 31, 42, 30, and 23 upstream miRNAs were identified at 1, 4, 7, and 14 days after rat sciatic nerve injury, respectively. Temporal expression patterns and biological involvement of commonly involved upstream miRNAs (miR-21, let-7, miR-223, miR-10b, miR-132, miR-15b, miR-127, miR-29a, miR-29b, and miR-9) were then determined at multiple time points. Expression levels of miR-21, miR-132, miR-29a, and miR-29b were robustly increased after sciatic nerve injury. Biological processes involving these miRNAs include multicellular organismal response to stress, positive regulation of the epidermal growth factor receptor signaling pathway, negative regulation of epithelial cell differentiation, and regulation of myocardial tissue growth. Moreover, we constructed mechanistic networks of let-7, miR-21, and miR-223, the most significantly involved upstream miRNAs. Our findings reveal that multiple upstream miRNAs (i.e., let-7, miR-21, and miR-223) were associated with gene expression changes in rat sciatic nerve stumps after nerve injury, and these miRNAs play an important role in peripheral nerve regeneration. This study was approved by the Experimental Animal Ethics Committee of Jiangsu Province of China (approval No. 20190303-18) on March 3, 2019.

Transcriptome analysis of molecular mechanisms underlying facial nerve injury repair in rats

Although the transcriptional alterations inside the facial nucleus after facial nerve injury have been well studied, the gene expression changes in the facial nerve trunk after injury are still unknown. In this study, we established an adult rat model of facial nerve crush injury by compressing the right lateral extracranial nerve trunk. Transcriptome sequencing, differential gene expression analysis, and cluster analysis of the injured facial nerve trunk were performed, and 39 intersecting genes with significant variance in expression were identified. Gene Ontology annotation and Kyoto Encyclopedia of Genes and Genomes pathway analyses of the 39 intersecting genes revealed that these genes are mostly involved in leukocyte cell-cell adhesion and phagocytosis and have essential roles in regulating nerve repair. Quantitative real-time polymerase chain reaction assays were used to validate the expression of pivotal genes. Finally, nine pivotal genes that contribute to facial nerve recovery were identified, including Arhgap30, Akr1b8, C5ar1, Csf2ra, Dock2, Hcls1, Inpp5d, Sla, and Spi1. Primary Schwann cells were isolated from the sciatic nerve of neonatal rats. After knocking down Akr1b8 in Schwann cells with an Akr1b8-specific small interfering RNA plasmid, expression levels of monocyte chemoattractant protein-1 and interleukin-6 were decreased, while cell proliferation and migration were not obviously altered. These findings suggest that Akr1b8 likely regulates the interaction between Schwann cells and macrophages through regulation of cytokine expression to promote facial nerve regeneration. This study is the first to reveal a transcriptome change in the facial nerve trunk after facial nerve injury, thereby revealing the potential mechanism underlying repair of facial nerve injury. This study was approved by the Animal Ethics Committee of Nantong University, China in 2018 (approval No. S20180923-007).

Characteristics of cytokines in the sciatic nerve stumps and DRGs after rat sciatic nerve crush injury

BACKGROUND

Cytokines are essential cellular modulators of various physiological and pathological activities, including peripheral nerve repair and regeneration. However, the molecular changes of these cellular mediators after peripheral nerve injury are still unclear. This study aimed to identify cytokines critical for the regenerative process of injured peripheral nerves.

METHODS

The sequencing data of the injured nerve stumps and the dorsal root ganglia (DRGs) of Sprague-Dawley (SD) rats subjected to sciatic nerve (SN) crush injury were analyzed to determine the expression patterns of genes coding for cytokines. PCR was used to validate the accuracy of the sequencing data.

RESULTS

A total of 46, 52, and 54 upstream cytokines were differentially expressed in the SNs at 1 day, 4 days, and 7 days after nerve injury. A total of 25, 28, and 34 upstream cytokines were differentially expressed in the DRGs at these time points. The expression patterns of some essential upstream cytokines are displayed in a heatmap and were validated by PCR. Bioinformatic analysis of these differentially expressed upstream cytokines after nerve injury demonstrated that inflammatory and immune responses were significantly involved.

CONCLUSIONS

In summary, these findings provide an overview of the dynamic changes in cytokines in the SNs and DRGs at different time points after nerve crush injury in rats, elucidate the biological processes of differentially expressed cytokines, especially the important roles in inflammatory and immune responses after peripheral nerve injury, and thus might contribute to the identification of potential treatments for peripheral nerve repair and regeneration.

Gene set enrichment analysis and protein-protein interaction network analysis after sciatic nerve injury

Background

Peripheral nerves are able to regenerate spontaneously after injury. An increasing number of studies have investigated the mechanism of peripheral nerve regeneration and attempted to find potential therapeutic targets. The various bioinformatics analysis tools available, gene set enrichment analysis (GSEA) and protein-protein interaction (PPI) networks can effectively screen the crucial targets of neuroregeneration.

Methods

GSEA and PPI networks were constructed through ingenuity pathway analysis and sequential gene expression validation ex vitro to investigate the molecular processes at 1, 4, 7, and 14 days following sciatic nerve transection in rats.

Results

Immune response and the activation of related canonical pathways were classified as crucial biological events. Additionally, neural precursor cell expressed developmentally downregulated 4-like (NEDD4L), neuregulin 1 (NRG1), nuclear factor of activated T cells 2 (NFATC2), midline 1 (MID1), GLI family zinc finger 2 (GLI2), and ventral anterior homeobox 1 (VAX1), which were jointly involved in both immune response and axonal regeneration, were screened and their mRNA and protein expressions following nerve injury were validated. Among them, the expression of VAX1 continuously increased following nerve injury, and it was considered to be a potential therapeutic target.

Conclusions

The combined use of GSEA and PPI networks serves as a valuable way to identify potential therapeutic targets for neuroregeneration.

Protein phosphorylation profiling of peripheral nerve regeneration after autologous nerve grafting

Autologous nerve grafting is the golden standard therapeutic approach of peripheral nerve injury. However, the clinical effect of autologous nerve grafting is still unsatisfying. To achieve better clinical functional recovery, it is of an impending need to expand our understanding of the dynamic cellular and molecular changes after nerve transection and autologous nerve transplantation. To address this aim, in the current study, rats were subjected to sciatic nerve transection and autologous nerve grafting. Rat sciatic nerve segments were collected at 4, 7, and 14 days after surgery and subjected to antibody array analysis to determine phosphoprotein profiling patterns. Compared with rats that underwent sham surgery, a total of 48, 19, and 75 differentially expressed phosphoproteins with fold changes > 2 or < -2 were identified at 4, 7, and 14 days after autologous nerve grafting, respectively. Several phosphoproteins, including STAM2 (Phospho-Tyr192) and Tau (Phospho-Ser422), were found to be differentially expressed at multiple time points, suggesting the importance of the phosphorylation of these proteins. Western blot validation of the expression patterns of STAM2 (Phospho-Tyr192) indicated the accuracy of antibody array assay. Bioinformatic analysis of these differentially expressed proteins suggested that cellular behavior and organ morphology were significantly involved biological functions while cell behavior and immune response-related signaling pathways were significantly involved canonical signaling pathways. These outcomes contributed to the illumination of the molecular mechanisms underlying autologous nerve grafting from the phosphoprotein profiling perspective.

Sequencing analysis of matrix metalloproteinase 7-induced genetic changes in Schwann cells

Previous research revealed the positive activity of matrix metalloproteinase 7 (MMP7) on migration and myelin regeneration of Schwann cells (SCs). However, understanding of the molecular changes and biological activities induced by increased amounts of MMP7 in SCs remains limited. To better understand the underlying molecular events, primary SCs were isolated from the sciatic nerve stump of newborn rats and cultured with 10 nM human MMP7 for 24 hours. The results of genetic testing were analyzed at a relatively relaxed threshold value (fold change ≥ 1.5 and P-value < 0.05). Upon MMP7 exposure, 149 genes were found to be upregulated in SCs, whereas 133 genes were downregulated. Gene Ontology analysis suggested that many differentially expressed molecules were related to cellular processes, single-organism processes, and metabolic processes. Kyoto Enrichment of Genes and Genomes pathway analysis further indicated the critical involvement of cell signaling and metabolism in MMP7-induced molecular regulation of SCs. Results of Ingenuity Pathway Analysis (IPA) also revealed that MMP7 regulates biological processes, molecular functions, cellular components, diseases and functions, biosynthesis, material metabolism, cell movement, and axon guidance. The outcomes of further analysis will deepen our comprehension of MMP7-induced biological changes in SCs. This study was approved by the Laboratory Animal Ethics Committee of Nantong University, China (approval No. 20190225-004) on February 27, 2019.

Characterising cellular and molecular features of human peripheral nerve degeneration

Nerve regeneration is a key biological process in those recovering from neural trauma. From animal models it is known that the regenerative capacity of the peripheral nervous system (PNS) relies heavily on the remarkable ability of Schwann cells to undergo a phenotypic shift from a myelinating phenotype to one that is supportive of neural regeneration. In rodents, a great deal is known about the molecules that control this process, such as the transcription factors c-Jun and early growth response protein 2 (EGR2/KROX20), or mark the cells and cellular changes involved, including SOX10 and P75 neurotrophin receptor (p75NTR). However, ethical and practical challenges associated with studying human nerve injury have meant that little is known about human nerve regeneration.The present study addresses this issue, analysing 34 denervated and five healthy nerve samples from 27 patients retrieved during reconstructive nerve procedures. Using immunohistochemistry and Real-Time quantitative Polymerase Chain Reaction (RT-qPCR), the expression of SOX10, c-Jun, p75NTR and EGR2 was assessed in denervated samples and compared to healthy nerve. Nonparametric smoothing linear regression was implemented to better visualise trends in the expression of these markers across denervated samples.It was found, first, that two major genes associated with repair Schwann cells in rodents, c-Jun and p75NTR, are also up-regulated in acutely injured human nerves, while the myelin associated transcription factor EGR2 is down-regulated, observations that encourage the view that rodent models are relevant for learning about human nerve injury. Second, as in rodents, the expression of c-Jun and p75NTR declines during long-term denervation. In rodents, diminishing c-Jun and p75NTR levels mark the general deterioration of repair cells during chronic denervation, a process thought to be a major obstacle to effective nerve repair. The down-regulation of c-Jun and p75NTR reported here provides the first molecular evidence that also in humans, repair cells deteriorate during chronic denervation.

Genetic changes in rat proximal nerve stumps after sciatic nerve transection

Background

Peripheral nerves can self-regenerate after traumatic injury, although their self-regeneration ability is limited after severe nerve injury. After peripheral nerve injury, the distal nerve stumps undergo Wallerian degeneration while the proximal nerve stumps undergo a regeneration process.

Methods

Here, to decipher genetic changes and involved biological processes in the proximal nerve stumps after peripheral nerve injury, microarray data (GSE30165) were analyzed. Differentially expressed genes in the proximal nerve stumps at 0.5 h, 1 h, 3 h, 6 h, 9 h, 1 d, 4 d, 7 d, and 14 d after rat sciatic nerve transection were subjected to Ingenuity pathway analysis (IPA) bioinformatic analysis.

Results

Cytokine signaling, cellular immune response, nuclear receptor signaling, disease-specific pathways, and organismal growth and development were significantly activated in the proximal nerve stumps after nerve transection. Organ development, inflammation and immune response, diseases and organ abnormalities, and cellular behavior-related biological functions were highly involved.

Conclusions

The expression levels of differentially expressed genes in biological function "Organismal Injury and Abnormalities" were displayed and validated. Our current study helps to obtain a better understanding of the biological processes of peripheral nerve regeneration, especially the regeneration process in the proximal nerve stumps, and thus may help to discover new therapeutic methods that can promote nerve regeneration.

Protein microarray analysis of cytokine expression changes in distal stumps after sciatic nerve transection

A large number of chemokines, cytokines, other trophic factors and the extracellular matrix molecules form a favorable microenvironment for peripheral nerve regeneration. This microenvironment is one of the major factors for regenerative success. Therefore, it is important to investigate the key molecules and regulators affecting nerve regeneration after peripheral nerve injury. However, the identities of specific cytokines at various time points after sciatic nerve injury have not been determined. The study was performed by transecting the sciatic nerve to establish a model of peripheral nerve injury and to analyze, by protein microarray, the expression of different cytokines in the distal nerve after injury. Results showed a large number of cytokines were up-regulated at different time points post injury and several cytokines, e.g., ciliary neurotrophic factor, were downregulated. The construction of a protein-protein interaction network was used to screen how the proteins interacted with differentially expressed cytokines. Kyoto Encyclopedia of Genes and Genomes pathway and Gene ontology analyses indicated that the differentially expressed cytokines were significantly associated with chemokine signaling pathways, Janus kinase/signal transducers and activators of transcription, phosphoinositide 3-kinase/protein kinase B, and notch signaling pathway. The cytokines involved in inflammation, immune response and cell chemotaxis were up-regulated initially and the cytokines involved in neuronal apoptotic processes, cell-cell adhesion, and cell proliferation were up-regulated at 28 days after injury. Western blot analysis showed that the expression and changes of hepatocyte growth factor, glial cell line-derived neurotrophic factor and ciliary neurotrophic factor were consistent with the results of protein microarray analysis. The results provide a comprehensive understanding of changes in cytokine expression and changes in these cytokines and classical signaling pathways and biological functions during Wallerian degeneration, as well as a basis for potential treatments of peripheral nerve injury. The study was approved by the Institutional Animal Care and Use Committee of the Chinese PLA General Hospital, China (approval number: 2016-x9-07) in September 2016.

Emergence of SARM1 as a Potential Therapeutic Target for Wallerian-type Diseases

Wallerian degeneration is a neuronal death pathway that is triggered in response to injury or disease. Death was thought to occur passively until the discovery of a mouse strain, i.e., Wallerian degeneration slow (WLD^S), which was resistant to degeneration. Given that the WLD^S mouse encodes a gain-of-function fusion protein, its relevance to human disease was limited. The later discovery that SARM1 (sterile alpha and toll/interleukin receptor [TIR] motif-containing protein 1) promotes Wallerian degeneration suggested the existence of a pathway that might be targeted therapeutically. More recently, SARM1 was found to execute degeneration by hydrolyzing NAD⁺. Notably, SARM1 knockdown or knockout prevents neuron degeneration in response to a range of insults that lead to peripheral neuropathy, traumatic brain injury, and neurodegenerative disease. Here, we discuss the role of SARM1 in Wallerian degeneration and the opportunities to target this enzyme therapeutically.

The Effects of Surgical Antiseptics and Time Delays on RNA Isolated From Human and Rodent Peripheral Nerves

Peripheral Nerve Injury (PNI) is common following blunt or penetrating trauma with an estimated prevalence of 2% among the trauma population. The resulting economic and societal impacts are significant. Nerve regeneration is a key biological process in those recovering from neural trauma. Real Time-quantitative Polymerase Chain Reaction (RT-qPCR) and RNA sequencing (RNA seq) are investigative methods that are often deployed by researchers to characterize the cellular and molecular mechanisms that underpin this process. However, the ethical and practical challenges associated with studying human nerve injury have meant that studies of nerve injury have largely been limited to rodent models of renervation. In some circumstances it is possible to liberate human nerve tissue for study, for example during reconstructive nerve repair. This complex surgical environment affords numerous challenges for optimizing the yield of RNA in sufficient quantity and quality for downstream RT-qPCR and/or RNA seq applications. This study characterized the effect of: (1) Time delays between surgical liberation and cryopreservation and (2) contact with antiseptic surgical reagents, on the quantity and quality of RNA isolated from human and rodent nerve samples. It was found that time delays of greater than 3 min between surgical liberation and cryopreservation of human nerve samples significantly decreased RNA concentrations to be sub-optimal for downstream RT-qPCR/RNA seq applications (<5 ng/μl). Minimizing the exposure of human nerve samples to antiseptic surgical reagents significantly increased yield of RNA isolated from samples. The detrimental effect of antiseptic reagents on RNA yield was further confirmed in a rodent model where RNA yield was 8.3-fold lower compared to non-exposed samples. In summary, this study has shown that changes to the surgical tissue collection protocol can have significant effects on the yield of RNA isolated from nerve samples. This will enable the optimisation of protocols in future studies, facilitating characterisation of the cellular and molecular mechanisms that underpin the regenerative capacity of the human peripheral nervous system.

Novel microRNA, miR-sc6, modulates Schwann cell phenotype via targeting ErbB4

MicroRNAs (miRNAs) are non-coding RNAs that regulate various tissues and organs, including the nervous system. Peripheral nerve injury is a common pathology of the nervous system and leads to differential expressions of a variety of miRNAs. Previously, a group of novel miRNAs have been identified in rat proximal nerve segments after sciatic nerve transection. However, the biological functions of these novel miRNAs remain undetermined. The aim of the current study was therefore to identify the function of a novel miRNA, miR-sc6, following nerve injury. Its target genes and effects on phenotypic modulation of Schwann cells were determined using a miR-sc6 mimic transfection. These observations contribute to the understanding of miRNA involvement in peripheral nerve injury and the cognition of regulatory mechanisms in peripheral nerve regeneration.

Melanoma-Induced Reprogramming of Schwann Cell Signaling Aids Tumor Growth

The tumor microenvironment has been compared with a nonhealing wound involving a complex interaction between multiple cell types. Schwann cells, the key regulators of peripheral nerve repair, have recently been shown to directly affect nonneural wound healing. Their role in cancer progression, however, has been largely limited to neuropathic pain and perineural invasion. In this study, we showed that melanoma activated otherwise dormant functions of Schwann cells aimed at nerve regeneration and wound healing. Such reprogramming of Schwann cells into repair-like cells occurred during the destruction and displacement of neurons as the tumor expanded and via direct signaling from melanoma cells to Schwann cells, resulting in activation of the nerve injury response. Melanoma-activated Schwann cells significantly altered the microenvironment through their modulation of the immune system and the extracellular matrix in a way that promoted melanoma growth in vitro and in vivo. Local inhibition of Schwann cell activity following cutaneous sensory nerve transection in melanoma orthotopic models significantly decreased the rate of tumor growth. Tumor-associated Schwann cells, therefore, can have a significant protumorigenic effect and may present a novel target for cancer therapy. SIGNIFICANCE: These findings reveal a role of the nerve injury response, particularly through functions of activated Schwann cells, in promoting melanoma growth.

The microRNAs let-7 and miR-9 down-regulate the axon-guidance genes Ntn1 and Dcc during peripheral nerve regeneration

Axon guidance helps growing neural axons to follow precise paths to reach their target locations. It is a critical step for both the formation and regeneration of neuronal circuitry. Netrin-1 (Ntn1) and its receptor, deleted in colorectal carcinoma (Dcc) are essential factors for axon guidance, but their regulation in this process is incompletely understood. In this study, using quantitative real-time RT-PCR (qRT-PCR) and biochemical and reporter gene assays, we found that the Ntn1 and Dcc genes are both robustly up-regulated in the sciatic nerve stump after peripheral nerve injury. Moreover, we found that the microRNA (miR) let-7 directly targets the Ntn1 transcript by binding to its 3'-untranslated region (3'-UTR), represses Ntn1 expression, and reduces the secretion of Ntn1 protein in Schwann cells. We also identified miR-9 as the regulatory miRNA that directly targets Dcc and found that miR-9 down-regulates Dcc expression and suppresses the migration ability of Schwann cells by regulating Dcc abundance. Functional examination in dorsal root ganglion neurons disclosed that let-7 and miR-9 decrease the protein levels of Ntn1 and Dcc in these neurons, respectively, and reduce axon outgrowth. Moreover, we identified a potential regulatory network comprising let-7, miR-9, Ntn1, Dcc, and related molecules, including the RNA-binding protein Lin-28 homolog A (Lin28), SRC proto-oncogene nonreceptor tyrosine kinase (Src), and the transcription factor NF-κB. In summary, our findings reveal that the miRs let-7 and miR-9 are involved in regulating neuron pathfinding and extend our understanding of the regulatory pathways active during peripheral nerve regeneration.

Novel miRNA, miR-sc14, promotes Schwann cell proliferation and migration

MicroRNAs refer to a class of endogenous, short non-coding RNAs that mediate numerous biological functions. MicroRNAs regulate various physiological and pathological activities of peripheral nerves, including peripheral nerve repair and regeneration. Previously, using a rat sciatic nerve injury model, we identified many functionally annotated novel microRNAs, including miR-sc14. Here, we used real-time reverse transcription-polymerase chain reaction to examine miR-sc14 expression in rat sciatic nerve stumps. Our results show that miR-sc14 is noticeably altered following sciatic nerve injury, being up-regulated at 1 day and diminished at 7 days. EdU and transwell chamber assay results showed that miR-sc14 mimic promoted proliferation and migration of Schwann cells, while miR-sc14 inhibitor suppressed their proliferation and migration. Additionally, bioinformatic analysis examined potential target genes of miR-sc14, and found that fibroblast growth factor receptor 2 might be a potential target gene. Specifically, our results show changes of miR-sc14 expression in the sciatic nerve of rats at different time points after nerve injury. Appropriately, up-regulation of miR-sc14 promoted proliferation and migration of Schwann cells. Consequently, miR-sc14 may be an intervention target to promote repair of peripheral nerve injury. The study was approved by the Jiangsu Provincial Laboratory Animal Management Committee, China on March 4, 2015 (approval No. 20150304-004).

Systemic functional enrichment and ceRNA network identification following peripheral nerve injury

Peripheral nerve injury is a worldwide clinical issue that impacts patients' quality of life and causes huge society and economic burden. Injured peripheral nerves are able to regenerate by themselves. However, for severe peripheral nerve injury, the regenerative abilities are very limited and the regenerative effects are very poor. A better understanding of the mechanisms following peripheral nerve injury will benefit its clinical treatment. In this study, we systematically explored the dynamic changes of mRNAs and long non-coding RNAs (lncRNAs) in the injured sciatic nerve segments after nerve crush, identified significantly involved Gene ontology (GO) terms and Kyoto Enrichment of Genes and Genomes (KEGG) pathways, and innovatively analyzed the correlation of differentially expressed mRNAs and lncRNAs. After the clustering of co-expressed mRNAs and lncRNAs, we performed functional analysis, selected GO term "negative regulation of cell proliferation", and constructed a competing endogenous RNA (ceRNA) network of LIF and HMOX1 gene in this GO term. This study is the first to provide a systematic dissection of mRNA-microRNA (miRNA)-lncRNA ceRNA network following peripheral nerve injury and thus lays a foundation for further investigations of the regulating mechanisms of non-coding RNAs in peripheral nerve repair and regeneration.

Transcriptional landscape of alternative splicing during peripheral nerve injury

Alternative splicing (AS) regulates a variety of biological activities in numerous tissues and organs, including the nervous system. However, the existence and specific roles of AS events during peripheral nerve repair and regeneration remain largely undetermined. In the current study, by mapping splice-crossing sequence reads, we identified AS events and relevant spliced genes in rat sciatic nerve stumps following sciatic nerve crush. AS-related genes at 1, 4, 7, and 14 days post nerve crush were compared with those at 0 day to discover alternatively spliced genes induced by sciatic nerve crush. These injury-induced alternatively spliced genes were then categorized to diseases and biological functions, genetic networks, and canonical signaling pathways. Bioinformatic analysis indicated that these alternatively spliced genes were mainly correlated to immune response, cellular growth, and cellular function maintenance. Our study elucidated AS events following peripheral nerve injury and might help deepen our understanding of the molecular mechanisms underlying peripheral nerve regeneration.

Pathophysiological Changes of Physical Barriers of Peripheral Nerves After Injury

Peripheral nerves are composed of complex layered anatomical structures, including epineurium, perineurium, and endoneurium. Perineurium and endoneurium contain many physical barriers, including the blood-nerve barrier at endoneurial vessels and the perineurial barrier. These physical barriers help to eliminate flux penetration and thus contribute to the establishment of a stable microenvironment. In the current review, we introduce the anatomical compartments and physical barriers of peripheral nerves and then describe the cellular and molecular basis of peripheral physical barriers. We also specifically explore peripheral nerve injury-induced changes of peripheral physical barriers, including elevated endoneurial fluid pressure, increased leakage of tracer, decreased barrier-type endothelial cell ratio, and altered distributions and expressions of cellular junctional proteins. The understanding of the pathophysiological changes of physical barriers following peripheral nerve injury may provide a clue for the treatment of peripheral nerve injury.

Comparative transcriptomic profiling of peripheral efferent and afferent nerve fibres at different developmental stages in mice

Peripheral nerve injury impairs motor and sensory function in humans, and its functional recovery largely depends on the axonal outgrowth required for the accurate reinnervation of appropriate targets. To better understand how motor and sensory nerve fibres select their terminal pathways, an unbiased cDNA microarray analysis was conducted to examine differential gene expression patterns in peripheral efferent and afferent fibres at different developmental stages in mice. Gene ontology (GO) and Kyoto Enrichment of Genes and Genomes (KEGG) analyses revealed common and distinct features of enrichment for differentially expressed genes during motor and sensory nerve fibre development. Ingenuity Pathway Analysis (IPA) further indicated that the key differentially expressed genes were associated with trans-synaptic neurexin-neuroligin signalling components and a variety of gamma-aminobutyric acid (GABA) receptors. The aim of this study was to generate a framework of gene networks regulated during motor and sensory neuron differentiation/maturation. These data may provide new clues regarding the underlying cellular and molecular mechanisms that determine the intrinsic capacity of neurons to regenerate after peripheral nerve injury. Our findings may thus facilitate further development of a potential intervention to manipulate the therapeutic efficiency of peripheral nerve repair in the clinic.

miR-3075 Inhibited the Migration of Schwann Cells by Targeting Cntn2

Peripheral nerve injury is a complex biological process that involves the expression changes of various coding and non-coding RNAs. Previously, a number of novel miRNAs that were dysregulated in rat sciatic nerve stumps after peripheral nerve injury were identified and functionally annotated by Solexa sequencing. In the current study, we studied one of these identified novel miRNAs, miR-3075, in depth. Results of transwell-based cell migration assay showed that increased expression of miR-3075 suppressed the migration rate of Schwann cells while decreased expression of miR-3075 elevated the migration rate of Schwann cells, demonstrating that miR-3075 inhibited Schwann cell migration. Results of BrdU cell proliferation assay showed that neither miR-3075 mimic nor miR-3075 inhibitor would affect Schwann cell proliferation. We further studied candidate target genes of miR-3075 by using bioinformatic tools and analyzing gene expression patterns and found that miR-3075 might target contactin 2 (Cntn2). Previous study showed that Cntn2 regulated cell migration and myelination. Our current observation suggested that the biological effects of miR-3075 on Schwann cell phenotype might by through the negative regulation of Cntn2. Overall, our study revealed the function of a novel miRNA, miR-3075, and expanded our current understanding of the molecular mechanisms underlying peripheral nerve injury and regeneration.

Soluble Neuregulin1 Down-Regulates Myelination Genes in Schwann Cells

Peripheral nerves are characterised by the ability to regenerate after injury. Schwann cell activity is fundamental for all steps of peripheral nerve regeneration: immediately after injury they de-differentiate, remove myelin debris, proliferate and repopulate the injured nerve. Soluble Neuregulin1 (NRG1) is a growth factor that is strongly up-regulated and released by Schwann cells immediately after nerve injury. To identify the genes regulated in Schwann cells by soluble NRG1, we performed deep RNA sequencing to generate a transcriptome database and identify all the genes regulated following 6 h stimulation of primary adult rat Schwann cells with soluble recombinant NRG1. Interestingly, the gene ontology analysis of the transcriptome reveals that NRG1 regulates genes belonging to categories that are regulated in the peripheral nerve immediately after an injury. In particular, NRG1 strongly inhibits the expression of genes involved in myelination and in glial cell differentiation, suggesting that NRG1 might be involved in the de-differentiation (or "trans-differentiation") process of Schwann cells from a myelinating to a repair phenotype. Moreover, NRG1 inhibits genes involved in the apoptotic process, and up-regulates genes positively regulating the ribosomal RNA processing, thus suggesting that NRG1 might promote cell survival and stimulate new protein expression. This in vitro transcriptome analysis demonstrates that in Schwann cells NRG1 drives the expression of several genes which partially overlap with genes regulated in vivo after peripheral nerve injury, underlying the pivotal role of NRG1 in the first steps of the nerve regeneration process.

Transcriptome analysis of adherens junction pathway-related genes after peripheral nerve injury

The neural regeneration process is driven by a wide range of molecules and pathways. Adherens junctions are critical cellular junctions for the integrity of peripheral nerves. However, few studies have systematically characterized the transcript changes in the adherens junction pathway following injury. In this study, a rat model of sciatic nerve crush injury was established by forceps. Deep sequencing data were analyzed using comprehensive transcriptome analysis at 0, 1, 4, 7, and 14 days after injury. Results showed that most individual molecules in the adherens junctions were either upregulated or downregulated after nerve injury. The mRNA expression of ARPC1B, ARPC3, TUBA8, TUBA1C, CTNNA2, ACTN3, MET, HGF, NME1 and ARF6, which are involved in the adherens junction pathway and in remodeling of adherens junctions, was analyzed using quantitative real-time polymerase chain reaction. Most of these genes were upregulated in the sciatic nerve stump following peripheral nerve injury, except for CTNNA2, which was downregulated. Our findings reveal the dynamic changes of key molecules in adherens junctions and in remodeling of adherens junctions. These key genes provide a reference for the selection of clinical therapeutic targets for peripheral nerve injury.

Analysis of biological functional networks during sciatic nerve repair and regeneration

Peripheral nerve injury is a common disease with a low recovery rate. A better understanding of the molecular changes underlying peripheral nerve injury and regeneration may contribute to the development of novel therapies for the treatment of peripheral nerve injury. In the current study, we analyzed differentially expressed genes in rat sciatic nerve stumps at 1, 4, 7, and 14 days post nerve crush and built biological functional networks at each time point. Our outcomes suggested that "Neurological Disease" involved networks were significant at 1 day post nerve crush, "Cellular Assembly and Organization" involved networks were important at 4 and 7 days post nerve crush, while "Tissue Morphology" involved networks were important at 14 days post nerve crush. We also investigated the temporal expression patterns as well as central genes of these critical networks. Taken together, our study revealed genetic networks and gene-gene interactions in the injured nerve stumps and thus might enhance our understanding of peripheral nerve regeneration.

Critical signaling pathways during Wallerian degeneration of peripheral nerve

Wallerian degeneration is a critical biological process that occurs in distal nerve stumps after nerve injury. To systematically investigate molecular changes underlying Wallerian degeneration, we used a rat sciatic nerve transection model to examine microarray analysis outcomes and investigate significantly involved Kyoto Enrichment of Genes and Genomes (KEGG) pathways in injured distal nerve stumps at 0, 0.5, 1, 6, 12, and 24 hours, 4 days, 1, 2, 3, and 4 weeks after peripheral nerve injury. Bioinformatic analysis showed that only one KEGG pathway (cytokine-cytokine receptor interaction) was significantly enriched at an early time point (1 hour post-sciatic nerve transection). At later time points, the number of enriched KEGG pathways initially increased and then decreased. Three KEGG pathways were studied in further detail: cytokine-cytokine receptor interaction, neuroactive ligand-receptor interaction, and axon guidance. Moreover, temporal expression patterns of representative differentially expressed genes in these KEGG pathways were validated by real time-polymerase chain reaction. Taken together, the above three signaling pathways are important after sciatic nerve injury, and may increase our understanding of the molecular mechanisms underlying Wallerian degeneration