Role of transcription factors in peripheral nerve regeneration

Engineering the Future of Restorative Clinical Peripheral Nerve Surgery

Peripheral nerve injury is a significant clinical challenge, often leading to permanent functional deficits. Standard interventions, such as autologous nerve grafts or distal nerve transfers, require sacrificing healthy nerve tissue and typically result in limited motor or sensory recovery. Nerve regeneration is complex and influenced by several factors: 1) the regenerative capacity of proximal neurons, 2) the ability of axons and support cells to bridge the injury, 3) the capacity of Schwann cells to maintain a supportive environment, and 4) the readiness of target muscles or sensory organs for reinnervation. Emerging bioengineering solutions, including biomaterials, drug delivery systems, fusogens, electrical stimulation devices, and tissue-engineered products, aim to address these challenges. Effective translation of these therapies requires a deep understanding of the physiology and pathology of nerve injury. This article proposes a comprehensive framework for developing restorative strategies that address all four major physiological responses in nerve repair. By implementing this framework, we envision a paradigm shift that could potentially enable full functional recovery for patients, where current approaches offer minimal hope.

Absence of T-box transcription factor 21 limits neuromuscular junction recovery after nerve injury in T-bet-knockout mice

Introduction

Terminal Schwann cells (tSCs), at the neuromuscular junction (NMJ), play critical roles in the repair of motor axon terminals at muscle, and rebuild neuronal signaling following nerve injury. Knowledge of mediators impacting tSCs post-nerve injury and in disease may guide beneficial therapies to improve motor outcomes. We previously found T-box transcription factor 21 (TBX21/TBET), classically associated with T-helper1 cells and immune cell recruitment, is expressed in tSCs at the mouse NMJ. The purpose of this study was to examine effects of Tbx21 absence during NMJ regeneration following peripheral nerve injury.

Methods

Wildtype (WT) and Tbet-knockout (Tbet-KO) mice underwent sciatic nerve transection and immediate repair. Functional muscle recovery assessment was performed with muscle force testing on mice at 2-, 3-, 4-, and 6-week (wks) and 6 months after nerve injury repair. Morphometric analyses of NMJ reinnervation, tSC number, and tSC processes were evaluated. Full NMJ reinnervation was defined as ≥75% coverage of endplates by axons. A minimum of three mice were evaluated in each group, and 50-100 NMJs were evaluated per mouse.

Results

Tbet-KO mice had significantly diminished muscle function compared to WT mice at every time point beyond 3 weeks. Tbet-KO mice showed just over half of the muscle force generated by WT mice at 4 weeks and 6 weeks post-injury and repair. By 6 months, Tbet-KO mice generated only 84.1% the muscle force of WT mice. Tbet-KO mice showed significantly decreased levels of fully reinnervated NMJs compared to WT mice at each time point tested. Tbet-KO mice also showed a lower number of tSCs with reduced cytoplasmic processes beyond NMJ area and lower number of immune cells during process of NMJ regeneration.

Discussion

Our findings show that the Tbx21 transcription factor promotes NMJ reinnervation to regain muscle function following nerve injury.

Modulation of pain sensitivity by Ascl1- and Lhx6-dependent GABAergic neuronal function in streptozotocin diabetic mice

Painful diabetic neuropathy commonly affects the peripheral nervous system in individuals with diabetes. However, the pathological processes and mechanisms underlying diabetic neuropathic pain remain unclear. We aimed to identify the overall profiles and screen for genes potentially involved in pain mechanisms using transcriptome analysis of the dorsal root ganglion of diabetic mice treated with streptozotocin (STZ). Using RNA sequencing, we identified differentially expressed genes between streptozotocin-treated diabetic mice and controls, focusing on altered GABAergic neuron-related genes and inflammatory pathways. Behavioral and molecular analyses revealed a marked reduction in GABAergic neuronal markers (GAD65, GAD67, VGAT) and increased pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) in the diabetic group compared with controls. Intrathecal administration of lentiviral vectors expressing transcription factors Ascl1 and Lhx6 reversed pain hypersensitivity and restored normal expression of GABAergic genes and inflammatory mediators. Protein-protein interaction network analysis revealed five key proteins influenced by Ascl1 and Lhx6 treatment, including those in the JunD/FosB/C-fos signaling pathway. These findings suggest that Ascl1 and Lhx6 mitigate diabetic neuropathic pain by modulating GABAergic neuronal function, pro-inflammatory responses, and pain-related channels (TRPV1, Nav1.7). These results provide a basis for developing transcription factor-based therapies targeting GABAergic neurons for diabetic neuropathic pain relief.

Chromatin accessibility: biological functions, molecular mechanisms and therapeutic application

The dynamic regulation of chromatin accessibility is one of the prominent characteristics of eukaryotic genome. The inaccessible regions are mainly located in heterochromatin, which is multilevel compressed and access restricted. The remaining accessible loci are generally located in the euchromatin, which have less nucleosome occupancy and higher regulatory activity. The opening of chromatin is the most important prerequisite for DNA transcription, replication, and damage repair, which is regulated by genetic, epigenetic, environmental, and other factors, playing a vital role in multiple biological progresses. Currently, based on the susceptibility difference of occupied or free DNA to enzymatic cleavage, solubility, methylation, and transposition, there are many methods to detect chromatin accessibility both in bulk and single-cell level. Through combining with high-throughput sequencing, the genome-wide chromatin accessibility landscape of many tissues and cells types also have been constructed. The chromatin accessibility feature is distinct in different tissues and biological states. Research on the regulation network of chromatin accessibility is crucial for uncovering the secret of various biological processes. In this review, we comprehensively introduced the major functions and mechanisms of chromatin accessibility variation in different physiological and pathological processes, meanwhile, the targeted therapies based on chromatin dynamics regulation are also summarized.

Types of Short-Duration Electrical Stimulation-Induced Efficiency in the Axonal Regeneration and Recovery: Comparative in Vivo Study in Rat Model of Repaired Sciatic Nerve and its Tibial Branch after Transection Injury

The restoration of adequate function and sensation in nerves following an injury is often insufficient. Electrical stimulation (ES) applied during nerve repair can promote axon regeneration, which may enhance the likelihood of successful functional recovery. However, increasing operation time and complexity are associated with limited clinical use of ES. This study aims to better assess whether short-duration ES types (voltage mode vs. current mode) are able to produce enhanced regenerative activity following peripheral nerve repair in rat models. Wistar rats were randomly divided into 3 groups: no ES (control), 30-minute ES with a current pulse, and 30-minute ES with a voltage pulse. All groups underwent sciatic nerve transection and repair using a silicone tube to bridge the 6-mm gap between the stumps. In the 2 groups other than the control, ES was applied after the surgical repair. Outcomes were evaluated using electrophysiology, histology, and serial walking track analysis. Biweekly walking tracks test over 12 weeks revealed that subjects that underwent ES experienced more rapid functional improvement than subjects that underwent repair alone. Electrophysiological analysis of the newly intratubular sciatic nerve at week 12 revealed strong motor function recovery in rats that underwent 30-minute ES. Histologic analysis of the sciatic nerve and its tibial branch at 12 weeks demonstrated robust axon regrowth in all groups. Both types of short-duration ES applied during nerve repair can promote axon regrowth and enhance the chances of successful functional recovery.

MSC-derived mitochondria promote axonal regeneration via Atf3 gene up-regulation by ROS induced DNA double strand breaks at transcription initiation region

BACKGROUND

The repair of peripheral nerve injury poses a clinical challenge, necessitating further investigation into novel therapeutic approaches. In recent years, bone marrow mesenchymal stromal cell (MSC)-derived mitochondrial transfer has emerged as a promising therapy for cellular injury, with reported applications in central nerve injury. However, its potential therapeutic effect on peripheral nerve injury remains unclear.

METHODS

We established a mouse sciatic nerve crush injury model. Mitochondria extracted from MSCs were intraneurally injected into the injured sciatic nerves. Axonal regeneration was observed through whole-mount nerve imaging. The dorsal root ganglions (DRGs) corresponding to the injured nerve were harvested to test the gene expression, reactive oxygen species (ROS) levels, as well as the degree and location of DNA double strand breaks (DSBs).

RESULTS

The in vivo experiments showed that the mitochondrial injection therapy effectively promoted axon regeneration in injured sciatic nerves. Four days after injection of fluorescently labeled mitochondria into the injured nerves, fluorescently labeled mitochondria were detected in the corresponding DRGs. RNA-seq and qPCR results showed that the mitochondrial injection therapy enhanced the expression of Atf3 and other regeneration-associated genes in DRG neurons. Knocking down of Atf3 in DRGs by siRNA could diminish the therapeutic effect of mitochondrial injection. Subsequent experiments showed that mitochondrial injection therapy could increase the levels of ROS and DSBs in injury-associated DRG neurons, with this increase being correlated with Atf3 expression. ChIP and Co-IP experiments revealed an elevation of DSB levels within the transcription initiation region of the Atf3 gene following mitochondrial injection therapy, while also demonstrating a spatial proximity between mitochondria-induced DSBs and CTCF binding sites.

CONCLUSION

These findings suggest that MSC-derived mitochondria injected into the injured nerves can be retrogradely transferred to DRG neuron somas via axoplasmic transport, and increase the DSBs at the transcription initiation regions of the Atf3 gene through ROS accumulation, which rapidly release the CTCF-mediated topological constraints on chromatin interactions. This process may enhance spatial interactions between the Atf3 promoter and enhancer, ultimately promoting Atf3 expression. The up-regulation of Atf3 induced by mitochondria further promotes the expression of downstream regeneration-associated genes and facilitates axon regeneration.

Calcium-Associated Proteins in Neuroregeneration

The dysregulation of intracellular calcium levels is a critical factor in neurodegeneration, leading to the aberrant activation of calcium-dependent processes and, ultimately, cell death. Ca2+ signals vary in magnitude, duration, and the type of neuron affected. A moderate Ca2+ concentration can initiate certain cellular repair pathways and promote neuroregeneration. While the peripheral nervous system exhibits an intrinsic regenerative capability, the central nervous system has limited self-repair potential. There is evidence that significant variations exist in evoked calcium responses and axonal regeneration among neurons, and individual differences in regenerative capacity are apparent even within the same type of neurons. Furthermore, some studies have shown that neuronal activity could serve as a potent regulator of this process. The spatio-temporal patterns of calcium dynamics are intricately controlled by a variety of proteins, including channels, ion pumps, enzymes, and various calcium-binding proteins, each of which can exert either positive or negative effects on neural repair, depending on the cellular context. In this concise review, we focus on several calcium-associated proteins such as CaM kinase II, GAP-43, oncomodulin, caldendrin, calneuron, and NCS-1 in order to elaborate on their roles in the intrinsic mechanisms governing neuronal regeneration following traumatic damage processes.

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

Assessment of the relationship between the dopaminergic pathway and severe acute respiratory syndrome coronavirus 2 infection, with related neuropathological features, and potential therapeutic approaches in COVID-19 infection

Dopamine is a known catecholamine neurotransmitter involved in several physiological processes, including motor control, motivation, reward, cognition, and immune function. Dopamine receptors are widely distributed throughout the nervous system and in immune cells. Several viruses, including human immunodeficiency virus and Japanese encephalitis virus, can use dopaminergic receptors to replicate in the nervous system and are involved in viral neuropathogenesis. In addition, studies suggest that dopaminergic receptors may play a role in the progression and pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. When SARS-CoV-2 binds to angiotensin-converting enzyme 2 receptors on the surface of neuronal cells, the spike protein of the virus can bind to dopaminergic receptors on neighbouring cells to accelerate its life cycle and exacerbate neurological symptoms. In addition, recent research has shown that dopamine is an important regulator of the immune-neuroendocrine system. Most immune cells express dopamine receptors and other dopamine-related proteins, indicating the importance of dopaminergic immune regulation. The increase in dopamine concentration during SARS-CoV2 infection may reduce immunity (innate and adaptive) that promotes viral spread, which could lead to neuronal damage. In addition, dopaminergic signalling in the nervous system may be affected by SARS-CoV-2 infection. COVID -19 can cause various neurological symptoms as it interacts with the immune system. One possible treatment strategy for COVID -19 patients could be the use of dopamine antagonists. To fully understand how to protect the neurological system and immune cells from the virus, we need to study the pathophysiology of the dopamine system in SARS-CoV-2 infection.

The Dynamics of Nerve Degeneration and Regeneration in a Healthy Milieu and in Diabetes

Appropriate animal models, mimicking conditions of both health and disease, are needed to understand not only the biology and the physiology of neurons and other cells under normal conditions but also under stress conditions, like nerve injuries and neuropathy. In such conditions, understanding how genes and different factors are activated through the well-orchestrated programs in neurons and other related cells is crucial. Knowledge about key players associated with nerve regeneration intended for axonal outgrowth, migration of Schwann cells with respect to suitable substrates, invasion of macrophages, appropriate conditioning of extracellular matrix, activation of fibroblasts, formation of endothelial cells and blood vessels, and activation of other players in healthy and diabetic conditions is relevant. Appropriate physical and chemical attractions and repulsions are needed for an optimal and directed regeneration and are investigated in various nerve injury and repair/reconstruction models using healthy and diabetic rat models with relevant blood glucose levels. Understanding dynamic processes constantly occurring in neuropathies, like diabetic neuropathy, with concomitant degeneration and regeneration, requires advanced technology and bioinformatics for an integrated view of the behavior of different cell types based on genomics, transcriptomics, proteomics, and imaging at different visualization levels. Single-cell-transcriptional profile analysis of different cells may reveal any heterogeneity among key players in peripheral nerves in health and disease.

βPix Guanine Nucleotide Exchange Factor Regulates Regeneration of Injured Peripheral Axons

Axon regeneration is essential for successful recovery after peripheral nerve injury. Although growth cone reformation and axonal extension are crucial steps in axonal regeneration, the regulatory mechanisms underlying these dynamic processes are poorly understood. Here, we identify βPix (Arhgef7), the guanine nucleotide exchange factor for Rac1 GTPase, as a regulator of axonal regeneration. After sciatic nerve injury in mice, the expression levels of βPix increase significantly in nerve segments containing regenerating axons. In regrowing axons, βPix is localized in the peripheral domain of the growth cone. Using βPix neuronal isoform knockout (NIKO) mice in which the neuronal isoforms of βPix are specifically removed, we demonstrate that βPix promotes neurite outgrowth in cultured dorsal root ganglion neurons and in vivo axon regeneration after sciatic nerve crush injury. Activation of cJun and STAT3 in the cell bodies is not affected in βPix NIKO mice, supporting the local action of βPix in regenerating axons. Finally, inhibiting Src, a kinase previously identified as an activator of the βPix neuronal isoform, causes axon outgrowth defects in vitro, like those found in the βPix NIKO neurons. Altogether, these data indicate that βPix plays an important role in axonal regrowth during peripheral nerve regeneration.

Integrin-Driven Axon Regeneration in the Spinal Cord Activates a Distinctive CNS Regeneration Program

The peripheral branch of sensory dorsal root ganglion (DRG) neurons regenerates readily after injury unlike their central branch in the spinal cord. However, extensive regeneration and reconnection of sensory axons in the spinal cord can be driven by the expression of α9 integrin and its activator kindlin-1 (α9k1), which enable axons to interact with tenascin-C. To elucidate the mechanisms and downstream pathways affected by activated integrin expression and central regeneration, we conducted transcriptomic analyses of adult male rat DRG sensory neurons transduced with α9k1, and controls, with and without axotomy of the central branch. Expression of α9k1 without the central axotomy led to upregulation of a known PNS regeneration program, including many genes associated with peripheral nerve regeneration. Coupling α9k1 treatment with dorsal root axotomy led to extensive central axonal regeneration. In addition to the program upregulated by α9k1 expression, regeneration in the spinal cord led to expression of a distinctive CNS regeneration program, including genes associated with ubiquitination, autophagy, endoplasmic reticulum (ER), trafficking, and signaling. Pharmacological inhibition of these processes blocked the regeneration of axons from DRGs and human iPSC-derived sensory neurons, validating their causal contributions to sensory regeneration. This CNS regeneration-associated program showed little correlation with either embryonic development or PNS regeneration programs. Potential transcriptional drivers of this CNS program coupled to regeneration include Mef2a, Runx3, E2f4, and Yy1. Signaling from integrins primes sensory neurons for regeneration, but their axon growth in the CNS is associated with an additional distinctive program that differs from that involved in PNS regeneration.SIGNIFICANCE STATEMENT Restoration of neurologic function after spinal cord injury has yet to be achieved in human patients. To accomplish this, severed nerve fibers must be made to regenerate. Reconstruction of nerve pathways has not been possible, but recently, a method for stimulating long-distance axon regeneration of sensory fibers in rodents has been developed. This research uses profiling of messenger RNAs in the regenerating sensory neurons to discover which mechanisms are activated. This study shows that the regenerating neurons initiate a novel CNS regeneration program which includes molecular transport, autophagy, ubiquitination, and modulation of the endoplasmic reticulum (ER). The study identifies mechanisms that neurons need to activate to regenerate their nerve fibers.

The Role of Hydrogen Sulfide in Regulation of Cell Death following Neurotrauma and Related Neurodegenerative and Psychiatric Diseases

Injuries of the central (CNS) and peripheral nervous system (PNS) are a serious problem of the modern healthcare system. The situation is complicated by the lack of clinically effective neuroprotective drugs that can protect damaged neurons and glial cells from death. In addition, people who have undergone neurotrauma often develop mental disorders and neurodegenerative diseases that worsen the quality of life up to severe disability and death. Hydrogen sulfide (H₂S) is a gaseous signaling molecule that performs various cellular functions in normal and pathological conditions. However, the role of H₂S in neurotrauma and mental disorders remains unexplored and sometimes controversial. In this large-scale review study, we examined the various biological effects of H₂S associated with survival and cell death in trauma to the brain, spinal cord, and PNS, and the signaling mechanisms underlying the pathogenesis of mental illnesses, such as cognitive impairment, encephalopathy, depression and anxiety disorders, epilepsy and chronic pain. We also studied the role of H₂S in the pathogenesis of neurodegenerative diseases: Alzheimer's disease (AD) and Parkinson's disease (PD). In addition, we reviewed the current state of the art study of H₂S donors as neuroprotectors and the possibility of their therapeutic uses in medicine. Our study showed that H₂S has great neuroprotective potential. H₂S reduces oxidative stress, lipid peroxidation, and neuroinflammation; inhibits processes associated with apoptosis, autophagy, ferroptosis and pyroptosis; prevents the destruction of the blood-brain barrier; increases the expression of neurotrophic factors; and models the activity of Ca²⁺ channels in neurotrauma. In addition, H₂S activates neuroprotective signaling pathways in psychiatric and neurodegenerative diseases. However, high levels of H₂S can cause cytotoxic effects. Thus, the development of H₂S-associated neuroprotectors seems to be especially relevant. However, so far, all H₂S modulators are at the stage of preclinical trials. Nevertheless, many of them show a high neuroprotective effect in various animal models of neurotrauma and related disorders. Despite the fact that our review is very extensive and detailed, it is well structured right down to the conclusions, which will allow researchers to quickly find the proper information they are interested in.

Regulation of chromatin organization during animal regeneration

Activation of regeneration upon tissue damages requires the activation of many developmental genes responsible for cell proliferation, migration, differentiation, and tissue patterning. Ample evidence revealed that the regulation of chromatin organization functions as a crucial mechanism for establishing and maintaining cellular identity through precise control of gene transcription. The alteration of chromatin organization can lead to changes in chromatin accessibility and/or enhancer-promoter interactions. Like embryogenesis, each stage of tissue regeneration is accompanied by dynamic changes of chromatin organization in regeneration-responsive cells. In the past decade, many studies have been conducted to investigate the contribution of chromatin organization during regeneration in various tissues, organs, and organisms. A collection of chromatin regulators were demonstrated to play critical roles in regeneration. In this review, we will summarize the progress in the understanding of chromatin organization during regeneration in different research organisms and discuss potential common mechanisms responsible for the activation of regeneration response program.

Preservation of KCC2 expression in axotomized abducens motoneurons and its enhancement by VEGF

The potassium chloride cotransporter 2 (KCC2) is the main Cl- extruder in neurons. Any alteration in KCC2 levels leads to changes in Cl- homeostasis and, consequently, in the polarity and amplitude of inhibitory synaptic potentials mediated by GABA or glycine. Axotomy downregulates KCC2 in many different motoneurons and it is suspected that interruption of muscle-derived factors maintaining motoneuron KCC2 expression is in part responsible. In here, we demonstrate that KCC2 is expressed in all oculomotor nuclei of cat and rat, but while trochlear and oculomotor motoneurons downregulate KCC2 after axotomy, expression is unaltered in abducens motoneurons. Exogenous application of vascular endothelial growth factor (VEGF), a neurotrophic factor expressed in muscle, upregulated KCC2 in axotomized abducens motoneurons above control levels. In parallel, a physiological study using cats chronically implanted with electrodes for recording abducens motoneurons in awake animals, demonstrated that inhibitory inputs related to off-fixations and off-directed saccades in VEGF-treated axotomized abducens motoneurons were significantly higher than in control, but eye-related excitatory signals in the on direction were unchanged. This is the first report of lack of KCC2 regulation in a motoneuron type after injury, proposing a role for VEGF in KCC2 regulation and demonstrating the link between KCC2 and synaptic inhibition in awake, behaving animals.

Engineered neuronal microtissue provides exogenous axons for delayed nerve fusion and rapid neuromuscular recovery in rats

Nerve injury requiring surgical repair often results in poor functional recovery due to the inability of host axons to re-grow long distances and reform meaningful connections with the target muscle. While surgeons can re-route local axon fascicles to the target muscle, there are no technologies to provide an exogenous source of axons without sacrificing healthy nerves. Accordingly, we have developed tissue engineered neuromuscular interfaces (TE-NMIs) as the first injectable microtissue containing motor and sensory neurons in an anatomically-inspired architecture. TE-NMIs provide axon tracts that are intended to integrate with denervated distal structures and preserve regenerative capacity during prolonged periods without host innervation. Following implant, we found that TE-NMI axons promoted Schwann cell maintenance, integrated with distal muscle, and preserved an evoked muscle response out to 20-weeks post nerve transection in absence of innervation from host axons. By repopulating the distal sheath with exogenous axons, TE-NMIs also enabled putative delayed fusion with proximal host axons, a phenomenon previously not achievable in delayed repair scenarios due to distal axon degeneration. Here, we found immediate electrophysiological recovery after fusion with proximal host axons and improved axon maturation and muscle reinnervation at 24-weeks post-transection (4-weeks following delayed nerve fusion). These findings show that TE-NMIs provide the potential to improve functional recovery following delayed nerve repair.

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.

Impaired disassembly of the axon initial segment restricts mitochondrial entry into damaged axons

The proteasome is essential for cellular responses to various physiological stressors. However, how proteasome function impacts the stress resilience of regenerative damaged motor neurons remains unclear. Here, we develop a unique mouse model using a regulatory element of the activating transcription factor (Atf3) gene to label mitochondria in a damage‐induced manner while simultaneously genetically disrupting the proteasome. Using this model, we observed that in injury‐induced proteasome‐deficient mouse motor neurons, the increase of mitochondrial influx from soma into axons is inhibited because neurons fail to disassemble ankyrin G, an organizer of the axon initial segment (AIS), in a proteasome‐dependent manner. Further, these motor neurons exhibit amyotrophic lateral sclerosis (ALS)‐like degeneration despite having regenerative potential. Selectively vulnerable motor neurons in SOD1^G93A ALS mice, which induce ATF3 in response to pathological damage, also fail to disrupt the AIS, limiting the number of axonal mitochondria at a pre‐symptomatic stage. Thus, damage‐induced proteasome‐sensitive AIS disassembly could be a critical post‐translational response for damaged motor neurons to temporarily transit to an immature state and meet energy demands for axon regeneration or preservation.

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.

Effect of treadmill exercise and bone marrow stromal cell engraftment on activation of BDNF-ERK-CREB signaling pathway in the crushed sciatic nerve

The effect of combined approach of exercise training and bone marrow stromal cell (BMSC) engraftment on activation of brain-derived neurotrophic factor (BDNF)-extracellular signal-regulated kinase 1 and 2 (ERK1/2)-cyclic adenosine monophosphate response element-binding protein (CREB) signaling pathway after sciatic nerve injury (SNI) was investigated. Sixty male Sprague-Dawley rats divided into the normal control, nonexercise (NEX), exercise training (EX), BMSC transplantation (TP), and exercise training+BMSC transplantation (EX+TP) groups 4 weeks after SCI. Exercise training was carried out on the treadmill device at 5-10 m/min for 20 min for 4 weeks. Single dose of 5×10⁶ harvested BMSC was injected into the injury area of the injured sciatic nerve. In order to evaluate induction levels of BDNF-ERK1/2-CREB signaling molecules in the whole cell and nuclear cell lysates of the injured sciatic nerve, we applied Western blot analysis. BDNF was significantly increased only in EX+TP compared to NEX, EX, and TP groups. Phosphoinositide-dependent kinase-1 was more increased in EX, TP, and EX+TP groups than NEX group, but EX+TP group showed the most upregulation of phosphorylated protein kinase B compared to other groups. In addition, in the whole cell lysate, phosphorylated ERK1/2, but not activating transcription factor-3 (ATF-3) and phosphorylated CREB, was significantly increased in TP and EX+TP groups. In the nuclear cell lysate, ATF-3 and phosphorylated CREB were strongly activated in EX+TP group compared to EX group. Regular exercise training combined with BMSC engraftment would seem to be more effective in controlling activation of regeneration-related signaling pathway after SNI.

Single-Cell Regulatory Network Inference and Clustering Identifies Cell-Type Specific Expression Pattern of Transcription Factors in Mouse Sciatic Nerve

Advances in single-cell RNA sequencing technologies and bioinformatics methods allow for both the identification of cell types in a complex tissue and the large-scale gene expression profiling of various cell types in a mixture. In this report, we analyzed a single-cell RNA sequencing (scRNA-seq) dataset for the intact adult mouse sciatic nerve and examined cell-type specific transcription factor expression and activity during peripheral nerve homeostasis. In total, we identified 238 transcription factors expressed in nine different cell types of intact mouse sciatic nerve. Vascular smooth muscle cells have the lowest number of transcription factors expressed with 17 transcription factors identified. Myelinating Schwann cells (mSCs) have the highest number of transcription factors expressed, with 61 transcription factors identified. We created a cell-type specific expression map for the identified 238 transcription factors. Our results not only provide valuable information about the expression pattern of transcription factors in different cell types of adult peripheral nerves but also facilitate future studies to understand the function of key transcription factors in the peripheral nerve homeostasis and disease.

Meta-Analysis Reveals Transcription Factor Upregulation in Cells of Injured Mouse Sciatic Nerve

Following peripheral nerve injury, transcription factors upregulated in the distal nerve play essential roles in Schwann cell reprogramming, fibroblast activation and immune cell function to create a permissive distal nerve environment for axonal regrowth. In this report, we first analysed four microarray data sets to identify transcription factors that have at least twofold upregulation in the mouse distal nerve stump at day 3 and day 7 post-injury. Next, we compared their relative mRNA levels through the analysis of an available bulk mRNA sequencing data set at day 5 post-injury. We then investigated the expression of identified TFs in analysed single-cell RNA sequencing data sets for the distal nerve at day 3 and day 9 post-injury. These analyses identified 55 transcription factors that have at least twofold upregulation in the distal nerve following mouse sciatic nerve injury. Expression profile for the identified 55 transcription factors in cells of the distal nerve stump was further analysed on the scRNA-seq data. Transcription factor network and functional analysis were performed in Schwann cells. We also validated the expression pattern of Jun, Junb, Runx1, Runx2, and Sox2 in the mouse distal nerve stump by immunostaining. The findings from our study not only could be used to understand the function of key transcription factors in peripheral nerve regeneration but also could be used to facilitate experimental design for future studies to investigate the function of individual TFs in peripheral nerve regeneration.

Editing SOX Genes by CRISPR-Cas: Current Insights and Future Perspectives

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and its associated proteins (Cas) is an adaptive immune system in archaea and most bacteria. By repurposing these systems for use in eukaryote cells, a substantial revolution has arisen in the genome engineering field. In recent years, CRISPR-Cas technology was rapidly developed and different types of DNA or RNA sequence editors, gene activator or repressor, and epigenome modulators established. The versatility and feasibility of CRISPR-Cas technology has introduced this system as the most suitable tool for discovering and studying the mechanism of specific genes and also for generating appropriate cell and animal models. SOX genes play crucial roles in development processes and stemness. To elucidate the exact roles of SOX factors and their partners in tissue hemostasis and cell regeneration, generating appropriate in vitro and in vivo models is crucial. In line with these premises, CRISPR-Cas technology is a promising tool for studying different family members of SOX transcription factors. In this review, we aim to highlight the importance of CRISPR-Cas and summarize the applications of this novel, promising technology in studying and decoding the function of different members of the SOX gene family.

Topoisomerase I inhibition and peripheral nerve injury induce DNA breaks and ATF3-associated axon regeneration in sensory neurons

Although axonal damage induces rapid changes in gene expression in primary sensory neurons, it remains unclear how this process is initiated. The transcription factor ATF3, one of the earliest genes responding to nerve injury, regulates expression of downstream genes that enable axon regeneration. By exploiting ATF3 reporter systems, we identify topoisomerase inhibitors as ATF3 inducers, including camptothecin. Camptothecin increases ATF3 expression and promotes neurite outgrowth in sensory neurons in vitro and enhances axonal regeneration after sciatic nerve crush in vivo. Given the action of topoisomerases in producing DNA breaks, we determine that they do occur immediately after nerve damage at the ATF3 gene locus in injured sensory neurons and are further increased after camptothecin exposure. Formation of DNA breaks in injured sensory neurons and enhancement of it pharmacologically may contribute to the initiation of those transcriptional changes required for peripheral nerve regeneration.

Genome-wide chromatin accessibility analyses provide a map for enhancing optic nerve regeneration

Retinal Ganglion Cells (RGCs) lose their ability to grow axons during development. Adult RGCs thus fail to regenerate their axons after injury, leading to vision loss. To uncover mechanisms that promote regeneration of RGC axons, we identified transcription factors (TF) and open chromatin regions that are enriched in rat embryonic RGCs (high axon growth capacity) compared to postnatal RGCs (low axon growth capacity). We found that developmental stage-specific gene expression changes correlated with changes in promoter chromatin accessibility. Binding motifs for TFs such as CREB, CTCF, JUN and YY1 were enriched in the regions of the chromatin that were more accessible in embryonic RGCs. Proteomic analysis of purified rat RGC nuclei confirmed the expression of TFs with potential role in axon growth such as CREB, CTCF, YY1, and JUND. The CREB/ATF binding motif was widespread at the open chromatin region of known pro-regenerative TFs, supporting a role of CREB in regulating axon regeneration. Consistently, overexpression of CREB fused to the VP64 transactivation domain in mouse RGCs promoted axon regeneration after optic nerve injury. Our study provides a map of the chromatin accessibility during RGC development and highlights that TF associated with developmental axon growth can stimulate axon regeneration in mature RGC.

Loc680254 regulates Schwann cell proliferation through Psrc1 and Ska1 as a microRNA sponge following sciatic nerve injury

Peripheral nerve injury triggers sequential phenotype alterations in Schwann cells, which are critical for axonal regeneration. Long noncoding RNAs (lncRNAs) are long transcripts without obvious coding potential. It has been reported that lncRNAs participate in diverse biological processes and diseases. However, the role of lncRNA in Schwann cells and peripheral nerve regeneration is unclear. Here, we identified an lncRNA, loc680254, which is upregulated in rat sciatic nerve after peripheral nerve injury. The loc680254 knockdown inhibits Schwann cell proliferation, enhances apoptosis, and hinders cell cycle, while loc680254 overexpression has the opposite effect. Mechanically, we found that loc680254 might act as a microRNA sponge to regulate the expression of mitosis-related gene, spindle and kinetochore associated complex subunit 1 (Ska1) and proline/serine-rich coiled-coil 1 (Psrc1). Silencing of Psrc1 or Ska1 attenuates the effect of loc680254 overexpression on Schwann cell proliferation. Finally, we repaired the rat sciatic nerve gap with chitosan scaffolds loaded with loc680254-overexpressing Schwann cells and evaluated axon regeneration and functional recovery. Our results indicated that loc680254 is a new potential modulator for Schwann cell proliferation, which could be targeted to develop novel therapeutic strategies for peripheral nerve repair.

Prepubertal exposure to high dose of cadmium induces hypothalamic injury through transcriptome profiling alteration and neuronal degeneration in female rats

Cadmium (Cd) is a toxic metal, which seems to be crucial during the prepubertal period. Cd can destroy the structural integrity of the blood-brain barrier (BBB) and enters into the brain. Although the brain is susceptible to neurotoxicity induced by Cd, the effects of Cd on the brain, particularly hypothalamic transcriptome, are still relatively poorly understood. Therefore, we investigated the molecular effects of Cd exposure on the hypothalamus by profiling the transcriptomic response of the hypothalamus to high dose of Cd (25 mg/kg bw/day cadmium chloride (CdCl₂)) during the prepubertal period in Sprague-Dawley female rats. After sequencing and annotation, differential expression analysis revealed 1656 genes that were differentially expressed that 108 of them were classified into 37 transcription factor (TF) families. According to gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, these differentially expressed genes (DEGs) were involved in different biological processes and neurological disorders including Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD), prolactin signaling pathway, PI3K/Akt signaling, and dopaminergic synapse. Five transcripts were selected for further analyses with Real-time quantitative PCR (RT-qPCR). The RT-qPCR results were mostly consistent with those from the high throughput RNA sequencing (RNA-seq). Cresyl violet staining clearly showed an increased neuronal degeneration in the dorsomedial hypothalamus (DMH) and arcuate (Arc) nuclei of the CdCl₂ group. Overall, this study demonstrates that prepubertal exposure to high doses of Cd induces hypothalamic injury through transcriptome profiling alteration in female rats, which reveals the new mechanisms of pathogenesis of Cd in the hypothalamus.

Cyclic Stretch of Either PNS or CNS Located Nerves Can Stimulate Neurite Outgrowth

The central nervous system (CNS) does not recover from traumatic axonal injury, but the peripheral nervous system (PNS) does. We hypothesize that this fundamental difference in regenerative capacity may be based upon the absence of stimulatory mechanical forces in the CNS due to the protective rigidity of the vertebral column and skull. We developed a bioreactor to apply low-strain cyclic axonal stretch to adult rat dorsal root ganglia (DRG) connected to either the peripheral or central nerves in an explant model for inducing axonal growth. In response, larger diameter DRG neurons, mechanoreceptors and proprioceptors showed enhanced neurite outgrowth as well as increased Activating Transcription Factor 3 (ATF3).

Transcriptional Reprogramming of Distinct Peripheral Sensory Neuron Subtypes after Axonal Injury

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

Growth factors-based therapeutic strategies and their underlying signaling mechanisms for peripheral nerve regeneration

Peripheral nerve injury (PNI), one of the most common concerns following trauma, can result in a significant loss of sensory or motor function. Restoration of the injured nerves requires a complex cellular and molecular response to rebuild the functional axons so that they can accurately connect with their original targets. However, there is no optimized therapy for complete recovery after PNI. Supplementation with exogenous growth factors (GFs) is an emerging and versatile therapeutic strategy for promoting nerve regeneration and functional recovery. GFs activate the downstream targets of various signaling cascades through binding with their corresponding receptors to exert their multiple effects on neurorestoration and tissue regeneration. However, the simple administration of GFs is insufficient for reconstructing PNI due to their short half‑life and rapid deactivation in body fluids. To overcome these shortcomings, several nerve conduits derived from biological tissue or synthetic materials have been developed. Their good biocompatibility and biofunctionality made them a suitable vehicle for the delivery of multiple GFs to support peripheral nerve regeneration. After repairing nerve defects, the controlled release of GFs from the conduit structures is able to continuously improve axonal regeneration and functional outcome. Thus, therapies with growth factor (GF) delivery systems have received increasing attention in recent years. Here, we mainly review the therapeutic capacity of GFs and their incorporation into nerve guides for repairing PNI. In addition, the possible receptors and signaling mechanisms of the GF family exerting their biological effects are also emphasized.

The Expression of E2F1, p53, and Caspase 3 in the Rat Dorsal Root Ganglia After Sciatic Nerve Transection

Neurotrauma is among the main causes of human disability and mortality. Nerve injury impairs not only neurons but also causes death of satellite glial cells remote from the injury site. We studied the dynamics of expression of different proapoptotic proteins (E2F1, p53, caspase 3) in the dorsal root ganglia (DRG) of a rat after sciatic nerve transection. TUNEL staining and immunoblotting were used for analysis of cell apoptosis and axotomy-induced biochemical changes. Apoptosis of glial cells was observed at 24 h after sciatic nerve transection and increased on day 7, when apoptosis of some neurons only started. The earliest proapoptotic event in the injured DRG was overexpression of transcription factor E2F1 at 4 h after sciatic nerve transection. This preceded the induction of p53 and cleavage of caspase 3 at 24-h post-axotomy. The nerve injury marker amyloid precursor protein and the nerve regeneration marker GAP-43 were overexpressed in DRG on day 7 after sciatic nerve transection. We also developed a novel fluorescence method for differential visualization of the rat DRG and nerves by means of double staining with propidium iodide and Hoechst 33342 that impart red and blue-green fluorescence, respectively. The present experiments showed that glial cells remote from the nerve transection site were more vulnerable to axotomy than DRG neurons. E2F1 and p53 may be considered promising molecular targets for development of potential neuroprotective agents.

Treadmill exercise activates ATF3 and ERK1/2 downstream molecules to facilitate axonal regrowth after sciatic nerve injury

The purpose of this study was to investigate the effect of treadmill exer-cise on activating transcription factors such as activating transcription factor 3 (ATF3) and extracellular signal-regulated kinase (ERK1/2) sig-naling pathway to facilitate axonal regrowth after sciatic nerve injury (SNI). The experimental rats divided into the normal control (n=10), sedentary groups for 7 (n=10) and 14 days (n=10) post crush, exercise group for 7 (n=10) and 14 days (n=10) post crush (dpc). The rats in ex-ercise groups run on treadmill device at a speed of 8 m/min for 20 min once a day according to exercise duration. In order to evaluate specific regeneration markers and axonal elongation in injured sciatic nerve, we applied immunofluorescence staining and western blot techniques. Treadmill exercise further increased growth-associated protein (GAP-43) expression and axonal regrowth at 7 and 14 dpc than those in sed-entary group. Among mitogen-activated protein kinase downstream molecules, phospho-ERK1/2 (p-ERK1/2) was enhanced by treadmill ex-ercise at only 7 dpc and decreased to basal level 14 days later. But c-Jun N-terminal kinase, c-Jun, and phospho-cyclic adenosine mono-phosphate response element-binding protein showed a tendency to in-crease continuously until 14 dpc by exercise. ATF3 expression in exer-cise group was upregulated at both 7 and 14 dpc compared to the sed-entary group. These results indicate that treadmill exercise had benefi-cial effect on expression of regeneration-related proteins after SNI, suggesting that exercise might be one of various therapeutic strategies for sciatic nerve regeneration.

Dysregulated Transcription Factor TFAP2A After Peripheral Nerve Injury Modulated Schwann Cell Phenotype

Transcription factors regulate the transcriptions and expressions of numerous target genes and direct a variety of physiological and pathological activities. To obtain a better understanding of the involvement of transcription factors during peripheral nerve repair and regeneration, significantly differentially expressed genes coding for transcription factors in rat sciatic nerves after sciatic nerve crush injury were identified. A total of 9 transcription factor genes, including GBX2, HIF3A, IRF8, LRRC63, SNAI3, SPIB, TBX21, TFAP2A, and ZBTB16 were identified to be commonly differentially expressed at 1, 4, 7, and 14 days after nerve injury. TFAP2A, a gene encoding transcription factor activating enhancer binding protein 2 alpha, was found to be critical in the regulatory network. PCR validation and immunohistochemistry staining of injured rat sciatic nerves showed that TFAP2A expression was significantly up-regulated in the Schwann cells after nerve injury for at least 2 weeks. Schwann cells transfected with TFAP2A-siRNA exhibited elevated proliferation rate and migration ability, suggesting that TFAP2A suppressed Schwann cell proliferation and migration. Collectively, our study provided a global overview of the dynamic changes of transcription factors after sciatic nerve injury, discovered key transcription factors for the regeneration process, and deepened the understanding of the molecular mechanisms underlying peripheral nerve repair and regeneration.

Epigenomic signatures underpin the axonal regenerative ability of dorsal root ganglia sensory neurons

Axonal injury results in regenerative success or failure, depending on whether the axon lies in the peripheral or the CNS, respectively. The present study addresses whether epigenetic signatures in dorsal root ganglia discriminate between regenerative and non-regenerative axonal injury. Chromatin immunoprecipitation for the histone 3 (H3) post-translational modifications H3K9ac, H3K27ac and H3K27me3; an assay for transposase-accessible chromatin; and RNA sequencing were performed in dorsal root ganglia after sciatic nerve or dorsal column axotomy. Distinct histone acetylation and chromatin accessibility signatures correlated with gene expression after peripheral, but not central, axonal injury. DNA-footprinting analyses revealed new transcriptional regulators associated with regenerative ability. Machine-learning algorithms inferred the direction of most of the gene expression changes. Neuronal conditional deletion of the chromatin remodeler CCCTC-binding factor impaired nerve regeneration, implicating chromatin organization in the regenerative competence. Altogether, the present study offers the first epigenomic map providing insight into the transcriptional response to injury and the differential regenerative ability of sensory neurons.

Long Non-coding RNA in CNS Injuries: A New Target for Therapeutic Intervention

CNS injuries, such as traumatic brain injury (TBI), subarachnoid hemorrhage (SAH), intracerebral hemorrhage (ICH), and cerebral ischemic stroke, are important causes of death and long-term disability worldwide. As an important class of pervasive genes involved in many pathophysiological processes, long non-coding RNAs (lncRNAs) have received attention in the past decades. Multiple studies indicate that lncRNAs are abundant in the CNS and have a key role in brain function as well as many neurological disorders, especially in CNS injuries. Several investigations have deciphered that regulation of lncRNAs exert pro-angiogenesis, anti-apoptosis, and anti-inflammation effects in CNS injury via different molecules and pathways, including microRNA (miRNA), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), phosphatidylinositol-4,5-bisphosphate 3-kinase/protein kinase B (PI3K/AKT), Notch, and p53. Thus, lncRNAs show great promise as molecular targets in CNS injuries. In this article, we provide an updated review of the current state of our knowledge about the relationship between lncRNAs and CNS injuries, highlighting the specific roles of lncRNAs in CNS injuries.

Role of shortwave and microwave diathermy in peripheral neuropathy

Objective

This study was performed to review the current evidence for the efficacy of shortwave and microwave diathermy in promoting nerve regeneration after peripheral nerve injuries in both animal models and human patients.

Methods

An extensive literature search was conducted without publication data restrictions. Studies including the intervention and outcome in animal or human models were selected. Non-English studies, reviews, letters, and case reports were excluded.

Results

Eleven articles were included in this study. Shortwave diathermy at the frequency of 27.12 or 40.68 MHz was used in six of seven animal studies, while only one study utilized microwave diathermy at 915 MHz. Seven animal experiments demonstrated that shortwave or microwave diathermy produces an increased myelinated nerve fiber number, myelin sheath thickness, and axon diameter as well as improved electrophysiological parameters and locomotion. A total of 128 patients (207 wrists) were enrolled in four clinical studies. The clinical use of diathermy in human patients with carpal tunnel syndrome showed positive effects on pain, hand function, and electrophysiological findings.

Conclusions

Shortwave or microwave diathermy can improve the electrophysiological parameters, myelinated fiber number, and axon diameter of the injured nerve.

Nociceptor Deletion of Tsc2 Enhances Axon Regeneration by Inducing a Conditioning Injury Response in Dorsal Root Ganglia

Neurons of the PNS are able to regenerate injured axons, a process requiring significant cellular resources to establish and maintain long-distance growth. Genetic activation of mTORC1, a potent regulator of cellular metabolism and protein translation, improves axon regeneration of peripheral neurons by an unresolved mechanism. To gain insight into this process, we activated mTORC1 signaling in mouse nociceptors via genetic deletion of its negative regulator Tsc2. Perinatal deletion of Tsc2 in nociceptors enhanced initial axon growth after sciatic nerve crush, however by 3 d post-injury axon elongation rate became similar to controls. mTORC1 inhibition prior to nerve injury was required to suppress the enhanced axon growth. Gene expression analysis in purified nociceptors revealed that Tsc2-deficient nociceptors had increased activity of regeneration-associated transcription factors (RATFs), including cJun and Atf3, in the absence of injury. Additionally, nociceptor deletion of Tsc2 activated satellite glial cells and macrophages in the dorsal root ganglia (DRG) in a similar manner to nerve injury. Surprisingly, these changes improved axon length but not percentage of initiating axons in dissociated cultures. The pro-regenerative environment in naïve DRG was recapitulated by AAV8-mediated deletion of Tsc2 in adult mice, suggesting that this phenotype does not result from a developmental effect. Consistently, AAV8-mediated Tsc2 deletion did not improve behavioral recovery after a sciatic nerve crush injury despite initially enhanced axon growth. Together, these data show that neuronal mTORC1 activation induces an incomplete pro-regenerative environment in the DRG that improves initial but not later axon growth after nerve injury.

A Conditioning Sciatic Nerve Lesion Triggers a Pro-regenerative State in Primary Sensory Neurons Also of Dorsal Root Ganglia Non-associated With the Damaged Nerve

The primary sensory neurons of dorsal root ganglia (DRG) are a very useful model to study the neuronal regenerative program that is a prerequisite for successful axon regeneration after peripheral nerve injury. Seven days after a unilateral sciatic nerve injury by compression or transection, we detected a bilateral increase in growth-associated protein-43 (GAP-43) and superior cervical ganglion-10 (SCG-10) mRNA and protein levels not only in DRG neurons of lumbar spinal cord segments (L4-L5) associated with injured nerve, but also in remote cervical segments (C6-C8). The increase in regeneration-associated proteins in the cervical DRG neurons was associated with the greater length of regenerated axons 1 day after ulnar nerve crush following prior sciatic nerve injury as compared to controls with only ulnar nerve crush. The increased axonal regeneration capacity of cervical DRG neurons after a prior conditioning sciatic nerve lesion was confirmed by neurite outgrowth assay of in vitro cultivated DRG neurons. Intrathecal injection of IL-6 or a JAK2 inhibitor (AG490) revealed a role for the IL-6 signaling pathway in activating the pro-regenerative state in remote DRG neurons. Our results suggest that the pro-regenerative state induced in the DRG neurons non-associated with the injured nerve reflects a systemic reaction of these neurons to unilateral sciatic nerve injury.

Histone deacetylase inhibitors prevent persistent hypersensitivity in an orofacial neuropathic pain model

Chronic orofacial pain is a significant health problem requiring identification of regulating processes. Involvement of epigenetic modifications that is reported for hindlimb neuropathic pain experimental models, however, is less well studied in cranial nerve pain models. Three independent observations reported here are the (1) epigenetic profile in mouse trigeminal ganglia (TG) after trigeminal inflammatory compression (TIC) nerve injury mouse model determined by gene expression microarray, (2) H3K9 acetylation pattern in TG by immunohistochemistry, and (3) efficacy of histone deacetylase (HDAC) inhibitors to attenuate development of hypersensitivity. After TIC injury, ipsilateral whisker pad mechanical sensitization develops by day 3 and persists well beyond day 21 in contrast to sham surgery. Global acetylation of H3K9 decreases at day 21 in ipsilateral TG . Thirty-four genes are significantly (p < 0.05) overexpressed in the ipsilateral TG by at least two-fold at either 3 or 21 days post-trigeminal inflammatory compression injury. The three genes most overexpressed three days post-trigeminal inflammatory compression nerve injury are nerve regeneration-associated gene ATF3, up 6.8-fold, and two of its regeneration-associated gene effector genes, Sprr1a and Gal, up 174- and 25-fold, respectively. Although transcription levels of 25 of 32 genes significantly overexpressed three days post-trigeminal inflammatory compression return to constitutive levels by day 21, these three regeneration-associated genes remain significantly overexpressed at the later time point. On day 21, when tissues are healed, other differentially expressed genes include 39 of the top 50 upregulated and downregulated genes. Remarkably, preemptive manipulation of gene expression with two HDAC inhibitors (HDACi's), suberanilohydroxamic acid (SAHA) and MS-275, reduces the magnitude and duration of whisker pad mechanical hypersensitivity and prevents the development of a persistent pain state. These findings suggest that trigeminal nerve injury leads to epigenetic modifications favoring overexpression of genes involved in nerve regeneration and that maintaining transcriptional homeostasis with epigenetic modifying drugs could help prevent the development of persistent pain.

Expression of a novel versican variant in dorsal root ganglia from spared nerve injury rats

The size and modular structure of versican and its gene suggest the existence of multiple splice variants. We have identified, cloned, and sequenced a previously unknown exon located within the noncoding gene sequence downstream of exon 8. This exon, which we have named exon 8β, specifies two stop-codons. mRNAs of the versican gene with exon 8β are predicted to be constitutively degraded by nonsense-mediated RNA decay. Here, we tested the hypothesis that these transcripts become expressed in a model of neuropathic pain.

Regulation of Neuroregeneration by Long Noncoding RNAs

In mammals, neurons in the peripheral nervous system (PNS) have regenerative capacity following injury, but it is generally absent in the CNS. This difference is attributed, at least in part, to the intrinsic ability of PNS neurons to activate a unique regenerative transcriptional program following injury. Here, we profiled gene expression following sciatic nerve crush in mice and identified long noncoding RNAs (lncRNAs) that act in the regenerating neurons and which are typically not expressed in other contexts. We show that two of these lncRNAs regulate the extent of neuronal outgrowth. We then focus on one of these, Silc1, and show that it regulates neuroregeneration in cultured cells and in vivo, through cis-acting activation of the transcription factor Sox11.

Neuroinflammation after Traumatic Brain Injury Is Enhanced in Activating Transcription Factor 3 Mutant Mice

Traumatic brain injury (TBI) induces a neuroinflammatory response resulting in astrocyte and microglia activation at the lesion site. This involves upregulation of neuroinflammatory genes, including chemokines and interleukins. However, so far, there is lack of knowledge on transcription factors (TFs) modulating this TBI-associated gene expression response. Herein, we analyzed activating transcription factor 3 (ATF3), a TF encoding a regeneration-associated gene (RAG) predominantly studied in peripheral nervous system (PNS) injury. ATF3 contributes to PNS axon regeneration and was shown before to regulate inflammatory processes in other injury models. In contrast to PNS injury, data on ATF3 in central nervous system (CNS) injury are sparse. We used Atf3 mouse mutants and a closed-head weight-drop-based TBI model in adult mice to target the rostrolateral cortex resulting in moderate injury severity. Post-TBI, ATF3 was upregulated already at early time points (i.e,. 1-4 h) post-injury in the brain. Mortality and weight loss upon TBI were slightly elevated in Atf3 mutants. ATF3 deficiency enhanced TBI-induced paresis and hematoma formation, suggesting that ATF3 limits these injury outcomes in wild-type mice. Next, we analyzed TBI-associated RAG and inflammatory gene expression in the cortical impact area. In contrast to the PNS, only some RAGs (Atf3, Timp1, and Sprr1a) were induced by TBI, and, surprisingly, some RAG encoding neuropeptides were downregulated. Notably, we identified ATF3 as TF-regulating proneuroinflammatory gene expression, including CCL and CXCL chemokines (Ccl2, Ccl3, Ccl4, and Cxcl1) and lipocalin. In Atf3 mutant mice, mRNA abundance was further enhanced upon TBI compared to wild-type mice, suggesting immune gene repression by wild-type ATF3. In accord, more immune cells were present in the lesion area of ATF3-deficient mice. Overall, we identified ATF3 as a new TF-mediating TBI-associated CNS inflammatory responses.

Defining the Transcriptional Targets of Leptin Reveals a Role for Atf3 in Leptin Action

Leptin acts via its receptor (LepRb) to modulate gene expression in hypothalamic LepRb-expressing neurons, thereby controlling energy balance and glucose homeostasis. Despite the importance of the control of gene expression in hypothalamic LepRb neurons for leptin action, the transcriptional targets of LepRb signaling have remained undefined because LepRb cells contribute a small fraction to the aggregate transcriptome of the brain regions in which they reside. We thus employed translating ribosome affinity purification followed by RNA sequencing to isolate and analyze mRNA from the hypothalamic LepRb neurons of wild-type or leptin-deficient (Lepob/ob) mice treated with vehicle or exogenous leptin. Although the expression of most of the genes encoding the neuropeptides commonly considered to represent the main targets of leptin action were altered only following chronic leptin deprivation, our analysis revealed other transcripts that were coordinately regulated by leptin under multiple treatment conditions. Among these, acute leptin treatment increased expression of the transcription factor Atf3 in LepRb neurons. Furthermore, ablation of Atf3 from LepRb neurons (Atf3LepRbKO mice) decreased leptin efficacy and promoted positive energy balance in mice. Thus, this analysis revealed the gene targets of leptin action, including Atf3, which represents a cellular mediator of leptin action.

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.

Axonal regeneration in zebrafish spinal cord

In the present review we discuss two interrelated events-axonal damage and repair-known to occur after spinal cord injury (SCI) in the zebrafish. Adult zebrafish are capable of regenerating axonal tracts and can restore full functionality after SCI. Unlike fish, axon regeneration in the adult mammalian central nervous system is extremely limited. As a consequence of an injury there is very little repair of disengaged axons and therefore functional deficit persists after SCI in adult mammals. In contrast, peripheral nervous system axons readily regenerate following injury and hence allow functional recovery both in mammals and fish. A better mechanistic understanding of these three scenarios could provide a more comprehensive insight into the success or failure of axonal regeneration after SCI. This review summarizes the present understanding of the cellular and molecular basis of axonal regeneration, in both the peripheral nervous system and the central nervous system, and large scale gene expression analysis is used to focus on different events during regeneration. The discovery and identification of genes involved in zebrafish spinal cord regeneration and subsequent functional experimentation will provide more insight into the endogenous mechanism of myelination and remyelination. Furthermore, precise knowledge of the mechanism underlying the extraordinary axonal regeneration process in zebrafish will also allow us to unravel the potential therapeutic strategies to be implemented for enhancing regrowth and remyelination of axons in mammals.

Bilateral activation of STAT3 by phosphorylation at the tyrosine-705 (Y705) and serine-727 (S727) positions and its nuclear translocation in primary sensory neurons following unilateral sciatic nerve injury

Unilateral sciatic nerve compression (SNC) or complete sciatic nerve transection (CSNT), both varying degrees of nerve injury, induced activation of STAT3 bilaterally in the dorsal root ganglia (DRG) neurons of lumbar (L4-L5) as well as cervical (C6-C8) spinal cord segments. STAT3 activation was by phosphorylation at the tyrosine-705 (Y705) and serine-727 (S727) positions and was followed by their nuclear translocation. This is the first evidence of STAT3(S727) activation together with the well-known activation of STAT3(Y705) in primary sensory neurons upon peripheral nerve injury. Bilateral activation of STAT3 in DRG neurons of spinal segments anatomically both associated as well as non-associated with the injured nerve indicates diffusion of STAT3 activation inducers along the spinal cord. Increased levels of IL-6 protein in the CSF following nerve injury as well as activation and nuclear translocation of STAT3 in DRG after intrathecal injection of IL-6 shows that this cytokine, released into the subarachnoid space can penetrate the DRG to activate STAT3. Previous results on increased bilateral IL-6 synthesis and the present manifestation of STAT3 activation in remote DRG following unilateral sciatic nerve injury may reflect a systemic reaction of the DRG neurons to nerve injury.

Transcription factor networks involved in cell death in the dorsal root ganglia following peripheral nerve injury

The peripheral nervous system has the potential to regenerate after nerve injury owing to the intrinsic regrowth ability of neurons and the permissive microenvironment. The regenerative process involves numerous gene expression changes, in which transcription factors play a critical role. Previously, we profiled dysregulated genes in dorsal root ganglion neurons at different time points (0, 3 and 9 hours, and 1, 4 and 7 days) after sciatic nerve injury in rats by RNA sequencing. In the present study, we investigated differentially expressed transcription factors following nerve injury, and we identified enriched molecular and cellular functions of these transcription factors by Ingenuity Pathway Analysis. This analysis revealed the dynamic changes in the expression of transcription factors involved in cell death at different time points following sciatic nerve injury. In addition, we constructed regulatory networks of the differentially expressed transcription factors in cell death and identified some key transcription factors (such as STAT1, JUN, MYC and IRF7). We confirmed the changes in expression of some key transcription factors (STAT1 and IRF7) by quantitative reverse transcription-polymerase chain reaction. Collectively, our analyses provide a global overview of transcription factor changes in dorsal root ganglia after sciatic nerve injury and offer insight into the regulatory transcription factor networks involved in cell death.

Translocator protein agonist Ro5-4864 alleviates neuropathic pain and promotes remyelination in the sciatic nerve

Our previous study reported the translocator protein to play a critical role in neuropathic pain and the possible mechanisms in the spinal cord. However, its mechanism in the peripheral nervous system is poorly understood. This study was undertaken to explore the distribution of translocator protein in the dorsal root ganglion and the possible mechanisms in peripheral nervous system in a rat model of spared nerve injury. Our results showed that translocator protein was activated in dorsal root ganglion after spared nerve injury. The translocator protein signals were primarily colocalized with neurons in dorsal root ganglion. A single intrathecal (i.t.) injection of translocator protein agonist (7-chloro-5-4-chlorophenyl)-1,3-dihydro-1-methyl-2-H-1,4-benzodiaze-pine-2) (Ro5-4864) exerted remarkable analgesic effect compared with the spared nerve injury group ( P < 0.01). After i.t. administration of 2 µg Ro5-4864 on day 3, the expression of translocator protein in ipsilateral dorsal root ganglion was significantly increased on day 7( P < 0.01) but decreased on day 14 ( P < 0.05) compared with the same point in time in the control group. The duration of translocator protein activation in dorsal root ganglion was remarkably shortened. Ro5-4864 also inhibited the activation of phospho-extracellular signal-regulated kinase 1(p-ERK1) ( P < 0.01), p-ERK2 (D7: P < 0.01, D14: P < 0.05), and brain-derived neurotrophic factor ( P < 0.05) in dorsal root ganglion. Meanwhile, i.t. administration of 2 µg Ro5-4864 on day 3 further accelerated the expression of myelin protein zero(P0) and peripheral myelin protein 22 (PMP22). Our results suggested Ro5-4864 could alleviate neuropathic pain and attenuate p-ERK and brain-derived neurotrophic factor activation in dorsal root ganglion. Furthermore, Ro5-4864 stimulated the expression of myelin regeneration proteins which may also be an important factor against neuropathic pain development. Translocator protein may present a novel target for the treatment of neuropathic pain both in the central and peripheral nervous systems.

Sustained cell body reactivity and loss of NeuN in a subset of axotomized bulbospinal neurons after a chronic high cervical spinal cord injury

Following central nervous system lesion, the ability of injured axons to regrowth may depend on the level and duration of the injured cell body response (CBR). Therefore, to investigate whether axotomized brainstem neurons maintain a durable growth-competent state after spinal cord injury, we studied the effect of a chronic C2 hemisection in rats on the expression of various CBR markers involved in axon regeneration, such as c-Jun, ATF-3, HSP27, NO synthase (NOS), and also of the neural mature phenotype marker NeuN, in the bulbospinal respiratory neurons as compared to the gigantocellularis nucleus. Both at 7 and 30 days post-lesion (DPL), c-Jun and HSP27 were present in, respectively, ~60 and ~20% of the axotomized respiratory neurons, whereas the apoptotic factor caspase 3 was not detected in these cells. NOS appeared belatedly, and it was detected in ~20% of the axotomized respiratory neurons at 30DPL. At 30DPL, these different CBR markers were strongly colocalized in a sub-population of axotomized respiratory neurons and also in a sub-population of injured neurons within the gigantocellularis nucleus. Such CBR was also accompanied by a sustained alteration of the neural mature phenotype, as indicated by a loss of NeuN immunoreactivity selectively in HSP27⁺ bulbospinal neurons at 7DPL and 30DPL. Altogether, this study shows that a subset of axotomized medullary respiratory neurons remains in a growth-competent state after a chronic injury, suggesting that they may play a preferential role in long-lasting respiratory neuroplasticity processes.