Axotomy-induced Smad1 activation promotes axonal growth in adult sensory neurons

The Transcription Factor Ets1 Influences Axonal Growth via Regulation of Lcn2

Transcription factors are essential for the development and regeneration of the nervous system. The current study investigated key regulatory transcription factors in rat spinal cord development via RNA sequencing. The hub gene Ets1 was highly expressed in the spinal cord during the embryonic period, and then its expression decreased during spinal cord development. Knockdown of Ets1 significantly increased the axonal growth of cultured spinal cord neurons. Luciferase reporter assays and chromatin immunoprecipitation assays indicated that Ets1 could directly bind to the Lcn2 promoter and positively regulate Lcn2 transcription. In conclusion, these findings provide the first direct evidence that Ets1 regulates axon growth by controlling Lcn2 expression, and Ets1 may be a novel therapeutic target for axon regeneration in the central nervous system.

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.

Aryl hydrocarbon receptor restricts axon regeneration of DRG neurons in response to injury

Injured neurons sense environmental cues to balance neural protection and axon regeneration, but the mechanisms are unclear. Here, we unveil aryl hydrocarbon receptor (AhR), a ligand-activated bHLH-PAS transcription factor, as molecular sensor and key regulator of acute stress response at the expense of axon regeneration. We demonstrate responsiveness of DRG sensory neurons to ligand-mediated AhR signaling, which functions to inhibit axon regeneration. Ahr deletion mimics the conditioning lesion in priming DRG to initiate axonogenesis gene programs; upon peripheral axotomy, Ahr ablation suppresses inflammation and stress signaling while augmenting pro-growth pathways. Moreover, comparative transcriptomics revealed signaling interactions between AhR and HIF-1α, two structurally related bHLH-PAS α units that share the dimerization partner Arnt/HIF-1β. Functional assays showed that the growth advantage of AhR-deficient DRG neurons requires HIF-1α; but in the absence of Arnt, DRG neurons can still mount a regenerative response. We further unveil a link between bHLH-PAS transcription factors and DNA hydroxymethylation in response to peripheral axotomy, while neuronal single cell RNA-seq analysis revealed a link of the AhR regulon to RNA polymerase III regulation and integrated stress response (ISR). Altogether, AhR activation favors stress coping and inflammation at the expense of axon regeneration; targeting AhR can enhance nerve repair.

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.

Circadian clock regulator Bmal1 gates axon regeneration via Tet3 epigenetics in mouse sensory neurons

Axon regeneration of dorsal root ganglia (DRG) neurons after peripheral axotomy involves reconfiguration of gene regulatory circuits to establish regenerative gene programs. However, the underlying mechanisms remain unclear. Here, through an unbiased survey, we show that the binding motif of Bmal1, a central transcription factor of the circadian clock, is enriched in differentially hydroxymethylated regions (DhMRs) of mouse DRG after peripheral lesion. By applying conditional deletion of Bmal1 in neurons, in vitro and in vivo neurite outgrowth assays, as well as transcriptomic profiling, we demonstrate that Bmal1 inhibits axon regeneration, in part through a functional link with the epigenetic factor Tet3. Mechanistically, we reveal that Bmal1 acts as a gatekeeper of neuroepigenetic responses to axonal injury by limiting Tet3 expression and restricting 5hmC modifications. Bmal1-regulated genes not only concern axon growth, but also stress responses and energy homeostasis. Furthermore, we uncover an epigenetic rhythm of diurnal oscillation of Tet3 and 5hmC levels in DRG neurons, corresponding to time-of-day effect on axon growth potential. Collectively, our studies demonstrate that targeting Bmal1 enhances axon regeneration.

Promising application of a novel biomaterial, light chain of silk fibroin combined with NT3, in repairment of rat sciatic nerve defect injury

Autologous nerve transplantation is the gold standard for treating peripheral nerve defects, but it is associated with defects such as insufficient donor and secondary injury. Artificial nerve guidance conduits (NGCs) are now considered promising alternatives for bridging long nerve gaps, although exploring new biomaterials to construct NGCs remains challenging. Silk fibroin (SF) has good biocompatibility and can self-assemble in aqueous solutions¹. However, the lack of proximal neurotrophic factors after nerve injury is a major concern, leading to incomplete nerve regeneration. In this study, NT-3, a neurotrophin that promotes neuronal survival and differentiation, was bound to the light chain of silk fibroin (FIBL) in two ways: one was directly bound to FIBL (FIBL-NT3) and the other was a polypeptides-linker (FIBL-Linker-NT3). The design aimed to take advantage of silk fiber's character of self-assembly of heavy-light chains and test whether a flexible linker with NT3 molecule is easy to be a NT3 dimer, the active form. In vitro studies indicated that FIBL-Linker-NT3 combined with SF membranes promoted axon growth in adult rat dorsal root ganglion (DRG) neurons. Then we tested if FIBL-Linker-NT3 could self-assemble with the SF heavy chain (SFH). DTT (Dithiothreitol) was used to break the disulfide bonds between the SF light and heavy chains, and the light-chain protein was removed via dialysis. SFH was assembled using FIBL-Linker-NT3, as evidenced by the western blotting results that showed a high molecular band corresponding to SFH-FIBL-Linker-NT3. Chitosan scaffolds have been identified to provide a suitable microenvironment, so a chitosan/SF-FIBL-Linker-NT3 conduit was also constructed. Nerve transplantation of this conduit was evaluated in vivo in a rat sciatic nerve defect model. Immunohistochemical assays showed that the chitosan/SF-FIBL-Linker-NT3 group was superior to the chitosan/PBS, SF, PBS + FIBL-Linker-NT3 groups in nerve regeneration. In addition, the chitosan/SF-FIBL-Linker-NT3 conduit-transplanted group exhibited better recovery in terms of neurite length, sciatic functional index value, sensitivity to heat, time on the rotarod, wet weight ratio, cross-sectional area, compound muscle action potential, number of myelin layers, and myelin thickness in the nerve. Taking together, our study identified that FIBL-Linker-NT3 could promote axonal growth and regeneration in vivo and in vitro and is a promising candidate biomaterial for artificial NGCs.

Axonal Regeneration: Underlying Molecular Mechanisms and Potential Therapeutic Targets

Axons in the peripheral nervous system have the ability to repair themselves after damage, whereas axons in the central nervous system are unable to do so. A common and important characteristic of damage to the spinal cord, brain, and peripheral nerves is the disruption of axonal regrowth. Interestingly, intrinsic growth factors play a significant role in the axonal regeneration of injured nerves. Various factors such as proteomic profile, microtubule stability, ribosomal location, and signalling pathways mark a line between the central and peripheral axons' capacity for self-renewal. Unfortunately, glial scar development, myelin-associated inhibitor molecules, lack of neurotrophic factors, and inflammatory reactions are among the factors that restrict axonal regeneration. Molecular pathways such as cAMP, MAPK, JAK/STAT, ATF3/CREB, BMP/SMAD, AKT/mTORC1/p70S6K, PI3K/AKT, GSK-3β/CLASP, BDNF/Trk, Ras/ERK, integrin/FAK, RhoA/ROCK/LIMK, and POSTN/integrin are activated after nerve injury and are considered significant players in axonal regeneration. In addition to the aforementioned pathways, growth factors, microRNAs, and astrocytes are also commendable participants in regeneration. In this review, we discuss the detailed mechanism of each pathway along with key players that can be potentially valuable targets to help achieve quick axonal healing. We also identify the prospective targets that could help close knowledge gaps in the molecular pathways underlying regeneration and shed light on the creation of more powerful strategies to encourage axonal regeneration after nervous system injury.

CBP/p300 activation promotes axon growth, sprouting, and synaptic plasticity in chronic experimental spinal cord injury with severe disability

The interruption of spinal circuitry following spinal cord injury (SCI) disrupts neural activity and is followed by a failure to mount an effective regenerative response resulting in permanent neurological disability. Functional recovery requires the enhancement of axonal and synaptic plasticity of spared as well as injured fibres, which need to sprout and/or regenerate to form new connections. Here, we have investigated whether the epigenetic stimulation of the regenerative gene expression program can overcome the current inability to promote neurological recovery in chronic SCI with severe disability. We delivered the CBP/p300 activator CSP-TTK21 or vehicle CSP weekly between week 12 and 22 following a transection model of SCI in mice housed in an enriched environment. Data analysis showed that CSP-TTK21 enhanced classical regenerative signalling in dorsal root ganglia sensory but not cortical motor neurons, stimulated motor and sensory axon growth, sprouting, and synaptic plasticity, but failed to promote neurological sensorimotor recovery. This work provides direct evidence that clinically suitable pharmacological CBP/p300 activation can promote the expression of regeneration-associated genes and axonal growth in a chronic SCI with severe neurological disability.

Analysis of neuronal injury transcriptional response identifies CTCF and YY1 as co-operating factors regulating axon regeneration

Injured sensory neurons activate a transcriptional program necessary for robust axon regeneration and eventual target reinnervation. Understanding the transcriptional regulators that govern this axon regenerative response may guide therapeutic strategies to promote axon regeneration in the injured nervous system. Here, we used cultured dorsal root ganglia neurons to identify pro-regenerative transcription factors. Using RNA sequencing, we first characterized this neuronal culture and determined that embryonic day 13.5 DRG (eDRG) neurons cultured for 7 days are similar to e15.5 DRG neurons in vivo and that all neuronal subtypes are represented. This eDRG neuronal culture does not contain other non-neuronal cell types. Next, we performed RNA sequencing at different time points after in vitro axotomy. Analysis of differentially expressed genes revealed upregulation of known regeneration associated transcription factors, including Jun, Atf3 and Rest, paralleling the axon injury response in vivo. Analysis of transcription factor binding sites in differentially expressed genes revealed other known transcription factors promoting axon regeneration, such as Myc, Hif1α, Pparγ, Ascl1a, Srf, and Ctcf, as well as other transcription factors not yet characterized in axon regeneration. We next tested if overexpression of novel candidate transcription factors alone or in combination promotes axon regeneration in vitro. Our results demonstrate that expression of Ctcf with Yy1 or E2f2 enhances in vitro axon regeneration. Our analysis highlights that transcription factor interaction and chromatin architecture play important roles as a regulator of axon regeneration.

Transcription factor network analysis identifies REST/NRSF as an intrinsic regulator of CNS regeneration in mice

The inability of neurons to regenerate long axons within the CNS is a major impediment to improving outcome after spinal cord injury, stroke, and other CNS insults. Recent advances have uncovered an intrinsic program that involves coordinate regulation by multiple transcription factors that can be manipulated to enhance growth in the peripheral nervous system. Here, we use a systems genomics approach to characterize regulatory relationships of regeneration-associated transcription factors, identifying RE1-Silencing Transcription Factor (REST; Neuron-Restrictive Silencer Factor, NRSF) as a predicted upstream suppressor of a pro-regenerative gene program associated with axon regeneration in the CNS. We validate our predictions using multiple paradigms, showing that mature mice bearing cell type-specific deletions of REST or expressing dominant-negative mutant REST show improved regeneration of the corticospinal tract and optic nerve after spinal cord injury and optic nerve crush, which is accompanied by upregulation of regeneration-associated genes in cortical motor neurons and retinal ganglion cells, respectively. These analyses identify a role for REST as an upstream suppressor of the intrinsic regenerative program in the CNS and demonstrate the utility of a systems biology approach involving integrative genomics and bio-informatics to prioritize hypotheses relevant to CNS repair.

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

Silencing the enhancer of zeste homologue 2, Ezh2, represses axon regeneration of dorsal root ganglion neurons

Recovery from injury to the peripheral nervous system is different from that of the central nervous system in that it can lead to gene reprogramming that can induce the expression of a series of regeneration-associated genes. This eventually leads to axonal regeneration of injured neurons. Although some regeneration-related genes have been identified, the regulatory network underlying axon regeneration remains largely unknown. To explore the regulator of axon regeneration, we performed RNA sequencing of lumbar L4 and L5 dorsal root ganglion (DRG) neurons at different time points (0, 3, 6, 12 hours, 1, 3 and 7 days) after rat sciatic nerve crush. The isolation of neurons was carried out by laser capture microscopy combined with NeuN immunofluorescence staining. We found 1228 differentially expressed genes in the injured sciatic nerve tissue. The hub genes within these differentially expressed genes include Atf3, Jun, Myc, Ngf, Fgf2, Ezh2, Gfap and Il6. We verified that the expression of the enhancer of zeste homologue 2 gene (Ezh2) was up-regulated in DRG neurons after injury, and this up-regulation differed between large- and small-sized dorsal root ganglion neurons. To investigate whether the up-regulation of Ezh2 impacts axonal regeneration, we silenced Ezh2 with siRNA in cultured DRG neurons and found that the growth of the newborn axons was repressed. In our investigation into the regulatory network of Ezh2 by interpretive phenomenal analysis, we found some regulators of Ezh2 (including Erk, Il6 and Hif1a) and targets (including Atf3, Cdkn1a and Smad1). Our findings suggest that Ezh2, as a nerve regeneration-related gene, participates in the repair of the injured DRG neurons, and knocking down the Ezh2 in vitro inhibits the axonal growth of DRG neurons. All the experimental procedures approved by the Administration Committee of Experimental Animals of Jiangsu Province of China (approval No. S20191201-201) on March 21, 2019.

Phenotypic Screening Following Transcriptomic Deconvolution to Identify Transcription Factors Mediating Axon Growth Induced by a Kinase Inhibitor

After injury to the central nervous system (CNS), both neuron-intrinsic limitations on regenerative responses and inhibitory factors in the injured CNS environment restrict regenerative axon growth. Instances of successful axon regrowth offer opportunities to identify features that differentiate these situations from that of the normal adult CNS. One such opportunity is provided by the kinase inhibitor RO48, which dramatically enhances neurite outgrowth of neurons in vitro and substantially increased contralateral sprouting of corticospinal tract neurons when infused intraventricularly following unilateral pyramidotomy. The authors present here a transcriptomic deconvolution of RO48-associated axon growth, with the goal of identifying transcriptional regulators associated with axon growth in the CNS. Through the use of RNA sequencing (RNA-seq) and transcription factor binding site enrichment analysis, the authors identified a list of transcription factors putatively driving differential gene expression during RO48-induced neurite outgrowth of rat hippocampal neurons in vitro. The 82 transcription factor motifs identified in this way included some with known association to axon growth regulation, such as Jun, Klf4, Myc, Atf4, Stat3, and Nfatc2, and many with no known association to axon growth. A phenotypic loss-of-function screen was carried out to evaluate these transcription factors for their roles in neurite outgrowth; this screen identified several potential outgrowth regulators. Subsequent validation suggests that the Forkhead box (Fox) family transcription factor Foxp2 restricts neurite outgrowth, while FoxO subfamily members Foxo1 and Foxo3a promote neurite outgrowth. The authors' combined transcriptomic-phenotypic screening strategy therefore allowed identification of novel transcriptional regulators of neurite outgrowth downstream of a multitarget kinase inhibitor.

C-terminal domain small phosphatase 1 (CTDSP1) regulates growth factor expression and axonal regeneration in peripheral nerve tissue

Peripheral Nerve Injury (PNI) represents a major clinical and economic burden. Despite the ability of peripheral neurons to regenerate their axons after an injury, patients are often left with motor and/or sensory disability and may develop chronic pain. Successful regeneration and target organ reinnervation require comprehensive transcriptional changes in both injured neurons and support cells located at the site of injury. The expression of most of the genes required for axon growth and guidance and for synapsis formation is repressed by a single master transcriptional regulator, the Repressor Element 1 Silencing Transcription factor (REST). Sustained increase of REST levels after injury inhibits axon regeneration and leads to chronic pain. As targeting of transcription factors is challenging, we tested whether modulation of REST activity could be achieved through knockdown of carboxy-terminal domain small phosphatase 1 (CTDSP1), the enzyme that stabilizes REST by preventing its targeting to the proteasome. To test whether knockdown of CTDSP1 promotes neurotrophic factor expression in both support cells located at the site of injury and in peripheral neurons, we transfected mesenchymal progenitor cells (MPCs), a type of support cells that are present at high concentrations at the site of injury, and dorsal root ganglion (DRG) neurons with REST or CTDSP1 specific siRNA. We quantified neurotrophic factor expression by RT-qPCR and Western blot, and brain-derived neurotrophic factor (BDNF) release in the cell culture medium by ELISA, and we measured neurite outgrowth of DRG neurons in culture. Our results show that CTDSP1 knockdown promotes neurotrophic factor expression in both DRG neurons and the support cells MPCs, and promotes DRG neuron regeneration. Therapeutics targeting CTDSP1 activity may, therefore, represent a novel epigenetic strategy to promote peripheral nerve regeneration after PNI by promoting the regenerative program repressed by injury-induced increased levels of REST in both neurons and support cells.

Bone Morphogenetic Protein-7 (BMP-7) Promotes Neuronal Differentiation of Bone Marrow Mesenchymal Stem Cells (BMSCs) In Vitro

This study is aimed at investigating the effects of bone morphogenetic protein-7 (BMP-7) on the differentiation of bone marrow mesenchymal stem cells (BMSCs) into neuron-like cells in vitro. The rat BMSCs were isolated and identified, which were divided into the control, empty, recombinant rhBMP-7 transfection, and Lv-BMP-7 transfection groups. BMSCs were induced under different conditions. CCK-8 assay was performed to detect cell proliferation. ALP was used to detect cell activity. Cellular morphology after induction was observed. Immunofluorescence was conducted to detect the expression and location of nerve cell markers. Quantitative real-time PCR and Western blot analysis were performed to detect the mRNA and protein expression levels, respectively. The rhBMP-7 and Lv-BMP-7 promoted the proliferation of BMSCs, accompanied with increased ALP activities. Morphological observations revealed that rhBMP-7 and Lv-BMP-7 induced BMSCs to differentiate into neuron-like cells. Immunofluorescence revealed that the rhBMP-7 and Lv-BMP-7 groups showed positive expression of MAP-2 and Nfh in BMSCs. MAP-2 was mainly distributed in the cell body and cellular protrusion, while Nfh was mainly distributed in the cytoplasm and cell protrusion. Positive mRNA and protein expressions of MAP-2 and Nfh were observed in the cells of the rhBMP-7 and Lv-BMP-7 groups, and the expression levels were significantly higher than the control and empty groups. Both exogenous BMP-7 (rhBMP-7) and endogenous BMP-7 (Lv-BMP-7) can induce BMSCs to differentiate into neuron-like cells highly expressing the neuronal markers MAP-2 and Nfh.

The intrinsic axon regenerative properties of mature neurons after injury

Thousands of nerve injuries occur in the world each year. Axon regeneration is a very critical process for the restoration of the injured nervous system's function. However, the precise molecular mechanism or signaling cascades that control axon regeneration are not clearly understood, especially in mammals. Therefore, there is almost no ideal treatment method to repair the nervous system's injury until now. Mammalian axonal regeneration requires multiple signaling pathways to coordinately regulate gene expression in soma and assembly of the cytoskeleton protein in the growth cone. A better understanding of their molecular mechanisms, such as axon regeneration regulatory signaling cascades, will be helpful in developing new treatment strategies for promoting axon regeneration. In this review, we mainly focus on describing these regeneration-associated signaling cascades, which regulate axon regeneration.

Experimental Model Systems for Understanding Human Axonal Injury Responses

Neurons are structurally unique and have dendrites and axons that are vulnerable to injury. Some neurons in the peripheral nervous system (PNS) can regenerate their axons after injuries. However, most neurons in the central nervous system (CNS) fail to do so, resulting in irreversible neurological disorders. To understand the mechanisms of axon regeneration, various experimental models have been utilized in vivo and in vitro. Here, we collate the key experimental models that revealed the important mechanisms regulating axon regeneration and degeneration in different systems. We also discuss the advantages of experimenting with the rodent model, considering the application of these findings in understanding human diseases and for developing therapeutic methods.

The Mechanisms of Peripheral Nerve Preconditioning Injury on Promoting Axonal Regeneration

Two major factors contribute to the failure of axonal regrowth in the central nervous system (CNS), namely, the neuronal intrinsic regenerative capacity and the extrinsic local inhibitory microenvironments. However, a preconditioning peripheral nerve lesion could substantially enhance the regeneration of central axons following a subsequent spinal cord injury. In the present review, we summarize the molecular mechanisms of the preconditioning injury effect on promoting axonal regeneration. The injury signal transduction resulting from preconditioning peripheral nerve injury regulates the RAG expression to enhance axonal regeneration. Importantly, preconditioning peripheral nerve injury triggers interactions between neurons and nonneuronal cells to amplify and maintain their effects. Additionally, the preconditioning injury impacts mitochondria, protein, and lipid synthesis. All these coordinated changes endow axonal regeneration.

TGFβ1 Induces Axonal Outgrowth via ALK5/PKA/SMURF1-Mediated Degradation of RhoA and Stabilization of PAR6

Transforming growth factor (TGF)β1 has repeatedly been associated with axonal regeneration and recovery after injury to the CNS. We found TGFβ1 upregulated in the stroke-denervated mouse spinal cord after ischemic injury to the motor cortex as early as 4 d postinjury (dpi) and persisting up to 28 dpi. Given the potential role of TGFβ1 in structural plasticity and functional recovery after stroke highlighted in several published studies, we investigated its downstream signaling in an in vitro model of neurite outgrowth. We found that in this model, TGFβ1 rescues neurite outgrowth under growth inhibitory conditions via the canonical TGFβR2/ALK5 signaling axis. Thereby, protein kinase A (PKA)-mediated phosphorylation of the E3 ubiquitin ligase SMURF1 induces a switch of its substrate preference from PAR6 to the Ras homolog A (RhoA), in this way enhancing outgrowth on the level of the cytoskeleton. This proposed mechanism of TGFβ1 signaling could underly the observed increase in structural plasticity after stroke in vivo as suggested by the temporal and spatial expression of TGFβ1. In accordance with previous publications, this study corroborates the potential of TGFβ1 and associated signaling cascades as a target for future therapeutic interventions to enhance structural plasticity and functional recovery for stroke patients.

AMPK controls the axonal regenerative ability of dorsal root ganglia sensory neurons after spinal cord injury

Regeneration after injury occurs in axons that lie in the peripheral nervous system but fails in the central nervous system, thereby limiting functional recovery. Differences in axonal signalling in response to injury that might underpin this differential regenerative ability are poorly characterized. Combining axoplasmic proteomics from peripheral sciatic or central projecting dorsal root ganglion (DRG) axons with cell body RNA-seq, we uncover injury-dependent signalling pathways that are uniquely represented in peripheral versus central projecting sciatic DRG axons. We identify AMPK as a crucial regulator of axonal regenerative signalling that is specifically downregulated in injured peripheral, but not central, axons. We find that AMPK in DRG interacts with the 26S proteasome and its CaMKIIα-dependent regulatory subunit PSMC5 to promote AMPKα proteasomal degradation following sciatic axotomy. Conditional deletion of AMPKα1 promotes multiple regenerative signalling pathways after central axonal injury and stimulates robust axonal growth across the spinal cord injury site, suggesting inhibition of AMPK as a therapeutic strategy to enhance regeneration following spinal cord injury.

NeuroD1 overexpression in spinal neurons accelerates axonal regeneration after sciatic nerve injury

Neurogenic differentiation 1 (NeuroD1) is mainlyexpressed in developing neurons where it plays critical roles in neuronal maturation and neurite elongation. The potential role and mechanism of NeuroD1 in adult axonal regeneration is not clear. The present study used synapsin (SYN) Cre and AAV9-Flex vectors to conditionally overexpress NeuroD1 in adult spinal neurons and found that NeuroD1 overexpression significantly accelerated axonal regeneration and functional recovery after sciatic nerve injury. Further in vitro and in vivo experiments suggested that the mechanism of NeuroD1 promotion on axonal regeneration was related to its regulation of the expression of neurotrophin BDNF and its receptor TrkB as well as a microtubule severing protein spastin.

Decoding epigenetic codes: new frontiers in exploring recovery from spinal cord injury

Spinal cord injury that results in severe neurological disability is often incurable. The poor clinical outcome of spinal cord injury is mainly caused by the failure to reconstruct the injured neural circuits. Several intrinsic and extrinsic determinants contribute to this inability to reconnect. Epigenetic regulation acts as the driving force for multiple pathological and physiological processes in the central nervous system by modulating the expression of certain critical genes. Recent studies have demonstrated that post-SCI alteration of epigenetic landmarks is strongly associated with axon regeneration, glial activation and neurogenesis. These findings not only establish a theoretical foundation for further exploration of spinal cord injury, but also provide new avenues for the clinical treatment of spinal cord injury. This review focuses on the epigenetic regulation in axon regeneration and secondary spinal cord injury. Together, these discoveries are a selection of epigenetic-based prognosis biomarkers and attractive therapeutic targets in the treatment of spinal cord injury.

BMP-7/Smad expression in dedifferentiated Schwann cells during axonal regeneration and upregulation of endogenous BMP-7 following administration of PTH (1-34)

PURPOSE:

To determine the expression and distribution of bone morphogenetic protein (BMP)-7 and related molecules during peripheral nerve regeneration and to assess whether administration of parathyroid hormone (PTH) drug (1-34) potentiates the intrinsic upregulation of BMP-7/Smad signaling.

METHODS:

The rat sciatic nerves were crushed with an aneurysm clip resulting in axonal degeneration. In the normal nerve, and at 1, 2, 4, and 8 weeks after injury, BMP-7, BMP receptors, p-Smad 1/5/8, and Noggin, the endogenous BMP antagonist, were evaluated. Additionally, the distribution of BMP-7 was assessed by fluorescent double immunostaining. In vitro studies were also performed to examine the effect of BMP-7 and PTH (1-34) administration on rat Schwann cells (SCs).

RESULTS:

Aneurysm clip made reliable animal model of the nerve injury with recovery at 8 weeks after the injury. BMP-7/Smad protein and mRNA were significantly upregulated on axon-SCs units at 1 week after injury, and this upregulated expression was maintained for 4 weeks. Besides, significant upregulation of Noggin's expression was observed on axon-SCs units at 2 weeks after injury. Moreover, fluorescent double immunostaining showed co-localization between expression of BMP-7 and p75NTR during axonal regeneration. In the in vitro study, administration of BMP-7 induced significant proliferation of SCs. Application of PTH (1-34) upregulated BMP-7 on SCs.

DISCUSSION/CONCLUSION:

BMPs were reported to be involved in protection and recovery after injury as well as in neurogenesis. Our current study showed that BMP/Smad signaling molecules were upregulated on dedifferentiated SCs after peripheral nerve injury and that administration of BMP-7 increased SC viability in vitro. These results suggested that axonal regeneration could be induced via upregulation of endogenous BMP-7 on SCs by PTH (1-34) administration.

Activation of the BMP4/Smad1 Pathway Promotes Retinal Ganglion Cell Survival and Axon Regeneration

Purpose

We investigate if the BMP4/Smad1 intracellular signaling pathway is neuroprotective and axogenic in adult rodent retinal ganglion cells (RGC) in vivo and in vitro.

Methods

Adult retinal cultures were prepared from intact and after optic nerve crush (ONC) injured rats that have been stimulated to survive and regenerate using an intravitreal peripheral nerve (PN) graft. Laser capture microdissection (LCM) then was used to isolate RGC with and without neurites. Quantitative RT-PCR determined changes in BMP4/Smad1 signaling pathway mRNA. Immunohistochemistry confirmed localization of BMP4 and activation of Smad1 in ONC+PN-stimulated RGC in vivo. BMP4 peptide was used to stimulate RGC survival and neurite/axon regeneration in vitro and in vivo. Finally, the rapamycin sensitivity of the effects was determined in BMP4-stimulated RGC in vitro and in vivo.

Results

In retinal cultures prepared from intact and ONC+PN-stimulated rats, RGC with neurites had upregulated regeneration-related and BMP4/Smad1 signaling pathway mRNA levels, while low levels of these mRNAs were present in RGC isolated without neurites. An optimal dose of 200 ng/mL BMP4 peptide in vitro promoted approximately 30% RGC survival and disinhibited RGC neurite outgrowth, despite the presence of inhibitory CNS myelin extracts. BMP4 also promoted approximately 30% RGC survival in vivo and stimulated significant RGC axon regeneration at 100, 200, and 400 μm beyond the lesion site. Finally, the response of RGC to BMP4 treatment in vitro and in vivo was rapamycin-insensitive.

Conclusions

Activation of the BMP4/Smad1 pathway promotes survival and axon regeneration independent of mTOR and, therefore, may be of therapeutic interest.

Intrinsic mechanisms of neuronal axon regeneration

Permanent disabilities following CNS injuries result from the failure of injured axons to regenerate and rebuild functional connections with their original targets. By contrast, injury to peripheral nerves is followed by robust regeneration, which can lead to recovery of sensory and motor functions. This regenerative response requires the induction of widespread transcriptional and epigenetic changes in injured neurons. Considerable progress has been made in recent years in understanding how peripheral axon injury elicits these widespread changes through the coordinated actions of transcription factors, epigenetic modifiers and, to a lesser extent, microRNAs. Although many questions remain about the interplay between these mechanisms, these new findings provide important insights into the pivotal role of coordinated gene expression and chromatin remodelling in the neuronal response to injury.

Epigenetic Regulation Of Axon Regeneration and Glial Activation in Injury Responses

Injury to the nervous system triggers a multicellular response in which epigenetic mechanisms play an important role in regulating cell type-specific transcriptional changes. Here, we summarize recent progress in characterizing neuronal intrinsic and extrinsic chromatin reconfigurations and epigenetic changes triggered by axonal injury that shape neuroplasticity and glial functions. We specifically discuss regeneration-associated transcriptional modules comprised of transcription factors and epigenetic regulators that control axon growth competence. We also review epigenetic regulation of neuroinflammation and astroglial responses that impact neural repair. These advances provide a framework for developing epigenetic strategies to maximize adaptive alterations while minimizing maladaptive stress responses in order to enhance axon regeneration and achieve functional recovery after injury.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

BMP4/Smad1 Signalling Promotes Spinal Dorsal Column Axon Regeneration and Functional Recovery After Injury

Signalling through the BMP4/Smad1 pathway promotes corticospinal tract axon regeneration and functional recovery in mice. However, unlike humans and rats, mice do not cavitate. Here, we investigated if activation of the BMP4/Smad1 pathway promotes axon regeneration and functional recovery in a rat model that cavitates. We show that dorsal root ganglion neurons (DRGN) in injury models, including the non-regenerating dorsal column (DC) and the regenerating sciatic nerve (SN) crush and preconditioning (p) SN + DC (pSN + DC) paradigms, regulate the BMP4/Smad1 signalling pathway. For example, mRNA expression of positive regulators of the BMP4/Smad1 pathway was highly up-regulated whilst negative regulators were significantly down-regulated in DRGN in the regenerating SN and pSN + DC models compared to non-regenerating DC models, matched by concomitant changes in protein expression detected in DRGN by immunohistochemistry. BMP4 peptide promoted significant DRGN survival and disinhibited neurite outgrowth in vitro, whilst AAV-BMP4 delivery in vivo stimulated DC axon regeneration and functional recovery in a model that cavitates. Our results show that activation of the BMP4/Smad1 pathway is a potential therapeutic target in the search for axon regenerative signalling pathways in the CNS.

An In Vitro Model for Conditioning Lesion Effect

Axons of a peripheral nerve grow faster after an axotomy if it attains a prior injury a few days earlier. This is called conditioning lesion effect (CLE) and very much valued since it may provide new insights into neuron biology and axonal regeneration. There are established in vivo experimental paradigms to study CLE, however, there is a need to have an in vitro conditioning technique where CLE occurs in a maximally controlled environment. Mouse primary sensory neurons were isolated from lumbar 4-5 dorsal root ganglia and incubated at 37 °C on a silicon-coated watch glass that prevents cell attachment. After this conditioning period they were transferred to laminin coated culture dishes. Similar cultures were set up with freshly isolated neurons from control animals and from the animals that received a sciatic nerve cut 3 days earlier. All preparations were placed on a live cell imaging microscopy providing physiological conditions and photographed for 48 h. Axonal regeneration and neuronal survival was assessed. During the conditioning incubation period neurons remained in suspended aggregates and did not grow axons. The regeneration rate of the in vitro conditioned neurons was much higher than the in vivo conditioned and control preparations during the first day of normal incubation. However, higher regeneration rates were compromised by progressive substantial neuronal death in both types of conditioned cultures but not in the control preparations. By using neutralizing antibodies, we demonstrated that activity of endogenous leukemia inhibitory factor is essential for induction of CLE in this model.

HSP90 is a chaperone for DLK and is required for axon injury signaling

Peripheral nerve injury induces a robust proregenerative program that drives axon regeneration. While many regeneration-associated genes are known, the mechanisms by which injury activates them are less well-understood. To identify such mechanisms, we performed a loss-of-function pharmacological screen in cultured adult mouse sensory neurons for proteins required to activate this program. Well-characterized inhibitors were present as injury signaling was induced but were removed before axon outgrowth to identify molecules that block induction of the program. Of 480 compounds, 35 prevented injury-induced neurite regrowth. The top hits were inhibitors to heat shock protein 90 (HSP90), a chaperone with no known role in axon injury. HSP90 inhibition blocks injury-induced activation of the proregenerative transcription factor cJun and several regeneration-associated genes. These phenotypes mimic loss of the proregenerative kinase, dual leucine zipper kinase (DLK), a critical neuronal stress sensor that drives axon degeneration, axon regeneration, and cell death. HSP90 is an atypical chaperone that promotes the stability of signaling molecules. HSP90 and DLK show two hallmarks of HSP90-client relationships: (i) HSP90 binds DLK, and (ii) HSP90 inhibition leads to rapid degradation of existing DLK protein. Moreover, HSP90 is required for DLK stability in vivo, where HSP90 inhibitor reduces DLK protein in the sciatic nerve. This phenomenon is evolutionarily conserved in Drosophila Genetic knockdown of Drosophila HSP90, Hsp83, decreases levels of Drosophila DLK, Wallenda, and blocks Wallenda-dependent synaptic terminal overgrowth and injury signaling. Our findings support the hypothesis that HSP90 chaperones DLK and is required for DLK functions, including proregenerative axon injury signaling.

The effect of Jun dimerization on neurite outgrowth and motif binding

Axon regeneration is a necessary step toward functional recovery after spinal cord injury. The AP-1 transcription factor c-Jun has long been known to play an important role in directing the transcriptional response of Dorsal Root Ganglion (DRG) neurons to peripheral axotomy that results in successful axon regeneration. Here we performed ChIPseq for Jun in mouse DRG neurons after a sciatic nerve crush or sham surgery in order to measure the changes in Jun's DNA binding in response to peripheral axotomy. We found that the majority of Jun's injury-responsive changes in DNA binding occur at putative enhancer elements, rather than proximal to transcription start sites. We also used a series of single polypeptide chain tandem transcription factors to test the effects of different Jun-containing dimers on neurite outgrowth in DRG, cortical and hippocampal neurons. These experiments demonstrated that dimers composed of Jun and Atf3 promoted neurite outgrowth in rat CNS neurons as well as mouse DRG neurons. Our work provides new insight into the mechanisms underlying Jun's role in axon regeneration.

BMP signaling downstream of the Highwire E3 ligase sensitizes nociceptors

A comprehensive understanding of the molecular machinery important for nociception is essential to improving the treatment of pain. Here, we show that the BMP signaling pathway regulates nociception downstream of the E3 ubiquitin ligase highwire (hiw). hiw loss of function in nociceptors caused antagonistic and pleiotropic phenotypes with simultaneous insensitivity to noxious heat but sensitized responses to optogenetic activation of nociceptors. Thus, hiw functions to both positively and negatively regulate nociceptors. We find that a sensory reception-independent sensitization pathway was associated with BMP signaling. BMP signaling in nociceptors was up-regulated in hiw mutants, and nociceptor-specific expression of hiw rescued all nociception phenotypes including the increased BMP signaling. Blocking the transcriptional output of the BMP pathway with dominant negative Mad suppressed nociceptive hypersensitivity that was induced by interfering with hiw. The up-regulated BMP signaling phenotype in hiw genetic mutants could not be suppressed by mutation in wallenda suggesting that hiw regulates BMP in nociceptors via a wallenda independent pathway. In a newly established Ca²⁺ imaging preparation, we observed that up-regulated BMP signaling caused a significantly enhanced Ca²⁺ signal in the axon terminals of nociceptors that were stimulated by noxious heat. This response likely accounts for the nociceptive hypersensitivity induced by elevated BMP signaling in nociceptors. Finally, we showed that 24-hour activation of BMP signaling in nociceptors was sufficient to sensitize nociceptive responses to optogenetically-triggered nociceptor activation without altering nociceptor morphology. Overall, this study demonstrates the previously unrevealed roles of the Hiw-BMP pathway in the regulation of nociception and provides the first direct evidence that up-regulated BMP signaling physiologically sensitizes responses of nociceptors and nociception behaviors.

Although pain is a universally experienced sensation that has a significant impact on human lives and society, the molecular mechanisms of pain remain poorly understood. Elucidating these mechanisms is particularly important to gaining insight into the clinical development of currently incurable chronic pain diseases. Taking an advantage of the powerful genetic model organism Drosophila melanogaster (fruit flies), we unveil the Highwire-BMP signaling pathway as a novel molecular pathway that regulates the sensitivity of nociceptive sensory neurons. Highwire and the molecular components of the BMP signaling pathway are known to be widely conserved among animal phyla, from nematode worms to humans. Since abnormal sensitivity of nociceptive sensory neurons can play a critical role in the development of chronic pain conditions, a deeper understanding of the regulation of nociceptor sensitivity has the potential to advance effective therapeutic strategies to treat difficult pain conditions.

Advances and Limitations of Current Epigenetic Studies Investigating Mammalian Axonal Regeneration

Axonal regeneration relies on the expression of regenerative associated genes within a coordinated transcriptional programme, which is finely tuned as a result of the activation of several regenerative signalling pathways. In mammals, this chain of events occurs in neurons following peripheral axonal injury, however it fails upon axonal injury in the central nervous system, such as in the spinal cord and the brain. Accumulating evidence has been suggesting that epigenetic control is a key factor to initiate and sustain the regenerative transcriptional response and that it might contribute to regenerative success versus failure. This review will discuss experimental evidence so far showing a role for epigenetic regulation in models of peripheral and central nervous system axonal injury. It will also propose future directions to fill key knowledge gaps and to test whether epigenetic control might indeed discriminate between regenerative success and failure.

miR-129 controls axonal regeneration via regulating insulin-like growth factor-1 in peripheral nerve injury

The microenvironment of peripheral nerve regeneration consists of multiple neurotrophic factors, adhesion molecules, and extracellular matrix molecules, secreted by unique glial cells in the peripheral nerve system (PNS)-Schwann cell (SCs). Following peripheral nerve injury (PNI), local IGF-1 production is upregulated in SCs and denervated muscle during axonal sprouting and regeneration. Regulation of IGF-1/IGF-1R signaling is considered as a potentially targeted therapy of PNI. We previously identified a group of novel miRNAs in proximal nerve following rat sciatic nerve transection. The present work focused on the role of miR-129 in regulation of IGF-1 signaling after sciatic nerve injury. The temporal change profile of the miR-129 expression was negatively correlated with the IGF-1 expression in proximal nerve stump and dorsal root ganglion (DRG) following sciatic nerve transection. An increased expression of miR-129 inhibited proliferation and migration of SCs, and axonal outgrowth of DRG neurons, which was inversely promoted by silencing of the miR-129 expression. The IGF-1 was identified as one of the multiple target genes of miR-129, which exerted negative regulation of IGF-1 by translational suppression. Moreover, knockdown of IGF-1 attenuated the promoting effects of miR-129 inhibitor on proliferation and migration of SCs, and neurite outgrowth of DRG neurons. Overall, our data indicated that miR-129 own the potential to regulate the proliferation and migration of SCs by targeting IGF-1, providing further insight into the regulatory role of miRNAs in peripheral nerve regeneration. The present work not only provides new insight into miR-129 regulation of peripheral nerve regeneration by robust phenotypic modulation of neural cells, but also opens a novel therapeutic window for PNI by mediating IGF-1 production. Our results may provide further experimental basis for translation of the molecular therapy into the clinic.

The Dorsal Column Lesion Model of Spinal Cord Injury and Its Use in Deciphering the Neuron-Intrinsic Injury Response

The neuron‐intrinsic response to axonal injury differs markedly between neurons of the peripheral and central nervous system. Following a peripheral lesion, a robust axonal growth program is initiated, whereas neurons of the central nervous system do not mount an effective regenerative response. Increasing the neuron‐intrinsic regenerative response would therefore be one way to promote axonal regeneration in the injured central nervous system. The large‐diameter sensory neurons located in the dorsal root ganglia are pseudo‐unipolar neurons that project one axon branch into the spinal cord, and, via the dorsal column to the brain stem, and a peripheral process to the muscles and skin. Dorsal root ganglion neurons are ideally suited to study the neuron‐intrinsic injury response because they exhibit a successful growth response following peripheral axotomy, while they fail to do so after a lesion of the central branch in the dorsal column. The dorsal column injury model allows the neuron‐intrinsic regeneration response to be studied in the context of a spinal cord injury. Here we will discuss the advantages and disadvantages of this model. We describe the surgical methods used to implement a lesion of the ascending fibers, the anatomy of the sensory afferent pathways and anatomical, electrophysiological, and behavioral techniques to quantify regeneration and functional recovery. Subsequently we review the results of experimental interventions in the dorsal column lesion model, with an emphasis on the molecular mechanisms that govern the neuron‐intrinsic injury response and manipulations of these after central axotomy. Finally, we highlight a number of recent advances that will have an impact on the design of future studies in this spinal cord injury model, including the continued development of adeno‐associated viral vectors likely to improve the genetic manipulation of dorsal root ganglion neurons and the use of tissue clearing techniques enabling 3D reconstruction of regenerating axon tracts. © 2018 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 00: 000–000, 2018

The intriguing nature of dorsal root ganglion neurons: Linking structure with polarity and function

Dorsal root ganglion (DRG) neurons are the first neurons of the sensory pathway. They are activated by a variety of sensory stimuli that are then transmitted to the central nervous system. An important feature of DRG neurons is their unique morphology where a single process -the stem axon- bifurcates into a peripheral and a central axonal branch, with different functions and cellular properties. Distinctive structural aspects of the two DRG neuron branches may have important implications for their function in health and disease. However, the link between DRG axonal branch structure, polarity and function has been largely neglected in the field, and relevant information is rather scattered across the literature. In particular, ultrastructural differences between the two axonal branches are likely to account for the higher transport and regenerative ability of the peripheral DRG neuron axon when compared to the central one. Nevertheless, the cell intrinsic factors contributing to this central-peripheral asymmetry are still unknown. Here we critically review the factors that may underlie the functional asymmetry between the peripheral and central DRG axonal branches. Also, we discuss the hypothesis that DRG neurons may assemble a structure resembling the axon initial segment that may be responsible, at least in part, for their polarity and electrophysiological features. Ultimately, we suggest that the clarification of the axonal ultrastructure of DRG neurons using state-of-the-art techniques will be crucial to understand the physiology of this peculiar cell type.

Metformin promotes neuronal differentiation and neurite outgrowth through AMPK activation in human bone marrow-mesenchymal stem cells

Metformin is an AMP-activated kinase (AMPK) activator that plays a role in glucose energy metabolism and cell protection. It is widely used to treat several diseases, including type 2 diabetes, cardiovascular diseases, cancer, and metabolic diseases. In this study, we investigated whether AMPK activation upon treatment with metformin may promote neurite outgrowth during the progression of neuronal differentiation in human bone marrow-mesenchymal stem cells (hBM-MSCs). Differentiation of metformin-treated MSCs (Met-MSCs to Met-diMSCs) in the neuronal induction media resulted in an increase in the number of differentiated cells in a metformin concentration dependent manner. The differentiation rate reached its maximum at 3 H after the initial treatment with neuronal induction media. At 3 H of induction, the neurite length increased significantly in Met-diMSCs as compared with control cells without metformin treatment (diMSCs). diMSCs showed a significant increase in the expression of neuronal-specific marker genes; however, the expression of dendrite-specific markers MAP-2 and Tuj-1 was significantly increased in Met-diMSCs as compared to diMSCs, as confirmed by immunoblotting. This effect was abolished upon treatment with the AMPK inhibitor, compound C, as evident by quantitative PCR, immunoblotting, and immunocytochemical staining. Thus, metformin treatment promotes neuronal differentiation and neurite outgrowth in hBM-MSCs through AMPK activation.

Functional integration of complex miRNA networks in central and peripheral lesion and axonal regeneration

New players are emerging in the game of peripheral and central nervous system injury since their physiopathological mechanisms remain partially elusive. These mechanisms are characterized by several molecules whose activation and/or modification following a trauma is often controlled at transcriptional level. In this scenario, microRNAs (miRNAs/miRs) have been identified as main actors in coordinating important molecular pathways in nerve or spinal cord injury (SCI). miRNAs are small non-coding RNAs whose functionality at network level is now emerging as a new level of complexity. Indeed they can act as an organized network to provide a precise control of several biological processes. Here we describe the functional synergy of some miRNAs in case of SCI and peripheral damage. In particular we show how several small RNAs can cooperate in influencing simultaneously the molecular pathways orchestrating axon regeneration, inflammation, apoptosis and remyelination. We report about the networks for which miRNA-target bindings have been experimentally demonstrated or inferred based on target prediction data: in both cases, the connection between one miRNA and its downstream pathway is derived from a validated observation or is predicted from the literature. Hence, we discuss the importance of miRNAs in some pathological processes focusing on their functional structure as participating in a cooperative and/or convergence network.

Identification of miRNAs involved in DRG neurite outgrowth and their putative targets

Peripheral neurons regenerate their axons after injury. Transcriptional regulation by microRNAs (miRNAs) is one possible mechanism controlling regeneration. We profiled miRNA expression in mouse dorsal root ganglion neurons after a sciatic nerve crush, and identified 49 differentially expressed miRNAs. We evaluated the functional role of each miRNA using a phenotypic analysis approach. To predict the targets of the miRNAs we employed RNA-Sequencing and examined transcription at the isoform level. We identify thousands of differentially expressed isoforms and bioinformatically associate the miRNAs that modulate neurite growth with their putative target isoforms to outline a network of regulatory events underlying peripheral nerve regeneration. MiR-298, let-7a, and let-7f enhance neurite growth and target the majority of isoforms in the differentially expressed network.

Serotonin axons in the neocortex of the adult female mouse regrow after traumatic brain injury

It is widely held that injured neurons in the central nervous system do not undergo axonal regrowth. However, there is mounting evidence that serotonin axons are a notable exception. Serotonin axons undergo long-distance regrowth in the neocortex after amphetamine lesion, and, following a penetrating stab injury, they can regrow from cut ends to traverse the stab rift. Traumatic brain injury (TBI) is clinically prevalent and can lead to pathologies, such as depression, that are related to serotonergic dysfunction. Thus, whether serotonin axons can regrow after TBI is an important question. We used two models for TBI-a persistent open skull condition and controlled cortical impact-to evoke injury in adult female mouse neocortex, and assessed serotonin axon density 1 week, 1 month, and 3 months after injury by serotonin transporter immunohistochemistry. We found that after both forms of TBI, serotonin axon density is decreased posterior but not anterior to the injury site when measured in layer 1 at 1 week post surgery, and that serotonin axons are capable of regrowing into the distal zone to increase density by 1 month post surgery. This pattern is consistent with the anterior-to-posterior course of serotonin axons in the neocortex. TBI in these models is associated with significant reactive astrogliosis both anterior and posterior to the impact, but the degree of reactive astrogliosis is not correlated with serotonin axon density when measured 1 week after TBI. Microglial density remains constant following both types of injuries, but microglial condensation was detected 1 week after controlled cortical impact.

Dual leucine zipper kinase-dependent PERK activation contributes to neuronal degeneration following insult

The PKR-like endoplasmic reticulum kinase (PERK) arm of the Integrated Stress Response (ISR) is implicated in neurodegenerative disease, although the regulators and consequences of PERK activation following neuronal injury are poorly understood. Here we show that PERK signaling is a component of the mouse MAP kinase neuronal stress response controlled by the Dual Leucine Zipper Kinase (DLK) and contributes to DLK-mediated neurodegeneration. We find that DLK-activating insults ranging from nerve injury to neurotrophin deprivation result in both c-Jun N-terminal Kinase (JNK) signaling and the PERK- and ISR-dependent upregulation of the Activating Transcription Factor 4 (ATF4). Disruption of PERK signaling delays neurodegeneration without reducing JNK signaling. Furthermore, DLK is both sufficient for PERK activation and necessary for engaging the ISR subsequent to JNK-mediated retrograde injury signaling. These findings identify DLK as a central regulator of not only JNK but also PERK stress signaling in neurons, with both pathways contributing to neurodegeneration.

The application of CRISPR technology to high content screening in primary neurons

Axon growth is coordinated by multiple interacting proteins that remain incompletely characterized. High content screening (HCS), in which manipulation of candidate genes is combined with rapid image analysis of phenotypic effects, has emerged as a powerful technique to identify key regulators of axon outgrowth. Here we explore the utility of a genome editing approach referred to as CRISPR (Clustered Regularly Interspersed Palindromic Repeats) for knockout screening in primary neurons. In the CRISPR approach a DNA-cleaving Cas enzyme is guided to genomic target sequences by user-created guide RNA (sgRNA), where it initiates a double-stranded break that ultimately results in frameshift mutation and loss of protein production. Using electroporation of plasmid DNA that co-expresses Cas9 enzyme and sgRNA, we first verified the ability of CRISPR targeting to achieve protein-level knockdown in cultured postnatal cortical neurons. Targeted proteins included NeuN (RbFox3) and PTEN, a well-studied regulator of axon growth. Effective knockdown lagged at least four days behind transfection, but targeted proteins were eventually undetectable by immunohistochemistry in >80% of transfected cells. Consistent with this, anti-PTEN sgRNA produced no changes in neurite outgrowth when assessed three days post-transfection. When week-long cultures were replated, however, PTEN knockdown consistently increased neurite lengths. These CRISPR-mediated PTEN effects were achieved using multi-well transfection and automated phenotypic analysis, indicating the suitability of PTEN as a positive control for future CRISPR-based screening efforts. Combined, these data establish an example of CRISPR-mediated protein knockdown in primary cortical neurons and its compatibility with HCS workflows.

Rapamycin-Resistant mTOR Activity Is Required for Sensory Axon Regeneration Induced by a Conditioning Lesion

Neuronal mammalian target of rapamycin (mTOR) activity is a critical determinant of the intrinsic regenerative ability of mature neurons in the adult central nervous system (CNS). However, whether its action also applies to peripheral nervous system (PNS) neurons after injury remains elusive. To address this issue unambiguously, we used genetic approaches to determine the role of mTOR signaling in sensory axon regeneration in mice. We showed that deleting mTOR in dorsal root ganglion (DRG) neurons suppressed the axon regeneration induced by conditioning lesions. To establish whether the impact of mTOR on axon regeneration results from functions of mTOR complex 1 (mTORC1) or 2 (mTORC2), two distinct kinase complexes, we ablated either Raptor or Rictor in DRG neurons. We found that suppressing mTORC1 signaling dramatically decreased the conditioning lesion effect. In addition, an injury to the peripheral branch boosts mTOR activity in DRG neurons that cannot be completely inhibited by rapamycin, a widely used mTOR-specific inhibitor. Unexpectedly, examining several conditioning lesion–induced pro-regenerative pathways revealed that Raptor deletion but not rapamycin suppressed Stat3 activity in neurons. Therefore, our results demonstrate that crosstalk between mTOR and Stat3 signaling mediates the conditioning lesion effect and provide genetic evidence that rapamycin-resistant mTOR activity contributes to the intrinsic axon growth capacity in adult sensory neurons after injury.

Comprehensive mapping of 5-hydroxymethylcytosine epigenetic dynamics in axon regeneration

In contrast to central nervous system neurons, dorsal root ganglia (DRG) neurons can switch to a regenerative state after peripheral axotomy. In a screen for chromatin regulators of the regenerative responses in this conditioning lesion paradigm, we identified Tet methylcytosine dioxygenase 3 (Tet3) as upregulated in DRG neurons, along with increased 5-hydroxymethylcytosine (5hmC). We generated genome-wide 5hmC maps in adult DRG, which revealed that peripheral and central axotomy (leading to no regenerative effect) triggered differential 5hmC changes that are associated with distinct signaling pathways. 5hmC was altered in a large set of regeneration-associated genes (RAGs), including well-known RAGs, such as Atf3, Bdnf, and Smad1, that regulate axon growth potential of DRG neurons, thus supporting its role for RAG regulation. Our analyses also predicted HIF-1, STAT, and IRF as potential transcription factors that may collaborate with Tet3 for 5hmC modifications. Intriguingly, central axotomy resulted in widespread 5hmC modifications that had little overlap with those of peripheral axotomy, thus potentially constituting a roadblock for regeneration. Our study revealed 5hmC dynamics as a previously unrecognized epigenetic mechanism underlying the divergent responses after axonal injury.

KLF7-transfected Schwann cell graft transplantation promotes sciatic nerve regeneration

Our former study demonstrated that Krüppel-like Factor 7 (KLF7) is a transcription factor that stimulates axonal regeneration after peripheral nerve injury. Currently, we used a gene therapy approach to overexpress KLF7 in Schwann cells (SCs) and assessed whether KLF7-transfected SCs graft could promote sciatic nerve regeneration. SCs were transfected by adeno-associated virus 2 (AAV2)-KLF7 in vitro. Mice were allografted by an acellular nerve (ANA) with either an injection of DMEM (ANA group), SCs (ANA+SCs group) or AAV2-KLF7-transfected SCs (ANA+KLF7-SCs group) to assess repair of a sciatic nerve gap. The results indicate that KLF7 overexpression promoted the proliferation of both transfected SCs and native SCs. The neurite length of the dorsal root ganglia (DRG) explants was enhanced. Several beneficial effects were detected in the ANA+KLF7-SCs group including an increase in the compound action potential amplitude, sciatic function index score, enhanced expression of PKH26-labeling transplant SCs, peripheral myelin protein 0, neurofilaments, S-100, and myelinated regeneration nerve. Additionally, HRP-labeled motoneurons in the spinal cord, CTB-labeled sensory neurons in the DRG, motor endplate density and the weight ratios of target muscles were increased by the treatment while thermal hyperalgesia was diminished. Finally, expression of KLF7, NGF, GAP43, TrkA and TrkB were enhanced in the grafted SCs, which may indicate that several signal pathways may be involved in conferring the beneficial effects from KLF7 overexpression. We concluded that KLF7-overexpressing SCs promoted axonal regeneration of the peripheral nerve and enhanced myelination, which collectively proved KLF-SCs as a novel therapeutic strategy for injured nerves.

Intrinsic Axonal Growth and the Drive for Regeneration

Following damage to the adult nervous system in conditions like stroke, spinal cord injury, or traumatic brain injury, many neurons die and most of the remaining spared neurons fail to regenerate. Injured neurons fail to regrow both because of the inhibitory milieu in which they reside as well as a loss of the intrinsic growth capacity of the neurons. If we are to develop effective therapeutic interventions that promote functional recovery for the devastating injuries described above, we must not only better understand the molecular mechanisms of developmental axonal growth in hopes of re-activating these pathways in the adult, but at the same time be aware that re-activation of adult axonal growth may proceed via distinct mechanisms. With this knowledge in hand, promoting adult regeneration of central nervous system neurons can become a more tractable and realistic therapeutic endeavor.

Single-cell RNA-seq reveals distinct injury responses in different types of DRG sensory neurons

Peripheral nerve injury leads to various injury-induced responses in sensory neurons including physiological pain, neuronal cell death, and nerve regeneration. In this study, we performed single-cell RNA-sequencing (scRNA-seq) analysis of mouse nonpeptidergic nociceptors (NP), peptidergic nociceptors (PEP), and large myelinated sensory neurons (LM) under both control and injury conditions at 3 days after sciatic nerve transection (SNT). After performing principle component and weighted gene co-expression network analysis, we categorized dorsal root ganglion (DRG) neurons into different subtypes and discovered co-regulated injury-response genes including novel regeneration associated genes (RAGs) in association with neuronal development, protein translation and cytoplasm transportation. In addition, we found significant up-regulation of the genes associated with cell death such as Pdcd2 in a subset of NP neurons after axotomy, implicating their actions in neuronal cell death upon nerve injury. Our study revealed the distinctive and sustained heterogeneity of transcriptomic responses to injury at single neuron level, implicating the involvement of different gene regulatory networks in nerve regeneration, neuronal cell death and neuropathy in different population of DRG neurons.

miR-148b-3p promotes migration of Schwann cells by targeting cullin-associated and neddylation-dissociated 1

MicroRNAs (miRNAs) are small, non-coding RNAs that negatively adjust gene expression in multifarious biological processes. However, the regulatory effects of miRNAs on Schwann cells remain poorly understood. Previous microarray analysis results have shown that miRNA expression is altered following sciatic nerve transaction, thereby affecting proliferation and migration of Schwann cells. This study investigated whether miR-148b-3p could regulate migration of Schwann cells by directly targeting cullin-associated and neddylation-dissociated 1 (Cand1). Up-regulated expression of miR-148b-3p promoted Schwann cell migration, whereas silencing of miR-148b-3p inhibited Schwann cell migration in vitro. Further experiments confirmed that Cand1 was a direct target of miR-148b-3p, and Cand1 knockdown reversed suppression of the miR-148b-3p inhibitor on Schwann cell migration. These results suggested that miR-148b-3p promoted migration of Schwann cells by directly targeting Cand1 in vitro.