Measuring nerve regeneration in the mouse. Exp Neurol. 2010;223:60-71. [PMID: 20080088 DOI: 10.1016/j.expneurol.2009.12.033]

Schwann Cells as Orchestrators of Nerve Repair: Implications for Tissue Regeneration and Pathologies

Peripheral nerves exist in a stable state in adulthood providing a rapid bidirectional signaling system to control tissue structure and function. However, following injury, peripheral nerves can regenerate much more effectively than those of the central nervous system (CNS). This multicellular process is coordinated by peripheral glia, in particular Schwann cells, which have multiple roles in stimulating and nurturing the regrowth of damaged axons back to their targets. Aside from the repair of damaged nerves themselves, nerve regenerative processes have been linked to the repair of other tissues and de novo innervation appears important in establishing an environment conducive for the development and spread of tumors. In contrast, defects in these processes are linked to neuropathies, aging, and pain. In this review, we focus on the role of peripheral glia, especially Schwann cells, in multiple aspects of nerve regeneration and discuss how these findings may be relevant for pathologies associated with these processes.

Exogenous Hsp70 exerts neuroprotective effects in peripheral nerve rupture model

Hsp70 is the main molecular chaperone responsible for cellular proteostasis under normal conditions and for restoring the conformation or utilization of proteins damaged by stress. Increased expression of endogenous Hsp70 or administration of exogenous Hsp70 is known to exert neuroprotective effects in models of many neurodegenerative diseases. In this study, we have investigated the effect of exogenous Hsp70 on recovery from peripheral nerve injury in a model of sciatic nerve transection in rats. It was shown that recombinant Hsp70 after being added to the conduit connecting the ends of the nerve at the site of its extended severance, migrates along the nerve into the spinal ganglion and is retained there at least three days. In animals with the addition of recombinant Hsp70 to the conduit, a decrease in apoptosis in the spinal ganglion cells after nerve rupture, an increase in the level of PTEN-induced kinase 1 (PINK1), an increase in markers of nerve tissue regeneration and a decrease in functional deficit were observed compared to control animals. The obtained data indicate the possibility of using recombinant Hsp70 preparations to accelerate the recovery of patients after neurotrauma.

Kinesin-5 inhibition improves neural regeneration in experimental autoimmune neuritis

BACKGROUND

Autoimmune neuropathies can result in long-term disability and incomplete recovery, despite adequate first-line therapy. Kinesin-5 inhibition was shown to accelerate neurite outgrowth in different preclinical studies. Here, we evaluated the potential neuro-regenerative effects of the small molecule kinesin-5 inhibitor monastrol in a rodent model of acute autoimmune neuropathies, experimental autoimmune neuritis.

METHODS

Experimental autoimmune neuritis was induced in Lewis rats with the neurogenic P2-peptide. At the beginning of the recovery phase at day 18, the animals were treated with 1 mg/kg monastrol or sham and observed until day 30 post-immunisation. Electrophysiological and histological analysis for markers of inflammation and remyelination of the sciatic nerve were performed. Neuromuscular junctions of the tibialis anterior muscles were analysed for reinnervation. We further treated human induced pluripotent stem cells-derived secondary motor neurons with monastrol in different concentrations and performed a neurite outgrowth assay.

RESULTS

Treatment with monastrol enhanced functional and histological recovery in experimental autoimmune neuritis. Motor nerve conduction velocity at day 30 in the treated animals was comparable to pre-neuritis values. Monastrol-treated animals showed partially reinnervated or intact neuromuscular junctions. A significant and dose-dependent accelerated neurite outgrowth was observed after kinesin-5 inhibition as a possible mode of action.

CONCLUSION

Pharmacological kinesin-5 inhibition improves the functional outcome in experimental autoimmune neuritis through accelerated motor neurite outgrowth and histological recovery. This approach could be of interest to improve the outcome of autoimmune neuropathy patients.

Enhanced axonal regeneration of ALS patient iPSC-derived motor neurons harboring SOD1A4V mutation

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease, characterized by degeneration of upper and lower motor neurons that leads to muscle weakness, paralysis, and death, but the effects of disease-causing mutations on axonal outgrowth of neurons derived from human induced pluripotent stem cells (iPSC)-derived motor neurons (hiPSC-MN) are poorly understood. The use of hiPSC-MN is a promising tool to develop more relevant models for target identification and drug development in ALS research, but questions remain concerning the effects of distinct disease-causing mutations on axon regeneration. Mutations in superoxide dismutase 1 (SOD1) were the first to be discovered in ALS patients. Here, we investigated the effect of the SOD1A4V mutation on axonal regeneration of hiPSC-MNs, utilizing compartmentalized microfluidic devices, which are powerful tools for studying hiPSC-MN distal axons. Surprisingly, SOD1+/A4V hiPSC-MNs regenerated axons more quickly following axotomy than those expressing the native form of SOD1. Though initial axon regrowth was not significantly different following axotomy, enhanced regeneration was apparent at later time points, indicating an increased rate of outgrowth. This regeneration model could be used to identify factors that enhance the rate of human axon regeneration.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

Neuropathic pain caused by miswiring and abnormal end organ targeting

Nerve injury leads to chronic pain and exaggerated sensitivity to gentle touch (allodynia) as well as a loss of sensation in the areas in which injured and non-injured nerves come together1-3. The mechanisms that disambiguate these mixed and paradoxical symptoms are unknown. Here we longitudinally and non-invasively imaged genetically labelled populations of fibres that sense noxious stimuli (nociceptors) and gentle touch (low-threshold afferents) peripherally in the skin for longer than 10 months after nerve injury, while simultaneously tracking pain-related behaviour in the same mice. Fully denervated areas of skin initially lost sensation, gradually recovered normal sensitivity and developed marked allodynia and aversion to gentle touch several months after injury. This reinnervation-induced neuropathic pain involved nociceptors that sprouted into denervated territories precisely reproducing the initial pattern of innervation, were guided by blood vessels and showed irregular terminal connectivity in the skin and lowered activation thresholds mimicking low-threshold afferents. By contrast, low-threshold afferents-which normally mediate touch sensation as well as allodynia in intact nerve territories after injury4-7-did not reinnervate, leading to an aberrant innervation of tactile end organs such as Meissner corpuscles with nociceptors alone. Genetic ablation of nociceptors fully abrogated reinnervation allodynia. Our results thus reveal the emergence of a form of chronic neuropathic pain that is driven by structural plasticity, abnormal terminal connectivity and malfunction of nociceptors during reinnervation, and provide a mechanistic framework for the paradoxical sensory manifestations that are observed clinically and can impose a heavy burden on patients.

Sortilin Modulates Schwann Cell Signaling and Remak Bundle Regeneration Following Nerve Injury

Peripheral nerve regeneration relies on the ability of Schwann cells to support the regrowth of damaged axons. Schwann cells re-differentiate when reestablishing contact with the sprouting axons, with large fibers becoming remyelinated and small nociceptive fibers ensheathed and collected into Remak bundles. We have previously described how the receptor sortilin facilitates neurotrophin signaling in peripheral neurons via regulated trafficking of Trk receptors. This study aims to characterize the effects of sortilin deletion on nerve regeneration following sciatic crush injury. We found that Sort1^–/– mice displayed functional motor recovery like that of WT mice, with no detectable differences in relation to nerve conduction velocities and morphological aspects of myelinated fibers. In contrast, we found abnormal ensheathment of regenerated C-fibers in injured Sort1^–/– mice, demonstrating a role of sortilin for Remak bundle formation following injury. Further studies on Schwann cell signaling pathways showed a significant reduction of MAPK/ERK, RSK, and CREB phosphorylation in Sort1^–/– Schwann cells after stimulation with neurotrophin-3 (NT-3), while Schwann cell migration and myelination remained unaffected. In conclusion, our results demonstrate that loss of sortilin blunts NT-3 signaling in Schwann cells which might contribute to the impaired Remak bundle regeneration after sciatic nerve injury.

Effects of exercise on muscle reinnervation and plasticity of spinal circuits in rat sciatic nerve crush injury models with different numbers of crushes

INTRODUCTION/AIMS

Motor function recovery is frequently poor after peripheral nerve injury. The effect of different numbers of nerve crushes and exercise on motor function recovery is unknown. We aimed to examine how different numbers of crushes of the rat sciatic nerve affects muscle reinnervation and plasticity of spinal circuits and the effect of exercise intervention.

METHODS

Single and multiple sciatic nerve crush models with different numbers of crushes were created in rats. Treadmill exercise was performed at 10 m/min for 60 min, five times a week. Muscle reinnervation and synaptic changes in L4-5 motor neurons were examined by immunofluorescence staining. Behavioral tests were the sciatic functional index (SFI) and the pinprick tests.

RESULTS

The percentage of soleus muscle reinnervation was not significantly increased by the presence of exercise in single or multiple crushes. Exercise after a single crush increased the contact of motor neurons with VGLUT1-containing structures (Exercised vs. Unexercised, 12.9% vs. 8.7%; P < 0.01), but after multiple crushes, it decreased with or without exercise (8.1% vs. 8.6%). Exercise after a single crush significantly improved SFI values from 14 to 24 days, and exercise after multiple crushes from 21 to 35 days (all P < 0.05). The pinprick test showed no difference in recovery depending on the number of crushes or whether or not exercised.

DISCUSSION

Different numbers of sciatic nerve crushes affect muscle reinnervation and motor neuron synaptic changes differently, but motor function recovery may improve with exercise regardless of the number of crushes. This article is protected by copyright. All rights reserved.

Baculoviral inhibitor of apoptosis protein repeat-containing protein 3 delays early Wallerian degeneration after sciatic nerve injury

Wallerian degeneration is a complex biological process that occurs after nerve injury, and involves nerve degeneration and regeneration. Schwann cells play a crucial role in the cellular and molecular events of Wallerian degeneration of the peripheral nervous system. However, Wallerian degeneration regulating nerve injury and repair remains largely unknown, especially the early response. We have previously reported some key regulators of Wallerian degeneration after sciatic nerve injury. Baculoviral inhibitor of apoptosis protein repeat-containing protein 3 (BIRC3) is an important factor that regulates apoptosis-inhibiting protein. In this study, we established rat models of right sciatic nerve injury. In vitro Schwann cell models were also established and subjected to gene transfection to inhibit and overexpress BIRC3. The data indicated that BIRC3 expression was significantly up-regulated after sciatic nerve injury. Both BIRC3 upregulation and downregulation affected the migration, proliferation and apoptosis of Schwan cells and affected the expression of related factors through activating c-fos and ERK signal pathway. Inhibition of BIRC3 delayed early Wallerian degeneration through inhibiting the apoptosis of Schwann cells after sciatic nerve injury. These findings suggest that BIRC3 plays an important role in peripheral nerve injury repair and regeneration. The study was approved by the Institutional Animal Care and Use Committee of Nantong University, China (approval No. 2019-nsfc004) on March 1, 2019.

The Neuropeptide Cortistatin Alleviates Neuropathic Pain in Experimental Models of Peripheral Nerve Injury

Neuropathic pain is one of the most severe forms of chronic pain caused by the direct injury of the somatosensory system. The current drugs for treating neuropathies have limited efficacies or show important side effects, and the development of analgesics with novel modes of action is critical. The identification of endogenous anti-nociceptive factors has emerged as an attractive strategy for designing new pharmacological approaches to treat neuropathic pain. Cortistatin is a neuropeptide with potent anti-inflammatory activity, recently identified as a natural analgesic peptide in several models of pain evoked by inflammatory conditions. Here, we investigated the potential analgesic effect of cortistatin in neuropathic pain using a variety of experimental models of peripheral nerve injury caused by chronic constriction or partial transection of the sciatic nerve or by diabetic neuropathy. We found that the peripheral and central injection of cortistatin ameliorated hyperalgesia and allodynia, two of the dominant clinical manifestations of chronic neuropathic pain. Cortistatin-induced analgesia was multitargeted, as it regulated the nerve damage-induced hypersensitization of primary nociceptors, inhibited neuroinflammatory responses, and enhanced the production of neurotrophic factors both at the peripheral and central levels. We also demonstrated the neuroregenerative/protective capacity of cortistatin in a model of severe peripheral nerve transection. Interestingly, the nociceptive system responded to nerve injury by secreting cortistatin, and a deficiency in cortistatin exacerbated the neuropathic pain responses and peripheral nerve dysfunction. Therefore, cortistatin-based therapies emerge as attractive alternatives for treating chronic neuropathic pain of different etiologies.

Activation of the regeneration-associated gene STAT3 and functional changes in intact nociceptors after peripheral nerve damage in mice

BACKGROUND

In the context of neuropathic pain, the contribution of regeneration to the development of positive symptoms is not completely understood. Several efforts have been done to described changes in axotomized neurons, however, there is scarce data on changes occurring in intact neurons, despite experimental evidence of functional changes. To address this issue, we analysed by immunohistochemistry the presence of phosphorylated signal transducer and activator of transcription 3 (pSTAT3), an accepted marker of regeneration, within DRGs where axotomized neurons were retrogradely labelled following peripheral nerve injury. Likewise, we have characterized abnormal electrophysiological properties in intact fibres after partial nerve injury.

METHODS/RESULTS

We showed that induction of pSTAT3 in sensory neurons was similar after partial or total transection of the sciatic nerve and to the same extent within axotomized and non-axotomized neurons. We also examined pSTAT3 presence on non-peptidergic and peptidergic nociceptors. Whereas the percentage of neurons marked by IB4 decrease after injury, the proportion of CGRP neurons did not change, but its expression switched from small- to large-diameter neurons. Besides, the percentage of CGRP+ neurons expressing pSTAT3 increased significantly 2.5-folds after axotomy, preferentially in neurons with large diameters. Electrophysiological recordings showed that after nerve damage, most of the neurons with ectopic spontaneous activity (39/46) were non-axotomized C-fibres with functional receptive fields in the skin far beyond the site of damage.

CONCLUSIONS

Neuronal regeneration after nerve injury, likely triggered from the site of injury, may explain the abnormal functional properties gained by intact neurons, reinforcing their role in neuropathic pain.

SIGNIFICANCE

Positive symptoms in patients with peripheral neuropathies correlate to abnormal functioning of different subpopulations of primary afferents. Peripheral nerve damage triggers regenerating programs in the cell bodies of axotomized but also in non-axotomized nociceptors which is in turn, develop abnormal spontaneous and evoked discharges. Therefore, intact nociceptors have a significant role in the development of neuropathic pain due to their hyperexcitable peripheral terminals. Therapeutical targets should focus on inhibiting peripheral hyperexcitability in an attempt to limit peripheral and central sensitization.

MiR-34a regulates Schwann cell proliferation and migration by targeting CNTN2

The proliferation and migration of Schwann cells contribute to axonal outgrowth and functional recovery after peripheral nerve injury. Previously, several microRNAs were abnormally expressed after peripheral nerve injury and they played important roles in peripheral nerve regeneration. However, the role and underlying mechanism of miR-34a in peripheral nerve injury remain largely unknown. The levels of miR-34a and contactin-2 (CNTN2) were detected by quantitative real-time PCR. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide and transwell assays were used to examine cell proliferation and migration, respectively. The protein level of CNTN2 was measured by western blot. The binding sites of miR-34a and CNTN2 were predicted by the online software and confirmed by dual-luciferase reporter assay and RNA immunoprecipitation (RIP) assay. Following sciatic nerve injury, the expression of miR-34a was downregulated in the crushed nerve segment, reaching a minimum at the seventh day. Knockdown of miR-34a enhanced the axon outgrowth of dorsal root ganglion neurons. Moreover, miR-34a overexpression evidently inhibited the proliferation of Schwann cells, whereas its knockdown showed the opposite effects. In addition, CNTN2 was a direct target of miR-34a and its expression was negatively regulated by miR-34a in the crushed nerve segment. Besides, CNTN2 overexpression or knockdown could reverse the effects of miR-34a upregulation or downregulation on proliferation and migration of Schwann cells, respectively. Collectively, miR-34a inhibited the proliferation and migration of Schwann cells via targeting CNTN2, which might provide a new approach to peripheral nerve regeneration.

Human Wharton's Jelly Mesenchymal Stem Cell-Mediated Sciatic Nerve Recovery Is Associated with the Upregulation of Regulatory T Cells

The acceleration of peripheral nerve regeneration is crucial for functional nerve recovery. Our previous study demonstrated that human Wharton's jelly-derived mesenchymal stem cells (hWJ-MSC) promote sciatic nerve recovery and regeneration via the direct upregulation and release of neurotrophic factors. However, the immunomodulatory role of hWJ-MSC in sciatic nerve recovery remains unclear. The effects of hWJ-MSC on innate immunity, represented by macrophages, natural killer cells, and dendritic cells, as well as on adaptive immunity, represented by CD4+ T, CD8+ T, B, and regulatory T cells (Tregs), were examined using flow cytometry. Interestingly, a significantly increased level of Tregs was detected in blood, lymph nodes (LNs), and nerve-infiltrating cells on POD7, 15, 21, and 35. Anti-inflammatory cytokines, such as IL-4 and IL-10, were significantly upregulated in the LNs and nerves of hWJ-MSC-treated mice. Treg depletion neutralized the improved effects of hWJ-MSC on sciatic nerve recovery. In contrast, Treg administration promoted the functional recovery of five-toe spread and gait stance. hWJ-MSC also expressed high levels of the anti-inflammatory cytokines TGF-β and IL-35. This study indicated that hWJ-MSC induce Treg development to modulate the balance between pro- and anti-inflammation at the injured sciatic nerve by secreting higher levels of anti-inflammatory cytokines.

Pharmacological BACE Inhibition Improves Axonal Regeneration in Nerve Injury and Disease Models

While the peripheral nervous system is able to repair itself following injury and disease, recovery is often slow and incomplete, with no available treatments to enhance the effectiveness of regeneration. Using knock-out and transgenic overexpressor mice, we previously reported that BACE1, an aspartyl protease, as reported by Hemming et al. (PLoS One 4:12, 2009), negatively regulates peripheral nerve regeneration. Here, we investigated whether pharmacological inhibition of BACE may enhance peripheral nerve repair following traumatic nerve injury or neurodegenerative disease. BACE inhibitor-treated mice had increased numbers of regenerating axons and enhanced functional recovery after a sciatic nerve crush while inhibition increased axonal sprouting following a partial nerve injury. In the SOD1G93A ALS mouse model, BACE inhibition increased axonal regeneration with improved muscle re-innervation. CHL1, a BACE1 substrate, was elevated in treated mice and may mediate enhanced regeneration. Our data demonstrates that pharmacological BACE inhibition accelerates peripheral axon regeneration after varied nerve injuries and could be used as a potential therapy.

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.

Red fluorescent AuNDs with conjugation of cholera toxin subunit B (CTB) for extended-distance retro-nerve transporting and long-time neural tracing

A retrograde transportation nerve probe, Au nanodots-cholera toxin B subunit (AuNDs-CTB), are prepared and fully characterized, which emit bright red fluorescence and show high quantum yield (7.2%) and good stability. The fluorescence emitted by the AuNDs is constant across a wide pH range (4-10) and after prolonged UV irradiation (>4 h). Previously, CTB has shown targeting characteristic for nerve cells with high sensitivity and effectiveness. After linking CTB to AuNDs through amidation reactions, AuNDs-CTB are obtained with excellent fluorescence property, nerve target characteristic, and, particularly, neural retrograde transportation feature. The red emission of the AuNDs-CTB is well distinguished from the blue autofluorescence of normal tissues, which provides potential for detection by naked eyes. Further, the fluorescence emission intensity maintains for 10 days in vivo, suggesting great utility for long-time monitoring and sensing of the nerve tissue. Furthermore, the AuNDs-CTB with bright red fluorescence can travel through the peripheral nerve to the spinal cord rapidly by retrograde transportation. The transportation occurs for a long distance (>5 cm) within only 2 days after injection of the AuNDs-CTB into the sciatic nerve. The present study exhibits a novel method for nerve visualization and drug delivery. STATEMENT OF SIGNIFICANCE: Au nanodots (AuNDs) conjugated with cholera toxin subunit B (CTB) have been developed for nerve labeling and neural retro-transporting. The red fluorescence from AuNDs-CTB is stable in vitro (pH 4-10 and 4 h UV irradiation) and in vivo (for a long time, more than 10 days). When injecting AuNDs-CTB into the sciatic nerve located at the midpiece of the thigh, the targeted nerve emits bright red fluorescence under UV light. Furthermore, the nerve can retrograde transport the AuNDs-CTB to the spinal cord for a distance of more than 5 cm just in 2 days. This work exhibits a novel method for nerve visualization by naked eyes and demonstrates the potential for intraoperative navigation.

Topical Application of Human Wharton's Jelly Mesenchymal Stem Cells Accelerates Mouse Sciatic Nerve Recovery and is Associated with Upregulated Neurotrophic Factor Expression

Peripheral nerve regeneration following injury is often slow and impaired, which results in weakened and denervated muscle with subsequent atrophy. Human Wharton's jelly mesenchymal stem cells (hWJ-MSC) have potential regenerative properties which, however, remain unknown in mouse nerve recovery. This study investigated the effect of the topical application of hWJ-MSC onto repairing transected sciatic nerves in a mouse model. Human adipocyte-derived stem cells (hADSC) were used as a positive control. The sciatic nerve of BALB/c mice was transected at a fixed point and repaired under the microscope using 10-0 sutures. hWJ-MSC and hADSC were applied to the site of repair and mice were followed up for 1 year. The hWJ-MSC group had significantly better functional recovery of five-toe spread and gait angles compared with the negative control and hADSC groups. hWJ-MSC improved sciatic nerve regeneration in a dose-dependent fashion. The hWJ-MSC group had a better quality of regenerated nerve with an increased number of myelinated axons throughout. hWJ-MSC appear to be safe in mice after 1 year of follow-up. hWJ-MSC also expressed higher levels of neurotrophic factor-3, brain-derived neurotrophic factor, and glial-derived neurotrophic factor than hADSC. hWJ-MSC may promote better nerve recovery than hADSC because of this upregulation of neurotrophic factors.

Essential References for Structural Analysis of the Peripheral Nervous System for Pathologists and Toxicologists

Toxicologic neuropathology for the peripheral nervous system (PNS) is a vital but often underappreciated element of basic translational research and safety assessment. Evaluation of the PNS may be complicated by unfamiliarity with normal nerve and ganglion biology, which differs to some degree among species; the presence of confounding artifacts related to suboptimal sampling and processing; and limited experience with differentiating such artifacts from genuine disease manifestations and incidental background changes. This compilation of key PNS neurobiology, neuropathology, and neurotoxicology references is designed to allow pathologists and toxicologists to readily access essential information that is needed to enhance their proficiency in evaluating and interpreting toxic changes in PNS tissues from many species.

Axonotmesis-evoked plantar vasodilatation as a novel assessment of C-fiber afferent function after sciatic nerve injury in rats

Quantitative assessment of the recovery of nerve function, especially sensory and autonomic nerve function, remains a challenge in the field of nerve regeneration research. We previously found that neural control of vasomotor activity could be potentially harnessed to evaluate nerve function. In the present study, five different models of left sciatic nerve injury in rats were established: nerve crush injury, nerve transection/suturing, nerve defect/autografting, nerve defect/conduit repair, and nerve defect/non-regeneration. Laser Doppler perfusion imaging was used to analyze blood perfusion of the hind feet. The toe pinch test and walking track analysis were used to assess sensory and motor functions of the rat hind limb, respectively. Transmission electron microscopy was used to observe the density of unmyelinated axons in the injured sciatic nerve. Our results showed that axonotmesis-evoked vasodilatation in the foot 6 months after nerve injury/repair recovered to normal levels in the nerve crush injury group and partially in the other three repair groups; whereas the nerve defect/non-regeneration group exhibited no recovery in vasodilatation. Furthermore, the recovery index of axonotmesis-evoked vasodilatation was positively correlated with toe pinch reflex scores and the density of unmyelinated nerve fibers in the regenerated nerve. As C-fiber afferents are predominantly responsible for dilatation of the superficial vasculature in the glabrous skin in rats, the present findings indicate that axonotmesis-evoked vasodilatation can be used as a novel way to assess C-afferent function recovery after peripheral nerve injury. This study was approved by the Ethics Committee for Laboratory Animals of Nantong University of China (approval No. 20130410-006) on April 10, 2013.

Morphological and functional evaluation of the effect of novel pyrimidine derivatives on regeneration of the sciatic nerve in rats

Two novel pyrimidine derivatives, RG2 and RG6, were studied using a rat's model of peripheral nerve injury. Toe-spreading reflex and skin sensitivity to pinch in the foot were monitored to follow recovery of motor and sensory functions in the treated animals. The remyelation rate in the distal segment of the damaged nerve was also studied using morphological analysis of cross-sections of the nerve stained with methylene blue. The obtained data demonstrate a high stimulating effect of RG2 and RG6 on the restoration of motor and sensory functions of the sciatic nerve, as well as on the post-traumatic regeneration of myelin fibers. Possible mechanisms of the observed effects are discussed.

Engineering a platform for nerve regeneration with direct application to nerve repair technology

The development of effective treatment options for repair of peripheral nerves is complicated by lack of knowledge concerning the interactions between cells and implants. A promising device, the multichannel scaffold, incorporates microporous channels, aligning glia and directing axonal growth across a nerve gap. To enhance clinical outcomes of nerve repair, a platform, representative of current implant technology, was engineered which 1) recapitulated key device features (porosity and linearity) and 2) demonstrated remyelination of adult neurons. The in vitro platform began with the study of Schwann cells on porous polycaprolactone (PCL) and poly(lactide co-glycolide) (PLGA) substrates. Surface roughness determined glial cell attachment, and an additional layer of topography, 40 μm linear features, aligned Schwann cells and axons. In addition, direct co-culture of sensory neurons with Schwann cells significantly increased neurite outgrowth, compared to neurons cultured alone (naive or pre-conditioned). In contrast to the control substrate (glass), on porous PCL substrates, Schwann cells differentiated into a mature myelinating phenotype, expressing Oct-6, MPZ and MBP. The direct applicability of this platform to nerve implants, including its response to physiological cues, allows for optimization of cell-material interactions, close observation of the regeneration process, and the study of therapeutics, necessary to advance peripheral nerve repair technology.

Nerve Fiber Regeneration in Toxic Peripheral Neuropathy

There is a striking difference in the potential for regeneration of injured axons in the central and peripheral nervous systems, which is important in neurotoxicologic studies. In contrast to the former, there is a ready mechanism for replacement of peripheral nerve axons that have degenerated following exposure to toxins, where long-distance axon regeneration and substantial functional recovery can occur. This relates at least in part to the nature of the glial and other supporting cells of the peripheral nerve. To provide background for these events, data on regeneration following traumatic injury to peripheral nerve are reviewed. This is followed by descriptions of nerve fiber regeneration after experimental exposure to 3 peripheral nerve axonopathic toxins, organophosphate tri-ortho-tolyl phosphate, the industrial chemical carbon disulfide, and the antituberculosis drug isoniazid.

Lysophosphatidic Acid and Glutamatergic Transmission

Signaling through bioactive lipids regulates nervous system development and functions. Lysophosphatidic acid (LPA), a membrane-derived lipid mediator particularly enriched in brain, is able to induce many responses in neurons and glial cells by affecting key processes like synaptic plasticity, neurogenesis, differentiation and proliferation. Early studies noted sustained elevations of neuronal intracellular calcium, a primary response to LPA exposure, suggesting functional modifications of NMDA and AMPA glutamate receptors. However, the crosstalk between LPA signaling and glutamatergic transmission has only recently been shown. For example, stimulation of presynaptic LPA receptors in hippocampal neurons regulates glutamate release from the presynaptic terminal, and excess of LPA induce seizures. Further evidence indicating a role of LPA in the modulation of neuronal transmission has been inferred from animal models with deficits on LPA receptors, mainly LPA₁ which is the most prevalent receptor in human and mouse brain tissue. LPA₁ null-mice exhibit cognitive and attention deficits characteristic of schizophrenia which are related with altered glutamatergic transmission and reduced neuropathic pain. Furthermore, silencing of LPA₁ receptor in mice induced a severe down-regulation of the main glutaminase isoform (GLS) in cerebral cortex and hippocampus, along with a parallel sharp decrease on active matrix-metalloproteinase 9. The downregulation of both enzymes correlated with an altered morphology of glutamatergic pyramidal cells dendritic spines towards a less mature phenotype, indicating important implications of LPA in synaptic excitatory plasticity which may contribute to the cognitive and memory deficits shown by LPA₁-deficient mice. In this review, we present an updated account of current evidence pointing to important implications of LPA in the modulation of synaptic excitatory transmission.

Fas Ligand Gene (Faslg) Plays an Important Role in Nerve Degeneration and Regeneration After Rat Sciatic Nerve Injury

Wallerian degeneration (WD) is associated with changes in the expression levels of a large number of genes. However, the effects of these up- or down-regulated genes are poorly understood. We have reported some key factors that are differentially regulated during WD in our previous research. Here, we explored the roles of Fas ligand gene (Faslg) in WD after rat sciatic nerve injury. The data showed that Faslg was up-regulated in injured nerves. Expression changed of Faslg in Schwann cells (SCs) resulted in alterations in the release of related factors. Silencing or overexpression of Faslg affected SC proliferation, migration, and apoptosis through β-catenin, nuclear factor-κB (NF-κB), and caspase-3 pathways in vivo and in vitro. Our data suggest that Faslg is a key regulatory gene that affects nerve repair and regeneration in peripheral nerve injury. This study sheds new light on the effects of Faslg on peripheral nerve degeneration and/or regeneration.

Validation of a microRNA target site polymorphism in H3F3B that is potentially associated with a broad schizophrenia phenotype

Despite much progress, few genetic findings for schizophrenia have been assessed by functional validation experiments at the molecular level. We previously reported evidence for genetic linkage of broadly defined schizophrenia to chromosome 17q25 in a sample of 24 multiplex families. 2,002 SNPs under this linkage peak were analyzed for evidence of linkage disequilibrium using the posterior probability of linkage (PPL) framework. SNP rs1060120 produced the strongest evidence for association, with a PPLD|L score of 0.21. This SNP is located within the 3'UTR of the histone gene H3F3B and colocalizes with potential gene target miR-616. A custom miRNA target prediction program predicted that the binding of miR-616 to H3F3B transcripts would be altered by the allelic variants of rs1060120. We used dual luciferase assays to experimentally validate this interaction. The rs1060120 A allele significantly reduced luciferase expression, indicating a stronger interaction with miR-616 than the G allele (p = 0.000412). These results provide functional validation that this SNP could alter schizophrenia epigenetic mechanisms thereby contributing to schizophrenia-related disease risk.

Schwann cells regulate sensory neuron gene expression before and after peripheral nerve injury

Sensory neurons in the PNS demonstrate substantial capacity for regeneration following injury. Recent studies have identified changes in the transcriptome of sensory neurons, which are instrumental for axon regeneration. The role of Schwann cells (SCs) in mediating these changes remains undefined. We tested the hypothesis that SCs regulate expression of genes in sensory neurons before and after PNS injury by comparing mice in which LDL Receptor-related Protein-1 (LRP1) is deleted in SCs (scLRP1^-/- mice) with wild-type (scLRP1^+/+ ) littermates. LRP1 is an endocytic and cell-signaling receptor that is necessary for normal SC function and the SC response to nerve injury. scLRP1^-/- mice represent a characterized model in which the SC response to nerve injury is abnormal. Adult DRG neurons, isolated from scLRP1^-/- mice, with or without a conditioning nerve lesion, demonstrated increased neurite outgrowth when cultured ex vivo, compared with neurons from wild-type mice. Following sciatic nerve crush injury, nerve regeneration was accelerated in vivo in scLRP1^-/- mice. These results were explained by transcriptional activation of RAGs in DRG neurons in scLRP1^-/- mice prior to nerve injury. Although the presence of abnormal SCs in scLRP1^-/- mice primed DRG neurons for repair, nerve regeneration in scLRP1^-/- mice resulted in abnormalities in ultrastructure, principally in Remak bundles, and with the onset of neuropathic pain. These results demonstrate the importance of SCs in controlling RAG expression by neurons and the potential for this process to cause chronic pain when abnormal. The SC may represent an important target for preventing pain following PNS injury.

Tubulation repair mitigates misdirection of regenerating motor axons across a sciatic nerve gap in rats

The repair of peripheral nerve laceration injury to obtain optimal function recovery remains a big challenge in the clinic. Misdirection of regenerating axons to inappropriate target, as a result of forced mismatch of endoneurial sheaths in the case of end-to-end nerve anastomosis or nerve autografting, represents one major drawback that limits nerve function recovery. Here we tested whether tubulation repair of a nerve defect could be beneficial in terms of nerve regeneration accuracy and nerve function. We employed sequential retrograde neuronal tracing to assess the accuracy of motor axon regeneration into the tibial nerve after sciatic nerve laceration and entubulation in adult Sprague-Dawley rats. In a separate cohort of rats with the same sciatic nerve injury/repair protocols, we evaluated nerve function recovery behaviorally and electrophysiologically. The results showed that tubulation repair of the lacerated sciatic nerve using a 3-6-mm-long bioabsorbable guidance conduit significantly reduced the misdirection of motor axons into the tibial nerve as compared to nerve autografting. In addition, tubulation repair ameliorated chronic flexion contracture. This study suggests that tubulation repair of a nerve laceration injury by utilizing a bioresorbable nerve guidance conduit represents a potential substitute for end-to-end epineurial suturing and nerve autografting.

Controlled Release of Growth Factors from Multilayered Fibrous Scaffold for Functional Recoveries in Crushed Sciatic Nerve

In this study, we designed and fabricated a multilayered fibrous scaffold capable of the controlled release of multiple growth factors for sciatic nerve regeneration in rats. The scaffold consists of three layers prepared by sequential electrospinning, where the first layer is fabricated using polycaprolactone (PCL)-aligned electrospun nanofibers for the attachment and differentiation of cells toward the direction of the sciatic nerve. The second and third layers are fabricated using poly(lactic-co-glycolic acid) 6535 (PLGA 6535) and 8515 (PLGA 8515), respectively. The resultant three nanofiber layers were stacked and fixed by incorporating hydrogel micropatterns at both ends of nanofiber scaffold, which also facilitated the surgical handling of the multilayered scaffolds. The PLGA layers acted as reservoirs to release growth factors neurotrophin (NT-3), brain-derived neurotrophic factor (BDNF), and platelet-derived growth factor (PDGF). The different biodegradation rate of each PLGA layer enabled the controlled release of multiple growth factors such as NT-3, BDNF, and PDGF with different patterns. In a rat model, the injured nerve was rolled up with the multilayered scaffold loading growth factors, and behavior tests were performed five weeks after surgery. Sciatic functional index (SFI) and mechanical allodynia analysis revealed that the fast release of NT-3 and BDNF from PLGA 6535 and subsequent slow release of PDGF from PLGA 8515 proved to be the greatest aid to neural tissue regeneration. In addition to the biochemical cues from growth factors, the aligned PCL layer that directly contacts the injured nerve could provide topographical stimulation, offering practical assistance to new tissue and cells for directional growth parallel to the sciatic nerve. This study demonstrated that our multilayered scaffold performs a function that can be used to promote locomotor activity and enhance nerve regeneration in combination with align-patterned topography and the controlled release of growth factors.

Therapeutic Effect of Vinorine on Sciatic Nerve Injured Rat

Vinorine is a monoterpenoid indole alkaloid, a type of natural alkaloids. Growing reports exhibited the numerous pharmacology activities of vinorine such as anti-inflammation, anti-bacterial and anti-tumor. In this study, the effect of vinorine injection (7.5, 15 and 30 mg/kg) on motor function, sensation and nerve regeneration in sciatic nerve crush injury rat was investigated. The results of behavioral analysis, electrophysiological analysis and muscle histological analysis suggested that vinorine promoted the motor function recovery after sciatic nerve injury. The results of mechanical withdrawal thresholds assay and hot plate test demonstrated that vinorine improved the sensation recovery after sciatic nerve injury. The results of Fluoro-gold retrograde labeling, transmission electron microscope assay, toluidine blue and HE staining showed that vinorine attenuated the nerve damage caused by sciatic nerve injury and promoted the nerve regeneration. Furthermore, nerve growth factor (NGF) and its downstream extracellular signal-regulated kinase (ERK) signaling pathway participated in the neuro-recovery effect of vinorine after crush. In conclusion, vinorine treatment accelerated the sciatic nerve regeneration, motor function recovery and sensation recovery after crush injury via regulation of NGF and ERK activity. These results suggested that vinorine is a promising agent for never injury therapy.

Beta secretase activity in peripheral nerve regeneration

While the peripheral nervous system has the capacity to regenerate following a nerve injury, it is often at a slow rate and results in unsatisfactory recovery, leaving patients with reduced function. Many regeneration associated genes have been identified over the years, which may shed some insight into how we can manipulate this intrinsic regenerative ability to enhance repair following peripheral nerve injuries. Our lab has identified the membrane bound protease beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1), or beta secretase, as a potential negative regulator of peripheral nerve regeneration. When beta secretase activity levels are abolished via a null mutation in mice, peripheral regeneration is enhanced following a sciatic nerve crush injury. Conversely, when activity levels are greatly increased by overexpressing beta secretase in mice, nerve regeneration and functional recovery are impaired after a sciatic nerve crush injury. In addition to our work, many substrates of beta secretase have been found to be involved in regulating neurite outgrowth and some have even been identified as regeneration associated genes. In this review, we set out to discuss BACE1 and its substrates with respect to axonal regeneration and speculate on the possibility of utilizing BACE1 inhibitors to enhance regeneration following acute nerve injury and potential uses in peripheral neuropathies.

Increased BACE1 activity inhibits peripheral nerve regeneration after injury

Axons of the peripheral nervous system possess the capacity to regenerate following injury. Previously, we showed that genetically knocking out Beta-Site APP-Cleaving Enzyme 1 (BACE1) leads to increased nerve regeneration. Two cellular components, macrophages and neurons, contribute to enhanced nerve regeneration in BACE1 knockout mice. Here, we utilized a transgenic mouse model that overexpresses BACE1 in its neurons to investigate whether neuronal BACE1 has an inverse effect on regeneration following nerve injury. We performed a sciatic nerve crush in BACE1 transgenic mice and control wild-type littermates, and evaluated the extent of both morphological and physiological improvements over time. At the earliest time point of 3days, we observed a significant decrease in the length of axonal sprouts growing out from the crush site in BACE1 transgenic mice. At later times (10 and 15days post-crush), there were significant reductions in the number of myelinated axons in the sciatic nerve and the percentage of re-innervated neuromuscular junctions in the gastrocnemius muscle. Transgenic mice had a functional electrophysiological delay in the recovery up to 8weeks post-crush compared to controls. These results indicate that BACE1 activity levels have an inverse effect on peripheral nerve repair after injury. The results obtained in this study provide evidence that neuronal BACE1 activity levels impact peripheral nerve regeneration. This data has clinical relevance by highlighting a novel drug target to enhance peripheral nerve repair, an area which currently does not have any approved therapeutics.

Enhanced in vivo survival of Schwann cells by a synthetic oxygen carrier promotes sciatic nerve regeneration and functional recovery

Local hypoxia in the early stages of peripheral nerve injury is a challenge for axonal regeneration. To address this issue, perfluorotributylamine (PFTBA)-based oxygen carrying fibrin hydrogel was prepared and injected into Schwann cell (SC)-seeded collagen-chitosan conduits to increase oxygen supply to SCs within the conduits. The conduit containing PFTBA-SC gel was then applied to bridge a 15-mm sciatic nerve defect in rats. It was observed that most of the GFP-labeled SCs initially seeded in the PFTBA hydrogel remained alive for approximately 28 days after their in vivo implantation. The number of SCs was significantly higher in the PFTBA-SC scaffold than that in the SC scaffold without PFTBA. In addition, nerve regeneration and functional recovery were examined after nerve injury repair. We found that the PFTBA-SC scaffold was capable of promoting axonal regeneration and remyelination of the regenerated axons. Further studies showed the PFTBA-SC scaffold was able to accelerate the recovery of motor and sensory function of the regenerating nerves. Electrophysiological analysis showed area under the curve of compound muscle action potential and nerve conduction velocity were also improved, and gastrocnemius muscle atrophy was partially reversed by PFTBA-SC scaffold. Furthermore, microvessel density analysis showed PFTBA-SC composites were beneficial for microvascular growth, which provided sustained oxygen for regenerating nerve in the later stages of nerve regeneration. In conclusion, enhanced survival of SCs by PFTBA is capable of promoting sciatic nerve regeneration and functional recovery, which provides a new avenue for achieving better functional recovery in the treatment of peripheral nerve injuries. Copyright © 2016 John Wiley & Sons, Ltd.

Episomal Induced Pluripotent Stem Cells Promote Functional Recovery of Transected Murine Peripheral Nerve

Traumatic peripheral nerve neurotmesis occurs frequently and functional recovery is often slow and impaired. Induced pluripotent stem cells (iPSCs) have shown much promise in recent years due to its regenerative properties similar to that of embryonic stem cells. However, the potential of iPSCs in promoting the functional recovery of a transected peripheral nerve is largely unknown. This study is the first to investigate in vivo effects of episomal iPSCs (EiPSCs) on peripheral nerve regeneration in a murine sciatic nerve transection model. Episomal iPSCs refer to iPSCs that are generated via Oct3/4-Klf4-Sox2 plasmid reprogramming instead of the conventional viral insertion techniques. It represents a relatively safer form of iPSC production without permanent transgene integration which may raise questions regarding risks of genomic mutation. A minimal number of EiPSCs were added directly to the transected nerve. Functional recovery of the EiPSC group was significantly improved compared to the negative control group when assessed via serial five-toe spread measurement and gait analysis of ankle angles. EiPSC promotion of nerve regeneration was also evident on stereographic analysis of axon density, myelin thickness, and axonal cross-sectional surface area. Most importantly, the results observed in EiPSCs are similar to that of the embryonic stem cell group. A roughly ten-fold increase in neurotrophin-3 levels was seen in EiPSCs which could have contributed to peripheral nerve regeneration and recovery. No abnormal masses or adverse effects were noted with EiPSC administration after one year of follow-up. We have hence shown that functional recovery of the transected peripheral nerve can be improved with the use of EiPSC therapy, which holds promise for the future of nerve regeneration.

Paclitaxel causes degeneration of both central and peripheral axon branches of dorsal root ganglia in mice

Background

Peripheral neuropathy is a common and dose-limiting side effect of many cancer chemotherapies. The taxane agents, including paclitaxel (Taxol^®), are effective chemotherapeutic drugs but cause degeneration of predominantly large myelinated afferent sensory fibers of the peripheral nervous system in humans and animal models. Dorsal root ganglia (DRG) neurons are sensory neurons that have unipolar axons each with two branches: peripheral and central. While taxane agents induce degeneration of peripheral axons, whether they also cause degeneration of central nervous system axons is not clear. Using a mouse model of paclitaxel-induced neuropathy, we investigated the effects of paclitaxel on the central branches of sensory axons.

Results

We observed that in the spinal cords of paclitaxel-intoxicated mice, degenerated axons were present in the dorsal columns, where the central branches of DRG axons ascend rostrally. In the peripheral nerves, degenerated myelinated fibers were present in significantly greater numbers in distal segments than in proximal segments indicating that this model exhibits the distal-to-proximal degeneration pattern generally observed in human peripheral nerve disorders.

Conclusions

We conclude that paclitaxel causes degeneration of both the peripheral and central branches of DRG axons, a finding that has implications for the site and mode of action of chemotherapy agents on the nervous system.

Spatiotemporal Expression of Poly(rC)-Binding Protein PCBP2 Modulates Schwann Cell Proliferation After Sciatic Nerve Injury

Poly(C)-binding proteins (PCBPs), also known as RNA-binding proteins, interact in a sequence-specific fashion with single-stranded poly(C). It was reported that PCBP2 contributed to gastric cancer proliferation and survival through miR-34a, and knockdown of PCBP2 inhibited glioma proliferation through inhibition of cell cycle progression. In addition, PCBP2 might play a critical role in the regulation of cortical neurons apoptosis induced by hypoxia or ischemia. Because of the essential role of PCBP2 in nervous system and cell growth, we investigated the spatiotemporal expression of PCBP2 in a rat sciatic nerve crush (SNC) model. We detected the upregulated expression of PCBP2 in Schwann cell after SNC. Besides, the peak expression of PCBP2 was in parallel with proliferation cell nuclear antigen. In vitro, we observed increased expression of PCBP2 during the process of TNF-α-induced Schwann cell proliferation. Specially, PCBP2-specific siRNA-transfected Schwann cell showed significantly decreased ability for proliferation. Together, all these data indicated that the change of PCBP2 protein expression was associated with Schwann cell proliferation after the trauma of the peripheral nervous system.

Spontaneous activity in C-fibres after partial damage to the saphenous nerve in mice: Effects of retigabine

BACKGROUND

Spontaneous pain is the most devastating positive symptom in neuropathic pain patients. Recent data show a direct relationship between spontaneous discharges in C-fibres and spontaneous pain in neuropathic patients. Unfortunately, to date there is a lack of experimental animal models for drug testing.

METHODS

We recorded afferent fibres from a new experimental model in vitro. The preparation contains a neuroma formed in a peripheral branch of the saphenous nerve together with the undamaged branches, which maintain intact terminals in a skin flap.

RESULTS

Fibres with stable rates of ectopic spontaneous discharges were found among axotomized (5 A- and 18 C-fibres, mean discharge 0.48 ± 0.08 Hz) and 'putative intact' fibres (12 C-fibres, mean discharge 0.28 ± 0.08 Hz). A proportion (~9%) of axotomized fibres had mechanical receptive fields in the skin far beyond the site of injury. Collision experiments demonstrated that action potentials evoked from neuroma and skin travelled by the same fibre, indicating functional cross-talk between neuromatose and putative intact fibres. Retigabine, the specific Kv7 channel opener, depressed spontaneous discharges by 70% in 15/18 units tested. In contrast, responses to mechanical stimulation of the skin were unaltered by retigabine.

CONCLUSIONS

Partial damage to a peripheral nerve may increase the incidence of spontaneous activity in C-fibres. Retigabine reduced spontaneous activity but not stimulus-evoked activity, suggesting an important role for ion channels in the control of spontaneous pain and demonstrating the utility of the model for the testing of compounds in clinically relevant variables. WHAT DOES THIS STUDY ADD?: Our in vitro experimental model of peripheral neuropathy allows for pharmacological characterization of spontaneously active fibres. Using this model, we show that retigabine inhibits aberrant spontaneous discharges without altering physiological responses in primary afferents.

Treatment with analgesics after mouse sciatic nerve injury does not alter expression of wound healing-associated genes

Animal models of sciatic nerve injury are commonly used to study neuropathic pain as well as axon regeneration. Administration of post-surgical analgesics is an important consideration for animal welfare, but the actions of the analgesic must not interfere with the scientific goals of the experiment. In this study, we show that treatment with either buprenorphine or acetaminophen following a bilateral sciatic nerve crush surgery does not alter the expression in dorsal root ganglion (DRG) sensory neurons of a panel of genes associated with wound healing. These findings indicate that the post-operative use of buprenorphine or acetaminophen at doses commonly suggested by Institutional Animal Care and Use Committees does not change the intrinsic gene expression response of DRG neurons to a sciatic nerve crush injury, for many wound healing-associated genes. Therefore, administration of post-operative analgesics may not confound the results of transcriptomic studies employing this injury model.

Length-dependent axo-terminal degeneration at the neuromuscular synapses of type II muscle in SOD1 mice

In motor neuron diseases, there is a prolonged period of time before any clinical symptoms begin to appear. During this time, distal axonal degeneration, or "dying back" axonopathy, begins to occur before the onset of clinical symptoms and motor neuron death. This preclinical degeneration is a hallmark of motor neuron diseases in both animal models and human patients. Generally, in muscles with mixed fiber types, distal degeneration occurs in fast-fatigable α-motor axons innervating type IIb muscle fibers before axons innervating slow, type I muscle fibers. We investigated whether the "dying back" axonopathy in a pure fast-fatigable α-motor axon nerve is a length-dependent process. The lateral thoracic nerve (LTN) exclusively consists of motor nerves that innervate the very thin cutaneous maximus muscle (CMM) that solely contains type II neuromuscular synapses. We characterized the LTN and CMM synapses both morphologically and physiologically in the superoxide dismutase 1 (SOD1) mutant mouse model of amyotrophic lateral sclerosis (ALS). By 60days of age, there was a significant "dying back" phenomenon at the caudal region while the rostral region remained intact. The longer axons innervating the caudal region appear to be more susceptible to degeneration in the SOD1 mouse indicating that the axonal degeneration of motor neurons innervating type II fibers is a length-dependent process. Additionally, we identified how the simplicity of the LTN-CMM system offers a better method to investigate axon degeneration in an ALS mouse model and may be used to investigate possible therapeutic compounds for axon protection and regeneration.

Lack of motor recovery after prolonged denervation of the neuromuscular junction is not due to regenerative failure

Motor axons in peripheral nerves have the capacity to regenerate after injury. However, full functional motor recovery rarely occurs clinically, and this depends on the nature and location of the injury. Recent preclinical findings suggest that there may be a time after nerve injury where, while regrowth to the muscle successfully occurs, there is nevertheless a failure to re-establish motor function, suggesting a possible critical period for synapse reformation. We have now examined the temporal and anatomical determinants for the re-establishment of motor function after prolonged neuromuscular junction (NMJ) denervation in rats and mice. Using both sciatic transection-resuture and multiple nerve crush models in rats and mice to produce prolonged delays in reinnervation, we show that regenerating fibres reach motor endplates and anatomically fully reform the NMJ even after extended periods of denervation. However, in spite of this remarkably successful anatomical regeneration, after 1 month of denervation there is a consistent failure to re-establish functional recovery, as assessed by behavioural and electrophysiological assays. We conclude that this represents a failure in re-establishment of synaptic function, and the possible mechanisms responsible are discussed, as are their clinical implications.

Neurotrauma and inflammation: CNS and PNS responses

Traumatic injury to the central nervous system (CNS) or the peripheral nervous system (PNS) triggers a cascade of events which culminate in a robust inflammatory reaction. The role played by inflammation in the course of degeneration and regeneration is not completely elucidated. While, in peripheral nerves, the inflammatory response is assumed to be essential for normal progression of Wallerian degeneration and regeneration, CNS trauma inflammation is often associated with poor recovery. In this review, we discuss key mechanisms that trigger the inflammatory reaction after nervous system trauma, emphasizing how inflammations in both CNS and PNS differ from each other, in terms of magnitude, cell types involved, and effector molecules. Knowledge of the precise mechanisms that elicit and maintain inflammation after CNS and PNS tissue trauma and their effect on axon degeneration and regeneration is crucial for the identification of possible pharmacological drugs that can positively affect the tissue regenerative capacity.

Biotinylated dextran amine anterograde tracing of the canine corticospinal tract

In this study, biotinylated dextran amine (BDA) was microinjected into the left cortical motor area of the canine brain. Fluorescence microscopy results showed that a large amount of BDA-labeled pyramidal cells were visible in the left cortical motor area after injection. In the left medulla oblongata, the BDA-labeled corticospinal tract was evenly distributed, with green fluorescence that had a clear boundary with the surrounding tissue. The BDA-positive corticospinal tract entered into the right lateral funiculus of the spinal cord and descended into the posterior part of the right lateral funiculus, close to the posterior horn, from cervical to sacral segments. There was a small amount of green fluorescence in the sacral segment. The distribution of BDA labeling in the canine central nervous system was consistent with the course of the corticospinal tract. Fluorescence labeling for BDA gradually diminished with time after injection. Our findings indicate that the BDA anterograde tracing technique can be used to visualize the localization and trajectory of the corticospinal tract in the canine central nervous system.

Hypoglossal-facial nerve reconstruction using a Y-tube-conduit reduces aberrant synkinetic movements of the orbicularis oculi and vibrissal muscles in rats

The facial nerve is the most frequently damaged nerve in head and neck trauma. Patients undergoing facial nerve reconstruction often complain about disturbing abnormal synkinetic movements of the facial muscles (mass movements, synkinesis) which are thought to result from misguided collateral branching of regenerating motor axons and reinnervation of inappropriate muscles. Here, we examined whether use of an aorta Y-tube conduit during reconstructive surgery after facial nerve injury reduces synkinesis of orbicularis oris (blink reflex) and vibrissal (whisking) musculature. The abdominal aorta plus its bifurcation was harvested (N = 12) for Y-tube conduits. Animal groups comprised intact animals (Group 1), those receiving hypoglossal-facial nerve end-to-end coaptation alone (HFA; Group 2), and those receiving hypoglossal-facial nerve reconstruction using a Y-tube (HFA-Y-tube, Group 3). Videotape motion analysis at 4 months showed that HFA-Y-tube group showed a reduced synkinesis of eyelid and whisker movements compared to HFA alone.