Neurotrophins, neurones and peripheral nerve regeneration

Main Morphological Characteristics of Tubular Polymeric Scaffolds to Promote Peripheral Nerve Regeneration-A Scoping Review

The "nerve guide conduits" (NGC) used in nerve regeneration must mimic the natural environment for proper cell behavior.

OBJECTIVE

To describe the main morphological characteristics of polymeric NGC to promote nerve regeneration.

METHODS

A scoping review was performed following the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) criteria in the PubMed, Web of Science, Science Direct, and Scientific Electronic Library Online (SciELO) databases. Primary studies that considered/evaluated morphological characteristics of NGC to promote nerve regeneration were included.

RESULT

A total of 704 studies were found, of which 52 were selected. The NGC main morphological characteristics found in the literature were: (I) NGC diameter affects the mechanical properties of the scaffold. (II) Wall thickness of NGC determines the exchange of nutrients, molecules, and neurotrophins between the internal and external environment; and influences the mechanical properties and biodegradation, similarly to NGC (III) porosity, (IV) pore size, and (V) pore distribution. The (VI) alignment of the NGC fibers influences the phenotype of cells involved in nerve regeneration. In addition, the (VII) thickness of the polymeric fiber influences neurite extension and orientation.

CONCLUSIONS

An NGC should have its diameter adjusted to the nerve with wall thickness, porosity, pore size, and distribution of pores, to favor vascularization, permeability, and exchange of nutrients, and retention of neurotrophic factors, also favoring its mechanical properties and biodegradability.

Endogenous automatic nerve discharge promotes nerve repair: an optimized animal model

Exogenous electrical nerve stimulation has been reported to promote nerve regeneration. Our previous study has suggested that endogenous automatic nerve discharge of the phrenic nerve and intercostal nerve has a positive effect on nerve regeneration at 1 month postoperatively, but a negative effect at 2 months postoperatively, which may be caused by scar compression. In this study, we designed four different rat models to avoid the negative effect from scar compression. The control group received musculocutaneous nerve cut and repair. The other three groups were subjected to side-to-side transfer of either the phrenic (phrenic nerve group), intercostal (intercostal nerve group) or thoracodorsal nerves (thoracic dorsal nerve group), with sural nerve autograft distal to the anastomosis site. Musculocutaneous nerve regeneration was assessed by electrophysiology of the musculocutaneous nerve, muscle tension, muscle wet weight, maximum cross-sectional area of biceps, and myelinated fiber numbers of the proximal and distal ends of the anastomosis site of the musculocutaneous nerve and the middle of the nerve graft. At 1 month postoperatively, compound muscle action potential amplitude of the biceps in the phrenic nerve group and the intercostal nerve group was statistically higher than that in the control group. The myelinated nerve fiber numbers in the distal end of the musculocutaneous nerve and nerve graft anastomosis in the phrenic nerve and the intercostal nerve groups were statistically higher than those in the control and thoracic dorsal nerve groups. The neural degeneration rate in the middle of the nerve graft in the thoracic dorsal nerve group was statistically higher than that in the phrenic nerve and the intercostal nerve groups. At 2 and 3 months postoperatively, no significant difference was detected between the groups in all the assessments. These findings confirm that the phrenic nerve and intercostal nerve have a positive effect on nerve regeneration at the early stage of recovery. This study established an optimized animal model in which suturing the nerve graft to the distal site of the musculocutaneous nerve anastomosis prevented the inhibition of recovery from scar compression.

One-pot construction of a twice-condensed pDNA polyplex system for peripheral nerve crush injury therapy

Non-viral vector gene delivery is generally limited by its potential toxicity problems, poor transfection abilities, serum stability, or relatively complex construction processes of modified polyplexes. Thus, we develop an efficient and stable polyplex system through convenient construction methods. Here, polyethyleneimine (PEI) 1.8 kDa and glutaraldehyde (GA) are used to construct a novel twice-condensed pDNA polyplex system using a one-pot construction method, including pH-responsive C[double bond, length as m-dash]N linkages by which different PEI molecules on one single polyplex can link with each other. In this system, smaller particle sizes, higher zeta potentials and better serum stabilities are achieved without PEGylation or other chemical modifications using lyophobic segments, but via pH-responsive linkages that ensure the escape of nucleic acids. This polyplex system is used to deliver the pDNA of vascular endothelial growth factor (VEGF) whose half-life period in vivo is only around 30 minutes. Compared with polyplexes prepared using PEI 25 kDa, cells and rats treated with twice-condensed VEGF pDNA polyplexes express significantly more VEGF or myelin basic protein (MBP), and this new polyplex system showed fewer adverse effects in vitro and in vivo. In addition, revascularization and neurogenesis are also discovered in the rat sciatic nerve crush injury model.

Adult Stem Cell-Based Strategies for Peripheral Nerve Regeneration

Peripheral nerve injuries (PNI) occur as the result of sudden trauma and can lead to life-long disability, reduced quality of life, and heavy economic and social burdens. Although the peripheral nervous system (PNS) has the intrinsic capacity to regenerate and regrow axons to a certain extent, current treatments frequently show incomplete recovery with poor functional outcomes, particularly for large PNI. Many surgical procedures are available to halt the propagation of nerve damage, and the choice of a procedure depends on the extent of the injury. In particular, recovery from large PNI gaps is difficult to achieve without any therapeutic intervention or some form of tissue/cell-based therapy. Autologous nerve grafting, considered the "gold standard" is often implemented for treatment of gap formation type PNI. Although these surgical procedures provide many benefits, there are still considerable limitations associated with such procedures as donor site morbidity, neuroma formation, fascicle mismatch, and scarring. To overcome such restrictions, researchers have explored various avenues to improve post-surgical outcomes. The most commonly studied methods include: cell transplantation, growth factor delivery to stimulate regenerating axons and implanting nerve guidance conduits containing replacement cells at the site of injury. Replacement cells which offer maximum benefits for the treatment of PNI, are Schwann cells (SCs), which are the peripheral glial cells and in part responsible for clearing out debris from the site of injury. Additionally, they release growth factors to stimulate myelination and axonal regeneration. Both primary SCs and genetically modified SCs enhance nerve regeneration in animal models; however, there is no good source for extracting SCs and the only method to obtain SCs is by sacrificing a healthy nerve. To overcome such challenges, various cell types have been investigated and reported to enhance nerve regeneration.In this review, we have focused on cell-based strategies aimed to enhance peripheral nerve regeneration, in particular the use of mesenchymal stem cells (MSCs). Mesenchymal stem cells are preferred due to benefits such as autologous transplantation, routine isolation procedures, and paracrine and immunomodulatory properties. Mesenchymal stem cells have been transplanted at the site of injury either directly in their native form (undifferentiated) or in a SC-like form (transdifferentiated) and have been shown to significantly enhance nerve regeneration. In addition to transdifferentiated MSCs, some studies have also transplanted ex-vivo genetically modified MSCs that hypersecrete growth factors to improve neuroregeneration.

Loss of Schwann cell plasticity in chronic inflammatory demyelinating polyneuropathy (CIDP)

Background

Chronic inflammatory demyelinating polyneuropathy (CIDP) is often associated with chronic disability, which can be accounted to incomplete regeneration of injured axons. We hypothesized that Schwann cell support for regenerating axons may be altered in CIDP, which may account for the poor clinical recovery seen in many patients.

Methods

We exposed human and rodent Schwann cells to sera from CIDP patients and controls. In a model of chronic nerve denervation, we transplanted these conditioned Schwann cells intraneurally and assessed their capacity to support axonal regeneration by electrophysiology and morphometry.

Results

CIDP-conditioned Schwann cells were less growth supportive for regenerating axons as compared to Schwann cells exposed to control sera. The loss of Schwann cell support was associated with lower levels of granulocyte-macrophage colony-stimulating factor (GM-CSF) in CIDP sera and correlated with altered expression of c-Jun and p57kip2 in Schwann cells. The inactivation of these regulatory factors resulted in an altered expression of neurotrophins including BDNF, GDNF, and NGF in CIDP-conditioned Schwann cells in vitro.

Conclusions

Our study provides evidence that pro-regenerative functions of Schwann cells are affected in CIDP. It thereby offers a possible explanation for the clinical observation that in many CIDP patients recovery is incomplete despite sufficient immunosuppressive treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0711-7) contains supplementary material, which is available to authorized users.

Effects of age and insulin-like growth factor-1 on rat neurotrophin receptor expression after nerve injury

INTRODUCTION

Neurotrophin receptors, such as p75(NTR) , direct neuronal response to injury. Insulin-like growth factor-1 receptor (IGF-1R) mediates the increase in p75(NTR) during aging. The aim of this study was to examine the effect of aging and insulin-like growth factor-1 (IGF-1) treatment on recovery after peripheral nerve injury.

METHODS

Young and aged rats underwent tibial nerve transection with either local saline or IGF-1 treatment. Neurotrophin receptor mRNA and protein expression were quantified.

RESULTS

Aged rats expressed elevated baseline IGF-1R (34% higher, P = 0.01) and p75(NTR) (68% higher, P < 0.01) compared with young rats. Post-injury, aged animals expressed significantly higher p75(NTR) levels (68.5% above baseline at 4 weeks). IGF-1 treatment suppressed p75(NTR) gene expression at 4 weeks (17.2% above baseline, P = 0.002) post-injury.

CONCLUSIONS

Local IGF-1 treatment reverses age-related declines in recovery after peripheral nerve injuries by suppressing p75(NTR) upregulation and pro-apoptotic complexes. IGF-1 may be considered a viable adjuvant therapy to current treatment modalities. Muscle Nerve 54: 769-775, 2016.

Neurotrophic Enhancers as Therapy for Behavioral Deficits in Rodent Models of Huntington's Disease: Use of Gangliosides, Substituted Pyrimidines, and Mesenchymal Stem Cells

The interest in using neurotrophic factors as potential treatments for neurodegenerative disorders, such as Huntington's disease, has grown in the past decade. A major impediment for the clinical utility of neurotrophic factors is their inability to cross the blood-brain barrier in therapeutically significant amounts. Although several novel mechanisms for delivering exogenous neurotrophins to the brain have been developed, most of them involve invasive procedures or present significant risks. One approach to circumventing these problems is using therapeutic agents that can be administered systemically and have the ability to enhance the activity of neurotrophic factors. This review highlights the use of gangliosides, substituted pyrimidines, and mesenchymal stem cells as neurotrophic enhancers that have significant therapeutic potential while avoiding the pitfalls of delivering exogenous neurotrophic factors through the blood-brain barrier. The review focuses on the potential of these neurotrophic enhancers for treating the behavioral deficits in rodent models of Huntington's disease.

Electrospun silk fibroin nanofibers promote Schwann cell adhesion, growth and proliferation

In this study, Schwann cells, at a density of 1 × 10(5) cells/well, were cultured on regenerated silk fibroin nanofibers (305 ± 84 nm) prepared using the electrospinning method. Schwann cells cultured on the silk fibroin nanofibers appeared more ordered, their processes extended further, and they formed more extensive and complex interconnections. In addition, the silk fibroin nanofibers had no impact on the proliferation of Schwann cells or on the secretion of ciliary neurotrophic factor, brain-derived neurotrophic factor or nerve growth factor. These findings indicate that regenerated electrospun silk fibroin nanofibers can promote Schwann cell adhesion, growth and proliferation, and have excellent biocompatibility.

Time-course effect of electrical stimulation on nerve regeneration of diabetic rats

BACKGROUND

Electrical stimulation (ES) has been shown to promote nerve regeneration in rats with experimental diabetes induced using streptozotocin (STZ). However, the time-course effect of ES on nerve regeneration of diabetic animals has not been reported in previous studies. The present study attempted to examine the effect of different timing of ES after peripheral nerve transection in diabetic rats.

METHODOLOGY/FINDINGS

Fifty Sprague-Dawley rats were used in the study. They were classified into five groups. STZ-induced diabetes was created in groups A to D. Normal animals in group E were used as the non-diabetic controls. The sciatic nerve was transected and repaired using a silicone rubber conduit across a 10-mm gap in all groups. Groups A to C received ES for 15 minutes every other day for 2 weeks. Stimulation was initiated on day 1 following the nerve repair for group A, day 8 for group B, and day 15 for group C. The diabetic control group D and the normal control group E received no ES. At 30 days after surgery in group A, histological evaluations showed a higher success percentage of regeneration across the 10-mm nerve gap, and the electrophysiological results showed significantly larger mean values of evoked muscle action potential area and amplitude of the reinnervated gastrocnemius muscle compared with group D.

CONCLUSIONS/SIGNIFICANCE

It is concluded that an immediate onset of ES may improve the functional recovery of large nerve defect in diabetic animals.

CRMP1 Interacted with Spy1 During the Collapse of Growth Cones Induced by Sema3A and Acted on Regeneration After Sciatic Nerve Crush

CRMP1, a member of the collapsin response mediator protein family (CRMPs), was reported to regulate axon outgrowth in Sema3A signaling pathways via interactions with its co-receptor protein neuropilin-1 and plexin-As through the Fyn-cyclin-dependent kinase 5 (CDK5) cascade and the sequential phosphorylation of CRMP1 by lycogen synthase kinase-3β (GSK-3β). Using yeast two-hybrid, we identified a new molecule, Speedy A1 (Spy1), a member of the Speedy/RINGO family, with an interaction with CRMP1. Besides, for the first time, we observed the association of CRMP1 with actin. Based on this, we wondered the association of them and their function in Sema3A-induced growth cones collapse and regeneration process after SNC. During our study, we constructed overexpression plasmid and short hairpin RNA (shRNA) to question the relationship of CRMP1/Spy1 and CRMP1/actin. We observed the interactions of CRMP1/Spy1 and CRMP1/actin. Besides, we found that Spy1 could affect CRMP1 phosphorylation actived by CDK5 and that enhanced CRMP1 phosphorylation might disturb the combination of CRMP1 and actin, which would contribute to abnormal of Sema3A-induced growth cones collapse and finally lead to influent regeneration process after rat sciatic nerve crush. Through rat walk footprint test, we also observed the variance during regeneration progress, respectively. We speculated that CRMP1 interacted with Spy1 which would disturb the association of CRMP1 with actin and was involved in the collapse of growth cones induced by Sema3A and regeneration after sciatic nerve crush.

Rat sciatic nerve reconstruction across a 30 mm defect bridged by an oriented porous PHBV tube with Schwann cell as artificial nerve graft

An oriented poly(3-hydroxybutyrate-co-3-hydroxyvalerate) nerve conduit has been used to evaluate its efficiency based on the promotion of peripheral nerve regeneration in rats. The oriented porous micropatterned artificial nerve conduit was designed onto the micropatterned silicon wafers, and then their surfaces were modified with oxygen plasma to increase cell adhesion. The designed conduits were investigated by cell culture analyses with Schwann cells (SCs). The conduits were implanted into a 30 mm gap in sciatic nerves of rats. Four months after surgery, the regenerated nerves were monitored and evaluated by macroscopic assessments and histology and behavioral analyses. Results of cellular analyses showed suitable properties of designed conduit for nerve regeneration. The results demonstrated that in the polymeric graft with SCs, the rat sciatic nerve trunk had been reconstructed with restoration of nerve continuity and formatted nerve fibers with myelination. Histological results demonstrated the presence of Schwann and glial cells in regenerated nerves. Functional recovery such as walking, swimming, and recovery of nociceptive function was illustrated for all the grafts especially conduits with SCs. This study proves the feasibility of the artificial nerve graft filled with SCs for peripheral nerve regeneration by bridging a longer defect in an animal model.

Mechanisms of nerve capping technique in prevention of painful neuroma formation

Nerve capping techniques have been introduced as a promising treatment modality for the treatment of painful neuroma with varied outcomes; however, its exact mechanism is still unknown. RhoA is one of the members of the RAS superfamily of GTPases that operate as molecular switches and plays an important role in peripheral nerve regeneration. Our aim was to investigate the structural and morphologic mechanisms by which the nerve capping technique prevents the formation of painful neuromas after neuroectomy. We also hoped to provide a theoretical basis for this treatment approach. An aligned nanofiber conduit was used for the capping procedure and the sciatic nerve of Sprague-Dawley rats was selected as the animal model. Behavioral analysis, extent of neuroma formation, histological assessment, expressions of pain markers of substance P and c-fos, molecular biological changes as well as ultrastructural features were investigated and compared with the findings in a no-capping control group. The formation of traumatic neuromas was significantly inhibited in the capping group with relatively “normal” structural and morphological features and no occurrence of autotomy and significantly lower expression of pain markers compared to the no-capping group. The gene expression of RhoA was consistently in a higher level in the capping group within 8 weeks after surgery. This study shows that capping technique will alter the regeneration state of transected nerves and reduce painful neuroma formation, indicating a promising approach for the treatment of painful neuroma. The initiation of the “regenerative brake” induced by structural as well as morphological improvements in the severed nerve is theorized to be most likely a key mechanism for the capping technique in the prevention of painful neuroma formation.

Efficacy of nanofibrous conduits in repair of long-segment sciatic nerve defects

Our previous studies have histomorphologically confirmed that nanofibrous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) conduit can be used to repair 30-mm-long sciatic nerve defects. However, the repair effects on rat behaviors remain poorly understood. In this study, we used nanofibrous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) conduit and autologous sciatic nerve to bridge 30-mm-long rat sciatic nerve gaps. Within 4 months after surgery, rat sciatic nerve functional recovery was evaluated per month by behavioral analyses, including toe out angle, toe spread analysis, walking track analysis, extensor postural thrust, swimming test, open-field analysis and nociceptive function. Results showed that rat sciatic nerve functional recovery was similar after nanofibrous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) conduit and autologous nerve grafting. These findings suggest that nanofibrous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) conduit is suitable in use for repair of long-segment sciatic nerve defects.

Comparison of different biogenic matrices seeded with cultured Schwann cells for bridging peripheral nerve defects

Tissue-engineering as laboratory based alternative to human autografts and allografts provides "custom made organs" cultured from patient's material. To overcome the limited donor nerve availability different biologic nerve grafts were engineered in a rat sciatic nerve model: cultured isogenic Schwann cells were implanted into acellular autologous matrices: veins, muscles, nerves, and epineurium tubes. Autologous nerve grafts, and the respective biogenic material without Schwann cells served as control. After 6 weeks regeneration was assessed clinically, histologically and morphometrically. The PCR analysis showed that the implanted Schwann cells remain within all the grafts. A good regeneration was noted in the muscle-Schwann cell-group, while regeneration quality in the other groups (with or without Schwann cells) was impaired. The muscle-Schwann cell graft showed a systematic and organized regeneration including a proper orientation of regenerated fibers. All venous and epineurium grafts had a more disorganized regeneration. Seemingly, the lack of endoneural tube like structures in vein grafts lead to impaired regeneration. And, apparently, the beneficial effects of implanted Schwann cells into a large luminal structure can only be demonstrated to a limited extent if endoneural like structures are lacking. A tube offers less area for Schwann cell adhesion and it is more likely to collapse. This underlines the role of the basal lamina, or at least an inner structure acting as scaffold in axonal regeneration. Although the conventional nerve graft remains the gold standard, the implantation of Schwann cells into an acellular muscle provides a biogenic graft with basal lamina tubes as pathway for regenerating axons and the positive effects of Schwann cells producing neurotrophic and neurotropic factors, and thus, supporting axonal regeneration.

Growth hormone treatment enhances the functional recovery of sciatic nerves after transection and repair

INTRODUCTION

Although nerves can spontaneously regenerate in the peripheral nervous system without treatment, functional recovery is generally poor, and thus there is a need for strategies to improve nerve regeneration.

METHODS

The left sciatic nerve of adult rats was transected and immediately repaired by epineurial sutures. Rats were then assigned to one of two experimental groups treated with either growth hormone (GH) or saline for 8 weeks. Sciatic nerve regeneration was estimated by histological evaluation, nerve conduction tests, and rotarod and treadmill performance.

RESULTS

GH-treated rats showed increased cellularity at the lesion site together with more abundant immunoreactive axons and Schwann cells. Compound muscle action potential (CMAP) amplitude was also higher in these animals, and CMAP latency was significantly lower. Treadmill performance increased in rats receiving GH.

CONCLUSION

GH enhanced the functional recovery of the damaged nerves, thus supporting the use of GH treatment, alone or combined with other therapeutic approaches, in promoting nerve repair.

Neurotrophic effects of perfluorocarbon emulsion gel: a pilot study

Background

Positive neurotrophic effects of hyperbaric oxygen treatment may be more easily achieved by applying a Perflourocarbon (PFC) emulsion gel to the repair site. PFCs are halogen substituted carbon oils with unique oxygen transport potentials that are capable of increasing oxygen availability in local tissues. The purpose of this study was to determine if the application of a PFC emulsion to a repaired nerve would improve recovery.

Materials and methods

The left tibial nerve of 21 immature female Sprague-Dawley rats was transected, immediately repaired, and then circumferentially coated with PFC gel (Group A, n = 7), PFC-less gel (Group B, n = 7), or nothing (suture only, Group C, n = 7). At eight weeks post surgery, electrophysiological testing and histological and morphological analysis was performed.

Results

No statistically significant differences between experimental groups were found for muscle size and weight, axon counts, or nerve conduction velocity. Group A had a significantly smaller G-ratio than Groups B and C (p < .0001).

Conclusion

Overall results do not indicate a functional benefit associated with application of a PFC emulsion gel to rodent tibial nerve repairs. A positive effect on myelination was seen.

Repair of nerve defect with acellular nerve graft supplemented by bone marrow stromal cells in mice

The acellular nerve graft that can provide internal structure and extracellular matrix components of the nerve is an alternative for repair of peripheral nerve defects. However, results of the acellular nerve grafting for nerve repair still remain inconsistent. This study aimed to investigate if supplementing bone marrow mesenchymal stromal cells (MSCs) could improve the results of nerve repair with the acellular nerve graft in a 10-mm sciatic nerve defect model in mice. Eighteen mice were divided into three groups (n = 6 for each group) for nerve repairs with the nerve autograft, the acellular nerve graft, and the acellular nerve graft by supplemented with MSCs (5 × 10(5)) fibrin glue around the graft. The mouse static sciatic index was evaluated by walking-track testing every 2 weeks. The weight preservation of the triceps surae muscles and histomorphometric assessment of triceps surae muscles and repaired nerves were examined at week 8. The results showed that the nerve repair by the nerve autografting obtained the best functional recovery of limb. The nerve repair with the acellular nerve graft supplemented with MSCs achieved better functional recovery and higher axon number than that with the acellular nerve graft alone at week 8 postoperatively. The results indicated that supplementing MSCs might help to improve nerve regeneration and functional recovery in repair of the nerve defect with the acellular nerve graft.

The effect of aligned core-shell nanofibres delivering NGF on the promotion of sciatic nerve regeneration

Recent bioengineering strategies for peripheral nerve regeneration have been focusing on the development of alternative treatments for nerve repair. In this study, we incorporated nerve growth factor (NGF) into aligned core-shell nanofibres by coaxial electrospinning, and reeled the scaffold into aligned fibrous nerve guidance conduits (NGCs) for nerve regeneration study. This aligned PLGA/NGF NGC combined physical guidance cues and biomolecular signals to closely mimic the native extracellular matrix (ECM). The effect of this aligned PLGA/NGF NGC on the promotion of nerve regeneration was evaluated in a 13-mm rat sciatic nerve defect using functional and morphological analysis. After 12 weeks implantation, the results of electrophysiological and muscle weight examination demonstrated that the functional recovery of the regenerated nerve in the PLGA/NGF NGC group was significantly better than that in the PLGA group, yet had no significant difference compared with the autograft group. The toluidine blue staining study showed that more nerve fibres were regenerated in the PLGA/NGF group, while the electron microscopy study indicated that the regenerated nerve in the PLGA/NGF group was more mature than that in the PLGA group. This study demonstrated that the aligned PLGA/NGF could greatly promote peripheral nerve regeneration and have a potential application in nerve regeneration.

The role of neurotrophic factors in nerve regeneration

This review considers the 2 sources of neurotrophic factors in the peripheral nervous system (PNS), the neurons and the nonneuronal cells in the denervated distal nerve stumps, and their role in axon regeneration. Morphological assessment of regenerative success in response to administration of exogenous growth factors after nerve injury and repair has indicated a role of the endogenous neurotrophic factors from Schwann cells in the distal nerve stump. However, the increased number of axons may reflect more neurons regenerating their axons and/or increased numbers of axon sprouts from the same number of neurons. Using fluorescent dyes to count neurons that regenerated their axons across a suture site and into distal nerve stumps, brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF) were found not to increase the number of neurons that regenerated their axons after immediate nerve repair. Nevertheless, the factors did reverse the deleterious effect of delayed nerve repair, indicating that the axons that regenerate into the distal nerve stump normally have access to sufficient levels of endogenous neurotrophic factors to sustain their regeneration, while neurons that do not have access to these factors require exogenous factors to sustain axon regeneration. Neurons upregulate neurotrophic factors after axotomy. The upregulation is normally slow, beginning after 7 days and occurring in association with a protracted period of axonal regeneration in which axons grow out from the proximal nerve stump across a suture site over a period of 1 month in rodents. This staggered axon regeneration across the suture site is accelerated by a 1-hour period of low-frequency electrical stimulation that simultaneously accelerates the expression of BDNF and its trkB receptor in the neurons. Elevation of the level of BDNF after 2 days to > 3 times that found in unstimulated neurons was accompanied by elevation of the level of cAMP and followed by accelerated upregulation of growth-associated genes, tubulin, actin, and GAP-43 and downregulation of neurofilament protein. Elevation of cAMP levels via rolipram inhibition of phosphodiesterase 4 mimicked the effect of the low-frequency electrical stimulation. In conclusion, the enhanced upregulation of neurotrophic factors in the electrically stimulated axotomized neurons accelerates axon outgrowth into the distal nerve stumps where endogenous sources of growth factors in the Schwann cells support the regeneration of the axons toward the denervated targets. The findings provide strong support for endogenous neurotrophic factors of axotomized neurons and of denervated Schwann cells playing a critical role in supporting axon regeneration in the PNS.

Types of neural guides and using nanotechnology for peripheral nerve reconstruction

Peripheral nerve injuries can lead to lifetime loss of function and permanent disfigurement. Different methods, such as conventional allograft procedures and use of biologic tubes present problems when used for damaged peripheral nerve reconstruction. Designed scaffolds comprised of natural and synthetic materials are now widely used in the reconstruction of damaged tissues. Utilization of absorbable and nonabsorbable synthetic and natural polymers with unique characteristics can be an appropriate solution to repair damaged nerve tissues. Polymeric nanofibrous scaffolds with properties similar to neural structures can be more effective in the reconstruction process. Better cell adhesion and migration, more guiding of axons, and structural features, such as porosity, provide a clearer role for nanofibers in the restoration of neural tissues. In this paper, basic concepts of peripheral nerve injury, types of artificial and natural guides, and methods to improve the performance of tubes, such as orientation, nanotechnology applications for nerve reconstruction, fibers and nanofibers, electrospinning methods, and their application in peripheral nerve reconstruction are reviewed.

Synergistic effects of NGF, CNTF and GDNF on functional recovery following sciatic nerve injury in rats

PURPOSE

To investigate the synergistic effects of nerve growth factor (NGF), ciliary neurotrophic factor (CNTF) and glia cell line-derived neurotrophic factor (GDNF) on survival and growth of sensory neurons and motoneurons, as well as on the functional recovery following sciatic nerve injury in rats.

METHODS

Experimental rats and neurons were randomized into 8 groups: NGF group, CNTF group, GDNF group, NGF+CNTF group, CNTF+GDNF group, NGF+GDNF group, NGF+CNTF+GDNF group and control group. Each group received local intramuscular injection of indicated NTFs according to the treatment protocol. The sciatic nerve function index (SFI), nerve conduction velocity and wet weight recovery rate of gastrocnemius muscle were tested to evaluate the functional recovery in vivo. A 2 (presence or absence of NGF) x 2 (presence or absence of CNTF) x 2 (presence or absence of GDNF) analysis of variance (ANOVA) was used to examine the main effects and interactions among NGF, CNTF and GDNF, and one-way ANOVA was calculated for multiple comparison.

RESULTS

NGF and GDNF acted significantly on the survival of sensory neuron and motoneuron, respectively. CNTF was a dominant factor promoting cell body development, and GDNF had the most powerful effect on neurite outgrowth and elongation of sensory neurons and motoneurons. Combined administration of the three factors resulted in optimal functional recovery following sciatic nerve injury in rats.

CONCLUSIONS

It is demonstrated that differential and complementary biological effects of various neurotrophic factors contribute to synergistic promotion of nervous function recovery.

The effect of the alignment of electrospun fibrous scaffolds on Schwann cell maturation

Peripheral nerve regeneration can be enhanced by the stimulation of formation of bands of Büngner prior to implantation. Aligned electrospun poly(epsilon-caprolactone) (PCL) fibers were fabricated to test their potential to provide contact guidance to human Schwann cells. After 7 days of culture, cell cytoskeleton and nuclei were observed to align and elongate along the fiber axes, emulating the structure of bands of Büngner. Microarray analysis revealed a general down-regulation in expression of neurotrophin and neurotrophic receptors in aligned cells as compared to cells seeded on two-dimensional PCL film. Real-time-PCR analyses confirmed the up-regulation of early myelination marker, myelin-associated glycoprotein (MAG), and the down-regulation of NCAM-1, a marker of immature Schwann cells. Similar gene expression changes were also observed on cells cultured on randomly oriented PCL electrospun fibers. However, up-regulation of the myelin-specific gene, P0, was observed only on aligned electrospun fibers, suggesting the propensity of aligned fibers in promoting Schwann cell maturation.

Hyperbaric oxygenation in peripheral nerve repair and regeneration

Peripheral nerves are essential connections between the central nervous system and muscles, autonomic structures and sensory organs. Their injury is one of the major causes for severe and longstanding impairment in limb function. Acute peripheral nerve lesion has an important inflammatory component and is considered as ischemia-reperfusion (IR) injury. Surgical repair has been the standard of care in peripheral nerve lesion. It has reached optimal technical development but the end results still remain unpredictable and complete functional recovery is rare. Nevertheless, nerve repair is not primarily a mechanical problem and microsurgery is not the only key to success. Lately, there have been efforts to develop alternatives to nerve graft. Work has been carried out in basal lamina scaffolds, biologic and non-biologic structures in combination with neurotrophic factors and/or Schwann cells, tissues, immunosuppressive agents, growth factors, cell transplantation, principles of artificial sensory function, gene technology, gangliosides, implantation of microchips, hormones, electromagnetic fields and hyperbaric oxygenation (HBO). HBO appears to be a beneficial adjunctive treatment for surgical repair in the acute peripheral nerve lesion, when used at lower pressures and in a timely fashion (<6 hours).

Mathematical modeling of guided neurite extension in an engineered conduit with multiple concentration gradients of nerve growth factor (NGF)

Neurotrophic factors such as nerve growth factor (NGF) provide essential cues to navigate growing axon toward their targets. Concentration and concentration gradient of NGF are key parameters affecting the growth rate and direction of neurites and axons. However, the maximum distance for guided nerve growth under stimulation of a single concentration gradient is limited and is thus unfavorable in nerve regeneration. Since the sensitivity of PC12 cells to NGF signals is restorable even after brief removal of the factors, exposure to multiple concentration gradients of the factor can achieve longer distances and greater rates of guided growth. In this study, a mathematical model simulating nerve growth in a virtually constructed nerve conduit incorporating multiple NGF concentration gradients is established. Using a genetic algorithm, optimized initial profiles of NGF able to achieve 4.5 cm of guided growth with a significantly improved growth rate has been obtained. The model also predicts an inverse relationship between the diffusion coefficient of the factor and the neurite growth rate. This model provides a useful tool for evaluating various conduit designs before fabrication and evaluation.

Early effects of neuropathy-inducing organophosphates on in vivo concentrations of three neurotrophins

Exposure to OP compounds that inhibit neurotoxic esterase (NTE) induces a delayed neuropathy (OPIDN) characterized by Wallerian-like degeneration of long axons in certain animals, including humans. Pope et al. (Toxicol. Lett. 75:111-117, 1995) found that neurite outgrowth occurred following the addition of spinal cord extracts from chickens with active OPIDN to neuroblastoma cells, suggesting growth factor expression during the neuropathy. We hypothesized that, shortly after exposure to a neuropathic OP compound, the central nervous system (CNS) attempts to recover from the toxic insult through upregulation of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) in susceptible regions of the nervous system. We hypothesized that such upregulation is transient and cannot be sustained. To test this hypothesis, we exposed 10-week-old chickens to a neuropathic OP compound (PSP, 2.5 mg/kg), a non-neuropathic OP compound (paraoxon, 0.10 mg/kg), and vehicle (DMSO, 0.5 ml/kg) intramuscularly. By day 8, all PSP-treated birds demonstrated clinical signs of OPIDN. We sacrificed chickens by pentobarbital overdose at 4, 8, 24, and 48 hours, and 5 and 10 days post-exposure and confirmed NTE inhibition in birds treated with PSP 4 and 24 hours earlier. Enzyme-linked immunosorbant assays indicated that NGF, BDNF, and NT-3 are found in chicken lumbar spinal cord after exposure to a neuropathic OP compound. However, exposure to the neuropathic OP compound, PSP, did not preferentially elevate levels of NGF, BDNF, and NTE compared to the non-neuropathic OP compound, paraoxon. This suggests that these neurotrophins alone do not contribute to a sustained regenerative effort in the CNS.

Biocompatibility evaluation of silk fibroin with peripheral nerve tissues and cells in vitro

Silk-based materials have been used in the field of bone or ligament tissue engineering. In order to explore the feasibility of using purified silk fibroin to construct artificial nerve grafts, it is necessary to evaluate the biocompatibility of silk fibroin material with peripheral nerve tissues and cells. We cultured rat dorsal root ganglia (DRG) on the substrate made up of silk fibroin fibers and observed the cell outgrowth from DRG during culture by using light and electron microscopy coupled with immunocytochemistry. On the other hand, we cultured Schwann cells from rat sciatic nerves in the silk fibroin extract fluid and examined the changes of Schwann cells after different times of culture. The results of light microscopy, MTT test and cell cycle analysis showed that Schwann cells cultured in the silk fibroin extract fluid showed no significant difference in their morphology, cell viability and proliferation as compared to that in plain L15 medium. Furthermore, no significant difference was found in expression of the factors secreted by Schwann cells, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and S-100, between Schwann cells cultured in the silk fibroin extraction fluid and in plain L15 medium by the aid of immunocytochemistry, RT-PCR and Western analysis. Collectively, these data indicate that silk fibroin has good biocompatibility with DRG and is also beneficial to the survival of Schwann cells without exerting any significant cytotoxic effects on their phenotype or functions, thus providing an experimental foundation for the development of silk fibroin as a candidate material for nerve tissue engineering applications.

Hand function after nerve repair

Treatment of injuries to major nerve trunks in the hand and upper extremity remains a major and challenging reconstructive problem. Such injuries may cause long-lasting disabilities in terms of lost fine sensory and motor functions. Nowadays there is no surgical repair technique that can ensure recovery of tactile discrimination in the hand of an adult patient following nerve repair while very young individuals usually regain a complete recovery of functional sensibility. Post-traumatic nerve regeneration is a complex biological process where the outcome depends on multiple biological and environmental factors such as survival of nerve cells, axonal regeneration rate, extent of axonal misdirection, type of injury, type of nerve, level of the lesion, age of the patient and compliance to training. A major problem is the cortical functional reorganization of hand representation which occurs as a result of axonal misdirection. Although protective sensibility usually occurs following nerve repair, tactile discriminative functions seldom recover--a direct result of cortical remapping. Sensory re-education programmes are routinely applied to facilitate understanding of the new sensory patterns provided by the hand. New trends in hand rehabilitation focus on modulation of central nervous processes rather than peripheral factors. Principles are being evolved to maintain the cortical hand representation by using the brain capacity for visuo-tactile and audio-tactile interaction for the initial phase following nerve injury and repair (phase 1). After the start of the re-innervation of the hand (phase 2), selective de-afferentation, such as cutaneous anaesthesia of the forearm of the injured hand, allows expansion of the nerve-injured cortical hand representation, thereby enhancing the effects of sensory relearning. Recent data support the view that training protocols specifically addressing the relearning process substantially increase the possibilities for improved functional outcome after nerve repair.

Effects of local continuous release of brain derived neurotrophic factor (BDNF) on peripheral nerve regeneration in a rat model

The purpose of this study was to evaluate the effect of continuously released BDNF on peripheral nerve regeneration in a rat model. Initial in vitro evaluation of calcium alginate prolonged-release-capsules (PRC) proved a consistent release of BDNF for a minimum of 8 weeks. In vivo, a worst case scenario was created by surgical removal of a 20-mm section of the sciatic nerve of the rat. Twenty-four autologous fascia tubes were filled with calcium alginate spheres and sutured to the epineurium of both nerve ends. The animals were divided into 3 groups. In group 1, the fascial tube contained plain calcium alginate spheres. In groups 2 and 3, the fascial tube contained calcium alginate spheres with BDNF alone or BDNF stabilized with bovine serum albumin, respectively. The autocannibalization of the operated extremity was clinically assessed and documented in 12 additional rats. The regeneration was evaluated histologically at 4 weeks and 10 weeks in a blinded manner. The length of nerve fibers and the numbers of axons formed in the tube was measured. Over a 10-week period, axons have grown significantly faster in groups 2 and 3 with continuously released BDNF compared to the control. The rats treated with BDNF (groups 2 and 3) demonstrated significantly less autocannibalization than the control group (group 1). These results suggest that BDNF may not only stimulate faster peripheral nerve regeneration provided there is an ideal, biodegradable continuous delivery system but that it significantly reduces the neuropathic pain in the rat model.

Structural parameters of collagen nerve grafts influence peripheral nerve regeneration

Large nerve defects require nerve grafts to allow regeneration. To avoid donor nerve problems the concept of tissue engineering was introduced into nerve surgery. However, non-neuronal grafts support axonal regeneration only to a certain extent. They lack viable Schwann cells which provide neurotrophic and neurotopic factors and guide the sprouting nerve. This experimental study used the rat sciatic nerve to bridge 2 cm nerve gaps with collagen (type I/III) tubes. The tubes were different in their physical structure (hollow versus inner collagen skeleton, different inner diameters). To improve regeneration Schwann cells were implanted. After 8 weeks the regeneration process was monitored clinically, histologically and morphometrically. Autologous nerve grafts and collagen tubes without Schwann cells served as control. In all parameters autologous nerve grafts showed best regeneration. Nerve regeneration in a noteworthy quality was also seen with hollow collagen tubes and tubes with reduced lumen, both filled with Schwann cells. The inner skeleton, however, impaired nerve regeneration independent of whether Schwann cells were added or not. This indicates that not only viable Schwann cells are an imperative prerequisite but also structural parameters determine peripheral nerve regeneration.

Role of MAPK/ERK in Neurotrophin-4 Potentiation of Necrotic Neuronal Death

Neurotrophic factors have been proposed for the treatment of a variety of neurological diseases. However, to this point they have failed in clinical trials. One potential problem is that while neurotrophic factors attenuate apoptosis, they have the potential to enhance necrosis. In this study we show that neurotrophin-4 (NT-4) attenuated apoptotic neuronal death while potentiating necrotic neuronal death in cortical cultures. The protective effects of NT-4 were not blocked by the mitogen-activated protein kinase kinase (MEK) inhibitors PD098059 or U0126, while the injury potentiation by NT-4 was blocked by these inhibitors. NT-4 stimulated the phosphorylation of ERK1/2 and this phosphorylation was attenuated by U0126 and PD098059. The results indicate a disassociation between the pathway by which NT-4 potentiates necrosis, and that by which it attenuates apoptosis, and suggest that addition of a MEK inhibitor may enhance the beneficial effects of NT-4 in treating complex injuries such as occur in vivo.

Pathophysiology of peripheral nerve injury: a brief review

Clinicians caring for patients with brachial plexus and other nerve injuries must possess a clear understanding of the peripheral nervous system's response to trauma. In this article, the authors briefly review peripheral nerve injury (PNI) types, discuss the common injury classification schemes, and describe the dynamic processes of degeneration and reinnervation that characterize the PNI response.