Repair with collagen tubules linked with brain-derived neurotrophic factor and ciliary neurotrophic factor in a rat sciatic nerve injury model

Focal Application of Neurotrophic Factors Augments Outcomes of Nerve-Muscle-Endplate Grafting Technique for Limb Muscle Reinnervation

BACKGROUND

 We have developed a novel muscle reinnervation technique called "nerve-muscle-endplate grafting (NMEG) in the native motor zone (NMZ)." This study aimed to augment the outcomes of the NMEG-NMZ (NN) by focal application of exogenous neurotrophic factors (ENFs) for limb reinnervation.

METHODS

 Adult rats were used to conduct NN plus ENF (NN/ENF) and autologous nerve grafting (ANG, technique control). The nerve innervating the left tibialis anterior (TA) muscle was resected and the denervated TA was immediately treated with NN/ENF or ANG. For NN procedure, an NMEG pedicle was taken from the lateral gastrocnemius muscle and transferred to the NMZ of the denervated TA. For ANG, the nerve gap was bridged with sural nerve. Three months after treatment, the extent of functional and neuromuscular recovery was assessed by measuring static toe spread, maximal muscle force, wet muscle weight, regenerated axons, and innervated motor endplates (MEPs).

RESULTS

 NN/ENF resulted in 90% muscle force recovery of the treated TA, which is far superior to ANG (46%) and NN alone (79%) as reported elsewhere. Toe spread recovered up to 89 and 49% of the control for the NN/ENF and ANG groups, respectively. The average wet muscle weight was 87 and 52% of the control for muscles treated with NN/ENF and ANG, respectively. The mean number of the regenerated axons was 88% of the control for the muscles treated with NN/ENF, which was significantly larger than that for the ANG-repaired muscles (39%). The average percentage of the innervated MEPs in the NN/ENF-treated TA (89%) was higher compared with that in the ANG-repaired TA (48%).

CONCLUSION

 ENF enhances nerve regeneration and MEP reinnervation that further augment outcomes of NN. The NN technique could be an alternative option to treat denervated or paralyzed limb muscles caused by traumatic nerve injuries or lesions.

Engineering Silk Fibroin-Based Nerve Conduit with Neurotrophic Factors for Proximal Protection after Peripheral Nerve Injury

Artificial nerve conduits capable of adequately releasing neurotrophic factors are extensively studied to bridge nerve defects. However, the lack of neurotrophic factors in the proximal area and their visible effects in axonal retrograde transport following nerve injury is one of the factors causing an incomplete nerve regeneration. Herein, an advanced conduit made of silk fibroin is produced, which can incorporate growth factors and promote an effective regeneration after injury. For that, enzymatically crosslinked silk fibroin-based conduits are developed to be used as a platform for the controlled delivery of neurotrophic factors. Nerve growth factor and glial-cell line derived neurotrophic factor (GDNF) are incorporated using two different methodologies: i) crosslinking and ii) absorption method. The release profile is measured by ELISA technique. The bioactivity of the neurotrophic factors is evaluated in vitro by using primary dorsal root ganglia. When implanted in a 10 mm sciatic nerve defect in rats, GDNF-loaded silk fibroin conduits reveal retrograde neuroprotection as compared to autografts and plain silk fibroin conduit. Therefore, the novel design presents a substantial improvement of retrograde trafficking, neurons' protection, and motor nerve reinnervation.

Dose-Dependent Differential Effect of Neurotrophic Factors on In Vitro and In Vivo Regeneration of Motor and Sensory Neurons

Although peripheral axons can regenerate after nerve transection and repair, functional recovery is usually poor due to inaccurate reinnervation. Neurotrophic factors promote directional guidance to regenerating axons and their selective application may help to improve functional recovery. Hence, we have characterized in organotypic cultures of spinal cord and dorsal root ganglia the effect of GDNF, FGF-2, NGF, NT-3, and BDNF at different concentrations on motor and sensory neurite outgrowth. In vitro results show that GDNF and FGF-2 enhanced both motor and sensory neurite outgrowth, NGF and NT-3 were the most selective to enhance sensory neurite outgrowth, and high doses of BDNF selectively enhanced motor neurite outgrowth. Then, NGF, NT-3, and BDNF (as the most selective factors) were delivered in a collagen matrix within a silicone tube to repair the severed sciatic nerve of rats. Quantification of Fluorogold retrolabeled neurons showed that NGF and NT-3 did not show preferential effect on sensory regeneration whereas BDNF preferentially promoted motor axons regeneration. Therefore, the selective effects of NGF and NT-3 shown in vitro are lost when they are applied in vivo, but a high dose of BDNF is able to selectively enhance motor neuron regeneration both in vitro and in vivo.

Approaches to Peripheral Nerve Repair: Generations of Biomaterial Conduits Yielding to Replacing Autologous Nerve Grafts in Craniomaxillofacial Surgery

Peripheral nerve injury is a common clinical entity, which may arise due to traumatic, tumorous, or even iatrogenic injury in craniomaxillofacial surgery. Despite advances in biomaterials and techniques over the past several decades, reconstruction of nerve gaps remains a challenge. Autografts are the gold standard for nerve reconstruction. Using autografts, there is donor site morbidity, subsequent sensory deficit, and potential for neuroma development and infection. Moreover, the need for a second surgical site and limited availability of donor nerves remain a challenge. Thus, increasing efforts have been directed to develop artificial nerve guidance conduits (ANCs) as new methods to replace autografts in the future. Various synthetic conduit materials have been tested in vitro and in vivo, and several first- and second-generation conduits are FDA approved and available for purchase, while third-generation conduits still remain in experimental stages. This paper reviews the current treatment options, summarizes the published literature, and assesses future prospects for the repair of peripheral nerve injury in craniomaxillofacial surgery with a particular focus on facial nerve regeneration.

Evaluation of the effectiveness of biodegradable electrospun caprolactoneand poly(lactic acid-ε-caprolactone) nerve conduits for peripheral nerveregenerations in a rat sciatic nerve defect model

BACKGROUND/AIM

The aim of this study was to compare electrospun caprolactone (EC) and poly(lactic acid-ε-caprolactone) (PLCL) nerve conduits with nerve graft in a rat sciatic nerve defect model.

MATERIALS AND METHODS

A total of 32 male Wistar albino rats were divided into 4 groups, with 8 rats in each group. A nerve defect of 1 cm was constructed in the left sciatic nerve of the rats. These defects were left denuded in the sham group, and reconstructed with nerve grafts, PLCL, and EC nerve conduits in the other groups. After 3 months, nerve regenerations were evaluated macroscopically, microscopically, and electrophysiologically. The numbers of myelinated axons in the cross-sections of the nerves were compared between the groups.

RESULTS

Macroscopically, all nerve coaptations were intact and biodegradation was detected in nerve conduits. Electromyographic assessment and count of myelinated axons in the cross-sections of the nerves displayed the best regeneration in the nerve graft group (P < 0.001) and similar results were obtained in the PLCL and EC nerve conduit groups (P = 0.79). Light and electron microscopy studies demonstrated nerve regeneration in both nerve conduit groups.

CONCLUSION

EC nerve conduits and PLCL nerve conduits yielded similar results and may be alternatives to nerve grafts as they biodegrade.

Peripheral Nerve Regeneration by Secretomes of Stem Cells from Human Exfoliated Deciduous Teeth

Peripheral nerve regeneration across nerve gaps is often suboptimal, with poor functional recovery. Stem cell transplantation-based regenerative therapy is a promising approach for axon regeneration and functional recovery of peripheral nerve injury; however, the mechanisms remain controversial and unclear. Recent studies suggest that transplanted stem cells promote tissue regeneration through a paracrine mechanism. We investigated the effects of conditioned media derived from stem cells from human exfoliated deciduous teeth (SHED-CM) on peripheral nerve regeneration. In vitro, SHED-CM-treated Schwann cells exhibited significantly increased proliferation, migration, and the expression of neuron-, extracellular matrix (ECM)-, and angiogenesis-related genes. SHED-CM stimulated neuritogenesis of dorsal root ganglia and increased cell viability. Similarly, SHED-CM enhanced tube formation in an angiogenesis assay. In vivo, a 10-mm rat sciatic nerve gap model was bridged by silicon conduits containing SHED-CM or serum-free Dulbecco's modified Eagle's medium. Light and electron microscopy confirmed that the number of myelinated axons and axon-to-fiber ratio (G-ratio) were significantly higher in the SHED-CM group at 12 weeks after nerve transection surgery. The sciatic functional index (SFI) and gastrocnemius (target muscle) wet weight ratio demonstrated functional recovery. Increased compound muscle action potentials and increased SFI in the SHED-CM group suggested sciatic nerve reinnervation of the target muscle and improved functional recovery. We also observed reduced muscle atrophy in the SHED-CM group. Thus, SHEDs may secrete various trophic factors that enhance peripheral nerve regeneration through multiple mechanisms. SHED-CM may therefore provide a novel therapy that creates a more desirable extracellular microenvironment for peripheral nerve regeneration.

Peripheral nerve regeneration: experimental strategies and future perspectives

Peripheral nerve injuries represent a substantial clinical problem with insufficient or unsatisfactory treatment options. This review summarises all the events occurring after nerve damage at the level of the cell body, the site of injury and the target organ. Various experimental strategies to improve neuronal survival, axonal regeneration and target reinnervation are described including pharmacological approaches and cell-based therapies. Given the complexity of nerve regeneration, further studies are needed to address the biology of nerve injury, to improve the interaction with implantable scaffolds, and to implement cell-based therapies in nerve tissue engineering.

Nerve conduits for peripheral nerve surgery

Autologous nerve grafts are the current criterion standard for repair of peripheral nerve injuries when the transected nerve ends are not amenable to primary end-to-end tensionless neurorrhaphy. However, donor-site morbidities such as neuroma formation and permanent loss of function have led to tremendous interest in developing an alternative to this technique. Artificial nerve conduits have therefore emerged as an alternative to autologous nerve grafting for the repair of short peripheral nerve defects of less than 30 mm; however, they do not yet surpass autologous nerve grafts clinically. A thorough understanding of the complex biological reactions that take place during peripheral nerve regeneration will allow researchers to develop a nerve conduit with physical and biological properties similar to those of an autologous nerve graft that supports regeneration over long nerve gaps and in large-diameter nerves. In this article, the authors assess the currently available nerve conduits, summarize research in the field of developing these conduits, and establish areas within this field in which further research would prove most beneficial.

Assisted peripheral nerve recovery by KMUP-1, an activator of large-conductance Ca(2+)-activated potassium channel, in a rat model of sciatic nerve crush injury

BACKGROUND

Axonal regeneration in peripheral nerves after injury is a complicated process. Numerous cytokines, growth factors, channels, kinases, and receptors are involved, and matrix metalloproteinase-9 (MMP-9) has been implicated in the pathogenesis subsequent to nerve injury. In this study, the effect of KMUP-1, an activator of large-conductance Ca(2+)-activated potassium channel, on functional recovery, myelinated axon growth, and immunoreactivity of MMP-9 was evaluated in rats subjected to sciatic nerve crush injury.

METHOD

A total of 144 male Sprague-Dawley rats were divided into the following six groups (n = 24/group): group 1, sham-operated; group 2, sciatic nerve injury without treatment; group 3, injured and vehicle-treated; group 4, injured and treated with 1 mM KMUP-1 by topical application; group 5, injured and treated with 10 mM KMUP-1; group 6, injured and treated with 50 mM KMUP-1. Functional recovery was evaluated using walking track analysis at 1, 2, 3, and 4 weeks (n = 6/group at each time point) after injury. In addition, the number of myelinated axons and MMP-9 in the nerve was also examined.

FINDINGS

Animals subjected to sciatic nerve crush injury had decreased motor function, a reduced number of myelinated axons, and increased MMP-9 in the nerve. Treatment with KMUP-1 concentration-dependently improved functional recovery, increased the number of myelinated axons, and decreased MMP-9.

CONCLUSIONS

These results suggest that KMUP-1 may be a novel agent for assisting peripheral nerve recovery after injury. The beneficial effect is probably due to known ability of the compound in activating the nitric oxide/cGMP/protein kinase G pathway.

Standardized criterion to analyze and directly compare various materials and models for peripheral nerve regeneration

Progress in understanding conditions for optimal peripheral nerve regeneration has been stunted due to lack of standardization of experimental conditions and assays. In this paper we review the large database that has been generated using the Lundborg nerve chamber model and compare various theories for their ability to explain the experimental data. Data were normalized based on systematic use of the critical axon elongation, the gap length at which the probability of axon reconnection between the stumps is just 50%. Use of this criterion has led to a rank-ordering of devices or treatments and has led, in turn, to conclusions about the conditions that facilitate regeneration. Experimental configurations that have maximized facilitation of peripheral nerve regeneration are those in which the tube wall comprised degradable polymers, including collagen and certain synthetic biodegradable polymers, and was cell-permeable rather than protein-permeable. Tube fillings that showed very high regenerative activity were suspensions of Schwann cells, a solution either of acidic or basic fibroblast growth factor, insoluble ECM substrates rather than solutions or gels, polyamide filaments oriented along the tube axis and highly porous, insoluble analogs of the ECM with specific structure and controlled degradation rate. It is suggested that the data are best explained by postulating that the quality of regeneration depends on two critical processes. The first is compression of stumps and regenerating nerve by a thick myofibroblast layer that surrounds these tissues and blocks synthesis of a nerve of large diameter (pressure cuff theory). The second is synthesis of linear columns of Schwann cells that serve as tracks for axon elongation (basement membrane microtube theory). It is concluded that experimental configurations that show high regenerative activity suppress the first process while facilitating the second.

Growth factor delivery systems and repair strategies for damaged peripheral nerves

Artificial nerve conduits (NCs) are, in certain cases, instrumental for repairing damaged peripheral nerves, although therapeutic efficacy remains often suboptimal. Considerable efforts have been made to improve the therapeutic performance of NCs. This article reviews recent developments in NC-technology for peripheral nerve regeneration with a main focus on growth factors delivery systems and repair strategies. Commonly used materials for NC fabrication include collagen, silk fibroin, and biodegradable aliphatic polyesters. The basic NC structure, i.e., a hollow tube, can be manufactured by diverse methods: spinning mandrel technology, sheet rolling, injection-molding, freeze-drying, and electro-spinning. Polymeric and cellular delivery systems for growth factors can be integrated in the NC wall or within luminal structures such as gels, fibers, or biological materials providing binding affinity for the bioactive compounds. NCs with sustained growth factor delivery generally improve significantly the axonal outgrowth in nerve defect models, although restoration of sensory and motor functions remains inferior to that achieved with autologous nerve grafts. To improve therapeutic outcomes, further biofunctionalization of NCs will be needed, i.e., adjusting degradation kinetics of NC scaffolding to be compatible with axonal regeneration; delivering multiple growth factors at individually optimized kinetics; incorporating modality specific glial cells and furnishing NCs with guiding nanostructures.

Construction of tissue engineered nerve grafts and their application in peripheral nerve regeneration

Surgical repair of severe peripheral nerve injuries represents not only a pressing medical need, but also a great clinical challenge. Autologous nerve grafting remains a golden standard for bridging an extended gap in transected nerves. The formidable limitations related to this approach, however, have evoked the development of tissue engineered nerve grafts as a promising alternative to autologous nerve grafts. A tissue engineered nerve graft is typically constructed through a combination of a neural scaffold and a variety of cellular and molecular components. The initial and basic structure of the neural scaffold that serves to provide mechanical guidance and optimal environment for nerve regeneration was a single hollow nerve guidance conduit. Later there have been several improvements to the basic structure, especially introduction of physical fillers into the lumen of a hollow nerve guidance conduit. Up to now, a diverse array of biomaterials, either of natural or of synthetic origin, together with well-defined fabrication techniques, has been employed to prepare neural scaffolds with different structures and properties. Meanwhile different types of support cells and/or growth factors have been incorporated into the neural scaffold, producing unique biochemical effects on nerve regeneration and function restoration. This review attempts to summarize different nerve grafts used for peripheral nerve repair, to highlight various basic components of tissue engineered nerve grafts in terms of their structures, features, and nerve regeneration-promoting actions, and finally to discuss current clinical applications and future perspectives of tissue engineered nerve grafts.

Repairing injured peripheral nerves: Bridging the gap

Peripheral nerve injuries that induce gaps larger than 1-2 cm require bridging strategies for repair. Autologous nerve grafts are still the gold standard for such interventions, although alternative treatments, as well as treatments to improve the therapeutic efficacy of autologous nerve grafting are generating increasing interest. Investigations are still mostly experimental, although some clinical studies have been undertaken. In this review, we aim to describe the developments in bridging technology which aim to replace the autograft. A multi-disciplinary approach is of utmost importance to develop and optimise treatments of the most challenging peripheral nerve injuries.

Current applications and future perspectives of artificial nerve conduits

Artificial nerve guide conduits have the advantage over autografts in terms of their availability and ease of fabrication. However, clinical outcomes associated with the use of artificial nerve conduits are often inferior to that of autografts, particularly over long lesion gaps. There have been significant advances in the designs of artificial nerve conduits over the years. In terms of materials selection and design, a wide variety of new synthetic polymers and biopolymers have been evaluated. The inclusion of nerve conduit lumen fillers has also been demonstrated as essential to enable nerve regeneration across large defect gaps. These lumen filler designs have involved the integration of physical cues for contact guidance and biochemical signals to control cellular function and differentiation. Novel conduit architectural designs using porous and fibrous substrates have also been developed. This review highlights the recent advances in synthetic nerve guide designs for peripheral nerve regeneration, and the in vivo applicability and future prospects of these nerve guide conduits.

Post-injury regeneration in rat sciatic nerve facilitated by neurotrophic factors secreted by amniotic fluid mesenchymal stem cells

Amniotic fluid mesenchymal stem cells have the ability to secrete neurotrophic factors that are able to promote neuron survival in vitro. The purpose of this study was to evaluate the effects of neurotrophic factors secreted by rat amniotic fluid mesenchymal stem cells on regeneration of sciatic nerve after crush injury. Fifty Sprague-Dawley rats weighing 250-300 g were used. The left sciatic nerve was crushed with a vessel clamp. Rat amniotic fluid mesenchymal stem cells embedded in fibrin glue were delivered to the injured nerve. Enzyme-linked immunosorbent assay (ELISA) and immunocytochemistry were used to detect neurotrophic factors secreted by the amniotic fluid mesenchymal stem cells. Nerve regeneration was assessed by motor function, electrophysiology, histology, and immunocytochemistry studies. Positive CD29/44, and negative CD11b/45, as well as high levels of expression of brain-derived neurotrophic factor, glia cell line-derived neurotrophic factor, ciliary neurotrophic factor (CNTF), nerve growth factor, and neurotrophin-3 (NT-3) were demonstrated in amniotic fluid mesenchymal stem cells. Motor function recovery, the compound muscle action potential, and nerve conduction latency showed significant improvement in rats treated with amniotic fluid mesenchymal stem cells. ELISA measurement in retrieved nerves displayed statistically significant elevation of CNTF and NT-3. The immunocytochemical studies demonstrated positive staining for NT-3 and CNTF in transplanted cells. The histology and immunocytochemistry studies revealed less fibrosis and a high level of expression of S-100 and glial fibrillary acid protein at the crush site. Rat amniotic fluid mesenchymal stem cells may facilitate regeneration in the sciatic nerve after crush injury. The increased nerve regeneration found in this study may be due to the neurotrophic factors secreted by amniotic fluid mesenchymal stem cells.

The role of evaluation methods in the assessment of peripheral nerve regeneration through synthetic conduits: a systematic review

Object

A number of evaluation methods that are currently used to compare peripheral nerve regeneration with alternative repair methods and to judge the outcome of a new paradigm were hypothesized to lack resolving power. This would too often lead to the conclusion that the outcome of a new paradigm could not be discerned from the outcome of the current gold standard, the autograft. As a consequence, the new paradigm would incorrectly be judged as successful.

Methods

An overview of the methods that were used to evaluate peripheral nerve regeneration after grafting of the rat sciatic nerve was prepared. All articles that were published between January 1975 and December 2004 and concerned grafting of the rat sciatic nerve (minimum graft length 5 mm) and in which the experimental method was compared with an untreated or another grafted nerve were included. The author scored the presence of statistically significant differences between paradigms.

Results

Evaluation of nerve fiber count, nerve fiber density, N-ratio, nerve histological success ratio, compound muscle action potential, muscle weight, and muscle tetanic force are methods that were demonstrated to have resolving power.

Conclusions

A number of evaluation methods are not suitable to demonstrate a significant difference between experimental paradigms in peripheral nerve regeneration. It is preferable to apply a combination of evaluation methods with resolving power to evaluate nerve regeneration properly.

Nerve conduits and growth factor delivery in peripheral nerve repair

Peripheral nerves possess the capacity of self-regeneration after traumatic injury. Transected peripheral nerves can be bridged by direct surgical coaptation of the two nerve stumps or by interposing autografts or biological (veins) or synthetic nerve conduits (NC). NC are tubular structures that guide the regenerating axons to the distal nerve stump. Early synthetic NC have primarily been made of silicone because of the relative flexibility and biocompatibility of this material and because medical-grade silicone tubes were readily available in various dimensions. Nowadays, NC are preferably made of biodegradable materials such as collagen, aliphatic polyesters, or polyurethanes. Although NC assist in guiding regenerating nerves, satisfactory functional restoration of severed nerves may further require exogenous growth factors. Therefore, authors have proposed NC with integrated delivery systems for growth factors or growth factor-producing cells. This article reviews the most important designs of NC with integrated delivery systems for localized release of growth factors. The various systems discussed comprise NC with growth factors being released from various types of matrices, from transplanted cells (Schwann cells or mesenchymal stem cells), or through genetic modification of cells naturally present at the site of injured tissue. Acellular delivery systems for growth factors include the NC wall itself, biodegradable microspheres seeded onto the internal surface of the NC wall, or matrices that are filled into the lumen of the NC and immobilize the growth factors through physical-chemical interactions or specific ligand-receptor interactions. A very promising and elegant system appears to be longitudinally aligned fibers inserted in the lumen of a NC that deliver the growth factors and provide additional guidance for Schwann cells and axons. This review also attempts to appreciate the most promising approaches and emphasize the importance of growth factor delivery kinetics.

Bioengineered nerve regeneration and muscle reinnervation

The peripheral nervous system has the intrinsic capacity to regenerate but the reinnervation of muscles is often suboptimal and results in limited recovery of function. Injuries to nerves that innervate complex organs such as the larynx are particularly difficult to treat. The many functions of the larynx have evolved through the intricate neural regulation of highly specialized laryngeal muscles. In this review, we examine the responses of nerves and muscles to injury, focusing on changes in the expression of neurotrophic factors, and highlight differences between the skeletal limb and laryngeal muscle systems. We also describe how artificial nerve conduits have become a useful tool for delivery of neurotrophic factors as therapeutic agents to promote peripheral nerve repair and might eventually be useful in the treatment of laryngeal nerve injury.

Enhanced regeneration in injured sciatic nerve by human amniotic mesenchymal stem cell

OBJECTIVE

Amniotic fluid mesenchymal stem cells (MSCs) have the potential to differentiate into neuronal stem cells in vitro. We evaluated using amniotic fluid MSCs to support or enhance the ability of the injured sciatic nerve to cross a nerve gap.

MATERIALS AND METHODS

We created a 5 mm nerve defect in Sprague Dawley rats. One group received therapy with MSCs embedded into woven oxidised regenerated cellulose gauze (Surgical; Ethicon, Somerville, NJ) and fibrin glue, while a control group received woven Surgicel and fibrin glue only. Evaluation methods included behavioural, electrophysiological and immunohistochemical studies.

RESULTS

In gait analysis, the angle of the ankles in the treatment and control group were 46.4 degrees (standard deviation [SD]=15 degrees) and 36 degrees (SD=8.2 degrees), respectively, which was statistically significant (p=0.045). Five of 10 treated rats (50%) demonstrated partial foot movement, while none of the control group had any movement. The percentage amplitude of muscle compound action potential in the experimental group was 43% (SD=12.5%) compared to 29% (SD=8.8%) in the control group (p=0.038). The conduction latencies in the control and experimental groups was 2.5 ms (SD=0.45) and 1.7 ms (SD=0.47), respectively (p=0.005). Histological examination demonstrated that 70% of the treatment group achieved a maximum axon diameter percentage across the nerve gap of greater than 50%, compared with 0% in the control group. There were no differences in direction of fibre growth and fibrotic reaction between the two groups.

CONCLUSION

Amniotic fluid MSC can augment growth of injured nerve across a nerve gap. This effect may be due to neurotrophic or induction effects of the MSC interacting with Schwann cells. Further study is required to determine the underlying mechanism of this effect.

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.

Oxidized galectin-1 advances the functional recovery after peripheral nerve injury

Oxidized galectin-1 has been shown to promote axonal regeneration from transected-nerve sites in an in vitro dorsal root ganglion (DRG) explant model as well as in in vivo peripheral nerve axotomy models. The present study provides evidence that oxidized galectin-1 advances the restoration of nerve function after peripheral nerve injury. The sciatic nerve of adult rats was transected and the distal nerve was frozen after being sutured into a proximal site with four epineurial stitches. An osmotic pump delivered oxidized galectin-1 peripherally to the surgical site. Functional recovery was assessed by measurement of the degree of toe spread of the hind paw for 3 months after the sciatic nerve lesion. The recovery curves of toe spread in the test group showed a statistically significant improvement of functional recovery after day 21 by the application of oxidized recombinant human galectin-1 (rhGAL-1/Ox) compared to the control group. This functional recovery was supported by histological analysis performed by light microscopic examination. The regenerating myelinated fibers at the site 21 mm distal to the nerve-transected site were quantitatively examined at 100 days after the operation. The frequency distribution of myelinated fiber diameters showed that exogenous rhGAL-1/Ox increased the number and diameter of regenerating myelinated fibers; the number of medium-sized (6-11 microm in diameter) fibers increased significantly (P<0.05). These results indicate that oxidized galectin-1 promotes the restoration of nerve function after peripheral nerve injury. Thus, rhGAL-1/Ox may be a factor for functional restoration of injured peripheral nerves.

The effect of brain-derived neurotrophic factor on sensory and autonomic function after lingual nerve repair

Brain-derived neurotrophic factor (BDNF) is important in the response to peripheral nerve injury and may enhance regeneration. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat. Six months after entubulation repair, with or without the incorporation of BDNF at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair were undertaken to characterise mechanosensitive, thermosensitive, and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in receptor sensitivity and spontaneous discharge, and persistent reductions in conduction velocity, proportion of gustatory and thermosensitive units, rate of salivary secretion, and vasomotor responses. Comparisons between the outcome of repair with or without BDNF revealed few differences. In the BDNF group, fewer units in the chorda tympani responded to gustatory or thermal stimuli and the sensitivity of the gustatory units was lower. The conduction velocity of afferents in the lingual nerve was also lower, but the mechanoreceptive field size was higher. Thus, despite its known trophic role in the gustatory system, BDNF had not enhanced recovery of these or other fibre populations. We conclude that the application of BDNF to a site of lingual nerve repair has a negative effect on the long-term outcome.

Immobilized concentration gradients of nerve growth factor guide neurite outgrowth

Axons are guided to their targets by a combination of haptotactic and chemotactic cues. We previously demonstrated that soluble neurotrophic factor concentration gradients guide axons in a model system. In an attempt to translate this model system to a device for implantation, our goal was to immobilize a stable neurotrophic concentration gradient for axonal (or neurite) guidance. Nerve growth factor (NGF) was immobilized within poly(2-hydroxyethylmethacrylate) [p(HEMA)] microporous gels using a gradient maker. The NGF was stably immobilized, with only approximately 0.05% of the amount originally incorporated into the gel released over an 8-day period. Immobilized NGF was bioactive: the percent of PC12 cells extending neurites on NGF-immobilized p(HEMA) gels was 16 +/- 2%, which was statistically the same as those exposed to soluble NGF (22 +/- 6%). We were able to predict and reproducibly create stable NGF concentration gradients in the gel. At an NGF concentration gradient of 357 ng/mL/mm, PC12 cell neurites were guided up the gradient. The facile, flexible, and reproducible nature of this method allowed us to translate soluble growth factor gradient models to stable growth factor gradient devices that may ultimately enhance axonal guidance and regeneration in vivo.

Peripheral nerve regeneration in RGD peptide incorporated collagen tubes

This paper describes the regeneration of lesioned sciatic nerve with collagen tubes incorporated with RGD cell-adhesive peptide. Collagen implants of 14 mm were grafted to bridge a gap length of 10 mm nerve defect in a rat model. The regenerated tissues were analyzed histomorphologically. The number of myelinated axons in the regenerated mid-graft of the RGD peptide incorporated groups was statistically significant (p<0.05) than control collagen tube and autograft control after 30 days postoperatively. After 90 days of implantation, the mean counts were still statistically significant in the case of RGD peptide group than control collagen and autograft groups. Immunofluorescence studies demonstrated the staining of S100 proteins in the peripherally located cells indicating the proliferation of Schwann cells in the early days of regeneration. The staining pattern of integrin-alphaV was observed mostly in the perineurial regions in close proximity to the RGD peptide incorporated collagen tubes. Other studies like sciatic functional index, conduction velocity at 90 days postoperatively suggest complete regeneration of lesioned nerves with RGD incorporated collagen implants.

Neurotrophic factors and their receptors in axonal regeneration and functional recovery after peripheral nerve injury

Over a half a century of research has confirmed that neurotrophic factors promote the survival and process outgrowth of isolated neurons in vitro. The mechanisms by which neurotrophic factors mediate these survival-promoting effects have also been well characterized. In vivo, peripheral neurons are critically dependent on limited amounts of neurotrophic factors during development. After peripheral nerve injury, the adult mammalian peripheral nervous system responds by making neurotrophic factors once again available, either by autocrine or paracrine sources. Three families of neurotrophic factors were compared, the neurotrophins, the GDNF family of neurotrophic factors, and the neuropoetic cytokines. Following a general overview of the mechanisms by which these neurotrophic factors mediate their effects, we reviewed the temporal pattern of expression of the neurotrophic factors and their receptors by axotomized motoneurons as well as in the distal nerve stump after peripheral nerve injury. We discussed recent experiments from our lab and others which have examined the role of neurotrophic factors in peripheral nerve injury. Although our understanding of the mechanisms by which neurotrophic factors mediate their effects in vivo are poorly understood, evidence is beginning to emerge that similar phenomena observed in vitro also apply to nerve regeneration in vivo.

Peripheral nerve regeneration with sustained release of poly(phosphoester) microencapsulated nerve growth factor within nerve guide conduits

Prolonged delivery of neurotrophic proteins to the target tissue is valuable in the treatment of various disorders of the nervous system. We have tested in this study whether sustained release of nerve growth factor (NGF) within nerve guide conduits (NGCs), a device used to repair injured nerves, would augment peripheral nerve regeneration. NGF-containing polymeric microspheres fabricated from a biodegradable poly(phosphoester) (PPE) polymer were loaded into silicone or PPE conduits to provide for prolonged, site-specific delivery of NGF. The conduits were used to bridge a 10 mm gap in a rat sciatic nerve model. Three months after implantation, morphological analysis revealed higher values of fiber diameter, fiber population and fiber density and lower G-ratio at the distal end of regenerated nerve cables collected from NGF microsphere-loaded silicone conduits, as compared with those from control conduits loaded with either saline alone, BSA microspheres, or NGF protein without microencapsulation. Beneficial effects on fiber diameter, G-ratio and fiber density were also observed in the permeable PPE NGCs. Thus, the results confirm a long-term promoting effect of exogenous NGF on morphological regeneration of peripheral nerves. The tissue-engineering approach reported in this study of incorporation of a microsphere protein release system into NGCs holds potential for improved functional recovery in patients whose injured nerves are reconstructed by entubulation.

Peripheral nerve regeneration using acellular nerve grafts

Long gap peripheral nerve injuries usually require a graft to facilitate axonal regeneration into the distal nerve stump. The use of autografts is often limited because of graft availability and donor-site morbidity. We investigated whether acellular nerve allografts would provide an appropriate channel for the promotion and induction of sciatic nerve regeneration in rats. Axons sprouted from the proximal portion and reached the distal portion in the 1 cm-long grafts by 1 month. The number of axons in the regenerated nerves was similar to that of normal nerves at 1 month. Loading the grafts with betaNGF and VEGF increased the number and mean diameter of axons and neovascularization in the regenerated nerves at 1 month. The motor conduction velocity increased over time and reached 63 +/- 10% of that of normal nerves at 6 months. The nerve injuries treated with the acellular grafts had a significant improvement in motor, nociception, and proprioception function compared to untreated nerves. The results from this study suggest that acellular nerve allografts may be a useful biomaterial for functional peripheral nerve regeneration.

Effects of brain-derived neurotrophic factor (BDNF) on compression-induced spinal cord injury: BDNF attenuates down-regulation of superoxide dismutase expression and promotes up-regulation of myelin basic protein expression

Neurotrophins enhance the survival of cells in the nervous system under both physiological and pathological conditions, such as those caused by disease or trauma. We recently demonstrated that expression of brain-derived neurotrophic factor (BDNF) was up-regulated in neurons and glia after compression-induced spinal cord injury (SCI). We show here the effects of BDNF on the oligodendrocyte survival and functional recovery after SCI. The effects of intrathecally administered BDNF on both Cu/Zn superoxide dismutase (CuZnSOD) and myelin basic protein (MBP) expression were examined using rats that had received compression-induced spinal cord injury. CuZnSOD expression in the spinal cord was down-regulated within 24 h of compression-induced injury and then recovered. Continuous infusion of BDNF inhibited the acute down-regulation of CuZnSOD expression. In situ hybridization showed that CuZnSOD was expressed in both neurons and glia. Although MBP expression was greatly reduced after injury, BDNF administration promoted the recovery of MBP expression nearly to a control level after 2 wk. Furthermore, BDNF administration also prompted behavioral recovery. These results suggest BDNF's usefulness in human clinical applications. The attenuation of CuZnSOD down-regulation may be related to a protective effect of BDNF and the promotion of MBP up-regulation may be related to a long-lasting restorative effect.

A dose-dependent facilitation and inhibition of peripheral nerve regeneration by brain-derived neurotrophic factor

The time-dependent decline in the ability of motoneurons to regenerate their axons after axotomy is one of the principle contributing factors to poor functional recovery after peripheral nerve injury. A decline in neurotrophic support may be partially responsible for this effect. The up-regulation of BDNF after injury, both in denervated Schwann cells and in axotomized motoneurons, suggests its importance in motor axonal regeneration. In adult female Sprague-Dawley rats, we counted the number of freshly injured or chronically axotomized tibial motoneurons that had regenerated their axons 1 month after surgical suture to a freshly denervated common peroneal distal nerve stump. Motor axonal regeneration was evaluated by applying fluorescent retrograde neurotracers to the common peroneal nerve 20 mm distal to the injury site and counting the number of fluorescently labelled motoneurons in the T11-L1 region of the spinal cord. We report that low doses of BDNF (0.5-2 microg/day for 28 days) had no detectable effect on axonal regeneration after immediate nerve repair, but promoted axonal regeneration of motoneurons whose regenerative capacity was reduced by chronic axotomy 2 months prior to nerve resuture, completely reversing the negative effects of delayed nerve repair. In contrast, high doses of BDNF (12-20 microg/day for 28 days) significantly inhibited motor axonal regeneration, after both immediate nerve repair and nerve repair after chronic axotomy. The inhibitory actions of high dose BDNF could be reversed by functional blockade of p75 receptors, thus implicating these receptors as mediators of the inhibitory effects of high dose exogenous BDNF.

Expression of Schwann cell-specific proteins and low-molecular-weight neurofilament protein during regeneration of sciatic nerve treated with neurotrophin-4

Neurotrophin-4 acts as a potent survival factor for subpopulations of motoneurons. To investigate its effect on Schwann cell sheath and axonal proteins during peripheral nerve regeneration, sciatic nerves in adult rats were transected and repaired, and fibrin glue containing neurotrophin-4 injected around the repair site. At 5, 15, 30 and 60 days after repair, 5-mm nerve segments distal to the repair were collected, and western blotting was used to measure myelin-associated glycoprotein, myelin basic protein and low-molecular-weight neurofilament protein. In control groups these dramatically declined at 5 and 15 days then increased from 30 and 60 days. However, in the neurotrophin-4 group there was a significant increase (to several times basal values) in myelin-associated glycoprotein and myelin basic protein at 5-15 days. The relatively small increases (<7%) in Schwann cell numbers suggest that this is mainly due to increased synthesis per cell. The neurotrophin-4 group also showed a small but significant increase at 15 days in low-molecular-weight neurofilament protein, which however remained much lower than basal. We conclude that neurotrophin-4 regulates the expression of myelin-associated glycoprotein, myelin basic protein, and to a lesser extent low-molecular-weight neurofilament protein, during peripheral nerve regeneration.

Reinnervation of a denervated skeletal muscle by spinal axons regenerating through a collagen channel directly implanted into the rat spinal cord

In the present study, the continuity between the central nervous system (CNS) and the peripheral nervous system (PNS) was restored by mean of a collagen channel in order to reinnervate a skeletal muscle. Three groups of animals were considered. In the first group, one end of the collagen channel was implanted in the cervical spinal cord of adult rats. The other end was connected to a 30-mm autologous peripheral nerve graft (PNG) implanted into the denervated biceps brachii muscle. The gap between the spinal cord and the proximal nerve stump varied from 3 to 7 mm. In the second group of animals, the distal end of the PNG graft was ligatured in order to compare the survival of the growing axons in the presence and in the absence of a muscular target. In the third group of animals, the extraspinal stump of the collagen channel was ligatured. Our study demonstrates that spinal neurons and dorsal root ganglion (DRG) neurons can grow long axons through the collagen channel over a 7-mm gap and reinnervate a denervated skeletal muscle. The results also indicate that the presence of a PNG at the extraspinal stump of the collagen channel is essential for axonal regrowth and that the muscle target contributes to the long-term maintenance of the regenerating axons. These data might be interesting for clinical application when the continuity between the CNS and PNS is interrupted such as in root avulsion.

The regrowth of axons within tissue defects in the CNS is promoted by implanted hydrogel matrices that contain BDNF and CNTF producing fibroblasts

In this study we demonstrate the potential for combining biocompatible polymers with genetically engineered cells to elicit axon regrowth across tissue defects in the injured CNS. Eighteen- to 21-day-old rats received implants of poly N-(2-hydroxypropyl)-methacrylamide (HPMA) hydrogels containing RGD peptide sequences that had been infiltrated with control (untransfected) fibroblasts (n = 8), fibroblasts engineered to express brain-derived neurotrophic factor (BDNF) (n = 5), ciliary neurotrophic factor (CNTF) (n = 5), or a mixture of BDNF and CNTF expressing fibroblasts (n = 11). Fibroblasts were prelabeled with Hoechst 33342. Cell/polymer constructs were inserted into cavities made in the left optic tract, between thalamus and superior colliculus. After 4-8 weeks, retinal projections were analyzed by injecting right eyes with cholera toxin (B-subunit). Rats were perfused 24 h later and sections were immunoreacted to visualize retinal axons, other axons (RT97 antibody), host astrocytes and macrophages, donor fibroblasts, and extracellular matrix molecules. The volume fraction (VF) of each gel that was occupied by RT97(+) axons was quantified. RT-PCR confirmed expression of the transgenes prior to, and 5 weeks after, transplantation. Compared to control rats (mean VF = 0.02 +/- 0.01% SEM) there was increased ingrowth of RT97(+) axons into implants in CNTF (mean VF = 0.33 +/- 0.19%) and BDNF (mean VF = 0.62 +/-0.19%) groups. Axon growth into hydrogels in the mixed BDNF/CNTF group (mean VF = 3.58 +/- 0.92%) was significantly greater (P < 0.05) than in the BDNF or CNTF fibroblast groups. Retinal axons exhibited a complex branching pattern within gels containing BDNF or BDNF/CNTF fibroblasts; however, they regrew the greatest distances within implants containing both BDNF and CNTF expressing cells.

Neurotrophin-4 delivered by fibrin glue promotes peripheral nerve regeneration

Neurotrophin-4 (NT-4) is a recently identified neurotrophic factor with potential trophic effects on subpopulations of neurons. Little is known about its role in peripheral nerve regeneration following nerve injury. To investigate this, 48 Sprague-Dawley rats underwent left sciatic nerve transection and immediate repair. Fibrin glue mixed with either NT-4 or vehicle (control) was injected around the nerve repair site. Nerve regeneration was assessed both functionally and histomorphometrically. The results showed that the NT-4-treated group had a significant increase compared with the control in the regeneration distance at 5 days. The sciatic function index was significantly greater in the NT-4 group from 40 to 60 days after nerve repair. Morphometric analysis revealed that nerves treated with NT-4 had significant improvement in the number of regenerated axons, axonal diameter, and myelin thickness. These results suggest that NT-4 is a potent factor improving rat sciatic nerve regeneration.

Engineering strategies for peripheral nerve repair

Tissue engineering in the peripheral nervous system unites efforts by physicians, engineers, and biologists to create either natural or synthetic tubular nerve guidance channels as alternatives to nerve autografts for the repair of peripheral nerve defects. Guidance channels help direct axons sprouting from the regenerating nerve end, provide a conduit for diffusion of neurotropic and neurotrophic factors secreted by the damaged nerve stumps, and minimize infiltration of fibrous tissue. In addition to efforts to control these physical characteristics of nerve guidance channels, researchers are optimizing the incorporation of biologic factors and engineering interactive biomaterial that can specifically stimulate the regeneration process. Current and future research will ultimately result in biologically active and interactive nerve guidance channels that can support and enhance peripheral nerve regeneration over longer, more clinically relevant defect lengths.

R, Silva CFD, Lainetti RD. Estudo experimental comparativo da ação das neurocinas cardiotrofina-1 e oncostatina-m na regeneração nervosa periférica

Os avanços das técnicas microcirúrgicas e o conhecimento detalhado do microambiente da regeneração podem contribuir significativamente na melhoria dos resultados das reparações nervosas periféricas. Nos últimos anos vários autores têm utilizado uma série de tecidos e substâncias interpostos entre os cotos de um nervo periférico seccionado, buscando estimular o crescimento axonal no local da lesão. Através da técnica de tubulização, os autores estudam o efeito de duas neurocinas, a cardiotrofina-1 (CT-1) e a oncostatina-M (OsM), no crescimento axonal e na sobrevida dos neurônios sensitivos nos gânglios da raiz dorsal de L5, após a lesão de nervos ciáticos em camundongos C57BL/6J. Utilizam 3 grupos de 7 animais que tiveram seus nervos seccionados e tubulizados com próteses de polietileno preenchidas com cardiotrofina-1, oncostatina-M e citocromo-C, associadas a um extrato de colágeno. Um quarto grupo de 3 animais, não operados, foi considerado por nós como grupo controle de normalidade. Após 4 semanas da cirurgia, os camundongos foram sacrificados, e realizada a contagem das fibras mielínicas nos cabos de regeneração retirados. Os gânglios das raizes dorsais de L5 também foram dissecados possibilitando a contagem dos neurônios sensitivos. Os dados foram analisados estatisticamente, permitindo concluir que as duas substâncias, utilizadas por nós, foram efetivas no estímulo ao brotamento axonal, porém, as mesmas não conseguiram impedir a morte dos neurônios sensitivos no gânglio da raiz dorsal de L5.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.