Neurotrophin-3 delivered locally via fibronectin mats enhances peripheral nerve regeneration

Comparable morphometric data of pathological nerve obtained using the three-window sampling method and total fiber quantification

Morphometric analysis of nerve biopsy provides data of structural changes that are essential for early detection of peripheral neuropathy. Because of the laborious work associated with the total fiber quantification, various sampling methods have been introduced with controversial accuracy. Three-window sampling technique has been recently proposed to provide the accurate morphometric data of normal human sural nerve. However, its application in the diseased nerve has not been validated. This study, therefore, compared the morphometric data of nerve biopsies from 12 patients with various neuropathies obtained using this sampling method and those obtained by total fiber analysis. Total number, density, and diameter of myelinated fibers including myelin thickness and g ratio were analyzed. Intraclass correlation coefficients ranging from 0.95 to 0.99 indicate the high agreement between the data derived from the two methods in these parameters. This finding suggests the accuracy of the three-window sampling technique in the morphometric study of nerve with pathological alterations.

Nerve repair: experimental and clinical evaluation of neurotrophic factors in peripheral nerve regeneration

Neurotrophic factors are a family of polypeptides required for survival of discrete neuronal populations. In the normal state such factors are mostly synthesised by target tissues and are used for the viability of the nerve-cell bodies. After nerve injury, neurotrophic factors (NFs) are synthesised by non-neuronal (Schwann cells and fibroblasts) in the nerve trunk, and act to support the outgrowth of axons. NFs can be classified into three major groups: (1) neurotrophins; (2) neurokines; and (3) the transforming growth factor beta (TGF)-beta superfamily.

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.

Herpes simplex virus-thymidine kinase-based suicide gene therapy as a "molecular switch off" for nerve growth factor production in vitro

Tissue-engineered constructs offer a new hope to patients suffering from functional impairment after nerve injury. An effort has been made to focus on delivery, regulation, and "molecular shutoff" of nerve growth factor (NGF) in tissue-engineered constructs. We have previously demonstrated that human embryonic kidney (HEK-293) cells can be genetically modified to secrete NGF at varying time points upon up regulation with Ponasterone A (PonA) both in vitro and in vivo. In the present study, HEK-293 cells that stably and inducibly produce NGF were further stably transfected with herpes simplex virus-thymidine kinase gene as a suicide gene (hNGF-EcR-293-TK) in order to shut off the NGF secretion and kill the cells upon treatment with ganciclovir (GCV). These cells following induction with PonA secreted NGF levels of 6659.2 +/- 489.4 pg/mL at day 10 postbooster dose at day 5, which was significantly higher than the control noninduced cells. The NGF secreted by these cells was bioactive as determined by a rat adrenal pheochromocytoma (PC-12) cell bioassay. Treatment of these cells with GCV significantly reduced the NGF levels to 645.3 +/- 16.2 pg/mL at day 10 and live cell numbers dropped to 7.95 x 10(3) +/- 278 compared to 2.73 x 10(5) +/- 6.1 x 10(4). GCV-treated cell media when transferred to the PC-12 cell bioassay demonstrated less than 10% cells differentiating into neurite-like extensions. We conclude that hNGF-EcR-293-TK cells can inducibly secrete bioactive NGF when treated with the inducing agent and can also be killed upon treatment with GCV. This double-gene transfection for gene expression and molecular shutoff mechanism will be a useful tool in tissue-engineered nerve constructs.

NT-3 Expression in Spared DRG and the Associated Spinal Laminae as well as Its Anterograde Transport in Sensory Neurons Following Removal of Adjacent DRG in Cats

Neurotrophin-3 plays an important role in survival and differentiation of sensory and sympathetic neurons, sprouting of neurites, synaptic reorganization, and axonal growth. The present study evaluated changes in expression of NT-3 in the spinal cord and L6 dorsal root ganglion (DRG), after ganglionectomy of adjacent dorsal roots in cats. NT-3 immunoreactivity increased at 3 days post-operation (dpo), but decreased at 10 dpo in spinal lamina II after ganglionectomy of L1-L5 and L7-S2 (leaving L6 DRG intact). Conversely, NT-3 immunoreactivity decreased on 3 dpo, but increased on 10 dpo in the nucleus dorsalis. Very little NT-3 mRNA signal was detected in the spinal cord, despite the changes in NT-3 expression. The above changes may be related to changes in NT-3 expression in the DRG. Ganglionectomy of L1-L5 and L7-S2 resulted in increase in NT-3 immunoreactivity and mRNA in small and medium-sized neurons, but decreased expression in large neurons of L6 DRG at 3 dpo. It is possible that increased NT-3 in spinal lamina II is derived from anterograde transport from small- and medium-sized neurons of L6 DRG, whereas decreased NT-3 immunoreactivity in the nucleus dorsalis is due to decreased transport of NT-3 from large neurons in the DRG at this time. This notion is supported by observations on NT-3 distribution in the dorsal root of L6 after ligation of the nerve root. The above results indicate that DRG may be a source of neurotrophic factors such as NT-3 to the spinal cord, and may contribute to plasticity in the spinal cord after injury.

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.

Effect of neurotrophin-3 on reinnervation of the larynx using the phrenic nerve transfer technique

Current techniques for reinnervation of the larynx following recurrent laryngeal nerve (RLN) injury are limited by synkinesis, which prevents functional recovery. Treatment with neurotrophins (NT) may enhance nerve regeneration and encourage more accurate reinnervation. This study presents the results of using the phrenic nerve transfer method, combined with NT-3 treatment, to selectively reinnervate the posterior cricoarytenoid (PCA) abductor muscle in a pig nerve injury model. RLN transection altered the phenotype and morphology of laryngeal muscles. In both the PCA and thyroarytenoid (TA) adductor muscle, fast type myosin heavy chain (MyHC) protein was decreased while slow type MyHC was increased. These changes were accompanied with a significant reduction in muscle fibre diameter. Following nerve repair there was a progressive normalization of MyHC phenotype and increased muscle fibre diameter in the PCA but not the TA muscle. This correlated with enhanced abductor function indicating the phrenic nerve accurately reinnervated the PCA muscle. Treatment with NT-3 significantly enhanced phrenic nerve regeneration but led to only a small increase in the number of reinnervated PCA muscle fibres and minimal effect on abductor muscle phenotype and morphology. Therefore, work exploring other growth factors, either alone or in combination with NT-3, is required.

Repair of neural pathways by olfactory ensheathing cells

Damage to nerve fibre pathways results in a devastating loss of function, due to the disconnection of nerve fibres from their targets. However, some recovery does occur and this has been correlated with the formation of new (albeit abnormal) connections. The view that an untapped growth potential resides in the adult CNS has led to various attempts to stimulate the repair of disconnectional injuries. A key factor in the failure of axonal regeneration in the CNS after injury is the loss of the aligned glial pathways that nerve fibres require for their elongation. Transplantation of cultured adult olfactory ensheathing cells into lesions is being investigated as a procedure to re-establish glial pathways permissive for the regeneration of severed axons.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

Biologically inspired approaches to drug delivery for nerve regeneration

As the biological processes governing nerve regeneration have become elucidated over the past decades, interest has developed in manipulating these processes to improve nerve regeneration. Drug delivery to the regenerating nerve has the potential for major clinical applications in neurodegenerative diseases, spinal cord injury and peripheral nerve injury or sacrifice. This article reviews the evolution of the field of drug delivery to the regenerating nerve, from simple local applications of neurotrophic agents in solution and osmotic pump delivery, to the existing approaches involving novel biomaterials and genetically manipulated cell populations. A discussion of the various known nerve growth-promoting agents, and the chemical considerations involved in their delivery, is included. A perspective on the role of tissue engineering approaches for nerve regeneration in the future is offered.

Accuracy of the three-window sampling method in morphometric analysis of human sural nerve

Morphometry has an important role in the assessment of sural nerve biopsies as a part of early detection of structural abnormalities in peripheral nerve. Various sampling methods have been used to reduce time and effort needed in the analysis of total nerve fibers but their accuracy remains controversial. We examined the accuracy of three-window sampling method in the morphometric evaluation of human sural nerve biopsies by comparing with the total fiber quantification. Three windows (0.012 mm(2) each) were placed in every possible fascicle in the sections and data from all windows were pooled and analyzed for the number of myelinated axons, myelinated fiber diameter, axon diameter, myelin thickness, g ratio as well as myelinated fiber density. Means and ranges of the data from the two techniques were similar and the agreement was further confirmed by intraclass correlation analysis. These findings indicate that the three-window sampling method can be used to evaluate human sural nerve with accuracy.

Tissue engineered nerve constructs: where do we stand?

Driven by enormous clinical need, interest in peripheral nerve regeneration has become a prime focus of research and area of growth within the field of tissue engineering. While using autologous donor nerves for bridging peripheral defects remains today's gold standard, it remains associated with high donor site morbidity and lack of full recovery. This dictates research towards the development of biomimetic constructs as alternatives. Based on current concepts, this review summarizes various approaches including different extracellular matrices, scaffolds, and growth factors that have been shown to promote migration and proliferation of Schwann cells. Since neither of these concepts in isolation is enough, although each is gaining increased interest to promote nerve regeneration, various combinations will need to be identified to strike a harmonious balance. Additional factors that must be incorporated into tissue engineered nerve constructs are also unknown and warrant further research efforts. It seems that future directions may allow us to determine the "missing link".

Biomaterials and Strategies for Nerve Regeneration

Nerve regeneration is a complex biological phenomenon. Once the nervous system is impaired, its recovery is difficult and malfunctions in other parts of the body may occur because mature neurons do not undergo cell division. To increase the prospects of axonal regeneration and functional recovery, researches have focused on designing "nerve guidance channels" or "nerve conduits." When developing ideal tissue-engineered nerve conduits, several components come to mind. They include a biodegradable and porous channel wall, the ability to deliver bioactive growth factors, incorporation of support cells, an internal oriented matrix to support cell migration, intraluminal channels to mimic the structure of nerve fascicles, and electrical activities. This article reviews the factors that are critical for nerve repair, and the advanced technologies that are explored to fabricate nerve conduits. To more accurately mimic natural repair in the body, recent studies have focused on the use of various advanced approaches to create ideal nerve conduits that combine multiple stimuli in an effort to better mimic the complex signals normally found in the body.

Bioengineered Strategies for Spinal Cord Repair

This article reviews bioengineered strategies for spinal cord repair using tissue engineered scaffolds and drug delivery systems. The pathophysiology of spinal cord injury (SCI) is multifactorial and multiphasic, and therefore, it is likely that effective treatments will require combinations of strategies such as neuroprotection to counteract secondary injury, provision of scaffolds to replace lost tissue, and methods to enhance axonal regrowth, synaptic plasticity, and inhibition of astrocytosis. Biomaterials have major advantages for spinal cord repair because of their structural and chemical versatility. To date, various degradable or non-degradable biomaterial polymers have been tested as guidance channels or delivery systems for cellular and non-cellular neuroprotective or neuroregenerative agents in experimental SCI. There is promise that bioengineering technology utilizing cellular treatment strategies, including Schwann cells, olfactory ensheathing glia, or neural stem cells, can promote repair of the injured spinal cord. This review is divided into three parts: (1) degradable and non-degradable biomaterials; (2) device design; and (3) combination strategies with scaffolds. We will show that bioengineering combinations of cellular and non-cellular strategies have enhanced the potential for experimental SCI repair, although further pre-clinical work is required before this technology can be translated to humans.

The effects of delayed nerve repair on neuronal survival and axonal regeneration after seventh cervical spinal nerve axotomy in adult rats

It has been proposed clinically that delayed surgery after traumatic brachial plexus injury may adversely affect functional outcome. In the present experimental study the neuroprotective and growth-promoting effects of early and delayed nerve grafting following proximal seventh cervical spinal nerve (C7) axotomy were examined. The ventral branch of C7 spinal nerve was transected and axons projecting out of the proximal nerve stump were labelled with Fast Blue (FB). At the same time, the biceps brachii muscle was denervated by transecting the musculocutaneous nerve at its origin. Neuronal survival and muscle atrophy were then assessed at 1, 4, 8 and 16 weeks after permanent axotomy. In the experimental groups, a peripheral nerve graft was interposed between the transected C7 spinal nerve and the distal stump of the musculocutaneous nerve at 1 week [early nerve repair (ENR)] or 8 weeks [delayed nerve repair (DNR)] after axotomy. Sixteen weeks after nerve repair had been performed, a second tracer Fluoro-Ruby (FR) was applied distal to the graft to assess the efficacy of axonal regeneration. Counts of FB-labelled neurons revealed that axotomy did not induce any significant cell loss at 4 weeks, but 15% of motoneurons and 32% of sensory neurons died at 8 weeks after injury. At 16 weeks, the amount of cell loss in spinal cord and dorsal root ganglion (DRG) reached 29 and 50%, respectively. Both ENR and DNR prevented retrograde degeneration of spinal motoneurons and counteracted muscle atrophy, but failed to rescue sensory neurons. Due to substantial cell loss at 8 weeks, the number of FR-labelled neurons after DNR was significantly lower when compared to ENR. However, the proportion of regenerating neurons among surviving motoneurons and DRG neurons remained relatively constant indicating that neurons retained their regenerative capacity after prolonged axotomy. The results demonstrate that DNR could protect spinal motoneurons and reduce muscle atrophy, but had little effect on sensory DRG neurons. However, the efficacy of neuroprotection and axonal regeneration will be significantly affected by the amount of cell loss already presented at the time of nerve repair.

Neurotrophin-3 prevents mitochondrial dysfunction in sensory neurons of streptozotocin-diabetic rats

Sensory neurons from streptozotocin (STZ)-diabetic rats exhibit depolarization of mitochondria and the related induction of reactive oxygen species has been proposed to contribute to the etiology of sensory polyneuropathy in diabetes. There is deficient neurotrophin-3 (NT-3)-dependent neurotrophic support of sensory neurons in diabetes and treatment of STZ-diabetic rats with NT-3 prevents neuropathological alterations in peripheral nerve. Therefore, we hypothesized that loss of NT-3 may contribute to mitochondrial dysfunction in sensory neurons in diabetic sensory neuropathy. The specific aim of this study was to determine whether treatment of STZ-diabetic rats with systemic NT-3 could prevent depolarization of the mitochondrial inner membrane potential (Deltapsi(m)). In vitro studies with cultured DRG neurons from control rats revealed that treatment with 50 ng/ml NT-3 for 6 h enhanced the Deltapsi(m), e.g., a higher polarized membrane potential, compared to untreated neurons (P < 0.05). Studies on DRG sensory neurons from control vs. STZ-diabetic rats demonstrated that NT-3 therapy prevented the diabetes-induced depolarization of Deltapsi(m) (P < 0.05) in parallel with normalization of diabetes-dependent deficits in sensory nerve conduction velocity. Furthermore, alterations in mitochondrial function in vitro and in vivo correlated with the level of activation/expression of Akt in DRG neurons.

Alpha7 integrin mediates neurite outgrowth of distinct populations of adult sensory neurons

The successful regeneration of peripheral branches of sensory neurons following injury is attributed to the presence of neurotrophins and interaction of regenerating axons with the extracellular matrix. Here, we show that the laminin receptor, alpha7beta1 integrin is a crucial mediator of neurite outgrowth from distinct populations of sensory neurons. Following sciatic nerve crush, alpha7 integrin is expressed by medium-large diameter, NF200-immunoreactive (IR), and medium diameter, CGRP-IR, neurons, but very few small diameter non-peptidergic neurons. The functional significance of alpha7 integrin expression following injury was addressed using dissociated adult rat and mouse sensory neurons. By using function-blocking antibodies and neurons isolated from alpha7 integrin null mice, we demonstrate that NGF- and NT-3-stimulated neurite outgrowth is reduced in the absence of alpha7 integrin signaling. In contrast, GDNF-stimulated neurite outgrowth is less dependent on alpha7 integrin. These results define an essential interaction between alpha7 integrin and laminin for mediating neurite outgrowth of subpopulations of injured adult sensory neurons.

In vivo induction and delivery of nerve growth factor, using HEK-293 cells

Tissue-engineering strategies offer hope to patients facing functional impairment after nerve injury. We have previously demonstrated that HEK-293 cells can release nerve growth factor (NGF) in vitro, using an inducible system of expression. In this study, our objective was to assess the efficacy of the NGF delivery system in vivo, using nude rats. HEK-293 cells were transfected with human NGF cDNA. Ponasterone A (PonA) was used as the inducing agent. NGF collection chambers were implanted subcutaneously in nude rats. Sealed chambers were filled with one of the following: (1) DMEM, (2) untransfected 293 cells (EcR-293) plus PonA, (3) untransfected EcR-293 without PonA, (4) transfected 293 cells (hNGF-EcR-293) plus PonA, or (5) transfected hNGF-EcR-293 without PonA. Chambers were aspirated 24, 48, and 120 h postimplantation. NGF secretion was analyzed in the following ways: (1) NGF protein expression bioactivity was assessed in a PC-12 cell bioassay, and (2) the concentration of secreted NGF was quantified by NGF ELISA. NGF quantification by ELISA reached a maximal release of 12.9 +/- 3.57 ng/mL at 120 h. PC-12 cells exposed to media from induced transfected HEK-293 cell chambers demonstrated higher levels of differentiation compared with controls. We conclude that hNGF-EcR-293 cells can inducibly secrete bioactive NGF when exposed to the induction agent PonA. This regulated delivery system can secrete bioactive NGF for up to 5 days in vivo. We believe this regulated delivery system will be useful for tissue-engineered nerve constructs.

Neurotrophin-3-induced production of nerve growth factor is suppressed in Schwann cells exposed to high glucose: involvement of the polyol pathway

Development of hypesthesia, a loss of sensitivity to stimulation, is associated with impaired regeneration of peripheral sensory fibers, in which Schwann cells play a key role by secreting nerve growth factor (NGF). Recent clinical trials indicated that an inhibitor of aldose reductase (AR), the rate-limiting enzyme in the polyol pathway, significantly improved hypesthesia in diabetic patients. The fact that AR is localized in Schwann cells led us to investigate the role of the polyol pathway in NGF production of isolated Schwann cells. Among various endogenous factors examined, increased production of NGF was demonstrated in the cells treated with neurotrophin-3 (NT-3) for 24 h. NT-3-induced NGF production was significantly suppressed when cells were cultured in the medium containing high glucose. In these cells, the levels of glutathione (GSH) and cAMP-response element binding protein (CREB) were reduced, whereas the level of activated nuclear factor-kappaB (NF-kappaB) was elevated. These changes were abolished when an AR inhibitor fidarestat was included in the medium. NT-3-induced NGF production was further attenuated in the cells treated with an inhibitor of GSH synthesis. Together, the enhanced polyol pathway activity under high-glucose conditions seems to elicit reduced NT-3-induced NGF production in Schwann cells. Enhanced oxidative stress linked to the polyol pathway activity may mediate this process.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.

Characterization of cortical neuron outgrowth in two- and three-dimensional culture systems

To improve the ability of regeneration by grafting living cells or by adding growth factor to a lesion site, it is important to find good biomaterials for neuron survival and regeneration. This study focused on two- and three-dimensional cultures in a matrix using biomaterials such as agarose, collagen, fibrin, and their mixtures, because these are considered to be suitable biomaterials for neuron outgrowth. Cortical neurons were dissected from E17 rat embryos and cultured in agarose gel, collagen gel, fibrin glue, and mixtures of collagen and fibrin. Results showed that neurons cultured in collagen gel and fibrin glue had longer periods of survival (more than 3 weeks) and better neurite extension than those observed in agarose gels. As to the survival rate according to the MTT and lactate dehydrogenase assays, fibrin glue was the most suitable biomaterial for neuron survival among the biomaterials examined. With two-dimensional fibrin plating, neuron cells exhibited cell aggregation and stress fibers, but the same results were not observed with collagen gel. There were no differences in neurite extension and survival in the mixtures of collagen and fibrin. The results suggest that collagen and fibrin can provide a suitable substrate for a three-dimensional culture matrix for neuronal survival and differentiation.

An electrophysiological study into the effect of neurotrophin-3 on functional recovery after lingual nerve repair

OBJECTIVE

Neurotrophin-3 (NT-3) is known to ameliorate central changes that result from peripheral nerve injury and may promote regeneration of myelinated axons. 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.

DESIGN

Six months after entubulation repair, with or without the incorporation of NT-3 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 allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents.

RESULTS

When compared with data from uninjured control animals, both repair groups showed persistent reductions in conduction velocity, receptor sensitivity, spontaneous discharge, proportion of gustatory and thermosensitive units, and rate of salivary secretion. Comparisons between the two repair groups revealed that in the NT-3 group, salivary secretion rate was lower and the activity evoked in the chorda tympani by gustatory or thermal stimuli was lower, but the spontaneous discharge rate was higher. Mechanosensitive units in the lingual nerve had slower conduction velocities but the mechanoreceptive field size, adaptation time and discharge frequency had increased.

CONCLUSIONS

Despite its known trophic role in the lingual somatosensory system, NT-3 did not enhance functional recovery from injury and had a negative effect on the long-term outcome for sensory and autonomic fibres.

Laryngeal abductor muscle reinnervation in a pig model

OBJECTIVE

To develop a large animal model for studies of laryngeal abductor reinnervation.

MATERIAL AND METHODS

Six minipigs underwent unilateral anastomosis of the phrenic nerve-abductor branch of the recurrent laryngeal nerve (RLN). Polyhydroxybutyrate (PHB) conduits were used for repair. At each of 30, 60 and 120 days, 2 animals underwent video laryngeal endoscopy (VLE) and were then killed. VLE was also performed in the 120-day pair at 60 days. Nerve-conduit-nerve-muscle samples were fixed for light and immunofluorescence (pan-neurofilaments, S-100) microscopy. Laryngeal muscles were harvested (myosin heavy chain analysis).

RESULTS

VLE showed recovery of abductor function in 1 animal at 60 days and in 1 at 120 days. Haematoxylin-eosin staining demonstrated a complex inflammatory response. Eosinophil recruitment was observed. Stepwise regeneration and reorganization of the distal nerve between 30 and 120 days was observed with pan-NF staining. The mean minimum diameter in the reinnervated posterior crico-arytenoids tended to increase for up to 120 days.

CONCLUSIONS

Anastomosis of the phrenic nerve-abductor branch of the RLN with a PHB conduit in a pig can result in functional and histological recovery within 2-4 months and appears to at least sustain abductor muscle fibre morphology. Recovery occurs despite a complex inflammatory response, which may be an essential part of healing rather than inhibitory.

Fluid shear in viscous fibronectin gels allows aggregation of fibrous materials for CNS tissue engineering

Fibronectin (Fn) materials prepared from human plasma have been used in various forms as substrates for tissue engineering. Such purposes require that the soluble protein aggregates into insoluble fibrous structures which encourage the attachment and migration of cells. The method of aggregation due to mechanical shear was investigated by applying fluid shear forces directly to a viscous solution of Fn. Structural analysis revealed that mechanical shear resulted in the formation of an orientated fibrous protein material that was less soluble than its non-sheared counterpart. The suitability of this shear aggregated Fn material for CNS repair purposes was assessed in vitro where it supported the growth of fibroblasts, S100 immunoreactive Schwann cells and GFAP immunoreactive astrocytes. Implantation of the shear aggregated Fn material into a rat model of spinal cord injury provided a permissive environment for axonal growth. This was extended using an impermeable coating to improve orientation and straightness of axonal growth.

Local Effects on Triiodothyronine-Treated Polyglactin Sutures on Regeneration across Peripheral Nerve Defects

We have previously described a new and simple method for nerve repair in which continuous longitudinal polyglactin sutures alone are used to bridge limited nerve defects in rats. Here we examined whether such sutures could be used to deliver a growth-promoting substance, triiodothyronine (T(3)), and enhance regeneration of the rat sciatic nerve. Sutures were pretreated in highly concentrated solutions of T(3) for 24 h. In vitro measurements showed that such sutures released T(3) with an initial rapid phase followed by a slow-release phase lasting at least 3 weeks. Bilateral sciatic nerve defects (7 mm) in rats were bridged by either T(3)- or saline-incubated sutures. Immunocytochemistry for Schwann cells and axons at 2 weeks showed no differences in Schwann cell distribution or axonal outgrowth length. Morphometric analysis 4 and 12 weeks after the repair revealed a slight but significant (p < 0.05) increase in the mean myelin area in T(3)-treated nerves. No differences were seen in the number of axons or return of force in the gastrocnemius muscle at 12 weeks. The results show that sutures can be used both for the bridging of defects in peripheral nerves and for the delivery of a growth-promoting substance to regenerating nerve structures.

A composite poly-hydroxybutyrate-glial growth factor conduit for long nerve gap repairs

There is considerable evidence that peripheral nerves have the potential to regenerate in an appropriate microenvironment. We have developed a novel artificial nerve guide composed of poly 3-hydroxybutyrate (PHB) filled with glial growth factor (GGF) suspended in alginate hydrogel. Gaps of 2-4 cm in rabbit common peroneal nerve were bridged using a PHB conduit containing either GGF in alginate hydrogel (GGF) or alginate alone (Alginate), or with an empty PHB conduit (Empty). Tissues were harvested 21, 42 and 63 days post-operatively. Schwann cell and axonal regeneration were assessed using quantitative immunohistochemistry. At 21 days, addition of GGF increased significantly the distance of axonal and Schwann cells regeneration in comparison with that observed in Alginate and Empty conduits for both gap lengths. The axons bridged the 2-cm GGF conduits gap by 63 days, with a comparable rate of regeneration seen in 4-cm conduits. Schwann cells and axonal regeneration quantity was similar for both gap lengths in each group. However, at all time points the quantity of axonal and Schwann cells regeneration in GGF grafts was significantly greater than in both Alginate and Empty conduits, the latter showing better regeneration than Alginate conduits. The results indicate an inhibitory effect of alginate on regeneration, which is partially reversed by the addition of GGF to the conduits. In conclusion, GGF stimulates a progressive and sustainable regeneration increase in long nerve gap conduits.

Neural tissue engineering: strategies for repair and regeneration

Nerve regeneration is a complex biological phenomenon. In the peripheral nervous system, nerves can regenerate on their own if injuries are small. Larger injuries must be surgically treated, typically with nerve grafts harvested from elsewhere in the body. Spinal cord injury is more complicated, as there are factors in the body that inhibit repair. Unfortunately, a solution to completely repair spinal cord injury has not been found. Thus, bioengineering strategies for the peripheral nervous system are focused on alternatives to the nerve graft, whereas efforts for spinal cord injury are focused on creating a permissive environment for regeneration. Fortunately, recent advances in neuroscience, cell culture, genetic techniques, and biomaterials provide optimism for new treatments for nerve injuries. This article reviews the nervous system physiology, the factors that are critical for nerve repair, and the current approaches that are being explored to aid peripheral nerve regeneration and spinal cord repair.

The effects of treatment with antibodies to transforming growth factor β1 and β2 following spinal cord damage in the adult rat

We recently showed axonal ingrowth into fibronectin (FN) mats implanted into the spinal cord. However, little axonal growth was found from FN mats into intact spinal cord. Previous research has shown that this is due in part to astrocytosis around an area of CNS damage. Antibodies to transforming growth factor beta (TGFbeta) can diminish this astrocytosis. TGFbeta also has effects on macrophages and Schwann cells, both of which infiltrate the spinal cord following damage. We examined the axonal, Schwann cell, and macrophage infiltration into FN mats as well as the level of astrocytosis and chondroitin sulfate proteoglycan NG2 around FN implants incubated in TGFbeta antibodies and implanted into a lesion cavity in the spinal cord. We also examined the effects of applying TGFbeta antibodies to a spinal cord hemisection site. Anti-TGFbeta1 within FN mats resulted in extensive cavitation, with the area of damage being larger than the original lesion. Cavitation was also seen following application of anti-TGFbeta1 to a spinal cord hemisection site. No cavitation was seen following saline, non-immune IgG or anti-TGFbeta2 treatment. However, anti-TGFbeta2 treatment did result in diminished axonal growth and Schwann cell and macrophage infiltration. Around the implant site, anti-TGFbeta2 treatment resulted in a reduction in the level of astrocytosis but had not effect on levels of NG2. Similar effects were seen following anti-TGFbeta2 application to spinal cord hemisection sites. The results suggest that anti-TGFbeta1 exacerbates secondary damage by preventing the anti-inflammatory effect of endogenous TGFbeta1. Anti-TGFbeta2 did not enhance axonal regeneration in this model but did slightly reduce astrocytosis.

Effect of NT-4 and BDNF delivery to damaged sciatic nerves on phenotypic recovery of fast and slow muscles fibres

We investigated whether neurotrophin-4 (NT-4) and brain-derived neurotrophic factor (BDNF) affected the reinnervation of slow and fast motor units. Neurotrophin-impregnated or plain fibronectin (FN) conduits were inserted into a sciatic nerve gap. Fast extensor digitorum longus (EDL) and slow soleus muscles were collected 4 months postsurgery. Muscles were weighed and fibre type proportion and mean fibre diameters were derived from muscle cross-sections. All fibre types in muscles from FN animals were severely atrophied and this correlated well with type 1 fibre loss and atrophy in soleus and type 2b loss and atrophy in EDL. Treatment with NT-4 reversed soleus but not EDL mass loss above the FN group by significantly restoring type 1 muscle fibre proportion and diameters towards those of normal unoperated animals. BDNF did not increase muscle mass but did have minor effects on fibre type and diameter. Thus, NT-4 significantly improved slow motor unit recovery, and provides a basis for therapies intended to aid the functional recovery of muscles after denervating injury.

Approaches to tissue engineered peripheral nerve

The late 1980s and early 1990s brought excitement to the idea that we would be able to replace body tissues and organs through the field of tissue engineering. This enthusiasm was soon replaced by the realization of the limitations in our knowledge for specific tissue types and replication efforts. Such is the case with nerve tissue. We have progressed in this field of knowledge; however, full elucidation to the complex interactions of nerve repair falls short.

Neurotrophin-3 prevents the proximal accumulation of neurofilament proteins in sensory neurons of streptozocin-induced diabetic rats

The relation between neurofilament expression and/or phosphorylation in the proximal versus distal components of the sensory peripheral neuraxis was studied and related to disorders in structure and function of the distal axon of streptozocin (STZ)-induced diabetic rats studied for 14 weeks. The ability of neurotrophin-3 (NT-3) to prevent abnormalities in neurofilament biology was also investigated. Compared with age-matched controls, neurofilament heavy (NF-H) (3.3-fold) and neurofilament medium (NF-M) (2.5-fold), but not neurofilament light (NF-L), subunits accumulated in the proximal axon of sensory neurons of the lumbar dorsal root ganglia (DRG) in untreated diabetic rats. Neurofilament accumulation was prevented by NT-3. Small- and large-diameter sensory neurons exhibited elevated levels of NF-H protein accumulation and phosphorylation in the DRG of untreated diabetic rats, levels that were ameliorated by NT-3. The sural nerve of untreated diabetic rats showed a 50% decrease in the levels of NF-H and NF-M, but not NF-L, subunits; NT-3 only partially normalized the defect in NF-M expression. These observations were associated with significant lowering of motor and sensory nerve conduction velocity but no alteration in the mean axonal diameter of myelinated axons in the sural nerve in untreated diabetic rats. It is proposed that the accumulation of NF-H and NF-M subunits in the proximal axon is an etiologic factor in the distal axon degeneration observed in diabetes.

Growth factor enhancement of peripheral nerve regeneration through a novel synthetic hydrogel tube

OBJECT

The authors' long-term goal is repair of peripheral nerve injuries by using synthetic nerve guidance devices that improve both regeneration and functional outcome relative to an autograft. They report the in vitro processing and in vivo application of synthetic hydrogel tubes that are filled with collagen gel impregnated with growth factors.

METHODS

Poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (PHEMA-MMA) porous 12-mm-long tubes with an inner diameter of 1.3 mm and an outer diameter of 1.8 mm were used to repair surgically created 10-mm gaps in the rat sciatic nerve. The inner lumen of the tubes was filled with collagen matrix alone or matrix supplemented with either neurotropin-3 at 1 microg/ml, brain-derived neurotrophic factor at 1 microg/ml, or acidic fibroblast growth factor (FGF-1) at 1 or 10 microg/ml. Nerve regeneration through the growth factor-enhanced tubes was assessed at 8 weeks after repair by histomorphometric analysis at the midgraft level and in the nerve distal to the tube repair. The tubes were biostable and biocompatible, and supported nerve regeneration in more than 90% of cases. Nerve regeneration was improved in tubes in which growth factors were added, compared with empty tubes and those containing collagen gel alone (negative controls). Tubes filled with 10 microg/ml of FGF-1 dispersed in collagen demonstrated regeneration comparable to autografts (positive controls) and showed significantly better regeneration than the other groups.

CONCLUSIONS

The PHEMA-MMA tubes augmented with FGF-1 in their lumens appear to be a promising alternative to autografts for repair of nerve injuries. Studies are in progress to assess the long-term biocompatibility of these implants and to enhance regeneration further.

Mats made from fibronectin support oriented growth of axons in the damaged spinal cord of the adult rat

A variety of biological as well as synthetic implants have been used to attempt to promote regeneration into the damaged spinal cord. We have implanted mats made from fibronectin (FN) into the damaged spinal cord to determine their effectiveness as a substrate for regeneration of axons. These mats contain oriented pores and can take up and release growth factors. Lesion cavities 1 mm in width and depth and 2 mm in length were created on one side of the spinal cord of adult rats. FN mats containing neurotrophins or saline were placed into the lesion. Mats were well integrated into surrounding tissue and showed robust well-oriented growth of calcitonin gene-related peptide, substance P, GABAergic, cholinergic, glutamatergic, and noradrenergic axons into FN mats. Transganglionic tracing using cholera toxin B indicated large-diameter primary afferents had grown into FN implants. Schwann cells had also infiltrated FN mats. Electron microscopy confirmed the presence of axons within implants sites, with most axons either ensheathed or myelinated by Schwann cells. Mats incubated in brain-derived neurotrophic factor and neurotrophin-3 showed significantly more neurofilament-positive and glutamatergic fibers compared to saline- and nerve growth factor-incubated mats, while mats incubated with nerve growth factor showed more calcitonin gene-related peptide-positive axons. In contrast, neurotrophin treatment had no effect on PGP 9.5-positive axons. In addition, in some animals with neurotrophin-3-incubated mats, cholera toxin B-labelled fibers had grown from the mat into adjoining intact areas of spinal cord. The results indicate that FN mats provide a substrate that is permissive for robust oriented axonal growth in the damaged spinal cord, and that this growth is supported by Schwann cells.

Nerve compound action current (NCAC) measurements and morphometric analysis in the proximal segment after nerve transection and repair in a rabbit model

In the evaluation of nerve regeneration using magneto-neurography (MNG), the proximal segment showed a reproducible decrease in peak-peak amplitude of the nerve compound action current's (NCAC) of 60%. To explain these changes, morphometry of myelinated axons in the proximal segment is compared to the MNG signals. A standardised nerve transection and reconstruction was performed in rabbits. NCACs were measured approximately 5 cm proximal to the lesion from operated and control nerves after 12 weeks. Histological samples were taken from the same area of the nerve where the NCACs were obtained. Results showed a decrease of the peak-peak amplitude of the NCAC of 57% compared to the control. Conduction velocity decreased 15% (not significant). Morphometry elicited a decrease in larger (10-15 microm) axons (284 +/- 134 vs 82 +/- 55) and an increase in smaller (2-5 microm) axons (1445 +/- 360 vs 1921 +/- 393). A strong correlation existed between the decrease in amplitude and the decrease in larger axons (0.85). Peak-peak amplitude varies approximately with the square of the diameter axon. Therefore, because peak-peak amplitude is mainly dependent on the larger-diameter axons, the decrease in peak-peak amplitude of the NCACs may be explained by a decrease in numbers of 10-15-microm axons.

Repair of peripheral nerve transections with fibrin sealant containing neurotrophic factors

Peripheral nerve injury is often followed by incomplete recovery of function and sometimes associated with neuropathic pain. There is, therefore, need for therapies which improve the speed of recovery and the final functional outcome after peripheral nerve injuries. In addition, neuropathic pain is not easily dealt with clinically and should preferably be eliminated. Neurotrophic factors have well-documented abilities to support neuron survival and stimulate neurite outgrowth, making them excellent candidates for use in repairing injured nerves. We investigated the possible beneficial effects of repairing the transected rat sciatic nerve by local application of a fibrin sealant containing nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), or acidic fibroblast growth factor (aFGF). Fibrin sealant was used in conjunction with sutures. Evaluation of motor and sensory function, autotomy, and histological parameters was carried out from 1 to 12 weeks after injury. We demonstrate that NGF cotreatment decreased the occurance of autotomy, suggesting a reduction of neuropathic pain, and improved the performance in motor and sensory tests. In addition, the number of regenerating motoneurons was significantly increased after NGF administration. GDNF increased the speed of sensory recovery, but also markedly increased autotomy, indicating an increased degree of neuropathic pain. aFGF did not alter the outcome of the motor or sensory tests. Fibrin sealant could easily be used in conjunction with sutures to deliver neurotrophic substances locally to the damaged nerve and to enhance recovery of nerve function.

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.

Addition of fibronectin to alginate matrix improves peripheral nerve regeneration in tissue-engineered conduits

Schwann cell (SC) transplantation has been proposed to encourage peripheral nerve regeneration, but an optimal SC-carrying matrix would be needed. The aim of this study was to characterize how the addition of fibronectin to alginate would affect the outcome of nerve regeneration promoted by Schwann cells embedded in this matrix. Genetically labeled rat SCs were obtained by lacZ gene transduction. SCs were suspended in alginate hydrogel matrix with/without addition of liquid fibronectin, and their viability and growth in the different types of matrices were assessed in vitro by AlamarBlue assay. In vivo assessment of SC transplantation in the matrix was carried out with poly-3-hydroxybutyrate (PHB) conduits to bridge a sciatic nerve gap. The grafted conduits were harvested at 2, 3, and 6 weeks and assessed for the presence of labeled SCs in relation to regrowing axons. The amount and rate of axonal regeneration were assessed by quantitative immunohistochemistry. Addition of fibronectin to alginate hydrogel improved SC viability and growth profile in vitro. X-Gal staining confirmed that SCs transplanted in PHB conduits were viable throughout the time course, and that the labeled SCs were clearly associated with regenerating axons. The regeneration rate was enhanced when liquid fibronectin was added to the alginate matrix. Furthermore, the presence of SCs also enhanced regeneration and there was an additive effect when both SCs and fibronectin were combined with alginate. In conclusion, the addition of fibronectin to alginate hydrogel matrix contributed to improve nerve regeneration, supporting SC viability and augmenting their effect on axonal growth when transplanted in a bioengineered nerve conduit.

Nerve guide material made from fibronectin: assessment of in vitro properties

We have previously used orientated mats of fibronectin as conduits to repair short gaps in peripheral nerves. Here we describe the in vitro properties of a new material in the form of large cables produced from a fibronectin-enriched solution with potential as a conduit for longer nerve defects. Large cables of fibronectin were made up to 14 cm long x 1.5 cm in diameter. When freeze dried, scanning electron microscopy revealed a predominant fiber orientation. Dried cables hydrated rapidly to 1.6 and 4.8 times their original length and diameter, respectively. Once hydrated these cables had pores that ranged from 10 to 100 microm through which Schwann cells and fibroblasts were able to grow in vitro and align with the axis of the fibrils by contact guidance. Furthermore, the porosity of the cable was enhanced by the natural dissolution of protein over a 3-week duration in culture with cells, such that 50- to 200-microm pores were observed. This study suggests that large fibronectin cables are a suitable alternative to the original fibronectin mats to guide components of the peripheral nerves and so to act as conduits with potential use in guiding regeneration across long nerve defects.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

An injectable nerve regeneration chamber for studies of unstable soluble growth factors

Modern surgical techniques cannot guarantee functional recovery following peripheral nerve injuries. Research into factors that may influence nerve regeneration has therefore assumed a prominent potential therapeutic role. We report here on the development of an approach to allow for direct manipulation of the microenvironment of regenerating peripheral nerve axons. We show that solutions can be delivered directly to this local milieu in vivo and that such a delivery can be performed multiple times over an extended period, potentially facilitating studies of multiple molecular players that act locally. We also demonstrate that the bundle of regenerated axons are amenable to morphological analysis by 21 days and that the injection system remains patent for at least 21 days.

Glial cell line-derived neurotrophic factor released by synthetic guidance channels promotes facial nerve regeneration in the rat

Regeneration of the human facial nerve after lesion is often limited, leading to severe functional impairments, in particular when repair is delayed for several months, when cross-facial nerve grafts have to be performed, or in elderly patients. To improve the outcome, the potential accelerating and maturating effects of the neurotrophic factors glial cell line-derived neurotrophic factor (GDNF) and neurotrophin-3 (NT-3) on nerve regeneration were assessed using an axotomy model of the rat facial nerve. One-centimeter-long synthetic guidance channels releasing the neurotrophic factors over several weeks were used to bridge an 8 mm nerve gap, a distance that does not allow regeneration in the absence of growth factors. Nerve cables regenerated in the presence of GDNF showed a large number of myelinated axons 6 weeks after grafting (871 +/- 373, n = 5), whereas only 106 +/- 86 (n = 5) myelinated axons were counted in the presence of NT-3. Retrograde labeling with fluorogold revealed 981 +/- 450 (n = 5) and 53 +/- 38 (n = 5) retrogradely labeled motoneurons in the facial nucleus in the presence of GDNF and NT-3, respectively. No regenerated axons or retrogradely labeled cells were observed in the absence of growth factors (n = 6). These results demonstrate that GDNF, as previously described for the sciatic nerve, a mixed sensory and motor nerve, is also very efficient in promoting regeneration of the facial nerve, an essentially pure motor nerve. GDNF may therefore be useful in improving facial nerve regeneration in the clinic.

A novel biodegradable implant for neuronal rescue and regeneration after spinal cord injury

After spinal cord injury, the severed neuronal pathways fail to regenerate spontaneously. This study describes a biodegradable implant using poly-beta-hydroxybutyrate (PHB) fibers as carrier scaffold for matrix components and cell lines supporting neuronal survival and regeneration after spinal cord injury. After cervical spinal cord injury in adult rats, a graft consisting of PHB fibers coated with alginate hydrogel + fibronectin was implanted in the lesion cavity. In control groups, PHB was omitted and only alginate hydrogel or fibronectin, or their combination, were used for grafting. In addition, comparisons were made with animals treated intrathecally after spinal cord injury with the neurotrophic factors BDNF or NT-3. The neurons of the rubrospinal tract served as experimental model. In untreated animals, 45% of the injured rubrospinal neurons were lost at 8 weeks postoperatively. Implantation of the PHB graft reduced this cell loss by 50%, a rescuing effect similar to that obtained after treatment with BDNF or NT-3. In the absence of PHB support, implants of only alginate hydrogel or fibronectin, or their combination, had no effect on neuronal survival. After addition of neonatal Schwann cells to the PHB graft, regenerating axons were seen to enter the graft from both ends and to extend along its entire length. These results show that implants using PHB as carrier scaffold and containing alginate hydrogel, fibronectin and Schwann cells can support neuronal survival and regeneration after spinal cord injury

Building a bridge: engineering spinal cord repair

Injuries to the spinal cord that result in disruption of axonal continuity have devastating consequences for injured patients. Current therapies that use biologically active agents to promote neuronal survival and/or growth have had modest success in allowing injured neurons to regrow through the area of the lesion. Strategies for successful regeneration will require an engineering approach. We propose the design of cell-free grafts of biocompatible materials to build a bridge across the injured area through which axons can regenerate. There are three critical regions of this bridge: the on-ramp, the surface of the bridge itself, and the off-ramp. Each of these regions has specific design requirements, which, if met, can promote regeneration of axons in the injured spinal cord. These requirements, and proposed solutions, are discussed.