Bone-nerve crosstalk: a new state for neuralizing bone tissue engineering-A mini review

Neuro bone tissue engineering is a multidisciplinary field that combines both principles of neurobiology and bone tissue engineering to develop innovative strategies for repairing and regenerating injured bone tissues. Despite the fact that regeneration and development are considered two distinct biological processes, yet regeneration can be considered the reactivation of development in later life stages to restore missing tissues. It is noteworthy that the regeneration capabilities are distinct and vary from one organism to another (teleost fishes, hydra, humans), or even in the same organism can vary dependent on the injured tissue itself (Human central nervous system vs. peripheral nervous system). The skeletal tissue is highly innervated, peripheral nervous system plays a role in conveying the signals and connecting the central nervous system with the peripheral organs, moreover it has been shown that they play an important role in tissue regeneration. Their regeneration role is conveyed by the different cells' resident in it and in its endoneurium (fibroblasts, microphages, vasculature associated cells, and Schwann cells) these cells secrete various growth factors (NGF, BDNF, GDNF, NT-3, and bFGF) that contribute to the regenerative phenotype. The peripheral nervous system and central nervous system synchronize together in regulating bone homeostasis and regeneration through neurogenic factors and neural circuits. Receptors of important central nervous system peptides such as Serotonin, Leptin, Semaphorins, and BDNF are expressed in bone tissue playing a role in bone homeostasis, metabolism and regeneration. This review will highlight the crosstalk between peripheral nerves and bone in the developmental stages as well as in regeneration and different neuro-bone tissue engineering strategies for repairing severe bone injuries.

Application of neurotrophic and proangiogenic factors as therapy after peripheral nervous system injury

The intrinsic ability of peripheral nerves to regenerate after injury is extremely limited, especially in case of severe injury. This often leads to poor motor function and permanent disability. Existing approaches for the treatment of injured nerves do not provide appropriate conditions to support survival and growth of nerve cells. This drawback can be compensated by the use of gene therapy and cell therapy-based drugs that locally provide an increase in the key regulators of nerve growth, including neurotrophic factors and extracellular matrix proteins. Each growth factor plays its own specific angiotrophic or neurotrophic role. Currently, growth factors are widely studied as accelerators of nerve regeneration. Particularly noteworthy is synergy between various growth factors, that is essential for both angiogenesis and neurogenesis. Fibroblast growth factor 2 and vascular endothelial growth factor are widely known for their proangiogenic effects. At the same time, fibroblast growth factor 2 and vascular endothelial growth factor stimulate neural cell growth and play an important role in neurodegenerative diseases of the peripheral nervous system. Taken together, their neurotrophic and angiogenic properties have positive effect on the regeneration process. In this review we provide an in-depth overview of the role of fibroblast growth factor 2 and vascular endothelial growth factor in the regeneration of peripheral nerves, thus demonstrating their neurotherapeutic efficacy in improving neuron survival in the peripheral nervous system.

Brief electrical nerve stimulation enhances intrinsic repair capacity of the focally demyelinated central nervous system

Our lab has shown that brief electrical nerve stimulation (ES) has a dramatic impact on remyelination of lysophosphatidyl choline (LPC)-induced focally demyelinated rat peripheral nerves, while also inducing an axon-protective phenotype and shifting macrophages from a predominantly pro-inflammatory toward a pro-repair phenotype. Whether this same potential exists in the central nervous system is not known. Thus, for proof of principle studies, the peripheral nerve demyelination and ES model was adapted to the central nervous system, whereby a unilateral focal LPC-induced demyelination of the dorsal column at the lumbar enlargement where the sciatic nerve afferents enter was created, so that subsequent ipsilateral sciatic nerve ES results in increased neural activity in the demyelinated axons. Data reveal a robust focal demyelination at 7 days post-LPC injection. Delivery of 1-hour ES at 7 days post-LPC polarizes macrophages/microglia toward a pro-repair phenotype when examined at 14 days post-LPC; results in smaller LPC-associated regions of inflammation compared to non-stimulated controls; results in significantly more cells of the oligodendroglial lineage in the demyelinated region; elevates myelin basic protein levels; and shifts the paranodal protein Caspr along demyelinated axons to a more restricted distribution, consistent with reformation of the paranodes of the nodes of Ranvier. ES also significantly enhanced levels of phosphorylated neurofilaments detected in the zones of demyelination, which has been shown to confer axon protection. Collectively these findings support that strategies that increase neural activity, such as brief electrical stimulation, can be beneficial for promoting intrinsic repair following focal demyelinating insults in demyelinating diseases such as multiple sclerosis. All animal procedures performed were approved by the University of Saskatchewan's Animal Research Ethics Board (protocol# 20090087; last approval date: November 5, 2020).

Elucidating the Pivotal Neuroimmunomodulation of Stem Cells in Spinal Cord Injury Repair

Spinal cord injury (SCI) is a distressing incident with abrupt onset of the motor as well as sensory dysfunction, and most often, the injury occurs as result of high-energy or velocity accidents as well as contact sports and falls in the elderly. The key challenges associated with nerve repair are the lack of self-repair as well as neurotrophic factors and primary and secondary neuronal apoptosis, as well as factors that prevent the regeneration of axons locally. Neurons that survive the initial traumatic damage may be lost due to pathogenic activities like neuroinflammation and apoptosis. Implanted stem cells are capable of differentiating into neural cells that replace injured cells as well as offer local neurotrophic factors that aid neuroprotection, immunomodulation, axonal sprouting, axonal regeneration, and remyelination. At the microenvironment of SCI, stem cells are capable of producing growth factors like brain-derived neurotrophic factor and nerve growth factor which triggers neuronal survival as well as axonal regrowth. Although stem cells have proven to be of therapeutic value in SCI, the major disadvantage of some of the cell types is the risk for tumorigenicity due to the contamination of undifferentiated cells prior to transplantation. Local administration of stem cells via either direct cellular injection into the spinal cord parenchyma or intrathecal administration into the subarachnoid space is currently the best transplantation modality for stem cells during SCI.

Biomaterials for Neural Tissue Engineering

The therapy of neural nerve injuries that involve the disruption of axonal pathways or axonal tracts has taken a new dimension with the development of tissue engineering techniques. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. To improve the limitations of purely cell-based therapies, the neural tissue engineering philosophy has emerged. Efforts are being made to produce an ideal scaffold based on synthetic and natural polymers that match the exact biological and mechanical properties of the tissue. Furthermore, through combining several components (biomaterials, cells, molecules), axonal regrowth is facilitated to obtain a functional recovery of the neural nerve diseases. The main objective of this review is to investigate the recent approaches and applications of neural tissue engineering approaches.

Comparison of the Efficacy of Optogenetic Stimulation of Glia versus Neurons in Myelination

Increasing evidence demonstrates that optogenetics contributes to the regulation of brain behavior, cognition, and physiology, particularly during myelination, potentially allowing for the bidirectional modulation of specific cell lines with spatiotemporal accuracy. However, the type of cell to be targeted, namely, glia vs neurons, and the degree to which optogenetically induced cell activity can regulate myelination during the development of the peripheral nervous system (PNS) are still underexplored. Herein, we report the comparison of optogenetic stimulation (OS) of Schwann cells (SCs) and motor neurons (MNs) for activation of myelination in the PNS. Capitalizing on these optogenetic tools, we confirmed that the formation of the myelin sheath was initially promoted more by OS of calcium translocating channelrhodopsin (CatCh)-transfected SCs than by OS of transfected MNs at 7 days in vitro (DIV). Additionally, the level of myelination was substantially enhanced even until 14 DIV. Surprisingly, after OS of SCs, > 91.1% ± 5.9% of cells expressed myelin basic protein, while that of MNs was 67.8% ± 6.1%. The potent effect of OS of SCs was revealed by the increased thickness of the myelin sheath at 14 DIV. Thus, the OS of SCs could highly accelerate myelination, while the OS of MNs only somewhat promoted myelination, indicating a clear direction for the optogenetic application of unique cell types for initiating and promoting myelination. Together, our findings support the importance of precise cell type selection for use in optogenetics, which in turn can be broadly applied to overcome the limitations of optogenetics after injury.

Gene Therapy Options as New Treatment for Inherited Peripheral Neuropathy

Inherited peripheral neuropathy (IPN) is caused by heterogeneous genetic mutations in more than 100 genes. So far, several treatment options for IPN have been developed and clinically evaluated using small molecules. However, gene therapy-based therapeutic strategies have not been aggressively investigated, likely due to the complexities of inheritance in IPN. Indeed, because the majority of the causative mutations of IPN lead to gain-of-function rather than loss-of-function, developing a therapeutic strategy is more difficult, especially considering gene therapy for genetic diseases began with the simple idea of replacing a defective gene with a functional copy. Recent advances in gene manipulation technology have brought novel approaches to gene therapy and its clinical application for IPN treatment. For example, in addition to the classically used gene replacement for mutant genes in recessively inherited IPN, other techniques including gene addition to modify the disease phenotype, modulations of target gene expression, and techniques to edit mutant genes have been developed and evaluated as potent therapeutic strategies for dominantly inherited IPN. In this review, the current status of gene therapy for IPN and future perspectives will be discussed.

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.

ERK1/2 and AKT are vital factors in regulation of the migration of rat Schwann cells

After peripheral nerve injury, Wallerian degeneration (WD) occurs in the distal nerve segment. During the process of degeneration, Schwann cells (SCs) dedifferentiate, proliferate and migrate to align in “bands of Büngner”, providing structural guidance and growth-promoting substrates to regenerating axons. The molecular signals that trigger SCs migration remain unclear. Here, we explored the molecular characteristics of the migration of cultured SCs prepared from rat sciatic nerves that had degenerated. The results revealed that elevated p-ERK1/2 was coupled with the migration of SCs, activated either by nerve degeneration or the addition of placenta growth factor. However, the inhibition of ERK1/2 activity, which activated the PI3K pathway, did not show a significant negative effect on SC migratory potential. Combined inhibition of ERK1/2 and AKT activity resulted in a significant decrease in SCs motility. These molecular characteristics suggest that both the ERK1/2 and AKT signals are involved in the migratory potential of SCs. It may be helpful to understand the process of nerve regeneration and perspective on promoting peripheral nerve regeneration.

Dose-dependent facilitation of peripheral nerve regeneration by bone marrow-derived mononuclear cells: a randomized controlled study: laboratory investigation

OBJECT

Bone marrow-derived stem cells enhance the rate of regeneration of neuronal cells leading to clinical improvement in nerve injury, spinal cord injury, and brain infarction. Recent experiments in the local application of bone marrow-derived mononuclear cells (BM-MNCs) in models of sciatic nerve transection in rats have suggested their beneficial role in nerve regeneration, although the effects of variable doses of stem cells on peripheral nerve regeneration have never been specifically evaluated in the literature. In this paper, the authors evaluated the dose-dependent role of BM-MNCs in peripheral nerve regeneration in a model of sciatic nerve transection in rats.

METHODS

The right sciatic nerve of 60 adult female Wistar rats (randomized into 2 test groups and 1 control group, 20 rats in each group) underwent transection under an operating microscope. The cut ends of the nerve were approximated using 2 epineural microsutures. The gap was filled with low-dose (5 million BM-MNCs/100 μl phosphate-buffered saline [PBS]) rat BM-MNCs in one group, high-dose (10 million BM-MNCs/100 μl PBS) rat BM-MNCs in another group, and only PBS in the control group, and the approximated nerve ends were sealed using fibrin glue. Histological assessment was performed after 30 days by using semiquantitative and morphometric analyses and was done to assess axonal regeneration, percentage of myelinated fibers, axonal diameter, fiber diameter, and myelin thickness at distal-most sites (10 mm from site of repair), intermediate distal sites (5 mm distal to the repair site), and site of repair.

RESULTS

The recovery of nerve cell architecture after nerve anastomosis was far better in the high-dose BM-MNC group than in the low-dose BM-MNC and control groups, and it was most evident (p < 0.02 in the majority of the parameters [3 of 4]) at the distal-most site. Overall, the improvement in myelin thickness was most significant with incremental dosage of BM-MNCs, and was evident at the repair, intermediate distal, and distal-most sites (p = 0.001).

CONCLUSIONS

This study emphasizes the role of BM-MNCs, which can be isolated easily from bone marrow aspirates, in peripheral nerve injury and highlights their dose-dependent facilitation of nerve regeneration.

Dual simulated childbirth injury delays anatomic recovery

A dual childbirth injury model, including vaginal distension (VD) and pudendal nerve crush (PNC), may best represent the injuries seen clinically. The objective of this study was to investigate urethral function, anatomy, and neurotrophin expression after several simulated childbirth injuries. Groups of 140 rats underwent PNC, VD, PNC+VD, or neither (C). Four days after injury, all injury groups had significantly decreased leak-point pressure (LPP) compared with C rats. Ten days after injury, LPP in PNC and PNC+VD rats remained significantly lower than C rats. Three weeks after injury, LPP in all injury groups had recovered to C values. Histological evidence of injury was still evident in the external urethral sphincter (EUS) after VD and PNC+VD 10 days after injury. Three weeks after injury, the EUS of PNC+VD rats remained disrupted. One day after VD, brain-derived neurotrophic factor (BDNF) expression in the EUS was reduced, while neurotrophin-4 (NT-4) and nerve growth factor (NGF) expression was unchanged. BDNF, NT-4, and NGF expression was dramatically upregulated in the EUS after PNC. After PNC+VD, NGF expression was upregulated, and BDNF and NT-4 expression was upregulated somewhat but not to the same extent as after PNC. Ten days after injury, PNC+VD had the least number of normal nerve fascicles near the EUS, followed by PNC and VD. Twenty-one days after injury, all injury groups had fewer normal nerve fascicles, but without significant differences compared with C rats. PNC+VD therefore provides a more severe injury than PNC or VD alone.

Effect of progesterone on recovery from nerve injury during leg lengthening in rats

We investigated the effect of progesterone on the nerve during lengthening of the limb in rats. The sciatic nerves of rats were elongated by leg lengthening for ten days at 3 mm per day. On alternate days between the day after the operation and nerve dissection, the progesterone-treated group received subcutaneous injections of 1 mg progesterone in sesame oil and the control group received oil only. On the fifth, tenth and 17th day, the sciatic nerves were excised at the midpoint of the femur and the mRNA expression level of myelin protein P0 was analysed by quantitative real time polymerase chain reaction. On day 52 nodal length was examined by electron microscopy, followed by an examination of the compound muscle action potential (C-MAP) amplitude and the motor conduction velocity (MCV) of the tibial nerve on days 17 and 52. The P0 (a major myelin glycoprotein) mRNA expression level in the progesterone-treated group increased by 46.6% and 38.7% on days five and ten, respectively. On day 52, the nodal length in the progesterone-treated group was smaller than that in the control group, and the MCV of the progesterone-treated group had been restored to normal. Progesterone might accelerate the restoration of demyelination caused by nerve elongation by activating myelin synthesis.

Peripheral regeneration

Whereas the central nervous system (CNS) usually cannot regenerate, peripheral nerves regenerate spontaneously after injury because of a permissive environment and activation of the intrinsic growth capacity of neurons. Functional regeneration requires axon regrowth and remyelination of the regenerated axons by Schwann cells. Multiple factors including neurotrophic factors, extracellular matrix (ECM) proteins, and hormones participate in Schwann cell dedifferentiation, proliferation, and remyelination. We describe the current understanding of peripheral axon regeneration and focus on the molecules and potential mechanisms involved in remyelination.

Why Is Wallerian Degeneration in the CNS So Slow?

Wallerian degeneration (WD) is the set of molecular and cellular events by which degenerating axons and myelin are cleared after injury. Why WD is rapid and robust in the PNS but slow and incomplete in the CNS is a longstanding mystery. Here we review current work on the mechanisms of WD with an emphasis on deciphering this mystery and on understanding whether slow WD in the CNS could account for the failure of CNS axons to regenerate.

BDNF contributes to animal model neuropathic pain after peripheral nerve transection

The outcome of peripheral nerve injury is often impaired by neuropathic pain, which is resistant to most analgesics and presents a serious clinical problem. The mechanisms underlying post-traumatic neuropathic pain remain unclear, but they are likely associated with the regeneration processes. Brain-derived neurotrophic factor (BDNF) is known to enhance peripheral nerve regeneration and is also considered to be an endogenous modulator of nociceptive responses following spinal cord lesion. The aim of this work was to examine the local effect of BDNF in a neuropathic pain model. Sciatic nerves of adult male rats were transected and supplied with connective tissue chambers filled with (1) fibrin only, (2) fibrin with BDNF, or (3) fibrin with antibodies against BDNF. In control animals the nerve was transected and no chamber was applied. During follow-up, autotomy behavior was assessed. Seven weeks after the operation, the number of surviving and regenerating neurons in dorsal root ganglia was counted and the neuroma incidence was examined. We found that local inactivation of BDNF decreased the incidence as well as severity of autotomy and neuroma formation, but did not influence neuron regeneration into the chambers. These results indicate that BDNF plays a locally crucial role in neuropathic pain development after peripheral nerve 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.

The quest for remyelination: a new role for neurotrophins and their receptors

The formation of myelin is dependent on a reciprocal and intimate relationship between neurons and the myelin-forming glia. Recently, the neurotrophin family of growth factors has been shown to regulate the complex cell-cell interactions that control myelination. Neurotrophins and their receptors influence myelin formation via two distinct mechanisms, either by acting on the neurons, changing the axonal signals that control myelination, or by acting directly on the myelin-forming glia. In this review, we will discuss research highlighting the ability of neurotrophins to both promote and inhibit the myelination process. As reflected in the work presented here, these effects are dependent on a delicate balance of which neurotrophins are expressed, and what receptors are activated. Additionally, we examine an emerging model in which the growth factors that promote the early survival and differentiation of particular sets of neurons later play important roles as key regulators in glial development. Characterizing the temporal expression and the cellular targets of neurotrophins, both during development and after injury, represents a pivotal step in developing a greater understanding of the myelination process, contributing to the development of effective treatments for demyelinating conditions. We conclude this review by discussing the potential for neurotrophins as therapeutics in the quest for remyelination.

Activation of MAPK ERK in peripheral nerve after injury

Background

Activation of extracellular signal-regulated protein kinase (ERK), a member of mitogen-activated protein kinase (MAPK) family, has been proposed to mediate neurite outgrowth-promoting effects of several neurotrophic factors in vitro. However, the precise activity of ERK during axonal regeneration in vivo remains unclear. Peripheral axotomy has been shown to activate ERK in the cell bodies of primary afferent neurons and associated satellite cells. Nevertheless, whether ERK is also activated in the axons and surrounded Schwann cells which also play a key role in the regeneration process has not been clarified.

Results

Phosphorylation of ERK in the sciatic nerve in several time-points after crush injury has been examined. Higher phosphorylation of ERK was observed in the proximal and distal nerve stumps compared to the contralateral intact nerve from one day to one month after crush. The activation of ERK was mainly localized in the axons of the proximal segments. In the distal segments, however, active ERK was predominantly found in Schwann cells forming Bungner's bands.

Conclusion

The findings indicate that ERK is activated in both the proximal and distal nerve stumps following nerve injury. The role of activated ERK in Wallerian degeneration and subsequent regeneration in vivo remains to be elucidated.

Different expressions of BDNF, NT3, and NT4 in muscle and nerve after various types of peripheral nerve injuries

The changes in the expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) in the rat neuromuscular system as a result of three different types of sciatic nerve injuries have been evaluated. The changes in mRNA and protein levels for BDNF, NT-3, and NT-4 in the soleus muscle and sciatic nerve were assessed 4-28 days after sciatic nerve transection (neurotmesis), sciatic nerve crush (axonotmesis), and mild acute compression (neurapraxia). BDNF mRNA levels increased dramatically with nerve transection in the soleus muscle and the sciatic nerve 7-14 days after injury, whereas the changes were low in other types of injury. The changes of protein levels for BDNF were also similar. The mRNA and the protein levels of NT-3 in the soleus muscle did not show any significant difference. The mRNA for NT-4 in the soleus muscle decreased from 4 to 14 days after sciatic nerve transection, and the protein level was also minimum 14 days after sciatic nerve transection. Our results indicate that the neurotrophic factors in the neuromuscular system could play a role in differentiating peripheral nerve injury.

Knockout of p75(NTR) impairs re-myelination of injured sciatic nerve in mice

Remyelination is an important aspect of nerve regeneration after nerve injury but the underlying mechanisms are not fully understood. The neurotrophin receptor, p75(NTR), in activated Schwann cells in the Wallerian degenerated nerve is up-regulated and may play a role in the remyelination of regenerating peripheral nerves. In the present study, the role of p75(NTR) in remyelination of the sciatic nerve was investigated in p75(NTR) mutant mice. Histological results showed that the number of myelinated axons and thickness of myelin sheath in the injured sciatic nerves were reduced in mutant mice compared with wild-type mice. The myelin sheath of axons in the intact sciatic nerve of adult mutant mice is also thinner than that of wild-type mice. Real-time RT-PCR showed that mRNA levels for myelin basic protein and P0 in the injured sciatic nerves were significantly reduced in p75(NTR) mutant animals. Western blots also showed a significant reduction of P0 protein in the injured sciatic nerves of mutant animals. These results suggest that p75(NTR) is important for the myelinogenesis during the regeneration of peripheral nerves after injury.

NGF, BDNF, NT-3, and GDNF mRNA expression in rat skeletal muscle following denervation and sensory protection

Poor muscle and nerve functional recovery after nerve damage is a serious clinical problem, particularly if there is prolonged delay before nerve-muscle contact is reestablished. Our previous studies showed that sensory nerve cross-anastomosis (sensory protection) provides support to the denervated muscle. In the present study, we analyzed neurotrophic factor mRNA expression by RT-PCR in denervated rat gastrocnemius muscle receiving sensory protection with the saphenous nerve, compared to normal innervated muscle, to denervated muscle, and to denervated muscle repaired immediately with the peroneal (motor) nerve, after periods of 3 days to 3 months. No significant differences in mRNA levels of beta-actin, nerve growth factor, brain-derived neurotrophic factor or neurotrophin-3 were found between the sensory protection treatment and the denervated or the motor repair groups. However, sensory protection resulted in levels of muscle glial cell line-derived neurotrophic factor mRNA expression that were lower than in denervated muscle and higher than in muscle given immediate motor repair. These results demonstrate that glial cell line-derived neurotrophic factor mRNA is elevated following denervation but is partially down-regulated by sensory protection. Our study suggests that sensory protection provides a modified trophic environment by modulating neurotrophic factor synthesis in muscle.

Molecular mechanisms in Schwann cell survival and death during peripheral nerve development, injury and disease

The mechanisms determining the fate of Schwann cells during disease and injury of the adult mammalian peripheral nervous system (PNS) are becoming defined by current advances in molecular neurobiology. It is now apparent that the molecular pathways which regulate the production of the mature myelinating Schwann cell during development may also apply to degenerative and regenerative mechanisms following PNS disease. This review outlines neurobiological responses of Schwann cells during development, injury and disease in order to define the molecular pathways which regulate these crucial events. These mechanisms have implications for our attempts to intervene pharmacologically during pathologies of the PNS.

P0 mRNA expression increases during gradual nerve elongation in adult rats

Leg lengthening with nerve elongation is a common clinical treatment. We investigated morphological and molecular changes in peripheral nerves associated with femoral lengthening using animal models. Sciatic nerves of 13 week old male Wistar rats (n = 35) were elongated indirectly by leg lengthening for 14 days at 1 mm/day. At 3, 7, 14, 21, and 35 days following initiation of elongation, sciatic nerves on the elongated side and contralateral (control) side were excised at the midpoint of the femur. Internodal length was increased by 17%. Light and electron microscopic observation of transverse sections at 14 days showed elongated nerves appearing similar to control nerves with no degenerating axons and normal myelin thickness. We next examined changes of mRNA expression of a major myelin glycoprotein, P0, in elongated nerves using a quantitative reverse transcription-polymerase chain reaction and in situ hybridization. P0 mRNA expression in elongated nerves was increased during the first 3 weeks, with expression reaching 160% of control nerve expression at 14 days. Results of in situ hybridization were confirmatory. We concluded that myelin synthesis occurred during gradual nerve elongation. In adulthood, Schwann cells retain ability to synthesize myelin in response to nerve stretching.

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.

Schwann cell apoptosis in the postnatal axotomized sciatic nerve is mediated via NGF through the low-affinity neurotrophin receptor

Schwann cell death is a developmentally regulated phenomenon and is also induced after peripheral nerve axotomy in neonatal rodents. In this study, we explored whether ligand-induced activation of the low-affinity neurotrophin receptor (p75(NTR)) is responsible for inducing Schwann cell death in vivo. Administration of exogenous nerve growth factor (NGF) to the axotomized nerve site in wild-type animals resulted in a 2.6-fold increase in Schwann cell apoptosis in the distal nerve stumps compared to axotomy alone. No increase in apoptosis, above baseline levels, was seen in p75(NTR)-mutant mice either with or without NGF When anti-NGF antibodies were administered to the site of the peripheral nerve lesion in wild-type mice there was a reduction in the percentage of Schwann cell apoptosis to levels seen in both the quiescent state and in the axotomized nerves of the p75(NTR)-mutant mice. These results demonstrate that apoptosis of Schwann cells in axotomized peripheral nerve is mediated predominantly through p75(NTR) signaling and initiated via endogenously produced NGF.