Molecular basis of interactions between regenerating adult rat thalamic axons and Schwann cells in peripheral nerve grafts. II. Tenascin-C

Aligned fibrous PVDF-TrFE scaffolds with Schwann cells support neurite extension and myelination in vitro

OBJECTIVE

Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), which is a piezoelectric, biocompatible polymer, holds promise as a scaffold in combination with Schwann cells (SCs) for spinal cord repair. Piezoelectric materials can generate electrical activity in response to mechanical deformation, which could potentially stimulate spinal cord axon regeneration. Our goal in this study was to investigate PVDF-TrFE scaffolds consisting of aligned fibers in supporting SC growth and SC-supported neurite extension and myelination in vitro.

APPROACH

Aligned fibers of PVDF-TrFE were fabricated using the electrospinning technique. SCs and dorsal root ganglion (DRG) explants were co-cultured to evaluate SC-supported neurite extension and myelination on PVDF-TrFE scaffolds.

MAIN RESULTS

PVDF-TrFE scaffolds supported SC growth and neurite extension, which was further enhanced by coating the scaffolds with Matrigel. SCs were oriented and neurites extended along the length of the aligned fibers. SCs in co-culture with DRGs on PVDF-TrFE scaffolds promoted longer neurite extension as compared to scaffolds without SCs. In addition to promoting neurite extension, SCs also formed myelin around DRG neurites on PVDF-TrFE scaffolds.

SIGNIFICANCE

This study demonstrated PVDF-TrFE scaffolds containing aligned fibers supported SC-neurite extension and myelination. The combination of SCs and PVDF-TrFE scaffolds may be a promising tissue engineering strategy for spinal cord repair.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

Review : The Role of Schwann cells in Neural Regeneration

The difference in regenerative capacity between the PNS and the CNS is not due to an intrinsic inability of central neurons to extend fibers. Rather, it is probably related to the environment in the CNS that is either repulsive to axonal outgrowth and/or nonsupportive of axonal elongation. In contrast, the PNS both supports and allows for axonal elongation after injury. The Schwann cell, which is the glial cell of the PNS, is strictly required for peripheral regeneration. Here we discuss recent work describing the biology of Schwann cell- dependent regeneration, discuss what is known of the molecular basis of this phenomenon, and how it might apply to the damaged CNS. NEUROSCIENTIST 5:208-216, 1999

Peripheral nerve injury: principles for repair and regeneration

Peripheral Nerve Injuries are one of the most common causes of hand dysfunction caused by upper limb trauma but still current management has remained suboptimal. This review aims to explain the traditional view of pathophysiology of nerve repair and also describe why surgical management is still inadequate in using the new biological research that has documented the changes that occur after the nerve injury, which, could cause suboptimal clinical outcomes. Subsequently presentation and diagnosis will be described for peripheral nerve injuries. When traditional surgical repair using end-to-end anastomosis is not adequate nerve conduits are required with the gold standard being the autologous nerve. Due to associated donor site morbidity and poor functional outcome documented with autologous nerve repair several new advancements for alternatives to bridge the gap are being investigated. We will summarise the new and future advancements of non-biological and biological replacements as well as gene therapy, which are being considered as the alternatives for peripheral nerve repair.

Expression change of beta-1,4 galactosyltransferase I, V mRNAs and Galbeta1,4GlcNAc group in rat sciatic nerve after crush

Glycosylation is one of the most important post-translational modifications. It is clear that the single step of beta-1,4-galactosylation is performed by a family of beta-1,4-galactosyltransferases (beta-1,4-GalTs), and that each member of this family may play a distinct role in different tissues and cells. beta-1,4-GalT I and V are involved in the biosynthesis of N-linked oligosaccharides. In the present study, Real-time PCR revealed that the beta-1,4-GalT I and V mRNAs reached peaks at 2 w after sciatic nerve crush. In situ hybridization showed that at 1 d after sciatic nerve crush, the expression levels of beta-1,4-GalT I and V mRNAs were strong at the crush site, and decreased gradually from crush site to the distal segments. In addition, combined in situ hybridization for beta1,4-GalT I and V mRNAs and immunohistochemistry for S100 showed that beta1,4-GalT I and V mRNAs were mainly located in Schwann cells. Lectin blot showed that the expression of Galbeta1,4GlcNAc group increased at 6 h immediately, reached a peak at 12 h and remained elevated up to 4 w after sciatic nerve crush. In conclusion, beta1,4-GalT I and V might play important roles in the regeneration of the injured sciatic nerve, and upregulation of Galbeta1,4GlcNAc group might be correlated with the process of the sciatic nerve injury.

Can end-to-side neurorrhaphy bridge large defects? An experimental study in rats

This study was undertaken to evaluate collateral sprouting capability in an end-to-side repair model with long regenerative distance. Forty-five rats were used and divided into four groups, according to the reparative procedure following peroneal nerve division: (A) "double" end-to-side neurorrhaphy with a regenerative distance of 0.6 cm; (B) "double" end-to-side neurorrhaphy with a regenerative distance of 1.2 cm; (C) end-to-end neurorrhaphy; and (D) nerve stumps buried into neighboring muscles. In all animals the contralateral healthy side served as a control. Functional assessment of nerve regeneration was performed at intervals up to 5 months using the Peroneal Function Index (PFI). Evaluation 150 days after surgery included peroneal and tibial nerve histologic and morphometric examination and wet weights of the tibialis anterior muscle. Functional evaluation and axonal counting data demonstrated that there was no statistically significant difference between groups A and B, or between groups A and C. There was no functional or histologic evidence of donor nerve deterioration. In conclusion, the present study confirms that "double" end-to-side neurorrhaphy may be useful for the repair of divided human nerves with long gaps.

Analysis of axonal regeneration in the central and peripheral nervous systems of the NG2-deficient mouse

Background

The chondroitin sulphate proteoglycan NG2 blocks neurite outgrowth in vitro and has been proposed as a major inhibitor of axonal regeneration in the CNS. Although a substantial body of evidence underpins this hypothesis, it is challenged by recent findings including strong expression of NG2 in regenerating peripheral nerve.

Results

We studied axonal regeneration in the PNS and CNS of genetically engineered mice that do not express NG2, and in sex and age matched wild-type controls. In the CNS, we used anterograde tracing with BDA to study corticospinal tract (CST) axons after spinal cord injury and transganglionic labelling with CT-HRP to trace ascending sensory dorsal column (DC) axons after DC lesions and a conditioning lesion of the sciatic nerve. Injury to these fibre tracts resulted in no difference between knockout and wild-type mice in the ability of CST axons or DC axons to enter or cross the lesion site. Similarly, after dorsal root injury (with conditioning lesion), most regenerating dorsal root axons failed to grow across the dorsal root entry zone in both transgenic and wild-type mice.

Following sciatic nerve injuries, functional recovery was assessed by analysis of the toe-spreading reflex and cutaneous sensitivity to Von Frey hairs. Anatomical correlates of regeneration were assessed by: retrograde labelling of regenerating dorsal root ganglion (DRG) cells with DiAsp; immunostaining with PGP 9.5 to visualise sensory reinnervation of plantar hindpaws; electron microscopic analysis of regenerating axons in tibial and digital nerves; and by silver-cholinesterase histochemical study of motor end plate reinnervation. We also examined functional and anatomical correlates of regeneration after injury of the facial nerve by assessing the time taken for whisker movements and corneal reflexes to recover and by retrograde labelling of regenerated axons with Fluorogold and DiAsp. None of the anatomical or functional analyses revealed significant differences between wild-type and knockout mice.

Conclusion

These findings show that NG2 is unlikely to be a major inhibitor of axonal regeneration after injury to the CNS, and, further, that NG2 is unlikely to be necessary for regeneration or functional recovery following peripheral nerve injury.

Tissue engineering in head and neck reconstructive surgery: what type of tissue do we need?

Craniofacial tissue loss due to congenital defects, disease or injury is a major clinical problem. The head and neck region is composed of several tissues. The most prevalent method of reconstruction is autologous grafting. Often, there is insufficient host tissue for adequate repair of the defect side, and extensive donor site morbidity may result from the secondary surgical procedure. The field of tissue engineering has the potential to create functional replacements for damaged or pathologic tissues.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Use of cold-preserved allografts seeded with autologous Schwann cells in the treatment of a long-gap peripheral nerve injury

BACKGROUND

Limitations in autogenous tissue have inspired the study of alternative materials for repair of complex peripheral nerve injuries. Cadaveric allografts are one potential reconstructive material, but their use requires systemic immunosuppression. Cold preservation (> or =7 weeks) renders allografts devoid of antigens, but these acellular substrates generally fail in supporting regeneration beyond 3 cm. In this study, the authors evaluated the reconstruction of extensive nonhuman primate peripheral nerve defects using 7-week cold-preserved allografts repopulated with cultured autologous Schwann cells.

METHODS

Ten outbred Macaca fascicularis primates were paired based on maximal genetic disparity as measured by similarity index assay. A total of 14 ulnar nerve defects measuring 6 cm were successfully reconstructed using autografts (n = 5), fresh allografts (n = 2), cold-preserved allografts (n = 3), or cold-preserved allografts seeded with autogenous Schwann cells (n = 4). Recipient immunoreactivity was evaluated by means of enzyme-linked immunosorbent spot assay, and nerves were harvested at 6 months for histologic and histomorphometric analysis.

RESULTS

Cytokine production in response to cold-preserved allografts and cold-preserved allografts seeded with autologous Schwann cells was similar to that observed for autografts. Schwann cell-repopulated cold-preserved grafts demonstrated significantly enhanced fiber counts, nerve density, and percentage nerve (p < 0.05) compared with unseeded cold-preserved grafts at 6 months after reconstruction.

CONCLUSIONS

Cold-preserved allografts seeded with autologous Schwann cells were well-tolerated in unrelated recipients and supported significant regeneration across 6-cm peripheral nerve defects. Use of cold-preserved allogeneic nerve tissue supplemented with autogenous Schwann cells poses a potentially safe and effective alternative to the use of autologous tissue in the reconstruction of extensive nerve injuries.

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".

Collagen matrix in spinal cord injury

The fibrous scar that develops after central nervous system (CNS) injury is considered a major impediment for axonal regeneration. It consists of a dense collagen IV meshwork, which serves as a binding matrix for numerous other extracellular matrix components and inhibitory molecules like proteoglycans and semaphorins, but also growth-promoting factors. Inhibition of collagen matrix formation in brain and spinal cord lesions leads to axonal regeneration and functional recovery, although collagen IV per se is not inhibitory for axonal outgrowth. This review focuses on the molecular properties of the collagen IV matrix and its interactions with various molecules that are expressed after CNS lesion. Moreover, studies on collagen expression and matrix formation after injury of regenerating versus non-regenerating nervous systems are reviewed. Major differences in collagen deposition in the CNS and the peripheral nervous system (PNS) and differences in specific cell responses to extracellular matrix deposition in the lesion area are discussed. Therapeutic treatments aiming at suppression of fibrous scarring have been shown to promote axon regeneration in various lesion paradigms of the mammalian CNS.

ATF3 upregulation in glia during Wallerian degeneration: differential expression in peripheral nerves and CNS white matter

BACKGROUND

Many changes in gene expression occur in distal stumps of injured nerves but the transcriptional control of these events is poorly understood. We have examined the expression of the transcription factors ATF3 and c-Jun by non-neuronal cells during Wallerian degeneration following injury to sciatic nerves, dorsal roots and optic nerves of rats and mice, using immunohistochemistry and in situ hybridization.

RESULTS

Following sciatic nerve injury--transection or transection and reanastomosis--ATF3 was strongly upregulated by endoneurial, but not perineurial cells, of the distal stumps of the nerves by 1 day post operation (dpo) and remained strongly expressed in the endoneurium at 30 dpo when axonal regeneration was prevented. Most ATF3+ cells were immunoreactive for the Schwann cell marker, S100. When the nerve was transected and reanastomosed, allowing regeneration of axons, most ATF3 expression had been downregulated by 30 dpo. ATF3 expression was weaker in the proximal stumps of the injured nerves than in the distal stumps and present in fewer cells at all times after injury. ATF3 was upregulated by endoneurial cells in the distal stumps of injured neonatal rat sciatic nerves, but more weakly than in adult animals. ATF3 expression in transected sciatic nerves of mice was similar to that in rats. Following dorsal root injury in adult rats, ATF3 was upregulated in the part of the root between the lesion and the spinal cord (containing Schwann cells), beginning at 1 dpo, but not in the dorsal root entry zone or in the degenerating dorsal column of the spinal cord. Following optic nerve crush in adult rats, ATF3 was found in some cells at the injury site and small numbers of cells within the optic nerve displayed weak immunoreactivity. The pattern of expression of c-Jun in all types of nerve injury was similar to that of ATF3.

CONCLUSION

These findings raise the possibility that ATF3/c-Jun heterodimers may play a role in regulating changes in gene expression necessary for preparing the distal segments of injured peripheral nerves for axonal regeneration. The absence of the ATF3 and c-Jun from CNS glia during Wallerian degeneration may limit their ability to support regeneration.

Neurite outgrowth by the alternatively spliced region of human tenascin-C is mediated by neuronal alpha7beta1 integrin

The region of tenascin-C containing only alternately spliced fibronectin type-III repeat D (fnD) increases neurite outgrowth by itself and also as part of tenascin-C. We previously localized the active site within fnD to an eight amino acid sequence unique to tenascin-C, VFDNFVLK, and showed that the amino acids FD and FV are required for activity. The purpose of this study was to identify the neuronal receptor that interacts with VFDNFVLK and to investigate the hypothesis that FD and FV are important for receptor binding. Function-blocking antibodies against both alpha7 and beta1 integrin subunits were found to abolish VFDNFVLK-mediated process extension from cerebellar granule neurons. VFDNFVLK but not its mutant, VSPNGSLK, induced clustering of neuronal beta1 integrin immunoreactivity. This strongly implicates FD and FV as important structural elements for receptor activation. Moreover, biochemical experiments revealed an association of the alpha7beta1 integrin with tenascin-C peptides containing the VFDNFVLK sequence but not with peptides with alterations in FD and/or FV. These findings are the first to provide evidence that the alpha7beta1 integrin mediates a response to tenascin-C and the first to demonstrate a functional role for the alpha7beta1 integrin receptor in CNS neurons.

Plasticity following Injury to the Adult Central Nervous System: Is Recapitulation of a Developmental State Worth Promoting?

The adult central nervous system (CNS) appears to initiate a transient increase in plasticity following injury, including increases in growth-related proteins and generation of new cells. Recent evidence is reviewed that the injured adult CNS exhibits events and patterns of gene expression that are also observed during development and during regeneration following damage to the mature peripheral nervous system (PNS). The growth of neurons during development or regeneration is correlated, in part, with a coordinated expression of growth-related proteins, such as growth-associated-protein-43 (GAP-43), microtubule-associated-protein-1B (MAP1B), and polysialylated-neural-cell-adhesion-molecule (PSA-NCAM). For each of these proteins, evidence is discussed regarding its specific role in neuronal development, signals that modify its expression, and reappearance following injury. The rate of adult hippocampal neurogenesis is also affected by numerous endogenous and exogenous factors including injury. The continuing study of developmental neurobiology will likely provide further gene and protein targets for increasing plasticity and regeneration in the mature adult CNS.

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.

FKBP12 mRNA expression is upregulated by intrinsic CNS neurons regenerating axons into peripheral nerve grafts in the brain

We have examined the expression of the immunophilin FKBP12 in adult rat intrinsic CNS neurons stimulated to regenerate axons by the implantation of segments of autologous tibial nerve into the thalamus or cerebellum. After survival times of 3 days to 6 weeks, the brains were fresh-frozen. In some animals the regenerating neurons were retrogradely labelled with cholera toxin subunit B 1 day before they were killed. Sections through the thalamus or cerebellum were used for in situ hybridization with digoxygenin-labelled riboprobes for FKBP12 or immunohistochemistry to detect cholera toxin subunit B-labelled neurons. FKBP12 was constitutively expressed by many neurons, and was very strongly expressed in the hippocampus and by Purkinje cells. Regenerating neurons were found in the thalamic reticular nucleus and deep cerebellar nuclei of animals that received living grafts. Neurons in these nuclei upregulated FKBP12 mRNA; such neurons were most numerous at 3 days post grafting but were most strongly labelled at 2 weeks post grafting. Regenerating neurons identified by retrograde labelling were found to have upregulated FKBP12 mRNA. No upregulation was seen in neurons in animals that received freeze-killed grafts, which do not support axonal regeneration. We conclude that FKBP12 is a regeneration-associated gene in intrinsic CNS neurons.

Development of the regenerative capacity of postnatal axotomized rat spinal motoneurons

The present study examined whether a peripheral nerve (PN) graft can rescue developing motoneurons from degeneration and determined when immature motoneurons begin to express a regenerative capacity. Transplantation of a PN graft was unable to rescue motoneurons from degeneration if spinal root avulsion was performed in animals younger than P14. However, this procedure did enhance motoneuron survival when root avulsion was performed at P14 or later. Immature (P1 or P7) motoneurons were unable to regenerate their axons into the transplanted PN graft following root avulsion, whereas in older animals (P14-P28) motoneurons were able to regenerate axons into the PN graft. The percentage of regenerated motoneurons increased from P21 to P28 and was similar to that of adult animals. Therefore, the regenerative capacity of rat spinal motoneurons first begins at about P14, which seems to be critical.

Bioactive poly(L-lactic acid) conduits seeded with Schwann cells for peripheral nerve regeneration

This study attempted to enhance the efficacy of peripheral nerve regeneration using our previously tested poly(L-lactic acid) (PLLA) conduits by incorporating them with allogeneic Schwann cells (SCs). The SCs were harvested, cultured to obtain confluent monolayers and two concentrations (1 x 10(4) and 1 x 10(6) SC/ml) were combined with a collagen matrix (Vitrogen) and injected into the PLLA conduits. The conduits were then implanted into a 12 mm right sciatic nerve defect in rats. Three control groups were used: isografts, PLLA conduits filled with collagen alone and empty silicone tubes. The sciatic functional index (SFI) was calculated monthly through four months. At the end of second and fourth months, the gastrocnemius muscle was harvested and weighed for comparison and the graft conduit and distal nerve were harvested for histomorphologic analysis. The mean SFI demonstrated no group differences from isograft control. By four months, there was no significant difference in gastrocnemius muscle weight between the experimental groups compared to isograft controls. At four months, the distal nerve demonstrated a statistically lower number of axons mm2 for the high and low SC density groups and collagen control. The nerve fiber density was significantly lower in all of the groups compared to isograft controls by four months. The development of a "bioactive" nerve conduit using tissue engineering to replace autogenous nerve grafts offers a potential approach to improved patient care. Although equivalent nerve regeneration to autografts was not achieved, this study provides promising results for further investigation.

Peripheral nerve injury: a review and approach to tissue engineered constructs

Eleven thousand Americans each year are affected by paralysis, a devastating injury that possesses associated annual costs of $7 billion (American Paralysis Association, 1997). Currently, there is no effective treatment for damage to the central nervous system (CNS), and acute spinal cord injury has been extraordinarily resistant to treatment. Compared to spinal cord injury, damage to peripheral nerves is considerably more common. In 1995, there were in excess of 50,000 peripheral nerve repair procedures performed. (National Center for Health Statistics based on Classification of Diseases, 9th Revision, Clinical Modification for the following categories: ICD-9 CM Code: 04.3, 04.5, 04.6, 04.7). These data, however, probably underestimate the number of nerve injuries appreciated, as not all surgical or traumatic lesions can be repaired. Further, intraabodominal procedures may add to the number of neurologic injuries by damage to the autonomic system through tumor resection. For example, studies assessing the outcome of impotency following radical prostatectomy demonstrated 212 of 503 previously potent men (42%) suffered impotency when partial or complete resection of one or both cavernosal nerve(s). This impotency rate decreased to 24% when the nerves were left intact (Quinlan et al., J. Urol. 1991;145:380-383; J. Urol. 1991;145:998-1002).

Correlation between putative inhibitory molecules at the dorsal root entry zone and failure of dorsal root axonal regeneration

The molecular mechanisms involved in preventing regenerating dorsal root axons from entering the spinal cord at the dorsal root entry zone (DREZ) are obscure. We used immunohistochemistry, in situ hybridization, and electron microscopy to study axonal regeneration after dorsal rhizotomy in adult rats and its relationship to cellular changes and the distribution of putative growth inhibitory molecules in this region. Astrocyte processes, ending as bulb-shaped expansions, grew up to 700 microm into the basal lamina tubes of injured roots, where regenerating axons were also present. Some of these axons approached or reached the DREZ but grew no further; others turned back toward the ganglion, suggesting the presence of repulsive cues in or near the DREZ. Tenascin-C mRNA and protein and CSPG stub immunoreactivity were strongly upregulated in the roots after rhizotomy, but were only weakly expressed in the DREZ. Tenascin-R immunoreactivity was confined to CNS tissue, and unaffected by rhizotomy. Large, rounded GFAP-negative, NG2-immunoreactive cells, a few of which were OX42 positive, were found in the DREZ following rhizotomy. Astrocyte processes projecting into the roots were tenascin-R and NG2 negative. Hence, only NG2-expressing cells and tenascin-R were appropriately situated to inhibit regeneration through the DREZ.

Challenges to nerve regeneration

Peripheral nerve injuries can result from mechanical, thermal, chemical, congenital, or pathological etiologies. Failure to restore these damaged nerves can lead to the loss of muscle function, impaired sensation, and painful neuropathies. Current surgical strategies for the repair of critical nerves involve the transfer of normal donor nerve from an uninjured body location. However, these "gold standard" methods for tissue restoration frequently are limited by tissue availability, risk of disease spread, secondary deformities, and potential differences in tissue structure and size. One possible alternative to autogenous tissue replacement is the development of engineered constructs to replace those elements necessary for axonal proliferation, including a scaffold, support cells, induction factors, and extracellular matrices. Despite advances and contributions in the field of tissue engineering, results to date with nerve conduits have failed to equal the nerve regeneration achieved with autogenous grafts for large distances. We review the current challenges to tissue-engineered constructs. Each of the four components is reviewed and approaches are outlined. Semin. Surg. Oncol. 19:312-318, 2000.

Long-term effect of vital labelling on mixed Schwann cell cultures

Schwann cell transplantation following neuronal injury could encourage regeneration of spinal cord as well as improving peripheral nerve gap repair. In order to gain a better understanding of the role of transplanted Schwann cells in vivo, it is essential to be able to follow their behaviour after transplantation. Our aim was to evaluate the suitability of two vital fluorescent labels on the proliferation rate and phenotypic stability of Schwann cells, in either pure culture or mixed co-culture. Primary cultures of Schwann cells were obtained from Dark Agouti and Lewis neonatal rats and labelled with H33342 and PKH26, respectively. In mixed cultures, a 50: 50 mixture of Dark Agouti and Lewis Schwann cells was present. Labelled cultured cells were examined at 1, 2 and 4 weeks for viability and phenotypic marker expression of S100, GFAP, p75, MHC I, MHC II and compared with corresponding unlabelled cells. The results showed that although there was no deleterious interaction in the mixed cultures, the viability was reduced by the labelling after 2 weeks. Labelled cells could be distinguished up to 4 weeks, but there was leakage of H33342 label after 2 weeks. Labelled Schwann cells showed reduced expression of phenotypic markers, especially p75 when labelled with H33342. In conclusion, H33342 and PKH26 can be used as fluorescent markers of Schwann cells for short-term studies, for a maximum of 2 weeks, but different markers may be needed for longer experiments.

Cellular reactivity to mechanical axonal injury in an organotypic in vitro model of neurotrauma

An in vitro model of traumatic brain injury is described that is based on organotypic cocultures (OTCs) of rat neocortex and thalamus connected by reciprocal axonal projections. Localized mechanical compression of this projection was inflicted with a mechanical device, and the effects on cell viability, axonal morphology, and protein expression levels were analyzed. Within 24 h after insult, major cell damage occurred in infragranular cortical layers containing the corticothalamic projection neurons and in thalamic regions adjacent to the mechanical impact as was assessed through the use of the vital stain Syto 21, and propidium iodide labeling. A small, but significant number of calretinin-positive interneurons in cortical and thalamic areas displayed symptoms of injury. Axonal elements, as revealed by neurofilament (NF-H/M) immunohistochemistry, in the corticothalamic transition zone displayed pathomorphological changes, such as axonal bulbs and swellings, already 4 h after insult. Densitometric analysis revealed that MAP-2a,b expression was not significantly changed within 4 h after injury. A significant reduction in MAP-2a,b amount was evident at 20 h after injury in thalamus (by 31.6%) and cortex (by 30%) maintained for 12 days in vitro (DIV), but not in OTCs aged 20 DIV. The axonally localized form MAP-2c significantly increased in cortex of 12-DIV OTCs at 4 and 20 h after insult (65.6% and 33.4%, respectively). MAP-2c levels in cortex of 20 DIV initially increased by 47.7% and declined below control values 20 h after injury. Thalamic areas revealed a delay in MAP-2c reactivity, in that expression was significantly elevated only at 20 h after injury (by 84.4% in 12-DIV and by 39.6% in 20-DIV OTCs, respectively). These data may reflect the regenerative ability of juvenile, but not of older neurons in response to mechanical axonal injury.

Cellular and molecular correlates of the regeneration of adult mammalian CNS axons into peripheral nerve grafts

Studies of the regeneration of CNS axons into peripheral nerve grafts have provided information crucial to our understanding of the regenerative potential of CNS neurons. Injured axons in the thalamus and corpus striatum produce regenerative sprouts within a few days of graft implantation, apparently in response to living cells in the grafts. The regenerating axons often grow directly towards the grafts, and enter Schwann cell columns where they elongate surrounded by Schwann cell processes. The regenerating CNS axons, and the Schwann cell processes along which they grow, initially express the cell adhesion molecules NCAM, and L1. The axons also express polysialic acid and, unlike regenerating peripheral axons, bind tenascin-C derived from Schwann cells. Wherever peripheral nerve grafts are implanted into the CNS they appear to promote the differential regeneration of CNS axons. Most of the axons which grow into grafts in the thalamus are derived from the thalamic reticular nucleus (TRN), whereas grafts in the striatum promote regeneration of axons from the substantia nigra pars compacta (SNpc) and grafts in the cerebellum promote regeneration from deep cerebellar nuclei (DCN) and brainstem precerebellar neurons. In contrast most thalamocortical projection neurons, striatal projection neurons and Purkinje cells in the cerebellar cortex are poor at regenerating. There are patterns to the expression of regeneration-related molecules by axons injured by nerve grafts in the CNS. Most neurons which regenerate well (e.g. TRN and DCN neurons) upregulate GAP-43, L1 and the transcription factor c-jun in response to a graft, whereas those neurons which do not regenerate well (e.g. Purkinje cells, thalamocortical and striatal projection neurons) do not upregulate these molecules. These observations suggest that some classes of CNS neurons may be intrinsically unable to regenerate axons and the repair of injuries in the brain and spinal cord may consequently require some form of gene therapy for axotomised neurons.

Regeneration in the central nervous system: pharmacological intervention, xenotransplantation, and stem cell transplantation

The factors inhibiting regeneration in the central nervous system (CNS) have been elaborated, debated, and studied for the past 70 years. Recent work has pointed to the fine balance that exists between repair and regeneration following CNS injury. Growth factors have featured prominently in this debate. In attempts to tip the scales toward regeneration and functional reconnection to damaged neurons, pharmacological intervention has come to the fore. However, a perennial concern has been that much of regeneration may be aberrant, although there is now evidence to suggest that this fear may have been exaggerated. In searching for additional avenues for achieving therapeutic reconstruction of damaged neural pathways, transplantation studies occupy a prominent place in the literature. Various principles have become established, and these have proved relevant for all approaches utilizing grafts. Xenotransplantation and stem cell transplantation are approaches with exciting potential. Circuitry can be effectively restored by xenotransplantation, including early indications of integration of pig dopaminergic neurons in Parkinson's disease. The considerable possibilities offered by the differentiation of neural stem cells into progenitor cells and then into neurons and glia are explored.

Peripheral nerve grafts exert trophic and tropic effects on anterior thalamic neurons

Peripheral nerve grafting into the central nervous system (CNS) has been used to study the regenerative capabilities of central neurons given access to a peripheral nervous system (PNS) environment. It is well documented that many CNS neurons regenerate axons along peripheral nerve grafts placed in close proximity to their cell bodies and that these grafts can ameliorate axotomy-induced retrograde degeneration. In the present study, we placed peripheral nerve grafts in proximity to axotomized neurons of the anterior thalamus. Standard histological and retrograde tracing techniques were used to examine these preparations 2 months after grafting. Three effects of these grafts were observed: amelioration of retrograde degeneration of axotomized anterior thalamic neurons, hypertrophy of many thalamic neurons in the local environment of the graft, and ingrowth of axons of axotomized anterior thalamic neurons as well as nonaxotomized neurons from surrounding nuclei. We conclude from these studies that peripheral nerve grafts not only provide a matrix for axonal outgrowth but also exert marked trophic and tropic effects on axotomized anterior thalamic neurons.

Comparison of neurite outgrowth induced by intact and injured sciatic nerves: a confocal and functional analysis

Mechanisms regulating axon growth in the peripheral nervous system have been studied by means of an in vitro bioassay, the tissue section culture, in which regenerating neurons are grown on substrata made up of tissue sections. Sections from intact and degenerated sciatic nerves proved to be different in their ability to support neurite outgrowth of embryonic chick sensory neurons from both qualitative and quantitative points of view. On denervated nerve sections, the total length of neurites elaborated per neuron was almost twice that found on intact nerve sections. In addition, confocal microscopy revealed a striking difference between intact and denervated nerve substrata: on denervated nerve sections, neurites grew inside the internal structures of endoneurial Schwann cell tubes, within the underlying tissue sections, whereas on intact nerve sections neurites extended along endoneurial basal laminae but never entered Schwann cell tubes. Perturbation experiments were used to analyze some of the molecular determinants that control neurite outgrowth in this system. Antibodies directed against the beta1-integrin subunit inhibited neurite extension on both normal and degenerated rat sciatic nerve tissue. Strikingly, however, differential inhibition was observed using antibodies directed against extracellular matrix molecules. Anti-laminin-2 (merosin) antibodies drastically reduced both the percentage of growing neurons and the total length of neurites on denervated nerve sections, but they did not modify these parameters on sections of normal nerve. Taken together, these results suggest that laminin-2/merosin promotes neurite outgrowth in peripheral nerve environments but only after Wallerian degeneration, which is when axons are allowed to extend within endoneurial tubes.

Long and short splice variants of human tenascin differentially regulate neurite outgrowth

Tenascin-C has been implicated in regulation of neurite outgrowth both during development and after injury; however, its role as permissive vs inhibitory remains controversial. We report that different tenascin splice variants may have dramatically different impacts on neuronal growth. In a cell culture model, the largest and smallest splice variants (TN.L and TN.S) of human tenascin both promoted process extension when surface-bound. In contrast, soluble TN.S inhibited outgrowth, whereas soluble TN.L had no inhibitory effect. Perturbation experiments with antibodies, and outgrowth experiments with recombinant tenascin fragments, indicate that the differential properties of these molecules can be attributed to their distinctive array of FN-III repeats. Monoclonal antibodies were used to demonstrate at least two distinct neurite outgrowth promoting domains within the alternatively spliced region. These results suggest that the effect of tenascin on axon growth is a function of splice variants, as well as the form or conformation of those variants.

Chondroitin sulfate proteoglycan and tenascin in the wounded adult mouse neostriatum in vitro: dopamine neuron attachment and process outgrowth

Extracellular matrix (ECM) molecules, including chondroitin-4 or chondroitin-6 sulfate proteoglycans (CSPGs) and tenascin, are upregulated in and around wounds and transplants to the adult CNS. In the present study, striatal wounds from adult mice were used in a novel in vitro paradigm to assess the effects of these wound-associated molecules on embryonic dopamine cell attachment and neurite outgrowth. Light and electron microscopic immunocytochemistry studies have shown that astroglial scar constituents persist in cultured explants for at least 1 week in vitro, and despite the loss of neurons from adult striatal explants, there is a retention of certain structural features suggesting that the wound explant-neuron coplant is a viable model for analysis of graft-scar interactions. Explants from the wounded striatum taken at different times after a penetrating injury in vivo were used as substrates for embryonic ventral mesencephalon neurons that were plated on their surfaces. Dopamine cell attachment is increased significantly in relation to the expression of both CSPG and tenascin. The increase in neuronal attachment in this paradigm, however, is accompanied by a postlesion survival time-dependent significant decrease in neuritic growth from these cells. In vitro ECM antibody treatment suggests that CSPG may be responsible for heightened dopamine cell attachment and that tenascin simultaneously may support cell attachment while inhibiting neurite growth. The present study offers a new approach for the in vitro analysis of cell and molecular interactions after brain injury and brain grafting, in essence acting as a nigrostriatal transplant-in-a-dish.

Effects of NMDA receptor blockade in the developing rat somatosensory cortex on the expression of the glia-derived extracellular matrix glycoprotein tenascin-C

The patterning of synaptic connections during development is thought to be influenced by the correlation of neuronal impulse activity. N-methyl-D-aspartate (NMDA) receptors have been implicated in the reorganization of thalamocortical afferents in the visual system. The topographic mapping of the periphery of sensory systems onto the somatosensory cortex in the whisker-barrel field of rodents has served as another important paradigm in the study of extrinsic influences on synaptic rearrangements. In a search for the molecular cues that may contribute to synaptic plasticity, we have investigated the distribution of the glia-derived extracellular matrix glycoprotein tenascin-C, which is highly expressed during the formation of the barrel field map around birth and delineates the boundaries between barrel fields after segregation of afferent inputs. Here we show that systemic and local application of NMDA receptor antagonists at postnatal day 2 inhibited the down-regulation of tenascin mRNA and protein by postnatal day 6 and prevented the appearance of tenascin-positive barrel field boundaries. Furthermore, barrels were not distinguishable by Nissl staining, and segregation of thalamocortical afferents as monitored by anterograde Dil tracing and acetylcholinesterase histochemistry was not complete. These observations indicate that expression of tenascin-C and segregation of afferent inputs are modified by NMDA receptor-dependent neuronal activity.

Disruption of the gene for the myelin-associated glycoprotein improves axonal regrowth along myelin in C57BL/Wlds mice

The myelin-associated glycoprotein (MAG) has been shown to be inhibitory for certain neurons in vitro (Mukhopadhyay et al., 1994; McKerracher et al., 1994). To investigate whether MAG is an inhibitory component in peripheral myelin in vivo, MAG-deficient mutant mice were cross-bred with C57BL/Wlds mice that have delayed lesion-induced myelin degeneration and axon regrowth. While in crushed nerves of C57BL/Wlds mice expressing MAG, only 16% of myelin sheaths were associated with regrowing axons, this number was doubled in MAG-deficient C57BL/Wlds mice. These observations suggest that the absence of MAG may contribute to the improved axonal regrowth in the double mutants. Therefore, degeneration of MAG-containing myelin might be an important prerequisite to optimize axonal regrowth after peripheral nerve injury.

Kainic acid activates transient expression of tenascin-C in the adult rat hippocampus

Kainic acid-induced limbic seizures enhance expression of tenascin-C (TN) in the hippocampus of adult rats. TN mRNA was detectable by in situ hybridization in many granule cells in the dentate gyrus 4.5 hr after kainic acid injection but not in saline-injected animals (controls) or in animals killed 2 or 24 hr after injection. Thirty days after kainic acid injection, TN mRNA was detectable only in pyramidal cells of CA3 and CA1. At the protein level, TN was detectable by immunocytochemistry in control animals in the strata oriens and lacunosum moleculare of CA1, in the molecular layer, and within a narrow area at the inner surface of the granule cell layer in the dentate gyrus. Twenty-four hours after kainic acid injection, TN immunoreactivity was enhanced in these areas and throughout the granule cell layer. Thirty days after kainic acid injection, TN immunoreactivity was downregulated in these areas, while it was prominent in the stratum oriens and in clusters of immunoreactivity in the stratum lucidum of CA3. Western blot analysis of the hippocampus showed a peak of TN expression 24 hr after kainic acid injection. These observations show that TN expression is upregulated in predominantly neuronal cells already by 4.5 hr after kainic acid injection, coincident with activation of granule cells and sprouting of axon terminals, whereas the remaining TN expression 30 days after injection relates to pyramidal cells in CA1 and CA3, coincident with an astroglial response, as marked by a strong expression of glial fibrillary acidic protein.

Molecular basis of interactions between regenerating adult rat thalamic axons and Schwann cells in peripheral nerve grafts I. Neural cell adhesion molecules

To gain insight into the possible molecular mechanisms underlying axonal regeneration of neurons of the adult central nervous system (CNS), we have investigated, by in situ hybridization and by immunocytochemistry, the localization and sites of synthesis of the neurite outgrowth-promoting cell surface molecules L1, N-CAM and its highly sialylated form, N-CAM-PSA, in and around peripheral nerve grafts implanted into the thalamus of adult rats. Normal unoperated adult rat thalamus contains N-CAM and L1 but no N-CAM-PSA immunoreactive axons. Between 7 days and 13 weeks after graft implantation, L1, N-CAM and N-CAM-PSA were all present at the surface of axonal sprouts in the brain parenchyma close to grafts and in the central parts of Schwann cell columns within grafts. Schwann cell membranes were L1 and N-CAM positive at all postgraft survival times, more strongly at 2-4 weeks than other times, but were associated with N-CAM-PSA reaction product only where they abutted N-CAM-PSA positive axons. Schwann cell membranes apposed to basal laminae (which were avoided by regenerating CNS axons) were L1, N-CAM and N-CAM-PSA negative. Between 3 days and 8 weeks after grafting, N-CAM and L1 mRNA were generally weakly upregulated in neurons of the ipsilateral thalamus, but, most conspicuously, L1 mRNA was strongly upregulated in the neurons of the thalamic reticular nucleus; these neurons are known to regenerate axons very effectively into peripheral nerve grafts and are the probable source of most of the axons which enter thalamic grafts. N-CAM and L1 mRNA were also strongly upregulated in presumptive Schwann cells in the graft. These results show that regenerating CNS axons (re)express N-CAM-PSA and upregulate L1 and N-CAM, suggesting that all of these molecules may play a role in cellular interactions during the regeneration of CNS axons. Furthermore L1 synthesis appears to be particularly well correlated with the ability of CNS neurons to regenerate axons into peripheral nerve grafts.

Tenascin-C expression by neurons and glial cells in the rat spinal cord: changes during postnatal development and after dorsal root or sciatic nerve injury

We have used in situ hybridization with a digoxigenin-labelled probe for tenascin-C mRNA and immunocytochemistry with antibodies against tenascin-C, glial fibrillary acidic protein, OX-42 and the 200 kDa neurofilament protein to study the expression, distribution and cellular relationships of tenascin-C mRNA and protein in the developing (postnatal) and adult spinal cord of rat, and the effects thereon of dorsal root, ventral root and sciatic nerve injuries. The most interesting finding was that on postnatal day 7 (P7), P14 and in the adult, but not on P0 or P3, a group of neurons in the lumbar ventral horn expressed the tenascin-C mRNA gene. They represented about 5% of ventral horn neurons in the adult and were among the smaller such neurons. Since 40-60% of such cells were lost at P13 following sciatic nerve crush on P0, some were almost certainly motor neurons. In addition, we found that at P0 and P3, mRNA-containing glial cells were widespread in grey and white matter but sparse in the developing dorsal columns; tenascin-C immunofluorescence showed a similar distribution. By P7 there were fewer mRNA-containing cells in the ventral horns and in the area of the dorsal columns containing the developing corticospinal tract where immunofluorescence was also weak. At P14 there were no glial-like mRNA-containing cells in the grey matter; such cells were confined to the periphery of the lateral and ventral white columns but were present throughout the dorsal columns where tenascin-C immunofluorescence was also strong. No glial-like mRNA-containing cells were present in the adult lumbar spinal cord and tenascin-C immunofluorescence was confirmed to irregular patches in the ventral horn, especially around immunonegative cell bodies of small neurons, a zone around the central canal, and a thin zone adjacent to the glia limitans. Thus the expression of tenascin-C is differentially developmentally regulated in the grey matter and in different parts of the white matter. Three days after injury of dorsal roots L4-6, many cells containing tenascin-C mRNA, some identified as glial fibrillary acidic protein-positive astrocytes, were present in the ipsilateral dorsal column, but were rare after longer survivals. Immunoreactivity, however, was elevated in the ipsilateral dorsal column at 3 days, remained high for several months and disappeared at 6.5 months. Dorsal root injury had no effect on tenascin-C mRNA or protein in the grey matter. Sciatic nerve or ventral root injury had no effect on these molecules in any part of the spinal cord.