cAMP and Schwann cells promote axonal growth and functional recovery after spinal cord injury

Central neurons regenerate axons if a permissive environment is provided; after spinal cord injury, however, inhibitory molecules are present that make the local environment nonpermissive. A promising new strategy for inducing neurons to overcome inhibitory signals is to activate cAMP signaling. Here we show that cAMP levels fall in the rostral spinal cord, sensorimotor cortex and brainstem after spinal cord contusion. Inhibition of cAMP hydrolysis by the phosphodiesterase IV inhibitor rolipram prevents this decrease and when combined with Schwann cell grafts promotes significant supraspinal and proprioceptive axon sparing and myelination. Furthermore, combining rolipram with an injection of db-cAMP near the graft not only prevents the drop in cAMP levels but increases them above those in uninjured controls. This further enhances axonal sparing and myelination, promotes growth of serotonergic fibers into and beyond grafts, and significantly improves locomotion. These findings show that cAMP levels are key for protection, growth and myelination of injured CNS axons in vivo and recovery of function.

Combining Schwann cell bridges and olfactory-ensheathing glia grafts with chondroitinase promotes locomotor recovery after complete transection of the spinal cord

Numerous obstacles to successful regeneration of injured axons in the adult mammalian spinal cord exist. Consequently, a treatment strategy inducing axonal regeneration and significant functional recovery after spinal cord injury has to overcome these obstacles. The current study attempted to address multiple impediments to regeneration by using a combinatory strategy after complete spinal cord transection in adult rats: (1) to reduce inhibitory cues in the glial scar (chondroitinase ABC), (2) to provide a growth-supportive substrate for axonal regeneration [Schwann cells (SCs)], and (3) to enable regenerated axons to exit the bridge to re-enter the spinal cord (olfactory ensheathing glia). The combination of SC bridge, olfactory ensheathing glia, and chondroitinase ABC provided significant benefit compared with grafts only or the untreated group. Significant improvements were observed in the Basso, Beattie, and Bresnahan score and in forelimb/hindlimb coupling. This recovery was accompanied by increased numbers of both myelinated axons in the SC bridge and serotonergic fibers that grew through the bridge and into the caudal spinal cord. Although prominent descending tracts such as the corticospinal and reticulospinal tracts did not successfully regenerate through the bridge, it appeared that other populations of regenerated fibers were the driving force for the observed recovery; there was a significant correlation between numbers of myelinated fibers in the bridge and improved coupling of forelimb and hindlimb as well as open-field locomotion. Our study tests how proven experimental treatments interact in a well-established animal model, thus providing needed direction for the development of future combinatory treatment regimens.

A combination of BDNF and NT-3 promotes supraspinal axonal regeneration into Schwann cell grafts in adult rat thoracic spinal cord

We previously demonstrated that Schwann cells (SCs) in semipermeable guidance channels promote axonal regeneration in adult rat spinal cord transected at the mid-thoracic level. Propriospinal but not supraspinal axons grew into these channels. Here, we tested the ability of exogenous brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) to promote axonal regeneration in this novel model. The two neurotrophins were delivered simultaneously into the channel by an Alzet minipump at a rate of 12 micrograms/day for each neurotrophin for 14 of 30 days tested; phosphate-buffered saline, the vehicle solution, was used as a control. Significantly more myelinated nerve fibers were present in SC/neurotrophin grafts than in SC/vehicle grafts (1523 +/- 292 vs 882 +/- 287). In the graft, at least 5 mm from the rostral cord-graft interface, some nerve fibers were immunoreactive for serotonin, a neurotransmitter specific to raphe-derived axons in rat spinal cord. Fast blue retrograde tracing from SC/neurotrophin grafts revealed labeled neurons in 10 nuclei of the brain stem, 67% of these being in the lateral and spinal vestibular nuclei. The mean number of labeled brain stem neurons in the SC/neurotrophin group (92; n = 3) contrasted with the mean in the SC/vehicle group (6; n = 4). Our results clearly demonstrate that BDNF and NT-3 infusion enhanced propriospinal axonal regeneration and, more significantly, promoted axonal regeneration of specific distant populations of brain stem neurons into grafts at the mid-thoracic level in adult rat spinal cord.

Schwann cell basal lamina and nerve regeneration

Nerve segments approximately 7 mm long were excised from the predegenerated sciatic nerves of mice, and treated 5 times by repetitive freezing and thawing to kill the Schwann cells. Such treated nerve segments were grafted into the original places so as to be in contact with the proximal stumps. The animals were sacrificed 1, 2, 3, 5, 7 and 10 days after the grafting. The grafts were examined by electron microscopy in the middle part of the graft, i.e. 3-4 mm distal to the proximal end and/or near the proximal and distal ends of the graft. In other instances, the predegenerated nerve segments were minced with a razor blade after repetitive freezing and thawing. Such minced nerves were placed in contact with the proximal stumps of the same nerves. The animals were sacrificed 10 days after the grafting. Within 1-2 days after grafting, the dead Schwann cells had disintegrated into fragments. They were then gradually phagocytosed by macrophages. The basal laminae of Schwann cells, which were not attacked by macrophages, remained as empty tubes (basal lamina scaffolds). In the grafts we examined, no Schwann cells survived the freezing and thawing process. The regenerating axons always grew out through such basal lamina scaffolds, being in contact with the inner surface of the basal lamina (i.e. the side originally facing the Schwann cell plasma membrane). No axons were found outside of the scaffolds. One to two days after grafting, the regenerating axons were not associated with Schwann cells, but after 5-7 days they were accompanied by Schwann cells which were presumed to be migrating along axons from the proximal stumps. Ten days after grafting, proliferating Schwann cells observed in the middle part of the grafts had begun to sort out axons. In the grafts of minced nerves, the fragmented basal laminae of the Schwann cells re-arranged themselves into thicker strands or small aggregations of basal laminae. The regenerating axons, without exception, attached to one side of such modified basal laminae. Collagen fibrils were in contact with the other side, indicating that these modified basal laminae had the same polarity in terms of cell attachment as seen in the ordinary basal laminae of the scaffolds.(ABSTRACT TRUNCATED AT 400 WORDS)

A polymer foam conduit seeded with Schwann cells promotes guided peripheral nerve regeneration

Alternatives to autografts have long been sought for use in bridging neural gaps. Many entubulation materials have been studied, although with generally disappointing results in comparison with autografts. The purpose of this study was to design a more effective neural guidance conduit, to introduce Schwann cells into the conduit, and to determine regenerative capability through it in an in vivo model. A novel, fully biodegradable polymer conduit was designed and fabricated for use in peripheral nerve repair, which approximates the macro- and microarchitecture of native peripheral nerves. It comprised a series of longitudinally aligned channels, with diameters ranging from 60 to 550 microns. The lumenal surfaces promoted the adherence of Schwann cells, whose presence is known to play a key role in nerve regeneration. This unique channel architecture increased the surface area available for Schwann cell adherence up to five-fold over that available through a simple hollow conduit. The conduit was composed of a high-molecular-weight copolymer of lactic and glycolic acids (PLGA) (MW 130,000) in an 85:15 monomer ratio. A novel foam-processing technique, employing low-pressure injection molding, was used to create highly porous conduits (approximately 90% pore volume) with continuous longitudinal channels. Using this technique, conduits were constructed containing 1, 5, 16, 45, or more longitudinally aligned channels. Prior to cellular seeding of these conduits, the foams were prewet with 50% ethanol, flushed with physiologic saline, and coated with laminin solution (10 microg/mL). A Schwann cell suspension was dynamically introduced into these processed foams at a concentration of 5 X 10(5) cells/mL, using a simple bioreactor flow loop. In vivo regeneration studies were carried out in which cell-laden five-channel polymer conduits (individual channel ID 500 microm, total conduit OD 2.3 mm) were implanted across a 7-mm gap in the rat sciatic nerve (n = 4), and midgraft axonal regeneration compared with autografts (n = 6). At 6 weeks, axonal regeneration was observed in the midconduit region of all five channels in each experimental animal. The cross-sectional area comprising axons relative to the open conduit cross sectional area (mean 26.3%, SD 10. 1%) compared favorably with autografts (mean 23.8%, SD 3.6%). Our methodology can be used to create polymer foam conduits containing longitudinally aligned channels, to introduce Schwann cells into them, and to implant them into surgically created neural defects. These conduits provide an environment permissive to axonal regeneration. Furthermore, this polymer foam-processing method and unique channeled architecture allows the introduction of neurotrophic factors into the conduit in a controlled fashion. Deposition of different factors into distinct regions within the conduit may be possible to promote more precisely guided neural regeneration.

Bridging Schwann cell transplants promote axonal regeneration from both the rostral and caudal stumps of transected adult rat spinal cord

Transplantation of cellular components of the permissive peripheral nerve environment in some types of spinal cord injury holds great promise to support regrowth of axons through the site of injury. In the present study, Schwann cell grafts were positioned between transected stumps of adult rat thoracic spinal cord to test their efficacy to serve as bridges for axonal regeneration. Schwann cells were purified in culture from adult rat sciatic nerve, suspended in Matrigel: DMEM (30:70), and drawn into polymeric guidance channels 8 mm long at a density of 120 x 10(6) cells ml-1. Adult Fischer rat spinal cords were transected at the T8 cord level and the next caudal segment was removed. Each cut stump was inserted 1 mm into the channel. One month later, a bridge between the severed stumps had been formed, as determined by the gross and histological appearance and the ingrowth of propriospinal axons from both stumps. Propriospinal neurons (mean, 1064 +/- 145 SEM) situated as far away as levels C3 and S4 were labelled by retrograde tracing with Fast Blue injected into the bridge. Near the bridge midpoint there was a mean of 1990 +/- 594 myelinated axons and eight times as many nonmyelinated, ensheathed axons. Essentially no myelinated or unmyelinated axons were observed in control Matrigel-only grafts. Brainstem neurons were not retrogradely labelled from the graft, consistent with growth of immunoreactive serotonergic and noradrenergic axons only a short distance into the rostral end of the graft, not far enough to reach the tracer placed at the graft midpoint. Anterograde tracing with PHA-L introduced rostral to the graft demonstrated that axons extended the length of the graft but essentially did not leave the graft. This study demonstrates that Schwann cell grafts serve as bridges that support (1) regrowth of both ascending and descending axons across a gap in the adult rat spinal cord and (2) limited regrowth of serotonergic and noradrenergic fibers from the rostral stump. Regrowth of monoaminergic fibres into grafts was not seen in an earlier study of similar grafts placed inside distally capped rather than open-ended channels. Additional intervention will be required to foster growth of the regenerated axons from the graft into the distal cord tissue.

Skin-derived precursors generate myelinating Schwann cells for the injured and dysmyelinated nervous system

Although neural stem cells hold considerable promise for treatment of the injured or degenerating nervous system, their current human sources are embryonic stem cells and fetally derived neural tissue. Here, we asked whether rodent and human skin-derived precursors (SKPs), neural crest-related precursors found in neonatal dermis, represent a source of functional, myelinating Schwann cells. Specifically, cultured SKPs responded to neural crest cues such as neuregulins to generate Schwann cells, and these Schwann cells proliferated and induced myelin proteins when in contact with sensory neuron axons in culture. Similar results were obtained in vivo; 6 weeks after transplantation of naive SKPs or SKP-derived Schwann cells into the injured peripheral nerve of wild-type or shiverer mutant mice (which are genetically deficient in myelin basic protein), the majority of SKP-derived cells had associated with and myelinated axons. Naive rodent or human SKPs also generated Schwann cells that myelinated CNS axons when transplanted into the dysmyelinated brain of neonatal shiverer mice. Thus, neonatal SKPs generate functional neural progeny in response to appropriate neural crest cues and, in so doing, provide a highly accessible source of myelinating cells for treatment of nervous system injury, congenital leukodystrophies, and dysmyelinating disorders.

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.

Designing ideal conduits for peripheral nerve repair

Nerve tubes, guides, or conduits are a promising alternative for autologous nerve graft repair. The first biodegradable empty single lumen or hollow nerve tubes are currently available for clinical use and are being used mostly in the repair of small-diameter nerves with nerve defects of < 3 cm. These nerve tubes are made of different biomaterials using various fabrication techniques. As a result these tubes also differ in physical properties. In addition, several modifications to the common hollow nerve tube (for example, the addition of Schwann cells, growth factors, and internal frameworks) are being investigated that may increase the gap that can be bridged. This combination of chemical, physical, and biological factors has made the design of a nerve conduit into a complex process that demands close collaboration of bioengineers, neuroscientists, and peripheral nerve surgeons. In this article the authors discuss the different steps that are involved in the process of the design of an ideal nerve conduit for peripheral nerve repair.

Transplantation of Schwann cells and/or olfactory ensheathing glia into the contused spinal cord: Survival, migration, axon association, and functional recovery

Schwann cells (SCs) and olfactory ensheathing glia (OEG) have shown promise for spinal cord injury repair. We sought their in vivo identification following transplantation into the contused adult rat spinal cord at 1 week post-injury by: (i) DNA in situ hybridization (ISH) with a Y-chromosome specific probe to identify male transplants in female rats and (ii) lentiviral vector-mediated expression of EGFP. Survival, migration, and axon-glia association were quantified from 3 days to 9 weeks post-transplantation. At 3 weeks after transplantation into the lesion, a 60-90% loss of grafted cells was observed. OEG-only grafts survived very poorly within the lesion (<5%); injection outside the lesion led to a 60% survival rate, implying that the injury milieu was hostile to transplanted cells and or prevented their proliferation. At later times post-grafting, p75(+)/EGFP(-) cells in the lesion outnumbered EGFP(+) cells in all paradigms, evidence of significant host SC infiltration. SCs and OEG injected into the injury failed to migrate from the lesion. Injection of OEG outside of the injury resulted in their migration into the SC-injected injury site, not via normal-appearing host tissue but along the pia or via the central canal. In all paradigms, host axons were seen in association with or ensheathed by transplanted glia. Numerous myelinated axons were found within regions of grafted SCs but not OEG. The current study details the temporal survival, migration, axon association of SCs and OEG, and functional recovery after grafting into the contused spinal cord, research previously complicated due to a lack of quality, long-term markers for cell tracking in vivo.

Skin-derived precursors generate myelinating Schwann cells that promote remyelination and functional recovery after contusion spinal cord injury

Transplantation of exogenous cells is one approach to spinal cord repair that could potentially enhance the growth and myelination of endogenous axons. Here, we asked whether skin-derived precursors (SKPs), a neural crest-like precursor that can be isolated and expanded from mammalian skin, could be used to repair the injured rat spinal cord. To ask this question, we isolated and expanded genetically tagged murine SKPs and either transplanted them directly into the contused rat spinal cord or differentiated them into Schwann cells (SCs), and performed similar transplantations with the isolated, expanded SKP-derived SCs. Neuroanatomical analysis of these transplants 12 weeks after transplantation revealed that both cell types survived well within the injured spinal cord, reduced the size of the contusion cavity, myelinated endogenous host axons, and recruited endogenous SCs into the injured cord. However, SKP-derived SCs also provided a bridge across the lesion site, increased the size of the spared tissue rim, myelinated spared axons within the tissue rim, reduced reactive gliosis, and provided an environment that was highly conducive to axonal growth. Importantly, SKP-derived SCs provided enhanced locomotor recovery relative to both SKPs and forebrain subventricular zone neurospheres, and had no impact on mechanical or heat sensitivity thresholds. Thus, SKP-derived SCs provide an accessible, potentially autologous source of cells for transplantation into and treatment of the injured spinal cord.

The ability of human Schwann cell grafts to promote regeneration in the transected nude rat spinal cord

Advances in the purification and expansion of Schwann cells (SCs) from adult human peripheral nerve, together with biomaterials development, have made the construction of unique grafts with defined properties possible. We have utilized PAN/PVC guidance channels to form solid human SC grafts which can be transplanted either with or without the channel. We studied the ability of grafts placed with and without channels to support regeneration and to influence functional recovery; characteristics of the graft and host/graft interface were also compared. The T9-T10 spinal cord of nude rats was resected and a graft was placed across the gap; methylprednisolone was delivered acutely to decrease secondary injury. Channels minimized the immigration of connective tissue into grafts but contributed to some necrotic tissue loss, especially in the distal spinal cord. Grafts without channels contained more myelinated axons (x = 2129 +/- 785) vs (x = 1442 +/- 514) and were larger in cross-sectional area ( x = 1.53 +/- 0.24 mm2) vs (x = 0.95 +/- 0.86 mm2). The interfaces formed between the host spinal cord and the grafts placed without channels were highly interdigitated and resembled CNS-PNS transition zones; chondroitin sulfate proteoglycans was deposited there. Whereas several neuronal populations including propriospinal, sensory, motoneuronal, and brainstem neurons regenerated into human SC grafts, only propriospinal and sensory neurons were observed to reenter the host spinal cord. Using combinations of anterograde and retrograde tracers, we observed regeneration of propriospinal neurons up to 2.6 mm beyond grafts. We estimate that 1% of the fibers that enter grafts reenter the host spinal cord by 45 days after grafting. Following retrograde tracing from the distal spinal cord, more labeled neurons were unexpectedly found in the region of the dextran amine anterograde tracer injection site where a marked inflammatory reaction had occurred. Animals with bridging grafts obtained modestly higher scores during open field [(x = 8.2 +/- 0.35) vs (x = 6.8 +/- 0.42), P = 0.02] and inclined plane testing (x = 38.6 +/- 0. 542) vs (x = 36.3 +/- 0.53), P = 0.006] than animals with similar grafts in distally capped channels. In summary, this study showed that in the nude rat given methylprednisolone in combination with human SC grafts, some regenerative growth occurred beyond the graft and a modest improvement in function was observed.

Nerve guides seeded with autologous schwann cells improve nerve regeneration

This study evaluates the ability of Schwann cells (SCs) transplanted into a nerve guide to improve regeneration and reinnervation after sciatic nerve resection and repair, leaving a 6-mm gap, in the mouse. SCs were isolated from predegenerated adult sciatic nerves and expanded in culture using a chemically defined medium. Syngeneic, isogeneic, and autologous SCs were suspended in Matrigel and seeded in resorbable, permeable poly(l-lactide-co-epsilon-caprolactone) guides at 150,000 cells/tube. Guides containing SCs were compared to guides filled with Matrigel alone and with peroneal nerve autografts. Functional reinnervation was assessed by noninvasive methods to determine recovery of sweating, nociceptive, sensory, and motor functions in the hindpaw during 4 months postoperation. Morphological analysis of the regenerated nerves was performed at the end of follow-up. The group with an autograft achieved faster and higher levels of reinnervation and higher number of regenerated myelinated fibers than groups repaired by tubulization. The immunogenicity of transplanted SCs influenced the outcome of nerve regeneration. Transplants of autologous SCs resulted in slightly lower levels of reinnervation than autografts, but higher recovery and number of regenerated fibers reaching the distal nerve than transplants of isologous and syngeneic SCs, although most of the differences were not statistically significant. Syngeneic SCs did not improve regeneration with respect to acellular guides. Prelabeled transplanted SCs were found to survive into the guide 1-3 months after implantation, to a larger number when they were autologous than syngeneic. Cellular prostheses composed of a resorbable guide seeded with autologous SCs appear as an alternative for repairing long gaps in injured nerves, approaching the success of autografts.

Regeneration of the rat sciatic nerve into allografts made acellular through chemical extraction

The aim of this study was to develop a procedure by which myelin and Schwann cells could be removed from a peripheral nerve while the basal lamina tubes, remained intact, and to test if such preparations could be used as allografts for the repair of a gap in the continuity of the rat sciatic nerve. We found that extraction with the detergents Triton X-100 and deoxycholate resulted in acellular nerve segments with preserved basal lamina tubes, here defined as the tubes which surrounds the axon/Schwann cell units. The morphology of the acellular nerve segments was revealed by scanning electron microscopy, teasing, immunohistochemistry and electrophoresis. Such grafts when allografted between two outbred rat strains, were found to support outgrowth of axons and migration of Schwann cells, which reoccupied the empty basal lamina tubes without excessive signs of inflammation. This new paradigm offers a possible solution to the major shortcomings of autologous nerve grafts, i.e., the requirement to sacrifice a healthy nerve and the shortage of graft material available for repair.

Multiple-channel scaffolds to promote spinal cord axon regeneration

As molecular, cellular, and tissue-level treatments for spinal cord injury are discovered, it is likely that combinations of such treatments will be necessary to elicit functional recovery in animal models or patients. We describe multiple-channel, biodegradable scaffolds that serve as the basis for a model to investigate simultaneously the effects on axon regeneration of scaffold architecture, transplanted cells, and locally delivered molecular agents. Poly(lactic-co-glycolic acid) (PLGA) with copolymer ratio 85:15 was used for these initial experiments. Injection molding with rapid solvent evaporation resulted in scaffolds with a plurality of distinct channels running parallel along the length of the scaffolds. The feasibility of creating scaffolds with various channel sizes and geometries was demonstrated. Walls separating open channels were found to possess void fractions as high as 89%, with accessible void fractions as high as 90% through connections 220 microm or larger. Scaffolds degraded in vitro over a period of 30 weeks, over which time-sustained delivery of a surrogate drug was observed for 12 weeks. Primary neonatal Schwann cells were distributed in the channels of the scaffold and remained viable in tissue culture for at least 48 h. Schwann-cell containing scaffolds implanted into transected adult rat spinal cords contained regenerating axons at one month post-operation. Axon regeneration was demonstrated by three-dimensional reconstruction of serial histological sections.

The role of the Schwann cell in trophic support and regeneration

Schwann cells show remarkable versatility in undertaking a broad repertoire of functions. Ensheathment and myelination are specifically regulated by contact with axons, and the Schwann cell is centrally involved in extracellular matrix production in the peripheral nerve trunk. Additional Schwann cell functions include the promotion of both peripheral and central nervous system regeneration, provision of a versatile source of trophic factors, the capacity to remyelinate central nervous system axons, and the restoration of electrophysiological conduction. Since it is now possible to isolate Schwann cells both from neonatal and adult human peripheral nerve, their ability to promote regenerative efforts by many central neurons suggests a role for Schwann cell autografts in influencing central nervous system repair.

Regrowth of axons into the distal spinal cord through a Schwann-cell-seeded mini-channel implanted into hemisected adult rat spinal cord

Schwann cells (SCs) have been shown to be a key element in promoting axonal regeneration after being grafted into the central nervous system (CNS). In the present study, SC-supported axonal regrowth was tested in an adult rat spinal cord implantation model. This model is characterized by a right spinal cord hemisection at the eighth thoracic segment, implantation of a SC-containing mini-channel and restoration of cerebrospinal fluid circulation by suturing the dura. We demonstrate that a tissue cable containing grafted SCs formed an effective bridge between the two stumps of the hemicord 1 month after transplantation. Approximately 10 000 myelinated and unmyelinated axons (1 : 9) per cable were found at its midpoint. In addition to propriospinal axons and axons of peripheral nervous system (PNS) origin, axons from as many as 19 brainstem regions also grew into the graft without additional treatments. Most significantly, some regenerating axons in the SC grafts were able to penetrate through the distal graft-host interface to re-enter the host environment, as demonstrated by anterograde axonal labelling. These axons coursed toward, and then entered the grey matter where terminal bouton-like structures were observed. In channels containing no SCs, limited axonal growth was seen within the graft and no axons penetrated the distal interface. These findings further support the notion that SCs are strong promotors of axonal regeneration and that the mini-channel model may be appropriate for further investigation of axonal re-entry, synaptic reconnection and functional recovery following spinal cord injury.

Effect of allogeneic Schwann cell transplantation on peripheral nerve regeneration

Transplantation of allogeneic Schwann cells (SC) would make it feasible to reconstruct immediately peripheral nerve defects, compared to using autologous SC; however, this treatment modality has not been adequately evaluated. The aim of this study was to characterize and compare the effects of allogeneic versus syngeneic SC transplantation following peripheral nerve injury. Polyhydroxybutyrate conduits were used to bridge a 10-mm gap in the rat sciatic nerve. The conduits were filled with alginate hydrogel with or without cultured allogeneic or syngeneic genetically labeled SC, without the use of immunosuppressive therapy, and examined after 2, 3, and 6 weeks with 5-bromo-4-chloro-3-indoyl-beta-D-galactosidase chemical staining and immunohistochemistry to quantify SC migration into the conduit, axonal regeneration, the state of SC differentiation, and the expression of major histocompatibility complexes (MHC) I and II, as well as to quantify macrophage and B- and T-lymphocyte infiltration. Allogeneic SC were rejected by 6 weeks, whereas syngeneic SC could still be identified. Allogeneic and syngeneic SC equally enhanced the axonal regeneration distance but the quantity of axons was greater using syngeneic SC. The ingrowth of SC into the conduits containing allogeneic SC was similar to that observed in the presence of syngeneic SC, indicating the absence of deleterious immune response. SC continued to express phenotypic markers of nonmyelination and these were highest in conduits with allogeneic SC. Expression of MHC I and II was higher in the conduits with allogeneic SC at 3 weeks and without significant difference in the number of macrophages and lymphocytes, except at 6 weeks, when there was a larger number of lymphocytes using syngeneic SC. In conclusion, allogeneic SC enhanced axonal regeneration distance and did not induce a deleterious immune response. In a clinical setting the immediate availability of allogeneic SC for transplantation may compensate for the better outcome achieved by the use of autologous SC that require a longer preparation time in culture.

Schwann cell transplantation for repair of the adult spinal cord

The Schwann cell is one of the most widely studied cell types for repair of the spinal cord. These cells play a crucial role in endogenous repair of peripheral nerves due to their ability to dedifferentiate, migrate, proliferate, express growth promoting factors, and myelinate regenerating axons. Following trauma to the spinal cord, Schwann cells migrate from the periphery into the injury site, where they apparently participate in endogenous repair processes. For transplantation into the spinal cord, large numbers of Schwann cells are necessary to fill injury-induced cystic cavities. Several culture systems have been developed that provide large, highly purified populations of Schwann cells. Importantly, the development of in vitro systems to harvest human Schwann cells presents a unique opportunity for autologous transplantation in the clinic. In animal models of spinal cord injury (SCI), grafting Schwann cells or peripheral nerve into the lesion site has been shown to promote axonal regeneration and myelination. However, axons do not regenerate beyond the transplant due to the inhibitory nature of the glial scar surrounding the injury. To overcome the glial scar inhibition, additional approaches such as increasing the intrinsic capacity of axons to regenerate and/or removal of the inhibitory molecules associated with reactive astrocytes and/or oligodendrocyte myelin should be incorporated. Clearly, Schwann cells have great potential for repair of the injured spinal cord, but they need to be combined with other interventions to maximize axonal regeneration and functional recovery.

Neurotrophic factors, cellular bridges and gene therapy for spinal cord injury

Injury to the adult mammalian spinal cord results in extensive axonal degeneration, variable amounts of neuronal loss, and often severe functional deficits. Restoration of controlled function depends on regeneration of these axons through an injury site and the formation of functional synaptic connections. One strategy that has emerged for promoting axonal regeneration after spinal cord injury is the implantation of autologous Schwann cells into sites of spinal cord injury to support and guide axonal growth. Further, more recent experiments have shown that neurotrophic factors can also promote axonal growth, and, when combined with Schwann cell grafts, can further amplify axonal extension after injury. Continued preclinical development of these approaches to neural repair may ultimately generate strategies that could be tested in human injury.

Transplantation of olfactory ensheathing cells or Schwann cells restores rapid and secure conduction across the transected spinal cord

Olfactory ensheathing cells (OECs) or Schwann cells were transplanted into the transected dorsal columns of the rat spinal cord to induce axonal regeneration. Electrophysiological recordings were obtained in an isolated spinal cord preparation. Without transplantation of cells, no impulse conduction was observed across the transection site; but following cell transplantation, impulse conduction was observed for over a centimeter beyond the lesion. Cell labelling indicated that the regenerated axons were derived from the appropriate neuronal source, and that donor cells migrated into the denervated host tract. As reported in previous studies, the number of regenerated axons was limited. Conduction velocity measurements and morphology indicated that the regenerated axons were myelinated, but conducted faster and had larger axon areas than normal axons. These results indicate that the regenerated spinal cord axons induced by cell transplantation provide a quantitatively limited but rapidly conducting new pathway across the transection site.

Olfactory ensheathing cells induce less host astrocyte response and chondroitin sulphate proteoglycan expression than Schwann cells following transplantation into adult CNS white matter

Both Schwann cells and olfactory ensheathing cells (OECs) create an environment favorable to axon regeneration when transplanted into the damaged CNS. However, transplanted cells can also exert an effect on the host tissue that will influence the extent to which regenerating axons can grow beyond the transplanted area and reenter the host environment. In this study equivalent numbers of Lac-Z-labeled Schwann cells and OECs have been separately transplanted into normal white matter of adult rat spinal cord and the host astrocyte response to each compared. Schwann cell transplantation resulted in a greater area of increased glial fibrillary acidic protein (GFAP) expression compared to that associated with OEC transplantation. This was accompanied by a greater increase in the expression of axon growth inhibitory chrondroitin sulfate proteoglycans (CSPGs) following Schwann cell transplantation compared to OEC transplantation. However, no differences were detected in the increased expression of the specific CSPG neurocan following transplantation of the two cell types. These results mirror differences in the interactions between astrocytes and either Schwann cells or OECs observed in tissue culture models and reveal one aspect of the complex biology of creating regeneration-promoting environments by cell transplantation where transplanted OECs have favorable properties compared to transplanted Schwann cells.

Transplants of Schwann cell cultures promote axonal regeneration in the adult mammalian brain

Transplantation of embryonic brain tissue or mature peripheral nerves into the adult mammalian central nervous system promotes axonal regrowth from axotomized central nervous system neurons; however, the cellular origin and molecular nature of the factors promoting axonal growth in vivo are unknown. To further characterize cellular environments that facilitate regeneration of central nervous system axons, we developed a methodology whereby cultured cell preparations can be transplanted into the brain of mature mammals. For this procedure, lesions are produced in the septal-hippocampal system of adult rats, and selected regions from collagen-supported Schwann cell/neuron cultures (consisting of Schwann cells, extracellular matrix, and degenerating neuronal processes and myelin but devoid of neuronal perikarya and fibroblasts) are positioned within the intracephalic cavity so that they bridge the lesion gap (approximately 3 mm) separating the septum and hippocampus. At various time up to 3 weeks after transplantation, specimens were prepared for acetylcholinesterase histochemistry and the immunocytochemical localization of laminin (an extracellular matrix protein) and C-4 (a Schwann cell membrane antigen). All specimens (from uninjured controls and from animals with either acellular collagen or mature Schwann cell/extracellular matrix transplants) contained laminin immunoreactivity associated with the meninges, choroid plexus, ependyma, and cerebral blood vessels. All animals with transplants showed prominent laminin staining on astrocytic processes along the intracephalic cavity, but only the Schwann cell/extracellular matrix transplants exhibited dense laminin and C-4 immunoreactivity within the cellular portion of the transplants. Regeneration of acetylcholinesterase-positive septal fibers occurred only in animals containing Schwann cell/extracellular matrix transplants. By 6 days after transplantation, acetylcholinesterase-positive fibers were observed both on laminin-positive cellular tissue strands connecting the septum and the Schwann cell/extracellular matrix transplants and on the initial portions of the transplants. By day 14, acetylcholinesterase-positive fibers traversed the entire lesion cavity in intimate association with the laminin- and C-4-positive cellular layer of the transplants and reinnervated the host hippocampus. However, cholinergic fibers were not associated with all laminin-containing processes along the lesion cavity nor did they grow along acellular collagen transplants. These results indicate the presence of factors in transplants of cultured Schwann cells and their associated extracellular matrix that promote rapid regeneration of central nervous system cholinergic axons in vivo.

Chondroitinase ABC enhances axonal regrowth through Schwann cell-seeded guidance channels after spinal cord injury

Grafting of Schwann cell-seeded channels into hemisected adult rat thoracic spinal cords has been tested as a strategy to bridge the injured cord. Despite success in guiding axonal growth into the graft, regeneration across the distal graft-host interface into the host spinal cord was limited. We hypothesized that chondroitin sulfate (CS) glycoforms deposited at the gliotic front of the interface constitute a molecular barrier to axonal growth into the host cord. Because CS glycoforms deposited by purified astrocytes in vitro were removable by digestion with chondroitinase ABC, we attempted to achieve likewise by infusion of the enzyme to the host side of the interface. By 1 month post-treatment, significant numbers of regenerating axons crossed an interface that was subdued in macrophage/microglia reaction and decreased in CS-immunopositivity. The axons extended as far into the caudal cord as 5 mm, in contrast to nil in vehicle-infused controls. Fascicular organizations of axon-Schwann cell units within the regenerated tissue cable were better-preserved in enzyme-treated cords than in vehicle-infused controls. We conclude that CS glycoforms deposited during gliosis at the distal graft-host interface could be cleared by the in vivo action of chondroitinase ABC to improve prospects of axonal regeneration into the host spinal cord.

Transplantation of oligodendrocytes and Schwann cells into the spinal cord of the myelin-deficient rat

Transplantation of oligodendrocytes or Schwann cells into the spinal cord of the newborn myelin-deficient (md) rat, an X-linked myelin mutant, was carried out and the extent of myelination of CNS axons studied. Dissociated glial cell suspensions, prepared from the spinal cords of female litter-mates, were injected into the lumbar spinal cord of 15 md rats and 5 normal litter-mates. In eight of the md rats examined 12 to 21 days post-transplantation patches of myelin produced by the transplanted oligodendrocytes were found in the dorsal or ventral columns. In two rats, small patches of myelination were found in more than one site. The myelin in these patches was positive on immunocytochemical staining for proteolipid protein. These observations were interpreted as evidence of the origin of this myelin from donor oligodendrocytes, as the md rat has an abnormality in synthesis of this protein. In addition, this myelin differed in its ultrastructure from host myelin, having a normal intraperiod line. Injection of cultured Schwann cells also resulted in extensive myelination of axons in the dorsal columns by these cells.