Nerve growth factor (NGF)-treated nitrocellulose enhances and directs the regeneration of adult rat dorsal root axons through intraspinal neural tissue transplants

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Sprouting, regeneration and circuit formation in the injured spinal cord: factors and activity

Central nervous system (CNS) injuries are particularly traumatic, owing to the limited capabilities of the mammalian CNS for repair. Nevertheless, functional recovery is observed in patients and experimental animals, but the degree of recovery is variable. We review the crucial characteristics of mammalian spinal cord function, tract development, injury and the current experimental therapeutic approaches for repair. Regenerative or compensatory growth of neurites and the formation of new, functional circuits require spontaneous and experimental reactivation of developmental mechanisms, suppression of the growth-inhibitory properties of the adult CNS tissue and specific targeted activation of new connections by rehabilitative training.

Cellular repair strategies for spinal cord injury

The implantation of exogenous cells or tissues has been a popular and successful strategy to overcome physical discontinuity and support axon growth in experimental models of spinal cord injury (SCI). Cellular therapies exhibit a multifarious potential for SCI restoration, providing not only a supportive substrate upon which axons can traverse the injury site, but also reducing progressive tissue damage and scarring, facilitating remyelination repair, and acting as a source for replacing and re-establishing lost neural tissue and its circuitry. The past two decades of research into cell therapies for SCI repair have seen the progressive evolution from whole tissue strategies, such as peripheral nerve grafts, to the use of specific, purified cell types from a diverse range of sources and, recently, to the employment of stem or neural precursor cell populations that have the potential to form a full complement of neural cell types. Although the progression of cell therapies from laboratory to clinical implementation has been slow, human SCI safety and efficacy trials involving several cell types within the US appear to be close at hand.

Experimental strategies to promote spinal cord regeneration--an integrative perspective

Detailed pathophysiological findings of secondary damage phenomena after spinal cord injury (SCI) as well as the identification of inhibitory and neurotrophic proteins have yielded a plethora of experimental therapeutic approaches. Main targets are (i) to minimize secondary damage progression (neuroprotection), (ii) to foster axon conduction (neurorestoration) and (iii) to supply a permissive environment to promote axonal sprouting (neuroregenerative therapies). Pre-clinical studies have raised hope in functional recovery through the antagonism of growth inhibitors, application of growth factors, cell transplantation, and vaccination strategies. To date, even though based on successful pre-clinical animal studies, results of clinical trials are characterized by dampened effects attributable to difficulties in the study design (patient heterogeneity) and species differences. A combination of complementary therapeutic strategies might be considered pre-requisite for future synergistic approaches. Here, we line out pre-clinical interventions resulting in improved functional neurological outcome after spinal cord injury and track them on their intended way to bedside.

The enhancement of cell adherence and inducement of neurite outgrowth of dorsal root ganglia co-cultured with hyaluronic acid hydrogels modified with Nogo-66 receptor antagonist in vitro

Hyaluronic acid hydrogels modified with polyclonal anti-Nogo-66 receptor antibody were developed in order to promote regeneration in the injured CNS. These modified hydrogels were intended not only to deliver antibodies, but also to serve as a scaffold for neural regeneration following their implantation into injured tissue. Since unmodified hyaluronic acid-hydrogels do not support cell attachment, the gels were modified with polyclonal anti-Nogo-66 receptor with the aim of altering the surface properties of the gels in such a way as to improve neuronal adherence and survival. After evaluating the immobilization efficiency of the system, chicken dorsal root ganglia and dorsal root ganglia cells were planted on the surface of the modified gels to determine cell viability. Dorsal root ganglia were also cultured close to the gels in order to assay the inducement of neurite outgrowth. In dorsal root ganglia and cell viability assay, dorsal root ganglia and neuron cells could adhere to the modified hydrogels and survive well, but it did not happen to unmodified hydrogels. After 72 h, these attached cells were stained positively with immuno-staining for neurofilament. Neurite outgrowth inducement assay showed that the number and length of dorsal root ganglia neurites on the side toward modified hydrogels were significantly more than that on the opposite side (both P<0.01). The results reveal that hyaluronic acid-hydrogels modified with anti-Nogo-66 receptor can support neural cell attachment and survival in vitro. Furthermore, this system can greatly induce neurite outgrowth. The results also indicate that this modified hydrogels have potential to repair injury in the CNS.

Controlled release of anti-inflammatory agent alpha-MSH from neural implants

Si-multi-electrode arrays implanted into brain tissue for long-term recording lose electrical connectivity due to the post-implantation inflammatory reaction. This inflammatory reaction creates a physical and electrical gap between the electrode and the surrounding neurons. In this study, novel nitrocellulose-based coatings were developed for the sustained delivery of the anti-inflammatory neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH). alpha-MSH was incorporated in micron-scale nitrocellulose coatings and slow, sustained release over 21 days was attained in vitro. The alpha-MSH released on day 21 was still bioactive, and successfully inhibited nitric oxide (NO) production by LPS-stimulated microglia. The amount of initial drug loading directly affected the release rate, with higher initial loading increasing the mass released but not the percent of drug released. The surface morphology and thickness of the coatings were examined by scanning electron microscopy (SEM) and profilometry. In addition, impedance measurement showed that the alpha-MSH loaded nitrocellulose coatings reduced the magnitude of electrode impedance at the biologically relevant frequency of 1 kHz. In conclusion, nitrocellulose-based, bioactive coatings that release anti-inflammatory agents without increasing the impedence of the electrode were successfully fabricated. These coatings have the potential to reduce inflammation at the electrode-brain interface in vivo, and facilitate long-term recordings from Si-multi-electrode arrays.

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

Here we review mechanisms and molecules that necessitate protection and oppose axonal growth in the injured spinal cord, representing not only a cast of villains but also a company of therapeutic targets, many of which have yet to be fully exploited. We next discuss recent progress in the fields of bridging, overcoming conduction block and rehabilitation after spinal cord injury (SCI), where several treatments in each category have entered the spotlight, and some are being tested clinically. Finally, studies that combine treatments targeting different aspects of SCI are reviewed. Although experiments applying some treatments in combination have been completed, auditions for each part in the much-sought combination therapy are ongoing, and performers must demonstrate robust anatomical regeneration and/or significant return of function in animal models before being considered for a lead role.

Acidic FGF enhances functional regeneration of adult dorsal roots

It has been well documented that the regeneration of sensory axons severed in the dorsal roots into the spinal cord is largely inhibited in adult mammals. We investigated whether peripheral nerve grafts combined with acidic fibroblast growth factor (aFGF) could induce the regeneration of transected dorsal roots in adult rats, as evaluated by cortical somatosensory evoked potentials (SEPs). Median nerve (forelimb) stimuli produced consistent responses in the primary somatosensory cortex of normal rats, but these were completely eliminated after the transection of cervical 6th - 8th roots. The dorsal root stumps were immediately anastomosed to the cord with intercostal nerve grafts. Subsequently, aFGF in fibrin glue was administered to the grafted area. Four to twenty weeks after rhizotomy, six of the seven rats receiving such reconstruction had recovery of SEPs. The reappearing SEPs typically showed similar waveforms and latencies as normal ones. They were eliminated by retransection of the repaired roots, thus verifying their source as the regenerated roots. We present here substantial evidence that aFGF enhances the functional restoration of cut dorsal roots. Cortical SEPs is considered a useful tool in evaluating such regeneration. These results may offer therapeutic potential in the treatment of dorsal root injuries.

Neurotrophic effects on dorsal root regeneration into the spinal cord

Dorsal root ganglion neurons exhibit a robust and generally successful regenerative response following injury of their peripheral processes. Regeneration fails, however, after section of their central processes in the dorsal roots or dorsal columns. Experiments characterizing the attenuated response of these neurons to injury, and the inhibition of regeneration exerted by astrocytes and oligodendrocytes within the dorsal root entry zone and spinal cord, have contributed important insights into the failure of regeneration after injury to the central nervous system (CNS). Interventions that have enhanced the metabolic response of injured dorsal root ganglion neurons, and altered the inhospitable environment, have increased sensory afferent regeneration and recovery. There is reason to expect that these strategies will help to develop clinically applicable treatments of CNS injuries.

NT-3 gene delivery elicits growth of chronically injured corticospinal axons and modestly improves functional deficits after chronic scar resection

Nervous system growth factors promote axonal growth following acute spinal cord injury. In the present experiment, we examined whether delivery of neurotrophic factors after chronic spinal cord injury would also promote axonal growth and influence functional outcomes. Adult Fischer 344 rats underwent mid-thoracic spinal cord dorsal hemisection lesions. Three months later, primary fibroblasts genetically modified to express human neurotrophin-3 (NT-3) were placed in, and distal to, the lesion cavity. Upon sacrifice 3 months later (6 months following the initial lesion), NT-3-grafted animals exhibited significant growth of corticospinal axons up to 15 mm distal to the lesion site and showed a modest but significant 1.5-point improvement in locomotor scores (P < 0.05) on the BBB scale, compared to control-grafted animals. Thus, growth factor gene delivery can elicit growth of corticospinal axons in chronic stages of injury and improves functional outcomes compared to non-growth-factor-treated animals.

Stimulating regeneration in the damaged spinal cord

Great progress has been made in recent years in experimental strategies for spinal cord repair. In this review we describe two of these strategies, namely the use of neurotrophic factors to promote functional regeneration across the dorsal root entry zone (DREZ), and the use of synthetic fibronectin conduits to support directed axonal growth. The junction between the peripheral nervous system (PNS) and central nervous system (CNS) is marked by a specialized region, the DREZ, where sensory axons enter the spinal cord from the dorsal roots. After injury to dorsal roots, axons will regenerate as far as the DREZ but no further. However, recent studies have shown that this barrier can be overcome and function restored. In animals treated with neurotrophic factors, regenerating axons cross the DREZ and establish functional connections with dorsal horn cells. For example, intrathecal delivery of neurotrophin 3 (NT3) supports ingrowth of A fibres into the dorsal horn. This ingrowth is revealed using a transganglionic anatomical tracer (cholera toxin subunit B) and analysis at light and electron microscopic level. In addition to promoting axonal growth, spinal cord repair is likely to require strategies for supporting long-distance regeneration. Synthetic fibronectin conduits may be useful for this purpose. Experimental studies indicate that fibronectin mats implanted into the spinal cord will integrate with the host tissue and support extensive and directional axonal growth. Growth of both PNS and CNS axons is supported by the fibronectin, and axons become myelinated by Schwann cells. Ongoing studies are aimed at developing composite conduits and promoting axonal growth from the fibronectin back into the spinal cord.

Pegylated brain-derived neurotrophic factor shows improved distribution into the spinal cord and stimulates locomotor activity and morphological changes after injury

The neurotrophin brain-derived neurotrophic factor (BDNF) shows promise for the treatment of central nervous system (CNS) trauma and disease. Effective delivery methods are required, however, for BDNF to be useful as a therapeutic agent. To this end, we examined the penetration of intrathecally infused N-terminal pegylated BDNF (peg-BDNF) compared to similar infusion of native BDNF after spinal cord injury (SCI). Pegylation dramatically improved delivery of BDNF to the spinal cord and induced the expression of Fos in spinal cord neurons. To test whether enhanced delivery would improve the modest effects on behavioral recovery and axonal outgrowth observed with native BDNF infusion, we assessed the efficacy of 2-week 25 microg/day peg-BDNF treatment, beginning 12-24 h (early) or 15 days (delayed) after midthoracic spinal contusion. Similar to native BDNF, early treatment with peg-BDNF accelerated the recovery of stepping in the open-field and acutely stimulated locomotor central pattern generator activity, as seen by the activation of hindlimb airstepping during either period of administration. The infusion of peg-BDNF, regardless of the timing of delivery, was related to enhanced sprouting of putative cholinergic fibers, like that observed after high dose native BDNF treatment. Despite improved delivery, however, neither axonal responses nor the extent of locomotor recovery were enhanced compared to native BDNF treatment. This suggests that alternative strategies, such as neurotrophin treatment in conjunction with cell transplantation techniques, or treatment nearer the cell bodies of target neurons might be employed in an attempt to effect significant repair after SCI.

NT-3 promotes growth of lesioned adult rat sensory axons ascending in the dorsal columns of the spinal cord

The regeneration capacity of spinal cord axons is severely limited. Recently, much attention has focused on promoting regeneration of descending spinal cord pathways, but little is known about the regenerative capacity of ascending axons. Here we have assessed the ability of neurotrophic factors to promote regeneration of sensory neurons whose central axons ascend in the dorsal columns. The dorsal columns of adult rats were crushed and either brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), neurotrophin-3 (NT-3) or a vehicle solution was delivered continuously to the lesion site for 4 weeks. Transganglionic labelling with cholera toxin beta subunit (CTB) was used to selectively label large myelinated Abeta fibres. In lesioned rats treated with vehicle, CTB-labelled fibres were observed ascending in the gracile fasciculus, but these stopped abruptly at the lesion site, with no evidence of sprouting or growth into lesioned tissue. No CTB-labelled terminals were observed in the gracile nucleus, indicating that the lesion successfully severed all ascending dorsal column axons. Treatment with BDNF did not promote axonal regeneration. In GDNF-treated rats fibres grew around cavities in caudal degenerated tissue but did not approach the lesion epicentre. NT-3, in contrast, had a striking effect on promoting growth of lesioned dorsal column axons with an abundance of fibre sprouting apparent at the lesion site, and many fibres extending into and beyond the lesion epicentre. Quantification of fibre growth confirmed that only in NT-3-treated rats did fibres grow into the crush site and beyond. No evidence of terminal staining in the gracile nucleus was apparent following any treatment. Thus, although NT-3 promotes extensive growth of lesioned axons, other factors may be required for complete regeneration of these long ascending projections back to the dorsal column nuclei. The intrathecal delivery of NT-3 or other neurotrophic molecules has obvious advantages in clinical applications, as we show for the first time that dorsal column axonal regeneration can be achieved without the use of graft implantation or nerve lesions.

Carbon filaments direct the growth of postlesional plastic axons after spinal cord injury

The effect of implantation of carbon filaments and fetal tissues on the axonal regeneration following contusion injury in a rat model was investigated by in situ immunofluorescence. Female Sprague-Dawley rats were subjected to severe contusion injury to the spinal cord at T9-T10. All animals were divided into 5 groups (N = 5/group): normal controls. surgical controls, with carbon filament implants, with fetal tissue implants and with implants consisting of fetal tissue cocultured with carbon filaments. After a 10-week survival period, the astroglial response was assessed by immunoreactive glial fibrillary acidic protein and the neuro-axonal profile by immunoreactive phosphorylated and nonphosphorylated neurofilament proteins. The contusion injury resulted in: (a) dramatically increased immunoreactivity of glial fibrillary acidic protein indicating injury-associated reactive astrogliosis, (b) increase in immunoreactive phosphorylated neurofilament protein indicating upregulated phosphorylation of neurofilament protein, (c) with no change in the highly differentiated nonphosphorylated neurofilament protein which normally occur in the nonregenerating mature neurons. Implantation of fetal tissues alone following contusion injury did not show any appreciable change with regard to the immunoreactivities for the glial and neuronal markers studied, compared to the injury controls. However, the implantation of carbon filaments alone or together with fetal tissues directed the growth of glial fibrillary acidic protein-positive astroglia and phosphoneurofilament-positive neurites along the carbon fibers, with no effect on nonphosphoneurofilament protein. In conclusion, implantation of carbon filaments appears to be critical for facilitating the attachment of astroglia forming a substrate and scaffolding that can further support and direct the growth of postlesional plastic axons across the lesion. In addition, carbon filament prostheses in combination with fetal tissue implants provides an improved combinational approach to promote regrowth of injured neurons following injury.

Focally administered nerve growth factor suppresses molecular regenerative responses of axotomized peripheral afferents in rats

Effects of delivery of nerve growth factor, from a catheterized osmotic mini-pump to the proximal stump of a transected sciatic nerve, were compared with the effects of normal saline. A pilot measured retrograde axonal transport of nerve growth factor to determine a pump concentration which raised axonal transport ipsilaterally, but not contralaterally. The effects of this delivery over 12 days were then determined on expression of growth-associated protein-43, trkA, p75NTR and preprotachykinin A ipsilateral and contralateral to the pump in dorsal root ganglia at L4 and L5 (pooled). Ganglionic expression was measured both as messenger RNA and protein. Axotomy (saline pumps) increased growth-associated protein-43 messenger RNA (318 +/- 14%: all changes are percent of contralateral, non-axotomized ganglia with saline pumps) and immunoreactivity (431 +/- 43%). The increase was significantly less (P < 0.001) ipsilateral to nerve growth factor pumps (191 +/- 45%). Axotomy reduced expression of p75NTR (messenger RNA: 52 +/- 17%, P < 0.01; immunoreactivity: 74 +/- 3%, P < 0.05). These decreases were converted to increases by nerve growth factor delivery (respectively 143 +/- 40% and 281 +/- 67%; both P < 0.01). With trkA, axotomy decreased the expression of the messenger RNA (68 +/- 40%, P < 0.01) and of the primary translation product--110,000 mol. wt protein (55 +/- 12%, P < 0.01)--but not the fully glycosylated trkA protein (mol. wt 145,000). Nerve growth factor delivery did not affect trkA expression. Axotomy reduced messenger RNA for the substance P precursor, preprotachykinin A, to 42 +/- 17% (P < 0.01) and this reduction was prevented by nerve growth factor treatment. We suggest that the primary effect of nerve growth factor on axotomized C-fibres is not to promote regeneration, although that may be its secondary effect via an action on Schwann cells. It is possible that reduced neuronal sensitivity to nerve growth factor during regeneration is advantageous in suppressing nociception.

NGF levels decrease in the spinal cord and dorsal root ganglion after spinal hemisection

To examine changes in nerve growth factor (NGF) levels in spinal cord and dorsal root ganglia (DRG) after spinal injury, male Sprague-Dawley rats weighing 150-175 g were given spinal hemisections. NGF content was measured at various post-surgical times and compared with naive controls (n = 4 per time point) in the spinal cord, DRG and blood serum by ELISA techniques (Promega). Levels of NGF in the blood serum were significantly increased 8-fold at 48h but were significantly decreased in the spinal cord and DRG by 2- to 4-fold until 7 days postsurgery (ANOVA, p < 0.05). Contrary to accepted dogma, spinal injury results in decreased levels of NGF in the spinal cord and DRG following spinal injury.

Neurotrophic agents in fibrin glue mediate adult dorsal root regeneration into spinal cord

OBJECTIVE

The aim of the present study was to determine whether neurotrophic factors (NTFs) exogenously administered in fibrin glue assisted cut dorsal root axons of adult rats to regenerate into the spinal cord.

METHODS

Rats received intraspinal implants of fibrin glue containing neurotrophin-3, brain-derived NTF, ciliary NTF, or Dulbecco's modified Eagle's medium (control) into left dorsal quadrant cavities aspirated in the lumbar enlargement. The transected L5 dorsal root stump was placed at the bottom of the lesion cavity and was secured between the fibrin glue and the spinal cord. Regenerated dorsal root axons were subsequently labeled with immunohistochemical methods to demonstrate those that contained calcitonin gene-related peptide.

RESULTS

Calcitonin gene-related peptide-immunoreactive dorsal root axons regenerated across the dorsal root-spinal cord interface of rats with fibrin glue containing neurotrophin-3, brain-derived NTF, or ciliary NTF, entered the spinal cord, and frequently arborized within clusters of motoneuronal cell bodies. Only a few axons regenerated into the spinal cord of animals with fibrin glue implants that lacked NTF, and their growth within the spinal cord was extremely limited. The results of quantitative studies confirmed these observations.

CONCLUSION

Our results indicate that neurotrophin-3, brain-derived NTF, and ciliary NTF enhance dorsal root regeneration into spinal cord and that fibrin glue is an effective medium for intraspinal delivery of NTF. This method of delivering NTF may therefore provide a strategy for restoring injured spinal reflex arcs.

Ensheathing glia transplants promote dorsal root regeneration and spinal reflex restitution after multiple lumbar rhizotomy

Previously, we have shown that transplants of olfactory bulb ensheathing cells promoted regeneration of transected dorsal roots into the spinal cord. In this study, we assessed the ability of regenerating axons to make functional connections in the cord. Dorsal roots L3 to L6 were sectioned close to their entrance into the spinal cord and reapposed after injecting a suspension of ensheathing cells into each dorsal root entry zone (Group G). Afferent regeneration into the cord and recovery of spinal reflexes were compared with animals that received no injection (Group S) or culture medium without cells (Group C). Electrophysiological tests, to measure nerve conduction and spinal reflexes (H response and withdrawal reflex) evoked by stimulation of afferents of the sciatic nerve, were performed. At 14 days after surgery, H response was found in only 1 of 7 rats of Group G, and withdrawal reflexes were absent from all animals. At 60 days, the H response reappeared in 7 of 10 rats of Group G, and 1 of 5 of each of Groups C and S. The withdrawal reflex recovered in 4 of 10 rats of Group G, but in none of Groups C and S. Immunohistochemical labeling for calcitonin gene-related peptide (CGRP) in rats of Group G showed immunoreactive fibers entering the dorsal horn from sectioned roots, although at lower density than in the contralateral side. In conclusion, transplanted ensheathing cells promoted central regeneration and functional reconnection of regenerating sensory afferents.

The transitional zone and CNS regeneration

Most nerves are attached to the neuraxis by rootlets. The CNS-PNS transitional zone (TZ) is that length of rootlet containing both central and peripheral nervous tissue. The 2 tissues are separated by a very irregular but clearly defined interface, consisting of the surface of the astrocytic tissue comprising the central component of the TZ. Central to this, myelin sheaths are formed by oligodendrocytes and the supporting tissue is astrocytic. Peripheral to it, sheaths are formed by Schwann cells which are enveloped in endoneurium. The features of transitional nodes are a composite of those of central and peripheral type. The interface is penetrated only by axons. It is absent at first. It is formed by growth of processes into the axon bundle from glial cell bodies around its perimeter. These form a barrier across the bundle which fully segregates prospectively myelinated axons. Rat spinal dorsal root TZs have been used extensively to study CNS axon regeneration. The CNS part of the TZ responds to primary afferent axon degeneration and to regenerating axons in ways which constitute a satisfactory model of the gliotic tissue response which occurs in CNS lesions. It undergoes gliosis and the gliotic TZ tissue expands distally along the root. In mature animals axons can regenerate satisfactorily through the endoneurial tubes of the root but cease growth on reaching the gliotic tissue. The general objective of experimental studies is to achieve axon regeneration from the PNS through this outgrowth and into the dorsal spinal cord. Since immature tissue has a greater capacity for regeneration than that of the adult, one approach includes the transplantation of embryonic or fetal dorsal root ganglia into the locus of an extirpated adult ganglion. Axons grow centrally from the transplanted ganglion cells and some enter the cord. Other approaches include alteration of the TZ environment to facilitate axon regeneration, for example, by the application of tropic, trophic, or other molecular factors, and also by transplantation of cultured olfactory ensheathing cells (OECs) into the TZ region. OECs, by association with growing axons, facilitate their extensive regeneration into the cord. Unusually, ventral motoneuron axons may undergo some degree of unaided CNS regeneration. When interrupted in the spinal cord white matter, some grow out to the ventral rootlet TZ and thence distally in the PNS. The DRTZ is especially useful for quantitative studies on regeneration. Since the tissue is anisometric, individual parameters such as axon numbers, axon size and glial ensheathment can be readily measured and compared in the CNS and PNS environments, thereby yielding indices of regeneration across the interface for different sets of experimental conditions.

Electromagnetic fields influence NGF activity and levels following sciatic nerve transection

Pulsed electromagnetic fields (PEMF) have been shown to increase the rate of nerve regeneration. Transient post-transection loss of target-derived nerve growth factor (NGF) is one mechanism proposed to signal induction of early nerve regenerative events. We tested the hypothesis that PEMF alter levels of NGF activity and protein in injured nerve and/or dorsal root ganglia (DRG) during the first stages of regeneration (6-72 hr). Rats with a transection injury to the midthigh portion of the sciatic nerve on one side were exposed to PEMF or sham control PEMF for 4 hr/day for different time periods. NGF-like activity was determined in DRG, in 5-mm nerve segments proximal and distal to the transection site and in a corresponding 5-mm segment of the contralateral nonoperated nerve. NGF-like activity of coded tissue samples was measured in a blinded fashion using the chick DRG sensory neuron bioassay. Overall, PEMF caused a significant decrease in NGF-like activity in nerve tissue (P < 0.02, repeated measures analysis of variance, ANOVA) with decreases evident in proximal, distal, and contralateral nonoperated nerve. Unexpectedly, transection was also found to cause a significant (P=0.001) 2-fold increase in DRG NGF-like activity between 6 and 24 hr postinjury in contralateral but not ipsilateral DRG. PEMF also reduced NGF-like activity in DRG, although this decrease did not reach statistical significance. Assessment of the same nerve and DRG samples using ELISA and NGF-specific antibodies confirmed an overall significant (P < 0.001) decrease in NGF levels in PEMF-treated nerve tissue, while no decrease was detected in DRG or in nerve samples harvested from PEMF-treated uninjured rats. These findings demonstrate that PEMF can affect growth factor activity and levels, and raise the possibility that PEMF might promote nerve regeneration by amplifying the early postinjury decline in NGF activity.

Adenoviral nerve growth factor and beta-galactosidase transfer to spinal cord: a behavioral and histological analysis

OBJECT

The present study characterizes the time course and loci of gene expression induced by the administration of adenoviral vectors into spinal cord. Although a marked inflammatory response to these vectors occurred, no effect on spinal cord function was seen in the 1st postoperative week. The expression of transgenic genes delivered by viral vectors is being exploited throughout the nervous system. The present study utilized adenoviral vectors containing the Rous sarcoma virus (RSV) promoter and a nuclear localization signal to achieve transgenic expression in mammalian spinal cord.

METHODS

Initial experiments utilizing the vector Ad.RSVlacZ (10(12) particles/ml) injected into the region of the central canal resulted in viral gene expression stretching over approximately 1.2 cm of spinal cord. Gene expression was first detected 3 days following viral administration and lasted until postinjection Day 14 with peak expression at Day 7. A variety of cell types in both white and gray matter expressed lacZ. Transgenic expression of the neurotrophin nerve growth factor (NGF) was achieved using injections of Ad.RSVNGF. On histological examination mononuclear inflammatory infiltrate and gliosis were revealed surrounding the injection sites of spinal cords receiving adenovirus but not vehicle. To assess spinal cord function during viral gene expression, animals previously trained in an operant runway task were tested at 7 days postinjection (the peak of viral gene expression) and demonstrated no changes in spinal cord function.

CONCLUSIONS

Results of this study using adenoviral neurotrophic gene transfer indicate that it provided an effective tool for the delivery of potentially therapeutic proteins to the injured or diseased spinal cord.

Schwann cells genetically modified to secrete human BDNF promote enhanced axonal regrowth across transected adult rat spinal cord

The infusion of BDNF and NT-3 into Schwann cell (SC) grafts promotes regeneration of brainstem neurones into the grafts placed in adult rat spinal cord transected at T8 (Xu et al., 1995b). Here, we compared normal SCs with SCs genetically modified to secrete human BDNF, grafted as trails 5 mm long in the cord distal to a transection site and also deposited in the transection site, for their ability to stimulate supraspinal axonal regeneration beyond the injury. SCs were infected with the replication-deficient retroviral vector pL(hBDNF)RNL encoding the human preproBDNF cDNA. The amounts of BDNF secreted (as detected by ELISA) were 23 and 5 ng/24 h per 106 cells for infected and normal SCs, respectively. Biological activity of the secreted BDNF was confirmed by retinal ganglion cell bioassay. The adult rat spinal cord was transected at T8. The use of Hoechst prelabelled SCs demonstrated that trails were maintained for a month. In controls, no SCs were grafted. One month after grafting, axons were present in SC trails. More 5-HT-positive and some DbetaH-positive fibres were observed in the infected vs. normal SC trails. When Fast Blue was injected 5 mm below the transection site (at the end of the trail), as many as 135 retrogradely labelled neurones could be found in the brainstem, mostly in the reticular and raphe nuclei (normal SCs, up to 22, mostly in vestibular nuclei). Numerous neurones were labelled in the ventral hypothalamus (normal SCs, 0). Also, following Fast Blue injection, a mean of 138 labelled cells was present in dorsal root ganglia (normal SCs, 46) and spinal cord (39 vs. 32) rostral to the transection. No labelled spinal neurones rostral to the transection were seen when SCs were not transplanted. Thus, the transplantation of SCs secreting increased amounts of BDNF improved the regenerative response across a transection site in the thoracic cord. Moreover, the enhanced regeneration observed with infected SCs may be specific as the largest response was from neurones known to express trkB.

Activated macrophage/microglial cells can promote the regeneration of sensory axons into the injured spinal cord

A prominent role for phagocytic cells in the regenerative response to CNS or PNS injury has been suggested by numerous studies. In the present work we tested whether increasing the presence of phagocytic cells at a spinal cord injury site could enhance the regeneration of sensory axons from cut dorsal roots. Nitrocellulose membranes treated with TGF-beta or coated with microglial cells were cotransplanted with fetal spinal cord tissue into an injured adult rat spinal cord. Cut dorsal roots were apposed to both sides of the nitrocellulose. Four weeks later, animals were sacrificed and spinal cord tissue sections were processed for immunocytochemical detection of calcitonin gene-related peptide (CGRP-ir) to identify regenerated sensory axons. Adjacent sections were processed with the antibody ED-1 or the lectin GSA-B4 for detection of macrophage/microglial cells in association with the regrowing axons. Qualitative and quantitative data indicate a correlation between the pattern and extent of axonal regeneration and the presence of phagocytic cells along the nitrocellulose implant. Axonal regeneration could be experimentally limited by implanting a nitrocellulose strip treated with macrophage inhibitory factor. These results indicate that increasing the presence of activated macrophage/microglial cells at a spinal cord injury site can provide an environment beneficial to the promotion of regeneration of sensory axons, possibly by the release of cytokines and interaction with other nonneuronal cells in the immediate vicinity.

A combination of insulin-like growth factor-I and platelet-derived growth factor enhances myelination but diminishes axonal regeneration into Schwann cell grafts in the adult rat spinal cord

Insulin-like growth factor-I (IGF-I) promotes axonal regeneration in the peripheral nervous system and this effect is enhanced by platelet-derived growth factor (PDGF). We decided, therefore, to study the effects of these factors on axonal regeneration in the adult rat spinal cord. Semipermeable polymer tubes, closed at the distal end, containing Matrigel mixed with cultured rat Schwann cells and IGF-I/PDGF, were placed at the proximal stump of the spinal cord after removal of the thoracic T9-11 segments. Control animals received implants of only Matrigel and Schwann cells or only Matrigel and IGF-I/PDGF. Four weeks after implantation, electron microscopic analysis showed that the addition of IGF-I/PDGF resulted in an increase in the myelinated:unmyelinated fiber ratio from 1:7 to 1:3 at 3 mm in the Schwann cell graft, and that myelin sheath thickness was increased 2-fold. The reduced number of unmyelinated axons was striking in electron micrographs. These results suggested that IGF-I/PDGF enhanced myelin formation of regenerated axons in Schwann cell implants, but there was a 36% decrease in the total number of myelinated axons at the 3 mm level of the graft. This finding and the altered myelinated:unmyelinated fiber ratio revealed that the overall fiber regeneration into Schwann cell implants was diminished up to 63% by IGF-I/PDGF. Histological evaluation revealed that there were more larger cavities in tissue at the proximal spinal cord-graft interface in animals receiving a Schwann cell implant with IGF-I/PDGF. Such cavitation might have contributed to the reduction in axonal ingrowth. In sum, the results indicate that whereas the combination of IGF-I and PDGF enhances myelination of regenerating spinal cord axons entering implants of Matrigel and Schwann cells after midthoracic transection, the overall regeneration of axons into such Schwann cell grafts is diminished.

Multiple neurotrophic factors including NGF-like activity in nerve regeneration chamber fluids

Silicone nerve regeneration chambers were implanted between the cut ends of the sciatic nerve of adult rats. Neurotrophic activities in cell-free fluids collected from the chambers were determined using bioassays for survival of embryonic chick ciliary and sympathetic neurons in culture. Separation by molecular exclusion HPLC of the components of fluids collected 1, 2 or 3 days after implantation revealed the presence of a multitude of neurotrophic factors differing in their molecular weights, specificity towards the two types of neurons, and time course. Antiserum to nerve growth factor partially blocked sympathetic activity of fluids collected at 1 day. Affinity purified antibody was also effective and completely eliminated bioactivity of HPLC fractions corresponding to the molecular weight of nerve growth factor. The presence in the fluids of 13-18 and 20-32 kD components active towards ciliary neurons is consistent with the release of fibroblast growth factor and ciliary neurotrophic factor respectively. The stimulation of sympathetic neurons by the 13-18 kD material, and also by 4-6 and 7-11 kD components cannot be entirely accounted for by known factors. This study demonstrates that a number of neurotrophic factors, which differ in their specificity towards sympathetic and parasympathetic neurons, are made available to the region of axonal regrowth over the first few days of regeneration. Contrary to earlier reports, nerve growth factor-like activity was shown to be present in nerve regeneration chambers.

The migration and intermixing of donor and host glia on nitrocellulose polymers implanted into cortical lesion cavities in adult mice and rats

The fate of neonatal glia (mostly glial fibrillary acidic protein-positive astrocytes), cultured on nitrocellulose papers and implanted into cortical lesion cavities, was examined in adult mice and rats. In mice, a Y-chromosome-specific probe and in situ hybridization techniques were used to identify male cells. Male-female grafts allowed visualization of donor glia and their behaviour after transplantation; female-male grafts allowed an analysis of how host cells responded to the presence of the implants. There was substantial intermixing of cells, with many donor glia migrating away from the implants and host cells migrating onto both sides of the nitrocellulose paper. In rats, donor glia were labelled with fluorescein-conjugated latex microspheres prior to transplantation on nitrocellulose polymers. The rat data were broadly consistent with those obtained from the mouse; moreover, immunohistochemical studies in rats suggested that the majority of host cells migrating onto the previously cell-coated papers were astrocytes. In a number of studies, glia-coated polymers have been used in an attempt to promote the regrowth of axons across lesion sites in the brain and spinal cord. The present work suggests that both transplanted and host glia may influence the regenerative growth seen in such implants.

Regeneration of dorsal root axons is related to specific non-neuronal cells lining NGF-treated intraspinal nitrocellulose implants

The regeneration of sensory axons from severed dorsal roots can be enhanced by the presence of nerve growth factor (NGF)-treated nitrocellulose strips implanted into an intraspinal lesion cavity. Rather than being directly apposed to the transplant, most regenerating axons are separated from the nitrocellulose by several layers of non-neuronal cells, suggesting that these cells may have a role in the promotion of axonal regrowth. The cellular layers associated with untreated nitrocellulose strips or NGF-treated implants were examined in this study to determine if there were differences in their arrangement or orientation along the implant which might explain some of the possible effects of substrate-bound NGF on axonal regrowth. Into a hemisection lesion cavity created in the adult rat lumbar spinal cord NGF-treated or untreated strips of nitrocellulose were placed vertically, with intact pieces of fetal spinal cord (FSC) tissue transplanted along each side. The distal ends of cut dorsal rootlets were apposed to the fetal tissue. Immunocytochemical and electron microscopic examination 30-60 days post-transplantation revealed a distinct layering of cell types along the NGF-treated strips. Closest to the nitrocellulose was a single layer of macrophages, followed by a separate layer of fibroblasts with dense collagen bundles, then a layer of astroglial cells, before reaching the neuropil of the fetal spinal cord tissue. A thickened basal lamina formed between the fibroblast and astrocytic cell layers and bundles of regenerated sensory axons extended along the interface between these two layers. In contrast, non-neuronal cells along untreated nitrocellulose strips were not as well organized, with an intermixing of fibroblasts and astroglial cells and only scattered macrophage-like cells. Axons rarely were found in conjunction with this mixed population of cells and, overall, fewer regenerated axons extended into transplants with untreated nitrocellulose. The results demonstrate consistent differences in the composition and organization of non-neuronal cells adjacent to NGF-treated nitrocellulose implants, compared to untreated implants. This suggests that the presence of bound NGF influences the recruitment of various cells from the surrounding transplant tissue as well as from the previously injured dorsal rootlets. The capacity for NGF to promote the regeneration of sensory axons may be an indirect effect that is mediated or potentiated by the non-neuronal cell population that gathers in response to the presence of bound NGF.

Fetal cell grafts into resection and contusion/compression injuries of the rat and cat spinal cord

This article reviews recent findings concerning the feasibility, basic neurobiology, and potential functional benefits of fetal CNS tissue grafts into acute and chronic lesions of the adult spinal cord. In the rat, neuro-anatomical observations suggest that transplants into resection cavities establish neuritic projections that could functionally reunite separated rostral and caudal segments of the host spinal cord. Furthermore, some complementary electrophysiological evidence has been obtained for synaptic connectivity between host and graft neurons. In these studies, extracellular single-unit activity was evoked in fetal spinal cord (FSC) transplants by stimulating host dorsal roots that had been juxtaposed to donor tissue at the time of transplantation. In other investigations, we examined whether grafts could also establish axonal projections to appropriate areas of gray matter in the chronically injured spinal cord. For this purpose, fetal serotoninergic (5-HT) neurons were injected caudal to complete spinal cord transections that had been made 1-3 months earlier. Immunocytochemistry revealed that these cells projected their axons into gray matter regions normally innervated by bulbospinal 5-HT neurons. To investigate transplantation in a more clinically relevant lesion model, a third group of experiments involved injection of dissociated cell suspensions into acute [less than 24 h postinjury (p.i.)]), subchronic (7-10 days p.i), and chronic (greater than or equal to one month, p.i.) contusion lesions. Such grafts routinely filled areas that otherwise would have been regions of cavitation extending rostral-caudal distances of approximately 7 mm. FSC transplants in such injuries also appeared to influence some aspects of motoneuron excitability and hindlimb locomotion. More recent studies of the cat spinal cord have extended these findings in the rat by showing long-term survival (greater than 2 years) of fetal CNS allografts in recipients with either subtotal transection or compression lesions. Preliminary studies of connectivity have also shown host-graft projection patterns similar to those seen in the rat. Behavioral analyses are currently underway to examine the effects of fetal grafts in cats with chronic postcompression lesions. These observations in the rat and cat are discussed in the general context of basic biological and clinical issues relevant to the long-term objective of promoting functional improvement in the damaged spinal cord.

Peripheral nerve injury down-regulates CNTF expression in adult rat sciatic nerves

Ciliary neurotrophic factor (CNTF) is a 200-amino acid protein expressed in high concentrations by peripheral nerves and is thought to be important for the survival and regeneration of injured motoneurons (Lin et al., J Biol Chem 265:8942-8947, 1990). To better understand CNTF's role in nerve injury we have characterized the effects of crush injury on the expression of CNTF in adult rat sciatic nerves using specific antibody and RNA probes. Following a crush injury, both the protein and mRNA levels undergo pronounced decreases distal to the crush. These changes in CNTF expression were qualitatively distinct from changes in the expression of the low-affinity NGF receptor (p75NGFR), which increases following crush. Thus, the changes in CNTF levels do not reflect an overall down-regulation of mRNA during degeneration, and are inconsistent with the proposed role of CNTF in neuronal injury, since its levels are decreasing at the same time as the requirement for neurotrophic support is increasing.

Blot and culture analysis of neuronotrophic factors in nerve regeneration chamber fluids

The fluid accumulating in silicone nerve regeneration chambers implanted between the cut ends of rat sciatic nerve contains neuronotrophic activities towards embryonic chick ciliary and sympathetic neurons. The blot and culture technique of Carnow et al. was used to determine if part of the neuronotrophic activities is due to ciliary neuronotrophic factor, which supports the survival of both types of neurons in vitro. The technique involves separating the fluid proteins by SDS-polyacrylamide gel electrophoresis, Western transfer, and then culturing of purified neurons on the nitrocellulose blots. After 24 hr surviving neurons are restricted to regions of the blot where neuronotrophic factor is present. Analysis of 1 and 2 day fluids showed that a multitude of factors are present, particularly in the 19-30 kD molecular weight range, with discrete peaks of activity at molecular weights consistent with those reported for ciliary neuronotrophic factor. There were several other peaks of activity present in the fluids in addition to these.

Nerve growth factor promotes CNS cholinergic axonal regeneration into acellular peripheral nerve grafts

Peripheral nerve grafts promote vigorous regeneration of adult mammalian CNS axons. Elimination of nerve-associated cells by freeze-thawing abolishes this promoting quality, possibly by creating inhibitory cellular debris and/or destroying the production of stimulatory factors by living Schwann or other cells. Here, debris-free acellular peripheral nerve segments placed between the disconnected septum and the hippocampal formation acquired almost no cholinergic axons after 1 month. However, such acellular nerve grafts treated before implantation with purified beta-nerve growth factor (NGF) contained nearly as many longitudinally oriented cholinergic axons as did fresh cellular nerve grafts. These results suggest that (i) NGF is required for the regeneration of adult CNS cholinergic axons into nerve grafts and (ii) an important function of living cells within peripheral nerve may be the production of neuronotrophic factors such as NGF.

Intraspinal transplants

Transplants of embryonic central nervous system tissue have long been used to study axon growth during development and regeneration, and more recently to promote recovery in models of human diseases. Transplants of embryonic substantia nigra correct some of the deficits found in experimental Parkinson's disease, for example, by mechanisms that are thought to include release of neurotransmitter and reinnervation of host targets, as well as by stimulating growth of host axons. Similar mechanisms appear to allow intraspinal transplants of embryonic brainstem to reverse locomotor and autonomic deficits due to experimental spinal cord injuries. Embryonic spinal cord transplants offer an additional strategy for correcting the deficits of spinal cord injury because, by replacing damaged populations of neurons, they may mediate the restoration of connections between host neurons. We have found that spinal cord transplants permit regrowth of adult host axons resulting in reconstitution of synaptic complexes within the transplant that in many respects resemble normal synapses. Transplants of fetal spinal cord may also contribute to behavioral recovery by rescuing axotomized host neurons that otherwise would have died. Electrophysiological and behavioral investigations of functional recovery after intraspinal transplantation are preliminary, and the role of transplants in the treatment of human spinal cord injury is uncertain. Transplants are contributing to our understanding of the mechanisms of recovery, however, and are likely to play a role in the development of rational treatments.

Astrocyte-polymer implants promote regeneration of dorsal root fibers into the adult mammalian spinal cord

To overcome obstacles to the regeneration of crushed dorsal root fibers at the dorsal root entry zone, we have employed specially designed Millipore implants coated with embryonic astrocytes to serve as a substrate for axonal growth. This strategy was successful in promoting the growth of crushed dorsal root axons into the grey matter of the adult mammalian spinal cord in a small number of animals. Fiber ingrowth into the spinal cord was closely associated with the surface of the polymer implant. In addition, unique terminal arbor malformations, not normally present, were seen in several animals. A consistent finding was the presence of a limited inflammatory response in regions immediately adjacent to the implant where axons penetrate the spinal cord. Our findings suggest that providing the dorsal root entry zone with an embryonic milieu can stimulate a limited amount of axonal regeneration into the adult mammalian spinal cord.

A comparison of the regeneration potential of dorsal root fibers into gray or white matter of the adult rat spinal cord

To assess the role of white matter inhibition as a barrier to neurite outgrowth in vivo, we unilaterally transected three consecutive lumbar dorsal roots (L4-L6), incised the spinal cord, and transplanted the peripheral stump of L4 either medially onto the white matter of the dorsal columns or laterally, just superficial to the gray matter of the dorsal horn at the level of L5. Three weeks to seven months later, the translocated root was retransected, and its central stump was anterogradely labeled with HRP. The staining pattern demonstrated that regenerating sensory axons had entered the spinal cord from both medially and laterally placed roots. Axonal staining from medially placed dorsal roots (onto the white matter of the dorsal columns) was sparse and limited to the white matter. Staining of laterally placed roots revealed a small subpopulation of regenerating axons which had entered the gray matter and formed terminal arbors. Successful axonal regeneration into the gray matter, albeit minimal, was associated with a localized and limited inflammatory response near the sites of axonal ingrowth.