Regeneration of adult rat CNS axons into peripheral nerve autografts: ultrastructural studies of the early stages of axonal sprouting and regenerative axonal growth

Astrocytes and microglia in the coordination of CNS development and homeostasis

Glia have emerged as important architects of central nervous system (CNS) development and maintenance. While traditionally glial contributions to CNS development and maintenance have been studied independently, there is growing evidence that either suggests or documents that glia may act in coordinated manners to effect developmental patterning and homeostatic functions in the CNS. In this review, we focus on astrocytes, the most abundant glia in the CNS, and microglia, the earliest glia to colonize the CNS highlighting research that documents either suggestive or established coordinated actions by these glial cells in various CNS processes including cell and/or debris clearance, neuronal survival and morphogenesis, synaptic maturation, and circuit function, angio-/vasculogenesis, myelination, and neurotransmission. Some molecular mechanisms underlying these processes that have been identified are also described. Throughout, we categorize the available evidence as either suggestive or established interactions between microglia and astrocytes in the regulation of the respective process and raise possible avenues for further research. We conclude indicating that a better understanding of coordinated astrocyte-microglial interactions in the developing and mature brain holds promise for developing effective therapies for brain pathologies where these processes are perturbed.

Engineering Fiber-Based Nervous Tissue Constructs for Axon Regeneration

Biomaterial-based scaffolds used in nerve conduits including channels for confining regenerating axons and 3-dimensional (3D) gels as substrates for growth have made improvements in models of nerve repair. Many biomaterial strategies, however, continue to fall short of autologous nerve grafts, which remain the current gold standard in repairing severe nerve lesions (<20 mm). Intraluminal nerve conduit fibers have also shown considerable promise in directing regenerating axons in vitro and in vivo and have gained increasing interest for nerve repair. It is unknown, however, how growing axons respond to a fiber when encountered in a 3D environment. In this study, we considered a construct consisting of a compliant collagen hydrogel matrix and a fiber component to assess contact-guided axon growth. We investigated preferential axon outgrowth on synthetic and natural polymer fibers by utilizing small-diameter microfibers of poly-L-lactic acid and type I collagen representing 2 different fiber stiffnesses. We found that axons growing freely in a 3D hydrogel culture preferentially attach, turn and follow fibers with outgrowth rates and distances that far exceed outgrowth in a hydrogel alone. Wet-spun type I collagen from rat tail tendon performed the best, associated with highly aligned and accelerated outgrowth. This study also evaluated the response of dorsal root ganglion neurons from adult rats to provide data more relevant to axon regenerative potential in nerve repair. We found that ECM treatments on fibers enhanced the regeneration of adult axons indicating that both the physical and biochemical presentation of the fibers are essential for enhancing axon guidance and growth.

Pharmacological, Cell, and Gene Therapy Strategies to Promote Spinal Cord Regeneration

In this review, recent studies using pharmacological treatment, cell transplantation, and gene therapy to promote regeneration of the injured spinal cord in animal models will be summarized. Pharmacological and cell transplantation treatments generally revealed some degree of effect on the regeneration of the injured ascending and descending tracts, but further improvements to achieve a more significant functional recovery are necessary. The use of gene therapy to promote repair of the injured nervous system is a relatively new concept. It is based on the development of methods for delivering therapeutic genes to neurons, glia cells, or nonneural cells. Direct in vivo gene transfer or gene transfer in combination with (neuro)transplantation (ex vivo gene transfer) appeared powerful strategies to promote neuronal survival and axonal regrowth following traumatic injury to the central nervous system. Recent advances in understanding the cellular and molecular mechanisms that govern neuronal survival and neurite outgrowth have enabled the design of experiments aimed at viral vector-mediated transfer of genes encoding neurotrophic factors, growth-associated proteins, cell adhesion molecules, and antiapoptotic genes. Central to the success of these approaches was the development of efficient, nontoxic vectors for gene delivery and the acquirement of the appropriate (genetically modified) cells for neurotransplantation. Direct gene transfer in the nervous system was first achieved with herpes viral and E1-deleted adenoviral vectors. Both vector systems are problematic in that these vectors elicit immunogenic and cytotoxic responses. Adeno-associated viral vectors and lentiviral vectors constitute improved gene delivery systems and are beginning to be applied in neuroregeneration research of the spinal cord. Ex vivo approaches were initially based on the implantation of genetically modified fibroblasts. More recently, transduced Schwann cells, genetically modified pieces of peripheral nerve, and olfactory ensheathing glia have been used as implants into the injured spinal cord.

Microglial responses around intrinsic CNS neurons are correlated with axonal regeneration

Background

Microglia/macrophages and lymphocytes (T-cells) accumulate around motor and primary sensory neurons that are regenerating axons but there is little or no microglial activation or T-cell accumulation around axotomised intrinsic CNS neurons, which do not normally regenerate axons. We aimed to establish whether there was an inflammatory response around the perikarya of CNS neurons that were induced to regenerate axons through a peripheral nerve graft.

Results

When neurons of the thalamic reticular nucleus (TRN) and red nucleus were induced to regenerate axons along peripheral nerve grafts, a marked microglial response was found around their cell bodies, including the partial enwrapping of some regenerating neurons. T-cells were found amongst regenerating TRN neurons but not rubrospinal neurons. Axotomy alone or insertion of freeze-killed nerve grafts did not induce a similar perineuronal inflammation. Nerve grafts in the corticospinal tracts did not induce axonal regeneration or a microglial or T-cell response in the motor cortex.

Conclusions

These results strengthen the evidence that perineuronal microglial accumulation (but not T-cell accumulation) is involved in axonal regeneration by intrinsic CNS and other neurons.

Axon regeneration through scaffold into distal spinal cord after transection

We employed Fast Blue (FB) axonal tracing to determine the origin of regenerating axons after thoracic spinal cord transection injury in rats. Schwann cell (SC)-loaded, biodegradable, poly(lactic-co-glycolic acid) (PLGA) scaffolds were implanted after transection. Scaffolds loaded with solubilized basement membrane preparation (without SCs) were used for negative controls, and nontransected cords were positive controls. One or 2 months after injury and scaffold implantation, FB was injected 0-15 mm caudal or about 5 mm rostral to the scaffold. One week later, tissue was harvested and the scaffold and cord sectioned longitudinally (30 microm) on a cryostat. Trans-scaffold labeling of neuron cell bodies was identified with confocal microscopy in all cell-transplanted groups. Large (30-50 microm diameter) neuron cell bodies were predominantly labeled in the ventral horn region. Most labeled neurons were seen 1-10 mm rostral to the scaffold, although some neurons were also labeled in the cervical cord. Axonal growth occurred bidirectionally after cord transection, and axons regenerated up to 14 mm beyond the PLGA scaffolds and into distal cord. The extent of FB labeling was negatively correlated with distance from the injection site to the scaffold. Electron microscopy showed myelinated axons in the transverse sections of the implanted scaffold 2 months after implantation. The pattern of myelination, with extracellular collagen and basal lamina, was characteristic of SC myelination. Our results show that FB labeling is an effective way to measure the origin of regenerating axons.

Fabrication of semipermeable hollow fiber membranes with highly aligned texture for nerve guidance

In order to improve the guidance potential of a nerve entubulation bridging device, highly aligned textures were formed on the inner surface of semipermeable hollow fiber membranes (HFMs) during the wet phase inversion process. By precisely controlling the fabrication parameters, such as polymer solution flow rate, coagulant solution flow rate, and the air-gap distance, also called drop height, different-sized aligned grooves can be fabricated on the inner surface of HFMs. Preliminary studies using in vitro dorsal root ganglion (DRG) regeneration assay showed that both the alignment and outgrowth rate of regenerating axons increased significantly on HFMs with aligned textures compared to those on HFMs with a smooth inner surface. Studies in progress are evaluating axonal outgrowth and regeneration using in vivo sciatic-nerve and spinal-cord-injury models.

Effect of filament diameter and extracellular matrix molecule precoating on neurite outgrowth and Schwann cell behavior on multifilament entubulation bridging devicein vitro

At present there is no clinically effective treatment for injuries or pathological processes that disrupt the continuity of axons in the mature central nervous system. However, a number of studies suggest that a tremendous potential exists for developing biomaterial based therapies. In particular, biomaterials in the form of bridging substrates have been shown to support at least some level of axonal regeneration across the lesion site, but display a limited capacity for directing axons toward their targets. To improve the directionality and outgrowth rate of the axonal regeneration process, filaments and tubes appear promising, but the technology is far from optimized. As a step toward optimization, the influence of filament diameter and various extracellular matrix coatings on nerve regeneration was evaluated in this article using a dorsal root ganglion (DRG) explant model. An increasing pattern of alignment and outgrowth of neurites in the direction parallel the long axis of the packed filament bundles with decreasing filament diameters ranging from supracellular and beyond (500 to 100 mum), cellular (30 mum), down to subcellular size (5 mum) was observed. Such effects became most prominent on filament bundles with individual filament diameters in the range of cellular size and below (5 and 30 mum) where highly directional and robust neuronal outgrowth was achieved. In addition, laminin-coated filaments that approached the size of spinal axons support significantly longer regenerative outgrowth than similarly treated filaments of larger diameter, and exceed outgrowth distance on similarly sized filaments treated with fibronectin. These data suggested the feasibility of using a multifilament entubulation bridging device for supporting directional axonal regeneration.

Acute implantation of an avulsed lumbosacral ventral root into the rat conus medullaris promotes neuroprotection and graft reinnervation by autonomic and motor neurons

Trauma to the conus medullaris and cauda equina may result in autonomic, sensory, and motor dysfunctions. We have previously developed a rat model of cauda equina injury, where a lumbosacral ventral root avulsion resulted in a progressive and parallel death of motoneurons and preganglionic parasympathetic neurons, which are important for i.e. bladder control. Here, we report that an acute implantation of an avulsed ventral root into the rat conus medullaris protects preganglionic parasympathetic neurons and motoneurons from cell death as well as promotes axonal regeneration into the implanted root at 6 weeks post-implantation. Implantation resulted in survival of 44+/-4% of preganglionic parasympathetic neurons and 44+/-4% of motoneurons compared with 22% of preganglionic parasympathetic neurons and 16% of motoneurons after avulsion alone. Retrograde labeling from the implanted root at 6 weeks showed that 53+/-13% of surviving preganglionic parasympathetic neurons and 64+/-14% of surviving motoneurons reinnervated the graft. Implantation prevented injury-induced atrophy of preganglionic parasympathetic neurons and reduced atrophy of motoneurons. Light and electron microscopic studies of the implanted ventral roots demonstrated a large number of both myelinated axons (79+/-13% of the number of myelinated axons in corresponding control ventral roots) and unmyelinated axons. Although the diameter of myelinated axons in the implanted roots was significantly smaller than that of control roots, the degree of myelination was appropriate for the axonal size, suggesting normal conduction properties. Our results show that preganglionic parasympathetic neurons have the same ability as motoneurons to survive and reinnervate implanted roots, a prerequisite for successful therapeutic strategies for autonomic control in selected patients with acute conus medullaris and cauda equina injuries.

Novel repair strategies to restore bladder function following cauda equina/conus medullaris injuries

Trauma to the thoracolumbar junction or lumbosacral spine may result in a conus medullaris or cauda equina syndrome. In both conditions, symptoms typically include paraparesis or paraplegia, sensory impairment, pain, as well as bladder, bowel, and sexual dysfunctions. We present in this review a series of neural repair strategies that have been developed to address the unique features and challenges of subjects with a conus medullaris or cauda equina syndrome. We address, in particular, neural repair strategies that may have a translational research potential to restore bladder function. Recent animal injury models have suggested that a progressive retrograde death of both autonomic and motor neurons may contribute to the neurological deficits in subjects with conus medullaris and cauda equina injuries. For subjects with acute injuries, we present novel strategies to promote neuroprotection, axonal regeneration, and functional reinnervation of the lower urinary tract. For subjects with chronic injuries, we discuss new approaches to replace lost autonomic and motor neurons. A brief discussion on a variety of outcome measures that may be suitable to evaluate the function of the lower urinary tract in rodent neural repair models is also provided.

Glial reactions in a rodent cauda equina injury and repair model

In the adult rat, an avulsion injury of lumbosacral ventral roots results in a progressive and pronounced loss of the axotomized motoneurons. A subsequent acute implantation of an avulsed ventral root into the spinal cord has neuroprotective effects. However, it has not been known whether a surgical implantation of an avulsed ventral root into the spinal cord for neural repair purposes affects intramedullary glial and microglial reactions. Here, adult female Sprague-Dawley rats underwent a unilateral L5-S2 ventral root avulsion injury with or without acute implantation of the L6 ventral root into the spinal cord. At 4 weeks postoperatively, immunohistochemistry using primary antibodies to GFAP (astrocytes), Ox-42 (microglia), and ED-1 (macrophages) was performed at the L6 spinal cord segment, and quantified using densitometry. Our results show that a lumbosacral ventral root avulsion injury induces an activation of astrocytes, microglia, and macrophages in the ventral horn. Interestingly, an acute implantation of an avulsed root into the white matter does not significantly affect the activation of glial cells or macrophages in the ventral horn. We speculate that neuroprotective and axonal growth promoting benefits of the combined glial and microglial/ macrophage responses may outweigh their potential negative effects, as previous studies have shown that implantation of avulsed roots is a successful strategy in promoting reinnervation of peripheral targets.

A single re-implanted ventral root exerts neurotropic effects over multiple spinal cord segments in the adult rat

Spinal cord injuries, particularly traumatic injuries to the conus medullaris and cauda equina, are typically complex and involve multiple segmental levels. Implantation of avulsed ventral roots into the spinal cord as a repair strategy has been shown to be neuroprotective and promote axonal regeneration by spinal cord neurons into an implanted root. However, it is not well known over what distance in the spinal cord an implanted ventral root can exert its neurotropic effect. Here, we investigated whether an avulsed L6 ventral root acutely implanted into the rat spinal cord after a four level (L5-S2) unilateral ventral root avulsion injury may exert neurotropic effects on autonomic and motor neurons over multiple spinal cord segments at 6 weeks postoperatively. Using retrograde labeling techniques and stereological quantification methods, we demonstrate that autonomic and motor neurons from all four lesioned spinal cord segments, spanning more than an 8 mm rostro-caudal distance, reinnervated the one implanted root. The rostro-caudal distribution suggested a gradient of neurotropism, where the axotomized neurons closest to the implanted site had the highest probability of root reinnervation. These results suggest that implantation of a single ventral root may provide neurotropic effects to injured neurons at the site of lesion as well as in the adjacent spinal cord segments. Our findings may be of translational research interest for the development of surgical repair strategies after multi-level conus medullaris and cauda equina injuries, in which fewer ventral roots than spinal cord segments may be available for implantation.

Growth-associated protein GAP-43 and L1 act synergistically to promote regenerative growth of Purkinje cell axons in vivo

Neuronal expression of growth-associated protein 43 (GAP-43) and the cell adhesion molecule L1 has been correlated with CNS axonal growth and regeneration, but it is not known whether expression of these molecules is necessary for axonal regeneration to occur. We have taken advantage of the fact that Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts to test the importance of these molecules for axonal regeneration in vivo. Transgenic mice were generated in which Purkinje cells constitutively express L1 or both L1 and GAP-43 under the Purkinje cell-specific L7 promoter, and regeneration of Purkinje cell axons into peripheral nerve grafts implanted into the cerebellum was examined. Purkinje cells expressing GAP-43 or L1 showed minor enhancement of axonal sprouting. Purkinje cells expressing both GAP-43 and L1 showed more extensive axonal sprouting and axonal growth into the proximal portion of the graft. When a predegenerated nerve graft was implanted into double-transgenic mice, penetration of the graft by Purkinje cell axonal sprouts was strongly enhanced, and some axons grew along the entire intracerebral length of the graft (2.5-3.0 mm) and persisted for several months. The results demonstrate that GAP-43 and L1 coexpressed in Purkinje cells can act synergistically to switch these regeneration-incompetent CNS neurons into a regeneration-competent phenotype and show that coexpression of these molecules is a key regulator of the regenerative ability of intrinsic CNS neurons in vivo.

Upregulation of activating transcription factor 3 (ATF3) by intrinsic CNS neurons regenerating axons into peripheral nerve grafts

The expression of the transcription factor ATF3 in the brain was examined by immunohistochemistry during axonal regeneration induced by the implantation of pieces of peripheral nerve into the thalamus of adult rats. After 3 days, ATF3 immunoreactivity was present in many cells within approximately 500 mum of the graft. In addition, ATF3-positive cell nuclei were found in the thalamic reticular nucleus (TRN) and medial geniculate nuclear complex (MGN), from which most regenerating axons originate. CNS cells with ATF3-positive nuclei were predominantly neurons and did not show signs of apoptosis. The number of ATF3-positive cells had declined by 7 days and further by 1 month after grafting when most ATF3-positive cells were found in the TRN and MGN. 14 days or more after grafting, some ATF3-positive nuclei were distorted and may have been apoptotic. In some experiments of 1 month duration, neurons which had regenerated axons to the distal ends of grafts were retrogradely labeled with DiAsp. ATF3-positive neurons in these animals were located in regions of the TRN and MGN containing retrogradely labeled neurons and the great majority were also labeled with DiAsp. SCG10 and c-Jun were found in neurons in the same regions as retrogradely labeled and ATF3-positive cells. Thus, ATF3 is transiently upregulated by injured CNS neurons, but prolonged expression is part of the pattern of gene expression associated with axonal regeneration. The co-expression of ATF3 with c-jun suggests that interactions between these transcription factors may be important for controlling the program of gene expression necessary for regeneration.

Cell death or survival: Molecular and connectional conditions for olivocochlear neurons after axotomy

We aimed to determine whether rat olivocochlear neurons survive axotomy inflicted through cochlear ablation, or if they degenerate. To estimate their intrinsic potential for axonal regeneration, we investigated the expression of the transcription factor c-Jun and the growth-associated protein-43 (GAP43). Axonal tracing studies based on application of Fast Blue into the cochlea and calcitonin gene-related peptide immunostaining revealed that many, but not all, lateral olivocochlear neurons in the ipsilateral lateral superior olive degenerated upon cochleotomy. A decrease of their number was noticed 2 weeks after the lesion, and 2 months postoperative the population was reduced to approximately one quarter (27-29%) of its original size. No further reduction took place at longer survival times up to 1 year. Most or all shell neurons and medial olivocochlear neurons survived axotomy. Following cochleotomy, 56-60% of the lateral olivocochlear neurons in the ipsilateral lateral superior olive were found to co-express c-Jun and GAP43. Only a small number of shell and medial olivocochlear neurons up-regulated c-Jun expression, and only a small number of shell neurons expressed GAP43. Up-regulation of c-Jun and GAP43 in lateral olivocochlear neurons upon axotomy suggests that they have an intrinsic potential to regenerate after axotomy, but cell counts based on the markers Fast Blue and calcitonin gene-related peptide indicate that this potential cannot be exploited and degeneration is induced instead. The survival of one quarter of the axotomized lateral olivocochlear neurons and of all, or almost all, shell and medial olivocochlear neurons appeared to depend on connections of these cells to other regions than the cochlea by means of axon collaterals, which remained intact after cochleotomy.

Viral vector-mediated gene transfer of neurotrophins to promote regeneration of the injured spinal cord

Injuries to the adult mammalian spinal cord often lead to severe damage to both ascending (sensory) pathways and descending (motor) nerve pathways without the perspective of complete functional recovery. Future spinal cord repair strategies should comprise a multi-factorial approach addressing several issues, including optimalization of survival and function of spared central nervous system neurons in partial lesions and the modulation of trophic and inhibitory influences to promote and guide axonal regrowth. Neurotrophins have emerged as promising molecules to augment neuroprotection and neuronal regeneration. Although intracerebroventricular, intrathecal and local protein delivery of neurotrophins to the injured spinal cord has resulted in enhanced survival and regeneration of injured neurons, there are a number of drawbacks to these methods. Viral vector-mediated transfer of neurotrophin genes to the injured spinal cord is emerging as a novel and effective strategy to express neurotrophins in the injured nervous system. Ex vivo transfer of neurotrophic factor genes is explored as a way to bridge lesions cavities for axonal regeneration. Several viral vector systems, based on herpes simplex virus, adenovirus, adeno-associated virus, lentivirus, and moloney leukaemia virus, have been employed. The genetic modification of fibroblasts, Schwann cells, olfactory ensheathing glia cells, and stem cells, prior to implantation to the injured spinal cord has resulted in improved cellular nerve guides. So far, neurotrophic factor gene transfer to the injured spinal cord has led to results comparable to those obtained with direct protein delivery, but has a number of advantages. The steady advances that have been made in combining new viral vector systems with a range of promising cellular platforms for ex vivo gene transfer (e.g., primary embryonic neurons, Schwann cells, olfactory ensheating glia cells and neural stem cells) holds promising perspectives for the development of new neurotrophic factor-based therapies to repair the injured nervous system.

Different effects of astrocytes and Schwann cells on regenerating retinal axons

Following a crush injury of the optic nerve in adult rats, the axons of retinal ganglion cells, stimulated to regenerate by a lens injury and growing within the optic nerve, are associated predominantly with astrocytes: they remain of small diameter (0.1-0.5 microm) and unmyelinated for > or = 2 months after the operation. In contrast, when the optic nerve is cut and a segment of a peripheral nerve is grafted to the ocular stump of the optic nerve, the regenerating retinal axons are associated predominantly with Schwann cells: they are of larger diameter than in the previous experiment and include unmyelinated axons (0.2-2.5 microm) and myelinated axons (mean diameter 2.3 microm). Thus, the grafted peripheral nerve, and presumably its Schwann cells, stimulate enlargement of the regenerating retinal axons leading to partial myelination, whereas the injured optic nerve itself, and presumably its astrocytes, does not. The result points to a marked difference of peripheral (Schwann cells) and central (astrocytes) glia in their effect on regenerating retinal axons.

Facial nerve regeneration through progesterone-loaded chitosan prosthesis. A preliminary report

Biodegradable nerve guides have represented new treatment alternatives for nerve repairing. They are gradually biodegradable, exert biological effects directly to the injured nerve, and act as drug- or cell-delivery devices. Furthermore, progesterone (PROG) has been demonstrated to promote injured peripheral nerve regeneration. In this study, it was hypothesized that PROG delivered from chitosan prostheses provides better facial nerve regenerative response than chitosan prostheses with no PROG. As there are no reports on the use of the former as nerve-guide material in the regeneration of injured nerves, this is the main objective of the present work. Chitosan prostheses containing PROG were used to bridge 10-mm gaps in rabbit facial nerves. The regenerated nerves were evaluated 45 days after implantation in animals with the use of light microscopy and morphometric analysis. Gas chromatography was used in order to quantify PROG content in prosthesis prior to and after implantation in subcutaneous tissue at different periods of up to 60 days. In addition, the prosthesis walls were evaluated with histological techniques in order to assess their integrity and the surrounding tissue reaction. Chitosan prostheses allowed PROG release during the time needed for nerve regeneration. At 45 days myelinated nerve fibers were observed in both the proximal and distal stumps. This parameter and the N ratio were higher in the progesterone-treated group when compared to that of the vehicle control. Findings indicate that chitosan prostheses were useful in nerve regeneration, acting as a long-lasting PROG delivery device a faster nerve regeneration.

Differential effects of neurotrophins on neuronal survival and axonal regeneration after spinal cord injury in adult rats

Spinal cord injury (SCI) induces retrograde cell death in descending pathways, which can be prevented by long-term intrathecal infusion of neurotrophins (Novikova et al. [2000] Eur J Neurosci 12:776-780). The present study investigates whether the same treatment also leads to improved regeneration of the injured tracts. After cervical SCI in adult rats, a peripheral nerve graft was attached to the rostral wall of the lesion cavity. The animals were treated by local application into the cavity of Gelfoam soaked in (1) phosphate buffered saline (untreated controls) or (2) a mixture of the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) (local treatment), or by intrathecal infusion of BDNF + NT-3 for (3) 2 weeks (short-term treatment) or (4) 5-8 weeks (long-term treatment). Despite a very strong survival effect, long-term treatment failed to stimulate ingrowth of descending tracts into the nerve graft. In comparison with untreated controls, the latter treatment also caused 35% reduction in axonal sprouting of descending pathways rostral to the lesion site and 72% reduction in the number of spinal cord neurons extending axons into the nerve graft. Local and short-term treatments neither prevented retrograde cell death nor enhanced regeneration of descending tracts, but induced robust regeneration of spinal cord neurons into the nerve graft. These results indicate that the signal pathways promoting neuronal survival and axonal regeneration, respectively, in descending tracts after SCI respond differently to neurotrophic stimuli and that efficient rescue of axotomized tract neurons is not a sufficient prerequisite for regeneration.

Characterization of the optic nerve and retinal ganglion cell layer in the dysmyelinated adult Long Evans Shaker rat: evidence for axonal sprouting

Myelin in the central nervous system (CNS) is hypothesized to help guide the growth of developing axons by inhibiting sprouting of aberrant neurites. Previous studies using animal models lacking CNS myelin have reported that increasing capacity for sprouting axons is negatively correlated with the degree of myelination. In the present study, we investigated the optic nerves of the recently identified Long Evans Shaker (LES) rat with prolonged dysmyelination of adult axons to determine whether the lack of myelin basic protein (MBP) in adult LES rats could manifest as increases in the population of CNS axons. We observed numerous small, unmyelinated axon profiles (<0.3 microm in diameter) clustered in bundles alongside normal caliber axons in dysmyelinated LES rats but not in normal myelinated Long Evans (LE) rats. These putative axon profiles resembled sprouting axons previously described in the CNS. Moreover, the high number of small putative axon profiles could not be accounted for by any significant increases in the number of ganglion cells and displaced amacrine cells in the ganglion cell layer when compared with normal rats as evaluated by using a variety of techniques. This finding suggests that the observed clusters of putative axon profiles were not due to developmental abnormalities in the retina but to the lack of myelin in the optic nerves of LES rats. The adult LES rat, therefore, may serve as a useful model to study the role of myelin in regulating axon development or axon regeneration after CNS injury in the adult mammalian system.

AFGF promotes axonal growth in rat spinal cord organotypic slice co-cultures

This study developed a slice culture model system to study axonal regeneration after spinal cord injury. This model was tested in studies of the roles of acidic fibroblast growth factor (aFGF) and peripheral nerve segments in axonal growth between pieces of spinal cord. Transverse sections of P15-P18 Sprague-Dawley rat spinal cord were collected for organotypic slice cultures. Group I consisted of two slices of spinal cord in contact with each other during the culture period. Group II consisted of two slices that were separated by 3 mm and connected by two segments of intercostal nerves. Group III consisted of single slices for studies of neuron survival. Some cultures from each group included aFGF in the culture medium. Bromodeoxyuridine (BrdU) was included in the medium for some cultures. The results showed three principal findings. First, counts of neurofilament-positive cells demonstrated that treatment with aFGF significantly increased the number of surviving neurons in culture. Second, neurofilament immunostaining and DiI tracing demonstrated axons crossing the junction between the two pieces of spinal cord or growing through the intercostal nerve segments, and these axons were seen only in cultures with aFGF treatment. Third, few cells were double stained for neurofilament and BrdU, and these were found only with aFGF treatment. These results demonstrate that (1) organotypic slice cultures present a useful model to study regeneration from spinal cord injury, (2) aFGF rescues neurons and promotes axonal growth in these cultures, and (3) segments of intercostal nerves promote axon growth between slices of spinal cord.

Marrow stromal cells form guiding strands in the injured spinal cord and promote recovery

Marrow stromal cells (MSC) can be expanded rapidly in vitro and differentiated into multiple mesodermal cell types. In addition, differentiation into neuron-like cells expressing markers typical for mature neurons has been reported. To analyze whether such cells, exposed to differentiation media, could develop electrophysiological properties characteristic of neurons, we performed whole-cell recordings. Neuron-like MSC, however, lacked voltage-gated ion channels necessary for generation of action potentials. We then delivered MSC into the injured spinal cord to study the fate of transplanted MSC and possible effects on functional outcome in animals rendered paraplegic. MSC given 1 week after injury led to significantly larger numbers of surviving cells than immediate treatment and significant improvements of gait. Histology 5 weeks after spinal cord injury revealed that MSC were tightly associated with longitudinally arranged immature astrocytes and formed bundles bridging the epicenter of the injury. Robust bundles of neurofilament-positive fibers and some 5-hydroxytryptamine-positive fibers were found mainly at the interface between graft and scar tissue. MSC constitute an easily accessible, easily expandable source of cells that may prove useful in the establishment of spinal cord repair protocols.

Reinnervation of a denervated skeletal muscle by spinal axons regenerating through a collagen channel directly implanted into the rat spinal cord

In the present study, the continuity between the central nervous system (CNS) and the peripheral nervous system (PNS) was restored by mean of a collagen channel in order to reinnervate a skeletal muscle. Three groups of animals were considered. In the first group, one end of the collagen channel was implanted in the cervical spinal cord of adult rats. The other end was connected to a 30-mm autologous peripheral nerve graft (PNG) implanted into the denervated biceps brachii muscle. The gap between the spinal cord and the proximal nerve stump varied from 3 to 7 mm. In the second group of animals, the distal end of the PNG graft was ligatured in order to compare the survival of the growing axons in the presence and in the absence of a muscular target. In the third group of animals, the extraspinal stump of the collagen channel was ligatured. Our study demonstrates that spinal neurons and dorsal root ganglion (DRG) neurons can grow long axons through the collagen channel over a 7-mm gap and reinnervate a denervated skeletal muscle. The results also indicate that the presence of a PNG at the extraspinal stump of the collagen channel is essential for axonal regrowth and that the muscle target contributes to the long-term maintenance of the regenerating axons. These data might be interesting for clinical application when the continuity between the CNS and PNS is interrupted such as in root avulsion.

Regrowth of acute and chronic injured spinal pathways within supra-lesional post-traumatic nerve grafts

The present work investigates the extent to which mature central neurons acutely or chronically axotomized by a spinal lesion still maintained the potential to regenerate an axon following post-traumatic nerve grafting within supra-lesional spinal structures. In adult rats, a C3 cervical hemisection (injury) was made and an autologous segment of the peroneal nerve was implanted 2mm rostrally into the ventrolateral part of the ipsilateral C2 spinal cord. Nerve graft implantations were carried out acutely at the time of injury (group I, acute conditions) or chronically, three weeks post-injury (group II, chronic conditions). Central neurons axotomized by the spinal lesion were labeled by True Blue injected at the lesion site at the time of trauma. Central neurons regenerating axons within the nerve grafts were labeled with either horseradish peroxidase (only in group I, n=4) or Nuclear Yellow (group I, n=3 and group II, n=6) applied two to four months post-grafting to the distal cut end of the nerve grafts. Neurons with dual staining (True Blue/Nuclear Yellow) represented central regenerating neurons which were previously axotomized by the spinal lesion and which had retained the capacity for axonal regeneration for a delayed period after injury. In group I (acute injury conditions), all types of labeled cells were found to be scattered with a clear bimodal distribution within the spinal cord and the brainstem. No labeled cells were found within the motor cortex. There was no statistically significant difference between horseradish peroxidase and all cells containing Nuclear Yellow (Nuclear Yellow and True Blue/Nuclear Yellow). In group II (chronic injury conditions), Nuclear Yellow- and True Blue/Nuclear Yellow-labeled cells had a similar dual distribution to that of group I, but were found to be significantly less represented (P=0.019). These differences are discussed in terms of capacity for cell survival and axonal regrowth after acute and chronic injury. The main conclusion is based on the evidence of dual staining of central neurons in both groups, which demonstrates that brainstem and spinal neurons involved in acute and chronic axotomy after spinal C3 lesion can survive the trauma and still maintain the capacity to regenerate lesioned axons within nerve grafts inserted rostrally (C2 spinal cord) to the primary site of injury. Although exhibited to a lesser extent in chronic than in acute conditions, this capacity was found to occur for as long as three weeks post-injury. These results indicate that supra-lesional post-traumatic nerve grafts may constitute an efficient delayed strategy for inducing axonal regrowth of chronically axotomized adult central neurons. We suggest that surgical intervention, which is not always possible immediately after a spinal cord injury, may be satisfactorily carried out after an appropriate delay.

Axonal regeneration from CNS neurons in the cerebellum and brainstem of adult rats: correlation with the patterns of expression and distribution of messenger RNAs for L1, CHL1, c-jun and growth-associated protein-43

Some neurons in the brain and spinal cord will regenerate axons into a living peripheral nerve graft inserted at the site of injury, others will not. We have examined the patterns of expression of four molecules thought to be involved in developmental and regenerative axonal growth, in the cerebellum and brainstem of adult rats, following the implantation into the cerebellum of peripheral nerve grafts. We also determined how the expression patterns observed correlate with the abilities of neurons in these regions to regenerate axons. Three days to 16 weeks after insertion of living tibial nerve autografts, neurons which had regenerated axons into the graft were retrogradely labelled from the distal extremity of the graft with cholera toxin conjugated to horseradish peroxidase, and sections through the cerebellum and brainstem were processed for visualization of transported tracer and/or hybridized with riboprobes to detect messenger RNAs for the cell recognition molecules L1 and CHL1 (close homologue of L1), growth-associated protein-43 and the cellular oncogene c-jun. Retrogradely labelled neurons were present in cerebellar deep nuclei close to the graft and in brainstem nuclei known to project to the cerebellum. Neurons in these same nuclei were found to have up-regulated expression of all four messenger RNAs. Individual retrogradely labelled neurons also expressed high levels of L1, CHL1, c-jun or growth-associated protein-43 messenger RNAs (and vice versa), and every messenger RNA investigated was co-localized with at least one other messenger RNA. Purkinje cells did not regenerate axons into the graft or up-regulate L1, CHL1 or growth-associated protein-43 messenger RNAs, but there was increased expression of c-jun messenger RNA in some Purkinje cells close to the graft. Freeze-killed grafts produced no retrograde labelling of neurons, and resulted in only transient and low levels of up-regulation of the tested molecules, mainly L1 and CHL1. These findings show that cerebellar deep nucleus neurons and precerebellar brainstem neurons, but not Purkinje cells, have a high propensity for axon regeneration, and that axonal regeneration by these neurons is accompanied by increased expression of L1, CHL1, c-jun and growth-associated protein-43. Furthermore, although the patterns of expression of the four molecules investigated are not identical in regenerating neuronal populations, it is probable that all four are up-regulated in all neurons whose axons regenerate into the grafts and that their up-regulation may be required for axon regeneration to occur. Finally, because c-jun up-regulation is seen in Purkinje cells close to the graft, unaccompanied by up-regulation of the other molecules investigated, c-jun up-regulation alone cannot be taken to reliably signify a regenerative response to axotomy.

Reinnervation of the biceps brachii muscle following cotransplantation of fetal spinal cord and autologous peripheral nerve into the injured cervical spinal cord of the adult rat

In order to compensate the loss of motoneurons resulting from severe spinal cord injury and to reestablish peripheral motor connectivity, solid pieces of fetal spinal cord, taken from embryonic day 14 rat embryos, were transplanted into unilateral aspiration lesions of the cervical spinal cord of adult rats. Concomitantly, one end of a 3.5-cm autologous peripheral nerve graft was put in close contact with the embryonic graft; the other end was sutured to the distal stump of the musculocutaneous nerve which innervate the biceps brachii muscle. The animals were examined 3 and 6 months after surgery. Following intramuscular injection of horseradish peroxidase, retrograde axonal labeling studies indicated that both transplanted and host spinal neurons were able to extend axons all the way through the peripheral nerve graft and nerve stump, up to the reconnected muscles. The labeled cells in the transplant were generally observed close to the intraspinal tip of the peripheral nerve graft. Retrograde axonal tracing, as well as electrophysiological and histological data, demonstrated the sensory and motor reinnervation of the reconnected muscles. This muscular reinnervation was able to reverse the atrophic changes observed in the denervated muscle. In control experiments, the extraspinal end of the peripheral nerve graft was ligatured in order to compare the differentiation of the transplanted neurons and the survival of their growing axons with or without their muscular targets. Six months after both types of surgery, large-size grafted neurons, identified as motoneurons by immunocytochemistry for peripherine and calcitonin gene-related peptide, were only observed in fetal spinal cord transplants which were connected to denervated muscles, thus demonstrating the trophic influence of the muscle target on the survival and differentiation of the transplanted neurons and on the maintenance of the axons they had grown into the peripheral nerve graft.

Columns of Schwann cells extruded into the CNS induce in-growth of astrocytes to form organized new glial pathways

Our previous work showed that stereotaxic microextrusion of columns of purified peripheral nerve-derived Schwann cells into the thalamus of syngeneic adult rats induces host axons to grow into the column and form a new fiber tract. Here we describe the time course of cellular events that lead to the formation of this new tract. At 2 h postoperation, numerous OX42-positive microglia accumulated at the graft-host interface, after which donor columns became progressively and heavily infiltrated by microglia/macrophages that took on an elongated morphology in parallel with the highly orientated processes of the donor Schwann cells. The penetration of host astrocytic processes into the Schwann cell columns was substantially slower in onset, being first detected at 4 days postoperation. This event was contemporaneous with the in-growth of host thalamic axons. Between 7 and 14 days postoperation, GFAP-positive astrocytes became fully incorporated into the transplants, where they too adopted an elongated form, orientated in parallel with the longitudinal axis of the graft. Thus, the columns became a mosaic of elongated and highly orientated donor Schwann cells intimately mingled with host microglia, astrocytes, and numerous, largely unbranched 200-kDa neurofilament-positive axons from the adjacent thalamus. Electron microscopy demonstrated that the processes of donor Schwann cells and host astrocytes within the column formed tightly packed bundles that were surrounded by a partial or complete basal lamina. Control columns, formed by extruding freeze-thaw-killed Schwann cells or purified peripheral nerve fibroblasts induced a reactive injury response by the adjacent host microglia and astrocytes, but neither host astrocytes nor neurofilament-positive axons were incorporated into the columns. A better understanding of the mechanisms that regulate the interactions between donor and host glia should facilitate improved integration of such grafts and enhance their potential for inducing tissue repair.

A new type of biocompatible bridging structure supports axon regrowth after implantation into the lesioned rat optic tract

We have developed a new type of polymer/cell/matrix implant and tested whether it can promote the regrowth of retinal ganglion cell (RGC) and other axons across surgically induced tissue defects in the CNS. The constructs, which consisted of 2-2.5-mm-long polycarbonate tubes filled with lens capsule-derived extracellular matrix coated with cultured neonatal Schwann cells, were implanted into lesion cavities made in the left optic tract (OT) of 18-21-day-old rats. In one group, to promote Schwann cell proliferation and perhaps also to stimulate axon regrowth, basic fibroblast growth factor (bFGF) was added to the lens capsule matrix prior to implantation. In another group, to determine whether application of growth factors to the somata of cells enhances the regrowth of distally injured axons, the neurotrophin NT-4/5 was injected into the eye contralateral to the OT lesion. NT-4/5 and bFGF treatments were combined in some rats. After medium-term (4-10 weeks) or long-term (15-20 weeks) survivals, axon growth into implants was assessed immunohistochemically using a neurofilament (RT97) antibody. RGC axons were visualized after injection of WGA/HRP into the right eye. Viable Schwann cells were present in implants at all times after transplantation. Large numbers of RT97+ axons were consistently found within the bridging implants, often associated with the peripheral glia. Axons were traced up to 1.7 mm from the nearest CNS neuropil and there was immunohistochemical evidence of myelination by Schwann cells and by host oligodendrocytes. There were fewer RGC axons in the implants, fibers growing up to 1.6 mm from the thalamus. Neither NT-4/5 nor bFGF, alone or in combination, significantly increased the extent of RGC axon growth within the implants. A group of OT-lesioned rats was implanted with polymer tubes filled with 2-2.5-mm-long pieces of predegenerate peripheral nerve. Surprisingly, polymer/cell/matrix constructs contained comparatively greater numbers of RGC and other axons and supported more extensive axon elongation. Thus, implants of this type may potentially be useful in bridging large tissue defects in the CNS.

The cell recognition molecule CHL1 is strongly upregulated by injured and regenerating thalamic neurons

Close homologue of L1 (CHL1) is a cell recognition molecule known to promote axonal growth in vitro. We have investigated the expression of CHL1 mRNA by regenerating central nervous system (CNS) neurons, by using in situ hybridisation 3 days to 10 weeks following the implantation of living and freeze-killed peripheral nerve autografts into the thalamus of adult rats. At all survival times after implantation of living grafts, neurons of the thalamic reticular nucleus (TRN), close to the graft tip and up to 1 mm away from it, displayed strong signal for CHL1 mRNA, even though TRN neurons show very low levels of CHL1 mRNA expression in unoperated animals. When the cell bodies of regenerating neurons were identified by retrograde labelling from the distal portion of the grafts, 4-6 weeks after operation, most of the labelled cells were found in the TRN and could be shown to haveupregulated CHL1 mRNA. In addition, some neurons in dorsal thalamic nuclei near the graft tip transiently upregulated CHL1 mRNA during the first 3 weeks after graft implantation, and glial cells showing CHL1 mRNA expression were present at the brain/graft interface 3 days to 2 weeks after operation. Freeze-killed grafts, into which axons do not regenerate, caused a transient upregulation of CHL1 in very few TRN neurons near the graft tip and in glial cells at the brain/graft interface but did not produce prolonged CHL1 mRNA expression. CHL1 can therefore be added to the list of molecules (including GAP-43, L1, and c-jun) strongly expressed by CNS neurons that regenerate their axons into nerve grafts, but not by those neurons that fail to regenerate their axons.

Temperature dependence of membrane sealing following transection in mammalian spinal cord axons

Using an in vitro sucrose-gap recording chamber, sealing of cut axons in isolated strips of white matter from guinea pig spinal cord was measured by recording the "compound membrane potential". This functional sealing was found to correlate well with anatomical resealing, measured by a horseradish peroxidase uptake assay. Near-complete functional and anatomical recovery of the axonal membrane occurred routinely within 60 min following transection at 37 degrees C in regular Krebs' solution. The rate of membrane potential recovery is exponential, with a time-constant of 20+/-5 min. The sealing process at 31 degrees C was similar to that at 37 degrees C, and was effectively blocked at 25 degrees C, under which condition most axons continued to take up horseradish peroxidase for more than 1h, and failed to substantially recover their membrane potential. Seventy-five percent of the cords transected at 40 degrees C had similar sealing behavior to those at 37 degrees C and 31 degrees C. The balance failed to seal the cut end. Two-dimensional morphometric analysis has shown that raising the temperature from 25 degrees C to above 31 degrees C significantly decreases axonal permeabilization to horseradish peroxidase (increases the sealing of transected ends) across all areas of a transverse section of spinal cord. Moreover, this enhancement of sealing exists across all axon calibers. Since severe cooling compromises membrane resealing, caution needs to be taken when hypothermic treatment (below 25 degrees C) is applied within the first 60 min following mechanical injury. In summary, we have found that at normal temperature (37 degrees C), nerve fibers repair their damaged membrane following physical injury with an hour. This is similar at mildly lower (31 degrees C) and relatively higher (40 degrees C) temperature, although some fibers tend to collapse under this febrile temperature. Moreover, severely low temperature (25 degrees C) hindered the repair of damaged membranes. Based on our study, caution is needed in treating spinal cord injury with low temperatures.

Intercostal nerve implants transduced with an adenoviral vector encoding neurotrophin-3 promote regrowth of injured rat corticospinal tract fibers and improve hindlimb function

Following injury to central nervous tissues, damaged neurons are unable to regenerate their axons spontaneously. Implantation of peripheral nerves into the CNS, however, does result in axonal regeneration into these transplants and is one of the most powerful strategies to promote CNS regeneration. In the present study implantation of peripheral nerve bridges following dorsal hemisection is combined with ex vivo gene transfer with adenoviral vectors encoding neurotrophin-3 (Ad-NT-3) to examine whether this would stimulate regeneration of one of the long descending tracts of the spinal cord, the corticospinal tract (CST), into and beyond the peripheral nerve implant. We chose to use an adenoviral vector encoding NT-3 because CST axons are sensitive to this neurotrophin and Schwann cells in peripheral nerve implants do not express this neurotrophin. At 16 weeks postimplantation of Ad-NT-3-transduced intercostal nerves, approximately three- to fourfold more of the anterogradely traced corticospinal tract fibers had regrown their axons through gray matter below the lesion site when compared to control animals. Regrowth of CST fibers occurred over more than 8 mm distal to the lesion site. No regenerating CST fibers were, however, observed into the transduced peripheral implant. Animals with a peripheral nerve transduced with Ad-NT-3 also exhibited improved function of the hindlimbs when compared to control animals treated with an adenoviral vector encoding LacZ. Thus, transient overexpression of NT-3 in peripheral nerve tissue bridges is apparently sufficient to stimulate regrowth of CST fibers and to promote recovery of hindlimb function, but does not result in regeneration of CST fibers into such transplants. Taken together, combining an established neurotransplantation approach with viral vector-gene transfer promotes the regrowth of injured CST fibers through gray matter and improves the recovery of hindlimb function.

Expression of CHL1 and L1 by neurons and glia following sciatic nerve and dorsal root injury

Cell adhesion molecules (CAMs), particularly L1, are important for axonal growth on Schwann cells in vitro. We have used in situ hybridization to study the expression of mRNAs for L1 and its close homologue CHL1, by neurons regenerating their axons in vivo, and have compared CAM expression with that of GAP-43. Adult rat sciatic nerves were crushed (allowing functional regeneration), or cut and ligated to maintain axonal sprouting but prevent reconnection with targets. In other animals lumbar dorsal roots were transected to produce slow regeneration of the central axons of sensory neurons. In unoperated animals L1 and CHL1 mRNAs were expressed at moderate levels by small- to medium-sized sensory neurons and L1 mRNA was expressed at moderate levels by motor neurons. Many large sensory neurons expressed neither L1 nor CHL1 mRNAs and motor neurons expressed little or no CHL1 mRNA. Neither motor nor sensory neurons showed any obvious upregulation of L1 mRNA after axotomy. Increased CHL1 mRNA was found in motor neurons and small- to medium-sized sensory neurons 3 days to 2 weeks following sciatic nerve crush, declining toward control levels by 5 weeks when regeneration was complete. Cut and ligation injuries caused a prolonged upregulation of CHL1 mRNA (and GAP-43 mRNA), indicating that reconnection with target tissues may be required to signal the return to control levels. Large sensory neurons did not upregulate CHL1 mRNA after axotomy and thus regenerated within the sciatic nerve without producing CHL1 or L1. Dorsal root injuries caused a modest, slow upregulation of CHL1 mRNA by some sensory neurons. CHL1 mRNA was also upregulated by many presumptive Schwann cells in injured nerves and by some satellite cells around large sensory neurons after sciatic nerve injuries and was transiently upregulated by some astrocytes in the degenerating dorsal columns after dorsal rhizotomy.

The use of adenoviral vectors and ex vivo transduced neurotransplants: towards promotion of neuroregeneration

Regeneration of injured axons following injury depends on a delicate balance between growth-promoting and growth-inhibiting factors. Overexpression of neurotrophin genes seems a promising strategy to promote regeneration. Trophic genes can be overexpressed at the site of injury at the axonal stumps, or at the perikaryal level of the injured neuron. Transduction of the neural cells can be achieved by applying adenoviral vectors, either directly in vivo or-in the case of neurotransplantation as an ex vivo approach. In both cases it would create a more permissive environment for axonal growth and therefore in functional regeneration. In this article, the feasibility of the use of adenoviral vectors in several neuroregeneration models--in particularly in spinal cord lesion models and the biological clock transplantation model--is illustrated. The results show that the adenoviral vectors can be a powerful tool to study the effects of overexpression of genes in an in vivo paradigm of nerve regeneration or nerve outgrowth. The potential use of adenoviral vectors and ex vivo transduced neurotransplants is discussed.

Adenoviral vector-mediated expression of a foreign gene in peripheral nerve tissue bridges implanted in the injured peripheral and central nervous system

Axons of the CNS do normally not regenerate after injury, in contrast to axons of the PNS. This is due to a different microenvironment at the site of the lesion as well as a particular intrinsic program of axonal regrowth. Although transplantation of peripheral nerve tissue bridges is perhaps the most successful approach to promoting regeneration in the CNS, ingrowth of CNS nerve fibers with such transplants is limited. Genetic modification of peripheral nerve bridges to overexpress outgrowth-promoting proteins should, in principle, improve the permissive properties of peripheral nerve transplants. The present study shows that pieces of peripheral intercostal nerve, subjected to ex vivo adenoviral vector-mediated gene transfer and implanted as nerve bridges in transected sciatic nerve, avulsed ventral root, hemi-sected spinal cord and intact brain, are capable of expressing a foreign gene. In vitro studies showed expression of the reporter gene LacZ up to 30 days in Schwann cells. After implantation, LacZ expression could be detected at 7 days postimplantation, but had virtually disappeared at 14 days. Schwann cells of the transduced nerve bridges retained the capacity of guiding regenerative peripheral and central nerve fiber ingrowth. Transduction of intercostal nerve pieces prior to implantation should, in principle, enable enhanced local production of neurotrophic factors within the transplant and has the potential to improve the regeneration of injured axons into the graft.

Regeneration of dorsal column fibers into and beyond the lesion site following adult spinal cord injury

Regeneration is abortive following adult mammalian CNS injury. We have investigated whether increasing the intrinsic growth state of primary sensory neurons by a conditioning peripheral nerve lesion increases regrowth of their central axons. After dorsal column lesions, all fibers stop at the injury site. Animals with a peripheral axotomy concomitant with the central lesion show axonal growth into the lesion but not into the spinal cord above the lesion. A preconditioning lesion 1 or 2 weeks prior to the dorsal column injury results in growth into the spinal cord above the lesion. In vitro, the growth capacity of DRG neurite is also increased following preconditioning lesions. The intrinsic growth state of injured neurons is, therefore, a key determinant for central regeneration.

NT-3 delivered by an adenoviral vector induces injured dorsal root axons to regenerate into the spinal cord of adult rats

Sensory axons interrupted in the dorsal roots of adult mammals are normally unable to regenerate into the spinal cord. We have investigated whether the introduction of a neurotrophin gene into the spinal cord might offer an approach to otherwise intractable spinal root injuries. The dorsal roots of the 4th, 5th, and 6th lumbar spinal nerves of adult rats were severed and reanastomosed. Fourteen to nineteen days later, adenoviral vectors containing either the LacZ or NT-3 genes were injected into the ventral horn of the lumbar spinal cord, resulting in strong expression of the transgenes in glial cells and motor neurons between 4 and 40 days after injection. When dorsal root axons were transganglionically labelled with HRP conjugated to cholera toxin subunit B, 16 to 37 days after dorsal root injury, large numbers of labelled axons could be seen to have regenerated into the cord, but only in those animals injected with vector carrying the NT-3 gene. The regenerated axons were found at the injection site, mainly in the grey matter, and had penetrated as deep as lamina V. Gene therapy with adenoviral vectors encoding a neurotrophin has therefore been shown to be capable of enhancing and directing the regeneration of a subpopulation of dorsal root axons (probably myelinated A fibres), into and through the CNS environment.

Effects of prelesioned peripheral nerve graft on nerve regeneration in the rat spinal cord

OBJECTIVE

The aim of this study was to examine the effects of prelesioned peripheral nerve grafts on central nerve regeneration compared with the freshly transected peripheral nerve grafts in the dorsal funiculus of the rat spinal cord.

METHODS

The experimental paradigm consisted of ligating the common peroneal nerve at the midthigh level for 7 days, while the adjacent tibial nerve was left intact. Numerous Schwann cells appeared accompanying regenerating axons in the proximal stump of the ligated nerve. The proximal stumps of the ligated (prelesioned) common peroneal nerve and the intact (untreated) tibial nerve were excised as one tissue block and autografted into the dorsal funiculi of the upper cervical cord. The graft was placed so that the prelesioned common peroneal nerve was positioned on the left dorsal funiculus and the untreated tibial nerve was positioned to the right of the midsagittal plane. Nerve regeneration was examined by light and transmission electron microscopy 1 to 16 weeks after grafting, comparing the effectiveness of prelesioned and untreated nerve grafts.

RESULTS

Numerous regenerating axons were observed in the caudal border of both grafts 1 to 2 weeks after grafting. Astrocyte proliferation was suppressed in the prelesioned grafts compared to the untreated grafts. Four to 16 weeks later, the number of regenerating axons was approximately 10-fold as large in the prelesioned grafts as in the untreated grafts. The regenerating axons were myelinated by Schwann cells. Astrocytic glial scar formation was inconspicuous in the prelesioned grafts, whereas it was prominent in the untreated grafts. Schwann cells were contiguous with astrocytes along regenerating axons, forming a continuous conduit from the central to peripheral nerve microenvironments for the outgrowth of regenerating axons.

CONCLUSION

The prelesioned peripheral nerve graft is more effective than the untreated graft in suppressing astrocytic scar formation and in supporting the outgrowth of regenerating axons in the dorsal funiculus of rat spinal cord.

Regeneration of neurosecretory axons into various types of intrahypothalamic grafts is promoted by the absence of blood brain barrier: fine structural analysis

Isogenous grafts of neural lobe and optic nerve and autologous grafts of sciatic nerve were placed into contact with the intrahypothalamically transected hypothalamo-neurohypophysial tract, and their fine structural characteristics examined at various time periods thereafter. The vascular bed of neural lobe grafts is composed primarily of fenestrated capillaries, that are permeable to blood-borne HRP throughout the entire experimental period. The microvasculature of sciatic nerve grafts consists of continuous, as well as fenestrated capillaries, which are similarly permeable to HRP. Fenestrated capillaries and HRP leakage in optic nerve grafts are observed at 10 days, but only in grafts located ventrally in the hypothalamus at 30 days. Neurosecretory axon regeneration is seen only in grafts or adjacent hypothalamus where the blood-brain barrier is breached. Regenerating axons are closely associated with the specific glial cells of the respective graft. Based on these observations, we conclude that blood-borne factors are necessary to initiate and sustain regeneration of transected neurosecretory axons, and that such regeneration occurs only in the presence of glial cells.

BDNF and NT-4/5 prevent atrophy of rat rubrospinal neurons after cervical axotomy, stimulate GAP-43 and Talpha1-tubulin mRNA expression, and promote axonal regeneration

Rubrospinal neurons (RSNs) undergo a marked atrophy in the second week after cervical axotomy. This delayed atrophy is accompanied by a decline in the expression of regeneration-associated genes such as GAP-43 and Talpha1-tubulin, which are initially elevated after injury. These responses may reflect a deficiency in the trophic support of axotomized RSNs. To test this hypothesis, we first analyzed the expression of mRNAs encoding the trk family of neurotrophin receptors. In situ hybridization revealed expression of full-length trkB receptors in virtually all RSNs, which declined 7 d after axotomy. Full-length trkC mRNA was expressed at low levels. Using RT-PCR, we found that mRNAs encoding trkC isoforms with kinase domain inserts were present at levels comparable to that for the unmodified receptor. TrkA mRNA expression was not detected in RSNs, and the expression of p75 was restricted to a small subpopulation of axotomized cells. In agreement with the pattern of trk receptor expression, infusion of recombinant human BDNF or NT-4/5 into the vicinity of the axotomized RSNs, between days 7 and 14 after axotomy, fully prevented their atrophy. This effect was still evident 2 weeks after the termination of BDNF treatment. Moreover, BDNF or NT-4/5 treatment stimulated the expression of GAP-43 and Talpha1-tubulin mRNA and maintained the level of trkB expression. Vehicle, NGF, or NT-3 treatment had no significant effect on cell size or GAP-43 and Talpha1-tubulin expression. In a separate experiment, infusion of BDNF also was found to increase the number of axotomized RSNs that regenerated into a peripheral nerve graft. Thus, in BDNF-treated animals, the prevention of neuronal atrophy and the stimulation GAP-43 and Talpha1-tubulin expression is correlated with an increased regenerative capacity of axotomized RSNs.

Intrastriatal grafts of rat colonic smooth muscle lacking myenteric ganglia stimulate axonal sprouting and regeneration

Grafts of living or freeze-killed freshly dissected colonic smooth muscle from young inbred Fischer rats were implanted into the corpus striatum of adult Fischer rats. Sections of brain were examined electron microscopically 3 and 6 wk after implantation. At both times, living grafts were vascularised and contained healthy differentiated smooth muscle cells, fibroblasts, interstitial cells of Cajal and some macrophages. Large bundles of small nonmyelinated axons, identified as CNS axonal sprouts, could be observed in the brain at and near the interface between the living smooth muscle and the CNS tissue. Bundles of regenerating CNS axons, often associated with astrocyte processes, had grown into the grafts. Some axons within the grafts had matured, enlarged and become myelinated by oligodendrocyte processes or Schwann cells. In some cases, smooth muscle cells were observed in close and intricate association with axons. In contrast to the living grafts, grafts of freeze-killed smooth muscle, examined 3 and 6 wk after implantation, contained macrophages, fibroblasts, collagen and large amounts of cellular debris, but no living muscle cells, astrocytes or Schwann cells. The striatal neuropil around freeze-killed grafts did not contain large bundles of CNS axonal sprouts and bundles of axons were not observed within the freeze-killed graft. This study demonstrates that cells from the smooth muscle layers of the colon, in the absence of myenteric ganglia, can stimulate a vigorous regenerative response from CNS axons when implanted into the corpus striatum of adult rats.

c-Jun expression in adult rat dorsal root ganglion neurons: differential response after central or peripheral axotomy

The response of the mature central nervous system (CNS) to injury differs significantly from the response of the peripheral nervous system (PNS). Axotomized PNS neurons generally regenerate following injury, while CNS neurons do not. The mechanisms that are responsible for these differences are not completely known, but both intrinsic neuronal and extrinsic environmental influences are likely to contribute to regenerative success or failure. One intrinsic factor that may contribute to successful axonal regeneration is the induction of specific genes in the injured neurons. In the present study, we have evaluated the hypothesis that expression of the immediate early gene c-jun is involved in a successful regenerative response. We have compared c-Jun expression in dorsal root ganglion (DRG) neurons following central or peripheral axotomy. We prepared animals that received either a sciatic nerve (peripheral) lesion or a dorsal rhizotomy in combination with spinal cord hemisection (central lesion). In a third group of animals, several dorsal roots were placed into the hemisection site along with a fetal spinal cord transplant. This intervention has been demonstrated to promote regrowth of severed axons and provides a model to examine DRG neurons during regenerative growth after central lesion. Our results indicated that c-Jun was upregulated substantially in DRG neurons following a peripheral axotomy, but following a central axotomy, only 18% of the neurons expressed c-Jun. Following dorsal rhizotomy and transplantation, however, c-Jun expression was upregulated dramatically; under those experimental conditions, 63% of the DRG neurons were c-Jun-positive. These data indicate that c-Jun expression may be related to successful regenerative growth following both PNS and CNS lesions.

Corticospinal tract regrowth

The natural ability of the adult central nervous system of higher vertebrates to recover from injury is highly limited. This limitation is most likely due to an inhospitable environment and/or intrinsic incapacities of the neurons to re-extend their neurites after injury or axotomy. The rat corticospinal tract is the largest tract leading from brain to spinal cord and is often used as a model in developmental and regeneration studies. The extensive know-how of factors involved in the development of the corticospinal tract did provide the foundation for many studies on corticospinal tract regrowth after injury in the adult spinal cord. The results of these experiments, as discussed in this review, have led to important contributions to the further understanding of central nervous system regeneration.

Integration of transplanted cultured Schwann cells into the long myelinated fiber tracts of the adult spinal cord

A suspension of about 10,000 purified Schwann cells cultured from the neonatal rat sciatic nerve was transplanted into a discrete site in the upper cervical level of the corticospinal tract of one side in adult rats. From 4 days after transplantation immunostaining for p75 (low-affinity neurotrophin receptor) showed that the transplants consisted of a central mass of Schwann cells and cuffs of elongated Schwann cells along the perivascular space of curving blood vessels (most of which had been formed in response to the transplantation). Schwann cells leaving the central mass and perivascular cuffs migrated in strictly linear orientation along the rostrocaudal axis of the host corticospinal tract. According to the territory through which they migrated, the transplanted Schwann cells adopted two quite different forms: (1) The row Schwann cells, which migrated singly or in groups within the rows of host oligodendrocytic and astrocytic cell bodies, were non-process-bearing, rather cuboidal, brick-like cells (about 8 x 12 microm in size). (2) In contrast, the interfascicular Schwann cells, which migrated singly or intertwined in rope-like small groups interspersed among the axons of the host corticospinal tract, were larger, symmetrically bipolar cells, with processes about 100-120 microm long and 2 microm wide and bulging, ovoid nuclei, located in centrally placed cell bodies about 10 microm across. After about 6 weeks, the p75 immunoreactivity of the interfascicular Schwann cells had become down-regulated. However, from as early as 10 days after transplantation, immunostaining for the peripheral myelin protein, P0, semithin sections, and electron microscopy showed that these Schwann cells were not lost, but that they had myelinated the segments of the host corticospinal axons in the region of the transplant. In contrast, the row Schwann cells did not express P0 or form myelin. They retained their p75 immunoreactivity at long survivals (presumably because they were secluded from contacting the tract axons). The row Schwann cells also migrated farther than the interfascicular Schwann cells (possibly a function of their maintained p75 expression), becoming dispersed singly for at least 8 mm from the original transplant site. Our previous study of corticospinal tract lesions had shown the formation of a "closed" scar formed by hypertrophic astrocytic processes, which walled off a central astrocyte-free region and totally disrupted the normal longitudinal alignment of the tract astrocytic processes. In contrast, while the present Schwann cell transplants induced a comparable astrocytic hypertrophy over the same time course, the astrocytic processes remained able to penetrate the transplant site, which was not walled off, so that the longitudinal arrangement of the host corticospinal tract astrocytic skeleton was preserved intact across the region of the transplant. These observations show that Schwann cells can be intimately integrated into the cytoarchitecture of the myelinated adult host corticospinal tract. This integration is not a random dispersal in damaged areas: it involves direct interaction with the cell elements present in the host tract, it respects the complex and regular organization of the host tract glial cells, and it results in the formation of a precisely arranged mosaic of central and peripheral tissue.

Regeneration of neurosecretory axons into various types of intrahypothalamic graft is promoted by the absence of the blood-brain barrier: a neurophysin-immunohistochemical and horseradish peroxidase-histochemical study

In order to test the hypothesis that neurosecretory axon regeneration occurs only in the presence of specific vascular, perivascular, and glial microenvironments, isografts of neural lobe and optic nerve and autografts of sciatic nerve were transplanted into the hypothalamo-neurohypophysial tract at the lateral retrochiasmatic area of adult male rats. The integrity of the blood-brain barrier (BBB) to intravenously administered horseradish peroxidase (HRP), the regenerative process of neurosecretory axons, and functional recovery from lesion-induced diabetes insipidus were analyzed at 18 hr, 36 hr, 10 days, 30 days, and 80 days postsurgery. Neurophysin-positive axons invaded all grafts, as well as perivascular spaces of the adjacent hypothalamus. Wherever neurosecretory axon regeneration occurred, the BBB was breached. Reestablishment of the BBB was paralleled by a decrease in both density and staining intensity of regenerated neurophysin-positive axons. These observations illustrate that neurosecretory axon regeneration is tributary of the absence of BBB. It is speculated that blood-borne factors, provided when the BBB is breached, initiate and sustain neurosecretory axon regeneration. In addition, products of glial elements may enhance or complement the above stimulatory processes.

Comparison of neurodegenerative pathology in transgenic mice overexpressing V717F beta-amyloid precursor protein and Alzheimer's disease

Overexpression of mutated human amyloid precursor protein (hAPP717V-->F) under control of platelet-derived growth factor promoter (PDAPP minigene) in transgenic (tg) mice results in neurodegenerative changes similar to Alzheimer's disease (AD). To clarify the pathology of these mice, we studied images derived from laser scanning confocal and electron microscopy and performed comparisons between PDAPP tg mice and AD. Similar to AD, neuritic plaques in PDAPP tg mouse contained a dense amyloid core surrounded by anti-hAPP- and antineurofilament-immunoreactive dystrophic neurites and astroglial cells. Neurons were found in close proximity to plaques in PDAPP tg mice and, to a lesser extent, in AD. In PDAPP tg mice, and occasionally in AD, neuronal processes contained fine intracellular amyloid fibrils in close proximity to the rough endoplasmic reticulum, coated vesicles, and electron-dense material. Extracellular amyloid fibrils (9-11 nm in diameter) were abundant in PDAPP tg and were strikingly similar to those observed in AD. Dystrophic neurites in plaques of PDAPP tg mouse and AD formed synapses and contained many dense multilaminar bodies and neurofilaments (10 nm). Apoptotic-like figures were present in the tg mice. No paired helical filaments have yet been observed in the heterozygote PDAPP tg mice. In summary, this study shows that PDAPP tg mice develop massive neuritic plaque formation and neuronal degeneration similar to AD. These findings show that overproduction of hAPP717V-->F in tg mice is sufficient to cause not only amyloid deposition, but also many of the complex subcellular degenerative changes associated with AD.

Change in the molecular phenotype of Schwann cells upon transplantation into the central nervous system: down-regulation of c-jun

Activated Schwann cells such as those in the distal stump of a cut peripheral nerve, or those cultured in vitro, develop a molecular phenotype very different from that of quiescent Schwann cells, and express high levels of the transcription factor c-jun. We studied the expression of c-jun messenger RNA, by in situ hybridization, and Jun-like immunoreactivity of Schwann cells in segments of peripheral nerve, or in cell suspensions grafted into the adult rat brain. Schwann cells rapidly lost their Jun immuno-positivity, and down-regulated expression of c-jun messenger RNA once implanted into the brain, and only the Schwann cells contained in the portion of peripheral nerve which remained outside the brain maintained Jun-like immunopositivity. c-jun messenger RNA was also down-regulated in the grafts, but more slowly than the protein; however, a proximodistal gradient in the level of expression of c-jun messenger RNA along the graft, comparable to that found for Jun immunoreactivity, was not detected. Schwann cells transplanted into the lesioned central nervous system promote regeneration of some injured central nervous system axons, but this regenerative response is always much more limited than peripheral nervous system regeneration. We suggest a correlation between the limited regeneration of central nervous system axons into peripheral nerve grafts and the loss of c-jun expression in Schwann cells following exposure to the central nervous system environment.

Intrinsic versus extrinsic factors in determining the regeneration of the central processes of rat dorsal root ganglion neurons: the influence of a peripheral nerve graft

The relative contribution of intrinsic growth capacity versus extrinsic growth-promoting factors in determining the capacity of transected dorsal root axons to regenerate long distances was studied. L4 dorsal root axons regenerating into 4-cm peripheral nerve grafts on transected dorsal roots were counted. Few dorsal root myelinated axons regenerated to the distal end of the grafts by 10 weeks unless the sciatic nerve was also crushed. Regeneration of unmyelinated axons was also increased by peripheral lesions. Crush or transection of the dorsal roots without grafting did not alter GAP-43 mRNA expression in L4 dorsal root ganglion (DRG) cells. Grafting a peripheral nerve onto the cut end of an L4 dorsal root doubled the number of DRG cells expressing high levels of GAP-43 mRNA after a delay of several weeks. Peripheral nerve crush at the time of nerve grafting resulted in a very rapid rise in GAP-43 mRNA expression, which then declined to a steady level, twice that of controls, by 7 weeks. Thus, the rapid increase in the number of DRG neurons expressing high levels of GAP-43 mRNA after peripheral but not central axotomy correlates with the regeneration of central axons through nerve grafts. Because GAP-43 mRNA is slowly upregulated in a subpopulation of sensory neurons in response to exposure of their central axons to a peripheral nerve environment, environments favourable for axonal growth may act by increasing the intrinsic growth response of neurons. Lack of intrinsic growth capacity may contribute to the failure of dorsal root axons to regenerate into the spinal cord.

Neuronal cell death in the mammalian nervous system: the calmortin hypothesis

1. This review is concerned with the calcium-dependent mechanisms involved in neuronal cell death. To this end, it provides definitions of the major types of cell death and then describes what is known of their occurrence during development and degeneration of the mammalian nervous system. 2. An analysis is presented of the different sources and compartments of calcium in neurons and of how these are related to the known calcium-dependent enzymes whose excess activation will lead to cell death. 3. The review uses the relatively large amount of pertinent information now available for other cell types, especially thymocytes, to reveal our limited knowledge of how calcium controls neuronal cell death. 4. In the final section, consideration is given to the identification of those factors that may mitigate against the calcium-dependent pathways leading to neuronal degeneration.