Regrowth of severed axons in the neonatal central nervous system: establishment of normal connections

Augmenting fibronectin levels in injured adult CNS promotes axon regeneration in vivo

In an attempt to repair injured central nervous system (CNS) nerves/tracts, immune cells are recruited into the injury site, but endogenous response in adult mammals is insufficient for promoting regeneration of severed axons. Here, we found that a portion of retinal ganglion cell (RGC) CNS projection neurons that survive after optic nerve crush (ONC) injury are enriched for and upregulate fibronectin (Fn)-interacting integrins Itga5 and ItgaV, and that Fn promotes long-term survival and long-distance axon regeneration of a portion of axotomized adult RGCs in culture. We then show that, Fn is developmentally downregulated in the axonal tracts of optic nerve and spinal cord, but injury-activated macrophages/microglia upregulate Fn while axon regeneration-promoting zymosan augments their recruitment (and thereby increases Fn levels) in the injured optic nerve. Finally, we found that Fn's RGD motif, established to interact with Itga5 and ItgaV, promotes long-term survival and long-distance axon regeneration of adult RGCs after ONC in vivo, with some axons reaching the optic chiasm when co-treated with Rpl7a gene therapy. Thus, experimentally augmenting Fn levels in the injured CNS is a promising approach for therapeutic neuroprotection and axon regeneration of at least a portion of neurons.

Cadherin-12 Regulates Neurite Outgrowth Through the PKA/Rac1/Cdc42 Pathway in Cortical Neurons

Cadherins play an important role in tissue homeostasis, as they are responsible for cell-cell adhesion during embryogenesis, tissue morphogenesis, and differentiation. In this study, we identified Cadherin-12 (CDH12), which encodes a type II classical cadherin, as a gene that promotes neurite outgrowth in an in vitro model of neurons with differentiated intrinsic growth ability. First, the effects of CDH12 on neurons were evaluated via RNA interference, and the results indicated that the knockdown of CDH12 expression restrained the axon extension of E18 neurons. The transcriptome profile of neurons with or without siCDH12 treatment revealed a set of pathways positively correlated with the effect of CDH12 on neurite outgrowth. We further revealed that CDH12 affected Rac1/Cdc42 phosphorylation in a PKA-dependent manner after testing using H-89 and 8-Bromo-cAMP sodium salt. Moreover, we investigated the expression of CDH12 in the brain, spinal cord, and dorsal root ganglia (DRG) during development using immunofluorescence staining. After that, we explored the effects of CDH12 on neurite outgrowth in vivo. A zebrafish model of CDH12 knockdown was established using the NgAgo-gDNA system, and the vital role of CDH12 in peripheral neurogenesis was determined. In summary, our study is the first to report the effect of CDH12 on axonal extension in vitro and in vivo, and we provide a preliminary explanation for this mechanism.

PI 3-kinase delta enhances axonal PIP3 to support axon regeneration in the adult CNS

Peripheral nervous system (PNS) neurons support axon regeneration into adulthood, whereas central nervous system (CNS) neurons lose regenerative ability after development. To better understand this decline whilst aiming to improve regeneration, we focused on phosphoinositide 3-kinase (PI3K) and its product phosphatidylinositol (3,4,5)-trisphosphate (PIP3 ). We demonstrate that adult PNS neurons utilise two catalytic subunits of PI3K for axon regeneration: p110α and p110δ. However, in the CNS, axonal PIP3 decreases with development at the time when axon transport declines and regenerative competence is lost. Overexpressing p110α in CNS neurons had no effect; however, expression of p110δ restored axonal PIP3 and increased regenerative axon transport. p110δ expression enhanced CNS regeneration in both rat and human neurons and in transgenic mice, functioning in the same way as the hyperactivating H1047R mutation of p110α. Furthermore, viral delivery of p110δ promoted robust regeneration after optic nerve injury. These findings establish a deficit of axonal PIP3 as a key reason for intrinsic regeneration failure and demonstrate that native p110δ facilitates axon regeneration by functioning in a hyperactive fashion.

Heterogeneity in the regenerative abilities of central nervous system axons within species: why do some neurons regenerate better than others?

Some neurons, especially in mammalian peripheral nervous system or in lower vertebrate or in vertebrate central nervous system (CNS) regenerate after axotomy, while most mammalian CNS neurons fail to regenerate. There is an emerging consensus that neurons have different intrinsic regenerative capabilities, which theoretically could be manipulated therapeutically to improve regeneration. Population-based comparisons between “good regenerating” and “bad regenerating” neurons in the CNS and peripheral nervous system of most vertebrates yield results that are inconclusive or difficult to interpret. At least in part, this reflects the great diversity of cells in the mammalian CNS. Using mammalian nervous system imposes several methodical limitations. First, the small sizes and large numbers of neurons in the CNS make it very difficult to distinguish regenerating neurons from non-regenerating ones. Second, the lack of identifiable neurons makes it impossible to correlate biochemical changes in a neuron with axonal damage of the same neuron, and therefore, to dissect the molecular mechanisms of regeneration on the level of single neurons. This review will survey the reported responses to axon injury and the determinants of axon regeneration, emphasizing non-mammalian model organisms, which are often under-utilized, but in which the data are especially easy to interpret.

The Rise of Molecules Able To Regenerate the Central Nervous System

Injury to the adult central nervous system (CNS) usually leads to permanent deficits of cognitive, sensory, and/or motor functions. The failure of axonal regeneration in the damaged CNS limits functional recovery. The lack of information concerning the biological mechanism of axonal regeneration and its complexity has delayed the process of drug discovery for many years compared to other drug classes. Starting in the early 2000s, the ability of many molecules to stimulate axonal regrowth was evaluated through automated screening techniques; many hits and some new mechanisms involved in axonal regeneration were identified. In this Perspective, we discuss the rise of the CNS regenerative drugs, the main biological techniques used to test these drug candidates, some of the most important screens performed so far, and the main challenges following the identification of a drug that is able to induce axonal regeneration in vivo.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth-Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

Injury to the brain and spinal cord has devastating consequences because adult central nervous system (CNS) axons fail to regenerate. Injury to the peripheral nervous system (PNS) has a better prognosis, because adult PNS neurons support robust axon regeneration over long distances. CNS axons have some regenerative capacity during development, but this is lost with maturity. Two reasons for the failure of CNS regeneration are extrinsic inhibitory molecules, and a weak intrinsic capacity for growth. Extrinsic inhibitory molecules have been well characterized, but less is known about the neuron-intrinsic mechanisms which prevent axon re-growth. Key signaling pathways and genetic/epigenetic factors have been identified which can enhance regenerative capacity, but the precise cellular mechanisms mediating their actions have not been characterized. Recent studies suggest that an important prerequisite for regeneration is an efficient supply of growth-promoting machinery to the axon; however, this appears to be lacking from non-regenerative axons in the adult CNS. In the first part of this review, we summarize the evidence linking axon transport to axon regeneration. We discuss the developmental decline in axon regeneration capacity in the CNS, and comment on how this is paralleled by a similar decline in the selective axonal transport of regeneration-associated receptors such as integrins and growth factor receptors. In the second part, we discuss the mechanisms regulating selective polarized transport within neurons, how these relate to the intrinsic control of axon regeneration, and whether they can be targeted to enhance regenerative capacity. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Gene Manipulation Strategies to Identify Molecular Regulators of Axon Regeneration in the Central Nervous System

Limited axon regeneration in the injured adult mammalian central nervous system (CNS) usually results in irreversible functional deficits. Both the presence of extrinsic inhibitory molecules at the injury site and the intrinsically low capacity of adult neurons to grow axons are responsible for the diminished capacity of regeneration in the adult CNS. Conversely, in the embryonic CNS, neurons show a high regenerative capacity, mostly due to the expression of genes that positively control axon growth and downregulation of genes that inhibit axon growth. A better understanding of the role of these key genes controlling pro-regenerative mechanisms is pivotal to develop strategies to promote robust axon regeneration following adult CNS injury. Genetic manipulation techniques have been widely used to investigate the role of specific genes or a combination of different genes in axon regrowth. This review summarizes a myriad of studies that used genetic manipulations to promote axon growth in the injured CNS. We also review the roles of some of these genes during CNS development and suggest possible approaches to identify new candidate genes. Finally, we critically address the main advantages and pitfalls of gene-manipulation techniques, and discuss new strategies to promote robust axon regeneration in the mature CNS.

MASH1/Ascl1a leads to GAP43 expression and axon regeneration in the adult CNS

Unlike CNS neurons in adult mammals, neurons in fish and embryonic mammals can regenerate their axons after injury. These divergent regenerative responses are in part mediated by the growth-associated expression of select transcription factors. The basic helix-loop-helix (bHLH) transcription factor, MASH1/Ascl1a, is transiently expressed during the development of many neuronal subtypes and regulates the expression of genes that mediate cell fate determination and differentiation. In the adult zebrafish (Danio rerio), Ascl1a is also transiently expressed in retinal ganglion cells (RGCs) that regenerate axons after optic nerve crush. Utilizing transgenic zebrafish with a 3.6 kb GAP43 promoter that drives expression of an enhanced green fluorescent protein (EGFP), we observed that knock-down of Ascl1a expression reduces both regenerative gap43 gene expression and axonal growth after injury compared to controls. In mammals, the development of noradrenergic brainstem neurons requires MASH1 expression. In contrast to zebrafish RGCs, however, MASH1 is not expressed in the mammalian brainstem after spinal cord injury (SCI). Therefore, we utilized adeno-associated viral (AAV) vectors to overexpress MASH1 in four month old rat (Rattus norvegicus) brainstem neurons in an attempt to promote axon regeneration after SCI. We discovered that after complete transection of the thoracic spinal cord and implantation of a Schwann cell bridge, animals that express MASH1 exhibit increased noradrenergic axon regeneration and improvement in hindlimb joint movements compared to controls. Together these data demonstrate that MASH1/Ascl1a is a fundamental regulator of axonal growth across vertebrates and can induce modifications to the intrinsic state of neurons to promote functional regeneration in response to CNS injury.

Neuritogenic activity of a genipin derivative in retinal ganglion cells is mediated by retinoic acid receptor β expression through nitric oxide/S-nitrosylation signaling

Genipin, a herbal iridoid, is known to have both neuroprotective and neuritogenic activity in neuronal cell lines. As it is structurally similar to tetrahydrobiopterin, its activity is believed to be nitric oxide (NO)-dependent. We previously proposed a novel neuroprotective activity of a genipin derivative, (1R)-isoPropyloxygenipin (IPRG001), whereby it reduces oxidative stress in RGC-5, a neuronal precursor cell line of retinal origin through protein S-nitrosylation. In the present study, we investigated another neuritogenic property of IPRG001 in RGC-5 cells and retinal explant culture where in we focused on the NO-cGMP-dependent and protein S-nitrosylation pathways. IPRG001 stimulated neurite outgrowth in RGC-5 cells and retinal explant culture through NO-dependent signaling, but not NO-dependent cGMP signaling. Neurite outgrowth with IPRG001 requires retinoic acid receptor β (RARβ) expression, which is suppressed by an RAR blocking agent and siRNA inhibition. Thereby, we hypothesized that RARβ expression is mediated by protein S-nitrosylation. S-nitrosylation of histone deacetylase 2 is a key mechanism in chromatin remodeling leading to transcriptional gene activation. We found a parallelism between S-nitrosylation of histone diacetylase 2 and the induction of RARβ expression with IPRG001 treatment. The both neuroprotective and neuritogenic activities of genipin could be a new target for the regeneration of retinal ganglion cells after glaucomatous conditions.

Developmental changes of gene expression after spinal cord injury in neonatal opossums

Changes in gene expression have been measured 24h after injury to mammalian spinal cords that can and cannot regenerate. In opossums there is a critical period of development when regeneration stops being possible: at 9 days postnatal cervical spinal cords regenerate, at 12 days they do not. By the use of marsupial cDNA microarrays, we detected 158 genes that respond differentially to injury at the two ages critical for regeneration. For selected candidates additional measurements were made by real-time PCR and sites of their expression were shown by immunostaining. Candidate genes have been classified so as to select those that promote or prevent regeneration. Up-regulated by injury at 8 days and/or down-regulated by injury at 13 days were genes known to promote growth, such as Mitogen-activated protein kinase kinase 1 or transcription factor TCF7L2. By contrast, at 13 days, up-regulation occurred of inhibitory molecules, including annexins, ephrins, and genes related to apoptosis and neurodegenerative diseases. Certain genes such as calmodulin 1 and NOGO, changed expression similarly in animals that could and could not regenerate without any additional changes in response to injury. These findings confirmed and extended changes of gene expression found in earlier screens on 9 and 12 ay preparations without lesions and provide a comprehensive list of genes that serve as a basis for testing how identified molecules, singly or in combination, promote and prevent central nervous system regeneration.

The ability of axons to regenerate their growth cones depends on axonal type and age, and is regulated by calcium, cAMP and ERK

The processes activated at the time of axotomy and leading to the formation of a new growth cone are the first step in regeneration, but are still poorly characterized. We investigated this event in an in vitro model of axotomy performed on dorsal root ganglia and retinal explants. We observed that the dorsal root ganglion axons and retinal ganglion cell axons, which had grown out on a poly d-lysine/laminin substrate at the time of culture preparation greatly differed in their regenerative response after a subsequent in vitro lesion made far from the cell body. The majority of axons of adult dorsal root ganglia but only a small percentage of axons of adult retinal ganglion cells regenerated new growth cones within four hours after in vitro axotomy, though both kinds of axons were growing before the lesion. The depletion of extracellular calcium and the inhibition of extracellular-signal regulated kinase 1,2 (ERK) and protein kinase A (PKA) at the time of injury significantly impaired the capacity of dorsal root ganglia axons to re-initiate growth cones without affecting growth cone motility. Pharmacological treatments directed at increasing the level of cAMP promoted growth cone regeneration in adult retinal ganglion cell axons in spite of the low regenerative potential exhibited in normal conditions. Understanding the cellular mechanisms activated at the time of lesion and leading to the formation of a new growth cone is necessary for devising treatments aimed at enhancing the regenerative response of injured axons.

Inhibition of collagen synthesis overrides the age-related failure of regeneration of nigrostriatal dopaminergic axons

To investigate the mechanism of the age-related failure of regeneration of transected axons, nigrostriatal dopaminergic axons were unilaterally transected in the lateral hypothalamus in adult mice and in immature mice aged postnatal days 7, 14, and 21. Ten days after the transection, tyrosine hydroxylase-immunoreactive axons had regenerated from caudally to rostrally across the lesion site in mice transected at postnatal day 7, whereas they stopped and did not extend across the lesion site in mice transected at postnatal day 14 or older. Reactive astrocytes bearing chondroitin sulfate proteoglycans were observed around the lesion in mice transected at all ages. However, a fibrotic scar containing type IV collagen-immunoreactive deposits, which was consistently formed at the lesion site in mice transected at postnatal day 14 or older, was not formed in mice lesioned at postnatal day 7. When 2,2'-dipyridyl, an inhibitor of collagen synthesis, was injected into the lesion site at the time of transection in both postnatal day 14 and adult mice, the deposition of type IV collagen and the formation of a fibrotic scar were completely prevented, and large numbers of tyrosine hydroxylase-immunoreactive axons extended across the lesion and reinnervated the striatum. These results imply that the fibrotic scar formed in the lesion site is a crucial impediment to the regeneration of ascending dopaminergic axons in adult mice and suggest that type IV collagen is required for the development of the fibrotic response to adult brain injury.

Higher calpastatin levels correlate with resistance to calpain-mediated proteolysis and neuronal apoptosis in juvenile rats after spinal cord injury

While the average age for patients admitted with spinal cord injury is 32 years, patients under the age of 16 account for 5% of spinal cord injured persons. For these younger patients, an increased mortality up to 24 h post-injury has been reported, however, survivors may regain more function than their adult counterparts, suggesting that age may play a role in injury tolerance. While the use of growth factors as a therapy for spinal cord injury is well researched, the response of the developing cord to secondary injury has not been thoroughly investigated. Following spinal cord injury, Ca(2+) influx can activate enzymes such as calpain, a Ca(2+)-dependent protease, which plays a role in the pathogenesis of spinal cord injury in rats. The present investigation revealed that following spinal cord injury, calpain upregulation was significantly less (15.3%) in the 21-day-old rats than in either 45-day-old (70%) or 90-day-old (99.6%) rats, as shown by Western blot and in situ immunofluorescent studies. Expression of the endogenous calpain inhibitor, calpastatin, was significantly higher in juvenile rats than adult rats. Juvenile rats with spinal cord injury also showed a reduced Bax:Bcl-2 ratio (4:1 vs. 6:1), reduced caspase-3 staining, reduced myelin loss (3% vs. 18%), and less neuronal DNA damage, as compared to older rats. These results suggest that increased calpastatin levels found in juvenile rats muted calpain activity and neuronal apoptosis, following spinal cord injury.

High rates of neurological improvement following severe traumatic pediatric spinal cord injury

STUDY DESIGN

Retrospective single-center study

OBJECTIVES

To determine the long-term outcome of pediatric spinal cord injuries

SUMMARY OF BACKGROUND DATA

Spinal cord injuries are uncommon events in the pediatric population. In the few large series reported in the literature, recovery of neurologic function was demonstrated after mild injuries but was rare after severe injuries.

METHODS

A total of 4,876 cases of pediatric trauma treated at the Children's Hospital of Los Angeles over a 9-year period (1993-2001) were reviewed. During the study period, 91 cases of spinal cord or spinal column injury were identified, and 30 cases involving a spinal cord injury were identified. Cauda equina injuries were excluded. Seven craniocervical, 12 cervical, 5 thoracic, and 6 thoracolumbar cases were identified. There were 6 cases of spinal cord injury without radiographic abnormality. Eight of the 30 patients received methylprednisolone at the time of admission. Follow-up ranged from 2 to 54 (mean = 19) months.

RESULTS

Twenty patients presented with complete injuries (ASIA grade A). Of these, 7 died, 7 had no neurologic recovery, and 6 experienced neurologic improvement. Five of these six eventually became ambulatory with functional gains occurring over a 4- to 50-week period. None of these 5 patients was found to have spinal cord injury without radiographic abnormality. Of the remaining 10 patients (grades B-D), 8 experienced improvements in neurologic function. Cervical dislocation injuries were associated with a low likelihood of neurologic improvement and atlanto-occipital injuries were associated with early death.

CONCLUSIONS

Recovery of neurologic function following severe traumatic spinal cord injury occurs with a significantly greater incidence in children than adults, and these improvements can occur over a prolonged postinjury period.

Long-lasting sprouting and gene expression changes induced by the monoclonal antibody IN-1 in the adult spinal cord

Lesion-induced plasticity of the rat corticospinal tract (CST) decreases postnatally, simultaneously with myelin appearance. In adult rats, compensatory sprouting can be induced by the monoclonal antibody (mAb) IN-1 raised against the growth inhibitory protein Nogo-A. In this study, we examined separately the fate of sensory and motor corticospinal fibers after mAb IN-1 application. Intact adult rats treated with the IN-1 antibody exhibited an increase of aberrant CST projections, i.e., sensory fibers projecting into the ventral horn and motor fibers projecting dorsally. Unilateral lesion of the CST [pyramidotomy (PTX)] in the presence of mAb IN-1 triggered a progressive reorganization of the sprouting of the remaining CST across the midline, with sensory fibers projecting gradually into the denervated dorsal horn and motor fibers projecting into the denervated ventral horn. In unilaterally denervated spinal cords, aberrant sprouts were only transient and disappeared by 6 weeks, whereas midline crossing fibers ending in the appropriate target region were stabilized and persisted over the entire study period. Within the spinal cord, IN-1 antibody treatment was associated with upregulation of growth factors (BDNF, VEGF), growth-related proteins (actin, myosin, GAP-43), and transcription factors (STATs), whereas pyramidotomy induced an enhanced expression of guidance molecules (semaphorins and slits) as well as neurotrophic factors (BDNF, IGFs, BMPs). These gene expression changes may contribute to attraction, guidance, and stabilization of sprouting CST fibers.

Is the capacity for optic nerve regeneration related to continued retinal ganglion cell production in the frog?

In the central nervous system of fish and frogs, some, but not all, axons can regenerate. Retinal ganglion cells are among those that can. The retinae of fish and frogs produce new retinal neurons, including ganglion cells, for months or years after hatching. We have evaluated the hypothesis that retinal axonal regeneration is obligatorily linked to continued production of new ganglion cells. We used bromodeoxyuridine immunocytochemistry to assess retinal neurogenesis in juvenile, yearling, and 10 year old Xenopus laevis. Retinal ganglion cell genesis was vigorous in the marginal retina of the juveniles, but in the yearlings and the 10 year olds, no new ganglion cells were produced there. Cellular proliferation in the central retina was evident at all three ages, but none of the cells produced centrally were in the ganglion cell layer. Regeneration was examined in vivo by cutting one optic nerve and then, weeks later, injecting the eye with tritiated proline. Autoradiographs of brain sections showed that the optic nerves of all three ages regenerated. Regeneration in vitro was assessed using retinal explants from frogs of all three ages. In all cases, the cultures produced neurites, with some age-specific differences in the patterns of outgrowth. We conclude that retinal axonal regeneration is not linked obligatorily to maintained neurogenesis.

The critical role of basement membrane-independent laminin gamma 1 chain during axon regeneration in the CNS

We have addressed the question of whether a family of axon growth-promoting molecules known as the laminins may play a role during axon regeneration in the CNS. A narrow sickle-shaped region containing a basal lamina-independent form of laminin exists in and around the cell bodies and proximal portion of the apical dendrites of CA3 pyramidal neurons of the postnatal hippocampus. To understand the possible function of laminin in axon regeneration within this pathway, we have manipulated laminin synthesis at the mRNA level in a slice culture model of the lesioned mossy system. In this model early postnatal mossy fibers severed near the hilus can regenerate across the lesion and elongate rapidly within strata lucidum and pyramidale. In slice cultures of the postnatal day 4 hippocampus, 2 d before lesion and then continuing for 1-5 d after lesion, translation of the gamma1 chain product of laminin was reduced by using antisense oligodeoxyribonucleotides and DNA enzymes. In the setting of the lesioned organotypic hippocampal slice, astroglial repair of the lesion and overall glial patterning were unperturbed by the antisense or DNA enzyme treatments. However, unlike controls, in the treated, lesioned slices the vast majority of regenerating mossy fibers could not cross the lesion site; those that did were very much shorter than usual, and they took a meandering course. In a recovery experiment in which the DNA enzyme or antisense oligos were washed away, laminin immunoreactivity returned and mossy fiber regeneration resumed. These results demonstrate the critical role of laminin(s) in an axon regeneration model of the CNS.

Reinnervation of the superior colliculus delays down-regulation of ephrin A2 in neonatal rat

Although the adult mammalian optic nerve does not regenerate following lesion, in the neonatal rat, retinal ganglion cell (RGC) axons retain the capacity to grow across lesion sites in the brain. Following a brachial lesion at postnatal day 2 (P2), some RGC axons, together with ingrowing cortico-tectal axons, cross the lesion to reinnervate the superior colliculus (SC). Here we use immunohistochemistry to examine expression of the guidance cue ephrin A2 following a brachial lesion. Normal animals show a steady decrease in ephrin A2 immunoreactivity between P5 and P31, with a low rostral to high caudal gradient being evident only at P5. By contrast, after brachial lesion, values are significantly elevated rostrally at P5 and caudally at P12; moreover, a steep rostro-caudal gradient is present at both ages. By P31 values fall to normal levels. Following unilateral enucleation at P2, levels are not significantly different from normal. Our results show that innervation but not denervation triggers increased ephrin A2 expression after a brachial lesion.

Corticostriatal plasticity is restricted by myelin-associated neurite growth inhibitors in the adult rat

After unilateral cortical lesions in neonatal rats, the spared unablated hemisphere is known to demonstrate remarkable neuroanatomical plasticity in corticofugal connectivity. This same type of structural plasticity is not seen after similar lesions in adult rats. One possibility for the lack of such a plastic response in the adult central nervous system may be the presence of myelin-associated neurite growth inhibitory proteins NI-35/NI-250. These proteins have previously been found to play a crucial role in preventing axotomized fibers from regenerating after adult rat spinal cord lesions. The aim of this study was to determine if blocking these inhibitory proteins by the application of the specific monoclonal antibody IN-1 would enhance corticostriatal plasticity from the spared hemisphere after unilateral cortical lesions in adult rats. Six- to 8-week-old Lewis rats underwent unilateral aspiration lesion of the sensorimotor cortex. Animals were immediately treated with either monoclonal antibody IN-1 or a control antibody released from hybridoma cells in Millipore filter capsules. After a survival period of 12 weeks, the opposite sensorimotor cortex was stereotaxically injected with the anterograde tracer biotinylated dextran amine, and biotinylated dextran amine-positive corticostriatal fibers were analyzed. The monoclonal antibody IN-1-treated animals showed an increase in corticostriatal fibers in the dorsolateral striatum contralateral to the injection site compared with control antibody-treated animals or normal controls, indicating a specific sprouting response in the deafferented zone. These results support the idea that through blockade of myelin-associated neurite inhibitory proteins, lesion-induced corticofugal plasticity is possible even in the adult central nervous system.

Expression of the gene encoding the chemorepellent semaphorin III is induced in the fibroblast component of neural scar tissue formed following injuries of adult but not neonatal CNS

This study evaluates the expression of the chemorepellent semaphorin III (D)/collapsin-1 (sema III) following lesions to the rat CNS. Scar tissue, formed after penetrating injuries to the lateral olfactory tract (LOT), cortex, perforant pathway, and spinal cord, contained numerous spindle-shaped cells expressing high levels of sema III mRNA. The properties of these cells were investigated in detail in the lesioned LOT. Most sema III mRNA-positive cells were located in the core of the scar and expressed proteins characteristic for fibroblast-like cells. Neuropilin-1, a sema III receptor, was expressed in injured neurons with projections to the lesion site, in a subpopulation of scar-associated cells and in blood vessels around the scar. In contrast to lesions made in the mature CNS, LOT transection in neonates did not induce sema III mRNA expression within cells in the lesion and was followed by vigorous axonal regeneration. The concomitant expression of sema III and its receptor neuropilin-1 in the scar suggests that sema III/neuropilin-1-mediated mechanisms are involved in CNS scar formation. The expression of the secreted chemorepellent sema III following CNS injury provides the first evidence that chemorepulsive semaphorins may contribute to the inhibitory effects exerted by scars on the outgrowth of injured CNS neurites. The vigorous regrowth of injured axons in the absence of sema III following early neonatal lesions is consistent with this notion. The inactivation of sema III in scar tissue by either antibody perturbation or by genetic or pharmacological intervention could be a powerful means to promote long-distance regeneration in the adult CNS.

Brainstem inputs to the ferret medial geniculate nucleus and the effect of early deafferentation on novel retinal projections to the auditory thalamus

Following specific neonatal brain lesions in rodents and ferrets, retinal axons have been induced to innervate the medial geniculate nucleus (MGN). Previous studies have suggested that reduction of normal retinal targets along with deafferentation of the MGN are two concurrent factors required for the induction of novel retino-MGN projections. We have examined, in ferrets, the relative influence of these two factors on the extent of the novel retinal projection. We first characterized the inputs to the normal MGN, and the most effective combination of neonatal lesions to deafferent this nucleus, by injecting retrograde tracers into the MGN of normal and neonatally operated adult ferrets, respectively. In a second group of experiments, newborn ferrets received different combinations of lesions of normal retinal targets and MGN afferents. The resulting extent of retino-MGN projections was estimated for each case at adulthood, by using intraocular injections of anterograde tracers. We found that the extent of retino-MGN projections correlates well with the extent of MGN deafferentation, but not with extent of removal of normal retinal targets. Indeed, the presence of at least some normal retinal targets seems necessary for the formation of retino-MGN connections. The diameters of retino-MGN axons suggest that more than one type of retinal ganglion cells innervate the MGN under a lesion paradigm that spares the visual cortex and lateral geniculate nucleus. We also found that, after extensive deafferentation of MGN, other axonal systems in addition to retinal axons project ectopically to the MGN. These data are consistent with the idea that ectopic retino-MGN projections develop by sprouting of axon collaterals in response to signals arising from the deafferented nucleus, and that these axons compete with other sets of axons for terminal space in the MGN.

Neurite growth inhibitors restrict plasticity and functional recovery following corticospinal tract lesions

Anatomical plasticity and functional recovery after lesions of the rodent corticospinal tract (CST) decrease postnatally in parallel with myelin formation. Myelin-associated neurite growth inhibitory proteins prevent regenerative fiber growth, but whether they also prevent reactive sprouting of unlesioned fibers is less clear. Here we show that after unilateral CST lesion in the adult rat brainstem, both intact and lesioned tracts show topographically appropriate sprouting after treatment with a monoclonal antibody that neutralizes these inhibitory proteins. Antibody-treated animals showed full recovery in motor and sensory tests, whereas untreated lesioned rats exhibited persistent severe deficits. Neutralization of myelin-associated neurite growth inhibitors thus restores in adults the structural plasticity and functional recovery normally found only at perinatal ages.

Spontaneous regeneration of the pyramidal tract after transection in young rats

Spontaneous regeneration of the pyramidal tract after transection of the medullary pyramid was examined in young rats by the anterograde tracing method with wheat germ agglutinin-conjugated horseradish peroxidase. Care was taken to cut the tract as sharply as possible to minimize traumatic injuries. A very sharp cut produced edema-free lesions without subsequent formation of either cysts or scars, whereas a relatively blunt cut produced edema and later scars and/or cysts in the lesion. Regenerated projections in the latter cases were sparse, short, dispersed and largely aberrant as described in previous reports. By contrast, regenerated projections in the former cases were very much similar to normal in various respects: the amount, extension, path, formation of a compact bundle and termination. There was, however, a decisive difference from normal, that is, the additional aberrant projections.

A comparison of postlesion growth of retinotectal and corticotectal axons after superior colliculus transections in neonatal rats

We examined, in neonatal rats, the postinjury response of two different axonal systems that project to a common target area in the visual system. Transections across the rostral part of the left superior colliculus (SC) were made in 2- or 6-day-old rats (P2, P6). Lesioned animals were randomly selected into short- or long-term groups. The short-term group was used to determine the efficacy of the lesion technique; 2-6 days after transections, right (contralateral) eyes were injected with horseradish peroxidase (HRP). Complete deafferentation of the SC was achieved in 73% of P2 (n = 22) and 53% of P6 (n = 10) short-term animals. In the long-term group (examined 2-7 months after transection), retinotectal and corticotectal projections were assessed in each animal by using [3H]proline and wheat germ agglutin-HRP, respectively. Examination of a series of sagittal sections revealed that the cut had extended across the entire SC in 63% of P2 (n = 19) and 55% of P6 (n = 12) long-term rats. Despite this, retinal and cortical axons were seen in appropriate layers in postlesion SC in all P2 lesioned animals. Cortical projections caudal to the cut were seen in all P6 rats; however, in these animals, the retinal projection was sparse and not always present. Differences in lesion geometry led to consistent differences in the pattern and extent of ingrowth of retinal and cortical axons into postlesion SC neuropil. The two axonal populations also followed different paths as they grew between prelesion and postlesion SC. It is likely that a number of factors influenced the patterns of postlesion growth, including the relative maturity of the axons and the neuropil into which they were growing. There was also, however, clear evidence of competitive interactions between retinal and cortical axons in postlesion SC that consistently led to greater than normal segregation of the two populations and hence restricted their terminal distributions.

Regeneration in the developing optic nerve: correlating observations in the opossum to other mammalian systems

Regeneration of severed axons within the central nervous system of adult mammals does not normally occur with any degree of success. During development, however, newly forming projections must send axons to distant sites and form appropriate connections with their targets: successful regeneration has been observed during this critical period. The opossum central nervous system develops during early postnatal life and has provided a useful experimental model to investigate this specialized mode of axonal regeneration in mammals. The presence of a clear decision point at the optic chiasm has also provided a useful site at which to investigate the navigational capacity of retinal ganglion cells regenerating along the optic nerve during this critical period. Regeneration failure occurs as the central nervous system progresses from this permissive, developing state to a mature, non-permissive adult state. Studies into the behaviour of glial and neuronal elements around this transition period can help elucidate some of the factors that need to be overcome if regeneration is ever to become successful in adult mammals. The regeneration characteristics of a lesioned projection are dependent upon its developmental stage and are also related to the proximity of axotomy along its pathway. A system of staging is proposed to correlate observations in the opossum optic nerve to other mammalian systems.

Early development and developmental plasticity of the fasciculus gracilis in the North American opossum (Didelphis virginiana)

The first objective of the present study was to ask when axons of the fasciculus gracilis reach the nucleus gracilis in the North American opossum (Didelphis virginiana). When Fast Blue (FB) was injected into the lumbar cord on postnatal day (PD) 1 and the pups were killed 2 days later, labeled axons were present within a distinct fasciculus gracilis at thoracic and cervical levels of the cord. When comparable injections were made at PD3 or 5 and the pups were allowed to survive for the same time period, a few labeled axons could be followed to the caudal medulla where they were located dorsal to the presumptive nucleus gracilis. In order to verify these observations and to determine if any of the axons which innervate the nucleus gracilis early in development originate within dorsal root ganglia, we also employed cholera toxin conjugated to horseradish peroxidase (CT-HRP) to label dorsal root axons transganglionically. When CT-HRP was injected into the hindlimb on PD1 and the pups were maintained for 1 day prior to death and HRP histochemistry, labeled axons were present within the fasciculus gracilis at thoracic and cervical levels, but they could not be traced into the medulla. When comparable injections were made on PD3, and the pups were maintained for 2 days, labeled axons were present within the caudal medulla. Our second objective was to determine whether axons of the fasciculus gracilis grow through a lesion of their spinal pathway during early development. In one group of animals, the thoracic cord was transected at PD5, 8, 12, 20 and 26 and bilateral injections of Fast Blue (FB) were made four segments caudal to the lesion 30-40 days later. After a 3-5 day survival, the pups were killed and perfused so that the spinal cord and brainstem could be removed and sectioned for fluorescence microscopy. In all of the cases lesioned at PD5, axons of the fasciculus gracilis were labeled rostral to the site of transection and they could be followed to the nucleus gracilis. Evidence for growth of fasciculus gracilis axons into the caudal medulla was also seen in cases lesioned at PD8. In contrast, labeled axons were not observed rostral to the lesion when it was made at PD12 or at later stages of development. In order to verify that some of the axons which crossed the lesion originated within dorsal root ganglia, the thoracic cord was transected at PD5 in another group of animals and 7 days later, injections of CT-HRP were made into one of the hindlimbs. After a 3 day survival, labeled axons could be traced through the lesion site and into the caudal medulla. We conclude that axons of the fasciculus gracilis reach the nucleus gracilis by at least PD5 in the opossum and that they grow through a lesion of their spinal pathway when it is made at the same age or shortly thereafter. The critical period for such growth appears to end between PD8 and PD12.

Growth of axons through a lesion in the intact CNS of fetal rat maintained in long-term culture

The ability of neurons in the central nervous system (CNS) to grow through a lesion and restore conduction has been analysed in developing spinal cord in vitro. The preparation consists of the entire CNS of embryonic rat, isolated and maintained in culture. Conduction of electrical activity and normal morphological appearance (light microscopical and electron microscopical) were maintained in the spinal cord of such preparations for up to 7 d in culture. A complete transverse crush of the spinal cord abolished all conduction for 2 d. After 3-5 d, clear recovery had occurred: electrical conduction across the crush was comparable with that in uninjured preparations. Furthermore, the spinal cord had largely regained its gross normal appearance at the crush site. Axons stained in vivo by carbocyanine dyes had, by 5 d, grown in profusion through the lesion and several millimetres beyond it. These experiments, like those made in neonatal opossum (Treherne et al. 1992) demonstrate that central neurons of immature mammals, unlike those in adults, can respond to injury by rapid and extensive outgrowth of nerve fibres in the absence of peripheral nerve bridges or antibodies that neutralize inhibitory factors. However, unlike the opossum, in which outgrowth occurred at 24 degrees C, although there was prolonged survival of rat spinal cords at this temperature, outgrowth of axons across the lesion required a temperature of 29 degrees C. With rapid and reliable regeneration in vitro it becomes practicable to assay the effects of molecules that promote or inhibit restoration of functional connections.

Expression of myelin proteins in the opossum optic nerve: late appearance of inhibitors implicates an earlier non-myelin factor in preventing ganglion cell regeneration

The pattern of appearance of myelin-associated proteins in the visual system of the Brazilian opossum Monodelphis domestica is described. Whole mounts of optic nerve, chiasm, and optic tract were sectioned horizontally and incubated with antibodies to myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), "Rip," and the neurite inhibitory protein (IN-1), followed by visualization with diaminobenzidine and a peroxidase-conjugated secondary antibody. PLP is first detectable 24 days after birth (P24) at the centre of the optic chiasm. MBP, MAG, Rip, and IN-1 appear first in the same area at P26. By P28 the distribution of all proteins is similar, occupying the entire chiasm, optic tracts, and prechiasmatic portion of the optic nerves. Protein expression progresses along the optic nerve to reach the lamina cribrosa by P34, coincident with the time of eye opening. A critical period in which the retinofugal pathway has a regenerative capacity has recently been observed in Monodelphis. This period ends at P12, 2 weeks before the appearance of the myelin-associated inhibitory proteins MAG and IN-1. These results therefore suggest that regeneration in the developing retinofugal projection of the opossum is restricted by an earlier non-myelin factor, which is in contrast to current literature on the spinal cord.

Growth of dorsal spinocerebellar axons through a lesion of their spinal pathway during early development in the North American opossum, Didelphis virginiana

Supraspinal axons grow around or through lesions of their spinal pathway during specific critical periods of mammalian development, but comparable plasticity has not been documented for axons which form ascending tracts. In the present study, we asked whether axons of the dorsal spinocerebellar tract (DSCT) are capable of such growth. The spinal cord of the North American opossum, Didelphis virginiana, was hemisected at mid-thoracic levels between postnatal day (PD) 5 and 68 and after varying survival times, bilateral injections of Fluoro-Gold or Fast Blue were made into the anterior lobe of the cerebellum, the major target of DSCT axons. Seven days later, the pups were sacrificed and their spinal cord processed for fluorescence microscopy. In animals lesioned between PD5 and 9, and allowed to survive for 37-269 days, neurons were labeled bilaterally in Clarke's nucleus (CN) caudal to the lesion, but they were fewest in number and smallest in size on the lesioned side. Since the DSCT originates almost entirely within CN on the ipsilateral side, we conclude that the neurons labeled ipsilateral and caudal to the lesion supported axons which grew around or through it. Histological examination revealed that recognizable spinal cord was present at the lesion site and that labeled spinocerebellar axons were located in their normal position ipsilateral to the lesion. It appears, therefore, that growth occurred through the lesion. In animals lesioned between PD13 and 68, labeled neurons were not found in CN caudal and ipsilateral to the lesion although they were present on the contralateral (control) side. We conclude that DSCT axons, like axons which form descending tracts, grow through a lesion of their spinal pathway if it is made early in development.

Development and role of retinal glia in regeneration of ganglion cells following retinal injury

AIMS/BACKGROUND

Recent observations have shown that the glial scar resulting from a surgical lesion of the immature retina differs from elsewhere in the central nervous system, in that it permits the through growth and reconnection of regenerating axons. This study in the opossum examines in detail the development and reaction to injury of retinal glia at different developmental stages, and specifically examines the distribution of the gliosis related inhibitory molecule, chondroitin sulphate proteoglycan (CSPG), making comparisons with a control site of gliosis in the cerebral cortex.

METHODS

A linear slit was cut into the retina or cortex with a fine tungsten probe. After a variable time delay, immunocytochemistry of the resulting gliosis was employed to detect astrocytes with glial fibrillary acidic protein (GFAP), Müller cells with vimentin, and CSPG with CS-56 antibodies. GFAP was also used at different ages to examine the normal development of astrocytes in the retina of this species.

RESULTS

Astrocytes entered the retina 12 days after birth (P12), closely associated with blood vessels in the nerve fibre layer. In experiments at all ages studied, cellular continuity was re-established across the lesioned retina, which did not result in a significant astrocyte proliferation or CSPG expression. In contrast, cortical injury led to the development of a cystic cavity surrounded by astrocytes and CSPG. Müller cells expressed GFAP but not CSPG in the lesioned retina.

CONCLUSION

Successful regrowth of ganglion cells through a retinal lesion may be partly the result of the scarcity of astrocytes in the retina, which results in minimal gliosis, or of their apparent inability to express inhibitory molecules.

The effect of fetal neocortical transplants on lesion-induced cerebral cortex plasticity

Sensorimotor cortical lesions in newborn rats lead to the formation of abnormal projections from the opposite intact sensorimotor cortex. In the present study the influence of fetal neocortical transplants on this lesion-induced plasticity was examined. Newborn rats received unilateral frontal neocortical lesions. One experimental group received grafts of fetal neocortical tissue (E14-E16) into the lesion cavities. Another group served as lesion-only animals, while a third group was left unlesioned and without grafts as normal controls. At 3 mo of age, the animals received injections of the anterograde tracer biotinylated dextran amine (BDA) into the sensorimotor cortex contralateral to the lesion/transplantation area. After sacrifice 2 wk later, the brains were processed histochemically for detection of BDA-labeled cells and fibers. As a measure of the lesion-induced axonal sprouting response, corticothalamic and corticopontine fibers crossing the midline were counted. Significantly fewer cortical efferent fibers crossed the thalamic midline in the transplanted rats compared to the lesion-only controls. In contrast, the presence of transplants did not reduce the corticopontine sprouting response. These results therefore indicate that fetal neocortical grafts have a modulatory, yet variable effect on the lesion-induced axonal sprouting of contralateral sensorimotor cortical neurons.

The Effect of Fetal Neocortical Transplants on Lesion-Induced Cerebral Cortex Plasticity

Sensorimotor cortical lesions in newborn rats lead to the formation of abnormal projections from the opposite intact sensorimotor cortex. In the present study the influence of fetal neocortical transplants on this lesion-induced plasticity was examined. Newborn rats received unilateral frontal neocortical lesions. One experimental group received grafts of fetal neocortical tissue (E14–E16) into the lesion cavities. Another group served as lesion-only animals, while a third group was left unlesioned and without grafts as normal controls. At 3 mo of age, the animals received injections of the anterograde tracer biotinylated dextran amine (BDA) into the sensorimotor cortex contralateral to the lesion/transplantation area. After sacrifice 2 wk later, the brains were processed histochemically for detection of BDA-labeled cells and fibers. As a measure of the lesion-induced axonal sprouting response, corticothalamic and corticopontine fibers crossing the midline were counted. Significantly fewer cortical efferent fibers crossed the thalamic midline in the transplanted rats compared to the lesion-only controls. In contrast, the presence of transplants did not reduce the corticopontine sprouting response. These results therefore indicate that fetal neocortical grafts have a modulatory, yet variable effect on the lesion-induced axonal sprouting of contralateral sensorimotor cortical neurons.

Accurate synapse regeneration despite ablation of the distal axon segment

In each body ganglion of the leech Hirudo medicinalis there is a single S-cell. After an S-cell axon is severed, it regenerates along its surviving distal segment and reconnects with its synaptic target, the axon of the neighbouring S-cell. In approximately half the cases the regenerating axon forms a temporary electrical synapse specifically with the distal segment, which remains active and connected to the target, thereby functioning as a splice until regeneration is complete. To determine whether the distal axon segment is required for successful regeneration, distal segments of severed S-cell axons were ablated by intracellular injection of bacterial protease. Fifty-seven preparations were examined from 2 to 212 days after injection of the axon segment. The extent of S-cell axon regeneration was assessed electrophysiologically by intracellular and extracellular recording, and anatomically by intracellular injection of markers followed by light microscopy and electron microscopy. The S-cell axons regenerated successfully in almost 90% of animals examined after 2 weeks or more. In a further four animals the target S-cell was ablated in addition to the distal axon segment, permanently disrupting conduction along the S-cell pathway. Nevertheless, the regenerating axon grew along its usual pathway and there was no evidence that alternative connections were formed. It is concluded that, although the distal axon segment can provide a means for rapid functional repair, the segment is not required for reliable regeneration of the axon along its usual pathway and accurate formation of an electrical synapse.

A critical period for axon regrowth through a lesion in the developing mammalian retina

Although the central nervous system of mature mammals is incapable of regeneration, certain elements present in the developing system must permit and promote the growth of new axons to their initial targets. We investigate whether the environment of a developing visual system is capable of supporting regeneration in the Brazilian opossum Monodelphis domestica, in which the retinofugal system develops postnatally. Retinae were lesioned up to the 16th postnatal day and analysed for regeneration after a further 7-10 days. Anterograde tracing with Dil showed axons to have regrown from the axotomized area of retina directly through the lesion. Retrograde tracing with horseradish peroxidase injected into the superior colliculus confirmed that axons from the lesioned area of retina had grown to an appropriate position in the midbrain. The proportion of retinae in which axonal continuity was restored across the lesion decreased as the visual system matured, falling to zero after the 12th postnatal day. Thus a critical period exists in the postnatal opossum in which a retinal lesion permits axon passage. Correlating these results to the known pattern of retinofugal pathway development provides an insight into factors that may restrict this critical period to the 12th postnatal day, and suggests that at least some of the axotomized neurons are regenerating.

Anatomy, development and lesion-induced plasticity of rodent corticospinal tract

In this review the current knowledge of the anatomy, development and plasticity of the rodent corticospinal tract is summarised. Recent technical advancements, especially in neuronal tracing methods, have provided much new data concerning the anatomy of the corticospinal tract. The rodent corticospinal axons project to the subcortical nuclei via collateral branches. These collateral branches of corticospinal axons are formed by delayed interstitial budding during early postnatal periods. Corticospinal neurons are generated in the ventricular zone during a short time lag, migrate into the cortical plate, and settle in layer V of the cerebral cortex. The migration of corticospinal neurons is experimentally deranged by prenatal exposure to alcohol or genetically affected by the reeler genetic locus (rl), resulting in generation of ectopic corticospinal neurons. Such experimentally or genetically induced ectopic corticospinal neurons are a good model for examining whether target recognition and path finding are affected by the intracortical position of corticospinal neurons. Some chemical molecules (e.g. L1 and B-50/GAP43) are transiently expressed in the corticospinal tract during the perinatal period, while others (e.g. protein kinase C gamma subspecies and alpha CaM kinase II) are permanently expressed in the adult corticospinal tract. The only chemical marker specific for layer V corticofugal neurons is an antibody to a soluble protein, protein 35. Since the corticospinal tract in the rodent is an easily identified group of fibers situated in the most ventral portion of the dorsal funiculus of the spinal cord and exhibits considerable postnatal development, it has often been utilized in the neurological studies on plasticity and regenerative capacity of the lesioned central nervous system. Recently, it has been clarified that growing corticospinal fibers have the ability to penetrate and traverse across the lesion sites under certain special conditions.

Developmental plasticity of reticulospinal and vestibulospinal axons in the north American opossum, Didelphis virginiana

We have shown previously that rubral axons grow around a lesion of their spinal pathway in the North American opossum if it is made at early stages of development. In the present experiments, we have asked whether reticular and vestibular axons have the same ability. The spinal cord was hemisected at postnatal day 20, 12, or 5, well within the critical period for rubrospinal plasticity, and, approximately 30 days later, bilateral injections of fast blue were made about four segments caudal to the lesion. The pups were killed 4 or 5 days after the injections. In most of the animals lesioned on postnatal day 20, labeled neurons were not found in the medial part of the pontine reticular nucleus or the dorsal part of the lateral vestibular nucleus ipsilateral to the lesion. The spinal projections from both areas are exclusively ipsilateral. When the lesions were made at postnatal day 12 or 5, however, labeled neurons were present in both areas, suggesting that they supported axons that had grown caudal to the lesion. As was expected from previous studies, rubral neurons were labeled contralateral to the lesion in all three groups. In the opossum, as in other species, the red nucleus projects contralaterally. We conclude that reticular and vestibular axons, like axons from the red nucleus, grow around a lesion of their pathway during development and that the critical period for their plasticity ends earlier than that for rubrospinal axons.

Biceps vs extensor carpi radialis recovery in Frankel grades A and B in spinal cord injury patients

Clinical literature suggests that the wrist extensors show a trend of achieving functional strength earlier than the biceps after spinal cord injury (SCI). Basic research, however, demonstrated that proximal muscles recover earlier than distal muscles after partial denervation. The purpose of this study was to compare biceps to extensor carpi radialis (ECR) recovery of muscle strength in 39 motor complete cervical SCI patients. Biceps (n = 19) and ECR (n = 20) with a 72 hour or 1 week motor grade of 1/5 were compared. Testing was performed weekly for 1 month, and again at 2, 3, 6 and 12 months post injury. The median recovery times to increase one motor grade were: biceps = 2 months and ECR = 2.5 months (p < 0.3). The median recovery times to increase two motor grades were: biceps = 2 months and ECR = 3 months (p < 0.4). In conclusion, there was no significant difference between the rates of recovery of the biceps and the ECR up to 12 months post SCI.

Regeneration of axons into the trochlear rootlet after anterior medullary lesions in the rat is specific for ipsilateral IVth nerve motoneurones

The fibre projection from the IVth nerve nucleus to the superior oblique muscle was determined quantitatively in the normal rat by defining fibre numbers in transverse sections of the IVth nerve, and neurone numbers after retrograde labelling by horseradish peroxidase (HRP) injection into the muscle. There were 183 +/- 27 (S.E.) labelled neurones in the nucleus contralateral to the injected muscle and only 2 +/- 1 ipsilateral. The ipsilateral fibre number was 234 +/- 7 and the cell/axon ratio 0.8 +/- 0.1. Extensive analysis of all HRP retrogradely labelled material revealed no central fibre contribution to the IVth nerve other than from neurones resident in the trochlear nucleus. The central portion of the trochlear nerve tract was severed at its point of decussation in the anterior medullary velum. Ninety days after lesion, 10 +/- 4 (6% of control) neurones were labelled in the ipsilateral trochlear nucleus; none were labelled in the contralateral nucleus or in any other part of the midbrain, pons, medulla, or cerebellum. The number of myelinated fibres in the IVth nerve had decreased to 21 +/- 5 (9% of control) so that the cell/axon ratio was 0.4 +/- 0.2, thus suggesting that a single motoneurone has more fibres after lesion. In electron micrographs of the IVth nerve, larger than normal numbers of unmyelinated fibres were seen. Many myelinated fibres displayed signs of abnormal myelination. After regeneration, the projection was exclusively ipsilateral and not crossed as in the normal. These findings establish that there is a high degree of specificity after regeneration since no myelinated central nervous system axons other than trochlear fibres select the IVth nerve root as a trajectory over which to regenerate.

Restoration of function by replacement of spinal cord segments in the rat

Reconstruction of a severed mammalian spinal cord with restoration of function has so far not been achieved, although structural and functional restitution after spinal transection has been successful in some lower vertebrates. In quail-chick and chick-chick chimaeras, spinal cord segments were found to be functional after replacement by isotopic and isochronic grafting of the neural tube. Here we achieve such a replacement in neonatal rats under less restricted topological and temporal conditions than were necessary for the avian chimaeras. The replaced segments united with the host spinal cord and promoted robust growth and regrowth of axons across the graft, enabling neural connections to be reconstructed that were hardly distinguishable from normal. The animals with replaced segments could walk, run and climb with almost normal hind-forelimb coordination. This functional restoration in these animals appeared to be permanent, raising the possibility of therapeutic application in humans.

The early development of major projections from caudal levels of the spinal cord to the brainstem and cerebellum in the gray short-tailed Brazilian opossum, Monodelphis domestica

The Brazilian short-tailed opossum, Monodelphis domestica, is born 14-15 days after copulation and is available for experimentation at stages of development corresponding to those which occur in utero in placental mammals. In the present study, we took advantage of the opossum's embryology to study the development of projections from caudal levels of the spinal cord to the brainstem and cerebellum using axonal tracing methods. In all cases, a 2-3 day survival time was used for axonal transport. When injections of Fast blue (FB) were made into caudal levels of the thoracic cord at postnatal day (PD) 1 or 2, axonal labeling could not be identified at supraspinal levels. When injections were made at PD3, however, labeled axons were found in the fasciculus gracilis at caudal medullary levels, within the ventrolateral medulla and pons, within an incipient inferior cerebellar peduncle, and within the cerebellar anlage. The dorsal root origin of at least some of the axons within the fasciculus gracilis was evidenced by the transganglionic transport of cholera toxin conjugated to horseradish peroxidase from the hindlimbs. After FB injections at PD7, a few labeled axons could be traced from the fasciculus gracilis into the nucleus gracilis and from the ventrolateral pathway to the inferior olive. Generally comparable results were obtained using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). In cases injected with FB at PD9, the pattern of brainstem labeling was adult-like. Although labeled axons were present within the cerebellum of animals injected with FB on PD3, they were limited to the marginal zone. Axonal labeling was present within an identifiable internal granular layer in cases injected with either FB or WGA-HRP at PD16, and it appeared to be limited to specific bands which foreshadowed those seen at later stages of development and in the adult animal. In some cases, labeled axons were present within the molecular layer where they were not seen in the adult animal. Our results provide a timetable for the normal development of projections from caudal levels of the spinal cord to the brainstem and cerebellum in Monodelphis and show that such development occurs postnatally rather than prenatally, as in placental mammals.

Protection from 1-methyl-4-phenylpyridinium (MPP+) toxicity and stimulation of regrowth of MPP(+)-damaged dopaminergic fibers by treatment of mesencephalic cultures with EGF and basic FGF

Several peptide growth factors can maintain survival or promote recovery of injured central neurons. In the present study, the effects of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) on the toxicity produced by the dopaminergic neurotoxin, 1-methyl-4-phenylpyridinium (MPP+), were investigated in rat mesencephalic dopaminergic neurons in culture. High affinity [3H]DA uptake and morphometric analyses of tyrosine hydroxylase immunostained neurons were used to assess the extent of MPP+ toxicity, dopaminergic neuronal survival and growth of neurites. Consistent with previous reports, EGF and bFGF treatments stimulated neuritic outgrowth in dopaminergic neurons, increased DA uptake and enhanced their long-term survival in vitro. These growth factors also stimulated proliferation of astrocytes. The time course of EGF and bFGF effects on dopaminergic neurons coincided with the increase in glial cell density, suggesting that proliferation of glia mediates their trophic effects. Several findings from our study support this possibility. When MPP+ was applied to cultures at 4 days in vitro, before glial cells had proliferated, the damage to dopaminergic neurons was not affected by EGF or bFGF pretreatments. However, when cultures maintained in the presence of the growth factors for 10 days were exposed to MPP+, after they had become confluent with dividing glial cells, the MPP(+)-induced decreases in DA uptake and cell survival were significantly attenuated. Furthermore, when glial cell proliferation was inhibited by 5-fluoro-2'-deoxyuridine, the protective effects of EGF and bFGF against MPP+ toxicity were abolished. Continuous treatment of MPP(+)-exposed cultures with EGF or bFGF resulted in the stimulation of process regrowth of damaged dopaminergic neurons with concomitant recovery of DA uptake, suggesting that the injured neurons are able to respond to the trophic effects of EGF and bFGF. In summary, our study shows that the trophic effects of EGF and bFGF on mesencephalic dopaminergic neurons include protection from the toxicity produced by MPP+ and promotion of recovery of MPP(+)-damaged neurons. Stimulation of glial cell proliferation is necessary for these effects.

The origins of descending projections to the lumbar spinal cord at different stages of development in the North American opossum

We have employed the retrograde transport of fast blue (FB) to identify the origins of descending projections to the lumbar cord of the opossum from postnatal day (PD)1, 12-13 days after conception, to maturity. When FB injections were made into the lumbar cord at PD1, supraspinal labeling was sparse and limited to the hypothalamus, the reticular formation, the coeruleus complex, the caudal raphe, and, in one case, the interstitial nucleus of the medial longitudinal fasciculus and the lateral vestibular nucleus. Only a few propriospinal neurons were labeled at cervical and thoracic levels. By PD3, however, supraspinal and propriospinal labeling was abundant and present in most of the areas labeled in the adult animal. A notable exception was the red nucleus which was not labeled until approximately PD10. Our results have been compared with those described in other species and discussed in light of their relevance to the development of descending control over hindlimb movement and developmental plasticity of descending spinal pathways.

Immature corticospinal neurons respond to axotomy with changes in tubulin gene expression

We have examined the expression of two different tubulin mRNAs in hamster corticospinal neurons that were axotomized at three different developmental stages; postnatal day 8 (P8), P20, and adult. In situ hybridization of histological sections of the sensorimotor cortex was done with 35S-labeled cDNA probes specific to alpha 1-tubulin and beta III-tubulin mRNAs at 2-14 days following unilateral transection of the corticospinal tract in the caudal medulla. Both film and emulsion autoradiography were used to detect changes in tubulin mRNA levels. Qualitative assessment indicated substantial decreases in both alpha 1-tubulin and beta III-tubulin mRNA levels in layer V neurons of the sensorimotor cortex following axotomy. The changes were apparent as early as 2 days postinjury for P20 and adult operates, but not for P8 operates. However, by 14 days postinjury, decreases in alpha 1-tubulin and beta III-tubulin gene expression were apparent in animals operated at all three developmental stages. These findings indicate that both immature and adult corticospinal neurons respond to axonal injury in a manner that is distinctly different from the peripheral neuron response.

The response of rubrospinal neurons to axotomy at different stages of development in the North American opossum

Rubral axons can grow around a lesion of their pathway in the thoracic spinal cord of developing opossums and a critical period exists for that plasticity. The critical period probably begins when rubral axons first grow into the thoracic cord, and it extends until approximately postnatal day 30. We previously noted that most rubrospinal neurons die after transection of their axon during the critical period, suggesting that plasticity results primarily from growth of axons not damaged by the lesion (Xu and Martin, J. Comp. Neurol. 279, 368-381, 1989). That observation led us to study the response of rubrospinal neurons to axotomy in more detail and at additional stages of development, using a prelabeling paradigm. We first injected fast blue (FB) into the caudal thoracic or rostral lumbar spinal cord in animals ranging from estimated postnatal day 9 to 50 and, about 4 days later, lesioned the rubrospinal tract several segments rostral to the injection. Approximately 30 days later, the animals were killed so that the red nucleus could be searched for labeled neurons. During the critical period for plasticity, rubrospinal neurons showed signs of degeneration 1 week after their axon was cut. When animals were killed 2-3 weeks after lesioning, there was an obvious decrease in axotomized neurons within the red nucleus, and by 4 weeks, more than 75% of them had degenerated. The marked susceptibility of rubrospinal neurons to axotomy during the critical period for plasticity is consistent with the hypothesis that developmental plasticity of the rubrospinal tract results primarily from growth of axons that were not damaged by the lesion. Our results also suggest that survival of axotomized rubrospinal neurons increases with age.

Preferential regeneration of spinal axons through the scar in hemisected lamprey spinal cord

Axons of lamprey spinal cord can regenerate across a complete spinal transection. Thus, unlike the scar of injured mammalian spinal cords, the scar in the lamprey is not an absolute impediment to regeneration. However, it is still not known whether the scar is a relative impediment or whether it provides a favorable environment for regeneration compared to the spinal cord parenchyma. In order to answer this question, the cords of 12 large larval sea lampreys (4-5 years old) were hemisected at the level of the third gill and the animals allowed to recover for 10 weeks. The large reticulospinal neurons (Müller and Mauthner cells) or their giant axons were injected intracellularly with HRP and their regenerating neurites visualized in central nervous system (CNS) wholemounts. Forty-five of seventy-one regenerating neurites (64%) grew beyond the level of the hemisection. Of these, 36 (82%) regenerated through the scar and remained on the same side of the cord as their parent axons, while only 8 (18%) crossed the midline and grew around the scar. Thus, regenerating neurites of giant reticulospinal axons tended to grow through the hemisection scar rather than around it. Once they passed the level of injury, they continued to elongate in their appropriate paths. It is possible that this tendency for axons to regenerate through the scar reflects the greater amount of empty spaces on the hemisected side. In order to rule this out, 13 animals received contralateral simultaneous hemisections at the level of the 3rd and 7th gills. This procedure created large numbers of degenerating axons and potential empty spaces both rostral and caudal to the scars within both hemicords; 92 of 158 neurites (58%) regenerated beyond the level of their respective hemisections. All of these grew through the scar and none crossed to the contralateral side. Distal to either hemisection, neurites remained on their correct side regardless of whether the contralateral cord contained normal CNS parenchyma or axonal debris and empty spaces produced by Wallerian degeneration. Moreover, in hemisected and double hemisected animals, as well as in completely transected control animals, neurites regenerating in their correct direction grew further than those that were misrouted. Because lamprey spinal axons grow preferentially through a scar rather than around it, the scar may play a positive role in supporting axonal regeneration.

Evidence for new growth and regeneration of cut axons in developmental plasticity of the rubrospinal tract in the North American opossum

We have shown previously that rubral axons can grow around a lesion of their spinal pathway in the developing opossum and that a critical period exists for that plasticity (Martin and Xu, Dev Brain Res 39:303, 1988). Since most rubrospinal neurons degenerate after axotomy during the critical period, we have proposed that plasticity results primarily from growth of late arriving axons around the lesion rather than regeneration of cut axons (Xu and Martin, J Comp Neurol 279:368, 1989). In the present study, we used a double-labeling paradigm to test that hypothesis. Four groups of pouch young opossums received bilateral or unilateral injections of Fast Blue (FB) into the caudal thoracic or rostral lumbar cord (T12-L2) at different ages in order to label rubrospinal neurons. Three or 4 days later, the rubrospinal tract was transected unilaterally, four to five segments rostral to the injection(s). If the injection was unilateral, the lesion was made ipsilateral to it. The animals were maintained for about 1 month before a second marker, Diamidino Yellow (DY), was injected, usually bilaterally, between the FB injection(s) and the lesion. The animals were maintained for about 5 days before sacrifice and sections through the red nucleus and spinal cord were examined with a fluorescence microscope. During the critical period for plasticity, only a few rubral neurons contralateral to the lesion were labeled by FB alone, supporting our previous contention that most axotomized neurons degenerate. In contrast, many neurons were labeled by DY alone, indicating that their axons were not present in the caudal cord at the time of the FB injection and that they grew around the lesion during the 1 month survival to incorporate DY. A few double-labeled neurons were also found. One interpretation of such neurons is that they survived axotomy, as evidenced by the presence of FB, and supported axons which grew around the lesion to take up DY. Another interpretation is that they supported late growing axons which incorporated residual FB as well as DY. In order to choose between these alternatives, a similar double-labeling paradigm was carried out, but with removal of FB at the time of the lesion. Since a few neurons were still double labeled, we conclude that regeneration of cut axons also contributed to rubrospinal plasticity. Our results support our previous suggestion that developmental plasticity of the rubrospinal tract results primarily from growth of late arriving axons around the lesion, but they also suggest that regeneration of cut axons occurs.