Increased lesion-induced sprouting of corticospinal fibres in the myelin-free rat spinal cord

The Role of Tissue Geometry in Spinal Cord Regeneration

Unlike peripheral nerves, axonal regeneration is limited following injury to the spinal cord. While there may be reduced regenerative potential of injured neurons, the central nervous system (CNS) white matter environment appears to be more significant in limiting regrowth. Several factors may inhibit regeneration, and their neutralization can modestly enhance regrowth. However, most investigations have not considered the cytoarchitecture of spinal cord white matter. Several lines of investigation demonstrate that axonal regeneration is enhanced by maintaining, repairing, or reconstituting the parallel geometry of the spinal cord white matter. In this review, we focus on environmental factors that have been implicated as putative inhibitors of axonal regeneration and the evidence that their organization may be an important determinant in whether they inhibit or promote regeneration. Consideration of tissue geometry may be important for developing successful strategies to promote spinal cord regeneration.

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

Upregulation of the zebrafish Nogo-A homologue, Rtn4b, in retinal ganglion cells is functionally involved in axon regeneration

Background

In contrast to mammals, zebrafish successfully regenerate retinal ganglion cell (RGC) axons after optic nerve section (ONS). This difference is explained on the one hand by neurite growth inhibitors in mammals (including Nogo-A), as opposed to growth-promoting glial cells in the fish visual pathway, and on the other hand by the neuron-intrinsic properties allowing the upregulation of growth-associated proteins in fish RGCs but not in mammals.

Results

Here, we report that Rtn4b, the zebrafish homologue of mammalian Nogo-A/RTN4-A, is upregulated in axotomized zebrafish RGCs and is primarily associated with the endoplasmic reticulum (ER). Rtn4b functions as a neuron-intrinsic determinant for axon regeneration, as was shown by downregulating Rtn4b through retrogradely transported morpholinos (MOs), applied to the optic nerve at the time of ONS. MO1 and MO2 reduced the number of axons from retina explants in a concentration-dependent manner. With MO1, the reduction was 55% (70 μM MO1) and 74% (140 μM MO1), respectively, with MO2: 59% (70 μM MO2) and 73% (140 μM MO2), respectively (compared to the control MO-treated side). Moreover, regenerating axons 7d after ONS and MO1 or MO2 application were labeled by Alexa488, applied distal to the first lesion. The number of Alexa488 labeled RGCs, containing the Rtn4b MO1 or MO2, was reduced by 54% and 62%, respectively, over control MO.

Conclusions

Thus, Rtn4b is an important neuron-intrinsic component and required for the success of axon regeneration in the zebrafish visual system. The spontaneous lesion-induced upregulation of Rtn4b in fish correlates with an increase in ER, soma size, biosynthetic activity, and thus growth and predicts that mammalian neurons require the same upregulation in order to successfully regenerate RGC axons.

Genetic deletion of paired immunoglobulin-like receptor B does not promote axonal plasticity or functional recovery after traumatic brain injury

The rewiring of neural networks is a fundamental step in recovering behavioral functions after brain injury. However, there is limited potential for axonal plasticity in the adult CNS. The myelin-associated proteins Nogo, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp) are known to inhibit axonal plasticity, and thus targeting the inhibitory pathways they participate in is a potential means of promoting plasticity and functional recovery. Each of Nogo, MAG, and OMgp interacts with both the Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB). Here, we determined whether blocking PirB activity enhances axonal reorganization and functional recovery after cortical injury. We found that axons of the contralesional corticospinal tract sprouted into the denervated side of the cervical spinal cord after unilateral injury of the motor cortex. The extent to which this axonal reorganization occurred was far greater in mice lesioned during early postnatal days than in mice lesioned at an age when myelin had begun to form. This suggests that myelin-associated proteins might limit axonal remodeling in vivo. However, the number of sprouting fibers within either the corticospinal or corticorubral tract was not enhanced in PirB(-/-) mice. Blocking PirB signaling also failed to enhance functional recovery with three motor tests. Our results suggest that blocking the function of PirB is not sufficient to promote axonal reorganization or functional recovery after cortical injury.

Differential changes in axonal conduction following CNS demyelination in two mouse models

Transgenic and disease model mice have been used to investigate the molecular mechanisms of demyelinating diseases. However, less attention has been given to elucidating changes in nerve conduction in these mice. We established an experimental system to measure the response latency of cortical neurons and examined changes in nerve conduction in cuprizone-induced demyelinating mice and in myelin basic protein-deficient shiverer mice. Stimulating and recording electrodes were placed in the right and left sensori-motor cortices, respectively. Electrical stimulation of the right cortex evoked antidromic responses in left cortical neurons with a latency of 9.38 +/- 0.31 ms (n = 107; mean +/- SEM). While response latency was longer in mice at 7 days and 4 weeks of cuprizone treatment (12.35 +/- 0.35 ms, n = 102; 11.72 +/- 0.29 ms, n = 103, respectively), response latency at 7 days and 4 weeks after removal of cuprizone was partially restored (10.72 +/- 0.45 ms, n = 106; 10.27 +/- 0.34 ms, n = 107, respectively). Likewise, electron microscopy showed cuprizone-induced demyelination in the corpus callosum and nearly complete remyelination after cuprizone removal. We also examined whether the myelin abnormalities in shiverer mice affected their response latencies. But there were no significant differences in response latencies in shiverer (9.83 +/- 0.24 ms, n = 103) and wild-type (9.33 +/- 0.22 ms, n = 112) mice. The results of these electrophysiological assessments imply that different demyelinating mechanisms, differentially affecting axon conduction, are present in the cuprizone-treated and shiverer mice, and may provide new insights to understanding the pathophysiology of demyelination in animal models in the CNS.

Screening of genes associated with termination of the critical period of visual cortex plasticity in rats

PURPOSE

To investigate the molecular mechanism involved in the termination of the critical period of visual cortex plasticity in the rat.

METHODS

The rats were divided into two groups, one group was dark-reared for 67 days after birth, while the other group was dark-reared for 60 days and then put under a normal light/dark cycle for 7 days. A subtracted cDNA library was constructed from the visual cortex, and differentially expressed genes were screened by nested PCR, reverse Northern hybridization, sequencing, and homology analysis.

RESULTS

Fourteen genes were found to be up-regulated in the visual cortex. These included 13 known genes and a novel fragment (Genbank submission EB174193). Of the known genes, three genes encoding beta -tubulin, myelin basic protein, and cyclophilin were previously reported to be associated with visual cortex plasticity.

CONCLUSION

A set of candidate genes related to the termination of the critical period was identified using the subtracted cDNA library. This work provides an important basis for understanding the molecular mechanisms involved in the termination of plasticity in the visual cortex.

In SilicoModeling of Axonal Reconnection within A Discrete Fiber Tract after Spinal Cord Injury

Following spinal cord injury (SCI), descending axons that carry motor commands from the brain to the spinal cord are injured or transected, producing chronic motor dysfunction and paralysis. Reconnection of these axons is a major prerequisite for restoration of function after SCI. Thus far, only modest gains in motor function have been achieved experimentally or in the clinic after SCI, identifying the practical limitations of current treatment approaches. In this paper, we use an ordinary differential equation (ODE) to simulate the relative and synergistic contributions of several experimentally-established biological factors related to inhibition or promotion of axonal repair and restoration of function after SCI. The factors were mathematically modeled by the ODE. The results of our simulation show that in a model system, many factors influenced the achievability of axonal reconnection. Certain factors more strongly affected axonal reconnection in isolation, and some factors interacted in a synergistic fashion to produce further improvements in axonal reconnection. Our data suggest that mathematical modeling may be useful in evaluating the complex interactions of discrete therapeutic factors not possible in experimental preparations, and highlight the benefit of a combinatorial therapeutic approach focused on promoting axonal sprouting, attraction of cut ends, and removal of growth inhibition for achieving axonal reconnection. Predictions of this simulation may be of utility in guiding future experiments aimed at restoring function after SCI.

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

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

Targeting myelin to optimize plasticity of spared spinal axons

Functional re-innervation of target neurons following neurological damage such as spinal cord injury is an essential requirement of potential therapies. There are at least two avenues by which this can be achieved: (a) through the regeneration of injured axons and (b) through promoting plasticity of those spared by the initial insult. There are several reasons why the latter approach may be more feasible, not the least of which are the inhibitory character of the glial scar, the often long distances over which injured axons must regrow, and the fact that spared axons are often already in the vicinity of denervated targets. The challenge is to unveil the well-recognized intrinsic plasticity of spared axons in a way that avoids complications, such as pain or autonomic dysfunction. One approach that we as well as others have taken is to target growth-suppressing signaling pathways initiated in spared axons by myelin-derived proteins. This article reviews models used for the study of spinal axon plasticity and describes the anatomical and behavioral effects of interfering with myelinderived proteins, their receptors, and components of their intracellular signaling cascades.

Changes within maturing neurons limit axonal regeneration in the developing spinal cord

Embryonic birds and mammals display a remarkable ability to regenerate axons after spinal injury, but then lose this ability during a discrete developmental transition. To explain this transition, previous research has emphasized the emergence of myelin and other inhibitory factors in the environment of the spinal cord. However, research in other CNS tracts suggests an important role for neuron-intrinsic limitations to axon regeneration. Here we re-examine this issue quantitatively in the hindbrain-spinal projection of the embryonic chick. Using heterochronic cocultures we show that maturation of the spinal cord environment causes a 55% reduction in axon regeneration, while maturation of hindbrain neurons causes a 90% reduction. We further show that young neurons transplanted in vivo into older spinal cord can regenerate axons into myelinated white matter, while older axons regenerate poorly and have reduced growth cone motility on a variety of growth-permissive ligands in vitro, including laminin, L1, and N-cadherin. Finally, we use video analysis of living growth cones to directly document an age-dependent decline in the motility of brainstem axons. These data show that developmental changes in both the spinal cord environment and in brainstem neurons can reduce regeneration, but that the effect of the environment is only partial, while changes in neurons by themselves cause a nearly complete reduction in regeneration. We conclude that maturational events within neurons are a primary cause for the failure of axon regeneration in the spinal cord.

Evidence that serotonin reuptake modulators increase the density of serotonin innervation in the forebrain

The mechanism of action of commonly used antidepressants remains an issue of debate. In the experiments reported here we studied the effects of three representative compounds, the selective serotonin reuptake inhibitor fluoxetine, the selective serotonin reuptake enhancer tianeptine and the selective norepinephrine reuptake inhibitor desipramine on the structure of central serotonin pathways after a 4-week administration. We found that the serotonin modulators fluoxetine and tianeptine, but not desipramine, increase the density of 5-HT and serotonin transporter (SERT)-immunoreactive axons in the neocortical layer IV and certain forebrain limbic areas, such as piriform cortex and the shell region of nucleus accumbens. These changes were noted in the absence of a significant effect of serotonin antidepressants on the expression of tryptophan hydroxylase (TPH-2), i.e. the rate-limiting enzyme for 5-HT biosynthesis and of SERT at the mRNA level. In addition, we found that anterogradely filled terminal axons from injections of biotinylated dextran amine into the dorsal raphe showed significantly more branching in animals treated with fluoxetine compared with animals treated with liposyn vehicle. Our findings suggest that antidepressants may exert very selective structural effects on their cognate monoamine systems in normal animals and raise the possibility that neurotrophic mechanisms may play a role in their clinical efficacy.

Assessing behavioural function following a pyramidotomy lesion of the corticospinal tract in adult mice

We have developed a pyramidotomy model in mice to lesion the corticospinal tract at the level of the brainstem pyramidal tract, and evaluated the resultant impairments in motor function in a series of behavioural tests. Adult C57BL/6 mice received a unilateral pyramidotomy and a control group of mice underwent sham surgery. We studied the effects of this lesion on forepaw function using five behavioural paradigms, some of which have been widely used in rat studies but have not been fully explored in mice. The tests used were: a rearing test, which assesses forepaw use for weight support during spontaneous vertical exploration of a cylinder; a grid walking test, which assesses the ability to accurately place the forepaws during exploration of an elevated grid; a tape-removal test, which measures both sensory and motor function of the forepaw; a CatWalk automated gait analysis, which provides a number of quantitative measures including stride length and stride width during locomotion; and a staircase reaching task, which assesses skilled independent forepaw use. All tests revealed lesion effects on forepaw function with the tape removal, grid walking, rearing and CatWalk tests demonstrating robust effects throughout the testing period. The development of a pyramidotomy lesion model in mice, together with behavioural tests which can reliably measure functional impairments, will provide a valuable tool for assessing therapeutic strategies to promote regeneration and plasticity.

Ephrin-B3 is a myelin-based inhibitor of neurite outgrowth

The inability of CNS axons to regenerate after traumatic spinal cord injury is due, in part, to the inhibitory effects of myelin. The three major previously identified constituents of this activity (Nogo, myelin-associated glycoprotein, and oligodendrocyte myelin glycoprotein) were isolated based on their potent inhibition of axon outgrowth in vitro. All three myelin components transduce their inhibitory signals through the same Nogo receptor/p75 neurotrophin receptor/LINGO-1 (NgR1/p75/LINGO-1) complex. In this study, we considered that molecules known to act as repellants in vertebrate embryonic axonal pathfinding may also inhibit regeneration. In mice, ephrin-B3 functions during development as a midline repellant for axons of the corticospinal tract. We therefore investigated whether this repellant was expressed in the adult spinal cord and retained inhibitory activity. We demonstrate that ephrin-B3 is expressed in postnatal myelinating oligodendrocytes and, by using primary CNS neurons, show that ephrin-B3 accounts for an inhibitory activity equivalent to that of the other three myelin-based inhibitors, acting through p75, combined. Our data describe a known vertebrate axon guidance molecule as a myelin-based inhibitor of neurite outgrowth.

The dorsolateral corticospinal tract in mice: an alternative route for corticospinal input to caudal segments following dorsal column lesions

In rodents, the main contingent of corticospinal tract (CST) axons descends in the ventral part of the dorsal column. There is, however, a contingent of CST axons that descends in the dorsolateral column (the "dorsolateral corticospinal tract," or DLCST). Here, we define some of the features of the DLCST by tracing CST projections following injections of biotinylated dextran amine into the sensorimotor cortex, assessing the distribution of DLCST axons and terminal arborizations in intact mice and in mice in which the main contingent of CST axons in the dorsal column had been transected. Axons of the DLCST diverge from the main tract at the pyramidal decussation, gather in fascicles in the dorsolateral gray matter below the spinomedullary junction, and project in a gradual trajectory laterally toward the dorsolateral column over the first few cervical segments. DLCST axons then project along the dorsolateral column to sacral levels, giving rise to collaterals that project into the gray matter. Labeled DLCST axons were most abundant in cervical segments, where they were often collected in fascicles, and progressively decreased in number in more caudal segments. Tracing of DLCST axons in mice with selective lesions of the dorsal column revealed that DLCST axons arborize extensively throughout the dorsal and ventral horns and that the overall territory that the DLCST axons invade is similar to the territory innervated by the CST axons in the main tract. Some DLCST axon arbors with varicosities are seen near large neurons in the ventral horn (presumed motoneurons). Substantial numbers of DLCST axons project across the midline to the gray matter on the contralateral side. Thus, the DLCST provides an alternate route for CST input to caudal segments, which is of particular relevance for studies of CST distribution and function following partial spinal cord injuries.

Neurotrophic factors expressed in both cortex and spinal cord induce axonal plasticity after spinal cord injury

We reported recently that overexpression of neurotrophin-3 (NT-3) by motoneurons in the spinal cord of rats will induce sprouting of corticospinal tract (CST) axons (Zhou et al. [2003] J. Neurosci. 23:1424-1431). We now report that overexpression of brain-derived neurotrophic factor (BDNF) or glial cell-derived neurotrophic factor (GDNF) in the rat sensorimotor cortex near the CST neuronal cell bodies together with overexpression of NT-3 in the lumbar spinal cord significantly increases axonal sprouting compared to that induced by NT-3 alone. Two weeks after unilaterally lesioning the CST at the level of the pyramids, we injected rats with saline or adenoviral vectors (Adv) carrying genes coding for BDNF (Adv.BDNF), GDNF (Adv.GDNF) or enhanced green fluorescent protein (Adv.EGFP) at six sites in the sensorimotor cortex, while delivering Adv.NT3 to motoneurons in each of these four groups on the lesioned side of the spinal cord by retrograde transport from the sciatic nerve. Four days later, biotinylated dextran amine (BDA) was injected into the sensorimotor cortex on the unlesioned side to mark CST axons in the spinal cord. Morphometric analysis of axonal sprouting 3 weeks after BDA injection showed that the number of CST axons crossing the midline in rats treated with Adv.BDNF or Adv.GDNF were 46% and 52% greater, respectively, than in rats treated with Adv.EGFP or PBS (P < 0.05). These data demonstrate that sustained local expression of neurotrophic factors in the sensorimotor cortex and spinal cord will promote increased axonal sprouting after spinal cord injury, providing a basis for continued development of neurotrophic factor therapy for central nervous system damage.

X‐irradiation of adult spinal cord increases its capacity to support neurite regeneration in vitro

Previous in vitro studies have shown that X-irradiation during early postnatal life can change the environment of CNS tissue in later adult life such that it becomes more supportive of neurite regeneration from adult dorsal root ganglion (DRG) neurons than non-irradiated tissue. The question arises whether or not x-irradiation during adult life can alter the CNS environment such that it also becomes more supportive of neurite regeneration. This was investigated by exposing portions of the spinal cord of adult rats to 10, 20 or 40 Gray of X-irradiation and later using this tissue to prepare cryosections suitable for use as a substrate in a cryoculture assay. Fixed cryocultures were immunolabelled using anti-glial fibrillary acidic protein (GFAP) to visualise the tissue sections and anti-growth associated protein (GAP-43) to visualise the regenerating neurites. Tissue sections from sham-irradiated animals and from those irradiated with 10 Gray did not support the regeneration of neurites. However, sections of spinal cords from rats treated with either 20 or 40 Gray of X-irradiation 4 or 32 days prior to sampling were found to support a certain degree of neurite regeneration. It is concluded that X-irradiation of adult CNS tissue can alter its environment such that it becomes more supportive of neurite regeneration and it is speculated that this change may be the result of alterations in the glial cell populations in the post-irradiated tissues.

Cell-autonomous mechanisms and myelin-associated factors contribute to the development of Purkinje axon intracortical plexus in the rat cerebellum

The highly specific connection patterns of the mature CNS are shaped through finely regulated processes of axon growth and retraction. To investigate the relative contribution of cell-autonomous mechanisms and extrinsic cues in these events, we examined the development of Purkinje axon intracortical plexus in the rat cerebellum. During the first postnatal week, several new processes sprout from focal swellings along the initial portion of the Purkinje neurite and spread in the granular layer. Intense structural plasticity occurs during the following week, with pruning of collateral branches and remodeling of terminal arbors. The mature distribution of the Purkinje infraganglionic plexus, confined within the most superficial portion of the granular layer, is attained at approximately postnatal day 15. A similar neuritic branching pattern is also developed by Purkinje cells grown in cultures of dissociated cerebellar cells or transplanted to extracerebellar CNS regions, suggesting that cell-autonomous mechanisms contribute to determining the Purkinje axon phenotype. The structural remodeling of Purkinje intracortical plexus is concomitant with the development of cerebellar myelin. To ask whether myelin-associated factors contribute to the morphological maturation of Purkinje neurites, we prevented normal myelinogenesis by killing oligodendrocyte precursors with 5'-azacytidine or by applying neutralizing antibodies against the myelin-associated neurite growth inhibitor Nogo-A. In both conditions, Purkinje axons retained exuberant branches, and the terminal plexus spanned the entire extent of the granular layer. Thus, the formation of Purkinje axon collaterals is, in part, controlled by intrinsic determinants, but their growth and distribution are regulated by environmental signals, among which are myelin-derived cues.

Neurotrophin-3 expressed in situ induces axonal plasticity in the adult injured spinal cord

The mammalian CNS lacks the ability to effectively compensate for injury by the regeneration of damaged axons or axonal plasticity of intact axons. However, reports suggest that molecular or cellular manipulations can induce compensatory processes that could support regeneration or plasticity after trauma. We tested whether local, sustained release of the neurotrophic factor neurotrophin-3 (NT-3) would support axonal plasticity in the spinal cord distal to the site of injury in rats. The corticospinal tract (CST) was cut unilaterally at the level of the medulla. This avoided excessive inflammation, secondary cell death, vascular disruption, and the release of inhibitory molecules in the lumbar spinal cord. A replication-defective adenoviral vector (Adv) carrying the NT-3 gene (Adv.EFalpha-NT3) was delivered to the spinal motoneurons by retrograde transport through the sciatic nerve. Retrograde transport of the adenoviral vectors avoided the inflammatory response that would be associated with direct injection into the spinal cord. Transduction of spinal motoneurons with Adv.EFalpha-NT3 resulted in a significant increase in the concentration of NT-3 in the L3-L6 region of the spinal cord for up to 3 weeks. In animals with a CST lesion, this local expression of NT-3 induced growth of axons from the intact CST across the midline to the denervated side. If the CST remained intact, overexpression of NT-3 did not lead to an increase in the number of axons crossing the midline. These data demonstrate that local, sustained expression of NT-3 will support axonal plasticity of intact CST axons after trauma-induced denervation.

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.

Improving axonal growth and functional recovery after experimental spinal cord injury by neutralizing myelin associated inhibitors

Injuries of the spinal cord often result in an irretrievable loss of motor and sensory functions of all body parts situated below the lesion site. Functional recovery is restricted mainly due to the limited regeneration and plasticity of injured axons in the adult central nervous system. Over the last few years different experimental approaches have led to axonal growth and functional benefits in animal models. This review focuses on the effects of the neutralization of myelin-associated neurite growth inhibitors, in particular Nogo-A, using the monoclonal antibody IN-1. Acute mAb IN-1 treatment of adult CNS lesioned rats results in extensive plastic changes of neuronal connections and regenerative fiber growth. In two different lesion paradigms (i.e. pyramidal tract lesion and incomplete spinal cord lesion in adult rats), the mAb IN-1-treated animals always showed a higher degree of recovery in various behavioral tests. These observations, together with electrophysiological results, suggest that neuronal CNS circuits of mAb IN-1-treated animals can be rearranged, and that sprouting and regenerating axons form functionally meaningful connections.

Reactive glia support and guide axon growth in the rat thalamus during the first postnatal week. A sharply timed transition from permissive to non-permissive stage

The present study demonstrates a supportive and guiding effect of the reactive glia on the postlesional axon growth in vivo, and offers a model system to compare permissive and non-permissive forms of the glial reaction. After stab wounds in early postnatal (P2-P9) rats, the reactive glia and the nerve fibers were detected by the immunohistochemical staining of glial fibrillary acidic protein (GFAP) and neurofilament protein, respectively. In the thalamus of the animals lesioned at P5 or earlier, an extraordinary bundle of fibers immunoreactive to neurofilament protein was found, corresponding to the lesion track marked by reactive glia. This bundle persisted up to 2 months, as shown by electron microscopy. When the animals were lesioned at P7 or later, the lesion track was immunonegative to neurofilament protein. Following P6 lesions, an intermediate situation was found, the strip of immunoreactive neurofilament protein was missing, or short and weak. GFAP immunostaining demonstrated a typical reactive glia in every case. As a result of the same operation, reactive glia plus a deficiency of neurofilament protein immunostaining was found in every animal in the cortex and the corpus callosum, independently from the age at lesion. The results demonstrate that the permissive nature of the glial reaction depends on the lesioned area as well, and changes to a non-permissive effect in a short time interval.

Nogo: a molecular determinant of axonal growth and regeneration

Following injury, axons of the adult mammalian central nervous system (CNS) fail to regenerate. As a result, CNS trauma generally results in severe and persistent functional deficits. The inability of CNS axons to regenerate is largely associated with nonneuronal aspects of the CNS environment that are inhibitory to axonal elongation. This inhibition is mediated by the glial scar, including reactive astrocytes, and by the myelin-associated neurite outgrowth inhibitors chondroitin sulfate proteoglycans, myelin-associated glycoprotein, and Nogo. Nogo is an integral membrane protein that localizes to CNS, but not peripheral nervous system, myelin. In vitro characterization of Nogo has demonstrated its function as a potent inhibitor of axon elongation. In vivo neutralization of Nogo activity results in enhanced axonal regeneration and functional recovery following CNS injury as well as increased plasticity in uninjured CNS fibers. These findings suggest that Nogo may be a major contributor to the nonpermissive nature of the CNS environment.

Comparison of wheat germ agglutinin-horseradish peroxidase and biotinylated dextran for anterograde tracing of corticospinal tract following spinal cord injury

Established methods for monitoring regeneration of the corticospinal tract involve anterograde labelling of the cortical motor neuron. While wheat germ agglutinin-horseradish peroxidase conjugate has been used to anterogradely label these neurons, we demonstrate that this technique may not completely label the whole axon and fine terminal processes when this tracer is administered in dried form. An alternative method is described for anterograde labelling of cortical motor neurons using biotinylated dextran. This tracer may be applied by either microinjection of 10% biotinylated dextran or implanting small globules of the dried tracer into the motor cortex. While more laborious, microinjection results in better anterograde labelling than implantation of dried biotinylated dextran. A procedure is also described for preparing serial coronal sections through the entire spinal cord and thaw-mounted on a minimum number of slides. The labelled nerve processes in these tissue sections can be visualised in the spinal cord under a fluorescent microscope following incubation with cy3-streptavidin complex. Permanent labelling of the biotinylated nerve processes is achieved by incubation of tissue sections with streptavidin-horseradish peroxidase conjugate followed by stringent washes and staining with tetramethylbenzidine. Use of tetramethylbenzidine allows resolution of a greater number of finer labelled processes than diaminobenzindine and allows clear visualisation of individual regenerating corticospinal tract processes. Using these procedures, we demonstrate that the corticospinal tract is completely lesioned by a standardised contusion spinal cord injury produced by the New York University weight-drop device.

Plasticity of motor systems after incomplete spinal cord injury

Although spontaneous regeneration of lesioned fibres is limited in the adult central nervous system, many people that suffer from incomplete spinal cord injuries show significant functional recovery. This recovery process can go on for several years after the injury and probably depends on the reorganization of circuits that have been spared by the lesion. Synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibres are important components of this recovery process. These reorganization processes might occur in cortical and subcortical motor centres, in the spinal cord below the lesion, and in the spared fibre tracts that connect these centres. Functional and anatomical evidence exists that spontaneous plasticity can be potentiated by activity, as well as by specific experimental manipulations. These studies prepare the way to a better understanding of rehabilitation treatments and to the development of new approaches to treat spinal cord injury.

Proteoglycans in the developing brain: new conceptual insights for old proteins

Proteoglycans are a heterogeneous class of proteins bearing sulfated glycosaminoglycans. Some of the proteoglycans have distinct core protein structures, and others display similarities and thus may be grouped into families such as the syndecans, the glypicans, or the hyalectans (or lecticans). Proteoglycans can be found in almost all tissues being present in the extracellular matrix, on cellular surfaces, or in intracellular granules. In recent years, brain proteoglycans have attracted growing interest due to their highly regulated spatiotemporal expression during nervous system development and maturation. There is increasing evidence that different proteoglycans act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. This review summarizes the most recent data on structures and functions of brain proteoglycans and focuses on new physiological concepts for their potential roles in the developing central nervous system.

Application of neutralizing antibodies against NI-35/250 myelin-associated neurite growth inhibitory proteins to the adult rat cerebellum induces sprouting of uninjured purkinje cell axons

The myelin-associated proteins NI-35/250 exert a powerful inhibition on axon regeneration, but their function exerted on intact neurons is still unclear. In the adult CNS these proteins are thought to regulate axon growth processes to confine plasticity within restricted regions and to prevent the formation of aberrant connections. We have recently shown that application of neutralizing IN-1 antibody Fab fragment against NI-35/250 proteins to the adult cerebellum induces the expression of injury/growth-associated markers in intact Purkinje cells. Here, we asked whether these cellular modifications are accompanied by growth phenomena of Purkinje neurites. A single intraparenchymal application of IN-1 Fab fragment to the adult cerebellum induces a profuse sprouting of Purkinje axons along their intracortical course. The newly formed processes spread to cover most of the granular layer depth. A significant axon outgrowth is evident 2 d after injection; it tends to increase at 5 and 7 d, but it is almost completely reversed after 1 month. No axonal modifications occur in control Fab-treated cerebella. The IN-1 Fab fragment-induced cellular changes and axon remodeling are essentially reproduced by applying affinity-purified antibody 472 raised against a peptide sequence of the recombinant protein NI-220, thus confirming the specificity of the applied treatments on these myelin-associated molecules. Functional neutralization of NI-35/250 proteins induces outgrowth from uninjured Purkinje neurites in the adult cerebellum. Together with previous observations, this suggests that these molecules regulate axonal plasticity to maintain the proper targeting of terminal arbors within specific gray matter regions.

Outgrowth-promoting molecules in the adult hippocampus after perforant path lesion

Lesion-induced neuronal plasticity in the adult central nervous system of higher vertebrates appears to be controlled by region- and layer-specific molecules. In this study we demonstrate that membrane-bound hippocampal outgrowth-promoting molecules, as present during the development of the entorhino-hippocampal system and absent or masked in the adult hippocampus, appear 10 days after transection of the perforant pathway. We used an outgrowth preference assay to analyse the outgrowth preference of axons from postnatal entorhinal explants on alternating membrane lanes obtained from hippocampus deafferented from its entorhinal input taken 4, 10, 20, 30 and 80 days post-lesion and from adult control hippocampus. Neurites from the entorhinal cortex preferred to extend axons on hippocampal membranes disconnected from their entorhinal input for 10 days in comparison with membranes obtained from unlesioned adult animals. Membranes obtained from hippocampi disconnected from their entorhinal input for 10 days were equally as attractive for growing entorhinal cortex (EC) axons as membranes from early postnatal hippocampi. Further analysis of membrane properties in an outgrowth length assay showed that entorhinal axons extended significantly longer on stripes of lesioned hippocampal membranes in comparison with unlesioned hippocampal membranes. This effect was most prominent 10 days after lesion, a time point at which axonal sprouting and reactive synaptogenesis are at their peak. Phospholipase treatment of membranes obtained from unlesioned hippocampi of adult animals strongly promoted the outgrowth length of entorhinal axons on these membranes but did not affect their outgrowth preference for deafferented hippocampal membranes. Our results indicate that membrane-bound outgrowth-promoting molecules are reactivated in the adult hippocampus following transection of the perforant pathway, and that neonatal entorhinal axons are able to respond to these molecules. These findings support the hypothesis of a temporal accessibility of membrane-bound factors governing the layer-specific sprouting of remaining axons following perforant path lesion in vivo.

Bovine CNS myelin contains neurite growth-inhibitory activity associated with chondroitin sulfate proteoglycans

The absence of fiber regrowth in the injured mammalian CNS is influenced by several different factors and mechanisms. Besides the nonconducive properties of the glial scar tissue that forms around the lesion site, individual molecules present in CNS myelin and expressed by oligodendrocytes, such as NI-35/NI-250, bNI-220, and myelin-associated glycoprotein (MAG), have been isolated and shown to inhibit axonal growth. Here, we report an additional neurite growth-inhibitory activity purified from bovine spinal cord myelin that is not related to bNI-220 or MAG. This activity can be ascribed to the presence of two chondroitin sulfate proteoglycans (CSPGs), brevican and the brain-specific versican V2 splice variant. Neurite outgrowth of neonatal cerebellar granule cells and of dorsal root ganglion neurons in vitro was strongly inhibited by this myelin fraction enriched in CSPGs. Immunohistochemical staining revealed that brevican and versican V2 are present on the surfaces of differentiated oligodendrocytes. We provide evidence that treatment of oligodendrocytes with the proteoglycan synthesis inhibitors beta-xylosides can strongly influence the growth permissiveness of oligodendrocytes. beta-Xylosides abolished cell surface presentation of brevican and versican V2 and reversed growth cone collapse in encounters with oligodendrocytes as demonstrated by time-lapse video microscopy. Instead, growth cones were able to grow along or even into the processes of oligodendrocytes. Our results strongly suggest that brevican and versican V2 are additional components of CNS myelin that contribute to its nonpermissive substrate properties for axonal growth. Expression of these CSPGs on oligodendrocytes may indicate that they participate in the restriction of structural plasticity and regeneration in the adult CNS.

Spinal cord injury in small animals 2. Current and future options for therapy

Although there can be substantial spontaneous improvements in functional status after a spinal cord injury, therapeutic intervention is desirable in many patients to improve the degree of recovery. At present only decompressive surgery and the neuroprotective drug methylprednisolone sodium succinate are effective and in widespread clinical use. There are limitations to the efficacy of these therapies in some clinical cases and they cannot restore satisfactory functional status to all patients. Many drugs have been investigated experimentally to assess their potential to preserve injured tissue and promote functional recovery in clinically relevant settings, and several of them would be suitable for assessment in future veterinary clinical trials. In addition, experimental techniques designed to mould the response of the CNS to injury, by the promotion of axonal regeneration across the lesion and the encouragement of local sprouting of undamaged axons, have recently been successful, suggesting that effective therapy for even very severe spinal cord injury may soon be available.

Neuroglial activation repertoire in the injured brain: graded response, molecular mechanisms and cues to physiological function

Damage to the central nervous system (CNS) leads to cellular changes not only in the affected neurons but also in adjacent glial cells and endothelia, and frequently, to a recruitment of cells of the immune system. These cellular changes form a graded response which is a consistent feature in almost all forms of brain pathology. It appears to reflect an evolutionarily conserved program which plays an important role in the protection against infectious pathogens and the repair of the injured nervous system. Moreover, recent work in mice that are genetically deficient for different cytokines (MCSF, IL1, IL6, TNFalpha, TGFbeta1) has begun to shed light on the molecular signals that regulate this cellular response. Here we will review this work and the insights it provides about the biological function of the neuroglial activation in the injured brain.

Cortical neurite outgrowth and growth cone behaviors reveal developmentally regulated cues in spinal cord membranes

Corticospinal axon outgrowth in vivo and the ability to sprout or regenerate after injury decline with age. This developmental decline in growth potential has been correlated with an increase in inhibitory myelin-associated proteins in older spinal cord. However, previous results have shown that sprouting of corticospinal fibers after contralateral lesions begins to diminish prior to myelination, suggesting that a decrease in growth promoting and/or an increase in inhibitory molecules in spinal gray matter may also regulate corticospinal axon outgrowth. To address this possibility, we carried out in vitro experiments to measure neurite outgrowth from explants of 1-day-old hamster forelimb sensorimotor cortex that were plated onto membrane carpets or membrane stripe assays prepared from white or gray matter of 1-to 22-day-old cervical spinal cord. On uniform carpets and in the stripe assays cortical neurites grew robustly on young but not older membranes from both white and gray matter. Mixtures of membranes from 1- and 15-day spinal cord inhibited neurite outgrowth, suggesting that the presence of inhibitory molecules in the 15-day cord overwhelmed permissive or growth promoting molecules in membranes from 1-day cord. Video microscopic observations of growth cone behaviors on membrane stripe assays transferred to glass coverslips supported this view. Cortical growth cones repeatedly collapsed at borders between permissive substrates (laminin or young membrane stripes) and nonpermissive substrates (older membrane stripes). Growth cones either turned away from the older membranes or reduced their growth rates. These results suggest that molecules in both the gray and white matter of the developing spinal cord can inhibit cortical neurite outgrowth.

Enhanced neurotrophin-induced axon growth in myelinated portions of the CNS in mice lacking the p75 neurotrophin receptor

Axonal growth in the adult mammalian CNS is limited because of inhibitory influences of the glial environment and/or a lack of growth-promoting molecules. Here, we investigate whether supplementation of nerve growth factor (NGF) to the CNS during postnatal development and into adulthood can support the growth of sympathetic axons within myelinated portions of the maturing brain. We have also asked whether p75(NTR) plays a role in this NGF-induced axon growth. To address these questions we used two lines of transgenic mice overexpressing NGF centrally, with or without functional expression of p75(NTR) (NGF/p75(+/+) and NGF/p75(-/-) mice, respectively). Sympathetic axons invade the myelinated portions of the cerebellum, beginning shortly before the second week of postnatal life, in both lines of NGF transgenic mice. Despite the presence of central myelin, these sympathetic axons continue to sprout and increase in density between postnatal days 14 and 100, resulting in a dense plexus of sympathetic fibers within this myelinated environment. Surprisingly, the growth response of sympathetic fibers into the cerebellar white matter of NGF/p75(-/-) mice is enhanced, such that both the density and extent of axon ingrowth are increased, compared with age-matched NGF/p75(+/+) mice. These dissimilar growth responses cannot be attributed to differences in cerebellar levels of NGF protein or sympathetic neuron numbers between NGF/p75(+/+) and NGF/p75(-/-) mice. Our data provide evidence demonstrating that growth factors are capable of overcoming the inhibitory influences of central myelin in the adult CNS and that neutralization of the p75(NTR) may further enhance this growth response.

Sprouting and regeneration after pyramidotomy and blockade of the myelin-associated neurite growth inhibitors NI 35/250 in adult rats

After a selective unilateral lesion of the corticospinal tract (CST) at the level of the brainstem (pyramidotomy) and neutralization of the myelin associated neurite growth inhibitors NI-35/250 with the monoclonal antibody (mAb) IN-1, we had previously observed a strong behavioural recovery in parallel with an enhanced structural plasticity of the lesioned as well as the unlesioned CST. The present study focuses on the regenerative response of the cut CST axons at the lesion site in these adult rats. The results show a strong enhancement of regenerative sprouting of CST fibres by treatment with the mAb IN-1. Successful elongation of these sprouts through the pyramidal decussation and into the cervical spinal cord was also dependent on the presence of this antibody. In the spinal cord, regenerating fibres were rarely found in the position of the former CST; most of the fibres were distributed seemingly randomly over the entire lateral extent of the spinal cord.

Thrombospondin expression in nerve regeneration II. Comparison of optic nerve crush in the mouse and goldfish

Expression of the extracellular matrix molecule thrombospondin (TSP) was examined following retrobulbar crush injury of the goldfish and mouse optic nerve. TSP was present within the glia limitans and surrounding axon fascicles of the control normal goldfish optic nerve, but was absent from the normal mouse optic nerve. Following crush injury of the goldfish optic nerve, TSP expression increased dramatically along the path of regenerating axons and returned to near normal levels following axonal outgrowth. In contrast, during the unsuccessful attempt at regeneration following crush injury of the mouse optic nerve, TSP expression was present only in glial fibrillary acidic protein (GFAP)-negative, macrophage-rich regions distal to ganglion cell axons. These results indicate that TSP expression is increased in a temporal pattern along the path of regenerating goldfish optic nerve axons and therefore may be involved in successful central nervous system regeneration. The absence of TSP in the environment encountered by damaged mouse optic nerve axons may correlate with the lack of regeneration observed in the mouse optic nerve.

Myelin does not influence the choice behaviour of entorhinal axons but strongly inhibits their outgrowth length in vitro

Myelin is crucial for the stabilization of the entorhinohippocampal projection during late development and is a non-permissive substrate for regrowing axons after lesion in the adult brain. We used two in vitro assays to analyse the impact of myelin on rat entorhinohippocampal projection neurons. A stripe assay was used to study the impact of myelin on the choice behaviour of axons from the entorhinal cortex (EC). Given a choice between alternating hippocampal membrane lanes from developmental stages ranging from early postnatal to adult, EC axons preferred to extend on early postnatal hippocampal membranes. Neither the neutralization of myelin-associated factors by a specific antibody (IN-1) nor the separation of myelin from membranes interfered with the axons' choice behaviour. The entorhinal axons showed no preference in the membrane combination of adult and myelin-free adult hippocampal membranes. These stripe assay experiments demonstrate that support for EC axon choice in the developing hippocampus is maturation-dependent and is not influenced by myelin. The application of IN-1 in the outgrowth assay and the separation of myelin from membranes, enhanced elongation of outgrowing entorhinal axons on adult hippocampal membranes, whereas a control antibody did not. This shows that myelin-associated factors have a strong inhibitory effect on the outgrowth length of entorhinal axons. In conclusion, we suggest that axonal elongation in the entorhinohippocampal system during development is strongly influenced by myelin-associated growth inhibition factors and that specific target finding of entorhinal axons is regulated by a different mechanism.

Biological interventions for spinal cord injury

Spinal cord injury is frequently followed by the loss of supraspinal control of sensory, autonomus and motor functions at sublesional level. To enhance recovery in patients with spinal cord injuries, three fundamental strategies have been developed in experimental models. These strategies involve three different time points for postlesional intervention in the spinal cord. Neuroprotection soon after injury uses pharmacological tools to reduce the progressive secondary injury processes that follow during the first week after the initial lesion occurs, in order to limit tissue damage. A second strategy, which is initiated shortly after the lesion occurs, aims at promoting axonal regeneration by acting pharmacologically on inhibitors or barriers of regeneration, or by the application of cell or gene therapy as a source of neurotrophic factors or as a bridge or support to enhance the regeneration of lesioned axons. Finally, a mid-term substitutive strategy is the management of the sublesional spinal cord by sensorimotor stimulation or the supply of missing key afferents, such as monoaminergic systems. These three strategies are reviewed. Only a combination of these different approaches can provide an optimal basis for potential therapeutic interventions aimed at functional recovery after spinal cord injury.

Regeneration in the spinal cord

Important advances have been made in our understanding of conditions that influence the intrinsic capacity of mature CNS neurons to initiate and maintain a regrowth response. The combination of exogenous neurotrophic support with strategies to alter the terrain at the injury site itself suggests that there are important interactions between them that lead to increased axonal regeneration. The ability of chronically injured neurons to initiate a regeneration response is unexpected. Our view of the role that inhibitors play in restricting axonal growth has also expanded. The findings indicate that the windows of opportunity for enhancing growth after spinal cord injury may be more numerous than previously thought.

Functional recovery and enhanced corticofugal plasticity after unilateral pyramidal tract lesion and blockade of myelin-associated neurite growth inhibitors in adult rats

After a lesion of the mature CNS, structural plasticity and functional recovery are very limited, in contrast to the developing CNS. The postnatal decrease in plasticity is correlated in time with the formation of myelin. To investigate the possible role of an important myelin-associated neurite growth inhibitor (NI-250; IN-1 antigen), one pyramidal tract of adult Lewis rats was lesioned (pyramidotomy), and the rats were treated with the antibody IN-1, a control antibody, or no antibody. Functional recovery was studied from postoperative day 14 until day 42 using a food pellet reaching task, rope climbing, and a grid walk paradigm. The corticofugal projections to the red nucleus and basilar pontine nuclei were analyzed after survival times of 2 and 16 weeks. Treatment with the monoclonal antibody IN-1 resulted in almost complete restoration of skilled forelimb use, whereas all the control groups showed severe and chronic impairments. This functional recovery was paralleled by sprouting of the corticorubral and the corticopontine fibers across the midline, thus establishing a bilateral, anatomically specific projection.

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.