Differential regenerative response of Purkinje cell and inferior olivary axons confronted with embryonic grafts: environmental cues versus intrinsic neuronal determinants

Neuroprotective actions of peripherally administered insulin-like growth factor I in the injured olivo-cerebellar pathway

Exogenous administration of insulin-like growth factor I (IGF-I) restores motor function in rats with neurotoxin-induced cerebellar deafferentation. We first determined that endogenous IGFs are directly involved in the recovery process because infusion of an IGF-I receptor antagonist into the lateral ventricle blocks gradual recovery of limb coordination that spontaneously occurs after partial deafferentation of the olivo-cerebellar circuitry. We then analysed mechanisms whereby exogenous IGF-I restores motor function in rats with complete damage of the olivo-cerebellar pathway. Treatment with IGF-I normalized several markers of cell function in the cerebellum, including calbindin, glutamate receptor 1 (GluR1), gamma-aminobutyric acid (GABA) and glutamate, which are all depressed after 3-acetylpyridine (3AP)-induced deafferentation. IGF-I also promoted functional reinnervation of the cerebellar cortex by inferior olive (IO) axons. In the IO, increased expression of bax in neurons and bcl-X in astrocytes after 3AP was significantly reduced by IGF-I treatment. On the contrary, IGF-I prevented the decrease in poly-sialic-acid neural cell adhesion molecule (PSA-NCAM) and GAP-43 expression induced by 3AP in IO cells. IGF-I also significantly increased the number of neurons expressing bcl-2 in brainstem areas surrounding the IO. Altogether, these results indicate that subcutaneous IGF-I therapy promotes functional recovery of the olivo-cerebellar pathway by acting at two sites within this circuitry: (i) by modulating death- and plasticity-related proteins in IO neurons; and (ii) by impinging on homeostatic mechanisms leading to normalization of cell function in the cerebellum. These results provide insight into the neuroprotective actions of IGF-I and may be of practical consequence in the design of new therapeutic approaches for neurodegenerative diseases.

Developmental plasticity of ascending spinal axons studies using the North American opossum, Didelphis virginiana

The objectives of the present study were to determine if axons of all ascending tracts grow through the lesion after transection of the thoracic spinal cord during development in the North American opossum, and if so, whether they reach regions of the brain they normally innervate. Opossum pups were subjected to transection of the mid-thoracic cord at PD5, PD8, PD12, PD20, or PD26 and injections of Fast Blue (FB) into the lower thoracic or upper lumbar cord 30-40 days or 6 months later. In the PD5 transected cases, labeled axons were present in all of the supraspinal areas labeled by comparable injections in unlesioned, age-matched controls. In the experimental cases, however, labeled axons appeared to be fewer in number and in some areas more restricted in location than in the controls. When lesions were made at PD8, labeled axons were present in the brain of animals allowed to survive 30-40 days prior to FB injections but they were not observed in those allowed to survive 6 months. When lesions were made at PD12 or later, labeled axons were never found rostral to the lesion. It appears, therefore, that axons of all ascending spinal pathways grow though the lesion after transection of the thoracic cord in developing opossums and that they innervate appropriate areas of the brain. Interestingly, the critical period for such growth is shorter than that for most descending axons, suggesting that factors which influence loss of developmental plasticity are not the same for all axons.

Retrograde regulation of growth-associated gene expression in adult rat Purkinje cells by myelin-associated neurite growth inhibitory proteins

Axon regeneration requires that injured neurons reinitiate long-distance growth and upregulate specific genes. To address the question of whether inhibitory environmental cues along the axon could exert a negative, tonic downregulation of growth-associated genes, we have examined adult rat Purkinje cells, which are endowed with poor regenerative capabilities. First we have compared their response to axotomy with that of neurons of the inferior olive, lateral reticular nucleus, and deep cerebellar nuclei, all of which vigorously regenerate into growth-permissive transplants. These injured neurons upregulate the transcription factors c-Jun and JunD, GAP-43, and NADPH diaphorase. In contrast, most axotomized Purkinje cells fail to express any of these markers, showing that the strength of this response parallels the regenerative potential of the examined neuron populations. However, strong upregulation of the same genes can be induced in Purkinje cells after colchicine injection into the uninjured adult cerebellum, indicating that their expression could be controlled by retrograde signals. To assess whether myelin-associated neurite growth inhibitory proteins contribute to this regulation, we applied the neutralizing antibodies IN-1 against one of the main inhibitory components of central myelin (NI-250) either in vivo or in vitro to organotypic cerebellar cultures. Application of IN-1 antibodies induces the upregulation of c-Jun, JunD, and NADPH diaphorase in Purkinje cells, showing that their expression is suppressed constitutively by myelin-associated neurite growth inhibitors. Thus, the inhibitory activity of the IN-1 antigen on axon growth is not restricted to the control of growth cone motility but also involves a retrograde regulation of gene expression in adult central neurons.

Axonal injury and peripheral nerve grafting in the thalamus and cerebellum of the adult rat: upregulation of c-jun and correlation with regenerative potential

The protooncogene c-jun is highly expressed for long periods in axotomized PNS neurons. This may be related to their growth and regeneration. In contrast, axotomized CNS neurons show only a small and transient upregulation of c-jun. It has been suggested that there may be a correlation between this failure to maintain high levels of c-jun expression after axotomy and abortive CNS axonal regeneration. We have studied, by in situ hybridization and immunohistochemistry, the c-jun response after stab wound lesion, and after peripheral nerve grafting in the thalamus and cerebellum of the adult rat. A lesion elicits upregulation of c-jun in thalamic neurons ipsilateral to the lesion. This is most evident and prolonged in neurons such as those of the thalamic reticular nucleus, which have an established propensity to regenerate. After peripheral nerve grafting, the c-jun response in thalamic neurons is enhanced, mostly in neurons which have axons regenerating along the grafts. These neurons also upregulate growth-associated protein 43 (GAP-43). By comparison, injured Purkinje cells of the cerebellum which do not regenerate their axons along a graft, do not upregulate either c-jun or GAP-43, although they increase their expression of p75. Thus CNS neurons able to regenerate their axons along a peripheral nerve graft are those in which c-jun is induced after injury, and c-jun may play a critical role in the control of gene programs for axonal regeneration. Moreover, the observed differences in the ability of CNS neurons to regenerate their axons may relate to a difference in their intrinsic molecular response to axotomy.

Degenerative phenomena and reactive modifications of the adult rat inferior olivary neurons following axotomy and disconnection from their targets

Adult olivocerebellar axons are capable of vigorous regeneration when provided with growth-permissive environmental conditions. To elucidate the contribution of intrinsic properties to the regenerative capabilities of inferior olivary neurons, we have examined the cellular modifications occurring in these neurons following axotomy and target deprivation in the absence of exogenous growth-promoting influences. Axotomized inferior olivary neurons undergo perikaryal shrinkage, dendritic atrophy and a loss of anti-calbindin immunoreactivity. A conspicuous cell death occurs during the first few weeks after lesion, but about 35% of the affected neurons survive up to 60 days. Coincidentally, a subset of the injured nerve cells become strongly reactive for NADPH diaphorase histochemistry, and this expression is correlated with survival in the medial accessory olive and in the principal olive. In addition, the affected neurons express or maintain the expression of several markers related to regenerative processes, including transcription factors c-Jun, JunD and Krox-24, the growth-associated protein GAP-43 and the developmentally regulated calcitonin gene-related peptide (CGRP). The expression of all these markers is sustained up to two months after lesion, the longest survival time examined. These results show that although adult axotomized inferior olivary neurons undergo severe regressive modifications leading to a conspicuous cell loss, at least a subset of them is resistant to the lesion. In addition, the long-lasting expression of several axon-growth associated markers expressed in these neurons in response to injury reveals that they are endowed with a strong intrinsic regenerative potential.

Targeted overexpression of the neurite growth-associated protein B-50/GAP-43 in cerebellar Purkinje cells induces sprouting after axotomy but not axon regeneration into growth-permissive transplants

B-50/GAP-43 is a nervous tissue-specific protein, the expression of which is associated with axon growth and regeneration. Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (; ). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities. Thus, transgenic mice were produced in which B-50/GAP-43 overexpression was driven by the Purkinje cell-specific L7 promoter. Uninjured transgenic Purkinje cells displayed normal morphology, indicating that transgene expression does not modify the normal phenotype of these neurons. By contrast, after axotomy numerous transgenic Purkinje cells exhibited profuse sprouting along the axon and at its severed end. Nevertheless, despite these growth phenomena, which never occurred in wild-type mice, the severed transgenic axons were not able to regenerate, either spontaneously or into embryonic neural or Schwann cell grafts placed into the lesion site. Finally, although only a moderate Purkinje cell loss occurred in wild-type cerebella after axotomy, a considerable number of injured transgenic neurons degenerated, but they could be partially rescued by the different transplants placed into the lesion site. Thus, B-50/GAP-43 overexpression substantially modifies Purkinje cell response to axotomy, by inducing growth processes and decreasing their resistance to injury. However, the presence of this protein is not sufficient to enable these neurons to accomplish a full program of axon regeneration.

Olivocerebellar axon regeneration and target reinnervation following dissociated Schwann cell grafts in surgically injured cerebella of adult rats

The ability of Schwann cells to induce the regeneration of severed olivocerebellar and Purkinje cell axons across an injury up to their deafferented targets was tested by transplanting freshly dissociated cells from newborn rat sciatic nerves into surgically lesioned adult cerebella. The grafted glial cells consistently filled the lesion gap and migrated into the host parenchyma. Transected olivocerebellar axons vigorously regenerated into the graft, where their growth pattern and direction followed the arrangement of Schwann cell bundles. Although some of these axons terminated within the transplant, many of them rejoined the cerebellar parenchyma beyond the lesion. Here, their fate depended on the territory encountered. No growth occurred in the white matter. Numerous fibres penetrated into the granular layer and formed terminal branches that remained confined within this layer. A few of them, however, regenerated up to the molecular layer and formed climbing fibres on Purkinje cell dendrites. By contrast, the growth of transected Purkinje cell axons into the grafts was very poor. These results underscore the different intrinsic responsiveness of Purkinje cell and olivocerebellar axons to the growth-promoting action of Schwann cells, and show that the development and outcome of the regenerative phenomena is strongly conditioned by the spatial organization and specific features of the environmental cues encountered by the outgrowing axons along the course they follow. However, Schwann cells effectively bridge the lesion gap, induce the regeneration of olivocerebellar axons, and direct their growth up to the deafferented host cortex, where some of them succeed in reinnervating their natural targets.

Regenerating and sprouting axons differ in their requirements for growth after injury

After spinal cord injury at birth, axotomized brainstem-spinal and corticospinal neurons are capable of permanent regenerative axonal growth into and through a fetal spinal cord transplant placed into the site of either a spinal cord hemisection or transection. In contrast, if fetal tissue which is not a normal target of the axotomized neurons (embryonic hippocampus or cortex) is placed into a neonatal spinal cord hemisection, brainstem-spinal serotonergic axons transiently innervate the transplant, but subsequently withdraw. The first set of experiments was designed to test the hypothesis that after spinal cord transection, serotonergic axons would cross the nontarget transplant, reach normal spinal cord targets caudal to the transection, and gain access to requisite target-derived cues, permitting permanent maintenance. Surprisingly, after a complete spinal cord transection, brainstem-spinal axons failed to grow into an inappropriate target even transiently. These observations suggest that the transient axonal ingrowth into nontarget transplants may represent lesion-induced axonal sprouting by contralateral uninjured axons. We have used double-labeling with fluorescent dyes, to test directly whether axonal sprouting of neurons which maintain collaterals to uninjured spinal cord targets (1) provide the transient ingrowth of brainstem-spinal axons into a nontarget transplant and (2) contribute to permanent ingrowth into target-specific transplants. Uninjured red nucleus, raphe nucleus, and locus coeruleus neurons extend axons into the nontarget transplant while maintaining collaterals to the host spinal cord caudal to the transplant. The lesion-induced sprouting by uninjured axons was also observed with a target-specific transplant. Taken together, these studies suggest that sprouting and regenerating axons may differ in their requirements for growth after injury.

Insulin-like growth factor 1 induces climbing fibre re-innervation of the rat cerebellum

The effect of insulin-like growth factor 1 (IGF-1) on neonatal plasticity was studied using the rat olivocerebellar projection as a model. Unilateral removal of climbing fibres in the rat before postnatal day 7 induces re-innervation of the deafferented hemi-cerebellum, which does not occur after postnatal day 10. Rats aged 11 or 12 days underwent climbing fibre transection followed by IGF-1 injection into the denervated cerebellar cortex 24 h later. The exogenous IGF-1 induced climbing fibre re-innervation of the denervated hemicerebellum in a pattern similar to that seen in the immature rat. Thus IGF-1 can extend the window of neonatal plasticity of the brain and therefore may be of potential therapeutic use post-trauma.

Intrinsic neuronal determinants that promote axonal sprouting and elongation

Nerve processes elongate, branch and form synaptic contacts in a highly regulated and specific manner. Long-distance axon elongation is restricted to the main phase of axon formation during development, but can be reinduced upon lesions in the adult (regeneration). It correlates with the expression of defined genes, including proteins involved in signalling (e.g. src, NCAM, integrins), transcription factors (e.g. c-jun) and structural proteins (e.g. actin and tubulin isoforms). Activation of an exon elongation program may require bcl-2. The formation and growth of local branches (sprouting) is controlled by mechanisms in the target region. In addition, the expression of growth-associated proteins such as GAP-43 and CAP-23 in neurons lowers the threshold for nerve sprouting and potentiates its vigour. Recent studies suggest that nerve sprouting and long-distance elongation depend on the expression of different intrinsic components in neurons. One implication of these findings is that the differential expression of genes facilitating local branching may affect structural plasticity in the intact adult nervous system.

Path and target finding of afferents in cerebellar anlagen grafted in the cerebellum of adult rats: a Phaseolus vulgaris leucoagglutinin study

Cerebellar anlagen from rat embryos were grafted into the cerebellum of intact adult rats. Most of the grafts survived and formed 'minicerebella'. The location of the grafts were varied, which provided various types of host/graft interface in laminar configuration: Hw/Gg, Hw/Gp, Hw/Gm, Hg/Gw, Hg/Gg, Hm/Gw, Hm/Gg, Hm/Gp, and Hm/Gm that comprised of the granule cell layer (g), the Purkinje cell layer (p), the molecular layer (m), or the white matter (w) of the host (H) or graft (G). The manner of entrance of mossy and climbing fibers through the host/graft interface and their outgrowth in the graft which has various subset of laminar organization of the cerebellum were examined 28-158 days after grafting by means of anterograde Phaseolus vulgaris leucoagglutinin (PHA-L) labeling. Each cerebellar afferent entered the graft through specific types of interface and grew in specific layers. Mossy fibers passed through Hg/Gg and Hw/Gg, grew in Gg, and mostly terminated there like normal fibers. Fibers in Gg, though rarely, grew further outside Gg like in development. Climbing fibers passed through Hm/Gg, Hw/Gg, and Hw/Gm, proceeded in cortical layers, and terminated in Gm. The outgrowth of climbing fibers in Gm showed selectivity for the direction of Gm with respect to the polarity of Purkinje cells; they permeated Gm to form terminal arbors similar to normal in the direction from the side of Purkinje cell somata to dendrites but not in the reverse direction. Occasionally a single fiber innervated neighboring multiple Purkinje cells. These results indicate that mature cerebellar afferents have potential to regrow and innervate the extraneous cerebellar anlage by finding paths and targets in a manner similar to normal ontogenesis.

Purkinje cell survival and axonal regeneration are age dependent: an in vitro study

Purkinje cells are among the most resistant neurons to axotomy and the most refractory to axonal regeneration. By using organotypic cultures, we have studied age- and environment-related factors implicated in Purkinje cell survival and axonal regeneration. Most Purkinje cells taken from 1- to 5-d-old rats, the period in which these neurons are engaged in intense synaptogenesis and dendritic remodeling, die 1 week after plating, whereas if cultured before or after this period, Purkinje cells survive, even in the absence of deep nuclear neurons, their postsynaptic targets. Cerebellar slices taken from 10-d-old rats and kept in vitro for 1 week acquire a cellular composition resembling mature cerebellum. Their Purkinje cells are resistant to axotomy, but even when confronted with permissive environments (sciatic nerves or fetal cerebellar slices), their axons do not regenerate. In contrast, fetal rat and mouse Purkinje cells are able to regenerate their axons on mature cerebellar slices. This regeneration is massive, and the regrowing axons invade all cerebellar regions of the apposed mature slices, including white matter. These results show that Purkinje cell survival and axonal regeneration are age-related and independent from environmental constraints. Moreover, our observations suggest strongly that the onset of synaptogenesis of Purkinje cell axons could provide a signal to turn off their growth program and that, thereafter, permissive microenvironment alone is unable to reestablish such a program.

Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum

In the adult cerebellum both the climbing fibre arbour and the Purkinje cell are very plastic and each element is able to exert a remarkable action on the other one. The adult phenotype of the Purkinje cell is strictly dependent on the presence of its climbing fibre arbour. When the climbing fibre is missing, the Purkinje cell undergoes a hyperspiny transformation and becomes hyperinnervated by the parallel fibres. However, this change is fully reversible. The climbing fibre-deprived Purkinje cell is able to elicit sprouting of nearby located intact climbing fibres and the new arbour is able to fully restore synaptic connections which appear normal both morphologically and functionally. Multiple climbing fibre innervation of a single Purkinje cell persists in the adult hypogranular cerebellum. The different fibres are distributed to separate dendritic regions, suggesting a local competition between the different arbours for their territory. It is postulated that in the intact rat, an activity dependent mechanism of the parallel fibre favours the predominance of one arbour with the elimination of its competitors. When the Purkinje cell is deleted, the climbing fibre arbour becomes heavily atrophic and reduced in size. The analysis of the pattern of this atrophy indicates that the climbing fibre arbour is made by two compartments: a proximal one, whose survival depends on the integrity of the inferior olive, and a distal one, which represents the true pre-synaptic site, which strictly depends on the target. The climbing fibre terminal arbour is able to extend its territory of innervation not only when adult intact climbing fibres are confronted with nearby denervated Purkinje cells, but also when an embryonic cerebellum is grafted onto the surface of an adult unlesioned cerebellum. In this case, collaterals of intact climbing fibre arbours elongate through the pial surface, enter the graft to innervate the Purkinje cells. This growth is likely under the influence of a tropic signal released by the embryonic Purkinje cells. This suggests that the sprouting observed in the adult rat following a subtotal inferior olive lesion is also triggered by a similar factor. The axonal elongation and the consequent synaptogenesis are likely guided by local cues. In this condition, the distribution of the new collateral reinnervation occurs within its projectional map. In addition, when the inferior cerebellar peduncle is sectioned at birth, the climbing fibres of the non-deafferented hemicerebellum emit collaterals which cross the midline and innervate cerebellar strips which are symmetrically positioned relative to the intact side. In the grafting experiments, both the migrated and non-migrated Purkinje cells show the typical electrophysiological properties of the mature cerebellum. These data show that the disappearance of neuronal elements is not a necessary prerequisite to allow new neurones to become fully morphologically and functionally integrated into an adult brain. The reciprocal trophic influence between the climbing fibres and the Purkinje cells shown in the present series of experiments are likely operative in the adult brain not only in pathological conditions and they could give a basic contribution to the synaptic plasticity underlying learned behaviour.

Intrinsic properties and environmental factors in the regeneration of adult cerebellar axons

The success of axon regeneration in the adult mammalian brain depends on the presence of growth-permissive environmental conditions as well as on specific properties of the affected neurons. To investigate the relative contribution of extrinsic cues and intrinsic determinants to reparative processes we have investigated the regenerative properties of olivocerebellar and Purkinje cell axons. When these axon populations are severed in the cerebellar white matter and confronted with embryonic neural grafts of cerebellar or extracerebellar origin, the former vigorously regenerate into the transplant, whereas the latter invariably fail to do so (Rossi et al., 1995). The same response occurs when dissociated Schwann cells are implanted in the lesion site: Purkinje cell axons fail to regrow, whereas olivocerebellar fibres regenerate for considerable distances. Within the graft, regenerating fibres follow tortuous courses along Schwann cell bundles and sometimes end with poorly developed terminal plexuses. Some of them, however, succeed in crossing the graft and grow further into the host cortex, where they break into fine terminal branches confined to the granular layer. The remarkable regenerative response of olivocerebellar axons revealed by these experiments might be an intrinsic reaction of the affected neurons to axon injury or it might be elicited by growth promoting cues derived from the grafts. To elucidate this point we have undertaken the investigation of cellular changes occurring in adult inferior olivary neurons following the transection of the inferior cerebellar peduncle. Our results show that axotomy induces a series of cellular changes, or reparative and regressive character, which ultimately lead to cell death. Interestingly, however, these modifications are not uniformly distributed throughout the whole inferior olive. (i) Neuronal atrophy and degeneration progress more rapidly in the PO and DAO than in the MAO. (ii) A subpopulation of inferior olivary neurons become reactive for NADPH-diaphorase histochemistry, and their preferential localisation in the MAO suggests that this modification is related to the longer survival of these cells after axotomy. (iii) The developmentally regulated calcitonin gene-related peptide (CGRP) is reexpressed by a subset of neurons in the caudal nuclear compartments. These results further emphasise the conclusion that the dissimilar regenerative response of Purkinje cell and olivocerebellar axons confronted with permissive environmental conditions is due to different intrinsic properties of these neuronal populations. The reexpression of developmentally regulated substances by axotomised inferior olivary neurons suggests that their reparative reaction is triggered by axon injury. However, the pattern of growth of regenerating olivocerebellar axons is strongly conditioned by environmental constraints, which, in the present experimental conditions, do not allow them to reattain denervated Purkinje cells.