Embryonic Purkinje Cells Grafted on the Surface of the Cerebellar Cortex Integrate in the Adult Unlesioned Cerebellum

Quantification of Solid Embryonic Cerebellar Graft Volume in a Degenerative Ataxia Model

Substitution of lost neurons by neurotransplantation would be a possible management of advanced degenerative cerebellar ataxias in which insufficient cerebellar reserve remains. In this study, we examined the volume and structure of solid embryonic cerebellar grafts in adult Lurcher mice, a model of olivocerebellar degeneration, and their healthy littermates. Grafts taken from enhanced green fluorescent protein (EGFP)-positive embryos were injected into the cerebellum of host mice. Two or six months later, the brains were examined histologically. The grafts were identified according to the EGFP fluorescence in frozen sections and their volumes were estimated using the Cavalieri principle. For gross histological evaluation, graft-containing slices were processed using Nissl and hematoxylin-eosin staining. Adjustment of the volume estimation approach suggested that it is reasonable to use all sections without sampling, but that calculation of values for up to 20% of lost section using linear interpolation does not constitute substantial error. Mean graft volume was smaller in Lurchers than in healthy mice when examined 6 months after the transplantation. We observed almost no signs of graft destruction. In some cases, compact grafts disorganized the structure of the host's cerebellar cortex. In Lurchers, the grafts had a limited contact with the host's cerebellum. Also, graft size was of greater variability in Lurchers than in healthy mice. The results are in compliance with our previous findings that Lurcher phenotype-associated factors have a negative effect on graft development. These factors can hypothetically include cerebellar morphology, local tissue milieu, or systemic factors such as immune system abnormalities.

Maturation, Refinement, and Serotonergic Modulation of Cerebellar Cortical Circuits in Normal Development and in Murine Models of Autism

The formation of the complex cerebellar cortical circuits follows different phases, with initial synaptogenesis and subsequent processes of refinement guided by a variety of mechanisms. The regularity of the cellular and synaptic organization of the cerebellar cortex allowed detailed studies of the structural plasticity mechanisms underlying the formation of new synapses and retraction of redundant ones. For the attainment of the monoinnervation of the Purkinje cell by a single climbing fiber, several signals are involved, including electrical activity, contact signals, homosynaptic and heterosynaptic interaction, calcium transients, postsynaptic receptors, and transduction pathways. An important role in this developmental program is played by serotonergic projections that, acting on temporally and spatially regulated postsynaptic receptors, induce and modulate the phases of synaptic formation and maturation. In the adult cerebellar cortex, many developmental mechanisms persist but play different roles, such as supporting synaptic plasticity during learning and formation of cerebellar memory traces. A dysfunction at any stage of this process can lead to disorders of cerebellar origin, which include autism spectrum disorders but are not limited to motor deficits. Recent evidence in animal models links impairment of Purkinje cell function with autism-like symptoms including sociability deficits, stereotyped movements, and interspecific communication by vocalization.

Sonic hedgehog patterning during cerebellar development

The morphogenic factor sonic hedgehog (Shh) actively orchestrates many aspects of cerebellar development and maturation. During embryogenesis, Shh signaling is active in the ventricular germinal zone (VZ) and represents an essential signal for proliferation of VZ-derived progenitors. Later, Shh secreted by Purkinje cells sustains the amplification of postnatal neurogenic niches: the external granular layer and the prospective white matter, where excitatory granule cells and inhibitory interneurons are produced, respectively. Moreover, Shh signaling affects Bergmann glial differentiation and promotes cerebellar foliation during development. Here we review the most relevant functions of Shh during cerebellar ontogenesis, underlying its role in physiological and pathological conditions.

Origin, lineage and function of cerebellar glia

The glial cells of the cerebellum, and particularly astrocytes and oligodendrocytes, are characterized by a remarkable phenotypic variety, in which highly peculiar morphological features are associated with specific functional features, unique among the glial cells of the entire CNS. Here, we provide a critical report about the present knowledge of the development of cerebellar glia, including lineage relationships between cerebellar neurons, astrocytes and oligodendrocytes, the origins and the genesis of the repertoire of glial types, and the processes underlying their acquisition of mature morphological and functional traits. In parallel, we describe and discuss some fundamental roles played by specific categories of glial cells during cerebellar development. In particular, we propose that Bergmann glia exerts a crucial scaffolding activity that, together with the organizing function of Purkinje cells, is necessary to achieve the normal pattern of foliation and layering of the cerebellar cortex. Moreover, we discuss some of the functional tasks of cerebellar astrocytes and oligodendrocytes that are distinctive of cerebellar glia throughout the CNS. Notably, we report about the regulation of synaptic signalling in the molecular and granular layer mediated by Bergmann glia and parenchymal astrocytes, and the functional interaction between oligodendrocyte precursor cells and neurons. On the whole, this review provides an extensive overview of the available literature and some novel insights about the origin and differentiation of the variety of cerebellar glial cells and their function in the developing and mature cerebellum.

Structural plasticity of climbing fibers and the growth-associated protein GAP-43

Structural plasticity occurs physiologically or after brain damage to adapt or re-establish proper synaptic connections. This capacity depends on several intrinsic and extrinsic determinants that differ between neuron types. We reviewed the significant endogenous regenerative potential of the neurons of the inferior olive (IO) in the adult rodent brain and the structural remodeling of the terminal arbor of their axons, the climbing fiber (CF), under various experimental conditions, focusing on the growth-associated protein GAP-43. CFs undergo remarkable collateral sprouting in the presence of denervated Purkinje cells (PCs) that are available for new innervation. In addition, severed olivo-cerebellar axons regenerate across the white matter through a graft of embryonic Schwann cells. In contrast, CFs undergo a regressive modification when their target is deleted. In vivo knockdown of GAP-43 in olivary neurons, leads to the atrophy of their CFs and a reduction in the ability to sprout toward surrounding denervated PCs. These findings demonstrate that GAP-43 is essential for promoting denervation-induced sprouting and maintaining normal CF architecture.

The genesis of cerebellar GABAergic neurons: fate potential and specification mechanisms

All cerebellar neurons derive from progenitors that proliferate in two germinal neuroepithelia: the ventricular zone (VZ) generates GABAergic neurons, whereas the rhombic lip is the origin of glutamatergic types. Among VZ-derivatives, GABAergic projection neurons, and interneurons are generated according to distinct strategies. Projection neurons (Purkinje cells and nucleo-olivary neurons) are produced at the onset of cerebellar neurogenesis by discrete progenitor pools located in distinct VZ microdomains. These cells are specified within the VZ and acquire mature phenotypes according to cell-autonomous developmental programs. On the other hand, the different categories of inhibitory interneurons derive from a single population of Pax-2-positive precursors that delaminate into the prospective white matter (PWM), where they continue to divide up to postnatal development. Heterotopic/heterochronic transplantation experiments indicate that interneuron progenitors maintain full developmental potentialities up to the end of cerebellar development and acquire mature phenotypes under the influence of environmental cues present in the PWM. Furthermore, the final fate choice occurs in postmitotic cells, rather than dividing progenitors. Extracerebellar cells grafted to the prospective cerebellar white matter are not responsive to local neurogenic cues and fail to adopt clear cerebellar identities. Conversely, cerebellar cells grafted to extracerebellar regions retain typical phenotypes of cerebellar GABAergic interneurons, but acquire type-specific traits under the influence of local cues. These findings indicate that interneuron progenitors are multipotent and sensitive to spatio-temporally patterned environmental signals that regulate the genesis of different categories of interneurons, in precise quantities and at defined times and places.

Laminar fate and phenotype specification of cerebellar GABAergic interneurons

In most CNS regions, the variety of inhibitory interneurons originates from separate pools of progenitors residing in discrete germinal domains, where they become committed to specific phenotypes and positions during their last mitosis. We show here that GABAergic interneurons of the rodent cerebellum are generated through a different mechanism. Progenitors for these interneurons delaminate from the ventricular neuroepithelium of the embryonic cerebellar primordium and continue to proliferate in the prospective white matter during late embryonic and postnatal development. Young postmitotic interneurons do not migrate immediately to their final destination, but remain in the prospective white matter for several days. The different interneuron categories are produced according to a continuous inside-out positional sequence, and cell identity and laminar placement in the cerebellar cortex are temporally related to birth date. However, terminal commitment does not occur while precursors are still proliferating, and postmitotic cells heterochronically transplanted to developing cerebella consistently adopt host-specific phenotypes and positions. However, solid grafts of prospective white matter implanted into the adult cerebellum, when interneuron genesis has ceased, produce interneuron types characteristic of the donor age. Therefore, specification of cerebellar GABAergic interneurons occurs through a hitherto unknown process, in which postmitotic neurons maintain broad developmental potentialities and their phenotypic choices are dictated by instructive cues provided by the microenvironment of the prospective white matter. Whereas in most CNS regions the repertoire of inhibitory interneurons is produced by recruiting precursors from different origins, in the cerebellum it is achieved by creating phenotypic diversity from a single source.

Axonal competition in the synaptic wiring of the cerebellar cortex during development and in the mature cerebellum

Purkinje cell (PC) dendrites are made by a proximal dendritic domain, which is provided with scattered clusters of spines innervated by a single climbing fiber (CF) and by a distal domain with a high density of spines innervated by parallel fibers (PFs). Following block of electrical activity a spine increase occurs in the proximal domain and the new spines are innervated by the PFs while the number of synaptic contacts formed by the CF is reduced. Also the GABAergic input expands its territory of innervation on the proximal domain, which undergoes a profound restructuring of the glutamate and GABA receptors. Excitatory-like postsynaptic assemblies appear not only on the new spines, but also on the smooth region of the dendrite and both of them may be innervated by GABAergic terminals. In this case GABA receptors coexist with the glutamate receptors leading to the formation of hybrid synapses. In contrast, PF synapses contain solely glutamate receptors. Thus, the expression of glutamate receptors appears to be an intrinsic property of the PC, while the expression of the GABA receptors is induced by the presence of GABAergic terminals. The data highlight an important feature of the CF input; its electrical activity, in addition to inducing a powerful phasic excitation and a tonic inhibition, controls the finer architecture of the cerebellar cortex.

Cerebellar granule cells transplanted in vivo can follow physiological and unusual migratory routes to integrate into the recipient cortex

CNS repair by cell transplantation requires new neurons to integrate into complex recipient networks. We assessed how the migratory route of transplanted granule neurons and the developmental stage of the host rat cerebellum influence engraftment. In both embryonic and postnatal hosts, granule cells can enter the cerebellar cortex and achieve correct placement along their natural migratory pathway. Donor neurons can also reach the internal granular layer from the white matter and integrate following an unusual developmental pattern. Although the frequency of correct positioning declines in parallel with cortical development, in mature recipients correct homing is more frequent through the unusual path. Following depletion of granule cell precursors in the host, more granule neurons engraft, but their ability for achieving correct placement is unchanged. Therefore, while the cerebellar environment remains receptive for granule cells even after the end of development, their full integration is partially hindered by the mature cortical architecture.

Reinnervation of late postnatal Purkinje cells by climbing fibers: neosynaptogenesis without transient multi-innervation

Synaptic partner selection and refinement of projections are important in the development of precise and functional neuronal connections. We investigated the formation of new synaptic connections in a relatively mature system to test whether developmental events can be recapitulated at later stages (i.e., after the mature synaptic organization has been established), using a model of postlesional reinnervation in the olivo-cerebellar pathway. During the development of this pathway, synaptic connections between climbing fibers (CFs) and Purkinje cells (PCs) are diffuse and redundant before synapse elimination refines the pattern. The regression of CFs during the first 2 postnatal weeks in the rat leads to mono-innervation of each PC. After unilateral transection of the rat olivo-cerebellar pathway and intracerebellar injection of BDNF 24 h after lesion, axons from the remaining inferior olive can sprout into the deafferented hemicerebellum and establish new contacts with denervated PCs at later developmental stages. We found that these contacts are first established on somatic thorns before the CFs translocate to the PC dendrites, recapitulating the morphological steps of normal CF-PC synaptogenesis, but on a relatively mature PC. However, electrophysiology of PC reinnervation by transcommissural CFs in these animals showed that each PC is reinnervated by only one CF. This mono-innervation contrasts with the reinnervation of grafted immature PCs in the same cerebellum. Our results provide evidence that relatively mature PCs do not receive several olivary afferents during late reinnervation, suggesting a critical role of the target cell state in the control of CF-PC synaptogenesis. Thus, synapse exuberance and subsequent elimination are not a prerequisite to reach a mature relationship between synaptic partners.

Neuronal replacement and integration in the rewiring of cerebellar circuits

Repair of CNS injury or degeneration by cell replacement may lead to significant functional recovery only through faithful reconstruction of the original anatomical architecture. This is particularly relevant for point-to-point systems, where precisely patterned connections have to be re-established to regain adaptive function. Despite the major interest recently drawn on cell therapies, little is known about the mechanisms and the potentialities for specific integration of new neurons in the mature CNS. Major findings and concepts about this issue will be reviewed here, with special focus on work dealing with the Purkinje cell transplantation in the rodent cerebellum. These studies show that the adult CNS may provide some efficient information to direct cell engraftment and process outgrowth. On their side, immature cells may be able to induce adaptive changes in their adult partners to facilitate their incorporation in the recipient network. Despite the rather high degree of specific integration achieved in several different CNS regions, these processes are usually defective and long-distance connections are not rewired. Thus, although some potentialities for cell replacement exist in the mature CNS, full incorporation of new neurons in adult circuits is rarely observed. Indeed, intrinsic mechanisms for growth control as well as injury-induced changes in the properties and architecture of the nervous tissue contribute to hamper repair processes. As a consequence, crucial to obtain successful cell replacement and integration in the mature CNS is a deep understanding of the basic biological mechanisms that regulate the interactions between newly added elements and the recipient environment.

Selective rather than inductive mechanisms favour specific replacement of Purkinje cells by embryonic cerebellar cells transplanted to the cerebellum of adult Purkinje cell degeneration (pcd) mutant mice

Cell replacement after neuronal degeneration in the adult CNS depends on the availability of specific cues to direct specification, differentiation and integration of newly born neurons into mature circuits. Following recent reports indicating that neurogenic signals may be reactivated in the adult injured CNS, here we asked whether such signals are expressed in the cerebellum after Purkinje cell degeneration. Thus, we compared the fate of embryonic cerebellar cells transplanted to the cerebella of adult wild-type and Purkinje cell degeneration (pcd) mutant mice. Donor cells were dissected from beta-actin-enhanced green fluorescent protein (EGFP) transgenic mice and transplanted as a single cell suspension. In both hosts, grafted cells generated all major cerebellar phenotypes, with a precise localization in the recipient cortex or white matter. Nevertheless, the phenotypic distributions showed striking quantitative differences. Most notably, in the pcd cerebellum there was a higher amount of Purkinje cells, while other phenotypes were less frequent. Analysis of cell proliferation by 5-bromo-2'-deoxyuridine (BrDU) incorporation revealed that in both hosts mitotic activity was strongly reduced shortly after transplantation, and virtually all donor Purkinje cells were actually generated before grafting. Together, these results indicate that some compensatory mechanisms operate in the pcd environment. However, the very low mitotic rate of transplanted cells suggests that the adult cerebellum, either wild-type or mutant, does not provide instructive neurogenic cues to direct the specification of uncommitted progenitors. Rather, specific replacement in mutant hosts is achieved through selective mechanisms that favour the survival and integration of donor Purkinje cells at the expense of other phenotypes.

Fate restriction and developmental potential of cerebellar progenitors. Transplantation studies in the developing CNS

The generation of cell diversity from undifferentiated progenitors is regulated by interdependent mechanisms, including cell intrinsic programs and environmental cues. This interaction can be investigated by means of heterochronic/heterotopic transplantation, which allows to examine the behaviour of precursor cells in an unusual environment. The cerebellum provides an ideal model to study cell specification, because its neurons originate according to a well-defined timetable and they can be are readily recognised by morphological features and specific markers. Cerebellar progenitors transplanted to the embryonic cerebellum develop fully mature cerebellar neurons, which often integrate in the host circuitry in a highly specific manner. In extracerebellar locations, cerebellar progenitors preferentially settle in caudal CNS regions where they exclusively acquire cerebellar identities. By contrast, neocortical precursors preferentially settle in rostral regions and fail to develop hindbrain phenotypes. The phenotypic repertoire generated by transplanted cerebellar progenitors is strictly dependent on their age. Embryonic progenitors originate all mature cerebellar cells, whereas postnatal ones exclusively generate later-born types, such as molecular layer interneurons and granule cells. Together, these observations foster the hypothesis that neural progenitors are first specified towards region-specific phenotypes along the rostro-caudal axis of the neural tube. Thereafter, the developmental potential of progenitor cells is progressively restricted towards later generated types. Such a progressive specification of precursor cells in space and time is stably transmitted to their progeny and it cannot be modified by local cues, when these cells are confronted with heterotopic and/or heterochronic environments.

Reparative mechanisms in the cerebellar cortex

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.

Target-specific innervation of embryonic cerebellar transplants by regenerating olivocerebellar axons in the adult rat

The reestablishment of topographically organized connections is a necessary prerequisite to obtain a full anatomical repair following brain injury. One system where such an issue can be addressed is the olivocerebellar system, where, normally, clusters of inferior olive neurons project to neurochemically heterogeneous Purkinje cell compartments defined by the expression of cell-specific markers, such as zebrin II. To assess whether adult injured olivocerebellar axons that regenerate into cerebellar transplants are able to establish target-specific innervation of grafted Purkinje cells, we made surgical transections in the white matter of adult rat cerebella and placed solid grafts from the embryonic cerebellar anlage into the lesion site. The transplanted tissue developed highly organized minicerebella, in which Purkinje cells were distributed into distinct clusters of zebrin II-immunopositive or -immunonegative neurons, mimicking the cortical compartments present in the normal adult cerebellum. Olivocerebellar axons, labeled by biotinylated dextran amine tracing, regenerated into the transplants where they formed discrete patches made of several terminal arbors impinging upon Purkinje cell dendrites. Among 401 such climbing fiber patches, 96% exclusively innervated Purkinje cells of either phenotype and stopped at the border of the zebrin II(+/-) Purkinje cell clusters, whereas only 4% were extended across this boundary and innervated both zebrin II-positive and -negative Purkinje cells. The results obtained support the view that the embryonic cerebellar tissue provides target-specific information that can be decoded by ingrowing adult olivocerebellar axons in order to establish appropriate innervation patterns with zebrin II-positive or -negative Purkinje cell compartments.

Phenotype changes of inferior olive neurons following collateral reinnervation

Inferior olive neurons are able to enlarge or retract their axonic terminal fields in response to changes in the extension of their target domain. Following Purkinje cell loss, the retraction of target-deprived climbing fibres is accompanied by a size reduction in the inferior olive neuron cell bodies. Here, we asked whether perikaryal modifications also occur when inferior olivary neurons enlarge their terminal fields to innervate supernumerary targets. To achieve this aim, we carried out a morphometric analysis on the somatic compartment of inferior olive neurons in two experimental conditions known to induce an expansion of their terminal field, i.e. a subtotal 3-acetylpyridine inferior olive lesion in the adult and a unilateral transection of the inferior cerebellar peduncle in newborn rats. In both experimental conditions, the inferior olive neurons that survived the lesion showed a remarkable increase in cell body and nuclear size, although the latter change was less pronounced in the 3-acetylpyridine-treated animals. These results show that both developing and mature inferior olive neurons are capable of adjusting their perikaryal phenotype to match the modifications of their target size.

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.

Postsynaptic currents and short-term synaptic plasticity in Purkinje cells grafted onto an uninjured adult cerebellar cortex

It has been shown recently that embryonic Purkinje cells grafted extraparenchymally into an intact cerebellum, in the absence of any sign of damage, are able to migrate into the host molecular layer where they receive a climbing fibre innervation. Using the same technique, we investigated the development of the electrophysiological properties of the synapses between the grafted cells and their main afferents. Purkinje cells either in the graft or having migrated into the molecular layer of the host were recorded using the whole-cell patch-clamp method in acutely prepared slices 17-112 days after grafting. Spontaneous postsynaptic currents with a single-exponential decay and mediated by GABAA receptors were very similar to those described in normal Purkinje cells. Excitatory postsynaptic currents (EPSCs) evoked by climbing fibre and by parallel fibre stimulation were blocked by an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate antagonist, and displayed the linear current-voltage relation typical of postnatal Purkinje cells. The attainment of normal functional properties by the adult axons at the newly formed synaptic sites was shown by the expression of short-term facilitation of parallel fibre EPSCs and of short-term depression of climbing fibre EPSCs. The grafted Purkinje cells showed climbing fibre polyinnervation 17-20 days after grafting which evolved to monoinnervation at 23-45 days, confirming the completion of the developmental programme up to maturation. Our experiments support the view that the adult intact brain is able to accept and integrate an additional number of neurons which show fully mature electrophysiological properties which are electrophysiologically indistinguishable from those of the host neurons.

Regeneration of immature mammalian spinal cord after injury

In this review we describe the growth of regenerating fibres through lesions in immature mammalian spinal cord. In newborn opossums and foetal rats, repair occurs rapidly and reliably without antibodies, implants or bridges of undamaged spinal cord. In the neonatal opossum one can compare recovery from lesions made to the CNS at various stages of development in the animal and in culture. As the CNS matures, the capacity for regeneration ceases abruptly. In particular, the extracellular matrix and molecules associated with glia have been shown to play a role in promoting and inhibiting regeneration. Major problems concern the precision with which regenerating axons become reconnected to their targets, and the specificity needed for recovery of function.

The cerebellar model of neural grafting: structural integration and functional recovery

A synopsis is presented of the recent history of cerebellar tissue transplantation over the past 25 years. The properties of growth and differentiation of cerebellar grafts placed intraocularly or intracranially are reviewed, as well as the interaction of heterotopic and orthotopic grafts with the host brain. Particular emphasis is placed on the use of ataxic mouse mutants as recipients of donor cerebellar tissue for the correction of their structural deficits and the functional recovery of behavioural responses.

Cellular plasticity at the climbing fibre-Purkinje cell synapse as a model of plasticity in adulthood and ageing

In the adult brain there is a high degree of plasticity which is documented here for climbing fibre terminal arborization impinging upon the proximal Purkinje cell dendrites. Such an arborization presents a more distal compartment, including 90% of branches and varicosities, whose integrity depends on the target and a more proximal compartment which survives to target degeneration. The same terminal arborization enlarges its territory of innervation through collateral sprouting when nearby Purkinje cells are deprived of their climbing fibres or when embryonic Purkinje cells are placed on the surface of an intact cerebellum. In the latter case axonal elongation occurs in the absence of degeneration products and of macrophage/microglia or astrocyte activation. In addition, the embryonic Purkinje cells are able to invade the intact host molecular layer where they develop dendritic arbours which become also innervated by climbing fibre collaterals.

Embryonic Purkinje cells grafted on the surface of the adult uninjured rat cerebellum migrate in the host parenchyma and induce sprouting of intact climbing fibres

By grafting solid pieces of cerebellar anlage onto the surface of the adult rat cerebellum, we have investigated the problem of the interactions between embryonic and adult neurons in an intact brain. A few days after grafting, embryonic astrocytic processes crossed the graft--host interface and radiated into the recipient molecular layer. Several grafted Purkinje cells also migrated into the host brain along such processes as well as adult Bergmann glia. Adult climbing fibres, labelled by means of Phaseolus vulgaris leucoagglutinin (PHA-L), sprouted new collateral branches which terminated on embryonic Purkinje cells at both extra- and intraparenchymal levels. No sign of activation of host astroglia or microglia was evident in the host cerebellum in relation to these processes. Embryonic Purkinje cells which migrated into the host cerebellum developed an adult-like morphology. Intraparenchymal grafts of neocortical embryonic tissue induced conspicuous growth of host olivary axons, characterized by a pattern which was different from that observed following cerebellar grafts. By contrast, when neocortical tissue was placed onto the surface of the recipient cerebellum, graft--host interactions were limited and climbing fibre sprouting was rarely seen. These results show that (i) supernumerary Purkinje cells can penetrate and settle in the adult intact cerebellar cortex, (ii) adult climbing fibres are able to innervate these new targets in the absence of any injury or activation of non-neuronal cells of the adult brain, and (iii) in the absence of damage to the adult brain, the plasticity of adult olivary axons is specifically elicited and controlled by embryonic Purkinje cells.

Ectopic Purkinje cells in the adult rat: olivary innervation and different capabilities of migration and development after grafting

The abnormal location of large numbers of neurones is characteristic of genetic mutations which impair the migratory processes of developing nerve cells. Nevertheless, the presence of small amounts of ectopic neurones is a fairly common finding even in normal adult animals. The first aim of this study was to investigate a series of features of ectopic Purkinje cells in normal adult rats and particularly to assess whether these cells are still capable of interacting with their normal afferents. Several displaced Purkinje cells, identified by anti-D28k calbindin immunolabelling as well as by typical morphological features, were present in the brainstem and cerebellum of all the examined animals. Two distinct morphological types of such cells could be recognized: 1) noncortical Purkinje cells, located in several areas of the dorsal brainstem and cerebellum, characterised by poorly developed and randomly oriented dendrites; and 2) cortical Purkinje cells, exclusively located in the dorsal cochlear nucleus, characterised by large dendritic trees oriented along parasagittal planes. Tracing experiments, in which Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into the inferior olive, revealed that several ectopic Purkinje cells, belonging to both types, were contacted by the terminal arbours of olivary axons, structurally similar to cerebellar climbing fibres. On the basis of the observation that ectopic Purkinje cells were more frequent in the dorsal cochlear nucleus than in any other of the examined regions, we tested the hypothesis that this nucleus might represent a particularly favourable environment for the survival and development of Purkinje cells. By grafting embryonic cerebellar tissue in the fourth ventricle, only minimal migration of Purkinje cells into the recipient parenchyma was observed when the transplant was placed on the brainstem surface caudal to the cochlear nucleus. By contrast, when the graft was apposed to the latter nucleus, large numbers of Purkinje cells migrated and developed in its superficial layers. These Purkinje cells passed through all the different phases which characterise the normal ontogenesis of this neuronal population and finally developed mature structural features similar to those displayed by cortical ectopic Purkinje cells. This study demonstrates that at least some of the ectopic Purkinje cells receive their physiological olivary input. This fact indicates that Purkinje cells are able to attract olivary axons and establish specific connections, even if they are displaced in an abnormal environment. In addition, we show that the dorsal cochlear nucleus represents a particularly favourable environment for the survival and the development of Purkinje cells.

Urokinase-type plasminogen activator expression by neurons and oligodendrocytes during process outgrowth in developing rat brain

The expression of tissue- and urokinase-type plasminogen activators has been studied in developing cerebellum, hippocampus, cerebral cortex, olfactory bulb and olfactory mucosa of the rat by in situ hybridization. All identifiable neurons express urokinase mRNA from an early stage in their development, and this expression appears to coincide with the onset of axogenesis. For cerebellar granule cells, both axonal growth and urokinase expression are initiated before they migrate from the external granule layer; for the majority of neocortical neurons, however, both processes are commenced after the cells have migrated to the cortical plate. Neurons continue to express this protease in the adult. The large projection neurons exhibit the highest levels of message, the smaller interneurons having much lower levels except for hippocampal granule cells, which have notably high levels of expression. Glial cells generally do not express urokinase message, except for transient expression by oligodendrocytes in developing fibre tracts during the period of myelination. Thus for both neurons and oligodendrocytes, the onset of urokinase-type plasminogen activator expression coincides with their initiation of major process outgrowth, although neurons maintain this expression in the adult, possibly to retain a degree of synaptic plasticity. In contrast, although high levels of message for the related protease, tissue plasminogen activator, are found in the embryonic floor plate, in postnatal brain it is abundantly expressed only by ventricular ependymal cells and by cells in connective tissue surrounding the olfactory nerve.

Regressive modifications of climbing fibres following Purkinje cell degeneration in the cerebellar cortex of the adult rat

The role of postsynaptic neurons in the maintenance of adult terminal axon arbours was investigated in the rat olivocerebellar system. The degeneration of Purkinje cells, the main target of olivary axons in the cerebellar cortex, was obtained by intraparenchymal application of kainate. The structural features of target-deprived climbing fibres, visualized by Phaseolus vulgaris leucoagglutinin tracing, were examined from two days to six months after the lesion. Following the degeneration of its Purkinje cell, the climbing fibre underwent remarkable regressive modifications involving the disappearance of most of the terminal arborization. Never the less, atrophic arbours still spanned through the molecular layer six months after the lesion. Morphometric evaluations showed that, one week after kainate application, total arbour length was already reduced to 52% of control, whereas the number of branches and of varicosities had both dropped around 40%. This retraction process progressed in the following stages to reach its maximum at about one month after the lesion, when total length was 30% of control and only 10% of branches and varicosities were still present. Only a slight tendency to a further decrease of the values could be detected at longer survival times. Branching pattern analysis revealed that such regressive phenomena mainly involved the distal compartment of the climbing fibres, the one made of fine varicose branchlets, while sparing the proximal thick branches. In addition, the whole process appeared to follow some rather strict guiding principles leading to an ordered branch retraction, from the periphery of the arbour inwards. Finally, in order to rule out the possibility that the observed changes could be due to a direct action of kainate on climbing fibres, we designed an alternative method of killing Purkinje cells by intraparenchymal injection of propidium iodide. The structural features of climbing fibres deprived of their target by such a procedure were very similar to those shown by arbours from time-matched kainate-lesioned animals at both qualitative and quantitative levels. Our results show that target deprivation induces remarkable structural modifications in the climbing fibre, leading to the retraction of most of the arbour. Never the less, the integrity of the Purkinje cell is not necessary for the maintenance of the whole arborization since its proximal compartment is maintained in the molecular layer for several months after target degeneration. It is proposed that the Purkinje cell, most likely by acting through a contact factor, directly controls the formation and the maintenance of the distal climbing fibre branches with their varicosities, which represent the presynaptic compartment of the axonal arbour.