Regional and cellular patterns of reelin mRNA expression in the forebrain of the developing and adult mouse

The reelin gene encodes an extracellular protein that is crucial for neuronal migration in laminated brain regions. To gain insights into the functions of Reelin, we performed high-resolution in situ hybridization analyses to determine the pattern of reelin expression in the developing forebrain of the mouse. We also performed double-labeling studies with several markers, including calcium-binding proteins, GAD65/67, and neuropeptides, to characterize the neuronal subsets that express reelin transcripts. reelin expression was detected at embryonic day 10 and later in the forebrain, with a distribution that is consistent with the prosomeric model of forebrain regionalization. In the diencephalon, expression was restricted to transverse and longitudinal domains that delineated boundaries between neuromeres. During embryogenesis, reelin was detected in the cerebral cortex in Cajal-Retzius cells but not in the GABAergic neurons of layer I. At prenatal stages, reelin was also expressed in the olfactory bulb, and striatum and in restricted nuclei in the ventral telencephalon, hypothalamus, thalamus, and pretectum. At postnatal stages, reelin transcripts gradually disappeared from Cajal-Retzius cells, at the same time as they appeared in subsets of GABAergic neurons distributed throughout neocortical and hippocampal layers. In other telencephalic and diencephalic regions, reelin expression decreased steadily during the postnatal period. In the adult, there was prominent expression in the olfactory bulb and cerebral cortex, where it was restricted to subsets of GABAergic interneurons that co-expressed calbindin, calretinin, neuropeptide Y, and somatostatin. This complex pattern of cellular and regional expression is consistent with Reelin having multiple roles in brain development and adult brain function.

staggerer phenotype in retinoid-related orphan receptor alpha-deficient mice

Retinoid-related orphan receptor alpha (RORalpha) is a member of the nuclear receptor superfamily. To study its physiological role we generated null-mutant mice by targeted insertion of a lacZ reporter gene encoding the enzyme beta-galactosidase. In heterozygous RORalpha+/- mice we found beta-galactosidase activity, indicative of RORalpha protein expression, confined to the central nervous system, skin and testis. In the central nervous system, the RORalpha gene is expressed in cerebellar Purkinje cells, the thalamus, the suprachiasmatic nuclei, and retinal ganglion cells. In skin, RORalpha is strongly expressed in the hair follicle, the epidermis, and the sebaceous gland. Finally, the peritubular cells of the testis and the epithelial cells of the epididymis also strongly express RORalpha. Recently, it was reported that the ataxic mouse mutant staggerer (sg/sg) is caused by a deletion in the RORalpha gene. The analysis of the cerebellar and the behavioral phenotype of homozygous RORalpha-/- mice proves identity to sg/sg mice. Although the absence of RORalpha causes dramatic developmental effects in the cerebellum, it has no apparent morphological effect on thalamus, hypothalamus, and retina. Similarly, testis and skin of RORalpha-/- mice display a normal phenotype. However, the pelage hair of both sg/sg and RORalpha-/- is significantly less dense and when shaved shows reluctance to regrow.

Development of the olivocerebellar projection

The establishment of orderly axonal projections is one of the essential steps in the formation of central networks. In this review, we discuss several of the current hypotheses on the mechanisms and molecules which govern this developmental process, using the olivocerebellar system as a model. During the formation of the olivocerebellar projection, there is a simultaneous and independent process of parcellation of the inferior olive and of the cerebellum. During embryogenesis, Purkinje cells in the cerebellar cortex and inferior olivary neurons are subdivided into small subsets of biochemically distinct compartments. We propose that this parcellation is involved in matching groups of olivary neurons to their corresponding subsets of target Purkinje cells. In vitro, the rotation of the anteroposterior axis of the cerebellum is followed by an equivalent inversion of the olivocerebellar projection. Olivary axons still project to the same Purkinje cells, suggesting that the formation of the olivocerebellar projection is regulated by positional information shared between pre- and postsynaptic neurons. We suggest that, in the chick embryo, the cell adhesion molecule BEN/SC1/DM-GRASP could be one of the target recognition molecules controlling the development of the olivocerebellar projection. These results also emphasize that coarse grained projection maps can form through chemoaffinity mechanisms, independent of the activity of the interacting neurons.

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.

Cajal-Retzius cells regulate the radial glia phenotype in the adult and developing cerebellum and alter granule cell migration

Studies on the reeler mutation have shown that pioneer Cajal-Retzius (CR) cells are involved in neuronal migration in the developing cortex. Here, we use grafting and coculture experiments to investigate the mechanisms by which CR cells govern migration. We show that transplantation of embryonic CR cells, but not other cortical neurons, into adult cerebella induces a transient rejuvenation of host Bergmann glia into a radial glia phenotype. Similarly, CR cells sustain the phenotype of developing radial glia in postnatal cerebellar slices and induce the organization of a glial scaffold inside the CR cell explants. Studies with semipermeable inserts show that these effects are mediated by diffusible signals. We also show that CR cells adjacent to the surface of cerebellar slices reverse the direction of the migration of granule cells. Finally, CR cells from reeler mutant embryos elicited similar effects. These observations imply a role for CR cells in the regulation of the radial glia phenotype, a key step for neuronal migration, and suggest that these pioneer neurons may also exert a chemoattractive influence on migrating neurons.

Distribution pattern and ultrastructural localization of Rxt1, an orphan Na+/Cl(-)-dependent transporter, in the central nervous system of rats and mice

The cellular and subcellular localization of Rxt1 protein, an orphan Na+/Cl(-)-dependent transporter, was investigated in the central nervous system of rats and mice, with rabbit polyclonal antibodies specifically directed against its C-terminal region. At the light microscope level, the distribution of Rxt1, visualized by the immunoperoxidase method, was found to be similar in rats and mice. Labelled elements were present in numerous gray matter regions of the central nervous system, from the olfactory bulb to the spinal cord. In all labelled regions, immunoreactivity was confined to the neuropil where both a diffuse labelling of low intensity and an intense punctate staining were noted. To further identify the nature of the cellular elements bearing the punctate staining, possible changes in this labelling pattern were investigated: (i) in deep cerebellar nuclei and lateral vestibular nucleus of the Lurcher mutant mouse, in which all cerebellar Purkinje cells are missing and (ii) in the rat cervical spinal cord, 10 days after multiple resections of dorsal roots. The vast majority of the punctate structures, delineating the neuronal perikaryal and stem dendritic contours, had disappeared in the mutant mouse, providing evidence that they belong to Purkinje cell axon terminals. In rhizotomized rats, the intense labelling in laminae I and III had disappeared, demonstrating that it occurred in subclasses of axonal projections of primary afferent fibres. These results strongly suggest that Rxt1 is present in presynaptic axon terminals. The electron microscopic study was carried out in the hippocampus, cerebellum and lateral vestibular nucleus of control mice, where Rxt1-labelled punctate structures were found to be abundant. Immunostaining was confined to axon terminals, particularly in hippocampal and cerebellar mossy fibres and in Purkinje cell axonal terminations of the cerebellar deep nuclei and lateral vestibular nucleus. In the cerebellar cortex, axon terminals belonging to inhibitory local circuit neurons (basket and Golgi cells), were free of labelling. The observations reported in this study have shown that: (1) The Rxt1 transporter is neuron-specific, and is expressed by only some classes or even subclasses of neuronal systems. (2) This transporter can be encountered in excitatory axons using glutamate as neurotransmitter (hippocampal and cerebellar mossy fibres: primary afferent fibres), as well as in inhibitory axons known by their GABAergic nature (Purkinje cell axon terminals) where it might be involved in the re-uptake process of one or several molecules released from corresponding terminals.

The embryonic cerebellum contains topographic cues that guide developing inferior olivary axons

The formation of the olivocerebellar projection is supposed to be regulated by positional information shared between pre- and postsynaptic neurons. However, experimental evidence to support this hypothesis is missing. In the chick, caudal neurons in the inferior olive project to the anterior cerebellum and rostral ones to the posterior cerebellum. We here report in vitro experiments that strongly support the existence of anteroposterior polarity cues in the embryonic cerebellum. We developed an in vitro system that was easily accessible to experimental manipulations. Large hindbrain explants of E7.5-E8 chick embryos, containing the cerebellum and its attached brainstem, were plated and studied using axonal tracing methods. In these cultures, we have shown that the normal anteroposterior topography of the olivocerebellar projection was acquired, even when the cerebellar lamella was detached from the brainstem and placed again in its original position. We also found that, following various experimental rotations of the anteroposterior axis of the cerebellum, the rostromedian olivary neurons still project to the posterior vermis and the caudolateral neurons to the anterior vermis, that now have inverted locations. Thus, the rotation of the target region results in the rotation of the projection. In addition, we have shown that the formation of the projection map could be due to the inability of rostromedian inferior olivary axons to grow in the anterior cerebellum. All these experiments strongly indicate that olivocerebellar fibers recognize within their target region polarity cues that organize their anteroposterior topography, and we suggest that Purkinje cells might carry these cues.

A rat mutation producing demyelination (dmy) maps to chromosome 17

A recessive mutation exhibiting severe myelin breakdown, mainly at the level of the lumbar segments of the spinal cord and without any associated inflammation, was discovered in a partially inbred rat colony. Analysis of the segregation patterns of a set of polymorphic microsatellite markers in two inter-strain crosses allowed the mapping of this autosomal recessive mutation to rat Chromosome (Chr) 17, very close to the prolactin (Prl) locus, in a region homologous to human Chr 6p21.2-22.3 and mouse Chr 13. The pathology of the demyelination process and the chromosomal localization indicate that this mutation has no known equivalent in either mouse or human.

Neuronal precursors in the postnatal mouse cerebellum are fully committed cells: evidence from heterochronic transplantations

Neural progenitors are thought to be multipotent cells whose adult phenotype is determined by extrinsic influences acting during and immediately after their last mitosis. To test this hypothesis, postnatal cerebellar precursor cells were placed in the heterochronic cellular environment of the embryonic mouse cerebellar anlage and the resulting phenotypes were determined. To identify the cells arising from postnatal precursors, tissue fragments taken from 3- to 8-day-old cerebellum of several transgenic mouse lines (each expressing the lacZ reporter gene in different sets of neuronal populations) were mixed with fragments taken from the wild-type cerebellar primordium of 12- or 13-day-old embryos. The fragments were dissociated and grafted into the cerebellum of adult mice. The phenotype acquired by postnatal precursors in the mixed grafts was determined by their morphology and ultrastructural features and by the expression of specific markers. Only two adult phenotypes were generated by these precursors: granule cells and molecular layer interneurons. Most granule cells were well integrated in the trilaminated cortex of the graft, being positioned in their proper layer both during development and after complete maturation. By contrast, basket and stellate cells were always ectopic, remaining outside the molecular layer. These results indicate that at least two distinct progenitor cells are present in the postnatal cerebellar cortex under the experimental conditions of this study. Both progenitors appear to be strictly specified at the time of grafting, and neither their identify nor the expression of their major distinctive features are significantly influenced by local signals emerging from the cellular environment of the embryonic cerebellar anlage.

Subventricular zone-olfactory bulb migratory pathway in the adult mouse: cellular composition and specificity as determined by heterochronic and heterotopic transplantation

To gain insight into cellular and molecular mechanisms subserving neuronal cell migration in the adult mouse forebrain, we have first investigated the cellular composition of the subventricular zone-olfactory bulb pathway (SVZ-OB). The pathway was essentially composed of cells with neuronal and astrocytic identities, neuronal cells being four times more numerous than astrocytes. Neuronal cells (precursors and some young postmitotic neurons) formed continuous cellular strands of migratory cells from the anterior horn of the lateral ventricle to the olfactory bulb. These chains of migrating cells moved within channels formed by the processes of a special subpopulation of astrocytes. The neuronal cells expressed the embryonic form of polysialic acid neural cell adhesion molecule, and the astrocytes were tenascin-C positive, thus preserving an embryonic cellular environment. Through transplantation experiments, the second part of this study attempted to analyze the functional properties of the adult SVZ-OB pathway. Early postnatal (P2-13) cerebellar progenitor cells, taken from a transgenic mouse line in which cerebellar granule cells and molecular layer interneurons (basket/stellate cells) expressed the reporter gene lacZ, were implanted in the SVZ-OB pathway of adult wild-type mice. Unlike grafted SVZ cells that migrate all along the pathway, none of the cerebellar precursors reached the olfactory bulb, although some of them were able to migrate along the caudal one-third of the pathway. The majority (over 67%) of the migrating cells were progenitors that acquired the phenotype of basket/stellate cells. Granule cell progenitors and most granule cells did not survive transplantation. These results show that the adult SVZ-OB pathway is not a "passive generic guidance" for all classes of premigratory neurons. From the two types of grafted cerebellar progenitors, only those with migratory capability and that do not follow radial glial axes are able to translocate along the SVZ-OB pathway. Furthermore, the basket/stellate cell progenitors are specified at the time of grafting: Neither their identity nor the pace of expression of their major distinctive features are influenced by local signals emanating from the adult forebrain.

BEN as a presumptive target recognition molecule during the development of the olivocerebellar system

It has been shown previously that in the chick embryo the cell adhesion molecule BEN/SC1/DM-GRASP is expressed by neurons in the inferior olive (IO) and by their terminal axonal arbors in the cerebellar cortex, the climbing fibers (Porquié et al., 1992b). Here, new information on the expression of BEN during the formation of the olivocerebellar projection adds the important notion that BEN is also expressed by the cerebellar targets of inferior olivary axons, Purkinje cells (PCs) and deep nuclear neurons. This expression is transient, starting at E7-E8 and vanishing shortly after hatching. More importantly, BEN expression is restricted to precise subsets of IO neurons and PCs. In the cerebellar cortex, BEN-immunoreactive (BEN-IR) structures are not found randomly but are distributed according to a reproducible pattern of parasagittal stripes. A maximum of four distinct sagittal stripes is found in each lobule, along the whole rostrocaudal extent of the cerebellum. Moreover, BEN-expressing stripes belong to two classes; one contains BEN-IR climbing fibers terminating on BEN-IR PCs and the other, more frequent class is solely composed of BEN-IR climbing fibers. Organotypic cultures of isolated cerebella have shown that the expression of BEN in the IO and in the cerebellum arise independently, probably because of an intrinsic developmental program. Thus, the cell adhesion molecule BEN meets all criteria for a recognition molecule involved in the formation of the olivocerebellar projection.

Lack of barrels in the somatosensory cortex of monoamine oxidase A-deficient mice: role of a serotonin excess during the critical period

In a transgenic mouse line (Tg8) deficient for the gene encoding monoamine oxidase A (MAOA), we show that the primary somatosensory cortex (S1) lacks the characteristic barrel-like clustering of layer IV neurons, whereas normal pattern formation exists in the thalamus and the trigeminal nuclei. No barrel-like patterns were visible with tenascin or serotonin immunostaining or with labeling of thalamocortical axons. An excess of brain serotonin during the critical period of barrel formation appears to have a causal role in these cortical abnormalities, since early administration of parachlorophenylalanine, an inhibitor of serotonin synthesis, in Tg8 pups restored the formation of barrels in S1, whereas inhibition of catecholamine synthesis did not. Transient inactivation of MAOA in normal newborns reproduced a barrelless phenotype in parts of S1.

Molecular heterogeneity of progenitors and radial migration in the developing cerebral cortex revealed by transgene expression

We have analyzed the developmental pattern of beta-galactosidase (beta-gal) expression in the cerebral cortex of the beta 2nZ3'1 transgenic mouse line, which was generated using regulatory elements of the beta 2-microglobulin gene and shows ectopic expression in nervous tissue. From embryonic day 10 onward, beta-gal was expressed in the medial and dorsal cortices, including the hippocampal region, whereas lateral cortical areas were devoid of labeling. During the period of cortical neurogenesis (embryonic days 11-17), beta-gal was expressed by selective precursors in the proliferative ventricular zone of the neocortex and hippocampus, as well as by a number of migrating and postmigratory neurons arranged into narrow radial stripes above the labeled progenitors. Thus, the transgene labels a subset of cortical progenitors and their progeny. Postnatally, radial clusters of beta-gal-positive neurons were discernible until postpartum day 10. At this age, the clusters were 250 to 500 microns wide, composed of neurons spanning all the cortical layers and exhibiting several neuronal phenotypes. These data suggest molecular heterogeneity of cortical progenitors and of the cohorts of postmitotic neurons originating from them, which implies intrinsic molecular mosaicism in both cortical progenitors and developing neurons. Furthermore, the data show that neurons committed to the expression of the transgene migrate along very narrow, radial stripes.

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

Regeneration of severed central axons is supposed to depend on two factors: a permissive local environment and the particular intrinsic properties of axotomized neurones. To assess the role of each of these factors in axonal regeneration, the capability of two particular axon populations of the adult mouse cerebellum to grow into target-specific (cerebellum) and target-unspecific (neocortex) embryonic grafts was determined. Purkinje cell and inferior olivary axons were transected by passing a microscalpel through the axial white matter of the cerebellar folia, particularly those of the anterior lobe. Immediately after the injury, solid transplants were placed in the lesion cavity. Purkinje cell axons were labelled by using anticalbindin immunocytochemistry, and olivocerebellar fibres were visualized by biotinylated dextran amine anterograde axonal tracing. Following axotomy, Purkinje cell axons appeared as thickened processes ending with large terminal clubs. Their morphology and number did not change up to the longest survival time considered (2 months), thereby confirming previous demonstrations that Purkinje cells survive axon injury (I. Dusart and C. Sotelo, 1994, J. Comp. Neurol. 347:211-232). Inferior olivary axons were thinner and bore smaller terminal bulbs. When embryonic cerebellar grafts, containing cortical and deep nuclear precursors, were placed close to the injured axons, olivocerebellar fibres vigorously regenerated into the transplants and ended in new climbing fibres along the dendrites of grafted Purkinje cells. By contrast, host Purkinje cell axons never showed any outgrowth towards the graft. Similarly, these axons failed to regenerate into grafts containing solely the rostromedial portion of the cerebellar anlage, mostly consisting of deep nuclear neurones, their main targets. Comparable results were obtained by transplanting embryonic neocortical tissue: inferior olivary axons also regenerated into the grafts, although with distinct terminal arbours without the climbing fibre phenotype, whereas Purkinje cell axons always failed to grow. These results provide the first direct demonstration that severed inferior olivary axons are able to regenerate. In addition, they show that the growth-permissive/-promoting conditions created by embryonic nervous tissue are not sufficient to induce the regeneration of every axonal type and allow us to hypothesise that successful regeneration depends on the interplay between environmental cues and intrinsic properties of the axotomized neurones.

Target neuron controls the integrity of afferent axon phenotype: a study on the Purkinje cell-climbing fiber system in cerebellar mutant mice

The effects of target loss on adult axonal arbors were investigated by comparing the morphological changes of adult climbing fibers in several mutant mouse strains where Purkinje cells slowly degenerate (namely, Lurcher, nervous, Purkinje cell degeneration, and tambaleante), with those occurring after a fast Purkinje cell death induced by mechanical lesions of the adult mouse cerebellum. In each of the different mutations, Purkinje cells displayed distinctive structural modifications. However, a set of regressive changes common to all strains could be disclosed, mostly dendritic atrophy and a progressive axonal retraction with the hypertrophy of recurrent collaterals. Climbing fibers that contacted such degenerating neurons also showed abnormal morphological features, consisting in the presence of extensive perisomatic plexuses, whereas peridendritic branches were atrophic or absent. In Lurcher mice, target-deprived climbing fibers were strictly confined around the granular-molecular layer interface and never penetrated into the molecular layer. Similar terminal plexuses at the level of the former Purkinje cell layer were observed in the other mutants. However, in the latter cases, atrophic terminal arbors were also present in the molecular layer, being confined to the deep portions in nervous, while spanning its whole extent in Purkinje cell degeneration and tambaleante mice. Following mechanical lesions, atrophic target-deprived climbing fibers were exclusively located in the molecular layer. In addition, some of the Purkinje cells that survived after the injury displayed regressive modifications similar to those observed in mutant mice, and their climbing fibers were characterized by perisomatic plexuses. These results show that the normal relationship between the climbing fiber and its Purkinje cell is already disrupted during the slow degeneration of the target neuron. As a consequence, the phenotypic pattern of target-deprived climbing fibers reflects the preceding interactions with their postsynaptic neurons and it is determined by the onset time and progression rate of Purkinje cell degeneration.

Initial tract formation in the brain of the chick embryo: selective expression of the BEN/SC1/DM-GRASP cell adhesion molecule

This study reports the spatio-temporal pattern of BEN expression (a molecule of the immunoglobulin superfamily) during early stages of the first axonal tract formation, in the fore- and midbrain of chick embryos [Hamburger and Hamilton (HH) stages 12-22]. The expression of BEN has been analysed using immunohistochemistry and non-radioactive in situ hybridization. Furthermore, double labelling experiments (combining anti-class III beta-tubulin, a pan-neuronal marker, and anti-BEN antibodies) have been carried out to determine whether BEN is expressed by all first axonal tracts. The first neurons expressing BEN appear around stage HH13-14, in the caudal diencephalon. They belong to the interstitial nucleus of Cajal, and their axons are the first components of the medial longitudinal fasciculus. By HH14, two other early axonal tracts appear: the tract of the postoptic commissure and the descending root of the mesencephalic nucleus of the trigeminal nerve. Only the latter expresses BEN. At later stages of development numerous new axonal tracts appear in the telencephalic, diencephalic and mesencephalic domains. Only a few of them (the fourth nerve, the lemniscus lateralis, the tectobulbar and habenulopeduncular tracts) express BEN. In all BEN positive systems, the cell bodies, axons and growth cones are uniformly labelled by the antibody. We have found that none of the early axonal tracts grows preferentially at interneuromeric boundaries. Moreover, each tract is formed by several thin fascicles rather than a single one. The expression of BEN is transient and disappears shortly before hatching. These results suggest that BEN may serve to promote axonal outgrowth of precise neuronal systems involved in 'axonal scaffolding'.

Parasagittal compartmentation of adult rat Purkinje cells expressing the low-affinity nerve growth factor receptor: changes of pattern expression after a traumatic lesion

The pattern of expression of p75, the low affinity nerve growth factor receptor, in the adult rat cerebellum and its fate after a traumatic lesion were analysed using immunohistochemical localization of this receptor. A subset of Purkinje cells was immunoreactive for low affinity nerve growth factor receptor in the intact adult cerebellum. These cells were arranged in alternating positive and negative parasagittal compartments along the cerebellar cortex. This pattern of expression had 90% homology with zebrin I. After a traumatic lesion, the specific pattern of expression of zebrin I remained unchanged, whereas the low affinity nerve growth factor receptor pattern changed as early as one day: Purkinje cells near the lesion site, independent of zebrin I staining, became immunoreactive. During the first week, the increase in immunoreactivity remained high. Thereafter, there was a short, fast decrease followed by a long period in which a faint immunostaining on lesioned Purkinje cells is maintained for up to one year. The increase in the expression of the low affinity nerve growth factor receptor by all traumatically affected Purkinje cells suggests a correlation between this specific up-regulation and the high resistance of these neurons to axotomy or other traumatic injuries.

Lack of Purkinje cell loss in adult rat cerebellum following protracted axotomy: degenerative changes and regenerative attempts of the severed axons

The cerebellar Purkinje cells, due to their geometrical disposition and their high calbindin content, offer an optimal system in which to test the adequacy of current opinions on axotomy effects. We have, therefore, analyzed with calbindin immunostaining the morphological changes of Purkinje cells from 1 day to 6 months after axonal section in the cerebellar white matter. This method allows us to study the morphological changes in their dendrites, cell bodies, and axons. We have also searched for simultaneous changes in glial cells and vascularization by using cell type-specific markers. In addition, an ultrastructural study of Purkinje cells, 7 days after large electrolytic lesions affecting the white matter and the overlying granular layer, was carried out to determine whether amputation of the recurrent collateral system provokes a fast neuronal death. Neither the Purkinje cells axotomized close to their cell bodies (electrolytic lesions) nor those axotomized in the white matter (cerebellar transection) degenerated. Thus, this study demonstrates that Purkinje cells are extremely resistant to axotomy; those severed in the white matter at distances varying from 100 microns to 3 mm remain alive for as long as 6 months. At all survival times studied, axotomized Purkinje cells exhibited few changes in their somata and dendrites, as well as in their glial microenvironment. The major changes occurred in the axonal compartment. Axonal alterations, namely the presence of torpedoes and hypertrophy of the recurrent collateral system, were early events already noticeable 24 hours after the lesion, although they later differed in their time course and spatial distribution. It is remarkable that the distal segments of the central stumps of the cut axons survived in large numbers without any apparent retraction, with their terminal varicosities apposed to the wall of the wound cavity even 6 months after the lesion. Nevertheless, these segments were thinner than normal Purkinje cell axons (axonal atrophy). Despite this apparent immutability, some regenerative attempts did occur in the severed axons, such as axonal sprouts penetrating the deeper region of the granular layer in zones close to the lesion, presence of arciform axons, and hypertrophy of the recurrent collateral system. However, the Purkinje cell axons did not regenerate, and these neurons remained separated from their targets by a cavity in virtually all cases.

The pontocerebellar projection: longitudinal zonal distribution of fibers from discrete regions of the pontine nuclei to vermal and parafloccular cortices in the rat

A longitudinal parasagittal organization (alternating labeled and unlabeled stripes) of mossy fiber terminals in the paraflocculus and in the vermal lobule VII of the cerebellum was found after small injections (less than 50 nl) of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into discrete regions of the basilar pontine nuclei (BPN) of rats. Up to three stripes were found within the paraflocculus of both sides, following injections (of about 500 microns in diameter) in either the medial or lateral region of the caudal half of the BPN. Up to five stripes were found in the vermal lobule VII after similar size injections into the rostro-ventral region of the BPN. These results emphasize the possibility that the parasagittal zonal arrangement could be a common pattern of organization shared by climbing and mossy fiber afferents.

Molecular plasticity of adult Bergmann fibers is associated with radial migration of grafted Purkinje cells

Embryonic Purkinje cells (PCs) from cerebellar primordia grafted in adult pcd mutant cerebellum replace missing PCs of the host, and become synaptically integrated into the defective cerebellar circuit. This process of neuronal replacement starts with the invasion of grafted PCs into the host cerebellum, and their radial migration through its molecular layer. The present study is aimed at determining whether the glial axes for this migration are embryonic radial glial cells that comigrate with the grafted PCs, or adult Bergmann fibers of the host, transiently reexpressing the molecular cues needed for their guidance of the migration. Transplants from a transgenic mouse line (Krox-20/lacZ14) in which Bergmann fibers could be identified by lacZ expression reveal that, despite the presence of X-gal-stained Bergmann fibers in the graft remnants and of grafted PCs in the host molecular layer, all Bergmann fibers in the host cerebellum lack of beta-galactosidase activity. Thus, these migratory axes belong to the host, not to the donor. Transplants from normal isogenic mouse embryos show that during the radial migration of grafted PCs (7 d after grafting) the involved host Bergmann fibers reexpress nestin (identified with monoclonal antibody Rat-401 immunostaining), normally expressed only by immature Bergmann fibers. Five days later, when grafted PCs have arrested their migration, host Bergmann fibers again become Rat-401 negative. These results indicate that embryonic PCs can trigger in adult cerebellum the molecular changes necessary for their own migration and ultimate synaptic integration in the host cortical circuitry.

The 'creeper stage' in cerebellar climbing fiber synaptogenesis precedes the 'pericellular nest'--ultrastructural evidence with parvalbumin immunocytochemistry

In perinatal rats, neurons in the dorsal cap of the inferior olivary complex transiently express parvalbumin-immunoreactivity (PA-IR). Their terminal axonic fields, particularly in the flocculonodular lobe, appear as precise bands of fine and convoluted immunostained fibers extending over the Purkinje plate and the nascent molecular layer. Thicker PA-IR fibers, corresponding to vestibular fibers, are observed only under the Purkinje plate. Electron microscopic analysis of the PA-IR climbing fibers within the bands allowed us to study their synaptogenesis with Purkinje cells. At birth (P0), thin PA-IR climbing fiber axons creep over these immature neurons following the contours of their perikarya and dendrites. They establish a few synaptic contacts, some of them of mature appearance, upon the smooth surface of Purkinje cell apical dendrites. PA-IR axonal growth cones are observed in the upper portion of the molecular layer. This precocious stage of climbing fiber/Purkinje cell synaptogenesis has been named here the 'creeper' stage. After the regression of the Purkinje cell apical dendrites (by P5), PA-IR climbing fibers are perisomatically located and synapse on Purkinje cell somatic protrusions, forming the classical pericellular 'nests'. The presence of mature synapses at P0 indicates the precocity of climbing fiber/Purkinje cell synaptogenesis and suggests its fetal onset. Therefore, this process of synaptogenesis occurs in two steps: (i) an early transient one, simultaneous with the initiation of the formation of the olivocerebellar map, that could be involved in the maintenance of the nascent topography of the projection and (ii) a latter step which concerns the refinement of the projection within a given PA-IR band through the regression of multiple innervation of Purkinje cells by climbing fibers, step which brings the required synaptic specificity to the adult olivocerebellar system.

The dorsal cochlear nucleus of the adult lurcher mouse is specifically invaded by embryonic grafted Purkinje cells

The fate of embryonic Purkinje cells grafted over the brainstem surface of the adult Lurcher mouse was analyzed using anti-calbindin (CaBP) immunocytochemistry. Purkinje cells are able to migrate specifically into the molecular layer of the host dorsal cochlear nucleus (DCoN) and develop dendritic trees that are practically isoplanar, suggesting synaptic interactions with the parallel fibres of the DCoN. These results provide a new argument in favour of the homology between the cerebellum and the DCoN.

Chick/quail chimeras with partial cerebellar grafts: an analysis of the origin and migration of cerebellar cells

Chick/quail chimeras with partial cerebellar grafts have been performed to obtain further information about the origin and migratory movements of cerebellar cortical neurons. The grafts were performed by exchanging between these two species a precise, small portion of the E2 cerebellar primordium, as defined in Martinez and Alvarado-Mallart (Eur. J. Neurosci. 1:549-560, 1989). All grafts were done unilaterally. The chimeric cerebella, fixed at various developmental stages, were analyzed in serial Feulgen-stained preparations to map the distribution of donor and host cells in the ependymal layer (considered to be reminiscent of the primary germinative neuroepithelium) and in the various cortical layers. In some of the oldest cases, we also used antiquail immunostaining to recognize quail cells. In the ependymal layer, it has been possible to conclude that each hemicerebellar primordium undergoes a morphogenetic rotation that changes its rostrocaudal axis to a rostromedio-caudolateral direction. However, important individual variations were observed among the chimeric embryos with respect to the ependymal area expected to be formed by donor cells. These variations cannot be explained solely on the basis of microsurgical procedure; however, they suggest the existence of important reciprocal interaction between host and grafted neuroepithelia. Therefore, it was not possible to draw a precise fate map of the E2 cerebellar primordium. Nevertheless, the dispersion of grafted cells in the cerebellar cortex, when compared to the real extent of the ependymal grafted area in each particular case, provided important data: (1) The external granular layer (EGL), the secondary germinative epithelium, seems not to originate exclusively from the "germinative trigone," as is usually considered the case. It emerges from a larger but restricted portion of the primary cerebellar matrix extending about the caudal fourth or third of the ventricular epithelium, as defined after its morphogenetic rotation. (2) The Purkinje cells (PCs) develop from all areas of the cerebellar epithelium. Although the distribution of donor PCs parallels the grafted ventricular layer mediolaterally, donor PCs extend more in the rostrocaudal dimension. The PC layer is formed mainly by donor cells in the lobules underlain by the grafted ependymal layer. However, donor PCs are also observed in cortical lobules surmounting the host ventricular layer. In these lobules, the donor PCs form clusters of various widths interrupting the host PCs. Reciprocally, clusters of host PCs are also found in the lobules formed mainly by donor PCs. The alternate small clusters of donor or host PCs are surrounded by Bergmann fibers of the other species' origin.(ABSTRACT TRUNCATED AT 400 WORDS)

Partial reconstruction of the adult Lurcher cerebellar circuitry by neural grafting

Solid cerebellar grafts, taken from normal mouse embryos (gestational day 12-14), were transplanted into the cerebellum of adult Lurcher mice. The degree of Purkinje cell replacement was analysed one to three months after transplantation by means of immunocytochemistry (antibodies against calbindin, cGMP-dependent protein kinase and neurofilament proteins) and electron microscopy. Grafted Purkinje cells succeed in moving out of the graft and migrate into the host cerebellar cortex. They are present next to the graft in the granule cell and molecular layers, and far from the graft remnant, only in the molecular layer, indicating that, although both layers subserve Purkinje cell migration, the molecular layer is the ultimate target. In the host molecular layer, axons of transplanted Purkinje cells form thick bundles running in the frontal plane over long distances. Most of them terminate in the upper granule cell layer by enlarged bulbs resembling collapsed growth cones. Axons reaching their normal targets (the neurons of the deep cerebellar nuclei) are observed only in cases where the granule cell layer is disrupted and/or grafted Purkinje cells remain in the white matter. The projection is massive only from grafts lying in the close vicinity of the target neurons. Electron-microscopic analysis of grafted Purkinje cells populating the host cerebellar cortex reveals that their synaptic investment is abnormal. In the molecular layer, where the normal inputs are reduced, the compartmentation in proximal and distal dendritic segments is severely affected, climbing fibre synapses only form on a minority of grafted cells and "pinceau" formations are absent. In the granule cell layer, the synaptic investment is similar to that of Purkinje cells in agranular cerebellum, and even heterelogous synapses with mossy fibres have been observed. These results, compared to those previously obtained with grafting experiments in Purkinje cell degeneration mutant mouse, allow us to conclude that: (i) the Purkinje cell-deficient molecular layer of the host, despite its severe atrophy and reactive gliosis, still exerts a positive neurotropism specific for grafted Purkinje cells; (ii) the unlesioned host granule cell layer underlying the molecular layer containing grafted Purkinje cells, even if almost depleted of granule cells, remains an obstacle for the re-establishment of a corticonuclear projection; and (iii) the degree of synaptic integration of grafted Purkinje cells is directly related to the nearby presence of available host axon terminals. Hence, owing to the atrophy of the Lurcher cerebellum, the postgrafting restoration of the cerebellar cortical circuit is much less complete in this mutant.

Cell interactions underlying Purkinje cell replacement by neural grafting in the pcd mutant cerebellum

The results obtained with neuronal grafting in an animal model of heredo-degenerative ataxia (the pcd mutant mouse) have been extremely useful to unmask new aspects of neural plasticity. The grafted embryonic Purkinje cells invade the deficient molecular layer of the host by migrating radially through adult Bergmann fibers. There, they start building their dendritic trees and, by promoting the axonal sprouting of specific adult neuronal population in a timed sequence, they receive appropriate synaptic contacts, starting ten days after grafting. Twenty-one days after grafting, the grafted Purkinje cells have acquired their adult dendritic pattern and synaptic investment. Both the detailed timetable and the nature of the cellular interactions between embryonic and adult neural cells are remarkably similar to those occurring during normal development. These results raise the possibility that embryonic Purkinje cells can induce in adult neural cells a new type of plasticity, that of recreating a permissive microenvironment for the synaptic integration of the grafted neurons, leading to the anatomical restoration of the cortical circuit of the mutant cerebellum.