Bergmann glia require continuous association with Purkinje cells for normal phenotype expression

Bergmann Glia, Long-Term Depression, and Autism Spectrum Disorder

Bergmann glia (BG), a specific type of radial astrocytes in the cerebellum, play a variety of vital functions in the development of this structure. However, the possible role of BG in the development of abnormalities observed in individuals with autism spectrum disorder (ASD) seems to be underestimated. One of the most consistent findings observed in ASD patients is loss of Purkinje cells (PCs). Such a defect may be caused by dysregulation of glutamate homeostasis, which is maintained mainly by BG. Moreover, these glial cells are involved in long-term depression (LTD), a form of plasticity which can additionally subserve neuroprotective functions. The aim of presented review is to summarize the current knowledge about interactions which occur between PC and BG, with special emphasis on those which are relevant to the survival and proper functioning of cerebellar neurons.

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.

Down-regulation of astroglial proteins in the rat cerebellum after portacaval anastomosis

The effect of short-term portacaval anastomosis (PCA) on the expression of specific astroglial markers [glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS)] in the rat cerebellum was examined to determine the influences of PCA on astroglial cells. The results suggest that PCA directly interferes with astroglial cytoskeleton, as indicated by the irregular distribution and reduced expression of GFAP observed after 1 month. PCA also decreased GS immunoreactivity in the Bergmann glial processes of the molecular layer, as well as in astrocytes of the granule cell layer. It might also modulate glutamatergic nervous activity as GS expression was reduced in 1 month post-PCA brains. Moreover, the GFAP and GS levels in PCA-exposed rats were lower than in control rats. This might contribute to the appearance of encephalopathy by increasing extracellular glutamate and/or ammonia concentrations. These results show that short-term PCA interferes with astroglial protein expression, with both GFAP and GS levels falling in astroglial cells.

Dual control of neurogenesis by PC3 through cell cycle inhibition and induction of Math1

Growing evidence indicates that cell cycle arrest and neurogenesis are highly coordinated and interactive processes, governed by cell cycle genes and neural transcription factors. The gene PC3 (Tis21/BTG2) is expressed in the neuroblast throughout the neural tube and inhibits cell cycle progression at the G1 checkpoint by repressing cyclin D1 transcription. We generated inducible mouse models in which the expression of PC3 was upregulated in neuronal precursors of the neural tube and of the cerebellum. These mice exhibited a marked increase in the production of postmitotic neurons and impairment of cerebellar development. Cerebellar granule precursors of PC3 transgenic mice displayed inhibition of cyclin D1 expression and a strong increase in the expression of Math1, a transcription factor required for their differentiation. Furthermore, PC3, encoded by a recombinant adenovirus, also induced Math1 in postmitotic granule cells in vitro and stimulated the Math1 promoter activity. In contrast, PC3 expression was unaffected in the cerebellar primordium of Math1 null mice, suggesting that PC3 acts upstream to Math1. As a whole, our data suggest that cell cycle exit of cerebellar granule cell precursors and the onset of cerebellar neurogenesis are coordinated by PC3 through transcriptional control of cyclin D1 and Math1, respectively.

Cytodifferentiation of Bergmann glia and its relationship with Purkinje cells

The Bergmann glia is composed of unipolar protoplasmic astrocytes in the cerebellar cortex. Bergmann glial cells locate their cell bodies around Purkinje cells, and extend radial or Bergmann fibers enwrapping synapses on Purkinje cell dendrites. During development, Bergmann fibers display a tight association with migrating granule cells, from which the concept of glia-guided neuronal migration has been proposed. Thus, it is widely known that the Bergmann glia is associated with granule cells in the developing cerebellum and with Purkinje cells in the adult cerebellum. As the information on how Bergmann glial cells are related structurally and functionally with differentiating Purkinje cells is quite fragmental, this issue has been investigated using cytochemical techniques for Bergmann glial cells. This review classifies the cytodifferentiation of Bergmann glial cells into four stages, that is, radial glia, migration, transformation and protoplasmic astrocytes, and then summarizes their structural relationship with Purkinje cells at each stage. The results conclude that the cytodifferentiation of Bergmann glial cells proceeds in correlation with the migration, dendritogenesis, synaptogenesis and maturation of Purkinje cells. Furthermore, morphological and molecular plasticity of this neuroglia appears to be regulated depending on the cytodifferentiation of nearby Purkinje cells. The functional relevance of this intimate neuron-glial relationship is also discussed with reference to recent studies in cell biology, cell ablation and gene knockout.

Growth arrest specific gene 1 is a positive growth regulator for the cerebellum

Postnatal cerebellum development involves the generation of granule cells and Bergmann glias (BGs). The granule cell precursors are located in the external germinal layer (EGL) and the BG precursors are located in the Purkinje layer (PL). BGs extend their glial fibers into the EGL and facilitate granule cells' inward migration to their final location. Growth arrest specific gene 1 (Gas1) has been implicated in inhibiting cell-cycle progression in cell culture studies (G. Del Sal et al., 1992, Cell 70, 595--607). However, its growth regulatory function in the CNS has not been described. To investigate its role in cerebellar growth, we analyzed the Gas1 mutant mice. At birth, wild-type and mutant mice have cerebella of similar size; however, mature mutant cerebella are less than half the size of wild-type cerebella. Molecular and cellular examinations indicate that Gas1 mutant cerebella have a reduced number of granule cells and BG fibers. We provide direct evidence that Gas1 is required for normal levels of proliferation in the EGL and the PL, but not for their differentiation. Furthermore, we show that Gas1 is specifically and coordinately expressed in both the EGL and the BGs postnatally. These results support Gas1 as a common genetic component in coordinating EGL cell and BG cell proliferation, a link which has not been previously appreciated.

Cerebellar alterations induced by chronic hypoxia: an immunohistochemical study using a chick embryonic model

A model of fetal aerogenic hypoxia was developed in which fertilized chicken eggs were half-painted with melted wax and incubated under normal conditions. The cerebellum of the hypoxic chick embryos at a later stage of development (E18-20) was analyzed immunochemically. Hypoxic insult resulted in considerable neurocytological deficits of the Purkinje cells and altered glial fibrillary acid protein (GFAP) immunoreactivity in the fetal cerebellum. Purkinje cells in the hypoxic embryos were marked by small cell size, poorly developed dendrites, low cell density, deletion and ectopia. On the other hand, enhanced GFAP immunoreactivity was found in astrocytes and Bergmann glia of the hypoxic embryos. Our results indicate that chronic hypoxia in the chick fetus can cause severe disorders of neuronal development as well as glial activation. We suggest that our hypoxic model of chick embryos could be an accessible animal model for further elucidating fetal hypoxia.

Down-regulated expression of glutamate transporter GLAST in Purkinje cell-associated astrocytes of reeler and weaver mutant cerebella

The glutamate transporter plays an important role in rapid removal of glutamate from the synaptic cleft. Glutamate transporter GLAST is highly expressed in the Bergmann glia (BG), a unipolar cerebellar astrocyte associated structurally and functionally with Purkinje cells (PCs). Here we investigated the expression and localization in the reeler and weaver mutant cerebella with disorganized cytoarchitecture and disrupted synaptic circuitry. In the cortex of both cerebella, GLAST-expressing cells were astrocytes associating PCs; they were located around PC somata and primary dendrites, and extended glial fibrillary acidic protein (GFAP)-immunopositive processes surrounding PC somata and dendrites. Additional signals were detected in astrocytes of the reeler subcortex; they were dispersed among ectopic PCs and had GFAP-positive processes apposing to PC somata and stunted dendrites. Therefore, GLAST expression in PC-associated astrocytes was conserved in these mutants. Compared to the wild-type BG, however, the transcription level in individual mutant astrocytes was significantly reduced to about one-third level in the reeler and weaver cortex or one-sixth level in the reeler subcortex. Taking previous results on remarkable up-regulation during dendritogenic/synaptogenic stages and down-regulation following experimental glutamatergic denervation, it is suggested that GLAST expression in cerebellar astrocytes is regulated correlatively with cytological and/or synaptic differentiation of neighboring PCs.

Sonic hedgehog regulates the growth and patterning of the cerebellum

The molecular bases of brain development and CNS malignancies remain poorly understood. Here we show that Sonic hedgehog (Shh) signaling controls the development of the cerebellum at multiple levels. SHH is produced by Purkinje neurons, it is required for the proliferation of granule neuron precursors and it induces the differentiation of Bergmann glia. Blocking SHH function in vivo results in deficient granule neuron and Bergmann glia differentiation as well as in abnormal Purkinje neuron development. Thus, our findings provide a molecular model for the growth and patterning of the cerebellum by SHH through the coordination of the development of cortical cerebellar cell types. In addition, they provide a cellular context for medulloblastomas, childhood cancers of the cerebellum.

Transplanted neurons alter the course of neurodegenerative disease in Lurcher mutant mice

Embryonic cerebellar, neocortical, and striatal tissues derived from NSE-LacZ transgenic mice were transplanted into the right cerebellar hemisphere of 8- to 10-day-old Lurcher or wild-type mice. Host mice survived for 30-90 days and the transplanted tissue was examined by light microscopy using Nissl staining, X-gal histochemistry, and immunohistochemistry for calcium binding protein and glutamic acid decarboxylase. Transplantation of cerebellar tissue, but not neocortical or striatal progenitors, resulted in robust infiltration of the lurcher mutant host cerebellar cortex by transgenic Purkinje neurons. Deep to the infiltrated molecular layer, the host granular layer was thicker and denser than the mutant granular layer, but transgenic cells did not contribute to the spared granular layer. The host inferior olivary complex consistently exhibited a noticeable bilateral asymmetry in Nissl-stained sections. A quantitative analysis of the olivary complex was performed in 10 90-day-old host mice. The results indicate that the left inferior olivary complex of 90-day-old host mice contained more neurons than the right inferior olive of the host mice and contained more neurons than was observed in 90-day-old Lurcher control mice. Analysis by olivary subdivision indicates that increased neuron numbers were present in all subdivisions of the host left inferior olive. These studies confirm the specific attractive effect of the mutant cerebellar cortex on transplanted Purkinje neuron progenitors and indicate that neural transplants may survive the neurodegenerative period to interact with developing host neural systems. The unilateral rescue of Lurcher inferior olivary neurons in cerebellar transplant hosts indicates that transplanted neurons may interact with diseased host neural circuits to reduce transneuronal degeneration in the course of a neurodegenerative disease.

Neonatal noradrenaline depletion prevents the transition of Bergmann glia in the developing cerebellum

The influence of neonatal administration of 6-hydroxydopamine (6-OHDA) on the cell proliferation in cerebellum was studied using 10-30 days-old rats. Compared to their littermates, treated rats had poor ability in searching, skills performance and orienting in the new environment. Elimination of noradrenergic terminals by 6-OHDA led to a delay in granular cell migration. The secondary foliation in neo-cerebellum was absent. The Bergmann glial cells were abnormally located, structurally different and did not form the intimate association with Purkinje cells. Our findings indicate that without noradrenergic influence neurones and glial cells do not proliferate normally and noradrenaline may act as an important trophic factor also for Bergmann glial cells.

Localization of transferrin binding protein in relation to iron, ferritin, and transferrin receptors in the chicken cerebellum

We have demonstrated that transferrin binding protein (TfBP), ferritin, and iron, are specifically localized in Bergmann glia, while the transferrin receptor is confined to Purkinje cells in the chicken cerebellum. The results of this study suggest that Bergmann glia have previously undescribed functions related to iron regulation such as sequestration of iron and the maintenance of iron homeostasis in the cerebellum.

Developmental analysis of GFAP immunoreactivity in the cerebellum of the meander tail mutant mouse

It is thought that Bergmann glial fibers assist in the inward migration of granule cells. Model systems in which there is a perturbation of either the migrating cells or the glial cell population have been useful in understanding the migratory process. In the meander tail mutant mouse, the anterior cerebellar region is agranular, whereas the posterior cerebellum is relatively unaffected by the mutation. This study presents a qualitative analysis of the development of cerebellar radial glia in mea/mea and +/mea mice aged from postnatal day 0 to adult, using an antibody against the glia specific antigen, glial fibrillary acidic protein. The results indicate a slight delay in the onset of immunoreactivity in the mea/mea cerebellum and abnormal glial formation in the anterior and posterior regions by postnatal day 5. At postnatal day 11, the full complement of labeled fibers appears to be present and although they appear abnormal in formation, they eventually reach the surface and terminate in oddly shaped and irregularly spaced endfeet. In adult mea/mea and +/mea mice, as compared to the early postnatal stages, there is a significant reduction in GFAP immunoreactive fibers. Cresyl violet stained adult mea/mea sections revealed the presence of ectopic granule cells in radial columns and small clumps at the surface of and within the molecular layer of the caudal cerebellum. Quantitative analyses revealed a 4- to 5-fold increase in the number of ectopic granule cells in lobule VIII of the mea/mea when compared with the +/mea cerebellum. These results suggest that the radial glia in the mea/mea cerebellum exhibit some uncharacteristic morphologies, but that these abnormalities are most likely the consequence of environmental alterations produced by the mutant gene.

5'-nucleotidase activity as a synaptic marker of parasagittal compartmentation in the mouse cerebellum

In the molecular layer of the mouse cerebellum, the histochemical activity of the adenosine-producing ectoenzyme 5'-nucleotidase discloses a parasagittal pattern of alternating enzyme-rich and enzyme-poor bands. In the rat, 5'-nucleotidase activity transiently labels cerebellar synapses during postnatal development and shifts later on towards an exclusive glial location in the molecular layer. We therefore asked whether different ultrastructural expression of 5'-nucleotidase would account for the light microscopic pattern seen in the adult mouse cerebellum. Using an enzyme cytochemical method, we localized 5'-nucleotidase activity on the glial cells and at the main types of asymmetrical synapses in the developing and mature cerebellum of the mouse. The percentage of labelled synapses increased until adulthood within the 5'-nucleotidase-positive bands. Here, the vast majority (86%) of the synapses were labelled against only 27% within the negative bands in the adult. Thus, 5'-nucleotidase appears as a marker of glia and of Purkinje cell synapses across cerebellar compartments. Changes in purinergic neuromodulation and/or cell adhesion mediated by 5'-nucleotidase across bands might participate in the functional differentiation of the cerebellar parasagittal subsets.