Neuronal regulation of astroglial morphology and proliferation in vitro.

To analyze the interdependence of neurons and astroglia during central nervous system development, a rapid method for purifying early postnatal cerebellar neurons and astroglia, and recombining them in vitro, has been developed. The influence of neurons on astroglial shape and proliferation has been evaluated with an in vitro model system previously used to describe the role of cerebellar astroglia in neuronal migration and positioning (Hatten, M. E., and R. K. H. Liem, 1981, J. Cell Biol., 90:622-630; and Hatten, M. E., R. K. H. Liem, and C. A. Mason, 1984, J. Cell Biol., 98:193-204. Cerebellar tissue harvested from C57Bl/6J mouse cerebellum on the third or fourth day postnatal was dissociated into a single cell suspension with trypsin, and enriched glial and neuronal fractions were separated with a step gradient of Percoll. Highly purified astroglial and neuronal fractions resulted from subsequently preplanting the cells on a polylysine-coated culture surface. In the absence of neurons, astroglia, identified by staining with antisera raised against purified glial filament protein, assumed a flattened shape and proliferated rapidly. In the absence of astroglia, cerebellar neurons, identified by staining with antisera raised against the nerve growth factor-inducible large external (NILE) glycoprotein and by electron microscopy, formed cellular reaggregates, had markedly impaired neurite outgrowth, and survived poorly. When purified neurons and isolated astroglia were recombined, astroglial proliferation slowed markedly and the flattened shape expressed in the absence of neurons transformed into highly elongated profiles that resembled embryonic forms of cerebellar astroglia. After longer periods (48-72 h) in the presence of neurons, astroglia had "Bergmann-like" or "astrocyte-like" shapes and neurons commonly associated with them. These results suggest that neurons influence the differentiation of astroglia.

Neuronal inhibition of astroglial cell proliferation is membrane mediated.

Previously we have used a microwell tissue culture assay to show that early postnatal mouse cerebellar astroglia have a flattened morphology and proliferate rapidly when they are cultured in the absence of neurons, but develop specific cell-cell contacts and undergo morphological differentiation when they are co-cultured with purified granule neurons (Hatten, M. E., 1985, J. Cell Biol., 100:384-396). In these studies of cell binding between neurons and astroglia, measurement with light and fluorescence microscopy or with [35S]methionine-labeled cells indicated that the kinetics of the binding of the neurons to astroglial cells are rapid, occurring within 10 min of the addition of the neurons to the growing glia. 6 h after neuronal attachment, astroglial DNA synthesis decreases, as shown by a two- to fivefold decrease in [3H]thymidine incorporation, and glial growth ceases. No effects on astroglial cell growth were seen after adding medium conditioned by purified cerebellar neurons cultured in the absence of astroglia, by astroglia cultured in the absence of neurons, or by a mixed population of cerebellar cells. This result was unchanged when any of these media were concentrated up to 50-fold, or when neurons and astroglia were cultured in separate chambers with confluent medium. Two groups of experiments suggest that membrane-membrane interactions between granule neurons and astroglia control astroglial cell growth. First, neurons fixed with dilute amounts of paraformaldehyde (0.5%) bound to the astroglia with the same kinetics as did living cells, inhibited DNA synthesis, and arrested glial growth within hours. Second, a cell membrane preparation of highly purified granule neurons also bound rapidly to the glia, decreased [3H]thymidine incorporation two- to fivefold and inhibited astroglial cell growth. The rate of the decrease in glial growth depended on the concentration of the granule neural membrane preparation added. A similar membrane preparation from purified cerebellar astroglial cells, PC12 cells, 3T3 mouse fibroblasts, or PTK rat epithelial cells did not decrease astroglial cell growth rates. Living neurons were the only preparation that both inhibited glial DNA synthesis and induced the astroglial cells to transform from the flat, epithelial shapes they have when they are cultured without neurons to highly differentiated forms that resemble Bergmann glia or astrocytes seen in vivo. These results suggest that membrane-membrane interactions between neurons and astroglia inhibit astroglial proliferation in vitro, and raise the possibility that membrane elements involved in glial growth regulation include neuron-glial interaction molecules.

Cell assembly patterns of embryonic mouse cerebellar cells on carbohydrate-derivatized polylysine culture substrata.

Four carbohydrate derivatives of poly-D-lysine have been synthesized and assayed as substrates for the tissue culture of embryonic mouse cerebellar cells. On poly-beta-(D-glucopyranosyl)-poly-D-lysine and on poly-beta-(N-acetyl-D-glucosaminyl)-poly-D-lysine, dissociated cerebellar cells formed a monolayer. On poly-beta-(D-galactopyranosyl)-poly-D-lysine, cellular aggregates were formed and cables of processes were extended between the aggregates. On poly-beta-(L-fucosyl)-poly-D-lysine, cerebellar cells failed to attach and died within 24 h. On poly-(N-acetyl)-poly-D-lysine, cell attachment was identical to that on poly-D-lysine. At low concentrations of underivatized poly-D-lysine (0.5-2.0 microgram/ml) dissociated embryonic cerebellar cells formed cellular aggregates, whereas at higher concentrations of poly-D-lysine monolayering was extensive.

Targeted deletion of the PEX2 peroxisome assembly gene in mice provides a model for Zellweger syndrome, a human neuronal migration disorder.

Zellweger syndrome is a peroxisomal biogenesis disorder that results in abnormal neuronal migration in the central nervous system and severe neurologic dysfunction. The pathogenesis of the multiple severe anomalies associated with the disorders of peroxisome biogenesis remains unknown. To study the relationship between lack of peroxisomal function and organ dysfunction, the PEX2 peroxisome assembly gene (formerly peroxisome assembly factor-1) was disrupted by gene targeting. Homozygous PEX2-deficient mice survive in utero but die several hours after birth. The mutant animals do not feed and are hypoactive and markedly hypotonic. The PEX2-deficient mice lack normal peroxisomes but do assemble empty peroxisome membrane ghosts. They display abnormal peroxisomal biochemical parameters, including accumulations of very long chain fatty acids in plasma and deficient erythrocyte plasmalogens. Abnormal lipid storage is evident in the adrenal cortex, with characteristic lamellar-lipid inclusions. In the central nervous system of newborn mutant mice there is disordered lamination in the cerebral cortex and an increased cell density in the underlying white matter, indicating an abnormality of neuronal migration. These findings demonstrate that mice with a PEX2 gene deletion have a peroxisomal disorder and provide an important model to study the role of peroxisomal function in the pathogenesis of this human disease.

Astroglial cells provide a template for the positioning of developing cerebellar neurons in vitro.

Indirect immunocytochemical staining with antisera raised against purified glial filament protein and a neurofilament polypeptide was used to study cell interactions between astrocytes and neurons dissociated from embryonic and early postnatal cerebellum. Staining with antibodies raised against purified glial filament protein revealed that greater than 99% of all processes present in cerebellar cultures during the 1st wk in vitro were glial in origin. After 1 wk in culture, unstained processes that were presumably neuronal were observed. Stained astroglial processes formed a dense network that served as a template for cerebellar neurons, identified by indirect immunocytochemical localization of tetanus toxin. More than 90% of neurons from postnatal days 1 or 7 were positioned within one cell diameter of a glial process. In contrast, less than 40% of the neurons dissociated from early embryonic cerebellum were located adjacent to a glial process. Staining with antibodies raised against purified glial filament protein also revealed differences in astroglial morphology that were under developmental regulation. Astroglial cells from embryonic cerebellum were fewer in number and had thick, unbranched processes. Those from postnatal day 1 were more slender, branched, and stellate. Those from postnatal day 7 were highly branched and stellate. Some veil-like astroglial processes were also observed in cells from postnatal animals. These morphological changes were also observed when cells from embryonic day 13 were maintained for a week in vitro. No specific staining of embryonic or postnatal cerebellum cells was observed with antibodies raised against purified neurofilament polypeptides.

Two forms of cerebellar glial cells interact differently with neurons in vitro.

Specific interactions between neurons and glia dissociated from early postnatal mouse cerebellar tissue were studied in vitro by indirect immunocytochemical staining with antisera raised against purified glial filament protein, galactocerebroside, and the NILE glycoprotein. Two forms of cells were stained with antisera raised against purified glial filament protein. The first, characterized by a cell body 9 microns diam and processes 130-150 microns long, usually had two to three neurons associated with them and resembled Bergmann glia. The second had a slightly larger cell body with markedly shorter arms among which were nestled several dozen neuronal cells, and resembled astrocytes of the granular layer. Staining with monoclonal antisera raised against purified galactocerebroside revealed the presence of immature oligodendroglia in the cultures. These glial cells constituted approximately 2% of the total cell population in the cultures and, in contrast to astroglia, did not form specific contacts with neurons. Staining with two neuronal markers, antisera raised against purified NILE glycoprotein and tetanus toxin, revealed that most cells associated with presumed astroglia were small neurons (5-8 microns). After 1-2 d in culture, some stained neurons had very fine, short processes. Nearly all of the processes greater than 10-20 micron long were glial in origin. Electron microscopy also demonstrated the presence of two forms of astroglia in the cultures, each with a different organizing influence on cerebellar neurons. Most neurons associated with astroglia were granule neurons, although a few larger neurons sometimes associated with them. Time-lapse video microscopy revealed extensive cell migration (approximately 10 microns/h) along the arms of Bergmann-like astroglia. In contrast, cells did not migrate along the arms of astrocyte-like astroglia, but remained stationary at or near branch points. Growth cone activity, pulsating movements of cell perikarya, and ruffling of the membranes of glial and neuronal processes were also seen.

Astrotactin: a novel neuronal cell surface antigen that mediates neuron-astroglial interactions in cerebellar microcultures.

A microculture system for mouse cerebellar cells has been used to identify an immune activity, raised in rabbits against postnatal cerebellar cells, that blocks neuron-glial interactions in vitro. In the presence of blocking antibodies, stable neuron-glial contacts did not form and neuronal induction of glial process outgrowth did not occur. Subsequently, neurons were randomly arranged in the cultures rather than organized along the arms of astroglia. We have named the immune activity that blocks neuron-astroglial interactions anti-astrotactin. Partial purification of the anti-astrotactin blocking antibodies was obtained by cellular absorption with PC12 cells, a clonal cell line which expresses both the N-CAM and NILE (Ng-CAM, L1) glycoproteins. Subsequent absorption with purified cerebellar granule cells, but not with astroglial cells, removed the blocking activity, suggesting that the antigen(s) bound by blocking antibodies are neuronal. Immunoprecipitation of [35S]methionine- or [3H]fucose-radiolabeled Triton extracts of early postnatal cerebellar cells showed that the unabsorbed antiserum recognized a large number of proteins. Among these were bands with apparent molecular masses of N-CAM (180 and 140 kD) and NILE (230 kD). After absorption of the immune serum with PC12 cells, the number of bands recognized by the antiserum was reduced to a prominent band at 100 kD and a diffuse smear of material between 80 and 90 kD. The prominent band at 100 kD was removed by subsequent absorption of the immune serum with granule cells, a step which removed the blocking activity in the cerebellar microculture assay. Further evidence suggests that the astrotactin activity is missing or defective on granule cells from the neurological mutant mouse weaver, an animal that suffers a failure of glial-guided neuronal migration. When anti-astrotactin Fab fragments were pre-absorbed with weaver cerebellar neurons and then tested in the functional assay of neuron-glial interactions, the immune blocking activity was not removed. In contrast, wild-type cerebellar neurons removed the anti-astrotactin blocking activity under the same conditions. Subsequently, when [3H]fucose-radiolabeled Triton extracts of weaver and normal cells were immunoprecipitated with whole or PC12-absorbed anti-astrotactin antiserum, the intensity of the band at 100 kD was reduced by 95% in weaver cells.

CNS gene encoding astrotactin, which supports neuronal migration along glial fibers.

Vertebrate central nervous system (CNS) histogenesis depends on glia-guided migration of postmitotic neurons to form neuronal laminae. Previous studies have established that the neuronal protein astrotactin functions in murine cerebellar granule cell migration in vitro. The gene encoding astrotactin predicts a protein with three epidermal growth factor repeats and two fibronectin type III repeats. Astrotactin messenger RNA is expressed in postmitotic neuronal precursors in the cerebellum, hippocampus, cerebrum, and olfactory bulb, where migration establishes laminar structures. Fab fragments of antibodies to a recombinant astrotactin peptide blocked migration of cerebellar granule neurons in vitro along astroglial fibers. Transfection of astrotactin complementary DNA into 3T3 cells indicated that astrotactin acts as a ligand for neuron-glia binding during neuronal migration.

Neuronal differentiation rescued by implantation of Weaver granule cell precursors into wild-type cerebellar cortex.

The migration of postmitotic neurons away from compact, germinal zones is a critical step in neuronal differentiation in the developing brain. To study the molecular signals necessary for cerebellar granule cell migration in situ, precursor cells from the neurological mutant mouse weaver, an animal with phenotypic defects in migration, were implanted into the external germinal layer (EGL) of wild-type cerebellar cortex. In this region, labeled weaver precursor cells of the EGL progressed through all stages of granule neuron differentiation, including the extension of parallel fibers, migration through the molecular and Purkinje cell layers, positioning in the internal granule cell layer, and extension of dendrites. Thus, the weaver gene acts nonautonomously in vivo, and local cell interactions may induce early steps in neuronal differentiation that are required for granule cell migration.

Brain lipid-binding protein (BLBP): a novel signaling system in the developing mammalian CNS.

Using a polyclonal antibody against postnatal cerebellar cells, we have isolated a new, brain-specific member of the lipid-binding protein family (BLBP). Members of this family, such as cellular retinoic acid-binding protein, have been shown to carry small hydrophobic signaling molecules between cellular compartments. The expression of BLBP is spatially and temporally correlated with neuronal differentiation in many parts of the mouse CNS, including postnatal cerebellum, embryonic spinal cord, and cerebral cortex. In situ hybridization and immunocytochemistry show that BLBP is transiently expressed in radial glia in both the embryonic ventricular zone and the postnatal cerebellum. Subcellular localization studies by immunoelectron microscopy demonstrate that BLBP is present in the nucleus as well as the cytoplasm. Affinity-purified anti-BLBP antibodies block glial and neuronal differentiation in primary cell cultures, but have no effect on cell proliferation or adhesion. Based on these results, we propose that BLBP is required for the establishment of the radial glial fiber system in developing brain, a system that is necessary for the migration of immature neurons to establish cortical layers.

Antibodies that recognize astrotactin block granule neuron binding to astroglia.

To provide a rapid, specific assay for receptor systems involved in the binding of cerebellar granule neurons to astroglia, granule cells, purified from early postnatal mice, or from E15-E16 chicks, were radiolabeled with [35S]methionine and plasma membranes were prepared. The kinetics of binding of radiolabeled material to primary mouse or chick glia or to the mouse G26-24 astrocytoma cell line was measured in the presence or absence of antibodies against astrotactin, neural cell adhesion molecules, cadherins, or integrins. Addition of Fab fragments of astrotactin antibodies reduced the amount of granule cell membrane binding to astroglia by 70%. In contrast, Fab fragments of antibodies against the neural adhesion molecules N-CAM, L1, and N-cadherin and against integrin did not reduce the level of granule cell membrane binding to astroglia. Combinations of antibodies against N-CAM, L1, N-cadherin, and integrin also did not impair neuron binding to glia.

Embryonic precursor cells from the rhombic lip are specified to a cerebellar granule neuron identity.

The specification of diverse classes of neurons is critical to the development of the cerebellar cortex. Here, we describe the purification of early embryonic precursors of cerebellar granule neurons from the rhombic lip, the dorsal aspect of the midbrain/hindbrain region. Isolation of rhombic lip cells reveals a homogenous population of precursor cells that express general neuronal markers and the granule cell marker RU49, but fail to extend neurites or express differentiation markers. Differentiation is induced by coculture with external germinal layer (EGL) cells, or their membranes, suggesting that a local inducing factor acts after formation of the EGL. Thus, proliferating precursors within the rhombic lip are specified to be granule cells very early, with the availability of an inducing factor increasing over the course of development.

Cerebellar granule cell neurogenesis is regulated by cell-cell interactions in vitro.

When CNS precursor cells purified from the external germinal layer of the early postnatal mouse cerebellum are cultured in cellular reaggregates, DNA synthesis increased 10-fold above that of cells dispersed in a monolayer or embedded in a collagen matrix. Dividing precursor cells gave rise to neurons immunopositive for the neural antigens N-CAM, L1, and TAG-1, but not to astroglial cells immunopositive for glial filament protein. Moreover, proliferating precursor cells did not generate other types of cerebellar neurons, as judged by the lack of expression of glutamic acid decarboxylase, the synthetic enzyme for gamma-amino-n-butyric acid. By contrast, the addition of astroglial cells, or astroglial cell membranes, to cellular reaggregates of granule cell neuroblasts arrested precursor cell DNA synthesis in a dose-dependent manner. These results suggest that homotypic contact interactions among CNS neural progenitors control precursor cell proliferation and fate in generative zones of developing brain.

Cell-cell interactions influence survival and differentiation of purified Purkinje cells in vitro.

To determine the role of cell-cell interactions in Purkinje cell survival and dendritic differentiation, perinatal mouse Purkinje cells were purified, and their development was analyzed in vitro. In isolation at low density, Purkinje cell survival was poor, improved by neuronal contacts, either with purified granule neurons or with Purkinje cells themselves. Moreover, coculture with specific cell populations led to widely different degrees of Purkinje cell differentiation. Purified Purkinje cells cultured alone or with an inappropriate afferent, the mossy fibers, did not progress beyond immature forms. With astroglia, Purkinje cells had thin smooth processes. Proper Purkinje cell differentiation was driven only by coculture with granule cells, resulting in dendrites with spines receiving synapses. These results suggest that Purkinje cell differentiation is regulated by local epigenetic factors, provided in large part by the granule neuron.

Activated Notch2 signaling inhibits differentiation of cerebellar granule neuron precursors by maintaining proliferation.

In the developing cerebellar cortex, granule neuron precursors (GNPs) proliferate and commence differentiation in a superficial zone, the external granule layer (EGL). The molecular basis of the transition from proliferating precursors to immature differentiating neurons remains unknown. Notch signaling is an evolutionarily conserved pathway regulating the differentiation of precursor cells of many lineages. Notch2 is specifically expressed in proliferating GNPs in the EGL. Treatment of GNPs with soluble Notch ligand Jagged1, or overexpression of activated Notch2 or its downstream target HES1, maintains precursor proliferation. The addition of GNP mitogens Jagged1 or Sonic Hedgehog (Shh) upregulates the expression of HES1, suggesting a role for HES1 in maintaining precursor proliferation.

A mouse serine/threonine kinase homologous to C. elegans UNC51 functions in parallel fiber formation of cerebellar granule neurons.

The formation of the cerebellar circuitry depends on the outgrowth of connections between the two principal classes of neurons, granule neurons and Purkinje neurons. To identify genes that function in axon outgrowth, we have isolated a mouse homolog of C. elegans UNC51, which is required for axon formation, and tested its function in cerebellar granule neurons. Murine Unc51.1 encodes a novel serine/threonine kinase and is expressed in granule cells in the cerebellar cortex. Retroviral infection of immature granule cells with a dominant negative Unc51.1 results in inhibition of neurite outgrowth in vitro and in vivo. Moreover, infected neurons fail to express TAG-1 or neuron-specific beta-tubulin, suggesting that development is arrested prior to this initial step of differentiation. Thus, Unc51.1 signals the program of gene expression leading to the formation of granule cell axons.

Functional analysis of the weaver mutant GIRK2 K+ channel and rescue of weaver granule cells.

In the neurological mutant mouse weaver, granule cell precursors proliferate normally in the external germinal layer of the cerebellar cortex, but fail to differentiate. Granule neurons purified from weaver cerebella have greatly reduced G protein-activated inwardly rectifying K+ currents; instead, they display a constitutive Na+ conductance. Expression of the weaver GIRK2 channel in oocytes confirms that the mutation leads to constitutive activation, loss of monovalent cation selectivity, and increased sensitivity to three channel blockers. Pharmacological blockade of the Na+ influx in weaver granule cells restores their ability to differentiate normally. Thus, Na+ flux through the weaver GIRK2 channel underlies the failure of granule cell development in situ.

Generation of cerebellar granule neurons in vivo by transplantation of BMP-treated neural progenitor cells.

Cerebellar granule neurons, the most abundant class of CNS neurons, have a critical role in cerebellar function. Granule neurons are generated at the dorsal border of the mesencephalon and metencephalon, the rhombic lip. In the mouse embryo, rhombic lip cells express a number of granule neuron markers, notably the bHLH transcription factor Math1. Dorsal midline cells adjacent to the rhombic lip express Bmp6, Bmp7 and Gdf7, three genes encoding peptide growth factors of the bone morphogenetic protein (BMP) family. These BMPs induced the expression of granule neuron markers in cultured neural tissue. Moreover, BMP-treated neural cells formed mature granule neurons after transplantation into the early postnatal cerebellum, suggesting that BMPs initiate the program of granule cell specification.

Riding the glial monorail: a common mechanism for glial-guided neuronal migration in different regions of the developing mammalian brain.

In vitro studies from our laboratory indicate that granule neurons, purified from early postnatal mouse cerebellum, migrate on astroglial fibers by forming a 'migration junction' with the glial fiber along the length of the neuronal soma and extending a motile 'leading process' in the direction of migration. Similar dynamics are seen for hippocampal neurons migrating along hippocampal astroglial fibers in vitro. In heterotypic recombinations of neurons and glia from mouse cerebellum and rat hippocampus, neurons migrate on astroglial processes with a cytology and neuron-glia relationship identical to that of homotypic neuronal migration in vitro. In all four cases, the migrating neuron presents a stereotyped posture, speed and mode of movement, suggesting that glial fibers provide a generic pathway for neuronal migration in developing brain. Studies on the molecular basis of glial-guided migration suggest that astrotactin, a neuronal antigen that functions as a neuron-glia ligand, is likely to play a crucial role in the locomotion of the neuron along glial fibers. The navigation of neurons from glial fibers into cortical layers, in turn, is likely to involve neuron-neuron adhesion ligands.

The role of migration in central nervous system neuronal development.

The past year has seen the emergence of significant new information on the control of neurogenesis and migration, and the establishment of neuronal identity in three systems: developing cerebellum, cortex, and optic tectum. These findings have important implications for the role of glial-guided migrations in central nervous system neuronal development.