Role of astroglial cell clones in the survival and differentiation of cerebellar embryonic neurons

Multiple retinol and retinal dehydrogenases catalyze all-trans-retinoic acid biosynthesis in astrocytes

All-trans-retinoic acid (atRA) stimulates neurogenesis, dendritic growth of hippocampal neurons, and higher cognitive functions, such as spatial learning and memory formation. Although astrocyte-derived atRA has been considered a key factor in neurogenesis, little direct evidence identifies hippocampus cell types and the enzymes that biosynthesize atRA. Here we show that primary rat astrocytes, but not neurons, biosynthesize atRA using multiple retinol dehydrogenases (Rdh) of the short chain dehydrogenase/reductase gene family and retinaldehyde dehydrogenases (Raldh). Astrocytes secrete atRA into their medium; neurons sequester atRA. The first step, conversion of retinol into retinal, is rate-limiting. Neurons and astrocytes both synthesize retinyl esters and reduce retinal into retinol. siRNA knockdown indicates that Rdh10, Rdh2 (mRdh1), and Raldh1, -2, and -3 contribute to atRA production. Knockdown of the Rdh Dhrs9 increased atRA synthesis ∼40% by increasing Raldh1 expression. Immunocytochemistry revealed cytosolic and nuclear expression of Raldh1 and cytosol and perinuclear expression of Raldh2. atRA autoregulated its concentrations by inducing retinyl ester synthesis via lecithin:retinol acyltransferase and stimulating its catabolism via inducing Cyp26B1. These data show that adult hippocampus astrocytes rely on multiple Rdh and Raldh to provide a paracrine source of atRA to neurons, and atRA regulates its own biosynthesis in astrocytes by directing flux of retinol. Observation of cross-talk between Dhrs9 and Raldh1 provides a novel mechanism of regulating atRA biosynthesis.

In Vitro Development of Rat Cerebellar Neurons of Early Embryonic Origin. An Anatomical and Electrophysiological Study

The development of the major morphological and electrophysiological properties of presumptive Purkinje cells (PCs) was studied in primary cultures of rat cerebellum dissociated on the 14th embryonic day, when PCs are minimally differentiated and migrate in vivo. PCs were identified with a specific antibody to calbindin D-28K (CaBP), which allowed visualization of the different morphological types of PCs between 3 and 29 days in vitro (DIV). CaBP-immunopositive cells were first detected at 3 DIV. Thereafter, the shape of these cells resembled some of those described in vivo. After 20 DIV, 95% of the CaBP-immunopositive cells had characteristic PC dendritic trees, although they were very atrophic. Glial cells immunopositive for the glial fibrillary acidic protein (GFAP) were first seen at 3 DIV. Thereafter GFAP-immunopositive cells resembled Bergmann cells or velate astrocytes. Neurons regarded as PCs were studied electrophysiologically using the patch-clamp whole-cell configuration. Voltage-dependent, tetrodotoxin-sensitive fast inward currents were virtually absent at 2 - 4 DIV, but increased between 7 and 14 DIV to reach two-thirds of the amplitude obtained after 15 DIV. These currents were large enough to give rise to overshooting spikes as early as 7 DIV in the current-clamp mode. This time schedule is in keeping with that of PCs developed in situ. The tetraethylammonium-sensitive, slowly inactivating outward currents had reached two-thirds of the amplitude obtained after 15 DIV by 3 - 4 DIV. Their amplitude remained stable between 4 and 7 DIV, and increased to their maximal value during 7 - 14 DIV, with a marked shortening of action potentials. 4-Aminopyridine-sensitive, fast-inactivating outward currents might also be associated with development, since they were present in 66% of the cells between 7 and 14 DIV but in only 39% from 15 to 29 DIV; however, their amplitude did not vary with time. Presumptive PCs bore l-glutamate-activated receptors, which preceded the emergence of kynurenate-sensitive, spontaneous synaptic currents at 7 DIV. These currents were sometimes intermingled with inhibitory currents, although presumptive PCs were sensitive to gamma-aminobutyrate at 7 DIV. The present model represents some unequivocal features of PC development, although the PCs used had undergone minimal differentiation in vivo and were cultured in a very disturbed cellular environment.

Restriction of high CD15 expression to a subset of rat cerebellar astroglial cells can be overcome by transduction with adenoviral vectors expressing the rat alpha 1,3-fucosyltransferase IV gene

Glycoconjugates bearing the epitope 3-fucosyl-N-acetyllactosamine (CD15) are believed to be involved in cell-cell interactions and are temporally and spatially regulated in the brain. In the rat postnatal cerebellum, CD15 is predominantly expressed in the molecular layer by Bergmann glial cells, but little CD15 expression is seen in other astroglia, and the basis for this restricted expression is not known. Adenoviral vectors were shown to efficiently deliver transgenes to cerebellar glial cells and were used to determine whether manipulation of glycosyltransferase activities could enhance the expression of CD15 in these cells. In dissociated cerebellar cell cultures, few glial cells normally express CD15. However, transduction of these cells with an adenoviral vector (AdGFPCMVFucT) that expressed both green fluorescent protein (GFP) and FLAG-tagged rat alpha 1, 3-fucosyltransferase IV (rFuc-TIV) resulted in high CD15 expression on the surface of all transduced glial cells. Likewise, infection of cerebellar slice cultures caused the appearance of CD15-positive transduced cells of glial cell morphology in the internal granule cell layer. Thus, enhancement of Fuc-T activity caused robust CD15 expression in cerebellar glial cells that normally show little expression of CD15, suggesting a role for Fuc-T levels in regulating CD15 expression in this cell type. The manipulation of levels of glycosyltransferases using adenoviral vectors may prove a useful tool to investigate questions of glycoconjugate regulation in glial cells in the developing rodent cerebellum.

Upregulation of GABAA current by astrocytes in cultured embryonic rat hippocampal neurons

Embryonic rat hippocampal neurons were cultured on poly-D-lysine (PDL) or a monolayer of postnatal cortical astrocytes to reveal putative changes in neuronal physiology that involve astrocyte-derived signals during the first 4 d of culture, GABA-induced Cl- current (IGABA) was quantified using outside-out and whole-cell patch-clamp recordings beginning at 30 min, when cells had become adherent. The amplitude and density (current normalized to membrane capacitance) of IGABA in neurons grown on astrocytes became statistically greater than that recorded in neurons grown on PDL after 2 hr in culture (HIC). Although the current density remained unchanged in neurons on astrocytes, that in neurons on PDL decreased and became statistically lower beginning after 2 HIC. The differences in amplitude and density of IGABA in the two groups of neurons were maintained during the 4 d experiment. The upregulation effect of astrocytes on neuronal IGABA required intimate contact between the neuronal cell body and underlying astrocytes. Suppression of spontaneous Cac2+ elevations in astrocytes by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid that was loaded intracellularly decreased their modulatory effects on IGABA. IGABA in all cells was blocked completely by bicuculline and exhibited virtually identical affinity constants, Hill coefficients, and potentiation by diazepam in the two groups. Outside-out patch recordings revealed identical unitary properties of IGABA in the two groups. More channels per unit of membrane area could explain the astrocyte enhancement of IGABA. The results reveal that cortical astrocytes potentiate IGABA in hippocampal neurons in a contact-dependent manner via a mechanism involving astrocyte Cac2+ elevation.

Astrocyte differentiation is enhanced in chick embryos treated with ethanol during early neuroembryogenesis

In this study, we examined the effects of ethanol administered to chick embryos, on the maturation of astrocytes, using glutamine synthetase (GS) activity as an astrocyte marker. Ethanol (50 mM) was administered in ovo via the air sac, embryos were sacrificed at various days of embryonic development and GS activity was determined in cerebral hemispheres and cerebellum. We found that in both cerebral hemispheres and cerebellum, GS activity was higher in the ethanol-treated embryos, as compared to controls, during the embryonic periods, E6 to E10 in the cerebral hemispheres and E10 to E14 in the cerebellum. These periods are characterized by increased neuronal differentiation in these CNS areas. The increase in GS activity in the ethanol-treated embryos is speculated to reflect either a transient reactive gliosis and/or an enhancement in the differentiation of radial glia, immature glia, to more mature astrocytes.

Expression of neuronal and glial polypeptides during histogenesis of the human cerebellar cortex including observations on the dentate nucleus

In order to gain a more complete understanding of the sequential pattern of gene expression during neurogenesis and gliogenesis in humans, we followed the expression of well-characterized, developmentally regulated polypeptides in the cerebellar cortex and dentate nucleus by immunohistochemistry using monoclonal antibodies of highly defined specificity. At 8-10 weeks gestational age (GA), progenitor cells and their immediate progeny in the rhombencephalic ventricular zone expressed vimentin and nestin and, to a lesser extent, microtubule-associated protein 5 (MAP5) and glial fibrillary acidic protein (GFAP), but not the low affinity nerve growth factor receptor (NGFR). In contrast, postmitotic, migrating immature neurons in the intermediate zone gave strong reactions for MAP2, tau, and a nonphosphorylated form of middle molecular weight neurofilament (NF) protein (NF-M) and weak reactivity for NGFR. At 15 weeks GA, proliferating cells of the superficial part of the cerebellar external granular layer stained only for NGFR, while more deeply situated cells of the external granular layer stained positively for NGFR, MAP2, MAP5, tau, and chromogranin A, which correlates with the early outgrowth of parallel fibers. All phosphoisoforms of NF-M as well as the low (NF-L) and high (NF-H) molecular weight NF proteins and alpha-internexin were expressed in the somatodendritic domain of Purkinje cells and dentate nucleus neurons from about 20 weeks GA with a gradual compartmentalization of highly phosphorylated forms of NF-M and NF-H into axons by the end of gestation. Alpha-internexin was also expressed strongly in axons of the deep white matter from 20 weeks GA to adulthood. MAP2, synaptophysin, and NGFR showed early, transient expression in the somatodendritic domain of Purkinje cells followed by the appearance of a 220 kDa nestin-like peptide that continued to be expressed in adult Purkinje cells. Notably, developing dentate nucleus neurons expressed many of these proteins in a similar temporal sequence. Early in the developing cerebellar cortex, the expression of NF protein and synaptophysin occurred in discrete patches or columns similar to those described for other antigens (i.e., zebrins). Finally, radial glia were positive for vimentin, GFAP, and nestin from 8 weeks GA to 8 months postnatal. This study describes the distinct molecular programs of lineage commitment in cerebellar progenitor cells and in differentiating neurons and astrocytes of the human cerebellum. The acquisition of a mature molecular neuronal phenotype correlates with the establishment of structural polarity in cerebellar neurons.

Neuronal survival and neurite extension supported by astrocytes co-cultured in transwells

The influence of astrocytes on the development of cerebral cortical neurons was studied in a co-culture system using transwells with chemically defined medium. Cerebral cortical neurons from 15- or 16-day-old mouse embryo were cultured in the lower wells which were separated by a porous membrane from the upper transwells where cerebral cortical astrocytes from newborn mouse were cultured. Neurons co-cultured with astrocytes for 7 days formed a network-like web and maintained a slightly better survival from 4 to 7 days. However, neurons cultured in conditioned medium obtained from astrocytes did not form any network after 7 days even though they maintained a better cell survival at 4 days.

Development and maintenance of the neuronal cytoskeleton in aggregated cell cultures of fetal rat telencephalon and influence of elevated K+ concentrations

Serum-free aggregating cell cultures of fetal rat telencephalon were examined by biochemical and immunocytochemical methods for their development-dependent expression of several cytoskeletal proteins, including the heavy- and medium-sized neurofilament subunits (H-NF and M-NF, respectively); brain spectrin; synapsin I; beta-tubulin; and the microtubule-associated proteins (MAPs) 1, 2, and 5 and tau protein. It was found that with time in culture the levels of most of these cytoskeletal proteins increased greatly, with the exceptions of the particular beta-tubulin form studied, which remained unchanged, and MAP 5, which greatly decreased. Among the neurofilament proteins, expression of M-NF preceded that of H-NF, with the latter being detectable only after approximately 3 weeks in culture. Furthermore, MAP 2 and tau protein showed a development-dependent change in expression from the juvenile toward the adult form. The comparison of these developmental changes in cytoskeletal protein levels with those observed in rat brain tissue revealed that protein expression in aggregate cultures is nearly identical to that in vivo during maturation of the neuronal cytoskeleton. Aggregate cultures deprived of glial cells, i.e., neuron-enriched cultures prepared by treating early cultures with the antimitotic drug cytosine arabinoside, exhibited pronounced deficits in M-NF, H-NF, MAP 2, MAP 1, synapsin I, and brain spectrin, with increased levels of a 145-kDa brain spectrin breakdown product. These adverse effects of glial cell deprivation could be reversed by the maintenance of neuron-enriched cultures at elevated concentrations of KCl (30 mM). This chronic treatment had to be started at an early developmental stage to be effective, a finding suggesting that sustained depolarization by KCl is able to enhance the developmental expression and maturation of the neuronal cytoskeleton.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.

Basic fibroblast growth factor regulates the ability of astrocytes to support hypothalamic neuronal survival in vitro

The putative neurotrophic effects of basic fibroblast growth factor (bFGF) were tested on embryonic hypothalamic neurons in dissociated cell culture. Basic FGF dramatically increased the survival of embryonic hypothalamic astrocytes plated on a poly-L-lysine (PLL) substrate. Basic FGF treatment also increased the number of hypothalamic neurons surviving in vitro; however, no neurotrophic effects were observed when astrocyte proliferation was prevented by using serum-free N2 medium or by using the mitotic inhibitor cytosine arabinoside. In contrast to effects when PLL was used as a substrate, bFGF reduced the survival of hypothalamic neurons plated on a confluent, contact-inhibited monolayer of astrocytes. This effect appears to be due to the direct actions of bFGF on astrocytes: treatment of confluent astrocytes with 5 ng/ml bFGF caused the protoplasmic astrocytes to develop a fibrillar morphology and reduced the ability of the astrocyte monolayer to promote neuronal survival after a further 24 hr in bFGF-free medium. It is concluded that in addition to its mitogenic effects, bFGF acts as a differentiation factor for protoplasmic astrocytes in vitro, and these morphological and functional changes may reflect the process of normal astrocytic development and response to brain injury in vivo.

Alpha isoform of smooth muscle actin is expressed in astrocytes in vitro and in vivo

We have previously reported that astroglial cell lines derived from spontaneously immortalized mouse cerebellar cultures as well as primary astrocyte cultures express the mRNA of the alpha isoform of smooth muscle actin. In this report, we have used an antiserum specific for the alpha smooth muscle actin protein to investigate the presence and the pattern of expression of alpha smooth muscle actin protein at the cellular level with immunocytochemical methods. The results show that an anti-smooth muscle vessels alpha actin antiserum labels a typical actin network in the D19 astroglial cell clone and in flat astrocytes of primary cultures derived from various CNS regions of embryonic and postnatal mice. Furthermore, this antiserum labels distinct populations of astrocytes in the adult mouse brain, in particular in the corpus callosum and the fornix. However, in the corpus callosum, astrocytic processes are strongly labeled by anti-SMV alpha actin antibodies only in parasagittal planes. Thus, alpha smooth muscle actin represents a new marker for subsets of astrocytes.

Microglial progenitors with a high proliferative potential in the embryonic and adult mouse brain

Single cell suspensions, prepared from brain stem, cerebellum, and forebrain parenchyma of embryonic and adult mice, were plated on monolayers of an astroglial cell line derived from a spontaneously immortalized mouse cerebellar culture, the D19 clone. A few of the brain cells adhering to the D19 monolayers were immunoreactive to the Mac-1 antibody, which labels all cells of the monocytic and granulocytic lineages. The Mac-1-positive cells proliferated vigorously and later most of them acquired the F4/80 epitope specific for macrophages and microglia cells. Studies in clonal conditions allowed development of large colonies of about 2 x 10(5) cells that expressed typical microglia markers. Bone marrow Mac-1-positive cells cocultured on D19 monolayers were also induced to proliferate, whereas peritoneal macrophages were not. D19 astrocytes express macrophage colony-stimulating factor (CSF-1) activity at a high level, and their conditioned media induced the proliferation of brain and bone marrow Mac-1-positive cells. A specific anti-CSF-1 antiserum completely blocked bone marrow macrophage progenitor proliferation and significantly reduced the multiplication of microglial precursors induced by the D19-conditioned medium. These data indicate that the embryonic and adult mouse brain parenchyma contains potential progenitors for microglial cells.

Metabolism and release of glutamate in cerebellar granule cells cocultured with astrocytes from cerebellum or cerebral cortex

Cerebellar granule cells were cocultured with astrocytes from either cerebral cortex or cerebellum in two different systems. In one system the cells were plated next to each other only sharing the culture medium (separated cocultures) and in the other system the granule cells were plated on top of a preformed layer of astrocytes (sandwich cocultures). Using astrocytes from cerebellum, granule cells developed morphologically and functionally showing a characteristic high activity of the glutamate synthesizing enzyme aspartate aminotransferase (AAT) as well as a high stimulus-coupled transmitter release regardless of the culture system, i.e., granule cells could grow on top of cerebellar astrocytes as well as next to these cells. In the case of cerebral cortex astrocytes it was found that cerebellar granule cells did not develop (11% survival) when seeded on top of these astrocytes. This was indicated by the morphological appearance of the cultures as well as by a negligible difference between the AAT activity in sandwich cocultures and astrocytes cultured alone. On the other hand, granule cells in separated cocultures with cerebral cortex astrocytes exhibited a normal morphology and a high activity of AAT as well as a large stimulus-coupled transmitter release. Cerebellar and cortical astrocytes expressed the astrocyte specific enzyme glutamine synthetase in a glucocorticoid-inducible form regardless of the culture system. The results show that under conditions of direct contact between granule cells and astrocytes, regional specificity exists with regard to neuron-glia contacts. This specificity does not seem to involve soluble factors present in the culture medium because in separated cocultures the cerebellar granule cells developed normally regardless of the regional origin of the astrocytes.

Isolation of cDNAs from a mouse astroglial cell line by a subtracted cDNA library

Astrocytes belong to the glial cell population and represent a major subclass of the CNS. Although different subtypes of astrocytes have been described according to their morphological characteristics, biochemical markers of each subtype of astrocytes are not yet available. We have thus undertaken to compare gene expression pattern of different astroglial subtypes. In this study we have taken advantage of two astroglial cell clones derived from 8 day postnatal mouse cerebellar explants and which might be the in vitro equivalents of the velate protoplasmic (D19) and of the Golgi-Bergmann (C8S) astrocytes (Alliot and Pessac, Brain Res., 306: 283-291, 1984). We have constructed a subtracted cDNA library derived from cytoplasmic poly(A)+ RNAs of the D19 cell line. This library was enriched 12-fold for D19 specific sequences by subtractive hybridization with an excess of cytoplasmic poly(A)+ RNAs purified from the C8S astroglial clone. This subtracted library was differentially screened with cDNA probes derived from D19 and C8S cell lines; both probes were subtracted with C8S poly(A)+ RNAs. Eight cDNA clones corresponding to transcripts overexpressed in D19 were selected. Three cDNAs encode for smooth muscle actin, one for fibronectin and one for polyadenylate binding protein. The three other gene products have not been previously reported. The in vivo distribution pattern of these sequences in various mouse adult tissues shows that all these transcripts are expressed in the cerebellum and/or in the brain.

Neuronal-glial interactions: complexity of neurite outgrowth correlates with substrate adhesivity of serotonergic neurons

To study the interactions between neurons of known transmitter phenotype and non-neuronal cells of glial or fibroblastic origin, serotonergic (5-HT) neurons were tested for their strength of adhesion and neurite outgrowth patterns on substrates of astrocytes or fibroblasts using a cell adhesion assay for transmitter-identified neurons, and morphometry of immunocytochemically stained neurons in dissociated cell cultures. Both the strength of adhesion and the rate and complexity of neurite outgrowth by 5-HT neurons were significantly greater on substrates of astrocytes compared to fibroblasts. These results provide evidence that 5-HT neurons can interact selectively with glia via cell surface determinants, and that this process may be important for the development of complex (dendrite-like) neuritic arbors. The methods developed in this study will be useful for future studies of interactions between transmitter-identified neurons and glial cells during ontogeny of the embryonic brain.

Retinoids increase perinatal spinal cord neuronal survival and astroglial differentiation

In this report we demonstrate that retinol and retinoic acid (RA) increase the survival and morphological differentiation of rat spinal cord neurons in vitro. Micromolar amounts of retinol and RA increased the number of surviving neurons by 2- to 3-fold and affected neuritic density resulting in increased secondary and tertiary processes compared to untreated sister cultures. A marked morphological differentiation of the astrocyte population in conjunction with an antiproliferative effect in the presence of retinoids were apparent. These trophic effects occurred mainly after 5 days in vitro, a time that corresponds to the time of birth in vivo. Retinoic acid exerted a direct trophic effect on spinal cord neurons in the absence of glial cells while retinol lost its effectiveness. Metabolic labeling suggested that retinol is converted to the biologically active RA within astrocytes but not in neurons. Taken together, our results have demonstrated direct trophic effects of RA on spinal cord neurons and have suggested another role for astrocytes in the maintenance of normal neural physiology by regulating RA concentrations through the oxidation of retinol.