Glutamate transporter GLAST is expressed in the radial glia-astrocyte lineage of developing mouse spinal cord

An attempt to generate neurons from an astrocyte progenitor cell line FBD-104

In the present study, a clonal astrocyte progenitor cell line derived from p53-deficient fetal brains, named FBD-104, was characterized in monolayer and suspension culture. In monolayer culture with medium containing 10% serum, FBD-104 cells expressed some markers of astrocytes, such as glial fibrillary acidic protein (GFAP), S100beta, and glutamate aspartate transporter (GLAST). They never expressed any markers of neurons or oligodendrocytes. Thus the cell line appears to be restricted to the astroglial lineage. However, in suspension culture in serum-free medium supplemented with EGF and FGF2, FBD-104 cells proliferated and formed neurospheres expressing mRNAs for Mash1 and Math3, generating cells expressing neuron specific beta-III tubulin. Re-plating the spheres onto an adhesive substrate and withdrawal of the growth factors induced the expression of mRNAs for NeuroD and Olig2 and generated more beta-III tubulin-positive cells. The present study demonstrated that neurosphere culture is an efficient method to induce neurogenesis from the astrocyte progenitor cell line FBD-104. We also determined that pretreatment with FGF2 caused a significant increase in yield of neurospheres. Thus, the FBD-104 line is an interesting in vitro model to study effect of trophic factors and adhesive substrates on lineage determination of neural progenitor cells.

Expression patterns of erythropoietin and its receptor in the developing spinal cord and dorsal root ganglia

Recombinant human erythropoietin (EPO) is neuroprotective in animal models of adult spinal cord injury, and reduces apoptosis in adult dorsal root ganglia after spinal nerve crush. The present work demonstrates that spinal cord and dorsal root ganglia share dynamic expression patterns of EPO and its receptor (EPOR) during development. C57Bl mice from embryonic days (E) 8 (E8) to E19 were studied. In spinal cord and dorsal root ganglia, EPOR expression in all precursor cells preceded the expression of EPO in subsets of neurons. On E11, EPO-immunoreactive spinal motoneurons and ganglionic sensory neurons resided adjacent to EPOR-expressing radial glial cells and satellite cells, respectively. From E12 onwards, EPOR-immunoreactivity decreased in radial glial cells and, transiently, in satellite cells. Simultaneously, large-scale apoptosis of motoneurons and sensory neurons started, and subsets of neurons were labelled by antibodies against EPOR. Viable neurons expressed EPO and EPOR. Up to E12.5, apoptotic cells were EPOR-immunopositive, but variably EPO-immunonegative or EPO-immunopositive. Thereafter, EPO-immunonegative and EPOR-immunopositive apoptotic cells predominated. Our findings suggest that EPO-mediated neuron-glial and, later, neuron-neuronal interactions promote the differentiation and/or the survival of subsets of neurons and glial cells in central as well as in peripheral parts of the embryonic nervous system. Correspondingly, expression of phospho-Akt-1/protein-kinase B extensively overlapped expression sites of EPO and EPOR, but was absent from apoptotic cells. Identified other sites of EPO and/or EPOR expression include radial glial cells that transform to astrocytes, cells of the floor plate and notochord as well as neural crest-derived boundary cap cells at motor exit points and cells of the primary sympathetic chain.

The fate of neural progenitor cells expressing astrocytic and radial glial markers in the postnatal rat dentate gyrus

In the dentate gyrus neurons continue to be generated from late embryonic to adult stage. Recent extensive studies have unveiled several key aspects of the adult neurogenesis, but only few attempts have so far been made on the analysis of the early postnatal neurogenenesis, a transition state between the embryonic and adult neurogenesis. Here, we focus on the early postnatal neurogenesis and examine the nature and development of neural progenitor cells in Wistar rats. Immunohistochemistry for Ki67, a cell cycle marker, and 5-bromo-2-deoxyuridine (BrdU) labelling show that cell proliferation occurs mainly in the hilus and partly in the subgranular zone. A majority of the proliferating cells express S100beta and astrocyte-specific glutamate transporter (GLAST) and the subpopulation are also positive for glial fibrillary acidic protein (GFAP) and nestin. Tracing with BrdU and our modified retrovirus vector carrying enhanced green fluorescent protein (GFP) indicate that a substantial population of the proliferating cells differentiate into proliferative neuroblasts and immature neurons in the hilus, which then migrate to the granule cell layer (66.8%), leaving a long axon-like process behind in the hilus, and the others mainly become star-shaped astrocytes (12.0%) and radial glia-like cells (4.7%) in the subgranular zone. These results suggest that the progenitors of the granule cells expressing astrocytic and radial glial markers, proliferate and differentiate into neurons mainly in the hilus during the early postnatal period.

Distribution of Aquaporin 4 in rodent spinal cord: relationship with astrocyte markers and chondroitin sulfate proteoglycans

Water balance between cells and extracellular compartments is essential for proper functioning of the central nervous system, as demonstrated by its perturbations in pathological conditions. Aquaporin 4 (AQP4) is the predominant water channel in brain and spinal cord, where it is present mainly on astrocytic endfeet contacting vessels. A role in water homeostasis control has been proposed also for the extracellular matrix, that in brain consists mainly of chondroitin sulfate proteoglycans (CSPGs). Using cytochemical and immunocytochemical techniques, we investigated their distribution in rodent spinal cord, to better understand the role of these two classes of molecules. The results show that in spinal gray matter AQP4 labeling is intense in all perivascular profiles and (1) displays a marked dorsoventral gradient in the neuropil; and (2) coexists extensively with glial glutamate transporter-1 (GLT-1) but scarcely with glial fibrillary acidic protein (GFAP). In white matter the overlap between AQP4, GLT-1, and GFAP is almost complete. Ultrastructural examination shows that AQP4-labeled astrocytic processes surround blood vessels, neuronal perikarya and processes, and both asymmetric and symmetric synapses, indicating that the protein may be involved in the regulation of water fluxes around both inhibitory and excitatory synapses. CSPGs, visualized by labeling with Wisteria floribunda agglutinin, show a distribution complementary to that of AQP4, being absent or weekly expressed in AQP4-enriched areas. These findings suggest that different mechanisms may contribute to the regulation of water homeostasis in different spinal cord regions.

Plasticity of cultured mesenchymal stem cells: switch from nestin-positive to excitable neuron-like phenotype

Bone marrow mesenchymal stem cells (MSCs) can differentiate into several types of mesenchymal cells, including osteocytes, chondrocytes, and adipocytes, but, under appropriate experimental conditions, can also differentiate into nonmesenchymal cells--for instance, neural cells. These observations have raised interest in the possible use of MSCs in cell therapy strategies for various neurological disorders. In the study reported here, we addressed the question of in vitro differentiation of MSCs into functional neurons. First, we demonstrate that when they are co-cultured with cerebellar granule neurons, adult MSCs can express neuronal markers. Two factors are needed for the emergence of neuronal differentiation of the MSCs: the first one is nestin expression by MSCs (nestin is a marker for the responsive character of MSCs to extrinsic signals), and the second one is a direct cell-cell interaction between neural cells and MSCs that allows the integration of these extrinsic signals. Three different approaches suggest that neural phenotypes arise from MSCs by a differentiation rather than a cell fusion process, although this last phenomenon can also coexist. The expression of several genes--including sox, pax, notch, delta, frizzled, and erbB--was analyzed by quantitative reverse transcription polymerase chain reaction (RT-PCR) in order to further characterize the nestin-positive phenotype compared to the nestin-negative one. An overexpression of sox2, sox10, pax6, fzd, erbB2, and erbB4 is found in nestin-positive MSCs. Finally, electrophysiological analyses demonstrate that MSC-derived neuron-like cells can fire single-action potentials and respond to several neurotransmitters such as GABA, glycine, and glutamate. We conclude that nestin-positive MSCs can differentiate in vitro into excitable neuron-like cells.

Distinct spatio-temporal expression of ABCA and ABCG transporters in the developing and adult mouse brain

Using in situ hybridization for the mouse brain, we analyzed developmental changes in gene expression for the ATP-binding cassette (ABC) transporter subfamilies ABCA1-4 and 7, and ABCG1, 2, 4, 5 and 8. In the embryonic brains, ABCA1 and A7 were highly expressed in the ventricular (or germinal) zone, whereas ABCA2, A3 and G4 were enriched in the mantle (or differentiating) zone. At the postnatal stages, ABCA1 was detected in both the gray and white matter and in the choroid plexus. On the other hand, ABCA2, A3 and A7 were distributed in the gray matter. In addition, marked up-regulation of ABCA2 occurred in the white matter at 14 days-of-age when various myelin protein genes are known to be up-regulated. In marked contrast, ABCA4 was selective to the choroid plexus throughout development. ABCG1 was expressed in both the gray and white matters, whereas ABCG4 was confined to the gray matter. ABCG2 was diffusely and weakly detected throughout the brain at all stages examined. Immunohistochemistry of ABCG2 showed its preferential expression on the luminal membrane of brain capillaries. Expression signals for ABCG5 and G8 were barely detected at any stages. The distinct spatio-temporal expressions of individual ABCA and G transporters may reflect their distinct cellular expressions in the developing and adult brains, presumably, to regulate and maintain lipid homeostasis in the brain.

Differentiation of radial glia from radial precursor cells and transformation into astrocytes in the developing rat spinal cord

Radial glial cell origins and functions have been studied extensively in the brain; however, questions remain relating to their origin and fate in the spinal cord. In the present study, radial glia are investigated in vivo using the neuroepithelial markers nestin and vimentin and the gliogenic markers GLAST, BLBP, 3CB2, and glial fibrillary acidic protein (GFAP). This has revealed heterogeneity among nestin/vimentin-positive precursor cells and suggests a lineage progression from neuroepithelial cell through to astrocyte in the developing spinal cord. A population of self-renewing radial cells, distinct from an earlier pseudo-stratified neuroepithelium, that resemble radial glial cells in morphology but do not express GLAST, BLBP, or 3CB2, is revealed. These radial cells arise directly from the spinal cord neuroepithelium and are probably the progenitors of neurons and the earliest appearing radial glial cells. GLAST/BLBP-positive radial glia first appear in the ventral cord at E14, and these cells gradually transform through one or more intermediate stages into differentiated astrocytes. Few if any neurons appear to be derived from radial glial cells, which are instead the major sources of astrocytes in the spinal cord. Evidence for the nonradial glial cell origins of some white matter astrocytes is also presented.

A bipotent neural progenitor cell line cloned from a cerebellum of an adultp53-deficient mouse generates both neurons and oligodendrocytes

Here we report developmental characteristics of a clonal cell line 2Y-3t established from a multifocal neoplasm that arose in a cerebellum of an adult p53-deficient mouse. The tumorigenicity of the line was not observed in soft agar assay or in nude mouse assay. In serum-containing medium, 2Y-3t cells were epithelial-like in morphology and were mitotic. When they were cultured in serum-free medium, the expressions of neural stem and/or progenitor cell markers were decreased. Concomitantly, the expressions of neuronal and oligodendrocyte markers were increased in concert with morphological differentiation, and DNA synthesis ceased. None of astrocyte markers were detected under these culture conditions. Double-labelling studies revealed that two cell populations coexisted, expressing neuronal or oligodendrocyte markers. Triiodothyronine (T3) increased the oligodendrocyte population when 2Y-3t cells were cultured in serum-free medium. Recloning of the line gave rise to three types of subclones. Sixteen subclones were capable of generating both neurons and oligodendrocytes, four subclones were capable of generating only neurons and one subclone was capable of generating only oligodendrocytes. Thus, 2Y-3t cells have characteristics of bipotent neural progenitor cells capable of generating both neurons and oligodendrocytes. In addition, the line expressed mRNA for Pax-2 and had GAD67-positive cells when cultured in serum-free medium. However, none of the mRNAs for Zic-1, Math1, zebrin or Calbindin-D28k were detected, suggesting that the 2Y-3t line might generate the GABAergic interneuron lineage of the mouse cerebellum.

Asymmetric distribution of EGFR receptor during mitosis generates diverse CNS progenitor cells

It has been debated whether asymmetric distribution of cell surface receptors during mitosis could generate asymmetric cell divisions by yielding daughters with different environmental responsiveness and, thus, different fates. We have found that in mouse embryonic forebrain ventricular and subventricular zones, the EGFR can distribute asymmetrically during mitosis in vivo and in vitro. This occurs during divisions yielding two Nestin+ progenitor cells, via an actin-dependent mechanism. The resulting sibling progenitor cells respond differently to EGFR ligand in terms of migration and proliferation. Moreover, they express different phenotypic markers: the EGFRhigh daughter usually has radial glial/astrocytic markers, while its EGFRlow sister lacks them, indicating fate divergence. Lineage trees of cultured cortical glioblasts reveal repeated EGFR asymmetric distribution, and asymmetric divisions underlie formation of oligodendrocytes and astrocytes in clones. These data suggest that asymmetric EGFR distribution contributes to forebrain development by creating progenitors with different proliferative, migratory, and differentiation responses to ligand.

Fibroblast growth factor 2 is required for maintaining the neural stem cell pool in the mouse brain subventricular zone

Cells within the subventricular zone (SVZ) express basic Fgf (Fgf2) and Fgf receptor proteins. We show that the absence of Fgf2 gene products reduces by 50% the dividing progenitor population of the anterior SVZ (SVZa) without changing their cell cycle time. Every 2-3 cell cycles of the SVZa progenitor cell population, 30,000 newly generated neurons capable of long-term survival are added to the glomerular layer of the olfactory bulb. Fgf2 knockout mice have smaller olfactory bulbs due to decreased output of these newly generated cells into the bulbs. A population of slow-dividing neural stem cells (NSCs) residing in the SVZa is identified by its slow cell cycle kinetics (cell cycle approx. 20 days); these cells, called 'S' cells, are negative for glial fibrillary acidic protein and occasionally express brain-lipid-binding protein, a molecular marker of radial glia. The number of these dividing NSCs is reduced from about 13,000 in wild-type to 8,500 cells in Fgf2 knockout mice. Thus, FGF2 regulates the number of proliferative cells and olfactory bulb neurogenesis by maintaining a slow-dividing stem cell pool within the SVZa.

Expression of glutamate transporter subtypes during normal human corticogenesis and type II lissencephaly

Glutamate transporters are thought to have an important role in central nervous system (CNS) development. We investigated the expression of the sodium-dependent high-affinity glutamate transporters EAAT1, EAAT2, and EAAT3 in 11 human autopsied cases without neurological disorders and in four cases with type II lissencephaly including Walker Warburg's syndrome (WWS) and Fukuyama-type congenital muscular dystrophy (FCMD), both of which are classified as migration disorders of the human brain. Expression of glutamate transporter subtypes was differentially regulated during normal human corticogenesis. Although EAAT1 and EAAT2 were mainly localized to the cortical astrocytes in the postnatal brain, EAAT1 was enriched in the proliferative zones and radial glia from 13 gestational weeks (GW) to 20 GW. EAAT2 was abundant in the intermediate zone until 23 GW, and transiently expressed in the radial fibers of the transitional form of radial glia into mature astrocytes as well as partly in the corticofugal axonal bundles. EAAT3 immunoreactivity was robust in the apical dendrites of the pyramidal neurons in the marginal zone and cortical plate during corticogenesis, and decreased postnatally. In the individuals with type II lissencephaly, glutamate transporters were expressed in the extrusion of neuroglial tissue. Bundles of EAAT2-immunoreactive radial fibers were prominent in the specimens at 20 GW. Thus, glutamate transporters are differentially regulated during normal and impaired corticogenesis. Altered glutamate transporter expression in type II lissencephaly suggests that glutamate metabolism is involved in the formation of the normal cortex and contributes to the disorganized cortex seen in migration disorders.

BM88 is an early marker of proliferating precursor cells that will differentiate into the neuronal lineage

Progression of progenitor cells towards neuronal differentiation is tightly linked with cell cycle control and the switch from proliferative to neuron-generating divisions. We have previously shown that the neuronal protein BM88 drives neuroblastoma cells towards exit from the cell cycle and differentiation into a neuronal phenotype in vitro. Here, we explored the role of BM88 during neuronal birth, cell cycle exit and the initiation of differentiation in vivo. By double- and triple-labelling with the S-phase marker BrdU or the late G2 and M-phase marker cyclin B1, antibodies to BM88 and markers of the neuronal or glial cell lineages, we demonstrate that in the rodent forebrain, BM88 is expressed in multipotential progenitor cells before terminal mitosis and in their neuronal progeny during the neurogenic interval, as well as in the adult. Further, we defined at E16 a cohort of proliferative progenitors that exit S phase in synchrony, and by following their fate for 24 h we show that BM88 is associated with the dynamics of neuron-generating divisions. Expression of BM88 was also evident in cycling cortical radial glial cells, which constitute the main neurogenic population in the cerebral cortex. In agreement, BM88 expression was markedly reduced and restricted to a smaller percentage of cells in the cerebral cortex of the Small eye mutant mice, which lack functional Pax6 and exhibit severe neurogenesis defects. Our data show an interesting correlation between BM88 expression and the progression of progenitor cells towards neuronal differentiation during the neurogenic interval.

Direct evidence for expression of dopamine receptors in astrocytes from basal ganglia

Expression of dopamine receptors (DA-Rs) in astrocytes was examined in vitro and in vivo using primary cultured astrocytes and brain slices from rat basal ganglia. Astrocytes from basal ganglia expressed DA D1-, D3-, D4- and D5-receptors and D4-mediated signal transduction in response to DA, suggesting possible involvement of astrocytes in the pharmacological action of atypical antipsychotic drugs and in DA response in some neurological diseases.

Members of the NF-kappaB family expressed in zones of active neurogenesis in the postnatal and adult mouse brain

The Rel/NF-kappaB family of transcription factors is implicated in cell proliferation, cell death, cell migration and cell interactions. Here, we examined by immunohistochemistry the expression pattern of various members of this family during postnatal telencephalon development and during adulthood, and we used neuronal and glial markers to identify the cells types where they are expressed. Distinct Rel/NF-kappaB proteins are highly expressed postnatally in the subventricular zone and in the rostral migratory stream. In particular, Rel A and p50 are expressed in radial glial cells, in migrating neuron precursors and in a population belonging to the astrocytic lineage. Rel B, on the other hand, is only expressed in migrating neuron precursors, whereas c-Rel is present in a few cells located at the edges of the rostral migratory stream. The expression of Rel A and p50 persists into adulthood, particularly in subventricular zone astrocyte-like cells and in migrating neuron precursors, respectively. The selective expression of NF-kappaB members in the postnatal subventricular zone and rostral migratory stream and their persistence into adulthood in regions of ongoing neurogenesis suggests possible mechanisms linking NF-kappaB expression with cell proliferation and migration. Their presence in actively proliferating progenitor cells, detected by BrdU staining, further suggests that NF-kappaB may be part of a signaling pathway that is important for neurogenesis.

Oligodendrocyte development and myelination in GFP-transgenic zebrafish

Green fluorescent protein (GFP) transgenic zebrafish technology has been employed to directly visualize and analyze dynamic developmental processes, such as cell migration and morphogenesis. Stable transgenic zebrafish that express GFP in oligodendrocytes can be a valuable tool to visualize complex myelination processes in vivo, as well as to conduct rapid mutagenesis screens for defective myelination mutants. We investigated whether two myelin gene promoters, the zebrafish P0 promoter and the mouse proteolipid protein (PLP) promoter, drive GFP expression in zebrafish oligodendrocytes. Transiently, both promoters drive enhanced GFP (EGFP) expression in morphologically identifiable oligodendrocytes, premyelinating oligodendrocytes, and possible oligodendrocyte precursors. We have established a stable transgenic zebrafish line, tg(plp:EGFP) zebrafish, at the F1 generation, which expresses enhanced GFP (EGFP) driven by the mouse PLP promoter. In this transgenic line, EGFP-expressing cells are visually detectable around 24-hr postfertilization (hpf), and later at 54 hpf, these cells start exhibiting the clear morphologic characteristics of oligodendrocytes. Shortly afterward, EGFP-expressing oligodendrocytes establish a ventral dominant distribution pattern throughout the central nervous system. This transgenic zebrafish line is likely to serve as a useful tool, in which normal myelination as well as abnormal myelination can be recorded under time-lapse confocal microscopy. Furthermore, it has the potential to greatly facilitate mutagenesis screening for novel dysmyelinating mutants.

Cellular distribution of glutamate transporters in the gastrointestinal tract of mice. An immunohistochemical and in situ hybridization approach

L-Glutamate transport by intestinal epithelial cells is an initial step of the entire glutamate metabolism pathway in the gut mucosa. The present study examined the cellular distribution of glutamate transporters in the digestive tract of adult mice using immunohistochemistry and in situ hybridization technique. Expression of EAAC1 mRNA was more intense in the ileum, where the epithelium in crypts and the basal half of intestinal villi showed high levels of transcripts, suggesting an essential role of EAAC1 in differentiating or premature epithelial cells. Electron-microscopically, EAAC1 immunoreactivity was predominantly localized in the striated border of enterocytes. Immunoreactivity for GLT-1 was found in the lateral membrane of epithelial cells at the bottom of gastric glands and at the intestinal crypts, and also in the lateral membrane of secretory cells at the duodenal gland. GLAST immunoreactivity was restricted to the fundic and pyloric glands, and was especially intense in the neck portion of both glands. However, in situ hybridization analysis failed to confirm the expression of GLT-1 and GLAST at the mRNA level, possibly due to limited sensitivity. The strong and specific luminal localization of EAAC1 in the intestinal epithelium suggests that EAAC1 is a predominant transporter of glutamate, at least in the lower part of the small intestine.

Gliotoxicity in hippocampal cultures is induced by transportable, but not by nontransportable, glutamate uptake inhibitors

Extracellular glutamate is kept below a toxic level by glial and neuronal glutamate transporters. Here we show that the transportable glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (t-PDC) induced cell death in mature, but not in immature, hippocampal neuron-enriched cultures. The cell death produced by a 24-hr treatment with t-PDC was dose-dependent and reached 85% of the cell population at a 250 microM concentration at 23 days in vitro (DIV). Immunocytochemistry experiments showed that, under these experimental conditions, t-PDC killed not only neurons as expected but also glial cells. The N-methyl-D-aspartate (NMDA) antagonist D-2-aminophosphonovalerate (D-APV; 250 microM) only partially reversed this toxicity, completely protecting the neuronal cell population but not the glial population. The antioxidant compounds alpha-tocopherol or Trolox, used at concentrations that reverse the oxidative stress-induced toxicity, did not block the gliotoxicity specifically produced by t-PDC in the presence of D-APV. The nontransportable glutamate uptake inhibitor DL-threo-beta-benzyloxyaspartate (TBOA) elicited cell death only in mature, but not in immature, hippocampal cultures. The TBOA toxic effect was dose dependent and reached a plateau at 100 microM in 23-DIV cultures. About 50% of the cell population died. TBOA affected essentially the neuronal population. D-APV (250 microM) completely reversed this toxicity. It is concluded that nontransportable glutamate uptake inhibitors are neurotoxic via overactivation of NMDA receptors, whereas transportable glutamate uptake inhibitors induce both an NMDA-dependent neurotoxicity and an NMDA- and oxidative stress-independent gliotoxicity, but only in mature hippocampal cultures.

Neural regeneration in the chick retina

In warm-blooded vertebrates, possibilities for retinal regeneration have recently become reality with the discovery of neural stem cells in the mature eye. A number of different cellular sources of neural stem cells have been identified. These sources include stem cells at the retinal margin, pigmented cells in the ciliary body and iris, non-pigmented cells in the ciliary body and Müller glia within the retina. This review focuses on recent reports of neural stem cells and regeneration in the postnatal chicken retina. In the chicken eye sources of neurogensis and regeneration include: (1) retinal stem cells at the peripheral edge of the retina; (2) Müller glia in central regions of the retina; (3) non-pigmented epithelial cells in the posterior portion of the ciliary body; and (4) possibly pigmented cells in the pars plana of the ciliary body. This review discusses the similarities between the retinal progenitor cells in the postnatal eye and those found in the embryo. In addition, I discuss combinations of growth factors, (insulin, IGF-I, EGF and FGF2) that are capable of stimulating the proliferation and production of neurons from neural progenitors, non-neural epithelial cells, and postmitotic support cells in the avian eye. In summary, the mechanisms that regulate the proliferation and differentiation of cells with neurogenic potential are beginning to be understood and the postnatal chicken eye has proven to be a useful model system to study retinal regeneration.

Specific characteristic of radial glia in the human fetal telencephalon

Phenotypic characteristics of cells in the developing human telencephalic wall were analyzed using electron microscopy and immunocytochemistry with various glial and neuronal cell markers. The results suggest that multiple defined cell types emerge in the neocortical proliferative zones and are differentially regulated during embryonic development. At 5-6 weeks gestation, three major cell types are observed. Most proliferating ventricular zone (VZ) cells are labeled with radial glial (RG) markers such as vimentin, glial fibrillary acidic protein (GFAP), and glutamate astrocyte-specific transporter (GLAST) antibodies. A subpopulation of these RG cells also express the neuronal markers beta III-tubulin, MAP-2, and phosphorylated neurofilament SMI-31, in addition to the stem cell marker nestin, indicating their multipotential capacity. In addition, the presence of VZ cells that immunoreact only with neuronal markers indicates the emergence of restricted neuronal progenitors. The number of multipotential progenitors in the VZ gradually decreases, whereas the number of more restricted progenitors increases systematically during the 3-month course of human corticogenesis. These results suggest that multipotential progenitors coexist with restricted neuronal progenitors and RG cells during initial corticogenesis in the human telencephalon. Since the multipotential VZ cells disappear during the major wave of neocortical neurogenesis, the RG and restricted neuronal progenitors appear to serve as the main sources of cortical neurons. Thus, the diversification of cells in human VZ and overlying subventricular zone (SVZ) begins earlier and is more pronounced than in rodents.

Isolation of Cystatin C via Functional Cloning of Astrocyte Differentiation Factors

We screened for factors upregulating glial fibrillary acidic protein (GFAP) promoter activity by functional cloning with an immature astrocyte cell line (HB108-10) harboring a GFAP-lacZ construct. One cDNA clone that repeatedly upregulated lacZ expression encoded cystatin C (CysC), a cysteine protease inhibitor. TGF-beta induced CysC and GFAP expression in AP-16 cells, an astrocyte progenitor-like cell line expressing GLAST (a glutamate transporter subtype specifically expressed in immature astrocytes). CysC gene expression started earlier than that of GFAP in the mouse forebrain. It started in the ventricular zone at a similar period as (or slightly after) GLAST expression, but before GFAP expression. Although previous data showed that CysC is involved in the maintenance of adult neural stem cells, our data indicate that it is involved in astrocyte differentiation during mouse brain development.

Glutamate-dependent transcriptional regulation of GLAST: role of PKC

The Na+-dependent glutamate/aspartate transporter GLAST plays a major role in the removal of glutamate from the synaptic cleft. Short-term, as well as long-term changes in transporter activity are triggered by glutamate. An important locus of regulation is the density of transporter molecules present at the plasma membrane. A substrate-dependent change in the translocation rate of the transporter molecules accounts for the short-term effect, whereas the long-term modulation apparently involves transcriptional regulation. Using cultured chick cerebellar Bergmann glial cells, we report here that glutamate receptors activation mediate a substantial reduction in the transcriptional activity of the chglast promoter through the Ca2+/diacylglicerol-dependent protein kinase (PKC) signaling cascade. Overexpression of constitutive active PKC isoforms of mimic the glutamate effect. Accordingly, increased levels of c-Jun or c-Fos, but not Jun-B, Jun-D or Fos-B, lower the chglast promoter activity. Serial deletions and electrophorectic mobility shift assays were used to define a specific region within the 5' proximal region of the chglast promoter, associated with transcriptional repression. A putative glutamate response element could be defined in the proximal promoter stretch more likely between nts -40 and -78. These results demonstrate that GLAST is under glutamate-dependent transcriptional control through PKC, and support the notion of a pivotal role of this neurotransmitter in the regulation of its own removal from the synaptic cleft, thereby modulating, mainly in the long term, glutamatergic transmission.

Mammalian septin Sept2 modulates the activity of GLAST, a glutamate transporter in astrocytes

Sept2 is a member of the septin family of GTPases. Septins form filaments in a GTP-form dependent manner, and are involved in cytokinesis from yeast to mammals; however, some mammalian septins, including Sept2, are expressed in the brain, a tissue in which almost all the cells are postmitotic. Recently, some functions of mammalian septin other than cytokinesis such as vesicle transport have been reported. However, mammalian septin's physiological functions are still unclear. The present study revealed that Sept2 co-localizes with the astrocyte glutamate transporter GLAST in the Bergmann glial processes facing axons and synapses. Biochemical analyses demonstrated that Sept2 bound directly to the carboxy-terminal region of GLAST in a GDP-form dependent manner. Expression of constitutive GDP-form Sept2 mutant reduced the glutamate uptake activity of GLAST via internalization of GLAST from cell surface. Thus Sept2 may regulate GLAST-mediated glutamate uptake by astrocytes, which is important for appropriate transmitter signalling in the cerebellum.

Converse control of oligodendrocyte and astrocyte lineage development by Sonic hedgehog in the chick spinal cord

In the developing spinal cord, oligodendrocyte progenitors (OLPs) originate from the ventral neuroepithelium and the specification of this lineage depends on the inductive activity of Sonic hedgehog (Shh) produced by ventral midline cells. On the other hand, it has been shown that OLP identity is acquired by the coexpression of the transcription factors olig2 and nkx2.2. Although initially expressed in adjacent nonoverlapping domains of the ventral neuroepithelium, these transcription factors become coexpressed in the pMN domain at the time of OLP specification through dorsal extension of the Nkx2.2 domain. Here we show that Shh is sufficient to promote the coexpression of Olig2 and Nkx2.2 in neuroepithelial cells. In addition, Shh activity is necessary for this coexpression since blocking Shh signalling totally abolishes Olig2 expression and impedes dorsal extension of Nkx2.2. Although Shh at these stages affects neuroepithelial cell proliferation, the dorsal extension of the Nkx2.2 domain is not due to progenitor proliferation but to repatterning of the ventral neuroepithelium. Finally, Shh not only stimulates OLP specification but also simultaneously restricts the ventral extension of the astrocyte progenitor (AP) domain and reduces astrocyte development. We propose that specification of distinct glial lineages is the result of a choice that depends on Shh signalling.

Specific induction of neuronal cells from bone marrow stromal cells and application for autologous transplantation

Bone marrow stromal cells (MSCs) have the capability under specific conditions of differentiating into various cell types such as osteocytes, chondrocytes, and adipocytes. Here we demonstrate a highly efficient and specific induction of cells with neuronal characteristics, without glial differentiation, from both rat and human MSCs using gene transfection with Notch intracellular domain (NICD) and subsequent treatment with bFGF, forskolin, and ciliary neurotrophic factor. MSCs expressed markers related to neural stem cells after transfection with NICD, and subsequent trophic factor administration induced neuronal cells. Some of them showed voltage-gated fast sodium and delayed rectifier potassium currents and action potentials compatible with characteristics of functional neurons. Further treatment of the induced neuronal cells with glial cell line-derived neurotrophic factor (GDNF) increased the proportion of tyrosine hydroxylase-positive and dopamine-producing cells. Transplantation of these GDNF-treated cells showed improvement in apomorphine-induced rotational behavior and adjusting step and paw-reaching tests following intrastriatal implantation in a 6-hydroxy dopamine rat model of Parkinson disease. This study shows that a population of neuronal cells can be specifically generated from MSCs and that induced cells may allow for a neuroreconstructive approach.

Spatiotemporal heterogeneity of CNS radial glial cells and their transition to restricted precursors

Radial glia are among the first cells that develop in the embryonic central nervous system. They are progenitors of glia and neurons but their relationship with restricted precursors that are also derived from neuroepithelia is unclear. To clarify this issue, we analyzed expression of cell type specific markers (BLBP for radial glia, 5A5/E-NCAM for neuronal precursors and A2B5 for glial precursors) on cortical radial glia in vivo and their progeny in vitro. Clones of cortical cells initially expressing only BLBP gave rise to cells that were A2B5+ and eventually lost BLBP expression in vitro. BLBP is expressed in the rat neuroepithelium as early as E12.5 when there is little or no staining for A2B5 and 5A5. In E13.5-15.5 forebrain, A2B5 is spatially restricted co-localizing with a subset of the BLBP+ radial glia. Analysis of cells isolated acutely from embryonic cortices confirmed that BLBP expression could appear without, or together with, A2B5 or 5A5. The numbers of BLBP+/5A5+ cells decreased during neurogenesis while the numbers of BLBP+/A2B5+ cells remained high through the beginning of gliogenesis. The combined results demonstrate that spatially restricted subpopulations of radial glia along the dorsal-ventral axis acquire different markers for neuronal or glial precursors during CNS development.

Changes of high-affinity choline transporter CHT1 mRNA expression during degeneration and regeneration of hypoglossal nerves in mice

The high-affinity choline transporter CHT1 works for choline uptake in the presynaptic terminals of cholinergic neurons. We examined its expression in the hypoglossal nucleus after unilateral hypoglossal nerve transection in mice by fluorescent in situ hybridization. One week after axotomy, CHT1 mRNA expression was lost in all hypoglossal motoneurons in the lesioned side. Two weeks after axotomy, CHT1 mRNA started to be re-expressed in a few motoneurons that recovered connections to tongue muscles as revealed by retrograde labeling with Fast Blue. After 4 weeks, most of axotomized hypoglossal motoneurons were reconnected and re-expressed CHT1 mRNA as strongly as control neurons, and the regenerating cholinergic axons established mature neuromuscular junctions. These results suggest that the establishment of motor innervation is critical for CHT1 mRNA expression in hypoglossal neurons after axotomy.

Glycine cleavage system in neurogenic regions

The glycine cleavage system (GCS) is the essential enzyme complex for degrading glycine and supplying 5,10-methylenetetrahydrofolate for DNA synthesis. Inherited deficiency of this system causes nonketotic hyperglycinemia, characterized by severe neurological symptoms and frequent association of brain malformations. Although high levels of glycine have been considered to cause the above-mentioned problems, the detailed pathogenesis of this disease is still unknown. Here we show that GCS is abundantly expressed in rat embryonic neural stem/progenitor cells in the neuroepithelium, and this expression is transmitted to the radial glia-astrocyte lineage, with prominence in postnatal neurogenic regions. These data indicate that GCS plays important roles in neurogenesis, and suggest that disturbance of neurogenesis induced by deficiency of GCS may be the main pathogenesis of nonketotic hyperglycinemia.

Expression of glutamate transporter GLAST in the developing mouse cochlea

The immunohistochemical localization of glutamate transporter GLAST in the developing mouse cochlea was studied at different ages between 0 and 30 days after birth (DAB). In the adult mouse cochlea, intense GLAST-like immunoreactivity was found in the supporting cells adjacent to the inner hair cells of the organ of Corti, the type II and suprastrial fibrocytes of the cochlear lateral wall, the fibrocytes of the spiral limbus and the satellite cells surrounding the spiral ganglion cells. At 0 DAB, weak GLAST-like immunoreactivity was found in the supporting cells around the immature inner hair cells. Immature fibrocytes in the cochlea were also positively immunostained. At 3 DAB, weak immunostaining of GLAST appeared in the immature satellite cells in the spiral ganglion. The GLAST-like immunoreactivity in the supporting cells around the inner hair cells, in the fiborocytes in the spiral ligament and the spiral limbus and in the satellite cells in the spiral ganglion increased progressively during the second postnatal week, and reached the adult level at 15 DAB. This time course correlates with the electrophysiological onset and maturation of the mouse auditory function, which is mediated by glutamatergic neurotransmission. These results suggest that the expression of GLAST may be needed for the efficient removal and metabolism of the released glutamate in the cochlea and may play important roles in the onset and maturation of the auditory system.

Early acquisition of typical metabolic features upon differentiation of mouse neural stem cells into astrocytes

Specific metabolic features, such as glutamate reuptake, have been associated with normal functions of mature astrocytes. In this study, we examined whether these characteristics are acquired together with classical phenotypic markers of differentiated astrocytes. Differentiation of E14 mouse neurospheres into astrocytes was induced by the addition of fetal bovine serum (FBS). Degree of differentiation was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence for both GFAP and nestin. Neural stem cells expressed nestin but not GFAP, while differentiated astrocytes were immunopositive for GFAP but displayed low levels of nestin expression. A strong increase in the expression of the glutamate transporter GLAST and the monocarboxylate transporter MCT1 accompanied phenotypic changes. In addition, active glutamate transport appeared in differentiated astrocytes, as well as their capacity to increase aerobic glycolysis in response to glutamate. Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor, but not interleukin-6, triggered the expression of phenotypic and morphological characteristics of astrocytes. In addition, exposure to LIF led to the appearance of metabolic features typically associated with astrocytes. Altogether, our results show that acquisition of some specific metabolic features by astrocytes occurs early in their differentiation process and that LIF represents a candidate signal to induce their expression.

Functional analysis of mouse 3-phosphoglycerate dehydrogenase (Phgdh) gene promoter in developing brain

D-3-Phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95) is a necessary enzyme for de novo L-serine biosynthesis via the phosphorylated pathway. Targeted disruption of the mouse Phgdh gene has been shown to result in embryonic lethality, accompanied by severe abnormalities in brain development. Phgdh is expressed exclusively by neuroepithelium and radial glia in developing brain and later mainly by astrocytes. To elucidate the molecular mechanism that regulates such cell-type-specific expression of Phgdh in developing brain, an upstream 3.5-kilobase-pair (kbp) region of the gene harboring the promoter was characterized in primary cultures and transgenic mice. Analysis of Phgdh 5'-nested deletions in transfected cultures indicated that overall reporter luciferase levels were higher in glial cultures than those in neuronal cultures. Although basal promoter activity of the gene appeared to depend on an Sp1 binding sequence residing between -193 and -184 in both glial and neuronal cultures, an upstream 5'-flanking region between -1,794 and -1,095 contributed to up-regulation of Phgdh transcription in a glial-cell-specific manner. In the cerebral cortex of transgenic mouse embryos, the Phgdh promoter-LacZ transgene DNA containing -1,794/+4 promoter sequences directed beta-galactosidase (beta-Gal) expression mainly to Phgdh-positive neuroepithelium and radial glia. This glial preference diminished when beta-Gal expression was driven solely by the upstream 0.2-kbp minimal promoter. However, glial preference of beta-Gal expression was restored by placing the 700-base-pair 5'-DNA segment upstream of the minimal promoter. These observations suggest the presence of cis-acting elements that confer the cell type specificity of Phgdh transcription in the distal promoter region.

Targeted disruption of the mouse 3-phosphoglycerate dehydrogenase gene causes severe neurodevelopmental defects and results in embryonic lethality

D-3-Phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95) is the first committed enzyme of L-serine biosynthesis in the phosphorylated pathway. To determine the physiological importance of Phgdh-dependent L-serine biosynthesis in vivo, we generated Phgdh-deficient mice using targeted gene disruption in embryonic stem cells. The absence of Phgdh led to a drastic reduction of L-serine metabolites such as phosphatidyl-L-serine and sphingolipids. Phgdh null embryos have small bodies with abnormalities in selected tissues and died after days post-coitum 13.5. Striking abnormalities were evident in the central nervous system in which the Phgdh null mutation culminated in hypoplasia of the telencephalon, diencephalon, and mesencephalon; in particular, the olfactory bulbs, ganglionic eminence, and cerebellum appeared as indistinct structures. These observations demonstrate that the Phgdh-dependent phosphorylated pathway is essential for normal embryonic development, especially for brain morphogenesis.

The glial identity of neural stem cells

Glia are the most numerous cells in the brain, and their many diverse functions highlight their essential role in the nervous system. Recent studies have revealed an unexpected new role for glia in a wide variety of species, that of stem cells/progenitors in the adult and embryonic brain. Differentiation along the glial lineage may be a default state of development reflected in the progression of stem cells along the neuroepithelial-->radial glia-->astrocyte lineage.

The potential to induce glial differentiation is conserved between Drosophila and mammalian glial cells missing genes

Drosophila glial cells missing (gcm) is a key gene that determines the fate of stem cells within the nervous system. Two mouse gcm homologs have been identified, but their function in the nervous system remains to be elucidated. To investigate their function, we constructed retroviral vectors harboring Drosophila gcm and two mouse Gcm genes. Expression of these genes appeared to influence fibroblast features. In particular, mouse Gcm1 induced the expression of astrocyte-specific Ca(2+)-binding protein, S100beta, in those cells. Introduction of the mouse Gcm1 gene in cultured cells from embryonic brains resulted in the induction of an astrocyte lineage. This effect was also observed by in utero injection of retrovirus harboring mouse Gcm1 into the embryonic brain. However, cultures from mouse Gcm1-deficient mouse brains did not exhibit significant reductions in the number of astrocytes. Furthermore, in situ hybridization analysis of mouse Gcm1 mRNA revealed distinct patterns of expression in comparison with other well-known glial markers. The mammalian homolog of Drosophila gcm, mouse Gcm1, exhibits the potential to induce gliogenesis, but may function in the generation of a minor subpopulation of glial cells.

Disruption of postnatal progenitor migration and consequent abnormal pattern of glial distribution in the cerebrum following administration of methylmercury

Transplacental administration of methylmercury (MeHg) induces disruption of neuronal migration in the developing cerebral cortex. However, the effects of MeHg on glial progenitor migration remain unclear. To understand this, we performed double administration of MeHg and 5-bromo-2-deoxyuridine (BrdU) to neonatal rat pups on postnatal day 2 (P2), when glial cells are generated from progenitors in the subventricular zone (SVZ). Histopathological examination of a proportion of the MeHg-treated rats on P28 revealed no apparent abnormalities of cytoarchitecture or neuron count in either the primary motor or primary somatosensory cortex of the cerebrum. BrdU immunohistochemistry revealed abnormal accumulation of the labeled cells in the deeper layers of the cortices and underlying white matter of both areas, where an excessive number of astrocytes (glial fibrillary acidic protein- or S-100beta-immunolabeled cells) and oligodendrocytes (2',3'-cyclic-nucleotide 3'-phosphohydrolase-labeled cells) were located. Next, to investigate the migration of individual progenitors from the forebrain SVZ of P2 neonates, we labeled them in vivo with a retrovirus encoding green fluorescent protein (GFP), following administration of MeHg, and then examined the distribution pattern of the GFP-labeled cells in the P28 cerebrum. We found that the labeled cells developed into astrocytes and oligodendrocytes and were accumulated abnormally in the lateral white matter as well as in the adjacent deeper layer of the lateral cortex and lateral side of the striatum. Thus, exposure to MeHg in the gliogenic period induced irregular distribution of glia as a consequence of abnormal migration of the postnatal progenitors.

Development of midline glial populations at the corticoseptal boundary

Three midline glial populations are found at the corticoseptal boundary: the glial wedge (GW), glia within the indusium griseum (IGG), and the midline zipper glia (MG). Two of these glial populations are involved in axonal guidance at the cortical midline, specifically development of the corpus callosum. Here we investigate the phenotypic and molecular characteristics of each population and determine whether they are generated at the same developmental stage. We find that the GW is derived from the radial glial scaffold of the cortex. GW cells initially have long radial processes that extend from the ventricular surface to the pial surface, but by E15 loose their pial attachment and extend only part of the way to the pial surface. Later in development the radial morphology of cells within the GW is replaced by multipolar astrocytes, providing supportive evidence that radial glia can transform into astrocytes. IGG and MG do not have a radial morphology and do not label with the radial glial markers, Nestin and RC2. We conclude that the GW and IGG have different morphological and molecular characteristics and are born at different stages of development. IGG and MG have many phenotypic and molecular characteristics in common, indicating that they may represent a common population of glia that becomes spatially distinct by the formation of the corpus callosum.

Reelin signaling directly affects radial glia morphology and biochemical maturation

Radial glial cells are characterized, besides their astroglial properties, by long radial processes extending from the ventricular zone to the pial surface, a crucial feature for the radial migration of neurons. The molecular signals that regulate this characteristic morphology, however, are largely unknown. We show an important role of the secreted molecule reelin for the establishment of radial glia processes. We describe a significant reduction in ventricular zone cells with long radial processes in the absence of reelin in the cortex of reeler mutant mice. These defects were correlated to a decrease in the content of brain lipid-binding protein (Blbp) and were detected exclusively in the cerebral cortex, but not in the basal ganglia of reeler mice. Conversely, reelin addition in vitro increased the Blbp content and process extension of radial glia from the cortex, but not the basal ganglia. Isolation of radial glia by fluorescent-activated cell sorting showed that these effects are due to direct signaling of reelin to radial glial cells. We could further demonstrate that this signaling requires Dab1, as the increase in Blbp upon reelin addition failed to occur in Dab1-/- mice. Taken together, these results unravel a novel role of reelin signaling to radial glial cells that is crucial for the regulation of their Blbp content and characteristic morphology in a region-specific manner.

Ephrin-B2 and EphB1 mediate retinal axon divergence at the optic chiasm

In animals with binocular vision, retinal ganglion cell (RGC) axons either cross or avoid the midline at the optic chiasm. Here, we show that ephrin-Bs in the chiasm region direct the divergence of retinal axons through the selective repulsion of a subset of RGCs that express EphB1. Ephrin-B2 is expressed at the mouse chiasm midline as the ipsilateral projection is generated and is selectively inhibitory to axons from ventrotemporal (VT) retina, where ipsilaterally projecting RGCs reside. Moreover, blocking ephrin-B2 function in vitro rescues the inhibitory effect of chiasm cells and eliminates the ipsilateral projection in the semiintact mouse visual system. A receptor for ephrin-B2, EphB1, is found exclusively in regions of retina that give rise to the ipsilateral projection. EphB1 null mice exhibit a dramatically reduced ipsilateral projection, suggesting that this receptor contributes to the formation of the ipsilateral retinal projection, most likely through its repulsive interaction with ephrin-B2.

The human NTERA2 neural cell line generates neurons on growth under neural stem cell conditions and exhibits characteristics of radial glial cells

NTERA2 cells are a human neural cell line generating neurons after exposure to retinoic acid and, as such, are widely used as a model of neurogenesis. We report that these cells form spheres when grown in serum-free medium supplemented with basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). These spheres were found to express markers of radial glial cells such as, Pax6, glutamate transporter (GLAST), tenascin C, brain lipid-binding protein (BLBP), and the 3CB2 antigen. On plating on an adhesive substrate, NTERA2 spheres generate a large percentage of immature neurons (30-50%) together with a minority of cells of the oligodendrocyte lineage. Thus NTERA2 cells share properties with neural stem cells. However, at variance with the latter, we found that they produce their own bFGF implicated in an autocrine or paracrine proliferative loop and that they do not generate astrocytes after differentiation. These results provide an interesting model to study radial glial cells and their role in human neurogenesis.

Reduced expression of glutamate transporter EAAT2 and impaired glutamate transport in human primary astrocytes exposed to HIV-1 or gp120

L-Glutamate is the major excitatory neurotransmitter in the brain. Astrocytes maintain low levels of synaptic glutamate by high-affinity uptake and defects in this function may lead to neuronal cell death by excitotoxicity. We tested the effects of HIV-1 and its envelope glycoprotein gp120 upon glutamate uptake and expression of glutamate transporters EAAT1 and EAAT2 in fetal human astrocytes in vitro. Astrocytes isolated from fetal tissues between 16 and 19 weeks of gestation expressed EAAT1 and EAAT2 RNA and proteins as detected by Northern blot analysis and immunoblotting, respectively, and the cells were capable of specific glutamate uptake. Exposure of astrocytes to HIV-1 or gp120 significantly impaired glutamate uptake by the cells, with maximum inhibition within 6 h, followed by gradual decline during 3 days of observation. HIV-1-infected cells showed a 59% reduction in V(max) for glutamate transport, indicating a reduction in the number of active transporter sites on the cell surface. Impaired glutamate transport after HIV-1 infection or gp120 exposure correlated with a 40-70% decline in steady-state levels of EAAT2 RNA and protein. EAAT1 RNA and protein levels were less affected. Treatment of astrocytes with tumor necrosis factor-alpha (TNF-alpha) decreased the expression of both EAAT1 and EAAT2, but neither HIV-1 nor gp120 were found to induce TNF-alpha production by astrocytes. These findings demonstrate that HIV-1 and gp120 induce transcriptional downmodulation of the EAAT2 transporter gene in human astrocytes and coordinately attenuate glutamate transport by the cells. Reduction of the ability of HIV-1-infected astrocytes to take up glutamate may contribute to the development of neurological disease.

Signaling to and from radial glia

Radial glia represent the major glial cell type in the developing CNS and perform many essential functions, which range from acting as neural precursors to providing physical substrates for newborn neurons to migrate on. Previous work has shown that cell-cell signaling is important for the development of the radial glial phenotype. In particular, signals from newborn neurons appear to contribute significantly to the formation of this cell type. In addition, radial glia may be involved in reciprocal signaling roles that contribute to regional patterning and differentiation in the developing CNS.

Gliogenic and neurogenic progenitors of the subventricular zone: who are they, where did they come from, and where are they going?

The subventricular zone (SVZ) of the perinatal forebrain gives rise to both neurons and glia. The mechanisms governing the phenotypic specification of progenitors within this heterogeneous germinal zone are unclear. However, the characterization of subpopulations of SVZ cells has given us a better understanding of the basic architecture of the SVZ and presents us with the opportunity to ask more detailed questions regarding phenotype specification and cell fate. Recent work demonstrating the embryonic origins of SVZ cells is summarized, and a model describing the formation of the perinatal SVZ, noting contributions of cells from pallial as well as subpallial germinal zones, is presented. We further address differences among classes of SVZ cells based on molecular profile, phenotype, and migration behavior and present a model summarizing the organization of perinatal SVZ cells along coronal, sagittal, and horizontal axes. A detailed description of the SVZ in the adult, outlining classes of cells based on morphology, molecular profile, and proliferative behavior, was recently reported by Doetsch et al. (Proc Natl Acad Sci USA 93:14895-14900, 1997). Potential relationships among cells within the perinatal and adult SVZ will be discussed. GLIA 43:52-61, 2003.

Spatiotemporal distribution of neuronal calcium sensor-1 in the developing rat spinal cord

The present study revealed the localization of neuronal calcium sensor (NCS)-1 immunoreactivity (IR) in the developing rat spinal cord. The NCS-1 IR first appeared at embryonic day 12 in the peripheral nerves and their somata. Intense NCS-1 IR was expressed in ascending and descending tracts in the white matter during the late prenatal period, which gradually decreased to the faint level during postnatal development. Intense NCS-1 IR was colocalized with growth associated protein (GAP)-43 IR in the marginal zone and with the glutamate-aspartate transporter (GLAST) IR in the radial processes traversing the marginal zone. In the adult rat white matter, radially oriented astrocytes and astrocytes in the glia limitans were double-labeled for NCS-1 and glial fibrillary acidic protein (GFAP), whereas small dots on finger-like dendritic projections were double-labeled for NCS-1 and synaptophysin. In the developing gray matter, the NCS-1 IR appeared at embryonic day 12 and gradually increased in the neuronal somata and neuropil, reaching a plateau after the end of the 4th postnatal week. The small dots in neuropil were colabeled for NCS-1 and GFAP or NCS-1 and synaptophysin in the adult rat gray matter. These results strongly suggest that NCS-1 is involved in axogenesis and synaptogenesis in the developing rat spinal cord. NCS-1 can serve as a Ca(2+)-sensor not only in neurons but also in radial glial cells or even in radially oriented astrocytes in the adult rat spinal cord.

Novel neuroglial and glioglial relationships mediated by L-serine metabolism

L-Serine is a non-essential amino acid that can be synthesized in the body. It derives from an intermediate of the glycolytic pathway, 3-phosphoglycerate, and utilized for the syntheses of proteins, other amino acids, membrane lipids, heme, and nucleotides. Emerging evidence indicates that L-serine functions as a glia-derived trophic factor, which strongly promotes the survival and differentiation of cultured neurons. L-Serine biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH) and small neutral amino acid transporter ASCT1 have been revealed to be expressed preferentially in the radial glia-astrocyte lineage and olfactory ensheathing glia of both adult and developing rodent brains. In contrast, these biosynthetic and transporter molecules for L-serine are faint or undetectable in neurons and phagocytic cells. In this review, we summarize recent progress to propose that L-serine synthesis in these glial cells and its supply to nearby neurons and other glia constitute a novel metabolic unit in the brain. Based on these neuroglial and glioglial relationships, glucose in neurons and phogocytes can be strategically used for energy production, while a variety of L-serine-derived biomolecules required for their proliferaton, survival, differentiation, and function are synthesized in and supplied from the radial glia-astrocyte lineage and olfactory ensheathing glia. A transient capillary expression of ASCT1 in fetal and neonatal brains further suggests that, in addition to the glia-borne L-serine, an active transport of blood-borne L-serine would play an essential role in neural development.

Some glial progenitors in the neonatal subventricular zone migrate through the corpus callosum to the contralateral cerebral hemisphere

The great majority of glial cells of the mammalian forebrain are generated in the perinatal period from progenitors in the subventricular zone (SVZ). We investigated the migration of progenitors from the neonatal (postnatal day 0, P0) rat forebrain SVZ by labeling them in vivo with a green fluorescence protein (GFP) retrovirus and monitoring their movements by time-lapse video microscopy in P3 slices. We identified a small number of progenitors that migrated tangentially within the corpus callosum (CC) and crossed the midline. These cells retained a relatively uniform morphology: the leading process was extended toward the contralateral side but showed no process branching or turning away from the migratory direction. Net migration requires the elongation of the leading process and nuclear translocation, and the migrating cells in the CC showed both modes. We confirmed the presence of unmyelinated axon bundles within the P3 CC, but failed to detect any radially directed glial processes (vimentin- or GLAST-immunolabeled fibers) spanning through the CC. Confocal images showed a close proximity between neurofilament-immunolabeled axons and the leading process of the GFP-expressing progenitors in the CC. The destination of the callosal fibers was examined by applying DiI to the right cingulum; the labeled fibers ran throughout the CC and reached the left cingulate and motor areas. The distribution and final fates of the retrovirus-labeled cells were examined in P28 brains. A small proportion of the labeled cells were found in the contralateral hemisphere, where, as oligodendrocytes and astrocytes, they colonized predominantly the cortex and the underlying white matter of the cingulate and secondary motor areas. The distribution pattern appears to coincide well with the projection direction of the callosal fibers. Thus, glial progenitors migrate across the CC, presumably in conjunction with unmyelinated axons, to colonize the contralateral hemisphere.

Neuregulin 1-erbB2 signaling is required for the establishment of radial glia and their transformation into astrocytes in cerebral cortex

Radial glial cells and astrocytes function to support the construction and maintenance, respectively, of the cerebral cortex. However, the mechanisms that determine how radial glial cells are established, maintained, and transformed into astrocytes in the cerebral cortex are not well understood. Here, we show that neuregulin-1 (NRG-1) exerts a critical role in the establishment of radial glial cells. Radial glial cell generation is significantly impaired in NRG mutants, and this defect can be rescued by exogenous NRG-1. Down-regulation of expression and activity of erbB2, a member of the NRG-1 receptor complex, leads to the transformation of radial glial cells into astrocytes. Reintroduction of erbB2 transforms astrocytes into radial glia. The activated form of the Notch1 receptor, which promotes the radial glial phenotype, activates the erbB2 promoter in radial glial cells. These results suggest that developmental changes in NRG-1-erbB2 interactions modulate the establishment of radial glia and contribute to their appropriate transformation into astrocytes.

Glutamate levels and transport in cat (Felis catus) area 17 during cortical reorganization following binocular retinal lesions

Glutamate is known to play a crucial role in the topographic reorganization of visual cortex after the induction of binocular central retinal lesions. In this study we investigated the possible involvement of the glial high-affinity Na+/K+-dependent glutamate transporters in cortical plasticity using western blotting and intracortical microdialysis. Basal extracellular glutamate levels and the re-uptake activity for glutamate have been determined by comparing the extracellular glutamate concentration before and during the blockage of glutamate removal from the synaptic cleft with the potent transporter inhibitor l-trans-pyrrolidine-3,4-dicarboxylic acid. In cats with central retinal lesions we observed increased basal extracellular glutamate concentrations together with a decreased re-uptake activity in non-deprived, peripheral area 17, compared with the sensory-deprived, central cortex of the same animal as well as the topographically matching regions of area 17 in normal subjects. Western blotting experiments revealed a parallel decrease in the expression level of the glial glutamate transporter proteins GLT-1 and GLAST in non-deprived cortex compared with sensory-deprived cortex of lesion cats and the corresponding regions of area 17 of normal subjects. This study shows that partial sensory deprivation of the visual cortex affects the removal of glutamate from the synaptic cleft and implicates a role for glial-neuronal interactions in adult brain plasticity.

Glial glutamate transporters mediate a functional metabolic crosstalk between neurons and astrocytes in the mouse developing cortex

Neuron-glia interactions are essential for synaptic function, and glial glutamate (re)uptake plays a key role at glutamatergic synapses. In knockout mice, for either glial glutamate transporters, GLAST or GLT-1, a classical metabolic response to synaptic activation (i.e., enhancement of glucose utilization) is decreased at an early functional stage in the somatosensory barrel cortex following activation of whiskers. Investigation in vitro demonstrates that glial glutamate transport represents a critical step for triggering enhanced glucose utilization, but also lactate release from astrocytes through a mechanism involving changes in intracellular Na(+) concentration. These data suggest that a metabolic crosstalk takes place between neurons and astrocytes in the developing cortex, which would be regulated by synaptic activity and mediated by glial glutamate transporters.

Neutral amino acid transporter ASCT1 is preferentially expressed in L-Ser-synthetic/storing glial cells in the mouse brain with transient expression in developing capillaries

Nonessential amino acid L-Ser plays an essential role in neuronal survival and differentiation, through preferential expression of the L-Ser biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH), in particular in glial cells but not in neurons. To seek the molecular candidates responsible for glia-borne L-Ser transport, we performed histochemical analyses on amino acid transporter ASCT1, which prefers small neutral amino acids, such as Ala, Ser, Cys, and Thr, and mediates their obligatory exchange. At early developmental stages, neuroepithelial cells constituting the ventricular zone expressed ASCT1 mRNA and protein ubiquitously. Thereafter, ASCT1 expression was gradually downregulated in neuronal populations during the late embryonic and neonatal periods, whereas its high expression was transmitted to radial glial cells and then to astrocytes. High levels of ASCT1 were also detected in the olfactory ensheathing glia. The preferential glial expression of ASCT1 was consistent with that of 3PGDH, and their extensive colocalization was demonstrated at the cellular level. Moreover, high cellular contents of L-Ser were revealed in these glial cells by using a specific antibody to L-Ser. These results strongly suggest that a large amount of L-Ser is synthesized and stored in these glial cells and is released through ASCT1 in exchange for other extracellular substrates. In addition, we observed prominent expression of ASCT1 in capillary endothelial cells of embryonic and neonatal brains. Therefore, ASCT1 appears to be regulated to meet metabolic demands by differentiating and mature neurons through the transport of glia- and blood-borne small neutral amino acids.

Differential regulation of fibroblast growth factor receptors in the regenerating amphibian spinal cord in vivo

Unlike mammals, adult urodele amphibians can regenerate their spinal cord and associated ganglia, but the molecular mechanisms controlling regeneration are not fully understood. We have recently shown that expression of FGF2, a member of the fibroblast growth factor family, is induced in the progenitor cells of the regenerating spinal cord and appears to play a role in their proliferation and possibly in their differentiation. In order to investigate which receptor(s) may mediate FGF2 signaling and their role in regeneration, we have studied expression of the four fibroblast growth factor receptors, FGFR1, FGFR2, FGFR3 and FGFR4, and of the spliced variants, sFGFR and KGFR, in the regenerating spinal cord of the adult urodele, Pleurodeles waltl, following tail amputation. We show that all FGFRs are expressed in normal and regenerating spinal cord, with the exception of the spliced variants that are expressed only in non-neural tissues of the tail. FGFR1 and 4 show the more interesting spatio-temporal patterns of expression. They are not detectable in the ependymal cells of normal cords, from which neural progenitors for regeneration are believed to originate, though they are expressed in some mature neurons. During regeneration, significant up-regulation of FGFR1 precedes that of FGFR4 in the ependymal tube from which the new cord will form. FGFR4 is highly expressed in these cells at later stages of regeneration, when neuronal differentiation is becoming apparent, and like FGFR1 is also expressed in some newborn neurons. In addition to the known form of FGFR1, the antibody against this receptor reacts also with a non-phosphorylated protein that appears to be present only during regeneration, and might represent a yet undescribed variant of the receptor. Altogether this study shows that fibroblast growth factor signaling is finely modulated during tail and spinal cord regeneration, and points to FGFR1 and FGFR4 as key players in this process, suggesting that FGFR1 is primarily associated with proliferation of progenitor cells and FGFR4 with early stages of neuronal differentiation.