Glutamine synthetase expression in rat oligodendrocytes in culture: regulation by hormones and growth factors

Effects of adult male rat feminization treatments on brain morphology and metabolomic profile

Body feminization, as part of gender affirmation process of transgender women, decreases the volume of their cortical and subcortical brain structures. In this work, we implement a rat model of adult male feminization which reproduces the results in the human brain and allows for the longitudinal investigation of the underlying structural and metabolic determinants in the brain of adult male rats undergoing feminization treatments. Structural MRI and Diffusion Tensor Imaging (DTI) were used to non-invasively monitor in vivo cortical brain volume and white matter microstructure over 30 days in adult male rats receiving estradiol (E₂), estradiol plus cyproterone acetate (CA), an androgen receptor blocker and antigonadotropic agent (E₂ + CA), or vehicle (control). Ex vivo cerebral metabolic profiles were assessed by ¹H High Resolution Magic Angle Spinning NMR (¹H HRMAS) at the end of the treatments in samples from brain regions dissected after focused microwave fixation (5 kW). We found that; a) Groups receiving E₂ and E₂ + CA showed a generalized bilateral decrease in cortical volume; b) the E₂ + CA and, to a lesser extent, the E₂ groups maintained fractional anisotropy values over the experiment while these values decreased in the control group; c) E₂ treatment produced increases in the relative concentration of brain metabolites, including glutamate and glutamine and d) the glutamine relative concentration and fractional anisotropy were negatively correlated with total cortical volume. These results reveal, for the first time to our knowledge, that the volumetric decreases observed in trans women under cross-sex hormone treatment can be reproduced in a rat model. Estrogens are more potent drivers of brain changes in male rats than anti-androgen treatment.

Distribution of immunoreactive glutamine synthetase in the adult human and mouse brain. Qualitative and quantitative observations with special emphasis on extra-astroglial protein localization

Glutamine synthetase catalyzes the ATP-dependent condensation of ammonia and glutamate to form glutamine, thus playing a pivotal role in glutamate and glutamine homoeostasis. Despite a plethora of studies on this enzyme, knowledge about the regional and cellular distribution of this enzyme in human brain is still fragmentary. Therefore, we mapped fourteen post-mortem brains of psychically healthy individuals for the distribution of the glutamine synthetase immunoreactive protein. It was found that glutamine synthetase immunoreactivity is expressed in multiple gray and white matter astrocytes, but also in oligodendrocytes, ependymal cells and certain neurons. Since a possible extra-astrocytic expression of glutamine synthetase is highly controversial, we paid special attention to its appearance in oligodendrocytes and neurons. By double immunolabeling of mouse brain slices and cultured mouse brain cells for glutamine synthetase and cell-type-specific markers we provide evidence that besides astrocytes subpopulations of oligodendrocytes, microglial cells and neurons express glutamine synthetase. Moreover, we show that glutamine synthetase-immunopositive neurons are not randomly distributed throughout human and mouse brain, but represent a subpopulation of nitrergic (i.e. neuronal nitric oxide synthase expressing) neurons. Possible functional implications of an extra-astrocytic localization of glutamine synthetase are discussed.

Neurofilament-tubulin binding site peptide NFL-TBS.40-63 increases the differentiation of oligodendrocytes in vitro and partially prevents them from lysophosphatidyl choline toxiciy

During multiple sclerosis (MS), the main axon cystoskeleton proteins, neurofilaments (NF), are altered, and their release into the cerebrospinal fluid correlates with disease severity. The role of NF in the extraaxonal location is unknown. Therefore, we tested whether synthetic peptides corresponding to the tubulin-binding site (TBS) sequence identified on light NF chain (NFL-TBS.40-63) and keratin (KER-TBS.1-24), which could be released during MS, modulate remyelination in vitro. Biotinylated NFL-TBS.40-63, NFL-Scramble2, and KER-TBS.1-54 (1-100 μM, 24 hr) were added to rat oligodendrocyte (OL) and astrocyte (AS) cultures, grown in chemically defined medium. Proliferation and differentiation were characterized by using specific antibodies (A2B5, CNP, MBP, GFAP) and compared with untreated cultures. Lysophosphatidyl choline (LPC; 2 × 10(-5) M) was used to induce OL death and to test the effects of TBS peptides under these conditions. NFL-TBS.40-63 significantly increased OL differentiation and maturation, with more CNP(+) and MBP(+) cells characterized by numerous ramified processes, along with myelin balls. When OL were challenged with LPC, concomitant treatment with NFL-TBS.40-63 rescued more than 50% of OL compared with cultures treated with LPC only. Proliferation of OL progenitors was not affected, nor were AS proliferation and differentiation. NFL-TBS.40-63 peptide induces specific effects in vitro, increasing OL differentiation and maturation without altering AS fate. In addition, it partially protects OL from demyelinating injury. Thus release of NFL-TBS.40-63 caused by axonal damage in vivo could improve repair through increased OL differentiation, which is a prerequisite for remyelination.

First survey and functional annotation of prohormone and convertase genes in the pig

Background

The pig is a biomedical model to study human and livestock traits. Many of these traits are controlled by neuropeptides that result from the cleavage of prohormones by prohormone convertases. Only 45 prohormones have been confirmed in the pig. Sequence homology can be ineffective to annotate prohormone genes in sequenced species like the pig due to the multifactorial nature of the prohormone processing. The goal of this study is to undertake the first complete survey of prohormone and prohormone convertases genes in the pig genome. These genes were functionally annotated based on 35 gene expression microarray experiments. The cleavage sites of prohormone sequences into potentially active neuropeptides were predicted.

Results

We identified 95 unique prohormone genes, 2 alternative calcitonin-related sequences, 8 prohormone convertases and 1 cleavage facilitator in the pig genome 10.2 assembly and trace archives. Of these, 11 pig prohormone genes have not been reported in the UniProt, UniGene or Gene databases. These genes are intermedin, cortistatin, insulin-like 5, orexigenic neuropeptide QRFP, prokineticin 2, prolactin-releasing peptide, parathyroid hormone 2, urocortin, urocortin 2, urocortin 3, and urotensin 2-related peptide. In addition, a novel neuropeptide S was identified in the pig genome correcting the previously reported pig sequence that is identical to the rabbit sequence. Most differentially expressed prohormone genes were under-expressed in pigs experiencing immune challenge relative to the un-challenged controls, in non-pregnant relative to pregnant sows, in old relative to young embryos, and in non-neural relative to neural tissues. The cleavage prediction based on human sequences had the best performance with a correct classification rate of cleaved and non-cleaved sites of 92% suggesting that the processing of prohormones in pigs is similar to humans. The cleavage prediction models did not find conclusive evidence supporting the production of the bioactive neuropeptides urocortin 2, urocortin 3, torsin family 2 member A, tachykinin 4, islet amyloid polypeptide, and calcitonin receptor-stimulating peptide 2 in the pig.

Conclusions

The present genomic and functional characterization supports the use of the pig as an effective animal model to gain a deeper understanding of prohormones, prohormone convertases and neuropeptides in biomedical and agricultural research.

Is adolescence a sensitive period for depression? Behavioral and neuroanatomical findings from a social stress model

OBJECTIVES

Sex differences in depressive symptoms emerge during adolescence, with females more at risk than males. However, adverse life events during development have greater impact on males. An animal model that incorporates behavioral and anatomical changes following adolescent stress is needed.

EXPERIMENTAL DESIGN

Sprague-Dawley rats were exposed to social stress (SS; isolation housing during P30-35) or remained group-housed (GRP) and tested in the forced swim test (FST), the triadic learned helplessness model (LH), and the elevated plus maze. Western immunoblots of myelin basic protein (MBP) and synaptophysin (SVP) and spinophillin indexed synaptic and dendritic plasticity, respectively.

PRINCIPAL OBSERVATIONS

At P36, SS increased climbing behavior in both sexes, and decreased the latency to immobility in females following a 15 min inescapable swim in the FST. Depressive-like behaviors were differentially elevated in both sexes 24 h later. GRP females exhibited higher levels of depression-related behaviors than GRP males in both FST and LH paradigms. SS significantly increased depressive behaviors in the FST in males, and impaired their ability to escape shock previously conditioned to be controllable. SS decreased open arm time in females only. The greatest reductions in synaptic plasticity proteins were observed in the prefrontal cortex: spinophillin (19.1%), SVP (7.9%), and MBP (48.7%, males only). Smaller reductions in spinophillin were observed in the hippocampus and amygdala.

CONCLUSIONS

Adolescent separation produces both behavioral and neural changes associated with stress-related depression and anxiety. Additional work is needed to improve our understanding of stress as it relates to depression during this vulnerable period of development.

Glutathione peroxidase-1 contributes to the protection of glutamine synthetase in astrocytes during oxidative stress

Glutamine synthetase (GS) is an astrocytic enzyme that is essential for the glutamate-glutamine cycle between neurons and astrocytes. To measure the effects of oxidative stress on the activity of GS in astrocytes, astrocyte-rich primary cultures from the brains of wild-type and glutathione peroxidase-1 deficient mice (GPx1(-/-)) were exposed to a chronic hydrogen peroxide-generating system consisting of xanthine oxidase, hypoxanthine and superoxide dismutase. The specific activity of GS was strongly diminished by chronic exposure to hydrogen peroxide in astrocytes cultured from both mouse lines. After 60 min of oxidative stress in the presence of 5 mU/mL, 10 mU/mL and 20 mU/mL of xanthine oxidase, the specific GS activity of wild-type astrocytes was reduced to 47%, 22% and 13% of the initial activity, respectively. For all activities of xanthine oxidase applied, astrocytes from GPx1(-/-) mice experienced a significantly greater rate of GS inactivation compared to their wild-type counterparts. These results confirm that GS is sensitive to inactivation by chronic peroxide stress in viable astrocytes and show that glutathione peroxidase-1 helps to protect GS from inactivation by oxidative stress.

Expression of thyroid hormone receptor isoforms in the oligodendrocyte lineage

Thyroid hormone (T3) regulates brain development and function and in particular ensures normal myelination. Animal models and in vitro systems have been employed to demonstrate the effects of T3, which acts via nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression, such as myelin basic protein (MBP), in a hormone-dependent or -independent fashion. Two distinct genes, TR alpha and TR beta, encode several receptor isoforms with specific functions. This overview summarizes current knowledge on the cellular expression and the role of these isoforms and also examines the action of T3 on oligodendrocyte lineage cell types at defined developmental stages. Re-expression of TRs and also that of other transcription factors in oligodendrocytes may constitute some of the metabolic changes required for succesfull remyelination in the adult central nervous system after demyelinating lesions.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

GFAP mRNA positive glia acutely isolated from rat hippocampus predominantly show complex current patterns

Electrophysiologically complex glial cells have been widely identified from different regions of the central nervous system and constitute a dominant glial type in juvenile mice or rats. As these cells express several types of ion channels and neurotransmitter channels that were thought to be only present in neurons, this glial cell type has attracted considerable attention. However, the actual classification of these electrophysiologically complex glial cells remains unclear. They have been speculated to be an immature astrocyte because, although these cells show positive staining for the predominantly astrocytic marker S 100beta, it has not been possible to show staining for the commonly accepted mature astrocytic marker, glial fibrillary acidic protein (GFAP). To address the question of whether these cells might express GFAP at the transcript level, we combined patch-clamp electrophysiological recording with single cell RT-PCR for GFAP mRNA in glial cells acutely isolated from 4 to 12 postnatal day rats. In fresh cell suspensions from the CA1 region, complex glial cells were found to be the dominant cell type (65% total cells). We found that the majority of these electrophysiologically complex cells (74%) were positive for GFAP mRNA. We also showed that the complex cells responded to AMPA and GABA application, and these were also GFAP mRNA positive. We also fixed and stained the preparations for GFAP without electrophysiological recording to better preserve GFAP immunoreactively. In agreement with other studies, only 1.5% of these presumed electrophysiologically complex cells, based on morphology, showed immunoreactivity for GFAP. The expression of GFAP at the transcript level indicates GFAP (-)/GFAP mRNA (+) glial cells have an astrocytic identity. As single cell RT-PCR is able to detect both GFAP (-)/GFAP mRNA (+) and GFAP (+)/GFAP mRNA (+) astrocytic subtypes, the present study also suggests it is a feasible approach for astrocytic lineage studies.

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Glutamine synthetase activity in patients with Parkinson's disease

According to current concepts, the excitatory amino acid glutamate is involved in the pathogenesis of Parkinson's disease (PD). Overactivity of glutamatergic projection neurons and beneficial effect of antiglutamatergic substances in animal experiments suggest that excess supply of glutamate might contribute to the pathophysiology of PD. Reduced activity of the glutamate metabolizing enzyme glutamine synthetase (GS) leads to decreased uptake of glutamate and thus abundant glutamate. Here we report that PD patients and age-matched controls are comparable with respect to GS activity in peripheral blood mononuclear cells (PBMC). These results imply no systemic dysregulation of the enzyme GS in patients with PD.

Ependymal and choroidal cells in culture: characterization and functional differentiation

During the past 10 years, our teams developed long-term primary cultures of ependymal cells derived from ventricular walls of telencephalon and hypothalamus or choroidal cells (modified ependymal cells) derived from plexuses dissected out of fetal or newborn mouse or rat brains. Cultures were established in serum-supplemented or chemically defined media after seeding on serum-, fibronectin-, or collagen-laminin-coated plastic dishes or semipermeable inserts. To identify and characterize cell types growing in our cultures, we used morphological features provided by phase contrast, scanning, and transmission electron microscopy. We used antibodies against intermediate filament proteins (vimentin, glial fibrillary acidic protein, cytokeratin, desmin, neurofilament proteins), actin, myosin, ciliary rootlets, laminin, and fibronectin in single or double immunostaining, and monoclonal antibodies against epitopes of ependymal or endothelial cells, to recognize ventricular wall cell types with immunological criteria. Ciliated or nonciliated ependymal cells in telencephalic cultures, tanycytes and ciliated and nonciliated ependymal cells in hypothalamic cultures always exceeded 75% of the cultured cells under the conditions used. These cells were characterized by their cell shape and epithelial organization, by their apical differentiations observed by scanning and transmission electron microscopy, and by specific markers (e.g., glial fibrillary acidic protein, ciliary rootlet proteins, DARPP 32) detected by immunofluorescence. All these cultured ependymal cell types remarkably resembled in vivo ependymocytes in terms of molecular markers and ultrastructural features. Choroidal cells were also maintained for several weeks in culture, and abundantly expressed markers were detected in both choroidal tissue and culture (Na+-K+-dependent ATPase, DARPP 32, G proteins, ANP receptors). In this review, the culture models we developed (defined in terms of biological material, media, substrates, duration, and subculturing) are also compared with those developed by other investigators during the last 10 years. Focusing on morphological and functional approaches, we have shown that these culture models were suitable to investigate and provide new insights on (1) the gap junctional communication of ependymal, choroidal, and astroglial cells in long-term primary cultures by freeze-fracture or dye transfer of Lucifer Yellow CH after intracellular microinjection; (2) some ionic channels; (3) the hormone receptors to tri-iodothyronine or atrial natriuretic peptides; (4) the regulatory effect of tri-iodothyronine on glutamine synthetase expression; (5) the endocytosis and transcytosis of proteins; and (6) the morphogenetic effects of galactosyl-ceramide. We also discuss new insights provided by recent results reported on in vitro ependymal and choroidal expressions of neuropeptide-processing enzymes and neurosecretory proteins or choroidal expression of transferrin regulated through serotoninergic activation.

Heterogeneous immunoreactivity of glial cells in the mesencephalon of a lizard: a double labeling immunohistochemical study

Astrocytes and radial glia coexist in the adult mesencephalon of the lizard Gallotia galloti. Radial glia and star-shaped astrocytes express glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). The same cell markers are also expressed by round or pear-shaped cells that are therefore astrocytes with unusual morphology. Other round or pear-shaped cells, also scattered in the tegmentum and the tectum, display only GS. Electron microscopy reveals that these cells may be oligodendrocytes. In this lizard, the GS is expressed in some oligodendrocytes while this does not occur in the central nervous system of mammals in situ. These results confirm that the cellular specificity of GS is different in various species and suggest that ependymal cells are also immunoreactive for GS but they do not contain GFAP.

Oligodendrocyte maturation and progenitor cell proliferation are independently regulated by thyroid hormone

The development of oligodendrocyte progenitor cells is regulated by epigenetic factors which control their proliferation and differentiation. When oligodendrocyte progenitor cells, purified on a Percoll centrifugation gradient from neonate rat brain, are cultured in serum-free medium in the presence of platelet-derived-growth factor (PDGF), they divide and their differentiation is delayed. Triiodothyronine (T3) treatment of progenitor cells blocks their proliferation and induces their differentiation into oligodendrocytes. T3 also induces morphological differentiation of oligodendrocytes as indicated by the marked increase in the length of oligodendrocyte processes. To determine whether the effects of T3 on progenitor cell proliferation and oligodendrocyte maturation are causally related, or instead, are independent, we examined the influence of T3 on secondary cultures of postmitotic oligodendrocytes. We show that T3 increases morphological and functional maturation of postmitotic oligodendrocytes as indicated by a well developed network of branched processes and by the expression of myelin/oligodendrocyte glycoprotein (MOG) and glutamine synthetase (GS). T3 increases glutamine synthetase activity and its message level after a lag period of 24-48 h, and these levels increase through a posttranscriptional event. In contrast, no effect of T3 was observed on myelin basic protein (MBP) gene expression as determined by Northern blot analysis. Our results indicate that thyroid hormones participate in the control of the progenitor cell proliferation and differentiation as well as in oligodendrocyte maturation and that these two T3-regulated events are independent.

Role of thyroid hormone in the morphological differentiation and maturation of astrocytes: temporal correlation with synthesis and organization of actin

Morphological changes and the molecular mechanisms associated with the maturation of astrocytes were studied under normal and thyroid hormone-deficient conditions using long-term (30 days) primary cultures derived from the neonatal rat brain. Immunocytochemical staining of cells with a monoclonal antibody specific to glial fibrillary acidic protein demonstrated for the first time that, similar to their maturation in vivo, astrocytes maintained in normal serum-containing medium can undergo complete maturation involving two distinct stages of morphological differentiation (from radial glia to flat polygonal cells with epithelioid morphology and then to mature process-bearing cells with stellate morphology). Deficiency of thyroid hormone delays the first step and totally blocks the second stage of differentiation in the maturation process. Comparative staining of normal and thyroid hormone-deficient astrocytes with filamentous actin-specific fluorescein isothiocyanate-phalloidin and quantitation of the various forms of intracellular actin using an improved DNase I assay demonstrated that maturation of astroglial cells is associated with characteristic alterations in the level of cytoskeletal and noncytoskeletal filamentous (F) actin. In particular, the maintenance of the epithelioid form of the hypothyroid astrocytes is associated with a progressive increase in the level of cytoskeletal F-actin and a concomitant decline in the level of non-cytoskeletal F-actin. Quantitation of actin mRNA by Northern blot analysis and studies on the rate of actin synthesis at various stages of differentiation showed that the initial transformation into the epithelioid form is associated with an increase in the rate of synthesis of actin and the expression of its mRNA, while the final transformation into the nature process-bearing form is correlated with a decline in these parameters. The results indicates that thyroid hormone plays an obligatory role in promoting the differentiation and maturation of astrocytes, and that during this process the hormone regulates the expression of actin and its intracellular organization in a way conducive to morphological differentiation.

Differentiation of serum-free mouse embryo cells into astrocytes is accompanied by induction of glutamine synthetase activity

Serum-free mouse embryo (SFME) cells derived in a defined serum-free medium have been cultured for more than 200 generations and display properties of neural progenitor cells. SFME cells express the neuroepithelial stem cell marker nestin in defined serum-free medium. Exposure of SFME cells to transforming growth factor beta (TGF-beta) or serum decreases nestin expression and induces the astrocyte marker glial fibrillary acidic protein, suggesting that SFME cells differentiate into astrocytes upon exposure to TGF-beta or serum. We examined the expression by SFME cells of the functional central nervous system (CNS) astrocyte marker glutamine synthetase (GS). GS activity is induced in SFME cells upon exposure to TFG-beta or serum. The induction of GS activity was dose- and time-dependent and was reversible. Retinoic acid, hydrocortisone, and dibutyryl cyclic AMP also induced GS expression. The induction of GS activity was accompanied by an increase in the level of GS mRNA and protein. This work provides further evidence that SFME cells represent neural progenitor cells which differentiate into functional astrocytes upon exposure to TGF-beta or serum.

Effect of dibutyryl cyclic AMP and dexamethasone on glutamine synthetase gene expression in rat astrocytes in culture

Astrocytes are the primary site of glutamate conversion to glutamine in the brain. We examined the effects of treatment with either dibutyryl cyclic AMP and/or the synthetic glucocorticoid dexamethasone on glutamine synthetase enzyme activity and steady-state mRNA levels in cultured neonatal rat astrocytes. Treatment of cultures with dibutyryl cyclic AMP alone (0.25 mM-1.0 mM) increased glutamine synthetase activity and steady state mRNA levels in a dose-dependent manner. Similarly, treatment with dexamethasone alone (10(-7)-10(-5) M) increased glutamine synthetase mRNA levels and enzyme activity. When astrocytes were treated with both effectors, additive increases in glutamine synthetase activity and mRNA were obtained. However, the additive effects were observed only when the effect of dibutyryl cyclic AMP alone was not maximal. These findings suggest that the actions of these effectors are mediated at the level of mRNA accumulation. The induction of glutamine synthetase mRNA by dibutyryl cyclic AMP was dependent on protein synthesis while the dexamethasone effect was not. Glucocorticoids and cyclic AMP are known to exert their effects on gene expression by different molecular mechanisms. Possible crosstalk between these effector pathways may occur in regulation of astrocyte glutamine synthetase expression.

Retinoic acid regulates the development of oligodendrocyte precursor cells in vitro

Cultures of oligodendrocyte precursor cells can be grown from brain hemispheres of newborn rats. These cells, also called O-2A progenitor cells, can differentiate in vitro into oligodendrocytes or type 2 astrocytes. Basic FGF and PDGF are known to stimulate their proliferation and delay their differentiation. Lack or excess of retinoic acid (RA) has been known for a long time to alter brain development suggesting that this compound is involved in normal brain development. Here we report that RA partially inhibits both the proliferation and the differentiation of oligodendrocyte precursor cells. It also down-regulates the mitogenic effect of bFGF on these cells while keeping them in an immature stage. RA is more effective than bFGF in inhibiting myelin basic protein mRNA expression in these cells, and like bFGF, it preserves their bipotential character. RA nuclear receptors RAR-alpha and their transcripts are expressed in oligodendrocyte precursor cells as seen by Western blot, Northern blot and in situ hybridization. The expression of RAR-alpha transcripts is stimulated transiently by RA alone or associated to bFGF. The expression of RAR-beta transcripts is not constitutive and is induced by RA alone or associated to bFGF and to a lesser extent by bFGF alone. These results suggest that retinoids participate in the control of the development of glial cells of the oligodendrocyte lineage.

Glucocorticoids enhance the potency of Schwann cell mitogens

Previous studies have documented that cultured Schwann cells require serum-containing medium to respond maximally to mitogens. We now report that Schwann cells are able to proliferate to a mitogenic response in a serum-free defined medium termed oligodendrocyte defined media (ODM). Glucocorticoids are the essential component of ODM which allow Schwann cell proliferation in the serum-free medium. Charcoal treatment of the fetal calf serum decreases the mitogenic potency of the axolemma-enriched fraction (AEF) by 50%. The addition of 2 microM hydrocortisone to charcoal-treated fetal calf serum restores 75% of the lost mitogenicity. These observations are consistent with the view that glucocorticoids present in fetal calf serum are potent co-mitogens essential for AEF-induced Schwann cell proliferation. The synthetic glucocorticoid, dexamethasone, is a more potent co-mitogen than hydrocortisone, with a maximal effect at concentrations less than 10 nM. In contrast, other steroids including aldosterone, progesterone, testosterone, and 17 beta-estradiol have no effect on enhancing the mitogenic response of Schwann cells to the AEF. The glucocorticoid antagonists RU 486 and dehydroepiandrosterone (DHEA), but not the antiestrogenic compound tamoxifen, block AEF-induced Schwann cell proliferation. These results suggest that glucocorticoid-induced Schwann cell proliferation is mediated through a glucocorticoid receptor (GR) mechanism. We detected immunoreactivity to the GR in the cytoplasm, but not in the nuclei of Schwann cells grown in ODM lacking dexamethasone. The addition of 100 nM dexamethasone to these cultures resulted in immunoreactivity in the nucleus. This data suggests that glucocorticoids working through the GR are potent co-mitogens for Schwann cell proliferation.

Transient expression of 3,5,3'-triiodothyronine nuclear receptors in rat oligodendrocytes: in vivo and in vitro immunocytochemical studies

It is generally accepted that the action of thyroid hormones is mediated through specific nuclear receptors. Recent studies have demonstrated the homology of the thyroid receptor with the cellular product of the oncogen v-erbA. So far, two genes have been identified and classified as alpha and beta subtypes. In this study, the expression of nuclear triiodothyronine (T3) receptors (NT3Rs) was examined in secondary cultures containing 85-90% oligodendrocytes (OL) prepared from newborn rat brain primary cultures enriched in OL. These cultures, which are able to produce myelin membranes, were examined by double immunolabelling with a monoclonal antibody (2B3) raised against purified rat liver NT3Rs and with antibodies against two maturation markers of OL: an early marker, galactocerebroside (GC), and myelin basic protein (MBP), which is expressed later than GC. 2B3 recognized three nuclear proteins with the same molecular weights as beta 1, alpha 1, and alpha 2 subtypes with different capacities for binding T3. In 5-day-old OL secondary cultures (25 days, total time in culture), 2B3-NT3R immunoreactivity was located in 77% of morphologically immature OL (GC)+ cells, whereas only 44% of morphologically mature OL were immunoreactive. Only 35% of the MBP+ cells co-expressed NT3Rs. In the corpus callosum of developing rat brain, at all ages studied from 7-60 days postnatal, the total absence of NT3Rs in dark OL (morphologically mature), confirmed by ultrastructural immunocytochemistry, indicates an even more dramatic decrease during maturation. Furthermore, the percentage of medium OL (less mature) stained by 2B3 is reduced by approximately half in 60- compared to 20-day-old rat brain. It is of interest to note that the in vitro observation with maturation markers mirrors the in vivo decrease of NT3R expression during development. It is interesting that NT3Rs are absent in vivo before the critical period of active myelination. These data indicate the presence of a nuclear T3 binding protein in the nuclei of OL at the time of myelination both in vitro and in vivo. The transient expression of these NT3Rs during active myelination argues in favour of a direct effect of thyroid hormones on OL.

The alpha and beta thyroid receptors are expressed by cultured ependymal cells. Correlation with the effect of L-3,5,3'-triiodothyronine on glutamine synthetase mRNAs

It is generally accepted that L-3,5,3'-triiodothyronine (L-T3) acts at the genomic level through an interaction with specific nuclear L-T3 receptors (NT3R). Using antibodies raised against different peptides of NT3R, we report here the immunocytochemical localization of the alpha, alpha 2, beta 1 NT3R subtypes in ependymal cell primary cultures. The alpha and beta thyroid hormone receptors are both expressed. While the alpha and alpha 2 subtypes are found in almost all cells, the beta 1 receptors are present in few cells only. The possibility that alpha and beta receptors are colocalized is discussed. We also demonstrate that ependymal cells respond to L-T3 with a marked increase of the expression of the glutamine synthetase messenger RNAs.