Ontogeny of glutamine synthetase in rat brain

Vitamin E-Induced Changes in Glutamate and GABA Metabolizing Enzymes of Chick Embryo Cerebrum

Vitamin E exists in eight different forms, four tocopherols and four tocotrienols. It forms an important component of our antioxidant system. The structure of Vitamin E makes it unique and indispensable in protecting cell membranes. α -tocopherol, one of the forms of Vitamin E, is also known to regulate signal transduction pathways by mechanisms that are independent of its antioxidant properties. Vitamin E compounds reduce the production of inflammatory compounds such as prostaglandins. Swollen, dystrophic axons are considered as the hallmark of Vitamin E deficiency in the brains of rats, monkeys, and humans. The present work aimed to study the Vitamin E- ( α -tochopherol acetate-) induced alterations of enzymes involved in metabolism of Glutamate and GABA during developmental neurogenesis of cerebrum. Therefore, cytosolic and crude mitochondrial enzyme activities of glutamine synthetase, aspartate transaminase, alanine transaminase, GABA transaminase, succinic Semialdehyde dehydrogenase, glutamic dehydrogenase, and α -Ketoglutarate dehydrogenase were analysed. Vitamin E induced significant changes in these enzymes thus altering the normal levels of glutamate and GABA during developmental neurogenesis. Such changes are surely to disturb the expression and/or intensity of neurotransmitter signaling during critical periods of brain development.

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.

Induction of glutamine synthetase by L-alpha-aminoadipate in developmental stages of cultured astrocytes

Effect of L-alpha-aminoadipate (L alpha AA), a gliotoxic L-glutamate analogue, on glutamine synthetase (GS) activity of rat cultured astrocytes was examined, L alpha AA at sub-toxic concentrations (less than 0.5 mM, for 48 h) increased GS activity of cultured astrocytes. This increase was prevented by 10 microM cycloheximide, an inhibitor of protein synthesis. L alpha AA increased GS activities of astrocytes cultured in three different conditions, i.e. 12-day-old, 5-233k-old and dibutyryl cAMP(DBcAMP)-induced differentiated cultures. Insulin (10 micrograms/ml) and hydrocortisone (10 microM) increased GS activity of 12-day-old cultured cells, but not that of 5-weeks-old and DBcAMP-treated cells. The increase in GS activity was observed after a transient treatment with L alpha AA for 4 h. These results show that the induction of GS by L alpha AA is not related to developmental stages of astrocytes in culture.

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.

Glutamine synthetase gene expression in a glioblastoma cell-line of clonal origin: regulation by dexamethasone and dibutyryl cyclic AMP

We investigated the expression of glutamine synthetase (GS), an enzyme involved in astroglial metabolism and marker of astroglial functional maturity, in a glioblastoma cell-line (GL-15) of clonal origin. In spite of their phenotypic immaturity, evidenced in a mosaic fashion by a poor glial fibrillary acidic protein (GFAP) expression, the level of GS-mRNA is high in GL15 cells and the considerable amount of GS biological activity can be further induced and stabilized by glucocorticoids. A correlation between the induction by dexamethasone of the GS-mRNA level and the GS biological activity suggests a transcriptional regulation of GS expression by the aforesaid hormone. Under this hormonal action, changes in cell morphology occur and they are correlated with an overexpression of the GFAP, a marker of astroglial differentiation. On the contrary, dibutyryl cyclic AMP (dbc AMP) down-regulates the GS-mRNA expression and decreases GS activity. These results suggest that GL-15 cells have a common glucocorticoid dependent mechanism able to induce GS and GFAP as well as morphological changes. However in these cells AMPc responsive elements are involved in the negative modulation of the GS expression, contrary to what occurs in normal astroglial cells.

The ontogeny and localization of glutamine synthetase gene expression in rat brain

A 2.4 kb cDNA clone containing the coding sequence for glutamine synthetase (GS) was isolated from a rat brain cDNA library, and a probe constructed from this cDNA was utilized in Northern analysis of total RNA to study the tissue distribution and the ontogeny of GS mRNA expression in the rat brain from embryonic day 14 (E14) to adulthood. The levels of GS mRNA were highest in the brain, followed by kidney and liver. In the brain, the GS message was detected as early as E14, earlier than it can be detected by either enzymatic assays of GS activity or by immunocytochemical localization of GS. The relatively low levels of GS mRNA seen at E14 increase to a peak around the time of birth, and in the second postnatal week another rise in GS message occurs approaching adult levels by P15. Localization of GS to astrocytes in the brain was confirmed using both immunocytochemistry and in situ hybridization.

Development and characterization of five monoclonal antibodies against neuronal cell surface antigens--evaluation of their use in cell separation by affinity chromatography

Five monoclonal antibodies directed against rat neuronal cell surface proteins have been isolated and characterized following fusion of mouse SP2/O-Ag-14 myeloma cells and spleen cells from BALB/c mice immunized with neuron-specific membrane proteins. The antibodies displayed cell specificity in binding and elicited precipitation of different polypeptides from total neuronal cell surface proteins. These antibodies are of potential use in the isolation of a neuronal subpopulation by immunoaffinity chromatography. The work represents the application of hybridoma technology for the isolation of specific types of neuronal populations.

Synergistic action of thyroid hormone, insulin and hydrocortisone on astrocyte differentiation

We report here on the synergistic regulation of astrocyte development by 3 hormones: thyroid hormone (TH), insulin, and hydrocortisone (HC). Their effect, in a defined serum-free media, on astrocyte morphology, on glia fibrillary acidic protein (GFAP) immunostaining pattern, and on glutamine synthetase (GS) was investigated. TH transformed the flat, polygonal astrocytes into process-bearing cells. This effect was accentuated by insulin, which by itself had no effect on astrocyte morphology. The morphological transformations were accompanied by changes in the pattern of GFAP immunostaining which indicated a more organized and directed cytoskeleton arrangement in the TH-insulin treated cultures. Over 95% of the cells in the culture expressed GFAP. All 3 hormones regulated GS levels. TH increased GS levels by 50% and insulin raised its levels by 3-fold. While having no effect on astrocyte morphology, HC increased GS levels by 3.7-fold in both the hormone-free and insulin-supplemented medium. HC acted synergistically with insulin in its action on GS bringing about a 12-fold increase in the enzyme activity. In contrast, TH did not interact with insulin and was additive with HC in its action on GS. The continuous presence of insulin and TH was required to maintain their morphological and GS effect, suggesting that these hormones might not only be important for astrocyte differentiation, but later on for astrocyte function as well. Since astrocytes interact with and affect neurons and oligodendrocytes, the findings reported here might have bearing on the development and function of these other brain cells as well.

Comparative analysis of the membrane proteins and their specificities in neurons, protoplasmic astrocytes, and oligodendrocytes from rat brain

Plasma membranes from neuronal perikarya (N), protoplasmic astrocytes (A) and oligodendrocytes (O) of rat brains were analysed with respect to their protein and glycoprotein contents and specificities. SDS-polyacrylamide gel electrophoresis revealed a total number of 23, 17, and 17 major proteins in N, A, and O respectively. Periodate-Schiff's staining showed that approximately 40-60% of these proteins were glycoproteins. The reactivity of these glycoproteins to Con A and WGA was also studied. Selective iodination of whole cells followed by electrophoresis and autoradiography indicated that of the major proteins, only 25% of neuronal and 60% of astroglial and oligodendroglial membrane proteins were exposed outside the cell surface. The overall results suggest that membrane proteins of each of the three cell types studied here have characteristically different internal and external markers differing in size, glycoprotein content, and reactivity of the glycoproteins to lectins.

Thyroidal induction of tubulin in brain development--identification of the target cell

Exposure of organ cultures of newborn rat brains to tri-iodothyronine (T3) followed by cell fractionation as well as direct exposure of prefractionated neuronal (N) and glial (G) cells to the hormone results in an almost selective induction of tubulin in the glial cells. This is established from two independent assays of tubulin, viz. colchicine binding and vinblastin precipitation. In the newborn rat brain, the tubulin content of the G cells is almost 3-fold higher than that of the N cells. Treatment with T3 elicits 40-50% stimulation of tubulin in the G cells within 2 hr without any significant increase in the N cells. Brains from 8- or 50-day-old rats are irresponsive to induction to tubulin by T3. The rate of incorporation of [3H]leucine into total protein is very similar in both N and G cells of newborn rat brain but that into tubulin of G cells is about 3-fold higher than that of N cells. T3 promotes this incorporation by over 30% in the G cells with only a marginal 5% increase in the N cells. The overall results suggest that the glial cells represent the target cells for the T3-induced synthesis of tubulin, the major structural protein of the developing brain.