A Silencer Element in the Regulatory Region of Glutamine Synthetase Controls Cell Type-specific Repression of Gene Induction by Glucocorticoids

Abstract

Glutamine synthetase is a key enzyme in the recycling of the neurotransmitter glutamate. Expression of this enzyme is regulated by glucocorticoids, which induce a high level of glutamine synthetase in neural but not in various non-neural tissues. This is despite the fact that non-neural cells express functional glucocorticoid receptor molecules capable of inducing other target genes. Sequencing and functional analysis of the upstream region of the glutamine synthetase gene identified, 5′ to the glucocorticoid response element (GRE), a 21-base pair glutamine synthetase silencer element (GSSE), which showed considerable homology with the neural restrictive silencer element NRSE. The GSSE was able to markedly repress the induction of gene transcription by glucocorticoids in non-neural cells and in embryonic neural retina. The repressive activity of the GSSE could be conferred on a heterologous GRE promoter and was orientation- and position-independent with respect to the transcriptional start site, but appeared to depend on a location proximal to the GRE. Gel-shift assays revealed that non-neural cells and cells of early embryonic retina contain a high level of GSSE binding activity and that this level declines progressively with age. Our results suggest that the GSSE might be involved in the restriction of glutamine synthetase induction by glucocorticoids to differentiated neural tissues. Glutamine synthetase is a key enzyme in the recycling of the neurotransmitter glutamate. Expression of this enzyme is regulated by glucocorticoids, which induce a high level of glutamine synthetase in neural but not in various non-neural tissues. This is despite the fact that non-neural cells express functional glucocorticoid receptor molecules capable of inducing other target genes. Sequencing and functional analysis of the upstream region of the glutamine synthetase gene identified, 5′ to the glucocorticoid response element (GRE), a 21-base pair glutamine synthetase silencer element (GSSE), which showed considerable homology with the neural restrictive silencer element NRSE. The GSSE was able to markedly repress the induction of gene transcription by glucocorticoids in non-neural cells and in embryonic neural retina. The repressive activity of the GSSE could be conferred on a heterologous GRE promoter and was orientation- and position-independent with respect to the transcriptional start site, but appeared to depend on a location proximal to the GRE. Gel-shift assays revealed that non-neural cells and cells of early embryonic retina contain a high level of GSSE binding activity and that this level declines progressively with age. Our results suggest that the GSSE might be involved in the restriction of glutamine synthetase induction by glucocorticoids to differentiated neural tissues. Glutamine synthetase (GS 1The abbreviations used are: GS, glutamine synthetase; GRE, glucocorticoid response element; GSSE, glutamine synthetase silencer element; NRSE, neural restrictive silencer element; NRSF, neural restrictive silencer factor; CAT, chloramphenicol acetyltransferase; PCR, polymerase chain reaction; PMSF, phenylmethylsulfonyl fluoride; DTT, dithiothreitol; bp, base pair()s); kb, kilobase pair(s); DMEM, Dulbecco's modified Eagle's medium; TK, thymidine kinase; RSV, Rous sarcoma virus; REST, RE-1-silencing transcription factor.;l-glutamate:ammonia ligase (ADP-forming); EC 6.3.1.2) is a “housekeeping” enzyme that is expressed at a particularly high level in neural tissues (1Grossman R. Fox L.E. Gorovits R. Ben-Dror I. Reisfeld S. Vardimon Brain Res. Mol. Brain Res. 1994; 21: 312-320Crossref PubMed Scopus (32) Google Scholar, 2Patejunas G. Young A.P. Mol. Cell. Biol. 1987; 7: 1070-1077Crossref PubMed Scopus (35) Google Scholar). High levels of GS expression are restricted to glial cells (1Grossman R. Fox L.E. Gorovits R. Ben-Dror I. Reisfeld S. Vardimon Brain Res. Mol. Brain Res. 1994; 21: 312-320Crossref PubMed Scopus (32) Google Scholar, 3Linser P. Moscona A.A. Proc. Natl. Acad. Sci. U. S. A. 1979; 76: 6476-6480Crossref PubMed Scopus (210) Google Scholar, 4Martinez H.A. Bell K.P. Norenberg M.D. Science. 1977; 195: 1356-1358Crossref PubMed Scopus (985) Google Scholar, 5Linser P. Moscona A.A. Dev. Brain Res. 1981; 1: 103-119Crossref Scopus (53) Google Scholar) and are essential for the recycling of the neurotransmitter glutamate. Synaptically released glutamate is taken up into glial cells, where it is converted by GS into glutamine, which re-enters the neurons and is hydrolyzed by glutaminase to form glutamate again (6Van-den-Berg C.J. Lajtha A. Handbook of Neurochemistry. Plenum, New York1970: 355-379Google Scholar, 7Kennedy A.J. Voaden M.J. Marshall J. Nature. 1974; 252: 50-52Crossref PubMed Scopus (51) Google Scholar). In this way, the neurotransmitter pool is replenished and glutamate neurotoxicity is prevented. Studies in the chicken neural retina showed that GS expression is regulated by systemic glucocorticoids, which induce in this tissue a very high level of GS by directly stimulating the transcription of the gene (8Vardimon L. Fox L.E. Degenstein L. Moscona A.A. Proc. Natl. Acad. Sci. U. S. A. 1988; 85: 5981-5985Crossref PubMed Scopus (67) Google Scholar, 9Gorovits R. Yakir A. Fox L.E. Vardimon L. Brain Res. Mol. Brain Res. 1996; 43: 321-329Crossref PubMed Scopus (26) Google Scholar, 10Patejunas G. Young A.P. J. Biol. Chem. 1990; 265: 15280-15285Abstract Full Text PDF PubMed Google Scholar). The ability of glucocorticoids to induce GS expression is developmentally controlled. Glucocorticoids can induce a high level of GS expression in neural retina at late embryonic ages, but not in early embryonic retina (2Patejunas G. Young A.P. Mol. Cell. Biol. 1987; 7: 1070-1077Crossref PubMed Scopus (35) Google Scholar, 11Vardimon L. Fox L.E. Moscona A.A. Proc. Natl. Acad. Sci. U. S. A. 1986; 83: 9060-9064Crossref PubMed Scopus (59) Google Scholar, 12Moscona A.A. Osborne N. Chader G. Progress in Retinal Research. Pergamon, Oxford1983: 111-135Google Scholar, 13Moscona M. Moscona A.A. Differentiation. 1979; 13: 165-173Crossref PubMed Scopus (43) Google Scholar). The direct involvement of glucocorticoids in the control of GS gene transcription is evidenced by the nuclear run-on transcription assay, as well as by the finding that the upstream region of the GS gene contains a glucocorticoid response element (GRE) that can bind the glucocorticoid receptor protein and confer responsiveness to glucocorticoids on an attached reporter gene (8Vardimon L. Fox L.E. Degenstein L. Moscona A.A. Proc. Natl. Acad. Sci. U. S. A. 1988; 85: 5981-5985Crossref PubMed Scopus (67) Google Scholar, 9Gorovits R. Yakir A. Fox L.E. Vardimon L. Brain Res. Mol. Brain Res. 1996; 43: 321-329Crossref PubMed Scopus (26) Google Scholar, 10Patejunas G. Young A.P. J. Biol. Chem. 1990; 265: 15280-15285Abstract Full Text PDF PubMed Google Scholar,14Ben-Dror I. Havazelet N. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 1117-1121Crossref PubMed Scopus (33) Google Scholar, 15Zhang H.Y. Young A.P. J. Biol. Chem. 1991; 266: 24332-24338Abstract Full Text PDF PubMed Google Scholar). This mode of regulation, which is dependent on the presence of active glucocorticoid receptor molecules, explains the cell type specificity of GS expression in the neural retina; GS expression is always restricted to Müller glial cells, which are the only cells in the tissue that express a detectable level of the glucocorticoid receptor protein (1Grossman R. Fox L.E. Gorovits R. Ben-Dror I. Reisfeld S. Vardimon Brain Res. Mol. Brain Res. 1994; 21: 312-320Crossref PubMed Scopus (32) Google Scholar, 16Gorovits R. Ben-Dror I. Fox L.E. Westphal H.M. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 4786-4790Crossref PubMed Scopus (35) Google Scholar). It does not, however, explain why various non-neural cells, which contain functional glucocorticoid receptor molecules capable of inducing other target genes, are not responsive to the hormone and express only low levels of GS. Neural-specific gene expression is often achieved by the action of negative transcriptional regulation. A prominent example is the neural restrictive silencer element (NRSE/RE-1) found in the regulatory regions of a number of genes expressed exclusively in the nervous system, including SCG10 (17Mori N. Schoenherr C. Vandenbergh D.J. Anderson D.J. Neuron. 1992; 9: 45-54Abstract Full Text PDF PubMed Scopus (348) Google Scholar), type II sodium channel (NaII) (18Kraner G. Neuron. 1992; 9: Full Text PDF PubMed Scopus Google Scholar), L. N. P. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: PubMed Scopus Google Scholar), the of the receptor A. M. N. M. PubMed Scopus Google Scholar), the receptor A. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, M. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar), and the cell P. S. J. Biol. Chem. PubMed Scopus Google Scholar). This element to a transcriptional protein as neural restrictive silencer C.J. Anderson D.J. Science. PubMed Scopus Google Scholar) RE-1-silencing transcription J. S. A.J. G. Cell. Full Text PDF PubMed Scopus Google Scholar), which is expressed in non-neural tissues and in of the nervous of the gene of by a negative form of the protein that is in to repress neural genes in non-neural in neural tissues A.J. Anderson D.J. PubMed Scopus Google Scholar). In the that the regulatory region of the GS gene contains a and that this element can repress the induction of gene transcription by Our results suggest that a neural restrictive activity be directly involved in the of GS induction in non-neural tissues and in the of expression of GS in early embryonic neural retina. A of regions of the GS upstream to was by the GS L. Fox L.E. Moscona A.A. Proc. Natl. Acad. Sci. U. S. A. 1986; 83: 9060-9064Crossref PubMed Scopus (59) Google Scholar) into the The was by the chain S. Proc. Natl. Acad. Sci. U. S. A. 1977; PubMed Scopus Google Scholar) the II S. a of chloramphenicol that are by the upstream of the GS the region of was Mol. Cell. Biol. PubMed Scopus Google Scholar) and into in the region of the GS upstream to to to to The GS and GS by the as the GS was with which a and with which a 5′ in the region of the 5′ to the GS and the of the was with for up to taken at to and for at The of by ligase and into by the A Scholar), which of the of the The in the GS was by that to the 5′ and and contain restriction for into GS. The was by The was I. Havazelet N. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 1117-1121Crossref PubMed Scopus (33) Google Scholar). It contains of the GRE to the the the to was GS with with the of polymerase and into the of was by restriction enzyme of the GSSE of the of the SCG10 gene (17Mori N. Schoenherr C. Vandenbergh D.J. Anderson D.J. Neuron. 1992; 9: 45-54Abstract Full Text PDF PubMed Scopus (348) Google Scholar). a of the GSSE and of the GSSE The was by of the a of the GSSE into and The of the GSSE into the of to and into to The of the into the of to was by restriction The reporter gene J. M. S. Mol. Cell. Biol. 1987; 7: PubMed Scopus Google Scholar) and the expression R. S. S. Cell. 1986; Full Text PDF PubMed Scopus Google Scholar) are the transcriptional control of the Rous sarcoma was the Retinal tissue was of chicken at of embryonic The tissue was in in Dulbecco's modified Eagle's with on a at was into of tissue by a with and of and as G. Vardimon L. J. 1994; 13: PubMed Scopus Google Scholar). in the in and for in the presence of and cells in with cells in with and The cells in and with to the the cells for with and in tissue activity was as Mol. Cell. Biol. PubMed Scopus Google Scholar). In the was to an of activity activity was by of the with a and and of the of converted to the activity was as J. M. S. Mol. Cell. Biol. 1987; 7: PubMed Scopus Google Scholar) and by a and cells at to A. 1988; PubMed Scopus Google Scholar) with cells a and in of The cell was by for at cells in the of cell of A phenylmethylsulfonyl and and to on for by a by of A and of in a The cell was for in an and the nuclear was in the of cell of PMSF, and by on with for The nuclear was by for at and the was into and by with The at Retinal cell for the The tissue was by by of of A and of in a The nuclear was the nuclear in the in the of cell of low PMSF, and by of the of a cell of high PMSF, and in the nuclear was the assays in a of DTT, and of nuclear to the and with for on by for at The on a in for at The and for at with an used for and used as used for of the GSSE, of the NRSE, the of the of the and a M. PubMed Scopus Google Scholar). The of the region of the GS gene to by H.Y. Young A.P. J. Biol. Chem. 1991; 266: 24332-24338Abstract Full Text PDF PubMed Google Scholar) and by of of the revealed a considerable number of including and The however, was the fact that not the but in in the region and in and in the region and in The region proximal to the transcriptional start site, to in only in the and was in This in the of the regions with other to and the to of the M. Mol. Biol. 1987; Google Scholar). in the regions and which contain a high of found to a and the regions and which contain at In of the fact that can often functional and M. The of Google Scholar, Mol. Biol. 1987; Google Scholar), this finding the that the regulatory of the GS gene are in regions and and not in regions and of and in the regions of the GS upstream of number of in region was by of as is the of in the of is the number of the in is the number of in to and J. Scholar). The of is to the in a The number of in region was by of as is the of in the of is the number of the in is the number of in to and J. Scholar). The of is to the In to the location of regulatory to on regions and which a and a to contain functional that are directly involved in the responsiveness of the GS gene to a glucocorticoid response element (GRE) at to I. Havazelet N. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 1117-1121Crossref PubMed Scopus (33) Google Scholar, 15Zhang H.Y. Young A.P. J. Biol. Chem. 1991; 266: 24332-24338Abstract Full Text PDF PubMed Google Scholar) and an at to H.Y. Young A.P. J. Biol. Chem. 1991; 266: 24332-24338Abstract Full Text PDF PubMed Google Scholar, S. S. Mol. Cell. Biol. 1993; 13: PubMed Scopus Google Scholar). the in region might contain regulatory a of region of the GS gene to the reporter gene of the GS upstream which contain the GRE used as a The various into retina and the ability of to induce expression was In the activity was to the activity a control to for in the GS which does not contain the GRE was not by the GS which contains the GRE, was of the GS revealed that the upstream of the GRE contains a regulatory element that can markedly repress the this a of of the upstream and of the not the repressive the repressive activity to a upstream of the GRE The repressive activity of this was not restricted to the but was in In cells, however, of the GS promoter was dependent on of a glucocorticoid receptor expression in the a in activity in cells with the GS but not in with the the GS of the a of with considerable homology to the of the C.J. A.J. Anderson D.J. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar). This element is found in the regulatory region of genes and expression in non-neural this is for the of the GS promoter an of the in the of the GS the Our results that of the the GS in of the activity in and embryonic cells not that this a functional silencer The location of this and is region of the GS upstream the region which contains other regulatory and in which In region which contains a high of not regulatory suggest that the of the upstream of the GS gene the location of involved in gene that the regulatory region of the GS gene in to regulatory a element that can repress the the functional of the GSSE, ability to confer negative control a heterologous The the was upstream of a which contains of a GRE and is to the reporter gene I. Havazelet N. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 1117-1121Crossref PubMed Scopus (33) Google Scholar). The into cells, and activity in the presence of was The and the showed on but of the of of the GSSE not the repressive The repressive activity of the GSSE was but of the GSSE to a site, upstream of the GRE, the of the the GSSE considerable homology with the of the the of the GSSE the regulatory region of the GS gene was with that with and SCG10 genes, in which of the in a of promoter in cells (17Mori N. Schoenherr C. Vandenbergh D.J. Anderson D.J. Neuron. 1992; 9: 45-54Abstract Full Text PDF PubMed Scopus (348) Google Scholar, G. Neuron. 1992; 9: Full Text PDF PubMed Scopus Google Scholar). the activity of the GSSE with that of the NRSE, a in which of of the GSSE upstream of the The results of in the of the the of the and the GSSE suggest that the GSSE is a to the GRE, on but can repress induction by The activity of the is by a as C.J. Anderson D.J. Science. PubMed Scopus Google Scholar) J. S. A.J. G. Cell. Full Text PDF PubMed Scopus Google Scholar). This protein is expressed in non-neural cells and the expression of genes. the GSSE with a a of assays on nuclear cells the that contains of the GSSE as a The specificity of the was by a of was with a of the GSSE, but not with a of an a M. PubMed Scopus Google Scholar). the homology and functional the that contains of the (17Mori N. Schoenherr C. Vandenbergh D.J. Anderson D.J. Neuron. 1992; 9: 45-54Abstract Full Text PDF PubMed Scopus (348) Google Scholar) could the of the GSSE the was that with the in the of the (17Mori N. Schoenherr C. Vandenbergh D.J. Anderson D.J. Neuron. 1992; 9: 45-54Abstract Full Text PDF PubMed Scopus (348) Google G. Neuron. 1992; 9: Full Text PDF PubMed Scopus Google Scholar), the GSSE binding activity was cell This was by analysis of nuclear the non-neural cell the cell and the glial cell GSSE binding activity was in and was by of GSSE, but not by a of an GSSE binding activity was in cells results with the repressive activity of the GSSE in In the of the the GSSE a in induction in cells but only a and a in and cells, The repressive activity in and cells might be to low levels of NRSF, which are detectable by but by analysis of (18Kraner G. Neuron. 1992; 9: Full Text PDF PubMed Scopus Google Scholar, C.J. Anderson D.J. Science. PubMed Scopus Google Scholar, J. S. A.J. G. Cell. Full Text PDF PubMed Scopus Google Scholar). it is that the GSSE induction of gene transcription by binding to the The fact that is expressed in non-neural cells explain why glucocorticoids induce a high level of GS expression in neural tissues binding and repressive are cell assays nuclear non-neural cells cell and cell was used as a used and The binding is by and cells with the of with the of was by with the reporter of The cells for in the presence of and assays to an of The of was by of the with a was by the of the with that of the in cell The are the of at Expression of GS in the neural retina is developmentally It is low at early embryonic and progressively with age. the involvement of the activity in the control of GS the that with the GSSE in the neural retina and binding activity at a of assays on nuclear tissue at of embryonic a at which GS expression is very low was with a of the GSSE, but not with a of an with cell the could the of the GSSE and The by the GSSE in but in in the that GSSE activity is by and that and chicken in assays on nuclear tissues at in GSSE binding activity was high in early embryonic retina and progressively with age. binding activity was in retina. This is in to binding which not with The of GSSE binding activity with the level of GS expression as evidenced by GS and by GS activity suggest that of might be to the developmentally dependent in GS binding activity and GS expression in the tissue are Gel-shift assays nuclear and which contains of the used as the to the level of GSSE binding activity at the ages, with a and the of the was The level of binding activity in retina was the number of levels of GS R. Yakir A. Fox L.E. Vardimon L. Brain Res. Mol. Brain Res. 1996; 43: 321-329Crossref PubMed Scopus (26) Google Scholar) and GS activity R. Yakir A. Fox L.E. Vardimon L. Brain Res. Mol. Brain Res. 1996; 43: 321-329Crossref PubMed Scopus (26) Google Scholar, 12Moscona A.A. Osborne N. Chader G. Progress in Retinal Research. Pergamon, Oxford1983: 111-135Google Scholar) at the are GS is a key enzyme in the recycling of the neurotransmitter a that a high level of GS expression in neural tissues. genes that are expressed exclusively in neural GS is a enzyme that is expressed at a low level in non-neural tissues. to be only of the GS gene (2Patejunas G. Young A.P. Mol. Cell. Biol. 1987; 7: 1070-1077Crossref PubMed Scopus (35) Google Scholar, Young A.P. Dev. Biol. 1988; PubMed Scopus Google Scholar), it is to that GS is by a regulatory that expression in cell and induction in neural tissues. In an to the GS regulation, and the upstream region of the the of could a considerable number of the not but in regions that in and to of this type are often found and regions and and are to M. Mol. Biol. 1987; Google Scholar, M. The of Google Scholar, Mol. Biol. 1987; Google Scholar). analysis of the 5′ upstream region of the GS gene revealed that the region and which a high of not regulatory the region and in which identified, regulatory suggest that regions involved in the of gene transcription might a of to that of regions in the suggest that a analysis of might of the location of regulatory in the upstream of a The regulatory region of the GS gene was found to in to regulatory that confer responsiveness to a silencer element that can repress the and the location of the silencer to a region upstream of the GRE and upstream of the transcription start This GSSE is to the silencer element which is in genes, and expression in neural tissues and in non-neural The of that the GSSE is a that can bind the and of gene induction in a cell The GSSE was found to be capable of induction of gene transcription in embryonic tissue and in the non-neural The repressive activity of the GSSE was not restricted to the GS promoter but could be conferred on a heterologous GRE promoter of the GSSE in the of the with the of the of the SCG10 gene in a of the a assay, able to in and embryonic cells a that with the GSSE, and to that this with the NRSE. of a was cell it was in the non-neural cells, but not at in the cell the cell with GSSE repressive activity in cells was in A cell type specificity for other (17Mori N. Schoenherr C. Vandenbergh D.J. Anderson D.J. Neuron. 1992; 9: 45-54Abstract Full Text PDF PubMed Scopus (348) Google Scholar, G. Neuron. 1992; 9: Full Text PDF PubMed Scopus Google Scholar, L. N. P. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: PubMed Scopus Google Scholar, A. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, J. S. A.J. G. Cell. Full Text PDF PubMed Scopus Google Scholar). The ability of the GSSE to repress the induction of gene transcription and the expression of in non-neural cell that the GSSE might be directly involved in the restriction of GS induction to neural tissues. It is to that the cell type specificity of GS induction can be by the glucocorticoid receptor of gene expression a level of active glucocorticoid receptor that cells are not responsive to the hormone R. Mol. 1987; 1: PubMed Scopus Google Scholar). This is the in neurons that not express the receptor molecules, express only at low levels (1Grossman R. Fox L.E. Gorovits R. Ben-Dror I. Reisfeld S. Vardimon Brain Res. Mol. Brain Res. 1994; 21: 312-320Crossref PubMed Scopus (32) Google Scholar, 16Gorovits R. Ben-Dror I. Fox L.E. Westphal H.M. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 4786-4790Crossref PubMed Scopus (35) Google Scholar). In cells GS is not by glucocorticoids, despite the fact that is not receptor molecules however, expressed in non-neural cell and in cells the repressive activity of the GSSE might be for a low level of GS The GSSE might be involved in of the expression of GS in early embryonic retina. This is by the finding that the levels of GSSE binding activity and GS expression embryonic are GS expression with GSSE binding activity the repressive activity of the GSSE might an in the control of GS it is not the only involved in this and that in glucocorticoid receptor transcription activity a in the control of GS expression I. Havazelet N. Vardimon L. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 1117-1121Crossref PubMed Scopus (33) Google Scholar, G. Vardimon L. J. 1994; 13: PubMed Scopus Google Scholar, Young A.P. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). activity of the glucocorticoid which is by a the protein G. Vardimon L. J. 1994; 13: PubMed Scopus Google Scholar), is low in early embryonic retina and progressively with age. It is that a low level of active receptor molecules, in with a high level of in early embryonic that GS not be can be to explain protein might the of transcriptional can be might by binding directly to the might induction by directly with the glucocorticoid receptor protein by for of a in the In this the repressive activity of the GSSE in the of promoter The of the GSSE the GRE in the GS promoter and in the was and the transcription start was and in the GSSE a in gene This might suggest that the GSSE of gene induction in a position-independent of the GSSE, in the of the to a that is upstream of the GRE and upstream of the transcription start the of the it is that of the response is position-independent with respect to the transcription start site, but a location proximal to the GRE. Our that in the of a GRE the GSSE on the repressive activity of the GSSE is restricted to the the transcription to express a low level of GS. The regulatory of the GS gene might to the for a particularly high level of GS expression in glial cells in to recycling of the neurotransmitter and a low level in other cells for A mode for cell control of GS expression is in In non-neural cells, which express active glucocorticoid receptor molecules, induction and a low level of GS cells express a high level of active glucocorticoid receptor molecules and not express GS expression in cells is not express NRSF, express the glucocorticoid GS expression in neurons is cell type specificity of GS expression might be achieved by the and negative regulatory the GRE and the GSSE, which are not by but which a of GS expression for the M. for the and for with the and S. for