Tissue-specific regulation of avian glutamine synthetase expression during development and in response to glucocorticoid hormones.

Abstract

We have isolated a glutamine synthetase cDNA clone derived from chicken retinal RNA. The clone detects a 3.2-kilobase RNA in chicken retina, liver, and brain, based on Northern blotting analysis. The dramatic developmental rise observed for the retinal enzyme, assayed as glutamyl transferase activity, is accompanied by a corresponding rise in this RNA. Injection of hydrocortisone 21-phosphate into the yolk sac of day 10 embryos produces an increase in retinal glutamine synthetase mRNA and glutamyl transferase activity, assayed 4 days after injection. An increase in glutamine synthetase mRNA is also observed within 2 h of incubation of retinal organ cultures with hydrocortisone. Moreover, incubation of these cultures with cycloheximide at a concentration that inhibits protein synthesis by 93% affects neither the basal level nor the hydrocortisonemediated induction of glutamine synthetase mRNA. Although expression of this RNA is developmentally regulated in the brain, steroid hormone injection does not result in a substantial induction. Hepatic glutamine synthetase mRNA is expressed constitutively between embryonic day 10 and 6 days after hatching and is also not hormone inducible. Southern blotting data with chicken DNA digested with EcoRI, HindUI, and BamHI are best interpreted in terms of the cDNA clone detecting only one gene. If so, several cell-type-specific regulatory mechanisms must function to modulate expression of this gene during development.Research primarily by Moscona and colleagues has established that during terminal differentiation of the chick retina levels for the enzyme glutamine synthetase rise dramatically (27). This rise is confined to the Muller glial cells (24) and is temporally correlated with the appearance of circulating corticosteroid hormones (26). Moreover, injection of a glucocorticoid hormone into the yolk sacs of embryos (30) or incubation of retinal organ cultures with a glucocorticoid (28,30) induces production of the enzyme. These observations suggest a physiological role for the glucocorticoid hormone and indicate that the induction of glutamine synthetase in the chicken retina provides a model for the action of glucocorticoid hormones during development.

Glutamine synthetase enhances the clearance of extracellular glutamate by the neural retina

“…The upstream region of the GS gene contains a glucocorticoid response element (GRE), and glucocorticoids regulate GS expression at the transcriptional level (Vardimon et al 1988;Zhang and Young 1991;Gorovits et al 1996). Supply of glucocorticoids to the embryo or to isolated retinal tissue results in accumulation of GS mRNA and protein, and in a marked increase in GS activity (Moscona 1975;Vardimon et al 1986;Patejunas and Young 1987). The hormone-mediated increase in GS expression is celltype specific: it is restricted to Mü ller glial cells, which are the only glial cells in the chicken retina and also the only cells in this tissue that express the glucocorticoid receptor protein (Linser and Moscona 1979;Gorovits et al 1994;Grossman et al 1994).…”

Identification of a Xenopus glutamine synthetase gene abundantly expressed in the embryonic nervous system but not in adult brain

“…This suggests developmental switching between multiple forms of glutamine synthetases, because this enzymatic activity is known to be high in adult brain in higher vertebrates [7,30]. Thus, It is likely that the glutamine synthetase encoded by xGS gene is an embryonic Isozyme and mainly expressed in immature neurons and/or glial cells.…”

“…Injected glucocorticoid induces glutamine synthetase activity in embryos [6,33] by transactivating the genets) [30,34,35,36]. Thus, glucocorticoids may contribute to the induction of glutamine synthetase during chick development [6].…”

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

“…However, unlike many neural-specific genes that are expressed exclusively in neural tissues, GS is also a housekeeping enzyme that is expressed at a low basal level in non-neural tissues. Since there appears to be only one copy of the GS gene (2,40), it is reasonable to expect that GS is controlled by a modular regulatory mechanism that allows constitutive expression in most cell types and specific induction in neural tissues.…”

“…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 (2,(11)(12)(13). 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 (8 -10, 14, 15).…”