Exogenous, but not endogenous, glucocorticoid receptor induces glutamine synthetase gene expression in early stage embryonic retina.

Although glucocorticoid receptors are present throughout retinal development, the chicken glutamine synthetase (GS) gene becomes hormonally inducible between embryonic day 7.5 (E7.5) and E8. In this report we demonstrate that a transiently expressed GS-chloramphenicol (CAT) fusion gene is subject to temporal control resembling that experienced by the endogenous GS gene during in vitro retinal development. In addition, an enhancer located approximately 2 kilobases upstream of the GS transcription start site renders an SV40-CAT fusion plasmid hormonally inducible in E10, but not E5.5 retina, thereby implicating this element as the mediator of developmentally regulated expression. The enhancer contains a single glucocorticoid-response element juxtaposed to an essential ancillary site. Band shift assays demonstrate that nuclear extracts obtained from E5.5, E10, and E18 retina all contain similar levels of proteins that interact with the ancillary site, suggesting that developmental regulation of the hormonal response does not reflect the timed appearance of an ancillary factor. However, supplying exogenous glucocorticoid receptor through cotransfection is sufficient to produce hormonally inducible expression of the glutamine synthetase-CAT fusion gene in nonresponsive early stage retina. Based on these data, we postulate that the general glucocorticoid signaling pathway is compromised in early stage retina in a manner that is compensated by overexpression of the glucocorticoid receptor.