Constitutive and glucocorticoid-mediated activation of glutamine synthetase gene expression in the developing chicken...

This report correlates constitutive and glucocorticoid-mediated alterations in glutamine synthetase gene expression observed in chicken retinal organ cultures with activation of the gene during terminal differentiation. Because glucocorticoid hormones are potent inducers of glutamine synthetase in embryonic retinae, they have been considered as primary mediators of the developmental rise. However, unlike what might be expected of a developmental inducer, our data demonstrate that glucocorticoids act as reversible regulators of glutamine synthetase transcription. Moreover, long-term organ culture of embryonic retina in the absence of glucocorticoids leads to a dramatic increase in expression of glutamine synthetase mRNA. These studies argue that although glucocorticoids may potentiate transcription of glutamine synthetase, the retina has been programmed as early as embryonic day 6 to express this enzyme at a later time via a glucocorticoid-independent process. Interestingly, transcription of retinal glutamine synthetase increases by only approximately 10-fold during development, based on nuclear run-on measurements. Therefore, post-transcriptional control mechanisms might also contribute to the 130-fold increase in glutamine synthetase mRNA observed during terminal differentiation.