DEVELOPMENTAL-CHANGES IN THE EXPRESSION AND COMPARTMENTALIZATION OF THE GLUCOCORTICOID RECEPTOR IN EMBRYONIC RETINA

Inducibility by glucocorticoids of the glutamine synthetase gene in ch icken embryo retina and the transcriptional activity of the glucocorti coid receptor (GR) greatly increase between embryonic days 6 and 10 (E 6, E10), although the level of GR does not markedly change during that time. This apparent discrepancy was investigated by examining the pat tern of GR expression in undifferentiated E6 retina and in E10 retina, which consists mostly of differentiated cells. Two GR isoforms, 90 an d 95 kDa, were found to be expressed at both of these ages at a simila r total level but in different proportions: in E6 retina the level of the 90-kDa isoform was higher, whereas in E10 retina the 95-kDa recept or was higher. However, following treatment of the retinas with cortis ol, the 95-kDa isoform became the predominant receptor at both ages. I mmunohistochemical analysis revealed that the cellular localization of GR markedly changed in the course of development: in the undifferenti ated E6 retina GR was expressed in virtually all cells, whereas in the more differentiated E10 and E12 retina, GR was detected only in Mulle r glia cells. The latter represent approximate to 20% of the cells in this tissue and are the only cells in which glucocorticoid hormone ind uces the glutamine synthetase gene. We suggest that the compartmentali zation of GR in Muller glia is a major aspect of the mechanism that mo dulates receptor activity during retina development and results in the temporal increase in the inducibility of glutamine synthetase and its specific localization in Muller glia cells.