Cell contact-dependent regulation of hormonal induction of glutamine synthetase in embryonic neural retina.

Glutamine synthetase (GS) is a differentiation marker in the neural retina of the chick embryo. GS is localized specifically in Müller glia cells, and it can be precociously induced by adrenal corticosteroids (such as cortisol). The induction depends on cortisol-elicited gene expression and results in de novo synthesis of GS and in a multifold increase in its level. GS is inducible only when Müller cells are closely associated with retina neurons. When retina tissue from 10-day embryos is dissociated into single cells and these are maintained either in suspension or in monolayer culture, GS cannot be induced. However, if identically prepared cells are reaggregated and allowed to reconstruct retinotypic associations, they are inducible for GS. Measurements of cytoplasmic cortisol-receptors showed that cell dissociation results in a rapid and marked reduction in the level (or activity) of these receptors. Their low level persists if the cells are maintained in a dispersed state. However, if the cells are reaggregated and reestablish tissue-like contacts, the level of cortisol receptors increases, as does GS inducibility. The results indicate that, in the embryonic neural retina, histotypic cell contacts are involved in regulating the level of cytoplasmic cortisol receptors and of the responsiveness of Müller glia cells to the induction of GS. Whether the two aspects are causally related is a matter for future study.