Effects of cytosine arabinoside on differential gene expression in embryonic neural retina. II. Immunochemical studies on the accumulation of glutamine synthetase

Cytosine arabinoside (Ara-C) elicits a significant increase in the level of the enzyme glutamine synthetase (GS) while it markedly reduces overall RNA and protein synthesis in cultures of embryonic chick neural retina. This increase was analyzed by radioimmunochemical procedures and compared with the induction of GS by hydrocortisone (HC). Accumulation of GS in Ara-C-treated retinas was found to be due to de novo synthesis of the enzyme; however, unlike the induction of GS by HC, Ara-C caused no measurable increase in the rate of GS synthesis. The results indicate that Ara-C facilitates GS accumulation largely by preventing degradation of the enzyme. Even though Ara-C inhibits the bulk of RNA synthesis in the retina, it does not stop the formation of GS-specific RNA templates. However, the progressive accumulation of these templates does not result in an increased rate of GS synthesis unless Ara-C is withdrawn from such cultures under suitable experimental conditions. Thus, it is suggested that the continuous presence of Ara-C imposes a reversible hindrance at the translational level which limits the rate of GS synthesis. The results demonstrate that the increase in retinal GS elicited by Ara-C is achieved through mechanisms which are quite different from those involved in the hydrocortisone-mediated induction of this enzyme.     

Involvement of poly-A in selective gene expression: suppression of enzyme induction in neural retina by inhibitors of poly-A synthesis

The hormonal induction og glutamine synthetase (GS) in embryonic neural retina involves selective accumulation of stable and active RNA templates for GS synthesis. Cordycepin, (3′-deoxyadenosine) suppresses this induction in close correlation with its inhibition of poly-adenylate (poly-A) synthesis. Similarly ethidium bromide, which also reduces poly-A synthesis, suppresses the induction of GS. Both agents inhibit GS induction by acting at a pre-translational level. The overall results suggest that formation of poly-A is required for the induction of GS; they provide the first indication of a relationship between poly-A synthesis and a specific, inducible, gene-controlled aspect of cell differentiation.     

The induction of glutamine synthetase in cell aggregates of embryonic neural retina: correlations with differentation and multicellular organization

A rapid increase in the synthesis and accumulation of the enzyme glutamine synthetase (GS) in the neural retina of the chick embryo characterizes the functional differentiation and maturation of this tissue. A precocious increase of GS can be induced in the embryonic retina by hydrocortisone and related corticosteroids. This paper presents evidence that the responsiveness of neural retina cells to GS induction by the hormonal inducer is dependent on histotypic associations and organization. This was demonstrated, using retina from embryos of different ages, by comparing GS induction in cultures of intact retina tissue with that in aggregates of retina cells and in monolayer cultures of retina cells.