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The role of cyclic AMP and differentiation-inducing factor in stalk cell differentiation during the development of the cellular slime mold Dictyostelium discoideum Sobolewski, Andre


The role of cyclic AMP and a differentiation-indueing factor (DIF) in the differentiation of stalk cells was investigated in the cellular slime mold Dictyostelium discoideum. In this organism, starvation triggers the aggregation of amoebae into multicellular masses within which a simple, well-regulated pattern of partially differentiated cells is formed and which ultimately form fruiting bodies comprised of spore and stalk cells. In a monolayer system at low cell densities, stalk cell formation is dependent on the presence of both cyclic AMP and DIF. Both factors act within a short time of each other, induction by cyclic AMP preceding induction by DIF, beginning between 8 to 10 hours of incubation in monolayers, and progressively committing an increasing proportion of the cells in monolayer to form stalk cells. The relative effectiveness of analogues of cyclic AMP to induce stalk cell formation in monolayers indicates that the well-characterized cell surface cyclic AMP receptor most probably mediates the action of cyclic AMP. Although this receptor appears early during aggregation, it does not become activated until later during development in vivo, probably because the cyclic AMP concentrations within developing cell masses must build up to levels higher than those in aggregation streams. The finding that caffeine inhibits stalk cell formation in low density monolayers and that the permeable analogue 8-Bromo-cyclic AMP can partially reverse this inhibition suggests that activation of this receptor leads to an increase in internal cyclic AMP levels as one of the steps in stalk cell differentiation. The finding that the expression in low density monolayers of AP IV, a cell-type non-specific isozyme of acid phosphatase, was cyclic AMP-dependent is consistent with the view that cyclic AMP induces non-specific postaggregative gene expression during development in vivo. The findings that the expression of pre-stalk arid stalk cell specific antigens and of the pre-stalk cell specific isozyme AP II was DIF-dependent provide good evidence for the idea that both pre-stalk and stalk cell formation are induced by DIF. The fact that isolated pre-stalk cells require DIF for stalk cell formation in low density monolayers further supports this idea. Whereas cells independent of DIF for stalk cell formation in monolayers appear immediately after cyclic AMP-independent cells during differentiation in low density monolayers, DIF-independent cells appear considerably later during development in vivo. This evidence and the fact that developing cell masses contain elevated levels of DIF lead to the postulate that the factor(s) which triggers the formation of fruiting bodies also controls the pre-stalk to stalk cell conversion.

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