Extracellular cAMP Depletion Triggers Stalk Gene Expression inDictyostelium: Disparities in Developmental Timing and Dose Dependency Indicate That Prespore Induction and Stalk Repression by cAMP Are Mediated by Separate Signaling Pathways

Abstract

DuringDictyosteliumdevelopment, amoebae differentiate into spores and stalk cells. Earlier studies showed that extracellular cAMP is essential for induction of prespore differentiation and that cAMP represses stalk gene expressionin vitro.We show that the repressive pathway is operativein vivo,because activation of the stalk-specific promoter region of theecmBgene is strongly enhanced by overexpression of a phosphodiesterase that depletes extracellular cAMP. To test whether a single cAMP transduction pathway controls the choice between prespore or stalk cell differentiation, we compared the timing and dose dependency of the effects of cAMP on both responses. Cells acquire competence for cAMP repression ofecmBpromoter activity 4 hr later than for prespore gene induction. Half-maximal prespore induction requires 30 μMstable cAMP analog Sp-cAMPs, whileecmBinduction is half-maximally repressed by 200 nMSp-cAMPs, which is equivalent to about 3 to 13 nMcAMP. At concentrations exceeding 10 μM,Sp-cAMPs stimulatesecmBexpression from the intact promoter, but not from the stalk-specific subregion. These data suggest that distinct signaling pathways operating at different developmental stages control induction of prespore genes on one hand and repression of stalk genes on the other. Both stalk gene repression and prespore gene induction by Sp-cAMPs are antagonized by millimolar adenosine concentrations. However, an adenosine analog that is resistant to extracellular metabolism is active at 10 μM.Since adenosine inhibits cAMP binding to cAMP receptors, it may facilitate stalk gene expression by reducing the perceived cAMP concentration.