Extracellular cAMP depletion triggers stalk gene expression in Dictyostelium: disparities in developmental timing and dose dependency indicate that prespore induction and stalk repression by cAMP are mediated by separate signaling pathways.
During Dictyostelium development, 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 expression in vitro. We show that the repressive pathway is operative in vivo, because activation of the stalk-specific promoter region of the ecmB gene 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 of ecmB promoter activity 4 hr later than for prespore gene induction. Half-maximal prespore induction requires 30 microM stable cAMP analog Sp-cAMPs, while ecmB induction is half-maximally repressed by 200 nM Sp-cAMPs, which is equivalent to about 3 to 13 nM cAMP. At concentrations exceeding 10 microM, Sp-cAMPs stimulates ecmB expression 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 microM. Since adenosine inhibits cAMP binding to cAMP receptors, it may facilitate stalk gene expression by reducing the perceived cAMP concentration.
Soede
R D
RD
Cell Biology Section, Institute for Molecular Plant Sciences, University of Leiden, Wassenaarseweg 64, Leiden, 2333 AL, The Netherlands.
Hopper
N A
NA
Williams
J G
JG
Schaap
P
P
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology; D000818:Animals; D002454:Cell Differentiation / Q000235:genetics; D002460:Cell Line; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000172:deficiency*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004023:Dictyostelium / Q000235:genetics*; D004305:Dose-Response Relationship, Drug; D016326:Extracellular Matrix Proteins / Q000235:genetics*; D018507:Gene Expression Regulation, Developmental; D015800:Protozoan Proteins / Q000235:genetics*; D015290:Second Messenger Systems / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D013170:Spores / Q000235:genetics; D015533:Transcriptional Activation
D016326:Extracellular Matrix Proteins; C096301:HPSR2 protein, Giardia intestinalis; D015800:Protozoan Proteins; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000241:Adenosine