Opposite effects of adenosine on two types of cAMP-induced gene expression in Dictyostelium indicate the involvement of at least two different intracellular pathways for the transduction of cAMP signals

Analysis of 5' nucleotidase and alkaline phosphatase by gene disruption in Dictyostelium

In Dictyostelium discoideum a phosphatase with a high pH optimum is known to increase in activity during cell differentiation and become localized to a narrow band of cells at the interface of prespore and prestalk cells. However, it was not clear if this activity is due to a classical "alkaline phosphatase" with broad range substrate specificity or to a "5'nucleotidase" with high substrate preference for 5'AMP. We attempted to disrupt the genes encoding these two phosphatase activities in order to determine if the activity that is localized to the interface region resides in either of these two proteins. During aggregation of 5nt null mutants, multiple tips formed rather than the normal single tip for each aggregate. In situ phosphatase activity assays showed that the wt and the 5nt gene disruption clones had normal phosphatase activity in the area between prestalk and prespore cell types, while the alp null mutants did not have activity in this cellular region. Thus, the phosphatase activity that becomes localized to the interface of the prestalk and prespore cells is alkaline phosphatase.

Altered cell-type proportioning in Dictyostelium lacking adenosine monophosphate deaminase

The proportions of prespore and prestalk cells in Dictyostelium discoideum are regulated so that they are size invariant and can adjust when the ratio is perturbed. We have found that disruption of the gene amdA that encodes AMP deaminase results in a significantly increased proportion of prestalk cells. Strains lacking AMP deaminase form short, thick stalks and glassy sori with less than 5% the normal number of spores. The levels of prestalk-specific mRNAs in amdA(-) cells are more than twice as high as those in wild-type strains and prespore-specific mRNAs are reduced. Using an ecmA::lacZ construct to mark prestalk cells, we found that amdA(-) null slugs have twice the normal number of prestalk cells. The number of cells expressing an ecmO::lacZ construct was not affected by loss of AmdA, indicating that the mutation results in an increase in PST-A prestalk cells rather than PST-O cells. This alteration in cell-type proportioning is a cell-autonomous consequence of the loss of AMP deaminase since mutant cells developed together with wild-type cells still produced excess prestalk cells and wild-type cells carrying the ecmA::lacZ construct formed normal numbers of prestalk cells when developed together with an equal number of amdA(-) mutant cells.

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

Factors controlling prespore cell differentiation in Dictyostelium discoideum: minute amounts of differentiation-inducing factor promote prespore cell differentiation

Amoebae of strain V12M2 differentiate efficiently into prespore cells without cell contact in a salt solution containing cAMP, if the pH of the medium is maintained suitably acidic using a restricted buffer [31]. Under such conditions, most cells differentiate into prespore cells at pH 5.2. Using this developmental system, the elements controlling prespore-cell differentiation were analyzed. First, the dependence on cell density was examined. At a very low density (10(2) cells/cm2), most cells did not differentiate. As the density was increased the proportion of prespore cells differentiating increased, reaching a maximum at 5 x 10(3) cells/cm2. Conditioned medium could mimic the effects of cell density on cell differentiation. These findings suggest the presence of factor(s) released into the medium which are involved in inducing prespore-cell differentiation. The conditioned medium was found to contain at least two prespore-inducing components; one is a novel factor(s) and the other is DIF, which has previous only been considered to repress prespore-cell differentiation. These findings were supported by experiments using a DIF-deficient mutant, HM44.

Biochemical and genetic characterization of a rapid-development strain in Dictyostelium discoideum

We have examined the rates of development of six wild-type and rapid-development strains of Dictyostelium discoideum. Strains NC4 and HU1231, a derivative of V12, have very similar developmental profiles. In comparison to these two strains, amoebae of the rapid-development strain HT100 reach the aggregation stage about 6 h earlier. Also, intracellular cAMP levels in HT100 increase precociously during early development. Postaggregative morphogenesis of HT100 proceeds at the same rate as NC4 and HU1231. In addition we have assessed the progress of development by RNA hybridization blotting. The appearance in HT100 of mRNAs associated with aggregation is advanced by 6 h while the appearance of early gene products is not affected appreciably. These data suggest that HT100 differs from strains NC4 and HU1231 primarily in the process of aggregation. We have partially characterized the genetic background of strain HT100. Mating and cell fusion analyses suggest that HT100 is a derivative of V12. We constructed a growth-temperature-sensitive derivative of HT100, and fused it with HU1231 cells. The resulting diploids develop at the same rate as HT100, suggesting that the rapid-development phenotype in HT100 is dominant.

Control of cAMP-induced gene expression by divergent signal transduction pathways

A compilation of literature data and recent experiments led to the following conclusions regarding cyclic adenosine 3':5' monophosphate (cAMP) regulation of gene expression. Several classes of cAMP-induced gene expression can be discriminated by sensitivity to stimulation kinetics. The aggregation-related genes respond only to nanomolar cAMP pulses. The prestalk-related genes respond both to nanomolar pulses and persistent micromolar stimulation. The prespore specific genes respond only to persistent micromolar stimulation. The induction of the aggregation- and prestalk-related genes by nanomolar cAMP pulses may share a common transduction pathway, which does not involve cAMP, while involvement of the inositol 1,4,5-trisphosphate (IP3)/Ca2+ pathway is unlikely. Induction of the expression of prespore and prestalk-related genes by micromolar cAMP stimuli utilizes divergent signal processing mechanisms. cAMP-induced prespore gene expression does not involve cAMP and probably also not cyclic guanosine 3'.5' monophosphate (cGMP) as intracellular intermediate. Involvement of cAMP-induced phospholipase C (PLC) activation in this pathway is suggested by the observation that IP3 and 1,2-diacylglycerol (DAG) can induce prespore gene expression, albeit in a somewhat indirect manner and by the observation that Li+ and Ca2+ antagonists inhibit prespore gene expression. Cyclic AMP induction of prestalk-related gene expression is inhibited by IP3 and DAG and promoted by Li+, and is relatively insensitive to Ca2+ antagonists, which indicates that PLC activation does not mediate prestalk-related gene expression. Neither prespore nor prestalk-related gene expression utilizes the sustained cAMP-induced pHi increase as intracellular intermediate.

cAMP and cell sorting control the spatial expression of a developmentally essential cell-type-specific ras gene in Dictyostelium

The Dictyostelium ras gene (Dd-ras) is expressed at a low level in vegetative cells, is not expressed between the onset of development and aggregation, and is then re-expressed in the multicellular aggregate stages from the distal, now cAMP-responsive, promoter and from two more proximal promoters. Expression of activated Dd-ras (G12----T12) (Reymond et al. 1986) results in an abnormal developmental phenotype with the formation of aggregates having multiple tips and an inhibition of further development. In this report we investigate the spatial expression of Dd-ras by fusing the 5'-flanking region to the Escherichia coli lacZ gene and by staining aggregates for beta-galactosidase (beta-gal) activity. We show that fusions using 5'-flanking sequences that include all promoters are expressed in approximately 10-20% of the cells randomly scattered within the early aggregate. Our data indicate that these beta-gal-expressing cells migrate to newly formed tips of aggregates and localize in the region that becomes the prestalk zone. Staining is also seen in the very posterior of the organism. The anterior staining appears to be specific for the prestalk A population, and beta-gal activity is subsequently present in stalk cells as developmental proceeds. When only the two more proximal promoters are used to drive lacZ expression, localized staining is seen in the anterior prestalk region, although it is weaker than with the construct carrying all promoters. Moreover, staining is not seen in the posterior domain in the first finger stage, suggesting differences in the spatial expression from the different promoters. Staining is also observed in some cells within the prespore region, which could be anterior-like cells. The pattern of Dd-ras/lacZ staining during tip formation suggests a directed, spiral pattern of cell migration, possibly in response to the proposed spiral gradient of cAMP within the developing aggregate. The pattern of Dd-ras is consistent with the abnormal developmental phenotype caused by expressing an activated Dd-ras Thr12 gene and suggests an essential role for Dd-ras in controlling spatial differentiation.

A spatial gradient of expression of a cAMP-regulated prespore cell-type-specific gene in Dictyostelium

Previously, we identified a class of genes in Dictyostelium that are prespore cell-type specific in their expression in the multicellular aggregate and are inducible by cAMP acting through cell-surface cAMP receptors. In this paper, we report the cloning and analysis of the regulatory regions controlling the expression of one such gene that encodes a spore coat protein, SP60. By use of a fusion of the firefly luciferase gene and Escherichia coli lacZ [expresses beta-galactosidase (beta-gal)], we have identified cis-acting regions required for proper spatial and temporal expression in multicellular aggregates and for cAMP induction in shaking cell culture. Deletion analysis suggests that a CA-rich element (CAE) and surrounding sequences present three times within the 5'-flanking sequence are required for proper regulation. SP60-lacZ fusions that include all three of these regions express lacZ only in the posterior approximately 85% of migrating slugs (prespore zone). Studies show that SP60 is expressed during mid to late aggregation, and SP60-lacZ-positive cells are spatially localized as a doughnut-shaped ring within the forming aggregate. Cells within the skirt that surrounds the aggregate and that are still migrating into the aggregate do not stain. Sequential 5' deletions of CAEs and surrounding regions affect the expression level of SP60-luciferase in response to developmental signals and cAMP, as well as the spatial pattern of SP60-lacZ. Deletion of the first (most 5') of these regions restricts the spatial expression of SP60-lacZ fusions to the anterior of the prespore zone. When both the first and second regions are removed, the expression level drops, and the staining is restricted to the prespore/prestalk boundary. Furthermore, the staining pattern that is seen with these two deletions is present as a gradient from anterior to posterior within the prespore zone. Deletion of all three regions results in a loss of both cAMP and developmentally induced expression. These results suggest the presence of a gradient within the prespore zone that differentially affects the activity of promoters containing different numbers of response elements.

Lithium respecifies cyclic AMP-induced cell-type specific gene expression in Dictyostelium

We investigated the effect of LiCl on pattern formation and cAMP-regulated gene expression in Dictyostelium discoideum. In intact slugs, 5 mM LiCl induces an almost complete redifferentiation of prespore into prestalk cells. We found that LiCl acts by interfering with the transduction of extracellular cAMP to cell-type-specific gene expression; LiCl inhibits the induction of prespore-specific gene expression by cAMP, while it promotes the induction of prestalk-associated gene expression by cAMP. Our results indicate that two divergent pathways transduce the extracellular cAMP signal to, respectively, prestalk and prespore gene expression.