Differential cellular distribution of cAMP-dependent protein kinase during development of Dictyostelium discoideum

Role of PKA in the timing of developmental events in Dictyostelium cells

The cyclic AMP (cAMP)-dependent protein kinase, PKA, is dispensable for growth of Dictyostelium cells but plays a variety of crucial roles in development. The catalytic subunit of PKA is inhibited when associated with its regulatory subunit but is activated when cAMP binds to the regulatory subunit. Deletion of pkaR or overexpression of the gene encoding the catalytic subunit, pkaC, results in constitutive activity. Development is independent of cAMP in strains carrying these genetic alterations and proceeds rapidly to the formation of both spores and stalk cells. However, morphogenesis is aberrant in these mutants. In the wild type, PKA activity functions in a circuit that can spontaneously generate pulses of cAMP necessary for long-range aggregation. It is also essential for transcriptional activation of both prespore and prestalk genes during the slug stage. During culmination, PKA functions in both prespore and prestalk cells to regulate the relative timing of terminal differentiation. A positive feedback loop results in the rapid release of a signal peptide, SDF-2, when prestalk cells are exposed to low levels of SDF-2. The signal transduction pathway that mediates the response to SDF-2 in both prestalk and prespore cells involves the two-component system of DhkA and RegA. When the cAMP phosphodiesterase RegA is inhibited, cAMP accumulates and activates PKA, leading to vacuolation of stalk cells and encapsulation of spores. These studies indicate that multiple inputs regulate PKA activity to control the relative timing of differentiations in Dictyostelium.

PSF and CMF, autocrine factors that regulate gene expression during growth and early development of Dictyostelium

Throughout growth and development, Dictyostelium cells secrete autocrine factors that accumulate in proportion to cell density. At sufficient concentration, these factors cause changes in gene expression. Vegetative Dictyostelium cells continuously secrete prestarvation factor (PSF). The bacteria upon which the cells feed inhibit their response to PSF, allowing the cells to monitor their own density in relation to that of their food supply. At high PSF/bacteria ratios, which occur during late exponential growth, PSF induces the expression of several genes whose products are needed for cell aggregation. When the food supply has been depleted, PSF production declines, and a second density-sensing pathway is activated. Starving cells secrete conditioned medium factor (CMF), a glycoprotein of Mr 80 kDa that is essential for the development of differentiated cell types. Antisense mutagenesis has shown that cells lacking CMF cannot aggregate, and preliminary data suggest that CMF regulates cAMP signal transduction. Calculations indicate that a mechanism of simultaneously secreting and recognizing a signal molecule, as used by Dictyostelium to monitor cell density, could also be used to determine the total number of cells in a tissue.

Conditions that elevate intracellular cyclic AMP levels promote spore formation in Dictyostelium

We have been using sporogenous mutants of Dictyostelium discoideum strain V12M2 to study regulation of cell fate during terminal differentiation of spores and stalk cells. Analyses of intracellular cAMP accumulation, cAMP secretion, cAMP binding to cell surface receptors, and chemotactic sensitivity to exogenous cAMP during aggregation showed that all of these functions were identical in V12M2 and HB200, a sporogenous mutant. We used several methods of altering intracellular cAMP levels in HB200 cells to test the hypothesis that intracellular cAMP levels affect cell fate. First, HB200 amoebae were treated with 5 mM caffeine for 4 h during growth, washed, and allowed to develop in the absence of caffeine. Treated cells had normal levels of intracellular cAMP and adenylate cyclase activities at the beginning of differentiation; by 6 h development, they contained two to three times more intracellular cAMP and two times more GTP-dependent adenylate cyclase activity than untreated cells. However, their level of basal Mn++-dependent adenylate cyclase activity was the same as untreated controls. Thus, treatment of growing HB200 amoebae with caffeine for only 4 h leads to hyperinduction of a GTP-dependent regulator (or inhibition of a negative regulator) of adenylate cyclase during subsequent differentiation, without induction of basal activity. The fraction of amoebae forming spores increased twofold when HB200 amoebae were treated with caffeine during growth. Spore (but not stalk cell) differentiation by such treated cells was blocked by inhibitors of cAMP accumulation. Second, cells grown on nutrient agar accumulated higher levels of intracellular cAMP and formed more spores in vitro than cells grown in shaken suspension.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP-dependent protein kinase from Dictyostelium discoideum

The cAMP-dependent protein kinase (cAK) from Dictyostelium discoideum is an enzyme composed of one catalytic and one regulatory subunit. Upon binding of cAMP, the holoenzyme dissociates to liberate free active catalytic subunits. The cAK is developmentally regulated, ranging from very little activity in vegetative cells to maximal expression in postaggregative cells. Although there is no immunological cross-reaction between the subunits of cAKs from Dictyostelium and from other organisms, they share several biochemical properties. A complete cDNA for the regulatory subunit has been cloned and sequenced. Only one copy of the gene for the regulatory subunit is present per haploid genome. On the basis of the comparison of the structure of the cAK from Dictyostelium with its counterparts in yeast and higher eukaryotes, we propose a model for the evolution of cyclic-nucleotide-binding proteins.

Pharmacological characterization of cyclic AMP receptors mediating gene regulation in Dictyostelium discoideum

Extracellular molecules regulate gene expression in eucaryotes. Exogenous cyclic AMP (cAMP) affects the expression of a large number of developmentally regulated genes in Dictyostelium discoideum. Here, we determine the specificity of the receptor(s) which mediates gene expression by using analogs of cAMP. The order of potency with which these analogs affect the expression of specific genes is consistent with the specificity of their binding to a cell surface receptor and is distinct from their affinity for intracellular cAMP-dependent protein kinase. Dose-response curves with cAMP and adenosine 3',5'-monophosphorothioate, a nonhydrolyzable analog, revealed that the requirement for high concentrations of exogenous cAMP for regulating gene expression is due to the rapid degradation of cAMP by phosphodiesterase. The addition of low concentrations of cAMP (100 nM) or analogs in pulses also regulates gene expression. Both the genes that are positively regulated by exogenous cAMP and the discoidin gene, which is negatively regulated, respond to cAMP analogs to the same degree. Genes expressed in prespore or prestalk cells are also similarly regulated. These data suggest that the effects are mediated through the same receptor. The specificity of this receptor is indistinguishable from that of the well-characterized cell surface cAMP receptor.

cAMP induction of prespore and prestalk gene expression in Dictyostelium is mediated by the cell-surface cAMP receptor

Extracellular adenosine 3',5'-cyclic monophosphate (cAMP) is required for cell-type-specific gene expression in developing Dictyostelium discoideum. We have developed a microassay for the expression of these genes, using antibodies directed against their protein products. To characterize the transduction mechanism, we have used in this assay cAMP analogues that preferentially activate either the cell-surface cAMP receptor or the internal cAMP-dependent protein kinase. N6-(aminohexyl) cAMP activates the Dictyostelium cAMP-dependent protein kinase but does not bind to the cell-surface cAMP receptor and does not cause cell-type-specific gene expression. 2'-Deoxy-cAMP does not activate the cAMP-dependent protein kinase but binds to the receptor and causes cell-type-specific gene expression. Cyclic AMP-induced accumulation of prestalk mRNA in shaking cultures still occurs in the presence of caffeine, which blocks the receptor-coupled activation of adenyl cyclase. This suggests that the extracellular cAMP induction of cell-type-specific gene expression in developing Dictyostelium cells is mediated by the cell-surface cAMP receptor and that activating adenyl cyclase by this receptor is not essential. Using the N6-(aminohexyl) cAMP to competitively inhibit phosphodiesterase, we show that 30 nM cAMP is sufficient to induce prestalk or prespore gene expression.

Pharmacological characterization of cyclic AMP receptors mediating gene regulation in Dictyostelium discoideum

Extracellular molecules regulate gene expression in eucaryotes. Exogenous cyclic AMP (cAMP) affects the expression of a large number of developmentally regulated genes in Dictyostelium discoideum. Here, we determine the specificity of the receptor(s) which mediates gene expression by using analogs of cAMP. The order of potency with which these analogs affect the expression of specific genes is consistent with the specificity of their binding to a cell surface receptor and is distinct from their affinity for intracellular cAMP-dependent protein kinase. Dose-response curves with cAMP and adenosine 3',5'-monophosphorothioate, a nonhydrolyzable analog, revealed that the requirement for high concentrations of exogenous cAMP for regulating gene expression is due to the rapid degradation of cAMP by phosphodiesterase. The addition of low concentrations of cAMP (100 nM) or analogs in pulses also regulates gene expression. Both the genes that are positively regulated by exogenous cAMP and the discoidin gene, which is negatively regulated, respond to cAMP analogs to the same degree. Genes expressed in prespore or prestalk cells are also similarly regulated. These data suggest that the effects are mediated through the same receptor. The specificity of this receptor is indistinguishable from that of the well-characterized cell surface cAMP receptor.

A gene expressed in undifferentiated vegetative Dictyostelium is repressed by developmental pulses of cAMP and reinduced during dedifferentiation

We describe the gene M4-1, whose unique pattern of developmental expression will allow us to study the molecular mechanisms controlling expression in undifferentiated cells in addition to repression in response to cAMP during development and reinduction during dedifferentiation. M4-1 is a Dictyostelium gene expressed in the undifferentiated cell. We have shown that M4-1 continues to be expressed very early during the developmental cycle but is repressed at a later stage of development, at a time coincident with the establishment of oscillations in the cAMP pool. Studies on the expression of the M4-1 gene in shaking culture, under conditions that mimic early development, have established that pulsatile stimulation of cells with cAMP is sufficient to repress M4-1 expression. Consistent with this, cells that are exposed to high levels of cAMP are unable to respond normally to cAMP oscillations and continue to express M4-1 at vegetative levels. These data indicate that low-level oscillations of cAMP are required for the repression of M4-1 expression rather than the continuous high levels of cAMP responsible for the regulation of a different class of Dictyostelium genes. We suggest that cAMP may mediate developmental expression of the Dictyostelium genome by different mechanisms. We also show that cell-cell interaction, a developmental event that occurs subsequent to the cAMP pulse, does not normally influence the regulation of M4-1. Finally, we have shown that when cAMP-pulsed cells are induced to dedifferentiate, M4-1 RNA sequences rapidly reappear in nuclei and cytoplasm, suggesting that regulation of M4-1 expression is primarily mediated at the level of transcription.

A secreted factor and cyclic AMP jointly regulate cell-type-specific gene expression in Dictyostelium discoideum

We are studying cell differentiation in Dictyostelium discoideum by examining the regulation of genes that are preferentially expressed in different cell types. A system has been established in which prestalk- and prespore-cell-specific genes are expressed in single cells in response to culture conditions. We confirm our previous results showing that cyclic AMP induces prestalk genes and now show that it is also required for prespore gene induction. The expression of both classes of genes is additionally dependent on the presence of a factor(s) secreted by developing cells which we call conditioned medium factor(s). An assay for conditioned medium factor(s) shows that it is detectable within 2.5 h after the onset of development. Conditioned medium factor(s) also promotes the expression of genes induced early in development, but has no detectable effect on the expression of actin genes and a gene expressed maximally in vegetative cells. In the presence of conditioned medium factor(s), exogenous cyclic AMP at the onset of starvation fails to induce the prespore and prestalk genes. The addition of cyclic AMP between 2 and 12 h of starvation results in rapid prestalk gene expression, whereas prespore genes are induced at an invarient time (approximately 18 h after the onset of starvation). These data suggest that cyclic AMP and conditioned medium factor(s) are sufficient for prestalk gene induction, whereas an additional parameter(s) is involved in the control of prespore gene induction. In contrast to several previous studies, we show that multicellularity is not essential for the expression of either prespore or prestalk genes. These data indicate that prespore and prestalk genes have cell-type-specific as well as shared regulatory factors.

A secreted factor and cyclic AMP jointly regulate cell-type-specific gene expression in Dictyostelium discoideum

We are studying cell differentiation in Dictyostelium discoideum by examining the regulation of genes that are preferentially expressed in different cell types. A system has been established in which prestalk- and prespore-cell-specific genes are expressed in single cells in response to culture conditions. We confirm our previous results showing that cyclic AMP induces prestalk genes and now show that it is also required for prespore gene induction. The expression of both classes of genes is additionally dependent on the presence of a factor(s) secreted by developing cells which we call conditioned medium factor(s). An assay for conditioned medium factor(s) shows that it is detectable within 2.5 h after the onset of development. Conditioned medium factor(s) also promotes the expression of genes induced early in development, but has no detectable effect on the expression of actin genes and a gene expressed maximally in vegetative cells. In the presence of conditioned medium factor(s), exogenous cyclic AMP at the onset of starvation fails to induce the prespore and prestalk genes. The addition of cyclic AMP between 2 and 12 h of starvation results in rapid prestalk gene expression, whereas prespore genes are induced at an invarient time (approximately 18 h after the onset of starvation). These data suggest that cyclic AMP and conditioned medium factor(s) are sufficient for prestalk gene induction, whereas an additional parameter(s) is involved in the control of prespore gene induction. In contrast to several previous studies, we show that multicellularity is not essential for the expression of either prespore or prestalk genes. These data indicate that prespore and prestalk genes have cell-type-specific as well as shared regulatory factors.