Requirement for cell differentiation in Dictyostelium discoideum

Preparation of an antibody that recognizes and neutralizes Dictyostelium differentiation-inducing factor-1

In the development of the cellular slime mold Dictyostelium discoideum, the differentiation-inducing factor-1 (DIF-1; 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one) plays an important role in the regulation of cell differentiation and chemotaxis; however, the cellular signaling systems involving DIF-1 remain to be elucidated. To obtain a probe for DIF-1, we synthesized a DIF derivative (DIF-1-NH(2); 6-amino-1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one), and prepared an anti-DIF-1 antibody using a DIF-1-NH(2)-conjugated macromolecule as the immunogen. A 100-fold dilution of the antibody bound to DIF-1-NH(2)-conjugated resin, and this binding was inhibited by co-addition of 20 microM DIF-1 or DIF-1-NH(2). In a monolayer culture of HM44 cells, a DIF-deficient D. discoideum strain, 0.5 nM exogenous DIF-1 induced stalk cell formation in approximately 60% of the cells; this induction was dose-dependently inhibited by the antibody (diluted 12.5- or 25-fold). Furthermore, this inhibition by the antibody was recovered by co-addition of 2.5 or10 nM DIF-1. The results indicate that the anti-DIF-1 antibody recognizes DIF-1 and neutralizes its function.

Dictyopyrones, novel alpha-pyronoids isolated from Dictyostelium spp., promote stalk cell differentiation in Dictyostelium discoideum

Dictyopyrones A and B (DpnA and B), whose function(s) is not known, were isolated from fruiting bodies of Dictyostelium discoideum. In the present study, to assess their function(s), we examined the effects of Dpns on in vitro cell differentiation in D. discoideum monolayer cultures with cAMP. Dpns at 1-20 microM promoted stalk cell formation to some extent in the wild-type strain V12M2. Although Dpns by themselves could hardly induce stalk cell formation in a differentiation-inducing factor (DIF)-deficient strain HM44, both of them dose-dependently promoted DIF-1-dependent stalk cell formation in the strain. In the sporogenous strain HM18, Dpns at 1-20 microM suppressed spore formation and promoted stalk cell formation in a dose-dependent manner. Analogs of Dpns were less effective in affecting cell differentiation in both HM44 and HM18 cells, indicating that the activity of Dpns should be chemical structure specific. It was also shown that DpnA at 2-20 microM dose-dependently suppressed spore formation induced with 8-bromo cAMP and promoted stalk cell formation in V12M2 cells. Interestingly, it was shown by the use of RT-PCR that DpnA at 10 microM slightly promoted both prespore- and prestalk-specific gene expressions in an early phase of V12M2 and HM18 in vitro differentiation. The present results suggest that Dpns may have functions (1) to promote both prespore and prestalk cell differentiation in an early stage of development and (2) to suppress spore formation and promote stalk cell formation in a later stage of development in D. discoideum.

GSK3 is a multifunctional regulator of Dictyostelium development

Glycogen synthase kinase 3 (GSK3) is a central regulator of metazoan development and the Dictyostelium GSK3 homologue, GskA, also controls cellular differentiation. The originally derived gskA-null mutant exhibits a severe pattern formation defect. It forms very large numbers of pre-basal disc cells at the expense of the prespore population. This defect arises early during multicellular development, making it impossible to examine later functions of GskA. We report the analysis of a gskA-null mutant, generated in a different parental strain, that proceeds through development to form mature fruiting bodies. In this strain, Ax2/gskA-, early development is accelerated and slug migration greatly curtailed. In a monolayer assay of stalk cell formation, the Ax2/gskA- strain is hypersensitive to the stalk cell-inducing action of DIF-1 but largely refractory to the repressive effect exerted by extracellular cAMP. During normal development, apically situated prestalk cells express the ecmB gene just as they commit themselves to stalk cell differentiation. In the Ax2/gskA- mutant, ecmB is expressed throughout the prestalk region of the slug, suggesting that GskA forms part of the repressive signalling pathway that prevents premature commitment to stalk cell differentiation. GskA may also play an inductive developmental role, because microarray analysis identifies a large gene family, the 2C family, that require gskA for optimal expression. These observations show that GskA functions throughout Dictyostelium development, to regulate several key aspects of cellular patterning.

Prespore-to-stalk conversion involves the production of a pathway-specific glycoprotein, wst25, in the cellular slime mould Dictyostelium discoideum

We have previously identified a stalk-specific wheat germ agglutinin (WGA)-binding protein, wst34, in the cellular slime mould Dictyostelium discoideum [Biochem. Cell Biol. 68 (1990) 699]. Here, we found another stalk-specific WGA-binding protein, wst25, which was detected with two antisera that recognize wst34. Using the two marker proteins, we then analyzed and compared the pathways of prestalk-to-stalk maturation and prespore-to-stalk conversion in vitro and in vivo. Prestalk cells isolated from normally formed slugs can be converted to stalk cells (designated StI) in vitro with 8-bromo-cAMP (Br-cAMP), whereas prespore cells isolated from slugs can be converted to fully vacuolated stalk cells (designated StII) in vitro with Br-cAMP and DIF-1. During the process of prespore-to-stalk conversion, prespore-specific mRNAs, D19 and 2H3, disappeared rapidly, while prestalk-specific mRNAs, ecmA and ecmB, appeared at 2h of incubation and increased thereafter. Most importantly, however, the StII cells thus formed were biochemically different from the StI cells originated from prestalk cells; that is, StI cells expressed wst34 but not wst25, while StII cells expressed wst25 but not wst34. When prespore cells isolated from slugs were allowed to develop on a substratum, they differentiated into spores and stalk cells and formed fruiting bodies, and the stalk cells formed from prespore cells in vivo expressed wst25 but not wst34. The present results indicate that there are two types of stalk cells, StI (prestalk-origin) and StII (prespore-origin), and that wst34 and wst25 are the specific markers for StI and StII, respectively.

Taking the plunge. Terminal differentiation in Dictyostelium

During the last stage of Dictyostelium development a motile, cylindrical slug transforms into an immotile, stalked fruiting body and the constituent cells change from amoebae to either refractile spores or vacuolated stalk cells. Analysis of this process using genetics and simple culture techniques is becoming a powerful way of investigating a number of conserved signal transduction processes. A common pathway activating cAMP-dependent protein kinase (PKA) triggers the maturation of spore cells and those stalk cells forming the stalk. It uses a eukaryotic version of the 'bacterial' two-component phospho-relay system to control cAMP breakdown. A second pathway, inhibiting the GSK3 protein kinase, might control the maturation of a distinct set of stalk cells at the base of the fruiting body.

Identification of the cell fate gene stalky in Dictyostelium

Using insertional mutagenesis, we have isolated a "stalky" mutant in which cells destined to become spores end up as stalk cells. Similar mutants were previously observed after chemical mutagenesis, but the affected gene could not be isolated. Our mutant, like the previous ones, is in stkA. Its defect is cell-autonomous and not overcome by overexpressing cAMP-dependent protein kinase. stkA is strongly expressed in the prespore region of aggregates but not in the anterior prestalk zone. The mutant expresses normal levels of prespore-cell transcripts but fails to produce the spore transcript spiA. stkA encodes a predicted 99 kDa protein (STKA) with two putative C4 zinc fingers, one of which is a GATA-type finger, indicating that it may be a transcription factor. This conclusion is supported by localization of STKA in the nucleus.

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.

Morphogenesis in Dictyostelium: new twists to a not-so-old tale

Prespore differentiation requires both cAMP-dependent protein kinase and the transcription factor GBF, and for one class of prespore genes the two form part of a single pathway. It seems that differentiation-inducing factor, the inducer of prestalk cell differentiation, may operate via a calcium signalling pathway, and terminal stalk cell differentiation is in part regulated by glycogen synthase kinase 3.

Comparison of the Dictyostelium rasD and ecmA genes reveals two distinct mechanisms whereby an mRNA may become enriched in prestalk cells

The Dictyostelium ras gene, rasD, encodes an mRNA that is more abundant in prestalk than prespore cells in the migratory slug. Its expression is inducible by extracellular cAMP but is not inducible by the prestalk and stalk cell morphogen differentiation inducing factor (DIF). We show that a rasD-lacZ fusion gene is first expressed in approximately one half of the cells in the aggregate, including some cells that also express a prespore-specific marker. The amount of rasD-lacZ fusion protein in prespore cells then diminishes as the slug is formed. Analysis of a rasD-lacZ fusion protein with an N terminal substitution that reduces protein stability within the cell provides strong confirmatory evidence that the ras gene product becomes enriched in prestalk cells by selective repression of gene expression in prespore cells. In contrast, the DIF-inducible ecmA gene is expressed only in those cells that will become prestalk cells in the migratory slug. These results show that there are two different ways in which an mRNA may become enriched in prestalk cells and support the view that DIF is the inducer of prestalk cell differentiation.

Glycogen synthase kinase 3 regulates cell fate in Dictyostelium

Extracellular cyclic AMP (cAMP) induces the formation of prespore cells in Dictyostelium but inhibits stalk cell formation. We have cloned gskA, which encodes the Dictyostelium homolog of glycogen synthase kinase 3 (GSK-3), and discovered that it is required for both cAMP effects. Disruption of gskA creates a mutant that aggregates but forms few spores and an abnormally high number of stalk cells. These stalk cells probably arise from an expanded prestalk B (pstB) cell population, which normally produces the basal disc of the fruiting body. In cultured mutant cells, cAMP neither inhibits pstB cell differentiation nor induces efficient prespore cell differentiation. We propose that cAMP acts through a common pathway that requires GSK-3 and determines the proportion of prespore and pstB cells.

Chemotactic sorting to cAMP in the multicellular stages of Dictyostelium development

Dictyostelium transformants that overproduce the extracellular form of cyclic nucleotide phosphodiesterase and so accumulate a reduced amount of cAMP are blocked in development after aggregation in the form of a tight mound, prior to formation of the apical tip. In such mounds, prespore cell differentiation is repressed, and the apical accumulation of prestalk cells is greatly retarded. When a source of cAMP is placed below the arrested mounds, prestalk cells that would normally migrate in an apical direction instead sort downwards to the substratum. Thus, by acting as the chemoattractant that draws prestalk cells to the apex, cAMP signaling directs the formation of a patterned structure.

Morphogenesis and differentiation of Dictyostelium cells interacting with immobilized glucosides: dependence on DIF production

Previous work has shown that multicellular morphogenesis of submerged Dictyostelium cells is inhibited when they bind to glucosides covalently linked to polyacrylamide gels. The amoebae aggregate normally, but then the aggregates repeatedly disperse and reaggregate, whereas control cells go on to form tight aggregates. We have investigated the role of the stalk cell differentiation inducing factors (DIFs) in this process. In the presence of cyclic AMP, amoebae submerged at high cell density accumulate DIF and differentiate into stalk cells. We find that stalk cell differentiation is inhibited by interaction of the cells with glucoside gels in these conditions, but can be restored by the addition of exogenous DIF-1. Since the responsiveness of cells to DIF-1 is not altered, it appears likely that the effect of the glucoside gel is to block DIF-1 production. Further, the addition of DIF-1 or DIF-2 stimulates the formation of tight aggregates by cells developing on glucoside gels in the absence of cyclic AMP, thus preventing the rounds of aggregation and disaggregation otherwise seen. This suggests a role for DIF in morphogenesis as well as in controlling cell differentiation. We propose a model in which immobilized glucosides activate a specific receptor ("food sensor") which drives the amoebae toward the vegetative state and inhibits DIF accumulation. DIF, on the other hand, induces tight aggregate formation and so locks the amoebae into the developmental program.

DC6, a novel type of Dictyostelium discoideum gene regulated by secreted factors but not by cAMP

We have isolated a gene, DC6, which is induced in the early aggregative stages of development in Dictyostelium discoideum. The increase in DC6 expression is dependent on high cell density, indicating that cellular interactions are required for DC6 induction. In low-cell-density cultures, the induction of DC6 occurs if supplied with conditioned medium of developing cells, suggesting that secreted factors are involved in DC6 induction. The expression of DC6 is not affected (1) in the presence of caffeine or adenosine, which block the production or the action of cAMP pulses, (2) in the presence of high concentrations of cAMP, or (3) in mutant strains (Synag7 and FrigidA), which are defective in transduction pathways of cAMP pulse signals. These results indicate that the induction of DC6 does not require extracellular cAMP pulse signals, which are known to regulate the expression of many genes in the early development. Independence of cAMP signals and dependence on other unknown cellular interactions are prominent characteristics of DC6.

Conditions that alter intracellular cAMP levels affect expression of the cAMP phosphodiesterase gene in Dictyostelium

We examined expression of the Dictyostelium cAMP phosphodiesterase (PDE) gene under conditions that alter intracellular cAMP levels during in vitro differentiation of wild-type strain V12M2 and a sporogenous derivative, HB200. In control cultures, cellular PDE activity peaked at 6 hr and declined by 8 hr, while secreted PDE activity continued to increase through 8 hr. Lowering intracellular cAMP levels with caffeine or progesterone increased cellular and secreted PDE activities 2-fold, increased stalk cell differentiation, and inhibited spore differentiation. In contrast, exposure to 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP; a membrane-permeable cAMP analog) or ammonia (which promotes intracellular cAMP accumulation in V12M2 and HB200 cells) lowered PDE activities by as much as 45%, decreased stalk cell differentiation, and increased spore differentiation. Simultaneous exposure to 8-Br-cAMP and caffeine gave intermediate PDE activities as would be expected if 8-Br-cAMP entered the cell and bypassed the caffeine-mediated block to adenylate cyclase activation. In all cases, we observed commensurate changes in developmental PDE transcript levels. The developmental time course of expression was not significantly altered by these treatments. These results suggest that the magnitude of PDE gene expression is negatively regulated by intracellular cAMP levels and provide evidence for one of the earliest changes in gene expression that is consistent with cell-type specificity. These results are discussed in terms of a bistable switch employing intracellular cAMP as a regulator of cell fate.

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)

The effect of ammonia on cell-type-specific enzyme accumulation in Dictyostelium discoideum

We have shown that ammonia inhibits stalk cell formation and promotes spore formation during cyclic AMP induced differentiation of monolayers of sporogenous amoebae (Gross, J.D. et al., Nature (London), 303, 244-245, 1983). Here we show that exposure to ammonia favours the accumulation of prespore- over prestalk-specific enzymes and related products. We conclude that ammonia switches cells from the prestalk to the prespore pathway rather than simply inhibiting stalk cell maturation and hence may act as a morphogen during development.

Identification of the cell surface molecule involved in sexual cell fusion of Dictyostelium discoideum

Dictyostelium discoideum was used as a model system for elucidating the molecular mechanism of sexual cell fusion. In heterothallic strains NC4 and HM1 of D. discoideum, complements in mating type, amoeboid cells acquire fusion competence only under certain environmental conditions, such as the presence of excess water and a certain period of darkness, to fuse sexually. The surface of cells which acquired fusion competence was found to possess specific antigens. Monovalent antibodies prepared from rabbit antiserum against fusion-competent NC4 cells inhibit the sexual cell fusion of these cells completely. Two specific antigenic proteins, 39 and 138 k Da in relative molecular mass and specific for fusion-competent cells, were detected. Only one, the 138-k Da protein, was capable of neutralizing the fusion-inhibitory activity of the monovalent antibody. These results show that the 139-k Da protein is the one involved in the sexual cell fusion of NC4 and HM1 strains in D. discoideum.

Plasma membrane proton pump inhibition and stalk cell differentiation in Dictyostelium discoideum

The choice of the stalk cell differentiation pathway in Dictyostelium is promoted by an endogenous substance, DIF-1, which is 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)-1-hexanone. It is also favoured by weak acids and two inhibitors of the plasma membrane proton pumps of fungi and plants, diethylstilbestrol (DES) and zearalenone, and antagonised by ammonia and other weak bases, which promote spore differentiation. These observations led to the proposal that the choice of differentiation pathway is regulated by intracellular pH. They also prompted the conjecture that DIF-1 itself is a plasma membrane proton pump inhibitor. We report here experiments showing that DIF-1 is not a plasma membrane proton pump inhibitor. We demonstrate that diethylstilbestrol and zearalenone do inhibit the plasma membrane proton pump of Dictyostelium and we show that there is an excellent qualitative and quantitative correlation between the inhibitory activity of these agents, and of a number of other substances, and their ability to divert differentiation from the spore to the stalk pathway. We conclude that inhibition of the plasma membrane proton pump does shift the choice of differentiation pathway in Dictyostelium towards the stalk pathway, but that DIF does not act by this route, and we propose a model for the actions of DIF and plasma membrane proton pump inhibitors in which the differentiation pathway is controlled by the pH of intracellular vesicles rather than by intracellular pH itself. The model invokes a DIF- and proton-activated vesicular chloride channel whose opening permits acidification of the vesicles and lowers cytosolic Ca++ concentration.

Cyclic AMP is an inhibitor of stalk cell differentiation in Dictyostelium discoideum

Cyclic AMP and DIF-1 (1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)-1-hexanone) together induce stalk cell differentiation in vitro in Dictyostelium discoideum strain V12M2. The induction can proceed in two stages: in the first, cyclic AMP brings cells to a DIF-responsive state; in the second, DIF-1 alone can induce stalk cell formation. We report here that during the DIF-1-dependent stage, cyclic AMP is a potent inhibitor of stalk cell differentiation. Addition of cyclic AMP at this stage to V12M2 cells appreciably delays, but does not prevent, stalk cell formation. In contrast, stalk cell differentiation in the more common strain NC4 is completely suppressed by the continued presence of cyclic AMP. This fact explains earlier failures to induce stalk cells in vitro in NC4. We now consistently obtain efficient stalk cell induction in NC4 by removing cyclic AMP in the DIF-1-dependent stage. Cyclic AMP also inhibits the production of a stalk-specific protein (ST310) in both NC4 and a V12M2 derivative. Adenosine, a known antagonist of cyclic AMP action, does not relieve this inhibition by cyclic AMP and does not itself promote stalk cell formation. Finally, stalk cell differentiation of NC4 cells at low density appears to require factors in addition to cyclic AMP and DIF-1, but their nature is not yet known. The inhibition of stalk cell differentiation by cyclic AMP may be important in establishing the prestalk/prespore pattern during normal development, and in preventing the maturation of prestalk into stalk cells until culmination.

Signals controlling cell differentiation and pattern formation in Dictyostelium

The major inducers of cell differentiation in Dictyostelium appear to be cyclic AMP and DIF-1. Recently we have chemically identified DIF-1, together with the closely related DIF-2 and -3. They represent a new chemical class of potent effector molecules, based on a phenyl alkanone with chloro, hydroxy, and methoxy substitution of the benzene ring. Previous work has shown that DIF-1 can induce prestalk-specific gene expression within 15 min, whereas it suppresses prespore differentiation. Hence, DIF-1 can control the choice of pathway of cell differentiation in Dictyostelium and is therefore likely to be involved in establishing the prestalk/prespore pattern in the aggregate. In support of this, we show that DIF treatment of slugs results in an enlarged prestalk zone. Cyclic AMP seems less likely to have such a pathway-specific role, but later in development it becomes inhibitory to stalk cell differentiation. This inhibition may be important in suppressing terminal stalk cell differentiation until culmination. Spore differentiation can be induced efficiently by high levels of Br-cyclic AMP, a permeant analogue of cyclic AMP. In this, it phenocopies certain spore-maturation mutants, and we propose that during normal development spore differentiation is triggered by an elevation in intracellular cyclic AMP levels. How this elevation in cyclic AMP levels is brought about is not known. The experiments with Br-cyclic AMP also provide the first direct evidence that elevated levels of intracellular cyclic AMP induce differentiation in Dictyostelium.

Stalk cell formation in monolayers of Dictyostelium discoideum V12-M2

Stalk cell formation in low-cell-density monolayers of Dictyostelium discoideum, strain V12-M2, occurs following the sequential addition of cyclic AMP and the differentiation-inducing factor (DIF). Both cyclic AMP and DIF are essential for the appearance of the prestalk-specific isozyme alkaline phosphatase-II, which suggests that both factors are necessary for prestalk cell formation. The available evidence suggests that the cyclic AMP requirement for stalk cell formation is mediated through the cell surface cyclic AMP receptor. However, stalk cell formation is inhibited by caffeine and this inhibition is reversed by the cell-permeable analogue 8-Br-cyclic AMP, which suggests in addition a possible involvement for elevated intracellular cyclic AMP concentrations in stalk cell formation. During in vivo development cells first become independent of cyclic AMP at the tipped aggregate stage, but the acquisition of cyclic AMP independence is advanced by several hours when cells are incubated in the presence of cyclic AMP for 2 hours. Cells do not become independent of DIF until the culmination stage of development, which suggests the possibility that DIF is required for the conversion of prestalk cells to stalk cells. There is an absolute requirement for DIF for stalk cell formation in low-density monolayers of prestalk cells but only part of population exhibits a requirement for cyclic AMP, which suggests that the prestalk cell population consists of two distinct cell types. Stalk cell formation from prespore cells is totally dependent on both cyclic AMP and DIF.(ABSTRACT TRUNCATED AT 250 WORDS)

Contact alters cAMP metabolism in aggregation-competent Dictyostelium amoebae

cAMP and cell-cell contact are involved in the coordination of differentiation and morphogenesis in Dictyostelium discoideum. The experiments described in this paper establish a relationship between cAMP and cell-cell contact. Contact between Enterobacter aerogenes and aggregation-competent Dictyostelium amoebae and contact between Dictyostelium amoebae themselves results in the transient secretion of cAMP and an alteration in the amount of cAMP secreted in response to subsequent stimulation by cAMP, i.e., an alteration in magnitude of a cAMP relay response. The subsequent cAMP relay response can be enhanced or diminished depending upon the number of contacts formed and the concentration of cAMP present at the time of contact. Latex beads are capable of evoking cAMP secretion. However, the bead/amoebal contact is unable to alter the magnitude of a subsequent response to cAMP. This suggests that a nonspecific interaction via cell-cell contact elicits transient cAMP secretion in aggregation-competent Dictyostelium amoebae. The two responses to cell-cell contact are distinct from each other and distinct from the cAMP relay response. 1) The dose-response curves for the responses to Enterobacter contact are clearly different. 2) Contact with latex beads can elicit cAMP secretion but not alter the magnitude of a subsequent cAMP relay response. 3) The temperature dependences of the contact-induced responses and the cAMP relay response show that only the contact-induced cAMP secretion is inhibited at 12 and 15 degrees C, while only the cAMP relay response is inhibited at 28 degrees C. A 4-second application of cAMP at the time that contact is initiated enhances both contact-induced responses. Whether the relationship between these two developmental regulators is important for the regulation of Dictyostelium development has yet to be established.

Structural and functional characterization of a Dictyostelium gene encoding a DIF inducible, prestalk-enriched mRNA sequence

The pDd56 mRNA sequence is highly enriched in prestalk over prespore cells and is inducible by DIF, the putative Dictyostelium stalk-specific morphogen. We show that the pDd56 gene is composed of forty one copies of a twenty four amino acid, cysteine rich repeat. This is highly homologus to a repeat which we have previously shown to compose the major fraction of the pDd63 mRNA, another DIF inducible, prestalk-enriched sequence. The predicted pDd56 protein contains a putative signal peptide but does not appear to contain a transmembrane segment. In combination these features suggest it to be an extrinsic protein and we confirm this elsewhere by showing that the pDd56 gene encodes a known, extracellular protein of the stalk. The pDd56 mRNA is dependent upon exogenous DIF for its accumulation. We show that this control is exerted at the transcriptional level and that a restriction fragment containing 1.7Kb of upstream sequence directs temporally-regulated expression of the gene.

Direct induction of Dictyostelium prestalk gene expression by DIF provides evidence that DIF is a morphogen

We have isolated a gene that is very rapidly induced at the transcriptional level by DIF--a low molecular weight, diffusible factor necessary for stalk cell differentiation in Dictyostelium cells developing in vitro. The gene encodes a protein containing an N-terminal signal peptide preceding approximately 70 tandem repeats of a highly conserved 24 amino acid sequence with a high cysteine content. These features suggest it is an extracellular structural protein. During normal development, the gene is maximally expressed in the slug, in which the mRNA is very highly enriched in prestalk over prespore cells. The gene is not detectably expressed until the tipped aggregate stage, several hours later than prespore genes, suggesting that prespore cell differentiation precedes prestalk cell differentiation. The demonstration that DIF induces a gene normally only expressed in the prestalk zone of the slug provides strong evidence that DIF is a Dictyostelium morphogen.

Multiple forms of acid phosphatase and their differential secretion during stalk formation under submerged monolayer incubation of Dictyostelium discoideum

The electrophoretic pattern of intracellular and secreted acid phosphatases (AcPases) in Dictyostelium discoideum was examined during incubation of the cells as a submerged monolayer. Three distinct forms of the enzyme were observed in the cell during differentiation; one was detected throughout development (AcPase 1), whereas the others including AcPase 2 were stage-specific. AcPase 1 was released in the medium predominantly in early development and AcPase 2, a prestalk specific form, was secreted during stalk formation. When cells were incubated under conditions where stalk cells did not form, only AcPase 1 was recognized both in the cell and in the medium.

Changes during differentiation in requirements for cAMP for expression of cell-type-specific mRNAs in the cellular slime mold, Dictyostelium discoideum

A number of genes encoding developmentally regulated mRNAs in the cellular slime mold, Dictyostelium discoideum, have been described. Many of these are regulated by cAMP. Analysis of the earliest time at which elevated levels of cAMP can induce the expression of these mRNAs reveals a more complex pattern of regulation in which genes change in their ability to be induced in response to cAMP with developmental stage. A prestalk mRNA (C1/D11) previously thought not be regulated by elevated levels of cAMP is inducible by cAMP between aggregation and loose mound stage; later in development its expression becomes independent of elevated cAMP. The early prespore genes (prespore class I) also show two modes of regulation; early in development they are induced independently of continuous elevated levels of cAMP, while later in development their expression is dependent upon elevated cAMP. The period during development when the prestalk genes are cAMP inducible precedes by 2 hr the first time at which either the early prespore class I or late prespore class II mRNAs are inducible by continuous elevated levels of cAMP. Previous analysis of these mRNAs has been carried out using Dictyostelium cells grown axenically. In this report we have studied the developmental expression of these mRNAs in cells grown on bacteria. A substantial shutoff of the class I prestalk and early prespore (class I) mRNAs not seen in axenically grown cells is observed when bacterially grown cells are plated for development. Less than 10% of the maximal level of these mRNAs remains in the cells at the time of mature spore and stalk differentiation. Additionally, in the bacterially grown cells two distinct patterns of developmental regulation are observed for mRNAs which in axenically growing cells appear to be constitutively expressed throughout growth and development.

The regulatory subunit of cAMP-dependent protein kinase from Dictyostelium discoideum: cellular localization and developmental regulation analyzed by immunoblotting

The level of the regulatory subunit of the cAMP-dependent protein kinase from Dictyostelium discoideum was analyzed in subcellular fractions of cells at various stages of development by Western blotting. The protein was found only in the cytosolic fraction. A small amount of regulatory subunit was present in vegetative cells, and its level increased sharply during the first hours of aggregation; a further increase also occurred during culmination. Analysis of mature spores and of the stalky mutant HL 65 revealed that the protein is present only in prespore cells.

Differentiation of Dictyostelium discoideum in monolayer cultures and its modification by ionic conditions

We have compared the pattern of enzyme expression in cyclic AMP-induced monolayer cultures of Dictyostelium discoideum with that found during normal development. We find that both the temporal and quantitative pattern of enzyme expression are initially similar in the two situations, although the developmental sequence is more protracted and terminal cell differentiation is delayed in the monolayer situation. We describe differentiation conditions that permit the expression of only one terminal phenotype, which may be useful for further biochemical studies. Enzyme accumulation patterns under these conditions indicate that UDP gal transferase is not required for stalk cell differentiation (i.e., it is a prespore enzyme). We have shown that, when cell monolayers are incubated with cAMP, the presence of a weak acid at low extracellular pH favors stalk-cell differentiation, while a weak base at high extracellular pH favors spore differentiation. Finally, we show that variations in the monovalent cation content of the buffer, or the addition of an ion transport inhibitor (scillaren), or an ionophore (valinomycin) all affect the ratio of stalk cells to spores. Taken together, these results suggest that intracellular H+ and/or other cations may play an important role in regulating differentiation of specific cell types in D. discoideum.

Effects of cAMP on single cell motility in Dictyostelium

The motility of individual, aggregation-competent amebae of Dictyostelium has been analyzed at different concentrations of cAMP under both nongradient and gradient conditions. The following is demonstrated: (a) concentrations of cAMP greater than 10(-8) M inhibit motility in a concentration-dependent fashion, decrease the frequency but not the degree of turning, and cause rounding in cell shape; (b) no concentration of cAMP stimulates motility, or positive chemokinesis; (c) concentrations of cAMP that stimulate a maximal chemotactic response do not affect motility and concentrations of cAMP that maximally inhibit motility do not stimulate chemotaxis under gradient conditions; and (d) the concentrations of cAMP that inhibit motility are identical under gradient and nongradient conditions.

A novel cyclic AMP metabolism exhibited by giant cells and its possible role in the sexual development of Dictyostelium discoideum

In Dictyostelium discoideum cyclic AMP (cAMP) metabolism during macrocyst development, i.e., the sexual cycle of this organism, and in giant cells, i.e., fusion products from opposite mating-type cells, was investigated. The pattern of change in cAMP levels during macrocyst development differed considerably from that observed during fruiting-body formation, i.e., the asexual cycle. Giant cells produced and excreted considerable amounts of cAMP. Adenylate cyclase activity catalyzing cAMP production in giant cells was comparable to that of unfused cells. However, the activity of membrane-bound phosphodiesterase in giant cells was extremely low, and no extracellular phosphodiesterase was excreted. A phosphodiesterase inhibitory protein was secreted in excess by giant cells.

The spore cell induction activity of conditioned media and subcellular fractions of Dictyostelium discoideum

While certain sporagenous mutants of Dictyostelium discoideum do not display a density dependence for spore cell formation under in vitro differentiating conditions, other sporagenous mutants (HM18, HM18-2 and FR17) do exhibit density dependence. In addition, pre-spore cell formation in the wild-type strain V12M2 is density-dependent. Spore cell formation in HM18 and HM18-2 is stimulated at low cell densities by a membrane fraction from pseudoplasmodial cells. Stimulation is also effected by extracts of these membranes or conditioned media from starving cells and these preparations also stimulate pre-spore cell formation in V12M2. The active factor in conditioned media is of low molecular weight, heat-stable and insensitive to treatment by pronase and glycosidase; the factor is not extractable by hexane. The available evidence suggests that the active factor in conditioned media and in membrane extracts is the same molecule and, although ammonia also stimulates spore cell formation under these conditions, the active factor is not ammonia. More activity is produced extracellularly by cells in shake suspension than by cells in monolayers, and some strains produce more activity than others.

Synthesis of spore proteins during development of Dictyostelium discoideum

The pattern of synthesis of the spore coat proteins during development of Dictyostelium discoideum has been determined by using immunoprecipitation with spore protein antibody. SP170, SP103, 'SP94', SP82, SP76 and SP55 are all first synthesized just prior to the 'Mexican hat' stage of development (16-18h), but the synthesis of SP72 is delayed. This protein is apparently synthesized as a precursor, P66, which is modified during spore maturation to yield SP72. The nature of the modification is unknown. At their peak period of synthesis during early culmination (18-20h), the spore coat proteins account for 5-9% of total protein synthesis. Shortly after synthesis, these proteins are inserted into the spore coat, where all except SP103 become disulphide-cross-linked during the period 24-30h. SP3 does not accumulate until disulphide-cross-linking of the major spore coat proteins occurs and is itself disulphide-cross-linked into the spore coat. Several additional proteins that are accumulated during development have also been identified, namely P31, P25, P21 and P18. P25 first appears at 18-20h and then continues to be made throughout development. P31 synthesis begins at 12-14h and then largely ceases after approx. 20 h of development. The genes for both P21 and P18 are first expressed early in development, starting at 9-12h. P21 synthesis ceases at approx. 14h, but P18 continues to be synthesized throughout the rest of development. The marked differences in the time period of accumulation of these proteins compared with the co-ordinated syntheses of SP170, SP103, 'SP94', SP82, SP76 and SP55 provide a useful system for analysis of the mechanism of temporal gene expression during development.