Dictyostelium mutants lacking DIF, a putative morphogen

Derivatives of Differentiation-Inducing Factor 1 Differentially Control Chemotaxis and Stalk Cell Differentiation in Dictyostelium discoideum

Differentiation-inducing factors 1 and 2 (DIF-1 and DIF-2) are small lipophilic signal molecules that induce stalk cell differentiation but differentially modulate chemotaxis toward cAMP in the cellular slime mold Dictyostelium discoideum; DIF-1 suppresses chemotactic cell movement in shallow cAMP gradients, whereas DIF-2 promotes it. The receptor(s) for DIF-1 and DIF-2 have not yet been identified. We examined the effects of nine derivatives of DIF-1 on chemotactic cell movement toward cAMP and compared their chemotaxis-modulating activity and stalk cell differentiation-inducing activity in wild-type and mutant strains. The DIF derivatives differentially affected chemotaxis and stalk cell differentiation; for example, TM-DIF-1 suppressed chemotaxis and showed poor stalk-inducing activity, DIF-1(3M) suppressed chemotaxis and showed strong stalk-inducing activity, and TH-DIF-1 promoted chemotaxis. These results suggest that DIF-1 and DIF-2 have at least three receptors: one for stalk cell induction and two for chemotaxis modulation. In addition, our results show that the DIF derivatives can be used to analyze the DIF-signaling pathways in D. discoideum.

Homeodomain-like DNA binding proteins control the haploid-to-diploid transition in Dictyostelium

Homeodomain proteins control the developmental transition between the haploid and diploid phases in several eukaryotic lineages, but it is not known whether this regulatory mechanism reflects the ancestral condition or, instead, convergent evolution. We have characterized the mating-type locus of the amoebozoan Dictyostelium discoideum, which encodes two pairs of small proteins that determine the three mating types of this species; none of these proteins display recognizable homology to known families. We report that the nuclear magnetic resonance structures of two of them, MatA and MatB, contain helix-turn-helix folds flanked by largely disordered amino- and carboxyl-terminal tails. This fold closely resembles that of homeodomain transcription factors, and, like those proteins, MatA and MatB each bind DNA characteristically using the third helix of their folded domains. By constructing chimeric versions containing parts of MatA and MatB, we demonstrate that the carboxyl-terminal tail, not the central DNA binding motif, confers mating specificity, providing mechanistic insight into how a third mating type might have originated. Finally, we show that these homeodomain-like proteins specify zygote function: Hemizygous diploids, formed in crosses between a wild-type strain and a mat null mutant, grow and differentiate identically to haploids. We propose that Dictyostelium MatA and MatB are divergent homeodomain proteins with a conserved function in triggering the haploid-to-diploid transition that can be traced back to the last common ancestor of eukaryotes.

Evidence that differentiation-inducing factor-1 controls chemotaxis and cell differentiation, at least in part, via mitochondria in D. discoideum

Differentiation-inducing factor-1 [1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one (DIF-1)] is an important regulator of cell differentiation and chemotaxis in the development of the cellular slime mold Dictyostelium discoideum However, the entire signaling pathways downstream of DIF-1 remain to be elucidated. To characterize DIF-1 and its potential receptor(s), we synthesized two fluorescent derivatives of DIF-1, boron-dipyrromethene (BODIPY)-conjugated DIF-1 (DIF-1-BODIPY) and nitrobenzoxadiazole (NBD)-conjugated DIF-1 (DIF-1-NBD), and investigated their biological activities and cellular localization. DIF-1-BODIPY (5 µM) and DIF-1 (2 nM) induced stalk cell differentiation in the DIF-deficient strain HM44 in the presence of cyclic adenosine monosphosphate (cAMP), whereas DIF-1-NBD (5 µM) hardly induced stalk cell differentiation under the same conditions. Microscopic analyses revealed that the biologically active derivative, DIF-1-BODIPY, was incorporated by stalk cells at late stages of differentiation and was localized to mitochondria. The mitochondrial uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP), at 25-50 nM, and dinitrophenol (DNP), at 2.5-5 µM, induced partial stalk cell differentiation in HM44 in the presence of cAMP. DIF-1-BODIPY (1-2 µM) and DIF-1 (10 nM), as well as CCCP and DNP, suppressed chemotaxis in the wild-type strain Ax2 in shallow cAMP gradients. These results suggest that DIF-1-BODIPY and DIF-1 induce stalk cell differentiation and modulate chemotaxis, at least in part, by disturbing mitochondrial activity.

c-di-GMP induction of Dictyostelium cell death requires the polyketide DIF-1

Two inducers, DIF-1 and c-di-GMP, each separately shown to play a major role in Dictyostelium cell death induction in vitro, in fact cooperate. A similar cooperation with polyketides might occur for c-di-GMP effects in other situations and organisms, in particular in innate immunity and cell death in animal cells.

Cell death in the model organism Dictyostelium, as studied in monolayers in vitro, can be induced by the polyketide DIF-1 or by the cyclical dinucleotide c-di-GMP. c-di-GMP, a universal bacterial second messenger, can trigger innate immunity in bacterially infected animal cells and is involved in developmental cell death in Dictyostelium. We show here that c-di-GMP was not sufficient to induce cell death in Dictyostelium cell monolayers. Unexpectedly, it also required the DIF-1 polyketide. The latter could be exogenous, as revealed by a telling synergy between c-di-GMP and DIF-1. The required DIF-1 polyketide could also be endogenous, as shown by the inability of c-di-GMP to induce cell death in Dictyostelium HMX44A cells and DH1 cells upon pharmacological or genetic inhibition of DIF-1 biosynthesis. In these cases, c-di-GMP–induced cell death was rescued by complementation with exogenous DIF-1. Taken together, these results demonstrated that c-di-GMP could trigger cell death in Dictyostelium only in the presence of the DIF-1 polyketide or its metabolites. This identified another element of control to this cell death and perhaps also to c-di-GMP effects in other situations and organisms.

Novel chlorinated dibenzofurans isolated from the cellular slime mold, Polysphondylium filamentosum, and their biological activities

Cellular slime molds are expected to have the huge potential for producing secondary metabolites including polyketides, and we have studied the diversity of secondary metabolites of cellular slime molds for their potential utilization as new biological resources for natural product chemistry. From the methanol extract of fruiting bodies of Polysphondylium filamentosum, we obtained new chlorinated benzofurans Pf-1 (4) and Pf-2 (5) which display multiple biological activities; these include stalk cell differentiation-inducing activity in the well-studied cellular slime mold, Dictyostelium discoideum, and inhibitory activities on cell proliferation in mammalian cells and gene expression in Drosophila melanogaster.

Initial cell type choice in Dictyostelium

Much remains to be understood about how a group of cells break symmetry and differentiate into distinct cell types. The simple eukaryote Dictyostelium discoideum is an excellent model system for studying questions such as cell type differentiation. Dictyostelium cells grow as single cells. When the cells starve, they aggregate to develop into a multicellular structure with only two main cell types: spore and stalk. There has been a longstanding controversy as to how a cell makes the initial choice of becoming a spore or stalk cell. In this review, we describe how the controversy arose and how a consensus developed around a model in which initial cell type choice in Dictyostelium is dependent on the cell cycle phase that a cell happens to be in at the time that it starves.

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.

Autophagic cell death: analysis in Dictyostelium

Autophagic cell death (ACD) can be operationally described as cell death with an autophagic component. While most molecular bases of this autophagic component are known, in ACD the mechanism of cell death proper is not well defined, in particular because in animal cells there is poor experimental distinction between what triggers autophagy and what triggers ACD. Perhaps as a consequence, it is often thought that in animal cells a little autophagy is protective while a lot is destructive and leads to ACD, thus that the shift from autophagy to ACD is quantitative. The aim of this article is to review current knowledge on ACD in Dictyostelium, a very favorable model, with emphasis on (1) the qualitative, not quantitative nature of the shift from autophagy to ACD, in contrast to the above, and (2) random or targeted mutations of in particular the following genes: iplA (IP3R), TalB (talinB), DcsA (cellulose synthase), GbfA, ugpB, glcS (glycogen synthase) and atg1. These mutations allowed the genetic dissection of ACD features, dissociating in particular vacuolisation from cell death.

Autophagic or necrotic cell death triggered by distinct motifs of the differentiation factor DIF-1

Autophagic or necrotic cell death (ACD and NCD, respectively), studied in the model organism Dictyostelium which offers unique advantages, require triggering by the same differentiation-inducing factor DIF-1. To initiate these two types of cell death, does DIF-1 act through only one or through two distinct recognition structures? Such distinct structures may recognize distinct motifs of DIF-1. To test this albeit indirectly, DIF-1 was modified at one or two of several positions, and the corresponding derivatives were tested for their abilities to induce ACD or NCD. The results strongly indicated that distinct biochemical motifs of DIF-1 were required to trigger ACD or NCD, and that these motifs were separately recognized at the onset of ACD or NCD. In addition, both ACD and NCD were induced more efficiently by DIF-1 than by either its precursors or its immediate catabolite. These results showed an unexpected relation between a differentiation factor, the cellular structures that recognize it, the cell death types it can trigger and the metabolic state of the cell. The latter seems to guide the choice of the signaling pathway to cell death, which in turn imposes the cell death type and the recognition pattern of the differentiation factor.

Biological activities of novel derivatives of DIF-1 isolated from Dictyostelium

The differentiation-inducing factor-1 (DIF-1) is a lipophilic signal molecule (chlorinated alkylphenone) that induces stalk cell differentiation in the cellular slime mold Dictyostelium discoideum. In addition, DIF-1 and its derivatives have been shown to possess anti-leukemic activity and glucose consumption-promoting activity in vitro in mammalian cells. In this study, to assess the chemical structure-effect relationship of DIF-1, we synthesized eight derivatives of DIF-1 and investigated their stalk cell-inducing activity in Dictyostelium cells and pharmacological activities in mammalian cells. Of the derivatives, two amide derivatives of DIF-1, whose hydrophobic indexes are close to that of DIF-1, induced stalk cell differentiation as strongly as DIF-1 in Dictyostelium cells. It was also found that some derivatives suppressed cell growth in human K562 leukemia cells and promoted glucose consumption in mouse 3T3-L1 cells. These results give us valuable information as to the chemical structure-effect relationship of DIF-1.

DIF-1 induces the basal disc of the Dictyostelium fruiting body

The polyketide DIF-1 induces Dictyostelium amoebae to form stalk cells in culture. To better define its role in normal development, we examined the phenotype of a mutant blocking the first step of DIF-1 synthesis, which lacks both DIF-1 and its biosynthetic intermediate, dM-DIF-1 (des-methyl-DIF-1). Slugs of this polyketide synthase mutant (stlB(-)) are long and thin and rapidly break up, leaving an immotile prespore mass. They have approximately 30% fewer prestalk cells than their wild-type parent and lack a subset of anterior-like cells, which later form the outer basal disc. This structure is missing from the fruiting body, which perhaps in consequence initiates culmination along the substratum. The lower cup is rudimentary at best and the spore mass, lacking support, slips down the stalk. The dmtA(-) methyltransferase mutant, blocked in the last step of DIF-1 synthesis, resembles the stlB(-) mutant but has delayed tip formation and fewer prestalk-O cells. This difference may be due to accumulation of dM-DIF-1 in the dmtA(-) mutant, since dM-DIF-1 inhibits prestalk-O differentiation. Thus, DIF-1 is required for slug migration and specifies the anterior-like cells forming the basal disc and much of the lower cup; significantly the DIF-1 biosynthetic pathway may supply a second signal - dM-DIF-1.

Analysis of autophagic and necrotic cell death in Dictyostelium

Non-apoptotic cell death types can be conveniently studied in Dictyostelium discoideum, an exceptionally favorable model not only because of its well-known genetic and experimental advantages, but also because in Dictyostelium there is no apoptosis machinery that could interfere with non-apoptotic cell death. We show here how to conveniently demonstrate, assess, and study these non-apoptotic cell death types. These can be generated by use of modifications of the monolayer technique of Rob Kay et al., and either wild-type HMX44A Dictyostelium cells, leading to autophagic cell death, or the corresponding atg1- autophagy gene mutant cells, leading to necrotic cell death. Methods to follow these non-apoptotic cell death types qualitatively and quantitatively will be reported.

Pharmacological evidence that stalk cell differentiation involves increases in the intracellular Ca(2+) and H(+) concentrations in Dictyostelium discoideum

Differentiation-inducing factors (DIFs) are required for stalk cell formation in Dictyostelium discoideum. In the present study, in order to support our hypothesis that DIFs may function via increases in [Ca(2+)](c) and [H(+)](c), we investigated the combined effects of 5,5-dimethyl-2,4-oxazolidinedione (DMO, a [H(+)](c)-increasing agent), thapsigargin (Tg) and BHQ ([Ca(2+)](c)-increasing agents) on in vitro stalk cell formation in several strains. DMO, in combination with Tg or BHQ, induced stalk cell formation in a DIF-deficient mutant HM44. Although the rates of stalk cell induction by the drugs were low in the presence of cerulenin (an inhibitor of endogenous DIF production) in HM44 and V12M2 (a wild-type strain), the drugs succeeded in inducing sufficient stalk cell formation when a small amount of DIF-1 was supplied. Furthermore, co-addition of DMO, BHQ and a small amount of DIF-1 also induced sufficient stalk cell formation in AX-4 (an axenic strain) and HM1030 (dmtA(-)) but not in CT15 (dimA(-)). The drugs suppressed spore formation and promoted stalk cell formation in both HM18 (a sporogenous mutant) and 8-bromo-cAMP-stimulated V12M2. The present results suggest that DIFs function, at least in part, via increases in [Ca(2+)](c) and [H(+)](c) in D. discoideum.

Anti-leukemic activities of Dictyostelium secondary metabolites: a novel aromatic metabolite, 4-methyl-5-n-pentylbenzene-1,3-diol, isolated from Dictyostelium mucoroides suppresses cell growth in human leukemia K562 and HL-60 cells

It has previously been shown that DIF-1, a differentiation-inducing factor of the cellular slime mold Dictyostelium discoideum, possesses antitumor activities in mammalian tumor cells and that neuronal differentiation of PC12 cells can be induced with furanodictines (FDs), aminosugar analogs found in D. discoideum, or dictyoglucosamines (DGs), N-acetyl glucosamine derivatives (DG-A from D. purpureum and DG-B from D. discoideum). Thus, cellular slime molds are attractive natural resources that may provide valuable lead compounds to be utilized in the field of pharmacology and medicine. In this study, we have isolated a novel aromatic compound, 4-methyl-5-n-pentylbenzene-1,3-diol (MPBD), from fruiting bodies of the cellular slime mold D. mucoroides and assessed the in vitro antiproliferative activities of MPBD, FDs, and DGs in human leukemia K562 and HL-60 cells. MPBD at 20-80 microM dose-dependently suppressed cell growth in both K562 and HL-60 cells. While FDs at 10-80 microM did not affect cell growth, DGs at 10-40 microM dose-dependently suppressed cell growth in the cells. Although we failed to find the roles of FDs and DGs in the original organisms, MPBD at 5-20 microM was found to promote stalk cell formation in D. discoideum. The present results indicate that MPBD, DGs or their derivatives may have therapeutic potential in the treatment of cancer and confirm our expectations regarding cellular slime molds as drug resources.

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.

Developmental decisions in Dictyostelium discoideum

Dictyostelium discoideum is an excellent system in which to study developmental decisions. Synchronous development is triggered by starvation and rapidly generates a limited number of cell types. Genetic and image analyses have revealed the elegant intricacies associated with this simple development system. Key signaling pathways identified as regulating cell fate decisions are likely to be conserved with metazoa and are providing insight into differentiation decisions under circumstances where considerable cell movement takes place during development.

Structural requirements of Dictyostelium differentiation-inducing factors for their stalk-cell-inducing activity in Dictyostelium cells and anti-proliferative activity in K562 human leukemic cells

The differentiation-inducing factor-1 (DIF-1) is a lipophilic signal molecule (chlorinated alkylphenone) that induces stalk-cell differentiation in the cellular slime mould Dictyostelium discoideum. It has also been shown that DIF-1 and its derivative (DIF-3) suppress cell growth in mammalian tumor cells. In the present study, in order to assess the chemical structure-effect relationship of DIF derivatives and to develop useful agents for the study of both Dictyostelium development and cancer biology, we synthesized 28 analogues of DIF-1 and DIF-3 and investigated their stalk-cell-inducing activity in Dictyostelium HM44 cells (mutant strain) and anti-proliferative activity in human leukemia K562 cells. HM44 cells are defective in endogenous DIF-1 production and should be suitable for the assay for stalk-cell-inducing activity of DIF analogues. DIF-1 and some of its derivatives at nanomolar levels were good stalk-cell inducers in HM44 cells, whereas DIF-3 and some DIF-3 derivatives at micromolar levels were potent anti-proliferative agents in K562 cells. We also tried to search for antagonistic molecules against DIF-1 and DIF-3 but failed to find such molecules from the analogues used here. The present findings would give us hints for identifying the target molecule(s) of DIFs and also for developing novel anti-cancer drugs.

New prestalk and prespore inducing signals in Dictyostelium

The differentiation-inducing signals (DIFs) currently known in Dictyostelium appear unable to account for the full diversity of cell types produced in development. To search for new signals, we analyzed the differentiation in monolayers of cells expressing prestalk (ecmAO, ecmA, ecmO, ecmB and cAR2) and prespore (psA) markers. Expression of each marker drops off as the cell density is reduced, suggesting that cell interaction is required. Expression of each marker is inhibited by cerulenin, an inhibitor of polyketide synthesis, and can be restored by conditioned medium. However, the known stalk-inducing polyketide, DIF-1, could not replace conditioned medium and induce the ecmA or cAR2 prestalk markers, suggesting that they require different polyketide inducers. Polyketide production by fungi is stimulated by cadmium ions, which also dramatically stimulates differentiation in Dictyostelium cell cultures and the accumulation of medium factors. Factors produced with cadmium present were extracted from conditioned medium and fractionated by HPLC. A new factor inducing prespore cell differentiation, called PSI-2, and two inducing stalk cell differentiation (DIFs 6 and 7) were resolved. All are distinct from currently identified factors. DIF-6, but not DIF-7 or PSI-2, appears to have an essential carbonyl group. Thus Dictyostelium may use extensive polyketide signaling in its development.

Dictyostelium cell death: early emergence and demise of highly polarized paddle cells

Cell death in the stalk of Dictyostelium discoideum, a prototypic vacuolar cell death, can be studied in vitro using cells differentiating as a monolayer. To identify early events, we examined potentially dying cells at a time when the classical signs of Dictyostelium cell death, such as heavy vacuolization and membrane lesions, were not yet apparent. We observed that most cells proceeded through a stereotyped series of differentiation stages, including the emergence of "paddle" cells showing high motility and strikingly marked subcellular compartmentalization with actin segregation. Paddle cell emergence and subsequent demise with paddle-to-round cell transition may be critical to the cell death process, as they were contemporary with irreversibility assessed through time-lapse videos and clonogenicity tests. Paddle cell demise was not related to formation of the cellulose shell because cells where the cellulose-synthase gene had been inactivated underwent death indistinguishable from that of parental cells. A major subcellular alteration at the paddle-to-round cell transition was the disappearance of F-actin. The Dictyostelium vacuolar cell death pathway thus does not require cellulose synthesis and includes early actin rearrangements (F-actin segregation, then depolymerization), contemporary with irreversibility, corresponding to the emergence and demise of highly polarized paddle cells.

Cross-induction of cell types in Dictyostelium: evidence that DIF-1 is made by prespore cells

To investigate how cell type proportions are regulated during Dictyostelium development, we have attempted to find out which cell type produces DIF-1, a diffusible signal molecule inducing the differentiation of prestalk-O cells. DIF-1 is a chlorinated alkyl phenone that is synthesized from a C12 polyketide precursor by chlorination and methylation, with the final step catalysed by the dmtA methyltransferase. All our evidence points to the prespore cells as the major source of DIF-1. (1) dmtA mRNA and enzyme activity are greatly enriched in prespore compared with prestalk cells. The chlorinating activity is also somewhat prespore-enriched. (2) Expression of dmtA is induced by cyclic-AMP and this induction is inhibited by DIF-1. This regulatory behaviour is characteristic of prespore products. (3) Short-term labelling experiments, using the polyketide precursor, show that purified prespore cells produce DIF-1 at more than 20 times the rate of prestalk cells. (4) Although DIF-1 has little effect on its own synthesis in short-term labelling experiments, in long-term experiments, using 36Cl– as label, it is strongly inhibitory (IC50 about 5 nM), presumably because it represses expression of dmtA; this is again consistent with DIF-1 production by prespore cells. Inhibition takes about 1 hour to become effective.

We propose that prespore cells cross-induce the differentiation of prestalk-O cells by making DIF-1, and that this is one of the regulatory loops that sets the proportion of prespore-to-prestalk cells in the aggregate.

The role of DIF-1 signaling in Dictyostelium development

We have constructed a mutant blocked in the biosynthesis of DIF-1, a chlorinated signal molecule proposed to induce differentiation of both major prestalk cell types formed during Dictyostelium development. Surprisingly, the mutant still forms slugs retaining one prestalk cell type, the pstA cells, and can form mature stalk cells. However, the other major prestalk cell type, the pstO cells, is missing. Normal pstO cell differentiation and their patterning in the slug are restored by development on a uniform concentration of DIF-1. We conclude that pstO and pstA cells are in fact induced by separate signals and that DIF-1 is the pstO inducer. Positional information, in the form of DIF-1 gradients, is evidently not required for pstO cell induction.

A novel role of differentiation-inducing factor-1 in Dictyostelium development, assessed by the restoration of a developmental defect in a mutant lacking mitogen-activated protein kinase ERK2

It has been previously reported that the differentiating wild-type cells of Dictyostelium discoideum secrete a diffusible factor or factors that are able to rescue the developmental defect in the mutant lacking extracellular signal-regulated kinase 2 (ERK2), encoded by the gene erkB. In the present study, it is demonstrated that differentiation-inducing factor-1 (DIF-1) for stalk cells can mimic the role of the factor(s) and the mechanism of the action of DIF-1 in the erkB null mutant is also discussed. The mutant usually never forms multicellular aggregates, because of its defect in cyclic adenosine monophosphate (cAMP) signaling. In the presence of 100 nM DIF-1, however, the mutant cells formed tiny slugs, which eventually developed into small fruiting bodies. In contrast, DIF-1 never rescued the developmental arrest of other Dictyostelium mutants lacking adenylyl cyclase A (ACA), cAMP receptors cAR1 and cAR3, heterotrimeric G-protein, the cytosolic regulator of ACA, or the catalytic subunit of cAMP-dependent protein kinase (PKA-C). Most importantly, it was found that DIF-1 did not affect the cellular cAMP level, but rather elevated the transcriptional level of pka during the development of erkB null cells. These results suggest that DIF-1 may rescue the developmental defect in erkB null cells via the increase in PKA activity, thus giving the first conclusive evidence that DIF-1 plays a crucial role in the early events of Dictyostelium development as well as in prestalk and stalk cell induction.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.

DIF signalling and cell fate

The DIFs are a family of secreted chlorinated molecules that control cell fate during development of Dictyostelium cells in culture and probably during normal development too. They induce stalk cell differentiation and suppress spore cell formation. The biosynthetic and inactivation pathways of DIF-1 (the major bioactivity) have been worked out. DIF-1 is probably synthesised in prespore cells and inactivated in prestalk cells, by dechlorination. Thus, each cell type tends to alter DIF-1 level so as to favour differentiation of the other cell type. This relationship leads to a model for cell-type proportioning during normal development.

Signalling pathways that direct prestalk and stalk cell differentiation in Dictyostelium

Prestalk cell differentiation in Dictyostelium is induced by DIF and two DIF-induced genes, ecmA and ecmB, have revealed the existence of multiple prestalk and stalk cell sub-types. These different sub-types are defined by the pattern of expression of subfragments derived from the ecmA and ecmB promoters. These markers have been utilised in three ways; for fate mapping in vivo, to investigate the molecular mechanisms underlying DIF signalling and to explore the relative requirement for DIF and other signalling molecules for prestalk and stalk cell differentiation in vitro. The heterogeneity of the prestalk and stalk populations seems to be reflected in differences in the cell signalling pathways that they utilise.

Production and activity of spore differentiation factors (SDFs) in Dictyostelium

SDF-1 and SDF-2 are peptides that promote terminal spore differentiation under submerged conditions. The present study shows that they accumulate differentially and are released during the development of wild-type cells and can promote spore formation in cells disaggregated from wild-type culminants. SDF-1 accumulates during the slug stage and is released in a single burst at the onset of culmination while SDF-2 accumulates during early culmination and is released in a single burst from mid-culminants. The effects of SDF-1 and SDF-2 on stalk cell formation in cell monolayers were investigated. SDF-1 by itself induces stalk cell formation in some strains and also synergizes with the stalk-cell-inducing factor, DIF-1. cAMP has an inhibitory effect on stalk cell formation when either DIF-1 or SDF-1 are present on their own but is almost not inhibitory when both are present. SDF-2 alone does not induce stalk cell formation and appears to inhibit the response to DIF-1. At the same time, it increases the extent of vacuolization of the stalk cells that are produced. We propose that the release of SDF-1 and then of SDF-2 may mark irreversible steps in the developmental programme associated, respectively, with culmination and spore maturation.

The biosynthesis of differentiation-inducing factor, a chlorinated signal molecule regulating Dictyostelium development

Differentiation-inducing factor (DIF)-1 is a chlorinated alkyl phenone released by developing Dictyostelium amoebae, which induces them to differentiate into stalk cells. A biosynthetic pathway for DIF-1 is proposed from labeling, inhibitor, and enzymological experiments. Cells incorporate 36Cl- into DIF-1 during development, showing that the chlorine atoms originate from chloride ions; peak incorporation is at the first finger stage. DIF-1 synthesis can be blocked by cerulenin, a polyketide synthase inhibitor, suggesting that it is made from a polyketide. This is most likely the C12 polyketide (2,4,6-trihydroxyphenyl)-1-hexan-1-one (THPH). Feeding experiments confirm that living cells can convert THPH to DIF-1. Conversion requires both chlorination and methylation of THPH, and enzymatic activities able to do this exist in cell lysates. The chlorinating activity, assayed using 36Cl-, is stimulated by H2O2 and requires both soluble and particulate components. It is specific for THPH and does not use this compound after O-methylation. The methyltransferase is soluble, uses S-adenosyl-L-methionine as a co-substrate, has a Km for dichloro-THPH of about 1.1 microM, and strongly prefers this substrate to close analogues. Both chlorinating and methyltransferase activities increase in development in parallel with DIF-1 production, and both are greatly reduced in a mutant strain that makes little DIF-1. It is proposed that DIF-1 is made by the initial assembly of a C12 polyketide skeleton, which is then chlorinated and methylated.

A mutational analysis of Dictyostelium discoideum multicellular development

We have collectedDictyosteliummutants that arrest in development after aggregation, but before first finger formation. A total of 118 mutant strains were isolated and are referred to as mound(mnd)mutants. Nine complementation groups(mndA-mndl), containing 46 of the mutant strains, were defined by parasexual methods. A statistical analysis suggested that there are about 118 genes which, when mutated, confer the mound phenotype. Of these genes, about 60 are predicted to be mutated in our collection: the 9 assigned to complementation groups and another 51 unassigned mutants.mndA, G, HandIwere assigned to linkage groups VII, IV, II and VI, respectively. Development of the mutant strains was characterized by terminal morphology, neutral red staining and expression of marker mRNAs for prespore and prestalk cells. Three broad classes were recognized. (1) Postaggregative mutants - those blocked early in multicellular development. They did not express any of the prestalk or prespore marker mRNAs and generally arrested as low mounds or ridges. (2) Pathway mutants - those blocked specifically in either prestalk or prespore differentiation. They expressed either prestalk or prespore marker mRNAs, but not both, and generally proceeded further morphologically than post-aggregative mutants. (3) Morphogenesis mutants - those apparently blocked in morphogenesis rather than cell differentiation. They expressed all the cell-type marker mRNAs tested. Most arrested as tight mounds lacking a tip and of defined upper size, but some mutants produced aberrant tips. The majority of mutants tested synergized with wild-type: 24/28 strains which cannot make spores when developed alone, were able do so when allowed to develop with an equal number of wild-type cells. We suggest that some of the morphogenesis mutants have a cytoskeletal defect which prevents first finger formation and that these mutants can be physically carried through development by the wildtype (synergy by ‘piggy-backing’).

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.

Evidence for positional differentiation of prestalk cells and for a morphogenetic gradient in Dictyostelium

We present evidence that Dictyostelium slug tip cells, the pstA cells, may arise by positional differentiation, but at a site remote from that which they will eventually occupy. When first detectable, the pstA cells form a peripheral ring surrounding the other prestalk cell subtype, the pstO cells, but subsequently move above the pstO cells to form the tip. Because pstA cell differentiation requires a 10-fold higher concentration of differentiation-inducing factor, the stalk cell inducer, the initial patterning seems likely to reflect the existence of a morphogenetic gradient. The subsequent redistribution of the two cell types is explicable by their different rates of chemotaxis to cyclic AMP. These results help reconcile the two apparently opposing views of pattern formation in Dictyostelium, that there is positional differentiation and that pattern formation occurs by cell sorting.

Programmed cell death in Dictyostelium

Programmed cell death (PCD) of Dictyostelium discoideum cells was triggered precisely and studied quantitatively in an in vitro system involving differentiation without morphogenesis. In temporal succession after the triggering of differentiation, PCD included first an irreversible step leading to the inability to regrow at 8 hours. At 12 hours, massive vacuolisation was best evidenced by confocal microscopy, and prominent cytoplasmic condensation and focal chromatin condensation could be observed by electron microscopy. Membrane permeabilization occurred only very late (at 40–60 hours) as judged by propidium iodide staining. No early DNA fragmentation could be detected by standard or pulsed field gel electrophoresis. These traits exhibit some similarity to those of previously described non-apoptotic and apoptotic PCD, suggesting the hypothesis of a single core molecular mechanism of PCD emerging in evolution before the postulated multiple emergences of multicellularity. A single core mechanism would underly phenotypic variations of PCD resulting in various cells from differences in enzymatic equipment and mechanical constraints. A prediction is that some of the molecules involved in the core PCD mechanism of even phylogenetically very distant organisms, e.g. Dictyostelium and vertebrates, should be related.

Developmental decisions in Dictyostelium discoideum

A few hours after the onset of starvation, amoebae of Dictyostelium discoideum start to form multicellular aggregates by chemotaxis to centers that emit periodic cyclic AMP signals. There are two major developmental decisions: first, the aggregates either construct fruiting bodies directly, in a process known as culmination, or they migrate for a period as "slugs." Second, the amoebae differentiate into either prestalk or prespore cells. These are at first randomly distributed within aggregates and then sort out from each other to form polarized structures with the prestalk cells at the apex, before eventually maturing into the stalk cells and spores of fruiting bodies. Developmental gene expression seems to be driven primarily by cyclic AMP signaling between cells, and this review summarizes what is known of the cyclic AMP-based signaling mechanism and of the signal transduction pathways leading from cell surface cyclic AMP receptors to gene expression. Current understanding of the factors controlling the two major developmental choices is emphasized. The weak base ammonia appears to play a key role in preventing culmination by inhibiting activation of cyclic AMP-dependent protein kinase, whereas the prestalk cell-inducing factor DIF-1 is central to the choice of cell differentiation pathway. The mode of action of DIF-1 and of ammonia in the developmental choices is discussed.

Differanisole A, an inducer of the differentiation of Friend leukemic cells, induces stalk cell differentiation in Dictyostelium discoideum

Differanisole A isolated from the conditioned medium of a soil microorganism, Chaetomium strain RB-001, is an inducer of the differentiation of the Friend leukemic cells (mouse leukemia cells). The chemical structure of this substance is very similar to that of stalk cell differentiation-inducing factor (DIF) isolated from the cellular slime mould, Dictyostelium discoideum. We examined the effects of differanisole A on Dictyostelium HM44 cells, a mutant strain which is defective in DIF production, and found this substance to be an inducer of stalk cell differentiation in D. discoideum.

Developmental regulation of transcription of a novel prespore-specific gene (Dp87) in Dictyostelium discoideum

The Dp87 is a novel prespore specific gene of Dictyostelium discoideum which has a long open reading frame of 555 amino acids. The entire amino acid sequence had low but significant homology to the spore coat proteins, SP96 and SP70, of this organism. When a chimeric gene, containing a 1380 bp of the 5′ upstream region of this gene fused with CAT gene, as reporter, was introduced into cells of this organism, it was expressed only in prespore cells of the slug. Transformation experiments, using chimeric genes, containing a series of 5′ deletions of the upstream region, showed that −447 bp to −357 bp is an important cis-acting regulatory region for transcription. A nuclear factor(s) that specifically bind to this cis-acting region were detected from slug cell nuclei. Transformation experiments using a chimeric gene consisting of the 5′ region between −666 bp and +149 bp of this gene, a beta-galactosidase reporter and an actin 8 terminator, showed that the reporter gene was expressed as early as in aggregation streams, indicating that Dp87 become transcribed a few hours earlier than the other prespore-specific genes so far reported. This was confirmed by northern hybridization detected using an image plate analyzer. The fact that cells expressing Dp87 appeared at random in aggregation streams gives solid support to the idea that position-independent differentiation of prespore and prestalk cells, followed by their sorting, brings about pattern formation in this organism.

A localized differentiation-inducing-factor sink in the front of the Dictyostelium slug

Differentiation-inducing factor 1 [DIF-1; 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)-hexan-1-one] induces stalk cell differentiation during Dictyostelium development. It is present as a gradient in the multicellular slug, its lowest concentration being in the anterior. Here we demonstrate the existence of a localized sink for DIF-1, also in the anterior of the slug, which could be responsible for generating the DIF-1 gradient. DIF-1 is metabolized extensively by developing cells, initially by a mono-dechlorination. We used an enzyme assay for DIF-1 dechlorinase to examine its distribution in the slug. DIF-1 dechlorinase activity is 30-fold higher in prestalk cells (largely anterior) compared with prespore cells (posterior) when these are separated from each other on Percoll density gradients. Dissection experiments showed that DIF-1 dechlorinase is 25-fold enriched in the anterior 13% of the slug compared with the rest. These experiments also showed that DIF-1 dechlorinase is more anterior-enriched than the standard prestalk markers, the ecmA and ecmB mRNAs. When cut from a slug, both prestalk and prespore fragments regulate to restore the missing cell type. Prespore fragments rapidly regain (by 30 min) a DIF-1 sink in their anteriors, and prestalk fragments restore a posterior zone with low DIF-1 dechlorinase by 4 hr after cutting. The reappearance of the DIF-1 sink in the anterior of prespore fragments is accomplished without detectable cell sorting and may, therefore, be in response to positional signals. Finally, a localized sink may provide a general way of producing a gradient of a signal substance in a developing embryo.

Differentiation-inducing-factor dechlorinase, a novel cytosolic dechlorinating enzyme from Dictyostelium discoideum

Differentiation-inducing factor 1 (DIF-1) is a dichlorinated alkyl phenone (1-[(3,5-dichloro-2,6-dihydroxy-4-methoxy)phenyl]hexan-1-one) from Dictyostelium discoideum, that induces amoebae to differentiate into stalk cells. It was shown previously that DIF-1 is rapidly metabolized into a series of more polar compounds by living cells [Traynor, D. & Kay, R.R. (1991) J. Biol. Chem. 266, 5291-5297]. The first step in DIF-1 metabolism is the formation of DIF metabolite 1 (now known to be DIF-3) by a monodechlorination. We report here the discovery of the enzyme activity catalyzing this dechlorination. A very sensitive enzyme assay was developed, using [3H]DIF-1 and a TLC system to separate DIF-1 from the product, DIF-3. DIF-1 3(5)-dechlorinase is present in the high-speed supernatant of cell lysates, and uses glutathione, at physiological concentrations, as cofactor. Kinetic measurements indicate a Km for DIF-1 of about 70 nM. The enzyme activity is inhibited by DIF-2 (the pentan-1-one analogue of DIF-1), with a median inhibitor concentration (IC50) of 1 microM, and DIF-3 (IC50 = 5 microM), which presumably act as substrates, but other compounds structurally related to DIF-1 were much less effective. Aurothioglucose, an inhibitor of selenocysteine enzymes, inhibited DIF-1 3(5)-dechlorinase with IC50 = 100 nM. DIF-1 3(5)-dechlorinase activity is developmentally regulated. It is essentially absent from growing cells and increases at the end of aggregation to reach a first peak of activity at the first finger stage, with a further rise at culmination.

A simple method of generating axenic derivatives of Dictyostelium strains

We describe a generally applicable method of adapting Dictyostelium from growth on a bacterial food source to axenic growth. Cells are initially selected by growth on a plastic substratum but subsequently acquire the ability to grow in suspension culture. These strains can be transformed efficiently by DNA-mediated gene transfer.

Chlorine-containing compounds produced during Dictyostelium development. Detection by labelling with 36Cl

DIF-1 [Differentiation-Inducing Factor 1; 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one] is a novel chlorinated signal molecule that induces stalk-cell differentiation during development of Dictyostelium discoideum. Here we introduce the use of the radioisotope 36Cl to label DIF-1 and other low-Mr chlorinated compounds produced during development. H.p.l.c. and t.l.c. were used to resolve the labelled compounds. We find the following. (1) At least 14 dialysable 36Cl-labelled compounds are released into the medium by cells labelled continuously through development with Na36Cl. (2) The compounds can be classified into two major groups according to their times of accumulation in development. The early group of compounds starts accumulating at the end of aggregation, co-ordinately with DIF-1; the late group is only made at the end of development, by mature fruiting bodies. There may also be an intermediate group made during culmination. (3) The early group of compounds has been identified as comprising DIF-1 and seven of its metabolites by co-chromatography with the authentic compounds. These metabolites had previously only been recognized in suspensions of living cells incubated with exogenous DIF-1. Their detection here, from cells undergoing normal development, suggests that endogenous DIF-1 is metabolized in normal development in much the same way as is DIF-1 added to cells in suspension. (4) The intermediate and late groups of compounds are not obvious DIF-1 metabolites. They may have some role unconnected with DIF signalling. (5) A group of 36Cl-labelled late compounds remain cell-associated after washing of the fruiting bodies, and these are greatly enriched in stalk, compared with spore, cells. (6) Other slime-mould species were labelled with 36Cl. All three tested, namely D. mucoroides, D. vinaceo-fuscum and P. violaceum, also produced chloro compounds. D. mucoroides produced DIF-1 by the criterion of h.p.l.c. co-elution with authentic DIF-1. A developmentally regulated metabolism of chlorinated compounds may therefore be widespread amongst slime moulds. To our knowledge, labelling with 36Cl in vivo has not been reported before and provides a powerful general method for investigating chlorinated compounds in diverse organisms.

A possible role for DIF-2 in the formation of stalk cells during Dictyostelium development

The differentiation inducing factor (DIF) is essential for stalk cell formation in monolayers of Dictyostelium discoideum and is necessary for the expression of several prestalk cell-specific genes. DIF activity has been fractionated into a major species, designated DIF-1, and several minor species, including DIF-2. Although DIF-1 is an excellent inducer of stalk cell formation from vegetative cells, it is a poor inducer of stalk cell formation from prestalk cells. In contrast, DIF-2 is more active for the conversion of prestalk cells into stalk cells, than for the conversion of vegetative cells to stalk cells. The same results were obtained regardless of whether chemically synthesized or naturally occurring components were utilized. In addition, stalk cell formation was three- to fourfold higher when vegetative cells were incubated with DIF-1 for a suboptimal period and then subsequently incubated with DIF-2, than when cells were incubated with DIF-2 first and then subsequently with DIF-1. These results indicate a distinct role for DIF-2 during stalk cell formation and suggest the possibility that DIF-1 and DIF-2 act sequentially.