Purification of stalk-cell-inducing morphogens from Dictyostelium discoideum

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.

ABC transporters in Dictyostelium discoideum development

ATP-binding cassette (ABC) transporters can translocate a broad spectrum of molecules across the cell membrane including physiological cargo and toxins. ABC transporters are known for the role they play in resistance towards anticancer agents in chemotherapy of cancer patients. There are 68 ABC transporters annotated in the genome of the social amoeba Dictyostelium discoideum. We have characterized more than half of these ABC transporters through a systematic study of mutations in their genes. We have analyzed morphological and transcriptional phenotypes for these mutants during growth and development and found that most of the mutants exhibited rather subtle phenotypes. A few of the genes may share physiological functions, as reflected in their transcriptional phenotypes. Since most of the abc-transporter mutants showed subtle morphological phenotypes, we utilized these transcriptional phenotypes to identify genes that are important for development by looking for transcripts whose abundance was unperturbed in most of the mutants. We found a set of 668 genes that includes many validated D. discoideum developmental genes. We have also found that abcG6 and abcG18 may have potential roles in intercellular signaling during terminal differentiation of spores and stalks.

The polyketide MPBD initiates the SDF-1 signaling cascade that coordinates terminal differentiation in Dictyostelium

Dictyostelium uses a wide array of chemical signals to coordinate differentiation as it switches from a unicellular to a multicellular organism. MPBD, the product of the polyketide synthase encoded by stlA, regulates stalk and spore differentiation by rapidly stimulating the release of the phosphopeptide SDF-1. By analyzing specific mutants affected in MPBD or SDF-1 production, we delineated a signal transduction cascade through the membrane receptor CrlA coupled to Gα1, leading to the inhibition of GskA so that the precursor of SDF-1 is released. It is then processed by the extracellular protease of TagB on prestalk cells. SDF-1 apparently acts through the adenylyl cyclase ACG to activate the cyclic AMP (cAMP)-dependent protein kinase A (PKA) and trigger the production of more SDF-1. This signaling cascade shows similarities to the SDF-2 signaling pathway, which acts later to induce rapid spore encapsulation.

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.

Differentiation inducing factor-1 (DIF-1) induces gene and protein expression of the Dictyostelium nuclear calmodulin-binding protein nucleomorphin

The nucleomorphin gene numA1 from Dictyostelium codes for a multi-domain, calmodulin binding protein that regulates nuclear number. To gain insight into the regulation of numA, we assessed the effects of the stalk cell differentiation inducing factor-1 (DIF-1), an extracellular signalling molecule, on the expression of numA1 RNA and protein. For comparison, the extracellular signalling molecules cAMP (mediates chemotaxis, prestalk and prespore differentiation) and ammonia (NH(3)/NH(4)(+); antagonizes DIF) were also studied. Starvation, which is a signal for multicellular development, results in a greater than 80% decrease in numA1 mRNA expression within 4 h. Treatment with ammonium chloride led to a greater than 90% inhibition of numA1 RNA expression within 2 h. In contrast, the addition of DIF-1 completely blocked the decrease in numA1 gene expression caused by starvation. Treatment of vegetative cells with cAMP led to decreases in numA1 RNA expression that were equivalent to those seen with starvation. Western blotting after various morphogen treatments showed that the maintenance of vegetative levels of numA1 RNA by DIF-1 in starved cells was reflected in significantly increased numA1 protein levels. Treatment with cAMP and/or ammonia led to decreased protein expression and each of these morphogens suppressed the stimulatory effects of DIF-1. Protein expression levels of CBP4a, a calcium-dependent binding partner of numA1, were regulated in the same manner as numA1 suggesting this potential co-regulation may be related to their functional relationship. NumA1 is the first calmodulin binding protein shown to be regulated by developmental morphogens in Dictyostelium being upregulated by DIF-1 and down-regulated by cAMP and ammonia.

Factors influencing the expression of dental extracellular matrix biomineralization

The forming tooth organ provides a number of opportunities to investigate the cellular and molecular biology of cell-mediated extracellular matrix (ECM) biomineralization. Regulatory processes associated with tooth formation are being investigated by identifying when and where cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs), dentine phosphoprotein and enamel gene products are expressed during sequential developmental stages. In vitro organotypic culture studies in serumless, chemically-defined medium, have shown that instructive and permissive signalling are required for both morphogenesis and cytodifferentiation. Intrinsic developmental instructions (autocrine and paracrine factors) act independently of long-range hormonal or exogenous growth factors and mediate morphogenesis from the initiation of the dental lamina to the crown stages of tooth development. This review summarizes the results of studies using experimental embryology, recombinant DNA technology and immunocytology to elucidate mechanisms responsive to instructive epithelial-mesenchymal interactions associated with ameloblast differentiation, odontoblast differentiation, and dentine and enamel ECM biomineralization.

Convergence of hormones and autoinducers at the host/pathogen interface

Most living organisms possess sophisticated cell-signaling networks in which lipid-based signals modulate biological effects such as cell differentiation, reproduction and immune responses. Acyl homoserine lactone (AHL) autoinducers are fatty acid-based signaling molecules synthesized by several Gram-negative bacteria that are used to coordinate gene expression in a process termed "quorum sensing" (QS). Recent evidence shows that autoinducers not only control gene expression in bacterial cells, but also alter gene expression in mammalian cells. These alterations include modulation of proinflammatory cytokines and induction of apoptosis. Some of these responses may have deleterious effects on the host's immune response, thereby leading to increased bacterial pathogenesis. Prokaryotes and eukaryotes have cohabited for approximately two billion years, during which time they have been exposed to each others' soluble signaling molecules. We postulate that organisms from the different kingdoms of nature have acquired mechanisms to sense and respond to each others signaling molecules, and we have named this process interkingdom signaling. We further propose that autoinducers, which exhibit structural and functional similarities to mammalian lipid-based hormones, are excellent candidates for mediating this interkingdom communication. Here we will compare and contrast bacterial QS systems with eukaryotic endocrine systems, and discuss the mechanisms by which autoinducers may exploit mammalian signal transduction pathways.

Signal relay during the life cycle of Dictyostelium

A fundamental property of multicellular organisms is signal relay, the process by which information is transmitted from one cell to another. The integration of external information, such as nutritional status or developmental cues, is critical to the function of organisms. In addition, the spatial organizations of multicellular organisms require intricate signal relay mechanisms. Signal relay is remarkably exhibited during the life cycle of the social amoebae Dictyostelium discoideum, a eukaryote that retains a simple way of life, yet it has greatly contributed to our knowledge of the mechanisms cells use to communicate and integrate information. This chapter focuses on the molecules and mechanisms that Dictyostelium employs during its life cycle to relay temporal and spatial cues that are required for survival.

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.

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.

Breaking symmetries: regulation of Dictyostelium development through chemoattractant and morphogen signal-response

Dictyostelium discoideum grow unicellularly, but develop as multicellular organisms. At two stages of development, their underlying symmetrical pattern of cellular organization becomes disrupted. During the formation of the multicellular aggregate, individual non-polarized cells re-organize their cytoskeletal structures to sequester specific intracellular signaling elements for activation by and directed movement within chemoattractant gradients. Subsequently, response to secreted morphogens directs undifferentiated populations to adopt different cell fates. Using a combination of cellular, biochemical and molecular approaches, workers have now begun to understand the mechanisms that permit Dictyostelium (and other chemotactic cells) to move directionally in shallow chemoattractant gradients and the transcriptional regulatory pathways that polarize cell-fate choice and initiate pattern formation.

Novel development rescuing factors (DRFs) secreted by the developing Dictyostelium cells, that are involved in the restoration of a mutant lacking MAP-kinase ERK2

We found novel development rescuing factors (DRFs) secreted from developing Dictyostelium cells, by using a mutant (erkB-) which is missing MAP-kinase ERK2 as a test strain for bioassay. The mutant erkB- fails to undergo multicellular morphogenesis due to impaired cAMP signaling. However, such developmental defect can be restored by the presence of low-molecular weight DRFs that are secreted from developing wild-type cells. We previously showed that DIF-1 (Differentiation-Inducing Factor 1 for stalk cells) possesses this activity, indicating a newly discovered role of DIF-1. Surprisingly, however, the mutant dmtA-, which is incapable of DIF-1 synthesis still exerts a strong inducing activity of the multicellular morphogenesis of erkB-. After analysis of HPLC fractions of conditioned media prepared from both wild type Ax2 and dmtA- strains revealed that both strains secrete at least two novel DRF activities with DIF-like mobility. However, these activities were not derived from other DIFs such as DIF-2 and DIF-3. Identification of these DRFs found in this study would provide insight into the mechanism by which the development of the erkB- mutant is restored and how these factors act in the normal development of Dictyostelium.

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.

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.

Chemo-orientation of echinostome cercariae towards their snail hosts: the stimulating structure of amino acids and other attractants

The cercariae of Pseudechinoparyphium echinatum and Echinostoma revolutum locate their host snails by turning back when swimming in decreasing gradients of the small molecular weight fraction (< 500) of snail conditioned water. Fractionation and chemical modifications of snail conditioned water from Lymnaea stagnalis showed that amino acids are necessary for the stimulating activity of snail conditioned water. A complete mixture of amino acids in concentrations determined from snail conditioned water had a high attraction. However, differently composed mixtures of amino acids and even single amino acids also had the same attraction as this complete mixture when used in concentrations corresponding to the total concentration of amino acids in snail conditioned water. Experiments with analogues and derivatives of amino acids showed that the primary alpha-amino group and the alpha-carboxyl group are necessary for the full effectiveness of amino acids. The highest effect was elicited by L-amino acids with a primary alpha-amino group, whereas the amino acid type and the chain length seemed to be unimportant. However, the full attraction of snail conditioned water was not achieved by amino acids alone. Chemical modifications of snail conditioned water suggested that the additional stimuli were neither inorganic ions nor organic acids or lipids. As the full attraction of snail conditioned water was obtained when the amino acid mixture of snail conditioned water was combined with its content of urea and ammonia, we conclude that the cercariae use only these excretory products as additional signals for their chemo-orientation. Chemo-orientation to amino acids, urea and ammonia seems to reflect a strategy to locate a broad spectrum of aquatic hosts.

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.

The proximal pathway of metabolism of the chlorinated signal molecule differentiation-inducing factor-1 (DIF-1) in the cellular slime mould Dictyostelium

Stalk cell differentiation during development of the slime mould Dictyostelium is induced by a chlorinated alkyl phenone called differentiation-inducing factor-1 (DIF-1). Inactivation of DIF-1 is likely to be a key element in the DIF-1 signalling system, and we have shown previously that this is accomplished by a dedicated metabolic pathway involving up to 12 unidentified metabolites. We report here the structure of the first four metabolites produced from DIF-1, as deduced by m.s., n.m.r. and chemical synthesis. The structures of these compounds show that the first step in metabolism is a dechlorination of the phenolic ring, producing DIF metabolite 1 (DM1). DM1 is identical with the previously known minor DIF activity, DIF-3. DIF-3 is then metabolized by three successive oxidations of its aliphatic side chain: a hydroxylation at omega-2 to produce DM2, oxidation of the hydroxy group to a ketone group to produce DM3 and a further hydroxylation at omega-1 to produce DM4, a hydroxyketone of DIF-3. We have investigated the enzymology of DIF-1 metabolism. It is already known that the first step, to produce DIF-3, is catalysed by a novel dechlorinase. The enzyme activity responsible for the first side-chain oxidation (DIF-3 hydroxylase) was detected by incubating [3H]DIF-3 with cell-free extracts and resolving the reaction products by t.l.c. DIF-3 hydroxylase has many of the properties of a cytochrome P-450. It is membrane-bound and uses NADPH as co-substrate. It is also inhibited by CO, the classic cytochrome P-450 inhibitor, and by several other cytochrome P-450 inhibitors, as well as by diphenyliodonium chloride, an inhibitor of cytochrome P-450 reductase. DIF-3 hydroxylase is highly specific for DIF-3: other closely related compounds do not compete for the activity at 100-fold molar excess, with the exception of the DIF-3 analogue lacking the chlorine atom. The Km for DIF-3 of 47 nM is consistent with this enzyme being responsible for DIF-3 metabolism in vivo. The two further oxidations necessary to produce DM4 are also performed in vitro by similar enzyme activities. One of the inhibitors of DIF-3 hydroxylase, ancymidol (IC50 67 nM) is likely to be particularly suitable for probing the function of DIF metabolism during development.

A repressor controls the timing and spatial localisation of stalk cell-specific gene expression in Dictyostelium

The ecmA and ecmB genes of Dictyostelium encode related extracellular matrix proteins and both are induced by DIF, the stalk cell-specific morphogen. The ecmA gene is expressed throughout the prestalk region of the migrating slug but only later, at culmination, do the prestalk cells express the ecmB gene. Expression of the ecmB gene is induced at the entrance to the stalk tube and we have identified two, apparently redundant, promoter elements that control this process. They act as repressors, preventing transcription in the tip of the migrating slug and the apical papilla of the culminant. They have a semi-palindromic consensus sequence TTGnCAA, where n is in one case 2 and in the other 4 bp. Either element alone is able to repress ecmB promoter activity in prestalk cells. Introduction of a single repressor element into the promoter of the ecmA gene changes its expression pattern to resemble that of the ecmB gene. Mutant elements, where n is altered, cause repression during the slug stage but allow premature ecmB expression during culmination; suggesting that the effective strength of the inductive signal may increase during culmination. Inhibition of cAMP-dependent protein kinase (PKA) in prestalk cells blocks both stalk cell maturation and ecmB gene expression. We show that the block to gene expression correlates precisely with the presence of a functional repressor element and this is consistent with the notion that expression of the ecmB gene is controlled by a PKA-dependent release from transcriptional repression.

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.

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.

New roles for DIF? Effects on early development in Dictyostelium

The DIFs are unusual, chlorinated molecules which induce stalk cell differentiation during the later, multicellular phase of Dictyostelium development. Here we provide evidence that one or more DIFs have a role during early development, when small amounts are known to be made. Initial indications came from an optical technique which detects changes in shape or cohesion of cells in suspension (Gerisch and Hess, PNAS 71, 2118, 1974). After a period of optical inactivity at the start of development, cell suspensions normally produce spontaneous spike-shaped light-scattering oscillations synchronised by oscillations in extracellular cAMP levels, followed by sinusoidal oscillations where the synchroniser is not known. DIFs 1 and 2 produce optical responses from cells at all these early stages of development. The phase of both spiked and sinusoidal oscillations can be shifted, indicating an effect on the oscillator in each case. We find further: (1) cAMP oscillations and cAMP relay during spiked oscillations are transiently inhibited by DIF-1. (2) DIF-1 causes a transient decrease in cellular cGMP levels in cells taken before oscillations commence and likewise inhibits the cGMP response to a cAMP stimulus in cells taken later in development. Cytoskeletal organization and hence cell shape might be affected by DIF-1 by this indirect route. (3) The effects of DIF-1 are transient, even though it is essentially stable in the cell suspension. Cells somehow adapt to DIF-1. (4) The effects are chemically specific: DIF-1 and DIF-2 are active at 10(-7) to 10(-8) M, with DIF-2 being the more active, whereas related compounds have little or no activity at 10(-6) M. These results indicate that cells are responsive to DIFs 1 and 2 from the start of development and suggest a wider role for the DIFs. This role might involve effects on cAMP signalling and on intracellular second messengers.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum contains three promoters specific for growth, aggregation, and late development

The cyclic nucleotide phosphodiesterase (phosphodiesterase) plays essential roles throughout the development of Dictyostelium discoideum. It is crucial to cellular aggregation and to postaggregation morphogenesis. The phosphodiesterase gene is transcribed into three mRNAs, containing the same coding sequence connected to different 5' untranslated sequences, that accumulate at different times during the life cycle. A 1.9-kilobase (kb) mRNA is specific for growth, a 2.4-kb mRNA is specific for aggregation, and a 2.2-kb mRNA is specific for late development and is only expressed in prestalk cells. Hybridization of RNA isolated from cells at various stages of development with different upstream regions of the gene indicated separate promoters for each of the three mRNAs. The existence of specific promoters was confirmed by fusing the three putative promoter regions to the chloramphenicol acetyltransferase reporter gene, and the analysis of transformants containing these constructs. The three promoters are scattered within a 4.1-kilobase pair (kbp) region upstream of the initiation codon. The late promoter is proximal to the coding sequence, the growth-specific promoter has an initiation site that is 1.9 kbp upstream of the ATG codon, and the aggregation-specific promoter has an initiation site 3 kbp upstream.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum contains three promoters specific for growth, aggregation, and late development

The cyclic nucleotide phosphodiesterase (phosphodiesterase) plays essential roles throughout the development of Dictyostelium discoideum. It is crucial to cellular aggregation and to postaggregation morphogenesis. The phosphodiesterase gene is transcribed into three mRNAs, containing the same coding sequence connected to different 5' untranslated sequences, that accumulate at different times during the life cycle. A 1.9-kilobase (kb) mRNA is specific for growth, a 2.4-kb mRNA is specific for aggregation, and a 2.2-kb mRNA is specific for late development and is only expressed in prestalk cells. Hybridization of RNA isolated from cells at various stages of development with different upstream regions of the gene indicated separate promoters for each of the three mRNAs. The existence of specific promoters was confirmed by fusing the three putative promoter regions to the chloramphenicol acetyltransferase reporter gene, and the analysis of transformants containing these constructs. The three promoters are scattered within a 4.1-kilobase pair (kbp) region upstream of the initiation codon. The late promoter is proximal to the coding sequence, the growth-specific promoter has an initiation site that is 1.9 kbp upstream of the ATG codon, and the aggregation-specific promoter has an initiation site 3 kbp upstream.

C-factor: a cell-cell signaling protein required for fruiting body morphogenesis of M. xanthus

During fruiting body development, the product of the csgA gene is necessary for cellular aggregation, for spore differentiation, and for gene expression that is initiated after 6 hr of starvation. From nascent wild-type fruiting bodies we have purified a polypeptide of 17 kd called C-factor, which, at approximately 1 to 2 nM, restores normal development to csgA mutant cells. C-factor activity is not recovered from extracts of unstarved, growing cells or csgA mutant cells. The amino acid sequence from purified C-factor demonstrates that it is the product of the csgA gene. C-factor is active over a narrow range of concentration and has properties of a morphogenetic paracrine signal.

The autoinhibitor of cell fusion in Dictyostelium inhibits calmodulin

During early sexual development in Dictyostelium discoideum cell and pronuclear fusion are negatively regulated by an endogenous autoinhibitor. Here, the autoinhibitor was partially purified from the culture medium and found to inhibit both cell and pronuclear fusion while augmenting gamete numbers. These developmental effects suggested that calmodulin might be an intracellular target for the autoinhibitor. In support of this data, the partially purified autoinhibitor inhibited the calmodulin-dependent activation of phosphodiesterase in a dose-dependent manner, but had no effect on either a calmodulin-insensitive form of phosphodiesterase or the calmodulin-independent enzymes acid and alkaline phosphatase. Thus, the autoinhibitor of sexual development in Dictyostelium discoideum appears to regulate cell and pronuclear fusion at least in part by a direct effect on calmodulin.

Differentiation-inducing factor from the slime mould Dictyostelium discoideum and its analogues. Synthesis, structure and biological activity

Previous work has led to the identification of a novel class of effector molecules [DIFs (differentiation-inducing factors) 1-3] released from the slime mould Dictyostelium discoideum. These substances induce stalk-cell differentiation in Dictyostelium discoideum and are thought to act as morphogens in the generation of the prestalk/prespore pattern during development. The DIFs are phenylalkan-1-ones, with chloro, hydroxy and methoxy substitution on the benzene ring. DIFs 1-3 and a number of their analogues have been synthesized by using a simple two-step procedure, and each analogue has been characterized by m.s., u.v. and n.m.r. spectroscopy. The crystal structure of synthetic DIF-1 [1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one, was investigated. The specific biological activity of each analogue was determined in a bioassay, where isolated Dictyostelium amoebae are induced to differentiate into stalk cells. The major biologically active substance, DIF-1, caused 50% stalk-cell differentiation at 1.8 x 10(-10) M; the C4 alkyl homologue (DIF-2) and C6 homologue possessed 40 and 16% of the activity of DIF-1 respectively. Further increase or decrease in the alkyl chain length resulted in a marked decrease in specific activity. The pattern of substitution on the benzene ring is a major determinant of bioactivity, since the specific activities of the 2,4-dihydroxy-6-methoxy and trihydroxy analogues were less than 1% of that of DIF-1. Substitution of bromine in DIF-1 had little effect on bioactivity; in contrast the activity of monochloro-DIF-1 (DIF-3) was diminished. There was no evidence for antagonism or synergy between DIF-1 and any of its analogues. This series of analogues will facilitate further studies in the biological effects and mode of action of DIF-1.

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.

The expression of glycoproteins in the extracellular matrix of the cellular slime mold Dictyostelium discoideum

In this report we examine the accumulation of glycoconjugates in the extracellular medium and insoluble matrices surrounding developing cells of the cellular slime mold Dictyostelium discoideum. Conditions were employed which permitted advanced development (slug stage and beyond) in suspension culture. Under these conditions, up to one-third of the total culture protein appeared as non-sedimentable, extracellular material over the course of 48 h of incubation. Most of the secreted molecules expressed carbohydrate antigens (glycoantigens) as detected by Western blotting, using a panel of six monoclonal antibodies. Since the glycoantigens are secreted, immunoelectron microscopy was used to localize the glycoantigens in the extracellular matrices surrounding normally developing cells, including the slime sheath, stalk tube, inner spore coat, outer spore coat, and intercellular fluid between spores. Each glycoantigen had a characteristic distribution, and each extracellular matrix space contained a unique combination of glycoantigens. Thus, although each of these matrices (except inter-spore fluid) contains cellulose as a primary component, they could be distinguished on the basis of their glycoantigen and, by inference, glycoprotein compositions. Furthermore, there were differences between anterior and posterior regions of both slime sheaths and stalk tubes. These observations show that secretion as detected in suspension culture occurs under normal conditions as a part of the process of depositing extracellular matrices around the cells. The distributions show that the cell aggregate positionally regulates the expression and deposition of secretory glycoproteins; the resultant patterns of expression of unique protein-linked carbohydrate structures imply a functional role in matrix organization and possibly cell activity which can now be explored.

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.

Structure elucidation of two differentiation inducing factors (DIF-2 and DIF-3) from the cellular slime mould Dictyostelium discoideum

Two endogenous differentiation-inducing factors (DIF-2 and DIF-3), which induce stalk-cell differentiation in the cellular slime mould Dictyostelium discoideum, have been identified as the pentan-1-one and monochloro analogues respectively of (1-[(3,5-dichloro-2,6-dihydroxy-4-methoxy)phenyl]hexan-1-one). These compounds represent a new chemical class of effector molecules.

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.

Identification and localization of proteins encoded by two DIF-inducible genes of Dictyostelium

We show that pDd56 and pDd63, two related DIF-inducible genes of Dictyostelium, respectively encode the ST310 and ST430 polypeptides identified by Morrissey, Devine, and Loomis (1984, Dev. Biol. 103, 414-424). We localize the two proteins by immunoelectron microscopy to the extracellular matrix surrounding the stalk cells and the stalk tube. Coupled with their predicted amino acid sequence and biochemical properties, this suggests that they are structural proteins of the stalk.

Purification, characterization, and partial structure of D factor from Polysphondylium violaceum

The A component of D factor (DfA) was overproduced during development of wild type Polyspondylium violaceum strain China after starvation in liquid medium. Crude DfA excreted by strain China was partially purified by ultrafiltration using Amicon YM10 and YM2 filters with DfA extracted from the filtrate by absorption onto a preparative grade C-18 resin. The concentrated material was further purified on a C-18 analytical column using both acetonitrile:water and methanol:water gradients. This highly purified fraction was a single component with a final specific activity of greater than 10(6) units per mg dry weight. Purified DfA is red having a broad visible absorbance at 500 nm and a ultraviolet (uv) absorbance at 290-300 nm. The red chromophore is sensitive to pH and to oxidation-reduction. 1H and 13C nmr studies with purified DfA indicate that it is a C11 compound with both polar and non-polar regions. The non-polar region has been identified as a hexanone and is the same as the side chain of DIF from Dictyostelium discoideum. Purified DfA has been used in studies with the D factor non-producing mutant, tsg-119 cyc-1 aggA586 (A586), to show that neither production of glorin nor chemotactic sensitivity to glorin are affected by D factor. However, founder cells develop in A586 mutant populations only after addition of D factor. These data suggest that DfA may be necessary for induction of aggregate formation by aggregation-competent amoebae.

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.