myo-inositol pentakisphosphates. Structure, biological occurrence and phosphorylation to myo-inositol hexakisphosphate

1. Standard and high-performance anion-exchange-chromatographic techniques have been used to purify myo-[3H]inositol pentakisphosphates from various myo-[3H]inositol-prelabelled cells. Slime mould (Dictyostelium discoideum) contained 8 microM-myo-[3H]inositol 1,3,4,5,6-pentakisphosphate, 16 microM-myo-[3H]inositol 1,2,3,4,6-pentakisphosphate and 36 microM-D-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate [calculated intracellular concentrations; Stephens & Irvine (1990) Nature (London) 346, 580-583]; germinating mung-bean (Phaseolus aureus) seedlings contained both D- and L-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate (which was characterized by 31P and two-dimensional proton n.m.r.) and D- and/or L-myo-[3H]inositol 1,2,3,4,5-pentakisphosphate; HL60 cells contained myo-[3H]inositol 1,3,4,5,6-pentakisphosphate (in a 500-fold excess over the other species), myo-[3H]inositol 1,2,3,4,6-pentakisphosphate and D- and/or L-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate; and NG-115-401L-C3 cells contained myo-[3H]inositol 1,3,4,5,6-pentakisphosphate (in a 100-fold excess over the other species), D- and/or L-myo-[3H]inositol 1,2,4,5,6-pentakisphosphate, myo-[3H]inositol 1,2,3,4,6-pentakisphosphate and D- and/or L-myo-[3H]inositol 1,2,3,4,5-pentakisphosphate. 2. Multiple soluble ATP-dependent myo-inositol pentakisphosphate kinase activities have been detected in slime mould, rat brain and germinating mung-bean seedling homogenates. In slime-mould cytosolic fractions, the three myo-inositol pentakisphosphates that were present in intact slime moulds could be phosphorylated to myo-[3H]inositol hexakisphosphate: the relative first-order rate constants for these reactions were, in the order listed above, 1:8:31 respectively (with first-order rate constants in the intact cell of 0.1, 0.8 and 3.1 s-1, assuming a cytosolic protein concentration of 50 mg/ml), and the Km values of the activities for their respective inositol phosphate substrates (in the presence of 5 mM-ATP) were 1.6 microM, 3.8 microM and 1.4 microM. At least two forms of myo-inositol pentakisphosphate kinase activity could be resolved from a slime-mould cytosolic fraction by both pharmacological and chromatographic criteria. Rat brain cytosol and a soluble fraction derived from germinating mung-bean seedlings could phosphorylate myo-inositol D/L-1,2,4,5,6-, D/L-1,2,3,4,5-, 1,2,3,4,6- and 1,3,4,5,6-pentakisphosphates to myo-inositol hexakisphosphate: the relative first-order rate constants were 57:27:77:1 respectively for brain cytosol (with first-order rate constants in the intact cell of 0.0041, 0.0019, 0.0056 and 0.000073 s-1 respectively, assuming a cytosolic protein concentration of 50 mg/ml) and 1:11:12:33 respectively for mung-bean cytosol (with first-order rate constants in a supernatant fraction with a protein concentration of 10 mg/ml of 0.0002, 0.0022, 0.0024 and 0.0066 s-1 respectively).

The DIF-1 signaling system in Dictyostelium. Metabolism of the signal

DIF-1 is a novel, chlorinated alkyl phenone from Dictyostelium which, at very low concentrations, induces amoebae to differentiate into stalk cells and may act as a morphogen in the formation of the prestalkprespore pattern during development. We report here the existence of a developmentally regulated metabolic pathway which inactivates DIF-1. Radioisotopically labeled DIF-1 was synthesized, incubated with developing cells, the metabolites recovered, and then analyzed by high pressure liquid chromatography and TLC. At least 12 metabolites are produced and the early steps of a complex metabolic pathway have been deduced by following the flow of counts from one metabolite to another and by determining the fate of purified metabolites when they are incubated with cells. The first metabolite, DM1, is largely cell-associated whereas the more distal ones are found mainly in the medium. Metabolism inactivates DIF-1, since DM1 retains only 7% of the specific activity of DIF-1 in the stalk cell differentiation bioassay and later metabolites possess even less activity. Metabolism is developmentally regulated, increasing toward the end of aggregation to reach maximal levels at the tipped mound stage, as endogenous DIF-1 levels are themselves rising. Cells at this stage of development possess the capacity to metabolize their endogenous DIF-1 with a half-life of a few minutes. We suggest that DIF-1 metabolism is important to prevent the DIF-1 receptor system from becoming saturated by excess ligand, thus allowing cells to respond to changes in DIF-1 production. Metabolism may also produce other effector molecules from DIF-1 or produce DIF-1 gradients in the aggregate by the localized destruction of DIF-1.

A myo-inositol D-3 hydroxykinase activity in Dictyostelium

A soluble ATP-dependent enzyme which phosphorylates myo-inositol has been characterized in Dictyostelium. The myo-inositol kinase activity was partially purified from amoebae by chromatography on DEAE-Sepharose and phenyl-Sepharose columns. The product of both the partially purified activity and of a crude cytosolic fraction was myo-inositol 3-phosphate. The partially purified preparations of myo-inositol kinase (a) possessed a Km for myo-inositol of 120 microM (in the presence of 5 mM-ATP) and for ATP of 125 microM (in the presence of 1 microM-myo-inositol), (b) did not recognize allo-, epi-, muco-, neo-, scyllo-, 1 D-chiro or 1 L-chiro-inositol as substrates, (c) were competitively inhibited by three naturally occurring analogues of myo-inositol: 1 L-chiro-inositol (Ki 49.5 +/- 0.7 microM: the structural equivalent of myo-inositol, except that the D-3 hydroxy moiety is axial), D-3-deoxy-myo-inositol [Ki 103 +/- 1 microM: (-)-viburnitol], and sequoyitol (Ki 271 +/- 7 microM; unlike 1 L-chiro-inositol and D-3-deoxy-myo-inositol, this was a substrate for the kinase), and finally (d) were apparently non-competitively inhibited by myo-inositol 3-phosphate. The product of myo-inositol kinase could be detected in intact amoebae and was a substrate for the first in a series of inositol polyphosphate kinases present in Dictyostelium which ultimately yield myo-inositol hexakisphosphate. The activity of myo-inositol D-3-hydroxykinase in Dictyostelium lysates showed evidence of developmental regulation.

Combinatorial control of cell differentiation by cAMP and DIF-1 during development of Dictyostelium discoideum

At least three distinct types of cell arise from a population of similar amoebae during Dictyostelium development: prespore, prestalk A and prestalk B cells. We report evidence suggesting that this cellular diversification can be brought about by the combinatorial action of two diffusible signals, cAMP and DIF-1. Cells at different stages of normal development were transferred to shaken suspension, challenged with various combinations of signal molecules and the expression of cell-type-specific mRNA markers measured 1–2 h later. pDd63, pDd56 and D19 mRNAs were used for prestalk A, prestalk B and prespore cells respectively. We find the following results. (1) Cells first become responsive to DIF-1 for prestalk A differentiation and to cAMP for prespore differentiation at the end of aggregation, about 2 h before these cell types normally appear. (2) At the first finger stage of development, when the rate of accumulation of the markers is maximal, the expression of each is favoured by a unique combination of effectors: prespore differentiation is stimulated by cAMP and inhibited by DIF-1; prestalk A differentiation is stimulated by both cAMP and DIF-1 and prestalk B differentiation is stimulated by DIF-1 and inhibited by cAMP. (3) Half-maximal effects are produced by 10–70 nM DIF-1, which is in the physiological range. (4) Ammonia and adenosine, which can affect cell differentiation in other circumstances, have no significant pathway-specific effect in our conditions. These results suggest that cell differentiation could be brought about in normal development by the localized action of cAMP and DIF-1.

A specific DIF binding protein in Dictyostelium

Differentiation Inducing Factor (DIF-1), a small chlorinated organic molecule which is produced during Dictyostelium development, is believed to be the morphogen that controls the stalk-specific pathway of differentiation. We describe the identification and characterization of a protease-sensitive activity from cell lysates which binds tritiated DIF-1 with the properties expected of a DIF receptor. Scatchard and linear subtraction plots show a single class of binding sites, of high affinity (Kd = 1.8 nM) and low abundance (1100 sites/cell). The activity elutes from heparin-agarose as a single peak. Various DIF-1 analogues compete for binding in proportion to their activities in a stalk cell differentiation bioassay. The amount of binding activity is developmentally regulated, peaking shortly before the appearance of the prestalk-prespore pattern and before the developmental rise in DIF concentration; the rise occurs at the same time that prestalk-specific genes become DIF inducible. Addition of cyclic AMP to aggregated cells, which induces post-aggregative gene expression in general, also induces the binding activity.

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.

Origins of the prestalk-prespore pattern in Dictyostelium development

Using cell-autonomous markers we have traced the origins of prespore cells and two types of prestalk cells (pstA and pstB cells) during slug formation. We show that cell sorting and positional information both contribute to Dictyostelium morphogenesis. The initial pattern established at the mound stage is topologically quite different from that of the slug. Confirming previous studies, we find that prespore cells occupy most of the aggregate but are absent from a thin layer at the base and from the emerging tip. PstB cells are almost entirely localized to the basal region during the early stages of tip formation. Thus prespore and pstB cell differentiation appear to occur in response to localized morphogenetic signals. In the case of pstB cells, these signals presumably emanate from the base and not, as might be expected, from the tip. When first detectable, pstA cells are scattered throughout the aggregate. They then appear to migrate to the apex, where the tip forms.

The molecular basis of positional signalling: introduction

The importance of cell–cell interactions in embryonic development was first described by Driesch (1891), who showed that any of the blastomeres of the 2-cell or 4-cell sea-urchin embryo is capable of forming a complete embryo if cultured in isolation; this implied that in normal development each blastomere is aware of the other and will only form a half- or quarter-embryo, as appropriate. And it was only ten years later that Spemann (1901) discovered the phenomenon of embryonic induction, recently reviewed by Gurdon (1987) and defined as an interaction in which the differentiation of one group of cells is affected by a signal from an adjacent group. Thus the significance of cell signalling during development has been appreciated for almost a century, but, as has frequently been remarked, progress in the molecular analysis of the phenomenon has been slow compared with that in the younger disciplines of, for example, immunology and molecular biology.

Morphogen hunting in Dictyostelium

A highly regulative pattern of prestalk and prespore tissue is formed during Dictyostelium development, starting from separate amoebae. Potential morphogens controlling this process have been hunted biochemically, using bioassays to monitor activity. All those discovered to date are low MW diffusible compounds: cAMP, adenosine, NH3 and DIFs 1–3. The DIFs are assayed by their ability to induce isolated amoebae to differentiate into stalk cells and have been identified as a family of chlorinated phenyl alkanones.

The diversification of amoebae into prestalk and prespore cells seems to be brought about by cAMP and DIF-1. cAMP is necessary for both pathways of differentiation but DIF-1 specifically induces the differentiation of prestalk cells while suppressing that of prespores.

When DIF-1 is added to intact slugs, it causes a substantial enlargement of the prestalk tissue at physiological concentrations in the time previously shown to be required for pattern regulation.

DIF-1 is a dynamic molecule and we have found that it is metabolized along a pathway involving at least 8 compounds. Metabolism is developmentally regulated and may be important in producing DEF gradients or other effector molecules from DIF.

Although we almost certainly have some of the central actors, it is difficult to formulate a satisfactory theory of pattern formation in Dictyostelium at the moment. We suspect that at least one important actor is missing.

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.

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.

Chemical structure of the morphogen differentiation inducing factor from Dictyostelium discoideum

Morphogens are signal molecules presumed to exist in embryos and to be involved in establishing the spatial pattern of cells during development. Differentiation inducing factor (DIF) has the properties of a morphogen required for producing the prestalk/prespore pattern in the aggregate formed by cells of the slime mould Dictyostelium in response to starvation. DIF-1, the major bioactive species after purification, has now been identified using a combined microchemical, spectroscopic and synthetic approach. The structure is defined as 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)-1-hexanone, and represents a new class of effector molecule. The availability of relatively large quantities of synthetic and isotopically labelled materials should now allow progress towards a detailed understanding of the pattern-forming processes in Dictyostelium development.

Two distinct classes of prestalk-enriched mRNA sequences in Dictyostelium discoideum

We have isolated cDNA clones derived from three mRNA sequences which are inducible by DIF, the putative stalk-specific morphogen of Dictyostelium. The three mRNA sequences are selectively expressed in cells on the stalk cell pathway of differentiation and we have compared them with previously characterized prestalk-enriched mRNA sequences. We find these latter sequences are expressed without a dependence on DIF, are much less highly enriched in prestalk over prespore cells and are expressed earlier during development than the DIF-inducible mRNA sequences. We propose two distinct mechanisms whereby a mRNA may become enriched in prestalk cells. An apparently small number of genes, represented by those we have isolated, is inducible by DIF and accumulates only in prestalk cells. We suggest that a second class of prestalk-enriched mRNA sequences are induced by cAMP to accumulate in all cells during aggregation and then become enriched in prestalk cells by selective loss from prespore cells.

Nature and distribution of the morphogen DIF in the Dictyostelium slug

The Dictyostelium slug contains a simple anterior-posterior pattern of prestalk and prespore cells. It is likely that DIF, the morphogen which induces stalk cells, is involved in establishing this pattern. Previous work has shown that a number of distinct species of DIF are released by developing cells and that cell-associated DIF activity increases rapidly during the slug stage of development. In this paper we describe a comparison of the DIF extracted from slugs with the DIF released into the medium. Analysis by high-pressure liquid chromatography (HPLC) using different solvent systems shows that the major species of DIF activity extracted from slugs coelutes with DIF-1, the major species of released DIF and is similarly sensitive to sodium borohydride reduction. Since DIF specifically induces the differentiation of prestalk cells, the anterior cells of the slug, it could be anticipated that DIF is localized in the prestalk region. We have therefore determined the distribution of DIF within the slug. Migrating slugs from strain V12M2 were manually dissected into anterior one-third and posterior two-third fragments and the DIF activity extracted. Surprisingly, we found that DIF was not restricted to the prestalk fragment. Instead there appears to be a reverse gradient of DIF in the slug with at least twice the specific activity of total DIF in the prespore region than in the prestalk region.

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.

Intracellular pH in Dictyostelium: a 31P nuclear magnetic resonance study of its regulation and possible role in controlling cell differentiation

Intracellular pH (pHi) has been measured in Dictyostelium discoideum cells by 31P nuclear magnetic resonance. Ax2 cells, newly harvested from growth medium, maintained a pHi of 7.33 +/− 0.04 (17) at an extracellular pH ranging from 3.5 to 6.5. Below pH 3.5 the cells tend to lyse, whereas at pH values above 6.5 their pHi rises though they remain viable. pHi regulation in acid medium is not dependent on external Na+ or any other inorganic ion and so most probably involves the electrogenic plasma membrane proton pump. No significant change in pHi was detected during development through to the slug stage. Mature stalk cells gave a very acidic phosphate signal (pH less than or equal to 5.5) which was probably vacuolar in origin. Indirect experiments had suggested that pHi might regulate the development of Dictyostelium cells, with low pHi favouring stalk cell and high pHi favouring spore cell differentiation. In particular, two inhibitors of the plasma membrane proton pump, diethylstilbestrol and zearalenone, had been shown to be stalk cell inducers. In the present studies measurements of pHi of cells exposed to these inducers failed to detect the expected drop in pHi. In addition, DIF-1 (a low Mr factor), the natural inducer of stalk cell formation, caused, if anything, a slight alkalinization of the cells. Thus the original theory linking pHi and cell differentiation is not supported by these results and therefore appears to require some modification. Finally, extract experiments revealed the existence of two unidentified abundant phospho-compounds with resonant frequencies close to inorganic phosphate. The existence of these compounds can complicate the interpretation of spectra gained from living Dictyostelium cells.

Selective induction of stalk-cell-specific proteins in Dictyostelium

We compared the proteins synthesized and accumulated by Dictyostelium discoideum amoebae in response to the morphogenetic factor termed differentiation-inducing factor (DIF) to assess the proposed ability of DIF to regulate the choice of differentiation pathway. When amoebae of a mutant strain with low endogenous DIF levels were given DIF, they dramatically increased the expression of 21 of 23 proteins preferentially found in stalk cells, but drastically repressed 4 major spore-specific proteins. Most of the induced proteins were also expressed in amoebae of a developmentally competent strain developing at low cell densities and exposed to DIF, low extracellular pH, or the proton pump inhibitor diethylstilbestrol; this suggests that an intracellular acidification may be a key part of the mechanism of DIF action. We conclude from the similar morphology and extensive homology of proteins of DIF-induced and stalk cells that most stalk-pathway functions are DIF dependent.

Purification of stalk-cell-inducing morphogens from Dictyostelium discoideum

We have shown previously that developing amoebae of Dictyostelium discoideum release one or more low-Mr factors, which can induce isolated cells to differentiate into stalk cells in the presence of cyclic AMP [Town, C. D., Gross, J. D. and Kay, R. R. (1976) Nature (Lond.) 262, 717-719; Town, C. D. and Stanford, E. (1979) Proc. Natl Acad. Sci. USA, 76, 308-312]. These differentiation-inducing factors (DIF) have now been purified by a procedure involving binding to and elution from XAD-2 resin, extraction into hexane and two steps of reverse-phase high-pressure liquid chromatography (HPLC). Our results show the following. HPLC resolves a major stalk-cell-inducing activity (DIF-1) and at least four minor and more polar activities (DIFs 2-5). DIF-1 has been purified at least 3000-fold over the starting dialysed medium with a recovery of about 2%. This low recovery of DIF-1 can be explained in part by the loss of non-specific stimulatory ('helper') factors during the purification. A few micrograms purified DIF-1 were obtained from 10(12) cells. This material could induce stalk cell differentiation in the standard assay at less than 0.2 nM. The biological activity of DIFs 1, 2 and 3 was sensitive to borohydride reduction, suggesting the presence of an essential carbonyl group. DIF-5 was partially sensitive and DIF-4 resistant. Other properties of DIF-1 suggest that it is a non-polar molecule of Mr less than 500, which becomes charged in alkaline solution, and that it is neither a peptide nor has essential sugar moieties. The purification of DIF should make possible its eventual identification by sensitive physical techniques, such as mass spectroscopy, and will allow further investigation of its biological effects.

Dictyostelium mutants lacking DIF, a putative morphogen

DIF is an endogenous extracellular signal that may control differentiation of D. discoideum cells. It is a dialyzable, lipid-like factor that induces stalk cell formation among isolated amebae incubated in vitro with cAMP. To examine the consequences of DIF deprivation, we have isolated several mutant strains that are impaired in DIF accumulation, and whose inability to make stalk cells in vitro and during normal development on agar can be corrected by the addition of exogenous DIF. Little DIF is made by the mutants, and morphological development on agar stops after the cells have aggregated, but before a slug forms. In these DIF-deprived conditions, prespore cells can differentiate, but prestalk cells cannot.

Intracellular pH and the control of cell differentiation in Dictyostelium discoideum

During development in the cellular slime mould Dictyostelium discoideum starved amoebae aggregate to form multicellular structures that display a simple antero-posterior pattern: prestalk cells occupy the front 20% of the aggregate, and prespore cells occupy the remainder. We have attempted to elucidate the nature of the mechanism regulating the proportions of the two cell types by examining the factors that influence the pathway of differentiation of amoebae in vitro. Amoebae of D. discoideum strain V12 M2 form stalk cells efficiently in appropriate conditions and 'sporogenous' derivatives produce spores as well as stalk cells. Mature spores are formed in a medium containing only cyclic AMP and salts, whereas formation of stalk cells requires, in addition, a low molecular weight hydrophobic factor (DIF). Recent observations have led us to propose that DIF is a morphogen responsible for activating stalk cell differentiation. Here we present evidence that ammonia is a second morphogen, that acts antagonistically to DIF, and that the choice of differentiation pathway is mediated by intracellular pH.

A possible morphogen controlling differentiation in Dictyostelium

The complex morphology of a higher organism is generated partly by such developmental processes as cell movement and cohesion but also by a social interaction between cells in small areas of embryonic tissue known as morphogenetic fields. The initially similar cells within such a field organize themselves and differentiate, forming a discrete spatial pattern which is remarkably independent of field size and which can regenerate after some part is removed. Although it is believed that a cell signalling system must underlie this behaviour, the putative signals--or morphogens--have so far proved elusive. Perhaps the simplest known morphogenetic field arises within the multicellular aggregate formed by developing cells of the slime mould Dictyostelium discoideum. As the amorphous aggregate transforms into a cylindrical slug, a simple pattern emerges, with prestalk cells differentiating in the anterior and prespores in the posterior. One great difficulty in identifying any morphogen has been to predict properties that could form the basis of a bioassay. However, in Dictyostelium it is almost essential that the morphogens should dictate to cells their choice of differentiation pathway. We have described previously a crude factor termed DIF which stimulates the differentiation of isolated amoebae into stalk cells. We now show that purified DIF also inhibits spore formation and so switches cells to stalk cell formation. Thus, we believe that DIF is a morphogen which regulates the choice of differentiation pathway of cells in the Dictyostelium slug.