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.     

Identification of new differentiation inducing factors from Dictyostelium discoideum

Developing Dictyostelium discoideum amoebae form a stalked fruiting body in which individual cells differentiate into either stalk cells or spores. The major known inducer of stalk cell differentiation is the chlorinated polyketide DIF-1 (1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one); however a mutant blocked in the terminal step of DIF-1 biosynthesis still produces one of the prestalk cell subtypes – the pstA cells – as well as some mature stalk cells. We therefore searched for additional stalk cell-inducing factors in the medium supporting development of this mutant. These factors were purified by solvent extraction and HPLC and identified by mass spectroscopy and NMR. The mutant lacked detectable DIF-2 and DIF-3 (the pentanone and deschloro homologues of DIF-1) but four major stalk cell-inducing activities were detected, of which three were identified. Two compounds were predicted intermediates in DIF-1 biosynthesis: the desmethyl, and desmethyl-monochloro analogues of DIF-1 (dM-DIF-1 and Cl-THPH, respectively), supporting the previously proposed pathway of DIF-1 biosynthesis. The third compound was a novel factor and was identified as 4-methyl-5-pentylbenzene-1,3-diol (MPBD) with the structure confirmed by chemical synthesis. To investigate the potential roles of these compounds as signal molecules, their effects on morphological stalk and spore differentiation were examined in cell culture. All three induced morphological stalk cell differentiation. We found that synthetic MPBD also stimulated spore cell differentiation. Now that these factors are known to be produced and released during development, their biological roles can be pursued further.     

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.     

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 ∼ 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.     

DIF-1 induces its own breakdown in Dictyostelium.

DIF-1 is a novel chlorinated alkyl phenone which induces differentiation of prestalk cells in Dictyostelium discoideum. It is broken down and inactivated by a cytoplasmic enzyme, DIF-1 3(5)-dechlorinase (hereafter referred to as DIF-1 dechlorinase), which is found only in prestalk cells. We show that DIF-1 dechlorinase levels are induced at least 50-fold when cells are treated with DIF-1. This response is rapid--enzyme activity doubles within 15 min and is fully induced within an hour--and occurs early in development, before other prestalk markers can be induced by DIF-1. Maximum inducibility is seen towards the end of aggregation, when DIF-1 dechlorinase is barely detectable in uninduced cells. The dose-dependence reveals a threshold concentration of DIF-1 (15 nM) below which almost no response is seen. Cyclic AMP, which is the chemoattractant during aggregation and plays a key role in later development, suppresses the induction of DIF-1 dechlorinase by DIF-1. We conclude that induction of DIF-1 dechlorinase is one of the first steps on the developmental pathway which leads to prestalk cell differentiation, and suggest that the resulting negative feedback on DIF-1 levels is an important part of the mechanism by which cells decide whether to become prestalk or prespore cells.