cAMP and spore differentiation in Dictyostelium discoideum

The individual amoebae composing of Dictyostelium aggregate can differentiate into either stalk or spore cells according, it is believed, to the extracellular signals they receive. By inducing the differentiation of isolated cells we hope to identify these signals. It is shown here that wild-type cells can differentiate into prespore cells in a solution of cAMP plus salts supplemented by conditioned medium. Cell-to-cell contact is not required. More important, isolated cells of various sporogenous mutant strains form spores in similar conditions without needing conditioned medium at all. For these strains at least, cAMP is the sole inducer necessary for spore formation. Earlier work has shown that stalk cells are induced by a combination of cAMP and a low molecular weight factor, differentiation inducing factor (DIF). DIF now appears to be the only pathway-specific inducer involved in the differentiation of sporogenous amoebae and DIF levels in the aggregate may therefore determine whether an amoeba becomes a stalk or a spore cell. In suitable conditions some sporogenous mutants form migrating slugs having an anterior/posterior pattern of prestalk and prespore cells. This pattern could be generated by the localized activity of DIF.

Cell patterning in Dictyostelium

We summarize studies on stalk and spore cell formation in D. discoideum cell monolayers, aimed at revealing factors involved in controlling the prestalk:prespore pattern in this organism. We propose that there are no cell interactions dependent on cell contact per se. Formation of mature stalk cells from isolated amoebae incubated in a buffered salts medium requires only cyclic AMP and a lipid-like factor (DIF) released by cells developing at high density. In addition, a variety of sporogenous mutants can form spores rapidly and efficiently when incubated at low density in tissue culture dishes containing a similar cyclic AMP and salts medium. In some cases spore formation is improved by the addition of one or other of a variety of protective agents such as bovine serum albumin. Wild-type amoebae at low density form prespore cells under the same conditions. We present some evidence that DIF is the activator of prestalk cell formation in a two-component patterning mechanism of the kind proposed by Wolpert et al. (Symp. Soc. exp. Biol. 25, 391-415 (1971)) and Gierer & Meinhardt (Kybernetik 12, 30-39 (1972)). We also provide data indicating that the role of inhibitor is played by ammonia, an idea first mooted by Sussman & Schindler (Differentiation 10, 1-5 (1978)).

Dictyostelium amoebae can differentiate into spores without cell-to-cell contact

Amoebae of sporogenous mutants of Dictyostelium discoideum can differentiate into stalk cells and spores in the absence of normal morphogenesis when spread on agar containing cyclic-AMP. The efficiency of differentiation is improved when the amoebae are incubated as submerged monolayers in plastic petri dishes. Under these conditions spore formation is density dependent and hence requires some form of cellular interaction. To determine whether this interaction involves direct cell-cell contact we have made time-lapse films of cells differentiating at intermediate density. These films show that amoebae can develop into spores without making contact with any other cells. In addition, although some cells do divide during incubation, division is not necessary for spore formation. At higher densities small aggregates form which give rise to mixtures of stalk cells and spores. There is no detectable patterning of the two cell types within such aggregates.

Gene expression in Dictyostelium discoidium: mutually antagonistic roles of cyclic-AMP and ammonia

Cyclic-AMP and ammonia have been previously identified as extracellular signals during Dictyostelium development. Both are important in controlling morphological movements and cyclic-AMP also in inducing gene expression. The work in this paper is concerned with their effects on developmental gene expression. Cyclic-AMP was found to act as an inducer during the aggregative (as exemplified by phosphodiesterase) and the post-aggregative (glycogen phosphorylase, UDP-galactose polysaccharide transferase, prespore vacuoles and stalk cells) phases of gene expression. Ammonia inhibited the appearance of each of the above markers and antagonized the inductive effects of cyclic-AMP on them. This inhibition by ammonia of cyclic-AMP inducible gene expression may involve a step linking elevated intracellular cyclic-AMP levels to gene activation.

It has been suggested that the specification of cells within the aggregate into the stalk and spore pathways of differentiation might be controlled by cyclic-AMP and ammonia. In this model for pattern formation cyclic-AMP would induce stalk cell differentiation and ammonia spore formation. The present results argue against this idea since cyclic-AMP induces and ammonia inhibits differentiation along both pathways. The function of these agents may rather be to coordinate the rates of biochemical differentiation of individual cells and link them to the overall morphological changes occurring during development.

Gene expression in Dictyostelium discoidium: mutually antagonistic roles of cyclic-AMP and ammonia

Cyclic-AMP and ammonia have been previously identified as extracellular signals during Dictyostelium development. Both are important in controlling morphological movements and cyclic-AMP also in inducing gene expression. The work in tis paper is concerned with their effects on developmental gene expression. Cyclic-AMP was found to act as an inducer during the aggregative (as exemplified by phosphodiesterase) and the post-aggregative (glycogen phosphorylase, UDP-galactose polysaccharide transferase, prespore vacuoles and stalk cells) phases of gene expression. Ammonia inhibited the appearance of each of the above markers and antagonized the inductive effects of cyclic-AMP on them. This inhibition by ammonia of cyclic-AMP inducible gene expression may involve a step linking elevated intracellular cyclic-AMP levels to gene activation. It has been suggested that the specification of cells within the aggregate into the stalk and spore pathways of differentiation might be controlled by cyclic-AMP and ammonia. In this model for pattern formation cyclic-AMP would induce stalk cell differentiation and ammonia spore formation. The present results argue against this idea since cyclic-AMP induces and ammonia inhibits differentiation along both pathways. The function of these agents may rather be to coordinate the rates of biochemical differentiation of individual cells and link them to the overall morphological changes occurring during development.

Effects of BUdR on developmental functions of Dictyostelium discoideum

The development of Dictyostelium discoideum cells, as measured by spore yield, is somewhat more sensitive to the presence of BUdR during vegetative growth than is growth itself. Observations on the development of control and BUdR-grown cells, their protein labelling patterns and assays of 4 developmentally regulated proteins all reveal a consistent picture. BUdR appears to block spore formation by partially inhibiting several or many different earlier events during development. The relative sensitivity of development compared to growth to inhibition by the drug may be a consequence of the nature of the developmental process rather than of some unique specificity of the inhibitor.