The role of cyclic nucleotides and cell agglomeration in postaggregative enzyme synthesis in Dictyostelium discoideum

Phosphorylation of spore coat proteins during development of Dictyostelium discoideum

Immunological evidence is presented which confirms that pp95, one of the major phosphoproteins accumulated in development of the cellular slime mould Dictyostelium discoideum, is identical with spore coat protein SP13. The site of phosphorylation is identified as phosphoserine. The second major phosphorylated component, pp74, corresponds to two co-migrating spore coat proteins known collectively as SP74.

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.

Transcriptional control of gene expression during development of Dictyostelium discoideum

During development of the cellular slime mold Dictyostelium discoideum, approximately 2,000 to 3,000 regulated mRNAs are induced when amoebae enter multicellular aggregates. We used in vitro transcription in isolated nuclei to follow the synthesis of individual mRNA precursors during development; these were quantitated by hybridization to cloned cDNAs or genomic DNAs. Those RNAs that are present at all stages of development--the common RNAs--were transcribed by nuclei from cells at all stages of development. By contrast, those RNAs that are present only after cells begin to aggregate--here called aggregation stage RNAs--were transcribed only by nuclei from cells at the aggregation and postaggregation stages of development. The temporal pattern of in vitro transcription correlated well with the time course of accumulation of different aggregation stage mRNAs. Continued expression of aggregation stage genes normally depends upon cell-to-cell contact or cyclic AMP (cAMP); when cells are disaggregated, the regulated mRNAs are rapidly and specifically degraded. When cAMP is subsequently added to the disaggregated cells, most of the mRNAs reaccumulate. We show here that disaggregation reduced 2- to 10-fold the relative transcription of several aggregation stage RNAs, whereas addition of cAMP to disaggregated cells reinduced the level of regulated gene transcription to values approximating those found in normal postaggregation cells. These results indicate that a representative set of Dictyostelium aggregation stage genes are under transcriptional control; both the transcription and the stability of these mRNAs require either continued cell-to-cell interactions or cAMP.

A new class of rapidly developing mutants in Dictyostelium discoideum: implications for cyclic AMP metabolism and cell differentiation

Rapidly developing (rde) mutants of Dictyostelium discoideum, in which cells precociously differentiated into stalk and spore cells without normal morphogenesis, were investigated genetically and biochemically. Genetic complementation tests demonstrated that the 16 rde mutants isolated could be classified into at least two groups (groups A and C) and that the first described rde mutant FR17 (D. R. Sonneborn, G. J. White, and M. Sussman, 1963, Dev. Biol. 7, 79-93) belongs to group A. Morphological studies revealed several differences in development and final morphology between group A and group C mutants. In group A mutants, the time required for cell differentiation from vegetative cells to aggregation competent cells is reduced, whereas the time required for spore and stalk cell differentiation following the completion of aggregation is shortened in group C mutants. This suggests that group C mutants represent a new class of rde mutants and that there exist at least two mechanisms involved in regulating the timing of development in D. discoideum. Measurements of cell-associated and extracellular phosphodiesterase activities, and intracellular and total cAMP levels revealed that cAMP metabolism in both groups is significantly altered during development. Group A mutants showed precocious and excessive production of phosphodiesterase and cAMP during the entire course of development; intracellular cAMP levels in group C mutants were extremely low, and spore and stalk cell differentiation occurred without an apparent increase in these levels. Thus, while cAMP metabolism is abnormal in all the rde mutants studied, there exist several distinct types of derangement, not necessarily involving the overproduction of cAMP.

Transcriptional control of gene expression during development of Dictyostelium discoideum

During development of the cellular slime mold Dictyostelium discoideum, approximately 2,000 to 3,000 regulated mRNAs are induced when amoebae enter multicellular aggregates. We used in vitro transcription in isolated nuclei to follow the synthesis of individual mRNA precursors during development; these were quantitated by hybridization to cloned cDNAs or genomic DNAs. Those RNAs that are present at all stages of development--the common RNAs--were transcribed by nuclei from cells at all stages of development. By contrast, those RNAs that are present only after cells begin to aggregate--here called aggregation stage RNAs--were transcribed only by nuclei from cells at the aggregation and postaggregation stages of development. The temporal pattern of in vitro transcription correlated well with the time course of accumulation of different aggregation stage mRNAs. Continued expression of aggregation stage genes normally depends upon cell-to-cell contact or cyclic AMP (cAMP); when cells are disaggregated, the regulated mRNAs are rapidly and specifically degraded. When cAMP is subsequently added to the disaggregated cells, most of the mRNAs reaccumulate. We show here that disaggregation reduced 2- to 10-fold the relative transcription of several aggregation stage RNAs, whereas addition of cAMP to disaggregated cells reinduced the level of regulated gene transcription to values approximating those found in normal postaggregation cells. These results indicate that a representative set of Dictyostelium aggregation stage genes are under transcriptional control; both the transcription and the stability of these mRNAs require either continued cell-to-cell interactions or cAMP.

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.

Membrane sites regulating developmental gene expression in Dictyostelium discoideum

Postaggregative gene expression in Dictyostelium discoideum requires cell contact. Polyspecific monovalent antibodies (Fab) prepared from sera raised against membranes of aggregation- and postaggregation-stage cells were used to probe the cell interactions that induce rapid postaggregative synthesis of UDP-glucose pyrophosphorylase. When cells of strain V12M2 were dissociated after 8 hr of development and replated in the presence of immune Fab, both reaggregation and pyrophosphorylase synthesis were blocked. Fab neutralized by incubation with EDTA-high salt extracts of cells developed for 3 hr blocked pyrophosphorylase synthesis but not reaggregation. Therefore, some cell-surface components that regulate pyrophosphorylase synthesis (called E sites) are antigenically distinct from those required for reaggregation. The Fab provides a means to assay E sites during their purification. Addition of 10(-3) M cyclic AMP or cyclic GMP enabled the cells to bypass the blocking of E sites by Fab; pyrophosphorylase was synthesized in the absence of reaggregation. We hypothesize that E sites function by raising the level of intracellular cyclic AMP.

Synthesis and stability of developmentally regulated dictyostelium mRNAs are affected by cell--cell contact and cAMP

Postaggregation Dictyostelium discoideum cells contain 3000 mRNA species that are absent from preaggregation cells; these aggregation-dependent sequences comprise 30% of the mass of mRNA in these cells. We show that the synthesis and stability of these regulated mRNA sequences are affected by both cell--cell contact and cAMP. Three independent assays are used to quantitate these mRNAs: in vitro translation followed by two-dimensional gel analysis of the protein products; hybridization of gel-separated RNAs to cloned DNAs; and hybridization of mRNA to a cDNA probe specific for the population of regulated sequences. In postaggregation cells, the half-life of both the developmentally regulated mRNAs and the constitutive mRNAs present throughout growth and differentiation is the same--about 4 hr. Following disaggregation, all of the late mRNA sequences are degraded and decay with a half-life of 25 to 45 min. The constitutive species are unaffected; 2.5 hr after disaggregation, the ratio of late to constitutive mRNAs is about 6% that of normal plated cells. Addition of cAMP to cells that have been disaggregated for 2.5 hr (or longer) restores the level of most late mRNAs within 3 hr. We conclude that cAMP stimulates the synthesis of these mRNAs and may also act to stabilize them in the cytoplasm. This effect of cAMP is dependent on the cells having been in contact with other cells; cAMP has no effect on the levels of mRNA in suspension-starved, aggregation-competent cells that have never formed cell--cell aggregates.

Developmental regulation of glycoprotein biosynthesis in Dictyostelium

We have examined the glycoprotein-linked oligosaccharides assembled during the life cycle of Dictyostelium discoideum, and found their expression to be dramatically dependent upon the stage of development. During early development mature glycans have a high mannose character, and a substantial proportion acquire a fucose residue that correlates with endo-H resistance. One-third of the glycans also acquire sulfate residues. These glycans diminish in importance during aggregation. The mature glycans expressed during aggregation. The mature glycans expressed during late development contain fewer mannose residues, from five to ten mannose residues, and are characterized by the absence of sulfate residues and by the presence of fucose residues on endo-H-sensitive glycans. These glycans make their appearance coincident with the construction of tips on tight cell mounds. At this stage glycans characteristic of both early and late stages occur simultaneously. Developmental regulation of the wide array of protein-linked glycans expressed during the life cycle of Dictyostelium discoideum may be as simple as the controlled transition from a group of structures that are assembled by the vegetative cells to a group of structures that are assembled by the terminally differentiating cells. The potential biological significance of this transition is discussed.

A change in the rate of transcription of a eukaryotic gene in response to cyclic AMP

The plasmid pDd812 contains the DNA copy of an mRNA sequence from Dictyostelium discoideum that undergoes first an increase and then a decrease in concentration during the first few hours of differentiation. We have recently shown that the mRNA sequence complementary to pDd812 encodes discoidin I, a developmentally regulated lectin that may play a role in cellular cohesion. By using pDd812 as a hybridization probe, we found that addition of cyclic AMP during the first few hours of development inhibited the accumulation of discoidin I mRNA. By measuring the rate of transcription in isolated nuclei, we showed that, at least in part, this inhibition results from a rapid and specific reduction in the rate of transcription of the discoidin I gene. Addition of cyclic AMP during the first few hours of development inhibited the accumulation of discoidin I mRNA. By measuring the rate of transcription in isolated nuclei, we showed that, at least in part, this inhibition results from a rapid and specific reduction in the rate of transcription of the discoidin I gene. Addition of cyclic AMP during the first few hours of development inhibited the accumulation of discoidin I mRNA. By measuring the rate of transcription in isolated nuclei, we showed that, at least in part, this inhibition results from a rapid and specific reduction in the rate of transcription of the discoidin I gene. Addition of high external concentrations of cAMP is known to increase the intracellular concentration to a level normally found later in development. This natural increase in cAMP concentration occurs at the time during development when transcription of the discoidin I gene ceases. We suggest, therefore, that changes in the intracellular concentration of cAMP act at the level of transcription to control gene expression during development. This hypothesis is supported by our observation that several poly(A)+RNA sequences that normally accumulate after transcription of the discoidin I gene has ceased are synthesized prematurely in cells exposed to exogenous cAMP.

Control of developmental transitions in the cyclic AMP signalling system of Dictyostelium discoideum

In the first few hours after starvation, the developing cAMP secretory system in Dictyostelium discoideum has been observed to be successively in one of four states: (a) quiescent, (b) excitable (capable of relay), (c) autonomously oscillating, and (d) secreting at a high steady level. A theoretical model is presented which demonstrates that the proximal cause of the transitions between different types of behavior may be slow changes in the activities of the enzymes adenylate cyclase and phosphodiesterase. These changes affect the stability properties of the steady state admitted by the cAMP signalling system. Sustained oscillations develop when the steady state is unstable, whereas relay of cAMP signals occurs upon perturbation of a stable steady state for parameter values close to those which produce oscillations. The developmental path suggested in the adenylate cyclase-phosphodiesterase space for the sequential transitions compares with the time course observed for the synthesis of these enzymes after starvation. It is suggested that there is general significance for the understanding of differentiation in the example given of a state-point following a developmental path in parameter space, moving from one behavioral domain to another, and thereby bringing about shifts in qualitative behavior.

A role for cyclic AMP in expression of developmentally regulated genes in Dictyostelium discoideum

Starved cells of Dictyostelium discoideum begin to synthesize a new class of developmentally regulated proteins at about 13 hr of the 24-hr developmental program, concomitant with the formation of tips on the tight cell aggregates [Alton, T. H. & Lodish, H. F. (1977) Dev. Biol. 60, 180--206]. Continued synthesis of these proteins is normally dependent upon the integrity of the multicellular aggregates, because cells that have been disaggregated at 13 hr and shaken in suspension for 5 hr do not make these proteins. We show here that addition of 20 microM cyclic AMP to suspension cultures of disaggregated 13-hr cells caused synthesis of most of these late proteins to be maintained. Translation in an in vitro wheat germ system of total cellular RNA isolated from these cyclic AMP-stimulated suspension cells, or from normal aggregates, generated several proteins that were not encoded by the RNA isolated from equivalent suspension cells which had not been treated with cyclic AMP or from preaggregation cells. We conclude that cyclic AMP has a direct role in maintaining the synthesis of aggregation-dependent Dictyostelium proteins and in maintaining the level of the corresponding mRNAs.

Cyclic AMP induces the synthesis of developmentally regulated plasma membrane proteins in Dictyostelium

Dictyostelium discoideum slugs (pseudoplasmodia) were disaggregated and the resynthesis of developmentally regulated plasma membrane proteins examined. The synthesis of the majority of these proteins was inhibited when cells were overlaid with Cellophane and maintained as a monolayer. However, cell contact and movement did occur under the Cellophane. The inhibition of differentiation may result from the inability of the cells to organize specifically into multicellular aggregates. The addition of cyclic AMP (1--5 mM) induced the synthesis of certain developmentally regulated plasma membrane proteins in cells overlaid with Cellophane. Hence, this confirms other work showing that cyclic AMP is required for at least some post-aggregative gene expression. Specific cell organisation and interactions are apparently required for an increase in or maintenance of intracellular cyclic AMP levels.