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

Specific cell-cell contact serves as the developmental signal to deactivate discoidin I gene expression in Dictyostelium discoideum

Specific cell-cell contact is a major regulatory signal controlling cell differentiation in Dictyostelium discoideum, causing dramatic changes in the developmental program of gene expression. In this report, we focus on the relationships between specific cell-cell contact and the activity of the genes for discoidin I, an endogenous lectin that has been implicated in the cell-cell cohesion process. By performing quantitative RNA dot-hybridization assays and RNA gel blot-hybridization analyses, using as a probe a recombinant plasmid containing a discoidin I cDNA insert, we have measured changes in discoiding I mRNA levels during normal development and in response to specific manipulations of the state of cellular aggregation. Our major findings are as follows. (i) During normal development on filters, there is a close temporal correspondence between the establishment of specific cell-cell contacts and the decline in discoidin I mRNA levels. By the tight-aggregate stage, discoidin I mRNA is barely detectable. (ii) When tight aggregates are disaggregated and the cells are maintained in the disaggregated state, there is a dramatic rise in discoidin I mRNA content. (iii) When cells are developed in suspension (conditions that interfere with the establishment of tight cell-cell contacts), discoidin I mRNA accumulates to abnormally high levels, and these persist well after the levels in filter-developed cells have declined. Taken together, these results strongly suggest that cell-cell contact is the normal developmental signal to deactivate discoidin I gene expression; thus, a contact-deactivated gene for which a recombinant DNA probe is available has now been identified. Furthermore, we demonstrate that exogenous cAMP almost completely blocks the disaggregation-induced reactivation of discoidin I gene expression. Possible mechanistic relationships between specific cell-cell contact, intracellular cAMP levels, and developmental gene expression are discussed.

Ethanolamine base exchange enzymatic activities in spontaneous transformed glial cell lines. Effect of dibutyryl cyclic AMP treatment

Cultured astrocytes derived from neonatal rats (normal cells) displayed maximal ethanolamine base exchange enzymatic activity (EBEE) when cultures reached confluency and cells almost ceased to divide. At this stage, ethanolamine phosphotransferase (EPT) and choline base exchange enzyme (CBEE) activities reached a plateau. In spontaneously transformed glial cells, no differential activity variation either between EPT and CBEE, or between EPT and EBEE was observed. The EBEE activity was mainly localized in the microsomal fraction and was completely absent from plasma membranes. Dibutyryl cyclic AMP (db-cAMP) treatment of the transformed cells reversed the pattern of these activities to that of normal cells. Moreover, treatment of the transformed cells with medium conditioned by normal astroblasts markedly increased EBEE activity. This study demonstrates that (i) variation of EBEE activity during cell growth differs in normal and in transformed cultured glial cells. (ii) EBEE activity may be modulated via both db-cAMP and normal cell conditioned medium. Our findings suggest a possible implication of EBEE in the maturation and contact inhibition of cell growth.

Role of cyclic AMP and ammonia in induction and maintenance of post-aggregative differentiation in a suspension culture of Dictyostelium discoideum

The effects of ammonia and cAMP on prespore and prestalk differentiation of Dictyostelium discoideum were investigated by monitoring eight developmentally regulated proteins as differentiation markers under the shake culture conditions in glucose/albumin medium. In the medium containing cAMP, cells form small agglomerates and undergo prespore differentiation [19]. Under the conditions where agglomeration was prevented, ammonia induced four marker proteins out of eight tested in the presence of cAMP, which included not only a prespore specific enzyme but also cell-type non-specific proteins. No inhibitory effect of ammonia was observed in presumptive cell differentiation. These results suggest that ammonia is an inducer of differentiation at the protein level as well as the mRNA level as found previously [24]. The effects of cAMP were examined with special attention to the difference between induction of differentiation and maintenance of differentiated state in this specific medium. The induction of differentiation from early aggregative cells was cAMP-dependent with all the marker proteins tested. This agrees with the observations so far obtained in other culture systems. However, when already differentiated cell masses (slugs) were dissociated and shaken in this specific medium, only two enzymes required cAMP to maintain the activity while five out of eight kinds of the proteins continued to be expressed as in undisturbed slugs even without cAMP. This suggests that for the maintenance of the differentiated state after slug disaggregation cAMP may not be required with respect to the majority of proteins, if cells are provided with some favorable conditions such as glucose/albumin medium.

Modulation of the cAMP relay in Dictyostelium discoideum by ammonia and other metabolites: possible morphogenetic consequences

Using a perfusion technique (P.N. Devreotes, P.L. Derstine, and T.L. Steck, 1979, J. Cell Biol. 80, 291-299), it has been shown that cAMP secretion by aggregation-competent cells in response to an exogenous cAMP signal is significantly reduced by exposure to NH4Cl or any of a set of carboxylic acids that includes propionate, succinate, pyruvate, and acetate. The effects of NH4Cl and any of the carboxylic acids are additive and the combinations restrict cAMP secretion to barely detectable or insignificant levels. The inhibitions are rapidly expressed, and are reversible. The activity of NH4Cl is marked at pH 7.2 and undetectable at pH 6.2. Hence, NH3 is presumably the active molecular species. Propionate activity is significantly greater at pH 6.2 than 7.2, indicating that the un-ionized acid is the active species. The data presented herein indicate that these effects are exerted via two separate and independent routes. During exposure of cAMP-stimulated cells to NH4Cl, the decrease in intracellular cAMP accumulation was even greater than the decrease in extracellular accumulation. Hence, NH3 appears to act as a cAMP accumulation inhibitor (CAI). In contrast, exposure to carboxylic acid concentrations that drastically reduce extracellular cAMP accumulation can actually enhance or, at worst, only slightly reduce intracellular accumulation. Hence, the carboxylic acids appear to act as cAMP release inhibitors (CRI). Stationary phase cells incubated on solid substratum in the presence of NH4Cl plus succinate (or propionate) for 18 hr failed to exhibit even the earliest signs of aggregation. If then harvested and redeposited in the absence of the metabolites, they proceeded through the morphogenetic sequence with approximately normal kinetics, suggesting that no significant morphogenetic competence had been achieved during their previous tenure. The morphogenetic implications of cAMP relay modulation are discussed.

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.