Defective cell cohesivity expressed late in the development of a Dictyostelium discoideum mutant

Specific cell-cell contacts are essential for induction of gene expression during differentiation of Dictyostelium discoideum

Postaggregation Dictyostelium discoideum cells contain 2000-3000 mRNA species that are absent from pre-aggregation cells. These aggregation-dependent sequences compose 30% of the mass of the late mRNA and represent the transcription products of an additional 11% of the single-copy genome. By analysis of mutants that are blocked at different stages of differentiation, we show that induction of expression of these genes is correlated with the formation of tight cell-cell contacts that resist EDTA. In particular, mutants that exhibit chemotaxis and aggregate to form loose mounds but do not form cell-cell contacts that resist EDTA fail to induce these late mRNA and protein species. By contrast, mutants that form normal contacts but progress no further through development do express the late mRNA species. Thus, interactions at the cell surface are involved in developmental induction of a large group of coregulated mRNAs. We have employed two independent assays for these developmentally regulated mRNAs: hybridization of gel-separated RNAs to cloned nuclear DNAs and hybridization of mRNA to a cDNA probe specific for the population of 2000-3000 regulated sequences.

Aggregation-related and post-aggregation-related cohesive systems in Dictyostelium discoideum involve stage-specific, Ca2+-dependent, di- or polymeric associations of membrane-bound moieties

The cells of D. discoideum acquire developmentally regulated cohesive properties during aggregation and fruiting body construction. On the bases of genetic, serological, and physiological evidence, it has been suggested that two distinct cohesive systems operate: an aggregation-related (AR) system that facilitates the formation of multicellular aggregates and post-aggregation-related (PAR) system that maintains the integrity of the aggregate thereafter. We had previously demonstrated that ghosts and membrane fragments retain the cohesive properties of the cell from which they were derived. Here, we describe a two-phase assay involving the Ca2+-dependent binding of 125I-labeled cell ghosts in suspension to their unlabeled counterparts immobilized on plastic surfaces. Using this assay we show that the ghosts of newly aggregation-competent (8 h) cells and of cells from the 'Mexican hat (18 h) stage' of fruit construction can bind, each to its immobilized counterpart, but not heterologously. Furthermore, neither binds to the immobilized ghosts of vegetative cells. This provides direct, functional evidence demonstrating the existence of the two stage-specific cohesive systems. It also suggests that both cohesive acts involve at least dimeric associations of molecules or molecular complexes located within or on juxtaposed membranes. Using immobilized 8 and 18 h ghosts, the specific binding activities of ghosts prepared from cells harvested at stages throughout the morphogenetic sequence were assayed in order to describe the developmental kinetics of the two cohesive systems. The binding data suggest that the AR system appears soon after the start of the morphogenetic sequence, peaks during early aggregation and is progressively diminished thereafter. The PAR system makes its appearance after aggregation and accumulates thereafter. Both systems are present in migrating slugs.

Analysis of gene expression in rapidly developing mutants of Dictyostelium discoideum

Developmentally regulated gene expression has been analyzed in the wild-type D. discoideum strain NC-4 and a series of temporally deranged mutants. The mutants include representatives from each class of rapid development mutation, Fr17(rdeA-) and HT506(rdeC-), and strain HIfm-1, which appears to be defective in the timing of events early in development. We have monitored four prespore-specific genes, three of which show coordinate expression in the wild type. The coordination is maintained in each of the mutant strains though the specific expression pattern varied from strain to strain. Likewise, a series of prestalk-specific genes have been analyzed. They also show coordinated expression in the wild type and in all of the mutants. The timing of expression, however, is different between the prestalk-specific and the prespore-specific with the overall pattern of expression being unique for each strain examined. These results confirm our previous suggestion that the major classes of prestalk- and prespore-specific genes are coordinately regulated and show that a great deal of tolerance is allowed in the timing of specific gene expression as it relates to terminal differentiation. In addition we have analyzed the expression of actin, discoidin I, and I42. These genes, or gene families, are preferentially expressed in either vegetatively growing cells or in cells during the early stages of development. As with the cell-type-specific genes, the pattern of expression of the three early gene classes is unique for each strain examined.

Regulation of late gene expression in a temperature-sensitive cohesion-defective mutant of Dictyostelium discoideum

We have analyzed the expression of a series of developmentally regulated genes in the Dictyostelium discoideum strain JC-5. This strain has been previously described as a temperature-sensitive, cohesion-defective derivative of FR17, itself a temporally deranged mutant of wild-type NC-4. At restrictive temperature (27 degrees C), JC-5 initially acquires EDTA-resistant cell contacts but at the time of tip formation (12 hr) loses the ability to make specific cell-cell associations and regresses to an amorphous mound of cells. WE have found that genes preferentially expressed in either prespore or prestalk cells are expressed prior to the appearance of the cohesion defect in JC-5; the specific cell contact system defective in this strain is necessary for neither the proper initiation nor maintenance of expression of either prespore of prestalk genes. We have also found, by use of an in vitro cell suspension system, that JC-5 is temperature-sensitive with respect to gene expression several hours before the defect in cell cohesion is observable. Our data suggest that the defect in JC-5 is due to a specific lesion not in the late cohesion system but rather in a more general component that is required earlier in the developmental process.

Cell surface carbohydrates and cell recognition in Dictyostelium

Carbohydrate ligands and complementary receptors have been detected on the surface of Dictyostelium cells, using lectins, monoclonal antibodies, and immobilized sugar probes. They have been implicated in cell recognition processes, such as phagocytosis and intercellular adhesion, and could act as membrane signals for differentiation. Specific glycoproteins have been proposed to mediate intercellular adhesion in Dictyostelium discoideum and Polysphondylium pallidum and to account for the species-specificity of adhesion displayed by these species. Recent studies with the inhibitor of N-glycosylation, tunicamycin, and with glycosylation defective mutants suggest that some carbohydrate groups in these glycoproteins play a role in cell adhesion.

Stage-specific cohesion of cell ghosts and plasma-membrane fragments from Dictyostelium discoideum

During fruiting-body construction by Dictyostelium discoideum, the formation and subsequent maintenance of the multicellular assembly involve two stage-specific cohesive systems that are acquired sequentially and are distinguishable on serological and genetic grounds. We demonstrated that both systems, termed aggregation related (AR) and postaggregation related (PAR), can function in vitro. Ghosts prepared from cells of the wild-type and of a cohesion-defective mutant that were harvested during growth and at aggregation and postaggregative stages of fruiting-body construction exhibited the same cohesive properties as the cells from which they were derived. Membrane fragments prepared from the ghosts by mechanical disruption retained these cohesive properties.

Reversible inhibition of aggregation-related cohesivity in Dictyostelium discoideum by diffusible metabolites

Evidence presented elsewhere (G.B. Williams, E.M. Elder, and M. Sussman 1984, Dev. Biol. 105, 377-388) indicates that NH3 and certain carboxylic acids including propionate, succinate, and acetate modulate the cAMP relay in Dictyostelium discoideum. The former appears to act as a cAMP accumulation inhibitor, the latter as cAMP release inhibitors. The cohesive properties of aggregation competent cells have been assayed quantitatively in the presence of these modulators. The following results were obtained: (1) At pH 7.5, EDTA-resistant cohesivity was greatly inhibited by NH4C within the concentration range tested (30-3.8 mM). Even at the higher concentrations the effect was not immediate but required ca. 10 min for full expression. At the lower concentrations, the inhibitory level was only slightly reduced but the time for full expression progressively increased. At pH 6.5, the level of inhibition was marginal, indicating that NH3 is the active molecular species. By themselves, neither ambient pH nor ionic strength appeared to affect cohesive performance within the ranges employed. The inhibition was immediately and completely reversed upon removal of NH4Cl or a shift of ambient pH from 7.5 to 6.5. The presence of cycloheximide did not affect the recovery of cohesivity after NH4Cl removal. (2) The presence of 15 mM succinate, propionate, or acetate also reduced cell cohesivity. The timing and extent of the inhibition were identical at pH 7.5 and 6.5. The inhibition was expressed immediately and was reversible. Each of the acids acted synergistically with NH4Cl. The relative potencies of these metabolites acting singly or in combination as inhibitors of cohesivity corresponded roughly to their potencies as modulators of the cAMP relay (Williams et al., 1984). (3) The sensitivity to the metabolites was stage specific, being maximal during and shortly after aggregation and disappearing abruptly at 11-12 hr. This corresponds to the time at which this cohesive system, responsible for the end-to-end cell associations evident during aggregation (H. Beug, G. Gerisch, S. Kempff, V. Riedel, and G. Cremer, 1970, Exp. Cell. Res. 63, 147-158) is supplanted by a newly arisen, serologically and genetically distinct system which thereafter maintains the integrity of the aggregate (C. Steinemann and R.W. Parish, 1980, Nature (London) 286, 721-724; D.K. Wilcox and M. Sussman, 1981, Dev. Biol. 82, 102-112, and Proc. Natl. Acad. Sci. USA 78, 358-362; C.L. Saxe III and M. Sussman, 1982, Cell 29, 755-759). The activities of the metabolites, detailed above, are discussed in relation to their previously demonstrated activities as morphogens.

Monoclonal antibodies: use to detect developmentally regulated antigens on D. discoideum amebae

We have used monoclonal antibodies to detect developmentally regulated cell surface antigens on D. discoideum amebae. A study of an antigen detected using an antibody produced by a hybridoma line implicates a previously undescribed component in the process of cell aggregation. This antigen (consisting of a doublet of 69,000 and 73,000 molecular weight) is first detected during the early hours of cell starvation and is present until cells begin slug formation. The developmental appearance of the antigen is not controlled by cAMP pulses and is distinct from that of Contact A sites. Fab fragments directed against the antigen are potent inhibitors of aggregation but do not inhibit the differentiation of cells to aggregation competence.