[Platelet-derived growth factor].

Throughout growth and development,Dictyostelium cells secrete autocrine factors that accumulate in proportion to cell density. At sufficient concentration, these factors cause changes in gene expression. VegetativeDictyostelium cells continuously secrete prestarvation factor (PSF). The bacteria upon which the cells feed inhibit their response to PSF, allowing the cells to monitor their own density in relation to that of their food supply. At high PSF/bacteria ratios, which occur during late exponential growth, PSF induces the expression of several genes whose products are needed for cell aggregation. When the food supply has been depleted, PSF production declines, and a second density-sensing pathway is activated. Starving cells secrete conditioned medium factor (CMF), a glycoprotein of Mr 80 kDa that is essential for the development of differentiated cell types. Antisense mutagenesis has shown that cells lacking CMF cannot aggregate, and preliminary data suggest that CMF regulates cAMP signal transduction. Calculations indicate that a mechanism of simultaneously secreting and recognizing a signal molecule, as used byDictyostelium to monitor cell density, could also be used to determine the total number of cells in a tissue.

PSF and CMF, autocrine factors that regulate gene expression during growth and early development of Dictyostelium.

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Cellular and Molecular Mechanisms of Sexual Incompatibility in Plants and Fungi

Diffusible polypeptide pheromones (formerly referred to as mating-type factors, sex factors or gamones), which distinguish otherwise morphologically identical vegetative cell (mating) types from one another, are produced by some species of ciliates. Their most striking effect can be observed by exposing cells of one type to a pheromone secreted by another co-specific cell type. In the presence of this 'non-self' signal, these cells interrupt their vegetative life to unite temporarily in mating pairs. Thus ciliate pheromones have traditionally been associated only with mating induction. However, the identification of autocrine pheromone receptors suggests a broader role, which is supported by the hypothesis that ciliates evolved their mating-type mechanism for pursuing self-recognition. We now report studies, in the cosmopolitan marine sand-dwelling protozoan ciliate Euplotes raikovi, demonstrating that these molecules promote the vegetative reproduction (mitogenic proliferation or growth) of the same cells from which they originate. As, understandably, such autocrine pheromone activity is primary to that of targeting and inducing a foreign cell to mate (paracrine functions), this finding provides an example of how the original function of a molecule can be obscured during evolution by the acquisition of a new one.

Autocrine mitogenic activity of pheromones produced by the protozoan ciliate Euplotes raikovi

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed Comparisons have been made with both bacteria and metazoa The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena) There are two novel aspects in this review First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed These results shed light on control mechanisms and their cooperation inside the living cell Intracellular systems have many common features in unicellular and multicellular organisms

Signaling in unicellular eukaryotes.

Polypeptide hormones, recognized for their ability to regulate cell growth and differentiation, have been classified as growth factors. These growth factors have been extensively described in higher eukaryotic organisms and cell lines [Hedin and Westermark, Cell 37:9-20, 1984]. Here we report the identification and partial characterization of a putative growth factor present in vegetative amoebae of the cellular slime mold Dictyostelium discoideum. A mutant was selected and found to be temperature sensitive due to the absence of an extracellular protein suggestive of a growth factor. The putative growth factor (DGF) is a protein resistant to both heat and strong detergent treatment but sensitive to reducing agents. The physiological significance of DGF is as yet unknown. DGF is of interest both in relation to understanding the events which control cell proliferation in Dictyostelium and in its relationship to other known growth factors.

Evidence for the presence of a growth factor in Dictyostelium discoideum.

https://www.future-science.com/doi/pdf/10.2144/98251bm03

Expression vector containing an N-terminal epitope tag for Dictyostelium discoideum.

Three stage-specific cohesive systems operate in D. discoideum: VEG, elaborated by vegetative cells: AR, by aggregation competent cells; and PAR, by post aggregation stage cells. Previous study of a mutant strain JC-5 had shown the stability of its PAR system (but not the AR) to be temperature sensitive. However, the phenotypic expression of this mutation termed Coh A is complicated by the presence in that strain of a preexisting mutant gene Rde A, which accelerates developmental events generally and alters the pattern of morphogenesis. Genetic evidence presented here indicates that the two mutations have been separated by parasexual recombination yielding a Coh A, Rde A+ segregant class of which strain JC-36 is a prototype.



At the permissive temperature, JC-36 follows a morphogenetic sequence like that of the wild type in respect to timing, morphogenetic pattern, and spore appearance. At the restrictive temperature, it forms normal aggregates at the usual time but exhibits two morphogenetic aberrancies during post aggregative development. First, fruit construction is arrested at a stage approximating the 16 hr “Bottle” stage of the wild type, though more squat and blunt tipped, and then the aggregate regresses. Cytodifferentiation into spores and stalk cells is also blocked. Second, a shift of slugs migrating normally at the permissive temperature to the restrictive causes the latter to disintegrate progressively as they leave clumps of cells behind them within the flattened sheath.



JC-36 cells developing at the restrictive temperature also exhibited a decrease in EDTA resistant cohesivity attributable on two grounds to the sensitivity of the PAR system. In addition, the disappearance of the AR system completed in the wild type by the Mexicanhat (18–19 hr) stage is indefinitely arrested at an intermediate level in JC-36.

Coh A, a mutation affecting the post aggregative related (par) cohesive system of Dictyostelium discoideum

Spontaneous nystatin resistance mutations inDictyostelium discoideum fall into three complementation groups;nys A. nys B andnys C. We demonstrate three methods for rapidly distinguishing mutations in the three complementation groups. In the first methodnys B andnys C mutations are identified by their sensitivity to the sterol biosynthesis inhibitors azasterol A25822B and fenarimol respectively. The second method exploits the differential sensitivities of thenys mutations to the polyene antibiotic pimaricin. In the last method we show thatnys C and non-nys C mutants can be distinguished on the basis of the Lieberman-Burchard color reaction for sterols.

Rapid identification of non-allelic nystatin resistance mutations inDictyostelium discoideum

Publisher Summary This chapter discusses the genetic and biochemical methods for analyzing both tubulin and the function of microtubules during the Dictyostelium life cycle. A two-dimensional (2D) gel system capable of resolving single charge differences in Dictyostelium polypeptides is developed. α- and β-tubulin is identified on high-resolution 2D gels, and microtubules is isolated in cytoskeleton preparations from amoebae and the gel system is used to screen potential microtubule mutants for electrophoretically altered microtubule proteins. Microtubules from Dictyostelium amoebae are localized by indirect immunofluorescence. The structure and nature of the microtubule system are defined in both growth and development. Conditions are established for preserving of Dictyostelium microtubules in vitro . This allowed the microtubules to be isolated and analyzed biochemically. Methods for isolating mutants with potentially altered microtubule proteins are devised. Once mutants are obtained, tubulin and the microtubule system from them are examined and the mutant phenotypes are characterized. A mutant possessing aberrations in cytoplasmic and mitotic microtubule systems is described.

Eugene R. Katz

Papers

Evidence for the presence of a growth factor in Dictyostelium discoideum.

Expression vector containing an N-terminal epitope tag for Dictyostelium discoideum.

Coh A, a mutation affecting the post aggregative related (par) cohesive system of Dictyostelium discoideum

Rapid identification of non-allelic nystatin resistance mutations inDictyostelium discoideum

Chapter 14 Biochemical and Genetic Approaches to Microtubule Function in Dictyostelium discoideum