Keith E. Lewis

Ca2+ and cell fusion during sexual development in liquid cultures of Dictyostelium discoideum☆

Abstract Cell fusion resulting in zygote giant cell formation is the first observable event of sexual development in D. discoideum . The results reported here show that this process is Ca 2+ -dependent and that by increasing the level of Ca 2+ in the medium the number of cell fusions can be increased 57-fold over control cultures. The data also suggest that Ca 2+ has both an early and late function in the development of zygotes and these functions are mediated at the cell surface. These results plus the availability of a liquid culture for generating large volumes of cells make sexual development in D. discoideum an excellent system for the analysis of membrane fusion in eukaryotes.

Phagocytosis in Dictyostelium: Nibbling, Eating and Cannibalism

ABSTRACT. Phagocytosis is a highly conserved biological process that serves numerous functions in a wide variety of organisms. Over the past few decades Dictyostelium has proven to be an excellent organism for investigations in cell biology and this is certainly no less the case for a study of phagocytosis. This review examines three distinct phagocytic activities which have been characterized in Dictyostelium. The first, “vegetative phagocytosis,” represents the classical eukaryotic microbial uptake of food particles (bacteria). The second, a predatory form of phagocytosis, arises when one species such as Dictyostelium caveatum attacks another species of slime mold, engulfing small pieces of the target prey. This has been termed “cell nibbling.” The third phagocytic process is “sexual cannibalistic phagocytosis.” In this situation a zygote giant cell, having arisen from the fusion of gametic amoebae, attracts unfused nonzygotic amoebae of the same species and engulfs them as a food source. While cell nibbling has not been actively studied, vegetative and sexual cannibalistic phagocytosis have received varying amounts of attention leading to the idea that some of the elements (e.g., glycoprotein receptors and a Gαs subunit) involved in certain of these phagocytic events may be the same. On the other hand, some unique events (e.g., filopodial induction in prey by D. caveatum) are also worthy of further investigation. Among other things, the presence of self‐nonself recognition, the existence of opsonin‐like substances and the presence of signal transduction elements (e.g., an A2‐like receptor that negatively modulates sexual phagocytosis) once considered to be extant only in higher organisms suggest that much can be learned about phagocytosis in general by further studies in the classic, eukaryotic microbe Dictyostelium discoideum and related species.

Signal transduction during cannibalistic sexual phagocytosis: Calcium is not the trigger but GTP-binding protein function is essential

After fertilization, the zygote giant cell of Dictyostelium discoideum chemoattracts and subsequently engulfs hundreds of amoebae of the same species and strains from which it was derived. A pharmacological approach indicates that, while it may have some role, calcium is not the trigger for this cannibalistic phagocytic process. Of several agents that perturb intracellular calcium levels [A23187, LaCl, 8-diethylamino-octyl-3,4,5-trimethoxylbenzoate (TMB-8), and chlorotetracycline], only A23187 had an effect in reducing amoebal ingestion. In keeping with this, agents which interfered with downstream effectors of calcium function did not alter sexual phagocytosis. Calmidazolium and trifluoperazine, which inhibit calmodulin function, were ineffective, as were a protein kinase C inhibitor (staurosporine) and activator (phorbol 12-myristate 13-acetate). On the other hand, the nucleotide analogues GTP gamma S and GDP beta S both inhibited sexual phagocytosis indicating a role for GTP-binding protein activity at some stage in the process. Sub-fractionation of cells from non-phagocytic and phagocytic stage cell cultures followed by immunolocalization after SDS-PAGE and western blotting revealed a number of GTP-binding proteins in both the cell membrane and intracellular membrane fractions that might function during the events of sexual phagocytosis.