Inhibition of multicellular development switches cell death of Dictyostelium discoideum towards mammalian-like unicellular apoptosis

The multicellular development of the single celled eukaryote Dictyostelium discoideum is induced by starvation and consists of initial aggregation of the isolated amoebae, followed by their differentiation into viable spores and dead stalk cells. These stalk cells ret ain their structural integrity inside a stalk tube that support the spores in the fruiting body. Terminal differentiation into stalk cells has been shown to share several features w ith programmed cell death (Cornillon et al. (1994), J. Cell Sci. 107, 2691- 2704). Here we report that, in the absence of aggregation and differentiati on, D. discoideum can undergo another form of programmed cell death that cl osely resembles apoptosis of most mammalian cells, involves loss of mitocho ndrial transmembrane potential, phosphatidylserine surface exposure,and eng ulfment of dying cells by neighboring D. discoideum cells. This death has b een studied by various techniques (light microscopy and scanning or transmi ssion electron microscopy, flow cytometry, DNA electrophoresis), in two dif ferent conditions inhibiting D. discoideum multicellular development. The f irst one, corresponding to an induced unicellular cell death, was obtained by starving the cells in a "conditioned" cell-free buffer; prepared by prev ious starvation of another D. discoideum cell population in potassium phosp hate buffer (pH 6.8). The second one, corresponding to death of D, discoide um after axenic growth in suspension, was obtained by keeping stationary ce lls in their culture medium. In both cases of these unicellular-specific ce ll deaths, microscopy revealed morphological features known as hallmarks of apoptosis for higher eukaryotic cells and apoptosis was further corroborat ed by now cytometry. The occurrence in D. discoideum of programmed cell dea th with two different phenotypes, depending on its multicellular or unicell ular status, is further discussed.