Correlates of developmental cell death in Dictyostelium discoideum
We have studied the correlates of cell death during stalk cell differentiation in Dictyostelium discoideum. Our main findings are four. (i) There is a gradual increase in the number of cells with exposed phosphatidyl serine residues, an indicator of membrane asymmetry loss and increased permeability. Only presumptive stalk cells show this change in membrane asymmetry. Cells also show an increase in cell membrane permeability under conditions of calcium-induced stalk cell differentiation in cell monolayers. (ii) There is a gradual fall in mitochondrial membrane potential during development, again restricted to the presumptive stalk cells. (iii) The fraction of cells showing caspase-3 activity increases as development proceeds and then declines in the terminally differentiated fruiting body. (iv) There is no internucleosomal cleavage of DNA, or DNA fragmentation, in D. discoideum nor is there any calcium- and magnesium-dependent endonucleolytic activity in nuclear extracts from various developmental stages. However, nuclear condensation and peripheralization does occur in stalk cells. Thus, cell death in D. discoideum shows some, but not all, features of apoptotic cell death as recognized in other multicellular systems. These findings argue against the emergence of a single mechanism of 'programmed cell death (PCD)' before multicellularity arose during evolution.