Cell density-sensing in Dictyostelium by means of the accumulation rate, diffusion coefficient and activity threshold of a protein secreted by starved cells.
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The simple eukaryote Dictyostelium discoideum grows as an amoeba on leaf and soil surfaces. When starved, the amoebae aggregate and differentiate. The amoebae can also be induced to differentiate as isolated cells submerged in buffer, if the buffer contains a sufficiently high concentration of a protein (CMF) secreted by starved cells. CMF is also necessary for aggregation and differentiation on surfaces. This indicated that CMF has either an autocrine function or is part of a density-sensing system. To distinguish between these two possible functions, we first examined the rate at which CMF is accumulated and the activity threshold of cells for CMF, since both parameters will affect whether a cell can provide enough CMF to self-stimulate. We find that CMF potentiates its own accumulation, and that otherwise the accumulation rate and activity threshold are affected very little by a variety of physiological conditions. We then use diffusion calculations to show that even after many hours of continuous secretion, the CMF concentration adjacent to an isolated starved cell on a leaf or soil surface will be too low to allow differentiation, whereas an extracellular concentration of CMF sufficiently high to allow differentiation will occur when starved cells are at high densities. We find a close match between the predicted and experimentally observed density necessary for differentiation. The theoretical and observed behavior of cells at different cell densities suggests that due to its accumulation rate, diffusion coefficient, and activation threshold, CMF can function as part of a cell density-sensing system which allows Dictyostelium cells in the wild to co-ordinate their development.
