Analysis of optical density wave propagation and cell movement during mound formation in Dictyostelium discoideum

Aggregation fields of Dictyostelium amoebae are organized by propagating concentric or spiral waves of cAMP. These waves coordinate cell movement directed toward the aggregation center. We now systematically investigated dark-field wave propagation and chemotactic cell movement during late aggregation and mound formation. The period and the signal propagation velocity decreased continuously during aggregation leading to a 15-fold decrease of the chemical wavelength. By analyzing the behavior of single GFP-labeled cells in aggregates and mounds we measured cell movement velocity, changes in cell shape, periodicity of cell movement, and cell trajectories. In early mounds of strain AX-3 dark-field waves propagated frequently as multiarmed (high-frequency) spirals. During the high-frequency waves observed in the early mound stage, cell movement speed is low and cell movement rather undirected. During tip formation the wave period decreased again and the cells started to rotate in the mound at unusually high average speeds of 40 microns/min. The rotation was almost monotonic with no clear periodicity. Since at this time the majority of the cells had already differentiated into prespore cells, this implies that prespore cells moved faster than aggregation stage cells. At 12 hr of development cell movement velocity dropped again and became highly periodic. These measurements show that the relay system is characterized by a specific temporal evolution, which is closely correlated with cellular differentiation. The remarkable changes in cell movement speed and period indicate a qualitative change in signal and movement parameters which might well be caused by the observed switch from high- to low-affinity cAMP receptors during mound formation. This switch might be required to copy with the increase in cell density and most likely plays a crucial role in the process of cell sorting.