8 - Chemotaxis and Aggregation

Summary

Free-living Dictyostelium amoebae must be able to find their way toward prey or, in the face of starvation, toward each other. Dictyostelium amoebae share a chemotactic capacity with leukocytes and many other motile cells, and employ many of the same mechanisms during the detection of the chemotactic molecule, the activation of signal transduction pathways, and the mobilization of the cytoskeleton (Devreotes and Zigmond, 1988; Parent et al., 1998; Parent and Devreotes, 1999). Chemotactic molecules bind to cell-surface receptors and stimulate G protein-mediated signal transduction pathways in amoebae and in mammalian cells. Agonists are degraded to steepen gradients and to overcome the effects of adaptation. Despite evolutionary distance, the cytoskeletons of leukocytes and Dictyostelium employ similar cytoskeletal rearrangements to move in the right direction. The advantage of Dictyostelium in the study of chemotaxis, motility and aggregation is that the gene products involved in each event can be eliminated by mutation, and the contribution of each element can be studied. The biochemical advantages that stem from synchrony of development and quantities of material have also been invaluable. This is not to say that we understand completely how a cell perceives that a gradient exists or how it moves toward higher concentrations of cAMP, but we are beginning to understand how this complex process works.