Cell behavior and actomyosin organization in Dictyostelium during substrate exploration.

The behavior of individual Dictyostelium amebae was quantitatively analyzed with the computer-assisted "Dynamic Morphology System" (Soll, Voss, Varnum-Finney and Wessels, (1988) J. Cell. Biochem., 37: 177-192.). The same amoebae were then fixed and analyzed for filamentous (F-) actin and myosin (myosin-II, or "conventional" myosin) by fluorescence microscopy using the "agar-overlay method" (Yumura, and Fukui (1985) Nature, 314: 194-196.). This procedure provides a novel description of the behavior and morphometric changes preceding the static analysis of cytoskeletal organization in the same cell. It is demonstrated that when translocating cells make contact with an etched-smooth glass interface, 14% cross the interface, 20% either reverse direction or migrate along the interface, and the remaining 45% stay at the site. Cells contacting the interface from the smooth or etched side show equivalent behavioral responses. Upon contact with the interface, they project numerous lamellipodia and pseudopodia. While the lamellipodial projections exhibit cycles of spreading and retraction, the pseudopodia show lateral scanning motion, analogous to "substrate exploration" in fibroblasts (Albrecht-Buehler (1976) J. Cell Biol., 69: 275-286.). F-actin is localized in the lamellipodia and pseudopodia of amoebae contacting the interface. There is also discernable cortical F-actin, while conventional myosin appears to be excluded from the cortex and dispersed throughout the cytoplasm. The myosin displays a transient filamentous lattice at the base of newly forming lamellipodia. The ultrastructural study suggests that the new lamellipodia are formed on the dorsal surface and subsequently make contact with the substrate, indicating the dorsoventral sequence of polarity of the motile/sensory cellular organs. The present study demonstrates substrate exploration in Dictyostelium amoebae, and suggests its coupling to dynamic reorganization of the actomyosin cytoskeleton. The possible role of single-headed small myosin(s) (myosin-I, or mini-myosin) is discussed.