Shear flow-induced motility of Dictyostelium discoideum cells on solid substrate.

Application of a mild hydrodynamic shear stress to Dicytostelium discoideum cells, unable to detach cells passively from the substrate, triggers a cellular response consisting of steady membrane peeling at the rear edge of the cell and periodic cell contact extensions at its front edge. Both processes require an active actin cytoskeleton. The cell movement induced by the hydrodynamic forces is very similar to amoeboid cell motion during chemotaxis, as for its kinematic parameters and for the involvement of phosphatidylinositol(3,4,5)-trisphosphate internal gradient to maintain cell polarity. Inhibition of phosphoinositide 3-kinases by LY294002 randomizes the orientation of cell movement with respect to the flow without modifying cell speed. Two independent signaling pathways are, therefore, induced in D. discoideum in response to external forces. The first increases the frequency of pseudopodium extension, whereas the second redirects the actin cytoskeleton polymerization machinery to the edge opposite to the stressed side of the cell.

Peeling process in living cell movement under shear flow.

We present a direct optical observation of the behavior of the contact area between a living cell (Dictyostelium discoideum) and a solid substrate under shear flow. It is shown that the membrane is peeled off the substrate. The relationship between the peeling velocity and the applied force is obtained experimentally and explained from the behavior of individual adhesion bridges. The dissipation occurring during the peeling process is explicitly calculated in terms of out-of-equilibrium thermodynamics.

Shear flow-induced detachment kinetics of Dictyostelium discoideum cells from solid substrate.

Using Dictyostelium discoideum as a model organism of specific and nonspecific adhesion, we studied the kinetics of shear flow-induced cell detachment. For a given cell, detachment occurs for values of the applied hydrodynamic stress above a threshold. Cells are removed from the substrate with an apparent first-order rate constant that strongly depends on the applied stress. The threshold stress depends on cell size and physicochemical properties of the substrate, but is not affected by depolymerization of the actin and tubulin cytoskeleton. In contrast, the kinetics of cell detachment is almost independent of cell size, but is strongly affected by a modification of the substrate and the presence of an intact actin cytoskeleton. These results are interpreted in the framework of a peeling model. The threshold stress and the cell-detachment rate measure the local equilibrium energy and the dissociation rate constant of the adhesion bridges, respectively.

Dictyostelium discoideum adhesion and motility under shear flow: experimental and theoretical approaches.

Among the different assays to measure cell adhesion, shear-flow detachment chambers offer the advantage to study both passive and active aspects of the phenomena on large cell numbers. Mathematical modeling allows full exploitation of the data by relating molecular parameters to cell mechanics. Using D. discoideum as a model system, we explain how cell detachment kinetics gives access to the rate constants describing the passive association or dissociation of the cell membrane to a given substrate.