SummaryWe have demonstrated that the two-and three-dimensional motility of the human pathogenic parasite Entamoeba histolytica (Eh) depends on sustained instability of the intracellular hydrostatic pressure. This instability drives the cyclic generation and healing of membrane blebs, with typical protrusion velocities of 10-20 m/second over a few hundred milliseconds and healing times of 10 seconds. The use of a novel micro-electroporation method to control the intracellular pressure enabled us to develop a qualitative model with three parameters: the rate of the myosin-driven internal pressure increase; the critical disjunction stress of membrane-cytoskeleton bonds; and the turnover time of the F-actin cortex. Although blebs occur randomly in space and irregularly time, they can be forced to occur with a defined periodicity in confined geometries, thus confirming our model. Given the highly efficient bleb-based motility of Eh in vitro and in vivo, Eh cells represent a unique model for studying the physical and biological aspects of amoeboid versus mesenchymal motility in two-and three-dimensional environments.

Bleb-based cell motility proceeds by the successive inflation and retraction of large spherical membrane protrusions ("blebs") coupled with substrate adhesion. In addition to their role in motility, cellular blebs constitute a remarkable illustration of the dynamical interactions between the cytoskeletal cortex and the plasma membrane. Here we study the bleb-based motions of Entamoeba histolytica in the constrained geometry of a micropipette. We construct a generic theoretical model that combines the polymerization of an actin cortex underneath the plasma membrane with the myosin-generated contractile stress in the cortex and the stress-induced failure of membrane-cortex adhesion. One major parameter dictating the cell response to micropipette suction is the stationary cortex thickness, controlled by actin polymerization and depolymerization. The other relevant physical parameters can be combined into two characteristic cortex thicknesses for which the myosin stress (i) balances the suction pressure and (ii) provokes membrane-cortex unbinding. We propose a general phase diagram for cell motions inside a micropipette by comparing these three thicknesses. In particular, we theoretically predict and experimentally verify the existence of saltatory and oscillatory motions for a well-defined range of micropipette suction pressures.cell motility | contractility | cytoskeleton T he dynamical properties of the actin cytoskeleton (CSK) and its interaction with the plasma membrane (PM) control crucial aspects of cellular shape change and motility. Actin-based motility can result from the polarization of the cell, with actin polymerization at the leading edge, in a flat cellular extension called lamellidopium, and myosin contraction at the rear end of the cell (1). Renewed attention is currently being paid to an alternative form of actin-myosin-based cell motility, where the contraction of the actomyosin cortex leads to cortex unbinding from the PM and the pressure-driven inflation of micron-size spherical membrane protrusions called "blebs" (2). Blebs are often the sign of apoptotis, but are also used for motility by several cell types, including amoebae and possibly cancer cells (3-5).The life cycle of a bleb is a remarkable illustration of the highly dynamical interplay between the CSK and the PM. Blebs are usually initiated by local rupture of the CSK cortex (6) or its local detachment from the PM (7), followed by the inflation of a CSK-free membrane blister. A new actin cortex containing myosin often repolymerizes under the bare membrane, and myosin contraction brings the bleb and the cell body back together. Blebbing generally occurs in a stochastic fashion and may be harnessed for cell motility through the cell's interaction with the external matrix. This example illustrates the importance of the mechanical interaction between the CKS and the PM for the conversions of a contractile stress into forward cellular protrusion in bleb-based motility. More generally, it calls for a quantitative understanding of the role of...

To study the mechanisms controlling front-rear polarity in migrating cells, we used zebrafish primordial germ cells (PGCs) as an in vivo model. We find that polarity of bleb-driven migrating cells can be initiated at the cell front, as manifested by actin accumulation at the future leading edge and myosin-dependent retrograde actin flow toward the other side of the cell. In such cases, the definition of the cell front, from which bleb-inhibiting proteins such as Ezrin are depleted, precedes the establishment of the cell rear, where those proteins accumulate. Conversely, following cell division, the accumulation of Ezrin at the cleavage plane is the first sign for cell polarity and this aspect of the cell becomes the cell back. Together, the antagonistic interactions between the cell front and back lead to a robust polarization of the cell. Furthermore, we show that chemokine signaling can bias the establishment of the front-rear axis of the cell, thereby guiding the migrating cells toward sites of higher levels of the attractant. We compare these results to a theoretical model according to which a critical value of actin treadmilling flow can initiate a positive feedback loop that leads to the generation of the front-rear axis and to stable cell polarization. Together, our in vivo findings and the mathematical model, provide an explanation for the observed nonoriented migration of primordial germ cells in the absence of the guidance cue, as well as for the directed migration toward the region where the gonad develops.