Dynamical organization of the cytoskeletal cortex probed by micropipette aspiration

Abstract

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...

Model for Probing Membrane-Cortex Adhesion by Micropipette Aspiration and Fluctuation Spectroscopy

“…Whenever the equilibrium stress exceeds the critical value s*, we expect the cell membrane to detach spontaneously. Micropipette aspiration (12,16,(27)(28)(29), among other techniques (11,30,31), allows us to apply pressure perturbations of controlled intensity and area. Pressure perturbations can be supplemented with perturbations on relevant cell parameters such as myosin activity and link or cortex density, by genetics (27)(28)(29) or direct drug treatment (10,11,31).…”

“…In this article, we present a model for adhesion based on the kinetics of the membrane-cortex ligands (13)(14)(15)(16). We describe the stability of adhesion by coupling the kinetics of the ligands to the stress exerted on them and to physical properties of the membrane.…”

Identification, Localization, and Functional Implications of the Microdomain-Forming Stomatin Family in the Ciliated Protozoan Paramecium tetraurelia

“…Stomatin may serve as a scaffold for the assembly of unknown mechanosensors in this case, and actin coating may serve to protect the organelle membrane. In fact, mechanical stress as an inducer of local actin polymerization is well established, at least for the cell surface (86,87), and actin covers phagosomes in Paramecium (88).…”

Shiver me timbers: Pulsatile contractility in model tissues

“…Micropipette aspiration applies controlled, negative pressure to draw material into the hollow micropipette, while imaging under a microscope; this technique is elegant and powerful, and it has been welldeveloped for sensitive measurements of isolated membranes and single cells (16,17). Using this approach, a constant stress is applied, and the strain (which may vary with time) is measured to infer viscoelastic material properties.…”