The role of actin and myosin II in the cell cortex of adhered and suspended cells

Abstract

Adhesion induces dramatic morphological and mechanical changes to cells, which are reflected by changes to the actin cortex. Among the many different proteins involved in this sub-membranous layer, motor proteins (e.g., nonmuscle myosin II [NMII]) and actin nucleators (e.g., Arp2/3, formins) are known to have significant influences on its dynamics and structure. The different roles of NMII, Arp2/3, and formins in the dynamics, structure, and mechanics of the actin cortex depend on the adhesion state of the cell. In this study, we unravel the interplay between the dynamics, structure, and mechanics of the actin cortex in adhered cells and in cells in suspension. We show that treatments with extrinsic cellular perturbants lead to alterations of all three properties that are correlated. However, intrinsic actin cortex variations between different cell adhesion states lead to unexpected correlations. Surprisingly, we find that NMII minifilaments have a minor influence on the actin cortex. Using new microscopy techniques, we show that NMII minifilaments are not localized within the actin cortex, as previously thought, but concentrated in a layer beneath it. Our treatments affecting Arp2/3 and formin reveal correlations between the actin cortex characteristics. Our data build towards a comprehensive understanding of the actin cortex. This understanding allows the prediction and control of cortical changes, which is essential for the study of general cellular processes, such as cell migration, metastasis, and differentiation.

Mechanical Implications of Cellular Viscoelasticity, Cortex Polarity, Superelasticity, and Cell-Cell Junctions in Curved Tissues

“…This also explains why bending can be neglected as a major contribution to the Hamiltonian as this cortex is extremely thin, on the order of only 100-200 nm. 11 In hemicysts, the cell layer is partially compressed before the indenter reaches the basolateral membrane/cortex, which explains the soft response at low indentation depth compared to cysts. Superelasticity further reduces the apparent stiffness measured in force relaxation experiments as additional surface is created at the expense of shell thickness.…”

“…5F shows how the stiffness of the epithelia K 0 A derived from hemicysts (blue) and cysts (green) changes as a function of the measured pre-stress T 0 . Assuming a linear relationship between this pre-stress and area dilatation α 0 we can describe the data with equations (10,11). The apparent stiffness (scaling factor) K 0 A of hemicysts increases strongly with pre-tension (T 0 ), while the apparent stiffness of cysts is almost constant.…”

“…Comparing equation (11) with equations ( 12) and ( 13) permits us to conclude that λ represents the constant curvature of the cyst:…”

“…6 On a single cell level, excess surface area in conjunction with the contractile actomyosin cortex permits to absorb external stress and to dissipate the associated energy reminiscent of a soft glassy material. [11][12][13][14][15] However, on tissue length scales, the number of experiments is sparse and our knowledge how cells organized in epithelia respond to lateral, in-plane, forces and their ability to dissipate the elastic energy remains limited. [4][5][6] Here, we examine how cysts 16,17 and hemicysts (domes) with opposing polarity generated from MDCK-II cells respond to external deformation with micrometer-sized colloidal probes on short time scales of seconds.…”