Rheology of the Active Cell Cortex in Mitosis

Abstract

The cell cortex is a key structure for the regulation of cell shape and tissue organization. To reach a better understanding of the mechanics and dynamics of the cortex, we study here HeLa cells in mitosis as a simple model system. In our assay, single rounded cells are dynamically compressed between two parallel plates. Our measurements indicate that the cortical layer is the dominant mechanical element in mitosis as opposed to the cytoplasmic interior. To characterize the time-dependent rheological response, we extract a complex elastic modulus that characterizes the resistance of the cortex against area dilation. In this way, we present a rheological characterization of the cortical actomyosin network in the linear regime. Furthermore, we investigate the influence of actin cross linkers and the impact of active prestress on rheological behavior. Notably, we find that cell mechanics values in mitosis are captured by a simple rheological model characterized by a single timescale on the order of 10 s, which marks the onset of fluidity in the system.

Pre-stress of actin cortices is important for the viscoelastic response of living cells

“…Viscoelasticity of the 2D area compressibility modulus is assumed to follow a power law. As demonstrated previously, the experimental setup ensures small curvature of the cell cortex therefore permitting to neglect bending contributions to the Hamiltonian [18,24,25]. Minimizing free energy assuming constant volume leads to minimal surfaces of constant curvature with the force balance f at the equatorial radius ( Fig.…”

EMT-induced cell mechanical changes enhance mitotic rounding strength

“…We probed actin cortex mechanics in mitotic and interphase cells through a previously established cell confinement setup based on atomic force microscopy (AFM). In this assay, initially round cells are dynamically confined between two parallel plates using a wedged cantilever [32][33][34][35] (Figure 2a,b, Materials and Methods). This method was shown to measure the contractility and the viscoelastic stiffness of the actin cortex 33,34 .…”

“…Furthermore, we also determine mechanical changes of the actin cortex of interphase cells upon EMT; mechanics of interphase cells may critically influence mitotic rounding as interphase cells are a major constituent of the surrounding of a mitotic cell which needs to be deformed in the process of rounding ( Figure 1). For our cortex-mechanical measurements, we use an established dynamic cell confinement assay based on atomic force microscopy 9,24 . We report striking cell-cycledependent cortex-mechanical changes upon EMT that are accompanied by a strong change in the activity of the actomyosin master regulators Rac1 and RhoA.…”

Time-resolved nanomechanics of a single cell under the depolymerization of the cytoskeleton

“…In a force oscillation experiment, the tip is indented on the cell to a given depth and a subsequent sinusoidal perturbation is applied. 28,33,38 Multi-harmonic AFM experiments 8,36,39 could also be included in this category although in multi-harmonic experiments there is not a distinction between the indentation and the oscillation stages. Force oscillation approaches require the use of an elaborated theoretical framework.…”

“…1,3 (3) On the other hand, there is not an established AFM protocol to perform time or frequency-dependent experiments. [33][34][35][36][37][38] In addition, there are several competing theoretical models to interpret the data. [40][41][42][43] Let's summarize the AFM studies devoted to measure the viscoelastic properties of cells.…”