The Poisson ratio of the cellular actin cortex is frequency-dependent

TL;DR

This study investigates how the Poisson ratio of the cellular actin cortex varies with frequency, revealing a decrease at higher frequencies likely due to actin turnover, challenging the common assumption of incompressibility in cell mechanics models.

Contribution

The paper introduces a frequency-dependent Poisson ratio model for the actin cortex, supported by simulations and experimental data, challenging the traditional incompressibility assumption.

Findings

Poisson ratio decreases with frequency

Actin cortex turnover influences mechanical response

Frequency dependence resembles glassy materials behavior

Abstract

Cell shape changes are vital for many physiological processes such as cell proliferation, cell migration and morphogenesis. They emerge from an orchestrated interplay of active cellular force generation and passive cellular force response - both crucially influenced by the actin cytoskeleton. To model cellular force response and deformation, cell mechanical models commonly describe the actin cytoskeleton as a contractile isotropic incompressible material. However, in particular at slow frequencies, there is no compelling reason to assume incompressibility as the water content of the cytoskeleton may change. Here we challenge the assumption of incompressibility by comparing computer simulations of an isotropic actin cortex with tunable Poisson ratio to measured cellular force response. Comparing simulation results and experimental data, we determine the Poisson ratio of the cortex in a frequency-dependent manner. We find that the Poisson ratio of the cortex decreases with frequency likely due to actin cortex turnover leading to an over-proportional decrease of shear stiffness at larger time scales. We thus report a trend of the Poisson ratio similar to that of glassy materials, where the frequency-dependence of jamming leads to an analogous effect.