Theory of active self-organization of dense nematic structures in the actin cytoskeleton

TL;DR

This paper presents a theoretical model explaining how uniform actin gels self-organize into dense nematic bundles through active patterning mechanisms influenced by active gel parameters.

Contribution

It introduces an active gel model that predicts the conditions for nematic bundle self-assembly and validates these with discrete network simulations.

Findings

Active tension perpendicular to nematic direction promotes bundle formation.

Active forces conjugate to nematic order are crucial for pattern development.

Model predicts reconfigurable dense nematic structures in actin gels.

Abstract

The actin cytoskeleton is remarkably adaptable and multifunctional. It often organizes into nematic bundles such as contractile rings or stress fibers. However, how a uniform and isotropic actin gel self-organizes into dense nematic bundles is not fully understood. Here, using an active gel model accounting for nematic order and density variations, we identify an active patterning mechanism leading to localized dense nematic structures. Linear stability analysis and nonlinear finite element simulations establish the conditions for nematic bundle self-assembly and how active gel parameters control the architecture, orientation, connectivity and dynamics of self-organized patterns. Finally, we substantiate with discrete network simulations the main requirements for nematic bundle formation according to our theory, namely increased active tension perpendicular to the nematic direction and generalized active forces conjugate to nematic order. Our work portrays actin gels a reconfigurable active materials with a spontaneous tendency to develop patterns of dense nematic bundles.