How Does the Interplay between Semiflexible Polymers Determine Composite Network Mechanics?

Abstract

The semiflexible polymers actin and intermediate filaments (IF) intertwine in a complex network within the cell, and together are key determinants of cellular stiffness. While the mechanics of actin networks together with stiff microtubules have been characterized, the interplay between actin and IF networks is largely unknown, necessitating the study of composite networks using mixtures of semiflexible biopolymers. We employ bulk rheology in a simplified in vitro system to uncover the fundamental mechanical interactions between networks of the two semiflexible polymers, actin and vimentin IF. Surprisingly, co polymerization of actin and vimentin can produce composite networks either stronger or weaker than pure actin networks. We show that this effect occurs through steric constraints imposed by IF on actin during network formation and filament crosslinking, highlighting novel emergent behavior in composite semiflexible networks.In addition, we find that vimentin IF have little effect on actin network mechanics when actin is crosslinked with alpha-actinin. However, when using Filamin-A as an actin crosslinker, co-polymerization with vimentin creates a stiffer network able to better maintain its elasticity under larger strains, but with a lower yield stress.