Vimentin Affects Actin Network Percolation and Mechanics

Abstract

The mechanics of the cytoskeleton are determined largely by actin, microtubules, and intermediate filaments. In vitro, composite actin/microtubule networks and actin/myosin networks have revealed unexpected emergent features, suggesting that cytoskeletal mechanical behavior is greater than the sum of its parts.Here, we construct in vitro composite networks of actin and the intermediate filament vimentin, and study the composite network mechanics using bulk rheology. When using biotin-neutravidin as an actin crosslinker, we find that vimentin can have opposing effects on network mechanics, depending on the actin and crosslinker concentrations. Vimentin strengthens actin networks with abundant biotinylation, and weakens composite networks when actin biotinylation is sparse, suggesting that vimentin can either add to or subtract from the number of effective crosslinks in the composite system. The crossover between these two effects occurs when the actin crosslinker spacing is roughly comparable to the actin network mesh size, pointing to a reduction of actin crosslinking through steric constraints by vimentin as the origin of the mechanical changes.When actin networks are crosslinked using Filamin-A or alpha-actinin, vimentin causes a mechanical weakening of the composite network, suggesting that the effective degree of crosslinking is reduced. This is confirmed by confocal fluorescence imaging, which shows actin networks reaching an earlier state of dynamic arrest in the presence of vimentin.Together, these data suggest that vimentin affects the formation of an interpenetrating actin network through steric interactions between the polymers. These indirect interactions affect both the final percolation state and the mechanical strength of the composite network, indicating that the polymers must be considered together as a composite system in determining the properties of the network.Support: NIH (P01GM096971)