Active restructuring of cytoskeleton composites leads to increased mechanical stiffness, memory, and heterogeneity

TL;DR

This study demonstrates that motor protein activity actively restructures cytoskeleton composites, significantly increasing their mechanical stiffness, memory, and heterogeneity, with stable, robust network formations observed after activity ceases.

Contribution

It reveals how motor-driven activity induces active restructuring in cytoskeleton composites, enhancing their mechanical properties and stability, a novel insight into active polymer networks.

Findings

Motor activity increases viscoelasticity and elastic storage.

Restructured networks are stable and robust to nonlinear forces.

Motor-driven restructuring suppresses filament bending.

Abstract

The composite cytoskeleton, comprising interacting networks of semiflexible actin and rigid microtubules, actively generates forces and restructures using motor proteins such as myosins to enable key mechanical processes including cell motility and mitosis. Yet, how motor-driven activity alters the mechanics of cytoskeleton composites remains an open challenge. Here, we perform optical tweezers microrheology on actin-microtubule composites driven by myosin II motors to show that motor activity increases the linear viscoelasticity and elastic storage of the composite by active restructuring to a network of tightly-packed filament clusters and bundles. Our nonlinear microrheology measurements performed hours after cessation of activity show that the motor-contracted structure is stable and robust to nonlinear forcing. Unique features of the nonlinear response include increased mechanical stiffness, memory and heterogeneity, coupled with suppressed filament bending following motor-driven restructuring. Our results shed important new light onto the interplay between viscoelasticity and non-equilibrium dynamics in active polymer composites such as the cytoskeleton.