Extensile to contractile transition in active microtubule-actin composites generates layered asters with programmable lifetimes

TL;DR

This study demonstrates how active microtubule-actin composites undergo a transition from turbulent flows to layered asters with programmable lifetimes, driven by viscoelasticity and active stresses, revealing new insights into active matter self-organization.

Contribution

It introduces a mechanically driven mechanism for self-organized layered asters in active composites, challenging biochemical regulation models.

Findings

Layered asters form at intermediate F-actin concentrations.

Asters have a microtubule-rich cortex with layered internal structure.

Asters are metastable with lifetimes controlled by material properties.

Abstract

We study a reconstituted composite system consisting of an active microtubule network interdigitated with a passive network of entangled F-actin filaments. Increasing viscoelasticity of the F-actin network controls the emergent dynamics, inducing a transition from turbulent-like flows to bulk contractions. At intermediate F-actin concentrations, where the active stresses change their symmetry from anisotropic extensile to isotropic contracting, the composite separates into layered asters that coexist with the background turbulent fluid. Contracted onion-like asters have a radially extending microtubule-rich cortex that envelops alternating layers of microtubules and F-actin. The self-regulating layered organization survives aster merging events, which are reminiscent of droplet coalescence, and suggest the presence of effective surface tension. Finally, the layered asters are metastable structures. Their lifetime, which ranges from minutes to hours, is encoded in the material properties of the composite. Taken together, these results challenge the current models of active matter. They demonstrate that the self-organized dynamical states and patterns, which are evocative of those observed in the cytoskeleton, do not require precise biochemical regulation but can arise due to purely mechanical interactions of actively driven filamentous materials.