Multiscale modeling and simulation of microtubule/motor protein assemblies

TL;DR

This paper develops a multiscale theoretical framework combining microscopic simulations and continuum modeling to understand the complex active flows and defect dynamics in microtubule-motor assemblies, revealing key active stress mechanisms.

Contribution

It introduces a combined multiscale approach and a continuum Doi-Onsager model to explain how microscopic motor-microtubule interactions lead to large-scale active liquid-crystalline behaviors.

Findings

Active stresses drive turbulent flows and defect dynamics.

Polarity-sorting and tether relaxation are key active stress sources.

The model captures formation of polar lanes and active turbulence.

Abstract

Microtubules and motor proteins self organize into biologically important assemblies including the mitotic spindle and the centrosomal microtubule array. Outside of cells, microtubule-motor mixtures can form novel active liquid-crystalline materials driven out of equilibrium by ATP-consuming motor proteins. Microscopic motor activity causes polarity-dependent interactions between motor proteins and microtubules, but how these interactions yield such larger-scale dynamical behavior such as complex flows and defect dynamics is not well understood. We develop a multiscale theory for microtubule-motor systems in which Brownian dynamics simulations of polar microtubules driven by motors are used to study microscopic organization and stresses created by motor-mediated microtubule interactions. We identify polarity-sorting and crosslink tether relaxation as two polar-specific sources of active destabilizing stress. We then develop a continuum Doi-Onsager model that captures polarity sorting and the hydrodynamic flows generated by these polar-specific active stresses. In simulations of active nematic flows on immersed surfaces, the active stresses drive turbulent flow dynamics and continuous generation and annihilation of disclination defects. The dynamics follow from two instabilities, and accounting for the immersed nature of the experiment yields unambiguous characteristic length and time scales. When turning off the hydrodynamics in the Doi-Onsager model, we capture formation of polar lanes as observed in the Brownian dynamics simulation.