Multiscale modeling and simulation of microtubule-motor-protein assemblies

Area of Science:

• Physics
• Biophysics
• Materials Science

Background:

• Microtubules and motor proteins self-organize into essential biological structures like the mitotic spindle.
• Outside cells, these mixtures form active liquid crystals driven by ATP-consuming motors.
• Microscopic motor activity creates polarity-dependent interactions, but large-scale dynamics remain unclear.

Purpose of the Study:

• To develop a multiscale theory explaining the self-organization and emergent behaviors in microtubule-motor systems.
• To identify the microscopic origins of active stresses and their contribution to macroscopic dynamics.
• To model the resulting hydrodynamic flows and defect dynamics in active nematic systems.

Main Methods:

• Brownian dynamics simulations of polar microtubules driven by motors to study microscopic organization and stresses.
• Development of a continuum Doi-Onsager model to capture polarity sorting and hydrodynamic flows.
• Simulations of active nematic flows on immersed surfaces to observe turbulence and defect dynamics.

Main Results:

• Identified polarity-sorting and crosslink tether relaxation as key sources of active destabilizing stress.
• The continuum model captures polarity sorting and generates hydrodynamic flows consistent with simulations.
• Active stresses drive turbulent flow dynamics and continuous generation/annihilation of disclination defects on immersed surfaces.
• In the absence of hydrodynamics, polar lane formation was observed, matching Brownian dynamics simulations.

Conclusions:

• Microtubule-motor systems exhibit complex emergent behaviors driven by polarity-specific active stresses.
• A multiscale approach combining Brownian dynamics and continuum modeling effectively describes these active materials.
• The study provides insights into the fundamental principles governing self-organization in active matter and biological assemblies.