Microscopic origins of anisotropic active stress in motor-driven nematic liquid crystals

Abstract

The cytoskeleton, despite comprising relatively few building blocks, drives an impressive variety of cellular phenomena ranging from cell division to motility. These building blocks include filaments, motor proteins, and static crosslinkers. Outside of cells, these same components can form novel materials exhibiting active flows and nonequilibrium contraction or extension. While dipolar extensile or contractile active stresses are common in nematic motor-filament systems, their microscopic origin remains unclear. Here we study a minimal physical model of filaments, crosslinking motors, and static crosslinkers to dissect the microscopic mechanisms of stress generation in a two-dimensional system of orientationally aligned rods. We demonstrate the essential role of filament steric interactions which have not previously be considered to significantly contribute to active stresses. With this insight, we are able to tune contractile or extensile behavior through control of motor-driven filament sliding and crosslinking. This work provides a roadmap for engineering stresses in active liquid crystals. The mechanisms we study may help explain why flowing nematic motor-filament mixtures are extensile while gelled systems are contractile.

Physical determinants of bipolar mitotic spindle assembly and stability in fission yeast

“…We computed the full filament pair partition function to ensure that cross-linker/motor kinetics followed the correct statistical mechanical rules ( 115 , 116 , 118 ). Attachment/detachment occurs for either head individually, so the motors/cross-linkers can have zero, one, or two heads bound.…”

Physical Determinants of Bipolar Mitotic Spindle Assembly and Stability in Fission Yeast

“…Because steric interactions between MTs may contribute to MT alignment and the establishment of bipolarity, we modeled short-range hard-wall repulsion between MTs rather than neglecting MT-MT interactions or using a spring-like repulsion. We also built on our previous work (115,116,118) to compute the statistical mechanics of motor and cross-linker binding and unbinding kinetics accurately, ensuring that detailed balance is maintained at the level of single binding/unbinding events. Finally, we used quantitative comparison of our simulated spindles' length and structure to data from light and electron microscopy to optimize the model's poorly constrained parameters.…”

“…Kinetic Monte Carlo methods model stochastic events that change the state of molecules in the system, including motor/cross-linker binding/unbinding and MT dynamic instability. We previously used these techniques to model MT-motor mixtures and kinetochore capture (115)(116)(117)(118)(119).…”

“…The effects of kinetochore-MT attachments on MT dynamics (23,25,114) are implicitly modeled through our optimization of MT dynamics for spindle stability. We use a hybrid Brownian dynamics-kinetic Monte Carlo simulation scheme, as previously described (115)(116)(117)(118)(119). Bipolar spindles robustly form (Fig.…”

“…We computed the full filament pair partition function to ensure that crosslinker/motor kinetics followed the correct statistical mechanical rules (115,116,118). Attachment/detachment occurs for either head individually, so the motors/cross-linkers can have zero, one, or two heads bound.…”