Spindles and active vortices in a model of confined filament-motor mixtures

Abstract

Robust self-organization of subcellular structures is a key principle governing the dynamics and evolution of cellular life. In fission yeast cells undergoing division, the mitotic spindle spontaneously emerges from the interaction of microtubules, motor proteins and the confining cell walls, and asters and vortices have been observed to self-assemble in quasi-two dimensional microtubule-kinesin assays. Their is no clear microscopic picture of the role of the active motors driving this pattern formation, and the relevance of continuum modeling to filament-scale structures remains uncertain. Here we present results of numerical simulations of a discrete filament-motor protein model confined to a pressurised cylindrical box. Stable spindles, nematic configurations, asters and high-density semi-asters spontaneously emerge, the latter pair having also been observed in cytosol confined within emulsion droplets. State diagrams are presented delineating each stationary state as the pressure, motor speed and motor density are varied. We further highlight a parameter regime where vortices form exhibiting collective rotation of all filaments, but have a finite life-time before contracting to a semi-aster. Quantifying the distribution of life-times suggests this contraction is a Poisson process. Equivalent systems with fixed volume exhibit persistent vortices with stochastic switching in the direction of rotation, with switching times obeying similar statistics to contraction times in pressurised systems. Furthermore, we show that increasing the detachment rate of motors from filament plus-ends can both destroy vortices and turn some asters into vortices. Based on our findings we argue the need for a deeper understanding of the microscopic activities underpinning macroscopic self-organization in active gels and urge further experiments to help bridge these lengths.

10 pages, 12 figures

References in corpus (6)

• The Mechanics and Statistics of Active Matter
• Shearing active gels close to the isotropic-nematic transition
• Sheared active fluids: thickening, thinning and vanishing viscosity
• Nonlinear competition between asters and stripes in filament-motor-systems
• Non-local fluctuation correlations in active gels
• Microscopic basis for pattern formation and anomalous transport in two-dimensional active gels

Cited by in corpus (4)

• Physics of Microswimmers - Single Particle Motion and Collective Behavior
• Non-equilibrium structure and dynamics in a microscopic model of thin film active gels
• In silico modeling of the rheological properties of covalently crosslinked collagen triple helices
• Filamentous Active Matter: Band Formation, Bending, Buckling, and Defects