Simulation of actin cytoskeleton structure and rheology

Abstract

Structures consisting of G-actin or other filament-forming monomers show a variety of morphologies with widely different properties in regard to pore size, degree of isotropy, and extent of cross-linking. These characteristics are primarily determined by the concentration and feature of proteins which cross-link filaments, but little is known how the filament-forming monomers and cross-linking proteins are organized in order to produce various network morphologies. In addition, it's generally known that mechanical force plays an important role in the physiology of eukaryote cells whose major structural component in cortex is actin cytoskeleton. Thus, understanding the origin of viscoelasticity of cross-linked networks should be crucial to figure out the exact role of cytoskeletal behaviors in many cellular functions. Here, we introduce a Brownian dynamics (BD) simulation model in three dimensions in which actin monomers polymerize into a filament and become cross-linked by two types of cross-linking molecules that constitute either perpendicular or parallel cross-links. We evaluate the influences of system parameters on the morphology of resultant networks. Some scaling behaviors that are independent of the specific choice of most parameters appear.

(cont.) Additionally, the modified model is employed to investigate the viscoelastic property of actin-like network by tracking the trajectories of filaments. This method is theoretically more direct and more precise than micro-bead rheology used in experiments. The viscoelastic property appears to be highly affected by characteristics of cross-linking molecules, average filament length, and concentration of actin monomers. Our model has the high potential as a BD model that can be applied for investigating a variety of actin-related phenomena after further refinement and modification.