Nonuniform filament turnover, contractility, and bundle formation in disordered actomyosin networks

We use mathematical modeling and simulation to investigate how filament turnover and mechanics influence contractility and bundle formation in disordered actomyosin networks. Using a two-dimensional agent-based model for an actomyosin network, we investigate four simplified models for filament turnover: uniform, biased, branching, and treadmilling. With no turnover, over time contractility decreases and bundle formation increases, and networks eventually form stationary patterns that cannot contract. Introducing turnover allows contractility to persist longer compared with the no-turnover scenario. Uniform turnover, where new filaments have random positions and orientations, disrupts bundle formation and enables persistent contractility. In biased turnover, branching, and treadmilling, the positions and orientations of new filaments depend on the existing network. These nonuniform turnover models increase bundle formation compared with uniform turnover, while still allowing long-term contractility. Branching at 70° disrupts bundle formation to enable prolonged contractility, whereas filament treadmilling disrupts the trade-off between bundle formation and contractility. Biased turnover places new filaments near existing ones, which promotes bundle formation but is less effective at maintaining contractility. Simulations showed that more bendable filaments accelerate bundle formation, while higher effective cross-linking friction enhances contractility. Our results suggest that cells can tune contractility and bundle formation in disordered actomyosin networks by varying actin turnover pathways.