Range and strength of mechanical interactions of force dipoles in elastic fiber networks

TL;DR

This study models the elastic fiber networks of the cytoskeleton to understand how force dipoles, like molecular motors, transmit mechanical signals over distances, revealing the influence of network heterogeneity and fiber properties on force propagation.

Contribution

It introduces a detailed model of actin cytoskeleton as a fiber network to analyze the range and heterogeneity of force transmission by molecular motors.

Findings

Long-range force transmission is hindered by fiber bending in diluted networks.

Tensile and compressive forces propagate differently in the network.

Force dipole interactions depend on their separation and orientation, matching continuum theory in homogeneous networks.

Abstract

Mechanical forces generated by myosin II molecular motors drive diverse cellular processes, most notably shape change, division and locomotion. These forces may be transmitted over long range through the cytoskeletal medium - a disordered, viscoelastic network of biopolymers. The resulting cell size scale force chains can in principle mediate mechanical interactions between distant actomyosin units, leading to self-organized structural order in the cell cytoskeleton. To investigate this process, we model the actin cytoskeleton on a percolated fiber lattice network, where fibers are modeled as linear elastic elements that can both bend and stretch, whereas myosin motors exert contractile force dipoles. We quantify the range and heterogeneity of force transmission in these networks in response to a force dipole, showing how these depend on varying bond dilution and fiber bending-to-stretching stiffness ratio. By analyzing clusters of nodes connected to highly strained bonds, as well as the decay rate of strain energy with distance from the force dipole, we show that long-range force transmission is screened out by fiber bending in diluted networks. We further characterize the difference in the propagation of tensile and compressive forces. This leads to a dependence of the mechanical interaction between a pair of force dipoles on their mutual separation and orientation. In more homogeneous networks, the interaction between force dipoles recapitulates the power law dependence on separation distance predicted by continuum elasticity theory, while in diluted networks, the interactions are short-ranged and fluctuate strongly with local network configurations. Altogether, our work suggests that elastic interactions between force dipoles in disordered, fibrous media can act as an organizing principle in biological materials.