Abstract
Movement within eukaryotic cells largely originates from localized forces exerted by myosin motors on scaffolds of actin filaments. Although individual motors locally exert both contractile and extensile forces, large actomyosin structures at the cellular scale are overwhelmingly contractile, suggesting that the scaffold serves to favor contraction over extension. While this mechanism is well understood in highly organized striated muscle, its origin in disordered networks such as the cell cortex is unknown. Here, we develop a mathematical model of the actin scaffold’s local two- or three-dimensional mechanics and identify four competing contraction mechanisms. We predict that one mechanism dominates, whereby local deformations of the actin break the balance between contraction and extension. In this mechanism, contractile forces result mostly from motors plucking the filaments transversely rather than buckling them longitudinally. These findings shed light on recent in vitro experiments and provide a new geometrical understanding of contractility in the myriad of disordered actomyosin systems found in vivo.Received 14 April 2014DOI:https://doi.org/10.1103/PhysRevX.4.041002This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society
Highlights
The structure and motion of living cells is largely controlled by the continuous remodeling of their cytoskeleton, which crucially involves the contractility of networks of actin filaments (F-actin) and myosin molecular motors
While the biochemical processes inducing the relative motion of the motors and filaments are similar to the ones involved in striated muscle, here the geometrical mechanisms used to convert this relative motion into contraction in the absence of organization are less clear
We note that in vitro parallel bundles of actin filaments contract considerably less than antiparallel bundles [23], in contradiction with a robust prediction of the position-dependent stall force model [24]; this supports our finding that the position-dependent stall force has little effect on contractility
Summary
The structure and motion of living cells is largely controlled by the continuous remodeling of their cytoskeleton, which crucially involves the contractility of networks of actin filaments (F-actin) and myosin molecular motors. Question by assuming from the onset that motors either induce an average contractile stress in the actomyosin medium [9] or, in more detailed descriptions, that they give rise to localized contractile force dipoles [10] These studies typically move on to consider the macroscopic consequences of such mesoscopic behaviors. In this paper, we adopt a different focus and ask how the contractility emerges from the networks’ microscopic components in the first place This question is most discussed in onedimensional actomyosin assemblies, i.e., actomyosin bundles. Because geometry in one dimension is very simple, there are strong geometrical constraints on the type of mechanisms that can lead to such symmetry breaking [12] Combining these theoretical constraints with further experiments, we have recently shown that F-actin buckling under longitudinal compression enables contraction by favoring local filament collapse in the absence of sarcomerelike organization [13]. Filament deformation is found to play a crucial role in most relevant regimes
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