Centering and symmetry breaking in confined contracting actomyosin networks.

Niv Ierushalmi,Maya Malik-Garbi,Alex Mogilner,Kinneret Keren,Bruce L Goode,Enas Abu Shah,Angelika Manhart

doi:10.7554/elife.55368

Abstract

Centering and decentering of cellular components is essential for internal organization of cells and their ability to perform basic cellular functions such as division and motility. How cells achieve proper localization of their organelles is still not well-understood, especially in large cells such as oocytes. Here, we study actin-based positioning mechanisms in artificial cells with persistently contracting actomyosin networks, generated by encapsulating cytoplasmic Xenopus egg extracts into cell-sized 'water-in-oil' droplets. We observe size-dependent localization of the contraction center, with a symmetric configuration in larger cells and a polar one in smaller cells. Centering is achieved via a hydrodynamic mechanism based on Darcy friction between the contracting network and the surrounding cytoplasm. During symmetry breaking, transient attachments to the cell boundary drive the contraction center to a polar location. The centering mechanism is cell-cycle dependent and weakens considerably during interphase. Our findings demonstrate a robust, yet tunable, mechanism for subcellular localization.

Highlights

The proper localization of cellular components is essential for a variety of cell functions (Rafelski and Marshall, 2008)
The system exhibits two stable configurations; a centered configuration in which the contraction center is actively maintained at the middle of the cell and a decentered one where the contraction center is near the boundary
While the contracting network dynamics are largely determined by the interplay between internal forces, namely the myosin induced contractile force and the opposing force which is primarily due to the viscosity of the network (Malik-Garbi et al, 2019), the size-dependent localization of the contraction center is determined by the smaller friction forces between the network and the fluid it is immersed in and the interactions between the network and the droplet boundary

Summary

Introduction

The proper localization of cellular components is essential for a variety of cell functions (Rafelski and Marshall, 2008). Depending on the cellular context, some components need to be positioned at the center of the cell, whereas other components must assume a polar, decentered localization (van Bergeijk et al, 2016). While much research has been devoted to the study of the various cellular positioning mechanisms, many basic questions remain open. This is true in large cells such as oocytes (Mitchison, 2012), where movements must be coordinated over large scales and it is often still unclear what drives movement in the first place and how the proper localization is stabilized

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Results

Conclusion