Symmetry breaking in reconstituted actin cortices

RESEARCH ARTICLE elifesciences.org Symmetry breaking in reconstituted actin cortices Enas Abu Shah1,2, Kinneret Keren1,2,3* Department of Physics, Technion–Israel Institute of Technology, Haifa, Israel; 2The Russell Berrie Nanotechnology Institute, Technion–Israel Institute of Technology, Haifa, Israel; 3Network Biology Research Laboratories, Technion–Israel Institute of Technology, Haifa, Israel 1 Abstract The actin cortex plays a pivotal role in cell division, in generating and maintaining cell polarity and in motility. In all these contexts, the cortical network has to break symmetry to generate polar cytoskeletal dynamics. Despite extensive research, the mechanisms responsible for regulating cortical dynamics in vivo and inducing symmetry breaking are still unclear. Here we introduce a reconstituted system that self-organizes into dynamic actin cortices at the inner interface of water-in-oil emulsions. This artificial system undergoes spontaneous symmetry breaking, driven by myosin-induced cortical actin flows, which appears remarkably similar to the initial polarization of the embryo in many species. Our in vitro model system recapitulates the rich dynamics of actin cortices in vivo, revealing the basic biophysical and biochemical requirements for cortex formation and symmetry breaking. Moreover, this synthetic system paves the way for further exploration of artificial cells towards the realization of minimal model systems that can move and divide. DOI: 10.7554/eLife.01433.001 Introduction *For correspondence: kinneret@ ph.technion.ac.il Competing interests: The authors declare that no competing interests exist. Funding: See page 13 Received: 27 August 2013 Accepted: 26 March 2014 Published: 29 April 2014 Reviewing editor: Mohan Balasubramanian, University of Warwick, United Kingdom Copyright Abu Shah and Keren. This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited. The actin cytoskeleton plays a central role in many cellular processes including polarization, cell shape determination, intracellular transport, cell division and movement (Pollard and Cooper, 2009). The structure and function of the cytoskeleton arise from the self-organized dynamics of numerous molecular building blocks. This self-organization spans several orders of magnitude in space and time and involves a complex interplay between biochemical and biophysical processes; A myriad of proteins interact with the actin cytoskeleton and influence its behavior, in a manner that is dependent on the global mechanical properties of the network but at the same time determines it (Lecuit and Lenne, 2007; Pollard and Cooper, 2009; Mullins and Hansen, 2013). Despite the significant progress in uncovering the molecular details underlying cytoskeletal dynamics, the principles governing largescale coordination and polarization of the cytoskeleton are still not well-understood. The realization of biomimetic systems that reconstitute cellular processes in vitro, detached from the complexity of the cell, is a powerful approach for dissecting complex cellular phenomena. In particular, in vitro experiments have significantly advanced our understanding of the molecular requirements and the biophysical principles underlying actin-based motility and cytoskeletal organization in bulk (Welch et al., 1998; Cameron et al., 1999; Loisel et al., 1999; Gardel et al., 2004; Van Der Gucht et al., 2005; Bendix et al., 2008; Field et al., 2011; Kohler et al., 2012), and more recently in cell-sized compartments (Pontani et al., 2009; Stachowiak et al., 2009; Pinot et al., 2012; Sanchez et al., 2012; Carvalho, 2013b). However, we are still far from understanding the complexity of cytoskeletal dynamics in vivo, and recapitulating even basic cellular phenomena such as polarization, division and directed movement in synthetic systems remains an outstanding challenge. The actin cytoskeleton undergoes continuous turnover and remodeling which are essential for its ability to perform its cellular tasks (Pollard and Cooper, 2009). In particular, the thin cortical actin shell Abu Shah and Keren. eLife 2014;3:e01433. DOI: 10.7554/eLife.01433 1 of 15 Research article Biophysics and structural biology | Cell biology eLife digest Cells are extremely complex because they have to perform a vast number of processes. However, this also makes it difficult for researchers to figure out how the individual parts of the cell work. There is interest, therefore, in developing simple artificial cells that can accurately mimic how specific parts of a cell behave. An important process for a cell is called polarization. This is where the contents of the cell arrange themselves in a way that is not symmetrical. Polarization is necessary for many cellular functions, and is particularly important during embryonic development where it helps to form the complex shape of the developing embryo. The cytoskeleton—a dynamic structure that supports the cell and enables it to move—is crucial for polarization. An important part of the cytoskeleton is the actin cortex. This is a thin active sheet made up of a network of tiny filaments of a protein called actin that assembles at the inner face of the cell membrane. Many aspects of the structure and behavior of the actin cortex are not understood. Abu Shah and Keren have now developed an artificial cell system using aqueous droplets surrounded by oil that can reproduce the behavior of actin cortices in real cells. An actin cortex forms upon the localization of specific nucleation factors at the inner surface of the droplets. The artificial cortices are capable of spontaneous symmetry breaking, similar to the initial polarization in embryonic cells during development. This symmetry breaking is driven by molecular motors called myosins and depends on the connectivity of the actin network in the cortex. Experiments on the artificial cells also rule out several other mechanisms that have been proposed to explain symmetry breaking. The work of Abu Shah and Keren represents a further step towards the goal of creating simple artificial cells that can move and divide. DOI: 10.7554/eLife.01433.002 underneath the cell membrane undergoes continuous assembly and disassembly processes, catalyzed by nucleation-promoting factors localized at the membrane and disassembly factors (Fritzsche et al., 2013). Among the nucleation-promoting factors, Arp2/3 which nucleates branched networks localizes to cortical actin networks (Machesky et al., 1994) and is essential for cortex formation (Bovellan, 2012). Formins, which nucleate linear filaments, were also found to localize to cortical actin networks, yet their role is still not entirely clear (Bovellan, 2012; Fritzsche et al., 2013). A host of actin binding proteins, including myosin motors, tethering proteins and various crosslinkers, further c (...truncated)