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Centering and symmetry breaking in confined contractile actin networks


  Niv Ierushalmi [1]  ,  Maya Malik-Garbi [1]  ,  Kinneret Keren [1]  ,  Alex Mogilner [2]  
 [1] Technion
[2] Courant Institute and New York University

Contractile actin network have an essential role in many cellular processes including cell division and cell motility, through mechanisms that are not completely understood. We reconstitute contractile actin networks in cell-sized compartments by encapsulating Xenopus egg extract, which contains all the components of the actin machinery, within water-in-oil droplets. Actin turnover in our system allows these networks to maintain persistent contractile flows. Depending on the conditions and the size of the droplets, we observe either a spherically symmetric state of contraction towards the droplets' center, or a polar state in which the contraction center is localized near the droplet's boundary. In the symmetric state, the contraction center is actively maintained near the middle of the droplet, reminiscent of actin-based centering mechanisms found in living cells. During symmetry breaking, the system transitions from the symmetric to a polar state, mimicking biological processes of cellular symmetry breaking. These findings serve as a basis for modeling the localization of the contraction center using active gel theory. Within this model, we show that centering can arise from friction between the contracting network and the surrounding fluid, whereas symmetry breaking can be induced by an attractive interaction with the boundary.