Spontaneous symmetry breaking as a route to cellular locomotion

Michael Cates, University of Edinburgh

We study a simple model of a "cell extract" comprising an actomyosin droplet. (Such extracts are used in experiment biophysics to model the cytoskeleton, which is responsible for cellular movement.) In actomyosin, actin fibres are organized into a polar liquid crystal with not only a preferred (nematic) axis, but also a preferred sense of (polar) orientation along that axis. Myosin motors, walking along the oriented fibres, create an active stress which is invariant under polarity inversion, and so does not lead to locomotion of a uniformly polarized droplet. For cells on a 2D surface, locomotion is known instead to be caused by a 'treadmilling' process in which each fibre moves along its own polarization vector, creating a crawling motion of the droplet as a whole. Here we show that treadmilling-free locomotion, caused solely by myosin activity, can nonetheless arise through the spontaneous symmetry breaking of parity inversion symmetry. The result is more like swimming than crawling, and could dominate over the treadmilling mechanism in low friction environments, which possibly include 3D tissues.