Cell Edge Dynamics During Polarization

Abstract

Upon contact with a flat substrate, most eukaryotic cells first attach and spread radially, then break symmetry and start directed migration, a phenomenon known as polarization. We analyzed the dynamics of the cell edge during polarization using the model system of fish epidermal keratocytes, which exhibit consistent polarization and a simple, constant shape during migration. We have developed an automatic method based on a level set formalism to perform accurate cell edge segmentation from phase-contrast images and to obtain protrusion/retraction maps with high accuracy and resolution in space and time. Analysis of the resulting protrusion/retraction maps demonstrated that polarization was often preceded by a transient oscillatory state in which relatively large protruding regions were separated by retracting regions that traveled around the cell periphery as rotating “blades” or waves. Depending on cell size, three to five of these “blades” were observed per cell during the oscillatory phase. Convergence of the “blades” eventually led to a polarized state with only one blade (protrusion and retraction were hence segregated to opposite sides of the cell). We have extracted the parameters of the oscillations, such as wavelength, period, and propagation velocity, and characterized the dynamics of cell area, as well as perimeter and edge velocity during polarization. In order to investigate how the protrusion/retraction switch at the tip of the “blade” was operated, we analyzed actin dynamics and the traction force pattern on elastic substrates. The obtained quantitative data will be used to develop a biophysical model of the convergence of cell edge oscillations into a polarized state. This work was supported by an NCCBI interdisciplinary PhD fellowship.