Molecular Regulation of Actin Turnover at the Leading Edge of Migrating Cells

Abstract

Actin couples molecular motors to extracellular adhesions and its turnover regulates cell polarization and protrusion efficiency. Actin filament turnover is regulated by polymerization and depolymerization at the ends, which is increased by severing filaments internally. To determine how these reactions affect F-actin turnover rate we simulated the molecular mechanisms revealed by experiments and determined the ODEs for changes in the average filament length and number of filaments that determine total F-actin. These results reveal the conditions that end dynamics and severing strongly cooperate to increase actin turnover and steady-state behavior. Although the molecular rate constants that regulate actin turnover have been measured in vitro in the presence of various actin regulatory proteins these constants are still unknown in vivo for most cell processes regulated by actin turnover. Importantly, these constants that regulate actin turnover at the leading edge of migrating cells remain unknown. Therefore, we modeled actin turnover as a system of these molecular mechanisms at the leading edge to investigate how actin distributions emerge and actin dynamics are regulated by migrating U251 glioma cells. We used the model to fit leading edge actin distributions of cells transfected with GFP-actin and aligned on one-dimensional tracks using model convolution microscopy. Best fits of the model yield estimates of the molecular rate constants that regulate actin turnover in vivo. We then correlated the molecular rates with the velocities of actin-network retrograde flow, cell protrusion and cell displacement of expanding, contracting and migrating cells. The quantitative analyses of our model provide in vivo estimates for the molecular rate constants that regulate actin turnover at the leading edge of cells and presents insights into the molecular mechanisms that regulate actin-based cell migration.