UC San Diego

Fast amoeboid migration requires cells to apply mechanical forces on their surroundings via transient adhesions. However, the role these forces play in controlling cell migration speed remains largely unknown. We use three- dimensional force microscopy to examine the mechanics underlying the chemotactic migration of wild-type Dictyostelium cells, as well as mutant strains with defects in contractility, internal F-actin cross-linking and cortical integrity. We show that cells pull on their substrate adhesions using two distinct, yet interconnected mechanisms: axial actomyosin contractility and cortical tension. The three-dimensional pulling forces generated by both mechanisms are internally balanced by an increase in cytoplasmic pressure that allows cells to push on their substrate downward without adhering to it. These compressive pressure-induced forces are not associated to adhesion sites, and may allow amoeboid cells to push off surrounding structures when migrating in complex three- dimensional environments. We find a relationship between the strength of these three-dimensional forces and the migration speed and we show that the cell migration speed increases with the ratio of the tangential to normal forces. This finding indicates that the migration speed increases when axial contractility balances cortical tension, allowing the cells to modulate their three- dimensional shape and move faster. Additionally, we develop a new methodology for the calculation of the three -dimensional forces exerted by migrating cells improved by a Lagrange multipliers optimization that provides a stress field in equilibrium and equal to zero outside the region in which the cell is localized. Furthermore, we design a novel elastometry technique based on the exact solution of the elastic equation of equilibrium, the measurement of the deformation exerted by cells when moving and the application of an optimization algorithm for solving a non -linear least-squares problem. This novel method enables the characterization of the Poisson ratio of polymer-based substrates on real time, which is essential for a precise calculation of the traction forces. The value of the Poisson ratio that we obtain for the polyacrylamide gels used in our experiments is 0.45. A similar methodology could be applied to calculate the mechanical properties and constitutive equations for other extracellular environments, which are not perfectly elastic