UC San Diego

Cell motility plays an essential role in many physiological and pathological processes, yet we still lack information about the spatio-temporal coordination between regulatory biochemical processes and mechanics of cell migration. This dissertation has investigated the mechanics of amoeboid cell migration through intensive analysis of the traction forces exerted and shapes adopted by single Dictyostelium discoideum cells migrating chemotactically, focusing on wild-type (WT) and contractility-defective cells lacking either protein myosin II (mhcA⁻) or the myosin II essential light chains (mlcE⁻). We have developed an improved traction force cytometry method to calculate cell traction stresses which considers the finite thickness of the substrate. We have shown that the strain energy exerted by locomoting cells on the substrate evolves quasi-periodically and correlates with cell length, and thus it can be used as a quantitative indicator of the cell motility cycle. The periodicity (T) of the oscillations in the traction forces correlates strongly with the average velocity of migration (V) of cells according to the hyperbolic law V T=[lambda], where the constant [lambda] is independent of the strain analyzed and corresponds to the average distance a cell travels per cycle. Given the quasi-periodic character of both cell length and strain energy, we have performed a phase statistical analysis to obtain a spatio-temporal representation of the canonical motility cycle divided into four phases: protrusion, contraction, retraction, and relaxation. This analysis has elucidated the role that protein myosin II plays in enhancing the kinetics of the four stages of the cycle and in controlling the spatial distribution of the traction forces regulating that process. We have used principal component analysis to dissect the mechanics of locomotion of amoeboid cells into a reduced set of dominant components of cellular traction forces and shape changes. The dominant traction force component accounts for 40% of the strain energy performed by these cells, and its temporal evolution correlates with the quasi-periodic variations of cell length and strain energy exerted on the substrate. Finally, we have developed two analytic assays for the calculation of cell traction stresses in configurations of interest to further understand the mechanosensing machinery of cells