UC San Diego

Directed cell migration is involved in a broad spectrum of biological phenomena, ranging from the metastatic spreading of cancer to the active migration of neutrophils in response to bacterial infection. It requires a tightly regulated, spatiotemporal coordination of underlying biochemical processes. However, our understanding of how these processes are spatiotemporally coordinated with the mechanics of cell migration is poor. The objective of this Dissertation is, therefore, to investigate the mechanics of cell migration using Fourier Traction Force Microscopy (FTFM) measurements to shed light onto the key role of specific cytoskeletal proteins in the migration process. Using an improved traction force cytometry method, we calculated the traction stresses of wild-type cells and a range of mutants with cytoskeletal deficiencies. We confirmed that for both wild type and mutants, the strength of the traction stresses and the cell length followed a quasi-periodic temporal evolution, supporting the existence of a motility cycle. In addition, we found that the cells with misregulated SCAR/WAVE-mediated, dendritic F-actin polymerization at the cell's leading edge were unable to move periodically, rendering their efficiency in migrating poor. We were able to demonstrate that wild type and cells lacking the SCAR/WAVE complex protein PIR121 (pirA⁻) or SCAR (scrA⁻) exert stresses of different strength that correlate with their levels of F- actin, suggesting that the amount of F-actin present within a cell is a determinant of its stress strength. In addition to FTFM, we also constructed traction tension kymographs to obtain a space-time representation of traction stresses. Kymographic representation of the traction stresses allowed us to determine, for the first time, how the formation and disassembly of adhesions are coupled with the generation of axial and lateral traction stresses to control cell migration. Our findings revealed that wild-type cells migrate by switching between two motility modes with distinct adhesion and contractility dynamics. These two modes are not conserved when wild-type cells migrate on highly adhesive substrates, where cells implement modes that rely on lateral contractility. Finally, we observed that cells with cytoskeletal crosslinking defects (mhcA⁻ and abp120⁻ cells) also move by developing increased lateral contractility. However, under conditions of increased adherence, they were unable to move due to their inability to augment the strength of their traction stresses, break their back adhesions, and migrate forward