Cell migration: mechanisms of rear detachment and the formation of migration tracks.

Cell migration is central to many biological and pathological processes, including embryogenesis, tissue repair and regeneration as well as cancer and the inflammatory response. In general, cell migration can be usefully conceptualized as a cyclic process. The initial response of a cell to a migration-promoting agent is to polarize and extend protrusions in the direction of migration. These protrusions can be large, broad lamellipodia or spike-like filopodia, are usually driven by actin polymerization, and are stabilized by adhering to the extracellular matrix (ECM) via transmembrane receptors of the integrin family linked to the actin cytoskeleton. These adhesions serve as traction sites for migration as the cell moves forward over them, and they must be disassembled at the cell rear, allowing it to detach. The mechanisms of rear detachment and the regulatory processes involved are not well understood. The disassembly of adhesions that is required for detachment depends on a coordinated interaction of actin and actin-binding proteins, signaling molecules and effector enzymes including proteases, kinases and phosphatases. Originally, the biochemically regulated processes leading to rear detachment of migrating cells were thought not to be necessarily accompanied by any loss of cell material. However, it has been shown that during rear detachment long tubular extensions, the retracting fibers, are formed and that "membrane ripping" occurs at the cell rear. By this process, a major fraction of integrin-containing cellular material is left behind forming characteristic migration tracks that exactly mark the way a cell has taken.