Ratchetaxis in Channels: Entry Point and Local Asymmetry Set Cell Directions in Confinement.

Cell motility is essential in a variety of biological phenomena ranging from early development to organ homeostasis and diseases. This phenomenon has mainly been studied and characterized on flat surfaces in vitro, whereas such conditions are rarely observed in vivo. Recently, cell motion in three-dimensional microfabricated channels was reported to be possible, and it was shown that confined cells push on walls. However, rules setting cell directions in this context have not yet been characterized. Here, we show by using assays that ratchetaxis operates in three-dimensional ratchets in fibroblasts and epithelial cancerous cells. Open ratchets rectify cell motion, whereas closed ratchets impose direct cell migration along channels set by the cell orientation at the channel entry point. We also show that nuclei are pressed in constriction zones through mechanisms involving dynamic asymmetries of focal contacts, stress fibers, and intermediate filaments. Interestingly, cells do not pass these constricting zones when they contain a defective keratin fusion protein implicated in squamous cancer. By combining ratchetaxis with chemical gradients, we finally report that cells are sensitive to local asymmetries in confinement and that topological and chemical cues may be encoded differently by cells. Overall, our ratchet channels could mimic small blood vessels in which cells such as circulating tumor cells are confined; cells can probe local asymmetries that determine their entry into tissues and their subsequent direction. Our results shed light on invasion mechanisms in cancer.