Most cells migrate under the appropriate conditions or stimuli; understanding the mechanisms of migration, the players involved, and their regulation, is pivotal to tackle the pathological situations where migration becomes an undesired effect. While largely overshadowed by the study of single cell migration, collective cell migration is a very relevant process that takes place during development as well as in adult life. Collective migration is very relevant for the formation and maintenance of epithelial layers: extensive migratory processes occur during the shape of the embryo, as well as during the healing of a skin incision in the adult. When openings or discontinuities appear in the epithelia, it is crucial that the appropriate mechanisms are activated.In the present work we attempt at deciphering what are the mechanisms involved in gap closure. Until now, most of the literature concerning the subject has reported contradictory results, mainly arising from the complexity of the process and the lack of systematic analysis. We have designed a novel approach to address epithelial gap closure under well-defined and controlled conditions. By using our gap patterning method, we have observed that epithelial cells extend lamellipodia when exposed to a newly available space. Interestingly, we found that the closure of such gap depends on the size: small gaps are closed by a passive physical mechanism, while large gaps are closed through a Rac-dependent cell crawling mechanism, in a collective migration-like manner. 11Abstract (English)Most cells migrate under the appropriate conditions or stimuli; understanding the mechanisms of migration, the players involved, and their regulation, is pivotal to tackle the pathological situations where migration becomes an undesired effect. While largely overshadowed by the study of single cell migration, collective cell migration is a very relevant process that takes place during development as well as in adult life. Collective migration is very relevant for the formation and maintenance of epithelial layers: extensive migratory processes occur during the shape of the embryo, as well as during the healing of a skin incision in the adult. When openings or discontinuities appear in the epithelia, it is crucial that the appropriate mechanisms are activated.In the present work we attempt at deciphering what are the mechanisms involved in gap closure. Until now, most of the literature concerning the subject has reported contradictory results, mainly arising from the complexity of the process and the lack of systematic analysis. We have designed a novel approach to address epithelial gap closure under well-defined and controlled conditions. By using our gap patterning method, we have observed that epithelial cells extend lamellipodia when exposed to a newly available space. Interestingly, we found that the closure of such gap depends on the size: small gaps are closed by a passive physical mechanism, while large gaps are closed through a Rac-dependent cell crawling mechanism, in a collective migration-like manner. Next, we also addressed the mechanical component of epithelial gap closure. In this study, we took advantage of a laser-ablation system to disrupt some cells within an epithelial monolayer, and study how the remaining cells sealed that gap. By measuring the traction forces that cells exert on the substrate along the closure, we observed that cells first pulled on the substrate to propel themselves. By the last steps of closure, there is a transition in the direction of the force, so that cells are pulled to the center of the gap due to the assembly of a supracellular actin cable. Altogether, this work provides valuable knowledge on the current understanding of the mechanisms accounting for epithelial gap closure. We believe that a better comprehension of these mechanisms can help to shed light in clinically relevant situations where epithelial gap closure is impaired.
Dynamics of epithelial gap closure using microfabrication and micromechanical approaches