Signaling pathways and cell mechanics involved in wound closure by epithelial cell sheets

Background: Sheets of cells move together as a unit during wound healing an d embryonic tissue movements, such as those occurring during gastrulation a nd neurulation. We have used epithelial wound closure as a model system for such movements and examined the mechanisms of closure and the importance o f the Rho family of Ras-related small GTPases in this process. Results: Wounds induced in Madin-Darby canine kidney (MDCK) epithelial cell monolayers close by Rac- and phosphoinositide-dependent cell crawling, wit h formation of lamellipodia at the wound margin, and not by contraction of a perimarginal actomyosin purse-string. Although Rho-dependent actin bundle s usually form at the margin, neither Rho activity nor formation of these s tructures is required for wound closure to occur at a normal rate. Cdc42 ac tivity is also not required for closure. Inhibition of Rho or Cdc42 results , however, in statistically significant decreases in the regularity of woun d closure, as determined by the ratio of wound margin perimeter over the re maining denuded area at different times. The Pac-dependent force generation for closure is distributed over several rows of cells from the wound margi n, as inhibition of motility in the first row of cells alone does not inhib it closure and can be compensated for by generation of motile force in cell s behind the margin. Furthermore, we observed high levels of Rac-dependent actin assembly in the first few rows of cells from the wound margin. Conclusions: Wounds in MDCK cell sheets do not close by purse-string contra ction but by a crawling behavior involving Pac, phosphoinositides and activ e movement of multiple rows of cells, This finding suggests a new distribut ed mode of signaling and movement that, nevertheless, resembles individual cell motility, Although Rho and Cdc42 activities are not required for closu re, they have a role in determining the regularity of closure.