Sub-Marginal Endothelial Cells Dynamically Regulate Endothelial Sheet Migration.

Migration of endothelial cells as a sheet in fully differentiated blood vessels is essential for reducing vascular permeability during wound healing. Indeed, loss of collective endothelial sheet migration contributes to increased vascular permeability in tumor angiogenesis and several vascular proliferative disorders. Despite this significance, mechanisms responsible for keeping migrating endothelial cells in a monolayer, sheet or tube are poorly understood. To unravel the basis for collective endothelial cell migration, we used time-lapse video microscopy to study early events of wound closure in confluent monolayers of primary microvascular endothelial cells in a live cell chamber. Immediately after wounding (0– 20 min), endothelial cells at the margin of the wound (marginal cells) retracted away from the wounded area and showed no visible lamellipodia extensions. The next phase of early wound healing (20 min – 6 hours) revealed extensive lamellipodia formation and migration of marginal cells into the wounded region. Remarkably, sub-marginal endothelial cells that were several microns away from the wound edge protruded lamellipodia that formed dynamic cell-cell contacts with the substratum of marginal cells at the wound edge. In several instances sub-marginal cells physically and coordinately pulled back endothelial cells at wound edge to maintain regularity of the endothelial sheet front. Cell-tracking measurements revealed autonomous and yet coordinated migration of marginal and sub-marginal endothelial cells culminating in net protrusion of the endothelial sheet into the wound. This study provides in real-time evidence of retraction of endothelial cells at the wound edge by several microns prior to the initiation of forward migration. In addition, we show for the first time that endothelial cells several microns away from the wound edge actively participate in sheet migration through the extention of lamellipodia into the substratum of cells at the wound edge. These findings highlight an important role in endothelial sheet migration for the Rho family of GTPases given their intimate control of cell retraction and lamelipodia extensions. Future studies will directly evaluate the influence of Rac, Cdc42 and RhoA in retraction of marginal cells and formation of lamellipodia by sub-marginal cells in endothelial sheet migration.