Membrane traffic as a coordinator of cell migration and junction remodeling.

The change in the overall shape of developing organs is a consequence of the cumulative movement, reshaping, and proliferation of the individual mural cells that make up the walls of these organs. Recent observations suggest that the shape and the position of endothelial cells (ECs) in growing blood vessels are highly dynamic, implying that these cells remodel their junctions extensively and do not preserve their initial relative positions. In order to determine the mechanisms that confer the dynamic behavior of mural ECs, we tracked the trafficking of a cell junction protein complex that consists of the RhoA-specific guanine exchange factor (GEF) Syx, the scaffold protein Mupp1, and the phospholipid binding protein Amot.1 We found that RhoA co-trafficked with this complex on the same endocytic vesicles, and that its cellular activity pattern was determined by Rab13-dependent trafficking. The vesicles were targeted by a Rab13-associated protein complex to Tyr(1175)-phosphorylated VEGFR2 at the leading edge of ECs migrating under a VEGF gradient. These results indicate that the dynamic behavior of ECs in sprouting vessels is conferred by using the same protein complex for the regulation of both cell junctions and cell motility. Together with previous studies that demonstrated regulation of Rac signaling by Rab5-dependent trafficking,(2) it appears now that membrane traffic is tightly coupled to the regulation of Rho GTPases, and, consequently, to the regulation of the actin cytoskeleton, cell junctions, and cell migration.