Junctional and Actomyosin Dynamics Drive Endothelial Cell Rearrangements during Vascular Tube Formation

Lumen formation is a key process during the morphogenesis of tubular organs such as the vertebrate vascular network. At the cellular level, lumen formation can be achieved by cell shape changes or cell rearrangements. We have previously shown that such cell rearrangements are driven by oscillating membrane protrusions, called junction-based lamellipodia (JBL), which provide a ratchet mechanism driving convergent cell movements to join local lumens and generate vascular patency. By performing in vivo time-lapse imaging at high spatiotemporal resolution, we have analyzed the cytoskeletal and junctional dynamics, which underlie JBL formation and function. We show that JBL formation requires the activity of the F-actin nucleation complex Arp2/3. We further show that a novel junction is formed at the distal end of the JBL from a pool of VE-cadherin, originating from outside the initial JBL area.Subsequently, proximal and distal junctions merge and fuse, a process driven by actomyosin contractility. Prior to this fusion we observe a specific recruitment Myl9 within the interjunctional space. Furthermore, inhibition of actomyosin contractility abrogates junctional merging. Taken together, our analyses demonstrate that JBL constitute a module, which by alternate generation of pushing forces (JBL formation) and pulling forces (junctional merging) provide the physical means of endothelial cells to elongate and to rearrange thereby generating a continuous vascular lumen.