Rasip 1 regulates vertebrate vascular endothelial junction stability through Epac 1-Rap 1 signaling

A central question in vascular development is how blood vessels form and maintain patent lumens. During vertebrate embryonic vasculogenesis, endothelial cells (ECs) aggregate into cords followed by de novo lumenogenesis, leading to the formation of lumenized vessels such as the paired dorsal aortae (DA) and yolk sac vascular plexus, which serve as the basis for further vascular expansion and angiogenesis. The current understanding of cellular mechanisms of de novo endothelial lumen formation is influenced by principles underlying formation of epithelial tubes or cysts. Vascular lumenogenesis requires membrane rearrangements and trafficking, apicalbasal partitioning of repulsive and adhesive proteins, rearrangement of junctional components, and cytoskeletal remodeling. Stabilizing vessels is critical to continued growth and development of an embryo. In murine development, onset of circulation coincides with active growth of the DA and yolk sac vascular plexus. Subsequent angiogenesis and remodeling involve extensive cell movement and rearrangement. Nascent vessels must be stable to permit circulation and withstand increasing shear stress, while allowing dynamic growth and remodeling. Vascular permeability is tightly regulated throughout development and adult life as changes in vessel permeability contribute to pathologies including hemorrhage and edema. EC-EC junctions are essential in control of vascular remodeling and permeability, and the mechanisms involved in their assembly and disassembly are under intense investigation. Key components of EC-EC tight junctions (TJs) and adherens junctions (AJs), such as zona occludens 1 (ZO-1) and vascular endothelial cadherin (VE-cadherin), critically influence vascular development and lumen stabilization in vivo, yet the complete molecular ensemble that regulates EC junctions remains to be defined. A key regulator of junction formation is the small GTPase Rasrelated protein 1 (RAP1). Membrane signaling events trigger the formation of cyclic adenosine monophosphate (cAMP), which binds to the RAP1 guanine nucleotide exchange factor 3 (RAPGEF3 or exchange protein directly activated by cAMP 1 [EPAC1]) to activate RAP1, triggering a cascade leading to stabilization and linkage of cortical actin to AJ and TJ complexes. Modulation of RAP1 activity affects endothelial barrier function. Disruption of RAP1 signaling in ECs leads to increased permeability and a failure to form stable endothelial tubes. Furthermore, activation of RAP1 signaling increases density of and cross-linking of cortical actin with VEcadherin, and promotes stable cell-cell contacts. RAP1 signaling has also been linked to the recruitment of nonmusclemyosin II (nmMHCII)