Abstract 327: Blood Vessels Need Ras Signaling to Maintain the Structure Integrity

The localization of prenylated Ras at the plasma membrane promotes activation of Ras by receptor tyrosine kinases, such as VEGF and FGF receptors. Although Ras has been implicated in angiogenesis, the exact regulatory mechanisms controlling Ras translocation and activation are currently unclear because little is known regarding molecules that control Ras translocation. Nogo-B receptor (NgBR) was identified as a receptor specific for Nogo-B, a cell surface ligand involved in blood vessel remodeling. Our recent study demonstrated that NgBR has a conserved hydrophobic pocket that promotes the membrane accumulation of Ras by directly binding prenylated Ras at the plasma membrane. As we expected, NgBR knockdown in endothelial cells diminishes the membrane localization of Ras and consequently abolishes VEGF/FGF-stimulated activation of Ras and Ras-mediated signalings such as phosphorylation of Akt and ERK. Therefore, NgBR knockout mouse is a unique animal model for examining the effects of Ras plasma membrane localization and Ras signaling on the morphogenesis of endothelial cells. Genetic deletion of NgBR in endothelial cells resulted in embryonic lethality and dilated cerebral blood vessels with fewer pericytes, which resembles the vascular lesion happened in cerebral cavernous malformation (CCM). CCM is characterized by an abnormal cluster of enlarged blood vessels in the brain and spinal cord and caused by dysfunction of three CCM genes (CCM1/2/3), which are required for maintaining endothelial cell (EC) junctions and pericyte recruitment. Our studies showed that NgBR transcript levels decrease in human CCM lesion, and NgBR endothelial specific knockout in mice results in decreased transcription of CCM1/2 in the yolk sac. Additional support for NgBR-CCM1/2 connections comes from studies using cultured human brain microvascular ECs, where loss of NgBR expression also decreases CCM1/2 transcription via NgBR-mediated Ras pathway, which is required for the expression of key transcription factors that are involved in regulating transcription of CCM1/2 genes. Our findings suggest that NgBR-Ras signaling pathway regulates CCM1/2 expression, and that disrupting this signaling pathway results in cerebrovascular malformation.