CXXC5 is a transcriptional activator of Flk-1 and mediates bone morphogenic protein-induced endothelial cell differentiation and vessel formation

CXXC5 is a member of a small subset of proteins containing CXXC-type zinc-finger domain. Here, we show that CXXC5 is a transcription factor activating Flk-1, a receptor for vascular endothelial growth factor. CXXC5 and Flk-1 were accmulated in nucli and membrane of mouse embryonic stem cells (mESCs), respectively, during their endothelial differentiation. CXXC5 overexpression induced Flk-1 transcription in both endothelium-differentiated mESCs and human umbilical vein endothelial cells (HUVECs). In vitro DNA binding assay showed direct interaction of CXXC5 on the Flk-1 promoter region, and mutation on its DNA-binding motif abolished transcriptional activity. We showed that bone morphorgeneic protein 4 (BMP4) induced CXXC5 transcription in the cells, and inhibitors of BMP signaling suppressed the CXXC5 induction and the consequent Flk-1 induction by BMP4 treatment. CXXC5 knockdown resulted in suppression of BMP4-induced stress fiber formation (56.8±1.3% decrease, P<0.05) and migration (54.6±1.9% decrease, P<0.05) in HUVECs. The in vivo roles of CXXC5 in BMP-signaling-specific vascular development and angiogenesis were shown by specific defect of caudal vein plex vessel formation (57.9±11.8% decrease, P<0.05) in cxxc5 morpholino-injected zebrafish embryos and by supression of BMP4-induced angigogensis in subcutaneously injected Matrigel plugs in CXXC5^-/- mice. Overall, CXXC5 is a transcriptional activator for Flk-1, mediating BMP signaling for differentiation and migration of endothelial cell and vessel formation.