Objective-Apelin is an endogenous ligand for the G protein-coupled receptor, APJ, and participates in multiple physiological processes. To identify the roles of endogenous apelin, we investigated the phenotype of apelin-deficient (apelin-KO) mice. Methods and Results-Apelin-KO mice showed impaired retinal vascularization and ocular development, which were analyzed by histology, immunohistochemistry, real-time polymerase chain reaction, and the mouse corneal micropocket assay. Apelin-KO mice showed significantly impaired retinal vascularization in the early postnatal period. Retinal apelin/APJ mRNAs were transiently upregulated during the first 2 postnatal weeks but were undetectable in adults. There were no differences in VEGF or FGF2 mRNA expression, or in the morphology and localization of GFAP-positive astrocytes, in the apelin-KO retinas at P5. The corneal pocket assay showed that angiogenic responses to VEGF and FGF2 were remarkably decreased in apelin-KO mice. The reduced responses to VEGF and FGF2 in apelin-KO mice were partially restored by apelin, but apelin alone did not induce angiogenesis.
Conclusions-Our
See accompanying article on page 1687Apelin/APJ mRNAs are expressed in multiple tissues, primarily in vascular endothelial cells, where they may contribute to angiogenesis. 8,9 Apelin stimulates the proliferation and migration of RF/6A cells, an endothelial cell line of monkey retina, thereby demonstrating angiogenic-like effects. 9 Similarly, apelin is a potent angiogenic factor required for the normal vascular development of frog embryos. 8 During mouse and frog embryonic development, the APJ receptor is highly expressed in endothelial precursor cells and in nascent vascular structures. Apelin is a potent angiogenic factor in 2 in vivo angiogenesis assays, the frog embryo and the chicken chorioallantoic membrane (CAM) assay. 8 More recently, Kidoya et al showed that the apelin/APJ system is involved in the regulation of blood vessel diameter during angiogenesis. 10 In the mouse, the formation of retinal vessels begins at birth and follows a centrifugal extension of retinal vessels from the optic disc to the periphery of the retina. Interestingly, retinal expression of APJ mRNA is observed during the formation of retinal vessels and traces the centrifugal extension of the superficial vasculature bed, whereas that of apelin mRNA is localized in "tip cells," an endothelial subpopulation that forms cell protrusions and directs the polarized extension of the vascular network. 11 In a mouse model of hypoxia-induced retinopathy of prematurity, APJ mRNA transcripts are localized in endothelial cells, and their expression traces the centripetal extension of the retinal network from the periphery of the retina to the optic disc. 12 These findings suggest that apelin/APJ signaling is involved in regulating retinal vascularization.Ocular developmental vascularization is a complex process, which requires the strict coordination of numerous molecular and cellular interactions. Retinal vascularization is contro...
