Apelin-APJ Signaling in Retinal Angiogenesis

he G protein-coupled receptor (GPCR) APJ (X-msr, angiotensin receptor like 1b) was cloned in several laboratories by homology screening, with the goal of identi- fying new members of this class of cell surface receptor.1 This gene attracted attention because of its sequence similar- ity to the important angiotensin II type 1 receptor and its highly restricted pattern of expression in endothelial cells. Five years later, this GPCR was deorphanized with cloning of the APJ-endogenous ligand apelin. Apelin was found to be synthesized as a 77-aa preproprotein and cleaved to a number of active peptides, including those of 36, 17, and 13 amino acids. Interestingly, apelin was also found to be highly expressed in endothelial cells. See accompanying article on page 1717 Detailed evaluation of APJ and apelin expression patterns in embryogenesis, and in the developing retina, suggested the hypothesis that autocrine signaling of this pathway in endo- thelial cells provides a mechanism for regulating new blood vessel growth, or angiogenesis.2,3 Loss of function experi- ments in frog embryos using morpholino knockdown exper- iments have consistently shown vascular developmental ab- normalities, varying from perturbed intersomitic vessel branching to more fundamental developmental defects, in- cluding loss of the posterior cardinal vein, and decreased numbers of endothelial cells.2,4 Overexpression of Xapelin led to disorganized expression of endothelial precursor mark- ers at the neurula stage. In zebrafish, knockout studies of one of the APJ homologues (angiotensin receptor like 1b) failed to perturb vascular development, but APJ receptor knock- down was found to decrease the hypoxia-induced vessel regeneration in the Fli-1 transgenic zebrafish model.5,6 Re- cent studies with an apelin null mouse model have suggested that this pathway is downstream of the angiopoietin-tek/tie pathway, and regulates blood vessel caliber.7 Activation of this pathway in cultured endothelial cells has been shown to promote migration and proliferation, and blood vessel growth-promoting functions of apelin have been demon- strated in the Matrigel plug assay in the mouse and chick chorioallantoic membrane assay.2 because of the highly programmed pattern of development that allows easy study of angiogenesis in vivo in the mouse, and the relationship to vascular proliferation and visual impairment in human diseases such as diabetes. These inves- tigators found Apelin-APJ mRNA expression in the retina to be transiently upregulated in the period of early postnatal vascular development, with expression disappearing in adult mice, after completion of angiogenesis. Retinal vasculariza- tion in apelin null mice (apelin-KO) appeared to be tempo- rally delayed in the early stages of vascular development compared to that in wild-type mice, but the pattern and extent of vascularization was the same in adult mice. Interestingly, in apelin-KO mice, the angiogenic response to VEGF and FGF introduced in the corneal pocket assay was reduced. Although apelin alone had no effect on angiogenesis in this assay, apelin treatment of the apelin-KO mice restored the angiogenic response of VEFG and FGF, showing an angio- genic effect of apelin when working cooperatively with