The Wnt inhibitor Apcdd1 controls retinal vasculature by modulating β-catenin signaling

The coordination of angiogenesis with acquisition of specialized properties by endothelial cells that line the blood vessels in distinct organs is essential for proper function. An important example of this principle is found in the CNS endothelium, namely the blood-brain barrier (BBB), that restricts paracellular diffusion of molecules into the brain by forming high resistance tight junctions between endothelial cells. Despite its importance for the central nervous system (CNS) function, the mechanisms that regulate angiogenesis and development of this barrier remain poorly characterized. We have previously identified that Wnt/b-catenin signaling is essential for CNS angiogenesis and acquisition of barrier properties by endothelial cells. Here, we report that Apcdd1, a downstream effector of Wnt/b-catenin signaling, is highly expressed in brain and retina endothelial cells during angiogenesis when endothelial cells acquire barrier properties. Moreover, Apcdd1 is selectively expressed in CNS, but not, peripheral endothelial cells and it can interact with both Wnt ligands (e.g. Wnt7a) that induce barrier properties in endothelial cells as well as non-canonical Wnt ligands (e.g. Wnt5a). We propose that Apcdd1 inhibits Wnt/b-catenin signalling in CNS endothelium to allow endothelial cells to mature and acquire BBB properties. In order to test if Apcdd1 is necessary for various aspects of angiogenesis and barrier properties, we have generated Apcdd1 knockout mice using gene targeting and have examined vascular density in the brain and retina at various developmental stages. By E16.5 of embryonic development Apccd1-/- mice have normal brain vascular density similar to wild-type littermates, whereas by postnatal day P5, the mutants have increased brain vascularization. Similarly, while the initial stages of retinal angiogenesis are normal, the Apcdd1-deficient mice have an increased vascularization by P10-P12 when vascular pruning occurs in the retina. Furthermore, Apcdd1 -/- mice show no increase in endothelial cell proliferation, but they have deficits in vascular pruning, suggesting that Apcdd1 plays an essential role in pruning and maturation of the CNS vasculature. We are currently analyzing whether Apcdd1 mice have deficits in BBB formation.