0048 : Frizzled7 controls postnatal vascular formation through Wnt/β-catenin canonical signaling in a Dll4/Notch dependent and independent way

Growing evidence has shown that Wnt/Fzd proteins are expressed in vascular cells and are directly involved in blood vessel development by regulating endothelial cell (EC) properties. In this study, we analyzed the effects of frizzled7 deficiency in EC on vessel formation in the postnatal mouse retina. We generated fzd7F/F mice and crossed them with Pdgfb-iCre mice to generate fzd7iECKO mice. In the fzd7iECKO retinas at P7 dpn, we observed a delay in vascular network formation, a strong increased in tip cells and filopodia numbers and a decrease in proliferation compared to WT. Interestingly, we also analyzed the retinal vascular development in mice KO for Dishevelled1 (DVL1), a partner involved in Wnt/Fzd signaling downstream Fzd receptor. DVL1KO mice showed a similar but mildly vascular phenotype compared with fzd7iECKO mice. Moreover, intraocular injection of siRNADVL3 in DVL1KO mice increased DVL1KO phenotype and mimics fzd7iECKO mice phenotype showing that Fzd7 may control angiogenesis through DVL1 and 3. To a better understanding of how fzd7 regulates vessels formation in vivo, we analysed the effects of LiCl injection (activator of β-catenin pathway) or Dll4 intraocular injection in fzd7iECKO and WT mice on vascular retinal phenotype. Our results showed that LiCl totally overrules the effects of Fzd7 deficiency in EC whereas Dll4 only partly rescues the phenotype observed in fzd7iECKO mice. The tip cells impairment in fzd7iECKO mice is overruled by Dll4 but not the delay of vascular growth. We then assessed whether fzd7 deletion impaired the expression of partners of β- catenin and Notch pathways by RT-qPCR. In vivo, deletion of fzd7 in EC induces a decrease in both partners of β-catenin and Notch pathways expression compared to control. Moreover this impairment of β-catenin and Notch pathways in vivo is totally rescue by LiCl injection. This study shows that fzd7 plays a crucial role during angiogenesis in postnatal mouse retina. Fzd7 controls vascular growth through a β-catenin signaling in a Notch independent way but also regulates the tip cells selection through a β-catenin signaling in a Notch dependent manner