Molecular Mechanisms of Sprouting Angiogenesis in the Retina: Crosstalk between Notch and Norrin-Wnt signaling

Abstract

Norrie disease is an X-linked recessive disorder that presents with congenital blindness, progressive deafness and rarely with mental retardation. It is caused by mutations in the Norrie Disease Pseudoglioma (NDP) gene, which may also lead to milder diseases like exudative vitreoretinopathy, retinopathy of prematurity or Coats’ disease. In the eye, mutations in NDP lead to a delayed and incomplete development of the superficial retinal vascular plexus (SRVP) and they prevent the development of the two deep retinal vascular plexuses. This is accompanied with persisting hyaloid vessels around the vitreous. It has been shown that Norrin, the protein product of NDP, binds the coreceptor complex consisting of Fz-4, LRP-5 and Tspan-12 which leads to the downstream activation of canonical Wnt signaling. Despite the phenotypic and molecular knowledge about Norrin, the intrinsic function of Norrin during retinal blood vessel development remains a mystery. This thesis aimed to gain more insight about the role of Norrin during retinal blood vessel development from Ndphy/- mice. This work suggests that Norrin-Wnt signaling and Notch signaling may not be directly linked. Further, endomucin was identified as a marker for central thorn-like aligned filopodia in Ndphy/- mice and extensive mural cell coverage of the SRVP from Ndphy/- retinas was found after P9. Arterial/vein crossing within the SRVP of Ndphy/- retinas was described for the first time. This indicates that MAPK signaling might be altered in Ndphy/- retinas since knockout mice with altered MAPK signaling also display vessel crossing. Norrin was further identified to be a mitogen for cells of the SRVP which is in concert with in vitro data from micro vascular endothelial cells. Finally, equal numbers of hyaloid vascular associated macrophages was found between P7 and P12 in Ndphy/- mice, suggesting that hyaloid vessels fail to regress despite the presence of a constant number of macrophages. In sum, it was shown that the loss of Norrin signaling might primarily affect MAPK signaling rather than Notch signaling.