Low-Density Lipoprotein Receptor-Related Protein 5 (LRP5) Regulates Retinal Blood Vessel Development and Formation of Pathologic Neovascularization

Abstract

Introduction: :

Purpose: : A fundamental problem in many blinding pediatric vascular eye diseases such as retinopathy of prematurity, familial exudative vitreoretinopathy (FEVR) and Norrie disease is inadequate blood vessel growth in the retina. This poor vascularization causes low tissue oxygen in the retina which then stimulates subsequent abnormal and sight-threatening vessel proliferation. Abnormalities of Wnt signaling pathways which involve Wnt receptor Frizzled4, co-receptor low-density lipoprotein receptor-related protein 5 (LRP5) and Wnt ligand Norrin have been associated with these diseases in humans. Studies in Wnt signaling are likely to lead us to the cellular and molecular mechanisms of the common disease phenotype of poor retinal vascular growth and to the design of potential therapies. In this study, we investigated the role of Wnt signaling in the development of retinal vasculature and in pathological retinal vessel proliferation using a LRP5 knockout (KO) mouse model deficient in Wnt signaling.

Methods: : Retinal vasculature was examined in LRP5 KO and wild type (WT) mice at various time points during development. Pathological retina neovascularization was modeled by exposing neonatal mice to 75% oxygen from P7 to P12. Gene expression was quantified with real time RT-PCR in whole retina and on laser-captured retinal vessels. Wnt receptors and ligands were localized in retina with immunohistochemistry.

Results: : LRP5 KO mice have significantly delayed growth of the primary retinal vascular plexus, and fail to form deeper networks of retinal vessels. This is associated with a decreased level of endothelial cell sprouting. LRP5 protein is localized specifically in developing neovessels in the retina. In WT mice exposed to oxygen-induced retinopathy, mRNA of Wnt receptors Frizzled4 and LRP5 is significantly upregulated (3-5 fold) in pathologic neovascularization compared with normal vessels. Wnt activity is also found specifically in pathologic neovascularization using Wnt reporter TOPGAL mice. LRP5 KO mice exposed to oxygen-induced retinopathy have significantly decreased level of pathologic neovascularization in the retina.

Conclusions: : Wnt signaling is required for both the normal development of retinal vasculature and the formation of pathologic neovessels in the retina, suggesting that components in Wnt pathway might be potentially useful targets for pharmaceutical intervention for treating retinopathy.

Keywords: retinal development • retinal neovascularization