Norrin Protects Photoreceptors From Apoptotic Cell Death

Abstract

Purpose: : Norrin is a signaling molecule, which binds to Fz4 and activates canonical Wnt/beta catenin signaling. We were interested to learn, if overexpression of Norrin in the retina of mice is a mechanism to protect photoreceptors from light-induced damage.

Methods: : Transgenic Rpe65-Norrin mice which overexpress Norrin in the retinal pigment epithelium were generated, characterized, exposed to light damage and analyzed by Western blotting, real time RT-PCR, semithin sectioning, immunohistochemistry and electroretinography (ERG). TOP-Gal Wnt pathway reporter mice were used to detect activation of canonical Wnt signaling.

Results: : Rpe65-Norrin mice do not express an obvious retinal phenotype, but show high amounts of transgenic Norrin in the outer retina and strong activation of Wnt/beta-catenin signaling. 30 h after light damage, significantly fewer TUNEL-positive cells were detected in the outer nuclear layer (ONL) of Rpe65-Norrin mice as compared to wild-type littermates. 7 and 14 days after light damage, the ONL was significantly thinner in control littermates than in Rpe65-Norrin mice. Structural changes correlated with functional changes as detected by ERG. Light damage was enhanced after blocking Wnt-signaling with Dickkopf-1. Untreated Rpe65-Norrin mice showed a significant upregulation of mRNA for GFAP and endothelin-2 , while light damage induced a significant upregulation of BDNF and LEDGF expression. Inhibition of endothelin-2 signaling in Rpe65-Norrin mice caused a significant increase in photoreceptor apoptosis 30h after light damage and prevented the increase in BDNF expression.

Conclusions: : Norrin protects photoreceptors from light-induced apoptotic cell death indicating a neuroprotective role of Norrin in a signaling network that involves the canonical Wnt-pathway, endothelin-2 and upregulation of BDNF.

Keywords: neuroprotection • photoreceptors • retinal degenerations: cell biology