Fine Tuning of an Oxidative Stress Model with Sodium Iodate Revealed Protective Effect of NF-κB Inhibition and Sex-Specific Difference in Susceptibility of the Retinal Pigment Epithelium

Abstract

Oxidative stress of the retinal pigment epithelium (RPE) is a major risk factor for age-related macular degeneration (AMD). As a dry AMD model via oxidative stress, sodium iodate (NaIO3), which is primarily toxic to the RPE, has often been used at a high dose to cause RPE death for studying photoreceptor degeneration. Thus, characterization of RPE damage by a low dose of NaIO3 is still limited. To quantify RPE damage caused by NaIO3 in mice, we recently developed a morphometric method using RPE flat-mounts. Here, we report that NaIO3 has a narrow range of dose–effect correlation at 11–18 mg/kg body weight in male C57BL/6J mice. We evaluated the usefulness of our quantification method in two experimental settings. First, we tested the effect of NF-κB inhibition on NaIO3-induced RPE damage in male C57BL/6J mice. IKKβ inhibitor BAY 651942 suppressed upregulation of NF-κB targets and protected the RPE from oxidative stress. Second, we tested sex-specific differences in NaIO3-induced RPE damage in C57BL/6J mice using a low dose near the threshold. NaIO3 caused more severe RPE damage in female mice than in male mice. These results demonstrate the usefulness of the quantification method and the importance of fine-tuning of the NaIO3 dose. The results also show the therapeutic potential of IKKβ inhibition for oxidative stress-related RPE diseases, and reveal previously-unrecognized sex-specific differences in RPE susceptibility to oxidative stress.

SIRT6 overexpression in the nucleus protects mouse retinal pigment epithelium from oxidative stress

“…Therefore, the earliest time point to analyze RPE flat-mounts with ease and consistency was day 7, which we chose as the universal time point for analyzing the extent of RPE damage throughout this study. During these preliminary studies, we also observed that RPE damage by NaIO 3 was not evenly distributed as previously noted ( Xia et al, 2011 ; Ma et al, 2020 ; Upadhyay et al, 2020 ; Wolk et al, 2020 ; Yang et al, 2021 ; Zhang et al, 2021 ). Although RPE in the periphery was well preserved, RPE in the center (posterior) around the optic nerve head was severely damaged, either degenerated or completely lost, compared with control mice with the vehicle ( Fig 1a and b ).…”

“…Using RPE flat-mounts with phalloidin staining that outlines the RPE cell shape, Xia et al reported that NaIO 3 injection in mice produced three regions with distinct RPE morphologies: normal cobblestone-like RPE (periphery), atrophic damaged RPE (center), and irregular elongated cells between the two regions (transitional zone) ( Xia et al, 2011 ). Recently, several others and we also observed similar RPE morphologies induced by NaIO 3 in mice ( Ma et al, 2020 ; Upadhyay et al, 2020 ; Wolk et al, 2020 ; Yang et al, 2021 ; Zhang et al, 2021 ).…”

“…Thus, we confirmed that RPE changes caused by NaIO 3 could be divided into three categories (regions): normal cobblestone-like RPE (periphery), elongated and enlarged RPE (transitional zone), and severely damaged or lost RPE (center) (A, B, and C, respectively, in Fig 1e ). In a separate study, we also noticed that a higher dose of NaIO 3 caused a larger area of severely damaged RPE, and a lower dose of NaIO 3 resulted in a larger area of normal-appearing RPE ( Yang et al, 2021 ), suggesting that these three regions might represent the different degrees of RPE damage, that is, no or mild damage in the periphery, moderate damage in the transitional zone, and severe damage in the center. In this separate study, we found that although NaIO 3 at 10 mg/kg body weight (BW) did not produce RPE morphological damage, NaIO 3 at 20 mg/kg BW caused severe RPE damage in the nearly entire RPE in male C57BL/6J mice ( Yang et al, 2021 ).…”

“…In a separate study, we also noticed that a higher dose of NaIO 3 caused a larger area of severely damaged RPE, and a lower dose of NaIO 3 resulted in a larger area of normal-appearing RPE ( Yang et al, 2021 ), suggesting that these three regions might represent the different degrees of RPE damage, that is, no or mild damage in the periphery, moderate damage in the transitional zone, and severe damage in the center. In this separate study, we found that although NaIO 3 at 10 mg/kg body weight (BW) did not produce RPE morphological damage, NaIO 3 at 20 mg/kg BW caused severe RPE damage in the nearly entire RPE in male C57BL/6J mice ( Yang et al, 2021 ). Because NaIO 3 at 15 mg/kg BW was between these two doses and caused severe RPE damage in roughly a half of the RPE, we assumed that this dose was more sensitive to modulating conditions and therefore used it in most of the experiments in the present study.…”

Tissue-specific sex difference in mouse eye and brain metabolome under fed and fasted states

“…The decrease of these amino acids may predispose the female RPE to oxidative damage. Consistently, Female mice show more severe RPE damage and reduced retinal thickness under oxidative damage induced by sodium iodate 63,64 . RPE mitochondria prefer to oxidize succinate from the retina to produce malate and fumarate for the retina [65][66][67] .…”