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Amit Patel, Abigail Hackam; Toll-Like Receptor 3 (TLR3) protects the retina from oxidative stress. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1192. doi: https://doi.org/.
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A leading cause of visual impairment is age-related macular degeneration (AMD). Elevated oxidative stress and abnormal innate immune regulation are major risk factors in disease progression. Genetic polymorphisms in TLR3 are associated with AMD but the precise role of TLR3 in AMD is unknown. Although several reports suggest that TLR3 leads to retinal cell death, other studies indicate TLR3 can be cytoprotective. Here, we tested the hypothesis that TLR3 signaling increases RPE and photoreceptor survival during oxidative stress.
The viability of primary RPE cultures derived from wild-type and TLR3 knock-out mice and the ARPE-19 cell line was measured using Cell Titer Blue assays. TLR3 signaling was activated by 100 µg/ml Poly (I:C). Cell cultures were exposed to oxidative stress using 0.4-0.8 mM paraquat. STAT3 expression was measured by IHC and Western blotting using antibodies against total and phosphorylated STAT3. Adult mice were injected subretinally with 1 µg Poly (I:C) and/or 1 µl of 1 mM paraquat and were examined by OCT and ERG.
TLR3 activation increased survival of RPE cells exposed to oxidative stress by 50% in both primary RPE cultures (n=3, p<0.05) and the ARPE-19 cell line (n=5, p<0.05). TLR3 significantly induced STAT3 signaling (n=3, p<0.05) and knockdown of STAT3 using siRNA abolished the protective effect of TLR3 during oxidative stress (n=4, p<0.05). TLR3 activation also protected photoreceptors from oxidative stress injury in vivo. Poly (I:C) injected in the presence of paraquat-induced oxidative stress in wild type mice increased photoreceptor layer thickness by 20%, compared with paraquat alone. In contrast, poly (I:C) injections in TLR3 knock-out mice did not result in photoreceptor protection from oxidative stress. Furthermore, there was no difference in photoreceptor function between adult wild type and TLR3 knock-out mice in the absence of oxidative stress, measured by ERG, indicating that TLR3 presence or absence does not alter the normal retinal phenotype.
We demonstrated that the innate immunity receptor TLR3 has a novel pro-survival role for RPE and photoreceptors in the presence of oxidative stress in vitro and in vivo. Furthermore, TLR3 induced protection is mediated by the STAT3 pathway. TLR3 signaling and related downstream pathways could be further investigated as targets for developing novel therapeutic strategies for AMD.
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