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Amit K Patel, Abigail S Hackam; A novel protective role for innate immunity Toll-Like Receptor 3 (TLR3) in retinal degeneration. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5755.
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Age-related macular degeneration (AMD) is one of the leading causes of blindness in the aged population. Risk factors of AMD include oxidative stress and abnormal innate immune activation, but how these two processes interact during retinal disease is still unknown. The toll-like receptor (TLR) branch of innate immunity responds to endogenous ligands released during injury and can be protective or pathogenic. Although TLR3 was recently demonstrated to induce retinal death, the consequences of TLR3 activation during concurrent AMD-like injury are still unknown. Here, we examined the role of TLR3 activation on photoreceptor survival during oxidative stress injury.
Wild type and TLR3 knockout mice were subretinally injected with 1 mM paraquat (PQ) to simulate AMD-like oxidative stress injury and 2µg of Poly (I:C) to activate TLR3 signaling. PBS was the vehicle control. Photoreceptor survival and function was assessed 2 weeks post-injection using OCT and ERG and optokinetic evaluation of visual acuity. Stat3 activation was measured by Western blotting using antibodies against total and phosphorylated Stat3.
TLR3 activation in the presence of oxidative stress increased photoreceptor layer thickness by 25%, compared with the injury alone group (PQ injected) (n=5, p<0.05) whereas TLR3 activation alone induced photoreceptor death. Photoreceptor function was also increased by TLR3 activation, shown in mice co-injected with Poly (I:C) and PQ that had greater than 3-fold higher ERG responses and 4-fold higher visual acuity compared with mice with only oxidative injury (n=4-5, p<0.05). There was no effect of Poly (I:C) in TLR3 knockout mice, indicating that increased survival was TLR3 dependent (n=4). Additionally, Stat3 signaling increased by 1.75 fold in retinas co-injected with Poly (I:C) and PQ, compared with mice injected with either compound alone (n=5, p<0.05). Knockdown of Stat3 using lentiviral delivery of Stat3 shRNA eliminated TLR3-dependent retinal protection during oxidative stress, implicating Stat3 as an essential mediator of protection (n=2).
We identified a novel neuroprotective role for TLR3 in the retina during injury. Furthermore, we discovered STAT3 signaling is a necessary component of TLR3-induced protection. These results may lead to new therapeutic approaches for AMD and other retinal degenerations.
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