A novel protective role for the innate immunity Toll-Like Receptor 3 (TLR3) in the retina via Stat3

Abstract

The innate immune system and inflammatory pathways play key roles in numerous diseases of the central nervous system (CNS). Recent evidence indicates that innate immunity induces both pathogenesis and protection during neuronal injury. To test the possibility that the conflicting roles of innate immunity in the CNS depends on the cellular environment in which innate immunity is stimulated, we analyzed the effect of toll-like receptor 3 (TLR3) activation on neuronal survival in the presence and absence of oxidative injury in a mouse model system. We demonstrated that activation of TLR3 by the double stranded RNA activator, Poly (I:C), during paraquat induced oxidative stress, significantly protected mouse photoreceptors, as measured by increased retinal structure, function, and improved visual acuity. In contrast, TLR3 activation without concurrent oxidative injury was neurotoxic. The neurotoxic and protective effects of Poly (I:C) stimulation were absent in TLR3 knockout animals, which indicates that protection by Poly (I:C) is dependent on the TLR3 signaling pathway. Furthermore, we identified the pro-survival transcription factor Stat3 as a necessary mechanism for protection. Knockdown of Stat3 using lentivirally delivered shRNA abolished the protective effects of TLR3 signaling in the retina during oxidative stress. Therefore, TLR3 activation in the context of oxidative stress triggers protective instead of pathogenic signaling, suggesting that TLR3 is a potential therapeutic target for neurodegeneration where oxidative stress is a significant contributor.

Protective effects of a grape-supplemented diet in a mouse model of retinal degeneration

“…The mice were first anesthetized by intraperitoneal injection of a xylazine-ketamine mixture and the eyes were locally anesthetized with eye drops containing 0.5% proparacaine hydrochloride (Akorn, Lake Forest, IL). Mice were then subretinally injected with either 2 μl of paraquat (PQ, 1mM) or saline control, as described [13]. Briefly, an incision was made in the conjunctiva and the sclera and injections were performed using a 1.5 cm 33-gauge Hamilton needle (Hamilton Company, Reno, NV) between the RPE and neural retina.…”

“…Retinal function was analyzed two weeks after paraquat injury using electroretinography (ERG), as described [13]. The mice were dark adapted and then anesthetized with a ketamine/xylazine cocktail and the pupils were dilated with topical 10% phenylepherine hydrochloride.…”

“…Paraquat is processed in the mitochondria into free radicals [12] and induces oxidative damage to the retina when injected directly into the retina [11]. We previously reported that paraquat induced retinal degeneration in mice, and reduced retinal thickness, lower photoreceptor counts, attenuated ERG responses and decreased visual acuity, which could all be prevented by delivery of specific molecules [13]. …”

Toll-Like Receptor 3 Activation Initiates Photoreceptor Cell Death In Vivo and In Vitro

“…It has been reported that STAT3 28,29 and c-Jun N-terminal kinases (JNK3) 30 are key factors in TLR3 activation in retinal cells. We found significant activation of JAK-STAT signaling pathway, but not upregulation of JNK3 in poly I:C-treated retina and 661W cells.…”

“…29,44,45 Photoreceptor cell death has been observed in a poly I:C-injected mouse model; however, whether photoreceptor cells respond to dsRNA directly or are affected by RPE dysfunction remains unexplained. Here we have shown that poly I:C treatment induced photoreceptor cell apoptosis, although the underlying mechanisms have not been fully elucidated (Fig.…”