Ischemic injury to retinal tissue is a clinical condition that frequently leads to visual impairments and blindness, affecting patients' quality of life and functional status. But a more rigorous understanding of the mechanisms of ischemia-induced retinal injury will be required if new therapies for the management of retinal disease are to be developed. Previously, we demonstrated the deleterious role of toll-like receptor 4 (Tlr4) signaling in retinal inflammation and damage triggered by ischemic conditions. Since Tlr4 signaling consists of two distinct signaling cascades, Myd88- and Trif-dependent, we individually evaluated the role of these cascades in ischemic retinae. Available evidence appears to indicate that the effects mediated by Trif and Myd88 signaling in ischemic retinal tissue are retina-specific and asymmetric. Whereas inactivation of either Trif or Myd88 resulted in significantly reduced inflammation in ischemic retinal tissue, ischemic retinae of Myd88-deficient animals demonstrated significantly higher levels of damage than ischemic retinae of Trif-deficient animals. We also noted that ischemia-induced Trif signaling directly facilitates necrotic retinal ganglion cell (RGC) death. Furthermore, we demonstrated that RGC necrosis, which exacerbates retinal injury by promoting more inflammation, can be regulated in some cases (termed ?necroptosis?). Since Trif signaling can mediate cell necroptosis, we hypothesize in Aim 1 that Trif signaling mediates ischemia-induced retinal damage by promoting RGC necroptosis. In addition, Trif-dependent signaling differs from the Myd88 signaling cascade in its ability to activate type I interferon (IFN) signaling. It was previously shown that Trif, via its activation of IFN, activates caspase-11 (Casp11), which can in turn mediate cell death both directly and indirectly (via NLRP3 inflammasome activity and interleukin-1b [Il1b] release). In light of our finding of high levels of Casp11 and inflammasome activity in ischemic RGCs, we hypothesize in Aim 2 that ischemia-induced Trif signaling mediates Casp11 activation, which in turn mediates RGC death both directly and indirectly (via elevated inflammatory responses). Finally, since Mller glia (MG) activity is associated with neuroprotection in nearly every pathological condition in the retina, we hypothesize in Aim 3 that ischemia-induced, MG-specific, Tlr4- dependent neuroprotective activity prevails over neurotoxic activity to facilitate survival of ischemic RGCs. If the proposed hypotheses are found to be correct, we will be able to explain the asymmetric roles of Trif and Myd88 in ischemic retinal tissue: while Myd88 regulates glial toxicity (astrocytes and microglia) and neuroprotection (MG) in ischemic retinae, Trif activates both aforementioned phenomena while also directly mediating RGC death, thus promoting more significant retinal damage than Myd88. To evaluate our hypotheses, we will use animal models and employ a wide range of biochemical, molecular, and cell biological techniques. By completing this project, we will assemble an intellectual foundation for new retina-specific therapeutic strategies that can more effectively treat ischemic retinal diseases.
Ischemic injury is implicated in a large array of pathological conditions in the retina. Although the cause of ischemic retinal injury is multifactorial, increasing experimental evidence suggests an important role of the toll- like receptor signaling cascade in the inflammation and retinal damage triggered by ischemia. A more detailed understanding of the neurotoxic effects of toll-like receptor signaling in ischemic retinal tissue will aid in the development of novel therapeutic approaches for treating retinal pathologies.
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