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Excess iron induces tissue damage and is implicated in age-related macular degeneration (AMD). Iron toxicity is widely attributed to hydroxyl radical formation through Fenton's reaction. We report that excess iron, but not other Fenton catalytic metals, induces activation of the NLRP3 inflammasome, a pathway also implicated in AMD. Additionally, iron-induced degeneration of the retinal pigmented epithelium (RPE) is suppressed in mice lacking inflammasome components caspase-1/11 or Nlrp3 or by inhibition of caspase-1. Iron overload increases abundance of RNAs transcribed from short interspersed nuclear elements (SINEs): Alu RNAs and the rodent equivalent B1 and B2 RNAs, which are inflammasome agonists. Targeting Alu or B2 RNA prevents iron-induced inflammasome activation and RPE degeneration. Iron-induced SINE RNA accumulation is due to suppression of DICER1 via sequestration of the co-factor poly(C)-binding protein 2 (PCBP2). These findings reveal an unexpected mechanism of iron toxicity, with implications for AMD and neurodegenerative diseases associated with excess iron., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

Alu RNA accumulation due to DICER1 deficiency in the retinal pigmented epithelium (RPE) is implicated in geographic atrophy (GA), an advanced form of age-related macular degeneration that causes blindness in millions of individuals. The mechanism of Alu RNA-induced cytotoxicity is unknown. Here we show that DICER1 deficit or Alu RNA exposure activates the NLRP3 inflammasome and triggers TLR-independent MyD88 signaling via IL18 in the RPE. Genetic or pharmacological inhibition of inflammasome components (NLRP3, Pycard, Caspase-1), MyD88, or IL18 prevents RPE degeneration induced by DICER1 loss or Alu RNA exposure. These findings, coupled with our observation that human GA RPE contains elevated amounts of NLRP3, PYCARD, and IL18 and evidence of increased Caspase-1 and MyD88 activation, provide a rationale for targeting this pathway in GA. Our findings also reveal a function of the inflammasome outside the immune system and an immunomodulatory action of mobile elements., (Copyright © 2012 Elsevier Inc. All rights reserved.)

The discovery of sequence-specific gene silencing by endogenous double-stranded RNAs (dsRNA) has propelled synthetic short-interfering RNAs (siRNAs) to the forefront of targeted pharmaceutical engineering. The first clinical trials utilized 21-nucleotide (nt) siRNAs for the treatment of neovascular age-related macular degeneration (AMD). Surprisingly, these compounds were not formulated for cell permeation, which is required for bona fide RNA interference (RNAi). We showed that these "naked" siRNAs suppress neovascularization in mice not via RNAi but via sequence-independent activation of cell surface Toll-like receptor-3 (TLR3). Here, we demonstrate that noninternalized siRNAs induce retinal degeneration in mice by activating surface TLR3 on retinal pigmented epithelial cells. Cholesterol conjugated siRNAs capable of cell permeation and triggering RNAi also induce the same phenotype. Retinal degeneration was not observed after treatment with siRNAs shorter than 21-nts. Other cytosolic dsRNA sensors are not critical to this response. TLR3 activation triggers caspase-3-mediated apoptotic death of the retinal pigment epithelium (RPE) via nuclear translocation of interferon regulatory factor-3. While this unexpected adverse effect of siRNAs has implications for future clinical trials, these findings also introduce a new preclinical model of geographic atrophy (GA), a late stage of dry AMD that causes blindness in millions worldwide.