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Yoshio Hirano, Valeria Tarallo, Bradley Gelfand, Sami Dridi, Younghee Kim, Nagaraj Kerur, Hiroki Kaneko, Benjamin Fowler, Sasha Bogdanovich, Jayakrishna Ambati; Alu Rna-induced Cytotoxicity In Age-related Macular Degeneration Is Mediated By Myd88, But Not By A Variety Of Rna Sensors. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2588.
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© ARVO (1962-2015); The Authors (2016-present)
We previously reported that a dramatic and specific reduction of the RNase DICER1 results in accumulation of Alu RNA transcripts in the RPE of human eyes with geographic atrophy (Kaneko et al., 2011). Still, the precise mechanisms of Alu RNA-induced cytotoxicity are unknown. Numerous innate immune receptors have been identified in the RPE (Kumar et al., 2004), and a variety of exogenous substances can induce retinal inflammation (Kleinman et al., 2011). However, it is not known whether this surveillance machinery recognizes or responds to host endogenous RNAs. In this study, we determined whether innate immune machinery would also recognize cytotoxic Alu RNA in age-related macular degeneration.
Subretinal administration of a plasmid coding for Alu RNA (pAlu) was tested in mice deficient in toll-like receptor-3 (TLR3), TLR4, toll / IL-1 receptor domain-containing adapter inducing IFN-β (TRIF, encoded by Ticam1), TLR7, melanoma-differentiation-associated gene 5 (MDA5), protein kinase R (PKR, encoded by Prkr), mitochondrial antiviral signaling protein (MAVS), or in Unc93b1 mutant mice to confirm whether Alu RNA is mediated via a TLR signaling or other dsRNA sensors. Furthermore, a custom TLR ligand screen was performed by InvivoGen using HEK293 cells over-expressing individual TLR family members coupled with an AP-1/NF-ΚB reporter system.
Alu RNA induced RPE degeneration in Tlr3-/-, Tlr4-/-, Ticam1-/-, Tlr7-/-, and Unc93b1 mutant mice just as in wild-type mice, indicating that Alu RNA does not activate these nucleic acid sensing TLRs in a redundant fashion. Moreover, two different in vitro transcribed Alu RNAs did not activate TLR-2, 3, 4, 5, 7, 8, or 9 in a reporter assay system based in human embryonic kidney 293 cells expressing each of the TLRs. pAlu also induced RPE degeneration in Mda5-/-, Prkr-/-, and Mavs-/- mice, indicating that Alu RNA does not activate other RNA sensors. However, neither Alu RNA nor two different pAlu plasmids induced RPE degeneration in mice deficient in myeloid differentiation factor 88 (MyD88) mice.
These results provide a novel mechanism of Alu RNA-induced RPE degeneration mediated by MyD88, but not by a wide range of canonical RNA sensors.
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