title: Protective responses to sublytic complement in the retinal pigment epithelium.
abstract: The retinal pigment epithelium (RPE) is a key site of injury in inherited and age-related macular degenerations. Abnormal activation of the complement system is a feature of these blinding diseases, yet how the RPE combats complement attack is poorly understood. The complement cascade terminates in the cell-surface assembly of membrane attack complexes (MACs), which promote inflammation by causing aberrant signal transduction. Here, we investigated mechanisms crucial for limiting MAC assembly and preserving cellular integrity in the RPE and asked how these are compromised in models of macular degeneration. Using polarized primary RPE and the pigmented Abca4(-/-) Stargardt disease mouse model, we provide evidence for two protective responses occurring within minutes of complement attack, which are essential for maintaining mitochondrial health in the RPE. First, accelerated recycling of the membrane-bound complement regulator CD59 to the RPE cell surface inhibits MAC formation. Second, fusion of lysosomes with the RPE plasma membrane immediately after complement attack limits sustained elevations in intracellular calcium and prevents mitochondrial injury. Cholesterol accumulation in the RPE, induced by vitamin A dimers or oxidized LDL, inhibits these defense mechanisms by activating acid sphingomyelinase (ASMase), which increases tubulin acetylation and derails organelle traffic. Defective CD59 recycling and lysosome exocytosis after complement attack lead to mitochondrial fragmentation and oxidative stress in the RPE. Drugs that stimulate cholesterol efflux or inhibit ASMase restore both these critical safeguards in the RPE and avert complement-induced mitochondrial injury in vitro and in Abca4(-/-) mice, indicating that they could be effective therapeutic approaches for macular degenerations.
authors.affiliation: Department of Ophthalmology and Visual Sciences, School of Medicine and Public Health, University of Wisconsin, Madison, WI 53706; Division of Pharmaceutical Sciences, School of Pharmacy, University of Wisconsin, Madison, WI 53706;
authors.forename: Li Xuan
authors.initials: LX
authors.lastname: Tan
authors.affiliation: Department of Ophthalmology and Visual Sciences, School of Medicine and Public Health, University of Wisconsin, Madison, WI 53706; McPherson Eye Research Institute, University of Wisconsin, Madison, WI 53706.
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: Toops
authors.affiliation: Department of Ophthalmology and Visual Sciences, School of Medicine and Public Health, University of Wisconsin, Madison, WI 53706; Division of Pharmaceutical Sciences, School of Pharmacy, University of Wisconsin, Madison, WI 53706; McPherson Eye Research Institute, University of Wisconsin, Madison, WI 53706 lakkaraju@wisc.edu.
authors.forename: Aparna
authors.initials: A
authors.lastname: Lakkaraju
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism; D000818:Animals; D018957:CD59 Antigens / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism; D015938:Complement Membrane Attack Complex / Q000378:metabolism*; D003165:Complement System Proteins / Q000378:metabolism*; D006801:Humans; D008247:Lysosomes / Q000378:metabolism; D008268:Macular Degeneration / Q000151:congenital / Q000235:genetics / Q000378:metabolism*; D018345:Mice, Knockout; D008928:Mitochondria / Q000378:metabolism; D018384:Oxidative Stress; D055213:Retinal Pigment Epithelium / Q000166:cytology / Q000378:metabolism*; D013108:Sphingomyelin Phosphodiesterase / Q000378:metabolism; D000080362:Stargardt Disease; D013552:Swine
chemical_list: D018528:ATP-Binding Cassette Transporters; C512841:Abca4 protein, mouse; D018957:CD59 Antigens; D015938:Complement Membrane Attack Complex; D003165:Complement System Proteins; D002784:Cholesterol; D013108:Sphingomyelin Phosphodiesterase; D002118:Calcium
keywords: cholesterol; drug targets; inflammation; macular degeneration; mitochondria