title: Lipofuscin and N-retinylidene-N-retinylethanolamine (A2E) accumulate in retinal pigment epithelium in absence of light exposure: their origin is 11-cis-retinal.
abstract: The age-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE) has been associated with the development of retinal diseases, particularly age-related macular degeneration and Stargardt disease. A major component of lipofuscin is the bis-retinoid N-retinylidene-N-retinylethanolamine (A2E). The current model for the formation of A2E requires photoactivation of rhodopsin and subsequent release of all-trans-retinal. To understand the role of light exposure in the accumulation of lipofuscin and A2E, we analyzed RPEs and isolated rod photoreceptors from mice of different ages and strains, reared either in darkness or cyclic light. Lipofuscin levels were determined by fluorescence imaging, whereas A2E levels were quantified by HPLC and UV-visible absorption spectroscopy. The identity of A2E was confirmed by tandem mass spectrometry. Lipofuscin and A2E levels in the RPE increased with age and more so in the Stargardt model Abca4(-/-) than in the wild type strains 129/sv and C57Bl/6. For each strain, the levels of lipofuscin precursor fluorophores in dark-adapted rods and the levels and rates of increase of RPE lipofuscin and A2E were not different between dark-reared and cyclic light-reared animals. Both 11-cis- and all-trans-retinal generated lipofuscin-like fluorophores when added to metabolically compromised rod outer segments; however, it was only 11-cis-retinal that generated such fluorophores when added to metabolically intact rods. The results suggest that lipofuscin originates from the free 11-cis-retinal that is continuously supplied to the rod for rhodopsin regeneration and outer segment renewal. The physiological role of Abca4 may include the translocation of 11-cis-retinal complexes across the disk membrane.
authors.affiliation: Department of Ophthalmology, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
authors.forename: Nicholas P
authors.initials: NP
authors.lastname: Boyer
authors.forename: Daniel
authors.initials: D
authors.lastname: Higbee
authors.forename: Mark B
authors.initials: MB
authors.lastname: Currin
authors.forename: Lorie R
authors.initials: LR
authors.lastname: Blakeley
authors.forename: Chunhe
authors.initials: C
authors.lastname: Chen
authors.forename: Zsolt
authors.initials: Z
authors.lastname: Ablonczy
authors.forename: Rosalie K
authors.initials: RK
authors.lastname: Crouch
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Koutalos
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000378:metabolism; D000818:Animals; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003116:Color; D008027:Light; D008062:Lipofuscin / Q000737:chemistry*; D008268:Macular Degeneration / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008856:Microscopy, Fluorescence / Q000379:methods; D008954:Models, Biological; D012160:Retina / Q000378:metabolism; D055213:Retinal Pigment Epithelium / Q000378:metabolism*; D012172:Retinaldehyde / Q000494:pharmacology; D012176:Retinoids / Q000737:chemistry*; D012374:Rod Cell Outer Segment / Q000378:metabolism
chemical_list: C112040:A2-E (N-retinylidene-N-retinylethanolamine); D018528:ATP-Binding Cassette Transporters; C512841:Abca4 protein, mouse; D008062:Lipofuscin; D012176:Retinoids; D012172:Retinaldehyde