Lipofuscin and A2E Accumulate with Age in the Retinal Pigment Epithelium of Nrl−/− Mice†

Abstract

Lipofuscin is a fluorescent material with significant phototoxic potential that accumulates with age in the retinal pigment epithelium (RPE) of the eye. It is thought to be a factor in retinal degeneration diseases. The most extensively characterized lipofuscin component, N-retinylidene-N-retinylethanolamine (A2E), has been proposed to be a byproduct of reactions involving the visual pigment chromophore. To examine the impact of the visual pigment and photoreceptor cell type on lipofuscin accumulation, we analyzed the RPE from Nrl(-/-) mice of various ages for lipofuscin fluorescence and A2E levels. The photoreceptor cells of the Nrl(-/-) retina contain only cone-like pigments, and produce cone-like responses to photostimulation. The cone-like nature of these cells was confirmed by the presence of RPE65. Lipofuscin was measured with fluorescence imaging, whereas A2E was quantified by UV/VIS absorbance spectroscopy coupled to HPLC. The identity of A2E was corroborated with tandem mass spectrometry. Lipofuscin and A2E accumulated with age, albeit to lower levels compared with wild type mice. The emission spectra of RPE lipofuscin granules from Nrl(-/-) mice were similar to those from wild type mice, with λ(max) ca 610 nm. These results demonstrate that cone visual pigments can contribute to the production of lipofuscin and A2E.