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M. Zareba, T. J. Sarna, J. M. Burke; Role of Lipofuscin in Differential Sensitivity of RPE Cells to Sub-Lethal and Lethal Photic Stress. Invest. Ophthalmol. Vis. Sci. 2010;51(13):468.
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Photic stress induced by sub-lethal visible light slows organelle movement in cultured RPE cells, especially for organelles made experimentally photoreactive. Here we asked whether endogenous human RPE lipofuscin granules and melanosomes, both of which are photoreactive (though lipofuscin is more so), are differentially sensitive to light-induced motility slowing. We also asked whether variable lipofuscin content predicts the susceptibility of cells to killing by higher light doses.
Confluent primary cultures of human RPE containing both granule types were treated with blue light (405 nm) delivered by the light source of a microscope equipped for live cell imaging. Movement of the same granules was tracked before and after sub-lethal light treatment using MetaMorph software and bright field images captured at 1 s intervals over 3 min. Lipofuscin granules were discriminated from melanosomes in the same cells by fluorescence imaging. Cell death on higher light doses was determined by adding propidium iodide (PI) to the medium and recording nuclear PI fluorescence in real time using images captured at 30 s intervals over 6 hrs.
Sub-lethal blue light stress slowed the movement of both melanosomes and lipofuscin granules, with greater slowing for lipofuscin granules within the same cells. Cell death induced by higher doses of blue light varied with lipofuscin content; cells with abundant endogenous lipofuscin died first while those with fewer lipofuscin granules showed delayed death or survived irradiation.
RPE cells are exposed throughout life to blue light. The results here suggest that photic stress impairs the trafficking and placement of both melanosomes and lipofuscin granules, and especially the latter. With aging as lipofuscin accumulates and melanin declines, not only may granule movement become more sluggish, but individual RPE cells may differ in their vulnerability to photic stress depending on their variable lipofuscin content. Since oxidative stress, including photic stress, is believed to diminish RPE support for the retina, those photoreceptors adjacent to RPE cells with high lipofuscin may be preferentially at risk for degeneration.
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