Melanosomes degrade lipofuscin and precursors that are derived from photoreceptor membrane turnover in the retinal pigment epithelium—an explanation for the origin of the melanolipofuscin granule

Abstract

The accumulation of the age pigment lipofuscin within the retinal pigment epithelium (RPE) is one the most remarkable changes observed in association with age-related macular degeneration (AMD) and Stargardt disease. Both aging and pathological processes lead to the accumulation of melanolipofuscin (MLF) granules, which have been reported to reflect the onset of AMD more accurately than lipofuscin. The underlying mechanism by which MLF forms is still not understood. We investigate the potential role that melanin plays in the degradation of lipofuscin and MLF in pigmented Abca4-/- mice following treatment with several NO generating drugs. Abca4-/- mice are generally used as models for lipofuscin-related eye diseases. We also induced melanogenesis in albino Abca4-/- mice via the over-expression of tyrosinase, the key enzyme involved in melanogenesis. We compared the ultrastructure of lipofuscinogensis in the RPE of pigmented and albino Abca4-/- mice. Fluorescence microscopy was employed for the quantification of lipofuscin. We found high amounts of unique thin (3-4 nm) lamellar membranes (TLMs) that were left over from the degradation of photoreceptor disc membranes by high-resolution electron microscopy. Accumulated TLMs were significantly more frequent in the RPE cells of the albinos than the pigmented mice, indicating that melanin plays a role in removing TLMs. The intravitreal injection of several NO generating drugs was found to reduce the amount of autofluorescent lipofuscin in the cytoplasm of RPE cells, particularly the MLF granules of pigmented Abca4-/- mice. No effect was observed in terms of lipofuscin removal in NO-exposed albino Abca4-/- mice. However, transfection with tyrosinase led to a reduction in the lipofuscin levels of artificially pigmented RPE cells in albino Abca4-/- mice following the formation of melanin. The results show for the first time that melanin plays an important, if not a key, role in the degradation of lipofuscin in RPE cells.

Retinal Pigment Epithelium Pigment Granules: Norms, Age Relations and Pathology

“…(3.) In a granule containing all three components, namely melanin, lipofuscin (bisretinoids) and PMD, light and ROS activate the transition of melanin to a high-energy state (melanin*) in which the excited pigment causes the degradation of lipofuscin [164,165].…”

“…It has recently been shown that when melanin is exposed to exogenous free radicals (superoxide, nitric oxide), it enters a high-energy state ("chemiexcitation") in which the pigment causes the degradation of lipofuscin [164,165]. Such mechanisms for removing lipofuscin using activated melanin can lead to the degradation of the latter [165].…”

Chemiexcitation: Mammalian Photochemistry in the Dark†

“…The Abca4 À/À mouse model for AMD accumulates lipofuscin faster and develops retinal degeneration earlier in albino than pigmented strains (166)(167)(168). Transfecting the eyes of albino mice with tyrosinase to generate new RPE melanin reduces lipofuscin accumulation (169). Strikingly, the lipofuscin component of melanolipofuscin can be degraded following intravitreal injection with generators of O 2 ÁÀ or Á NO, in both monkeys and Abca4 À/À mice, with some even rescuing retinal degeneration in an advanced Stargardt mouse model.…”

“…Strikingly, the lipofuscin component of melanolipofuscin can be degraded following intravitreal injection with generators of O 2 ÁÀ or Á NO, in both monkeys and Abca4 À/À mice, with some even rescuing retinal degeneration in an advanced Stargardt mouse model. The radical generators used include drugs, riboflavin, and light (169)(170)(171)(172)(173)(174). Lipofuscin removal from cultured human RPE was blocked by a radical scavenger (172).…”

“…Lipofuscin removal from cultured human RPE was blocked by a radical scavenger (172). Yet, · NO was ineffective in albino mice (169); if radicals per se were the effector, the absence of melanin would not matter. Horseradish peroxidase can also degrade melanolipofuscin (175), raising the possibility that these turnover pathways share the common property of chemiexciting melanin and using the excitation energy to degrade lipofuscin.…”