Aberrant Buildup of All-Trans-Retinal Dimer, a Nonpyridinium Bisretinoid Lipofuscin Fluorophore, Contributes to the Degeneration of the Retinal Pigment Epithelium.

Purpose: Nondegradable fluorophores that accumulate as deleterious lipofuscin of RPE are involved in pathological mechanisms leading to the degeneration of RPE in AMD. A2E, a major component of RPE lipofuscin, could cause damage to RPE cells. Nevertheless, all-trans-retinal dimer (atRAL dimer) was found to be much more abundant than that of A2E in eyes of Abca4-/-Rdh8-/- double-knockout (DKO) mice, a rodent model showing the typical characteristics of retinopathies in AMD patients. Our aim was to elucidate the effect and mechanism of atRAL dimer-induced RPE degeneration.

Methods: Eyes harvested from C57BL/6J wild-type (WT) and Abca4-/-Rdh8-/- DKO mice were examined by HPLC. Cellular uptake, subcellular localization, 5-bromo-2-deoxyuridine (BrdU), Cdc25C, DNA strand breaks, mitochondrial membrane potential (ΔΨm), and cytochrome c were analyzed by fluorescence microscopy. Cellular toxicity was assayed by lactate dehydrogenase (LDH) assay and dead cell staining. Apoptosis and cell-cycle stages were detected by flow cytometry. Furthermore, in vitro and in vivo expression of proteins associated with cell cycle and apoptosis was measured by immunoblot assays.

Results: All-trans-retinal dimer clearly could damage RPE cell membrane and inhibit the proliferation of RPE cells as well as induce DNA damage and cell-cycle arrest at the G2/M phase via activating the ATM/ATR-Chk2-p53 signaling pathway. Moreover, this di-retinal adduct triggered mitochondrion-associated apoptosis in RPE cells. Evidence from the cell-based experiments was also corroborated by a remarkable abnormality in expression of proteins associated with cell cycle (Cyclin B1 and Cdc2) and apoptosis (p53, Bcl-2 and Bax) in the RPE of Abca4-/-Rdh8-/- DKO mice.

Conclusions: These findings suggest that atRAL dimer that accumulates beyond a critical level, facilitates age-dependent RPE degeneration.