Tam V. Bui

Rpe65 Is a Retinyl Ester Binding Protein That Presents Insoluble Substrate to the Isomerase in Retinal Pigment Epithelial Cells

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of β-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Lebers congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.

Investigation of oral fenretinide for treatment of geographic atrophy in age-related macular degeneration.

Background: Excessive accumulation of retinol-based toxins has been implicated in the pathogenesis of geographic atrophy (GA). Fenretinide, an orally available drug that reduces retinol delivery to the eye through antagonism of serum retinol-binding protein (RBP), was used in a 2-year trial to determine whether retinol reduction would be effective in the management of geographic atrophy. Methods: The efficacy of fenretinide (100 and 300 mg daily, orally) to slow lesion growth in geographic atrophy patients was examined in a 2-year, placebo-controlled double-masked trial that enrolled 246 patients at 30 clinical sites in the United States. Results: Fenretinide treatment produced dose-dependent reversible reductions in serum RBP-retinol that were associated with trends in reduced lesion growth rates. Patients in the 300 mg group who achieved serum retinol levels of ⩽1 &mgr;M (⩽2 mg/dL RBP) showed a mean reduction of 0.33 mm2 in the yearly lesion growth rate compared with subjects in the placebo group (1.70 mm2/year vs. 2.03 mm2/year, respectively, P = 0.1848). Retinol-binding protein reductions <2 mg/dL correlated with further reductions in lesion growth rates (r2 = 0.478). Fenretinide treatment also reduced the incidence of choroidal neovascularization (approximately 45% reduction in incidence rate in the combined fenretinide groups vs. placebo, P = 0.0606). This therapeutic effect was not dose dependent and is consistent with anti-angiogenic properties of fenretinide, which have been observed in other disease states. Conclusion: The findings of this study and the established safety profile of fenretinide in chronic dosing regimens warrant further study of fenretinide in the treatment of geographic atrophy.

Characterization of Native Retinal Fluorophores Involved in Biosynthesis of A2E and Lipofuscin-associated Retinopathies

Mutations in the photoreceptor-specific ABCA4 gene are associated with several inherited retinal and macular degenerations. A prominent phenotype of these diseases is the accumulation of cytotoxic lipofuscin fluorophores such as A2E within the retinal pigment epithelium. Another compound, dihydro-N-retinylidene-N-retinylphosphatidyl-ethanolamine (A2PE-H2), also accumulates in retinas of mice and humans harboring ABCA4 mutations and was proposed to be a precursor of A2E. The role of A2PE-H2 in the biogenesis of A2E and its relationship to other retinal fluorophores has not been previously investigated. We report spectral properties and structural relationships of the principal retinal fluorophores that accumulate in retina and retinal pigment epithelium of abca4–/– mice. A long wavelength fluorescence emission intrinsic to abca4–/– retinal explants is shown to emanate from A2PE-H2. All-trans retinal dimer conjugates, which were also identified in the retinal explants, possessed distinct fluorescence and structural properties and, unlike A2PE-H2, did not accumulate in an age-dependent manner. Derivative absorbance and fluorescence spectroscopy revealed that A2PE-H2, A2E, and N-retinylidene-N-retinyl-phosphatidylethanolamine (A2PE), a known precursor of A2E, share common electronic and resonant structures. Importantly, collision-induced dissociation of A2PE-H2 produced daughter ions that were identical to authentic A2E and its daughter ions. Finally, intravitreal administration of A2PE-H2 to wild-type mice resulted in the formation of A2PE and A2E. These data validate a previously hypothesized biosynthetic pathway for A2E and implicate A2PE-H2 as a precursor in this pathway. Fluorescence properties of A2PE-H2 and other related fluorophores characterized in this report have significance for evaluation of human retinal diseases characterized by aberrant fundus autofluorescence.