Reduction of Serum Retinol Binding Protein Arrests the Accumulation of Toxic Retinal Fluorophores: A Nonpharmacological Approach

Abstract

Purpose:: Excessive accumulation of lipofuscin is observed in numerous degenerative retinal diseases. A toxic retinal fluorophore present within lipofuscin (A2E) has been implicated in the death of RPE and photoreceptor cells. We have previously shown that the accumulation of A2E can be profoundly reduced in the ABCA4 null mutant mouse following treatment with 4-hydroxyphenylretinamide (4-HPR). 4-HPR competes with retinol for binding to retinol binding protein (RBP) and effectively lowers serum retinol and visual cycle retinoids. Although 4-HPR is not believed to affect processes of the visual cycle, it has not been determined whether reduction of serum RBP alone is effective to halt the accumulation of lipofuscin fluorophores. We have addressed this issue by studying mice which express varied levels of RBP on an ABCA4-deficient background.

Methods:: We mated ABCA4 null mutant mice with RBP4 null mutant mice to generate a line which is heterozygous for both ABCA4 and RBP4. Appropriate heterozygous control lines (abca4+/-, rbp4+/+ and abca4+/+, rbp4+/-) were also generated to compare levels of RBP and lipofuscin fluorophores. Genotypes of the generated lines were confirmed by PCR. Serum RBP, retinol, retinoid flux within the visual cycle and levels of retinal fluorophores were measured by HPLC. Histological analyses were also performed in order to evaluate morphology of the retina and RPE.

Results:: abca4/rbp4 heterozygous mice demonstrated significant reductions in serum retinol and RBP. Levels of visual cycle retinoids, A2E and autofluorescence within the RPE were also dramatically lower in this line. Chromophore regeneration, light sensitivity of photoreceptors, and phototransduction processes were normal. Histologic examinations showed no alteration of retina cytostructure or morphology.

Conclusions:: These findings validate modulation of serum RBP as a therapeutic approach to reduce the accumulation of toxic retinal fluorophores. The fact that visual cycle retinoids and A2E can be reduced by lowering serum RBP indicates that interference of processes within the visual cycle is not a prerequisite for therapeutic efficacy.

Keywords: retinoids/retinoid binding proteins • vitamin A deficiency • age-related macular degeneration