Reduced phagosomal content of the retinal pigment epithelium in response to retinoid deprivation

Abstract

Previous investigations have shown that lipofuscin accumulation in the retinal pigment epithelium (RPE) is reduced greatly as a consequence of vitamin A deprivation. The mechanism by which vitamin A regulates RPE lipofuscin deposition remains to be determined. It is possible that retinoids are direct precursors of this substance. Alternatively, vitamin A deficiency may reduce the uptake and processing of other potential precursors. In retinas lacking photoreceptor cells, RPE lipofuscin accumulation is decreased substantially. This finding suggested that components of phagocytosed photoreceptor outer segments may be precursors for RPE lipofuscin. The effect of vitamin A deprivation on RPE lipofuscin content therefore could be the result of reduced outer segment phagocytosis by the RPE of vitamin A-deprived animals. To evaluate this possibility, experiments were conducted to determine whether vitamin A deprivation altered the phagosomal content of the RPE. Rats were fed diets containing or lacking retinoid precursors of 11-cis retinal. Retinoic acid was included in the diets of the vitamin A-deprived animals. After both 10 and 26 weeks, the RPE phagosomal contents were determined in animals from each dietary group. Photoreceptor cell densities also were measured in these rats. At both time points, the RPE phagosomal content was lower significantly in the retinoid-deprived animals than in those fed a vitamin A precursor of the visual pigment chromophore. This reduction was not the result of photoreceptor cell death; the density of these cells was not affected significantly by dietary vitamin A. Thus, it appears that retinoid deprivation reduces the rate of photoreceptor outer segment turnover and, consequently, outer segment phagocytosis by the RPE.(ABSTRACT TRUNCATED AT 250 WORDS)