Development of lipofuscin-like fluorescence in the retinal pigment epithelium in response to protease inhibitor treatment

Abstract

Experiments were conducted to determine whether photoreceptor outer segment components contain precursors for lipofuscin fluorophores that accumulate in the retinal pigment epithelium (RPE) during senescence. Intravitreal injection of the lysosomal protease inhibitor leupeptin caused a rapid accumulation of inclusions with lipofuscin-like autofluorescence in the RPE of albino rats. These inclusions appeared to be derived from photoreceptor outer segments, which are normally phagocytosed and degraded by the RPE. The tripeptide leu-gly-gly, which is similar to leupeptin except that it does not inhibit proteolysis, had no effect on RPE autofluorescent pigment content. Likewise, netilmicin, a purported inhibitor of lysosomal phospholipases, did not enhance autofluorescent pigment deposition in the RPE. These findings are consistent with other experiments suggesting that RPE age-pigment fluorophores are derived from molecular components of the photoreceptor outer segments. The effect of leupeptin suggests that outer segment proteins may be among the precursors for these fluorophores. The RPE appears to differ from other cell types that accumulate lipofuscin in that the majority of the precursors for this pigment in the RPE are taken up by phagocytosis rather than being generated within the cells themselves.