Phagosome maturation during endosome interaction revealed by partial rhodopsin processing in retinal pigment epithelium

Abstract

Defects in phagocytosis and degradation of photoreceptor outer segments (POS) by the retinal pigment epithelium (RPE) are associated with aging and retinal disease. The daily burst of rod outer segment (ROS) phagocytosis by the RPE provides a unique opportunity to analyse phagosome processing in vivo. In mouse retinae phagosomes containing stacked rhodopsin-rich discs were identified by immuno-electron microscopy. Early apical phagosomes stained with both cytoplasmic and intradiscal domain rhodopsin antibodies. During phagosome maturation a remarkably synchronised loss of the cytoplasmic epitope coincided with movement to the cell body and preceded phagosome:lysosome fusion and disc degradation. Loss of the intradiscal rhodopsin epitope and disc digestion occurred upon fusion with cathepsin D-positive lysosomes. The same sequential stages of phagosome maturation were identified in cultured RPE and macrophages challenged with isolated POS. Loss of the cytoplasmic rhodopsin epitope was insensitive to pH but sensitive to protease inhibition and co-incided with interaction of phagosomes with endosomes. Thus, during prelysosomal maturation of ROS-containing phagosomes limited rhodopsin processing occurs upon interaction with endosomes. This potentially provides a sensitive readout of phagosome:endosome interactions applicable to multiple phagocytes.