Phagosome maturation during endosome interaction revealed by partial rhodopsin processing in retinal pigment epithelium.
Bottom Line: Loss of the cytoplasmic rhodopsin epitope was insensitive to pH but sensitive to protease inhibition and coincided with the interaction of phagosomes with endosomes.Thus, during pre-lysosomal maturation of ROS-containing phagosomes, limited rhodopsin processing occurs upon interaction with endosomes.This potentially provides a sensitive readout of phagosome-endosome interactions that is applicable to multiple phagocytes.
Affiliation: Molecular Medicine Section, National Heart and Lung Institute, Imperial College London, London SW7 2AZ, UK UCL Institute of Ophthalmology, University College London, London EC1V 9EL, UK.Show MeSH
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Mentions: Phagocytosis of shed ROS by the RPE occurs during the first 1–2 h after light onset. We have previously shown 2.5 h after light onset to be a time-point at which both apical and basal phagosomes are present and when phagosomes accumulate if phagosome degradation is impaired (Wavre-Shapton et al., 2013). Thus, at this time, all stages of phagosome processing are likely to be present. It has previously been shown that antibodies against specific rhodopsin epitopes might stain only a subset of ROS-containing phagosomes (Esteve-Rudd et al., 2014; Law et al., 2009). In order to determine whether antibodies against specific rhodopsin epitopes can be used to identify sequential stages of phagosome maturation, we monitored rhodopsin processing by cryo-immuno-electron microscopy using two different rhodopsin antibodies. The RET-P1 antibody binds to the N-terminal intradiscal domain of rhodopsin, whereas 1D4 recognises the C-terminal cytoplasmic domain of the protein. ROS and some phagosomes located in the apical region very close to the ROS (early phagosomes) contained both epitopes (insets, Fig. 1A,B, respectively). Outer segment discs were clearly visible in these 1D4- and RET-P1-positive phagosomes. Interestingly, these specimens also contained phagosomes in the apical region and in the cell body that were strongly positive for RET-P1 staining but contained very little 1D4 cytoplasmic epitope staining (maturing phagosomes) or, more commonly, no 1D4 staining (late phagosomes), as illustrated in Fig. 1C,D, respectively. Despite the scarcity of 1D4 staining, most of these phagosomes contained clearly visible discs. To determine whether phagosomes staining for both rhodopsin epitopes (early and maturing phagosomes) and phagosomes that had lost the cytoplasmic 1D4 epitope (late phagosomes) represented sequential stages in phagosome maturation, the two types of phagosome were quantified at 1 h and 2.5 h after light onset (8am and 9.30am, respectively). As shown in Fig. 1E, at 8am, almost 80% of the phagosomes were double-labelled with RET-P1 and 1D4, compared with 55% at 9.30am. This indicates a progression over time from double-labelled to single-labelled phagosomes during the maturation process.
Affiliation: Molecular Medicine Section, National Heart and Lung Institute, Imperial College London, London SW7 2AZ, UK UCL Institute of Ophthalmology, University College London, London EC1V 9EL, UK.