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Jing Cao, Chris Murat, Carol Anne Ogden, Sandra Santulli-Marotto, Xiang Yao, Ian R Harris, George Inana; Human umbilical tissue-derived cells secrete bridge molecules and rescue phagocytosis in cultured retinal pigment epithelial cells from Royal College of Surgeons rat. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1728.
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© ARVO (1962-2015); The Authors (2016-present)
Royal College of Surgeons (RCS) rat, a model of retinal degeneration, carries a mutation in Mertk resulting in diminished phagocytosis of rod outer segment (ROS) and leading to photoreceptor cell death. Retinal pigment epithelium (RPE) cells from RCS rats showed reduced phagocytosis in vitro. Interestingly, these cells, when fed with ROS pretreated with human umbilical tissue-derived cell (hUTC) conditioned medium (CM), showed significantly restored phagocytosis of ROS, suggesting that hUTC CM may prime ROS in a way that enhances its phagocytosis. It was reported that isolated ROS possess externalized phosphatidylserine (PS), whose blockade or removal reduces their binding and engulfment by RPE in culture. PS exposure is a hallmark of apoptotic cells. This leads to our hypothesis that hUTC could regulate RCS RPE phagocytosis in a similar manner to apoptotic cell clearance used by other phagocytes through secretion of bridge molecules.
Transcriptome profiling (RNA-Seq) was performed to identify gene expression in hUTC and RCS RPE cells. Levels of bridge molecules, including milk-fat-globule-EGF-factor 8 (MFG-E8), growth arrest-specific 6 (Gas6), thrombospondin (TSP)-1, TSP-2 in hUTC CM were measured by ELISA, and their effects on phagocytosis were examined by pre-incubating ROS with the corresponding recombinant human proteins followed by addition of ROS to RCS RPE cells for phagocytosis assay in the absence of the respective bridge molecule protein.
Transcriptome profile analysis of RCS RPE cells identified multiple receptor genes involved in apoptotic clearance, such as scavenger receptors and integrins. hUTC expresses a number of bridge molecule genes including MFG-E8, Gas6, TSP-1 and TSP-2. We detected high levels of MFG-E8, TSP-1, TSP-2 and low amount of Gas6 in hUTC CM. Moreover, RCS RPE cells, fed with ROS pretreated with the corresponding recombinant human bridge molecule protein, showed significantly restored phagocytosis.
These results suggest that hUTC could rescue RCS RPE cell phagocytosis in a Mertk-independent manner, through secretion of bridge molecules.
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