Mesd extrinsically promotes phagocytosis by retinal pigment epithelial cells

Phagocytosis is a critical process to maintain tissue homeostasis. In the retina, photoreceptor cells renew their photoexcitability by shedding photoreceptor outer segments (POSs) in a diurnal rhythm. Shed POSs are phagocytosed by retinal pigment epithelial (RPE) cells to prevent debris accumulation, retinal degeneration, and blindness. Phagocytosis ligands are the key to understanding how RPE recognizes shed POSs. Here, we characterized mesoderm development candidate 2 (Mesd or Mesdc2), an endoplasmic reticulum (ER) chaperon for low-density lipoprotein receptor-related proteins (LRPs), to extrinsically promote RPE phagocytosis. The results showed that Mesd stimulated phagocytosis of fluorescence-labeled POS vesicles by D407 RPE cells. Ingested POSs were partially degraded within 3 h in some RPE cells to dispense undegradable fluorophore throughout the cytoplasm. Internalized POSs were colocalized with phagosome biomarker Rab7, suggesting that Mesd-mediated engulfment is involved in a phagocytosis pathway. Mesd also facilitated phagocytosis of POSs by primary RPE cells. Mesd bound to unknown phagocytic receptor(s) on RPE cells. Mesd was detected in the cytoplasm, but not nuclei, of different retinal layers and is predominantly expressed in the ER-free cellular compartment of POSs. Mesd was not secreted into medium from healthy cells but passively released from apoptotic cells with increased membrane permeability. Released Mesd selectively bound to the surface of POS vesicles and apoptotic cells, but not healthy cells. These results suggest that Mesd may be released from and bind to shed POSs to facilitate their phagocytic clearance.