Mesd extrinsically promotes phagocytosis by retinal pigment epithelial cells

Abstract

Phagocytosis is a critical process to maintain tissue homeostasis. In the retina, photoreceptor cells renew their photo-excitability 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.

Overexpression of miR‐181a‐5p inhibits retinal neovascularization through endocan and the ERK1/2 signaling pathway

“…Western blot analyses were performed as previously described (X. P. Chen et al, 2016) with the following primary antibodies: anti‐endocan (0.1 μg/ml; R&D Systems); anti‐ERK1/2 (1:1,000; Cell Signaling Technology); anti‐p‐ERK1/2 (1:1,000; Cell Signaling Technology); anti‐VEGF (1:1,000; Abcam); anti‐VEGFR1 (1:1,000; Abcam); anti‐VEGFR2 (1:1,000; Cell Signaling Technology); and GAPDH (1:1,000; Cell Signaling Technology).…”

Mechanisms that minimize retinal impact of apolipoprotein E absence

“…8E) (95,96), whose functions are similar to those of LRPAP1, i.e., facilitation of receptor folding and regulation of ligand-receptor interactions. MESDC2 was only shown to be released from the shed OSs and bind to these shed segments to facilitate their phagocytic clearance by the RPE (97). The identification of LRP1, LRPAP1, and MESDC2 as abundant cholesterol-related retinal proteins enhances our knowledge about retinal cholesterol maintenance and links retinal cholesterol to other retinal processes, some of which have not yet been considered for association with this lipid.…”

Capsule-epithelium-fibre unit ultrastructure in the human lens

“…This specific morphological feature of the capsule in the present study might represent the progressive degradation of the protruding lens capsule, or there might be a form of communication between the capsule and epithelial cells. Many epithelia have the potential to transform into non-professional phagocytes and engulf and degrade material; for example, shedding photoreceptor outer segments are phagocytosed by retinal pigment epithelial cells to prevent debris accumulation and thereby maintain tissue homeostasis 16 , 17 . The question is raised whether the lens epithelial cells have the same potential of transforming into non-professional phagocytes like other epithelia.…”