The Extracellular Part Of MerTK Is Shed By RPE Cells Both In Vitro And In Vivo

Abstract

Purpose: : The Mer tyrosine kinase (MerTK) receptor is expressed at the apical surface of retinal pigment epithelial (RPE) cells. MerTK is activated at the time of the phagocytic peak and is required to internalize bound photoreceptor outer segments (POS), and is therefore crucial for vision. We showed previously that the alphavbeta5 integrin receptor and its secreted ligand MFG-E8 stimulate MerTK and synchronize phagocytosis in vivo. Strikingly, in the absence of alphavbeta5 or MFG-E8, phagocytosis still occurs but loses its rhythm, thus implying that MerTK can be activated to perform internalization independently from this integrin-linked pathway. A few studies have shown that MerTK and Axl, from the same family of receptors, can be cleaved and detected in the conditioned media of cultured cells. In this study, we investigated if a soluble form of MerTK (sMerTK) is released by RPE cells during phagocytosis.

Methods: : We incubated rat RPE-J cells in medium alone or medium plus POS, with and without the addition of ligands, for different time lengths. We then analyzed proteins present in the conditioned media of RPE cells, and quantified protein levels using immunoblots of cell lysates and immunofluorescence. In addition, we studied the circadian release of sMerTK in mouse retina.

Results: : As previously observed, MFG-E8 was secreted by RPE cells. We identified a 120-kDa band of sMerTK that increased with time and was more marked in POS-challenged cells. Conversely, we detected reduced cellular levels of MerTK in the same wells. sMerTK was detected in mouse retina in vivo at different times of the light:dark cycle.

Conclusions: : These results suggest that sMerTK could represent a way of downregulating the phagocytic activity of RPE cells.

Keywords: retinal degenerations: cell biology • retinal pigment epithelium • photoreceptors