Abstract
Purpose:
Daily clearance of aged photoreceptor outer segment (POS) tips by retinal pigment epithelial (RPE) cells is crucial for retinal health and function. 2 main receptors have been identified, alphavbeta5 integrin and MerTK, that rhythmically tether and internalize POS fragments to be eliminated, respectively. However, many other receptors are present at the RPE cell surface, among which the scavenger receptor family. The only family member explored so far, CD36, seems to intervene in the speed rate of phagocytosis. Other receptors from this family have been suggested to play a role in POS phagocytosis by RPE cells or in macrophage elimination of apoptotic cells. Thus, we investigated the potential implication of these receptors in POS phagocytosis in vitro.
Methods:
The candidates investigated are class A scavenger receptors SR-A and MARCO, and class B receptor SR-BI. We analyzed the expression of the candidates before and during POS challenge using immunoblots. We tested their co-localization with POS at different times of phagocytosis using immunofluorescence assays. We inhibited expression of the candidates in RPE-J using siRNA or blocked the proteins function using antibodies, and then analyzed the potential impact on the cells phagocytic capabilities using in vitro phagocytosis assays.
Results:
All receptors are expressed by RPE-J cells, and their total cellular expression levels do not seem to vary extensively with POS inbubation. SR-A and MARCO receptors co-localized partially with POS, while the association was less observed for SR-BI after 1.5 hours of POS challenge. Inhibition of the candidates’ function using siRNAs showed an overall decrease in POS phagocytosis that was more marked after 3 hours of POS challenge. The decrease was more pronounced for the binding step of phagocytosis for SR-BI, while SR-A inhibition affected mostly the internalization step. Blocking the candidates’ function using specific antibodies confirmed the decreased phagocytic capabilities of RPE cells.
Conclusions:
These results indicate that scavenger receptors might participate in POS clearance by RPE cells, possibly in different steps of the phagocytic process. Studies are under investigation in order to identify how they intervene in POS phagocytosis, either directly or by signaling to the cognate receptors alphavbeta5 integrin or MerTK.
Keywords: 701 retinal pigment epithelium •
645 phagocytosis and killing •
695 retinal degenerations: cell biology