Loss of ABCB5 leads to progressive visual loss due to sphingolipid accumulation in Retinal Pigment Epithelium

Abstract

Purpose : Retinal pigment epithelium (RPE) has a central role in eye development and is critical for the maintenance of the blood-retinal barrier, and homeostasis and survival of photoreceptors during adult life. Progressive loss of RPE is thought to be a major contributor to age-related macular degeneration (AMD). Currently, the general consensus is that RPE is maintained during aging via proliferation and enlargement of mature adult cells. Recently, however, this idea was challenged when a small population of cells was identified within the human RPE layer that displayed stem cell characteristics in vitro. We hypothesize that the stem cell gene ABCB5 is expressed by a subpopulation of RPE cells which retain the ability to self-renew and differentiate and are essential for RPE homeostasis.

Methods : Genetically engineered ABCB5 KO mice and ABCB5 GFP/Cre reporter mice were used to analyze ABCB5 function in retinal maintenance and to monitor murine ABCB5+ RPE cells in situ. Mice were evaluated by immunofluorescence, RNA in situ hybridization, FACS, and liquid chromatography/mass spectrometry (LC/MS). In addition, in vivo evaluation of visual function was performed by comparative full-field electroretinography (ERG) on young and aged ABCB5 KO and ABCB5 WT mice. Two-tailed Student’s t-test was used for statistical analysis.

Results : In the adult eye, we found specific GFP (ABCB5) expression in the RPE of ABCB5 GFP/Cre mice, which was further corroborated by ABCB5 mRNA in situ hybridization and FACS studies. Moreover, up to 50% of cells within the ABCB5 (+) RPE subpopulation did not express the differentiation marker RPE65. ERG analyses revealed reduction in both the amplitude and the sensitivity of the flash responses, as well as longer implicit times in the one-year-old KO animals compared to age-matched WT mice, indicating that loss of ABCB5 results in progressive functional visual abnormalities. There were associated alterations in lipid metabolism revealed by LC/MS analyses, such as accumulation of pro-apoptotic ceramide and its metabolite GM3 ganglioside.

Conclusions : Our results demonstrate that ABCB5 identifies an RPE65-negative progenitor cell subpopulation within adult RPE. In addition, we show that intact ABCB5 function is important for normal RPE homeostasis and maturation as demonstrated by its role in sphingolipid metabolism.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.