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Tamara L Lenis, Shanta Sarfare, Jane Hu, Zhichun Jiang, Marcia Lloyd, Dean Bok, Gabriel H Travis, Roxana A Radu; Expression of ABCA4 in Retinal Pigment Epithelium cells and its implications for Stargardt disease. Invest. Ophthalmol. Vis. Sci. 2017;58(8):607.
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© ARVO (1962-2015); The Authors (2016-present)
Recessive Stargardt disease (STGD1) is an inherited macular degeneration caused by mutations in ABCA4, a membrane protein thought to be exclusively expressed in photoreceptor outer-segment (OS) discs. Loss of ABCA4 results in retinal pigment epithelium (RPE) deposition of visual cycle toxic byproducts and late-onset photoreceptor degeneration. We previously demonstrated ABCA4 expression in normal human and murine RPE cells by qRT-PCR and immunoblotting. We also showed that RPE cells from Mertk1-/- mice, which do not phagocytose photoreceptor OS, also contained Abca4 mRNA and protein. We hypothesized that RPE-expressed ABCA4 performs a similar protective function as in photoreceptor OS discs.
We previously generated transgenic mice using a construct that contains the normal mouse Abca4 coding-region downstream of the RPE-specific promoter. We crossed this transgene onto the Abca4-/- background, to obtain a line that expresses ABCA4 in the RPE, but not the retina (Rpe65-Abca4-Tg/Abca4-/- mice). Expression of ABCA4 was determined by qRT-PCR, immunoblotting, and immunohistochemistry. Retinoids and bis-retinoids were measured by high-performance liquid chromatography (HPLC). Retinal morphology, RPE lipofuscin burden and RPE autofluorescence were assessed by light microscopy, electron microscopy, and confocal microscopy respectively. Complement C3b levels were measured by immunoblotting and co-localization studies were performed using immunohistochemistry.
Characterization of a transgenic mouse line that expresses ABCA4 in the RPE, but not in the retina (Rpe65-Abca4-Tg/Abca4-/-) revealed decreased autofluorescence and decreased lipofuscin granules, in addition to greater photoreceptor preservation, suggesting an active role for ABCA4 in the RPE. Furthermore, A2E levels and complement C3b accumulation were reduced in the RPE of Rpe65-Abca4-Tg/Abca4-/- mice compared to Abca4-/- mice, demonstrating that RPE-expressed ABCA4 may protect against bis-retinoid mediated damage by curbing complement reactivity. Finally, endogenous RPE-expressed ABCA4 appears to co-localize with endolysosomal structures in human and murine RPE.
ABCA4 plays a similar role in RPE internal membranes as it does in photoreceptor OS discs. This finding introduces a cell-autonomous pathway in the pathophysiology of STGD1 and may direct future cell-targeted therapies for ABCA4-associated maculopathies.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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