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Sriganesh Ramachandra Rao, Bruce A Pfeffer, Meerim Choi, Kevin Francis, Forbes D Porter, Steven J Fliesler; Understanding the molecular mechanism of RPE pathology in Smith-Lemli-Opitz Syndrome. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2389.
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© ARVO (1962-2015); The Authors (2016-present)
We previously showed that the retinal pigmented epithelium (RPE) in the AY9944 rat model of Smith-Lemli-Opitz syndrome (SLOS) exhibits marked accumulation of phagosomes, lipofuscin and bis-retinoids compared to age-matched controls. Lysosomal pH and function are sensitive to altered cholesterol homeostasis and oxysterol levels. We tested the hypothesis that SLOS patient-derived RPE cells in vitro would exhibit dysfunctional lysosomal molecular signatures, and also would have decreased autophagic flux and an elevated ubiquitinated protein load due to hindered autophagic clearance.
Inducible pluripotent stem cells (iPSCs) derived from SLOS patient skin fibroblasts were reprogrammed to RPE cells (SLOS RPE) (Neural Stem Cell Institute, Rensselaer, NY); control human embryonic stem cell-derived RPE (nhRPE) cells were used as controls. Sterols (cholesterol (Chol), 7-dehydrocholesterol (7DHC)) were quantified by RP-HPLC (N=3 ea.). Protein levels of lysosomal markers (mature Cathepsin-D, LAMP2), upstream (Beclin1) and downstream (p62 and LC3-I/II) markers of autophagy, and ubiquitinated protein levels were assessed by Western blot/densitometry (N=3 ea.), normalized to GAPDH levels. Mean/S.D. values were statistically compared using Student’s t-test (significance criterion, p ≤ 0.05).
SLOS RPE and nhRPE cells were validated by morphological criteria (hexagonal, melanized, polarized epithelial monolayers) and RPE protein markers (cRAlBP, cytokeratin-8, ZO-1). SLOS RPE cells exhibited 20-fold higher 7DHC/Chol mole ratios [0.697 ± 0.064] than nhRPE [0.035 ± 0.009] (p<0.01). Protein levels of lysosome and autophagic markers in SLOS RPE (expressed as % change compared to normal RPE) were as follows (p < 0.02): LAMP2, -42%; mature Cathepsin-D, -98%; Beclin-1, 388%; p62, >+1500%; LC3II, -71%; ubiquitinated proteins, +440%.
These findings are indicative of marked lysosomal dysfunction and compromised autophagic flux in SLOS RPE compared to control cells (nhRPE), consistent with the RPE pathology observed in the AY9944 rat model of SLOS. These changes may be due to the increased 7DHC levels (or oxysterols derived therefrom) in SLOS RPE cells, which may perturb lysosomal membrane structure and function.
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