Light-induced exacerbation of retinal degeneration in a rat model of Smith–Lemli–Opitz syndrome

Abstract

Potentiation of retinal degeneration by intense light exposure, and its amelioration by an antioxidant, were studied in a rat model of Smith-Lemli-Opitz syndrome (SLOS), in comparison with normal (control) Sprague-Dawley rats. The SLOS model is created by treating rats with AY9944, a selective inhibitor of cholesterol synthesis at the level of 3β-hydroxysterol-Δ 7 -reductase. A subset of rats was treated with dimethylthiourea (DMTU), a synthetic antioxidant, 24 and 1 hr prior to light exposure. Half of the animals (±DMTU) were exposed to intense, constant, green light (24 hr, 1700 lx, 490-580 nm), while the others were maintained in darkness. Subsequently all animals were returned to dim cyclic light (20-40 lx, 12 hr light-12 hr dark) for 2 weeks, after which electroretinograms were recorded. One eye from each rat was taken for histological and quantitative morphometric analyses; sterol analysis was performed on retinas from contralateral eyes. HPLC analysis confirmed the accumulation of 7-dehydrocholesterol (7DHC) in retinas of AY9944-treated rats; cholesterol represented >99% of the sterol in control retinas. Histology of retinas from unexposed, AY9944-treated rats (6-week-old) was normal. In contrast, age-matched, light-exposed rats exhibited massive photoreceptor cell loss in both the superior and inferior hemispheres, and concomitant rod and cone dysfunction. The severity and geographic extent of the damage was far greater than that observed in normal albino rats exposed to the same conditions. DMTU pre-treatment largely prevented these degenerative changes. These findings indicate that the AY9944-induced rat SLOS model is hypersensitive to intense light-induced retinal damage, relative to normal rats. DMTU protection against light-induced damage implicates free radical-based oxidation in the retinal degeneration process. Furthermore, the use of green light (corresponding to the absorption maxima of rhodopsin) implicates rhodopsin in the initiation of the pathobiological mechanism. We propose that generation of cytotoxic oxysterols (by-products of 7DHC oxidation) is an integral part of retinal cell death in the SLOS rat model, which is exacerbated by intense light. Furthermore, the results predict lightdependent potentiation of retinal degeneration in SLOS patients, and the possible ameliorative effects of antioxidant therapy.

Partial Rescue of Retinal Function and Sterol Steady-State in a Rat Model of Smith-Lemli-Opitz Syndrome

“…Photoreceptor outer segment membranes are highly enriched in PUFA (32). We have shown recently (18,19) that: a) the SLOS rat model is more susceptible to intense light-induced retinal degeneration than normal albino rats; b) the histologic damage correlates with levels of lipid peroxides; c) ERG deficits and levels of lipid peroxides in retinas of SLOS rats under normal ambient lighting conditions are comparable to those in retinas of light-damaged (but otherwise normal) albino rats; and d) pretreatment of rats with a free radical scavenger (dimethylthiourea) affords substantial protection from light-induced retinal damage in both normal albino and SLOS rats. In fact, lipid peroxidation has been linked to retinal photodamage by other investigators (33)(34)(35).…”

“…The methods used for histologic and quantitative morphometric analysis of retinas were as described in detail previously (11,18).…”

“…Taken together, these findings predict that SLOS has an associated retinal degeneration involving biochemical, histologic, and functional abnormalities in rods and cones. Furthermore, constant bright light exposure exacerbates photoreceptor degeneration in the rat SLOS model (18), far more so than in normal, age-matched albino rats, and the severity of the histologic degeneration correlates with the levels of lipid hydroperoxides present in the retina (19).…”

Biological activities of 7-dehydrocholesterol-derived oxysterols: implications for Smith-Lemli-Opitz syndrome

“…For example, a cholesterolderived oxysterol that is structurally analogous to compound 10 has been reported to be present in ozone-exposed lung lavage and in a human bronchial epithelial cell line, and this oxysterol appears to be toxic at micromolar concentrations ( 69 ). Light has been reported to exacerbate retina degeneration in a rat model of SLOS and this observation could well be due to accelerated free radical oxidation of 7-DHC ( 22,70 ). In addition, the formation and accumulation of 7-DHC oxysterols might be modifi ed by enzymatic transformations, and therefore different tissues may show specifi c 7-DHC-derived oxysterol profi les.…”

Novel oxysterols observed in tissues and fluids of AY9944-treated rats: a model for Smith-Lemli-Opitz syndrome

“…This was necessary in order to be able to characterize the onset and progression of the biochemical, morphological, and electrophysiological features of the retinal degeneration that occurs in this model, since the rodent retina undergoes substantial development over the fi rst several weeks of postnatal life ( 41,42 ). It has been demonstrated that the levels of lipid hydroperoxides in retinas of AY9944-treated rats are comparable to those found in photo-damaged retinas of control rats, and that exposure of AY9944-treated rats to intense visible light greatly accelerates the formation and steady-state accumulation of such oxidized lipids in the retina, which correlates with markedly increased severity and geographic extent of the associated retinal degeneration in this model ( 43,44 ).…”