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

Abstract

ABSTRACT:The Smith-Lemli-Opitz syndrome (SLOS) is the firstdescribed in a growing family of hereditary defects in cholesterol biosynthesis, and presents with a spectrum of serious abnormalities, including multiple dysmorphologies, failure to thrive, cognitive and behavioral impairments, and retinopathy. Using a pharmacologically induced rat model of SLOS that exhibits key hallmarks of the disease, including progressive retinal degeneration and dysfunction, we show that a high-cholesterol diet can substantially correct abnormalities in retinal sterol composition, with concomitant improvement of visual function, particularly within the cone pathway. Although histologic degeneration still occurred, a high-cholesterol diet reduced the number of pyknotic photoreceptor nuclei, relative to animals on a cholesterol-free diet. These findings demonstrate that cholesterol readily crosses the blood-retina barrier (unlike the blood-brain barrier) and suggest that cholesterol supplementation may be efficacious in treating SLOS-associated retinopathy. H ereditary abnormalities in cholesterol biosynthesis underlie a family of human birth defects associated with specific enzyme defects and phenotypes (1-3). The first described and most common of these is SLOS (4), a multiple congenital anomalies syndrome involving the inability to efficiently convert 7DHC to Chol (5,6). The enzyme responsible for catalyzing this biosynthetic step (3␤-hydroxysterol-⌬ 7 -reductase; DHCR7; EC 1.3.1.21) is encoded by the DHCR7 gene localized to human chromosome 11q12-q13 (7-9). More than 100 mutations in DHCR7 have been identified in association with SLOS; many result in abnormally low levels of cholesterol and abnormally high levels of 7DHC in bodily tissues (8,9). With an estimated global incidence of 1 in 20,000 to 1 in 60,000 live births, SLOS is likely the fourth most common human recessive disease, after cystic fibrosis, phenylketonuria, and hemochromatosis (8,9). However, a recent study (10) estimates the SLOS carrier frequency to be 1 in 30, suggesting a much higher actual disease frequency, e.g. approximately 1 in 1,590 to 1 in 13,500. The SLOS phenotype displays a spectrum of clinical manifestations, from mild to lethal, and may include such findings as moderate to severe mental retardation, autism, dysmorphic craniofacial and skeletal malformations, and failure to thrive (1-3).We previously reported progressive retinal degeneration in a pharmacological rat model of SLOS (11) characterized by shortening and eventual loss of retinal ROS, pyknosis and thinning of the ONL, accumulation of membranous inclusions and lipid droplets in the retinal pigment epithelium (RPE), defects in rod and cone photoresponses, together with SLOSlike accumulation of 7DHC and diminished cholesterol levels in retina, brain, liver, and serum. The SLOS rat model (11-13) is induced by treatment of pregnant rats and their progeny with AY9944, a selective inhibitor of 3␤-hydroxysterol-⌬ 7 -reductase (14,15), the enzyme defective in SLOS. The clinical relevance of the S...

Alteration of retinal rod outer segment membrane fluidity in a rat model of Smith-Lemli-Opitz syndrome

“…It should be noted that our in vitro studies of rhodopsin regeneration were performed in the presence of excess exogenous 11-cis retinal; hence, the decreased regeneration cannot be due to insufficient retinoid availability. Reduction in the regenerability of rhodoposin also would be expected to negatively impact the efficiency of phototransduction and, hence, photoreceptor function, consistent with our prior in vivo observations with the SLOS rat model (7,8,15). We speculate that opsinʼs ability to assume a conformational state requisite for optimal regeneration is impaired as a consequence of alterations in membrane lipid composition and biophysical properties (e.g., fluidity), such alterations being more extensive by 3 months than by 1 month of exposure of rats to AY9944.…”

“…It is tempting to speculate that faced with a metabolic insult (such as disrupting normal Chol biosynthesis), the cell responds by globally remodeling its membrane lipidome in an effort to preserve cellular integrity and function, including preservation of membrane fluidity that is critical to the function of a host of membrane-associated enzymes, receptors, and ion channels (40). However, although the retinal photoreceptor cells may be able to tolerate this level of stress for at least 1 month, eventually they lose physiological competence and cellular integrity, undergoing degeneration and cell death by 3 months of exposure to AY9944 (8,15). At that point, rod and cone electrophysiology and structure are severely compromised, and this also correlates temporally with the dramatic losses in the DHA content of ROS membranes documented in this study, as well as those recently reported under virtually identical conditions for whole retina (9).…”

“…In-house analysis confirmed that the chow was essentially Chol-free (data not shown). Rats were treated with AY9944 as previously described (8,15). In brief, pregnant rats were implanted subcutaneously with Alzet® osmotic pumps containing AY9944 (in PBS solution, 1.5 mg/ml), so as to provide continuous delivery of AY9944 at a uniform rate (0.37 mg/kg/day, at 2.5 ml/h), beginning on gestational day 7 and continuing through the second postnatal week.…”

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

“…Kolf-Clauw et al (35)(36)(37)(38) and Xu et al ( 39 ) generated animal models of SLOS by treating rats with AY9944 or with a related DHCR7 inhibitor, BM15766; however, postnatal viability was limited. Subsequently, the AY9944-induced SLOS rat model was improved by Fliesler and coworkers, such that postnatal viability was extended to at least three months (40)(41)(42). 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 ).…”

The ins and outs of cholesterol in the vertebrate retina

“…However, the same is not true for the retina, at least under conditions where normal cholesterol metabolism has been perturbed. In fact, in an aforementioned rat model of SLOS, feeding a high-cholesterol diet (2%, by weight) was effective in nearly normalizing the cholesterol content of the retina, with a concomitant normalizing trend in the electrophysiological competence of the retina, particularly as involves cone-mediated visual transduction ( 13 ). Attempts to "load" the retina of normal rats by similar dietary cholesterol supplementation have not proved successful (S.J.…”