Abstract
Purpose :
Autosomal recessive Stargardt disease, caused by mutations in the ABCA4 gene, is the most common form of inherited macular degeneration, affecting ~1 in 5,000 people. Loss of ABCA1 function leads to the accelerated accumulation of toxic vitamin A bisretinoids in the retinal pigment epithelium (RPE). Using Abca4-/- mice, we discovered that bisretinoids cause cholesterol accumulation in RPE lysosomes. This activates acid sphingomyelinase (ASM), the enzyme that hydrolyzes sphingomyelin to ceramide. Excess cholesterol and ceramide compromise critical homeostatic functions in the RPE, leading to inflammation and photoreceptor dysfunction. Here, we evaluated clinically approved drugs that inhibit ASM for their ability to preserve RPE health and restore photoreceptor function in Stargardt disease
Methods :
Pigmented Abca4-/- mice and age-matched wildtypes were administered drugs intraperitoneal injections 3 times/week for 8 weeks. In vitro studies were performed on primary polarized porcine RPE cultures with the bisretinoid A2E. Immunostaining of RPE flatmounts, immunoblotting, and high-speed live imaging were used to assess ceramide and cholesterol accumulation, exosome secretion, complement activation, autophagy, mitochondrial dynamics, and recruitment of microglia to the subretinal space. ERGs were used to assess retinal function.
Results :
ASM inhibitors decreased ceramide, restored autophagy, and protected the RPE from complement-induced mitochondrial injury in Abca4-/- mice. Lowering RPE ceramide also prevented the secretion of pro-inflammatory exosomes by Abca4-/- RPE, which in turn prevented sub-retinal recruitment of microglia. 18-month-old Abca4-/- mice exhibit significant decreases in scotopic a- and c-waves. An 8-week treatment regimen at 16 months restored photoreceptor (a-wave) and RPE (c-wave) function in aged Abca4-/- mice.
Conclusions :
Using mechanism-based approaches, we have identified pathological accumulation of cholesterol and ceramide as triggers that compromise several homeostatic functions of the RPE eventually leading to photoreceptor degeneration and vision loss. Our recent studies have successfully repositioned two FDA-approved drugs that lower RPE ceramide and prevent complement activation, lipid accumulation, and mitochondrial injury. These drugs safeguard RPE integrity and restore vision loss in macular degeneration models.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.