Abstract
The retinoid cycle is a metabolic process in the vertebrate retina that continuously regenerates 11-cis-retinal (11-cisRAL) from the all-trans-retinal (atRAL) isomer. atRAL accumulation can cause photoreceptor degeneration and irreversible visual dysfunction associated with incurable blinding retinal diseases, such as Stargardt disease, retinitis pigmentosa (RP), and atrophic age-related macular degeneration (AMD). The underlying cellular mechanisms leading to retinal degeneration remain uncertain, although previous studies have shown that atRAL promotes calcium influx associated with cell apoptosis. To identify compounds that mitigate the effects of atRAL toxicity, here we developed an unbiased and robust image-based assay that can detect changes in intracellular calcium levels in U2OS cells. Using our assay in a high-throughput screen of 2,400 compounds, we noted that selective estrogen receptor modulators (SERMs) potently stabilize intracellular calcium and thereby counteract atRAL-induced toxicity. In a light-induced retinal degeneration mouse model (Abca4-/-Rdh8-/-), raloxifene (a benzothiophene-type scaffold SERM) prevented the onset of photoreceptor apoptosis and thus protected the retina from degeneration. The minor structural differences between raloxifene and one of its derivatives (Y 134) had a major impact on calcium homeostasis after atRAL exposure in vitro, and we verified this differential impact in vivo In summary, the SERM raloxifene has structural and functional neuroprotective effects in the retina. We propose that the highly sensitive image-based assay developed here could be applied for the discovery of additional drug candidates preventing photoreceptor degeneration.
Highlights
The retinoid cycle is a metabolic process in the vertebrate retina that continuously regenerates 11-cis-retinal (11-cisRAL) from the all-trans-retinal isomer. atRAL accumulation can cause photoreceptor degeneration and irreversible visual dysfunction associated with incurable blinding retinal diseases, such as Stargardt disease, retinitis pigmentosa (RP), and atrophic age-related macular degeneration (AMD)
Vision at the molecular level relies on a metabolic pathway that continuously regenerates 11-cis-retinal (11-cisRAL)4 from its all-trans-retinal isomer [2]. atRAL is a photoproduct released from bleached rhodopsin, and its efficient clearance from retinal rod outer segment (ROS) discs and isomerization to 11-cis-retinal by the visual cycle enzymes are essential for the effective renewal of light-sensitive visual pigments and maintenance of rod photoreceptor homeostasis [3, 4]
Humans affected by age-related macular degeneration (AMD), Stargardt disease, retinitis pigmentosa (RP), or other retinal diseases associated with the abnormal accumulation of atRAL condensation products undergo retinal degeneration, which eventually culminates in vision loss [5, 6]
Summary
11-cisRAL, 11-cis-retinal; AM, acetoxymethyl; atRAL, all-trans-retinal; atROL, all-trans-retinol; CCB, calcium channel blocker; 9-cisRAL, 9-cis-retinal; A2E, diretinoid-pyridinium-ethanolamine; ERG, electroretinography; ER␣, estrogen receptor ␣ (gene Esr1); ER, estrogen receptor  (gene Esr2); ER, estrogen receptor; GPER30, G protein– coupled estrogen receptor 1 (gene Gper1); HTS, high-throughput screen; i.p., intraperitoneal; mER, membrane estrogen receptor; OCT, optical coherence tomography; SD-OCT, spectral domain OCT; ONL, outer nuclear layer; RPE, retinal pigmented epithelial; RP, retinitis pigmentosa; ROS, rod outer segment; SLO, scanning laser ophthalmoscopy; SERM, selective estrogen receptor modulator; DMEM, Dulbecco’s modified Eagle’s medium; S/B, signal/background; scRNA, single-cell RNA. Our findings likely will yield new targets for drug intervention in the prevention and/or progression of retinal degenerative diseases
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