Involvement of All-trans-retinal in Acute Light-induced Retinopathy of Mice

Exposure to bright light can cause visual dysfunction and retinal photoreceptor damage in humans and experimental animals, but the mechanism(s) remain unclear. We investigated whether the retinoid cycle (i.e. the series of biochemical reactions required for vision through continuous generation of 11-cis-retinal and clearance of all-trans-retinal, respectively) might be involved. Previously, we reported that mice lacking two enzymes responsible for clearing all-trans-retinal, namely photoreceptor-specific ABCA4 (ATP-binding cassette transporter 4) and RDH8 (retinol dehydrogenase 8), manifested retinal abnormalities exacerbated by light and associated with accumulation of diretinoid-pyridinium-ethanolamine (A2E), a condensation product of all-trans-retinal and a surrogate marker for toxic retinoids. Now we show that these mice develop an acute, light-induced retinopathy. However, cross-breeding these animals with lecithin:retinol acyltransferase knock-out mice lacking retinoids within the eye produced progeny that did not exhibit such light-induced retinopathy until gavaged with the artificial chromophore, 9-cis-retinal. No significant ocular accumulation of A2E occurred under these conditions. These results indicate that this acute light-induced retinopathy requires the presence of free all-trans-retinal and not, as generally believed, A2E or other retinoid condensation products. Evidence is presented that the mechanism of toxicity may include plasma membrane permeability and mitochondrial poisoning that lead to caspase activation and mitochondria-associated cell death. These findings further understanding of the mechanisms involved in light-induced retinal degeneration.The retinoid cycle is a fundamental metabolic process in the vertebrate retina responsible for continuous generation of 11-cis-retinal from its all-trans-isomer (1-3). Because 11-cis-retinal is the chromophore of rhodopsin and cone visual pigments (4), disabling mutations in genes encoding proteins of the retinoid cycle can cause a spectrum of retinal diseases affecting sight (3). Moreover, the efficiency of the mammalian visual system and health of photoreceptors and retinal pigment epithelium (RPE)² decrease significantly with age. Even in the presence of a functional retinoid cycle, A2E, retinal dimer (RALdi), and other toxic all-trans-retinal condensation products (5-7) can accumulate as a consequence of aging (8). Under experimental conditions, these compounds can produce toxic effects on RPE cells (9-11). Patients affected by age-related macular degeneration, Stargardt disease, or other retinal diseases associated with accumulation of surrogate markers, such as A2E, all develop retinal degeneration (12). Thus, elucidating the fundamental causes of these age-dependent changes is of increasing importance. Encouragingly, our understanding of both retinoid metabolism outside the eye and production of 11-cis-retinal unique to the eye has accelerated recently (Scheme 1) (1-3), and genetic mouse models are readily available to study these processes and their potential aberrations in vivo (13). Thus, a central question can be addressed, namely what initiates the death of photoreceptor cells and the underlining RPE?Open in a separate windowSCHEME 1.Retinoid flow and all-trans-retinal clearance in the visual cycle. After diffusion from the RPE, the visual chromophore, 11-cis-retinal, combines with rhodopsin and then is photoisomerized to all-trans-retinal. Most of the all-trans-retinal dissociates from opsin into the cytoplasm, where it is reduced to all-trans-retinol by RDHs, including RDH8. The fraction of all-trans-retinal that dissociates into the disc lumen is transported by ABCA4 into the cytoplasm (23) before it is reduced. All-trans-retinol then is translocated to the RPE, esterified by LRAT, and recycled back to 11-cis-retinal. Mutations of ABCA4 are associated with human macular degeneration, Stargardt disease, and age-related macular degeneration (55, 56).Several mechanisms associated with retinoid metabolism may contribute to different retinopathies (1). For example, lack of retinoids in LRAT (lecithin:retinol acyltransferase) or chromophore in retinoid isomerase knock-out (Rpe65^-/-) mice leads to rapid degeneration of cone photoreceptors and slowly progressive death of rods (14). Such mice do not produce toxic condensation products from all-trans-retinal. Instead, their retinopathies have been attributed to continuous activation of visual phototransduction (15) due to either the basal activity of opsin (16-18) or disordered vectorial transport of cone visual pigments without bound chromophore (19). Paradoxically, an abnormally high flux of retinoids through the retinoid cycle can also lead to retinopathy in other mouse models (20, 21). Animal models featuring anomalies in the retinoid cycle illustrate the importance of chromophore regeneration and provide an approach to elucidating mechanisms involved in human retinal dysfunction and disease.Recently, we showed that mice carrying a double knock-out of Rdh8 (retinol dehydrogenase 8), one of the main enzymes that reduces all-trans-retinal in rod and cone outer segments (22), and Abca4 (ATP-binding cassette transporter 4), which transports all-trans-retinal from the inside to the outside of disc membranes (23), rapidly accumulate all-trans-retinal condensation products and exhibit accentuated RPE/photoreceptor dystrophy at an early age (24). Although these studies suggest retinoid toxicity, it is still unclear if the elevated levels of retinal and/or its condensation products, such as A2E, are the cause of this retinopathy or merely a nonspecific reflection of impaired retinoid metabolism. Here, we report that spent chromophore, all-trans-retinal, is most likely responsible for photoreceptor degeneration in Rdh8^-/-Abca4^-/- mice. Toxic effects of all-trans-retinal include caspase activation and mitochondria-associated cell death.