Retinoids and the Visual Cycle: New Actors for an "Old" Function

Vision, the process that acquires most of the brain's sensory input, is a paramount example of the complex interactions of our body with the environment. In order to sustain vision, vertebrates have further evolved pathways that regenerate the visual chromophore, 11-cis-retinal, from the photoproduct all-trans-retinal. This process takes place in ocular cell types and is referred to as the visual or retinoid cycle. Gene therapy in animal models of blinding diseases, such as retinitis pigmentosa, has successfully replaced enzymes involved in chromophore regeneration and restored vision. Currently, dystrophies resulting from impaired chromophore synthesis have been shown to respond to supplementation with a readily available chromophore analog precursor (9-cis-retinyl acetate), and those derived from accumulation of toxic retinoid derivatives can be treated by inhibiting the visual cycle or limiting the supply of vitamin A to the eyes via pharmacological intervention.