[Retinal degeneration diseases and the retinoid cycle]

The retinoid cycle is a vitamin A metabolic circuit which occurs between the photoreceptor outer segment and the retinal pigmented epithelium to maintain visual function. Abnormalities in the retinoid cycle are involved in some hereditary retinal degenerative diseases and age-related retinal degeneration. Here, we summarize the entire process of the retinoid cycle and the pathophysiology of those retinal diseases due to molecular abnormalities of the cycle. Gene transfer and pharmacologic treatment have been developed based on recent studies revealing characteristic clinical features initiated by each aberrant reaction in the retinoid cycle. The molecular mechanisms of retinal degeneration caused by an abnormal retinoid cycle have been investigated intensively, and practical therapies have been introduced for currently incurable retinal diseases.