Abstract
Vitamin A is an essential fat-soluble vitamin that occurs in various chemical forms. It is essential for several physiological processes. Either hyper- or hypovitaminosis can be harmful. One of the most important vitamin A functions is its involvement in visual phototransduction, where it serves as the crucial part of photopigment, the first molecule in the process of transforming photons of light into electrical signals. In this process, large quantities of vitamin A in the form of 11-cis-retinal are being isomerized to all-trans-retinal and then quickly recycled back to 11-cis-retinal. Complex machinery of transporters and enzymes is involved in this process (i.e., the visual cycle). Any fault in the machinery may not only reduce the efficiency of visual detection but also cause the accumulation of toxic chemicals in the retina. This review provides a comprehensive overview of diseases that are directly or indirectly connected with vitamin A pathways in the retina. It includes the pathophysiological background and clinical presentation of each disease and summarizes the already existing therapeutic and prospective interventions.
Highlights
Vitamin A is a group of unsaturated organic compounds which is mostly associated with retinol
There are specific biochemical pathways involved in vitamin A transfer to and storage in the retinal pigment epithelium (RPE), transfer to the photoreceptors, conversion to its active form (11-cis-retinal), recycling of the inactive forms and removal of the toxic by-products; which will be reviewed in the 1st part of this paper
For treatment of age-related macular degeneration (AMD), the original Age-Related Eye Disease Study (AREDS) formulation that included beta-carotene was replaced with the AREDS2 formula, where beta-carotene was substituted with lutein and zeaxanthin [23]
Summary
Vitamin A is a group of unsaturated organic compounds which is mostly associated with retinol All-trans-retinoic acid is an essential signalling molecule in cell differentiation, proliferation and apoptosis, and participates in gene transcription. It is crucial for reproduction, growth and development of the embryo, normal brain function, immune system functioning, skeletal development, growth of hair and nails, and epithelial cell RNA synthesis including the conjunctiva and cornea. There are specific biochemical pathways involved in vitamin A transfer to and storage in the retinal pigment epithelium (RPE), transfer to the photoreceptors, conversion to its active form (11-cis-retinal), recycling of the inactive forms and removal of the toxic by-products; which will be reviewed in the 1st part of this paper. Pathogenic variants in genes, encoding the proteins involved in these processes, have been implicated in several retinal diseases, which will be reviewed in 2nd part of this paper
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