Abstract
Driven by the energy of a photon, the visual pigments in rod and cone photoreceptor cells isomerize 11-cis-retinal to the all-trans configuration. This photochemical reaction initiates the signal transduction pathway that eventually leads to the transmission of a visual signal to the brain and leaves the opsins insensitive to further light stimulation. For the eye to restore light sensitivity, opsins require recharging with 11-cis-retinal. This trans-cis back conversion is achieved through a series of enzymatic reactions composing the retinoid (visual) cycle. Although it is evident that the classical retinoid cycle is critical for vision, the existence of an adjunct pathway for 11-cis-retinal regeneration has been debated for many years. Retinal pigment epithelium (RPE)-retinal G protein-coupled receptor (RGR) has been identified previously as a mammalian retinaldehyde photoisomerase homologous to retinochrome found in invertebrates. Using pharmacological, genetic, and biochemical approaches, researchers have now established the physiological relevance of the RGR in 11-cis-retinal regeneration. The photoisomerase activity of RGR in the RPE and Müller glia explains how the eye can remain responsive in daylight. In this review, we will focus on retinoid metabolism in the eye and visual chromophore regeneration mediated by RGR.
Highlights
Supplementary key words vision vitamin A retina retinal pigment epithelium visual pigments visual chromophore photoisomerase retinal pigment epithelium-retinal G proteincoupled receptor opsin
These findings demonstrate that RPE-specific 65 kDa (RPE65)-mediated regeneration of visual chromophore occurred in the dark, whereas in conditions resembling daylight, retinal G protein-coupled receptor (RGR) activity was required to augment the regeneration of visual chromophore to normal levels
Compared with the invertebrate eye, the vertebrate eye is more complicated because it involves multiple cell types, numerous enzymes, and the intercellular flow of retinoids facilitated by different binding proteins
Summary
One of the characteristics of vitamin A is that it cannot be synthesized by the human body and must be obtained from a dietary source. Dietary vitamin A is mainly available in two forms, preformed vitamin A and provitamin A. Preformed vitamin A is generally found in animal sources including liver, eggs, fish, and dairy products. Each individual has a different extent of conversion efficiency, approximately half of provitamin A carotenoids are converted into retinol in the human intestine and absorbed. The retinol derived either from preformed vitamin A or provitamin A carotenoids can be reesterified to REs by an enzyme called lecithin:retinol acyltransferase (LRAT) [14,15,16]. The chylomicron remnants containing most of the REs are directed mainly to the liver [17, 18]. The remaining REs in the circulation are taken up by peripheral tissues including the adipose, heart, muscle, and lungs [19]
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