Rhodopsin Formation inDrosophilaIs Dependent on the PINTA Retinoid-Binding Protein

Abstract

Retinoids participate in many essential processes including the initial event in photoreception. 11-cis-retinal binds to opsin and undergoes a light-driven isomerization to all-trans-retinal. In mammals, the all-trans-retinal is converted to vitamin A (all-trans-retinol) and is transported to the retinal pigment epithelium (RPE), where along with dietary vitamin A, it is converted into 11-cis-retinal. Although this cycle has been studied extensively in mammals, many questions remain, including the specific roles of retinoid-binding proteins. Here, we establish the Drosophila visual system as a genetic model for characterizing retinoid-binding proteins. In a genetic screen for mutations that affect the biosynthesis of rhodopsin, we identified a novel CRAL-TRIO domain protein, prolonged depolarization afterpotential is not apparent (PINTA), which binds to all-trans-retinol. We demonstrate that PINTA functions subsequent to the production of vitamin A and is expressed and required in the retinal pigment cells. These results represent the first genetic evidence for a role for the retinal pigment cells in the visual response. Moreover, our data implicate Drosophila retinal pigment cells as functioning in the conversion of dietary all-trans-retinol to 11-cis-retinal and suggest that these cells are the closest invertebrate equivalent to the RPE.

Phototransduction and retinal degeneration in Drosophila

“…Mutations in Drosophila crumbs and its human homolog lead to light-induced retinal degeneration, but the mechanisms underlying these retinal degenerations are poorly understood [238,239]. The Drosophila retinal pigment cells appear to function similarly to human RPE cells in the generation of the chromophore [81], and many genetic defects in human RPE cells are known to cause photoreceptor cell degeneration. Thus, further characterization of the Drosophila retinal pigment cells is warranted.…”

“…The pinta (PDA is not apparent) locus [81] encodes a retinoid binding protein, which binds preferentially to all-trans-retinol, and is required in retinal pigment cells for production of the chromophore ( Model for the conversion of dietary β-carotene to the chromophore. The NINAD scavenger receptor is required for absorption of β-carotene into the midgut, which is subsequently taken up into extraretinal neurons and glia in the head via the SANTA MARIA scavenger receptor and cleaved by the NINAB BCO.…”

“…The NINAB BCO subsequently functions in the same neurons and glia as SANTA MARIA in the centric cleavage of β-carotene to retinal [78]. The all-trans-retinol is then transferred to the retinal pigment cells, where it is converted into the chromophore, through a process involving the PINTA retinoid binding protein and the NINAG oxidoreductase [81,82]. This proposed pathway is not yet complete as many factors are still unknown.…”

Molecular evolution and expression of the CRAL_TRIO protein family in insects

“…PINTA or prolonged depolarization afterpotential (PDA) is not apparent was discovered in a screen for Drosophila eye mutants that are deficient in rhodopsin biogenesis (Wang and Montell, 2005). Visual pigments or rhodopsins are composed of an opsin protein covalently linked to derivatives of Vitamin A (all-trans-retinol), such as 11-cis-retinal.…”

“…Humans that carry mutations in the CRALBP gene may develop retinitis pigmentosa or retinitis punctata albescence (Fishman et al, 2004). In Drosophila, PINTA is localized to the retinal pigment cells, where all-trans 3-hydroxyretinol is converted to 11-cis 3-hydroxyretinal (Pak et al, 2012;Wang and Montell, 2005). Ligand-binding assays indicate that PINTA preferentially binds all-trans-retinol (Wang and Montell, 2005).…”