Drosophila NinaB and NinaD Act Outside of Retina to Produce Rhodopsin Chromophore

Guie Gu,Jing Yang,Kathleen A Mitchell,Joseph E O'Tousa

doi:10.1074/jbc.m400323200

Abstract

The Drosophila ninaB gene encodes a beta,beta-carotene-15,15'-oxygenase responsible for the centric cleavage of beta-carotene that produces the retinal chromophore of rhodopsin. The ninaD gene encodes a membrane receptor required for efficient use of beta-carotene. Despite their importance to the synthesis of visual pigment, we show that these genes are not active in the retina. Mosaic analysis shows that ninaB and ninaD are not required in the retina, and exclusive retinal expression of either gene, or both genes simultaneously, does not support rhodopsin biogenesis. In contrast, neuron-specific expression of ninaB and ninaD allows for rhodopsin biogenesis. Additional directed expression studies failed to identify other tissues supporting ninaB activity in rhodopsin biogenesis. These results show that nonretinal sites of NinaB beta,beta-carotene-15,15'-oxygenase activity, likely neurons of the central nervous system, are essential for production of the visual chromophore. Retinal or another C(20) retinoid, not members of the beta-carotene family of C(40) carotenoids, are supplied to photoreceptors for rhodopsin biogenesis.

Highlights

Mutants in eight Drosophila genes, designated ninaA through ninaH, are characterized by reduced rhodopsin levels in photoreceptors and altered electroretinograms (1)
Rescue of ninaB and ninaD by Dietary Vitamin A—We examined the rhodopsin content in ninaB and ninaD mutant flies fed on diets supplemented with either ␤-carotene or all-transretinal as the form of vitamin A (Fig. 1)
The ninaB mutant was unable to use ␤-carotene or all-trans-retinal as the source of rhodopsin chromophore when reared in the dark but could use all-trans-retinal when reared in a 12-h light/dark cycle

Summary

Introduction

Mutants in eight Drosophila genes, designated ninaA through ninaH, are characterized by reduced rhodopsin levels in photoreceptors and altered electroretinograms (1). The ninaB gene encodes a ␤,␤-carotene-15,15Јoxygenase (BCO) responsible for the centric cleavage of ␤-car-. This enzyme was originally named as a ␤-carotene dioxygenase. A human BCO is expressed in the retinal pigment epithelium and in the kidney, intestine, liver, brain, stomach, and testis (12, 13), suggesting that the processing of dietary carotenoids occurs in a variety of vertebrate tissues. We used targeted expression studies to show that the ninaB and ninaD genes did not act within photoreceptors to produce rhodopsin chromophore. Only nonretinal neuronal cells were identified as being capable of supporting the conversion of ␤-carotene to retinal These results showed that, despite a requirement for retinal only within photoreceptors, other cell types are responsible for carotenoid metabolism

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Conclusion