Cone survival despite rod degeneration in XOPS-mCFP transgenic zebrafish.

Vertebrate rod photoreceptors are highly specialized retinal neurons that mediate visual responses under scotopic conditions. Light capture by rhodopsin in the rod outer segment (ROS) initiates a phototransduction cascade that results in synaptic transmission in the outer plexiform layer to the second-order retinal neurons. Given its central role in rod phototransduction, it is not surprising that mutations in the rhodopsin gene are the most frequently identified cause of retinitis pigmentosa (RP), a heritable retinal degenerative disease characterized by a progressive loss of vision. Patients with RP initially experience night blindness and tunnel vision, due to loss of rod photoreceptors, but eventually become completely blind after secondary degeneration of the cones. Although it has been 15 years since the first rhodopsin mutations that cause RP were identified,1 the question of how mutations in a gene expressed specifically in rods lead ultimately to loss of cones remains unresolved. Much has been learned about the molecular events leading to retinal degeneration through studies in animal models. In the mouse, several naturally occurring mutations, as well as targeted mutations and gene knockouts, display retinal degeneration phenotypes analogous to human RP. As in the human disease, rod photoreceptor degeneration is invariably followed by degeneration of the cones. The current research seeks to characterize what, if any, interactions between rods and cones are needed to promote cone photoreceptor survival. However, the applicability of this work to human retinal disease is limited by the fact that the mouse, a nocturnal animal, has a rod-dominated retina with only 3% cones, whereas humans have a retina that is more adapted to color vision and diurnal activity. Therefore, studies of the mechanism of photoreceptor degeneration would benefit from additional and complementary animal models of RP. The zebrafish, a small, freshwater teleost, has good color vision and a cone-dense retina. Zebrafish retinal architecture is well conserved with that of other vertebrates, and the optical transparency of both the embryo and free-swimming larvae greatly facilitate studies of the visual system. Zebrafish possess one type of rod photoreceptor and four distinct classes of cones maximally sensitive to ultraviolet, short, medium, and long wavelengths of light. Over the past 10 years, large- and small-scale genetic screens conducted by various laboratories have led to the isolation of numerous mutant lines of zebrafish displaying retinal degenerations.2–6 Although most mutations affect both rods and cones or are lethal (suggesting the mutated genes are not rod specific), two dominant mutations demonstrate an initial degeneration of rod photoreceptors,5,7 somewhat similar to RP and other human rod dystrophies. However, in homozygous embryos, these mutations display pleiotropic affects outside the retina and are lethal. A genetic approach that specifically targets the rod photoreceptors in zebrafish would therefore be beneficial. With this goal in mind, we and others have generated lines of transgenic zebrafish that demonstrate rod-photoreceptor–specific expression of fluorescent reporter proteins such as green fluorescent protein (GFP).8–11 All show patterns of expression similar to the endogenous rhodopsin gene, and none cause any functional or structural abnormalities in the photoreceptors. Genetic manipulation of these lines should allow a more detailed study of the processes of rod development and degeneration. Furthermore, the rod-specific promoters may facilitate genetic ablation of the rods, which would further studies of the impact of rod loss on cone survival and may provide useful models for human RP. In this report we describe a transgenic line of zebrafish expressing a membrane-targeted variant of cyan fluorescent protein (mCFP) driven by a portion of the Xenopus rhodopsin promoter that unexpectedly displays rod photoreceptor degeneration. Of interest, cones of transgenic animals appear to be morphologically and physiologically unaffected by the loss of the rods, at least up to 7 months of age.