Rod Photoreceptor Development Research Articles

Retinoid-related orphan nuclear receptor RORβ is an early-acting factor in rod photoreceptor development

Rods and cones are morphologically and developmentally distinct photoreceptor types with different functions in vision. Cones mediate daylight and color vision and in most mammals express M and S opsin photopigments for sensitivity to medium-long and short light wavelengths, respectively. Rods mediate dim light vision and express rhodopsin photopigment. The transcription factor networks that direct differentiation of each photoreceptor type are incompletely defined. Here, we report that Rorb(-/-) mice lacking retinoid-related orphan nuclear receptor beta lose rods but overproduce primitive S cones that lack outer segments. The phenotype reflects pronounced plasticity between rod and cone lineages and resembles that described for Nrl(-/-) mice lacking neural retina leucine zipper factor. Rorb(-/-) mice lack Nrl expression and reexpression of Nrl in Rorb(-/-) mice converts cones to rod-like cells. Thus, Rorb directs rod development and does so at least in part by inducing the Nrl-mediated pathway of rod differentiation.

NR2E3mutations in enhanced S-cone sensitivity syndrome (ESCS), Goldmann-Favre syndrome (GFS), clumped pigmentary retinal degeneration (CPRD), and retinitis pigmentosa (RP)

NR2E3, also called photoreceptor-specific nuclear receptor (PNR), is a transcription factor of the nuclear hormone receptor superfamily whose expression is uniquely restricted to photoreceptors. There, its physiological activity is essential for proper rod and cone photoreceptor development and maintenance. Thirty-two different mutations in NR2E3 have been identified in either homozygous or compound heterozygous state in the recessively inherited enhanced S-cone sensitivity syndrome (ESCS), Goldmann-Favre syndrome (GFS), and clumped pigmentary retinal degeneration (CPRD). The clinical phenotype common to all these patients is night blindness, rudimental or absent rod function, and hyperfunction of the "blue" S-cones. A single p.G56R mutation is inherited in a dominant manner and causes retinitis pigmentosa (RP). We have established a new locus-specific database for NR2E3 (www.LOVD.nl/eye), containing all reported mutations, polymorphisms, and unclassified sequence variants, including novel ones. A high proportion of mutations are located in the evolutionarily-conserved DNA-binding domains (DBDs) and ligand-binding domains (LBDs) of NR2E3. Based on homology modeling of these NR2E3 domains, we propose a structural localization of mutated residues. The high variability of clinical phenotypes observed in patients affected by NR2E3-linked retinal degenerations may be caused by different disease mechanisms, including absence of DNA-binding, altered interactions with transcriptional coregulators, and differential activity of modifier genes.

Activin Inhibits Cell Growth and Induces Differentiation in Human Retinoblastoma Y79 Cells

Purpose: Activin is a member of the transforming growth factor-beta (TGF-β) superfamily and exerts certain effects on differentiation and apoptosis. We investigated the effects of activin on retinoblastoma cell line. Materials and Methods: We used retinoblastoma cell line Y79. Intracellular signal transduction of activin was investigated with RT-PCR, immunofluorescence study, and luciferase reporter assay. The effect of activin on cell growth was examined with fluorescence cell viability assays. To determine the effect of activin on apoptosis, a TUNEL assay and an immunofluorescence study of cleaved PARP were performed. The effect of activin on cell differentiation was examined with RT-PCR and Western blotting. Results: Intracellular signal transduction of activin was confirmed in Y79 cells. Activin inhibited Y79 cell growth. Activin induced the expression of neural retina leucine zipper (Nrl) at the mRNA and protein levels. Conclusions: Nrl is a specific gene in rod photoreceptor development and is a gene indispensable to differentiation into rod photoreceptors, so the present results suggest that activin affects the differentiation of retinoblastoma cells into rod photoreceptor cells.

Pias3-Dependent SUMOylation Directs Rod Photoreceptor Development

Specification of retinal rod photoreceptors is determined by several different transcription factors that activate expression of rod-specific genes and repress expression of cone photoreceptor-specific genes. The mechanism by which this dual regulation occurs is unclear. We have found that Pias3, a transcriptional coregulator and E3 SUMO ligase that is selectively expressed in developing photoreceptors, promotes the differentiation of rod photoreceptors while preventing rods from adopting cone photoreceptor-like characteristics. Pias3 binds the photoreceptor-specific transcription factors Crx and Nr2e3 and is specifically targeted to the promoters of photoreceptor-specific genes. Pias3 SUMOylates Nr2e3, converting it into a potent repressor of cone-specific gene expression. Rod- and cone-specific promoters are bound by hyperSUMOylated proteins in rod photoreceptors, and blocking SUMOylation in photoreceptors results in cells with morphological and molecular features of cones and an absence of rod-specific markers. Our data thus identify Pias3-mediated SUMOylation of photoreceptor-specific transcription factors as a key mechanism of rod specification.

Analysis of the involvement of the NR2E3 gene in autosomal recessive retinal dystrophies

The nuclear receptor protein NR2E3 is postulated to play an important role in rod and cone photoreceptor development. NR2E3 gene mutational analyses were carried out in 103 unrelated subjects with different retinal diseases. A total of 14 different sequence variants were identified, including 3 mutations, 6 rare sequence variants and five polymorphisms. One of three mutations is novel (a frameshift mutation: c.1034_1038del5bp). Five of the six rare sequence variants and one of the polymorphisms identified are novel. Splice prediction programs and functional splicing assays were performed to study three of these variants. The c.119-2 A>C mutant allele construction produces, in addition to the normal one, an abnormal transcript of 180 bp resulting from an aberrant splicing with skipping of exon 2 and the generation of a premature stop codon in exon 3. These experimental data confirm the splice predictions made by the computer programs. The obtained results reinforce the idea that NR2E3 gene is involved in several retinal diseases without a clear genotype-phenotype correlation.

Requirement of histone deacetylase activity for the expression of critical photoreceptor genes

BackgroundHistone deacetylases (HDACs) play a major role in the regulation of gene transcription, often leading to transcriptional repression, as well as other effects following deacetylation of non-histone proteins.ResultsTo investigate the role of HDACs in the developing mammalian retina, a general inhibitor of HDACs, trichostatin-A (TSA), was used to treat newborn murine retinae in explant cultures. Inhibition of HDAC activity resulted in a reduction in RNA levels for genes that regulate retinal development, as well as cell cycle regulators. Several of the genes encode transcription factors essential for rod photoreceptor development, Otx2, Nrl, and Crx. Using luciferase reporter assays, the promoter activity of both Nrl and Crx was found to be compromised by HDAC inhibition. Furthermore, downregulation of gene expression by HDAC inhibition didn't require de novo protein synthesis, and was associated with hyperacetylation of histones and non-histone proteins. Finally, HDAC inhibition in retinal explant cultures resulted in increased cell death, reduction in proliferation, a complete loss of rod photoreceptors and Müller glial cells, and an increase in bipolar cells.ConclusionHDAC activity is required for the expression of critical pro-rod transcription factors and the development of rod photoreceptor cells.

The transcription factorNr2e3functions in retinal progenitors to suppress cone cell generation

The transcription factor Nr2e3 is an essential component for development and specification of rod and cone photoreceptors; however, the mechanism through which it acts is not well understood. In this study, we use Nr2e3(rd7/rd7) mice that harbor a mutation in Nr2e3, to serve as a model for the human retinal disease Enhanced S Cone Syndrome. Our studies reveal that NR2E3 is expressed in late retinal progenitors and differentiating photoreceptors of the developing retina and localized to the cell bodies of mature rods and cones. In particular, we demonstrate that the abnormal increase in cone photoreceptors observed in Nr2e3(rd7/rd7) mice arise from ectopic mitotic progenitor cells that are present in the outer nuclear layer of the mature Nr2e3(rd7/rd7) retina. A prolonged phase of proliferation is observed followed by abnormal retinal lamination with fragmented and disorganized photoreceptor synapses that result in a progressive loss of rod and cone function. An extended and pronounced wave of apoptosis is also detected at P30 and temporally correlates with the phase of prolonged proliferation. Approximately twice as many apoptotic cells were detected compared to proliferating cells. This wave of apoptosis appears to affect both rod and cone cells and thus may account for the concurrent loss of rod and cone function. We further show that Nr2e3(rd7/rd7) cones do not express rod specific genes and Nr2e3(rd7/rd7) rods do not express cone specific genes. Our studies suggest that, based on its temporal and spatial expression, NR2E3 acts simultaneously in different cell types: in late mitotic progenitors, newly differentiating post mitotic cells, and mature rods and cones. In particular, this study reveals the function of NR2E3 in mitotic progenitors is to repress the cone generation program. NR2E3 is thus one of the few genes known to influence the competency of retinal progenitors while simultaneously directing the rod and cone differentiation.

Characterization of Mice with Targeted Deletion of Glycine Receptor Alpha 2

Glycine receptors are ligand-gated chloride channels that mediate inhibitory neurotransmission in the adult nervous system. During development, glycine receptor alpha 2 (GlyRalpha2) is expressed in the retina, in the spinal cord, and throughout the brain. Within the cortex, GlyRalpha2 is expressed in immature cells and these receptors have been shown to be active and excitatory. In the developing retina, inhibition of glycine receptor activity prevents proper rod photoreceptor development. These data suggest that GlyRalpha2, the developmentally expressed glycine receptor, may play an important role in neuronal development. We have generated mice with a targeted deletion of glycine receptor alpha 2 (Glra2). Although these mice lack expression of GlyRalpha2, no gross morphological or molecular alterations were observed in the nervous system. In addition, the cerebral cortex does not appear to require glycine receptor activity for proper development, as Glra2 knockout mice did not show any electrophysiological responses to glycine.

Mosaic deletion of Rb arrests rod differentiation and stimulates ectopic synaptogenesis in the mouse retina

The retinoblastoma gene (Rb) regulates neural progenitor cell proliferation and cell fate specification and differentiation. For the developing mouse retina, two distinct functions of Rb have been described: regulation of retinal progenitor cell proliferation and rod photoreceptor development. Cells that would normally become rods fail to mature and remain as immature cells in the outer nuclear layer in the adult. By using Chx10-Cre;Rb(Lox/-) mice, we generated a chimeric retina with alternating apical-basal stripes of wild-type and Rb-deficient tissue. This provides a unique model with which to study synaptogenesis at the outer plexiform layer within regions that lack differentiated rods. In regions where rods failed to differentiate, the outer plexiform layer (OPL) was disrupted. Horizontal cells formed, and their somata were appropriately aligned, but their neurites did not project laterally. Instead many horizonal cell neurites extended apically, forming ectopic synapses with photoreceptors at all levels of the outer nuclear layer. These ectopic photoreceptor terminals contained synaptic ribbons, horizontal cell processes with synaptic vesicles, and a single mitochrondrion characteristic of rod spherules. Rb-deficient bipolar cells differentiated normally, extended dendrites to the OPL, and formed synapses that were indistinguishable from adjacent wild-type cells. In contrast to OPL-positioned synapses, ectopic synapses did not contain bipolar dendrites. This finding suggests that horizontal cells and photoreceptors can form stable synapses that are devoid of bipolar dendrites outside the normal boundaries of the OPL. Finally, analysis of P4, P7, P12, and P15 retinae suggests that the apical horizontal cell processes result from their failure to establish their normal lateral projections during development.

Studying rod photoreceptor development in zebrafish

The zebrafish has rapidly become a favored model vertebrate organism, well suited for studies of developmental processes using large-scale genetic screens. In particular, zebrafish morphological and behavioral genetic screens have led to the identification of genes important for development of the retinal photoreceptors. This may help clarify the genetic mechanisms underlying human photoreceptor development and dysfunction in retinal diseases. In this review, we present the advantages of zebrafish as a vertebrate model organism, summarize retinal and photoreceptor cell development in zebrafish, with emphasis on the rod photoreceptors, and describe zebrafish visual behaviors that can be used for genetic screens. We then describe some of the photoreceptor cell mutants that have been isolated in morphological and behavioral screens and discuss the limitations of current screening methods for uncovering mutations that specifically affect rod function. Finally, we present some alternative strategies to target the rod developmental pathway in zebrafish.

Developmental defects in Rb-deficient retinae

We recently found that the Rb protein is important for the regulation of retinal progenitor cell proliferation and rod photoreceptor development in the mouse retina. These two functions are separate for Rb and in this study we further characterize the role of Rb in retinal development. At postnatal day 12 in the retinae of Chx10- Cre; Rb Lox/− mice, immature cells are found in the outer nuclear layer where rods normally are differentiating. This results in alternating patches of the outer nuclear layer (ONL) that are lacking rod inputs. At this stage of development, horizontal cell processes at the outer plexiform layer do not mature appropriately and they extend into the outer nuclear layer. These disruptions in horizontal cell differentiation can persist for several weeks into the adult stage. While there are several secondary effects of the loss of Rb on retinal development including, limited cell death in the ONL, Müller glial cell activation, persistence of immature cells in the ONL, and altered nuclear morphology of cells in the ONL, we suggest that the defect in horizontal cell synapse formation at the OPL results from fewer rod inputs. Mice with other developmental defects in photoreceptor cell fate specification or glial cell activation do not exhibit a similar alteration in horizontal cell differentiation. Therefore, the retinae from Chx10- Cre; Rb Lox/− mice represent a unique model to study the role of rod photoreceptor inputs in horizontal cell differentiation and synapse formation.

STAT3-mediated signaling in the determination of rod photoreceptor cell fate in mouse retina.

The purpose of this study was to determine the intracellular pathways by which ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) negatively regulate the development of rod photoreceptors in the mouse retina. Retina explant cultures derived from timed-pregnant CD-1 mice were used to monitor rod photoreceptor differentiation. CNTF was used to activate the signal transducer and activator of transcription (STAT)-3 and mitogen-activated protein kinase (MAPK) signal transduction pathways. Activation of STAT3 and MAPK were manipulated by using dominant-negative STAT3 recombinant adenoviruses and a specific inhibitor of MAPK, respectively. Explanted retinas were harvested at distinct time points and processed for immunohistochemistry. Blocking of the MAPK pathway by the MAPK inhibitor PD98059 did not affect normal development of rods in retina explants or the suppression of their appearance by treatment with CNTF. In contrast, activated STAT3 was necessary for suppression of the rod cell fate decision. A deficiency of the STAT3 pathway induced by a dominant negative STAT3 abolished inhibition of rod development by CNTF. These results indicate that STAT3, but not MAPK, can critically regulate photoreceptor development during mouse retina development.

Taurine Provides Signals for Sight

Young and Cepko investigated the signals involved in cell fate specification in the developing retina. They found that in retinas cultured from postnatal day 0 (P0) mice, the addition of taurine, a ligand for glycine and ionotropic γ-aminobutyric acid type A (GABA A ) receptors, stimulated rod cell production through a mechanism that could be inhibited by application of drugs that blocked glycine or GABA A receptors. Furthermore, only the application of glycine and GABA together was able to stimulate rod cell production to an extent similar to that produced by taurine application. In situ mRNA analysis indicated that glycine receptor α2 (GlyRα2) subunits and GABA α6 subunits were present in retinas from P0 mice. If GlyRα2 was overexpressed in retina isolated from E16 embryos that were cultured for 12 days in the presence of taurine, then there was an increase in rod cells, cone cells, and amacrine cells and a decrease in progenitor and Muller glial cells (identified as positive for glutamine synthetase) compared with amounts of those cell types in wild-type retinas. Evidence that taurine activation of the receptors was important for these changes in cell specification came from experiments showing that mutant forms of the GlyRα2 that could not bind taurine did not promote similar changes. In vivo treatment of P0 rat retina with RNAi to knock down GlyRα2 resulted in an increase in the number of bipolar cells and Muller glial cells and a decrease in the number of rod cells. Further studies will be needed to understand if and how the abundance of taurine, which is an amino acid derived from cysteine that can be synthesized locally or taken up from the circulation, is controlled. Rentería et al. discuss these findings. T. L. Young, C. L. Cepko, A role for ligand-gated ion channels in rod photoreceptor development. Neuron 41 , 867-879 (2004). [Online Journal] R. C. Rentería, J. Johnson, D. R. Copenhagen, Need rods? Get glycine receptors and taurine. Neuron 41 , 839-841 (2004). [Online Journal]

A Role for Ligand-Gated Ion Channels in Rod Photoreceptor Development

Neurotransmitter receptors are central to communication at synapses. Many components of the machinery for neurotransmission are present prior to synapse formation, suggesting a developmental role. Here, evidence is presented that signaling through glycine receptor α2 (GlyRα2) and GABA(A) receptors plays a role in photoreceptor development in the vertebrate retina. The signaling is likely mediated by taurine, which is present at high levels throughout the developing central nervous system (CNS). Taurine potentiates the production of rod photoreceptors, and this induction is inhibited by strychnine, an antagonist of glycine receptors, and bicuculline, an antagonist of GABA receptors. Gain-of-function experiments showed that signaling through GlyRα2 induced exit from mitosis and an increase in rod photoreceptors. Furthermore, targeted knockdown of GlyRα2 decreased the number of photoreceptors while increasing the number of other retinal cell types. These data support a previously undescribed role for these ligand-gated ion channels during the early stages of CNS development.

Caspase-3 in postnatal retinal development and degeneration.

The primary purpose of this study was to evaluate the impact of caspase-3 ablation on photoreceptor degeneration in the rd-1 mouse. Concurrently, the role of caspase-3 in postnatal retinal development was evaluated. Caspase-3 is an important effector caspase that mediates many of the terminal proteolytic events of apoptosis. Its activation has been demonstrated in rodent models of photoreceptor degeneration and its ablation results in exencephaly and neonatal death. Retinal morphometry was performed at the light microscopic level in caspase-3 mutant mice from PN0 through PN23, and in rd-1/caspase-3 double mutant mice at PN14, -16, and -18. This was supplemented by terminal dUTP transferase nick end labeling (TUNEL) and immunohistochemical staining for activated caspase-3, rhodopsin, factor VII-related antigen and proliferating cell nuclear antigen (PCNA). Caspase-3-deficient animals display marginal microphthalmia, peripapillary retinal dysplasia, delayed regression of vitreal vasculature, and retarded apoptotic kinetics of the inner nuclear layer. Ablation of caspase-3 provided transient photoreceptor protection in rd-1, but TUNEL-positive rod death proceeded, despite the absence of caspase-3 activation. In vivo, caspase-3 is not critical for rod photoreceptor development, nor does it play a significant role in mediating pathologic rod death. Peripapillary dysplastic lesions suggest that there is delayed fusion of the optic fissure, and inner nuclear layer abnormalities indicate a cell-specific dependency on the mitochondria-caspase axis during development. The temporal nature of apoptotic retardation in the absence of caspase-3 implies the presence of caspase-independent mechanisms of developmental and pathologic cell death.

Rb regulates proliferation and rod photoreceptor development in the mouse retina.

The retinoblastoma protein (Rb) regulates proliferation, cell fate specification and differentiation in the developing central nervous system (CNS), but the role of Rb in the developing mouse retina has not been studied, because Rb-deficient embryos die before the retinas are fully formed. We combined several genetic approaches to explore the role of Rb in the mouse retina. During postnatal development, Rb is expressed in proliferating retinal progenitor cells and differentiating rod photoreceptors. In the absence of Rb, progenitor cells continue to divide, and rods do not mature. To determine whether Rb functions in these processes in a cell-autonomous manner, we used a replication-incompetent retrovirus encoding Cre recombinase to inactivate the Rb1(lox) allele in individual retinal progenitor cells in vivo. Combined with data from studies of conditional inactivation of Rb1 using a combination of Cre transgenic mouse lines, these results show that Rb is required in a cell-autonomous manner for appropriate exit from the cell cycle of retinal progenitor cells and for rod development.

The expression of the Leber congenital amaurosis protein AIPL1 coincides with rod and cone photoreceptor development.

The Leber congenital amaurosis (LCA) protein AIPL1 is present only in the rod photoreceptors of the adult human retina and is excluded from the cone photoreceptors. LCA, however, is characterized by an absence of both rod and cone function at birth or shortly thereafter. Therefore, this study was conducted to determine whether AIPL1 is present in the rod and cone photoreceptors of the developing human retina. In addition, the expression of NUB1, a putative AIPL1-interacting partner, was examined. A comprehensive spatiotemporal examination of AIPL1 distribution during development was performed by immunohistochemistry, using a previously characterized AIPL1 anti-serum. Immunofluorescence confocal microscopy was used to examine the coexpression of AIPL1 with the long/medium (L/M) and short (S) wavelength-sensitive cone photoreceptors in the developing human retina. The spatiotemporal distribution of NUB1 was also examined by immunohistochemistry, using a newly developed anti-serum to the C terminus of NUB1. AIPL1 protein was detected by 11.8 fetal weeks in the central fetal human retina. With continued development, AIPL1 expression spread gradually toward peripheral retina. AIPL1 was expressed in the L/M and S cone photoreceptors in addition to the rods of the developing human retina. NUB1 was localized in cell nuclei throughout the human fetal and adult eye at all time points. The pattern of AIPL1 expression closely follows the centroperipheral gradient in photoreceptor development. The data suggest that AIPL1 is essential for the normal development of both rod and cone photoreceptor cells and that mutations in the AIPL1 gene cause the death or dysfunction of photoreceptors early in development resulting in blindness or severely impaired vision at birth.

GDNF regulates chicken rod photoreceptor development and survival in reaggregated histotypic retinal spheres.

To investigate the role of glial-cell-line-derived neurotrophic factor (GDNF) on proliferation, differentiation, and apoptosis of different retinal cell types--in particular, photoreceptor cells. Reaggregated histotypic spheres, derived from retinal cells of the E6 chicken embryo were used. Under rotation, so-called rosetted spheroids arose by aggregation of dissociated retinal cells, followed by the proliferation, migration, differentiation and programmed cell death of particular cell types. Rosetted spheroids were cultured under serum-reduced conditions, either in the absence or presence of 50 ng/mL GDNF. At appropriate stages, rosetted spheroids were analyzed by using conventional staining and immunolabeling with antibodies against different retinal cell types. At early stages of culture, the application of GDNF to rosetted spheroids significantly increased and sustained the rate of proliferation. In particular, a de novo production of rod photoreceptors was observed, whereas cone photoreceptors and amacrine, horizontal, ganglion, and Müller cells were not affected. In addition, in GDNF-treated cultures, rod photoreceptors differentiated earlier than in nontreated cultures. In older rosetted spheroids raised in absence of GDNF, rod but not cone photoreceptors underwent apoptosis. By supplementation with GDNF, the percentage of dying rod photoreceptors was dramatically reduced (31%-6% at 8 days in culture, 71%-3% at 10 days in culture). Both the mitogenic and survival promoting effect of GDNF were dose dependent. The results strongly suggest that GDNF, at least in vitro, affects rod photoreceptors. Depending on the developmental stage, GDNF regulates their proliferation, differentiation, and survival.

Visualization of rod photoreceptor development using GFP-transgenic zebrafish.

Zebrafish retina contains five morphologically distinct classes of photoreceptors, each expressing a distinct type of opsin gene. Molecular mechanisms underlying specification of opsin expression and differentiation among the cell types are largely unknown. This is partly because mutants affected with expression of a particular class of opsin gene are difficult to find. In this study we established the transgenic lines of zebrafish carrying green fluorescent protein (GFP) gene under the 1.1-kb and 3.7-kb upstream regions of the rod-opsin gene. In transgenic fish, GFP expression initiated and proceeded in the same spatiotemporal pattern with rod-opsin gene. The retinal section from adult transgenic fish showed GFP expression throughout the rod cell layer. These results indicate that the proximal 1.1-kb region is sufficient to drive gene expression in all rod photoreceptor cells. These transgenic fish should facilitate screening of mutants affected specifically with rod-opsin expression or rod cell development by visualization of rod cells by GFP.

Transgenic expression of a GFP-rhodopsin COOH-terminal fusion protein in zebrafish rod photoreceptors.

To facilitate the identification and characterization of mutations affecting the retina and photoreceptors in the zebrafish, a transgene expressing green fluorescent protein (GFP) fused to the C-terminal 44 amino acids of Xenopus rhodopsin (Tam et al., 2000) under the control of the 1.3-kb proximal Xenopus opsin promoter was inserted into the zebrafish genome. GFP expression was easily observed in a ventral patch of retinal cells at 4 days postfertilization (dpf). Between 45-50% of the progeny from the F1, F2, and F3 generations expressed the transgene, consistent with a single integration event following microinjection. Immunohistochemical analysis demonstrated that GFP is expressed exclusively in rod photoreceptors and not in the UV, blue, or red/green double cones. Furthermore, GFP is localized to the rod outer segments with little to no fluorescence in the rod inner segments, rod cell bodies, or rod synapse regions, indicating proper targeting and transport of the GFP fusion protein. Application of exogenous retinoic acid (RA) increased the number of GFP-expressing cells throughout the retina, and possibly the level of expressed rhodopsin. When bred to a zebrafish rod degeneration mutant, fewer GFP-expressing rods were seen in living mutants as compared to wild-type siblings. This transgenic line will facilitate the search for recessive and dominant mutations affecting rod photoreceptor development and survival as well as proper rhodopsin expression, targeting, and transport.