Subfunctionalization of a retinoid-binding protein provides evidence for two parallel visual cycles in the cone-dominant zebrafish retina

Dicer Loss in Müller Glia Leads to a Defined Sequence of Pathological Events Beginning With Cone Dysfunction

Purpose

The loss of Dicer in Müller glia (MG) results in severe photoreceptor degeneration, as it occurs in retinitis pigmentosa or age-related macular degeneration; however, the sequence of events leading to this severe degenerative state is unknown. The aim of this study was to conduct a chronological functional and structural characterization of the pathological events in MG-specific Dicer-conditional knockout (cKO) mice in vivo and histologically.

Methods

To delete Dicer and mature microRNAs (miRNAs) in MG, two conditional Dicer1 knockout mouse strains (Rlbp-CreER:tdTomato:Dicer-cKOMG and Glast-CreER:tdTomato:Dicer-cKOMG) were created. Optical coherence tomography (OCT), electroretinograms (ERGs), and histological analyses were conducted to investigate structural and functional changes up to 6 months after Dicer deletion.

Results

Dicer/miRNA loss in MG leads to (1) impairments of the area spanning from the external limiting membrane (ELM) to the retinal pigment epithelium (RPE), (2) cone photoreceptor dysfunction, and (3) retinal remodeling and functional loss of the inner retina at 1, 3, and 6 months after Dicer loss, respectively, in both of the knockout mouse strains. Furthermore, in the Rlbp-CreER:tdTomato:Dicer-cKOMG strain, rod photoreceptor impairment was found 4 months after Dicer depletion (4) accompanied by alteration of RPE integrity (5).

Conclusions

MG Dicer loss in the adult mouse retina impacts cone function prior to any measurable changes in rod function, suggesting a pivotal role for MG Dicer and miRNAs in supporting cone health. A partially impaired RPE, however, seems to accelerate rod degeneration and overall degenerative events.

Dicer loss in Müller glia leads to a defined sequence of pathological events beginning with cone dysfunction

Purpose

The loss of Dicer in Müller glia (MG) results in severe photoreceptor degeneration as it occurs in retinitis pigmentosa or AMD. However, the sequence of events leading to this severe degenerative state is unknown. The aim of this study was to conduct a chronological functional and structural characterization of the pathological events in MG-specific Dicer-cKO mice in vivo and histologically.

Methods

To delete Dicer and mature microRNAs (miRNAs) in MG, two conditional Dicer1 knock-out mouse strains namely RlbpCre:Dicer-cKO MG and GlastCre:Dicer-cKO MG, were created. Optical coherence tomography (OCT), electroretinograms (ERGs) as well as histological analyses were conducted to investigate structural and functional changes up to six months after Dicer deletion.

Results

Dicer/miRNA loss in MG leads to 1) impairments of the external limiting membrane (ELM) - retinal pigment epithelium (RPE), 2) cone photoreceptor dysfunction and 3) retinal remodeling and functional loss of the inner retina, 1, 3 and 6 months after Dicer loss, respectively, in both strains. Furthermore, in the Rlbp:Dicer-cKO MG strain, rod photoreceptor impairment was found 4 months after Dicer depletion (4) accompanied by alteration of RPE integrity (5).

Conclusions

MG Dicer loss in the adult mouse retina impacts cone function prior to any measurable changes in rod function, suggesting a pivotal role for MG Dicer and miRNAs in supporting cone health. A partially impaired RPE however seems to accelerate rod degeneration and overall degenerative events.

Vitamin A supply in the eye and establishment of the visual cycle

Animals perceiving light through visual pigments have evolved pathways for absorbing, transporting, and metabolizing the precursors essential for synthesis of their retinylidene chromophores. Over the past decades, our understanding of this metabolism has grown significantly. Through genetic manipulation, researchers gained insights into the metabolic complexity of the pathways mediating the flow of chromophore precursors throughout the body, and their enrichment within the eyes. This exploration has identified transport proteins and metabolizing enzymes for these essential lipids and has revealed some of the fundamental regulatory mechanisms governing this process. What emerges is a complex framework at play that maintains ocular retinoid homeostasis and functions. This review summarizes the recent advancements and highlights future research directions that may deepen our understanding of this complex metabolism.

The Retina-Based Visual Cycle

The continuous function of vertebrate photoreceptors requires regeneration of their visual pigment following its destruction upon activation by light (photobleaching). For rods, the chromophore required for the regeneration of rhodopsin is derived from the adjacent retinal pigmented epithelium (RPE) cells through a series of reactions collectively known as the RPE visual cycle. Mounting biochemical and functional evidence demonstrates that, for cones, pigment regeneration is supported by the parallel supply with chromophore by two pathways-the canonical RPE visual cycle and a second, cone-specific retina visual cycle that involves the Müller glial cells in the neural retina. In this article, we review historical information that led to the discovery of the retina visual cycle and discuss what is currently known about the reactions and molecular components of this pathway and its functional role in supporting cone-mediated vision.

Dominant role for pigment epithelial CRALBP in supplying visual chromophore to photoreceptors

Cellular retinaldehyde-binding protein (CRALBP) supports production of 11-cis-retinaldehyde and its delivery to photoreceptors. It is found in the retinal pigment epithelium (RPE) and Müller glia (MG), but the relative functional importance of these two cellular pools is debated. Here, we report RPE- and MG-specific CRALBP knockout (KO) mice and examine their photoreceptor and visual cycle function. Bulk visual chromophore regeneration in RPE-KO mice is 15-fold slower than in controls, accounting for their delayed rod dark adaptation and protection against retinal phototoxicity, whereas MG-KO mice have normal bulk visual chromophore regeneration and retinal light damage susceptibility. Cone pigment regeneration is significantly impaired in RPE-KO mice but mildly affected in MG-KO mice, disclosing an unexpectedly strong reliance of cone photoreceptors on the RPE-based visual cycle. These data reveal a dominant role for RPE-CRALBP in supporting rod and cone function and highlight the importance of RPE cell targeting for CRALBP gene therapies.

Update on Viral Gene Therapy Clinical Trials for Retinal Diseases

In 2001, the first large animal was successfully treated with a gene therapy that restored its vision. Lancelot, the Briard dog that was treated, suffered from a human childhood blindness called Leber's congenital amaurosis type 2. Sixteen years later, the gene therapy was approved by the U.S. Food and Drug Administration. The success of this gene therapy in dogs led to a fast expansion of the ocular gene therapy field. By now every class of inherited retinal dystrophy has been treated in at least one animal model and many clinical trials have been initiated in humans. In this study, we review the status of viral gene therapies for the retina, with a focus on ongoing human clinical trials. It is likely that in the next decade we will see several new viral gene therapies approved.

Disturbed retinoid metabolism upon loss of rlbp1a impairs cone function and leads to subretinal lipid deposits and photoreceptor degeneration in the zebrafish retina

The RLBP1 gene encodes the 36 kDa cellular retinaldehyde-binding protein, CRALBP, a soluble retinoid carrier, in the visual cycle of the eyes. Mutations in RLBP1 are associated with recessively inherited clinical phenotypes, including Bothnia dystrophy, retinitis pigmentosa, retinitis punctata albescens, fundus albipunctatus, and Newfoundland rod–cone dystrophy. However, the etiology of these retinal disorders is not well understood. Here, we generated homologous zebrafish models to bridge this knowledge gap. Duplication of the rlbp1 gene in zebrafish and cell-specific expression of the paralogs rlbp1a in the retinal pigment epithelium and rlbp1b in Müller glial cells allowed us to create intrinsically cell type-specific knockout fish lines. Using rlbp1a and rlbp1b single and double mutants, we investigated the pathological effects on visual function. Our analyses revealed that rlbp1a was essential for cone photoreceptor function and chromophore metabolism in the fish eyes. rlbp1a-mutant fish displayed reduced chromophore levels and attenuated cone photoreceptor responses to light stimuli. They accumulated 11-cis and all-trans-retinyl esters which displayed as enlarged lipid droplets in the RPE reminiscent of the subretinal yellow-white lesions in patients with RLBP1 mutations. During aging, these fish developed retinal thinning and cone and rod photoreceptor dystrophy. In contrast, rlbp1b mutants did not display impaired vision. The double mutant essentially replicated the phenotype of the rlbp1a single mutant. Together, our study showed that the rlbp1a zebrafish mutant recapitulated many features of human blinding diseases caused by RLBP1 mutations and provided novel insights into the pathways for chromophore regeneration of cone photoreceptors.

Function of mammalian M-cones depends on the level of CRALBP in Müller cells

Cone photoreceptors mediate daytime vision in vertebrates. The rapid and efficient regeneration of their visual pigments following photoactivation is critical for the cones to remain photoresponsive in bright and rapidly changing light conditions. Cone pigment regeneration depends on the recycling of visual chromophore, which takes place via the canonical visual cycle in the retinal pigment epithelium (RPE) and the Müller cell-driven intraretinal visual cycle. The molecular mechanisms that enable the neural retina to regenerate visual chromophore for cones have not been fully elucidated. However, one known component of the two visual cycles is the cellular retinaldehyde-binding protein (CRALBP), which is expressed both in the RPE and in Müller cells. To understand the significance of CRALBP in cone pigment regeneration, we examined the function of cones in mice heterozygous for Rlbp1, the gene encoding CRALBP. We found that CRALBP expression was reduced by ∼50% in both the RPE and retina of Rlbp1+/- mice. Electroretinography (ERG) showed that the dark adaptation of rods and cones is unaltered in Rlbp1+/- mice, indicating a normal RPE visual cycle. However, pharmacologic blockade of the RPE visual cycle revealed suppressed cone dark adaptation in Rlbp1+/- mice in comparison with controls. We conclude that the expression level of CRALPB specifically in the Müller cells modulates the efficiency of the retina visual cycle. Finally, blocking the RPE visual cycle also suppressed further cone dark adaptation in Rlbp1-/- mice, revealing a shunt in the classical RPE visual cycle that bypasses CRALBP and allows partial but unexpectedly rapid cone dark adaptation.

Abnormal Cone and Rod Photoreceptor Morphogenesis in gdf6a Mutant Zebrafish

Purpose

Analysis of photoreceptor morphology and gene expression in mispatterned eyes of zebrafish growth differentiation factor 6a (gdf6a) mutants.

Methods

Rod and cone photoreceptors were compared between gdf6a mutant and control zebrafish from larval to late adult stages using transgenic labels, immunofluorescence, and confocal microscopy, as well as by transmission electron microscopy. To compare transcriptomes between larval gdf6a mutant and control zebrafish, RNA-Seq was performed on isolated eyes.

Results

Although rod and cone photoreceptors differentiate in gdf6a mutant zebrafish, the cells display aberrant growth and morphology. The cone outer segments, the light-detecting sensory endings, are reduced in size in the mutant larvae and fail to recover to control size at subsequent stages. In contrast, rods form temporarily expanded outer segments. The inner segments, which generate the required energy and proteins for the outer segments, are shortened in both rods and cones at all stages. RNA-Seq analysis provides a set of misregulated genes associated with the observed abnormal photoreceptor morphogenesis.

Conclusions

GDF6 mutations were previously identified in patients with Leber congenital amaurosis. Here, we reveal a unique photoreceptor phenotype in the gdf6a mutant zebrafish whereby rods and cones undergo abnormal maturation distinct for each cell type. Further, subsequent development shows partial recovery of cell morphology and maintenance of the photoreceptor layer. By conducting a transcriptomic analysis of the gdf6a larval eyes, we identified a collection of genes that are candidate regulators of photoreceptor size and morphology.

Yap haploinsufficiency leads to Müller cell dysfunction and late-onset cone dystrophy

Hippo signalling regulates eye growth during embryogenesis through its effectors YAP and TAZ. Taking advantage of a Yap heterozygous mouse line, we here sought to examine its function in adult neural retina, where YAP expression is restricted to Müller glia. We first discovered an unexpected temporal dynamic of gene compensation. At postnatal stages, Taz upregulation occurs, leading to a gain of function-like phenotype characterised by EGFR signalling potentiation and delayed cell-cycle exit of retinal progenitors. In contrast, Yap^+/- adult retinas no longer exhibit TAZ-dependent dosage compensation. In this context, Yap haploinsufficiency in aged individuals results in Müller glia dysfunction, late-onset cone degeneration, and reduced cone-mediated visual response. Alteration of glial homeostasis and altered patterns of cone opsins were also observed in Müller cell-specific conditional Yap-knockout aged mice. Together, this study highlights a novel YAP function in Müller cells for the maintenance of retinal tissue homeostasis and the preservation of cone integrity. It also suggests that YAP haploinsufficiency should be considered and explored as a cause of cone dystrophies in human.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Non-photopic and photopic visual cycles differentially regulate immediate, early, and late phases of cone photoreceptor-mediated vision

Cone photoreceptors in the retina enable vision over a wide range of light intensities. However, the processes enabling cone vision in bright light (i.e. photopic vision) are not adequately understood. Chromophore regeneration of cone photopigments may require the retinal pigment epithelium (RPE) and/or retinal Müller glia. In the RPE, isomerization of all-trans-retinyl esters to 11-cis-retinol is mediated by the retinoid isomerohydrolase Rpe65. A putative alternative retinoid isomerase, dihydroceramide desaturase-1 (DES1), is expressed in RPE and Müller cells. The retinol-isomerase activities of Rpe65 and Des1 are inhibited by emixustat and fenretinide, respectively. Here, we tested the effects of these visual cycle inhibitors on immediate, early, and late phases of cone photopic vision. In zebrafish larvae raised under cyclic light conditions, fenretinide impaired late cone photopic vision, while the emixustat-treated zebrafish unexpectedly had normal vision. In contrast, emixustat-treated larvae raised under extensive dark-adaptation displayed significantly attenuated immediate photopic vision concomitant with significantly reduced 11-cis-retinaldehyde (11cRAL). Following 30 min of light, early photopic vision was recovered, despite 11cRAL levels remaining significantly reduced. Defects in immediate cone photopic vision were rescued in emixustat- or fenretinide-treated larvae following exogenous 9-cis-retinaldehyde supplementation. Genetic knockout of Des1 (degs1) or retinaldehyde-binding protein 1b (rlbp1b) did not eliminate photopic vision in zebrafish. Our findings define molecular and temporal requirements of the nonphotopic or photopic visual cycles for mediating vision in bright light.

Differential expression of PKCα and -β in the zebrafish retina

The retina is a complex neural circuit, which processes and transmits visual information from light perceiving photoreceptors to projecting retinal ganglion cells. Much of the computational power of the retina rests on signal integrating interneurons, such as bipolar cells. Commercially available antibodies against bovine and human conventional protein kinase C (PKC) α and -β are frequently used as markers for retinal ON-bipolar cells in different species, despite the fact that it is not known which bipolar cell subtype(s) they actually label. In zebrafish (Danio rerio) five prkc genes (coding for PKC proteins) have been identified. Their expression has not been systematically determined. While prkcg is not expressed in retinal tissue, the other four prkc (prkcaa, prkcab, prkcba, prkcbb) transcripts were found in different parts of the inner nuclear layer and some as well in the retinal ganglion cell layer. Immunohistochemical analysis in adult zebrafish retina using fluorescent in situ hybridization and PKC antibodies showed an overlapping immunolabeling of ON-bipolar cells that are most likely of the BON s6 and BON s6L or RRod type. However, comparison of transcript expression with immunolabeling, implies that these antibodies are not specific for one single zebrafish conventional PKC, but rather detect a combination of PKC -α and -β variants.

Retinoid isomerase inhibitors impair but do not block mammalian cone photoreceptor function

RPE65 is a retinoid isomerase essential for rod function, but its contribution to cone vision is enigmatic. Using selective RPE65 inhibitors, Kiser et al. demonstrate that cone function depends only partially on continuous RPE65 activity, providing support for cone-specific regeneration mechanisms.

Visual function in vertebrates critically depends on the continuous regeneration of visual pigments in rod and cone photoreceptors. RPE65 is a well-established retinoid isomerase in the pigment epithelium that regenerates rhodopsin during the rod visual cycle; however, its contribution to the regeneration of cone pigments remains obscure. In this study, we use potent and selective RPE65 inhibitors in rod- and cone-dominant animal models to discern the role of this enzyme in cone-mediated vision. We confirm that retinylamine and emixustat-family compounds selectively inhibit RPE65 over DES1, the putative retinoid isomerase of the intraretinal visual cycle. In vivo and ex vivo electroretinography experiments in Gnat1^−/− mice demonstrate that acute administration of RPE65 inhibitors after a bleach suppresses the late, slow phase of cone dark adaptation without affecting the initial rapid portion, which reflects intraretinal visual cycle function. Acute administration of these compounds does not affect the light sensitivity of cone photoreceptors in mice during extended exposure to background light, but does slow all phases of subsequent dark recovery. We also show that cone function is only partially suppressed in cone-dominant ground squirrels and wild-type mice by multiday administration of an RPE65 inhibitor despite profound blockade of RPE65 activity. Complementary experiments in these animal models using the DES1 inhibitor fenretinide show more modest effects on cone recovery. Collectively, these studies demonstrate a role for continuous RPE65 activity in mammalian cone pigment regeneration and provide further evidence for RPE65-independent regeneration mechanisms.

Enhancing Understanding of the Visual Cycle by Applying CRISPR/Cas9 Gene Editing in Zebrafish

During the vertebrate visual cycle, all-trans-retinal is exported from photoreceptors to the adjacent RPE or Müller glia wherein 11-cis-retinal is regenerated. The 11-cis chromophore is returned to photoreceptors, forming light-sensitive visual pigments with opsin GPCRs. Dysfunction of this process perturbs phototransduction because functional visual pigment cannot be generated. Mutations in visual cycle genes can result in monogenic inherited forms of blindness. Though key enzymatic processes are well characterized, questions remain as to the physiological role of visual cycle proteins in different retinal cell types, functional domains of these proteins in retinoid biochemistry and in vivo pathogenesis of disease mutations. Significant progress is needed to develop effective and accessible treatments for inherited blindness arising from mutations in visual cycle genes. Here, we review opportunities to apply gene editing technology to two crucial visual cycle components, RPE65 and CRALBP. Expressed exclusively in the human RPE, RPE65 enzymatically converts retinyl esters into 11-cis retinal. CRALBP is an 11-cis-retinal binding protein expressed in human RPE and Muller glia. Loss-of-function mutations in either protein results in autosomal recessive forms of blindness. Modeling these human conditions using RPE65 or CRALBP murine knockout models have enhanced our understanding of their biochemical function, associated disease pathogenesis and development of therapeutics. However, rod-dominated murine retinae provide a challenge to assess cone function. The cone-rich zebrafish model is amenable to cost-effective maintenance of a variety of strains. Interestingly, gene duplication in zebrafish resulted in three Rpe65 and two Cralbp isoforms with differential temporal and spatial expression patterns. Functional investigations of zebrafish Rpe65 and Cralbp were restricted to gene knockdown with morpholino oligonucleotides. However, transient silencing, off-target effects and discrepancies between knockdown and knockout models, highlight a need for more comprehensive alternatives for functional genomics. CRISPR/Cas9 in zebrafish has emerged as a formidable technology enabling targeted gene knockout, knock-in, activation, or silencing to single base-pair resolution. Effective, targeted gene editing by CRISPR/Cas9 in zebrafish enables unprecedented opportunities to create genetic research models. This review will discuss existing knowledge gaps regarding RPE65 and CRALBP. We explore the benefits of CRISPR/Cas9 to establish innovative zebrafish models to enhance knowledge of the visual cycle.

A Brain-Derived Neurotrophic Factor Mimetic Is Sufficient to Restore Cone Photoreceptor Visual Function in an Inherited Blindness Model

Controversially, histone deacetylase inhibitors (HDACi) are in clinical trial for the treatment of inherited retinal degeneration. Utilizing the zebrafish dye^ucd6 model, we determined if treatment with HDACi can rescue cone photoreceptor-mediated visual function. dye exhibit defective visual behaviour and retinal morphology including ciliary marginal zone (CMZ) cell death and decreased photoreceptor outer segment (OS) length, as well as gross morphological defects including hypopigmentation and pericardial oedema. HDACi treatment of dye results in significantly improved optokinetic (OKR) (~43 fold, p < 0.001) and visualmotor (VMR) (~3 fold, p < 0.05) responses. HDACi treatment rescued gross morphological defects and reduced CMZ cell death by 80%. Proteomic analysis of dye eye extracts suggested BDNF-TrkB and Akt signaling as mediators of HDACi rescue in our dataset. Co-treatment with the TrkB antagonist ANA-12 blocked HDACi rescue of visual function and associated Akt phosphorylation. Notably, sole treatment with a BDNF mimetic, 7,8-dihydroxyflavone hydrate, significantly rescued dye visual function (~58 fold increase in OKR, p < 0.001, ~3 fold increase in VMR, p < 0.05). In summary, HDACi and a BDNF mimetic are sufficient to rescue retinal cell death and visual function in a vertebrate model of inherited blindness.

Transcriptional and Behavioral Responses of Zebrafish Larvae to Microcystin-LR Exposure

Microcystins are cyclic heptapeptides that constitute a diverse group of toxins produced by cyanobacteria. One of the most toxic variants of this family is microcystin-LR (MCLR) which is a potent inhibitor of protein phosphatase 2A (PP2A) and induces cytoskeleton alterations. In this study, zebrafish larvae exposed to 500 μg/L of MCLR for four days exhibited a 40% reduction of PP2A activity compared to the controls, indicating early effects of the toxin. Gene expression profiling of the MCLR-exposed larvae using microarray analysis revealed that keratin 96 (krt96) was the most downregulated gene, consistent with the well-documented effects of MCLR on cytoskeleton structure. In addition, our analysis revealed upregulation in all genes encoding for the enzymes of the retinal visual cycle, including rpe65a (retinal pigment epithelium-specific protein 65a), which is critical for the larval vision. Quantitative real-time PCR (qPCR) analysis confirmed the microarray data, showing that rpe65a was significantly upregulated at 50 μg/L and 500 μg/L MCLR in a dose-dependent manner. Consistent with the microarray data, MCLR-treated larvae displayed behavioral alterations such as weakening response to the sudden darkness and hypoactivity in the dark. Our work reveals new molecular targets for MCLR and provides further insights into the molecular mechanisms of MCLR toxicity during early development.

Zebrafish models of human eye and inner ear diseases

Eye and inner ear diseases are the most common sensory impairments that greatly impact quality of life. Zebrafish have been intensively employed to understand the fundamental mechanisms underlying eye and inner ear development. The zebrafish visual and vestibulo-acoustic systems are very similar to these in humans, and although not yet mature, they are functional by 5days post-fertilization (dpf). In this chapter, we show how the zebrafish has significantly contributed to the field of biomedical research and how researchers, by establishing disease models and meticulously characterizing their phenotypes, have taken the first steps toward therapies. We review here models for (1) eye diseases, (2) ear diseases, and (3) syndromes affecting eye and/or ear. The use of new genome editing technologies and high-throughput screening systems should increase considerably the speed at which knowledge from zebrafish disease models is acquired, opening avenues for better diagnostics, treatments, and therapies.

cis Retinol oxidation regulates photoreceptor access to the retina visual cycle and cone pigment regeneration

KEY POINTS

This study explores the nature of the cis retinol that Müller cells in the retina provide to cones for the regeneration of their visual pigment. We report that the retina visual cycle provides cones exclusively with 11-cis chromophore in both salamander and mouse and show that this selectivity is dependent on the 11-cis-specific cellular retinaldehyde binding protein (CRALBP) present in Müller cells. Even though salamander blue cones and green rods share the same visual pigment, only blue cones but not green rods are able to dark-adapt in the retina following a bleach and to use exogenous 9-cis retinol for pigment regeneration, suggesting that access to the retina visual cycle is cone-specific and pigment-independent. Our results show that the retina produces 11-cis retinol that can be oxidized and used for pigment regeneration and dark adaptation selectively in cones and not in rods.

ABSTRACT

Chromophore supply by the retinal Müller cells (retina visual cycle) supports the efficient pigment regeneration required for cone photoreceptor function in bright light. Surprisingly, a large fraction of the chromophore produced by dihydroceramide desaturase-1, the putative all-trans retinol isomerase in Müller cells, appears to be 9-cis retinol. In contrast, the canonical retinal pigment epithelium (RPE) visual cycle produces exclusively 11-cis retinal. Here, we used the different absorption spectra of 9-cis and 11-cis pigments to identify the isoform of the chromophore produced by the visual cycle of the intact retina. We found that the spectral sensitivity of salamander and mouse cones dark-adapted in the isolated retina (with only the retina visual cycle) was similar to that of cones dark-adapted in the intact eye (with both the RPE and retina visual cycles) and consistent with pure 11-cis pigment composition. However, in mice lacking the cellular retinaldehyde binding protein (CRALBP), cone spectral sensitivity contained a substantial 9-cis component. Thus, the retina visual cycle provides cones exclusively with 11-cis chromophore and this process is mediated by the 11-cis selective CRALBP in Müller cells. Finally, despite sharing the same pigment, salamander blue cones, but not green rods, recovered their sensitivity in the isolated retina. Exogenous 9-cis retinol produced robust sensitivity recovery in bleached red and blue cones but not in red and green rods, suggesting that cis retinol oxidation restricts access to the retina visual cycle to cones.

Retinoids and Retinal Diseases

Recent progress in molecular understanding of the retinoid cycle in mammalian retina stems from painstaking biochemical reconstitution studies supported by natural or engineered animal models with known genetic lesions and studies of humans with specific genetic blinding diseases. Structural and membrane biology have been used to detect critical retinal enzymes and proteins and their substrates and ligands, placing them in a cellular context. These studies have been supplemented by analytical chemistry methods that have identified small molecules by their spectral characteristics, often in conjunction with the evaluation of models of animal retinal disease. It is from this background that rational therapeutic interventions to correct genetic defects or environmental insults are identified. Thus, most presently accepted modulators of the retinoid cycle already have demonstrated promising results in animal models of retinal degeneration. These encouraging signs indicate that some human blinding diseases can be alleviated by pharmacological interventions.

Function Over Form: Modeling Groups of Inherited Neurological Conditions in Zebrafish

Zebrafish are a unique cell to behavior model for studying the basic biology of human inherited neurological conditions. Conserved vertebrate genetics and optical transparency provide in vivo access to the developing nervous system as well as high-throughput approaches for drug screens. Here we review zebrafish modeling for two broad groups of inherited conditions that each share genetic and molecular pathways and overlap phenotypically: neurodevelopmental disorders such as Autism Spectrum Disorders (ASD), Intellectual Disability (ID) and Schizophrenia (SCZ), and neurodegenerative diseases, such as Cerebellar Ataxia (CATX), Hereditary Spastic Paraplegia (HSP) and Charcot-Marie Tooth Disease (CMT). We also conduct a small meta-analysis of zebrafish orthologs of high confidence neurodevelopmental disorder and neurodegenerative disease genes by looking at duplication rates and relative protein sizes. In the past zebrafish genetic models of these neurodevelopmental disorders and neurodegenerative diseases have provided insight into cellular, circuit and behavioral level mechanisms contributing to these conditions. Moving forward, advances in genetic manipulation, live imaging of neuronal activity and automated high-throughput molecular screening promise to help delineate the mechanistic relationships between different types of neurological conditions and accelerate discovery of therapeutic strategies.

Vitamin A and Vision

Visual systems detect light by monitoring the effect of photoisomerization of a chromophore on the release of a neurotransmitter from sensory neurons, known as rod and cone photoreceptor cells in vertebrate retina. In all known visual systems, the chromophore is 11-cis-retinal complexed with a protein, called opsin, and photoisomerization produces all-trans-retinal. In mammals, regeneration of 11-cis-retinal following photoisomerization occurs by a thermally driven isomerization reaction. Additional reactions are required during regeneration to protect cells from the toxicity of aldehyde forms of vitamin A that are essential to the visual process. Photochemical and phototransduction reactions in rods and cones are identical; however, reactions of the rod and cone visual pigment regeneration cycles differ, and perplexingly, rod and cone regeneration cycles appear to use different mechanisms to overcome the energy barrier involved in converting all-trans- to 11-cis-retinoid. Abnormal processing of all-trans-retinal in the rod regeneration cycle leads to retinal degeneration, suggesting that excessive amounts of the retinoid itself or its derivatives are toxic. This line of reasoning led to the development of various approaches to modifying the activity of the rod visual cycle as a possible therapeutic approach to delay or prevent retinal degeneration in inherited retinal diseases and perhaps in the dry form of macular degeneration (geographic atrophy). In spite of great progress in understanding the functioning of rod and cone regeneration cycles at a molecular level, resolution of a number of remaining puzzling issues will offer insight into the amelioration of several blinding retinal diseases.

Zebrafish Models of Retinal Disease

Visual defects affect a large proportion of humanity, have a significant negative impact on quality of life, and cause significant economic burden. The wide variety of visual disorders and the large number of gene mutations responsible require a flexible animal model system to carry out research for possible causes and cures for the blinding conditions. With eyes similar to humans in structure and function, zebrafish are an important vertebrate model organism that is being used to study genetic and environmental eye diseases, including myopia, glaucoma, retinitis pigmentosa, ciliopathies, albinism, and diabetes. This review details the use of zebrafish in modeling human ocular diseases.

Structure of zebrafish IRBP reveals fatty acid binding

Interphotoreceptor retinoid-binding protein (IRBP) has a remarkable role in targeting and protecting all-trans and 11-cis retinol, and 11-cis retinal during the rod and cone visual cycles. Little is known about how the correct retinoid is efficiently delivered and removed from the correct cell at the required time. It has been proposed that different fatty composition at that the outer-segments and retinal-pigmented epithelium have an important role is regulating the delivery and uptake of the visual cycle retinoids at the cell-interphotoreceptor-matrix interface. Although this suggests intriguing mechanisms for the role of local fatty acids in visual-cycle retinoid trafficking, nothing is known about the structural basis of IRBP-fatty acid interactions. Such regulation may be mediated through IRBP's unusual repeating homologous modules, each containing about 300 amino acids. We have been investigating structure-function relationships of Zebrafish IRBP (zIRBP), which has only two tandem modules (z1 and z2), as a model for the more complex four-module mammalian IRBP's. Here we report the first X-ray crystal structure of a teleost IRBP, and the only structure with a bound ligand. The X-ray structure of z1, determined at 1.90 Å resolution, reveals a two-domain organization of the module (domains A and B). A deep hydrophobic pocket with a single bound molecule of oleic acid was identified within the N-terminal domain A. In fluorescence titrations assays, oleic acid displaced all-trans retinol from zIRBP. Our study, which provides the first structure of an IRBP with bound ligand, supports a potential role for fatty acids in regulating retinoid binding.

The Rho-GTPase binding protein IQGAP2 is required for the glomerular filtration barrier

Podocyte dysfunction impairs the size selectivity of the glomerular filter, leading to proteinuria, hypoalbuminuria, and edema, clinically defined as nephrotic syndrome. Hereditary forms of nephrotic syndrome are linked to mutations in podocyte-specific genes. To identify genes contributing to podocyte dysfunction in acquired nephrotic syndrome, we studied human glomerular gene expression data sets for glomerular-enriched gene transcripts differentially regulated between pretransplant biopsy samples and biopsies from patients with nephrotic syndrome. Candidate genes were screened by in situ hybridization for expression in the zebrafish pronephros, an easy-to-use in vivo assay system to assess podocyte function. One glomerulus-enriched product was the Rho-GTPase binding protein, IQGAP2. Immunohistochemistry found a strong presence of IQGAP2 in normal human and zebrafish podocytes. In zebrafish larvae, morpholino-based knockdown of iqgap2 caused a mild foot process effacement of zebrafish podocytes and a cystic dilation of the urinary space of Bowman's capsule upon onset of urinary filtration. Moreover, the glomerulus of zebrafish morphants showed a glomerular permeability for injected high-molecular-weight dextrans, indicating an impaired size selectivity of the glomerular filter. Thus, IQGAP2 is a Rho-GTPase binding protein, highly abundant in human and zebrafish podocytes, which controls normal podocyte structure and function as evidenced in the zebrafish pronephros.

Transducin duplicates in the zebrafish retina and pineal complex: differential specialisation after the teleost tetraploidisation

Gene duplications provide raw materials that can be selected for functional adaptations by evolutionary mechanisms. We describe here the results of 350 million years of evolution of three functionally related gene families: the alpha, beta and gamma subunits of transducins, the G protein involved in vision. Early vertebrate tetraploidisations resulted in separate transducin heterotrimers: gnat1/gnb1/gngt1 for rods, and gnat2/gnb3/gngt2 for cones. The teleost-specific tetraploidisation generated additional duplicates for gnb1, gnb3 and gngt2. We report here that the duplicates have undergone several types of subfunctionalisation or neofunctionalisation in the zebrafish. We have found that gnb1a and gnb1b are co-expressed at different levels in rods; gnb3a and gnb3b have undergone compartmentalisation restricting gnb3b to the dorsal and medial retina, however, gnb3a expression was detected only at very low levels in both larvae and adult retina; gngt2b expression is restricted to the dorsal and medial retina, whereas gngt2a is expressed ventrally. This dorsoventral distinction could be an adaptation to protect the lower part of the retina from intense light damage. The ontogenetic analysis shows earlier onset of expression in the pineal complex than in the retina, in accordance with its earlier maturation. Additionally, gnb1a but not gnb1b is expressed in the pineal complex, and gnb3b and gngt2b are transiently expressed in the pineal during ontogeny, thus showing partial temporal subfunctionalisation. These retina-pineal distinctions presumably reflect their distinct functional roles in vision and circadian rhythmicity. In summary, this study describes several functional differences between transducin gene duplicates resulting from the teleost-specific tetraploidisation.

CRALBP supports the mammalian retinal visual cycle and cone vision

Mutations in the cellular retinaldehyde-binding protein (CRALBP, encoded by RLBP1) can lead to severe cone photoreceptor-mediated vision loss in patients. It is not known how CRALBP supports cone function or how altered CRALBP leads to cone dysfunction. Here, we determined that deletion of Rlbp1 in mice impairs the retinal visual cycle. Mice lacking CRALBP exhibited M-opsin mislocalization, M-cone loss, and impaired cone-driven visual behavior and light responses. Additionally, M-cone dark adaptation was largely suppressed in CRALBP-deficient animals. While rearing CRALBP-deficient mice in the dark prevented the deterioration of cone function, it did not rescue cone dark adaptation. Adeno-associated virus-mediated restoration of CRALBP expression specifically in Müller cells, but not retinal pigment epithelial (RPE) cells, rescued the retinal visual cycle and M-cone sensitivity in knockout mice. Our results identify Müller cell CRALBP as a key component of the retinal visual cycle and demonstrate that this pathway is important for maintaining normal cone-driven vision and accelerating cone dark adaptation.

The role of 11-cis-retinyl esters in vertebrate cone vision

A cycle of cis-to-trans isomerization of the chromophore is intrinsic to vertebrate vision where rod and cone photoreceptors mediate dim- and bright-light vision, respectively. Daylight illumination can greatly exceed the rate at which the photoproduct can be recycled back to the chromophore by the canonical visual cycle. Thus, an additional supply pathway(s) must exist to sustain cone-dependent vision. Two-photon microscopy revealed that the eyes of the zebrafish (Danio rerio) contain high levels of 11-cis-retinyl esters (11-REs) within the retinal pigment epithelium. HPLC analyses demonstrate that 11-REs are bleached by bright light and regenerated in the dark. Pharmacologic treatment with all-trans-retinylamine (Ret-NH2), a potent and specific inhibitor of the trans-to-cis reisomerization reaction of the canonical visual cycle, impeded the regeneration of 11-REs. Intervention with 11-cis-retinol restored the regeneration of 11-REs in the presence of all-trans-Ret-NH2. We used the XOPS:mCFP transgenic zebrafish line with a functional cone-only retina to directly demonstrate that this 11-RE cycle is critical to maintain vision under bright-light conditions. Thus, our analyses reveal that a dark-generated pool of 11-REs helps to supply photoreceptors with the chromophore under the varying light conditions present in natural environments.

Identification of differentially expressed genes during development of the zebrafish pineal complex using RNA sequencing

We describe a method for isolating RNA suitable for high-throughput RNA sequencing (RNA-seq) from small numbers of fluorescently labeled cells isolated from live zebrafish (Danio rerio) embryos without using costly, commercially available columns. This method ensures high cell viability after dissociation and suspension of cells and gives a very high yield of intact RNA. We demonstrate the utility of our new protocol by isolating RNA from fluorescence activated cell sorted (FAC sorted) pineal complex neurons in wild-type and tbx2b knockdown embryos at 24 hours post-fertilization. Tbx2b is a transcription factor required for pineal complex formation. We describe a bioinformatics pipeline used to analyze differential expression following high-throughput sequencing and demonstrate the validity of our results using in situ hybridization of differentially expressed transcripts. This protocol brings modern transcriptome analysis to the study of small cell populations in zebrafish.

Whole-genome duplication in teleost fishes and its evolutionary consequences

Whole-genome duplication (WGD) events have shaped the history of many evolutionary lineages. One such duplication has been implicated in the evolution of teleost fishes, by far the most species-rich vertebrate clade. After initial controversy, there is now solid evidence that such event took place in the common ancestor of all extant teleosts. It is termed teleost-specific (TS) WGD. After WGD, duplicate genes have different fates. The most likely outcome is non-functionalization of one duplicate gene due to the lack of selective constraint on preserving both. Mechanisms that act on preservation of duplicates are subfunctionalization (partitioning of ancestral gene functions on the duplicates), neofunctionalization (assigning a novel function to one of the duplicates) and dosage selection (preserving genes to maintain dosage balance between interconnected components). Since the frequency of these mechanisms is influenced by the genes' properties, there are over-retained classes of genes, such as highly expressed ones and genes involved in neural function. The consequences of the TS-WGD, especially its impact on the massive radiation of teleosts, have been matter of controversial debate. It is evident that gene duplications are crucial for generating complexity and that WGDs provide large amounts of raw material for evolutionary adaptation and innovation. However, it is less clear whether the TS-WGD is directly linked to the evolutionary success of teleosts and their radiation. Recent studies let us conclude that TS-WGD has been important in generating teleost complexity, but that more recent ecological adaptations only marginally related to TS-WGD might have even contributed more to diversification. It is likely, however, that TS-WGD provided teleosts with diversification potential that can become effective much later, such as during phases of environmental change.

A self-renewing division of zebrafish Müller glial cells generates neuronal progenitors that require N-cadherin to regenerate retinal neurons

Müller glia function as retinal stem cells in adult zebrafish. In response to loss of retinal neurons, Müller glia partially dedifferentiate, re-express neuroepithelial markers and re-enter the cell cycle. We show that the immunoglobulin superfamily adhesion molecule Alcama is a novel marker of multipotent retinal stem cells, including injury-induced Müller glia, and that each Müller glial cell divides asymmetrically only once to produce an Alcama-negative, proliferating retinal progenitor. The initial mitotic division of Müller glia involves interkinetic nuclear migration, but mitosis of retinal progenitors occurs in situ. Rapidly dividing retinal progenitors form neurogenic clusters tightly associated with Alcama/N-cadherin-labeled Müller glial radial processes. Genetic suppression of N-cadherin function interferes with basal migration of retinal progenitors and subsequent regeneration of HuC/D(+) inner retinal neurons.

Retina, retinol, retinal and the natural history of vitamin A as a light sensor

Light is both the ultimate energy source for most organisms and a rich information source. Vitamin A-based chromophore was initially used in harvesting light energy, but has become the most widely used light sensor throughout evolution from unicellular to multicellular organisms. Vitamin A-based photoreceptor proteins are called opsins and have been used for billions of years for sensing light for vision or the equivalent of vision. All vitamin A-based light sensors for vision in the animal kingdom are G-protein coupled receptors, while those in unicellular organisms are light-gated channels. This first major switch in evolution was followed by two other major changes: the switch from bistable to monostable pigments for vision and the expansion of vitamin A's biological functions. Vitamin A's new functions such as regulating cell growth and differentiation from embryogenesis to adult are associated with increased toxicity with its random diffusion. In contrast to bistable pigments which can be regenerated by light, monostable pigments depend on complex enzymatic cycles for regeneration after every photoisomerization event. Here we discuss vitamin A functions and transport in the context of the natural history of vitamin A-based light sensors and propose that the expanding functions of vitamin A and the choice of monostable pigments are the likely evolutionary driving forces for precise, efficient, and sustained vitamin A transport.

Vitamin A metabolism in rod and cone visual cycles

The chromophore of all known visual pigments consists of 11-cis-retinal (derived from either vitamin A1 or A2) or a hydroxylated derivative, bound to a protein (opsin) via a Schiff base. Absorption of a photon results in photoisomerization of the chromophore to all-trans-retinal and conversion of the visual pigment to the signaling form. Regeneration of the 11-cis-retinal occurs in an adjacent tissue and involves several enzymes, several water-soluble retinoid-binding proteins, and intra- and intercellular diffusional processes. Rod photoreceptor cells depend completely on the output of 11-cis-retinal from adjacent retinal pigment epithelial (RPE) cells. Cone photoreceptors cells can use 11-cis-retinal from the RPE and from a second more poorly characterized cycle, which appears to involve adjacent Müller (glial) cells. Recent progress in the characterization of rod and cone visual cycle components and reactions will result in the development of approaches to the amelioration of blinding eye diseases associated with visual cycle defects.

The visual system of zebrafish and its use to model human ocular diseases

Free swimming zebrafish larvae depend mainly on their sense of vision to evade predation and to catch prey. Hence, there is strong selective pressure on the fast maturation of visual function and indeed the visual system already supports a number of visually driven behaviors in the newly hatched larvae.The ability to exploit the genetic and embryonic accessibility of the zebrafish in combination with a behavioral assessment of visual system function has made the zebrafish a popular model to study vision and its diseases.Here, we review the anatomy, physiology, and development of the zebrafish eye as the basis to relate the contributions of the zebrafish to our understanding of human ocular diseases.

New insights into retinoid metabolism and cycling within the retina

The retinoid cycle is a series of biochemical reactions within the eye that is responsible for synthesizing the chromophore, 11-cis retinal, for visual function. The chromophore is bound to G-protein coupled receptors, opsins, within rod and cone photoreceptor cells forming the photosensitive visual pigments. Integral to the sustained function of photoreceptors is the continuous generation of chromophore by the retinoid cycle through two separate processes, one that supplies both rods and cones and another that exclusively supplies cones. Recent findings such as RPE65 localization within cones and the pattern of distribution of retinoid metabolites within mouse and human retinas have challenged previous proposed schemes. This review will focus on recent findings regarding the transport of retinoids, the mechanisms by which chromophore is supplied to both rods and cones, and the metabolism of retinoids within the posterior segment of the eye.

Novel expression patterns of metabotropic glutamate receptor 6 in the zebrafish nervous system

The metabotropic glutamate receptor 6 (mGluR6 or GRM6) belongs to the class III of the metabotropic glutamate receptor family. It is the only known mGluR that mediates direct synaptic transmission in the nervous system and is thought to mediate the ON-response in the ON-pathway of the vertebrate retina. Phylogenetic and gene structure analysis indicated that the zebrafish genome harbours two mglur6 paralogs, mglur6a and mglur6b. Besides expression in the inner nuclear layer and distinct regions in the brain, both mglur6 paralogs are expressed in ganglion cells of the retina, an expression pattern which can also be observed in the downstream effector molecules gnaoa and gnaob. This unexpected expression pattern is consistent with immunohistological labeling using a peptide antibody specific for the mGluR6b paralog. These expression patterns contradict the existing view that mGluR6 is solely located on ON-bipolar cells where it functions in signal transmission. Consistent with expression in ON-bipolar cells, we report a decreased b-wave amplitude in the electroretinogram after morpholino-based downregulation of mGluR6b, showing a function in the ON response. Our data suggest more widespread functions of mGluR6 mediated signaling in the central nervous system, possibly including sign reversing synapses in the inner retina.

Mechanism of all-trans-retinal toxicity with implications for stargardt disease and age-related macular degeneration

Compromised clearance of all-trans-retinal (atRAL), a component of the retinoid cycle, increases the susceptibility of mouse retina to acute light-induced photoreceptor degeneration. Abca4(-/-)Rdh8(-/-) mice featuring defective atRAL clearance were used to examine the one or more underlying molecular mechanisms, because exposure to intense light causes severe photoreceptor degeneration in these animals. Here we report that bright light exposure of Abca4(-/-)Rdh8(-/-) mice increased atRAL levels in the retina that induced rapid NADPH oxidase-mediated overproduction of intracellular reactive oxygen species (ROS). Moreover, such ROS generation was inhibited by blocking phospholipase C and inositol 1,4,5-trisphosphate-induced Ca(2+) release, indicating that activation occurs upstream of NADPH oxidase-mediated ROS generation. Because multiple upstream G protein-coupled receptors can activate phospholipase C, we then tested the effects of antagonists of serotonin 2A (5-HT(2A)R) and M(3)-muscarinic (M(3)R) receptors and found they both protected Abca4(-/-)Rdh8(-/-) mouse retinas from light-induced degeneration. Thus, a cascade of signaling events appears to mediate the toxicity of atRAL in light-induced photoreceptor degeneration of Abca4(-/-)Rdh8(-/-) mice. A similar mechanism may be operative in human Stargardt disease and age-related macular degeneration.

Loss of daylight vision in retinal degeneration: are oxidative stress and metabolic dysregulation to blame?

Retinitis pigmentosa is characterized by loss of night vision, followed by complete blindness. Over 40 genetic loci for retinitis pigmentosa have been identified in humans, primarily affecting photoreceptor structure and function. The availability of excellent animal models allows for a mechanistic characterization of the disease. Metabolic dysregulation and oxidative stress have been found to correlate with the loss of vision, particularly in cones, the type of photoreceptors that mediate daylight and color vision. The evidence that these problems actually cause loss of vision and potential therapeutic approaches targeting them are discussed.

Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches

Activation of the visual pigment by light in rod and cone photoreceptors initiates our visual perception. As a result, the signaling properties of visual pigments, consisting of a protein, opsin, and a chromophore, 11-cis-retinal, play a key role in shaping the light responses of photoreceptors. The combination of pharmacological, physiological, and genetic tools has been a powerful approach advancing our understanding of the interactions between opsin and chromophore and how they affect the function of visual pigments. The signaling properties of the visual pigments modulate many aspects of the function of rods and cones, producing their unique physiological properties.

Metabolism of carotenoids and retinoids related to vision

All animals endowed with the ability to detect light through visual pigments must have evolved pathways in which dietary precursors for the involved chromophore are absorbed, transported, and metabolized. Knowledge about this metabolism has exponentially increased over the past decade. Genetic manipulation of animal models provided insights into the metabolic flow of these compounds through the body and in the eyes, unraveling their regulatory aspects and aberrant side reactions. The scheme that emerges reveals a common origin of key components for chromophore metabolism that have been adapted to the specific requirements of retinoid biology in different animal classes.

An alternative isomerohydrolase in the retinal Müller cells of a cone-dominant species

Cone photoreceptors have faster light responses than rods and a higher demand for 11-cis retinal (11cRAL), the chromophore of visual pigments. RPE65 is the isomerohydrolase in the retinal pigment epithelium (RPE) that converts all-trans retinyl ester to 11-cis retinol, a key step in the visual cycle for regenerating 11cRAL. Accumulating evidence suggests that cone-dominant species express an alternative isomerase, likely in retinal Müller cells, to meet the high demand for the chromophore by cones. In the present study, we describe the identification and characterization of a novel isomerohydrolase, RPE65c, from the cone-dominant zebrafish retina. RPE65c shares 78% amino acid sequence identity with RPE-specific zebrafish RPE65a (orthologue of human RPE65) and retains all of the known key residues for the enzymatic activity of RPE65. Similar to the other RPE-specific RPE65, RPE65c was present in both the membrane and cytosolic fractions, used all-trans retinyl ester as its substrate and required iron for its enzymatic activity. However, immunohistochemistry detected RPE65c in the inner retina, including Müller cells, but not in the RPE. Furthermore, double-immunostaining of dissociated retinal cells using antibodies for RPE65c and glutamine synthetase (a Müller cell marker), showed that RPE65c co-localized with the Müller cell marker. These results suggest that RPE65c is the alternative isomerohydrolase in the intra-retinal visual cycle, providing 11cRAL to cone photoreceptors in cone-dominant species. Identification of an alternative visual cycle will contribute to the understanding of the functional differences of rod and cone photoreceptors.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Cone arrestin confers cone vision of high temporal resolution in zebrafish larvae

Vision of high temporal resolution depends on careful regulation of photoresponse kinetics, beginning with the lifetime of activated photopigment. The activity of rhodopsin is quenched by high-affinity binding of arrestin to photoexcited phosphorylated photopigment, which effectively terminates the visual transduction cascade. This regulation mechanism is well established for rod photoreceptors, yet its role for cone vision is still controversial. In this study we therefore analyzed arrestin function in the cone-dominated vision of larval zebrafish. For both rod (arrS ) and cone (arr3 ) arrestin we isolated two paralogs, each expressed in the respective subset of photoreceptors. Labeling with paralog-specific antibodies revealed subfunctionalized expression of Arr3a in M- and L-cones, and Arr3b in S- and UV-cones. The inactivation of arr3a by morpholino knockdown technology resulted in a severe delay in photoresponse recovery which, under bright light conditions, was rate-limiting. Comparison to opsin phosphorylation-deficient animals confirmed the role of cone arrestin in late cone response recovery. Arr3a activity partially overlapped with the function of the cone-specific kinase Grk7a involved in initial response recovery. Behavioral measurements further revealed Arr3a deficiency to be sufficient to reduce temporal contrast sensitivity, providing evidence for the importance of arrestin in cone vision of high temporal resolution.

Investigating the genetics of visual processing, function and behaviour in zebrafish

Over the past three decades, the zebrafish has been proven to be an excellent model to investigate the genetic control of vertebrate embryonic development, and it is now also increasingly used to study behaviour and adult physiology. Moreover, mutagenesis approaches have resulted in large collections of mutants with phenotypes that resemble human pathologies, suggesting that these lines can be used to model diseases and screen drug candidates. With the recent development of new methods for gene targeting and manipulating or monitoring gene expression, the range of genetic modifications now possible in zebrafish is increasing rapidly. Combined with the classical strengths of the zebrafish as a model organism, these advances are set to substantially expand the type of biological questions that can be addressed in this species. In this review, we outline how the potential of the zebrafish can be harvested in the context of eye development and visual function. We review recent technological advances used to study the formation of the eyes and visual areas of the brain, visual processing on the cellular, subcellular and molecular level, and the genetics of visual behaviour in vertebrates.

The cone-specific visual cycle

Cone photoreceptors mediate our daytime vision and function under bright and rapidly-changing light conditions. As their visual pigment is destroyed in the process of photoactivation, the continuous function of cones imposes the need for rapid recycling of their chromophore and regeneration of their pigment. The canonical retinoid visual cycle through the retinal pigment epithelium cells recycles chromophore and supplies it to both rods and cones. However, shortcomings of this pathway, including its slow rate and competition with rods for chromophore, have led to the suggestion that cones might use a separate mechanism for recycling of chromophore. In the past four decades biochemical studies have identified enzymatic activities consistent with recycling chromophore in the retinas of cone-dominant animals, such as chicken and ground squirrel. These studies have led to the hypothesis of a cone-specific retina visual cycle. The physiological relevance of these studies was controversial for a long time and evidence for the function of this visual cycle emerged only in very recent studies and will be the focus of this review. The retina visual cycle supplies chromophore and promotes pigment regeneration only in cones but not in rods. This pathway is independent of the pigment epithelium and instead involves the Müller cells in the retina, where chromophore is recycled and supplied selectively to cones. The rapid supply of chromophore through the retina visual cycle is critical for extending the dynamic range of cones to bright light and for their rapid dark adaptation following exposure to light. The importance of the retina visual cycle is emphasized also by its preservation through evolution as its function has now been demonstrated in species ranging from salamander to zebrafish, mouse, primate, and human.