Differential immunogold-dextran labeling of bovine and frog rod and cone cells using monoclonal antibodies against bovine rhodopsin

Light regulation of rhodopsin distribution during outer segment renewal in murine rod photoreceptors

Vision under dim light relies on primary cilia elaborated by rod photoreceptors in the retina. This specialized sensory structure, called the rod outer segment (ROS), comprises hundreds of stacked, membranous discs containing the light-sensitive protein rhodopsin, and the incorporation of new discs into the ROS is essential for maintaining the rod's health and function. ROS renewal appears to be primarily regulated by extrinsic factors (light); however, results vary depending on different model organisms. We generated two independent transgenic mouse lines where rhodopsin's fate is tracked by a fluorescently labeled rhodopsin fusion protein (Rho-Timer) and show that rhodopsin incorporation into nascent ROS discs appears to be regulated by both external lighting cues and autonomous retinal clocks. Live-cell imaging of the ROS isolated from mice exposed to six unique lighting conditions demonstrates that ROS formation occurs in a periodic manner in cyclic light, constant darkness, and artificial light/dark cycles. This alternating bright/weak banding of Rho-Timer along the length of the ROS relates to inhomogeneities in rhodopsin density and potential points of structural weakness. In addition, we reveal that prolonged dim ambient light exposure impacts not only the rhodopsin content of new discs but also that of older discs, suggesting a dynamic interchange of material between new and old discs. Furthermore, we show that rhodopsin incorporation into the ROS is greatly altered in two autosomal recessive retinitis pigmentosa mouse models, potentially contributing to the pathogenesis. Our findings provide insights into how extrinsic (light) and intrinsic (retinal clocks and genetic mutation) factors dynamically regulate mammalian ROS renewal.

Hyperglycemia and circadian disruption lead to retinal dysfunction in a stabilized colony of the fat sand rat Psammomys obesus

PURPOSE

The Fat Sand Rat (Psammomys obesus) recapitulates several features of human pre-proliferative diabetic retinopathy, but data are restricted to wild animals, incompatible with stringent biomedical research criteria. To overcome this barrier, we characterized retinal changes in a colony of P. obsesus maintained under strictly controlled housing conditions.

METHODS

Animals were maintained on low or high caloric energy diets, and raised under either standard (12 h light/12 h dark) or shortened (5 h light/5 h dark) photoperiods. Visual responses were tested by electroretinography, while structural/molecular changes were assayed by immunochemistry and molecular biology (RNAseq and qPCR).

RESULTS

Whereas high calorie diet alone did not induce hyperglycemia, coupled with short photoperiod >80 % animals developed severe hyper-insulinemia by 15 weeks, and 16 % animals further developed hyperglycemia. In these groups, electroretinography showed significant declines in visual responses in both hyper-insulinemic and hyperglycemic animals, especially in photopic (cone) responses. Transcriptomics analysis of hyperglycemic compared to low caloric controls revealed major upregulation in pathways involved in glial activation, extracellular matrix remodeling, inflammation, cytokine production, partial ischemic responses and angiogenesis. Western blotting against rhodopsin and cone opsin also showed decreased levels in both groups, overall decreases being greater for cones than rods in hyperglycemic animals.

CONCLUSIONS

P. obesus maintained in rigorously monitored captive conditions, albeit showing attenuated responses to dietary overload compared to wild counterparts, nevertheless do develop some retinal features of diabetic retinopathy-like degeneration. Such a colony with known sanitary status opens their broader use for biomedical research.

Aggregation of rhodopsin mutants in mouse models of autosomal dominant retinitis pigmentosa

Mutations in rhodopsin can cause it to misfold and lead to retinal degeneration. A distinguishing feature of these mutants in vitro is that they mislocalize and aggregate. It is unclear whether or not these features contribute to retinal degeneration observed in vivo. The effect of P23H and G188R misfolding mutations were examined in a heterologous expression system and knockin mouse models, including a mouse model generated here expressing the G188R rhodopsin mutant. In vitro characterizations demonstrate that both mutants aggregate, with the G188R mutant exhibiting a more severe aggregation profile compared to the P23H mutant. The potential for rhodopsin mutants to aggregate in vivo was assessed by PROTEOSTAT, a dye that labels aggregated proteins. Both mutants mislocalize in photoreceptor cells and PROTEOSTAT staining was detected surrounding the nuclei of photoreceptor cells. The G188R mutant promotes a more severe retinal degeneration phenotype and greater PROTEOSTAT staining compared to that promoted by the P23H mutant. Here, we show that the level of PROTEOSTAT positive cells mirrors the progression and level of photoreceptor cell death, which suggests a potential role for rhodopsin aggregation in retinal degeneration.

Three different seasonally expressed opsins are present in the brain of the Eared Dove, an opportunist breeder

Birds living at high latitudes perceive the photoperiod through deep-brain photoreceptors (DBP) located in deep-brain neurons. During long photoperiods the information transmitted by these photoreceptors increases the activity of the hypothalamic-pituitary-gonadal (HPG) axis, leading to gonadal development. The presence of photopigments such as VA-Opsin, Opn4, Opn5 and Opn2 in brain areas implicated in reproductive behaviors has been firmly established in several avian species with seasonal breeding, whereas their existence in opportunistic breeding birds remains unconfirmed. The Eared Dove is an urban and peri-urban dove that breeds throughout the year. Males of this species do not exhibit the typical gonadal regression/recrudescence cycle, thus posing the question of what occurs upstream of the HPG axis. We addressed this issue by first studying the presence of diverse opsins located in DBP in the brains of Eared Dove males and whether these photopigments changed their expression throughout the year. We carried out an immunohistochemistry analysis on three different opsins: Opn2 (rhodopsin), Opn3 and Opn5. Our results demonstrate the discrete neuroanatomical distribution of these opsins in the brain of Eared Dove males and strongly indicate different seasonal expressions. In the anterior region of the hypothalamus, Opn2-positive cells were detected throughout the year. By contrast, Opn5 was found to be strongly and seasonally expressed during winter in the anterior and the hypothalamic region. Opn3 was also found to be significantly and seasonally expressed during winter in the hypothalamic region. We thus demonstrate for the first time that males of the Eared Dove, have three different deep-brain opsin-expressing photoreceptors with differential location/distribution in the anterior and hypothalamic region and differential seasonality. The persistence of Opn2 and the strong seasonal expression of nonvisual photopigments Opn3 and Opn5 in two areas of the avian brain, which are associated with reproduction, could be the primary distinction between seasonal and opportunistic breeders.

Super-resolution mapping in rod photoreceptors identifies rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway

Photoreceptor cells in the vertebrate retina have a highly compartmentalized morphology for efficient phototransduction and vision. Rhodopsin, the visual pigment in rod photoreceptors, is densely packaged into the rod outer segment sensory cilium and continuously renewed through essential synthesis and trafficking pathways housed in the rod inner segment. Despite the importance of this region for rod health and maintenance, the subcellular organization of rhodopsin and its trafficking regulators in the mammalian rod inner segment remain undefined. We used super-resolution fluorescence microscopy with optimized retinal immunolabeling techniques to perform a single molecule localization analysis of rhodopsin in the inner segments of mouse rods. We found that a significant fraction of rhodopsin molecules was localized at the plasma membrane, at the surface, in an even distribution along the entire length of the inner segment, where markers of transport vesicles also colocalized. Thus, our results collectively establish a model of rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway in mouse rod photoreceptors.

Mapping rhodopsin trafficking in rod photoreceptors with quantitative super-resolution microscopy

Photoreceptor cells in the vertebrate retina have a highly compartmentalized morphology for efficient long-term phototransduction. Rhodopsin, the visual pigment in rod photoreceptors, is densely packaged into the rod outer segment sensory cilium and continuously renewed through essential synthesis and trafficking pathways housed in the rod inner segment. Despite the importance of this region for rod health and maintenance, the subcellular organization of rhodopsin and its trafficking regulators in the mammalian rod inner segment remain undefined. We used super-resolution fluorescence microscopy with optimized retinal immunolabeling techniques to perform a single molecule localization analysis of rhodopsin in the inner segments of mouse rods. We found that a significant fraction of rhodopsin molecules was localized at the plasma membrane in an even distribution along the entire length of the inner segment, where markers of transport vesicles also colocalized. Thus, our results collectively establish a model of rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway in mouse rod photoreceptors.

Intrinsic differences in rod and cone membrane composition: implications for cone degeneration

PURPOSE

In many retinal pathological conditions, rod and cone degeneration differs. For example, the early-onset maculopathy Stargardts disease type 1 (STGD1) is typified by loss of cones while rods are often less affected. We wanted to examine whether there exist intrinsic membrane differences between rods and cones that might explain such features.

METHODS

Abca4 mRNA and protein levels were quantified in rod- and cone-enriched samples from wild-type and Nrl^-/- mice retinas; rod- and cone-enriched outer segments (ROS and COS respectively) were prepared from pig retinas, and total lipids were analyzed by flame ionization, chromatography, and tandem mass spectrometry. Immunohistochemical staining of cone-rich rodent Arvicanthis ansorgei retinas was conducted, and ultra-high performance liquid chromatography of lipid species in porcine ROS and COS was performed.

RESULTS

Abca4 mRNA and Abca4 protein content was significantly higher (50-300%) in cone compared to rod-enriched samples. ROS and COS displayed dramatic differences in several lipids, including very long chain poly-unsaturated fatty acids (VLC-PUFAs), especially docosahexaenoic acid (DHA, 22:6n-3): ROS 20.6% DHA, COS 3.3% (p < 0.001). VLC-PUFAs (> 50 total carbons) were virtually absent from COS. COS were impoverished (> 6× less) in phosphatidylethanolamine compared to ROS. ELOVL4 ("ELOngation of Very Long chain fatty acids 4") antibody labelled Arvicanthis cones only very weakly compared to rods. Finally, there were large amounts (905 a.u.) of the bisretinoid A2PE in ROS, whereas it was much lower (121 a.u., ~ 7.5-fold less) in COS fractions. In contrast, COS contained fivefold higher amounts of all-trans-retinal dimer (115 a.u. compared to 22 a.u. in rods).

CONCLUSIONS

Compared to rods, cones expressed higher levels of Abca4 mRNA and Abca4 protein, were highly impoverished in PUFA (especially DHA) and phosphatidylethanolamine, and contained significant amounts of all-trans-retinal dimer. Based on these and other data, we propose that in contrast to rods, cones are preferentially vulnerable to stress and may die through direct cellular toxicity in pathologies such as STGD1.

Loss of the Bardet-Biedl protein Bbs1 alters photoreceptor outer segment protein and lipid composition

Primary cilia are key sensory organelles whose dysfunction leads to ciliopathy disorders such as Bardet-Biedl syndrome (BBS). Retinal degeneration is common in ciliopathies, since the outer segments (OSs) of photoreceptors are highly specialized primary cilia. BBS1, encoded by the most commonly mutated BBS-associated gene, is part of the BBSome protein complex. Using a bbs1 zebrafish mutant, we show that retinal development and photoreceptor differentiation are unaffected by Bbs1-loss, supported by an initially unaffected transcriptome. Quantitative proteomics and lipidomics on samples enriched for isolated OSs show that Bbs1 is required for BBSome-complex stability and that Bbs1-loss leads to accumulation of membrane-associated proteins in OSs, with enrichment in proteins involved in lipid homeostasis. Disruption of the tightly regulated OS lipid composition with increased OS cholesterol content are paralleled by early functional visual deficits, which precede progressive OS morphological anomalies. Our findings identify a role for Bbs1/BBSome in OS lipid homeostasis, suggesting a pathomechanism underlying retinal degeneration in BBS.

Primary cilia are key sensory organelles whose dysfunction leads to ciliopathy disorders such as Bardet-Biedl syndrome (BBS). Here they identify a role for Bbs1 in lipid homeostasis of photoreceptor outer segments in zebrafish, which may contribute to vision loss in patients with Bardet-Biedl syndrome.

Studying the morphology, composition and function of the photoreceptor primary cilium in zebrafish

Vision is one of our dominant senses and its loss has a profound impact on the life quality of affected individuals. Highly specialized neurons in the retina called photoreceptors convert photons into neuronal responses. This conversion of photons is mediated by light sensitive opsin proteins, which are found in the outer segments of the photoreceptors. These outer segments are highly specialized primary cilia, explaining why retinal dystrophy is a key feature of ciliopathies, a group of diseases resulting from abnormal and dysfunctional cilia. Therefore, research on ciliopathies often includes the analysis of the retina with special focus on the photoreceptor and its outer segment. In the last decade, the zebrafish has emerged as an excellent model organism to study human diseases, in particular with respect to the retina. The cone-rich retina of zebrafish resembles the fovea of the human macula and thus represents an excellent model to study human retinal diseases. Here we give detailed guidance on how to analyze the morphological and ultra-structural integrity of photoreceptors in the zebrafish using various histological and imaging techniques. We further describe how to conduct functional analysis of the retina by electroretinography and how to prepare isolated outer segment fractions for different -omic approaches. These different methods allow a comprehensive analysis of photoreceptors, helping to enhance our understanding of the molecular and structural basis of ciliary function in health and of the consequences of its dysfunction in disease.

Structure and function of ABCA4 and its role in the visual cycle and Stargardt macular degeneration

ABCA4 is a member of the superfamily of ATP-binding cassette (ABC) transporters that is preferentially localized along the rim region of rod and cone photoreceptor outer segment disc membranes. It uses the energy from ATP binding and hydrolysis to transport N-retinylidene-phosphatidylethanolamine (N-Ret-PE), the Schiff base adduct of retinal and phosphatidylethanolamine, from the lumen to the cytoplasmic leaflet of disc membranes. This ensures that all-trans-retinal and excess 11-cis-retinal are efficiently cleared from photoreceptor cells thereby preventing the accumulation of toxic retinoid compounds. Loss-of-function mutations in the gene encoding ABCA4 cause autosomal recessive Stargardt macular degeneration, also known as Stargardt disease (STGD1), and related autosomal recessive retinopathies characterized by impaired central vision and an accumulation of lipofuscin and bis-retinoid compounds. High resolution structures of ABCA4 in its substrate and nucleotide free state and containing bound N-Ret-PE or ATP have been determined by cryo-electron microscopy providing insight into the molecular architecture of ABCA4 and mechanisms underlying substrate recognition and conformational changes induced by ATP binding. The expression and functional characterization of a large number of disease-causing missense ABCA4 variants have been determined. These studies have shed light into the molecular mechanisms underlying Stargardt disease and a classification that reliably predicts the effect of a specific missense mutation on the severity of the disease. They also provide a framework for developing rational therapeutic treatments for ABCA4-associated diseases.

Eye-Transcriptome and Genome-Wide Sequencing for Scolecophidia: Implications for Inferring the Visual System of the Ancestral Snake

Molecular genetic data have recently been incorporated in attempts to reconstruct the ecology of the ancestral snake, though this has been limited by a paucity of data for one of the two main extant snake taxa, the highly fossorial Scolecophidia. Here we present and analyze vision genes from the first eye-transcriptomic and genome-wide data for Scolecophidia, for Anilios bicolor, and A. bituberculatus, respectively. We also present immunohistochemistry data for retinal anatomy and visual opsin-gene expression in Anilios. Analyzed in the context of 19 lepidosaurian genomes and 12 eye transcriptomes, the new genome-wide and transcriptomic data provide evidence for a much more reduced visual system in Anilios than in non-scolecophidian (=alethinophidian) snakes and in lizards. In Anilios, there is no evidence of the presence of 7 of the 12 genes associated with alethinophidian photopic (cone) phototransduction. This indicates extensive gene loss and many of these candidate gene losses occur also in highly fossorial mammals with reduced vision. Although recent phylogenetic studies have found evidence for scolecophidian paraphyly, the loss in Anilios of visual genes that are present in alethinophidians implies that the ancestral snake had a better-developed visual system than is known for any extant scolecophidian.

Core circadian clock genes Per1 and Per2 regulate the rhythm in photoreceptor outer segment phagocytosis

Retinal photoreceptors undergo daily renewal of their distal outer segments, a process indispensable for maintaining retinal health. Photoreceptor outer segment (POS) phagocytosis occurs as a daily peak, roughly about 1 hour after light onset. However, the underlying cellular and molecular mechanisms which initiate this process are still unknown. Here we show that, under constant darkness, mice deficient for core circadian clock genes (Per1 and Per2) lack a daily peak in POS phagocytosis. By qPCR analysis, we found that core clock genes were rhythmic over 24 hours in both WT and Per1, Per2 double mutant whole retinas. More precise transcriptomics analysis of laser capture microdissected WT photoreceptors revealed no differentially expressed genes between time points preceding and during the peak of POS phagocytosis. In contrast, we found that microdissected WT retinal pigment epithelium (RPE) had a number of genes that were differentially expressed at the peak phagocytic time point compared to adjacent ones. We also found a number of differentially expressed genes in Per1, Per2 double mutant RPE compared to WT ones at the peak phagocytic time point. Finally, based on STRING analysis, we found a group of interacting genes that potentially drive POS phagocytosis in the RPE. This potential pathway consists of genes such as: Pacsin1, Syp, Camk2b, and Camk2d among others. Our findings indicate that Per1 and Per2 are necessary clock components for driving POS phagocytosis and suggest that this process is transcriptionally driven by the RPE.

The route of the visual receptor rhodopsin along the cilium

The photoreceptor outer segment is the most elaborate primary cilium, containing large amounts of rhodopsin (RHO) in disk membranes that grow from a connecting cilium. The movement of RHO along the connecting cilium precedes formation of the disk membranes. However, the route that RHO takes has not been clearly determined; some reports suggest that it follows an intracellular, vesicular route along the axoneme, possibly as an adaptation for the high load of delivery or the morphogenesis of the disk endomembranes. We addressed this question by studying RHO in cilia of IMCD3 cells and mouse rod photoreceptors. In IMCD3 cilia, fluorescence recovery after photobleaching (FRAP) experiments with fluorescently tagged RHO supported the idea of RHO motility in the ciliary plasma membrane and was inconsistent with the hypothesis of RHO motility within the lumen of the cilium. In rod photoreceptors, FRAP of RHO-EGFP was altered by externally applied lectin, supporting the idea of plasmalemmal RHO dynamics. Quantitative immunoelectron microscopy corroborated our live-cell conclusions, as RHO was found to be distributed along the plasma membrane of the connecting cilium, with negligible labeling within the axoneme. Taken together, the present findings demonstrate RHO trafficking entirely via the ciliary plasma membrane.This article has an associated First Person interview with the first author of the paper.

The rod signaling pathway in marsupial retinae

The retinal rod pathway, featuring dedicated rod bipolar cells (RBCs) and AII amacrine cells, has been intensely studied in placental mammals. Here, we analyzed the rod pathway in a nocturnal marsupial, the South American opossum Monodelphis domestica to elucidate whether marsupials have a similar rod pathway. The retina was dominated by rods with densities of 338,000-413,000/mm². Immunohistochemistry for the RBC-specific marker protein kinase Cα (PKCα) and the AII cell marker calretinin revealed the presence of both cell types with their typical morphology. This is the first demonstration of RBCs in a marsupial and of the integration of RBCs and AII cells in the rod signaling pathway. Electron microscopy showed invaginating synaptic contacts of the PKCα-immunoreactive bipolar cells with rods; light microscopic co-immunolabeling for the synaptic ribbon marker CtBP2 confirmed dominant rod contacts. The RBC axon terminals were mostly located in the innermost stratum S5 of the inner plexiform layer (IPL), but had additional side branches and synaptic varicosities in strata S3 and S4, with S3-S5 belonging to the presumed functional ON sublayer of the IPL, as shown by immunolabeling for the ON bipolar cell marker Gγ13. Triple-immunolabeling for PKCα, calretinin and CtBP2 demonstrated RBC synapses onto AII cells. These features conform to the pattern seen in placental mammals, indicating a basically similar rod pathway in M. domestica. The density range of RBCs was 9,900-16,600/mm2, that of AII cells was 1,500-3,260/mm2. The numerical convergence (density ratio) of 146-156 rods to 4.7-6.0 RBCs to 1 AII cell is within the broad range found among placental mammals. For comparison, we collected data for the Australian nocturnal dunnart Sminthopsis crassicaudata, and found it to be similar to M. domestica, with rod-contacting PKCα-immunoreactive bipolar cells that had axon terminals also stratifying in IPL strata S3-S5.

The topography of rods, cones and intrinsically photosensitive retinal ganglion cells in the retinas of a nocturnal (Micaelamys namaquensis) and a diurnal (Rhabdomys pumilio) rodent

We used immunocytochemistry to determine the presence and topographical density distributions of rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) in the four-striped field mouse (Rhabdomys pumilio) and the Namaqua rock mouse (Micaelamys namaquensis). Both species possessed duplex retinas that were rod dominated. In R. pumilio, the density of both cones and rods were high (cone to rod ratio: 1:1.23) and reflected the species' fundamentally diurnal, but largely crepuscular lifestyle. Similarly, the ratio of cones to rods in M. namaquensis (1:12.4) reflected its nocturnal lifestyle. Similar rod density peaks were observed (R. pumilio: ~84467/mm2; M. namaquensis: ~81088/mm2), but a density gradient yielded higher values in the central (~56618/mm2) rather than in the peripheral retinal region (~32689/mm2) in R. pumilio. Two separate cone types (S-cones and M/L-cones) were identified implying dichromatic color vision in the study species. In M. namaquensis, both cone populations showed a centro-peripheral density gradient and a consistent S- to M/L-cone ratio (~1:7.8). In R. pumilio, S cones showed a centro-peripheral gradient (S- to M/L-cone ratio; central: 1:7.8; peripheral: 1:6.8) which appeared to form a visual streak, and a specialized area of M/L-cones (S- to M/L-cone ratio: 1:15) was observed inferior to the optic nerve. The number of photoreceptors per linear degree of visual angle, estimated from peak photoreceptor densities and eye size, were four cones and 15 rods per degree in M. namaquensis and 11 cones and 12 rods per degree in R. pumilio. Thus, in nocturnal M. namaquensis rods provide much finer image sampling than cones, whereas in diurnal/crepuscular R. pumilio both photoreceptor types provide fine image sampling. IpRGCs were comparably sparse in R. pumilio (total = 1012) and M. namaquensis (total = 862), but were homogeneously distributed in M. namaquensis and densest in the dorso-nasal quadrant in R. pumilio. The adaptive significance of the latter needs further investigation.

Modeling Dominant and Recessive Forms of Retinitis Pigmentosa by Editing Three Rhodopsin-Encoding Genes in Xenopus Laevis Using Crispr/Cas9

The utility of Xenopus laevis, a common research subject for developmental biology, retinal physiology, cell biology, and other investigations, has been limited by lack of a robust gene knockout or knock-down technology. Here we describe manipulation of the X. laevis genome using CRISPR/Cas9 to model the human disorder retinitis pigmentosa, and to introduce point mutations or exogenous DNA sequences. We introduced and characterized in-frame and out-of-frame insertions and deletions in three genes encoding rhodopsin by co-injection of Cas9 mRNA, eGFP mRNA, and single guide RNAs into fertilized eggs. Deletions were characterized by direct sequencing and cloning; phenotypes were assessed by assays of rod opsin in retinal extracts, and confocal microscopy of cryosectioned and immunolabeled contralateral eyes. We obtained germline transmission of editing to F1 offspring. In-frame deletions frequently caused dominant retinal degeneration associated with rhodopsin biosynthesis defects, while frameshift phenotypes were consistent with knockout. We inserted eGFP or point mutations into rhodopsin genes by co-injection of repair fragments with homology to the Cas9 target sites. Our techniques can produce high frequency gene editing in X. laevis, permitting analysis in the F0 generation, and advancing the utility of X. laevis as a subject for biological research and disease modeling.

Opposing Effects of Valproic Acid Treatment Mediated by Histone Deacetylase Inhibitor Activity in Four Transgenic X. laevis Models of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is an inherited retinal degeneration (RD) that leads to blindness for which no treatment is available. RP is frequently caused by mutations in Rhodopsin; in some animal models, RD is exacerbated by light. Valproic acid (VPA) is a proposed treatment for RP and other neurodegenerative disorders, with a phase II trial for RP under way. However, the therapeutic mechanism is unclear, with minimal research supporting its use in RP. We investigated the effects of VPA on Xenopus laevis models of RP expressing human P23H, T17M, T4K, and Q344ter rhodopsins, which are associated with RP in humans. VPA ameliorated RD associated with P23H rhodopsin and promoted clearing of mutant rhodopsin from photoreceptors. The effect was equal to that of dark rearing, with no additive effect observed. Rescue of visual function was confirmed by electroretinography. In contrast, VPA exacerbated RD caused by T17M rhodopsin in light, but had no effect in darkness. Effects in T4K and Q344ter rhodopsin models were also negative. These effects of VPA were paralleled by treatment with three additional histone deacetylase (HDAC) inhibitors, but not other antipsychotics, chemical chaperones, or VPA structural analogues. In WT retinas, VPA treatment increased histone H3 acetylation. In addition, electron microscopy showed increased autophagosomes in rod inner segments with HDAC inhibitor (HDACi) treatment, potentially linking the therapeutic effects in P23H rhodopsin animals and negative effects in other models with autophagy. Our results suggest that the success or failure of VPA treatment is dependent on genotype and that HDACi treatment is contraindicated for some RP cases.SIGNIFICANCE STATEMENT Retinitis pigmentosa (RP) is an inherited, degenerative retinal disease that leads to blindness for which no therapy is available. We determined that valproic acid (VPA), currently undergoing a phase II trial for RP, has both beneficial and detrimental effects in animal models of RP depending on the underlying disease mechanism and that both effects are due to histone deacetylase (HDAC) inhibition possibly linked to autophagy regulation. Off-label use of VPA and other HDAC inhibitors for the treatment of RP should be limited to the research setting until this effect is understood and can be predicted. Our study suggests that, unless genotype is accounted for, clinical trials for RP treatments may give negative results due to multiple disease mechanisms with differential responses to therapeutic interventions.

Specialized photoreceptor composition in the raptor fovea

The retinae of many bird species contain a depression with high photoreceptor density known as the fovea. Many species of raptors have two foveae, a deep central fovea and a shallower temporal fovea. Birds have six types of photoreceptors: rods, active in dim light, double cones that are thought to mediate achromatic discrimination, and four types of single cones mediating color vision. To maximize visual acuity, the fovea should only contain photoreceptors contributing to high-resolution vision. Interestingly, it has been suggested that raptors might lack double cones in the fovea. We used transmission electron microscopy and immunohistochemistry to evaluate this claim in five raptor species: the common buzzard (Buteo buteo), the honey buzzard (Pernis apivorus), the Eurasian sparrowhawk (Accipiter nisus), the red kite (Milvus milvus), and the peregrine falcon (Falco peregrinus). We found that all species, except the Eurasian sparrowhawk, lack double cones in the center of the central fovea. The size of the double cone-free zone differed between species. Only the common buzzard had a double cone-free zone in the temporal fovea. In three species, we examined opsin expression in the central fovea and found evidence that rod opsin positive cells were absent and violet-sensitive cone and green-sensitive cone opsin positive cells were present. We conclude that not only double cones, but also single cones may contribute to high-resolution vision in birds, and that raptors may in fact possess high-resolution tetrachromatic vision in the central fovea.

Rev-Erbα modulates retinal visual processing and behavioral responses to light

The circadian clock is thought to adjust retinal sensitivity to ambient light levels, yet the involvement of specific clock genes is poorly understood. We explored the potential role of the nuclear receptor subfamily 1, group D, member 1 (REV-ERBα; or NR1D1) in this respect. In light-evoked behavioral tests, compared with wild-type littermates, Rev-Erbα^-/- mice showed enhanced negative masking at low light levels (0.1 lx). Rev-Erbα^-/- mouse retinas displayed significantly higher numbers of intrinsically photosensitive retinal ganglion cells (ipRGCs; 62% more compared with wild-type) and more intense melanopsin immunostaining of individual ipRGCs. In agreement with a pivotal role for melanopsin, negative masking at low light intensities was abolished in Rev-Erbα^-/- Opn4^-/- (melanopsin gene) double-null mice. Rev-Erbα^-/- mice showed shortened latencies of both a and b electroretinogram waves, modified scotopic and photopic b-wave and scotopic threshold responses, and increased pupillary constriction, all of which suggested increased light sensitivity. However, wild-type and Rev-Erbα^-/- mice displayed no detectable differences by in vivo fundus imaging, retinal histology, or expression of cell type-specific markers for major retinal cell populations. We conclude that REV-ERBα plays a major role in retinal information processing, and we speculate that REV-ERBα and melanopsin set sensitivity levels of the rod-mediated ipRGC pathway to coordinate activity with ambient light.-Ait-Hmyed Hakkari, O., Acar, N., Savier, E., Spinnhirny, P., Bennis, M., Felder-Schmittbuhl, M.-P., Mendoza, J., Hicks, D. Rev-Erbα modulates retinal visual processing and behavioral responses to light.

MERTK signaling in the retinal pigment epithelium regulates the tyrosine phosphorylation of GDP dissociation inhibitor alpha from the GDI/CHM family of RAB GTPase effectors

Photoreceptor outer segments (OS) in the vertebrate retina undergo a process of continual renewal involving shedding of disc membranes that are cleared by phagocytic uptake into the retinal pigment epithelium (RPE). In dystrophic Royal College of Surgeons (RCS) rats, OS phagocytosis is blocked by a mutation in the gene encoding the receptor tyrosine kinase MERTK. To identify proteins tyrosine-phosphorylated downstream of MERTK in the RPE, MALDI-mass spectrometry with peptide-mass fingerprinting was used in comparative studies of RCS congenic and dystrophic rats. At times corresponding to peak phagocytic activity, the RAB GTPase effector GDP dissociation inhibitor alpha (GDI1) was found to undergo tyrosine phosphorylation only in congenic rats. In cryosections of native RPE/choroid, GDI1 colocalized with MERTK and the intracellular tyrosine-kinase SRC. In cultured RPE-J cells, and in transfected heterologous cells, MERTK stimulated SRC-mediated tyrosine phosphorylation of GDI1. In OS-fed RPE-J cells, GDI1 colocalized with MERTK and SRC on apparent phagosomes located near the apical membrane. In addition, both GDI1 and RAB5, a regulator of vesicular transport, colocalized with ingested OS. Taken together, these findings identify a novel role of MERTK signaling in membrane trafficking in the RPE that is likely to subserve mechanisms of phagosome formation.

Light regulates the ciliary protein transport and outer segment disc renewal of mammalian photoreceptors

The outer segment (OS) of the rod photoreceptor is a light-sensing cilium containing ~1,000 membrane-bound discs. Each day, discs constituting the distal tenth of the OS are shed, whereas nascent discs are formed at the base of the OS through the incorporation of molecules transported from the inner segment. The mechanisms regulating these processes remain elusive. Here, we show that rhodopsin preferentially enters the OS in the dark. Photoexcitation of post-Golgi rhodopsins retains them in the inner segment. Disc-rim protein peripherin2/rds enters the OS following a rhythm complementary to that of rhodopsin. Light-dark cycle-regulated protein trafficking serves as a mechanism to segregate rhodopsin-rich and peripherin2/rds-rich discs into alternating stacks, which are flanked by characteristic cytoplasmic pockets. This periodic cytostructure divides the OS into approximately ten fractions, each containing discs synthesized in a single day. This mechanism may explain how the rod photoreceptor balances the quantity of discs added and removed daily.

Ultrastructural visualization of trans-ciliary rhodopsin cargoes in mammalian rods

Background

Cilia are vital to various cellular and sensory functions. The pathway by which ciliary membrane proteins translocate through the transition zone is not well understood. Direct morphological characterization of ciliary cargoes in transit remains lacking. In the vertebrate photoreceptor, rhodopsin is synthesized and transported from the inner segment to the disc membranes of the outer segment (OS), which is a modified cilium. To date, the membrane topology of the basal OS and the mechanisms by which rhodopsin is transported through the transition zone (i.e., connecting cilium) and by which nascent disc membranes are formed remain controversial.

Results

Using an antibody recognizing its cytoplasmic C-terminus, we localize rhodopsin on both the plasma membrane and lumen of the connecting cilium by immuno-electron microscopy (EM). We also use transmission EM to visualize the electron-dense enzymatic products derived from the rhodopsin-horseradish peroxidase (HRP) fusion in transfected rodent rods. In the connecting cilium, rhodopsin is not only expressed in the plasma membrane but also in the lumen on two types of membranous carriers, long smooth tubules and small, coated, filament-bound vesicles. Additionally, membrane-bound rhodopsin carriers are also found in close proximity to the nascent discs at the basal OS axoneme and in the distal inner segment. This topology-indicative HRP-rhodopsin reporter shows that the nascent basalmost discs and the mature discs have the same membrane topology, with no indication of evagination or invagination from the basal OS plasma membranes. Serial block face and focus ion beam scanning EM analyses both indicate that the transport carriers enter the connecting cilium lumen from either the basal body lumen or cytoplasmic space between the axonemal microtubules and the ciliary plasma membrane.

Conclusions

Our results suggest the existence of multiple ciliary gate entry pathways in rod photoreceptors. Rhodopsin is likely transported across the connecting cilium on the plasma membrane and through the lumens on two types of tubulovesicular carriers produced in the inner segment. Our findings agree with a previous model that rhodopsin carriers derived from the cell body may fuse directly onto nascent discs as they grow and mature.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-015-0013-1) contains supplementary material, which is available to authorized users.

Numb regulates the polarized delivery of cyclic nucleotide-gated ion channels in rod photoreceptor cilia

The development and maintenance of protein compartmentalization is essential for neuronal function. A striking example is observed in light-sensing photoreceptors, in which the apical sensory cilium is subdivided into an inner and outer segment, each containing specific proteins essential for vision. It remains unclear, however, how such polarized protein localization is regulated. We report here that the endocytic adaptor protein Numb localizes to the inner, but not the outer segment of mouse photoreceptor cilia. Rod photoreceptor-specific inactivation of numb in vivo leads to progressive photoreceptor degeneration, indicating an essential role for Numb in photoreceptor cell biology. Interestingly, we report that loss of Numb in photoreceptors does not affect the localization of outer segment disk membrane proteins, such as rhodopsin, Peripherin-rds, Rom-1, and Abca4, but significantly disrupts the localization of the rod cyclic nucleotide-gated (Cng) channels, which accumulates on the inner segment plasma membrane in addition to its normal localization to the outer segments. Mechanistically, we show that Numb interacts with both subunits of the Cng channel and promotes the trafficking of Cnga1 to the recycling endosome. These results suggest a model in which Numb prevents targeting of Cng channels to the inner segment, by promoting their trafficking through the recycling endosome, where they can be sorted for specific delivery to the outer segment. This study uncovers a novel mechanism regulating polarized protein delivery in light-sensing cilia, raising the possibility that Numb plays a part in the regulation of protein trafficking in other types of cilia.

Glycosylation of rhodopsin is necessary for its stability and incorporation into photoreceptor outer segment discs

Rhodopsin, a G-protein coupled receptor, most abundant protein in retinal rod photoreceptors, is glycosylated at asparagines-2 and 15 on its N-terminus. To understand the role of rhodopsin's glycosylation in vivo, we generated and characterized a transgenic mouse model that expresses a non-glycosylated form of rhodopsin. We show that lack of glycosylation triggers a dominant form of progressive retinal degeneration. Electron microscopic examination of retinas at postnatal day 17 revealed the presence of vacuolar structures that distorted rod photoreceptor outer segments and became more prominent with age. Expression of non-glycosylated rhodopsin alone showed that it is unstable and is regulated via ubiquitin-mediated proteasomal degradation at the base of outer segments. We observed similar vacuolization in outer segments of transgenic mice expressing human rhodopsin with a T17M mutation (hT17M), suggesting that the mechanism responsible for the degenerative process in mice expressing the non-glycosylated rhodopsin and the RHO(hT17M) mice is likely the cause of phenotype observed in retinitis pigmentosa patients carrying T17M mutation.

Mutant ELOVL4 that causes autosomal dominant stargardt-3 macular dystrophy is misrouted to rod outer segment disks

PURPOSE

Autosomal dominant Stargardt macular dystrophy caused by mutations in the Elongation of Very Long Chain fatty acids (ELOVL4) gene results in macular degeneration, leading to early childhood blindness. Transgenic mice and pigs expressing mutant ELOVL4 develop progressive photoreceptor degeneration. The mechanism by which these mutations cause macular degeneration remains unclear, but have been hypothesized to involve the loss of an ER-retention dilysine motif located in the extreme C-terminus. Dominant negative mechanisms and reduction in retinal polyunsaturated fatty acids also have been suggested. To understand the molecular mechanisms involved in disease progression in vivo, we addressed the hypothesis that the disease-linked C-terminal truncation mutant of ELOVL4 exerts a dominant negative effect on wild-type (WT) ELOVL4, altering its subcellular localization and function, which subsequently induces retinal degeneration and loss of vision.

METHODS

We generated transgenic Xenopus laevis that overexpress HA-tagged murine ELOVL4 variants in rod photoreceptors.

RESULTS

Tagged or untagged WT ELOVL4 localized primarily to inner segments. However, the mutant protein lacking the dilysine motif was mislocalized to post-Golgi compartments and outer segment disks. Coexpression of mutant and WT ELOVL4 in rods did not result in mislocalization of the WT protein to outer segments or in the formation of aggregates. Full-length HA-tagged ELOVL4 lacking the dilysine motif (K308R/K310R) necessary for targeting the WT ELOVL4 protein to the endoplasmic reticulum was similarly mislocalized to outer segments.

CONCLUSIONS

We propose that expression and outer segment mislocalization of the disease-linked 5-base-pair deletion mutant ELOVL4 protein alters photoreceptor structure and function, which subsequently results in retinal degeneration, and suggest three possible mechanisms by which mutant ELOVL4 may induce retinal degeneration in STGD3.

Phospholipid flippase ATP8A2 is required for normal visual and auditory function and photoreceptor and spiral ganglion cell survival

ATP8A2 is a P4-ATPase that is highly expressed in the retina, brain, spinal cord and testes. In the retina, ATP8A2 is localized in photoreceptors where it uses ATP to transport phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the exoplasmic to the cytoplasmic leaflet of membranes. Although mutations in ATP8A2 have been reported to cause mental retardation in humans and degeneration of spinal motor neurons in mice, the role of ATP8A2 in sensory systems has not been investigated. We have analyzed the retina and cochlea of ATP8A2-deficient mice to determine the role of ATP8A2 in visual and auditory systems. ATP8A2-deficient mice have shortened photoreceptor outer segments, a reduction in photoresponses and decreased photoreceptor viability. The ultrastructure and phagocytosis of the photoreceptor outer segment appeared normal, but the PS and PE compositions were altered and the rhodopsin content was decreased. The auditory brainstem response threshold was significantly higher and degeneration of spiral ganglion cells was apparent. Our studies indicate that ATP8A2 plays a crucial role in photoreceptor and spiral ganglion cell function and survival by maintaining phospholipid composition and contributing to vesicle trafficking.

Retinal cone photoreceptors of the deer mouse Peromyscus maniculatus: development, topography, opsin expression and spectral tuning

A quantitative analysis of photoreceptor properties was performed in the retina of the nocturnal deer mouse, Peromyscus maniculatus, using pigmented (wildtype) and albino animals. The aim was to establish whether the deer mouse is a more suitable model species than the house mouse for photoreceptor studies, and whether oculocutaneous albinism affects its photoreceptor properties. In retinal flatmounts, cone photoreceptors were identified by opsin immunostaining, and their numbers, spectral types, and distributions across the retina were determined. Rod photoreceptors were counted using differential interference contrast microscopy. Pigmented P. maniculatus have a rod-dominated retina with rod densities of about 450.000/mm² and cone densities of 3000 - 6500/mm². Two cone opsins, shortwave sensitive (S) and middle-to-longwave sensitive (M), are present and expressed in distinct cone types. Partial sequencing of the S opsin gene strongly supports UV sensitivity of the S cone visual pigment. The S cones constitute a 5-15% minority of the cones. Different from house mouse, S and M cone distributions do not have dorsoventral gradients, and coexpression of both opsins in single cones is exceptional (<2% of the cones). In albino P. maniculatus, rod densities are reduced by approximately 40% (270.000/mm²). Overall, cone density and the density of cones exclusively expressing S opsin are not significantly different from pigmented P. maniculatus. However, in albino retinas S opsin is coexpressed with M opsin in 60-90% of the cones and therefore the population of cones expressing only M opsin is significantly reduced to 5-25%. In conclusion, deer mouse cone properties largely conform to the general mammalian pattern, hence the deer mouse may be better suited than the house mouse for the study of certain basic cone properties, including the effects of albinism on cone opsin expression.

P23H opsin knock-in mice reveal a novel step in retinal rod disc morphogenesis

Retinal rod photoreceptor cells have double membrane discs located in their outer segments (ROS) that are continuously formed proximally from connecting cilia (CC) and phagocytized distally by the retinal pigmented epithelium. The major component of these rod discs, the light-sensitive visual pigment rhodopsin (Rho), consists of an opsin protein linked to 11-cis-retinal. The P23H mutation of rod opsin (P23H opsin) is the most common cause of human blinding autosomal dominant retinitis pigmentosa (adRP). A mouse model of adRP with this mutation (Rho(P23H/+)) shows low levels of P23H opsin protein, partial misalignment of discs and progressive retinal degeneration. However, the impact of mutant P23H opsin on the formation of abnormal discs is unclear and it is still unknown whether this mutant pigment can mediate phototransduction. Using transretinal ERG recordings, we demonstrate that P23H mutant Rho can trigger phototransduction but Rho(P23H/P23H) rods are ∼17 000-fold less sensitive to light than Rho(+/+) rods and produce abnormally fast photo-responses. By analyzing homozygous Rho(P23H/P23H) knock-in mice, we show that P23H opsin is transported to ciliary protrusions where it forms sagittally elongated discs. Transmission electron microscopy of postnatal day (PND) 14 Rho(P23H/+) mouse retina revealed disordered sagittally oriented discs before the onset of retinal degeneration. Surprisingly, we also observed smaller, immature sagittally oriented discs in PND14 Rho(+/)(-) and Rho(+/+) mice that were not seen in older animals. These findings provide fundamental insights into the pathogenesis of the P23H mutant opsin and reveal a novel early sagittally aligned disc formation step in normal ROS disc expansion.

Expression and Evolution of Short Wavelength Sensitive Opsins in Colugos: A Nocturnal Lineage That Informs Debate on Primate Origins

A nocturnal activity pattern is central to almost all hypotheses on the adaptive origins of primates. This enduring view has been challenged in recent years on the basis of variation in the opsin genes of nocturnal primates. A correspondence between the opsin genes and activity patterns of species in Euarchonta-the superordinal group that includes the orders Primates, Dermoptera (colugos), and Scandentia (treeshrews)-could prove instructive, yet the basic biology of the dermopteran visual system is practically unknown. Here we show that the eye of the Sunda colugo (Galeopterus variegatus) lacks a tapetum lucidum and has an avascular retina, and we report on the expression and spectral sensitivity of cone photopigments. We found that Sunda colugos have intact short wavelength sensitive (S-) and long wavelength sensitive (L-) opsin genes, and that both opsins are expressed in cone photoreceptors of the retina. The inferred peak spectral sensitivities are 451 and 562 nm, respectively. In line with adaptation to nocturnal vision, cone densities are low. Surprisingly, a majority of S-cones coexpress some L-opsin. We also show that the ratio of rates of nonsynonymous to synonymous substitutions of exon 1 of the S-opsin gene is indicative of purifying selection. Taken together, our results suggest that natural selection has favored a functional S-opsin in a nocturnal lineage for at least 45 million years. Accordingly, a nocturnal activity pattern remains the most likely ancestral character state of euprimates.

Mice lacking Period 1 and Period 2 circadian clock genes exhibit blue cone photoreceptor defects

Many aspects of retinal physiology are modulated by circadian clocks, but it is unclear whether clock malfunction impinges directly on photoreceptor survival, differentiation or function. Eyes from wild-type (WT) and Period1 (Per1) and Period2 (Per2) mutant mice (Per1(Brdm1) Per2(Brdm1) ) were examined for structural (histology, in vivo imaging), phenotypical (RNA expression, immunohistochemistry) and functional characteristics. Transcriptional levels of selected cone genes [red/green opsin (Opn1mw), blue cone opsin (Opn1sw) and cone arrestin (Arr3)] and one circadian clock gene (RORb) were quantified by real-time polymerase chain reaction. Although there were no changes in general retinal histology or visual responses (electroretinograms) between WT and Per1(Brdm1) Per2(Brdm1) mice, compared with age-matched controls, Per1(Brdm1) Per2(Brdm1) mice showed scattered retinal deformations by fundus inspection. Also, mRNA expression levels and immunostaining of blue cone opsin were significantly reduced in mutant mice. Especially, there was an alteration in the dorsal-ventral patterning of blue cones. Decreased blue cone opsin immunoreactivity was present by early postnatal stages, and remained throughout maturation. General photoreceptor differentiation was retarded in young mutant mice. In conclusion, deletion of both Per1 and Per2 clock genes leads to multiple discrete changes in retina, notably patchy tissue disorganization, reductions in cone opsin mRNA and protein levels, and altered distribution. These data represent the first direct link between Per1 and Per2 clock genes, and cone photoreceptor differentiation and function.

Aging-like circadian disturbances in folate-deficient mice

The elderly population shows various circadian disturbances, including dampened amplitude of rhythmicity and decreased responsiveness to light. The common poor folate status in the elderly might account for these aging-related circadian disturbances. To test this hypothesis, we investigated whether folate deficiency in mice affects circadian oscillations of the master clock in the suprachiasmatic nuclei, and the shifting responses to light. Mice fed a diet without folate for 6 weeks displayed markedly reduced (4.5-fold) erythrocyte folate concentration and increased (2.3-fold) homocysteinemia compared with control mice. Folate deficiency decreased the circadian amplitude of vasopressin and the clock protein PERIOD 2 (PER2) in the master clock, slowed the rate of re-entrainment of behavioral rhythms after delayed light-dark cycle and reduced light-induced phase-delays, without detectable morphologic changes in the retina, such as the number of melanopsinergic ganglion cells, that might have impaired photodetection. In conclusion, folate deficiency and consecutive hyperhomocysteinemia led to dampened PER2 and vasopressin oscillations in the master clock and reduced responsiveness to photic resetting, which constitute hallmarks of aging effects on circadian rhythmicity.

Requirement for zebrafish ataxin-7 in differentiation of photoreceptors and cerebellar neurons

The expansion of a polyglutamine (polyQ) tract in the N-terminal region of ataxin-7 (atxn7) is the causative event in spinocerebellar ataxia type 7 (SCA7), an autosomal dominant neurodegenerative disorder mainly characterized by progressive, selective loss of rod-cone photoreceptors and cerebellar Purkinje and granule cells. The molecular and cellular processes underlying this restricted neuronal vulnerability, which contrasts with the broad expression pattern of atxn7, remains one of the most enigmatic features of SCA7, and more generally of all polyQ disorders. To gain insight into this specific neuronal vulnerability and achieve a better understanding of atxn7 function, we carried out a functional analysis of this protein in the teleost fish Danio rerio. We characterized the zebrafish atxn7 gene and its transcription pattern, and by making use of morpholino-oligonucleotide-mediated gene inactivation, we analysed the phenotypes induced following mild or severe zebrafish atxn7 depletion. Severe or nearly complete zebrafish atxn7 loss-of-function markedly impaired embryonic development, leading to both early embryonic lethality and severely deformed embryos. More importantly, in relation to SCA7, moderate depletion of the protein specifically, albeit partially, prevented the differentiation of both retina photoreceptors and cerebellar Purkinje and granule cells. In addition, [1–232] human atxn7 fragment rescued these phenotypes showing strong function conservation of this protein through evolution. The specific requirement for zebrafish atxn7 in the proper differentiation of cerebellar neurons provides, to our knowledge, the first in vivo evidence of a direct functional relationship between atxn7 and the differentiation of Purkinje and granule cells, the most crucial neurons affected in SCA7 and most other polyQ-mediated SCAs. These findings further suggest that altered protein function may play a role in the pathophysiology of the disease, an important step toward the development of future therapeutic strategies.

Neurite outgrowth and synaptophysin expression of postnatal CNS neurons on GaP nanowire arrays in long-term retinal cell culture

We have established long-term cultures of postnatal retinal cells on arrays of gallium phosphide nanowires of different geometries. Rod and cone photoreceptors, ganglion cells and bipolar cells survived on the substrates for at least 18 days in vitro. Glial cells were also observed, but these did not overgrow the neuronal population. On nanowires, neurons extended numerous long and branched neurites that expressed the synaptic vesicle marker synaptophysin. The longest nanowires (4 μm long) allowed a greater attachment and neurite elongation and our analysis suggests that the length of the nanowire per se and/or the adsorption of biomolecules on the nanowires may have been important factors regulating the observed cell behavior. The study thus shows that CNS neurons are amenable to gallium phosphide nanowires, probably as they create conditions that more closely resemble those encountered in the in vivo environment. These findings suggest that gallium phosphide nanowires may be considered as a material of interest when improving existing or designing the next generation of implantable devices. The features of gallium phosphide nanowires can be precisely controlled, making them suitable for this purpose.

An S-opsin knock-in mouse (F81Y) reveals a role for the native ligand 11-cis-retinal in cone opsin biosynthesis

In absence of their natural ligand, 11-cis-retinal, cone opsin G-protein-coupled receptors fail to traffic normally, a condition associated with photoreceptor degeneration and blindness. We created a mouse with a point mutation (F81Y) in cone S-opsin. As expected, cones with this knock-in mutation respond to light with maximal sensitivity red-shifted from 360 to 420 nm, consistent with an altered interaction between the apoprotein and ligand, 11-cis-retinal. However, cones expressing F81Y S-opsin showed an ∼3-fold reduced absolute sensitivity that was associated with a corresponding reduction in S-opsin protein expression. The reduced S-opsin expression did not arise from decreased S-opsin mRNA or cone degeneration, but rather from enhanced endoplasmic reticulum (ER)-associated degradation of the nascent protein. Exogenously increased 11-cis-retinal restored F81Y S-opsin protein expression to normal levels, suggesting that ligand binding in the ER facilitates proper folding. Immunohistochemistry and electron microscopy of normal retinas showed that Mueller cells, which synthesize a precursor of 11-cis-retinal, are closely adjoined to the cone ER, so they could deliver the ligand to the site of opsin synthesis. Together, these results suggest that the binding of 11-cis-retinal in the ER is important for normal folding during cone opsin biosynthesis.

Retinal remodeling in the Tg P347L rabbit, a large-eye model of retinal degeneration

Retinitis pigmentosa (RP) is an inherited blinding disease characterized by progressive loss of retinal photoreceptors. There are numerous rodent models of retinal degeneration, but most are poor platforms for interventions that will translate into clinical practice. The rabbit possesses a number of desirable qualities for a model of retinal disease including a large eye and an existing and substantial knowledge base in retinal circuitry, anatomy, and ophthalmology. We have analyzed degeneration, remodeling, and reprogramming in a rabbit model of retinal degeneration, expressing a rhodopsin proline 347 to leucine transgene in a TgP347L rabbit as a powerful model to study the pathophysiology and treatment of retinal degeneration. We show that disease progression in the TgP347L rabbit closely tracks human cone-sparing RP, including the cone-associated preservation of bipolar cell signaling and triggering of reprogramming. The relatively fast disease progression makes the TgP347L rabbit an excellent model for gene therapy, cell biological intervention, progenitor cell transplantation, surgical interventions, and bionic prosthetic studies.

Gene therapy in ophthalmology: validation on cultured retinal cells and explants from postmortem human eyes

Gene therapy studies in primates can provide important information regarding vector tropism, specific cellular expression, biodistribution, and safety prior to clinical trials. In this study, we report the assessment of transduction efficiency of recombinant adeno-associated virus (rAAV) vectors using human postmortem retina. Transductions were performed using two in vitro models prepared from human tissue: dissociated cell cultures and retinal explants. These models were used to assess cellular tropism and selectivity of rAAV vectors encoding for fluorescent proteins under the control of different promoters. These promoters were a ubiquitous cytomegalovirus promoter and a cell type-specific promoter targeting expression in ON bipolar cells. The results demonstrate that this in vitro approach can limit the use of living primates for the validation of gene therapy in vision and ophthalmology.

Circadian rhythms in the pineal organ persist in zebrafish larvae that lack ventral brain

Background

The mammalian suprachiasmatic nucleus (SCN), located in the ventral hypothalamus, is a major regulator of circadian rhythms in mammals and birds. However, the role of the SCN in lower vertebrates remains poorly understood. Zebrafish cyclops (cyc) mutants lack ventral brain, including the region that gives rise to the SCN. We have used cyc embryos to define the function of the zebrafish SCN in regulating circadian rhythms in the developing pineal organ. The pineal organ is the major source of the circadian hormone melatonin, which regulates rhythms such as daily rest/activity cycles. Mammalian pineal rhythms are controlled almost exclusively by the SCN. In zebrafish and many other lower vertebrates, the pineal has an endogenous clock that is responsible in part for cyclic melatonin biosynthesis and gene expression.

Results

We find that pineal rhythms are present in cyc mutants despite the absence of an SCN. The arginine vasopressin-like protein (Avpl, formerly called Vasotocin) is a peptide hormone expressed in and around the SCN. We find avpl mRNA is absent in cyc mutants, supporting previous work suggesting the SCN is missing. In contrast, expression of the putative circadian clock genes, cryptochrome 1b (cry1b) and cryptochrome 3 (cry3), in the brain of the developing fish is unaltered. Expression of two pineal rhythmic genes, exo-rhodopsin (exorh) and serotonin-N-acetyltransferase (aanat2), involved in photoreception and melatonin synthesis, respectively, is also similar between cyc embryos and their wildtype (WT) siblings. The timing of the peaks and troughs of expression are the same, although the amplitude of expression is slightly decreased in the mutants. Cyclic gene expression persists for two days in cyc embryos transferred to constant light or constant dark, suggesting a circadian clock is driving the rhythms. However, the amplitude of rhythms in cyc mutants kept in constant conditions decreased more quickly than in their WT siblings.

Conclusion

Our data suggests that circadian rhythms can be initiated and maintained in the absence of SCN and other tissues in the ventral brain. However, the SCN may have a role in regulating the amplitude of rhythms when environmental cues are absent. This provides some of the first evidence that the SCN of teleosts is not essential for establishing circadian rhythms during development. Several SCN-independent circadian rhythms have also been found in mammalian species. Thus, zebrafish may serve as a model system for understanding how vertebrate embryos coordinate rhythms that are controlled by different circadian clocks.

Retinal photoreceptors of two subterranean tuco-tuco species (Rodentia, Ctenomys): morphology, topography, and spectral sensitivity

Traditionally, vision was thought to be useless for animals living in dark underground habitats, but recent studies in a range of subterranean rodent species have shown a large diversity of eye features, from small subcutaneous eyes to normal-sized functional eyes. We analyzed the retinal photoreceptors in the subterranean hystricomorph rodents Ctenomys talarum and Ctenomys magellanicus to elucidate whether adaptation was to their near-lightless burrows or rather to their occasional diurnal surface activity. Both species had normally developed eyes. Overall photoreceptor densities were comparatively low (95,000-150,000/mm(2) in C. magellanicus, 110,000-200,000/mm(2) in C. talarum), and cone proportions were rather high (10-31% and 14-31%, respectively). The majority of cones expressed the middle-to-longwave-sensitive (L) opsin, and a 6-16% minority expressed the shortwave-sensitive (S) opsin. In both species the densities of L and S cones were higher in ventral than in dorsal retina. In both species the tuning-relevant amino acids of the S opsin indicate sensitivity in the near UV rather than the blue/violet range. Photopic spectral electroretinograms were recorded. Unexpectedly, their sensitivity profiles were best fitted by the linear summation of three visual pigment templates with lambda(max) at 370 nm (S pigment, UV), at 510 nm (L pigment), and at 450 nm (an as-yet unexplained mechanism). Avoiding predators and selecting food during the brief aboveground excursions may have exerted pressure to retain robust cone-based vision in Ctenomys. UV tuning of the S cone pigment is shared with a number of other hystricomorphs.

Arrestin-1 expression level in rods: balancing functional performance and photoreceptor health

In rod photoreceptors, signaling persists as long as rhodopsin remains catalytically active. Phosphorylation by rhodopsin kinase followed by arrestin-1 binding completely deactivates rhodopsin. Timely termination prevents excessive signaling and ensures rapid recovery. Mouse rods express arrestin-1 and rhodopsin at ∼0.8:1 ratio, making arrestin-1 the second most abundant protein in the rod. The biological significance of wild type arrestin-1 expression level remains unclear. Here we investigated the effects of varying arrestin-1 expression on its intracellular distribution in dark-adapted photoreceptors, rod functional performance, recovery kinetics, and morphology. We found that rod outer segments isolated from dark-adapted animals expressing arrestin-1 at wild type or higher level contain much greater fraction of arrestin-1 than previously estimated, 15-25% of the total. The fraction of arrestin-1 residing in the outer segments (OS) in animals with low expression (4-12% of wild type) is much lower, 5-7% of the total. Only 4% of wild type arrestin-1 level in the outer segments was sufficient to maintain near-normal retinal morphology, whereas rapid recovery required at least ∼12%. Supra-physiological arrestin-1 expression improved light sensitivity and facilitated photoresponse recovery, but was detrimental for photoreceptor health, particularly in the peripheral retina. Thus, physiological level of arrestin-1 expression in rods reflects the balance between short-term functional performance of photoreceptors and their long-term health.

Structural and physiological responses to prolonged constant lighting in the cone-rich retina of Arvicanthis ansorgei

Cone photoreceptor death is a leading cause of blindness in industrialised countries. Despite this, there are few mammalian models available to study cone pathophysiology. The diurnal rodent Arvicanthis possesses a high cone percentage and ease of maintenance. We recently described the effect of ambient light conditions on cyclic disc shedding, and observed that 24 h of constant illumination ("LL") completely disrupted the normal rhythmic process and increased cone shedding fourfold. The current study was undertaken to see whether protracted constant illumination (7 days LL) would further perturb cone (and rod) turnover, and possibly lead to photoreceptor degeneration. Whereas control (cyclic lighting) retinas exhibited a typical early morning burst in phagosomes, LL retinas exhibited only low uniform numbers of rod and cone phagosomes across 24 h, with no peak of shedding at any time. Morphometric and immunohistochemical analyses of Arvicanthis retinas after 7 days LL (300 lux) showed no structural changes compared to control retinas.

Cone loss is delayed relative to rod loss during induced retinal degeneration in the diurnal cone-rich rodent Arvicanthis ansorgei

Cone photoreceptor breakdown underlies functional vision loss in many blinding diseases. Cone loss is often secondary to that of rods, but little experimental data are available on the relationship between the two populations. Because of its high cone numbers, we used the diurnal rodent Arvicanthis ansorgei to explore changes in rod and cone survival and function during chemically-induced retinal degeneration. Adult animals received intraperitoneal injections of N-methyl-N-nitrosourea (MNU), and changes in retinal fundus appearance, histology, phenotype, apoptosis (TUNEL staining) and functionality (scotopic and photopic electroretinography) were monitored as a function of post-treatment time and retinal topography. Relative to control animals injected with vehicle only, MNU-injected animals showed time-, region- and population-specific changes as measured by morphological and immunochemical criteria. Histological (gradual thinning of photoreceptor layer) and phenotypical (reduced immunostaining of rhodopsin and rod transducin, and mid wavelength cone opsin and cone arrestin) modifications were first observed in superior central retina at 11 days post-injection. These degenerative changes spread into the superior peripheral and inferior hemisphere during the following 10 days. Rod loss preceded that of cones as visualized by differential immunolabelling and presence of apoptotic cells in rod but not cone cells. By 3 months post-injection, degeneration of the photoreceptor layer was complete in the superior hemisphere, but only partial in the inferior hemisphere. Despite the persistence of cone photoreceptors, scotopic and photopic electroretinography performed at 90 days post-treatment showed that both rod and cone function were severely compromised. In conclusion, MNU-induced retinal degeneration in Arvicanthis follows a predictable spatial and temporal pattern allowing clear separation of rod- and cone-specific pathogenic mechanisms. Compared to other rodents in which MNU has been used, Arvicanthis ansorgei demonstrates pronounced resistance to photoreceptor cell loss.

Retinal photoreceptor arrangement, SWS1 and LWS opsin sequence, and electroretinography in the South American marsupial Thylamys elegans (Waterhouse, 1839)

We studied the retinal photoreceptors in the mouse opossum Thylamys elegans, a nocturnal South American marsupial. A variety of photoreceptor properties and color vision capabilities have been documented in Australian marsupials, and we were interested to establish what similarities and differences this American marsupial showed. Thylamys opsin gene sequencing revealed two cone opsins, a longwave-sensitive (LWS) opsin and a shortwave-sensitive (SWS1) opsin with deduced peak sensitivities at 560 nm and 360 nm (ultraviolet), respectively. Immunocytochemistry located these opsins to separate cone populations, a majority of LWS cones (density range 1,600-5,600/mm(2)) and a minority of SWS1 cones (density range 100-690/mm(2)). With rod densities of 440,000-590,000/mm(2), the cones constituted 0.4-1.2% of the photoreceptors. This is a suitable adaptation to nocturnal vision. Cone densities peaked in a horizontally elongated region ventral to the optic nerve head. In ventral-but not dorsal-retina, roughly 40% of the LWS opsin-expressing cones occurred as close pairs (double cones), and one member of each double cone contained a colorless oil droplet. The corneal electroretinogram (ERG) showed a high scotopic sensitivity with a rod peak sensitivity at 505 nm. At mesopic light levels, the spectral ERG revealed the contributions of a UV-sensitive SWS1 cone mechanism and an LWS cone mechanism with peak sensitivities at 365 nm and 555 nm, respectively, confirming the tuning predictions from the cone opsin sequences. The two spectral cone types provide the basis for dichromatic color vision, or trichromacy if the rods contribute to color processing at mesopic light levels.

Analysis of retinal cell development in chick embryo by immunohistochemistry and in ovo electroporation techniques

Background

Retinal cell development has been extensively investigated; however, the current knowledge of dynamic morphological and molecular changes is not yet complete.

Results

This study was aimed at revealing the dynamic morphological and molecular changes in retinal cell development during the embryonic stages using a new method of targeted retinal injection, in ovo electroporation, and immunohistochemistry techniques. A plasmid DNA that expresses the green fluorescent protein (GFP) as a marker was delivered into the sub-retinal space to transfect the chick retinal stem/progenitor cells at embryonic day 3 (E3) or E4 with the aid of pulses of electric current. The transfected retinal tissues were analyzed at various stages during chick development from near the start of neurogenesis at E4 to near the end of neurogenesis at E18. The expression of GFP allowed for clear visualization of cell morphologies and retinal laminar locations for the indication of retinal cell identity. Immunohistochemistry using cell type-specific markers (e.g., Visinin, Xap-1, Lim1+2, Pkcα, NeuN, Pax6, Brn3a, Vimentin, etc.) allowed further confirmation of retinal cell types. The composition of retinal cell types was then determined over time by counting the number of GFP-expressing cells observed with morphological characteristics specific to the various retinal cell types.

Conclusion

The new method of retinal injection and electroporation at E3 - E4 allows the visualization of all retinal cell types, including the late-born neurons, e.g., bipolar cells at a level of single cells, which has been difficult with a conventional method with injection and electroporation at E1.5. Based on data collected from analyses of cell morphology, laminar locations in the retina, immunohistochemistry, and cell counts of GFP-expressing cells, the time-line and dynamic morphological and molecular changes of retinal cell development were determined. These data provide more complete information on retinal cell development, and they can serve as a reference for the investigations in normal retinal development and diseases.

A battery of cell- and structure-specific markers for the adult porcine retina

The pig is becoming an increasingly used non-primate model in experimental studies of human retinal diseases and disorders. The anatomy, size, and vasculature of the porcine eye and retina closely resemble their human counterparts, which allows for application of standard instrumentation and diagnostics used in the clinic. Despite many reports that demonstrate immunohistochemistry as a useful method for exploring neuropathological changes in the mammalian central nervous system, including the pig, the porcine retina has been sparsely described. Hence, to facilitate further immunohistochemical analysis of the porcine retina, we report on the successful use of a battery of antibodies for staining of paraformaldehyde-fixed cryosectioned retina. The following antibodies were evaluated for neuronal cells and structures: recoverin (cones and rods), Rho4D2 (rods), transducin-gamma (cones), ROM-1 (photoreceptor outer segments), calbindin (horizontal cells), PKC-alpha (bipolar cells), parvalbumin (amacrine and displaced amacrine cells), and NeuN (ganglion cells and displaced amacrines). For detecting synaptic connections in fiber layers, we used an antibody against synaptobrevin. For detecting retinal pigment epithelium, we studied antibodies against cytokeratin and RPE65, respectively. The glial cell markers used were bFGF (Müller cells and displaced amacrine cells), GFAP (Müller cells and astrocytes), and vimentin (Müller cells). Each staining effect was evaluated with regard to its specificity, sensitivity, and reproducibility in the identification of individual cells, specific cell structures, and fiber layers, respectively. The markers parvalbumin and ROM-1 were tested here for the first time for the porcine retina. All antibodies tested resulted in specific staining of high quality. In conclusion, all immunohistochemical protocols presented here will be applicable in fixed, cryosectioned pig retina.

The role of rhodopsin glycosylation in protein folding, trafficking, and light-sensitive retinal degeneration

Several mutations in the N terminus of the G-protein-coupled receptor rhodopsin disrupt NXS/T consensus sequences for N-linked glycosylation (located at N2 and N15) and cause sector retinitis pigmentosa in which the inferior retina preferentially degenerates. Here we examined the role of rhodopsin glycosylation in biosynthesis, trafficking, and retinal degeneration (RD) using transgenic Xenopus laevis expressing glycosylation-defective human rhodopsin mutants. Although mutations T4K and T4N caused RD, N2S and T4V did not, demonstrating that glycosylation at N2 was not required for photoreceptor viability. In contrast, similar mutations eliminating glycosylation at N15 (N15S and T17M) caused rod death. Expression of T17M was more toxic than T4K to transgenic photoreceptors, further suggesting that glycosylation at N15 plays a more important physiological role than glycosylation at N2. Together, these results indicate that the structure of the rhodopsin N terminus must be maintained by an appropriate amino acid sequence surrounding N2 and may require a carbohydrate moiety at N15. The mutant rhodopsins were rendered less toxic in their dark inactive states, because RD was abolished or significantly reduced when transgenic tadpoles expressing T4K, T17M, and N2S/N15S were protected from light exposure. Regardless of their effect on rod viability, all of the mutants primarily localized to the outer segment and Golgi and showed little or no endoplasmic reticulum accumulation. Thus, glycosylation was not crucial for rhodopsin biosynthesis or trafficking. Interestingly, expression of similar bovine rhodopsin mutants did not cause rod cell death, possibly attributable to greater stability of bovine rhodopsin.