Delayed dark adaptation in 11-cis-retinol dehydrogenase-deficient mice: a role of RDH11 in visual processes in vivo

17β-hydroxysteroid dehydrogenases in the progression of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has become a worldwide epidemic and a major public health problem, with a prevalence of approximately 25%. The pathogenesis of NAFLD is complex and may be affected by the environment and susceptible genetic factors, resulting in a highly variable disease course and no approved drugs in the clinic. Notably, 17β-hydroxysteroid dehydrogenase type 13 (HSD17B13), which belongs to the 17β-hydroxysteroid dehydrogenase superfamily (HSD17Bs), is closely related to the clinical outcome of liver disease. HSD17Bs consists of fifteen members, most related to steroid and lipid metabolism, and may have the same biological function as HSD17B13. In this review, we highlight recent advances in basic research on the functional activities, major substrates, and key roles of HSD17Bs in the progression of NAFLD to develop innovative anti-NAFLD drugs targeting HSD17Bs.

A large animal model of RDH5-associated retinopathy recapitulates important features of the human phenotype

Pathogenic variants in retinol dehydrogenase 5 (RDH5) attenuate supply of 11-cis-retinal to photoreceptors leading to a range of clinical phenotypes including night blindness because of markedly slowed rod dark adaptation and in some patients, macular atrophy. Current animal models (such as Rdh5-/- mice) fail to recapitulate the functional or degenerative phenotype. Addressing this need for a relevant animal model we present a new domestic cat model with a loss-of-function missense mutation in RDH5 (c.542G > T; p.Gly181Val). As with patients, affected cats have a marked delay in recovery of dark adaptation. In addition, the cats develop a degeneration of the area centralis (equivalent to the human macula). This recapitulates the development of macular atrophy that is reported in a subset of patients with RDH5 mutations and is shown in this paper in seven patients with biallelic RDH5 mutations. There is notable variability in the age at onset of the area centralis changes in the cat, with most developing changes as juveniles but some not showing changes over the first few years of age. There is similar variability in development of macular atrophy in patients and while age is a risk factor, it is hypothesized that genetic modifying loci influence disease severity, and we suspect the same is true in the cat model. This novel cat model provides opportunities to improve molecular understanding of macular atrophy and test therapeutic interventions for RDH5-associated retinopathies.

The Role of Vitamin A in Retinal Diseases

Vitamin A is an essential fat-soluble vitamin that occurs in various chemical forms. It is essential for several physiological processes. Either hyper- or hypovitaminosis can be harmful. One of the most important vitamin A functions is its involvement in visual phototransduction, where it serves as the crucial part of photopigment, the first molecule in the process of transforming photons of light into electrical signals. In this process, large quantities of vitamin A in the form of 11-cis-retinal are being isomerized to all-trans-retinal and then quickly recycled back to 11-cis-retinal. Complex machinery of transporters and enzymes is involved in this process (i.e., the visual cycle). Any fault in the machinery may not only reduce the efficiency of visual detection but also cause the accumulation of toxic chemicals in the retina. This review provides a comprehensive overview of diseases that are directly or indirectly connected with vitamin A pathways in the retina. It includes the pathophysiological background and clinical presentation of each disease and summarizes the already existing therapeutic and prospective interventions.

A new phenotype of syndromic retinitis pigmentosa with myopathy is caused by mutations in retinol dehydrogenase 11

Retinol dehydrogenase 11 (RDH11) is an 11-cis-retinol dehydrogenase that has a well-characterized, albeit auxiliary role in the retinoid cycle. Diseases caused by mutations in the RDH11 gene are very rare, and only one affected family with eye and intelligence involvement has been reported. In the present study, we describe the clinical and genetic findings in a Chinese patient with retinitis pigmentosa (RP), juvenile cataracts, intellectual disability, and myopathy. Trio-based whole-exome sequencing (Trio-WES) and whole genomic copy number variation (CNV) detection were performed in this family, and compound heterozygous mutations were identified in RDH11 of the patient: c.938T>C (p.Leu313Pro) derived from the father and c.75-3C>A derived from the mother. Variant c.75-3C>A was confirmed to be a splice site mutation by cDNA sequencing. It caused exon 2 skipping, resulting in a frameshift mutation and premature translation termination (p.Lys26Serfs*38). Moreover, we found mislocalization of RDH11 protein in muscle cells of the patient by using immunofluorescence staining. This is the first case reported in the Chinese population harboring mutations in RDH11 and revealing a new phenotype of syndromic RP with myopathy. This article is protected by copyright. All rights reserved.

RNA-seq analysis of ageing human retinal pigment epithelium: Unexpected up-regulation of visual cycle gene transcription

Ageing presents adverse effects on the retina and is the primary risk factor for age‐related macular degeneration (AMD). We report the first RNA‐seq analysis of age‐related transcriptional changes in the human retinal pigment epithelium (RPE), the primary site of AMD pathogenesis. Whole transcriptome sequencing of RPE from human donors ranging in age from 31 to 93 reveals that ageing is associated with increasing transcription of main RPE‐associated visual cycle genes (including LRAT, RPE65, RDH5, RDH10, RDH11; pathway enrichment BH‐adjusted P = 4.6 × 10⁻⁶). This positive correlation is replicated in an independent set of 28 donors and a microarray dataset of 50 donors previously published. LRAT expression is positively regulated by retinoid by‐products of the visual cycle (A2E and all‐trans‐retinal) involving modulation by retinoic acid receptor alpha transcription factor. The results substantiate a novel age‐related positive feedback mechanism between accumulation of retinoid by‐products in the RPE and the up‐regulation of visual cycle genes.

The iTRAQ-based quantitative proteomics reveals metabolic changes in Scylla paramamosain under different light intensities during indoor overwintering

Light intensity is one of the ecological factors that appreciably affects the metabolism of Scylla paramamosain during overwintering. This study adopted the isobaric tag for relative and absolute quantitation (iTRAQ) method to investigate metabolic changes of S. paramamosain under three illumination levels (0, 1.43 and 40.31 μmol m^-2·s^-1) for four months during indoor overwintering. The iTRAQ identified 3282 proteins, among which 267 exhibited significant differential expression (122 upregulated and 145 downregulated) in the low light group, and 299 with significant differential expression (252 upregulated and 47 downregulated) in the high light group. Analysis of these results showed that there were different metabolic regulatory patterns under different light intensities. Low light is more conducive to the survival of S. paramamosain, which needs to produce and consume relatively less energy to sustain physiological activities. Thus, the essential proteins associated with physiological activities were significantly upregulated, while those related to energy production were significantly downregulated. In contrast, high light exerts a certain stress on the survival of S. paramamosain and required more energy to cope with this stress, which forced a significant upregulation of proteins related to stress response and energy production. The findings of this study highlighted the metabolic regulatory mechanisms of S. paramamosain under different light intensities.

A spectral-domain optical coherence tomographic analysis of Rdh5-/- mice retina

PURPOSE

To investigate the longitudinal findings of spectral-domain optical coherence tomography (SD-OCT) in relation to the morphologic features in Rdh5 knockout (Rdh5-/-) mice.

MATERIALS AND METHODS

The mouse retina was segmented into four layers; the inner retinal (A), outer plexiform and outer nuclear (B), rod/cone (C), and retinal pigment epithelium (RPE)/choroid (D) layers. The thickness of each retinal layer of Rdh5-/- mice was longitudinally and quantitatively measured at six time points from postnatal months (PM) 1 to PM6 using SD-OCT. Age-matched C57BL/6J mice were employed as wild-type controls. The data were statistically compared using Student's t-test. The fundus appearance was assessed, histologic and ultrastructural examinations were performed in both groups.

RESULTS

Layers A and B were significantly thinner in the Rdh5-/- mice than in the wild-type C57BL/6J mice during the observation periods. Layers C and D became thinner in the Rdh5-/- mice than in the wild-type mice after PM6. Although no abnormalities corresponding to whitish fundus dots were detected by SD-OCT or histologic examinations, the intracellular accumulation of low-density vacuoles was noted in the RPE of the Rdh5-/- mice by electron microscopy. The photoreceptor nuclei appeared less dense in the Rdh5-/- mice than in the wild-type mice.

DISCUSSION

The results from the present study suggest that although it is difficult to detect qualitative abnormalities, SD-OCT can detect quantitative changes in photoreceptors even in the early stage of retinal degeneration induced by the Rdh5 gene mutation in mice.

Large Animal Models of Inherited Retinal Degenerations: A Review

Studies utilizing large animal models of inherited retinal degeneration (IRD) have proven important in not only the development of translational therapeutic approaches, but also in improving our understanding of disease mechanisms. The dog is the predominant species utilized because spontaneous IRD is common in the canine pet population. Cats are also a source of spontaneous IRDs. Other large animal models with spontaneous IRDs include sheep, horses and non-human primates (NHP). The pig has also proven valuable due to the ease in which transgenic animals can be generated and work is ongoing to produce engineered models of other large animal species including NHP. These large animal models offer important advantages over the widely used laboratory rodent models. The globe size and dimensions more closely parallel those of humans and, most importantly, they have a retinal region of high cone density and denser photoreceptor packing for high acuity vision. Laboratory rodents lack such a retinal region and, as macular disease is a critical cause for vision loss in humans, having a comparable retinal region in model species is particularly important. This review will discuss several large animal models which have been used to study disease mechanisms relevant for the equivalent human IRD.

Retinal dehydrogenase 5 (RHD5) attenuates metastasis via regulating HIPPO/YAP signaling pathway in Hepatocellular Carcinoma

Retinal dehydrogenase 5 (RDH5) is an important enzyme in the visual cycle. Several studies have reported that the RDH family may play crucial roles in tumor prognosis. However, the role of RDH5 in tumor prognosis is still unclear. We examined the mRNA level of RDH5 by using q-PCR in hepatocellular carcinoma (HCC) and adjacent non-cancerous tissues. The proliferation rate of HCC cells was detected by MTS assay, and the invasive ability was examined by transwell and scratch wound assays. The YAP protein localization and expression were visualized by immunofluorescence in two different cell lines. CpG islands in the promoter region were predicted by using the methprimer database. Clinical characteristics of a patient cohort data came from The Cancer Genome Atlas database. RDH5 was significantly downregulated in hepatocellular carcinoma tissues, and low RDH5 expression was associated with metastasis and poor patient prognosis. Functional assays revealed that the RDH5 promoter is methylated in HCC cell lines. Moreover, overexpressing RDH5 can suppress metastasis by reversing the epithelial-mesenchymal transition (EMT) process, and RDH5 also inhibits cell proliferation in HCC cell lines. Furthermore, suppressing RDH5 can activate the Hippo/YAP signaling pathway and promote the nuclear translocation of YAP. Clinical data demonstrated that RDH5 is an independent prognostic factor in HCC. In our study, we provided the first evidence that RDH5 plays a crucial role in suppressing proliferation and metastasis, and the RDH5 promoter is methylated in hepatocellular carcinoma. And as an important regulator, RDH5 can suppress the Hippo/YAP signaling pathway. Taken together, it revealed that RDH5 might be a potential therapeutic target in HCC patients.

Generation of Retinaldehyde for Retinoic Acid Biosynthesis

The concentration of all-trans-retinoic acid, the bioactive derivative of vitamin A, is critically important for the optimal performance of numerous physiological processes. Either too little or too much of retinoic acid in developing or adult tissues is equally harmful. All-trans-retinoic acid is produced by the irreversible oxidation of all-trans-retinaldehyde. Thus, the concentration of retinaldehyde as the immediate precursor of retinoic acid has to be tightly controlled. However, the enzymes that produce all-trans-retinaldehyde for retinoic acid biosynthesis and the mechanisms responsible for the control of retinaldehyde levels have not yet been fully defined. The goal of this review is to summarize the current state of knowledge regarding the identities of physiologically relevant retinol dehydrogenases, their enzymatic properties, and tissue distribution, and to discuss potential mechanisms for the regulation of the flux from retinol to retinaldehyde.

Retinyl esters are elevated in progeny of retinol dehydrogenase 11 deficient dams

Retinol dehydrogenase 11 (RDH11) is an NADPH-dependent retinaldehyde reductase that was previously reported to function in the visual cycle. Recently, we have shown that RDH11 contributes to the maintenance of retinol levels in extraocular tissues under conditions of vitamin A deficiency or reduced vitamin A availability. RDH11 is also expressed in the embryo. Rdh11 knockout animals do not display embryonic defects and appear to develop normally to the adult stage, but the exact function of RDH11 during development is not yet known. In contrast to RDH11-null mice, animals that lack dehydrogenase/reductase 3 (DHRS3), the enzyme that functions as a retinaldehyde reductase and is essential for the maintenance of retinoid homeostasis during embryogenesis, rarely survive until birth. Here, we investigated whether inactivation of RDH11 together with DHRS3 exacerbates the severity of retinoid homeostasis disruption in embryos that lack both enzymes compared to DHRS3-null mice. The results of this study indicate that in vitamin A sufficient animals, the loss of RDH11 in addition to DHRS3 does not appear to significantly impact the total levels of retinoic acid, free retinol, or retinyl esters in Rdh11^-/-/Dhrs3^-/-embryos in comparison to Dhrs3^-/- embryos. Surprisingly, Rdh11^-/- single gene knockout embryos obtained from breeding of Rdh11^-/- dams display elevated levels of embryonic retinyl esters compared to wild type embryos. The mechanism of the maternal effect of Rdh11 status on fetal retinoid stores remains to be elucidated.

Aberrant RNA splicing is the major pathogenic effect in a knock-in mouse model of the dominantly inherited c.1430A>G human RPE65 mutation

Human RPE65 mutations cause a spectrum of retinal dystrophies that result in blindness. While RPE65 mutations have been almost invariably recessively inherited, a c.1430A>G (p.(D477G)) mutation has been reported to cause autosomal dominant retinitis pigmentosa (adRP). To study the pathogenesis of this human mutation, we have replicated the mutation in a knock-in (KI) mouse model using CRISPR/Cas9-mediated genome editing. Significantly, in contrast to human patients, heterozygous KI mice do not exhibit any phenotypes in visual function tests. When raised in regular vivarium conditions, homozygous KI mice display relatively undisturbed visual functions with minimal retinal structural changes. However, KI/KI mouse retinae are more sensitive to light exposure and exhibit signs of degenerative features when subjected to light stress. We find that instead of merely producing a missense mutant protein, the A>G nucleotide substitution greatly affects appropriate splicing of Rpe65 mRNA by generating an ectopic splice site in comparable context to the canonical one, thereby disrupting RPE65 protein expression. Similar splicing defects were also confirmed for the human RPE65 c.1430G mutant in an in vitro Exontrap assay. Our data demonstrate that a splicing defect is associated with c.1430G pathogenesis, and therefore provide insights in the therapeutic strategy for human patients.

Structural biology of 11-cis-retinaldehyde production in the classical visual cycle

The vitamin A derivative 11-cis-retinaldehyde plays a pivotal role in vertebrate vision by serving as the chromophore of rod and cone visual pigments. In the initial step of vision, a photon is absorbed by this chromophore resulting in its isomerization to an all-trans state and consequent activation of the visual pigment and phototransduction cascade. Spent chromophore is released from the pigments through hydrolysis. Subsequent photon detection requires the delivery of regenerated 11-cis-retinaldehyde to the visual pigment. This trans-cis conversion is achieved through a process known as the visual cycle. In this review, we will discuss the enzymes, binding proteins and transporters that enable the visual pigment renewal process with a focus on advances made during the past decade in our understanding of their structural biology.

Noninvasive Electroretinographic Procedures for the Study of the Mouse Retina

Overall retinal function can be monitored by recording the light-evoked response of the eye at the corneal surface. The major components of the electroretinogram (ERG) provide important information regarding the functional status of many retinal cell types including rod photoreceptors, cone photoreceptors, bipolar cells, and the retinal pigment epithelium (RPE). The ERG can be readily recorded from mice, and this unit describes procedures for mouse anesthesia and the use of stimulation and recording procedures for measuring ERGs that reflect the response properties of different retinal cell types. Through these, the mouse ERG provides a noninvasive approach to measure multiple aspects of outer retinal function, including the status of the initial rod and cone pathways, rod photoreceptor deactivation, rod dark adaptation, the photoreceptor-to-bipolar cell synapse, and the RPE. © 2018 by John Wiley & Sons, Inc.

Intracrine Regulation of Estrogen and Other Sex Steroid Levels in Endometrium and Non-gynecological Tissues; Pathology, Physiology, and Drug Discovery

Our understanding of the intracrine (or local) regulation of estrogen and other steroid synthesis and degradation expanded in the last decades, also thanks to recent technological advances in chromatography mass-spectrometry. Estrogen responsive tissues and organs are not passive receivers of the pool of steroids present in the blood but they can actively modify the intra-tissue steroid concentrations. This allows fine-tuning the exposure of responsive tissues and organs to estrogens and other steroids in order to best respond to the physiological needs of each specific organ. Deviations in such intracrine control can lead to unbalanced steroid hormone exposure and disturbances. Through a systematic bibliographic search on the expression of the intracrine enzymes in various tissues, this review gives an up-to-date view of the intracrine estrogen metabolisms, and to a lesser extent that of progestogens and androgens, in the lower female genital tract, including the physiological control of endometrial functions, receptivity, menopausal status and related pathological conditions. An overview of the intracrine regulation in extra gynecological tissues such as the lungs, gastrointestinal tract, brain, colon and bone is given. Current therapeutic approaches aimed at interfering with these metabolisms and future perspectives are discussed.

Rdh13 deficiency weakens carbon tetrachloride-induced liver injury by regulating Spot14 and Cyp2e1 expression levels

Mitochondrion-localized retinol dehydrogenase 13 (Rdh13) is a short-chain dehydrogenase/reductase involved in vitamin A metabolism in both humans and mice. We previously generated Rdh13 knockout mice and showed that Rdh13 deficiency causes severe acute retinal light damage. In this study, considering that Rdh13 is highly expressed in mouse liver, we further evaluated the potential effect of Rdh13 on liver injury induced by carbon tetrachloride (CCl₄). Although Rdh13 deficiency showed no significant effect on liver histology and physiological functions under regular culture, the Rdh13^-/- mice displayed an attenuated response to CCl₄-induced liver injury. Their livers also exhibited less histological changes and contained lower levels of liver-related metabolism enzymes compared with the livers of wild-type (WT) mice. Furthermore, the Rdh13^-/- mice had Rdh13 deficiency and thus their liver cells were protected from apoptosis, and the quantity of their proliferative cells became lower than that in WTafter CCl₄ exposure. The ablation of Rdh13 gene decreased the expression levels of thyroid hormone-inducible nuclear protein 14 (Spot14) and cytochrome P450 (Cyp2e1) in the liver, especially after CCl₄ treatment for 48 h. These data suggested that the alleviated liver damage induced by CCl₄ in Rdh13^-/- mice was caused by Cyp2e1 enzymes, which promoted reductive CCl₄ metabolism by altering the status of thyroxine metabolism. This result further implicated Rdh13 as a potential drug target in preventing chemically induced liver injury.

Retinol dehydrogenase 11 is essential for the maintenance of retinol homeostasis in liver and testis in mice

Retinol dehydrogenase 11 (RDH11) is a microsomal short-chain dehydrogenase/reductase that recognizes all-trans- and cis-retinoids as substrates and prefers NADPH as a cofactor. Previous work has suggested that RDH11 contributes to the oxidation of 11-cis-retinol to 11-cis-retinaldehyde during the visual cycle in the eye's retinal pigment epithelium. However, the role of RDH11 in metabolism of all-trans-retinoids remains obscure. Here, we report that microsomes isolated from the testes and livers of Rdh11^-/- mice fed a regular diet exhibited a 3- and 1.7-fold lower rate of all-trans-retinaldehyde conversion to all-trans-retinol, respectively, than the microsomes of WT littermates. Testes and livers of Rdh11^-/- mice fed a vitamin A-deficient diet had ∼35% lower levels of all-trans-retinol than those of WT mice. Furthermore, the conversion of β-carotene to retinol via retinaldehyde as an intermediate appeared to be impaired in the testes of Rdh11^-/-/retinol-binding protein 4^-/-(Rbp4^-/-) mice, which lack circulating holo RBP4 and rely on dietary supplementation with β-carotene for maintenance of their retinoid stores. Together, these results indicate that in mouse testis and liver, RDH11 functions as an all-trans-retinaldehyde reductase essential for the maintenance of physiological levels of all-trans-retinol under reduced vitamin A availability.

Modeling Retinal Diseases Using Genetic Approaches in Mice

Genetic mouse models mimicking human diseases have been developed and utilized for retinal research in various topics, involving anatomy, physiology, biochemistry, and pathology. The main reasons why mouse models are important for retinal research include that rodents share a key retinal homology with humans and that genetic manipulation is relatively easily applicable for mice. Here, we describe genetic mouse models, which are categorized with functions in the retina and relationship with human diseases.

Progranulin increases phagocytosis by retinal pigment epithelial cells in culture

Retinal pigment epithelium (RPE) cells take part in retinal preservation, such as phagocytizing the shed photoreceptor outer segments (POS), every day. The incomplete phagocytic function accelerates RPE degeneration and formation of the toxic by-product lipofuscin. Excessive lipofuscin accumulation is characteristic of various blinding diseases in the human eye. Progranulin is a cysteine-rich protein that has multiple biological activities, and it has a high presence in the retina. Progranulin has been recognized to be involved in macrophage phagocytosis in the brain. The purpose of this study is to determine whether progranulin influences phagocytosis by RPE cells. All experiments were performed on primary human RPE (hRPE) cells in culture. pHrodo was used to label the isolated porcine POS, and quantification of pHrodo fluorescence was used to determine the degree of phagocytosis. Western blotting and immunohistochemistry of key proteins involved in phagocytosis were used to clarify the mechanism of progranulin. Progranulin increased RPE phagocytosis in hydrogen peroxide-treated and nontreated RPE cells. The phosphorylated form of Mer tyrosine kinase, which is important for POS internalization, was significantly increased in the progranulin-exposed cells. This increase was attenuated by SU11274, an inhibitor of hepatic growth factor receptor. Under the oxidative stress condition, exposure to progranulin led to an approximately twofold increase in integrin alpha-v, which is associated with the first step in recognition of POS by RPE cells. These results suggest that progranulin could be an effective stimulator for RPE phagocytosis and could repair RPE function. © 2017 Wiley Periodicals, Inc.

A Dominant Mutation in Rpe65, D477G, Delays Dark Adaptation and Disturbs the Visual Cycle in the Mutant Knock-In Mice

RPE65 is an indispensable component of the retinoid visual cycle in vertebrates, through which the visual chromophore 11-cis-retinal (11-cis-RAL) is generated to maintain normal vision. Various blinding conditions in humans, such as Leber congenital amaurosis and retinitis pigmentosa (RP), are attributed to either homozygous or compound heterozygous mutations in RPE65. Herein, we investigated D477G missense mutation, an unprecedented dominant-acting mutation of RPE65 identified in patients with autosomal dominant RP. We generated a D477G knock-in (KI) mouse and characterized its phenotypes. Although RPE65 protein levels were decreased in heterozygous KI mice, their scotopic, maximal, and photopic electroretinography responses were comparable to those of wild-type (WT) mice in stationary condition. As shown by high-performance liquid chromatography analysis, levels of 11-cis-RAL in fully dark-adapted heterozygous KI mice were similar to that in WT mice. However, kinetics of 11-cis-RAL regeneration after light exposure were significantly slower in heterozygous KI mice compared with WT and RPE65 heterozygous knockout mice. Furthermore, heterozygous KI mice exhibited lower A-wave recovery compared with WT mice after photobleaching, suggesting a delayed dark adaptation. Taken together, these observations suggest that D477G acts as a dominant-negative mutant of RPE65 that delays chromophore regeneration. The KI mice provide a useful model for further understanding of the pathogenesis of RP associated with this RPE65 mutant and for the development of therapeutic strategies.

Enzymatic Metabolism of Vitamin A in Developing Vertebrate Embryos

Embryonic development is orchestrated by a small number of signaling pathways, one of which is the retinoic acid (RA) signaling pathway. Vitamin A is essential for vertebrate embryonic development because it is the molecular precursor of the essential signaling molecule RA. The level and distribution of RA signaling within a developing embryo must be tightly regulated; too much, or too little, or abnormal distribution, all disrupt embryonic development. Precise regulation of RA signaling during embryogenesis is achieved by proteins involved in vitamin A metabolism, retinoid transport, nuclear signaling, and RA catabolism. The reversible first step in conversion of the precursor vitamin A to the active retinoid RA is mediated by retinol dehydrogenase 10 (RDH10) and dehydrogenase/reductase (SDR family) member 3 (DHRS3), two related membrane-bound proteins that functionally activate each other to mediate the interconversion of retinol and retinal. Alcohol dehydrogenase (ADH) enzymes do not contribute to RA production under normal conditions during embryogenesis. Genes involved in vitamin A metabolism and RA catabolism are expressed in tissue-specific patterns and are subject to feedback regulation. Mutations in genes encoding these proteins disrupt morphogenesis of many systems in a developing embryo. Together these observations demonstrate the importance of vitamin A metabolism in regulating RA signaling during embryonic development in vertebrates.

Retinol Dehydrogenases Regulate Vitamin A Metabolism for Visual Function

The visual system produces visual chromophore, 11-cis-retinal from dietary vitamin A, all-trans-retinol making this vitamin essential for retinal health and function. These metabolic events are mediated by a sequential biochemical process called the visual cycle. Retinol dehydrogenases (RDHs) are responsible for two reactions in the visual cycle performed in retinal pigmented epithelial (RPE) cells, photoreceptor cells and Müller cells in the retina. RDHs in the RPE function as 11-cis-RDHs, which oxidize 11-cis-retinol to 11-cis-retinal in vivo. RDHs in rod photoreceptor cells in the retina work as all-trans-RDHs, which reduce all-trans-retinal to all-trans-retinol. Dysfunction of RDHs can cause inherited retinal diseases in humans. To facilitate further understanding of human diseases, mouse models of RDHs-related diseases have been carefully examined and have revealed the physiological contribution of specific RDHs to visual cycle function and overall retinal health. Herein we describe the function of RDHs in the RPE and the retina, particularly in rod photoreceptor cells, their regulatory properties for retinoid homeostasis and future therapeutic strategy for treatment of retinal diseases.

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.

The Silk-protein Sericin Induces Rapid Melanization of Cultured Primary Human Retinal Pigment Epithelial Cells by Activating the NF-κB Pathway

Restoration of the retinal pigment epithelial (RPE) cells to prevent further loss of vision in patients with age-related macular degeneration represents a promising novel treatment modality. Development of RPE transplants, however, requires up to 3 months of cell differentiation. We explored whether the silk protein sericin can induce maturation of primary human retinal pigment epithelial (hRPE) cells. Microarray analysis demonstrated that sericin up-regulated RPE-associated transcripts (RPE65 and CRALBP). Upstream analysis identified the NF-κB pathway as one of the top sericin-induced regulators. ELISA confirmed that sericin stimulates the main NF-κB pathway. Increased levels of RPE-associated proteins (RPE65 and the pigment melanin) in the sericin-supplemented cultures were confirmed by western blot, spectrophotometry and transmission electron microscopy. Sericin also increased cell density and reduced cell death following serum starvation in culture. Inclusion of NF-κB agonists and antagonists in the culture medium showed that activation of the NF-κB pathway appears to be necessary, but not sufficient, for sericin-induced RPE pigmentation. We conclude that sericin promotes pigmentation of cultured primary hRPE cells by activating the main NF-κB pathway. Sericin’s potential role in culture protocols for rapid differentiation of hRPE cells derived from embryonic or induced pluripotent stem cells should be investigated.

Recent advances in the dark adaptation investigations

Dark adaptation is a highly sensitive neural function and may be the first symptom of many status including the physiologic and pathologic entity, suggesting that it could be instrumental for diagnose. However, shortcomings such as the lack of standardized parameters, the long duration of examination, and subjective randomness would substantially impede the use of dark adaptation in clinical work. In this review we summarize the recent research about the dark adaptation, including two visual cycles-canonical and cone-specific visual cycle, affecting factors and the methods for measuring dark adaptation. In the opinions of authors, intensive investigations are needed to be done for the widely use of this significant visual function in clinic.

Conditional Ablation of Retinol Dehydrogenase 10 in the Retinal Pigmented Epithelium Causes Delayed Dark Adaption in Mice

Regeneration of the visual chromophore, 11-cis-retinal, is a crucial step in the visual cycle required to sustain vision. This cycle consists of sequential biochemical reactions that occur in photoreceptor cells and the retinal pigmented epithelium (RPE). Oxidation of 11-cis-retinol to 11-cis-retinal is accomplished by a family of enzymes termed 11-cis-retinol dehydrogenases, including RDH5 and RDH11. Double deletion of Rdh5 and Rdh11 does not limit the production of 11-cis-retinal in mice. Here we describe a third retinol dehydrogenase in the RPE, RDH10, which can produce 11-cis-retinal. Mice with a conditional knock-out of Rdh10 in RPE cells (Rdh10 cKO) displayed delayed 11-cis-retinal regeneration and dark adaption after bright light illumination. Retinal function measured by electroretinogram after light exposure was also delayed in Rdh10 cKO mice as compared with controls. Double deletion of Rdh5 and Rdh10 (cDKO) in mice caused elevated 11/13-cis-retinyl ester content also seen in Rdh5(-/-)Rdh11(-/-) mice as compared with Rdh5(-/-) mice. Normal retinal morphology was observed in 6-month-old Rdh10 cKO and cDKO mice, suggesting that loss of Rdh10 in the RPE does not negatively affect the health of the retina. Compensatory expression of other retinol dehydrogenases was observed in both Rdh5(-/-) and Rdh10 cKO mice. These results indicate that RDH10 acts in cooperation with other RDH isoforms to produce the 11-cis-retinal chromophore needed for vision.

Cone Health and Retinoids

Cones are photoreceptor cells used for bright light and color vision. Retinoids are vitamin A derivatives, one of which is the 11-cis aldehyde form that serves as the chromophore for both cone and rod visual pigments. In the visual disease, Type 2 Leber congenital amaurosis (LCA2), 11-cis-retinal generation is inhibited or abolished. Work by others has shown that patients with LCA2 have symptoms consistent with degenerating cones. In mouse models for LCA2, early cone degeneration is readily apparent: cone opsins and other proteins associated with the outer segment are delocalized and cell numbers decline rapidly within the first month. Rods would appear normal morphologically and functionally, if not for the absence of chromophore. Supplementation of mouse models of LCA2 with cis-retinoids has been shown to slow loss of cone photoreceptor cells if mice were maintained in darkness. Thus, 11-cis-retinal appears not only to have a role in the light response reaction but also to promote proper trafficking of the cone opsins and maintain viable cones.

Divergence of dim-light vision among bats (order: Chiroptera) as estimated by molecular and electrophysiological methods

Dim-light vision is present in all bats, but is divergent among species. Old-World fruit bats (Pteropodidae) have fully developed eyes; the eyes of insectivorous bats are generally degraded, and these bats rely on well-developed echolocation. An exception is the Emballonuridae, which are capable of laryngeal echolocation but prefer to use vision for navigation and have normal eyes. In this study, integrated methods, comprising manganese-enhanced magnetic resonance imaging (MEMRI), f-VEP and RNA-seq, were utilized to verify the divergence. The results of MEMRI showed that Pteropodidae bats have a much larger superior colliculus (SC)/ inferior colliculus (IC) volume ratio (3:1) than insectivorous bats (1:7). Furthermore, the absolute visual thresholds (log cd/m(2)•s) of Pteropodidae (-6.30 and -6.37) and Emballonuridae (-3.71) bats were lower than those of other insectivorous bats (-1.90). Finally, genes related to the visual pathway showed signs of positive selection, convergent evolution, upregulation and similar gene expression patterns in Pteropodidae and Emballonuridae bats. Different results imply that Pteropodidae and Emballonuridae bats have more developed vision than the insectivorous bats and suggest that further research on bat behavior is warranted.

Mouse model of human RPE65 P25L hypomorph resembles wild type under normal light rearing but is fully resistant to acute light damage

Human RPE65 mutations cause a spectrum of blinding retinal dystrophies from severe early-onset disease to milder manifestations. The RPE65 P25L missense mutation, though having <10% of wild-type (WT) activity, causes relatively mild retinal degeneration. To better understand these mild forms of RPE65-related retinal degeneration, and their effect on cone photoreceptor survival, we generated an Rpe65/P25L knock-in (KI/KI) mouse model. We found that, when subject to the low-light regime (∼100 lux) of regular mouse housing, homozygous Rpe65/P25L KI/KI mice are morphologically and functionally very similar to WT siblings. While mutant protein expression is decreased by over 80%, KI/KI mice retinae retain comparable 11-cis-retinal levels with WT. Consistently, the scotopic and photopic electroretinographic (ERG) responses to single-flash stimuli also show no difference between KI/KI and WT mice. However, the recovery of a-wave response following moderate visual pigment bleach is delayed in KI/KI mice. Importantly, KI/KI mice show significantly increased resistance to high-intensity (20 000 lux for 30 min) light-induced retinal damage (LIRD) as compared with WT, indicating impaired rhodopsin regeneration in KI/KI. Taken together, the Rpe65/P25L mutant produces sufficient chromophore under normal conditions to keep opsins replete and thus manifests a minimal phenotype. Only when exposed to intensive light is this hypomorphic mutation manifested physiologically, as its reduced expression and catalytic activity protects against the successive cycles of opsin regeneration underlying LIRD. These data also help define minimal requirements of chromophore for photoreceptor survival in vivo and may be useful in assessing a beneficial therapeutic dose for RPE65 gene therapy in humans.

Fundus albipunctatus: review of the literature and report of a novel RDH5 gene mutation affecting the invariant tyrosine (p.Tyr175Phe)

Fundus albipunctatus (FA) is a rare, congenital form of night blindness with rod system impairment, characterised by the presence of numerous small, white-yellow retinal lesions. FA belongs to a heterogenous group of so-called flecked retina syndromes. This disorder shows autosomal recessive inheritance and is caused mostly by mutations in the RDH5 gene. This gene encodes the enzyme that is a part of the visual cycle, the 11-cis retinol dehydrogenase. This study is a brief review of the literature on FA and a report of the first molecular evidence for RDH5 gene mutation in a Polish patient with this rare disorder. We present a novel pathogenic RDH5 gene mutation in a 16-year-old female patient with symptoms of night blindness. The patient underwent ophthalmological examinations, including colour vision testing, fundus photography, automated visual field testing, full-field electroretinography (ERG) and spectral optical coherent tomography (SOCT). The patient showed typical FA ERG records, the visual field was constricted and fundus examination revealed numerous characteristic, small, white-yellowish retinal lesions. DNA sequencing of the RDH5 gene coding sequence (exons 2–5) enabled the detection of the homozygous missense substitution c.524A > T (p.Tyr175Phe) in exon 3. This is the first report of RDH5 gene mutation that affects the invariant tyrosine, one of the most conserved amino acid residues in short-chain alcohol dehydrogenases/reductases (SDRs), crucial for these enzymes’ activity. The location of this substitution, together with its predicted influence on the protein function, indicate that the p.Tyr175Phe mutation is the cause of FA in our patient.

Structural organization and function of mouse photoreceptor ribbon synapses involve the immunoglobulin protein synaptic cell adhesion molecule 1

Adhesive interactions in the retina instruct the developmental specification of inner retinal layers. However, potential roles of adhesion in the development and function of photoreceptor synapses remain incompletely understood. This contrasts with our understanding of synapse development in the CNS, which can be guided by select adhesion molecules such as the Synaptic Cell Adhesion Molecule 1 (SynCAM 1/CADM1/nectin-like 2 protein). This immunoglobulin superfamily protein modulates the development and plasticity of classical excitatory synapses. We show here by immunoelectron microscopy and immunoblotting that SynCAM 1 is expressed on mouse rod photoreceptors and their terminals in the outer nuclear and plexiform layers in a developmentally regulated manner. Expression of SynCAM 1 on rods is low in early postnatal stages (P3-P7) but increases after eye opening (P14). In support of functional roles in the photoreceptors, electroretinogram recordings demonstrate impaired responses to light stimulation in SynCAM 1 knockout (KO) mice. In addition, the structural integrity of synapses in the OPL requires SynCAM 1. Quantitative ultrastructural analysis of SynCAM 1 KO retina measured fewer fully assembled, triadic rod ribbon synapses. Furthermore, rod synapse ribbons are shortened in KO mice, and protein levels of Ribeye, a major structural component of ribbons, are reduced in SynCAM 1 KO retina. Together, our results implicate SynCAM 1 in the synaptic organization of the rod visual pathway and provide evidence for novel roles of synaptic adhesion in the structural and functional integrity of ribbon synapses.

Retinol dehydrogenases: membrane-bound enzymes for the visual function

Retinoid metabolism is important for many physiological functions, such as differenciation, growth, and vision. In the visual context, after the absorption of light in rod photoreceptors by the visual pigment rhodopsin, 11-cis retinal is isomerized to all-trans retinal. This retinoid subsequently undergoes a series of modifications during the visual cycle through a cascade of reactions occurring in photoreceptors and in the retinal pigment epithelium. Retinol dehydrogenases (RDHs) are enzymes responsible for crucial steps of this visual cycle. They belong to a large family of proteins designated as short-chain dehydrogenases/reductases. The structure of these RDHs has been predicted using modern bioinformatics tools, which allowed to propose models with similar structures including a common Rossman fold. These enzymes undergo oxidoreduction reactions, whose direction is dictated by the preference and concentration of their individual cofactor (NAD(H)/NADP(H)). This review presents the current state of knowledge on functional and structural features of RDHs involved in the visual cycle as well as knockout models. RDHs are described as integral or peripheral enzymes. A topology model of the membrane binding of these RDHs via their N- and (or) C-terminal domain has been proposed on the basis of their individual properties. Membrane binding is a crucial issue for these enzymes because of the high hydrophobicity of their retinoid substrates.

New syndrome with retinitis pigmentosa is caused by nonsense mutations in retinol dehydrogenase RDH11

Retinitis pigmentosa (RP), a genetically heterogeneous group of retinopathies that occur in both non-syndromic and syndromic forms, is caused by mutations in ∼100 genes. Although recent advances in next-generation sequencing have aided in the discovery of novel RP genes, a number of the underlying contributing genes and loci remain to be identified. We investigated three siblings, born to asymptomatic parents of Italian-American descent, who each presented with atypical RP with systemic features, including facial dysmorphologies, psychomotor developmental delays recognized since early childhood, learning disabilities and short stature. RP-associated ophthalmological findings included salt-and-pepper retinopathy, attenuation of the arterioles and generalized rod-cone dysfunction as determined by almost extinguished electroretinogram in 2 of 3 siblings. Atypical for RP features included mottled macula at an early age and peripapillary sparing of the retinal pigment epithelium. Whole-exome sequencing data, queried under a recessive model of inheritance, identified compound heterozygous stop mutations, c.C199T:p.R67* and c.C322T:p.R108*, in the retinol dehydrogenase 11 (RDH11) gene, resulting in a non-functional protein, in all affected children. In summary, deleterious mutations in RDH11, an important enzyme for vision-related and systemic retinoic acid metabolism, cause a new syndrome with RP.

The retinaldehyde reductase activity of DHRS3 is reciprocally activated by retinol dehydrogenase 10 to control retinoid homeostasis

The retinoic acid-inducible dehydrogenase reductase 3 (DHRS3) is thought to function as a retinaldehyde reductase that controls the levels of all-trans-retinaldehyde, the immediate precursor for bioactive all-trans-retinoic acid. However, the weak catalytic activity of DHRS3 and the lack of changes in retinaldehyde conversion to retinol and retinoic acid in the cells overexpressing DHRS3 undermine its role as a physiologically important all-trans-retinaldehyde reductase. This study demonstrates that DHRS3 requires the presence of retinol dehydrogenase 10 (RDH10) to display its full catalytic activity. The RDH10-activated DHRS3 acts as a robust high affinity all-trans-retinaldehyde-specific reductase that effectively converts retinaldehyde back to retinol, decreasing the rate of retinoic acid biosynthesis. In turn, the retinol dehydrogenase activity of RDH10 is reciprocally activated by DHRS3. At E13.5, DHRS3-null embryos have ∼4-fold lower levels of retinol and retinyl esters, but only slightly elevated levels of retinoic acid. The membrane-associated retinaldehyde reductase and retinol dehydrogenase activities are decreased by ∼4- and ∼2-fold, respectively, in Dhrs3(-/-) embryos, and Dhrs3(-/-) mouse embryonic fibroblasts exhibit reduced metabolism of both retinaldehyde and retinol. Neither RDH10 nor DHRS3 has to be itself catalytically active to activate each other. The transcripts encoding DHRS3 and RDH10 are co-localized at least in some tissues during development. The mutually activating interaction between the two related proteins may represent a highly sensitive and conserved mechanism for precise control over the rate of retinoic acid biosynthesis.

STRA6 is critical for cellular vitamin A uptake and homeostasis

Vitamin A must be adequately distributed within the body to maintain the functions of retinoids in the periphery and chromophore production in the eyes. Blood transport of the lipophilic vitamin is mediated by the retinol-binding protein, RBP4. Biochemical evidence suggests that cellular uptake of vitamin A from RBP4 is facilitated by a membrane receptor. This receptor, identified as the Stimulated by retinoic acid gene 6 (Stra6) gene product, is highly expressed in epithelia that constitute blood-tissue barriers. Here we established a Stra6 knockout mouse model to analyze the metabolic basis of vitamin A homeostasis in peripheral tissues. These mice were viable when bred on diets replete in vitamin A, but evidenced markedly reduced levels of ocular retinoids. Ophthalmic imaging and histology revealed malformations in the choroid and retinal pigmented epithelium, early cone photoreceptor cell death, and reduced lengths of rod outer segments. Similar to the blood-retina barrier in the RPE, vitamin A transport through the blood-cerebrospinal fluid barrier in the brain's choroid plexus was impaired. Notably, treatment with pharmacological doses of vitamin A restored vitamin A transport across these barriers and rescued the vision of Stra6(-/-) mice. Furthermore, under conditions mimicking vitamin A excess and deficiency, our analyses revealed that STRA6-mediated vitamin A uptake is a regulated process mandatory for ocular vitamin A uptake when RBP4 constitutes the only transport mode in vitamin A deficiency. These findings identifying STRA6 as a bona fide vitamin A transporter have important implications for disease states associated with impaired blood vitamin A homeostasis.

Comparison between the behavior of different hydrophobic peptides allowing membrane anchoring of proteins

Membrane binding of proteins such as short chain dehydrogenase reductases or tail-anchored proteins relies on their N- and/or C-terminal hydrophobic transmembrane segment. In this review, we propose guidelines to characterize such hydrophobic peptide segments using spectroscopic and biophysical measurements. The secondary structure content of the C-terminal peptides of retinol dehydrogenase 8, RGS9-1 anchor protein, lecithin retinol acyl transferase, and of the N-terminal peptide of retinol dehydrogenase 11 has been deduced by prediction tools from their primary sequence as well as by using infrared or circular dichroism analyses. Depending on the solvent and the solubilization method, significant structural differences were observed, often involving α-helices. The helical structure of these peptides was found to be consistent with their presumed membrane binding. Langmuir monolayers have been used as membrane models to study lipid-peptide interactions. The values of maximum insertion pressure obtained for all peptides using a monolayer of 1,2-dioleoyl-sn-glycero-3-phospho-ethanolamine (DOPE) are larger than the estimated lateral pressure of membranes, thus suggesting that they bind membranes. Polarization modulation infrared reflection absorption spectroscopy has been used to determine the structure and orientation of these peptides in the absence and in the presence of a DOPE monolayer. This lipid induced an increase or a decrease in the organization of the peptide secondary structure. Further measurements are necessary using other lipids to better understand the membrane interactions of these peptides.

Signatures of selection in the Iberian honey bee (Apis mellifera iberiensis) revealed by a genome scan analysis of single nucleotide polymorphisms

Understanding the genetic mechanisms of adaptive population divergence is one of the most fundamental endeavours in evolutionary biology and is becoming increasingly important as it will allow predictions about how organisms will respond to global environmental crisis. This is particularly important for the honey bee, a species of unquestionable ecological and economical importance that has been exposed to increasing human-mediated selection pressures. Here, we conducted a single nucleotide polymorphism (SNP)-based genome scan in honey bees collected across an environmental gradient in Iberia and used four FST -based outlier tests to identify genomic regions exhibiting signatures of selection. Additionally, we analysed associations between genetic and environmental data for the identification of factors that might be correlated or act as selective pressures. With these approaches, 4.4% (17 of 383) of outlier loci were cross-validated by four FST -based methods, and 8.9% (34 of 383) were cross-validated by at least three methods. Of the 34 outliers, 15 were found to be strongly associated with one or more environmental variables. Further support for selection, provided by functional genomic information, was particularly compelling for SNP outliers mapped to different genes putatively involved in the same function such as vision, xenobiotic detoxification and innate immune response. This study enabled a more rigorous consideration of selection as the underlying cause of diversity patterns in Iberian honey bees, representing an important first step towards the identification of polymorphisms implicated in local adaptation and possibly in response to recent human-mediated environmental changes.

Enzymology of retinoic acid biosynthesis and degradation

All-trans-retinoic acid is a biologically active derivative of vitamin A that regulates numerous physiological processes. The concentration of retinoic acid in the cells is tightly regulated, but the exact mechanisms responsible for this regulation are not completely understood, largely because the enzymes involved in the biosynthesis of retinoic acid have not been fully defined. Recent studies using in vitro and in vivo models suggest that several members of the short-chain dehydrogenase/reductase superfamily of proteins are essential for retinoic acid biosynthesis and the maintenance of retinoic acid homeostasis. However, the exact roles of some of these recently identified enzymes are yet to be characterized. The properties of the known contributors to retinoid metabolism have now been better defined and allow for more detailed understanding of their interactions with retinoid-binding proteins and other retinoid enzymes. At the same time, further studies are needed to clarify the interactions between the cytoplasmic and membrane-bound proteins involved in the processing of hydrophobic retinoid metabolites. This review summarizes current knowledge about the roles of various biosynthetic and catabolic enzymes in the regulation of retinoic acid homeostasis and outlines the remaining questions in the field.

Retinal degeneration in animal models with a defective visual cycle

Continuous generation of visual chromophore through the visual (retinoid) cycle is essential to maintain eyesight and retinal heath. Impairments in this cycle and related pathways adversely affect vision. In this review, we summarize the chemical reactions of vitamin A metabolites involved in the retinoid cycle and describe animal models of associated human diseases. Development of potential therapies for retinal disorders in these animal models is also introduced.

Fundus albipunctatus: novel mutations and phenotypic description of Israeli patients

PURPOSE

To characterize the genetic defects associated with fundus albipunctatus (FAP) in patients in Israel.

METHODS

Twenty patients with FAP from diverse ethnicities underwent ophthalmic and electroretinogram tests following the International Society for Clinical Electrophysiology of Vision protocol. Genomic DNA was extracted from peripheral blood. Mutation analysis of the 11-cis retinol dehydrogenase (RDH5) gene was performed with direct sequencing of PCR-amplified exons.

RESULTS

Four novel RDH5 gene mutations were identified. Of them, the null mutations c.343C>T (p.R54X) and c.242delTGCC were most prevalent. Macular involvement was present in two patients who carry different mutation types.

CONCLUSIONS

Mutation analysis of the RDH5 gene in the present series revealed four novel mutations and a previously reported one. No significant genotype-phenotype correlation was found.

Retinol dehydrogenase 13 protects the mouse retina from acute light damage

PURPOSE

To investigate whether retinol dehydrogenase 13 (RDH13) can protect the retina from acute light-induced damage.

METHODS

We generated Rdh13 knockout mice using molecular biologic methods and assessed the associated morphological and functional changes under room-light conditions by hematoxylin-eosin (H&E), transmission electron microscopy (TEM), and scotopic electroretinography. Then, the light-damage model was established by exposure to diffuse white light (3,000 lx) for 48 h. Twenty-four h after light exposure, H&E was used for the histological evaluation. The thickness of the outer-plus-inner-segment and the outer nuclear layer was measured on sections parallel to the vertical meridian of the eye. An electroretinography test was performed to assess the functional change. Furthermore, the impairment of mitochondria was detected by TEM. Finally, the expression of cytochrome c (CytC) and other apoptosis-related proteins was detected by western blot.

RESULTS

We found that there was no obvious difference in phenotype or function between Rdh13 knockout and wild-type mice. In Rdh13(-/-) mice subjected to intense light exposure, the photoreceptor outer-plus-inner-segment and outer nuclear layer were dramatically shorter, and the amplitudes of a- and b-waves under scotopic conditions were significantly attenuated. Distinctly swollen mitochondria with disrupted cristae were observed in the photoreceptor inner segments of Rdh13(-/-) mice. Increased expression levels of CytC, CytC-responsive apoptosis proteinase activating factor-1 (Apaf-1) and caspases 3, and other mitochondria apoptosis-related genes (nuclear factor-kappa B P65 [P65] and B-cell lymphoma 2-associated X protein [Bax]) were observed in Rdh13(-/-) mice.

CONCLUSIONS

Rdh13 can protect the retina against acute light-induced retinopathy. The mechanism may involve inhibition of the mitochondrial apoptosis pathway.

174delG mutation in mouse MFRP causes photoreceptor degeneration and RPE atrophy

PURPOSE

The authors have identified a recessive mutation causing progressive retinal degeneration, white fundus flecks, and eventual retinal pigment epithelium (RPE) atrophy. The goal of these studies was to characterize the retinal phenotype, to identify the causative locus, and to examine possible functions of the affected gene.

METHODS

SNP mapping, DNA sequencing, and genetic complementation were used to identify the affected locus. Histology, electroretinography, immunohistochemistry, Western blot analysis, fundus photography, electron microscopy, and in vitro phagocytosis assays were used to characterize the phenotype of the mouse.

RESULTS

Gene mapping identified a single base pair deletion in membrane-type frizzled related protein (MFRP), designated Mfrp(174delG). MFRP is normally expressed in the RPE and ciliary body but was undetectable by Western blot in mutants. CTRP5, a binding partner of MFRP, was upregulated at the mRNA level and at the protein level in most patients. Assays designed to test the integrity of retinoid cycling and phagocytic pathways showed no deficits in Mfrp(174delG) or rd6 animals. However, the RPE of both Mfrp(174delG) and rd6 mice exhibited a dramatic increase in the number of apical microvilli. Furthermore, evidence of RPE atrophy was evident in Mfrp(174delG) mice by 21 months.

CONCLUSIONS

The authors have identified a novel null mutation in mouse Mfrp. This mutation causes photoreceptor degeneration and eventual RPE atrophy, which may be related to alterations in the number of RPE microvilli. These mice will be useful to identify a function of MFRP and to study the pathogenesis of atrophic macular degeneration.

Evolution of retinoid and steroid signaling: vertebrate diversification from an amphioxus perspective

Although the physiological relevance of retinoids and steroids in vertebrates is very well established, the origin and evolution of the genetic machineries implicated in their metabolic pathways is still very poorly understood. We investigated the evolution of these genetic networks by conducting an exhaustive survey of components of the retinoid and steroid pathways in the genome of the invertebrate chordate amphioxus (Branchiostoma floridae). Due to its phylogenetic position at the base of chordates, amphioxus is a very useful model to identify and study chordate versus vertebrate innovations, both on a morphological and a genomic level. We have characterized more than 220 amphioxus genes evolutionarily related to vertebrate components of the retinoid and steroid pathways and found that, globally, amphioxus has orthologs of most of the vertebrate components of these two pathways, with some very important exceptions. For example, we failed to identify a vertebrate-like machinery for retinoid storage, transport, and delivery in amphioxus and were also unable to characterize components of the adrenal steroid pathway in this invertebrate chordate. The absence of these genes from the amphioxus genome suggests that both an elaboration and a refinement of the retinoid and steroid pathways took place at the base of the vertebrate lineage. In stark contrast, we also identified massive amplifications in some amphioxus gene families, most extensively in the short-chain dehydrogenase/reductase superfamily, which, based on phylogenetic and genomic linkage analyses, were likely the result of duplications specific to the amphioxus lineage. In sum, this detailed characterization of genes implicated in retinoid and steroid signaling in amphioxus allows us not only to reconstruct an outline of these pathways in the ancestral chordate but also to discuss functional innovations in retinoid homeostasis and steroid-dependent regulation in both cephalochordate and vertebrate evolution.