Reduced level of interphotoreceptor retinoid-binding protein (IRBP), a possible cause for retinal degeneration in the Abyssinian cat

CryoEM structure and small-angle X-ray scattering analyses of porcine retinol-binding protein 3

The vertebrate visual cycle hinges on enzymatically converting all-trans-retinol (at-ROL) into 11-cis-retinal (11c-RAL), the chromophore that binds to opsins in photoreceptors, forming light-responsive pigments. When struck by a photon, these pigments activate the phototransduction pathway and initiate the process of vision. The enzymatic isomerization of at-ROL, crucial for restoring the visual pigments and preparing them to receive new light stimuli, relies on various enzymes found in both the photoreceptors and retinal pigment epithelium cells. To function effectively, retinoids must shuttle between these two cell types. Retinol-binding protein 3 (RBP3), located in the interphotoreceptor matrix, probably plays a pivotal role in this transport mechanism. Comprised of four retinoid-binding modules, RBP3 also binds fatty acids, potentially aiding retinal function by facilitating the loading and unloading of different retinoids at specific cell types thereby directing the cycle. In this study, we present a 3.67 Å cryoEM structure of porcine RBP3, along with molecular docking analysis and corroborative in-solution small-angle X-ray scattering data for titration of RBP3 with relevant ligands, that also give insights on RBP3 conformational adaptability.

Towards a New Biomarker for Diabetic Retinopathy: Exploring RBP3 Structure and Retinoids Binding for Functional Imaging of Eyes In Vivo

Diabetic retinopathy (DR) is a severe disease with a growing number of afflicted patients, which places a heavy burden on society, both socially and financially. While there are treatments available, they are not always effective and are usually administered when the disease is already at a developed stage with visible clinical manifestation. However, homeostasis at a molecular level is disrupted before visible signs of the disease are evident. Thus, there has been a constant search for effective biomarkers that could signal the onset of DR. There is evidence that early detection and prompt disease control are effective in preventing or slowing DR progression. Here, we review some of the molecular changes that occur before clinical manifestations are observable. As a possible new biomarker, we focus on retinol binding protein 3 (RBP3). We argue that it displays unique features that make it a very good biomarker for non-invasive, early-stage DR detection. Linking chemistry to biological function and focusing on new developments in eye imaging and two-photon technology, we describe a new potential diagnostic tool that would allow rapid and effective quantification of RBP3 in the retina. Moreover, this tool would also be useful in the future to monitor therapeutic effectiveness if levels of RBP3 are elevated by DR treatments.

Interphotoreceptor Retinol-Binding Protein Ameliorates Diabetes-Induced Retinal Dysfunction and Neurodegeneration Through Rhodopsin

Patients with diabetes often experience visual defects before any retinal pathologies are detected. The molecular mechanism for the visual defects in early diabetes has not been elucidated. Our previous study reported that in early diabetic retinopathy (DR), rhodopsin levels were reduced due to impaired 11-cis-retinal regeneration. Interphotoreceptor retinol-binding protein (IRBP) is a visual cycle protein and important for 11-cis-retinal generation. IRBP levels are decreased in the vitreous and retina of DR patients and animal models. To determine the role of IRBP downregulation in the visual defects in early DR, we induced diabetes in transgenic mice overexpressing IRBP in the retina. IRBP overexpression prevented diabetes-induced decline of retinal function. Furthermore, IRBP overexpression also prevented decreases of rhodopsin levels and 11-cis-retinal generation in diabetic mice. Diabetic IRBP transgenic mice also showed ameliorated retinal oxidative stress, inflammation, apoptosis, and retinal degeneration compared with diabetic wild-type mice. These findings suggest that diabetes-induced IRBP downregulation impairs the regeneration of 11-cis-retinal and rhodopsin, leading to retinal dysfunction in early DR. Furthermore, increased 11-cis-retinal-free opsin constitutively activates the phototransduction pathway, leading to increased oxidative stress and retinal neurodegeneration. Therefore, restored IRBP expression in the diabetic retina may confer a protective effect against retinal degeneration in DR.

Interphotoreceptor Retinoid-Binding Protein (IRBP) in Retinal Health and Disease

Interphotoreceptor retinoid-binding protein (IRBP), also known as retinol binding protein 3 (RBP3), is a lipophilic glycoprotein specifically secreted by photoreceptors. Enriched in the interphotoreceptor matrix (IPM) and recycled by the retinal pigment epithelium (RPE), IRBP is essential for the vision of all vertebrates as it facilitates the transfer of retinoids in the visual cycle. It also helps to transport lipids between the RPE and photoreceptors. The thiol-dependent antioxidant activity of IRBP maintains the delicate redox balance in the normal retina. Thus, its dysfunction is suspected to play a role in many retinal diseases. We have reviewed here the latest research on IRBP in both retinal health and disease, including the function and regulation of IRBP under retinal stress in both animal models and the human retina. We have also explored the therapeutic potential of targeting IRBP in retinal diseases. Although some technical barriers remain, it is possible that manipulating the expression of IRBP in the retina will rescue or prevent photoreceptor degeneration in many retinal diseases.

ADIPOR1 is essential for vision and its RPE expression is lost in the Mfrprd6 mouse

The knockout (KO) of the adiponectin receptor 1 (AdipoR1) gene causes retinal degeneration. Here we report that ADIPOR1 protein is primarily found in the eye and brain with little expression in other tissues. Further analysis of AdipoR1 KO mice revealed that these animals exhibit early visual system abnormalities and are depleted of RHODOPSIN prior to pronounced photoreceptor death. A KO of AdipoR1 post-development either in photoreceptors or the retinal pigment epithelium (RPE) resulted in decreased expression of retinal proteins, establishing a role for ADIPOR1 in supporting vision in adulthood. Subsequent analysis of the Mfrprd6 mouse retina demonstrated that these mice are lacking ADIPOR1 in their RPE layer alone, suggesting that loss of ADIPOR1 drives retinal degeneration in this model. Moreover, we found elevated levels of IRBP in both the AdipoR1 KO and the Mfrprd6 models. The spatial distribution of IRBP was also abnormal. This dysregulation of IRBP hypothesizes a role for ADIPOR1 in retinoid metabolism.

Immune mechanisms in inflammatory and degenerative eye disease

It has recently been recognized that pathology of age-associated degenerative eye diseases such as adult macular degeneration (AMD), glaucoma and diabetic retinopathy, have strong immunological underpinnings. Attempts have been made to extrapolate to age-related degenerative disease insights from inflammatory processes associated with non-infectious uveitis, but these have not yet been sufficiently informative. Here we review recent findings on the immune processes underlying uveitis and those that have been shown to contribute to AMD, discussing in this context parallels and differences between overt inflammation and para-inflammation in the eye. We propose that mechanisms associated with ocular immune privilege, in combination with paucity of age-related antigen(s) within the target tissue, dampen what could otherwise be overt inflammation and result in the para-inflammation that characterizes age-associated neurodegenerative disease.

Neurodegeneration in the diabetic eye: new insights and therapeutic perspectives

Diabetic retinopathy (DR), one of the leading causes of preventable blindness, has been considered a microcirculatory disease of the retina. However, there is emerging evidence to suggest that retinal neurodegeneration is an early event in the pathogenesis of DR, which participates in the development of microvascular abnormalities. Therefore, the study of the underlying mechanisms leading to neurodegeneration and the identification of the mediators in the crosstalk between neurodegeneration and microangiopathy will be essential for the development of new therapeutic strategies. In this review, an updated discussion of the mechanisms involved in neurodegeneration, as well as the link between neurodegeneration and microangiopathy, is presented. Finally, the therapeutic implications and new perspectives based on identifying those patients with retinal neurodegeneration are given.

Interphotoreceptor retinoid-binding protein (IRBP) is downregulated at early stages of diabetic retinopathy

AIMS/HYPOTHESIS

Interphotoreceptor retinoid-binding protein (IRBP) plays a major role in the visual cycle and is essential to the maintenance of photoreceptors. The aim of this study was to determine whether a decrease in IRBP production exists in the early stages of diabetic retinopathy.

METHODS

Vitreous samples from diabetic patients with proliferative and non-proliferative diabetic retinopathy (PDR, NPDR), and from non-diabetic patients with macular hole (control group) were selected for IRBP quantitative assessment by proteomic analysis (fluorescence-based difference gel electrophoresis) and western blot. Human post mortem eyes (n = 16) from diabetic donors without clinically detectable retinopathy and from non-diabetic donors (n = 16) were used to determine IRBP (also known as RBP3) mRNA levels (RT-PCR) and protein content (western blot and confocal microscopy). Retinal neurodegeneration was assessed by measuring glial fibrillar acidic protein (GFAP) and the apoptotic rate. Y79 human retinoblastoma cells were used to test the effects of glucose, TNF-alpha and IL-1beta on IRBP expression and IRBP levels.

RESULTS

Intravitreous IRBP concentration was significantly lower in PDR < NPDR < control in proteomic and western blot analysis. IRBP mRNA levels and IRBP protein content were significantly lower in the retinas from diabetic donors than in those from non-diabetic donors. Increased GFAP and a higher degree of apoptosis were observed in diabetic retinas compared with non-diabetic retinas. A dose-dependent downregulation of IRBP mRNA expression and IRBP content was detected with glucose, TNF-alpha and IL-1beta in cultures of Y79 human retinoblastoma cells.

CONCLUSIONS/INTERPRETATION

Underproduction of IRBP is an early event in the human diabetic retina and is associated with retinal neurodegeneration. The mechanisms leading to this deficit deserve further investigation.

Immunohistochemical studies of cone photoreceptors and cells of the inner retina in feline rod-cone degeneration

Previous studies using electroretinography and immunohistochemistry have shown normal cone function and structure in early stages of hereditary rod-cone degeneration of Abyssinian cats. To further investigate the cone photoreceptors and the inner retina of dystrophic cats, antibodies against green- and blue-sensitive cones and specific cell types of inner retina were used in seven cats with the recessively inherited rod-cone degeneration, and three normal European short-haired cats. There was a reduction in number of both types of cones early in the disease. Changes at early stages of disease also occurred among horizontal cells in which there was an extension and a thickening of their lateral processes. The regular configuration of bipolar cells was changed in the more advanced stages of disease and their apical dendrites were lost. Abnormalities were not observed in the amacrine cells and in the ganglion cell layer in any of the present cases. This study shows that the cone system is morphologically abnormal in young cats at an earlier stage of disease than previously shown. The present findings also support the assumption that the inner retina is largely preserved throughout the disease process.

Studies of vitamin A metabolism in mouse model systems

Over the past several years, discoveries from mouse genetics have had direct impact on our understanding of vitamin A metabolism. Although the metabolism of vitamin A in the mouse does have some special features (for example very large stores of liver and pulmonary retinyl esters), the ability to construct knockout and transgenic mouse models has yielded an impressive amount of information directly relevant to understanding the general principles of vitamin A transport, storage and degradation. We discuss below the metabolism of vitamin A through a number of genetically engineered mouse strains with alterations in genes that affect this metabolism. The novelty of this experimental approach is evidenced by the fact that the oldest of these strains was first reported only eight years ago.1)

The zinc finger transcription factor, MOK2, negatively modulates expression of the interphotoreceptor retinoid-binding protein gene, IRBP

The human and murine MOK2 orthologue genes encode Krüppel/TFIIIA-related zinc finger proteins, which are factors able to recognize both DNA and RNA through their zinc finger motifs. MOK2 proteins have been shown to bind to the same 18-base pair (bp)-specific sequence in duplex DNA. This MOK2-binding site was found within introns 7 and 2 of human PAX3 and interphotoreceptor retinoid-binding protein (IRBP) genes, respectively. As these two genes are expressed in the brain as MOK2, we have suggested that PAX3 and IRBP genes are two potentially important target genes for the MOK2 protein. In this study, we focused our attention on IRBP as a potential MOK2 target gene. Sequence comparison and binding studies of the 18-bp MOK2-binding sites present in intron 2 of human, bovine, and mouse IRBP genes show that the 3'-half sequence is the essential core element for MOK2 binding. Very interestingly, 8-bp of this core sequence are found in a reverse orientation, in the IRBP promoter. We demonstrate that MOK2 can bind to the 8-bp sequence present in the IRBP promoter and repress its transcription when transiently overexpressed in retinoblastoma Weri-RB1 cells. In the IRBP promoter, it appears that the TAAAGGCT MOK2-binding site overlaps with the photoreceptor-specific CRX-binding element. We suggest that MOK2 represses transcription by competing with the cone-rod homeobox protein (CRX) for DNA binding, thereby decreasing transcriptional activation by CRX. Furthermore, we show that Mok2 expression in the developing mouse and in the adult retina seems to be concordant with IRBP expression.

Early onset photoreceptor abnormalities induced by targeted disruption of the interphotoreceptor retinoid-binding protein gene

Vision in all vertebrates is dependent on an exchange of retinoids between the retinal pigment epithelium and the visual photoreceptors. It has been proposed that the interphotoreceptor retinoid-binding protein (IRBP) is essential for this intercellular exchange, and that it serves to prevent the potentially cytotoxic effects of retinoids. Although its precise function in vivo has yet to be defined, the early expression of IRBP suggests that it may also be required for normal photoreceptor development. To further assess the biological role of IRBP, we generated transgenic mice with targeted disruption of the IRBP gene (IRBP-/- mice). Specifically, homologous recombination was used to replace the first exon and promoter region of the IRBP gene with a phosphoglycerate kinase-promoted neomycin-resistant gene. Immunocytochemical and Western blot analyses demonstrated the absence of IRBP expression in the IRBP-/- mice. As early as postnatal day 11, histological examination of the retinas of IRBP-/- mice revealed a loss of photoreceptor nuclei and changes in the structural integrity of the receptor outer segments. At 30 d of age, the photoreceptor abnormalities in IRBP-/- mice were more severe, and electroretinographic recordings revealed a marked loss in photic sensitivity. In contrast, no morphological or electrophysiological changes were detected in age-matched heterozygotes. These observations indicate that normal photoreceptor development and function are highly dependent on the early expression of IRBP, and that in the absence of IRBP there is a slowly progressive degeneration of retinal photoreceptors.

An early decrease in interphotoreceptor retinoid-binding protein gene expression in Abyssinian cats homozygous for hereditary rod-cone degeneration

Levels of interphotoreceptor retinoid-binding protein (IRBP) protein and message in retinas of Abyssinian cats homozygous for progressive rod-cone degeneration were determined at early ages, well before the onset of clinical retinal degeneration. IRBP gene expression was assessed by immunochemical quantitation of IRBP protein, and by Northern blotting and slot-blotting of total RNA using a human IRBP cDNA probe. Morphology was assessed by electron microscopy and immunocytochemistry. Levels of both IRBP protein and message in affected Abyssinian cat retinas were significantly reduced below normal as early as 4 weeks of age at the earliest stage of retinal disorientation. Opsin mRNA was more abundant in affected Abyssinian cat retinas than in control retinas. This was at least 1 year before the onset of clinical symptoms. The reduction in IRBP gene expression to levels significantly below normal well before the onset of retinal degeneration in affected Abyssinian cat retinas indicates that this represents a primary defect or at least an early problem that could itself cause adverse effects.

Expression of opsin and IRBP genes in mutant RCS rats

The retinal pigment epithelium of RCS rats bearing the autosomal recessive rdy mutation fails to ingest shed rod outer segment tips. Accumulation of disk debris in the subretinal space of the maturing mutant retina causes a secondary degeneration of photoreceptor cells. Two hypotheses have been offered as possible explanations of the death of photoreceptor cells in this disorder: (1) photoreceptors are starved for amino acids, retinal, oxygen, etc; and (2) that IRBP levels and synthesis may be decreased and interfere with retinal transport and this deficiency is lethal to these cells. To test these hypotheses, we have studied the effect of this mutation on the levels of expression of opsin and IRBP genes, and gene products and on rates of synthesis at various ages in dystrophic RCS p+ rats and compared the results to those obtained with normal Long Evans rats. The mutant rats and normal controls had comparable amounts of opsin and IRBP mRNA transcripts and rates of synthesis up to post-natal day 45 (P45) but opsin transcripts were barely detectable at P60 and thereafter. IRBP mRNA levels were also very low after P62 although somewhat higher than opsin mRNA. Opsin could be detected immunochemically, albeit at lower levels, at all the ages studied up to P310, but IRBP levels fell below detection after P45. We localized opsin and IRBP in the retina by post-embedding EM immunocytochemical procedures and found that opsin is present in the remnants of rod outer segment debris, even at P390, long after detectable opsin synthesis had ceased. These data suggest that expression of opsin and IRBP genes is not influenced by the shape and state of the outer segments, and that the rdy mutation does not influence the expression of the opsin and IRBP in these retinas until the photoreceptor cells are profoundly damaged. Thus, neither hypothesis about the causes of cell death in this disorder is supported.

Cellular immune responses to retinal antigens in retinitis pigmentosa

Patients with retinitis pigmentosa and a group of controls were tested for their cellular immune response toward two retinal proteins, S-antigen and interphotoreceptor retinoid-binding protein (IRBP), as well as their reaction against two synthetic peptides ("M" and "N") derived from the sequence of S-antigen and peptide "R14", derived from IRBP. Positive responses to the retinal antigens were found in larger proportions and with higher levels in the patient group than in the controls. The difference between the two groups was statistically significant in their response to S-antigen, but the patients reacted better than the controls against the other antigens as well. Of particular interest was the finding that several patients responded to both retinal proteins and/or to their peptides. These patients suffered from severe retinal changes and the data are thus interpreted as suggesting that the responses to the retinal antigens are secondary to these changes and to nonphysiological release of retinal antigens.

Retina-specific expression from the IRBP promoter in transgenic mice is conferred by 212 bp of the 5'-flanking region

IRBP is a photoreceptor-specific glycoprotein that has been suggested as a retinoid carrier in the visual process. Previous research has shown that 1.3 kb of 5'-flanking sequence from the human IRBP gene is sufficient to promote photoreceptor-specific expression of reporter genes in transgenic mice. To define more narrowly the sequences that promote tissue-specific expression, chimeric constructs with shorter promoters were used to generate transgenic mice. The bacterial CAT gene was fused to fragments of 706 bp or 212 bp from the 5' end of the human IRBP gene. Analysis of the three transgenic families bearing the 706 bp IRBP promoter revealed that CAT expression was confined to the neuro-retina and the pineal gland. Analysis of the four transgenic families bearing the 212 bp IRBP promoter revealed the same tissue-specific CAT expression in three families. These results establish that tissue-specific expression of IRBP can be regulated by a short 212 bp promoter which has been conserved between humans and mice.

Interphotoreceptor retinoid-binding protein (IRBP) in progressive rod-cone degeneration (prcd)--biochemical, immunocytochemical and immunologic studies

Interphotoreceptor retinoid-binding protein (IRBP) is synthesized and secreted by photoreceptor cells and is thought to facilitate the transport of retinoids during the visual cycle as well as fatty acids essential to the maintenance of normal outer segment membranes. Proteins such as IRBP, which are unique to the photoreceptor cells in the retina, are prime candidates in the consideration of biochemical defects which could contribute to photoreceptor cell degeneration in man and animals. In this study, the association between IRBP and retinal degeneration was examined using the progressive rod-cone degeneration (prcd) mutant retina in dogs as an animal model. This study shows that loss of IRBP is not an early occurrence in prcd. IRBP is present in relatively normal amounts and distribution even at 1.7 years of age, a time when there is extensive visual cell disease and degeneration. By 2.7-3.0 years of age, IRBP loss correlates with the severity of the disease and concomitant loss of photoreceptor cells. IRBP immunoreactivity was present in the interphotoreceptor matrix (IPM) as long as inner segments were present to a significant degree. The late loss of IRBP immunoreactivity seems to be, therefore, the result of advanced degeneration and end-stage atrophy of the retina. In addition, immunological studies were carried out in order to examine the possible role of an autoimmune response against IRBP in the disease cascade. Normal, heterozygote and prcd-affected dogs had measurable antibody titers to IRBP, but there was no correlation between disease state and antibody levels.

Photoreceptor degeneration and loss of immunoreactive GABA in the Abyssinian cat retina

GABA (gamma-amino butyric acid) and its synthesizing enzyme, GAD (glutamate decarboxylase; EC 4.1.1.15) were localized in the retina of Abyssinian cats homozygous for a recessively inherited retinal degenerative disorder which in several respects is similar to the human disease, retinitis pigmentosa. Clinically normal mongrel cats and heterozygous Abyssinian cats were studied for comparison. The GABA and GAD immunoreactive neurons of the heterozygous or young homozygous (clinically unaffected animals) had the same distribution and morphology as normal mongrel European type cats. The neuronal GABA immunoreactivity in both the inner and outer parts of the retina gradually disappeared in the course of the disease, with little or no loss of GAD immunoreactive neurons. Early in the disease, the changes were most severe in patches in the mid periphery of the eye and then spread both centrally and peripherally. Loss of photoreceptors was a prerequisite for the loss of GABA immunoreactivity. The observations show that retinal changes are not limited to the photoreceptors. The GABA loss is not likely to be due to a loss of neurons, because of the persistence of GAD immunoreactive neurons.