X-box binding protein 1 is essential for the anti-oxidant defense and cell survival in the retinal pigment epithelium

Damage to the retinal pigment epithelium (RPE) is an early event in the pathogenesis of age-related macular degeneration (AMD). X-box binding protein 1 (XBP1) is a key transcription factor that regulates endoplasmic reticulum (ER) homeostasis and cell survival. This study aimed to delineate the role of endogenous XBP1 in the RPE. Our results show that in a rat model of light-induced retinal degeneration, XBP1 activation was suppressed in the RPE/choroid complex, accompanied by decreased anti-oxidant genes and increased oxidative stress. Knockdown of XBP1 by siRNA resulted in reduced expression of SOD1, SOD2, catalase, and glutathione synthase and sensitized RPE cells to oxidative damage. Using Cre/LoxP system, we generated a mouse line that lacks XBP1 only in RPE cells. Compared to wildtype littermates, RPE-XBP1 KO mice expressed less SOD1, SOD2, and catalase in the RPE, and had increased oxidative stress. At age 3 months and older, these mice exhibited apoptosis of RPE cells, decreased number of cone photoreceptors, shortened photoreceptor outer segment, reduced ONL thickness, and deficit in retinal function. Electron microscopy showed abnormal ultrastructure, Bruch's membrane thickening, and disrupted basal membrane infolding in XBP1-deficient RPE. These results indicate that XBP1 is an important gene involved in regulation of the anti-oxidant defense in the RPE, and that impaired activation of XBP1 may contribute to RPE dysfunction and cell death during retinal degeneration and AMD.

ELOVL4 protein preferentially elongates 20:5n3 to very long chain PUFAs over 20:4n6 and 22:6n3

We hypothesized that reduction/loss of very long chain PUFAs (VLC-PUFAs) due to mutations in the ELOngase of very long chain fatty acid-4 (ELOVL4) protein contributes to retinal degeneration in autosomal dominant Stargardt-like macular dystrophy (STGD3) and age-related macular degeneration; hence, increasing VLC-PUFA in the retina of these patients could provide some therapeutic benefits. Thus, we tested the efficiency of elongation of C20-C22 PUFA by the ELOVL4 protein to determine which substrates are the best precursors for biosynthesis of VLC-PUFA. The ELOVL4 protein was expressed in pheochromocytoma cells, while green fluorescent protein-expressing and nontransduced cells served as controls. The cells were treated with 20:5n3, 22:6n3, and 20:4n6, either individually or in equal combinations. Both transduced and control cells internalized and elongated the supplemented FAs to C22-C26 precursors. Only ELOVL4-expressing cells synthesized C28-C38 VLC-PUFA from these precursors. In general, 20:5n3 was more efficiently elongated to VLC-PUFA in the ELOVL4-expressing cells, regardless of whether it was in combination with 22:6n3 or with 20:4n6. In each FA treatment group, C34 and C36 VLC-PUFAs were the predominant VLC-PUFAs in the ELOVL4-expressing cells. In summary, 20:5n3, followed by 20:4n6, seems to be the best precursor for boosting the synthesis of VLC-PUFA by ELOVL4 protein.

Alpha-phenyl-N-tert-butylnitrone (PBN) prevents light-induced degeneration of the retina by inhibiting RPE65 protein isomerohydrolase activity

α-Phenyl-N-tert-butylnitrone (PBN), a free radical spin trap, has been shown previously to protect retinas against light-induced neurodegeneration, but the mechanism of protection is not known. Here we report that PBN-mediated retinal protection probably occurs by slowing down the rate of rhodopsin regeneration by inhibiting RPE65 activity. PBN (50 mg/kg) protected albino Sprague-Dawley rat retinas when injected 0.5-12 h before exposure to damaging light at 2,700 lux intensity for 6 h but had no effect when administered after the exposure. PBN injection significantly inhibited in vivo recovery of rod photoresponses and the rate of recovery of functional rhodopsin photopigment. Assays for visual cycle enzyme activities indicated that PBN inhibited one of the key enzymes of the visual cycle, RPE65, with an IC(50) = 0.1 mm. The inhibition type for RPE65 was found to be uncompetitive with K(i) = 53 μm. PBN had no effect on the activity of other visual cycle enzymes, lecithin retinol acyltransferase and retinol dehydrogenases. Interestingly, a more soluble form of PBN, N-tert-butyl-α-(2-sulfophenyl) nitrone, which has similar free radical trapping activity, did not protect the retina or inhibit RPE65 activity, providing some insight into the mechanism of PBN specificity and action. Slowing down the visual cycle is considered a treatment strategy for retinal diseases, such as Stargardt disease and dry age-related macular degeneration, in which toxic byproducts of the visual cycle accumulate in retinal cells. Thus, PBN inhibition of RPE65 catalytic action may provide therapeutic benefit for such retinal diseases.

Retinal sphingolipids and their very-long-chain fatty acid-containing species

PURPOSE

Recent evidence suggests that ceramide metabolism plays an important role in retinal photoreceptor cell survival and apoptosis. The purpose of this study was to characterize sphingolipids in the retina with special emphasis on the very-long-chain-containing saturated (VLC-FA) and polyunsaturated (VLC-PUFA) fatty acid-containing species. The VLC-FAs and VLC-PUFAs are synthesized by the ELOVL4 protein, which is involved in human Stargardt's macular dystrophy type 3 (STGD3).

METHODS

Total lipids were extracted from retina and other tissues, and different sphingolipid classes were isolated and purified using various combinations of liquid- and solid-phase separation. Purified sphingolipids were analyzed by high-performance thin layer chromatography (HPTLC), gas chromatography (GC), and GC-MS (GC-mass spectrometry).

RESULTS

Nonsialylated sphingolipids (NSLs) comprised approximately 3.5% of total retinal lipids of which 70% was sphingomyelin. Ceramide and glycosylceramides (GCs) constituted<or=1% of total retinal lipids. Gangliosides (GGs), on the other hand, comprised approximately 3.0% of total retinal lipids. Fatty acid analysis of retinal NSLs indicated an abundance of saturated fatty acids, with the presence of VLC-FAs but not of VLC-PUFAs beyond 24 carbons. However, GG had significant levels of unsaturated, polyunsaturated, and VLC-PUFAs. Retinal rod outer segments (ROS) contained approximately 1% each of NSL and GG, and their fatty acid profile was not very different from whole retinal NSL and GG, respectively.

CONCLUSIONS

Retina has a total of 6% to 7% fatty acids that are N-linked to a sphingosine, which would be 11 to 13 mole % in comparison to phospholipids. The presence of VLC-FAs and VLC-PUFAs in retinal sphingolipids indicates that they may play role in ELOVL4-mediated Stargardt 3.

Retinal very long-chain PUFAs: new insights from studies on ELOVL4 protein

Compared with other mammalian tissues, retina is highly enriched in PUFA. Long-chain PUFA (LC-PUFA; C18-C24) are essential FAs that are enriched in the retina and are necessary for maintenance of normal retinal development and function. The retina, brain, and sperm also contain very LC-PUFA (VLC-PUFA; >C24). Although VLC-PUFA were discovered more than two decades ago, very little is known about their biosynthesis and functional roles in the retina. This is due mainly to intrinsic difficulties associated with working on these unusually long polyunsaturated hydrocarbon chains and their existence in small amounts. Recent studies on the FA elongase elongation of very long chain fatty acids-4 (ELOVL4) protein, however, suggest that VLC-PUFA probably play some uniquely important roles in the retina as well as the other tissues. Mutations in the ELOVL4 gene are found in patients with autosomal dominant Stargardt disease. Here, we review the recent literature on VLC-PUFA with special emphasis on the elongases responsible for their synthesis. We focus on a novel elongase, ELOVL4, involved in the synthesis of VLC-PUFA, and the importance of these FAs in maintaining the structural and functional integrity of retinal photoreceptors.

Retinol dehydrogenase 12 detoxifies 4-hydroxynonenal in photoreceptor cells

Mutations of the photoreceptor retinol dehydrogenase 12 (RDH12) gene cause the early onset retinal dystrophy Leber congenital amaurosis (LCA) by mechanisms not completely resolved. Determining the physiological role of RDH12 in photoreceptors is the focus of this study. Previous studies showed that RDH12, and the closely related retinol dehydrogenase RDH11, can enzymatically reduce toxic lipid peroxidation products such as 4-hydroxynonenal (4-HNE), in vitro. To explore the significance of this activity, we investigated the ability of RDH11 and RDH12 to protect stably transfected HEK-293 cells against the toxicity of 4-HNE. Both enzymes protected against 4-HNE modification of proteins and 4-HNE-induced apoptosis in HEK-293 cells. In the retina, exposure to bright light induced lipid peroxidation, 4-HNE production, and 4-HNE modification of proteins in photoreceptor inner segments, where RDH11 and RDH12 are located. In mouse retina, RDH12-but not RDH11-protected against adduct formation, suggesting that 4-HNE is a physiological substrate of RDH12. RDH12-but not RDH11-also protected against light-induced apoptosis of photoreceptors. We conclude that in mouse retina RDH12 reduces 4-HNE to a nontoxic alcohol, protecting cellular macromolecules against oxidative modification and protecting photoreceptors from light-induced apoptosis. This activity is of particular significance to the understanding of the molecular mechanisms of RDH12-induced LCA.

Curcumin protects retinal cells from light-and oxidant stress-induced cell death

Age-related macular degeneration (AMD) is a complex disease that has potential involvement of inflammatory and oxidative stress-related pathways in its pathogenesis. In search of effective therapeutic agents, we tested curcumin, a naturally occurring compound with known anti-inflammatory and antioxidative properties, in a rat model of light-induced retinal degeneration (LIRD) and in retina-derived cell lines. We hypothesized that any compound effective against LIRD, which involves significant oxidative stress and inflammation, would be a candidate for further characterization for its potential application in AMD. We observed significant retinal neuroprotection in rats fed diets supplemented with curcumin (0.2% in diet) for 2 weeks. The mechanism of retinal protection from LIRD by curcumin involves inhibition of NF-kappaB activation and down-regulation of cellular inflammatory genes. When tested on retina-derived cell lines (661W and ARPE-19), pretreatment of curcumin protected these cells from H(2)O(2)-induced cell death by up-regulating cellular protective enzymes, such as HO-1, thioredoxin. Since, curcumin with its pleiotropic activities can modulate the expression and activation of many cellular regulatory proteins such as NF-kappaB, AKT, NRF2, and growth factors, which in turn inhibit cellular inflammatory responses and protect cells; we speculate that curcumin would be an effective nutraceutical compound for preventive and augmentative therapy of AMD.

Age-related retinal degeneration (arrd2) in a novel mouse model due to a nonsense mutation in the Mdm1 gene

We observed that a naturally occurring mouse strain developed age-related retinal degeneration (arrd2). These mice had normal fundi, electroretinograms (ERGs) and retinal histology at 6 months of age; vessel attenuation, RPE atrophy and pigmentary abnormalities at 14 months, which progressed to complete loss of photoreceptors and extinguished ERG by 22 months. Genetic analysis revealed that the retinal degeneration in arrd2 segregates in an autosomal recessive manner and the disease gene localizes to mouse chromosome 10. A positional candidate cloning approach detected a nonsense mutation in the mouse double minute-1 gene (Mdm1), which results in the truncation of the putative protein from 718 amino acids to 398. We have identified a novel transcript of the Mdm1 gene, which is the predominant transcript in the retina. The Mdm1 transcript is localized to the nuclear layers of neural retina. Expression of Mdm1 in the retina increases steadily from post-natal day 30 to 1 year, and a high level of Mdm1 are subsequently maintained. The Mdm1 transcript was found to be significantly depleted in the retina of arrd2 mice and the transcript was observed to degrade by nonsense-mediated decay. These results indicate that the depletion of the Mdm1 transcript may underlie the mechanism leading to late-onset progressive retinal degeneration in arrd2 mice. Analysis of a cohort of patients with age-related macular degeneration (AMD) wherein the susceptibility locus maps to chromosome 12q, a region bearing the human ortholog to MDM1, did not reveal association between human MDM1 and AMD.

CTRP5 is a membrane-associated and secretory protein in the RPE and ciliary body and the S163R mutation of CTRP5 impairs its secretion

PURPOSE

In a prior study, a S163R mutation in the complement-1q tumor necrosis factor-related protein 5 (CTRP5/ C1QTNF5) was reported to be associated with early-onset long anterior zonules (LAZ) and late-onset retinal degeneration (L-ORD). The ocular tissues involved in the phenotype are the retinal pigment epithelium (RPE) in the posterior segment and ciliary epithelium (CE) and lens in the anterior segment. The purpose of this study was to characterize the spatial and temporal expression of the mouse Ctrp5 gene, determine tissue and subcellular localization, and study the effect of the S163R mutation.

METHODS

The expression of the Ctrp5 gene in the mouse was studied by quantitative (q)RT-PCR and in situ hybridization. CTRP5 protein expression and distribution were studied by Western blot analysis, immunohistochemistry, and immunoelectron microscopy. Cellular location of wild-type and mutant CTRP5 in MDCK and COS-7 cells was determined by immunofluorescence and immunoblot analysis.

RESULTS

A significant level of Ctrp5 expression was detected in the adult mouse in the ciliary body (CB) and RPE, and expression started at a very early stage of embryogenesis. Immunohistochemical analysis showed CTRP5 protein in the apical processes of the RPE and forming a hexagonal lattice associated with the RPE lateral membranes. In the ciliary body, CTRP5 was localized to the apical aspects of the CE, the region between the bilayered ciliary epithelial cells. The membrane association of CTRP5 in the RPE and CE was further confirmed by immunoelectron microscopy. Furthermore, cultured cells were used to show that the CTRP5 is a secretory protein and that its secretion is impaired by the S163R mutation.

CONCLUSIONS

CTRP5, a secretory and membrane-associated protein, is localized to the lateral and apical membranes of the RPE and CB. Impaired secretion of the mutant protein may underlie the pathophysiology of L-ORD and LAZ.

Spatial and temporal expression of MFRP and its interaction with CTRP5

PURPOSE

Mutations in the membrane frizzled-related protein (MFRP) gene cause nanophthalmos in humans, and a splice site mutation causes recessive retinal degeneration in the rd6 mouse. In human and mouse genomes, the MFRP gene lies adjoining to the complement 1q tumor necrosis factor-related protein 5 (CTRP5/C1QTNF5) gene involved in causing retinal degeneration and abnormal lens zonules in human. The purpose of this study was to characterize the spatial and temporal expression of the mouse Mfrp gene, determine tissue and subcellular localization of MFRP protein, and study its interaction with CTRP5.

METHODS

Expression of the Mfrp gene in the mouse was studied by quantitative (q)RT-PCR. MFRP protein expression and distribution were studied by Western blot analysis, immunohistochemistry, and immunoelectron microscopy. Interaction with CTRP5 was studied by immunoprecipitation and immunoblot analysis, using mouse eye and human retinal pigmented epithelium (RPE) choroid extracts and by expressing full-length CTRP5 and MFRP in a heterologous system.

RESULTS

The Mfrp gene is specifically expressed in RPE and ciliary body (CB), and its expression starts during early stages of embryogenesis. In the albino mouse eye, MFRP is localized to the apical and basal membranes of RPE and ciliary epithelium (CE). In addition, MFRP and CTRP5 were found to colocalize in RPE, CE, and MDCK cells, a general model of polarized epithelia. These proteins interact with each other in ocular tissues and also in a heterologous system.

CONCLUSIONS

MFRP is localized to the plasma membrane of CE and RPE, and colocalizes and interacts with CTRP5 indicating a functional relationship between these two proteins.

Complement factor H: spatial and temporal expression and localization in the eye

PURPOSE

Complement factor H (CFH) is a component of the mammalian complement system, which regulates the alternative pathway of complement activation and protects the host cell from inappropriate complement activation. CFH is a key regulator of innate immunity, and CFH deficiency leads to membranoproliferative glomerulonephritis type II. A variation in human CFH, Y402H, has been shown to be associated with an increased risk for age-related macular degeneration. The authors describe studies on the spatial and temporal expression of the CFH gene and localization of this protein in ocular tissues to gain insight into its role in the eye.

METHODS

CFH expression in human and mouse tissues was studied by quantitative RT-PCR and Western blot analysis, and localization of CFH was studied by immunohistochemical analysis followed by fluorescence microscopy.

RESULTS

In human and mouse, CFH expression was found to be similar to the highest level of expression in the liver. In ocular tissue, CFH was detected in the distalmost optic nerve (3 mm) cut from the scleral surface of the eyeball, sclera, RPE-choroid, retina, lens, and ciliary body. In mouse, Cfh expression was observed from early embryonic stages, and in the eye its expression increased with age.

CONCLUSIONS

A significant level of CFH expression is maintained in different ocular tissues during development and aging. Sustained high levels of CFH expression in eye tissues suggest that this protein may play a role in protecting these tissues from indiscriminate complement activation and inflammatory insult.

Elovl45-bp–Deletion Knock-in Mice Develop Progressive Photoreceptor Degeneration

PURPOSE

To develop and characterize a heterozygous knock-in mouse model carrying the 5-bp deletion in Elovl4 (E_mut+/-) and to study the pathology underlying Stargardt-like macular degeneration (STGD3).

METHODS

E_mut+/- mice were generated by targeting a 5-bp deletion (AACTT) in the Elovl4 gene by homologous recombination. E_mut+/- mice of age 2 to 18 months and age-matched wild-type (Wt) littermate control animals were analyzed for the expression of Elovl4 transcript, ELOVL4 protein, photoreceptor-specific genes, and retinal fatty acid composition. Functional retinal changes were evaluated by electroretinography (ERG) and by morphologic and ultrastructural criteria.

RESULTS

E_mut+/- mice retinas showed the presence of both Wt and mutant Elovl4 transcripts and proteins. Morphologic evaluation revealed cone photoreceptor ultrastructural abnormalities as early as 2 months of age, accumulation of lipofuscin in retinal pigment epithelium (RPE), and subretinal deposits at later ages. Shortening of rod outer segments (OS) was observed at approximately 10 months of age. Both cone and rod changes progressed with age. Unlike rod-specific genes, expression of selected cone specific genes was significantly reduced by 7 months of age. Mixed rod-cone and light-adapted b-waves were higher than normal at both 8 and 15 months. Levels of the fatty acids 20:5 (P = 0.027), 22:5 (P = 0.040) and 24:6 (P = 0.005) were found to be significantly lower in the retinas of E_mut+/- mice than in retinas of control subjects.

CONCLUSIONS

E_mut+/- animals display characteristic features associated with Stargardt-like macular degeneration and serve as a model for the study of the mechanism underlying STGD3.

Haploinsufficiency is not the key mechanism of pathogenesis in a heterozygous Elovl4 knockout mouse model of STGD3 disease

PURPOSE

Autosomal dominant Stargardt-like (STGD3) disease results from mutations in the ELOVL4 gene (elongation of very-long-chain fatty acids). This study was undertaken to characterize a mouse model with a targeted deletion of Elovl4 and to explore the role of this gene in retinal/macular degeneration.

METHODS

A construct targeted to exon 2 of the Elovl4 gene was used to suppress expression of the gene. Elovl4 homozygous pups were nonviable and were not available for study. Hence, the analysis was performed on heterozygous Elovl4(+/-) mice 16 to 22 month of age and littermate wild-type (WT) control mice of the same age. Characterization included examining gene message and protein levels, electroretinogram (ERG), retinal morphology and ultrastructure, and plasma and retinal fatty acid composition.

RESULTS

Although the level of Elovl4 mRNA was reduced in Elovl4(+/-) retinas, only minimal morphologic abnormalities were found, and the retinal (ERG) function was essentially normal in Elovl4(+/-) retinas compared with the WT control retinas. Systemic fatty acid profiles of Elovl4(+/-) mice were unremarkable, although the concentration of several fatty acids was significantly lower in Elovl4(+/-) retinas, particularly the monounsaturated fatty acids.

CONCLUSIONS

The detailed characterization of this animal model provides the first in vivo evidence that Elovl4 haploinsufficiency is not the underlying key disease mechanism in STGD3. The results are consistent with a dominant negative mechanism for the deletion mutation. The Elovl4 knockout mouse is one of three complementary animal models that will help elucidate the disease mechanism.

Sequencing arrays for screening multiple genes associated with early-onset human retinal degenerations on a high-throughput platform

PURPOSE

To develop and apply microarray-based resequencing technology to detect sequence alterations in multiple autosomal recessive retinal disease genes on a single high-throughput platform.

METHODS

Oligonucleotides corresponding to both strands of the target exons and the flanking intron sequences of 29,214 bp from 11 genes associated with autosomal recessive retinitis pigmentosa (arRP) were tiled on 20 x 25-microm microarrays (arRP-I arrays). A total of 155 exons were amplified from 35 arRP patient DNA samples, with each sample being sequenced on an arRP-I chip by hybridization.

RESULTS

With the arRP-I arrays, 97.6% of the tiled sequence were determined with more than 99% accuracy and reproducibility. Of the 2.4% unread sequence, 89.5% involved stretches of G or C. In analyzing the 903,140-bp sequence from the 35 patient samples, 506 sequence changes have been detected in which 386 are previously reported alterations, and 120 are novel. In addition to four known causative mutations, six novel sequence changes that are potentially pathogenic were observed. Additional analysis is needed to determine whether these changes are responsible for arRP in these patients.

CONCLUSIONS

The use of microarray for sequencing is a novel approach, and the arRP-I chip is the first successful application of this technology for determining sequence alteration in multiple disease-related genes. These arrays can be used for high-throughput genotyping of patients with relevant retinal conditions. In addition, these arrays offer a unique opportunity to interrogate complex patterns of inheritance due to the involvement of more than one gene by screening multiple genes on a single platform.

Late-onset macular degeneration and long anterior lens zonules result from a CTRP5 gene mutation

PURPOSE

To identify the gene responsible for a complex ocular phenotype of late-onset macular degeneration, long anterior zonules (LAZ), and elevated intraocular pressure (IOP) and to study its expression.

METHODS

Ocular examination, visual field, fluorescein angiography, and electrophysiology testing were performed. One affected individual was treated with vitamin A. DNA from 55 family members (UM:H389) was used for linkage, mapping, and mutation analysis. Linkage analysis of macular degeneration and LAZ phenotypes was performed independently. Mutations in candidate genes were screened by sequencing. mRNA expression of CTRP5 and MFRP, which are bicistronic genes, was studied by semiquantitative RT-PCR (qRT-PCR) in various human tissues. CTRP5 expression was also evaluated by in situ hybridization.

RESULTS

Affected members had LAZ detectable by the third decade and/or macular degeneration by the fourth to fifth decade. A six-month treatment with vitamin A shortened dark adaptation considerably in one affected member. Both conditions mapped independently with zero recombination to 11q23, with maximum lod scores of 3.31 for macular degeneration and 5.41 for LAZ. The same CTRP5 missense mutation was identified in all affected individuals. Retinal pigment epithelium (RPE) and ciliary epithelium (CE) showed highest CTRP5 transcript expression, which was also true for MFRP. CTRP5 tissue expression was confirmed by in situ hybridization.

CONCLUSIONS

A single locus at 11q23 is implicated in a complex ocular phenotype involving RPE and CE, tissues of neuroectodermal origin. All individuals with either LAZ and/or macular degeneration carry the same CTRP5 S163R mutation, which is transmitted in autosomal dominant manner.

Characterization of mouse orthologue of ELOVL4: genomic organization and spatial and temporal expression

Mutations in ELOVL4 are associated with dominant macular degeneration (adMD/STGD3). This gene is highly expressed in the retina and is conserved through evolution. Here we report the genomic organization of the mouse orthologue of ELOVL4 and its temporal and spatial expression. A significant amount of ELOVL4 mRNA expression is detected in the adult retina, brain, skin, testis, and lens. During development, expression is first noted at embryonic day 7 (E7). A significant level of the mRNA is observed both in brain and in eyes at postnatal day 1 (P1), after which levels decrease in the brain and increase in the retina until they stabilize at P30. ELOVL4 protein is evident in the ocular tissues by E10.5 and becomes restricted predominantly to the photoreceptor layer in the mature retina. These observations suggest that ELOVL4 may play an important role in embryonic development and in maintaining normal physiology of retina and brain at later stages of development.