Expression of protein kinase C isoenzymes in cultured hooded rat retinal pigmented epithelial cells: comparison with dystrophic Royal College of Surgeons rat

PURPOSE

To determine the isoenzymes of Protein Kinase C (PKC) that are present in cultured retinal pigmented epithelial (RPE) cells from both the Lister hooded rat and the Royal College of Surgeons (RCS) rat.

METHODS

PKC isoenzymes were determined using immunocytochemistry and electrophoresis / Western blotting.

RESULTS

Analyses showed the presence of six isoenzymes of PKC (PKCalpha, beta1, beta2, delta, epsilon and zeta) within cultured RPE cells derived from Lister hooded rats. Significantly, however, cultures of cells derived from dystrophic Royal College of Surgeons rats differed in PKCdelta and PKCalpha isoenzyme expression as compared with cultured hooded rat RPE cells.

CONCLUSIONS

Rat RPE cells express several PKC isoenzymes. Differences in expression of PKCalpha and PKCdelta in RCS rat RPE cells may relate to the decreased phagocytic capability of these cells as compared with those of the Lister hooded rat.

Differential expression of nitric oxide synthase isoforms in form-deprived chick eyes

PURPOSE

To clarify whether nitric oxide synthase (NOS) is involved in development of myopia, we examined the influence of form deprivation on the expressions of NOS isoform mRNA.

METHODS

NOS isoform cDNAs were amplified from total RNA extracted from control and 7-day-form-deprived chick retina-RPE (retinal pigment epithelium)-choroid, using competitive RT-PCR (reverse-transcription polymerase chain reaction). Each NOS isoform protein was also analyzed by Western blotting and immunohistochemistry.

RESULT

Expression of inducible NOS (iNOS) mRNA was highest in the control chick retina-RPE-choroid, followed by the expression of brain NOS (bNOS) mRNA. Expression of endothelial NOS (eNOS) mRNA was faint. The iNOS protein level, however, was only slightly higher than the levels of the bNOS and eNOS proteins and was found mainly in the outer part of the photoreceptor layer and inner and outer parts of RPE and choroid. bNOS alone was found in the outer nuclear layer. Although form deprivation reduced the iNOS and bNOS mRNA expressions, only the iNOS protein showed significant reduction.

CONCLUSION

All three NOS isoforms were expressed in chick retina-RPE-choroid. Predominant expression of iNOS, instead of bNOS and eNOS, suggested the existence of ocular tissue-specific regulation of the iNOS gene. In addition to differences in expression level, bNOS displayed regional differential expression. Moreover, only iNOS was reduced in response to form deprivation. It is suggested that NOS isoforms may be differentially involved in the mechanisms regulating the posterior eye tissues, including myopic eye growth.

Changes in the redox state in the retina and brain during the onset of diabetes in rats

Diabetic retinopathy is thought to result from chronic changes in the metabolic pathways of the retina. Hyperglycemia leads to increased intracellular glucose concentrations, alterations in glucose degradation and an increase in lactate/pyruvate ratio. We measured lactate content in retina and other ocular and non-ocular tissues from normal and diabetic rats in the early stages of streptozotocin-induced diabetes. The intracellular redox state was calculated from the cytoplasmic [lactate]/[pyruvate] ratio. Elevated lactate concentration were found in retina and cerebral cortex from diabetic rats. These concentrations led to a significant and progressive decrease in the NAD+/NADH ratio, suggesting that altered glucose metabolism is an initial step of retinopathy. It is thus possible that tissues such as cerebral cortex have mechanisms that prevent the damaging effect of lactate produced by hyperglycemia and/or alterations of the intracellular redox state.

The visual cycle retinol dehydrogenase: possible involvement in the 9-cis retinoic acid biosynthetic pathway

The 11-cis-retinol dehydrogenase (11-cis-RoDH) gene encodes the short-chain alcohol dehydrogenase responsible for 11-cis-retinol oxidation in the visual cycle. The structure of the murine 11-cis-RoDH gene was used to reinvestigate its transcription pattern. An 11-cis-RoDH gene transcript was detected in several non-ocular tissues. The question regarding the substrate specificity of the enzyme was therefore addressed. Recombinant 11-cis-RoDH was found capable of oxidizing and reducing 9-cis-, 11-cis- and 13-cis-isomers of retinol and retinaldehyde, respectively. Dodecyl-beta-1-maltoside used to solubilize the enzyme was found to affect the substrate specificity. This is the first report on a visual cycle enzyme also present in non-retinal ocular and non-ocular tissues. A possible role in addition to its role in the visual cycle is being discussed.

Recombinant adenovirus-mediated gene delivery into the rat retinal pigment epithelium in vivo

PURPOSE

The present paper describes changes following the subretinal injection of a recombinant replication-deficient adenovirus carrying the beta-galactosidase reporter gene construct into the rat retina.

METHODS

Ad.RSV.betagal-mediated transduction of rat retinal pigment epithelium (RPE) cells in vitro and in vivo was examined following X-gal staining.

RESULTS

There was a low level of beta-galactosidase expression in the RPE cells at 4 days postinjection. At 7 days postinjection, a strong transgene expression was present in RPE cells and the expression was maintained at 14 days postinjection. Except for the accumulation of cells at the site of the injection, the morphology of the rest of the retina remained normal.

CONCLUSION

These results demonstrate that the RPE layer can be successfully targeted for gene delivery in the rat.

[Effects of Hirudo therapy on lipid peroxidation and catalase activity of the rabbit retina after light-induced eye injuries]

Effect of Hirudo therapy on the level of lipid peroxidation (LPO) products and catalase activity in the retina and pigmented epithelium (PE) after exposure to intensive illumination is assessed. Light injury to the eye (100,000 lux in 90 min) leads to accumulation of LPO products and suppression of catalase activity. Hirudo therapy after potent light exposure of the eye decreased the level of hydroperoxide and malonic dialdehyde and normalized retinal and PE catalase activity, thus exerting a therapeutic antioxidant effect. Hirudo therapy before illumination did not notably protect the eye as regards the content of LPO products and catalase activity.

Expression of tyrosinase and the tyrosinase related proteins in the Mitfvit (vitiligo) mouse eye: implications for the function of the microphthalmia transcription factor

Mitf (Microphthalmia transcription factor), a basic-helix-loop-helix zipper protein, encoded at the microphthalmia (Mitf) locus, regulates the transcription of the gene encoding tyrosinase, the rate-limiting enzyme in melanin biosynthesis, by binding the DNA sequence CATGTG. This binding site is present also in the genes encoding two tyrosinase related proteins, TRP-1 and TRP-2. To gain insight into the function of Mitf in vivo, we determined whether there was a difference in the levels of these proteins in the RPE/choroid of the vitiligo (Mitfvit) mouse, in which there is a mutation of the Mitf gene. This mouse has alteration of RPE pigmentation and function that presumably leads to slow progressive loss of photoreceptor cells. The RPE/choroid was dissected from eyes of vitiligo and C57BL/6 wild-type mice at postnatal ages 2, 4, 7, 10, 14, 21 and 42 days. Extracts of pooled tissues were subjected to electrophoresis and immunoblotting. The levels of tyrosinase, TRP-1 and TRP-2 were determined densitometrically following immunodetection with rabbit antipeptide antisera. In addition, the tyrosine hydroxylase activity of tyrosinase as assayed radiometrically. Levels of TRP-1 were 3-7 fold greater in control RPE/choroid compared with mutants. This marked difference in protein level was observed at the earliest age examined (P2) and persisted throughout the first two weeks. Tyrosinase levels in mutants were similar to controls at P2 and P4, but were reduced at P10 and beyond. Tyrosinase activity was diminished also in mutants by P10. Levels of TRP-2 were similar between mutants and controls, although the typical decrease seen in controls after P14 was attenuated in the mutant mice. There is a significant reduction in the level of TRP-1 in the RPE/choroid of the Mitfvit mouse. The data suggests that transcription of the gene encoding TRP-1 is extremely dependent upon functional Mitf. It provides in vivo evidence that Mitf regulates the transcription of the gene encoding TRP-1 as well as tyrosinase.

Culture of human retinal pigment epithelial cells from peripheral scleral flap biopsies

PURPOSE

We studied various methods for harvesting retinal pigment epithelium (RPE) biopsies from cadaver human eyes of donors over age 60 years. Our goal was to harvest cells for possible autologous RPE cell transplantation in patients with age-related macular degeneration and to test the viability of the RPE after isolation by evaluating explant growth in culture.

METHODS

Choroid-RPE biopsies were excised from enucleated human eyes. The RPE was separated from the choroid by treatment with type IV collagenase. RPE patches were cultured. After 100-500 cells had grown out from the explant, the primary cultures were passaged.

RESULTS

There was no clear effect of donor age on the ability to establish primary RPE cultures with good morphology from biopsies 2 x 2-10 x 10 mm2 in size. Biopsies 6 x 6 mm2 or larger produced satisfactory primary cultures more than 70% of the time. The number of viable RPE cells (defined as the number of cells adherent to the culture dish 24 h after plating) obtained after enzymatic separation of the RPE and choroid was an important determinant of our ability to establish primary cultures and passage the cells. Primary cultures with good cellular morphology were obtained 100% of the time when RPE explants > 4 mm2 in size were obtained from the biopsy specimen. Seventy-three percent of the biopsies yielding explants > 4 mm2 in size were successfully passaged.

CONCLUSIONS

These results suggest that peripheral scleral flap biopsies in aging donors can be used to establish RPE explant primary cultures. These cultures may be suitable as a source for autologous RPE transplantation in patients.

Prevention of ornithine cytotoxicity by proline in human retinal pigment epithelial cells

PURPOSE

To investigate the relationship between ornithine-delta-aminotransferase (OAT) deficiency and ornithine accumulation and the specific degeneration of retinal pigment epithelial (RPE) cells in gyrate atrophy.

METHODS

Human RPE cells, human hepatoma cells, and human fibroblast cells were treated with 5-fluoromethylornithine (5-FMOrn), a specific irreversible inhibitor of OAT. Ornithine cytotoxicity was determined by using a [3H]thymidine incorporation assay and immunohistochemical staining for cytokeratin. The effects of various metabolites of ornithine and arginine, such as creatine, creatine phosphate, I-delta 1-pyrroline-5-carboxylic acid (L-P5C), and proline, which may be deficient in gyrate atrophy on RPE cell damage by ornithine, were determined by the same procedures.

RESULTS

When the human RPE cells, HepG2 hepatoma cells, and WI-38 fibroblast cells were treated with 0.5 mM 5-FMOrn for 30 minutes, which inactivated OAT, ornithine exhibited severe time- and dose-dependent inhibition of DNA synthesis in the human RPE cells but not in the HepG2 hepatoma cells or WI-38 fibroblast cells. The inhibition of DNA synthesis was accompanied by drastic changes in morphologic appearance, disorganization of the cytoskeleton, and cell death. Ornithine or 5-FMOrn alone did not exhibit such cytotoxicity to the RPE cells. Proline prevented the cytotoxicity of ornithine.

CONCLUSIONS

These findings suggest that an elevated level of ornithine combined with an increased sensitivity to ornithine as a result of OAT deficiency may be crucial to the specific RPE degeneration in gyrate atrophy. They suggest also that abnormalities of proline metabolism may be involved in the progress of gyrate atrophy.

Substrate specificity of retinyl ester hydrolase activity in retinal pigment epithelium

In the eye, hydrolysis of stored retinyl esters is catalyzed by retinyl ester hydrolase (REH) activities in retinal pigment epithelium (RPE) membranes. In the present study, biochemical analyses were conducted to determine the substrate specificity of these activities. Specific activities determined for hydrolysis of various retinol isomers of retinyl palmitate (9-cis-, 11-cis-, 13-cis-, and all-trans-retinyl palmitates) indicated that 11-cis-retinyl palmitate is preferentially hydrolyzed (1.7 nmol/min/mg) compared to the other isomers (0.1-0.3 nmol/min/mg). Examination of the specificity of REH activity for 11-cis-retinyl esters of varied acyl chain length (-myristate, -palmitate, and -stearate) and degree of saturation (-oleate and -linoleate) further demonstrated that palmitate is the preferred fatty acyl moiety. Notably, retinyl esters possessing chain lengths which more closely approximate that of the palmitate ester exhibited higher rates of hydrolysis. Similar results were obtained in retinyl ester-plasma membrane fusion studies in which hydrolysis took place within the membrane domain rather than at the lipid-water interface. REH substrate specificity was further assessed in competition studies in which 11-cis-retinyl palmitate hydrolysis was monitored in the presence of 13-cis-, 9-cis-, or all-trans-retinyl palmitate. Results show that addition of these retinyl palmitate isomers does not affect the rate of hydrolysis of 11-cis-retinyl palmitate. However, the hydrolytic rates associated with other retinyl palmitate isomers were significantly reduced in the presence of 11-cis-retinyl palmitate. Finally, cholesterol ester hydrolase activity was found to be distinct from the observed 11-cis-REH activity and the presence of cholesterol oleate did not affect the rate of 11-cis-retinyl palmitate hydrolysis. Collectively, these data support the hypothesis that a distinct, membrane-associated, 11-cis-retinyl palmitate-specific retinyl ester hydrolase activity exists in the retinal pigment epithelium.

Regulation of isomerohydrolase activity in the visual cycle

While the overall biosynthetic pathway leading from all-trans-retinoids to 11-cis-retinoids in the visual cycle is understood, little is known about which step(s) may be rate-limiting and how control is exerted. One possible target for control is the isomerohydrolase, which processes all-trans-retinyl esters into 11-cis-retinol. The basal rate of 11-cis-retinol synthesis from all-trans-retinyl esters is extremely slow using bovine retinal pigment epithelial membranes [3.5 pmol of 11-cis-retinol min-1 (mg of protein)-1], and only small amounts of 11-cis-retinyl ester are formed. However, the addition of retinol binding proteins stimulates 11-cis-retinol formation by a factor of approximately 13. Specific protein-protein interactions are probably unimportant because bovine serum albumin and the physiologically relevant cellular retinaldehyde binding protein (CRALBP) both stimulate 11-cis-retinol formation to the same extent, although CRALBP does so at much lower concentrations. The relatively rapid rate of isomerization in the presence of binding proteins [44.3 pmol of 11-cis-retinol min-1 (mg of protein)-1] suggests that the rate-limiting enzyme in the visual cycle need not be the isomerohydrolase. Also, 11-cis-retinol is shown to inhibit isomerohydrolase, providing a simple mechanism for regulation of the visual cycle and the stimulating effect of binding proteins.

Localization of lipocalin-type prostaglandin D synthase (beta-trace) in iris, ciliary body, and eye fluids

PURPOSE

Prostaglandin (PG) D synthase is present in neural tissues and cerebrospinal fluid (beta-trace). This enzyme belongs to the lipocalin family which consists of transporter proteins for lipophilic substances in the extracellular space. PGD synthase is found in retinal pigment epithelium, from where it is secreted into the interphotoreceptor matrix. The authors have undertaken the localization of this unique enzyme within the tissues and spaces of the anterior segment of the eye.

METHODS

Iris, ciliary body, lens, and aqueous and vitreous humors were collected from adult rats and mice. PGD synthase activity was determined, and the protein was quantified by Western blot analysis and localized immunohistochemically. Finally, in situ hybridization was performed to localize PGD synthase mRNA.

RESULTS

PGD synthase was most abundant in the aqueous and vitreous humors. It was less abundant in tissue cytosolic fractions; these fractions had almost 10-fold as much as their corresponding membrane-bound fractions. Lens tissue had the lowest amount observed. PGD synthase was localized to the epithelial cells of the iris and the ciliary body and to the adjacent extracellular chambers, but PGD synthase mRNA was found only within the epithelial cells. Several glycosylated forms of PGD synthase were also detected.

CONCLUSIONS

PGD synthase was synthesized within the epithelial cells of the iris and the ciliary body and was then secreted into the aqueous and vitreous humors, where it accumulated as an active enzyme.

"Oxidative protector" enzymes in the macular retinal pigment epithelium of aging eyes and eyes with age-related macular degeneration

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Inhibition of inducible nitric oxide synthase expression by interferons alpha and beta in bovine retinal pigmented epithelial cells

Bovine retinal pigmented epithelial (RPE) cells express an inducible nitric oxide synthase (NOS-2) after activation with interferon (IFN)-gamma and lipopolysaccharide (LPS). Experiments were performed to investigate the effects of IFN-alpha and IFN-beta on NOS-2 activity. These types of interferons did not aid LPS in the production of nitrite, but markedly inhibited in a concentration-dependent manner the nitrite release due to LPS/IFN-gamma. Analysis by Western and Northern blots showed that RPE cells co-stimulated with IFN-alpha or IFN-beta with LPS/IFN-gamma accumulated lower levels of NOS-2 protein and mRNA than in the presence of LPS/IFN-gamma alone. The presence of IFN-alpha or IFN-beta did not accelerate mRNA degradation, implying that these interferons did not affect NOS-2 mRNA stability, but more probably NOS-2 gene expression. Furthermore, IFN-gamma binding studies demonstrated that the inhibitory effect of IFN-alpha and IFN-beta is not caused by a blocking of IFN-gamma receptors. Analysis of NF-kappaB activation by electrophoretic mobility shift assay demonstrated that LPS/IFN-gamma-induced NF-kappaB binding was not changed by the presence of IFN-alpha. However, similar experiments revealed that the activation of interferon regulatory factor-1 (IRF-1) by LPS/IFN-gamma was decreased by IFN-alpha. This phenomenon could be due to the decline of IRF-1 mRNA and the up-regulation of IRF-2 mRNA, an IRF-1 repressor, by IFN-alpha. These results suggest that the inhibitory effect of IFN-alpha and -beta on NOS-2 induction could be partially explained by their effect on the induction of the IRFs, which were involved in NOS-2 gene transcription.

Control of nitric oxide production by endogenous TNF-alpha in mouse retinal pigmented epithelial and Muller glial cells

Since the induction of nitric oxide synthase (NOS) by lipopolysaccharide (LPS) has been suggested to be partially dependent of the synthesis of tumor necrosis factor alpha (TNF alpha), we have investigated in vitro the production of NO in retinal cells from mice deficient in Lymphotoxin alpha (LT alpha)/TNF alpha. Treatment of retinal Müller glial (RMG) and retinal pigmented epithelial (RPE) cells from both wild-type and knockout mice with LPS and interferon gamma (IFN gamma) induced NO synthesis as determined by nitrite release into the media and was correlated to an increase in NOS-2 mRNA levels, evaluated by RT-PCR. However, the level of nitrite and the accumulation of mRNA was always less in cells from LT alpha/TNF alpha knockout mice than in wild type mice. Simultaneous addition of TNF alpha restored the level of NO synthesis by RMG and RPE cells from LT alpha/TNF alpha knockout mice stimulated with LPS and IFN gamma to wild type levels. Transforming growth factor beta (TGF beta) blocked LPS/IFN gamma-induced NO production is RMG and RPE cells from wild-type and LT alpha/TNF alpha knockout mice. Our results demonstrate that induction of NO synthesis in RMG and RPE cells by LPS and IFN gamma is dependent in part on endogenous TNF alpha while inhibition of NO production by TGF beta does not require a modulation of TNF alpha synthesis.

Analysis of esterification of retinoids in the retinal pigmented epithelium of the Mitf-vit (vitiligo) mutant mouse

PURPOSE

Mice homozygous for the vitiligo mutation of the microphthalmia (Mitf) gene have a retinal degeneration characterized by slow loss of photoreceptor cells and compromised retinal pigment epithelial (RPE) structure and function. The levels of retinyl esters, which are essential for generation of 11-cis-retinaldehyde for the formation of rhodopsin, were reported previously to be elevated by 6 weeks postnatally in the RPE of vitiligo mutant mice. The purpose of the present study was to determine whether this elevation was due to increased activity of lecithin:retinol acyl transferase (LRAT) the enzyme that converts all-trans-retinol to retinyl esters.

METHODS

Retinoids extracted from the RPE and neural retina of mutant and normal mice ages 2, 4, 6 and 8 weeks were analyzed by reversed-phase HPLC. The esterification capacity of the RPE to convert 3H-retinol to 3H-retinyl ester was determined by HPLC in mutant and normal mice at 3 and 9 weeks.

RESULTS

Retinyl ester levels were elevated significantly in the mutant RPE as early as postnatal week 2 and were four-fold greater by 8 weeks. The esterification assay indicated no significant differences between mutants and controls at 3 weeks. At 9 weeks, the esterification activity of the mutant RPE was significantly reduced compared to controls rather than elevated.

CONCLUSIONS

The data suggest that the accumulation of retinyl esters is not due to increased LRAT activity. Alternative explanations for the retinyl ester accumulation are discussed.

Bovine retinal pigment epithelium contains novel types of phospholipase A2

We have recently demonstrated the presence of phospholipase A2 (PLA2) activity in cells from bovine retinal pigment epithelium (RPE) [Jacob et al. (1996) J. Biol. Chem. 271, 19209-19218]. We report here our results on the characterization of this RPE-PLA2 activity. We show that RPE probably contains two types of PLA2 enzyme, as indicated by the results obtained with different PLA2-active fractions eluted from cation-exchange columns and treated with Ca2+/EGTA, dithiothreitol, p-bromophenacyl bromide or heat. These results, in addition to those from PLA2 assays using different substrates, also suggest that RPE-PLA2 enzymes are different from the well-known secretory, cytoplasmic and Ca2+-independent forms. Sequential extraction of RPE with (1) isotonic, (2) hypertonic and (3) detergent-containing PBS argues for the presence of weakly membrane-associated enzymes. Control experiments using 'back and forth' TLC allowed us to discriminate between PLA2 and phospholipase C/diacylglycerol lipase activity and confirmed that, in our assay conditions, the release of fatty acids was indeed due to PLA2 enzymes. These results, together with those obtained by treating RPE homogenates with H2SO4, guanosine 5'-[gamma-thio]triphosphate, ATP and different protease inhibitors, permitted us to make the first characterization of these RPE-PLA2 enzymes. We conclude that RPE contains novel types of PLA2 that are different from the secretory, cytoplasmic and Ca2+-independent forms.

Zinc induces catalase expression in cultured fetal human retinal pigment epithelial cells

PURPOSE

We have previously shown that an experimental, low-zinc environment decreased catalase activity in cultured human fetal retinal pigment epithelial (RPE) cells. The purpose of this study was to investigate the effect of zinc supplementation on catalase expression in cultured human fetal RPE cells.

METHODS

Confluent fetal RPE cells incubated in Coon's modified Ham's F12 (CMF-12) were treated (18 h) with zinc chloride (ZnCl2) (15, 30, or 100 microM) to assess changes in catalase enzyme activity or for 6 h to assess the induction of catalase mRNA by Northern analysis and in situ hybridization. RPE cells were also treated with 30 microM ZnCl2 for 2, 6, 24, 48 and 72 h to assess the time course of changes in catalase enzyme activity, changes in mRNA levels and status of the Sp1 transcription factor.

RESULTS

Catalase activity was increased above control by the addition of 15, 30 and 100 microM ZnCl2. Catalase gene expression was induced by 30 microM zinc in 6 h, but decreased to non-treated control levels by 24 h. The transcription factor Sp1 was also activated by zinc treatment (30 microM) which peaked at 2 h and declined to non-treated control levels by 24 h. Catalase enzyme activity peaked at 24 h and decreased to control levels by 72 h.

CONCLUSIONS

Our results demonstrate that zinc treatment of RPE cells increases catalase expression and activates the transcription factor Sp1. The results suggest zinc may play a role in the transcriptional regulation of catalase in RPE cells.

Modification of glutamine synthetase expression by mammalian Müller (glial) cells in retinal organ cultures

One of the key enzymes in glial-neuronal transmitter recycling is glutamine synthetase (GS). In the retina, GS is exclusively expressed by glial (Müller) cells where it serves to convert neuron-released active transmitter substances (glutamate and GABA) into glutamine. Experiments on avian retinae have shown that GS expression is developmentally regulated by glucocorticoid hormones and, to a lesser extent, by a non-hormonal control mechanism(s). Much less is known about GS regulation in mammalian retinae, although either increases or decreases of GS immunoreactivity have been observed in Müller cells in different forms of retinal pathologies. We studied GS expression in postnatal rabbit retinae both in vivo and explanted as wholemounts in vitro, using immunocytochemistry and Western immunoblotting. GS expression was detectable in vivo from the fourth postnatal day, and increased rapidly within the first weeks of life. Levels were lower in vitro than in vivo by an order of magnitude, and could be significantly stimulated (> 60-110%) in vitro by application of hydrocortisone, conditioned medium from cultured retinal pigment epithelium and glutamate or ammonia, but not GABA. It is concluded that GS expression in mammalian Müller cells is dependent on systemic control by glucocorticoid hormones, as observed in birds, but environmental (activity-dependent) factors may play a more important role in mammals.

[Metalloproteinase stromelysin. Expression in human retinal pigment epithelium (RPE)]

BACKGROUND

Under normal circumstances the retinal pigment epithelium (RPE) does not undergo cell division. After retinal detachment or the development of choroidal neovascularization (CNV) it can be induced to reenter the cell cycle. The RPE cells proliferate and dedifferentiate. A prerequisite for proliferation of RPE cells is degradation of the extracellular matrix (ECM), which may be induced by metalloproteinases. We investigated the potential role of human RPE cells in the expression of the metalloproteinase stromelysin.

MATERIALS AND METHODS

Human RPE cells were cultured from donor eyes. Stromelysin was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) from mRNA.

RESULTS

Human RPE cells in culture express stromelysin. Its expression is enhanced by tetraphorbolacetate (TPA).

CONCLUSIONS

Stromelysin generally degrades important constituents of the ECM. This may induce the detachment of RPE cells from the basement membrane and initiate RPE proliferation and dedifferentiation.

Membrane-associated carbonic anhydrase in cultured rabbit nonpigmented ciliary epithelium

PURPOSE

To measure the activity of membrane-associated carbonic anhydrase (CA) in cultured rabbit nonpigmented epithelial (NPE) cells, determine its identity and its sensitivity to extracellular trypsin, and compare the ability of acetazolamide and a cell-impermeant dextran-bound CA inhibitor to change cytoplasmic pH.

METHODS

Studies were conducted using a cell line derived from rabbit NPE. The cells were lysed and separated into soluble and insoluble fractions by differential centrifugation. CA activity in these fractions was determined using a CO2 hydration assay. In studies with intact cells, a membrane-impermeable high-molecular-weight dextran-bound inhibitor (DBI) was synthesized and used to selectively bind and inhibit the extracellular-facing membrane-bound CA. Measurements of CA activity in intact red blood cells were conducted to confirm DBI remains extracellular. Acetazolamide, a membrane-permeable CA inhibitor, was used to inhibit total CA activity. Intracellular pH was determined using the pH-dependent absorbance of the fluorescent dye BCECF-AM.

RESULTS

A low-speed pellet enriched with plasma membrane material accounted for 22.3 +/- 6.1% (n = 18) of the total CA activity in the cultured NPE. When intact cells were exposed to trypsin-EDTA, a 28% reduction of membrane-associated CA activity was observed; DBI inhibited this CA activity loss. Cytosolic CA activity was inhibited by 0.2% sodium dodecyl sulfate (SDS). In contrast, membrane-associated CA was SDS resistant, a characteristic of the CA-IV isozyme. By Western blot, CA-IV immunoreactive polypeptide was detected in the cultured cells and also in native rabbit and porcine ciliary epithelium. Inhibition of total CA activity with acetazolamide and inhibition of extracellular-facing membrane-associated CA with DBI caused an identical intracellular pH decrease in intact NPE cells.

CONCLUSIONS

Expression of the CA-IV isozyme could account for the significant fraction of CA activity in the cultured NPE, which is membrane associated and SDS resistant. Sensitivity to tryptic hydrolysis suggests the membrane-associated CA partially faces extracellularly. As judged by responses to an extracellular CA inhibitor, the membrane-associated CA has a functional role in maintaining cytoplasmic pH.

Age-related increase in activity of specific lysosomal enzymes in the human retinal pigment epithelium

Age related changes in the activity of lysosomal enzymes have been studied in the cultured human retinal pigment epithelium cells collected from 26-85 year old donors. Among four such enzymes studied, activities of cathepsin D and beta-glucuronidase increased with the age of the donors while no notable change in activity of arylsulfatase B and alpha-mannosidase was observed. Kinetic parameters of beta-glucuronidase was measured in retinal pigment epithelium cells isolated from donors of different ages. Similar kinetic parameters for beta-glucuronidase at different ages suggest that the observed increase in the activity of the enzyme with age is not due to post-translational modification of the enzyme. Western blot analysis provides evidence for increased synthesis of beta-glucuronidase with aging. Relative proportions of glycosaminoglycans, the natural substrates of beta-glucuronidase and arylsulfatase B, in the retinal pigment epithelium altered with the age of the donors. A significant decrease of dermatan sulfate levels with aging correlates well with the observed increase in the level of beta-glucuronidase activity.

The effects of elevated glucose on Na+/K(+)-ATPase of cultured bovine retinal pigment epithelial cells measured by a new nonradioactive rubidium uptake assay

The effects of stimulated hyperglycemia on the Na+/K(+)-ATPase activity of cultured bovine retinal pigment epithelial (RPE) cells were investigated. Total Rb+ uptake, measured by a chromatographic method, was decreased 20-30% by 55.5 mM glucose relative to 5.55 mM glucose for culture periods of 2 to 28 days. An acute hyperglycemic stress (< 1 week) had no effect on ouabain-inhibition of Rb+ uptake or ouabain binding to RPE cells (IC50 = 55 nM for both processes) and did not alter the IC50 value (near 10 nM) for binding of strophanthidin, another selective Na+/K(+)-ATPase inhibitor. A small increase in the apparent K(m) of Rb+ for Na+/K(+)-ATPase accompanied the decrease in maximal Rb+ uptake at 55.5 mM glucose. The continuous presence of AL-1576, an aldose reductase inhibitor (ARI), normalized the effect of severe hyperglycemia on Rb+ uptake in the chronic (28 days) but not the acute exposure protocols. Thus, decreased efficiency of Na+/K(+)-ATPase caused by chronic accumulation of intracellular sorbitol can account for previously reported functional and structural alterations in the RPE cell layer of diabetic rodents. The results of the present study suggest that hyperglycemia-induced loss of Na+/K(+)-ATPase function in RPE cells, which responds to aldose reductase inhibitor treatment, contributes to the pathogenesis of diabetic retinopathy.

Retrovirus-mediated transfer of the suicide gene into retinal pigment epithelial cells in vitro

PURPOSE

Human retinal pigment epithelial (HRPE) cells are a major cell component in proliferative vitreoretinopathy (PVR) membranes. We investigated the feasibility of killing HRPE cells by retroviral-mediated transfer of the herpes simplex virus-thymidine kinase (HSV-tk) gene, also known as the suicide gene, into HRPE cells followed by ganciclovir treatment, and to study the so-called bystander effect. Such a treatment plan might serve as a possible therapy for PVR.

METHODS

Transduction efficiency was determined using retroviral vectors encoding the beta-galactosidase reporter gene. To evaluate the efficacy of suicide gene therapy. HRPE cells were transduced with retroviral vectors encoding the HSV-tk gene (G1TkSvNa), with empty vectors or without vectors, and were treated with 5 micrograms/ml ganciclovir. Sensitivity of HSV-tk positive HRPE cells to various concentrations of ganciclovir was evaluated. To demonstrate the bystander effect, HSV-tk positive cells were cultured with HSV-tk negative cells at varying proportions.

RESULTS

Transduction efficiency in vitro was 15.1 +/- 4.8%. Cell growth was significantly inhibited after transduction with G1TkSvNa followed by ganciclovir treatment (P < 0.01). Ganciclovir showed dose- and time-dependent cytotoxicity only on HSV-tk positive cells. The concentration that resulted in 50% inhibited was 0.1 micrograms/ml. In terms of the bystander effect, after ganciclovir treatment the viability of co-cultured cells decreased with increasing populations of HSV-tk positive cells.

CONCLUSIONS

HRPE cells were successfully transduced with the HSV-tk gene via retroviral vectors and displayed a strong bystander effect after treatment with ganciclovir. These results suggest that retrovirus-mediated suicide gene therapy might be a feasible treatment strategy for PVR.

Modulation of cathepsin D activity in retinal pigment epithelial cells

This project used retinal pigment epithelial (RPE) cells to investigate the effects of up- and down-regulation of cathepsin D expression on the processing of cathepsin D and on the normal phagocytic and digestive function of these cells. RPE cells were transfected with a pHbetaApr-1-neo vector construct carrying the full-length sequence of the translated region of human cathepsin D in sense and antisense directions. Transfected cells were characterized for the presence and expression of the transgene by PCR amplification using transgene-specific primers. Total aspartic proteinase activity present in transformed RPE cells was measured by an enzyme assay using haemoglobin as substrate. Flow cytometry was used to quantify phagocytosis of fluorescein isothiocyanate-labelled rod outer segments (ROS), and lysosomal digestion of ROS was monitored by immunofluorescence. A 435 bp fragment was present in RPE cells carrying the cathepsin D transgene in sense and antisense orientations after PCR amplification. Expression of both 52 kDa procathepsin D and 34 kDa active cathepsin D was significantly up-regulated in sense cathepsin D-transfected RPE cells and down-regulated in RPE cells transfected with antisense cathepsin D. No other forms of cathepsin D were detected in the transfected cells, suggesting that, if pseudo-cathepsin D exists in RPE cells in vivo, it requires the presence of unknown specific regulatory elements. The up- and down-regulation of cathepsin D expression was further confirmed by enzyme assay. Transfected cells retained their phagocytosing ability after ROS challenge and maintained their ability to process ROS. The processing of ROS was significantly slower in RPE cells transfected with antisense than control vector or in sense-cathepsin D-transfected cells. These results demonstrate that cathepsin D is a major proteolytic enzyme participating in the lysosomal digestion of photoreceptor outer segments.

Matrix metalloproteinase-2 and tissue inhibitor of metalloproteinase-1 in the retinal pigment epithelium and interphotoreceptor matrix: vectorial secretion and regulation

Matrix metalloproteinases (MMPs) and the tissue inhibitors of metalloproteinases (TIMPs) play an essential role in both normal and pathological extracellular matrix degradation, and a TIMP has been associated with at least one type of retinal degeneration. We have studied expression of MMP-2 and TIMP-1 by zymography, immunocytochemistry, and immunoblotting in the retinal pigment epithelium (RPE) from normal, aged and diseased retinas. MMPs and TIMPs were found in the rat RPE, interphotoreceptor matrix (IPM), and in media conditioned by human and rat RPE in culture. In other polarized cells. MMPs and TIMP-2 are secreted vectorially towards the basal lamina. In the RPE, however, MMP-2 and TIMP-1 were secreted preferentially from the apical surface, the surface bordering the IPM. These findings provide new evidence that MMPs and TIMPs could play a role in the turnover of IPM components. Cell homogenates and conditioned media from RPE isolated from mutant Royal College of Surgeons (RCS) rats with inherited retinal dystrophy had similar amounts of MMP-2 and TIMP-1 as those from congenic control rats. The secretion of MMP-2 and TIMP-1 from RPE cell cultures isolated from young and aged human donors varied widely. However, with increasing cell passage number, secretion of MMPs and TIMPs from human RPE increased dramatically. Also, growing human RPE on bovine corneal endothelial cell-generated extracellular matrix instead of plastic reduced the secretion of both MMPs and TIMPs. These data suggest that the integrity of Bruch's membrane may serve to regulate RPE functions in MMP and TIMP secretion and that extracellular matrices contain signals that regulate MMP and TIMP synthesis and/or secretion by the RPE.

Expression of 17 beta-hydroxysteroid dehydrogenase type IV in chick retinal pigment epithelium

Retinal pigment epithelium (RPE) has essential roles that maintain functions of the neural retina. Previously, we described that the 3b5 monoclonal antibody (3b5 mAb) recognized the RPE cells in the chick embryonic eye. In the present study, a gene coding for the 3b5 antigen has been cloned and sequenced. A 10-day-old embryonic RPE cDNA library was constructed in lambda Uni-ZAP XR, and screened with the 3b5 mAb as a probe. The complete nucleotide sequence of the antigen recognized by the 3b5 mAb was obtained. The cDNA encodes a 735 amino acid protein with a calculated molecular mass of 80,147 Da. The deduced amino acid sequence shares 72% identity with the human 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) type IV. Northern blot analysis detected a 2.9 kb transcript. Immunohistochemical and in-situ hybridization studies revealed that 17 beta-HSD type IV was preferentially expressed in the RPE cells. The expression of steroid metabolizing enzymes in the RPE cells is able to create estrogenic environment, which may have a role of maintenance of the neural retina.

Immunolocalization of ubiquitin and related enzymes in human retina and retinal pigment epithelium

PURPOSE

To examine the localization of ubiquitin (Ub) and related enzymes in human retina with emphasis on the retinal pigment epithelium (RPE)-Bruch's membrane complex.

METHODS

Thirty human eyes enucleated for various disease processes were examined. Immunohistochemistry was performed on paraffin sections using antibodies against Ub, Ub-conjugating enzyme (E2), Ub carboxyl-terminal hydrolase (PGP 9.5), and, for comparison, arrestin (Arr). Immunoreactivity (IR) was tested using the avidin-biotin method.

RESULTS

Ub was present throughout retina but was particularly prominent in ganglion cells and RPE. Most intriguing was the presence of Ub IR in age-related, sub-RPE deposits such as drusen and basal laminar deposits (BLD). RPE immunolabeling was more intense in older tissue, but otherwise no pattern of Ub IR could be linked to specific diseases. E2 IR colocalized with Ub, with one exception; E2 IR was not found in drusen or BLD. PGP 9.5 IR was intense in nerve fibers, ganglion cells, and the inner nuclear and plexiform layers. RPE staining was faint and patchy; sub-RPE deposits were not labeled. Arr IR was present in photoreceptors but not within or beneath RPE cells.

CONCLUSION

The ubiquitination process is important in human retina and particularly in ganglion cells. Ub-related processes are also active in RPE and may be involved in the degradation and disposal of proteins from these cells. The presence of Ub in sub-RPE deposits--without related Ub-processing enzymes--raises the possibility that certain proteins become ubiquitinated within RPE but that further degradation of the Ub-protein complexes does not occur.

Indomethacin alters the Na,K-ATPase response to protein kinase C activation in cultured rabbit nonpigmented ciliary epithelium

PURPOSE

To test whether prostaglandin E2 (PGE2) is generated by cultured nonpigmented ciliary epithelial (NPE) cells treated with the phorbol ester, phorbol dibutyrate (PDBu), an activator of protein kinase C. In addition, the authors tested whether indomethacin, a cyclooxygenase inhibitor, influences the stimulation of active sodium-potassium transport observed in PDBu-treated cells.

METHODS

A cell line derived from rabbit NPE was used in this study. PGE2 was measured by an enzyme-linked immunosorbent assay technique. Ouabain-sensitive potassium (86Rb) uptake was measured as an index of active sodium-potassium (Na, K-ATPase-mediated) transport. Ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) also was measured. Cell sodium and potassium content was determined by atomic absorption spectrophotometry.

RESULTS

Marked PGE2 generation was observed in PDBu-treated cells. Indomethacin abolished the PGE2 response. Ouabain-sensitive potassium (86Rb) uptake was stimulated approximately 40% in cells exposed to PDBu, but a stimulation of > 100% was observed in cells exposed to PDBu in the presence of indomethacin. Added alone, indomethacin did not alter ouabain-sensitive potassium (86Rb) uptake. Neither nordihydroguaiaretic acid (a lipoxygenase inhibitor) nor ethoxyresorufin (a cytochrome P450 inhibitor) altered the 86Rb uptake response to PDBu. Sodium and cyclic adenosine monophosphate content was unchanged in cells treated with PDBu + indomethacin.

CONCLUSIONS

In PDBu-treated cells, there may be generation of cyclooxygenase metabolites of arachidonic acid that inhibit Na, K-ATPase activity, suppressing the stimulatory effect of PDBu on active sodium-potassium transport. Based on the observation that PGE2 can inhibit Na, K-ATPase activity and also inhibit ouabain-sensitive potassium (86Rb) uptake, the authors suggest PGE2 may influence the Na,K-ATPase response to the activation of protein kinase C in NPE cells.

Expression of carbonic anhydrase isozyme III in the ciliary processes and lens

PURPOSE

To determine whether carbonic anhydrase isozyme (CA) III is expressed in the ciliary processes and lens.

METHODS

Total RNA was isolated from rabbit ciliary epithelium and human ciliary processes and from the anterior lens of rabbit, cow, and human eyes. First-strand cDNA was synthesized, and the polymerase chain reaction (PCR) was performed using oligomer primers designed to amplify CA III sequences specifically. Selected PCR products were eluted from agarose gels, cloned, and sequenced. Northern blots were performed to confirm the presence of CA III in these tissues.

RESULTS

Polymerase chain reaction products of the predicted size were generated from rabbit ciliary epithelium and from rabbit, bovine, and human lens. The sequence of the PCR product from human lens was identical to the published sequence of the corresponding region of the human CA III gene. The sequence of the PCR products from rabbit ciliary epithelium and bovine lens showed 88% and 97% identity, respectively, with the corresponding sequences for human CA III, suggesting that the PCR products corresponded to the rabbit and bovine orthologs. Northern blots confirmed the presence of CA III mRNA in the rabbit ciliary epithelium and in rabbit and bovine lens.

CONCLUSIONS

mRNA for CA III is present in the intraocular tissues of rabbits, cows, and humans. The previous detection of CA III protein in the bovine lens is confirmed, and it extended to the lenses of other species, including humans. The detection of mRNA for CA III in the ciliary epithelium is new and suggests that the ciliary epithelium contains not only isozymes II and IV but isoenzyme III as well. Although the function of CA III in the eye is unknown, it may play a role in fluid transport and homeostasis.

Plasma membrane calcium-ATPase in cultured human retinal pigment epithelium

The present work demonstrates the presence of plasma membrane (Ca(2+) + Mg2+)-ATPase (PMCA) activity in cultured human retinal pigment epithelium (HRPE). Whole-cell HRPE homogenates exhibited Ca(2+)-stimulated ATPase activity that was inhibited with high affinity (IC50 = 60 nM) by eosin, a potent inhibitor of the erythrocyte PMCA. This activity was not inhibited by thapsigargin, a selective inhibitor of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA). A PMCA-specific monoclonal antibody exhibited staining in attached HRPE monolayers. By Western blot analysis of SDS-PAGE separations of whole cell lysates of HRPE, this antibody identified a single band at approximately 145 kD. Finally, a Ca(2+)-dependent, La(3+)-augmented phosphoprotein, which comigrated with anti-PMCA immunoreactivity, was also detected. Taken together, these results show that cultured HRPE express PMCA activity.

Dynamics of gene transfer to retinal pigment epithelium

PURPOSE

To examine the nature and dynamics of gene transfer to human retinal pigment epithelium (RPE) using an adenoviral vector and adjuvants that may enhance the uptake of recombinant adenoviruses.

METHODS

Human RPE cultures (HRPE7) were transfected in vitro with varying concentrations (4, 20, 40, 120, and 200 pfu/microliter) and for varying periods (1, 2, 4, 16, 24, 48, and 72 hours) with a replication-deficient adenovirus (Ad.RSV. beta gal) containing the bacterial beta-galactosidase transgene (beta gal). The expression of beta gal was monitored by counting after X gal staining. The transgene expression profiles were compared to those of human F2000 fibroblasts under the same conditions. The adjuvant effect of sodium hyaluronate (HA) on the expression of beta gal was tested in F2000 and early and late passage human RPE cells for differing concentrations of HA, viral titers, and incubation times. Immunofluorescent cytochemistry was carried on HRPE7 and F2000 cells for the HA receptors, homing receptor CD44 (CD44), intercellular adhesion molecule 1 (ICAM-1), and the receptor for hyaluronan mediated motility (RHAMM).

RESULTS

The number of HRPE7 and F2000 cells expressing the adenoviral transgene increased consistently with increasing incubation time and viral titer. There was a higher uptake of Ad.RSV. beta gal in HRPE7 cells compared to the F2000 fibroblasts under the same conditions. There was an increase of 28.1% and 41.4% in the number of RPE7 cells expressing adenoviral transgene and 16.2% and 15.8% F2000 fibroblast cells expressing the adenoviral transgene in the presence of 0.001% and 0.005% HA, respectively. Significant adjuvant effects on transgene expression also were shown in HRPE51 cells. It appears that the effects of increasing viral titer, length of incubation, and the presence of HA on transgene expression are at least additive. The appearance of CD44 and ICAM receptors on RPE7 and F2000 cells and RHAMM receptors on F2000 cells was similar. The RHAMM receptors in HRPE7 cells, however, were shown preferentially over the nucleus.

CONCLUSIONS

On the basis of these results, the authors propose that adenovirus transgene expression increases with increasing incubation time and viral titer in cell culture. The rate of increase of expression differs between human RPE cells and the F2000 fibroblast cells, which may offer a targeting opportunity. The authors propose that the use of HA can offer both an adjuvant effect and a targeting advantage in terms of transferring adenoviral transgenes to human RPE in culture.

Lipocalin-type prostaglandin D synthase (beta-trace) is located in pigment epithelial cells of rat retina and accumulates within interphotoreceptor matrix

Glutathione-Independent prostaglandin D synthase, identical to beta-trace, (a major CSF protein), is localized in the CNS. This enzyme, lipocalin-type prostaglandin D synthase, is a member of the lipocalin family of secretory proteins that transport small lipophilic substances. This enzyme's activity in adult rat retina was enriched sixfold in retinal pigment epithelium (RPE) and even more in interphotoreceptor matrix (IPM), all higher than brain. Western blots with anti-lipocalin-type prostaglandin D synthase showed three distinct immunoreactive bands. In the retinal cytosolic fraction, only one band was observed (M(r) 25,000); in IPM, the larger component occurred (M(r), 26,000). The RPE membrane-bound fraction showed two bands (M(r) 20,000 and 23,000), indicating synthesis, and the cytosolic fraction contained two bands (M(r) 23,000 and 26,000), indicating modification for release into IPM. At least two glycosylation sites occurred on the prostaglandin D synthase moiety, explaining the three immunoreactive bands in Western blots. Immunohistochemistry with polyclonal antibodies against this lipocalin-type enzyme showed intense localization in RPE, but less in photoreceptor outer and inner segments. In situ hybridization showed mRNA specifically expressed in RPE. Thus, lipocalin-type prostaglandin D synthase is predominantly expressed in RPE and actively accumulated in IPM. This may demonstrate gene sharing because, while catalyzing prostaglandin D2 synthesis, it may perform an additional, unrelated role in IPM. This enzyme is secreted from the RPE into IPM from which it is then taken up by photoreceptors. However, the nature of its ligand(s) is not known; they may be retinoids and/or docosahexanoic acid.

Comparison of retinyl ester hydrolase activities in bovine liver and retinal pigment epithelium

Various properties of retinyl ester hydrolysis in the liver and the retinal pigment epithelium (RPE) have been studied, yet the relationship between the retinyl ester hydrolase (REH) activities in these tissues of the same species is not known. In the present study, REH activities in bovine liver and RPE microsomes were compared to explore potential biochemical relationships of retinyl ester metabolism in these tissues. Rates of [3H]all-trans retinyl palmitate hydrolysis by liver and RPE were comparable (i.e., Vmaxapp approximately 300 pmol/min per mg; K(m)app approximately 30 microM), while hydrolysis of [3H]11-cis retinyl palmitate by RPE was significantly higher (Vmaxapp = 1,667 pmol/min per mg). When equimolar amounts (10 microM) of either [14C]triolein or unlabeled 11-cis retinyl palmitate were added to [3H]all-trans REH assays, all-trans REH activities in liver and RPE demonstrated similar time-dependent inhibition profiles. In contrast, hydrolysis of [3H]11-cis retinyl palmitate by RPE was relatively unaffected by addition of either [14C]triolein or unlabeled all-trans retinyl palmitate. Additionally, modification of the microsomal proteins with N-ethylmaleimide produced profound, dose-dependent alterations in K(m)app values for all-trans retinyl ester hydrolysis, whereas K(m)app for 11-cis REH in the RPE was not significantly altered. These results have elucidated common biochemical features of all-trans retinyl ester hydrolysis in liver and RPE. In contrast, hydrolysis of 11-cis retinyl ester in RPE is characterized by a distinctive substrate preference and unique biochemical properties.

Nitric oxide: a review of its role in retinal function and disease

Nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide from L-arginine, exists in three major isoforms, neuronal, endothelial, and immunologic. Neuronal and endothelial isoforms are constitutively expressed, and require calcium for activation. Both of these isoforms can be induced (i.e., new protein synthesis occurs) under appropriate conditions. The immunologic isoform is not constitutively expressed, and requires induction usually by immunologic activation; calcium is not necessary for its activation. Neuronal and immunologic NOS have been detected in the retina. Neuronal NOS may be responsible for producing nitric oxide in photoreceptors and bipolar cells. Nitric oxide stimulates guanylate cyclase of photoreceptor rod cells and increases calcium channel currents. In the retina of cats, NOS inhibition impairs phototransduction as assessed by the electroretinogram. Inducible nitric oxide synthase, found in Müller cells and in retinal pigment epithelium, may be involved in normal phagocytosis of the retinal outer segment, in infectious and ischemic processes, and in the pathogenesis of diabetic retinopathy. Nitric oxide contributes to basal tone in the retinal circulation. To date, findings are conflicting with respect to its role in retinal autoregulation. During glucose and oxygen deprivation, nitric oxide may increase blood flow and prevent platelet aggregation, but it may also mediate the toxic effects of excitatory amino acid release. This reactive, short-lived gas is involved in diverse processes within the retina, and its significance continues to be actively studied.

Chronic retinal effects by ultraviolet irradiation, with special reference to superoxide dismutases

Recently ultraviolet light (UV) reaching the Earth's surface has been gradually increasing in amounts by the destruction of the ozone layers. Large parts of UV are absorbed in the cornea and lens, and only a few amounts reached the retina; however, the effect on the retina is not fully elucidated. 38 rats were irradiated 0.5-5.0 J/cm2 UV from 6 to 50 times every 24 hours, and immunohistochemically and immunochemically for superoxide dismutases (SOD). Morphologically, the destruction of rod outer segments (ROS) and dissociation of cell membranes between the pigment epithelial cells (PE) were already observed by 6 times 0.5 J/cm2 UV irradiations. As the doses of UV increased, heterochromatins and lipid droplets increased in the PE. In normal retina, Cu/Zn SOD were mainly distributed from the inner limiting membrane (ILM) to the ganglion cell layer, and the PE; however, after 6 times 0.5 J/cm2 UV irradiations, the distribution became widened from inner to outer plexiform layer (OPL). At that time, the concentrations of Cu/Zn and Mn SOD increased in the retina. The present study reveals that the morphological damage caused by UV irradiation is observed in the ROS and PE, where no immunoreactivities could be detected to Cu/Zn and Mn SOD. However, morphological damage was not from the ILM to OPL, where the immunoreactivities to both Cu/Zn and Mn SOD were observed.

Expression of inducible nitric oxide synthase in the eye from endotoxin-induced uveitis rats

PURPOSE

Inducible nitric oxide (NO) synthase (iNOS) has been implicated in the pathogenesis of endotoxin-induced uveitis (EIU). This study was undertaken to localize the cells, in the eye, which express iNOS during EIU in the rat.

METHODS

EIU was induced in Lewis rats by a single foot pad injection of 150 micrograms lipopolysaccharide (LPS) from Salmonella typhimurium. At different time intervals after LPS injection, the authors evaluated ocular inflammation (slit lamp observation), iNOS localization by in situ hybridization, and comparison of OX-42- and ED1-positive cell appearance and of glial response by specific immunohistochemistry.

RESULTS

iNOS mRNA was not detected in the iris-ciliary body nor in the retina of control rats. It was detected strongly in the epithelial cells of the iris-ciliary body at 6 hours and also in stromal cells of the ciliary processes at 16 hours after LPS injection. In the neuroretina, iNOS mRNA was observed in the inner layers 16 hours after LPS injection. iNOS-positive cells were also present on the vitreous at this time. At 6 and approximately 16 hours after LPS injection, immunohistochemistry experiments revealed a large number of OX-42- and ED1-positive cells (microglia, macrophages, or polymorphonuclear leukocytes) colocalized in part with some iNOS-positive cells in the ciliary body and in the retina. Furthermore, expression of iNOS in Müller cells cannot be excluded.

CONCLUSIONS

These observations confirm that subcutaneous injection of endotoxin dramatically induces NOS mRNA expression in the eye, and they demonstrate that epithelial cells of the iris-ciliary body and cells infiltrating the anterior segment of the eye and the retina are the major source of NO. These results support the hypothesis that both inflammatory and resident ocular cells are involved in iNOS expression during EIU.

The cloning and characterization of chick tyrosinase from a novel embryonic cDNA library

Very little is known about the genes involved in the regulation of avian skin and feather pigmentation. In mammals, two gene families have been identified as being important for the regulation of melanin biosynthesis. To isolate the avian equivalents of these families, we have generated an embryonic chick melanocyte cDNA library. Neural crest cells from 500 black chick embryos were cultured under conditions supportive of melanocyte differentiation and proliferation. A cDNA library was constructed and screened with a mouse tyrosinase cDNA probe. Nineteen clones were obtained, seven of which cross-hybridized to a mouse tyrosinase cDNA on Southern blots. The longest of these clones, B8.3 (1.9 kb), was sequenced and found to share 99.7% nucleotide and 99.8% amino acid sequence homology to a reported chick tyrosinase cDNA. Both Northern blot analysis and in situ hybridization demonstrated that clone B8.3 was expressed in the retinal pigment epithelium of chick embryos. Our results suggest therefore that the cDNA library described here may allow the cloning of novel melanogenic genes.

Chromosomal localization of the mammalian peptide-methionine sulfoxide reductase gene and its differential expression in various tissues

Peptide methionine sulfoxide reductase (MsrA; EC 1.8.4.6) is a ubiquitous protein that can reduce methionine sulfoxide residues in proteins as well as in a large number of methyl sulfoxide compounds. The expression of MsrA in various rat tissues was determined by using immunocytochemical staining. Although the protein was found in all tissues examined, it was specifically localized to renal medulla and retinal pigmented epithelial cells, and it was prominent in neurons and throughout the nervous system. In addition, blood and alveolar macrophages showed high expression of the enzyme. The msrA gene was mapped to the central region of mouse chromosome 14, in a region of homology with human chromosomes 13 and 8p21.

Carbonic anhydrase activity is increased in retinal pigmented epithelium and choriocapillaris of RCS rats

BACKGROUND

In Royal College of Surgeons (RCS) rats the retinal pigmented epithelium (RPE) exhibits defective phagocytosis of rod outer segments, causing degeneration of the photoreceptor layer. It is not known whether another function of the RPE, ion and fluid transport, is also affected by the disease. One enzyme involved in modulation of RPE transport activities is carbonic anhydrase (CA). To clarify whether changes in CA activity are correlated with the process of retinal degeneration, the localization of CA activity in RCS rat eyes was investigated.

METHODS

Eyes of 12 RCS rats and 12 age-matched congenic controls of different ages were studied, using a modified histochemical method of Hansson for light and electron microscopy.

RESULTS

Control eyes showed CA staining in corneal endothelium, both layers of ciliary epithelium, Müller cells, inner segments of photoreceptors, and RPE cells. In RPE the apical membranes were most intensely stained. In RCS rats, changes in CA staining were seen only in the posterior segment of rats 6 and 7 months of age. Most of the RPE cells were more intensely stained than those of age-matched controls, especially due to increased CA activity in the basolateral membrane infoldings. Adjacent endothelial cells of the choriocapillaris and of retinal capillaries developed staining for CA activity.

CONCLUSION

Changes in CA activity in the RPE and adjacent capillary endothelium, together with previously described changes in RPE morphology in RCS rats, indicate changes in ion and fluid transport across the RPE. Since the retina was already impaired when the increase in CA activity occurred, we hypothesize that the causative factor is not the genetic defect per se, but the destruction of the retina.

Age-related changes of glycosidases in human retinal pigment epithelium

This study was undertaken to determine whether there are age-related changes in the specific activities of several glycosidases in fresh retinal pigment epithelial cells (RPE) isolated from the posterior pole of human donor eyes. One hundred and twenty-one pairs of eyes from human donors, between the ages of 43 and 95 years, were obtained from the National Disease Research Interchange (NDRI, Philadelphia, PA) and the Cleveland Ohio Eye Bank within 18 to 24 h of death. None had histories of diabetes, hepatitis, HIV infection, intraocular surgery, or documented age-related macular degeneration, although several older donors with evidence of drusen were included in the study. RPE cells were isolated from the posterior third of the retina using the conventional rush method and homogenized with a glass, Broeck tissue grinder. All post-nuclear supernatants were analyzed for glycosidase activity; a smaller number of nuclear pellets were assayed to verify that the majority of the enzyme activity was associated with the post-nuclear sypernatants. Glycosidase activity was quantitated fluorometrically by measuring the enzymatic release of umbelliferone from synthetic substrate preparations, specific for each enzyme. Total protein was determined by a micro BCA protein assay. Regression analysis revealed statistically significant age-related decreases for the specific activities of alpha-mannosidase (p = 0.0001), beta-galactosidase (p = 0.0001), N-acetyl-beta-glucosaminidase (p = 0.0001), and N-acetyl beta galactosaminidase (p = 0.0001) in fresh human donor RPE cells taken from the region of the posterior third of the retina that included the macula. Mannose and N-acetyl-glucosamine are major carbohydrate monomers of the oligosaccaride chains of human rhodopsin, and a relatively high percentage of the oligosaccharide chains are galactosylated. Defects in their degradation may lead to the accumulation of undigested residual material in the RPE.

Adenovirus-mediated gene transfer of ornithine aminotransferase in cultured human retinal pigment epithelium

PURPOSE

To evaluate the efficacy of adenovirus mediated transfer of ornithine delta-aminotransferase (OAT) into human retinal pigment epithelial (RPE) cells.

METHODS

Adenovirus-mediated gene transfer into primary cultures of human RPE was evaluated by measurement of enzyme activity in whole cell extracts and by Western blot analysis. To assess mitochondrial integrity, succinate dehydrogenase activity was measured in transduced RPE cells. Expression of adenovirus early genes was evaluated using reverse transcription-polymerase chain reaction.

RESULTS

OAT activity, which was 65 nmol/mg.hour in untransduced cells, could be increased to levels in excess of 20,000 nmol/mg.hour using an adenovirus vector carrying the OAT cDNA. There was, however, a significant reduction in succinate dehydrogenase activity associated with OAT activity greater than 12,000 nmol/mg.hour. Transduced human RPE displayed an altered morphology that appears to be a response to the vector because similar changes could be induced by an adenovirus vector that does not carry the OAT cDNA. Adenovirus early gene expression was detected in transduced RPE.

CONCLUSIONS

This study represents a first step in the development of intraocular gene replacement therapy for the treatment of gyrate atrophy. The authors demonstrate that adenovirus is an efficient vehicle for the delivery of OAT into human RPE and that RPE will tolerate greater than a 150-fold increase in OAT-specific activity. Evidence for disruption of mitochondria when OAT activity exceeds 12,000 nmol/mg.hour and vector-induced toxicity indicate that more controlled transgene expression and refinement of the vector systems is needed.

Hypericin inhibits cell growth and induces apoptosis in retinal pigment epithelial cells: possible involvement of protein kinase C

Proliferative vitreoretinopathy (PVR) is characterized by the proliferation and migration of retinal pigment epithelial (RPE) cells in the vitreous cavity. The drug hypericin, which is already in clinical use as an antidepressant, has shown promise as an antiviral and antineoplastic agent. To investigate the therapeutic potential of hypericin in PVR, we incubated RPE cells in standard medium with various serum concentrations containing 0.5 to 5 microM hypericin. In some experiments we studied the effects of hypericin in conjunction with the RPE growth stimulating cytokine tumor necrosis factor alpha (TNF-alpha). Dose-dependent inhibition of RPE cell proliferation with IC50 values of 0.7 microM and 3.3 microM in 1% and 5% serum respectively, was found. Even in conjunction with TNF-alpha, hypericin inhibited RPE proliferation with an IC50 value of 1.5 microM. The drug inhibited PKC activity in cells treated with a 2.5 microM dose by 72% after 30 min and by 100% after 180 min. Finally, hypericin induced RPE cells to undergo apoptotic cell death, as shown by the presence of DNA laddering. These results suggest that hypericin may have potential as a therapeutic drug for PVR and that its antiproliferative and apoptotic effects on RPE cells in vitro are in part mediated by PKC.

Lysosomal enzyme activities in cultured retinal pigment epithelial and glial cells of RCS rat

PURPOSE

To compare the activities of acid phosphatase, N-acetyl-beta-glucosaminidase and alpha-mannosidase in cultured retinal pigment epithelium (RPE) and glial cells of Royal College of Surgeons (RCS) rat with those in Long Evans (LE).

METHODS

The cultured RPE and glial cells of RCS and LE rat were plated into the same 96 well microtitre, and the biochemical method in microsystem were used for enzyme assays.

RESULTS

The activities of acid phosphatase and N-acetyl-beta-glucosaminidase are higher by, respectively, 30% and 46% in cultured RPE of RCS rat than LE rat. The activity of alpha-mannosidase has no significant difference. The activities of 3 enzymes in the retinal glial cells derived from RCS rats are higher than LE rat by 13% to 77%.

CONCLUSION

These results suggest that the high activities of lysosomal enzymes in RCS RPE and glial cells may play an important role in the pathogenesis of retinal dystrophy.

Cloning and expression of a bovine adenylyl cyclase type VII specific to the retinal pigment epithelium

A cDNA of a type 7 adenylyl cyclase isoform was cloned from a bovine retinal pigment epithelium cDNA library using oligonucleotides developed to conserved regions common to mammalian adenylyl cyclases. A 6.7 kb mRNA of very high abundance was uniquely present on Northern blots containing mRNA or total RNA from the pigment epithelium. This transcript was undetectable in all other tissues examined. The cDNA encoded a protein of 1,097 amino acids and exhibited the known doublet of 6 transmembrane-spanning regions in a hydrophobicity plot. The novel member of the type 7 adenylyl cyclase isoform was expressed in COS-1 cells. It was stimulated 10- and 20-fold by 10 microM GTP gamma S and 100 microM forskolin, respectively. The high expression rate exclusively in the retinal pigment epithelium suggests that this adenylyl cyclase isoform is involved in processes specific to this functionally exceedingly important subretinal cell layer.

Misrouting of tyrosinase with a truncated cytoplasmic tail as a result of the murine platinum (cp) mutation

Mice homozygous for the platinum (cp) allele at the albino locus manifest severe oculocutaneous albinism despite the presence in vitro of tyrosinase activity at 25% wild-type levels. We demonstrate that the cp allele results from an A-->T substitution, changing a lysine residue at position 489 to a termination codon, with truncation of tyrosinase's cytoplasmic tail. In choroidal melanocytes of neonatal mutant mice, tyrosinase activity could be detected in the trans Golgi network, but was absent from melanosomes. Instead, it was detected in vesicles in the cell periphery and dendrites, and on the extracellular surface. In the retinal pigment epithelium, activity was present on the extracellular apical and basolateral surfaces. Our results demonstrate misrouting of a mutant tyrosinase lacking its cytoplasmic tail, providing an explanation for the severe effect of this mutation on ocular and cutaneous pigmentation.

The distribution of Na+,K(+)-ATPase and 5A11 antigen in apical microvilli of the retinal pigment epithelium is unrelated to alpha-spectrin

The retinal pigment epithelium was used to study the relationship between the cortical cytoskeleton and two plasma membrane proteins that associate with it. These proteins were the Na+,K(+)-ATPase, an ion pump, and the 5A11 antigen, a member of the immunoglobulin superfamily of receptor proteins. The cytoskeleton was marked by two of its constituents, alpha-spectrin and ankyrin. Ankyrin links the Na+,K(+)-ATPase to spectrin in many cells. The RPE is of interest, because unlike most epithelia it distributes the Na+,K(+)-ATPase to the apical membrane. The development of polarity was studied during chick embryogenesis. On embryonic day 6 (E6), each of these proteins was observed in the apical and lateral plasma membranes. As development proceeded, only the Na+,K(+)-ATPase was removed from the lateral membranes. Beginning on E12, ankyrin, spectrin and 5A11 appeared together in patches along the basal plasma membrane. By E16, these patches coalesced into a uniform distribution along the basal membrane. At the apical pole, alpha-spectrin appeared near the base of the microvilli, but was undetected in the microvilli themselves. This distribution resembled the distribution of alpha-spectrin in the intestine and proximal kidney tubule. By contrast, a pool of ankyrin and 5A11 and nearly all the Na+,K(+)-ATPase appeared in the microvilli. Despite its segregation from alpha-spectrin, the Na+,K(+)-ATPase appeared to associate with a macromolecular complex, as judged by extraction with Triton X-100. Changes in spectrin distribution could not be related to changes in isoform expression, as only one isoform of beta-spectrin was detected by co-immunoprecipitation with alpha-spectrin. By contrast, multiple ankyrin-like peptides could be identified by immunoblotting. These data illustrate some of the unique properties of RPE microvilli. These properties prevent the Na+,K(+)-ATPase from complexing with the alpha-spectrin-based cytoskeleton by sequestering the enzyme into the compartment where its activity is required.

Transforming growth factor-beta inhibits the cytokine-mediated expression of the inducible nitric oxide synthase mRNA in human retinal pigment epithelial cells

Human retinal pigment epithelial (RPE) cells in culture respond to a mixture of cytokines (IFN-gamma, IL-1 beta, TNF-alpha) by producing large amounts of nitric oxide. Transforming growth factor-beta, unlike other growth factors, was found to inhibit this response by more than 75%. The expression of mRNA for the inducible form of nitric oxide synthase in RPE cells treated with cytokines was demonstrated by reverse transcription-polymerase chain reaction, sequencing of the PCR product and northern blotting. Transforming growth factor-beta was highly effective in inhibiting (by 75%) the cytokine-induced nitric oxide synthase mRNA expression. This response by RPE may play an important role in the etiology of infectious and inflammatory diseases affecting retina.

Cloning and expression of a cDNA encoding bovine retinal pigment epithelial 11-cis retinol dehydrogenase

PURPOSE

Identification of a 32-kd protein in the bovine retinal pigment epithelium.

METHODS

A bovine retinal pigment epithelium cDNA library was constructed in the bacteriophage lambda ZAP Express. A monoclonal antibody, designated 21-C3/AV, was used to isolate the cDNA encoding the 21-C3/AV antigen. A positive full-length clone, designated 21-C3RDH/CD, was sequenced. Northern blot analysis was used to determine the length of the mRNA and the tissue expression pattern. The entire open reading frame of clone 21-C3RDH/CD was used to isolate a recombinant baculovirus clone and expressed in Spodoptera frugiperda insect cells. Enzymatic activity toward 11-cis retinaldehyde was investigated.

RESULTS

The complete nucleotide sequence of 21-C3RDH/CD was obtained. The deduced amino acid sequence reveals homology with short-chain alcohol dehydrogenases. Northern blot analysis detected a 1.2-kb transcript. Although the monoclonal antibody used to isolate 21-C3RDH/CD also reacts with other ocular and nonocular tissues, the authors were unable to demonstrate any reactivity with RNA samples isolated from different (non)ocular tissues. Recombinant baculovirus-infected insect cells synthesized the 21-C3/AV antigen. This protein showed 11-cis retinol dehydrogenase activity.

CONCLUSIONS

Homology to the human D-beta-hydroxybutyrate dehydrogenase precursor and other alcohol dehydrogenases shows that 21-C3RDH/CD encodes a short-chain alcohol dehydrogenase. Furthermore, tissue specificity and molecular weight of the antigen suggest that 21-C3RDH/CD encodes the bovine retinal pigment epithelial 11-cis retinol dehydrogenase. Direct proof came from experiments in which we used the baculovirus-based expression system for in vitro synthesis of the protein encoded by 21-C3RDH/CD. Protein extracts obtained from recombinant baculovirus-infected insect cells were found capable of reducing 11-cis retinaldehyde.