The possibility of gene therapy for the treatment of choroidal neovascularization

PURPOSE

Choroidal neovascularization (CNV) is responsible for most cases of severe visual loss in age-related macular degeneration. Recently, the possibility of gene therapy has been proposed for the treatment of CNV. The purpose of this study was to examine the feasibility of ex vivo and in situ gene therapy approaches for CNV.

DESIGN

Experimental study.

METHODS

Human retinal pigment epithelial (RPE) cells were transduced with a retroviral vector coding for beta-galactosidase. Transduced cells were grown on type II collagen sheets and transplanted under the retina of 20 rabbits. Animals were observed for 3 to 56 days, and transplanted cells were examined histologically and with X-gal staining. Bovine choroidal endothelial cells (CEC) were transduced with retroviral vectors coding for tissue inhibitor of metalloproteinase-2 (TIMP-2) or control vector. Production of TIMP-2 by transduced cells was determined by immunohistochemical analysis and enzyme-linked immunosorbent assay. Effect of transduction on in vitro proliferation, migration, and tube formation was examined in response to vascular endothelial growth factor (VEGF). Four CNV lesions were induced in one cynomolgus monkey by laser photocoagulation. Two days later, retroviral vector coding for TIMP-2 or control vector was injected into the subretinal space overlying the CNV lesions. The monkey was observed for 12 weeks using fluorescein angiography.

RESULTS

Transplantation of transduced RPE cells was technically achieved in 10 of 20 animals. In these animals, RPE cells at the site of transplantation formed a monolayer and expressed beta-galactosidase for 14 days. beta-Galactosidase-positive cells were not identified at 56 days. Choroidal endothelial cells transduced with TIMP-2 secrete TIMP-2 into the media and show decreased migration and tube formation in vitro. In the in vivo monkey model, the control CNV lesions (n = 2) showed prominent leakage, whereas the experimental lesions (n = 2) showed minimal hyperfluorescence.

CONCLUSIONS

Retrovirally transduced RPE cells survive in the subretinal space for at least 14 days and continue to express the gene product coded for by the vector. Choroidal endothelial cells retrovirally transduced for TIMP-2 produce TIMP-2 in vitro and show decreased angiogenic responses in vitro in response to VEGF. A preliminary study attempting in situ delivery of TIMP-2 vector to CNV lesions in a monkey eye supports the feasibility of this approach and encourages further study.

Involvement of the protein kinase pathway in melanin synthesis by chick retinal pigment epithelial cells

Protein kinases are involved in a variety of cellular functions and cell proliferation in eyes. We have explored the involvement of protein kinase C (PKC) in cell proliferation and melanin synthesis by chick retinal pigment epithelial (RPE) cells in vitro. This was achieved by incubation of confluent RPE cells with known inhibitors of protein kinase, H-7, W-7, H-8, and staurosporine. Chick RPE cells were cultured in the presence or absence of the protein kinase inhibitors for a 10-day period. Effects of the inhibitors on cell proliferation and melanin synthesis, as an indication of cell differentiation, were assessed by counting the number of surviving cells and by measuring the melanin content in the cells, respectively. H-7, W-7, and staurosporine inhibited cell proliferation and increased melanin synthesis in a concentration-dependent manner during culture; however, H-8 did not produce these cellular effects. These findings indicate that PKC and calcium/calmodulin-dependent kinase pathways are involved in the proliferation and differentiation of chick RPE cells.

Glutamine synthetase and marker enzymes of the blood-retina barrier in fetal bovine retinal pigment epithelial cells

BACKGROUND

Glutamine synthetase is involved in the recycling of synaptically released glutamate and GABA and in the detoxification of ammonia. It is present in the Müller cells of the neural retina but not in the retinal pigment epithelial (RPE) cells of adult mammals. In human retinal pathological conditions glutamine synthetase has also been detected in RPE cells. In this case glutamine synthetase may provide the cells with glutamine needed for proliferation. Proliferation is also intense during retinal development.

METHODS

We studied the distribution of glutamine synthetase immunoreactivity in fetal bovine retinae, especially in the RPE. The maturity of the RPE was demonstrated by histochemical detection of gamma-glutamyltranspeptidase, alkaline phosphatase and Na/K-ATPase as marker enzymes for the blood-retina barrier and by electron microscopy.

RESULTS

We found that in the first 3 months of gestation glutamine synthetase immunoreactivity is located exclusively in the RPE. During the 3rd month the marker enzymes of the blood-retina barrier begin to appear. From the 4th month on, RPE cells are glutamine synthetase immunonegative.

CONCLUSION

Glutamine in RPE cells in early development may supply glutamine for the intensely proliferating cells in the retina. Glutamine synthetase immunoreactivity in human retinal pathological conditions may indicate reinduction of an enzyme used in earlier development.

Synthesis pattern of matrix metalloproteinases (MMPs) and inhibitors (TIMPs) in human explant organ cultures after treatment with latanoprost and dexamethasone

PURPOSE

To determine changes in production of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs) in the ciliary body, the trabecular meshwork and the retinal pigment epithelium induced by both prostaglandins and corticosteroids.

METHODS

Explant organ cultures were removed by a scleral incision 3 mm posterior to the limbus. Retinal pigment epithelium was grown to confluence. Organ and cell cultures were treated with latanoprost and/or dexamethasone for 72 h. The activity of MMP-2, -3 and -9 was assessed using zymography. The synthesis pattern of MMPs and TIMP-1 and -2 was identified using immunostaining.

RESULTS

Treatment of explant organ cultures with 10 micrograms/ml of latanoprost induced a mean upregulation of MMP-2 by 36%, MMP-3 by 112% and MMP-9 by 156% as seen by zymography. Dexamethasone 500 nm reduced the amounts of secreted MMP-2 by 13%, MMP-3 by 69%. MMP-9 was not detectable in the media of corticosteroid-treated explant organ cultures. The addition of 10 micrograms/ml of latanoprost to dexamethasone-treated cultures increased MMP-2 by 14%, MMP-3 by 43% and MMP-9 by 49%. Using immunohistochemistry we found staining with antibodies against MMP-2, -3, -9 and TIMP-1 and -2 within the ciliary body, and only to a lesser degree in the trabecular meshwork. Latanoprost treatment caused an increase of 29% in MMP-2 (p < 0.0001), 98% in MMP-3 (p < 0.0001) and 108% in MMP-9 (p < 0.0001). Dexamethasone reduced the staining for MMP-2 by 32% (p < 0.0001), for MMP-3 by 33% (p < 0.0001) and for MMP-9 by 83% (p < 0.0001). Almost no change in staining for MMPs was detectable in the trabecular meshwork. Neither latanoprost treatment nor dexamethasone induced significant changes (p < 0.93) in the secretion of TIMPs. In the media of non-treated retinal pigment epithelium (RPE) cells the only MMP detected was MMP-2. RPE cells in culture did not respond to either treatment with a change in their MMP secretion.

CONCLUSION

We detected a profound upregulation of both MMP-3 and MMP-9 and a mild induction of MMP-2 through latanoprost in the ciliary body, but not the trabecular meshwork or RPE cells. Corticosteroids, on the other hand, downregulated MMP expression in both tissues. This inhibiting effect of corticosteroids on MMP production was reversed by latanoprost.

A novel immunoassay for the evaluation of rod outer segment digestion in cultured retinal pigment epithelial cells

In this work a novel enzyme-linked immunoabsorbent assay quantifying residual rod outer segments in the medium of rod outer segment-challenged retinal pigment epithelial cells is described. A retinal pigment epithelial cell line (D407) that produces low level of cathepsin D, and a primary human retinal pigment epithelial cell culture (HRPE51) that has normal cathepsin D levels, were challenged with bovine rod outer segments. At 3 days post-challenge, the amount of undigested or residual bovine rod outer segments left in the culture medium was quantified by an enzyme-linked immunoabsorbent assay. An antibody raised against bovine rod outer segments, which had been purified and labelled with nitroiodophenyl haptens, was used in the assay. The sensitivity of the immunoassay was less than 10(2) bovine rod outer segments per mL and the signal followed a linear curve, saturating around 10(6) bovine rod outer segments per mL. HRPE51 cells had no residual bovine rod outer segments present in the medium following a challenge with 10(4) bovine rod outer segments per mL. In the medium of D407 cells, residual bovine rod outer segment levels were higher at all bovine rod outer segment concentrations when compared to the residual bovine rod outer segment levels in HRPE51 cells, suggesting that D407 cells have a lower digestive capacity. These results demonstrated that the immunoassay for detecting bovine rod outer segments is a sensitive and reliable technique that can be used to quantify the amount of residual bovine rod outer segments, following bovine rod outer segment challenge of retinal pigment epithelial cells.

Regulation of indoleamine 2,3-dioxygenase, the first enzyme in UV filter biosynthesis in the human lens. Relevance for senile nuclear cataract

3-Hydroxykynurenine (3OHKyn), the precursor of UV filters in human lens, is highly autooxidizable, generates H2O2, and binds to lens proteins, yielding a tanned/yellow product resembling senile nuclear cataractous materials. Thus, if 3OHkyn can be shown to be the causative agent in cataract, it may be possible to prevent the disease by lowering the level of 3OHKyn. To this end, indoleamine 2,3-dioxygenase, the first enzyme in UV filter synthesis, was studied using lens epithelial cell lines. The results indicated that the IDO expression is mediated by IFN-gamma. Immuno-suppressants which inhibit production of IFN-gamma may act as anti-cataract agents. Another way to lower the level of 3OHKyn is to use specific inhibitors for IDO. A recombinant human IDO was expressed to develop the inhibitors.

Glucose-specific regulation of aldose reductase in human retinal pigment epithelial cells in vitro

PURPOSE

To test the hypothesis that pathophysiological levels of glucose regulate aldose reductase (AR2) gene expression, protein production, and activity in human retinal pigment epithelial (RPE) cells in vitro.

METHODS

Primary cultures of human RPE cells were grown for up to 72 hours in media supplemented with various concentrations of glucose (5, 20, or 75 mM), or in 5 mM glucose containing media supplemented with one of the following: galactose, the transported but nonmetabolized glucose analogue 3-O-methylglucose (3-OMG), or the impermeant hexitol mannitol-so that the final hexose concentrations were equimolar to those of the various glucose concentrations used. Changes in the transcript levels for AR2 mRNA, AR2 protein content, and AR2 enzyme activity were determined. RPE glucose utilization and lactate production were determined in media containing 5 and 20 mM glucose.

RESULTS

Glucose utilization and lactate production increased 4.8-fold and 4.4-fold, respectively, when RPE cells were grown in media containing 20 mM versus 5 mM glucose. Glucose was more effective than any other hexose in the induction of AR2 mRNA or increased AR2 protein expression. When RPE cells were grown in media containing 20 mM mannitol, 3-OMG, or galactose they had lower levels of AR2 mRNA expression than when cells were grown in medium containing 5 mM glucose. RPE cells grown in medium supplemented with 20 or 75 mM galactose did not show a greater increase in AR2 protein expression than cells grown in medium containing 5 mM glucose. Hyperosmotic induction of AR2 mRNA was the same in medium containing 75 mM glucose or 75 mM mannitol, but was at least 50% lower when RPE cells were grown in 75 mM galactose or 3-OMG. CONCLUSIONS. These data indicate that elevations in ambient glucose result in greater metabolism of glucose through glycolysis and polyol metabolism. Induction of AR2 was greatest when RPE cells were grown in pathophysiological concentrations of glucose. Hyperosmolar stress is not a necessary determinant of AR2 mRNA, AR2 protein, or AR2 protein activity in cells that form the outer blood-retinal barrier. Increased facilitative glucose transport or glucose metabolism appears to be requisite for glucose-specific and nonosmotic regulation of AR2 in the RPE cell in vitro.

Gene structure, expression analysis, and membrane topology of RDH4

The murine retinol dehydrogenase RDH4 oxidizes several cis-isomers of retinol into their corresponding aldehydes. We have determined the structure of the murine gene, investigated the temporal and spatial expression of the enzyme, and analyzed the membrane topology of the enzyme. The gene has four translated exons, and several alternatively spliced exons in the 5'-untranslated region were identified. Immunohistochemical analysis showed expression of RDH4 in developing and adult mouse eye, particularly in the retinal pigment epithelium. In nonocular adult tissues, including liver, kidney, lung, and skin, RDH4 expression was widespread. The results suggest that RDH4 may have a dual and tissue-specific role in oxidation of 9-cis- and 11-cis-isomers of retinol into 9-cis-retinal and 11-cis-retinal, respectively. Furthermore, the lumenal orientation of the enzyme domain in the ER suggests that oxidation of both cis-isomers of retinol occurs in the ER.

Melanin granules prevent the cytotoxic effects of L-DOPA on retinal pigment epithelial cells in vitro by regulation of NO and superoxide radicals

Inasmuch as the nitrogen cycle elicits the direct reduction of N2 to NH3 through enzymatic reactions, and inasmuch as L-DOPA (L-dihydroxyphentlalamine), a catecholamine, can be a source of nitric oxide (NO), it is possible that melanin granules in the eye affect the generation of NO, which causes damage to the retinal pigment epithelial (RPE) cells during the oxidation of L-DOPA. In order to confirm this possibility, we analyzed the correlations of NO generation, cell growth, and superoxide dismutase (SOD) activities in two types (melanotic and amelanotic) of bovine RPE cells following exposure to L-DOPA. NO generation from L-DOPA was determined using an NO detector that is reliant on redox currents. The concentration of NO was measured in terms of diffusion currents run between a working electrode and a counter electrode, both being set in culture medium placed in a Petri dish. For the assays, L-DOPA was added to the medium at various concentrations (5, 29.9, 79.4, 152.7 or 249 microM), and 6 min after addition, an NO-trapping agent 2,4-carboxyphenyl-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide (carboxy-PTIO) was also added. The melanotic and amelanotic types of RPE cells were cultured separately in medium with L-DOPA under an atmosphere containing 20, 10 or 5% oxygen. Cell numbers were counted using a Coulter counter, and SOD activities were determined following incubation for 24, 48 or 72 hr using a modification of the luminol assay. The results obtained indicated that: (a) NO was produced from L-DOPA in a concentration-dependent manner and was trapped quantitatively by carboxy-PTIO; (b) the generation of NO was inhibited more markedly in the melanotic cell line than in the amelanotic one, suggesting an increased tolerance to L-DOPA-derived cytotoxicity in the former; and (c) the SOD activities were more affected by oxygen concentration in the melanotic cells than in the amelanotic ones. From these results, it is concluded that melanin granules in RPE cells have a role in preventing the cytotoxicity derived from L-DOPA and in regulating the generation of NO and superoxide radicals.

Protein kinase C isoenzyme expression in retinal cells

Protein kinase C (PKC) is involved in both the physiological and pathophysiological processes in the retina and plays an important role in signal transduction. The aim of this study was to determine the PKC isoenzyme profile in three retinal cell types in culture, namely RPE cells, pericytes and retinal microvascular endothelial cells. Confluent cultures were lysed and isoenzyme expression detected by Western blotting. PKC isoenzymes alpha, beta(2) and delta were observed for all three cell types while beta(1) was specific for RPE cells. This study has characterised the PKC isoenzyme profile in three retinal cell types and suggests that defining the cell-specific isoenzyme pattern is an important step in understanding their precise physiological role and regulation in the retina.

The use of adenovirus-mediated gene transfer to develop a rat model for photoreceptor degeneration

PURPOSE

To investigate the effects of recombinant adenovirus-mediated downregulation of cathepsin S (CatS) on the retinal pigment epithelium and/or neural retina in vivo.

METHODS

The expression of green fluorescent protein (gfp) after subretinal injection of a recombinant adenovirus, Ad.gfp, into rat eyes was first established by in vivo fundus fluorescence photography and fluorescence microscopy. The autofluorescent debris accumulation in Ad.CatSAS (recombinant adenovirus carrying the antisense CatS gene)injected rat eyes was monitored by fluorescence microscopy, and the antisense CatS RNA expression was demonstrated by in situ hybridization. Changes in the retinal morphology were assessed by light microscopy. ResuLTS. The gfp expression was present in 30% to 90% of the injection area at 3 days and was absent 9 days after Ad.gfp injection. In Ad.CatSAS-injected eyes, the expression of antisense CatS RNA was demonstrated by in situ hybridization. Autofluorescent debris accumulation was significantly higher in Ad.CatSAS-injected eyes than in control eyes. The shortening of photoreceptor outer segments in Ad.CatSAS-injected eyes coincided with intense autofluorescent debris accumulation. The number of layers of photoreceptor cells decreased with time and were 11, 9, and 8 at 7, 14, and 28 days after Ad.CatSAS injection, respectively. In control eyes, the number of layers of photoreceptor cells (14) remained unchanged.

CONCLUSIONS

These results demonstrate that recombinant adenovirus-mediated transient modulation of gene expression in retinal pigment epithelial (RPE) cells could induce changes in the retina, and, in spite of the low expression of endogenous CatS in RPE cells, this enzyme plays an important role in maintenance of normal retinal function.

Creatine kinase in human retinal pigment epithelium

Retinal pigment epithelial ion transport activity, and consequent ATP consumption vary significantly as a function of photoreceptor activity. In a variety of cell types, ATP levels are maintained during high-energy usage by phosphocreatine hydrolysis, catalysed by the enzyme creatine kinase. The present work was designed to assess the importance of creatine kinase in retinal pigment epithelial cell metabolism. To this end, activity measurements, non-denaturing gel electrophoresis, Western blot analysis and immunohistochemistry were used to characterize creatine kinase in retinal pigment epithelium. Total creatine kinase activity in the retinal pigment epithelium is approximately 0.05 micromol ATP mg protein(-1) min(-1). The bulk of this activity was mediated by the B-CK isoform. However, by immunoblotting, non-denaturing gel electrophoresis and immunohistochemistry, the presence of the M-CK isoform of creatine kinase was also detected. The M-CK isoform was plasma membrane associated and predominately localized to the apical surface. Creatine kinase in the retinal pigment epithelium could function in a spatial energy shuttle that helps to sustain apical plasma membrane ion transport activity.

Visually induced changes in components of the retinoic acid system in fundal layers of the chick

Eye growth is visually regulated via messengers that are released from the retina. The retina involves a yet unknown algorithm to analyse the projected image so that the appropriate growth rates for the back of the eye are ensured. One biochemical candidate that could act as a growth controller, is retinoic acid (RA). Previous work (Seko, Shimokawa and Tokoro, 1996; Mertz et al., 1999) has shown that retinal and choroidal RA levels are indeed predictably changed by visual conditions that cause myopia or hyperopia, respectively. We have studied in which fundal tissues aldehyde dehydrogenase-2 (AHD2) and retinaldehyde dehydrogenase-2 (RALDH2), enzymes involved in RA synthesis, are expressed and at which levels the effects of vision on RA levels may be controlled. Using Northern blot analysis, we have found that the retinal mRNA level of the AHD2 is up-regulated after 3 days of treatment with negative lenses (negative lenses place the image behind the retina). The abundance of the retinal mRNA of a RA receptor, RAR-beta, was up-regulated already after 6 hr of treatment with positive lenses (positive lenses place the image in front of the retina). The up-regulation persisted for at least 1 week. Finally, we have studied the effects of an inhibitor of RA synthesis, disulfiram, on the visual control of eye growth. We found inhibition of myopia as induced by frosted goggles ('deprivation myopia') but no significant inhibitory effects on refractive errors induced by +7D or -7D lenses. Our results are in line with the hypothesis that RA may play a role in the visual control of eye growth. The RA system differs from a number of other candidates (dopamine, cholinergic agents, opiates) in that it distinguishes between positive and negative defocus, similar to the immediate early gene ZENK (Stell et al., 1999). The exact time kinetics of the changes have still to be worked out since it is possible that the changes in RA relate to already occurring changes in growth rather than to initial steps of the signaling cascade.

Heterogeneity in ornithine cytotoxicity of bovine retinal pigment epithelial cells in primary culture

Gyrate atrophy of the choroid and retina is a chorioretinal degeneration caused by hyperornithinemia and a deficiency of ornithine-delta-aminotransferase (OAT). We recently showed that ornithine exhibits cytotoxicity to human retinal pigment epithelial (RPE) cell lines treated with the OAT inhibitor, 5-fluoromethylornithine (5-FMOrn), and suggested that this system may be an in vitro model of gyrate atrophy. In the present study, in order to apply this system to primary cultured RPE cells, we freshly prepared RPE cells from bovine eyes and studied the effect of ornithine on cell damage. Two phenotypes, epithelioid and fusiform, which coexisted in the primary culture and epithelioid phenotype cells, but not fusiform ones, were severely damaged and partially detached from the substrate by 10 m m ornithine and 0.5 m m 5-FMOrn. Neither ornithine nor 5-FMOrn alone exhibited such cytotoxicity to both phenotypes of RPE cells. Proline significantly prevented the ornithine-induced cytotoxicity. Epithelioid and fusiform phenotypes isolated from the primary culture showed different distribution of actin filaments. A combination of ornithine and 5-FMOrn time-dependently inhibited [(3)H]thymidine incorporation in the epithelioid, but not fusiform, cells. Proline prevented the inhibition of [(3)H]thymidine incorporation by ornithine in 5-FMOrn-treated epithelioid cells. Furthermore, l -azetidine-2-carboxylic acid, a collagen synthesis inhibitor, reduced [(3)H]thymidine incorporation in epithelioid, but not fusiform, cells, which was reversed by proline. These results demonstrate that the epithelioid phenotype of bovine RPE cells becomes susceptible to ornithine following inactivation of OAT. The phenotypic cells and its prevention by proline may provide insight into biochemical triggers that induce gyrate atrophy.

Experimental gene therapy for an in vitro model of proliferative vitreoretinopathy

BACKGROUND

Proliferative vitreoretinopathy (PVR) is the leading cause of failure of retinal reattachment surgery. Since a key component of PVR is cell proliferation, we performed a study to examine whether the ribonucleotide-reductase-deficient herpes simplex virus type I (HSV-I) mutant hrR3 can be effective at destroying proliferating retinal pigment epithelial (RPE) cells and thus prevent epiretinal membrane formation and PVR, while sparing nondividing cells, such as neurons.

METHODS

Primary cultures of rat RPE cells and rat cortical neurons were infected with 300 microL of hrR3 HSV-I to achieve a multiplicity of infection of 1.0. After 1 hour at 37 degrees C, 700 microL of growth medium was added to raise the total volume of medium to 1 mL. At 0, 12, 24 and 36 hours the cultures were observed, and the ratio of dead cells to live cells was determined. HSV infection and protein expression were confirmed by a beta-galactosidase histochemical assay or an antihuman HSV-I immunoassay, or both.

RESULTS

At 24 hours more than 95% of the RPE cells and neurons stained positively for HSV infection, although beta-galactosidase was expressed predominantly in RPE cells. At 36 hours 72% (standard deviation 2.1%) of the RPE cells were dead. There was no noticeable cell death in the neuronal or mock-infected control cultures.

INTERPRETATION

The results suggest that the hrR3 mutant strain of HSV-I can be used to infect and selectively kill actively proliferating rat RPE cells while sparing normal, nonreplicating cells. This model may be used to explore potential therapies for PVR in humans.

Cellular and subcellular specification of Na,K-ATPase alpha and beta isoforms in the postnatal development of mouse retina

The Na,K-ATPase is a dominant factor in retinal energy metabolism, and unique combinations of isoforms of its alpha and beta subunits are expressed in different cell types and determine its functional properties. We used isoform-specific antibodies and fluorescence confocal microscopy to determine the expression of Na,K-ATPase alpha and beta subunits in the mouse and rat retina. In the adult retina, alpha1 was found in Müller and horizontal cells, alpha2 in some Müller glia, and alpha3 in photoreceptors and all retinal neurons. beta1 was largely restricted to horizontal, amacrine, and ganglion cells; beta2 was largely restricted to photoreceptors, bipolar cells, and Müller glia; and beta3 was largely restricted to photoreceptors. Photoreceptor inner segments have the highest concentration of Na,K-ATPase in adult retinas. Isoform distribution exhibited marked changes during postnatal development. alpha3 and beta2 were in undifferentiated photoreceptor somas at birth but only later were targeted to inner segments and synaptic terminals. beta3, in contrast, was expressed late in photoreceptor differentiation and was immediately targeted to inner segments. A high level of beta1 expression in horizontal cells preceded migration, whereas increases in beta2 expression in bipolar cells occurred very late, coinciding with synaptogenesis in the inner plexiform layer. Most of the spatial specification of Na,K-ATPase isoform expression was completed before eye opening and the onset of electroretinographic responses on postnatal day 13 (P13), but quantitative increase continued until P22 in parallel with synaptogenesis.

Downregulation of the human taurine transporter by glucose in cultured retinal pigment epithelial cells

In diabetes, activation of the aldose reductase (AR) pathway and alterations of glucose-sensitive signal transduction pathways have been implicated in depletion of intracellular taurine, an endogenous antioxidant and compatible osmolyte. Cellular taurine accumulation occurs by an osmotically induced, protein kinase C (PKC)-regulated Na(+)-taurine cotransporter (hTT). The effects of ambient glucose on taurine content, hTT activity, and hTT gene expression were therefore evaluated in low and high AR-expressing human retinal pigment epithelial cell lines. In low AR-expressing cells, 20 mM glucose decreased taurine content, hTT transporter activity, and mRNA levels, and these effects were unaffected by AR inhibition (ARI). In these cells, the inhibitory effects of high glucose on hTT appeared to be posttranscriptionally mediated, because 20 mM glucose decreased hTT mRNA stability without affecting hTT transcriptional rate. Inhibition of PKC overcame the decrease in hTT activity in high glucose-exposed cells. In high AR-expressing cells, prolonged exposure to 20 mM glucose resulted in intracellular taurine depletion, which paralleled sorbitol accumulation and was prevented by ARI. In these cells exposed to 5 mM glucose, hTT mRNA abundance was decreased and declined further in 20 mM glucose but was corrected by ARI. In 5 mM glucose, hTT transcriptional rate was markedly decreased in high AR-expressing cells, did not decline further in 20 mM glucose, but was increased by ARI to levels above those observed in low AR-expressing cells. Therefore, glucose rapidly and specifically decreases taurine content, hTT activity, and mRNA abundance by AR-unrelated and AR-related posttranscriptional and transcriptional mechanisms.

Distribution of cathepsin D in human eyes with or without age-related maculopathy

Cathepsin D is a ubiquitous enzyme which plays an important role in the catabolism of proteins. Enzymatic studies showed that cathepsin D is the most important lysosomal enzyme in the proteolysis of opsin. The importance of cathepsin D in the lysosomal digestion of phagocytosed photoreceptor outer segments by the retinal pigment epithelium suggests that a decrease in cathepsin D activity might contribute to the development of hyalinized drusen and to the development of age-related maculopathy. The aim of this project was to study the immunohistochemical localization of cathepsin D in human eyes and particularly to compare the immunoreactivity of cathepsin D normal retinal pigment epithelial cells and in cells surrounding hyalinized drusen or lesions of age-related maculopathy. Following clinicopathological examinations the eyes were fixed, paraffin embedded and individual sections were subjected to Picro-Mallory staining for histopathological examination. Bleaching was performed then immunohistochemistry was carried out using a monoclonal mouse anti-human cathepsin D antibody. On the basis of the appearance of basal laminar deposit the eyes were divided into five groups corresponding to levels of progression in age-related maculopathy development. Following optimization of bleaching cathepsin D immunostaining was clearly visible in the iris epithelium, ciliary body and the retinal pigment epithelial layer of all eyes with the highest immunoreactivity present in the RPE cells. Within the neural retina the ganglion cells demonstrated a weak signal. Retinal pigment epithelial cathepsin D immunoreactivity was not impaired by age, geographical location or by age-related maculopathy status. There was a small increase in cathepsin D immunoreactivity around hyalinized drusen. The maintenance of cathepsin D immunoreactivity in eyes with hyalinized drusen or in samples with age-related maculopathy suggest that down-regulation of cathepsin D expression in the affected locations does not precede the development of these conditions. However, further studies are required to establish if the immunoreactive cathepsin D represents the fully processed biologically active enzyme.

Induction of glutathione S-transferase hGST 5.8 is an early response to oxidative stress in RPE cells

PURPOSE

To delineate the role of the glutathione S-transferase (GST) isozyme hGST 5.8 in protection mechanisms against oxidative stress, the effect of low-level transient exposure of H2O2 to retinal pigmented epithelial (RPE) cells on hGST 5.8 and other enzymes involved in defense against oxidative stress was examined.

METHODS

Cultured human RPE cells were exposed to 50 microM H2O2 for 20 minutes. Subsequently, the cells were washed and resuspended in the culture media. The cells were pelleted and lysed, and the levels of lipid peroxidation products including thiobarbituric acid-reactive substances (TBARS), glutathione (GSH), glutathione peroxidase (GPX), glucose 6-phosphate dehydrogenase, glutathione reductase, GST, catalase (CAT), and superoxide dismutase (SOD) were determined and compared with levels in control cells. Total GSTs were purified by GSH-affinity chromatography, and the isozymes were separated by isoelectric focusing, characterized, and quantitated. hGST 5.8 was quantitated by an immunologic method as well as by determining activity toward its preferred substrate, 4-hydroxynonenal (4-HNE). Kinetic constants of hGST 5.8 purified from H2O2-treated cells were also determined and compared with those of control cells.

RESULTS

Exposure of RPE cells to 50 microM H2O2 for 20 minutes showed a significant increase in TBARS (1.8-fold) and gamma-glutamyl cysteine synthetase (gamma-GCS) activity (1.6-fold). A significant increase (1.2-fold) was also observed in GPX activity toward cumene hydroperoxide, but CAT and SOD activities remained unchanged. There was no significant increase in GST activity toward 1-chloro-2, 4-dinitrobenzene but GST activity toward 4-HNE was increased by 1.4- to 1.8-fold. The increase in GST activity toward 4-HNE was associated with a 2.8-fold increase in protein of the isozyme hGST 5.8, which uses 4-HNE as the preferred substrate.

CONCLUSIONS

Results of these studies show that the induction of hGST 5.8, which is involved in the detoxification of the lipid peroxidation products 4-HNE and hydroperoxides, may be an early adaptive response of RPE cells exposed to low levels of transient oxidative stress. It is suggested that this isozyme may be crucial for protecting the RPE from low levels of chronic oxidative stress. Observed increases in GPX and gamma-GCS activities are consistent with this idea, because GPX activity is also expressed by hGST 5.8, and gamma-GCS is the rate-limiting enzyme in biosynthesis of GSH, the substrate for hGST 5.8.

Senescence-associated beta-galactosidase histochemistry for the primate eye

PURPOSE

To develop a senescence-associated beta-galactosidase histochemistry and bleaching protocol for the primate posterior pole.

METHODS

Rhesus monkey eyes of different ages were enucleated after death, fixed in 4% paraformaldehyde for up to 16 hours, and cryoprotected using a graded sucrose infiltration technique. Ten-micrometer tissue sections were treated with beta-galactosidase, pH 4 (lysosomal) or pH 6 (senescence-associated) activity, for various times. Bleaching of retinal pigment epithelial (RPE) cell and choroidal melanocyte pigment was performed after beta-galactosidase histochemistry using 0.1% to 1% potassium permanganate incubation for 1 minute to 2 hours followed by 0.5% oxalic acid immersion.

RESULTS

A 6-hour incubation with beta-galactosidase, pH 4 or 6, demonstrated optimal staining of the RPE. Uniform staining of the RPE for pH 4 beta-galactosidase was seen in both young and old eyes. In contrast, senescence-associated beta-galactosidase (pH 6) staining was seen in the RPE of 16 and 29-year-old, but not 1- and 2-year-old eyes. Senescence-associated beta-galactosidase staining was evident in RPE cells adjacent to cuticular drusen. Optimal bleaching without loss of beta-galactosidase staining was obtained using a 25-minute incubation with 0.05% permanganate.

CONCLUSIONS

The senescence-associated beta-galactosidase histochemistry assay, adapted for use in the primate posterior pole, showed staining of RPE cells in older eyes. Visualization of beta-galactosidase activity in the RPE was enhanced by permanganate bleaching of melanin pigment. This technique could be valuable for identifying senescent RPE cells in human eyes.

Antioxidant enzymes in the macular retinal pigment epithelium of eyes with neovascular age-related macular degeneration

PURPOSE

To test the hypothesis that neovascular age-related macular degeneration is related to oxidative stress involving the macular retinal pigment epithelium. This study investigated, as a function of age, levels of enzymes that defend tissues against oxidative stress in the macular retinal pigment epithelium of human eyes with this disease.

METHODS

Surgical specimens of macular choroidal neovascular membranes from eyes with age-related macular degeneration and the macular regions of whole donor eyes with neovascular age-related macular degeneration or without evident ocular disease were studied by quantitative electron microscopic immunocytochemistry with colloidal gold-labeled second antibodies. Relative levels in retinal pigment epithelium cell cytoplasm and lysosomes were determined of five enzymes believed to protect cells from oxidative stress, as well as levels of the retinal pigment epithelium marker cytoplasmic retinaldehyde-binding protein, for comparison with the enzymes.

RESULTS

Copper, zinc superoxide dismutase immunoreactivity increased and catalase immunoreactivity decreased with age in cytoplasm and lysosomes from macular retinal pigment epithelium cells of normal eyes and eyes with age-related macular degeneration. Cytoplasmic retinaldehyde-binding protein immunoreactivity showed no significant relationship to age or the presence of neovascular age-related macular degeneration. Glutathione peroxidase immunoreactivity was absent from human retinal pigment epithelium cells. Both heme oxygenase-1 and heme oxygenase-2 had highly significantly greater immunoreactivity in retinal pigment epithelium cell lysosomes than in cytoplasm, differing from the much greater cytoplasmic immunoreactivity of the other proteins studied. This immunoreactivity decreased with age, particularly in the lysosomes of retinal pigment epithelium cells from eyes with neovascular age-related macular degeneration. These decreases were of borderline significance (P = .067 for heme oxygenase-1; P = .12 for heme oxygenase-2) when eyes with age-related macular degeneration were compared with normal eyes by multivariable logistic regression.

CONCLUSIONS

The high heme oxygenase-1 and heme oxygenase-2 lysosomal antigen levels in macular retinal pigment epithelium cells of eyes with neovascular age-related macular degeneration suggest that oxidative stress causes a pathologic upregulation of these enzymes. Increased lysosomal disposal may indicate that the reparative functions of these enzymes are accompanied by deleterious effects, necessitating their rapid removal from the cell. The much higher heme oxygenase-1 and heme oxygenase-2 antigen levels in macular retinal pigment epithelium cells from younger individuals suggest that protective mechanisms against oxidation and, hence, presumably to the development of age-related macular degeneration, decrease with age.

[The effect of Hirudo therapy on a change in the activity of retinal transport adenosinetriphosphatases in the rabbit with photic eye damage]

Effect of Hirudo therapy on the activities of transport adenosine triphosphatases (ATPases) of the retina and pigmented epithelium (PE) is studied in normal and albino rabbits whose eyes were exposed to potent illumination (100,000 Lux at the level of the cornea for 90 min). The exposure decreased the activities of ATPase, which did not recover in any of animal groups. Hirudo therapy immediately after illumination increased the enzymes activity in pigmented animals in comparison with intact control: by 20% in the retina and by 23% in PE; Mg-ATPase activity increased by 23% in the retina and decreased by 10% in PE. In subsequent 24 h, ATPase activities decreased, but in comparison with exposed retina the activity of Na,K-ATPase in the retina was 70% increased and in PE 78% increased; the activities of Mg-ATPase were increased by 33 and 8%, respectively. Complete recovery of ATPase activities was attained in 8 days. In albino animals, ATPase activities did not recover completely, but they were notably higher than in intact controls. Hirudo therapy before illumination had a marked protective effect.

The localization of glutathione peroxidase in the photoreceptor cells and the retinal pigment epithelial cells of Wistar and Royal College of Surgeons dystrophic rats

INTRODUCTION

If degenerating photoreceptor outer segments not phagocytized by RPE cells in the retina of Royal College Surgeons (RCS) rats were to undergo peroxidation, the distribution of glutathione peroxidase (GSH-PO) in the mitochondria or cytoplasm of the retina might be altered. We evaluated the immunocytochemical localization of GSH-PO to identify subcellular organelles in sections of the retinas of RCS rats.

METHODS

Immunoblot analysis confirmed the presence of GSH-PO molecules in the retinas of RCS and Wistar rats aged 3 weeks. Sections were reacted with the F(ab) fragment of anti-rat alphaGSH-PO and then examined by laser scanning microscopy (LSM) and transmission electron microscopy (TEM).

RESULTS

The size of the GSH-PO molecule in the retina was about 21 KD in the mitochondria and 23 KD in the cytosol in both strains of rats. LSM revealed fluorescent granules in the photoreceptor inner segments of the Wistar rats, and immunohistochemical TEM revealed GSH-PO in the mitochondria of their photoreceptor inner segments and retinal pigment epithelial (RPE) cells. In the RCS rats, the degenerating photoreceptor outer segments were clearly seen to be positive for anti-GSH-PO by conventional light microscopy (CLM). However, the photoreceptor inner segments of the RCS rats were negative for staining with anti-GSH-PO by LSM, and no GSH-PO could be detected in the mitochondria of the photoreceptor inner segments or RPE cells by immuno-TEM.

CONCLUSION

Degeneration of the photoreceptor outer segments induced mitochondrial damage in the photoreceptor inner segments, and as a result GSH-PO shifted from the photoreceptor inner segments to the degenerating outer segments.

Cloning of a major human retinal pigment epithelial lysosomal aspartic protease and mapping its transcriptional start sites

PURPOSE

It has been proposed that the major proteolytic enzyme responsible for the proteolysis of photoreceptor outer segments is an aspartic protease similar or identical to cathepsin D. The aim of this study was to determine if the major retinal pigment epithelial (RPE) aspartic protease was cathepsin D, to study its distribution and investigate its transcription start sites (TSS).

METHODS

An RPE cDNA library was screened at low stringency for the presence of aspartic proteases. DNA sequencing was used to analyze the identified clones. The expression of cathepsin D mRNA was analyzed by Northern blot while immunohistochemistry was used to demonstrate cathepsin D related immunoreactivity in an eye section. RNase protection assay was used to map the Cathepsin D TSS in RPE cells.

RESULTS

The aspartic protease identified in the RPE cDNA library was identical to the DNA sequence of cathepsin D found in other tissues. Northern blot analysis of a wide range of cell types showed that the highest level of cathepsin D mRNA expression was found in RPE cells which was similar only to cathepsin D expression in MCF7 breast cancer cells. Immunohistochemical staining of a human retina confirmed the inherent nature of high cathepsin D expression in RPE cells. An RNase protection assay demonstrated two major cathepsin D TSS in RPE cells. One of them was identical to the TSS responsible for the constitutive expression of cathepsin D and the other, a TATA box-controlled TSS, was identical to a TSS found in MCF7 cells. Cathepsin D expression was not enhanced by the phagocytosis and digestion of rod outer segments (ROS) and ROS phagocytosis did not induce the use of other cathepsin D TSS in the RPE cells.

CONCLUSION

The major aspartic protease identified in RPE cells was cathepsin D. In addition, it was demonstrated that the high level of cathepsin D expression in RPE cells is the intrinsic nature of these cells and that it is linked to a TATA box-controlled TSS. This TSS has previously been described as an estrogen regulated TSS and further studies are required to identify the RPE-specific inducer or indirect factors that may be responsible for the enhanced expression of cathepsin D in the RPE cells.

Differential regulation of nitric oxide synthase-II mRNA by growth factors in rat, bovine, and human retinal pigmented epithelial cells

Retinal pigmented epithelial cells (RPE cells) express the inducible isoform of NO synthase (NOS-II) after stimulation by cytokines and endotoxin. We have attempted to describe and compare the effect of fibroblast growth factor (FGF) and transforming growth factor beta (TGFbeta) on NOS-II mRNA level in three different cell species: rat, bovine, and human. Northern blot analysis demonstrated that, in the rat RPE cells, FGF upregulates and TGFbeta inhibits NOS-II mRNA accumulation, whereas in the bovine cells, the opposite effect appears. For the human RPE cells, RT-PCR analysis demonstrated that FGF upregulates and TGFbeta inhibits NOS-II mRNA accumulation, as in rat cells, even though the effect of TGFbeta was weaker. Thus, this study demonstrates that marked differences in growth factors regulating NOS-II occur between species, suggesting fundamental signal transduction differences at some level between rat, human, and cow.

A rapid method of preparing the plasma membrane enriched fraction of the bovine retinal pigment epithelial cells

PURPOSE

To devise a rapid method of isolating the plasma membrane enriched fraction (PMEF) of the bovine retinal pigment epithelial (RPE) cells with Percoll centrifugation medium.

METHODS

The bovine RPE was homogenised with a tight fit Dounce homogeniser and centrifuged in a 16.7% Percoll gradient for 20 minutes. The RPE particulate fractions were characterised in terms of their protein concentrations, Na/K-ATPase and bicarbonate stimulated ATPase activities.

RESULTS

The total protein recovery was 88.7% of the RPE homogenate. The nucleus layer was identified at the first band. The mitochondrial fraction was at the second layer according to its bicarbonate stimulated ATPase activity. The 3rd and 4th bands were enriched with plasma membranes and their Na/K-ATPase activities were 31.5 and 34.6 mumol/mg/h respectively. The Na/K-ATPase activities were about six times that of the RPE homogenate.

CONCLUSIONS

A rapid method of isolating the bovine RPE PMEF has been devised which involved a single centrifugation procedure in a Percoll gradient.

Nitric oxide synthase-containing cells in the retina, pigmented epithelium, choroid, and sclera of the chick eye

Nitric oxide is a nonconventional neurotransmitter that is produced as needed by the enzyme nitric oxide synthase (NOS). NOS has been detected in numerous neural structures, including distinct populations of retinal neurons in a variety of vertebrate species. The purpose of this study was to identify NOS-containing cells in the retina and extraretinal ocular tissues of hatched chicks. NOS was detected in frozen sections by using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and antisera to neuronal NOS. In the retina, NADPH-diaphorase and NOS immunolabelling were present in four subtypes of amacrine cells, some ganglion cells, efferent fibers, efferent target cells, and neuronal processes in both plexiform layers, whereas diaphorase alone was detected in photoreceptor ellipsoids and Müller cells. In addition, NADPH-diaphorase and immunoreactive NOS were detected in axon bundles and innervation to vascular smooth muscle in the choroid, whereas stromal and endothelial cells in the choroid, scleral chondrocytes, and the retinal pigmented epithelium contained only NADPH-diaphorase. The excitotoxin quisqualate destroyed all but one subtype of NOS-immunoreactive amacrine cell and caused increased NADPH-diaphorase activity in Müller cells. We conclude that nitric oxide is produced by many different cells in the chick eye, including retinal amacrine and ganglion cells, Müller cells, retinal pigmented epithelium, and cells in the choroid, and likely has a broad range of visual and regulatory functions.

Role of interferon regulatory factor-1 and mitogen-activated protein kinase pathways in the induction of nitric oxide synthase-2 in retinal pigmented epithelial cells

Bovine retinal pigmented epithelial cells express an inducible nitric oxide synthase (NOS-2) after activation with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS). Experiments were performed to investigate the involvement of interferon regulatory factor-1 (IRF-1) on NOS-2 induction and its regulation by NOS-2 inhibitors such as pyrrolidine dithiocarbamate (PDTC), an antioxidant, or protein kinase inhibitors. Analysis by transitory transfections showed that LPS, alone or with IFN-gamma, stimulated activity of the murine NOS-2 promoter fragment linked upstream of luciferase and its suppression by PDTC and by the different protein kinase inhibitors, genistein (tyrosine kinase inhibitor), PD98059 (mitogen-actived protein (MAP) kinase kinase inhibitor), and SB 203580 (p38 MAP inhibitor). Using specific antibodies, we have confirmed that extracellular signal-regulated kinases and p38 MAP kinase were activated by LPS and IFN-gamma in retinal pigmented epithelial cells. Analysis by reverse transcriptase-polymerase chain reaction, Western blot, and electrophoretic mobility shift assay demonstrated that IFN-gamma alone or combined with LPS induced an accumulation of IRF-1 mRNA and protein and IRF-1 DNA binding. Transfections assays with the IRF-1 promoter showed an induction of this promoter with IFN-gamma, potentiated by LPS. The decrease of LPS/IFN-gamma-induced IRF-1 promoter activity, IRF-1 synthesis, and IRF-1 activation, by PDTC, genistein, PD98059, and SB 203580, could explained in part the inhibition of the NOS-2 induction by these compounds. Our results demonstrate that IRF-1 is necessary for NOS-2 induction by LPS and IFN-gamma and that its synthesis requires the involvement of a redox-sensitive step, the activation of tyrosine kinases, and extracellular signal-regulated kinases 1/2 and p38 MAP kinases.

Activity of human 11-cis-retinol dehydrogenase (Rdh5) with steroids and retinoids and expression of its mRNA in extra-ocular human tissue

This report describes the activity of recombinant human Rdh5 (11-cis-retinol dehydrogenase) with steroids and retinoids and expression of the Rdh5 mRNA in extra-ocular human tissue. The data show that Rdh5 catalyses 9-cis-retinol metabolism equally efficiently as 11-cis-retinol metabolism and recognizes 5alpha-androstan-3alpha,17beta-diol and androsterone as substrates (3alpha-hydroxysteroid dehydrogenase activity), but not testosterone, dihydrotestosterone, oestradiol and corticosterone (lack of 17beta-hydroxysteroid and 11beta-hydroxysteroid dehydrogenase activities). Rdh5 mRNA expression was widespread in extra-ocular tissues with human liver (100% relative expression in extra-ocular tissues only) and mammary gland (97% relative to liver) showing the most intense signals. Other noteworthy relatively intense expression sites included colon (45%), thymus (43%), small intestine (39%), kidney (37%), bladder (29%), pancreas and spleen (28% each), heart (26%), uterus and ovary (25% each), testis (22%) and spinal cord (24%). Human fetal tissues also expressed Rdh5 with fetal liver showing the most intense expression among the fetal tissues (20%). Considered along with the identical nucleotide sequences in the untranslated regions of human Rdh5 and human 9-cis-retinol dehydrogenase cDNAs and the nearly identical nucleotide sequences overall (99% identity), the current results suggest that the two cDNAs represent a single gene product.

Intracellular localization and membrane topology of 11-cis retinol dehydrogenase in the retinal pigment epithelium suggest a compartmentalized synthesis of 11-cis retinaldehyde

11-cis retinol dehydrogenase (EC 1.1.1.105) catalyses the last step in the biosynthetic pathway generating 11-cis retinaldehyde, the common chromophore of all visual pigments in higher animals. The enzyme is abundantly expressed in retinal pigment epithelium of the eye and is a member of the short chain dehydrogenase/reductase superfamily. In this work we demonstrate that a majority of 11-cis retinol dehydrogenase is associated with the smooth ER in retinal pigment epithelial cells and that the enzyme is an integral membrane protein, anchored to membranes by two hydrophobic peptide segments. The catalytic domain of the enzyme is confined to a lumenal compartment and is not present on the cytosolic aspect of membranes. Thus, the subcellular localization and the membrane topology of 11-cis retinol dehydrogenase suggest that generation of 11-cis retinaldehyde is a compartmentalized process.

Changes in manganese superoxide dismutase expression after exposure of the retina to intense light

Manganese superoxide dismutase (Mn-SOD) is a naturally-occurring scavenger of superoxide, one of several reactive oxygen intermediates. To determine if Mn-SOD expression is enhanced as a defensive mechanism against oxidative challenges, such as intense light exposure, rats were exposed to cyclic light (80 lux) for 2 weeks, intense light (1,800 lux) for 24 h, and then again to cyclic light. Experimental and control (exposed to cyclic light only) eyes were enucleated 3 h, 1, 3, 7, and 14 days after light challenge. Protein expression was examined immunohistochemically using rabbit antisera against rat Mn-SOD. There was no significant difference between the light-exposed and the control groups in the thickness of the outer nuclear layers. Both retinal pigment epithelial cells and photoreceptor inner segments in the normal retina were labeled for Mn-SOD. Mn-SOD labeling was lost 3 h and day 1 after light challenge. It was re-expressed in the retinal pigment epithelial cells 3, 7, and 14 days after the light challenge, and in the photoreceptor inner segments after day 14. These results suggest that the retina might have a protective potential against light damage, in which Mn-SOD may play an important role.

Histochemical localisation of mitochondrial enzyme activity in human optic nerve and retina

AIMS

To demonstrate the quantitative distribution of mitochondrial enzymes within the human optic nerve and retina in relation to the pathogenesis of ophthalmic disease.

METHODS

Enucleations were performed at the time of multiple organ donation and the optic nerve and peripapillary retina immediately excised en bloc and frozen. Reactivities of the mitochondrial enzymes cytochrome c oxidase and succinate dehydrogenase were demonstrated in serial cryostat sections using specific histochemical assays.

RESULTS

In the optic nerve the unmyelinated prelaminar and laminar regions were rich in both cytochrome c oxidase and succinate dehydrogenase. Myelination of fibres as they exited the lamina cribrosa was associated with an abrupt reduction in enzyme activity. Within the retina, high levels of enzyme activity were found localised within the retinal ganglion cells and nerve fibre layer, the outer plexiform layer, inner segments of photoreceptors, and the retinal pigment epithelium.

CONCLUSIONS

Mitochondrial enzyme activity is preserved in human optic nerve and retina retrieved at the time of multiple organ donation. The distribution of enzyme activity within the eye has implications for the understanding of the pattern of ophthalmic involvement seen in mitochondrial diseases and the site of ganglion cell dysfunction in those patients with optic nerve involvement.

Role of IFN-gamma-induced indoleamine 2,3 dioxygenase and inducible nitric oxide synthase in the replication of human cytomegalovirus in retinal pigment epithelial cells

An in vitro model of human CMV infection of primary retinal pigment epithelial (RPE) cells was used to study the effects of cytokines on CMV replication in these cells, which are targets of CMV infection in vivo. IFN-gamma and IFN-beta were potent inhibitors of CMV replication in RPE cells, while TNF-alpha, IL-1beta, or TGF-beta2 did not affect viral replication. Inhibition by IFN-gamma, and to a lesser extent IFN-beta, was almost completely reversed by addition of L-tryptophan to the culture medium, strongly implicating the indoleamine 2,3 dioxygenase (IDO) pathway. Polyadenylated IDO mRNA accumulation was detected as early as 2 h after IFN stimulation. Furthermore, CMV blocked the production of nitric oxide by the inducible form of nitric oxide synthase. This inhibition depended on a functional viral genome. However, exogenous nitric oxide significantly inhibited viral protein expression in RPE cells. Thus, CMV infection blocks the inducible nitric oxide synthase pathway activated by IFN-gamma and IL-1beta, but cannot counteract the IFN-induced IDO pathway, which ultimately controls its replication in primary human RPE cells.

Beta-galactosidase histochemistry and telomere loss in senescent retinal pigment epithelial cells

PURPOSE

To investigate the relation of senescence-related beta-galactosidase activity and telomere shortening to replicative senescence in cultured human retinal pigment epithelial (RPE) cells.

METHODS

A human RPE cell line was serially passaged until 80% of cells were nondividing in a 72-hour 5-bromo-2'-deoxyuridine (BrdU) labeling study. Early- and late-passage cells were double-stained for BrdU and senescence-related beta-galactosidase activity (pH 6). The average chromosomal telomere length at several population doublings was estimated by Southern blot analysis after double digestion of DNA with RsaI and HinfI and using a telomere-specific probe.

RESULTS

BrdU-beta-galactosidase double-staining revealed an inverse correlation between the number of BrdU-labeled nuclei and beta-galactosidase-labeled cells as a function of population doubling level (PDL). At PDL 58, only 20% of all cells labeled for BrdU, whereas 57% stained for beta-galactosidase. The mean terminal restriction fragment length (TRF) was reduced from 10 kb in early (PDL 12) cultures to 4 kb in late (PDL 57) cultures.

CONCLUSIONS

Senescence-related beta-galactosidase activity and mean TRF length may prove useful in studying the senescence of RPE cells in vitro. These techniques may be valuable in determining senescence of the retinal pigment epithelium in vivo, where senescent RPE cells could be involved in the development of age-related maculopathy and age-related macular degeneration.

Na+,K+-ATPase activity in cultured bovine retinal pigment epithelium

PURPOSE

To characterize electrogenic Na+,K+-ATPase activity in cultured bovine retinal pigment epithelium (RPE).

METHODS

Cultured bovine RPE cells from passages 3 through 5 were dissociated enzymatically. Na+,K+-adenosine triphosphatase (ATPase)-activated currents (Ip) were measured by using a nystatin perforated-patch recording technique under voltage- clamp conditions. In the presence of suitable blockers for known voltage-dependent Na+, K+, and Ca2+ conductances, the Ip was activated in a concentration-dependent manner by adding K to the external solution.

RESULTS

The median effective concentration (EC50) and Hill coefficient for external K+ concentration ([K+]o) were 1.06 mM and 2.55, respectively. The Ip showed no significant voltage dependency. A large outward shift of holding current was observed when [Na+]o, was removed. In the presence of [Na+]o, the addition of K+ to the external solution induced Ip, even when the internal solution did not contain Na+, suggesting the existence of a continuous Na+ influx across the plasma membrane in the presence of [Na+]o,. When Na+ was removed from the external and internal solutions, a transient Ip was observed, indicating that the transient Ip was activated by the intracellular residual Na+. The Ip was concentration-dependently suppressed by ouabain. The 50% inhibitory concentration (IC50) value and Hill coefficient for ouabain were 5.98 microM and 1.12, respectively.

CONCLUSIONS

The present study is the first to reported the functional properties of electrogenic Na+,K+-ATPase activity in cultured bovine RPE.

[An analysis of parthogenetic cell clones in chimeric C57BL/6(PG)<-->BALB/c mice]

We studied the distribution of parthenogenetic cell clones in the retinal pigment epithelium and choroid of eyes on serial sections and in the brain, kidneys, and liver by electrophoretic analysis of glucose phosphate isomerase isozymes in 12 mouse chimeras C57BL/6(PG)<-->BALB/c obtained earlier. Asymmetry was noted in the distribution of the parthenogenetic cell clones in the eye structure, just as the earlier established asymmetry in the distribution of the parthenogenetic clones of epidermal melanoblasts. A high correlation was shown between the ratio of parthenogenetic to normal cells in the retinal pigment epithelium of the right or left eyes and epidermal melanoblasts in the hair cover of the corresponding body half of the chimera. These data suggest that there is a certain relationship between the processes leading to the characteristic distribution of the ectodermal parthenogenetic clones in the retinal pigment epithelium of the right and left eyes and epidermal melanoblasts in parthenogenetic chimeras. Electrophoretic analysis did not show parthenogenetic components in the liver or kidneys of any chimera, and the parthenogenetic component was found in the brain of only two chimeras, in which a high percentage of parthenogenetic cells of ectodermal origin was noted. In these cases, asymmetry was noted in the right and left cerebral hemispheres, just as in the retinal pigment epithelium of the right and left eyes. The data obtained suggest that, during the development of the chimeras, parthenogenetic C57BL/6 cells were actively eliminated from the tissues of endodermal and mesodermal origin. In adult chimeras C57BL/6(PG)<-->BALB/c, parthenogenetic cell clones of ectodermal origin are mostly preserved.

Dedifferentiation of the retinal pigment epithelium compared to the proliferative membranes of proliferative vitreoretinopathy

PURPOSE

To examine the gene expression for melanogenesis of retinal pigment epithelial cells during dedifferentiation and to compare the condition to that of eyes obtaining anatomical success after surgery for proliferative vitreoretinopathy.

METHODS

Gene expression for melanogenesis was determined by reverse transcriptase-polymerase chain reaction of tyrosinase and tyrosinase-related protein-1 genes in normal and cultured retinal pigment epithelial cells and in proliferative membranes in patients with proliferative vitreoretinopathy.

RESULTS

Gene expression for melanogenesis was classified into three types during dedifferentiation of retinal pigment epithelial cells: (1) tyrosinase-related protein-1 gene expression, (2) tyrosinase and tyrosinase-related protein-1 gene expression and (3) no expression of these genes. The expression of these genes were maintained better in mediums with basic fibroblast growth factor than in medium without basic fibroblast growth factor. Of the anatomically unsuccessful patients with proliferative vitreoretinopathy treated by surgery, 76.9% showed both tyrosinase and tyrosinase-related protein-1 gene expression; only 20% of the anatomically successful patients showed the gene expression.

CONCLUSIONS

We reported three different conditions of retinal pigment epithelial cells based on gene expression for melanogenesis during dedifferentiation. The different condition of the retinal pigment epithelial cells may have some relationship to the anatomical results for proliferative vitreoretinopathy surgery.

Effects of tamoxifen, toremifene and chloroquine on the lysosomal enzymes in cultured retinal pigment epithelial cells

Retinal pigment epithelial cells carry out phagocytosis and digestion of material shed from the photoreceptor outer segments. In this process, the integrity of lysosomal enzymes is of major importance. In the present study the effects of tamoxifen, toremifene and chloroquine on the activity of two lysosomal enzymes (cathepsin D and N-acetyl-beta-D-glucosaminidase) in the retinal pigment epithelial cells were studied. Retinal pigment epithelial cells from pig eyes were cultured for two weeks in Dulbecco's Modified Eagle Medium, after which the cells were exposed to 1-40 microM concentrations of tamoxifen citrate, toremifene citrate and chloroquine diphosphate. To eliminate possible medium-borne oestrogenic mechanisms, the test was repeated using phenol red-free medium with charcoal-stripped fetal calf serum. The exposure time was one week, after which the lysosomal enzymes cathepsin D and N-acetyl-beta-glucosaminidase were determined. Cellular injuries were assessed by quantifying the leakage of lactate dehydrogenase into the culture medium. Cathepsin D and N-acetyl-beta-D-glucosaminidase showed different sensitivities to tamoxifen, toremifene and chloroquine. The main lysosomal protease cathepsin D was more sensitive than N-acetyl-beta-D-glucosaminidase to the effects of tamoxifen and toremifene, possibly due to their antioestrogenic properties. The phenol red-free medium with charcoal-stripped serum seemed to make the drugs more effective than the reference medium. Chloroquine had only a minor effect on the lysosomal protease cathepsin D, but a clearer effect could be seen on N-acetyl-beta-glucosaminidase.

Melanosomes of retinal pigment epithelium--distribution, shape, and acid phosphatase activity

The distribution and shape of melanosomes of the retinal pigment epithelium (RPE), and acid phosphatase activity in melanosomes were studied in rabbits. The rabbit eyes were observed using electron microscopy and enzyme cytochemical electron microscopy. The majority of melanosomes were located near the apical region of the RPE. Melanosomes in the RPE were classified as two shapes, elliptical and spherical or oval. Elliptical melanosomes were located parallel to the apical process and spherical or oval melanosomes were arranged vertically or obliquely to the apical process. We think that the distribution and shape of melanosomes contributes to the effective absorption and blocking of light coming from all directions. Almost all of the mature and immature melanosomes we identified showed positive in acid phosphatase reaction, indicating that melanosomes are commonly incorporated into the lysosomal system of the RPE. However, a few melanosomes showed negative in acid phosphatase reaction, suggesting that some melanosomes are stable and inert. The observed premelanosome showed negative reaction. Two types of melanosome-related complex granules were identified; melanosomes with a cortex of enzyme-reactive material (melanolysosome) and melanosomes with a cortex of lipofuscin (melanolipofuscin). These findings indicate tha a relationship between melanosomes and the lysosomal system of the RPE exists, and suggest that melanosomes may undergo modification or degradation in the cytoplasm. Also, the observation of a premelanosome and the positive acid phosphatase activity in mature and immature melanosomes indicates that melanosomes of the RPE may continue to be synthesized at a low rate in adult eyes.

Distribution of 11-cis LRAT, 11-cis RD and 11-cis REH in bovine retinal pigment epithelium membranes

Our recent finding of the co-localization of 11-cis retinyl esters and 11-cis retinyl ester hydrolase (11-cis REH) activity in bovine retinal pigment epithelium (RPE) plasma membrane (PM) has led us to explore the possibility that the PM may provide 11-cis retinal for rhodopsin regeneration. In the RPE, visual chromophore is synthesized via a membrane associated 11-cis retinol dehydrogenase (11-cis RD). Accordingly, bovine RPE membranes enriched with either endoplasmic reticulum (ER) or plasma membrane (PM) enzyme markers were prepared and assayed for visual cycle enzyme activities. Pronounced 11-cis RD activity was associated with both ER- and PM-enriched membrane fractions. In contrast, 11-cis REH activity was mostly recovered in PM-enriched fractions while LRAT activity was found only in ER-enriched membranes. The finding that both 11-cis retinol and 11-cis retinal can be produced at the PM of the bovine RPE strongly suggests that 11-cis retinyl esters at this subcellular locale serve as a precursor of visual chromophore for pigment regeneration.

Expression of cathepsin S antisense transcripts by adenovirus in retinal pigment epithelial cells

PURPOSE

To show the production of sense or antisense transcripts by recombinant adenoviruses, to investigate whether the transcripts produced were suitable for downregulating the expression of the targeted gene, cathepsin S (CatS), and to examine the effect of antisense transcript production on the biologic function of retinal pigment epithelial (RPE) cells, including the regulation of endogenous aspartic protease expression.

METHODS

Ad.MLP.CatSAS, Ad.RSV.CatSAS, and Ad.MLP.CatSS recombinant viruses were produced by homologous recombination. The recombinant viruses were tested by restriction enzyme digestion to confirm the orientation of the inserts. The expression of antisense transcripts was tested by northern blot analysis. Western blot analysis was used to study the regulation of the endogenous CatS protein in ARPE19 cells. The biologic effect of CatS downregulation in ARPE19 cells was tested by proliferation and phagocytosis assays, de novo cathepsin D (CatD) synthesis, and measurement of aspartic protease activity.

RESULTS

After characterization of the recombinant adenovirus constructs, the production of antisense and sense CatS transcripts was shown in ARPE19 cells. The transcripts appeared at approximately 1.9 kb 48 hours after transduction, and the expression of the antisense transcripts was similar in constructs carrying either the MLP or the RSV promoter. Western blot analysis showed that ARPE19 cells transduced with Ad.MLP.CatSAS and Ad.RSV.CatSAS had no detectable CatS. In contrast, there was a strong signal appearing at 24 kDa in ARPE19 cells transduced with Ad.MLP.CatSS. ARPE19 cells were transduced to a high level. The transduction of ARPE19 cells with the recombinant adenoviruses did not affect the morphologic appearance of the cells, their proliferation, or their phagocytosing ability. However, ARPE19 cells transduced by Ad.MLP.CatSAS recombinant adenovirus showed a significant downregulation of de novo CatD synthesis and a twofold decrease in aspartic protease activity.

CONCLUSIONS

Recombinant adenoviruses were shown to be suitable for producing antisense CatS transcripts to modulate endogenous CatS expression in RPE cells. It is proposed that CatS may play an important role, directly or indirectly, in the lysosomal digestion of outer segments through the regulation of other lysosomal enzyme activity, such as the expression of CatD.

Polarization of the Na+, K(+)-ATPase in epithelia derived from the neuroepithelium

The neuroepithelium generates a fascinating group of epithelia. One of their intriguing properties is how they polarize the distribution of the Na+, K(+)-ATPase. Typically, this ion pump is concentrated in the basolateral membrane, but it is concentrated in the apical membranes of the retinal pigment epithelium and the epithelium of the choroid plexus. A comparison of their development with that of systemic epithelia yields insights into how cells polarize the distribution of this and other membrane proteins. The polarization of the Na+, K(+)-ATPase depends upon the interplay between different sorting signals and different types of polarity mechanisms. These include intracellular targeting signals that direct the delivery of newly synthesized proteins, and maintenance signals that stabilize proteins in the proper membrane domain. Conflicting signals appear to be arranged in a hierarchy that can be rearranged as cells respond to certain environmental stimuli. Part of this response is mediated by changes in the distribution and composition of the cortical cytoskeleton.

Molecular cloning and sequence analysis of a chicken cDNA encoding tyrosinase-related protein-2/DOPAchrome tautomerase

We have cloned and sequenced a chicken cDNA encoding an l-DOPAchrome tautomerase (DCT) from an embryonic melanocyte cDNA library. The chicken DCT gene encodes a deduced protein of 516 amino acids (aas) and shares 69.2% and 69.9% aa sequence identity with the deduced mouse and human DCT proteins, respectively. Northern blot hybridisation analysis reveals a DCT transcript of 3.5kb in RNA from the retinal pigment epithelium (RPE) of chick embryos. Genomic Southern blot hybridisation analysis suggests that the chicken DCT gene consists of several introns and spans between 15 and 30kb of the chicken genome. This study completes the sequencing of all the members of the chicken tyrosinase-related protein gene family and provides evidence that this gene family is conserved between avians and mammals.

Endogenous FGF1-induced activation and synthesis of extracellular signal-regulated kinase 2 reduce cell apoptosis in retinal-pigmented epithelial cells

Retinal-pigmented epithelial (RPE) cell survival is critical to the maintenance of the function of the neural retinal and in the development of various retina degenerations. We investigated molecular mechanisms involved in this function by assessing apoptosis in RPE cells following serum deprivation. Apoptosis induced by serum withdrawal is lower in aged RPE cells because of higher endogenous acidic fibroblast growth factor (FGF1) synthesis and secretion. These experiments examined several aspects of FGF signaling and the contribution of endogenous FGF1 to activation of the extracellular signal-regulated kinase 2 (ERK2). In aged RPE cells, FGFR1 was rapidly activated, and its autophosphorylation followed the kinetics of endogenous FGF1 secretion, before the onset of apoptosis. ERK2 phosphorylation, activity, and de novo synthesis increased at the same time. In marked contrast, no de novo JNK1 synthesis was observed. MEK1 inhibition resulted in lower levels of ERK2 activation and synthesis and higher levels of apoptosis. Treatment with neutralizing anti-FGF1 or blocking anti-FGFR1 antibodies mimics these effects. Thus, this study strongly suggests that the survival-increasing effect of FGF1 in aged RPE cells is because of an autocrine/paracrine loop in which the ERK2 cascade plays a pivotal role.

Retroviral cDNA transfer to the RPE: stable expression and modification of metabolism

PURPOSE

To determine the versatility of retroviral vector-mediated rat beta-glucuronidase cDNA expression in the normal retinal pigment epithelium (RPE) of eyes of various species and in RPE of eyes with three types of mucopolysaccharidosis (MPS types I, VI, and VII) and to evaluate the effect of multiple transductions and long-term stable expression in the RPE.

METHODS

A retroviral construct containing a rat beta-glucuronidase cDNA (NTK-BGEO) was used to infect RPE cells at subconfluence. The transduced cells were selected in G418, an antibiotic toxic to normal mammalian cells. Beta-glucuronidase activity was measured in transduced cells and media, using a fluorogenic substrate. Glycosaminoglycan profiles were examined by metabolically labeling RPE with Na2(35)SO4.

RESULTS

Transduced RPE cells, regardless of species or disease status, expressed rat beta-glucuronidase. The expressed enzyme restored normal levels of glycosaminoglycans in the RPE cells of homozygous MPS VII-affected dogs by metabolizing stored glycosaminoglycans. The expressed enzyme failed to metabolize stored glycosaminoglycans of MPS I and MPS VI, indicating that overexpression could not bypass the exoglycosidase restriction. Multiple transductions increased beta-glucuronidase activity several times in the cell layer and in the media. The expression was stable in vitro for at least 12 weeks.

CONCLUSIONS

A retroviral vector can mediate transfer of beta-glucuronidase in various species of normal and MPS-affected RPE. The expression is stable in vitro. The metabolism of stored glycosaminoglycans in MPS needs replacement of only the deficient enzyme to reverse the storage.

Galactosemic cataractogenesis disrupts intracellular interactions and changes the substrate specificity of choline/ethanolamine kinase

The functional characteristics of enzymes depends upon their environment, and within physiologically intact cells, many metabolic pathways are thought to involve multienzyme complexes and other enzyme-enzyme interactions that increase efficiency and specificity by mechanisms such as channeling of intermediates. A disease such as cataract may change the intracellular environment, but the effects of these changes on enzyme-enzyme interactions can be observed only in relatively intact cells, and in enzymes that have unambiguously different properties in different environments. In intact rat lenses, choline and ethanolamine are phosphorylated independently, with no competition between the two compounds, as the first step of phospholipid biosynthesis. However, disruption of lens structure and intracellular interactions by homogenization leads to a paradoxical change in enzymic properties, causing choline and ethanolamine to become competing alternative substrates of a single enzyme that resembles the purified choline/ethanolamine kinase from liver and other tissues. The properties of ethanolamine kinase in intact cataractous lenses from rats fed a 50% galactose diet for 7-14 days were intermediate between those of intact control lenses and those in lens homogenates. In monolayers of human and dog lens epithelial cells and human retinal pigment epithelial cells ethanolamine kinase was similar to that in intact tissue, showing that the kinetic differences between intact lenses and homogenates result from the intracellular environment, not from artifacts of diffusion, and that they are not exclusive to rats or to lens cells. Results with intact lenses from monkeys, rabbits, pigs and dogs showed some differences between species, but in every case, choline had little or no effect on the phosphorylation of radiolabeled ethanolamine. Further studies will be necessary to determine how the changes in intracellular environment during cataractogenesis affect other enzymes and whether other model systems for cataractogenesis cause similar changes.