The retinal pigment epithelium: a versatile partner in vision

Retinal pigment epithelium cell culture on thin biodegradable poly(DL-lactic-co-glycolic acid) films

Thin films of 50:50 and 75:25 poly(DL-lactic-co-glycolic acid) (PLGA) were manufactured with a controlled thickness of less than 10 microm. The effect of PLGA copolymer ratio on in vitro cell attachment, proliferation, morphology, and tight junction formation was evaluated using a human D407 retinal pigment epithelium (RPE) cell line. Almost complete cell attachment was achieved on both PLGA films after 8 h of cell seeding, which was comparable to that on tissue culture polystyrene (TCPS) controls. The initial cell seeding density affected attachment, and the optimal value for 50:50 PLGA was 25000 cells cm(-2). After 7 days of in vitro culture, cell density on 50:50 and 75:25 PLGA films increased 45 and 40 folds, respectively, and a 34-fold increase was observed on TCPS. The RPE cells cultured on PLGA films at confluence had a characteristic cobblestone morphology. Confluent RPE cells also developed normal tight junctions in vitro which were concentrated mainly at the apical surfaces of cell-cell junctions. These results demonstrated that thin biodegradable PLGA films can provide suitable substrates for human RPE cell culture, and may serve as temporary carriers for subretinal implantation of organized sheets of RPE.

Isolation and one-step preparation of A2E and iso-A2E, fluorophores from human retinal pigment epithelium

Age-related macular degeneration, a major cause of blindness for which no satisfactory treatments exist, leads to a gradual decrease in central high acuity vision. The accumulation of fluorescent materials, called lipofuscin, in retinal pigment epithelial cells of the aging retina is most pronounced in the macula. One of the fluorophores of retinal pigment epithelial lipofuscin has been characterized as A2E, a pyridinium bis-retinoid, which is derived from two molecules of vitamin A aldehyde and one molecule of ethanolamine. An investigation aimed at optimizing the in vitro synthesis of A2E has resulted in the one-step biomimetic preparation of this pigment in 49% yield, readily producing more than 50 mg in one step. These results have allowed for the optimization of HPLC conditions so that nanogram quantities of A2E can be detected from extracts of tissue samples. By using 5% of the extract from individual aged human eyes, this protocol has led to the quantification of A2E and the characterization of iso-A2E, a new A2E double bond isomer; all-trans-retinol and 13-cis-retinol also have been identified in these HPLC chromatograms. Exposure of either A2E or iso-A2E to light gives rise to 4:1 A2E:iso-A2E equilibrium mixtures, similar to the composition of these two pigments in eye extracts. A2E and iso-A2E may exhibit surfactant properties arising from their unique wedge-shaped structures.

Morphogenesis of the retinal pigment epithelium: toward understanding retinal degenerative diseases

The phenotype of an epithelial cell is defined by a unique combination of morphology, gene and protein expression, and protein localization. Results indicate that the terminal differentiation of the RPE cell can be described in part by changes in the polarity of its surface proteins alpha v beta 5 integrin, Na,K-ATPase, N-CAM, and EMMPRIN. Changes in protein/gene expression and protein localization in late stages of RPE development identify alpha v beta 5 integrin as a key player in RPE phagocytosis, and N-CAM and EMMPRIN as potentially important molecules in other RPE functions necessary for photoreceptor survival. By studying the trafficking of the later two proteins it is shown that entry into an apical or basolateral pathway in RPE cells cannot be predicted by the distribution of a given protein in other epithelial cells, and that this distribution may change through the course of RPE development. The mechanisms used by RPE and other epithelia to establish and maintain their specific polarity properties are fundamental to the formation and maintenance of their specific epithelial phenotype. The ability to therapeutically direct molecules incorporated into RPE by gene therapy into apical or basal surfaces requires an understanding of protein localization and expression. Furthermore, evidence is provided that assays capitalizing on changes in gene/protein expression and protein localization during the late stages of RPE development can prove a productive way of identifying proteins used by RPE for photoreceptor support. This approach can continue to be exploited to identify other proteins essential for the mission of the RPE cell, that may thus be likely candidates for participation in retinal degenerative disease.

Genomic structure and developmental expression of the chicken nonocarboxylate transporter MCT3 gene

MCT3 is a monocarboxylate transporter that is specifically expressed on the basolateral membrane of retinal pigment epithelial cells (RPE). In these studies the temporal expression of MCT3 during ocular development was examined using Northern blot analysis. A 2.2 kb transcript (MCT3b) was detected in RPE by embryonic day 7 (E7) and was present throughout embryonic development. A 2.45 kb transcript (MCT3a) was expressed at low levels before E11 but its expression increased between E11 and E17. Using 5'-RACE (rapid amplification of cDNA ends) it has demonstrated that MCT3a and MCT3b mRNA had distinct 5'-untranslated sequences but shared the same translation start site. To determine the exon-intron structure and to understand the elements that control the tissue specific and developmental expression of MCT3, the MCT3 gene was cloned and sequenced from a chicken genomic library. The MCT3 gene is distributed over 8 kb of DNA and is composed of 6 exons. Coding sequences for MCT3 are found on exon 2 through exon 5. Comparison of the 5'-RACE sequence with the genomic sequence reveals that the two 5'-untranslated regions of the mRNAs are encoded by distinct exons, 1a and 1b, which are alternatively spliced to exon 2. These data suggest that two forms of MCT3 mRNAs could be generated by two distinct promoters that may be regulated in response to changes in the metabolic activity of the retina during development.

Expression of the protective proteins hemopexin and haptoglobin by cells of the neural retina

The blood-retinal barrier, consisting of retinal pigment epithelial cells and retinal endothelial cells, prevents hemopexin and haptoglobin, anti-oxidant protective plasma proteins normally synthesized by the liver, from entering the neural retina. If present, these proteins must, therefore, be made locally. The cell types within the retina in which hemopexin and haptoglobin mRNAs are made have been investigated. RNA was extracted from both the neural retina and pigment epithelium obtained by dissection of human donor eyes as well as from cultured pigment epithelial and photoreceptor cells. The mRNAs for both haptoglobin and hemopexin were detected, using reverse-transcriptase polymerase chain reaction, in the neural retina and cultured photoreceptors but not in pigment epithelial cells. The cellular location of these mRNAs was determined using in situ hybridization of sections of human retina which revealed that haptoglobin mRNA was located principally in the photoreceptor cells, cells of the inner nuclear layer and some cells of the ganglion cell layer. Hemopexin mRNA, previously shown to be made in the human neural retina (Hunt et al., 1996. Journal of Cellular Physiology 168: 71-80), is expressed by most of the cells of neural retina including the photoreceptors and, notably, the ganglion cells.

Monocarboxylate transporter MCT1 is located in the apical membrane and MCT3 in the basal membrane of rat RPE

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier and regulates the movement of nutrients, water, and ions between the choroidal blood supply and the retina. The transport properties of the RPE maintain retinal adhesion and regulate the pH and osmolarity in the space surrounding the photoreceptor cell outer segments. In this report we identify two monocarboxylate transporters, MCT1 and MCT3, expressed in rat RPE. On the basis of Northern and Western blot analyses, MCT1 is expressed in both the neural retina and the RPE, whereas the expression of MCT3 is restricted to the RPE. Using indirect immunolocalization we show that the two transporters are polarized to distinct membrane domains. MCT1 antibody labels the apical surface and the apical processes of the RPE. A polyclonal antibody produced against the carboxy terminus of rat MCT3 labels only the basolateral membrane of the RPE. The demonstration of MCT1 on the apical membrane and MCT3 on the basal membrane identifies specific proteins involved in the discriminate and critical regulation of water and lactate transport from the retina to the choroid.

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.

Critical role of TrkB and brain-derived neurotrophic factor in the differentiation and survival of retinal pigment epithelium

In the vertebrate eye, the retinal pigment epithelium (RPE) and the neural retina arise from a single layer of neuroectoderm. Factors influencing the differentiation of retinal neurons have been identified; however, little is known about molecules directing the differentiation of the RPE. Here we have found that the neurotrophin brain-derived neurotrophic factor (BDNF) plays an autocrine role in the differentiation and survival of Xenopus laevis RPE. Fluorescent in situ hybridization studies showed a precise co-expression of BDNF and its receptor trkB in the retinal neuroepithelium and actively differentiating RPE; in vitro studies demonstrated survival- and differentiation-promoting effects in serum-free explants and dissociated cultures. When a dominant negative mutant of the trkB receptor was expressed in developing embryos, severe arrest of RPE differentiation was seen with persistence of nestin- and Notch-positive neuroblasts.

Sodium-calcium exchanger in cultured human retinal pigment epithelium

Regulation of intracellular free Ca2+ concentration ([Ca2+]i) by an Na+/Ca2+ exchanger was studied in cultures of human retinal pigment epithelial cells using Ca(2+)-indicator dyes (fura-2 and fluo-3) and digital fluorescence imaging. Mean resting [Ca2+]i of cultured RPE in a control Ringer solution was 189 +/- 16 nM. Replacing extracellular Na+ with N-methyl-D-glucamine elicited a two-fold rise in [Ca2+]i; the magnitude of the [Na+]o-free-induced rise in [Ca2+]i varied as a function of extracellular [Ca2+]. The [Na+]o-free response was not significantly affected by the Ca2+ channel blocker nifedipine, or by pretreatment with thapsigargin which depletes intracellular Ca2+ stores. By contrast, the [Na+]o-free-induced rise in [Ca2+]i was significantly reduced by CBDMB, an amiloride derivative that is highly selective for Na+/Ca2+ exchange inhibition. These findings indicate that removal of extracellular Na+ promotes net [Ca2+]i gain via Na+/Ca2+ exchange. Western and Northern blot analyses, respectively, confirmed the presence of Na+/Ca2+ exchanger protein and mRNA in cultures of human RPE. Specifically, Western blot analysis of whole cell lysates of cultured RPE using a polyclonal antibody made against the canine cardiac exchanger identified a major band at approximately 126 kD. Northern blot analysis of total human RPE RNA using a restriction fragment cRNA probe coding for the canine cardiac Na+/Ca2+ exchanger showed that the major exchanger-related transcript was approximately 6.8 kb. In sum, our findings demonstrate the presence of a cardiac-exchanger-related transcript was approximately 6.8 kb. In sum, our findings demonstrate the presence of a cardiac-type Na+/Ca2+ exchanger in cultures of human RPE.

Phagocytosis of rod outer segments by retinal pigment epithelial cells requires alpha(v)beta5 integrin for binding but not for internalization

Phagocytosis of shed photoreceptor rod outer segments (ROS) by the retinal pigment epithelium (RPE) is essential for retinal function. Here, we demonstrate that this process requires alpha(v)beta5 integrin, rather than alpha(v)beta3 integrin utilized by systemic macrophages. Although adult rat RPE expressed both alpha(v)beta3 and alpha(v)beta5 integrins, only alpha(v)beta3 was expressed at birth, when the retina is immature and phagocytosis is absent. Expression of alpha(v)beta5 was first detected in RPE at PN7 and reached adult levels at PN11, just before onset of phagocytic activity. Interestingly, alpha(v)beta5 localized in vivo to the apical plasma membrane, facing the photoreceptors, and to intracellular vesicles, whereas alpha(v)beta3 was expressed basolaterally. Using quantitative fluorimaging to assess in vitro uptake of fluorescent particles by human (ARPE-19) and rat (RPE-J) cell lines, alpha(v)beta5 function-blocking antibodies were shown to reduce phagocytosis by drastically decreasing (85%) binding of ROS but not of latex beads. In agreement with a role for alpha(v)beta5 in phagocytosis, immunofluorescence experiments demonstrated codistribution of alpha(v)beta5 integrin with internalized ROS. Control experiments showed that blocking alpha(v)beta3 function with antibodies did not inhibit ROS phagocytosis and that alpha(v)beta3 did not colocalize with phagocytosed ROS. Taken together, our results indicate that the RPE requires the integrin receptor alpha(v)beta5 specifically for the binding of ROS and that phagocytosis involves internalization of a ROS-alpha(v)beta5 complex. Alpha(v)beta5 integrin does not participate in phagocytosis by other phagocytic cells and is the first of the RPE receptors involved in ROS phagocytosis that may be specific for this process.

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.

Implication of OTX2 in pigment epithelium determination and neural retina differentiation

The expression pattern of Otx2, a homeobox-containing gene, was analyzed from the beginning of eye morphogenesis until neural retina differentiation in chick embryos. Early on, Otx2 expression was diffuse throughout the optic vesicles but became restricted to their dorsal part when the vesicles contacted the surface ectoderm. As the optic cup forms, Otx2 was expressed only in the outer layer, which gives rise to the pigment epithelium. This early Otx2 expression pattern was complementary to that of PAX2, which localizes to the ventral half of the developing eye and optic stalk. Otx2 expression was always observed in the pigment epithelium at all stages analyzed but was extended to scattered cells located in the central portion of the neural retina around stage 22. The number of cells expressing Otx2 transcripts increased with time, following a central to peripheral gradient. Bromodeoxyuridine labeling in combination with immunohistochemistry with anti-OTX2 antiserum and different cell-specific markers were used to determine that OTX2-positive cells are postmitotic neuroblasts undergoing differentiation into several, if not all, of the distinct cell types present in the chick retina. These data indicate that Otx2 might have a double role in eye development. First, it might be necessary for the early specification and subsequent functioning of the pigment epithelium. Later, OTX2 expression might be involved in retina neurogenesis, defining a differentiation feature common to the distinct retinal cell classes.

Identification of a unique monocarboxylate transporter (MCT3) in retinal pigment epithelium

The retinal pigment epithelium transports lactate between two tissue compartments, the interphotoreceptor matrix and the choriocapillaris. In this report we describe a 2.45-kb cDNA isolated from a chick cDNA RPE library that encodes a membrane protein found only in RPE cells. The deduced protein has 542 amino acids with twelve putative membrane spanning domains. The cDNA has been designated MCT3 based on its 45% identity in amino acid sequence and structural similarity with the monocarboxylate transporters MCT1 and MCT2. Stable transfectants (pCl-neo/MCT3), made in a rat thyroid epithelial cell line (FRTL-5), express MCT3 RNA. Transfectants had enhanced pyruvate uptake (used as a measure of lactate uptake) which was proton-dependent and inhibited by alpha-cyano-4-hydroxycinnamate. In summary, MCT3's unique expression in RPE cells, multiple potential phosphorylation sites, and basolateral distribution suggest that MCT3 may regulate lactate levels in the interphotoreceptor space.

Dark adaptation in vertebrate photoreceptors

Exposure of the eye to bright light bleaches a significant fraction of the photopigment in rods and cones and produces a prolonged decrease in the sensitivity of vision, which recovers slowly as the photopigment is regenerated. This sensitivity decrease is larger than would be expected merely from the decrease in the concentration of the pigment. Recent experiments have shown that the decrease in sensitivity is produced largely by an excitation of the phototransduction cascade by bleached pigment; even in darkness, it produces an equivalent background similar to that produced by real steady background illumination. Thus, excitation produced by a form of rhodopsin thought previously to be inactive has a profound effect on the physiology of the photoreceptor. This raises the possibility that forms of other G protein-coupled receptors thought to be inactive might also play an important role in signal transduction and disease.

Light-mediated retinoic acid production

Retinoids serve two main functions in biology: retinaldehyde forms the chromophore bound to opsins, and retinoic acid (RA) is the activating ligand of transcription factors. These two functions are linked in the vertebrate eye: we describe here that illumination of the retina results in an increase in RA synthesis, as detected with a RA bioassay and by HPLC. The synthesis is mediated by retinaldehyde dehydrogenases which convert some of the chromophore all-trans retinaldehyde, released from bleached rhodopsin, into RA. As the eye contains high levels of retinaldehyde dehydrogenases, and as the oxidation of retinaldehyde is an irreversible reaction, RA production has to be considered an unavoidable by-product of light. Through RA synthesis, light can thus directly influence gene transcription in the eye, which provides a plausible mechanism for light effects that cannot be explained by electric activity. Whereas the function of retinaldehyde as chromophore is conserved from bacteria to mammals, RA-mediated transcription is fully evolved only in vertebrates. Invertebrates differ from vertebrates in the mechanism of chromophore regeneration: while in the invertebrate visual cycle the chromophore remains bound, it is released as free all-trans retinaldehyde from illuminated vertebrate rhodopsin. RA synthesis occurring as corollary of dark regeneration in the vertebrate visual cycle may have given rise to the expansion of RA-mediated transcriptional regulation.

Serum retinoids in retinitis pigmentosa patients treated with vitamin A

BACKGROUND

Patients with retinitis pigmentosa have been suggested to benefit from treatment with moderate doses of retinyl palmitate. Retinyl palmitate is not an active retinoid in itself but is metabolised to active components in the body. To find out which metabolites of retinyl palmitate were formed and at which concentrations, we measured the concentrations of retinol, retinyl palmitate, retinoic acids and tocopherol in serum of patients treated with oral retinyl palmitate for retinitis pigmentosa.

METHODS

Nine male patients and one female diagnosed as having retinitis pigmentosa after a complete ophthalmological examination including a full-field electroretinogram were given vitamin A at their own request as one daily morning dose of 16600 IU vitamin A. Blood samples were obtained before and after > 2 weeks of treatment. The concentrations of retinoids and tocopherol were measured with established methods.

RESULTS

The patients were not deficient in vitamin A or vitamin E as judged from the serum vitamin concentrations. Treatment with retinyl palmitate significantly increased the serum concentration of retinyl palmitate and of 13-cis-retinoic acid but not of retinol, tocopherol or all-trans-retinoic acid.

CONCLUSIONS

Neither retinyl palmitate nor 13-cis-retinoic acid, are known to be biologically active. However, 13-cis-retinoic acid can isomerise to the active vitamin A derivative, all-trans-retinoic acid. It is suggested that patients may be treated with a small dose of 13-cis-retinoic acid instead, to avoid the relatively long metabolic detour from retinyl palmitate.

Heme-mediated reactive oxygen species toxicity to retinal pigment epithelial cells is reduced by hemopexin

Catalysis of the formation of reactive oxygen species (RO2S) by low molecular weight complexes of iron has been implicated in several pathological conditions in the retina since photoreceptors and retinal pigment epithelial cells are likely to be especially sensitive to RO2S. Since protective proteins cannot cross the blood-retinal barrier, it is likely that the retina performs its own protective functions by synthesizing proteins that bind iron and nonprotein iron complexes, the major catalysts of RO2S generation. Investigations were carried out to determine whether pigment epithelial cells are themselves sensitive to iron-generated RO2S and whether apo-transferrin and apo-hemopexin, known to be made locally in the retina, can perform a protective function. In 51Cr release assays, the toxicity of exogenous RO2S including hydrogen peroxide or superoxide (generated by xanthine oxidase/hypoxanthine) to human retinal pigment epithelial cells was inhibited by the iron chelators, desferrioxamine and apo-transferrin. Free but not protein-bound ferric iron and heme exacerbated the toxic effect. The toxic effect of heme was abolished by the heme-scavenging, extracellular antioxidant, apo-hemopexin, and also by exogenous bovine serum albumin. In addition, heme toxicity was inhibited by a 3 h preincubation of cells with either heme, apo-hemopexin, or heme-hemopexin 24 h prior to the toxicity assay. It is concluded, first, that toxic effects of iron and heme can be prevented by apo-transferrin or apo-hemopexin and, second, that exposure of RPE cells to free heme or hemopexin sets in motion a series of biochemical events resulting in protection against oxidative stress. It is probable that these include heme oxygenase induction.

Retinal dysplasia and degeneration in RARbeta2/RARgamma2 compound mutant mice

The eye is the organ whose development is the most frequently altered in response to maternal vitamin A deficiency [VAD; Warkany, J. and Schraffenberger, S. (1946). Archs Ophthalmol. 35, 150–169]. With the exception of prenatal retinal dysplasia, all the ocular abnormalities of the fetal VAD syndrome are recapitulated in mouse mutants lacking either RARalpha and RARbeta2, RARalpha and RARgamma, RARgamma and RARbeta2, or RXRalpha [Lohnes, D., Mark, M., Mendelsohn, C., Dolle, P., Dierich, A., Gorry, P., Gansmuller, A. and Chambon, P. (1994) Development 120, 2723–2748; Mendelsohn, C., Lohnes, D., Decimo, D., Lufkin, T., LeMeur, M., Chambon, P. and Mark, M. (1994) Development 120, 2749–2771; Kastner, P., Grondona, J. Mark, M., Gansmuller, A., LeMeur, M., Decimo, D., Vonesch, J.L., Dolle, P. and Chambon, P. (1994) Cell 78, 987–1003], thus demonstrating that retinoic acid (RA) is the active vitamin A metabolite during prenatal eye morphogenesis. Whether retinoids are also involved in postnatal eye development could not be investigated, as VAD newborns are not viable and the above RAR double null mutants and RXRalpha null mutants died in utero or at birth. We report here the generation of viable RARbeta2/RARgamma2 double null mutant mice, which exhibit several eye defects. The neural retina of newborn RARbeta2gamma2 mutants is thinner than normal due to a reduced rate of cell proliferation, and from day 4 shows multiple foci of disorganization of its layers. These RARbeta2gamma2 mutants represent the first genetically characterized model of retinal dysplasia and their phenotype demonstrates that RARs, and therefore RA, are required for retinal histogenesis. The RARbeta2gamma2 retinal pigment epithelium (RPE) cells display histological and/or ultrastructural alterations and/or fail to express cellular retinol binding protein I (CRBPI). Taken altogether, the early onset of the RPE histological defects and their striking colocalisation with areas of the neural retina displaying a faulty laminar organization, a reduced neuroblastic proliferation, and a lack of photoreceptor differentiation and/or increased apoptosis, make the RPE a likely target tissue of the RARbeta2gamma2 double null mutation. A degeneration of the adult neural retina, which may similarly be secondary to a defective RPE, is also observed in these mutants, thus demonstrating an essential role of RA in the survival of retinal cells. Moreover, all RARbeta2gamma2 mutants display defects in structures derived from the periocular mesenchyme including local agenesis of the choroid and of the sclera, small eyelids, and a persistence of the primary mesenchymal vitreous body. A majority of the RARbeta2 single null mutants also exhibit this latter defect, thus demonstrating that the RARbeta2 isoform plays a unique role in the formation of the definitive vitreous body.

Hemopexin in the human retina: protection of the retina against heme-mediated toxicity

The existence of the blood-retinal barrier means that proteins that protect the retina from damage by reactive oxygen species must either be made locally or specifically transported across the barrier cells; however, such transepithelial transport does not seem to occur. Among the circulatory proteins that protect against iron-catalyzed production of free radicals are apo-transferrin, which binds ferric iron and has previously been shown to be made by cells of the neural retina (Davis and Hunt, 1993, J. Cell Physiol., 156:280-285), and the extracellular antioxidant, apo-hemopexin, which binds free heme (iron-protoporphyrin IX). Since hemorrhage and heme release can be important contributing factors in retinal disease, evidence of a hemopexin-based retinal protection system was sought. The human retina has been shown to contain apo-hemopexin which is probably synthesized locally since its mRNA can be detected in retinal tissue dissected from human donor eyes. It is likely that the retina contains a mechanism for the degradation of hemopexin-bound heme since the blood-retinal barrier also precludes the exit of heme-hemopexin from the retina. Retinal pigment epithelial cells have been found to bind and internalize heme-hemopexin in a temperature-dependent, saturable, and specific manner, analogous to the receptor-mediated endocytic system of hepatoma cells. Moreover, the binding of heme-hemopexin to the cells stimulates the expression of heme oxygenase-1, metallothionein-1, and ferritin.

Immunodetection of an arrestin-like protein in human retinal pigment epithelium

The goal of this study was to determine if an arrestin/S-antigen-like protein is produced by human retinal pigment epithelial (HRPE) cells maintained in tissue culture. Arrestin immunoreactivity was examined in fixed, monolayer cultures of HRPE and on immunoblots of SDS-PAGE separations of whole cell lysates of HRPE using five monoclonal antibodies (mAbs A2G5, A9C6, 3C4.2, 3D1.2 and 5c6.47) that bind to different epitopes in bovine retinal S-antigen. Monolayers of HRPE cells showed immunoreactivity with four of the mAbs though the relative staining intensity varied among mAbs and donors. For example, mAb A2G5 which historically shows very limited crossreactivity among species, reacted strongly with cells from one donor, moderately with cells from a second donor and only weakly with other donor cultures examined. mAb 3D1.2 showed no reactivity with HRPE cells. Immunoblots of SDS-PAGE separations of whole cell lysates of HRPE established from ten different donors confirmed the presence of an arrestin-related polypeptide that comigrated with retinal arrestin. These results demonstrate the presence of an arrestin-like protein in HRPE cells which have been maintained in tissue culture. Though the function of this arrestin homologue in HRPE is not yet established, it could play a role in the downregulation of receptor and/or transport protein activity.

The retinal pigmented epithelium is required for development and maintenance of the mouse neural retina

BACKGROUND

During development of the vertebrate eye, there is a series of reciprocal cellular interactions that determine the fate of the eye components. Although evidence from organ culture suggests that the retinal pigmented epithelium (RPE) organizes the laminar structure of the differentiated neural retina, no role has been identified for the RPE in early eye development, nor has the later function of RPE been demonstrated in vivo.

RESULTS

To investigate the role of RPE cells in eye development, we generated transgenic mice that carry the attenuated diphtheria toxin-A gene; this transgene was driven by the promoter of the gene encoding the tyrosinase-related protein-1, which is specifically expressed in pigment cells. Depending on the expression level of the transgene, the retinal epithelium was ablated before or after its differentiation into a pigmented cell layer. We show that an early ablation (embryonic day E10-11) resulted in disorganization of the retinal layer, immediate arrest of eye growth and subsequent eye resorption. A later ablation (E11.5-12.5) allowed the eye to be maintained during embryogenesis, but the laminar structure of the retina became disrupted by the end of gestation, the vitreous failed to accumulate the adults were anophthalmic or severely microphthalmic. In some microphthalmic eyes, a number of RPE cells escaped ablation and formed patches of pigmented cells; the laminar structure of the retina was maintained immediately adjacent to such pigmented areas but disrupted elsewhere. In both cases--early or late ablation of the RPE--the retina appears to be the primary affected tissue.

CONCLUSIONS

We conclude that presence of the RPE is required for the normal development of the eye in vivo. Its presence early in development is necessary for the correct morphogenesis of the neural retina. After the neural retina has started to differentiate, the RPE is still necessary, either directly or indirectly, to maintain the organization of the retinal lamina.

Microfilament organization and wound repair in retinal pigment epithelium

Several systems of microfilaments (MF) associated with adherens-type junctions between adjacent retinal pigment epithelial (RPE) cells and between these cells and the substratum play an important role in maintaining the integrity and organization of the RPE. They include prominent, contractile circumferential MF bundles that are associated with the zonula adherens (ZA) junctions. In chick RPE, these junctions are assembled from smaller subunits thus giving greater structural flexibility to the junctional region. Because the separation of the junctions requires trypsin and low calcium, both calcium-dependent and -independent mechanisms are involved in keeping adjacent RPE cells attached to one another. Another system of MF bundles that crosses the cell at the level of ZA junctions can be induced to form by stretching the epithelium. The MF bundles forming this system are oriented in the direction in which the RPE is stretched, thereby preventing the overextension of the cell in any one direction. The system may be useful as an indicator of the direction in which tension is experienced by RPE during development of the eye, in animal models of disease and during repair of experimentally induced wounds. Numerous single-cell wounds resulting from death of RPE cells by apoptosis at various stages of repair are normally present in developing chick and adult mammalian RPE. These wounds are repaired by the spreading of adjacent RPE cells and by the contraction of MF bundles oriented parallel to the wound edge, which develop during this time. As a result of the spreading in the absence of cell proliferation, the RPE cells increase in diameter with age. Experimentally induced wounds made by removing 5-10 RPE cells are repaired by a similar mechanism within 24 h. In repair of larger wounds, over 125 microns in width, the MF bundles oriented parallel to the wound edge characteristic of spreading cells are later replaced by stress fibers (SFs) that run perpendicularly to the wound edge and interact with the substratum at focal contacts (FCs) as RPE cells start to migrate. Cell proliferation is induced in cells along the wound edge only when the wounds are wide enough to require cell migration. In the presence of antibodies to beta-1-integrins, a component of FCs, cell spreading is not prevented but both cell migration and cell proliferation are inhibited. Thus, only the organization of the cytoskeleton characteristic of migrating RPE cells that have SFs that interact with the substratum at FCs, is associated with the induction of cell proliferation.

Thermal control of rod outer segment length and shedding in a fish, Fundulus zebrinus

The effects of temperature on rod outer segment (ROS) length and membrane shedding were studied in a cyprinodont fish, Fundulus zebrinus. After 30 days in 14L/10D cyclic light and 17 degrees C, ROS length averaged 41.2 microns. Fish were then exposed to 7, 17 or 27 degrees C for 10 and 25 days before being sampled 5 hr before and 1-4 hr after light onset. In 7 degrees C ROS shortened to 83.5% of initial controls within 10 days, then only 4.1% further, to 79.4% by day 25 (34.4, 32.7 microns). ROS length did not change significantly in fish remaining at 17 degrees C (39.7 and 40.7 microns at day 10 and 25) or in fish moved to 27 degrees C (41.7 and 41.6 microns). Phagosomes were most numerous in 7 degrees C and least numerous in 17 degrees C, but varied in overall size among the largest phagosomes being more common after light onset. After light onset at day 25, the estimated volume per phagosome was 1.14, 4.73 and 5.75 microns 3 in 7, 17 and 27 degrees C. Total phagosome volume per 100 microns RPE at 27 degrees C was generally double that at 17 degrees C. Apparently, in F. zebrinus, the number of disks shed from ROS is adjusted during thermal acclimation to stabilize ROS length.

Expression of kinesin heavy chain isoforms in retinal pigment epithelial cells

To examine the possible role of kinesin in pigment granule migration in the retinal pigment epithelium (RPE) of teleosts, we investigated the expression and distribution of kinesin heavy chain (KHC) in RPE. Blots of fish RPE lysates probed with two well-characterized antibodies to KHC (H2 and HD) displayed a prominent band at 120 kD. A third KHC antibody (SUK4) recognized a band at 118 kD. The 118 kD band was also occasionally present in blots probed with H2, suggesting the presence of two KHC isoforms in teleost RPE. Reverse transcriptase-polymerase chain reaction (RT-PCR) of mRNA from RPE using primers homologous to conserved regions of the KHC motor domain resulted in the identification of two putative KHC genes (FKIF1 and FKIF5) based on partial amino acid sequences. Previous studies had demonstrated a requirement for microtubules in pigment granule aggregation in RPE. In addition, the reported microtubule polarity orientation in RPE apical projections is consistent with a role for kinesin in pigment granule aggregation. Immunofluorescent localization of KHC in isolated RPE cells using H2 revealed a mottled distribution over the entire cell body, with no detectable selective association with pigment granules, even in cells fixed while aggregating pigment granules. Microinjected KHC antibodies had no effect on pigment granule aggregation or dispersion, although each of the three antibodies has been shown to block kinesin function in other systems. Thus we found no evidence for KHC function in RPE pigment granule aggregation. However, the two KHC isoforms may participate in other microtubule-dependent processes in RPE.