Acid phosphatase localization in normal and dystrophic retinal pigment epithelium

Summary In this study acid phosphatase (ACPase) was localized in the retinal pigment epithelium (RPE) of normal and Royal College of Surgeons (RCS) rats pink-eyed and pigmented with inherited retinal dystrophy to determine differences in staining during the post-engulfment stages of phagocytosis using two substrates, Na--glycerophosphate and cytidine-5-monophosphate. Staining was similar using either substrate and in the normal RPE the Golgi system, lysosomes and phagosomes were ACPase-positive. In the dystrophic RPE, which has a diminished capacity to phagocytose photoreceptor rod outer segments, ACPase staining was localized on melanosomes in the pigmented dystrophic and on the apical microvillous membranes in the pink-eyed dystrophic, but was not localized on phagosomes in either the pink-eyed or pigmented dystrophic RPE. Since only a few phagosomes were seen at any given time in dystrophic RPE in vivo, a tissue expiant system was used to examine the number of latex beads phagocytosed by normal and RCS RPE, as well as the number of phagosomes containing both beads and ACPase activity in the normal and mutant RPE. Our findings indicate that in the dystrophic, fewer phagosomes are ACPase-positive than in the normal, and that some enzyme may be inappropriately shunted to either the apical microvilli or to melanosomes instead of to phagolysosomes.     

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.     

Claudin localization in cilia of the retinal pigment epithelium

Using immunocytochemistry and confocal microscopy we demonstrate that claudin-immunoreactivity is a novel marker for retinal pigment epithelial cilia. Claudin-immunoreactivity obtained by polyclonal anti-claudin 1 antibody, which could crossreact with claudin 3, was colocalized with acetylated tubulin-immunoreactivity in cultured human retinal pigment epithelial cells. Claudin-immunoreactivity associated with the retinal pigment epithelium (RPE) cilia was more intense than was claudin-immunoreactivity in the junctional complex. Approximately two-thirds of the RPE cells in the rat contain cilia that are immunoreactive with acetylated tubulin on postnatal day 1, and a significant portion of these cilia label with the anti-claudin 1 antibody. Cilia decrease in frequency over subsequent postnatal days, and are absent by postnatal day 30. As RPE cilia decrease in number during postnatal rat development, claudin-immunoreactivity is lost earlier than acetylated tubulin, suggesting that the loss of claudin may initiate RPE cilium degeneration. Claudin-immunoreactivity was not evident in cilia of photoreceptor cells, epithelia of nasal mucosa, small intestine, or colon, suggesting that claudin may be a unique molecule in RPE cilia. These data suggest that cilia of the RPE, unlike cilia on other cell types, contain claudin, and that this molecule may play an important and specific role in the function and/or maintenance of RPE cilia.     

Light-evoked responses of the mouse retinal pigment epithelium

In response to light, the retinal pigment epithelium (RPE) generates a series of slow potentials that can be recorded as the c-wave, fast oscillation (FO), and light peak (LP) of the electroretinogram (ERG). As these potentials can be related to specific cellular events, they provide information about RPE function and how that may be altered by disease or experimental manipulation. In the present study we describe a noninvasive means for recording the light-evoked responses of the mouse RPE and use this to define the stimulus-response properties of the major components in three inbred strains of mice (BALBc/ByJ, C57BL/6J, and 129/SvJ) and two mouse mutants that reduce activity in the rod pathway. All of the major ERG components generated by the RPE are readily measured in the mouse. In albino strains (BALBc/ByJ and 129/SvJ) the intensity-response functions for the c-wave, FO, and LP are shifted toward lower intensities in comparison to those for C57BL/6J mice. Each of these components was markedly reduced in mice lacking transducin in which rod phototransduction is interrupted, indicating that they reflect primarily rod photoreceptor activity. All components were observed in no b-wave (nob) mutant mice, indicating that inner retinal activity does not make a major contribution to these potentials. Further studies of mutant mice will allow us to define the functional consequences of gene manipulation on RPE function and to evaluate specific hypotheses regarding the generation of ERG components.     

糖尿病大鼠视网膜色素上皮细胞超微结构

目的 :探讨糖尿病大鼠视网膜色素上皮细胞超微结构病变 ,为糖尿病视功能障碍提供有关的行态学依据。方法 :选择清洁级SD大鼠随机分成正常对照组、糖尿病 1月、3月和 6月。腹腔内注射链脲佐菌素 (STZ)诱导大鼠糖尿病 ,每组 1 2只 ,取视网膜制备超薄切片 ,透射电镜观察。处死前每月测体重、血糖 1次。结果 :糖网病大鼠视网膜色素上皮细胞的微绒毛和基底部的质膜内褶随血糖的增高病情加重而逐渐稀疏、低矮直至脱落、低平。胞质内细胞器以线粒体的脱嵴、水肿和内质网的扩张为主要损害 ,而相邻色素上皮细胞之间的连接复合体始终存在。结论 :糖网病大鼠视网膜色素上皮细胞的损害主要表现于微绒毛、质膜内褶及细胞器的损害 ,并无脉络膜与视网膜之间屏障功能的损害。本研究为糖网病的临床研究提供形态学依据     

Ferroxidase hephaestin's cell-autonomous role in the retinal pigment epithelium

Hephaestin (Heph) is a ferroxidase protein that converts ferrous to ferric iron to facilitate cellular iron export by ferroportin. Many tissues express either Heph or its homologue, ceruloplasmin (Cp), but the retina expresses both. In mice, a combined systemic mutation of Heph and systemic knockout of Cp (Cp(-/-), Heph(sla/sla)) causes retinal iron accumulation and retinal degeneration, with features of human age-related macular degeneration; however, the role of Heph and Cp in the individual retinal cells is unclear. Herein, we used conditional knockout mice to study Heph's role in retinal pigment epithelial (RPE) and photoreceptor cells. Loss of both Heph and Cp from RPE cells alone results in RPE cell iron accumulation and degeneration. We found, however, that RPE iron accumulation in these conditional knockout mice is not as great as in systemic knockout mice. Photoreceptor-specific Heph knockout indicates that the additional iron in the RPE cells does not result from loss of ferroxidases in the photoreceptors, and Cp and Heph play minor roles in photoreceptors. Instead, loss of ferroxidases in other retinal cells causes retinal iron accumulation and transfer of iron to the RPE cells. Cp and Heph are necessary for iron export from the retina but are not essential for iron import into the retina. Thus, our studies, revise how we think about iron import and export from the retina.     

Immunocytochemical characterisation of proteins secreted by retinal pigment epithelium in retinas of normal and Royal College of Surgeons dystrophic rats

In a previous study, an antigen consisting of proteins secreted by retinal pigment epithelial (RPE) cells was injected into a sheep and the specificity of the resulting antiserum was shown by Western blotting and its effects on retinal development were determined in vitro and in vivo. In the present study, the distribution of these secreted proteins was determined by light microscopy immunocytochemistry in cultured neonatal rat RPE cells and retinas of normal and Royal College of Surgeons (RCS) dystrophic rats and cerebrum of normal adult rats. Immunolabelling for these RPE-secreted proteins was detected in cytoplasmic vesicles surrounding nuclei and within processes of cultured normal and transformed rat RPE. In retinas of late postnatal and adult rats, dense immunostaining was found in the cytoplasm of RPE cells and ganglion cell bodies. In addition to RPE and ganglion cells, scattered photoreceptors within the thin outer nuclear layer and small structures within the debris zone were also densely immunoreactive in retinas of 2-mo-old RCS dystrophic rats. The numbers of immunostained ganglion cells appeared to decrease in retinas of older RCS rats, although the immunoreactivity within the RPE appeared to increase in density. No other neuron within the retina, i.e. bipolar, amacrine or horizontal, was immunoreactive for RPE-secreted proteins. In the cerebral cortex of adult rats, immunoreactivity for RPE-secreted proteins was primarily detected within large perikarya of pyramidal neurons and smaller granule neurons. In conclusion, we report an immunocytochemical analysis of an antiserum raised against secreted proteins of rat RPE. This antiserum recognised proteins within secretory-like vesicles of cultured neonatal normal and transformed rat RPE and showed a specificity for RPE and ganglion cells in normal rat retinas, that appeared to be developmentally regulated, and neuron perikarya in adult rat cerebrum.     

视网膜色素上皮腺瘤的病理形态学观察

目的　探讨视网膜色素上皮腺瘤临床及形态学特征 ,为其诊断及鉴别提供依据。方法　常规石蜡切片HE染色、组织化学PAS和VG染色、透射电镜及用SP法做免疫组化S 10 0、Cytokeratin和Vimentin检测。结果　该瘤细胞呈椭圆形、立方形 ,部分瘤细胞排列成腺管状 ,瘤细胞团周围见均匀红染的条状物质 ;上述条状物质PAS染色阳性 ,VG染色大部分呈黄色 ,少量呈红色 ;透射电镜显示细胞的紧密连接 ;免疫组化S 10 0和Cytokeratin阳性、vimentin阴性。结论　视网膜色素上皮腺瘤的超微结构及免疫组化特征均同视网膜色素上皮相似 ,可通过形态学检查确诊。     

Lectin-ferritin binding on dystrophic and normal retinal pigment epithelial membranes

Summary Phagocytosis in the rat retina is a process which involves uptake of shed photoreceptor outer segments by the overlying retinal pigment epithelium (RPE). In rats with inherited retinal degeneration, there is a defect in phagocytosis. One aspect of this phagocytic defect may be an alteration in glycoconjugate-containing membrane components on RPE membranes which mediate this phagocytic interaction. Lectin binding sites have been studied in order to localize the distribution of glycoconjugates on pigment epithelial microvilli in normal and dystrophic retinas and to determine if there are differences in the dystrophic retinas which would provide a clue about the defect.The following ferritin-conjugated lectins were used in this study: Concanavalin A (Con A-fe) or lens culinaris haemagglutinin (LcH-fe) for mannosyl-containing glycoconjugates; and wheat germ agglutinin (WGA-fe) forn-acetylglucosamine and sialic acid-containing glycoconjugates. Control tissue was incubated in the lectin in the presence of its competitor sugar. The number of ferritin particles or lectin-ferritin binding sites per micrometre of microvillous membrane was quantified from electron micrographs using a computer and a digitizing tablet. The number of WGA-fe binding sites on normal RPE microvillous membranes (56.0/m) was statistically equivalent to the dystrophic membranes (48.8/m). The number of Con A-fe binding sites on normal (27.3/m) and dystrophic RPE (26.7/m) was also the same. A dramatic difference in LcH-fe binding sites was demonstrated on normal (1.5/m) as compared to dystrophic RPE (19.1/m). Our results indicate that more mannosyl residues are accessible on dystrophic microvillous membranes and, based on what is currently known about LcH binding, that these residues belong to glycoconjugates having fucosyl-containing carbohydrate cores. The data also suggest that in normal animals without a phagocytic defect such fucosyl-containing glycoconjugates are not as accessible and may be masked by other sugar residues in the oligosaccharide chain.     

Voltage-dependent currents in isolated cells of the turtle retinal pigment epithelium

The electrophysiological properties of isolated turtle retinal pigment epithelial cells (RPE cells) were investigated using the whole-cell patch-clamp technique. Most RPE cells exhibited a voltage-dependent outward current activated by depolarization beyond about –43 mV that inactivated during a 500-ms voltage step. Tail current measurements indicated that the conductance underlying this current was potassium selective. This current inactivated with prolonged depolarization and was abolished or reduced by extracellular quinidine, barium, tetraethylammonium (TEA) and 4-aminopyridine (4-AP). Steady-state inactivation of the voltage-dependent outward current revealed a time-independent outwardly rectifying current/voltage relationship in many cells. In addition, many cells had an outward current that activated slowly upon depolarization beyond about +40 mV and appeared to reverse near 0 mV in both 3 mM KCl and 30 mM KCl external solutions. This current was often observed in the presence of potassium channel blockers. Hyperpolarizing pulses commonly evoked inward currents that activated slowly and did not inactivate. These currents were commonly observed when fluoride was absent from the pipette, and only occasionally when fluoride was the major pipette anion. Tail current measurements indicated that this current was somewhat anion selective.These currents may play important roles in the homeostatic and phagocytic functions of RPE cells in their interactions with the neural retina.     

Hyperpigmented lesions of the retinal pigment epithelium in familial adenomatous polyposis

Ophthalmic examinations were performed on 56 patients with validated familial adenomatous polyposis (FAP) for hyperpigmented defects of the retinal pigment epithelium. Such lesions were seen bilaterally in 29 patients (52%) and unilaterally in 8 patients (14%). Of the 56 patients, 33 had one or more of the extracolonic expressions associated with Gardner syndrome. We found retinal lesions in 8 patients without any of the expressions of Gardner syndrome. No association was found between Gardner syndrome and the retinal lesions when these patients were compared to patients without any stigmata of Gardner syndrome, nor was any significant association found when each of the expressions was compared individually with the presence of the pigmented retinal lesions. The presence or absence of eye findings were seen to cluster within families. There was no association with sex. Fundus lesions are apparently a variable expression of the FAP gene and are not specifically associated with Gardner syndrome.     

Lectin binding pattern in human retinal pigment epithelium.

A comparative lectin histochemical study of human retinal pigment epithelium (RPE) was performed to investigate the lectin binding pattern of normal, reactive and proliferating RPE. Normal RPE with attached sensory retina was found to bind the lectins Con A, WGA, PNA and RCA I. Reactive and proliferating RPE in retinal detachment and in photocoagulation scars revealed the same lectin binding pattern although its cellular topography changed. RPE-macrophages showed an additional reaction with SBA. In periretinal membranes of human PVR the typical lectin binding pattern of Con A, WGA, PNA and RCA I was found in pigmented and in a subpopulation of non-pigmented cells, suggesting that these lectin-positive elements were of RPE-origin. Additionally, single pigmented cells positive for SBA were found indicating macrophage differentiation. Thus lectin histochemistry provides a tool for cytochemical identification of RPE and its morphologic variants by revealing a specific combination of sugar-binding sites.     

Mitochondrial alterations of retinal pigment epithelium in age-related macular degeneration

Mitochondrial dysfunctions have been implicated in the pathophysiology of several age-related diseases including age-related macular degeneration (AMD), a progressive neurodegenerative disease affecting primarily the retinal pigment epithelium (RPE). The aims of our electron microscopic and morphometric studies were to reveal qualitative and quantitative alterations of mitochondria in human RPE from AMD and from age- and sex-matched controls. With increasing age a significant decrease in number and area of mitochondria, as well as loss of cristae and matrix density were found in both AMD and control specimens. These decreases were significantly greater in AMD than in normal aging. Alterations of mitochondria were accompanied by proliferation of peroxisomes and lipofuscin granules in both AMD and control specimens, although the difference between groups was significant only for peroxisomes. Unexpectedly, morphometric data showed that the RPE alterations seen in AMD may also develop in normal aging, 10-15 years after appearing in AMD patients. These findings suggest that (i) the severity of mitochondrial and peroxisomal alterations are different between AMD and normal aging, and (ii) the timing of damage to RPE may be critical for the development of AMD. We conclude that besides the well-documented age-related changes in mitochondrial DNA, alterations of mitochondrial membranes may also play a role in the pathogenesis of AMD. These membranes could be a new target for treatment of AMD and other age-related diseases.     

Relationship between dietary retinol and lipofuscin in the retinal pigment epithelium

A variety of evidence suggests that autoxidation of cellular components probably plays a significant role in the age-related accumulation of lipofuscin, or age-pigment, in the mammalian retinal pigment epithelium (RPE). Among the likely candidates for conversion into RPE lipofuscin fluorophores via autoxidative mechanisms are vitamin A compounds, which are present in the retina and RPE in high concentrations. Vitamin E, an important lipid antioxidant, is likely to inhibit vitamin A autoxidation. Experiments were conducted to evaluate the significance of vitamin A autoxidation in the deposition of lipofuscin in the RPE. Albino rats were fed diets either supplemented with or lacking vitamin E. Each of these two groups of animals was further subdivided into three groups which were fed different levels of vitamin A palmitate: none, 14.0 mumol/kg diet, and 80.5 mumol/kg diet. After 26 weeks, the animals were killed and the RPE lipofuscin contents were determined by both fluorescence measurements and quantitative ultrastructural morphometry. Vitamin A palmitate deficiency led to significant reductions in RPE lipofuscin deposition, relative to the amounts of this pigment present in the groups receiving vitamin A palmitate in their diets. The relative magnitude of the vitamin A effect was greater in the vitamin E-supplemented groups than in the groups fed the diets deficient in vitamin E. This finding suggests that vitamin E interacts with vitamin A ester metabolites in vivo in a more complex manner than simply acting as an antioxidant protectant. Rats fed the diets containing the higher level of vitamin A palmitate failed to display elevated RPE lipofuscin contents relative to those in the rats fed 14.0 mumol of vitamin A palmitate/kg diet. Failure of high vitamin A intake to enhance RPE lipofuscin deposition may have been due to the fact that intake of vitamin A above normal levels did not lead to an elevation in vitamin A content of the retinal tissue. Establishing an effect of vitamin A deficiency on RPE lipofuscin deposition and characterization of the interactions between vitamins E and A are important steps toward defining precisely the molecular and cellular mechanisms underlying age-pigment accumulation in the RPE.     

Retinal degeneration triggered by inactivation of PTEN in the retinal pigment epithelium

Adhesion between epithelial cells mediates apical–basal polarization, cell proliferation, and survival, and defects in adhesion junctions are associated with abnormalities from degeneration to cancer. We found that the maintenance of specialized adhesions between cells of the retinal pigment epithelium (RPE) requires the phosphatase PTEN. RPE-specific deletion of the mouse pten gene results in RPE cells that fail to maintain basolateral adhesions, undergo an epithelial-to-mesenchymal transition (EMT), and subsequently migrate out of the retina entirely. These events in turn lead to the progressive death of photoreceptors. The C-terminal PSD-95/Dlg/ZO-1 (PDZ)-binding domain of PTEN is essential for the maintenance of RPE cell junctional integrity. Inactivation of PTEN, and loss of its interaction with junctional proteins, are also evident in RPE cells isolated from ccr2^−/− mice and from mice subjected to oxidative damage, both of which display age-related macular degeneration (AMD). Together, these results highlight an essential role for PTEN in normal RPE cell function and in the response of these cells to oxidative stress.     

Ocular toxoplasmosis: the role of retinal pigment epithelium migration in infection

Our aim was to study the migration of retinal pigmented epithelium (RPE) into the retinal layer during infection of C57BL/6 mice with Toxoplasma gondii. Eyes from infected and non-infected animals were analyzed on the 60th day of infection by light and transmission electron microscopy. Non-infected eyes showed a normal morphology. In contrast, we observed free parasites in the retinal vasculature, the presence of mononuclear inflammatory infiltrate (MNII) and parasites in the vasculature of choroids in infected eyes. No inflammatory infiltrate was observed; RPE cells were identified near the MNII in nuclear and plexiforme layers. RPE cells were also found on the ganglion cell layer and in the outer segments of the photoreceptor. The morphology showed that RPE cells caused a discontinuity in the nuclear and plexiforme layers. Clusters of parasites were found surrounded by RPE cells and MNII in the inner plexiforme layers. Ultrastructural analysis showed that RPE cells migrated through the epithelium into the inner retinal layers. We did not observe Toxoplasma cysts in many eyes in which pathological changes were detected. Only 8.3% of the animals had Toxoplasma cysts in the inner nuclear layer in the absence of inflammatory cells. The migration of RPE cells can be triggered by a disruption of the RPE monolayer or injury to the neural retina, as in the case of toxoplasmosis.     

Lipofuscin can be eliminated from the retinal pigment epithelium of monkeys

Lipofuscin is a cytologic hallmark of aging in metabolically active postmitotic cells including neurons, cardiac muscle cells, and the retinal pigment epithelium (RPE). High levels of lipofuscin are involved in the pathogenesis of age-related macular degeneration (AMD), the main cause of blindness in the elderly population in the western world. Degradation and exocytosis of lipofuscin by RPE cells have not been observed in vivo until now, and no drug is known to eliminate the intracellular amount of lipofuscin. Here, we show that in monkeys treated with a small molecule belonging to the tetrahydropyridoethers class (n = 36 of 48 monkeys), RPE cells significantly release lipofuscin. In 4 eyes, macrophages were detected which had taken up lipofuscin. They were located between the Bruch's membrane and the RPE, and in the choroid. The quantification of pigment granules was performed by transmission electron microscopy. Our findings open the way to develop therapeutic strategies to remove lipofuscin from RPE cells, which may have implications for the treatment of age-related macular degeneration in which lipofuscin accumulation in cells is a causative factor.     

Adenocarcinoma of the retinal pigment epithelium in the guppy Poecilia reticulata Peters.

A single case of adenocarcinoma of the retinal pigment epithelium occurred in a guppy, Poecilia reticulata, Peters. This is the first such tumour reported from fishes. The left eye of the affected fish was severely exophthalmic because of a large intraocular tumour mass. The tumour, which displaced normal retina anteriorly, consisted mainly of melanin-containing epithelial cells. Neoplastic cells were bilayered and arranged in a tubular pattern. The tumour was confined to the orbit. Although the specimen was from a group exposed to a mixture of halogenated organic compounds, the lesion was not considered to have been chemically induced because of its rare occurrence within the group as a whole.