Immortalization of polarized rat retinal pigment epithelium

Bipolar assembly of caveolae in retinal pigment epithelium

Caveolae and their associated structural proteins, the caveolins, are specialized plasmalemmal microdomains involved in endocytosis and compartmentalization of cell signaling. We examined the expression and distribution of caveolae and caveolins in retinal pigment epithelium (RPE), which plays key roles in retinal support, visual cycle, and acts as the main barrier between blood and retina. Electron microscopic observation of rat RPE, in situ primary cultures of rat and human RPE and a rat RPE cell line (RPE-J) demonstrated in all cases the presence of caveolae in both apical and basolateral domains of the plasma membrane. Caveolae were rare in RPE in situ but were frequent in primary RPE cultures and in RPE-J cells, which correlated with increased levels in the expression of caveolin-1 and -2. The bipolar distribution of caveolae in RPE is striking, as all other epithelial cells examined to date (liver, kidney, thyroid, and intestinal) assemble caveolae only at the basolateral side. This might be related to the nonpolar distribution of both caveolin-1 and 2 in RPE because caveolin-2 is basolateral and caveolin-1 nonpolar in other epithelial cells. The bipolar localization of plasmalemmal caveolae in RPE cells may reflect specialized roles in signaling and trafficking important for visual function.

Cell culture models of the ocular barriers

The presence of tight barriers, which regulate the environment of ocular tissues in the anterior and posterior part of the eye, is essential for normal visual function. The development of strategies to overcome these barriers for the targeted ocular delivery of drugs, e.g. to the retina, remains a major challenge. During the last years numerous cell culture models of the ocular barriers (cornea, conjunctiva, blood-retinal barrier) have been established. They are considered to be promising tools for studying the drug transport into ocular tissues, and for numerous other purposes, such as the investigation of pathological ocular conditions, and the toxicological screening of compounds as alternative to in vivo toxicity tests. The further development of these in vitro models will require more detailed investigations of the barrier properties of both the cell culture models and the in vivo ocular barriers. It is the aim of this review to describe the current status in the development of cell culture models of the ocular barriers, and to discuss the applicability of these models in pharmaceutical research.

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.

Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Excitatory amino acid transporters (EAATs) are involved in regulating extracellular glutamate levels at synaptic regions in the CNS. EAAT1, 2, 3, and 5 have been found in the mammalian retina, but the presence of EAAT4 has remained controversial. Recently, we found a high level of EAAT4 mRNA in the human retina, and this observation lead us to examine whether EAAT4 was expressed in the mammalian retina. Immunoblotting studies showed the presence of EAAT4-immunoreactive proteins in human and mouse retinas, corresponding to EAAT4 monomers and dimers. Immunohistochemistry revealed that EAAT4 was localized in rod and cone photoreceptor outer segments in the human retina, and in the outer and inner segments of mouse and ground squirrel retinas. In no case was EAAT4 found in the outer plexiform layer or in any other layer in the retina. EAAT4 expression by photoreceptors was confirmed by immunoblotting a purified rod outer segment preparation, which showed the presence of a 50-kDa EAAT4-immunoreactive protein. In addition, the EAAT4-associated protein, GTRAP41, was found in the human, mouse, and squirrel retinas as well as in the rod outer segment preparation. Further immunocytochemical and co-immunoprecipitation experiments demonstrated that GTRAP41 was colocalized and interacted in vivo with EAAT4. Importantly, glutamate uptake and drug inhibition experiments showed that an EAAT4-like glutamate uptake system is present in the rod outer segments. Finally, we examined whether glutamate signaling mediated by EAAT4 can modulate rod outer segment phagocytosis by the retinal pigment epithelium. Results of the present study show that EAAT4 is present in the outer segments, a nonsynaptic region of photoreceptors, where it might provide a feedback mechanism for sensing extracellular glutamate or serve as an outer barrier to prevent glutamate from escaping from the retina.

Amniotic membrane maintains the phenotype of rabbit retinal pigment epithelial cells in culture

The success of surgical removal of choroidal neovascularisation followed by transplantation of autologous retinal pigment epithelial cells (RPE) for age-related macular degeneration (ARMD) may be limited by damage in Bruch's membrane. We investigated whether amniotic membrane (AM) might be used as an alternative basement membrane-containing matrix to support RPE growth and differentiation. Primary RPE plastic cultures were established from freshly enucleated Dutch belted rabbit eyes in DMEM/F12 containing 0.1 mM Ca(++) and 10% dialysed FBS. Upon subconfluence, cells were subcultured at 5000-9000 cells cm(-2) in the above-mentioned culture medium on intact AM (iAM), epithelially denuded AM (dAM) or plastic. After confluence, the Ca(++) concentration in the medium was increased to 1.8 mm for 4 weeks. Growth and morphology were monitored by phase contrast microscopy, and the phenotype by immunostaining with antibodies against cytokeratin 18, tight junction protein ZO-1, and RPE65 protein, and by transepithelial resistance (TER) measurement. Immunostaining to cytokeratin 18 confirmed the epithelial origin of isolated cells in both primary culture and subcultures. Compared to plastic cultures, RPE increased pigmentation within 24 hr after seeding on AM, with iAM being more pronounced than dAM. RPE adopted a hexagonal epithelial phenotype with more organised pigmentation, strong expression of ZO-1 and RPE65, and a significantly higher TER 4 weeks after Ca(++) switch on dAM. Our results indicate that AM may be used as a basement membrane-containing matrix to maintain RPE phenotype in vitro, and may facilitate subsequent transplantation to treat ARMD.

Protein s-glutathionylation in retinal pigment epithelium converts heat shock protein 70 to an active chaperone

A disulfide bond between key redox-sensitive cysteine residues and glutathione is one mechanism by which redox related allosteric effectors can regulate protein structure and function. Here we test the hypothesis that glutaredoxin-1 (Grx-1), a member of the oxidoreductase family of enzymes, may be a critical component of redox-sensitive molecular switches by mediating reversible protein S-glutathionylation and enzymatic catalysis of thiol/disulfide exchange. Deglutathionylation of a 70 kDa protein by Grx-1 was detected using a monoclonal antibody specific to protein S-glutathionylation. Heat shock cognate protein 70 (Hsc70) was identified as a substrate of Grx-1 through mass spectrometry. Recombinant Hsc70 was glutathionylated in vitro, and protein S-glutathionylation reversed by Grx-1. Glutathionylated Hsc70 was more effective in preventing luciferase aggregation at 43 degrees C than reduced Hsc70 in a dose dependent fashion. ATP did not effect the chaperone activity of Hsc70-SG but did increase the activity of reduced Hsc70-SG. Reversible glutathionylation of Hsc70 may provide a mechanism for post-translation regulation of chaperone activity.

Adenovirus type 5 pseudotyped with adenovirus type 37 fiber uses sialic acid as a cellular receptor

For purposes of gene therapy, the tropism of adenovirus (Ad) serotype 5 vectors can be altered with fibers derived from alternative serotypes. However, there is currently limited information available on the cellular receptors used by the approximately 51 known Ad serotypes. Recently, alpha(2-->3)-linked sialic acid (2,3-SA) has been implicated as the cellular receptor for wild-type Ad37. However, some studies have demonstrated that wild-type Ad37 uses a 50-kDa protein and not sialic acid as its primary receptor for binding of human conjunctival cells. The sialic acid receptor has also been shown not to play a major role in the infection of these cells by an Ad5 virion pseudotyped with Ad37 fiber (Ad5.GFP.DeltaF/37F). In this study, we demonstrate that a similar virus (Ad5F37) can indeed use alpha(2-->3)-linked sialic acid as a cellular receptor. We also find that the receptor used by Ad5F37 is sensitive to proteases and that Ad5F37 can use integrin more efficiently than sialic acid for cell entry. Unlike Ad5 vectors, Ad5F37 does not efficiently employ the coxsackie and adenovirus receptor (CAR) to infect cells. Similar to Ad5, Ad5F37 infection of cells that form tight junctions can be enhanced by ethylenediaminetetraacetic acid (EDTA). These results have implications in the design of pseudotyped adenovirus vectors for gene therapy and may have particular use in the treatment of diseases involving breakdown of the blood-retinal barrier.

Intercellular Ca(2+)-transient propagation in normal and high glucose solutions in rat retinal epithelial (RPE-J) cells during mechanical stimulation

We investigated the effect of high glucose and modulation of protein kinase C (PKC) on the intercellular propagation of Ca(2+)-waves in a rat retinal pigment epithelial cell line (RPE-J cells) in order to compare its properties with the properties previously investigated in primary LE-RPE cells. The intercellular propagation of the Ca(2+)-waves in RPE-J cells was analyzed by fluorescence imaging confocal microscopy and fluorescence recovery after photobleaching (FRAP). In control conditions the maximal normalized fluorescence in the mechanically stimulated (MS) cell and the propagation towards the neighboring RPE-J cells were similar to LE-RPE cells. As in LE-RPE cells, the propagation was reduced by the gap junction (GJ) blocker halothane, and FRAP experiments demonstrated the presence of functional GJ coupling. Similar to the effect in LE-RPE cells, the propagation of the Ca(2+)-transient was reduced by 25 mM glucose. However, unlike LE-RPE cells, the neighboring RPE-J cells presented a Ca(2+)-rise of amplitude similar to that in normal glucose levels. PKC activation with 1 microM PMA for 30 min resulted in inhibition of the Ca(2+)-wave propagation, which could be overcome by PKC downregulation as in LE-RPE cells. Cells grown for 72 h in a high glucose solution in which PKC activity was downregulated, did not develop the inhibitory effect on Ca(2+)-wave propagation that was induced by elevated glucose levels. However, the effects were not as pronounced as in LE-RPE cells. We concluded that despite marked similarities, the transduction and the modulation of intercellular propagation of the Ca(2+)-transients in RPE-J cells are not identical to the mechanisms in primary LE-RPE cells.

Evidence of protein transduction but not intercellular transport by proteins fused to HIV tat in retinal cell culture and in vivo

The human immunodeficiency virus type-1 Tat protein is known to exit virally infected cells and enter the nucleus of adjacent uninfected cells. This property has been mapped to an 11-amino-acid protein transduction domain (PTD). When the PTD of Tat is fused to heterologous proteins and added exogenously to cells, the fusion peptide is able to demonstrate protein transduction across plasma membranes. Recent reports indicate that endogenously expressed Tat fusion peptides can demonstrate intercellular transport and improve biodistribution of therapeutic protein in the context of adenovirus vectors. Intercellular transport and protein transduction have not been observed in some studies and in the former have been attributed to an artifact of fixation. We have attempted to resolve these studies using an approach that unambiguously distinguishes cells that express Tat fusion protein from those that receive it from their environment. We find no evidence of intercellular transport in the context of an adenovirus vector in cell culture or in vivo. Instead, we find that Tat fusion peptides are down regulated in terms of expression not only in the context of adenovirus vectors, but also when expressed from transfected plasmid DNA. However, when Tat fusion peptides are released from cells by degradation of the plasma membrane, the fusion peptides demonstrate protein transduction without the need for cell fixation, indicating a unidirectional transport of Tat fusion proteins across the plasma membrane. Our data are consistent with previously reported studies and help to explain the apparently different results obtained from several different laboratories.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Reversal of protein S-glutathiolation by glutaredoxin in the retinal pigment epithelium

Protein cysteines can serve both sensory and activation roles in the regulation of protein function. The modulation of mixed disulfides with glutathione may promise to be a broad mechanism of redox signalling. Using both protein extract and intact RPE cells, we have generated covalent adduction of glutathione to protein cysteines and further show that glutaredoxin (Grx-1) is able to remove glutathione from protein S-glutathiolated substrates. Our data demonstrate that glutathione can modify a wide range of RPE proteins in intact cells, but that the reversal of this process--deglutathiolation and thiol bond restoration--may require a specific catalytic reaction with glutaredoxin. More generally, our experiments support the hypothesis that glutathione can non-specifically become adducted to protein cysteines during oxidative stress, but that the specific, functional reconstitution of protein thiols depends on recognition by an oxidoreductase such as glutaredoxin. This concept offers the idea that redox signalling involves both adduction of a non-specific non-protein reducing equivalent such as glutathione and specific protein based removal by glutaredoxin.

Induction of citrulline-nitric oxide (NO) cycle enzymes and NO production in immunostimulated rat RPE-J cells

Nitric oxide (NO) has been implicated in many physiological and pathological conditions in the eyes. The induction of inducible NO synthase (iNOS) and NO production have been noted in immunostimulated retinal pigment epithelial (RPE) cells. Cellular NO production depends on the availability of arginine, a substrate for NOS. Arginine can be regenerated from citrulline, another product of the NOS reaction, by argininosuccinate synthetase and argininosuccinate lyase, forming the citrulline-NO cycle. When rat RPE-J cells were treated with interferon-gamma (IFNgamma), tumor necrosis factor-alpha (TNFalpha) and lipopolysaccharide (LPS), and expression of the citrulline-NO cycle enzymes and related enzymes was analyzed, iNOS and argininosuccinate synthetase were highly induced at both mRNA and protein levels. On the other hand, argininosuccinate lyase was not induced. Among other related enzymes and transporters, mRNA for cationic amino acid transporter (CAT)-1 was weakly induced, whereas those for CAT-2, arginase I and II, ornithine aminotransferase and ornithine decarboxylase remained little changed. NO was produced by cells after stimulation with TNFalpha, IFNgamma and LPS. The induction of iNOS mRNA and the production of NO by these immunostimulated cells was further enhanced by cAMP. NO was produced from citrulline as well as from arginine. Our findings indicate that in activated RPE-J cells citrulline-arginine recycling is important for NO production.

Intercellular trafficking of adenovirus-delivered HSV VP22 from the retinal pigment epithelium to the photoreceptors--implications for gene therapy

Adenovirus (Ad)-mediated gene transfer is a promising technology for therapy of a wide variety of genetic disorders of the retina. The tropism of Ad vectors limits their utility to cells that express the coxsackie-adenovirus receptor. Upon ocular delivery, Ad vectors primarily infect the retinal pigment epithelium (RPE) and the Müller cells of the retina. However, the most frequent blinding diseases such as retinitis pigmentosa and age-related macular degeneration are associated with the expression of mutant proteins in the photoreceptors. In this study we demonstrate that adenovirus-delivered heterologous proteins fused to the herpes simplex virus tegument protein VP22 can translocate from infected cells to uninfected cells in culture and in vivo. We tested three different ocular cell lines, specifically Y79, RPE-J, and Chang C. We show that there is a 3.25-fold increase in the number of Y79 cells that take up GFP mediated by the intercellular trafficking properties of VP22. Our data are based on FACS analysis of living cells and there was no need for cell fixation for the effect to be observed. When adenovirus expressing a VP22-GFP fusion was injected into the subretinal space of adult mice, the VP22-GFP fusion peptides translocated from the RPE to all of the other layers of the retina, including the outer nuclear layer, which contains the photoreceptor cell bodies. Our study has significant implications for a wide variety of diseases of the retina and other organ systems.

Retinal pigment epithelial cells exhibit unique expression and localization of plasma membrane syntaxins which may contribute to their trafficking phenotype

The SNARE membrane fusion machinery controls the fusion of transport vesicles with the apical and basolateral plasma-membrane domains of epithelial cells and is implicated in the specificity of polarized trafficking. To test the hypothesis that differential expression and localization of SNAREs may be a mechanism that contributes to cell-type-specific polarity of different proteins, we studied the expression and distribution of plasma-membrane SNAREs in the retinal pigment epithelium (RPE), an epithelium in which the targeting and steady-state polarity of several plasma membrane proteins differs from most other epithelia. We show here that retinal pigment epithelial cells both in vitro and in vivo differ significantly from MDCK cells and other epithelial cells in their complement of expressed t-SNAREs that are known - or suggested - to be involved in plasma membrane trafficking. Retinal pigment epithelial cells lack expression of the normally apical-specific syntaxin 3. Instead, they express syntaxins 1A and 1B, which are normally restricted to neurons and neuroendocrine cells, on their apical plasma membrane. The polarity of syntaxin 2 is reversed in retinal pigment epithelial cells, and it localizes to a narrow band on the lateral plasma membrane adjacent to the tight junctions. In addition, syntaxin 4 and the v-SNARE endobrevin/VAMP-8 localize to this sub-tight junctional domain, which suggests that this is a region of preferred vesicle exocytosis. Altogether, these data suggest that the unique polarity of many retinal pigment epithelial proteins results from differential expression and distribution of SNAREs at the plasma membrane. We propose that regulation of the expression and subcellular localization of plasma membrane SNAREs may be a general mechanism that contributes to the establishment of distinct sorting phenotypes among epithelial cell types.

Bestrophin interacts physically and functionally with protein phosphatase 2A

Bestrophin is a 68-kDa basolateral plasma membrane protein expressed in retinal pigment epithelial cells (RPE). It is encoded by the VMD2 gene, which is mutated in Best macular dystrophy, a disease characterized by a depressed light peak in the electrooculogram. Recently it was proposed that bestrophin is a chloride channel responsible for generating the light peak. To investigate its function further, we immunoaffinity purified a bestrophin complex from RPE lysates and identified bestrophin and the beta-catalytic subunit of protein phosphatase 2A (PP2A) as members of the complex by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Protein-protein interaction between bestrophin and PP2Ac and the structural subunit of PP2A, PR65, was confirmed by reciprocal immunoprecipitation. The C-terminal cytoplasmic domain of bestrophin was sufficient for the interaction with PP2A as demonstrated by a pulldown assay using a fusion of this domain with glutathione S-transferase. Bestrophin was phosphorylated when expressed in RPE-J cells and this phosphorylation was sensitive to okadaic acid. Purified PP2A effectively dephosphorylated bestrophin in vitro. These data suggest that bestrophin is in the signal transduction pathway that modulates the light peak of the electrooculogram, that it is regulated by phosphorylation, and that phosphorylation of bestrophin is in turn regulated by PP2A.

Mertk triggers uptake of photoreceptor outer segments during phagocytosis by cultured retinal pigment epithelial cells

The RCS rat is a widely studied model of recessively inherited retinal degeneration. The genetic defect, known as rdy (retinal dystrophy), results in failure of the retinal pigment epithelium (RPE) to phagocytize shed photoreceptor outer segment membranes. We previously used positional cloning and in vivo genetic complementation to demonstrate that Mertk is the gene for rdy. We have now used a rat primary RPE cell culture system to demonstrate that the RPE is the site of action of Mertk and to obtain functional evidence for a key role of Mertk in RPE phagocytosis. We found that Mertk protein is absent from RCS, but not wild-type, tissues and cultured RPE cells. Delivery of rat Mertk to cultured RCS RPE cells by means of a recombinant adenovirus restored the cells to complete phagocytic competency. Infected RCS RPE cells ingested exogenous outer segments to the same extent as wild-type RPE cells, but outer segment binding was unaffected. Mertk protein progressively co-localized with outer segment material during phagocytosis by primary RPE cells, and activated Mertk accumulated during the early stages of phagocytosis by RPE-J cells. We conclude that Mertk likely functions directly in the RPE phagocytic process as a signaling molecule triggering outer segment ingestion.

The lipofuscin component A2E selectively inhibits phagolysosomal degradation of photoreceptor phospholipid by the retinal pigment epithelium

Daily phagocytosis of spent photoreceptor outer segments is a critical maintenance function performed by the retinal pigment epithelium (RPE) to preserve vision. Aging RPE accumulates lipofuscin, which includes N-retinylidene-N-retinylethanolamine (A2E) as the major autofluorescent component. We studied the effect of physiological levels of A2E in RPE cultures on their ability to phagocytose outer segments. A2E localized to lysosomes in cultured RPE as well as in human RPE in situ. A2E-loaded RPE cells in culture bound and internalized identical numbers of outer segments as control RPE indicating that A2E does not alter early steps of phagocytosis. A2E-loaded RPE degraded outer segment proteins efficiently but, strikingly, failed to completely digest phospholipids within 24 h. Because of the circadian rhythm of RPE phagocytosis in the eye, a delay in lipid degradation would likely result in a build up of undigested material in RPE that could contribute to the development of age-related macular degeneration.

Differential roles of CD36 and alphavbeta5 integrin in photoreceptor phagocytosis by the retinal pigment epithelium

Retinal pigment epithelial (RPE) cells employ alphavbeta5 integrin and CD36 receptors to phagocytose photoreceptor outer segment fragments (OS). We explored special properties of RPE phagocytosis to identify the contribution of CD36 to RPE phagocytosis measuring effects of CD36 antibodies on OS binding and internalization kinetics. Early, CD36 antibodies had no effect on OS binding or internalization. Both control and CD36 antibody treated RPE initiated internalization approximately 2 hours after OS challenge. Later, bivalent CD36 IgG accelerated OS engulfment while monovalent Fab fragments inhibited engulfment. Cross-linking Fab fragments restored the accelerating activity of intact IgG. Strikingly, antibodies were effective even if added to OS already bound by RPE. alphavbeta5 blocking antibody reduced OS binding equally well in the presence of CD36 antibodies but CD36 antibodies accelerated internalization of remaining bound OS. Furthermore, CD36 ligation at either apical or basal RPE surface partially substituted for soluble factors that are required for internalization but not for binding of OS at the RPE apical surface. Our results demonstrate that CD36 ligation is necessary and sufficient to activate the OS internalization mechanism of RPE. They suggest that CD36 acts as a signaling molecule in postbinding steps of RPE phagocytosis independently of the OS binding receptor alphavbeta5 integrin.

Proteome survey of proliferating and differentiating rat RPE-J cells

The suitability of the rat derived SV-40T immortalized RPE-J cell line for identifying proteome changes associated with RPE differentiation was evaluated by surveying changes in protein expression levels. Rat RPE-J cells were induced to undergo differentiation in culture by growth at the nonpermissive temperature of 40 degrees C in the presence of retinoic acid. Total proteins were extracted from cells grown under proliferating or differentiating conditions and separated by 1D and 2D gel electrophoresis. Gel spots were excised, digested in situ with trypsin, and analysed by mass spectrometry to identify proteins. Computer assisted image analysis was used to align gel patterns and quantify spot intensities. Neither proliferating nor differentiating RPE-J cell cultures exhibited detectable levels of cellular retinaldehyde-binding protein, RPE65, 11- cis -retinol dehydrogenase or lecithin retinol acyl transferase, suggesting that RPE-J cells are not appropriate for visual cycle studies. About 18% of the 61 identified proteins appear to change expression levels with the cell growth conditions. Seven proteins appeared to be up-regulated and four proteins down-regulated when the cells were changed from proliferating to differentiating culture conditions. The majority of the apparent changes in protein expression levels were associated with stress response genes. Significant changes in the apparent mass and charge properties of proteins were also observed and for select proteins, the modifications appeared to be correlated with cell growth conditions. The results demonstrate that proteome differences in RPE-J cells associated with growth conditions can be identified and support the suitability of RPE-J cells for more targeted and/or more global proteome analysis of RPE differentiation.

Bestrophin, the product of the Best vitelliform macular dystrophy gene (VMD2), localizes to the basolateral plasma membrane of the retinal pigment epithelium

Best vitelliform macular dystrophy is a dominantly inherited, early onset, macular degenerative disease that exhibits some histopathologic similarities to age-related macular degeneration. Although the vitelliform lesion is common in the fundus of individuals with Best disease, diagnosis is based on a reduced ratio of the light peak to dark trough in the electrooculogram. Recently, the VMD2 gene on chromosome 11q13, encoding the protein bestrophin, was identified. The function of bestrophin is unknown. To facilitate studies of bestrophin, we produced both rabbit polyclonal and mouse monoclonal antibodies that proved useful for Western blotting, immunoprecipitation, and immunocytochemistry. To characterize bestrophin, we initially probed the retinal pigment epithelium (RPE)-derived cell lines ARPE-19, D407, and RPE-J. All of the cell lines expressed bestrophin mRNA by reverse transcription-PCR, but not on Western blots. Bestrophin in human RPE partitioned in the detergent phase during Triton X-114 extraction and could be modified by biotin in intact cells, indicative of a plasma membrane localization. Immunocytochemical staining of macaque and porcine eyes indicated that bestrophin is localized at the basolateral plasma membrane of RPE cells. When expressed in RPE-J cells by adenovirus-mediated gene transfer, bestrophin again was determined by confocal microscopy and cell surface biotinylation to be a basolateral plasma membrane protein. The basolateral plasma membrane localization of bestrophin suggests the possibility that bestrophin plays a role in generating the altered electrooculogram of individuals with Best disease.

Purification and characterization of beta-actin-rich tumor cell pseudopodia: role of glycolysis

The MSV-MDCK-INV invasive variant of Moloney sarcoma virus (mos) transformed MDCK cells express multiple beta-actin-rich pseudopodia (P. U. Le et al., Cancer Res. 58, 1631-1635, 1998). We show here that the tips of these actively protruding cellular domains are morphologically distinct presenting numerous blebs and selectively pass through 1-microm-pore filters. The pseudopodia were purified from the underside of the filters and a major protein component was identified as the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). By confocal microscopy, GAPDH colocalized with actin in MSV-MDCK-INV pseudopodia localizing this glycolytic enzyme to this site of active actin polymerization. Inhibition of glycolysis with 2-deoxyglucose or oxamate induced a rapid transformation of beta-actin-rich pseudopodia into extended lamellipodia and prevented cell motility. A localized glycolytic supply of energy therefore regulates the formation of beta-actin-rich pseudopodial protrusions and thereby the motility of invasive tumor cells.

Distal upstream tyrosinase S/MAR-containing sequence has regulatory properties specific to subsets of melanocytes

A distal upstream regulatory element of the mouse tyrosinase gene has locus control region (LCR)-like activity and is required for position-independent expression of linked genes. It consists of a DNAse I hypersensitive site, which has enhancer activity in neural crest-derived melanocytes, embedded within a scaffold/matrix attachment region (S/MAR), both of which are necessary for LCR activity. To address the role of the S/MAR in position-independent expression, we assessed the ability of a fragment containing most of the S/MAR to insulate a transgene from position effects. The S/MAR sequence showed a striking cell type specificity in its function in all six multicopy transgenic lines, dampening position effects considerably in cutaneous melanocytes while allowing no expression in other neural crest-derived melanocytes, and causing elevated expression in ocular melanocytes derived from the neural tube. The specificity of transgene expression in the eye suggested the presence of both positive and negative regulatory elements in this enhancer/S/MAR region, which was confirmed by transient transfection analyses. This is the first known regulatory element to exhibit different activities in melanocytes of different developmental origins.

Ezrin promotes morphogenesis of apical microvilli and basal infoldings in retinal pigment epithelium

Ezrin, a member of the ezrin/radixin/moesin (ERM) family, localizes to microvilli of epithelia in vivo, where it bridges actin filaments and plasma membrane proteins. Here, we demonstrate two specific morphogenetic roles of ezrin in the retinal pigment epithelium (RPE), i.e., the formation of very long apical microvilli and of elaborate basal infoldings typical of these cells, and characterize the role of ezrin in these processes using antisense and transfection approaches. In the adult rat RPE, only ezrin (no moesin or radixin) was detected at high levels by immunofluorescence and immunoelectron microscopy at microvilli and basal infoldings. At the time when these morphological differentiations develop, in the first two weeks after birth, ezrin levels increased fourfold to adult levels. Addition of ezrin antisense oligonucleotides to primary cultures of rat RPE drastically decreased both apical microvilli and basal infoldings. Transfection of ezrin cDNA into the RPE-J cell line, which has only trace amounts of ezrin and moesin, sparse and stubby apical microvilli, and no basal infoldings, induced maturation of microvilli and the formation of basal infoldings without changing moesin expression levels. Taken together, the results indicate that ezrin is a major determinant in the maturation of surface differentiations of RPE independently of other ERM family members.

Macrophage and retinal pigment epithelium phagocytosis: apoptotic cells and photoreceptors compete for alphavbeta3 and alphavbeta5 integrins, and protein kinase C regulates alphavbeta5 binding and cytoskeletal linkage

Noninflammatory monocyte macrophages use alphavbeta3 integrin to selectively bind apoptotic cells, initiating their phagocytic removal. In a related process, the retinal pigment epithelium (RPE) employs alphavbeta5 integrin to recognize spent photoreceptor outer segment particles (OS). Here, we show that apoptotic cells and OS compete for binding to these receptors, indicating that OS and apoptotic cells expose surface signals recognizable by alphavbeta3 and alphavbeta5. Particle binding to alphavbeta5 required protein kinase C (PKC) activation. In RPE, alphavbeta5 binding was maximally activated even before any phagocytic challenge and was reduced by PKC inhibitors. In macrophages, it was dormant but became activated upon PKC stimulation. PKC-activated alphavbeta5-mediated binding in macrophages differed from constitutive binding to the same integrin receptor in RPE cells in that the former followed much faster kinetics, similar to particle binding mediated by alphavbeta3. Activation of alphavbeta5 for particle binding correlated with its recruitment into a detergent-insoluble fraction, a process sensitive to pharmacological modulation of PKC in both types of phagocytes. Furthermore, alphavbeta5 but not alphavbeta3 particle binding required actin microfilaments. These data constitute the first evidence that noninflammatory phagocytes actively regulate the earliest phase of phagocytic clearance, particle binding, by controlling receptor activity.

Calreticulin affects focal contact-dependent but not close contact-dependent cell-substratum adhesion

We used two cell lines expressing fast (RPEfast) and slow (RPEslow) attachment kinetics to investigate mechanisms of cell-substratum adhesion. We show that the abundance of a cytoskeletal protein, vinculin, is dramatically decreased in RPEfast cells. This coincides with the diminished expression level of an endoplasmic reticulum chaperone, calreticulin. Both protein and mRNA levels for calreticulin and vinculin were decreased in RPEfast cells. After RPEfast cells were transfected with cDNA encoding calreticulin, both the expression of endoplasmic reticulum-resident calreticulin and cytoplasmic vinculin increased. The abundance of other adhesion-related proteins was not affected. RPEfast cells underexpressing calreticulin displayed a dramatic increase in the abundance of total cellular phosphotyrosine suggesting that the effects of calreticulin on cell adhesiveness may involve modulation of the activities of protein tyrosine kinases or phosphatases which may affect the stability of focal contacts. The calreticulin and vinculin underexpressing RPEfast cells lacked extensive focal contacts and adhered weakly but attached fast to the substratum. In contrast, the RPEslow cells that expressed calreticulin and vinculin abundantly developed numerous and prominent focal contacts slowly, but adhered strongly. Thus, while the calreticulin overexpressing RPEslow cells "grip" the substratum with focal contacts, calreticulin underexpressing RPEfast cells use close contacts to "stick" to it.

Retinal pigmented epithelium cultures on thermally responsive polymer porous substrates

A cross-linkable co-polymer of UV-sensitive 4-(N-cinnamoylcarbamide)methylstyrene (CCMS) and N-isopropylacrylamide (NIPAAm), was applied to porous tissue culture inserts. Surface chemical analyses of the inserts show an introduction of a thermally responsive polymer comparable to that on similarly incorporated non-porous polystyrene surfaces. Contact angle measurements as well as atomic force microscopy show a surface change in response to changing temperature in an aqueous environment, from hydrophilic, extended polymer chains below 32 degrees C to a dense hydrophobic film above 32 degrees C. Cell growth on porous inserts allowed measurement of cell expression, such as transepithelial resistance and fluid transport, which are not observable on cells from non-porous surfaces. Cultures of retinal pigmented epithelium (RPE) were able to restore an environment similar to in vivo by forming a tight junction barrier membrane upon confluence at 37 degrees C, as observed by changes in morphology, transepithelial resistance, and directionally-specific fluid transport. In addition, cells cultured on these surfaces detached as an oriented polarized sheet when the inserts were brought to 20 degrees C. This cell sheet was transplanted to other tissue culture surface without polymer detachment or dissolution, or cell damage caused by traditional detachment methods using proteolytic enzymes.

Role of the glycosyl-phosphatidylinositol anchor in the intracellular transport of a transmembrane protein in Madin-Darby canine kidney cells

In order to compare the trafficking of proteins with different membrane anchors, we have constructed and expressed three different recombinant forms of neutral endopeptidase (NEP) in MDCK cells. The wild type form of NEP (WT-NEP) is attached to the plasma membrane by a single N-terminal membrane spanning domain, whereas the glycosylphosphatidylinositol-anchored form of the protein (GPI-NEP) contains a C-terminal GPI anchor. A double anchored form of NEP (DA-NEP) was also constructed, that contains both the original N-terminal membrane spanning domain and a C-terminal GPI anchor. We show here that WT-NEP, GPI-NEP and DA-NEP, which are all apically targeted in MDCK cells, behave differently when subjected to Triton X-100 solubilisation: despite the presence of the transmembrane anchor DA-NEP behaves as a GPI-anchored protein. This suggests that the GPI anchor of DA-NEP is dominant over the transmembrane anchor of the native protein to determine its pattern of solubility in Triton X-100.

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.

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.

The extent of polylactosamine glycosylation of MDCK LAMP-2 is determined by its Golgi residence time

The increased polylactosamine glycosylation of LAMP-2 in MDCK cells cultured for 1 day relative to cells cultured for 3 days has been correlated with its slower rate of Golgi transit (Nabi and Rodriguez-Boulan, 1993, Mol. Biol. Cell., 4, 627-635). To determine if the differential polylactosamine glycosylation of LAMP-2 is a consequence of glycosyltransferase expression levels, the activities of beta1-6GlcNAc-TV, beta1-3GlcNAc-T(i), beta1-2GlcNAc-TI, beta1, 4Gal-T, alpha2-6sialyl-T, and alpha2-3sialyl-T were assayed and no significant differences in the activities of these enzymes in 1 and 3 day cell extracts were detected. During MDCK epithelial polarization, the Golgi apparatus undergoes morphological changes and apiconuclear Golgi networks were more evident in 3 day cells. Treatment with nocodazole disrupted Golgi networks and generated numerous Golgi clusters in both 1 day and 3 day cells. In the presence of nocodazole the differential migration of LAMP-2 in 1 and 3 day MDCK cells was maintained and could be eliminated by treatment with endo-beta-galactosidase, indicating that gross Golgi morphology did not influence the extent of LAMP-2 polylactosamine glycosylation. Nocodazole treatment did, however, result in the faster migration of LAMP-2 which was not due to modification of core N-glycans as the precursor form of the glycoprotein migrated with an identical molecular size. Following incubation at 20 degrees C, which prevents the exit of proteins from the trans-Golgi network, the molecular size of LAMP-2 increased to a similar extent in both 1 and 3 day MDCK cells. Extending the time of incubation at 20 degrees C did not influence the size of LAMP-2, demonstrating that its glycosylation is modified not by its retention within the Golgi but rather by its equivalent slower Golgi passage at the lower temperature in both 1 and 3 day cells. An identical effect was observed in nocodazole treated cells, demonstrating that Golgi residence time determines the extent of LAMP-2 polylactosamine glycosylation, even in isolated Golgi clusters.

Apical polarity of N-CAM and EMMPRIN in retinal pigment epithelium resulting from suppression of basolateral signal recognition

Retinal pigment epithelial (RPE) cells apically polarize proteins that are basolateral in other epithelia. This reversal may be generated by the association of RPE with photoreceptors and the interphotoreceptor matrix, postnatal expansion of the RPE apical surface, and/or changes in RPE sorting machinery. We compared two proteins exhibiting reversed, apical polarities in RPE cells, neural cell adhesion molecule (N-CAM; 140-kD isoform) and extracellular matrix metalloproteinase inducer (EMMPRIN), with the cognate apical marker, p75-neurotrophin receptor (p75-NTR). N-CAM and p75-NTR were apically localized from birth to adulthood, contrasting with a basolateral to apical switch of EMMPRIN in developing postnatal rat RPE. Morphometric analysis demonstrated that this switch cannot be attributed to expansion of the apical surface of maturing RPE because the basolateral membrane expanded proportionally, maintaining a 3:1 apical/basolateral ratio. Kinetic analysis of polarized surface delivery in MDCK and RPE-J cells showed that EMMPRIN has a basolateral signal in its cytoplasmic tail recognized by both cell lines. In contrast, the basolateral signal of N-CAM is recognized by MDCK cells but not RPE-J cells. Deletion of N-CAM's basolateral signal did not prevent its apical localization in vivo. The data demonstrate that the apical polarity of EMMPRIN and N-CAM in mature RPE results from suppressed decoding of specific basolateral signals resulting in randomized delivery to the cell surface.

Role of tight junctions in establishing and maintaining cell polarity

The tight junction (TJ) is not randomly located on the cell membrane, but occupies a precise position at the outermost edge of the intercellular space and, therefore, is itself considered a polarized structure. This article reviews the most common experimental approaches for studying this relationship. We then discuss three main topics. (a) The mechanisms of polarization that operate regardless of the presence of TJs: We explore a variety of polarization mechanisms that operate at stages of the cell cycle in which TJs may be already established. (b) TJs and polarity as partners in highly dynamic processes: Polarity and TJs are steady state situations that may be drastically changed by a variety of signaling events. (c) Polarized distribution of membrane molecules that depend on TJs: This refers to molecules (mainly lipids) whose polarized distribution, although not the direct result of TJs, depends on these structures to maintain such distribution.

A splice variant of trkB and brain-derived neurotrophic factor are co-expressed in retinal pigmented epithelial cells and promote differentiated characteristics

There is evidence suggesting reciprocal trophic interactions between photoreceptors and the retinal pigmented epithelium (RPE), but the factors involved have not been identified. In this study, we investigated the hypothesis that one or more known neurotrophic factors act upon the RPE. Cultured human and freshly isolated bovine RPE cells demonstrated saturable specific binding for [125I]labeled BDNF, NT-4/5 and NT-3 with little specific binding for CNTF and none for NGF. Cross-competition experiments showed that BDNF is the preferred ligand and cross-linking of [125I]BDNF resulted in a doublet at 160 kd that was increased in RPE cells incubated in all-trans retinoic acid. There was basal phosphorylation of a 145 kd protein recognized by an anti-trk antibody that was increased in RPE cells pulsed with BDNF. RT-PCR with primers spanning the transmembrane domain demonstrated that RPE cells express trkB mRNA lacking a region homologous to exon 9 of chicken trkB, a splice variant that has been demonstrated to preferentially interact with BDNF. Northern blots demonstrated that cultured RPE cells also express mRNA for BDNF. BDNF did not stimulate proliferation or increase survival of RPE cells in serum-free medium, but promoted a differentiated morphology and increased the expression of cellular retinaldehyde binding protein, a marker of the differentiated state in RPE cells. An RPE cell line that spontaneously shows differentiated features showed a high level of BDNF mRNA. These data demonstrate that RPE cells express a short splice variant of trkB whose activation correlates with expression of differentiated characteristics and the cells themselves are capable of producing a ligand for the receptors. Signaling through trkB could play a role in differentiation of RPE cells during development and maintenance of the differentiated state in adult 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.

Cell-cell adhesion molecules and the development of an epithelial phenotype in cultured human retinal pigment epithelial cells

For most epithelial cells, the adherens junction protein E-cadherin is an epithelial morphogen, inducing the development of an epithelial phenotype in vitro after cell contact at confluency. Here retinal pigment epithelial cells (RPE), which lack E-cadherin but express a cadherin that is also found in many non-epithelial cells (N-cadherin), were examined for the ability to produce an epithelial phenotype in vitro. Subpopulations of grossly epithelioid or fusiform cells were selected for analysis from RPE cultures derived from adult human donors. After confluency, epithelioid RPE cells were observed to undergo time-dependent changes that were similar to those previously found in epithelial cells expressing E-cadherin: the cadherin gradually developed a zonular distribution of detergent-resistant protein that co-localized with forming circumferential actin bundles; Na/K ATPase accumulated at cell contact sites, then polarized to its tissue-specific domain (the apical membrane for RPE); the cells formed elevated domes on the impermeant culture substrate. In contrast to cells expressing E-cadherin, these events in RPE required weeks rater than days at confluency. Additional proteins were examined in epithelioid RPE cells revealing that cytokeratins reorganized after confluency producing a zonular array, and several other adhesion proteins (alpha5beta1 integrin, ICAM-1, PECAM-1, NCAM) became enriched at cell-cell contact sites, each developing a distinct pattern at a distinct postconfluency interval. In contrast to epithelioid RPE, in fusiform RPE the adhesion molecules did not develop discrete distribution patterns after confluency, although the same complement of adhesion proteins was expressed. In cells expressing E-cadherin, the absence of epithelial properties is often due to underexpression of the cadherin or of the catenins, adherens junction proteins that link the cadherin to actin. Fusiform RPE, however, were not deficient in these proteins, expressing amounts of N-cadherin, alpha-catenin, beta-catenin, plakoglobin, p120, alpha-actinin and vinculin that were equivalent to epithelioid cells. It appears, therefore, that a subset of epithelial cells that express N-cadherin can produce a highly-developed epithelial phenotype in vitro through a slow morphogenetic process. However, the expression alone of adhesion molecules, including those with a morphoregulatory function in other cells, is insufficient to produce an epithelial phenotype in all cells derived from the pigment epithelium.

Protein insolubility and late-stage morphogenesis in long-term postconfluent cultures of MDCK epithelial cells

Epithelial morphogenesis in vitro has been studied in cultures soon after cell-cell contact at confluency when several actin-associated proteins (fodrin, adherens junction molecules E-cadherin and catenins) localize to specific subcellular domains and become resistant to extraction with non-ionic detergents. Here we demonstrate that early confluency is followed by a long postconfluent period of several weeks during which these proteins and actin itself become progressively enriched in the detergent-resistant fraction of MDCK epithelial cells. Cultures from another tissue (human retinal pigment epithelium) which produces weakly epithelialized monolayers in culture do not exhibit similar late-stage increases in protein insolubility. After confluency some cells in MDCK cultures undergo additional morphogenetic changes giving rise to cord-like structures, and the MDCK adherens junction becomes more stable to disrupting agents. These results indicate that in vitro morphogenesis is not restricted to early confluency in MDCK cells but rather molecular stabilization and dynamic changes in cell shape occur over a protracted postconfluent interval.

Differentiation and transdifferentiation of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lies between the retina and the choroid of the eye and plays a vital role in ocular metabolism. The RPE develops from the same sheet of neuroepithelium as the retina and the two derivatives become distinguished by different expression patterns of a number of transcription factors during embryonic development. As the RPE layer differentiates it expresses a set of unique molecules, many of which are restricted to certain regions of the cell. PRE cells undergo both a loss of polarity and a loss of expression of many of these cell type-specific molecules when placed in monolayer culture. The RPE of many species, including mammals, can be induced to transdifferentiate by growth factors such as basic fibroblast growth factor. Under the influence of such factors the RPE is triggered to alter expression of a wide array of molecules and to take on a retinal epithelium fate, from which differentiated retinal cell types including rod photoreceptors can be produced.

SV40 large T immortalised cell lines of the rat blood-brain and blood-retinal barriers retain their phenotypic and immunological characteristics

In the central nervous system the blood-brain and blood-retinal barriers (BBB and BRB respectively) are instrumental in maintaining homeostasis of the neural parenchyma and controlling leucocyte traffic. These cellular barriers are formed primarily by the vascular endothelium of the brain and retina although in the latter the pigmented epithelial cells also form part of the barrier. From primary cultures of rat brain endothelium, retinal endothelium and retinal pigment epithelium (RPE) we have generated temperature sensitive SV40 large T immortalised cell lines. Clones of brain (GP8.3) and retinal (JG2.1) endothelia and RPE (LD7.4) have been derived from parent lines that express the large T antigen at the permissive temperature. The endothelial cell (EC) lines expressed P-glycoprotein, GLUT-1, the transferrin receptor, von Willebrand factor and the RECA-1 antigen and exhibited high affinity uptake of acetylated LDL and stained positive with the lectin Griffonia simplicifolia. The RPE cell line was positive for cytokeratins and for the rat RPE antigen RET-PE2. All the cell lines expressed major histocompatibility complex (MHC) class 1 and intercellular adhesion molecule (ICAM)-1 constitutively and could be induced to express MHC class II and vascular cell adhesion molecule (VCAM)-1 following cytokine activation. The EC also expressed platelet endothelial cell adhesion molecule (PECAM)-1. Monolayers of these cells could support the migration of antigen-specific T cell lines. The generation of immortalised cell lines derived from the rat BBB and BRB should prove to be useful tools for the study of these specialised cellular barriers.

Cadherin cell adhesion molecules in differentiation and embryogenesis

The cadherin gene superfamily of calcium-dependent cell-cell adhesion molecules contains more than 40 members. We summarize functions attributed to these proteins, especially their roles in cellular differentiation and embryogenesis. We also describe hierarchies of protein-protein interactions between cadherins and cadherin-associated proteins (catenins). Several signal transduction pathways converge on, and diverge from, the cadherin/catenin complex to regulate its function; we speculate on roles of these signaling processes for cell structure and function. This review provides a framework for interpretation of developmental functions of cadherin cell adhesion molecules.

Hypoxic induction of vascular endothelial growth factor (VEGF) in human epithelial cells is mediated by increases in mRNA stability

Vessel growth is often associated with ischemia. VEGF, a potent angiogenic factor, has been shown to be induced by low oxygen concentrations. These studies were conducted to investigate the molecular basis of the hypoxia-induced increase in VEGF mRNA. Run-on analysis of VEGF revealed a minimal increase in the rate of gene transcription in a human retinal epithelial cell line grown under hypoxic conditions. Examination of VEGF mRNA stability revealed that the half-life of VEGF transcripts under normoxia was short, 30-45 min, but was dramatically increased to 6-8 h in cells grown under hypoxia. Cobalt chloride, which elevates VEGF and has been suggested to be similar to hypoxia in its mechanism of action, had only a slight effect on decay rate. We postulate that hypoxia-induced increases in mRNA stability provide the sustained increases in VEGF mRNA levels necessary to support a neovascular response.

Development of the blood-retinal barrier in vitro: formation of tight junctions as revealed by occludin and ZO-1 correlates with the barrier function of chick retinal pigment epithelial cells

To elucidate the molecular mechanisms of the blood-retinal barrier (BRB), we examined chick retinal tissues histochemically using antibodies against tight junction proteins such as ZO-1, 7H6 antigen, and occludin. Retinal pigment epithelial (RPE) cells in situ in chickens and late chick embryos expressed all of the tight junctional proteins examined, showing that tight junctions seal the cell borders of chick RPE cells in vivo. On the other hand, RPE cells isolated from late chick embryos and transferred in vitro did not express occludin, ZO-1 and 7H6 antigen. The effects of differentiation-inducing agents, such as retinoic acid, dexamethasone and dimethyl sulfoxide (DMSO) were tested. Only DMSO induced an increase in transepithelial electrical resistance (TER) in a time-dependent manner. Under supplementation with DMSO, immunofluorescently demonstrable occludin and ZO-1 were induced progressively at cell borders in parallel with the increase in TER that occurred with decreases in inulin and dextran permeability. Electron microscopically tight junction-like junctional apparatus were induced in RPE cells. These results indicated that tight junctions of RPE cells play an important role in the formation of the BRB.

Generation of cell lines from embryonic quail retina capable of mature neuronal differentiation

The avian embryonic retina is widely used as a model system for cellular and molecular studies on central nervous system neurons. We aimed at the generation of cell lines from the early embryonic quail retina by retroviral oncogene transduction. For this, we made use of the retina organ culture system which exhibits both proliferation, necessary for stable oncogene transduction, and initial neuronal differentiation, a prerequisite for the generation of cell lines with mature neuronal properties. The oncogene myc was chosen as it is both proliferation-inducing and differentiation-compatible. A chimeric gene, mycER, containing v-myc and the hormone-binding domain of the estrogen receptor, was used for transduction in order to allow for hormone regulation of myc activity. Transduced organ-cultured cells from temporal and nasal retina were passaged into sparse single cell cultures. From these, colonies of rapidly dividing cells were isolated and the progeny expanded as cell lines. The lines contained cells with features of neuroepithelial cells, showing vimentin and A2B5. They also contained spontaneously differentiated neuronal cells showing neurofilament L and N-CAM180. A subpopulation of the neuronal cells exhibited the morphological characteristics of retinal ganglion cells, i.e., large pear-shaped somata each emitting one long process with a distinct growth cone. In addition, they showed the marker profile of retinal ganglion cells, i.e., expression of Thy-1, G4, DM-GRASP, Nr-CAM, neurofilament H, and tau. Neuronal differentiation could be induced by the addition of db cAMP and retinoic acid. The mature neuronal features of the lines open new possibilities to study properties of retinal neurons, including ganglion cells, in a defined and manipulable experimental system.

SV40-immortalized and primary cultured human retinal pigment epithelial cells share similar patterns of cytokine-receptor expression and cytokine responsiveness

Retinal pigment epithelial (RPE) cells produce and respond to a variety of cytokines; however, molecular and biochemical studies are restricted by the limited access to large numbers of pure cells and the variability associated with different donor sources. Despite success in establishing primary human RPE (HRPE) cell cultures, the inability to sustain consistent proliferation rates and morphology over several passages remains a concern. This problem was approached by using an immortalized line of simian virus (SV)40 transformed fetal HRPE cells (SVRPE). Cytokine production, receptor expression and responsiveness in the SVRPE cell line was analyzed to determine the usefulness of this model for studying HRPE-cytokine interactions. Using reverse transcriptase polymerase chain reaction (RT-PCR), HRPE and SVRPE cells demonstrated an identical pattern of interleukin-1 receptor (IL-1R), IL-2R (alpha sub-unit), IL-6R, interferon (IFN)-gamma R and tumor necrosis factor-alpha (TNF)R p55 expression. No amplification products for TNFR p75 or granulocyte/macrophage colony stimulating factor (GM-CSF)R were demonstrated in either population. IFN-gamma stimulation induced surface human leukocyte antigen (HLA)-DR in both SVRPE and HRPE, while TNF treatment induced surface expression of intercellular adhesion molecule (ICAM)-1 on SVRPE and upregulated ICAM from basal levels on HRPE. Both cell types showed amplification products for interleukin (IL)-1 beta, IL-6 and transforming growth factor (TGF)-beta 1 using RT-PCR. The bioassays demonstrated that both populations of unstimulated cells constitutively secrete very low levels of TGF-beta and no IL-6.(ABSTRACT TRUNCATED AT 250 WORDS)