Phagocytosis by retinal pigment epithelium explants in culture

Clearance phagocytosis by the retinal pigment epithelial during photoreceptor outer segment renewal: Molecular mechanisms and relation to retinal inflammation

Mammalian photoreceptor outer segment renewal is a highly coordinated process that hinges on timed cell signaling between photoreceptor neurons and the adjacent retinal pigment epithelial (RPE). It is a strictly rhythmic, synchronized process that underlies in part circadian regulation. We highlight findings from recently developed methods that quantify distinct phases of outer segment renewal in retinal tissue. At light onset, outer segments expose the conserved "eat-me" signal phosphatidylserine exclusively at their distal, most aged tip. A coordinated two-receptor efferocytosis process follows, in which ligands bridge outer segment phosphatidylserine with the RPE receptors αvβ5 integrin, inducing cytosolic signaling toward Rac1 and focal adhesion kinase/MERTK, and with MERTK directly, additionally inhibiting RhoA/ROCK and thus enabling F-actin dynamics favoring outer segment fragment engulfment. Photoreceptors and RPE persist for life with each RPE cell in the eye servicing dozens of overlying photoreceptors. Thus, RPE cells phagocytose more often and process more material than any other cell type. Mutant mice with impaired outer segment renewal largely retain functional photoreceptors and retinal integrity. However, when anti-inflammatory signaling in the RPE via MERTK or the related TYRO3 is lacking, catastrophic inflammation leads to immune cell infiltration that swiftly destroys the retina causing blindness.

Large-scale purification of porcine or bovine photoreceptor outer segments for phagocytosis assays on retinal pigment epithelial cells

Analysis of one of the vital functions of retinal pigment epithelial (RPE) cells, the phagocytosis of spent aged distal fragments of photoreceptor outer segments (POS) can be performed in vitro. Photoreceptor outer segments with stacks of membranous discs containing the phototransduction machinery are continuously renewed in the retina. Spent POS are eliminated daily by RPE cells. Rodent, porcine/bovine and human RPE cells recognize POS from various species in a similar manner. To facilitate performing large series of experiments with little variability, a large stock of POS can be isolated from porcine eyes and stored frozen in aliquots. This protocol takes advantage of the characteristic of photopigments that display an orange color when kept in the dark. Under dim red light, retinae are collected in a buffer from opened eyecups cut in halves. The retinal cell suspension is homogenized, filtered and loaded onto a continuous sucrose gradient. After centrifugation, POS are located in a discrete band in the upper part of the gradient that has a characteristic orange color. POS are then collected, spun, resuspended sequentially in wash buffers, counted and aliquoted. POS obtained this way can be used for phagocytosis assays and analysis of protein activation, localization or interaction at various times after POS challenge. Alternatively, POS can be labeled with fluorophores, e.g., FITC, before aliquoting for subsequent fluorescence quantification of POS binding or engulfment. Other possible applications include the use of modified POS or POS challenge combined with stress conditions to study the effect of oxidative stress or aging on RPE cells.

Understanding photoreceptor outer segment phagocytosis: use and utility of RPE cells in culture

RPE cells are the most actively phagocytic cells in the human body. In the eye, RPE cells face rod and cone photoreceptor outer segments at all times but contribute to shedding and clearance phagocytosis of distal outer segment tips only once a day. Analysis of RPE phagocytosis in situ has succeeded in identifying key players of the RPE phagocytic mechanism. Phagocytic processes comprise three distinct phases, recognition/binding, internalization, and digestion, each of which is regulated separately by phagocytes. Studies of phagocytosis by RPE cells in culture allow specifically analyzing and manipulating these distinct phases to identify their molecular mechanisms. Here, we compare similarities and differences of primary, immortalized, and stem cell-derived RPE cells in culture to RPE cells in situ with respect to phagocytic function. We discuss in particular potential pitfalls of RPE cell culture phagocytosis assays. Finally, we point out considerations for phagocytosis assay development for future studies.

Age-related maculopathy susceptibility 2 participates in the phagocytosis functions of the retinal pigment epithelium

AIM

Age-related macular degeneration (AMD) is a multifactorial disease and a prevalent cause of visual impairment in developed countries. Many studies suggest that age-related maculopathy susceptibility 2 (ARMS2) is a second major susceptibility gene for AMD. At present, there is no functional information on this gene. Therefore, the purpose of the present study was to detect the expression of ARMS2 in retinal pigment epithelium (RPE) cells and to investigate the effect of ARMS2 on the phagocytosis function of RPE cells.

METHODS

Immunofluorescence and reverse transcriptase PCR were used to demonstrate the presence and location of ARMS2 in ARPE-19 (human retinal pigment epithelial cell line, ATCC, catalog No.CRL-2302) cells. siRNA was used to knock down ARMS2 mRNA, and the effects of the knockdown on the phagocytosis function of the ARPE-19 cells were evaluated via Fluorescence Activated Cell Sorting (FACS).

RESULTS

ARMS2 was present in ARPE-19 cells, localized in the cytosol of the perinuclear region. The expression of ARMS2 mRNA (messenger RNA) in ARPE-19 cells transfected with ARMS2-siRNA (small interfering RNA, 0.73±0.08) was decreased compared with normal cells (1.00±0.00) or with cells transfected with scrambled siRNA (0.95±0.13) (P<0.05). After incubation of RPE cells with a latex beads medium for 12, 18, or 24 hours, the fluorescence intensities were 38.04±1.02, 68.92±0.92, and 78.00±0.12 in the ARMS2-siRNA-transfected groups, respectively, and 77.98±5.43, 94.87±0.60, and 98.30±0.11 in the scrambled siRNA-transfected groups, respectively. The fluorescent intensities of the same time points in the two groups were compared using Student's t-test, and the p values were all less than 0.001 at the three different time points.

CONCLUSION

There is endogenous expression of ARMS2 in ARPE-19 cells. ARMS2 plays a role in the phagocytosis function of RPE cells, and this role may be one of the mechanisms that participates in the development of AMD.

Morphological Recovery in the Reattached Retina of the Toad Bufo marinus: A New Experimental Model of Retinal Detachment

BACKGROUND

The retinal pigment epithelium (RPE) of the toad (Bufo marinus) has been used in many studies as a model for understanding its role and interaction with the neural retina. The toad's retina has been used to establish a new in vitro model of experimental retinal detachment (RD) and replacement . It has been shown that the electrophysiological measures of retinal function recovered following complete RD. The toad was chosen because its RPE is similar to the mammalian RPE . In this report, light microscopy was used to characterize the morphologic changes that occur in the RPE and neural retina following RD/replacement and to correlate these findings with recovery of electrophysiologic function.

METHODS

Retinas from Bufo marinus were studied in vitro. The neural retina completely detached from the RPE and then replaced. At various times after replacement, neural retina-RPE tissues were processed for light microscopy.

RESULTS

At 30 min after replacement, the subretinal space was greatly expanded, and the apical processes that normally ensheath the rod outer segments were short and no longer contacted the rod outer segments. The RPE was swollen, contained many vacuoles and the apical surface was rounded. By 2 h after replacement, the subretinal space was significantly resorbed and contained many shredded rod outer segments; RPE cells were still swollen, although less. During the next 5-10 h, the number of phagosomes in the RPE cytoplasm increased and the number of shredded rod outer segments in the subretinal space decreased. RPE cells regained their normal size and interdigitation of apical processes and rod outer segments were observed.

CONCLUSIONS

These results demonstrate the re-establishment of morphological interactions between the RPE and neural retina within hours following RD/replacement. Morphological recovery coincides with recovery of electrophysiologic parameters. This is a good model to investigate the retinal pigment epithelium (RPE) and neural retina mechanisms involved in retinal adhesion and recovery from retinal detachment.

Chloroquine causes lysosomal dysfunction in neural retina and RPE: implications for retinopathy

Chronic use of chloroquine has been shown to induce numerous pathophysiological defects in the retina. This drug has the ability to alter pH of intracellular compartments and lysosomal function of the retinal pigment epithelium (RPE) and retinal neurons may constitute the basis of chloroquine retinopathy. The aim of the current study was to investigate pathogenic alterations in retinal cells continuously exposed to chloroquine using appropriate in vivo and in vitro models. Male hooded Lister rats were implanted with osmotic mini pumps which released chloroquine continuously over a period of seven days. The eyes were processed for electron microscopy and ultrastructural abnormalities determined in the neural retina and quantified using stereology in the retinal pigment epithelium (RPE). RPE were also exposed to chloroquine in vitro and lysosomal pH changes were investigated using a pH sensitive probe. Degradative capacity was also analysed using FITC labeled rod outer segments (ROS). Chloroquine-treated animals displayed several ultrastructural abnormalities including numerous membranous cytoplasmic bodies (MCBs) in retinal neurons. Cone photoreceptors displayed numerous MCBs although rods did not. The RPE of the treated groups all showed significantly higher numbers of lysosomal associated organelles (LAO) than the control group (p < 0.001). The in vitro experiments demonstrated chloroquine-mediated rises in lysosomal pH and an increase in lysosome/phagosome accumulation of ROS in the chloroquine treated group (p < 0.01). The current study demonstrates that chloroquine disrupts lysosomal function in retinal neurons and RPE. The evidence presented provides a clear pathogenic basis for the functional defects experienced by patients with chloroquine retinopathy.

Homozygous deletion in the coding sequence of the c-mer gene in RCS rats unravels general mechanisms of physiological cell adhesion and apoptosis

The RCS rat presents an autosomal recessive retinal pigment epithelium dystrophy characterized by the outer segments of photoreceptors being phagocytosis-deficient. A systematic genetic study allowed us to restrict the interval containing the rdy locus to that between the markers D3Mit13 and D3Rat256. We report the chromosomal localization of the rat c-mer gene in the cytogenetic bands 3q35-36, based on genetic analysis and radiation hybrid mapping. Using a systematic biocomputing analysis, we identified two strong related candidate genes encoding protein tyrosine kinase receptors of the AXL subfamily. The comparison of their expression patterns in human and mice tissues suggested that the c-mer gene was the best gene to screen for mutations. RCS rdy- and RCS rdy+ cDNAs were sequenced. The RCS rdy- cDNAs carried a significant deletion in the 5' part of the coding sequence of the c-mer gene resulting in a shortened aberrant transcript encoding a 20 amino acid peptide. The c-mer gene contains characteristic motifs of neural cell adhesion. A ligand of the c-mer receptor, Gas6, exhibits antiapoptotic properties.

Photoreceptor phagocytosis selectively activates PPARgamma expression in retinal pigment epithelial cells

Phagocytosis of tips of rod outer segments (ROS) by retinal pigment epithelial (RPE) cells is vitally important for maintaining structural and functional integrity of the retina. We previously reported that receptor-mediated specific phagocytosis of ROS induces expression of early response genes coding for transcription factors. Here we study the expression of peroxisome proliferator-activated receptors (PPAR) -alpha, -delta (beta) and -gamma during ROS phagocytosis of rat RPE cells in primary cell culture, using competitive quantitative RT-PCR. During phagocytosis of ROS (but not of latex particles) by RPE cells, RT-PCR revealed a transient increase in PPARgamma mRNA expression, that peaked at 4-6 hr. We sequenced and described two alternatively spliced variants of rat PPARgamma: rPPARgamma1a and rPPARgamma1b. Both of these, along with the recently described rPPARgamma2 were induced by ROS phagocytosis. PPARalpha and PPARdelta mRNA expression was also detected in RPE cells, but the level of expression did not change during ROS phagocytosis. All-trans-retinoic acid and prostaglandin E(2) (PGE(2)) selectively potentiated both basal and ROS-phagocytosis-induced PPARgamma expression. All-trans-retinoic acid had the opposite inhibitory effect on PPARalpha and PPARdelta expression. Cycloheximide had a dual action on PPARgamma expression in RPE cells: it enhanced expression under basal conditions but repressed expression induced by ROS phagocytosis. It also stimulated expression of PPARalpha but had no effect on PPARdelta. Selective activation of PPARgamma may play an important role in regulating the expression of target genes that are involved in lipid and fatty acid metabolism in the photoreceptor renewal process.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

Temperature-sensitive interactions between RPE and rod outer segment surface proteins

Phagocytosis of rod outer segments by the retinal pigment epithelium is distinguished by the two distinct temperature-dependent steps of binding and ingestion. This study was designed to see if retinal pigment epithelial (RPE) plasma membrane proteins interact with ROS plasma membrane proteins at temperatures favoring either binding or ingestion. A modified blot overlay assay was used whereby Western blots of RPE plasma membrane proteins were overlaid with biotinylated ROS plasma membrane proteins. RPE/ROS interactions were detected by streptavidin-HRP and the ECL method at 25 degrees C (ingestion), 15 degrees C (binding), and 4 degrees C (little or no binding or ingestion). Unlabeled ROS proteins served as the negative control. Competition with excess unlabeled ROS proteins were used to test the specificity of the protein interactions. Some protein interactions were somewhat temperature dependent. For example, two RPE plasma membrane proteins (200 kDa and 173 kDa) interacted with ROS plasma membrane proteins at both 25 degrees C and 15 degrees C, but not at 4 degrees C. A strongly labeled protein at 50 kDA protein was present at 25 degrees C but weakly labeled at 15 degrees C and at 4 degrees C. Other protein interaction were more clearly temperature dependent. For example, a 110 kDa RPE protein interacted with ROS proteins only at 25 degrees C. Another RPE protein (55 kDa) interacted only at 15 degrees C. These latter data provide correlations between binding events in the assay and previously described stages of phagocytosis.

Rat messenger RNA for the retinal pigment epithelium-specific protein RPE65 gradually accumulates in two weeks from late embryonic days

The RPE65 protein appears late during the retinal development. To study the basis for this regulation, the rat RPE65 cDNA was sequenced and the mRNA subsequently quantitated at various stages by competitive RT-PCR. RPE65 mRNA was detected as early as E18 (36 copies/ng of whole eye total RNA). It gradually accumulates up to P12 (27000 copies/ng) at which point it reaches a steady state level. This increase is interrupted for 3 days (P2-P4) during which the levels of mRNA remain stable. This timing and rate of accumulation parallels that of rat and mouse opsin mRNA and suggests that common factors may control the activation of genes in photoreceptors and retinal pigment epithelium cells.

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.

Iris pigment epithelial cells of long evans rats demonstrate phagocytic activity

The phagocytic activities of iris pigment epithelial (IPE) cells and retinal pigment epithelial (RPE) cells of Long Evans rats towards latex beads and rod outer segments (ROS) were compared in vitro. IPE and RPE cells of Long Evans rats were isolated and pure cultures obtained. The cultures were incubated with latex beads, fixed, and analysed computer morphometrically, IPE and RPE cell cultures were also incubated with isolated ROS and examined using transmission electron microscopy. IPE cells were able to ingest latex beads. There was no significant difference between the number of latex particles phagocytized by IPE and RPE cells. After incubation with isolated ROS, IPE cells also recognized and ingested the ROS particles. However, the specific phagocytic capacity of IPE cells was 76% of that of RPE cells. The autologous IPE cells might have the potential to be used as an alternative to RPE cells for transplantation in the subretinal space.

Selective transcription factor induction in retinal pigment epithelial cells during photoreceptor phagocytosis

Expression of early response genes during rod outer segment phagocytosis by normal Long Evans and Royal College of Surgeons-rdy+p+ rats and by dystrophic Royal College of Surgeons-p+ rat retinal pigment epithelial cells was studied in primary cell culture. Northern analysis revealed that the abundance of zif-268 (egr-1), c-fos, and tis-1 (NGF1-B) mRNA was rapidly and transiently increased in normal retinal pigment epithelial cells during rod outer segment phagocytosis but not during phagocytosis of latex particles. No increase in gene expression was found in Royal College of Surgeons-p+ dystrophic retinal pigment epithelial cells challenged with rod outer segments. As shown by electrophoretic mobility shift assay, a prominent short term increase in the intensity of the gel-shifted band was detected using nuclear protein extracts derived from rod outer segment-challenged, control retinal pigment epithelial cells and zif-268, AP-1, AP-2, or tis-1 consensus oligonucleotides. No such increase was detected when using nuclear factor kappaB consensus oligonucleotide or when the early response gene prostaglandin H synthase-2 mRNA was measured over the time course studied. The results suggest that in retinal pigment epithelial cells, rod outer segment-specific phagocytosis is accompanied by the selective expression of early response genes coding for transcription factors. The specific pattern of the induction of these transcription factors is predicted to modulate the expression of gene cascades.

CD36 participates in the phagocytosis of rod outer segments by retinal pigment epithelium

Mechanisms of phagocytosis are complex and incompletely understood. The retinal pigment epithelium provides an ideal system to study the specific aspects of phagocytosis since an important function of this cell is the ingestion of packets of membranous discs that are normally discarded at the apical ends of rod and cone cells during outer segment renewal. Here we provide evidence that rod outer segment phagocytosis by retinal pigment epithelium is mediated by CD36, a transmembrane glycoprotein which has been previously characterized on hematopoietic cells as a receptor for apoptotic neutrophils and oxidized low density lipoprotein. Immunocytochemical staining with monoclonal and polyclonal antibodies demonstrated CD36 expression by both human and rat retinal pigment epithelium in transverse cryostat sections of normal retina and in primary cultured cells. By western blot analysis of retinal pigment epithelial cell lysates, polyclonal and monoclonal antibodies to CD36 recognized an 88 kDa protein which comigrated with platelet CD36. Furthermore, the synthesis of CD36 mRNA by retinal pigment epithelium was confirmed by reverse transcriptase-PCR using specific CD36 oligonucleotides. The addition of CD36 antibodies to cultured retinal pigment epithelial cells reduced the binding and internalization of 125I-labeled rod outer segments by 60%. Immunofluorescence confocal microscopy confirmed that outer segment uptake was significantly diminished by an antibody to CD36. Moreover, we found that transfection of a human melanoma cell line with CD36 cDNA enabled these cells to bind and internalize isolated photoreceptor outer segments as seen by double immunofluorescent staining for surface bound and total cell-associated rod outer segments, and by measurement of cell-associated 125I-labeled rod outer segments. We conclude that the multifunctional scavenger receptor CD36 participates in the clearance of photoreceptor outer segments by retinal pigment epithelium and thus, participates in the visual process.

Effects of extracellular matrix and Bruch's membrane on retinal outer segment phagocytosis by cultured human retinal pigment epithelium

The goal of this study was to determine the effect of extracellular matrix components on the phagocytic function of the retinal pigment epithelial (RPE) cell. Cultured human fetal RPE cells were established in culture and plated on three commercially-prepared substrates: collagen IV, fibronectin and laminin and on three native matrices: bovine corneal endothelial cell matrix (BCEM) denuded bovine Bruch's membrane and denuded human Bruch's membrane. Cultured cells were allowed to become confluent and maintained for an additional two weeks before uptake of fluorescent bovine retinal outer segments (ROS) was measured by flow cytometry. Morphology by phase contrast microscopy and melanization was also determined as measures of differentiation. The results showed that morphology, melanization and ROS uptake by cells on collagen IV, laminin and fibronectin were not different from control cells plated on tissue culture plastic. However, ROS uptake by cells plated on BCEM was significantly less than that of cells cultured on plastic and melanization was greater. ROS uptake by cells plated on both types of Bruch's membrane was also significantly less than control cells. Treatment of cells plated on tissue culture plastic with 44 mM NaHCO3, which increased melanization, also reduced ROS uptake. We conclude that native matrices seem to contain components that significantly depress ROS uptake in culture. The inhibition is not mimicked by collagen, laminin or fibronectin coated wells. The ECM may play a significant role in controlling phagocytosis of ROS either by determining morphology, increasing differentiation or by directly influencing intracellular metabolism, and thus serve as another level of control for this RPE function which may not occur in cells plated on tissue culture plastic. These results may also have implications for the effects of aging or disease in which there are changes in the ECM.

A developmentally regulated microsomal protein specific for the pigment epithelium of the vertebrate retina

In the vertebrate retina, the retinal pigment epithelium (RPE) performs specific functions critical to the normal process of vision. Although some of these functions are well documented, molecular data are still scarce. Using the RPE-specific monoclonal antibody RPE9, raised against human RPE cells, we have identified a novel 65 kD protein, conserved in mammals, birds, and frogs. This RPE-specific protein was found to be nonglycosylated. It was most effectively solubilized in the presence of detergent suggesting that it is associated with the RPE cell membranes. Its partitioning in the detergent phase of Triton X-114 and its solubilization in 0.75 M and 1.0 M KCl suggest that it interacts with the membrane either through a polypeptide anchor or charged amino acids. Cell fractionation by differential solubilization and differential centrifugation demonstrated that the protein was preferentially associated with the microsomal membrane fraction, where it is the major protein. Developmental expression of this 65 kD protein was examined in neonatal rats. Morphologically well-differentiated RPE cells did not express the 65 kD protein at birth. However, expression was detectable at postnatal day 4, that is, one to two days before the photoreceptors develop their outer segments, suggesting that the expression of the 65 kD protein may be coordinated with other developmental events in the intact retina. This is further supported by the fact that RPE cells in confluent culture lose the expression of this protein within two weeks, while they maintain their characteristic epithelial morphology. Because of its specificity, its evolutionary conservation, and its timing of expression, it is possible that this protein may be involved in one of the key roles of RPE and as such is an important molecular marker for RPE differentiation.

Reconstitution of the photoreceptor-pigment epithelium interface: l-glutamate stimulation of adhesive interactions and rod disc shedding after recombination of dissociated Xenopus laevis eyecups

In order to investigate adhesive interactions between photoreceptor and pigment epithelial cells, we have mechanically separated neural retinas from Xenopus laevis eyecups and then recombined the tissues in vitro. When tissue pairs are incubated in a defined medium, cell-cell contact is achieved within 3 hr. However, the average proportion of reassembled eyecups in which photoreceptor outer segments interdigitate with epithelial microvilli is limited. Furthermore, rod disc shedding does not take place in these cultures, even following a dark to light transition. When recombined tissues are placed in medium supplemented with 12 mM L-glutamate, retinal reattachment is enhanced and there is a four-fold increase in epithelial phagosome content. The positive effect of excitatory amino acid exposure on shedding, however, is restricted to regions where visual and epithelial cells interdigitate. These results indicate that re-establishment of cell contact may be necessary for shedding of apical disc membranes prior to their engulfment by the epithelium. While reattachment is not affected by pre-incubation of separated tissues in normal medium, rod photoreceptors fail to undergo membrane turnover in response to L-glutamate if a delay of 1 hr or more is interposed between isolation of the retina and its recombination with the pigment epithelium. This is probably due to a decline in retinal function in culture, since a similar preincubation of the pigment epithelium prior to reassembly with a freshly isolated retina does not inhibit the shedding response.(ABSTRACT TRUNCATED AT 250 WORDS)

The phagocytosis of ROS by RPE cells is inhibited by an antiserum to rat RPE cell plasma membranes

A polyclonal antiserum to a rat retinal pigment epithelium (RPE) plasma membrane-enriched fraction has been utilized to identify candidate receptor proteins which may be involved in the phagocytosis of rod outer segments (ROS) by the RPE. Immunoblots of RPE cell extracts show that the the antiserum recognizes a number of glycoproteins, including two with M(r)s of 174 and 75 kDa. The antiserum also recognizes their non-glycosylated counterparts, with M(r)s of 169 and 65 kDa, respectively, which are synthesized after treatment of the cells with tunicamycin B2. Immuno-precipitation of [35S]-methionine-labeled RPE cell extracts also demonstrates the presence of antibodies to these same glycoproteins as well as to other proteins. The antiserum inhibits the binding of ROS to the RPE, which subsequently results in a decrease in the ingestion of ROS. ROS phagocytosis by the RPE is inhibited by 97% in the presence of a 1:10 dilution of the IgG fraction of the antiserum. Phagocytosis recovers to normal levels after 4-6 hr of chase in the absence of antibodies. After sequential adsorption of the IgG fraction to monolayers of fixed RPE cells, which removes RPE surface-specific IgGs, the extent of inhibition of ROS phagocytosis produced by the IgG fraction is reduced. Using immunoblotting we have identified a number of surface-specific immunoreactive bands which are adsorbed out of the antiserum, including the 174 and 75 kDa bands. These data give further support to the hypothesis that ROS phagocytosis is a receptor-mediated process, which occurs via specific cell surface glycoprotein receptors.

Characterization and application of an in vitro detection system for studying the binding and phagocytosis of rod outer segments by retinal pigment epithelial cells

Direct and indirect radioactivity and fluorescent assays have been developed to study the interaction of rod outer segments (ROS) with retinal pigment epithelial (RPE) cells. In the direct assays ROS labelled with 125I or fluorescein isothiocyanate (FITC) have been used to measure total phagocytosis, i.e. surface binding and ingestion. In the indirect assays RPE cells were first treated with unlabelled ROS or biotinylated ROS and subsequently probed with [125I]Rho 4D2 antirhodopsin antibody or [125I]streptavidin for radioactivity measurements or with the Rho 4D2 antibody and FITC-goat anti-mouse Ig or FITC-streptavidin for fluorescent counting. In these indirect methods the number of surface bound ROS were distinguished from the number of ingested ROS by comparative labelling of non-permeabilized and permeabilized ROS-treated RPE cells. Using these assays, we have studied the binding and ingestion of bovine ROS with cultured bovine RPE cells. As in the case of newborn cultured rat RPE cells [Hall and Abrams (1987) Exp. Eye Res. 45, 907-22], binding and ingestion of bovine ROS by bovine RPE cells was saturable with respect to ROS concentration and time. At 37 degrees C ROS binding reached a saturating concentration at 1 x 10(7) ROS per well; the number of bovine ROS ingested by bovine RPE cells, however, was less than the number of rat ROS ingested by rat RPE cells. When 1 x 10(7) ROS per well was used, maximal surface binding of bovine ROS to bovine RPE cells was obtained after 2-3 hr, whereas after an initial delay, ingestion rapidly increased to a maximum at 1-2 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Phagocytic challenge induces changes in phosphorylation of retinal pigment epithelium proteins

Changes in protein phosphorylation induced by phagocytic challenge were identified in cultured rat retinal pigment epithelium (RPE) following exposure to isolated rat rod outer segments (ROS) or to polystyrene latex microspheres (PSL). RPE phosphoproteins were characterized based on molecular weight and isoelectric point and 32P incorporation into phosphoproteins was quantified by digitized image analysis of two-dimensional gel autoradiograms. Changes in the phosphorylation of RPE proteins were determined by comparing 32P gel data from phagocytically challenged cultures with control cultures. ROS-specific changes were defined as those occurring only in response to ROS while nonspecific changes were those associated with either ROS or PSL phagocytosis. A parallel study was conducted to identify those proteins which also show increased phosphorylation following protein kinase C (PKC) activation by phorbol-12-myristate-13-acetate. ROS-specific increases in the phosphorylation of 2 RPE proteins were found, 1 of which also showed an increase with PKC activation. Nonspecific increases included the phosphorylation of 11 RPE proteins, 10 of which were also phosphorylated with PKC activation. ROS-specific decreases were observed in 12 RPE phosphoproteins while 3 proteins showed nonspecific decreases in their phosphorylation. These findings demonstrate that phagocytic challenge of the RPE with either specific or nonspecific particles is linked to the activation of phosphatases and kinases and that activation of PKC may play a role in phagocytosis of both particle types. The identification of two distinct groups of changes in phosphorylation supports the hypothesis that different pathways exist for phagocytosis of ROS-specific and nonspecific particles by the RPE.

The effect of swainsonine on the phagocytosis of rod outer segments by rat RPE

From studies using inhibitors such as tunicamycin and castanospermine, it has been suggested that plasma membrane glycoproteins may function as receptors in the phagocytosis of rod outer segments (ROS) by the retinal pigment epithelium (RPE). The exact structure of the oligosaccharide side chain of the glycoprotein may not be critical for this process. We have employed another inhibitor, swainsonine, which inhibits mannosidase II, a terminal enzyme in the protein glycosylation pathway, which results in membrane glycoproteins having hybrid-type oligosaccharide chains and fewer complex oligosaccharide chains. We have examined the ability of cultured rat RPE explants to phagocytize fluorescein isothiocyanate (FITC) labelled ROS or latex beads in the presence and absence of swainsonine. A significant (p less than 0.05) reduction in the phagocytosis of FITC-ROS was found between the swainsonine treated (37.7 +/- 4.1%) and untreated (85.4 +/- 2.7%) RPE explants. The nonspecific uptake of latex beads in both swainsonine treated (85.3 +/- 2.4%) and untreated (89.3 +/- 2.0%) RPE explants indicate that the RPE cells retained their ability to phagocytize. Major differences in spectrophotometric analysis of WGA-stained blots were an absence of a peak at 201 kD, a doublet at 86 kD and an overall reduction in all peak absorbances in the swainsonine treatments as compared to the untreated controls. These results suggest that the alterations in RPE glycoprotein formation due to swainsonine alter the ability of RPE to phagocytize ROS.

Effect of glycoconjugates on rod outer segment phagocytosis by retinal pigment epithelial explants in vitro assessed by a specific double radioimmunoassay procedure

Rod outer segment (ROS) phagocytosis by explanted bovine retinal pigment epithelium (RPE) was evaluated by a procedure using an indirect double radioimmunoassay which distinguished between ROS attached to the RPE cell surface and those which had been ingested. This approach has been used to investigate the effect of a variety of glycoconjugates on the phagocytic process. Inclusion of the glycosaminoglycans (GAGs) chondroitin sulphate type-A (CS-A) and type-C (CS-C), hyaluronic acid (HA) or dermatan sulphate (DS) in the incubation medium significantly inhibited the ingestion phase of ROS phagocytosis, whereas the binding phase was inhibited to a lesser extent. The interphotoreceptor matrix (IPM), containing these GAGs as part of proteoglycans, also had an inhibitory effect on phagocytosis. The free monosaccharides mannose, fucose and galactose all stimulated the ingestion of ROS by RPE cells. These findings support the suggestion that glycoconjugates may have a physiological role in the photoreceptor renewal process.

Retinal pigment epithelium of new-born rats is capable of phagocytosis of rod outer segments in vitro

The ability of retinal pigment epithelium (RPE) cells of rats at 0, 1, 3, 4, 5, 8, 10, 12 and 15 days after birth towards phagocytosis of rod outer segments (ROS) has been studied in short-term explant culture of the scleral part of the eye. The neural retina was removed before cultivation. The suspension of ROS isolated from the retina of adult rats was layered on top of RPE cells. The phagocytic activity was evaluated by visualization of phagosomes in the cytoplasm of RPE cells after 4-5 hr incubation, using electron microscopy. It has been shown that RPE cells at all the stages studied, including new-born rats, are capable of ROS phagocytosis.

Phagocytic interactions of sialated glycoprotein, sugar, and lectin coated beads with rat retinal pigment epithelium

Latex beads coated with mucin (a sialic acid containing glycoconjugate), N-acetylglucosamine (GlcNAc), or with the lectins, succinylated wheat germ agglutinin (sWGA) (binds to GlcNAc) or Limax flavus agglutinin (LFA) (binds to sialic acid residues) were used as phagocytic particles. Phagocytosis of these coated beads by retinal pigment epithelial (RPE) explants was determined by bead uptake in normal (Long Evans) and dystrophic (Royal College of Surgeons, RCS/p+) rat retinas. Electron microscopy showed that beads coated with mucin or LFA lectin were not phagocytized by either dystrophic or normal RPE. sWGA-coated beads were phagocytized by both dystrophic and normal RPE, while GlcNAc-coated beads were taken up by dystrophic RPE only. Specificity of uptake for sWGA and GlcNAc bead coatings was shown by the reduction in the number of beads phagocytized in the presence of the appropriate competing sugar or lectin. The lack of phagocytic uptake of beads coated with a sialated glycoprotein or a sialic acid binding lectin suggests that sialic acid residues are not recognized as particulate ligands in this phagocytic assay. The data which show the uptake of beads coated with sWGA (binds only GlcNAc) together with results which showed WGA (binds both GlcNAc and sialic acid)-coated beads were not taken up, further suggest that GlcNAc residues may be involved in bead phagocytosis. The most striking difference between normal and dystrophic RPE engulfment of coated beads is the uptake of GlcNAc-coated beads by dystrophic RPE only. These results suggest that the receptor molecules on dystrophic RPE cell surface membranes may be different from those on normal RPE membranes.

Circadian rhythms in the green sunfish retina

We investigated the occurrence of circadian rhythms in retinomotor movements and retinal sensitivity in the green sunfish, Lepomis cyanellus. When green sunfish were kept in constant darkness, cone photoreceptors exhibited circadian retinomotor movements; rod photoreceptors and retinal pigment epithelium (RPE) pigment granules did not. Cones elongated during subjective night and contracted during subjective day. These results corroborate those of Burnside and Ackland (1984. Investigative Ophthalmology and Visual Science. 25:539-545). Electroretinograms (ERGs) recorded in constant darkness in response to dim flashes (lambda = 640 nm) exhibited a greater amplitude during subjective night than during subjective day. The nighttime increase in the ERG amplitude corresponded to a 3-10-fold increase in retinal sensitivity. The rhythmic changes in the ERG amplitude continued in constant darkness with a period of approximately 24 h, which indicates that the rhythm is generated by a circadian oscillator. The spectral sensitivity of the ERG recorded in constant darkness suggests that cones contribute to retinal responses during both day and night. Thus, the elongation of cone myoids during the night does not abolish the response of the cones. To examine the role of retinal efferents in generating retinal circadian rhythms, we cut the optic nerve. This procedure did not abolish the rhythms of retinomotor movement or of the ERG amplitude, but it did reduce the magnitude of the nighttime phases of both rhythms. Our results suggest that more than one endogenous oscillator regulates the retinal circadian rhythms in green sunfish. Circadian signals controlling the rhythms may be either generated within the eye or transferred to the eye via a humoral pathway.

Surface modification of retinal pigment epithelial cells: effects on phagocytosis and glycoprotein composition

Proteases have been used as a tool to investigate the role of cell-surface molecules of cultured retinal pigment epithelial cells (RPE) in the phagocytosis of rod outer segments (ROS). Proteolytic digestion of RPE cells by pronase, thermolysin and Staphylococcus aureus V8 protease (V8 protease) inhibited the phagocytosis of ROS without affecting the viability of the RPE cells. A particular feature of RPE cell proteolysis was that those macromolecules responsible for ROS ingestion were susceptible, while those macromolecules that mediated ROS binding were resistant to cleavage by all three proteases. By taking advantage of this phenomenon, ROS were used as affinity particles to obtain a plasma membrane-enriched fraction of RPE cells before and after proteolytic digestion. All three proteases partially or completely removed several glycoproteins from the cell surfaces. Removal of these glycoproteins was correlated with a loss in phagocytic ability by RPE cells. Two high-molecular-weight (MW) glycoproteins of MWs 160,000 and 214,000 were consistently removed by all proteases tested. Protease-treated RPE cells restored their phagocytic capabilities and normal glycoprotein composition within 24 hr after proteolytic treatment. These data suggest that glycoproteins located on the surfaces of RPE cells may be involved in mediating the phagocytosis of ROS by these cells.

The influence of carbohydrates on the binding of rod outer-segment (ROS) disc membranes and intact ROS by the cells of the retinal pigment epithelium of the embryonic chick

The role of carbohydrates in mediating the interaction of rhodopsin-containing membranes with retinal pigment epithelium (RPE) cells was investigated by studying the influence of various monosaccharides on their binding by RPE cells of the embryonic chick maintained in cell culture. Rod outer-segment (ROS) disc membranes were selected as a model rhodopsin-containing membrane system for these studies in view of their high concentration of rhodopsin and the relative purity with which they can be isolated. Disc membranes, frozen and thawed in order to expose the carbohydrate groups of rhodopsin which are oriented intraluminally in situ, were incubated with monolayers of RPE cells under various conditions, and the binding of the membranes by the cells was quantitated by radioimmunoassay for rhodopsin. Cell-membrane association was also verified by indirect immunofluorescence microscopy. The surface accessibility of the sugars in frozen-thawed discs was verified by succinyl concanavalin A-binding studies. From 15- to 20-fold increase in carbohydrate-reactive sites was obtained after freezing and thawing the discs. The RPE cell-membrane binding process was saturable, and time- and temperature-dependent. By means of competition studies carried out in the presence of high concentrations of various monosaccharides, and also by comparing the binding of disc membranes whose carbohydrate groups were either exposed (frozen-thawed) on the surface or inaccessible (native), it was concluded that the carbohydrates of rhodopsin, mannose and N-acetylglucosamine, were not involved in the interaction with the RPE. The possibility was also examined that enzymatically galactosylated rhodopsin might serve as a site for recognition by the RPE cell. The binding of ROS disc membranes modified in this manner was not enhanced, indicating that the presence of galactose groups on rhodopsin did not serve as a site for recognition by the RPE. The influence of monosaccharides on the binding of intact ROS by the RPE cells was also investigated. Similar to the results with the disc membranes, the process was not blocked by the presence in the incubation medium of high concentrations (up to 30,000-fold higher than that of rhodopsin) of mannose or GlcNAc, as with the disc membranes, or by glucose or galactose. Thus, from these studies it is concluded that a lectin-like carbohydrate-recognition process may not be involved in the interaction between rhodopsin-containing membranes and the RPE cells.

Short-term organ culture of the retinal pigment epithelium in microtitration plates: ultrastructural studies

Microtitration plates were used to culture simultaneously multiple, small (6 mm diameter) explants of bovine retinal pigment epithelium (RPE). Evaluation of tissue by light microscopy and by scanning and transmission electron microscopy after various incubation periods up to 6 h showed that RPE maintained in this system retains near normal morphology. Initially, the explanted RPE lacks apical microvilli, but during the first 2-3 h in culture recovery of apical microvilli occurs. The results suggest that the system is suitable for short-term maintenance of RPE for experimental purposes. Moreover, the ability to culture up to 16 explants from one bovine eye aids statistical evaluation of RPE behaviour under varying experimental conditions.

Rat retinal pigment epithelial cells show specificity of phagocytosis in vitro

The retinal pigment epithelial (RPE) cell of the eye normally phagocytozes only retinal rod outer segments (ROS). The specificity of this phagocytic process was examined by incubating RPE cells with a variety of particle types. Confluent RPE cell cultures were incubated for 3 h at 37 degrees C in the presence of rat ROS, rat red blood cells (RBC), algae, bacteria, or yeast. Other cell cultures were incubated with equal numbers of ROS and one other particle type. Quantitative scanning electron microscopy was used to determine the numbers and morphology of particles bound to RPE cells, while double immunofluorescence labeling (Chaitin, M. H., and M. O. Hall, 1983, Invest. Ophthalmol. Vis. Sci., 24:812-820) was used to quantitate particle binding and ingestion. Both assays demonstrated phagocytosis to be a highly specific process. RPE cells bound 40-250 X more ROS than RBC, 30 X more ROS than algae, and 5 X more ROS than bacteria or yeast. Ingestion was more specific than binding; RPE cells ingested 970 X more ROS than RBC, 140 X more ROS than bacteria, and 35 X more ROS than yeast. The phagocytic preference for ROS was maintained in competition experiments with other particle types. Serum was found to be essential for phagocytosis. This study demonstrates that both the binding and ingestion phases of phagocytosis are highly specific processes.

Phagocytosis of light- and dark-adapted rod outer segments by cultured RPE cells: a reassessment

Rod outer segments (ROS) isolated from adult rat retinas are phagocytized by cultured rat retinal pigment epithelial (RPE) cells. Using a double immunofluorescent labeling procedure, we have compared the binding and ingestion of ROS isolated at different times of the day. After 2 hr of incubation, approximately 98% of the ROS are ingested, while 2% are still attached to the RPE cell surface, irrespective of the time of day or lighting conditions under which the ROS are isolated. These findings differ from those reported earlier, using a radioactive method for quantitating ROS phagocytosis (Hall, 1978).

Rod photoreceptor disc shedding in eye cups: relationship to bicarbonate and amino acids

We previously demonstrated that disc shedding in eye cups from Xenopus laevis depends on medium bicarbonate concentration, but also found that other unidentified medium components influenced the response. Here, in order to determine which medium components influence shedding, we used high bicarbonate salt solutions supplemented with medium components in amphibian tissue culture medium. Compared to the high bicarbonate salt solution alone, addition of fetal bovine serum or whole egg ultrafiltrate enhanced disc shedding. However, a response fully comparable to that in intact animals was obtained on addition of a mixture of 14 amino acids. The full light-evoked response depended on the simultaneous presence of high bicarbonate (35 mM) and amino acids. In the presence of amino acids, photoreceptor tips and RPE showed tight interdigitation and less tendency for in vitro separation, suggesting that these medium components promote such interdigitation. Using the defined medium we have additionally shown that shedding is related to HCO3- concentration under conditions of controlled medium pH, Cl- content and osmolality.

The use of sugar-coated beads to study phagocytosis in normal and dystrophic retina

In this study, a technique is described for coupling the sugars to carboxylated latex beads for use in phagocytic studies of dystrophic and normal retinal pigment epithelium. The presence and distribution of the sugars, mannose and fucose, on the bead surfaces are visualized by transmission electron microscopy using the lectin-Ferritin conjugates. Concanavalin A-Ferritin (Con A-Fe) and Ulex-europeus-Ferritin (Ulex-Fe), respectively. Both lectin-Ferritin conjugates label the surfaces of the sugar coated beads. Control experiments to determine the extent of nonspecific lectin-Ferritin binding using Con A-Fe with fucose-coated beads, Ulex-Fe with mannose-coated beads, or beads without coupling reagent show little to no labeling. Lectin-Ferritin conjugates are also incubated with the appropriate sugar-coated beads and their competing sugars and little to no labeling is observed. Our data shows that when sugar-coated beads are presented to retinal pigment epithelial explants, differences in phagocytic uptake are observed. Uncoated beads are avidly taken up by normal and dystrophic tissue, but fucose-coated beads are not taken up by either. Twice as many mannose beads are phagocytized by the normal as compared to dystrophic tissue. It remains to be established whether these differences in bead uptake indicate a difference in ligand-receptor interactions between normal and dystrophic retinal pigment epithelium.