How good is the evidence to suggest that phagocytosis of ROS by RPE is receptor mediated?

Outer segment phagocytosis by cultured retinal pigment epithelial cells requires Gas6

The function and viability of vertebrate photoreceptors requires the daily phagocytosis of photoreceptor outer segments (OS) by the adjacent retinal pigment epithelium (RPE). We demonstrate here a critical role in this process for Gas6 and by implication one of its receptor protein tyrosine kinases (RTKs), Mertk (Mer). Gas6 specifically and selectively stimulates the phagocytosis of OS by normal cultured rat RPE cells. The magnitude of the response is dose-dependent and shows an absolute requirement for calcium. By contrast the Royal College of Surgeons (RCS) rat RPE cells, in which a mutation in the gene Mertk results in the expression of a truncated, non-functional receptor, does not respond to Gas6. These data strongly suggest that activation of Mertk by its ligand, Gas6, is the specific signaling pathway responsible for initiating the ingestion of shed OS. Moreover, photoreceptor degeneration in the RCS rat retina, which lacks Mertk, and in humans with a mutation in Mertk, strongly suggests that the Gas6/Mertk signaling pathway is essential for photoreceptor viability. We believe that this is the first demonstration of a specific function for Gas6 in the eye.

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.

Retinal pigment epithelial cell cultures as a tool for evaluating retinal toxicity in vitro

This article reviews in vitro testing of retinal toxicity in retinal pigment epithelium (RPE) cell cultures. It is based on the literature on RPE cell cultures and on our recent studies on the retinal toxicity of selected amphiphilic drugs. The RPE plays a major role in maintaining the homeostasis and health of the retina. Various pharmacological agents are known to cause adverse effects in RPE cells. For example, long-term treatment with chloroquine in patients with rheumatoid arthritis has induced retinopathy, and tamoxifen, a drug that is commonly used in the treatment of advanced breast cancer and in the prevention of breast cancer among high-risk women, has been reported to cause retinal changes and impaired vision. During our research, we have developed novel in vitro methods for evaluating the retinal toxicity of xenobiotics. We have used a pig RPE primary culture and a human RPE cell line (D407), which retain epithelial cell characteristics. They form a layer of hexagonal cells with intercellular junctions, and possess a keratin-containing cytoskeleton. They are both good models for determining the retinal cell toxicity of test compounds. Further studies on phagocytic activity, lysosomal enzyme activity and glutamate uptake might generate new methods for the toxicological evaluation of the retinal side-effects of drugs in vitro.

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.

X-ray microanalysis and phagocytotic activity of cultured retinal pigment epithelial cells in hypoxia

PURPOSE

To investigate the influence of the functional and morphological changes induced in retinal pigment epithelial (RPE) cells by retinal ischemia, we evaluated the phagocytotic activity, the concentration of various elements, and ultrastructure in cultured RPE cells in hypoxia.

METHODS

The concentrations of oxygen in incubators were adjusted to 20, 10, and 1% by the addition of nitrogen for 72 hr. To observe phagocytotic activity and its relationship to actin filaments, the filaments of RPE cells incubated with fluoresbrite carboxylate YG microspheres were stained with rhodamine phalloidin. Some of the specimens were subjected to X-ray microanalysis by scanning electron microscope after being fixed, freeze-dried, and coated with carbon to investigate the cytoplasmic concentration of elements. A part of the latter specimens was also observed by transmission electron microscope after being embedded in epon and cut into ultrathin sections to see the ultrastructural changes inside cell.

RESULTS

Lowering oxygen concentrations from 20% to 1% swelled RPE cells and decreased the number of fluoresbrite carboxylate YG microspheres phagocytized by RPE cells. Phagocytosis of a large amount of latex beads (30 microl) for 24 hr in 1% oxygen caused a disruption of RPE cells. Na, S, and P were detected in RPE cells cultured in 20% oxygen. Reducing the oxygen concentration from 20 to 10 or 1% significantly decreased Na and increased S. Mitochondria were observed in RPE cells in 20 and 10% oxygen, but many vacuoles were observed in the cytoplasm in 1% oxygen.

CONCLUSION

Hypoxia as low as 1% oxygen induced malfunction of phagocytosis and the fragility of RPE cells. We could speculate the imbalance of the electrolytes such as Na or a decrease of antioxidants such as glutathione containing S as a reason of disturbance of cell viability.

Inherited retinal degeneration: basic FGF induces phagocytic competence in cultured RPE cells from RCS rats

In RCS rats, the retinal pigment epithelium (RPE) is defective in phagocytosis of photoreceptor membranes. We have previously shown reduced expression of basic fibroblast growth factor (bFGF) in the RPE of 7-10-day-old RCS rats. This study using primary RPE cultures from rats of this age demonstrates that the phagocytic defect in the mutant RPE can be overcome by treatment with bFGF, by a mechanism involving gene transcription and that normal RPE phagocytosis, also requiring transcription, is blocked by a bFGF neutralizing antibody. The combined data point to a role for bFGF in the normal mechanism of RPE phagocytosis and the RCS defect.

Binding of anionic phospholipids to retinal pigment epithelium may be mediated by the scavenger receptor CD36

The specific recognition of negatively charged phospholipids in cell membranes has been suggested to play an important role in a variety of physiological and pathophysiological processes. Recent work (Rigotti, A., Acton, S. L., and Krieger, M. (1995) J. Biol. Chem. 270, 16221-16224) has described specific and tight binding of anionic phospholipids, such as phosphatidylserine (PS) and phosphatidylinositol (PI), to the class B scavenger receptors, CD36 and SR-B1. We have previously reported that CD36 is present on retinal pigment epithelium (RPE) and plays a role in the phagocytosis of photoreceptor outer segments (ROS), a function critical to the normal visual process (Ryeom, S. W., Sparrow, J. R., and Silverstein, R. L. (1996) J. Cell Sci. 109, 387-395). We now report that phospholipid liposomes PS and PI, but not phosphatidylethanolamine, bind specifically to RPE. Cross-competition experiments suggest that PS and PI recognize the same receptor on RPE, while immunoinhibition studies indicate that the receptor is CD36. RPE cells isolated from a mutant rat strain, the RPE of which does not express CD36 ( Sparrow, J. R., Ryeom, S. W. , Abumrad, N., Ibrahimi, A., and Silverstein, R. L. (1996) Exp. Eye Res., in press), did not bind PS or PI, further confirming the role of CD36. We also showed that purified ROS blocked binding and uptake of anionic phospholipid liposomes by RPE and that PS and PI liposomes blocked ROS uptake by RPE, suggesting that PS and PI on the ROS membrane may be the ligands on ROS recognized by CD36. This is the first demonstration that CD36-phospholipid interactions may play a role in normal physiology.

Natural, high-mannose glycoproteins inhibit ROS binding and ingestion by RPE cell cultures

Previous studies have suggested that a mannose receptor mediates the phagocytic uptake of effete rod outer segments by retinal pigment epithelial cells. In the present study, the effect of adding a soluble ligand for the mannose receptor, horseradish peroxidase, was examined. Cultured retinal pigment epithelial cells from Long Evans rats were preincubated with various concentrations of horseradish peroxidase for 20 min followed by a challenge of FITC-labeled bovine rod outer segments for 3 h. Both counts of total rod outer segments (bound and ingested) and ingested rod outer segments were determined. Rod outer segment uptake was reduced, in a concentration-dependent fashion, by an average of 60% of control values when horseradish peroxidase was added to retinal pigment epithelial cultures. Similarly, total rod outer segment values were reduced to 50% of controls in the presence of at least a 10 micrograms ml-1 horseradish peroxidase concentration. Horseradish peroxidase inhibition of retinal pigment epithelial phagocytic capacity was reversible. Other high mannose glycoproteins, such as invertase, beta-glucoronidase, and ovalbumin, were equally effective in preventing rod outer segment ingestion by retinal pigment epithelial cells. These data further support the hypothesis that a mannose receptor on the retinal pigment epithelial apical surface facilitates phagocytosis of rod outer segments.