Excitatory amino acid receptors in membranes from cultured human retinal pigment epithelium

Glutamate accelerates RPE cell proliferation through ERK1/2 activation via distinct receptor-specific mechanisms

The proliferation and migration of Retinal Pigment Epithelium cells resulting from an epithelial-mesenchymal transition plays a key role in proliferative vitreoretinopathy, which leads to retinal detachment and the loss of vision. In neurons, glutamate has been shown to activate the Ras/Raf/MEK/ERK cascade, which participates in the regulation of proliferation, differentiation, and survival processes. Although glutamate-stimulation and the activation of ERK1/2 by different stimuli have been shown to promote RPE cell proliferation, the signaling pathway(s) linking these effects has not been established. We analyzed the molecular mechanisms leading to glutamate-induced proliferation by determining ERK1/2 and CREB phoshporylation in chick RPE cells in primary culture and the human-derived RPE cell line ARPE-19. This study shows for the first time, that glutamate promotes RPE cell proliferation by activating two distinct signaling pathways linked to selective glutamate receptor subtypes. Results demonstrate that glutamate stimulates RPE cell proliferation as well as ERK and CREB phosphorylation. These effects were mimicked by the mGluR agonist ACPD and by NMDA, and were prevented by the respective receptor inhibitors MCPG and MK-801, indicating a cause-effect relationship between these processes. Whereas mGluR promoted proliferation by activating the MEK/ERK/CREB cascade, NMDA stimulated proliferation through the MEK-independent activation of Ca(2+)/calmodulin-dependent kinases. The blockage of both signaling pathways to proliferation by KN-62 suggests the involvement of CaMKs in the control of glutamate-induced proliferation at a common step, downstream of CREB, possibly the regulation of cell cycle progression. Based on these findings, the participation of glutamate in the development of PVR can be considered.

The activation of MEK-ERK1/2 by glutamate receptor-stimulation is involved in the regulation of RPE proliferation and morphologic transformation

Retinal pigment epithelial (RPE) cells are the main cell type involved in the pathogenesis of proliferative vitreoretinopathy (PVR). As a result from retinal detachment or surgical procedures, RPE comes in contact with glutamate from serum, glial release and the injured retina. The purpose of this study was to explore a possible role for glutamate in the development of PVR, mediated by the receptor-stimulated activation of the ERK1/2 MAPK pathway, the alteration of cell proliferation and the transdifferentiation of RPE cells, using rat RPE cells in culture as a model system. We demonstrated the expression in these cells of Group I metabotropic-and ionotropic AMPA/KA and NMDA glutamate receptors (GluRs), predominantly of the NMDA subtype, which are targeted to the membrane, and exhibit pharmacological and biochemical characteristics equivalent to those previously established in brain tissue. Proliferation was measured by MTS-reduction colorimetric assay, and actin cytoskeleton dynamics was visualized by immunoflurescence using alpha-sma specific antibodies. Activation of metabotropic, AMPA and NMDA receptors by glutamate induced the time-and dose-dependent phosphorylation of ERK1/2, assessed by Western blot analysis, in parallel to a significant increase in cell proliferation and a decrease in alpha-sma expression and its recruitment into stress fibers. These effects were all prevented by the inhibition of MEK. Hence, results suggest that glutamate could be involved in the generation of PVR, through a GluR-mediated increase in proliferation and phenotypic transformation, cause-effect related to the activation of ERK1/2.

Iron alters glutamate secretion by regulating cytosolic aconitase activity

Glutamate has many important physiological functions, including its role as a neurotransmitter in the retina and the central nervous system. We have made the novel observations that retinal pigment epithelial cells underlying and intimately interacting with the retina secrete glutamate and that this secretion is significantly affected by iron. In addition, iron increased secretion of glutamate in cultured lens and neuronal cells, indicating that this may be a common mechanism for the regulation of glutamate production in many cell types. The activity of the iron-dependent enzyme cytosolic aconitase (c-aconitase) is increased by iron. The conversion of citrate to isocitrate by c-aconitase is the first step in a three-step process leading to glutamate formation. In the present study, iron increased c-aconitase activity, and this increase was associated with an increase in glutamate secretion. Inhibition of c-aconitase by oxalomalate decreased glutamate secretion and completely inhibited the iron-induced increase in glutamate secretion. Derangements in both glutamate secretion and iron metabolism have been noted in neurological diseases and retinal degeneration. Our results are the first to provide a functional link between these two physiologically important substances by demonstrating a significant role for iron in the regulation of glutamate production and secretion in mammalian cells resulting from iron regulation of aconitase activity. Glutamatergic systems are found in many nonneuronal tissues. We provide the first evidence that, in addition to secreting glutamate, retinal pigment epithelial cells express the vesicular glutamate transporter VGLUT1 and that regulated vesicular release of glutamate from these cells can be inhibited by riluzole.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Zinc and energy requirements in induction of oxidative stress to retinal pigmented epithelial cells

In age-related macular degeneration (AMD), retinal pigmented epithelium (RPE) cells are believed to be detrimentally affected. It is thought that zinc may play a part in this process. In the past, therefore, zinc supplementation has been suggested as a treatment for AMD. Experimental data shown here confound this view by indicating that whereas low amounts of zinc do protect RPE cells in culture from stress-induced effects, greater amounts of zinc have the opposite influence. These effects are partly dependent upon the "health status" of the cells. Experimental data presented herein also show that zinc-induced death of RPE cells can, however, be attenuated by compounds such as antioxidants (alpha-tocopherol, trolox, and metipranolol), or cellular energy substrates (pyruvate and oxaloacetate). It is therefore concluded that a combination of zinc and antioxidants or energy substrates rather that zinc alone should provide a safer and more effective way to treat a disease such as AMD.

Glutamate uptake is inhibited by tamoxifen and toremifene in cultured retinal pigment epithelial cells

The systemic drugs chloroquine and tamoxifen have caused retinal defects in human eye. The aim of our study was to investigate the effects of the amphiphilic drug tamoxifen, of its homologue toremifene, and of chloroquine on the glutamate uptake in retinal pigment epithelial (RPE) cells. Cultured human RPE cell line D407 and pig RPE cells were used in the study. Glutamate uptake was characterised and the glutamate transporters of pig RPE cells and the human RPE cell line D407 were compared to each other. The uptake of glutamate was studied using L-[3H]glutamate as a tracer. The radioactivity in the solubilised RPE was measured with a liquid scintillation counter. In the uptake experiments, the cells were exposed to the test drugs, to the selected glutamate receptor antagonists, and to the glutamate transporter inhibitors. Both RPE cell types exhibited a high-affinity transport system for glutamate. The glutamate transporter in RPE exhibited features characteristic of the uptake systems of neurotransmitters. The transport was Na+-dependent, and L- and D-aspartate were transported into the cell by the same transporter. Chloroquine had no effect on glutamate uptake, but tamoxifen and toremifene decreased the glutamate uptake of RPE cells dose-dependently both in pig RPE cells and in human RPE cell line. The IC50 values of tamoxifen and toremifene were lower for pig RPE cells, compared to the human RPE cell line D407. The glutamate uptake was a sensitive target for the effects of tamoxifen and toremifene, and disturbances in this function could be considered as one of the possible mechanisms of retinal defects.

Effects of mercuric chloride exposure on the glutamate uptake by cultured retinal pigment epithelial cells

The cytotoxicity of mercuric chloride and the effects of mercuric chloride on glutamate and calcium uptake and the factors regulating glutamate uptake were studied in retinal pigment epithelium (RPE) cell cultures. RPE cells isolated from pig eyes and human RPE cell line (D407) cells were cultured to confluency and further subcultured according to the test protocol in question. The cytotoxicity caused by 15 min of exposure to mercuric chloride (0.01--1000 microM) was evaluated by WST-1 assay based on the activity of mitochondrial dehydrogenases. [(3)H]Glutamate uptake was measured after the cells were exposed to 0.1--100 microM mercuric chloride and the selected regulators of protein kinase C (PKC) pathway: PKC activator SC10, PKC inhibitor chelerythrine chloride, phospholipase A(2)/C inhibitor manoalide, tyrosine kinase inhibitor lavendustin A, competitive NMDA receptor antagonist AP7 and IP(3) receptor antagonist heparin. Intracellular calcium was monitored with Fluo-3 probe starting immediately after the exposure to 1--1000 microM mercuric chloride. Mercuric chloride showed concentration-dependent effects on cell viability, on glutamate uptake and on intracellular calcium concentration. The results give some support to the concept that glutamate uptake is affected by PKC. The PKC inhibitor chelerythrine chloride decreased glutamate uptake by 25%, but the PKC activator SC10 could partly prevent the inhibitory effect of mercuric chloride. Lavendustin A, manoalide and heparin had smaller, but statistically significant, effects. All these substances act on mediators which can regulate the activity of PKC. However, PKC is not likely to be the only regulator of glutamate uptake. The rise observed in [Ca(2+)](i) may initiate various cellular events during mercury intoxication.

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.

Glutamate-stimulated proliferation of rat retinal pigment epithelial cells

We investigated the effects of glutamate on cell proliferation and the expression of basic fibroblast growth factor (bFGF) and its receptor (FGF-R1) mRNA in cultured rat retinal pigment epithelial (RPE) cells. The number of primary RPE cells was significantly higher after treatment with 0.2 to 1.0 mM glutamate (maximum at 1.0 mM) for 7 days than in controls. Glutamate-stimulated cell proliferation was abolished by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), but not by 6,7-dinitroquinoxaline-2,3-dione or L(+)-2-amino-3-phosphonopropionic acid. Proliferation was increased to a similar extent by N-methyl-D-aspartate (NMDA), but not by kainate, alpha-amino-3-hydroxy-3-methyl-4-isoxazolepropionic acid or trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid. NMDA-receptor-like immunoreactivity was detected in most cells cultured. Treatment of cells with glutamate increased the level of bFGF mRNA and, to a lesser extent, that of FGF-R1 mRNA, which peaked 2 and 4 days, respectively, after glutamate was added. The increase in bFGF mRNA induced by glutamate was inhibited by MK-801. These findings suggest that glutamate might stimulate proliferation of RPE cells through activation of NMDA receptors and expression of bFGF and further suggest that glutamate may be involved in the proliferative changes of RPE cells in retinal wound healing.

Neuroprotective therapy for argon-laser induced retinal injury

Laser photocoagulation treatment of the central retina is often complicated by an immediate side effect of visual impairment, caused by the unavoidable laser-induced destruction of the normal tissue lying adjacent to the lesion and not affected directly by the laser beam. Furthermore, accidental laser injuries are at present untreatable. A neuroprotective therapy for salvaging the normal tissue might enhance the benefit obtained from treatment and allow safe perifoveal photocoagulation. We have developed a rat model for studying the efficacy of putative neuroprotective compounds in ameliorating laser-induced retinal damage. Four compounds were evaluated: the corticosteroid methylprednisolone, the glutamate-receptor blocker MK-801, the anti-oxidant enzyme superoxide dismutase, and the calcium-overload antagonist flunarizine. The study was carried out in two steps: in the first, the histopathological development of retinal laser injuries was studied. Argon laser lesions were inflicted in the retinas of 18 pigmented rats. The animals were killed after 3, 20 or 60 days and their retinal lesions were evaluated under the light microscope. The laser injury mainly involved the outer layers of the retina, where it destroyed significant numbers of photoreceptor cells. Over time, evidence of two major histopathological processes was observed: traction of adjacent normal retinal cells into the central area of the lesion forming an internal retinal bulging, and a retinal pigmented epithelial proliferative reaction associated with subretinal neovascularization and invasion of the retinal lesion site by phagocytes. The neuroprotective effects of each of the four compounds were verified in a second step of the study. For each drug tested, 12 rats were irradiated with argon laser inflictions: six of them received the tested agent while the other six were treated with the corresponding vehicle. Twenty days after laser exposure, the rats were killed and their lesions were subjected to image-analysis morphometry. The extent of retinal destruction was assessed by measuring the lesion diameter and the amount of photoreceptor cell loss in the outer nuclear layer. Methylprednisolone and MK-801 were shown to ameliorate laser-induced retinal damage, whereas both superoxide dismutase and flunarizine were ineffective. Furthermore, MK-801 diminished the proliferative reaction of the retinal pigment epithelial cells. On the basis of our results we suggest that the pigmented rat model is suitable for studying and screening various compounds for their neuroprotective efficacy in treating retinal laser injury. We further suggest that glutamate might play a key role in mediating retinal injury induced by laser irradiation.

Effects of quisqualic acid on the corneal and intraretinal direct-current electroretinogram and on the standing potential of the rabbit eye

Quisqualic acid, an excitatory amino acid agonist, has been shown to stimulate inositol phosphate production in the rabbit retina. Inositol trisphosphate serves as a second messenger and increases intracellular calcium. We investigated the influence of quisqualic acid on the direct-current electroretinogram and on the standing potential of the rabbit eye. After unilateral vitrectomy, the corneal direct-current electroretinogram and the standing potential were recorded from both eyes of albino rabbits during simultaneous unilateral intravitreal perfusion with quisqualic acid alternating with control solution. The contralateral eye was used as a control. Intravitreal perfusion with 100-microM and 200-microM quisqualic acid elevated the standing potential significantly. This elevation was accompanied by a significant increase in c-wave amplitude and a significant decrease in b-wave amplitude. Quisqualic acid at 200-microM concentration decreased the a-wave amplitude also. In vivo intraretinal recordings showed that intravitreal perfusion with quisqualic acid at 200-microM concentration significantly increased the retinal pigment epithelial component of the c-wave. We conclude that quisqualic acid influences the direct-current electroretinogram and the standing potential apparently through its action on the retinal pigment epithelium. A possible mode of action is increased production of inositol trisphosphate, followed by an increase in intracellular release of calcium ions and an increase in basal chloride conductance. The decrease in a- and b-wave amplitudes indicates direct effects of quisqualic acid also on the neural retina.

Glycine stimulation of glutamate binding to chick retinal pigment epithelium

The effect of glycine (Gly) and taurine (Tau) on the biochemical and pharmacological properties of [3H]L-glutamate ([3H] Glu) binding to membranes from primary cultures of chick retinal pigment epithelium (RPE), as well as from intact tissue during development was studied. Gly and Tau increase Bmax of [3H]Glu binding to a high affinity site (KB = 300 nM) in membranes from 16 days in vitro (immature) cultures; additionally, Gly discloses a low affinity Glu-binding site (KB = 970 nM) at this stage. In membranes from 25 days in vitro (mature) cultures, the high affinity site is no longer present and Tau has no effect on Glu-binding; Gly still stimulates binding to the low affinity site by four fold, with an EC50 = 200 microM. Pharmacological profile using specific excitatory amino acid (EAA) receptor agonists and antagonists suggests that at 16 days in vitro Glu binds preferentially to metabotropic Glu receptors (mGluRs), and at 25 days in vitro to ionotropic receptors different from neuronal ones. The stimulatory effect of Gly and Tau was also observed in intact RPE, and decreased with increasing embryonic age. Glu binding was also stimulated in membranes from chick retina, but not in those from rat brain. Results support the possibility of EAA participation in several aspects of RPE physiology, including phagocytosis and cell division.