Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

question of how the RPE could help prevent this accumulation of metabolic acid and water in the SRS.

In in vivo studies of rabbit eye, it was estimated that70% of fluid absorption across the RPE is linked to metabolite transport to the choroidal blood supply. In addition, in vitro studies of frog RPE showed that steady-state fluid absorption decreased by 70%, following the removal of HCO3 from both bathing solutions, implicating HCO3 transport in a regulatory role on fluid transport. The RPE functionally expresses several different HCO3 transport proteins at the apical and basolateral membranes as illustrated in Figure 5. In an earlier study, a 4,40-diisothiocyano-2,20-stillbene-disulfonic acid (DIDS)- sensitive electrogenic Na/2HCO3 co-transporter was localized to the apical membrane of frog and bovine RPE; DIDS is a bicarbonate transport inhibitor. At the basolateral membrane, HCO3 is transported out of the RPE through a pH-sensitive Cl/HCO3 exchanger with a possible contribution from a Na/HCO3 co-transporter. These HCO3 transporters in the RPE are linked to Na and Cl transport, which are major driving forces for

fluid transport. Recently, the identities of some of these HCO3 transporters have been characterized in our laboratory and by other groups. NBC1 (Na/2HCO3 cotransporter) and NBC3 (NBCn1; electroneutral Na/ HCO3 co-transporter) were localized to the apical membrane. AE2 (Cl/HCO3 exchanger) mRNA transcripts were detected, but protein expression in the RPE remains to be determined. The identity of the basolateral membrane Na/nHCO3 co-transporter (NBC) is still unknown.

In vitro, we mimic the increased retinal CO2 production, following the transition from light to dark by increasing apical bath CO2 level from 5% to 13%. This maneuver increased NaCl uptake at the apical membrane and can enhance CA-mediated Na/HCO3 co-transport across the RPE, thus increasing net NaHCO3 absorption. This increase in solute transport would drive additional fluid across the RPE as observed in in vitro experiments. The transport of metabolic waste products from the SRS to the choroidal blood supply by the RPE helps maintain ionic and pH homeostasis of the SRS. The RPE handles the increased metabolic load by transporting

CO2 across the RPE in the form of HCO3 through HCO3- transporters, and this process is mediated by the catalytic activity of CAs. This increase in Na and HCO3 absorption provides the driving force for increased net fluid absorption across the RPE, which dehydrates the SRS and creates retinal adhesion, thus allowing the RPE to maintain proper anatomical relationship with the photoreceptors.

In the retina, 95% of glucose consumption is metabolized through glycolysis into lactic acid, which is subsequently deposited into the SRS. In addition to the high glycolytic activity of the retina, several other mechanisms cause additional lactic acid to be released by the retina following light–dark transition: (1) increased glucose metabolism at the outer retina; (2) reduced retinal oxygen level in the dark-adapted eye, leading to an increased anaerobic lactate production; and (3) glutamate-induced lactate release from Mu¨ller cells. The RPE disposes of this metabolic load by transporting lactic acid to the choroid

through monocarboxylate transporters (MCTs) of the MCT family. We previously demonstrated that the RPE is extremely resistant to pH change compared to other epithelia and that part of this regulation comes from H/Lac cotransporters at the apical and basolateral membranes.

In human RPE, Philp and colleagues showed that monocarboxylate transporter 1 (MCT1), a H/Lac cotransporter, is immunolabeled at the apical membrane (Figure 5). They also showed that MCT3, a H/Lac cotransporter expressed exclusively in the RPE and choroid plexus basolateral membranes, meditates lactate efflux from the RPE into the choroidal blood supply (Figure 5). In addition, a Cl/Lac exchanger, possibly anion exchanger 2 (AE2), has been shown to transport lactate at the basolateral membrane. The importance of lactate transport in the mammalian eye has also been demonstrated in mice lacking MCT1, MCT3, and MCT4 expression – the mutant mice gradually lose photoreceptor function and

were completely blind after 41 weeks. Further, altered visual function in MCT3-null mice demonstrates the importance of lactate transport specifically in the RPE.

Oxidative Stress

The RPE encounters significant levels of oxidative stress on a daily basis and this onslaught promotes mitochondrial (mt) damage and decreases in mt potential and respiration, which may contribute to inflammation and the onset of age-related diseases such as AMD (summarized by Jarrett and colleagues). In opposition to these oxidative stresses, there exist mt protective mechanisms that provide direct antioxidant protection and those that enhance glutathione (GSH) production; furthermore, there is evidence that all of these protective mechanisms weaken with age. Different cell types can exert different levels of protection; for example, it has been shown that hfRPE monolayers are significantly more resistant to oxidative stress than ARPE19 cells. Voloboueva and colleagues used the hfRPE primary cultures to examine mt and other pathways that are putative targets for therapeutic intervention against oxidative stress. In one set of experiments they studied the protective effects of a-lipoic acid (R-form), a potent intracellular antioxidant that has been shown in other systems, to induce all three cellular protective mechanisms. The R form of lipoic acid is a coenzyme in mt that has

been shown to reverse the age-related decrease in mt function. Measurements of cell viability, mt potential, cell death, oxidative stress, apoptosis, and GSH/GSSH show that lipoic acid can protect hfRPE cells in three ways: (1) directly scavenge reactive oxidative species;

(2) repair and protect mt enzymes; and (3) activate antioxidant defenses through phase 2 enzymes. Our results suggest that (R)-a-lipoic acid can be used as an all-purpose therapeutic intervention against the slow accumulation of oxidative damage that can occur in AMD.

Cigarette smoke is an important risk factor for AMD and causes significant oxidative damage in RPE that also can be mitigated by (R)-a-lipoic acid. RPE mitochondria are themselves a main generation site of oxidants and a critical and sensitive target of specific cigarette-smoke components. Acrolein is present in the gas phase of cigarettes (25–140 mg per cigarette), and it is estimated that the gas phase of one cigarette reaches a concentration of 80 mM in the airway surface fluid. Acrolein has a high hazard risk in cigarette smoke and causes oxidative stress in cells by reacting with sulfhydryl groups. In the physiological range (0.1–100 mM), it causes significant mt damage in hfRPE that can be ameliorated by (R)-a-lipoic acid, for example, by inducing GSH and other phase-2 antioxidant protective enzymes. Pretreatment by (R)-alpha-lipoid acid has a protective effect against peroxide induced mt oxidative stress in several ways: (1) by lowering cell calcium; (2) by increasing mt electron chain complexes I, II, and III

activity levels; (3) by increasing mt membrane potential; and (4) by increasing total antioxidant power as well as GSH peroxidase/GSH/superoxide dismutase levels. Collectively, these data indicate that lipoic acid may be an effective therapeutic strategy against age-related, oxidantinduced RPE degeneration.

As summarized recently by Dunaief, AMD patients have elevated levels of iron within the RPE that also can lead to oxidative damage to mitochondria. Based on that observation, Voloboueva and colleagues showed that ferric ammonium citrate increased intracellular iron and oxidant production and decreased GSH and mt complex IV activity in human fetal cultured RPE. They also showed that N-tert-butyl hydroxylamine (Nt-BHA), a known mt antioxidant, reduced oxidative stress, mt damage, and age-related iron accumulation. These data show that the application of Nt-BHA may be an effective therapeutic strategy against AMD.

RPE–Immune System Interactions in and around the SRS

The integrated effect of proinflammatory molecules on RPE function depends on the polarized location of the cognate receptors and the access of their ligands (cytokines and chemokines) to the apical and basolateral membranes, and the interactions of downstream signaling pathways. For the experiments summarized in Table 1, we used a mixture of three proinflammatory cytokines, interleukin 1 beta (IL-1b), interferon gamma (IFNg), and tumor necrosis factor-alpha (TNF-a) to stimulate confluent monolayers of hfRPE. These proinflammatory cytokines are elevated in patients with uveitis and are detected in the vitreous and blood of patients with proliferative diabetic retinopathy (PDR) and AMD with choroidal neovascularization (CNV).

As a first step in understanding how the RPE in vivo can actively control the inflammatory environment in the SRS and choroid, Shi and colleagues used confluent monolayers of human fetal RPE primary cultures to (1) measure the constitutive and polarized secretion of angiogenic/angiostatic cytokines by the RPE; (2) determine how this pattern of polarized secretion changes in the inflammatory state; and (3) demonstrate that the inflammatory state alters RPE physiology. Constitutively, the human RPE secretes massive amounts of monocyte chemoattractant protein 1 (MCP-1) to the SRS and lesser amounts of IL-6 and IL-8 (Table 1), all of which contribute to the ongoing downregulation of the immune environment of the retina. RPE activation was achieved using a cocktail of IL-1b, TNF-a, and INFg with similar concentrations as that detected in the diseased eye. We showed that IL-1b receptors are mainly localized to the apical membrane and TNF-a and INFg (subunit 1)

receptors are mainly localized at the basolateral membrane. This cocktail significantly increased the secretion of various cytokines/chemokines to both baths, but significantly more to the apical bath. The increase in angiogenic cytokine secretion exceeds the increase in angiostatic cytokine secretion. However, two chemokines generally thought to be angiostatic, interferon-inducible T-cell a-chemoattractant (I-Tac) and monokine induced by g interferon (MIG), were secreted to the apical bath in significant quantities. The mechanisms by which these chemokines exert their effects and their role in eye physiology are not yet known. Similarly intriguing and not understood are the secretions into the apical bath of interferon-inducible protein 10 (IP-10), monocyte chemoattractant protein 3 (MCP-3), and the Rantes chemokine. In animal model experiments from Charlotte Reme’s group, blue-light-induced oxidative damage induces the invasion of blood-borne monocytes and activation of retinal microglia, thus stimulating the secretion of cytokines to induce an inflammatory response. Our experiments strongly suggest that the RPE is a significant source of cytokines and chemokines. Thus both retinal microglia and RPE can contribute to the inflammatory response in a diseased eye. Our further demonstration that basolateral addition of the cocktail acutely increases fluid absorption across the RPE (Figure 6), from the apical to basal baths (retina to choroidal side of tissue), and significantly decreases transepithelial resistance after a 24-h treatment is important because of the possibility that with age or accumulated oxidative stress these changes can alter chemokine/cytokine gradients across the RPE. These gradients regulate the attraction of monocytes to the RPE basement membrane and, thus, play a role in the accumulation of drusen with age. We believe that this concept is important for understanding early events that underlie chronic disease processes, such as AMD, a notion revisited below.

Modulation of RPE Proliferation and Migration by Cytokines and Growth Factors

Breakdown of the inner or outer blood–retinal barrier can lead to significant alterations in the chemical composition of the SRS, including cytokines and growth factors, which trigger the activation of normally quiescent RPE cells. In proliferative vitreoretinopathy (PVR), RPE cells proliferate and migrate to the vitreous cavity along with other types of cells (e.g., glial cells, fibroblasts, and macrophages) and form fibrocellular membranes on the retinal surface or in the vitreous. These newly formed membranes, if left untreated, eventually contract, resulting in retinal detachment and eventual vision loss.

Several isoforms of platelet-derived growth factor (PDGF) are present in retinal membrane from patients

Table 1 Inflammatory cytokine mixture alters polarized secretion of chemokines and cytokines by cultured human fetal retinal pigment epithelium (hfRPE)

Physiology Epithelial of Modulation Cytokine Epithelium: Pigment Retinal 768

Jv (μl·cm–2·h–1)

TEP (mV)

(a)

30

20

10

0

Jv (μl·cm−2 · hr−1)

TEP (mV)

(b)

ICM (Ap + Ba)

20

10

0

Probe out

4

3

2

1

0

10 min

with PVR and PDR and are elevated in the vitreous of PVR eyes. Recently we showed that PDGF-C, -D are highly expressed in human fetal and adult RPE and that the mRNA levels of these two isoforms are up to 100-fold higher than PDGF-A and -B. PDGF-C and -D have been implicated in PVR and lens epithelial cell proliferation, but relatively little is yet known about their function in RPE. In other systems, they play an important role in angiogenesis and wound healing.

PDGFR-a and PDGFR-b, the receptors for PDGF-C and -D, respectively, are mainly localized to the apical membrane of human fetal RPE as shown in Figure 7. PDGF-CC, -DD, and -BB significantly stimulated hfRPE cell proliferation, while PDGF-DD, -BB, and - AB significantly stimulated cell migration. Furthermore, the stimulatory effects of PDGF were abrogated by a proinflammatory cytokine cocktail composed of TNF-a, IL-1b, and IFNg. Comparison of the component effects

showed that IFNg was more effective in suppression than the entire cocktail or any subset of the cocktail, indicating that the downstream cytokine signaling pathways are interactive. Identifying the elements of this putative network and the specific nature of these interactions could provide targets for therapeutic intervention. For example, the proinflammatory cocktail may activate PDGF secretion by the RPE. In preliminary experiments, we showed that IFNg increased the polarized secretion of PDGF-AA to the apical bath, providing a possible autocrine signal mediating RPE proliferation/migration. We have shown that the cytokine cocktail induces cell apoptosis, alters cytoskeleton distribution, and significantly decreases transepithelial resistance, which can help mediate leukocyte traffic to the SRS. The cytokine cocktail-induced in hibition of RPE proliferation/migration indicates a potential therapeutic role against proliferative responses at the retina/RPE/choroid interfaces.

Since IFNg has a strong inhibitory effect on RPE proliferation and migration, it is natural to ask how this signaling pathway might provide the basis for inhibition. Native human adult RPE and hfRPE cells constitutively express two transcription factors, interferon regulatory factors 1 and 2 (IRF-1 and IRF-2), which are well-characterized members of the IFN regulatory family and key factors in the regulation of cell growth through their effects on cell cycle. In hfRPE, we showed that stimulation by IFNg significantly increased IRF-1 protein levels with no effect on IRF-2. If these two transcription factors are mutually antagonistic, as shown in other systems, this may explain the strong inhibitory effect of IFNg on RPE proliferation

and migration, which we hypothesize is caused by an increase in the ratio of IRF-1/IRF-2.

IFNg Regulation of RPE Fluid Transport

Immunoblots, immunofluorescence, intracellular recordings, pharmacology, and fluid transport data indicate a basolateral location of cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel, in native adult and fetal human RPE. As a first step in unraveling the network of cytokine interactions, we focused on INFg since it is a main determinant of several key effects produced by the inflammatory cocktail. IFNg has been implicated in the pathogenesis of a number of inflammatory diseases of infectious or presumed autoimmune origin and it has been detected in vitreous aspirates of patients with uveitis, PVR, and other inflammatory ocular diseases. In human RPE, IFNg activates several intracellular signaling pathways, including the canonical janus-activated kinase and signal transducers and activators of transcription protein (JAK/STAT) pathway and P38 mitogenactivated protein kinase (MAPK), leading to the elevation of cyclic adenosine monophosphate (cAMP) and the subsequent activation of protein kinase A-dependent chloride channels – CFTRs. This results in a significant increase in net fluid absorption across the epithelium (Figure 8). These data and the data summarized below provide a possible basis for the etiology of chronic inflammatory diseases, such as posterior uveitis and AMD. In the diseased eye, the IFNg-induced dehydration of the SRS

ns

80

60

40

20

Figure 9 IL-8 and MCP-1 regulated transmigration of leukocytes. (a) Basolateral to apical transepithelial migration of

could increase the concentration of the already-accumu- lating chemokines and thereby help draw monocytes and neutrophils to the RPE basement membrane or across the RPE to the SRS. This helps control the continuing accumulation of debris from incompletely digested photoreceptors, oxidative stress, and accumulation of drusen that normally occur in and around the RPE over the first five decades of life. Based on a variety of risk factors that activate the immune system (e.g., monocytes), these protective gradients may dissipate with age, aided perhaps by the loss of RPE barrier function and the steady buildup of an immunologically hostile environment.

Leukocyte Migration across the RPE: A Model of Disease Progression

The integrity of the RPE monolayer depends on the inter-epithelial junctions that include tight and adherens junctions and desmosomes. The main constituents of tight junctions are three families of transmembrane proteins: occludins, claudins, and junctional adhesion molecules

granulocytes. The number of transmigrating granulocytes is shown as % of control. Exogenous addition of modified junctional adhesion molecule Fc-JAM-C competes with JAM-C/ JAM-C interactions between adjacent cells, significantly reducing IL-8 induced transmigration of granulocytes. (b) In contrast, Fc-JAM-C had no significant effect on the basolateral to apical transepithelial migration of monocytes toward MCP-1. Addition of another RPE JAM isoform Fc-JAM-A, did not alter transmigration of granulocytes or monocytes. From Economopoulou, M., et al. (2009). Investigative Ophthalmology and Visual Science 50(3): 1454–1463. ã Association for Research in Vision and Ophthalmology.

( JAMs). The third member of the JAM family, JAM-C, has been identified in various cell types and implicated in inflammatory processes and shown to participate in the transmigration of leukocytes through endothelial and gut epithelial cells. Economopoulou and colleagues found that JAM-C is localized at the tight junctions of intact monolayers of adult and fetal human RPE, it is found at the initial cell–cell contacts of newly forming junctions, and that it helps initiate hfRPE junction formation and polarization. JAM-C also promotes the transepithelial migration of granulocytes through intact monolayers of cultured hfRPE driven by physiological gradients of interleukin 8 (IL-8). Thus, in the intact eye, JAM-C may be an important determinant of RPE initial junction formation, cell polarization, and immune-system-mediated pathophysiology at the retina–RPE interface (Figure 9). Recent animal model studies have implicated monocyte chemoattractant protein 1 (MCP-1) and fractalkine receptor in retinal microglia as critical regulators of drusen accumulation, local inflammation, and the development of AMD. As demonstrated by Shi and colleagues, the RPE secretes significant amounts of MCP-1 and IL-8 to the apical side in a polarized manner (Table 1). Both chemokines could form gradients across the RPE that coordinate monocyte and neutrophil movement to the RPE basement membrane. This could provide local surveillance/protection against the accumulation of immunologically active debris (drusen). The transformation over time of monocytes into macrophages would slowly degrade the RPE’s ability to maintain protective chemokine gradients for the removal of immunologically active debris and eventually lead to degeneration/disease.

See also: Injury and Repair: Light Damage; Phototransduction: The Visual Cycle; Secondary Photoreceptor Degenerations: Age-Related Macular Degeneration.

Further Reading

Adijanto, J., Banzon, T., Jalickee, S., and Miller, S. S. (2009). CO2-induced ion and fluid transport in human retinal pigment epithelium. Journal of General Physiology 133(6): 603–622.

Blaug, S., Quinn, R., Quong, J., Jalickee, S., and Miller, S. S. (2003). Retinal pigment epithelial function: A role for CFTR? Documenta Ophthalmologica 106: 43–50.

Bryant, D. M. and Mostov, K. E. (2008). From cells to organs: Building polarized tissue. Nature Reviews. Molecular Cell Biology 9(11): 887–901.

Daniele, L. L., Sauer, B., Gallagher, S. M., Pugh, E. N., Jr., and Philp, N. J. (2008). Altered visual function in monocarboxylate transporter 3 (Slc16a8) knockout mice. American Journal of Physiology Cell Physiology 295: C451–C457.

Donoso, L. A., Kim, D., Frost, A., Callahan, A., and Hageman, G. (2006). The role of inflammation in the pathogenesis of age-related macular degeneration. Surveys of Ophthalmology 51: 137–152.

Dunaief, J. L. (2006). Iron induced oxidative damage as a potential factor in age-related macular degeneration: The Cogan lecture.

Investigative Ophthalmology and Visual Science 47: 4660–4664. Economopoulou, M., Hammer, J., Wang, F., et al. (2009). Expression,

localization, and function of junctional adhesion molecule-C (JAM-C) in human retinal pigment epithelium. Investigative Ophthalmology and Visual Science. 50: 1454–1463.

Fisher, S. K., Lewis, G. P., Linberg, K. A., and Verardo, M. R. (2005). Cellular remodeling in mammalian retina: Results from studies of experimental retinal detachment. Progress in Retinal and Eye Research 24: 395–431.

Gehrs, K. M., Anderson, D. H., Johnson, L. V., and Hageman, G. S. (2006). Age-related macular degeneration – emerging pathogenetic and therapeutic concepts. Annals of Medicine

38: 450–471.

Illek, B., Fu, Z., Schwarzer, C., et al. (2008). Flagellin activates inflammatory response and cystic fibrosis transmembrane conductance regulator-dependent Cl secretion: Role for p38.

American Journal of Physiology Lung Cell Molecular Physiology.

295: L531–L542.

Jarrett, S. G., Lin, H., Godley, B. F., and Boulton, M. E. (2008). Mitochondrial DNA damage and its potential role in retinal degeneration. Progress Retinal Eye Research 6: 596–607.

Jia, L., Liu, Z., Sun, L., et al. (2007). Acrolein, a toxicant in cigarette smoke, causes oxidative damage and mitochondrial dysfunction in RPE cells: Protection by (R)-alpha-lipoic acid. Investigative Ophthalmology and Visual Science 48: 339–348.

Li, R., Maminishkis, A., Wang, F. E., and Miller, S. S. (2007). PDGF-C and-D induced proliferation/migration of human RPE is abolished by inflammatory cytokines. Investigative Ophthalmology and Visual Science 48: 5722–5732.

Maminishkis, A., Chen, S., Jalickee, S., et al. (2006). Confluent monolayers of cultured human fetal retinal pigment epithelium exhibit morphology and physiology of native tissue. Investigative Ophthalmology and Visual Science 47: 3612–3624.

Nussenblatt, R. B. and Ferris, F., 3rd (2007). Age-related macular degeneration and the immune response: Implications for therapy.

American Journal of Ophthalmology 144: 618–626.

Philp, N. J., Ochrietor, J. D., Rudoy, C., et al. (2003). Loss of MCT1, MCT3, and MCT4 expression in the retinal pigment epithelium and neural retina of the 5A11/basigin-null mouse. Investigative Ophthalmology and Visual Science 44: 1305–1311.

Shi, G., Maminishkis, A., Banzon, T., et al. (2008). Control of chemokine gradients by the retinal pigment epithelium. Investigative Ophthalmology and Visual Science 49: 4620–4630.

Strauss, O. (2005). The retinal pigment epithelium in visual function.

Physiological Reviews 85: 845–881.

Voloboueva, L. A., Killilea, D. W., Atamna, H., and Ames, B. N. (2007). N-tert-butyl hydroxylamine, a mitochondrial antioxidant, protects human retinal pigment epithelial cells from iron overload: Relevance to macular degeneration. FASEB Journal 21: 4077–4086.

Voloboueva, L. A., Liu, J., Suh, J. H., Ames, B. N., and Miller, S. S. (2005). (R)-alpha-lipoic acid protects retinal pigment epithelial cells from oxidative damage. Investigative Ophthalmology and Visual Science 46: 4302–4310.

Wangsa-Wirawan, N. D. and Linsenmeier, R. A. (2003). Retinal oxygen: Fundamental and clinical aspects. Archives of Ophthalmology 121: 547–557.

Winkler, B. S., Starnes, C. A., Twardy, B. S., Brault, D., and Taylor, R. C. (2008). Nuclear magnetic resonance and biochemical measurements of glucose utilization in the cone-dominant

ground squirrel retina. Investigative Ophthalmology and Visual Science 49: 4613–4619.

Glossary

Adherens junctions – A component of the apical junctional complex that provides mechanical strength to cell–cell adhesions and, together with the tight junction, regulates cell size, shape, and proliferation.

Apical junctional complex – An assembly of tight, adherens, and gap junctions that join neighboring cells of an epithelial monolayer together. The junctions form a belt that completely encircles each cell at the apical end of the lateral membranes. Apical membrane – The region of the plasma membrane that interdigitates with the photoreceptors of the neural retina. It is separated from the basolateral membranes by the apical junctional complex. Basolateral membrane – The region of the plasma membrane that rests on Bruch’s membrane and faces the choroid. It is separated from the apical membrane by the apical junctional complex. Claudins – A family of proteins that forms tight junctional strands and determines the selectivity and permeability of the tight junctions.

Paracellular space – The space between the neighboring cells of an epithelial monolayer. Subretinal space – The thin space that lies between the apical membrane of the retinal pigment epithelium and the photoreceptors. It becomes a wide space with retinal edema and detachment. Tight junctions – A component of the apical junctional complex that regulates transepithelial diffusion through the paracellular space, retards diffusion of lipids and membrane proteins between the apical and basolateral membranes, and, together with the adherens junction, regulates cell size, shape, and proliferation.

Transepithelial electrical resistance (TER) – An amalgam of the electrical resistances of the apical membrane, basolateral membrane, and paracellular space. It is commonly used as a reflection of the electrical resistance of tight junctions. When the sum of the membrane resistances greatly exceeds the paracellular (shunt) resistance, the TER approximates the electrical resistance of the tight junctions.

Introduction

Blood–tissue barriers were first revealed by the inability of injected proteins, or protein-bound dyes, to move from the blood into certain tissues. Only the brain, testes, and placenta shared this property. The cells that formed the barrier exhibited reduced transcytosis and were bound together by seemingly impermeable tight junctions. Transcytosis is one mechanism to move serum solutes across the cells of the barrier, whereas tight junctions partially occlude the paracellular spaces to reduce transepithelial diffusion between the cells. This initial conception has since been expanded to include all mechanisms of transcellular transport and the metabolic and catabolic pathways that alter solutes during transport.

The blood–retinal barrier has two divisions. The inner layers of the retina are supplied by a vascular bed, whose endothelia form the inner blood–retinal barrier. This endothelial barrier typifies most of the blood–brain barrier. This article focuses on the outer blood–retinal barrier, which is more similar to the choroid plexus and testes blood–tissue barriers. These barriers are a collaboration of a fenestrated capillary bed with an epithelium. In the outer retina, the collaboration is between the choriocapillaris and the retina pigment epithelium (RPE). As fenestrae make the capillaries porous, the RPE forms the barrier to serum components (Figure 1). Certainly, the Bruch’s membrane that separates the capillaries and RPE serves as a filter; however, this aspect of the outer blood–retinal barrier is discussed elsewhere in the encyclopedia. A good example of a metabolic pathway that participates in barrier function is the visual cycle. Vitamin A, transported by the serum, is endocytosed and transformed into cis-retinal as it is transported across the cell and exported to the photoreceptors. Ultimately, the paracellular and transcellular transport of ions and small organic solutes should be considered as a unit. Because explorations of how these pathways interact remain in their infancy, the two topics are discussed separately in this encyclopedia. This article focuses on the paracellular pathway, but includes the transcellular pathway whenever a connection between the two can be made. We discuss the assembly of RPE tight junctions, their retina-specific properties, and how the RPE and its tight junctions are regulated by the surrounding tissues.