Electron microscopic immunocytochemical demonstration of blood-retinal barrier breakdown in human diabetics and its association with aldose reductase in retinal vascular endothelium and retinal pigment epithelium

Summary Light-microscopic immunohistochemical staining for albumin has been used to localize sites of blood-retinal barrier (BRB) breakdown in ocular disorders, but the mechanism for BRB compromise cannot be resolved at this level. Using eyes up to 2 days post-mortem from normal patients or from patients with diabetic retinopathy, or other disorders known to cause BRB failure, electron-microscopic immunocytochemistry reveals focal breakdown of the inner BRB, comprised of the retinal vascular endothelium (RVE), which appears to be mediated by diffuse permeation of the RVE cells and by vesicular transport. Permeation of the retinal pigmented epithelial (RPE) cells that comprise the outer BRB also occurs, but there is no evidence of opening of tight junctions between RVE or RPE in any of the disorders evaluated. Increased aldose reductase (AR) expression in the RVE and RPE cells of diabetics as well as in the perivascular retinal astrocytes, which interact with RVE cells to establish the inner BRB, suggests that AR activity and the subsequent intracellular accumulation of sorbitol in these cell types may impair the function of the BRB in diabetes.     

Ultrastructural localization of blood-retinal barrier breakdown in diabetic and galactosemic rats

Breakdown of the blood-retinal barrier (BRB) is an early event in diabetic and galactosemic rats, but the location and nature of the specific defect(s) are controversial. Using an electron microscopic immunocytochemical technique, the retinas of normal, diabetic, and galactosemic rats were immunostained for endogenous albumin. Normal rats showed little evidence of BRB breakdown at either the inner barrier (retinal vasculature) or the outer barrier (retinal pigment epithelium) (RPE). In diabetic and galactosemic rats, as was true in human diabetics, BRB breakdown occurred predominantly at the inner BRB, but in some cases at the outer barrier as well. Treatment with the aldose reductase inhibitor sorbinil largely prevented BRB failure in galactosemic rats. In the inner retina of diabetic and galactosemic rats, albumin was frequently demonstrated on the abluminal side of the retinal capillary endothelium (RCE) in intercellular spaces, basal laminae, pericytes, ganglion cells, astrocytes, and the perinuclear cytoplasm of cells in the inner nuclear layer. Albumin did not appear to cross RCE cell junctions; however, it was occasionally seen in RCE cytoplasm of galactosemic rats. In the outer retina, albumin was frequently detected in the subretinal space, in the intercellular space between photoreceptors, and in the perinuclear cytoplasm of photoreceptor cells, but was only infrequently found in the RPE cells constituting the barrier. Albumin derived from the choroidal vasculature did not appear to cross the tight junctions of the RPE. These findings suggest that specific sites of BRB compromise are infrequent but that once albumin has crossed the RCE or RPE it freely permeates the retinal tissue by filling intercellular spaces and permeating the membranes of cells not implicated in BRB formation. The diffuse cytoplasmic staining of some RCE and RPE cells suggests that the predominant means of BRB breakdown in diabetes and galactosemia involves increased focal permeability of the surface membranes of the RCE and RPE cells rather than defective tight junctions or vesicular transport.     

Implication of the hypoxia response element of the Vegf promoter in mouse models of retinal and choroidal neovascularization, but not retinal vascular development

Retinal neovascularization (NV) and macular edema, resulting from blood-retinal barrier (BRB) breakdown, are major causes of visual loss in ischemic retinopathies. Choroidal NV (CNV) occurs in diseases of the retinal pigmented epithelium/Bruch's membrane complex and is another extremely prevalent cause of visual loss. We used mice in which the hypoxia response element (HRE) is deleted from the vascular endothelial growth factor (vegf) promoter (Vegf(delta/delta) mice) to explore the role of induction of VEGF through the HRE in these disease processes. Compared to wild type (Vegf+/+) mice with oxygen-induced ischemic retinopathy (OIR) in which vegf mRNA levels were increased and prominent retinal NV and BRB breakdown occurred, Vegf(delta/delta) littermates with OIR failed to increase vegf mRNA levels in the retina and had significantly less retinal NV and BRB breakdown, but showed prominent dilation of some superficial retinal vessels. Vegf(+/delta) littermates with ischemic retinopathy developed comparable retinal NV to Vegf+/+ mice, exhibited intermediate levels of BRB breakdown, and did not show vasodilation. In a mouse model of CNV, due to laser-induced rupture of Bruch's membrane, the area of CNV at Bruch's membrane rupture sites was more than tenfold greater in Vegf+/+ mice than in Vegf(delta/delta) littermates. In contrast to these dramatic differences in pathologic ocular NV, Vegf(delta/delta) mice showed subtle differences in retinal vascular development compared to Vegf+/+ mice; it was slightly delayed, but otherwise normal. These data suggest that induction of VEGF through the HRE in its promoter is critical for retinal and CNV, but not for retinal vascular development.     

Hyperactivation of retina by light in mice leads to photoreceptor cell death mediated by VEGF and retinal pigment epithelium permeability

Light toxicity is suspected to enhance certain retinal degenerative processes such as age-related macular degeneration. Death of photoreceptors can be induced by their exposure to the visible light, and although cellular processes within photoreceptors have been characterized extensively, the role of the retinal pigment epithelium (RPE) in this model is less well understood. We demonstrate that exposition to intense light causes the immediate breakdown of the outer blood–retinal barrier (BRB). In a molecular level, we observed the slackening of adherens junctions tying up the RPE and massive leakage of albumin into the neural retina. Retinal pigment epithelial cells normally secrete vascular endothelial growth factor (VEGF) at their basolateral side; light damage in contrast leads to VEGF increase on the apical side – that is, in the neuroretina. Blocking VEGF, by means of lentiviral gene transfer to express an anti-VEGF antibody in RPE cells, inhibits outer BRB breakdown and retinal degeneration, as illustrated by functional, behavioral and morphometric analysis. Our data show that exposure to high levels of visible light induces hyperpermeability of the RPE, likely involving VEGF signaling. The resulting retinal edema contributes to irreversible damage to photoreceptors. These data suggest that anti-VEGF compounds are of therapeutic interest when the outer BRB is altered by retinal stresses.     

Oxidized low density lipoprotein-induced senescence of retinal pigment epithelial cells is followed by outer blood-retinal barrier dysfunction

Age-related macular degeneration is the most common cause of vision loss in the elderly, which starts from aging processes of retinal pigment epithelial cells. Among variable risk factors in occurrence and progression of age-related macular degeneration, oxidized low density lipoprotein could be causally involved in pathobiological changes of RPE cells. Herein we showed that oxidized low density lipoprotein-induced senescence of retinal pigment epithelial cells is followed by outer blood-retinal barrier dysfunction. Under sub-lethal concentration, oxidized low density lipoprotein could promote advanced senescence of retinal pigment epithelial cells. Interestingly expression of CRALBP and RPE 65, indicators of retinal pigment epithelial cell differentiation, was decreased by oxidized low density lipoprotein. In addition, oxidized low density lipoprotein induced reactive oxygen species production and up-regulated inflammatory factors such as tumor necrosis factor-α and vascular endothelial growth factor, when β-catenin, a critical mediator of the canonical Wnt pathway, was also elevated. Oxidized low density lipoprotein increased paracellular permeability of retinal pigment epithelial cells, when zonula occludens-1 at intercellular junctions markedly decreased as well. Furthermore, in retinal pigment epithelial cells and choriocapillaris of human apolipoprotein E2 transgenic mouse eye, increased vascular endothelial growth factor and decreased zonula occludens-1 expression was observed. Therefore, our results suggest that oxidized low density lipoprotein could promote senescence of retinal pigment epithelial cells which leads to induce outer blood-retinal barrier dysfunction as an early pathogenesis of age-related macular degeneration.     

Inhibition of protein kinase C decreases prostaglandin-induced breakdown of the blood-retinal barrier

Breakdown of the blood-retinal barrier (BRB) occurs in several retinal diseases and is a major cause of visual loss. Vascular endothelial growth factor (VEGF) has been implicated as a cause of BRB breakdown in diabetic retinopathy and other ischemic retinopathies, and there is evidence to suggest that other vasopermeability factors may act indirectly through VEGF. In this study, we investigated the effect of several receptor kinase inhibitors on BRB breakdown resulting from VEGF, tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), insulin-like growth factor-1 (IGF-1), prostaglandin E1 (PGE(1)), or PGE(2). Inhibitors of VEGF receptor kinase, including PKC412, PTK787, and SU1498, decreased VEGF-induced breakdown of the BRB. None of the inhibitors blocked leakage caused by TNF-alpha, IL-1beta, or IGF-1 and only PKC412, an inhibitor of protein kinase C (PKC) as well as VEGF and platelet-derived growth factor (PDGF) receptor kinases, decreased leakage caused by prostaglandins. Since the other inhibitors of VEGF and/or PDGF receptor kinases that do not also inhibit PKC had no effect on prostaglandin-induced breakdown of the BRB, these data implicate PKC in retinal vascular leakage caused by prostaglandins. PKC412 may be useful for treatment of post-operative and inflammatory macular edema, in which prostaglandins play a role, as well as macular edema associated with ischemic retinopathies.     

RPE barrier breakdown in diabetic retinopathy: seeing is believing

Diabetic retinopathy (DR) is a major complication of diabetes and a leading cause of blindness in working-age Americans. DR is traditionally regarded as a disorder of blood–retina barriers, and the leakage of blood content is a major pathological characteristic of the disease. While the breakdown of the endothelial barrier in DR has been investigated extensively, the vascular leakage through the retinal pigment epithelium (RPE) barrier in the disease has not been widely acknowledged. As the blood content leaked through the RPE barrier causes excessive water influx to the retina, the breakdown of the RPE barrier is likely to play a causative role in the development of some forms of diabetic macular edema, a major cause of vision loss in DR. In this article, we will discuss the clinical evidences of the diabetes-induced RPE barrier breakdown, the alteration of the RPE in diabetes, the molecular and cellular mechanism of RPE barrier breakdown, and the research tools for the analysis of RPE barrier leakage. Finally, we will discuss the methodology and potential applications of our recently developed fluorescent microscopic imaging for the diabetes- or ischemia-induced RPE barrier breakdown in rodents.     

Altered retinal metabolism in diabetes. II. Measurement of sodium-potassium ATPase and total sodium and potassium in individual retinal layers

Pathological changes in retinas of diabetics include specific morphological, biochemical, and functional abnormalities. As biochemical manifestations of the disease, increased sorbitol and decreased myo-inositol were found in retinas of experimentally diabetic animals. Similar alterations in polyol metabolism have been associated in nerves of diabetics with a reduction of Na+-K+-ATPase activity. To determine whether this association extends to the retinas of diabetic animals, we applied quantitative histochemical techniques to measure ATPase activities and the amounts of sodium and potassium in samples from nine individual layers of cryostat sections of rabbit retina. ATPase activities were determined fluorimetrically, and the ions were measured by atomic absorption with a carbon rod atomizer. The activity of Na+-K+-ATPase was reduced in the retinal pigmented epithelium (retinal pigment epithelium) and in selected layers of the neural retina, and total sodium in the retinal pigment epithelium layer was elevated in diabetes. The retinal pigment epithelium forms the outer component of the blood-retinal barrier and partly determines the composition of the retinal interstitial fluid. Changes in retinal pigment epithelium biochemistry and function might alter the intraretinal environment, predisposing neural retina or retinal blood vessels to disease. The morphologically and functionally well defined retinal pigment epithelium may provide a useful model for studying the pathogenesis of diabetic complications.     

VEGF-TRAP(R1R2) suppresses choroidal neovascularization and VEGF-induced breakdown of the blood-retinal barrier

Vascular endothelial growth factor (VEGF) plays a central role in the development of retinal neovascularization and diabetic macular edema. There is also evidence suggesting that VEGF is an important stimulator for choroidal neovascularization. In this study, we investigated the effect of a specific inhibitor of VEGF, VEGF-TRAP(R1R2), in models for these disease processes. VEGF-TRAP(R1R2) is a fusion protein, which combines ligand binding elements taken from the extracellular domains of VEGF receptors 1 and 2 fused to the Fc portion of IgG1. Subcutaneous injections or a single intravitreous injection of VEGF-TRAP(R1R2) strongly suppressed choroidal neovascularization in mice with laser-induced rupture of Bruch's membrane. Subcutaneous injection of VEGF-TRAP(R1R2) also significantly inhibited subretinal neovascularization in transgenic mice that express VEGF in photoreceptors. In two models of VEGF-induced breakdown of the blood-retinal barrier (BRB), one in which recombinant VEGF is injected into the vitreous cavity and one in which VEGF expression is induced in the retina in transgenic mice, VEGF-TRAP(R1R2) significantly reduced breakdown of the BRB. These data confirm that VEGF is a critical stimulus for the development of choroidal neovascularization and indicate that VEGF-TRAP(R1R2) may provide a new agent for consideration for treatment of patients with choroidal neovascularization and diabetic macular edema.     

PKCζ Mediates Breakdown of Outer Blood-Retinal Barriers in Diabetic Retinopathy

Aims/hypothesis

Diabetic macular edema represents the main cause of visual loss in diabetic retinopathy. Besides inner blood retinal barrier breakdown, the role of the outer blood retinal barrier breakdown has been poorly analyzed. We characterized the structural and molecular alterations of the outer blood retinal barrier during the time course of diabetes, focusing on PKCζ, a critical protein for tight junction assembly, known to be overactivated by hyperglycemia.

Methods

Studies were conducted on a type2 diabetes Goto-Kakizaki rat model. PKCζ level and subcellular localization were assessed by immunoblotting and immunohistochemistry. Cell death was detected by TUNEL assays. PKCζ level on specific layers was assessed by laser microdissection followed by Western blotting. The functional role of PKCζ was then evaluated in vivo, using intraocular administration of its specific inhibitor.

Results

PKCζ was localized in tight junction protein complexes of the retinal pigment epithelium and in photoreceptors inner segments. Strikingly, in outer segment PKCζ staining was restricted to cone photoreceptors. Short-term hyperglycemia induced activation and delocalization of PKCζ from both retinal pigment epithelium junctions and cone outer segment. Outer blood retinal barrier disruption and photoreceptor cone degeneration characterized long-term hyperglycemia. In vivo, reduction of PKCζ overactivation using a specific inhibitor, restored its tight-junction localization and not only improved the outer blood retinal barrier, but also reduced photoreceptor cell-death.

Conclusions

In the retina, hyperglycemia induced overactivation of PKCζ is associated with outer blood retinal barrier breakdown and photoreceptor degeneration. In vivo, short-term inhibition of PKCζ restores the outer barrier structure and reduces photoreceptor cell death, identifying PKCζ as a potential target for early and underestimated diabetes-induced retinal pathology.     

Melanin protects choroidal blood vessels against light toxicity

Low ocular pigmentation and high long-term exposure to bright light are believed to increase the risk of developing age-related macular degeneration (ARMD). To investigate the role of pigmentation during bright light exposure, cell damage in retinae and choroids of pigmented and non-pigmented rats were compared. Pigmented Long Evans (LE) rats and non-pigmented (albino) Wistar rats were exposed to high intensity visible light from a cold light source with 140,000 lux for 30 min. Control animals of both strains were not irradiated. The animals had their pupils dilated to prevent light absorbance by iris pigmentation. 22 h after irradiation, the rats were sacrificed and their eyes enucleated. Posterior segments, containing retina and choroid, were prepared for light and electron microscopy. Twenty different sections of specified and equal areas were examined in every eye. In albino rats severe retinal damage was observed after light exposure, rod outer segments (ROS) were shortened and the thickness of the outer nuclear layer (ONL) was significantly diminished. Choriocapillaris blood vessels were obstructed. In wide areas the retinal pigment epithelium (RPE) was absent in albino rats after irradiation. In contrast, LE rats presented much less cell damage in the RPE and retina after bright light exposure, although intra-individual differences were observed. The thickness of the ONL was almost unchanged compared to controls. ROS were shortened in LE rats, but the effect was considerably less than that seen in the albinos. Only minimal changes were found in choroidal blood vessels of pigmented rats. The RPE showed certain toxic damage, but cells were not destroyed as in the non-pigmented animals. The number of melanin granules in the RPE of LE rats was reduced after irradiation. Ocular melanin protects the retina and choroid of pigmented eyes against light-induced cell toxicity. Physical protection of iris melanin, as possible in eyes with non-dilated pupils, does not seem to play a major role in our setup. Biochemical mechanisms, like reducing oxidative intracellular stress, are more likely to be responsible for melanin-related light protection in eyes with dilated lens aperture.     

Vitronectin is a constituent of ocular drusen and the vitronectin gene is expressed in human retinal pigmented epithelial cells.

Age-related macular degeneration (AMD) leads to dysfunction and degeneration of retinal photoreceptor cells. This disease is characterized, in part, by the development of extracellular deposits called drusen. The presence of drusen is correlated with the development of AMD, although little is known about drusen composition or biogenesis. Drusen form within Bruch's membrane, a stratified extracellular matrix situated between the retinal pigmented epithelium and choriocapillaris. Because of this association, we sought to determine whether drusen contain known extracellular matrix constituents. Antibodies directed against a battery of extracellular matrix molecules were screened on drusen-containing sections from human donor eyes, including donors with clinically documented AMD. Antibodies directed against vitronectin, a plasma protein and extracellular matrix component, exhibit intense and consistent reactivity with drusen; antibodies to the conformationally distinct, heparin binding form of human vitronectin are similarly immunoreactive. No differences in vitronectin immunoreactivity between hard and soft drusen, or between macular and extramacular regions, have been observed. RT-PCR analyses revealed that vitronectin mRNA is expressed in the retinal pigmented epithelium (RPE)-choroidal complex and cultured RPE cells. These data document that vitronectin is a major constituent of human ocular drusen and that vitronectin mRNA is synthesized locally. Based on these data, we propose that vitronectin may participate in the pathogenesis of AMD.     

Glycoproteins of drusen and drusen-like lesions

Drusen are a marker of age-related macular degeneration (AMD). Lesions similar to drusen, both in histology and their clinical appearance, are also seen in choroidal tumours, chronic inflammatory and degenerative conditions of the eye, and in mesangiocapillary glomerulonephritis type II (MCGN-II). This study aims to compare the saccharide composition of these drusen-like lesions in the various ocular pathological groups and in MCGN-II. Formalin fixed and paraffin wax embedded tissue from 21 eyes was studied. The histological diagnoses included AMD, retinal detachment, phthisis bulbi following failed retinal detachment surgery, malignant melanoma, long-standing uveitis, glaucoma and MCGN II. Glycosylation was examined using a panel of twenty biotinylated lectins and an avidin-peroxidase DAB-cobalt revealing system, with and without neuraminidase pre-treatment. High mannose, bi/tri-nonbisected and bisected complex N-glycan, N-acetyl glucosaminyl, galactosyl and sialyl residues were found to be expressed by drusen, while treatment with neuraminidase exposed subterminal N-acetyl galactosamine and galactosyl residues. Similar binding patterns were found in the various pathological groups studied. As there was no significant difference in the lectin-binding pattern in drusen in different pathologies, a common pathogenesis or at least a final common pathway for the elaboration of carbohydrate components of drusen is suggested.     

Pluripotent human stem cells for the treatment of retinal disease

Despite advancements made in our understanding of ocular biology, therapeutic options for many debilitating retinal diseases remain limited. Stem cell-based therapies are a potential avenue for treatment of retinal disease, and this mini-review will focus on current research in this area. Cellular therapies to replace retinal pigmented epithelium (RPE) and/or photoreceptors to treat age-related macular degeneration (AMD), Stargardt's macular dystrophy, and retinitis pigmentosa are currently being developed. Over the past decade, significant advancements have been made using different types of human stem cells with varying capacities to differentiate into these target retinal cell types. We review and evaluate pluripotent stem cells, both human embryonic stem cells and human induced pluripotent stem cells, as well as protocols for differentiation of ocular cells, and culture and transplant techniques that might be used to deliver cells to patients.     

Retinal pigment epithelial cell proliferation

The human retinal pigment epithelium forms early in development and subsequently remains dormant, undergoing minimal proliferation throughout normal life. Retinal pigment epithelium proliferation, however, can be activated in disease states or by removing retinal pigment epithelial cells into culture. We review the conditions that control retinal pigment epithelial proliferation in culture, in animal models and in human disease and interpret retinal pigment epithelium proliferation in context of the recently discovered retinal pigment epithelium stem cell that is responsible for most in vitro retinal pigment epithelial proliferation. Retinal pigment epithelial proliferation-mediated wound repair that occurs in selected macular diseases is contrasted with retinal pigment epithelial proliferation-mediated fibroblastic scar formation that underlies proliferative vitreoretinopathy. We discuss the role of retinal pigment epithelial proliferation in age-related macular degeneration which is reparative in some cases and destructive in others. Macular retinal pigment epithelium wound repair and regression of choroidal neovascularization are more pronounced in younger than older patients. We discuss the possibility that the limited retinal pigment epithelial proliferation and latent wound repair in older age-related macular degeneration patients can be stimulated to promote disease regression in age-related macular degeneration.     

Localization of 2-[125I]Iodomelatonin Binding Sites in Mammalian Retina

Specific melatonin binding sites were localized in the mammalian retina using the selective radioligand 2-[125I]iodomelatonin. Frozen sections obtained from both pigmented and albino rabbit eyes and albino mouse eyes were incubated with 2-[125I]iodomelatonin in the absence and presence of competing agents. In eyecups from albino rabbits, the highest density of specific 2-[125I]iodomelatonin binding sites was localized over the inner plexiform layer. Approximately 40-60% of the binding was specific, as determined with both the agonist 6-chloromelatonin and the antagonist luzindole. A high density of binding sites was observed over the choroid and retinal pigmented epithelium, but no statistical difference between total and nonspecific binding was detected. Results were similar with eyecups from pigmented rabbits. Albino mice showed a significant extent of 2-[125I]iodomelatonin binding in both the inner plexiform and the outer and inner segment layers. The specific binding of 2-[125I]iodomelatonin in retinas from albino rabbits maintained in the light for 24 h before decapitation was increased in the inner retina compared with the control. The distribution of 2-[125I]iodomelatonin binding sites in the various layers of the mammalian retina is consistent with the described functions for this hormone in retinal physiology.     

mosaic eyes: a zebrafish gene required in pigmented epithelium for apical localization of retinal cell division and lamination

For proper function of the retina, the correct proportions of retinal cell types must be generated, they must be organized into cell-specific laminae, and appropriate synaptic connections must be made. To understand the genetic regulation of retinal development, we have analyzed mutations in the mosaic eyes gene that disrupt retinal lamination, the localization of retinal cell divisions to the retinal pigmented epithelial surface and retinal pigmented epithelial development. Although retinal organization is severely disrupted in mosaic eyes mutants, surprisingly, retinal cell differentiation occurs. The positions of dividing cells and neurons in the brain appear normal in mosaic eyes mutants, suggesting that wild-type mosaic eyes function is specifically required for normal retinal development. We demonstrate that mosaic eyes function is required within the retinal pigmented epithelium, rather than in dividing retinal cells. This analysis reveals an interaction between the retinal pigmented epithelium and the retina that is required for retinal patterning. We suggest that wild-type mosaic eyes function is required for the retinal pigmented epithelium to signal properly to the retina.     

Novel atypical PKC inhibitors prevent vascular endothelial growth factor-induced blood-retinal barrier dysfunction

Pro-inflammatory cytokines and growth factors such as VEGF (vascular endothelial growth factor) contribute to the loss of the BRB (blood-retinal barrier) and subsequent macular oedema in various retinal pathologies. VEGF signalling requires PKCβ [conventional PKC (protein kinase C)] activity; however, PKCβ inhibition only partially prevents VEGF-induced endothelial permeability and does not affect pro-inflammatory cytokine-induced permeability, suggesting the involvement of alternative signalling pathways. In the present study, we provide evidence for the involvement of aPKC (atypical PKC) signalling in VEGF-induced endothelial permeability and identify a novel class of inhibitors of aPKC that prevent BRB breakdown in vivo. Genetic and pharmacological manipulations of aPKC isoforms were used to assess their contribution to endothelial permeability in culture. A chemical library was screened using an in vitro kinase assay to identify novel small-molecule inhibitors, and further medicinal chemistry was performed to delineate a novel pharmacophore. We demonstrate that aPKC isoforms are both sufficient and required for VEGF-induced endothelial permeability. Furthermore, these specific, potent, non-competitive, small-molecule inhibitors prevented VEGF-induced tight junction internalization and retinal endothelial permeability in response to VEGF in both primary culture and in rodent retina. The results of the present study suggest that aPKC inhibition with 2-amino-4-phenyl-thiophene derivatives may be developed to preserve the BRB in retinal diseases such as diabetic retinopathy or uveitis, and the BBB (blood-brain barrier) in the presence of brain tumours.     

Primary cilia in retinal pigment epithelium development and diseases

Retinal pigment epithelium (RPE) is a highly polarized epithelial monolayer lying between the photoreceptor layer and the Bruch membrane. It is essential for vision through participating in many critical activities, including phagocytosis of photoreceptor outer segments, recycling the visual cycle-related compounds, forming a barrier to control the transport of nutrients, ions, and water, and the removal of waste. Primary cilia are conservatively present in almost all the vertebrate cells and acts as a sensory organelle to control tissue development and homeostasis maintenance. Numerous studies reveal that abnormalities in RPE lead to various retinal diseases, such as age-related macular degeneration and diabetic macular oedema, but the mechanism of primary cilia in these physiological and pathological activities remains to be elucidated. Herein, we summarize the functions of primary cilia in the RPE development and the mutations of ciliary genes identified in RPE-related diseases. By highlighting the significance of primary cilia in regulating the physiological and pathological processes of RPE, we aim to provide novel insights for the treatment of RPE-related retinal diseases.