Retinal pigment epithelium

Retinal Pigment epithelial cells perform a multitude of functions to protect the retina and maintain normal vision. Any kind of RPE malfunction leads to a variety of ocular diseases.
This PPT explains the various functions performed by RPE to maintain normal vision.

1. EPITHELIAL TISSUE Large sheets of cells covering all the surfaces of the body. Hollow organs and body cavities which do not connect to the exterior of the body are lined by endothelium. Highly cellular with little or no extracellular material between the cells. Apical surface-exposed to outer environment ; Basal surface- close to underlying body structures. Exhibits polarity and has differences in structure and function between the exposed apical surface and basal surface. Cells are closely connected and are not separated by intercellular material. Three types of connections are seen based on their degree of interaction. They are : tight junction, gap junction, anchoring junctions. Tight junctions- separates the cells into apical and basal compartments Gap junctions- connexons which allow communication between cells such as ion and nutrient transport.(electrical and metabolic coupling). Anchoring junctions-forms lateral and basal connections via, desmosomes, hemidesmosomes and adherens . Desmosomes-protein patches on inner surface of cell membrane. These embed cadherins which adhere to cadherins of adjacent cell. Hemidesmosomes – link the epithelial cells to the basal lamina via integrins . (look similar to desmosomes). Note: protein connecting two adjacent cells- cadherins whereas those connecting cell with basal lamina are integrins .

2. POLARITY IN EPITHELIAL CELLS Differences in shape, structure and functions of cells. Characterized by structure orientation and protein localisation patterns. Apical and basolateral membranes have unique protein and phospholipid compositions. Polarization allows the transport of molecules in a directional manner. When epithelial cell polarity is lost, EMT occurs and epithelial cells acquire mesenchymal migratory phenotype.

3. RETINAL PIGMENT EPITHELIUM Light detecting organs made up of two types of cells ; photoreceptors and pigment cells Interaction between both cell types essential for vision. RPE and photoreceptors depend on each other. Mutation in photoreceptor genes led to RPE diseases and vice versa. RPE is located between POS and blood supply of the choroid. Hexagonally packed, connected by tight junctions and contain pigment granules. Apical membrane faces the subretinal space containing ECM which enables POS-RPE interaction. Basolateral membrane connected to Bruch’s membrane which enhances RPE-CC interaction.

4. FUNCTIONS OF RPE

5. ABSORPTION OF LIGHT Improves the quality of light by absorbing scattered light. Light is focussed on macula resulting in high concentration of photo-oxidative energy in the retina. Retinal side- Photooxidative stress ; blood side- oxygen overflow. Photo-oxidation damages POS tips and therefore need constant renewal. RPE phagocytose destroyed and shed POS. This process generates a load of free radicals. RPE has to be capable of maintaining structural integrity of retina by defence against free radicals, photo-oxidative stress etc. HOW?????? Through melanosomes . Depending upon the light-dark cycle , they move inside the cytoplasm to the apical processes and absorbs light. Other defense mechanisms are anti-oxidants which scavenge free radicals. Human retina accumulates lipofuscin , an end product of phagosome break down. This accumulates and debilitates RPE cells and their lines of defence becomes weaker. This leads to AMD in older population.

6. EPITHELIAL TRANSPORT RPE connected to each other by tight junctions and act as blood-retinal barrier; isolates the inner retina from systemic influences at the choroidal side. Transport from blood side to the retina Photoreceptors require: 1. Glucose for energy metabolism 2.Retinal for the visual cycle of the opsins 3.omega-3- fatty acids for membranes of photoreceptors GLUT-1 and GLUT-3 are expressed in both apical and basal membrane which transport glucose to photoreceptors. Retinol is taken up from the blood which is a major component in visual cycle. omega-3-fatty acid is taken and incorporated into glycerolipids for synthesis, storage and subsequent delivery.

7. Transport from subretinal space to blood side : Water, ions, metabolic end products are transported to the blood side. Retina has the highest cell density in the body which results in high metabolic turn-over rate which produces metabolic water which accumulates in the retina. How water is eliminated???? Active transport Transepithelial transport of Cl - ions from subretinal space to blood side This drives water through aquaporins from subretinal space into the choriocapillaries . Metabolism of cells in retinal space builds up lactic acid which has to be eliminated. Mediated by lac-/H + cotransporter . pH regulation occurs through Na+/H+ exchanger and Na+/HCo 3 exchanger in apical membrane and Cl -/HCo 3 - in the basolateral membrane.

8. VISUAL CYCLE Absorption of photon by chromophore of rhodopsin, 11-cis retinal. This changes the conformation to All- transretinal and rhodopsin is converted to meta-rhodopsin. Rhodopsin requires 11-cis retinal for its visual function. Therefore all-trans retinal has to be re-isomerised to give 11-cis retinal. But photoreceptors lack re- isomerase enzyme. The re-isomerization of all-trans retinal takes place in RPE. All-trans retinal is transported to RPE where it is converted to 11-cis retinal and is delivered back to photoreceptors. This process is called visual cycle. All- trans retinal is transferred from intra- discal space to cytosolic space of POS by ATP-driven flippase ABCR. In the next step, all-trans retinal is converted to all- trans retinol by membrane bound retinol dehydrogenase. All-trans retinol is delivered to sub-retinal space where it is loaded into IRBP (Interstitial Retinal Binding Protein)and delivered to a protein complex in RPE. This complex consists of LRAT, RPE65, RDH 5 which catalyse a three step reaction converting all-trans retinol to 11- cis retinal.

9. PHAGOCYTOSIS OF POS POS gets damaged due to photo-oxidative stress. Destroyed POS needs to be replaced to maintain vision. Phagocytosis of POS is done by RPE. This is a diurnal process and is triggered by light. It takes 11 days to renew the whole length of POS. Coordination between POS and RPE required. Three receptors on RPE surface involved in regulation of POS phagocytosis. CD36 - POS internalisation ; MerTK - activation of phagocytosis ; α v β 5 integrin -binding of POS. Process starts with binding of POS to α v β 5 integrin. Secreted glycoprotein milk fat globule activates integrin. Activated integrin produces intracellular signalling cascade which involves Focal Adhesion Kinase(FAK). FAK phosphorylates Mer -TK which is now activated. Gas-6 protein is strongly expressed by RPE which is autocrine stimulator of phagocytosis by RPE. Activation of Mer -TK produces intracellular signalling cascade which generates inositol-1,4,5- triphosphate followed by a subsequent increase of intracellular Ca 2+ . Phagocytosis is enabled by macrophage receptor CD36. Loss of Mer -TK leads to retinitis pigmentosa . Loss of β 5 - integrin leads to loss of circadian regulation o f phagocytosis which leads to lipofuscin accumulation in RPE.

10. SECRETION In order to communicate with neighbouring tissues, RPE secretes a variety of factors and signalling molecules. RPE secretes ATP, fas -ligand, FGF-1,4,5 , IGF-1, TGF- β , CNTF, PDGF, VEGF, LEDGF, PEDF, members of interleukin family. These factors are constantly released and help to maintain the structural integrity of the neighbouring tissues. PEDF- a neurotrophic factor secreted to the apical side of RPE to stabilise neuronal retina. VEGF and TIMP are secreted to the basolateral side stabilise the fenestrated structure of endothelium of the choroid. To maintain the secretory activity, RPE expresses a number of Ca2+-dependent regulatory mechanisms. In order to regulate secretion specific for the different factors, the RPE expresses a large number of different ion channels or transporters which increase intracellular free Ca2+. In the RPE the L-Type channel is regulated by either cytosolic tyrosine kinases, such as src -type kinase , or by FGF receptor FGFR-2. L-type channel regulates of number of different factors . Another Ca2+ channel calledTRPV2 channel is involved in the regulation of growth-factor secretion by the RPE. TRPV2 channels are activated by heat. Laser treatment causes an increase in the secretion of growth-factors from the RPE and it may be that TRPV2 activation is permitting this secretion . This technique is used in treatment of degenerations such as AMD.

11. IMMUNE PRIVILEGE Inner eye has an immune privileged space which is disconnected from the immune system of the blood stream. Provides mechanical and tight barrier which separates the inner space of the eye from the blood stream . RPE is able communicate with the immune system in order to silence immune reaction in the healthy eye or, on the other hand, to activate the immune system in the case of disease. For these purposes the RPE can secrete immune modulatory factors such as interleukin-8 (IL-8 ), complement factor H (CFH ) or monocyte chemotactic protein-1 (MCP1 ). Also the RPE is able to react to different factors of immune signalling cascades by expressing corresponding receptors. The receptors are MHC receptors , toll-like receptors or receptors for signalling molecules such as tumor -necrosis factor- α(TNF-α ). Many types of retinal degeneration seem to be associated with an insufficiency to silence the immune reactions, for example, in age-related macular degeneration. Immune privilege of the RPE is going to be helpful in the treatment of retinal degenerations by injected peptides, gene-therapy via viruses or in approaches using transplantation.