Lecture 16: Learning Objectives on RPE PDF

‭Learning Objectives‬ ‭Locate on a diagram:‬ ‭‬ ‭RPE (including apical and basolateral‬ ‭sides)‬ ‭‬ ‭Photoreceptors‬ ‭‬ ‭Bruch’s membrane (BM)‬ ‭‬ ‭Choriocapillaris (CC) with fenestrated‬ ‭endothelial cells‬ ‭Describe the functions of the...

‭Learning Objectives‬ ‭Locate on a diagram:‬ ‭‬ ‭RPE (including apical and basolateral‬ ‭sides)‬ ‭‬ ‭Photoreceptors‬ ‭‬ ‭Bruch’s membrane (BM)‬ ‭‬ ‭Choriocapillaris (CC) with fenestrated‬ ‭endothelial cells‬ ‭Describe the functions of the RPE‬ ‭‬ ‭Blood-retina barrier‬ ‭○‬ ‭RPE forms part of the blood-retina barrier → forms tight-junction epithelium between‬ ‭choroid and outer segment of photoreceptors‬ ‭‬ ‭No paracellular movement of water or solutes (cannot get BETWEEN cells →‬ ‭must go THROUGH cells)‬ ‭‬ ‭Efficient isolation of inner retina from systemic influences at the choroidal side‬ → ‭ immune privilege‬ ‭‬ ‭Light absorption (RPE catches and absorbs photons & dissipates heat)‬ ‭○‬ ‭Prevention of reflected photons‬ ‭‬ ‭Photons (of light) that are not absorbed by rhodopsin can be absorbed by‬ ‭melanin → prevents unabsorbed light from reflecting off the back of the retina‬ ‭○‬ ‭Dissipation of heat energy‬ ‭‬ ‭Energy from absorbed photons → (converted) heat‬ ⇒ ‭‬ ‭Heat is transported away in the blood due to high perfusion of the choroid‬ ‭‬ ‭Heat diffuses into blood → blood “takes it away”‬ ‭‬ ‭Epithelial transport (including mechanisms of transport and names of transporters)‬ ‭○‬ ‭No paracellular movement of water or solutes‬ ‭○‬ ‭Nutrients‬ ‭‬ ‭Glucose (energy metabolism) → transporters = GLUT1 and GLUT3 (basic‬ ‭transport) *facilitated transport‬ ⇒ ‭‬ ‭Metabolized immediately always low [glucose] in photoreceptor cells‬ ‭‬ ‭Omega-3 fatty acids → simple diffusion‬ ‭‬ ‭Constructs membranes‬  ‭ ‬ ‭Moves down concentration gradient‬ ‭○‬ ‭Waste products from retinal cells to blood‬ ‭‬ ‭Lactic acid, monocarboxylate transporters (MCT), facilitated diffusion‬ ‭‬ ‭Diffuses out as long as there is a transporter‬ ‭‬ ‭CO2 → simple diffusion to blood/combines with water to form carbonic acid‬ ‭(H2CO3)‬ ‭○‬ ‭Maintenance of pH‬ ‭‬ ‭Carbonic acid buffer‬ ‭‬ ‭Cotransporters for HCO3- and H+‬ ‭○‬ ‭Water‬ ‭‬ ‭Water transported through‬‭aquaporins‬‭(from subretinal‬‭space to‬ ‭choriocapillaris) (RPE apical and basolateral membrane)‬ ‭‬ ‭Water follows localized hypertonic solution (from photoreceptors to the‬ ‭blood)‬ ‭‬ ‭Driving force = Na+/K+ -ATPase (K+ in, Na+ out) (RPE apical membrane)‬ ‭‬ ‭Creates electrochemical gradient‬ ‭‬ ‭NKCC cotransporter → uses Na+ gradient to Cl- into cell (RPE apical membrane)‬ ‭‬ ‭Cl- channels (RPE basolateral membrane)‬ ‭‬ ‭High concentration in blood‬ ‭ ‬ ‭Ion buffering in the interphotoreceptor matrix‬ ‭○‬ ‭RPE maintains ion homeostasis of subretinal space by epithelial transport of ions‬ ‭‬ ‭Buffers change by providing K+ when needed and removes when no longer‬ ‭needed → minimized large ion changes‬ ‭○‬ ‭Dark current‬ ‭‬ ‭Influx of Na+ and Ca2+ through gated ion channels (outer segment) are‬ ‭counterbalanced by outflow of K+ at inner segment‬ ‭‬ ‭5 mM [K+]‬ ‭○‬ ‭Light‬ ‭‬ ‭Gated ion channels (cGMP-dependent) are closed; outflow of K+ at inner‬ ‭segment is smaller‬ ‭‬ ‭Decrease in K+ concentration compensated by RPE‬ ‭‬ ‭Leads to hyperpolarization of apical RPE membrane → activation of‬ ⇒ ‭inward K+ channels efflux of K+ into subretinal space, leads to‬ ‭increase of subretinal K+ concentration back to normal values‬ ⇒ ‭‬ ‭Na+ cannot go back in but continued to be pumped out LOTS of + charge‬ ‭outside of the cell‬ ‭‬ ‭Hyperpolarization slows down Na+ pumped out, [K+] decreases (5 mM to 2 mM)‬ ‭‬ ‭RPE releases K+/opens K+ channels to increase [K+]‬ ‭○‬ ‭Compensatory pathway (by RPE) enables fast reactions of RPE to decrease or increase‬ ‭subretinal K+ concentration‬ ‭‬ ‭Fast coupling of K+ concentration with apical transmembrane potential and ion‬ ‭conductance → RPE able to respond to changes in subretinal [K+} as they occur‬  ‭ ‬ ‭Fast reaction, adds to sustained transepithelial transport by RPE‬ ‭‬ ‭Phagocytosis of photoreceptor outer segments‬ ‭○‬ ‭Process‬ ‭‬ ‭Microvilli surround and seal off phagosome → phagosome fuses with‬ ‭endosome, then lysosome → contents of phagolysosome digested → some‬ ‭molecules produced by digestion are recycled to photoreceptors‬ ‭‬ ‭Regulated by circadian rhythm and coordination between RPE and‬ ‭photoreceptors‬ ‭‬ ‭Outer segment tips are sloughed off due to extensive photooxidative damage‬ ‭(light ALWAYS entering the eye; light energy + O2 → ROS)‬ ‭○‬ ‭Timing‬ ‭‬ ‭Takes place in morning‬ ‭‬ ‭Triggered by light‬ ‭○‬ ‭Consequences (positive and negative)‬ ‭‬ ‭Destroyed tips of photoreceptor outersegments are shed and phagocytosed by‬ ‭RPE‬ ‭‬ ‭Fully replenished in ~2 weeks‬ ‭‬ ‭7-10% of mass of outer segment eliminated daily‬ ‭‬ ‭Maintains excitability of photoreceptors → outer segments are newly built using‬ ‭energy (ATP) and materials from inner segment‬ ‭ ‬ ‭Secretion‬ ‭○‬ ‭Prevent photoreceptors from undergoing apoptosis (due to photooxidative damage)‬ ‭○‬ ‭PEDF function → pigment epithelium-derived factor‬ ‭‬ ‭Neurotrophic factor that stabilizes neuronal retina by preventing apoptosis‬ ‭‬ ‭Secreted TOWARDS photoreceptors‬ ‭○‬ ‭VEGF function → vascular endothelial growth factor‬ ‭‬ ‭Stabilizes the fenestrated structure of the choroid capillaries‬ ‭‬ ‭Fenestrated = things move OUT‬