Biochemistry and Metabolism- RPE Flashcards
What is the function of the RPE
- vitamin A regeneration, which is integral to sustaining vision
- phagocytosis of shed photoreceptor outer-segment discs
- biological filter for the neurosensory retina through transport of necessary nutrients
and ions to photoreceptor cells and removal of waste products from photoreceptors - absorption of scattered and out-of-focus light via pigmentation
- adhesion of the retina
- secretion of humoral and growth factors
What does mutation of gene RPE65 lead to
Mutation of the gene RPE65, which encodes the enzyme (RPE65 isomerohydro- lase) that converts all-trans-retinyl ester to 11-cis-retinol, causes Leber congenital amaurosis (LCA). RPE65 is the target of a treatment approved by the US Food and Drug Administration (FDA) that uses an adeno associated virus to deliver the gene to the RPE of LCA patients
What is autophagy
cell degrades its own damaged components and recycles the products. In the RPE, this is essential for management of phagocytosed outer segments as well as for turnover of its components
What embrylogical layer is the RPE derived from
monolayer of neuroectoderm-derived epithelial cells located between the highly vascular choriocapillaris and the photoreceptor outer segments
How many RPE cells are there per eye
4–6 million RPE cells per eye
What forms the outer blood retinal barrier
On the apical surface of RPE cells are long microvilli that interdigitate with the outer segments of photoreceptor cells. These cells are joined near their apical side by tight junctions that establish polarity, block the passage of water and ions, and constitute the outer blood–retina barrier
What is Bruch Membrane composed of
an extracellular matrix between the RPE and the choriocapillaris), has many infoldings that increase the surface area available for the exchange of solutes
What key cells do RPE contain
Melanin and Phagosomes reflecting their role in light absorption and phagocytosis
Pathways used to generate energy in RPE
glycolysis, the Krebs cycle, and the pentose phosphate pathway
What is the majority of wet weight of RPE contributed by and how much?
Water- >80%.
Proteins, lipids and nucleic acids contribute the rest
How many proteins are identified in RPE
Nearly 850
What kind of proteins are found in RPE
Hydrolytic enzymes such as glutathione, peroxidase, catalase and superoxide dismutase important for detoxification
Cytoskeletal proteins such as actin, myosin, alpha actinin, fodrin and vinculin are also present in RPE and other cells
Where is the Na+/K+/ATPase found found on RPE
On the apical membrane in contrast to most polarized cells
Which RPE protein is exclusively expressed on the RPE and its function
RPE-specific protein 65 kDa (RPE65), is an obligate component of the isomerization and hydrolysis of vitamin A, which is required for regeneration of visual pigment (described later in Vitamin A Regeneration)
What % weight do lipids contribute of the weight of the RPE
3%- half are phospholipids
Which organ contains the highest concentration of vitamin A in the body
Liver, then RPE
What is RPE role in vitamin A regeneration
RPE supplies the photoreceptor outer segments with vitamin A, which is tethered to rhodopsin in rods and to the 3 different cone opsins (red, green, and blue). Regenerates all-trans retinal to cis-retinal for activation by light
How is free all-trans-retinal cleared from rod discs
ABCA4. After transport to cytosol from rod discs, all-trans-retinal is reduced to all-trans-retinol by retinol dehydrogenease
What is the phototransduction cascade
Retinal attaches to a lysine residue on rhodopsin. When the com- plex absorbs light, 11-cis-retinal transforms into all-trans-retinal via a process known as photo- isomerization. This induces a conformational change in the attached rhodopsin molecule, activating the second-messenger system and initiating the phototransduction cascade within the photoreceptor. The all-trans-retinal is shed from rhodopsin and transported by ABCA4 from the rod disc to the cytosol, where it is converted to all-trans-retinol. Then, all-trans-retinol is delivered to the RPE via interphotoreceptor retinoid-binding protein (IRBP), which acts as a shuttle and also shields the cell membranes from the membranolytic retinoid molecules. Once in the RPE, this molecule is esterified by lecithin retinol acyltransferase (LRAT). The resultant retinyl ester is converted to 11-cis-retinol by the isomerohydrolase RPE65. 11-cis-Retinol is then oxidized to 11-cis-retinal by retinol dehydrogenase (RDH) and shuttled back to the photoreceptor outer-segment discs by IRBP to participate in another visual cycle
What conditions are associated with mutation of ABCR gene
ABCR gene encodes ABCA4 transporter protein leads to Stargadt disease.
What conditions are associated with mutation of RPE65 gene
Leber congenital amaurosis
How many outer segment discs are shed per day
100 from each photoreceptor cell. Many photoreceptors interdigitate with a single RPE cell, each RPE cell digests more than 4000 discs daily
When are discs shed
Shedding most vigorous at dawn in rods
Shedding most vigorous at dusk in cones
What happens to the shed outer segment discs
encapsulated in phagosomes which in turn fuse with lysosomes and are digested. During degradation of the discs, building blocks are recycled into photoreceptors for use in the synthesis and assembly of new discs
What happens in Stargadt disease
the free all-trans-retinal is not read- ily cleared from the outer-segment discs by ABCA4. The excess all-trans-retinal combines with phosphatidylethanolane (PE) in the disc lipid bilayer, forming N-retinylidene-PE (N-retPE). Elevated N-ret-PE and all-trans-retinal undergo a secondary nonenzymatic condensation in the outer segments to yield A2PE-H2. he distal outer segments contain- ing A2PE-H2 and elevated all-trans-retinal and N-ret-PE are phagocytosed by the RPE as part of the normal disc-renewal process, but the RPE is unable to fully degrade the nonphysiologic load. This leads to the accumulation of toxic retinal fluorophores like A2E (derived from A2PE-H2), which damage the RPE
Why is a signal detected in autoflouresence
The lipofuscin characteristic of the RPE is derived from the photosensitive membranes and is responsible for generating the signal detected in fundus autofluorescence imaging
What proteins determine cell polarity
Cytoskeletal proteins
Where in the RPE are pigment granules primarily found
Pigment granules are abundant in the cytoplasm of adult RPE cells, predominantly in the apical and midportions of the cell
What triggers the onset of melanogenesis in RPE cells during developement
activation of the tyrosinase promoter and marks the commitment of the neuroectoderm to become RPE. Although most melanogenesis occurs before birth, melanin production in the RPE occurs throughout life, albeit at a slow rate.
Why is the fundus of an older person less pigmented than that of a younger person
As humans age, the melanin granules fuse with lysosomes; thus, the fundus of an older person is less pigmented than that of a young person most evident in the peripheral fundus
What are visual problems in albinism attributed to
foveal hypoplasia, not optical scatter
What disease states are found when melanin levels are below a critical level
as in oculocutaneous albinism, there is aberrant neuronal migration in the visual pathway (more contralateral projections of ganglion cells), incomplete foveal development, low vision, nystagmus, and strabismus.
Function of melanin
neutral density filter preventing light scatter. Also a free-radical stabilizer and can bind many toxins and drugs (such as chloroquine and hydroxychloroquine). Some regard this feature as protective; others think that it contributes to tissue toxicity.
How does retinal laser work
melanin within the RPE absorbs the light delivered to the eye during laser photocoagulation of the retina. the absorbed energy is transferred to the surrounding tissues as heat. the outer retina is damaged, and the ensuing inflammatory reaction creates an adhesion between the retina and the RPE. Because of the high blood flow of the choroid, the heat typically dissipates, with minimal to no damage to the choroid
How is retinal adhesion maintained
The subretinal space is never bridged by tissue, and yet the neural retina remains firmly attached to the RPE throughout life. This is achieved by passive hydrostatic forces, interdigitation of outer segments and RPE microvilli, active transport of subretinal fluid, and the complex structure of the interphotoreceptor matrix and its binding properties (van der Waals forces)
What cells are involved in autophagy
In RPE cells, autophagic machinery, which includes phagosomes and lysosomes. Because RPE cells do not divide under normal conditions, autophagy is also important for quality control of intracellular components.
What conditions are seen in dysregulated autophagy
age-related macular degeneration, glaucoma, and photoreceptor loss in retinal detachment. Drugs that inhibit autophagy (eg, chloroquine) lead to RPE and photoreceptor damage.
What is proliferative vitreoretinopathy
he metaplastic RPE cells form contractile membranes on the surface of the retina, leading to retinal detachment
What condition is seen in Bestrophin protein mutation
heterozygous missense mutations of the bestrophin gene (BEST1) produce Best disease.the encoded protein bestrophin functions as a chloride channel, found on the basolateral surface of the RPE
What condition is seen in TIMP protein mutation
Sorsby macular dystrophy.
The TIMp3 protein is an inhibitor of a metalloproteinase that regulates the extracellular matrix, where it acts as an antiangiogenesis factor
