Biochemistry and Metabolism- RPE

Question 1

What is the function of the RPE

Answer 1

• 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

Question 2

What does mutation of gene RPE65 lead to

Answer 2

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

Question 3

What is autophagy

Answer 3

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

Question 4

What embrylogical layer is the RPE derived from

Answer 4

monolayer of neuroectoderm-derived epithelial cells located between the highly vascular choriocapillaris and the photoreceptor outer segments

Question 5

How many RPE cells are there per eye

Answer 5

4–6 million RPE cells per eye

Question 6

What forms the outer blood retinal barrier

Answer 6

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

Question 7

What is Bruch Membrane composed of

Answer 7

an extracellular matrix between the RPE and the choriocapillaris), has many infoldings that increase the surface area available for the exchange of solutes

Question 8

What key cells do RPE contain

Answer 8

Melanin and Phagosomes reflecting their role in light absorption and phagocytosis

Question 9

Pathways used to generate energy in RPE

Answer 9

glycolysis, the Krebs cycle, and the pentose phosphate pathway

Question 10

What is the majority of wet weight of RPE contributed by and how much?

Answer 10

Water- >80%.
Proteins, lipids and nucleic acids contribute the rest

Question 11

How many proteins are identified in RPE

Answer 11

Nearly 850

Question 12

What kind of proteins are found in RPE

Answer 12

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

Question 13

Where is the Na+/K+/ATPase found found on RPE

Answer 13

On the apical membrane in contrast to most polarized cells

Question 14

Which RPE protein is exclusively expressed on the RPE and its function

Answer 14

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)

Question 15

What % weight do lipids contribute of the weight of the RPE

Answer 15

3%- half are phospholipids

Question 16

Which organ contains the highest concentration of vitamin A in the body

Answer 16

Liver, then RPE

Question 17

What is RPE role in vitamin A regeneration

Answer 17

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

Question 18

How is free all-trans-retinal cleared from rod discs

Answer 18

ABCA4. After transport to cytosol from rod discs, all-trans-retinal is reduced to all-trans-retinol by retinol dehydrogenease

Question 19

What is the phototransduction cascade

Answer 19

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

Question 20

What conditions are associated with mutation of ABCR gene

Answer 20

ABCR gene encodes ABCA4 transporter protein leads to Stargadt disease.

Question 21

What conditions are associated with mutation of RPE65 gene

Answer 21

Leber congenital amaurosis

Question 22

How many outer segment discs are shed per day

Answer 22

100 from each photoreceptor cell. Many photoreceptors interdigitate with a single RPE cell, each RPE cell digests more than 4000 discs daily

Question 23

When are discs shed

Answer 23

Shedding most vigorous at dawn in rods
Shedding most vigorous at dusk in cones

Question 24

What happens to the shed outer segment discs

Answer 24

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

Question 25

What happens in Stargadt disease

Answer 25

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

Question 26

Why is a signal detected in autoflouresence

Answer 26

The lipofuscin characteristic of the RPE is derived from the photosensitive membranes and is responsible for generating the signal detected in fundus autofluorescence imaging

Question 27

What proteins determine cell polarity

Answer 27

Cytoskeletal proteins

Question 28

Where in the RPE are pigment granules primarily found

Answer 28

Pigment granules are abundant in the cytoplasm of adult RPE cells, predominantly in the apical and midportions of the cell

Question 29

What triggers the onset of melanogenesis in RPE cells during developement

Answer 29

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.

Question 30

Why is the fundus of an older person less pigmented than that of a younger person

Answer 30

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

Question 31

What are visual problems in albinism attributed to

Answer 31

foveal hypoplasia, not optical scatter

Question 32

What disease states are found when melanin levels are below a critical level

Answer 32

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.

Question 33

Function of melanin

Answer 33

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.

Question 34

How does retinal laser work

Answer 34

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

Question 35

How is retinal adhesion maintained

Answer 35

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)

Question 36

What cells are involved in autophagy

Answer 36

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.

Question 37

What conditions are seen in dysregulated autophagy

Answer 37

age-related macular degeneration, glaucoma, and photoreceptor loss in retinal detachment. Drugs that inhibit autophagy (eg, chloroquine) lead to RPE and photoreceptor damage.

Question 38

What is proliferative vitreoretinopathy

Answer 38

he metaplastic RPE cells form contractile membranes on the surface of the retina, leading to retinal detachment

Question 39

What condition is seen in Bestrophin protein mutation

Answer 39

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

Question 40

What condition is seen in TIMP protein mutation

Answer 40

Sorsby macular dystrophy.
The TIMp3 protein is an inhibitor of a metalloproteinase that regulates the extracellular matrix, where it acts as an antiangiogenesis factor