2 RPE and PR Layer Flashcards
how are cones oriented?
- nuclei line up in a row vitread to ELM
- inner and outer segments protrude into subretinal space RPE
in fovea - nuclei in oblique columns
where are the rod nuclei located?
- ONL (fill up space between larger cones)
in periphery, what are the diameters of rod and cone inner segments
- rod 2um
- cone 6um
in fovea, what are the diameters of rod and cone inner segments
- cone 1.5um
- no rods
- rods are usually thinner than cones except at fovea
why are there only cones in fovea? implications of having small cones?
- high acuity - need to be able to squish everything in
- better discrimination between 2 different edges if receptive field is small
- if receptive field is large, discrimination decreases
where are photon receptors located in a photoreceptor?
outer segments
how does packing differ between inner and outer segment
- hexagonal at inner segment
- irregular at outer segment
what cell types are involved with converting photons into electrical signals
- photoreceptors
- RPE (for support)
describe photoreceptors
- photon detectors
- discs made of bilayered membranes
- specialised transmembrane light-sensitive molecules
what is opsin?
visual pigment
what is retinal and what’s it derived from?
a chromophore derived from vitamin A
opsin incorporation
- where’s it synthesized?
- how is it inserted?
- synthesized in inner segment
- inserted into membrane by vesicular usion, then diffuses to outfoldings then discs
are discs unattached/independent to each other in rods or cones?
rods
opsins
- 7 transmembrane protein
- 4 forms in humans: rhodopsin (rods), S/M/L cone opsins (cones)
rhodopsin sensitivity and peak sensitivity?
- sensitive to blue to green light
- peak sensitivity 500nm
S/M/L cone opsin senstiivity?
to S/M/L wavelengths
cone differences
- red and green similar (peak sensitivities and amino acid composition)
- blue AA comp and absorptions quite different
- peak sensitivities (S 419, M 531, L 558)
photoisomerisation of opsin-bound 11-cis retinal
- vitamin A is precursor for 11-cis retinal
- vitamin not produced by body
- 11-cis retinal is part of rhodopsin that captures photons
- several conformations before becoming all-trans retinal
phototransduction cascade
- rhodopsin activated by photon absorption
- activated rhodopsin activates G protein
- activated G protein activates cGMP phosphodiesterase (PDE)
- activated cGMP PDE converts cGMP to GMP
- reduction in cGMP binding causes channels to close –> hyperpolarization
phototransduction amplification
- amplification needed for individual quanta to elicit noticeable event
- R* interacts with G* 800x
- no amplification of PDE by G*
- 1 PDE* converts ~6cGMP to GMP
- 1R causes 200 cGMP gated channels to close
- dark adapted cone amplification not as great as for rods dues to cone properties and wiring
in the dark, is the Na+ current carried into the cell?
by which channels?
is it depolarized or hyperpolarized?
- yes
- by cGMP -gated channels
- depolarized
in the light, is the Na+ current carried into the cell?
by which channels?
is it depolarized or hyperpolarized?
- no
- hyperpolarized (no inward Na+ current)
- photocurrent reduced by light
after an inward flux of positive ions…
- you get depolarization
- try to negate this by closing off channels
- then you get hyperpolarization
one photon can produce…
- a rod signal
- changes in ion fluxes detected by pipette
what are the similarities and differences in photocurrent magnitudes and intensities between rods and cones?
- both responses increase with increasing light
- rod is positive only
- cone is biphasic (looks like sine wave)
- cone responses are shorter and saturate later
- rod responses saturate before cone responses by 50%
- rod and cone responses overlap; no break so you can detect everything
how does calcium modulate photocurrent?
- closing off cGMP channels decreases Ca stores
- Ca+ can enter by light-induced reflux to restore Ca+
- small hyperpolarisation
what is rhodopsin made of?
opsin and 11-cis-retinal
how is phototransduction ended (how’s rhodopsin inactivated)?
- R* phosphorylated by rhodopsin kinase (GRK1)
- arrestin (Arr1) binds to phosphorylated R* to block rhodopsin-transducin interaction
aside from preventing rhodopsin-transducin interaction, what else does Arr1 do?
- promotes separation of all-trans retinal from opsin so opsin stays in disc membrane (gets recycled) and all-trans retinal diffuses into cytoplasm
- where does inactivated rhodopsin get regenerated?
- transducin and PDE regeneration in outer segment
- 11-cis retinal regeneration in RPE, slower than G-protein regeneration
how does transducin and PDE get regenerated?
- when G* activates PDE, GTP hydrolysed
- G* inactivated and dissociated from PDE
- Ga combines with by to become Gaby
- Gaby-GDP (inactive transducin formed)
- PDE activates with G* and is inactive without it
describe the start of 11-cis retinal regeneration
- in outer segment, all-trans retinal (atRAL) reduced to atROL by retinol dehydrogenase
- all-trans retinol transported to interphotoreceptor matrix (IPM)
- interstitial retinoid-binding protein (IRBP) transports all-trans retinol to RPE
- retinol transferred to cellular retinoid-binding protein (CRBP) in RPE
how do retinals and retinols prevent degradation? give an example
- bind to proteins
- interstitial retinoid binding protein (IRBP)
functions of RPE?
- secretion
- phagocytosis
- visual cycle
- glia
- epithlial transport
- light transportation
describe the visual cycle (retinoid cycle)
- lecithin retinol acyltransferase (LRAT) esterifies atROL to all-trans retinyl ester (atRE)
- RPE-65 hydrolyses and isomerises atRE to 11-cis retinol (11cROL)
- 11-cis retinol dehydrogenase (11-cis-RDH) oxidises 11cROL to 11-cis retinal (11cRAL)
11-cis retinal transporter
11cRAL transported by cellular retinal-binding protein (CRALBP) to IPM, the by IBRP to outer segments
- in outer segment, 11cRAL recombines with apo-opsin to form rhodopsin
draw the visual cycle
http://www.molvis.org/molvis/v18/a108/wang-fig1.html
what’s another source for retinol?
- new retinol from liver stores and digestive tract are transported to RPE via blood stream by retinol-binding protein and transthyretin complex (RBP-TTR)
- complex diffuses out of choriocapillaris
- transported into RPE by RBP receptor
describe RPE involvement of outer segment phagocytosis
- old discs in tip engulfed by RPE
- discs distally displaced toward RPE
- phagocytosis is cyclical
describe the 3 steps in the signalling of phagocytosis
- recognition - phosphatidylserine is the “eat me” signal
- engulfment - reorganisation of cytoskeleton, Myosin II important in cup formation
- degradation - phagosome, endosom, lysosom, phagolysosome
cone-specific visual cycle
- independent of RPE
- involves Muller cells
- more rapid - important for cones in bright light and rapid dark adaptation after exposure to light
- bleaching too long decreases rod responses, not cone responses
how does the cone visual cycle differ than than the rod visual cycle?
it has an additional source of 11-cis retinol (Muller cells)
what are other Muller cell functions?
- synthesize retinoic acid from retinol (for dev of eye and NS)
- metabolic support (anaerobic metabolism)
- protect neurons from excess exposure to NT
- phagocytosis of neuronal debris
- control homeostasis and protect from deleterious changes - steadies K+ current (ERG wave)
