Biochemistry of the Visual System (Kinde) Flashcards
1
Q
- photoreceptor cell for light vision, 100 mil
- opsin: rhodopsin (cannot detect color)
- high sensitivity and low spatial resolution
A
rods
2
Q
- photoreceptor cell for color detection, 7 mil
- 3 opsins: red, green, blue
- low sensitivity and high spatial resolution
A
cones
3
Q
What is the difference in convergence between rods and cones?
A
- rods respond to single photon, cones need ~100 photons to respond
- many rods converge into a single bipolar cell, many bipolar cells contact single amacrine cell (ganglion cell), wich allows for high sensitivity but sacrifices resolution
- one cone contacts one bipolar cell, this allows for high resolution but sacrifices sensitivity
(these cells are located on the posterior side of the retina)
4
Q
How is the outer segment of photoreceptor cells organized?
A
- multiple disc membranes (vesicles) are stacked on top of one another in the outer membrane w/ very little (nm) distance between them
- contains the GPCR system (opsin, transducin, phosphodiesterase) that initiates signal transduction
- also contains structural protein, peripherin (retinitis pigmentosa)
5
Q
How is signal transduction initiated in rods and cones
A
- light enters eye and is focused to the back of the eye, permeates the 3 retinal layers, and activates the receptor proteins (opsins)
- signal transduction is initiated by GPCR system when the receptor (opsin) amd chromophore (retinal), activate G-protein (transducin, inherent GTPase activity)
- the effector protein is cGMP phosphodiesterase which cleaves cGMP to GMP (secondary messenger)
6
Q
What happens when cGMP phosphodiesterase (PDE) hydrolyzes cGMP, reducing its concentration in the outer segment?
A
- in dark conditions, cGMP-gated Na+ channels (Ca2+ leak channels) on the membrane of outer segment that are open, contribute to partial depolarization in dark conditions
- in light conditions, when PDE is activated and reduces cGMP conc, the Na+ channels are closed
- Na+/Ca2+-exchanger allows for Ca2+ efflux after illumination
- guanylate cyclase recycles cGMP back to depolarized state (GTP > cGMP + PPi)
- desensitization occurs through rhodopsin kinase (phosphorylation, first step in signal term) and B-arrestin (signal term through blocking interaxn of rhodopsin w/ transducin)
7
Q
What is the electrochemical state of a photoreceptor cell in dark conditions?
A
- cGMP-gated Na+ channels (Ca2+ leak channels) are open
- cell is depolarized
- default state (corresponds to high metabolic rates and highest respiratory rate in body than any other tissue), active all the time, inhibitory NT’s (glutamate) are constantly released
- “dark current”
8
Q
What is the electrochemical state of photoreceptor cells in light conditions?
A
- Na+ channels are closed
- cell is hyperpolarized, due to rapid hydrolysis of cGMP by PDE
- transduction system is unique in that stimuli (light: photons) cause hyperpolarization and reduce release of NT’s (glutamate)
- TLDR: light > decreases glutamate presence, hyperpolarizes cells
9
Q
- also referred to as a GPCR
- human genome codes for over 800 of these proteins
- 1/3 of all drug targets
- rhodopsin is a type of this protein
A
7TM receptor
10
Q
What are the associated structures of rhodopsin (GPCR) and how is it activated?
A
- rhodopsin is homologous to beta-adrenergic receptors
- lysine-296 located in center of 7TM, and covalently binds to 11-retinal which is derived from vitamin A
- the aldehyde of retinal forms a Schiff base w/ amine of lysine
- the Schiff base lysine is protonated and absorbs light at >440 nm (free retinal: 370 nm; unprotonated Schiff base: 380 nm)
- photon (light) induced isomerization causes atomic motion in the form of: 11-cis-retinal > 11-trans-retinal (5A conformational shift of Schiff-base nitrogen)
- this conformational change upon light-induction mimics conformational change that occurs upon ligand binding in other 7TMs
- activated rhodopsin is metarhodopsin II (R*)
11
Q
- homologues of rod receptor opsins
- 3 types: 460nm (blue, chromosome 7), 530nm (green, X chrom), >560nm (red, X chrom)
- mutations in genes encoding for these structures are responsible for maternally inherited red/green color blindness
A
cone opsins
(photo: open circles are rod/cones identical residues; filled circles are different residues; 3 black circles are responsible for most of the differences in absorption spectra)
12
Q
Explain the process of signal transduction within photoreceptor cells :)
A
(occuring in outer membrane disc of a rod)
- photon (hν) is absorbed and interacts w/ retinal which isomerizes from 11-cis to all-trans config > activates rhodopsin by conformational change in disc membrane to R*
- R* makes repeated contacts w/ transducin molecules, catalyzing its activation to G* by release of bound GDP in exchange for cytoplasmic GTP, which disassociates w/ its Gβ and Gγ subunits
(G* = Gα subunit + GTP)
- G* binds inhibitory γ subunits of the phosphodiesterase (PDE) activating its α and β subunits
- activated PDE hydrolyzes cGMP (cGMP > GMP), lowering conc of cGMP which causes Na+ channels to close > hyperpolarization of cell due to efflux of K+ which causes voltage-gated Ca2+ channels to close too
- guanylyl cyclase (GC) synthesizes cGMP (2nd messenger in cascade); reduced levels of cGMP cause cyclic nucleotide gated channels to close preventing further influx of Na2+ and Ca2+
- Ca2+ level drops causing NT (glutamate) release to drop b/c Ca2+ is req for glutamate-containing vesicles to fuse w/ cell and release contents
- decrease is glutamate released by photoreceptors causes depolarization of on-center bipolar cells (rod and cone on bipolar cells) and hyperpolarization of cone off-center bipolar cells
13
Q
How is the signal terminated in photoreceptor cells?
A
- light-activated rhodopsin becomes blocked from activating transducin: rhodopsin kinase phosphorylates C-terminus of metarhodopsin II (R*) at Thr and Ser, allowing binding by β-arrestin (prevents interaction w/ transducin)
- rapid hydrolysis of GTP to GDP causes dissociation of α-subunit from PDE and reassociation w/ β- and γ-subunits (innate GTPase activity of transducin)
- elevated cGMP levels re-open cGMP-gated Na+ channels (guanylate cyclase synthesizes cGMP from GTP)
(low Ca2+ levels induce recovery (termination of phototransduction cascade))
14
Q
How does Ca2+ control the rate at which the phototransduction system is restored?
A
- during activation, cGMP levels decrease
- decreased levels of cGMP close cGMP-gated Na+ and Ca2+ channels
- this decreases intracellular Na+ and Ca2+
- decreased Ca2+ levels induces recovery, where low levels of Ca2+ increase guanylate cyclase activity
- increased GC activity leads to increased levels of cGMP (opening Na+/Ca2+ channels and depolarizing the cell)
15
Q
Summarize the conformational changes of rhodopsin in the phototransduction system:
A
- Rh is inactive: 11-cis-retinal
- light hits 11-cis-retinal, converting it to all-trans-retinal (Rh*)
- Rh* repeatedly contacts transducin (G-protein), activating it and causing signaling within cell
- intracellular processes and recovery gives rise to P-Rh*-arrestin-1 (prevents rhodopsin binding to transducin)
16
Q
What vitamin is retinal produced from?
A
- retinal produced in retina from vitamin A (dietary pro-vit A carotenoids)
- 3 different vitamin A structures: carboxyl (retinoic acid), aldehyde (retinal), hydroxyl (retinol)
19
Q
What are the conditions a/w vitamin A deficiency, excessive intake, and in-utero exposure?
A
- night blindness (carrots)
- xerophthalmia aka dry eye condition (leafy greens)
- keratinization of epithelium in GI/resp (sweet potatoes)
- dry, scaly skin (squash)
- AMD (broccoli)
- leading cause of preventable blindness in children (animal prods, eggs, dairy, fish, liver)
- Bitot’s spots (keratin debris in conjunctiva)
- growth impairment, failure of wounds to heal well, dry skin, follicular hyperkeratosis, alopecia, lung conditions
- excess: liver tox and joint pain
- in-utero: isotretinoin (accutane) cleft palates and heart abnormalities
20
Q
- abnormal dryness of the conjunctiva and cornea of the eye, typically a/w vitamin A deficiency
- cornea can undergo complete necrosis
- affects ~5.2 mil children worldwide, of which 250,000-500,000 become blind, and 1/2 will die within a year of losing vision
A
xerophthalmia
22
Q
What are the 2 systems that the retinoid cycle takes place between?
A
- photoreceptors: located in deepest retinal layer, outer segments of cells are farthest retinal layer from light entering pupil, in close proximity to specialized monolayer of RPE cells
- retinal pigmented epithelium (RPE): the second deepest layer, adjacent to the photoreceptor cells; not a part of the neural retina, but essential for nml function/survival of photoreceptors; principal site for 11-cis-retinal regeneration in retinoid cycle
23
Q
What is the first step in the retinoid cycle?
A
in the rod cell:
- light-induced conformation of 11-cis-retinal to all-trans-retinal
- all-trans-retinal released from activated opsin into inner leaflet of disc bilayer; believed to complex w/ phosphatidylethanolamine, resulting in N‐retinylidine‐phosphatidylethanolamine
- all-trans-retinal (N‐retinylidine‐phosphatidylethanolamine) transported to cytoplasmic disc surface by retina specific ATP binding cassette transporter (ABC transporter), and released into cytoplasm of outer segment as all-trans-retinal
- all-trans-retinal is reduced to all-trans-retinol (vit A) by all-trans retinol dehydrogenase (aRDH) in NADPH-dependent rxn
- all-trans-retinol transported from photoreceptor, crosses sub-retinal space bound to inter-photoreceptor retinoid binding protein (iRBP), and enters retinal pigmented epithelium (RPE)
25
Q
What occurs within step 2 of the retinoid cycle?
A
in the retinal pigmented epithelium (RPE):
- all-trans-retinol is transferred to cellular retinoid binding protein (CRBP)
- esterification to all-trans-retinyl ester by lecithin retinol acyl transferase (LRAT)
- isomerization and hydrolysis to 11-cis-retinol by the isomerohydrolase, RPE65 protein (essential for regeneration of 11-cis retinoids, no isomerohydrolase activity w/o it)
- 11-cis-retinol binds cellular retinaldyhyde binding protein (CRALBP)
- CRALBP delivers 11-cis-retinol to 11-cis retinol dehydrogenase (11-cis RDH) for 3rd/final enzymatic step: oxidation to 11-cis-retinal (NAD is cofactor)
- 11-cis-retinal transported back to photoreceptors by iRBP
TLDR: in RPE, all-trans-retinol is converted to 11-cis-retinal by 3 enzymes in smooth ER
26
Q
- oxidative and inflammatory changes in RPE and retina due to combo of environmental factors and genetic predisposition
- retinoid cycle’s unique photochemistry maintains vision, however high flux of photons by light can lead to elevated levels of toxic retinal metabolites, and accum of retinoid metabolites throughout life due to dysfunctional clearance can induce photoreceptor degeneration
- leads to progressive loss of rod cells, eventual loss of cone cells
- bony spicules form in RPE
- sx: decreased night vision, loss of peripheral vision, loss of central vision
- cause: LRAT (esterification of ATR > all-trans-retinyl ester) and RPE65 (ATRE > 11-cis-retinol) dysfunction
A
retinitis pigmentosa
(LRAT and RPE65 dysfunction)
28
Q
- condition that results in loss of central field vision due to impaired clearance of toxins and retinoid metabolites
- mutations in ABC transporters (esp ABCA4) are known to cause this condition and other conditions a/w severe vision loss
- risk factors: old age (esp >70), hx of smoking within past 20 yr, diet (low intake of antioxidants, zinc, and omega-3 fatty acids; high fat intake), obesity, caucasian
- 15 mil ppl in North America have non-exudative type, 0.7 mil have neovascular type, projected to increase by >50% in 2020
- leading cause of irreversible blindness in ppl >50 in developed world
- ~5% of ppl >50 have signs of condition, 30% of ppl >75 have some form
- males and females equally affected, 15% of white women >80 have severe form
A
macular degeneration
