Lecture 10: Biochemistry of Vision Flashcards
What are the 3 main cell types of the eye?
- Photoreceptors
- Interneurons (bipolar cells, horizontal cells, amacrine cells)
- Ganglion cells
What are the 3 main components of the retinal circuit for the processing of visual signals?
Photoreceptors —> Interneurons —-> Ganglion cells
What are the output cells of the retina and what do their axons form?
- Ganglion cells
- Axons form the optic nerve
- Project to the brain
- Information transmitted via AP’s
How do the photoreceptors differ in rods vs. cones; what is the sensitivity and resolution like in each?
Rods (night vision)
- Rhodopsin (cannot detect color)
- High sensitivity and low spatial resolution
Cones (color detection)
- Three opsins (red, green, and blue)
- Low sensitivity and high spatial resolution
What are the 2 components of Rhodopsin?
Opsin (protein) + 11 cis-retinal (derived from Vitamin A)
The structure of Rhodopsin is very similar to what receptor?
β2-adrenergic receptor
How is retinal able to form the protonated schiff base of functional rhodopsin?
- Lysine-296 in opsin (located in the 7th TM of the protein) covalently bound to 11-cis retinal
- Aldehyde of retinal forms Schiff base with amine of lysine
- Schiff base becomes protonated
What is the absorption wavelengths of free retinal vs. un-protonated schiff base retinal, and protonated schiff base retinal?
Free retinal: 370 nm
Un-protonated: 380 nm
Protonated: 440 nm +
*Rhodopsin absrobs maximally at 500 nm
What occurs once a photon hits rhodopsin?
- 11-cis-retinal —> 11-trans-retinal (isomerization)
- Causes 5Å conformational change of Schiff-base Nitrogen
What is the activated form of Rhodopsin called?
Metarhodopsin II
Explain the visual signal transduction pathway after the photon is absorbed by Rhodopsin in a photoreceptor cell?
- Light absorbed by rhodopsin in photoreceptor cell, which interacts with the retinal causing 11-cis —> 11-trans
- Conformational change of rhodopsin —> Metarhodopsin or R*
- R* interacts w/ G protein transducin, catalyzing its activation by the release of bound GDP in exchange for GTP
- The alpha subunit of transducin disassociates from its β and γ subunits and activates phosphodiesterase, which hydrolyzes cGMP
- Lowered cGMP levels close the cGMP-gated Na+ channels leading to hyperpolarization of the cell and neuronal signaling
How does each step of the visual signal transduction contribute to the sensitivity of our eyes to light?
- At each step of the process, there is significant amplification
What are the signal termination steps which block light-activated rhodopsin from activating transducin?
- Rhodopsin kinase phosphorylates COOH terminus of Metarhodopsin II at Thr and Ser allowing binding by Arrestin and preventing the interaction with Transducin
- Transducin has intrinsic GTPase activity and hydrolyzes GTP to GDP causing dissociation of transducin from PDE and reassociation with the βγ subunits
- Guanylate cyclase synthesizes cGMP from GTP
- Elevated cGMP levels re-open cGMP-gated ion channels
Ca2+ inhibits the activity of what enzyme in the signal transduction pathway?
Guanylate cyclase
What is the movement of Ca2+ in the rod during dark conditions?
- Ca2+ and Na+ enter the rod OS through cGMP-gated ion channels
- Ca2+ influx is balances by its efflux through a Na+/K+/Ca2+ exchanger
What is the movement of Ca2+ in the rod during light conditions?
- Ca2+ influx through the cGMP channel stops but exchanger transport continues
- Reduces intracellular Ca2+ from 500 nM to 50 nM
- This STIMULATES the activity of guanylate cyclase, restoring [cGMP] and re-opening cGMP-gated ion channels
Rods and cones release what inhibitory NT in the dark when depolarized; why?
- Glutamate
- Inhibits the optic nerve bipolar cells
- Quiets the information to the brain
When you open your eyes, what occurs to the cell and what effect does this have?
- Causes hyperpolarization
- The inhibitory NT, glutamate is removed
Cone cells are homologues of ______, members of ________ family, use ______ as chromophore.
Cone cells are homologues of rhodopsin, members of 7TM family, use 11-cis-retinal as chromophore
What are the 3 varieties of cone receptors and what wavelength does each correspond to?
1) Blue (460 nm)
2) Green (530 nm)
3) Red (560 nm)
When someone is color-blind they cannot distinguish between what 2 colors; what chromosome are the genes for these 2 colors found on?
- Red and green
- X chromosome
Rearrangement of the genes for color during DNA replication may lead to what?
1) Loss of visual pigment genes
2) Formation of hybrid pigment genes that encode photoreceptors with anomalous absorption spectra
The AA’s most important for determining absorption spectra are in what half of each photoreceptor protein?
- The carboxyl-terminal half of each photoreceptor protein
- The part of the gene that encodes this region most strongly affects the absorption charateristics of hybrid receptors
The purpose of the retinoid cycle is the regeneration of what?
11-cis-retinal
Explain the basics of the Retinoid cycle from rod cell –> RPE –> rod cell
In the rod cell:
- Light-induced change from 11-cis to all-trans-retinal
- Release of all-trans-retinal from opsin
- Enzymatic reduction of all-trans-retinal to all-trans-retinol
- Export of all-trans-retinol (with help from iRBP)
In the retinal pigmented epithelium (RPE):
- Uptake into the RPE and translocation to ER for enzymatic processing to 11-cis-retinal
- Export of 11-cis-retinal
In the rod cell:
- Uptake of 11-cis retinal into rod cell
- Covalent attachment (Schiff base) to opsin forming a functional rhodopsin
Why are photoreceptors of the outer segment (POS) particularly vulnerable to damage and why is this significant?
- Contains highly reactive retinoids and high levels of unsaturated PLs
- Rod and cones terminally differentiated post-mitotic cells (DO NOT DIVIDE)
How have photoreceptors of the outer segment developed a unique mechanism for renewal?
- Shedding tips which get phagocytosed by RPE
- In mammals, 10% rods shed/day and same amount of membrane and protein components made ever day
- Disruption in renewal leads to degeneration
OS disk recycling occurs in what type of manner; when is peak rod versus peak cone shedding?
- Occurs in a circadian manner
- Peak rod shedding in morning
- Peak cone shedding after dark
What occurs when a photoreceptor gets shed, how does the RPE deal with it?
- The retinal pigmented epithelium (RPE) ingests the POS, which is surrounded by membrane to make a phagosome
- Series of fusion events with endosome and lysosome for degradation
- Some components recycled and reused
What is Retinitis Pigmentosa; caused by; affects; and characterized by; ultimately what does this lead to?
- Group of inherited retinopathies
- Caused by mutations in rhodopsin and other photoreceptors protein (peripherin, PDE)
- Affect disk morphology, photoreceptor structure and function, and renewal
- Characterized by loss of night vision followed by peripheral vision
- Leads to degeneration, disease called retinitis pigmentosa
Retinitis pigmentosa is characterized by?
Loss of night vision, followed by complete blindness
What are some of the consequences of Vitamin A deficiency?
- Night blindness
- Xerophtalmia (dry eye syndrome)
- Bitot’s spots (due to keratin debris in conjunctiva)
- Visual impairment
What is the mainstay of therapy for Vitamin A deficiency; is too much Vitamin A a bad thing?
- Mainstay of therapy for a deficiency is Vitamin A supplementation
- Excess Vitamin A due to copious intake of supplements causes liver toxicity and joint pain
Night blindness can be caused by?
- Deficiency of Vitamin A
- Glaucoma
- Cataracts
- Retinitis pigmentosa
Age-related macular degeneration (AMD) effects which part of the retina?
Effecys the macular region of the retina
What plays a key role in the development of age-related macular degeneration (AMD)?
Pathological processes in lipid metabolism, oxidative stress and inflammation
Age-related macular degeneration (AMD) is clinically divided into what 2 forms and what are the characteristics of each; which of the 2 is most common?
1) Dry Form: characterized by accumulation of lipid rich extracellular deposits, degenration of RPE, and secondary photoreceptor loss
2) Wet Form:associated withchoroidal neovascularization. Less common, results in severe vision loss
Mutations in what enzyme is related to many vision diseases and what are they?
- Mutation in ATP binding cassette transporter or ABC transporter
- Stargardt’s disease (autosomal recessive forms of juvenile degeneration)
- Cone-rod dystrophy
- Retinitis pigmentosa
- AMD
Explain what effect mutations in ABC transporters play in the development of vision diseases?
- Elevated levels of diretinoid-pyridinium-ethanolamine (A2E), the ultimate product of condensation of 2 molecules of all-tran-retinal and one molecule of phosphatidylethanolamine
- Also accumulation of all-trans-retinal
- Accumulation of these 2 produces cellular debris, which in turn generates oxidative stress
What are 2 macular carotenoids that have shown to lower the risk of AMD?
- Lutein
- Zeaxanthin
*Both are free-radical scavenging agents and anti-oxidant compounds
