Lecture 10: Biochemistry of Vision

Question 1

What are the 3 main cell types of the eye?

Answer 1

1. Photoreceptors
2. Interneurons (bipolar cells, horizontal cells, amacrine cells)
3. Ganglion cells

Question 2

What are the 3 main components of the retinal circuit for the processing of visual signals?

Answer 2

Photoreceptors —> Interneurons —-> Ganglion cells

Question 3

What are the output cells of the retina and what do their axons form?

Answer 3

• Ganglion cells
• Axons form the optic nerve
• Project to the brain
• Information transmitted via AP’s

Question 4

How do the photoreceptors differ in rods vs. cones; what is the sensitivity and resolution like in each?

Answer 4

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

Question 5

What are the 2 components of Rhodopsin?

Answer 5

Opsin (protein) + 11 cis-retinal (derived from Vitamin A)

Question 6

The structure of Rhodopsin is very similar to what receptor?

Answer 6

β2-adrenergic receptor

Question 7

How is retinal able to form the protonated schiff base of functional rhodopsin?

Answer 7

• 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

Question 8

What is the absorption wavelengths of free retinal vs. un-protonated schiff base retinal, and protonated schiff base retinal?

Answer 8

Free retinal: 370 nm

Un-protonated: 380 nm

Protonated: 440 nm +

*Rhodopsin absrobs maximally at 500 nm

Question 9

What occurs once a photon hits rhodopsin?

Answer 9

• 11-cis-retinal —> 11-trans-retinal (isomerization)
• Causes 5Å conformational change of Schiff-base Nitrogen

Question 10

What is the activated form of Rhodopsin called?

Answer 10

Metarhodopsin II

Question 11

Explain the visual signal transduction pathway after the photon is absorbed by Rhodopsin in a photoreceptor cell?

Answer 11

• 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

Question 12

How does each step of the visual signal transduction contribute to the sensitivity of our eyes to light?

Answer 12

• At each step of the process, there is significant amplification

Question 13

What are the signal termination steps which block light-activated rhodopsin from activating transducin?

Answer 13

• 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

Question 14

Ca2+ inhibits the activity of what enzyme in the signal transduction pathway?

Answer 14

Guanylate cyclase

Question 15

What is the movement of Ca2+ in the rod during dark conditions?

Answer 15

• 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

Question 16

What is the movement of Ca2+ in the rod during light conditions?

Answer 16

• 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

Question 17

Rods and cones release what inhibitory NT in the dark when depolarized; why?

Answer 17

• Glutamate
• Inhibits the optic nerve bipolar cells
• Quiets the information to the brain

Question 18

When you open your eyes, what occurs to the cell and what effect does this have?

Answer 18

• Causes hyperpolarization
• The inhibitory NT, glutamate is removed

Question 19

Cone cells are homologues of ______, members of ________ family, use ______ as chromophore.

Answer 19

Cone cells are homologues of rhodopsin, members of 7TM family, use 11-cis-retinal as chromophore

Question 20

What are the 3 varieties of cone receptors and what wavelength does each correspond to?

Answer 20

1) Blue (460 nm)
2) Green (530 nm)
3) Red (560 nm)

Question 21

When someone is color-blind they cannot distinguish between what 2 colors; what chromosome are the genes for these 2 colors found on?

Answer 21

• Red and green
• X chromosome

Question 22

Rearrangement of the genes for color during DNA replication may lead to what?

Answer 22

1) Loss of visual pigment genes
2) Formation of hybrid pigment genes that encode photoreceptors with anomalous absorption spectra

Question 23

The AA’s most important for determining absorption spectra are in what half of each photoreceptor protein?

Answer 23

• 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

Question 24

The purpose of the retinoid cycle is the regeneration of what?

Answer 24

11-cis-retinal

Question 25

Explain the basics of the Retinoid cycle from rod cell --\> RPE --\> rod cell

Answer 25

**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-retin**al** to all-trans-retin**ol** - Export of all-trans-retin**ol** (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

Question 26

Why are photoreceptors of the outer segment (POS) particularly vulnerable to damage and why is this significant?

Answer 26

- Contains highly reactive retinoids and high levels of unsaturated PLs - Rod and cones **terminally differentiated post-mitotic cells (DO NOT DIVIDE)**

Question 27

How have photoreceptors of the outer segment developed a unique mechanism for renewal?

Answer 27

- **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

Question 28

OS disk recycling occurs in what type of manner; when is peak rod versus peak cone shedding?

Answer 28

- Occurs in a **circadian manner** - Peak **rod** shedding in **morning** - Peak **cone** shedding after **dark**

Question 29

What occurs when a photoreceptor gets shed, how does the RPE deal with it?

Answer 29

- 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

Question 30

What is Retinitis Pigmentosa; caused by; affects; and characterized by; ultimately what does this lead to?

Answer 30

- 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**

Question 31

Retinitis pigmentosa is characterized by?

Answer 31

Loss of night vision, followed by complete blindness

Question 32

What are some of the consequences of Vitamin A deficiency?

Answer 32

- Night blindness - Xerophtalmia (dry eye syndrome) - Bitot's spots (due to **keratin debris in conjunctiva**) - Visual impairment

Question 33

What is the mainstay of therapy for Vitamin A deficiency; is too much Vitamin A a bad thing?

Answer 33

- 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

Question 34

Night blindness can be caused by?

Answer 34

- Deficiency of **Vitamin A** - **Glaucoma** **- Cataracts** **- Retinitis pigmentosa**

Question 35

Age-related macular degeneration (AMD) effects which part of the retina?

Answer 35

Effecys the **macular region** of the **retina**

Question 36

What plays a key role in the development of age-related macular degeneration (AMD)?

Answer 36

Pathological processes in **lipid metabolism, oxidative stress and inflammation**

Question 37

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?

Answer 37

**1) Dry Form:** characterized by accumulation of **lipid rich extracellular deposits,** degenration of RPE, and secondary photoreceptor loss **2) Wet Form****:**associated with**choroidal neovascularization.** Less common, results in severe vision loss

Question 38

Mutations in what enzyme is related to many vision diseases and what are they?

Answer 38

- Mutation in **ATP binding cassette transporter** or **ABC** transporter - Stargardt's disease (autosomal **recessive** forms of juvenile degeneration) - Cone-rod dystrophy - Retinitis pigmentosa - AMD

Question 39

Explain what effect mutations in ABC transporters play in the development of vision diseases?

Answer 39

- 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**

Question 40

What are 2 macular carotenoids that have shown to lower the risk of AMD?

Answer 40

- Lutein - Zeaxanthin **\*Both are free-radical scavenging agents and anti-oxidant compounds**