Download - Lecture #17
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Lecture #17
The visual cycle + eye disease4/2/13
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Spatial resolution – last timeB/W did have highest resolution
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Patrick – what do photoreceptors see?
• Visualization
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tan θ and θ
0.1
10
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EEEEEEEEEE
• Calculate the image size on your retina for one line of the small E in the front of the room
• Write the image size of one line on the board according to where you are sitting
• How many receptors does one line of the small E cover on your retina?
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Next few lectures
• Visual cycle - todayMuch of this pathway has been identified through disease studiesVisual cycle in rods Visual cycle in cones
• Eye diseases – next 3 lecturesRetinal diseaseOther diseases (glaucoma, cataracts etc)Treatments
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Visual cycle: recycling 11-cis retinal through isomerization and pigment regeneration
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George Wald showed 11-cis retinal isomerizes to start vision
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G protein pathway in rod disc
R + hv g R* Rhodopsin absorbs photon g excitedR* + Gαβγ g R* + Gα*-GTP + Gβγ Rhodopsin activates G protein
Rhodopsin G protein
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R* shutoff: phosphorylation and arrestin binding
Inactive R has all trans retinal bound to it – need to regenerate 11-cis retinal and reform visual pigment
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Photoreceptors at the very back of the retina next to retinal pigment epithelium (RPE)
http://webvision.med.utah.edu/
Pigment epithelium
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Some facts about pigment regeneration after pigment absorbs light (bleached)
Rods Cones
Retina plus RPE Regenerate Regenerate
Remove RPE None Still regenerate
Remove cones from retina
None
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Phototransduction: 11-cis + photon = all transVisual cycle: all trans + enzymes = 11-cis
Visual cycle Phototransduction
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Retinal pigment epithelium (RPE) - plays a big role in 11-cis retinal regeneration in rods
Retinal dissociates from opsin, diffuses out of photoreceptor, goes to RPE, gets regenerated and comes back
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Both 11-cis and all-trans retinal are isomers of the same aldehyde : retinaldehyde
R-CHO R H
C
O
11
11
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Retinoid biochemistry
• Aldehyde RCHO -al
• Alcohol ROH -ol
• Ester R-CO-O-R’
R HC
O
R OH
R OC
O
R’
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Opsin + retinal = Schiff base
Schiff base is a carbon nitrogen double bond, C=NR3
In the case of rhodopsin, the N comes from the lysine and C comes from the retinal
+ X-(CH2)4NH2Lysine
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Schiff base
Schiff base can often be protonated NH+
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Visual pigment =opsin + retinal
11-cis retinal
membrane
In rod, visual pigment is called rhodopsin
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11-cis in retinal binding pocket
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Phototransduction occurs in retina: rods and cones
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Phototransduction occurs in retinaVisual cycle occurs in retina and RPE
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Retinoids(Bound to opsin)
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Moving retinoids around takes chaperones
They protect retinoids from oxidation and isomerizationThey help transfer fat soluble retinoids across aqueous layers
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Moving retinoids around takes chaperones
IRBP - interphotoreceptor retinoid binding proteinCRBP - cellular retinol binding proteinCRALBP - cellular retinaldehyde binding protein
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Slightly different version
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First steps to retinal regeneration
ABCR = ATP binding cassette transporter
ABCR binds to all trans retinal (RAL) and helps it diffuse out of discs
This leaves free opsin which is ready to make new visual pigment
Disc
Apo-protein = protein without prosthetic group which is needed for its function
Rod
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First steps to retinal regeneration
Disc
All-trans retinol dehydrogenase (RDH) converts all trans retinal to all trans retinol
Rod
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Retinal(dehyde) retinol
RDH12
C=O
H
C-OH
HHNote: all-trans-ROL=Vitamin A!!
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Moving retinoids around takes chaperones
IRBP - interphotoreceptor retinoid binding protein
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Transport of retinol to RPE
RPEInterphotoreceptor retinol binding protein (IRBP) helps transport all-trans retinol to the RPE
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Moving retinoids around takes chaperones
In RPE, CRBP binds retinol 100x greaterCRBP = cellular retinol binding protein
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Conversion of all trans retinolLRAT = lecithin retinol acyl transferase
Converts all trans retinol to all trans retinal ester
Works best on retinol bound to CRBP
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Convert to ester
LRAT = adds acyl group
O
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IsomerizationIsomerase RPE65 converts all trans retinyl ester to 11 cis retinol
RPE65 is 65 kilodalton protein which occurs in RPE
RGR - retinal G protein coupled receptor is enzyme that may help in isomerization
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Forms 11-cis retinalRetinal dehydrogenase converts 11-cis retinol to 11-cis retinal
CRALBP is cellular retinal binding protein
Can bind ROL or RAL to help move either
Can store 11-cis ROL as ester till needed.
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Conversion
RDH5
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Conversion
RDH5 RDH12
In RPE In rod
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Summary
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IRBP takes 11cis-RAL back to photoreceptor
Reforms the active visual pigment
RPE65
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Visual cycle in rods
• Needs RPEAll trans retinol leaves retina and goes to RPE to be regeneratedWithout RPE, no isomerization and no visual pigment regenerationTakes time for retinal to be transported around so slow process
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Retinal regeneration in cones
• Cones do NOT require RPEIsolated retina can regenerate pigment
• Cones can use 11 cis retinol to regenerate pigmentIf add 11 cis retinol, cones can convert this to 11 cis retinal and regenerate pigment
• In bright sun, need more 11-cis than can regenerate with known rod visual cycle
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Cone visual cycle is different - uses Muller cells
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Cone visual cycle
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Cone visual cycleDon’t yet know the isomerase
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Use ARAT to form retinal ester
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BIG DIFFERENCE - 11cis ROL goes back to cone outer segment where it is converted to 11-cis RAL
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Why have two visual cycles in same retina?
11-cis RAL to rods 11-cis ROL to cones
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Theories
• Deficiency / survival - independence of rod and cone pathways
• Sensitivity - so rods can detect lower levels of light
• Energy needs of cones are greater• Cones / rods process different information• Cones function at hi light, rods at low
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Cone pathway
• Cones work at higher light levelsOpsin bleached more rapidlyCone visual cycle is 20 times faster
• Muller cells send 11-cis retinol to conesRods can’t convert this so can’t “steal” from cone pathwayImportant for rod dominated retina
20-100x more rods than cones
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Macula - pigmented layers coincident with fovea
Macula is key to high quality vision
Where all the cones are!
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Gene mutations cause disease
ABCA4 mutations = Stargardt macular degeneration
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Stargardt macular degeneration
RecessiveShows up in people 7-12 yrs old
Most common juvenile macular degeneration
Loss of central vision Eventual blindness
Affects 1 : 10,000 children
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Gene mutations cause disease
RDH12 mutants = Leber congenital amaurosis
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Leber congenital amaurosis
• Discovered by Leber in 1800sCongenital - present from birthAmaurosis - vision loss without abnormal tissuePhotoreceptors don’t develop
• Affects 3 : 100,000 newbornsCaused by several genesRDH12
- 13 known mutations
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Gene mutations cause disease
LRAT mutations = retinitis pigmentosa or Leber congenital amaurosis
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Many causes of Retinitis pigmentosa
Pigmentation and loss of photoreceptors through time
Usually starts with night blindness
2 LRAT mutations S175R 2 bp del 396AA
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Gene mutations cause disease
RPE65 mutations = Leber congenital amaurosis
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Gene mutations cause disease
RGR mutations = recessive retinitis pigmentosa
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Gene mutations cause disease
CRALBP mutations = recessive retinitis pigmentosa
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Mutations in visual cycle genes cause 3 types of retinal disease
• Stargardt macular degenerationABCA4
• Leber congenital amaurosisRDH12, RPE65, LRAT
• Retinitis pigmentosaLRAT, RGR, CRALBP
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Online mendelian inheritance in man (OMIM)
• NCBI database of genetic basis of disease http://www.ncbi.nlm.nih.gov/sites/entrez?db=omim
• Describes gene causing diseaseHow discoveredGene function
• Keep track of mutations identified to cause disease
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Mouse knockouts for all key genes
• Stargardt macular degenerationabca4-/-
• Leber congentical amaurosisRdh12-/- rpe65-/-
• Retinitis pigmentosaLrat-/- rgr-/- cralbp-/-
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Retinal disease
• Have isolated gene mutants in rod pathway• Developed animals models
Have gene mutation built inUseful to follow progression of diseaseUseful to test different treatments
• Will discuss treatment options in a few lectures