Exam 4: Lecture 5 Flashcards
1
Q
Photoreceptor Cells In Humans
A
- rods: responsible for detecting objects and motion
- cones: responsible for detecting color
2
Q
Opsins
A
- proteins expressed in rods and cones
- belong to superfamily of proteins containing several transmembrane domains
- capable of capturing photons of light and in turn initiate transduction of light into electrical signals
- number of different opsin genes in human genome and each will encode opsin protein that can capture light of particular wavelengths
- contain extracellular domain, seven transmembrane segments and an intracellular domain.
3
Q
Opsins in Cones
A
- capture light in red, blue, and green wavelengths
- each cone receptor will express only one type of opsin molecule
4
Q
Ratio of Cone Cell Colors
A
- not 1:1:1
- always more red cones than green cones than blue cones
- stochastic ratio of cone cell but they’re positioned randomly
5
Q
Photoreceptor Domains
A
- structurally divided into domains
- region of cell responsible for capturing light is outer segment which is made of hundreds of membrane stacks
6
Q
Opsin Protein and Capturing Light
A
- light capturing protein opsin located in sacks of outer segment
- after translation opsin protein folded within ER, is modified by addition of sugar residues within Golgi and targeted to outer segment.
- needs cofactor to capture light
- bound to chromophore molecule called retinal.
7
Q
Rhodopsin
A
- opsin-retinal complex
- can absorb photons of different light wavelengths
- loss of individual opsin genes can lead to various forms of color blindness
8
Q
Phototransduction
A
- outer segments contain entire machinery required to transduce photons of light into electrical signals
- includes opsin proteins and attached chromophores, set of membrane associated and cytoplasmic factors and several ion channels
- capture of light by rhodopsin triggers opening and closing of channels changing electrical potential across membrane
- change in potential communicated to neurons in brain
- brain reconstructs electrical signals into image
9
Q
Photoreceptors in low light
A
- rhodopsin molecules not interacting with downstream phototransduction machinery
- downstream sodium channels are open allowing Na+ into cell
- balanced by movement of K+ out of cell va K channels
- more K+ exported than Na+ imported
- gives 40 mV voltage difference across membrane
10
Q
Photoreceptors Exposed to Light
A
- rhodopsin molecules activated and interact with phototransduction machinery
- cyclic GMP binds and closes Na channel thus blocking Na+ to enter cell
- K+ still allowed to exit cell
- causes a -70 mV difference across membrane
11
Q
Rhodopsin Genes in Drosophila
A
-insect retina=simple nervous system with stereotyped developmental pattern and limited number of cell types
12
Q
Charles Zuker and Joseph O’Tousa
A
- coned first invertebrae rhodopsin (Rh1)
- absorbs light in orange spectrum
- essentially used as motion detector
13
Q
Charles Zuker, Karl Fryxell, and Craig Montell
A
- cloned Rh2, Rh3, and Rh4 rhodopsin genes
- absorb light in ultraviolet spectrum
14
Q
Claude Desplan and Steve Britt
A
- cloned Rh5 and Rh6 rhodopsins
- absorb blue and green light
- have shown that flies can see in color
- don’t see well in red, but rather well in blue and green
15
Q
Structure of Drosophila Retina
A
- compound eye contains approximately 800 identical unit eyes (ommatida) packaged into hexagonal array
- each ommatidium contains ~20 cells divided into 8 photoreceptors (4 cones, 8 pigment cells)
- photoreceptor cells contain rhodopsin protein and are responsible for capturing and transducing light
- cone cells secret overlying lens and pigment cells optically insulate each ommatidium from its neighbors
- interspersed between ommatidia are mechanosensory bristles used to sense changes in air velocity and detect presence of foreign objects
16
Q
Photoreceptors of Fly Retina
A
- each ommatidium contains 8 photoreceptor (R) cells
- you will only see 7 in any one given plane
- R1-R6 cells extend entire depth of retina and R7 sits on top of R8.
- R7 is distal R8 is proximal
- occupy stereotyped positions within ommatidium and are arranged as asymmetric trapezoid
17
Q
Rhabdomeres
A
- part of insect photoreceptors
- homologous to rod outer segments of vertebrate photoreceptors
- contain all phototransduction machinery including rhodopsin proteins
18
Q
Rh1 protein
A
- expressed in six outer photoreceptors
- absorbs light in orange spectrum
- required for motion detection
19
Q
Rh2
A
- not expressed in photoreceptors of compound eye
- found within photoreceptors of three simple eyes (ocelli) that sit at vertex of fly head
20
Q
Rh3 and Rh4
A
- expressed in non-overlapping sets of R7 cells
- 70% of ommatids will have R7 cell that expresses R3
- remaining 30% of ommatidia will have R7 cell that contains Rh4 protein
- capture light in ultraviolet spectrum
21
Q
Rh5 and Rh6
A
- expressed in non-overlapping sets of R8 cells
- 70% of ommatidia will have R8 cell that expresses Rh5
- remaining 30% of ommatidia have R8 cell that contains Rh6 protein
22
Q
Pattern of Rhodopsin Expression
A
- if ommatidium contains R7 cell that expresses Rh3, then R8 cell will express Rh5
- if ommatidium contains R7 cell that expresses Rh4 then R8 cell will express Rh6
23
Q
Color Blindness
A
-loss of individual rhodopsin genes can lead to various forms of color blindness
24
Q
Pseudo-Isochromatic Plate (PIP) Test
A
-used to determine if patient is color blind
25
Retinal Degeneration
- other phenotype
- followed by an expansion of overlying retinal pigment epithelium
- called retinis pigmentosa
- characterized by pigmentation of eye
- decreases field of view due to invasion of retinal pigment epithelium ultimately leading to blindness
26
Loss of Rh1 in Flies
- leads to retinal degenration of R1-R cells
| - rhabdomeres completely lost in such mutants
27
Sevenless Pathway and R7 Development (Experiment)
- R7 cell expresses rhodopsins that can absorb light from ultraviolet spectrum
- Donald Ready set up T maze and let flies chose different paths
- end of one path, normal visible light
- end of other path, ultraviolet light
- wild type flies choose ultraviolet light 100% of time
- interested in genes controlloing light prefrence
- introduced random mutations and allowed them to choose again
- most mutants went to ultraviolet indicating that flies were not affected in ability to discriminate between light types
- few mutant flies chose visible light thus indicating they are insensitive to UV light (can't detect it)
28
Thoughts on Light Experiement
- thought these mutations would be in genes that encoded members of phototransduction machinery
- noticed that R7 cell was completely missing
- called mutant sevenless
29
Sevenless RTK Pathway
- after discovery that sevenless mutation resulted in failure of R7 cell to develop, large number of research groups set out to clone sevenless gene and determine which protein is encoded
- Utpal Baneriee, Gerald Rubin and Ernst Hafen determined sevenless protein was a RTK
- wanted to identify all cytoplasmic and nuclear proteins downstream of receptor
- did screnes similar to Don Rady
- over decade identified Ras, Raf, MEK, MAPK and several TFs
- each case, loss of these genes leads to loss of R7
- each of these genes subsequently shown to be expressed in R7 cell
30
Bride of Sevenless (Boss) Ligand
- Larry Zipursky identified mutation like sevenless that resulted in loss of R7 cell
- cloned gene and determined expression pattern
- unlike other pathway members of this gene not expressed in R7 cell
- expressed in R8 cell
- suggests that this gene is actually a ligand called Boss since ligand is bound to sevenless receptor
- comparison of sevenless expression indicated that it is expressed in not just R7 cell but also R3, R4 and the four cone cells
- only presumptive R7 cell actually becomes an R7. Why?
- of sevenless expressing cells the only one that is contact with R8 cell is presumptive R7
- since only cell that becomes R7 suggests that Boss ligand is tethered and functions through justacrine signaling
- biochemically proven
