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Photoreceptor Cells In Humans

-rods: responsible for detecting objects and motion
-cones: responsible for detecting color



-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.


Opsins in Cones

-capture light in red, blue, and green wavelengths
-each cone receptor will express only one type of opsin molecule


Ratio of Cone Cell Colors

-not 1:1:1
-always more red cones than green cones than blue cones
-stochastic ratio of cone cell but they're positioned randomly


Photoreceptor Domains

-structurally divided into domains
-region of cell responsible for capturing light is outer segment which is made of hundreds of membrane stacks


Opsin Protein and Capturing Light

-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.



-opsin-retinal complex
-can absorb photons of different light wavelengths
-loss of individual opsin genes can lead to various forms of color blindness



-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


Photoreceptors in low light

-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


Photoreceptors Exposed to Light

-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


Rhodopsin Genes in Drosophila

-insect retina=simple nervous system with stereotyped developmental pattern and limited number of cell types


Charles Zuker and Joseph O'Tousa

-coned first invertebrae rhodopsin (Rh1)
-absorbs light in orange spectrum
-essentially used as motion detector


Charles Zuker, Karl Fryxell, and Craig Montell

-cloned Rh2, Rh3, and Rh4 rhodopsin genes
-absorb light in ultraviolet spectrum


Claude Desplan and Steve Britt

-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


Structure of Drosophila Retina

-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


Photoreceptors of Fly Retina

-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



-part of insect photoreceptors
-homologous to rod outer segments of vertebrate photoreceptors
-contain all phototransduction machinery including rhodopsin proteins


Rh1 protein

-expressed in six outer photoreceptors
-absorbs light in orange spectrum
-required for motion detection



-not expressed in photoreceptors of compound eye
-found within photoreceptors of three simple eyes (ocelli) that sit at vertex of fly head


Rh3 and Rh4

-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


Rh5 and Rh6

-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


Pattern of Rhodopsin Expression

-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


Color Blindness

-loss of individual rhodopsin genes can lead to various forms of color blindness


Pseudo-Isochromatic Plate (PIP) Test

-used to determine if patient is color blind


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


Loss of Rh1 in Flies

-leads to retinal degenration of R1-R cells
-rhabdomeres completely lost in such mutants


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)


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


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


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