Phototransduction Flashcards
What is contain in the outer segment of a retinal rod cell?
Pigment containing discs
What channels are important in rod cells?
Na+ K+ ATPase
Na+ K+ Ca2+ exchange protein
CGMP regulated ion channel
What does the Na+K+Ca2+ exchange channel do?
Moves sodium in and potassium and calcium out
Decreases intracellalar calcium
Powered by sodium potassium ATPase gradient set up
What does the CGMP channel regulate?
When cGMP bound it permits sodium and calcium influx = depolarisation
Wat does the balance of calcium influx and efflux achieve?
Intracellaul are calcium level of about 500nM
Memebrane is slightly depolarised (-40mV) allowing glutamate release to bipolar neurons
The cells in the dark state is depolorised…what does this allow form?
Tonic release of glutamate
What I’d the GPCR which in embedded into the discs?
Rhodopsin
Rhodopsin is made from what?
The GPCR opsin
Covalently bound prosthetic group (11-cis-retinal)
What does light do to Rhodopsin?
Converts the inverse agonists (11-cis-retinal) to the full agonist (trans-retinal)
Causes a conformational change in GPCR which increases the guanine nucleotide exchange factor activity
The is the G protein associated with rhodopsin?
Transducin (Gt)
Describe the pathway why which a photon decreases the amount of cGMP in a cell
Activation of Rhodopsin causes GDP for GTP exchange on transducin which causes alpha subunit to lose affinity for BY subunits.
Gat then activates PDE6 which was sequestered by an inhibitory gamma 2 subunit and converts it to PDE(alpha,beta)
Activated PDE6 then cleaves cGMP to 5’GMP
What is the effect of cGMP cleavage?
Decreases cGMP gated ion channel activation. This then causes the channel to close which decreases sodium and calcium influx = hyperpolarization
What does the concentration of calcium drop to following cGMP regulated ion channel closure? What does this do no NT release
100nM
Decreases it
How is the signal amplified in this system?
Depends on the ambient level of light (low light = low amplification)
1Rh can activate >10 transducin molecules
Gat can then activate several PDE6 (1500 fold increase in PDE6 activity)
Therefore, 1 photon can hydrolysed >10^5 cGMP molecules
How long does it take for phototransduction recovery
200ms
What 3 processes control recovery of phototransduction?
Inactivation of rhodopsin
Inactivation of transducin
Resynthesis of cGMP
How long is Rh* activated for?
Approx 40ms therefore can only activate 10-15 transducins
What GRK phosphorylates rhodopsin and where does this occur?
Disc membrane bound GRK1
C terminal
How many time of Rh* phosphorylated and what effect does this have?
Three times
Each time partially decreases the rate at which Rh* can activate transducin
What occurs following GRK1 phosphorylation?
Arrestin 1 binds which sterically inhibits further GEF activity
How is retinal replaced?
All transretinal moved into another cell which contains enzyme for 11 cis retinal formation.
Then moved back into rod cell for covalent attachment to opsin
What does RGS stand for?
Regulators of G protein signalling
What do RGS proteins do?
They have GTPase activating activity so increase the intrinsic GTPase activity of the G alpha subunit
What RGS protein aids in the deactivation of transducin?
RGS9-1 which is found in high concentrations in rod cells
When RGS9-1 causes hydrolysis of GTP what occurs then?
Alpha subunit regains affinity for beta gamma subunit
CGMP synthesis is prototeional to what?
1/(Ca2+)i
What stimulates particulate guanylyl cyclase?
Guanylyl cyclase activating protein (GCAP)
What inhibits GCAP?
High intracellular calcium
What a re the common GCAP isoforms?
GCAP 1+2
GCAP 1 has low affinity for calcium then GCAP 2. therefore calcium dissociates quicker from GCAP 1 =activated first
How does light regulate GCAP function?
Light causes a decrease in intracellaul calcium
Calcium released by GCAP therefor is release from inhibition
GCAP activates pGC
Makes cGMP which opens cGMP gated ion channels which increases calcium again
GCAP inhibited
What is Rod cell adaption?
Process by which rod cells adapt to different level of background illumination. This allows us to detect/report visual stimuli over a range of background illumination
How does Rod cell adaption occur?
Likely to involved 3 processes
- Changes in Ca2+ in outersegment
- Changes in phosphorylation state of rhodopsin
- Translocation of key phototransuction components between inner and outer segments
What proteins are involved in regulating basal calcium in outer segment?
Recoverin
GCAP
Calmodulin
What is the role of recoverin?
When recoverin is associated with calcium it is bound of rhodopsin kinase which prevents rhodopsin kinase activity
How does recoverin permit adaptation?
Light = decrease in calcium = recoverin not bound to calcium and does not inhibit GRK1 = Rh phosphorylated
Longer and larger changes in light cause a larger decrease in calcium therefore more recoverin inhibition = more GRK1 activity
How does GCAP permit adaption?
Levels of calcium dictate activity of GCAP. The greater the extent of calcium decrease the more pGC activation and the more cGMP made
What is the role of calmodulin in rod cells?
When abound of calcium, calmodulin calcoum complx binds to cGMP gated ion channel and inhibits it
How does calmodulin regulate adaption?
Light decreases Ca2+
This means calmodulin releases its bound calcium and unbinds from cGMP gated ion channels
This increase channel sensitive to cGMP. Thus, light adapted cells can operate cGMP regulated channels at lower cGMP levels
What is the sequence of residues phosphorylated by GRK1?
S343
S338
S334
How does the phosphorylation state of Rh reflect adaptation?
Increases phospho reflects levels of illumination which means there are varying levels of transducin activation for different phosphorylation states (less phospho = more Gat activation)
Dictates level of amplification
Rh may be mono/di/tri phosphorylated
What protein may move into the outer segment following light?
Arrestin
RGS9-1
These act to decrease amplification
Which proteins move out of outersegment?
Gat (10 fold decrease in application)
Recoverin (GSK1 not inhibited)
This will decrease application
What are some characteristics of protein translocation?
Occurs via diffusion
Takes serval minutes
Permits change Ina biliary of phototransduction cascade to amplify and sufficiently shut off signal
In terms of stimulus what is the difference between GPCRs involved in neurotransmission and GPCRs involve din vision?
Neurotransmitter vs photon
In terms of receptor activation what is the difference between GPCRs involved in neurotransmission and GPCRs involved in vision?
Change in confirmation for both except one is activate by attached prosthetic group
In terms of G protein responce activation what is the difference between GPCRs involved in neurotransmission and GPCRs involved in vision?
Both bind GTP
In terms of secondary messenger change what is the difference between GPCRs involved in neurotransmission and GPCRs involved in vision?
Increase in secondary messenger for NT GPCRs
Decrease in secondary messenger for visual GPCR
In terms of ion channel responce what is the difference between GPCRs involved in neurotransmission and GPCRs involved in vision?
Increase or decrease in conductance for NT GPCR
Decrease in na+ conductance for vision GPCR
In terms of GPCR characteristics what is the difference between GPCRs involved in neurotransmission and GPCRs involved in vision?
NT GPCR have no group attached which are important for activation
Prosthetic group act as full agonist
