Lecture 12 - Photosensation and Vision

Question 1

What is phototransduction?

Answer 1

‘The transduction of light into a sense’

• light hits neurons (or equivulent) and an action potential is induced
• many species respond to light/it’s absense (plants, bacteria, animals)
• can be occular or extraoccular

Question 2

What is light required for in an organism?

Answer 2

• circadian rhythm
• vitamin production
• metabolism
• escape responses

Question 3

What is vision?

Answer 3

‘The faculty or state of being able to see’

• includes colour, shape, movement, and dimension
• usually occular and a decidcated structure is required (e.g. lens)

Question 4

What animals use basic photosensation and what does this involve?

Answer 4

Barnacles

• if a shadow is cast over they retract (predator evasion)
• vision confined to the perception of this shade
• involves a primitive eye

Question 5

What are the features of a primitive eye?

Answer 5

• simple
• often respond to the strength of light for a simple reflex response or phototascic responses
• light response is mediated through a reticular cillia structure

Question 6

What is the reticular cillia structure?

Answer 6

structure through with a light response is mediated in a primitive eye
-through a membrane rich cillium or rhobdmere, i.e. the light collecting point of a villa (with neurons extending below)

Question 7

What is the structure of photoreceptors?

Answer 7

-similar to neuroepithelial cells
-2 major differences (based on the orientation of polarity):
Rhabdomeric or Cilliary
-drosophila are rhabdomeric
-normally embedded within larger structure in multiples to give complex vision

Question 8

What is the conserved gene between humans and insects for initiating eye development?

Answer 8

Pax6(human)/eyeless(insects)

Question 9

How was the conserved natures of Pax6 and eyelesss exhibited experimentally?

Answer 9

• Pax6 expressed in the imaginal disks (discs of epithelial tissue that generate all external structures of the adult drosphila in metamorphesis) of drosophila larvae
• RESULT
• all external structures of the fly contained ectopic eye related structures

Concl. = Pax6 and eyeless are developmentally conserved and all eyes have been derived from similar primitive evolutionary tract

Question 10

What anatomical structures of the eye are likely to have been conserved between fly and human? And what differences are there in the structure?

Answer 10

• cornea, iris cell and cone cells
• photoreceptor cells
• both have optic nerves at the back

Differences
Insect-light is channeled down through the eye
Vertberates- light focused at the back

Question 11

As organisms have evolved to be more complex, what is shown by photoreceptor anatomy?

Answer 11

Photo receptors show an increasing development of sophistication

• can see in different dimensions
• convex eyes can harvest more light than concave (but most eyes are concave - essential to deal with intensity, to gain control and saliency of a signal)

Question 12

What is Pax6 and what is its mutant phenotype?

Answer 12

homeobox gene in mammals

• mutated pax6=> no iris or lens
• eyeless = homolog in insects

Question 13

What was the experiment on drosophila that exhibited rescue of sight in organisms with mutated eyeless?

Answer 13

When pax6 was expressed when a drosophila’s eyeless gene was mutated this could rescue the eye
-shows ancientness (sorry) - both eyes arose from same simple structure, with an ancester gene of eyeless/pax6

Question 14

What is rhodopsin?

Answer 14

AG protein coupled receptor

-light sensing molecule (ancient)

Question 15

What are the two types of opsin?

Answer 15

cilliary opsins

rhabdomeric opsins

Question 16

What are melanopsins?

Answer 16

light harvesting sensory proteins

-for the maintenance of the circadian clock not for vision

Question 17

What type of opsins do cridona use?

Answer 17

C-opsins

Question 18

What is are some of the main differences in vertebrate and insect vision?

Answer 18

Vertebrate
-rods hyperpolarise to light
-this inhibts the activity of the assicated neurons
Insects
-depolarise to light, turning the neuron ON
-rhodopsins are tuned to different frequencies and are expressed in different receptors

Question 19

What are the features of rods and cones in vertebrates?

Answer 19

Rods
-majority of cells in the vertberate eye
-detect the degree of light
-bleached by light
-hyperpolaraise to light (turns neuron off)
-rod sensitivity is determined by the amount of rhodopsin
-generally low sensitivity, used by vertebrates in dim situations
Cones
-sensitive to light but maintain function in high illumination (use pigment iodopsins - in iris)
-iris filters out light to see ambient light

Question 20

What are the features of -R and -C opsins?

Answer 20

• g protein coupled recptors
• r opsins act through phospholipase C
• C opsins act through phosphodiesterase
• both phopho- are downstream of the rhodopsins (are receptors themselves)
• both contain arrestins (2, 3 in r-opsins; 1 in c-opsins) which have inhibitory functions - block cycle of vision

Question 21

What are the features of drosophila phototransduction?

Answer 21

• TRP channels
• has lots of pigment to protect it from light
• photoreceptors cluster in groups of 7 (with one responding to UV) and are sensitive to different wavelengths
• contains rhadbomeres and submircrovillus C beneath (where Ca2+ released from to depolarise the photoreceptor)

Question 22

What experiment showed the involvement of DAG/glyerol and TRP channels in drosophila phototransduction?

Answer 22

• light sent to the photoreceptor under a cell ptch clamp to record the output
• 1 photon and 1 Ca2+ molcule was released and measured as a ‘bump’ - 20ms in duration and 10pA in amplitutde

This is the time for DAG/glycerol to accumulate and activate TRP channels

Question 23

What are the features of TRP channels involvedin drosophila phototransduction?

Answer 23

• normally present on the villi
• in light, TRPs shuttle down microtubules into submicrovilli (towards the cell body)
• TRPs in the form of heterodimers (TRPγ and TRPL)
• tightly associated with PKC (anchors)
• results in the initiation of a closely coupled molecular cascade

Question 24

What is the molecular casecade of phototransduction in drosophila?

Answer 24

1-photoisomerisation of rhodopsin to melarhodopsin activates heteromeric Gq - Gαq is released
2-Gαq activates phospholipase C, generating IP3 and DAG from PIP2. DAG also releases PUFAS by activation of DAG lipase
3-TRP and TRPL are activated by PUFAS and/or DAG. TRPs, PKC, PLC (+NINAC/inaS) form a signalplex
4-SMC Ca2+ stores are activated and an action potential is generated
5-DAG is converted to PA via DAG kinases and CDP-DAG by CD synthesis.
6-PI regenerated and transported back to microvillar membrane by PI transfer protein and converted to PIP2 which silences DAG.
7-Arrestin is then activated to silence Rhodopsin
This results in the cessation of signal

Question 25

What is the detailed process of silencing phototransduction?

Answer 25

1-20ms after absorption of a photo, metarhodopsin activates the G protein, which then activated PLC generating a membrane 2nd messenger and consequently reaching the threshold for activation of one channel (this is the ‘bump’)
2-then Ca2+ influxsensitises other channels, risiing the phase of the ‘bump’
3- then Ca2+ flood the submicrovillus leading to rapid inactivation and a refractory period
4-Ca2+ returns to normal within 100ms via Ca2+/Na+ exchanger system
5-TRP brings Ca2+ in, CalX takes it out as Na+ channel brings Na+ in
6-M, Gq and PLC are deactivated, PIP2 synthesised

Question 26

Explain how photosensation is actually caused by mechanotransduction, and what the evidence is for this

Answer 26

DAG enters the membrane and gates the TRP channel by changing the membrane structure (aka is mechanical, force activates the transduction and opens the channel)
-if the membrane is prevented from changing shape then the force generated is blocked and photoresponse is blocked

Question 27

Do vertebrates use TRP channels in vision?

Answer 27

No. No TRP channels in mammalian vision

Question 28

What is the mammalian phototransduction cascade?

Answer 28

1. Rhodopsin in the cones and rods catalyses the only light sensitive step in the reaction, through a 11-cis retinal chromophore, which lies in a pocket of rhodopsin and is isomerised to all-trans-retinal when light is absorbed
   2-the isomerisation leads to a change in the shape of rhodopsin which triggers a cascade of reactions that lead to a nerve impulse
   3-nerve impulse is transmitted to the brain via the optic nerve

Question 29

What are the features of the mammalian photoreceptors in the light and the dark?

Answer 29

• activated rhodopsins bind to transductin (a trimeric G protein)
• activated αtransductin can remove the inhibitory subunit of phosphodiesterase E
• PDE hydrolyses cGMP to GMP

Dark
-cGMP is high
-binds to cyclic nucletide gated channels
-gating these channels so they’re constantly open
-‘dark’ current is able to flow, releasing glutamate into horizontal and bipolar cells
Light
-light hits the receptor and hyperpolarises it
-cGMP is hydrolysed by PDE and so it can no longer gate the CNG channel so the channels close
-inhibits glutamate release
-bipolar cells relay this to the ganglian cells

Question 30

What are some variations in phototreansduction in different organisms?

Answer 30

• some organisms using the same opsins are depolariaed rather than hyperpolarised (channels getd differently)
• some use cAMP some cGMP
• some use chemicals e.g. PDE in different manners
• althrough the neuron maps are similarly comparable, in vertebrates there is more cross talk
• however in fly there is cross talk between the 2nd order neurons