Visual Cycle And Phototransuction

Visual cycle (the conversion of 11-cis-retinal to all-trans-retinal and all-trans-retinol)

• Light exposure initiates the visual cycle (sensory transduction of the visual system). It acts like a second messenger system in reverse

• In the resting state (dark): Na channels are held open by cGMP, keeping the photoreceptor outer segment in a state of depolarisation

• Photon energy is absorbed by 11-cis-retinal (the ligand of rhodopsin) which converts to the stable all-trans-retinal form

• All-trans-retinal activates rhodopsin which is converted to bathorhodopsin and then lumirhodopsin

• The isomerisation of 11-cis-retinal to all-trans-retinal is known as bleaching

• All-trans-retinal converts to all-trans retinol which does not fit within the rhodopsin transmembrane loops and thus rhodopsin is converted to opsin during bleaching. 

• All-trans-retinol diffuses away to be absorbed by the RPE

• Lumirhodopsin becomes metarhodopsin I and then metarhodopsin II
• The conversion of metarhodopsin I to metarhodopsin II is the only reversible step
• Metarhodopsin II decays to metarhodopsin III by releasing all-trans-retinal

Phototransduction

• Occurs within photoreceptor outer segments
• 1 photon is needed to trigger the process
• Outer segments remain depolarised by open cGMP controlled Na channels
• Sodium and calcium enter the cell
• Light stimulation starts the visual cycle producing metarhodopsin (aka enzyme R*)

• A quantum of light breaks the 11-cis double bond of retinal and opsin undergoes conformational changes

• The metarhodopsin II molecule is the activated state and begins a cascade that controls cation flow into the outer segment

• One molecule of metarhodopsin II activates hundreds of molecules of transducin (thus amplifying the reaction)

• Transducin: a G-protein which dissociates into subunits
• Alpha subunit activates rod phosphodiesterase (rod PDE)
• Rod PDE hydrolyses cGMP to 5’-GMP
• One molecule of PDE can hydrolyse 600 molecules of cGMP (further amplification)
• The target is the cGMP-gated cationic channel on the outer segment membrane.
• cGMP is hydrolysed and its concentration decreases 
• Therefore Na channels close, preventing sodium entry
• This leads to hyperpolarisation (the cell develops negative charge)

• Hyperpolarisation leads to closure of voltage-regulated calcium gates and reduced calcium influx

• Hyperpolarization prevents glutamate release from the synaptic terminal

Inhibition of photocascade

• Can occur at many stages
• When the light goes off, the rod returns to its dark state
• Rhodopsin can be inactivated by phosphorylation or binding to arrestin
• Activated PDE can recombine with its gamma subunits

• The transducin alpha subunit is inactivated by hydrolysis and rejoins its beta-gamma complex

• Reduced calcium influx during phototransduction in light conditions stimulates recoverin activity which activates guanylate cyclase

• Guanylate cyclase replenishes cGMP to reopen ion channels

Cone phototransduction

• Cone phototransduction is comparatively insensitive compared to rod, but faster and can adapt to ambient illumination

• Greater light means faster and more accurate cone response
• Cones show neurally-mediated negative feedback
• Horizontal cells synapse antagonistically back to cones
• They release GABA: an inhibitory neurotransmitter 
• This protects the cone from being overloaded

• Hence cones have a greater flicker fusion frequency by turning off rapidly so they can respond to new stimuli faster

Vitamin A/retinol metabolism and recycling

• After releasing from opsin, all-trans-retinal is recycled differently by rods vs cones

• Cone chromophores are reisomerised within the retina to regenerate 11-cis-retinal which recombines with bleached rhodopsin

• Rod pigments are converted to all-trans-retinol within the retina by retinol dehydrogenase

• All-trans-retinol is then transported by interphotoreceptor retinoid binding protein (IRBP) to the RPE

• IRBP produced y photoreceptors accounts for 70% of soluble protein in the interphotoreceptor matrix (space between outer segments and RPE)

• In RPE, all-trans-retinol is isomerised (in the dark) to 11-cis-retinol, then bound to CRALBP to become 11-cis-retinal-CRALBP and transferred back to the photoreceptors on IRBP to reattach to rhodopsin

• IRBP’s main function is the transport of retinoids between photoreceptors and the RPE and protects the plasma membranes from damaging from high retinoid concentrations

• Note: all-trans-retinol can also enter the RPE from the choriocapillaris

• Vitamin A:

• A provitamin in the yellow and red carotenoid pigments in vegetables (eg. carrots), liver, fish oils, dairy

• Fat-soluble (thereforereduced availability in malabsorption syndromes)
• Other fat-soluble vitamins are: D, E and K.
• Stored in the liver as retinyl ester and hydrolyzed to retinol
• Retinol is combined with serum retinol binding protein to deliver to RPE
• Essential for corneal and conjunctival health
• Required for epithelial keratin expression and glycoprotein synthesis
• Essential for the inhibition of proteolytic enzymes

• Bitot’s spot: corynebacterium xerosis release gas as a byproduct and produce foamy conjunctival lesions