Post on 24-Feb-2016
description
Lecture 15
3/26/13Invertebrate phototransduction
Drosophila visual transduction
Invertebrate phototransduction
• Invertebrate vision starts with absorption of photon by opsin
Drosophila eye• 800 ommatidia
• Single ommatidiumCO corneal lenspsC pseudoconePC pigment cellCC cone cellR photoreceptor cells
Rhabdomeres are microvillar cells containing different opsins: R-opsin
R1-6 like rods: sensitive+slow R7-8 like cones: high acuity, color
Drosophila mutant screen
• Model systemScreen for mutants which have visual defectFind gene causing defectPiece together genes that are important part of visual pathway
Earliest mutant screensPhototaxis behavior
Fly goes to the light if eyes are functioningMutants do not
Earliest mutant screensPhototaxis behavior
Fly goes to the light if eyes are functioningMutants do not
Electroretinograms, ERGs• Send light pulse to eye
• Wire outside of retina records voltage coming from whole eye response
• Sum of all neural signals in eye
Light coincident receptor potential
Electrical response to light flash
5 s pulse
Light coincident receptor potential
Norp - no receptor potential
Trp - transient receptor potential
Drosophila mutants
5 mV
5 s pulse
Identified > 40 genes from Drosophila mutants
A few are:
• norpA - phospholipase C• ninaA - rhodopin transporter• ninaC - protein kinase• ninaE - rhodopsin• Trp - cation channel
Response begins with opsin bound to 11-cis retinal
Rhodopsin gets turned off by being bound by arrestinSame as vert phototransduction
Phototransduction pathway
Phototransduction pathwayTRP-TRPL likely heteromeric channels
Comparison of vert and invert phototransduction
Vertebrate / Ciliary
Invertebrate / Microvillar
Initiation Light activates opsin which turns on G protein
G protein Gt Gq
Effector Phosphodiesterase
Phospholipase C
Channel Closes in light Opens in light
Ca+2 levels Decreases Increases
Cell Hyperpolarizes Depolarizes
Neurotransmitter
Decreases Increases
Where do these pathways come from?
Relationship of bilateria
Vert and invert vision
• Did they evolve at same time?
Did ciliary and rhabdomeric pathways evolve at same time?
Science 2004
Both ciliary and rhabdomeric cells in worm
C-opsin in brain
R-opsin in eyes
ae = adult eye
le = larval eye
Brain with cPRC – cilated photoreceptor cells
So worm has both types of photoreceptors!!
Vertebrate camera eye
Rods and cones
C-opsin and r-opsin at least to here