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