Rhodopsin Expression Level Affects Rod Outer Segment Morphology and Photoresponse Kinetics

Clint L Makino,Norman A Michaud,Xiao-Hong Wen,Emmanuele Dibenedetto,Henry I Covington,Heidi E Hamm,Janis Lem,Giovanni Caruso,Karl-Wilhelm Koch

doi:10.1371/journal.pone.0037832

Abstract

BackgroundThe retinal rod outer segment is a sensory cilium that is specialized for the conversion of light into an electrical signal. Within the cilium, up to several thousand membranous disks contain as many as a billion copies of rhodopsin for efficient photon capture. Disks are continually turned over, requiring the daily synthesis of a prodigious amount of rhodopsin. To promote axial diffusion in the aqueous cytoplasm, the disks have one or more incisures. Across vertebrates, the range of disk diameters spans an order of magnitude, and the number and length of the incisures vary considerably, but the mechanisms controlling disk architecture are not well understood. The finding that transgenic mice overexpressing rhodopsin have enlarged disks lacking an incisure prompted us to test whether lowered rhodopsin levels constrain disk assembly.Methodology/Principal FindingsThe structure and function of rods from hemizygous rhodopsin knockout (R+/−) mice with decreased rhodopsin expression were analyzed by transmission electron microscopy and single cell recording. R+/− rods were structurally altered in three ways: disk shape changed from circular to elliptical, disk surface area decreased, and the single incisure lengthened to divide the disk into two sections. Photocurrent responses to flashes recovered more rapidly than normal. A spatially resolved model of phototransduction indicated that changes in the packing densities of rhodopsin and other transduction proteins were responsible. The decrease in aqueous outer segment volume and the lengthened incisure had only minor effects on photon response amplitude and kinetics.Conclusions/SignificanceRhodopsin availability limits disk assembly and outer segment girth in normal rods. The incisure may buffer the supply of structural proteins needed to form larger disks. Decreased rhodopsin level accelerated photoresponse kinetics by increasing the rates of molecular collisions on the membrane. Faster responses, together with fewer rhodopsins, combine to lower overall sensitivity of R+/− rods to light.

Highlights

The outer segment of a retinal rod is an elaborate sensory cilium that is highly specialized for transducing light into an electrical signal, reviewed in [1]
We studied the rods of hemizygous rhodopsin knockout (R+/2) mice, which express half the normal amount of rhodopsin [8], [12]
In the same field, some rods were sectioned at the inner segment while others were sectioned at the outer segment

Summary

Introduction

The outer segment of a retinal rod is an elaborate sensory cilium that is highly specialized for transducing light into an electrical signal, reviewed in [1]. R+/2 mouse rods, expressing half the normal amount of rhodopsin, had outer segments that differed from those of WT in three ways.

Results

Conclusion