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754<br />

PHYSIOLOGY<br />

Opsins: Not Just for Eyes<br />

Studies in invertebrates are enriching our sense of how versatile and ancient these<br />

light-sensitive proteins are<br />

When researchers got their fi rst glimpse of<br />

the sea urchin genome in 2006, they were surprised<br />

to fi nd genes for opsins, light-sensitive<br />

proteins without which vision as we know it<br />

today would be impossible. Living in the subtid<strong>al</strong><br />

zones, sea urchins are not only eyeless,<br />

but <strong>al</strong>so headless, and, ostensibly brainless.<br />

They seemed to lack the speci<strong>al</strong>ized photoreceptor<br />

cells that house opsins in the eyes<br />

of other anim<strong>al</strong>s. “Nobody knew what [the<br />

opsins] were for,” rec<strong>al</strong>ls Maria Ina Arnone,<br />

a development<strong>al</strong> biologist who has long<br />

studied sea urchins at the Stazione Zoologica<br />

Anton Dohrn in Naples, It<strong>al</strong>y, one of the<br />

world’s oldest marine labs.<br />

Arnone’s preliminary an<strong>al</strong>yses suggested<br />

an opsin gene was active at the base<br />

of the tube fe<strong>et</strong>, the tiny projections located<br />

in and around urchin spines. And in 2011,<br />

her group showed that these tube fe<strong>et</strong> were<br />

loaded with photoreceptor cells that had<br />

been missed because they lack pigment typic<strong>al</strong>ly<br />

associated with opsins.<br />

That work and other recent studies have<br />

driven home the fact that a wide vari<strong>et</strong>y<br />

of organisms don’t need tradition<strong>al</strong> eyes<br />

to make use of opsins, and that opsins can<br />

likely sense more than light. Last month, at<br />

the annu<strong>al</strong> me<strong>et</strong>ing of the Soci<strong>et</strong>y for Integrative<br />

and Comparative Biology (SICB) in<br />

San Francisco, Arnone and other researchers<br />

reve<strong>al</strong>ed the rich history and unexpectedly<br />

broad utility of these proteins. In fruit fl ies,<br />

for example, they may be involved in hearing.<br />

“Opsins can be expressed in many more<br />

tissues than the simple eye,” Arnone says.<br />

Beyond eyes<br />

Hints that opsins existed outside the eye<br />

started dribbling in <strong>al</strong>most 25 years ago, with<br />

suggestions that fish skin and dove brains<br />

contained the molecules. Among<br />

the first to pin down an extraocular<br />

opsin protein were Ignacio<br />

Provencio and Mark Rollag<br />

at the Uniformed Services University<br />

of the He<strong>al</strong>th Sciences in<br />

B<strong>et</strong>hesda, Maryland, and their<br />

colleagues. They knew that pigment<br />

cells in amphibian skin reacted to light<br />

and eventu<strong>al</strong>ly isolated an opsin in frog skin<br />

that they named melanopsin in 1998. Until<br />

then, researchers thought there was one kind<br />

of opsin in vertebrates, c<strong>al</strong>led ciliary opsin,<br />

and another, more ancient kind, rhabdomeric<br />

opsin, in invertebrates. But though melan-<br />

15 FEBRUARY 2013 VOL 339 SCIENCE www.sciencemag.org<br />

Published by AAAS<br />

opsin was found in a frog, it looked<br />

more like the invertebrate opsin.<br />

Provencio and Rollag <strong>al</strong>so found<br />

melanopsin in the frog eye and brain<br />

and over the next few years, a fl urry<br />

of papers teased out which other vertebrates<br />

possess the protein and provided<br />

clues to melanopsin’s function.<br />

Researchers have found it in mouse<br />

and human neur<strong>al</strong> tissue, for example,<br />

and in some anim<strong>al</strong>s, it helps<br />

establish circadian rhythms (Science,<br />

20 December 2002, p. 2297).<br />

Melanopsin hinted at an underappreciated<br />

complexity of opsins.<br />

And a 2003 survey of opsins throughout<br />

the anim<strong>al</strong> kingdom by D<strong>et</strong>lev<br />

Arendt from the European Molecular<br />

Biology Laboratory in Heidelberg,<br />

Germany, drove home that<br />

both rhabdomeric and ciliary opsins<br />

are ancient and that the invertebrate/<br />

vertebrate divide for these types<br />

doesn’t hold up. But the search for<br />

opsins in anim<strong>al</strong>s other than vertebrates<br />

and insects and in places other<br />

than eyes didn’t re<strong>al</strong>ly take off until recent<br />

advances in DNA sequencing made it possible<br />

to probe the genomes of a wide vari<strong>et</strong>y<br />

of organisms, such as the sea urchin.<br />

Arnone has slowly homed in on how<br />

opsins help this simple creature use light<br />

and “see.” These spiny echinoderms tend<br />

to avoid direct sunshine. Some will cover<br />

themselves with debris; others move under<br />

rocks in search of shadow. Researchers<br />

have shown some that sea urchins can even<br />

distinguish different shaped objects. Last<br />

month at the SICB me<strong>et</strong>ing, Arnone proposed<br />

that the opsin photoreceptor cells in<br />

the sea urchin are positioned at the base of<br />

the tube fe<strong>et</strong> such that they lie parti<strong>al</strong>ly in the<br />

shadow of its c<strong>al</strong>cite skel<strong>et</strong>on,<br />

Online<br />

sciencemag.org<br />

Podcast interview<br />

with author Elizab<strong>et</strong>h<br />

Pennisi (http://scim.ag/<br />

pod_6121).<br />

Light sensors. Opsin-laden photoreceptor<br />

cells (green, red) provide juvenile sea<br />

urchins a means to d<strong>et</strong>ect light.<br />

<strong>al</strong>lowing the skel<strong>et</strong>on to serve<br />

the same purpose as pigment<br />

in typic<strong>al</strong> eyes—most opsins<br />

co-occur with pigment, which<br />

shields part of a photoreceptor<br />

cell so it can register the direction<br />

of incoming light. She has<br />

<strong>al</strong>so shown that the photoreceptor cells connect<br />

to the fi ve radi<strong>al</strong> nerves in the brainless<br />

urchin, which may enable the input from the<br />

different photoreceptor cells to be compiled,<br />

much like an insect’s compound eye does.<br />

In echinoderms, the opsin story is complex.<br />

The opsin at the base of the tube fe<strong>et</strong><br />

CREDIT: MARIA INA ARNONE/STAZIONE ZOOLOGICA ANTON DOHRN<br />

on February 14, 2013<br />

www.sciencemag.org<br />

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