The Scientist November 2018

58 THE SCIENTIST | the-scientist.com

LAB TOOLS

MISHRA ET AL.,

SCI

REP

, 8:901, 2018

T

ools that use light, drugs, or tem-

perature to make neurons fire or

rest on command have become a

mainstay in neuroscience. Thermogenet-

ics, which enables neurons to respond to

temperature shifts, first took off with fruit

flies about a decade ago, but is emerging as

a new trick to manipulate the neural func-

tioning of other model organisms. That’s

due to some advantages it affords over

optogenetics—the light-based technique

that started it all.

Genetic toolkits such as thermogenetics

and optogenetics follow a basic recipe: sci-

entists pick a receptor that responds to an

external cue such as temperature or light,

express the receptor in specific neurons as a

switch that changes the cell’s voltage—trig-

gering or inhibiting firing—and then use

the cue to turn the neural switch on or off.

Optogenetics revolutionized our

understanding of how the brain’s wir-

ing affects animal behavior. But it comes

with drawbacks. For one, delivering light

into the deepest regions of the brains of

nontransparent animals is a challenge.

In mice, this requires surgically insert-

ing optical fibers into the brain, tethering

the animal to the light source. Research-

ers working with adult fruit flies can cut

a window through the head cuticle to

access the brain. In both cases, the neces-

sary experimental setups are invasive and

often time and effort intensive.

Additionally, the light intensity

required for optogenetics tends to damage

tissue. “ Yo u pump a lot of light through

the optical fiber to activate neurons,” says

Vsevolod Belousov, a biochemist at the

Russian Academy of Sciences in Moscow

who develops thermogenetic tools. “In

general, this is not avoidable.”

Thermogenetics allows neuroscien-

tists to sidestep these issues by harness-

ing proteins that respond to changes in

temperature with stronger levels of acti-

vation than light-triggered switches, with

less-invasive stimulus delivery. Unlike the

light receptors of optogenetics, however,

most of the thermoreceptors currently in

use only allow researchers to turn neurons

on, but not off; and their use in rodents is

still evolving.

Here, The Scientist lays out the current

state of the thermogenetic toolbox for dif-

ferent model organisms.

TURNING THE HEAT ON

FRUIT FLIES

Thermogenetics owes its humble begin-

nings to Drosophila research and is by far

most developed for use in fruit flies. In the

early 2000s, Toshihiro Kitamoto first used

a mutated form of a protein called shibire

to shut down synaptic communication in

specific fly neurons at temperatures above

29° C. Shibire is an enzyme in the dyna-

min superfamily, which is involved in ves-

icle formation; its mutant version inhibits

chemical transmission in a wide range of

neurons within a few minutes of a temper-

ature hike. But because dynamins affect

many cellular processes, the use of shibire

can have far-reaching, nonspecific effects.

Another class of proteins called thermo-

TRPs is more suitable for thermogenet-

ics. ThermoTRPs are cation channels of

the transient receptor potential family

that normally mediates temperature pref-

erences, both in the brain and elsewhere

in the body. These channels respond dra-

matically to temperature shifts as small as

1°–2° C. The Drosophila TrpA1, for exam-

ple, turns on slightly above 25° C, and the

rat TRPM8 turns on just below 25° C.

When scientists first expressed

TrpA1 in the motor neurons of fruit flies,

they found that heating up the cells par-

alyzed the animals. “We got warm water

and started dunking them in and that

just made them pass out,” says Paul Gar-

rity, a biologist at Brandeis University

who pioneered the use of thermoTRPs.

“It was like a magic trick.”

TRP channels also conduct ions

very efficiently, says Belousov—at about

1,000-fold higher flux than the ion chan-

nels used in optogenetics. This means

that thermoTRPs can drive robust acti-

vation at low expression levels, reducing

toxic effects of overexpressing proteins.

Fruit fly researchers have yet another

option: Gr28bD, a gustatory receptor that

was recently found to respond to heat in

Thermogenetics brings neural circuits into focus.

BY DEVIKA G. BANSAL

Temperature as Tool

A TAST E OF TEMPERATURE: Adult Drosophila

ventral nerve cord motor neurons expressing the

gustatory thermoreceptor Gr28bD (left); in gray

is a 3D reconstruction of the motor neurons, in

green is neural activity in response to heat, with

the green trace showing calcium currents in a

single neuron (right).