“The anatomical definition is
superb,” says Rockefeller University’s Peter Mombaerts, watching a computer monitor as tiny blobs of green and
red appear, then intensify over a black-and-white image of a mouse
brain. “We can identify each of the structures
beautifully.”
Mombaerts and research associate Thomas Bozza
are reviewing a movie of the brain of a living mouse as it is
exposed to odors. Each time the mouse gets a whiff of a chemical
called hexanal, which smells something like freshly cut grass,
specific sections of the mouse’s brain light up. Mombaerts
and Bozza are watching as mice, quite literally, process scent.
Making this possible is a technique, developed
by Mombaerts, Bozza, and colleagues at Boston University, to
monitor the flow of information from one brain cell to the next.
The scientists can essentially see the tiny bursts of light as
messages pass from one neuron to the next. The technique allows
researchers to see 10 to 20 percent of the 2,000 structures in the
brain’s olfactory bulb, which receives smell information from
the nose (the other structures are too deep within the mouse brain
to be seen).
The system promises to advance research on how
animals, as well as humans, sense odors, and it may also help
scientists develop new drugs that target a family of molecules in the brain known as G
protein-coupled receptors (GPCRs). About a third of drugs on the
market today, from Zyprexa (for schizophrenia) to Claritin (for
allergy relief), act on GPCRs.
“Look,” Bozza says, pointing to
his monitor, “it’s easy to see which specific parts of
the olfactory bulb are being used as the mouse responds to the
odors.”
Mombaerts, who is head of the Laboratory of
Developmental Biology and Neurogenetics, and Bozza collaborated
with Boston University researchers John P. McGann and Matt
Wachowiak. The mouse strain used is genetically modified to produce
a molecule called synapto-pHluorin in its olfactory sensory
neurons. The molecule is a fusion of a pH-sensitive green
fluorescent protein — the same type used by many scientists
to track gene activity in cells — and a nerve cell protein
called VAMP2. Bozza developed the mouse strain to study olfactory
physiology in a live animal.
The researchers anaesthetized the mice, then
surgically thinned their skulls until they were virtually
transparent. They then placed the mice under a microscope and
shined blue light — the wavelength that causes the green
fluorescent protein to glow — onto them. The microscope
snapped a series of digital images through each mouse’s skull
as a nozzle sprayed short bursts of hexanal nearby.
The initial imaging was done by Wachowiak at
Boston University. Bozza is reproducing the system in the Mombaerts
lab in the Bronk building.
The technique allows the researchers to return
to the same mouse repeatedly to track changes in brain activity.
This option is not available with other imaging techniques because
the animals must be euthanized before their brains can be analyzed.
(Anticipating a wide demand, the genetically
engineered mice have been sent to The Jackson Laboratory in Bar
Harbor, Maine, which will distribute them to researchers later this
year.)
The olfactory system is very suitable for this
type of imaging because of its unusual organization, Mombaerts
explains. Each neuron releases information into the synapse through
the cell’s structure called an axon. “Several thousand
axons, all of the same specificity, terminate in the same area of
the brain, so the density of synapses is extraordinarily high.
That’s probably why we can see the signals so clearly,”
says Bozza.
To understand the sense of smell, scientists
relate molecules, called ligands, to odorant receptors, which are
either stimulated or blocked by the ligands. Currently, scientists
have identified only a handful of ligands that bind to odorant
receptors in mice, rats and people.
“Ideally, we would like to have an
enormous data set with 100,000 chemicals and 1,000 odorant
receptors — a hundred million combinations — and figure
out exactly at a given concentration what receptor is stimulated by
what odorant,” says Mombaerts. “At that point, we will
be able to understand the sense of smell because we will be able to
predict the quality of an odor, which, apart from a few exceptions,
no chemist can do now.”
The next step is to transfer that knowledge to
drug developers who need better data about the locations of
specific receptors in order to design drugs that target them.
Ligand-less receptors are known as orphans in the pharmaceutical
industry. The technique of visualizing the brain as it senses odors
also may help clarify the roles these orphan receptors play in many
body processes.
“As we find more ligands and better
understand the structure and function of GPCRs, in the long run
that’s going to be useful for drug development,” says
Mombaerts.
May 14, 2004
