“Badass” new method uses a magnetised protein to activate brain cells rapidly, reversibly, and non-invasively
Researchers in the United States have developed a new method for
controlling the brain circuits associated with complex animal
behaviours, using genetic engineering to create a magnetised protein
that activates specific groups of nerve cells from a distance.
Understanding how the brain generates behaviour is one of the
ultimate goals of neuroscience – and one of its most difficult
questions. In recent years, researchers have developed a number of
methods that enable them to remotely control specified groups of neurons
and to probe the workings of neuronal circuits.
The most powerful of these is a method called optogenetics,
which enables researchers to switch populations of related neurons on
or off on a millisecond-by-millisecond timescale with pulses of laser
light. Another recently developed method, called chemogenetics, uses engineered proteins that are activated by designer drugs and can be targeted to specific cell types.
Although powerful, both of these methods have drawbacks. Optogenetics
is invasive, requiring insertion of optical fibres that deliver the
light pulses into the brain and, furthermore, the extent to which the
light penetrates the dense brain tissue is severely limited.
Chemogenetic approaches overcome both of these limitations, but
typically induce biochemical reactions that take several seconds to
activate nerve cells.
The new technique, developed in Ali Güler’s lab at the University of Virginia in Charlottesville, and described in an advance online publication in the journal Nature Neuroscience, is not only non-invasive, but can also activate neurons rapidly and reversibly.
Several earlier studies have shown that nerve cell proteins which are
activated by heat and mechanical pressure can be genetically engineered
so that they become sensitive to radio waves and magnetic fields,
by attaching them to an iron-storing protein called ferritin, or to
inorganic paramagnetic particles. These methods represent an important
advance – they have, for example, already been used to regulate blood glucose levels in mice – but involve multiple components which have to be introduced separately.
The new technique builds on this earlier work, and is based on a protein called TRPV4, which is sensitive to both temperature and stretching forces.
These stimuli open its central pore, allowing electrical current to
flow through the cell membrane; this evokes nervous impulses that travel
into the spinal cord and then up to the brain.
Güler and his colleagues reasoned that magnetic torque (or rotating)
forces might activate TRPV4 by tugging open its central pore, and so
they used genetic engineering to fuse the protein to the paramagnetic
region of ferritin, together with short DNA sequences that signal cells
to transport proteins to the nerve cell membrane and insert them into
it.
When they introduced this genetic construct into human embryonic
kidney cells growing in Petri dishes, the cells synthesized the
‘Magneto’ protein and inserted it into their membrane. Application of a
magnetic field activated the engineered TRPV1 protein, as evidenced by
transient increases in calcium ion concentration within the cells, which
were detected with a fluorescence microscope.
Next, the researchers inserted the Magneto DNA sequence into the
genome of a virus, together with the gene encoding green fluorescent
protein, and regulatory DNA sequences that cause the construct to be
expressed only in specified types of neurons. They then injected the
virus into the brains of mice, targeting the entorhinal cortex, and
dissected the animals’ brains to identify the cells that emitted green
fluorescence. Using microelectrodes, they then showed that applying a
magnetic field to the brain slices activated Magneto so that the cells
produce nervous impulses.
To determine whether Magneto can be used to manipulate neuronal
activity in live animals, they injected Magneto into zebrafish larvae,
targeting neurons in the trunk and tail that normally control an escape
response. They then placed the zebrafish larvae into a specially-built
magnetised aquarium, and found that exposure to a magnetic field induced
coiling manouvres similar to those that occur during the escape
response. (This experiment involved a total of nine zebrafish larvae,
and subsequent analyses revealed that each larva contained about 5
neurons expressing Magneto.)
In one final experiment, the researchers injected Magneto into the
striatum of freely behaving mice, a deep brain structure containing
dopamine-producing neurons that are involved in reward and motivation,
and then placed the animals into an apparatus split into magnetised a
non-magnetised sections. Mice expressing Magneto spent far more time in
the magnetised areas than mice that did not, because activation of the
protein caused the striatal neurons expressing it to release dopamine,
so that the mice found being in those areas rewarding. This shows that
Magneto can remotely control the firing of neurons deep within the
brain, and also control complex behaviours.
Neuroscientist Steve Ramirez of Harvard University, who uses optogenetics to manipulate memories in the brains of mice, says the study is “badass”.
“Previous attempts [using magnets to control neuronal activity]
needed multiple components for the system to work – injecting magnetic
particles, injecting a virus that expresses a heat-sensitive channel,
[or] head-fixing the animal so that a coil could induce changes in
magnetism,” he explains. “The problem with having a multi-component
system is that there’s so much room for each individual piece to break
down.”
“This system is a single, elegant virus that can be injected
anywhere in the brain, which makes it technically easier and less likely
for moving bells and whistles to break down,” he adds, “and their
behavioral equipment was cleverly designed to contain magnets where
appropriate so that the animals could be freely moving around.”
‘Magnetogenetics’ is therefore an important addition to
neuroscientists’ tool box, which will undoubtedly be developed further,
and provide researchers with new ways of studying brain development and
function.
Reference
Wheeler, M. A., et al. (2016). Genetically targeted magnetic control of the nervous system. Nat. Neurosci., DOI: 10.1038/nn.4265 [Abstract]
