This STED image of a nerve cell in the upper brain layer of a living mouse shows in previously impossible detail the very fine dendritic protrusions of a nerve cell, the so-called spines, at which the synapses are located. The inset shows the mushroom-shaped head of such a dendritic spine at which the nerve cells receive information from their peers. Image: Max Planck Institute for Biophysical Chemistry |
To
explore the most intricate structures of the brain in order to decipher
how it functions—Stefan Hell’s team of researchers at the Max Planck
Institute for Biophysical Chemistry in Göttingen has made a significant
step closer to this goal. Using stimulated emission depletion microscopy
developed by Hell, the scientists have, for the first time, managed to
record detailed live images inside the brain of a living mouse. Captured
in the previously impossible resolution of less than 70 nm, these
images have made the minute structures visible which allow nerve cells
to communicate with each other. This application of STED microscopy
opens up numerous new possibilities for neuroscientists to decode
fundamental processes in the brain.
Every
day a huge quantity of information travels not only over our
information superhighways; our brain must process an enormous amount of
data as well. In order to do this, each of the approximately hundred
billion nerve cells establishes contact with thousands of neighboring
nerve cells. The entire data exchange takes place via contact sites—the
synapses. Only if the nerve cells communicate via such contact sites at
the right time and at the right place can the brain master its complex
tasks: We play a difficult piece of piano, learn to juggle, or remember
the names of people we have not seen for years.
We
can learn most about these important contact sites in the brain by
observing them at work. When and where do new synapses form and why do
they disappear elsewhere? This is not easy to determine, since details
in living nerve cells can only be observed with optical microscopes. Due
to the diffraction of light, however, structures located closer
together than 200 nm appear as a single blurred spot. The STED
microscopy developed by Stefan Hell and his team at the Max Planck
Institute for Biophysical Chemistry is a groundbreaking method devised
to surpass this resolution limit. They use a simple trick:
Closely-positioned elements are kept dark under a special laser beam so
that they emit fluorescence sequentially one after the other, rather
than simultaneously, and can therefore be distinguished. Using this
technique, Hell’s team has been able to increase the resolution by
approximately tenfold compared to conventional optical microscopes.
STED
microscopy has already found wide application in fields ranging from
materials research to cell biology. Under this microscope, cell cultures
and histological preparations have offered unique insights into the
cellular nanocosmos. The first real-time video clips from the interior
of a nerve cell have demonstrated how tiny transmitter vesicles migrate
within the long nerve cell endings.
A vision becomes reality
What
was only an ambitious vision a year ago has now become reality: to also
study higher living organisms at this sharp resolution in the nanometer
range. By looking directly into the brains of living mice using a STED
microscope, Hell and his team were the first ones to image nerve cells
in the upper brain layer of the rodent with resolution far beyond the
diffraction limit.
“With
our STED microscope we can clearly see the very fine dendritic
structures of nerve cells at which the synapses are located in the brain
of a living mouse. At a resolution of 70 nm, we easily recognize these
so-called dendritic spines with their mushroom- or button-shaped heads,”
explains Hell. They are the clearest images of these fundamental
contact sites in the brain to date.
“To
make these visible, we take genetically modified mice that produce
large quantities of a yellow fluorescing protein in their nerve cells.
This protein migrates into all the branches of the nerve cell, even into
smallest, finest structures,” adds Katrin Willig, a postdoctoral
researcher in Hell’s department. The genetically modified mice for these
experiments originated from the group of Frank Kirchhoff at the
Göttingen Max Planck Institute for Experimental Medicine. Images of the
nerve cells taken seven to eight minutes apart revealed something
surprising: The dendritic spine heads move and change their shape. “In
the future, these super-sharp live images could even show how certain
proteins are distributed at the contact points,” adds Hell. With such
increasingly detailed images of structures in the brain, Hell’s team
hopes to shed light onto the composition and function of the synapses on
the molecular level.
Such
insights could also help to better understand illnesses that are caused
by synapse malfunction. Among these so-called synaptopathies are, for
example, autism and epilepsy. As Hell explains, “Through STED microscopy
and its application in living organisms, we should now be able to gain
optical access of such illnesses on the molecular scale for the first
time.”
As
one of the two representatives of the Göttingen Research Center
Molecular Physiology of the Brain funded by the German Research
Foundation, he is committed to collaboration in his further research.
Together with neurobiologists and neurologists, he and his team plan to
transfer the progress made in imaging technology into fundamental
knowledge about the functioning of our brains.