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by Austin Hou
“W
hat’s your name?” I was slightly confused. Didn’t she
already know? “Austin,” I replied. “That’s a nice name,”
she said. “It’s nice to meet you.” Then she turned away from me.
“She is your grandmother’s sister,” my mother had reminded me, “and she is very forgetful, so be patient.” I pondered this fact as I sat in the bright, sunny room of the nursing home. My
thoughts were interrupted as my great-aunt turned back to me. “What’s your name?” she asked.
This was my first encounter with Alzheimer’s disease,
a debilitating condition that causes its victims to eventually
lose their memories of everything they know, even the names
and faces of close family. It is always fatal. I was young when I’d
visited my great-aunt at the nursing home, but because of my
encounter with her, I grew up fascinated with the inner workings of the brain, our thoughts, and memories. So when, in
eleventh grade, I had an opportunity to work on a project that
might one day unravel the mysteries of the processes behind
memory, I jumped at the chance.
seeking the right Fit
The bright spots in the
neuron represent dendritic
spines in this confocal
microscope image,
courtesy of Dr. Xiaobing
Chen.
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I sought an internship within the National Institutes of Health
(NIH) on the advice of friends who had previously worked
there. The National Institute of Neurological Disorders and
Stroke (NINDS), an institute within the NIH, offers a student
training program in brain and nervous system research. The
Summer Program in the Neurological Sciences, open to academically talented high school, undergraduate, graduate, and
medical students, seemed like a perfect fit. Once I had applied
to the program, I began contacting researchers and scheduling
meetings to see if I was a good match for the lab. This somewhat informal process, in which prospective interns contact
researchers to discuss their work, is typical.
At NINDS, I met Dr. Xiaobing Chen, an NIH scientist
specializing in biophysics. Dr. Chen’s research dealt with
the postsynaptic density (PSD), a macromolecular structure
composed of hundreds of specialized proteins located on the
dendritic spines of the neurons of synapses. In essence, it is
the organizational structure that receives and processes signals between neurons. The PSD plays an integral role in the
processes behind learning and memory, including long-term
potentiation—the synchronization of firing of nerve pulses
between neurons that is the basis for new memory formation.
I was thrilled when, after an interview and several meetings
with Dr. Chen, I was accepted into the NINDS Laboratory of
Neurobiology’s Summer Internship Program. It was an amazing opportunity to work alongside great scientists, performing
cutting-edge research that seeks to understand the core of what
makes us human: our brains.
How to build an unknown object
While we know that the PSD is important in learning and memory,
we know very little about how it actively influences these processes.
And although we can determine the proteins that compose the PSD,
we don’t know much about its organizational structure and the role
this structure plays in its function. Dr. Chen’s research focused on
decoding this complex structure with the goal of understanding the
way we learn—and maybe, one day, developing drugs that can cure
memory-related diseases such as Alzheimer’s.
The size of the PSD—only around 30 nanometers thick and
300 nanometers wide—is an inherent obstacle to understanding it. (By comparison, a hair is about 100,000 nanometers
wide.) So how do you decipher the organization of a nanoscale
structure composed of thousands of proteins? The process
we used, electron microscopic (EM) tomography, generates
sept/oct 2012
A raw image from the electron
microscope depicting one of the
“silhouettes” of a rat hippocampal neuron.
Many images would be assembled to
create a three-dimensional tomogram.
a three-dimensional model of the object—in this case the
PSD—through the use of back projections: First, silhouettes of
the object are imaged at various angles with an electron microscope. Then, through computation, the images are assembled
to create a complete three-dimensional model.
The process is not completely straightforward, though.
Using an electron microscope involves bombarding the
sample (in this case, slices of hippocampal neuron from a rat
brain) with a high-intensity electron beam, which causes the
sample to degrade extremely quickly. To address this problem,
we used freeze substitution, wherein the sample undergoes
high-pressure freezing to prevent the formation of large ice
crystals that can damage the cellular structures. The cellular
H₂O is then removed and replaced with a durable resin,
which is more resistant to irradiation by an electron beam.
This process allows the cellular structures to withstand the
stress that occurs when hundreds of images are taken with
an electron microscope.
Once the problem of preserving the cellular structures was
solved, we used EM tomography to study the core structural
organization of the PSD, and in particular, the molecule PSD95. This molecule was chosen because it had been detected
in high concentrations within the PSD, and also because Dr.
Chen’s previous work showed that PSD-95 tends to organize
in a network of membrane-associated, vertically oriented filaments—that is, filaments perpendicular to the cell membrane.
Our goal was to see how these filaments interacted with other
proteins within the PSD.
that formed the foundation of the PSD,
we identified several other structural
networks that associated with the vertical filaments. Together, these networks
created a cascading framework of scaffolding proteins that formed the core of
the PSD. We found clear trends in how
these networks interacted, both with
each other and with the PSD’s general
structure. Perhaps most surprisingly,
some of these proteins bound to each
other in highly regular patterns. The PSD
is incredibly complex, and the fact that
it is highly ordered is fascinating. More
interesting still, the proteins themselves
are organized very regularly. We are currently working to establish the identities
of these proteins in hopes of creating a
more realistic molecular picture of the
fundamental structure of the PSD.
I am truly thankful for the chance
to work with Dr. Chen on neuroscience research at NINDS. Through my
experience, I’ve learned that within great
complexity lies simplicity. I’m thrilled that
I have been able to contribute to research
that may one day help decipher the mysteries behind memory and hopefully lead to a
cure for diseases such as Alzheimer’s.
A surprising discovery
Over a two-month period, we examined the tomograms closely,
identifying and classifying structures by their structural and
associative properties. Slice by slice, cross-section by crosssection, we pored over the tomograms in order to observe
the relationships between proteins in three dimensions. Because
the work was progressing well, I returned the following summer
to continue the research with Dr. Chen. By the end of the second
summer, we had identified a number of trends in the models.
And as we worked, we made a surprising discovery.
In addition to the core vertical network of filaments
Austin Hou won a
2011 Exceptional
summer student
Award for his work at ninds. he is
currently studying physics and electrical
engineering at the University of maryland
under a Banneker-key scholarship, and is
continuing his work in the lab with dr.
chen. Austin plans to continue to do research in the future, most
likely in the engineering field.
A cross-section of the
tomogram produced
from the raw EM
images. The transposed
colored model shows
the filaments of PSD-95
(red) interacting with
other proteins in the
PSD.
For more information on the ninds summer Program in the
neurological sciences, see www.ninds.nih.gov/jobs_and_training/summer.
www.cty.jhu.edu/imagine
imagine
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