Study Using Robotic Microscope Shows How Mutant Huntington's Disease Protein Affects Neurons
Using a specially designed robotic microscope to study cultured
cells, researchers have found evidence that abnormal protein clumps
called inclusion bodies in neurons from people with Huntington's
disease (HD) prevent cell death. The finding helps to resolve a
longstanding debate about the role of these inclusion bodies in
HD and other disorders and may help investigators find effective
treatments for these diseases. The study was funded primarily by
the NIH's National Institute of Neurological Disorders and
Stroke (NINDS) and appears in the October 14, 2004, issue of Nature1.
Inclusion bodies are common to many neurodegenerative disorders,
including HD, Parkinson's disease, Alzheimer's disease,
and amyotrophic lateral sclerosis (ALS). The role of inclusion bodies
in these diseases has long been controversial. Some studies suggest
that they may be a critical part of the disease process, while others
indicate that they may help protect the cells from toxic proteins
or that they are merely bystanders in the disease process.
One problem in identifying how inclusion bodies influence disease
is that researchers have been unable to track changes in individual
neurons over time. "It was like viewing pictures of a football
game and trying to imagine the score," says Steven Finkbeiner,
M.D., Ph.D., of the Gladstone Institute of Neurological Disease
and the University of California, San Francisco. "Much was
happening that we couldn't see."
To overcome this problem, Dr. Finkbeiner and his colleagues wrote
a computer program that allows a microscope to match images in a
culture dish to images it has stored and to manipulate its controls
to look at the same neurons over and over again like time-lapse
photography. This allowed the investigators to follow changes in
a single neuron or a group of neurons over a period of days. They
used this automated microscope to study neurons that contained a
version of the huntingtin protein that causes HD. The huntingtin
was fused to green fluorescent protein, a widely used marker that
allows researchers to see where proteins accumulate.
Many neurons with the mutated HD gene died without forming inclusion
bodies, the researchers found. The formation of inclusion bodies
actually prolonged neurons' survival and lowered their overall
risk of death. The rate of cell death was higher in neurons with
larger gene mutations, but the death rate for each set of cells
remained constant over time.
The researchers also examined the level of mutant huntingtin protein
spread throughout the neurons, outside of inclusion bodies. They
found that neurons with larger amounts of mutant huntingtin spread
throughout the cell died more rapidly than cells with less of this
protein. The amount of mutated protein decreased in other parts
of the cell when inclusion bodies formed. Taken together, these
findings suggest that inclusion bodies lock up mutant huntingtin
and keep it from interfering with the rest of the neuron in ways
that can trigger cell death.
These findings provide evidence that inclusion bodies in HD, and
possibly other neurodegenerative diseases, help neurons cope with
toxic proteins and avoid neurodegeneration. Many researchers have
been working to develop ways of interfering with inclusion body
formation as potential treatments for HD and other disorders. This
study suggests that finding ways to remove mutant proteins diffused
throughout the cell might be a more effective approach.
"This approach provides a way to connect cellular changes
to fate," says Dr. Finkbeiner. The automated microscope system
could be applied to sort out many important questions about how
cellular changes or abnormalities affect disease, he adds. He and
his colleagues are now planning studies to examine the role of proteasomes
enzyme-filled compartments that break down and recycle proteins
The NINDS is a component of the National Institutes of Health within
the Department of Health and Human Services and is the nation'
primary supporter of biomedical research on the brain and nervous
1 Arrasate M, Mitra S, Schweitzer ES, Segal
MR, Finkbeiner S. "Inclusion body formation reduces levels of mutant
huntingtin and the risk of neuronal death."Nature, October 13, 2004,
Vol. 431, No. 7010, pp. 805-810.