Penn Researchers Reveal Inner Workings
of Transcription Factor Protein
In Neuronal Cell Dendrites
Cell-Death Protein Could Play a Role in Neurodegeneration
(Philadelphia, PA) - Researchers at the University
of Pennsylvania School of Medicine discovered that a protein
called Elk-1 interacts with mitochondria, the energy storehouse of a cell,
suggesting that this protein - typically active in the nucleus - could
play a role in cell death, and mitochondria-related diseases such as neurodegeneration
The neuron is a particular type of cell in the brain that is responsible
for, among other tasks, learning and memory, cognitive function, and other
higher order physiologies. The neuronal cell exhibits a complex structure
where fine hairlike structures called dendrites receive signals from other
neurons. These signals are transferred to the soma, or body, of the cell
and result in neuronal responsiveness to stimulation.
The researchers found that mRNA (messenger RNA) and protein encoding Elk-1,
a transcription factor, were localized in the dendrites of intact rodent
neurons. “Transcription factors normally only function in the nucleus
and to find a transcription factor in the dendrite is pretty unique,”
says senior author James Eberwine, PhD, Professor of
Pharmacology. “These factors are proteins that bind to DNA and play
a role in the regulation of gene expression by promoting transcription.
Our lab and others showed that Elk-1 is present in the dendrites of nerve
cells.” Transcription is the process of translating the DNA code
Along with Eberwine, co-authors Lindy Barrett, PhD, a
student from the Eberwine lab who was recently awarded her doctorate in
philosophy; Philip Haydon, PhD, Professor of Neuroscience;
Jai Yoon Sul, PhD, a postdoctoral fellow in the Haydon
lab; and colleagues published their findings in the June issue of Nature
Methods and a March issue of the Proceedings of the National
Academy of Sciences.
In the series of experiments (described in the PNAS study) to discern
the nature of Elk-1’s role in the dendrite, the investigators first
characterized some of the proteins with which Elk-1 interacts, and found
that Elk-1 associates with mitochondria proteins. Mitochondria are distributed
throughout cells, including in the dendrites, and are important in maintaining
the energy stores and regulating viability and death of the cell.
The researchers then overexpressed Elk-1 in rat neurons to see if there
was an effect on cell viability. “We thought that through interaction
with Elk-1, the mitochondria would be able to regulate cell death,”
says Eberwine. “By overexpressing Elk-1, we found that we did decrease
cell viability, achieving more cell death. Conversely, when we knock-down
Elk-1 expression, the survivability of neurons increased, which indicates
that Elk-1 plays a role in neuron viability.”
Cell-death is a component of a number of psychiatric and neurological
diseases such as schizophrenia and those that involve neurodegeneration.
For many of these diseases dysfunction of the dendrite is also associated
with the disease process. “Therefore, anything that impacts dendrite
function might be associated with illness,” surmises Eberwine. “The
fact that Elk-1 RNA and protein are present in dendrites, and the fact
that Elk-1 can modulate cell viability, potentially through the mitochondria,
suggests that Elk-1 could play a role in these diseases perhaps through
modulation of mitochondrial activity.”
To more precisely understand the role of Elk-1 RNA in the dendrite, the
researchers developed a method called phototransfection, which was described
in their June Nature Methods paper, to focally introduce Elk-1 RNA into
the dendrite. In this technique, a laser light beam is used to create
small transient pores in the membrane of intact rat nerve cells, into
which a known amount of RNA molecules are introduced by diffusion.
The introduction and translation of Elk-1 mRNA in dendrites by phototransfection
also elicited cell death whereas introduction and translation of Elk-1
mRNA in the cell soma did not produce cell death. The Elk-1 proteins translated
in the dendrites were transported to the nucleus and cell death depended
on subsequent transcription. These results compliment and expand upon
the PNAS study in which Elk-1’s involvement in cell death and association
with mitochondria were elucidated.
“This is the first formal proof that RNA can be translated and made
into protein in an intact neuronal dendrite,” explains Eberwine.
“We have seen this with isolated dendrites before, but not in an
In the mouse model, Elk-1 mRNA is being made into protein on ribosomes
located at the periphery of the nerve cell in the dendrite. Protein is
also being made in the cell body, but there may be a difference in the
Elk-1 protein made in the cell body. “We speculate that the dendrite’s
environment comes into play, with kinases and phosphatases that modify
the Elk-1 proteins made in the dendrite,” explains Eberwine. “We
suggest that there’s a different phosphorylation signal on the protein
made in the dendrite versus in the cell body.
“These studies highlight the importance of the dendritic environment
in modifying proteins after they have been made,” concludes Eberwine.
“There is a clear link between the nucleus and mitochondria via
Elk-1, and it’s rapid. We don’t know exactly what that is,
but it’s a very interesting signal in terms of neurodegeneration.
These data provide intriguing new avenues for research, including determining
the role of localized protein synthesis and protein modifications in dendrite-related
pathologies, including Fragile X disease, schizophrenia, and autism. It
is likely that not only are particular proteins going to be important
in these diseases, which has been proven by genetics, but also the environment
in which they are synthesized.”
Study co-authors on the PNAS and Nature Methods paper are Hajime Takano
from Penn and Elisabeth J. Van Bockstaele from Thomas Jefferson University,
Philadelphia. This research was funded by the National Institutes on Aging
and the National Institutes of Mental Health.
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