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Researchers Discover "Doorways" Into Brain Cells

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Duke Health News 919-660-1306

DURHAM, N.C. -- Duke University Medical Center researchers
have discovered that neurons take in receptors and other
molecules from their surface membranes through discrete
"doorways" -- specialized domains on the surface of nerve cells
that regulate such entry.

The discovery of such entry points drastically revises a
long-held theory that surface molecules such as receptors are
enveloped right where they rest in the fatty membrane, to be
drawn into the cell's interior.

This uptake process, called endocytosis, is part of the
constant cycling of receptors to and from the membrane surface.
The balance of this cycling is the principal means by which
neurons regulate the number of surface receptors for such
chemical triggers as neurotransmitters and drugs -- thereby
controlling neuronal sensitivity to such external chemical
triggers. The newly discovered "endocytic zones" are also entry
points for nutrients and pathogens such as viruses.

The researchers said their discovery of the zones raises the
possibility that drugs affecting receptor transport to and
through the zones could prove useful in treating addiction,
depression, stroke, epilepsy, and other neurological disorders
involving abnormal transport of receptors into the neuron's
interior.

The researchers, led by neurobiologist Michael Ehlers,
reported their discoveries in the October 24, 2002, issue of
the journal Neuron. Co-authors of the paper are neurobiologists
Thomas Blanpied and Derek Scott. The research was sponsored
principally by the National Institutes of Health.

"We have found that the nerve cell is in a way like a room
with only certain entry points or doorways into that room,"
said Ehlers. "Before, it had been thought that the cell
membrane might be like one big curtain that substances could
move through at any point."

In their studies, Ehlers and his colleagues concentrated on
the "postsynaptic" regions of the neuron -- the parts of
neurons that receive chemical signals, called
neurotransmitters, from neighboring neurons that trigger
impulses in the receiving neuron. These postsynaptic regions of
neurons are also sensitive to drugs that plug into the same
protein receptors.

The brain establishes memory pathways by adjusting the
strength of the connections, called synapses, among certain
neurons. Synapses are located on doorknob-shaped spines that
extend from neuronal branches called dendrites.

"We first began to look for specific endocytic zones,
because studies over the past five years had indicated that
synaptic strength could be controlled by removing or inserting
postsynaptic receptors from the membrane," said Ehlers. "And a
major unresolved question was where exactly do these receptors
get removed from that membrane."

While microscopic studies of neurons had revealed that
receptors tend to cluster in "postsynaptic densities" in the
membrane, there was no evidence for specialized doorways
through which they would enter the neuronal cell. To detect
such regions, Ehlers and his colleagues first attached
fluorescent molecules to a molecule called clathrin that is
part of the structural mold that shapes the infinitesimal
bubbles in the membrane that carry receptors and other protein
cargoes into the cell. Specifically, clathrin forms a coat on
the membrane surface, pinching off the cargo-carrying bubble,
or vesicle.

Infusing the tagged clathrin molecules into cultures of
neurons, the researchers then used high-resolution imaging to
see where the tagged clathrin molecules migrated in carrying
out their vesicle-coating duties. These imaging studies
revealed specific "hot spots" where clathrin coats tended to
cluster. What's more, the researchers found that these hot
spots tended to change in character as neurons aged.

"Initially, we found a lot of dynamic behavior of the hot
spots in young neurons as they were growing and forming their
synapses," said Ehlers. "But still mysterious is that, as the
neurons in culture mature and age, these hot spots seem to
stabilize and specialize. They become much more well-defined in
location and not to appear and disappear as often as they do in
young cells," he said.

"We believe that this change with maturation provides
important clues about how nerve cells differentiate and
specialize during brain development," said Ehlers. "And, these
changes give some new insight into the diminished plasticity of
the brain with aging."

Since entry of nutrients and pathogens such as viruses is
through such clathrin-coated vesicles, further understanding of
the endocytic zones could lead to better understanding of
nutrient uptake by nerve cells and the process by which they
are invaded by pathogens, said Ehlers.

Importantly, he said, the finding could also lead to new
strategies for treating drug addiction and maintaining
sensitivity of patients to therapeutic drugs.

"It seems quite reasonable to imagine developing techniques
to control drug tolerance or sensitivity to a therapeutic agent
such as an antidepressant by preventing the relevant receptors
from migrating to or entering the endocytic zones to undergo
endocytosis," said Ehlers. More broadly, he said, the discovery
of endocytic zones could represent the beginning of a new
paradigm for understanding neuronal cell membranes.

"These zones could represent the first of a series of
specialized membrane structures dedicated to anchoring, sorting
and trafficking of proteins in the postsynaptic membrane or the
dendritic spines," said Ehlers. "We could end up developing an
entirely new 'microanatomy' of neurons."

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