http://www.nationalmssociety.org/articles.asp?SMContentIndex=0&SMContentSet=0
November 20, 2001
SAN DIEGO - One of the great dramas
of human thought is the story of figuring out how humans think.
During the 20th century, neuroscientists
developed a story featuring nerve cells, or neurons, as the main characters.
Neurons connect to each other in elaborate networks, sending signals back
and forth through a clever combination of electricity and chemistry. Any
given neuron receives thousands of messages from its neighbors, and then
decides whether to send out a message of its own.
While neurons are the main actors
in the drama of brain activity, they are vastly outnumbered by the stagehands,
known as glial cells. The glia offer structural support, provide nutrients,
and otherwise hang around and watch as the neurons talk to each other.
But lately scientists have discovered
that glial cells do much more than watch - they also listen in to neuron
chatter and toss in some comments on their own.
"Glial cells are among the most intriguing
and mysterious cells in the brain," says neurobiologist Fiona Doetsch of
Harvard University.
Doetsch and colleagues at Harvard
are among the researchers who have recently shown that some glial cells
can transform themselves into neurons. Scientists had previously found
reservoirs in the brain of "stem cells" that could change identities. Now
the identity of those stem cells themselves has been revealed to be glia.
Some researchers have focused on
the young developing brain, where "radial" glial cells project (or radiate)
long arms that serve sort of like railroad tracks to guide newborn neurons
to their proper brain location. It turns out that the parent cells of the
newborn neurons are the radial glia themselves.
Radial glial aren't found in the
adult brain, but that's because they mature into astrocytes, another type
of glial cell. And Doetsch and others have shown that astrocytes in at
least two brain regions can also become neurons.
"There is no evidence yet that astrocytes
throughout the brain have this capability," Doetsch said in San Diego last
week at the annual meeting of the Society for Neuroscience.
But if most or all astrocytes can
become neurons, new strategies arise for treating brain disorders caused
by neuron death, such as Alzheimer's disease.
"The hope is maybe that all astrocytes
in the brain might have the potential to generate neurons," says Magdalena
Gotz, another leading researcher in the field.
Doetsch points out that the research
on glial cells has only begun to show how important they might be.
"We've just uncovered one novel function
for them, which is to give rise to new neurons," she said at the neuroscience
meeting. "This just hints at how much more there is to discover about their
function elsewhere in the brain as well."
Other recent research has already
turned up new roles for astrocytes. Some astrocytes wind their arms around
the communication links, or synapses, where chemical signals swim from
one neuron to another. The same astrocyte may have one arm around one synapse
and another arm around a second synapse linking two other neurons. And
one astrocyte can communicate with neighboring astrocytes, via chemicals
or by electrical particles traveling through cell-to-cell tubes called
gap junctions.
It turns out that chemicals released
from a neuron can induce chemical changes inside one astrocyte. That astrocyte
can then signal other astrocytes to change as well. Down the line, an excited
astrocyte may signal a nearby neuron, perhaps retarding its inclination
to fire a further signal. In this way, the firing of one neuron might alter
the messages sent by a second, faraway neuron, because of the influence
of the astrocytes.
"Signaling between astrocytes might
form the basis of a long-range signaling pathway in the nervous system,"
writes Philip Haydon, of the zoology and genetics department at Iowa State
University in Ames.
So far, such astrocyte signaling
has been detected only in some parts of the brain. But the process may
be more widespread. If so, it provides an entirely new level of brain interaction
to consider in trying to figure out how different parts of the brain communicate.
The rapid-fire messages sent by neurons would be subtly altered by the
slow-moving signaling between astrocytes.
Scientists are still working out
the chemical details of astrocyte signaling. Long-range signals may use
a different strategy from short-range signals. But in any case, it may
be that communication among astrocytes is a method for the brain to coordinate
the action of different groups of neurons, Haydon suggests in a recent
issue of Nature Reviews/Neuroscience.
In his view, synapses should be considered
not just as the link between two neurons but as a "tripartite" communication
junction - the sending neuron, the receiving neuron, and the eavesdropping
astrocyte.
"It is time to rethink our understanding
of the wiring diagram of the nervous system," he writes.
The role of glia in the brain's wiring
may help explain the human mental capacity compared with other animals.
In primitive worms, neurons outnumber glia 5-to-1. But in humans and other
primates, glia outnumber neurons 10-to-1.
"The more technically sophisticated
the performance," writes Haydon, "the greater the number of backstage people
needed."
(c) 2001, The Dallas Morning News
Tom Siegfried
Knight Ridder/Tribune