DYING BRAIN CELLS CUE NEW BRAIN CELLS TO GROW IN SONGBIRD
Brain cells that
multiply to help birds sing their best during breeding season are known to die
back naturally later in the year. For the first time researchers have described
the series of events that cues new neuron growth each spring, and it all
appears to start with a signal from the expiring cells the previous fall that
primes the brain to start producing stem cells
If scientists can
further tap into the process and understand how those signals work, it might
lead to ways to exploit these signals and encourage replacement of cells in
human brains that have lost neurons naturally because of aging, severe
depression or Alzheimer's disease, said Tracy Larson, a University of
Washington doctoral student in biology. She's lead author of a paper in the
Sept. 23 Journal of Neuroscience on brain cell birth that
follows natural brain cell death.
Neuroscientists have
long known that new neurons are generated in the adult brains of many animals,
but the birth of new neurons - or neurogenesis - appears to be limited in
mammals and humans, especially where new neurons are generated after there's
been a blow to the head, stroke or some other physical loss of brain cells,
Larson said. That process, referred to as "regenerative"
neurogenesis, has been studied in mammals since the 1990s.
This is the first
published study to examine the brain's ability to replace cells that have been
lost naturally, Larson said.
"Many
neurodegenerative disorders are not injury-induced," the co-authors write,
"so it is critical to determine if and how reactive neurogenesis occurs
under non-injury-induced neurodegenerative conditions."
The researchers worked
with Gambel's white-crowned sparrows, a medium-sized species 7 inches (18
centimeters) long that breeds in Alaska, then winters in California and Mexico.
Sometimes in flocks of more than 100 birds, they can be so plentiful in parts
of California that they are considered pests. The ones in this work came from
Eastern Washington.
Like most songbirds,
Gambel's white-crowned sparrows experience growth in the area of the brain that
controls song output during the breeding season when a superior song helps them
attract mates and define their territories. At the end of the season, probably
because having extra cells exacts a toll in terms of energy and steroids they
require, the cells begin dying naturally and the bird's song degrades.
Gambel's white-crowned
sparrows are particularly good to work with because their breeding cycle is
closely tied to the amount of sunlight they receive. Give them 20 hours of
light in the lab, along with the right increase of steroids, and they are ready
to breed. Cut the light to eight to 12 hours and taper the steroids, the
breeding behavior ends.
"As the hormone
levels decrease, the cells in the part of the brain controlling song no longer
have the signal to 'stay alive,'" Larson said. "Those cells undergo
programmed cell death - or cell suicide as some call it. As those cells die it
is likely they are releasing some kind of signal that somehow gets transmitted
to the stem cells that reside in the brain. Whatever that signal is then
triggers those cells to divide and replace the loss of the cell that sent the
signal to begin with."
The next spring, all
that's needed is for steroids to ramp up and new cells start to proliferate in
the song center of the brain.
"This paper
doesn't describe the exact nature of the signals that stimulate
proliferation," Larson said. "We're just describing the phenomenon
that there is this connection between cells dying and this stem cell
proliferation. Finding the signal is the next step."
"Tracy really
nailed this down by going in and blocking cell death at the end of the breeding
season," said Eliot Brenowitz, UW professor of psychology and of biology,
and co-author on the paper. "There are chemicals you can use to turn off
the cell suicide pathway. When this was done, far fewer stem cells divided. You
don't get that big uptick in new neurons being born. That's important because
it shows there's something about the cells dying that turns on the replacement
process.'
"There's no
reason to think what goes on in a bird brain doesn't also go on in mammal
brains, in human brains," Brenowitz says. "As far as we know, the
molecules are the same, the pathways are the same, the hormones are the same.
That's the ultimate purpose of all this, to identify these molecular mechanisms
that will be of use in repairing human brains."
In mammals, the area
of the brain that controls the sense of smell and the one that is thought to
have a role in memories can produce tiny numbers of new brain cells but it is
not understood how or why. The numbers of new cells is so low that trying to
identify and quantify if dying cells are being replaced and if so, the steps
that are involved, is much more difficult than when using a songbird like
Gambel's white-crowned sparrow, Larson and Brenowitz said.
The other co-authors
on the paper are Nivretta Thatra, who started working with Larson while still
in high school, continued while earning her UW undergraduate degree and is now
at the Allen Institute for Brain Science; and Brian Lee, who worked in
Brenowitz's UW lab while earning his undergraduate degree from Johns Hopkins
University. The work was supported by the National Institutes of Health and the
UW Department of Biology.
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