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New nerve cells for the brain -- even in old
age
May
7, 2010--After birth the brain looses many
nerve cells and this continues throughout
life – most neurons are formed before birth,
after which many excess neurons degenerate.
However, there are some cells that are still
capable of division in old age – in the
brains of mice, at least.
According to scientists from the Max Planck
Institute of Immunobiology in Freiburg,
different types of neuronal stem cells exist
that can create new neurons.
While they divide continuously and create
new neurons in young animals, a large
proportion of the cells in older animals
persist in a state of dormancy. However, the
production of new cells can be reactivated,
for example, through physical activity or
epileptic seizures.
What happens in mice could also be
applicable to humans as neurons that are
capable of dividing also occur in the human
brain into adulthood. (Cell Stem Cell,
May 7th 2010)
You can't teach an old dog new tricks. The
corresponding view that the brain loses
learning and memory capacity with advancing
age prevailed for a long time. However,
neuronal stem cells exist in the hippocampus
– a region of the brain that plays a central
role in learning and memory functions – that
can produce new nerve cells throughout life.
It is known from tests on mice that the
newly formed cells are integrated into the
existing networks and play an important role
in the learning capacity of animals.
Nonetheless, the formation of new cells
declines with age and the reasons for this
were unknown up to now.
Together with colleagues from Dresden and
Munich, the Freiburg researchers have now
succeeded in explaining for the first time
why fewer new neurons are formed in the
adult mouse brain.
They managed to identify different
populations of neuronal stem cells, thereby
demonstrating that the hippocampus has
active and dormant or inactive neuronal stem
cells.
"In young mice, the stem cells divide four
times more frequently than in older animals.
However, the number of cells in older
animals is only slightly lower. Therefore,
neuronal stem cells do not disappear with
age but are kept in reserve," explains
Verdon Taylor from the Max Planck Institute
of Immunobiology.
The precise factors that influence the
reactivation of dormant stem cells are not
yet clear. The cells can, however, be
stimulated to divide again. The scientists
observed more newborn hippocampal neurons in
physically active mice than in their
inactive counterparts.
"Consequently, running promotes the
formation of new neurons," says Verdon
Taylor. Pathological brain activity, for
example that which occurs during epileptic
seizures, also triggers the division of the
neuronal stem cells.
Horizontal and radial stem cells
The different stem cell populations are easy
to distinguish under the microscope. The
first group comprises cells which lie
perpendicular to the surface of the
hippocampus.
Most of these radial stem cells are
dormant. As opposed to this, over 80% of the
cells in the group of horizontal stem cells
– cells whose orientation runs parallel to
the hippocampus surface – continuously form
new cells; the remaining 20% are dormant but
sporadically become activated. The activity
of genes such as Notch, RBP-J and Sox2 is
common to all of the cells.
Radial and horizontal stem cells differ not
only in their arrangement, apparently they
also react to different stimuli.
When the animals are physically active, some
radial stem cells abandon their dormant
state and begin to divide, while this has
little influence on the horizontal stem
cells.
The result is that more radial stem cells
divide in active mice. The horizontal stem
cells, in contrast, are also influenced by
epileptic seizures.
It would appear that neuronal stem cells are
not only found in the brains of mice. The
presence of neurons that are formed over the
course of life has also been demonstrated in
the human hippocamus.
Therefore, scientists suspect that different
types of active and inactive stem cells also
arise in the human brain. It is possible
that inactive stem cells in humans can also
be activated in a similar way to inactive
stem cells in mice. "There are indicators
that the excessive formation of new neurons
plays a role in epilepsy. The use of
neuronal brain stem cells in the treatment
of brain injuries or degenerative diseases
like Alzheimers may also be possible one
day," hopes Verdon Taylor.
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