http://unisci.com/stories/20014/1221014.htm
21-Dec-2001
Stem cells give rise to all of the
cells in an organism’s body. They have the unique ability to produce cells
that differentiate into specialized structures, such as tissue and bone,
and create new stem cells to replenish the stem cell population.
Scientists have long been investigating
the mechanisms that allow these special cells to perform this unmatched
double duty.
Studying the common fruit fly Drosophila,
Erika Matunis and Natalia Tulina of the Department of Embryology at the
Carnegie Institution of Washington, have identified a key gene that initiates
the signaling process that instructs a stem cell to renew itself instead
of differentiating into another type of cell. Their results are published
in today's issue of Science.
The Carnegie researchers study the
stem cells that lead to sperm in the fruit fly. These stem cells attach
to a niche called the hub, which consists of a cluster of support cells
in the Drosophila testes. Upon cell division, some of the cells move away
from the hub and differentiate into sperm, while others stay near the hub
and remain stem cells.
Recently, researchers identified
genes that are needed for stem cells to produce differentiating cells (see
Nature 407, p. 754). In the Carnegie study, the investigators searched
for an opposing signal that is responsible for instructing stem cells to
self-renew.
They found that a gene called Unpaired
(Upd) is expressed in hub cells and that it activates a cascade of signals
in a signaling pathway known as JAK-STAT, which promotes self-renewal.
The JAK-STAT signaling pathway was first identified in humans.
When this pathway is activated, the
STAT transcription factor -- a gene that controls the transcription of
other genes -- binds to target genes to change the cell's pattern of gene
expression.
The researchers think that cells
closest to the hub receive high enough levels of Unpaired to activate STAT,
resulting in the expression of stem cell-specific genes. In contrast, cells
distant from the hub do not receive enough Unpaired to activate STAT, and
instead they differentiate.
According to Matunis, “The fact that
all organisms have so many genes in common suggests that the identification
of the unpaired gene's function will help others find similar genes that
program stem cell self-renewal in more complex organisms.”
After identifying the Upd gene as
the first step in the complicated signaling process, the researchers turned
their attention to determining whether Upd instructs a stem cell to self-renew
or simply maintain viability. They found that, in fact, the cascade of
signaling events promotes the self-renewal of these unique cells.
[Contact: Tina McDowell]
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