A Princeton University-led research team has discovered that
protein competition over an important enzyme provides a mechanism
to integrate different signals that direct early embryonic
development. The work suggests that these signals are combined long
before they interact with the organism's DNA, as was previously
believed, and also may inform new therapeutic strategies to fight
cancer.
The fought-over enzyme, known as the mitogen-activated protein
kinase (MAPK), is found in all complex organisms, ranging from
yeast to humans. MAPK signaling pathways, or chemical networks that
involve the enzyme, are critical for normal development, and
defects in these pathways can lead to severe developmental
disorders and cancer.
During early embryonic development, a single undifferentiated
cell becomes a complex and highly specialized organism containing a
variety of different cell types arranged in very precise patterns.
These patterns, which ensure that the body structures from
head-to-tail and front-to-back develop correctly and in the
appropriate places, are created when cells respond to a series of
chemical signals from different signaling pathways. The different
patterning signals received by any given cell are ultimately
combined to govern its future fate and tell it what kind of cell it
should become.
Until now, scientists believed these pathways operated largely
independently of one another to produce protein signals that
traveled to the nuclei of the embryo's cells where DNA is stored.
There, coordination of these signals was thought to occur when they
interacted with cell DNA to influence and control the expression of
genes. Results published March 9 in the journal Current
Biology, however, suggest that competition for the MAPK enzyme
among proteins in different pathways influences which signals are
sent to cells, establishing a biochemical mode of signal
integration that adds a previously unrecognized layer of complexity
and control to embryonic development.
"It appears that different proteins in different pathways are
competing for the MAPK enzyme inside these living organisms," said
Stanislav Shvartsman, associate professor in the Department of
Chemical Engineering and the Lewis-Sigler Institute for Integrative
Genomics who earned his Ph.D. from Princeton in 1999. "Since these
proteins are fighting for the same limited resource -- the enzyme
-- they indirectly control one another, which in turn coordinates
the developmental signals."
Conventional biology teaches that enzymes like MAPK act on
certain molecules, called substrates, to regulate chemical
reactions. The new findings are surprising because it appears that,
through competition with one another, the substrates of MAPK are,
in fact, influencing the enzyme's activity.
"In a way, it's like the tail wagging the dog," Shvartsman said.
"The substrates are regulating the enzyme, and, by extension,
mediating the chemical reactions."
Eric Wieschaus, Princeton's Squibb Professor in Molecular
Biology who received the 1995 Nobel Prize in medicine for his
pioneering work in developmental biology, said, "Their results
argue convincingly that these signaling molecules are interacting
with each other in a competitive way such that even before anything
gets to the DNA, they've already made decisions. Essentially the
decisions aren't just made in terms of DNA, but also in terms of
proteins working together. This is, in a way, revolutionary."
The research team, led by Princeton chemical engineering
graduate student Yoosik Kim, focused its attention on the
interaction between MAPK and two proteins involved in two different
signaling pathways for head-to-tail pattern formation. The first of
these proteins is part of the pathway that governs the development
of the head. The second protein plays a significant role in the
chemical circuit that controls the development of the ends of the
embryo, including the tail.
Using special techniques to visualize whether the proteins had
interacted with the MAPK enzyme, the team found that the relative
amount of the first protein controlled how much enzyme was
available to interact with the second protein. For example, in the
portion of the embryo that would become the head, where the
concentration of the first protein was high, much less enzyme was
available to act on the second protein than at the other end of the
embryo, where the tail would ultimately develop.
"This competition makes sure that the same enzyme signals are
interpreted differently in the head and in the tail, thereby
allowing for the integration of multiple signals," Shvartsman
said.
Based on how the enzyme interacted with the proteins in the head
region of the embryo, the team predicted that a third protein also
might be competing for the MAPK enzyme in that area. To test the
hypothesis, research team members at the Institute for Medical
Research Israel-Canada at Hebrew University in Jerusalem used a
series of experimental techniques to verify that their proposed
protein could bind to the enzyme, an ability that was previously
unknown. These findings suggest that the competition model may
provide a novel way to identify proteins that are involved in
signaling pathways.
Beyond advancing the fundamental understanding of mechanisms
that control embryonic patterning, the work has implications for
how to target cancer cells, which often exhibit hyperactive MAPK
signaling.
"According to our substrate competition idea, MAPK signaling
activity directed toward any given substrate decreases when you
introduce a competing substrate," Kim said. "In theory, you can
lower the activity of MAPK if you introduce a protein whose sole
function is to bind to MAPK and thus act as a competitive inhibitor
of MAPK signaling to all other substrates." This strategy might one
day allow scientists to slow or stop MAPK signaling pathways in
cancer cells by adding a protein that monopolizes the MAPK enzyme,
effectively disrupting the chemical circuitry of a cancer cell.
In future work, the researchers plan to conduct experiments to
investigate competition among other proteins that bind to MAPK and
to investigate how this competition for the MAPK enzyme manifests
itself in other organisms. The group also intends to explore how
certain proteins are able to outcompete other proteins for the
enzyme's attention, perhaps by binding more strongly or efficiently
to the molecule. In time, the group may expand its work to consider
whether similar competition models affect the activity of different
enzymes in other signaling pathways.
SOURCE