December
27, 2005
Researchers using a customized atomic force microscope
(AFM) have discovered new evidence for how the fibrous
scaffolding within our cells, which is made of the
protein actin, responds to obstacles in its environment.
The discovery demonstrates a technique for tracking
a cell's growth history, and if it proves valid outside
of the laboratory, researchers may one day look for
actin-growth clues while tracking the pathways of
spreading cancers, immune cells, and other free-moving
cells that crawl throughout the body.
National Science Foundation CAREER awardee
Daniel Fletcher of the University of California at
Berkeley, lead authors Sapun Parekh and Ovijit Chaudhuri,
also of Berkeley, and Julie Theriot of Stanford University
published their findings in the Dec. 2005 issue of
Nature Cell Biology.
"How do cells push in a particular direction when
they confront a barrier? That was the initial
question in this research," says Fletcher.
When faced with a barrier, the researchers suspect
the elongating matrix of filaments in a growing actin
network adds more branches to counter the resistance.
When the barrier is removed, they believe, the added
filaments remain and the network grows at a new,
faster rate.
Scientists
have known that actin networks, unlike many other
cell components, alter their growth in response
to forces, not just chemical signals. The
new findings help clarify that response and provide
new clues for how our cells stretch, change shape
and move around obstacles.
Among other responsibilities, actin provides the
structural support for cells and the growth force
necessary for certain cellular activities.
"The front of a cell extends forward during crawling,
and actin and its associated proteins are necessary
for powering that forward motion," says Fletcher. "Other
mechanical processes in cells, such as the 'Pac-Man'-like
action of an immune cell eating a bacterium, also
involve forces generated by growth of actin networks."
The networks are collections of proteins, so they
are more complicated and more difficult to study
than many other cellular components involved with
cell motion and force. To track the growth rate and
force generated by actin, the bioengineers modified
an atomic force microscope (AFM).
In most research, the business end of an AFM is
a miniscule, extremely sharp tip that is attached
to a thin silicon-nitride cantilever. Because the
tip is so slight, even features as tiny as individual
atoms can cause the cantilever to deflect as it passes
along, or slightly above, a surface. A laser bounces
off the cantilever and into a detector, registering
the tiny deflections and providing signals a computer
translates into an image.
For this study, the researchers created a specialized
AFM that uses two cantilevers and two lasers. Instead
of scanning a surface, the cantilevers served as
tiny springboards, one to bend as actin grew beneath
it and the other to stay as a reference point close
to the floor of the sample chamber. Using the two-cantilever
system, the researchers pushed longer on the filaments
than in any earlier study, and with more force --
in some cases to the point where the filaments stalled
and could grow no further.
In multiple experiments, the cantilevers applied
an initial force to a slurry of growing actin filaments,
then applied a larger force for as long as 30 minutes.
They then returned to the original load, at every
stage tracking how fast the network grew.
Each time, when the cantilever returned to its original
load, the growth velocity of the actin was faster.
When the fibers endured multiple load cycles, they
grew at a rate that was dependent upon all of the
cycles.
"We've found that the growth of actin is dependent
on its loading history -- not just on the load it
feels at one moment, as we previously thought," says
Fletcher. "This means the structure of a cell has
some 'memory' of its physical interactions."
The researchers suspect the effect may relate to
filament density, and the growth rate may be a function
of the network architecture, itself dependent upon
the entire load history.
"For a given load, proteins assume a certain network
architecture," says Fletcher. "This architecture
then remodels under a new load. So, if you go back
to the original load, the architecture is still tuned
for a higher load, resulting in explosive growth."
These are fundamental research findings, adds Fletcher,
but in the long term they may help scientists and engineers
understand cell crawling, potentially aiding future
treatments that help white blood cells work better
or stop tumor cells from moving to other parts of the
body.
-NSF-
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