Biology Reference
In-Depth Information
Magnetic tweezers were later developed to utilize magnetic ields to
generate forces on small paramagnetic beads with a typical size of 0.1-5
μm. Resulting displacements of the beads can then be used to deduce
rheological properties of living cells. Beads were functionalized and
bound to integrin receptors on the cell membrane to measure viscoelastic
properties of ibroblast cells
A series
of experiments 38 using magnetic twisting cytometry clariied that applied
force was transmitted through integrin receptors found at focal adhesions,
which are directly connected to the cytoskeleton. Cells with RGD-coated
ferromagnetic beads attached to integrin receptors experienced a force-
dependent increase in stiffness, while beads attached to other receptors
did not experience the same effect. It was also found that this effect was
proportional to an increased number of connections to the ECM. Together,
this indicates that integrins act as mechanoreceptors which transmit signals
to the cytoskeleton from the ECM and directly modulates cell rigidity.
Published evidence supports the transmission of force through focal
adhesions using a combination of micromanipulation with glass needles and
cells expressing green luorescent protein (GFP) conjugated to actin.
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and their response to deformation.
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Advances in optical technology have also led to several interesting
approaches to studying cell nanomechanics. Optical tweezers (laser
traps) are a highly sensitive technique in which dielectric spherical beads
are trapped at the focus of a laser beam.
The surface of the bead is
functionalized and can be attached to a cell membrane or other molecules.
The laser beam creates a ield that “traps” the bead at the focal point,
allowing measurement of forces acting on the bead. Using this method,
forces such as those generated by single molecules such as kinesin motors
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and cytoskeleton-integrin linkage 46 were successfully measured. The
ability to apply a controlled and localized force to a cell demonstrated that
increased force on focal adhesion complexes and stress ibres leads to an
increased calcium ion inlux near those focal adhesion complexes. This
supports the theory that mechanosensitive ion channels can be activated by
increased tension in the cytoskeleton.
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It is also possible to use a focused
laser with enough precision to sever a single cytoskeleton ilament, known
as laser ablation.
47
demonstrated
that stress ibres will mechanically retract (as opposed to depolymerization)
and force pseudo focal adhesions along the basal membrane as they slide
along it. Evidence also supports the presence of a “tension sensor” protein,
zyxin, which localizes to points of increased tension along the cytoskeleton
and at adhesion sites, both new and old, and disappears immediately
following a loss of tension. Finally, in a related technique, optical stretching
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A series of laser ablation experiments
20
has been demonstrated to be an extremely powerful tool in the study of
cell nanomechanics. Unlike an optical trap, the optical stretcher utilizes
two unfocussed lasers to trap and stretch suspended cells in solution.
49
 
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