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that resulted from AFM indentation. Images are coloured so that a red
(before indentation) and green (after indentation) overlay can be created.
As can be seen, changes in the actin ibres are visible at locations far from
the indentation point. Comparing the natural and the indented states, some
ilaments at the cell edge assume a curved state following indentation (green),
in comparison with their pre-indented stretched state (red) (
Fig. 18.7
).
Signiicant deformation is taking place throughout the actin network in
response to a point load over the nucleus. This is particularly important as we
postulated that mitochondria move in response to this type of deformation.
Moreover, the deformation is taking place over very short timescales (<5
seconds).
To investigate longer timescale (60 seconds) viscous deformation
and relaxation processes through the cytoskeleton we performed stress-
relaxation tests.
In these experiments, a cell was allowed to relax for
60 seconds under an initial contact force (2 nN) from the AFM tip (
Fig.
18.8
). We have shown previously
19,89
19
that the relaxation time and viscosity
(a)
(b)
(c)
(d)
(e)
(f)
Figure 18.7.
Deformation in the actin cytoskeleton following AFM indentation.
Images of actin-GFP-transfected cells were taken prior (a, green) and after (b, red)
AFM indentation, with 4 second interval between the two images. The overlayed
images are shown in (c). Local deformations of the actin cytoskeleton can be clearly
seen far from the indentation point (c, white cross). d, e and f are magniied areas,
marked by the white squares in (c). Scale bars are: a-c, 10
μ
m; d-f, 5
μ
m.
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