Biomedical Engineering Reference
In-Depth Information
cation of external forces on cells induced focal adhesion assembly and growth
[78]. At the same time, it is thought that high-tension forces promote adhe-
sion disassembly and rupture, which are essential for retraction of the cell
rear. The regulation and coordination of these processes in fast and smooth
moving cells such as keratocytes is still not clear.
Several experiments using deformable substrates or micromachined devices
to measure the traction forces generated by moving keratocytes have shown
that cell pulling on the substrate is negligible under the front part of the
lamellipodium but that keratocytes pull inward very strongly at the sides (see
Figure 2.7; References [36, 41]). This tension is in the same direction as the in-
ward actin network flow in that part of the cell (see Figure 2.4c,d). Keratocytes
moving on soft substrates such as gelatin demonstrate large inward traction
force and thereby generate wrinkles under the cell body, moving forward in a
cycle of fast and slow movements where each phase of rapid movement corre-
lates with a sudden release of adhesions to the substrate and a loss of surface
wrinkling as the elastic substrate snaps back to its initial shape [79, 80]. The
strong perpendicular traction forces are attributed to myosin contraction (see
Section 2.2.3). However since these forces are mostly directed perpendicular to
the direction of keratocyte motion, their importance to steady state motility
is unclear.
2.2.3 The Role of Myosin
As discussed earlier, substantial experimental evidence from a variety of sys-
tems established that actin polymerization alone is capable of generating pro-
trusive forces. However, this does not rule out contributions from other force-
generating molecular motors, such as myosins, in cell motility. Myosins are a
superfamily of molecular motors that bind filamentous actin, and use ATP
hydrolysis to generate force by stepping along the filament. Of the different
classes of myosins, non-muscle myosin II has been most strongly implicated
to play a role in actin-based cell motility. Individual myosin II molecules
can come together and form supramolecular assemblies called thick filaments
which are
0
.
4
μ
m long [38] and have multiple actin-binding heads that func-
tion cooperatively to locally contract the actin meshwork. The distribution of
myosin II in keratocytes is enhanced at the cell rear and is most pronounced
along the actin bundle in the back of the cell and in the transition zone be-
tween the lamellipodium and the cell body (see Figure 2.8; Reference [38]).
Discrete myosin clusters are apparent throughout the lamellipodium with their
size and density increasing toward the cell body. Live cell microscopy revealed
that these clusters assemble in the lamellipodium, are stationary with respect
to the substrate, and progressively grow as they move toward the cell body.
A similar distribution of myosin II was found in lamellipodial fragments [30].
Several models suggest that myosin activity is required for steady state
motility of keratocytes, both for forward translocation of the cell body and
for maintenance of cell polarity [37, 38, 39]. The distribution of myosin II
∼
