Biomedical Engineering Reference
In-Depth Information
The pattern of contacts a moving cell makes with the substrate can be
visualized by interference reflection microscopy in which image contrast is
determined by the distance between the ventral surface of the cell and the
substrate [74]. The pattern of adhesion in keratocytes typically includes a rim
of very close contact at the leading edge, which appears simultaneously with
lamellar extension (see Figure 2.6a; Reference [19]). The rest of the lamel-
lipodium also maintains relatively close contact to the substrate, although
the pattern and extent of regional variation differ among cells. Investigation
of the molecular composition of adhesion complexes in keratocytes showed
that
β
1 integrin adhesion molecules were localized within a narrow rim near
the leading edge (see Figure 2.6b; Reference [19]), and hence are probably
involved in the establishment of close contact at the rim (see Figure 2.6a-c).
Note that in order to maintain such localization, the interaction between
β
1
integrins and the substrate must be highly dynamic since the leading edge in
keratocytes advances forward rapidly.
β
1 integrin proteins were also found in
small foci throughout the lamellipodium. Vinculin, an adaptor protein that
associates actin filaments with integrin complexes at the cell membrane [3]
was not found near the leading edge, but rather in small foci throughout the
lamellipodium (see Figure 2.6e; References [19, 75]). These foci were shown
to be stationary with respect to the substrate [75]. Vinculin was also found in
larger foci at the cell rear (see Figure 2.6e), where it appeared to slide inward
from the sides toward the cell body. These larger foci were typically confined
to the retracting cell rear. The observed distribution of adhesion molecules is
consistent with a model in which formation of focal contacts is initiated at the
leading edge. These contacts remain stationary with respect to the substrate,
and therefore move away from the leading edge. As they move away, the focal
contacts mature with additional components such as vinculin being added to
them. Finally, upon reaching the cell rear, the focal contacts disassemble. At
the same time, new focal contacts are continuously formed at the leading edge
to maintain close contact there.
The formation of adhesions at the rim of the leading edge was recently
shown to be crucial for forward protrusion. The application of very weak
forces
10pN
/μ
m
2
by fluid flow at the leading edge locally stalled forward
protrusion [76]. As expected, these forces were much too weak to stall fila-
ment growth, and indeed actin polymerization continued nearly unaffected.
However, the weak forces appeared to disrupt the formation of nascent ad-
hesions at the leading edge, which resulted in local network growth upward
rather than forward. These results suggest a direct coupling between forward
protrusion at the leading edge and contact formation.
Mechanical force regulates adhesion size and stability [4]. The amount of
tension generated across adhesion complexes depends both on the intracellu-
lar force generation primarily by myosin contraction, and on the mechanical
properties of the substrate. Experiments in other cell types, such as fibrob-
lasts, showed that force enhanced focal adhesion assembly; the size of focal
adhesions correlated with the local traction force [77], and furthermore appli-
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