Biology Reference
In-Depth Information
Rac is a member of a family of GTPases, other members of which, such as cdc42, can also
lead to activation of Arp2/3. Although both Rac and cdc42 lead to activation of Arp2/3, Rac
activation tends to be associated with the formation of lamellipodia and cdc42 with filopodia.
Until recently, it was assumed that Rac and cdc42 acted in completely separate parallel path-
ways to drive formation of lamellipodia and filopodia, respectively, but a number of experi-
ments have suggested that both act on Arp2/3 in the same way to achieve the same
nucleation of actin branches.
31
The difference between the favoured actin morphologies
seems to arise mainly from other activities of Rac and cdc42 that have diverged between
the two proteins (
Figure 8.7
). Filopodia are favoured when growing actin filaments are pro-
tected from capping, and cdc42 can provide this protection by signalling via IRSp53 to cause
recruitment of Mena to the filament tips at the ends of filopodia: Mena protects the filament
ends from capping.
23,32,33
Some formins are also effectors of cdc42 and encourage elongation
rather than capping of actin filaments.
34
ADVANCE OF THE CELL BODY
Locomotion of crawling cells is possible only if they can exert force on their substratum; by
elementary Newtonian physics, they can move forward only by pushing their environment
backwards. The forces that cells exert on their environment can be visualized by growing
cells on very thin, deformable films.
35
The system worked well enough to indicate when
tension was being generated, but the large size of the wrinkles limited spatial resolution
and the highly non-linear physics of wrinkling made absolute measurements of force difficult
to obtain. Silicone has therefore been replaced by pre-tensed polyacrylamide films, with
a Young's modulus as low as 1
e
10 kN/m
2
, in which tiny fluorescent marker beads are
embedded. Up to the elastic limit of the film, movement of the beads is proportional to the
force exerted so that forces can be calculated from displacement of the beads in the
substrate.
35
The leading edge of advancing 3T3 fibroblast cells produces strong traction
directed inward towards the centre of the cell body, but inside the lamellipodium proper
is a zone with precisely the opposite orientation (so that the substrate is being tugged
towards the front of the cell;
Figure 8.8
).
Several things may be deduced from this observation. The transmission of these forces to the
substrate at all indicates that the cell must have strong mechanical connections with the
substrate at each side of the lamellipodium/cell body boundary. The convergence of force on
this boundary has been taken to indicate that much of the mechanical work done by the cell
takes place there.
36
From the point of view of tension-generating mechanisms that pull the
substrate towards this boundary, this must be so. It should be remembered, though, that the
compressive forces generated by extension of the lamellipodium against the surface tension
of the membrane system will tend to push the substrate backwards towards the boundary
and will also result in an inwards force on the substrate under the lamellipodium itself.
While actin filaments in the lamellipodium are all arranged with their barbed ends point-
ing outwards, filaments in the cell body are orientated both ways round.
37
This allows them
to line up in an antiparallel fashion cross-linked with myosin. The myosin will 'try' to slide
the filaments past each other if the filaments are free or, if the filaments are attached, it will
place the filaments under tension. Unlike the filaments in the lamellipodium, which show
