| When
the locomotion of tissue cultured vertebrate cells are studied using
time-lapse photomicrography, it can be appreciated that the most motile
part of the cell is typically the very leading edge which has been
described as "ruffling" (see Cell
Locomotion). It is now known that this is
driven by cycles of actin polymerization and depolymerization and that
actin "treadmills" as actin monomers add at the very leading
edge, move with respect to the leading edge toward the cell centre where
actin monomers are removed from the pointed ends. This flux of
F-actin from the leading edge to the cell body has been demonstrated by
a number of ingenious experimental approaches. In one of the
earliest such studies the cellular actin pool was labelled with
fluorescent actin by microinjection and after this has equilibrated with
the endogenous actin, a laser was used to bleach a spot in the leading
edge. This spot was then observed to move rearward as the cell
crawled forwards (Wang,
1985).
A similar experiment (Theriot
& Mitchison, 1991),
labelled cells with a fluorescent actin label which became fluorescent
after activation by a brief pulse of laser light. Again, the
marked actin population was seen to move rearward. Using the very large
flat lamella of the neurons (so called "bag cells") of Aplysia
(a nudibranch or sea-slug), and two fungal actin-binding drugs cytochalasin
and phalloidin (Forscher
& Smith, 1988)
it has been shown that actin tread-milling results from addition of
actin monomer onto the barbed ends of filaments that abut the membrane
at a high angle. This was shown by treating the cells with cytochalasin
that blocks actin polymerization at the barbed end, and then fixing and
staining the cells with rhodamine labelled phalloidin which binds
F-actin. This showed that filaments shrank back from the leading edge as
polymerization was blocked at the barbed end.
In cells the
situation is very much more complex than simply of actin filaments
tread-milling as the whole process is regulated by a huge number of
different types of actin binding proteins such as the ADF/Cofilins
and the Arp2/3 complex. |
|
References:-
Forscher, P. & Smith, S. (1988)
Actions of cytochalasins on the organization of actin filaments and
microtubules in a neuronal growth cone, J. Cell Biol. 107,
1505-1516.
Theriot, J. A. & Mitchison, T. J.
(1991) Actin microfilament dynamics in locomoting cells. Nature. 352,
126-131.
Wang, Y.-L. (1985) Exchange of actin
subunits at the leading edge of living fibroblasts: Possible role of
treadmilling, J.Cell Biol. 101, 597-602. |
