Biology Reference
In-Depth Information
a powerful self-organizing system; filaments near enough to the leading edge to be useful can
give rise to daughter filaments, and those that project to the leading edge at the optimum
angle grow most and last longest, and can in turn give rise to daughter filaments. Badly
placed or badly directed filaments are capped and fail to grow.
The association between branches and their mother filament is not stable forever;
in vitro
the half-life of a branch before it dissociates from the mother filament is about 500 seconds.
15
This is in remarkably close agreement with the half-life of the
g
-phosphate remaining on the
actin's bound ATP, and is altered by experimental treatments that alter the
g
-phosphate half-
life, in exactly the way one would expect if
g
-phosphate dissociation were to be the trigger for
branch dissocation.
15
In living cells, the half-life of the branched filaments is much shorter
(factors such as ADF/cofilins, which are located just behind the leading edge, accelerate
g
-
phosphate dissociation). The freed filaments are potentially able to join up by associating
pointed end to barbed end, to make long, unbranched filaments; this works
in vitro
although
it is not yet clear how capping would not make the barbed ends inaccessible to being rejoined
in vivo
. Either way, it is clear that a little way back from the leading edge of a lamellipodium,
the highly branched network of short actin filaments is remodelled into a zone of long and
unbranched filaments. Those filaments that do not survive to become long ones are broken
up by ADF/cofilin. Profilin binds the actin from dissociated filaments to enable it to bind
fresh ATP so that it can be used again.
With respect to the reference frame of the cell, specific pieces of actin therefore move back-
wards, becoming polymerized at the leading edge of the cell, being pushed back away from
the edge by their daughters and eventually becoming unpolymerized (or perhaps re-
arranged) at the trailing edge of the lamellipodium. This 'treadmilling', which can be demon-
strated by photo-bleaching and by birefringence imaging,
16,17
usually takes place with
respect to the reference frame of the substrate (Petri dish, and so on) as well as that of the
cell.
17
FILOPODIA IN CELL CRAWLING
The lamellipodium is not the only structure that may be found at the leading edge of
a migrating cell: some leading edges, particularly those of growth cones, also bear many filo-
podia (
Figure 8.6
). Filopodia are long spikes supported internally by parallel filaments of
actin, arranged in the barbed-end-outwards orientation typical of cellular protrusions (see
Chapter 5). Filopodia and lamellipodia are both based on actin, but filopodia have no
Arp2/3 and almost none of the capping proteins that are so common in lamellipodia.
They are, instead, relatively rich in fascin,
18
which is a protein that cross-links filaments in
a bundle. Although a leading edge or growth cone may possess filopodia for a long time,
the life of an individual filopodium is short and each generally follows a relatively rapid
course of protrusion and contraction.
19
The timing and rates of these phases are determined
locally and can vary between different filopodia, even between different filopodia that exist
simultaneously on the same growth cone of a single neuron.
20
Despite their morphological differences, lamellipodia and filopodia have much in
common. Both are based on actin (albeit arranged differently), and the actin in both shows
treadmilling. In filopodia, as in lamellipodia, actin moves backwards towards the main
