Biology Reference
In-Depth Information
FIGURE 8.6
A typical growth cone, showing both lamellipodia and also filopodia that contain long, parallel
bundles of actin cross-linked by fascin.
body of the cell as new filaments are added to the outer, barbed ends.
17,20
The speed of retro-
grade flow is more or less independent of the rate of advance of the cell as a whole. The two
structures also have an intimate relationship with each other in the cell; filopodia are rooted
in lamellipodia and seem to arise by modification of the lamellipodial network. The first sign
of filopodial growth is the emergence of small
-shaped groups of actin filaments, distinct at
their apices but melting into the general pool of lamellipodial filaments at their bases. The
apices of the
L
-precursors, as they are called,
21
are enriched in proteins associated with filo-
podia, such as Ena-VASP and fascin but are not associated with the branch-initiator, Arp2/3;
the
L
-precursor therefore seems to represent a gradual transition between lamellipodium-
like character at its base and filopodium-like character at its apex. Actin filaments then elon-
gate considerably, outwards from the apical ends of
L
-precursors and, as this happens, other
filaments cluster with them. This takes place in an initially fascin-independent manner, but
the filaments later become cross-linked with fascin.
21
Cross-linking of filaments gives the
assembly more strength and resistance to bending than is available from a single filament,
and the filaments can therefore support a long extension of the plasma membrane. As tread-
milling takes place, the original associations between the filopodium and the underlying
lamellipodial network are lost and the relationship becomes less obvious.
The formation of both lamellipodia-like and filopodia-like arrays of actin can be demon-
strated from the same brain extract
in vitro
. When beads coated in either of the Arp2/3 acti-
vators, WASP or SCAR, are placed in rat brain extracts, two distinct actin morphologies result
around beads. One, common in undiluted extracts, is a lamellipodium-like network of short
filaments while the other, more common in diluted extracts, is a star-shaped arrangement of
'arms' composed of long, parallel actin filaments arranged and orientated as they would be in
filopodia.
22
Both depend on the presence of Arp2/3. High-resolution electron microscopy of
the system reveals that a lamellipodium-like network forms initially, and filopodial bundles
form from this by addition of new monomers to the uncapped ends of filaments and by the
'zippering' of adjacent filaments. The key determinant of whether the short filaments of the
lamellipodium-like network are able to go on to produce filopodia seems to be the avail-
ability of capping protein; if more capping protein is added to the system, lamellipodial or-
ganization predominates whereas, if it is depleted, filopodial organization is seen.
L
