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A
D
Filopodium
Growth
cone
Actin linked to a
substrate as a clutch
Microtublues
Myosin
Actin
Filopodium
Axon
Axon
before
contact
Substrate
B
Myosin pulls body
of growth, release clutch
Laminin
Substrate
C
Filopodia grow by adding
actin at tips, engage clutch
Substrate
Guidepost
cells
FIGURE 5.13 Single filopodia can direct growth cones. A. A single filopodium from a growth cone exerts
tension and pulls on an axon it contacts in culture. B. A single filopodium touches a laminin-coated spot in
a culture dish and reorients. C. A single filopodium of a Ti1 cell contacts a guidepost cell, and by the process
of microtubule invasion becomes the new leading edge. D. In the hypothetical clutch mechanism, myosin at
the base of filopodia, pulls on actin cables that are attached to the substrate through a transmembrane clutch
and so pulls the main body of the growth cone forward. (After Mitchison and Kirschner, 1988)
associated with microtubules and actin. Several micro-
tubule-associated proteins (MAPs) are found in
growing neurites, and these proteins may regulate the
growth process. Some MAPs seem to be involved in
stabilizing microtubules and inducing them to form
bundles. Different MAPs are differentially localized:
MAP2 is mostly in dendrites, and Tau is in axons. Anti-
sense mediated depletion or knockouts of many of
these MAPs diminish neurite outgrowth (Letourneau,
1996). MAPs may also interact with actin fibers and
cause bundling of F-actin, or cross linking of actin
fibers to microtubules, thus leading to the stabilization
of actin-microtubule complexes. Microtubule destabi-
lizing proteins such as SCG10 and stathmin are also
important for growth cone function. For example,
SCG10 overexpression in growth cones leads to
enhanced dynamic instability of microtubules and
increased neurite outgrowth (Grenningloh et al., 2004).
There is also an array of actin-associated proteins (more
than 50 have been identified) involved in growth cone
function. Cofilin is an actin-binding protein found in
the growth cone that increases the rate of actin dissoci-
ation, working a bit like SCG10 does on increasing
dynamic instability, but on actin filaments rather than
microtubules. Arps promote the branching of actin fil-
aments, and the Ena/Vasp proteins that localize to
filopodial tips act as anticapping agents, encouraging
straight actin filaments to grow at the leading edge
 
 
 
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