Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 139

extending growth cone to sample its environment and respond appropri-
ately. The response of a single filopodium to a positive/attractant guidance
signal is sufficient to reorient the growth cone. However, for guidance
to occur, the filopodium must be invaded by one or more microtubules
( Gallo and Letourneau, 2000 ; Dent and Gertler, 2003 ), which provide
structural support for the development of a new segment of axon shaft in
the direction of the signal. Microtubules will also allow for the organelles in
the central domain of the growth cone to undergo vectorial redistribution
toward the signal ( Lin and Forscher, 1993 ).
Microtubules can penetrate individual growth cone filopodia ( Fig. 3.6 ,
arrowheads), and similarly, axonal filopodia ( Fig. 3.6 , arrow). When axons
are undergoing extension in the absence of guidance signals, the penetra-
tion of microtubules in filopodia is likely randomly distributed along the
growth cone perimeter. However, individual filopodia that make contact
with a source of a guidance cue (e.g. the surface of a cell) become invaded
by one or more microtubules and retain the microtubule(s) while the
growth cone reorients. Thus, there may be two major regulatory mecha-
nisms in this process: (i) regulation of the entry of microtubules in filopodia
and (ii) the retention/stabilization of microtubules in the filopodia. Both
these mechanisms likely impact the dynamic instability of microtubule tips,
which otherwise exhibit cycles of polymerization and depolymerization
occurring on a much smaller timescale (a few tens of seconds) than the
reorientation of the growth cone (minutes to tens of minutes).
Molecules that link the actin and microtubule cytoskeleton, physically
or functionally, have been identified and suggested to have roles in growth
cone guidance ( Vitriol and Zheng, 2012 ). However, direct analysis of the
Figure 3.6 Microtubules enter neuronal filopodia. Example of a cultured sensory axon
stained to reveal actin filaments (red) and microtubules (green) following combined
fixation and extraction, a protocol which removes soluble tubulin but retains tubulin in
polymeric form (i.e. microtubules) ( Gallo and Letourneau, 1999 ). Microtubules are found
in subsets of filopodia both at the growth cone (arrowheads) and along the axon shaft
(arrow). For interpretation of the references to color in this figure legend, the reader is
referred to the online version of this topic.