Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 141

general role of myosin II in maintaining the growth cone polarized. As dis-
cussed further in Section 5.3 , septin 7 localizes at the base of axonal filopo-
dia and promotes the localization of microtubules into filopodia, and septin
7 binds both microtubules and actin filaments ( Hu et al., 2012 ). Interestingly,
septin 7 is also found at the base of growth cone filopodia suggesting that it
may have a similar role in microtubule capture during guidance ( Fig. 3.7 ).
Collectively, these studies suggest that localized regulation of myosin II dur-
ing growth cone guidance could serve as a mechanism for controlling the
invasion of filopodia by microtubules, which may be captured by septins.
Microtubule motors may also have a role in regulating the interac-
tions between the actin and microtubule cytoskeleton during growth
cone-mediated guidance. The microtubule motor dynein may act opposite
to myosin II-based retrograde flow, thus allowing microtubules to enter
filopodia ( Myers et al., 2006 ). Kinesin 12 regulates aspects of the interac-
tions between microtubules and filopodial actin bundles ( Liu et al., 2010 ).
Similarly, kinesin 5 negatively regulates the advance of microtubule tips
in growth cone filopodia and contributes to the establishment of asym-
metries in microtubules across the growth cone when turning toward
beads coated with nerve growth factor ( Nadar et al., 2008 ). Growth cones
also turn toward the source of a pharmacological inhibitor of kinesin 5
Figure 3.7 Septin 7 (SEPT7) immunolocalization in sensory axons. SEPT7 targets to the
base of filopodia both along the axon shaft (left panel) and in growth cones (middle
panel). Accumulations of SEPT7 at the lateral sides of the base of filopodia are denoted
by green arrowheads. SEPT7 is also found at the base of established collateral branches
(right panel), indicating that it is retained there after the filopodium has matured into a
branch. These data were obtained through collaborative work with Dr E. Spiliotis (Drexel
University) ( Hu et al., 2012 ). For interpretation of the references to color in this figure
legend, the reader is referred to the online version of this topic.