Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 117

Gallo, 2010 ). In both cases, the emergence of the new filopodium is pre-
ceded by an accumulation of actin at the site of formation, as revealed by
live imaging of fluorescently tagged actin. While the biological significance
of the formation of new filopodia from a preexisiting filopodial shaft is not
clear, these observations demonstrate that the filopodial shaft is capable
of giving rise to additional protrusive activity, and thus, must contain the
relevant molecular mechanisms.
The tip of the filopodium represents a specialized subfilopodial domain
and atomic force microscopy measurements indicate that tips are thicker
than the shaft ( Grzywa et al., 2006 ). The tips of filopodia represent a domain
where the barbed ends of actin filaments are linked to the membrane, and
likely membrane spanning proteins ( Faix et al., 2009 ). Molecules that either
have been described to be specifically targeted to tips or have been shown
to strongly target to tips are discussed. A recent study investigating the
dynamics of the membrane spanning dopamine transporter (DAT) revealed
accumulation of the levels of surface DAT at the tips of filopodia ( Rao
et al., 2012 ). The localization of surface DAT at the tips of axonal filopodia
is consistent with the notion that neuronal filopodia are specialized to be
precursors to presynaptic structures (discussed in Section 2.4 and further
in Section 3.3 ). Fluorescence recovery from photobleaching experiments
revealed that surface DAT at the filopodial tip exhibits low surface mobility,
suggesting the existence of a mechanism that retains the DAT at the filo-
podial tip. The filopodial tip is generally considered to be the site of actin
filament barbed end polymerization that drives filopodial extension. Con-
sistent with this notion, Ena/VASP target to the tips of growth cone filo-
podia ( Lebrand et al., 2004 ). Similarly, the actin nucleation/polymerization
formins localize to the tips, and shafts, of growth cone filopodia ( Matusek
et al., 2008 ; Gonçalves-Pimentel et al., 2011 ). F-BAR proteins are involved
in membrane deformation and contribute to the formation of filopodia
( Heath and Insall, 2008 ). The F-BAR protein CIP4 has been detected to
accumulate at the tips of filopodia and the lamellipodial leading edge during
neuritogenesis ( Saengsawang et al., 2012 ).
Although the filopodium exhibits a striking degree of subcellular orga-
nization, much remains to be learned about the mechanisms that establish
this organization. It will be of interest in future studies to further detail the
time course of the recruitment of the complement of proteins that target to
filopodia and their subdomains, and how the dynamics of these proteins are
regulated during the different phases of filopodial initiation, extension and
retraction, and by extracellular signals.