Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 126

a striking enrichment of actin filaments, the axon and dendrites contain
much lower amounts of actin filaments. Along axons and dendrites, filo-
podia emerge from dynamic accumulations of actin filaments both in vitro
and in vivo ( Andersen et al., 2005 ; Loudon et al., 2006 ; Mingorance-Le
Meur and O'Connor, 2009 ; Ketschek and Gallo, 2010 ; Korobova and
Svitkina, 2008 , 2010 ; Spillane et al., 2011 ; Andersen et al., 2011 ; Spillane
et al., 2012 ), which will be referred to as actin patches ( Fig. 3.4 A,B). The
formation of actin patches is dependent on the Arp2/3 complex, and actin
patches exhibit actin filaments consistent with Arp2/3-mediated nucleation
( Korobova and Svitkina, 2008 ; Spillane et al., 2011 ). The emergence of
filopodia from actin patches requires the actin-filament-binding septin 6
( Hu et al., 2012 ). Through in vitro reconstitution assays, septin 6 was found
to target to sites of Arp2/3-medited filament branches along actin fila-
ments ( Hu et al., 2012 ). In axons, septin 6 in actin patches promotes the
recruitment of cortactin, a stabilizer of Arp2/3-nucleated filament branches.
Cortactin, in turn, promotes the formation of filopodia from actin patches
( Mingorance-Le Meur and O'Connor, 2009 ; Spillane et al., 2012 ).
A similar “actin accumulation as precursors to filopodia formation” phe-
nomenology was also reported in the growth cones of sympathetic neurons
and PC12 cells ( Goldberg et al., 2000 ), in which case structures that may
be analogous to axonal actin patches were termed splotches . In these experi-
ments, growth cones were deprived of nerve growth factor and were largely
immotile containing minimal levels of actin filaments. Treatment with nerve
growth factor rapidly induced filopodia formation, which was preceded by
the emergence of splotches. Splotches, like axonal actin patches, contained
Arp2/3 subunits. In our own work imaging eYFP-tagged β-actin dynamics
using chicken sensory neurons cultured in nerve growth factor, in which
case, the growth cones are highly motile, we have also observed instances
of filopodia formation being preceded by the formation of actin filament
splotches/patches (Fig. 3 . 4 C). In this example, by 6 s, an actin patch/splotch
has formed (arrowhead) and by 18 s, a filopodium has emerged (arrow).
A patch/splotch to filopodium emergence sequence was most readily
observed in growth cones that did not have prominent lamellipodia. In
growth cones with lamellipodia, the levels of actin filaments were usually
quite high, possibly obfuscating formation of splotches/patches. Rac1 is a
GTPase that contributes to the formation of lamellipodial actin networks,
and inhibition of Rac1 similarly decreased the number of filopodia at the
growth cone and along the shaft in sensory neurons ( Spillane et al., 2012 ).
Thus, there may be conceptual similarities between the mechanism that