Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 137

growth cone turning away from repellent signals following filopodial con-
tact. During development, extending motoneuron growth cones migrate
through sclerotomal tissue and avoid the posterior sclerotome. Motoneu-
ron growth cones making filopodial contact with posterior sclerotome cells
in vitro turn away from the site of contact ( Oakley and Tosney, 1993 ). Filo-
podia that contact posterior sclerotome cells do not undergo retraction and
are stabilized following contact. However, the filopodial contact impairs the
ability of growth cone lamellipodia to advance in the direction of contact.
This establishes an asymmetry in lamellipodial protrusion at the growth
cone, with more lamellipodia extending away from the site of filopodial
contact, and thus contributing to redirecting the growth cone away from
the contact. Inhibition of lamellipodial advance along filopodia was also
noted when filopodia were experimentally stabilized through the induction
of intrafilopodial calcium transients (Section 3.5 ). Thus, the stabilization of
a growth cone filopodium, in and of itself, is not a predictor of the response
of a growth cone to a guidance signal, and filopodia may be stabilized by
multiple mechanisms having opposite functional outcomes.
In the context of the stabilization of the filopodium during turning
toward an attractant, the stabilization of the filopodium may be an indirect
result of the entry of microtubules into the filopodium. However, inhibi-
tion of microtubule dynamics, and thus, the entry of microtubules in filo-
podia during growth cone guidance induced by contact with nerve growth
factor-coated beads does not prevent filopodial stabilization, although it
prevents turning ( Gallo and Letourneau, 2000 ). Conversely, treatment with
the kinase inhibitor KT5926 prevents growth cone turning toward nerve
growth factor-coated beads, but does not prevent the stabilization of the
contacting filopodium, indicating additional KT5926-sensitive mechanisms
are operative, and filopodial stabilization by itself is not sufficient for turning
( Gallo et al., 1997 ).
The cytoskeletal proteins that regulate the stabilization of filopodia dur-
ing guidance are not well understood. Gelsolin is an actin filament severing
protein that is found throughout axons and growth cones ( Lu et al., 1997 ).
Surprisingly, neurons from the gelsolin knockout mouse exhibit no obvi-
ous morphological abnormalities. However, a careful morphometric screen
revealed that gelsolin knockout neurons exhibit an increased number of
filopodia along their axon, and that this is due to retention of filopodia
that would normally retract (Lu et al., 1997). The tip of filopodia of gelso-
lin knockout neurons extended at the same rate as wild-type neurons, but
exhibited decreased rates of retraction and increased time spent pausing.