Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 140

roles of these proteins, or signaling mechanisms, in the cytoskeletal dynam-
ics underlying filopodia-mediated growth cone guidance is minimal. Sur-
prisingly, very little is known about how microtubule tips are targeted and
retained in filopodia. Thus, understanding the mechanisms that regulate the
targeting or retention of microtubules in filopodia during guidance remains
a frontier. Even less is known about whether microtubules are negatively
regulated in growth cones during guidance in response to repellent signals.
Bentley et al. established the fundamental sequence of events underlying
growth cone turning toward attractants through single filopodial contact
( Sabry et al., 1991 ; O'Connor and Bentley, 1993 ; Bentley and O'Connor,
1994 ). In a series of papers, they used live imaging approaches to describe
the cytoskeletal (actin and microtubule) reorganization underlying the
guidance of grasshopper Ti1 pioneer growth cones in the developing limb
bud. This guidance event is initiated and mediated as a filopodial contact
with specific guidepost cells. In filopodia that contact targets, actin filaments
initially accumulate at the base of the filopodium and also undergo a degree
of retrograde flow toward the body of the growth cone. In concert with
the ensuring alterations in the filopodial actin cytoskeleton, microtubules
were observed to undergo excursions into and out of filopodia, as well
as retention in filopodia that demarcated the future direction of growth
cone extension. A similar sequence of cytoskeletal reorganization has been
described in the growth cones of sensory neurons undergoing guidance
toward a source of nerve growth factor ( Gallo and Letourneau, 2000 ).
Using the growth cones of Aplysia bag cell neurons, Forscher et al. have
provided additional insights into the interactions between actin filaments
in filopodia and growth cone microtubules during guidance, again through
live imaging approaches. In these growth cones, microtubules often align
with filopodial actin filament bundles. Microtubules align and polymerize
along filopodial actin bundles, but also engage actin retrograde flow and are
in turn cleared from the peripheral domain ( Schaefer et al., 2002 ). Specific
depletion of filopodial actin bundles resulted in increased penetration of
microtubule tips in the growth cone peripheral domain, and decreased ret-
rograde flow of microtubules toward the central domain ( Burnette et al.,
2007 ). Actin filament retrograde flow is in part mediated by myosin II
( Lin et al., 1996 ). Consistently, inhibition of myosin II promotes the distance
that microtubules penetrate filopodia ( Ketschek et al., 2007 ). Finally, myosin
II is required for growth cone turning at substratum borders and in response
to nerve growth factor ( Turney and Bridgman, 2005 ; Loudon et al., 2006 ),
although the failure to undergo guidance may also be related to a more