Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 142

( Nadar et al., 2012 ), although a specific role for microtubule invasion of
filopodia was not addressed.
The identification of a molecule as required for guidance does not, in
and of itself, shed light on the underlying mechanism, it merely involves it
in guidance. Ultimately, it will be required to understand where and when
specific molecules act in the growth cone to coordinate the sequence of
localized signaling and cytoskeletal events that result in the coupling of the
actin- and microtubule-based cytoskeleton. Due to technical reasons, many
studies rely on inferences from the effects of depleting or overexpressing
a molecule of interest in neurons over prolonged time periods. The inter-
pretation of these types of experiments is clearly subject to many caveats as
molecules often have multiple roles in different domains of the neuron. For
example, chronic depletion of a motor protein is likely to have cell-wide
effects, and affect multiple cellular systems, which could in turn generate
nonrelevant effects when assayed in subcellular compartments such as filo-
podia. Experiments designed to directly address the issues of guidance will
require high spatiotemporal resolution imaging studies to determine how
cytoskeletal dynamics are affected during guidance under conditions where
the molecule of interest is acutely inhibited or activated ( Nadar et al., 2012 ).
5.3. Actin and Microtubules in the Formation of Axon
Collateral Branches
The entry and capture of a microtubule into an axonal filopodium are
required events in the maturation of a collateral branch (Section 2.3 ;
Gallo, 2011 ). The formation of axon collateral branches provides an excel-
lent model system for studying interactions between the actin and micro-
tubule cytoskeleton. Branches arise from specific sites along the axon and
thus represent a highly spatially restricted cellular process. Furthermore,
in axons, microtubules are aligned in parallel arrays and their tips tend to
polymerize in a most linear manner toward the distal axon. In contrast,
the base of most axonal filopodia is oriented orthogonal to the direction
of microtubule tip polymerization. Interestingly, the mechanisms of axon
branching and growth cone guidance have been suggested to be similar
by multiple reviews ( Dent and Gertler, 2003 ; Schmidt and Rathjen, 2010 ;
Gallo, 2011 ).
As noted in Section 3.2 , the base of the filopodium is the site where
microtubules and the actin filament bundle of axonal filopodia first come
into contact.Two proteins that interact with both actin filaments and micro-
tubules, or associated proteins, specifically target to the base or proximal