Mechanisms Underlying the Initiation and Dynamics of Neuronal Filopodia: From Neurite Formation to Synaptogenesis - International Review of Cell and Molecular Biology - page 122

away from the side where transients were elicited. The stabilization of
filopodia by calcium transients is in turn dependent of activation of the
protease calpain within filopodia, resulting in net decreased tyrosine phos-
phorylation of filopodia proteins ( Robles et al., 2003 ). A similar stabilizing
role for filopodial calcium signals has been reported for dendritic filopodia
( Heiman and Shaham, 2010 ). Some aspects of cyclic adenosine monophos-
phate (cAMP) signaling are also selectively regulated in growth cone filo-
podia. Han et al. (2007) found that the targeting of type II protein kinase A
to filopodia is required for the regulation of growth cone turning by cAMP.
Similarly, Nicol et al. (2011) described the presence of cAMP transients in
growth cone filopodia, which in turn regulate filopodial calcium transients.
Filopodia also respond to extracellular signals with increases in the levels of
the signaling lipid phosphatidylinositol (3,4,5)-triphosphate (PIP3), gen-
erated through activation of phosphatidylinositol 3-kinases (PI3K). Eleva-
tions of PIP3 have been reported in both dendritic and axonal filopodia in
response to neurotrophin signaling ( Luikart et al., 2008 ; Ketschek and Gallo,
2010 ). Elevations in the activity of the Rho-family GTPase Cdc42 have
also been detected in growth cone filopodia and found to correlate with
active filopodial extension ( Myers et al., in press ). These studies indicate that
filopodia serve as specialized signaling domains in the development of both
axons and dendrites.
3.6. Biomechanics of Filopodia
Atomic force microscopy (AFM) is a versatile imaging method that can
also be used to extract quantitative information on the mechanical and
volumetric properties of cells ( Müller and Dufrêne, 2011 ). AFM analysis
of growth cone filopodia of cultured invertebrate and vertebrate neurons
revealed that these filopodia are between 100-900 nm wide and 40-270 nm
in height ( Grzywa et al., 2006 ; Laishram et al., 2009 ; Xiong et al., 2009 ). As
noted in Section 3.2 , the tip of the filopodium is considered to be a special-
ized compartment for the active polymerization of actin filaments during
filopodial dynamics. Interestingly, AFM analysis revealed that the tip of filo-
podia is 2.5× and 1.5× higher and wider, respectively, than the shaft of the
filopodium ( Grzywa et al., 2006 ). Relative to the lamellipodial structures of
the growth cone peripheral domain, filopodial actin bundles are 30-50 nm
higher ( Xiong et al., 2009 ). Thus, filopodial actin bundles are physically
prominent features of the growth cone cytoplasm.
The filopodial actin filament bundles also exhibit different mechani-
cal properties than the adjacent lamellipodia. Young's modulus, also known