Actin Isoforms in Neuronal Development and Function - International Review of Cell and Molecular Biology - page 161

more rapidly than α-skeletal actin, but were virtually indistinguishable from
each other. However, when the individual isoforms were polymerized in
the Ca 2+ bound form, β-actin exhibited significantly more rapid filament
nucleation, elongation, phosphate release, and depolymerization compared
to Ca 2+ -bound γ-actin ( Bergeron et al., 2010 ). These results collectively
indicate that β-actin is the more dynamic cytoplasmic actin isoform, but
essentially only in the Ca 2+ -bound state. It was also definitively demon-
strated for the first time that β- and γ-actin can readily copolymerize, with
filament dynamics reflective of the actin isoform composition. Under basal
conditions in most cell types where Mg 2+ is readily available and Ca 2+ is
tightly sequestered, it can thus be expected that β- and γ-actin function
essentially equivalently.
The potential for distinct dynamics of β- and γ-actin is particularly high
in neurons, however, where localized Ca 2+ transients from 1 to 100 µM
can be elicited in response to chemotropic guidance cues in growth cones,
at presynaptic axon terminals preceding synaptic vesicle fusion, as well
as within postsynaptic dendritic spines following synaptic transmission
( Augustine et al., 2003 ; Henley and Poo, 2004 ; Sabatini et al., 2002 ). It is
perhaps no coincidence that actin, and specifically β-actin, is thought to
have a crucial role in nearly all of these processes. Because actin has a higher
affinity for Ca 2+ than Mg 2+ ( Carlier et al., 1986 ), it is possible that these
local transients could result in functionally relevant concentrations of Ca 2+ -
bound actin. Additionally, local translation of β-actin has been proposed to
occur in response to chemotropic guidance cues and synaptic transmission
( Leung et al., 2006 ; Sasaki et al., 2010 ; Tiruchinapalli et al., 2003 ; Welshhans
and Bassell, 2011 ; Yao et al., 2006 ), which both involve local Ca 2+ transients,
providing another opportunity for actin to complex with Ca 2+ . Neurons
may thus represent one of the few examples where the distinct polymeriza-
tion rates of β- and γ-actin may be able to influence cell development and
function.
4. REGULATION OF ACTIN ISOFORMS IN NEURONAL
DEVELOPMENT AND FUNCTION
Very little is known regarding the molecular regulation of actin iso-
forms in neurons. In the following section, we will highlight studies that
have identified sometimes distinct regulation of actin isoforms predomi-
nantly in cell lines, but also with a high likelihood of relevance for neuro-
nal development and function as well. In a few instances, specific neuronal