Actin Isoforms in Neuronal Development and Function - International Review of Cell and Molecular Biology - page 175

permanent photoconversion of the preexisting reporter protein, neurons
whose axon had been severed were exposed to netrin-1 and showed a sig-
nificant increase in newly synthesized Kaede-β-actin reporter in growth
cones, which was prevented by protein synthesis inhibitors or the expres-
sion of the reporter lacking the β-actin 3′ UTR zipcode sequence. Finally,
the local translation of β-actin was examined in response to local gradients
of netrin-1, which more closely mimic the guidance cue gradients neuronal
growth cones encounter and interpret in vivo. β-actin protein was found to
accumulate on the side of the growth cone proximal to the netrin-1 gradi-
ent, which could be blocked by treatment with protein synthesis inhibitors
and more importantly the antisense morpholino predicted to block β-actin
synthesis. It is interesting to note, however, that only a modest 23% decrease
in β-actin protein was detected in growth cones following treatment with
the morpholino. Nevertheless, that same morpholino completely blocked
attractive turning of growth cones toward a gradient of netrin-1, while it
had no effect on repulsive turning, demonstrating a specific, functional role
for the local translation of β-actin in growth cones.
In the second study, Xenopus spinal neurons were used to demonstrate
again that β-actin mRNA and ZBP3/VgRBP 1 colocalize in growth cones,
which could also be enhanced upon treatment with the guidance cue brain-
derived neurotrophic factor (BDNF) ( Yao et al., 2006 ). This colocalization
was significantly decreased by treatment with antisense morpholinos tar-
geted against the zipcode sequence of β-actin however. Thus, this group
chose to disrupt the interaction of β-actin mRNA with ZBP3/VgRBP
1 rather than block β-actin synthesis with morpholinos used by the first
group. The two distinct approaches demonstrated similar effects however,
as the second group also demonstrated that β-actin became preferentially
enriched on the side of the growth cone proximal to a gradient of an
attractive guidance factor. This finding on its own would not be entirely
unexpected, but what makes it more interesting and substantial is that the
asymmetrical distribution of β-actin was normalized to the distribution of
γ-actin, which did not exhibit as significant an enrichment on the proximal
side of the growth cone as β-actin. While it is well known that local actin
accumulation initiates the membrane protrusion underlying growth cone
turning, this was the first demonstration that β-actin specifically is locally
enriched within the growth cone which does not appear to be the case of
γ-actin, or at least not to the same extent. A similar significant asymmetric
distribution of β-actin compared to γ-actin was also found to occur in
growth cones encountering a repulsive cue, except this time with β-actin