Actin Isoforms in Neuronal Development and Function - International Review of Cell and Molecular Biology - page 178

The role for local translation in axons and growth cones remains con-
tentious, however, and more clarity would likely be provided with in vivo
studies to complement the predominantly in vitro work performed so far.
We recently generated a CNS-specific β-actin KO mouse (CNS- Actb KO),
where β-actin protein levels were rapidly depleted from the embryonic
mouse brain ( Cheever et al., 2012 ). Interestingly, we found normal position-
ing and morphology of the anterior commissure and rostral corpus callo-
sum, suggesting that axon elongation and guidance can occur normally in
the absence of local translation of β-actin.Yet, we cannot rule out the possi-
bility that preexisting β-actin mRNA and protein may be sufficient to facil-
itate the predominantly normal axonal development observed. However,
we feel several pieces of evidence strengthen the suitability of the CNS-
Actb KO model. First, most of the large axonal tracts we examined develop
relatively late embryonically, starting around E14.5-15.5 ( Rash and Rich-
ards, 2001 ). Importantly, in whole brain western blots from CNS- Actb KO
embryos, we observed a 50% decrease in β-actin protein levels just 3 days
after Cre recombinase expression at E13.5 ( Cheever et al., 2012 ), suggest-
ing that preexisting β-actin mRNA and protein were exhausted rapidly to
allow for protein turnover. Furthermore, full length β-actin mRNA tran-
scripts would have ceased to be made even prior to this with Cre expression
at E10.5 based on the gene targeting scheme employed ( Perrin et al., 2010 ).
While most of the axonal tracts examined appeared to have developed
normally, there was one striking exception. The caudal portion of the cor-
pus callosum failed to cross the midline in CNS- Actb KO brains ( Cheever
et al., 2012 ). What is intriguing about this phenotype is that the axons of
the rostral corpus callosum, which were able to navigate successfully, have
an arguably more complex path to follow at roughly the same developmen-
tal timepoint. Pioneer axons of the rostral corpus callosum must interpret
guidance cues and gradients established by a number of different glial and
support cell populations to successfully reach the midline, and then extend
beyond it to reach their synaptic targets in the contralateral side of the
brain ( Lindwall et al., 2007 ; Richards et al., 2004 ). In the caudal portion
of the corpus callosum, however, it is believed that axons simply follow a
preexisting tract of axons that forms 1 day earlier ( Livy and Wahlsten, 1997 ;
Richards et al., 2004 ). This preexisting tract lies immediately ventral to the
corpus callosum and is known as the dorsal hippocampal commissure , composed
of hippocampal neuron axons crossing the brain to synapse with the con-
tralateral hippocampus. A netrin-1 gradient localized to the midline of the