Actin Isoforms in Neuronal Development and Function - International Review of Cell and Molecular Biology - page 195

depends on bidirectional interactions between the motor axon and muscle
motor endplate ( Sanes and Lichtman, 1999 ), impaired or abnormal interac-
tions between these cells could be the primary mechanism of pathogenesis.
Interestingly, voltage-gated calcium channels, which function in axon guid-
ance and neurotransmitter vesicle release, were also found to be mislocalized
similar to β-actin in cultured SMA motor neurons ( Jablonka et al., 2007 ).
Thus, β-actin may still play a contributing factor in the pathogenesis of
SMA but perhaps not in early motor axon development or guidance.
We hypothesized that the mislocalization of β-actin in SMA motor
neurons leads to a loss-of-function of β-actin and motor neuron disease.
In order to test this hypothesis, we generated a motor-neuron-specific
β-actin KO mouse (MNs- Actb KO) by crossing a floxed β-actin allele to the
motor-neuron-specific transgenic Cre line Mnx1 (also known as Hb9). Cre
expression in this transgenic line is initiated at E9.5 corresponding with the
onset of motor neuron specification. Surprisingly, MNs- Actb KO mice were
viable and morphologically indistinguishable from controls ( Cheever et al.,
2011 ). MNs- Actb KO mice had normal numbers of motor neurons at both
6 and 12 months of age, and no indication of morphological NMJ abnor-
malities. This was corroborated by a number of behavioral and histological
assays, which revealed normal motor function and a lack of any neurogenic
atrophy in muscle histology. CNS- Actb KO mice, which also express Cre
recombinase in motor neurons in addition to neurons and glia throughout
the brain, were also subjected to the same motor function tests with com-
parable results ( Cheever et al., 2012 ).
The study described above suggests that β-actin has a nonessential role in
motor axon function; however, one caveat to this study is that it remains pos-
sible preexisting β-actin mRNA and protein were sufficient for the normal
motor axon development that occurs rapidly in vivo. In order to more directly
test whether β-actin is required for motor axon elongation as predicted by
the cell culture work described above, we performed a peripheral nerve
regeneration model in adult animals at a timepoint well after Cre-mediated
recombination of the β-actin locus. Additionally, β-actin protein and RNA
localization have also been directly implicated in the regenerative response
in several different adult nerve regeneration paradigms, providing even fur-
ther precedent for a critical role of β-actin in nerve regeneration ( Lund and
McQuarrie, 1996 ; Willis et al., 2005 , 2011 ; Zheng et al., 2001 ). Here again,
however, we found no significant differences in the functional or histological
regeneration of motor axons lacking β-actin compared to controls ( Cheever
et al., 2011 ). Thus, β-actin does not appear to be required for motor axon