Actin Isoforms in Neuronal Development and Function - International Review of Cell and Molecular Biology - page 199

light on whether synaptic transmission defects may be contributing to the
behavioral defects observed in CNS- Actb KO mice.
We have also stressed the potential role of Ca 2+ signals in promoting
distinct functions for the cytoplasmic actin isoforms in neuronal develop-
ment and function. Given that local Ca 2+ signals factor into nearly every
major function of neurons, and that actin in the Ca 2+ -bound form is the
only paradigm where functional distinctions between β- and γ-actin have
been directly demonstrated, we feel that this issue merits particular atten-
tion. Until a method can be devised to “lock” a particular actin isoform in
a Ca 2+ - or Mg 2+ -bound form, one experiment might be to knock β-actin-
coding sequences into the γ-actin locus and vice versa. By utilizing this
approach, Ca 2+ -dependent responses to guidance cues and synaptic stimuli
could be assessed without having to perturb global cytosolic Ca 2+ signals,
which undoubtedly interact with a large number of proteins besides actin.
One would thus be able to determine whether a neuron expressing only
γ-actin (but under the control of Actb and Actg1 genetic loci) exhibits
impaired morphological or functional responses to stimuli. If so, this might
suggest that having β-actin with its distinct polymerization rate in the pres-
ence of Ca 2+ may be functionally relevant. However, this interpretation is
working under the assumption that the four amino acid difference between
β- and γ-actin only affects the polymerization dynamics in the presence of
Ca 2+ , which may be an oversimplification.
7.2. In Vivo Studies
One commonality between our studies and those of other groups is that
perturbation of β-actin localization or levels in general rarely affects the basal
morphology of cultured neurons and many neurons in vivo for that matter.
These findings may have been initially surprising, but now appear to sug-
gest that β-actin may only be critical when a cellular response is stimulated.
While these conditions can sometimes be mimicked in vitro, it is likely that
in vivo experiments will be required particularly because there are essentially
no phenotypes currently known in cultured β-actin null neurons to attempt
to rescue. Although in vivo experiments can be much more time consuming,
it is promising that many useful transgenic mouse lines are already available.
In our opinion, the most pressing in vivo question is whether any of the
phenotypes observed in the CNS- Actb KO mouse model are due specifically
to the inability of neurons to locally translate β-actin. The most straightfor-
ward way to address this question would be to generate a transgenic mouse
expressing β-actin but lacking the 3′ UTR zipcode sequence, and determine