Actin Isoforms in Neuronal Development and Function - International Review of Cell and Molecular Biology - page 159

Figure 4.1 Higher vertebrate actin isoform family . Actin isoforms can be split into two
broad classes based on their relative expression patterns. Muscle actins are primarily
found in striated ( α -cardiac and α -skeletal-actin) and smooth muscle ( α -smooth and
γ -smooth-actin) while the cytoplasmic actins ( β - and γ -cytoplasmic or simply β - and
γ -actin) are found in all cells. Also shown is an alignment of the various actin isoform
N-termini. Amino acids conserved across all actin isoforms are highlighted in gray while
the four divergent amino acids between β - and γ -actin are highlighted in yellow. For
interpretation of the references to color in this figure legend, the reader is referred to
the online version of this topic.
3.1. Expression in Neurons and the Nervous System
Clues as to what distinct roles individual actin isoforms may perform have
been primarily derived from investigating potential differential expression
patterns. As their name suggests, the α-actin isoforms are primarily expressed
and localized to the contractile apparatus in striated and smooth muscle,
while the two cytoplasmic actin isoforms, β- and γ-actin, are found in all
tissues. Early work using 2D electrophoresis of proteins from whole brain
and cultured neuron lysates revealed the presence of both β- and γ-actin,
while no α-actin expression was detected ( Choo and Bray, 1978 ; Flana-
gan and Lin, 1979 ). There are reports of small amounts of α-actin mRNA
detected in brain preparations ( McHugh et al., 1991 ; Tondeleir et al., 2009 ),
but it is important to consider that this may be due to vasculature-derived
α-smooth actin integrated into the brain tissue. In one study, α-cardiac
actin was overexpressed in hippocampal neurons and found to form rod-
like inclusions within neurites, suggesting that neurons may lack some fac-
tor important for the proper regulation of α-actin isoforms ( Kaech et al.,
1997 ). Recent work in a central nervous system (CNS)-specific β-actin