Biology Reference
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mRNAs were identified, which is not surprising given the large repertoire
of muscle miRNAs. However, most of these misexpressed mRNAs contain
predicted binding sites for either
miR-1
or
miR-133
, suggesting that these
two muscle-specific miRNAs play a predominant role in the miRNA-
dependent control of muscle gene expression. Further, these muscle
miRNA targets are enriched for actin-related and acting-binding proteins,
providing a mechanistic explanation for the morphological defects displayed
by MZ
dicer
mutants.
Phenotypic analysis of
dicer
mutants has also been used to analyze the role
of miRNAs during muscle development in mice. Unlike MZ
dicer
-deficient
zebrafish embryos, however, mouse embryos that are depleted of zygotic
dicer
arrest early in development at embryonic day 7.5 (E7.5) and are
shrunken and malformed (
Bernstein
et al
., 2003
). These
dicer
mutant
embryos express the early mesodermal marker
brachyury,
indicating that
mesodermal cells form, but its expression domain is reduced, suggesting
that the progression of patterning is abnormal in these embryos (
Spruce
et al
., 2010
). Since cultured Dicer-deficient mouse embryonic stem cells
have proliferation and general differentiation defects due to the inability to
silence pluripotency factors (
Kanellopoulou
et al
., 2005
;
Murchison
et al
.,
2005
;
Sinkkonen
et al
., 2008
), they have not been useful for investigating
the requirement for miRNAs in cell fate specification of the mouse meso-
derm.
miR-1
and
miR-133
have been directly implicated in these early cell
fate decisions, however, since forced expression of either in mouse embry-
onic stem cells results in enhanced mesoderm gene expression and a
corresponding reduction in ectodermal and endodermal gene expression
(
Ivey
et al
., 2008
).
Subsequent roles for Dicer during the development of cardiac muscle,
skeletal muscle, or smooth muscle have been investigated by a series of
studies using a conditional knockout approach. These studies have made use
of different floxed
dicer
alleles in combination with tissue-specific Cre lines
that together disrupt
dicer
function in specific spatiotemporal and tissue-
specific patterns during mouse development. The results of these studies are
summarized below, but in general Dicer is implicated in the morphogenetic
events of muscle systems rather than the initial cell fate specification of
muscle subtypes. This result may be a consequence of the conditional
depletion strategy, however, since early cell fate specification may have
already occurred at
the time when Dicer depletion has
functional
consequences.
3.1.1. Mouse skeletal muscle
The role of Dicer in skeletal muscle development has been investigated
using a MyoD-Cre line, which is expressed in developing skeletal muscle in
the mandibular arch, somitic myotomes, developing limb buds, and trunk
musculature between E9.75 and E12.5 (
O'Rourke
et al
., 2007
). These mice
