Biology Reference
In-Depth Information
containing
miR-196b
and part of
HoxA-10
is expressed in humans (
Mainguy
et al
., 2007
). In chick, EST libraries reveal a transcript with alternate
polyadenylation sites, in which an antisense
miR-196
is present in one
isoform (
Benson
et al
., 2004
;
Kent, 2002
). Humans and mice both transcribe
antisense
miR-196a-1
as well
(
Mainguy
et al
., 2007
). It
is currently
unknown whether or what roles these transcripts play.
6.2. Integrating developmental expression with
target predictions
miR-196
expression analysis has revealed a strong consensus across many
species: expression is axially restricted and more caudal than
miR-10
,asis
expected given its chromosomal position. In zebrafish, robust
miR-196a
expression is observed by 24hpf throughout the posterior trunk including
somitic tissue and the CNS (
Wienholds
et al
., 2005
), similar to adjacent
Hox9
paralog expression
(
http://zfin.org
). With time, expression becomes more
restricted to the CNS, potentially spreading more anteriorly than some
hpf (
Wienholds
et al
., 2005
), with
miR-196a-1
specific pronephric expression
identified following primary transcript detection (
He
et al
., 2011a
). This
dynamic pattern of expression is mirrored quite closely in
Xenopus
(
Qiu
et al
., 2009
). In chick,
Asli and Kessel (2010)
provide evidence that
miR-
196
expression is not initiated prior to Hamburger and Hamilton Stage 15
(HH15), a time when robust expression of the validated
miR-196
target
Hoxb-8
is observed in the hindlimb field and posterior unsegmented paraxial
mesoderm. As development proceeds, the expression of
Hoxb-8
diminishes in
posterior tissue and
miR-196
expression was shown to increase, resolving into
a mutually exclusive pattern at the level of somite 23-24 (
Asli and Kessel,
2010
). This expression pattern is in keeping with either a classic “switch” or
“failsafe” mechanism whereby the miRNA acts at a more posterior location
to either actively repress or reinforce repression of anterior Hox gene expres-
sion and anterior morphological programs. However, these expression data
are not consistent with morphological and molecular observations following
loss-of-function studies in chick (
McGlinn
et al
., 2009
). Here, loss of cumu-
lative
miR-196
by antagomiR-based knockdown resulted in an expansion of
Hoxb-8
at HH12 and importantly, homeotic transformations of vertebral
morphology at the cervical-thoracic boundary (somite 11-12). Classic graft-
ing experiments (
Kieny
et al
., 1972
), complemented by more recent genetic
strategies in mouse (
Carapuco
et al
., 2005
), support the view that vertebral
identity is established in the PSM prior to somite segmentation, suggesting
that very low levels of
miR-196
may function at this time and place.
In mouse, combined analysis of
miR-196a-1
and
miR-196a-2
expression
using LNA probes (
Asli and Kessel, 2010
;
Kloosterman
et al
., 2006
) supports
early transgenic sensor approaches (
Mansfield
et al
., 2004
), highlighting
