Biology Reference
In-Depth Information
expression in the very posterior hindbrain and into the spinal cord. While
anterior hindbrain expression is more anterior to/mutually exclusive with
miR-10c
, forced
miR-10
expression can reduce both target transcripts at this
location and direct repression is supported by
in vivo
sensor assays
(
Woltering and Durston, 2008
). In the spinal cord where
miR-10
paralogs
are coexpressed with target genes,
miR-10
loss-of-function studies result in
upregulation and posterior expansion of
HoxB1a
and
HoxB3a
, supporting
the
in vivo
relevance of these target gene interactions.
HoxB1b
and
HoxA1a
are also predicted
miR-10
targets; however,
HoxA3a
did not respond to
miR-10
manipulation at least in this developmental context. The repression
of anterior Hox genes by
miR-10
, which was shown to synergize with more
classic Hox-mediated repressive mechanisms, lends strong support to a
multifaceted approach to posterior prevalence.
With this compelling molecular evidence, and the observation that
miR-
10
overexpression phenocopies loss of anterior
HoxB1a
as judged by mis-
patterning of rhombomere 4 and branchiomotor neuron migration, it was
perhaps surprising that no overt phenotype was observed in
miR-10
loss-of-
function fish (
Woltering and Durston, 2008
). This suggests that
miR-10
regulation is not a principal mechanism restricting target Hox gene levels in
the zebrafish embryo, or at least, that additional compensatory mechanisms
limit phenotypic outcomes. To date, loss-of-function studies have not been
performed in any other species and will be an important resource in
dissecting
miR-10
function.
5.
miR-iab-4/miR-iab-8
5.1. Transcript structure, processing, and upstream
regulatory mechanisms
The
Drosophila iab-4
and
iab-8
loci were originally defined through fine-
scale mutational analysis of the
BX-C
and mapped between
abd-A
and
Abd-
B
(reviewed in
Lewis, 1978
). Partially overlapping transcripts are generated
from opposite DNA strands within this region (
Bae
et al
., 2002
;
Cumberledge
et al
., 1990
), generating antisense miRNAs,
miR-iab-4
and
miR-iab-8
(
Aravin
et al
., 2003
;
Ruby
et al
., 2007
). The primary
miR-iab-4
transcript has alternate polyadenylation sites, one of which excludes the
miRNA precursor, although differential expression of these isoforms has
not been reported (
Bender, 2008
;
Cumberledge
et al
., 1990
).
miR-iab-8
is
processed from a very long (at least 120kb) transcription unit that initiates
within the
iab-8
locus and extends proximally through the
abd-A
coding
sequence, thus encompassing nearly the entire
abd-A
-
Abd-B
intergenic
region (
Bae
et al
., 2002
;
Bender, 2008
;
Zhou
et al
., 1999
). Mature transcripts
processed from both arms of each miRNA have been detected; however,
